STRAIN LOCALIZATION AND MEGATHRUST SLIP ACCOMMODATION MODES IN THE FRANCISCAN COMPLEX, CALIFORNIA

Numerical models of subduction predict that broad (multi-km thickness) zones of displacement that accommodate subduction megathrust displacement and exhumation of rocks along the subduction interface. Such displacement zones are commonly referred to as a “subduction channels”. Some suggest 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. These mélanges are commonly less than 1 km in thickness and are show evidence of genesis as variably deformed sedimentary packages in which sedimentary sliding mixed exotic blocks prior to burial and subsequent tectonic deformation. In contrast, localized (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 may have been accommodated in two modes, accretionary and non accretionary. In accretionary mode a series of imbricate faults accommodated the transfer of a rock unit from the subducting to the upper plate. Whereas such megathrust faults would occupy positions spanning the full structural thickness of the unit (up to several km), most of the rock shows little if any penetrative strain. This type of megathrust fault ranges from cm-scale brittle fault zones to networks of brittle faults that may collectively span tens of meters. Most of the ~13000 km of Franciscan subduction may have taken place in non-accretionary mode consistent with the nature of modern subduction zones. Such non-accreting megathrust 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). The exhumation-related faults appear to have been discrete features with thicknesses of 10s of m or less on the basis of sharp contrasts in metamorphic pressure across them instead of broader gradients.