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          The Southern Cordilleran System of North America

          Geological Time Chart, Paleozoic, GSC-1999

    The Cordilleran System forms a c. 500 km wide 'collage' of oceanic, arc and continental margin tectono-stratigraphic terranes accreted during the Phanerozoic to  the western margin of continental North America. Each terrane is a fault bounded entity of regional extent characterized by an internally homogeneous stratigraphy and geologic  history that is different from that of contiguous terranes.
       Terranes that are potentially far  traveled from their sites of origin are called 'suspect' terranes, whereas those that are clearly related to the continental margin against which they presently sit are called  'native' terranes.

Suspect Terranes of the Cordilleran Orogenic System
Simplified terrane map of the Cordilleran system.

    The welding of terranes to a continental mass is known as  'accretion', whereas the assembling of terranes to form composite units prior to accretion is known as 'amalgamation'. These processes take place along continental  margins without the intervention of continent - continent collision. Limits on the timing of amalgamation or accretion may be provided by the age  of plutonic bodies that cut the suture boundary between the terranes ('stitching  plutons'), the age of overlapping strata present in both terranes, or the age of shared  metamorphic-deformation events. Sedimentary basins that cross terrane boundaries are  commonly called 'successor basins'.

        The Southern Cordillera of California and the Klamath Mountains
        Southern California is composed of three tectonic elements:
       a) the Franciscan blueschist melange + ophiolite;
       b) the Great Valley sedimentary sequence, proximal and distal (Toro Formation);
       c) The Sierra Nevada batholith.

        Generalized geological map of Southern California
        Section illustrating the geology of southern California during the Jurrassic
        Geology of the Franciscan at San Luis Obispo, Page (1972)
        Tectonic models for the development of the Franciscan, Constenius et al., (2000, Fig 1)

          These three tectonic elements represent a Jurassic - Cretaceous arc system in which the Sierra Nevada and Great Valley represent an arc/fore-arc basin couple, and the  Franciscan a subduction zone melange of oceanic crust and regurgitated blueschist  material brought back to the surface from depths of about 30 km. Terrane boundaries  dip eastwards such that accretion was promoted by underthrusting of the Franciscan  beneath the Great Valley sequence.
        The accretionary history of the Klamath region of northern California and Oregon extends back to early Paleozoic  time, and involves four phases of accretion, all associated with easterly underthrusting of  arc systems beneath the continental margin. The most easterly terrane, the Eastern Klamaths, is composed of  Late -Silurian - Early Devonian  (431-398 Ma, zircon)oceanic crust and arc material, structurally underlain  by high grade amphibolitic rocks representing a dynamothermally metamorphosed sheet  of subducted oceanic crust. Lower Devonian volcanic rocks (Redding Fm) overlying the Trinity ophiolite were coeval with mafic complexes associated with the ophiolite. The ophiolite was formed above an easterly dipping subduction zone. Siluro-Devonian volcaniclastic, melange and quartzofeldspathic metasedimentary rocks of the Yreka terran structurally overlying the Trinity complex. The sediments have sources both in arc and continental material. The Trinity Eureka rocks are correlated with the Shoofly complex of the Sierra Nevada. Maturation of the arc was completed by the Late Devonian, and predates the Early Mississippian Antler orogeny.
        To the west the Ordovician rocks are underthrust by a unit of  blueschist-bearing rocks (Stuart Fork Formation) metamorphosed and accreted during the Triassic. Further west  again, the Western Paleozoic and Triassic belt is composed largely of arc and melange  units containing Permian age Tethyan fusulinid fossils. Accretion of these terranes to North America was complete by Mid-Jurassic time, prior to the intrusion of the early mid-Jurassic (c. 174 Ma) 'stitching' plutons intrusive into all the terranes.

Generalized tectonostratigraphic map of the Southern Cordillera, Ingersoll, (1986)
Map of the Klamath Mountains region, Irwin, (1972) - map1
Map of the North Fork, Hayfork and Rattlesnake Creek terranes, Irwin (1972) - map2
Genralized map of the Klamath Mountains region, Snoke (1977)
Age and Fauna types within the Paleozoic Triassic terranes of the Klamath Mountains, Gray (1986)
Recent Geological Time Chart for the Paleozoic, GSC 1999
Correlation chart of terranes in the Klamath Mountains, Gray (1986)
Photo of melange in the North Fork Terrane, Klamath Mountains, Irwin & Jones (1977, Fig. 2)
Plate tectonic history of the Klamath Mountains, Ernst (1999)

        The youngest accreted terrane  is known as the Western Jurassic belt. It is composed of younger Jurassic oceanic rocks (Josephine,  Rogue-Galice), accreted during the latest Jurassic, and is roughly equivalent to the Coast Range  ophiolites and arc rocks of California, and the Smartville ophiolite and  associated arc of the Western margin of the Sierra Nevada.

Model for the Rogue-Galice, Snoke (1977)
    Snoke (1977) represents the Rogue-Galice sediments as having been deposited in a back-arc basin. However they may have been rather deposited in a fore-arc basin above a westerly dipping subdcution zone.
Large scale map and comparative Late Jurassic tectonic history of the Coast Range, Sierra Nevada, and Klamath Mountains, Ingersoll (1986)
Tectonostratigraphic Map of the Southern Cordillera, Ingersoll, (1986)
Paleogeographic evolution of the West Coast of the USA during the Late Jurassic, Ingersoll (1986)

    Most recent publication:
    Soreghan, M.J. and Gehrels, G.E., 2000. Paleozoic and Triassic Paleogeography and Tectonics of Western Nevada and Northern California. GEological Society of America SPE347, 256 p.
    Metcalf, R.V. and Wallin, E.T., 2000, Mid-Paleozoic development of the Klamath-Sierra Forearc-arc-backarc subduction system, Northern California. GSA Abstr., Reno, p. A-497.
        In the case of the Klamaths, Southern California and the Sierra Nevada, it is  clear that the mass of continental North America has been increased since early  Phanerozoic times by the simple sequential accretionary underthrusting of ocean crust  and related arc systems.


Structural Provinces of North America.

Simplified terrane map of the Cordilleran system.


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