Elastic, Magnetic, and Electrical Properties of Exhumed Fault Mylonites: Exploring the Geophysical Anomalies Adjacent to the Alpine Fault, New Zealand

Abstract:

Geophysical measurements made during the South Island Geophysical Transect (SIGHT) and the Southern Alps Passive Seismic Experiment (SAPSE) identified a region of anomalously low elastic wave velocity at depth adjacent to New Zealand’s Alpine Fault. In the same area there is an anomaly of increased electrical conductivity, identified in magnetotelluric surveys across the Southern Alps. These anomalies have been assumed to relate to the presence of fluids. In particular, enhanced resistivity may result from interconnected fluid or graphite on the grain scale within ductilely shearing rock. These fluids were released from the lower crust as it metamorphosed during burial into the base of the thickened crust beneath New Zealand’s Southern Alps. Graphite, observed in the Alpine Schist and exhumed hanging wall mylonites, is hypothesized to be remobilized by and precipitated from these fluids in trace amounts to contribute to the high conductivity. Pore decorated grain boundaries, which impart dynamic permeability during shear, could allow upward migration of over pressured fluids and potentially graphite, until they reach an array of near vertical backshears adjacent to the Alpine Fault. Outcrops along the hanging wall of the Alpine Fault expose rock exhumed from subsurface regions. To identify the causes of the large scale geophysical anomalies, we investigated static rock elastic, magnetic and electrical properties of the exhumed rocks on a hand sample scale. We will present measurements of phase anisotropy with respect to foliation to verify the anomaly is present at hand sample scale. We consider how geophysical measurements vary with mineralogical content and distribution, determined by both XRD derived bulk mineralogy and thin section observations. We aim to identify the microscale source of the geophysical anomaly and determine the relative contribution of different mineral phases.