Late Cretaceous-early Paleogene extensional ancestry of the Harcuvar and
Buckskin-Rawhide metamorphic core complexes, western Arizona
Abstract
Metamorphic core complexes in the western North American Cordillera are
commonly interpreted as the result of a single phase of large-magnitude
extension during the middle to late Cenozoic. We present evidence that
mylonitic shear zones inthe Harcuvar and Buckskin-Rawhide core complexes
in west-central Arizona also accommodated an earlier phase of extension
during the Late Cretaceous to early Paleocene. Microstructural data
indicate substantial top-NE mylonitization occurred at
amphibolite-facies, and 40Ar/39Ar
thermochronology documents post-tectonic footwall cooling to
<500°C by the Paleocene to mid-Eocene. Amphibolite-facies
mylonites are spatially associated with voluminous and variably deformed
footwall leucogranites that were emplaced from ca. 74-64 Ma, and a late
kinematic ca. 63 Ma dike indicates this phase of mylonitization had
waned by the early Paleogene. Reconstruction of the footwall
architecture indicates that this latest Cretaceous – early Paleocene
deformation occurred within a NE-dipping extensional shear zone. The
leucogranites were likely the result of crustal melting due to orogenic
thickening, implying a model whereby crustal heating triggered
gravitational collapse of overthickened crust. Other tectonic processes,
such as the Laramide underplating of Orocopia Schist or mantle
delamination, may have also contributed to this episode of orogenic
extension. Miocene large-magnitude extension was superimposed on this
older shear zone and had similar kinematics, suggesting that the
location and geometry of Miocene extension was strongly influenced by
tectonic inheritance. We speculate that other Cordilleran core complexes
also experienced a more complex and polyphase extensional history than
previously recognized, but in many cases the evidence may be obscured by
later Miocene overprinting.