TY - JOUR
T1 - Tectonic underplating of trench sediments beneath magmatic arcs
T2 - The central California example
AU - Ducea, Mihai N.
AU - Kidder, Steven
AU - Chesley, John T.
AU - Saleeby, Jason B.
N1 - Funding Information:
Field work in the Coast Ranges was possible only by the generosity of the Dorrances, Violinis, P. Kephart at the Rana Creek Ranch, John Smiley at the Big Creek Reserve, and M. Stromberg at the University of California Hastings Reserve. This work was supported by NSF EAR 0229470. The manuscript benefited greatly from constructive criticism from Dave Scholl, Carl Jacobson, and Art Snoke.
PY - 2009/1
Y1 - 2009/1
N2 - We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting.
AB - We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting.
KW - California
KW - Subduction
KW - Tectonics
KW - Underplating
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U2 - 10.1080/00206810802602767
DO - 10.1080/00206810802602767
M3 - Article
AN - SCOPUS:67650677101
SN - 0020-6814
VL - 51
SP - 1
EP - 26
JO - International Geology Review
JF - International Geology Review
IS - 1
ER -