Subduction and exhumation slip accommodation at depths of 10–80 km inferred from field geology of exhumed rocks: Evidence for temporal-spatial localization of slip

Abstract

ABSTRACT Field relationships in the Franciscan Complex of California suggest localization of subduction slip in narrow zones (≤300 m thick) at the depths of ~10–80 km. Accretionary and non-accretionary subduction slip over the ca. 150 Ma of Franciscan history was accommodated across the structural thickness of the complex (maximum of ~30 km). During accretion of a specific unit (<5 Ma), subduction slip (accretionary subduction slip) deformed the full thickness of the accreting unit (≤5 km), primarily on discrete faults of <20 m in thickness, with the remainder accommodated by penetrative deformation. Some faults accommodating accretionary subduction slip formed anastomosing zones ≤200 m thick that resulted in block-in-matrix (tectonic mélange) relationships but did not emplace exotic blocks. Mélange horizons with exotic blocks range in thickness from 0.5 m to 1 km. These apparently formed by sedimentary processes as part of the trench fill prior to subsequent deformation during subduction-accretion. Accretionary subduction slip was localized within some of these mélanges in zones ≤300 m thick. Such deformation obscured primary sedimentary textures. Non-accretionary subduction faults separate units accreted at different times, but these <100-m-thick fault zones capture a small fraction of associated subduction slip because of footwall subduction and likely removal of hanging wall by subduction erosion. Most exhumation was accommodated by discrete faults ≤30 m thick. Structural, geochronologic, and plate motion data suggest that of the ~13,000 km of subduction during the ca. 150 Ma assembly of the Franciscan Complex, ~2000 km was associated with accretion.