CHARACTER AND ORIGIN OF CATACLASITE DEVELOPED ALONG THE LOW-ANGLE WHIPPLE DETACHMENT FAULT, WHIPPLE MOUNTAINS, CALIFORNIA

Jan-Claire Phillips Mobil Oil Corporation P. O. Box 5444 Denver, CO 80217

ABSTRACT ment fault in the Whipple Mountains of southeastern Cal iforni a. The \'/hipple detachment fault in the \'Jhipple Mountains, southeastern California, is characterized Previ ous \~ork by a thin layer of cataclasite, a green or bro\'m aphanitic rock containing randomly oriented myloni­ Two studies dealing with lithified random­ tic porphyroclasts derived from lower plate rocks. fabric fault rocks (cataclasites and microbreccias) At least two generations of cataclasites are are those of Brock and Engelder (1977) and Anderson present. The cataclasites are composed of randomly and Osborne (1980). Brock and Engelder determined oriented quartz and feldspar porphyroclasts in a that cataclastic flow was the mechanism responsible random-fabric matrix. Color banding can be seen in for the deformation they observed beneath the Muddy the matrices of some green and most brown samples, Mountain Thrust in the Muddy Mountains, Nevada. but does not appear to be associ ated with oriented Anderson and Osborne did not address the problem of fabrics, shear zones, or other indications of flow. what mechanism was responsible for the cataclasites Porphyroclast size distributions for both green and deve loped along the San Gabri e1 fault in Southern brown cataclasites are of logarithmic-extreme value California, but they did provide an extensive type. petrologic and textural characterization of those rocks. The \'lOrk of Brock and Engelder and of The cataclasite matrix is composed of very Anderson and Osborne, along with the experimental small, equant, interlocking grains with an average studies of Engelder (1974) and Sammis and others size of 0.004-0.006 mm, sho\'/ing little evidence of (1980), i ndi cate that both the random-fabri c faul t deformation. Predominant matrix minerals are quartz rocks studied to date and experimentally formed and alkali feldspar. Much of the cataclasite fault gouge yield logarithmic-normal particle size mineralogy is probably secondary, although both distributions. Engelder (1974) and Brock and upper and lower plate components are present. Field Engelder (1977) have suggested that this type of relationships indicate that each cataclasite matrix particle size distribution is characteristic of was highly mobile at the time of its formation. cataclasis and cataclastic flow. The presence of uniformly sized interlocking Heidrick and Wilkins (1980) have presented matrix grains favors a creep mechanism for cata­ bulk chemistry data for cataclasites collected in clasite formation rather than one involving only the \·Ihipple and Buckskin Mountains, including the high fluid pressures or cataclastic flow. Although area of this study. They concluded that the the temperature of cataclasite formation was lower cataclasites formed primarily from lower plate than that expected for structural superplastic flow, rocks, although they were slightly enriched in the matrix textures suggest structural superplasti­ potassium and depleted in sodium relative to the city as a possible deformation mechanism for the lower plate rocks analyzed. cataclasite matrices. Superplasticity at these temperatures may be possible if diffusional Geologic Setting processes are enhanced by the presence of water. The 1ack of rotated porphyrocl asts and of matri x The ~Ihipple Mountains are located in eastern fabrics and the porphyroclast size distributions San Bernardino County, California, adjacent to the suggest that cataclastic flow was not the dominant Colorado River near Parker, Arizona (F igure 1). deforma t ion proces s . It issugges ted that cata­ This range is part of a large ( 3000 km 2) area of clastic flow initiated the deformation process and extensional faulting in the Colorado River trough was responsible for most grain size reduction, but of south;astern Cali forni a, western Ari zona, and that superplasticity may have become dominant after southernmost Nevada. The detached terrane a very small grain size was ach i eved. Both mecha­ coincides in part with the zone of metamorphic core nisms may have been aided by the presence of water complexes that extends throughout the southern and by higher than normal fluid pressures. Cordillera. An Oligocene to mid-Miocene low-angle fault surface, called the Whipple detachment fault I NTRODUCTI ON in the Whipple Mountains area, separates autoch­ thonous lower-pl ate igneous and metamorphic rocks Interest in earthquake prediction has contri­ of Precambrian to Tertiary (?) age from alloch­ buted to a vari ety of studi es of faul t rocks and thonous upper-plate igneous, metamorphi c, and fault mechanisms in recent years. However, most sedi mentary rocks rangi ng in age from Precambri an studies of cataclastic rocks have dealt exclusively to mi ddl e ~~i ocene. Thi s fault underl i es much of with clay gouge (e.g., \~u and others, 1975), non­ the Colorado River trough area and is exposed about lithified gouge or breccia (e.g., Anderson and the margins of adjacent ranges, including the others, 1980), or mylonitic rocks. Few studies have Chemehuevi , Sacramento, Bucks ki n, and Rawhi de focused on lithified, random-fabric fault rocks such Mountains. Rocks now seen in the lower plate and as those found along the lO\'i-angle Whipple detach- in one upper-plate location have been subjected to

109 MIO.-PlIO. OSBORNE WASH FM.

OLlO.-MIO. OENE CANYON & COPPER BASIN FMS N UPPER - PLATE CRYSTALLINE 1 ROCKS LOWER - PLATE MYLONITIC ROCKS o , 2 '-----"---' LOWER - PL ATE NON - MYLONITIC km ROCKS

CALIFORNIA Figure 1. Generalized geologic map of the vlhipple tliountains after Davis and others (1980). Boxes enclose areas sampled for this study. an earlier (mid- Cretaceous to early Tertiary (7)) locally absent in the western part of the range mylonitization that preceded detachment faulting. I'lhere the 10l'ier-plate rocks are not mylonitized. The region has also been subjected to a I'larping The cataclasite thus appears to be preferen­ event that formed a series of asymmetric anti­ tially developed in the eastern part of the range, forms and synforms that control the exposure of where myl on iti crocks underl i e the detachment the detachment fault around the margins of the surface. ranges. The chlorite breccia zone is described in The Colorado River trough area, and the Davis and others (1979; 1980) and in Krass (1980). Whipple Mountains in particular, have been the Occasionally there is a sharp contact between the subject of ongoing study at the University of cataclasite and chlorite breccia zone, but more Southern California for the past several years. often the latter becomes progressively more Da vi s and others (1979; 1980) provi de the mos t shattered and brecciated as the fault surface is recent overvi ews of the structure, petrology, and approached, finally grading into the microbreccia geologic history of the Whipple, Buckskin, and or cataclasite. Brecciation of the upper-plate Rawhide Mountains. rocks is much less pervasive and is limited to the first few meters above the fault plane. Extensive \~hipple Detachment Fault alteration, including iron and occasionally copper mineralization, is common in the cataclasite and The Whipple detachment fault apparently adjacent upper-plate rocks (Heidrick and \~ilkins, developed within crystall ine rocks, but in some 1980) . places now separates upper-plate Tertiary sedimentary and volcanic rocks from lower-plate CATACLASITES cryste.lline rocks. It is along such contacts that the cataclasite appears to be best developed. The Sample Locations detachment faul tis characteri zed by a typi cally smooth, planar fault surface which is underlain by a Sampling for the present study was confined to thin layer of cataclasite and microbreccia that the eastern and southern \~hipple Mountains, in the 1 tends to form a distinct ledge (Figure 2). The \~hipple \~ash and \~hipple Mountains SV! 7 2 minute cataclasite is usually 2 to 25 cm thick, but in quadrangles. Most samples were collected in areas places its thickness can be as great as 1.5 to 2 m. where Tertiary volcanic and sedimentary rocks are The cataclasite layer is usually underlain by a the predominant upper-plate units. The areas where thick (up to 250-300 m) zone of sheared and sampl es were coll ected have been mapped by brecciated lower-plate rocks that are chloritized G. A. Davis, K. V. Evans, E. G. Frost, V. A. Krass, and appear pale to dark green in color. This zone, S. H. Lingrey, J. A. Lopez, and L. C. Thurn; no termed the chlorite brecci a zone, appears to be further mapping was done for this study. preferentially developed where the lower plate consists of mylonitic gneisses and is thinner and Field Descriptions

110 Figure 2. The Whipple detachment fault exposed in the south-central Whipple Mountains (area B in Figure 1). The prominent ledge is formed of cataclasite developed beneath the fault.

The cataclasite is an aphanitic, well-indurated of very low birefringence and appears to be a brown or green rock, often containing randomly fine-grained mass of quartz and feldspar, a conclu­ oriented floating porphyroclasts of 1 to 10 mm in sion verified by X-ray analysis. In places, the size with occas iona 1 severely fractured porphyro­ matrix is severely stained or opaque, particularly clasts up to 10 em in size. Porphyroclasts found in ~Iithin the brown cataclasite samples. Occasional the catacl asite above mylonitic lower-pl ate rocks porphyroclasts of alkali feldspar and epidote are are of three types: monomineralic quartz or feld­ also present. Identifiable plagioclase feldspar is spar, mylonitic rocks derived from the lower plate, rarely seen. Both euhedral (square and hexagonal) and fragments of older cataclasites. At least tlvO and irregularly shaped opaque minerals are common; generations of cataclasite are present. The younger the shapes of the euhedral opaques suggest that brown layer lies structurally above the older green they are pyrite and magnetite. Calcite veins are layer and locally intrudes it, producing thin seams, occasionally present, particularly in the brown shatter zones, and zones of green porphyrocl asts cataclasite. entrai ned withi n the brown ma tri x. Mos t porphyro­ clasts observed within the brown layer are fragments The green cataclasite usually appears very of older cataclasites. The green cataclasite matrix uniform in texture, whereas the brown cataclasite often intrudes shattered mylonitic porphyroclasts exhibits more textural variability. Color banding along fractures, resulting is large porphyroclasts is seen in one green cataclasite sample and is which are actually mosaics of smaller fragments. quite common among brown catacl asite sampl es; the These relationships indicate that both cataclasite color bands are usually parallel to the fault matrices were highly mobile when they were formed. surface. In the green cataclasite exhibiting color banding, the bands appear to represent composi­ Petrographic Descriptions tional differences in the matrix as well as dif­ ferences in porphyroclasts;ze and density. In In thin section, the cataclasite is composed of the brown cataclasites it is not po~sible to angular to sub-rounded porphyroclasts of quartz, discern any textural or compositional differences fragments of mylonitic gneiss, and cataclasite between bands of different colors. fragments that are suspended ina matri x formed of particles too small to be discerned optically Several small-scale ductile features have been (Figure 3). The matrix exhibits no pervasive, observed within the brown cataclasite matrix. oriented crystallographic fabric. It is commonly These features occur in small shear zones which are

111 found both parallel to and at moderate angles 0 (30-45 ) to the color bands, and include sheared porphyroclasts and ductilely disrupted color bands. In places the matrices of both green and brown cataclasites exhibit a vague fabric which is best seen under crossed nicols and may represent a preferred orientation of matrix grains.

Where the contact between green and brown cataclasite is seen in thin section it is very sharply defined. Small incursions of brO\vn matrix can be seen extending down into the underlying green layer. A similar relationship can be observed between two brown layers found in one sampl e, in whi ch seams of the 'upper 1ayer extend downwards into the lovler layer. These matrix incursions are usually at high angles to the contact between 1ayers and in some cases appear to follow what appears to be pre-existing fractures. Some of the seams or vei ns end abruptly; these do not appear to be following pre-existing fractures since no fractures extend from their terminations. At two locations, the brown seams extend several centimeters down into the green cataclasite.

Porphyroclast Size Distributions

Since the cataclasite is extremely well indurated, conventional textural analysis techniques of seiving and pipetting were not possible. As a result, a Zeiss TGZ particle size analyzer was used to determine porphyroclast size. The minimum measurable size using this technique was 0.0078 n@. Approximately 5000 porphyrocl asts were measured in each of three representati ve catac1as ite samples. The histogram and cumulative probability plot for one of these samples is shown in Figure 4. The cumulative probability plots for all three samples Figure 3. Photomicrograph of green cataclasite indicate that the porphyroclast size distributions showing typical matrix texture. Large porphyro­ are of logarithmic-extreme val ue type. The average clasts are quartz; linear feature at left center porphyroclast size (mode) for the three samples is a vein of brown cataclasite matrix injected varies from 0.0074 mm to 0.0084 mm. into the green cataclasite.

x-ray r~i neralogy cataclasite samples cannot be positively identified by X- ray di ffracti on; however, it can be determi ned Forty-three samples, including both green and to be an iron-rich chlorite on the basis of rela­ brown cataclasites, upper-plate volcanics, and tive peak intensities. Alkali feldspars in both lower-plate nvlonitic rocks, were analyzed by X-ray green and brown cataclasites exhibit structural powder diffraction. Each of the minerals identified states falling between the orthoclase series and can be assumed to represent 3% or more of the sanadine-high albite series. It was possible to sample, since minerals comprising less than 3% of a calculate a K-feldspar structural state for only sample are usually not identifiable by X-ray dif­ one upper-plate volcanic sample, but it also fell fracti on un 1es s mi nera1 separations are performed. midway between the orthoclase and sanidine-high The minerals identified in the green cataclasite are albite series. Since these feldspars are composi­ quartz, monoclinic alkali feldspar, chlorite, and tionally almost pure orthoclase, the structural albite. Quartz, monoclinic alkali feldspar, states approaching sanidine may indicate that a hematite, geothite, chlorite, calcite, and epidote volcanic component is present in the cataclasites. (?) were identified in the brown cataclasite. Quartz is the major component in all cataclasite It should be noted that when both brO\vn and samples. The major differences noted between green green cataclasite is present at a locality, the and brown catacl as ites are the presence of iron brown layer, which is structurally higher, consis­ oxides in the brown cataclasites and the greatly tently yields K-feldspars with structural states reduced intensities of the alkali feldspar peaks in closer to sanidine than those in the green layer. the brown samples relative to those in green A similar relationship is present in localities samples. It is inferred that all samples contain where there are tVIO 1ayers of brown catacl as ite; ch 1ori te, although in some cases the amount may be i.e., alkali feldspars from the upper layer are too small to be distinguished by X-ray diffraction. consistently structurally closer to sanidine. This The bulk mineralogy of the cataclasite relative to is not the case when a series of samples within a the upper and lower-plate samples analyzed indicates green cataclasite layer are compared. that much of the cataclasite mineralogy is secondary although both upper and lower plate components are Scanning Electron Microscopy probably present. Observations of a polished, etched surface of The type of chlorite contained in the a green cataclasite sample yielded an average

112 1.0 invoked for the formati on of randorn-fabri c cata­ clastic rocks is that of cataclastic flow, the E repeated fracture of individual grains coupled with ..s ~ rigid-body rotation and sl iding of the grains and ...... new fragments in the direction of shearing (Borg w .10 N and others, 1960; Engelder, 1974; Ashby and Verrall, 1977). This allows for great mobility of V) ...... ----.---- ...... -- the gouge produced . Z ...... ~ Cataclastic flow is characterized by pervasive - .01 ...... - 40 angularity and fractured nature of large porphyro­ U clasts in the cataclasite also attests to a cata­ Z clastic origin for at least the porphyroclast w fraction. However, there is little or no evidence :;) for cataclastic flow within the cataclasite mat­ 0 w rices as seen in thin sections or by SEM. In I:lI:::: 20 particular, the lack of cementing material and u.. abraded textures on grainsurfaces, as well as the interlocking nature of matrix grains observed by SEM, presents a case against cataclastic flow. Coup 1ed wi th a cons pi cuous 1ack of rotated grains and the absence of clear mi crofractures in mos t small porphyroclasts, these observations indicate GRAIN SIZE (mm x 10-1) that cataclastic flow was not the dominant deforma­ tion mechanism in the formation of the cataclasite. This conclusion is also supported by the logarith­ Figure 4. Histogram and cumulative probability mic-extreme value particle size distributions distribution for the porphyroclast fraction of obtained for the porphyrocl ast fractions of the a green cataclasite sample. An extreme value cataclasites in contrast to the logarithmic-normal probability scale was used. particle size distribution demonstrated for cata­ clastic flow (Engelder, 1974) and found in other studies of cataclasite and fault gouge (Brock and matrix grain size of roughly 0.004 to 0.006 mm. The Engelder, 1977; Anderson and others, 1980; Anderson minimum grain size observed for this sample was and Os borne, 1980; Sammi s and others, in prep.). about 0.002 mm. Matri x grains appear to be very uniform in size, and are roughly equant, showing no The presence of high fluid pressures has often fabri c or preferred ori entati on. The edges of the been suggested as a mechanism for the motion of low grains appear very clean, with no visible abraded angl e faults (Hubbert and Rubey, 1959; Rubey and material present. Signs of cement are rare; most Hubbert, 1959). The actual mechanism by which grains are in direct contact with other grains and fault motion principally accommodated by high pore in some areas the grains exhibit an interlocking fluid pressures could take place is that of texture (Figure 5). independent grain boundary sliding (Borradaille, 1979; 1981). This mechanism involves purely Fracture surfaces of another green cataclasite intergranular deformation occurring independently sample were also observed; it is difficult to of deformation within the grains, and is accom­ estimate an average or minimum grain size for this plished by relief of the normal stresses across sample because grain boundaries do not show clearly grain boundaries such that grain boundary sliding on the fracture surface, on which intragranular is not i nhi bited by the presence of asperiti es on fracture appears to be domi nant. Large porphyro­ irregular grain surfaces. Independent grain clasts appear very clean, exhibiting no signs of boundary sliding involves no grain size reduction abras i on and very abrupt contacts with the matri x and does not cause a preferred orientation of grains. No secondary mineralization has been grai ns. One expected characteri sti c of thi s observed in either sample. deformation mechanism is that the resultant deformed textures cannot be distinguished from the DISCUSSION AND CONCLUSIONS undeformed textures (Handin and others, 1963). Random-fabric fault rocks are usually thought The presence of injection features and shatter to have formed by purely brittle processes, since zones at the boundaries between different they show no oriented textural features indicating generations of cataclasite suggests the action of ductil e deforma t ion. The process mos t often high fluid pressures during their formation, or may

113 superplasticity is a deformation mechanism that was initially suggested for metals in which tensional strains of greater than 1000% were reached without necking or fracturing of the sample. Although the exact nature of the mechanism is uncertain, it is grain size dependent and may involve the process of grain boundary sliding accommodated by diffusional creep along the gra i n boundari es (Ashby and Verrall, 1973; Gueguen and Boullier, 1975; Boullier and Gueguen, 1975; Nicolas and Poirier, 1976; Twiss, 1976). The mechanism allows the achievement of very large ductile strains, usually at tempera­ tures above 1;; the melting temperature (Tm; Nicolas and Poirier, 1976; Twiss, 1976) or above 0.3 Tm for 10\'1 stresses (Gueguen and Soullier, 1975; Allison and others, 1979). High temperatures may not be required if the strain rate is very slo\'1 (llhite, 1977), the grain size is very small (Sibson, 1977), or diffusional processes are enhanced by the presence of water or during metamorphic reactions (White and Knipe, 1978; Allison and others, 1979). Structural superplasticity is characterized by a very small (generally less than 0.01 mm) grain size, a low stress, low dislocation densities, a lack of subgrains, and a consistent microstructure of equant grains (Boullier and Gueguen, 1975; Gueguen and Boullier, 1975; Nicolas and Poirier, 1976; White, 1977, Taplin and others, 1979). Gueguen and Boullier (1975) and Boullier and Gueguen (1975) also consider a lack of crystal­ lographic fabric or prefered orientation to represent a characteristic of structural super­ plasticity, although \·lhite (1977) argues that this is not necessarily the case. \~hite also states that ... a fine grain size is not conclusive evidence for superplasticity. However, Figure 5. Scanning electron micrograph of a green the presence of a consistent micro­ cataclasite sample. Note interlocking texture of structure based on equidimensional quartz matrix grains. grains, which must at least form a penetrative matrix, throughout large strains may be the best indicator for alternatively represent episodes of very rapid superplastic behavior in nature. seismic faulting. The extreme mobility indicated by (p. 167) the matrix intrusions implies the existence of a highly pressured system, driven either by very Structural superplasticity has been suggested abrupt motion or by high fluid pressures concen­ for several geologic materials, including trated along the fault zone. Although the faulted kimberlites (Gueguen and Soullier, 1975), some nature of the upper plate makes it unlikely that mylonites (Boullier and Gueguen, 1975; White, 1977; extremely high fluid pressures could have been All i son and others, 1979) , and fi ne-grai ned maintained -for any length of time, they may have limestones (Schmid, 1976; Schmid and others, 1977). contributed significantly to short episodes of It has not been suggested for random-fabric increased catacl as ite mobil ity and i ntrus i on along cataclastic rocks because the temperatures at which fractures. these rocks are known to form are generally much lower than 0.5 Tm. Even if high fluid pressures were present, the microstructures observed in thin secions and by SEM Severa1 features observed in thi n secti on and indicate that deformation did not take place solely by scanning electron microscopy favor structural by independent grain boundary sliding. It is superplasticity as a possible deformation mechanism obvious that significant grain size reduction has for the cataclasite developed along the Vlhipple taken place in these rocks, a process which would fault. These features i ncl ude the extremely fi ne not have occurred by grain boundary sliding alone. and fairly uniform matrix grain size (0.004 - 0.006 The interlocking textures and uniformity of matrix mm), the relative equidimensionality of matrix grains also argue against independent grain boundary grains and their polygonal, interlocking shapes, sliding assisted only by high fluid pressures. the 1ack of a preferred crysta11 ographi c ori enta­ tion within the matrix, and the general lack of The lack of evidence supporting either features indicating large strains (Vlhite, 1977). cataclastic flow or abnormally high fluid pressures However, the temperatures at which cataclasite makes it necessary to propose another method of formation took place are certainly lower than the cataclasite formation. One such method suggested by minimum 0.5 Tm that is often considered a require­ the textural features of the cataclasite matrix is ment for superplasticity. Even so, superplasticity that of structural superplasticity. Structural might remain a possibility at temperatures of 0.3

114 Tm or lOvler if the grain size is small enough to REFEREIKES offset the low temperature, or if diffusion is enhanced by some means. Allison, I., R. L. Barnett, and R. Kerich, 1979, Superplastic flow and changes in crystal The temperatures at which the cataclasi5e chemistry of feldspars: Tectonophysics, v. 53, formed can be estimated to be in the range 250-350 C p. T41-T45. on the basis of fluid inclusions data collected by Anderson, J. L., and R. H. Osborne, 1980, Schuiling (1978) in the Sacramento ~lountains. The Petrologic conlparison of cataclastic rocks upper limit of this range is approximately equal to from shallow and deeper crustal levels within 0.3 Tm for quartz, the main cons t ituent of the the San Andreas fault system of southern cataclasite. Although quartz does not normally California: U. S. Geol. Survey Open File Rep. behave ductilely or deform by diffusional processes 80-987, 120 pp. at this temperature, the presence of sufficient Anderson, J. L., R. H. Osborne, and D. F. Palmer, water can cause hydrolytic weakening of the quartz 1980, Petrogenesis of cataclastic rocks within (Griggs, 1967; 1974; Kirby, 1975) and allow it to the San Andreas fault zone of southern deform o plastically at temperatures as low as California: Tectonophysics, v. 67, 250-300 C (Jones, 1975). p. 221-249. Ashby, J. L., and R. A. Verrall, 1973, Diffusion­ A major problem presented by the assumption of accommodated flow and superplasticity: Acta. stl'uctura1 superp 1as t i city as the domi nant deforma­ ~'letall., v. 21, p. 149-163. tion mechanism in these rocks is the method of Ashby, ri. F., and R. A. Verrall, 1977, grain-size reduction. The average grain size for Micromechanisms of flow and fracture, and feldspars in the matrix of the lower-plate mylonites their relevance to the rheology of the upper is about 0.01 mm, while that for quartz is slightly mangle: Phil. Trans. Roy. Soc. London, ser. larger (Davis and others, 1979). The average grain A, V. 288, p. 59-95. size in the cataclasite matrix is roughly 0.004 to Borg, I., ~1. Friedman, J. Handin, and D. V. Higgs, 0.006 mm, requiring still further reduction in grain 1960, Experimental deformation of St. Peter size. Large grains or lithic fragments appear to Sand: a study of cataclastic flow: in Griggs, undergo grain size reduction cataclastically, but it D., and Handin, J., eds., Rock deformation, is uncertain whether cataclastic grain size Geol. Soc. Amer. Memoir 79, p. 133-192. reducti on conti nues for very small gra ins. Ell iott Borradaille, G. J., 1979, Strain study of the (1973) suggests that stress concentrations around a Caledonides in the Islay region, SW Scotland: pile- up of dislocations or an internal flovi could implications for strain histories and cause the nucleation of microcracks, breaking up deformation mechanisms in greenschist: Jour. gra ins and formi ng a gra in .s i ze sma 11 enough f?r Geol. Soc. London., v. 136, p. 77-88. diffusional processes to donllnate. He calls thlS Borradaille, G. J., 1981, Particulate flow of rock process "broken-grain diffusion flow" It is and the formation of cleavage: Tectono­ conceivable that this or a similar process of physics, v. 72, p. 305-321. cataclastically reducing grain size has taken place Boullier, A. M., and Y. Gueguen, 1975, S-P in this case, thus allowing structural superplastic mylonites: origin of son~e mylonites by flovi to occur. superplastic flO\'i: Contrib. ~lin. Pet., v. 50, p. 93-104. In concl usion, the deformation mechanism Brock, W. G., and T. Engelder, 1977, Deformation responsible for the cataclasite formation is associated with the movement of the Muddy possibly one that incorporates some aspects of Mountain overthrust in the Buffington window, cataclastic behavior with diffusion or southeastern Nevada: Geol. Soc. Amer. Bull., dislocation-accommodated grain boundary sliding, v. 88, p. 1667-1677. both of which may have been aided by high pore fluid Carr, W. J., 1981, Tectonic history of the pressures. Some cataclastic deformation has Vidal-Parker region, California and Arizona obviously occurred, as is documented by the rotated (abs.): in Howard, K. A., Carr, M. D., and and disrupted mylonitic. fabrics found in the Miller, ~ M., eds., Tectonic framework of the chlorite breccia zone and the angularity and brittle Mojave and Sonoran deserts, California and deformation of the 1arger porphyrocl asts, but Arizona, U. S. Geol. Survey Open File cataclastic flow does not appear to be the dominant Rep. 81-503, p. 18. deformation mechanism within the cataclasite matrix. Davis, G. A., J. L. Anderson, E. G. Frost, and Structural superplasticity at low temperatures is T. J. Shackelford, 1979, Regional Miocene possible only if diffusivity is enhanced by the detachment faulting and early Tertiary (7) presence of I'later or duri ng metamorphi c reacti ons, n~lonitization, Whipple-Buckskin-Rawhide and if grain size is effectivel,y re?uced and ke~t Mountains, southeastern California and western stable in order to favor dlffuslon and graln Arizona: in Abbott, P. L., ed., Geologic boundary sliding processes. However, micro­ excursionS-in the southern California area, San scopic characteristics of the cataclasite matrix Diego State University, San Diego, California, favor structural superplasticity as a possible p. 74-108. mechanism, and it dOES not seem unreasonable at the Davis, G. A., J. L. Anderson, E. G. Frost, and conditions suggested for faulting. Therefore it is T. J. Shackelford, 1980, Mylonitization and suggested that cataclasis or cataclastic flow detachment faulting in the Whipple-Buckskin­ initiated the deformation process and was Rawhide Mountains terrane, southeastern responsible for most grain size reduction,. but that California and western Arizona: in Crittenden, superplasticity became the dominant mec~anlsm af~er M. D., Jr., P. J. Coney, and G. ~Davis, eds., gra insi ze was effecti ve ly reduced. Th 1 S mechanl sm Tectonic significance of metamorphic core is similar to that of "broken-grain diffusion flow" complexes of the North American Cordillera, suggested by Elliott (1973) for some cataclastic Geol. Soc. Amer. Memoir 153, p. 79-129. rocks. Elliott, D., 1973, Diffusion flow laws in metamorphic rocks: Geol. Soc. Amer. Bull.,

115 v. 84, p. 2645-2664. materials: Il,nn. Rev. ~later. Sci., v. 9, Enge1der, J. T., 1974, Cataclasis and the generation p. 151-189. of fault gouge: Geol. Soc. Amer. Bull., v. 85, Twiss, R. J., 1976, Structural superplastic creep p. 1515-1522. and linear viscosity in the Earth's mantle: Griggs, D. T., 1967, Hydrolytic I'leakening of quartz Earth Planet, Sci. Lett., v. 33, p. 86-100. and other silicates: Geophys. Jour. Roy. White, S., 1977, Geological significance of Astron. Soc., v. 14, p. 19-31. recovery and recrys ta 11 i zation processes in Griggs, D. T., 1974, A model of hydrolytic weakening quartz: Tectonophysics, v. 39, p. 143-170. in quartz: Jour. Geophys. Res., v. 79, \~hite, S. H., and R. J. Knipe, 1978, p. 1655-1661. Transformation- and reaction-enhanced Gueguen, Y., and A. M. Boullier, 1975, Evidence of ductility in rocks: Jour. Geol. Soc. London, superp1asticity in mantle peridotites: in v. 135, p. 513-516. Proceedings of i~ATO petrophysics mtg., New Wu, F. T., L. Blatter, and H. Robertson, 1975, York, \~i1ey Academic Press, p. 19-33. Clay gouges in the San Andreas fault system and Handin, J., R. V. Hager, M. Friedman, and J. N. their possible implications: Pure Appl. Feather 1963, Experimental deformation of Geophys., v. 113, p. 87-95. sedimentary rocks under confining pressure: pore pressure effects: Amer. Assoc. Petroleum ACKNO\iLEDGn':ENTS Geol. Bull., v. 47, p. 717-755. Heidrick, T. L., and J. \~i1kins, 1930, Field guide This work was completed as a portion of the to the geology and ore deposits of the Buckskin author's r~.s. thesis at the University of Southern Mountains, Arizona: in Mylonitization, California and was partially supported by NSF detachment fau1ting,-and associated grants EAR 77-09695 to G. A. Davis and J. L. mineralization, Whipple Mountains, California, Anderson and EAR 79-04368 to C. G. Sammis. Mobil and Buckskin Mountains, Arizona: Arizona Oil Corporation funded the reproductions and Peggy Geol. Soc. 1980 spring field trip guide, Nitz kindly typed the manuscript. p. 31-51. Hubbert, M. K., and W. W. Rubey, 1959, Role of fluid pressure in mechanics of overthrust faulting. I. Mechanics of fluid-filled porous solids and its application to overthrust faulting: Geo1. Soc. Amer. Bull., v. 70, p. 115-166. Jones, ~1. E., 1975, Water weakening of quartz and its application to natural rock deformation: Jour. Geol. Soc. London, v. 131, p. 429-432. Kirby, S. H., 1975, Creep of synthetic alpha quartz: unpub. Ph.D. dissertation, University of California, Los Angeles, California. Krass, V. A., 1980, Petrology of upper-plate and lower-plate crystalline terranes, Bowmans Wash area of the Whipple Mountains, southeastern California: unpub. M.S. thesis, University of Southern California, Los Angeles, California. Nicolas, A., and J. P. Poirier, 1976, Crystalline plasticity and solid state flow in metamorphic rocks: New York, John Wiley &Sons, 462 pp. Rubey, W. W., and M. K. Hubbert, 1959, Role of fluid pressure in mechanics of overthrust faulting. II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypotheses: Geo1. Soc. Amer. Bull., v. 70, p. 167-205. Sammis, C. G., R. H. Osborne, J. L. Anderson, and M. Banerdt, 1980, Reading stress from cataclastic rocks: EOS, Trans. Amer. Geophys. Union, v. 61, p. 1118. Schmid, S. M., 1976, Rheological evidence for changes in the deformation mechanism of Solenhofen limestone towards low stresses: Tectonophysics, v. 43, p. T21-T28. Schmid, S. M., J. N. Boland, and r~. S. Paterson, 1977, Superp1astic flow in fine grained limestone Tectonophysics, v. 43, p. 257-291. Schuiling, W. T., 1978, The environment of formation of Cu + Ag - bearing calcite veins, Sacramento Mountains, California: unpub. M.S. thesis, University of Arizona, Tucson, Arizona. Sibson, R. H., 1977, Fault rocks and fault mechanisms: Jour. Geol. Soc. London, v. 133, p. 191-213. Taplin, D. M. R., G. W. Dunlop, and T. G. Langdon, 1979, Flow and failure of superplastic

116 GEOLOGY AND REGIONAL SETTING OF THE CASTLE DOME MOUNTAINS, SOUTHWESTERN ARIZONA

James T. Gutmann Department of Earth &Environmental Sciences Wesleyan University, Middletown, CT 06457

ABSTRACT

A voluminous and little-known pile of silicic 10 20 I I volcanic rocks is exposed in the Castle Dome Moun­ KILOMETERS 40 tains and surrounding ranges of Yuma County, Arizona. These rocks are of late Oligocene to early Miocene age in the Castle Domes, as shown by five new K-Ar mineral ages. Reconnaissance field studies indicate that the volcanic section locally has undergone large rotations that contrast in style with late Miocene, Basin-and-Range block faulting and resemble the thin- skinned rotational tectonics documented for earlier, mid-Tertiary extensional deformation in ranges to the north and northeast. zoic ~:~a~~~~~~~~a~~c~~c~~~t ~:~~~:~:~a~}yp~~_~:~~i_ ~ ary rocks in the region are distributed around the margins of a large negative residual Bouguer gravity anomaly but are absent within this gravity low. Dis­ position of pre-Tertiary outcrops, and to some extent the gravity anomaly itself, may reflect cauldron sub­ sidence associated with the copious silicic volcanism (Gutmann, 1981). Rotational deformation of the vol­ canic pile may have occurred during regionally control­ led extension of the roof over a large Oligocene­ Miocene batholithic complex that fed the volcanism and now contributes to the gravity anomaly. INTRODUCTION The Castle Dome Mountains are located in the Basin and Range province of southwestern Arizona, about 60 km northeast of Yuma. This area has at­ tracted interest as a possible source of geothermal energy. Regional geophysical studies (West, 1972; Aiken, 1976) demonstrated the presence here of a large negative gravity anomaly of the sort which could indicate geothermal enhancement (Aiken, 1976). Near the center of the gravity low, at Stone Cabin (Fig. 1), bodies of Quaternary rhyolite are shown on the Arizona State Geologic Map (Wilson et al., 1969). Furthermore, much of the area lies within a "quiet zone" of 1o~/-ampl i tude aeromagneti c anoma 1i es i nter­ Figure 1. Map of Castle Dome area showing location preted to possibly indicate a shallow depth to the of Castle Dome Mtns. (coarse stippled pattern), Kofa Curie isotherm (Sauck, 1972). Mtns., U~S. Rte. 95, Stone Cabin (1), Chocolate Mtns. (2) and Middle Mtns. (3). Also shown are areas of Outstanding syntheses of the geology and geo­ pre-Tertiary metamorphic or sedimentary rocks (fine chronology of west-central and southwestern Arizona stippled pattern) after Wilson (1960), and residual have been provided recently by Reynolds (1980) and Bouguer gravity contours (heavy hachured lines, values Shafiqullah et al. (1980). However, the region within in mgals) from Aiken (1976). The heavy dashed line the gravity low remains poorly understood, especially marks locations of relatively steep gravity gradient. as regards geochronology, for almost nothing has been published on its bedrock geology since the pioneering work of Wilson (1933, 1960). This paper summarizes results of studies by the writer in 1980 under the sided in several places, and as much as 30 mgals in aegis of the Hot Dry Rock geothermal energy program depth below the high st closed contour. It covers an at Los Alamos National Laboratory. These were re­ area of some 1500 km 2 that includes much of the Cas­ ported earlier in more detail (Gutmann, 1981), espe­ tle Dome Mountains together with parts of the adja­ cially as regards geothermal potential of the region. cent Chocolate, Middle, and Kofa Mountains and the intervening basins (Fig. 1). The gravity low is cen­ The negative residual Bouguer gravity anomaly tered in the northern Castle Domes, in ~he general (Aiken, 1976) is about 45 km in mean diameter, steep- vicinity of Stone Cabin, a very small settlement

117 along U.S. Route 95. Most of the area within the the Castle Dome mining district (Wilson, 1951). anomaly lies on either the Yuma Proving Ground (U.S. Army) or the Kofa National Wildlife Refuge (U.S. Fish This section of low-grade, metasedimentary rocks and Wildlife Service) and is closed to energy devel­ resembles thick Mesozoic clastic sequences as des­ opment. However, a belt of potentially open land cribed from the southern Dome Rock Mountains by extends north-south across nearly all of the gravity Marshak (1980) and from the Livingston Hills by low and passes through its central region at Stone Harding (1980). The section described by Marshak Cabin. reportedly is about 5 km thick where it crops out some 30 km north-northwest of Stone Cabin. Marshak PRE-TERTIARY ROCKS (1980) considers these rocks to be mid-Jurassic to Late Cretaceous in age. Harding (1980) indicates Granitic basement rocks of Precambrian age crop that the Livingston Hills Formation is 3.6 km thick out in the Yuma area about 60 km to the southwest in the Livingston Hills, about 30 km northeast of (Olmsted et al., 1973) and in the Plomosa Mountains, Stone Cabin, and presents paleomagnetic evidence near Quartzsite, a comparable distance to the north suggesting that the age of metamorphism is no younger (Miller and ~IcKee, 1971). Crystalline terrane in the than Late Jurassic. Muggins Mountains and Laguna Mountains, a few tens of km south of the Castle Domes, is shown as Precambrian TERTIARY VOLCANIC ROCKS in age by Shafiqullah et al. (1980, Fig. 2). Where examined by the writer near Gila City, these rocks Resting unconformably on these metamorphic rocks are tightly folded, heterogeneous, quartzofeldspathic is a sequence of intermediate to silicic volcanic biotite gneisses. In thin section they exhibit tex­ rocks which forms the bulk of the Castle Domes and tures suggestive of polymetamorphism involving retro­ adjacent ranges. The volcanics are separated from gression to biotite-muscovite assemblages from some the metasedimentary rocks 1oca 11 y by severa1 metres considerably higher metamorphic grade. Olmsted et al. of pale reddish to brown, poorly sorted, tuffaceous (1973) report locally migmatitic, hornblende-bearing sandstone rich in felsic volcanic clasts. Laharic(?) gneisses in this area. deposits also occur locally in paleotopographic lows along this contact, which evidently had relief of Perhaps the oldest rocks exposed within the 50 m or more in the southern Castle Domes. Total Castle Domes themselves are schists and gneisses preserved thickness of the volcanic pile appears to cropping out in the southern part of the range and be at least 0.5 km and probably is not substantially regarded by Wilson as Precambrian in age based on more than 1.5 km. Complications arising from numer­ their degree of metamorphism (Wilson, 1933, 1951) and ous faults preclude any more precise estimate of later as Mesozoic in age (Wilson, 1960) when it was stratigraphic thickness at this time. realized that the ~1esozoic sedimentary and metasedi­ mentary rocks of Yuma County can grade locally into With the exception of a group of rhyolites near schist (Wilson, 1962). Where examined by the writer, Stone Cabin, the age of these rocks is shown as Cre­ these are dominantly quartzofeldspathic muscovite taceous on the Arizona State Geologic Map (Wilson et schists containing chorite + epidote + garnet, cut by al., 1969). The rhyolites near Stone Cabin are shown quartz-stilpnomelane(?) veinlets, but bearing no evi­ as Quaternary in age O~ilson et al., 1969). In fact, dence of former high metamorphic grade (c.f., Pre­ these rocks range in age from late Oligocene to early cambrian rocks near Gila City noted above). Accord­ Miocene, as indicated by five K-Ar mineral ages (Gut­ ingly, the writer tends to concur with Wilson's (1960) mann, 1981) which evidently are the first radiometric Mesozoic age assignment for these rocks from the ages obtained from rocks of the Castle Domes. VOl+ southern part of the range. They may represent ex­ canism began in the Castle Dome region about 25.0 ­ posures of the deeper and more highly metamorphosed 1.0 m.y. ago, as indicated by the age of biotite from parts of a thick sequence of Mesozoic rocks whose a dacite on Honki dori Hi 11, in the southern part of Precambrian floor crops out yet further south, in the the range (sample CD-85 of Gutmann, 1981, Table 1); Muggins and Laguna Mountains. the sample was from a compound cooling unit of welded tuff at least 100 m thick and separated from the Meso­ Rocks of Paleozoic age occur in the Plomosa zoic metasedimentary rocks by no more than 10 m of Mountains about 40 km north of Stone Cabin (Miller sandstone. This date is considerably younger than the and McKee, 1971) but evidently are absent over a age of inception of mid-Tertiary volcanism (at least broad area south of that range. 32-35 m.y. ago) across the Colorado River in south­ eastern Cal iforni a (Crowe et a1., 1979). However, it Most rocks exposed beneath the mid-Tertiary vol­ is very similar to the age of basal tuffs to the east­ canics of the Castle Dome Mountains are gray to northeast in the Eagle Tail Mountains (24 m.y., Sha­ greenish-gray phyllites, impure quartzites, limey fiqullah et al., 1980) and Vulture Mountains (26 m.y., phyllites, and abundant gray, poorly sorted, feld­ Rehrig et al., 1980). spathic sandstones and conglomerates, commonly with weakly phyllitic matrices. Meta-andesite(?) flows Dips in the Castle Domes are generally east­ occur locally in the pile, as do maroon phyllites. northeastward. Biotite from rhyolitic ash-flow tuff Lithic clasts suggest a mixed provenance including at Little White Tanks, on the east side of the range, both a gneissic-plutonic terrane and a volcanic ter­ yields a K-Ar age of 23.5 ~ 0.9 m.y.c (sample CD-75 of rane. The volcanic clasts are chiefly of mafic rocks Gutmann, 1981, Table 1). Field evidence is ambiguous although felsic types also are present. Metamorphic as to the stratigraphic position of some isolated mineral assemblages typically include albite + epi­ rhyolite lava flows and tuffs exposed to the north, dote + chlorite and indicate low greenschist facies near Stone Cabin. These were assigned a Quaternary conditions. The more fine-textured of these rocks age by Wilson et al. (1969), as were some flow-banded are pervasively foliated. The metamorphic sequence rhyolites with a similar and distinctive phenocryst is cut by various intermediate and silicic intrusions, assemblage cropping out on the other (east) side of including numerous quartz porphyry dikes related to the range near its northern end. However, samples of the overlying mid-Tertiary volcanic pile; and it is two of these units collected by the writer at and the host rock for the lead-silver mineralization of south of Stone Cabin (locations in Gutmann, 1981,

118 Table 1) gave biotite K-Ar ages of 22.28 ~ 0.47 m.y. 57.65' WLong.; CD-85 is a dacite welded tuff from and 22.20 ~ 0.47 m.y. (M. Shafiqullah and P. Damon, base of volcanic sequence at 32° 58.78' NLat., 114° personal communication, 1980). Thus, these rocks are 09.08' WLong.; CD-106 is a rhyolite welded tuff at of earliest Miocene age (following Berggren, 1969). the mouth of Palm Canyon, Kofa Mtns.; CD-77 is a Evidently the volcanics exposed at Stone Cabin and rhyolite lava flow at Stone Cabin, 33° 15.99' N Lat., south along U.S. Rte. 95 have been downfaulted to 114° 14.67' WLong. their present positions relative to the main mass of the Castle Dome volcanic pile to the east. Wilson et al. (1969) maps several areas of Qua­ Major-element analyses of four of the volcanic ternary basalt on the east side of the Castle Domes rocks, including three of the radiometrically dated near their southern end. At least several if not all samples, are presented in Table 1 together with CIPW of these areas are underlain by flows of what may norms. These data are too few to adequately charac­ better be termed basaltic andesite. One such flow terize the suite but they suggest that the volcanics (sample CD-72 of Gutmann, 1981, Table 1), resting are generally calc-alkaline and similar to the 28 to with apparent unconformity on the underlying volcan­ 22 m.y. silicic volcanic sequence described by Crowe ics and dipping 10° ENE, yielded a K-Ar age of et al. (1979) from southeastern California. The 20.39 ~ 0.43 m.y. (M. Shafiqullah and P. Damon, per­ youngest of the Castle Dome rocks (sample CD-72) would sonal communication, 1980). This probably is among be classified as a basalt following Irvine and Bara­ the youngest volcanic rocks in the Castle Domes. It gar (1971, Fig. 7) but as an andesite following may represent an eastward extension of unit Cb of Streckeisen (1979). Probably this rock is best term­ Crowe et al. (1979), a group of mafic and intermedi­ ed a basaltic andesite, especially in view of its ate flows in southeastern California ranging in age borderline character with respect to criteria such as from 22 to 13 m.y. and separated by a distinct age silica content and plagioclase composition. Of the gap from the underlying silicic volcanic rocks. other three analyzed rocks, the 25 m.y. basal ash­ flow tuff (CD-85) is a dacite, and the other two are TABLE 1 rhyolites (following StrecKPisen, 1979) with a nota­ ble excess of K 0 over Na2C. MAJOR-ELEMENT COMPOS ITIONS 1 fINn CIPW NORMS 2 OF VOL­ 2 CANIC ROCKS FROM THE CASTLE DOME MTNS.3, SOUTHWESTERN Judging from these analyses, numerous thin sec­ ARIZONA tions, and several partial chemical analyses, the Oxide CD-72 CD-85 CD-106 CD-77 Castle Dome volcanic pile appears rhyodacitic or rhyolitic (nomenclature of Streckeisen, 1979) in Si02 53.6 67.2 72.0 70.2 overall composition. As in the mid-Tertiary volcan­ A1 0 17.2 15.8 14.9 15.2 i cs of southeastern Cal iforni a (Crowe et a1., 1979), 2 3 andesite is markedly subordinate in volume. Of the Ti02 1. 28 0.39 0.23 0.24 several dozens of volcanic units examined, no more FeO* 7.88 2.59 1.83 1. 63 than one fifth are andesite or basaltic andesite, another fifth are approximately dacitic, and three MgO 5.15 1.48 0.57 0.43 fifths appear more nearly rhyolitic. Among the fel­ CaO 8.79 4.23 1. 19 1.68 sic volcanic units, about two thirds are ash-flow tuffs, of which over half are welded to some degree. 3.85 Na 20 3.30 3.99 3.71 All are porphyritic, usually with plagioclase and K 0 1. 33 3.09 4.78 5.68 biotite. Over half of the felsic units carry pheno­ 2 crystic sanidine and nearly half contain quartz phe­ MnO 0.11 NO NO NO nocrysts. Hornblende also occurs in half of the Total 98.64 98.77 99.21 98.91 rocks examined. Clinopyroxene phenocrysts are pre­ sent in a few of the felsic units. A significant proporation of the Castle Dome vol­ q 3.8 21.7 28.4 21 .9 canic pile, perhaps as much as one third, consists or 7.79 18.4 28.4 33.4 not of ash flows but of ordinary lava flows of rhyo­ ab 27.8 33.6 31.5 32.5 litic composition. These boudinaged, flow-banded an 28.4 16.1 5.84 7.51 rhyolites must have been emplaced as flows of very c 1. 43 high viscosity. Such flows probably would not have wo 6.42 1.98 0.35 moved long distances from their vents and, together en 12.9 3.71 1.41 1.10 with the presence of especially coarse-textured, fs 5. 28 0.66 0.53 0.40 block-and-ash avalanche(?) deposits, their abundance mt 3.94 1.85 1.39 1.16 suggests the former occurrence of vents and magma il 2.43 0.76 0.46 0.46 sources in or near the Castle Domes. 100 an/ab + an 50.5 32.5 15.7 18.8 normative c. i. 30.9 8.96 3.79 3.47 The gross stratigraphy of the mid-Tertiary vol­ Ab + An : Or : Q 83:11:6 55:21 :24 40:30:30 42:35:23 canic pile of southwestern Arizona is poorly known. However, this stratigraphy has been well worked out *Total iron as FeO. NO = not detected in southeastern California by Crowe et al. (1979), Analyses by Jamie N. Gardner, Los Alamos National who recognize a 35-26 m.y. basal sequence of overall Laboratory, by electron microprobe techniques on dacitic composition and a generally younger (28-22 fused, powdered samples. m.y.) silicic sequence which is quartz latitic in 2 Calculated from analyses as reported and setting overall composition. According to Wilson (1962), the Fe 0 equal to the least of the following: 3.0%, or 2 3 volcanic section of the Castle Dome and Kofa Moun­ Ti02 + 1.5% (Irvine and Baragar, 1971), or tains is predominantly andesitic in its lower parts FeO*/2. and andesitic to rhyolitic in its upper portion. 3 Sample locations are as follows: CD-72 is a basal­ Wilson (1960) mapped andesitic and overlying ande­ tic andesite lava flow from 33° 03.45' N Lat., 113° sitic-to-rhyolitic units separately on the Yuma

119 County geologic map. The writer examined the ande­ the correspondence between its locally steep margins sitic unit (Ka of Wilson, 1960) in several places. and those of the region of concealed basement rocks Although it can contain relatively dark-colored lava suggests that cauldron subsidence may have occurred flows, locally in abundance, silicic welded tuffs (e. here (Gutmann, 1981). Indeed, the copious outpour­ g., rhyolite sample CD-106, Table 1) generally are ings implied by some of the ash-flow cooling units present as well, whereas andesite apparently can be exposed in the Castle Domes and northwestern Kofas entirely absent from the section in places. seemingly must have been associated with subsidence in some form, albeit perhaps distributed over a broad How extensive is Wilson's (1960) andesitic lower region. unit? Olmsted et al. (1973) note that the oldest rocks of the Tertiary volcanic sequence in the Yuma The edge of the gravity anomaly is steepest and area apparently are of andesitic composition. These most sharply defined in the northeastern third of its are exposed chiefly in the easternmost Chocolate periphe~y. Along most of this third, the edge of the Mountains of California, near the Colorado River, but gravity low lies over sediment filling what could be are missing in the Laguna Mountains to the southeast deep and steep-sided basins. However, it also cuts (Olmsted et al., 1973). It seems likely that a) at across the northwestern end of the Kofa Mountains least some of the rocks mapped as andesites by Wilson (Fig. 1). Where it does so, Wilson (1960) maps two are much more silicic rocks of dark color; b) the faults, each several km long. The geographic coinci­ lower parts of the volcanic pile, which demonstrably dence of these faults and the sharp edge of the gra­ are less silicic than the overlying rocks in Califor­ vity low may be fortuitous; but the orientation of nia (Crowe et al., 1979), may also be so in parts of and sense of motion on these faults, as indicated by westernmost Arizona (Wilson, 1960) but in otherplaces Wilson (1960), is appropriate if they represent part are not as mafic in overall composition as andesite; of a fracture system bounding a region of cauldron and c) this less silicic lower unit pinches out east­ subsidence to the south and west. ward and is thin or absent in the vicinity of the Castle Domes. Both overall rock composition and the The Castle Domes extend from the margins of the 25 m.y. age of inception of volcanism support corre­ gravity low northwestward to its center at Stone lation of volcanic rocks of the Castle Dome Mountains Cabin. Metasedimentary rocks exposed at their south (Kofa volcanics of Wilson, 1960) with the silicic end are succeeded northwestward by progressively youn­ sequence of Crowe et al. (1979) to the apparent ger volcanic strata, a succession that evidently re­ exclusion of the basal, less silicic sequence. flects relative subsidence of the Stone Cabin area. Volcanic rocks of the Castle Domes are cut by Other significant structural phenomena also are numerous rhyolitic dikes carrying phenocrysts of indicated by Castle Dome geology. This block-faulted quartz and one or two feldspars. The dikes occur range has a steep western face and its volcanic lay­ especially in north-northwesterly trending swarms ering dips predominantly east-northeastward at gentle near the crest of the range and in the Castle Dome to moderate angles. The principal strike direction mining district, along the west flank of the range. both of this layering and of the silicic dikes is N30­ A northeasterly trending swarm of thick dikes occurs 35W, parallel to the trend of the range and to its near the Keystone mine, on the east side of the range. chief morphologic lineaments. Trends of faults and Many of these intrusions were intensely fractured on prominent outcrop fracture sets in the volcanics fan closely spaced (ca. 1 mm) subparallel fractures and from N to WNW; few lie outside this directional range, were either sericitized or kaolinized. In at least although a significant population of cross fractures, one instance, strain appears to have developed not topographic lineaments and dikes trends approximately later than alteration/recrystallization at elevated northeast. Whereas layering in much of the Castle (300° C?) temperatures, i.e., fracturing occurred Domes is inclined east-northeastward, gentle south­ while the terrane still was warm. An intriguingly westerly dips occur over broad areas in the east­ similar set of dikes is described by Rehrig et al. central part of the range, as does apparent gentle (1980) from the Vulture Mountains, about 140 km to warping of the strata. The gently dipping areas ter­ the northeast. There, 26 to 16 m.y. calc-alkaline minate abruptly against terrane characterized by volcanics, also believed to have come from nearby moderate to steep northeasterly dips and local sharp sources, are cut by north- to north-northwest-trending reversals of dip. silicic porphyritic dikes; some of these dikes post­ date listric normal faults whereas others apparently The abrupt changes of dip through 70-80° within are cut by the faults (Rehrig et al., 1980). There, this volcanic pile imply large rotations along faults. too, the dikes locally trend northeast instead of Repetition and rotation of the mid-Tertiary volcanic north-northwest, evidently reflecting control of dike section in the Vulture Mountains to the northeast has orientation by a basement fracture fabric of Laramide been ascribed to listric faulting (Rehrig et al., age (Rehrig et al., 1980; Rehrig and Heidrick, 1976). 1980), a structural phenomenon the general importance The plutonic sources of these dikes are not exposed of which, in the Basin and Range province, has been either in the Vulture Mountains or in the Castle Domes. recognized recently (Stewart, 1978). The occurrence of listric faulting about 15-20 m.y. ago has been MAJOR MID-TERTIARY STRUCTURAL EVENTS documented to the north of the Castle Domes (Davis et Centered a few km south of Stone Cabin is a al., 1980) as well as to the northeast. Rehrig et al. crudely circular region about 45 km across within (1980) suggest that the shallow crustal extension and which pre-Tertiary basement rocks evidently are not north-northwest-trending listric normal faulting in exposed although they do crop out all around its peri­ the region was genetically related to and facilitated phery (Fig. 1). This region corresponds closely with by dike or pluton emplacement. Occurrence of swarms the position of the large negative Bouguer gravity of north-northwest-trending dikes in the Castle Domes, anomaly noted earlier. In many places the margins of some of which evidently were sheared while still warm, the gravity low are steep and sharply defined, sug­ fits with this model. Listric faulting in the Castle gesting lateral density contrast at relatively shal­ Domes seems weakly developed in comparision with that low depth. Although the gravity anomaly to some ex­ described from nearby areas (Rehrig et al., 1980; tent reflects Basin and Range fault block structures, Davis et al., 1980). These downward-flattening

120 structures were not directly observed in the Castle Domes than is the influence of any such discrete Domes during this regional study. They are exposed batholith. Nevertheless, the voluminous rhyolitic in the Vultures, but only rarely (Rehrig et al., ash flows and ordinary lava flows, the gravity low, 1980). Other studies in the Castle Domes may deter­ and the distributions of both Tertiary and pre-Terti­ mi ne the preci se nature of the north- northwest­ ary strata vis a vis that gravity low together sug­ trending faults. gest that a-very large silicic intrusive complex occurs at depth in the Castle Dome area and fed vol­ Whatever their geometry, it seems likely that canism accompanied by cauldron subsidence in late these faults were penecontemporaneous with mid­ Oligocene to early Miocene time. Tertiary magmatism. Only minor tilting of structural blocks has occurred in southwestern Arizona since 12 AC KNOWL EDG~1ENTS m.y. ago (Damon et al., 1973), the time of inception of Basin-Range block faulting in this region (Eberly Geothermal assessment of this area was suggested and Stanley, 1978). Yet some blocks of the volcanic by Fraser Goff while the writer was a visiting staff section in the Castle Domes dip at angles ranging member at Los Alamos National Laboratory in spring from 20 to at least 60 degrees. Several recent con­ 1980. The assistance and encouragement of F. Goff tributions (Rehrig and Heidrick, 1976; Shafiqullah and A. William Laughlin is greatly appreciated. This et al., 1980; Rehrig et al., 1980) emphasize the work was supported by the U.S. DOE Division of Geo­ distinction between the thin-skinned rotational tec­ thermal Energy. Three of the K-Ar ages were deter­ tonics of the mid-Tertiary orogeny, which dominated mined in the Laboratory of Isotope Geochemistry, this area until about 15 to 17 m.y. ago, and the University of Arizona, supported by NSF grant EAR 78­ vertical tectonics associated with the ensuing epi­ 11535 and by the Arizona Bureau of Geology and Miner­ sode of basin subsidence along steep normal faults. al Technology through the cooperation of W. Richard Evidence presented by Crowe (1978) indicates that Hahman, Sr. topographic disruption by motion along north-north­ westerly trending normal faults began at least 26 m.y. ago in southeastern California and probably REFERENCES CITED prior to 32 m.y. ago. Clearly this region was under­ going extension both before and after Castle Dome Aiken, C.L.V., 1976, The analysis of the gravity volcanic activity. Yet the relatively gentle dip of anomalies of Arizona: Ph.D. Dissert., Univ. of the 20.4 m.y. basaltic andesite from the eastern Arizona, Tucson, 127 p. Castle Domes, together with the flat-lying attitude Aiken, C.L.V., and Ander, M.E., 1981, Batholithic of a basaltic andesite from the Kofas dated at 18.3 complex beneath the Castle Dome area, southwest­ m.y. (Shafiqullah et al., 1980), indicate that rota­ ern Arizona: a geophysical study: EOS, v. 62, p. 384. tional events had largely ceased in this particular Anderson, R.E., 1971, Thin skin distension in Tertiary area by 18-19 m.y. These events evidently were rocks of southeastern Nevada: Geol. Soc. Amer. penecontemporaneous with Castle Dome volcanism. Bull., v. 82, p. 43-58. Berggren, W.A., 1969, Cenozoic chronostratigraphy, Anderson (1971) and Wright (1976) have suggested planktonic forminiferal zonation and the radio­ that Basin-Range extension accommodated by normal metric time scale: Nature, v. 224, p. 1072-1075. faulting at shallow crustal levels may have been Crowe, B.M., 1978, Cenozoic volcanic geology and accompanied by emplacement of plutons at depth. Mod­ probable age of inception of basin-range faulting elling of the source of the large negative Bouguer in the southeasternmost Chocolate Mountains, gravity anomaly centered in the northern Castle Domes Cal ifornia: Geol. Soc. Amer. Bull., v. 89, has been carried out by Carlos L. V. Aiken at the p. 251-264. University of Texas at Dallas. Following removal of Crowe, B.M., Crowell, J.C., and Krummenacher, D., the effects of very high-level density contrasts (e. 1979, Regional stratigraphy, K-Ar ages, and g., due to Basin-Range or cauldron-bounding faults), tectonic implications of Cenozoic volcanic rocks, the anomaly is closely modelled (± 1 mgal) by a low­ southeastern California: Am. Jour. Sci., density mass with a bottom 20 km down (base of the v. 279, p. 186-216. crust) and a top 8 km down (assuming .1 gr/cc density Damon, P.E., Shafiqullah, M., and Lynch, D.J., 1973, contrast) to 10 km down (assuming .2 gr/cc density Geochronology of block faulting and basin sub­ contrast) (C.L.V. Aiken, personal communication, sidence in Arizona: Geol. Soc. Amer. Abs. with 1980). Such a mass could be the silicic batholithic Programs, v. 5, p. 590. source of the mid-Tertiary volcanic pile. Aiken and Davis, G.A., Anderson, J.L., and Frost, E.G., 1980, Ander (1981) report that, when the gravity effect of Regional Miocene detachment faulting and early this deep feature is removed, a 10-mgal low remaining Tertiary(?) mylonitization, Whipple-Buckskin­ over the Castle Domes indicates that the volcanics Rawhide Mountains, southeastern California and thicken toward the center of the area and, thus, western Arizona: guide to Arizona Geological further supports the proposition of cauldron subsi­ Society 1980 Spring Field Trip, p. 4-12, Arizona dence. Geological Society, Tucson. Eberly, L.D., and Stanley, T.B., Jr., 1978, Cenozoic Deformation of the volcanic pile along listric(?) stratigraphy and geologic history of southwestern faults may reflect instabilities in the thick roof Arizona: Geol. Soc. Amer. Bull., v. 89, p. 921­ over the batholith but it must have been controlled 940. regionally. General attitudes of layering in the Frost, E.G., Cameron, T.E., Krummenacher, D., and Castle Domes and surrounding ranges fit the regular Martin, D.L., 1981, Possible regional interaction Cenozoic tilt patterns of southwestern Arizona (Ste­ of mid-Tertiary detachment faulting with the San wart, 1980). These patterns are of wide regional Andreas fault and the Vincent-Orocopia thrust extent, far wider than the gravity low, and are pre­ system, Arizona and California: Geol. Soc. Amer. sumably related to extensive mid-Tertiary detachment Abs. with Programs, v. 13, p. 455. faulting (Frost et al., 1981). The associated frac­ Gutmann, J.T., 1981, Geologic framework and Hot Dry turing and intrusions (Stewart, 1980; Rehrig et al., Rock geothermal potential of the Castle Dome 1980) are more evident in the geology of the Castle

121 area, Yuma County, Arizona: Los Alamos Scienti­ Wilson, E.D., Moore, R.T., and Cooper, J.R., 1969, fic Laboratory report LA-8723-HDR, 23 p. Geologic map of Arizona: U.S. Geol. Survey Map, Harding, L.E., 1980, Petrology and tectonic setting 1:500,000. of the Livingston Hills formation, Yuma County, Wright, L., 1976, Late Cenozoic fault patterns and Arizona: Ariz. Geol. Soc. Digest, v. 12, p. stress fields in the Great Basin and westward 135-145. displacement of the Sierra Nevada block: Geology Irv i ne, T. N., and Baragar, W.R.A., 1971, A guide to v. 4, p. 489-494. the chemical classification of the common vol­ canic rocks: Canad. Jour. Earth Sci., v. 8, p. 523-548. Marshak,S., 1980, A preliminary study of Mesozoic geology in the southern Dome Rock Mountains, southwestern Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 123-133. Miller, F.K., and McKee, E.H., 1971, Thrust and strike-slip faulting in the Plomosa Mountains, southwestern Arizona: Geol. Soc. Amer. Bull., v. 82, p. 717-722. Olmsted, F.H., Loeltz, O.J., and Irelan, B., 1973, Geohydrology of the Yuma area, Arizona and Cali­ fornia: U.S. Geol. Survey Prof. Pap. 486-H, 227 p. Rehrig, W.A., and Heidrick, T.L., 1976, Regional tectonic stress during the Laramide and late Tertiary intrusive periods, Basin and Range province, Arizona: Ariz. Geol. Soc. Digest, v. 10, p. 205 -228 . Rehrig, W.A., Shafiqullah, M., and Damon, P.E., 1980, Geochronology, geology, and listric normal faulting of the Vulture Mountains, Maricopa County, Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 89-110. Reynolds, S.J., 1980, Geologic framework of west­ central Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 1-16. Sauck, W.A., 1972, Compilation and preliminary inter­ pretation of the Arizona aeromagnetic map: Ph.D. Dissert., Univ. of Arizona, Tucson, 147 p. Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., Rehrig, W.A., and Raymond, R.H., 1980, K-Ar geochronology and geologic history of south­ western Arizona and adjacent areas: Ariz. Geol. Soc. Digest, v. 12, p. 201-260. Stewart, J.H., 1978, Basin and range structure in western North America- A review, in Smith, R.B., and Eaton, G.P., eds., Cenozoic tectonics and regional geophysics of the western Cordillera: Geol. Soc. Amer. Memoir 152, p. 1-31. ____~---' 1980, Regional tilt patterns of late Cenozoic basin-range fault blocks, western United States: Geol. Soc. Amer. Bull., Part I, v. 91, p. 460-464. Streckeisen, A., 1979, Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites, and melilitic rocks: recommendations and sug­ gestions of the lUGS subcommission on the syste­ matics of igneous rocks: Geology, v. 7, p. 331­ 335. West, R.E., 1972, A'regional Bouguer gravity map of Arizona: Ph.D. Dissert., Univ. of Arizona, Tucson, 186 p. Wilson, E.D., 1933, Geology and mineral deposits of southern Yuma County, Arizona: Univ. of Arizona, Ariz. Bur. Mines Bull. 134, 236 p. , 1951, Arizona zinc and lead deposits ­ ----~C~a-s~t~le Dome district (Yuma County): Univ. of Arizona, Ariz. Bur. Mines Bull. 158, p. 98-115. , 1960, Geologic map of Yuma County, Ari­ -----z-o-n-a-: Univ. of Arizona, Ariz. Bur. Mines, 1:375,000. , 1962, A resume of the geology of Arizona: ----'U'n~i-v-. of Arizona, Ariz. Bur. Mines Bull. 171. 140 p.

122 ORIGIN OF FOLDS OF TERTIARY LOW-ANGLE FAULT SURFACES, 1 SOUTHEASTERN CALIFORNIA AND WESTERN ARIZONA

Jon E. Spencer U.S. Geological Survey 345 Middlefield Road Menlo Park, CA 94025

Davis and others (1980) and Rehrig and Rey­ ABSTRACT nolds (1980) have helped resolve the form and distribution of these folds. Cameron and The lower Colorado River trough is a Frost (1981) documented two sets of folds, detachment fault terrane in which autochthon­ one striking northeast, and the other strik­ ous rocks have been denuded of as much as 4 ing north to northwest. Previously recog­ to 7 km of rock, and possibly more, by low­ nized domal culminations were reinterpreted angle (detachment) faulting. Detachment as representing positive interference by faul ts have been warped into two sets of ap­ antiforms of the two fold sets. Frost (1981) proximately perpendicular folds. Application recognized that the distribution of clastic of principles of mechanics to simple models sediments of mid-Tertiary age was influenced of the denudation process provides an explan­ by northeast trending folds in the Whipple ation for the origin of these folds. Folds Mountains, and interpreted this data as indi­ whose axes parallel the northeasterly to cating that folding was synchronous with easterly direction of allochthon transport movement on one of the major detachment can be explained as resulting from deviatoric faults, the Whipple basal detachment fault. stresses generated in autochthonous rocks by tectonic denudation. The amount of denuda­ The distribution of structures that are tion is modeled as increasing linearly in the interpreted as folds related to detachment direction of displacement of allochthonous faulting is shown in Figure 1. The northeast rocks. Greater isostatic uplift in areas of to east trending folds have axes parallel to greater denudation causes flexure of autoch­ the inferred direction of movement of alloch­ thonous rock masses. Stresses generated by thonous rocks, while the axes of northwest to flexure may have played an important role in north trending folds are roughly perpendicu­ producing the north to northwest trending lar to the movement direction. Where well­ folds. exposed, the northeast set of folds typically has wavelengths of 4 to 12 km and amplitudes INTRODUCTION of perhaps 100 to 600 m, whereas the north­ west to north trending folds have wavelengths Mountain ranges along the lower Colorado of 10 to 50 km and amp 1 i t u des 0 f as mu c has River have received much study during the perhaps one kilometer. The northeast set may past ten years due in part to the presence of be older (Cameron and Frost, 1981) but par­ enigmatic low-angle faults commonly called tial synchroneity in formation is entirely detachment faults. Detachment faults are possible. similar to low-angle faults in Nevada and Utah that typically place younger rocks over Beyond recognizing that the folds are in older rocks, and have been called denudation some way related to crustal extension (Frost, faults (Armstrong, 1972) and low-angle normal 1981; Otton, 1981), an adequate explanation faults. In the lower Colorado River region, for their formation has not been presented. these subhorizontal faults are of undoubted Rehrig and Reynolds (1980) recognized that Tertiary age as allochthonous rocks typically isostatic rebound following denudation might include moderately to steeply tilted Tertiary be a primary cause of arching. Hyndman volcanic and sedimentary rocks, as well as (1980), in studying an area in Idaho with Precambrian and Mesozoic crystalline rocks. notable similarities to the 101-ler Colorado Autochthonous rocks are primarily Precambrian Ri ver tro ugh, calculated the amount of iso­ and Phanerozoic crystalline rocks. Internal static rebound that should follow denudation extension of allochthons, by movement on nu­ of the Sapphire block from the Bitterroot merous listric and/or planar normal faul ts dome area. The actual form of the Bitterroot that merge with or are cut by basal detach­ dome matched well with the geometry calcu­ ment f aul ts, may be in the range of 50 to lated on the basis of isostatic rebound of 100% (Anderson, 1971; Davis and others, the denuded area. Whereas isostatic rebound 1980). Allochthonous fault blocks are typi- following tectonic denudation is probably an cally tilted or even overturned in the direc­ important fac tor in producing folds in the tion 0 pposi te to the generally inferred lower Colorado River region, the short wave­ direction of allochthon transport. length, steep limbs, and sinusoidal or wave­ like form of many folds contrasts sharply Warping of detachment fault surfaces with the broad domal form of the Bitterroot into arches and domes has been recognized in dome, suggesting that other factors are in­ many detachment fault terranes, and is excep­ volved in the Colorado River region. tionally well-developed along the lower Colo­ rado River trough in southern California and This study is an attempt to apply prin­ western Arizona (Fig. 1). Recent studies by ciples of mechanics to detachment faul t ter­ ranes in order to better understand the ori­ 1This report has not been reviewed for con­ gin of secondary structures such as folds of formity with U.S. Geological Survey editorial detachment fault surfaces, and ultimately to standards or stratigraphic nomenclature. help clarify factors leading to the formation

123 ~ of detachment faults themselves. Quantita­ I tive aspects of this study are based primari­ lyon the theory of flexure of elastic beams or plates as reviewed by Timoshenko and Young (1968) and Johnson (1970). CALIFORNIA NEVADA Assumptions

i Several basic assumptions are necessary i ./ prerequisites for the following analysis of processes leading to folding of detachment

I faults. i i The first assumption is that an approxi­ (,-...... ,< mately hydrostatic (±100 to 300 bars) state of stress existed in the crust of the lower Colorado River region prior to the inception of middle Tertiary extensional faulting. This was clearly not the case during much of Mesozoic and early Cenozoic time when a sig­ ,. nificant northeast- to east-directed compres­ sive stress (o-NE) led to widespread thrust .'i NEEDLES faulting (Burchfiel and Davis, 1971; Miller I i and McKee, 1971; HOHard and others, 1980; I I Reynolds and others, 1980). An approximately hydrostatic stress field was also clearly not ~\ present during middle to late Tertiary time as indicated by the Hidespread existence of low-angle normal faults of this age. At some time between these tHO periods of deforma­ tion, ~NE must have dropped through a range j!J of values close to (}NW and Civ (o-v = lith­ ostatic or vertically directed stress). It ARIZONA is at this time that the assumption of a hy­ ---'jj;.---.,;-!-- drostatic state of stress is justified. A second assumption is that the autoch­ thon, for some finite distance downward, may be approximated by a struqturally homogeneous and isotropic elastic (Hookeian) plate within a range of stresses large enough to cause folding. Au tochthonous rocks in many areas are crystalline rocks of Precambrian and Mes­ ozoic age that have no obvious regional structural anisotropy. In other areas, per­ vasive subhorizontal mylonitic gneisses occur in the autochthon (Davis and others, 1980; PARKER· Rehrig and Reynolds, 1980; Shackelford, 1980). Detachment faults, and folds of de- tachment fault surfaces, occur regardless of the presence or absence of these structurally anisotropic mylonitic gneisses, indicating that, at least with regard to fold formation, -+­ the assumption of structural homogeneity and * Figure 1. Location and orientation of folds inferred to be related to detachment faults. Folds east and north of Salome are from Rehrig and Reynolds (1980), Shackelford (1980), and E. Frost (pers. commun., 1981), and represent the Harquahalla, Harcuvar, Buckskin, and Rawhide Mountains. Folds north ----f- SYNFORM of Parker are from Davis and others (1980), and E. Frost (pers. commun., 1981), and rep­ resent the Whipple Mountains. Folds south, west and northwest of Needles are from unpub­ -+- ANTIFORM lished maps produced by the Southern Pacific Land Company, Davis and others (1980), Vol­ borth (1973), HeHett (1956), B. John (pers. • SALOME commun., 1981), Spencer and Turner (1982, --+- this volume), and Spencer and Turner (unpub­ MI. 10 20 lished mapping), and include the Chemihuevi, 0 I , I I , I i Sacramento, Dead, NeHberry, Homer, Piute, and KM 0 10 20 30 ~ Castle Mountains.

124 isotropy has some justification. 1\1 though represent shortening perpendicular to fold some workers have proposed that mylonitic axes. In other words, two points, located at gneisses represent ductile extension syn­ opposite ends of a rock mass, move closer chronous or nearly synchronous with brittle together during folding. The northeast extension in the immediately overlying al­ trending folds along the lower Colorado River lochthon (Davis and Coney, 1979; Rehrig and may have formed by a distinctly different Reynolds, 1980), field and geochronologic process, a process in which no shortening evidence presented by Davis and others (1980) occurs at all. Folding without shortening and Shackelford (980) indicate that myloni­ requires an increase in the surface area of a tization in the lower Colorado River area was folding rock mass, which is what is proposed early Tertiary or older. here. The increase in surface area is a con­ sequence of an increase in the volume of the A third assumption is that major detach­ rock mass. If volume increases, and the rock ment faults in the lower Colorado River re­ is not allowed to extend in one direction, gion had an initial east to northeast region­ folds may form to accommodate this continued al dip, and that transport of allochthonous extension. rocks was consistently in this direction. The present dip of detachment faults is high­ How would such a volume increase occur ly variable, but this is inferred to be pri­ in autochthonous rocks of the lower Colorado marily the result of folding and isostatic River region? Consider the stresses within a uplift of tectonically denuded autochthonous vertica 1, northwest striking plane ( 0- v rocks following detachment fault movement. and o-NW)' One of our basic assumptions is The assumption of a regional northeast dip is that, initially, 0- v 0NH at all depths. in part justified by the presence, at the The process of tectonic denudation, which was southwestern or western margin of the detach­ clearly operative in the lower Colorado River ment terrane, of an inferred headwall or region during middle to late Tertiary time, breakaway point of detached rocks in the led to a substantial reduction in the verti­ Mopah Range (Davis and others, 1980), the cally directed compressive stress (Qv) in Littl~ Piute Mountains (Keith Howard, pers. autochthonous or parautochthonous rocks. For commun., 1982) and at Homer Mountain (Spencer a Hookeian solid, reducing OIv is like remov­ and Turner, this volume). ing a weight from a spring, causing extesnion of the autochthonous rock in the vertical In addition, the general absence of old­ direction. If the northwest and southeast er over younger low-angle fault relationships boundaries of the denuded terrane are assumed of Tertiary age at the eastern margin of the to be stationary then no expansion or con­ detachment terrane (Lucchitta, 1981) supports traction can occur in the northwest-southeast the inference that detachment faults are direction, and 0NW remains constant during rooted low-angle normal faul ts that accom­ the denudation process (it actually decreases modate crustal extension. This conclusion slightly, depending on the value of Poisson's was reached by Anderson (1971) and Armstrong ratio). We are left with a situation in (1972) in areas farther north, al though in which o-v becomes less than O-NW by an amount detail, proposed fault geometry and mechan­ approximately equal to the weight of the de­ isms of extension in the middle and lower nuded rock (in this case, as much as 1 to 2 crust are highly variable. Davis and Coney kb and possibly more). It is important to (1979, Fig. 4), for example, envisioned a note that this stress difference, or devia­ reversal in dip of the basal detachment fault toric stress, developed in the autochthon at middle crustal levels, whereas Wernicke without any increase in 0NH' (1981) proposed that basal detachment faul ts maintain their dip all the way to the mantle. The hypothesis proposed here is that The assumption of a regional dip is entirely folding is the result of the deviatoric consistent with the model proposed by stress caused by tectonic denudation. Fold­ Wernicke (1981), but is only consistent with ing is thus a process that accommodates re­ the models of Davis and Coney'(1979) and Ham­ duction of ()NH and corresponding reduction ilton (1981) at upper crustal levels above of the deviatoric stress. Reduction of the basal detachment fault dip-reversals. deviatoric stress by folding, if it is com­ pletely effective, will cause the autochthon Let us now assume that a structurally to return to a hydrostatic state of stress isotropic hydrostatically stressed upper (o-v = C>NW)' In this final state of stress, crust is cut by a gently northeast dipping the stress in both directions «()v and o-NW) low-angle normal faul t. Movement on this has been reduced by the same amount from the fault and internal extension of the alloch­ initial state, and volume is increased by thon results in tectonic denudation of the elongation in both the vertical and northwest autochthon. The actual amount of tectonic directions. If the net stress drop is the denudation in the lower Colorado River trough same in each direction, then the percentage is difficult to determine. Values of up to 4 of elongation is the same in each direction. to 7 km of tectonic denudation, corresponding Elongation in the vertical direction is ac­ to vertically directed stress drops of 1 to 2 commodated by thickening. In other words, kb, seem quite reasonable, and as much as 10 points at the top and bottom of the autoch­ km of denudation may have occurred in some thon move farther apart. Elongation in the areas (Keith Howard, pers. commun., 1982). northwest direction is accommodated by fold­ ing since points at opposite ends of the de­ ORIGIN OF NORTHEAST TRENDING FOLDS nuded terrane do not move farther apart. The folds thus represent elongation of the au­ Most large scale folds are inferred to tochthon between constrained end points,

125 rather than shortening, and correspond to a reduction of 0' N\oI' not an increase. Folds formed in this manner Hill here in be called AULT I "decompression folds." KM 40 80 120 160 200 240

This hypothesis for fold formation does not require that large deviatoric stresses ever actually existed in the autochthon. Folding may have begun shortly after the be­ ginning of tectonic denudation and may have continued during tectonic denudation Hith deviatoric stresses never exceeding the rela­ tively small value needed to initiate fold­ ing. A small residual deviatoric stress may remain in the autochthon after denudation has stopped.

The maximum conceivable net stress drop in the northHest-southeast direction (()N\oI) occurs if the residual deviatoric stress is of negligible value. In this case, the amount of northHest-southeast elongation (ap­ parent shortening) represented by the north­ east trending folds should be dj~rectly pro­ portional to the net decrease in 0 N\oI' and this value should be roughly equal to the net _"oo:,:~:t~:t~{~:c::.:~_.:~~,,_ 0':-~~~~40--="'8=0='-'-~~1~20~-~-='-'16"'0"""~~2~0-0-~---.J240 decrease in O'v. The net drop in elv is rea­ o sonably assumed to be less than 2 kb, repre­ KM senting denudation of about 7.5 km.

An additional factor may be of critical Figure 2. Flexural rigidity of a 30 km importance in fold formation. The deviatoric thick, tabular, Hookeian rock mass cut by a stresses generated by tectonic denudation single, frictionless 10H-angle fault. The must overcome the flexural rigidity of the solid lines represent net flexural rigidity, autochthon and allochthon, or folding Hill the dashed lines l'epl'esen t the flexural ri­ not occur. The flexural rigidity (R) of an gidity of each fault block. The basic de­ ela,stic beam or plate is given by the equa­ tachmen t faul t geometry is that proposed by tion \oIernicke (1981). R = BT3/12 = ET3/12(1-v2) Hhere T = thicknes,s, E = Young's modulus, v = Poisson's ration, and B the modulus of elasticity (Johnson, 1970). It can be seen 100 from this equation that flexural rigidity is a function of the thickness cubed, and that small reductions in thickness Hill cause ES large reductions in flexural rigidity. r f- o Consider the hypothetical situation, o shoHn schematically at the top of Figure 2, ii: of a 30 km thick crust, behaving as a .J "0: Hookeian solid throughout, and cut by a sin­ ~ 50 gle 10H-angle normal fault. If this fault is w ..J considered to be a frictionless surface, the lL net flexural rigidity of the crust, or resis­ .J tance to folding, is 25% of its value before "'::: f­ fault formation Hhere the fault is at 15 km Z depth (Fig. 2). The development of multiple detachment faults Hill cause an even larger reduction in flexural rigidity, as Hill crustal attenuation resul ting from signifi­ o cant movement on these faults. o 10 15 KM DENUDATION The percent reduction in flexural rigid­ ity is dependent on: (1) the amount of tec­ Figure 3. Flexural rigidity of a tabular tonic denudation, (2) the presence and loca­ rock mass Hith no internal faults versus tion of 10H-angle faults, and (3) the depth amount of denudation for three values of ini­ to Hhich the crust behaves as a Hookeian tial thickness. Denudation is defined as solid. The effect of tectonic denudation on vertical thickness of matel'ial removed. In flexural rigidity is shoHn graphically in this case, detachment faulting has caused Figure 3 for three values of initial thick­ complete removal of allochthonous material. ness. For example, a 35 km thick crust re­ duced to betHeen 33 and 25 km thickness by 2 to 10 km of tectonic denudation is reduced to betHeen '84% to 36% of its original flexural

126 rigidity. Considering the lower crust to be and/or (3) original irregularities in the ductile (Newtonian) instead of elastic detachment fault surface that were locally (Hookeian) simply reduces the effective am p 1 i fie d by f oldi n g , and / 0 r (4) va ria b iIi t Y thickness of the crust in calculating the of fold amplitude and wavelength about an flexural rigidity. For the situation des­ average which is significantly less than the cribed above, but with a 15 km initial effec­ high values, and/or (5) superimposed regional tive thickness instead of 35 km (this might tectonic stresses which acted additively with be thought of as representing the brittle­ those generated by denudation. It is encour­ ductile transition at 15 km), 2 km of tecton­ aging, however, that most values are compati­ ic denudation will reduce flexural rigidity ble with stress drops of 2 to 3 kb. More to 65% of its original value, and 10 km of data on fold amplitude and wavelength are denudation causes a reduction to 4% of the needed to better test this hypothesis, as original flexural rigidity. This assumes well as experimental data to determine real­ that the elastic (brittle)-ductile transition istic values of Young's modulus. does not migrate downward during denudation, which it almost certainly does due to cooling of the autochthon as it is denuded and up­ lifted. Downward migration of the brittle­ 10.------r----, ductile transition will lessen the reduction of flexural rigidity. Z 0 9 i= These examples demonstrate that the (,) presence of low-angle faults causes drastic w 8 reduction of flexural rigidity of the crust. Q': However, these calculations are based on the Or assumption that detachment faults are fric­ ~ z tionless surfaces. With regard to the forma­ ~ 6 tion of these long wavelength, low amplitude CD folds, this assumption may be an ~ adequate approximation in that only small a. 5 amounts of slip are required between folding 0 tabular rock masses, and this slip need only Q': 4 occur during normal low-angle fault movement. a Thus, one meter of down-dip (normal) dis­ (/) (/) :3 placement during a single seismic event might w V·'l. be accompanied by 1 em of lateral displace­ Q': f- ment due to flexural slip. Folding need not (/) 2 initiate displacemen t on faul t surfaces, it f- need only utilize these surfaces for small W Z amounts of slip while much larger displace­ ments are occurring in a perpendicular direc­ tion. Frictional resistance to faul t move­ .5 1.0 1.5 2.0 ment may decrease sharply during movement due % ELONGATION to frictional heating and increased pore­ fluid pressure (Sibson, 1973; Raleigh, 1977). (APPARENT SHORTENING) Under these conditions, the assumption of zero friction is at least partially justi­ fied. Figure 4. Percent northwest-southeast elong­ ation (apparent shortening) versus net stress The percentage of elongation or apparent drop (IS"NW) for three reasonable values of shortening represented by the northeast Young's modulus (E). The graphical relation­ trending folds can be calculated given the ship depicted is that of Hooke's law. The amplitude and wavelength of the folds. For four circles represent estimated elongation folds of low amplitude and long wavelength, (apparent shortening) based on measurements the equation of the amplitude and wavelength of northeast trending folds in the Whipple Mountains (E. Frost, unpublished data). The "X" represents the estimated elongation (apparent shorten­ where a amplitude (one half the crest to ing) based on measurements of northeast trough elevation difference), and f.. = wave­ trending folds in the Chemehuevi Mountains 1 ength (c rest to crest distance), is an ade­ (8. John, pers. commun., 1981). A three kb quate approximation for calculating the per­ stress drop, representing 11 kilometers of cent elongation or apparent shortening (106 denudation, is probably the maximum conceiv­ s). Several calculated values of percent able value. This graph indicates that net elongation, based on actual fold geometry, stress drops in the northwest direction de­ are plotted against inferred net stress drop termined from measurment of fold amplitude in Figure Lf. At least two of these values and wavelength are comparable to somewhat suggest net drops of ClNW that are signifi­ larger than estimated net vertical stress cantly larger than inferred drops of () v drops due to tectonic denudation. Points based on reasonable amounts of tectonic denu­ indicating greater stress drops may indicate dation. This discrepancy can be explained low values of Young's modulus or other fac­ by: (1) a low value of Young's modulus for tors involved in fold formation. autochthonous rocks, and/or (2) larger amounts of tectonic denudation (7 to 12 km),

127 ORIGIN OF NORTHWEST TRENDING FOLDS is one of pure shear. Horizontally directed compressive stress (Ox) increases linearly In modeling stresses in the autochthon upHard from zero at the neutral plane to a leading to formation of the northwest trend­ maximum value equal to 01 immedia tely below ing folds, it is necessary to make some addi­ the detachment fault (in this case, horizon­ tional assumptions. If we assume that al­ tal should be taken to mean parallel to a lochthonous rocks are cut by numerous listric line extending radially outward from the and/or planar normal faul ts, that they are sharp tapered end of the autochthon (point uniformly extended, and have a planar basal T)). Below the neutral plane the state of detachment, then the amount of tectonic denu­ stress is one of horizontally directed ten­ da ti on shaul d inc rease Iinear ly in the down­ sion which increases linearly downward to a dip direction of the detachment fault from a value equal to ()3 at the base of the autoch­ zero value at the breakaway point. This will thon (Fig. 6A). Application of a regional be called a linear denudation model. In re­ tensional stress will elevate the neutral ality, detachment faults may be initially plane (Fig. 6A) and if large enough will sig­ concave or convex upward, or even undulating, nificantly alter the trajectory orientations. and the initial thickness and amount of ex­ tension within the allochthon may be highly Figure 5B shows the trajectories of the variable. For the purpose of determining the maximum shear stress. In the case of stress magnitude and orientation of stresses gener­ induced by flexure only, these trajectories ated in the autochthon by tectonic denuda­ will intersect the upper and lower surfaces tion, a linear model is a good starting point of the autochthon at 45°. The magnitude of and may suitably predict stress trajectories the horizontally directed shear stress (r yx) for many nonlinear denudation models. due to flexure reaches a maximum at the neu­ tral plane and decreases to zero at the upper Another assumption in this model is that and lower boundaries of the autochthon (Fig. the base of the autochthon can be considered 6B) • as an initially horizontal, planar boundary across which no shear stresses are transmit­ ted. Such a boundary might correspond to a deeper, regional detachment, the brittle-duc­ tile transition, the base of the crust, or the base of the lithosphere. Figure 5. Stress trajectories in a flexed tapered slab following tectonic denudation. One predictable effect of tectonic denu­ The cross sections represent a linear denuda­ dation is isostatic rebound. Depending on tion model in which the allochthon has uni­ rock density at the level of isostatic com­ formly undergone 50% extension. The style of pensation, for complete local isostatic re­ extensional faulting shown is purely schemat­ equi libration the amount of uplift of the ic, and represen ts only one of many pass ible autochthon should be about 80 to 100% of the structural styles. Point T, originally at 20 amount of tectonic denudation. For a linear km depth, has undergone 5 km of uplift to denduation model, the amount of denudation achieve isostatic equilibrium. No denudation and resultant isostatic uplift increase lin­ and therefore no uplift has occurred at the early in the down-dip direction of the- de­ left edge of the cross sections. The angle ¢ tachment, resul ting in upward flexure of the represents the amount of upward flexure, the autochthon. angle -B- represents the initial dip of the detachment fault. The stresses resulting from this flexure Stress trajectories in Figure 5A repre­ can be determined using formulas modified sent the maximum (0 ) and minimum (0- ) com­ 1 3 from those presented in many civil engi­ pressive stresses. Maximum shear stress tra­ neering textbooks (Timoshenko and Young, jectories in Figure 5B represent clockwise 1968) to calculate stresses in bending beams shear (r c) and coun te rclockwise shear (1:' or plates. Calculated stress trajectories cc)' To determine the configuration of the for a linear denudation model are shown in Figure 5. In this model, the base of the stress trajectories, the left edge of the triangular shaped autochthon is subjected to tapered slab (autochthon) is held fixed while an upward directed force at each point along a normal force is applied upward to the base its base that is equal in magnitude to the of the slab. The normal force increases lin­ weight of the rock denuded from the top of early from zero magnitude at the left edge to the autochthon above each point. For ex­ a maximum value at point T. No shear stres­ ample, 5 km of tectonic denudation would re­ ses were applied in Figure 5A. In Figure 5B sul t in an upward directed force equal to the a shear stress with a clockwise sense of ro­ weight of a 5 km thick section of rock. The tation was applied at the detachment fault. orientations of the stress trajectories are The magnitude of the applied shear stress is the same regardless of the magnitude of the proportional to the weight of the allochthon applied stress as long as the applied stress above any given point, and therefore increas­ increases linearly with horizontal distance. es linearly from zero at the breakaway point to a maximum value at T. This applied shear The orientations of the principal stres­ stress is assumed to die out linearly down­ ses are shown in Figure 5A. The solid line, ward to zero at the base of the tapered slab. representing the maximum principal stress An applied shear stress of the magnitude 0 trajectory ( (1)' always intersects the mini­ shown in Figure 6B caused an 8 clockwise mum principal stress trajectory (J'l) at 90°. rotation of stress trajectories below the The state of stress along the neufral plane detachment fault near point T.

128 ~8REAK-AWAY POINT O~ "'" "'"

~ , ::x:: ,, ,, , <:s;- ~ / , IO+P. , ,, ~

,, °l_,,-t- , DETACHMENT

•••">...... , •••• FAULT)

'" ..... :':U:~:::~1m---:-l:---_-:-n .'" " :."' '" .. ~t ~T _---1--- u__------: j __ u t ------I 2J_~ --_L__ t l¢ -l,"';------L--- L~- L~ 1 ...... - l\) <0 -v K M 40 50 60 70

O~

, ,

,, ~ ,, ::x:: , , \ , \ ,, ,, ~ \'\1\ , , , , ,, , DETACHMENT ,, , , , ,, ,, FAULT) " " or------,------:~--77'CJr;I--, Gravitationally induced shear stresses ,, at a detachment fault surface are proportion­ ,, al to the weight of the overlying allochthon ,, and the sine of the dip of the detachment fault. For a linear denudation model, the ;1-' shear stress increases linearly in the down­ ,, w , dip direction from a zero value at the break­ u ,, away point. Transmission of this shear z , <:( stress into the autochthon will produce a ~ rotation of principal and shear stress tra­ NEUTRAL PLANE,/ jectories (clockwise in Figs. 5A, 8). The , stress trajectories shown in Figure 58 repre­ -! , <:( , sent stresses caused by flexure and by an U , applied shear stress at the detachment fault I-­ , a: , / surface. The applied shear stress is assumed w , > , to die out linearly downward to the base of ,, the autochthon (Fig. 6B). In Figure 5B, the , I average magnitude of the applied shear stress , is comparable to the average shear stress / along the neutral plane induced by flexure. , CJ3 S L--T-E-N-'2.SLIO-N-('-_"-)-----;O~---;C;-;:O:-;-M;-;:P;-;:R;-;:E:;:;S-;:;S;-;:I O:;;:N~(+G)---'

Figure 6A. Vertical distribution of horizon­ tally directed compressive stress ( Ox), I:: dl 'Ld2 Solid line represents the case for pure bend­ o """------+------,~------, , , ing. Dashed line represents case for pure ,, ,, , bending with superimposed extensional stress. , , Note reduction in 0 x at detachment fault , , ,, , surface and elevation of neutral plane. D ,, detachment (top of autochthon), B :: base of I , autochthon. , 2 W ' , 3 U I ,, Z ' , Figure 6B. Vertical distribution of shear <:( ,, I-- ' stresses actin,g on a horizontal plane due to (fJ I o NEUTRALPLANi pure bending (curve 1, represen ting vertical , cross section below breakaway point), and to ...J , I , , from breakaway point in Fig. 5). The magni­ I tude of the applied shear stress is given by I I the dashed lines, with r d and 1:d repre­ sen ting the magnitude of ehe appliEfct shear stress at the detachment fault surface at points located 30 and 55 km, respectively, s"""------' from breakaway. Note that shear stresses L:yx generated by bending and those genera ted by gravitationally induced shear across the de­ tachment fault are complimentary.

Figure 6C. Hypothetical stress history of the autochthon due to combined effects of flexure and regionally superimposed exten­ sional stress. (1) Initial hydrostatic con­ dition, (2) extensional stress superimposed on hydrostatic stress causes reduction of 6 x at all depths and initiation of detachment w faulting, (3) stresses generated by tectonic U Z denudation and resultant flexure cause Cl x to <:( increase above the neutral plane. At the ~ NEUTRAL PLANE is highest levels in the crust, 6""x is greater than the stress due to overburden, leading to -' folding and reverse faul ting. (4) Termina­ faulting above the neutral plane. Lines (1) and (2) may actually be much closer together, as may be (3) and (4) (see text). Folding and faulting will allow rotation of line (4) BO:------o-:-X-----"---.:L---.:"'-~+ about the neutral plane and return to a hy­ drostatic state (1).

130 The resultant trajectories differ only imposed extension is shown graphically in slightly from the situation with no applied Figure 6C. Subhorizontal stresses in the shear stress. Rotation of stress trajector­ uppermost crust increase both during flexure ies reaches a maximum of 8° immediately below and during termination of extension. This the detachment fault near the tapered tip of may account for post-detachment reverse the autochthon. faulting (Shakelford 1980) and late stage folding. In reality, the magnitude of re­ Two inferences can be made from this gionally superimposed extensional stresses linear denudation model. First, if the may be small relative to those generated by stress in the third direction (i.e., into and flexure, so that lines 1 and 2 in Figure 6c out of the plane of the cross section) is the would be much closer together, as would lines intermediate stress, then any dike emplace­ 3 and 4. ment should follow the trajectory of the max­ imum compressive stress (Fig. 5A). Dikes Nonlinear denudation processes may also emplaced in the upper part of the autochthon play an important role in forming many of during low-angle faulting should dip gently these folds. For example, an tiforms may rep­ in the down-dip direction of the detachment resent areas that were overlain by north to faul t. Gently east-dipping dikes at Homer northwest trending mountain ranges and under­ Mountain may be interpreted as representing went more denudation than synforms which were this phenomena (Spencer and Turner, this vol­ overlain by topographic lows before detach­ ume). Second, the orientation of the maximum ment faul ting. North to northwest trending shear planes in Figure 5B indicates that warps in detachment faults would then reverse faults should form striking parallel represent, at least in part, variable iso­ to the strike of the detachment fault (north­ static rebound reflecting pre-detachment west to north). The Lincoln-Ranch fault map­ fault topography. ped by Shackelford (1980) is a good candidate for such a fault. Formation of north to northwest trending folds may thus be the result of the combined Folding may also be a consequence of the effects of: (1) stresses generated by dif­ stresses generated by flexure of the autoch­ ferential isostatic uplift and resultant thon. As shown in Figure 5A, (J 1 is or ien ted flexure, and (2) nonlinear denudation. Folds parallel to the detachment fault surface in with short wavelengths and steep limbs, such the upper part of the au toch thon, and may as the antiform forming the axis of the Dead therefore have been the driving force for the and Newberry Mountains, may be dominantly the development of the north to northwest trend­ resul t of stresses caused by flexure. This ing folds. The stresses generated by flexure type of fold might be associated with reverse of the autochthon could be transmitted for faul ts, especially on the fold limbs. Folds some distance laterally into adjacent ter­ with longer wavelengths, such as the north­ ranes that have not undergone tectonic denu­ west trending antiform through the Whipple dation. North-south trending anticlines de­ Mountains, may be dominantly the result of forming Tertiary rocks in areas that may not non-linear denudation. The amplitude and have undergone detachment faulting, such as wavelength of folds produced by these proces­ the Castle Range (Hewett, 1956) and Piute ses is thus, in part, a function of the rela­ Range (Spencer and Turner, unpub. mapping, tive significance of these two fold-forming 1981) may have originated by this mechanism. processes.

The subhorizontal compressive stresses One test of the hypothesis that crustal generated by autochthon flexure die out to flexure is the cause of folding is to deter­ zero at the neutral plane. Because of this, mine if the amount of shortening represented the amount of displacement, due to folding, by the northwest to north trending folds is of each point within the autochthon should within the range that could be produced by decrease from a maximum at the detachmen t reasonable amounts of flexure. In the most fault to zero at the neutral plane. If this simplistic model, all of the flexure occurs is an accurate way to represent folding, and by upward rotation of the autochthon about a the neutral plane remains planar during fold­ pivot point located in the neutral plane and ing, then the antiforms represent thickening directly below the breakaway point (point P and the synforms represent reduced thicken­ in Fig. 5A). The amount of shortening at the ing. Folds formed this way might more accur­ top of the autochthon above this point is ately be termed wrinkles. plotted against the amount of flexure for three values of initial thickness (To) in The magnitude of subhorizontal compres­ Figure 7. In reality, this shortening is sive stresses generated in the upper crust by distributed over a broad area, and actual autochthon flexure may be influenced by ex­ shortening will be somewhat less because ternally applied tectonic stresses. A re­ flexure occurs where the authochthon is thin­ gionally superimposed extensional tectonic ner than To' Preliminary values for the stress would have the effect of reducing the amount of shortening represented by the subhorizontal compressive stresses at all northwest trending fold through the Whipple levels in the crust (Fig. 6A). Similarly, Mountains, based on unpublished data provided termination of superimposed extensional by Eric Frost (pers. commun., 1981) indicate stresses may increase the subhorizontal com­ shortening of approximately 0.2 to 0.4 km pressive stresses throughout the crust. A which is well within the range of most values hypothetical stress history for the crust of To and yl (Fig. 7). based on interaction between stresses gener­ ated by flexure and those generated by super-

131 2.5 r------.,.------, leI to the overlying detachment fault and progressively farther from the neutral plane, then most of the elastic energy stored in the overlying rock by subhorizontal compressive stesses and some amount of gravitational po­ 2.0 tential energy as well would be available for -s.. low-angle fault formation and initial move­ z ment. Stresses generated by flexure may thus fJ) be a cause of the formation on new detachment ~IN_ 15 faults following initial detachment faulting

(9 and denudation. z W t­ REGIONAL IMPLICATIONS a:: o 1.0 I Evi dence for Tertiary tee tonic denuda­ fJ) tion is present in parts of the Cordillera from British Columbia to Sonora (Crittenden and others, 1978). The lower Colorado River 0.5 region is unique among these terranes in that detachment faults are both well-exposed and have not been modified by younger high-angle faulting. If tectonic denudation and iso­ statically induced flexure occurred in these 4° 6° 8° terranes, evidence of folding should be pre­ (AMOUNT OF FLEXURE) sent where younger deformation has not been too severe. Some notable candidates for de­ compression folds include the Tanque Verde Figure 7. Amount of shortening at the top of antiform and associated parallel folds near the autochthon versus amount of flexure (¢ in Tucson (Drewes, 1972; Davis, 1980), the Figure 5) for three values of initial thick­ northeast trending foliation arch of South ness (To)' The "+" represents the situation Moun tain near Phoenix (Reynolds and Rehrig, depicted in Figure 5. There is rarely more 1980), and the Raft River anticline (Compton than one northwest trending fold in any given and others, 1977). Candidates for folds northeast oriented cross section of the lower caused by differential denudation and result­ Colorado River trough area. The amount of ant flexure in some combination with non-lin­ shortening determined from measurement of ear denudation include the northwest trending fold amplitude and wavelength is less than Rincon-Cata·lina-Tortolita arch (Banks, 1980; 0.4 km in the Whipple Mountains (E. Frost, Keith and others, 1980, and references there­ pers. co mm un., 1 98 1) • This val u e is en t ire1 y in), the broad, northwest tt'ending at'ch of compatible with most values of To and %. South Mountain (Reynolds and Rehrig, 1980), and Kettle Dome (Cheney, 1980; Rhodes and Cheney, 1981). IMPLICATIONS FOR THE DEVELOPMENT OF DETACHMENT FAULTS Detachmen t faults are numerous along a corridor that extends for approximately 600 Based on Figures 5B and 6B, the highest km from southernmost Nevada to southeastern shear stresses occur near the tapered end of Arizona. This corridor lies between the the autochthon at some location between the southwestern margin of the Colorado Plateau neutral plane and the detachment fault. If a and the axis of the Mesozoic and early Ter­ fault developed parallel to a shear trajec­ tiary magmatic arc. The Colorado Plateau is tory with a clockwise sense of rotation now higher in elevation than the area to the ( ?: c), it would be.a reverse fault with a southwest, but the opposite was apparently gen tIe to modera te dlp away from the detach­ true before Oligocene or early Miocene time ment fault (to the left in Fig. 5), and would (Young and McKee, 1978; Peirce and others, flatten with depth into near parallelism with 1979). This inference is supported by the the neu tral plane. A small amount of move­ general absence of Mesozoic and early Tertia­ ment on this fault segment would greatly re­ ry volcanic and sedimentary rocks throughout duce the subhorizontal compressive stresses much of southeastern California and western in the overlying upper part of the autochthon and southern Arizona. Erosional denudation (i.e., the same stresses causing folding), and attendant isostatic uplift of the Meso­ and would lead to reorientation of local zoic and early Tertiary magmatic are, prior stress trajectories. to Oligocene to early Miocene time, may have caused upward flexure of the crust along the The concave upward, roughly subhorizon­ northeastern margin of the magmatic arc. tal reverse fault segment described above is During middle Tertiary initiation of exten­ very similar in form to the toe of an incipi­ sional tectonism, the presence of subhori­ ent landslide. Although the form of the re­ zontal compressive stresses generated by ero­ oriented local stress trajectories following sional denudation and resultant crustal flex­ movement on this fault segment have not been ure may have played a key role in the initial determined, the following speculation seems formation of low-angle normal faul ts along appropriate. If, due to reorientation of the flank of the Mesozoic and early Tertiary stress trajectories following fault movement, magmatic arc. this fault should propagate upward (to the left in Fig. 5) at a shallow angle subparal-

132 CONCLUSION Banks, N. G., 198o, Geology of a zone of met­ amorphic core complexes in southeastern Application of principles of mechanics Arizona,~ Crittenden, M. D., Jr., Coney, and the development of a linear denudation P. J., and Davis, G. A., eds., model provide a basis for modeling fold-for­ Cordilleran metamorphic core complexes: mation processes in detachment fault Geological Society of America Memoir 153, terranes. The decompression fold hypothesis p. 177-216. for the formation of folds whose axes paral­ Burchfiel, B. C., and Davis, G. A., 1971, lel the direction of allochthon transport Clark Mountain thrust complex in the (northeast trending folds) is completely in­ Cordillera of southeastern California, dependent of stresses in this direction. Geologic summary and field trip guide: Northeast directed stretching, a feature of University of California at Riverside models such as that proposed by Otton (1981), Museum Contributions, v. 1, p. 1-28. is not required. The decompression fold hy­ Cameron, T. E., and Frost, E. G., 1981, Re­ pothesis is compatible with several models gional development of major antiforms and for detachment fault geometry, including both synforms coincident Hith detachment the mega-gravity slide model of Davis and faulting in California, Arizona, Nevada, others (1980) and the rooted low-angle normal and Sonora: Geological Society of Amer­ fault model of l'1ernicke (1980. ica Abstracts with Programs, v. 13, no. 7, p. 421-422. Application of principles of mechanics Cheney, E. s., 1980, Kettle dome and related to a linear denudation model suggest that structures of northeastern Washington, ~ stresses induced by autochthon flexure may Crittenden, M. D., Jr., Coney, P. J., and play an important role in the formation of Davis, G. A., eds., Cordilleran met- folds with axes perpendicular to the direc­ amorphic core complexes: Geological tion of allochthon transport. Other observed Society of America Memoir 153, p. 463­ structural features such as reverse faults 484. and gently dipping dikes are predicted by Compton, R. R., Todd, V. R., Zartman, R. E., this analysis. Determination of the abun­ and Naeser, C. W., 1977, Oligocene and dance of such features by further field map­ Miocene metamorphism, folding, and low­ ping should help determine the applicability angle faulting in northHestern Utah: of these mechanical principles and models. Geological Society of America Bulletin, Perhaps the most important parameters needed v. 88, no. 9, p. 1237-1250. for accurate nonlinear denduation models are Crittenden, M. D., Jr., Coney, P. J., and constra ints on the geometry of major detach­ Davis, G. H., 1978, Penrose conference ment faults and of the surficial topography report: Tectonic significance of meta- at the time of detachment fault formation. morphic core complexes in the North American Cordillera: Geology, v. 6, p. Erosional denudation prior to tee tonic 79-80. denudation may have had an effect on stresses Davis, G. A., Anderson, ,]. L., Frost, E. G., in the crust that was almost identical to and Shackelford, T. J., 1980, Myloniti­ that of tectonic denudation. This may help zation and detachment faulting in the account for the large amplitude of some Whipple-Buckskin-RaHhide Mountains ter­ northeast trending folds as well as for the rane, southeastern California and western initial development of detachment faults. Arizona, 2Jl Crittenden, M. D., Jr., Coney, P. J., and Davis, G. H., eds., AC KNO WL EDGEME NT S Cordilleran metamorphic core complexes: Geological Society of America Memoir 153, I would especially like to thank Dave p. 79-130. Pollard of the U.S. Geological Survey for his Davis, G. H., 1980, Structural characteris­ time and interest in helping make this study tics of metamorphic core complexes, as quantitative as possible, and G. A. Davis southern Arizona, in Crittenden, M.D., and colleagues at the University of Southern Jr., Coney, P. J., and Davis, G. H" California for the inspirational field trips eds., Cordilleran metamorphic core cam­ they have led through the Whipple Mountains. p lexe s: Geo logical Soci et y 0 fAme ri ca This study has also benefited from conversa­ Memoir 153, p. 35-78. tions I'lith Keith Howard, Dave Miller, Barbara Davis, G. H., and Coney, P. J., 1979, Geo­ John, and Mark Brandon. I also thank Eric logic development of the Cordilleran Frost for the use of his unpublished contour metamorphic core complexes: Geology, v. map of the Whipple basal detachment fault, 7, no. 3, p. 120-124. and Keith Howard and Barbara John for access DreHes, H., 1978, Geologic map and sections to unpublished data. This paper was revieHed of the Rincon Valley quadrangle, Pima by Dave Pollard and Dave Miller. County, Arizona: U.S. Geological Survey Miscellaneous Investigations Map 1-997. REFERENCES CITED Frost, E. G., 1981, Mid-Tertiary detachment faulting in the Whipple Mtns., Calif., Anderson, R. E., 1971, Thin-skinned disten­ and Buckskin Mtns., Ariz., and its rela­ sion of Tertiary rocks of southeastern tionship to the development of major Nevada: Geological Society of America antiforms and synforms: Geological So­ Bulletin, v. 82, p. 43-58. ciety of America Abstracts Hith Programs, Armstrong, R. L., 1972, LOH-angie (denuda­ v. 13, no. 2, p. 57. tion) faults, hinterland of the Sevier Hamilton, W., 1981, "Core complexes" of the Hestern Utah: Geological Society of Cordillera, 2Jl Tectonic Framework of the America Bulletin, v. 83, p. 1729-1754. Mojave and Sonoran Deserts, California

133 and Arizona: U.S. Geological Survey Geological Society Digest, v. 12 p. 45­ Open-file Report 81-503, p. 41. 5" Hewett, D. F., 1956, Geology and mineral re­ Reynolds, S. J., and Rehrig, W. A., 1980, sources of the Ivanpah quadrangle, Cali­ Mid-Tertiary plutonism and mylonitiza­ fornia and Nevada: U.S. Geological Sur­ tion, South Mountains, central Arizona, vey Professional Paper 275, 172 p. ~ Crittenden, M. D., Jr., Coney, P. J., Howard, K. A., Miller, C. F., and Stone, P., and Davis, G. A., eds., Cordilleran met­ 1980, Mesozoic thrusting in the eastern amorphic core complexes: Geological Mojave Desert, California: Geological Society of America Memoir 153, p. 159­ Society of America Abstracts with Pro­ 176. grams, v. 12, p. 112. Rhodes, B. P., and Cheney, E. S., 1981, Low­ Hyndman, D. W., 1980, Bitterroot dome-Sap­ angle faulting and the origin of Kettle phire tectonic block, an example of a dome, a metamorphic core complex in plu toni c-core-gne iss-dome complex with northeastern Washington: Geology, v. 9, its detached suprastructure, ~ Critten­ no. 8, p. 366-369. den, M. D., Jr., Coney, P. J., and Davis, Shackelford, T. J., 1980, Tertiary tectonic G. A., eds., Cordilleran metamorphic core denudation of a Mesozoic-early Ter­ complexes: Geological Society of America tiary(?) gneiss complex, Rawhide Moun­ Memoir 153, p. 427-444. tains, western Arizona: Geology, v. 8, Johnson, A. M., 1970, Physical Processes in no. 4, p. 190-194. Geology: Freeman, Cooper and Company, Sibson, R. H., 1973, Interactions between San Francisco, Calif., 577 p. temperature and pore-fluid pressure dur­ Keith, S. B., Reynolds, S. J., Damon, P. E., ing earthquake faul ting and a mechanism Shafiqullah, M., Livingston, D. E., and for partial or total stress relief: Na­ Pushkar, P. D., 1980, Evidence for mul­ ture Physical Science, v. 243, p. 66-68. tiple intrusion and deformation within Spencer, J. E., and Turner, R., 1982, Dike the Santa Catalina-Rincon-Tortolita cry­ swarms and low-angle faul ts, Homer Moun­ stalline complex, southeastern Arizona, tains and the northwestern Sacramento in Crittenden, M. D., Jr., Coney, P. J., Mountains: Mesozoic-Cenozoic Tectonic a;;-d Davis, G. A., eds., Cordilleran me­ Evolution of the Colorado River Region tamorphic core complexes: Geological (this volume). Society of America Memoir 153, p. 217­ Timoshenko, S., and Young, D. H., 1968, Ele­ 268. ments of Strength of Materials, 5th edi­ Lucchitta, 1., 1981, On autochthonous upper tion: D. Van Nostrand Company, Inc. , plates and allochthonous lower plates: Pri nce ton, New Jersey, 377 p. core-complex underthrusting in west-cen­ Volborth, A., 1973, Geology of the granite tral Arizona: Geological Society of complex of the Eldorado, Newberry, and America Abstracts with Programs, v. 13, northern Dead Mountains, Clark County, no. 2, p. 68. Nevada: Nevada Bureau of Mines and Ge­ Miller, F. K., and McKee, E. H., 1971, Thrust ology Bulletin, no. 80,40 p. and strike-slip faulting in the Plomosa Werni cke, B., 1981, Low-angle normal faults Mountains, southwestern Arizona: Geo­ in the Basin and Range Province: nappe logical Society of America Bulletin, v. tectonics in an extending orogen: Na- 82, p. 717-722. ture, v. 291, p. 645-648. Otton, J. K., 1980, Complex Oligocene-Miocene Young, R. A., and McKee, E., 1978, Early and extensional and strike-slip tectonics in middle Cenozoic drainage and erosion in west-central Arizona: Geological Society west-central Arizona: Geological Society of America Abstracts with Programs, v. of America Bulletin, v. 89, p. 1745-1750. 13, no. 2, p. 99. Peirce, H. W., Damon, P. E., and Shafiqullah, M., 1979, An Oligocene (?) Colorado Pla­ teau edge in Arizona: Tectonophysics, v. 61, p. 1-24. Raleigh, C. B., 1977, Frictional heating, dehydration and earthquake stress drops, ~ Proceedings of Conference II: Exper­ imental Studies of Rock Friction with Application to Earthquake Prediction: U.S. Geological Survey, Menlo Park, Cal­ ifornia, p. 291-304. Rehrig, W. A., and Reynolds, S. J., 1980, Geologic and geochronologic reconnais­ sance of a northwest-trending zone of metamorphic complexes in southern Ari­ zona, in Crittenden, M. D., Jr., Coney, P. J., and Davis, G. A., eds., Cordil­ leran metamorphic core complexes: Geo­ logical Society of America Memoir 153, p. 131-158. Reynolds, S. J., Keith, S. B., and Coney, P. J., 1980, Stacked overthrusts of Precam­ brian crystalline basement and inverted Paleozoic sections emplaced over Mesozoic strata, west-central Arizona: Arizona

134 A PROGRESS REPORT ON THE TECTONIC SIGNIFICANCE OF THE McCOY MOUNTAINS FORMATION, SOUTHEASTERN CAlIFORNIA AND SOUTIDvESTERN ARIZONA

Lucy E. Harding Laboratory of Geotectonics Department of Geosciences University of Arizona Tucson, Arizona 85721

The rocks of the McCoy Mountains Formation are age. Sympathetic to the north- and south-dipping exposed as a large Mesozoic metasedimentary terrane cleavages respectively, are the northern and southern in the McCoy, Palen, and Coxcomb Mountains, Califor­ bounding thrust faults. The northern bounding thrust nia, and in the Dome Rock and southern Plomosa fault places Paleozoic and Precambrian rocks of the Mountains, and Livingston Hills, Arizona (Harding, North American craton onto the McCoy Mountains 1982). The McCoy Mountains Formation is at least Formation (?) and Jurassic volcanic rocks. The 7.3 km thick and very different from the classic southern bouoding thrust fault places crystalline Mesozoic sequence of the Colorado Plateau, 250 km to rocks of the Mojave-Sonora Complex onto overturned the northeast. Deposited in a narrow west-northwest Jurassic volcanic rocks interbedded with basal McCoy trending basin, the siliciclastic McCoy Mountains Mountains Formation. The McCoy Mountains Formation Formation rests disconformably and nonconformably on and underlying Jurassic volcanic rocks are thus over­ and is interbedded at its base with Jurassic volcan­ thrust along both the northern and southern margins. ic rocks. The McCoy Mountains Formation and under­ lying Jurassic volcanic rocks physically separate Age constraints on the McCoy Mountains Formation Paleozoic and Precambrian rocks of the North Ameri­ were confined to the upper Cretaceous and Paleocene can craton from rocks of the Mojave-Sonora Complex 00 the basis of poorly preserved fossil angiosperm (a composite terrane which includes the San Gabriel, (7) \lood collected throughout the sequence. Nel-l age Joshua Tree and Orocopia Schist terranes and the constraints come from paleomagnetic data which yield­ Vincent thrust, as suggested by L. Silver). A ed a metamorphic pole position on the North knerican Mesozoic clastic sequence laterally equivalent to D('lar wander path very near to a pole position from the McCoy Mountains Formation but much thinner (2 km the Callovian (upper Middle Jurassic) Summerville thick) is exposed in depositional contact on Paleo­ Formation of the Colorado plateau (Hardiog, Coney and zoic rocks on the northern margin of the McCoy Butler, 1980). Paleomagoetic data reported here Mountains Formation and underlying Jurassic volcan­ come entirely from the northern cleavage domain. ics basin, but not on the southern margin. Additional data from the southern cleavage donlain and overturned fold limb is being processed. The data The McCoy Mountains Formation consists of fail the fold test indicating that magnetization of sandstone, conglomerate, siltstone and mudstone and the McCoy Mountains Formation was acquired in re­ has been divided into six map units, each containing sponse to mild in situ metamorphism follm-ling deposi-, internally consistent petrology and stratigraphy. tion and deformati~~ The pole position of the McCoy Detailed petrographic analysis of sandstone meta­ Mountains Formation near the Summerville Formation morphosed to greenschist facies suggests that the pole indicates that magnetization took place at or McCoy Mountains Formation was derived from a geo­ near the time of Summerville deposition. Therefore logically heterogenous source area containing the data imply that at least some of the deformation plutonic, volcanic, and sedimentary rocks. The of the McCoy Mountains Formation took place during or proportions of these three source constituents earlier than Callovi.an time and that the protolith is changed irregularly with time. Paleocurrent indi­ older. cators consisting of high angle (>13°) cross-strata and asymmetric ripple marks, distributed laterally Harding, L.E., 1982, Tectonic significance of the and vertically within the McCoy Mountains Formation, McCoy Mtns. nn., southeastern California and show paleocurrent directions to the south. Sparse southwestern Arizona: ph.D. thesis, University sedimentary structures including cross-strata, of Arizona, Tucson, Arizona, 175 7. ripple marks, channels, wedge-shaped depositional units, and fining upl,ard sequences, along I,ith uni­ Harding, L.E., Coney, P.J., and Butler, R.F., 1980, directional paleocurrent patterns indicate a fluvial Paleomagnetic evidence for a Jurassic age environment of deposition. Application of models assignment on a 6 km thick sedimentary terrane of basin genesis suggest that the McCoy Mountains in SIV Ari zona and SE California: GeoI. Soc. Formation was deposited along a northl-lest trending America Abstr. with Programs, v. 12, no. 3, strike-slip fault in a series of rhombochasmic p. 109. basins.

The McCoy Mountains Formation is homoclinal and south- or southeast-dipping. The top of the section is folded into an overturned, north-verging syn­ cline. A penetrative south-dipping mineral cleavage sympathetic to the overturned syncline is present in the southern part of the McCoy Mountains Formation. The northern part of the McCoy Mountains Formation and underlying Jurassic volanic rocks contain a similarly penetrative, north-dipping mineral cleav-

135

MULTIPLE DEFORMATION IN THE HARCUVAR AND HARQUAHALA }!OUNTAINS, WEST-CENTRAL ARIZONA

Stephen J. Reynolds Arizona Bureau of Geology and Mineral Technology 845 N. Park Ave. Tucson, Arizona 85719

INTRODUCTION GENERAL GEOLOGIC RELATIONSHIPS

The Harcuvar and Harquahala Mountains are tHO of Each mountain range is composed of a central the most prominent mountain ranges in Hestern Arizona core of plutonic and metamorphic rocks that probably (Figure 1). They are distinct from adjacent ranges range in age from 1.7 b.y. to 25 m.y. (Figure 2). because of their anomalous northeast trend and rela­ Plutonic rocks vary in composition from granite to tively high elevation. In spite of their prominence diorite and are commonly foliated. Metamorphic rocks and uniqueness, little has been Hritten about the are primarily amphibolite-grade gneiss, schist, and geology of either range. migmatite, but also include lOHer-grade Paleozoic and Mesozoic metasedimentary units. In the Harquahala The most important early studies of the overall Mountains, metamorphosed Paleozoic rocks occur beneath region Here by Darton (1925) and Wilson (1960). More a large overthrust sheet of Precambrian crystalline recently, both mountain ranges Here discussed by rocks (Reynolds and others, 1980). Rehrig and Reynolds (1980) as part of their reconnais­ sance of metamorphic core complexes in Arizona. Several types of fabrics are present Hithin Additional articles have been published on specific metamorphic and foliated plutonic rocks. Crystallo­ aspects of one or both ranges (Kam, 1964; Varga, 1977; blastic fabrics are common in deeper structural level~ Keith, 1978; Reynolds, 1980; Reynolds and others, 1980). Hhereas mylonitic fabrics are more common in higher structural levels. Both types of foliation define The purpose of this report is to summarize the broad, doubly plunging anticlines that trend east­ structural geology of the Harcuvar and Harquahala northeast, parallel to the topographic axis of each Mountains, Hith special emphasis on geometry and age range (Figure 2). of each generation of structures. Conclusions presented here must be considered preliminary, because Along the northeast end of each range, metamorphic neither range has been completely mapped. and plutonic rocks are overlain by a major, IOH-angle

o 2 Miles ~I1iii=::;;;;;;=~~~~1 ~lrit Fiil ! 'I I I o I 2 3 Km ~ Q

Vulture Mtns. Black ~e

Martin Peak

Figure 1. Physiographic features of the Harcuvar and Harquahala Mountains and vicinity.

137 Q

Q ~~em

10 Miles pem / ~=-=~! \ ' S/ -/ ...... I 10 t

ROCK UNITS Q ®

~-UDPer Tertiary - Quaternary surficial deposits

II-Middle Tertiary volcanic and Q sedimentary rocks F>v,,>,.l-Lower Tertiary(?} muscovite-bearing granite; locally foliated !Tkiml-Tertiory-Cretaceous igneous and metamorphic rocks !.«,'::Il-upper Cretaceous granitej locally foliated

t~=~=-j-Mesozoic sed imentor y and volcan ic rocks; locally metamorphosed ~-Paleozoicsedimentary rocks; locally metamorphosed C{;~~-Precambriangranite; locally foliated

I PCm!-Precombrian metamorphic rocks

Q SYMBOLS

"'---_ -contact; dashed where inferred or approximately located -fault; dashed where inferred -thrust fault

-detachment fculfi dashed where inferred or covered -.strike and dip of foliation Figure 2. Reconnaissance geologic map of the Harcuvar and Harquahala clountains and vicinity. Sources of informa­ tion include Reynolds (1980), Reynolds and others (1980), Rehrig and Reynolds (1980), Wilson (1960), Coney and Reynolds (1980), Varga (1977), S. Richard (1982, pers. comm.), and unpublished geologic mapping by S. Reynolds.

detachment fault. Upper plate rocks include Precam­ Precambrian Deformation, Metamorphism and Plutonism brian crystalline rocks and strongly tilted, middle Tertiary volcanic and sedimentary rocks. The earliest deformational event is recorded in Bedrock exposures of the ranges are surrounded by Precambrian metamorphic and plutonic rocks that occur late Tertiary-Quaternary surficial deposits. near Merrit Pass, Ferra Ridge, and in the Vulture, Bighorn, and Harquahala Mountains (Figures 1 and 2). DEFORClATIONAL EVENTS These rocks consist of high-grade gneiss, schist, granofels, migmatite, quartzite, and foliated plutonic The Harcuvar and Harquahala Mountains contain rocks. They possess a crystalloblastic foliation de­ evidence for at least nine episodes of deformation. fined by compositional layering and by preferred orien­ They are, from oldest to youngest: tation of minerals, such as biotite and hornblende. 1) Precambrian deformation, accompanied by plutonism \fuere unaffected by later deformation, foliation usually and high-grade metamorphism; strikes northeast and is nearly vertical (Figures 3 and 2) Mesozoic, southeast-vergent folding of Paleozoic 4). Some granites have been injected parallel to folia­ and Mesozoic rocks in western Harquahala Mountains; tion, and are themselves foliated. The steep crystallo­ 3) Late Cretaceous, northeast-vergent, ductile defor­ blastic foliation is similar in style and orientation mation synchronous ,,,ith plutonism and high-grade to Precambrian fabrics elsewhere in Arizona. metamorphism; 4) Late Cretaceous-early Tertiary, north-directed N I overthrusting; 30 1.0% 30 5) Intrusion of early Tertiary (?) muscovite-bearing \ / granites into northeast-striking fractures; 6) Tertiary mylonitization which formed a gently dipping foliation and east-northeast-trending lineation; 7) Intrusion of middle Tertiary microdiorite dikes into northwest-striking fractures; 8) Northeast-directed detachment faulting, accompa­ nied by antithetic rotation of upper-plate rocks and by formation of the main northeast-trending anticlines; ___-'- ..,.=-'E 9) Late Tertiary Basin and Range (?) faulting. w'------'------1

Salient features of each deformational event are Figure 3. Strike of steep Precambrian foliation near summarized below. Herrit Pass.

138 cross-cut by undeformed dikes of early Tertiary (7) muscovite granite. Synkinematic hornblende from an N I amphibolite near Cunningham Pass gave a K-Ar age of 70 m.y. (Rehrig and Reynolds, 1980; Shafiqullah and others, 1980). K-Ar biotite ages on foliated and unfoliated granite in the western Harcuvar Mountains range from 44 to 51 m.y. (Rehrig and Reynolds, 1980; Shafiqullah and others, 1980).

N I 30 30 \ I W'------'------'l""-----'-----='E

Figure 4. Strike of steep Precambrian foliation in the southeastern half of Ferra Ridge.

w'-----'- -'----" "!'-__.....L..__-.l_-IE Mesozoic Southeast-Vergent Folding

The Belt of Paleozoic rocks between Martin Peak and Hhite Marble Mine contains numerous large-and small-scale folds that have overturned much of the Figure 6. Trend of mineral lineation associated with section. The folds trend northeast (Figure 5) and northeast-vergent ductile deformation. have consistent vergence to the southeast (Varga, 1977; Reynolds, 1980; Reynolds and others, 1980). Age of folding, though poorly constrained, is probably Late Cretaceous-Early Tertiary, North-Directed Mesozoic. The folds affect Mesozoic clastic and vol­ Overthrusts canic rocks (S. Richards, 1981, personal communication) and are discordantly cut by Late Cretaceous-early The Harquaha1a and Little Harquahala Mountains Tertiary thrust faults (Reynolds and others, 1980). contain stacked overthrusts that place Precambriun crystalline basement over Mesozoic and Paleozoic rocks 0Jilson, 1960; Reynolds and others, 1980). The N two largest thrusts are: 1) the Hercules thrust I which juxtaposes Precambrian metamorphic and granitic 30 30 \ / rocks over Mesozoic clastic and volcanic rocks; and

20% 2) the structurally higher, Harquahala thrust which places high-grade Precambrian gneiss over deformed Precambrian granite and overturned Paleozoic rocks (Reynolds and others, 1980; Keith and others, 1981). Both faults are associated with north-to north­ northeast-trending lineation and striae (Figur2s 7 and8).

WL--' --'- -! N I 30 30 , I

Figure 5. Axial trend of southeast-vergent folds in Paleozoic rocks near Socorro Peak, western Harquahala i"1ountains (from Varga, 1977).

Late Cretaceous, Northeast-Vergent, Ductile Deformation

Metamorphic and plutonic rocks of the Harquahala W'-----' ---L --!"- .l..- L--JE and Harcuvar Mountains experienced a major episode of n::: 54 ductile deformation and high-grade metamorphism that produced a gently dipping foliation and northwest-to north-trending mineral lineation (Figure 6). Symnetamorphic folds are usually aligned parallel to Figure 7. Trend of mylonitic lineation in Precambrian lineation and are overturned to the northeast or east. granite near Harquahala thrust (from Keith and others, In the Harcuvar Nountains, these fabrics occur in 1981). compositionally banded gneiss and interlayered granit-· ic sills. In the Harquahala Mountains, they occur in strongly deformed porphyritic granite of probable A northward sense of tectonic transport is indicated Precambrian age. Netamorphism and deformation are for the Harquahala thrust by east-west trending folds almost certainly Late Cretaceous in age because they that are overturned consistently to the north affected Late Cretaceous Tank Pass Granite (Rehrig (Figures 9 and 10). The sense of movement for the and Reynolds, 1980) and may have occurred during its Hercules thrust is less clear. The Harquahala thrust emplacement. These metamorphic fabrics are generally is probably late Cretaceous or early Tertiary because

139 it cuts late Cretaceous fabric described in the dikes locally exhibit a Hell-developed mylonitic previous section, but is intruded by muscovite granires foliation and east-to east-northeast-trending lineation. that are Eocene or older. The thrusts are similar to In a similar manner, dikes of muscovite granite in the late Cretaceous overthrusts in southHestern Arizona Harcuvar Mountains cross-cut Late Cretaceous ductile (Haxel and others, 1980; G. Haxel, 1981, personal fabrics, but are themselves cut by mylonitic foliation communication). that bears an east-northeast-trending lineation.

N I N I 30 20% 30 \ /

w'---' --'__-' C-.-__L-__....L--,-,-JE

WL- -L__-..m...:s '----..,..,-dE

Figure 8. Trend of striae on subsidiary thrust beloH Figure 11. Trend of early Tertiary (?) dikes of Harquahala thrust (from Keith and others, 1981). muscovite-bearing granite in central Harquahala Mountains.

N I The muscovite granites are probably early Tertiary 30 30 or late Cretaceous because they 1) have intruded Late \ I Cretaceous Tank Pass Granite; 2) post-date most Late 20% Cretaceous fabrics; and 3) have yielded an Eocene Rb-Sr muscovite age.

Tertiary Mylonitization

Tertiary mylonitization has extensively over­ printed pre-existing rocks and structures in higher structural levels of each range. Mylonitic foliation w E generally dips 30 degrees or less and contains an east-northeast-trending lineation (Figure 12). Gash fractures and other extensional features are usually Figure 9. Axial trend of north-vergent folds along oriented perpendicular to lineation. Mylonitization Harquahala thrust (from Keith and others, 1981). occurred after emplacement of early Tertiary (?) muscovite-bearing granites, but before emplacement of middle Tertiary microdiorite dikes. Mylonitic N I gneisses in the Harcuvar Mountains yield middle ~---20% Tertiary K-Ar biotite ages (Rehrig and Reynolds, 1980; 30 30 \ I Shafiqullah and others, 1980).

6~ ~o

N I 30 30 L.J. ...JE \ I 30%

Figure 10. Axial trend of north-vergent folds in Paleozoic rocks beloH Harquahala thrust (from Keith and others, 1981).

Emplacement of Early Tertiary (?) Muscovite-Bearing WL--L L--__----IfIIA '---_--'- J.-lE Granites

Muscovite-bearing granites, alaskites, and pegmatites are common in both ranges. In the central Harquahala Mountains, east-northeast-trending dikes of muscovite granite (Figure 11) have been emplaced across fabrics produced by Late Cretaceous ductile Figure 12. Trend of mylonitic lineation in eastern deformation and the Harquahala thrust. HOHever, the Harcuvar and Harquahala Mountains.

140 Intrusion of Microdiorite Dikes Northeast-directed movement on the fault is supported by minor structures and regional considerations The Harquahala and western Harcuvar Mountains (Rehrig and Reynolds, 1980). Rocks as young as 16m.y. contain an impressive swarm of middle Tertiary micro­ have been affected by movement on the fault diorite dikes. The dikes cross-cut all structures (Scarborough and Wilt, 1979, p. 76). thus far described and are locally associated with copper-gold mineralization. They generally strike northwest to north-north,vest and dip steeply (Figure 13). K-Ar biotite and hornblende ages average 25 m.y. N (Shafiqullah and others, 1980). I 30 30 \ ~---30% /

N I 30 30 \ /

30%

""r-----'------...L..JE

W'-_-'-- '---_--=",....__----'~____L_ __'E Figure 15. Strike of Tertiary volcanic and sedimentary rocks in northwestern half of Ferra Ridge and near Bullard Peak. Figure 13. Trend of middle Tertiary microdiorite dikes in western Harcuvar Mountains. There is some evidence that formation of the Harcuvar Mountains and Harquahala Mountains anticlines occurred during detachment faulting (as suggested by Tertiary Detachment Faulting Cameron and Frost, 1981, for similar anticlines in southeastern California). The anticlines are reflected Metamorphic and plutonic rocks of both ranges in the dips of 1) foliation formed during Late are overlain to the northeast by the Bullard Detach­ Cretaceous ductile deformation; 2) Late Cretaceous­ ment Fault (Figure 2). Crystalline rocks below the early Tertiary thrust faults; 3) Tertiary mylonitic fault have been extensively brecciated, fractured, foliation; and 4) the Bullard Detachment Fault. hydrothermally altered, and ultimately converted into Traditional geologic reasoning would suggest that a tabular zone of chloritic breccia and microbreccia. formation of the anticlines has affected, and there­ The zone of chloritic breccia and overlying detach­ fore postdates, all of these structures. However, ment fault usually dip gently to moderately off the the systematic change in strike of Tertiary units broad Harcuvar Mountains and Harquahala Mountains along Ferra Ridge suggests otherwise. The Tertiary anticlines. The detachment fault contains an upper units strike northwest and dip gently to the south­ plate of Precambrian and Mesozoic crystalline rocks west on the southeast end of the ridge. Toward the and middle Tertiary volcanic and sedimentary rocks. northwest, the rocks increase in dip and assume a Tertiary rocks zenerally strike northlvest and dip more westerly strike. The entire ridge may be a southwest at variable angles. Tertiary rocks along large-scale drag feature. It appears that the upper­ Ferra Ridge systematically change in strike from plate rocks were transported northeast on the detach­ northwest to east-west as they approach the Bullard ment fault, and were dragged against the southeast Detachment Fault from the southeast (Figures 14 and flank of the already formed Harcuvar Mountains anti­ 15). The rocks dip gently southwest where they strike cline. The left-lateral sense of drag is consistent northwest, but are nearly vertical where they strike with such a model. east-west. The southwesterly dip -of Tertiary upper­ plate rocks is most readily explained as a result of Basin and Range (?) Faulting antithetic rotation that accompanied northeast-directed transport above the detachment fault. The Harcuvar and Harquahala Mountains are traversed by major, northwest-striking faults that cut Tertiary mylonitic fabrics and have oblique slip. N I The faults may be related to 1) Basin and Range 30 30 block faulting; 2) northwest-trending, strike-slip \ / 30% faults in the Plomosa Mountains (Miller and McKee, 1971); or 3) northwest-striking reverse faults in the Rawhide and Buckskin Mountains (Shackelford, 1976). In many parts of Arizona, Late Tertiary block faulting outlined the major basins and ranges. However, the Harcuvar and Harquahala Mountains are probably not Basin and Range "horst" blocks. Instead both ranges are northeast-trending anticlines, and the intervening McMullen Valley is a syncline. The WL-...L-__----L_--l ..,..,=------''------'--,,-JE McMullen Valley syncline may have been modified by block faulting near Aguila, where Late Tertiary basin­ fill is thickest (Kam, 1964). Figure 14. Strike of Tertiary volcanic and sedimentary rocks in southeastern half of Ferra Ridge.

141 EPILOGUE Wilson, E.D., 1960, Geologic map of Yuma County, Arizona: Ariz. Bur. Mines Map, scale 1:375,000. The deformational events described above appear, at first, to be separate and unrelated. However, the events can probably be grouped into several tectonic regimes. For instance, events which occurred in the Late Cretaceous-early Tertiary, may all be related to a single episode of crustal compression. In contrast, the various middle Tertiary events probably represent a single regime of crustal extension. More definitive conclusions must await additional mapping.

REFERENCES CITED

Cameron, T.E., and Frost, E.G., 1981, Regional develop­ ment of major antiforms and synforms coincident with detachment faulting in California, Arizona, Nevada, and Sonora: Geol. Soc. Am. Abstracts with Programs, v. 13, p. 421-422. Coney, P.J., and Reynolds, S.J., 1980, Cordilleran metamorphic core complexes and their uranium favorability: U.S. Dept. of Energy Open-File Report GJBX-258(80), 627 p. Darton, N.H., 1925, A resume of Arizona geology: Ariz. Bur. Mines Bull. 119, 298 p. Haxel, G., Wright, J.E., May, D.J., and Tosdal, R.M., 1980, Reconnaissance geology of the Mesozoic and lower Cenozoic rocks of the southern Papago Indian Reservation, Arizona: A preliminary report: Ariz. GeoI. Soc. Digest, v. 12, p. 17-29. Kam, H., 1964, Geology and ground-water resources of McMullen Valley, Maricopa, Yavapai, and Yuma Counties, Arizona: U.S. Geol. Survey Hater­ Supply Paper 1665, 64 p. Keith, Stanley B., Reynolds, S.J., Rehrig, W.A., and Richard, S.M., 1981, Low-angle tectonic phenomena between Tucson and Salome, Arizona: Ariz. Bur. Geol. Min. Tech. Open-File Report 81-2, 114 p. Keith, Stanton B., 1978, Index of mining properties of Yuma County, Arizona: Ariz. Bur. GeoI. Min. Tech. Bull. 192, 185 p. Miller, F.K., and McKee, E.H., 1971, Thrust and strike­ slip faulting in the Plomosa Mountains, south­ western Arizona: Geol. Soc. Am. Bull. v. 82, p. 717-722. Rehrig, W.A., and Reynolds, S.J., 1980, Geologic and geochronologic reconnaissance of a northwest­ trending zone of metamorphic core complexes in southern and western Arizona: Geol. Soc. Am. Memoir 153, p. 131-157. Reynolds, S.J., 1980, Geologic framework of west­ central Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 1-16. Reynolds, S.J., Keith, S.B., and Coney, P.J., 1980, Stacked overthrusts of Precambrian crystalline basement and inverted Paleozoic sections emplaced over Mesozoic strata, west-central Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 45-51. Scarborough, R., and Wilt, J.C., 1979, A Study of uranium favorability of Cenozoic sedimentary rocks, Basin and Range Province, Arizona: Part I: General geology and chronology of pre-late Miocene Cenozoic sedimentary rocks: Ariz. Bur. Geol. Min. Tech. Open-File Report 79-1, 101 p. Shackelford, T.J., 1976, Structural geology of the RaI"hide Mountains, Mohave County, Arizona: Ph.D. thesis, Univ. Southern California, Los Angeles. Shafiqullah, M., and others, 1980, K-Ar geochronology and geologic history of southwestern Arizona and adjacent areas: Ariz. Geol. Soc. Digest, v. 12, p. 201-260. Varga, R.J., 1977, Geology of the Socorro Peak area, western Harquahala Mountains: Ariz. Bur. Geol. Min. Tech. Circ. 20, 20 p.

142 TERTIARY EXTENSIONAL TECrONICS AND ASSOCIATED VOLCANISM IN WEST-CENTRAL ARIZONA

James K. Otton U.S. Geological Survey MS 916, Box 25046 Denver Federal Center Denver, Colorado 80225

INTRODUCTION blocks where volcanism was less continuous, angular discordance may range up to 200 Recent work by Rehrig and others (1980), Suneson (1980), and Suneson and Lucchitta (1978, 1979, 1980 From west to east across the north edge of the and 1981, written commun.) has contributed Date Creek basin, the angle of maximum stratal tilt significantly to the body of major-element tends to decrease and the angle of the block­ geochemical and geochronologic data available for bounding fault tends to increase. Artillery the previously little known volcanic rocks of west­ Formation rocks in the Ester basin area bEB, Fi8' 1) central Arizona. In addition, geologic mapping by just west of the Date Creek basin dip 70 to 90 these workers, the author and others has revealed southwest and are bounded by the subhorizontal fault the structural setting of these rocks. In this mentioned above. In the Artillery Peak area (Fig. report an additional 56 major element analyses and 8 2; Fig. 3A) maximum stratal tilt in the basal part K-Ar ages are reported for volcanic rocks in this of the Artillery Formation (Tal' Fig. 3A) is about area, largely from the north edge of the Date Creek 70 0 and the section is offset By low-angle normal basin. These new data and that of the other workers faults that dip 200 to 300 northeast. Upsection show that significant changes in the composition but dips are progressively shallower. The upper not the petrogenetic affinity of the volcanic rocks lacustrine unit (Ta 4, Fig. 3A) of the Artillery occurred through the mid- to late Tertiary and that Formation dips 200 to 30 southwest. Between the the changes are clearly associated with the changing Artillery Peak and Anderson mine areas (AP and AM, character of mid- to late-Tertiary extensional Fig. 2) maximum dips range from 300 &0 600• tectonics in western Arizona. Shallowest dips range from 150 to 20 to the west (Fig. 3B) to essentially horizontal in a late This study benefited greatly by discussions with tectonic major stratovolcano that underlies the R. E. Anderson, W. A. Rehrig, Ivo Lucchitta, N. H. Black Mountains on the northeast flank of the basin Suneson, W. E. Brooks, Ken Hon, and P. W. Lipman. (BM, Fig. 1). This volcanic pile has been Janna Lishner assisted with computer manipulations informally named the McLendon volcanic rocks (W. of the geochemical data. R. F. Marvin and staff of Earl Brooks, oral commun., 1979). Throughout most the USGS obtained the new K-Ar ages reported here. of the Black Mountains these volcanic rocks are Data from Suneson (1980) and Suneson and Lucchitta generally flat-lying and are cut by northeast­ (1981, written commun.) are used with permission. trending high-angle faults of small displacement. In their westernmost exposures near the Anderson STRUCTURAL RELATIONS mine, however, they overlie with slight angular discordance older tilted andesites and dacites Geologic mapping by the author along the (Tova, Fig. 3C) and are themselves (Tovd, Fig. 3C) northern edge of the Date Creek basin (Fig. 1 and involved in stratal tilting. 2), by Rehrig and others ( 1980, Fig. 2) in the Vulture Mountains (Fig. 1) and by Suneson and Overlying this older, variably tilted volcanic Lucchitta (Plate I in Suneson, 1980) in the and sedimentary group is a thick succession of more Castaneda Hills (Fig. 1) has shown that the Tertiary gently dipping volcanic and basinal sedimentary volcanic and related sedimentary rocks of west­ rocks. The degree of angular discordance between central Arizona can be divided into two groups based these two sequences varies across the region and is on structural relations. The oldest group is dependent on how late in the early phase of faulting comprised of highly faulted, variably tilted and stratal tilting the last rocks of the older, volcanic and intertonguing sedimentary rocks that variably tilted group were deposited and whether predate or are coeval with early severe crustal parts of the section were removed by erosion. extension. Overlying these rocks is a younger group which range from gently tilted, folded and faulted In a manner similar to that in the older group, volcanic and interbedded sedimentary rocks to dips in the younger section are progressively untilted volcanic rocks that follow modern gentler and the amount of offset on faults is drainages. progressively less upsection. The youngest basalt flows, found in the Castaneda Hills area (Suneson The older, variably tilted group is generally and Lucchitta, 1979), are unfaulted and follow cut by closely spaced (1 to 2 km) normal faults of modern drainages. Maximum stratal tilts for these listric geometry. These faults are oriented younger rocks are about 350 in the Castaneda Hills northwest to north-northwest. In the western part area and 50 to 10 0 along the northeast side of the of the basin area, they merge downward into a Date Creek basin. The boundary between the older, regional low-angle fault surface (Shackelford, variably tilted and the younger, gently tilted 1980). Within many fault blocks progressively groups is more difficult to pick in the Castaneda gentler stratal tilts may be observed upsection Hills area because the younger units underwent (Fig. 3A, B, C). Minor angular discordance occurs significant ongoing stratal tilt. Faults are high between successive flows; however, in some fault angle (700 or more in the Date Creek basin) and

143 3

EXPLANATION QTal - Holocene to upper r'lio­ cene alluvium '" Tvs - r'lid-upper r'liocene to upper 01 igocene vol­ canic and sedimentary """ rocks r~zmi - ~1esozoic metamorphic and i9- V neous rocks (varying proto- , I ith age ) p€gn - Precambrian gneiss and Anticline. showing dip granite and plunge 10 10 Lines dashed v/here approx­

I, ! II ' I, " I ! imately located km Figure 1- Distribution of Tertiary volcanic and sedimentary rocks and selected Tertiary structural features, west-central Arizona. Modified from Wilson and others, 1969; Shackelford, 1976; Rehrig and others,1980; Suneson, 1980; and Reynolds, 1980. EB- Ester Basin area; AM- Anderson mine; AR- Aguila Ridge; AP­ Artillery Peak; BM- Black Mountains. Geochemical data described and discussed in text comes from the area of Fig. 2 and Aguila Ridge (this report), from the Castaneda Hills area (Suneson, 1980), and the Vulture Mountains area (Rehrig and others, 1980).

/ / Tmvs 0 ",...... ; 5 km ! II II .,,--_ ..

Figure 2- Geology of the north edge of the Date Creek basin. AM­ Anderson mine; AP- Artillery Peak. Sections A-A', B-B' and C-C' are shown in Fig. 3. Thin lines- contacts; heavy lines­ faults as designated in Fig. 1. Lines dashed where approxi­ mately located. Qal- Quaternary alluvium; QTal- Quaternary to upper Tertiary alluvium; Tmvs- Miocene volcanic and sedi­ mentary rocks; Ti- Tertiary silicic intrusives; Tcw- Cha­ pin Wash Formation; Tsb- slide blocks incorporated into the base of Tcw; Ta- Artillery Formation; Ta5- basalt member of the Artillery Formation; Tov- older volcanic rocks; Mzmi­ Mesozoic metamorphic and igneous rocks; pCgn- Precambrian granite gnei ss.

144 TERTIARY: A Ta Ta - Artillery Formation Seal e Tal - Basa I arkose, Ta - Lower lacustrine unit. 2 Ta - Upper conglomera te, 3 Ta 4 - Upper lacustrine unit, ~LJ§ Ta - "Basalt" member, alluvial­ S lacustrine facies p-Ggn 2000' PRECMIBRIAN: p€gn - Gran; te gnei ss A

QUATERNARY:

Qal Z - Youngest alluvium Qal - Older alluvium TERTIARY: l lew - Chapin Wash Formation Tcb - Cobwebb basal t Tcws - Alluvial facies lew - Chapin ~Jash FOI'mation rewa - Basaltic andesite TO/51 - Lower alluvial facies Tcwb - Basa It Tcwb - Basalt Tov - Older volcanic rocks C TOla - Basaltic andesite TOV - Upper dacites, l TCI'/SU - Upper alluvial-lacustrine­ andes i tes fae i es TOv - lower dacites, Z Tov - Older volcanics andes i tes loyd - Thick dacite flows Tov - Lower welded rhyolitic 34 lova - Thin andesite flows, , and lithic tuffs [ong 1orneI'd te Toar - Basal arkose

PRECAr1BRIAN: PRECAMBR I AN: pfgn - Grani te gnei ss pEqn - Gran; te Clnei 5S c Figure 3- Cross-sections, north edge of Date Creek basin (see Fig. 2). Ta5, Ta5b in Fig. 3A are approximate age equivalents of lower parts of Tcw in Figs. 3B and 3C. Vertical exaggeration- 2x. *- Rock samples with analyses in Table 2. Dips are measured and have also been exaggerated. Irregular lines- unconformities. faults with significant movement on them are widely first appearance of these breccia beds marks the spaced (Fig. 1). Total displacements on major tectonic transition where it cannot be discerned faults may exceed 1000 m (for example, the basin from structural relations. bounding fault along the southern edge of the Date Creek basin, Fig. 2, Otton, 1981a). Basin-fill AGE RELATIONS sediments associated with these faults show a growth fault relation suggesting continuous fault movement The age of most of the younger more gently during sedimentation. tilted group of volcanic and associated sedimentary rocks has been reasonably well established by K-Ar In the western part of the Date Creek basin, dating, especially in the Castaneda Hills area this tectonic transition is also marked by the first (Suneson and Lucchitta, 1979; Suneson, 1980). K-Ar appearance of large volumes of sedimentary breccia ages in that area range from 18.7± 0.3 m.y. for the basalts lowest in the section to 5.4± 0.6 m.y. for and slide blocks (incorporated in Ta 5, and base of Tcw, Fig. 2, Fig. 3A). These rocks lnclude megacryst basalts (young flows that follow present Mesozoic(?) metavolcanic rocks, metamorphosed drainages). Datable volcanic rocks are missing from Paleozoic sedimentary rocks, and Precambrian(?) the older, variably tilted group in the Castaneda granitic rocks. These sedimentary breccias and Hills area. In the western and central Date Creek slide blocks were likely derived from fault scarps basin, volcanic rocks in comparable stratigraphic on rising arches west of the basin area (Frost and and geomorphic positions yield ages (1, 2, 3, 4, 9, Otton, 1981). In the Castaneda Hills area, the 12 in Table 1) consistent with those obtained by

145 Suneson and Lucchitta (1979). Rehrig and others (1980) have reported three ages for highly tilted volcanic flows in the Vulture Establishing the age of the oldest part of the Mountains: 16.3±0.3 m.y. (whole rock on altered gently tilted section and the age of the older, porphyritic tuffaceous alkali rhyolite), 16.7±1.1 variably tilted group has been frustrated by m.y. (whole rock, basaltic andesite); and 26.0±0.6 contradictory ages for flows in the same sequence m.y. (biotite from alkali rhyolite). Although (compare 10, 11, and 15, Table 1, and note sample Rehrig and others (1980) accepted a 16 to 18 m.y. description for 8, Table 1) and inverted ages in a age for these flows (and related dikes) and for the stratigraphic succession (compare 5 and 6, Table approximate time of major crustal extension in the 1). The most reliable age for this part of the Vulture Mountains, the biotite age of 26.0 m.y. is section may be the Arikareean (early Miocene, 21 to preferred as the approximate age for these 26 m.y. land mammal age assigned to fauna found at moderately to steeply tilted flows. The younger the Anderson mine (AM, Fig. 2). This fauna includes flowages and similar young ages for dikes are Oxydactylus, a tall camelid, and Diceratherium, a believed to be related to the peculiar potassic primitive rhinoceros form (Lindsay and Tessman, alteration that has affected the section (see 1974). Lindsay and Tessman (1974) initially geochemistry data below). The older age for these assigned this assemblage to the Hemingfordian; highly tilted rocks is consistent with a 20.0 m.y. however, the assemblage is actually Arikareean (G. age reported for gently tilted andesitic volcanic Edward Lewis, 1980, written commun. following Wood rocks exposed at the west end of the Vulture and others, 1941). This age assignment is Mountains (Scarborough and Wilt, 1979) and these two consistent with a 24.7±0.7 m.y. K-Ar age on ages (26 and 20 m.y.) would be consistent with the plagioclase reported (N. H. Suneson, 1978, written relations seen near the Anderson mine. commun.) for one of the dacite flows in the Black Mountains immediately northeast of the mine and Structural and age relations analogous to the stratigraphically below the Anderson mine host Date Creek basin appear to exist in the Whipple rock s. Mountains (Frost, 1981) where the highly tilted Gene Canyon Formation of Davis and others, 1979, has This land mammal age and the K-Ar age bracket yielded K-Ar biotite ages of 26.0±2.1 and 31.8±3.2 the time of tectonic transition described above. m.y. whereas the overlying, less tilted Copper Basin The land mammal fauna occur in the upper alluvial­ Formation of Davis and others, 1979, has yielded 17 lacustrine part of an intertonguing sequence of to 18 m.y. ages. However the Copper Basin Formation mafic flows and basin beds correlated with the itself was involved in significant stratal tilting Chapin Wash Formation (Sherborne and others, 1979; and it seems probable that crustal extension Otton, 1981b). In the vicinity of the mine, these persisted longer in that area. Unfortunately little rocks rest unconformably on variably tilted biotite­ is yet known about the geochemistry of the volcanic hornblende-plagioclase dacite flows which, farther rocks of the Gene Canyon and Copper Basin to the northeast, yield the 24.7 m.y. K-Ar age. A Format ions. few kilometers to the west the Chapin Wash rests with angular unconformity on lower, more steeply Although evidence for the age of the older part tilted parts of the older volcanic section. of the Tertiary section in west-central Arizona is not conclusive, some tentative conclusions regarding These data suggest that three K-Ar ages reported the probable age for the entire section can be here for the older volcanics (5, 6, 7 in Table 1) reached. The older, variably tilted older volcanic must be considered minimal ages. The data do not sequence seems best interpreted as mid-Oligocene(?) allow us to define the maximum age of the older to early Miocene in age. The younger volcanic rocks volcanic and related sedimentary rocks, however, the and interbedded sediments were first deposited in youngest flows are about 24 to 25 m.y. in age. A the late(?) early Miocene and extended to the late 41.5 m.y. age has been reported for a steeply Miocene. dipping flow near the base of the older volcanic section (14, Table 1) northwest of the Anderson GEOCHEMISTRY OF VOLCANIC ROCKS mine. In an area where K-Ar ages are problematical, single ages are not reliable. Previously unreported major element compositions and normative minerals for 56 samples from volcanic Tracks of a large camelid no older than early rocks across the north edge of the Date Creek basin Miocene (G. Edward Lewis, 1981, written commun.) and from the northwest-trending ridge that lies were found a few meters beneath the sequence of between the eastern Date Creek basin and McMullen flows which have yielded the 16.2, 21, and 43.3 m.y. Valley to the south (hereafter referred to as Aguila ages rnent i oned above. These fl ows occur in the Ridge, Fig. 1) are shown in Table 2. Included are upper part of the gently di pping basalt member of 43 analyses for the older, variably tilted group (15 the Artillery Formation (Lasky and Webber, 1949). analyses of which are of the late tectonic McLendon The basalt member (Ta 5, Fig. 3A) rests with angular volcanic sequence), 10 analyses of gently tilted discordance on the variably dipping lower four mafic volcanic rocks interbedded with the Chapin members of the Artillery Formation. The Wash Formation, and 3 analyses of gently tilted paleontological age although imprecise suggests that young basalts. Major element geochemical data from the 21 m.y. age for the basalt flows is probably Rehrig and others (1980), Suneson (1980), and most accurate. This upper bracket for the tectonic Suneson and Lucchitta (1981, written commun.) are transition is consistent with that established in used in chemical variation diagrams for comparison the Anderson mine area. Unfortunately, there are no and discussion purposes. age definitive fossil materials in the lower four members of the Artillery Formation and the alkali The silica content of the older, variably tilted rhyolite ash-flow tuff near the base of member 3 has volcanic group ranges from 52 to 77 % with a sl ight yielded two diverse K-Ar ages (see 8, Table 1) both compositional gap from 60 to 65%. In most of the of which are probably minimal ages. analyses there is a slight to moderate excess of

146 Table 1.--K-Ar data for volcanic rocks in the Date Creek basin area.

Rad Atm Age, m.y. Sample No. Sample description 40 10 40 40 40 K2°x,Y Ar (10- moles/g) %Ar Ar /K +20 Source 1. AP76-186alJ Olivine-clinopyroxene 1.49 0.2369 32 0.000642 11. 0+0.3 This basalt, somewhat altered. (1.49, 1.49) report Flow near base of conglomerate capping section at Anderson mine. (340 18'30"N, 113 0 17'07"W)

2. AP76-37Y Quartz-bearing basalt, flow 1.12 0.2085 48 0.000748 12.8+0.3 This rests unconformably on Chapin (1:11, 1.14) report Wash Formation rocks along east side of Big Sandy River. (34°21 '52"N, 113'0 31 '55"W)

3. AP76-3aY Clinopyroxene basalt or basaltic 2.07 0.3628 41 0.000708 12.1+0.3 This andesite intrudes Chapin Wash (2.06, 2.08) report Formation rocks along east side of Big Sandy River. (34 0 21'39''N, 113 0 30'58"W)

~ ~ 4. AP76-4o.!! Olivine-clinopyroxene basalt 2.41 0.4513 61 0.000754 12.9+0.3 This "" or basaltic andesite, flow 2.40, 2.43) report rests unconformably on Chapin Wash Formation. (340 20'00"N., 113 0 27'47"W) 5. AP76-3z.Y Biotite, hornblende, pyroxene, 3.75 0.9546 83 0.0103 17.6+0. E0' This plagioclase dacite flow from (3.73, 3.78) report older volcanic section north­ west of the Anderson mine. Stratigraphically below section bearing early Miocene fauna and below AP76-35. (340 20'5.1''N, 1130 19'51"W) 6. AP76-30 Olivine-pyroxene basaltic 2.28 0.6330 80 0.00112 19.2+0JH This andesite (plagioclase pheno­ (2.26, 2.30) report crysts). Flow from Chapin Wash Formation. Stratigraph­ ically below section bearing early Miocene fauna at

-¥samPles 1-8, analysts-R. F. Marvin, H. H. Mehnert and V. M. Merrit, USGS, Denver. -1R~P?rted as K20 for 1 to 8, K for 9 to 15. 1 MlnlmUm age. Table 1, con't.

Rad Atm Age, m.y. 40 10 4O 4O Sample No. Sample description KZO% Ar (10- moles/g) %Ar Ar /K 4O +ZO Source

Anderson mine. (34 0 18'Z9.5''N, 1130 ZZ'5Z.9"W) 7. AP78-1l! Altered, very fine-grained 4.14 1. Z16 79 0.00118 20.3+0.721 This basaltic andesite from base of (4.14, 4.15) report McLendon volcanic pile east of the Anderson mine. Strati- graphically below section bearing early Miocene fauna at Anderson mine. (340Z1' 13.5"N, 113012'7.5"W) 8. AP78-2Y Altered ultrapotassic rhyolite 8.15 1. 609 85 0.000796 13.6+0.0 This welded tuff from Artillery (8.15, 8.16) report Formation about ZOO m above base. Interbedded with green altered tuff and conglomerate. Same flow yielded 19.~.5 m.y. whole rock age (Scarborough and ~ Wil , 1979). 00 (34bZl'26.Z"N, 113034'57"W) 9. Locality Cobwebb Basalt (Lasky and Unreported Unreported 13+Z.1 'Y 86 Webber, 1949). (34019'7"N, 113036'12"W) 10. Local ity Basalt in upper part of Un reported Unreported Zl+3.6 i/ 85 Artillery Formation (Lasky and Webber, 1949 , (340Z0'51"N, 113637'15"W - location corrected by Otton after oral. commun. with Eberly) • 11 and 15 below from same conformable sequence as this flow.

11. Basalt flow in upper part of 1.1Z5 .184 31.8 16.Z+0.4 '§j Artillery Formation (Lasky and .184 32.0 Webber, 1949) but now con- sidered part of Chapin Wash Formation. (340Z1'N, 113037'W). See 10.

~EberlY and Stan1ey, 1978 2 Shackelford, 1980 Table 1, con't-

Rad Atm Age, m.y. 40 10 4O 4O 4O Sample No. Sample description K2O% Ar (10- moles/g) %Ar Ar /K +20 Source

12. Mesa-forming basalt flow under- 1.329 .217 69.0 9.6+0.3 21 lying Manganese Mesa. .220 70.5 (340 22'N, 1130 42.5'N). .223 70.2 .225 67.7 13. JSG-74-373 Biotite from rhyolite glass 5.27 1.688 55.1 17.9+0.5 1/ in volcanic flows at Eagle Poi nt. Volcanic flows inter- tongue with upper part of Artillery Formation (also considered part of Chapin Wash Formation). (340 24'50 I N,1l30 40'19"W). 14. JSG-74-191 Biotite from andesite near 4.04 3.016 57.2 41. 5+1. 2 §j base of older volcanic ..,.~ sequence northwest of the CD Anderson mine. 15. JSG-74-38-1 Basalt flow in upper part of 0.280 .2199 78.9 43.3+3.3 §j Artillery Formation. Con- sidered by Gassaway (1977) as part of an Eocene sequence between the Artillery and Chapin Wash Formations, but herein believed to be lower part of Chapin Wash. See 10.

Y Gassaway, 1977 NazO over K?O; however, in 7 al kal i rhyol i tes, the 1) of either the quartz-bearing basalts or the mesa­ K 0-Na 0 raEio is greater than 1. An alkali-lime forming basalts. index2 of2 58 places these rocks within the calc­ alkalic class (Peacock, 1931). In the alkali versus The older basalts of Suneson (1980) and Suneson silica plot (Figure 4) virtually all of the samples and Lucchitta (1981, written commun.) and the fall within the subalkaline compositional field. basalts associated with the Chapin Wash Formation Note, however, that rocks of lower silica content (Table 2) fall within a relatively restricted silica approach alkali compositions close to the alkaline range of 50 to 53% (Figure 8). Most fall in the fi el d. alkaline field using the discriminator of Irvine and Baragar (1971); however, three are subalkaline. The The alkalic silicic volcanic rocks of Date Creek quartz-bearing and mesa-capping basalts are basin and Aguila Ridge form a decreasing linear subalkaline and show wide silica ranges. The trend across the high alkali and high silica part of megacryst basalts are largely alkaline and as a the variation diagram (Fig. 4). An additional 15 group contain the lowest silica percentage of all alkali rhyolites from the Vulture Mountains are the younger rocks. The young basalts (Table 2) show shown in Figure 4 (Rehrig and others, 1980: data wide silica variation and plot in both alkaline and recalculated to the same basis). These rhyolites subalkaline fields. The rhyolites are largely high from the Vulture Mountains exhibit a similar alkali-high silica types. chemical variation suggesting a common origin. Both groups include ultrapotassic rhyolites with K20/Na20 A comparison of the alkali-silica variation for greater than 10, most of the rest are potassic these younger volcanic rocks to the alkali-silica variation for the older volcanic suite (see dashed alkalai rhyolites with K20/Na 20 ranging from 1 to 5. This alkalic suite may represent an unusual high lines, Fig. 8) shows that the young rhyolites are s i1i ca -h i gh a1ka 1i magma predi s posed to alterati On geochemically similar to part of the older altered and mobilization of silica and potassium after alkali rhyolites; they also show moderate to high eruption, possibly much later. Alternatively, this K20/Na20 (Suneson, 1980). In addition, most of the entire suite may be derived from the more "normal" mafic volcanics of the younger suite plot within Or dacites and rhyolites by deuteric alteration. The along extensions of the trend for the older volcanic former interpretation is preferred because of the suite. distinctive separate linear character of the alkali rhyolite data in the alkali-silica variation plot. In the plot of magnesium versus total iron for the young mafic volcanic rocks (Fig. 9), the basalts In the AFM plot (Figure 5) the entire older, associated with the Chapin Wash Formation and the variably tilted volcanic group falls in the calc­ older basalts from the Castaneda Hills area show alkaline field. In the normative Ab-An-Or ternary wide variation in magnesium content but a very diagram (Figure 6), these volcanic rocks lie largely restricted iron content (8 to 9% total Fe for all within the average range for calc-alkaline suites but hlo analyses). The quartz-beari ng and mesa­ with the slight tendency for the more mafic end of forming basalts show similar Fe compositions; the suite to be enriched in potassium. The potassic however, the mesa-forming basalts are more and ultrapotassic rhyolites plot in the Or corner of magnesium-rich than the quartz-bearing basalts. The the diagram. Finally in a plot of MgO versus total young megacryst basalts show the highest iron iron (Figure 7), there is a roughly linear relation content overall. The young basalts (Table 2) between MgO and total iron that is subparallel to geochemically resemble either quartz-bearing or the Cascade trend but is slightly on the iron­ megacryst basalts (Figs. 8, and 9). The entire enriched side for rocks of intermediate to mafic suite of young volcanic rocks appears to lie upon composition. In Figures 4 through 7, the McLendon the continuation of the petrogenetic trend volcanic rocks fall within the range and trends for established by the older volcanic suite (Fig. 9). the entire older volcanic sequence, suggesting little significant difference between the late Both the older volcanic and the younger volcanic tectonic volcanics and the entire older volcanic suites show similar petrogenetic trends in variation suite. diagrams (Figs. 8 and 9), the principal difference is the more mafic compositions represented in the The younger, gently tilted to untilted volcanic basalts of the younger suite. The older volcanic rocks of western Arizona are, in part, composition­ suite was never more mafic than basaltic andesite ally distinct from the older calc-alkalic suite (52% Si02), whereas the younger volcanic suite extruded contemporaneously with early crustal lncludes rocks with silica contents as low as 46%. extension. These volcanic rocks are most completely The wide variation in silica content in the younger represented in the Castaneda Hills area where their volcanic rocks shows systematic changes through geochemistry and geochronology has been studied in time. The 21 (7) to 16 m.y. old Chapin Wash basalts detail by Suneson (1980) and Suneson and Lucchitta and older basalts of the Castaneda Hills are more (1978, 1979, 1980, and 1981, written commun.). They silica rich (50 to 53%) than any of the young (8.6 have divided the volcanic rocks of the Castaneda to 5.4 m.y.) megacryst basalts (46% to 49.5%). The Hills area into five units based on petrologic, mafic rocks ranging in age from 14 to 9 m.y. show a stratigraphic and geomorphic differences: older relatively wide silica variation (49 to 59%). basalts, quartz-bearing basalts, rhyolite lavas and tuffs, mesa-forming basalts, and megacryst-bearing DISCUSSION basalts. The petrologic, stratigraphic and geomorphic differences are also reflected in the The relation between volcanism and structural geochemistry (discussed following). The basalts events in Ivest-central Arizona is summarized in Fig. associated with the Chapin Wash Formation (Table 2) 10. Early crustal extension and accompanyi ng are in a stratigraphic position equivalent to the volcanism may have begun about 32 m.y. ago and seem older basalts of Suneson (1980) and are geochem­ to have ceased about 24 to 21 m.y. ago. The onset ically similar in many respects. The young basalts was initially accompanied by widespread alkalic (Table 2) are the age equivalents (1, 2, 3, 4, Table rhyolitic ash flow tuffs and subsequently superseded

150 FeO+0.8998 Fe203

Calc-alkaline

MgO Figure 4- Alkali-silica variation diagram for the ol­ der volcanic rocks. McLendon volcanic rocks in open circles; potassic rhyolites from Aguila Figure 5- AFM diagram for older volcanic rocks. Ridge in solid circles; Vulture Mountain data in McLendon volcanic rocks in open circles; potass­ triangles. Dashed lines outline compositional' ic rhyolites from Aguila Ridge in solid circles. fields for Fig. 8. Alkaline-subalkaline subdi­ Superposed data points underlined. Compositional vision after Irvine and Baragar (1971). field division after Irvine and Baragar (1971).

An :] /

o L

10 11 14 b~--'---.1..-----'------'-----'---=---'------'-----'--,,-""---' A Or FeD + 9 FeZ 03 (\oJeiqht percent)

Figure 6- Ternary diagram of normative Or-Ab-An for older volcanic rocks. McLendon volcanic rocks Figure 7- MgO versus total iron for the older volcanic in open circles; potassic rhyolites from Aguila rocks. McLendon volcanic rocks in open circles; Ridge in solid circles. Average calc-alkaline potassic rhyolites from Aguila Ridge in solid rocks enclosed by dashed lines (after Irvine and circles. CA- Cascade trend from Lowder and Car­ Baragar, 1971). Superposed data points are un­ michael (1970). Dashed line shows compositional derlined or circled. Three data points from field for Fig. 9. Figs. 4 and 5 are missing here.

by basaltic andesites and dacite-low silica contamination of them, true basalts did not reach rhyolites (1, 2, 3; Fig. 10). These rocks were the surface. The basaltic andesites may thus extruded through a warm, plastically extending crust represent contaminated basaltic materials or more with a brittly extending thin skin perhaps in a buoyant differentiates of the upper mantle-derived manner like that suggested by Eaton (1979, Figure magmas. The alkali rhyolites, which erupted first, 5). Magmas reachi ng the surface may have been may represent volatile-rich first melts of the lower derived from diapirs of upper mantle material moving crustal rocks. buoyantly upwards in the crust and melted lower crustal rocks (Elston and Bornhorst, 1979). Because About 24 to 21 m.y. ago the crust in the Date of their greater density or significant crustal Creek basin area apparently cooled and became more

151 Table 2.--Major element geochemical data for volcanic rocks, Date Creek basin ared.

Older volcanics (ori9inal analyse~/)

a b b a b a b C d d d Sampl e Sampl e Sample Sampl e Sample Sampl e Sampl e Sampl e Sampl e Sampl e Sample~ sam p ,d samp ,d samijle Sampled Sampled Sampled Oxi de 7630 7631 7632 7633 7635 7705 7722 7802 7908 7944 79481'.'/ 7948~4 7948C-:-4 7950.'/ 7976 7977 7978 ~ ------~---- _._._--_._------_._------_._------_._------SiD 64.60 69.90 66.30 60.40 54.20 75.20 56.70 75.60 51. 60 69.40 73.30 74.40 67.10 72.50 52.90 64.10 60.20 ~ I 2b3 17.10 15.30 15.80 16.50 17.00 12.70 16.30 1 .70 17.40 16.10 11.90 11. 30 16.40 13.10 17 .80 15.90 18.10 Fet3 3.90 2.00 3.00 6.00 6.10 1. 60 4.10 0.52 6.80 2.60 1.10 0.90 2.20 1. 70 5.80 3.80 5.20 Fe 0.16 0.04 0.24 0.16 0.80 0.08 3.00 0.02 1.10 0.06 0.12 0.36 0.12 0.12 1.80 0.40 0.12 M9 0 0.9Z 0.43 1. 20 2.10 3.40 0.07 4.00 0.05 4.10 0.30 0.20 0.30 0.30 0.40 4.00 1.60 1.80 CaD 3.20 1. 70 3.30 3.50 8.00 0.56 6.80 J.• 40 8.10 2.30 1. 30 1. 40 0.64 0.80 8.00 4.10 4.10 ~at 4.30 4.10 3.90 3.40 3.50 2.80 3.60 .40 3.30 4.50 0.40 0.20 0.80 0.00 3.10 3.90 4.40 3.20 4.20 3.60 4.50 2.50 6.80 2.50 7.90 2.20 4.20 10.00 10.10 12.50 9.70 2.10 3.40 2.60 H~O 2.14 1.15 1.12 3.10 1. 62 0.63 1. 20 0.52 2.30 0.52 0.65 1. 20 0.66 0.99 1. 70 1. 50 1. 70 T102 0.50 0.26 0.45 0.82 1.10 0.15 0.98 0.15 1.10 0.38 0.07 0.09 0.42 0.22 0.91 0.64 0.79 P2 05 0.20 0.11 0.16 0.31 0.55 0.04 0.30 0.09 0.37 0.12 0.05 0.07 0.11 0.14 0.28 0.21 0.24 MnO 0.03 0.05 0.03 0.08 0.06 0.03 0.10 0.03 0.09 0.04 0.03 0.02 0.04 0.06 0.13 0.11 0.10 CO 2 0.02 0.00 0.15 0.08 0.04 0.03 0.00 0.78 0.05 0.06 0.9Z 0.35 0.19 0.34 0.04 0.48 0.03 Tota 1 100.27 99.24 99.25 100.95 98.87 100.69 99.58 100.16 98.51 100.58 100.04 100.69 101. 48 100.07 98.56 100.14 99.38

Adjusted analyse'y Adjusted analyses

SI Db 65.83 71. 26 67.66 61. 78 55.75 75.17 57.63 77 .17 53.66 69.40 74.44 75.04 66.68 73.42 54.63 65.30 61.65 17.43 A12 3 15.60 16.12 16.87 17.49 12.70 16.57 11. 94 18.11 16.11 12.08 11.40 16.30 13.27 18.39 16.20 18.54 Fe 03 2.04 1. 79 1. 99 2.37 2.67 1.60 2.42 0.53 2.70 1.88 1.12 0.90 1.90 1.72 2.49 2.18 2.34 Fe b 2.10 0.29 1. 31 3.92 4.42 0.08 4.80 0.02 5.51 0.78 0.12 0.36 0.40 0.12 5.36 2.10 3.10 M90 0.94 0.44 1. 22 2.15 3.50 0.07 4.07 0.05 4.26 0.30 0.20 0.30 0.30 0.40 4.13 1. 63 1.84 CaD 3.26 1. 73 3.37 3.58 8.23 0.56 6.91 1. 43 8.42 2.30 1. 33 1. 41 0.63 0.81 8.26 4.18 4.20 Nabo 4.38 4.18 3.98 3.47 3.60 2.80 3.66 1.43 3.43 4.50 0.41 0.20 0.79 0.00 3.20 3.97 4.50 KZ 3.26 4.28 3.68 4.60 2.57 6.80 2.54 7.15 2.29 4.20 10.15 10.19 12.42 9.82 2.17 3.47 2.66 ~ h02 0.51 0.27 0.46 0.84 1.13 0.15 1.00 0.15 1.14 0.38 0.07 0.09 0.42 0.22 0.94 0.65 0.81 U1 P 0 2 5 0.20 0.11 0.16 0.32 0.57 0.04 0.30 0.09 0.38 0.12 0.05 0.07 0.11 0.14 0.29 0.21 0.25 '" MnO 0.03 0.05 0.03 0.08 0.06 0.03 0.10 0.03 0.09 0.04 0.03 0.02 0.04 0.06 0.13 0.11 0.10

Normative mineralJ../ Normative Minerals

q 19.48 26.51 22.22 12.34 4.72 31. 83 6.55 40.49 1.49 22.27 32.47 33.36 13.51 34.71 3.87 18.89 13.09 C 1. 29 1. 21 0.22 0.61 I. 31 0.40 '0.31 1.09 2.29 1. 27 or 19.27 25.30 21. 68 27 .18 15.19 40.16 15.02 41. 89 13.51 24.80 59.46 59.99 73.27 57.85 12.81 20.37 15.73 ab 37.08 35.37 33.63 29.40 30.45 23.68 30.96 12.00 29.02 38.06 3.41 1. 70 6.71 -- 27.08 33.46 38.12 an 14.72 7.87 14.65 15.16 23.95 2.00 21. 28 1. 45 27.20 10.24 0.31 0.11 1. 25 0.91 29.37 16.06 19.02 wo 5.39 0.10 4.60 4.90 1. 75 -- 3.95 0.04 -- en 2.34 1. 09 3.05 5.35 8.71 0.17 10.13 0.13 10.61 0.75 0.50 0.75 0.74 1. 01 10.29 4.04 4.59 fs 1. 39 0.07 4.02 4.16 5.35 6.17 -- 6.48 1.18 2.61 fo fa nit 2.96 0.32 2.88 3.44 3.88 3.51 3.92 1.54 0.28 0.97 0.20 3.61 3.15 3.40 hm 1. 58 1. 60 0.53 0.82 0.91 0.24 1.77 1. 72 -- ,I 0.97 0.50 0.87 1. 59 2.15 0.23 1.89 0.11 2.17 0.72 0.13 0.17 0.79 0.39 1. 78 1. 23 1. 54 tn O. 07 ru 0.10 0.19 ap 0.48 0.27 0.39 0.75 1. 34 0.10 0.72 0.22 0.91 0.28 0.12 0.17 0.26 0.34 0.69 0.50 0.58 cc 0.05 0.35 0.19 0.09 0.07 1.80 0.12 0.14 2.11 0.80 0.43 0.78 0.09 1.11 0.07

II -- Water values combined. ~Fe values adjusted, H20 and CO 2 deleted per Irvine and 8ara9ar (1971). llc~ I cui ated ~er ~tuTk I ess and Vag Tru,,? 1979 .4 U trapotass c r yo ites from rl ge no th of Aguila, probably altered, see text. Ana lysts: a-K. Coates, H. Smith; b-Z. A. Hamlin; (-N. Skinner; d-J. Reid, P. Hearn; e-H. Smith. Table 2 (Continued)

Older volcanics (ori9inal analyses) McLendon volcanic rocks (ori9inal analyses) d d d d b b a Sampled Sampled Sampl e Sampl e Sampl e Sampl e Sampled Sampled Sample Sample Sample Samolee Sampl ee Sampl ee Sampl ee Oxide 7979 7981 7982 7985 7988 7994 7997 7998 7713A 7711A 7721B 10277 11377 13177 13277

SiO 67.40 6B.40 68.50 51. 50 67.10 73.70 51. 50 51. 30 58.50 55.60 56.90 57.90 69.60 65.70 64.30 A1 16.70 15.90 15.80 17.90 16.50 12.10 17.80 17.10 16.BO 16.20 16.90 16.70 15.00 15.90 15.70 263 Fel3 2.50 2.70 2.70 7.10 2.60 1.20 7.50 6.50 3.30 4.40 6.60 3.40 2.10 2.20 1. 70 Fe 0.08 0.08 0.06 1.40 0.12 0.04 1.00 1. 30 1.10 1. 70 1.30 2.70 0.44 1. 30 1. 90 M90 0.50 0.60 0.70 3.30 0.70 0.20 4.10 4.40 4.50 4.00 3.10 3.20 1.00 1.40 1. 60 CaO 2.50 2.50 2.20 9.60 2.50 1.40 B.70 B.90 5.00 7.80 6.60 6.70 2.30 3.30 3.60 ~a6° 4.00 4.60 4.00 4.00 4.70 2.60 3.70 3.60 3.60 3.40 3.40 3.80 4.00 3.BO 4.30 3.90 3.BO 3.70 1.50 3.70 6.10 1. 70 1. 90 1.50 2.60 2.90 2.20 3.90 3.50 3.50 2 H O 1. 70 O.BO 1. 30 1. 70 1.00 0.63 2.20 2.20 4.50 2.44 2.00 LOB 1. 63 1.29 1. 76 2 T102 0.40 0.41 0.40 1. 20 0.40 0.13 1.10 1.20 0.58 1.00 1.10 0.7B 0.35 0.55 0.59 P205 0.16 0.14 0.14 0.44 0.15 0.06 0.49 0.40 0.20 0.49 0.30 0.25 0.12 0.20 0.22 MnO 0.04 0.05 0.05 0.11 0.38 0.17 0.10 0.09 0.08 0.06 0.09 0.11 0.03 0.06 O.OB 0.02 0.02 0.04 0.13 0.03 0.93 0.03 0.04 0.04 0.02 0.02 0.22 0.02 0.02 0.02 CO 2 Total 99.90 100.00 99.59 99.8B 99.BB 99.26 99.92 9B.93 99.70 99.71 101.21 99.04 100.49 99.22 99.27

Adjusted analyses Adjusted analyses

SiO 69.00 6B.96 69.71 52.52 67.8B 75.43 52.72 53.05 61. 47 57.17 57.36 59.24 70.41 67.10 65.95 A1 6 17 .10 16.04 16.0B 18.25 16.70 lB.23 17.69 17.66 16.66 17.04 2 3 12.39 17.09 15.17 16.24 16.10 Fe 03 1.43 1. 93 1. 93 2.75 1. 92 1. 23 2.66 2.79 2.1B 2.57 2.62 2.33 I.B7 2.09 1. 74 Fe6 0.70 O.BB O. B7 5.92 0.B3 0.04 6.04 5.27 2.44 3.70 5.34 3.91 0.70 1. 4B 1. 95 M90 0.51 0.60 0.71 3.37 0.71 0.20 4.20 4.55 4.73 4.11 3.12 3.28 1.01 1.43 1. 64 ~ CaO 2.56 2.52 2.24 9.79 2.53 1.43 8.90 9.21 5.25 8.02 6.65 6.B6 2.33 3.37 3.69 01 c.:> NagO 4.09 4.64 4.07 4.0B 4.75 2.67 3.79 3.72 3.78 3.50 3.43 3.89 4.05 3.88 4.41 3.99 2.67 K2 3.B3 3.76 1. 53 3.74 6.24 1. 74 1. 96 1.58 2.92 2.26 3.94 3.57 3.59 T102 0.41 0.41 0.41 1.22 0.40 0.13 1.13 1.24 0.61 1.03 1.11 0.80 0.35 0.56 0.61 P 0 0.16 2 5 0.14 0.14 0.45 0.15 0.06 0.50 0.41 0.21 0.50 0.30 0.26 0.12 0.20 0.23 MnO 0.04 0.05 0.05 0.11 0.3B 0.17 0.10 0.09 0.08 0.06 0.09 0.11 0.03 0.06 0.08

Normative Minerals Normative Minerals

q 24.17 21.47 26.27 -- 19.93 35.12 -- 15.00 6.49 7.33 9.68 25.6B 22.04 16.99 c 1. B2 0.06 1. 67 -- 0.65 0.99 0.77 -- -- 0.35 0.39 or 23.59 22.64 22.25 9.03 22.11 36.55 10.28 11. 61 9.31 15.80 17.27 13.27 23.31 21.12 21. 21 ab 34.64 39.24 34.44 34.47 40.22 22.31 32.04 31. 49 32.00 29.58 29.00 32.82 34.24 32.83 37.32 an 11.50 11.45 9.92 26.95 11. 36 0.68 27.57 25.73 24.42 21.86 22.46 22.47 10.62 15.25 13.54 wo ------7.43 -- -- 5.48 7.08 -- 6.06 3.52 3.50 -- I. 33 en 1. 28 1. 51 1.77 7.00 1. 76 0.51 9.87 10.86 11. 77 10.24 7.78 8.14 2.52 3.56 4.09 fs -- -- 5.66 -- - - 6.82 5.28 1.82 3.09 5.98 4.13 0.18 1. 29 fo ------0.96 -- 0.41 0.33 fa -- 0.86 -- 0.31 0.17 mt 1.19 1. 79 1.81 3.99 2.76 0.31 3.B6 4.05 3.17 3.73 3.38 1.32 3.04 2.53 hm 0.61 0.69 0.69 0.02 1.00 ------=~ 80 I -- 0.96 -- -- i1 0.78 0.79 0.77 2.32 0.77 0.25 2.14 2.36 1.16 1.95 2.11 1. 51 0.67 1. 07 1.15 tn ru ap 0.39 0.33 0.34 1.06 0.36 0.14 1.19 0.98 0.50 1. 19 0.72 0.60 0.29 0.48 0.53 cc 0.05 0.05 0.09 0.30 0.07 2.14 0.70 0.09 0.10 0.05 0.51 0.51 0.05 0.05 0.05 Table 2 (Continued)

Volcanics associated with McLendon volcanic rocks (original analyses) Chapin Wash Form. (original analyses) C a d d Sampl ee Samplee Sampl ee Sampl e Sampl e Sampl e Sampl e Sampled Sampled Sampled Sampled Sampled Samplea Sampled Sampled Sampled Oxide 13577 13777 14578 7801 7807 7907 7909 7913 7935A 7940 7974 7901 GSB 7920 7923 7946 ------~------SiO 60.40 57.40 68.20 65.20 68.00 64.40 67.00 51. 60 57.70 70.40 54.90 50.30 50.20 47.90 49.90 49.90 A1 203 15.60 16.80 14.70 15.10 15.50 17.10 16.70 18.. 00 17.90 14.90 17.80 17.10 15.80 15.30 17.70 18.20 Fe 03 5.00 3.90 1. 70 4.40 3.10 1. 50 2.20 6.20 5.40 2.00 4.40 5.80 3.70 4.90 6.50 4.60 Fee 1.00 2.20 0.96 0.16 0.28 1.10 0.60 2.30 0.12 0.24 2.70 2.70 5.40 6.70 2.00 3.70 MgO 1. 60 3.60 1.10 0.75 1. 30 1. 30 1. 20 3.60 2.60 0.80 3.70 5.80 7.80 6.60 5.50 6.00 CaO 3.90 7.00 2.50 3.00 3.20 2.90 2.70 8.30 5.20 2.10 6.70 8.30 8.10 8.00 8.40 8.10 ~aeO 4.20 3.70 3.80 4.20 3.90 4.90 4.10 3.90 4.60 3.40 3.90 3.90 3.90 2.90 3.60 4.60 2 3.60 2.50 3.60 4.00 3.10 2.00 4.00 2.10 2.60 3.90 2.10 1. 50 1. 50 1. 50 1. 50 1. 50 H O 2 2.20 1.85 2.44 1.42 0.73 3.93 0.76 1.40 1. 57 1. 40 1.71 1. 30 2.03 2.53 2.10 1.10 T102 1.20 0.86 0.37 0.86 0.50 0.42 0.41 1.20 0.88 0.35 1. 00 1. 30 1.40 0.97 1. 30 1. 30 0 0.45 P2 5 0.22 0.12 0.31 0.15 0.13 0.16 0.39 0.20 0.11 0.24 0.48 0.53 0.28 0.47 0.48 MnO 0.08 0.07 0.05 0.16 0.04 0.06 0.05 0.15 0.09 0.04 0.12 0.14 0.14 0.16 0.13 0.14 CO 2 0.01 0.04 0.01 0.04 0.02 0.03 0.05 0.03 0.04 0.04 0.02 0.06 0.02 0.86 0.06 0.03 Total 99.24 100.14 99.55 99.60 99.82 99.77 99.93 99.17 98.90 99.68 99.29 98.68 100.52 98.60 99.16 99.65 Adjusted analyses Adjusted analyses SiO 62.25 58.42 70.24 66.44 68.63 67.22 67.59 52.80 59.30 71. 66 56.27 51.68 50.98 50.31 51. 44 50.65 A1 b 2 3 16.08 17.10 15.14 15.39 15.65 17.85 16.85 18.42 18.40 15.17 18.25 17.58 16.05 16.07 18.26 18.48 2.78 Feb03 2.40 1. 75 2.40 2.02 1. 57 1. 93 2.76 2.45 1.88 2.56 2.88 2.94 2.59 2.88 2.84 Fe 3.40 3.81 0.99 2.24 1.39 1.15 0.90 5.93 3.23 0.40 4.71 5.85 6.30 9.60 5.87 5.58 MgO 1. 65 3.66 1.13 0.76 1. 31 1. 36 1.21 3.68 2.67 0.81 3.79 8.53 7.92 6.94 5.67 6.09 CaO 4.02 7.13 2.57 3.06 3.23 3.03 2.72 8.49 5.35 2.14 6.87 4.00 8.22 8.41 8.66 8.22 ~ 4.33 3.76 3.91 4.28 3.94 5.11 4.14 3.99 4.73 01 NaCO 3.46 4.00 1. 54 3.96 3.05 3.71 4.67 .g:,. 3.71 K2 2.54 3.71 4.07 3.13 2.09 4.03 2.15 2.67 3.97 2.15 1. 33 1. 52 1. 58 1. 55 1. 52 T102 1.24 0.87 0.38 0.88 0.50 0.44 0.41 1. 23 0.90 0.36 1. 02 0.49 1.42 1. 02 1. 34 1. 32 P205 0.46 0.22 0.12 0.32 0.15 0.14 0.16 0.40 0.21 0.11 0.25 0.14 0.54 0.29 0.48 0.49 MnO 0.08 0.07 0.05 0.16 0.04 0.06 0.05 0.15 0.09 0.04 0.12 0.06 0.14 0.17 0.13 0.14 Normative Minerals Normative Minerals

q 12.93 7.57 26.45 19.26 25.31 21. 34 21.03 -- 6.48 30.96 4.33 c -- -- 0.33 -- 0.32 2.07 1.23 -- -- 1.65 or 21. 92 15.03 21. 91 24.08 18.49 12.33 23.84 12.69 15.79 23.45 12.72 9.10 9.00 9.22 9.13 8.99 ab 36.62 31.85 33.11 36.20 33.30 43.26 34.98 33.75 39.99 29.27 33.82 33.89 33.51 25.54 31. 39 32.84 an 13.48 22.23 11.90 10.73 14.90 13.93 12.13 25.99 21.08 9.61 25.48 25.39 21. 50 25.29 28.55 24.94 ne ------3.61 wo 1.40 4.75 0.88 -- -- 5.57 1. 60 -- 2.86 5.55 6.54 3.53 4.52 5.20 en 4.11 9.12 2.82 1. 90 3.27 3.38 3.01 6.26 6.65 2.03 9.44 6.84 5.08 10.87 8.84 3.46 fs 2.06 3.69 -- 0.99 0.13 0.21 -- 4.69 2.58 -- 5.08 2.97 1.82 8.88 4.03 1. 37 fo ------2.04 ------5.60 10.26 4.37 3.69 8.20 fa ------I. 68 - - - - 2.68 4.04 3.94 1. 86 3.57 mt 4.03 3.48 2.25 3.49 2.93 2.27 1.86 4.00 3.55 0.38 3.72 4.17 4.27 3.73 4.18 4.12 hm -- -- 0.20 -- -- 0.64 -- -- I. 62 ------il 2.35 1. 66 0.72 1.66 0.96 0.83 0.79 2.33 1. 72 0.68 1. 95 2.54 2.70 1. 92 2.54 2.51 ap 1.10 0.53 0.29 0.75 0.36 0.32 0.38 0.95 0.49 0.27 0.58 1. 17 1. 28 0.69 1. 15 1. 15 cc 0.02 0.09 0.02 0.09 0.05 0.07 0.12 0.07 0.09 0.09 0.05 0.14 0.05 2.04 0.14 0.07 Table 2 (Continued)

Volcanics associated with Chapin Wash Young basalts Formation (ori gi nal analyses) (ori gi nal analyses) Sampled Sampled sampled Sampled Sampled Sample b Sample a Sample b Oxide 7956 7957 7958 7959 7991A 7614 7640 4221 ------~--_. __._------SiO 49.00 49.50 49.90 49.70 51.60 51.40 59.20 47.60 15.50 16.60 A1 2b3 15.00 17.00 17.10 17.20 17.70 16.40 Fe 03 7.40 5.30 7.00 8.10 7.60 2.30 1. 90 6.20 Fe b 1. 30 4.50 1.80 0.88 1.10 8.00 4.30 3.80 MgO 5.90 6.30 4.40 4.40 3.20 6.50 5.00 7.10 CaO 9.80 9.40 8.80 9.00 9.50 8.80 7.30 9.50 ~abo 3.10 3.80 3.80 3.70 4.20 3.10 3.50 3.30 2 2.10 0.85 1. 70 1. 60 1. 60 1. 00 2.60 1. 40 H O 2.70 2.28 2.40 2.50 1.40 0.85 0.50 2.21 2 1. 00 Tl02 1.40 1. 20 1.40 1.40 1.10 1.40 1.10 P2 05 0.85 0.21 0.47 0.46 0.38 0.22 0.34 0.59 MnO 0.14 0.15 0.14 0.15 0.11 0.14 0.09 0.12 CO 2 0.70 0.03 0.11 0.30 0.06 0.06 0.02 0.06 Total 99.39 100.52 99.02 99.39 99.55 100.17 101.35 99.48 Adjusted analyses SiD 51. 05 50.41 51. 71 51. 46 52.60 51. 73 58.71 48.97 A1 2B3 15.62 17.31 17.73 17.81 18.05 16.52 15.38 17.09 Fet3 3.02 2.75 3.00 3.00 2.65 2.31 1.88 2.57 Fe 6.04 7.23 6.12 6.30 6.21 8.05 4.26 7.71 MgO 6.14 6.41 4.55 4.55 3.26 6.54 4.96 7.30 CaD 10.21 9.57 9.12 9.32 9.69 8.86 7.24 9.78 Nat 3.23 3.87 3.93 3.83 4.28 3.12 3.47 3.39 K2 2.19 0.87 1.76 1.66 1.63 1.01 2.58 1. 44 T10 1.46 1. 22 1.45 1. 44 1.12 1. 51 1. 09 1. 03 2 0.34 P205 0.89 0.21 0.49 0.47 0.39 0.22 0.61 MnO 0.14 0.15 0.14 0.15 0.11 0.14 0.09 0.12 No rmat i ve Minerals

q 7.85 c or 12.83 5.11 10.40 9.76 9.63 5.94 15.24 8.51 ab 27 .13 32.46 33.28 32.31 36.12 26.38 29.37 25.73 an 21. 52 27.30 25.44 26.42 25.18 28.04 18.74 27.09 ne 0.15 0.05 1. 61 wo 7.70 7.76 6.62 6.10 8.32 5.86 6.20 7.10 en 11. 20 4.65 6.31 7.36 4.20 16.24 12.35 4.26 fs 4.73 2.70 3.68 4.52 3.92 10.61 4.64 2.47 fo 2.80 7.93 3.53 2.77 2.75 0.03 9.76 fa 1.30 5.07 2.27 1.88 2.83 0.02 6.25 mt 4.35 3.99 4.35 4.34 3.84 3.35 2.73 3.73 11m i1 2.75 2.32 2.75 2.74 2.13 2.87 2.07 1. 95 ap 2.08 0.51 1.15 1.12 0.92 0.52 0.80 1. 44 cc 1. 65 0.07 0.26 0.70 0.14 0.14 0.05 0.14 ------_._-_._._-----_._------rigid. Extension continued in the form of deeply upper mantle and lower crust and the late megacryst penetrating normal faults which progressively tapped basalts from the upper mantle. lower, more mafic magmas (4, 5, 7; Fig. 10) and zones of rhyolitic lower crustal magmas (6; Fig. The Date Creek basin lies at the northeast edge 10). This magmatism persisted until about 5 m.y. of the Basin and Range terrane of west-central ago. Arizona and southeastern California (Otton, 1981a). Athough the early phase of crustal These conclusions are supported, in part, by Rb­ extension ceased in the Date Creek basin area about Sr istopic and petrologic data for the basalts and 24 to 21 m.y. ago, this crustal extension appears to rhyolites belonging to the younger volcanic group in have persisted longer and total extension was the Castaneda Hills area (Suneson and Lucchitta, greater to the west. In the Whipple Mountains, for 1980). The older basalts and, by inference, the example, the end of severe crustal extension appears basalts associated with the Chapin Wash Formation, to be bracketed by the 17 and 18 m.y. ages for the appear to be derived from lower crustal material, moderately tilted volcanic rocks of the Copper Basin the basalts and rhyolites of intermediate age from Formation (Frost, 1981) and a 13.5 m.y. age for the

155 * Basalts, Chapin \·jash Fonnation -/( .. Young basalts -/( 1 CA A Olde)' basalts 1 o Quartz-bea)"ing basalts ill -/( 1 II Hesa-fonning basalts 1 ill 61 Heqacryst basalts ill OJ

* l~ilSil1t,;. Chapin '.Iash .. Young basalts A Ol,jer bilsalts 1 Subalk,'1 1 o nUdl'lz-bcilriwl LS

III 1'!eSil-fnl"lIliiFl ~H';'llt,;l l o .'·1('ailu'"st h'lS<1ltS 1 OPh'lolites

l-SIII1f'son (1980). Suneson and Lucchi tta (1981, \·wi ttell co:nlnunicat ion) Figure 8- Alkali-silica variation diagram for the younger group of volcanic rocks. Dashed lines Figure 9- MgO versus total iron for the younger group are field from Fig. 4. Compositional fields of volcanic rocks. Dashed line 1S field from Fig. divided per Irvine and Baragar (1971). 7. CA- Cascade trend from Lowder and Carmichael (1971) . Tectonic stabi:

, i- REFERENCES I - ~ -J ---- -f - - -f -- 'essatioo of exte"-.i",, Davis, G. A., Anderson, J. L., Frost, E. G., Shackelford, T. J., 1979, Regional Miocene 10 -V-0------0- BJsin-raoqe phase, 6 Clock faultinq, rerjiona1 detachment faulting and early Tertiary(?) folds mylonitization, Whipple-Buckskin-Rawhide Mountains, southeastern California and western ~ 20 Arizona: in Abbott, P. L., editor, Geological excursions~n the southern California area: San Diego, San Diego State University, p. 75-108. Eaton, G. P., 1979, Regional geophysics, Cenozoic 30 tectonics, and geologic resources of the Basin and Range Province and adjoining regions, in No vulcanislll recorded in Possihle !i1inOl" extensionrl1 Newman, G. W., and Goode, H. D., eds., 1979 i \'lest-centl"a1 flrizona normal L'lIJ1till f ! Basin and Range Symposium: Denver, Rocky 404LO---'5iO----6"O----7r'O---,p,in-- Mountain Association of Geologists, p. 11-39. Eberly, L. D., and Stanley, T. B., Jr., 1978, Cenozoic stratigraphy and geologic history of southwestern Arizona: Geol. Soc. America Bull., Figure 10- Relation between silica composition, time v. 89, p. 921-940. of eruption, and structural events associated Elston, VI. E., and Bornhorst, T. J., 1979, The Rio with volcanism in west-central Arizona. Approach Grande rift in context of regional post-40 m.y. here suggested by Elston and Bornhorst (1979). volcanic and tectonic events: in Riecker, R. Time of inception of eruption is approximate. E., Rio Grande Rift: Tectonics-atnd Magmatism: Older volcanic rocks: 1- basaltic andesite; 2­ Vlashington, D.C., American Geophysical Union, p. dacites and low-silica rhyolites; 3- potassic 416-438. rhyolites. Younger volcanic rocks: 4- silicic Frost, E. G., 1981, Mid-Tertiary detachment faulting basalts; 5- basalts and basaltic andesites; in the Whipple Mtns., Calif., and Buckskin 6- potassic rhyolites; 7- low-silica basalts. Mtns., Ariz., and its relationship to the development of major anti forms and synforms: Geol. Soc. America Abs. with Progs., v. 13, no. little- tilted basalt flows in the Osborne Wash 2, p. 57. Formation (Davis and others, 1979). The Date Creek Frost, E. G., and Otton, J. K., 1981, Mid-Tertiary basin thus appears to be at the edge of the terrane stratigraphic record produced during regional affected by extension and consequently may have detachment faulting and development of extended less and cooled earlier than the terrane to extensional arches and basins, southwest of the the west. Vlhether the transition in the composition Colorado Plateau margin: Geol. Soc. America of volcanic rocks to the west occurred later is Abs. with Progs., v. 12, n. 4, p. 197. unknown. Gassaway, J. S., 1977, A reconnaissance study of Cenozoic geology in west-central Arizona: San Diego, San Diego State University M.S. thesis, 117 p. Irvine, T. N., and Baragar, VI. R. A., 1971, A guide to the chemical classification of the commOn volcanic rocks: Canadian Jour. Earth Sci., v. 8, p. 523-548.

156 Lasky, S. G., and Webber, B. N., 1949, Manganese Geological Survey Open-File Report 81-503, p. resources of the Artillery Mountains region, 107-109. Mohave County, Arizona: U.S. Geological Survey ~~ilson, E. D., Moore, R. T., and Cooper, J. R., Bull. 961, 86 p. 1969, Geologic map of Arizona: Tucson, Arizona Lindsay, E. H., and Tessman, N. T., 1974, Cenozoic Bureau of Mines and U.S. Geological Survey. vertebrate localities and faunas in Arizona: \~ood, H. E., 2nd, Chaney, R. W., Clark, J., Colbert, Journal Arizona Academy of Science, v. 9, p. 3­ E. H., Jepsen, G. L., Reeside, J. B., Jr., and 24. Stock, C., Nomenclature and correlation of the Lowder, G. G., and Carmichael, 1. S. E., 1970, The North American continental Tertiary: Geol. Soc. volcanoes and caldera of Talasea, New Britain: America Bull., v. 52, p. 1-48. geology and petrology: Geol. Soc. America Bull., v. 81, p. 17-38. Otton, J. K., 1981a, Structural geology of the Date Creek basin area, west-central Arizona: in Howard, K. A., Carr, M. D., and Miller, D~M., eds., Tectonic framework of the Mojave and Sonoran Deserts, California and Aizona: U.S. Geological Survey Open-File Report 81-503, p. 82-84. Otton, J. K., 1981b, Geology and genesis of the Anderson mine, a carbonaceous lacustrine uranium deposit, western Arizona: a summary report: U.S. Geological Survey Open-File Report 81-780, 24 p. Peacock, M. A., 1931, Classification of igneous rock series: Jour. of Geology, v. 39, p. 54-67. Rehrig, W. A., Shafiquillah, M., and Damon, P. E., 1980, Geochronology, geology, and listric normal faulting of the Vulture Mountains, Maricopa County, Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 89-110. Reynolds, S. J., 1980, Geologic framework of west­ central Arizona, in Jenney, J. P. and Stone, Claudia, eds., Stllcfies in western Arizona: Ariz. Geol. Society Digest, v. 12, p. 1-16. Scarborough, R., and Wi It, J. C., 1979, A study of uranium favorability of Cenozoic sedimentary rocks, Basin and Range Province, Arizona: U.S. Geological Survey Open-File Report 79-1429, 101 p. Shackelford, T. J., 1976, Structural geology of the Rawhide Mountains, Mohave County, Arizona: Ph.D. thesis, University of Southern California, Los Angeles, 176 p. Shackelford, T. J., 1970, Tertiary tectonic denudation of a Mesozoic-early Tertiary(?) gneiss complex, Rawhide Mountains, western Arizona: Geology, v. 8, no. 4, p. 190-194. Sherborne, J. E., Jr., Buckovic, W. A., DeWitt, D. B., Hellinger, To S., and Pavlak, S. J., 1979, Major uranium discovery in volcaniclastic sediments, Basin and Range Province, Yavapai County, Arizona: Am. Assoc. of Petroleum Geol. Bull., v. 63, p. 621-646. Stuckless, J. S., and Van Trump, G., Jr., 1979, A revised version of Graphic Normative Analysis Program (GNAP) with examples of petrologic problem solving: U.S. Geological Survey Open­ File Report 79-1237, 74 p. Suneson, N. H., 1980, The origin of bimodal volcanism, west-central Arizona: Ph. D. thesis, University of California, Santa Barbara, 293 p. Suneson, N. H., and Lucchitta, 1., 1978, Bimodal volcanism along the eastern magin of the Basin and Range Province, western Arizona: Geol. Soc. America Abs. with Progs., v. 10, n. 3, p. 149. Suneson, N. H., and Lucchitta, I., 1979, K-Ar ages of Cenozoic volcanic rocks west-central Arizona: IsochronjWest, v. 24, p. 25-29. Suneson, N., and Lucchitta, I., 1980, The origin of a Miocene basalt-rhyolite suite, southern Basin and Range Province, western Arizona: in Howard, K. A., Carr, M. D., and Miller, D. M.,eds., Tectonic framework of the Mojave and Sonoran Deserts, California and Arizona: U.S.

157 158 mD-TERTIARY DETACIlHEHT FAULTlHG AND 11IllERALIZATION IN TilE TRIGO IIOUNTAIlIS, YUllA COUNTY, ARIZOlIA

\,elford E. Garner Eric G. Fros t

Susan E. Tanges Hark P. Gerrainario

Department of Geological Sciences San Diego State University San Diego, California 92182

ABSTRACT transport of the upper plate I"as to the southwest. Host faults appear to merge with the detachment fault, Exposed within the Trigo Hountains of southwest­ which is folded into several large-scale antiforms and ern Arizona is a well-developed, low-angle normal synforms. Significant silver-lead-zinc mineralization fault, or detachment fault, of mid-Tertiary age. within the south-central Trigos is concentrated along Below this detachment fault is an assemblage of Pre­ the normal faults in thick, open-space fillings, cambrian metasedimentary and meta-igneous rocks, which indicating a structural control for the ore-forming is intruded by rapakivi granite. Also contained solutions. within the lower plate are Laramide intrusive rocks and the Hesozoic Pelona-Orocopia Schist. Above the The presence of detachment faulting in the south­ detachment fault are Precambrian crystalline rocks western corner of Arizona significantly increases the that are depositionally overlain by a thick sequence size of the known detachment terrane. It also raises of Tertiary volcanic and sedimentary units of late several questions about the Tertiary overprint of Oligocene to Hiocene age. These upper-plate units are detachment faulting on the Hesozoic Pelona-Orocopia tilted rather uniformly to the northeast and strike Schist and Vincent-Chocolate Hountains Thrust System. northwest. They are repeated numerous times across The presence of detachment faulting in the Trigos also the range by northwest-striking, southwest-dipping raises the possibility that the effects of this mid­ normal faults. These normal faults progressively Tertiary extension may be present on the west side of extend the upper plate and indicate that relative the San Andreas fault system.

{/~~~ ~

AZe

20km

Figure 1. Location of the Trigo Mountains in southwest Arizona, and relation to nearby mountain ranges. Tertiary rocks in dot pattern, crystalline rocks as enclosed clear areas. Pronounced northlvest trend to the ranges is evident. Tertiary-crystalline contacts in this area are the prime candidates for detachment faults.

159 INTRODUCTION Partly because southwesternmost Arizona lies out­ side of the band of "core complexes," this region has Hid-Tertiary detachment faulting is exposed not been widely considered for its possible relation­ throughout much of Arizona and southeastern Califor­ ship to mid-Tertiary detachment faulting. \oIithin the nia. Such low-angle normal faulting has been de­ western and northern Trigo Hountains, just north of scribed within a northwest-trending band across Ari­ Yuma, Arizona (Fig. 1), a well-developed detachment zona in the zone of ranges termed "metamorphic core fault of mid-Tertiary age is spectacularly exposed complexes" (Crittenden and others, 1980; Davis and (Figs. 2 and 3), where it separates an upper plate of Coney, 1979; Rehrig and Reynolds, 1980; Davis and Precambrian and Tertiary rocks from a lower plate of others, 1979,1980; Coney, 1980). The terrane in this non-mylonitic Precambrian and Hesozoic crystalline zone is characterized by major extension of an upper units. Extension of the detachment terrane to this plate above a regionally developed detachment surface corner of Arizona significantly increases the size of that overlies a lower plate containing both mylonitic the known detachment terrane and reaffirms the non­ and non-mylonitic rocks. The mylonitic fabric exposed genetic link between mylonitization and detachment in many of these ranges has been interpreted by some faulting. \oIidespread mineralization present in the workers as being coeval with, and genetically related Trigos also appears to be genetically related to the to, detachment faulting. In central and western extensional regime. The economic ramifications of Arizona and eas tern California, however, detachment this association of mineralization with mid-Tertiary faulting of mid-Tertiary age appears to be super­ extensional tectonics are immense. The occurrence of imposed on late Hesozoic to earliest Tertiary myloni­ mid-Tertiary detac~lent faulting in this southwestern­ tization (Davis and others, 1979, 1980). In this most corner of Arizona also raises several intriguing region, detachment faulting and mylonitization do not questions about the regional geology of sOllth\;estern appear to be either temporally or genetically related Arizona and southeastern California. Among these are to each other (Shackelford, 1976, 1980; Davis and how detachment faulting and mid-Tertiary extension are others, 1979, 1980; Hartin and others, 1980). related to both the late tlesozoic Vincent-Chocolate

Figure 2. View of the northwestern Trigo Mountains looking north with the Big Maria Mountains in the distance. The upper-plate normal faults cut the section into a series of northwest-striking grabens and half-grabens. The Tertiary volcanic sequence is repeated several times, so that the same stratigraphy is seen several times across the Trigo Mountains. Units trend into the Trigo detachment surface "here they are truncated. Light­ colored rocks lie beneath the detachment fault.

160 Figure 3. View of the Trigo Mountains detachment surface in the northern section of the range, looking west toward the Chocolate Mountains of California in the distance. Dark, upper-plate Tertiary volcanic rocks and Precambrian to Mesozoic crystalline rocks are truncated by the detachment fault with Precambrian gneiss and Mesozoic intrusive rocks forming the lower plate.

~lountains thrust system and the late Tertiary San Detailed study of the south-central Trigo ~loun­ Andreas fault system. The presence of detachment tains has been prompted by the presence of several faulting in this corner of Arizona also suggests that large mineral deposits in the region. The largest of at least portions of the entire Arizona-Sonora border these include the Red Cloud, Black Rock, Clip, and region are detached, and indicates that further explo­ Dives mines. Because museum-quality wulfenite and ration for detachment faults in Sonora may prove very vanadinite are associated with some of these mines, fruitful. extensive mineralogic studies have been conducted within this district (Edson, 1980; Shannon, 1980). Previous \~ork Geologic study of these mineral deposits by Parker (1966) defined both the mineralogy and structural The overall rock distribution within the Trigo control of the silver-lead-zinc mineralization. Mountains region was first deciphered by Wilson Parker mapped numerous normal faults in the southern (1933). He described the range as consisting of Trigos, and clearly showed the spatial association, mostly tilted volcanic rocks separating isolated and perhaps control, that these faults had with the exposures of gneiss, schist, granite, and Laramide vein deposits. Distribution of the vein material as a intrusive rocks, all overlain by untilted basalts. He thick zone in the footwall of normal-fault blocks is interpreted the crystalline rocks to be Mesozoic and described briefly by Dohms and others (1980) in the the volcanic rocks to be Cretaceous in age on the Black Rock mine area. basis of his regional correlation with other ranges. Wilson recognized several of the major faults that South of the area of extensive mineralization, a juxtapose crystalline against Tertiary rocks in the broad antiformal exposure of Mesozoic schist has been south-central Trigos, where the major mining activity correlated to the Pelona-Orocopia Schist by Haxel and was located. However, he described most of the other Dillon (1978). This regionally developed schist is crystalline-volcanic contacts as unconformities. exposed in the southernmost Trigos and is contiguous

161 with large bodies of the schist exposed across the LITHOLOGY Colorado River in California. The origin of this Hesozoic schist is controversial, with both back-arc The Trigo Hountains consist of a sequence of Pre­ (Haxel and Dillon, 1978) and fore-arc (Yeats, 1968; cambrian through Recent rock units. Host of the Burchfiel and Davis, 1981) scenarios being propounded. crystalline rocks in the Trigo Hountains are Precam­ Overlying the Pelona-Orocopia Schist are Precambrian brian in age. In the northern part of the range the gneissic rocks, above a contact that is probably Precambrian rocks appear to be predominantly meta­ equivalent to the Hesozoic Vincent-Chocolate Hountains sedimentary gneiss and schist. These rocks appear to thrust (Haxel and Dillon, 1978; Ehlig, 1981). be the oldest unit in the T~igo Hountains and are well exposed in the region around the Hart Hine in the Overlying both the upper and lower plates of this west-central portion of the range. They are intensely thrust system is a thick sequence of Tertiary volcanic deformed and exhibit abundant small-scale folds. In­ rocks (Hilson, 1933), Ivhich have been studied region­ truded into these metasedimentary rocks are a variety ally by Eberly and Stanley (1978) and Shafiqullah and of meta-igneous rocks including a qua~tz monzodiorite, others (1980). The tilted volcanic rocks within the which is similar to that described to the no~th in the Trigos have been correlated to other tilted, late \~hipple Hountains by Anderson and others (1979). In­ Oligocene to Hiocene units on the basis of a 25 ± 1.7 truded into the p~eexisting metasedimentary and meta­ m.y.B.P. age obtained on a flow in the northl"estern igneous rocks is a la~ge body of rapakivi granite, portion of the range (Eberly and Stanley, 1978; which contains large potassium-feldspar phenocrysts Shafiqullah and others, 1980). As defined by the rimmed with plagioclase. In the northernmost Trigo geochronologic work of Heaver (1982), the volcanic Hountains, the crystalline rocks consist almost en­ rocks within the range vary from about 29 to about 20 tirely of this rapakivi granite. This rock body has m.y. old. Tilting of all these flows indicates the some primary flow foliation developed within it, but presence of a pronounced, mid-Tertiary, or younger, is largely unfoliated suggesting that the foliation in deformation that has affected the Trigo Hountains the metasedimentary rocks was formed prior to the in­ region. trusion of the rapakivi granite.

EXPLANATION ei~ Quaternary s-sediments b- basalt

Tert iary volcanics

Laramide intrusives

Mesozoic PelOnll schist

Precambriam crystalline rocks

Figure 4. Generalized geologic map of the Trigo Mountains area. Note location of Pelona-Orocopia Schist and possible Vincent-Chocolate Mountain Thrust in the southwestern portion of the map area. Adapted from Hilson, 1960.

162 The age relationship of the metasedimentary rocks Unconformably overlying all these crystalline and the rapakivi granite appears to correlate with rocks, except the Pelona-Orocopia Schist, are a se­ similar kinds of age relationships seen throughout the quence of Tertiary volcanic and sedimentary rocks. region (Reynolds, 1980). The metasedimentary gneiss These Tertiary rocks consist largely of volcanic and schist are probably equivalent to the 1.7-1.8 flows, tuffaceous rocks, and sedimentary rocks includ­ b.y.B.P. package of crystalline rocks seen throughout ing fanglomerate, debris flow deposits, and also this region (Reynolds, 1980; Anderson and Silver, lacustrine sediments (Figs. 5 and 6). These Tertiary 1979). The rapakivi granite is probably equivalent to volcanic and sedimentary rocks are well exposed in the the 1.4-1.5 b.y.B.P. suite of rapakivi granites seen northern Trigo Nountains in coherent sequences that all the way from Wisconsin to southern California strike northwest and dip northeast. They appear to be (Silver and others, 1977). correlative with similar volcanic and sedimentary rock sequences seen to the west of the Trigo Nountains in Intruded into the rapakivi granite and metasedi­ southeastern California (CrOlve and others, 1979) and mentary and meta-igneous rocks is a sequence of dia­ to the east of the Trigo Nountains in the Castle Dome base dikes and sills that have a distinctive ophitic and Kofa Nountains (Wilson, 1933). The Castle Dome texture. These rocks are probably equivalent to the and Kofa Nountains have sequences of what are probably 1.1 b.y.B.P. dike swarm that is seen throughout the caldera-related volcanic rocks as recently described western Cordillera (Burchfiel and Davis, 1981). by Gutmann (1981). Gutmann assigns these rocks an age of early to mid-Niocene on the basis of several K-Ar No Paleozoic rocks are known to exist in the ages. Trigo Nountains area but are present to the north in the Big Naria (Hamilton, 1964, 1971) and Plomosa The volcanic rocks in the Trigo Nountains may Nountains (Niller, 1970; Niller and NcKee, 1971) . also be correlative with large packages of tilted volcanic and sedimentary rock units that are seen In the southern Trigo Nountains a large body of throughout southwestern Arizona as described by Eberly Late Nesozoic Pelona-Orocopia Schist (Haxel and and Stanley (1978). Eberly and Stanley distinguish Dillon, 1978) is exposed in a north-plunging antiform. two major groups of Tertiary rocks: an older, tilted The Pelona-Orocopia rocks exposed in this antiform sequence and a younger, relatively untilted sequence. plunge beneath the Trigo Nountains and appear to have The volcanic and sedimentary rocks of the Trigo Noun­ Precambrian metasedimentary rocks overlying them along tains largely correlate with the older, tilted se­ what may be the Vincent-Chocolate Nountains thrust quence of Eberly and Stanley. fault. Also exposed in the Trigo Nountains is a nearly Intrusive into the other crystalline rocks in the flat-lying basaltic flow that Wilson (1960) mapped as central Trigo Nountains is a large body of Late Neso­ a Quaternary basalt. This basalt is probably correla­ zoic adamellite to granodiorite. These rocks were tive with the younger, untilted Tertiary sequence of assigned a Laramide age by Wilson (1960) and are so Eberly and Stanley (1978) and is probably actually shown on his map (Fig. 4). K-Ar dating by \veaver late Niocene or Pliocene in age as judged from re­ (1982) suggests that this is a valid age assignment. gional geochronologic relationships (Shafiqullah and Nost of the mineralization encountered in the Trigo others, 1980). This untilted volcanic rock unit is a Nountains occurs in the same central portion of the dark basaltic flow, which is compositionally distinct range as these Laramide intrusive rocks. from the older volcanic rocks.

Figure 5. View looking to the east of the northwest-striking,northeast-dipping Tertiary volcanic sequence in the northeasternmost Trigo Mountains. Note the repetition of the Tertiary sequence between the major ridges. The setion is repeated by northwest-striking, southwest-dipping normal faults that are inferred to join the detach­ ment fault. (Photo courtesy of J. B. Dahm.)

163 Along the wes tern edge of the Trigo Nountains, The detachment fault is exposed as a sharp con­ thick exposures of the late Niocene to early Pliocene tact (Figs. 2, 3, and 7) that, ,,,hen exhumed, forms a Bouse Formation (Netzger, 1968; Smith, 1970; Blair, ramp-like surface. This ramp-like surface consists of 1978) exist. The Bouse Formation records the Niocene lower-plate rocks that are highly chloritized, epido­ incursion of the Gulf of California into the area tized, and in places brecciated. This zone appears (Smith, 1970; Blair, 1978; Lucchitta and Suneson, correIative wi th the chlorite-breccia zone seen in 1980). Overlying the Bouse Formation is a thick other ranges further to the north (Davis and others, sequence of various alluvial units related to the 1980; Rehrig and Reynolds, 1980). erosion of the range. Rather than being a planar surface, the detach­ This overall package of Precambrian to Recent ment fault appears to be an undulating surface that is units appears to be correlative with similar sequences folded into a series of superposed northeast-trending of Precambrian to Recent rocks that are seen on a re­ and northwest-trending antiforms and synforms. Inter­ gional scale in southeastern California and western ference of the two superposed fold sets appears to re­ Arizona (Reynolds, 1980; Davis and others, 1979, 1980; sult in the "egg-carton" style of range culminations Anderson and others, 1979; Burchfiel and Davis, 1975, and basin depressions that typifies the upper crustal 1981). The rocks in the Trigo Nountains seem to pre­ structure of portions of this region. These antiforms sent a similar picture to that observed in most of the and synforms may largely account for the irregular other ranges within the region. In addition, the pattern of distribution of crystalline rocks and Ter­ Pelona-Orocopia Schist is present in the southern tiary rocks as shown on the geologic map of Yuma Trigo Nountains. Because the Pelona-Orocopia Schist County (Fig. 8). is such a widespread unit in southern California, the Trigo Nountains are significant in their joining Upper-Plate Faults together the regional geology of Arizona and southern California. Penetratively developed normal faulting is typi­ cal of the upper-plate structure of much of the detach­ STRUCTURE ment terrane (Davis and others, 1979, 1980). The Trigo Mountains are cut by a series of predominantly Detachment Fault Description northwest-striking normal faults, 'vhich dip to both the northeast and the southwest (Fig. 9). These The principal structural feature in the Trigo normal faults appear to record northeast- south,,,est Mountains is the regionally developed detachment extension of the range. The upper-plate normal faults fault of mid-Tertiary age. The detachment fault is (Fig. 3) cut the section into a series of very long best exposed in the west-central Trigo Mountains where and thin northwest-striking grabens and half-grabens, it separates an upper plate of Tertiary volcanic and repeating the Tertiary section and its upper-plate sedimentary rocks from a lower-plate assemblage that crystalline rocks many times. This style of extension consists largely of Precambrian metasedimentary and appears very similar to the kind of extension that is meta-igneous rocks. seen in other detachment terranes, such as in the

Figure 6. View of the Tertiary rock sequence in the vicinity of the Black Rock Mine in the central Trigo Mountains. The Tertiary rock units include a basal fanglomerate (Fg), a sequence of welded to non-welded tuffaceous rocks (Tf), and a basaltic rock unit (Bs). The Tertiary sequence is in fault contact with the Precambrian and Mesozoic crystalline rocks (Xl) of the area.

164 Figure 7. Closer view of the same (northwestern Trigo Mountains) fault surface as in Figure 3, showing well­ developed ra~p-like character of the detachment fault, highly chloritized lower plate, and tilted upper-plate volcanic rocks.

\Vhipple Hountains (Davis and others, 1979, 1980), HINERALIZATION where a distinctive corrugated pattern of alternating ridges that are repeated sequences of upper-plate The mineralization within the Trigo Hountains units is seen. These sequences progressively offset seems to be intimately related to the extensional the different Tertiary units in a northeast-southwest regime. In the south-central Trigos numerous mines, direction. which collectively make up the Silver District of Yuma County, have been worked since the last century In the northern Trigos most of the large faults (\olilson, 1933; Figs. 4 and 9). The largest among dip to the southwest with the overlying units rotated these include the Red Cloud Hine, the Black Rock Hine, so that they are now dipping to the northeast suggest­ and the Clip ~1ine. Some of these mines are either ing southwest-directed transport of the upper plate. currently in operation or being considered for reac­ In the southern Trigo Hountains the normal faults dip tivation. The mineralization usually occurs in brec­ both to the northeast and to the southwest, suggesting ciated fault zones in the crystalline rocks, within a both northeastward and southwestward motion of the few meters or tens of meters of the crystalline­ upper plate. volcanic contact (Fig. 10; from Thompson, 1925). These fault zones are often imbricate in the crystal­ Offset on most of the faults in both the northern line rocks and roughly parallel to the strike and dip and southern Trigo Hountains does not appear to be of the crystalline-volcanic contact (Fig. 11). great. Offset on individual faults appears to vary up to several hundred meters, suggesting that if trans­ Upper-plate mineralization of lead, zinc, and port across the detachment fault is the summation of silver is common in the Trigo Hountains (Wilson, all upper-plate motion, then the offset along the 1933). Hineralization at the Black Rock Hine is typi­ detachment fault may be on the order of kilometers, cal of mineralization in the range. Typical gangue but less than tens of kilometers. The amount of associations at the Black Rock Hine are vuggy quartz, apparent extension recorded in the upper plate of the blebs of ferruginous material, and abundant calcite. Trigo Hountains does not appear to be as great as that The calcite ranges from manganese stained and banded, seen elsewhere. to white and essentially featureless. It is exposed

165 Figure 8. Generalized geologic map of the Trigo Mountains showing the known and inferred location of the detachment fault. Lower-plate rocks are composed of Precambrian gneiss and rapakivi granite and Mesozoic intrusive rocks in the north with Pelona Schist to the south. Crystalline rocks are exposed in the cores of major northwest trending antiforms. Some of what is mapped as detachment faulting may be upper-plate structure. Modified from Wilson, 1960.

CllP " , . MH~E \~;// DCi I I

,/ \ -/ ! I DC I I \

Figure 9. Fault map of a portion of the central Trigo Mountains, showing general northwest trend of major normal faulting. Faults dip generally to both the northeast and southwest. These upper-plate normal faults form a series of grabens and half-grabens, and appear to record north­ east-southwest extension of the range. Adapted from Parker, 1966.

166 E

Figure 10. A. Diagrammatic cross section showing Red Cloud Mine developed along fault contact between crystalline and volcanic rocks. Volcanic rocks are shown in a graben­ like structure, related to mid-Tertiary extension. Adapted from Thompson, 1925. B. Vertical section view through the Red Cloud Mine perpendicular to the vein. Adapted from Wilson and others, 1951.

in veinlets of a few millimeters \vidth to layers of a high porosity and permeability, allowing the flow of several meters. A map illustrating some of the geo­ hydrothermal fluids through the system, thus con­ logic relationships at the Black Rock Nine has been centrating the mineralization along the major fault presented by the Arizona Geological Society (Dohms and zones. others, 1980). Faults in this area are normal and often form Important ore minerals consist of (argen­ graben and half-graben structures. The abundant tiferous?) galena, lead oxides, lead sulfates, and mineralized fault zones in the upper-plate crystalline carbonates of both lead and zinc (Wilson, 1933; rocks are oriented approximately parallel to each Parker, 1966). The mineralization appears to be low­ other. This geometry may be a localized expression of temperature hydrothermal alteration of the brecciated distributed shear felt in the upper plate due to the fault zones in the crystalline rock, and to be spa­ detaclunent event. Normal faults in the upper plate tially associated with nearby Hesozoic granodioritic probably join the detachment fault at depth, and could intrusions. Because the mineralization is Tertiary in form conduits for movement of mineralizing fluids. age, these Nesozoic plutons may have exerted primarily Thus the mineralization may be an indirect consequence a host-rock control on the mineralized areas. Frac­ of the detachment faulting. turing of the brecciated fault zones may have produced

167 Figure 11. View to northeast of the Black Rock }line, showing relationship of faulting and mineralization. Note positions of the: overlying volcanic rocks (Tvr), exposed crystalline-volcanic contact (Fault Surface), and mineralized zone in parallel fault in crystalline rocks, below contact (Black Rock Mine). Truck for scale courtesy of J.L. Corones.

INTERPRETATION Because the detachment fault is so well-developed in the Trigo Nountains, it "'ould seem that it should The presence of detachment faulting ",ithin the also be present in surrounding ranges, especially Trigo Nountains has several major ramifications for those between the Trigos and other areas of known the regional geology of Arizona and southern Cali­ detachment-related deformation. Recent study of many fornia. Extension of the detac~nent terrane to south­ of these ranges has confirmed this suggestion. Well­ ",esternmost Arizona significantly increases the kno",n developed detachment faults, or detachment-related extent of detachment-related deformation from the deformation (large-scale crustal folding and upper­ \~hipple and Bucks kin Nountains (Davis and others, plate extension) has been documented in the nearby 1979,1980; Dickey and others, 1980; Carr and others, Nohawk ~lountains (Haxel and others, 1982; Nueller and 1980), Ra\-Ihide Nountains (Shackelford, 1976, 1980), others, this volume), in the Baker Peaks-Copper Noun­ Riverside Nountains (Carr and Dickey, 1980; Hamilton, tains terrane (Pridmore and Craig, this volume), in pers. commun., 1980), and Harcuvar-Harquahala Noun­ the Castle Dome Nountains (Gutmann, 1981, this volume; tains (Rehrig and Reynolds, 1980). Because the detach­ Logan and Hirsch, this volume), in the Kofa Nountains ment fault in the Trigos is so similar to detachment (Dahm and Hankins, this volume), and in the Nidway faults present in these other areas, it suggests that Nountains of California (Berg and others, this vol­ the phenomenon of detachment faulting is much more ume). The presence of such well-developed detachment­ regionally developed than previously thought. The related deformation in these ranges indicates that presence of this style of mid-Tertiary deformation in this style of mid-Tertiary extension was alive and the south",estern corner of Arizona also raises the well in southwesterlooost Arizona and southeasternmost intriguing possibility that the detachment terrane may California. extend far into Sonora and also \-Iest of the late Tertiary San Andreas fault.

168 Hi thin this region, the area of known detachment I~here detachment-related deformation is present faulting (Fig. 12) extends from at least Las Vegas to in the Trigo Mountains region, offset between indi­ Sonora along the Colorado River and to southeastern vidual fault blocks does not appear to be great. Arizona and northern Sonora (Davis, 1980; Davis and Offset across the detachment fault does not appear to Coney, 1979). The detachment terrane may also be be great, therefore, if the collective offset of all related to extension within the Rio Grande Rift the upper-plate normal faults records the total offset (Riecker, 1979; Hawley, 1978). This style of mid­ on the detachment fault. Several adjacent ranges have Tertiary deformation is clearly present in numerous, opposing directions of stratal tilt as that present in isolated ranges and can be inferred to be present the Trigos (Berg and others, this volume; Logan and beneath valleys or over ranges where adequate expo­ Hirsch, this volume). Motion of npper-plate rocks sures exist. More and more such exposures, as in the toward each other does not allow for major amounts of Trigos, are continually being found, thus increasing tectonic transport. Having major rotation of the the known regional extent of the terrane. The region­ upper-plate rocks over an enormous region, together ality of this phenomenon suggests that this style of with opposing stratal tilts, may suggest that the extensional deformation may be penetratively developed upper plate has been rotated essentially in place by through the upper crus t of the entire region. The extensional motion of the lower plate. cause of detachment-related deformation would seem to be related to regional crustal extension, which oc­ The clear absence of any mylonitic rocks in curred spatially and temporally ",ithin the mid­ ranges such as the Trigos does not raise the question Tertiary arc. Ihthin the context of such crustal of the equivalence of detachment £:Wlting and myloniti­ extension, the extensional strain is probably better za tion that has plagued lvorkers in other regions. developed in some domains than others. Detachment­ Instead, the clear absence of a specific fabric ",ithin related deformation may not be present in some parts the lower plate suggests that 100"er-plate extension of Arizona, such as the Papago country (Haxe1, pers. took place by some other mechanism such as offset on a commun., 1981), but still present, or discernible, in multitude of lower-plate faults as seen in the other areas. The full extent of detachment-related Nel"berry ~lountains of Nevada Wathis, this volume). deformation within the southwestern United States and Another way of accommodating lower-plate extension northern Sonora is not, as yet, well known. would be by folding of the upper crust during detach-

Figure 12. Apparent extent of mid-Tertiary detachment faulting in California and Arizona. Detachment faults or their related upper-plate structures are well documented from south of the Las Vegas-Lake Mead Shear Zones to nearly the San Andreas Fault. The equivalence of detachment faulting to extension within the Rio Grande Rift and the presence of detachment faulting into Sonora is not, as yet, "'ell understood.

169 ment-related extension (Otton and Dokka, 1981; Cameron thesis: Am. Jour. Science, v. 275-A, p. 363-396. and Frost, 1981). HOle such folding is related to Burchfiel, B.C., and Davis, G.A., 1981, Hojave desert lower-plate extension and to the inferred multiplicity and environs, in The geotectonic development of of lower-plate faults is an unresolved question. California, Ernst, W.G., ed., p. 217-252. Cameron, T.E., and Frost, E.G., 1981, Regional devel­ The presence of well-developed folding of both opment of major antiforms and synforms coincident the detachment fault and the Pelona-Orocopia Schist in with detachment faulting in California, Arizona, the Trigos raises several interesting questions about Nevada, and Sonora: Geol. Soc. of America Ab­ the regional geology of southern California. If stracts with Programs, v. 13, no. 7. folding within the Pelona-Orocopia Schist is Carr, I';.J., and Dickey, D.D., 1980, Geologic map of detachment-related, then the widespread occurt'ence of the Vidal, California, and Parker SW, California­ such folds in southern California (Haxel and Dillon, Arizona quadrangles: U. S. Geol. Survey Hap 1978; Ehlig, 1981) may suggest that the mid-Tertiary 1-1125. extension seen in Arizona also affected parts of Carr, W.J., Dickey, D.D., Quinlivan, W.D., 1980, southern California, Subsequent offset along the San Geologic map of the Vidal NW, Vidal Junction, and Andreas system may have masked the mid-Tertiary defor­ parts of the Savahia Peak SW and Savahia Peak mation by making it attractive to equate folding of quadrangles, San Bernadino County, California: the Pelona-Orocopia Schist bodies to San Andreas­ U.S. Geol. Survey Hap 1-1126. related deformation. The degree of Tertiary over­ Coney, P. J., 1980, Cordilleran metamorphic core com­ printing on the Hesozoic Pelona-Orocopia Schist and plexes, in Tectonic significance of metamorphic Vincent-Chocolate Hountains thrust system is not well core complexes of the North American Cordillera, understood ei ther geologically or geochronologically. Crittenden, H.D., Jr., Coney, P.J., and Davis, I';ork in progress is aimed at determining the exact G.H., eds., Geol. Soc. America Hemoir 153. character of this mid-Tertiary deformation in southern Crittenden, H.D., Jr., Coney, P.J., and Davis, G.H., California and its effect on the interpretation of eds., 1980, Tectonic significance of metamorphic both Hesozoic structures (history of the Pelona Schist core complexes of the North American Cordillera: and Vincent Thrust) and late Tertiary deformation Geol. Soc. funerica Hemoir 153, 490 p. (amount of offset and character of deformation along CrOlee, B.E., Crowell, J.C., and Kummenacher, D., 1979, the San Andreas system). Regional stratigraphy, K-Ar ages, and tectonic implications of Cenozoic volcanic rocks, south­ ACKNOI';LEDGEHENTS eastern California: Am. Jour. Science, v. 279, p. 186-216. Our thanks to Jerry Dahm, Chris Holloway, and Dahm, J.B., and Hankins, D., 1982, Preliminary geology Vicki Stocker of San Diego State University for their of a portion of the southern Kofa Hountains, Yuma assistance in the field. \';e benefited greatly from County, Arizona: (this volume). discussion and debate with William Rehrig and Robert Davis, G.A., Anderson, J.L., Frost, E.G., and Wilson of Phillips Hinerals Group. Scott Fenby of San Shackelford, T.J., 1979, Regional Hiocene detach­ Diego State University drafted several of the figures. ment faulting and early Tertiary (?) myloni­ Huch support ''''as provided by Joe Corones and Lindee tization, Whipple-Buckskin-RaI"hide Hountains, Berg of San Diego State University. Gene Jensen of southeastern California and western Arizona, in Woodward-Clyde Consultants, San Diego assisted tremen­ Geologic excursions in the southern California dously in the preparation of the manuscript. Harty area, Abbott, P.L., ed., San Diego State Univer­ Fife of Harris Oil Consultants kindly provided the end sity, San Diego, California. graphic. Last, but certainly not least, we are espe­ Davis, G.A., Anderson, J.l., Frost, E.G., and cially grateful to Donna Hartin of San Diego State Shackelford, T.J., 1980, Hylonitization and University for her support of this project and her detachment faulting in the I';hipple-Buckskin­ efforts in producing this volume. Rawhide Hountains terrane, southeastern Califor­ nia and western Arizona, in Tectonic significance REFERENCES of metamorphic core complexes of the North Ameri­ can Cordillera, Crittenden, H.D., Jr., Coney, Anderson, J.L., Davis, G.A., and Frost, E.G., 1979, P.J., and Davis, G.H., eds., Geo!. Soc. 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170 Edson, G.M., 1980, The Red Cloud ~1ine, Yuma County, of the Silver district, Trigo Mountains, Yuma Arizona: Arizona Mineralogic Record, v. 11, County, Arizona: 1'1. S. thesis, San Diego State no. 3, p. 141-152. University. Ehlig, P.L., 1981, Origin and tectonic history of the Pridmoi'e, C., and Craig, C., 1982, Upper-plate struc­ basement terrane of the San Gabriel Mountains, ture and sedimentation of the Baker Peaks area, central Transverse Ranges, in The geotectonic Yuma County, Arizona: (this volume). development of California, W.G. Ernst, ed. 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Hamilton, W., 1971, Tectonic framework of southeastern Riecker, R.E., ed., 1979, Rio Grande Rift: tectonics California: Geol. Soc. of America Abstracts with and magmatism: American Geophysical Union. Programs, p. 130-131. Shackelford, T.J., 1976, Structural geology of the Haxel, G.B., and Dillon, J.T. ,1978, The Pelona­ Rawhide Mountains, Mohave Connty, Arizona: Orocopia Schist and Vincent-Chocolate Mountains Unpub. Ph.D. dissertation, University of Southern thrust system, southern California, ~Il Mesozoic California, Los Angeles, California, 175 p. paleogeography of the western United States, Shackelford, T.J., 1980, Tertiary tectonic denudation Howell, D.G., and McDougall, K.A., eds., Pacific of a Mesozoic-early Tertiary (1) gneiss complex, Section, Society Econ. Paleo. and Mineral., Rawhide Mountains, western Arizona: Geology, p. 453-469. v. 8, no. 4, p. 190-194. 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Yuma County, Arizona: Arizona !'lineralogical Logan, R., and Hirsch, D., 1982, Geometry of detach­ Record, v. 11, no. 3, p. 135-140. ment faulting and dike emplacement in the central Silver, L.T., Bickford, M.E., Van Schmus, W.R., Castle Dome Mountains, Yuma County, Arizona: Anderson, J.L., Anderson, T.H., and Medaris, (this volume). L.G., Jr., 1977, The 1.4-1.5 b.y. trans­ Lucchitta, I., and Suneson, N., 1980, The !'liocene continental anorogenic plutonic perforation of Arti llery and Chapin Wash Formations of \vest­ North America: GeoI. Soc. of America Abstracts central Arizona: identification and signi f­ with Programs, v. 9, no. 7, p. 1176-1177. icance: GeoI. Soc. of America Abstracts with Smith, P.B., 1970, New evidence for a Pliocene marine Programs, v. 12, no. 6, p. 279. embayment along the lower Colorado River area, Nartin, D.L., Barry, Iv.L., Krummenacher, D., and California and Arizona: Geol. Soc. America Frost, E.G., 1980, K-Ar dating of mylonitization Bull., v. 81, p. 1411-1420. and detachment faulting in the Ivhipple Hountains, Thompson, A.P., 1925, The Silver mining district in San Bernadino County, California, and the Yuma County: Arizona Mining Jour., v. 8, no. 16. Buckskin Mountains, Arizona: Geol. Soc. of Iveaver, B.F., 1982, Reconnaissance geology and K-Ar America Abstracts with Programs, v. 12, no. 3, geochronology of the Trigo Mountains detachment p. 118. terrane, Yuma County, Arizona: M.S. thesis, San Nathis, R.S., 1982, Nid-Tertiary detachment faulting Diego State University. in the southeastern Newberry ~lountains, Clark Ihlson, E.D., 1933, Geology and mineral deposits of County, Nevada: (this volume). southern Yuma County, Arizona: Arizona Bur. Metzger, D.G., 1968, The Bouse Formation (Pliocene) of Mines Bull. 134, 236p. the Parker-Blythe-Cibola area, Arizona and Cali­ Ihlson, E. D., 1951, Arizona zinc and lead deposits; fornia: U.S. GeoI. Survey Prof. Paper 600-D, Silver and Eureka districts: Arizona Bur. Mines p. D126-D136. Bull. 158. !'liller, F.K., 1970, Geologic map of the Quartzite Ivilson, E.D., 1960, Geologic map of Yuma County, Ari­ quadrangle, Yuma County, Arizona: U.S. Geol. zona: Arizona Bureau of Mines. Survey Map GQ 841. Yeats, R.S., 1968, Southern California structure, sea Miller, F.K:, and NcKee, E.H., 1971, Thrust and strike­ floor spreading, and history of the Pacific slip faulting in the Plomosa Nountains, south­ basin: Geol. Soc. America Bull., v. 79, western Arizona: Geol. Soc. America Bull. , p. 1693-1702. v. 82, p. 717-722. Nueller, K., Frost, E.G., and Haxel, G., 1982, Detach­ ment faulting in the Mohawk Mountains, Yuma County, Arizona: (this volume). Otton, J.K., and Dokka, R .. , 1981, The role of stretch folding in crustal extension in the Moj ave and Sonoran Deserts: a low strain rate model: Geol. Soc. of America Abstracts with Programs, v. 13, no. 7, p. 524. Parker, F.Z., 1966, The geology and mineral deposits

171

GEOLOGY OF GRANODIORITE IN THE COXCO~lB MOUNTAINS, SOUTHEASTERN CALIFORNIA

J. P. CALZIA U.S. Geological Survey 345 Middlefield Road, MS 80 Menlo Park, California 94025

ABSTRACT COUNTRY ROCKS

McCoy Mountains Formation of Miller (1944) The Coxcomb Granodiorite of Miller (1944) is exposed over 15S sq km in the Coxcomb Mountains of The McCoy Mountains Formation in the southern southeastern California. The Coxcomb Granodiorite and central Coxcomb Mountains consists of gray to intrudes the McCoy Mountains Formation of Miller green fine-grained meta-sandstone and meta-silt­ (1944) and includes pendants of meta-igneous rocks stone that locally becomes schist and phyllite. mapped as the Jurassic quartz monzonite of the Pinto The meta-sandstone includes lenticular beds of and Eagle Mountains by Hope (1966). The McCoy Moun­ pebble conglomerate near the base of the section tains Formation is deformed by at least two episodes and meta-limestone lenses throughout the section. of folding and is metamorphosed to greenschist. The meta-sandstone is overlain by maroon fine­ Hornblende hornfels assemblages reported along the grained tuffaceous meta-sandstone that is overlain contact between granodiorite and greenschist limits by meta-andesite. Contacts between the meta­ the P-T conditions during emplacement to greater sedimentary and meta-volcanic rocks generally are than 520°-540°C and 0.5 to 2.0 kbars. conformable with local slight angular discordance. Metamorphic foliation generally parallels the The Coxcomb Granodiorite is subdivided into original bedding of the clastic deposits. four intrusive facies by textural, modal, and chemi­ cal data. The intrusive facies include (oldest to The McCoy Mountains Formation tentatively is youngest) biotite-hornblende granodiorite, porphy­ assigned a Jurassic(?) age for this report. These ritic biotite granodiorite and monzogranite, and rocks unconformably overlie Jurassic rhyodacite biotite-muscovite monzogranite. The contact between porphyry in the Palen Mountains (Pelka, 1973, p. the oldest granodiorite and the porphyritic rocks is 101) and are intruded by the Coxcomb Granodiorite. gradational. The porphyritic rocks are intruded by Harding and others (1980) assign a pre-Upper monzogranite dikes. The monzogranite intrudes a Jurassic age to the McCoy Mountains Formation fourth facies of porphyritic biotite granodiorite. based on paleomagnetic evidence. Chemical data suggest that this granodiorite crysta­ llized from a more primitive magma and may be older The McCoy Mountains Formation is deformed than the other facies. Intrusive contacts to prove by at least two episodes of folding. The first this relation are lacking. episode is characteized by tight, nearly isoclinal folds. The axes of these folds are refolded Major element chemical data indicate that the around a broad anticline that plunges west. The Coxcomb Granodiorite crystallized from an alkali­ first-generation folds now represent minor folds calcic magma. Chemical variation between facies over the crest of this broad structure. The shows progressive enrichment of K20 and depletion Coxcomb Granodiorite was emplaced within the core of the other oxides with increasing Si02 • Succes­ of the anticline. sive facies become more peraluminous by fraction­ ation of hornblende and andesine. Greene (1968 p.23) reports that the meta-sedi­ mentary and meta-volanic rocks are regionally New Rb/Sr data indicate that the Coxcomb Grano­ metamorphosed to greenschist throughout the south­ diorite is in part Late Jurassic in age. Inital ern Coxcomb Mountains. A 90 to 120 m-wide contact 87Sr/ 86Sr is 0.7089. Scatter on the Rb/Sr isochron aureole between granodiorite and greenschist suggest that some of the monzogranite crystallized consists of quartz+plagioclase(An25-35)+biotite+ from a different magma. The age of this separate hornblende. This assemblage grades outward to magma is unknown. quartz+plagioclase+biotite+epidote in the meta­ clastic rocks and from quartz+plagioclase+ INTRODUCTION grossularite+diopside+epidote to quartz+plagio­ clase+calcite+epidote~tremolitein the calcareous The Coxcomb Granodiorite of Miller (1944) is lenses (Greene, 1968). exposed over 155 sq krn in the Coxcomb Mountains of southeastern California. These rocks include Creta­ Winkler (1967, p. 79) reports that the assem­ ceous granodiorite and granite that eroded to form blage diopside+quartz+calcite is diagnostic of the precipitous skyline that is the namesake of the hornblende hornfels facies. The beginning the range. The Coxcomb Granodiorite intrudes the of this facies may be defined by the reaction McCoy Mountains Formation of Miller (1944) and epidote+calcite+quartz * grossularite+H20 includes pendants of Jurassic meta-igneous rocks that is in equilibrium from 520°-540°C ~ 20°C and (fig. 1). These granitic rocks may be subdivided 0.5 to 2.0 kbars (Winkler, 1967, P. 72). These into four intrusive facies by petrographic and equilibrium data define the minurnum P-T conditions chemical data. This report describes the geology in the wall rocks during emplacement of the Coxcomb of the Coxcomb Granodiorite and the relation of Granodiorite. the granodiorite to the country rocks in the Coxcomb Mountains.

173 FIGURE 1. GENERALIZED GEOLOGIC MAP OF THE COXCOMB MOUNTAINS,

SOUTHEASTERN CALIFORNIA

LOCATION MAP 115""5' 117'" 115" 1 I

, G , Los " ",Vegas 3>0 , CALIFORNIA

OLos ArlgelB5

34°_1

...... ,...... ~ , '$cgm Jm " 34°00'_ , c DESCRIPTION OF MAP UNITS Jmi '25 "CO < ALLUVIAL DEPOSITS (Quaternary) - Unconsolidated stream terrace and alluvial fan deposits. " EJ 'Y'" VOLCANIC ASH (Quaternary) - Ash from Bishop eruption 8 described by Merriam and Bishoff (1975). FANGLOMERATE DEPOSITS (Quaternary or Tertiary) ­ Stratified cobbles to boulders of granitic and meta­ EJ sedimentary rocks. OLIVINE BASALT ( Quaternary or Tertiary) - Vesicular, a interbedded with fanglomerate deposits UncoILformity I COXCOMB GRANODIORITE OF MILLER (1944) / BIOTITE-MUSCOVITE MONZOGRANITE (Cretaceous and/or JKcm Jurassic) - Gray, fine- to medium-grained, equigranular. ~~ Jcgm PORPHYRITIC GRANODIORITE AND MONZOGRANITE (Jurassic) ­ Gray, medium- to coarse-grained. K-feldspar pheno­ crysts zoned with plagioclase inclusions. Jcg BIOTITE-HORNBLENDE GRANODIORITE (Jurassic) - Gray I 65 mediurn- to coarse-grained, equigranular. OTb-_ ) 26 OTD __ Jcgp PORPHYRITIC GRANODIORITE (Jurassic) - Gray, medium­ OTb 41 ___ ~o Dncan fonuity to coarse-grained, sheared. ,0 OTt 65~J_m \0 META-IGNEOUS BaCKS (Jurassic) - Dark, medium-grained. Unit :£1) =::J __ ~o B mapped as quartz monzonite of Pinto and Eagle Mountains ~ by Hope (1966).. K-Ar age of 167 m.y. from biotite. ~o~Q B META-TUFF AND META-ANDESITE (Jurassic?) .. 33°55'- OTt-V McCOY MOUNTAINS FORMATION OF MILLER (1944) (Jurassic?) ­ G schist and phyllite of meta-sedimentary rocks. "o v o ~ o 75 / Contact showing dip, dashed where approximately located. >/ ?3~ 63 •• ~ Fault showing dip dotted where concealed. 0, downthrown 9Y' side.

~ Slip fault of slide block, sawteeth on upper plate ''V'Pc-e>

~ 26 .." O"c-'-...... c.n Y Strike and dip of bedding

~y Strike and dip of foliation. Open arrow, metamorphic foliation; dark arrow, magmatic foliation.

12 /' Bearing and plunge of lineation

y Strike and direction of dip of joints and plunge of lineation SCALE J cg o 2 3 Miles ·~Jm I " I I a 2 3 4 Kilometers

33°50'_

A 6000'

4000 Oal Oal Oal 2000~

S.L Jeg m J Kem J egp Jcgm J egm

Jcgp • Jcgm Meta-igneous Rocks diorite dikes with K-feldspar+biotite+hornblende phenocrysts are mapped in an isolated outcrop of Pendants and fault blocks of meta-igneous rocks granodiorite southeast of the eastern adit of the in the northern Coxcomb Mountains consist of foli­ Colorado River Aqueduct. ated, medium-grained biotite-amphibole monzogranite. Foliation is expressed by the alignment of the long The biotite-hornblende granodiorite intrudes dimensions of biotite. Biotite is pleochroic from the McCoy Mountains Formation to the south and brown to very dark brown. Amphibole (hornblende?) grades into porphyritic granodiorite to monzogran­ has reacted with magma to form secondary biotite ite to the north. The contact with the meta­ and is completely altered to chlorite+epidote. sedimentary rocks generally is sharp and parallels the foliation within the older rocks. The west end The meta-igneous rocks in the Coxcomb Mountains of this contact is characterized by a 30 m-wide are mapped as the Jurassic quartz monzonite of the zone of en echelon granodiorite dikes in the meta­ Pinto and Eagle Mountains by Hope (19p6, Plate 1). sedimentary rocks. A contact facies of granodio­ K-Ar ages of 167 m.y. (Hope 1966, p.37) and 152.8 rite without mafic minerals locally is developed m.y. (Armstrong and Suppe 1973, spl. SC-69-12) are between the granodiorite and meta-sedimentary rocks obtained on biotite from this quartz monzonite in southeast of the aqueduct. the Pinto Mountains. Biotite-hornblende granodiorite is separated COXCOMB GRANODIORITE OF MILLER (1944) from porphyritic granodiorite and monzogranite to the north by a pendant of folded meta-sedimentary The Coxcomb Granodiorite of Miller (1944) con­ rocks. Where exposed, the contact between grano­ sists of four intrusive facies within the COXCO[@ diorite and porphyritic rocks is sinuous, grada­ ~\ountains. These facies include from south to north tional, and characterized by coarse-grained leuco­ (and probably oldest to youngest): biotite-horn­ granite that is free of mafic minerals and is blende granodiorite, porphyritic biotite granodio­ locally porphyritic. Intrusive relations between tite and monzogranite, and biotite-muscovite monzo­ the granodiorite, leucogranite, and porphyritic granite (fig 2). A fourth facies of locally rocks are confusing as evidenced by apophyses of porphyritic biotite granodiorite probably is older granodiorite in leucogranite, stoped blocks of than the other facies although intrusive contacts leucogranite in granodiorite and granodiorite in to prove this relation are lacking. porphyritic rocks, and dikes of porphyritic rocks in the granodiorite. Chemical and isotopic evidence suggest that the granodiorite and por­ phyritic rocks probably represent successive and nearly synchronous intrusive facies from a common magma.

The central third of the Coxcomb Granodiorite consist of porphyritic biotite granodiorite and Granodiorite Quartz monzogranite. A northeast-trending central band Diorite of porphyritic monzogranite grades both north and o south into granodiorite that is locally porphyritic. A single intrusive contact between these rocks o 0.(;) indicates that the central porphyritic monzogranite n CB crystallized later than the locally porphyritic . EB------'-"--- granodiorite along the margins.

Quartz Quartz Magmatic foliation in the porphyritic rocks MOf1zon de Monzodiorite or sou~h­ Monzogabbro locally is well developed and dips to the southwest. This foliation is expressed by com­ postional layers of biotite and quartz+K-feldspar Monzodiorite} [,,10rl!Unlte lYlonl0 lJobhro as well as by the alignment of biotite. Quartz+ K-feldspar pegmatites are found near the margins A p of the porphyritic rocks. A quartz+K-feldspar+ Figure 2. Modal mineral classification and biotite+muscovite pegmatite parallels the contact nomenclature of the Coxcomb Granodiorite.() =por­ with the biotite-muscovite monzogranite to the phyritic granodiorite, .. =biotite-hornblende grano­ north. diorite,E8 =porphyritic granodiorite and monzo­ granite,O =biotite-mu.5covite monzogranite. Q+P+A= Microcline phenocrysts in the central porphy­ 100% (from Streckeisen, 1973). ritic monzogranite are 2 to 7 em long. Zones of plagioclase inclusions are parallel to the edge of Field Relations these phenocrysts. Dickson and Sabine (1968) report that the phenocrysts are zoned with alternating The southern third of the Coxcomb Granodiorite Ba-rich and Ba-poor layers that parallel crystal consists of biotite-hornblende granodiorite that is faces. The Ba-rich layers contain approximately 3% remarkably uniform in texture and composition. This BaO (Marshall Reed, 1980, personal communication). rock generally is massive, although Miller (1944, p. 64) and Hope (1966, p. 43) describe a local, The porphyritic rocks are intruded by equi­ poorly developed foliation expressed by the align­ granular monzogranite. The contact between these ment of biotite. Quartz+K-feldspar+biotite peg­ rocks is characterized by a 3 km-wide zone of en matites are mapped near fault zones and near the echelon monzogranite dikes. The monzogranite margins of the granodiorite. En echelon grano- grades into biotite-muscovite monzogranite to the northwest.

176 Foliation and a well-developed lineation are Modal quartz and microcline increase at the expense mapped along the northern margins of the monzogran­ of plagioclase. Microcline crystals are perthitic ite. The foliation is expressed by layers of with inclusions of quartz and plagioclase. Plagio­ biotite. The lineation is expressed either by the clase crystals are twinned and zoned with more alignment of the short dimension of biotite, quartz, calcic cores locally altered to sericite. The and feldspar or by the long dimension of stretched anorthite content of the crystals as well as the quartz on polished joint surfaces. Well developed, inclusions remains within the andesine range for closly-spaced joints that are remarkably continuous all facies. along strike trend northwest through both the two-mica monzogranite and the porphyritic rocks. Mafic minerals include biotite and hornblende. Pegmatites of quartz+K-feldspar to the south and Biotite is ubiquitous to all intrusive facies and quartz+K-feldspar+biotite+muscovite to the north­ pleochroic from yellow-brown to dark brown and dark northwest are common in the monzogranite. olive-green. The ratio of biotite to hornblende is approximately 2.5 to 1 in the biotite-hornblende Biotite-muscovite monzogranite intrudes a granodiorite and the granodiorite dikes southeast locally porphyritic biotite granodiorite that is of the aqueduct. in fault contact with porphyritic rocks to the south. The biotite granodiorite is characterized Hornblende is altered to chlorite+epidote by zoned K-feldspar phenocrysts up to 1 em long and is resorbed without reaction rims in the grano­ and by pervasively sheared outcrops. Major­ diorite. Subhedral to euhedral hornblende in the element chemical data suggest that the sheared granodiorite dikes is altered to epidote and is granodiorite crystallized from a more primitive, pleochroic from green to yellow brown. These less differentiated magma than other facies of hornblende crystals have reacted with the melt to the Coxcomb Granodiorite. form groundmass biotite~rutile followed by magne­ tite as the reaction continues. The groundmass Petrography biotite is characterized by slight pleochroism from colorless to greenish brown and by anomalous inter­ The texture of the Coxcomb Granodiorite is ference colors. hypidiomorphic-granular in thin section. The biotite-hornblende granodiorite and biotite grano­ Both primary and secondary muscovite are found diorite and monzogranite facies are medium- to in the porphyritic rocks and in the northern monzo­ course-grained. The biotite-muscovite monzogranite granite. Primary muscovite is fine-grained, euhed­ is fine-grained and equigranular. Perthite is ral and represents 1% of the porphyritic rocks and seen in all facies. Myrmekite is restricted to 1-3% of the monzogranite. Secondary muscovite is the porhyritic rocks and the biotite-muscovite subhedral and forms as a reaction product between monzogranite. contiguous biotite and K-feldspar.

The modal mineralogy of the Coxcomb Granodio­ Zircon is found in all intrusive facies of the rite is listed in Table 1. Samples are listed from Coxcomb Granodiorite. Magnetite and apatite are south to north to recognize modal and/or chemical found in the biotite-hornblende granodiorite and variation within and between the intrusive facies. the porphyritic facies. Resorbed sphene is re­ In general, each successive facies becomes enriched stricted to the biotite-hornblende facies and the in the silicic minerals (quartz+K-feldspar+plagio­ sheared granodiorite. Miller (1944, p. 64) and clase) and deficient in the mafic (Fe-Mg) ;inerals. Hope (1966, p. 43) report allanite in the biotite­ hornblende granodiorite.

TABLE 1. MODAL ANAYLSES (VOLUME PERCENT) OF THE COXCOMB GRANODIORITE

Porphyritic Porphyritic Biotite-Muscovite Granodiorite Biotite-Hornblende Granodiorite Granodiorite and Monzogranite ~lonzongranite

Sample nO. B8 05 03 M3 K4 K5 Il 16 E4 D2A E2A B2 C3 24

Quartz 20.8 20.1 18.4 19.1 22.7 20;5 21.0 27.7 25.3 19.1 21.0 24.4 29.6 22.2

K-feldspar 1 19.5 20.2 17.4 19.5 19.8 19.8 15.7 23.6 21.9 24.9 28.2 35.1 31.7 33.4

Plagioclase 50.1 47.3 50.4 52.0 45.1 47.8 50.2 41.2 42.9 50.1 44.7 37.1 35.9 41.3 (An) (33-37) (32-35) (35-43) (34-37) (30-34) (33-35) (30-32) ( 38)

Fe-Mg Minerals1 9.6 12.1 12.3 9.3 1l.5 11.8 13.1 7.5 9.9 5.9 5.8 3.4 1.9 2.5

Other 2 0.3 1.5 0.1 0.9 0.1 0.3 0.9 0.6

Total points counted 907 622 1003 702 987 996 999 347 403 644 944 615 1098 888

1 See text for description. 2 Includes accessory and alteration minerals. See text for description.

177 Major Element Chemistry The Coxcomb Granodiorite becomes more peralu­ minous with magmatic differentiation. The molar Chemical analyses of the samples listed in ratio A1Z03/[CaO+NaZO+KzOj (abbreviated A/CNK) Table 1 indicate that the Coxcomb Granodiorite is increases from 0.97 in the biotite-hornblende enriched in A1Z03 and NaZO and deficient in FeO, granodiorite to 1.09 in the biotite-muscovite PZ05' and MnO when compared to average granodiorites monzogranite. This peraluminous trend may be and adamellites listed by Nockolds (1954). The explained by fractionation of hornblende and by percent FeO and MnO are approximiately 1/4 to l/Z the decrease in modal andesine during crystalliza­ the FeO and MnO content of average granitic rocks. tion. Fractionation of these phases in the ratio andesine/hornblende of Z/l controls the peralumi­ Chemical variation diagrams suggest that the nous trend shown in Figure 4. coxcomb Granodiorite represents continuous magmatic differentiation of an alkali-calcic magma from AGE the sheared porphyritic granodiorite to the biotite­ muscovite monzogranite (fig. 3). The percent The Coxcomb Granodiorite includes both Jurassic oxides are grouped tightly within successive facies. and Cretaceous rocks. The biotite-hornblende Chemical variation between facies shows progressive granodiorite and porphyritic rocks tentatively are enrichment of KZO and depletion of the other assigned a Late Jurassic age based on new Rb/Sr oxides with increasing SiOZ' The percent NaZO data. The biotite-muscovite monzogranite may be and MnO are constant, Jurassic, Creataceous, or may include both Jurassic and Cretaceous rocks.

, C 5.0 17(' AlzO, 0 No,O .0. 0 EB $B EB 0 .0... 0 0>- 4.0 16.C · EB * · · JiI,. * JiI,. ~ ... 0 r-----r. 1-----'- --.- r--- 1 •1 KzO 5.0 • 0 ... 0 71·~-'------~-[ ... • 0 1 I 1 I 1 DO 4.0 o ··0· ClI F 0 JiI,. LZ 3 ERB 1.0 EB .. JiI,. EB *0 ..· · 3.0 ffi\ EB 0.5 • DO 0

~ W I, ... 0 TiO z 0.6 0 0 8 . I 00 ", X ... d .A JiI,. X . FeO 0.4 0 0 ••0 ... · - • ... 10 ::!2 .' 0 ::!2 0 EB EB • l- · 2 l- I 0.5 - EB EB I .. DO (') ~ EB 0 - DO - W 0 w ~~-~'~f~-'--~- .-----r-----,-- 0.2 ~ ...... JiI,. PzOs ~ o . . JiI,. gO *0 . ... 10 - ... ·. . • 015 ... 0 0.5 - @ 01 ( EB DO EB 0 EB ffi Dr.! r.! ~T------, .-.------r .---'-- I I ! *~ . CoO 0 005 ... . JiI,. MnO 3.0 '* • · EB ffi 0 0 0.04 2.0 - EBEB ... 0 0 0 0000. EB Ef$ [) 003 1.0 - 0 n ~----- I+i e--- ~--T------r ~"--I 1 ---.-- 1 I I I 66 68 70 72 74 66 68 70 72 74

VVEIGHT%SiU 2 WEIGHT%Si0 2

Figure 3. Major element chemical variation diagrams of the Coxcomb Granodiorite. Same symbols as Figure Z plus * =average hornblende-biotite granodiorite, A=average biotite granodiorite, " =average biotite adamellite, • =average muscovite-biotite adamellite. Chemical data for average rocks from Nockolds (1954).

178 1.5

ANDESINE PERALUMINOUS 1.0 METALUMINOUS

0.5

HORNBLENDE

40 50 60 70 80 WEIGHT o/OSi02

Figure 4. Peraluminous trend of the Coxcomb Granodiorite. Same symbols as Figure 2.

A three point whole rock Rb/Sr isochron yields 0.712 a Late Jurassic age of 145.4 m.y. and an inital o o 87Sr/86sr of 0.7089 (fig. 5). Scatter shown on the isochron suggest different monzogranite magmas. Additional field, chemical, and isotopic data are required to subdivide the monzogranite and to ... (f) define the age(s) of this rock. ill 0.710 OJ t= 145.4m.y. K-Ar ages from the Coxcomb Granodiorite vary '-.... " from 65.1 to 70.8 m.y. (Table 2). These ages may (f) I'- rj =0.7089 not represent emplacement ages because the monzo­ OJ granite yields discordant K-Ar ages of 54.9 and 68.8 m.y. on biotite and muscovite for the same sample. 0.708 I Miller and Morton (1980, p. 18) report that dis­ 0.5 /.0 15 cordant K-Ar ages west of the Coxcomb Mountains 87Rb / 86Sr may be the result of a Late Cretaceous to early Tertiary thermal disturbance. K-Ar data in Table Figure 5. Rb/Sr isochron diagram of the Cox­ 2 suggest that this disturbance may extend into comb Granodiorite. Rb/Sr data from R. W. Kistler the Coxcomb Mountains. (1981, written communication).

TABLE 2. POTASSIUM-ARGON AGES OF THE COXCm1B GRANODIORITE

Location 40 Rock N. latitude, Mineral Ar Apparent -10 rad Description \'1. Longitude dated %K 2 O (10 moles/gm) %40Arrad Age (m.y. ) Reference

Biotite-muscovite 34°06'48", Biotite 8.99 8.59 85 65.1+2.0 Calzia and Morton monzogranite 115°31'35" (1980)

Biotite-muscovite 34°55'33", do 6.36. 5.10 73 54.9+1.5 Armstrong and granodiorite 115°24'28" Suppe (1973) Muscovite 9.92 10.01 78 68.8+1.0

11 Biotite-hornblende 33°55'32 , Biotite 8.10 8.42 79 70.8+1.0 Do granodiorite 115°18'07"

179 REFERENCES CITED

Armstrong, R. L., and Suppe, J., 1973, Potassium­ argon geochronometry of Mesozoic igneous rocks in Nevada, Utah, and southern California: Geological Society America Bulletin, v. 84, p. 1375-1392. calzia, J. P., and Morton, J. L., 1980, Compilation of isotopic ages within the Needles lOx2° Quadrangle, California and Arizona: U.S. Geological Survey Open-File Report 80-1303. Dickson, F. W., and Sabine, C., 1968, Barium-zoned large K-feldspars in quartz monzonites of eastern and southeastern California: Geological Society America Special Paper 115, p. 323. Greene, R. P., 1968, Metamorphosed McCoy Mountains Formation, Coxcomb Mountains, California: Santa Barbara, University of California: M.A. Thesis, 50 p. Harding, L. E., Coney, P. J., and Butler, R. F., 1980, Paleomagnetic evidence for a Jurassic age assignment on a 6 km thick sedimentary terrane in SW Arizona and SE California: Geological Society America Abstracts with Programs, v. 12, p. 109. Hope, R. A., 1966, Geology and structural setting of the eastern Transverse Ranges, southern california: Los Angles, University of California Ph.D. Dissertation, 158 p. Merriam, Richard and Bischoff, J. L., 1975, Bishop Ash: A widespread volcanic ash extended to southern California: Journal Sedimentary Petrology, v. 45, p. 207-211. Miller, F. K. and Morton, D. M., 1980, Potassium­ argon geochronology of the eastern Transverse Ranges and southern Mojave Desert, southern California: U.S. Geological Survey Professional Paper 1152, 30 p. Miller, W. J., 1944, Geology of the Palm Springs­ Blythe strip, Riverside County, California: California Journal Mines and Geology, v. 40, p. 11-72. Nockolds, S. R., 1954, Average chemical compositions of some igneous rocks: Geological Society America Bulletin, v. 65, p. 1007-1032. Pelka, G. J., 1973, Geology of the McCoy and Palen Mountains, southeasten California: Santa Barbara, University of California Ph.D. Dissertation, 162 p. Streckeisen, A. L., 1973, Plutonic rocks classifica­ tion and nomenclature recommended by the lUGS Subcommission on the Systematics of Igneous Rocks: Geotimes, v. 18, n. 10, p. 26-30. Winkler, H. G. F., 1967, Petrogenesis of metamorphic rocks: New York, Springer-verlag, 237 p.

180