Sedimentary thickness variations and deformation intensity during basin inversion in the Flinders Ranges, South Australia

The central and northern parts of the Adelaide fold belt in the Flinders Ranges, South Australia, consist of a sequence of Neo-Proterozoic–Cambrian sediments overlying a Meso-Proterozoic basement complex, both of which were mildly deformed in an intracratonic setting during the ∼500 Ma Delamerian orogeny. The fold belt lies within a prominent heat flow anomaly (average heat flows of ∼90 mWm−2) reflecting extraordinary enrichments in heat producing elements in the Meso-Proterozoic basement, suggesting that anomalous thermal regimes may have been significant in localising Delamerian deformation. However, spatial variations in deformation intensity correlate more closely with variations in the thickness of the sedimentary sequence than with observed variations in heat flow, suggesting that the thickness of the sedimentary blanket plays a crucial role in localising Delamerian deformation during basin inversion. We use simple numerical models of lithospheric strength to investigate the potential role of sedimentary thickness variations on the distribution and style of deformation, focussing on the impact of a variable thickness sediment pile deposited above a ‘radioactive’ basement. We show that for thermal parameters appropriate to the Flinders Ranges, Moho temperatures may vary by ∼25–30°C for every additional kilometre of sediment. For a ‘Brace–Goetze’ lithospheric rheology, controlled by a combination of temperature-dependent creep processes and frictional sliding, the observed variations in thickness of the sedimentary pile are sufficient to cause dramatic reductions in the vertically-integrated strength of the lithosphere (by many orders of magnitude), thereby providing a plausible explanation for observed correlation between sediment thickness and deformation intensity during basin inversion.