ABSTRACT TECTONIC SETTING OF THE EL PASO BASIN

Transcription

1 The stratigraphie evlutin f the El Pas basin, suthern Califrnia: Implicatins fr the Micene develpment f the Garlck fault and uplift f the Sierra Nevada DANA P. LOOMIS Department f Gelgy, University f Nrth Carlina, Chapel Hill, Nrth Carlina DOUGLAS W. BURBANK Department f Gelgical Sciences, University f Suthern Califrnia, Ls Angeles, Califrnia ABSTRACT The Ricard Grup is a 1,700-m-thick sequence f Micene vlcanic rcks and cntinental sedimentary rcks depsited between ~19 and 7 Ma in the El Pas basin, near the junctin f the Garlck fault and the Sierra Nevada frntal fault in suthern Califrnia. The cmbinatin f stratigraphic and structural data frm the Ricard Grup with chrnlgic and rtatinal histries derived frm magnetstratigraphic and radimetric studies prvides sme new cnstraints fr the mrphtectnic emergence f the Sierra Nevada, the initiatin f strike-slip mvement n the Garlck fault, and related east-west extensin in the suthern Basin and Range regin. The Micene sequence dcuments (1) vlcanism and nrth-suth tensin withut net extensin at Ma (2) relative uplift abut Ma, (3) the beginning f sinistral slip n the Garlck fault and east-west extensin in the suthwestern Basin and Range regin abut 10-9 Ma, and (4) emergence f the Sierra Nevada as a surce area and tpgraphic high by 8 Ma. Stratigraphic evidence frm the Ricard Grup des nt indicate significant Micene dip-slip mvement alng the large-scale, dwn-tthe-nrth fault that has been previusly hypthesized t have preceded the present Garlck fault in the same lcatin during the early Tertiary. The evlutin f this sequence, hwever, can be explained by tectnic mdels which prpse a time-space linkage between basin evlutin, east-west extensin in suthern Califrnia, and the Mendcin triple-junctin system. INTRODUCTION Majr structural bundaries frequently delimit tectnic prvinces exhibiting cntrasting histries and defrmatinal styles. An understanding f the gelgical develpment f such bundary znes can illuminate the timing and nature f events that caused the present juxtapsitins f cntrasting terranes. In suthern Califrnia, the Garlck fault and, t a lesser extent, the Sierra Nevada frntal fault represent this type f delimiting tectnic bundary. The late Cenzic gelgy f the Basin and Range regin nrth and east f these faults has been largely shaped by east-west extensin, whereas the Mjave prvince t the suth has been relatively unaffected by it. The El Pas basin is virtually astride the line dividing these gelgic regins, and the evlutin f the basin and its nearly 2 km f late Cenzic sedimentary and vlcanic fill have been directly influenced by the develpment f the Garlck fault and the uplift f the Sierra Nevada. This paper presents the results f stratigraphic, chrnlgic, and tectnic investigatins f the Micene Ricard Grup in the El Pas basin. Sedimentary and magnetstratigraphic data prvide evidence fr the nset f east-west extensin nrth f the Garlck fault abut 10 Ma and fr the emergence f the Sierra Nevada as a tpgraphic upland abut 8 Ma. In additin t these age cnstraints fr the Garlck fault and Sierra Nevada, this analysis als prvides an pprtunity t test sme plate-tectnic mdels fr the late Cenzic sedimentary and tectnic evlutin f suthern Califrnia. Sedimentary evidence frm the El Pas basin is largely cnsistent with prpsals that basin grwth and sedimentatin (Glazner and Lmis, 1984) and basin-and-range extensin (Ingersll, 1982; Glazner and Bartley, 1984) in suthern Califrnia are temprally assciated with the recnstructed latitude f the Mendcin triple-junctin system and can be explained by interactins between the Mendcin fracture zne and the Nrth American plate. TECTONIC SETTING OF THE EL PASO BASIN The El Pas basin is lcated n the nrth side f the El Pas Muntains 5-6 km nrth f the Garlck fault in Kern Cunty, Califrnia (Fig. 1). Its western bundary is the Sierra Nevada and the Sierra Nevada frntal fault system. Tilted and faulted Tertiary rcks f the basin are expsed in an east-nrtheast-trending belt abut 33 km by 7 km, and they are cvered n the nrth and east by Quaternary alluvial depsits. The El Pas Muntains are bunded n their suthern margin by the El Pas fault (Fig. 1). This fault appears t be a splay f the Garlck fault, but unlike the Garlck fault, it appears t be a dip-slip fault with little r n sinistral ffset. The Garlck fault separates the muntains f the Tehachapi, Sierra Nevada, and Basin and Range prvinces n the nrth frm the lwer-relief Mjave prvince t the suth (Fig. 1). It is a majr left-slip intracntinental transfrm fault which bunds the east-west extensin f the Basin and Range and allws the Mjave blck t remain relatively unextended (Hamiltn and Myers, 1966; Davis and Burchfiel, 1973). The ttal lateral displacement n the fault is estimated t be at least 64 km (Smith, 1962; Smith and Ketner, 1970; Davis and Burchfiel, 1973). Abundant gemrphic evidence indicates Quaternary left slip n the Garlck fault near the El Pas Muntains (Dibblee, 1952; Clark, 1973; Carter, 1980; La Vilette and thers, 1980), but the age f the beginning f left slip in this area has nt previusly been cnstrained t an interval mre precise than sme time during the Tertiary. It has ften been pstulated that the present strike-slip Garlck fault was preceded by a large, highangle dip-slip (Hewett, 1954; Smith, 1962) r strike-slip (Nilsen and Clarke, 1975) fault as early as Palecene time. The evidence fr bth Gelgical Sciety f America Bulletin, v. 100, p , 14 figs., 2 tables, January

2 STRATIGRAPHIC EVOLUTION OF EL PASO BASIN, CALIFORNIA 13 Figure 1. Generalized gelgic map f the El Pas basin and surrunding area. Tertiary sediments f El Pas basin include Tru, Ricard Grup undivided; Tmd, Dve Spring Frmatin, members 1-5; Tmc, Cudahy Camp Frmatin; Tpg, Gler Frmatin. El Pas Muntains basement cmplex: Mzg, Meszic granitid cmplex; Mzm, Mesquite schist; Pzm, Palezic metamrphic rcks, including the Garlck Frmatin. strike-slip and earlier dip-slip mtin n the Garlck fault will be examined here n the basis f sedimentary and tectnic evidence frm the El Pas basin. METHODS During this study, the Micene strata f the El Pas basin were remapped, and representative lithlgies were sampled fr petrgraphic study. Palecurrent directins were derived frm calculated vectr means f freset rientatins. Cnglmerate cmpsitins were determined by cunting apprximately 100 clasts in the pebble-t-cbble size range expsed n a single utcrp. Thin-sectin pint cunting (Dickinsn, 1970) was used t determine sandstne mdal cmpsitins, which are presented as Q:F:L ratis. Detailed discussins f these methds are given in Lmis (1984, Appendix III). A 1,500-m-thick magnetstratigraphic sectin was sampled thrugh the Dve Spring Frmatin f the Ricard Grup. Sampling sites were spaced at 10 t 20-m intervals, and at each site, 3-4 riented specimens were cllected. Based n thermal demagnetizatin studies f pilt specimens (discussed later), all specimens were demagnetized at 450 and 500 C. The results frm the multiple specimens at each site were evaluated using Fisher (1953) statistics and were classified fllwing Jhnsn and thers (1982) as "class I" if Fisher k >10; "class II" if k <10, but tw specimens were in clse agreement and their plarity was unambiguus; r "class III" if the results were t dispersed fr use. The mean site directins were used t calculate the latitude f the virtual gemagnetic ple (VGP) at each sampling level, and ags-errr envelpes were calculated fr each VGP latitude. The lcal magnetstratigraphy was defined by magnetic reversals inferred frm the changing VGP latitudes. STRATIGRAPHY OF THE EL PASO BASIN The fill f the El Pas basin includes clastic sediments f the Palecene Gler Frmatin (Dibblee, 1952, 1967; Cx, 1982) and cntinental

3 14 LOOMIS AND BURBANK Figure 2. Gegraphic names and majr faults in the El Pas basin area. The stippling dentes the utcrp area f the Ricard Grup. The enclsed bx indicates the regin encmpassed by the gelgic map f Figure 3. CANTIL VALLEY sediments and vlcanics f the Micene Ricard Grup (Merriam, 1919; Dibblee, 1952, 1967; Whistler, 1969; Lmis, 1984). Unnamed pst- Micene sediments, at least in part Pleistcene (Dibblee, 1952; Whistler, 1969; Carter, 1980), are als present. Tertiary rcks f the basin nlap suthward nt the erded pre-tertiary basement cmplex f the El Pas Muntains (Fig. 1) which includes metasediments, metavlcanics, and Meszic granitid intrusive rcks (Dibblee, 1952; Christiansen, 1961; Cx and Mrtn, 1980). The Gler Frmatin (Figs. 2 and 3) is a 4,300-m-thick assemblage f middle Palecene cnglmerate, sandstne, and mudrck derived primarily frm granitic-metasedimentary surces t the east (Cx, 1982). The Gler Frmatin was tilted t the nrth and erded smetime between middle Palecene and early Micene time. The Micene strata f the El Pas basin were first described by G. K. Gilbert (1875) and subsequently by Baker (1911, 1912), wh included them in the Rsamnd Series. The names "Ricard Grup" and "Ricard Frmatin" were first applied by Merriam (1913, 1919) t the rcks in Red Rck Canyn near the frmer twn f Ricard (Fig. 2). Dibblee (1952, 1967) mapped the area, divided the Ricard Frmatin int eight members, and designated a type area between Red Rck and Last Chance Canyns. A 10.3-m.y. K-Ar age near the base f the Ricard was determined by Evernden and thers (1964) in a study f the istpic ages f Nrth American land-mammal faunas. Whistler (1969) develped a detailed recrd f Clarendnian micrvertebrates frm the Ricard. The Black Muntain Basalt was named by Baker (1912) and mapped by Dibblee (1952, 1967); bth cnsidered it Quaternary. Cx and Diggles (1986), hwever, have determined an early Micene K-Ar age fr the Black Muntain Basalt. The Ricard Frmatin f Dibblee (1952,1967) is divisible int tw distinct lithstratigraphic units: a 350-m-thick, lwer vlcanic unit f andesite, tuff, basalt, and cnglmerate which is uncnfrmable n the Palecene Gler Frmatin and is equivalent t Dibblee's (1967) members 1 and 2, and a 2,000-m-thick upper unit f cnglmerate, sandstne, mudrck, chert, basalt, and tuff, cntaining Dibblee's (1967) members 3-8 (Fig. 4). K-Ar, fissin-track, and magnetstratigraphic dates indicate that these units differ significantly in age and that the discnfrmity between them represents a hiatus f at least 1.5 m.y. The discnfrmity r lwangle uncnfrmity separating these units is indicated by the fllwing features: (1) the base f the upper unit cntains channels incised int the lwer unit, (2) clasts derived frm the lwer unit are incrprated in cnglmerates at the base f the upper ne, and (3) the cntact between the tw units cuts stratigraphically dwnward t the basement alng strike. These tw units are sufficiently different lithlgically t warrant their definitin as separate frmatins. Cnsequently, we have elevated Dibblee's (1952,1967) Ricard Frmatin t grup status within which tw new frmatins are defined. The Cudahy Camp Frmatin (Table 1) supercedes Dibblee's (1967) members 1 and 2. This name is taken frm Cudahy Camp in Last Chance Canyn (Fig. 2), where the maximum thickness and type lcality (Dibblee, 1952,1967) f the frmatin is expsed. This unit cntains andesite, tuff, and cnglmerate, as well as the Black Muntain Basalt. Cx (1982) sampled the Black Muntain Basalt near the base and tp f a sequence f 15 flws expsed n the east slpes f Black Muntain and btained K-Ar ages f 17.1 ± 0.5 m.y. and 15.1 ± 0.5 m.y., respectively. This istpic age range places the Black Muntain Basalt in the early middle t early Micene. K-Ar ages have als been btained n tw andesite flws in the Cudahy Camp Frmatin (Cx and Diggles, 1986). The upper andesite,

4 LEGEND QUATERNARY {ALLUVIUM,TERRACES,LANDSLIDES RICARDO GROUP ; UNDIVIDED ma MIOCENE DOVE SPRING FM. MEMBER 5 MEMBERS 2-4 MEMBER 1 BLACK MTN. BASALT CUDAHY CAMP FM. BASALT DIKES MEMBERS 2-4 MEMBER t E57?1 PALEOCENE {GOLER FM. UNDIVIDED MESOZOIC {GRANITOID COMPLEX ES3 PAI Fnymr f-hltuiun, [METASEDIMENTARY AND {METAVOLCANIC ROCKS I IKM (14) 'u/ Figure 3. Gelgic map f the El Pas basin and the El Pas Muntains. The gegraphic lcatin f the map is shwn in Figure 2. The lcatin f the palemagnetic sectin is indicated by the dark stippled bxes, and the dtted lines represent alng-strike traverses between individual sectins. m

5 16 LOOMIS AND BURBANK Dibblee, MEMBER 8 MEMBER 6 This Paper Age is the same as that designated by Dibblee (1952, 1967) fr his Ricard members 3-8. The Dve Spring strata are expsed in a nrtheast-striking hmcline dipping nrthwest. They are verlain by fssiliferus lacustrine silt and clay f unknwn Pleistcene age (Whistler, 1969), by alluvial fanglmerates, and by elevated, dissected pst-micene terrace gravels als assumed t be Pleistcene (Dibblee, 1952). MAGNETOSTRATIGRAPHIC AGE OF THE DOVE SPRING FORMATION 1200 E - CO v> O) e -SC 1c 1- - c "+ > cd 900 E - c_ li MEMBER 7 MEMBER 6 "O c_ cd MEMBER Cd ; MEMBER 3 MEMBER 5 MEMBER 5 MEMBER 4 MEMBER 3 MEMBER 2 C O +-> <a E Li_ > e E Q- c <L> > O MEMBER MBR 5/BMB. n MEMBER 2 MEMBER H E cd O MEMBER 3 ->> < MEMBER 2 «J MEMBER 1 MEMBER 1 "O a. ( cd ir 8.4± 1.8 Ma O c_ 10.4± 1.6 Ma 1 1.8±0.9 Ma 15.1±0.5 Ma 17.2±0.8 Ma Ma Figure 4. Crrelatins between Dibblee's (1952,1967) lithstratigraphic terminlgy and the new definitins used in this paper. Radimetric dates are shwn at their apprpriate stratigraphie psitin. See text fr descriptin f each member. member 5, has been dated at 17.2 ± 0.5 m.y., and the age f the lwer andesite has been determined t be 18.1 ± 0.6 m.y. Bth ages are early Micene (Palmer, 1983; Berggren and thers, 1985). The age f the basal cnglmerate (member 1; see Table 1) is unknwn, but it is prbably als early Micene. The newly named Dve Spring Frmatin (Table 1) encmpasses Dibblee's members 3-8 and is uncnfrmable n the Cudahy Camp Frmatin. The name is taken frm Dve Spring Wash (Fig. 2) where the upper 800 m f the Ricard Grup is well expsed. The Dve Spring Frmatin is middle t late Micene in age, and it cnsists f cnglmerate, sandstne, mudrck, chert, basalt, and tuff. Member 6 f the Dve Spring Frmatin is cmpsed f prly expsed, weakly cnslidated, and gently dipping t flat-lying sedimentary rcks. A discnfrmity may be present between member 6 and the underlying 1,500 m f Ricard strata. The presence f well-develped caliches and silcretes near the tp f member 5 suggests that increasingly spradic sedimentatin preceded the initial depsitin f member 6. Due t its pr expsure, member 6 was nt included in ur analysis. The type sectin fr the Dve Spring Frmatin is lcated between Red Rck and Last Chance Canyns (Fig. 2) and Due t the abundance f vlcanic breccias, cnglmerates, and thick ash-flw tuffs, extensive magnetic sampling was nt undertaken in the Cudahy Camp Frmatin. Mre than 100 magnetic sampling sites (Fig. 3), hwever, were placed in the Dve Spring Frmatin in an attempt t develp a magnetstratigraphy fr these strata. Stepwise thermal demagnetizatin f numerus pilt specimens revealed similar magnetic behavir fr mst specimens. Typical results (Fig. 5) shw the fllwing characteristics: (1) large changes in intensity belw -250 C and, fr specimens with a reversed depsitinal remanence (DRM), an increase in intensity; (2) between 250 and 500 C, a slw decrease in the remanent intensity, and in sme cases (fr example, specimen R103A, Fig. 5), n significant change in intensity within this temperature range; and (3) between 550 and 600 C, a very rapid decrease in intensity. Fr all specimens, the characteristics remanence directins are displayed between 250 and 500 C. We interpret these data as indicating the presence f a lwtemperature nrmal verprint that is remved belw -200 C and is respnsible fr the increase in intensity bserved in the reversed specimens. The rapid decrease in intensity abve 550 C suggests that the primary magnetic carrier is magnetite. Furthermre, the slw t negligible decrease in intensity between 250 and 500 C suggests that this magnetite is primarily fine grained. Because this uncmplicated demagnetizatin behavir typifies mst Dve Spring samples, all ther specimens were demagnetized at 450 and 500 C in rder t reveal their characteristic remanence and t minimize the effects f secndary verprinting. Fllwing this treatment, the gruped magnetic data passed the reversal test and displayed a mean f 15 f cunterclckwise rtatin (Fig. 6). The magnetic plarity stratigraphy (MPS) based n the demagnetized and statistically evaluated data frm each site is shwn in Figure 7. The errr envelpe n the VGP latitude emphasizes the generally unambiguus plarity data thrughut the sectin. The mst striking feature f the Dve Spring MPS is the lng interval f nrmal plarity between 600 and 1,200 m (N8 and N9, Fig. 7). Because f the Clarendnian fssils present in the Dve Spring strata, this lng nrmal interval shuld crrelate with the nrmal segment f magnetic chrn C5N (Berggren and thers, 1985) spanning 8.92 t Ma (Fig. 7) in the magnetic plarity time scale (MPTS). Fissin-track dates f 8.4 ± 1.8 Ma and 10.4 ± 1.6 Ma (Cx and Diggles, 1986) als supprt this crrelatin. Given this crrelatin, magnetznes R1 t R8 can be crrelated with chrns C5AB t C5R ( Ma). The presence in the lcal Dve Spring MPS f all but ne f the well-dcumented magnetznes in this time span lends supprt t the interpretatin prpsed here. Statistical evaluatin based n the frmulatins f Jhnsn and McGee (1983) indicates that the prtin f the magnetstratigraphy belw the lng nrmal magnetzne (Chrn C5N) shuld encmpass at least 2.5 m.y. In additin, a fissin-track date f 11.8 ± 0.9 (2a) Ma n a vlcanic ash at 370 m (Fig. 7) is in basic agreement with this crrelatin. The interpretatin f magnetznes R10 t N13 is less straightfrward than in the underlying strata. The mst reasnable interpretatin f the MPS, hwever, crrelates magnetznes R10-N11 with chrns 9 and

6 TABLE 1. STRATIGRAPHY OF THE DOVE SPRING AND CUDAHY CAMP FORMATIONS Frmatin Member Thickness Descriptin Age Basal Lcatin (m) m.y. cntact Dve Spring Weakly cnslidated, unstratified clay; arksic sand; carse gravel; and bulders. Dve Spring Yellwish gray t pale range plutnicvlcanic pebble cnglmerate; carse t fine lithic arkse and arkse; gray, light brwn, and green mudrck; pinkish gray limestne; light gray t green, bedded and ndular chert; and vlcanic ash and tuff. Numerus, well-develped caliches near tp. <7 Discnfrmable Opal Canyn, Dve Spring Wash Cnfrmable Opal Canyn, Red Rck Canyn Dve Spring Basalt and massive, planar-bedded, and crss-bedded medium-t carse-grained lithic sandstne; gray t green mudrck, limestne, and chert; base and tp marked by pairs f basalt flws. Dve Spring Massive t planar-bedded medium-grained sandstne, siltstne, gray and green mudrck, and chert; prly srted vlcanic and plutnic cnglmerate and crss-bedded carse lithic sandstne. Dve Spring Pale red t light yellwish gray, prly srted, vlcanicplutnic pebble cnglmerate; massive t crss-bedded, carse, prly srted lithic sandstne, and tuffbreccia in suthwest; planar-bedded t massive lithic sandstne, green mudrck and chert, with limestne, tuff, and vlcanic ash in the nrtheast Cnfrmable Last Chance Canyn, Red Rck Canyn Cnfrmable Last Chance Canyn, Red Rck Canyn Cnfrmable Last Chance Canyn Dve Spring Yellwish gray t reddish purple, prly srted, massive t crudely stratified vlcanic-cbble rundstne cnglmerate, and massive and crss-bedded lithic sandstne Angular uncnfrmity Last Chance Canyn; pinches ut t nrtheast and suthwest. Cudahy Camp Black Muntain Basalt Olivine basalt, as many as 15 separate flws ± ±0.5 (Cx, 1982) Pssible uncnfrmity Black Hills and Black Muntain Cudahy Camp 5 Blackish t grayish red, massive and flaggy andesite flws ±0.5 (Cx and Diggles, 1986) Cnfrmable Last Chance Canyn t the Black Hills Cudahy Camp Grayish yellw t pinkish gray, massive tuff breccia. Cnfrmable Black Muntain t Cudahy Camp in Last Chance Canyn; pinches ut west f Last Chance Canyn Cudahy Camp Red-purple t dark reddish brwn andesite in brecciated and massive flws ± 0.6 (Cx and Diggles, 1986) Cnfrmable Last Chance Canyn; pinches ut suthwest and nrtheast alng strike. Cudahy Camp White t light yellwish gray-bedded, waterlain sandy tuff and pink r grayish yellw tuff-breccia. Base defined by light clred, bedded tuff up t 2 m thick with ccasinal ripple marks and shrinkage crack casts; cntains pink, massive tuff-breccia cntaining primarily 1- t 5-mm clasts f pumice and banded felsite. Cnfrmable Last Chance Canyn Cudahy Camp Red r gray diamictic cnglmerate f runded plylithlgic bulders and cbbles in carse arksic sand; prminent channels at base. Weakly cemented and prly expsed. >18.1 Angular uncnfrmity Maximum thickness at Black Muntain and in Last Chance Canyn; thins eastward (Dibblee, 1967; Cx and Diggles, 1986) and t the suthwest.

7 18 LOOMIS AND BURBANK Figure 5. Typical thermal demagnetizatin behavir fr siltstnes in the Dve Spring Frmatin. Slid symbls, nrmal plarity specimens; pen symbls, reversed plarity specimens. A. Magnetic inclinatin. Univectrial decay tward the rigin is shwn by all specimens at temperatures abve 200 C. B. Magnetic declinatins. Declinatin plts shw antipdal rientatins fr nrmal and reversed specimens, fllwing remval f a nrmal verprint. C. Magnetic intensity (J) during thermal demagnetizatin. The intensity increase in reversed specimens is due t remval f a nrmal verprint belw -200 C. The majr drp in intensity between 550 and 600 C indicates that the primary magnetic carrier is magnetite. Characteristic remanence is displayed between 200 and 550 C. 10 ( Ma, Berggren and thers, 1985) and magnetznes R12-N13 with chrns 7 and 8 ( Ma, Fig. 7). An imperfect match between the lcal MPS and the MPTS, such as displayed here, is nt unexpected in a cntinental envirnment (Jhnsn and McGee, 1983) due t bth the spradic nature f depsitin and the relatively large spacing (10-20 m) between sampling sites. Despite these caveats, the lcal MPS, the crrbrative radimetric dates, and statistical analysis f the data indicate that the Dve Spring Frmatin (members 1-5) spans an interval frm Ma t -7.0 Ma. This places the initiatin f Dve Spring sedimentatin -3 m.y. earlier than previusly believed (Evernden and thers, 1964) and extends it 1-2 m.y. later int the late Micene. The hiatus between eruptin f the upper flws f the Black Muntain Basalt and the inceptin f Dve Spring depsitin is -1.5 m.y., whereas the span between member 5 f the Cudahy Camp Frmatin and the Dve Spring is -3.5 m.y. Member 6 f the Dve Spring Frmatin is cnstrained t be yunger than 7 m.y., but at present, n reliable estimates f the time encmpassed by these largely unexpsed strata can be made. SEDIMENT-ACCUMULATION RATES Sedimentatin-accumulatin and subsidence histries fr the Dve Spring Frmatin are shwn in Figure 8. Packets f sediments defined by magnetzne bundaries have been decmpacted using the methds f Sclater and Christie (1980). During these calculatins, it was assumed that an additinal 500 m f strata were depsited prir t uplift f the sectin. Decmpacted, "instantaneus" sediment-accumulatin rates averaged ver 1- t 2-m.y.-lng intervals fr deeply buried strata are up t twice as large as the rates calculated frm presently preserved thicknesses. By decmpacting the sedimentary clumn after incremental remval f successive layers, it is pssible t develp a subsidence histry fr the basin during Dve Spring depsitin (Fig. 8). Each f these depictins (subsidence and cmpacted r decmpacted accumulatin) shw fur discrete intervals f sedimentatin. Accumulatin was rapid frm abut 13.5 t 12.0 Ma, crrespnding apprximately t depsitin f the cnglmerates and sandstnes f members 1 and 2. These rates slwed smewhat during

8 STRATIGRAPHIC EVOLUTION OF EL PASO BASIN, CALIFORNIA 19 W N / \ i: \ /DOVE SPRING FM. CLASS 1 SITE MEANS \ I I 1 1 \ 0 \ \ i 0 0 s O / _ / 0 Oq / Ê / Figure 6. Sterenet plt f demagnetized class I nrmal and reversed sites. The data are nearly antipdal and indicate that a mean f 15 f cunterclckwise rtatin has ccurred. The stippled areas indicate the «95-errr envelpe n the mean directins. the succeeding 2 m.y. (members 3 and 4) but increased sharply between 10.4 and 9 Ma, crrespnding t abut the lwer 600 m f member 5. The predminantly fine-grained upper 300 m accumulated at a slwer rate frm 9 Ma n. DETRITAL COMPOSITION The sandstnes and cnglmerates f the Ricard Grup were subdivided int fur majr cmpsitinal petrfacies, using pétrgraphie and field techniques fr determining detrital cmpsitin (Lmis, 1984). All f the sandstnes are texturally and cmpsitinally immature. The distributin f cmpsitinal petrfacies is in large part stratigraphically cntrlled (Fig. 9). In stratigraphie rder, these petrfacies are (1) plutnic-sedimentary-metamrphic cnglmerate and lithic sandstne, (2) vlcanic cnglmerate and lithic sandstne, (3) mixed vlcanic-plutnic cnglmerate and lithic arkse, and (4) granitid plutnic cnglmerate and alkali feldspar arkse (Table 2). The cmpsitins f these fur facies are shwn graphically in Figure 10. PALEOCURRENTS Palecurrent directins in the Dve Spring Frmatin were determined by statistical analysis f field measurements f the azimuths f 222 crss-beds. Measurements were taken at 44 lcatins thrughut the study area, and data frm individual statins were gruped by detrital petrfacies and by age (stratigraphie member) t test fr systematic changes in current directin with time and surce area. N ther current indicatrs were sufficiently abundant t prvide reliable data. Similarly, palecurrent directins were nt quantitatively determined fr the Cudahy Camp Frmatin, because current indicatrs are t rare fr statistical analysis. E Palecurrent directins shw a strng preferred rientatin tward the nrthwest t nrth-nrthwest thrughut the Dve Spring Frmatin, except in the uppermst strata f member 5 (Figs. 11a and 12), and there is essentially n systematic variatin in palecurrent directin with time (Figs. 1 lb 1 Id). Similarly, the described petrfacies, which might be expected t reflect gegraphically distinct surce areas, shw n significant palecurrent differences between them (Figs, lie, 1 If). The relatively small, pstdepsitinal, cunterclckwise rtatin f the Dve Spring strata shwn by the magnetic data (Fig. 6) indicates that nly a minr clckwise crrectin (~ 15 ) needs t be added t the presently bservable vectrs in rder t recnstruct frmer current directins. DETRITAL PROVENANCE Plutnic-Sedimentary-Metamrphic Clasts The basal unit f the Cudahy Camp Frmatin which cntains plylithlgic cnglmerate and lithic sandstne has few palecurrent indicatrs. An estimate f apprximately nrthward paleflw can be made n the basis f qualitative bservatins: (1) maximum clast size in the basal cnglmerate decreases nrthward, and (2) measurements frm pebble imbricatins in Cudahy Camp member 1 n the east slpes f Black Muntain (Cx and Diggles, 1986) indicate nrthward palecurrents in that area. Pssible surces fr all the distinctive detrital lithlgies in this petrfacies can be fund within the El Pas basin and El Pas Muntains t the suth. Many quartzite, black chert, metavlcanic, and metasedimentary clasts cmmn in the Cudahy Camp Frmatin appear t be recycled frm the underlying Gler Frmatin. Clasts derived frm the Garlck Frmatin, the Mesquite Schist, and granphyre expsed in the western El Pas Muntains are als cmmn in the Cudahy Camp basal cnglmerate. In sum, rapid fining t the nrth, the presence f clasts up t 1 m in diameter in the mre sutherly utcrps, and the nearby availability f apprpriate clast lithlgies suggest that the basal Cudahy Camp Frmatin was lcally derived frm the pre-micene rcks expsed alng the present-day El Pas Muntains. Vlcanic Oasts Palecurrent indicatrs in the vlcanic petrfacies clearly shw derivatin frm the suth-sutheast (Figs. 1 If and 12). Vlcanic-clast cnglmerates in the Dve Spring Frmatin are characterized by subrunded t runded clasts f prphyritic andesite, banded felsite, vesicular basalt, and pumiceus tuff. Althugh the Cudahy Camp Frmatin is nw expsed t the sutheast, vlcanic detritus in the Dve Spring (with the exceptin f the basal 0.5 m) is unlike the andesites and tuffs f the Cudahy Camp. Cnsequently, it is necessary t lk farther suth fr the surce f vlcanic clasts in the Dve Spring Frmatin. The Tertiary rcks f the Mjave blck, suth f the Garlck fault, cntain large vlumes f vlcanic rcks, including all f the majr clast lithlgies in Dve Spring cnglmerates. The ccurrence f similar Tertiary vlcanics ver large areas f the western Mjave Desert makes it difficult t lcate the specific surce area f clasts in the Dve Spring, and the task is further cmplicated by uncertainty abut the Micene psitin f the El Pas basin alng the Garlck fault. Nevertheless, the vlcanic detrital surce was clearly in the nrthern Mjave blck suth f the Garlck fault, n a line between the present lngitude f the El Pas basin and its lngitude in the late Micene, which may have been as much as 64 km farther east alng the Garlck fault.

9 LITHOLOGIC CHARACTER VIRTUAL GEOMAGNETIC POLE LATITUDE MAGNETIC POLARITY STRATIGRAPHY MAGNETIC POLARITY TIME SCALE 150C t (0 L- a> 0) CO c LU z * c cc < III > I NORMAL POLARITY 4.0 VOLCANIC ASH AAA REVERSED POLARITY MUDSTONE/SILTSTONE J}) SANDSTONE BASALT CONGLOMERATE

10 Figure 7. The lithstratigraphy, VGP latitudes, fissin-track dates, and magnetic plarity stratigraphy (MPS) f the Dve Spring Frmatin, members 1-5, and their crrelatin with the magneticplarity time scale (Berggren and thers, 1985). Only sandstne and cnglmerate units thicker than ~5 m are depicted in the lithstratigraphy. Generally unambiguus plarities are indicated by the a95-errr envelpe n the individual VGP latitudes. The lcal MPS is based n the VGP latitudes. The numbered magnetznes (Rl, Nl, etc.) are referred t in the text. Numerus vlcanic ashes (triangles) ccur in the sectin, and fissin-track dates f 11.8 ± 0.9 (2a) Ma at 370 m (this study), f 10.4 ± 1.6 Ma at 680 m (Cx and Diggles, 1986), and f 8.4 ± 1.8 Ma at 1,180 m (Cx and Diggles, 1986) aid in the crrelatin. The lng nrmal magnetzne frm 650-1,200 m (N8, FN9 f Fig. 7) is crrelated with marine anmaly 5 r magnetic chrn C5N. A reliable match f the MPS with the MPTS appears pssible fr magnetznes Rl t R8 (-13.5 t 10.4 Ma). The fissin-track dates at 370 and 680 m supprt this crrelatin. Crrelatin f magnetznes R10 t N13 is mre ambiguus, but the mst reasnable match places the tp f Dve Spring member 5 at ~7 Ma. RICARDO GROUP PETROFACIES GRANITOID PLUTONIC CONGLOMERATE AND ALKALAI FELDSPAR ARKOSE VOLCANIC-PLUTONIC CONGLOMERATE AND HIGH-QUARTZ LITHIC ARKOSE VOLCANIC-CLAST CONGLOMERATE AND LOW-OUARTZ LITHIC SANDSTONE PLUTONIC-SEDIMENTARY CONGLOMERATE AND LITHIC SANDSTONE I I I I VOLCANIC ROCKS TABLE 2. PETROFACIES DATA FROM THE RICARDO GROUP Cmpsitin Name N. Q:F:L* (sandstnes) P:V:S:M^ (cnglmerates) Plutnic-sedimentarymetamrphic cnglmerate and lithic sandstne Vlcanic cnglmerate and lithic sandstne 1 46:32:22 39:4:9: :25:64 2:95:2:1 Mixed plutnic-vlcanic cnglmerate and lithic arkse 3 35:44:21 22:63:2:13 Alkali feldspar arkse 4 40:59:1 n.a. Percentages f quartz (Q), feldspar (F), and lithic (L) grains, t Percentage f plutnic (P), vlcanic (V), sedimentary (S), and metamrphic (M) dasts. Figure 9. Sandstne and cnglmerate petrfacies in the Ricard Grup. Time markers shwn are 1, tuff-breccia in Dve Spring Frmatin member 2; 2, base f lwer basalt in member 4; 3 and 4, ash beds in member 5. CC, Cudahy Camp; R, Ricard. ^ Age (Ma) Figure 8. Sediment-accumulatin and subsidence curves fr the Dve Spring Frmatin based n the crrelatin f the lcal MPS with the MPTS. Open, small circles shw presently bserved thickness versus time relatinships. The slid circles represent decmpacted thicknesses based n the frmulatins f Sclater and Christie (1980) and assuming an additinal 500 m f pst-7 Ma sediments have been erded. After the ttal decmpacted thickness is nrmalized t the presently bserved thickness, the slpe is prprtinal t the lngterm "instantaneus" sediment-accumulatin rate, that is, the rate that prevailed at the time f depsitin. Decmpactin enhances the "instantaneus" rates in the mre deeply buried, mre cmpacted sediments. The large pen circles delineate the subsidence histry f the base f the Dve Spring Frmatin. By 7.2 Ma, mre than 1,600 m f sediments had accumulated since 13.5 Ma. Subsequent additin f 500 m f strata cmpacted these underlying strata t their present thickness. The inflectin pints n all curves at and -9 Ma can be interpreted as respnses t nearby tectnic events.

11 22 LOOMIS AND BURBANK Q Figure 10. Sandstne and cnglmerate mdal cmpsitins, Ricard Grup; see text fr discussin. Grup 1, plutnic-sedimentarymetamrphic petrfacies; grup 2, vlcanic petrfacies; grup 3, vlcanic-plutnic petrfacies; grup 4, granitid-plutnic petrfacies. Vlcanic-Plutnic Oasts Streams which depsited clastic rcks f the vlcanic-plutnic petrfacies flwed frm the suth-sutheast as in the vlcanic-clast petrfacies (Figs, lie and 12), and vlcanic detritus in this petrfacies is identical t that bserved in the mre vlcanic-rich facies. This facies, hwever, cntains a higher prprtin f plutnic, metamrphic, and sedimentary cbbles and f quartz and feldspar grains, which predminate ver vlcanic-rck fragments. The appearance f plutnic clasts and the greater abundance f quartz in this facies suggest a change t a mre plutnic-rich surce area, but it is nt accmpanied by a change in palecurrent directin r in the lithlgy f the vlcanic fractin f the sediment. Because this petrfacies is in general yunger than the vlcanic-rich facies described abve, this change in sediment cmpsitin may be due t eventual expsure f basement rcks in the surce area by deep ersin, while the vlcanic cver als cntinued t erde. Granitid plutnic rcks that culd cnstitute a pssible sediment surce are expsed beneath erded Tertiary cver ver wide areas f the Mjave Desert, but clasts frm the Dve Spring Frmatin are nt sufficiently distinctive t match t a unique surce. Granitid Plutnic Oasts Althugh the granitid plutnic petrfacies is expsed nly at the tp f the Dve Spring Frmatin in members 5 and 6, it is tectnically significant, because it is cmpsitinally unrelated t the rest f the Dve Spring strata. Few palecurrent data are available frm sandstnes in this petrfacies, but lithlgic and stratigraphic criteria suggest that the surce f sediment in this petrfacies was entirely different frm surces fr ther Dve Spring clastics. The absence f unstable lithic grains in this facies and the high prprtin f alkali feldspar and quartz indicate a prbable granitic surce fr this sediment. The interbedded cnglmerates are dminated by clasts f micrcline-bitite granite and grandirite cntaining dark, schistse xenliths. The bulders increase t mre than 1 m diameter in member 6 updip and west f the member 5 arkses. The westward carsening, large clast sizes, and available palecurrent data suggest that the arkses and assciated bulders were derived frm the west. Granites and xenlith-bearing grandirites f the Sierra Nevada (Samsel, 1962), expsed as clse as 4 km west f utcrps f the Dve Spring Frmatin, are the mst likely surce fr this petrfacies. STRUCTURAL GEOLOGY The structural gelgy f the El Pas basin is typical f the Basin and Range regin nrth f the Garlck fault. The western bundary f the basin, frmed by the Sierra Nevada fault, cincides with the western limit f basin-and-range structures. The spatial and tempral pattern f flding, dike injectin, and faulting in the study area (Fig. 3) prvide imprtant cntrls n the Micene histry f the El Pas basin. The Gler Frmatin was tilted hmclinally t the nrth and erded prir t the inceptin f Micene sedimentatin. Mre precise time cnstraints fr this defrmatin are nt presently available. Tw eastwest-trending faults east f Red Buttes (Fig. 2) are the nly faults f this rientatin in the mapped area (Fig. 3). These steeply dipping faults, which juxtapse a large blck f pre-tertiary basement against the Gler Frmatin, were active in the middle Palecene during depsitin f the Gler Frmatin (Cx, 1982). The western edge f the upfaulted basement blck is buried by the Cudahy Camp Frmatin, and s majr fault

12 STRATIGRAPHIC EVOLUTION OF EL PASO BASIN, CALIFORNIA 23 a) c) TOTAL DOVE SPRING FM VM = 323 L= 60% N = 222 P = 2.7 x IO" 35 MEMBER 4 VM = 307 L = 40% N = 28 b) d) MEMBER 2 VM = 310 L = 58 % N = 121 P= 2.4 x IO" 15 MEMBER 5 VM = 343 L = 75 % N = 73 be cnstrained t a precise time interval, and, therefre, they d nt ffer an independent check n the inferred stress regime. Within the study area, the Dve Spring Frmatin and lder strata are cut by several significant nrth-suth-striking, lw-angle, nrmal faults (Figs. 2 and 3). The slip surfaces f these faults generally dip at t the east, and m f demnstrable stratigraphic separatin is displayed by each fault. Dve Spring members 1-4 and the basal half f member 5 are cut by ne r mre f these faults. Strata as yung as 9-8 m.y. ld have therefre been extended in an east-west directin. We attribute the apparent lack f faults within the upper strata f member 5 t the limited expsures f these strata in the nrthwestern part f the study area rather than t an absence f faulting; therefre, the timing f mvement alng the nrth-suth nrmal faults can nly be cnstrained t include strata at least as yung as 9 m.y. This east-west extensin may have been ceval with activity n the nrtheast-trending faults. The largest such fault shws dwn-t-the-sutheast displacement and may als have ac- P = 1.2 x IO" 2 P= 55xlO" W 35" 30 H + e) f) VOLCANIC-PLUTONIC PETROFACIES VOLCANIC PETROFACIES VM = 326 L = 58% N = 154 I.Sx IO" 23 Figure 11. Palecurrent rse diagrams fr entire Dve Spring Frmatin (a), and separately fr members 2, 4, and 5 (b-d) and by petrfacies (e-f): see text fr discussin. VM, vectr mean azimuth; L, vectr length (percent); N, ppulatin size; p, Rayleigh test value. Tc displacement was pre-micene. Cx (1982), hwever, reprted that a large basalt dike f presumed early Micene age that is intruded alng the trace f the suthern fault is als sheared alng the fault, indicating sme reactivatin fllwing emplacement f the dike. Bth the Gler Frmatin and the early Micene Cudahy Camp Frmatin are cut by a series f vertical east-west-trending dikes (Fig. 3). These dikes cut units as high stratigraphically as member 4 f the Cudahy Camp Frmatin but d nt disrupt the Dve Spring Frmatin. Intrusin f the dikes apparently fllwed eruptin f member 4 and was apprximately cntempraneus with eruptin f the Black Muntain Basalt, which is itself underlain by similar dikes (Cx, 1984). The prximity and petrgraphic similarity f these dikes t the Black Muntain Basalt make them likely surces fr the extrusive basalt. Because dikes and fractures are mst likely t frm nrmal t <73 (Nakamura and thers, 1977), the dikes in the Cudahy Camp Frmatin prbably frmed in a nrth-suth tensinal regime. Althugh numerus ther lcal faults cut strata lder than the Black Muntain Basalt, nne can Td5 V /Td, TdP itd, Figure 12. Vectr mean palecurrent rientatin at 44 sample sites, Dve Spring Frmatin. N KM

13 24 LOOMIS A N D < RICAROO GROUP- Nrmal Sites O Reversed Sites > J A l p h a Errr Bar ss Errr Envelpe n Nrmal Sites Figure 13. Cunterclckwise rtatin f the Dve Spring Frmatin between 13.5 and 7 Ma. Nrmally and reversely plarized data are gruped separately fr successive age brackets. The errr envelpe represents a 2 a cnfidence interval n the nrmally plarized data. Strata lder than ~10 m.y., including thse f the underlying Cudahy Camp Frmatin, are rtated Yunger strata shw prgressively lesser amunts f rtatin, such that the 7- t 8-m.y.-ld beds appear unrtated. cmmdated left-slip ffset, bth f which are cnsistent with the gemetrical cnstraints impsed by nearby nrth-suth nrmal faults. The El Pas fault trends essentially east-west and merges with the Garlck fault near the east end f the El Pas Muntains. It appears t terminate west f the study area befre reaching the Sierra Nevada (Dibblee, 1952) (Fig. 3). It crps ut as a series f discntinuus traces, and its fault plane is vertical in the subsurface (Mabey, 1960). Althugh its utcrp pattern suggests that it is a strand f the Garlck fault, there is n evidence fr left-lateral slip n the El Pas fault, and displacement f Quaternary alluvial fans alng its trace shws that recent slip n the El Pas fault has been vertical and up n the nrth (Dibblee, 1952; Carter, 1980). The maximum thrw n the fault exceeds 6 km. Thrughut mst f its utcrp area, the Ricard Grup is tilted west-nrthwest tward the Sierra Nevada fault. The attitude f the tilted Dve Spring Frmatin is cnsistent with rtatin accmpanying fundamentally east-west extensin between the nrth-trending Sierra Nevada fault and the east-nrtheast Garlck fault. The Sierra Nevada fault dips BURBANK at the surface (Dibblee, 1967) but may flatten and underlie the Dve Spring Frmatin at depth. Early Micene strata n the nrth slpe f the El Pas Muntains dip generally Dips decrease systematically upward t 5-10 at the tp f member 5 f the Dve Spring, abve which expsures f member 6 appear essentially flat lying. The time f tilting is cnfined t late Micene t Pleistcene by the verlying untilted Pleistcene sediments. Althugh the systematic upward and nrthwestward dip decrease culd be suggestive f syndepsitinal basin grwth, it culd als reflect diminishing defrmatin due t increased distance frm the El Pas Muntains uplift. Uncnfrmities, which are typical f synsedimentary tilting, were nt bserved in the Dve Spring Frmatin, but there is abundant sedimentary evidence fr diastemic expsure in the upper part f member 5. The palemagnetic data indicate that the Dve Spring strata have been rtated cunterclckwise an average f 15 (Fig. 6). This rtatin can be examined in mre detail (Fig. 13) by viewing the magnetic data within separate time increments (D. W. Burbank and D. P. Whistler, unpub. data). We interpret these data t indicate that rtatin f the Dve Spring strata was in part syndepsitinal. It appears t have cmmenced abut 10 m.y. ag and t have prceeded systematically cunterclckwise abut between 10 and 7 Ma. Additinal studies f the rtatin bth f ischrnus ash beds within the Dve Spring and f the underlying Cudahy Camp strata are cnsistent with this interpretatin (D. W. Burbank and D. P. Whistler, unpub. data). This rtatin culd pssibly be related t cunterclckwise rtatins that are recrded at several lcalities in the central and western Mjave blck t the suth (Burke and thers, 1982; Luyendyk and thers, 1985; MacFadden and thers, 1987), but which have pr tempral cnstraints (pst-palegene t pst-middle Micene?). It appears much mre likely that this Ricard rtatin is a direct respnse t sinistral shear alng the nearby Garlck fault. As such, it suggests that mvement alng the Garlck fault cmmenced in this area at abut 10 Ma. Sequence f Defrmatin Structural crsscutting relatinships shw that the El Pas basin has been affected by at least fur episdes f structural defrmatin since the Gler Frmatin was depsited in the middle Palecene: (1) nrthward tilting f the Gler Frmatin between late Palecene and early Micene time; (2) nrth-suth extensin and intrusin f east-west dikes in the early r middle Micene, prbably Ma; (3) west-nrthwest tilting and related extensin invlving strata at least as yung as 9 m.y. ld; and (4) cunterclckwise rtatin beginning abut 10 Ma. In additin, the basin was translated westward alng the Garlck fault, prbably penecntempraneusly with the 10- t 7-m.y.-ld rtatin. Changes in the rientatin and style f structures f different ages are evidence that the basin was subjected t stresses which have varied significantly in rientatin with time, including a switch frm nrth-suth t east-west extensin between the early and late Micene. MIOCENE HISTORY OF THE EL PASO BASIN Early and Early Middle Micene Depsitin f the Cudahy Camp Frmatin began subsequent t nrthward tilting f the Gler Frmatin between the middle Palecene and early Micene. Tilting is mst likely t have been caused by mderate uplift f the ancestral El Pas Muntains alng an east-west trend. The basal cnglmerate f the Cudahy Camp Frmatin was depsited ver

14 STRATIGRAPHIC EVOLUTION OF EL PASO BASIN, CALIFORNIA 25 the erded basement cmplex and Gler Frmatin as a nrth-slping bajada n the suthern margin f the basin. Bulders as large as 1 m in diameter indicate that the surce area was nearby. The cnglmerates f the basal Cudahy Camp are dminated by clasts erded frm the intrabasinal pre-micene rcks nw expsed in the El Pas Muntains, suggesting that an elevated, basement-cred area blcked transprt f clasts frm the Mjave vlcanic prvince suth f the Garlck fault. Cnglmerate depsitin was fllwed by andesite and pyrclastic eruptins suth and east f the present basin frm 18 t 17 Ma. Subsequent basalt eruptins were accmpanied by minr nrth-suth extensin in the basin, which prduced east-west-trending dikes. Eruptin f the Black Muntain Basalt between 17 and 15 Ma was the last majr vlcanic event in the El Pas basin. The depsitinal basin f the Cudahy Camp need nt have been deep t accmmdate the preserved 300 m f sediments and vlcanic rcks. The Cudahy Camp Frmatin accumulated with n evidence f basin grwth thrugh synsedimentary faulting r tilting. Stratigraphic relatinships alng the suth margin f the El Pas basin suggest that the Cudahy Camp sediments and vlcanics were restricted t the eastern prtin f the El Pas basin, where the Palecene Gler Frmatin is als preserved. Cudahy Camp rcks are absent frm the western part f the basin, and the succeeding Dve Spring strata cntain n evidence that a substantial vlume f the Cudahy Camp strata was remved by late Micene ersin. The depcenter f the El Pas basin may have thus been farther east in the early Micene and may have shifted westward tward the Sierra Nevada fault during the late Micene. Middle and Late Micene The discnfrmity between the Cudahy Camp and Dve Spring frmatins spans the perid frm Ma. The lack f significant tilting between Cudahy Camp and Dve Spring strata suggests that mvement n basin-cntrlling faults ceased fllwing the event that caused tilting f the Gler Frmatin. The absence f clastic material derived frm the Cudahy Camp and pre-micene rcks f the El Pas basin in Dve Spring cnglmerates and sandstnes indicates that lder rcks did nt cnstitute a significant sediment-surce area. Sediment accumulatin resumed 13.5 m.y. ag with depsitin f carse alluvial fanglmerates in the lwer part f the Dve Spring Frmatin. In cntrast t the basal cnglmerates f the Cudahy Camp Frmatin, clasts in the basal Dve Spring are almst entirely vlcanic and f extra-basinal rigin. Althugh these clasts cannt be linked with any specific surce area, similar rcks are widely expsed in the western Mjave Desert. Sandstnes in member 1 and lwer member 2 f the Dve Spring reflect a predminantly basaltic r andesitic vlcanic prvenance cnsistent with that f the cnglmerates. The reappearance in the basal Dve Spring f extra-basinal clasts up t 0.5 m in diameter and the change in surce area can be cited as evidence f mderate uplift f the Mjave blck in the middle Micene, such that a sediment-transprt netwrk linking the Mjave blck with the El Pas basin became established. Basal alluvial-fan depsits, such as the Cudahy Camp vlcanics, are thickest in the central part f the basin, but by 13 Ma, the Dve Spring stream system spread westward tward the Sierra Nevada, and later Dve Spring depsits verstepped the El Pas Muntains basement cmplex n the suthwest edge f the basin. Middle t late Micene drainage in the basin was t the nrthnrthwest, transverse t basin-bunding tectnic elements. Vlcanic-clast gravels frm the Mjave blck were depsited n alluvial fans near the suth edge f the basin, and sand-sized sediment was transprted tward the center f the basin by ephemeral braided streams. A persistent lake ccupied the nrthern part f the basin fr mst f the late Micene. Depsitin f Dve Spring sediments was punctuated by pyrclastic and basaltic vlcanic eruptins with vents prbably lcated west f the basin. A systematic upward increase in the rati f quartz t lithic clasts in sandstnes and cnglmerates f the Dve Spring Frmatin was nt accmpanied by crrespnding changes in palecurrent directins. This suggests that prgressive denudatin f Tertiary vlcanic cver led t expsure f basement rcks as ersin succeeded eruptin in the Mjave blck surce area. Mst Tertiary vlcanism in the Mjave blck ccurred in the early t middle Micene. Therefre, the accumulatin f large vlumes f new vlcanic rcks in the surce area had prbably ended by Dve Spring time. Beginning -10 Ma, the El Pas basin began t be rtated cunterclckwise, prbably as a cnsequence f sinistral shear due t the initiatin f left-slip mtin alng the Garlck fault near the suthern margin f the basin. Such mvement is als likely t signal the beginning f basin-andrange-style extensin nrth f the fault. A sharp increase in apparent sediment-accumulatin rates t 44 cm/ka (Fig. 8) accmpanied the beginning f rtatin between 10-9 Ma. This suggests relative uplift f the Mjave blck surce and increased subsidence rates in the basin, due t crustal thinning as extensin began. At abut 8 Ma, a particularly significant cmpsitinal change ccurred in the yungest sandstnes and cnglmerates f the Dve Spring (upper members 5 and 6). In the sandstnes, a high prprtin f quartz and alkali feldspars and an absence f lithic grains indicate a silicic plutnic surce area. Granite and grandirite bulders and gravel dminate the cnglmerates. Althugh palecurrent indicatrs are sparse in these sandstnes, the Sierra Nevada can be identified as the surce f this detritus with cnsiderable cnfidence due t bservable clast-size trends, facies relatinships, and distinctive lithlgies. The pstulated mrphtectnic emergence f the Sierra Nevada as a surce fr the -8-m.y.-ld upper Dve Spring sediments is cnsistent with estimates f accelerated uplift f the range between 10 and 3 Ma (Christiansen, 1966; Huber, 1981; Chase and Wallace, 1986). The apparent decrease in sedimentaccumulatin rates between 9 and 8 Ma (Fig. 8) is difficult t interpret in this cntext. It may be a respnse t tectnic disruptin f the prximal basin margin due t precursr mvements alng the Sierra Nevada frntal fault, r it may result frm the remval f sme unknwn quantity f member 5 during expsure and tilting assciated with uplift f the Sierra Nevada. Fllwing majr uplift, rapid sedimentatin culd be anticipated adjacent t the frntal fault. Palemagnetic data frm restricted expsures in these prximal areas d suggest accelerated sedimentatin. There are, hwever, insufficient data t determine reliable accumulatin rates. The dramatic change frm extensin in a nrth-suth directin in the early Micene t east-west extensin in the late Micene is evidence f the initiatin f the system f basin-and-range faulting that has created present-day structures and tpgraphy nrth f the Garlck fault. The El Pas basin f Dve Spring time was cnsiderably larger than the basin f the Cudahy Camp, and its western margin reached t the Sierra Nevada. This expansin f the basin may als have resulted frm increased subsidence assciated with the nset f basin-and-range extensin. DISCUSSION The nset f the late Cenzic extensin which dminates the gelgy f the Basin and Range regin is recrded in the late Micene rcks f the Ricard Grup. Althugh this extensin is intimately related t the functin f the Garlck fault as a majr left-slip discntinuity (Davis and

15 EL PASO BASIN À& 10 II CO OC 12 < LU Z I 13 E= 5 i -^I-I 21 cm/ka 50 cm/ka 14 cm/ka 62 cm/ka / \ \ \ CALICHES, SILCRETES, PLUTONIC CONGLOMERATE OF SIERRAN SOURCE, DEPOSITED TO THE SOUTHEAST, VOLCANIC-PLUTONIC CONGLOMERATES, ARKOSE, MUDROCK, LIMESTONE, AND CHERT. SCATTERED VOLCANIC TUFFS. BASALT FLOWS 1NTEREEDDED WITH LIMESTONES, CHERT, AND MUDROCK. MINOR SANDSTONES. VOLCANIC-PLUTONIC CONGLOMERATES, SANDSTONES, SILTSTONES, MUDROCK, AND CHERT, SANDSTONES AND SILTSTONES, CHERT, LIMESTONE, AND TUFF-BRECCIA, LENSOID CONGLOMERATES, VOLCANIC TUFFS. BASAL VOLCANIC CONGLOMERATE, EXTRA- BASINAL IN ORIGIN, PALEOCURRENTS TO THE NORTH-NORTHWEST. g S T3 S O "O 5.-0 CO a) CC n D c <D O.M c» }2» 553 CO S C S «73 «8 13 CD Ss S S CO «m < _ 3 Eg.2«««.2 c GQ O L t CO > O 5; H C0 CC < D < > 1 1- a < OC Z» nr < O LU u. CO CO X z LU LL O O _j h- X 1- tr UJ U. < LU _l O CL z z < c a. Q CO z < < z IT CO H < CO m z CO 15 DEPOSITIONAL HIATUS did 0= i= MULTIPLE BASALT FLOWS IN EASTERN EL PASO BASIN. EAST-WEST-TRENDING DIKES: MOST LIKELY AS FEEDER DIKES FOR OVERLYING FLOWS, ANDESITE FLOWS, THINNING TO THE WEST, TUFF BRECCIA, THINNING TO THE WEST. BRECCIATED AND MASSIVE ANDEISITIC FLOWS TUFFS AND TUFF-BRECCIAS OF EXTRA- BASINAL ORIGIN. BASAL CONGLOMERATE, LOCALLY DERIVED, THINNING AND FINING TO EAST, NORTH, AND WEST,» <0 CO «t Q. 0) uj 5 13 O <B CO É «T3U_ OOT SCO I- gi 1 " a >«y Li. DC O CL Z 3 O I- i CO d

16 STRATIGRAPHIC EVOLUTION OF EL PASO BASIN, CALIFORNIA 27 Figure 14. Summary figure f the Micene histry f the El Pas basin. Chrnlgic cntrl frm magnetstratigraphic and radimetric studies is used t place sedimentlgic and tectnic events in a precise tempral framewrk. The sediment-accumulatin rates represent the mean, decmpacted "instantaneus" rates fr each interval. The vertical extent f lines assciated with the tectnic interpretatins is intended t indicate the time interval ver which each event is interpreted t have ccurred. Dashed lines indicate uncertainty cncerning the precise initiatin r terminatin f an event. Burchfiel, 1973), it has, as previusly nted, been pstulated many times that a large dwn-t-the-nrth, dip-slip r strike-slip fault ccupied the site f the present Garlck fault at sme earlier time in the Cenzic (Hewett, 1954; Nilsen and Clarke, 1975; Cx, 1982). The relatively thin (300 m), areally restricted, and lcally derived depsits f the Cudahy Camp Frmatin, hwever, are suggestive f depsitin in a small, restricted basin (Fig. 14), rather than in a reginal, fault-cntrlled depressin, such as that pstulated fr the Palegene Garlck fault zne. Althugh ur interpretatin f the Cudahy Camp Frmatin des nt supprt the idea f an active, majr pre-garlck fault in the early Micene, there are indicatins that tectnic stresses in the El Pas basin at this time wuld be at least cnsistent with dip-slip mtin n an east-west fault suth f the basin. East-west dikes cutting the Cudahy Camp and lder rcks shw that the El Pas basin was subjected t nrth-suth tensinal stresses smetime in the early t middle Micene, but there is n evidence f net extensin in that directin. Plate-tectnic interactins between the Nrth American plate and the Mendcin fracture zne may help t explain this early Micene tectnic event. Several investigatrs (Blake and thers, 1978; Dickinsn and Snyder, 1979a; Glazner and Lmis, 1984) have nted that late Cenzic events in many Califrnia sedimentary basins are assciated in time with the prgressive cnversin f the western margin f Nrth America frm a subductin zne t a transfrm fault which accmpanied nrthward passage f the Mendcin triple junctin up the Pacific Cast (Atwater, 1970). Glazner and Lmis (1984) prpsed a tectnic mdel which relates basin histry t flexure f the Nrth American plate ver the subducted Mendcin fracture zne, mdeled as a several-hundred-metrehigh, east-west-trending, tpgraphic step in the Farallón plate. Simulatin f the Nrth American plate's flexural prperties shws that it wuld underg nrth-suth tensin as it verrde the fracture zne in the Farallón plate (Glazner and Schubert, 1985). Accrding t plate recnstructins, the Mendcin fracture zne wuld have passed thrugh the latitude f the El Pas basin at abut 17 Ma (Glazner and Lmis, 1984), that is, at the same time as eruptin f the Black Muntain Basalt and prbably at the same time that the east-west basalt dikes were emplaced in the Ricard Grup (Fig. 14). These events in the El Pas basin tend t supprt predictins (Glazner and Lmis, 1984; Glazner and Schubert, 1985) that nrthsuth tensin withut net extensin shuld be assciated with passage f the Mendcin fracture zne. Cnsideratin f the effects f the Mendcin fracture zne n the Nrth American plate als predicts that tectnic uplift shuld fllw the fracture zne's passage thrugh a given area (Glazner and Lmis, 1984). Data frm the Ricard Grup als supprt this predictin. The middle Micene hiatus between the Cudahy Camp and Dve Spring frmatins ccurs shrtly after the calculated time f the fracture zne's passage and is indicative f at least relative uplift abve lcal base level (Fig. 14). Tectnic uplift, in the abslute sense, als ccurred farther west in the San Jaquin basin (Lmis and Glazner, 1986). This abslute uplift at the same time and latitude suggests that reginal uplift did take place ver a brad area after the Mendcin fracture zne passed thrugh, and that the uncnfrmity in the Ricard Grup is prbably a result f this reginal prcess. The middle t late Micene rcks f the El Pas basin, which verlie the uncnfrmity, are distinctly different frm the Cudahy Camp Frmatin and cntain increasing evidence f the east-west extensin which dminates the present gelgy f the regin. The nset f east-west extensin in suthern Califrnia may als be related t the nrthward migratin f the Mendcin triple-junctin system. Theretically, rerientatin f lithspheric stresses suth f the triple junctin (Ingersll, 1982) wuld prduce a reginal stress field cnducive t east-west extensin accmmdated by nrth-suth nrmal faults (Glazner and Bartley, 1984). Evidence frm the El Pas basin indicates that basin-and-range-style extensin began several millin years after the apparent passage f the Mendcin fracture zne thrugh the regin. The sedimentary recrd f this extensin during the Micene is marked initially by renewed basin grwth and rapid sedimentatin nrth f the Garlck fault and culminates with accumulatin in the El Pas basin f detritus frm the newly emergent Sierra Nevada beginning abut 8 Ma (Fig. 14). Sedimentary evidence frm the Dve Spring Frmatin des nt directly mark the beginning f strike-slip mvement n the Garlck fault, but because f the intimate relatinship between the Garlck fault and extensin east f the Sierra Nevada (Davis and Burchfiel, 1973), it seems that strike-slip displacement must have begun by the time the Sierra Nevada became manifest as a sediment-surce area in the late Micene. Mre precise specificatin f the time f inceptin f sinistral mvement n the Garlck fault derives frm the rtatinal data which pint t strike-slip mtin beginning 10 Ma and frm sediment-accumulatin-rate data that indicate a ceval interval f rapid subsidence (Figs. 8 and 14), which we attribute t crustal thinning due t extensin nrth f an active Garlck fault. CONCLUSIONS The Micene vlcanic and sedimentary rcks f the El Pas basin dcument the fllwing tectnic events: (1) vlcanism and nrth-suth tensin with n net extensin, abut Ma in the early Micene; (2) relative uplift abut Ma in the middle Micene at the same time as abslute tectnic uplift farther west in the San Jaquin basin; (3) initiatin f sinistral slip n the Garlck fault abut 10 Ma; (4) the nset f east-west extensin by at least 9 Ma; and (5) mrphtectnic emergence f the Sierra Nevada by 8 Ma. These events can be related thrugh plate-tectnic mdels t interactins between the Nrth American, Pacific, and Faralln plates at the western margin f Nrth America, and they prvide sme new age cnstraints fr the uplift f the Sierra Nevada and fr initial sinistral mtin alng the Garlck fault. ACKNOWLEDGMENTS This research was supprted by grants frm the Petrleum Research Fund (PRF) f the American Chemical Sciety t Allen F. Glazner (13940-G2), the University f Nrth Carlina Martin Fund fr Research in Gelgy and Sigma Xi t Lmis and frm PRF (15324-G2), NSF (EAR ), and the University f Suthern Califrnia Faculty Research and Innvatin Fund t Burbank. We thank Brett Cx and David Whistler fr willingly sharing their field experience in the area and fr many thughtful discussins. Allen Glazner, Jhn Bartley, Jhn Rgers, Steve Lund, and Greg Davis cntributed much appreciated advice and

17 28 LOOMIS AND BURBANK assistance n this prject. Special thanks are als due t the staff and residents f Red Rck Canyn State Park fr their supprt and t Nancy Williams, Trileigh Strh, and Miles Grant wh helped with the field wrk. REFERENCES CITED Atwater, T., 1970, Implicatins f plate tectnics fr the Cenzic tectnic evlutin f western Nrth America: Gelgical Sciety f America Bulletin, v. 81, p Baker, C. L., 191 1, Ntes n the later Cenzic histry f the Mhave Desert regin in sutheastern Califrnia: University f Califrnia Publicatins, Bulletin f the Department f Gelgy, v. 6, p , Physigraphy and structure f the western El Pas Range and the suthern Sierra Nevada: University f Califrnia Publicatins, Bulletin f the Department f Gelgy, v. 7, p Berggren, W. A., and van Cuvering, J. A., 1974, The late Negene: Bistratigraphy, gechrnlgy, and paleclimatlgy f the last 15 millin years in marine and cntinental sequences: Develpments in Palentlgy and Stratigraphy, v. 2,216 p. Berggren, W. A., Kent, D. 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MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 27,1986 REVISED MANUSCRIPT RECEIVED MAY 15,1987 MANUSCRIPT ACCEPTED MAY 15,1987