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Styles of underplating in the Marin Headlands terrane, Franciscan complex, California

By
Christine A. Regalla
Christine A. Regalla
Department of Earth and Environment, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, USA
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Christie D. Rowe
Christie D. Rowe
Earth & Planetary Sciences Department, McGill University, 3450 University Street, Montréal, Québec H3A 0E8, Canada
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Nicolas Harrichhausen
Nicolas Harrichhausen
Earth & Planetary Sciences Department, McGill University, 3450 University Street, Montréal, Québec H3A 0E8, Canada
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Matthew S. Tarling
Matthew S. Tarling
Earth & Planetary Sciences Department, McGill University, 3450 University Street, Montréal, Québec H3A 0E8, Canada
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Jyotsana Singh
Jyotsana Singh
Earth & Planetary Sciences Department, McGill University, 3450 University Street, Montréal, Québec H3A 0E8, Canada
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Publication history
25 September 201723 April 2018

ABSTRACT

Geophysical images and structural cross sections of accretionary wedges are usually aligned orthogonal to the subduction trench axis. These sections often reveal underplated duplexes of subducted oceanic sediment and igneous crust that record trench-normal shortening and wedge thickening facilitated by down-stepping of the décollement. However, this approach may underrecognize trench-parallel strain and the effects of faulting associated with flexure of the downgoing plate. New mapping of a recently exposed transect across a portion of the Marin Headlands terrane, California, United States, documents evidence for structural complexity over short spatio-temporal scales within an underplated system. We documented the geometry, kinematics, vergence, and internal architecture of faults and folds along ~2.5 km of section, and we identified six previously unmapped intraformational imbricate thrusts and 13 high-angle faults that accommodate shortening and flattening of the underthrust section. Thrust faults occur within nearly every lithology without clear preference for any stratigraphic horizon, and fold vergence varies between imbricate sheets by ~10°–40°. In our map area, imbricate bounding thrusts have relatively narrow damage zones (≤5–10 m) and sharp, discrete fault cores and lack veining, in contrast to the wide, highly veined fault zones previously documented in the Marin Headlands terrane. The spacing of imbricate thrusts, combined with paleoconvergence rates, indicates relatively rapid generation of new fault surfaces on ~10–100 k.y. time scales, a process that may contribute to strain hardening and locking within the seismogenic zone. The structural and kinematic complexity documented in the Marin Headlands is an example of the short spatial and temporal scales of heterogeneity that may characterize regions of active underplating. Such features are smaller than the typical spatial resolution of geophysical data from active subduction thrusts and may not be readily resolved, thus highlighting the need for cross-comparison of geophysical data with field analogues when evaluating the kinematic and mechanical processes of underplating.

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Contents

GSA Special Papers

Geology and Tectonics of Subduction Zones: A Tribute to Gaku Kimura

Geological Society of America
Volume
534
ISBN electronic:
9780813795348

GeoRef

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