BACKSTOP AND OROGENIC WEDGE COOLING HISTORIES IN THE ACTIVE YAKUTAT TERRANE COLLISION, SOUTHERN ALASKA

The coastal Chugach-St. Elias Mountains are an active orogenic belt, produced by the oblique collision of the Yakutat terrane (YT) into the southern Alaskan margin after ~25 Ma. The Chugach-St. Elias thrust fault (CSE), a major, seismically active shear-zone, marks the boundary between an active fold-and-thrust belt on the northern edge of the YT and the North American (NA) plate backstop. New apatite and zircon fission track dates have been obtained from samples located along a transect that parallels the YT-NA (Chugach terrane) convergence vector. These combined cooling ages extend a (U-Th)/He low-T cooling history from the same samples (Spotila et al., 2004) across the active, eroding, orogenic wedge to the inactive backstop in the north. Samples from the leading edge of deformation in the south are detrital, with ages older than the ~5 Ma depositional age of the syn-orogenic Yakataga Formation. Cross-sections suggest that these samples have experienced largely horizontal motions with minimum burial. A second population of samples, located slightly farther north within the actively deforming wedge of the Yakutat terrane, have AHe and AFT ages of ~1-3 and 4-6 Ma, respectively, but are not reset with respect to ZHe and ZFT, indicating a maximum burial of 6-7 km. Apatite data from this densely glaciated part of the range indicate moderate long-term exhumation rates of <1-3 mm/yr. Samples collected north of the CSE fault, in the backstop, have AHe and AFT ages between 8-15 Ma (exhumation rate <<1mm/yr), and ~30 Ma ZHe and ZFT cooling ages. Together, these cooling ages indicate that the locus of exhumation shifted southward, from the backstop into the active orogenic belt with time as the consequence of collision and subsequent foreshortening of the YT. That neither ZHe or ZFT from the YT have a cooling history coincident with collision argues that material at the present level of erosion in the orogen came from relatively shallow depths and followed paths with a large component of horizontal motion. Backstop exhumation early in the orogenic event is consistent with relatively efficient erosion of the southern windward flank by glaciers, although the long-term erosion rates are controlled by rates of rock influx and uplift.