Paper No. 4
Presentation Time: 8:50 AM


SIBSON, Richard H., Geology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand,

Subduction systems promote mineralization through critically interdependent magmatic, metamorphic, hydrothermal, and seismotectonic activity, all modulated by the stress state. Seismological and related studies of recent great earthquakes (e.g. the 2011 Mw9.0 Tohoku-oki earthquake), have resolved the fine structure of subduction interface shear zones (SISZ) and the physical conditions affecting megathrust rupture, illuminating potential structural pathways for fluid redistribution. SISZ serve as barriers to fluid released from metamorphic dewatering of the descending slab and appear to be strongly, if variably, overpressured to near-lithostatic values to the base of the seismogenic megathrust at c. 40±5 km. In some cases, the down-dip base of the seismogenic megathrust is defined by a band of episodic tremor which, with <4 % porosity, represents a significant repository (> 1 km3/km strike-length) of hydrothermal fluid with 350° < T < 450 °C. A plausible model for this tremor band is that it takes the form of a dilatant fault-fracture shear mesh within the SISZ.

Containment of overpressured fluid within and below the SISZ is precarious and critically dependent on both the stress state and structural permeability, both of which may change dramatically across much of an island arc following megathrust rupture. Aftershocks from the Tohoku earthquake reveal, for example, that the fore-arc stress regime switched from horizontal compression with reverse faulting to extension with normal faulting over an area of approximately 100,000 km2. Dramatic increases in permeability along the SISZ accompanying stress changes during rupture, together with local structural irregularities promote fluid loss from the interface into the fore-arc hangingwall – a form of trans-megathrust fault-valve action (cf. Husen & Kissling, 2001). Mud volcanoes and hydrothermal vein swarms testify to transitory fluid release through fore-arcs. Hangingwall dilation localized above the down-dip limits of megathrust ruptures may especially favor fluid release. It may not be a coincidence that in paleotectonic reconstructions, the Mother Lode belt of orogenic Au-quartz mineralization appears to overlie the down-dip end of the likely megathrust interface for California in the Early Cretaceous.