MIDCRETACEOUS THRUSTING IN THE SOUTHERN COAST BELT, BRITISH-COLUMBIA AND WASHINGTON, AFTER STRIKE-SLIP-FAULT RECONSTRUCTION

A major thrust system of mid-Cretaceous age is present along much of t he Coast Belt of northwestern North America. Thrusting was concurrent, and spatially coincided, with emplacement of a great volume of are in trusives and minor local strike-slip faulting. In the southern Coast B elt (52 degrees to 47 degrees N), thrusting was followed by major dext ral-slip faulting, which resulted in significant translational shuffli ng of the thrust system. In this paper, we restore the displacements o n major dextral-slip faults of the southern Coast Belt and then analyz e the mid-Cretaceous thrust system. Two reconstructions were made that use dextral faulting on the Yalakom fault (115 km), Castle Pass and R oss Lake faults (10 km), and Fraser fault (100 km). The reconstruction s differ in the amount of dextral offset on the Straight Creek fault ( 160 and 100 km) and how much the NE part of the Cascades crystalline c ore expanded (30 km and 0 km) during Eocene extension. Reconstruction A produces the best match of lithotectonic units and thrust systems. O ur synthesis shows that the southern Coast Belt thrust system was grea ter than or equal to 250 - 180 km wide after thrusting. The thrust sys tem was mainly southwest vergent but had a belt of northeast vergent b ack thrusts on the northeast side associated with the Tyaughton-Methow basin, which may indicate large-scale tectonic wedging. Thrust faults are commonly low to moderate angle, but high angle faults also occur, especially as late stage, out-of-sequence, structures involving pluto ns. The amount of thrust displacement across the system is unknown but must be at least 100 km and may be many hundreds of kilometers. Most thrusting occurred from similar to 100 to similar to 80 Ma and did not migrate systematically until after similar to 90 Ma, when thrusting a nd magmatism shifted to the northeast for a few million years. Widespr ead thrusting occurred both near plutons and where there are no (or sm all) plutons, which strongly suggests that thrust faulting was caused by regional- to plate-scale forces such as rapid plate convergence and /or are-continent collision.