The frictional strength of granular fault gouge: application of theory to the mechanics of low-angle normal faults
Published:January 01, 2009
Carolyn Boulton, Tim Davies, Mauri McSaveney, 2009. "The frictional strength of granular fault gouge: application of theory to the mechanics of low-angle normal faults", Extending a Continent: Architecture, Rheology and Heat Budget, U. Ring, B. Wernicke
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There is a persistent body of literature that suggests low-angle normal faults (LANF) form and slip seismically; if true, the effective friction coefficient is much lower (<0.3) than that found in laboratory tests of rock friction (c. 0.8) and in low-displacement faults that lack well-developed fault cores. This paper summarizes and discusses the mechanisms proposed to explain the low apparent friction of crustal-scale faults with low resolved shear stresses. Emphasis is placed on differentiating static weakening mechanisms, operating at strain rates c. 10−12 s−1–10−15 s−1, from dynamic weakening mechanisms, operating at strain rates >10−1 s−1. Previous published explanations for low fault friction do not appear to meet the key requirements of (i) reducing both static and dynamic frictional strength of LANF and (ii) operating only along crustal-scale faults. Fault rock assemblages in quartzo-feldspathic continental crust reveal that grain size reduction, or comminution, plays a fundamental role in fault zone development. As a fault accrues displacement, a fault core forms that contains granular material. We postulate that dynamic rock fragmentation occurs during the shearing of confined granular material; dynamic fragmentation is a volume-dependent mechanism responsible for reducing the static and dynamic frictional strengths of faults.
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Extending a Continent: Architecture, Rheology and Heat Budget
Over the last three decades, there has been a growing appreciation of the role of extensional tectonics in convergent orogens. The opening contribution to this book, by Brian Wernicke, provides a flavour of how this ‘detachment era’ has changed our views on tectonometamorphic relationships in mountain belts. This introduction provides a historical account on how our views on large-scale tectonic contacts in mountain belts have changed over the years. Wernicke concludes that controversy still persists over the existence and mechanics of slip on shallowly dipping extensional detachments, although incontrovertible field evidence indicates that slip on shallowly dipping extensional faults occurs in nature. Other papers in the volume provide a mix of new, innovative and controversial ideas that may help to solve the mechanical paradox on slip on shallowly dipping extensional detachments and quantitative case studies from New Zealand, the Aegean extensional province, the Alps and Finland.