Abstract

The Marble Mountain terrane of northern California constitutes a metamorphosed ophiolitic tectonic mélange occupying the lowest structural levels of the western Paleozoic and Triassic belt of the Klamath Mountains province. On the basis of lithology and structural position, it probably represents the metamorphosed equivalent of the Rattlesnake Creek terrane. The Marble Mountain terrane consists of sedimentary, oceanic mafic volcanic, and ultramafic rocks and is structurally and probably stratigraphically overlain by arc-derived meta-volcanic and volcaniclastic rocks of the western Hayfork terrane. The contact between the two terranes is interpreted to be a zone of tectonic and stratigraphic interlayering, but superimposed metamorphism has largely obscured its original character.

Metamorphism of the assembled mélange and overlying western Hayfork terrane took place under low-pressure amphibolite facies conditions, a conclusion based primarily on the presence of prograde andalusite in pelitic rocks. Metamorphism was accompanied by penetrative deformation, which produced two generations of northeast-trending, subhorizontal isoclinal recumbent folds and associated axial-planar foliation; mineral and stretching lineations also formed at this time. These events preceded the intrusion of the 162 ± 2-Ma Wooley Creek batholith. A later episode of deformation and recrystallization locally produced north-northeast–trending upright folds and steeply dipping crenulation cleavage. Greenschist to lower amphibolite facies metamarphic assemblages that were developed at this time overprint the earlier structures. The latter deformational event may reflect the westward thrusting of the western Paleozoic and Triassic belt over the subjacent western Jurassic belt during Late Jurassic time.

The Marble Mountain/Rattlesnake Creek terranes probably represent oceanic crust which lay to the west of an active subduction zone along the western margin of North America during Late Triassic and Early Jurassic time and which was trapped in the forearc region when subduction stepped westward in earliest Middle Jurassic time. Newly initiated subduction led to the inception of the western Hayfork arc, which was built upon the Marble Mountain/Rattlesnake Creek oceanic basement. The mélange may have been formed by imbrication of the oceanic lithosphere trapped between the new trench and the magmatic axis of the incipient arc. Continued convergence led to juxtaposition of oceanic and arc-derived rocks, to regional metamorphism, and to deformation of the mélange and overlying volcaniclastic rocks. The low-pressure character of the metamorphism supports an arc setting rather than a subduction-zone regime for this tectonism.

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