Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1 ABSTRACT The northern Snake Range metamorphic core complex has been studied in depth and numerous competing models have been proposed for its formation and evolution (e.g., Wernicke, 1981; Miller and Gans, 1983; Lee, 1995). Most of these models have utilized relations in lower plate rocks beneath the northern Snake Range decollement rather than an in depth analysis of upper plate stratigraphy and faulting, which provides important additional insights into how low-angle detachment faults form. This study utilizes previously unpublished geologic mapping data from the Sacramento Pass and Sixmile Canyon Quadrangles, an area of continuous exposure of upper plate geology, preserved in part due to a younger down-to-the-west normal fault. Map relations indicate at least two earlier generations of originally down-to-the-southeast normal faults that repeat section and together tilt bedding 30 to 70 degrees to the northwest. Younger second generation faults are spaced at least 1 km apart, dip between 20 and 30 degrees to the E to SE, and display between 1 and 4 km of offset. These faults cut a closely spaced and complex set of earlier first generation faults that currently dip 10 to 40 degrees to the W to NW, frequently splay off each other, and display maximum total offsets between 3.5 and 6 km. A third class of previously unidentified faults with apparent strike-slip offset, strike NW - SE and serve as accommodation faults between the highly extended northern Snake Range and the less extended southern Snake Range. These new maps and additional data highlight several problems and inconsistencies with earlier simplified models for upper plate faulting in the northern Snake Range that described two generations of rotating domino-style faults. Most notably, the amount of rotation required to rotate faults to their present positions is not consistently observed in the dips of the rock units involved in faulting, and the relative amount of Cambrian Pole Canyon Limestone at the base of the upper plate increases from west to east. The additional mapping and data suggest a new model for the evolution of the northern Snake Range decollement that follows the rolling hinge model for the development of metamorphic core complexes described by Buck (1988) and Wernicke and Axen (1988). In this new model, first generation faults represent a complex series of faults and fault splays related to an originally high angle master fault that soled listrically into a brittle ductile transition at depth. Increased offset and the isostatically induced 2 back-rotation of upper portions of the master fault lead to the formation of a series of second generation faults that soled into steeper portions of the master fault and allowed continued uplift of lower plate rocks along the master fault. This model explains the increase in abundance of Cambrian Pole Canyon Limestone from west to east, and is compatible with the tilts observed on fault planes and in stratigraphic units. INTRODUCTION Over the past twenty years, the term metamorphic core complex has been used to describe uplifted and often domed exposures of metamorphic rocks that occur in highly extended terranes. In the North American Cordillera, metamorphic core complexes occur in regions that have experienced previous crustal thickening, such as behind the Sevier thrust belt, and form a series of isolated metamorphic uplifts that extend in a belt from Mexico to Canada (Fig. 1) (Coney and Harms, 1984). Metamorphic core complexes are characterized by a subhorizontal to gently dipping or domed detachment fault that separates unmetamorphosed and complexly faulted upper plate rocks that have undergone brittle deformation from metamorphosed lower plate rocks that have deformed ductilely during extension. Lower plate rocks, which range from slightly metamorphosed breccias and protomylonites through mylonites and metamorphic tectonites, form by grain size reduction and plastic flow of minerals such as quartz which begins to deform ductilely at temperatures above about 300°C. The structural history and evolution of metamorphic core complexes present many geologic and kinematic problems. To preserve mylonitic fabrics in the lower plate, the rocks must cool quickly as deformation continues, indicating that uplift and deformation are closely linked processes. However, the mechanisms by which metamorphic rocks from the ductile crust are uplifted, brought to the surface and placed in direct contact with brittlely fauled rocks are not clear. In addition, rock fracture theory and modern earthquakes in extensional tectonic settings indicate that motion along low-angle faults like the detachment faults of metamorphic 3 Snake Range 0 300 kilometers Figure 1: Distribution of the major metamorphic core complexes in the Western Cordillera with respect to the frontal trace of the Sevier Thrust Belt (black barbs) and the Laramide uplifts (open barbs) (from Coney, 1979). 4 core complexes is not only mechanically difficult and unlikely, but also rarely observed in seismically active regions of contemporary extension (Jackson and White, 1989). As more metamorphic core complexes are being described in the literature, these problems have been addressed with several different kinematic models (Wernicke, 1981; Miller and Gans, 1983; Buck, 1988; Wernicke and Axen, 1988; Brun, 1994; Miller et al., 1999c). The northern Snake Range, located in east-central Nevada in the Basin and Range province and in the hinterland of the Sevier thrust belt (Fig. 2), is classic example of a metamorphic core complex. Here, upper and lower plate rocks are separated by a gently domed detachment, referred to as the northern Snake Range decollement (NSRD) (Misch, 1960) (Fig. 3-4). Through the years, different researchers have studied the northern Snake Range and used its geologic relationships to support various models for the formation of metamorphic core complexes. Wernike (1981) originally proposed that the NSRD was an example of a low-angle normal fault with up to 60 km of displacement. Miller et al. (1983) noted the striking lack of stratigraphic omission across the detachment, and suggested that the SRD was an exhumed brittle-ductile transition zone. Buck (1988) and Wernike and Axen (1988) developed the concept of the rolling hinge model for normal faults, in which detachment faults originate as listric high-angle normal faults that rotate to horizontal through time as they are uplifted to the surface of the earth. Lee (1995) suggested that the lower plate cooling history of the NSRD was compatible with the rolling hinge model. Similarities between the northern Snake Range and sand and silicone analogue models suggest metamorphic core complexes could be underlain by regions of locally hot and possibly rising material as noted by noted by Brun (1994). Miller et al. (1999c) suggested that the NSRD represents the top of a rising and extending mass or welt of heated crustal rocks. Most of these ideas build on the deformational history and strain geometry of lower plate rocks, placing less emphasis on relationships in 5 6 114 00' 114 15' 39 45' 30 Deep Creek Range Kern Mtns 113 45' Range Snake Valley n usio Con f 39 00' ? Southern 39 15' Sacramento Pass Transverse Zone Snake Range 39 30' ? Snake Range Kern Mountains Transverse Zone Northern N. Spring Valley ? Pleasant Valley Transverse Zone Figure 3: Snake Range Decollement Fault System 0 Footwall Precambrian-Paleozoic strata and Mesozoic plutons 25 kilometers Extent of Tertiary ductile deformation (dashes show stretching lineations) Hanging Wall Tilted Tertiary fanglomerates Upper Paleozoic + Tertiary strata Paleozoic miogeoclinal units Interpreted older portions of Snake Range decollement Younger normal fault systems and interperted younger portions of Snake Range decooement Mesozoic thrust faults Folds 50 7 Northern Snake Range Schell Creek Range pC-C 0 Schell Creek Fault Spring Valley Confusion Range Synclinorium Snake Range Decollement Snake Valley M-TR C-D pC-C 5 pC pC-C 10 5 pC-C Km 0 10 20 0 No vertical exaggeration Figure 4: Cross-section through Snake Valley, the northern Snake Range, Spring Valley, and the Schell Creek Range showing the northern Snake Range decollement cut by the Schell Creek Fault and extending beneath the Snake Valley and the Confusion Range Synclinorium (from Miller and Dumitru, 1999). 10 8 the upper plate that might provide significant insight into the development of the NSRD and metamorphic core complexes. The northern Snake Range has perhaps the best exposures of upper plate rocks of any core complex, and these rocks are characterized by a straightforward and easily mappable stratigraphy. Thus the northern Snake Range is an ideal location for a more in-depth analysis of upper plate faulting and its relation to the evolutionary history of metamorphic core complex detachment faults. The geology of most of the northern Snake Range has been mapped and published as a series of seven 1:24,000 quadrangle maps (Miller et al., 1999a; 1999b; Gans et al., 1999a; 1999b; Lee et al., 1999a; 1999b; 1999c) (see Plate I, Index map). This paper builds on this previous mapping and includes new geologic data and maps covering the Sacramento Pass Quadrangle and the southern part of the Sixmile Canyon Quadrangle (Plate I). New geologic mapping by Elizabeth Miller, Cynthia Martinez and myself was carried out on 1:24,000 scale topographic maps and airphotos, and this work in progress will be published as USGS quadrangle geologic maps. The project was partially funded by the EDMAP program within the USGS. The region covered by the new mapping spans the geologically poorly understood transition zone between the highly extended northern Snake Range and the less extended southern Snake Range. Good preservation of upper plate units and their fault relations in the Sacramento Pass region are related to a young west-dipping normal fault that cuts the NSRD and downdrops this region with respect to the rest of the range (Fig. 5). In an attempt to better understand the geometry of normal faults, two small areas were mapped in greater detail at 1:12,000 scale (Plate I). In addition, this study identifies a series of NW - SE striking faults and fault zones with apparent strike-slip offset that help accommodate differential strain between the northern and southern Snake Ranges. The new data underscore several major problems and inconsistencies with regard to earlier models for upper plate faulting, and have lead to the development of a new model. This model is compatible with the rolling hinge model for 9 Tertiary and Quarternary Basin Fill Cambrian to Permian Upper Plate Rocks Precambrian to Cambrian Lower Plate Rocks Young down-to-the-west fault Other major fault sets Northern Snake Range Decollement 0 1 Miles Figure 5: Relative location and outcrop pattern of the young down-to-the-west fault. 2 10 the formation of metamorphic core complexes, and suggests that similar processes may have played a major role in the development of the NSRD. GEOLOGIC SETTING The stratigraphy of the northern Snake Range and adjacent ranges is characterized by a relatively uniform15-km-thick miogeoclinal sequence of sediments that ranges in age and lithology from upper Precambrian siliciclastic rocks to a Cambrian through Triassic section that is dominated by carbonate rocks with lesser shale and quartzite. Detailed descriptions of the lithologies found in the study area are taken from reports completed by the Stanford Geological Survey and from descriptions in the published geologic maps of adjacent quadrangles (Miller et al., 1999a; 1999b; Lee et al., 1999c; Gans and Miller, 1983). These descriptions along with a stratigraphic column are provided in the Appendix to this thesis. During the late Mesozoic, several episodes of compression and crustal thickening affected the region and were coeval with the emplacement of plutons in the Jurassic and Late Cretaceous (Miller et al., 1989). Peak metamorphism and deformation of lower plate rocks occurred in the Late Cretaceous and was accompanied by the intrusion of crustally derived, two-mica granite plutons, metamorphic assemblages up through the staurolite zone and the regional development of penetrative west-dipping cleavages (Miller et al., 1989). The orientation of the cleavage and the presence of the plutons suggest that the crust in this region was ductilely thickened during east-vergent simple shear, a deep-seated equivalent of supracrustal deformation related to the Sevier thrust belt (Miller et al., 1989). Unlike the Sevier thrust belt, however, the lack of significant local or regional angular unconformities between Paleozoic and lower Tertiary rocks, as well as the ability to reconstruct the entire stratigraphic section within late Cenozoic 11 normal fault blocks, indicates that the upper crustal rocks in this region exhibit only broad folds and small displacement along late Mesozoic thrust faults (Miller et al., 1989). In the northern Snake Range, most of the Mesozoic metamorphic fabrics in lower plate rocks are overprinted by Cenozoic metamorphism and deformation related to extension and deformation along the NSRD. The NSRD forms part of a more extensive fault system that extends about 150 km along strike from the Deep Creek Range in the north through the northern and southern Snake Ranges, and has played a major role in the extensional history of the area (Fig. 3) (Miller et al., 1999c). Of the three ranges, the northern Snake Range is the most highly deformed with estimates of extension up to 330 - 500% (Gans and Miller, 1983) and is the only range that exposes vast tracts of highly mylonitized rocks in a lower plate or footwall position with respect to the NSRD. The NSRD is also inferred to extend underneath the Snake Valley to the east and die out underneath the Confusion Range, and it is seismically imaged beneath Spring Valley to the west where it is cut by the Schell Creek Fault (Fig. 4, Plate I: Index map) (Gans et al., 1985). Two major extensional events in this region are recorded in the uplift and cooling history of footwall rocks in the northern and southern Snake Ranges, and can be separated from each other based on geologic relations, using 40Ar/39Ar dating techniques and fission track analyses of apatites and zircons (Miller et al., 1999c). The first event began in the late Eocene and continued through the early Oligocene, and is characterized by a gradual migration of fault activity from west to east (Lee, 1995). The second event occurred much more quickly at about 17 Ma, and is responsible for at least 12 –15 km of rapid slip along the NSRD on the eastern front of the northern Snake Range (Miller et al., 1999c). This second event is coeval with continued arching and doming of the western portion of the NSRD as a result of reverse drag along the Schell Creek Fault, as well as by down-to-the-west faulting and folding that uplifted the western front of the northern Snake Range with respect to the adjacent valleys (Miller et al., 1999). 12 In the southern half of the northern Snake Range, stratigraphic units in the lower plate of the NSRD are mostly Precambrian through Cambrian siliciclastic rocks and the overlying Cambrian Pioche Shale with occasional scraps of marble belonging to the Cambrian Pole Canyon Limestone which overlies the Cambrian Pioche Shale. In the Old Mans Canyon Quadrangle, large portions of the lower plate are comprised of mylonitized Jurassic Silver Creek Granite and the Jurassic Old Mans Canyon Granite (Plate I) (Miller et al., 1999a). All of these units exhibit layering or foliation that is parallel to the NSRD, but they have been severely attenuated, and are increasingly deformed and thinned from west to east across the range (Plate I, II A) (Lee et al., 1999c; Miller et al., 1999a; Lee et al., 1987). Lineations associated with the metamorphic foliation in the mylonites of the lower plate indicate that the principle stretching direction in the northern Snake Range was N60W with evidence for a top to the east sense of shear increasing and becoming more conspicuous toward the east (Miller and Gans, 1983). On average, stratigraphic rock units in the lower plate have been reduced to one third of their original thickness, which yields an average total extension of 330% upon restoration of these lower plate rocks to their original thicknesses (Gans and Miller, 1983). Extensional deformation in the upper plate of the NSRD is characterized by multiple generations of highly complex NE-SW to N-S trending normal faults (Plate I). These faults cut and deform Paleozoic rocks down through the Cambrian Pole Canyon Limestone, but do not cut the Cambrian Pioche Shale or any of the Precambrian to Cambrian siliciclastics below it. Although all stratigraphic units can be found throughout the northern Snake Range, exposures of lower Paleozoic strata and especially the Cambrian Pole Canyon Limestone become distinctly more abundant moving from west to east across the range. As the different sets of faults evolved, they rotated and tilted the upper plate Paleozoic strata to their current orientations dipping between 0° and 90°to the W to NW (Gans and Miller, 1983). The oldest set of faults are closely spaced and 13 presumably represent originally down-to-the-east normal faults that have since been rotated into their current orientations dipping 10° to 30° to the W to NW (Gans and Miller, 1983). More widely spaced younger faults, dipping 20° to 30° to the east and abruptly soling into the NSRD, displace earlier generations of faults and upper plate rock units down to the east (Gans and Miller, 1983). Various methods for calculating total extension due to faulting in the upper plate of the NSRD have yielded estimates of between 450 and 500 % extension of upper plate rocks in the northern Snake Range (Gans and Miller, 1983). GEOLOGY OF THE SACRAMENTO PASS AND SIXMILE CANYON QUADRANGLES The Sacramento Pass and Sixmile Canyon Quadrangles are located on the southwestern flank of the northern Snake Range (Plate I). Highway 50 passes through the southern part of the Sacramento Pass Quadrangle, and the area is easily accessible by way of various Forest Service roads leading from Highway 50. The westernmost exposures of the Tertiary Sacramento Pass Basin succession are exposed in the southeastern corner of the Sacramento Pass Quadrangle. Here, Middle Miocene and younger conglomerate, lacustrine deposits and rare volcanic rocks lie disconformably upon upper Paleozoic rocks. This relationship is best exposed along Weaver Creek in the southern part of the Old Mans Canyon Quadrangle (Plate I). The Tertiary section also includes large breccia sheets composed of various Paleozoic lithologies that have been interpreted as landslide deposits shed into the basin from the surrounding ranges (Martinez, 1998; Grier, 1984). The Tertiary section has been tilted 30° to 60° to the northwest and repeated along a series of rotated low-angle normal faults that sole into the NSRD and can account for at least 200% extension of the basin (Miller, 1999a). 14 As topography rises to the north from the low-lying region underlain by sediments of the Sacramento Pass Basin, the resistant upper plate Paleozoic carbonates and shales form a series of rugged ridges that characterize most of the study area. In part, the spectacular preservation of upper plate rocks and their fault relations in this area are due to a west-dipping, high-angle normal fault that extends along the east side of the Sacramento Pass Quadrangle and into the Old Mans Canyon Quadrangle (Fig. 5, Plate I). This fault could have been localized along the western boundary of the large Jurassic granitic pluton that might have deformed more rigidly than the surrounding Precambrian to Cambrian quartzite country rocks. Offset along this fault gradually dies out along strike to the northeast. This fault cuts and offsets the NSRD, displacing upper plate rocks at least 300 meters down to the west (Plate II A, B, C2, D), and as such, is a fault related to the final doming and arching of the NSRD and the uplift and formation of the present mountain range. Locally, the NSRD underwent normal drag associated with this fault (Plate II A), and the rock units in the upper and lower plates may have experienced significant amounts of normal drag near the fault. In addition to this young, relatively minor, down-to-the-west fault, upper plate rocks in the Sacramento Pass and Sixmile Canyon Quadrangles are cut by at least two older generations of top to the southeast normal faults that repeat section and together tilt bedding 30° to 70° to the northwest (Plate I, Figs. 6, 7). Younger second generation faults are spaced at least 1 km apart, dip between 20° and 30° to the E to SE, and display between 1 and 4 km of offset (Fig. 6). These faults cut a closely spaced and complex set of earlier first generation faults that currently dip 10° to 40° to the W to NW, frequently splay off each other, and display maximum total offsets between 3.5 and 6 km (Fig. 7). A third class of faults within the study area strikes E–W to SE–NW and display apparent strike slip-offset (Fig. 8). In the following paragraphs, I will discuss the nature and style 15 Tertiary and Quarternary Basin Fill Cambrian to Permian Upper Plate Rocks Precambrian to Cambrian Lower Plate Rocks Second generation faults Other major fault sets Northern Snake Range Decollement 0 1 Miles Figure 6: Distibution and map pattern of second generation faults. 2 16 Tertiary and Quarternary Basin Fill Cambrian to Permian Upper Plate Rocks Precambrian to Cambrian Lower Plate Rocks First generatinon Faults Other major fault sets Northern Snake Range Decollement 0 1 Miles Figure 7 boxes. 2 17 Tertiary and Quarternary Basin Fill Cambrian to Permian Upper Plate Rocks Precambrian to Cambrian Lower Plate Rocks Accommodation faults Other major fault sets Northern Snake Range Decollement 0 1 Miles Figure sets in the upper plate of the northern Snake Range. 2 18 of these three different sets of faults within the upper plate of the NSRD, beginning with the second generation faults because they are the best exposed and most well understood. The geometry of second generation faults are well constrained by geologic mapping and are the most immediately obvious faults in cross-sections (Plate II A, B, C1, C2, D). These faults displace upper plate rocks down to the east, and repeat section as well as cut and offset first generation faults. The fault traces of the second generation faults show that they are relatively planar faults, and vary in length from 2 km to faults that span the entire study area, a distance of at least 11 km (Fig. 6, Plate I). Second generation faults are relatively widely spaced with at least 1 km between faults, and branches and splays are not common and are only observed in several locations (Fig. 6, Plate I). Exposures of second generation faults along Sixmile Canyon (Plate I) indicate that at depth, second generation faults sole abruptly into the NSRD, and truncated first generation faults are often placed in direct contact with the NSRD (Plate II A). This relationship suggests that slip along second generation faults was coeval with and related to an additional component of slip along the NSRD. Because these faults flatten into the subhorizontal NSRD, previous workers have interpreted these faults as originally high angle normal faults that rotated through time to their present shallowly dipping orientations as offsets along the faults grew (Gans and Miller, 1983). This interpretation is consistent with the orientation of earlier sets of normal faults that have been rotated through horizontal and the maximum dips of bedding, both of which can be achieved through extensive rotation along second generation faults. However, the relatively regular spacing of second generation faults, the abrupt intersections between the second generation faults and the NSRD, as well as evidence for simultaneous slip along second generation faults and the NSRD suggest that second generation faults within the study area may have been a series of faults that formed sequentially from west to east in a similar fashion to the fault slices predicted in the upper plate of the rolling hinge model. 19 In this preferred interpretation, originally high-angle second generation faults soled into a temporarily inclined NSRD, and rotated to their current shallow dips as a result of a negative isostatic load applied with increased offset and uplift along the NSRD (Plate II Inset: Fig. 16B, C). This interpretation is also consistent with the observed orientations of upper plate strata and earlier fault generations, and is described in more detail in subsequent discussions. The map pattern of first generation faults is very complex and is hard to generalize, but many important observations can still be made. Although first generation faults dip shallowly to the northwest, because bedding dips more steeply to the northwest than the faults, it is inferred after Gans (1983) that first generation faults were originally down-to-the-southeast normal faults that were subsequently cut and rotated through horizontal by younger faults. The first generation faults are very closely spaced, sometimes only tens of meters apart, and along the westernmost ridge in the study area, they intersect and splay off each other along strike (Fig. 7, Plate I, II A, B, C1, C2, D). Although individual faults cannot always be followed along strike, groups of first generation faults can be followed across the study area along strike and can be roughly correlated with each other across second generation faults. The fact that first generation faults do not match perfectly across the second generation faults suggests that their geometry with abundant splays is complex along strike, and also that they may have remained active even after they were cut by the younger set of faults (Gans and Miller, 1983). The first generation faults are approximately planar and display fault to bedding angles between 10° and 30° (Plate II). These low fault to bedding angles are at least partially the result of intense normal drag that rotated bedding between the closely spaced faults into near parallelism with the faults (Gans and Miller, 1983), but these relationships might also indicate that the first generation faults could have cut through upper plate strata at angles significantly less than 60°, especially in lower parts of the 20 stratigraphic section. The interaction between the first generation faults, as well as the ambiguity with respect to original fault to bedding angles, makes it difficult to estimate offset along the first generation faults. However, along cross section B-B’ (Plate II B) several first generation faults intersect placing Permian Ely Limestone on Cambrian Notch Peak Limestone, and bracket the maximum total offset along first generation faults in this part of the study area between 6 km and 3.5 km assuming 30° and 60° fault to bedding angles, respectively. First generation faults are best exposed in the southern part of the Sixmile Canyon Quadrangle along cross-section A - A’ (Plate I, II A). Here, first generation faults can be followed from the western side of the Sixmile Canyon Quadrangle and into the northwest corner of the Old Mans Canyon Quadrangle where they lie beneath an intact section of upper plate rocks at least 3 km thick that spans the Upper Ordovician to the Permian (Plate I, II A). This wide stretch of unfaulted section suggests that the closely spaced first generation faults may not have been penetrative throughout the section in the northern Snake Range, and may represent one, or a series of widely spaced fault zones or fault splays. These faults might have operated as distributed zones of shearing in the lower part of the stratigraphic section, above and along the evolving NSRD. At lower structural levels in certain locations, first generation faults are more difficult to map in the thick Cambrian section. Previous mapping showed some of these faults to bound units whose dips indicated anomalously thick stratigraphic sections. In an attempt to get a better understanding of this deformation, two different locations were mapped in detail. The first location, just to the south of Miller Basin (Fig. 7-9), has apparently been affected by a left-lateral, dip-slip accommodation fault that extends along the northeast corner of the area (see discussion on accommodation faults below), and as a consequence, stratigraphic contacts and strikes and dips of bedding have been bent and folded from their more regional orientations. Because similar faulting patterns are 21 Op Op OS Dg 6 24 34 34 Opd 34 G' Qol 38 26 34 32 Oe Qal 37 20 38 32 OS Op 42 28 37 32 Opk 39 36 32 33 42 44 Opb 31 Oe Cn 31 Op 40 26 27 38 31 31 36 Opc 40 26 Opd 22 26 34 44 39 Opd 8 15 Opc 16 65 61 49 24 29 49 24 Opl 46 Cn 40 Opk Opa 45 29 45 25 46 Cl 47 Opb 35 60 OS42 23 24 H 41 40 24 17 40 22 Opd Opl 25 Oe 22 32 51 34 48 27 42 45 33 27 26 32 35 28 22 OS 22 17 64 Oe Cn 39 22 G Contour Interval 20 meters 0 Meters H' Cl Opa Opb10 34 48 34 19 20 Cpc 45 33 25 Oe 44 50 33 42 24 Opc 43 29 16 29 30 Oe Cl 31 49 24 32 34 41 41 35 23 41 35 46 Oe 40 36 51 42 Dse 32 38 Opb 28 41 45 45 Opk Opl Cl 39 50 34 52 44 36 32 Cn 11 34 Opa Opl Oe 42 49 500 1000 Figure 9: Detailed mapping at 1:12,000 scale near Miller Basin in the Sacramento Pass Quadrangle. Here, stratigraphic contacts are preserved and repeated perpendicular to, and within a series of gently west dipping first generation faults. An accommodation fault with left-lateral downto-the-southwest sense of displacement is mapped in the northeast corner of this study area. For location, map explanation and rock descriptions, refer to Plate I and the Appendix. 22 observed in other locations in both quadrangles, it can be assumed that the original structural relationships have been preserved despite this younger rotation. Here, a series of rotated, top-to-the-east normal faults dip 10° to 40° to the west, and repeat intact section that strikes roughly east-west within the fault slivers. As these faults rotated, a series of faults with considerably less offset were also developed in the shaley Ordovician Pogonip Group and Cambrian Lincoln Peak Formation lithologies stratigraphically above and below the more resistant Cambrian Notch Peak Limestone (Fig. 9-10). These smaller faults helped to alleviate space problems that were created on either side of the more rigid Cambrian Notch Peak Limestone as offset along the larger faults grew. Similar styles of faulting are observed along the southern side of Sixmile Creek (Fig. 7, 11). In this area, the structurally lowest first generation fault dips 25° to the NW, and exposes an intact section of SW-dipping upper Cambrian Notch Peak Limestone and Ordovician Pogonip Group that has been downdropped onto stratigraphically lower sections of the Cambrian Notch Peak Limestone (Fig. 11-12). Unlike the area near Miller Basin, smaller displacement faults are not observed within the Op or Cl. However, a series of faults with offsets on the scale of meters are well developed within more resistant beds in the Cambrian Lincoln Peak Formation and suggest that space problems in this area may have been accommodated through a suite of unmappable microfaults on the meter scale or smaller. Immediately above this fault there are least three more closely spaced first generation faults as documented by klippe of Ordovician Eureka Quartzite and Ordovician Silurian Dolomite above the Ordovician Pogonip Group (Fig. 11-12). Two tight folds in the Ordovician Pogonip Group display fold axes that strike 199° and 232° (Fig. 13A, B), roughly parallel to the strike of the first generation faults. This deformation suggests that original bedding relationships may not be well preserved as a result of shear and normal drag operating between the closely spaced first generation 23 G G' Opl Oe Oe Opl Opl 9000 Opk Opd Opb Opd Opk Opc Opb Opa Cn Opk Opl Opd Opc Opb Opl Opc Opa 9000 Opk Opb Opa Opd Cn Cn Cn Qol Dg 8000 8000 Cl ? Cl Opk Opd 9000 Oe OS Dse Opl Opk Opd 8000 H' Opl H Opc 9000 Opl Opk Opd Opa Opb Cn Opc 8000 Cl Cl No vertical exaggeration. 0 1000 Feet Figure 10: Vertical cross-sections through the Miller Basin detailed study area in the Sacramento Pass Quadrangle. 2000 24 Cn Cpi Cpc 7 12 Cpc 10 28 7 6 28 22 Op 28 Qal 22 8 10 18 16 15 Cn Oe Os Cpi 30 25 Cpi 21 Opa Cl 32 51 26 Oe Dsi 13 27 46 28 21 21 17 25 Cpc Cpc 13 16 57 17 30 Opb 40 J 29 20 40 54 Os Mc Dg 24 18 54 56 22 54 26 Oe 17 14 69 76 10 36 39 35 Os 17 16 40 64 Dse 42 28 33 51 Os Opk 34 31 43 Opl Opk 12 36 25 70 59 15 39 16 30 49 Os Cl 26 49 25 31 19 61 35 Dse 41 29 Oe 24 26 35 45 Opk Opd 68 64 24 19 55 35 Figure 11: Detailed mapping at 1:12,000 scale near Sixmile Canyon in the Sixmile Canyon Quadrangle. Here, stratigraphic and fault contacts between the Cambrian Notch Peak and the Ordovician Pogonip Group are observed beneath a major set of first generation faults. Mylonitized lower plate rocks outcrop in Sixmile Canyon in the northern section of this study area. For location, map explanation and rock descriptions, refer to Plate I and the Appendix. 43 Opc 46 27 Qol 27 19 24 Opl 15 Os Cn Oe 44 30 Oe 20 Pe 42 39 27 46 28 Cn Dse Mc 44 Op 25 Contour Interval 20 meters 0 Meters 500 Mj 1000 J' 25 J J' Oe 9000 Mj Pe Mc Opl MDp Cn Cl Opd Cpc Oe Mc Dg 9000 Os NSRD Opk Opl 7000 Cpi 7000 Cpi Cpm Cpm No vertical exaggeration. 0 1000 Feet Figrue12: Vertical cross-section through the Sixmile Canyon detailed study area in the Sixmile Canyon Quadrangle. 2000 26 A. B. F.A. = 232, 38 SW F.A. = 200, 16 SW C. D. Figure 13: Structural data from the study area plotted on equal area stereonets. A - Broad fold in Ordocician Pogonip Group, Unit D. B - Tight fold in Ordovician Poganip Group, Lehman Formation. C - Poles to to fault plane surfaces preserved in jasperoid along an accommodation fault zone. D - Trend and plunge of lineations on fault surfaces. 27 faults. Although the exact nature of the first generation faults is still not fully understood, this more detailed mapping emphasizes the complexity of their geometry. First and second generation faults are both affected by a third set of faults that strike NW – SE and show apparent strike-slip sense of displacement. These faults are most prominent in map view (Fig. 8, Plate I), and are described here for the first time. Apparently serving as accommodation faults that separate areas of the upper plate that have experienced somewhat different amounts of extension, these accommodation faults are observed on a variety of scales and occur where first generation normal faults end abruptly or splay in slightly different directions (Fig. 8, Plate I). Stratigraphic units and structures observed near these accommodation faults are often bent and dragged into parallelism with the accommodation fault zones (Plate I). Because first generation faults cannot be easily restored or traced across them (Plate II E), these accommodation faults could have been synkinematic with the first generation faults, and some of them appear to have remained active and also provided boundaries for the termination of second generation faults. Two of the most conspicuous accommodation faults are observed in the middle of the Sacramento Pass Quadrangle between cross-section lines B-B’ and D – D’. Both faults juxtapose rock units in such a way as to suggest apparent left-lateral offset and down-to-the-SW sense of slip (Plates I, II E). This type of offset is best observed along the northernmost of these two faults. Here, first generation faults along B-B’ place Permian rocks against the middle Cambrian section, while more widely spaced fault splays along C-C’ to the south place Permian strata against Devonian strata indicating at least 2 km less offset. As this fault system evolved, second generation faults also splayed or branched off the accommodation fault to the south, and the entire system was gradually rotated and reoriented to show the apparent left-lateral sense of displacement that is presently observed (Plate I). The southernmost of these two accommodation faults is characterized by massive exposures of red jasperoid that occur 28 along the length of the fault (Plate I). Lineations on small fault surfaces in the resistant jasperoid are varied, but generally trend to the NE and SW with relatively high plunge values, and suggest that the sense of shear in this accommodation fault zone was normal and down-to-the-SW (Fig 13C, D). Another major accommodation zone striking NW – SE that indicates left-lateral and down-to-the-south sense of offset is exposed on the east side of the Sacramento Pass Quadrangle (Plate I). To the east, this fault extends across the southern edge of the Old Mans Canyon quadrangle where it serves as a major strike-slip fault juxtaposing upper plate and lower plate rocks. Here, this accommodation fault acts as the northern boundary of the Sacramento Pass Basin and as the terminus of the major faults that cut the Sacramento Pass Basin and gradually merge into this strike-slip zone (Plate I). The fact that the faults that cut the Sacramento Pass Basin merge with this accommodation fault suggests that the two fault systems were synkinematic or operated together, indicating that at depth, the NSRD may have remained active to the south of the accommodation fault while extension along the NSRD to the north of the accommodation fault had slowed or ceased. As this fault extends into the Sacramento Pass Quadrangle, it juxtaposes mylonitized lower plate rocks to the north against unmylonitized Cambrian Prospect Mountain Quartzite to the south (Plate I). The fact that Cambrian Prospect Mountain Quartzite normally resides in a lower plate position, suggests that this fault may have cut the NSRD and exhibits significant strike-slip offset. Further to the west, offset along this fault decreases and is presumably transferred to the series of accommodation faults discussed above, which suggests that they might have also cut the NSRD. As the region to the west of the younger normal fault is covered by upper plate rocks, the nature and location of the boundary between mylonitized and unmylonitized rocks is not well understood. The southernmost fault belonging to this set of accommodation faults lies concealed beneath Tertiary alluvium along Highway 50 (Plate 29 I). The existence of this fault is inferred by the fact that Cambrian carbonates to the north of Sacramento Pass strike into sections of similarly striking Precambrian siliciclastic rocks to the south of Sacramento Pass. Map scale drag of units as evidenced by rotation of stratigraphic boundaries as well as by rotation of bedding indicate that this fault displays a right-lateral sense of offset (Plate I). Although the kinematic history of these accommodation faults that cut through the study area are not fully understood, the geologic relationships as well as the fact that they are developed parallel to the direction of maximum stretching indicate that they are related to differential amounts of extension associated with the transition zone between the more extended northern Snake Range and the less extended southern Snake Range. KINEMATIC INTERPRETATIONS Previous models for upper plate faulting in the northern Snake Range have simplified the complex arrays of upper plate faults by describing them as two generations of planar, rotating, domino-style faults that soled into a brittle-ductile boundary at depth (Fig. 14, 15) (Miller and Gans, 1983; Gans and Miller, 1983). A closely spaced set of first generation faults originated at high angles (60°) and rotated 30° to 40° before they were cut by a second, more widely spaced set of second generation faults which, after moving, accomplished an additional 30° to 40° of rotation. Together, the two sets of faults produced the dramatic thinning observed in the upper plate, and resulted in total extension values estimated between 450% and 500% (Fig. 14). Because both sets of faults cut stratigraphic section down through, but no deeper than the Cambrian Pole Canyon Limestone and there was no omission of stratigraphic units across the NSRD, researchers supported an in situ stretching or pure shear model for the creation of the NSRD (Miller and Gans, 1983; Gans and Miller, 1983). 30 Figure 14: Schematic models of extension in the northern Snake Range showing bedding to fault relationships as a result of two generations of normal faults above a horizontal NSRD. This model also illustrates the space problems that are created at the bases of these faults as they rotate from high to low angles along a horizontal detachment that must be compensated by splays or smaller scale faults (from Miller and Gans, 1983). 31 Figure 15: Interpretive cross-sections of the evolution of the northern Snake Range utilizing the model for two generations of normal faults in the upper plate, and suggesting an overall in situ stretching or pure shear model for the development of the NSRD (from Gans, 1985). 32 However, this model for faulting in the upper plate presents several major problems. First, although 30° to 40° of rotation between normal fault generations is mechanically possible (Nur et al., 1986), in the Singatse, Wassuck and Egan Ranges of Nevada where this amount of rotation is observed between normal fault sets, the second generation faults are typically younger, steeper, and more widely spaced Basin and Range style faults, while first generation faults have not been rotated through horizontal (Proffet, 1977; Gans and Miller, 1983; Surpless, 1999). Thus, two sets of such closely spaced normal faults as proposed by the model for the northern Snake Range are not commonly observed. Second, because first generation faults within the study area dip between 10° and 40° to the west, the existing model requires that first generation faults must have rotated a total of 70° to 100°, but significantly lesser dips in the rock units involved in faulting suggest that the units may not have been rotated by a comparable amount. This discrepancy was previously explained by high amounts of normal drag along the first generation faults that strained units into parallelism with the closely spaced first generation faults, creating the low angles between faults and bedding that are currently observed. However, even in the best preserved, least faulted and most resistant units of the upper plate as observed in the northwest corner of the Old Mans Canyon Quadrangle, stratigraphic units have only been rotated 60° to 70°. In contrast, in the Singatse and Wassuck Ranges, comparable tilts ≥60° on rock units are achieved, even though first generation faults have not been rotated through horizontal (Proffet, 1977; Surpless, 1999). Third, in order to thin the crustal column and uplift a flat brittle-ductile transition zone, faults within individual fault sets must be closely spaced and synchronous across the width of the uplifted zone (Buck, 1988). In the northern Snake Range, however, thermochronologic and geochronologic data indicate an asymmetric uplift history with fault activity gradually moving from west to east (Lee, 1995). In addition, although first generation faults are closely spaced, there is no direct evidence that they were pervasive 33 over a region as wide as the current exposures of the NSRD. In fact, palinspastic reconstructions of the upper plate of the northern Snake Range indicate that first generation faults originally spanned only 1.7 km horizontally (Miller and Gans, 1983). Fourth, because all upper plate faults are assumed to cut down through the Cambrian Pole Canyon Limestone directly above the brittle-ductile transition zone, this model does not explain the relative increase of Cambrian Pole Canyon Limestone exposures from west to east across the range. Last, rigid fault block rotation above a flat detachment fault creates increasingly significant space problems where the faults contact the detachment (Fig. 14). In the northern Snake Range, space problems would be most prominent at the base of the widely spaced second generation faults, and are accounted for by suggesting that less resistant units were warped and folded, and that first generation faults were reactivated as fault splays off the second generation faults. Although possible, this seems highly unlikely as it requires that first generation faults remain active as normal splays even as they are rotated into thrust position. In addition, neither deformation nor first generation faults are consistently observed in the immediate hanging wall of second generation faults. The additional mapping and data presented in this paper, in conjunction with previously published maps of the northern Snake Range suggest a new model for upper plate faulting and the evolution of the NSRD. This model utilizes and builds on the rolling hinge model of Buck (1988) and Wernicke and Axen (1988) as shown in Fig. 16 (inset in Plate II). This model calls for the origin of the NSRD as a master fault (MF) that formed at high angles to bedding, and which soled in a listric fashion into a zone of decoupling along a subhorizontal brittle-ductile transition (BDT) (Fig. 16 A). As movement along the MF increased, a complex system of related faults and fault splays developed synthetically to it, and soled in a listric fashion into the basal BDT to help accommodate space problems created at depth (Fig. 16 B). Between these closely spaced 34 Plate II Inset: Figure 16: Interpretive cross-sections showing the evolution of the NSRD through time. Scale - no vertical exaggeration (km) Scale - no vertical exaggeration (km) 0 0 8 4 Scale - no vertical exaggeration (km) 0 B. A. MF MF 0 1 MF Pe Dg Dg Cpc 4 Cn Brittle Ductile Cpm Depth (km) Pe Dg Cn Cpc Cn Cpc 8 Scale - no vertical exaggeration (km) 8 4 Brittle Ductile Cpm 8 0 1 234 4 Brittle Ductile Cpm 8 8 0 Pe 4 4 C. 0 Depth (km) Depth (km) 8 4 Scale - no vertical exaggeration (km) 12 0 8 4 12 16 20 E. D. 2 1 MF 0 3456789 0 Pe 4 5 6 7 8 9 MF Pe Dg Zone of backrotation Dg Depth (km) Depth (km) 3 2 1 Listric Normal Fault 4 Cn 4 Cn Brittle Cpc Zone of ductile flow and stretching Cpm Zone of ductile flow and stretching Cpc Ductile Cpm 8 Brittle Ductile 8 Scale - no vertical exaggeration (km) 0 4 8 12 20 16 28 24 36 32 40 44 46 F. 9 Listric Normal Fault 0 1 8 2 3 MF Schell Creek Fault 10 11 12 13 14 15 16 17 18 19 20 21 7 6 Pe 6 Depth (km) Dg 5 4 4 Zone of backrotation Cn Cpc Cpm 8 Brittle NSRD Zone of ductile flow and stretching Brittle Ductile 35 faults, normal drag deformed and bent stratigraphic units to form low fault to bedding angles (Fig. 16 B). Continued extension and thinning of the upper plate to the east of the MF created a negative isostatic load on footwall rocks which lead to the gradual uplift and backrotation of the structurally highest and westernmost portions of the MF (Buck, 1988; Wernicke and Axen, 1988). Eventually, as this section of the fault continued to rotate, it reached a position unfavorable for slip, and the first in a series of younger second generation faults formed and soled into a deeper section of the MF that remained steep (Fig. 16 C). As displacement along this younger fault grew, upward truncations of first generation splays slid down onto the deeper portions of the MF, while the upper section of original MF and its associated faults and splays were tilted in the footwall of the second generation fault, and continued to be rotated toward horizontal (Fig. 16 C, D). It is likely that the lower portions of first generation faults and splays remained active even after they had been cut by the second generation faults and possible that additional first generation splays continued to form fanning out further onto the BDT (Fig. 16 D). Like the original MF, the first second generation fault was gradually back-rotated by negative isostatic load applied to the footwall, and a younger second generation fault was formed at high angles soling into a more steeply inclined portion of the original MF zone (Fig. 16 D). This process repeated itself many times, each time cutting through the first generation faults with slip occurring along new second generation faults, the lower portion of the original MF and along the basal BDT (Fig. 16 D, E, F). This model requires that mylonitized lower plate rocks are brought into contact with upper plate rocks in the footwall of the MF with increased slip along the fault system, and as the MF rotates to horizontal, it is eventually exposed as the NSRD, separating upper and lower plate rocks. In order to conserve rock unit volume in the footwall of the MF or NSRD, rock units near the BDT are thinned to some extent, and a series of listric normal faults are developed that tilt brittle rock units and migrate from west to east (Fig. 16 D, E). The 36 Schell Creek Fault represents the youngest of these listric normal faults as it cuts through the western portion of the NSRD (Fig 16 D, E). DISCUSSION This model for the evolution of the NSRD presents several problems related to the history of upper plate faulting. One major uncertainty lies in the mechanics and timing of the accommodation faults, the affect they had on first and second generation faults, and the role they played between the differentially extending northern and southern Snake Ranges. This is partially the result of poor exposures and a lack of detailed mapping, and as a result, a variety of interpretations for different offsets can be created depending on the timing and on the position of rocks involved with respect to the NSRD. However, it seems likely that major left-lateral down-to-the-south offsets observed throughout the study area were created as sections of the NSRD to the north of the accommodation faults became inactive due to rapid uplift and back-rotation, while sections of the NSRD to the south of the accommodation faults continued to drop upper plate rocks along the MF (Fig. 17). As lower plate rocks were brought to the surface in the footwall of the MF, they cooled, became brittle, and were eventually affected and cut by the accommodation faults as differential extension and uplift continued. This interpretation is consistent with observations in the considerably less extended southern Snake Range that suggest a similar style of extensional deformation along an original major down-to-the-west normal fault and a series of back-rotating second generation faults (McGrew, 1993), and explains the major left-lateral, down-to-the-south offsets observed at the northern boundary of the Sacramento Pass Basin. Another major uncertainty lies in the original coverage of the first generation faults and fault splays, and the extent to which they interacted with and splayed off the MF as well as second generation faults. Again, this uncertainty is related to the lack of 37 Slo w Ex ten sio n Ra pid Ex ten sio n North Figure 17: A cartoon portraying a proposed mechanism for the formation of major accommodation faults in the upper plate of the northern Snake generati the observed accommodation structures (shown in blue). 38 exposure and detailed mapping of upper plate rocks throughout the northern Snake Range. Mapping at 1:12,000 scale revealed the complexity of upper plate faulting and also pointed out the necessity for simplification even at this enlarged scale. These simplifications lead to difficulties when generalizing relations and creating a model for the evolution of the NSRD. At the same time, the model presented above is compatible with the data currently available based on geologic mapping of the upper plate rocks, and successfully addresses some of the problems that arose with earlier models for upper plate faulting. Because this preferred model describes the NSRD as a master high-angle normal fault that cut through upper plate stratigraphic units at angles near 60° and subsequently rotated to subhorizontal orientations, the least deformed upper plate rocks in contact with this fault would be expected to show dips only on the order of 60° to the northwest. The interpretation of first generation faults as a series of lower-angle splays from a master fault is compatible with an originally narrow horizontal zone of first generation faults. Also, second generation faults would not cut first generation faults at lower structural levels until several sets of second generation faults had run their course and significant uplift had occurred, which explains the relative increase in the abundance of Cambrian Pole Canyon Limestone outcrops from west to east across the range. This model is also compatible with the migration of uplift along the NSRD from west to east, and because second generation faults intersect the MF at low angles, they do not rotate with respect to the NSRD, thus eliminating space problems that are developed at the bases of rotating domino faults. CONCLUSION New mapping of excellently exposed upper plate rocks in the Sacramento Pass and Sixmile Canyon Quadrangles has revealed several major problems with the in situ pure shear model for extension that has previously been applied to upper plate rocks in 39 the northern Snake Range. Although more detailed mapping needs to be completed before the mechanics of some of these faults are fully understood, the geologic relations observed in the study area suggest that first generation faults represent a major system of splays related to a MF. A more widely spaced set of second generation faults soled into temporarily steep portions of the MF and formed sequentially as the MF was isostatically uplifted and back-rotated to horizontal. In addition, a set of dip-slip accommodation faults was formed as different sections of the NSRD experienced extension and uplift at different rates. This model for upper plate faulting is compatible with the rolling hinge model for the evolution of metamorphic core complexes. The interpretation of upper plate faults as part of, and the consequence of a rolling hinge system, brings the analysis of upper plate faulting in the northern Snake Range into agreement with related studies on lower plate rocks that suggest the rolling hinge model (Lee, 1995), and is compatible with models for the origin of the southern Snake Range decollement (McGrew, 1993) that also indicate large amounts of isostatically induced vertical uplift. 40 REFERENCES Bartley, J.M., Wernicke, B.P., 1984, The Snake Range decollement interpreted as a major extensional shear zone: Tectonics: v. 3, p. 647-657. Brun, Jean-Pierre, Sokoutis, D., Driessche, J.V.D., 1994, Analogue modeling of detachment fault systems and core complexes: Geology, v. 22, p. 319-322. Buck, W.R., 1988, Flexural rotation of normal faults: Tectonics, v. 7, p. 959-973. Coney, P.J., 1979, Tertiary evolution of Cordilleran metamorphic core complexes, in Armentrout, J.M., Cole, M.R., and TerBest, H., Jr., eds., Cenozoic paleogeography of the western United States: Third Pacific Coast Paleogeography Symposium: Pacific Section, Society of Economic Paleontologists and Mineralogists, Los Angeles, p. 14-28. Coney P.J., Harms, T.A., 1984, Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression: Geology v. 12, p. 550-554. Davis, G.H., 1983, Shear-zone model for the origin of metamorphic core complexes: Geology, v. 11, p. 342-347. Gans, P.B., Miller, E.L., McCarthy, J., Ouldcott, M.L., 1985, Tertiary extensional faulting and evolving ductile–brittle transition zones in the northern Snake Range and vicinity: New insights from seismic data: Geology, v. 13, p. 189-193. Gans, P.B., Miller, E.L., 1983, Style of mid–Tertiary extension in east–central Nevada: in Utah Geological and Mineral Survey Special Studies 59, 1983, Guidebook Part 1 – GSA Rocky Mountain and Cordilleran Sections Meeting, p. 107-144. Gans, P.B., Miller, E.L., Huggins, C.C., Lee, J., 1999a, Geologic map of the Little Horse Canyon Quadrangle, Nevada and Utah, Nevada Bureau of Mines and Geology Field Studies Map 20, (1-24,000), 12 pp. 41 Gans, P.B, Miller, E.L., Lee, J., 1999b, Geologic map of the Spring Mountain Quadrangle, Nevada and Utah, Nevada Bureau of Mines and Geology Field Studies Map 18, ( 1-24,000), 12 pp. Jackson, J.A., White, N.J., 1989, Normal faulting in the upper continental crust: Observations from regions of active extension: Journal of Structural Geology, v. 11, p. 15-36. Lee, J., Miller, E.L., Sutter, J.F., 1987 Ductile strain and metamorphism in an extensional tectonic setting: A case study from the northern Snake Range, Nevada, USA, in Coward, M.P. Dewey, J.F., and Hancock, P.L., eds., Continental Extensional Tectonics: Geological Society of London Special Publication 28, p. 267-298. Lee, J., 1995, Rapid uplift and rotation of mylonitic rocks from beneath a detachment fault: Insights from potassium feldspar Ar/Ar thermochronology, northern Snake Range, Nevada: Tectonics, v. 14, p. 54-77. Lee, J., Gans, P.B., Miller, E.L., 1999a, Geologic map of the Mormon Jack Pass Quadrangle, Nevada, Nevada Bureau of Mines and Geology Field Studies Map 17, (1-24,000), 12 pp. Lee, J., Gans, P.B., Miller, E.L., 1999b, Geologic map of the Third Butte East Quadrangle, Nevada, Nevada Bureau of Mines and Geology Field Studies Map 16, (1-24,000), 12 pp. Lee, J., Miller, E.L., Gans, P.B., Huggins, C.C., 1999c, Geologic map of the Mount Moriah Quadrangle, Nevada, Nevada Bureau of Mines and Geology Field Studies Map 19, (1-24,000), 12 pp. Lister, G.S., Baldwin, S.L., 1993, Plutonism and the origin of metamorphic core complexes: Geology, v. 21, p. 607-610. 42 Lister, G.S., and Davis, G.A., 1989, The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.: Journal of Structural Geology, v. 21, p. 65-94. Martinez, C.M., Miller, E.L., and Stockli, D.F., 1998, Miocene age rock avalanche deposits of the Sacramento Pass Basin, Basin and Range Province, Nevada: Geological Society of America Abstracts with Programs, v. 30, no. 5, p. 53. McGrew, A.J., 1993, The origin and evolution of the southern snake range decollement, east central Nevada: Tectonics, v. 12, p. 21-34. Miller, E.L., Gans, P. B., 1983, The Snake Range decollement: an exhumed mid-Tertiary ductile–brittle transition: Tectonics, v. 2, p. 239-263. Miller, E.L., Gans, P.B., 1989, Cretaceous crustal structure and metamorphism in the hinterland of the Sevier thrust belt, western U.S. Cordillera: Geology, v. 17, p. 5962. Miller, E.L., Gans, P.B., Grier, S.P., Huggins, C.C., Lee, J., 1999a, Geologic map of the Old Mans Canyon Quadrangle, Nevada, Nevada Bureau of Mines and Geology Field Studies Map 21, 12 pp. Miller, E.L., Gans, P.B., 1999b, Geologic map of The Cove Quadrangle, Nevada and Utah, Nevada Bureau of Mines and Geology Field Studies Map 22, 12 pp. Miller, E.L., Dumitru, T.A., Brown, R.W., Gans, P.B.,1999c, Rapid Miocene slip on the Snake Range – Deep Creek Range fault system, east-central Nevada: GSA Bulletin, v. 111, p. 886-905. Misch, P., 1960, Regional structural reconnaissance in central-northeast Nevada and some adjacent areas: Observations and interpretations, in Guidebook to the Geology of East-Central Nevada: International Association of petroleum Geology and Eastern Nevada Geological Society, Eleventh Annual Field Conference, Salt Lake City, Utah, p. 17-42. 43 Nur, A., Scotti, O., 1986, Fault mechanics and the kinematics of block rotations: Geology, v. 14, p. 746-749. Proffet, J.M., Jr., 1977, Cenozoic geology of the Yerington District, Nevada, and implications for the nature and origin of Basin and Range faulting: Geological Society of America Bulletin, v. 88, p. 247-266. Proffet, J.M., Jr., and Diles, J.H., 1984, Geologic map of the Yerington District, Nevada: Reno, Nevada, Nevada Bureau of Mines and Geology, Map 77. Surpless, B., 1999, Tectonic evolution of the northern Sierra Nevada-Basin and Range transition zone: A study of crustal evolution in extensional provinces, PhD Thesis, 186 p. Walker, C.T., Francis, R.D., Dennis, J.G., Lumsden, W.W., 1992, Cenozoic attenuation detachment faulting: A possible control on oil and gas accumulation in EastCentral Nevada: The American Association of Petroleum Geologist Bulletin, v. 76, p. 1665-1686. Wernicke, B. P., Axen, G. J., 1988, On the role of isostacy in the evolution of normal fault systems: Geology, v. 16, p. 848-851. Wernicke, B. P., 1981, Low angle normal faults in the Basin and Range Province: Nappe tectonics in an extending orogen: Nature, v. 291, p. 645-648.