Structure and relationships of late mesozoic schists of NW california and SW Oregon

Abstract

Abstract Cross sections through the South Fork Mountain Schist of north-western California and the Colebrooke Formation schist of south-western Oregon are described and the regional relationships of the rocks outlined. The schists are of the so-called blueschist facies, and have undergone intense synmetamorphic and less intense post-metamorphic folding and cataclasis. High pressure-low temperature conditions of metamorphism apparently preceded and then overlapped dynamic metamorphism. Three textural subzones result from progressive increase in deformation; mineral zonation related to increase in P-T conditions is less distinct. Axial planes of folds dip easterly and most of the structures probably “face” westerly towards the Pacific Ocean. Relations between mineral and textural isogradic surfaces are obscure, but they are almost certainly non-parallel. Textural zones in the South Fork Mountain Schist are apparently inverted and mineral isograds steepened by thrust-faulting, but neither is directly related to the faults as now exposed. K/Ar dates reveal a dose relationship between metamorphic grade and age of schists in an apparently progressive sequence. The inferred macroscopic structures, the textural and mineral isograd relationships, and the age-grade relation are attributed to regional metamorphism deep in the crust, accompanied by localised deformation on deep shear zones. A mechanism is proposed whereby movements during deformation elevated older schists beyond the limit of argon equilibration, and at the same time generated newer schist at depth in and beneath the inclined shear zones; at upper crustal levels the elevation resulted in erosion of older eugeosynclinal sediments and in local deposition of conglomerates with derived clasts.