Thermal evolution of the upper crust during large-scale crustal extension

Metamorphic core complexes evolve by rapid tectonic denudation along low-angle normal faults in response to crustal extension. While tectonic denudation may exhume mid-crustal rocks to shallow crustal levels, this process alone does not account for the fission-track ages and the topographic relief ({dollar}sim{dollar}2.0 km) observed in the Catalina metamorphic core complex, southeast Arizona. Fission-track analysis was combined with time-dependent thermal modeling to gain insight into crustal extension and the processes associated with the unroofing of the Catalina metamorphic core complex.; The thermal history of the hanging wall of detachment faults provides information regarding the amount of crustal thinning, the rate of displacement, and the timing of extension. Apatite fission-track ages of hanging-wall samples reflect transient heating that occurs during denudation of the footwall. Apatite fission-track data obtained from two hanging wall localities of the Catalina detachment fault yield reset fission-track ages of 15 Ma to 30 Ma. Mean confined track lengths for these samples are {dollar}<{dollar}14 {dollar}mu{dollar}m and have broad distributions, characteristic of slowly cooled samples. A series of numerical heat-transfer experiments were conducted using constraints based on field and thermochronologic studies of the footwall of the Catalina metamorphic core complex. Temperature-time paths determined from these simulations were used as input into an apatite fission-track annealing model. A comparison of observed vs. predicted apatite fission-track ages and track lengths from the hanging wall constrains the total amount of displacement to {dollar}geq{dollar}25 km and the displacement rate to {dollar}geq{dollar}5 km/m.y. The young apatite ages and shortened confined track lengths further suggest that these rocks were unroofed by {dollar}sim{dollar}2 to 4 km of post-tectonic erosion.; Fission-track thermochronology of the footwall also indicates rapid denudation followed by a period of slow cooling. Zircon fission-track ages from the footwall, as exposed in the Santa Catalina Mountains, decrease in the hanging-wall slip direction from 31.9 Ma in the main range to 22.3 Ma in the forerange. In contrast, apatite fission-track ages obtained from the same samples increase in the hanging-wall slip direction from 15.8 Ma in the main range to 20.2 Ma in the forerange. Mean confined track lengths from apatite samples range from 13.8 {dollar}mu{dollar}m to 12.6 {dollar}mu{dollar}m. These data suggest that the main range footwall cooled through the blocking temperature for fission tracks in zircon (240 {dollar}pm{dollar} 50{dollar}spcirc{dollar}C) at {dollar}sim{dollar}32 Ma, but remained in the partial annealing zone for fission tracks in apatite (110{dollar}spcirc{dollar}-60{dollar}spcirc{dollar}C) until {dollar}sim{dollar}16 Ma. The young apatite ages and shortened track lengths ({dollar}<{dollar}14 {dollar}mu{dollar}m) indicate a period of slow cooling and erosion followed mid-Tertiary detachment faulting.