Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 17-8
Presentation Time: 10:35 AM


RAYMOND, Loren A., Department of Geology, Sonoma State University, 1801 East Cotati Ave, Rohnert Park, CA 94928

The underplated Franciscan Complex core of the Diablo Range consists predominantly of blueschist facies metasandstones and metashales with locally abundant metacherts and metabasalts comprising mélanges, dismembered formations, and broken formations. Core rocks were subducted, underplated, and metamorphosed at a depth of about 23-30 km. Evidence indicates that uplift of the southern Diablo Range began with the passing of the Mendocino Triple Junction about 14-12 my ago, whereas in the north surface evidence indicates that uplift was delayed until about 7-6 my ago. Details of the uplift history are debatable. Passing of the triple junction, imposition of a strike-slip faulting regime on the western edge of the continent, folding, and development of core-bounding high-angle faults, accompanied uplift in Late Cenozoic time. Local evidence indicates that the Late Cenozoic deformation events were sporadic and intermixed temporally. Detailed mapping in the northeastern Diablo Range reveals five principal Franciscan Accretionary Units (AUs) that were affected by these events. Removing strike–slip separations on the Arroyo Mocho, Pegleg, Chicken Flat, and Ballard Flat faults spreads parts or all of the northernmost of these units to the south. Unfolding of the Mt. Oso anticline rotates some units from their current positions and orientations. These reconstructions yield three southern, folded units including a mélange, with WNW-striking fold axes and unit trends, plus three NW-trending AUs that are mélanges and dismembered formations (including extensions of the southern mélange). Unfolding the southern units further spreads the AU stack, leaving a series of E-W- to NW- trending AUs lying beneath a broad, EW-trending arch in the overlying Coast Range Ophiolite (CRO) plus sub-ophiolite mélange. Unfolding the arch leaves an imbricate, NE dipping AU stack of four units with an overlying mélange cap, all metamorphosed to blueschist facies grade, which requires that all Franciscan units be lowered in reconstruction by 23 - 30 km. The pressure gap revealed by metamorphism, between the CRO and the underlying Franciscan AUs, requires up to 15 km of Mesozoic and Cenozoic section be faulted out along the northeastern core-bounding fault(s), during uplift at a rate of 1.3 mm/yr over a period of 7 million years.
  • Raymond.2019.Cordilleran Talk 17-8.pdf (13.7 MB)