CAN CO-SEISMIC PORE PRESSURE OF ANCIENT CONTINENTAL EARTHQUAKES BE ESTIMATED FROM FAULT-RELATED INJECTION VEINS?

Developing tools for estimating paleo-pore pressure in faults is essential in understanding how the properties of the fault host rocks evolve during an earthquake cycle and the lifetime of the fault. High pore pressure represents a weakening mechanism needed for fault slip, earthquake triggering, heat transport, fluid and chemical cycling and potential a key to understanding the mechanical behaviour of low-angle thrust, which remains the greatest paradox in tectonics and fault mechanics. The Muddy Mountain Thrust in southern Nevada, USA, is part of the regional mid-late Cretaceous Sevier orogenic front and exposes evidence of paleoseismic slip. The thrust juxtaposed an imbricated Paleozoic carbonate sequence above Jurassic and Cretaceous sandstones, molasse and conglomerates. The thrust displays injections of gouge and breccia from the fault core into fractures in the hangingwall rocks. Crosscutting relations between principal slip surfaces and injections indicate multiple seismic pressurization events. Field observation and microstructure of the injection materials revealed the presence of poorly sorted assorted remnant clasts of quartz, microcline, foliated gouge, breccia, and cement. The mixed lithology clasts were sourced from the fault core below and pushed upward into the hangingwall injection site, possibly implying rapid emplacement and quenching of the injection materials by thermal pressurization and fluidization during seismic slip. Microstructural observations of cement and solution features will be used to infer the chemical characteristics and constrain density estimates and clast carrying capacity of the driving fluid. These observations will be used as boundary conditions to model the velocity and pressure of the driving fluid and the pore pressures required to drive injections and relate it to co-seismic stress conditions associated with large earthquakes in the continental fold and thrust settings. Our model attempts to understand the pore pressure limits on the fault, which we will compare to the pressurization required to propel thrust slip on the low angle fault and potentially provide the first quantitative observational constraints testing the pore pressure answer to the low angle thrust paradox.