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Petrotectonics, climate, crustal thickness, and evolution of geologically young orogenic belts

By
W.G. Ernst
W.G. Ernst
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA
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Published:
January 01, 2007

New and recycled intermediate to felsic crust in island arcs and continental margins is generated by magmatism surmounting convergent plate junctions. Paired Pacific-type orogens develop adjacent sites of long-lived subduction of oceanic lithosphere. They consist of (1) an outboard accretionary prism deposited in and continentward from the oceanic trench, the filling of which is derived mainly from (2) an inboard calc-alkaline volcanic-plutonic arc. The trench assemblage includes arc-sourced clastic mélanges, minor but widespread deepwater cherts and carbonates, and tectonically disaggregated ophiolites. These relatively incompetent sections recrystallize under high-pressure (HP) conditions and decouple from the descending oceanic plate at depths of ∼15–50 km. A massive, slightly older to coeval andesitic-granodioritic arc dominates the landward belt where new sialic crust is added from I-type magmas, and preexisting continental materials are recycled as S-type melts; high temperature (HT) characterizes local-regional metamorphism of the wall rocks. In contrast, Alpine-type orogens result from the underflow of an oceanic plate that transports island arcs, microcontinents, and/or continental salients into the subduction zone. Reflecting lithospheric coherence, the sialic crust may be carried down as much as 90–140 km, becomes thermally softened, and decouples from the sinking plate. Metamorphism of deeply subducted parts of Alpine belts ranges from high pressure to ultrahigh pressure (UHP), and is not paired with a HT calc-alkaline arc. In both types of subduction complex, outboard thrust faults dip landward and fold vergence is seaward, reflecting the polarity of underflow (and rollback) of the oceanic lithosphere. Antithetic thrusting typifies some contractional continental realms. Propelled by buoyancy, allochthonous nappes conduct relatively low-density HP and UHP sections to midcrustal levels. At convergent syntaxes (plate-margin cusps) of overthickened arcs, tectonic aneurysms may produce domical uplifts, further exhuming UHP units surfaceward.

The regional crustal thickness of an active mountain belt partly reflects climate as well as the main convergent-transform plate processes producing the orogen. The volcanogenic Chilean Cordillera parallels the seaward convergent plate junction and the eastward-sinking Nazca plate. The highest ranges and thickest continental crust (∼70 km) occur in the compressed north-central Andes at ∼25°S, a region of extremely low rain- and snowfall. Aridity and low erosion rates help account for the high-standing calc-alkaline volcanic-plutonic contractional arc and the elevated, internally drained Altiplano downwind. Thus erosional degradation is weakly developed. A very different climate typifies the orogen at ∼45°S, where abundant moisture-laden westerlies bring abundant precipitation to the Chilean margin. The rugged mountain belt is of lower elevation compared with the north-central Andes and is supported by a crust of only moderate thickness (35–40 km). Vigorous erosion supplies voluminous detritus to the offshore Chile-Peru Trench as well as eastward, so a plateau is lacking in the lee of the arc. The Himalayas and Tibetan Plateau, the Sierra Nevada and Colorado Plateau, the Japanese island arc, and New Zealand exhibit somewhat similar relationships between crustal thickness and precipitation-linked erosion.

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GSA Memoirs

4-D Framework of Continental Crust

Robert D. Hatcher, Jr.
Robert D. Hatcher, Jr.
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Marvin P. Carlson
Marvin P. Carlson
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John H. McBride
John H. McBride
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José R. Martínez Catalán
José R. Martínez Catalán
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Geological Society of America
Volume
200
ISBN print:
9780813712000
Publication date:
January 01, 2007

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