Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

Abstract

This paper investigates the causes of the Late Cretaceous transition from “Sevier” to “Laramide” orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction.

Geophysical and Geochemical Constraints on Neogene‐Recent Volcanism in the North American Cordillera

“…It has stabilized at the top of the spinel‐garnet transition depth of ∼65 km and may now be in nearly steady state. The lithosphere thinning in the eastern portion of the US Cordillera is estimated to have started mainly in the Late Cretaceous, ∼80 Ma (e.g., Saylor et al., 2020) and may have its origin in Cretaceous Laramide‐age flat slab subduction. Heating, and weakening of the eastern US Cordillera in Cretaceous‐Eocene, has been concluded by a number of authors (Fan & Carrapa, 2014; Gao et al., 2016; Liu & Currie, 2016; Saylor et al., 2020; Smith et al., 2014).…”

“…The lithosphere thinning in the eastern portion of the US Cordillera is estimated to have started mainly in the Late Cretaceous, ∼80 Ma (e.g., Saylor et al., 2020) and may have its origin in Cretaceous Laramide‐age flat slab subduction. Heating, and weakening of the eastern US Cordillera in Cretaceous‐Eocene, has been concluded by a number of authors (Fan & Carrapa, 2014; Gao et al., 2016; Liu & Currie, 2016; Saylor et al., 2020; Smith et al., 2014). The former thicker Eocene lithosphere is indicated by mantle xenoliths from over 150 km depth, a depth which is now in the asthenosphere (e.g., Butcher et al., 2017; Riter & Smith, 1996).…”

Late Cretaceous–Recent Low‐Temperature Cooling History and Tectonic Analysis of the Zuni Mountains, West‐Central New Mexico

“…One scenario is that flat-slab propagation drove Laramide upper crustal strain through basal traction, with or without the conjugate Shatksy Rise oceanic plateau (e.g., Copeland et al, 2017;Heller & Liu, 2016;Lawton, 2019;. A second scenario is that hydration-induced lithospheric weakening (e.g., Humphreys et al, 2003;Saylor et al, 2020) could have led to a time-transgressive pattern, whereby compressional stress established by a plate margin end load (e.g., Axen et al, 2018;Livaccari & Perry, 1993) sequentially overcame frictional resistance via weakening. It is not possible with our data alone to determine whether basal traction or hydration weakening is more likely.…”

“…However, current timing estimates outlined above may suggest an east-directed time transgressive sweep of tectonism across the southern Colorado Plateau-Rocky Mountain Laramide region, similar to easterly directed magmatic sweeps in southern Arizona-New Mexico (Coney & Reynolds, 1977) and Montana-Wyoming-South Dakota (Constenius et al, 2003). A deformation sweep would be consistent with Laramide upper crustal strain that was driven by the Farallon flat slab, at least in part, whether by basal traction (e.g., Copeland et al, 2017;Heller and Liu, 2016;Lawton, 2019) or progressive dehydration-weakening (e.g., Humphreys et al, 2003;Saylor et al, 2020). Later rollback of the Farallon flat slab is thought to have led to a westward sweep of tectonism (Fan & Carrapa, 2014).…”

“…The flat slab's role in imparting inboard tectonism is commonly considered to have occurred either by basal traction of the flat slab beneath western North America (e.g., Bird, 1998;Dickinson & Snyder, 1978;Lawton, 2019), perhaps driven by subduction of the conjugate Shatsky Rise oceanic plateau (Copeland et al, 2017;Heller & Liu, 2016;, or by an end load established at the plate margin (e.g., Livaccari & Perry, 1993) with possible scraping off of the sub-continental mantle lithosphere (Axen et al, 2018). Others have suggested the role of hydration weakening in destabilizing the North American craton to drive deformation far inland from the plate margin (e.g., Humphreys et al, 2003;Saylor et al, 2020), and eventual rollback of the Farallon flat slab as a mechanism for driving tectonism (Fan & Carrapa, 2014).…”