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Abstract: An Isotopic Provenance Analysis of the Methow Basin, North-central Washington

DEGRAAFF, KATHLEEN A., Stanford University, Department of Geological and Environmental Sciences, Stanford, CA

The Baja British Columbia controversy arises from tectonically incompatible interpretations of paleomagnetic and geologic data. At issue is the mid-Cretaceous location of the Insular and Intermontane superterranes, collectively Baja British Columbia (BBC).

Paleomagnetic data from BBC indicate a lower magnetic inclination than would be expected if these rocks were magnetized at their current latitude. The shallow inclinations require magnetization at approximately the latitude of present-day Baja California. Accretion of BBC to North America at this low latitude requires later northward displacement of as much as 3000 kilometers. However, geologists cite stratigraphic correlations and offset geologic markers as limiting superterrane transport to less than 1000 kilometers.

Situated between the Insular and Intermontane superterranes, the Methow basin is ideally located to resolve this controversy. The mid-Cretaceous strata of the basin could have potentially two very different sources for its continentally-derived sediment: the old, evolved rocks adjacent to the basin in its present location, or the younger, less evolved rocks of southern California and Mexico. Among the approaches to distinguish between these source terranes are U/Pb dating of detrital zircon and trace element geochemistry of detrital minerals. This study will integrate isotopic provenance techniques with traditional basin analysis to develop a characterization of the source of Cretaceous sediments and produce a model for the chronostratigraphic evolution of successor basins that could be applied to similar basins throughout the Cordillera.

AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid