Temporal and Spatial Shifts in Deformation Pattern of a Glacially Impacted Submarine Fold-Thrust Belt, Northern Gulf of Alaska

In this study we analyze the structural evolution of the submarine Pamplona Zone fold-thrust belt (PZ) which comprises the frontal thrust system of the St. Elias orogen in the northern Gulf of Alaska. Multi-channel seismic reflection data at a variety of scales image the northeastern extent of the PZ and are interpreted for signs of neotectonic deformation using stratal offset due to faulting and the presence of growth strata on the forelimb or backlimb of folds. These interpretations are compared with surface ruptures due to active faulting on a high-resolution bathymetric dataset in order to place the observed active offshore structures within a regional context. Previously defined as a broad zone of distributed deformation, we find instead that recent movement in the PZ is localized on the distal, southeastern-most fault-cored folds. Glacial erosion-deposition cycles are located within the seismic facies overlying currently inactive structures in the landward portion of the PZ, but are not evident near the active edge. Further, the total amount of shortening observed across structures within the PZ indicates that deformation within this portions of the margin accounts for only 15-20% of total Pacific-N. America convergence over the last 6 million years. Taking these observations together, we suggest that the bulk of neotectonic deformation is accommodated by either onshore structures or a decollement located below the depth of seismic penetration. Our study will help inform future regional models of uplift, convergence and tectonic-climatic interaction within the orogen.