THE HIT-AND-RUN MODEL: BAJA BRITISH COLUMBIA AND ITS OROGENIC CONSEQUENCES (Invited Presentation)

A persistent problem in understanding the tectonics of the North American Cordillera is the role of accreted terranes vs. subduction on mountain building. Paul Umhoefer was a long-time advocate for supporting large-scale displacements required by paleomagnetic data, even if they contradicted certain “known” aspects of the tectonic evolution. The original Hit-and-Run model for Cretaceous-Paleogene tectonism, in which Insular terrane collision occurred further south at ~100 Ma (the “hit”) and was subsequently moved northward (the “run”) along the coast, was motivated by Paul’s mobilistic viewpoint. More recently, detrital zircon analyses and reconstructions based on seismic tomography of subducted plates generally support long-range transport.

This contribution updates and advances the original hit-and-run model. The hit phase (100-85 Ma) resulted in a nearly simultaneous collision from central Mexico to central Idaho, and likely resulted from a 105-100 Ma worldwide plate reorganization. All the coastal magmatic arcs in this segment experience a rapid and intense episode of dextral transpressional/contractional deformation. The hit phase also records incipient plateau formation in the hinterland, major contraction in the Sevier fold-thrust belt, foreland block uplifts in the northern Rocky Mountains, and significant foreland sedimentation in adjacent North America. A 85-55 Ma “run” phase resulted in continued contraction throughout North America in addition to dextral strike-slip faulting of coastal blocks. The hit-to-run transition – and the more extreme contractional deformation in Wyoming and Montana is hypothesized to occur because of the clockwise rotation of a large lithospheric block (Blue Mountains terranes and adjacent Laurentia in western Idaho). This block rotation was accommodated by sinistral motion along the Lewis and Clark deformation zone, and resulted in significant shortening in SW Montana.