Cameron A. Snow

Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California

We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic–Cretaceous grains originating from the Klamath–Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100–160 Ma grains; the upper El Cerrito also contains several grains in the 1200–1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this melange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is significantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a significant Paleozoic detrital component. Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early–Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area.

Contrasting early and late Mesozoic petrotectonic evolution of northern California

Devonian–Middle Jurassic terrane assemblies in the Klamath Mountains and Sierra Nevada Foothills consist chiefly of ophiolite-chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas associated fine-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted arc origin for Klamath mafic metavolcanic units inter-layered with distal turbidites in the 170–200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic melange with North Fork petrotectonic affinities. The North Fork–Eastern Hayfork–Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafic belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt–Eastern Klamath–Feather River–Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. Klamath-Sierran terrane assemblies reflect ∼230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks. In contrast, the Late Jurassic to largely Cretaceous Klamath–Sierra Nevada quartzo-feldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underflow by the Farallon plate. The coeval Galice-Mariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are first-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ∼70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian–Middle Jurassic rifting, drifting, and stranding of ophiolite-chert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units.

Terranes of the Western Sierra Nevada Foothills Metamorphic Belt, California: A Critical Review

Previous classifications of the western Sierra Nevada Foothills belt have been modified into five geologic packages: the Northern Sierra terrane, Feather River terrane, Calaveras Complex, Jura-Triassic arc belt, and Middle—Late Jurassic arc sequence. The Northern Sierra terrane comprises the Shoo Fly Complex and an overlying Paleozoic succession of arc volcanics. The Feather River terrane is predominantly serpentinized ultramafics containing tectonic blocks of amphibolite, plagiogranite, and mafic schists. The Permian-Late Triassic Calaveras Complex is a subduction mélange comprising psuedostratigraphic sequences of both chert and argillite with incorporated blocks of limestone, amphibolite, greenschist, phyllite, and basalt. The Red Ant schist is also included in this terrane even though it is of higher metamorphic grade. The Jura-Triassic arc belt consists of a Paleozoic basement of disrupted ophiolite, serpentinite mélange, and ultramafic rocks overlain by uppermost Triassic—Early Jurassic arc volcanics and coeval 200 Ma intrusive rocks. This belt may comprise several small terranes. The Middle—Late Jurassic arc sequence comprises 165-155 Ma volcanic arc rocks, greenstones, and metasedimentary rocks of the Mariposa Formation, and rocks of the Smartville Complex. Coeval intrusive rocks are included within this sequence because they are probably comagmatic with the arc volcanics. The Northern Sierra terrane and Feather River terrane are bounded by pre-Nevadan structures that may represent original sutures or subduction zones and have geologic histories that are relatively decoupled from adjacent rocks; they are therefore designated as terranes. The Calaveras Complex formed either adjacent to the Cordilleran margin as a result of east-dipping subduction, adjacent to arc rocks of the Jura-Triassic arc belt from west-dipping subduction, or a combination thereof. Therefore, the Calaveras Complex has a common geologic history with adjacent rocks and is not designated as a terrane. The Jura-Triassic arc belt comprises several ophiolitic microterranes, which may have been either exotic or fringing relative to North America, and are therefore grouped as a belt instead of individual terranes. The Middle-Late Jurassic arc sequence unconformably overlies rocks of the Jura-Triassic arc belt and is therefore a cover sequence instead of a separate terrane. The accretionary history of the Mesozoic Cordilleran margin is complex. After emplacement of the Northern Sierra terrane and Feather River terrane, the system was dominated by arc volcanism and accretion. The entire Sierra Nevada Foothills metamorphic belt was likely accreted to the margin by the Late Jurassic. The Sierra Nevada Foothills comprise units that appear to be correlative with those in the Klamath Mountains. Possible correlations are drawn between the Northern Sierra terrane and Eastern Klamath terrane, Calaveras Complex + Red Ant Schist and Stuart Fork + Eastern Hayfork terranes, Jura-Triassic arc belt and Rattlesnake Creek terrane, and Middle-Late Jurassic arc sequence and Western Hayfork terrane + coeval intrusives.

Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?

We report a new 153 ± 2 Ma SIMS U-Pb date for zircons from the hypabyssal Guadalupe pluton which crosscuts and contact metamorphoses upper crustal Mariposa slates in the southern Sierra. A ~950 m thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 ± 2 Ma. Assuming a compacted depositional rate of ~120 m/Myr, accumulation of Mariposa volcanogenic sediments, which overlie previously stranded Middle Jurassic and older ophiolite + chert-argillite belts in the Sierran Foothills, began no later than ~160 Ma. Correlative Oxfordian-Kimmeridgian strata of the Galice Formation occupy a similar position in the Klamath Mountains. We speculate that the Late Jurassic was a time of transition from (1) a mid-Paleozoic–Middle Jurassic interval of mainly but not exclusively strike-slip and episodic docking of oceanic terranes; (2) to transpressive plate underflow, producing calcalkaline igneous arc rocks ± outboard blueschists at ~170–150 Ma, whose erosion promoted accumulation of the Mariposa-Galice overlap strata; (3) continued transpressive underflow attending ~200 km left-lateral displacement of the Klamath salient relative to the Sierran arc at ~150–140 Ma and development of the apparent polar wander path cusps for North and South America; and (4) then nearly orthogonal mid and Late Cretaceous convergence commencing at ~125–120 Ma, during reversal in tangential motion of the Pacific plate. After ~120 Ma, nearly head-on subduction involving minor dextral transpression gave rise to voluminous continent-building juvenile and recycled magmas of the Sierran arc, providing the erosional debris to the Great Valley fore arc and Franciscan trench.

Petrotectonic evolution and melt modeling of the Peñon Blanco arc, central Sierra Nevada foothills, California

The Penon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Penon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ∼900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Penon Blanco Formation, which is made up of ∼700 m of crystal-lithic basaltic tuff, 1–3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Penon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Penon Blanco arc against the inboard Calaveras Complex. New geochemical data from the Penon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Penon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30–60 km depth by low to moderate amounts of partial melting (∼3%–5% for Jasper Point and 5%–7.5% for Penon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Penon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America.

Chronology of Gold Mineralization in the Sierra Nevada Foothills from 40Ar/39Ar Dating of Mariposite

Mariposite (Cr-rich muscovite) crystals formed the during metasomatic replacement of serpentinite yield 40Ar/39Ar ages demonstrating that gold-quartz-carbonate mineralization in the Sierra Nevada Foothills Metamorphic Belt occurred as early as 152 Ma and continued until at least 122 Ma. The earliest gold mineralization is found in the Grass Valley District; mineralization subsequently migrated southward along the Melones fault zone, forming the Mother Lode Gold Belt. These ages correspond to a lull in Sierra Nevada plutonism, and with periods of ductile deformation along the Melones and related fault zones. Our data suggest that gold-quartz-carbonate mineralization was not associated with Sierra Nevada plutonism, but instead was related to changes in plate dynamics, and generation and migration of CO2-rich ore-forming metamorphic fluids along the continental margin of North America. In this model, shallow gold mineralization was produced by heating of the lower crust within the structural mélange of the Sierra Nevada Foothills Metamorphic Belt as a response to an increase in dip of the subducting slab and oceanward migration of the saw-tooth thermal structure.