DERIVED BEDROCK ELEVATIONS, STRAIN RATES AND STRESSES FROM MEASURED SURFACE ELEVATIONS AND VELOCITIES - JAKOBSHAVNS-ISBRAE, GREENLAND

Jakobshavns Isbrae (69 degrees 10'N, 49 degrees 5'W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The J akobshavns Isbrae basin of about 10 000 km(2) was mapped photogrammetr ically from four sets of aerial photography, two taken in July 1985 an d two in July 1986. Positions and elevations of several hundred natura l features on the ice surface were determined for each epoch by photog rammetric block-aerial triangulation, and surface velocity vectors wer e computed from the positions. The two flights in 1985 yielded the bes t results and provided most common points (716) for velocity determina tions and are therefore used in the modeling studies. The data from th ese irregularly spaced points were used to calculate ice elevations an d velocity vectors at uniformly spaced grid paints 3 km apart by inter polation. The field of surface strain rates was then calculated from t hese gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates, X, Y pointing eastward and northward. and curvilinear coordinates, L, T po inting longitudinally and transversely to the changing ice-flow direct ion. Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding . Our calculated ice thicknesses are in fair agreement with an ice-thi ckness map based on seismic sounding and supplied to us by K. Echelmey er. Ice thicknesses were subtracted from measured ice-surface elevatio ns to map bed topography. Our calculation shows that basal sliding is significant only in the 10-15 km before Jakobshavns Isbrae becomes afl oat in Jakobshavns IsfJord.