Rheology and stress in subduction zones around the aseismic/seismic transition

John P Platt,William Lamborn Schmidt,Haoran Xia

doi:10.1186/s40645-018-0183-8

Abstract

Subduction channels are commonly occupied by deformed and metamorphosed basaltic rocks, together with clastic and pelagic sediments, which form a zone up to several kilometers thick to depths of at least 40 km. At temperatures above ~ 350 °C (corresponding to depths of > 25–35 km), the subduction zone undergoes a transition to aseismic behavior, and much of the relative motion is accommodated by ductile deformation in the subduction channel. Microstructures in metagreywacke suggest deformation occurs mainly by solution-redeposition creep in quartz. Interlayered metachert shows evidence for dislocation creep at relatively low stresses (8–13 MPa shear stress). Metachert is likely to be somewhat stronger than metagreywacke, so this value may be an upper limit for the shear stress in the channel as a whole. Metabasaltic rocks deform mainly by transformation-assisted diffusional creep during low-temperature metamorphism and, when dry, are somewhat stronger than metachert. Quartz flow laws for dislocation and solution-redeposition creep suggest strain rates of ~ 10−12 s−1 at 500 °C and 10 MPa shear stress: this is sufficient to accommodate a 100 mm/yr. convergence rate within a 1 km wide ductile shear zone.The up-dip transition into the seismic zone occurs through a region where deformation is still distributed over a thickness of several kilometers, but occurs by a combination of microfolding, dilational microcracking, and solution-redeposition creep. This process requires a high fluid flux, released by dehydration reactions down-dip, and produces a highly differentiated deformational fabric with alternating millimeter-scale quartz and phyllosilicate-rich bands, and very abundant quartz veins. Bursts of dilational microcracking in zones 100–200 m thick may cause cyclic fluctuations in fluid pressure and may be associated with episodic tremor and slow slip events. Shear stress estimates from dislocation creep microstructures in dynamically recrystallized metachert are ~ 10 MPa.

Highlights

Seismicity along the subduction zone interface at shallow depths transitions downwards into a zone of aseismic creep at depths of 25–40 km (Tichelaar and Ruff 1993)
The purpose of this paper is to present field and microstructural data from two terranes in California that represent rocks exhumed from the subduction channel developed in the Late Mesozoic to Early Tertiary subduction zone along the western margin of the North American plate
The process we describe is likely to produce repeating events with very similar characteristics, which is what is observed for Low-frequency earthquake (LFE) (Shelly et al 2007; Frank et al 2014)

Summary

Introduction

Seismicity along the subduction zone interface at shallow depths transitions downwards into a zone of aseismic creep at depths of 25–40 km (Tichelaar and Ruff 1993). The character and properties of the creeping zone are poorly known: many analyses assume that it is a zone of stable slip along the interface (e.g., Holtkamp and Brudzinski 2010), but exhumed rocks from these depths suggest there may be a so-called subduction channel containing metasedimentary and volcanic rocks, which take up much or all of the displacement (Gerya 2002; Warren et al 2008; Beaumont et al 2009; Blanco-Quintero et al 2011; Behr and Platt 2013). The seismic-aseismic transition generally occurs in the depth range 25–40 km, corresponding to temperatures in the range 350–500 °C (Peacock et al 2011), above the lower temperature limit for crystal plasticity in quartz (Hirth et al 2001)

Objectives

Conclusion