New apatite and zircon fission track ages obtained from the footwall of the Catalina metamorphic core complex record a complicated cooling history associated with mid-Tertiary extension. Zircon fission track ages record the progressive unroofing of the Catalina metamorphic core complex along the Catalina detachment fault. Zircon fission track ages range from 31.9 to 19.4 Ma, generally decrease in the hanging-wall slip direction, and yield slip rates along the Catalina detachment fault ranging from 1.2 to 12 km Myr-1. In contrast, apatite fission track ages increase in the hanging wall slip direction. Samples from the main range of the Santa Catalina Mountains yield apatite fission track ages of 20.5 to 14.6 Ma; samples from the Santa Catalina Mountain forerange, located closer to the detachment fault, yield apatite fission track ages of 21.4 to 18.8 Ma. Rapid cooling (40¿ to 60 ¿C Myr-1) related to detachment faulting is best recorded by zircon fission track ages and higher-temperature thermochronometers in the main range and by nearly concordant zircon and apatite fission track ages in the forerange. Slower cooling (3¿--7 ¿C Myr-1) of the footwall is recorded by shortened mean confined fission track lengths (<14 μm) and is related to erosional unroofing. Approximately 2 km of late-Tertiary erosion played a significant role in the unroofing of the footwall of the Catalina metamorphic core complex in contrast to metamorphic core complexes in western Arizona, where detachment faulting is the dominant unroofing mechanism. ¿ 2000 American Geophysical Union