Localization of Intraplate Deformation through Fluid-Assisted Fault Reactivation in the Lower-Crust: The Flinders Ranges, South Australia

Abstract:

Within the stable continental region of Australia, the Flinders Ranges stands out as experiencing concentrated and prolonged seismic activity. Previous studies have shown that strain rates inferred from seismicity of 10^-16 —10^-15 s^-1 are similar to neotectonic slip rates inferred for range-bounding faults. It is also an area of relatively pronounced topography up to 1700 m and high heat flow averaging 90 mW/m². 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, with explanations including erosion-driven isostatic rebound, lithospheric flexure, stress concentration due to change in lithospheric strength, and thermal weakening. We present a hypothesis for localized, intraplate deformation in the continental crust of south-central Australia that involves fluid-assisted reactivation of faults in the mid- to lower crust. This study utilizes data from a temporary seismometer deployment in the Flinders Ranges from 2003—2005. We show that earthquakes in the region extend to depths of 20 km and are clustered in elongated low V_p/V_s anomalies. These anomalies suggest a highly fractured or deformed zone that is aligned with the axis of the Flinders Ranges and extends to the lower crust. We argue that the compressive earthquake focal mechanisms are consistent 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. These data reveal important constraints on structure, rheology, and stress that are crucial for understanding intraplate deformation in the Flinders Ranges, with possible implications for high-seismicity zones in stable continental regions elsewhere.