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

J. D. Kirkpatrick, Kate Dobson, D. F. Mark, Z. K. Shipton, E. E. Brodsky, F. M. Stuart

Research output: Contribution to journalArticle

18 Citations (Scopus)

Abstract

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 <= 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 +/- 0.3 Ma (2 sigma) from Ar-40/Ar-39 dating of pristine vein matrix. A suite of thermochronometers define the temperature-time path of the host rock granodiorite from similar to 550 to 60 degrees C. When the pseudotachylytes formed, the ambient temperature was 110 to 160 degrees C, implying a depth of similar to 2.4 to 6.0 km under typical geothermal gradients. At these depths, the failure stress on optimally oriented faults with Byerlee friction and hydrostatic pore pressure was <= 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 10 MPa), 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.

LanguageEnglish
Article numberB06406
Number of pages13
JournalJournal of Geophysical Research Atmospheres
Volume117
DOIs
Publication statusPublished - 23 Jun 2012

Fingerprint

pseudotachylite
thermochronology
rupture
melting
melt
rocks
cataclasite
hydrostatic pressure
geothermal gradient
hydrostatics
dating
veins
granodiorite
pore pressure
host rock
mechanics
ambient temperature
seismicity
faulting
friction

Keywords

  • pseudotachylyte formation
  • thermochronology
  • dynamic stress
  • Byerlee friction
  • hydrostatic pore pressure

Cite this

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abstract = "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 <= 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 +/- 0.3 Ma (2 sigma) from Ar-40/Ar-39 dating of pristine vein matrix. A suite of thermochronometers define the temperature-time path of the host rock granodiorite from similar to 550 to 60 degrees C. When the pseudotachylytes formed, the ambient temperature was 110 to 160 degrees C, implying a depth of similar to 2.4 to 6.0 km under typical geothermal gradients. At these depths, the failure stress on optimally oriented faults with Byerlee friction and hydrostatic pore pressure was <= 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 10 MPa), 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.",
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The depth of pseudotachylyte formation from detailed thermochronology and constraints on coseismic stress drop variability. / Kirkpatrick, J. D.; Dobson, Kate; Mark, D. F.; Shipton, Z. K.; Brodsky, E. E.; Stuart, F. M.

In: Journal of Geophysical Research Atmospheres, Vol. 117, B06406, 23.06.2012.

Research output: Contribution to journalArticle

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