Exhumation of High-Pressure Metamorphic Rocks in Accretionary Orogens


This is a research project in progress by John Platt, 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?

By pervasive shear during subduction at low temperature?

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

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 ?

Underplating and extension ?

Or return flow in a subduction channel (?


The nature of the Coast Range Fault

An out-of-sequence thrust?

A backthrust above a Tertiary blind thrust?  

Or an extensionally reactivated thrust?

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

of the Paskenta transect in the northern Coast Ranges