DEEP SEISMIC STRUCTURE AND TECTONICS OF NORTHERN ALASKA - CRUSTAL-SCALE DUPLEXING WITH DEFORMATION EXTENDING INTO THE UPPER-MANTLE

Seismic reflection and refraction and laboratory velocity data collect ed along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composi te picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made : (1) Northern Alaska is underlain by nested tectonic wedges, most wit h northward vergence (i.e., with their tips pointed north). (2) High r eflectivity throughout the crust above a basal decollement, which deep ens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these t ectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which ap pears to involve chiefly bending of a relatively rigid plate consistin g of the parautochthonous North Slope crust and a 10- to 15-km-thick s ection of mantle material. (4) This plate is interpreted as a southwar d verging tectonic wedge, with its tip in the lower crust or at the Mo ho beneath the southern Brooks Range. In this interpretation the middl e and upper crust, or all of the crust, is detached in the southern Br ooks Range by the tectonic wedge, or indentor: as a result, crust is u plifted and deformed above the wedge, and mantle is depressed and unde rthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle o f two different origins (and seismic velocities), giving rise to a pro minent sub-Moho reflector.