SUBDUCTION INTERFACE PROCESSES AT DEPTHS OF 10-30+ KM INFERRED FROM FIELD GEOLOGY OF EXHUMED ROCKS: EVIDENCE FOR LOCALIZATION OF SLIP ASSOCIATED WITH SUBDUCTION AND EXHUMATION

Subduction megathrust earthquakes and associated fault rupture can be studied by seismology, but the nature of subduction interface deformation over geologic timescales in the depth range of such earthquakes cannot be directly observed. Subduction complex rocks have been transferred from the subducting plate to the upper plate by megathrust slip during subduction accretion, which takes place when the subduction interface steps into the subducting plate. Examination of such rocks from the Franciscan Complex (FC) of California, where most rocks have been exhumed from depths of about 10 to 30 km, provides insight into the nature of deformation along the subduction interface in the depth range associated with great earthquakes. During accretion, subduction slip is accommodated across the thickness of the accreted unit (up to 2 to 3 km) on discrete faults <10 m in thickness, forming duplexes, and by penetrative deformation within the unit, as well as on out-of-sequence faulting cutting structural higher, previously accreted units. Locally, faulting results in tectonic mélange formation without exotic blocks, although the general structure is that of a coherent imbricated stack. Accreted rocks comprise trench fill clastic sediments with a variable component of the subducting oceanic plate, including pelagic sedimentary rocks (mostly chert), and/or the upper igneous part of the oceanic plate (mostly basalt). The trench fill includes bedded turbidites and submarine landslide deposits, the latter represented by sedimentary mélanges with exotic blocks. Non-accretionary megathrust horizons separate units accreted at different times and accommodate slip during periods of non-accretion or subduction erosion. These are preserved as fault zones <50 m in thickness that may include pseudotachylites. Contrasts in metamorphic pressure, recorded by metamorphic mineral assemblages, occur across discrete faults <50 m in thickness rather than broader metamorphic gradients, suggesting localization of strain during exhumation in addition to subduction. Most of the FC records processes associated with a cold subduction zone. However, imbricate structures within the highest-grade FC rocks, that record warmer geothermal gradients at depths of up to about 70 km, suggest similar slip localization.