The role of mantle delamination in widespread Late Cretaceous extension and magmatism in the Cordilleran orogen, western United States

Abstract

Extension during plate convergence and mountain building is widely recognized, yet the causes of synconvergent extension remain controversial. Here we propose that delamination of lithospheric mantle, aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating, incursion of fl uids, and partial melting, explains many enigmatic yet prevalent aspects of the metamorphic, magmatic, and kinematic history of the Sevier-Laramide orogen of the western United States during the Late Cretaceous. Extension, heating, anatexis, magmatism, and perhaps rock uplift were widespread during a restricted time interval in the Late Cretaceous (75-67 Ma) along the axis of maximum crustal thickening within the Mojave sector of the Sevier orogen, and to a lesser extent within the interior of the Idaho-Utah-Wyoming sector to the north; similar processes may have been active in the Peninsular Range, Sierran, western Mojave, and Salinian segments of the Mesozoic Cordilleran arc. These processes are viewed as predictable consequences of the thermal, rheological, and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts. The proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the Farallon plate during the inception of the Laramide orogeny. Following delamination, extension and anatexis of the North American crust were aided locally by egress of slab-derived fl uids from the low-angle Farallon slab. We suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry. Delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis. The Late Cretaceous delamination event proposed here for the Sevier-Laramide orogen occurred during protracted plate convergence and was synchronous with, and followed by, continued shortening in the external part of the orogen.

Age trends in garnet-hosted monazite inclusions from upper amphibolite facies schist in the northern Grouse Creek Mountains, Utah

“…The Grouse Creek Mountains are part of the larger Raft River-Albion-Grouse Creek metamorphic core complex and lie in the hinterland of the Late Mesozoic to early Cenozoic Sevier fold-thrust belt of western North America. These metamorphic rocks represent mid-crustal levels of the Sevier hinterland (Coney and Harms, 1984;Miller et al, 1988;McGrew et al, 2000;Hoisch et al, 2002) that were exhumed during protracted Cenozoic detachment faulting (Wernicke, 1992, and references therein) as well as during synconvergent Mesozoic extension (e.g., Wells et al, 1990Wells et al, , 1998Hodges and Walker, 1992;Wells and Hoisch, 2008).…”

The discovery of a low-angle normal fault in the Taurus Mountains: the İvriz detachment and implications concerning the Cenozoic geology of southern Turkey*

“…During the core complex formation, delamination of the lithosphere can create synand postexhumation magmatic activity (e.g. Cordilleran, western US - Wells and Hoisch, 2008;Liaonan, northern China -Ji et al, 2015).…”

Review of metamorphic and kinematic data from Internal Crystalline Massifs (Western Alps): PTt paths and exhumation history

“…Although evidence for extensional deformation has been found in many orogenic belts, it is still unclear whether extension represents a local exception to the dominant shortening regime, related to lithospheric thickening (e.g., Burg and Chen, 1984;Burchfield and Royden, 1985;Froitzheim et al, 1994;Schmid et al, 1996;Wheeler et al, 2001), or whether it can represent a transient dominant deformation mode at the scale of the whole orogen (e.g., Collins, 2002;Forster and Lister, 2005;Wells and Hoisch, 2008). Detailed studies in the internal zones have illustrated that many of the preserved structural features may not be related to NW-directed thrusting but developed during SE-directed extension (Wheeler and Butler, 1993;Reddy et al, 2003) that is linked to the exhumation of eclogite facies units formed by subduction.…”