Structural and Stratigraphic Evolution of the Calico Mountains

Structural and stratigraphic evolution of the Calico Mountains: Implications for early Miocene extension and Neogene transpression in the central Mojave Desert, California

John S. Singleton* Phillip B. Gans Department of Earth Science, University of California, Santa Barbara, California 93106, USA

ABSTRACT rapid unroofi ng of the central Mojave meta- (the Waterman Hills detachment fault) that morphic core complex, yet extension in the juxtaposes tilted early Miocene volcanic and New geologic mapping, structural data, Calico Mountains is minor and is overprinted sedimentary rocks in the hanging wall against and 40Ar/39Ar geochronology document early by dextral faulting and transpression. variably mylonitized basement rocks in the Miocene sedimentation and volcanism and Calico Member beds north of the Calico footwall. Based on apparent offsets of pre-Ter- Neogene deformation in the Calico Moun- fault are intensely folded into numerous tiary markers, several workers (Glazner et al., tains, located in a complexly deformed region east-west–trending, upright anticlines and 1989; Walker et al., 1990; Martin et al., 1993) of California’s central Mojave Desert. Across synclines that represent 25%–33% (up to proposed that 40–60 km of northeast-directed most of the Calico Mountains, volcaniclastic ~0.5 km) north-south shortening. Folds are normal slip occurred along the Waterman Hills sediments and dacitic rocks of the Pickhan- detached along the base of the Calico Mem- detachment fault. The distribution of exten- dle Formation accumulated rapidly between ber and thrust over the Pickhandle Forma- sion is controversial. Dokka (1989) argued that ca. 19.4 and 19 Ma. Overlying fi ne-grained tion, which dips homoclinally ~15–30°S to regional extension occurred within an east- lacustrine beds (here referred to as the Cal- SE. The geometry and distribution of folds west–trending belt across most of the Mojave ico Member of the Barstow Formation) are are most compatible with localized transpres- Desert region. In contrast, Glazner et al. (2002) bracketed between ca. 19 and 16.9 Ma, and sion between the Calico Member and the suggested that extension was largely confi ned are thus older than the type section of the Pickhandle Formation within a positive to an ~25-km-wide area centered around the Barstow Formation in the Mud Hills. Sev- fl ower structure. Transpressional folding and central Mojave metamorphic core complex. eral 17.1–16.8 Ma calc-alkaline dacite domes faulting in the Calico Mountains postdate the Currently there is no strong consensus on intrude the Calico Member and represent a ca. 17 Ma dacite intrusions and appear to be the precise timing of extension in the central previously unrecognized volcanic episode in largely restricted to the area along the Calico Mojave Desert. A few lines of evidence suggest this region. fault restraining bend. that deformation associated with the central In the southern Calico Mountains, the Cal- Mojave metamorphic core complex occurred ico fault (part of the Eastern California shear Keywords: Calico Mountains, Calico fault, Mo- between ca. 24 and 19 Ma. First, a dacite zone) forms a west-northwest–striking, trans- jave Desert, Barstow Formation, transpression. dike in the Mitchel Range and the Waterman pressional restraining bend with ~3 km of Hills granodiorite are interpreted to have been right-lateral slip and perhaps 1 km of reverse INTRODUCTION emplaced synkinematically into the footwall (north side up) throw distributed on two main of the central Mojave metamorphic core com- fault strands. Part of the Calico fault appears The central Mojave Desert region in south- plex (Walker et al., 1990; Fletcher and Bartley, to have originated as an early Miocene normal ern California records a complex deformation 1994); these intrusions have zircon U-Pb ages of fault that unroofed metavolcanic basement history that includes Cenozoic extension, con- 23.0 ± 0.9 Ma and 21.9 ± 0.8 Ma, respectively rocks in the footwall and created a hanging- traction, and strike-slip faulting. Early Mio- (Walker et al., 1990, 1995). Second, the Pick- wall basin in which Pickhandle Formation cene detachment faulting and extensional basin handle Formation volcanic and sedimentary strata accumulated. This extensional slip development generally preceded transform- rocks in the hanging wall of the central Mojave must have largely ceased prior to deposition dominated tectonics related to the Pacifi c–North metamorphic core complex are interpreted as of the Calico Member, which unconformably American plate boundary, yet the timing, mag- synextensional deposits ranging in age from overlies the Pickhandle Formation north of nitude, and tectonic signifi cance of these dispa- ca. 24 to 19 Ma (Fillmore and Walker, 1996). the Calico fault and directly overlies metavol- rate modes of deformation remain controversial Younger (ca. 17–13 Ma) fi ne-grained lacustrine canic rocks south of the Calico fault. Deposi- (see Glazner et al., 2002, for a review). rocks of the Barstow Formation are considered tion of the Pickhandle Formation and at least The central Mojave metamorphic core postextensional deposits. Thermochronologic part of the Calico Member was coeval with complex exposes a low-angle normal fault data of mylonitic rocks from the Mitchel Range

*Present address: Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, USA.

Geosphere; June 2008; v. 4; no. 3; p. 459–479; doi: 10.1130/GES00143.1; 13 ﬁ gures; 1 table; 1 plate; 1 supplemental ﬁ gure.

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

and Hinkley Hills indicate that the footwall of ley et al., 1990). Several strike-slip faults are plify synextensional deposition and transpres- the central Mojave metamorphic core complex considered to be active now (Jennings, 1994). sion in the central Mojave Desert. underwent rapid cooling (50–100 °C/m.y.) Neogene shortening in the Mojave Desert between ca. 21 and 17.5 Ma (Gans et al., 2005). region has primarily been attributed to local GEOLOGIC OVERVIEW OF THE This episode of cooling is interpreted to refl ect transpression along northwest-striking dextral CALICO MOUNTAINS exhumation of the footwall during slip on the faults (e.g., Dibblee, 1980b, 1994), or regional Waterman Hills detachment fault. north-south contraction (Bartley et al., 1990; Located ~15 km northeast of Barstow, the Cal- Strike-slip faulting associated with the Eastern Linn et al., 2002). The most common types ico Mountains form a 15-km-long, northwest- California shear zone appears to have been the of contractional structures are approximately trending range composed primarily of early dominant mode of postextensional deformation east-west–trending folds, many of which occur Miocene sedimentary and volcanic rocks in in the Mojave Desert region. Northwest-trending in Miocene lacustrine rocks. Folds are wide- the upper plate of the central Mojave metamor- right-lateral faults are ubiquitous and accom- spread across the region, suggesting that con- phic core complex (Fig. 1). Dacite and coarse modate a small percent of the relative motion traction is a regional phenomenon (Bartley et volcaniclastic sedimentary rocks of the Pick- between the Pacifi c and North American plates al., 1990). The magnitude of shortening repre- handle Formation compose most of the north- (Dokka and Travis, 1990b). East- to northeast- sented by these folds is not well documented, ern and central Calico Mountains (Fig. 1). The trending left-lateral faults are also common, par- and the timing of folding is unclear. Gently type locality of the Pickhandle Formation is in ticularly in the northeastern Mojave Desert. The folded Quaternary gravels indicate that some the northwestern Calico Mountains, where the cumulative amount of northwest-directed dex- folding is related to active strike-slip faults, and south- to southwest–dipping section is ~1500 m tral shear across the region is probably on the north-south shortening may play an important thick (McCulloh, 1952; Dibblee, 1994; Fig. 1). order of 50–75 km (Dokka and Travis, 1990a; role in present-day strain accumulation across Overlying the Pickhandle Formation are fi ne- Glazner et al., 2002). It is unclear when this the Eastern California shear zone (Oskin et al., grained lacustrine rocks generally considered faulting began, but some indirect evidence sug- 2007). The goal of this study is to understand part of the Barstow Formation and referred to in gests that northwest- trending dextral faults may the stratigraphic and structural evolution of the this study as the Calico Member of the Barstow have locally been active as early as 19 Ma (Bart- Calico Mountains, which arguably best exem- Formation. These lacustrine rocks are intruded

117o 00' Pc Explanation Mg lt 25 Barstow Fm. (ca. 17-13 Ma Garlock fau Tb lacustrine rocks) 50 Santa Barbara Tb Los Angeles San 45 Tp 45 Andr ca. 17 Ma dacitic intrusions 25 Tv eas 30 fault Mud Hills 25 45 Southern California Calico Member of Barstow Fm. Tbc (ca. 19-17 Ma lacustrine rocks

60 40 Tpv Pickhandle Fm. volcanic rocks Tbc? o 35 00' 40 35o 00' Pickhandle Fm. (early Miocene, 20 Tp Tp mostly coarse-grained Fort Irwin Rd. volcaniclastic rocks) 10 Mg TwgTTwwwgg Waterman Hills granodiorite 65 (early Miocene) Calico Mtns.

Calico fa

Mg Mesozoic plutonic rocks 65 ult 40 20 Metavolcanic rocks (Jurassic mv Sidewinder Fm.?) 35 40 Tbc ccm Tv Mylonitic rocks in the footwall 50 ccm of the central Mojave 45 40 45 metamorphic core complex Mitchel Range Study area mv Manix Paleozoic metasedimentary Hinkley Hills Lead Mtn. fault Pc and metavolcanic rocks 25 (Plate 1) Tp 70 = contact = bedding I-15 Yermo 25 attitude = anticline 30 mv = metamorphic Barstow n. 0 5 km = syncline foliation attitude and lineation trend = fault nt Mt

ha = Waterman Hills o o detachment fault 117 00' 55 Elep 116 50'

Figure 1. Geologic map of the central Mojave Desert region near Barstow, California, compiled from McCulloh (1960, northern Calico Moun- tains), Dibblee (1968, Mud Hills; 1970, Lead Mountain–Elephant Mountain area, central Calico Mountains), Cox and Wiltshire (1993, Ele- phant Mountain area), Fletcher et al. (1995, Hinkley Hills, Mitchel Range, Waterman Hills), and this study (southern Calico Mountains).

460 Geosphere, June 2008

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Structural evolution of the Calico Mountains, Mojave Desert, California

by several dacite domes that form most of the at 1:12,000. Axial traces of all folds with ampli- (Calico Member) directly overlies metavolcanic peaks in the southeastern Calico Mountains. tudes ≥1 m were mapped, and the orientations rocks and conglomerates and breccias composed The dominant structure in the Calico Moun- of fold axes and axial surfaces were determined largely of pre-Tertiary detritus (Fig. 3). The tains is the Calico fault, a northwest-trending by measuring bedding orientations around the metavolcanic basement rocks range from basalt right-lateral fault with a maximum displace- hinge of each fold and axial trace orientations to rhyolite, but most are andesitic. The green- ment of ~10 km in the Rodman Mountains on profi le views of folds. schist facies metamorphism that affected these (~30 km southeast of the Calico Mountains; The 40Ar/39Ar geochronology was used to rocks preserves volcanic textures, but resulted Dibblee, 1964; Glazner et al., 2000). In the determine the ages of Miocene sedimentation in widespread growth of metamorphic epidote, southern Calico Mountains, the Calico fault and volcanism in the Calico Mountains. Detailed chlorite, and albite. It is unclear whether these strikes west-northwest and forms a 10-km- step-heating experiments (typically 7–12 steps) metavolcanic rocks are part of the Jurassic Side- long restraining bend (Fig. 1). Lacustrine rocks were performed on pure separates of plagio- winder Formation (e.g., Schermer and Busby, north of the Calico fault are folded into numer- clase, biotite, and whole rock from 11 different 1994) or the upper Paleozoic Coyote Group ous anticlines and synclines, making the Calico volcanic samples (Table 1). Errors reported on described by McCulloh (1952). Similar rocks Mountains one of the type localities for Neo- plateau ages are ±2σ. The major and trace ele- have been described in the Elephant Mountain gene contractional deformation in the central ment geochemistry of selected volcanic rocks area to the west-southwest (Fig. 1; Cox and Mojave Desert. The overall east-west trend of was determined by X-ray fl uorescence (XRF). Wiltshire, 1993). the folds and their proximity to the restraining bend in the Calico fault have led most geolo- STRATIGRAPHY OF THE CALICO Nature of the Pickhandle Formation–Calico gists to interpret the folding as transpressional MOUNTAINS Member Contact (e.g., Tarman and McBean, 1994; Dibblee, The contact between the Pickhandle Forma- 1994; Glazner et al., 1994). Pickhandle Formation and Metavolcanic tion and directly overlying Calico Member is Basement Rocks generally conformable, and maroon sandstone PREVIOUS WORK beds of the Pickhandle Formation appear to The Pickhandle Formation consists primar- grade up into the white-gray volcaniclastic The geology of the Calico Mountains was ily of coarse-grained volcaniclastic deposits and sandstone beds at the base of the fi ne-grained mapped by McCulloh (1952, 1960, 1965) and silicic volcanic rocks that are generally inter- lacustrine section. However, on a larger scale, later by Dibblee (1970). Prior to this investiga- preted to have accumulated in extensional basins homoclinally southeast-dipping Pickhandle tion, the fi ne-grained lacustrine section in the during rapid slip along the Waterman Hills strata strike obliquely to the Calico beds, and Calico Mountains was correlated with the type detachment fault (e.g., Fillmore and Walker, the amount of section in the uppermost Pick-

locality of the Barstow Formation in the Mud 1996). The thickest and most widespread occur- handle Formation subunit Tpsu varies from Hills (McCulloh, 1952; Dibblee, 1980a). Reyn- rence of the Pickhandle Formation in the central ~170 m to <30 m beneath the contact with the olds (2000) argued that a sequence of three Mojave Desert is in the Calico Mountains. In the Calico Member (Plate 1). Volcaniclastic sand- marker beds suggest a stratigraphic correlation northern part of the study area, the Pickhandle stone beds at the base of the Calico Member are between the lacustrine sections in the Calico Formation forms a gently southeast- to south- consistently present above the contact east of Mountains and the Mud Hills. However, the dipping homocline, and is composed primarily Calico Ghost Town (Fig. 1), so there does not age of fi ne-grained lacustrine sedimentation in of volcaniclastic sandstone, tuff breccia, and appear to be any section omitted from the base the Mud Hills Barstow Formation is bracketed dacite domes (Plate 1). Most of the Pickhandle of the Calico Member. These stratigraphic rela- between ca. 17 and 13 Ma (MacFadden et al., Formation beds are channelized and were likely tionships indicate that the bedding-subparallel 1990), which is distinctly younger than what deposited by alluvial or fl uvial processes. Pickhandle Formation–Calico Member contact we determine to be the age of lacustrine beds A distinct characteristic of the Pickhandle is a subtle angular unconformity. The geom- in the southern Calico Mountains. In this paper, Formation in the southern Calico Mountains etry of this unconformity has most likely been the lacustrine section in the Calico Mountains is that it primarily contains clasts of dacite. In modifi ed by transpressional movement along is referred to as the Calico Member of the Bar- addition, a signifi cant part of the Pickhandle the contact. stow Formation. Formation consists of biotite ± hornblende Fletcher (1986) mapped a 6 × 1 km area along dacite domes and fl ows (unit Tpd; Plate 1). 40Ar/39Ar Geochronology the Calico fault west of Calico Ghost Town at In the study area the thickest exposed section of Previous geochronologic studies of the Pick- a scale of 1:4800, focusing primarily on silver Pickhandle sedimentary rocks is only ~300 m handle Formation in other parts of the central mineralization. Based on the apparent offset of because dacite domes are widespread lower in Mojave Desert suggest that volcanism and two similar mineralized deposits, he estimated the section. Some of these domes intrude Pick- coarse volcaniclastic sedimentation occurred that 1.9–3.2 km of right-lateral offset existed handle sedimentary beds, and others are depo- between ca. 24 and 19 Ma (Burke et al., along the Calico fault. sitionally overlain by volcaniclastic intervals. 1982; Fillmore and Walker, 1996). Three new The association of domes, fl ows, volcaniclas- 40Ar/39Ar dates on dacite from the Pickhandle METHODS tic breccias, and sandstone beds suggests that Formation in the Calico Mountains range from the Pickhandle Formation in the southern Cal- 19.35 ± 0.15 to 19.0 ± 0.1 Ma. An exogenous Detailed geologic mapping provided the basis ico Mountains represents a dacite lava-dome dacite dome at the base of the Calico Mem- for understanding the stratigraphy and structural fi eld and associated pyroclastic and epiclastic ber near Old Borate yielded 40Ar/39Ar plateau geology of the southern Calico Mountains. The apron deposits. ages of 19.13 ± 0.04 Ma and 19.0 ± 0.1 Ma on folded lacustrine section in the center of the Immediately south of the Calico fault, rocks biotite and plagioclase, respectively (Plate 1; study area (Plate 1; Fig. 2) was mapped at a resembling the Pickhandle Formation do not Table 1). Detritus of this dome is present in beds scale of 1:6000; surrounding areas were mapped exist, as the fi ne-grained lacustrine section at the base of the lacustrine section, indicating

Geosphere, June 2008 461

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans Tdb 70 80 47 sshl 1 D b U ine, 55’00’’ 57’30’’ Tbc o o 80 Tdi 34 34 21 Tdb sh 35 40 80 67 Tbc 64 60 80 60 35 40 80 70 65 20 25 25 30 75 Tdb OB3 30 35 60 5 Tbc 25 c 12 42 Tdi 12 65 50 45 47’30’’ 36 o s 65 Tdi 36 75

35 1m

40 c b 116 70 70

20 b

35 d 40 65 T Tdb OB2 85 34 70 45 45

50 Tdb 50 24 48 45 ssi 85 45 Tbc 80 60 60 10 Tdb 45 60 85 Tbc 50 + 60 15 10 CM-185 17.0 0.1 Ma 65 20 g 20 60 60 25 30 Tdi 55 20 37 15 55 65 65 + 15 mv 60 mv U D 80 CM-190 17.1 0.1 Ma 1 35 30 37 30 4 30 30 Tdb 50 35 Tc 22 sh* 30 30 25 45 70 50 16 32 50 U 5 50 60 Tbc 40 D 43 70 40 Tpd 40 25 40 23 Ttb 20 42 35 35 67 55 53 27 57 50 15 40 45 ing the PDF of this paper or reading it ofﬂ reading or of this paper ing the PDF 30 35 45 1 52 75 60 32 70 60 Tgb 72 46 55 28 Tc F' 53 55 34 55 60 10 65 6 70 60 45 50 65 62 62 Tdi 40 60 42 75 45 83 40 70 40 25 60 50 67 47 Tdi 32 35 50 26 63

55 60 20 50 E' 60 33 72 35 40 60 58 40 60 30 65 32 40 30 48 30 60 33 40 50 37 lsh 60 53 45 40 72 40 80 30 60 50 CM- 257 47 Tbc OB2 25 b 1 mile 75 45 + 5 45 46 45 40 55 CM-90 19.1 0.05 Ma Tbc Tdi F 55 47 40 U 28 D 18 63 25 45 55 43 21 80 45 44 48 21 35 57 52 37 36 15 57 10 OB1 Tps 31 20 24 50 42 25 70 + Tbc 30 46 10 CM-96 16.9 0.2 Ma 61 27 28 68 15 68 28 1 kilometer 35 30 56 78 25 28 D 38 27 45 36 U 56 49 32 D D U 76 60 U 47 35 18 20 19 20 50 Tdb 60 72 48 60 OB2 mv 56 82 58 Tbc

40 Tdb 0.5 70 30 20 15 50 50 50 47 5 70 48 75 85 15 43 25 55 57 8 23 2 40 55 60 40 18 5 50 35 10 30 62 Tc 57 53 50 20 80 E 60 10 65 D 27 73 55 U Tdi 30 62 50 60 80 20 58 43 75 35 50 D 28 62 30 U 65 35 60 28 17 23 60 40 35 50 45 37 25 53 38 Qs 23 65 60 30 70 60 40 sshl 57 50 70 50 78 85 85 85 10 40 + 50 58 Tbc 23 57 85 15 60 33 CM-80 54 57 85 16.8 0.2 Ma 73 60 70 58 38 75 D 0 7 80 D' 30 78 80 U 65 65 40 80 70 38 80 60 85 52 Contour interval = 20 feet Contour 40 50 75 73 85 80 58 20 50 60 70 45 35 85 60 60 45 65 D 30 60 68 70 0 55 37 30 20 U 60 60 40 45 48

53 35 sl 45 c b Tdi 68 55 70 23 D 70 80 25 20 U 5

75 8 Tbc

60 18 Tdb 20 50 35 45 55 7 50 45 30 35 43 62 20 23 25 1 85 13 42 28 30 55 25 24 20 55 14 25 72 Tdb 56 10 43 70 17 56 38 20 70 80 40 D 80 + 30 73 9 70 40 25 45 53 30 20 72 0.5 47 CM-98 83 15 13 16.9 0.1 Ma 45 9 30 15 25 50 D 70 40 60 U 55 54 55 50 10 73 65 65 58 49 26 50’00’’ Tpd 65 15 77 o Qs 35 30 10 40 40 12 60 65 46 + 30 23 25 12 40 57 116 c 20 35 CM-42 19.0 0.1 Ma 20 25 50

66 65 40

20 15 40 35 24 18 40 1 40 7 75 70 55 27 70 30 47 80 6 Td 17 70

28 35 49 22 77

65 58 20 50 10 75 Tbc 55 60 26 60 30 15 40 15 20 55 85 62 30 25 25 Tbc Qs 22 62 80 57 65 51 10 45 85 25 32 10 33 70 15 43 70 25 55 38 3 47 15 20 22 N 50 60 35 23 55 77 70 51 55 5 45 37 55 Tdb 17 75 83 37 20 27 56 68 25 Tc 70 0.5 73 73 35 48 20 35 20 29 27 50 40

10 42 56

37 66 40 b 40 70 l 25 l

85 35 U 40

50 24 20 20 ssh 65 ssh 45

D u

55 1

23 25 52 70 55

24 40 CM-CR 85 55 Tbc 73 Tbc Tdi 62 60 38 50 45 37 76

16 11

24 38 17 db 46

20 26 23 63

Tps 67 57 60 T 85 18 68 18 40 70 35 19 62 53 U D 20 75 15 46 33 43 41 73 17 58 50 68 73 23 32 75 55 44 45 35 72 4 55 3 5 C' C 67 43 20 15 20 20 l 70 62 75 62 60 30 73 67 + 30 35 45 30 38 31

63 60 20

17 22

Tbc 45 Tbc Tbc . CM-124 .

16.8 0.1 Ma

20 Tps

26 17 70 64 43 Rd Rd Rd Rd R 12 Rd

42 70 15

43 t

71 l

23 c 25

on on on on on on on o on 50 o 50

30 y y y y y y y y y y 20 y

60 40 20 83

60 n 23

an an an an an an an

fau an

63 25 52 32 45 43 C C

20 C 17

Tdi

45 60 33

25 55 25 82 55 le C C C C C C C C C C C le le le le le le le le le le le 60 C le 72 26

17 43 54 18

19 46

67 lico 45 16 Mu Mu Mu Mu Mu Mu Mu Mu Mu Mu 20 Mu 35 10 20 21 23

9 a 24 29

50 bc 56 85 Tdb

75 C 50 32 D 85 20 75 U

23 Tdb 25 35 30 18 55 40 55 85 20 35 17 17 6 60 20 73 30 60 54 23 26 80 26 48 2 40 co fault

45 i Tbc 30 45 65 23 62 28 16 15 18

c Cal 22 52 Tdi Tbc 59 55 77 33 55 24 S. 50 60 69 Td 37 38 43 37 75 55 25 2 sigma) 30 22 60 + B' 45 40 35 63 60 32 50 70 73 25 47 22 52 40 25 75 68 80 38 50 50 65 25 18 Tdi 50 45 70 80

22 sshl 60 30 38 15 B 50 5

30 dc 38 D 75 70 30 77 73

70 hl hl

10 45 s

28 s 35 b 50

s 49 s 84 45 45 28 60 76 30 25 20 43 20

Tdi 77 30 18 Tbc 20 Tbc 65 78 60 5 66 45 58 Ar geochronology & XRF geochemistryAr geochronology = volcanic conglomerate; predominantly clast-supported; ~2/3 clasts are basalt, basalt, ~2/3 clasts are clast-supported; predominantly conglomerate; = volcanic sandstone section coarse into fines upwards metavolcanics; ~1/3 clasts are matrix and thin limy brown clast-supported; predominantly breccia; = metavolcanic common platy interbeds limestone clast-supported monolithologic, breccia; = granite ashfall tuff with ~3% biotite of reworked consisting = breccia clasts include metavolcanic mostly clast-supported; = polymictic conglomerate, ashfall tuff reworked granite, rocks, muscovite hydrothermal plagioclase, quartz, k-feldspar, minerals: = leucogranite; and breccias andesitic lavas rhyolite; basalt to rocks, metavolcanic = undifferentiated and epidote with chlorite greenschist-facies altered, propylitically most common; U 15 70 13 40 39 75 28 D 32 17 37 82 67 + 1 35 Ar/ D 50 55 65 b 37 20 42 Geochronology sample location, name, & age name, Geochronology sample location, Geochemistry sample location & name sample location, name, & age ( name, sample location, Qs contact Geologic contact approxim. (dashed where concealed) where dotted or inferred, arrow dip of fault; (hash mark shows Fault and plunge of slip vector; trend shows side) U=up-thrown side, D=down-thrown detachment fault and fold Bedding-parallel (Tp) the Pickhandlebetween Formation and Member (Tbc)the Calico where of an anticline (dotted Axial trace shows arrow or concealed, approximate axis) plunge direction of fold of a syncline Axial trace 40 2 1 25 v 35 35 20 CM-200 40 5 80 17.1 0.1 Ma 60 70 2, 52 55 mv Tc Tc g Ttb 33 Tdb Tgb 32 TmvbT 70 63 45

65 56 20

48 60 58 20 + Pre-Tertiary + CM-209 u 40 22 55

Strike & dip of bedding Strike of vertical bedding Strike & dip of overturned bed strike & dip Approximate bedding Horizontal Strike foliation & dip of flow 60 45 CM-190 CM-42

17.1 0.1 Ma 19.0 0.1 Ma 25 75 fault Calico the of South 30 35 20 D 78 U Tps 60 87 LEGEND 70 85 35 80 25 45 38 50 50 65 u 45 1 58 70 55 82 64 30 25 bc U 30 D 28 Qs 35 Tpd 85 OB3 25 30 43 60 20 25 87 17 40 22 clasts CM-209 45 18 and Tbc and b 73 20 42 85 OB2 Tdi 50 c 50 38 40 U 39 Tdi D 25 44 47 75 30 45

O 27 55 25

C 25

I 72 and between Tbc and between

LICOL 40 quartz 60 A Tps below sst & breccia = Pickhandle Fm. l 6 + A A' C 20 65 48 50 70 80 22 3% hornblende) 80 45 < 35 35 hornblende 38 + 28 77 40 56 55 30 85 70 60 25 35 by a marker bed by l 77 37 = dacite breccia composed entirely of entirely composed breccia = dacite 25 b 45 85 50 23 68 26 67 Tdb 33 53 sshl 55 77 b; b; 72 40 73 70 50 = matrix-supported. 59 Tbc 48 25 80 68 1ms 63 70 35 = clast-supported dacite breccia; locally monolithologic; clasts mostly derived from from clasts mostly derived locally monolithologic; = clast-supported breccia; dacite Tdi andTdi = undifferentiated Quaternary sediment deposits; mostly gravel-dominated mostly gravel-dominated Quaternary sediment deposits; = undifferentiated Member the Calico talus mapped over and dacite colluvium alluvium; 4-7% hornblende 13-22% plagioclase, phenocrysts: intrusion; = hornblende dacite 20-25% alteration; celadonite intrusion with patchy = hornblende dacite <1% biotite ~5% hornblende, phenocrysts: 1-3% bio 4-7% hbl, 18-25% phenocrysts: intrusion; dacite = hornblende-biotite mafic is dominant; color sea-green alteration; with pervasive celadonite = dacite ( oxidized heavily minerals basalt clasts dominant, = clast- and matrix-supported and sandstone; conglomerate uncertain position stratigraphic granite; dacite, other clasts include metavolcanics, 3/4 lower granite; metavolcanics, dacite, basalt, clasts: = polymictic conglomerate; upper 1/4 is mostly clast-supported is matrix-supported w/ sst and sltst interbeds, and limestone shale, = sandstone, platy and shale limestone = brown and limestone = sandstone and shale sandstone (silica+barite) = weakly mineralized and siltstone = lithic sandstone and siltstone = claystone altered hydrothermally shale, calcareous = red/purple/yellow-brown Tdb overlies east of Old Borate; exposure limited and shale; = tan limestone and claystone siltstone green-gray = tan to chert near base beds common shale, platy and tan calcareous limestone = brown Tbc with interbedded breccia = dacite and claystone siltstone gray green = light tan to sandstone = light olive-brown = tan sandstone sandstone = light red-brown chert near base beds common and limestone; siltstone, = sandstone, sandstone volcaniclastic = white-gray mostly clast-supported; Member; (?) with the Calico interstratified breccia = dacite Tdb phenocrysts breccia; monolithologic dacite and coarse, flows, lava domes, = dacite of plagioclase + biotite Tps); sedimentary= uppermost Pickhandle (same lithology as rocks Formation Tps from separated volcaniclastic maroon/tan sedimentary (undifferentiated); rocks = Pickhandle Fm. Tps & sandy dacitic breccia; sandstone 38 1 30 78 u 6 5 4 3 2 1 sl c b m bc ssi 50 c lsh ssh Tps sh* 4 3

sshl OB3 OB2 OB1

pped a Qs Td Tc Tc 72 75 Tps

38 Tdb Tdi Tdi Tdi Tpd Tps Tbc Tbc Tbc Tbc Tbc Tbc 35

Tbc Tdb Tbc Tbc 30 m Tdb Tbc Tbc Tbc Tbc Tbc Tbc Tbc 60

18 ROCK UNITS

Old Borate Old Mule Canyon, N of the Calico fault Calico the of N Canyon, Mule 45 Member Calico undifferentiated

52 25 45 Formation

not not

40 Ma) (~17 center volcanic

57’30’’

m Pickhandle 40 53 Dacite of the Yermo Yermo the of Dacite 77 Formation Barstow the of Member Calico 34 75 25 52 20 60 12 Tbc 35 Plate 1. A Geologic map of the southern Calico Mountains, Yermo Quadrangle, central Mojave Desert, California. If you are view Quadrangle, central Mojave Desert, California. If you are Yermo Geologic map of the southern Calico Mountains, A Plate 1. please visit http://dx.doi.org/10.1130/GES00143.SP1 (Plate 1) or the full-text article on www.gsajournals.org to view Plate 1. the full-text article on www.gsajournals.org (Plate 1) or please visit http://dx.doi.org/10.1130/GES00143.SP1

462 Geosphere, June 2008

A' (N) B (S) B' (N) 3000 3000 A (S) Tpsu Elevation (ft.) Tbcslsh 2500 2500 Tdc 2500 ? Tbcslsh Tdbbc Tbc1 c 4 6 2

S. Calico fault Tb bc 5 Tbc T c 3 Calico f. 6 Tbc b Tpd Tdib T bc 2000 Tps 2000 T Tps 2000 Tdi c Tdi Tbcslsh Tdi

? S. Calico fault 1500 1500 1500 Cal Tpd

ico

ult 1000

C' (N) 3000 C (S) Elevation (ft.) Tdb1 Tdb 2500 bc 2500 6 Tbc Tbc ? 5 Tbc Tbc1 slsh Tdic Tbc4 Tbc3 Tpsu 2000 2 2000 Tbc ? Tc3 Tdi ? Tpd Tlc 1500 ssh 1500

S. Calico fault ? Tps Tc2 1000 1000 mv

Bend in cross section D (SSW) D' (N) Elevation (ft.) 3000 3000 Tdb Tbc5/TbcOB2 Qs

Tbc5 TbcOB1 Tdib Tbc4 2500 2500 Tdi Tbc Tdi Tbcsl (undifferentiated) Tdi Calic 2000 2000 ? o fau

lt Tc2 Tpd Tps 1500 S. Calico fault Tbcssh 1500 mv Tps

1000 1000

E (SSW) Elevation (ft.) E' (NNE) 3500 3500

3000 Tbc 3000 (undiff.) Tbc Tdb

Tc2 2500 TbcOB2 2500 Tdi Tdi Tbcssh TbcOB1 Tdi Tpd 2000 ? 2000 mv ? 1500 Calic 1500 Tps Tps o faul

F' (N) Bend in section 3500 Tdb F (S)

Elevation (ft.) 3000 3000 Tbc s no vertical exaggeration Tbclsh T Tdib Tdi bc 2500 ? sh* 2500 Tdi Tp 0 500 m 1000 m ? lsh Tc 1 Calico fau Tp b 2000 Tbc 2000 mv Tmv lt Tp mv ? 1500 1500

Figure 2. Geologic cross sections of the southern Calico Mountains. Section lines and map units are shown in Plate 1.

Geosphere, June 2008 463

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

TABLE 1. 40AR/39AR GEOCHRONOLOGY OF VOLCANIC ROCKS IN THE CALICO MOUNTAINS UTM Age Radiogenic yield Sample Description coordinates Separate (Ma) K/Ca ratio (%) CM-80 hbl dacite dome that intrudes the upper 0515958 E plag 16.9 ± 0.15 .04–.07 25–74 Calico Member north of the Calico fault 3866603 N WR 16.8 ± 0.2 1.1–3.4 18–52 CM-96 hbl dacite clast from breccia overlying Calico 0516844 E plag 16.85 ± 0.15 .033–.046 31–80 Member 3867615 N WR 17.0 ± 0.5 .46–1.1 47–65 CM-98 hbl dacite dome that intrudes Calico Member 0515214 E plag 16.9 ± 0.1 .037–.047 45–74 south of the Calico fault 3865298 N CM-124 celadonite-altered hbl dacite dome south of 0514621 E plag 16.85 ± 0.15 .03–.05 40–84 the Calico fault 3865596 N CM-185 celadonite-altered hbl dacite dome that 0518768 E plag 17.00 ± 0.08 .051–.063 66–84 intrudes Calico Member west of Sunrise 3864670 N Canyon CM-190 hbl dacite dome that intrudes Calico Member 0518420 E plag 17.10 ± 0.12 .027–.031 57–71 north of Sunrise Canyon 3866144 N CM-200 hbl-bio-qtz dacite that intrudes Calico 0513528 E biotite 17.11 ± 0.06 15–95 80–92.5 Member south of the Calico fault 3866290 N CM-42 hbl-bio dacite dome in Pickhandle Formation 0515063 E plag 19.0 ± 0.1 .059–.068 56–92 3868152 N CM-90 bio-qtz dacite flow and/or dome underlying 0517190 E biotite 19.13 ± 0.04 6–130 78–93 Calico Member near Tin Can Alley 3868238 N plag 19.0 ± 0.1 .055–.06 72–87 PGMJ-57 basal vitrophyre of dacite lava in Pickhandle 0509733 E plag 19.35 ± 0.15 .068–.077 42–79 Formation near Jackhammer Gap, 3876297 N northwestern Calico Mountains CM-298 breccia consisting of reworked ash-fall tuff; 0517906 E biotite 24.9 ± 0.1 13–450 74–95 underlies Calico Member south of the 3864832 N Calico fault Note: Universal Transverse Mercator (UTM) coordinates are based on the North American Datum, 1927 (NAD 27), zone 11. Mineral abbreviations: plag—plagioclase; bio—biotite; WR—whole rock; hbl—hornblende; qtz—quartz. Mineral separates were prepared and analyzed at University of California–Santa Barbara. Ages listed are calculated weighted mean plateau ages from incremental heating experiments and are relative to flux monitor Taylor Creek Rhyolite with an assigned age of 27.92 Ma. Uncertainties are ± 2σ. K/Ca ratios are based on 39Ar/37Ar ratios. For age spectra and isochron plots, see Supplemental Figure S11 .

that the oldest part of the Calico Member is Calico Member of the Barstow Formation thick and 35–40 m above the Pickhandle Forma- ca. 19 Ma or younger. A compositionally dif- tion near Calico Ghost Town (Fig. 4). ferent dacite dome in the north-central part of The Calico Member of the Barstow Forma- The Calico Member is generally considered the study area yielded a plagioclase plateau age tion consists primarily of siltstone, sandstone, part of the middle Miocene Barstow Forma- of 19.0 ± 0.1 Ma (Plate 1; Table 1). Locally and limestone (Plate 1; Fig. 4). Lateral and verti- tion, which is ~1000 m thick at its type locality this dome appears to have intruded Pickhandle cal facies changes are common, although some in the Mud Hills (Dibblee, 1968; Woodburne sedimentary beds ~150 m beneath the contact distinct groups of beds can be mapped for a dis- et al., 1990). The age of the Barstow Forma- with the Calico Member. In the northwestern tance of ~4 km (Plate 1). We have divided the tion and potential stratigraphic correlations are Calico Mountains, a dacite fl ow near the base internal stratigraphy of the Calico Member north important because the Barstow Formation in the of the Pickhandle Formation along Fort Irwin of the Calico fault into different subunits based Mud Hills provides the basis for the Barstovian Road yielded a plateau age of 19.35 ± 0.15 Ma on distinct lithostratigraphic and color char- land- mammal age. Although stratigraphic cor- on plagioclase. There is ~1 km of volcanic and acteristics (Plate 1; Fig. 4). In the center of the relations between the Mud Hills and Calico volcaniclastic sedimentary rocks between the study area the Calico Member is ~375 m thick Mountains have been suggested (Reynolds, top of this dacite fl ow and the base of the over- (near cross-section C–C'; Plate 1; Fig. 4). East 2000), there are a few clear lithostratigraphic lying Barstow Formation (McCulloh, 1952, of Old Borate, exposure of the section is not as differences between the Calico Mountains 1960). Assuming that the uppermost Pickhan- complete, but the thickness appears to be at least lacustrine rocks and the type section of the dle Formation here is ca. 19 Ma, as it is in the 450 m. South of the Calico fault, the minimum Barstow Formation. Lacustrine rocks in the Mud Hills (MacFadden et al., 1990) and east- thickness of the fi ne-grained lacustrine section is Calico Mountains contain less granitic detritus ern Calico Mountains, then the Calico Moun- ~300 m. The Calico Member north of the fault than the Mud Hills Barstow Formation, and the tains were the site of very rapid volcaniclas- decreases in thickness westward toward Calico prominent water-laid ash-fall tuff beds in the tic sedimentation (~2.5 mm/yr) from ca. 19.4 Ghost Town. For example, a distinct set of brown Mud Hills are not present in the Calico Moun- to 19.0 Ma. This rapid sedimentation is most sandstone beds that is ~50 m thick and ~130 m tains. Sandstone beds in the Calico Mountains likely a product of both abundant dacitic volca- stratigraphically above the top of the Pickhandle are typically dominated by dacitic detritus, and nism and extensional basin development. Formation near Mule Canyon is only ~20 m rare ash-rich beds are all strongly reworked. In

1If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00143.S1 (Fig. S1) or the full-text article on www.gsajournals.org to view Supplemental Figure S1.

464 Geosphere, June 2008

North of the Calico fault South of the Calico fault

DESCRIPTION Dacite breccia (Tdb) Dacite breccia (Tdb)Monolithologic; clast-supported; 800 m 800 m most clasts < 0.5 m; crudely 16.9 + 0.2 Ma stratified in parts; source = hornblende dacite intrusions Dacite intrusion (Tdi) Dacite intrusion (Tdi) phenocrysts: 13-22% plag, dacite breccia (Tdb ) phenocrysts: 13-22% plag, Polymictic; upper 40 m = mostly clast- 4-7% hbl, 0-2% biotite bc 4-7% hbl, 0-2% biotite supported; up to 40 cm clasts, clast types Conglomerate (Tc4) =basalt (~50%), hbl-bio dacite, metavolcanic 16.8 - 17.1 Ma siltstone and 16.8 - 17.1 Ma rocks, granite Calico Member claystone (lacustrine rocks) Limey sandstone and siltstone interbedded 600 m 600 m w/ matrix-supported conglomerate

sandstone Calico Member (lacustrine rocks) limestone, siltstone, and Dacite sill (Tdi) sandstone Primarily platy limestone interbedded w/ Conglomerate (Tc3) 400 m limey siltstone; near Mule Canyon Road = Predominantly clast- limey sandstone and siltstone supported; clasts up to DaciteDacite (Td)(Tpd) ? phenocrysts: 15-20% ~0.5 m; ~66% clasts plag, 4% biotite, 1% qtz = basalt; ~33% clasts = metavolcanic rocks Metavolcanic breccia (Tmvb) 19.13 19.13+ + 0.04 0.04 Ma Ma maroon to tan volcaniclastic sandstone, matrix- Granite breccia (Tgb) Clast-supported; monolithologic supported dacite breccia, unconformity and dacite domes/flows, Tuff breccia (Ttb) clasts commonly <3 cm, breccia consisting of reworked ash-fall tuff 200 m 200 m + channelized beds common w/ ~3% biotite; bio age = 24.9 0.1 Ma Pickhandle Fm. (Tps) >1 km thick Polymictic conglomerate (Tc1) mostly clast-supported; clasts include Hypabyssal granite (g) metavolcanic rocks, granite, reworked K-spar + quartz + plag ash-fall tuff + sparse muscovite; Metavolcanic Dacite (Tpd) granophyric texture basement (mv) Basalt to rhyolite lavas and breccias; andesitic Dacite (Td) rocks most common; propylitically altered, biotite + hornblende + qtz phenocrysts greenschist-facies with abundant chlorite & 0 m 0 m epidote; Jurassic Sidewinder Formation?

Figure 3. Composite stratigraphic columns north and south of the Calico fault in the southern Calico Mountains.

addition, borates are present in the upper part of approximately east-west–trending belt that crysts. The XRF analyses of eight samples from the lacustrine section in the Calico Mountains covers ~20 km2 in the southeastern Calico the Yermo volcanic center indicate an IUGS (Fig. 4), whereas the Mud Hills Barstow Forma- Mountains (Plate 1). This dome ﬁ eld is desig- classiﬁ cation range from dacite to trachydacite

tion apparently lacks borate mineralization. nated as the Yermo volcanic center (after the (Fig. 5), with a narrow range of SiO2 content The new 40Ar/39Ar data in this study (Table 1) nearby town of Yermo; Fig. 1). The domes (65.2–67.7 wt%) and generally calc-alkaline bracket the age of the Calico Member north of intrude Calico beds that are generally steep- geochemical signatures (Singleton, 2004). the Calico fault primarily between ca. 19 and ened and baked (hardened and oxidized) The Yermo volcanic center was active dur- 17 Ma, demonstrating that this section is older against the margins of the domes. At least 15 ing the ﬁ nal stages of lacustrine sedimentation than the ﬁ ne-grained Barstow Formation in different dacite intrusions are present (Fig. 1). preserved in the Calico Member of the Bar- the Mud Hills. However, the Calico Member Coarse, clast-supported, monolithologic brec- stow Formation. Clast-supported dacite breccia

overlaps in time with the Owl Conglomerate cias that overlie the Calico Member are com- sheets (unit Tdbbc) are locally interbedded with Member at the base of the Barstow Formation positionally identical to these dacite intrusions shale in the upper part of the lacustrine sec- (Dibblee, 1968; MacFadden et al., 1990). We and were most likely shed from the domes as tion (Plate 1; Fig. 4). The clasts in this breccia propose that the fi ne-grained lacustrine section rock avalanche deposits and block and ash are compositionally indistinguishable from the in the Calico Mountains be given a new designa- fl ows. In some areas it is diffi cult to distinguish Yermo domes that intrude the Calico Member tion, Calico Member of the Barstow Formation, these breccia deposits from the brecciated por- and do not resemble dacite from the Pickhandle to indicate that the Calico Mountains lacustrine tions of the domes. Some of the breccia sheets Formation. Assuming that this breccia sheet rocks are not age correlative to fi ne-grained appear to be as thick as 150 m, but these sheets was shed from a Yermo dacite dome, at least lacustrine rocks at the type locality of the Bar- are concentrated around domes and do not 60–80 m of fi ne-grained lacustrine beds were stow Formation. appear to be laterally extensive. deposited west and east of the Yermo volcanic The dacites of the Yermo volcanic fi eld are center after the initiation of volcanic activity. It Dacite of the Yermo Volcanic Center mineralogically and geochemically homoge- appears that dacitic volcanism locally shut off neous. All contain phenocrysts of plagioclase lacustrine sedimentation in the southeastern Dacite domes and dacite breccias that are (10%–22%) and hornblende (4%–7%), and Calico Mountains, but sedimentation distal to younger than the Calico Member form an some contain sparse (1%–3%) biotite pheno- the Yermo volcanic center continued and may

Geosphere, June 2008 465

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

C-C' 400 m (Mule Canyon Rd.)

WEST Tdb dacite breccia EAST

B-B' dacite breccia claystone Calico fault Tdbbc 300 m & siltstone limestone

dacite breccia borate horizon E-E' (howlite) siltstone (Old Borate Canyon) sandstone & claystone Tbc6 reworked ash-fall tuff dacite breccia 200 m A-A' (Calico Ghost Town) borate horizon gypsiferous limey siltstone (colemanite) Calico fault Tbc5

siltstone & limey siltstone claystone Tbc4 TbcOB2 100 m sst & sandy sltst Tbc3 limestone limestone

Tbc2 sandstone limestone & sltst, sst, & chert TbcOB1 limey siltstone Tbc lithic sst 0 lithic sandstone 1 chert Tps Pickhandle Formation dacite Pickhandle Fm. Pickhandle Fm. u mostly volcaniclastic sst & Tpd matrix-supported dacite breccia (Pickhandle Fm.) 1300 m 770 m 3000 m

Figure 4. Stratigraphic columns indicating west to east variations in thickness and lithology of the Calico Member of the Barstow Forma- tion, north of the Calico fault. Most thicknesses were measured from cross sections (Plate 1; Fig. 2) and illustrate the original vertical stratigraphic succession after folding is removed. Plate 1 map units are listed on the left side of columns C–C' and E–E'.

16 have been synchronous with deposition the Bar- stow Formation beds in the Mud Hills. 14 Phonolite 40Ar/39Ar Geochronology Tephri- 12 phonolite Nine mineral separates from seven different Trachyte Yermo dacite units yielded 40Ar/39Ar plateau 10 Phono- ages that range from 16.8 ± 0.1 to 17.1 ± 0.1 Ma Tephrite O Foidite Trachy- Trachydacite (Plate 1; Table 1), indicating that the Yermo vol- 2 andesite Rhyolite 8 Basaltic canic center was short-lived. This ca. 17 Ma

O+K Tephrite trachy- volcanic activity has not previously been recog- 2 6 Basanite Trachy- andesite nized in the central Mojave Desert. Prior to this Na basalt Dacite study, it was thought that most volcanism in the Andesite 4 Basaltic Barstow area occurred predominantly between Basalt andesite 24 and 20 Ma and had ceased by ca. 18 Ma 2 Picro- basalt (Glazner et al., 2002). These new 40Ar/39Ar data provide a clear 0 upper age bracket on lacustrine sedimentation 35 40 45 50 55 60 65 70 75 in the southern Calico Mountains. North of the

SiO2 Calico fault, a dacite dome that intrudes the upper part of the Calico Member yielded pla- Figure 5. Total alkalies-silica diagram showing International Union of teau ages of 16.8 ± 0.1 and 16.9 ± 0.1 Ma on Geological Sciences volcanic rock classiﬁ cation of Yermo dacite samples a whole-rock separate and plagioclase, respec-

(ca. 17.1–16.8 Ma). The most alkalic sample of the group contains 6.56% K2O tively. A clast from dacite breccia that caps the and has probably undergone minor potassic alteration. Geochemical analyses Calico Member near Old Borate yielded a pla- were determined by X-ray ﬂ uorescence at the Washington State University teau age of 16.9 ± 0.2 Ma (plagioclase). This GeoAnalytical Laboratory. particular breccia layer contains clasts of baked

466 Geosphere, June 2008

shale, indicating that the breccia clast age is formed by slip on the Manix fault, a major east- for at least 3 km of dextral slip (see following), younger than ﬁ ne-grained sedimentation. Two northeast–striking sinistral fault that is inferred and the cumulative amount of dextral shear in dacite domes that intrude the Calico Mem- to intersect the Calico fault at the southeastern the study area (across a northeast transect from ber near Sunrise Canyon yielded ages of 17.0 end of the Calico Mountains (Fig. 1). C–E'; Plate 1) is estimated to be ~4.1 km. The ± 0.1 and 17.1 ± 0.1 Ma. Two dacite intrusions The Calico Mountains are probably best largest northeast-trending fault in the study area south of the Calico fault yielded ages of 17.1 known for the folded lacustrine rocks that are is an oblique sinistral-reverse fault with ~650 m ± 0.1 and 16.9 ± 0.1 Ma. These ages indicate spectacularly exposed along Mule Canyon Road apparent heave. Other east- to northeast-trending that the most of the exposed lacustrine rocks in and in the parking lot of the Calico Ghost Town faults appear to have small offsets, and the cumu- the southern half of the Calico Mountains are (Plate 1; Fig. 7). These folds are located north of lative amount of sinistral slip across the study older than ca. 17 Ma, although post–17 Ma the restraining bend in the Calico fault and have area is probably ≤750 m. Locally the Yermo vol- lacustrine sedimentation continued both east been cited as an example of contraction associ- canic center is shortened by the combination of and west of the Yermo volcanic center. ated with a strike-slip fault (Tarman and McBean, dextral slip along the Calico fault and sinistral 1994; Dibblee, 1994; Glazner et al., 1994). slip along the largest east- northeast–trending STRUCTURAL GEOLOGY fault. However, the dominance of dextral and Faults dextral-reverse shearing over sinistral shear- The Miocene rocks in the southern Calico ing indicates that conjugate strike-slip fault- Mountains record a complex structural history The majority of faults in the Calico Moun- ing does not play a major role in north-south that involves extension, strike-slip faulting, tains strike northwest and dip steeply (Fig. 6). shortening. In addition, although the east- to and shortening. Northwest-striking, high-angle These northwest-striking faults include normal northeast- trending sinistral faults are oriented (≥45°) normal faults are common in the Pick- faults in the Pickhandle Formation and oblique as a conjugate set to the dextral faults, dextral handle Formation (Fig. 6). However, strike-slip dextral faults throughout the study area. East- to faulting appears to have postdated most of the and transpressional faulting strongly overprint northeast-striking faults are also present and typ- sinistral faulting. The Calico fault and Southern the extensional fault system and represent the ically have oblique sinistral slip. The intensity Calico fault are not cut by any east- or northeast- dominant style of deformation in the study area. of faulting is strongly dependent on rock type; trending faults, whereas the Calico fault cuts Northwest-striking dextral and oblique dextral the Pickhandle Formation and the Yermo dacite three northeast-trending faults (including the faults are particularly common. The most sig- rocks are highly faulted, whereas faults are rare largest oblique sinistral fault; Plate 1). Outside niﬁ cant structure in the area is the Calico fault, within the Calico Member (Plate 1). Northwest- of the Calico fault system, east- to northeast- which strikes west-northwest and forms a left striking dextral faults that cut the Pickhandle trending faults are commonly cut by northwest- bend in the southern Calico Mountains. Based on Formation and Yermo dacite appear to die out trending dextral faults (Plate 1). These timing the orientation of this apparent restraining bend, abruptly in the Calico Member. Individual Cal- relationships suggest that sinistral and dextral the Calico fault is thought to accommodate trans- ico beds that show little or no evidence of brittle faulting were generally not mutually active. pressional deformation, although the amounts of offset persist along strike for >2 km (Plate 1). heave and throw along this segment of the fault Dextral and oblique dextral-reverse shear- Faults within the Pickhandle Formation have not been previously estimated. Dibblee ing are clearly the dominant modes of brittle Most faults in the Pickhandle Formation (1994) suggested that this restraining bend was deformation. The Calico fault system accounts strike northwest, dip ≥45° (dominantly to the

Equal Area Equal Area Equal Area

AB C

Figure 6. Fault data from the southern Calico Mountains. (A) Poles to all measured fault planes. Boxes—fault planes in the Pickhandle Formation. Triangles—fault planes along the Calico fault system. Solid circles—all other fault planes (mostly in dacite rocks of the Yermo volcanic center). (B) Measured fault planes and striae in the Pickhandle Formation. The majority of faults strike northwest, dip >45° and have downdip striae. Approximately 40% of measured surfaces with downdip striae also have subhorizontal striae. (C) Measured fault planes and striae along the Calico fault system. The red fault represents an average of fault data collected from the main Calico fault northwest of the restraining bend (northwest of the study area). Most fault planes dip north-northeast and have striae that rake obliquely from the west-northwest, indicating dextral-reverse slip.

Geosphere, June 2008 467

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

S N

Calico fault

PICKHANDLEPICKHANDLE FORMAFORMATIONTION

Parking lot 0 ~25 m A

N N

1 m

B C

Figure 7. Selected views of folds in the Calico Member. (A) Folded lacustrine rocks in the Calico Ghost Town parking lot. The photo is a composite of three shots with distortion. The line drawing is a composite proﬁ le view sketch that is approximately true

to scale. View is looking west. (B) Anticline and syncline in chert beds near the base of Tbc2. (C) Anticline in lacustrine sandstone,

siltstone, and limestone beds (subunit Tbc2). 1.5 m Jacob’s staff for scale. (D) Anticline and syncline in subunits Tbc5 and Tbc6 along Mule Canyon Road. Folds have an amplitude of ~30 m and a north-vergent asymmetry. View is looking northwest.

468 Geosphere, June 2008

southwest), and have downdip striae (Fig. 6B). Geometry and Kinematics tive hydrothermally altered dacite (Plate 1). This Offset markers and fault plane kinematic indi- Southeast and northwest of the study area, the older dacite is highly fractured and weathered cators (e.g., Reidel shears) suggest that most of Calico fault has an average strike of ~N35–40W and has a distinct sea-green color from pervasive these dip-slip faults have normal offset, although (Fig. 1). Along the restraining bend in the south- celadonite alteration (unit Tdc). The northwest- the amount of normal throw along individual ern Calico Mountains, the Calico fault has an ern margins of both intrusive domes are faulted faults is typically <50 m and often only a few average strike of ~N70W and dips 45º–70°NNE against crudely stratifi ed, purple dacite breccia meters. The total magnitude of northeast-south- (Plate 1; Fig. 6C). Fault plane striae on the that appears to be interbedded with lacustrine west extension calculated from fault dips and Calico fault and subsidiary fault strands consis- rocks on the south side of the Calico fault (Plate apparent offsets is ~5% (~100 m over a distance tently rake obliquely from the west-northwest, 1). Two sets of northeast-striking faults that are of ~2 km). Northwest-striking normal faults are indicating oblique dextral-reverse slip (Fig. 6C). cut by the Calico fault northwest of the offset rare in the Calico Member and Yermo dacite, The reverse (north side up) component of slip domes are likely the same (Plate 1). These dis- suggesting that much of this faulting occurred can also be inferred from the north-dipping, tinct sequences of rocks on opposite sides of the prior to ca. 19 Ma and was related to extension in overturned beds on the south side of the Calico Calico fault match well if 1.1–1.2 km of right- the central Mojave metamorphic core complex. fault, west of Calico Ghost Town (Plate 1). This lateral slip along the Calico fault is restored. East However, the gentle south and southeast tilting fault geometry and fault slip data are compatible of Mule Canyon, the amount of slip most likely of Pickhandle Formation strata is not compatible with a transpressional strain regime in which the increases due to additional northwest-striking with northeast-directed extension, suggesting principal shortening direction is approximately faults that either merge with or are cut by the that the tilting may be the composite effect of north-south. Northwest of the restraining bend, Calico fault. A northeast-striking, northwest- both extensional and transpressive deformation. the Calico fault dips ≥70°NE and has subhori- dipping fault along Mule Canyon Road south of Of measured fault planes with downdip striae, zontal striae, suggesting that the transpressional the Calico fault may be the offset portion of a ~40% also have subhorizontal striae (Fig. 6B). slip regime is primarily restricted to the restrain- similarly oriented fault north of the Calico fault, On fault planes where the relative timing of ing bend in the southern Calico Mountains which would indicate ~1.5–1.6 km of cumula- slip could be determined, dextral movement (Fig. 6C). The offset Pickhandle Formation– tive right-lateral slip (Plate 1). overprints dip-slip (mostly normal) movement. Barstow Formation contact in the easternmost Exposure south of the Southern Calico fault These relationships are consistent with the idea Mud Hills does not appear to be uplifted across is poor, but two distinctive dacite domes crop that dextral faulting related to the Eastern Cali- the Calico fault, providing further evidence that out along opposite sides of the fault east of fornia shear zone was superimposed on early transpressional shortening across the Calico Ghost Town Road and west of Mule Canyon Miocene northeast-southwest extension. fault is localized along the restraining bend. Road (Plate 1). Mineralogically, these domes However, reverse slip has also been reported are similar to most dacite of the Yermo volca- Calico Fault along the Calico fault in the Rodman Moun- nic center (~5% hornblende phenocrysts, <1% The Calico fault is one of the largest strike- tains (Glazner et al., 2000; Oskin et al., 2007), biotite, no quartz), but both domes are distinct slip faults in the Mojave Desert. In the Rodman indicating that locally the Calico fault accom- in outcrop because of their patchy celadonite Mountains (~30 km southeast of the Calico modates shortening without the presence of a alteration and liesegang banding. Assuming that Mountains) the Calico fault has a maximum dis- restraining bend or stepover. these domes are the same, the southern Calico placement of 9.8 km (Dibblee, 1964; Oskin et Fault striae on the Southern Calico fault also fault has 1.8–1.9 km of right-lateral slip. Thus, al., 2007). Displacement decreases to the north- rake obliquely from the west-northwest, but the the Calico fault system in the southern Calico west, and the Calico fault appears to break into dip of this fault varies from near vertical to 48°N Mountains has 3 ± 0.1 km of cumulative right- several strands in the Mud Hills (Fig. 1). The over short distances, and near Mule Canyon lateral slip distributed between two faults, and segment of the fault northwest of the restrain- Road the fault makes an abrupt bend (Plate 1). restoring this right-lateral slip concentrates the ing bend in the Calico Mountains was estimated This irregular geometry suggests that the South- Yermo dacite dome fi eld into an approximately to have 1.9–3.2 km of dextral slip based on the ern Calico fault may have been an older strand east-west–trending ellipse. This offset estimate apparent offset of the Waterloo and Langtry of the Calico fault system that became inactive is similar to the 1.9–3.2 km of offset estimated barite-silver deposits (Fletcher, 1986). In the and was deformed. by Fletcher (1986) along the Calico fault northeastern Mud Hills the main strand of the Calico west of the study area. However, an offset of fault appears to have 1.3 km of slip, based on the Offset 3 km should be considered a minimum estimate apparent offset of a dacite dome at the top of the The clearest offset markers along the Cal- because additional strands of the Calico fault Pickhandle Formation. ico fault are found between Mule Canyon and may exist under alluvium southwest of the Cal- In the study area the Calico fault cuts lacus- Ghost Town Road. A distinctive 17.1 ± 0.1 Ma ico Mountains. trine rocks of the Calico Member, domes of the dacite dome south of the main Calico fault can The amount of reverse (north side up) slip Yermo volcanic center, and, in the southeastern be matched to a dacite dome north of the fault along the Calico fault system is ambiguous. end of the range, metavolcanic rocks (Plate 1). at Camp Rock, ~1.2 km to the east-southeast Based on average slickenline rakes, the main Two major strands of the Calico fault system (Plate 1). These domes contain 20%–25% strand of the Calico fault should have 0.2–1 km exist along the restraining bend, including a phenocrysts, including 3%–5% hornblende, of reverse slip, corresponding to ~100–500 m poorly exposed fault that fl anks the southern 2%–4% biotite, and ~1% anhedral quartz. No of approximately north-south shortening. The edge of the Calico Mountains east of Calico other dacite rocks near the Calico fault have Southern Calico fault should have 0.7–1.3 km of Ghost Town (Plate 1). This fault is referred to as more than 1% biotite phenocrysts or more than a north-side-up throw, but the amount of horizon- the Southern Calico fault. The fault previously trace amount of quartz. Both offset domes along tal shortening across this fault is unknown due identifi ed by McCulloh (1965) and Dibblee the Calico fault intrude a calcareous interval to the variable dip of the fault plane. Thus, if slip (1970) as the main Calico fault is referred to as within the Calico Member, and on their eastern vectors measured along the Calico fault system the Calico fault in this study. margin are in intrusive (?) contact with a distinc- are representative of the entire slip history of

Geosphere, June 2008 469

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

the fault, the cumulative amount of reverse slip structural high during Pickhandle deposition. blee, 1994; Glazner et al., 1994), yet based on along the Calico fault system is ~1–2 km. How- We favor the interpretation that an early Mio- fi eld work to the west of Calico Ghost Town, ever, the Calico fault juxtaposes fi ne-grained cene proto-Calico fault was a basin-bounding, Weber (1976) proposed that folding was due to lacustrine rocks <500 m thick that are presum- northeast- to north-northeast–dipping normal south-directed gravitational sliding of the Calico ably part of the same section, suggesting that fault that enabled >1 km of Pickhandle For- Member off of the underlying Pickhandle For- reverse slip along this strand is <500 m. Oblique mation rocks to accumulate to the north. The mation. This interpretation of folding was sup- dextral-reverse slip vectors may only refl ect the present ~45°–70°NNE dip of the Calico fault ported in part by Tarman and McBean (1994), most recent movement along the Calico fault is consistent with the inferred dip of this hypo- who noted that west of Calico Ghost Town, a system. It is possible that the Manix fault bent thetical proto-Calico normal fault, and in one south-dipping polished plane separates the Pick- the Calico fault into a restraining bend orienta- location in the southeastern Calico Mountains, handle Formation from Calico Member, and that tion after the Calico fault had been active as a the main Calico fault plane has downdip striae some beds appear to be missing from the base more northwest-trending, purely strike-slip fault that are overprinted by striae from oblique of the Calico Member, suggesting that gravity (Dibblee, 1994). Accordingly, 1–2 km would dextral-reverse movement. The sense of slip on slumping may have formed some folds. How- be an overestimate of the cumulative amount of the downdip striae is unclear, but this older dip- ever, Tarman and McBean (1994) argued that reverse slip along the Calico fault system. Given slip movement may record early Miocene nor- the upright axial surfaces of many of the folds that the amount of reverse slip along the Calico mal faulting. Northwest of the restraining bend are not compatible with gravity folding. Another fault is most likely <500 m, the total amount of in the Calico Mountains, Pickhandle Formation possible explanation for folding is that forceful north-side-up slip along the Calico fault system rocks are present on both sides of the Calico emplacement of dacite domes into the Calico is probably closer to 1 km, corresponding to fault (Fig. 1), suggesting that only the restrain- Member ca. 17 Ma folded the lacustrine rocks. ~500 ± 150 m of north-south shortening. ing bend segment of the fault was a Pickhandle Folding in the Calico Mountains is primar- basin-bounding normal fault. ily restricted to the Calico Member north of the Pretranspressional Slip on the Calico Fault Calico fault restraining bend. Folds are detached The signifi cant stratigraphic mismatch of Folds from the underlying Pickhandle Formation, which pre-lacustrine rocks across the Calico fault in homoclinally dips ~15–30°S to SE beneath the the southeastern Calico Mountains strongly Folds are common structures across the Calico Member (Plate 1; Fig. 8A). Calico beds suggests there was uplift on the south side of Mojave Desert, particularly in Miocene lacus- south of the Calico fault are steeply tilted but the fault prior to dextral and/or transpressional trine rocks (e.g., folds in the Calico Mountains, not folded into the same scale of anticlines and movement. Metavolcanic basement rocks Alvord Mountains, Lead Mountain area, Mud synclines that characterize deformation in the underlie the Calico Member south of the Calico Hills, Kramer Hills, and Black Canyon area; Calico beds north of the Calico fault (Fig. 8). fault, whereas a thick section of Pickhandle for locations see Bartley et al., 1990). In most Formation underlies the Calico Member north of these areas the age and structural signifi cance Geometry of the fault. Calico beds across the fault are of folding are poorly understood. The Calico The geometry of folds in the Calico Mem- most likely correlative, indicating that either Member rocks in the southern Calico Mountains ber varies considerably, but several important metavolcanic basement rocks were unroofed expose some of the tightest folds in the central generalizations can be made. Most folds have of Pickhandle deposits prior to the deposition Mojave Desert (Fig. 7). This folding is generally steeply dipping axial surfaces (>75°) and shal- of the Calico Member (beginning ca. 19 Ma), thought to be related to transpression along the lowly plunging axes that trend east-west ±30° or that the area south of the Calico fault was a Calico fault (Tarman and McBean, 1994; Dib- (Fig. 9). One exception to this fold orientation is

Equal Area Equal Area Equal Area

ABC

Figure 8. (A) Poles to measured bedding attitudes in the Pickhandle Formation. Beds generally dip 15–30°S to SE and are not folded. (B) Poles to bedding attitudes in the folded Calico Member north of the Calico fault. Figure only shows attitudes plotted in Plate 1 and does not include bedding from fold hinges. The greater number of south-dipping beds reﬂ ects the overall north-vergent asymmetry of folds in the Calico Member. (C) Poles to bedding attitudes in the Calico Member south of the Calico fault. The triangles represent poles to overturned Calico beds west of Calico Ghost Town (see Plate 1). Folds south of the Calico fault are rare, and west-dipping beds are common.

470 Geosphere, June 2008

o o o p T races parallel = fold axis = pole to fold axial surface = average fold interlimb Stereonet IA* angle: 1 = 45-60 3 = 75-90 2 = 65-75 IA*=2 Ty th of the axial trace. Note LEGEND nset stereonet A shows fold axes A nset stereonet tting planes to fold axes and axial = axial trace of anticline in the Calico Member = axial trace of syncline in the Calico Member 0 1 km = fault (dashed where approx. or concealed) = contact between Miocene rocks = Quaternary alluvium = Pickhandle Formation = ca.17 Ma Yermo dacite rocks Yermo = ca.17 Ma (intrusions and breccia) Tp Ty Qal IA*=1 tting planes to poles to bedding collected around tting planes to poles bedding collected around Ty IA*=3 IA*=2 IA*=1

Calico fault IA*=2 Pickhandle Formation (Tp) IA*=2 trend and subhorizontal plunge. (B) Poles to fold axial surfaces. Axial surfaces were determined by fi Axial surfaces were and subhorizontal plunge. (B) Poles to fold axial surfaces. trend ° ed map of the southern Calico Mountains illustrating geometry and map view pattern of folds in the Calico Member. Fold axial t ed map of the southern Calico Mountains illustrating geometry and view pattern folds in Member. A: fold axes fold A: axial surfaces B: N IA*=3 n = 132 n = 111 Figure 9. Simplifi 9. Figure and in any given structural domain, folds have consistent orientations. (A) I (Ty), dacite rocks Yermo the overall contact with determined by fi m. Most fold axes were with an amplitude >1 folds in the Calico Member all measured from than one point due to a change in fold geometry along the leng by more hinges. In some cases the same fold hinge is represented 30 overall east-west ± trace orientations. Note that most axial surfaces dip steeply north and south.

Geosphere, June 2008 471

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

a small set of north-south–trending folds on the approximately northeast-southwest to the north- and Odessa Canyon Roads the amount of northeast side of Mule Canyon Road (Plate 1; Fig. 9). west of the contact and northwest-southeast to south shortening ranges from 27% to 33%, and These north-south folds plunge into the core of the southwest of the contact (Plate 1; Fig. 9). the amount of north-northeast–south-southwest an east-west–trending syncline, suggesting that West of the Yermo volcanic center, axial traces shortening near Old Borate is 24%–29%. Along they have been refolded by the syncline. Folds are consistently more east-west oriented; thus transects where folding accounts for ~500 m are cylindrical and systematically oriented on the map view geometry of the folds appears north-south shortening (near C–C'; Plate 1; short length scales, although fold axis orienta- to have been infl uenced by the presence of the Fig. 2), folding appears to die out near the tion, axial surface attitude, and interlimb angle Yermo volcanic center. The map view pattern of Pickhandle Formation–Calico Member contact, commonly change along the axial trace of a fold axial traces is also refl ected by the contact with suggesting that 500 m is an approximate upper (Plate 1; Fig. 9). the underlying Pickhandle Formation (Plate 1; limit on shortening due to folding. One of the most distinct characteristics of the Fig. 9). North of the Yermo volcanic center, the Calico folds is their relatively small size. The Pickhandle Formation–Calico Member contact Stratigraphic Controls largest anticlines and synclines have amplitudes also forms a sigmoidal shape that is parallel to Stratigraphic controls on folding are signifi - up to ~30 m, but the majority of folds have ampli- the contact with the Yermo dacite rocks. cant in the Calico Mountains. The most obvious tudes of 2–12 m. Most fold sets have wavelengths Axial surface attitudes vary across the study contrast in deformation can be seen between the <100 m, and axial traces are mostly ≤0.75 km area, but folds with similar axial surface ori- Pickhandle Formation and the Calico Member. long. These small-scale folds are very different entations generally occur in groups (Fig. 9). The Pickhandle Formation is clearly not folded, than isolated, kilometer-scale folds such as the There does not appear to be a systematic spa- whereas Calico beds stratigraphically 5–15 m Barstow syncline in the Mud Hills (Dibblee, tial pattern of north-dipping or south-dipping above the Pickhandle Formation are folded into 1968), the Box Canyon syncline in the Rodman axial surfaces (Fig. 9). Neither fold interlimb a series of anticlines and synclines (Plate 1; Mountains (Dibblee, 1964), and the Lenwood angles nor the amount of shortening due to Figs. 2 and 8). Thus, there is a detachment hori- anticline near Barstow (Dibblee, 1967). folding vary systematically across the study zon between the top of the Pickhandle Forma- Many of the folds have a clear north-vergent area (Fig. 9). The folds with the largest aver- tion and lower part of the Calico Member. The asymmetry. Although axial surfaces dip nearly age interlimb angles (75°–90°) that represent homogeneous, massively bedded Pickhandle equally to the north and south (Fig. 9), north- the smallest percent of north-south shortening Formation lacks the marked planar mechanical dipping limbs are usually shorter than south- (20%–23%) are near Calico Ghost Town at anisotropy of the thinly bedded lacustrine sec- dipping limbs (Plate 1, Fig. 2). In certain areas the western end of the folded Calico Member tion and was rheologically less able to deform folds are symmetric, but south vergence is rare. (Fig. 9). The tightest folds (average interlimb by folding. The majority of folded sandstone, limestone, angle = 45°–60°) occur in an ~1.5-km-long The amount of shortening by folding and chert beds are classifi ed under Ramsay’s belt where folds trend northeast-southwest to decreases in the upper ~30–100 m of the Cal- class 1B (parallel folds), whereas low compe- east-west between the contacts with the under- ico Member. The dips of beds become con- tence shale layers are commonly thickened in lying Pickhandle Formation and overlying sistently shallower up toward the contact with the hinge area, producing class 2 and class 3 dacite breccias (Fig. 9). These folds represent the overlying dacite breccia (Fig. 10). Folds folds (Ramsay, 1967; Fig. 7). Angular, chevron- 30%–40% shortening. Between Mule Canyon rarely exist within 15 vertical m of this contact, like fold hinges are common in competent beds (Fig. 7). The presence of some parasitic folds and the strong planar mechanical anisotropy in the Calico Member indicate that folding was accommodated largely by fl exural slip. Most Tdb folds have an interlimb angle between ~45° and 90° (average = ~60°–70°). Based on eight restorable cross sections of the Calico beds, the average amount of north-south horizontal shortening represented by the Calico folds is ~25%–30% (see Singleton, 2004, for details). The total shortening due to folding between the Calico fault and the Pickhandle Formation is ~150–500 m.

Map View Patterns In the eastern half of the study area, folds parallel the contact with the dacite domes and breccias of the Yermo volcanic center (Plate 1; Fig. 9). This map view pattern is particularly evident north of the volcanic center, where axial traces form a sigmoidal shape that is parallel Figure 10. Contact between the Calico Member and overlying dacite brec- to the overall contact with the dacite domes cia (Tdb). Note that Calico beds become more shallowly dipping closer to and breccias. East of cross section C–C', axial the contact. The solid white line marks the Calico Member–dacite breccia traces wrap around the western edge of the contact. Dashed white lines are form lines in the Calico Member. View is Calico Member–dacite breccia contact, trending looking southwest.

472 Geosphere, June 2008

and shale beds typically dip <30° beneath the breccias suggests that folding is younger than fault are not tighter than northeast-southwest– breccia (Plate 1). The decrease in dip in the dome emplacement. trending folds, and fold interlimb angles do not uppermost part of the Calico Member appears Another explanation for the overall sigmoidal decrease along strike to the west (closer to the to be gradual, but in one location at the south pattern of fold axial traces involves the forma- Calico fault; Fig. 12). Other observations that end of Old Borate Canyon, the decrease in tion and subsequent rotation of originally north- argue against a wrench folding model include: shortening is accommodated by a discrete sub- east-southwest–trending folds by right-lateral (1) east-west– and east-southeast–west-north- horizontal detachment ~15 m below the Calico shear. If folds were originally oriented more west–trending folds north of the Yermo volca- Member–dacite breccia contact. Below this northeast-southwest, the east-west– trending nic center are not located near a zone of higher detachment shale beds are folded into a small- folds west of cross-section C–C' (Plate 1) could shear strain, and (2) east of cross-section C–C', scale anticline and syncline pair, whereas above have rotated clockwise due to higher dextral northwest-southeast–trending folds are located the detachment beds dip shallowly toward the shear strain adjacent to the Calico fault. This due south of northeast-southwest–trending folds dacite breccia. The shallowing of dips and model interprets the folds as wrench folds along (Plate 1; Fig. 9), resulting in a map view pattern decrease of shortening in the uppermost part the Calico fault. Most experimental models pre- that is incompatible with progressive clockwise of the Calico Member appear to be restricted dict that folds associated with wrench faults will rotation of folds. to beds near the Yermo volcanic center that are initially form at ~45° to the trace of the mas- The most likely explanation for the map view capped by dacite breccias (Plate 1). ter fault (i.e., perpendicular to the incremental pattern of fold axial traces is that the Yermo dac- shortening direction; Odonne and Vialon, 1983; ite domes and breccias resisted shortening, forc- Timing of Folding Jamison, 1991; Tikoff and Peterson, 1998). With ing the Calico beds to wrap around these rocks. The shallowing of dips in the uppermost progressive shear, folds rotate toward parallel- This buttressing inﬂ uence of the rigid Yermo Calico beds could be explained if sedimentation ism with the master fault, becoming tighter as volcanic center during north-south shortening were synchronous with folding; however, fold they rotate (Fig. 11). If folding in the southern caused folds to become oriented parallel to the growth strata do not appear to be present. Dacite Calico Mountains was produced by this mecha- overall contact of the dacite domes and brec- breccias related to the Yermo volcanic center are nism, folds that parallel the trace of the Calico cias. The decrease in shortening in the upper- overall conformable with the underlying Calico fault (~N70W) should have smaller interlimb most part of the Calico Member can be similarly Member and do not cut across any fold struc- angles than the northeast-southwest–trending explained by assuming that the thick breccia

tures. The dacite breccia sheet (Tdbbc) interbed- folds, and folds should become progressively sheets adjacent to domes resisted folding. Cal- ded with shale in the upper Calico Member is tighter westward (closer to the Calico fault). ico beds close to the dacite breccia contact were clearly folded. These observations indicate that However, there are no systematic relationships not able to fold due to the mechanical rigidity folding took place after deposition of the old- between the fold orientation, interlimb angle, of the overlying breccias. West of cross-section est Yermo breccias (ca. 17 Ma). Upper age con- and distance from the Calico fault (Figs. 9, 11, C–C' (west of the Yermo volcanic center; Plate straints on folding are poor due to the lack of and 12). Folds that are subparallel to the Calico 1), folds are consistently oriented approximately post–Calico Member rocks.

Interpretation of Folding Map view patterns and stratigraphic varia- Fold trend vs. interlimb angle

tion. The map view pattern of fold axial traces 160 30 suggests there is a geometric relationship hypothetical pattern due to tightening 60 30 of wrench folds with progressive rotation 50 140 63 between the folds and the dacite rocks of the (based on calculations by Jamison, 1991) wrench fault (strike = 110 Yermo volcanic center. One possible explana- 55 tion for the parallelism between the axial traces 120 o) and the contacts with the dacite domes and breccias is that folding was caused by the force- 100 ful intrusion of dacite into the Calico Member ca. 17 Ma. Calico beds adjacent to intrusions 80 are steepened and generally strike parallel to the margins of the intrusions, but this deforma- 60

tion appears to be largely restricted to an area (degrees) angle Interlimb within ~150 m of the intrusion contact (Plate 1). 40 Some small-scale folds occur in beds deformed by dome emplacement, but unlike most folds 20 in the Calico Member, the geometry of these 0 intrusion-parallel folds is highly irregular, 0 20 40 60 80E-W 100 120 140 160 180 and the axial traces are <150 m long. Dome N-S Fold trend average strike of the N-S emplacement as a mechanism for folding also Calico fault (range:100-130O) does not explain the presence of east-west– trending folds far west of the dome ﬁ eld, nor Figure 11. Relationship between fold trend and fold interlimb angle using data from does this mechanism account for the overall folds in the Calico Member north of the Calico fault. There is no systematic relationship lack of folding south of the Calico fault. In between fold trend and fold tightness. If folding was related to wrench faulting, folds that addition, evidence that folding took place fol- parallel the trace of the Calico fault theoretically should have smaller interlimb angles lowing deposition of the oldest Yermo dacite than the northeast-southwest–trending folds.

Geosphere, June 2008 473

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

Along strike variation in fold interlimb angle ber to be thrust over the Pickhandle Formation (Interlimb angle vs. E-W position) along the south-dipping contact–folding detachment. The dominant north-vergent asymmetry of E-W trending folds NE-SW folds E-W trending folds ESE-WNW trending folds folds is compatible with this sense of movement Distance from Calico fault (m): < 250 300-800 800-1200 1200-2000 > 2000 along the detachment. The total amount of north- directed reverse slip along the detachment must Tp Only folds shown in map inset plotted be at least equal to north-south shortening due 140 to folding in the hanging wall (up to ~500 m). In Ty Mule Canyon, there is evidence that folding dies 120 out just south of the contact (e.g., cross-section C–C'; Fig. 2), suggesting that the detachment 100 is a blind fault with slip progressively decreas- ing northward to where the Tp-Tbc detachment 80 becomes a normal stratigraphic contact. A right- lateral component of movement along the Tp- 60 Tbc contact can be inferred from the change in

Interlimb angle (degrees) angle Interlimb the orientation of Pickhandle beds at the con- 40 tact. West of Mule Canyon, southeast-dipping Pickhandle beds abruptly bend into parallelism 20 with south-dipping Calico beds adjacent to the

50' ° contact (Plate 1), consistent with drag due to

Calico Ghost Town Old Borate Canyon

116 0 right-lateral slip along the contact. 012345678 Along the Tp-Tbc contact to the east of Calico W E-W position (km) E Ghost Town, there does not appear to be a discrete slip surface along which reverse-dextral Figure 12. Along strike variation in fold interlimb angle. Only interlimb angles from the movement occurred, suggesting that movement relatively continuous belt of folds shown in the map inset were plotted. If the approxi- may have been largely distributed throughout the mately east-west–trending folds west of the Yermo dacite domes and breccias (Ty) rotated sandstone beds at the base of the Calico Member. from more northeast-southwest orientations, there should be a progressive tightening of West of Calico Ghost Town, the absence of the folds to the west (closer to the Calico fault). However, the tightest folds are located in a basal sandstone beds and the presence of a south- northeast-southwest– to east-west–trending belt between the dacite rocks and the Pick- dipping, bedding-parallel fault surface between handle Formation (Tp). the Pickhandle Formation and the Calico Mem- ber indicate that slip along the contact–folding detachment in this area probably occurred along east-west, and several close folds are present suggesting north-directed (reverse) transport a discrete fault. Two sets of striae are present on in the inferred uppermost part of the section. above the folding detachment. this surface; one set rakes 67° from the west and a These observations are consistent with the idea Transpression. The location of folds adjacent more subtle set rakes 36° from the east. The east- that the geometry of Calico beds in this area was to a transpressional restraining bend constitutes raking striae are compatible with dextral-reverse not inﬂ uenced by dacite intrusions and breccias the most basic evidence that folding is tectonic slip along the folding detachment. As the detach- from the Yermo volcanic center. and related to the Calico fault system. Calico ment approaches the Calico fault (e.g., near Cal- The presence of a folding detachment hori- Member beds northwest of the Calico fault ico Ghost Town), it steepens and is inferred to zon between the Calico Member and the Pick- restraining bend are generally not intensely root into the main Calico fault (Fig. 2), forming handle Formation might be used to argue that folded or overturned (Dibblee, 1970), sug- a fault zone geometry that resembles a positive south-directed gravity gliding was responsible gesting that shortening in the southern Calico ﬂ ower structure. for folding the Calico beds. If folding were due Mountains is fundamentally related to trans- The north-vergent asymmetry of folds and to the Calico Member sliding off of the south- pression within the restraining bend. the lack of numerous folds south of the Calico dipping Pickhandle Formation, folds would The Pickhandle Formation (Tp)–Calico fault indicate that the Tp-Tbc contact played a most likely have south-vergent asymmetry and Member (Tbc) contact represents an important fundamental role in folding. If slip along the axial surfaces that dip north, away from the break in the style and magnitude of deforma- main Calico fault was primarily responsible for direction of transport. Alternatively, if gravity tion. Calico beds north of the Calico fault have folding, folds might have a south-vergent asym- folding occurred when the Calico beds were still undergone 25%–33% north-south shortening metry, and lacustrine rocks south of the fault unconsolidated, folds might have highly irregu- due to folding, whereas the Pickhandle Forma- would most likely also be folded into small-scale lar geometries that are characteristic of soft sed- tion generally lacks folds and reverse faults. Slip anticlines and synclines. The presence of north- iment deformation and slumping. None of these along the Tp-Tbc contact is necessary in order vergent folds only north of the Calico fault can hypothetical geometries is consistent with folds to accommodate detachment folding. A few be explained with a model in which north-south in the Calico Mountains. Folds in the Calico observations suggest that this contact has both compression between the Calico Member and Member are systematically oriented and have a reverse and right-lateral component of move- the more rigid Pickhandle Formation above a upright axial surfaces that dip nearly equally to ment that is compatible with transpressional slip basal detachment drove folding of the lacustrine the north and south (Fig. 9). Moreover, the dom- along the Calico fault. North-south shortening rocks. Calico beds south of the Calico fault were inant sense of fold asymmetry is north vergent, would presumably have forced the Calico Mem- not thrust over a south-dipping basal detachment

474 Geosphere, June 2008

and therefore deformed differently than their fault (Fig. 13D). Transpressional faulting and and 17.5 Ma (Gans et al., 2005). Based on new counterparts north of the fault (Fig. 8). folding are fundamentally related to the west- geochronologic data from this study, the thick northwest–striking restraining bend in the Calico section of Pickhandle Formation in the Calico DISCUSSION fault system. It is possible that the proto-Calico Mountains accumulated rapidly from ca. 19.4 fault was west-northwest striking, which would to 19 Ma, and ~400 m of fi ne-grained lacustrine Interpretive Stratigraphic and Structural indicate that the restraining bend is an original rocks were deposited between ca. 19 and 17 Ma. History of the Southern Calico Mountains feature of the Calico fault system. If the restrain- Thus, the Pickhandle Formation and at least the ing bend is original, transpressional deforma- older part of the Calico Member of the Barstow Based on geologic mapping, fi eld observa- tion may have initiated early in the history of Formation accumulated during large-magnitude tions and 40Ar/39Ar geochronology, a schematic the right-lateral Calico fault system, perhaps extension in the central Mojave metamorphic Neogene geologic history of the southern Calico in the middle or late Miocene. Alternatively, core complex. The association of coarse-grained Mountains is presented in Figure 13. Between the restraining bend may be the result of coun- volcaniclastic deposits with extensional basin ca. 19.4 and 19 Ma, a thick (>1 km) section of terclockwise rotation due to movement on the development and fi ne-grained lacustrine depos- coarse volcaniclastic rocks and dacite domes east-northeast–striking, left-lateral Manix fault its with postextensional sedimentation can be and fl ows of the Pickhandle Formation accumu- (Dibblee, 1994), implying that transpressional misleading. Assuming that extension in the cen- lated. The absence of the Pickhandle Formation deformation may postdate some right-lateral tral Mojave metamorphic core complex did not south of the Calico fault suggests a model in movement on the Calico fault. Approximately begin until ca. 21 Ma (Gans et al., 2005), Pick- which a northeast- or north- northeast-dipping 3 km of right-lateral slip and perhaps 1 km of handle Formation rocks that range in age from proto-Calico normal fault uplifted metavol- reverse slip have occurred along two major 24 to 21 Ma may be preextensional deposits that canic basement rocks in the footwall and cre- strands of the Calico fault system. refl ect proximity to local volcanic centers rather ated a Pickhandle basin in the hanging wall Transpression along the Calico fault restrain- than extensional basin development. Similarly, (Fig. 13A). Alternatively, the proto-Calico fault ing bend forced the Calico Member north of the the coarse breccia sheets that overlie the Calico may have unroofed metavolcanic basement fault to detach along its base and move over the Member in the southeastern Calico Mountains rocks of Pickhandle deposits. In either scenario, south-dipping Pickhandle Formation–Calico apparently postdate extension and are instead a slip along this inferred proto-Calico fault must Member contact in a reverse-dextral sense consequence of steep volcanic topography cre- have ceased prior to the deposition of fi ne- (Fig. 13D). The thinly bedded Calico Mem- ated by the Yermo volcanic center. grained lacustrine rocks of the Calico Member, ber responded to this transpression by folding Although most of the Barstow Formation was which unconformably overlie the Pickhandle into numerous anticlines and synclines that deposited after extension in the central Mojave Formation north of the Calico fault and directly account for 25%–33% north-south shortening. metamorphic core complex had ended, synex- overlie footwall metavolcanic rocks south of the The Pickhandle Formation and dacite rocks of tensional, 19–17 Ma Barstow Formation lacus- Calico fault. Pickhandle dacite domes emplaced the Yermo volcanic center resisted folding and trine deposits appear to be more widespread ca. 19 Ma most likely formed topographic highs acted as rigid buttresses against which folds in than previously recognized. In the easternmost that marked the northern margin of the shallow the Calico Member developed. Post–16.8 Ma Mud Hills and northwestern Calico Mountains lake in which Calico Member sediments accu- deformation within the Pickhandle Formation the Pickhandle Formation grades into comform- mulated (Fig. 13A). Both the Pickhandle For- and Yermo dacite rocks consisted primarily of ably overlying fi ne-grained lacustrine beds (Sin- mation and at least the older part of the Calico strike-slip and oblique strike-slip-reverse fault- gleton, 2006, personal observ.), suggesting that Member were deposited during rapid slip along ing that was broadly synchronous with folding Calico Member sedimentation extended from the Waterman Hills detachment fault (Gans et in the Calico Member. the southeastern Calico Mountains to the east- al., 2005), although most normal faulting in the ernmost Mud Hills (Fig. 1). Van Pelt and Gans southern Calico Mountains appears to predate Miocene Stratigraphic Framework for the (2005) bracketed the timing of fi ne-grained deposition of the Calico Member. Central Mojave Desert lacustrine sedimentation in the Lead Mountain The Yermo volcanic center became active dur- area between ca. 19.3 and 17.2 Ma (Fig. 1), and ing the waning stages of lacustrine sedimentation. The prevailing tectonostratigraphic frame- north of Daggett Ridge several hundred meters Between 17.1 Ma and 16.8 Ma several dacite work of early to mid-Miocene rocks in the of fi ne-grained lacustrine beds comformably domes intruded Calico Member sediments north central Mojave Desert is based on northeast- overlie the ca. 18.5 Ma Peach Springs Tuff and south of the Calico fault (Figs. 13B, 13C). southwest extension associated with the central (Wells and Hillhouse, 1989; Dibblee, 1970). Coarse dacite breccias shed from the domes Mojave metamorphic core complex. Coarse Lacustrine deposits in the central Mojave Des- locally fi lled the shallow lake and precluded volcaniclastic rocks of the Pickhandle Forma- ert are commonly assumed to correlate to the lacustrine sedimentation. East and west of the tion have been interpreted as synextensional postextensional type section of the Barstow For- Yermo volcanic center, dacite breccia sheets are deposits ranging in age from ca. 24 to 19 Ma mation in the Mud Hills, yet many of these cor- overlain by as much as 60 m of Calico Member (Fillmore and Walker, 1996), whereas overly- relations appear to be inaccurate and overlook beds, indicating that lacustrine deposition con- ing, fi ne-grained Barstow Formation rocks are earlier synextensional lacustrine sedimentation. tinued locally during volcanic activity. considered postextensional deposits that infi lled Strike-slip and transpressional deformation remnant basins (Fillmore and Walker, 1996; Extensional Deformation in the southern Calico Mountains postdates the Ingersoll et al., 1996). This tectonostratigraphic formation of the Yermo volcanic center (post- model is fl awed in detail. Thermochronologic The paucity of large normal faults and the 16.8 Ma) and refl ects the dominant style of data indicate that rapid slip on the Waterman gentle tilts of strata in the Pickhandle Forma- post–early Miocene deformation in the central Hills detachment fault and extensional unroofi ng tion suggest that much of the Calico Mountains Mojave Desert. A portion of the proto-Calico of the central Mojave metamorphic core com- behaved as a fairly coherent block during exten- normal fault was reactivated as a dextral-reverse plex footwall occurred largely between ca. 21 sion in the central Mojave metamorphic core

Geosphere, June 2008 475

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 km Singleton and Gans 1 (SSW) 2 21-17.5 Ma NE-SW regional extension (NNE) 1' ca. 17.2 Ma ~19 Ma dacite intrusion (Tpd) Calico Member (Tbc) 1

Tc1 Pickhandle

proto-CalicoFm. (Tps) fault 0 metavolcanic rocks (mv) granite (gr) mv?

26 A) A 0 1 2 km

km 2 (SSW) (NNE) 2' 2 ca. 16.8 Ma 2' Tpd Tdb PickhandleTpdTpd Formation Tbc Tbc Tdi Tpd 1 Calico Member sediments 15 (beneath shallow lake) Tc1 Tps

dacite breccia prot o-Calico fault 0 mv gr Yermo volcanic center

mv N ca. 16.8 Ma 2

B C 012 km 0 1 2 km

3 (SSW) N-S shortening 3' (NNE) post-16 Ma folded Calico Member km 2 Tc Tpd 3 Tdi Tps

Tc2 1

Tc1

S. Calico fault Calico fault mv 0

D 012 km

Figure 13. Schematic reconstruction illustrating sequential stratigraphic and structural evolution of the southern Calico Mountains. (A) Cross-section 1–1': geometry of early Miocene sedimentation prior to the formation of the Yermo volcanic center (ca. 17.2 Ma). More than 1 km of dacite rocks and volcaniclastic sediments of the Pickhandle Formation accumulated rapidly in a basin formed by normal slip along a north-northeast–dipping proto-Calico fault. Extensional faulting within the Pickhandle Formation is minor. Beginning ca. 19 Ma, slip along the proto-Calico fault decreases or stops, and fi ne-grained lacustrine sedimentation begins. Approximately 200–400 m of lacustrine sandstone, shale, and limestone accumulate in a lake that extends into the footwall of the proto-Calico fault, where the Calico Member is deposited directly on metavolcanic basement rocks. (B, C) Schematic cross-section (2–2') and geologic map after the formation of the Yermo volcanic center (ca. 16.8 Ma). Note that the map scale is one-half the scale of cross-section 2–2'. Between 17.1 and 16.8 Ma, the approximately east-west–trending dacite dome fi eld erupted into a shallow lake. Breccia sheets shed from the domes were deposited on lacustrine sediments, locally fi lling up the lake. West of the Yermo volcanic center, fi ne-grained lacustrine sedimentation continued during dome emplacement. (D) Cross-section 3–3': schematic geometry of post 16.8 Ma transpressional deformation. The proto-Calico fault was reactivated as an oblique dextral- reverse fault within a restraining bend of the Calico fault system. Transpression resulted in reverse-dextral movement between the Calico Member and the south- to southeast-dipping Pickhandle Formation, folding the fi ne-grained lacustrine rocks into small-scale anticlines and synclines. Northwest-trending normal faults within the Pickhandle Formation were reactivated as dextral faults. See Plate 1 for key to rock unit symbols.

476 Geosphere, June 2008

complex. High-angle, northwest-striking normal northeast-southwest extension associated with progressive shear. Transpressional folding and faults of inferred early Miocene age are present the central Mojave metamorphic core complex faulting in the southern Calico Mountains is also within the Pickhandle Formation, yet northeast- most likely created numerous northwest-striking inconsistent with a kinematic model in which southwest extension associated with these faults normal faults, many of which may have been regional-scale shortening is accommodated by is minor (~5%), and the dominant sense of reactivated as right-lateral faults. conjugate dextral and sinistral faulting. Instead, normal slip (top to the southwest) is antithetic folds are localized north of the restraining bend to shear across the central Mojave metamor- Folding and Transpression in the Calico fault system and accommodate a phic core complex. In the study area the Pick- component of north-south shortening associated handle Formation generally dips <30°SE and S Several geologists have argued that north- with transpressional, dextral-reverse faulting. (Fig. 8A). Across the central Calico Mountains, south crustal contraction associated with strike- In the Calico Mountains transpression appears Pickhandle beds dip <20° in various directions slip faulting dominates post–early Miocene to be restricted to this restraining bend region, (Dibblee, 1970). With the exception of the sec- deformation across most of the Mojave Desert and is not indicative of regional contraction, tion along Fort Irwin Road, the Pickhandle region (e.g., Bartley et al., 1990; Glazner et al., as proposed by Bartley et al. (1990). Although Formation in the Calico Mountains never dips 2002). East-west–trending folds are generally north-south contraction may be a regional phe- homoclinally to the southwest, which would be thought to be a consequence of this regional, nomenon in the Mojave Desert region (Bartley expected for northeast-directed extension in the crustal contraction, yet there are very few areas et al., 1990), some of the best examples of short- upper plate of the central Mojave metamorphic where the detailed geometry, style, and timing ening in the central Mojave Desert may refl ect core complex. of folding have been documented. New geo- localized transpression more than regional con- The apparent lack of signifi cant extension logic mapping, structural data, and interpreta- traction. For example, the Su Casa basement may be somewhat surprising given the proxim- tions presented here illustrate several important arch and east-west–striking overturned beds are ity of the Calico Mountains to lower plate rocks characteristics of folds in the southern Calico localized adjacent to the restraining bend in the of the central Mojave metamorphic core com- Mountains, including the following. Camp Rock fault (Dibblee, 1970; Dokka, 1986). plex, but the geometry of Pickhandle Formation 1. The numerous approximately east-west– The Barstow syncline occurs in a stepover zone strata in several other areas is also incompatible trending, upright folds are largely restricted to between the Calico fault and the Blackwater fault with northeast-southwest extension. For exam- the fi ne-grained lacustrine beds north of the zone (Dibblee, 1968), and the Lenwood anticline ple, Pickhandle strata in the Lead Mountain area Calico fault and are detached from the under- is located along a left-stepping bend in the Len- are folded and generally strike northeast-south- lying Pickhandle Formation. Basement rocks wood fault (Dibblee, 1967; Glazner and Bartley, west (Fig. 1; Dibblee, 1970). The Pickhandle are clearly not involved in the folding and are 1994). Shortening does not appear to be homo- Formation in the Gravel Hills (northwest of instead shortened by transpressional faulting. geneously distributed across the Mojave Desert, the Mud Hills) dips gently to the south (Dib- 2. Folding in the Calico Member represents and localized transpression may play an impor- blee, 1968). It is possible that strike-slip and/or ~25%–33% north-south shortening (up to tant role generating contractional structures. transpressional deformation overprinted exten- 0.5 km), and thus does not account for several sion, or that some of these south- to southeast- kilometers of shortening, as proposed by Glazner CONCLUSIONS dipping strata rotated counterclockwise about a et al. (1994). Shortening due to reverse slip along vertical axis from previous southwest-dipping the Calico fault system is not as well constrained, The Neogene geologic history of the Calico orientations. However, most paleomagnetic but is estimated to be ~0.5 km. Thus, transpres- Mountains includes synextensional early Mio- data from the central Mojave Desert suggest sional folding and faulting across the southern cene sedimentation and volcanism, followed clockwise rotation (see Glazner et al., 2002, for Calico Mountains probably accounts for ~1 km by transpressional faulting and folding. New a review). More detailed mapping is needed to of shortening. This shortening most likely 40Ar/39Ar geochronology ages indicate that most shed light on the style and magnitude of upper absorbed some of the dextral shear along the of the type section of the Pickhandle Forma- plate extension in the central Mojave Desert. Calico fault system and may be partly respon- tion accumulated rapidly between ca. 19.4 and sible for the northward decrease of dextral offset 19.0 Ma. The lack of a thick section of Pickhan- Faulting along the Calico fault (Oskin et al., 2007). dle Formation beneath the Calico Member of 3. The map-scale geometry of folds in the the Barstow Formation south of the Calico fault Right-lateral slip along northwest-striking southern Calico Mountains is strongly infl u- suggests that a northeast- or north- northeast– faults represents the dominant style of post– enced by boundary conditions, including the dipping proto-Calico normal fault unroofed early Miocene deformation across the central presence of a volcanic dome fi eld (the Yermo metavolcanic basement rocks in the footwall Mojave Desert. New fi eld data from the south- volcanic center) that acted as a rigid buttress and created a Pickhandle Formation basin in ern Calico Mountains suggest that many of these and resisted folding. The interpretation of folds the hanging wall. This inferred extensional faults may have complex histories that date back in the Mojave Desert should take into consid- basin development must have ceased prior to to at least the early Miocene. The stratigraphic eration specifi c boundary conditions that may the deposition of fi ne-grained lacustrine rocks mismatch of pre–Calico Member rocks across affect deformation. of the Calico Member, which unconformably the Calico fault restraining bend cannot be 4. Folding in the southern Calico Mountains overlie the Pickhandle Formation north of the explained with dextral and/or reverse slip. Prior postdates the formation of the Yermo volcanic Calico fault and directly overlie metavolcanic to transpressional deformation, there appears to center (post ca. 17 Ma) and is temporally and rocks south of the Calico fault. The 40Ar/39Ar have been a signifi cant normal (southwest side spatially associated with transpressional slip geochronology brackets the age of fi ne-grained up) component of slip along the Calico fault. along the Calico fault system. However, this lacustrine sedimentation north of the Calico fault Similarly, northwest-striking, early Miocene (?) folding is not classic wrench folding, where folds between ca. 19 and 16.9 Ma, indicating that the normal faults in the Pickhandle Formation are initiate obliquely to a strike-slip fault and then Calico Member is older than the type section of overprinted by right-lateral slip. Early Miocene rotate toward parallelism with the fault during the Barstow Formation in the Mud Hills. The

Geosphere, June 2008 477

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021 Singleton and Gans

slip tectonics in southern California: Science, v. 248, Glazner, A.F., and Bartley, J.M., 1994, Eruption of Pickhandle Formation and at least the older p. 1398–1401, doi: 10.1126/science.248.4961.1398. alkali basalts during crustal shortening in south- part of the Calico Member were deposited dur- Burke, D.B., Hillhouse, J.W., McKee, E.H., Miller, S.T., ern California: Tectonics, v. 13, p. 493–498, doi: ing rapid slip along the Waterman Hills detach- and Morton, J.L., 1982, Cenozoic rocks in the Bar- 10.1029/93TC03491. stow basin area of southern California—Stratigraphic Glazner, A.F., Bartley, J.M., and Walker, J.D., 1989, Magni- ment fault, yet northeast-southwest extension relations, radiometric ages, and paleomagnetism: U.S. tude and signifi cance of Miocene crustal extension in within the Pickhandle Formation is only ~5%. Geological Survey Bulletin 1529-E, p. 1–16. the central Mojave Desert, California: Geology, v. 17, Cox, B.F., and Wiltshire, H.G., 1993, Geologic map of the p. 50–53, doi: 10.1130/0091-7613(1989)017<0050: The Yermo volcanic center in the southeastern area around the Nebo Annex, Marine Corps Logistics MASOMC>2.3.CO;2. Calico Mountains was the site of calc-alkaline Base, Barstow, California: U.S. Geological Survey Glazner, A.F., Walker, D.J., Bartley, J.M., Fletcher, J.M., dacite dome emplacement between 17.1 and Open-File Report 93–568, 36 p., scale 1:12,000. Martin, M.W., Schermer, E.R., Boettcher, S.S., Miller, Dibblee, T.W., 1964, Geologic map of the Rodman Moun- J.S., Fillmore, R.P., and Linn, J.K., 1994, Reconstruc- 16.8 Ma. This dacite volcanism was active dur- tains Quadrangle, San Bernardino County, California: tion of the Mojave Block, in McGill, S.F., and Ross, ing the late stages of lacustrine sedimentation. U.S Geological Survey Miscellaneous Geologic Inves- T.M., eds., Geological investigations of an active mar- Previous geochronologic investigations have tigations Map I-430, scale 1:62,000. gin: Geological Society of America, Cordilleran Sec- Dibblee, T.W., Jr., 1967, Areal geology of the western tion Guidebook, p. 3–30. not recognized ca. 17 Ma volcanism in the cen- Mojave Desert, California: U.S. Geological Survey Glazner, A.F., Bartley, J.M., and Sanner, W.K., 2000, Nature tral Mojave Desert. Professional Paper 522, 153 p. of the southwestern boundary of the central Mojave Dibblee, T.W., Jr., 1968, Geology of the Fremont Peak and Tertiary province, Rodman Mountains, California: Strike-slip faulting and transpression are the Opal Mountain quadrangles, California: California Geological Society of America Bulletin, v. 112, dominant styles of post–early Miocene defor- Division of Mines and Geology Bulletin, v. 188, 64 p. p. 34–44, doi: 10.1130/0016-7606(2000)112<0034: mation in the southern Calico Mountains. The Dibblee, T.W., Jr., 1970, Geologic map of the Daggett quad- NOTSBO>2.3.CO;2. rangle, San Bernadino County, California: U.S. Geo- Glazner, A.F., Walker, J.D., Bartley, J.M., and Fletcher, north-northeast–dipping Calico fault system logical Survey Miscellaneous Geologic Investigations J.M., 2002, Cenozoic evolution of the Mojave block of forms a restraining bend that accommodates Map I-592, scale 1:62,000. southern California, in Glazner, A.F., et al., eds., Geo- dextral-reverse slip. Based on the apparent off- Dibblee, T.W., Jr., 1980a, Cenozoic rock units of the Mojave logical evolution of the Mojave Desert and southwest- Desert, in Fife, D.L., and Brown, A.R., eds., Geology ern Basin and Range: Geological Society of America set of dacite domes of the Yermo volcanic cen- and mineral wealth of the California desert: Dibblee Memoir 195, p. 19–41. ter, the Calico fault restraining bend system has Volume: Santa Ana, California, South Coast Geologi- Ingersoll, R.V., Devaney, K., Geslin, J.K., Cavazza, W., cal Society, p. 41–68. Diamond, D.S., Heins, W.A., Jagiello, K.J., Marsaglia, ~3 km of right-lateral slip and perhaps 1 km of Dibblee, T.W., Jr., 1980b, Geologic structure of the Mojave K.M., Paylor, E.I., and Short, P.F., 1996, The Mud reverse slip distributed on two main faults. The Desert, in Fife, D.L., and Brown, A.R., eds., Geology Hills, Mojave Desert, California: Structure, stratigra- cumulative amount of dextral shear across the and mineral wealth of the California desert: Dibblee phy and sedimentology of a rapidly extended terrane, Volume: Santa Ana, California, South Coast Geologi- in Beratan, K.K., ed., Reconstructing the history of southern Calico Mountains is ~4.1 km. cal Society, p. 69–100. Basin and Range extension using sedimentology and Deformation within the Calico Member Dibblee, T.W., Jr., 1994, Geology of the Calico Mountains, stratigraphy: Geological Society of America Special north of the Calico fault is taken up primarily central Mojave Desert, California: South Coast Geo- Paper 303, p. 61–84. logic Society Annual Field Trip Guidebook, v. 22, Jamison, W.R., 1991, Kinematics of compressional by folding. Numerous approximately east-west– p. 80–99. fold development in convergent wrench ter- trending, upright folds represent 25%–33% Dokka, R.K., 1986, Patterns and modes of early Miocene ranes: Tectonophysics, v. 190, p. 209–232, doi: crustal extension, central Mojave Desert, California, 10.1016/0040-1951(91)90431-Q. north-south shortening (up to ~0.5 km). Dacite in Mayer, L., ed., 1986, Extensional tectonics of the Jennings, C.W., 1994, Fault activity map of California and domes and breccias of the Yermo volcanic cen- southwestern United States: A perspective on processes adjacent areas: California Division of Mines and Tech- ter resisted folding and deformed instead by and kinematics: Geological Society of America Special nology Geologic Data Map 6, scale 1:750,000. Paper 208, p. 75–95. Linn, J.K., Walker, J.D., and Bartley, J.M., 2002, Late Ceno- strike-slip and transpressional faulting. Folds Dokka, R.K., 1989, The Mojave extensional belt of southern zoic crustal contraction in the Kramer Hills, west- in the Calico Member are detached from the California: Tectonics, v. 8, p. 363–390, doi: 10.1029/ central Mojave Desert, California, in Glazner, A.F., et homoclinally south- to southeast-dipping Pick- TC008i002p00363. al., eds., Geological evolution of the Mojave Desert Dokka, R.K., and Travis, C.J., 1990a, Late Cenozoic strike- and southwestern Basin and Range: Geological Soci- handle Formation. We interpret the Pickhandle slip faulting in the Mojave Desert, California: Tecton- ety of America Memoir 195, p. 161–172. Formation–Calico Member contact as a reverse- ics, v. 9, p. 311–340, doi: 10.1029/TC009i002p00311. MacFadden, B.J., Swisher, C.C., Opdyke, N.D., and Wood- Dokka, R.K., and Travis, C.J., 1990b, Role of the Eastern burne, M.O., 1990, Paleomagnetism, geochronology, dextral fault zone that is part of a positive fl ower California shear zone in accommodating Pacifi c–North and possible tectonic rotation of the middle Miocene structure along the Calico fault restraining bend. American plate motion: Geophysical Research Letters, Barstow Formation, Mojave Desert, southern Califor- Transpression within this fl ower structure was v. 17, p. 1323–1326, doi: 10.1029/GL017i009p01323. nia: Geological Society of America Bulletin, v. 102, Fillmore, R.P., and Walker, J.D., 1996, Evolution of a supra- p. 478–493, doi: 10.1130/0016-7606(1990)102<0478: responsible for folding the Calico Member. The detachment extensional basin: The lower Miocene PGAPTR>2.3.CO;2. geometry and map view pattern of folds are not Pickhandle basin, central Mojave Desert, California, in Martin, M.W., Glazner, A.F., Walker, J.D., and Schermer, compatible with gravity folding, folding due to Beratan, K.K., ed., Reconstructing the history of Basin E.R., 1993, Evidence for right-lateral transfer faulting and Range extension using sedimentology and stratig- accommodating en echelon Miocene extension, Mojave dacite dome emplacement, or wrench folding. raphy: Geological Society of America Special Paper Desert, California: Geology, v. 21, p. 355–358, doi: 10.1 Transpressional folding and faulting in the Cal- 303, p. 107–126. 130/0091-7613(1993)021<0355:EFRLTF>2.3.CO;2. Fletcher, D.I., 1986, Geology and genesis of the Waterloo and McCulloh, T.H., 1952, Geology of the southern half of the ico Mountains is localized along the Calico fault Langtry silver-barite deposits, California [Ph.D. thesis]: Lane Mountain quadrangle, California [Ph.D. thesis]: restraining bend and is not indicative of regional Stanford, California, Stanford University, 158 p. Los Angeles, University of California, 182 p. north-south contraction. Fletcher, J.M., and Bartley, J.M., 1994, Constrictional strain McCulloh, T.H., 1960, Geologic map of the Lane Mountain in a non-coaxial shear zone: Implications for fold quadrangle, California: U.S. Geological Survey Open- and rock fabric development, central Mojave meta- File map, scale 1:48,000. ACKNOWLEDGEMENTS morphic core complex, California: Journal of Struc- McCulloh, T.H., 1965, Geologic map of the Nebo and Yermo tural Geology, v. 16, p. 555–570, doi: 10.1016/0191- quadrangles, San Bernardino County, California: U.S. This work was funded by Rio Tinto Industrial Min- 8141(94)90097-3. Geological Survey Open-File Map OFR-65-107, scale erals Exploration. We thank several Rio Tinto borate Fletcher, J.M., Bartley, J.M., Martin, M.W., Glazner, A.F., 1:24,000. exploration geologists, particularly John Reynolds, and Walker, J.D., 1995, Large-magnitude continen- Odonne, F., and Vialon, P., 1983, Analogue models of folds tal extension: An example from the Central Mojave above a wrench fault: Tectonophysics, v. 99, p. 31–46, for valuable discussions on Mojave geology. Thought- metamorphic core complex: Geological Society of Amer- doi: 10.1016/0040-1951(83)90168-3. ful reviews by Mike Oskin and John Fletcher have ica Bulletin, v. 107, p. 1468–1483, doi: 10.1130/0016-760 Oskin, M., Perg, L., Blumentritt, D., Mukhopadhyay, S., improved this manuscript and are greatly appreciated. 6(1995)107<1468:LMCEAE>2.3.CO;2. and Iriondo, A., 2007, Slip rate of the Calico fault: Gans, P., Devecchio, D., Singleton, J., Van Pelt, J., Wong, Implications for geologic versus geodetic rate dis- REFERENCES CITED M., and Reynolds, J., 2005, Cenozoic magmatic and crepancy in the Eastern California Shear Zone: Jour- structural evolution of the central Mojave desert, Cali- nal of Geophysical Research, v. 112, B03402, doi: fornia: New constraints from 40Ar/39Ar geochronology 10.1029/2006JB004451. Bartley, J.M., Glazner, A.F., and Schermer, E.R., 1990, and thermochronology: Geological Society of America Ramsay, J.G., 1967, Folding and fracturing of rocks: New North-south contraction of the Mojave block and strike- Abstracts with Programs, v. 37, no. 4, p. 103. York: McGraw-Hill, 568 p.

478 Geosphere, June 2008

Reynolds, R.E., 2000, Marker units suggest correlation Van Pelt, J.R., and Gans, P., 2005, Stratigraphy and struc- Wells, R.E., and Hillhouse, J.W., 1989, Paleomagnetism and between the Calico Mountains and the Mud Hills, cen- ture of the southern Mitchell Range, San Bernardino tectonic rotation of the lower Miocene Peach Springs tral Mojave Desert, California: San Bernardino County County, California: Implications for the southern mar- Tuff: Colorado Plateau, Arizona, to Barstow, Califor- Museum Association Quarterly, v. 47, no. 2, p. 21–24. gin of the central Mojave metamorphic core complex: nia: Geological Society of America Bulletin, v. 101, Schermer, E.R., and Busby, C.J., 1994, Jurassic magma- Geological Society of America Abstracts with Pro- p. 846–863, doi: 10.1130/0016-7606(1989)101<0846: tism in the central Mojave Desert: Implications for grams, v. 37, no. 7, p. 562. PATROT>2.3.CO;2. arc paleogeography and preservation of continental Walker, J.D., Bartley, J.M., and Glazner, A.F., 1990, Woodburne, M.O., Tedford, R.H., and Swisher, C.C., 1990, volcanic sequences: Geological Society of America Large-magnitude Miocene extension in the central Lithostratigraphy, biostratigraphy, and geochronology Bulletin, v. 106, p. 767–790, doi: 10.1130/0016- Mojave Desert: Implications for Paleozoic to Ter- of the Barstow Formation of the Mojave Desert, south- 7606(1994)106<0767:JMITCM>2.3.CO;2. tiary paleogeography and tectonics: Journal of Geo- ern California: Geological Society of America Bulletin, Singleton, J.S., 2004, Geologic evolution of the southeastern physical Research, v. 95, p. 557–569, doi: 10.1029/ v. 102, p. 459–477, doi: 10.1130/0016-7606(1990) Calico Mountains, central Mojave Desert, California [M.S. JB095iB01p00557. 102<0459:LBAGOT>2.3.CO;2. thesis]: Santa Barbara, University of California, 94 p. Walker, J.D., Fletcher, J.M., Fillmore, R.P., Martin, M.W., Tarman, D.W., and McBean, D.M., 1994, Folding of the Taylor, W.J., Glazner, A.F., and Bartley, J.M., 1995, Barstow Formation in the southern Calico Mountains, Connection between igneous activity and extension in San Bernardino County, California, in Murbach, D., the central Mojave metamorphic core complex: Journal and Baldwin, J., eds., Mojave Desert: Annual Field of Geophysical Research, v. 100, p. 10,477–10,494, Trip Guidebook, Volume 22: Santa Ana, California, doi: 10.1029/94JB03132. South Coast Geological Society, p. 488–499. Weber, F.H., Jr., 1976, Geology of the Calico silver dis- Tikoff, B., and Peterson, K., 1998, Physical experi- trict, San Bernardino County, California, in Geologic ments of transpressional folding: Journal of Struc- guidebook to the southwestern Mojave Desert region, MANUSCRIPT RECEIVED 26 JULY 2007 tural Geology, v. 20, p. 661–672, doi: 10.1016/ California: Irvine, California, South Coast Geological REVISED MANUSCRIPT RECEIVED 15 JANUARY 2008 S0191-8141(98)00004-2. Society, 1976 ﬁ eld trip, p. 83–94. MANUSCRIPT ACCEPTED 06 FEBRUARY 2008

Geosphere, June 2008 479

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/459/3337602/i1553-040X-4-3-459.pdf by guest on 30 September 2021