The Mahogany Peaks Fault, A Late Cretaceous‐Paleocene(?) Normal Fault in the Hinterland of the Sevier Orogen

Abstract

The contact separating Ordovician rocks from the underlying lower part of the Raft River Mountains sequence, northwestern Utah, is reinterpreted as a large-displacement low-angle normal fault, the Mahogany Peaks fault, that excised 4-5 km of structural section. High δ 13 C values identified in marble in the lower part of the Raft River Mountains sequence suggest a Proterozoic, rather than Cambrian age. Metamorphic conditions of hanging wall Ordovician and footwall Proterozoic strata are upper greenschist and middle amphibolite facies, respectively, and quantitative geothermometry indicates a temperature discontinuity of about 100°C. A discordance in muscovite 40 Ar/ 39 Ar cooling ages between hanging wall and footwall strata in eastern exposures, and the lack of a corresponding cooling age discordance in western exposures, suggest a component of west dip for the fault. The juxtaposition of younger over older and colder over hotter rocks, the muscovite cooling age discordance with older over younger, and top-to-the-west shearing down-structure are consistent with an extensional origin. The age of faulting is bracketed between 90 and 47 Ma, and may be synchronous with footwall cooling at about 60-70 Ma. Recognition of the Mahogany Peaks fault, its extensional origin, and its probable latest Cretaceous to Paleocene age provides further evidence that episodes of extension at mid-crustal levels in the hinterland of the Sevier orogenic belt were synchronous with protracted shortening in the foreland fold and thrust belt, and that the Sevier orogen acted as a dynamic orogenic wedge.

Synorogenic extension localized by upper-crustal thickening: An example from the Late Cretaceous Nevadaplano

“…In addition, multiple studies have documented initial exhumation of mid-crustal rocks in the Ruby-East Humboldt and Raft River-Grouse Creek-Albion core complexes as early as ca. 85-75 Ma, and have identified shear zones that accommodated this early exhumation (e.g., Hodges and Walker, 1992;Wells et al, 1998Wells et al, , 2012McGrew et al, 2000;Hallett and Spear, 2014).…”

Comparative anatomy of core-complex development in the northeastern Great Basin, U.S.A.

“…Elba Quartzite (Figs. 3B, 4) (Armstrong, 1968b;Compton et al, 1977;Todd, 1980;Miller, 1980;Wells et al, 1998). Within the Albion and Grouse Creek Mountains, fault-bounded Neoproterozoic through Ordovician rocks lie beneath the range-bounding detachment fault and are thus part of the autochthon as it is defined here (Figs.…”

“…Numerous studies indicate an episode of Late Cretaceous or early Tertiary extension in the A-RR-GC core complex that followed peak metamorphism and thrust-nappe emplacement Wells, 1997;Wolff, 1997;Wells et al, 1998). recognized ~82-90-Ma 40 Ar/ 39 Ar muscovite cooling ages in the lower structural levels of the allochthon in the Raft River Mountains and interpreted them to represent cooling during the final stages of layer-parallel extension postdating peak metamorphism.…”

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 garnet schists studied here are from the schist of Stevens Springs in Basin Creek canyon in the northern Grouse Creek Mountains, and are part of a sequence of alternating quartzite and psammitic, pelitic, and amphibolitic schists of Proterozoic age that unconformably overlie Archean basement (for a more complete description of the stratigraphy, see Compton, 1972;Wells, 1997;Wells et al, 1998;Hoisch et al, 2002). Prior studies in the Raft River-Albion-Grouse Creek core complex have unraveled a history of alternating shortening and extension during continued growth of the Sevier orogen (Wells, 1997;Hoisch et al, 2002).…”