Journal of Structural Geology, Vol. 20, No. 12, pp. 1721 to 1731, 1998 © 1998 Elsevier Science Ltd. All rights reserved PlI: 80191-8141(98)00088-1 0191-8141/98/$ - see front matter Sedimentary thickness variations and deformation intensity during basin inversion in the Flinders Ranges, South Australia MIKE SANDIFORD*, EIKE PAUL and THOMAS FLOTTMANN Department of Geology and Geophysics, University of Adelaide, South Australia (Rece/ved 22 December 1997; accepted /n rev/sed/arm 26 July 1998) Abstract-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 com- plex, 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 extra- ordinary enrichments in heat producing elements in the Meso-Proterozoic basement, suggesting that anoma- lous 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 dis- tribution 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. © 1998 Elsevier Science Ltd. All rights reserved INTRODUCTION The modern view of orogenie belts as manifestations of relative motion across active plate boundaries implies, at the largest scale, that deformation intensity is related to proximity to plate boundaries. However, it is well known that both ancient and modern oro- genie belts show significant regional variations in both style and intensity of deformation that are not readily explicable in terms of proximity to plate boundaries. One of the most spectacular examples of this in the modern Earth is the partitioning of active deformation evident around the Tarim Basin in Central Asia (e.g. Neil and Houseman, 1997). While such regional vari- ations in deformation intensity (and style) are likely to reflect variations in the mechanical response of the orogen, the specific controls that mediate the mechan- ical response of the continental lithosphere remain poorly understood. This is particularly true of defor- mations involving basement reactivation in intracra- tonic settings (e.g. Rogers, 1995). The Adelaide fold belt in South Australia (Fig. 1) consists of a Neo-Proterozoic to Cambrian sedimen- tary (cover) succession deformed along with its Meso- Proterozoic basement in the late Cambrian-early Ordovician (~SOO-490 Ma) Delamerian Orogeny. Like many fold belts, it shows systematic regional variations in style and intensity of deformation. At the largest *E-mail:msandifor@geology.adelaide.edu.au scale, the fold belt can be divided into three distinct zones characterised by different styles and intensities of deformation (Marshak and Flottmann, 1996); • a southern zone, including the southern Adelaide fold belt (with orogenie shortening of 30-S0%) and the Nackarra Arc (where orogenie shortening strains average > 6%-see Fig. 1b); • a central zone (the central Flinders zone), character- ised by shortening < S % (Fig. 1b); and • a northern zone (the northern Flinders zone) charac- terised by intermediate deformation intensity with shortening averaging ~ 11% (Fig. 1b). Late Cambrian palaeogeographical reconstructions place the active continental margin to the southeast of the preserved fragments of the fold belt (Coney et al., 1990), with the central and northern parts of the fold belt bounded by the older, relatively undeformed cra- tonic blocks of the Gawler Craton (to the west) and the Curnamona Craton (to the east). In parts of the fold belt, the deformation has involved the basement, which is now exposed as a series of inliers in the north (the Painter and Babbage Inliers), in the east (the Willyama Inliers) and in the south (the Houghton and Myponga Inliers). Elsewhere the deformation has detached the cover sequences from the underlying basement (e.g. the Nackarra Arc). Paul et al. (1998) have shown that the first order variations in the deformation intensity and the extent of basement reactivation in this central and northern 1721