THE RATE PROBLEM OF EMPLACEMENT OF THE ALTA STOCK, WASATCH MOUNTAINS

The favored emplacement model for the Alta stock and numerical modeling of its contact-metamorphic aureole imply emplacement durations that differ by 2-3 orders of magnitude. Vogel et al. (1997, 2000) inferred that tectonic dilation was a major space-making mechanism for the mid-Tertiary Wasatch plutons, and particularly for the 1-2 km wide tabular Alta stock owing to its spatial association with numerous dikes oriented parallel to it as well as regional evidence for synextensional intrusion. Local rates of tectonic dilation in continents (e.g., across the Wasatch fault) are of the order of 1 mm/yr, and thus a duration of the order of 1 m.y. would be needed to make the space for the Alta stock by tectonic dilation. Thermal models indicate that a pluton that grows this slowly must be internally sheeted (e.g., Hanson and Glazner, 1995). New magnetic-susceptibility mapping and outcrop studies of the Alta stock suggest that the stock indeed may reflect amalgamation of steep intrusive sheets, and preliminary new U-Pb zircon data suggest an emplacement duration of ~1 m.y. However, the very thick (~200 m) periclase zone in the Alta contact aureole requires rapid and efficient delivery of heat to the wall rocks if the observed pluton was the sole heat source. Cook et al. (1997) simulated the petrologic record of temperature and fluid exchange in the contact aureole by assuming instantaneous intrusion of the stock, followed by horizontally channeled hydrothermal fluid flow. In the best-fitting model, the metamorphic peak in the inner aureole is reached in 5000 years, followed by ~15,000 years of conductive heating to reproduce the tremolite zone. In practical terms, instantaneous intrusion in the model corresponds to pluton emplacement over a much shorter duration than the metamorphism and thus implies emplacement over no more than a few thousand years. If the pluton took 2-3 orders of magnitude more time to grow as suggested by the emplacement model, this would result in a massive shortfall of heat needed to generate the observed contact aureole. Either the Alta stock was emplaced vastly more rapidly than tectonic dilation permits, in spite of the evidence that favors that model, or a much larger unexposed heat source, presumably located beneath the exposed Alta stock, supplied the majority of the heat that produced the contact aureole.