Fission track analysis of Wind River Range basement rocks provides information about the timing, magnitude, and cooling that occurred in response to Laramide uplift. Eighty samples were collected from Precambrian rock along six traverses in the northern, central, and southern parts of the range over 5 km of elevation. Apatite fission track ages and track length data suggest that the range did not have a uniform thermal history during late Mesozoic and Cenozoic time. Zircon fission track ages are Precambrian, implying that these rocks have not been at temperatures higher than ~ 200øC (9-10 km burial depth) since Precambrian time. Apatite ages from the northeastern and southwestern flanks of the range suggesst that cooling may have initiated in the Wind River Range by about 85 Ma but was definitely occurring by 75 Ma. Data from the Green River Lakes area and Fremont Peak in the northern part of the range indicate that cooling was occurring by 62 Ma and was most rapid between 60 and 57 Ma. Apparent rates of exhumation vary from 94 m/m.y. to 246 m/m.y. in this part of the range. The central part of the range records this same period of rapid cooling, but structural and topographic relief are not as great. Ages from the southern part of the range suggest that uplift and erosion may have been initiated earlier there but continued at much slower rates (34 m/m.y.) over a longer period of time than in the north. Samples from a drill hole in the central Wind River Range have apatite ages ranging from 37 Ma to 86 Ma and suggest a significant cooling event at approximately 42 Ma involving nearly 2 km of the rock column. Timing, magnitudes, and apparent rates of exhumation have varied spatially in the Wind River Range. INTRODUCTION The Wind River Range is a basement-cored uplift in central western Wyoming, exposing Precambrian crystalline rocks over an area of about 6000 km 2. The purpose of this study is to determine the low-temperature thermal history of the Wind 1Now at Arco Exploration and Production Technology, Plano, Texas. River Range using fission track dating of apatite and zircon, and fission track length data from apatite. The data set was derived from surface samples of basement rocks of the Wind River Range and from core and cuttings from the Air Force "A" drill hole. Isotopic systems of low closure temperature, such as fission track dating of apatite and zircon, are sensitive to cooling events because they record the thermal history at shallow crustal depths. Where topographic relief is sufficient, the fission track ages will vary with elevation. If it is possible to collect a suite of samples over a range of elevations, an apparent rate of exhumation (cooling) may be determined [Wagner et al., 1977; Parrish, 1983; Gleadow and Fitzgerald, 1987]. Only a few fission track uplift/cooling studies have been completed in the ranges of the Cordilleran Foreland. These include the Beartooth Range [Giegengack et al., 1986; Omar et a1.,1989], the Front Range of Colorado [Bryant and Naeser, 1980; Lindsey et al., 1986], and ...
