The Rheology of Deep Slow Slip during Subduction: the View from the Rock Record

Cailey Condit, University of Washington
Melodie French, Rice University
William Hoover, University of Washington
Nicole Ferrie, University of Washington

Abstract

Episodic tremor and slow slip events occur down dip of the locked seismogenic zone in well instrumented subduction zones around the world. During these events, the plate interface slips faster than tectonic creeping rates but slower than earthquakes, and can readily load the locked seismogenic zone. Geophysical observations of the slow slip environment suggest it is a critically stressed, fluid-rich part of the plate interface. These constraints have led to a range of hypotheses for slow slip mechanisms from viscous creep to activation of frictional mechanisms by elevated pore fluid pressures. However, these hypotheses remain largely unconstrained in the rock record. We present geological, microstructural, and textural observations from two exhumed subduction terranes to examine the mechanisms that accommodated deformation at the depths of deep slow slip. We couple these observations with rheological analysis to link deformation features to slip modes during subduction. By targeting deformation microstructures from a range of typical subduction lithologies, we show that quartz and feldspar-rich rocks deform predominantly by pressure solution creep, and cannot accommodate slow slip strain rates unless stresses are 10 to 100x larger than estimated. In contrast, metasomatic schists containing talc and chlorite have microstructures consistent with rheological models suggesting these chemically hybrid lithologies can host slow slip strain rates frictionally during periods of elevated pore fluid pressures. Together, these observations provide key geologic constraints on the mechanisms and conditions of deep slow slip events during subduction, and suggest that chemical transformations and fluid infiltration play a key role in leading to these mechanical behaviors.

Session

Session 2: Rheology of the Lithosphere