Precarious Containment of Overpressured Fluids in Subduction Settings

Thursday, 18 December 2014
Richard H Sibson, University of Otago, Dunedin, New Zealand
Subduction forearcs are critically organized systems whose metastable state is governed by the configuration of boundary stresses and the internal distribution of fluid-pressure in pore/fracture space. In particular, seismogenic megathrusts lying within subduction interface shear zones (SISZ) appear overpressured to near-lithostatic values (i.e. λv = Pfv → 1.0) with substantial fluid repositories (<4% porosity) likely in tabular zones of non-volcanic tremor (NVT) defining the base of some megathrusts. Containment of overpressure is precarious because activation of brittle fault-fracture systems allows escape of overpressured fluids. This leads to a critical interdependence of differential stress and fluid-overpressure with overpressures more easily sustained in compressional as opposed to extensional stress regimes. Overpressures within SISZ are thus susceptible to abrupt stress changes and subsidiary fracturing that may occur locally around rupture heterogeneities or, on a broader scale, when total shear stress relief and stress field switching occurs along a megathrust, as occurred during the 2011 Mw9.0 Tohoku-oki earthquake. Massive fluid loss from a SISZ following megathrust rupture has been inferred from observed changes in the velocity structure of a fore-arc hangingwall. Paleodischarge sites in subduction fore-arcs exists in the form of diapiric roots to mud volcanoes and hydrothermal vein swarms. In some exhumed forearcs, extensive belts of Au-Quartz mineralization (orogenic gold) are plausibly related to episodic fluid redistribution from the subduction interface. Fluid loss from SISZ locally raises frictional strength along the megathrust, forming strength asperities. Subsequent failure of such ‘drainage asperities’ is then governed by the reaccumulation of fluid overpressure as well as shear stress within the SISZ. Populations of drainage asperities at various stages of overpressure restoration are likely along subduction interfaces.