LINKING FAULT-RELATED INJECTION VEINS TO PALEOEARTHQUAKE CYCLES: OBSERVATION FROM THE MUDDY MOUNTAIN THRUST, NV, USA

The mechanical behaviour of low-angle thrust remains the greatest paradox in tectonics and fault mechanics. Movement along a low angle basal detachment fault (dip angle < 30°) is unlikely without high pore pressure, which represents a weakening mechanism needed for fault slip, earthquake triggering, heat transport, fluid and chemical cycling. It is essential to develop reliable tools for estimating paleo-pore pressure and understanding how the properties of the fault host rocks evolve during an earthquake cycle and lifetime of the fault. During earthquake-induced heating, the slipping layer can have sufficiently low permeability to trap fluids and increase fluid pressure, and devolatilization can produce fluid internally. Subsequent cooling after an earthquake can permit for chemical alteration and precipitation, facilitating healing, impermeability and overpressurization. The Muddy Mountain Thrust in southern Nevada, USA displays injections of gouge and breccia from the fault core into fractures in the wallrocks, recording transient pressurization of the fault surface, presumably due to coseismic heating. Crosscutting relations between principal slip surfaces and injections indicate multiple seismic pressurization events. Clast source relationships and transport distances to the injection site will be used to constrain fluid properties and injection velocity, which in turn places limits on the pore pressure on the fault during slip. Microstructural observations of cements and solution features will be used to infer the chemical characteristics of the driving fluid. Our field observations demonstrate that geologic records of pore pressure transients are possible, and will constrain the magnitude of pore pressurization associated with large earthquakes in a continental fold and thrust setting, potentially providing the first quantitative observational constraints testing the pore pressure answer to the low angle thrust paradox.