Mid‐Cretaceous thrusting in the southern Coast Belt, British Columbia and Washington, after strike‐slip fault reconstruction

Abstract

A major thrust system of mid‐Cretaceous age is present along much of the Coast Belt of northwestern‐North America. Thrusting was concurrent, and spatially coincided, with emplacement of a great volume of arc intrusives and minor local strike‐slip faulting. In the southern Coast Belt (52° to 47°N), thrusting was followed by major dextral‐slip faulting, which resulted in significant translational shuffling of the thrust system. In this paper, we restore the displacements on major dextral‐slip faults of the southern Coast Belt and then analyze the mid‐Cretaceous thrust system. Two reconstructions were made that use dextral faulting on the Yalakom fault (115 km), Castle Pass and Ross Lake faults (10 km), and Fraser fault (100 km). The reconstructions 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 core expanded (30 km and 0 km) during Eocene extension. Reconstruction A produces the best match of lithotectonic units and thrust systems. Our synthesis shows that the southern Coast Belt thrust system was ≥250 – 180 km wide after thrusting. The thrust system was mainly southwest vergent but had a belt of northeast vergent back 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 plutons. 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 ∼100 to ∼80 Ma and did not migrate systematically until after ∼90 Ma, when thrusting and magmatism shifted to the northeast for a few million years. Widespread thrusting occurred both near plutons and where there are no (or small) plutons, which strongly suggests that thrust faulting was caused by regional‐ to plate‐scale forces such as rapid plate convergence and/or arc‐continent collision.