Late Cretaceous–early Eocene Laramide uplift, exhumation, and basin subsidence in Wyoming: Crustal responses to flat slab subduction

Abstract

Low-angle subduction of the Farallon oceanic plate during the Late Cretaceous-early Eocene is generally considered as the main driver forming the high Rocky Mountains in Wyoming and nearby areas. How the deformation was transferred from mantle to upper crust over the great duration of deformation (~40 Myr) is still debated. Here, we reconstruct basin subsidence and compile paleoelevation, thermochronology, and provenance data to assess the timing, magnitude, and rates of rock uplift during the Laramide deformation. We reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information. The amount (and rate) of rock uplift of individual Laramide ranges was less than 2.4-4.8 km (~0.21-0.32 mm/yr) during the early Maastrichtian-Paleocene (stage 1) and increased to more than~3 km (~0.38-0.60 mm/yr) during the late Paleocene-early Eocene (stage 2). Our quantitative constraints reveal a two-stage development of the Laramide deformation in Wyoming and an increase of rock uplift during stage 2, associated with enhanced intermontane basin subsidence. Exhumation and uplift during stage 1 is consistent with eastward migration of Cordilleran deformation associated with low-angle subduction, whereas the change in exhumation during stage 2 seems to follow a southwestward trend, which requires an alternative explanation. We here suggest that the increase of rock uplift rate during the late Paleocene-early Eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the Farallon oceanic slab.

Early Inception of the Laramide Orogeny in Southwestern Montana and Northern Wyoming: Implications for Models of Flat‐Slab Subduction

“…Previous studies of uplift, basin subsidence, and erosional histories of the Laramide in Wyoming indicate mostly early Cenozoic ages (e.g., Peyton et al, ; Stevens et al, ); these relatively young ages may represent thermal processes related to westward roll‐back of the subducting slab (Fan & Carrapa, ) or accretion of Shatsky conjugate depleted mantle lithosphere beneath Wyoming and the associated upper crustal deformation and subsequent removal (Humphreys et al, ; Stevens et al, ). Westward to southwestward migration of magmatism in the mid‐Cenozoic is consistent with the history of the stress field (Bird, ) and can be explained by westward migration of the Farallon plate hingeline and falling away of the slab (referred to as slab rollback ; Best et al, ; Coney & Reynolds, ; Constenius, ; Dickinson & Snyder, ).…”

“…Assessment of these and other models for the Laramide event hinges on the availability of accurate estimates for the timing of the Laramide uplifts. Unfortunately, the timing of Laramide structural events has been difficult to date directly because the rocks involved in the uplifts have not been metamorphosed since the Precambrian, the kinematic events involved in the Laramide were largely amagmatic, and few studies have been able to assess the low‐temperature thermochronological ages of Laramide block exhumation (e.g., Copeland et al, ; Fan & Carrapa, ; Kelley, ; Omar et al, ; Peyton et al, ; Peyton & Carrapa, ). Thus, the timing of Laramide flat‐slab subduction has been inferred from the timing and spatial distribution of regionally inboard migrating magmatism (e.g., Coney & Reynolds, ; Constenius et al, ) and the timing of Laramide synorogenic sedimentation and basin development (e.g., Amato et al, ; Crews & Ethridge, ; Dickinson et al, ; DeCelles et al, , ; Graham et al, ; Hoy & Ridgway, ; Ingersoll et al, ; Keefer, ; Lawton, , ; Nichols et al, ; Ryder & Scholten, ; Suttner et al, ).…”

Southwestward weakening of Wyoming lithosphere during the Laramide orogeny

“…Based on accelerated exhumation and elevation gain of the Laramide ranges, Fan and Carrapa [] proposed a two‐stage history of the Laramide deformation and suggested that the deformation during the Maastrichtian–early Paleocene may be attributed to the low‐angle subduction of the Farallon plate and the accelerated deformation during the late Paleocene–early Eocene may be due to slab removal through rollback [e.g., Coney and Reynolds , ; Constenius , ], or removal of ecologitized oceanic plateau or aseismic ridges [e.g., Liu et al , , ; Liu and Gurnis , ]. Our flexural modeling results agree with this two‐stage history of Laramide deformation by showing accelerated load height gain of the Laramide ranges during the early Eocene.…”

“…Although it is generally perceived that dynamic effect causes long‐wavelength uplift, it is not the case in the central Rockies maybe because that strain release preferentially occurred along preexisting faults. The slab rollback model has been suggested by many previous studies based on several lines of evidence, including the trenchward migration of igneous activity between ~55 Ma and ~47 Ma [ Constenius , ]; the uplift and exhumation of the Laramide ranges became younger southwestward during the late Paleocene‐early Eocene [ Fan and Carrapa , ]; and the westward hydrologic ponding and unconformity development in the Greater Green River Basin during the early Eocene [ Smith et al , ].…”

Recent craton growth by slab stacking beneath Wyoming

“…To calculate the effects of the erosion of pre-125 Ma sediment from the Colorado Plateau and the Laramide uplifts, we use Fan and Carrapa (2014) and Reed et al (2005) to estimate the amount of this sediment that was removed (again, averaging over areas one-degree on a side). The pre-erosion thickness of this sediment is estimated by extrapolating the mapped sediment thickness of the pre-125 Ma sediments Cook and Bally (1975).…”