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Extension in the Basin and Range Province and East Pacific Margin |
Department of Geological Sciences, Hoffman Laboratory, Harvard University, Cambridge, MA 02138, USA
We investigate the possibility that the onset and development of Cenozoic extension in western North America was governed by the potential energy contrast within, and mechanical properties of, lithosphere that was previously thickened during the Sevier and Laramide Orogenies. The strength of the lithosphere can be defined by its vertically averaged properties; to a first approximation, this strength is too great for geologically significant extension to occur unless the Moho temperature exceeds about 700°C (±100°). This means that there may be a delay between the end of compression and the beginning of extension, the length of which depends on the pre-thickening thermal structure and the compressional strain. Delays of up to 100 My may occur for the lowest initial Moho temperatures investigated (<450°C), or extension may follow immediately on release of compression if the initial Moho temperature exceeds about 700°C. The total extensional strain that is achieved depends on the potential-energy contrast between the thickened lithosphere and its surroundings. Partial melting of peridotite to produce basaltic magma is possible after small degrees of extension, but depends strongly on details of the initial temperature condition in the lower part of the lithosphere.
The results of these calculations agree with observations of the Cenozoic extensional history of North America: late-Mesozoic/early-Tertiary compression in the Pacific Northwest was accompanied by extensive calc-alkaline magmatic activity and was followed almost immediately by extension; in the northern and southern Great Basin—which show respectively, little and no evidence of syn-compressional igneous activity—the gap between compression and extension was 20–40 Ma (N) to about 70 Ma (S).
* Present address: Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
Present address: Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK.
Present address: United States Geological Survey, 245 Middlefield Road, Menlo Park, CA 94025, USA
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