Fluid-enhanced neotectonic faulting in the cratonic lithosphere of the Nullarbor Plain, Australia
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The Nullarbor Plain is underlain by a thick cratonic lithospheric mantle, which is thought to have a paucity of neotectonic faults and seismicity. Based on the analysis of high-resolution digital elevation models, identified neotectonic fault traces on the nearly flat karst landscape locally extend >100 km long, suggesting potential for hosting large (>7.3 to 7.5) moment magnitude earthquakes. The measured along-strike maximum displacement Dmax for each trace is not proportional to surface rupture length (L) but is correlated with the occurrence of crust-scale electrical conductors identified in magnetotelluric surveys. Two major conductors penetrate from the upper crust to the topmost mantle along crustal scale shear zones. The conductivity value in the topmost mantle is much higher than in the cratonic mantle, indicating serpentinization of the mantle with the addition of fluids. Lithospheric fluid localization may have weakened pre-existing faults and enhanced neotectonic faulting in the Nullarbor plain.Keywords:
Magnetotellurics
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[1] Although presence of weak layers due to hydration and/or metasomatism in the lithospheric mantle of cratons has been detected by both geophysical and geochemical studies, its influence on craton evolution remains elusive. Using a 2‒D thermomechanical viscoelastoplastic numerical model, we studied the craton extension of a heterogeneous lithospheric mantle with a rheologically weak layer. Our results demonstrate that the effect of the weak mantle layer is twofold: (1) enhances deformation of the overlying lithosphere and (2) inhibits deformation of the underlying lithospheric mantle. Depending on the weak‒layer depth, the Moho temperature and extension rate, three extension patterns are found (1) localized mantle necking with exposed weak layer, (2) widespread mantle necking with exposed weak layer, and (3) widespread mantle necking without exposed weak layer. The presence of the weak mantle layer reduces long‒term acting boundary forces required to sustain extensional deformation of the lithosphere.
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The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) has the goal of mapping the electrical resistivity of the Australian lithosphere to constrain the geodynamic framework of the continent. Between August 2014 and May 2015, 125 long-period magnetotelluric (MT) data will be collected across the Gawler Craton and the south-eastern part of South Australia at intervals of 50 km. Results will be compared to existing 3D models highlighting enhanced conductivity in the sub-lithospheric mantle of the Gawler Craton. Initial results of 1D depth transformations show a significant change in resistivity at mantle lithosphere depths (100 km). The results will also be tied to the newly acquired Eucla MT line and a new 1000 km NS MT profile extending north from the central Gawler Craton into the Arunta Province, NT.
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