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Geology; August 2008; v. 36; no. 8; p. 595-598; DOI: 10.1130/G24993A.1
© 2008 Geological Society of America
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Initiation of Franciscan subduction along a large-offset fracture zone: Evidence from mantle peridotites, Stonyford, California

Sung Hi Choi1, Samuel B. Mukasa*,2 and John W. Shervais3

1 Korea Polar Research Institute, Songdo Techno Park 7-50, Incheon 406-840, South Korea
2 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA
3 Department of Geology, Utah State University, Logan, Utah 84322-4505, USA

Correspondence: *E-mail: mukasa{at}umich.edu.

We report here on the radiogenic isotope characteristics of peridotites from one ophiolite locale, Stonyford, in the Coast Range ophiolite (CRO) of California, spanning the entire observed range of upper mantle peridotites from both abyssal and subduction-zone settings. Based on the Sr-Nd-Pb-Hf isotopic compositions and estimated equilibration temperatures, we suggest that the abyssal peridotite block represents a remnant of large-offset transform oceanic lithosphere at >172 Ma. East-dipping, proto-Franciscan subduction is likely to have been initiated along this fault zone during a plate reorganization event, which produced the supra-subduction zone (SSZ) peridotites in the CRO. We propose that the remnant abyssal peridotites represent a snapshot of the mantle wedge composition prior to overprinting of large sectors by hydrous melting during the subsequent subduction-controlled SSZ processes.

Key Words: abyssal peridotite • large-offset fracture zone • Sr-Nd-Pb-Hf isotope • SSZ peridotite • Coast Range ophiolite • California






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