Collaborative Research: RUI: Testing the Stress/Heat-Flow Paradox of the San Andreas Fault with Fission-Track and U+Th/He Data from Zircon from the SAFOD Drill Hole

Saint Louis University, Saint Louis MO

Investigators

Abstract

The strength of large faults such as the San Andreas fault must be known to better understand their behavior and hopefully to predict future damaging earthquakes. There has been a longstanding debate, however, regarding the strength of large faults in the San Andreas system: Do they have normal or anomalously low strengths? There have been numerous field- and laboratory-based studies focused on this problem, but the strength of these faults is still not well known. One of the most widely accepted and oft-quoted arguments in support of anomalously weak faults is the absence of high heat flow in the vicinity of these faults. At stress levels typical of the earth's crust, seismogenic slip on normal-strength faults generates significant heat due to frictional sliding; however, for weak faults, the amount of frictionally generated heat will be much less. This affect has been coined the "stress / heat-flow" paradox. This research provides new data directly related to resolving the stress / heat-flow paradox by providing information regarding the integrated time-temperature history (and ultimately, the strength) in the San Andreas fault near Parkfield. The data are from zircons collected from cuttings and cores of the SAFOD drill hole. Four evenly spaced samples are being collected from the vertical drill hole of phase one drilling; thirteen more samples are being collected across the fault in the inclined hole of phase two. One aliquot of the zircon separates is being analyzed in the fission-track laboratory at Union College to determine fission-track ages and track length distribution. Another aliquot is being analyzed in the geochronology laboratory at Yale University to determine the U-Th/He ages of the zircons. A third aliquot of the zircons are being chemically analyzed with an electron microprobe to determine the chemistry and chemical zonations of the zircons. The results of these analyses provide constraints for our finite-element computer model of the San Andreas fault. The computer model incorporates the strengthes of the seismogenic upper crust and creeping lower crust of the fault with the time-temperature history deciphered from the fission-track and U-Th/He data of the zircons. The results of the modeling provide insight into the strength of the San Andreas fault near Parkfield. Outcomes of this research and the SAFOD project will benefit society because we will better understand the seismic hazard associated with this and other active fault systems. Our results will help earthquake scientists gain a greater understanding of the mechanics of faulting and thus better predict the occurrence of damaging earthquakes along the San Andreas system.