The Sunda Shelf area is surrounded by a number of island groups, characterized by a very complicated relief of the sea bottom, such in contradistinction to the flat continental shelf. We will designate this garland of islands as the circum-Sunda Archi- pelago. It consists of heterogeneous elements, which will be outlined in the following pages.
3.1. THE SIN-COWE REEFS IN THE SOUTH CHINA SEA
The southern part of the South China Sea belongs to the Sunda Shelf up to the connecting line between the most southern point of Cochin China and the border between Sarawak and British North Borneo. North of this line the depth increases abruptly to more than 1000 m (2000 to 4000 m), This, however, does not apply to a rather extensive platform of less than 1000 m depth, which occupies the eastern part of the South China Sea (West of Palawan). On this flat a great number of coral reefs are found, which group might be called the "Sin- Cowe Reefs" after an islet in their centre (VAN BEMMELEN, 1933 f, p. 894).
Only a few soundings are available of this area, which is very dangerous for navigation. The chart by VAN RIEL (1934Y,'reproduced in the "Atlas van Tropisch Nederland" (1938, map 3), accepts a depth of the seafloor of about 1000-2000 m, from which the coral islands rise steeply to the surface. However, the interpretation given by the chart of the gravity expeditions at sea by VENING MEINESZ (1934) seems to be more probable, accepting that the mean depth of this part of the South China Sea is less than 1000 m. See the isobaths on fig. 78, plate 8.
3.2. THE PHILIPPINE ARCHIPELAGO (See fig. 17, and fig. 18)
 |
| Fig. 17. Relief map of the Philippine Archipelago |
This group lies between 21 ° 8' and 4 ° 35' northern latitude, and between 116°55' and 124°37' eastern longitude. The total number of islands is 7,083; 466 of them have an area of more than 2 sq krn, and 2,441 have a proper .name. The rest is formed by small rocky islets emerging from the sea. FIG. 17. (Opposite to page 3.) Reproduction of the relief map of the Philippine Archipelago by. Coast- and Geodetic Survey. The total area amounts to 296,000 sq km, that is some what less than Great Britain (314,713 sq km). The principal islands with an area of more than 1000 sq km are:
C.2.1.
GENERAL OUTLINES
The outline of this archipelago is roughly triangular with its apex in the North (Batan Islands) and its basis between Borneo and Palmas Island. It is bounded to the West by the China Basin, which attains a depth of over 5000 m, and to the East by the Philippine Basin of about 6000 m depth. The East coast of Samar and Mindanao is skirted by the narrow furrow of the Philippine Deep, which is the deepest trough of the world (the Emden Deep, -10,830 m).
Broadly speaking the foundation structure of the Philippines is a raised part of the ocean floor, which begins South of Formosa (Taiwan), gradually rising and broadening via the Batan and Babuyan Islands to northern Luzon. The distance between the southernmost point of Taiwan and the northernmost island of the Philippines is only 145 km.
South of the line from the Lingayen Gulf to Dingalan Bay, the framework of the Archipelago becomes suddenly much wider ramifying into a number of branches, which connect it with the Indonesian Archipelago.
The island festoon of Lubang-Mindoro-Cala- mian-Palawan-Balabac joins on to the Kinabalu Range of North Borneo; the submarine Sulu ridge, which forms the pedestal of several islands, links the Zamboanga Peninsula of Mindanao with the Northeast peninsula of Borneo; finally, the Sangihe and Talaud ridges form the connecting links, respectively with Sulawesi and the Northern Moluccas. These ridges are separated by two large basins, the Sulu Basin (-5,580 m) and the Sulawesi Basin (-6,220 m).
Where these ridges branch off from the Philippines the framework is broken into a number of blocks, separated by smaller basins, straits, isthmian narrowings or structural valleys. The islands are not always individual blocks; they are in places connected by submerged platforms, such as the Visayan Shelf in the Central Philippines.
It is as if the top part of the great Philippine undation, where it becomes broader, breaks down into a number of blocks, until it is finally engulfed into the deep intervening basins of Sulu and Sulawesi.
Analysing the Philippine framework more in detail these very characteristic and important physiographical trendlines will become apparent.
3.2.2.
LUZON
Northern Luzon measures about 300 km from S to N and about 200 km from E to W. Within this area occur several structural units, namely a complex mountain range in the western part, the Cagayan Valley in the centre and the cordillera skirting the East coast.
The western mountain complex can be divided into three ranges, viz. the Malaya Range, the Central Range and the Polis Range (respectively ± 1800 m, 1800-2400 m, and .1500-2100 m high). The highest summit is Mt Pulog (2,930 m) in the Central Range. The Cagayan Valley, drained by the Cagayan River (ca 300 km long), is a structural downwarp between the western and eastern cordilleras. The Polis Range gently slopes eastward, whilst the eastern cordillera is a block or strip which is highest along the East coast and slopes westward to this median downwarp. In the northern part of this Cagayan trough lies the young-quaternary Cagua volcano (1,159 m), which belongs to a volcanic zone which can be traced northward to the volcanoes of the eastern Babuyan and the Batan Islands.
The Cagayan Valley rises gradually southward to the mountains of Adams or "Central Knot", which constitutes the southeastern part of the northern peninsula of Luzon. Here the transition to the plain of Central Luzon is affected by a steep morphological flexure, trending NW-SE. BAILY WILLIS (1937, p. 20) remarks that the profile of this southern termination of North Luzon is not one of an eroded fault scarp, but the one of a flexure. The summits of the mountains carry remnants of the tertiary mature land and the front has the same topographical character, though the surface bends down towards the plain. Only in the eastern section, from Dingalan Bay on the East coast to Bongabon, there is a distinct fault. Elsewhere, from Rizal to Damortis on the Lingayen Gulf, the front is a steep bend. The character of the mountain front is best explained by assuming that the bulging up of the mountains of northern Luzon and the subsidence of the Central plain are still in progress.
South of this Lingayen-Dingalan flexure, the basement foundation of the Philippines widens considerably, increasing from ± 200 km to more than 500 km.
Central Luzon. The western cordilleras of Central Luzon or Zambales Range are displaced some dozens of kolimetres to the West with respect to those of Northern Luzon 1). The Zambales Range is a tilted block with a high eastern edge and a more gradual westward slope. The eastern descent is probably a faultscarp. Its southern section is capped by a series of quaternary volcanoes, the youngest of which is the Mariveles volcano on the Bataan Peninsula.
The Central Plain, extending from the Lingayen Gulf to Manila Bay, is a subsiding block, covered by more than 300 m of shallow water and alluvial delta deposits, as is attested by the logs of artesian wells (FAUSTINO, 1931).
From this Central Plain rise several, deeply eroded, plio-pleistocene volcanoes, such as the Cabarnan Hills, Mt Balungao, Mt Amorong and Mt Bancay in its northern part, and the conspicuous Mt Arayat in its centre.
The Central Plain is the southern continuation of the median Cagayan downwarp, though wider and more deeply depressed than the latter. It is bounded to the East by the eastern cordilleras. At the transition from the plain to the eastern mountains ALVIR (1929) mapped some longitudinal and transverse faults, which are partly very young, e.g. those bordering the Mariquina graben. Movements along these faults caused probably the destructive earth- quake of June 3, 1863.
The eastern Coast Range stretches along the eastern side of the Central Plain for nearly 200 krn, extending from Laur, NE of Cabanatuan, to the head of the Bondoc Peninsula. It is from 40 to 50 km in width and has the cross section of a flat, dissected arch. On both sides the marginal slopes are steep rather than gentle, but the eastern is the steeper one and is most abrupt where it descends into the sea. The drainage pattern is that of rivers which grew upon a plain or mature land at relatively low altitude and were captured by vigorous, consequent streams as a result of uplift and uparching of the terrain (BAILEY WILLIS, 1937, p. 23-24).
The median depression of Central Luzon extends utheastward from Manila Bay to Tayabas Bay. The intervening stretch is occupied by the southeastern volcanic district, with the active Taal volcano in the caldera of Lake Bombon, and the Gonzales, Maquiling, Malaraya, and Banahao volcanoes.
This volcanic district is separated from the Eastern Cordilleras by the great lake of Laguna de Bay, which is perhaps a former extension of Manila Bay, separated from it by a slight arching up of the pleistocene tuff layers (Guadalupe Formation). This can-be clearly observed near Fort William Mc Kinley at the Pasig, which river maintained an antecedent course, draining the lake towards Manila Bay.
The main physiographic trendlines of North and Central Luzon are the flariks of the Luzon undation, formed by the zones of the Eastern and Western Cordilleras, and its top part, occupied by a median depression (Cagayan Valley-Central Plain). This central depression zone will be called "Manila Zone". Besides being a young downwarp on the top of the undation, it is characterized by young volcanic activity.
It will appear from the following analysis that the zones of the eastern and western cordilleras can be traced south- and southwestward to North Borneo, whilst the intervening Manila Zone ends in the Sulu trough.
SE-Luzon or Camerines Peninsula represents a new structural element, separated from the Eastern Cordilleras by the Strait of Polillo and Ragay Gulf and attached to the main land by a narrow isthmian lirik. This southeastern peninsula of Luzon forms part of the eastern Philippine rim.
Camerines Norte and Camerines Sur are connected with Polillo Island and Catanduanes Island by' a submerged platform, called the Polillo Shelf, which has a gradual slope into the oceanic depths. Along the inner, or southwestern side of this block another depression zone is found, separating it from the zone of the Eastern Cordilleras of the Luzon undation. This intervening depression might be called "Ragay Zone". Like the Manila Zone, it is characterized by young volcanic activity, such as Mt Labo in Camerines Norte and the NW-SE row of volcanoes: Isarog, Iriga, Malinao, Mayon, Pocdol, and Bulusan on the Bicol Peninsula.
The Ragay Zone, or "Philippine Fault Zone" as it was called by BAILEY WILLIS (1937, p. 38-40), extends from the Ragay Gulf past Ticao Island, through the Island of Leyte, and into the intermontane depressions of Mindanao. It ends in the Sulawesi Basin as well as in the narrow trough between the Sangihe and Taland Islands. The Ragay Zone is bounded on the West by the Zone of the Eastern Cordilleras of the Luzon Arc and on the East by the "Samar-Diuata Zone", which forms the eastern rim of the Philippines. Thus we see that the main structural belts of the Philippine framework can already be distinguished in the island of Luzon. They belong either to the Luzon Arc, which is convex to the East, extending from Taiwan to Borneo, or to the Samar Arc along its eastern side, extending between Polillo Island in the North to Palmas (or Miangas) Island in the South.
3.2.3.
LUZON ARC
The elements of the Luzon Arc can be traced from this island to North Borneo.
The Zone of the Western Cordilleras, forming its West flank, passes via the Lubang Islands and Mindoro to the platform carrying the Calamian Islands, Cuyo Islands, Palawan and Balabac. The trend of the Zambales Range is NNW -SSE, whilst the Palawan block has a NNE-SSW direction. Mindoro forms the pivot of this change of direction. North and South of this island deep tranverse, trough- like channels are found, viz. the Verde Passage and the Mindoro Strait. These troughs occur at the bending points of the general trendlines, interrupting the physiographic connections. However, the geological analysis of the Philippine structure in chapter V makes it probable that Mindoro forms a link in the zone of Western Cordilleras.
Mindoro is a high point in the structural arc. It is a great tilted block, the crest of which extends in a direction N 20° W from Mt Patrick to Mt Wood. From this crest line there is a long slope to the SW -coast and a short, steep slope toward the NE. At the base of the latter is a longitudinal valley in which the Aglubang and Rosanna rivers flow respectively NW and SE. It is, according to BAILEY WILLIS (1937, p. 29), obviously a structural feature and presumably a fault valley. In southern Mindoro an almost East-and-West trending fault was observed by ALVIR (1926), which marks the breakdown of this block to the Mindoro Strait. In rock character, in physiographical aspects, and in structural tilt this mountain bloc resembles the northern part of the Zambales Range. The northern part of Mindoro consists largely of volcanic flows from the Mt Halcon (2,587 m), Mt Calavite, and intermediate vents. These are all extinct, being presumably of plio-pleistocene age, and they might be considered as the continuation of the volcanic Mariveles chain along the inner side of the Zambales Range. This zone of extinct volcanoes can be traced to the Cuyo Islands situated on the eastern edge of the Palawan platform, which consists also of basalts and andesites.
A younger, probably subrecent row of submarine volcanoes is found, according to BAILEY WILLIS (1937, p. 8), between CagayanSulu and the Cagayanes Islands, but this belongs to the young volcanic activity of the Manila Zone, to be discussed here after. However, it is possible that this Cagayan belt of volcanism, forming the southwestward extension of the volcanic belt of Mariveles-Halcon-Calavite- Cuyo, has structurally the same meaning as the latter. It is probably related with the fault system along the eastern and southeastern edge of the zone of Western Cordilleras, along which the latter broke down to the Manila Zone.
The older formations and plutonic rocks of the basement complex occur in the western part of the zone of Western Cordilleras formed by the Zambales Range, Lubang Islands, Central and West Mindoro, Calamian, Palawan, and Balabac.
Whilst the Zambales-Mindoro section rises from the more than 4000 m deep floor of the South China Sea, the Palawan platform is skirted by a rather narrow trench with a maximum depth of -2,890 m, which separates it from the platform of the Sin Cowe Reefs, discussed in a preceding paragraph.
The Manila Zone of Luzon has been traced already to Tayabas Bay. Here the central part of the Philippines is reached with the Sibuyan Sea and the Visayan Sea. In this area the cross section of the Manila Zone increases from 75-100 km to 250- 300 km, whilst it assumes a much more complicated appearance. Instead of being a simple structural downwarp, it ramifies into a number of irregular channels and basins separated by raised blocks or partly submerged platforms. Between Mindoro and the Bondoc Peninsula of Luzon we find the Island of Marinduque, an elevated block, with a south- ward tilt. South of this island the Manila Zone splits into three or four branches.
A western branch of the Manila depression runs in a southern direction, from Tayabas Bay along the channel between Mindoro and the Tablas Islands, and West of Panay, ending in the Sulu Basin. It is generally over 1000 m in depth and has a width of about 50 km. Next comes a relatively elevated, sinuous ridge, on which are the islands of Sibuyan and Tablas and the western range of Panay.
The following branch of the Manila-depression zone comprises the basins of the Sibuyan Sea. The one between Sibuyan Island and Bondoc Peninsula is more than 2000 m deep and the other, extending between Sibuyan Island and Panay, has a depth of over 1000 m. This branch extends across the central plain of Panay, which is a rather simple, structural downwarp, filled by thousands of metres of miocene and younger sediments, comparable with the Cayan Valley in North Luzon. This branch of the Manila Zone also ends via the Panay Gulf in the Sulu Basin. East of it extends the Visayan platform, which carries the islands of Masbate, eastern Panay, Guimaras, Bantayan Islands and Negros.
Negros consists of two distinct parts. The northern part is formed by elevated tertiary sediments, capped by a young volcanic range with the cones of Silay, Mandalagan, and Canlaon (2,465 m), The southern part of the island is formed by two volcanoes: Mt Malapantao and Mt Magaso or Cuernos de Negros (1,903 m). These cones stand on the edge of the Visayan block, where it breaks down into the Sulu trough.
Masbate consists also of two distinct parts, viz. a NW-SE cordillera, at the northern end of which are found the NW -SE trending gold veins of the Aroroy district, and a NE-SW spur, pointing to eastern Panay. It is possible that only the latter belongs to the Visayan block in a stricter sense, whilst the NW-SE cordillera curves southward into the NNE-SSW trending narrow crustal slice of Cebu.
Cebu is separated from the Visayan block (viz. Negros) by a very narrow trench of more than 500 m depth. The southern end of this graben is filled by the volcanic cone of the Cuernos de Negros, which masks its transition into the Sulu Basin.
The easternmost branch of the Manila Zone follows the narrow channel between the Islands of Burias and Ticao on the NE and Masbate on the SW -side, then curves southward into the passage between Leyte and the northern part of Cebu, where it attains a depth of over 1000 m. It curves farther into a SSW-direction between Bohol and southern Cebu, passes along the young volcanic island of Siquijor, and ends, like the other branches of the Manila Zone, in the large Sulu trough.
It appears that the Manila Zone has a very sinuous course. In northern Luzon it is slightly convex to the East, whilst in Central Luzon it is convex to the SW. It then forms a great bulge to the East, considerably wider and rarified into four branches, separated by relatively high block-slices. Finally, it assumes a SW-direction in the Sulu Basin, where the entire belt has been engulfed to considerable depth. Especially the southeastern part of this basin is a remarkable trough, 200-250 km broad and more than 500 km long, with steep sub- marine walls. This trough is deepest (more than 5000 m) at its NE side near Negros and Zamboanga. From there the floor shoals to about 3000 m at its southwestern end, where it rises again steeply towards the Bornean Shelf.
The Sulu trough is bordered to the SE by the Zamboanga-Pangutarang ridge and to the NW by the Cagayan Sulu-Cagayan ridge, which carries probably some submarine volcanoes. The four branches of the Manila Zone all end in this trough.
The NW part of the basin, between the Cagayan ridge and Palawan is probably not a separate trough. but its floor can be considered as the more or less gradual slope of the Palawan platform towards the edge of the Cagayan ridge, where it plunges down a depth of 2000 m to 4000 metres. Therefore, this NW part of the Sulu Basin might still be reckoned with the zone of Western Cordilleras, which forms the West flank of the Luzon Arc.
The Zone of the Eastern Cordilleras forms the East flank of the Luzon Arc. The structural connections in the inner part of the Philippine Archipelago are difficult to unravel. The geological analysis indicates as a probable solution that the zone of the Eastern Cordilleras extends from the Bondoc Peninsula of Luzon along Burias and Ticao, and the eastern edge of Masbate to the northeastern peninsula of Leyte. It then curves south and south- westward via the Camotes Islands to Bohol.
Between Bohol and Zamboanga it is interrupted by the Mindanao Sea of more than 1500 m depth, but it reappears in the NW-SE trending block of Zamboanga. The latter is connected by way of the Sulu Archipelago with the Darvel Bay area of North Borneo. The Sulu Archipelago consists of two parallel ridges; one stretching from Zamboanga to the peninsula North of Darvel Bay and carrying the coral reef islands of the Pangutarang group; the other extending from the Sibuguey Peninsula via Basilan, Jolo, Tawitawi to the peninsula South of Darvel Bay. The latter has a young volcanic nature and belongs to the young volcanic belts of the Ragay-Zone, to be discussed later. The shoals of both ridges are separated by an intervening, narrow depression of nearly 600 m depth. This double ridge is a repetition of the situation found at the NW side of the Sulu Trough. There, the Cagayan ridge forms a volcanic belt parallel with the non-volcanic Palawan ridge, in the same way as here the Jolo ridge is a volcanic belt parallel with the non-volcanic Pangutarang ridge. The Cagayan Sulu volcanoes stand on the edge of the Sulu Trough, and the Tawitawi-Basilan row of volcanoes is built upon the steep and straight slope which bounds the great Sulawesi Basin at its NW side. Both volcanic belts are probably related with fault zones along the en- gulfed blocks which form the floor of these basins. The only difference is that the distance between the Palawan ridge and the NW-side of the Sulu Trough is 150-200 km, whilst the two parallel ridges at the NW side of the Sulawesi Basin lie closely together.
3.2.4.
SAMAR ARC
East of the Luzon undation, outlined in the preceding paragraphs, a new element is welded to the framework of the Philippine Archipelago, causing a widening of its basement structure. We will call this element the Samar Arc. It is separated from the Luzon Arc by a depression zone, called "Ragay Zone".
The Ragay Zone begins in Strait Polillo, crosses the isthmus of the Camerines Peninsula to the Ragay Gulf. BAILEY WILLIS (1937, p. 39) writes about this isthmian link as follows:
"At the head of the Ragay Gulf, between the Bondoc Peninsula and Camerines Norte there is a strip about 16 kilometres wide that is cut from NW to SE apparently by several vertical faults. One skirts the NE coast of Caluag Bay; another defines the NE coast of Alabati Island; a third is inferred in Lopez Bay. All three extend across the isthmian strip to Ragay Gulf, as is indicated by river valleys, and reappear in the sharp-cut fault coast of that depression" .
It extends farther southeastward along the NE- sides of Burias and Ticao to the Samar Sea and forms the sound between Biliran and Leyte at the head of Leyte Bay. Along the northeastern margin of this Ragay Zone, young quaternary volcanoes are found, which has been mentioned already in the description of SE-Luzon (Mayon volcano, etc.). This row extends southeastward into the young volcanic and solfataric Biliran Island and the central volcanic cordillera of Leyte with the Amandiung and Cabalian (see description by MUSPER & NEUMANN VAN PADANG, 1937; p. 76-77).
In southern Leyte the Ragay Zone curves southward and becomes much wider in cross-section. Here begins a splitting up into several branches, divided by elevated blocks, like the ramification of the Manila Zone South of Tayabas Bay.
A western branch skirts the Zone of Eastern Cordilleras between southern Leyte and Bohol, forming the western part of the Mindanao Sea between Camiguin and Siquyor and then narrows down into the Panguil Bay of Mindanao. It then crosses the isthmus of the Zamboanga Peninsula and ends in the Illana Bay, which is an extension of the Sulawesi Basin, having a depth of more than 2000 m. The northern portion of this branch is bordered on the East by the SW Peninsula of Leyte, and its southern portion by the raised blocks of Bukidnon and Lanao in Mindanao.
The middle branch starts from the Mindanao Sea East of Camiguin and passes southward along the structural valleys of the Tagoloan and upper Pulangi to the wide, triangular basin of the Pulangi, Here it bifurcates into a western branch, ending in the Illana Bay near Cotabato, and a southeastern one, ending in Sarangani Bay. Thus both branches end also in the Sulawesi Basin. The intervening stretch is occupied by the elevated block of the Tiruray plateau, which rises as a horst between the Pulangi Basin and the Sulawesi Trough. It is concave to the NW and presents a convex front to the adjoining Sulawesi Basin (-6,220 m).
This Pulangi Basin is a former sea arm, as is attested by widely distributed raised coral-reefs. It is skirted by two ranges of young quaternary volcanoes.
The northwestern one is convex to the SE. It begins with the Catarman volcano on Camiguin Island. The eastern margin of the Bukidnon plateau is little known, but IcKES gave a topographical sketch of the divide area between the Tagoloan and the Pulangi rivers (reproduced by SMITH, 1924, p. 209) on the western side of which is distinguished the Katunlund, an extinct volcano of an estimated elevation of 6000-7000 feet. South of it follows the also problematical "Calayo" volcano (MUSPER & NEUMANN VAN PADANG, 1937, p. 80-81; MUSPER, 1939, p. 42-43). Here the volcano zone assumes definitely a westward trend across the active Ragay volcano and the extinct Makaturing Cone. This volcanic belt stands on the edge of the Bukidnon and Lanao plateaux, where they break down to the Pulangi trough.
The other volcanic belt forms the eastern margin of this trough, with the cones of the Apo, Magolo, and Matutum (MUSPER & NEUMANN VAN PADANG, 1937, p. 81-82).
This belt extends by way of the Sarangani Bay to the Balut on the Sarangani Islands, being linked by a submarine ridge to the active volcanoes of the Sangihe Islands and those of the Minahasa in North Sulawesi.
The median Pulangi branch of the Ragay Zone is seperated by .a N-S range from the eastern branch. The latter is a structural downwarp, be- ginning in the deep trench East of the SW-penin- sula of Leyte; it passes along the eastern margin of the Mindanao Sea and then joins on to the NNW- SSE trending, structural valley of the Agusan River, where it traverses a low dividing ridge into the Tagum Valley, finally ending via the Davao Gulf into the Sangihe Trough.
The group of raised blocks and intervening depressions in Central Mindanao bears a close resemblance in topography and geology to the group of islands in the Central Philippines which comprises Panay, Negros, Cebu, and some smaller islands. Both occupy a widened part of a median depression zone, respectively the Manila Zone and the Ragay Zone. Both consist of raised blocks alternating with intervening graben structures. Both are the site of young volcanic activity. And finally, both break down at their SE-side into basins of 5000-6000 m depth (respectively the Sulu- and the Sulawesi Basin).
In lower-quaternary time Central Mindanao was still a group of islands separated by straits, and the sea regressed only in late quaternary time owing to a general uplift of this area.
The Samar-Diuata Zone. The eastern units of the Philippine Archipelago, namely Camerines, Samar and the Diuata Range of East Mindanao, form an arc which is bordered by the Philippine Deep. The waters between them are generally shallow, considerably less than 200 metres deep as a rule. It descends abruptly from the marginal ranges, about 500-1000 m in height, into the adjoining trench, which is 8000-10000 m deep. This Samar-Diuata Zone forms a long and narrow platform with a sligthly convex face toward the East, and concave along the inner side. Its total length is 1.200 km, extending from Polillo Island, North of the Camerines, to the southern tip of Mindanao 1). Its width is rarely more than 75 km.
This elevated eastern flank of the Samar Arc consists of two portions. The northern one is the Polillo block whichhas a WNW-ESE trend. Its elevated edge lies at its southern side, in Camerines Norte and Sur and Catanduanes, whilst its northern flank gradually slopes down to the ocean floor of more than 5000 m depth. The Philippine Deep terminates off its eastern side.
The southern part or Samar-Diuata Zone (sensu stricto) is an almost straight block, extending from the northern margin of Samar to the southern point of the Eastern Cordilleras of Mindanao. The WNW- ESE trend of the Polillo block abruptly changes into the NNW -SSE trend of the Samar block. This change of direction occurs East of the Bicol Pen- insula, where Batan and some other small islands are found. Here the outer arc has been largely engulfed, so that a deep branch of the Philippine Deep penetrates into the Lagonoy Gulf. Such transverse trenches occur also in the zone of the Western Cordilleras, where its general direction changes from N-S to NE-SW, viz. the Verde Passage and the Strait of Mindoro, at both sides of Mindoro.
The inner boundary of the Samar-Diuata block is a zone of faulting, along which the downwarp of the Ragay Zone has occurred. The Southwest coast of Samar Island is an obvious fault of vertical attitude, extending between and beyond Calbayog and Catbalogan. It strikes about NW-SE. The valley of the Ulut River trends NE from Maqueda Bay as a structural valley under a NW facing scarp from near Catbalogan to the NE coast, according to BAILEY WILLIS (1937, p. 36). This transverse fault through Samar meets the above mentioned fault along the SW coast approximately at a right angle.
South of Samar the platform of the eastern rim is submerged for a distance, except for the Islands of Dinagat and Siargao. Continuing southward the platform rises and becomes the eastern mountain range of Mindanao, a composite upland. Its northeastern section, known as the Diuata Range, is the extension of the range of Dinagat Island. It is a tilted and deeply dissected tableland, with a crest near the W -side presenting a steep slope towards the West and descending with a longer, gentler grade to the drowned East coast.
Lake Mainit is a depression in the western slope; it is probably a fault-trough and not a volcanic centre, according to MUSPER and NEUMANN VAN PADANG (1937, p. 79). The inner boundary of this block is an almost straight fault zone with a NNW-SSE trend, whilst the outer boundary is formed by a drowned coastline, as though it has been submerged by a tilt towards the adjoining Philippine Deep.
At the southern end of the eastern cordilleras of Mindanao a long and narrow block can be distinguished which extends along the East coast of the Davao Gulf, being separated from the main block of the Eastern Cordilleras by a narrow structural valley. This block might be interpreted as a slice, which broke off from the main block. It continues southward via a narrow submarine ridge, carrying Palmas (or Miangas) Island, into the non-volcanic Talaud Ridge of the Northern Moluccas.
3.2.5.
SUMMARY OF THE MAIN STRUCTURAL TRENDLINES
The foregoing analysis shows that the seemingly chaotic distribution of the Philippine Islands can be grouped according to some general structural trend- lines. The basement structure of this group of islands consists of two crustal waves, the Luzon Arc, extending from Taiwan tot Borneo, and the Samar Arc, extending from Camerines to Sulawesi. The latter arc represents a branch of the former, splitting off in SE Luzon. The Luzon Arc consists of two distinct portions. The northern one extends from Taiwan to the transverse flexure of Lingayen- Dingalan, and the southern one from this line to North Borneo.
The frame work of the Philippines is thus formed by both flanks of the Luzon Arc (viz. the zones of the Western- and Eastern Cordilleras) and a median downwarp on its top, called "Manila Zone". This Luzon Arc is separated from the Samar Arc by another zone of partial engulfment, called "Ragay Zone".
The Manila Zone ends in the Sulu Trough and the Ragay Zone in the Sulawesi Basin. The ridges connecting the Luzon- and Samar Arcs with the Indonesian Archipelago are double ridges, consisting of a volcanic- and a non-volcanic belt.
These are: 1) The non-volcanic Palawan ridge and the volcanic Cagayan Sulu ridge between the Sulu Trough and the South China Sea; 2) the non- volcanic Pangutarang ridge and the volcanic Jolo ridge 1) between the Sulu- and the Sulawesi Troughs, and finally 3) the non-volcanic Palmas-Talaud ridge and the volcanic Sarangani-Sangihe-Minahasa ridge between the Sulawesi Basin and the Philippine Deep.
Thus the Philippines represent an instance of the transition of a single arc with volcanic activity on its crest (namely the northern part of the Luzon Arc), into systems of double-arcs, the outer ones being non-volcanic, whilst the inner ones have a volcanic nature.
The volcanoes stand on the edges of deep basins, formed by the engulfment of the top parts of crustal waves.
3.2.6.
ACTUAL SHORE LINES
The outlines of the Philippine Archipelago are the result of considerable vertical movements, which continued up to the present time. The present appearance of the islands appears to be chiefly the result of tectonical movements, which blockfaulted and warped the crust, though the eustatic movements of the sealevel in glacial and post-glacial times too have largely influenced the actual shore lines.
Extensive peneplains formed during and before the pleistocene glaciations, submerged after the melting of the ice caps. These submerged platforms now form the shelf-seas of Polillo, Palawan, Visa- yan, Leyte-Samar-East Mindanao, and Zamboango- Sulu, which are about 50 metres deep, according to FAUSTINO (1926).
This eustatic rise of the sealevel caused the formation of barrier-reefs, atolls (in the Sulu Archipelago), and coral shoals.
FAUSTINO (1931) somewhat modified the definition for coral-shoals or drowned coral reefs, given by NIERMEYER. According to FAUSTINO they do not rise above sealevel, they are surrounded by water and they are not connected with neighbouring reefs. The coral-shoals of the Philippines are situated on the above mentioned platforms.
The eustatic rise of the sea level is also responsible for the drowned and deeply indented coastlines, in so far as these are not the result of tectonic subsidence and tilting of crustal blocks. Besides these relative subsidences there are also numerous indications for late quaternary uplift. FAUSTINO (1926 b) mentions the following instances:
In the Batan group, North of Luzon, FERGUSON reported in Sabtan Island on its western side, at an elevation of about 7 metres, a raised beach, while on its eastern side the shore rises towards the centra of the island in a rather irregular series of marine terraces. The highest terrace occurs at an elevation of about 180 metres. At its southern end there is in Batan a series of terraces reaching a maximum elevation of about 275 m.
In general, the whole western coast of Luzon is a shore line of emergence. Along the western coast of Ilocos Sur, Ilocos Norte, and La Union Province there are two or three well marked marine terraces, attaining an elevation of 75 to 90 metres in the coastal region South of the town of Vigan. On the peninsula which forms the western side of Lingayen Gulf there is found a well developed system of marine terraces. Along the western coast of Zambales, ridges of coralline limestone to an elevation of at least 100 metres are known. On the southern coast of Batangas at Malbrigo Point, fairly broadstepped terraces in regular succession rise to an elevation of 180 metres.
Extensive marine terraces are found on Panay, Negros and Masbate. In the latter island the higher terraces attain an approximate elevation of about 200 metres. Along the northern coast of Mindanao several benches to a maximum elevation of 360 metres have been observed.
MOODY suggested (in the Philippine J. of Sci., 25, 1924, 22) that the present island of Mindanao was divided into five islands in early pleistocene time and that successive uplifts were in the main responsible for the joining together of these islands and the formation of a much larger island area.
These young nsmg movements of the islands have been certainly accompanied by subsidences of the crust in the intervening channels and basins. The combination of these vertical movements of the crust and the eustatic oscillations of the sea level have resulted in the formation of the present configuration of the archipelago.
3.3. SULAWESI (See fig. 182)
This island has an area of about 172,000 sq km, and together with surrounding islands it measures about 188,000 sq km, being the third in area of the Larger Sunda Islands. But for some narrow stretches of coastal lowland and intermontane plains, Sulawesi in entirely occupied by mountainland, being one of the most mountainous of the larger islands of the archipelago.
Its typical outline, like a huge K, has long drawn the attention of geographers and geologists. The island consists of four narrow branches or arms, separated by deep gulfs and uniting in a central trunk. This outline has some resemblance to Halmaheira. The distribution of volcanic rocks of the Pacific suite in the western and northern parts of both islands, whilst basic and ultra-basic ophiolites are widely distributed in their eastern parts, causes also some geological analogy. Nevertheless, the geological analysis of both units in chapter V will show that this similarity is largely accidental, their orogenic evolution being entirely different.
Sulawesi is surrounded by deep basins and troughs. Sulawesi and Borneo are separated by the Makassar Trough, which is about 2000-2500 m deep (max. depth 2,717 m). Between Sulawesi and the Philippine Islands we find the Sulawesi Basin, about 5000-5500 m deep (max. depth 6,220 m). Between North Sulawesi and the northern Moluccas extends the Moluccan Sea, which has depths up to more than 4000 m. Between South Sulawesi and the southern Moluccas stretches the North Banda Basin, about 4500-5500 m deep (max. depth 5,750 m). Finally, between South Sulawesi and the Lesser Sunda Islands we find the western part of the South Banda Basin (about -4,500 m) and the Flores Deep (max. -5,140 m). On the other hand the mountain ranges of Sulawesi reach altitudes of over 3000 m (max. Mt Rantemario in the Latimodjong Range with a height of 3,440 m).
This shows that there is at present a considerable crustal relief in the Sulawesi area. with differences of altitude in adjoining areas amounting to 7000- 8000 m. In this respect the Sulawesi area shows some analogy to the Philippine Archipelago. Sulawesi is connected with the latter by the island festoon of Sangihe-Wawio-Sarangani.
In the chapter on the geological evolution it will be shown that indeed the Sulawesi Oro gene can be considered as the southern end of the East-Asiatic island festoons, namely of the Samar Arc of the Philippines. Only in the southern parts of the South- and Southeast arms of Sulawesi occurs a convergence with elements of the Sunda Mountain System.
3.3.1.
NORTH ARM
The North arm of Sulawesi has a sinuous outline. The eastern end, with a NE-SW trend, is the highly volcanic Minahasa area. This part joins on to the volcanic Sangihe Ridge which connects Sulawesi with Mindanao. The rise of the submarine Sangihe Ridge towards the Minahasa is probably accompanied by transverse faults, one along the Northeast coast. one from the Manado Bay to Kema (with the hotsprings of Airmadidi). and one from the Amurang Bay to Belang. The latter NW-SE fault forms the boundary between the young volcanic Minahasa area and the Gorontalo section of the North arm. Between this fault and the Ongkag Dumora River the main trend curves from NE-SW to E- W. In this transitional section still some isolated young volcanoes and solfataric action are found (Mt Lolombulan, Ambang Mts). The central part of the North arm has an East- to-West direction. Here the volcanism is entirely extinct. The width of this central or Gorontalo Section of the North arm ranges from 35 km in the middle to 110 km at its western end, where it is also highest (Mt Maling, 2.707 m).
It then suddenly narrows down to 30 km between the Dondo Bay at the North coast and Tinombo at the South coast. Here the general trend sweeps round from E-W via NE-SW to N-S.
The narrow isthmian stretch between Tinombo and Parigi is called the "Neck" of the North arm. This neck is about 20-40 km wide. It culminates in Mt Ogoamas (2,565 m) at its northern end, and in Mt Sidole (2,199 m) at its southern end.
The Gorontalo section is traversed by a longitudinal median depression. This stretches between the mountain ranges of the North coast (U-Mountains with Mt Tentolomatinan, 2,207 m; T-Mountains, 1,960 m), and those of the South coast (W- Mountains, Dapi Mts, Southern Mts East of Gorontalo, culminating in Mt Nunuka, 1, 606 m).
The median depression is formed by the valleys of the Paguat River, Randangan River, Pagujaman River, Lake Limboto, Bone River, Ongkag Dumoga River. We will call this longitudinal depression the "Limboto Zone".
The Limboto Zone can also be traced into the Minahasa, where its presence is indicated by Lake Tondano at the West foot of the Lembean Range. However, if such a median depression is present in the Minahasa, it is largely masked by young volcanic cones.
The North arm is separated from the East arm by the Gulf of Tomini or Gorontalo, which is about 100 km wide at its eastern end, broadening to 200 km in the West between Tomini and Poso. This gulf is a western extension of the Gorontalo Trough, which lies in front of the transitional part of the North arm between the Minahasa and the Gorontalo section (max. depth -4,180 m). The seafloor gradually shoals westward in the Gulf of Gorontalo, being less than 2000 m deep in its western part.
Between this broad western part of the Gulf of Gorontalo and the Gorontalo Trough in the East a median submarine ridge is found, which carries the Togian or Schildpad Islands. This ridge is at present subsiding; atols and barrier reefs are being formed on its top (UMBGROVE, 1939).
The higher central hills of these islands are presumably extinct volcanoes. Offside the Togian Ridge lies the active Una-Una volcano, rising steeply from the seafloor at -2000 m to about 500 m above sealevel.
The Togian Ridge branches off from the Bualemo Peninsula of the East arm of Sulawesi, being separated from the latter by the Gulf of Poh.
3.3.2.
EAST ARM
The East arm of Sulawesi has a ENE- WSW trend. Three parts can be distinguished. The Bualemo Peninsula at its eastern end (with the Balantak Mts 1,590 m) is separated from the central part by the isthmian narrowing between the Gulf of Poh and the Besama Bay. The central part of the East arm gradually increases in width from about 20 km in the East to 80 km in North Bunku. The central axis is formed by the Batui Mts with Mt Bulutumpu (2,400 m), which traverses this part diagonally with a NE-SW trend. The western part of the East arm stretches between the line Cape Api - Kolokolo Bay in the East and the line Lemoro - Tomori Bay in the West. Its width ranges from 75 to 100 km. It is a highly mountainous area. The highest summit lies in the Tokala Mts near the South coast (2,628); Mt Lumut in the North reaches an altitude of 2,280 m. This mountain-land is dissected by the complicated drainage system of the Bongka River, which rises on the Tokala Mts and empties near Bongka at the NE coast.
3.3.3.
BANGGAI ARCHWELAGO
The Banggai Archipelago lies off the eastern part of the East arm, being separated from it by the Strait of Peleng (-920 m, 15-30 km wide). The largest island of this group is Peleng. Geologically this group belongs to a more stable crustal belt which extends eastward via the Sula Islands to the SW-part of the Vogelkop of New Guinea, forming a barrier between the Northern- and Southern Moluccas.
3.3.4.
SOUTHEAST ARM
The Southeast arm of Sulawesi is about 100 km wide between the Usu Bay and the Tomori Bay. This isthmus forms the connection with the central trunk of the island.
In the Southeast arm also three parts can be distinguished. The northern part is situated between the Bight of Palopo (northern end of the Gulf of Bone) and the Gulf of Tolo or Tomaiki. It is occupied by the large peridotite massif of the Verbeek Mts (with Mt Salura, 1,102 m, as highest point). In its centre two graben are found, Lake Matano (surface 382 m above sea, depth 590 m) and Lake Towuti (surface 293 m above sealevel, depth 203 m).
The central part of the Southeast arm is much wider (max. 170 km). In its western part the crystalline schists of the Mekongga or Mengkoka Range rise to an altitude of 2,790 m, whilst in its eastern part peridotites and mesozoic sediments are found. The boundary between both areas is formed by the Tangeasinua Range which has a NW-SE direction. It is highest at its NW-end (Mt Tangkeleboke, 1,782 m) and its axis gradually plunges southeastward to Kendari. The wide basin between this range and the Mekongga Range is drained by the Konaweha River, which flows through an extensive alluvial plain in its lower course, before it empties North of Kendari. The East coast of the central section is formed by another low, NW-SE trending, southeastward plunging range. This belt gradually submerges, forming a number of bights and islets. It can be traced farther southeastward in the submarine relief via the Salabangka Islands to Manui Island. The basin between the Tangeasinua Range and the eastern coast range is drained by the Lasolo River.
The southern part of the Southeast arm is separated from the central part by an East-to-West depression, extending between Kendari and Kolaka and occupied by alluvial swampy plains. The southern section is occupied by an irregular hilly mountainland with more East-to- West trendlines, less than 1000 m high (Mt Mendoke, 981 m).
3.3.5.
BUTON ARCHIPELAGO AND TUKANG BESI ISLANDS
The Southeast arm of Sulawesi crumbles at its end >'into a number of islands, forming the Buton Archipelago.
Buton (or Butung), Muna, Kabaena and Wowoni are the larger islands of this group. They are .separated from the Southeast arm by narrow straits.
The islands form a rising anticlinorium, concave to the NW. The truncated neogene folds are locally capped by a carapace of pleistocene coral. reefs, which rise, for example, in South Buton WIth 14 terraces to an altitude of 703 m above the present sealevel (Mt Kontu).
From this Buton Archipelago submerged crustal blocks radiate in eastern, southeastern, southern, and southwestern directions. From Wowoni a submarine ridge plunges eastward to the floor of the North Banda Basin. Then follows a deep of 5,100 m. Next come the block-slices of the Tukang Besi group, which point from central Buton to the SE. The physiography of this interesting island group has been discussed by ESCHER, MOLENGRAAFF, RUITEN, HET ZEL and KUENEN. We will postpone its treatment to Chapter V.
The Buton trough stretches parallel with the Tukang Besi blocks, separating them from a more or less triangular block, carrying Hagedis Island (Batuata) and Kabia Island. The latter block has its apex off the southernmost point of Buton ann 1tS base s\61:ts tne western enu 0\ \ne 'i)O\l\'n Banda Basin. This area has a depth of about 2000 m. In Batuata raised young coral reefs occur to an altitude of 193 m above sealevel. From South- west Buton several ridges plunge southwestward via the small islands Kadatuang and Siumpa towards the Bone trough.
The orogenic meaning of this peculiar radial arrangement of crustal slices (some of which are rising in the present time, whilst the intervening blocks are engulfed to depths of thousands of metres) will be discussed in Chapter V.
3.3.6.
SOUTH ARM
The South arm of Sulawesi is connected to the trunk along a NE-SW line from Palopo to the Gulf of Mandar. However, from a geological point of view, the southwestern part of the trunk with the Quarles Mts can better be discussed in relation with the South arm. So we consider the northern part of the South arm as the area comprised between the SE-NW line from Palopo to the mouth of the Karama River at the West coast of the trunk on the one side, and the Tempe depression on the other side. The latter extends along the SE-NW line from the mouth of the Tjenrana River via Lake Tempe to the mouth of the Sadang River.
This northern part of the South arm is one of the most mountainous areas of Sulawesi.
The promontory between Madjene and Mamuuju shows S-to-N trending ridges of tertiary strata WIth a capping of raised coral reefs near. Madjene. It is further more characterized by leucite-bearing volcanic rocks (Cape William). Farthher East, the granite massif of the Quarles Mts rises to an altitude of 3,107 m. The eastern part of the Quarles Mts, with Mt Kalando (2,963 m), consists largely of tertiary volcanic rocks of andesitic comp~sit~on, invaded by intrusions of diorites afold gran?dlOntes. The Karua massif (over 2,500 m high) at Its southeastern side is a centre of eruption of large dacito- liparitic tuff-flows which fill the valleys, being in their turn incised by deep canyons. The Quarles Mts are separated from the Latimodjong ~ange by the Sadang Valley. Large righthand tnbutanes from the Quarles Mts are the Masupu and the Mamaso.
Between the Sadang Valley and the Gulf of Bone, the N-to-S trending Latimodjong Range rises to altitudes of over 3000 m (Mt Rante-Mario, 3,440 m; Mt Latimodjong ± 3,300 m).
The northern part of the South arm is separated from its southern part by a notable NW-SE depress- ion. This depression has been a sea-strait until late in the geological history, as is attested by the presence cl young clays with recent marine shells around lake Tempe. The surface of this lake is situated 9 m above sea level and its depth is only 2 m. Northeast of it is Lake Sidenreng and North of it the small Crocodile Lake. These lakes are drained by the Tjenrana River.
The southern part of the South arm of Sulawesi has a much smaller average elevation than the northern part. There can be distinguished a Western and an Eastern Divide Range with the intervening valley of the WaIanae River.
The Western Divide Range rises to altitudes of over 1000 m (Peak of Maros ± 1,377 m, Tonrong Krambu + 1,660 m, Bulu Laposo ± 1,270 m), The Eastern Divide Range or Bone Range is only about 800 m high.
Both ranges unite in the South in the Bontorilni Mts (± 800 m) with Mt BohongLangieng (;1: 1,973 m). The latter is a young-tertiary volcanic boss. This southern mountain complex, from which the WaIanae River flows northward, is dominated by the large volcanic cone of the Lompobatang or Peak of Bonthain (2,871 m), which still has a recognizable crater rim.
Off the Makassar coast lies the Spermonde shelf with numerous coral reefs, and off the Watampone coast we find another shelf with coral reefs. These shelf -seas finally break down to the Makassar Trough in the West and the Bone Trough in the East.
The belt of the Western Divide Range can be traced southwest and westward along the Postiljon and 'paternoster coral reefs to the Maria Reigersbergen reefs. The belt of the Eastern Divide or Bone Range extends southward and then eastward via Salajar (or Salayer), to Tanah Djampea and Kalao. Between these two diverging belts the Flores Basin is intercalated and has a triangular outline. It is over 5000 m deep at its base, formed by the E-to- W . trending Flores Deep. Northward it shoals to its apex, which lies at the South coast of the South arm of Sulawesi. The WaIanae Depression in the Southarm is presumably a northern extension of the Flores Basin, being separated from it by the capping massif of the Lompobatang volcano. In Chapter V the tectonic meaning of these physio- graphic trendlines will be discussed.
3.3.7.
CENTRAL SULAWESI
The four branches of Sulawesi unite in the central trunk. The central part has a wedgelike shape with its base at the West coast and pointing to the Tomori Bay and the Gulf of Tolo in the East.
It is bounded to the NE by a NW -SE line from Dongala via Parigi and Lemoro to the Tomori Bay, which separates it from the North and East arm. To the SE it is limited by a SW-NE line from Madjene via Palopo to Dongi at the Tomori Bay. This line separated the trunk from the South and Southeast arms. We have pointed out already that the southwestern part of Central Sulawesi (with the Quarles Mts) might be considered as a part of the South arm.
BROUWER (1930, 1934, 1941) distinguishes three structural belts with a N-to-S direction:
1.
The western belt, which might be called "Palu Zone", lies between the West coast and BROUWER'S "Median line". The latter is a structural boundary extending from Masamba in the South to Malakosa at the Gulf of Tomini, along the West side of the Tawaelia depression or graben.
2.
The central belt, which will be called "Poso Zone" in this discussion, stretches between the median line and a N to-S line from Lemoro at the Bight of Poso, via Peleru, to the Verbeek Mts of the Southeast arm.
3.
The eastern belt is bounded by the lines Lemoro- Peleru, Lemoro-Tomori Bay, and the Verbeek Mts. This third area we will call the "Kolonodale Zone", after the place of this name at the Tomori Fay.
In connection with the tectonical analysis in chapter V a slightly different grouping is preferred. From West to East the following physiographic units may be distinguished:
1a. The western coastal belt and foothills of the Molengraaff Mts, consisting of folded tertiary strata (Mamudju - Doda embayment.)
1b. The Molengraaff Range, which runs from Dongala in a SSE-direction to Masamba (highest summits Mt Waukara 3,122 m, Mt Kasenturu 2,855 m, Mt Kambuno 2,950 m).
1c, The "Fossa Sarasina", a narrow graben along the East side of the Molengraaff Range, extending from the Palu Bay in the North in a SSE-direction to the intermontane plain of Leboni, where it meets the Median Line.
1d. The X-Mountains with Mt Nokilalaki (3,311 m) between the Fossa Sarasina and the Median Line. North of Parigi these mountains join on to the neck of the North arm.
2a. The Tawaelia depression or graben with the median fault along its western side, running N-S from Malakosa to Masamba.
2b. The Fennema Range between the Tawaelia depression and the Poso depression.
2c. The Poso depression, which is concave to the East, formed by the valley of the Poso River, Lake Poso (surface 510 m above sea, depth at least 440 m) and the valley of the Kodina River. The Takolekadju Mts (1,637 m) are a relatively high part of this structural depression.
2d. The Kruyt Mts and the Wanaripalu Mts, East of the Poso depression, 1300-1400 m high. 2e. The Pompangeo Range, a structural culmination (Mt Kajoga 2,563 m) between Lemoro and Majumba.
3a. The East foot of the Pompangeo Range is formed by a complex belt in which rocks of the Poso Zone (2) and the Kolonodale Zone (3) are tectonically mixed. This "Peleru subzone" extends between Lemoro and Peleru.
3b. The Era depression, extending from the plain of Malino southward along the valley of the Ban River to Era, and from there to Kolaka (in Central Sulawesi). It is characterized by extensive outcrops of mesozoic sediments.
3c. The hilly upland between the Era depression and the Tomori Bay, which comprises the Peleru- and Towi Mts, consisting largely of mesozoic sediments, and the peridotite and serpentine belt of the Makaleke- and Tiu Rivers to Mandowe.
3d. The depression, separating the trunk from the East arm, formed by the valley of the Sumara River and the Tomori Bay.
These structural elements can be grouped into larger units in the following way:
3.4. HALMAHERA AND BANDA ARC (MOLUCCAS or MALUKU)
The name Moluccas was first used by the Portuguese for the Spice Islands between Sulawesi and New Guinea. To these islands belong Halmaheira, Ternate, Tidore, Obi, Sula, Ceram, Buru, Ambon, and Banda. The name will be used in this book in a wider sense, comprising the whole complex of island-groups and -festoons, bordered by the Philippines in the North, New Guinea in the East, Australia in the Southeast, the Lesser Sunda Islands in the Southwest and Sulawesi in the West.
It is an area of considerable relief with alternating basins and ridges, in which the process of mountain-building is very active at present. The Northern Moluccas are partly connected with the East-Asiatic island festoons and partly with the Melanesian System, whilst the Southern Moluccas (or Banda Arcs) form a section of the Sunda Mountain System. As dividing barrier between the Northern and the Southern Moluccas we may consider the E-and-W trending ridge, which extends from the East arm of Sulawesi to the Vogelkop of New Guinea via the Banggai Archipelago, the Sula Islands, Gomumu (South of Obi) and Misool. The connection between the Sula Ridge and Misool is less pronounced. The axis plunges East of Mangola to the circa 2000 m deep threshold in the Strait of Lifamatola, which separates the Mangole Basin and the Buru Basin. It then rises again in a WoE trending ridge South of Obi major, which carries the Island of Gomumu. This narrow submarine ridge extends farther eastward, forming a barrier between the small basin South of Tobalai 1) (-2,080 m) and the eastern part of the Buru Basin. Finally it rises to the western edge of the shelf sea of New Guinea which carries the Island of Misool.
In the physiographic description of New Guinea it will be pointed out that Misool occupies the western end of a ridge which skirts the foredeep of the Banda Arcs and then curves eastward into the Merauke Ridge. This threshold between the Northern and the Southern Moluccas forms in geo- tectonic sense the dividing barrier between the oro- genic systems around the western Pacific, and the Sunda Mountain System, which belongs to the Tethys geosyncline.
Therefore, we will discuss separately the trendlines of the Northern- and Southern Moluccas.
3.4.1.
NORTHERN MOLUCCAS (See {jg. 19)
The Northern Moluccas form the connecting link between the Philippines in the North, New Guinea in the East and Sulawesi in the West. They are composed of a complex of submarine ridges and platforms carrying island-festoons and island-groups, separated by comparatively small basins and troughs. The latter are generally 2000-4000 metres in depth, and the mean level of this area may be roughly estimated at -1,500 m. The Northern Moluccas have approximately a triangular outline. The comers of the triangle are connected with more extensive land areas (viz. Mindanao, New Guinea, and Sulawesi).
Its sides are skirted by deep troughs, respectively, the Philippine Deep of 6000-9000 m depth along its NE-side, the Ceram Sea (-5,319 m) and the north- western Banda Basin (-5,800 m) along its southern side. and the Sulawesi Basin (-6.220 m) along its NW-side.
It thus appears that the Northern Moluccas form a strongly warped part of the crust which stands on theaverage some thousands of metres above the surrounding downwarps.
We will now consider the physiographic trendlines of this area more in detail.
The submarine ridge. connecting the southern- most point of Mindanao with the Minahasa (North- arm of Sulawesi) carries the volcanic Sarangani Islands (belonging to the Philippines). the Kawio Islands (a number of small coral reefs), and the volcanic Sangihe Islands. It is, therefore, a volcanic island-festoon, called the Sangihe Ridge, which connects the volcanic Ragay Zone with the volcanic North arm of Sulawesi. Next comes a depressed belt, extending from the Davao Gulf of Mindanao southward via the Sangihe Trough (-3,800 m) and via a narrow trench of 2500-3000 m depth to the Gorontalo Basin (-4,180 m).
The latter bends westward into the Gulf of Tomini, which separated the North- and the North- east arm of Sulawesi.
The following unit is again a crustal upwarp, which has a rather complicated relief. The Samar- Diuata Zone of the Philippine Islands, forming the eastern cordilleras of Mindanao, plunges southward into the Sangihe Trough and disappears as a physiographic unit at about 5t 0 N, latitude. It is, however, connected by a narrow ridge, carrying Palmas or Miangas Island, with the platform of the Talaud and Nanusa Islands. This ridge forms a threshold between the Philippine Deep and the Sangihe Trough.
The Samar-Diuata belt has a position "en echelon" with respect to the Talaud-Maju belt, outlined here under.
The Talaud platform joins on to a 75 km broad crustal upwarp, which extends southward in the bottom configuration of the Moluccan Sea. This upwarp might be called the Maju Ridge, after the islet of Maju in its centre. It is composed of several parallel ridges, giving it the appearance of an anticlinorium.
There are two axial depressions in it, one South of the Talaud Islands (between the Sangihe and the Morotai Basins), and the other near its southern end (between the Gorontalo and the Batjan Basins). The axial culmination lies in its middle part, between Manado and Ternate. The section through the Maju Ridge at this place is composed of the following units, from West to East: First the marginal trench, over 2500 m in depth, then a ridge of about 1200 metres below sealevel; this is separated by a trench of more than 2000 m depth from two interchanging ridges which carry the islets Maju or Mojau and Tifore or Tofure; next a trench, over 2500 m in depth, paralleled to the East by a sub- marine ridge of about 1500 m depth; finally, the sea floor plunges down to the Ternate Trough, which reaches a depth of 3200 m halfway between Maju and Ternate.
These ridges plunge southward to an axial depression, which is more than 2000 m in depth. Southward the sea floor rises again to a culmination, little over 1500 m in depth.
The Maju Ridge is terminated at its southern end by the E-W trending Mangole Basin (-3.510 m), which separates it from the Sula barrier. An indistinct threshold between the Mangole Basin and the Gorontalo Basin runs southwestward connecting the Maju Ridge with the East arm of Sulawesi. Another submarine threshold, between the Mangole Basin and the Batjan Basin, connects this ridge with the Obi group.
The Snellius Ridge forms a problematical part of the Talaud-Maju Ridge. It is a crustal upwarp along the southern end of the Philippine Deep (-8,710 m), rising to 360 m below sealevel. The SneIIius Ridge is separated from the Talaud Islands by the Talaud Trough (3,410 m), and from Morotai and the northern peninsula of Halmaheira by the Morotai Basin (-3,890 m), This submarine SneIIius Ridge extends northwestward from the northern end of Morotai and disappears as a distinct feature of the seafloor off the Nanusa Islands.
It looks like the reflected image of a similar rise of the sea floor, skirting this part of the Philippine Deep along its eastern side. As such it is a feature related with this Deep; the edges of the bordering crustal parts appear to be slightly turned up. The central culmination of the SneIIius Ridge is connected by a ridge of more than 2000 m depth with the Talaud-Maju Ridge. This connecting link is a distinct threshold between the Talaud and the Morotai Troughs. The central cuhnination of the SneIIius Ridge might, therefore, be considered also as a branch of the Talaud-Maju Ridge.
The complex rise of the floor of the Moluccan Sea is bordered along its eastern side by a down- warp, extending from the Morotai Basin (-3,890 m), by way of the Ternate Trough (-3,450 m) to the Batjan Basin (-4,810 m). An eastward branch of the latter, 1000-2000 m in depth, separates Batjan from Obi.
Next comes again a relatively high portion of the crust, carrying Halmaheira and the neighbouring is- lands. This northeastern part of the Moluccas, lying between the Moluccan Sea and the Carolinan Basin and joining on to the Vogelkop of New Guinea, will be called the Halmaheira group of islands. In its central part we find the Halmaheira Sea (-2,039 m).
Halmaheira is the largest island of the Moluccas (about 18.000 sq km). The form of this island resembles that of Sulawesi, but on a reduced scale. Its diameters are about one third of those of Sulawesi and its surface is about one tenth. Like Sulawesi it has fplJr arms, forming a capital K. The interjacent bays are the Kau Bay, the Buli Bay, and the Weda Bay.
The Kau Bay ends in a peculiar circular depression. some 500 m deep and 60 by 30 km in diameter. It is separated from the open sea by a broad threshold which exceeds nowhere 50 m in depth.
Morotai, off the northern peninsula, is largely composed of neogene volcanic rocks. Active volcanoes are found on the northern peninsula of Halmaheira. The highest cone is the Gamkonora 0,653 m), which is also the highest summit of Halmaheira. The most active volcano is at present the Dukono 0,335 m) near Tobelo.
This young volcanic chain is continued in the islets of the West coast of the main island: Hiri, Ternate, Tidore, Mare, Moti, Makian.
Makian is the southernmost volcano of this belt which has had eruptions in historical time. But young volcanic rocks are also known in the zone which curves eastward via Batjan to Kofiau (Kajoa, GuraItji Islands, Waidoba, Taneti, Latalata, Kasiruta, Mendioli, Batjan, Woka, Salo Islands, with Djoronga, Kekik, Lawin, Pisang, Kofiau).
This volcanic zone seems to extend farther east- ward via northern Salawati to the mountain range along the North coast of the Vogelkop, in which also young neogene and quaternary volcanic rocks are found (Tamrau Range and Arfak Range with Umsini volcano).
Thus we see that the Halmaheira group is bordered at its western and southern side by a belt which contains young neogene-quaternary volcanic rocks. It is strongly convex to the West and South. Active volcanoes are only found in its middle part, from Tobelo to Makian. We will call this volcanic belt the Temate Zone.
The Ternate Zone is separated from the inner part of the Halmaheira group by a discontinuous, depressed zone, consisting of the following elements: Kau Bay-Kau depression; Pajahe Bay; Patientie or
Patinti Strait (-2.048 m); the strait between the southernmost point of Halmaheira and Damar; the southern end of the Halmaheira Basin (1300-1400 m deep); the strait between Jef Doif and Kofiau, ending either in the Sagewin Strait between Batanta and Salawati or in the Dampier Strait between Batanta and Waigeo.
The islands of the central part of the Halmaheira group consist largely of a basement complex of basic and ultrabasic rocks, covered by marine tertiary strata, rich in detritus of igneous rocks. It is bordered to the Northeast by the southern end of the Philippine Deep, whilst it is limited to the West and South by the Ternate Zone.
The largest land-units of the inner part of the Halmaheira group are the arms of Halmaheira East and South of the Kau cauldron, and further the islands of Gebe, Waigeo, and Batanta. The islands between Halmaheira and the Vogelkop of New Guinea are called the Radja Ampat group. Salawati belongs also to this group, but physiographically it forms a part of theVogelkop area, being situated, like Misool, on the Vogelkop Shelf, and separated from Batanta by the narrow (4 km wide), trough-like Sagewin Strait (over 200 m deep). This strait is presumably a young graben, comparable with a similar graben farther East, on the NW edge of the Vogelkop, which is occupied by the Valley of the Warsamson.
The small Aju- and Asia Islands form a spur of the Halmaheira group which extends into the Carolinan Basin.
The centre of the Halmaheira group is a broad downwarp, viz. the Halmaheira Basin, which reaches a depth of more than 2000 m. General structural picture of the Northern Moluccas.
After this analysis of the physiographical trend- lines in the Northern Moluccas we will unite them again in a general picture.
The Northern Moluccas are formed by two converging systems of ridges, one bordering the Sulawesi Basin, being convex to the East. and the other skirting the central part of the Halmaheira group, being convex to the West. We will call them respectively the Sangihe System and the Ternate System. The Sangihe System is composed of the following units:
a.
Backdeep: Sulawesi Basin;
b.
Volcanic inner arc: Sangihe Ridge;
c.
Interdeep: Sangihe-Gorontalo Troughs;
d.
Non-volcanic outer arc: Talaud-Maju Ridge.
This system forms the connecting link between the Samar arc in the Philippines and the North and East arms of Sulawesi.
The Temate System consists of the following elements:
a.
Backdeep: general part of the Halmaheira group. only partially engulfed (Halmaheira Basin);
b.
Volcanic inner arc: Ternate Zone;
c.
Interdeep: Morotai-Ternate-Batjan Troughs;
d.
Non-volcanic outer arc: Snellius-Maju-Obi Ridge.
It appears that in the Maju Ridge of the Central Moluccan Sea both systems coincide, having a mutual outer arc. This is a fact of geotectonic importance. which will be amply dicussed in Chapter V.
3.4.2.
SOUTHERN MOLUCCAS OR BANDA ARCS (See fig. 20)
The Central Banda Basin is skirted by two parallel arcs; the inner one is crowned by active volcanoes. Where as the outer one is free of young volcanism. This Banda area clearly represents a coherent. orogenic system. which has often been considered as an example of a mountain system in statu nascendi. 1) (See Chapter V B).
The Banda Basin consists of a northern and a southern part. The first or"Northem Banda Basin" lies between Sulawesi and Buru, the second or "Southern Banda Basin" between Batu Tara (North of Lomblen) in the West and Manuk in the East. (See also the physiographic sketchmap of the Northern Banda Basin. fig. 187.)
The Southern Banda Basin. in its turn. is divided (by the Api volcano) into a western and an eastern section. The latter is surrounded by the Banda Arcs and might be called "Central Banda Basin". We will start the physiographical description of the Southern Moluccas from this centre.
The Central Banda Basin has a diameter of 400 km between Damar and Buru (SE-NW) and also between Api and Banda (SW-NE).
In the northern part of the Central Banda Basin some complicated SW-NE swells are found. The Luymes and Siboga Ridges do not reach the sea- level; only some coral reefs of the Lucipara- and Schildpad Islands emerge. Between the Luymes Ridge and Buru a depth of 5.330 m is attained; the floor of its southern part is about 5000 m deep. with a maximum of -5,400 m West of Damar.
In the western part of the Southern Banda Basin (see fig. 187) the Api Volcano (282 m) rises from a flat floor of about 4500 m depth. This western deep-sea platform ramifies west- and north- westward into a number of trenches. The main one parallels the inner arc from Alor westward. across a rise of -2,480 m North of Flores. to the Flores Deep (-5.130 m). The next branch curves. steadily decreasing in depth. into the Gulf of Bone between the South and Southeast arm of Sulawesi. A smaller one leads (across a rise to -3.850 m) to the Buton Trough (-4.180 m). Finally, a number of ill-defined. NW -SE trenches across the rise between the Tukang Besi Islands and the Luymes Ridge connect the western part of the Southern Banda Basin with the northern one.
The Northern Banda Basin. like the central part has a diameter of about 400 km. Its maximum depth is -5.800 m.
The Central Banda Basin is bordered on its southern. eastern and northern sides by the Banda Inner Arc. This arc consists of a number of ridges. which have an "en echelon" orientation according to the maps of the Snellius Expedition ").
In the Southwest this inner arc is not the direct continuation of the inner row of the Lesser Sunda Islands. The axis of the latter geanticlinal elevation of the crust plunges from Wetar via Romang east- ward to the submarine ridge between Damar and Moa, finally ending in the Weber Deep. The volcanic inner arc of the Banda System has an "en echelon" position with respect to this belt. First comes the SW-NE trending Damar Ridge. crowned by the volcanoes of Damar (868 m), Teon (655 m), Nila (781 m), and Serua (641 m). This ridge plunges northward, and separated from it by a trench of more than 3000 m deep we find the S-N trending Manuk Ridge (Manuk, 285 m). The latter is separated in turn from the Banda Dome (Api, 656 m) by a depth of more than 4000 m. A southeastern spur of the Banda group plunges down into the Weber Trough. while a northwestern one curves westward. ending South of the Uliassers and Ambon. Thus we see that the inner arc consists of some ridges and dome-like culminations which have an "en echelon" arrangement. This interchanging position occurs in that part of the geanticline, where its general trend shows an intensive curvature; from an East-to-West trend in the inner arc of the Lesser Sunda Islands it sweeps round to the NE and N. and finally back to the NW and W. In Ambon the direction becomes even ENE to WSW. extending (by way of the submerged volcano(?) in the Manipa Basin) to Amblau.
Between the Banda Inner- and Outer Arc an "interdeep" occurs, with a typical crescentic out- line bulging eastward, called Weber Deep (-7,440 m, max. width 150 km). It shoals northwestward finally passing into the rising axis of the ridge which carries the Uliassers and Ambon; and it becomes also shallower in a SW direction, joining on to the submarine ridge between Damar and Moa. The Weber Deep is separated from the Wetar Basin by this threshold of 1,480 m depth. The island of Kisar farther West is one of the very few islands occupying an intermediate position between the inner- and outer arc.
The Banda Outer Arc is a geanticlinal uplift of the crust, 100-200 km broad, in which geosyn- clinal deposits have been elevated into mountain ranges with overthrust structures, but without active volcanism (such in contrast to the young volcanic composition of the islands of the inner arc).
In Ceram the mean elevation above the fore- deep is about 5000 m and that above the inter- deep amounts to about 6500 m. In the eastern or Kai section these differences of mean altitude are respectively 4500 m and 7500 m. This is in rather close accordance with the elevation of the Ceram sector, notwithstanding the fact that the latter shows a mountain range up to 3000 m height, while the Kai group reaches only 800 m above sealevel.
The southern section of the Banda Outer Arc forms the continuation of the outer arc of the Lesser Sunda Islands. It begins East of Timor with the narrow Leti-Sermata Ridge. Then follows the Babar culmination from which low spurs radiate in several directions. The northeastern one plunges down into the Weber Trough and the southwestern one into the Timor foredeep; some smaller ones point to the Wand NW, while a broad rise of the seafloor unites the Babar Dome with the Tanimbar group. The general trend of the latter is SW and NE, whereas that between Leti and Sermata is more Wand E. This curvature is marked by an "en echelon" arrangement of the ridges, which also characterizes the inner arc.
The eastern sector of the Banda arcs comprises the Tanimbar-Kai (or Ewab) Islands. It has a steep inner slope to the Weber Trough. The width of the geanticline is 100 km in the Tanimbar group and increases to 200 km in the Kai Islands, narrowing again to 75 km in the SE-NW stretching submarine ridge which forms the link with Ceram. On the whole this eastern sector shows an eastward bulge like the crescentic Weber Deep. This broadening is accentuated by the presence of a rise of the seafloor (less than 1000 m deep) in the Aru foredeep, East of the Kai Group. .
Along the crest of this broad geanticline a longitudinal depression occurs, the width of which increases with the cross section of the geanticlinal arch. In the Tanimbar Group the median depression measures some dozens of kilometres across, widening to about 100 km in the Kai Group (between the "Drie Gebroeders" and Nuhutjut), then narrowing again to the Masiwang-Bobot graben of East Ceram. The eastern section of the outer arc is divided into an inner and an outer zone by this belt of relative subsidence on its crest. The inner zone runs from Wuliaru (188 m) along Molu (274 m) to the "Drie Gebroeders" and Kur (423 m), and farther across Tiur or Tioor (376 m), Kasiui (362 m), Watubela, Manawoke, Pulu Pandjang, and Ceram Laut with Geser, to the southeastern spur of Ceram. This garland of islets skirts the interdeep along its eastern side. The outer zone can be traced from Selaru via Jamdena and Sofiani to Nuhutjut or Great Kai. Its connection with Northeast Ceram is less clear, because here this outer festoon is interrupted by a westward extension of the foredeep (viz. Aru Trough).
The northern section of the Banda Outer Arc comprises the islands Ceram, Boano, Kellang, Manipa, and Buru.
Seram is the largest island of the Banda Outer Arc (17,152 sq km, that is more than half of the size of the Netherlands; 340 km long, about 70 km broad; highest summit Binaja 3,055 m). The median depression of the Banda Outer Arc is represented by the Masiwang-Bobot Graben in East Ceram, which can probably be traced westward along the series of depressions: Teluti Bay - Kawa Valley - Ruatan Valley - EIQa- putih Bay - Tala Valley. The inner festoon of the eastern section continues in Ceram across the low mountains South of the Masiwang - Bobot Graben (723 m), the Y-Mountain-range, and the Z- or Wallace Range (1,260 m). The outer garland is representend in Ceram by the ESE to WNW trending X- Mountains (Binaja 3,055 m) and the ENE to WSW trending Lumute Mts 1,373 m).
West of the Piru Bay the structural pattern of Ceram becomes more irregular. The Hoamoal Peninsula is bordered by more or less N-S directed faults.
The islands of Boano, Kellang (Kelang) and Manipa form a NE-SW row between Ceram and Buru. On the southern tip of Hoamoal and on Kellang young volcanic rocks, like those of Ambon, are found. Most geographers and geologists trace the western end of this northern section of the Banda Outer Arc from Ceram via the islets of Kellang and Manipa to the large islands of Buru.
Buru (9,599 sq km; 140 km long and 90 km broad; highest summit Kau Palatmada, 2,429 m).
The physiographic structure of Buru is less clear than that of Ceram. Three mountain blocks can be distinguished, separated by structural valleys. The western massif with the Kau Palatmada is over 2000 m high. It is bordered to the East by the NNE-SSW depression of Nibe River - Lake Rana - Wala River. The central block rises to about 1000 m, being situated between the above mentioned structural valley and an ENE-WSW depression formed by the Kajeli Bay and the Apu Valley. The southeastern block is formed by the Walua Range, which has an ENE-WSW trend, reaching an altitude of 1,731 m in Mt Batakbual. This range is separated from the row of Manipa, Kelang and Boano by the northern part of the Manipa Basin.
Buru forms a domelike elevation of the crust, which is surrounded by four compensative basins:
a.
the Manipa Basin, Southeast of Buru, 4,360 m deep, with a cone shaped elevation in its centre, presumably a submerged or submarine volcano;
b.
a basin between Buru and the Luymes Ridge, -5,330 m;
c.
the northern Banda Basin, reaching a depth of 5,290 m West of Buru;
d.
the Buru Basin, North of this island, with a maximum depth of 5,319 m.
The northwestern corner of the island is connected by a distinct, though locally more than 3000 m deep, submarine ridge with Sanana of the Sula Islands. The southwestern corner is connected with the. Luymes Ridge by a more than 3000 m deep rise of the sea floor.
Foredeep of the Banda Arcs. The Banda Outer Arc is skirted by a typical foredeep, which begins SE of the Tanimbar Islands with a narrow trough (30 km wide and 1,690 m deep), opening north card into the sickle-shaped Aru Trough (-3,680 m). Here the crescentic shape of the Weber Trough, convex to the East, is repeated in a still more pronounced form.
At its concave western side a rise of the seafloor to -530 m (East of Kai) interrupts its regular outline, and also at its northern side the Kumawa promontory of the Vogelkop of New Guinea with Adi Island form an inward bulge in the outline of the Aru Trough.
The foredeep of the NE- and N-sector of the Banda Outer Arc is formed by the Ceram Sea, a geosyncline of about 80 km width and more than 2000 m depth. Westward it joins on to the Buru Trough which has a depth of 5,319 m.
3.5.LESSER SUNDA ISLANDS (See figures 187, 212, 214)
These islands are situated on two geanticlinal belts, which form the westward extension of the Banda Arcs. The inner (i.e. northern) geanticline carries from East to West the islands Roma(ng), Wetar, Kambing, Alar (or Ombai), Pantar, Lomblen, Solor, Adonara, Flores, Rintja, Komodo, Sumbawa, Lombok, and Bali. The outer (i.e. southern) arc is formed by the islands Timor, Semau, Roti, Sawu (or Savu) Raidjua, and Dana. The geanticlinal ridge bifurcates in the Sawu area. One branch forms a spur, which plunges westward across Raidjua and Dana, pointing towards the submarine ridge in the trough South of Java; the other branch forms a connecting link with the inner arc across the island of Sumba. In this subchapter will be described successively the physiographical features of the Backdeep, the Inner Are, the Interdeep. the Outer Arc, and the Foredeep of the section of the Sunda Mountain System represented by the Lesser Sunda Islands.
3.5.1.
BACKDEEP
East of Flores the back deep of the Lesser Sunda Islands is formed by the western part of the Southern Banda Basin, which has been described with the Southern Moluccas. North of Flores and Sumbawa extends the Flores Sea. This sea consists of three parts:
a.
The NW Flores Sea is the broad and shallow platform which connects the South Arm of Sulawesi with the Sunda Shelf.
b.
The Central Flores Basin has a triangular form, its apex lying South of the Lompobatang volcano, which masks its supposed connection with the Walanae Depression of the South arm of Sulawesi, while its base along the North coast of Flores forms its deepest part (-5,140 m).
c.
The East Flores Sea comprises the ridges and interjacent troughs 1), which connect the South arm of Sulawesi with the submarine Batu Tara Ridge.
North of Bali and Lombok the back deep is formed by the Bali Sea (about 100 km wide and about 1500 m deep). Westward its floor gradually rises, till it joins on to the shelf sea in the Strait of Madura (about 100 m deep).
3.5.2.
INNER ARC
The Inner Arc of the Lesser Sunda Islands connects Java with the Banda Inner Arc, thus forming a sector of the long belt of fertile volcanic islands, which "winds around the equator like a girdle of emeralds" (a metaphor given by the author "Multatuli"). The Lesser Sunda Islands of the inner arc are all situated on a geanticlinal ridge, which has a width of about 100 km in its western end, gradually diminishing eastward (especially from E Flores) to about 40 km.
The straits between the islands are shallow in the western part, becoming deeper eastward.
Bali is separated from Java by the narrow Strait Bali, which originated, according to Hindu records, in 280 A. D. (STUTTERHEIM, 1922, pag. 22, Note 1). In this island the zones, distinguished in Java, can be retraced. The northern and largest part of the island consists of quaternary and active volcanoes, representing the continuation of the young volcanic complexes in the Solo Zone of Java. The fertile plain of Denpasar, extending on the southern foot of these volcanoes, belongs to the Blitar Subzone of Java. This plain is connected by an isthmian narrowing with the tertiary limestone hills of the "Tafelhoek" (213 m), which are comparable with those of the Blambangan Peninsula of Java (Southern Mountains). The same can be said of the islet of Nusa Penida (529 m) between Bali and Lombok.
Lombok. The general structure of the island is similar to that of Bali: in the North a volcanic zone with the active Rindjani (Solo Zone); then the lowland plain of Mataram (Blitar Subzone); in the South, the "Southern Mountains" with tertiary limestones and volcanic breccias (Maredje, 716 m).
Sumbawa. The physiographical trendlines of Java, which could also be traced through Bali and Lombok, are no longer present on this island.
A typical feature is the Saleh Bay which divides the island in a western and an eastern part. The bay is separated from the sea by the Island of Mojo (600 m) which gives it the character of an intermontane depression on the crest of the geanticline.
The northern side of the island is crowned by some young volcanoes. The Ngenges, Tambora and Soromandi have produced leucitic rocks. They might be compared with the alkaline volcanoes of the Ngawi Subzone on Java (Lurus and Ringgit Beser). For the rest, tertiary sediments and calc- alkaline volcanic rocks, like those of the southern mountains of Java, are widely distributed. This gives the impression that the zone of the Southern Mountains of Java occupies the whole island and that the median depression, called Solo Zone, is no longer present East of Lombok. The Saleh Bay might be considered as an isolated apical depression of this Solo Zone on Sumbawa. (See chapter V).
Flores. The island is separated from Sumbawa by Strait Sape, a narrow, more than, 200 m deep trench. Komodo and Rintja belong to the geanti- clinal culmination of West- and Central Flores, which consists of older (tertiary) volcanic rocks and igneous intrusions comparable with those of the Southern Mts of Java. Younger volcanoes occur along the South coast of West Flores. In Central Flores they are present not only along the South coast but also at the northern side of the island (Paluweh). In East Flores the geanticline shows an axial plunge so that the older volcanic rocks and granodioritic intrusions are no longer exposed; only young volcanoes are found, crowning its crest.
The geanticline continues along Solor, Adonara, Lomblen and Pantar, which islands also carry young and active volcanoes 1).
The axis then crosses Alor or Ombai, Kambing, Wetar and Romang. In this section of the inner arc active volcanoes are absent; the islands consist of late tertiary, partly submarine volcanic deposits.
BROUWER (since 1917) attaches a special importance to the extinction of the volcanism in this section of the inner arc from Alor to Romang. We will discuss this problem more in detail in the chapter on the geological evolution (Chapter V B).
3.5.3.
INTERDEEP WITH SUMBA
Interdeep of the Lesser Sunda Islands. Between the volcanic inner arc of Java-Bali-Lombok and the submarine ridge, South of Java (about -2000 m), there occurs a trough of 175 km width. Its greatest depth, South of Lombok, is 5,160 m. This trough bifurcates eastward into two branches North and South of the Island of Sumba (Strait Sumba -1,020m and Strait Sawu -1,160 m).
These branches form the connection between the trough South of Java and the Sawu Basin (-3,440m); e greatest width of the latter is about 200 km tween East Flores and Roti).
It then gradually narrows eastward to the threshold, 25 km in width and 1,815 m depth, between Kambing and Timor, which separated the Sawu Basin from the Wetar Trough. The latter is 60 km wide with a maximum depth of -3,460 m and is limited at its eastern end by Kisar.
The steep slopes of the Wetar- and Sawu Basins and their relatively flat bottoms give them the appearance of down-thrown crustal blocks which are bordered at their eastern and western end by the horst-like elevations of Kisar and Sumba. These islands belong physiographically to the interdeep zone.
Sumba is the most important geological link between the inner- and outer arc of the Sunda Mountain System. Young volcanoes are absent; an almost horizontal cover of marine neogene-quater nary deposits has a wide distribution, forming typical "bad land" sceneries, which are elsewhere almost lacking in the Indian Archipelago.
3.5.4.
OUTER ARC
The Lesser Sunda Islands belonging to the outer arc are Dana, Raidjua, Sawu, Roti, Semau, and Timor. The distinct submarine ridge South of Java, rising to -1,200 m, plunges eastward, to -4000 m. Its elevation above the adjacent interdeep is at most some hundreds of metres. Then the geanticlinal axis rises again to the Sawu Islands (Dana, Raidjua, Sawu).
Sawu has elevated coral reefs of 300 m above sealevel around a nucleus of pre-tertiary rocks. The Dana-Raidjua-Sawu Ridge has an "en echelon" position in respect to the Roti-Timor Ridge, from which it is separated by the Strait of Dao, which is more than 1000 m deep.
Roti has a core of strongly folded sediments, overlain by quaternary coral reefs up to 430 m above sealevel.
Timor. Like the other broad geanticIinal elevations in the Indian Archipelago, West Timor has a longitudinal depression on its crest, which can be traced from the Kupang Bay to Timor Leste border, where it seems to end at the Lois River.
3.5.5.
FOREDEEP
Between Christmas Island and the median sub- marine ridge South of Java, a prominent deep stretches West and East (-7,450 m). This fore- deep of the Java sector of the Sunda Mountain System extends eastward as a narrow trough of 6000-7000 m depth. Off Sumba its depth diminishes, and South of Sawu it curves northeastward, parallel to Timor. Off Roti ,a rise to -1,940 m separates this western part from the Timor Trough (-3,310 m). The South Java Trough is limited to the South by an ill-defined rise of the Ocean floor (passing from Christmas Island to the swell of 3000-4000 m depth between 108lo and 114lo eastern longitude, the isolated shoal on 1180 40' eastern long and 120 30' southern latitude). The bathymetrical data are yet insufficient to decide whether it forms a continuous• ridge on an ocean floor of 5000-6000 m depth, or belongs to a more complicated submarine relief of this part of the Indian Ocean. The eastern part of the foredeep of Timor is bordered by the Australian- or Sahul Shelf (Arafura Sea).
3.6. THE SUNDA MOUNTAIN SYSTEM IN THE JAVA-SUMATRA SECTOR (See fig, 11 on pl. 2)
The islands of Java and Sumatra, belonging to the central sector of the Sunda Mountain System, have been outlined under the heading of the Sunda area, because they are 'connected with the Asiatic Continent by the Sunda Shelf, whereas the outer arc of this Sumatra-Java sector belongs physiographically to the circum-Sunda Archipelago. The same general trendlines as in the eastern part can be discerned in the central sector.
The backdeep is represented by the oilbearing basins of East Sumatra and North Java. It differs physiographically from the backdeep in the eastern part of the Sunda Mountain System in being filled by folded neogene sediments (so that it forms at present a land surface which connects the inner arc with the central Sunda Land), whereas in the eastern part the backdeep is formed by sea basins.
The volcanic inner arc is marked by the Barisan Range on Sumatra and the geanticline of South Java.
The interdeep is a trench of 3000-4000 m depth South of Java ending off the Wijnkoops Bay of West Java. It is interrupted SW of Strait Sunda in the bending point between Java and Sumatra. Here the bathymetrical data show a gradual slope of the sea- bottom from Strait Sunda (-1,575 m) to the foredeep (-6,960 m).
Along the Southwest coast of Sumatra this inter- deep reappears as a series of troughs between the Barisan and the islands west of Sumatra. It is 2,260 m deep SW of Lake Ranau, and this basin is separated by a rise of -936 m from the Mentawei Basin (-1,760 m). The islet of Pini forms a threshold between the latter and the Nias Basin (-705 m), while the Banjak Islands in their turn are the separation between the Nias Basin and the basin SW of Atjeh (North Sumatra). The latter has a depth of more than 1000 m.
The outer are, South of Java, is a submarine geanticlinal ridge rising from -7,450 m to the South of it up to -1,200 m. It is not developed as a distinct ridge in the submarine relief SW of Strait Sunda, where the general trend of the Sunda Mountain System changes from East-West to SE-NW. But it reappears West of Sumatra as the island festoon from Enggano to Sirneulue (Simular).
The foredeep, South of Java, has been outlined in connection with the foredeep of the Sunda Islands; West of Sumatra it is represented by an uninterrupted, though hardly pronounced trough of 5000- 6000 m bordered by the floor of the Indian Ocean which has a mean depth of 4000-5000 m. This fore- deep ends West of Simeulue.
3.7. NICOBARS AND ANDAMANS (See fig. lIon pl. 2, and fig. 375)
The Nicobars (total area 1,645 sq km) consist of 10 small islands and 9 still smaller islets. The Andaman group (6,497 sq km) is formed by 4 larger, mountainous islands. These islands form the northern end of the Sunda Mountain System in so far as this is treated in this book, though this mountain system can be traced still farther northward to the Arakan Y oma of Burma on the Asiatic Continent.
The backdeep. The geosynclinal trough of Northeast Sumatra opens northward into the Andaman Sea which is 450 km wide and -3,600 m deep. This basin forms the connecting link between the Sumatran backdeep and the Sittang depression (Toungoo) in Burma. The Sittang River flows through the eastern part of the Central Belt of Burma (CHHIB- BER, 1934).
This Andaman Basin is connected with the Indian Ocean by a strait of 200 km width and over 2000 m deep between the Nicobar Islands and Sumatra. It is this interruption of the geanticlinal trend which makes it difficult to decide how the Andaman-Nicobar Arc is connected with the Sumatran sector.
The volcanic inner arc of North Sumatra plunges via the Island of Weh (with the harbour of Sabang) down into the Andaman Sea. Here, it cannot be traced clearly between 60 and 110 northern latitude, but it rises again to the Invisible Bank, and from there it runs across the volcanic islands of Barren (with an active volcano) and Narcondam to the volcanic zone between the Sittang and the Irawadi in Burma. This is the igneous line of the Central Belt of Burma, distinguished by CHIBBER (1934, p. 288).
The interdeep is ill defined between the northern spur of Sumatra and the Invisible Bank owing to the lack of bathymetrical data. Perhaps it can be traced northward via the trough of -3,136 m East of the Nicobar Islands. Its course is clear, however, between the Andaman Islands and the volcanic inner arc, and northward it passes into the Irawadi depression of Burma.
The outer arc is interrupted between Simeulue and the Nicobar Islands by the above mentioned passage between the Andaman Sea and the Indian Ocean. The Nicobar and Andaman Islands have often been pictured by geographers and geologists as the continuation of the Sumatran Range. For instance CHHIBBER (1934) in his "Geology of Burma" is of the opinion that the Arakan Yoma of Burma (with the Waga and the Manipur Hills) "continues southwards through the Andaman and Nicobar Islands to Sumatra and Java". UMBGROVE (1930, Geological History of the East Indies, p. 67) prefers the conception "that the western part of Sumatra (the so-called Barisan geanticline), as well as the series of islands West of Sumatra, did indeed during the Tertiary form one zone, which must be considered as the continuation of the Arakan Yoma". The above analysis of the trendlines of the Sunda System, indicates that the Andamans and Nicobars are physiographically an element of the outer arc of this system; in other words, they are the continuation of the chain of islands West of Sumatra. Their non-volcanic character is also in favour of this conception 1).
Narcondam and Barren Island, on the other hand, are situated on the inner arc, which is a continuation of the Barisan geanticline of Sumatra.
