Earth-Science Reviews 73 (2005) 149–176 www.elsevier.com/locate/earscirev Large-scale gravity sliding in the Miocene Shadow Valley Supradetachment Basin, Eastern Mojave Desert, California G.A. Davis a,*, S.J. Friedmann b a Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA b Energy and Environmental Directorate, MC L-640, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Accepted 1 April 2005 Abstract The Miocene Shadow Valley basin in the eastern Mojave Desert of California developed above the active west-dipping Kingston Range-Halloran Hills extensional detachment fault system between 13.5 and ca. 7 mybp. Although mass-wasting processes are common phenomena in supradetachment basins, the Shadow Valley basin is an exceptional locale for the study of such processes, especially rock-avalanches and gravity sliding. A score of megabreccias, interpreted as rock-avalanche deposits, and half that number of very large (N1km2, up to 200 m thick), internally intact gravity-driven slide sheets are interbedded with various sedimentary facies. The slide sheets, variably composed of Proterozoic crystalline rocks and Proterozoic, Paleozoic, and Tertiary sedimentary strata, moved across both depositional and erosional surfaces in the basin. Although the majority consist of Paleozoic carbonate rocks, the largest slide sheet, the Eastern Star crystalline allochthon, contains Proterozoic gneisses and their sedimentary cover and is now preserved as klippen atop Miocene lacustrine and alluvial fan deposits over an area N40 km2. Estimates of slide sheet runouts into the basin from higher eastern and northern source terranes range from approximately a few km to N10 km; in most cases the exact provenances of the slide blocks are not known. The basal contacts of Shadow Valley slide sheets are characteristically knife sharp, show few signs of lithologic mixing of upper- and lower-plate rocks, and locally exhibit slickensided and striated, planar fault-like bases. Pronounced folding of overridden Miocene lacustrine and fan deposits beneath the Eastern Star allochthon extends to depths up to 40 m at widely scattered localities. We conclude that this slow moving slide sheet encountered isolated topographic asperities (hills) and that stress transfer across the basal slide surface produced folding of footwall strata. Synkinematic gypsum veins in footwall playa sediments, with fibers up to 12 cm long, have trends and shear senses compatible with the direction and sense of displacement of the overriding crystalline allochthon. The undisturbed veins, which closely parallel the base of the slide sheet, attest to high fluid presence and pressure in the playa sediments—factors facilitating allochthon movement across them. The long length of the fibers, indicative of a protracted dilational process, is incompatible with a catastrophic rate of emplacement. We believe that the only explanation for slow displacement of this allochthon and other gravity driven slide sheets across the landscape is that they formed as slumps on high, steep bedrock slopes and that their elevated heads drove their toes across lower fan and playa deposits. Initial detachments from bedrock sources were facilitated by pre-existing structural and stratigraphic anisotropies. * Corresponding author. Tel.: +1 213 740 6726; fax: +1 213 740 8801. E-mail address: [email protected] (G.A. Davis). 0012-8252/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2005.04.008 150 G.A. Davis, S.J. Friedmann / Earth-Science Reviews 73 (2005) 149–176 Detachment of the Eastern Star allochthon from the bedrock of Shadow Mountain likely occurred by inversion along the playa- ward dip of a preexisting Mesozoic thrust fault within Proterozoic rock units. D 2005 Elsevier B.V. All rights reserved. Keywords: slide blocks; gravity tectonics; Shadow Valley; Kingston Wash; Mojave Desert 1. Introduction the corrugated geometry of the detachment fault and its breakaway zone, the emplacement of the shallow- The Miocene Shadow Valley basin lies within the seated Kingston Range pluton across the detachment southern portion of the Mesozoic foreland fold and in northern parts of the early Shadow Valley basin thrust belt in the easternmost Mojave Desert, Califor- (Davis et al., 1993), contemporaneous normal faulting nia (Fig. 1; Burchfiel and Davis, 1971, 1988). The of the allochthonous basin fill, and late stage folding basin formed above the active Kingston Range-Hal- associated with sinistral strike-slip faulting beneath loran Hills extensional detachment fault between 13.5 both Kingston Wash (Davis and Burchfiel, 1993) and ca. 7 mybp, during which time it was being and the Kingston Spring area. displaced and internally extended above the shallow- Four unconformity-bounded informal members dipping fault (Fig. 2; Davis et al., 1993; Friedmann et referred to as Member I, II, etc., by Friedmann al., 1994, 1996; Fowler et al., 1995; Friedmann, (1999) constitute its 2.5–3.5 km-thick fill (Figs. 2 1999). Sedimentation patterns in the basin were influ- and 3). Volcanic activity, primarily andesitic to rhyo- enced by a number of tectonic factors, among them litic, was concentrated in Members I and II from ca. Fig. 1. Location map of Kingston Range-Halloran Hills detachment fault, eastern San Bernardino County, California. G.A. Davis, S.J. Friedmann / Earth-Science Reviews 73 (2005) 149–176 151 13 to 11 Ma (Friedmann, 1999) Megabreccias occur range in present areal extent from approximately 0.5 to in Members I, II, and IV, and slide blocks occur in all N40 km2 and include Proterozoic crystalline rocks and four members, but most importantly in Members II Proterozoic, Paleozoic, and Tertiary strata. Some Sha- and III. Collectively, megabreccias and slide blocks dow Valley slide blocks (e.g. Francis, 1109, central and constitute 10–15% of the Shadow Valley basin fill, but southern Eastern Star, Valley Wells) moved into active mass-wasting processes account for 30–50% of the depositional environments, but others (e.g. northern total fill when lahars and debris flow conglomerates Eastern Star, Valjean, Carrara-Bonanza King) were are included. Most of the mass-wasting events in emplaced on erosional surfaces developed across pre- Shadow Valley probably occurred during semi-arid viously tilted and faulted basin units. Estimates of or arid conditions. The presence of relatively deep minimum block run-out from their source terranes perennial lakes between 12 and 11 Ma suggest a range from a few kilometers to N15 km, although in semi-arid to moderate climate, although an arid cli- most cases the exact provenances of the slide blocks mate has not been ruled out. Gypsiferous playa depos- are not known. its between 11 and 10 Ma indicate that Shadow Valley Throughout the western United States, many was arid or semi-arid during that time (Friedmann, internally intact slide blocks were originally mapped 1999). as thin thrust sheets. Lasky and Webber (1949), for Although mass-wasting processes are characteristic example, described the Artillery Mountains thrust phenomena in supradetachment basins (e.g. Yarnold, plate in western Arizona as a Laramide structure. In 1993; Yin and Dunn, 1992; Forshee and Yin, 1995; reality it is a slide block of Proterozoic gneiss encased Friedmann et al., 1994; Friedmann and Burbank, 1995; in a Miocene fanglomerate sequence above the Whip- Friedmann, 1997, 1999), the Shadow Valley basin is ple-Rawhide detachment fault (Eric Frost, personal an exceptional locale for the study of such processes, communication, 1985). Hewett (1955, 1956) first especially rock-avalanches and gravity sliding. A described the slide sheets of Precambrian rocks in score of megabreccias, interpreted as rock-avalanche the Shadow Mountains area, but interpreted them as deposits, and half that number of very large sheets of thrust-fault klippen of Tertiary age, an interpretation structurally intact rock (N1km2, up to 200 m thick), challenged by Burchfiel and Davis (1971). There are interpreted as gravity-driven slide (=glide) blocks, several reasons for such structural misinterpretations. crop out in transverse and longitudinal cross-sections First, many workers originally thought that thrust intercalated with various sedimentary facies. The Sha- plates were gravitationally driven, making the distinc- dow Valley basin megabreccias have been described tion between slide blocks and thrusts sheets essen- elsewhere (Friedmann, 1997), but the spectacular tially semantic and one of scale (e.g. Hubbert and gravity-driven blocks of the basin (Fig. 4) have Rubey, 1959; Kehle, 1970). Second, gravity-gliding received little previous detailed attention and some of rock units into active depositional sites always published misinterpretation. Their impressive scale places older rocks atop younger strata. Third, the and the excellence of their exposure warrant this basal contacts of the allochthonous blocks strongly account. The Shadow Valley basin provides an unpar- resemble shallow low-angle faults, complete with alleled opportunity to examine Neogene examples of gouge zones, brittle microfabrics, slickensided and grand-scale, well-preserved gravity slide blocks and to striated surfaces, and breccia (friction) carpets as learn about their transport history and mechanisms. described below for the Shadow Valley basin exam- ples. Fourth, the very large areal extent of some of supradetachment basin slide blocks (see below) makes 2. General attributes of Shadow Valley