HR: 0800h
AN: T11A-0336    [Abstracts]
TI: Grain Fragmentation: Dynamic Implications
AU: * Davies, T R
EM: tim.davies@canterbury.ac.nz
AF: Department of Geological Sciences, University of Canterbury, New Zealand, Private Bag 4800, Christchurch, 8140, New Zealand
AU: Boulton, C J
EM: cjb205@student.canterbury.ac.nz
AF: Department of Geological Sciences, University of Canterbury, New Zealand, Private Bag 4800, Christchurch, 8140, New Zealand
AU: McSaveney, M J
EM: m.mcsaveney@gns.cri.nz
AF: GNS Science, Lower Hutt, New Zealand, PO Box 30368, Lower Hutt, 5040, New Zealand
AU: Bowman, L T
EM: elisabeth.bowman@canterbury.ac.nz
AF: Department of Civil Engineering, University of Canterbury, New Zealand, Private Bag 4800, Christchurch, 8140, New Zealand
AB: Granular flow has long been recognised as the dominant process in many geological phenomena, from bedload sediment transport through debris flows to rock avalanches and fault motion. The high confining stresses occurring in large-scale geological phenomena cause intact grains in a grain-flow to fragment as they shear. Grain flow with fragmentation thus affects the dynamics of many large-scale geophysical phenomena. Brittle grain fragmentation has been much studied in the context of shear band formation and fault motion, and recent 3D micromechanical simulations have succeeded in representing the kinematics of comminution, leading to successful reproduction of catacalastically deformed grains, shear localisation and fracture sets. In these simulations, however, the dynamics of fragmentation were investigated only in the context of acoustic emissions and induced seismicity. The kinetic energy arising from fragmentation was artificially and arbitrarily damped, decreasing the energy available for further deformation and obscuring the dynamic effects. We demonstrate that grain fragmentation under sufficiently high confining stress causes instantaneous local dispersive pressures in the GPa range. At sufficiently high strain rate the spatial concentration of fragmentation events can generate stresses sufficient to dominate the dynamics of grain flow; this leads to low intergranular effective stresses and correspondingly low frictional resistance at normal (Byerlee) values of friction coefficient. We also show that the spatial concentration of fragmentation is self-regulating to a calculable value. During coseismic rupture, high-velocity sliding generates strain rates high enough to produce dynamically-fragmenting grain flow in a localised principal slip zone; kinetic energy release within this zone quantitatively explains the observed low frictional resistance to shearing motion on the San Andreas and other large faults. It also satisfactorily explains the motion of the Heart Mountain and Waikaremoana blockslides and the Socompa volcanic debris avalanche.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8122 Dynamics: gravity and tectonics
SC: Tectonophysics [T]
MN: 2007 Fall Meeting