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Book Chapter

Seismogenic fault-zone processes and heterogeneity recorded by pseudotachylyte: New insights from the Homestake shear zone, Colorado

By
Joseph L. Allen
Joseph L. Allen
Department of Physical Sciences, Concord University, Athens, West Virginia 24712-1000, USA
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Colin A. Shaw
Colin A. Shaw
Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA
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Published:
January 01, 2013

Abstract

This one-day field trip will examine Proterozoic pseudotachylyte and ultramylonite in the Homestake shear zone (HSZ) in the northeastern Sawatch Range of central Colorado. Our ongoing research and geologic mapping shows that the HSZ incorporates a 25-km-long, partitioned system of dip-slip mylonites and ultramylonites and strike-slip to oblique-slip pseudotachylytes and uniquely preserves details of earthquake rupture at the fault-system scale. The HSZ originated as a high-temperature structure during continental assembly at ~1.7 Ga, and was reactivated as a subvertical, transpressional system at ~1.4 Ga under lower temperatures in a mid-crustal, intracontinental setting. The shear zone was seismogenic in this later deformation cycle. The HSZ shows a lateral frictional-plastic strain gradient across a width of 3–4 km, from mylonite and ultramylonite with mutually crosscutting pseudotachylyte, to mylonitic and recrystallized pseudotachylyte, to a system of dispersed pseudotachylyte-bearing fault strands. The broad width of the shear zone and delocalization of seismogenic fault strands suggests the HSZ is an example of a strong-type seismogenic fault. This trip will examine outcrops demonstrating this frictional-plastic strain gradient and discuss implications for coeval plastic flow and earthquake rupture near the base of the seismogenic zone. We also review the petrology and geochemistry of pseudotachylytes as well as the depth and environment of their generation.

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Contents

GSA Field Guide

Classic Concepts and New Directions: Exploring 125 Years of GSA Discoveries in the Rocky Mountain Region

Lon D. Abbott
Lon D. Abbott
Department of Geological Sciences University of Colorado Boulder, Colorado 80305 USA
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Gregory S. Hancock
Gregory S. Hancock
Department of Geology College of William and Mary Williamsburg, Virginia 23187 USA
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Geological Society of America
Volume
33
ISBN electronic:
9780813756332
Publication date:
January 01, 2013

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