The Rudbar-Tarom Iran earthquake (June 20, 1990, MS=7.7) occurred in the high Alborz mountains south of the Caspian Sea in northwest Iran. Although the structural style of the region is dominated by reverse faulting and folding, the focal mechanisms of the events indicate pure right-lateral strike slip motion. We have studied the source parameters of the 11 largest aftershocks (MS>4.6) using three different methodologies: (1) joint inversion of teleseismic and regional distance body waves; (2) single-station moment tensor inversion (SSMT); and (3) comparison of entire wave trains to those of master events. The aftershocks show a clear bipolar distribution: thrusting earthquakes on NW-SE trending planes and strike slip mechanisms with fault geometry similar to that of the mainshock. We interpret the faulting behavior of the 1990 earthquake sequence to indicate slip partitioning. In northwest Iran the Arabian and Eurasian plates are converging in a NE direction and the Alborz mountains are undergoing shortening and left-lateral shear. This oblique motion results in dominantly thrusting earthquakes and infrequent strike slip earthquakes. ¿ American Geophysical Union 1995