Rigidity of the Australian Continental Lithosphere Across the Banda Orogen, Indonesia
(Australian Geological Survey Organisation, GPO Box 378, Canberra, Australia, 2601; e-mail: gobrien@agso.gov.au) Pliocene-Recent collision of the Australian plate along the Banda orogen from Sumba to the Kai Plateau (~ 120°-137° E) has created an underfilled, continental foredeep with abundant reactivated, seaward-dipping, normal faults on the outer slope. Rigidity of the continental lithosphere and acting boundary forces are investigated in areas (1) where convergence has ceased (Timor Island) and (2) where the collision is currently active (east and west of Timor Island). Preliminary results from elastic flexure modeling using a uniform broken-plate model flexed by a triangular load (approximating Banda orogen) and end-point load (idealizing subsurface loading) show a range of 35-75 km of Te (effective elastic thickness) from E-W for the Australian lithosphere, with highest values south of central Timor. If variable Te is included in the analysis, Te changes from ~ 80 km at the shelf to ~ 20 km beneath the island, SE of Timor. This Te gradient toward the orogen may result from inelastic yielding and crust-mantle decoupling via flexure of the foredeep. End-point loads vary between 1011-1012 N/ m2 except beneath central Timor where they reduce to zero. Subsurface loading in general may be caused by weak coupling between the end of the plate and the leading edge of the subducted continental plate. The northern end of the broken plate lies beneath the orogen at the eastern and western end of Timor Island and also at the Kai Plateau; these observations are similar to those made by others in the Himalayas and New Guinea Highlands.
Cenozoic-to-Recent convergence of northern Australia and South East Asia has produced a broad foreland basin system comprising Timor Island, the Timor Trough, and the outer North West Shelf of Australia. Advance of Timor Island over the Australian Plate weighs down and bends the lithosphere. If the bending is too great, the upper crust may break in a process known as flexural extension. Ideal elastic lithospheric models predict flexural uplift on the outer North West Shelf. However, flexural fault reactivation can subdue flexural uplift at the periphery of the foreland basin and also compromise hydrocarbon trap integrity. In the Timor Sea, flexural extension reactivates Jurassic rift basins under low strain conditions (~2%). Neogene sequence isopach maps suggest flexural basin reactivation was initiated prior to major Late Pliocene thrusting on Timor . Modern sea floor fault throws diminish away from Timor and are absent beyond ~300 km of the Trough axis, in accordance with realistic rheological models. In the Late Miocene/Pliocene, reactivation extended perhaps >400 km from the foreland basin axis. Current flexural uplift is absent at the sea floor on the outer NW Shelf and intra-Pliocene unconformties do not suggest past flexural uplift. Flexural faulting may also have collapsed the expected flexural uplift. J M Lorenzo, K Tandon, (Both at: Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803; ph. 504-388-4249; fax 504-388-2302; e-mail: juan@tellus.geol.lsu.edu) G W O'Brien (Australian Geological Survey Organisation, GPO Box 378, Canberra, Australia, 2601; e- mail: gobrien@selenite.agso.gov.au) Cenozoic-to-Recent convergence of northern Australia and South East Asia has produced a broad modern-day region of continent-arc collision comprising Timor Island, the Timor Trough, and the outer North West Shelf of Australia. Advance of Timor Island over the Australian Plate weighs down and bends the lithosphere. If the bending is too great, the upper crust may break in a process known as flexural extension. Flexural extension is the inelastic response to curvature-induced stresses and is inadequately explained by elastic idealization of the lithosphere. Multiple Phanerozoic rifting events have weakened and predisposed the shallow crust to failure. Over 2,000 km of Australian Geological Survey deep seismic reflection profiles (0-16 s) show flexurally reactivated Jurassic-age faults. The period of fault reactivation includes the major period of vertical growth on Timor Island in the Late Pliocene. Stratigraphic sequence isopach maps derived from 14,000 km of commercial seismic reflection profiles record reactivation starting in the Late Miocene. Elastic models predict a several-thousand-meter flexural downwarp in the Timor Trough and a several-hundred-meter flexural uplift or forebulge along the outer North West Shelf of Australia. Yet, a present-day forebulge is apparently missing along the N.W. Shelf. Plate-driving forces calculated by other workers may create in-plane tensional stresses that reduce flexural uplift and/or induce normal faulting. If faulting is caused by slab pull from the west, a westward increase in faulting is expected. Instead, fault patterns in the Timor Sea show a northward increase in throw. Brittle failure of the upper continental crust through loading by Timor to the north more readily explains the missing forebulge.
Pliocene-Recent extension in foredeeps on the (1) outer North West Shelf of Australia, south of Timor Island, (2) offshore southwestern Taiwan, and (3) Eratosthenes seamount, offshore southern Cyprus, is manifested by normal faults dipping toward the trench. In all these areas extension may be due to continental flexure during plate convergence. These cases are compared to oceanic lithosphere undergoing subduction in the Java Trench and Puerto Rico Trench. For continental cases, bending stresses estimated from flexure models predict rock failure envelopes which can be tested against changes in fault distribution patterns since the Pliocene. Elastic half-beam flexure models are derived by matching bathymetry and gravity. Regional Airy isostatic anomaly and Bouguer gravity anomaly maps imply Moho shape. Contrary to the predictions of flexure modeling, we do not observe continental forebulges. Forebulges may be destroyed as a consequence of extension. Positive Airy isostatic and Bouguer gravity anomalies indicate an upwarped Moho or thinned crust. Oceanic examples at the Java and Puerto Rico trenches show both forebulges and positive gravity anomalies. In flexure models, triangular loads idealise Timor Island and the Troodos ophiolite (Cyprus). End moments approximate the oceanic slab pull. South of central Timor, bathymetry can be matched using an effective elastic thickness (Te) for the lithosphere of ~50 km. In this area, previous studies indicate that the oceanic lithosphere has detached itself from the Australian continental lithosphere. However, south of eastern Timor a combination of triangular loading and an end moment (1017 Nm) is needed to match the bathymetry. Failure envelopes predict the observed extent of present-day seafloor faulting. Offshore southwestern Taiwan, bathymetry is matched by applying only an end moment (1016 - 1017 Nm ) with Te of 25-30 km. K. Tandon, J. M. Lorenzo, R. S. Boehme, D. A. Pyne (Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803; ph. (504) 388-3966; fax (504) 388-2302; email: kush@glass.geol.lsu.edu, juan@tellus.geol.lsu.edu, boehme@glass.geol.lsu.edu, dave@aapg.geol.lsu.edu) A study of flexural extension in the (1) Timor Sea area of the North West Shelf of Australia and (2) offshore southwestern Taiwan using flexure modeling, gravity modeling, seismicity mapping and seismic reflection data has been conducted. The Timor Sea area is a foreland basin created by the collision of Australian continental lithosphere with the Banda Island Arc since the Pliocene. The Tainan basin, offshore southwestern Taiwan also became a foreland basin in the Pliocene when the North Luzon Arc collided with the South China continental margin. We hypothesize that continental extension in these areas is caused by the flexure of the lithosphere during plate convergence. Elastic half-beam models predict a several hundred meter high forebulge on the continental margin centered 50-100 km from the trough. Neither the North West Shelf nor the Tainan basin show such a forebulge in the present-day bathymetry. This is in contrast to forebulges in the subducting oceanic lithosphere e.g., Java Trench. In Timor Sea area, seismic reflection data indicate no evidence of a paleo-forebulge. However, regional isostatic and free-air gravity highs in the offshore southwestern Taiwan and in the Timor Sea area possibly indicate a regionally flexed Moho. We suggest that the continental lithosphere in the North West Shelf and offshore southwestern Taiwan as a whole is flexed elastically. but that the continental lithosphere does not show a forebulge. It is possible that the forebulge in these areas is destroyed by the brittle failure of the outer veneer of continental lithosphere. BOEHME, Ricky S., LORENZO, Juan M., TANDON, Kush, PYNE, David A., Dept. of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803; O'BRIEN, Geoffrey W., MARSHALL, John, Australian Geological Survey Organisation, GPO Box 378, Canberra, Australia, 2601. The Timor Sea area of the Australian North West Shelf is an early-stage foreland basin created by Pliocene-to-Recent collision of Australian continental lithosphere with the Banda island arc. The Australian lithosphere is flexed downward to the north west by an advancing orogenic wedge which includes the Indonesian island of Timor. Elastic beam models for the lithosphere predict the presence of a 300 m high flexural forebulge near the tectonic shelf edge. The predicted forebulge is not observed in the present-day bathymetry. However, evidence of erosion or non-deposition on paleo-forebulges may be preserved in the rock record as unconformities. Seismic profiles from the Australian Geological Survey Organisation and the petroleum industry, and well data from the study area were examined for evidence of collision time (Pliocene-to-Recent) forebulge unconformities. A shelf edge-parallel, ~100 km wide belt of uplifted Tertiary reflectors is observed on the outer shelf, in the western part of the study area. An intra-Pliocene unconformity caps this sequence and is the most viable candidate for a paleo-forebulge unconformity. However, the uplifted region also lies over a pre-existing structural high that may have been reactivated by flexural stresses. In the eastern part of the study area, along-strike with this structural high, a basin of similar age has developed over a pre-existing structural low. Structure and isopach maps of Tertiary depositional sequences in the entire study area reveal no unconformities that can be attributed uniquely to a paleo-forebulge. We propose that the predicted flexural forebulge associated with this foreland basin does not exist or has been effectively obscured by reactivated structures during continent-island arc collision. [*J M Lorenzo*]; K Tandon; R. Boehme (Department of Geology and Geophysics, Louisiana State Univ, Baton Rouge, LA 70803-4101; ph. 504-388-3353; fax 504-388-2302; e-mail: juan@tellus.geol.lsu.edu, kush@glass.geol.lsu.edu, boehme@glass.geol.lsu.edu) Convergence between northern continental Australia and South East Asia since the Neogene has produced a broad zone of young continent- arc collision that includes the outer North West Shelf of Australia , the Timor Trough and the island of Timor. The outer North West Shelf represents the outer arch of a present-day collisional foredeep on an underriding continental plate. During the Pliocene, a series of narrow (150x50km) NE-SW oriented Jurassic rift basins, collectively known as the Vulcan Sub-Basin, were reactivated via oblique extension coeval with the major phase of folding and thrusting on Timor. We propose that continental extension occurred in response to bending of the downgoing plate when the present study area neared the Timor Trough sufficiently. Initial two-dimensional flexural modeling of the underriding Australian Plate using elastic beam theory matches trench-axis depths (2 km) and the position of the shelf break, about 100 km south of the Timor Trough axis. We use an effective continental elastic thickness of 25 km loaded by a "sediment" wedge 100 km wide and 10 km in maximum thickness. Modeling predicts a 300-m bathymetric arch which is not observed on the outer Shelf. Non-elastic behavior of the Australian plate such as brittle failure of the shallow crust may have subdued the expected topographic high. Maps of faults prepared from seismic data suggest that cumulative fault displacement increases toward the Trough as observed commonly in other foredeep settings. Stress estimates obtained from the elastic half-plate model were used to delineate regions of rock failure with a Coulomb and modified Griffith criterion. The estimated fracture envelope extends toward the continent 130 to 140 km from the axis of the Trough. The observed limit of present-day fault scarps southward of the Trough axis is also approximately 150 km. |
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