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ABSTRACT: Mesozoic Tectonics and Paleogeography of the Western U.S. and the Adjacent Pacific Basin

Yildirim Dilek

Recent geological, geochemical, and geochronological information from Jurassic and older ophiolite complexes and arc rocks in northern California provides new interpretations for Mesozoic tectonics of the western U.S. and the adjacent Pacific basin. Tectonostratigraphic units in the northern Sierra Nevada metamorphic belt, California, constitute parts of two nearly coeval but stratigraphically and structurally distinct arc terranes. The continental margin arc to the east is composed of successive sequences of Paleozoic and Jurassic arc complexes built on continentally derived lower Paleozoic rocks of the Shoo Fly complex. This continental margin arc is bounded on the west by an early Mesozoic subduction complex (Feather River belt) and a late Paleozoic-early Mesozoic accr tionary prism (Calaveras complex). West of the inferred accretionary prism are discontinuous exposures of multiply deformed and metamorphosed, late Paleozoic (?)-early Mesozoic ophiolite complexes, which are locally stratigraphically overlain by a disrupted marine sequence, and fragments of a Jurassic island-arc sequence. The arc fragments, including the Slate Creek, Lake Combie, and Smartville complexes, contain Lower Jurassic plutonic and volcanic rocks and tectonically and/or stratigraphically overlie older ophiolitic and marine rocks. Arc related rocks are coeval with boninitic, high-magnesiun basaltic andesite dikes that intrude ophiolitic and marine rocks throughout the metamorphic belt. These relations thus suggest that the arc fragments may be part of an ensimatic, Early Jurassic arc terrane that was built on and across the preexisting oceanic basement. The ensimatic arc terrane and its inferred oceanic basement were coalesced with the subduction-accretion complexes and the continental margin arc to the east during or prior to Middle Jurassic time as suggested by cross-cutting relations. This amalgamation event corresponds to a major episode of contractional deformation that probably affected the entire western U.S. Cordillera. The ensimatic arc terrane is in turn intruded at all stratigraphic levels by Upper Jurassic dikes and zoned plutons. Dikes, which have calc-alkaline affinity, locally form an extensive sheeted dike complex and are associated with widespread mineralization and epidotization both in the country rock and the dike complex. Development of the d ke complex and associated plutonism are interpreted to represent an intra-arc/back-arc rifting event within the ensimatic arc terrane in Late Jurassic time that is synchronous with generation of the Coast Range ophiolite occurring beneath (?) and west of the Great Valley sequence.

The outlined tectonic history is reminiscent of a paleogeography similar to that of the Melanesian region in the southwest Pacific Ocean, where island-arc systems, bounded by subduction zones and oceanic transform faults, and marginal basins are fringing the continental Australia, Papua New Guinea, and the surrounding shallow seas. The northward motion of the Australia-India plate caused by rifting along the Southeast Indian Ridge creates a compressive regime at the leading northern edge of the continental mass composed of Australia and Papua New Guinea and is likely to cause in the near geologic future lateral extrusion and translation as well as southward transportation and emplacement of island-arc systems, preexisting oceanic crust, and seamounts onto the continental margin. This cenario is similar to the one envisioned to have occurred along the western periphery of west-northwest moving North America during the opening stages of the Atlantic Ocean in Jurassic time. The fringing arc system composed of ensimatic arc terrane(s), small marginal basins floored with oceanic crust, fractures zones, and seamounts was first accreted into the outer continental margin of western North America and then dispersed along margin-parallel crustal shear zones in Jurassic to Early Cretaceous time.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990