The structure and composition of exhumed faults, and their implications for seismic processes

Evans, J.P., Shipton, Z., Pachell, M.A., Lim, S. and Robeson, K. (2000) The structure and composition of exhumed faults, and their implications for seismic processes. In: Proceedings of the 3rd Conference on Tectonic Problems of the San Andreas Fault System, Stanford University, 6-8 September 2000, pp. 67-81.

Abstract

Field studies of faults exhumed from seismogenic depths provide useful data to constrain seismologic models of fault zone processes and properties. Data collected on the San Andreas Fault in the San Gabriel Mountains has shown that large-displacement faults consist of one to several very narrow slip zones embedded in a cataclastically deformed sheared region several meters thick. However these faults have not been buried to depths greater than 5 km. Fault zones in the Sierra Nevada, California allow us to study the microstructures resulting from the deformation mechanisms active at seismogenic depths. Syn-fault mineralization shows that these left-lateral strike-slip faults formed at 5-12 km depth. Detailed microstructural analyses of the small faults reveal that they evolved from cooling joints filled by chlorite, epidote and quartz. These joints were then reactivated to form shear faults with accompanying brittle fracture and cataclastic deformation, ultimately developing very fined-grained cataclasites and ultracataclasites. The shear-induced microstructures are developed on faults with as little as several mm of slip showing that narrow slip-surfaces develop early in the lifetime of these faults. Subsequent slip has little effect on the microstructures. The inferred similarity of deformation mechanisms in faults 10 m to 10 km long indicates that basic slip processes on the faults are scale invariant, and may be a cause for the inferred constant b-value for small earthquakes. Analysis of map-scale fault linkages and terminations indicate that linkage zones are up to 400 m wide and 1 km long, and consist of altered and fractured rocks with numerous through-going slip surfaces. Terminations are regions of numerous splay faults that have cumulative offsets approaching those of the main faults. The slip distribution and structure of the terminations and linkage zones suggest that seismic slip may propagate into these zones of enhanced toughness, and that through-going slip can occur when a sufficient linkage of faults in the zone allow slip to be transmitted.

• Item Type: Conference Proceedings
• Status: Published
• Refereed: No
• Glasgow Author(s) Enlighten ID: Shipton, Dr Zoe
• Authors: Evans, J.P., Shipton, Z., Pachell, M.A., Lim, S., and Robeson, K.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Publisher: School of Earth Sciences, Stanford University
• First Published: First published in Proceedings of the 3rd Conference on Tectonic Problems of the San Andreas Fault System, 6-8 September 2000, Stanford University:67-81
• Publisher Policy: Reproduced with the permission of the Publisher