SEISMIC-REFLECTION EVIDENCE FOR A DEEP SUBGLACIAL TROUGH BENEATH JAKOBSHAVNS-ISBRAE, WEST GREENLAND

Seismic-reflection methods were used to determine the ice thickness an d basal topography of Jakobshavns Isbrae a large, fast-moving ice stre am/outlet glacier in West Greenland. A method of data analysis was dev eloped which involves the pointwise migration of data from a linear se ismic array and a single explosive source; the method yields the depth , horizontal position and slope of the basal reflector. A deep U-shape d subglacial trough was found beneath the entire length of the well-de fined ice stream. The trough is incised up tc, 1500 m into bedrock, an d its base lies 1200-1500 m below sea level for at least 70 km inland. Center-line ice thickness along most of the channel is about 2500 m, or about 2.5 times that of the surrounding ice sheet. This prominent b edrock trough was not apparent in existing radio-echo-sounding data. R eflection coefficients indicate that much of the basal interface is pr obably underlain by compacted, non-deforming sediment. The large ice t hickness, coupled with relatively steep surface slopes, leads to high basal shear stresses (200-300 kPa) along the ice stream. The large she ar stresses and lack of a deformable bed imply that internal deformati on plays a dominant role in the dynamics of Jakobshavns Isbrae.