THE EVOLVING TRANSFORM, OROGENIC COLLAPSE, AND “DE-ARCING” OF THE WESTERN NORTH AMERICAN CORDILLERA: IS THE WALKER LANE THE SITE OF THE FUTURE PLATE BOUNDARY?

Since ~30 Ma, western North America has been evolving from an Andean type margin to a transform boundary. Transform growth has been marked by progressive retreat of magmatic arcs, collapse of orogenic highlands, and periodic inland steps of the plate boundary. The distribution of Oligo-early Miocene ignimbrites indicates that orogenic highlands generally persisted into the mid-Tertiary and began collapsing roughly coincident with transform initiation. Collapse was initially accommodated by widely distributed major extension but is now focused in regions of transtension near or along parts of the plate boundary.

The eastward jump of the southern transform into the Gulf of California ~6 Ma placed the western Great Basin in a favorable position to accommodate plate motion and facilitated development of the Walker Lane-eastern California shear zone (WL), a dextral fault system that accommodates ~20% (~8-12 mm/yr) of the Pacific-North American motion. An apparent Riedel shear geometry (left-stepping dextral faults) in the northern WL, small cumulative displacements (0-80 km), and local timing constraints indicate that the WL is a young system that formed since the late Miocene. Slip rates and cumulative offsets decrease NW-ward in the WL as dextral shear diffuses into NW-SE extension in the Great Basin. The WL terminates into the Cascade arc in NE California near the current latitude of the Mendocino triple junction (MTJ) and north end of the transform, or San Andreas fault (SAF). This implies that the WL is migrating NW in concert with N-ward migration of the MTJ.

We suggest that the Pacific-North American boundary will jump inland to the WL. Continued N-ward migration of the MTJ puts it on a collision course with the NW-propagating WL somewhere off the southern Oregon coast. A new NW-trending plate boundary extending from the Gulf of California, through the WL, and to southern Oregon will accommodate NW-trending North American-Pacific dextral motion more readily than a more northerly trending segment of the SAF in northern California-southern Oregon. At the current rate of N-ward MTJ movement (~3 cm/yr), this realignment will likely occur ~8 Ma from now. Continued transform lengthening and transtension will further enhance orogenic collapse, de-arcing of western North America, and dismantling of the Andean type margin.