Seismic reflection images of the crust of the northern part of the Chugach terrane, Alaska: results of a survey for the Trans-Alaska Crustal Transect (TACT)

Deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the Aleutian trench. These data were collected across the Border Ranges fault system, a major suture zone that separates the Peninsular and Chugach tectonostratigraphic terranes. The shallow part of the seismic section (0-5 s) reveals only a few reflections, which undulate and interweave, indicating that some of them originate outside of the plane of the seismic section. Other undulating events could reveal early Cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults. We interpret divergent reflections within the uppermost band (5-5.5 s) as gently north dipping thrust faults, and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults. The upper and middle bands are separated by an area on the seismic section that reveals few reflections. This area correlates approximately in depth with a high velocity (7.5 km/s) layer evident in refraction models. The poorly reflective, high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the Chugach terrane. The deepest band of subparallel reflections (30-35 km) correlates closely in depth with the top of the Wadati-Benioff zone associated with the underthrusting Pacific plate. -from Authors