Skip to Main Content
Book Chapter

Fluid-related deformation processes at the up- and downdip limits of the subduction thrust seismogenic zone: What do the rocks tell us?

By
Åke Fagereng
Åke Fagereng
School of Earth & Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK
Search for other works by this author on:
Johann F.A. Diener
Johann F.A. Diener
Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
Search for other works by this author on:
Susan Ellis
Susan Ellis
GNS Science, P.O. Box 30-368, Avalon, Lower Hutt 5010, New Zealand
Search for other works by this author on:
Francesca Remitti
Francesca Remitti
Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Modena 41125, Italy
Search for other works by this author on:
Publication history
25 September 201714 June 2018
25 September 201714 June 2018
Gold Open Access: This paper is published under the terms of the CC-BY license.

ABSTRACT

The subduction thrust interface represents a zone of concentrated deformation coupled to fluid generation, flow, and escape. Here, we review the internal structure of the megathrust as exposed in exhumed accretionary complexes, and we identify a deformation sequence that develops as material entering the trench is subducted through the seismogenic zone. Initial ductile flow in soft sediment generates dismembered, folded, and boudinaged bedding that is crosscut by later brittle discontinuities. Veins formed along early faults, and filling hydrofractures with the same extension directions as boudins in bedding, attest to fluid-assisted mass transfer during the shallow transition from ductile flow to brittle deformation. In higher-metamorphic-grade rocks, veins crosscut foliations defined by mineral assemblages stable at temperatures beyond those at the base of the seismogenic zone. The veins are, however, themselves ductilely deformed by diffusion and/or dislocation creep, and thus they record fracture and fluid flow at a deeper brittle-to-ductile transition.

The results of numerical models and mineral equilibria modeling show that compaction of pore spaces may occur over a wide zone, as underconsolidated sediments carry water under the accretionary prism to the region where the last smectite breaks down at a temperature of ≤150 °C. However, at temperatures above clay stability, no large fluid release occurs until temperatures reach the zone where lawsonite and, subsequently, chlorite break down, i.e., generally in excess of 300 °C. In thermal models and strength calculations along overpressured subduction interfaces, where phyllosilicates form an interconnected network that controls rheology, as is generally observed, the deep brittle-viscous transition—analogous to the base of the seismogenic zone—occurs at temperatures less than 300 °C. We therefore suggest that the seismogenic zone does not produce fluids in significant volumes; however, major fluid release occurs at or near the base of the seismogenic zone. These deep fluids are either trapped, thus enabling embrittlement and features such as episodic tremor and slow slip, or flow updip along a permeable interface. Overall, we highlight fluid production as spatially intermittent, but fluid distribution as controlled also by the permeability of a deforming zone, where secondary porosity is both generated and destroyed, commonly in sync with the generation and movement of fluids.

Figures & Tables

Contents

GSA Special Papers

Geology and Tectonics of Subduction Zones: A Tribute to Gaku Kimura

Geological Society of America
Volume
534
ISBN electronic:
9780813795348

GeoRef

References

Related

A comprehensive resource of eBooks for researchers in the Earth Sciences

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal