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Deconstruction of the Franciscan Complex Central Terrane Mélange and re-evaluation of Franciscan mélanges and architecture of the northwestern San Francisco Bay Area, California, USA
Paleogeographic reconstruction of regional accretionary complex architecture, Franciscan Complex, northwestern San Francisco Bay Area, California, USA
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
Mappability, stratigraphic variation, and diagenetic problems in sedimentary map unit definition and field mapping
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.
Clastic lawsonite, glaucophane, and jadeitic pyroxene in Franciscan metagraywackes from the Diablo Range, California: Alternative interpretation
Metamorphic conditions in the Ashe Metamorphic Suite, North Carolina Blue Ridge
Comment and Reply on "Late Precambrian crustal extension preserved in Fries fault zone mylonites, southern Appalachians"
Mesoscopic to macroscopic block-in-matrix structures are widely distributed in the Blue Ridge belt of the southern Appalachian orogen. The belt is subdivided into four tectonostratigraphic terranes: (1) the eastern Upper Proterozoic Toe terrane, consisting of metasedimentary rocks, metabasites (amphibolites), and ultramafic rocks; (2) a western, Middle Proterozoic, cratonic terrane, the Sherwood terrane, consisting of metamorphosed granitoid rocks and structurally overlying Upper Proterozoic to Paleozoic metasedimentary and sedimentary rocks; (3) the enigmatic, intervening Cullowhee terrane, lithologically similar to the Toe terrane, largely of unknown age but yielding a Middle Proterozoic age from one area in the north; and (4) a Middle to Upper Proterozoic terrane, the Grandfather terrane, exposed beneath the Toe and Sherwood terranes, in the Grandfather Mountain window. Block-in-matrix structures occur principally in the Toe and Cullowhee terranes. Block-in-matrix structures are formed by a variety of sedimentary, igneous, diapiric, and tectonic (including metamorphic) processes. Notably, such structures characterize mélanges and migmatites. In the southern Appalachian orogen, inasmuch as the Toe and Cullowhee terrane rocks are metamorphic, where block-in-matrix structures have been recognized in the past, they have generally been assigned a metamorphic-tectonic origin involving tensile or compressive, ductile, penetrative strain. Specifically, they have been considered to be migmatites or the blocks to be boudins of metamorphic rock in a metamorphic matrix. Metamorphosed mélanges are identified primarily on the basis of: (1) block-in-matrix structures (with included exotic lithologies) and (2) paleogeographic position in the orogen. The wide distribution, paleogeographic position, exotic ultramafic rocks, and pre-peak metamorphic fragmentation history of Toe and Cullowhee terrane rock units, which exhibit block-in-matrix structure, suggest that their protoliths could have been mélanges.
Comment and Reply on "Bacterially mediated diagenetic origin for chert-hosted manganese deposits in the Franciscan Complex, California Coast Ranges"
Comment and Reply on “Fault-related rocks: Suggestions for terminology”: COMMENT
Preface
Melanges and related rock bodies have been defined, described, and classified in a variety of ways. Classifications, divisible into eight types, reflect specific definitions of the term melange . Although melanges were originally considered to be tectonic in origin, currently the term is used widely in a descriptive rather than a genetic sense. Melanges are here defined as bodies of rock mappable at a scale of 1:24000 or smaller and characterized both by the lack of internal continuity of contacts or strata and by the inclusion of fragments and blocks of all sizes, both exotic and native, in a fragmented matrix of finer-grained material. Matrix composition and fabric are rejected as definitive criteria. Progressive fragmentation and mixing of original sedimentary, igneous, and/or metamorphic protoliths give rise to broken formations, dismembered formations, and melanges—three classes of unit that, together with formations, represent a continuum of rock bodies ranging from coherent stratified units at one extreme to chaotically mixed masses at the other. Melanges may originate through sedimentary processes, diapiric processes, tectonic processes, or combinations of these processes, and are classified accordingly.
Early deformation in melange terranes of the Ghost Rocks Formation, Kodiak Islands, Alaska
The Ghost Rocks Formation represents the youngest phase of a Late Cretaceous to early Tertiary episode of accretion during which about 80 percent of Kodiak Island was added to the continental margin. The formation consists of both structurally coherent terranes and terranes of complexly deformed sandstone-shale melange. Two mesoscopically ductile and regionally occurring deformations and three younger, relatively brittle deformations have affected the melange. D 1 is characterized by a foliation, S 1 , which dips northwest and a lineation, L 1 , which plunges gently southwest. S 1 is defined by disrupted lithologic layering, pinch and swell structures, the alignment of inclusions, and an anastomosing cleavage in the shales. L 1 is defined by pinch and swell axes, the long axes of inclusions, mullions, and folds of phyllosilicates in the shales. D 2 is characterized by a foliation, S 2 , and a lineation, L 2 , which respectively dip and plunge parallel to the D 1 fabrics. S 2 is a closely spaced, roughly planar, slaty-like cleavage that commonly transposes, but only locally folds S 1 . L 2 is defined by D 2 fold axes and the intersection of S 1 and S 2 which locally is a pencil cleavage. D 1 significantly and substantially altered the original stratigraphy in the melange, and the distinctive disruptive style of the melange is probably a direct consequence of this deformation. The most important structural element that contributed to this deformation, and therefore to S 1 and L 1 , was a three-dimensional web structure of cataclastic shear zones. This structural element occurs in essentially every sandstone in the melange and suggests, but does not prove, that D 1 was technically induced. Other common D 1 structural features are calcite-filled veins, swirls and folds of detrital or diagenetic phyllosilicates, and planar shear zones in the shales. Asymmetric D 1 structures, especially disrupted tuff horizons, suggest D 1 occurred during northwest directed layer-parallel shear and underthrusting, although other noncoaxial strain histories are also possible. Layer-parallel simple shear, and coaxial plane, constrictional, and flattening strains appear to be ruled out, however, because they do not explain the general, mesoscopic strains observed in the melange.
Formation of melange in a foreland basin overthrust setting: Example from the Taconic Orogen
The Taconic melanges of eastern New York developed through the progressive deformation of a synorogenic flysch sequence deposited within a N-S elongate foreland basin. This basin formed in front of the Taconic Allochthon as it was emplaced onto the North American continental shelf during the medial Ordovician Taconic Orogeny. The flysch was derived from, and was subsequently overridden by the allochthon, resulting in the formation of belts of tectonic melange. An east to west decrease in deformation intensity allows interpretation of the structural history of the melange and study of the flysch-melange transition. The formation of the melange involved: isoclinal folding, boudinage and disruption of graywacke-shale sequences due to ductility contrasts; sub-aqueous slumping and deposition of olistoliths which were subsequently tectonized and incorporated into the melange; and imbrication of the overthrust and underthrust sedimentary sections into the melange. The characteristic microstructure of the melange is a phacoidal conjugate-shear cleavage, which is intimately associated with high strains and bedding disruption. Rootless isoclines within the melange have apparently been rotated into an east-west shear direction, consistent with fault, fold, and cleavage orientations within the flysch. The melange zones are best modeled as zones of high shear strain developed during the emplacement of the Taconic Allochthon. Total displacement across these melange zones is estimated to be in excess of 60 kilometers.
When subduction rates are several cm/yr or more, various geologic and heat flow considerations indicate the portion of the accretionary wedge beneath the overriding plate will consist of two structural “layers.” The upper layer is composed of schistose rocks directly in contact with, and heated by the base of the overriding plate. The lower layer is composed of fluid-rich, undercompacted, less heated mud and sand. With continued convergence, large shear strains are concentrated in the lower layer and tectonic flow melange develops. The portion of the accretionary wedge that accumulates in front of the overriding plate can be, as in the Franciscan of California, largely composed of subduction-generated melange blanketed by bedded slope sediments. Mappable units within an accreted melange wedge are defined by variations in the type and relative numbers of clasts. The population of clasts in flowing melange can vary where high-pressure metamorphic rocks are only locally plucked from the base of the overriding plate, where upflow ceases, and where features such as seamounts are dismembered as they locally impinge the accretionary complex. Melange intrusions, in the form of dikes and sills, can also form mappable melange units within an accretionary wedge.
Modern submarine sediment slides and their implications for melange and the Dunnage Formation in north-central Newfoundland
Modern submarine sediment slides produce two features: a slide scar that delineates a zone of removal and a deposit of slide material. The upturned edges of the slide scar form prominent scarps with relief of generally less than 100 m. The zone of deposition includes “hummocky,” “blocky,” and “debris flow” high-resolution (3.5–12 kHz) seismic fades. These echo types probably represent olistoliths, piles and/or folds of deformed sediment, and debris flow deposits, respectively. Cores and bottom photographs exhibit deformed, chaotic material typical of debris flow deposits. Sediment slides are common, not only on active margins, but also on passive margins. Slide complexes are not restricted to base-of-slope sites; rather, a single slide on a passive margin can stretch over 700 km from the continental shelf break to the abyssal plain. Giant submarine sediment slides have implications for studies of melange. Sediment slides provide an extremely important mechanism for generating the internal chaos characteristic of melange. Because sediment slides are not restricted to convergence zones, the presence of a chaotic unit in the rock record does not imply, of itself, a paleosubduction zone. Other characteristics of the melange must be observed and studied before proposing a paleotectonic site of formation. These characteristics include clast lithology and structural fabric history. The Cambro-Ordovician Dunnage Formation is a melange that crops out in north-central Newfoundland in the Dunnage tectono-stratigraphic zone. The Dunnage melange exhibits soft-sediment deformation features similar to those observed in cores raised from submarine sediment slides. These features are consistent with the interpretation that the chaotic nature of the Dunnage Formation formed initially by sediment sliding. The deformational features include pebbly mudstones with no cleavage and isoclinal folds that are overprinted at high angles by non-axial-planar cleavage. The original site of deposition of the Dunnage Formation is equivocal, but angular clasts in the Dunnage were derived from both an island arc and oceanic crust. This observation suggests that the heads of the slides were very close to subaerial exposures of a combined island arc and ophiolite terrane (perhaps similar to present-day Luzon). By Silurian times, the Dunnage Formation was involved in overthrusting which, considering regional relationships, was probably forearc thrusting.
Ophiolitic olistostromes in the basal Great Valley sequence, Napa County, northern California Coast Ranges
The basal Great Valley sequence in Napa and southern Lake Counties, California, is a mappable chaotic unit composed largely of ophiolitic debris. Serpentinite flows and breccias, mafic breccias and associated finer-grained clastic rocks, and blocks of extrusive greenstone, mafic breccia, chert, bedded and unbedded clastic sedimentary rocks, phyllites, actinolitic greenschists, and hornblende amphibolites are mixed with Great Valley sequence mudstone and serpentinous mudstone. The chaotic unit extends along strike for at least 50 km. Cross-sections indicate that it extends for at least 20 km across strike and is up to 1 km thick. It is involved in complex folds caused by imbricate thrust faulting. The unit lies directly above the serpentinite that represents the Coast Range Ophiolite within the study area and below the well-bedded Great Valley sequence of Upper Jurassic and Cretaceous age. Its lower contact is enigmatic but is probably depositional; the upper contact is sheared and gradational. Locally the unit represents the entire Tithonian Stage. Ophiolitic detritus in the lower Great Valley sequence is also found elsewhere in the Northern California Coast Ranges—near the Geysers, in Rice Valley, near Wilbur Springs, along the Bartlett Springs Road near Walker Ridge, near Cooks Springs, and at Crowfoot Point west of Paskenta. Other accumulations of ophiolitic debris are inter-layered in the Great Valley sequence at various stratigraphic levels in and near the study area. This detritus takes four forms, which may be mixed together: (1) sedimentary serpentinite debris flows; (2) mafic breccias; (3) basaltic sandstones; and (4) polymict, polymorphous chaotic units with blocks-in-matrix texture, like the rocks in Napa County described here. Widespread detrital textures and the common occurrence of spaced, rather than penetrative, shear foliation in its matrix demonstrate that the chaotic unit in Napa County is not a tectonic melange, and I interpret it to be an amalgam of olistostromes. These ophiolitic olistostromes are a facies distinct from the overlying turbidites. Thus, the basal Great Valley sequence in this area is composed of two different rock types: very proximal ophiolitic debris flows, and substantially more distal subsea-fan rocks derived from a volcanic arc. Ophiolites may form at mid-ocean ridges, in back-arc or forearc basins, or in island arcs. Ophiolitic detritus may be eroded and deposited on ophiolitic basement in any environment in which the oceanic crust is deformed. The geology of surrounding terranes and the petrologic features of the ophiolitic basement below the Great Valley sequence suggest that the basement was formed in a back-arc basin. The stratigraphy of the chaotic rocks that overlie the basement in Napa County suggests that they were deposited on deeply eroded basement in a technically active forearc basin. Large volumes of rock stuffed under the hanging-wall slab after the onset of subduction may have uplifted the forearc basin and subjected its basement to erosion. A wave of uplift may have passed across the basin, so that debris shed from eroding oceanic basement was deposited directly on freshly exposed harzburgite tectonite. Some blocks may have been carried completely across the forearc basin and into the trench, and incorporated into the Franciscan melange wedge, which is also rich in ophiolitic blocks. The change from back-arc to forearc basin was probably caused by collisional tectonics and the establishment of a new subduction zone off the western coast of California during the Late Jurassic Nevadan orogeny. Stratigraphic relationships in and above the Coast Range Ophiolite are unusual through much of the Northern Coast Ranges. Nearly complete ophiolites are the exception rather than the rule, and in many areas only serpentinite is present. In some areas, ophiolitic debris different from that described here overlies the serpentinite. In other areas, arc-derived submarine fan rocks of the Great Valley sequence directly overlie serpentinized harzburgite tectonite. The relationships described here suggest that many of these contacts are not tectonic, and that the Coast Range Ophiolite does not owe its fragmentary nature to tectonic dismemberment. Rather, it is likely that the ophiolitic basement below the Great Valley sequence was deeply eroded during Mesozoic time. Many of the contacts throughout the Coast Ranges along which sedimentary rocks overlie serpentinite—which must represent deep layers of the oceanic crust or the upper mantle—are in the main nonconformities.
The Kanar Melange is an ophiolitic melange underlain by Cretaceous shelf-slope argillites and overlain by a thrust sheet of Cretaceous ophiolites. The melange consists of a chaotic mixture of millimeter to kilometer-size clasts embedded in a dominantly argillaceous matrix. The affinities of the clasts are continental (e.g., Mesozoic shelf carbonate rocks, clastic and sedimentary rocks, mafic-ultramafic and alkali-volcanic rock bearing conglomerates) and oceanic (serpentinite, gabbro, and basalt). The matrix is mildly deformed to strongly folded and foliated. The melange was emplaced during the Paleocene in a trough located on the western margin of the Indo-Pakistan subcontinent. A two stage model for the origin of melange is proposed. It involves Paleocene oblique convergence between the western margin of the Indian Plate and the adjoining Neo-Tethys along a Cretaceous transform boundary. This resulted in the formation of a zone of tectonically mixed continental and oceanic rocks (proto-Kanar Melange) bounded by a trough (tectonic foredeep) on the continental margin. Debris from the mixed zone was shed into the trough, leading to the formation of the Kanar Melange. Tectonic mixing of contrasting rock types is considered an important step in the genesis of the Kanar Melange. However, the presence of sedimentary features in the matrix of the melange and its depositional relationship with the underlying Sembar Formation suggest that the emplacement of the melange may have been controlled by sedimentary processes. Oblique convergence culminated in the obduction of ophiolite nappes above the melange and regional-scale folding of the whole area. These events produced a tectonic overprint that obscured primary stratigraphic and structural relationships within the melange belt.