Introduction

This is a research project in progress by John Platt, in collaboration with Trevor Dumitru and Marty Grove (Stanford) and John Wakabayashi (California State University, Fresno). It is funded in part by NSF.

Some of the outstanding problems in the Franciscan Complex: California’s Mesozoic - Tertiary accretionary complex

Origin of mud-matrix melanges

Are these formed by superficial processes, as olistostromes, for example (Cowan, Wakabayashi)

By pervasive shear during subduction at low temperature?

Or by corner flow of water-saturated mud in a subduction channel (Cloos)?

Contractional deformation

Can we identify structures formed during subduction and accretion?

How do these structures relate in time to melange formation and to the high-pressure metamorphism?

Exhumation of high-P metamorphic rocks

Is this primarily a result of erosion (Brandon & Ring)?

Underplating and extension (wedge mechanics: Platt)?

Or return flow in a subduction channel (Cloos)?

The nature of the Coast Range Fault

An out-of-sequence thrust (Brandon & Ring)?

A backthrust above a Tertiary blind thrust (Wentworth & Zoback)?

Or an extensionally reactivated thrust (Platt, Jayko & Blake)?

We are trying to address these issues by a detailed structural analysis

of the Paskenta - Covelo transect in the northern Coast Ranges