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.

Abstract Earthquakes arise from frictional ‘stick–slip’ instabilities as elastic strain is released by shear failure, almost always on a pre-existing fault. How the faulted rock responds to applied shear stress depends on its composition, environmental conditions (such as temperature and pressure), fluid presence and strain rate. These geological and physical variables determine the shear strength and frictional stability of a fault, and the dominant mineral deformation mechanism. To differing degrees, these effects ultimately control the partitioning between seismic and aseismic deformation, and are recorded by fault-rock textures. The scale-invariance of earthquake slip allows for extrapolation of geological and geophysical observations of earthquake-related deformation. Here we emphasize that the seismological character of a fault is highly dependent on fault geology, and that the high frequency of earthquakes observed by geophysical monitoring demands consideration of seismic slip as a major mechanism of finite fault displacement in the geological record.

Abstract A microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces. The exhumed Chrystalls Beach Complex, in the Otago Schist, New Zealand, may be analogous to an actively deforming underthrust rock assemblage. The complex contains asymmetric competent lenses of sandstone, chert and basalt enclosed in a cleaved mudstone matrix. Continuous fabrics such as folds, boudins and asymmetric phacoids formed by distributed cataclasis and dissolution–precipitation creep. Discontinuous deformation is evident in an extensive fault-fracture mesh involving mutually cross-cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces. The Hikurangi margin provides an example of along-strike variations in seismic style, possibly related to heterogeneous fluid-pressure state and interface geology. In both the ancient and active subduction-related shear zone, fluid-pressure state appears to be a critical control on frictional failure, which primarily occurs on weak, fluid-overpressured discontinuities. Continuous, aseismic deformation occurs where other mineral deformation mechanisms, such as dissolution–precipitation creep, are preferred. The geometry and composition of the underthrust rock assemblage appear to be first-order controls on megathrust fluid-pressure distribution, bulk rheology and dominant deformation mechanism, and thus may be significant controls on megathrust seismic style.