FOREARC-BASIN FACIES DEVELOPMENT IN THE TALKEETNA FORMATION, SOUTH-CENTRAL ALASKA: TECTONIC IMPLICATIONS FOR LOWER JURASSIC TERRANE RECONSTRUCTION

The 7-km-thick Lower Jurassic Talkeetna Formation of south-central Alaska represents the most complete exposure of an accreted arc volcanic section yet documented. Sedimentary facies development within this sequence reflects the tectonic processes that accompanied the evolution of this arc. We present a detailed analysis of the Talkeetna volcanic and volcaniclastic sequence and compare these facies with modern intra-oceanic arc analogs such as Tonga and the Marianas. The dominance of reworked primary volcanic material and proximal mass-flow deposits within the Talkeetna Formation implies proximal forearc deposition rather than more distal trench-slope sedimentation. Forearc and backarc facies are inferred on the basis of stratigraphic evolution and geochemical characteristics.

The sedimentary history of the Talkeetna Arc is consistent with its evolution above a north-dipping subduction zone, prior to the accretion of this terrane to North America. The Talkeetna Trench is interpreted to have been tectonically erosive in order to explain juxtaposition of younger accretionary complex rocks of the Chugach terrane exposed to the south against the Talkeetna crust section, as well as the geochemistry of the units. There is no suggestion that much continental material was subducted to the magmatic roots of this arc. Coupled with the lack of a geochemical response to a collision event along its northern edge around 160 Ma, this suggests a south-facing arc developed over a north-dipping subduction zone. It is not yet clear whether this collisional event was with the North American continent or with Wrangellia.