Styles of underplating in the Marin Headlands Terrane, Franciscan Complex, California

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 under-recognize 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, USA documents evidence for structural complexity over short spatio-temporal scales within an underplated system. We document the geometry, kinematics, vergence and internal architecture of faults and folds along ~2.5 km of section, and identify six previously unmapped intra-formational imbricate thrusts and thirteen 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), 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 paleo-convergence rates indicates relatively rapid generation of new fault surfaces on ~10-100 ka timescales, a process which may contribute to strain hardening and locking within the seismogenic zone. The structural and kinematic complexity documented in the Marin Headlands are 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.