Franciscan Complex Research Articles

Subduction initiation along transform faults: The proto-Franciscan subduction zone

The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones present special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduction initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite melange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that reflect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that reflect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting. The Tehama-Colusa serpentinite melange consists of blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. The melange matrix represents hydrated refractory peridotites with forearc affinities, and blocks within the melange consist largely of upper-plate lithologies (harzburgite, arc volcanics, and arc-derived sediments). Lower-plate blocks within the melange include oceanic basalts and chert with rare blueschist and amphibolite. The abyssal peridotites have low pyroxene equilibration temperatures that are consistent with formation in a fracture-zone setting. However, the current melange 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 or fracture zone that evolved into a subduction-zone melange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems and for the Franciscan–Coast Range ophiolite system. Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite melange.

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

The Franciscan Complex of California is a type example of an accretionary prism with widespread high-P/T subduction-zone metamorphism. Low-T, fine-grained, lawsonite-bearing mafic blueschists encased in a shale-matrix mélange near San Simeon have been dated for the first time. 40Ar-39Ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks. The three blocks with lawsonite + epidote yield ages of between 154 and Ma, while the actinolitic rind yields an age of Ma. These ages are from the part of the Franciscan Complex west of the San Andreas Fault that moved northward at least 300 km with respect to the extensive Franciscan exposures east of the fault. The ages obtained in this study show that some fine-grained, low-T (lawsonite + epidote) Franciscan blueschist blocks formed at the same time as coarse-grained, high-T (garnet ± epidote) Franciscan blueschist blocks. These dated rocks indicate that both high- and low-T mafic blueschists are coeval and probably formed along 1000+ km of the North American Plate margin at ∼160–155 Ma. The episode of dynamic blueschist metamorphism was soon followed by high-Mg rind formation and a long period of nearly static high-P/T conditions.

Sandstone matrix olistostrome deposited on intra-subduction complex serpentinite, Franciscan Complex, western Marin County, California

Detailed field mapping in the Franciscan Complex in western Marin County, California, offers insight into the origins and development of mélanges, as well as accretionary complex architecture. From structurally high to low, the study area units comprise a structurally high sandstone matrix olistostrome, serpentinite, a structurally intermediate sandstone matrix olistostrome, Early Cretaceous Nicasio Reservoir Terrane greenstone, and coherent Cretaceous greywacke, all exhibiting prehnite–pumpellyite burial metamorphism. The structurally high sandstone matrix olistostrome includes volcanic blocks up to 700m in diameter and chert blocks up to 100m in diameter, whereas the structurally intermediate olistostrome unit comprises a lithic-rich sandstone matrix enclosing volcanic blocks 1–20m in diameter. The youngest detrital zircons in the high olistostrome and the coherent greywacke have U–Pb ages of ≈20m.y. older than the inferred depositional ages of the units themselves, suggesting that the bulk of the detritus comprising these units was derived by exposure and sedimentary recycling of significantly older, previously accreted sediments. The matrix of both olistostrome units exhibits minimal strain with weak local development of pressure solution cleavage. These findings demonstrate introduction of Franciscan exotic blocks and development of Franciscan block-in-matrix fabrics through sedimentary sliding and support sedimentary interpretations for more highly strained shale and serpentinite matrix mélanges. Additionally, these data suggest that sandstone matrix olistostromes/mélange may constitute a larger part of the Franciscan and other accretionary complexes than previously believed.The structurally high olistostrome preserves a depositional contact on serpentinite, verifying an intra-subduction complex origin for the serpentinite. The serpentinite lacks block-in-matrix structure and likely represents a tectonically emplaced unit derived from the subducting plate as a coherent nappe, rather than a sedimentary serpentinite deposit. Multi-kilometer scale west-vergent overturned folds with subhorizontal fold axes deform the nappes of the field area, and a later generation of open, northeast-trending folds overprints the earlier folds.

Asthenospheric flow and lithospheric evolution near the Mendocino Triple Junction

The migration of the Mendocino Triple Junction in northern California creates a complicated lithosphere–asthenosphere boundary system at shallow depths (<60km), following the progressive transition from Cascadia subduction to San Andreas transform motion. It provides a natural laboratory to examine lithospheric evolution associated with slab removal in an active tectonic setting. However, pathways of the asthenospheric sources that fill the slab-free region created by the triple junction migration remain unclear. Previous active source profiles and body-wave tomography are limited to either crustal or a deeper upper mantle scale, respectively. In this study, we present a three-dimensional shear velocity model from joint inversion of ambient noise (8–30s) and ballistic Rayleigh wave dispersion curves (22–100s), as well as Ps receiver functions, using 111 stations from the Flexible Array Mendocino Experiment, USArray Transportable Array and the regional Berkeley Digital Seismic Network. In the crust, we have observed the low-Vs Franciscan Complex in the Coast Ranges and the relatively high-Vs Great Valley ophiolite abutting the low-Vs Sierran basement. The low-Vs uppermost mantle imaged near the sudden steepening of the subducted oceanic slab extends seismic evidence for forearc mantle serpentinization further south along the Cascadia margin. In addition to the asthenosphere beneath the Gorda plate, the joint inversion Vs model further identifies three other young asthenospheres resulting from different partial melting mechanisms. Northward motion of the triple junction causes asthenospheric flow both from under the Gorda plate and from the cooling former mantle wedge left under the Great Valley and Sierra Nevada, imaged from the joint inversion as a relatively deep (>75km) low-Vs anomaly. These two mantle flows appear to begin mixing ~100km south of the southern edge of the Gorda plate in the slab window region. We speculate that the latter provides the wedge-type geochemical signature seen in the Coast Range volcanic rocks, reconciling slab window models and volcanic geochemistry. This ‘staggered’ upwelling model proposed here also explains the ~3Myr delay in onset of volcanism after triple junction migration.

Mantle–crust interactions in a paleosubduction zone: Evidence from highly siderophile element systematics of eclogite and related rocks

Substantial differences in 187Os/188Os and absolute and relative abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) in mantle peridotites compared to oceanic crust indicate that this suite of elements may prove useful in assessing mechanisms and pathways of mantle and slab mass transfer within the subduction channel. Currently, however, information regarding the mobility of the HSE in subduction zone environments is limited. To better understand the systematics of highly siderophile elements in subduction settings, we measured Os isotopic compositions and HSE abundances of cores and rinds of meter-scale blocks of eclogite, blueschist and garnet amphibolite from subduction-related mélanges within 1) the Franciscan Complex, CA, and a related terrane in Oregon; 2) the Catalina Schist, CA; and 3) the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Rinds composed of amphibole, chlorite, ± phengite, talc, titanite and rutile partly enclose blocks at each location. Mineralogic, petrologic and geochemical data suggest that these rinds are metasomatic features that formed by fluid-mediated interaction between mafic blocks and an ultramafic matrix.The cores of high-grade blocks are characterized by high (Pt+Pd)/(Os+Ir+Ru), and variably radiogenic Os; initial 187Os/188Os ranges from 0.197 to 4.30. These characteristics are consistent with the HSE compositions of subducted basalts. In contrast, (Pt+Pd)/(Os+Ir+Ru) of rinds are generally much lower than cores, approaching values typical of mantle peridotites. The initial 187Os/188Os of most rinds are also typical of upper mantle peridotites; values for most rinds fall between 0.125 and 0.14. The similarities of the 187Os/188Os and HSE abundances between rinds and mantle peridotites indicate that the HSE contained within rinds reflect a contribution by mantle peridotite, whether by fluid-mediated transfer or through mechanical processes. If the HSE signatures of the rinds were generated by fluid-mediated metasomatic processes, these data require remarkably efficient and uniform transference of mantle HSE from peridotitic mélange matrix to the exteriors of mélange blocks during rind formation. More likely, the rinds are coatings of materials that contain HSE derived from peridotite precursors. These results suggest that centimeter-scale mass transfer by physical mechanisms may be important for some elements within the subduction channel.

Matrix deformation mechanisms in HP-LT tectonic mélanges — Microstructural record of jadeite blueschist from the Franciscan Complex, California

Exhumed high pressure–low temperature metamorphic mélanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction. Rigid blocks of widely varying size are embedded in a weak matrix, which takes up the deformation and controls the rheology of the composite. The microfabrics of a highly deformed jadeite-blueschist from the Franciscan Complex, California, are investigated to help understand deformation mechanisms at depth. The specimen shows a transposed foliation with dismembered fold hinges and boudinage structures. Several generations of open fractures have been sealed to become veins at high-pressure metamorphic conditions. The shape of these veins, frequently restricted to specific layers, indicates distributed host rock deformation during and after sealing. Small cracks in jadeite and lawsonite are healed, with tiny quartz inclusions aligned along the former fracture surface. Large jadeite porphyroblasts show strain caps and strain shadows. Open fractures are sealed by quartz and new jadeite epitactically grown on the broken host. Monophase glaucophane aggregates consist of undeformed needles with a diameter between 0.1 and 2μm, grown after formation of isoclinal folds. Only quartz microfabrics indicate some stage of crystal-plastic deformation, followed by annealing and grain growth. Aragonite in the latest vein generation shows retrogression to calcite along its rims. The entire deformation happened under HP-LT metamorphic conditions in the stability field of jadeite and quartz, at temperatures between 300 and 450°C and pressures exceeding 1–1.4GPa. The microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism, accompanied by brittle failure and vein formation at quasi-lithostatic pore fluid pressure. This indicates low flow strength and, combined with high strain rates expected for localized deformation between the plates, a very low viscosity of material in the interplate shear zone at a depth>30–45km.

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.

A methodology for predicting future coastal hazards due to sea-level rise on the California Coast

Sea-level rise will increase the risks associated with coastal hazards of flooding and erosion. Along the active tectonic margin of California, the diversity in coastal morphology complicates the evaluation of future coastal hazards. In this study, we estimate future coastal hazards based on two scenarios generated from a downscaled regional global climate model. We apply new methodologies using statewide data sets to evaluate potential erosion hazards. The erosion method relates shoreline change rates to coastal geology then applies changes in total water levels in exceedance of the toe elevation to predict future erosion hazards. Results predict 214 km2 of land eroded by 2100 under a 1.4 m sea level rise scenario. Average erosion distances range from 170 m along dune backed shorelines, to a maximum of 600 m. For cliff backed shorelines, potential erosion is projected to average 33 m, with a maximum potential erosion distance of up to 400 m. Erosion along the seacliff backed shorelines was highest in the geologic units of Cretaceous marine (K) and Franciscan complex (KJf). 100-year future flood elevations were estimated using two different methods, a base flood elevation approach extrapolated from existing FEMA flood maps, and a total water level approach based on calculations of astronomical tides and wave run-up. Comparison between the flooding methods shows an average difference of about 1.2 m with the total water level method being routinely lower with wider variability alongshore. While the level of risk (actual amount of future hazards) may vary from projected, this methodology provides coastal managers with a planning tool and actionable information to guide adaptation strategies.

San Andreas Fault tremor and retrograde metamorphism

[1] Tectonic tremor is an enigmatic low-frequency seismic phenomenon mainly observed in subduction zones, but also documented along the deep extension of the central San Andreas Fault. The physical mechanisms behind this unusual seismic event are not yet determined for any tectonic setting; however, low effective stress conditions arising from metamorphic fluid production are commonly inferred for subduction-related tremor. We investigate the petrologic conditions at which the San Andreas tectonic tremor is inferred to occur through calculations of the pressure – temperature – time evolution of stable mineral assemblages and their water content in the dominant lithologies of the Franciscan Complex. We find that tremor locations around Parkfield and Cholame are currently experiencing retrograde metamorphic conditions. Within the temperature-depth conditions of observed tremor activity, at approximately 500°C and 20 km depth, several mineralogical transitions may occur in cooling greywacke and mafic rocks, leading to localised, significant removal of free water and an associated volume decrease. This indicates that, contrary to subduction-related tremor, tremor on the San Andreas Fault is not linked to prograde, crustal metamorphic fluid production within the fault zone; rather it might be related to mantle-derived fluids from below the tremor zone, and/or fault zone weakening that occurs as phyllosilicates replace more competent and granular mineral phases.

The influence of upper-crust lithology on topographic development in the central Coast Ranges of California

A fundamental geological tenet is that as landscapes evolve over graded to geologic time, geologic structures control patterns of topographic distribution in mountainous areas such that terrain underlain by competent rock will be higher than terrain underlain by incompetent rock. This paper shows that in active orogens where markedly weak and markedly strong rocks are juxtaposed along contacts that parallel regional structures, relatively high topography can form where strain is localized in the weak rock. Such a relationship is illustrated by the topography of the central Coast Ranges between the Pacific coastline and the San Andreas fault zone (SAFZ), and along the length of the Gabilan Mesa (the “Gabilan Mesa segment” of the central Coast Ranges). Within the Gabilan Mesa segment, the granitic upper crust of the Salinian terrane is in contact with the accretionary-prism mélange upper crust of the Nacimiento terrane along the inactive Nacimiento fault zone. A prominent topographic lineament is present along most of this lithologic boundary, approximately 50 to 65 km southwest of the SAFZ, with the higher topography formed in the mélange on the southwest side of the Nacimiento fault. This paper investigates factors influencing the pattern of topographic development in the Gabilan Mesa segment of the central Coast Ranges by correlating shortening magnitude with the upper-crust compositions of the Salinian and Nacimiento terranes. The fluvial geomorphology of two valleys in the Gabilan Mesa, which is within the Salinian terrane, and alluvial geochronology based on optically-stimulated luminescence (OSL) age estimates, reveal that the magnitude of shortening accommodated by down-to-the-southwest tilting of the mesa since 400 ka is less than 1 to 2 m. Our results, combined with those of previous studies, indicate that at least 63% to 78% of late-Cenozoic, northeast–southwest directed, upper-crustal shortening across the Gabilan Mesa segment has been accommodated within the Nacimiento terrane. This is significant because perpendicular to orogenic strike the Nacimiento terrane constitutes less than ¼ of the distance between the coast and the SAFZ, and the other ¾ (or greater) of the distance between the coast and the SAFZ is underlain by the granitic upper crust of the Salinian terrane. We propose that strain and mountain building are localized within the Nacimiento terrane because it consists predominantly of the relatively weak Franciscan Complex mélange, and because the upper crust of the Salinian terrane is composed of relatively strong granitic rocks. Our hypothesis is supported by the distribution of post-seismic surface uplift associated with the 2003, 6.5 M W San Simeon earthquake, which mimics the topography of the southwestern part of the Gabilan Mesa segment of the central Coast Ranges.

Elemental mobility in subduction metamorphism: insight from metamorphic rocks of the Franciscan Complex and the Feather River ultramafic belt, California

The degree of element mobility in subduction metamorphism has generated much debate; some workers advocate considerable mobility during metamorphism, whereas others postulate minimal mobility. We assess this issue by examination of major and trace element concentrations and Pb-, Nd-isotopic data for 39 mafic metavolcanic rocks from the Franciscan subduction complex, related units of coastal California, and the Feather River ultramafic belt of the northern Sierra Nevada, California; these samples span a wide range of metamorphic grade. We conclude that these rocks, despite their metamorphism up to eclogite facies, preserve protolith major and trace elemental compositions and isotopic ratios, with the exception of some mobile large ion lithophile elements such as Ba, Pb, and to a smaller extent La, U, and Sr. Thus subduction metamorphism of these metabasalts occurred in a largely closed system. Lack of light rare earth element enrichment in the rocks demonstrates lack of chemical exchange with subducted metasediments. Relatively low SiO2 content (<48 wt.%) of many of the metamorphic rocks and the lack of correspondence between silica depletion and metamorphic grade suggests that the silica depletion resulted from seafloor hydrothermal alteration before subduction. In spite of demonstrated mobility of Pb, and possible mobility of Nd, isotopic ratios of Pb and Nd were not modified during subduction metamorphism. In contrast to our results from metabasaltic rocks, our analysis of actinolite-rich rinds from high-grade Franciscan mélange blocks suggests some chemical exchange between metachert and the overlying mantle. The increasing enrichment in Ba and Pb with increasing metamorphic grade suggests that Ba- and Pb-rich fluids interacted more intensely with metabasalt at the higher grades of metamorphism. Comparison of these results with studies of the active Mariana forearc suggests that fluids interacting with the mantle wedge up-dip of the region of magma genesis are derived from subducting sediments overlying the down-going plate.

Sediment yield, spatial characteristics, and the long-term evolution of active earthflows determined from airborne LiDAR and historical aerial photographs, Eel River, California

In mountainous landscapes with weak, fine-grained rocks, earthflows can dominate erosion and landscape evolution by supplying sediment to channels and controlling hillslope morphology. To estimate the contribution of earthflows to regional sediment budgets and identify patterns of landslide activity, earthflow movement needs to be quantified over significant spatial and temporal scales. Presently, there is a paucity of data that can be used to predict earthflow behavior beyond the seasonal scale or over spatially extensive study areas. Across 226 km^2 of rapidly eroding Franciscan Complex rocks of the Eel River catchment, northern California, we used a combination of LiDAR (light detection and ranging) and orthorectified historical aerial photographs to objectively map earthflow movement between 1944 and 2006. By tracking the displacement of trees growing on earthflow surfaces, we find that 7.3% of the study area experienced movement over this 62 yr interval, preferentially in sheared argillaceous lithology. This movement is distributed across 122 earthflow features that have intricate, elongate planform shapes, a preferred south-southwesterly aspect, and a mean longitudinal slope of 31%. The distribution of mapped earthflow areas is well-approximated by a lognormal distribution with a median size of 36,500 m^2. Approximately 6% of the study area is composed of earthflows that connect to major channels; these flows generated an average sediment yield of 19,000 t km^(−2) yr^(−1) (rock erosion rate of ∼7.6 mm/yr) over the 62 yr study period, equating to a regional yield of 1100 t km^(−2) yr^(−1) (∼0.45 mm/yr) if distributed across the study area. As such, a small fraction of the landscape can account for half of the regional denudation rate estimated from suspended sediment records (2200 t km^(−2) yr^(−1) or ∼0.9 mm/yr). We propose a conceptual model for long-term earthflow evolution wherein earthflows experience intermittent activity and long periods of dormancy when limited by the availability of readily mobilized sediment on upper slopes. Ultimately, high-order river channels and ephemeral gully networks may serve to destabilize hillslopes, controlling the evolution of earthflow-prone terrain.

Accretion of the Franciscan Complex attending Jurassic-Cretaceous geotectonic development of northern and central California

Research Article| September 01, 2011 Accretion of the Franciscan Complex attending Jurassic–Cretaceous geotectonic development of northern and central California W.G. Ernst W.G. Ernst † Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA †E-mail: wernst@stanford.edu Search for other works by this author on: GSW Google Scholar Author and Article Information W.G. Ernst † Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA †E-mail: wernst@stanford.edu Publisher: Geological Society of America Received: 08 Aug 2010 Revision Received: 10 Nov 2010 Accepted: 29 Nov 2010 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (9-10): 1667–1678. https://doi.org/10.1130/B30398.1 Article history Received: 08 Aug 2010 Revision Received: 10 Nov 2010 Accepted: 29 Nov 2010 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W.G. Ernst; Accretion of the Franciscan Complex attending Jurassic–Cretaceous geotectonic development of northern and central California. GSA Bulletin 2011;; 123 (9-10): 1667–1678. doi: https://doi.org/10.1130/B30398.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Scattered igneous activity marks the Triassic margin of California, but an Andean volcanic-plutonic arc began to form in the Sierra Nevada–White-Inyo Range and Klamath Mountains at ca. 170 Ma during underflow of the oceanic lithosphere. This calc-alkaline belt supplied volcanogenic erosional debris to the Mariposa-Galice proximal overlap sequence by ca. 165–160 Ma. Transpression also generated high-pressure–low-temperature basaltic eclogites, garnet-blueschists, and amphibolites along the Middle and Late Jurassic convergent plate junction at ca. 170–155 Ma; most of these returned surfaceward as tectonic blocks only during the mid-Cretaceous. The high-grade metamafic (metamorphosed mafic) blocks that formed during construction of the emergent arc and derived Mariposa-Galice strata predate the onset of Franciscan trench deposition. Near the end of Jurassic time, the Klamath salient evidently migrated ∼200 km westward. An earliest Cretaceous westward stepout of the convergent junction apparently formed directly offshore from the Klamath imbricate orogen, but to the south, preexisting oceanic crust–capped lithosphere was trapped landward as the Coast Range ophiolite. Andean arc detritus began to accumulate on this ophiolitic basement within the Great Valley forearc and outboard Franciscan oceanic trench at ca. 145–140 Ma. Because earliest Cretaceous, relatively continuous Great Valley strata were deposited on the stable North American plate, protected from both surface and subcrustal erosion, forearc terrigenous sedimentation also signaled coeval deposition in the outboard trench ∼10–25 m.y. after Middle and Late Jurassic initiation of the continental margin calc-alkaline arc. Voluminous sedimentation and the accretion of Franciscan and San Joaquin sections of Great Valley rocks took place during mid- and Late Cretaceous time after a flare-up in igneous arc activity at ∼125–120 Ma. The youngest Sierran granites have ca. 85 Ma ages, reflecting extinction of the magmagenic zone beneath northern and central California, and presumably reflecting subhorizontal oceanic plate underflow attending the Laramide orogeny. Feebly zeolitized Upper Cretaceous and Lower Tertiary Franciscan Coastal belt rocks, derived from the now-inactive arc, underwent only low-pressure burial metamorphism. Reflecting shallow offloading and/or large-scale transpression, they were not appreciably subducted. Although Coastal belt units are members of the Franciscan lithotectonic assemblage, their origin contrasts with the rest of the subduction complex. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Reconciling patterns of interseismic strain accumulation with thermal observations across the Carrizo segment of the San Andreas Fault

The Carrizo segment of the San Andreas Fault separates rocks of the Salinian Block southwest of the fault characterized by high heat flow (~ 75–95 mW/m 2) and shallow seismicity (<~ 10 km depth), from rocks of the Franciscan Complex and Great Valley Group northeast of the fault associated with low heat flow (50–60 mW/m 2) and deeper seismicity (<~ 20 km depth). GPS data from this region suggest that the northeast side of the fault accommodates more interseismic strain than the southwest side. We show that by incorporating variations in depth to the brittle ductile transition inferred based on heat flow and seismicity data, we fit the geodetic data well and achieve agreement with geologically accepted long-term average slip rates with a ~ 20 km wide zone with low Young's modulus NE of the fault. This wide compliant zone is consistent with observations suggesting that the presence of large overpressures may be decreasing the elastic moduli in this region.

Fill Placement on Slopes Underlain by Franciscan Mélange

Distress in two housing developments was caused by a deep bedrock landslide triggered by placement of a large upslope fill. The large fill was placed to create a visual barrier between the upslope development and downslope housing developments and to balance the cut and fill quantities for the upslope site. This case history presents some of the ramifications of fill placement on natural slopes surrounded by urban areas, such as overstressing underlying weak material that may exist below the depth of borings typically conducted for single family residences and office complexes; the importance of surface and subsurface information in a formation known locally as Franciscan complex/ melange (which is a block-in-matrix rock formation common to the area); the shear behavior of serpentinite, which is part of the Franciscan complex; and the importance of natural and man-made changes to a slope, such as rainfall, surficial grading, home construction, and fill placement. It also illustrates the importance of locating the critical slope cross section before construction and the proper use of back-analyses in a landslide investigation. DOI: 10.1061/(ASCE)GT.1943-5606.0000394. © 2011 American Society of Civil Engineers. CE Database subject headings: Landslides; Shear strength; Slope stability; Subsurface investigations; Case studies; Fills. Author keywords: Landslides; Shear strength; Slope stability; Subsurface investigation; Serpentinite; Franciscan complex; Melange; Block-in-matrix rock.

Suprasubduction-zone ophiolite generation, emplacement, and initiation of subduction: A perspective from geochemistry, metamorphism, geochronology, and regional geology

Ophiolites are on-land remnants of oceanic lithosphere, and most of the more extensive ophiolites apparently formed above a subduction zone, a tectonic setting known as a suprasubduction-zone setting. Thin sheets of high-grade metamorphic rocks, known as metamorphic soles, crop out structurally beneath many suprasubduction-zone ophiolites. Such rocks may have formed during the inception of subduction beneath young and hot oceanic lithosphere. Disagreement exists as to whether suprasubduction-zone ophiolites are emplaced over the same subduction zone over which they once formed or over a later one. High-grade metamorphic rocks (blocks-in-melange and coherent sheets) from the Franciscan Complex may represent a metamorphic sole beneath the suprasubduction-zone Coast Range ophiolite. Trace-element and isotopic data indicate that the Franciscan high-grade metamorphic rocks formed in a suprasubduction-zone envi ronment, requiring the existence of a pre-Franciscan sub duction zone, whereas later-subducted, lower-grade oceanic rocks are of mid-ocean-ridge or oceanic-island basalt affi ni ties. The Coast Range ophiolite and Franciscan high-grade rock protoliths formed over a pre-Franciscan subduction zone that may have dipped westward. The high-grade Franciscan rocks were metamorphosed at the inception of east-dipping subduction beneath the Coast Range ophiolite, and the ophiolite was subsequently emplaced over this later subduction zone. Suprasubduction-zone protolith signatures have been obtained for other metamorphic soles beneath suprasubduction-zone ophiolites, suggesting that our proposed model of suprasubduction-zone ophiolite generation over one subduction zone and emplacement over a second one may be globally applicable. Regional geology suggests that this dual subduction-zone model may also apply to suprasubduction-zone ophiolites with midocean-ridge and/or oceanic-island basalt soles.

Lithium isotopes in Guatemalan and Franciscan HP–LT rocks: Insights into the role of sediment-derived fluids during subduction

High-pressure, low-temperature (HP–LT) rocks from a Cretaceous age subduction complex occur as tectonic blocks in serpentinite mélange along the Motagua Fault (MF) in central Guatemala. Eclogite and jadeitite among these are characterized by trace element patterns with enrichments in fluid mobile elements, similar to arc lavas. Eclogite is recrystallized from MORB-like altered oceanic crust, presumably at the boundary between the down-going plate and overlying mantle wedge. Eclogite geochemistry, mineralogy and petrography suggest a two step petrogenesis of (1) dehydration during prograde metamorphism at low temperatures (<500 °C) followed by (2) partial rehydration/fertilization at even lower T during exhumation. In contrast, Guatemalan jadeitites are crystallized directly from low- T aqueous fluid as veins in serpentinizing mantle during both subduction and exhumation. The overall chemistry and mineralogy of Guatemalan eclogites are similar to those from the Franciscan Complex, California, implying similar P– T– x paths. Li concentrations (⩽90 ppm) in mineral separates and whole rocks (WR) from Guatemalan and Franciscan HP–LT rocks are significantly higher than MORB (4–6 ppm), but similar to HP–LT rocks globally. Li isotopic compositions range from −5‰ to +5‰ for Guatemalan HP–LT rocks, and −4‰ to +1‰ for Franciscan eclogites, overlapping previous findings for other HP–LT suites. The combination of Li concentrations greater than MORB, and Li isotopic values lighter than MORB are inconsistent with a simple dehydration model. We prefer a model in which Li systematics in Guatemalan and Franciscan eclogites reflect reequilibration with subduction fluids during exhumation. Roughly 5–10% of the Li in these fluids is derived from sediments. Model results predict that the dehydrated bulk ocean crust is isotopically lighter (δ 7Li ⩽ +1 ± 3‰) than the depleted mantle (∼+3.5 ± 0.5‰), while the mantle wedge beneath the arc is the isotopic complement of the bulk crust. A subduction fluid with an AOC–GLOSS composition over the full range of model temperatures (50–600 °C) gives an average fluid δ 7Li (∼+7 ± 5‰ 1 σ) that is isotopically heavier than the depleted mantle. If the lowest temperature steps are excluded (50–260 °C) as too cold to participate in circulation of the mantle wedge, then the average subduction fluid (δ 7Li = +4 ± 2.3‰ 1 σ, is indistinguishable from depleted mantle. Because of the relatively compatible nature of Li in metamorphic minerals, the most altered part of the crust (uppermost extrusives), may retain a Li isotopic signature (∼+5 ± 3‰) heavier than the bulk crust. The range of Li isotopic values for OIB, IAB and MORB overlap, making it is difficult to resolve which of these components may contribute to the recycled component in the mantle using δ 7Li alone.

Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA

Abstract High-grade blocks of eclogite, blueschist, and garnet amphibolite found in melange terrains in the Franciscan Complex, CA display petrologic and geochemical evidence for a complicated history of interactions with aqueous fluids. These blocks have features (actinolitic rinds, a blueschist layer) that have petrologic and geochemical indicators of interaction with retrograde aqueous fluids. Lithium concentrations and isotopic compositions of different portions of three high-grade blocks are different from those of MORB (their presumed protolith) and show systematic variations associated with fluid-related features. Block cores and an eclogite layer have high whole-rock Li concentrations relative to MORB (17 to 39 ppm) and low to moderate δ7Li (0.4 to 2.1‰). This range of values is inconsistent with dehydration of MORB and is interpreted as the product of infiltration of aqueous fluids with an elevated Li concentration and low δ7Li. Rinds and a blueschist layer exhibit lower Li concentrations (7 to 24 ppm) and higher δ7Li (2.7 to 4.4‰). The differences between cores and rinds, and the eclogite and blueschist layers are interpreted as the product of retrograde infiltration of an aqueous fluid with low Li concentrations and high δ7Li. Results of a small-scale traverse across the boundary between the blueschist and eclogite layers show that the boundary is gradational over ∼ 0.5 cm, suggesting small-scale cross-layer diffusion/dispersion associated with the fluid infiltration. The high Li concentrations and low δ7Li of block cores suggests that the source of early fluids was likely subducted sedimentary rocks. The lower Li concentrations and higher δ7Li values of the rinds and blueschist layers do not fully constrain the source of retrograde fluids, but indicate that the fluid source likely had a lower Li concentration and higher δ7Li.

Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes

We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by multicollector inductively coupled plasma–mass spectrometry. Lawsonite 176 Lu/ 177 Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 ± 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins.

Multiple origins of zircons in jadeitite

Jadeitites form from hydrothermal fluids during high pressure metamorphism in subduction environments; however, the origin of zircons in jadeitite is uncertain. We report ion microprobe analyses of δ18O and Ti in zircons, and bulk δ18O data for the jadeitite whole-rock from four terranes: Osayama serpentinite melange, Japan; Syros melange, Greece; the Motagua Fault zone, Guatemala; and the Franciscan Complex, California. In the Osayama jadeitite, two texturally contrasting groups of zircons are identified by cathodoluminescence and are distinct in δ18O: featureless or weakly zoned zircons with δ18O = 3.8 ± 0.6‰ (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher δ18O = 5.0 ± 0.4‰. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar δ18O values to the latter from Osayama: Guatemala zircons are 4.8 ± 0.7‰, and the Syros zircons are 5.2 ± 0.5‰ in jadeitite and 5.2 ± 0.4‰ in associated omphacitite, glaucophanite and chlorite-actinolite rinds. The δ18O values for most zircons above fall within the range measured by ion microprobe in igneous zircons from oxide gabbros and plagiogranites in modern ocean crust (5.3 ± 0.8‰) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6‰). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of δ18O (whole-rock) ≅ δ18O (jadeite) and vary from 6.3 to 10.1‰ in jadeitites in all four areas.