Thermochronometric Constraints on the Timing and Geometry of Extensional Faulting in the Whipple and Iron Mountains, Colorado River Extensional Corridor

The metamorphic core complexes in the Colorado River extensional corridor of the Basin and Range province represent some of the world`s premier examples of large-magnitude crustal extension and have inspired many of the modern concepts of metamorphic core complex formation. Tertiary exhumation of middle-crustal rocks in the Whipple Moutains core complex and its proposed breakway zone in the Iron Mountains present an ideal opportunity to better understand the timing, rates, and structural processes of footwall exhumation using low-T thermochronometry. For this study, detailed apatite and zircon (U-Th)/He data were collected along a swath parallel to the regional extensional direction from the Iron Moutains to the NE-most extent of the Whipple Moutains. The zircon (U-Th)/He ages (ZHe) range from ~14-47 Ma in the Whipple Mountains and from ~20-46 Ma in the Iron Mountains. The apatite (U-Th)/He ages (AHe) progress from ~14-27 Ma and from ~19-45 Ma in the Whipple and Iron Mountains, respectively. In the Whipple Moutains ZHe data exhibit a marked inflection point, constraining the onset of rapid exhumation at ~22 Ma. Both AHe and ZHe ages below the inflection point smoothly decrease in down-dip direction, yielding apparent time-integrated fault slip rates of 1.4 (+0.6/-0.3) km/Myr (ZHe) and 3.1 (+1.4/-0.7) km/Myr (AHe), respectively. A detailed look reveals that AHe and ZHe ages define structurally-repeated age-distance arrays best interpreted as progressive transfer of footwall slivers to the hanging wall during exhumation. The data illustrate the structural complexity of the Whipple detachment and the presence of footwall incisement slices. New AHe and ZHe ages from the Iron Moutains show that the timing of dominant exhumation (21-24 Ma) is similar to the onset of rapid exhumation of the Whipple detachment footwall. The Iron Mountains experienced early Miocene exhumation due to flexural rotation, resulting in westward tilting, likely recording the earliest history of extension and thus the onset of fault slip along the Whipple detachment fault system in its breakaway zone.