CENOZOIC SEDIMENTARY RECORD OF ACCRETED TERRANE PROCESSES ALONG THE SOUTHERN ALASKA CONVERGENT MARGIN, YAKUTAT TERRANE, ST. ELIAS MOUNTAINS

The Yakutat terrane (YT) is an allochthonous terrane that has been transported and partially subducted along the continental margin of western North America; this terrane is currently colliding with the margin of southern Alaska. Geophysical studies show that both the subducted and unsubducted parts of the YT consist of thick oceanic crust. The subducted northwestern part of the YT is 11-22 km thick whereas the crust of the unsubducted southeastern part is 20-25 km thick. In this paper, we present stratigraphic and geochronologic data from the 10 km of Cenozoic strata that overlie the oceanic crust of the YT. These strata contain the record of the initial collision, strike-slip transport, partial subduction, and final collision of this terrane along the continental margin. The oldest strata on the YT are the accretionary prism strata of the Yakutat Group (YG). The YG had been previously assigned a Late Cretaceous age based on limited fossils; detrital zircon U-Pb age data show that deposition of the YG extended at least into the Paleocene. In our interpretation, the YG was thrust onto the oceanic crust of the YT when the terrane initially entered a subduction zone along the continental margin. The Eocene-Oligocene Kulthieth Formation (KF) was deposited over both the oceanic crust of the YT and the thrusted YG. The KF consists of 3 km of shallow marine and deltaic strata. Based on detrital zircons populations, the YG and overlying KF have a shared provenance. The Oligocene-Upper Miocene Poul Creek Formation (PCF) conformably overlies the KF and consists of 2 km of mainly submarine fan to nearshore strata. In our interpretation, the PCF was deposited during the northward transport of the YT and represents the distal foredeep basin related to an onshore transpressional thrust belt. The overlying 5 km thick Upper Miocene-Pleistocene Yakataga Formation (YF) represents the proximal part of the foredeep basin. We divide the YF into a lower and upper unit. The lower unit is conformable with the underlying PCF whereas the upper unit is unconformable with underlying units; in some areas over 5 km of strata have been eroded beneath the upper unit. In our interpretation the unconformity at the base of the upper unit is a product of focused exhumation related to the introduction of the thicker crust of the YT into the subduction zone.