HR: 0800h
AN: T41B-0584    [Abstracts]
TI: Fault Zone Weakening and Strain Localization Adjacent to the Alpine Fault in Fiordland, New Zealand: the Roles of Cohesion Loss and Elevated Fluid Pressure
AU: * Judge, P A
EM: paj42@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences Snee Hall, Ithaca, NY 14853, United States
AU: * Judge, P A
EM: paj42@cornell.edu
AF: University of Vermont, Geology Department 180 Colchester Ave, Burlington, VT 05405, United States
AU: Klepeis, K A
EM: kklepeis@uvm.edu
AF: University of Vermont, Geology Department 180 Colchester Ave, Burlington, VT 05405, United States
AB: The mechanisms by which continental transforms and major strike-slip faults localize deformation and accommodate large amounts of strain are unresolved problems in continental tectonics. We used fault-slip data, geological observations, and an analysis of paleostress tensors to empirically test the roles of cohesion loss and elevated fluid pressure in localizing strain adjacent to the Alpine Fault in northern Fiordland, New Zealand. A kinematic analysis of fault populations reveals the presence of a narrow, ~10 km wide zone on the southeast side of the Alpine Fault where deformation is dominated by reverse faulting and oblique-dextral slip. Cross cutting relationships and published geochronology suggest that most of these faults record motion since ~11 Ma, when changing relative plate motions resulted in an increased component of compression across the Alpine Fault. Pervasive hydrothermal alteration, veining, and mineralization indicate that this zone was preferentially infiltrated by metamorphic fluids during deformation. Outside of the 10 km wide zone, to the east and southeast, evidence of pervasive fluid infiltration and metasomatism is lacking, and faults record dominantly strike-slip motion. Stress inversions indicate that within 10 km of the Alpine Fault, compression axes are oriented at moderately high (~65°) angles to the dominantly northeasterly strike of the plate boundary. Outside of this 10 km zone, stress axes are oriented at low (~10°) angles. The high angles recorded near the Alpine Fault contrast with a 11-25° angle of oblique plate convergence and suggest that the Alpine fault zone in Fiordland is very weak, and slips in response to very low shear stresses. A quantitative test of this hypothesis using geometric and frictional constraints on stress inversions confirmed that the coefficient of friction on the faults near the Alpine Fault is very low (μ = 0.10). This result suggests that cohesion loss as a result of high fluid pressures, mineral transformations, and a reduction in the coefficient of friction on faults are primary causes of fault zone weakening up to 10 km away from the main trace of the Alpine Fault. A loss of cohesion also appears to contribute to the localization of the contractional component of deformation away from the main trace of the Alpine Fault in Fiordland. This style of kinematic partitioning, where the contractional component of deformation occurs elsewhere and the Alpine Fault itself records strike-slip motion, is unique to the southernmost segment of the plate boundary. Farther north the Alpine Fault records reverse-dextral slip and low degrees of strike-slip partitioning.
DE: 8045 Role of fluids
DE: 8108 Continental tectonics: compressional
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: 2007 Fall Meeting