ACTIVE TECTONICS OF THE WESTERN ST. ELIAS SYNTAXIS

The St. Elias orogen is the most actively deforming region in North America, and recent work suggests the most active part of the system is along the western fringe of the orogen where older thin-skinned fold-thrust systems are overprinted by folding about steeply dipping axes and the suture has been warped into a tight, 90 degree bend. Hundreds of surface ruptures are recognized within this segment of the orogen, and although some of these ruptures are gravitational (sacking and fissures) it is now known that many are fault scarps. We have mapped these features in detail and correlated the relative concordance versus discordance of ruptures to bedding plane orientation and terrain in attempts to separate tectonic from gravitational features. Field work in 2007 should further clarify our initial observations, but it appears that most of the scarps are subparallel to bedding and are probably related to flexural slip. Refolding also locally warps the suture into a right angle bend and is spatially tied to the region between the suture and a large glacial valley, the Bering Glacier. We infer that the Bering Glacier covers an active fault system that represents a structural boundary to the actively deforming area of refolding. Collectively, this geometry is surprisingly similar to the Himalayan syntaxes where researchers have shown that active structures are intimately linked to focused erosion by rivers. In the St. Elias region similar structural/erosional ties are indicated, but glaciers play the role of rivers in the exhumation. Offshore seismic reflection data suggest the development of the large ice stream of the Bering Glacier is a Pleistocene event, which is consistent with relatively recent development of the “syntaxis” and close ties between erosional processes and initiation of the Bering Glacier structure.