HR: 14:15h
AN: S33D-03 INVITED    [Abstracts]
TI: Involvement of Overpressured Fluids in the Nucleation of High-Angle Reverse Ruptures: Evidence from Fault-Hosted Hydrothermal Vein Systems
AU: * Sibson, R H
EM: rick.sibson@otago.ac.nz
AF: Department of Geology University of Otago, P.O. Box 56, Dunedin, 9054, New Zealand
AB: Dips of near-pure reverse-slip M>5.5 ruptures are bimodally distributed with a dominant peak at δ = 30±5°, a subordinate peak at δ = 50±5°, and no ruptures with δ > 60°. Assuming horizontal trajectories for maximum compressive stress (σ₁), the dominant peak corresponds to optimally oriented faults with Byerlee friction coefficients (μ_s = 0.6) for which frictional lock-up is expected at δ = 60°. In recent years, several compressional inversion earthquakes in the upper crust of Honshu, Japan (e.g. the 2003 M_w6.5 Northern Miyagi, the 2004 M_w6.6 Mid-Niigata Prefecture, and the 2007 M_w6.7 Noto-Hanto sequences) have involved high-angle reverse-slip with dips of 50-60° on inherited normal faults along the margins of Miocene extensional basins. Rupturing during these earthquakes thus took place on faults that were poorly oriented for frictional reactivation and close to lock-up. Frictional mechanics suggests that reshear of the steep reverse faults (in preference to the formation of new favorably oriented thrusts within intact crust) is allowable only under near- lithostatic fluid pressures with P_f approaching σ₃, and that reshear of severely misoriented faults (δ > c.60°) requires P_f > σ₃ (the hydrofracture condition). Notably, the 2004 Mid-Niigata sequence involved a criss-crossing network of high-angle and low-angle reverse ruptures, suggesting competition between reshear of steep inherited faults and the formation of more favorably oriented thrusts. A range of geophysical evidence, including local bright S-wave reflectors, indicates strong fluid overpressuring in the focal regions of these earthquakes. Mesozonal Au-quartz vein systems hosted in reverse faults exhumed from depths corresponding to the lower half of the seismogenic zone (P ~ 2-4 kbar; T ~ 250-400°C) occur throughout the geological record and provide additional evidence for the involvement of strongly overpressured fluids in reverse fault rupturing. Incrementally deposited fault-infill veins up to meters in thickness may extend for 1-2 km down-dip with comparable dimensions along-strike. For steeper faults, especially, these fault-veins are commonly in mutual cross-cutting relationships with arrays of flat-lying extension veins that are the product of hydraulic extension fracturing. Individual extension veins extend laterally for tens to hundreds of metres, tapering away from the reverse faults, but flat vein arays may extend over greater distances. The fault-related vein systems have been interpreted as the product of cyclical fault-valve action whereby failure on severely misoriented reverse faults (oriented at > 55-60° to σ₁) is triggered by the accumulation of overpressure to near-lithostatic values, the ensuing fault rupture then allowing postfailure discharge upwards along the fault. Fluid inclusion studies support the cycling of fluid-pressure between ~lithostatic prefailure and sublithostatic postfailure values. The flat-lying hydrofracture arrays provide an explanation for the bright-spot reflectors observed around the base of the seismogenic zone, while the fault-vein complex may represent rupture nucleation sites on steep reverse faults where failure is predominantly fluid-driven. Under such circumstances, near-total shear stress relief may accompany rupture. Net fluid volumes involved in the formation of these vein systems may be of the order of 1 km³ per kilometer strike-length, but the fluid volume involved in each fault-valve cycle is likely to be 2-4 orders of magnitude lower.
DE: 7215 Earthquake source observations (1240)
DE: 8045 Role of fluids
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: 2007 Fall Meeting