Constraints on heat generation along two (paleo)seismogenic faults in California from U-Th/He and fission-track data
Abstract
Although the stress / heat-flow paradox of the San Andreas fault has been debated for several decades, there is yet no widely accepted resolution to the problem. New results from the SAFOD drilling project on the San Andreas and similar drilling projects of other active faults around the world will help resolve this paradox. Similarly, our study and other detailed structural and thermochronologic studies of exhumed faults are providing important constraints on the evolution, thermal history, and strength of (paleo)seismogenic faults. We have obtained U-Th/He data for zircons from one inactive strike-slip fault in the San Andreas system (the San Gabriel) that has been exhumed approximately 2 to 5 kilometers. This data complements our current fission-track data set of 41 apatite and 8 zircon samples. The San Gabriel fault samples were collected in traverses near Whitaker Peak, the "Earthquake locality", and along Bear Creek. Samples were also collected from one traverse across the associated Punchbowl fault. Six zircon U-Th/He dates from the Bear Creek locality range from 18 to 23 Ma on the north side of the fault and from 36 to 51 Ma on south side of fault. Zircon FT dates from the same locality range from 39 to 46 Ma on the north side and 42 to 61 Ma on south side of the fault. Apatite FT dates from the same locality range from 9 to 22 Ma and 18 to 44 Ma (excluding one sample) from the north and south side of the fault, respectively. These data are consistent with an approximately 20 Ma offset in U-Th/He and FT dates across the fault and "cooling" rates between 3 and 6 oC / Ma. Apatite FT dates are between 51 and 25 Ma from Whitaker Peak, 54 and 24 Ma from the Earthquake locality, and 15 and 7 Ma at the Punchbowl fault. U-Th/He and FT dates vary systematically with distance from the fault cores along individual traverses; though, they do not young monotonically towards the faults, which would be expected if there had been sustained elevated temperature along the faults. Until we obtain more U-Th/He data, we are unable to definitively determine the factors controlling the variations in dates, though there is a moderately good correlation between dates and concentration of mobile elements in the fault rocks. Ultimately, this data set will help constrain the heat production and strength of these faults when they were active.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.T23A0568K
- Keywords:
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- 7209 Earthquake dynamics and mechanics;
- 1035 Geochronology