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AGU: Journal of Geophysical Research, Solid Earth

 

Keywords

  • 40Ar/39Ar
  • Dynamic stress drop
  • Low temperature thermochronology
  • Pseudotachylyte
  • Sierra Nevada California

Index Terms

  • 1115 - Radioisotope geochronology
  • 1140 - Thermochronology
  • 8010 - Fractures and faults
  • 8025 - Mesoscopic fabrics
  • 8163 - Rheology and friction of fault zones

Paper in Press

JOURNAL OF GEOPHYSICAL RESEARCH, doi:10.1029/2011JB008846

The depth of pseudotachylyte formation from detailed thermochronology and constraints on coseismic stress drop variability

Key Points
  • Pseudotachylytes formed at 2.4 to 6 km
  • Negligible shear resistance implies complete stress drop associated with melting
  • Dynamic stress drop varied spatially by at least an order of magnitude

Authors:

James David Kirkpatrick

Katherine Joanna Dobson

Darren Francis Mark

Zoe Kai Shipton

Emily E. Brodsky

Finlay Stuart

Pseudotachylytes are accepted as recording paleo-seismicity in the rock record. However, the interpretation of the mechanics of faulting based on pseudotachylyte generation is often hindered because the depth at which they form is poorly constrained. Here, we use thermochronology to determine the depth at which pseudotachylytes in the Sierra Nevada, California, formed. The pseudotachylytes formed in {less than or equal to}10 m long patches over a rupture surface, the rest of which comprised cataclasites that did not melt. The age of the pseudotachylytes is found to be 76.6 {plus minus} 0.3 Ma (2σ) from 40Ar/39Ar dating of pristine vein matrix. A suite of thermochronometers define the temperature-time path of the host rock granodiorite from ~550 to 60 ˚C. When the pseudotachylytes formed, the ambient temperature was 110 to 160 {degree sign}C, implying a depth of ~2.4 to 6.0 km under typical geothermal gradients. At these depths, the failure stress on optimally oriented faults with hydrostatic pore pressures was {less than or equal to}51 MPa. Following melting, the dynamic stress acting on the fault is the melt shear resistance, which we calculate to be <0.2 MPa, suggesting that the stress drop associated with melting was complete. To conform with seismologically observed dynamic stress drops averaged over an entire rupture (1 to 10MPa), dynamic stress drop must have varied by at least an order of magnitude between the parts of the fault that melted and those that did not. Constraining the depth of pseudotachylyte formation using thermochronology therefore provides a quantitative estimate of the degree and scale of coseismic stress heterogeneity.

Received 20 September 2011; accepted 17 April 2012.

Citation: Kirkpatrick, J. D., K. J. Dobson, D. F. Mark, Z. K. Shipton, E. E. Brodsky, and F. Stuart (2012), The depth of pseudotachylyte formation from detailed thermochronology and constraints on coseismic stress drop variability, J. Geophys. Res., doi:10.1029/2011JB008846, in press.