Dextral shear and north-directed crustal shortening defines the transition between extensional and contractional provinces in north-central Oregon

Graduation date: 2011Distributed deformation in the backarc of Cascadia is complex. Off the west\ud coast lies the Cascadia convergent margin. East of the plate boundary,\ud clockwise rotation of the Oregon Coast Range block with respect to stable\ud North America influences backarc deformation, causing extensional faults in\ud southeast Oregon, contraction folding in southeast Washington and a transition\ud between the two opposing styles in north-central Oregon. Folding of the\ud Mutton Mountain and Tygh Ridge anticlines commenced post 15.5 Ma and\ud was most active between ~8.4 and 2.6 Ma. From Mutton Mountain to Tygh\ud Ridge anticlines, minimum estimates of north-south shortening give an average\ud shortening rate of 0.14 mm/yr from 8.4 to 2.6 Ma. Change in strike and dextral\ud offsets of axial traces reflects a broad shear zone transferring motion between\ud two relatively stable blocks; the Ochoco Mountain block and the Cascade arc.\ud The transition region is characterized where dextral shear accommodates for an\ud west-east velocity gradient. GPS velocity vectors indicate a difference of 1.0\ud mm/yr from the northern Basin and Range to the north and ~0.8 mm/yr from\ud west to east