Plate tectonics of the Laramide orogeny

Abstract

In terms of plate tectonics, most orogenic belts are arc, collision, or transform orogens marked by regional batholiths, overthrust nappes, and en echelon fold trains, respectively. None of these models fits the crustal buckling of the classic Laramide orogeny, marked in the central Rocky Mountains by fault-bounded, basement-cored uplifts separated by intervening sediment-filled basins. Reported patterns of current seismicity, volcanism, and deformation in the modern central Andes document two modes of subduction; one involves plate descent at an abnormally shallow angle and may simulate Laramide conditions. In the more familiar mode, a plate descending steeply into the asthenosphere beneath the continental margin generates standard arc morphology with an active volcanic chain; crustal seismicity outside the subduction zone near the trench is confined mainly to a back-arc fold-thrust belt. In the unfamiliar mode, the descending slab of lithosphere slides along under the overriding plate of lithosphere, with which contact is maintained; crustal earthquakes are widespread across the dormant arc massif, within which local block uplifts bounded by reverse faults are prominent, and magmatism is meanwhile suppressed because the asthenosphere is never penetrated by the descending slab. The largely amagmatic Laramide style of deformation can be ascribed to the dynamic effects of an overlapped plate scraping beneath the Cordillera. That inference is strongly supported by the close correlation, in both space and time, between a prominent magmatic null or gap in the western Cordillera and the classic Laramide orogeny in the eastern Cordillera.