John S. Singleton

Miocene slip history of the Eagle Eye detachment fault, Harquahala Mountains metamorphic core complex, west-central Arizona

The structural and thermal evolution of major low-angle normal faults in the Colorado River extensional corridor has been a controversial topic since the pioneering studies of metamorphic core complexes in the early 1980s. We present new geo-thermochronometry data from the Harquahala Mountains in west-central Arizona to determine the timing of extension, displacement magnitude, and slip rates along the Eagle Eye detachment fault (EED) during large-magnitude Miocene extension. Zircon and apatite (U-Th)/He data (ZHe and AHe, respectively) from 31 samples along a ~55u2009km extension-parallel transect indicate active slip along the EED occurred between ~21u2009±u20091u2009Ma until ~14u2009Ma. The spatial extent of ZHe ages and exhumation of the zircon partial retention zone indicated ~44u2009±u20092u2009km of total displacement, whereas lithologic similarity and identical U-Pb ages between correlated footwall rocks in the Little Harquahala Mountains and breccia clasts at Bullard Peak in the NE Harcuvar Mountains indicated ~43-45u2009km of displacement across the EED. AHe and ZHe data indicated slip rates of ~6.7u2009+u20097.8/-2.3u2009km/Myr, and ~6.6u2009+u20097.1/-2.0u2009km/Myr, respectively, both consistent with the duration and displacement estimates. The EED initiated as a listric fault with an ~34u2009±u20099° dip that decreased to ~13u2009±u20095° below ~7u2009km depth. Secondary breakaway development and footwall exposure occurred by ~17u2009Ma, during active EED slip. Lithologic and geo-thermochronometric offset constraints show excellent agreement and provided a rare opportunity to fully resolve the timing, rates, and total displacement magnitudes along a major continental detachment fault.

The role of calcite-rich metasedimentary mylonites in localizing detachment fault strain and influencing the structural evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Although crystalline rocks dominate the footwall of the Buckskin-Rawhide detachment fault in west-central Arizona (USA), we estimate that thin (<1 to 100 m thick) calcite-rich metasedimentary mylonites were present along 25%–35% of the detachment fault, and in parts of the footwall they were continuous for ~30 km in the slip direction. New field observations, geochronology, and detailed microstructural data provide insight into the origin of these metasedimentary rocks and their role in the structural evolution of the detachment fault system. We propose that calc-mylonites along the Ives Peak footwall corrugation were derived from locally overturned Pennsylvanian–Permian strata that were buried to mid-crustal depths beneath a southeast-vergent Cretaceous thrust fault, which was reactivated in the Miocene by the parallel Buckskin detachment fault shear zone. In some areas these laterally persistent calc-mylonites were smeared out along the detachment fault during incisement into the crystalline footwall, forming a thin carapace of rheologically weak rocks structurally below the original weak zone. Metasedimentary mylonites consistently record top-to-the-northeast simple shear parallel to the detachment fault slip direction. Strain, synmylonitic veins, and paleostress recorded in these mylonites increase toward the detachment fault. Marble mylonites <1 m below the detachment fault preserve strong calcite crystallographic preferred orientations and lack cataclastic deformation that characterizes quartz-rich rocks along the detachment fault. In addition, unlike quartzofeldspathic mylonites, calc-mylonites typically lack extension via postmylonitic normal faults and associated horizontal axis rotation. Paleopiezometry and rheological modeling of the metasedimentary mylonites suggest that when quartzite layers were being sheared at ~100 MPa and 10−13 to 10−14 s−1 near the brittle-plastic transition, marble layers could have been strained ~100× faster at ~20 MPa. Detachment fault strain localized within the metasedimentary rocks, and the calcite marbles exerted significant control on the rheology of the footwall shear zone. This study highlights the important role that inherited weak zones may play in influencing the location, geometry, rheology, and style of deformation associated with detachment fault systems. LITHOSPHERE; v. 10; no. 2; p. 172–193; GSA Data Repository Item 2018080 | Published online 30 January 2018 https://doi.org/10.1130/L699.1