Ductile strain and metamorphism in an extensional tectonic setting: a case study from the northern Snake Range, Nevada, USA

Abstract

Summary In recent years considerable attention has focused on metamorphic core complexes of the Basin and Range Province of the western US Cordillera. These highly extended areas are characterized by an upper plate that has been brittlely attenuated by normal faults separated by a sub-horizontal detachment surface from a lower plate that has been ductilely thinned and stretched. A study of mesoscopic structures, finite strain, microstructures, quartz c -axis fabrics and 40 Ar/ 39 Ar geochronology was undertaken in order to characterize the nature, geometry, kinematic history and timing of ductile extension in the lower plate of the northern Snake Range metamorphic core complex in E-central Nevada. These data provide new insights into the processes of deep-seated ductile strain beneath supracrustal normal fault mosaics in highly extended regions. Mesoscopic structures and finite strain analyses indicate that the lower plate underwent plane strain with a sub-vertical Z-axis and a sub-horizontal WNW-ESE X -axis. The magnitude of strain increases dramatically from W-to-E, and is constant vertically. Bedding and foliation are everywhere parallel and bedding is thinned 30–90%. The nature and geometry of microstructures and c -axis fabrics changes progressively from W-to-E. Lower strain rocks on the W flank of the range are characterized by a single bedding-parallel foliation, defined by flattened detrital grains, globular grains which have their c -axes parallel to the Z -axis and symmetrical cross-girdle c -axis fabrics. Further E to slightly higher strains, the rocks are characterized by asymmetrical cross-girdle c -axis fabrics, although globular grains with their c -axes parallel to Z are still present. The high-strain rocks on the E flank of the range are characterized by C -planes, asymmetrical mica ‘fish’ and oblique quartz foliations, and asymmetrical single-girdle c -axis fabrics. These data suggest that the lower plate has not deformed entirely by either coaxial or non-coaxial strain, and we propose an evolutionary model whereby the lower plate deformed during an early period of coaxial strain followed by a later component of non-coaxial strain accompanied by coaxial strain on the E flank of the range. 40 Ar/ 39 Ar geochronology on lower-plate hornblendes and micas indicates that temperatures of deformation were >280°C, but <530°C, and increased with depth. Geochronological data suggest that lower-plate ductile strain is post-latest Cretaceous, and biotite and muscovite data indicate that ductile deformation was probably ongoing 22–26 Ma. Our data and regional geological relations suggest that ductile extensional deformation in the northern Snake Range occurred because of localized heat input to shallow levels of the crust, and together with seismic reflection data, indicate that lower-plate rocks probably represent the top of a regional metamorphic terrane of Tertiary age.

Oligo‐Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada

“…[8] With a predominant top to the east sense of shear, deformation along the NSRD and its mylonitic footwall was responsible for exhumation of metamorphosed Upper Proterozoic to Lower Cambrian quartzite and metapelite and Middle Cambrian to Ordovician marble relative to normalfaulted Paleozoic and Tertiary strata in the hanging wall [Wernicke, 1981;Lee et al, 1987;Miller et al, 1999a] (Figures 2 and 3). The Snake Range MCC has been at the heart of a major debate about the general style of lithospheric extension, and in particular on the role of pure shear versus simple shear in crustal deformation.…”

“…Wernicke [1981] interpreted the NSRD as a major Tertiary low-angle normal fault (décollement) that accommodated several tens and perhaps more than 60 km of extensional displacement [Bartley and Wernicke, 1984]. In contrast, Miller et al [1983], Gans and Miller [1983], Gans et al [1985], Lee et al [1987], Miller and Gans [1989], and Lee and Sutter [1991] depict the NSRD as a subhorizontal brittle-ductile transition zone with only limited (<10 km) slip. The brittlely extended upper plate (450%) rests on top of an equally stretched lower plate showing Cenozoic greenschist-facies metamorphism.…”

“…[5] The Northern Snake Range MCC (Nevada, USA) ( Figure 1) is an excellent locality to assess the role of fluids in detachment systems because 1) it is well studied [e.g., Wernicke, 1981;Miller et al, 1983;Bartley and Wernicke, 1984;Lee and Sutter, 1991;Lee, 1995;Lewis et al, 1999;Miller et al, 1999a;Lee et al, 1987;Gans et al, 1985Gans et al, , 1989, 2) the upper crust experienced more than 450% extension resulting in open brittle fractures that permit downward percolation of fluids [Losh, 1997], and 3) the region had a high geothermal gradient, which could drive fluid flow, because heat was advected within the footwall of the detachment, with some additional contribution from the emplacement of crustal melts (∼35 Ma, Eocene-Oligocene batholiths ).…”

“…[6] The eastern flank of the Northern Snake Range MCC exposes a complete section through a fossil detachment system that in its footwall is composed almost entirely of mylonitic quartzite and interlayered schist units [e.g., Lee et al, 1987;Lee, 1995;Miller et al, 1999a]. Oxygen and hydrogen isotope geochemistry represents an excellent tool to trace the fate of fluids in such brittle-ductile detachment systems [e.g., Wickham and Taylor, 1987;Fricke et al, 1992;Losh, 1997;Morrison and Anderson, 1998;Holk and Taylor, 2007;.…”

Rheological control on the initial geometry of the Raft River detachment fault and shear zone, western United States

“…The dip of the RRD at 12 Ma is estimated between 5 ø and 21ø for geothermal gradients of 25ø-35øC during the early part of the brittle slip history as recorded in the apatite fission track ages (Table 1) Figures 1 l a and 1 lb) or, alternatively, the result of shearing parallel to lithologic layering [e.g., Lee et al, 1987;McGrew, 1993] (Figures 11c and 1 ld). Field examples used to substantiate these interpretations, while they convincingly demonstrate a significant component of noncoaxial deformation, do not offer sufficient exposures beneath the shear zone to al!ow reconstruction of the shear zone boundary to the undefonned Figures 11c and 11d).…”