Fluid‐Enhanced Neotectonic Faulting in the Cratonic Lithosphere of the Nullarbor Plain in South‐Central Australia.

The Nullarbor Plain is underlain by thick cratonic lithospheric mantle that is almost devoid of contemporary seismicity. Analysis of high‐resolution digital elevation models indicates neotectonic fault‐propagation fold traces on the nearly flat karst landscape that locally extend to lengths of >100 km, suggesting potential for hosting large (>7.3–7.5) moment magnitude earthquakes. Along‐strike maximum displacements are not proportional to neotectonic fold surface trace length but are spatially associated with crust‐scale electrical conductors identified in magnetotelluric surveys. Two major conductors penetrate from the upper crust to the uppermost mantle (at depths < 60 km) along crustal scale shear zones. Conductivity in the uppermost mantle shear zones is higher than conductivity at increased depth, suggesting fluid‐enhanced enrichment with hydrogen and/or carbon. Lithospheric fluid localization associated with ancient slab subduction and/or hydrothermal alteration may have weakened pre‐existing faults and enhanced neotectonic faulting in the Nullarbor Plain. Plain Language Summary: Stable cratons on the Earth are generally considered to be immune from large earthquakes. Historical seismic quiescence and a paucity of evidence for geologically recent activity on large intraplate faults can result in an absence or low level of seismic hazard awareness, though intraplate earthquakes have caused more fatalities than interplate earthquakes. Long recurrence intervals commonly obscure evidence for paleo‐earthquakes because surface erosional processes may erode or bury surface ruptures and other neotectonic deformational features. Finding an ideal place that can provide sufficient observations of intracontinental active faults is challenging but pivotal for unraveling intracontinental fault dynamics and seismic hazard assessment. The nearly flat Nullarbor Plain in South‐Central Australia is covered by 100s‐m‐thick limestone, which is cut by reactivated reverse faults propagating upward from the Precambrian basement. The limestone has been exposed to the surface since the mid‐Miocene. The arid climate in this region has helped preserve these neotectonic features from surface erosion. Fault traces with high displacements can be well correlated with patches with high electrical conductivities, which are sensitive to fluids in the lithosphere. With co‐located seismic reflection profiles, gravity, and magnetic maps, we propose that enhanced neotectonic activity is associated with fluids ascending from deeper parts of the lithosphere. Key Points: The Nullarbor Plain in Australia is a cratonic region with neotectonic fault (fold) traces of 7–150 km lengthFault displacement is not positively related to fault length but is spatially linked to high electrical conductors in the lithosphereLithospheric fluid localization is proposed to have weakened pre‐existing faults and enhanced neotectonic faulting in the Nullarbor Plain [ABSTRACT FROM AUTHOR]