Geological Society of America Field Guide 2 2000 Late Cenozoic crustal extension and magmatism, southern Death Valley region, California J.P. Calzia U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA O.T.Ramo Department of Geology, University of Helsinki, P.O. Box 11, FTN-00014, Finland ABSTRACT The late Cenozoic geologic history of the southern Death Valley region is characterized by coeval crustal ex­ tension and magmatism. Crustal extension is accommodated by numerous listric and planar normal faults as well as right- and left-lateral strike slip faults. The normal faults dip 30°-50°W near the surface and flatteri and merge at depth into a detachment zone at or near the contact between Proterozoic cratonic rocks and Proterozoic and Pa­ leozoi_c miogeoclinal rocks; the strike-slip faults act as tear faults between crustal blocks that have extended at dif­ ferent times and at different rates. Crustal extension began 13.4-13.1 Ma and migrated northwestward with time; undef'ormed basalt flows and lacustrine deposits suggest that extension stopped in this region (but continued north of the Death Valley graben) between 5 and 7 Ma. Estimates of crustal extension in this region vary from 30-50 per­ cent to more than 100 percent. Magmatic rocks syntectonic with crustal extension in the southern Death Valley region include 12.4-6.4 Ma granjtic rocks as well as bimodal14.~.0 Ma volcanic rocks. Geochemical and isotopic evidence suggest that the granjtic rocks get younger and less alkalic from south to north; the volcanic rocks become more mafic with less evidence of crustal interaction as they get younger. The close spatial and temporal relation between crustal extension and magmatism suggest a genetic and prob­ ably a dynamic relation between these geologic processes. We propose a tectonic-magmatic model that requires heat be transported into the crust by mantle-derived mafic magmas. These magmas pond at lithologic or rheo­ logic boundaries, begin to crystallize, and partially melt the surrounding crustal rocks. With time, the thermaUy weakened crust is extended (given a regional extensional stress .field) concurrent with granitic magmatism and bi­ modal volcanism. INTRODUCTION tension and magmatism in the southern Death Valley region and introduces a tectonic-magmatic model that relates these geo­ The Death Valley region in southeastern California is one logic processes in an actively extending orogen. of the youngest regions of large-scale crustal extension within the Basin-Range province. Normal and associated strike-slip GEOLOGIC SETTING faulting accompanied by extensional basin formation began <L5 Ma and continues today (Wright et al., 1981; Stewart, The southern Death Valley region is bounded on the west 1980). Numerous granitic plutons, dikes, and sills as well as fel­ by the Panamint Range and the south by the Providence Moun­ sic to mafic volcanic fields are synchronous with extension tains and New York Mountains (Fig. 1). The pre-Cenozoic (Calzia and Finnerty, 1984; Wrightet al., 1991; Davis and Fleck, stratigraphy in this region consists of Early Proterozoic cratonic 1977). Previous workers have focused on the timing and kine­ rocks and Middle Proterozoic to Paleozoic sedimentary de­ matics of extensional faulting but few have considered the spa­ posits. The cratonic rocks include Early Proterozoic paragneiss, tial and temporal relations between extension and magmatism schist, and quartzite intruded by ca.l700 Ma orthogneiss in Death Valley. This paper describes late Cenozoic crustal ex- and 1400 Ma anorogenic granites (Wooden and MilJer, 1990; Calzia, J. P., and Ramo, O.T., 2000, Late Cenozoic crustal extension and magmatism, southern Death Valley region, California, in Lageson, D.R., Peters, S.G., and Lahren, M.M., eds., Great Basin and Sierra Nevada: Boulder, Colorado, Geological Society of America Field Guide 2, p. 135- 164. 135 117° 116° 36° Alexander Hills AH New York Mtns. NYM Amargosa River AR Noble Hills NH Amargosa Valley AV Nopah Range NR Avawatz. Mtns. AM Old Dad Mlns. OD Black Butte BB Dwlshead Mtns. OM Black Mtns. BM Provldence Mtns. PM Cima Dome CD Panamint Range PR Clark Mtns. CM Quail Mtns. OM Dumont Dunes DO Rainbow Mtn. RM Dublin Hills DH Resting Spring Range RSR Deadmans Pass OP Saddlepeak Hills SOH Death Valley Graben OVG Silurian Hills SH Greenwater Valley GV Soda Mtns. SDM Halloran Hills HH Spling Mlns. SM lvanpah M1ns. IM Sperry Hills SPA lvanpah Valley IV Salt Spring Hills SSH Kelso Mtn. KM Shadow Valley sv Kingston Range KR Tecopa T Tecopa Peak TP ~28~\~"u~a~~ ~~~ MCR Union Pacitic Railroad UP Mosquito Mtns. MM Mesquite Pass MP ~::!:~~ ~~~~~~v Mescal Range MR Warm Springs Canyon WSC Mud Hills MH 0 30km Scale a ==' 35o Fig\lre I A. Index map of the southern Death ValJey region showing the Kingston Range-Halloran Hills de­ tachment fault (KRHH), geographic features referred to in the text, major highways, and field trip stops. Late Cenozoic crustal extension and magmatism, southern Death Valley region, California 137 Lithologic sketch map of Southern Death Valley Region, California Tertiary Synextension Sedimentary Basins Tertiary Volcanic Rocks Tertiary Granitic Rocks Mesozoic Granitic Rocks Middle Proterozoic Pahrump Group Early Proterozoic Cratonic N A OCEAN Figure lB. Lithologic sketch map of the same area shown in Figure lA. Lanphere et al., 1964; Ramo and Calzia, 1998). The Middle Pro­ CRUSTAL EXTENSION terozoic Pahrump Group unconformably overlies the cratonic rocks and consists of -2100 rn of conglomerate, sandstone, Southwest and west-northwest-directed crustal extension shale, and carbonate rocks divided into the Crystal Spring For­ in the southern Death Valley region is accommodated by north­ mation, Beck Spring Dolomite, and Kingston Peak Formation northeast-trending listric and planar normal faults as well (Hewett, 1940). The Crystal Spring Formation is intruded by as northwest-trending right-lateral and northeast-trending left­ 1068 Ma and 1087 Ma (Heaman and Gretzinger, 1992) diabase lateral strike-slip faults (Wright et al., 1981). Most of the nor­ sills (Wright, 1968; Hammond, 1986). The Pahrump Group is mal faults dip 50°-30°W near the surface and flatten and overlain by 3000-5000 m of Late Proterozoic and Paleozoic merge at depth, forming a detachment zone at or near the con­ miogeoclinal deposits. Most of the Proterozoic and Paleozoic tact between the cratonic rocks and miogeoclinal deposits rocks are intruded by Mesozoic and Tertiary plutons (Calzia, (Wright and Troxel, 1973). COCORP (Consortium for Conti­ 1990; Ramo et al., 2000). All of these rocks are unconformably nental Reflection Profiling) lines across the southern Death overlain by later Tertiary sedimentary and volcanic rocks and Valley region show that most of the normal faults do not extend Quaternary alluvial deposits (Wright and Troxel, 1973). below a depth of 5 km (Serpa et al., 1986, 1988). The pattern, r 138 J.P Calzia and 0. T. Riimo geometry, and kinematics of these faults suggest that the mio­ ent generations of the northwest-trending faults at different geoclinal deposits are sliding off the cratonic rocks as mega­ structural levels. slumps into Tertiary basins formed during crustal extension The consistent strike of the tear faults indicates that the (Wright and Troxel, 1971). Deeper (5- 15 krn) reflectors show upper plate of the Kingston Range detachment fault was trans­ moderately dipping normal and vertical strike-slip faults in ported to the southwest (Burch:fiel et al., 1983, 1985). Displacement the cratonic rocks that are relatively straight and spaced more increases to the southwest as each generation of northwest­ widely apart than faults in the upper 5 krn of the crust. These trending faults added its displacement to the extending upper deeper faults bound tilted crustal blocks that have extended at plate. Reconstruction of contacts within the miogeoclinal sec­ different rates and at different times (Wright et al., 1984a). Es­ tion suggests that maximum horizontal displacement along the timates of crustal extension of the southern Death Valley re­ northeastemmost faults is about 1-2 km; cumulative horizontal gion range from 30-50% (Wright and Troxel, 1973) to >100% displacement of the upper plate is --6 km (Burchfiel, written (Wernicke et al., 1988). commun., 1989; Fowler and Calzia, 1999). The oldest Late Cenozoic extensional fault in the southern The Halloran Hills detachment fault is best exposed in Death Valley region is the J(jngston Range-Halloran Hills de­ Mesquite Pass and the Mescal Range (Fig. 1). The detachment tachment fault system (Fig. 1; Burchfiel et aL, 1983, 1985; fault in Mesquite Pass is mapped as an aoastamosing shear Davis et al., 1993). This fault system defines the east boundary zone, 10-20 m thick, that dips 20°W. The fault cuts lower Pale­ of the Death Valley extended terrain and is divided into north­ ozoic sandstone, a 13.4 Ma hypabyssal felsic sill, and Miocene ern and southern segments. The northern segment, the Kingston basin sediments; lower Paleozoic rocks are faulted over Ter­ Range detachment fault, is well exposed in the northern and tiary sedimentary breccias along several lateral ramps in the eastern J<j.ngston Range to about the latitude of Kingston Wash hanging wall (Friedmann et aL, 1994). Regional geologic rela­ (Fig. 1). The southern segment, the Halloran Hills detachment tions suggest that rocks in the hanging wall were transported fault, is discontinuously exposed along the west side of the 5-9 krn to the southwest during at least two phases of west­ Mesquite Mountains, Clark Mountains, and Mescal Range directed sliding (Fowler et al., 1995). A folded half graben in (Davis et al., 1993) and projects beneath the Halloran Hills the banging wall of the shear zone consists of rock avalanche (Bishop et al., 1991 ). deposits and gravity glide blocks cut by channel conglomer­ The Kingston Range detachment fault dips as much as ates.