Modes of active intraplate deformation, Flinders Ranges, Australia

The Flinders Ranges form one of the most seismically active zones within the Australian continent with seismogenic strain-rates over the last 30 years of ∼10^-16 s^-1. Active deformation in the region reflects late Neogene increases in stress levels in the Indo-Australian plate as a response to increased plate boundary forcing from collision zones with the neighboring Asian and Pacific plates. Geological and geophysical observations suggest two modes of active deformation in operation in the Flinders Ranges over the last several million years: (1) low-amplitude (∼200-500 m), long-wavelength (∼200 km) lithospheric flexure and (2) active reverse faulting along the range front with fault slip rates of up to ∼50 m Myr^-1. Numerical models are developed to explore the contribution of each of these deformation modes to the observed geophysical signals. An elastic mode of deformation is suggested by a distinctive long-wavelength positive correlation between gravity and topography in which the Flinders Ranges are bordered by anomalous topographic and gravity lows, now occupied by playa-lake systems, centered some 50 km outboard of range-bounding faults. Numerical models show that flexural instabilities localized by vertical loads arising from older tectonic structuring produce a first-order match with observed topography and gravity. Numerical models are also used to illustrate how the localized failure evident in the contemporary seismicity and Quaternary faulting record within the Flinders Ranges reflects thermal weakening associated with extraordinary concentrations of heat producing elements in the crust, as reflected in modern-day heat flows of ∼90 mW m^-2.