Abstract

The northwards movement of the Indian-Australian plate caused cratonic India to begin its collision with continental Eurasia in the Eocene and cratonic Australia to begin collision with the Indonesian island arcs in the Miocene. The Indian collision caused clockwise bending of the pre-Tertiary fabric of Sundaland, accomplished predominantly by right-lateral wrench faulting, mostly but not exclusively structurally conformable (e.g. Semangko and Sagaing faults). The Australian collision caused anti-clockwise bending, accomplished by left-lateral faulting (e.g. Sorong Fault).

The fracture systems displaced micro-continents southeastwards from Sundaland and westwards from Australia-Irian Jaya. Between 1880 and 1910, Alfred Russel Wallace moved his faunal-floral divide from west of to east of Sulawesi because of the island’s mixed affinities. Today we might agree to put the geological dividing line through central Sulawesi — the eastern arms having terranes displaced from Australia-Irian Jaya and the western arms forming an integral part of early Tertiary Sundaland.

Major rotations in Southeast Asia led Warren Carey to coin the terms orocline and sphenochasm (= rhombochasm) respectively for a bent orogen and a deep oceanic or marginal basin formed by rotational pull-apart tectonics. His elegant analysis of Southeast Asia was a product of his expanding Earth hypothesis, which undeservedly had only a limited following.

Before indentation or extrusion tectonics, it was standard practice to analyze the whole region in terms of a regional compression and orthogonal extension, using the strain ellipsoid, on the false assumption that the whole region, if it were miniaturized, would behave like a homogeneous sandstone core in a laboratory tri-axial cell. The popular E-W compression hypothesis requires a NW–SE and NE–SW conjugate fault system. The hypothesis is flawed because only the former direction is well developed; NE–SW faults are rare. If we could so shrink Southeast Asia, laboratory tri-axial tests would tell us that the fracture system produced would result in reactivation of the pre-existing zones of weakness of the pre-Tertiary fabric. Likewise, the energy of collision of India with Eurasia should preferentially be transmitted along pre-existing zones of weakness in extrusion tectonics, though P. Molnar argued it would be totally absorbed in Tibet.

The fracture system, which is now widely accepted to have resulted from the Eocene collision of India with Eurasia, in fact predominantly followed pre-existing zones of weakness (the fabric) of the basement (e.g. suture zones and fold axes), overwhelmingly established during the Triassic Indosinian Orogeny. The collision of Australia with the Indonesian island arcs cannot be analysed in the same way.

The wide acceptance that major faults propagated throughout Sundaland from the Yunnan–Assam syntaxis has naturally led to Cenozoic trans-extensional and trans-pressional tectonics becoming fashionable. The latter has resulted in transverse fold axes and ridges, with examples in the Mesozoic Tembeling Formation of Peninsular Malaysia and the Tertiary basins of Southeast Asia.

The NE–SW fabric of eastern China (Cathaysia) was established during the Devonian Caledonian Orogeny, accentuated during the Indosinian Orogeny and then largely overwhelmed by the structurally concordant Jurassic–Cretaceous Yenshanian ‘Orogeny’ with its vast production of high level granite and rhyolite. Tertiary basins and structures are conformable to the fabric of this basement.

The Trung Son (Annamitic) fold-belt also contains elements of the Caledonian Orogeny, but is dominated by NW–SE trending features such as the Song Ma suture and several Triassic basins. Metamorphic rocks of the Song Ma yield dates peaking in the Early Triassic. The zone has been re-activated with spectacular left-lateral wrench faulting along the Red River Fault, which experienced strong inversion during the Oligocene–Miocene. The present day motion is right-lateral. The NW–SE fabric also characterizes the Indochina Peninsula and the same trend may be inferred for the Malay Basin basement.

The E–W structural grain of Tibet has resulted from successive collisions of terranes which drifted from Gondwanaland, each causing further compression. India arrived in the Eocene and indented a great right-angled oroclinal bend into the pre-existing fabric, around the Yunnan syntaxis. Thus the E–W trending Lhasa Block correlates with the N–S West Burma Block and the E–W Qiangtang Block with N–S trending Sinoburmalaya (Sibumasu). The N–S fabric of the latter defines the geology of east Myanmar, west Thailand and Peninsular Malaysia and controls the Tertiary basins of the Gulf of Thailand. The collision of Sinoburmalaya with Indochina and Cathaysia followed narrowing of the Palaeo-Tethys Ocean, resuling in the Triassic Indosinian Orogeny. The extensive highlands so created remained elevated despite high Mesozoic eustatic sea levels: widespread inundation came only after the Eocene unconformity. The N–S Indosinian fabric through Peninsular Malaysia bends due east through Bangka and Billiton. The outer curvature of the orocline is seen in Sumatra, where the pre-Tertiary fabric is predominantly NW–SE. The bending controlled the fault and Tertiary basin geometry.

The orocline should continue eastwards from Billiton, but the pre-Cretaceous geology of Borneo is extremely fragmented and extrapolation cannot be successfully made because the geology and structuring of Borneo is overwhelmingly Cretaceous–Tertiary. However, Triassic correlation of northwestern Borneo is possible with eastern Vietnam.

Most but not all Cenozoic structures follow the pre-Tertiary fabric. Analysis of the Tertiary fracture pattern is accordingly difficult, if not impossible. Notable cross-cutting features are, for example, the Penyu–West Natuna basin system and the NW–SE and N–S faults of Peninsular Malaysia and Sumatra respectively, though they both extrapolate into the contiguous country as structures conformable with the pre-Tertiary basement.

Although the pre-Tertiary fabric of the lithosphere has dictated the Tertiary fracture pattern, the orientation of many parts of that fabric may have been rotated into their present position by Tertiary events. Unfortunately, palaeomagnetic research has not given a definitive solution to the exact nature of the Tertiary rotations, and the deduced anti-clockwise rotation of Borneo is disputed because it cannot be accommodated by the Indian collision, rather it should more logically be related to the collision of Australia, but the timing is too early.