ABSTRACT The basic stratigraphic and structural framework of Mount Diablo is described using a revised geologic map, gravity data, and aeromagnetic data. The mountain is made up of two distinct stratigraphic assemblages representing different depocenters that were juxtaposed by ~20 km of late Pliocene and Quaternary right-lateral offset on the Greenville-Diablo-Concord fault. Both assemblages are composed of Cretaceous and Cenozoic strata overlying a compound basement made up of the Franciscan and Great Valley complexes. The rocks are folded and faulted by late Neogene and Quaternary compressional structures related to both regional plate-boundary–normal compression and a restraining step in the strike-slip fault system. The core of the mountain is made up of uplifted basement rocks. Late Neogene and Quaternary deformation is overprinted on Paleogene extensional deformation that is evidenced at Mount Diablo by significant attenuation in the basement rocks and by an uptilted stepped graben structure on the northeast flank. Retrodeformation of the northeast flank suggests that late Early to early Late Cretaceous strata may have been deposited against and across a steeply west-dipping basement escarpment. The location of the mountain today was a depocenter through the Late Cretaceous and Paleogene and received shallow-marine deposits periodically into the late Miocene. Uplift of the mountain itself happened mostly in the Quaternary.

ABSTRACT Franciscan subduction complex rocks of Mount Diablo form a 8.5 by 4.5 km tectonic window, elongated E-W and fault-bounded to the north and south by rocks of the Coast Range ophiolite and Great Valley Group, respectively, which lack the burial metamorphism and deformation displayed by the Franciscan complex. Most of the Franciscan complex consists of a stack of lawsonite-albite–facies pillow basalt overlain successively by chert and clastic sedimentary rocks, repeated by faults at hundreds of meters to <1 m spacing. Widely distributed mélange zones from 0.5 to 300 m thick containing high-grade (including amphibolite and eclogite) assemblages and other exotic blocks, up to 120 m size, form a small fraction of exposures. Nearly all clastic rocks have a foliation, parallel to faults that repeat the various lithologies, whereas chert and basalt lack foliation. Lawsonite grew parallel to foliation and as later grains across foliation. The Franciscan-bounding faults, collectively called the Coast Range fault, strike ENE to WNW and dip northward at low to moderate average angles and collectively form a south-vergent overturned anticline. Splays of the Coast Range fault also cut into the Franciscan strata and Coast Range ophiolite and locally form the Coast Range ophiolite–Great Valley Group boundary. Dip discordance between the Coast Range fault and overlying Great Valley Group strata indicates that the northern and southern Coast Range fault segments were normal faults with opposite dip directions, forming a structural dome. These relationships suggest accretion and fault stacking of the Franciscan complex, followed by exhumation along the Coast Range fault and then folding of the Coast Range fault.

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.

The Cuesta Ridge ophiolite is a well-preserved remnant of the Middle Jurassic Coast Range ophiolite tectonically overlying rocks of the Franciscan complex. It is a nearly complete ophiolite section, consisting of over 1 km of serpentinized harz-burgite and dunite, sills of wehrlite, pyroxenite, and lherzolite, isotropic gabbro, a sheeted complex of quartz-hornblende diorite, an ∼1200-m-thick volcanic section, late-stage mafic dikes, and 5–10 m of tuffaceous radiolarian chert. The volcanic section at Cuesta Ridge has two chemically distinct volcanic groups. The lower volcanic section is characterized by low Ti/V ratios (11–21), enriched large ion lithophile element (LILE) concentrations, and depleted high field strength elements (HFSEs). Boninitic lavas with high MgO, Cr, and Ni abundances are present in this suite, along with arc tholeiites (basaltic andesites to dacites). Basalts of the upper volcanic section, which conformably overlie the lower volcanic section, and late-stage basaltic dikes that crosscut the hornblende–quartz diorite plutonic section are characterized by higher Ti/V ratios (20–27) and HFSE abundances and lower LILE abundances than the underlying section. These late-stage volcanic rocks have mid-ocean-ridge basalt–like chemistry. The field and geochemical data indicate formation in a suprasubduction-zone setting above an east-dipping proto-Franciscan subduction zone due to the onset of subduction and subsequent slab rollback. Multiple stages of magmatism ensued, until the emplacement of the late-stage dikes and uppermost flows. These late-stage dikes, which are present in several Coast Range ophiolite remnants, signify the end of ophio-lite formation and are interpreted to represent a Late Jurassic ridge collision.

Serpentinite matrix mélange represents a significant, if less common, component of many accretionary complexes. There are two principal hypotheses for the origin of serpentinite mélange: (1) formation on the seafloor in a fracture zone–transform fault setting, and (2) formation within a subduction zone with mixing of rocks derived from both the upper and lower plates. The first hypothesis requires that the sheared serpentinite matrix be derived from hydrated abyssal peridotites and that the block assemblage consist exclusively of oceanic rocks (abyssal peridotites, oceanic basalts, and pelagic sediments). The second hypothesis implies that the sheared serpentinite matrix is derived from hydrated refractory peridotites with supra-subduction zone affinities, and that the block assemblage includes rocks derived from both the upper plate (forearc peridotites, arc volcanics, sediments) and the lower plate (abyssal peridotites, oceanic basalts, pelagic sediments). In either case, serpentinite mélange may include true mélange, with exotic blocks derived from other sources, and serpentinite broken formation , where the blocks are massive peridotite. The Tehama-Colusa serpentinite mélange underlies the Coast Range ophiolite in northern California and separates it from high-pressure/temperature (P/T) metamorphic rocks of the Franciscan complex. It has been interpreted both as an accreted fracture zone terrane and as a subduction-derived mélange belt. Our data show that the mélange matrix represents hydrated refractory peridotites with forearc affinities, and that blocks within the mélange consist largely of upper plate lithologies (refractory forearc harzburgite, arc volcanics, arc-derived sediments, and chert with Coast Range ophiolite biostratigraphy). Lower plate blocks within the mélange include oceanic basalts and chert with rare blueschist and amphibolite. Hornblendes from three amphibolite blocks that crop out in serpentinite mélange and sedimentary serpentinite yield 40 Ar/ 39 Ar plateau ages of 165.6–167.5 Ma, similar to published ages of high-grade blocks within the Franciscan complex and to crystallization ages in the Coast Range ophiolite. Other blocks have uncertain provenance. It has been shown that peridotite blocks within the mélange have low pyroxene equilibration temperatures that are consistent with formation in a fracture zone setting. However, the current mélange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault–fracture zone that evolved into a subduction zone mélange complex. Mélange matrix was formed by the hydration and volume expansion of refractory forearc peridotite, followed by subsequent shear deformation. Mélange blocks were formed largely by the breakup of upper plate crust and lithosphere, with minor offscraping and incorporation of lower plate crust. We propose that the methods discussed here can be applied to serpentinite matrix mélange worldwide in order to understand better the tectonic evolution of the orogens in which they occur.

Geochronology, especially U-Pb zircon geochronology, has made important contributions to our understanding of the Jurassic Coast Range ophiolite of California. However, much of the older work is primitive by modern standards, and even some recent U-Pb work is limited in its precision and accuracy by a range of factors. We apply a new zircon analysis method, chemical abrasion–thermal ionization mass spectrometry (CA-TIMS), to generate high-precision, high-accuracy multistep 206 Pb*/ 238 U plateau ages for zircons from plagiogranites from the Point Sal (Coast Range ophiolite) and San Simeon (Coast Range ophiolite) ophiolite remnants. These remnants have been postulated to have been part of a single, contiguous remnant prior to offset along the San Gregorio–San Simeon–Hosgri fault system. Two fractions of zircon from a Point Sal Coast Range ophiolite plagiogranite, and one fraction of zircon from a San Simeon Coast Range ophiolite plagiogranite yield 206 Pb*/ 238 U plateau ages that are indistinguishable from one another—a mean age for the three determinations is 165.580 ± 0.038 Ma (95% confidence, mean square of weighted deviates [MSWD] = 0.47). The error quoted is an internal precision, which is appropriate for comparison of the ages to one another. The fact that the San Simeon and Point Sal ages are indistinguishable, even with such very small internal precision errors, is a remarkably robust confirmation of the correlation between the San Simeon and Point Sal ophiolite remnants.

Field mapping and structural analysis indicate that the Snowcamp remnant of the Coast Range ophiolite, in the vicinity of Game Lake, southwest Oregon, was thrust to the northeast over mafic phyllonite and amphibolite correlated with the Chetco complex. The contact between the upper ophiolitic and lower mafic sections is marked by a ∼5- to 10-m-thick serpentinite unit and paralleled by mylonitic foliations in the lower mafic section. Syndeformational tonalites intrude the lower plate. The fabric-forming metamorphic mineral assemblages (amphibolite facies) in the mafic section suggest a temperature range of ∼550–600+ °C and medium pressure, whereas quantitative thermobarometry indicates 615–720 °C and 3.5–6 kb (11.5–20 km depth). A mylonitic greenschist-facies mineral assemblage in phyllonites below the thrust overprints the amphibolite-facies fabric, indicating that thrusting continued during cooling. The 40 Ar/ 39 Ar analyses indicate that thrusting and syntectonic intrusion occurred by at least 154.2 ± 2.0 and 149.1 ± 0.4 Ma, respectively. The character of this thrust strongly resembles that of the Madstone Cabin thrust that emplaced the Josephine ophiolite over the Rogue–Chetco arc complex. The proposed correlation between the two faults implies that either the Snowcamp remnant of the Coast Range ophiolite has undergone a similar emplacement history as the Josephine ophiolite or that the ophiolitic rocks near Game Lake may be an outlier of the Josephine ophiolite juxtaposed by postemplacement faulting against a Coast Range remnant around Snowcamp Mountain to the south.