Abstract

Following cessation of contractional deformation, the Cordilleran foreland fold and thrust belt collapsed and spread west during a middle Eocene - early Miocene (ca. 49-18 Ma) episode of crustal extension and regional magmatism coeval with formation of metamorphic core complexes in the Cordilleran hinterland. The sedimentary and structural record of this event in the Cordilleran foreland is a network of grabens that extends from southern Canada to southern Utah and Nevada. Remarkably, foreland and hinterland extensional elements of the Cordillera that are normally widely geographically separated are co-located in central Utah where the thrust belt straddles the Archean-Proterozoic crustal suture. Here, the Charleston-Nebo salient, an immense leading-edge structural element of the foreland, collapsed during late Eocene-early Miocene time when its sole thrust was extensionally reactivated. Concurrent with normal faulting and collapse of the thrust sheet, the north margin of the salient was warped by flexural-isostatic rise of a Cheyenne belt crustal welt and its footwall was intruded by crustal melts of the Wasatch igneous belt. Thus, rejuvenation of the Archean-Proterozoic suture during Cretaceous-middle Eocene crustal contraction and later periods of late Eocene-early Miocene and middle Miocene-Recent extension gave rise to formation of a core complex uniquely situated near the leading-edge of the fold-thrust belt.

Extensional structures superposed on the thrust belt are rooted in the physical stratigraphy, structural relief and sole faults of preexisting thrust-fold structures. In the case of Charleston-Nebo thrust reactivation, normal faults that bound the half grabens follow the same stratigraphic units over long distances or cut discordantly across bedding to link segments of the bedding-plane faults. Thus, the same detachment surfaces and mechanically weak stratigraphic horizons that accommodated up to 100 km of crustal shortening also served as glide-planes when the Charleston-Neb salient collapsed and sustained 5–7 km of crustal extension. The resultant half grabens contain up to 2 km of late Eocene-early Miocene (ca. 39-18 Ma) basin-fill that catalogues the initial phase of salient collapse. West of the salient, in the Basin and Range Province, the thrust sheet has been broken by normal faults such as the Wasatch fault which have a compound extensional history: normal faulting caused by late Eocene-early Miocene orogenic collapse followed by middle Miocene to Recent (ca. 17-0 Ma) Basin and Range extension.

An intriguing aspect of thrust reactivation is that because the same detachment surfaces were used to accommodate both contractional and extensional deformation, an unambiguous chronology of tectonism for the foreland fold and thrust belt can established. Regional space-time variations in the transition from contractile to extensional deformation can be determined by dating latest thrusting and initial normal faulting linked to common detachment surfaces. Near the Charleston-Nebo salient, there were profound changes in the style of tectonism over a ~10 m.y. period beginning in early middle Eocene time (ca. 49 Ma). Thrusting ended and extension began about 49 Ma in the southernmost part of the Archean

Wyoming Province. Whereas, a short distance to the south, in the Proterozoic Yavapai-Mojave provinces, the switch from Charleston-Nebo thrust and Uinta Mountains uplift related shortening to collapse and synextensional magmatism began at about 39 Ma. Thus, from early middle Eocene to late Eocene time (ca. 49-39 Ma), terrains undergoing markedly different structural evolution (i.e., active thrusting in the south versus thrust sheet collapse in the north) were juxtaposed across the Archean-Proterozoic crustal boundary.

Grabens in the Charleston-Nebo salient, as well as throughout the Cordilleran foreland, preserve thick late Paleogene-Neogene stratal assemblages representative of two major episodes of crustal extension and associated magmatism: 1) gravitational collapse of the Cordilleran orogen, followed by, 2) the Basin and Range event. These extensional regimes were the product of changes in rate and style of plate convergence between North America and plates of the Pacific realm. Late Paleogene-early Neogene orogenic collapse and related “flareup” magmatism, was precipitated by steepening and rollback of the subducted Pacific plate, concomitant with a dramatic reduction in rate and orientation of plate convergence. Neogene Basin and Range extension and bimodal basaltic-rhyolitic magmatism can be linked to a second major change in plate convergence that corresponds with development of San Andreas transform. Regional tectono-stratigraphic analysis shows that evidence of profound changes in plate convergence and resultant mode of Cordilleran tectonism is not only found in basin-fill assemblages but is chronicled in Cenozoic continental deposits spread throughout the western United States.