T6 Feedbacks between Upper-Plate Deformation, Accretion and Alteration and Plate Boundary Processes in Subduction Systems

Sunday, 10 October 2021
8:00 AM - 12:00 PM
B111

TEMPORAL AND SPATIAL LOCALIZATION OF SUBDUCTION AND EXHUMATION SLIP AND ESTIMATES OF ACCRETIONARY VERSUS NON-ACCRETIONARY SLIP: VIEW FROM THE FRANCISCAN COMPLEX

Paper number: T6-12
Program: Technical Sessions
Time: 10:50 AM – 11:05 AM

Authors

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. Broad (>100 m thick) gradients of recorded metamorphic pressure are lacking. 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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-365334

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