A TRANSPRESSIONAL TERRANE OROGENIC MODEL FOR THE SEVIER-LARAMIDE OROGENY

The Sevier-Laramide orogeny is commonly considered to be a result of shallow slab subduction, a hypothesis that emerged because deformation in western North America appeared similar to collisional mountain belts but it lacked an obvious collider. We think shallow slab model is incorrect and were working with E. Moores on articulating an alternative model: Terranes of the Canadian and Alaskan sections of the Cordillera “collided” – in a strike-slip sense - before moving northward into their current location. This more mobilistic view was presented in Moores et al. (2002). The transcurrent, margin-parallel motion of the terranes likely initiated between ~100 Ma and 85 Ma, based on paleomagnetic evidence. Detrital zircon data also support the far-transport of these terranes. Because the margin of North America was segmented – as a result of late Precambrian rifting – the deformation was focused on areas where the Precambrian margin sticks out and impedes right-lateral motion: the Idaho and Mohave segments. Elsewhere during the middle-Late Cretaceous, deformation occurs along right-lateral transcurrent or transpressional structures without major uplift. The Idaho and Mohave regions are linked transpressional syntaxes, each one similar to the Yakutat collision in Alaska being driven by motion on the dextral Queen Charlotte fault system. The younger, more widespread Late Cretaceous-Paleogene mountain building results during wholesale northward dextral translation of the terranes. We lack a name for a juxtaposition (“collision”) of two continental bodies by strike-slip tectonism. Moreover, if juxtaposition occurs along a long margin, there will be zones of localized mountain building linked by strike-slip faults. The tranpressional terrane orogenic model simultaneously solves the problem of coastward transport of terranes required by the paleomagnetic data, the issue of the “missing” collider, and provides a more robust mobilist version for the development of the Sevier-Laramide orogeny.