Abstract We present two‐dimensional electrical resistivity models of two 40 km magnetotelluric (MT) profiles across the Frome Embayment to the east of the northern Flinders Ranges, South Australia. The lower crust shows low resistivity of 10 Ω m at around 30 km depth. The middle crust is dominated by resistive (>1000 Ω m) basement rocks underlying the Flinders Ranges. Adjacent to the ranges, conductive lower crust is connected to vertical zones of higher conductivity extending to just below the brittle‐ductile transition at ∼10 km depth. The conductive zones narrow in the brittle upper crust and dip at roughly 45° beneath the surface. Zones of enhanced conductivity coincide with higher strain due to topographic loading and sparse seismicity. We propose that fluids are generated through neotectonic metamorphic devolatilization. Low‐resistivity zones display areas of fluid pathways along either preexisting faults or an effect of crustal compression leading to metamorphic fluid generation. The lower crustal conductors are responding to long‐wavelength flexure‐induced strain, while the upper crustal conductors are responding to short wavelength faulting in the brittle regime. MT is a useful tool for imaging crustal strain in response to far‐field stresses in an intraplate setting and provides important constraints for geodynamic modeling and crustal rheology.
Instabilities in Geomechanics appear on multiple scales involving multiple physical processes. They appear often as planar features of localised deformation (faults), which can be relatively stable creep or display rich dynamics, sometimes culminating in earthquakes. To study those features, we propose a fundamental physics-based approach that overcomes the current limitations of statistical rule-based methods and allows a physical understanding of the nucleation and temporal evolution of such faults. In particular, we formulate the coupling between temperature and pressure evolution in the faults through their multiphysics energetic process(es). We analyse their multiple steady states using numerical continuation methods and characterise their transient dynamics by studying the time-dependent problem near the critical Hopf points. We find that the global system can be characterised by a homoclinic bifurcation that depends on the two main dimensionless groups of the underlying physical system. The Gruntfest number determines the onset of the localisation phenomenon, while the dynamics are mainly controlled by the Lewis number, which is the ratio of energy diffusion over mass diffusion. Here, we show that the Lewis number is the critical parameter for dynamics of the system as it controls the time evolution of the system for a given energy supply (Gruntfest number).
<p>Deformation bands are sub-seismic brittle structures found in granular materials. These structures exhibit two spatial distributions: [1] non-linear decay of spacing associated with the damage zone of a fault, and [2] periodic, constant spacing not associated with faults. Periodically spaced deformation bands are of interest as they can be pervasive through porous (>5% &#966;) formations and are known to impact fluid flow. Bands can act as conduits or barriers to fluid flow and are commonly identified in petroleum reservoirs. An understanding of the factors controlling their distribution is therefore of great importance.</p><p>Here, we test a novel mathematical theory postulating that material instabilities in solids with internal mass transfer associated with volumetric deformation are due to elastoviscoplastic p-waves termed cnoidal waves. The stationary cnoidal wave model for periodic compaction bands predicts that their spacing is controlled by important material properties: the permeability of the weak phase in the pores, the viscosity of the weak phase, and the inelastic volumetric viscosity (strength) of the solid grains. A semi-analytical parametric study of the dimensional non-linear governing equations yields a surprisingly simple scaling relationship, which requires testing in the field. Stronger units with higher permeability are predicted to exhibit a wider spacing between deformation bands.</p><p>We test the cnoidal-wave model on natural deformation bands from Castlepoint, North Island, New Zealand. These bands are hosted by Miocene turbidites of the Whakataki formation, which formed in tectonically controlled trench-slope basins associated with the onset of subduction of the Pacific plate beneath the Zealandian plate along the Hikurangi subduction margin. Adjacent sand- and siltstone beds exhibit significant differences in deformation band spacing. Spacing statistics derived from field mapping and laboratory measurements of host-rock permeability and strength are employed to test the scaling relation predicted by the cnoidal wave model. Inconsistencies between theoretical and observed spacing are discussed critically.</p>
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