The Muddy mountain thrust: louis Warnock
The Muddy Mountain Thrust was formed by a thrust sheet and is located in Southern Nevada, between Las Vegas and the Valley of Fire, in the Buffington Window (see fig. 1). It is oriented roughly in the east-west direction, overthrusting towards the east. The overthrust extends a minimum of 210 km, from the Clark Mountains to the Muddy Mountains. It is to be highlighted that its strike is offset by the Las Vegas shear zone. Palezoic carbonates are found thrusting above younger Mesozoic Sandstone (see fig. 2) and the thrust sheet itself is thought to be between 24 and 40 km in width, although some estimates are as large as 80 km (see fig 1). Its minimum thickness is constrained to 4-5 km; 2-2.5 km of dolomite/limestone and 2-2.5 km of sandstone. Therefore, considering the range of widths and thicknesses of the thrust sheet, its area is at least 3800 km2 (Brock & Engelder 1977).
Figure 1. Geological map of the Muddy Mountain thrust (Fossen, 2015)
Below the fault plane is Jurassic Aztec sandstone composed of medium-to-fine grained and well-rounded quartz (eolian sandstone). It is red-brick in color, although bleaching is observed in some instances near the fault. In some parts, the thrust moves across a sandstone which has filled topographic depression through surface erosion; the grain size of this sandstone is similar to that of the Aztec sandstone below it, although the sorting is poorer and the grains are show more angularity (Brock & Engelder, 1977). This sandstone is referred to as molasse by Cayeux (1929). Erosional pockets filled by the molasse and channel cuts in the Aztec sandstone is fluvial, indicated that the faulting must have been near-surface; however, the molasse is some distant away from the leading edge of the thrust, meaning erosional depressions must have been filled before the thrusting. The contact between the molasse and the erosional surface of the Aztec sandstone is parallel to the fault line.
Figure 2. Cross-sectional view of the Muddy Mountain overthrust
The thrust was measured to have a dip of 55˚ north in the northern sector (Brock & Engelder, 1977). Several elements of the structure are evidence for thrusting: an increase in intensity of microfractures parallel to and at small angle of contact, a cataclastic decrease in grain size as we approach the contact, an increase in the degree of induration, a loss of well-defined beddings and an increase color contrasts in the sandstone and shearing of dolomite in the upper crust. The zone of deformation (that is, fracturing and cataclasis) associated with the faulting extends as much as 75 m down into the Aztec Sandstone and more than 100 m up into the Goodsprings Dolomite. In the overthrust sheet, there is presence of brecciation and shattered carbonates measuring up to 5 m with gauge injections around them (Brock & Engelder, 1977). In the lower plate, cataclastic deformation has caused angular clasts with finer and more poorly sorted grains than the undeformed sandstone (Graph 1). Microfracturing positively correlates with cataclasis in the sandstone. The zones of induration (cemented sandstone) range from 1 to 5 m in thickness, and the cementation occurs at 10 to 15 m from the contact, although it is also shown in areas as far as 50 m away from the contact (Brock & Engelder, 1977). The induration is caused by lithostatic stress of the thrusting load above. Fractures with low-angle to contact line are associated with this induration. The shortening direction of the deformation bands in the Aztec Sandstone is analogous to that of the Cordilleran thrust belt.
(Brock & Engelder, 1977)
The deformation is strongly localized to the thrust zone. A planar contact with a narrow gouge zone is visible for several hundred meters of outcrop (see fig 3). This gauge has a thickness which ranges between 3 and 30 cm and was generated by cataclasis and a high mobility, allowing it to flow in cracks of upper layer. Less than 30% of the grains are larger than 100 micrometers; thus, the gauge is fine-grained (Brock & Engelder, 1977). As many as three foliated gauge layers appear within the more granular material, representing shear zones and indicating differential movement and relative slip in the upper plate; the microfracture orientations in the quartz gouge and the cataclastic sandstone next to a shear fracture gives an indication of the orientation of the stress field in which the gouge was formed (parallel to the maximum principal stress). The shear displacement along the fault is restricted to a gouge zone 5 to 20 cm thick at the fault contact. The fault gauge is injected into dolomite cracks of upper layer, which contains the largest percentage of fragments of less than 25 micrometers (Brock & Engelder, 1977). Either the injection promotes further cataclastic deformation, or this flow process acts as a barrier against coarser fractions. The quartz gauge strongly reduces frictional resistance, promoting strike-slip motion. The presence of widely distributed band populations and of compactional bands in the porous sandstone indicate the contractional nature of the deformation (Fossen, 2015) . | Figure 4. Principal stresses acting on the Aztec Sandstone (Fossen, 2015) |
The thrusting is related to the Cretaceous to early Paleogene Sevier orogeny of the North American Cordilleran thrust system. In the early stages of the Muddy Mountains development, contraction was occurring at shallow burial depths, causing compactional bands (see fig 4): shear-enhanced compaction bands (SECB) and pure compaction bands (PCB) in the porous parts of the sandstone. Later, thinner and more offset cataclastic shear bands (CSB) would reflect a higher degree of cataclasis associated with shearing in a less porous medium, where the shear stress is generated by friction on the fault (Fossen 2015). The presence of SECBs and PCBs supports the idea that low differential stress is related to this contractional regime. Experimentally, it has been shown that locally the deformation is lithologically controlled (i.e. grain size and porosity), whereas the stress path depends on the overburdern; it is the contraction (the compaction) that results in well distributed bands in the Aztec Sandstone (Soliva et al., 2013). Thrusting occurred slowly and was near-surface.
Works cited
Brock, William G., and Terry Engelder. "Deformation associated with the movement of the Muddy Mountain overthrust in the Buffington window, southeastern Nevada." Geological Society of America Bulletin 88.11 (1977): 1667-1677.
Fossen, H., Zuluaga, L.F., Ballas, G., Soliva, R., Rotevatn, A., Contractional
deformation of porous sandstone: insights from the Aztec Sandstone, SE Nevada, USA, Journal of Structural Geology (2015), doi: 10.1016/j.jsg.2015.02.014.
Soliva, R., Schultz, R.A., Ballas, G., Taboada, A., Wibberley, C., Saillet, E., Benedicto, A., 2013. A model of strain localization in porous sandstone as a function of tectonic setting, burial and material properties; new insight from Provence (southern France). Journal of Structural Geology 49, 50-63.
Brock, William G., and Terry Engelder. "Deformation associated with the movement of the Muddy Mountain overthrust in the Buffington window, southeastern Nevada." Geological Society of America Bulletin 88.11 (1977): 1667-1677.
Fossen, H., Zuluaga, L.F., Ballas, G., Soliva, R., Rotevatn, A., Contractional
deformation of porous sandstone: insights from the Aztec Sandstone, SE Nevada, USA, Journal of Structural Geology (2015), doi: 10.1016/j.jsg.2015.02.014.
Soliva, R., Schultz, R.A., Ballas, G., Taboada, A., Wibberley, C., Saillet, E., Benedicto, A., 2013. A model of strain localization in porous sandstone as a function of tectonic setting, burial and material properties; new insight from Provence (southern France). Journal of Structural Geology 49, 50-63.