Franciscan Metagraywacke Research Articles

Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino‐Garberville‐Covelo 30′ × 60′ quadrangles, northwest California

The Franciscan Complex is a classic subduction‐zone assemblage. In northwest California, it comprises a stack of west vergent thrust sheets: westernmost Eastern Belt outliers; Central Belt mélange; Coastal Belt Yager terrane; Coastal Belt Coastal terrane; Coastal Belt King Range/False Cape terranes. We collected samples and determined P‐T conditions of recrystallization for 88 medium‐fine‐grained metasandstones to assess their subduction‐exhumation histories and assembly of the host allochthons. Feebly recrystallized Yager, Coastal, and King Range strata retain clear detrital features. Scattered neoblastic prehnite occurs in several Coastal terrane metasandstones; traces of possible pumpellyite are present in three Yager metaclastic rocks. Pumpellyite ± lawsonite ± aragonite‐bearing Central Belt metasandstones are moderately deformed and reconstituted. Intensely contorted, thoroughly recrystallized Eastern Belt affinity quartzose metagraywackes contain lawsonite + jadeitic pyroxene ± aragonite ± glaucophane. We microprobed neoblastic phases in 23 rocks, documenting mineral parageneses that constrain the tectonic accretion and metamorphic P‐T evolution of these sheets. Quasi‐stable mineral assemblages typify Eastern Belt metasandstones, but mm‐sized domains in the Central and Coastal belt rocks failed to achieve chemical equilibrium. Eastern Belt slabs rose from subduction depths approaching 25–30 km, whereas structurally lower Central Belt mélanges returned from ∼15–18 km. Coastal Belt assemblages suggest burial depths less than 5–8 km. Eastern and Central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin; K‐feldspar‐rich Coastal Belt rocks were stranded along the continental edge without undergoing appreciable subduction, probably during Paleogene unroofing of the older, deeply subducted units of the Franciscan Complex in east‐vergent crustal wedges.

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.

Characterization of the geothermal system beneath the Northwest Geysers steam field, California, from seismicity and velocity patterns

A high-resolution seismic network at the Coldwater Creek Steam Field (CCSF) in the Northwest Geysers region has provided data with which to estimate three-dimensional P- and S-wave velocity models and to relocate hypocenters. The seismicity distribution and resulting seismic velocity structure, coupled with the known geology, are used to characterize the geothermal reservoir. The velocity structure is estimated using the progressive inversion technique of Thurber [ J. geophys. Res. 88, 8226–8236 (1983)]. Hypocenters within the CCSF lie between depths of 1.5 and 2.5 km and appear to be associated with the steam production zone. A deeper cluster of events is located beneath an injection well. High-velocity near-surface anomalies correspond to mapped sections of Franciscan metagraywacke and greenstone. Low-velocity anomalies at depth appear to represent sections of Franciscan melange. High Vp/Vs anomalies at depth suggest a recharge region or a condensation zone. The steam producing zone is marked by low Vp/Vs values, suggesting partial liquid saturation of the reservoir rocks.

Kinematic evidence for extensional unroofing of the Franciscan Complex along the Coast Range Fault, Northern Diablo Range, California

Franciscan metagraywacke immediately below the Del Puerto ophiolite, an outlier of the Coast Range ophiolite in the northern Diablo Range, was sheared during top‐to‐the‐east displacement on the Coast Range fault. This represents normal faulting and extensional offset. It was accompanied by attenuation of the Coast Range ophiolite and Great Valley sequence in the hanging wall along layer‐parallel normal faults that sole into the Coast Range fault. Extension occurred as the Franciscan Complex moved relatively west, out from under North American lithosphere and across the subducting ocean plate below. This effected a lengthening and thinning in the wedge of material above the down‐going plate, presumably in response to instability brought about by subduction shallowing (Krueger and Jones, 1989) and accretion of the Franciscan Central belt in the latest Cretaceous to early Paleocene. As a result, blueschist facies terranes of the uppermost part of the Franciscan Complex are now juxtaposed directly against hanging wall units that bear only low‐grade metamorphism.

Clastic lawsonite, glaucophane, and jadeitic pyroxene in Franciscan metagraywackes from the Diablo Range, California

Textural relationships and mineral chemistry identify a detrital origin for jadeitic pyroxene and some lawsonite and glaucophane in Franciscan metagraywackes in the Diablo Range. These clasts differ from the neometamorphic (post-depositional) assemblage by having larger grain sizes and deformational features, including bending, cracking, and optical strain polarization. Large strain-polarized jadeitic pyroxenes contain inclusions of strained lawsonite and glaucophane, but none of these minerals has grown across clast/matrix boundaries. By comparison, neometamorphic lawsonite forms small unstrained tablets crossing the margins of large clasts and is associated with quartz + albite + chlorite + white mica in recrystallized metagraywacke matrices. Similarly, authigenic glaucophane forms discrete small prisms or euhedral outgrowths on large clasts of sodic amphibole. Jadeitic clasts in contact with metagraywacke matrices may be deeply embayed and show marginal recrystallization to fine-grained mixtures of albite + chlorite + quartz or internally to pumpellyite + albite. Backscatter electron imaging and microprobe analyses consistently show that the pyroxenes have extremely variable Jd contents. Within a single clast, compositional heterogeneity can be as much as Jd 50 Ac 39 -Aug 11 to Jd 95 Ac 4 Aug 1 . Because their texture, habit, and chemical variability are similar to those of metasomatic jadeites from tectonic inclusions in serpentinite at New Idria, southern Diablo Range, or to those of some jadeite-quartz associations in extensively recrystallized metabasite knockers, such a provenance is inferred for pyroxene clasts in the Franciscan metasediments. In metaconglomerates, pebbles containing pyroxenes (Jd 59-98 ) formed original clasts that were overprinted later by the post-depositional lawsonitic metamorphism. Metabasites at Pacheco Pass have compositionally variable Na-pyroxenes (Jd 30-92 ) which indicate tectonic emplacement of the igneous bodies within enclosing (in some cases, pyroxenefree) metagraywackes. The Diablo Range metagraywackes are weakly developed blueschist-facies assemblages of lawsonite + white mica + albite + chlorite + titanite ± glaucophane ± aragonite ± pumpellyite, which indicate minimum metamorphic conditions of P = 3.5-5.0 ± 0.5 kb at T = 100-200 °C.

Mineral paragenesis in Franciscan metagraywackes of the Nacimiento Block, a subduction complex of the Southern California Coast Ranges

Geologic‐petrographic reconnaissance of Franciscan metagraywackes from the Nacimiento block of the southern California Coast Ranges has demonstrated a perceptible eastward increase in metamorphic grade. Most feebly recrystallized zone I metaclastic rocks bordering the Pacific Ocean are rich in calcite veins and patches and contain no new Ca‐Al hydrous silicates; abundant unstable detritus includes lithic fragments, biotite flakes, and epidote granules. Metasedimentary rocks of medial zone II are characterized by fine‐grained intergrowths of pumpellyite ±5 minor regenerated epidote‐clinozoisite. Easternmost zone III metasandstones contain lawsonite, rarely aragonite and/or jadeitic pyroxene. In general, proportions of calcite, rock fragments, epidote, and biotite decrease eastward, and white mica, chlorite, and stilpnomelane increase. On the basis of experimental phase equilibrium data, temperatures of 150±50°C are estimated to have attended recrystallization, with lithostatic pressures increasing eastward from about 2–3 kbar on the west to approximately 5 kbar, and locally approaching 8 kbar, on the east. Absence of zeolites in the lowest grade zone I metagraywackes may indicate dilution of the aqueous fluid phase by CO2 during the metamorphism. In combination with the northwest tectonic ‘grain’ of the Nacimiento block and its inferred imbricate structure, the assemblage of rock types, the general north‐eastward dip of bedding, and predominantly right‐side‐up nature of the section, this continental margin deposit is thought to have been recrystallized during a period of relatively rapid (and perhaps brief) Late Cretaceous plate convergence. The lack of eclogite and amphibolite as tectonic blocks may indicate less profound depths of recovered subduction compared to Franciscan basement lying northeast of the San Andreas fault. Metamorphosed Nacimiento lithic belts evidently were assembled over a brief period of subduction offscraping at the western edge of the American lithospheric plate and seem to mirror events which unfolded over a longer time interval in Franciscan rocks of the northern California Coast Ranges.

South Fork Mountain Schist at Black Butte and Cottonwood Creek, northern California

The South Fork Mountain Schist at Black Butte and Cottonwood Creek consists largely of fine-grained metasedimentary and voluminous mafic metavolcanic rocks. Contacts between the metasedimentary and metavolcanic rocks are commonly thrust faults that formed during or preceding blueschist-facies metamorphism. The contact between the South Fork Mountain Schist and “normal” Franciscan metagraywacke was mapped by Suppe (1973) in the Ball Mountain region as the Log Spring fault. This fault may be traced northward into the Yolla Bolly quadrangle where it diverges westward from the schist-graywacke contact. At Cottonwood Creek the schist apparently passes down gradationally into less-metamorphosed rocks. The primary rock types of the South Fork Mountain Schist have more similarities to the lower facies (Knoxville Formation) of the Great Valley sequence than to other units of the Franciscan Complex, in that its primary sedimentary rocks were mainly mudstone, rather than graywacke. The schist may represent a similar facies relationship to the Franciscan as the Knoxville does to the Great Valley, namely, an older distal portion; or it may have originated as part of the Knoxville. In either case, the nature and position of the schist are more compatible with a western rather than an eastern source for the Franciscan graywackes.

Stability relations of jadeite pyroxene in Franciscan metagraywackes near San Jose, California

Stability relations of jadeite pyroxene in Franciscan metagraywackes near San Jose, California

Petrologic Reconnaissance of Franciscan Metagraywackes from the Diablo Range, Central California Coast Ranges

Petrologic Reconnaissance of Franciscan Metagraywackes from the Diablo Range, Central California Coast Ranges