Can Competition Between Frictional Sliding and Viscous Creep Determine Megathrust Fault Slip Style?

Thursday, 18 December 2014: 3:10 PM
Ake Fagereng, University of Cape Town, Department of Geological Sciences, Cape Town, South Africa
In exhumed megathrust analogues, deformation is partitioned between continuous and discontinuous deformation structures, commonly reflecting partitioning between concurrent frictional and viscous shear. This partitioning is a function of material properties, strain rate, and fluid pressure distribution. Mineral strength and preferred deformation mechanism vary down-dip as a function of temperature and pressure; however, incoming sediment composition, roughness of the sea floor, and the relative proportions of competent and incompetent material, all affect bulk rheology, and may vary both with depth and along strike.

Fluid pressure varies with depth, but also along strike if fluid sources and/or permeability vary along the margin. At the locations of major dehydration reactions, localized peaks in fluid pressure occur if permeability is low. These zones of low effective stress may allow for frictional sliding in rocks normally deforming by viscous shearing flow, and could relate to zones of tremor and slow slip. Frictional sliding and possible associated tensile fractures would, however, allow fluid escape, resulting in fluid pressure fluctuations and a time-dependent interplay between continuous and discontinuous deformation.

Locally elevated effective stress increases frictional strength, promoting failure by viscous mechanisms. If this is true, and representative of large-scale megathrust behavior, then decreased fluid pressure may promote creep. In a fluid-saturated, tabular fault zone with small grain size, this creep can take place by pressure solution creep at subgreenschist conditions. If pressure solution is the active mineral deformation mechanism, and shear is distributed though a tabular zone, viscous shearing flow at plate boundary rates is possible at temperatures significantly less than required for the onset of dislocation creep in quartzofeldspathic rocks. In a wide shear zone, such viscous flow may occur at low differential stress. A hypothesis to consider, consistent with field observations of coexisting frictional sliding and pressure solution creep, is then that locked megathrust segments have relatively low effective stress and prefer frictional over viscous deformation, whereas creeping segments have higher effective stress and dominantly deform viscously.