Mass and fluid flux during accretion at the Alaskan margin

Abstract

Prestack depth migration of a seismic reflection transect across the Alaskan convergent margin clarified structure and provided an undistorted tectonic cross section. Between the trench floor and a backstop of eroded Eocene–Miocene rock, the accretionary prism displays three tectonic styles. Seaward of the deformation front, trench sediment is cut by incipient faults confined to the subsurface, and distributed deformation. In the 10 km landward of the deformation front, thrust faults develop into imbricate structures above a lower detachment. In the remaining 20 km, longer and more steeply dipping thrust units form a classical imbricate prism above an upper detachment. A velocity model at the scale of the thrust slices was derived from depth processing and seismic refraction data. The resulting true-scale tectonic cross section was balanced to estimate horizontal contraction. A porosity field was derived through the velocity-porosity relation in five nearby drill holes. It indicates porosity reduction patterns characteristic of each tectonic style. Seaward of the deformation front, the strain rate is high, and pore-fluid pressure elevates, but porosity reduction is minimal. In the 10 km landward of the deformation front, tectonism and dewatering are most rapid: sea-floor venting was observed only here. In the classic accretionary zone, strain rates decrease and dewatering is moderate. Here an upper detachment propagates across the lower parts of imbricate slices and doubles the underthrust mass. One-third of the trench sediment section is underthrust at the deformation front, whereas two-thirds of the sediment section is subducted beneath the backstop.