The composite California Coast Range ophiolite consists of remnants of Middle Jurassic oceanic lithosphere, a Late Jurassic deep-sea volcanopelagic sediment cover, and Late Jurassic intrusive sheets that invade the ophiolite and volcano-pelagic succession. The dismembered Middle Jurassic Coast Range ophiolite remnants (161–168 Ma) were parts of the axial sequence of an oceanic spreading center that consisted of basaltic submarine lava, subvolcanic intrusive sheets, and gabbro, and coeval but off-axis upper lava, dunite-wehrlite mantle transition zone, peridotite restite, and dikes rooted in the mantle transition zone that fed the upper lava. Hydrothermal metamorphism overprints the lavas, subvolcanic sheets, and part of the gabbro. The nearly complete magmatic pseudostratigraphy with minimal syngenetic internal deformation accords with a “hot” thermal structure and robust magma budget, indicative of fast spreading. Upper Jurassic volcanopelagic strata composed of tuffaceous radiolarian mud-stone and chert (volcanopelagic distal facies) overlie the ophiolite lava disconformably and grade up locally into arc-derived deep-marine volcaniclastics (volcanopelagic proximal facies). An ophiolitic breccia unit at northern Coast Range ophiolite localities caps shallow to deep levels of fault-disrupted Middle Jurassic oceanic crust. The Late Jurassic igneous rocks (ca. 152–144 Ma) are mafic to felsic subvolcanic intrusive sheets that invade the Middle Jurassic ophiolite, its Late Jurassic volcanopelagic cover, and locally the ophiolitic breccia unit. Hydrothermal metamorphism of volcanopelagic beds and underlying ophiolite meta-igneous rocks accompanied the Late Jurassic deep-sea magmatic events. The Middle Jurassic ophiolite formed at a spreading ocean ridge (inferred from its Jurassic plate stratigraphy). Intralava sediment and thin volcanopelagic strata atop the Coast Range ophiolite lava record an 11–16 m.y. progression from an open-ocean setting to the distant submarine apron of an active volcanic arc, i.e., the sediments accumulated upon oceanic lithosphere being drawn progressively closer to a subduction zone in front of an ocean-facing arc. Trace-element signatures of Coast Range ophiolite lavas that purportedly link ocean-crust formation to a suprasubduction-zone setting were influenced also by processes controlled by upper-mantle dynamics, especially the mode and depth of melt extraction. The polygenetic geochemical evidence does not decisively determine tectonic setting. Paleomagnetic and biostratigraphic evidence constrains the paleolatitudes of Coast Range ophiolite magmatism and volcanopelagic sedimentation. Primary remanent magnetism in ophiolite lavas at Point Sal and Llanada Coast Range ophiolite remnants records eruption within a few degrees of the Middle Jurassic paleoequator. The volcanopelagic succession at Coast Range ophiolite remnants consistently shows upward progression from Central Tethyan to Southern Boreal radiolarian assemblages, recording Late Jurassic northward plate motion from the warm-water paleo-equatorial realm. Northward seafloor spreading was interrupted by local Late Jurassic rift propagation through the Middle Jurassic oceanic lithosphere. Coast Range ophiolite crust with volcanopelagic soft-sediment cover that lay in the path of propagating rifts hosted rifting-related magmatic intrusions and hydrothermal metamorphism. The advancing broad deformation zone between propagating and failing rifts left paths of pervasive crustal deformation marked now by fault-disrupted ophiolite covered by depression-filling ophiolitic breccias, found at northern Coast Range ophiolite remnants. Coast Range ophiolite lithosphere that lay outside the propagating and failed rift zones lacks those features. The rift-related magmatism and crustal deformation took place at ephemeral spreading-center offsets along a transform fault. Late Jurassic seafloor spreading carried Middle Jurassic oceanic lithosphere northeastward toward a subduction zone in front of the Middle to Late Jurassic arc that fringed southwestern North America. Termination of oblique subduction during the late Kimmeridgian, replaced by dextral transform faulting, left a Coast Range ophiolite plate segment stranded in front (west) of the trench. The trench was then filled and locally bridged by the arc’s submarine sediment apron by the latest Jurassic, allowing coarse volcaniclastic (proximal volcanopelagic) deposits to lap onto earlier, plate-transported tuffaceous radiolarian chert (distal volcanopelagic) deposits. Deep-marine terrigenous muds and sands from southwestern Cordilleran sources then buried the stranded Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit oceanic crust during latest Jurassic northward dextral displacement, which proceeded offshore. Those basal Great Valley Group strata record lower continental-slope and basin-plain marine sedimentation on Jurassic oceanic basement, i.e., the Coast Range ophiolite and adjacent Franciscan oceanic lithosphere (Coast Range serpentinite belt). Forearc basin deposition did not begin until the mid–Early Cretaceous, when the inception of outboard Franciscan subduction lifted and tilted the Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit–basal Great Valley Group succession and Coast Range serpentinite belt to form a basin-bounding forearc ridge. Thereafter, Cretaceous Franciscan subduction and accretionary wedge growth operated in front (west) of the submerged ridge, and Great Valley Group forearc basin terrigenous sediments accumulated behind it.

The Fosdick Mountains, West Antarctica, form an 80 × 15 km migmatite dome comprising massive paragneisses that exhibit polyphase fabrics, nappe-scale folds that involve granodiorite to leucogranite intrusions, and diatexite. High strain zones developed on the NE flank of the dome. Multiple generations of leucogranite sheets, dikes and diatexite intrude the dome, and evidence for partial melt in structural sites is widespread. Macroscopic folds and the maximum anisotropy of magnetic susceptibility (AMS) direction are oriented NE-SW, generally parallel with the N65W regional finite strain axis determined from brittle faults and a mafic dike array outside the dome. The direction is oblique to the inherited fault that bounds the dome, to regional trends in the surrounding Ford Ranges, and to the nearby continental margin. Paragneiss assemblages yield thermobarometry results that indicate ≥18 km depth for growth of texturally early garnet and ∼10 km depth for growth of texturally late cordierite at the expense of biotite. Nodular and dendritic forms of cordierite that develop at shallow crustal depths completely overprint dynamic fabrics. The cordierite-K feldspar-sillimanite-garnet-biotite gneisses are determined by U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon analysis to contain inherited zircon populations of 1100–1000 Ma and 500 Ma age. The U-Pb distribution is characteristic of sediments shed from the Ross Orogen of the Paleozoic Gondwana margin, represented by Swanson Formation in the Ford Ranges. A granodiorite gneiss yields 375 Ma prismatic zircon grains characteristic of Ford Granodiorite in the region. Zircon rim ages in both rock types suggest a protracted growth history during polyphase high-temperature metamorphism. The peak of metamorphism was attained at 106–99 Ma, based on prior U-Pb monazite ages and regional relationships, followed by rapid cooling through the range of 40 Ar/ 39 Ar mineral systems between 101 and 94 Ma. The timing coincides with a change from convergent to divergent tectonics along the West Gondwana margin prior to breakup. Considered together, the partial melt evidence, decompression record and rapid thermal evolution of the partially molten rocks suggests diapiric processes in effect during emplacement the Fosdick Mountains dome along the Balchen Glacier fault. The consistent NE-SW orientation of folds, AMS strain axes, stretching direction from mafic and felsic dikes, kinematic axes from minor faults, and sparse mineral lineation attest to structural controls on dome emplacement. These are interpreted as evidence of dextral transcurrent strain across the region at ca. 100 Ma.

ABSTRACT Mafic rock samples from the northern Channel Islands (western Transverse Ranges) and the Peninsular Ranges and offshore islands were dated by 40 Ar/ 39 Ar incremental heating experiments on groundmass and plagioclase concentrates (24 experiments total). Many of the ages are substantially different from published K-Ar dates. Early Miocene 40 Ar/ 39 Ar dates on two lavas from San Miguel Island agree with the paleontology control there (Saucesian) and conflict with prior late Oligocene K-Ar dates from the same units. Hypabyssal intrusions from Santa Rosa Island are around 18 Ma. The top of the Santa Cruz Island volcanic section is 16.33 ± 0.26 Ma. Anacapa ages near 16 Ma are the youngest found for the northern Channel Islands. Published 40 Ar/ 39 Ar dates from the Tranquillon Volcanics are 17.8 Ma .suggesting a widespread volcanic event between 18 and 16 Ma. We interpret this as timing the beginning of rifting of the northern borderland. The apparent age progression of volcanism from west (18 to 17 Ma) to east (16 Ma) is suggestive of a propagating rift prior to major extension and rotation of the western Transverse Ranges. The El Modeno volcanics in the Peninsular Ranges give ages of about 11 Ma, several million years younger than published K-Ar dates. The Rosarito Beach Formation comprises Miocene basalts, tuffs, and sediments that accumulated in two basins between Tijuana and Ensenada. The section spans from 15.51 + 0.14 to 16.19 ± 0.28 Ma. This formation was derived from a western (offshore) source that has since been translated away, submerged, or eroded. It is possible that part of the northern Channel Islands platform was adjacent there before rifting and rotation. Ages from San Clemente Island are younger than the Rosarito section and span 14.5 to 16.0 Ma. The combined sections on San Clemente and at Rosarito span several reversed magnetic polarity intervals. More ages from these sections could aid in calibrating the magnetic reversal time scale.

Several papers in this volume deal with the structure beneath the Santa Barbara Channel from opposing view points. These are the thickskinned structural interpretations versus the thin-skinned ones. In the thick-skin interpretations, high angle reverse faults cut the crust to the base of the seismic zone (see Yeats, this volume). In the thin-skinned model these reverse faults are actually active axial surfaces of fault-bend-folds above detachment ramps in the lower crust (see Novoa, this volume). The detachments are necessarily blind, and they do not crop out.

The Hosgri fault zone (HFZ) is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, east-west-trending structures in the western Transverse Ranges north of the channel abut against the HFZ. The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.