Strain localization and megathrust slip accommodation modes in the Franciscan Complex, California
John Wakabayashi, California State University, Fresno
Numerical models of subduction produce broad (multi-km thickness) zones of displacement that accommodate subduction megathrust displacement and exhumation of subduction complex rocks. Such displacement zones are commonly referred to as a "subduction channels". Many assert that mélanges represent the exhumed analogs of such channels, and that subduction slip and exhumation results in tectonic mixing of the blocks in matrix. Field relationships in the Franciscan Complex of California contradict such models. Franciscan mélanges with exotic blocks that have been considered type examples of exhumed subduction channels show evidence for incorporation of blocks by sedimentary sliding prior to burial and subsequent tectonic deformation. These mélanges are commonly less than 1 km in thickness and are variably deformed sedimentary packages rather than shear zones. In contrast, more limited (tens of meters thick) fault zones show progressive deformation from imbricated ocean plate stratigraphy to block-in-matrix geometry; such zones lack exotic blocks. Megathrust slip appears to have been accommodated in two modes, accretionary and non accretionary. For the accretionary mode a series of faults accommodated the transfer of the unit from the subducting to the upper plate. Whereas the megathrust slip in such cases is accommodated on faults spanning the full structural thickness of the unit (several km), most of the rock shows little if any penetrative strain. The faults accommodating the accretion range from cm-scale brittle fault zones to networks of brittle faults that may collectively span tens of meters. Non-accretionary megathrust zones may have accommodated most of the ~13000 km of Franciscan subduction slip. Such zones separate accreted units with notable contrasts in lithology and have a thickness of <50 m. Most of the exhumation of Franciscan rocks was accommodated by upper plate extension, and cross-sectional extrusion (thrust fault below, normal fault above). Faults accommodating exhumation appear to have been discrete features of minimal thickness (also tens of meters or thinner). This is consistent with sharp contrasts in metamorphic grade across narrow (10s of m and thinner) faults instead of broader metamorphic gradients.