Quartz-in-garnet barometry constraints on formation pressures of eclogites from the Franciscan Complex, California

Determining pressure and temperature variations between high-pressure/low-temperature (HP–LT) eclogite blocks is crucial for constraining end-member exhumation models; however, it has historically been challenging to constrain eclogite pressures due to the high variance associated with this bulk-rock composition. In this work, we utilize quartz-in-garnet elastic barometry to constrain formation pressures of eclogites from the northern (Junction School, Ring Mountain, Jenner Beach) and southern Franciscan Complex (Santa Catalina Island). Multiple eclogite blocks from Jenner Beach are analyzed, and a single eclogite from the other localities. By comparing garnet growth conditions from within a single outcrop and between distinct outcrops, we evaluate the local and regional spatial distribution of P conditions recorded by eclogites. We compare the mean, median, and max pressures between different garnet zones and eclogites. Pressures sometimes exhibit systematic changes across garnet zones; however, some eclogites exhibit no systematic pressure variations across garnet zones. Pressures from northern Franciscan eclogites range from $$\sim $$ ∼ 1.4–1.8 GPa, at an estimated temperature of 500 $$^{\circ }$$ ∘ C; pressures from the Catalina eclogite range from $$\sim $$ ∼ 1.2–1.5 GPa, at an estimated temperature of 650 $$^{\circ }$$ ∘ C. Mean and maximum pressures of different eclogites from the northern Franciscan exhibit negligible differences (< 0.1 GPa). The results are inconsistent with models that propose exhumation of metamorphic blocks from different structural levels, and suggest that now exposed HP–LT eclogites from the northern Franciscan Complex may represent rocks that were coherently underplated, and exhumed from similar structural levels.

Middle Jurassic to Early Cretaceous tectonic evolution of the western Klamath Mountains and outboard Franciscan assemblages, northern California–southern Oregon, USA

ABSTRACT The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out the possibility that such materials were removed via subduction and/or strike slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice Formation–filled basin demonstrably occurred adjacent to North America; and (3) the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench assemblages underplated &gt;100 km inboard of the subduction margin, presumably during a previously unrecognized phase of shallow-angle subduction. In aggregate, these relations suggest that the Klamath Mountains and adjacent Franciscan complex represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic subduction zone, wherein the downgoing oceanic plate took a variety of trajectories into the mantle. We speculate that the downgoing plate contained alternating tracts of smooth and dense versus rough and buoyant lithosphere—the former gliding into the mantle (facilitating slab rollback and upper plate extension) and the latter enhancing basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific Ocean, with subduction of the Australian plate beneath New Guinea and adjacent island groups providing perhaps the best analog.

Subduction and exhumation slip accommodation at depths of 10–80 km inferred from field geology of exhumed rocks: Evidence for temporal-spatial localization of slip

ABSTRACT Field relationships in the Franciscan Complex of California suggest localization of subduction slip in narrow zones (≤300 m thick) at the depths of ∼10–80 km. Accretionary and non-accretionary subduction slip over the ca. 150 Ma of Franciscan history was accommodated across the structural thickness of the complex (maximum of ∼30 km). During accretion of a specific unit (&lt;5 Ma), subduction slip (accretionary subduction slip) deformed the full thickness of the accreting unit (≤5 km), primarily on discrete faults of &lt;20 m in thickness, with the remainder accommodated by penetrative deformation. Some faults accommodating accretionary subduction slip formed anastomosing zones ≤200 m thick that resulted in block-in-matrix (tectonic mélange) relationships but did not emplace exotic blocks. Mélange horizons with exotic blocks range in thickness from 0.5 m to 1 km. These apparently formed by sedimentary processes as part of the trench fill prior to subsequent deformation during subduction-accretion. Accretionary subduction slip was localized within some of these mélanges in zones ≤300 m thick. Such deformation obscured primary sedimentary textures. Non-accretionary subduction faults separate units accreted at different times, but these &lt;100-m-thick fault zones capture a small fraction of associated subduction slip because of footwall subduction and likely removal of hanging wall by subduction erosion. Most exhumation was accommodated by discrete faults ≤30 m thick. Structural, geochronologic, and plate motion data suggest that of the ∼13,000 km of subduction during the ca. 150 Ma assembly of the Franciscan Complex, ∼2000 km was associated with accretion.

Regional-scale correlations of accreted units in the Franciscan Complex, California, USA: A record of long-lived, episodic subduction accretion

ABSTRACT The Franciscan Complex of California, the type example of an exhumed accretionary complex, records a protracted history of voluminous subduction accretion along the western margin of North America. Recent geochronological work has improved our knowledge of the timing of accretion, but the details of the accretionary history are disputed, in part, due to uncertainties in regional-scale correlations of different units. We present new detrital zircon U-Pb ages from two sites on opposite sides of San Francisco Bay in central California that confirm previously proposed correlations. Both sites are characterized by a structurally higher blueschist-facies unit (Angel Island unit) underlain by a prehnite-pumpellyite-facies unit (Alcatraz unit). The Angel Island unit yields maximum depositional ages (MDAs) ranging from 112 ± 1 Ma to 114 ± 1 Ma (±2σ), and the Alcatraz unit yields MDAs between 94 ± 2 Ma and 99 ± 1 Ma. Restoration of post-subduction dextral displacement suggests these sites were originally 44–78 km apart and much closer to other Franciscan units that are now exposed farther south in the Diablo Range. Comparison with detrital zircon dates from the Diablo Range supports correlations of the Bay Area units with certain units in the Diablo Range. In contrast, correlations with Franciscan units in the northern Coast Ranges of California are not robust: some units are clearly older than those in the Bay Area whereas others exhibit distinct differences in provenance. Integration of age data from throughout the Franciscan Complex indicates long-lived and episodic accretion from the Early Cretaceous to Paleogene. Although minor, sporadic accretion began earlier, significant accretion occurred during the interval 123–80 Ma and was followed by minor accretion at ca. 53–49 Ma. Periods of accretion and nonaccretion were associated with arc magmatism in the Sierra Nevada–Klamath region, cessation of arc activity, and reorganization of paleodrainage systems, which implicates plate dynamics and sediment availability as major controls on the development of the Franciscan Complex.

Field and petrographic reconnaissance of Franciscan complex rocks of Mount Diablo, California: Imbricated ocean floor stratigraphy with a roof exhumation fault system

ABSTRACT Franciscan subduction complex rocks of Mount Diablo form an 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 &lt;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.

Mount Diablo mercury deposits

ABSTRACT The California Coast Ranges mercury deposits are part of the western North America mercury belt, in which mercury occurs most commonly as red cinnabar (α-HgS), sometimes associated with its high-temperature polymorph, metacinnabar (β-HgS). In the Coast Ranges, ores were deposited from hydrothermal solutions and range in age from Miocene to Holocene. Ore deposition at Mount Diablo generally occurred along active faults and associated extension fractures in the Franciscan complex, most often in serpentinite that had been hydrothermally altered to silica-carbonate rock. The Mount Diablo mine lies ~48 km (~30 miles) northeast of San Francisco in Contra Costa County and is mineralogically unique in California, because metacinnabar, the higher-temperature polymorph of mercury sulfide, is a major primary ore mineral in the deposit, while at all other mercury mines in California, it is quite rare. In addition, hydrothermal activity is so recent that sulfurous gases and methane continued to be released into the mine at least into the 1940s. Historically, long before active large-scale mining began in the 1800s, the Mount Diablo mercury deposits were known to the Indigenous people of the Ohlone tribes, who used the cinnabar in rituals as well as for red pigment to decorate their bodies, and as a prized trade item. The deposit was later rediscovered in 1863 and mined intermittently until 1958. The Mount Diablo mine and adjacent Rhyne (also variously spelled Ryne or Rhine) mine were the sites of most of the mercury operations in the region, and at both mines, mercury ore occurs in structurally controlled lenticular bodies of silica-carbonate rock and serpentinite. The total district production probably exceeded 12,300 flasks (at 76 pounds or ~34.5 kg per flask) at an estimated grade of 2711 g per metric ton. Low-grade ore reserves are believed to still exist, with 17,000 short tons of indicated and inferred ore. Other minor deposits of copper, silver, and gold occur on Mount Diablo, principally in and around Eagle Peak, but mercury is not associated with these deposits.

Quartz-in-garnet elastic barometry vs. conventional thermobarometers: a comparison across diverse tectonic settings

&lt;p&gt;In recent years, elastic thermobarometry has gained wider acceptance and utility within the petrologic community and beyond. In particular, quartz-in-garnet (qtz-in-grt) elastic barometry is widely used because of the ubiquity of garnet in metamorphic rocks. The technique is based on using Raman spectroscopy to quantify strains recorded by inclusions, and modeling the elastic evolution of the inclusion-host pair to constrain the initial conditions of inclusion entrapment. Recent studies have validated the technique experimentally by comparing pressures from the qtz-in-grt barometer with experimental conditions of garnet growth and entrapment of quartz, and have shown that the barometer can provide reliable pressure conditions of garnet growth. However, current experimental studies fail to capture the reliability of the technique under disparate pressure (P), temperature (T) and deformation conditions, and studies that systematically compare qtz-in-grt barometry and conventional thermobarometry are lacking.&amp;#160;&lt;/p&gt;&lt;p&gt;In this work, we compare P conditions from qtz-in-grt barometry and conventional thermobarometry from the following locations: spatially and temporally variant high P/T subduction zone eclogite blocks from the Franciscan Complex in California, high P/T subduction zone rocks of varying compositions from Syros, Greece, high P/T and low P/T rocks of varying compositions from the Betics system in Spain, low P/T schists from the Jajarkot and Karnali klippen in the Himalaya, high-P rocks from the Alps, and low P/T metapelites from northeast Nevada. Qtz-in-grt barometry constraints from the Franciscan and Syros show good agreement with some reference P-T conditions, but disagree with some thermodynamic equilibria constraints and subsets of multi-mineral thermobarometry calibrations. Qtz-in-grt barometry constraints from the Himalaya are in excellent agreement with reference P constraints. Measurements of samples from other localities are currently in progress. This set of quartz inclusion analyses further allows us to evaluate the effects of inclusion geometry, anisotropy, P and T conditions of garnet growth, and P and T paths on the ultimate P conditions recorded by the qtz-in-grt barometer. The data-set also provides insights into the possible limitations of other techniques (e.g., conventional thermobarometry).&lt;/p&gt;