Continental Margin Stratigraphy, Deformation, and Intraplate Stresses for the Indo-Australian Region

Abstract

The stratigraphic record along the northwestern Australian continental margin provides constraints on the tectonic evolution of the Indo-Australian region, which is characterized by an extremely high level of intraplate deformation, presumably reflecting high levels of regional stress. The patterns of folding and faulting in the northeastern Indian Ocean (shown by focal mechanisms, gravity, and seismic-reflection data) are consistent with predictions of a stress model based on age-dependent driving forces on the lithosphere. Separate compressional and extensional stress provinces are found, each characterized by regionally consistent stress orientations. For example, folding occurs only within the region of predicted deviatoric compression. The evolution of the stress field can be inferred from the stratigraphic data of ODP Leg 116. An unconformity dated at 7.5 Ma marks the end of the recent major phase of folding since which deformation has occurred primarily through faulting. Here, we examine the stratigraphic record of the northwestern margin of Australia using data from Legs 122 and 123 to investigate whether the continental margin stratigraphy records the spatial and temporal variations in stress inferred from the folding in the northeastern Indian Ocean. With the exception of the Pliocene unconformities, the major unconformities inferred from Leg 122 coincide with lowstands in sea level on regional sea-level curves proposed for the northwestern Australian margin, while the major unconformities found during Leg 116 show a lack of correlation with lowstands in these regional curves. Although the correlation of the unconformities from Legs 116 and 122 with lowstands in the Haq et al. (1987) eustatic curve is sometimes good, the major unconformities drilled during Legs 116 and 122 also coincide with plate-tectonic changes of a global character. The Neogene stratigraphic record of the Northwest Shelf can be explained by a paleostress field characterized by an overall tensional stress regime during middle Miocene time to a regime with an increasing level of intraplate compression from late Miocene time onward. The paleostress field inferred from the stratigraphic modeling of the Northwest Shelf is consistent with observed tectonic activity, including middle Miocene fault reactivation in the Browse and Carnarvon basins and Pliocene tilting of the Wombat Plateau. The inferred increase in the level of compression for the northwestern margin occurs at a time corresponding with the onset of compressional folding in the northeastern Indian Ocean, which was documented during Leg 116. The tectono-stratigraphic evolution of the northwestern Australian margin seems to reflect both the changes of intraplate tectonics associated with the unique dynamic situation of the Indo-Australian Plate, as well as the interplay between a major global plate reorganization in Neogene time with eustatic contributions to the sea-level record.

Borehole Instability on the Northwest Shelf of Australia

“…Recent breakout analyses on wells in the Vulcan Subbasin (Hillis and Williams, 1991) [8] show breakout and a minor horizontal stress oriented 130 0 -170 0 N, which is consistent with the numerically modelled stress orientations shown in Figure 7 (after Cloetingh et al, 1991) [9].…”

Delineating the Exmouth mantle plume (NW Australia) from denudation and magmatic addition estimates

“…Figure 9 depicts tectonic subsidence versus time for two locations in Figure 4 (Cape Range fracture zone and J1). While J1 is constrained by well analysis using standard backstripping techniques (e.g., Cloetingh et al, 1992), the Cape Range fracture zone location lacks a well but has similar Triassic seismic stratigraphy and tectonic subsidence history to J1. Tectonic uplift (U) at the Cape Range fracture zone is estimated from the amount of eroded Triassic section from Figure 4 (D = 2.6 km), calculated back into the tectonic subsidence curve (U = 545 m) using the relation from Chadwick (1985): U = D(r m -r s )/r m (Fig.…”

“…4 and 5), starts at the Cape Range fracture zone, runs SW-NE, and uses denudation data from Lorenzo et al (1991) Distance from CRFZ ( Delineating the Exmouth mantle plume (NW Australia) | RESEARCH and seismic-reflection data calibrated to offset well data (Fig. 3) up to ~60 km from the Cape Range fracture zone, while denudation at larger distances from the Cape Range fracture zone is based on backstripped exploration wells V1 (Vinck-1), I1 (Investigator-1), and J1 (Jupiter-1; Cloetingh et al, 1992). Relatively little denudation seems to have taken place at distances from 60 km to 200 km from the Cape Range fracture zone (V1 to J1; Fig.…”

Mesozoic/Cenozoic tectonic events around Australia

“…A spectacular example of oceanic intraplate deformation, resulting in an oceanic fold and thrust belt, can be found in the central Indian Ocean [Royer and Gordon, 1997]. Many tentative links between plate tectonic events and tectonic reactivation on the Australian plate have been proposed [Cloetingh et al, 1992;Etheridge et al, 1991]. Etheridge et al [1991] pointed out that in many Australian sedimentary basins nearly all oil and gas fields are associated with tectonic reactivation, highlighting the need to understand changing plate driving forces through time and their impact on intraplate stresses from an exploration point of view.…”