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Metamorphic evolution, partial melting and rapid exhumation above an ancient flat slab: insights from the San Emigdio Schist, southern California

Chapman, A. D. and Luffi, P. I. and Saleeby, J. B. and Petersen, S. (2011) Metamorphic evolution, partial melting and rapid exhumation above an ancient flat slab: insights from the San Emigdio Schist, southern California. Journal of Metamorphic Geology, 29 (6). pp. 601-626. ISSN 0263-4929.

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The San Emigdio and related Pelona, Orocopia, Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent southern California batholith along a shallow segment of the subducting Farallon plate in Late Cretaceous to early Tertiary time. These subduction accretion assemblages represent a regional, deeply exhumed, shallowly dipping domain from an ancient slab segmentation system and record the complete life cycle of the segmentation process from initial flattening and compression to final extensional collapse. An important unresolved question regarding shallow subduction zones concerns how the thermal structure evolves during the slab flattening process. New field relationships, thermobarometry, thermodynamic modelling and garnet diffusion modelling are presented that speak to this issue and elucidate the tectonics of underplating and exhumation of the San Emigdio Schist. We document an upsection increase in peak temperature (i.e. inverted metamorphism), from 590 to 700 °C, peak pressures ranging from 8.5 to 11.1 kbar, limited partial melting, microstructural evidence for large seismic events, rapid cooling (825–380 °C Myr^−1) from peak conditions and an ‘out and back’P–T path. While inverted metamorphism is a characteristic feature of southern California schists, the presence of partial melt and high temperatures (>650 °C) are restricted to exposures with maximum depositional ages between 80 and 90 Ma. Progressive cooling and tectonic underplating beneath an initially hot upper plate following the onset of shallow subduction provide a working hypothesis explaining high temperatures and partial melting in San Emigdio and Sierra de Salinas schists, inverted metamorphism in the schist as a whole, and the observed P–T trajectory calculated from the San Emigdio body. Lower temperatures in Pelona, Orocopia and Rand schists are likewise explained in the context of this overarching model. These results are consistent with an inferred tectonic evolution from shallow subduction beneath the then recently active Late Cretaceous arc to exhumation by rapid trench-directed channelized extrusion in the subducted schist.

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Luffi, P. I.0000-0001-9536-5405
Additional Information:© 2011 Blackwell Publishing Ltd. Received 9 August 2010; revision accepted 24 January 2011. Article first published online: 24 Feb. 2011. This research was supported by National Science Foundation grant EAR-0739071 and by the Gordon and Betty Moore Foundation. This is Caltech Tectonics Observatory Contribution 146. The manuscript benefitted from discussions with N. Borneman, E. Dachs, J. Eiler, Y. Guan, C. Jacobson, S. Kidder and D. Vielzeuf. A.Barth, J. Platt and D. Whitney are thanked for thoughtful reviews of the manuscript.
Group:Caltech Tectonics Observatory
Funding AgencyGrant Number
Gordon and Betty Moore FoundationUNSPECIFIED
Subject Keywords: diffusion modelling; garnet; inverted metamorphism; P–T pseudosections; San Emigdio Schist
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Caltech Tectonics Observatory146
Issue or Number:6
Record Number:CaltechAUTHORS:20110719-072052592
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Official Citation:Metamorphic evolution, partial melting and rapid exhumation above an ancient flat slab: insights from the San Emigdio Schist, southern California (pages 601–626) A. D. CHAPMAN, P. I. LUFFI, J. B. SALEEBY and S. PETERSEN Article first published online: 24 FEB 2011 | DOI: 10.1111/j.1525-1314.2011.00932.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24454
Deposited By: Ruth Sustaita
Deposited On:19 Jul 2011 15:20
Last Modified:09 Mar 2020 13:19

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