Abstract: Neogene Paleogeography and Tectonics of Northwesternmost California

K. R. Aalto, Kathy Moley, Linda Stone

A complex series of tectonic scenarios is invoked to explain the Neogene sedimentary tectonic history of northwestern California. Contrasting styles of tectonism include: (1) the evolution of the Late Oligocene to Early Miocene Weaverville fluvial deposystem via La Grange detachment faulting and coincident clockwise rotation of the northwestern Klamath Mountains and Coast Ranges; (2) the growth of the Neogene Cascadia accretionary wedge, which involved the subduction accretion of Cenozoic Franciscan Complex rocks (including the newly-discovered 'False Cape terrane'), accumulation of a thick, regionally extensive, Neogene shelf/slope sediment apron, evolution of a Pleistocene fold-and-thrust belt with coeval regional uplift of Klamath peneplain; and (3) coastal uplift and eformation that accompanied the migration of the of Mendocino triple junction into the region.

The Weaverville Formation accumulated syntectonically in upper plate grabens above the LaGrange detachment fault. Imbrication of Weaverville Formation fluvial clasts indicates westerly flow of a major braided river. However, the deltaic facies of this formation is missing, due either to Miocene erosional stripping or removal by faulting. Basal Late Miocene to Early Pliocene Wildcat Group and Wimer Formation sediments are fluvial to littoral. However, rapid submergence of the northwestern California to bathyal depths (2-3 km) occurred following the initiation of Wildcat-Wimer sedimentation. Ca. 5 Ma the Klamath peneplain was at sea level, based upon Wimer overlap. ⁴⁰Ar/³⁹Ar laser probe analyses of detrital micas indicate that Wildcat Group marine sediments are in art derived from the Idaho batholith. Sediment progradation and shoaling over a wide coastal shelf, onset of deposition of locally-derived fluvial sediments in the Coast Ranges and regional uplift during the mid-Pleistocene may relate to the decreased plate convergence rate, lessening of angle of subduction and increased normal convergence.

North of the Mendocino triple junction, a Neogene accretionary complex has been uplifted >2 km and tilted northward in response to the interaction of the Gorda plate with the older North American and Pacific plates. Plate interaction resulted in tectonic mixing of accreted Franciscan Complex rocks with those of the Neogene overlap assemblage in subvertical shear zones. Folding, tilting and shear zone development reflects ongoing north-south directed crustal shortening that occurred in response to plate coupling and wedge thickening in the deforming Miocene and younger accretionary complex that accompanied migration of the triple junction into the region.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California