MID-CRETACEOUS INTRAFORELAND TECTONISM IN THE WESTERN INTERIOR USA: THE ROLE OF THE FLAT SLAB ON ACCENTING THE NORTH AMERICAN CORDILLERAN SYSTEM

Late Cretaceous–Paleogene Laramide tectonism disrupted the foreland system from Montana to southern Arizona–New Mexico. Tectonism is thought to have resulted from flat-slab subduction of the Farallon plate, but what role the flat slab played remains unresolved. Two end member models prevail to explain Laramide orogenesis via the Farallon flat slab: basal traction and a plate margin end load. Deciphering which process drove tectonism requires knowledge of the timing of tectonic processes. Compiled geologic and geochronologic data presented in this work shows that strain was established across the Laramide foreland region by ca. 100 Ma (or earlier), 10–40 Ma prior to onset of classic Laramide tectonism. Recognized since the 1970’s, early tectonism is recorded from Montana–New Mexico, and possibly as far inboard as western North Dakota. Early signals appear to be subtle (m- to 10’s m-scale), suggesting low magnitude strain and/or the stable craton impeded appreciable strain. This was followed by a documented NE- or ENE-directed time-transgressive sweep of onset of Laramide tectonism from ca. 90–60 Ma across the region. The sweep pattern appears to agree with advancement of a strain front via basal traction as the flat slab progressed inboard. However, the rate of the sweep is 2–4 times slower than Farallon-North America convergence at that time, instead suggesting the plates were weakly coupled. Early phase and sweep phase intraforeland tectonism may represent a two-part role for the Farallon flat slab. First, a plate margin end load established far field mid-Cretaceous compression that allowed optimal crustal weaknesses to lightly fail. Second, advance of the dehydrating flat slab progressively destabilized North America’s western interior and facilitated sweeping onset of Laramide strain. As such, plate margin end loads (and/or critical taper) are envisioned as the primary source of far field compression, but weakening of the lithosphere is the fundamental control on whether foreland regions can accommodate significant strain that leads to broken (Laramide-style) foreland regions in Cordilleran ocean-continent convergent systems.