Pore fluid and seismogenic characteristics of fault rock at depth on the Wasatch Fault, Utah

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Abstract
Fluid inclusions and structural fabric in fault rock in the exhumed footwall of the Wasatch fault allow measurement of fluid pressure, temperature, and composition and estimation of rheological characteristics of fault behavior. Hydrothermally altered and deformed Oligocene quartz monzonite of the Little Cottonwood stock forms a partially preserved footwall carapace at the southern end of the Salt Lake segment of the Wasatch normal fault zone. Cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall. An early epidote‐chlorite‐sericite‐magnetite alteration assemblage developed in phyllonite and cataclasite, and a later laumontite‐prehnite‐hematite alteration assemblage developed during cataclasis. Fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains. Fluid inclusions that record the transition temperature from quasi‐plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite, epidote, and sericite and were trapped at minimum temperatures of 223°C–353°C and minimum fluid pressures of 1150–2800 bars. The fluids average 13 mol% CO2 and 8.2 wt % NaCl. A radiometric age of hydrothermal sericite suggests that the age of the alteration is 17.6±0.7 m.y. If maximum fluid pressure is near lithostatic, the minimum depth of formation of the alteration was 11 km at 337°C consistent with a thermal gradient of 30°C/km. The ratio of pore fluid pressure to lithostatic pressure varied from 0.52 to nearly 1.0 and indicates that effective normal stress on the fault at these depths may have approached zero. Structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault. Cleavage lineation and S‐C bands in the phyllonite indicate that deformation at temperatures above 350°C and depths greater than 11 km was dominated by aseismic creep. Unstable fracture propagation in cataclasite developed during seismic slip at shallower levels. The downdip, seismogenic length of the Salt Lake segment is estimated as 16 km using a fault zone dip of 45° and a minimum depth of 11 km for the quasi‐plastic/frictional transition. The segment strike length is 35 km, and the seismogenic area is about 600 km2, compatible with generation of M = 7.0 earthquakes.

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