AbstractsEarth & Environmental Science

The Talisker area, situated on the southern coast of Fleurieu Peninsula, comprises part of the southernmost mainland exposures of the Adelaide Fold Belt. The deformation within the area, developed during the compressional Delamerian Orogeny, is represented by tight, upright to steeply inclined and shallowly northeast plunging overturned folds that are transected by a series of high angle, west verging thrust faults. A prominent S1 foliation which occurs as a slaty cleavage or schistosity with an intense down-dip lineation, up to 60% crustal shortening and X/Z finite axial strain ratios of up to 7.8:1 have also developed during this compressional event. A microstructural analysis conducted east of the Coalinga Creek Fault defined the microstructural characteristics of the Coalinga Creek Shear Zone, situated adjacent to the fault, and showed how these characteristics change with distance east of the shear zone. The deformation mechanisms that have operated on the rock types within this area are dominated by diffusive mass transfer and fracture processes with the exception of the Coalinga Creek Shear Zone which displays microstructures that characterize crystal-plastic processes. A quantitative strain analysis, also conducted on the area east of the Coalinga Creek Fault, defined a bulk oblate strain geometry for the area with a trend to become more oblate with increasing magnitudes of strain. The analysis also suggests that the strain was accommodated predominantly by the rock matrix, that is, by the aforementioned deformation mechanisms, rather than by interlayer slip or folding. Restored cross sections of the Talisker area show a dramatic thickening of the Talisker Calc-siltstone across the Coalinga Creek Fault from fault footwall to fault hanging wall. This is interpreted as representing the development of syn-sedimentary historic normal faults within an evolving extensional basin and coincides with the early Cambrian lithospheric attenuation phase of Jenkins (1990). The subsequent, compressive Delamerian Oogeny resulted in the inversion of individual extensional flats to contractional geometries and has generated the present structural geometry of the Talisker area. Regionally, the Talisker area represents the western margin of a major duplex structure with the Talisker Fault forming the frontal ramp and the area west of the fault, the local foreland region.