STRUCTURAL EVOLUTION OF THE SUBMARINE PAMPLONA ZONE FOLD-THRUST BELT, ST. ELIAS OROGEN

In this study we describe a possible climatic driver for the structural evolution of the submarine Pamplona Zone fold-thrust belt (PZ) which comprises the offshore frontal thrust system of the St. Elias orogen in southern Alaska. Our dataset includes 3 multichannel seismic reflection surveys: a 2008 STEEP survey imaging up to 9 sec (twtt), a 1975 USGS survey imaging up to 6 sec, both at ~30 m vert. resolution, and a 2004 NSF-funded high resolution survey imaging up to 2.5 sec at ~5 m vert. resolution. The data provides constraints for quantifying total offshore Pleistocene deformation recorded in the glaciomarine Yakataga formation using stratal offset due to faulting, growth strata analysis and fault-bend fold analysis. The total amount of shortening observed varies from ~9 km near the Bering Trough to ~3 km along the northeastern deformation front giving a range of shortening across the shelf of ~2-5 mm/yr. This rate is only 7.6% of the GPS derived convergence rate and 24.7% of estimated convergence since the onset of glaciation, implying that much of the shortening occurs onshore within the core of the orogen, the locus of intense glacial erosion. Comparison of relative timing of fault activity reveals temporal and spatial shifting of deformational patterns within the margin. In the early Pleistocene, deformation was distributed across multiple structures mapped throughout PZ. From the early- to mid-Pleistocene, activity waned on faults within the interior of the PZ, initiating fault localization towards the inner St. Elias orogen onshore and farther offshore towards the distal extent of the wedge. From the mid-Pleistocene to Recent, faulting further localized adjacent to the coastline and at the current submarine deformation front. This bi-modal localization of fault activity during the Pleistocene provides a possible link with climatic processes in the St. Elias orogen. The abandoned, currently inactive, region is co-located with the major glacial depocenter in the region, the Bering Trough. These observations imply that glacial processes such as sediment loading and focused erosion during advance-retreat cycles has a direct effect on the evolution of individual faults within the PZ and the overall deformation pattern in the offshore St. Elias margin.