Geometry and kinematics of the Grant Range brittle detachment system, eastern Nevada, U.S.A.: An end‐member style of upper crustal extension

Abstract

Documenting the range of styles of normal faulting is fundamental to understanding crustal extension. Here geologic mapping, field relationships, and deformed and restored cross sections illustrate the geometry and kinematic development of a system of west-vergent detachment faults in the Grant Range in eastern Nevada. Faults exhibit brecciation and stratigraphic cutoff angles of 5-15°at all structural levels and deform a 10 km thick section of Paleozoic and Paleogene rocks. The fault system is folded across an anticlinal culmination, which grew during extension, as indicated by progressively increasing interlimb angles and incision in the axial zone. The eastern limb consists of an imbricate stack of faults that were emplaced from bottom to top. In the western limb, several faults exhibit apparent thrust relationships. The oldest faults are cut by a~29 Ma dike, and the highest preserved fault cuts~32 Ma volcanic rocks that restore to paleodepths of~1 km. Retrodeformation of folding and minimal structural relief and angularity across a Paleogene unconformity indicate the faults were active at 5-15°angles. Retrodeformation of offset indicates ≥49 km (98%) extension. We propose a model of stationary, sustained isostatic uplift and incision at the culmination axis (a "fixed hinge"), with updip excision producing bottom-to-top growth of the imbricate stack and downdip excision producing apparent thrust relationships. The fault system exhibits similarities to core complex detachment systems, though it is confined to upper crustal levels, and there are no preserved high-angle or listric normal faults, indicating a unique extension style dominated by low-angle excision.

Rapid Oligocene to Early Miocene Extension Along the Grant Range Detachment System, Nevada, USA: Insights From Multipart Cooling Histories of Footwall Rocks

“…In the central Grant Range, 4–6 km north of the studied transect, Long and Walker () documented that the set 1 fault system grew from bottom to top into an imbricate stack, through progressive excision. Isostatic rebound accompanying tectonic thinning resulted in syn‐extensional folding of the set 1 system across an anticlinal culmination, as indicated by progressively increasing interlimb angles observed on younger, structurally higher faults (Long & Walker, ). The final geometry consists of an imbricate stack of ~5–25° E‐dipping, back‐rotated faults in the eastern part of the range, and a series of ~5–30° W‐dipping faults in the western part (Lund et al, ).…”

“…These thrust faults are correlated with the Central Nevada thrust belt, a system of E‐vergent, Mesozoic contractional structures interpreted as a hinterland component of the Sevier thrust belt (e.g., Taylor et al, ). However, northward continuations of these thrust faults have not been mapped at the latitude of the cross section, or further to the north in the Grant Range (Long & Walker, ; Moores et al, ). The Schofield Canyon thrust can be traced as close as ~6 km south of the cross section line (Figure ), where it places Cambrian rocks over Ordovician rocks (Fryxell, , ).…”

“…Spatially isolated Late Cretaceous to Paleocene (~80-60 Ma) extension, which was contemporaneous with the final stages of shortening in the Sevier thrust belt, has been documented (e.g., Camilleri & Chamberlain, 1997;Druschke, Hanson, Wells, Rasbury, et al, 2009;Hodges & Walker, 1992;, and has been interpreted to have been initiated by lithospheric delamination (Wells & Hoisch, 2008). Eocene-Oligocene extension has also been documented (e.g., Gans et al, 1989Gans et al, , 2001Evans et al, 2015;Lee et al, 2017;Long & Walker, 2015), and was often associated spatially and temporally with the Great Basin ignimbrite flare-up, a NE to SW sweep of silicic volcanism interpreted to have accompanied post-Laramide slab rollback (Figure 1a; e.g., Dickinson, 2002;Humphreys, 1995). The initiation of widespread extension that formed the Basin and Range Province, which is attributed to establishment of the San Andreas transform system (e.g., Atwater, 1970), was not until the middle Miocene (e.g., Cassel et al, 2014;Colgan & Henry, 2009;Dickinson, 2002).…”

“…The final geometry consists of an imbricate stack of~5-25°E-dipping, back-rotated faults in the eastern part of the range, and a series of 5-30°W-dipping faults in the western part (Lund et al, 1993). Retro-deformation of folding indicates that set 1 faults were active at~5-15°dip angles (Long & Walker, 2015).…”

“…One site of likely pre‐middle Miocene extension is the Grant Range in east‐central Nevada (Figure ). The Grant Range is a core complex‐type range that has undergone high‐magnitude (~100%) extension accommodated by a system of brittle normal faults that were active at low (<20°) dip angles, which has exhumed granite and greenschist‐facies metasedimentary rocks from depths up to ~10 km (e.g., Fryxell, ; Long & Walker, ; Lund et al, ). Published geochronology indicates that extension on this low‐angle fault system was underway by ~29 Ma (Long & Walker, ); however, the duration of extension is not constrained.…”