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Large-Magnitude Late Tertiary Strike-Slip Faulting North of Lake Mead, Nevada

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Large-Magnitude Late Tertiary Strike-Slip Faulting North of Lake Mead, Nevada Large-Magnitude LateTertiary Strike-Slip Faulting North of Lake Mead, Nevada GEOLOGICAL SURVEY PROFESSIONAL PAPER 794 Large-Magnitude Late Tertiary Strike-Slip Faulting North of Lake Mead, Nevada By R. ERNEST ANDERSON GEOLOGICAL SURVEY PROFESSIONAL PAPER 794 Geologic mapping of 115-square-mile area indicates rnajor northeast-trending fault systent ·with estimated 4 0-ntile left-lateral displacement UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1973 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600343 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $1.65 (paper cover) Stock Number 2401-00285 CONTENTS Page Page Abstract______________________________________________________________________________________ 1 Structure___________________________________________________________________________________ 7 Introduction and acknowledgments_______________________________________ 1 Hamblin Bay fault____________________________________________________________ 8 StratigraphY-----------------------------------------------------------------------------· 3 Structures south of Hamblin Bay fault ........ ---------------- 8 Permian rocks____________________________________________________________________ 3 Structures north of Hamblin Bay fault_______________________ 9 Mesozoic rocks___________________________________________________________________ 3 Age and magnitude of strike-slip displacements________________ 12 Tertiary rocks____________________________________________________________________ 4 Relationships between strike-slip and normal faults_________ 13 Gale Hills Formation_______________________________________________ 4 Regional setting and implications__________________________________________ 14 Rocks of the Hamblin-Cleopatra volcano_____________ 5 References__________________________________________________________________________________ .,.17 Unassigned volcanic and intrusive rocks_____________ 6 Muddy Creek Formation_________________________________________ 7 ILLUSTRATIONS Page PLATE 1. Geologic map of the northern extremity of the Black Mountains, Nevada __________________________________________________________ Jn pocket FIGURE 1. Index maP---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 2 2. Photograph of strata of the Muddy Creek Formation______________________________________________________________________________________________________ 7 3. Block diagram of fault relationships at intersection of strike-slip and normal systems_________________________________________ 9 4. Sketch maps showing comparison between selected fold and tension structures------------------------------------------------------- 11 5. Photograph of folds in Gale Hills Formation·------------------------------------------------------------------------------------------------------------------- 12 6. Block diagram of faults in area of unevenly distributed extension______________________________________________________________________________ 14 7. Tectonic index maP----------------------------------------------------------------------------------------------------------------------------------------------------------------- 15 8. Generalized tectonic map of Lake Mead area·---------•--~------------------------------------------------------------------------------------------------------- 16 III LARGE-MAGNITUDE LATE TERTIARY STRIKE-SLIP FAULTING NORTH OF LAKE MEAD, NEVADA By R. ERNEST ANDERSON ABSTRACT is so, the rocks between the Las Vegas shear zone and the In late Miocene time, in what is now the Lake Mead area of shear zone that passes through the mapped area have been southern Nevada, an andesitic stratovolcano, the Hamblin­ shifted about 40 miles westward relative to the terrain to the Cleopatra volcano, formed at or near the northern boundary north and south. Such large-scale shift of a narrow crustal of a broad late-Cenozoic volcanic terrain that developed on a belt can be explained best by assuming their transport, piggy­ platform of Precambrian crystalline rock. The boundary may back, on a fast-moving· current of mantle and possibly lower have been a hinge line or a major fault zone prior to forma­ crustal plastic material that was banked against and deflected tion of the volcano. After the volcano formed, it was disrupted around the margin of the Colorado Plateau. An alternative into three principal parts by as much as 12 miles of left­ interpretation which does not take movement on the Las lateral displacement on a system of northeast-trending strike­ Vegas shear zone into account is that the northeast-trending slip and oblique-slip faults that now mark the boundary. left-lateral shear system is a structural feature that com­ Other faults of the same system displace Tertiary and pre­ pensates for contrasting amounts of extensional strain as the Tertiary sedimentary rocks in the area north and northwest Basin and Range structural province broadens northward of the disrupted volcano. The cumulative displacement on the around the Colorado Plateau. An analogous interpretation can entire fault zone is estimated at 40 miles. be made for the Garlock fault, which has similar trend and The area northwest of the disrupted volcano consists of sense of displacement. several large blocks that are tilted steeply east or southeast and are separated from one another by complex fault zones INTRODUCTION AND ACKNOWLEDGMENTS of the strike-slip system. Some strike-slip fault traces parallel the general strike of the strata, and in such areas small-scale Erosion by the Colorado River in the vicinity of folds and normal faults that are genetically related to the Lake Mead has removed much upper Tertiary and strike-slip faults are well developed. The small-scale struc­ Quaternary basin fill and locally has cut deeply into tures bear a geometric relationship to the strike-slip faults bedrock, thereby producing excellent exposures analogous to that reported for similar fold and fault struc­ tures associated with major strike-slip faults in New Zealand. within the Basin and Range structural province. The Where the strike-slip faults cross the trend of the tilted elevation of a pre-Colorado River fossil water table blocks, large-scale first-order drag folds with steeply to ver­ indicates as much as 1,800 feet of downcutting near tically plunging axes are developed. Some normal faults in Hoover Dam in the last 2.6 million years (Anderson, the northwest area are probably equal in rank to the strike­ 1969). The excellence of exposures, together with slip faults and are part of a single movement pattern anal­ ogous to that reported for the area south of Lake Mead. indications of exotic structural relationships re­ South of the northeast-trending shear zone, vertical dis­ vealed in reconnaissance mapping by Longwell placements of about 6,000 feet occurred on north-trending (1963) and Longwell, Pampeyan, Bowyer, and Rob­ normal faults that were active during at least the late stages erts (1965), led in 1969 to a mapping study of of strike-slip displacements. The largest of these normal transcurrent faults and other structures in the Lake faults bound the elevated Wilson Ridge mass of the Black Mead area. The present report results from those Mountains against the structurally depressed Detrital Valley block on the east and the Colorado River block on the west. studies, as have other published reports (Anderson, Where the strike-slip and normal faults intersect there is 1969, 1971a, 1971b; Anderson and others, 1972). complex .interaction between the two. Some strike-slip dis­ This report describes the results of geologic map­ placement seems to be translated into normal displacement, ping of about 115 square miles at the north end of and a simplistic model presented herein interprets the me­ chanics of that interaction. That area provides an excellent the Black Mountains north of Lake Mead (fig. 1). example of diverse types of simultaneous deformation that Much of the mapping was detailed reconnaissance are produced in the Western United States by strong local with some reliance on photogeologic interpretation, inhomogeneities in stress fields. but because the study emphasized the area's struc­ The Las Vegas shear zone must pass north of the mapped tural geology, detailed field mapping was done in area with possibly as much as 45 miles of right-lateral sepa­ ration to account for the abrupt southward termination of the numerous structurally critical areas, especially along compressional structures of the Sevier orogenic belt. If this major fault zones. 1 2 LATE TERTIARY STRIKE-SLIP FAULTING NORTH OF LAKE MEAD, NEVADA BS Bitter Spring EB Echo Bay CW Callville Wash CM Callville Mesa RG Rainbow Gardens FH Fortification Hill HD Hoover Dom r--------- 1 I l NEVADA I I RENO ' """ ', Area of I1 ~~ FIGURE 1.-Index map of Clark County, Nev., and part of Mohave County, Ariz., show­ ing location of mapped area (heavy line) and major cultural and physiographic fea­ tures. Hachured line marks boundary between thick Paleozoic, Mesozoic, and Tertiary sedimentary rocks to the north and Precambrian crystalline and Tertiary igneous rocks to the south. The area consists of Precambrian crystalline Geologic mapping was conducted on a part-time rocks, about 6,000 feet of Permian and Mesozoic
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