ABSTRACT Late Cenozoic growth of the Mount Diablo anticline in the eastern San Francisco Bay area, California, USA, has produced unique 3D exposures of stratigraphic relationships and normal faults that record Late Cretaceous uplift and early Tertiary extension in the ancestral California forearc basin. Several early Tertiary normal faults on the northeast flank of Mount Diablo have been correlated with structures that accommodated Paleogene subsidence of the now-buried Rio Vista basin north of Mount Diablo. Stepwise restoration of deformation at Mount Diablo reveals that the normal faults probably root into the “Mount Diablo fault,” a structure that juxtaposes blueschist-facies rocks of the Franciscan accretionary complex with attenuated remnants of the ophiolitic forearc basement and relatively unmetamorphosed marine forearc sediments. This structure is the local equivalent of the Coast Range fault, which is the regional contact between high-pressure Franciscan rocks and structurally overlying forearc basement in the northern Coast Ranges and Diablo Range, and it is folded about the axis of the Mount Diablo anticline. Apatite fission-track analyses indicate that the Franciscan rocks at Mount Diablo were exhumed and cooled from depths of 20+ km in the subduction zone between ca. 70−50 Ma. Angular unconformities and growth relations in the Cretaceous and Paleogene stratigraphic sections on the northeast side of Mount Diablo, and in the Rio Vista basin to the north, indicate that wholesale uplift, eastward tilting, and extension of the western forearc basin were coeval with blueschist exhumation. Previous workers have interpreted the structural relief associated with this uplift and tilting, as well as the appearance of Franciscan blueschist detritus in Late Cretaceous and early Tertiary forearc strata, as evidence for an “ancestral Mount Diablo high,” an emergent Franciscan highland bordering the forearc basin to the west. This outer-arc high is here interpreted to be the uplifted footwall of Coast Range fault. The stratigraphic and structural relations exposed at Mount Diablo support models for exposure of Franciscan blueschists primarily through syn-subduction extension and attenuation of the overlying forearc crust in the hanging wall of the Coast Range fault, accompanied by (local?) uplift and erosion of the exhumed accretionary prism in the footwall.

Abstract Phase relations in metabasic rocks are documented from the results of phase modelling, experiments and ACF analysis. A P–T pseudosection for a mid-ocean-ridge basalt (MORB) composition was calculated in the P–T range of 1–32 kbar and 400–1100°C using THERMOCALC 3.45. Phase relations in this pseudosection are mostly consistent with the results from experiments but complicated. ACF analyses suggest that the complicated phase relations can be simplified into 10 invariant assemblages involving orthopyroxene, clinopyroxene, hornblende, glaucophane, actinolite, garnet, chlorite, epidote, lawsonite, kyanite and plagioclase, assuming quartz, NaAlO 2 and H 2 O/melt being are excess. Phase relation analyses in the P–T projections that are constructed according to Schreinemakers’ rules and qualitative P–T pseudosections that are obtained for a MORB composition indicate that 22 subfacies assemblages are recognized for metabasic rocks. A four-fold classification of metamorphism is proposed on the basis of the phase relations for metabasic rocks, including low-, medium-, high- and very-high- P / T types, with apparent geothermal gradients >110°C/kbar, between 110 and 55°C/kbar, between 55 and 28°C/kbar, and <28°C/kbar, respectively. This four-fold classification of metamorphism is better able to match various tectonic settings.