A thermochronologic study of blueschists and related high pressure rocks from a subduction complex in west-central Baja California has provided constraints on the timing of subduction-related metamorphism and timing of subsequent uplift. Subduction-related metamorphism of coherent blueschists occurred in late Early Cretaceous time. One portion of the subduction complex was uplifted from a depth of 25 km to the surface of the Earth at an average rate of 0.1 mm/yr. The relatively slow uplift rate and the lack of any higher temperature overprinting assemblages in the coherent blueschists of the Western Baja terrane suggest that synsubduction uplift was gradual and proceeded through a dynamic accretionary wedge characterized by low geothermal gradients. An increase in uplift rate to 1 mm/yr during post-Miocene time coincides with a change from a convergent to a transform plate boundary.
Ages and mineral assemblages for exotic blocks within serpentinite-matrix melange indicate the blocks have experienced different P-T-t histories. Mid-Jurassic epidote amphibolite facies blocks are likely derived from oceanic crust and associated sediments that were metamorphosed during initiation of subduction. 40Ar/39Ar analyses of white micas from blueschist blocks indicate the blocks experienced subduction-related metamorphism at approximately the same time as the coherent blueschists. However, age spectra for white micas from blueschist blocks show evidence for varying degrees of diffusional loss of 40Ar* suggesting that the blocks remained in portions of the accretionary wedge where temperatures were high enough to cause partial outgassing of the white micas. Mid-Jurassic amphibolite facies blocks from East San Benito Island were partially overprinted by blueschist facies mineral assemblages and represent an intermediate type of block which records both events.
Results of isothermal, hydrothermal experiments on metamorphic hornblendes support a previously reported estimate of the activation energy of 40Ar in hornblende (~60 kcal/mol). However, phyllosilicate intergrowths and exsolution lamellae within metamorphic hornblende result in extremely small diffusion domains which lead to lower Ar retentivities and closure temperatures of 440ºC, assuming a cooling rate of 5ºC/Ma.
Preliminary results of thermal modeling of a subduction complex indicate that the temperature-time history of the accretionary wedge is strongly dependent on the choice of the angle between the subducting plate and the overlying wedge and not affected by low values (0.1 mm/yr) of the advection term.
Baldwin, S.L., 1988. Thermochronology of a subduction complex in
western Baja California. Unpublished PhD dissertation, State
University of New York at Albany. 247pp., +xiii
University at Albany Science Library call number: SCIENCE MIC Film QE 40 Z899 1988 B35
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