ABSTRACT
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
Copies of this PhD dissertation can be ordered
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Front matter (title,
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Photo pages in dissertation
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and greyscale photos with captions): - 7.1MB pdf file
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