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