GRAPHICAL METHOD THAT RELATES DISPLACEMENT FIELDS AND STRAIN FOR THE KINEMATIC ANALYSIS OF DEFORMING TECTONIC ZONES: EXAMPLES FROM THE WALKER LANE, WESTERN GREAT BASIN AND ALEUTIAN ISLAND ARC, ALASKA

High precision space based geodetic techniques allow the rapid determination of displacement fields within actively deforming tectonic zones. Kinematic reconstructions of these zones can be better understood if displacement fields are related to what is being recorded in the crust, strain. This relationship has been explored analytically, however a very accessible graphical construction also exists that can rapidly quantify the orientation and ratio of strain, angular shear strain, vorticity and the finite rotation of passive, material lines. This graphical method was originally developed by Simpson & De Paor (1993) for the analysis of ductile shear zones, but can be applied to tectonic problems as well. Fundamental inputs are (1) the average relative velocity within the deforming zone, or plate convergence vector and (2) the width of the zone. Convergence in the Aleutian Arc, Alaska varies along strike from nearly head on in the east to nearly parallel in the west. Strain orientations in the Aleutian Arc were calculated using published GPS velocities and were compared to both published earthquake P-axes and results obtained analytically. Active, transtensional deformation within the Walker Lane, western Great Basin is partitioned into zones of wrench, and pure dominated domains. The GPS velocity field is characterized by a rotation from west-northwest to northwest, increasing from 3 to 12-14 mm/yr toward the Sierra Nevada. Extensional strain axes rotate from west-northwest to west-southwest resulting in a deviation of the strain and velocity trajectories by as much as 50 degrees against the eastern Sierra Nevada. The orientation of strain found using the graphical method agrees with published results and analytically determined solutions.