THE PAMPLONA FOLD AND THRUST BELT, GULF OF ALASKA: CONTRACTION IN AN EVOLVING OROCLINAL BEND AT THE SUBDUCTION-TRANSFORM TRANSITION OF THE EASTERN ALEUTIAN ARC

The Pamplona zone is the deformation front of an active fold-thrust system developed at the leading edge of the Yakutat microplate during collision of the microplate with the eastern end of the Aleutian trench. Published geodetic and Quaternary geology studies onland suggest that the Yakutat microplate is converging with North America at essentially full North America-Pacific velocity, and thus, if the Pamplona zone were a typical fold-thrust belt it should be absorbing most, or all, of this convergence. This is clearly not the case. Using public domain and industry seismic data from the offshore segment between Icy Bay and the central part of the Pamplona zone, we produced balanced cross-sections across the two leading anticlines of the fold-thrust belt. Age control is limited, but two offshore exploration wells provide sufficient information to place at least approximate control on the timing of fold development through analysis of growth strata on the fold limbs. Analysis of growth strata suggest little, if any, hinge migration and folding via a detachment fold mechanism. Restoration of these folds indicates only 1km of shortening across these structures. At full North America-Pacific convergence rates this amount of contraction could have occurred within the last 20 Ka, yet the growth strata demonstrate that the folds developed over at least 600,000Ka. Thus, the Pamplona zone is absorbing only a small fraction, if any, of the direct convergence of Yakutat microplate. Instead, we suggest that most, or all, of the present contraction is either occurring onland or to the west in the Kayak Island zone; a band of previously recognized deformation that extends northeastward from the Aleutian trench into the core of the orogen. We interpret the Pamplona zone as a secondary effect of ?oroclinal bending?. In this model the Pamplona zone is produced in the contractional ?inside corner? where the Yakutat microplate is oroclinally bent as it is driven into the subduction-transform transition and forced to mould to the backstop. This model implies significant strike-slip within the core of the orogen along the Bagley-Seward glacier trough, and may resolve the origin of the peculiar geometry of the Pamplona zone, which cuts obliquely across the topographic/bathymmetric grain of the orogen.