Localization of Intraplate Deformation through Fluid-assisted Fault Reactivation in the Lower-Crust: The Flinders Ranges, South Australia (Balfour, Cummins and Pilia)

Understanding the interplay between continental intraplate seismicity, stress and deformation is important for elucidating the forces driving plate tectonics and the tectonic processes that shape the continents, as well as for reliably assessing earthquake hazard. But this interplay is still poorly understood. The Flinders Ranges region is unusual for an intraplate environment in experiencing concentrated and sustained seismic activity, relatively high neotectonic slip rates inferred for range-bounding faults, pronounced topography and high heat flow. For these reasons the Flinders Ranges have been the subject of many studies trying to understand why deformation of the Australian continent appears to be localized there. While many previous these studies have remarked on the high seismicity, none of them have considered the information provided by earthquake data - spatial and depth distribution of hypocentres, arrival time anomalies, and focal mechanisms - in any detail. Using data from a temporary seismometer deployment in the Flinders Ranges, we found that earthquakes cluster in elongated low vp/vs anomalies that extend to depths exceeding 20 km, and are aligned with the axis of the Flinders Ranges. In the northern Flinders Ranges these low vp/vs anomalies separate two cratonic blocks, the Gawler Craton to the west and the Curnamona Province in the east. We argue that the compressive earthquake focal mechanisms are compatible with the regional stress field, that there is no evidence for stress concentration, and that the occurrence of earthquakes at mid- to lower crustal depth in an area of high heat flow can only be explained by high pore fluid pressure in the lower crust.

Earthquake locations and characteristics

(a) Earthquake locations and focal mechanisms; (b) tomographic imaging of vp/vs, and (c) Flinders and non-Flinders earthquake depths vs. continental geotherm.