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Late Cenozoic Fault Patterns and Stress Fields in the Great Basin and Westward Displacement of the Sierra Nevada Block

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Late Cenozoic Fault Patterns and Stress Fields in the Great Basin and Westward Displacement of the Sierra Nevada Block Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block Lauren Wright ABSTRACT Department of Geosciences The patterns of late Cenozoic faulting in the Great Basin Pennsylvania State University apparently delimit two deformational fields, each extensional University Park, Pennsylvania 16802 but contrasting in magnitude and style of extension. The field of smaller magnitude, which shows about 10 percent extension, occupies the northern and most of the central part of the Great Basin. It is characterized by steeply dipping normal faults and fault zone and ruptures that delineate the group of crudely gently tilted blocks, with a preferred north to northeast trend. aligned valleys of the well-known Walker Lane (Fig. 1), follow the suggestion of Becker (1934), Carey (1958), and Wise (1963) and Evidence of greater extension occurs in the other defor- favor a "megashear" model in which the right-lateral faults of the mational field, which lies between Walker Lane and the Sierra Great Basin, together with the San Andreas fault zone, compose Nevada and extends across the narrow southern end of the a major part of a large-scale right-lateral shear system involving Great Basin. This field contains most of the complementary much or all of the North American Cordillera. In the context of strike-slip faults (northwest-striking right-lateral and northeast- plate tectonics these faults are commonly represented as involving striking left-lateral faults), long recognized as major components a zone of transform faulting between the Pacific and North of the structural framework. It also contains abundant normal American plates (Atwater, 1970). In the megashear model, the faults, most of which strike north to northeast. In certain areas, normal faults theoretically are second-order features analogous to extension of 50 percent or more is indicated in the association tension-gash fractures. They also are commonly cited as products of an "oblique extension" that combines components of westerly of strike-slip and normal faults and in the palinspastic restora- extension and right-lateral shear (Hamilton and Myers, 1966). tion of fault blocks that have been steeply tilted along gently dipping normal faults. These contrasts in structural pattern and The most enduring interpretation is that the normal faults are first-order features that record a generally westward, non- apparent percentage of extension may be related to westward rotational crustal spreading. This "simple extension" model is movement of the Sierra Nevada block and southward narrow- rooted in the work of G. K. Gilbert and W. M. Davis, whose ob- ing of the Great Basin. servations in the early 1900s (before the strike-slip faults were The faults along which strike-slip displacement occurred recognized) led to the conclusion that the typical basin-range in late Cenozoic time appear to have functioned as conjugate block is bounded by normal faults. Recent proponents of "simple shears, the shears and associated normal faults being first-order extension" (that is, Gilluly, 1970; Scholz and others, 1971; Davis extensional features. The fault pattern also invites a simplistic and Burchfiel, 1973) have stated or implied that the strike-slip interpretation that is based on the orientation of three mutually faults within the Great Basin are relatively unimportant. perpendicular directions of stress. The major normal faults, Still other workers have argued that the complementary nature of the strike-slip faults holds the key to late Cenozoic which strike north-northeast in most parts of the Great Basin, deformation in the Great Basin and have interpreted these faults suggest a pervasive horizontal minimum compressive (maximum as conjugate shears produced in a nonrotational stress field tensional) stress that is oriented west-northwest. Maximum (Allison, 1949; Donath, 1962; Shawe, 1965; Hill and Troxel, 1966). compressive stress would be oriented perpendicularly where the In this model, shearing is generally ascribed to compression along horst and graben structure predominates and horizontally to the a north- to northeast-trending axis, and the normal faults are east-northeast where the strike-slip faults are abundant. viewed as release features striking parallel with the compressional axis. Objections have been raised to each of the three interpreta- tions: principally, (1) the "simple extension" model inadequately accommodates the complementary strike-slip faults (Hill and INTRODUCTION Troxel, 1966); (2) the "megashear" model lacks evidence of right- The patterns of late Cenozoic (post-Oligocene) faulting, which lateral displacement along the proposed northeast margin of the characterize the Great Basin (Fig. 1) of the Basin and Range prov- shear system (Gilluly, 1970) and inadequately accounts for the left- ince, have become increasingly well documented in recent years. lateral faults (implied by Davis and Birchfiel, 1973); and (3) the However, geologists continue to puzzle, as they have done for a compression, apparently implicit in the "conjugate shear" model, century and more, over the relationship that these patterns bear should have produced thrusts oriented at low angles to the free to large-scale crustal motions. Investigators apparently agree that surface of the Earth rather than the strike-slip faults at high the crust of the Great Basin is spreading relatively west to west- angles to the surface (Gilluly, 1963). northwest, causing movement on normal faults, which, although The following discussion explores an alternative interpreta- variously oriented, strike predominantly north-northeast. Opinions tion, shown schematically in Figure 2, that the fault pattern differ, however, concerning the role of two sets of strike-slip faults records a deformational environment featured by markedly con- also active in late Cenozoic time, one right lateral and northwest trasting degrees of extension. This interpretation is supported by striking and the other left lateral and northeast striking. features that, although previously noted, seem to have been under- Most recent workers, impressed with evidence of right-lateral emphasized. These include (1) the restriction of most of the strike- displacement along the northern Death Valley-Furnace Creek slip faults to a generally symmetrical region that bounds the Sierra GEOLOGY, v. 4, p. 489-494 489 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4/8/489/3541379/i0091-7613-4-8-489.pdf?casa_token=BpatOfzDi0QAAAAA:I-cD_Xv5bIrFHXC8qZV_KLgSLVlog3hbNv7HXUpeHE94L-igR4lRplro2SnZbsuFoLXCXpSl by California Geological Survey, 19774 on 20 July 2019 Figure 1. Generalized map of late Cenozoic structural features of Great Basin and bordering regions, showing spatial relationship between Sierra Nevada block, distribution of normal (hachered) and strike-slip faults of Basin and Range province, and location of cross section in Figure 3. Synthesized and modified from Jennings (1973), King (1969), and Stewart and Carlson (1974). Stippled pattern west of Sierra Nevada represents Cenozoic sedimentary units; V-pattern north of Sierra Nevada represents Cenozoic volcanic rocks. Lettered features are as follows: (B) Brothers fault zone, (ED) Eugene-Denio fault zone, (G) Garlock fault zone, (HL) Honey Lake and Litchfield faults, (L) Likely fault, (LR) Lime Ridge and associated faults, (LV) Las Vegas shear zone, (ME FZ) Mendocino fracture zone, (ML) Mount Lassen, (MM) Mount McLaughlin fault zone, (MS) Mount Shasta, (MU FZ) Murray fracture zone, (NDV-FC) northern Death Valley-Furnace Creek fault zone, (P) Pahranagat shear system, (PL) Pyramid Lake, (SA) San Andreas fault, (TM) Timber Mountain and related calderas, and (WL) Walker Lane, northern part. Nevada on the east and spans the southern part of the Great Basin, where mountains of the Basin and Range province gen- Basin (Fig. 1), (2) the southward narrowing of the Great Basin erally trend north-northeast; this area is identified as deforma- between the Sierra Nevada block and the relatively rigid crust of tional field I in Figure 2. the Colorado Plateau and Middle Rockies, (3) indications of The region that extends westward from the Walker Lane to greater extensional strain in the southern Great Basin than in the the Sierra Nevada, however, has also been recognized as topo- much wider northern part, and (4) evidence that the Sierra Nevada graphically and structurally distinct from most of the rest of the block has moved considerably farther west, with reference to the Great Basin (Fig. 1). In that region a large proportion of the Colorado Plateau, than the areas north and south of the block. ranges trend northwestward, primarily as a result of the northwest- striking right-slip faults. Associated with these faults are abun- CONTRASTING PATTERNS OF LATE CENOZOIC dant, generally north- to northeast-striking normal faults and less DEFORMATION numerous east-northeast-striking left-lateral faults. That the Most of the Great Basin has long been recognized as a sys- region of prominent strike-slip faults extends eastward from the tem of horsts and grabens, marked by gently tilted fault blocks southern end of Walker Lane to the edge of the Colorado Plateau and steeply dipping normal faults. Such features are especially has been recently documented, mainly by Tschanz and Pampeyan characteristic of the central and northern parts of the Great (1970) in Lincoln County, Nevada, and by Anderson (1973) in the 490 AUGUST 1976 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4/8/489/3541379/i0091-7613-4-8-489.pdf?casa_token=BpatOfzDi0QAAAAA:I-cD_Xv5bIrFHXC8qZV_KLgSLVlog3hbNv7HXUpeHE94L-igR4lRplro2SnZbsuFoLXCXpSl
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