Thermobarometry for Cretaceous to mid-Tertiary plutonism and deformation in the lower plate of Whipple and Santa Catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust. Moreover, they are characterized by a striking acceleration of tectonic decompression coincident with middle Tertiary, low-angle detachment faulting leading to erosional and tectonic unroofing by mid-Miocene time.
Depth estimates for emplacement of five intrusive suites within the Whipple complex include (1) 33 ±4 km for the peraluminous, 89 Ma Whipple Wash plutonic suite; (2) 29 ±1 km for the 73 Ma Axtel quartz diorite; (3) 16 ±5 km for mylonitization and synkinematic plutonism at 26 Ma; (4) 6.2 ±1.9 km for the postkinematic, 19 Ma War Eagle gabbro-quartz diorite complex; and (5) 5.2 ±2.3 km for a 17 Ma postkinematic granodiorite. Decompression initially occurred at a low rate of 0.3 mm/yr from 89 to 26 Ma and increased to approximately 2 mm/yr during the late Oligocene to middle Miocene. Estimated depths for four pluton emplacement or deformational events in the Santa Catalina Mountains include (1) 21 ±1 km for the magmatic epidote-bearing, 68 Ma Leatherwood quartz diorite; (2) 15 ±3 km for the garnet, two-mica, 47 Ma Wilderness granite; (3) 9.3 ±1.9 km for post-Wilderness mylonitzation; and (4) 6.3 ±2.6 km for the 27 Ma Catalina monzogranite. Post-Laramide decompression, estimated at 0.3 mm/yr, accelerated to 1.3 mm/yr prior to the cessation of detachment faulting.
Whereas most batholithic terranes of the North American Cordillera are representative of an upper crustal setting, core complexes provide, as a consequence of their tectonic evolution, a petrological and structural view into middle crustal processes.