Loren A. Raymond

Tesla-Ortigalita Fault, Coast Range Thrust Fault, and Franciscan Metamorphism, Northeastern Diablo Range, California

Fault contacts in the northeastern Diablo Range, California, between the partially melanged late Mesozoic Franciscan Complex and the broadly coeval, less deformed sedimentary rocks of the Great Valley sequence have been called, by definition, the Tesla-Ortigalita fault. “Coast Range thrust” is the name applied by Bailey and others (1970a) to the fault of regional extent that originally separated subducted oceanic crust and sedimentary rock of the Franciscan Complex from structurally overlying ophiolite plus shelf-slope facies sedimentary rock of the Great Valley sequence. The two faults are not equivalent. High-angle Neogene segments of the Tesla-Ortigalita fault truncate older fault surfaces of the Coast Range thrust-fault system. Franciscan metamorphic rocks contain low-to high-pressure, low-temperature mineral parageneses characterized by the phases pumpellyite, prehnite, aragonite, lawsonite, glaucophane, and jadeitic pyroxene. Metamorphism has been variously ascribed to metastable re-crystallization, metasomatism by fluids generated in and around serpentinite, structural burial produced by subduction of an oceanic lithospheric plate, and burial plus tectonic overpressures generated beneath the Coast Range thrust fault. Suppe (1970) and Ernst (1971a) argued against metastable recrystallization and metasomatism on the basis of available field and laboratory evidence. If metamorphism is related to serpentinization, a metamorphic aureole should surround ultramafic bodies undergoing present-day ser-pentinization. Metamorphism resulting from tectonic overpressures generated beneath the Coast Range thrust fault will be revealed by an increase in metamorphic grade toward the thrust, whereas structural burial would result in an increase in metamorphic grade with structural depth. Distribution of mineral parageneses in the northeastern Diablo Range, revealed by analyses of more than 300 thin sections of metaclastic rocks, shows no spatial relation between highest grade rocks and either exposed segments of the Coast Range thrust fault or the margins of ultramafic masses undergoing present-day serpentinization. Thus the available evidence fails to support the metastable recrystallization, metasomatic, and tectonic overpressure concepts. Only the hypothesis of structural burial is not negated by the observed relations.

Metamorphic conditions in the Ashe Metamorphic Suite, North Carolina Blue Ridge

Taconian metamorphism of mafic rocks in the Ashe Metamorphic Suite can be characterized by reference to an isograd corresponding to the reaction bio + epi = hbl + gar, which separates rocks into two zones of low-variance assemblages. Temperatures and pressures estimated from mineral exchange geothermometers and a barometer suggest that this reaction occurred at approximately 600-650 °C and 7.5 kbar. Phase equilibria between biotite and hornblende, as well as the sharpness of the mapped isograd, indicate that the reaction is discontinuous. Inferred differences in metamorphic grade between Ashe amphibolites and mafic dikes in the underlying basement suggest that these units are in faulted contact. Isograd patterns in pelitic rocks suggest an elongated domal uplift that developed after metamorphism and thrusting, the core of which is exposed in the adjacent Grandfather Mountain window.

Designating tectonostratigraphic terranes versus mapping rock units in subduction complexes: perspectives from the Franciscan Complex of California, USA

Accretionary complex histories are broadly understood. Sedimentation in seafloor and trench environments on drifting subducting plates and in associated trenches, followed by (1) deformation and metamorphism in the subduction zone and (2) subsequent uplift at the overriding plate edge, result in complicated stratigraphic and structural sequences in accretionary complexes. Recognizing, defining, and designating individual terranes in subduction complexes clarify some of these complicated relationships within the resulting continent-scale orogenic belts. Terrane designation does not substitute for detailed stratigraphic and structural mapping. Stratigraphic and structural mapping, combined with radiometric and palaeontologic dating, are necessary for delineation of coherent, broken, and dismembered formations, and various mélange units, and for clarification of the details of subduction complex architecture and history. The Franciscan Complex is a representative subduction complex that has evolved through sedimentation, faulting, folding, and low-temperature metamorphism, followed by uplift, associated deformation, and later overprinted deformation. Many belts of Franciscan rocks are offset by strike-slip faults associated with the dextral San Andreas Fault System. In the Franciscan Complex, among the terrane names applied widely, are the ‘Yolla Bolly Terrane’ and the ‘Central Terrane’. Where detailed mapping and detrital zircon ages exist, data reveal that the two names have been applied to rocks of similar general character and age. In the northeastern Diablo Range, Franciscan Complex rocks include coherent units, broken and dismembered formations, and various types of mélanges, all assigned at various times to the Yolla Bolly and other terranes. The details of stratigraphic and structural history revealed by large-scale mapping and radiometric dating prove to be more useful in clarifying the accretionary complex history than assigning a terrane name to the rocks. That history will assist in resolving terrane assignment issues and allow discrimination of subduction-associated and post-subduction events, essential for understanding the overall history of the orogen.

Tectonite and Mélange— A Distinction

Metamorphic tectonite is a rock in which the fabric elements reflect the history of deformation. S- and B-tectonites are rocks characterized by planar and linear fabric elements, respectively, in which the fabric elements were produced by flow in the solid state. SF-tectonite (defined herein) is a metamorphic rock with planar fabric elements that were produced by fracture and (or) shear along a pervasive set of subparallel surfaces. Melange is a rock body; albeit many melanges contain tectonites, the presence of exotic blocks in a fragmented matrix–not the presence of tectonites–is the main criterion supported here for distinguishing a melange from other rock units.

Sandstone-matrix mélanges, architectural subdivision, and geologic history of accretionary complexes: A sedimentological and structural perspective from the Franciscan Complex of Sonoma and Marin counties, California, USA

Understanding details of accretionary complex architecture is essential to understanding construction of oceanic “outer” sides of orogens. The architecture of the Franciscan Complex (California), considered by many to be the “type” accretionary complex, is widely viewed in the context of terranes or belts delimited by reconnaissance mapping that reveals neither regional variations within terranes nor critical details of stratigraphy and structure. The architectural importance of Franciscan melanges is recognized, but the importance of sandstone-matrix melanges and olistostromal sandstones is not. Large-scale mapping in Sonoma and Marin counties, California, shows that Franciscan rocks are deformed, submarine-fan units of Facies A–E, plus Facies F olistolith-bearing submarine channel sandstones and olistostromal sandstone- and shale-matrix melanges. Some melanges are polygenetic with a sedimentary origin and a tectonic overprint. Glaucophane schists were recycled into conglomerates and olistostromes. Mappable units constitute members, broken and dismembered formations, and melanges. Considering the stratigraphy and structure evident at the 1:24,000 scale, accretion via a subduction channel mechanism is impossible. The Sonoma-Marin Central belt or Central terrane (melange) is not a monolithic shale-matrix melange and lacks this characteristic of rocks assigned the same name to the north. Franciscan rocks here structurally underlie thrust-faulted fragments of a regional ultramafic sheet and, locally, an underlying exotic block-bearing serpentinite-matrix melange. The detailed mapping shows that regional relations among and within Franciscan terranes and belts are poorly understood and suggests that such mapping is needed to clarify accretionary complex architecture and history. The implication for accretionary complex studies, in general, is that, while terrane or belt designations provide a general picture of the collage nature of accretionary complexes and clarification of regional relationships, only large-scale structural and stratigraphic studies can elucidate the architectural details of these orogenic complexes.

Possible Modern Analogs for Rocks of the Franciscan Complex, Mount Oso Area, California

The Deep Sea Drilling Project has sampled sedimentary sections from a wide variety of tectonic settings. Of those sampled, sections from the abyssal ocean floor combined with inferred sections from the oceanic trenches provide the best stratigraphic analogs for chert-graywacke, volcanic rock–chert–graywacke, and olistostrome-bearing sequences in the Franciscan Complex of the Mount Oso area, California.

Cr-spinel compositions, metadunite petrology, and the petrotectonic history of Blue Ridge ophiolites, Southern Appalachian Orogen, USA

Abstract Resolution of the petrotectonic history of Blue Ridge ophiolites of the Southern Appalachian Orogen has remained enigmatic because of metamorphism and tectonic fragmentation of ultramafic bodies. Understanding of this history is confounded by the presence of five partial metamorphic overprints and by similar Ti enrichments in spinels from Blue Ridge and modern mid-ocean ridge basalt ultramafic rocks that result from different processes. Chrome spinels from oceanic ultramafic lithosphere show increases in Ti caused by metasomatism induced by passing mafic melts, which create both dunite melt channels within harzburgite wall rocks and associated troctolite impregnation zones. In the Blue Ridge Belt, the oldest metadunite mineral association generally lacks high-Ti spinel, whereas the higher Ti spinels are relatively low in Al and Mg and occur in three amphibolite- to greenschist-facies retrograde metamorphic associations that occur in deformed, metasomatized ultramafic bodies with high aspect ratios. Some spinel compositions in the oldest mineral association are similar to those from arc-suprasubduction zone ultramafic lithosphere. Together, available data are consistent with the hypothesis that: (1) the Blue Ridge ophiolites are fragmented, metamorphosed, very slow-spreading ridge, Xigaze-type ophiolites, consisting of mafic rocks, minor plutonic rocks, and a sublithospheric ultramafic tectonite base; (2) the metadunites represent sublithospheric melt channels and zones of high melt flux, perhaps formed in a suprasubduction zone setting; (3) pre-Taconic subduction may have been west-directed rather than east-directed. The Taconic orogenesis deformed, fragmented, and metamorphosed the ophiolites; and later Taconic, Acadian, and Alleghenian metamorphism hydrated the bodies, while associated deformation exaggerated their elongation.

Episodic accretion and plutonism in southwestern Alaska

Abstract Paleontologic and radiometric dating of the accretionary prism and magmatic arc of southwestern Alaska reveal an history of episodic accretion and plutonism. Possible accretion events in the Triassic (220-195 m.y.) and Early Jurassic (184-176 m.y.) were followed by Middle Cretaceous (108-83 m.y.), earliest Paleogene (65-60 m.y.), Middle Paleogene (50-40 m.y.), and Neogene (25-0 m.y.) accretion episodes. Plutonic events, which alternate with the accretion events, occurred in the Early Jurassic (193-184 m.y.), Middle/Late Jurassic (176-145 m.y.), Late Cretaceous/Early Paleogene (83-50 m.y.), and Late Paleogene (38-26 m.y.). Episodicity of accretion events is an apparent cause of incomplete stratigraphic records in the accretionary prism and forearc basin.

A metasomatic setting, the Russian River Arch, and gravitational emplacement in the history of eclogites at the classic eclogite locality of Jenner, California, USA

ABSTRACT Blocks of metamorphic rock designated as ‘high-grade’ blocks, commonly less than 100 m in diameter, consisting of garnet-glaucophane- and hornblende-schists and gneisses and rare eclogite, are widely distributed within mélanges of the Franciscan (accretionary) Complex of California. Eclogite-glaucophane schist blocks present at Jenner, California, have been studied for petrographic, geochemical, structural, and age characteristics, but their relationship to associated Franciscan rocks is poorly understood. The studied blocks are not in situ, but rather occur in landslide deposits and beach sands. The landslide deposits overlie the low to middle slope exposures of sandstone-rich broken formations of the Franciscan Complex that are not known to contain high-grade blocks. Geochemical studies suggest a serpentinite host for the blocks. Upslope, a serpentinite-matrix mélange contains numerous high-grade blocks, including rare retrograded eclogite, and is the likely block source. The Jenner terrain as a whole was uplifted relative to rocks to the north and south near Annapolis and Freestone, respectively, by uplift along the post-Pliocene Russian River (anticlinal) Arch, as indicated by the regional distribution of arching, wave-cut, post-Franciscan surfaces with overlying Miocene/Pliocene marine sedimentary rocks. Local uplift increased landsliding and colluvial downslope movement of the blocks. In addition, local, wave-influenced transportation of smaller blocks, together with the downslope mass movements, brought the high-grade blocks to their present positions. The high-grade blocks are thus displaced from upslope exposures of the original serpentinite-matrix mélange host, in which the blocks likely experienced the metasomatism that converted eclogite to glaucophane schist. In general, the relationship of blocks to the original serpentinite host is a critical element of subduction zone architecture related to subduction zone processes and history, and should be analysed, in any studies that seek to explain the architecture and history of any accretionary complex with similar high-grade blocks.

What is Franciscan?: revisited

ABSTRACT The Franciscan Complex of California is better understood now than in 1972, when Berkland et al. defined it as a complex and divided it into three geographic belts. A re-evaluation is needed. Belts first served as major architectural units, but they have been abandoned by some and renamed as and subdivided into tectonostratigraphic terranes by others. The Franciscan Complex – considered to be the archetypical accretionary complex by many – is the folded, faulted, and stratally disrupted rock mass comprising the supramantle basement of the California-Southern Oregon Coast Ranges exposed east of the Salinian Block and west of and structurally below principal exposures of the Coast Range Fault, Coast Range Ophiolite, Great Valley Group, and Klamath Mountains. The Complex is dominated by sandstones and mudrocks, but contains mafic oceanic crustal fragments with chert, limestone, and other rock types, and zeolite, prehnite-pumpellyite, blueschist, and rare amphibolite and eclogite facies metamorphic rocks. Review of historical precedence, new data, available large-scale maps, and fundamental definitions suggest now (1) that the Belt terminology as applied to the entire Franciscan Complex conflicts with current knowledge of Franciscan rocks and architecture; and (2) that most named Franciscan terranes and nappes are inconsistent with basic definitions of those unit types. The major architectural units into which the Franciscan Complex can be divided are accretionary units – mélanges and underthrust sheets. Underthrust sheets can be subdivided into smaller units, e.g. broken formations and olistostromal mélanges, mappable using traditional lithostratigraphic and structural mapping techniques. Unresolved controversies in reconstruction of the nature and history of the accretionary complex relate to specific mélange origins; megathrust versus subduction channel mélange models; chert conundrums; delineation of the ages, subdivisions, and regional architecture of Franciscan units; palinspastic reconstruction of the pre-Late Cenozoic architecture; and reconstruction of the complete histories of accretionary units.