IMPLICATIONS FOR THE CRETACEOUS HISTORY OF THE SIERRA NEVADA

Most geochronological methods have a closure temperature (Dodson, 1973). However, in the apatite fission track method for example, there is not one distinct temperature, but a rather diffuse zone over which the geochronological system ``closes''. Nevertheless, for our purposes, the rather crude concept of a closure temperature is still useful. For the U/Pb system in zircon, the closure temperature is as high at $\sim$900$^oC$ (Dahl, 1997; Miller et al., 2003). The closure temperature of the zircon fission track method is somewhere between $\sim$230 and 310$^o$C (Wagner and Van den Haute, 1992; Tagami and Dumitru, 1996). For the apatite fission track method, a closure temperature of $\sim$100$^o$C can be used, although this value varies with apatite chemistry (e.g., Gleadow and Duddy, 1981).

A frequent practice in igneous and metamorphic geochronology is the simultaneous use of several dating techniques on the same sample. A graph of apparent age versus closure temperature is then used to estimate the cooling history of such a sample (e.g., Harrison and McDougall, 1980). A similar approach can be used for detrital samples. Conservatively assuming that the tops of the plutons in the southern Sierra Nevada were emplaced at 2-3km depth (Ague and Brimhall, 1988), Surpless (2001) argued argued that the relatively short minimum lag times between the U/Pb zircon ages and the depositional ages of 3-15Ma indicated rapid exhumation of the Cretaceous Sierra Nevada at rates of $\sim$ 0.6-1mm/yr. We can extend this method to the lower temperature thermochronometers of Table 1.

If we had access to double-dated grains, as in Rahl et al. (2003), estimating the probability distribution of provenance cooling rates would be a trivial exercise. However, because of the uncontroversial provenance of our samples and the fact that the southern Sierra Nevada can be considered a structurally more or less homogeneous fault block, we might be able to proceed without such data. Thus, we can get a first order estimate of the cooling rates by looking at the time lag between the mean (or central) ages of two thermochronological grain-age populations (e.g., ZFT and AFT), or at that between the mean (or central) age of a grain-age population and the depositional age of the sample.

Doing this for the thermally unreset data of Table 1 yields seven apparent cooling rates which do not show any systematic variation with depositional age. The weighted mean of these estimates yields an apparent cooling rate of $\sim$21$^o$C/Ma. Depending on the thermal gradient (22-40$^o$C/km; Rothstein and Manning, 2003), this then corresponds to exhumation rates of $\sim$0.5-1mm/yr, which agrees with the estimates of Surpless (2001) and Ague and Brimhall (1988). These are rather high rates, but this does not come as a surprise when we consider the amount of sediment deposited in the Great Valley Group at the time.