LATE CENOZOIC EXHUMATION OF THE CASCADIA ACCRETIONARY WEDGE IN THE OLYMPIC MOUNTAINS, NORTHWEST WASHINGTON-STATE

The apatite fission-track method is used to determine the exhumation h istory of the Olympic subduction complex, an uplifted part of the mode rn Cascadia accretionary wedge. Fission-track ages are reported for 35 sandstones from the Olympic subduction complex, and 7 sandstones and 1 diabase from the Coast Range terrane, which structurally overlies th e Olympic subduction complex. Most sandstone samples give discordant r esults, which means that the variance in grains ages is much greater t han would be expected for radioactive decay alone. Discordance in an u nreset sample is caused by a mix of detrital ages, and in a reset samp le is caused by a mix of annealing properties among the detrital apati tes and perhaps by U loss from some apatites. Discordant grain-age dis tributions can be successfully interpreted by using the minimum age, w hich is the pooled age of the youngest group of concordant fission-tra ck grain ages in a dated sample. The inference is that this fraction o f apatites has the lowest thermal stability, and will be the first to reset on heating and the last to close on cooling. Comparison of the m inimum ape with depositional age provides a simple distinction between reset samples (minimum age younger than deposition) and unreset sampl es (minimum age older than deposition). The success of the minimum-age approach is demonstrated by its ability to resolve a well-defined age -elevation trend for reset samples from the Olympic subduction complex . Microprobe data suggest that the apatites that make up the minimum-a ge fraction are mostly fluorapatite, which has the lowest thermal stab ility for fission tracks among the common apatites. Reset minimum ages are all younger than 15 Ma, and show a concentric age pattern; the yo ungest ages are centered on the central massif of the Olympic Mountain s and progressively older ages in the surrounding lowlands. Unreset lo calities are generally found in coastal areas, indicating relatively l ittle exhumation there. Using a stratigraphically coordinated suite of apatite fission-track ages, we estimate that prior to the start of ex humation, the base of the fluorapatite partial annealing zone was loca ted at similar to 100 degrees C and similar to 4.7 km depth. The tempe rature gradient at that time was 19.6 +/- 4.3 degrees C/km, similar to the modern gradient in adjacent parts of the Cascadia forearc high. A patite and previously published zircon fission-track data are used to determine the exhumation history of the central massif. Sedimentary ro cks exposed there were initially accreted during late Oligocene and ea rly Miocene time at depths of 12.1-14.5 km and temperatures of similar to 242-289 degrees C. Exhumation began at ca. 18 Ma. A rock currently at the local mean elevation of the central massif (1204 m) would have moved through the a-damaged zircon closure temperature at about 13.7 Ma and similar to 10.0 lan depth, and through the fluorapatite closure temperature at about 6.7 Ma and similar to 4.4 km depth. On the basis of age-elevation trends and paired cooling ages, we find that the exh umation rate in the central massif has remained fairly constant, simil ar to 0.75 km/m.y., since at least 14 Ma. Apatite fission-track data a re used to construct a contour map of long-term exhumation rates for t he Olympic Peninsula. The average rate for the entire peninsula is sim ilar to 0.28 km/m.y., which is comparable with modern erosion rates (0 .18 to 0.32 km/m.y.) estimated from sediment yield data for two major rivers of the Olympic Mountains. We show that exhumation of this part of the Cascadia forearc high has been dominated by erosion and not by extensional faulting. Topography and erosion appear to have been susta ined by continued accretion and thickening within the underlying Casca dia accretionary wedge. The rivers that drain the modern Olympic Mount ains indicate that most of the eroded sediment is transported into the Pacific Ocean, where it is recycled back into the accretionary wedge, either by tectonic accretion or by sedimentary accumulation in shelf and slope basins. The influx of accreted sediments is shown to be simi lar to the outflux of eroded sediment, indicating that the Olympic seg ment of the Cascadia margin is currently close to a topographic steady state. The record provided by our fission-track data, of a steady exh umation rate for the central massif area since 14 Ma, suggests that th is topographic steady state developed within several million years aft er initial emergence of the forearc high.