TERRANE RELATIONSHIPS ALONG THE EASTERN BOUNDARY OF THE TOGIAK ARC, SOUTHWESTERN ALASKA

Western Alaska exposes numerous allochthonous terranes that accreted to the North American Cordilleran during fluctuating phases of convergent and strike-slip deformation. However, the identity, age, kinematics, and tectonic role of terrane bounding structures remain cryptic for much of the region. We present and synthesize new and existing geologic mapping, structural, and geochronological data along a ~20-km-long transect across the East Kulukak fault- the NE-SW striking mapped boundary between the Togiak arc terrane to the northwest and Kulukak Bay greywacke to the southeast.

Mapping, structural, and petrographic data indicate that the previously mapped location of the East Kulukak fault juxtaposes petrographically similar sandstone units with abundant volcanic lithics along a subvertical fault. Detrital zircon U-Pb data from the lithic-rich sandstones on both sides of the fault are nearly identical, display a bimodal distribution of ca. 150 Ma and 185 Ma grains, and yield youngest statistical population maximum depositional ages ranging from ca. 145-150 Ma. To the southwest of our transect, published mapping shows the East Kulukak fault as splitting into ~N-striking splays that bound relatively flat-lying locally sourced boulder conglomeratic strata with Hauterivian-Valanginian fossils. Reinterpreting this map pattern to represent extensional splays associated with the East Kulukak fault implies left lateral kinematics on the fault during the Early Cretaceous.

Altogether, the synthesis of new and previously published data provides new insight to terrane relationships in western Alaska: 1) The Togiak Arc terrane is predominantly Jurassic in age. Furthermore, the similarity in ages between the Togiak arc terrane and Peninsular terrane (Talkeetna-Bonanza arc) may suggest some genetic relationship between the two arc terranes; 2) Similarities in age and petrography between Togiak arc rocks and Kulukak Bay greywacke support previous field studies suggesting that Kulukak Bay greywacke represents a backarc basin to the Togiak arc terrane; and 3) The East Kulukak fault is a site of Early Cretaceous left lateral motion that likely overprints an older arc-backarc boundary.