JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. B10, PAGES 10,422-10,426, SEPTEMBER 10, 1987
Geology of PanamintValley - SalineValley Pull-Apart System,California: PalinspasticEvidence for Low-Angle Geometryof a Neogene Range-BoundingFault
B.C. BURCHFIEL, K. V. HODGES AND L. H. ROYDEN
Departmentof Earth, Atmospheric,and Planetary Sciences,Massachusetts Institute of Technology,Cambridge
Geological studies support the interpretation that northern Panamint Valley and Saline Valley, southeastCalifornia, form paired pull-apart basins on opposite sides of the fight-slip Hunter Mountain Fault Zone. Eight to ten km of late Cenozoicnet slip can be establishedon the Hunter Mountain Fault Zone. Palinspasticreconstruction of northernPanamint Valley indicatesthat the valley was formed by movementon a shallow crustal,low-angle normal fault of 0-15 degreewest dip during the last 3.0 Ma. This interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low-angle faulting and that the most recent extension in the Basin and Range Province is accomodatedexclusively by high-angle faulting. Saline Valley, however, is interpretedto have formed by extensionon closely spaced,rotated planar normal faults. Thus, within one geometric systemof paired pull-apart basins,extension appear• to have been accommodatedin the shallow crust in two different ways.
INTRODUCTION equivalents that are part of the western Cordilleran miogeocline; and (2) Jurassic-lowerTertiary plutonic rocks The structure of the Death Valley Extended Area of intruded into the miogeoclinal section during the southeasternCalifornia (Figure 1) is dominatedby NNE to development of the Sierran batholithic complex at this NNW-trending normal fault systems and NW-trending latitude [MCAllister, 1956; Hall andMcKevitt, 1962;Hall, wrench fault systems. The late Cenozoic extensional 1971]. Polyphase compressional deformation of tectonics of this region has been characterized by the Mesozoic-early Cenozoic age has resulted in complex interplay of both systems,such that many of the valleys structural relationshipsin the ranges [e.g., Dunne et al., here (e.g., Death Valley itself) are "pull-apart" basins 1978; D unne, 1986; Wernicke et al., 1986]. All [Burchfiel and Stewart, 1966]. Two of these,Panmint and Precambrian-lowerTertiary rocks were eroded to a surface Saline valleys, are linked by the dominantly strike-slip of relatively low relief before being uncomformably Hunter Mountain fault (Figure 2). This structure overlain by upper Cenozoic sedimentary and volcanic maintained strain compatibility between the normal faults rocks. responsible for the opening of the valleys, serving much Pre-Cenozoic rocks relevent to this discussion are the the same functions as tear faults in compressionalregions. Hunter Mountain batholith(Figure 2) of early Jurassicage Simple transfer faults (terminology of Gibbs, [1984]) such [Chen and Moore, 1982] and its eastern wall rocks. These as this one place important controls on palinspastic rocks crop out extensively in the northern Panamint reconstructionsof extensional regions becausethe net slip Range and in smaller erosional windows through the on these structures corresponds to the direction and extensive Cenozoic basalt cover on the Darwin Plateau. magnitude of extension in adjoining basins. Our detailed mapping of the Panmint Butte area confirms We are currently conducting detailed geologic mapping a N7øE strikingnearly vertical contact for the southeast in the area of northern Panamint Valley and Saline Valley. side of the Hunter Mountain batholith (Figure 3a). Although our work there is far from complete,it has led to Mapping of part of the Darwin Plateau originally mapped the discovery of a unique linear element (formed by the by Hall and McKevitt [1962] has revealeda new exposure intersection of a high-angle intrusive contact and an of this contact near the northeastmargin of the plateau. overlappingbasalt flow) which occurson either side of the These contactsare similar in orientation,but are offset by Hunter Mountain fault and constrains its post-4 Ma net slip. Palinspastic reconstruction of northern Panamint the N6øW strikingHunter Mountain Fault Zone (Figures Valley based on these data indicates that the basin was 2 and 3a). developed by movement on a west dipping low-angle In the Panamint Butte area, pre-Cenozoic rocks are detachment. Coupled with geophysical data presentedin a unconformablyoverlain by Miocene-Pliocenesedimentary and basaltic volcanic rocks of the Nova Formation companionpaper [MIT 1985 Field GeophysicsCourse and Biehler, this issue], the geological evidence presents a [Hopper, 1947] These units representan older extensional compelling case for the importanceof shallow level, low- basin (which we call the Nova Basin) that has been angle detachmentsin the developmentof Basin and Range strandedin the Panmint Rangeby the subsequentopening physiography. of PanamintValley (Figure 2). Detailed mappingand K- Ar geochronologyin the Tucki Mountain area (Figure 1) GEOLOGIC FRAMEWORK [Wernicke et al., 1986; W. Hildreth, unpublisheddata, 1985] and SE of Panmint Butte [K. V. Hodges et al., The bedrock geology of ranges adjacent to northern unpublished data, 1985] indicate that the Nova Basin Panamint Valley is dominatedby (1) upper Precambrian- developed in the latest Miocene-early Pliocene time. One of the youngest basalts of the Nova Basin (K-Ar dated at Triassic sedimentary rocks and their metamorphic 4.0-4.3 Ma by Larson [1979]) lies unconformablydirectly Copyright 1987 by the American GeophysicalUnion. on miogeoclinal strata and the Hunter Mountain batholith, and caps PanamintButte itself. Paper number 6B6210. Remnants of the Nova Basin are also present on the 0148-0227/87/006B-6210505.00 west side of northernPanmint Valley in the Argus Range
10,422 BUq•CHFIELET AL.: PANAMINT-SALINEVALLEYS PULL-APARTSYSTEM 10,423
FIG 2 Tertiary Fault Systems ,•----______• ...... /California/--Neva-'•d•• ueal;nvalley • , ExtendedArea• • j
300 km
DARWIN PLATEAU
30 km
SIERRA NEVADA
ß ,
,
, , • ,
ß , - ß , ,
Fig. 1. Major faultsof the Death Valley extendedarea and locati• of the northemPanamint Valley and Saline Valley paired pull-apart basins. Location of Figure 2 is shown. and on the Darwin Plateau [Smith, 1976;Schweig, 1985]. normal-slip faults, some of which also have right-slip Much of the northern Darwin Plateau consists of basaltic displacement. NE-trending normal-slip faults are abundant volcanic rocks of Miocene-Pliocene age which rest in the Saline Range [Ross, 1967] and present in the Dry unconformably on bedrock identical to that in the Mountain area [Burchfiel, 1969]. These faults are parallel Panamint Butte area. Petrology and K-At ages of the to active normal fault scarps along the east side of Saline Darwin Plateau volcanics [Larson, 1979;Schweig, 1985] Valley [Zellrner, 1980, 1983]. indicate that the uppermost basalts on the plateau are We believe that formation of northern Panmint Valley correlative to the capping basalt on Panamint Butte. and Saline Valley was the result of developmentof a late These data indicate that the northern end of Panamint Pliocene to Recent extensional system that consists of Valley opened after developmentof the Nova Basin and paired pull-apart basins connectedby the Hunter Mountain eruption of the lower Pliocene basalts, and fault scarps transfer Fault. This interpretation of the kinematic cutting the alluvium on both sides of the valley indicate significance of the Hunter Mountain fault Zone is continuing extension. These latest structures show both supported by the following: (1) the zone does not extend right-slip and normal-slip components of displacement eastwardbeyond the easternboundary of Panamint Valley, (see Smith [1976] and our mapping). The valley ends nor does it extend westwardbeyond the westernmargin of abruptly in the north against a steep topographic slope, Saline Valley, and (2) there is no evidence for pre-late trending N6øW and having 1.0-1.5 km of relief, that Cenozoic movement on the zone. correspondsto the Hunter Mountain Fault Zone. Active subsidiary scarps along this margin of the valley also EXTENSION GEOMETRY IN NORTHERN PANAMINT VALLEY attest to modem extension. Saline Valley appearsto have formed during the same Given the transfer fault interpretation for the Hunter time period as northern PanamintValley. Although parts Mountain Fault Zone, we argue that the displacementon of the Saline Range may have been involved in earlier the central segmentof the structureis equal to the amount basin development [Blakely and McKee, 1985], the Saline of extension in both northern Panmint Valley and Saline Range to the north of the Saline Valley contains basalts Valley. The intersectionbetween the steep easternwall of that date from 3.8 to 1.7 Ma [Larson, 1979]. Mapping in the Hunter Mountain batholith and the unconformityat the the Saline Range and adjacent areas suggeststhat Saline base of the Miocene-Pliocenebasalt sequenceproduces a Valley did not begin to form until after extrusion of the N7øE, subhorizontalline that is offset by the Hunter oldest (3.8-2.8 Ma) basalts [Burchfiel, 1969; Ross, 1967, Mountain Fault Zone. 1968; Larson, 1979]. Saline Valley is bounded on its Figure 3b is a crosssection drawn parallel to the Hunter south side by the westward continuation of the Hunter Mountain Fault Zone (N6øW) and depicts relevant Mountain Fault Zone [Zellmer, 1980, 1983] The faults geological data projected from both sides of the structure. along the east side of the valley are marked by active The linear element described above is shown as two 10,424 BURCHFIEL ET AL.: PANAMINT-SALINE VALLEYS PULL-APARTSYSTEM
Fig. 2 Generalgeological framework of the northernPanamint Valley and SalineValley area. HMB, HunterMountain batholith;HMFZ, HunterMountain Fault Zone; DP, DarwinPlateau; PB, PanamintButte; SR, SalineRange; NB, Nova Basin. Random dash pattem covers area of late Cenozoicvolcanic rocks. piercingpoints in the section. The net slip indicatedby PanamintValley, then the valley fill cannotbe very deep. the offset of this element is 8-10 km of right-lateral A drill hole in the central part of the valley encountered strike-slip accompaniedby 0-2 km of down-to-the-south 370 ft (113 m) of Cenozoic valley fill before entering dip slip. The range of slip vectors shown on Figure 3 Paleozoic rocks [Smith and Pratt, 1957]. In addition, the reflects uncertainties in the projection of the linear low-anglenormal fault would have removedall the Tertiary elementonto the section. If we assignthe maximumage and Paleozoic rocks of the hanging wall from the footwall of Saline Valley (3.0 Ma) to the age of inceptionof the in the valley: the valley fill must have been deposited Hunter Mountain Fault Zone, the minimum averageslip directly onto a progressively broader expanse of the rate on the fault is 2-3.2 mm/y. Presumably,this figure footwall rocks as the valley formed. Subsequently,the also corresponds to the minimum rate of extension in valley fill must have moved relatively westward with the northernPanmint Valley and Saline Valley. hanging wall. Geophysical data gathered by the Northern Panamint Valley may be palinspastically MassachusettsInstitute of Technology(MIT) Geophysics reconstructed by aligning the two piercing points in Field Camp in 1985 in connectionwith our geological Figure 3b. This exercise completely restores the work [M/T 1985 Field GeophysicsCourse and Biehler, tl'ds uppermostbasalt flows on the Darwin Plateau againstthe issue] indicate that the valley fill is only a few hundred correlative caprocks of Panamint Butte, strongly feet thick and that no basalt underlies the valley. The suggestingthat opening of the basin was principally latter interpretationis important becauseit supportsthe accommodatedby movementon a detachmentfault dipping interpretation that hanging wall rocks of the postulated 0ø-15ø westward. The range-boundingfault along the low-angle normal fault were completely removed from easternmargin of PanamintValley (which is presumably footwall rocks beneath the valley. the surface expression of this detachment) dips approximately 42øW, indicating a broadly listtic TECTONIC IMPLICATIONS geometry. This conclusionis supportedby the gently curved, convex-upward geometry of the Darwin Plateau Structuralrelationships around northern Panmint Valley basalts, indicating "rollover" in the hanging wall of a and geophysicaldata for the valley itself stronglyindicate curved fault [Hamblin, 1965]. that low-angledetachment faulting has played a major role If a low-angie detachment fault underlies northern in the development of this P!iocene-Recent basin. BURCffFIELET AL.: PANAMINT-SALINEVALLEYS PULL-APART SYSTEM 10,425
SV
MOUNTAIN BATHOLITH
DARWIN
PANAMINT PLATEAU / BUTTE
NPV
0 I Okm
DARWIN PLATEAU p...... g point PANAMINT BUTTE n s•de p...... g pornt net 'ß :'..'...'...'.;_'...... ?.'..:., •. • ß
• •- possible low-angle
normal fault 8- I Okra
Fig. 3 (a) Detailedmap of the areaof the HunterMountain batholith. Uniqueline usedto establishto piercingpoints is the intersectionof the vertical southeastcontact of the Hunter Mountain batholith (heavy dark line) and the late Cenozoic volcanic rocks of the Darwin Plateau and PanamintButte. HMFZ, Hunter Mountain Fault Zone; SV, Saline Valley; NPV, NorthernPanamint Valley, Irregularline segmentpattern covers area of late Cenozoicvolcanic rocks. (b) Crosssection A-B parallelto the HunterMountain Fault Zone. Piercingpoints form both southside (left sideof section) and north side (right side of section)are projectedonto the line of section. Correspondinglines of net slip and possible low-angle normal faults are shown. The uncertaintyof the piercingpoint on the north side is due to a normal fault at the rangefront which dropsthe contactdown. We suggestthat the highestpiercing point is the correctone, but the lower one is also possible. Uncertaintyof horizontaloffset is mainly due to a 2-kin uncertaintyin projectingthe uniqueline to the line of sectionon the southside of the fault zone. Faultsin the PanamintButte area are from more detailedmaps than that of Figure 3a.
Although many geologistshave stressedthe importanceof Jackson, 1987], and (2) that the most recent extensionis low-anlge normal faults to Cenozoic extension in the accommodated exclusively by high-angle "Basin and North American Cordillera (e.g., Anderson, 1971; Range" faults [e.g., Zoback et al., 1981] The northern Armstrong, 1972; Wernicke, 1981], some controversy Panamint Valley detachmentfault clearly initiated in the exists concerningthe initial geometry of these structures. upper 5 km of the crust and apparentlyis still active at a Few examplesdemonstrate an initial low-angle geometry shallow level. Moreover, the detachment is directly as convincingly as the northern Panamint Valley responsiblefor the present Basin and Range topography situation. ReconstrUctionof the basin along a low-angle that characterizes the Panamint Butte-Darwin Plateau area. detachmentfault results in the restoration of an early We do not, however, wish to imply that all Basin and Pliocene volcanic plateau; no alternative reconstruction Range topographyis a consequenceof low-angle range involving large-scale rotation of an initially high-angle bounding structures. Immediately to the north, in Saline structure produces an acceptable initial orientation of the Valley, extension appears to have been accommodated volcanic units. differently. In the Saline Range and parts of Dry The geology of northern Panamint Valley seems to Mountain, there are numerous northeast striking, west contradict two widely held notions coneming Cenozoic dipping normal faults which project into Saline Valley, extension in the western United States: (1) that little of and Pliocene volcanic rocks are exposed continuosly the extensionin the uppermostcrust is accommodatedby across the ranges [Ross, 1967, 1968; B urchfiel, 1969; low-angle faulting [e.g., Eyidogan and Jackson, 1985; Larsen, 1979]. Moreover, gravity data [Chapman et al., 10,426 BURCHFIEL ET AL.: PANAMINT-SALINE VALLEYS PULL-APARTSYSTEM
1973] indicate that Saline Valley is an extremelydeep Hall, W. E., and E. N. McKevitt Jr., Geology and ore depositsof basin. Theselines of evidencepreclude a singlelow-angle the Darwin Quadrangle, !nyo County, California, U.S. Geol. detachmentmodel for extensionin Saline Valley; instead Surv. Prof. Pap. 368, 87 pp., 1962. they favor a model basedon closelyspaced rotated planar Hamblin, W. K., Origin of "reverse-drag"on the downthrownside of normal faults, Bull. Geol. Soc. Am., 76, 1145-1164, 1965. faults [Wernicke and Burchfiel, 1982]. These faults may Hopper, R. H., Geologic section from the Seirra Nevada to Death end at depth at a low-angle normal fault, but such a low- Valley, California, Geol. Soc. Amer. Bull., 58, 393-423, 1947. angle fault would not be directly responsible for the Jackson, J. A., Active normal faulting and crustal extension in shallow crustal extension. Thus, within one geometric Continental Extension Tectonics edited by J. F. Dewey and M.P. system of paired pull-apart basins, extension appears to Coward, J. Geol. Soc. London , Special Publication , 1987 have been accommodatedin the shallow crust in markedly Larson, R. W., Chronology of Late Cenozoic basaltic volcanism: different ways. The tectonic implications along a segmentof the Sierra Nevada and Basin and Range Province, Ph.D. thesis, 95 pp., Brigham Acknowledgments. This work has been supportedby National Young Univ., Provo, Utah, 1979 ScienceFoundation grants EAR-8314161 and EAR-8512283 to Kip McAllister, J. F., Geology of the Ubehebe Peak Quadrangle, V. Hodges, Chevron Oil Company, Shell Oil Company, Califomia, U.S. Geol. Surv. Quad. Map, GQ-95, 1956. SchlumbergerChair, and the Department of Earth, Atmospheric, M.IT 1985 Field GeohpysicsCourse and S. 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