EVIDENCE FOR TECTONIC INHERITANCE IN THE DEVELOPMENT OF THE HARCUVAR METAMORPHIC CORE COMPLEX, WEST ARIZONA

Cordilleran metamorphic core complexes are interpreted as unique modes of large-magnitude extension that result in the exhumation of mylonitic mid-crustal rocks along a currently low-angle normal (detachment) fault. The processes by which core complexes form remain controversial, including the initial dip of the detachment fault, the magnitude and rate of slip, and the role of mid-crustal flow. Although core complexes in the central and southern Basin and Range are largely interpreted as the product of Oligo-Miocene extension, these regions have a complex pre-Miocene history and the importance of tectonic inheritance in shaping Miocene extension has often been overlooked. We present evidence from the Harcuvar core complex in west Arizona that highlights the importance of tectonic inheritance in its evolution.

U-Pb titanite analyses from metamorphic footwall units yield maximum ages of ca. 145 Ma and minimum ages of 75-60 Ma, while U-Pb monazite analyses yield 72-74 Ma ages. These results suggest a significant phase of Late Cretaceous metamorphism that overlapped with the intrusion of the Tank Pass leucogranite and was likely the result of significant crustal thickening and partial melting. Southwest-vergent thrust faults and NE-SW trending folds in the footwall are manifestations of this crustal thickening event. Immediately following crustal thickening, an amphibolite-facies shear zone developed that accommodated top-NE extension, likely related to gravitational collapse of overthickened crust. This early extensional event was largely complete by ca. 63 Ma.

Miocene extension was superimposed on these earlier tectonic events and was likely influenced by this older history. Miocene mylonitization mainly occurred at greenschist-facies and was largely restricted to metasedimentary footwall units. The geometry of Miocene mylonitic fabrics parallel those of the latest Cretaceous extensional shear zone, suggesting the geometry of Miocene extension was strongly influenced by this pre-existing shear zone. Corrugations in the Miocene detachment fault parallel the NE extension direction and have been interpreted to be the result of Miocene processes. However, their geometry is similar to that of Late Cretaceous folds, suggesting that this distinctive fault geometry might also be inherited from older tectonic events.