THE ROLE OF TERRANE RHEOLOGY VS FAULT GEOMETRY FOR MOUNTAIN FORMATION AND EXHUMATION ALONG THE DENALI FAULT OF SOUTH-CENTRAL ALASKA

Southern Alaska is comprised of a number of tectonostratigraphic terranes, accreted against the former North American continental margin. The intracontinental Denali fault (DF) cuts an arc across south-central Alaska between two of the largest terranes, the Wrangellia and Yukon composite terranes (WCT, YCT). Partition of strain varies along the DF, shown by a decreasing (Holocene at least) slip rate from east to west. Strike-slip motion appears to be concentrated on the DF while fault-normal deformation is accommodated on thrusts oriented parallel and sub-parallel to the DF. Presently, and at least since the Late Miocene, the highest topography and greatest exhumation in the central and eastern Alaska Range, within suture zone rocks, is located near the McKinley and Hayes restraining bends. Episodes of rapid cooling and exhumation have occurred along the DF since the Eocene. Episodes beginning ~25 Ma and ~6 Ma, appear related with the onset of Yakutat microplate collision and plate motion change. Rapid cooling and exhumation also occurs in the mid-Miocene, perhaps at slightly different times in slightly different places. However, the asymmetry (polarity) of high topography and exhumation across the DF there is controlled by the proximity of the restraining bends to the stronger rheological terrane backstop, in this case the YCT. Overall the location of the highest topography and greatest exhumation along the DF is controlled by a combination of three factors: contrasting terrane and suture zone rheology, fault geometry, and partition of strain. Through time, how have these factors controlled the location of deformation (topographic expression, exhumation) as dextral slip modified fault geometry and juxtaposed terranes of contrasting rheologies? We are testing the relative importance of each along a relatively straight portion of the DF, the eastern DF corner east of the Richardson Highway. There, the YCT lies north of the DF and the WCT south, where it is cut by a number of thrusts, including the McCallum Creek thrust. By applying thermochronology to basement and cobbles (within small basins, deposition age constrained by tephras), and in combination with basin analysis, we seek to constrain the temporal and spatial cooling (exhumation) patterns relative to the juxtaposition of varying terranes and fault geometry.