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ABSTRACT: Passive Margin Formation, Timor Sea, Australia

Richard R. Hillis

Recent ODP data show that sea-floor spreading began in the Argo Abyssal Plain in the earliest Cretaceous, and not the Callovian-Oxfordian as had previously been believed. These data are now consistent with the Callovian-Valanginian rifting observed on seismic records over the adjacent continental shelf (Vulcan subbasin, western Timor Sea).

Tectonic subsidence plots have been constructed for well, extrapolated well, and significant off-well (seismically based) locations in the Vulcan subbasin and adjacent highs. The fully corrected plots show relatively little tectonic subsidence during the Callovian-Valanginian rift phase, even in the depocenter of the Swan Graben, where the Callovian-Valanginian interval reaches its maximum thickness. This is atypical for a passive margin basin.

Assuming an extensional origin for the margin, the absence of tectonic subsidence is considered to indicate that continental rifting in the area was "wet" (accompanied by major volcanic activity). Recent studies have shown that extensive volcanism may occur where rift zones cut through regions of anomalously hot mantle (100-200°C above normal). The addition to the crust of igneous material, the density of which has been modified by adiabatic decompression, inhibits syn-rift subsidence.

A "wet" rifting model also has implications for the origin of the nearby marginal plateaux such as the Scott Plateau. Their relatively thick crust and lack of subsidence may be due to igneous underplating associated with "wet" rifting. As such the plateaux may be regarded as transitional between oceanic and continental crust.

The post-Valanginian Cretaceous subsidence of the Vulcan subbasin and adjacent areas is consistent with typical post-rift thermal subsidence, the predicted exponentially decaying subsidence history for a "wet" rift being indistinguishable from that of a "dry" rift.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990