Fission-track thermochronologic data have the potential to reveal aspects of the timing, rate, and style of crustal extension tectonics. Previous fission-track studies in the footwalls of detachment faults in the Basin and Range Province of western North America have found monotonic decreases in apparent fission-track age in the slip direction of the faults (e.g., Fitzgerald and others, 1991; Foster and others, 1991, in press). This pattern has been interpreted as recording the time at which progressively deeper parts of the footwalls to the detachments have moved through the annealing zone for fission-tracks in apatite. In this study we analyzed fission-tracks in apatite and zircon from samples of Proterozoic crystalline rock from a south to north transect across the northern Plomosa Mountains, Arizona (Figure 1). The transect runs from about 100 meters structurally below the Tertiary unconformity (DF90-226) north to the deepest exposed structural levels in the range (DF90-220) beneath the Tertiary Plomosa detachment fault (Scarborough and Meader, 1981). In addition, one sample (DF90-221) is from a detached block of basement in the hanging wall of the Plomosa fault in the northern-most part of the range. The fission-track data from these samples constrain the low temperature (<250 C) thermal history of the northern Plomosa Mountains and have implications for the timing and rate of displacement on the Plomosa detachment. The data also have implications for nature of extension in this range which occupied a position near the breakaway region for detachments in the Whipple tilt domain (Spencer and Reynolds, 1991). ( 11 pages )