FAULT VALVE AND SUCTION PUMP FLUID FLOW MECHANISMS AT THE SEISMOGENIC BASE OF A DYING ARC

Mapping, structure, and stable isotope analyses of fault veins in the Saddlebag Lake roof pendant of the central Sierra Nevada arc, California, suggest that valve and seismic pump mechanisms directed Late Cretaceous fluid flow at the seismogenic base of the crust. The Sawmill Canyon area contains a tectonically transpressive, ductile-brittle shear zone with a complex fluid flow history. This shear zone overprints Mesozoic ductile shortening, and overprints and is overprinted by Late Cretaceous arc magmatism at ~8 km depth.

In fault valve systems, hydrothermal mineral deposits seal fluids in unfavorably oriented compressive faults, elevating fluid pressure, removing normal stresses, and weakening them. Sawmill Canyon preserves evidence that relates this fault-fluid system to similar gold-quartz deposits associated with the valve mechanism. An L-S tectonite fabric preserves a steeply dipping metamorphic foliation and steeply plunging lineations under Amphibolite to Greenschist facies conditions. Quartz and tourmaline flats and hydrofracture breccia veins are evident.

In a seismic pump system, jogs that connect strike-slip faults at their rupture termini are instantaneously dilated, inducing a suction effect that implodes host rock and draws in water at pressures conspicuously lower than hydrostatic. In Sawmill Canyon area, a strike-slip duplex with brecciated faults contains releasing bend-related imbricate fans and oblique faults connecting higher order faults. Quartz vein stable isotope values from recent studies indicate that the fluid provenance was meteoric and may have mixed with earlier arc magmatic fluids.

These structure and isotope analyses indicate both valve and suction pump mechanisms were operative. In the former, fluids are forced out of the fault, and in the latter, they are drawn in. Thus, this rock record may preserve evidence for a means by which fault mechanisms drive vertical transport of fluids and heat across the seismogenic crust.