MASS AND FLUID FLUX DURING ACCRETION AT THE ALASKAN MARGIN

Prestack depth migration of a seismic reflection transect across the A laskan convergent margin clarified structure and provided an undistort ed tectonic cross section. Between the trench floor and a backstop of eroded Eocene-Miocene rock the accretionary prism displays three tecto nic styles, Seaward of the deformation front, trench sediment is cut b y incipient faults confined to the subsurface, and distributed deforma tion. In the 10 km landward of the deformation front, thrust faults de velop into imbricate structures above a lower detachment., In the rema ining 20 km, longer and mon: steeply dipping thrust units form a ''cla ssical'' imbricate prism above an upper detachment, A velocity model a t the scale of the thrust slices was derived from depth processing and seismic refraction data. The resulting true-scale tectonic cross sect ion was balanced to estimate horizontal contraction. A porosity held w as derived through the velocity-porosity relation in five nearby drill holes, It indicates porosity reduction patterns characteristic of eac h tectonic style, Seaward of the deformation front, the strain rate is high, and pore-fluid pressure elevates, but porosity reduction is min imal. In the 10 lan landward of the deformation front, tectonism and d ewatering are most rapid: sea-floor venting was observed only here. In the classic accretionary zone, strain rates decrease and dewatering i s moderate Here an upper decrease propagates across the lower parts of imbricate slices and doubles the underthrust mass, One-third of the t rench sediment section is underthrust at the deformation front, wherea s two-thirds of the sediment section is subducted beneath the backstop .