Lower crustal involvement in upper crustal thrusting

Abstract

Summary. Basement structures mapped in the Devonian Adavale Basin, eastern Australia, indicate two styles of lower-crustal involvement in the formation of upper-crustal structures. The first style is typified by thrust features in the upper-crustal sedimentary section and basement, a response to lower-crustal shortening over a wide area. The second style includes lower-crustal thrusting and thickening in a limited region, with associated uplift of the upper crust. These two styles suggest that the upper and lower crust were mechanically decoupled during Palaeozoic compressive episodes. 1. Tectonic structures in the upper crust Basement involved thrust faults have been mapped in the southern Adavale Basin, Australia, using both Bureau of Mineral Resources and industry seismic data (Finlayson et al., in press). These faults were grouped into two sets, those striking predominantly north- south (Gumbardo and Corona) and those striking predominantly east-west (Como, Paradise and Grenfield), and form an orthogonal network of faults in this region (Fig. 1). Mechanical considerations dictate that these orthogonal thrust sets did not form synchronously. Relative thinning of stratigraphic units over basement faults has been used to date uplift movement on these faults. Retrodeformed sections produced by restoring the mid-Devonian Cooladdi Dolomite unit (or its lateral equivalent) were used to date initiation of thrusting. The sections show that the east-west striking thrusts developed first during the Lower Devonian in response to north-south crustal shortening. The thrust faults were separated by tear faults which transferred the relative strain between the fault blocks (Dalhstrom 1970). The second phase of tectonic activity commenced in the Late Devonian and continued into the Carboniferous. This phase is interpreted to be due to east-west crustal shortening and reversed the roles of the thrust and tear faults developed during the previous episode, (Leven et al., in press). The north-south striking Gumbardo and Corona faults were reactivated as thrusts, while previous thrusts acted as transfer faults for this deformation. During the latter portion of this second tectonic episode, sediments of the Adavale Basin were deformed into large-scale, predominantly north-south trending folds. The folding style of this later deformation contrasts with the faulting associated with the earlier episode. Carboniferous erosion, which Veevers et al. (1984) suggest was caused by an extensive ice sheet, effectively removed most of the sediment of the newly formed Adavale Basin, leaving only the remnant synclinal cores. These are represented in the southern Adavale Basin region by the Cooladdi, Quilpie, and Blackwater Troughs. The wavelength of this basin-wide folding is about 100 km and the amplitude (in the case of the Cooladdi Trough) exceeds 8 km, but diminishes towards the west. The Warrabin Trough to