TOPOGRAPHIC EVIDENCE FOR THE NEOTECTONIC FRAMEWORK OF THE TALKEENTA MOUNTAINS, SOUTH-CENTRAL ALASKA

The Talkeetna Mountains define an anomalous topographic high in south-central Alaska that extends across the trend of a long-lived forearc basin. The anomalous position of this topographic high raises some questions about its origin: Why do the Talkeetna Mountains extend across the trend of this basin as a topographic high, and do active tectonic structures contribute to its uplift? The position of the Talkeetna Mountains corresponds directly to where thick crust of the Yakutat block is subducting and causing flat-slab subduction due to its buoyancy and/or flexural strength. I hypothesize that the Talkeetna Mountains have been uplifted primarily as a result of buoyancy and/or flexure associated with flat-slab subduction, but local relief may reflect fold and fault structures resulting from westward rotation of the southern Alaska block south of the Denali fault.

Preliminary analysis of elevation and topography has yielded important observations. The southern Talkeetna Mountains define a gently domal uplift that has been partially dissected. To the north they define a plateau capped by an extensive uplifted erosion surface of unknown age. The erosion surface and the glacial valleys within it have been deeply incised by fluvial drainages. The Susitna River is the dominant drainage in the area cutting all the way across the mountains and is likely an antecedent drainage. The trend and location of several NE-trending elliptical topographic highs in the NW Talkeetna Mountains are consistent with their being anticlines resulting from convergence due to westward rotation of the southern Alaska block. No active faults are obviously expressed by topography or seismicity, but Broad Pass fault is the most likely candidate for a large-displacement fault based on the rock units it juxtaposes, its prominent topographic expression, and its linear trace.

This project may add to understanding of flat-slab subduction and transpression and their relation to topography and deformation. This project will evaluate the possible seismic hazards to the Susitna-Watana hydroelectric dam site proposed along the Susitna River, local towns in the area, and a major transportation corridor to interior Alaska.