STRUCTURAL FABRICS, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE REINDEER HILLS MELANGE, A SUTURE ZONE IN SOUTH-CENTRAL ALASKA

Observations of the Broad Pass region in south-central Alaska depict a history of N-directed subduction of oceanic crust in the Late Cretaceous followed by uplift, development of an erosional unconformity, and a shift in sediment source from NW to SE. Broad Pass separates the Alaska Range to the NW and the Talkeetna Mountains to the SE. On the north side of Broad Pass, the Reindeer Hills (RH) expose a mélange complex that includes a lower argillite-dominated mélange and an overlying klippe of older chert-dominated mélange that also includes greenstones and felsic tuffs. Lithologic and structural observations, combined with geochemical analyses of the tuffs, indicate that the RH mélange is an accretionary complex consisting of oceanic crust and hemipelagic sediments that formed prior to the collision, with tuff layers derived from an associated volcanic arc. The structural fabric of the mélange is dominated by a scaly fabric that strikes roughly E-W and dips steeply to the N. The sequence of microfabrics in the mélange indicates a compactive strain path with early particulate flow (dirty veins and development of a grain shape fabric) followed by layer-parallel shearing along an anastomosing array of scaly microfaults. The orientation and asymmetry of sandstone “fish” and shear bands indicate top to the south shearing consistent with N-directed subduction. The Jack River conglomerate unconformably overlies the RH mélange, and the unconformity cuts all deformation fabrics within the mélange. This unconformity indicates uplift and development of a regional erosion surface. The geochemistry of the trachytes and dacites from the chert-dominated mélange are consistent with a volcanic arc signature. They also exhibit compositional characteristics indicative of adakites and may have originated from an arc inboard of subducting, relatively young, oceanic crust. U-Pb ages of detrital zircons (DZ) from clasts within the RH mélange and clasts from the overlying Jack River conglomerate indicate distinct sources. In the RH mélange, there are peaks at 103, 190, and 377 Ma which are consistent with a North American source to the NW and 57% of the DZ grains are Precambrian. In the overlying Jack River conglomerate, there is a single peak at 105 Ma and 74% of the DZ grains are Cretaceous.