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New, much younger ages for the Yolla Bolly terrane and a revised time line for accretion in the Franciscan subduction complex, California

By
Trevor A. Dumitru
Trevor A. Dumitru
Jasper Canyon Research, Inc., 4100 Campana Drive, Palo Alto, California 94306, USA
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Jeremy K. Hourigan
Jeremy K. Hourigan
Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95064, USA
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William P. Elder
William P. Elder
Golden Gate National Recreation Area, U.S. National Park Service, Fort Mason, Building 201, San Francisco, California 94123, USA
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W.G. Ernst
W.G. Ernst
Department of Geological Sciences, Stanford University, Stanford, California 94305, USA
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Raymond Joesten
Raymond Joesten
Department of Chemistry and Center for Integrative Geosciences, University of Connecticut, Storrs, Connecticut 06269, USA
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Publication history
05 February 201829 October 2018

ABSTRACT

The Franciscan subduction complex formed over a protracted, ~150 m.y. period, during Late Jurassic to late Cenozoic subduction of oceanic lithosphere beneath the western margin of the North American continent. Growth of the complex occurred chiefly by progressive accretion, in which voluminous sediment was eroded from the magmatic arc and continent, deposited in the trench region, and then progressively subducted and accreted soon after deposition. The Yolla Bolly terrane, a major Franciscan subunit, has stood out as a possible exception to a progressive-accretion model. Yolla Bolly clastic rocks are almost barren of fossils, but there are ~13 localities with Late Jurassic and Early Cretaceous Buchia specimens, ~3 with mid-Cretaceous ammonites or Inoceramus, plus several with mid-Cretaceous youngest detrital-zircon populations. These ages had suggested that sediments may not have been deposited into an active trench, but instead were deposited into a relatively stable Yolla Bolly basin, which was both long-lived (ca. 150 Ma through ca. 95 Ma) and far-traveled (exotic). This basin was then accreted and metamorphosed at perhaps 92 Ma. It is surprising, however, that such a basin could have survived for ~50 m.y. along a subduction margin before being accreted. We determined detrital-zircon U-Pb ages from 31 new sandstone samples, including from key Buchia sites, and they indicate that Yolla Bolly clastic deposition actually occurred almost entirely between ca. 115 and 98 Ma. All of the Buchia specimens in the main parts of the Yolla Bolly terrane have been redeposited and the arc- and continent-sourced clastic rocks that comprise almost all of the terrane are much younger than once thought. This makes evolution of the Yolla Bolly terrane compatible with a progressive-accretion model, in which its constituent packets of clastic rocks were deposited in a native trench setting and then rapidly subducted, accreted, and metamorphosed.

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Contents

GSA Special Papers

Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson

Geological Society of America
Volume
540
ISBN electronic:
9780813795409

GeoRef

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