JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 85, NO. B5, PAGES , MAY 10, 1980

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 85, NO. B5, PAGES , MAY 10, 1980 STRUCTURE, TECTONICS, AND STRESS FIELD OF THE COSO RANGE, INYO COUNTY, CALIFORNIA Glenn Roquemore Department of Geological Sciences, University of Nevada-Reno, Reno, Nevada Earth and Planetary Sciences Division, Naval Weapons Center, China Lake, California Abstract. The tectonics of the Coso Range has been described as having arcuate and ring faults both suggesting the presence of a circumscribed subsidence bowl or calderalike feature. New information suggests the Coso Range is situated in an area of transition between the stress of the right slip San Andreas fault-plate interaction and the extensional tectonics of the Basin and Range. Arcuate faults in the Coso Range are interpreted to have been produced by the regional stress field rather than to have been of volcanogenic origin. Focal mechanisms of small-magnitude earthquakes support the stress directions indicated by local fault patterns. Fumeroles in the area are primarily associated with oblique slip faults rather than with arcuate or ring faults. The geothermal reservior is therefore much different from that of a caldera or subsidence bowl, and the overall geothermal potential is probably less than earlier estimates. Introduction This paper, based primarily on original geologic and geomorphic information, provides the basis for a synthesis of present-day tectonics in the Coso Range region of southeast California. This interpretation is relevant to the assessment of the potential of this known geothermal resource area [Goodwin et al., 1971]. Previous studies of Coso Range tectonics include discussions of arcuate faults. Austin et al. [1971] and Koenig et al. [1972] noted the presence of these features, which they believed may indicate a caldera; Duffield [1975] proposed ring faulting forming a circumscribed structural subsidence bowl which defines a calderalike feature as being of primary importance in understanding the tectonic-volcanic history of the Coso Range. Roquemore [1978a] described Basin and Range structures in the Coso Range and states that there is no clear evidence for ring faults or caldera formation. On the basis of seismicity and active faulting, Roquemore [1978b] proposed that the Coso Range lies in a complex zone of transition between the Sierra Nevada and the Basin and Range province, as predicted by Wright [1976]. This paper expands on this topic and considers the relationships between tectonism, arcuate faulting, and hot springs. Four major points that relate to the tectonic interpretation of the Coso Range are the following: (1) The structure is consistent with Basin and Range stress patterns. (2) The rate and style of vertical and horizontal displacement suggests transition between strike slip tectonics to the west and extensional tectonics to the east. (3) Arcuate faulting results from the strike slip component of regional stress in the Coso Range. (4) The fumerolic activity is controlled largely by faults with a significant oblique slip component and thus is probably unrelated to caldera formation. Each of these points is discussed in the following subsections. Structure Consistent With Basin and Range Stress Patterns Graben Valleys and Tilted Blocks Rose Valley (Figure 1) separates the Coso Range from the Sierra Nevada. According to Healy and Press [1964] it represents a southward extension of the Owens Valley graben, a 240-km zone along the Sierra front. Their interpretation is consistent with later work by D. B. Slemmons (private communication, 1979), who found that faults in the southern Owens Valley connect with those mapped in Rose Valley by Allen et al. [1965] and Roquemore [1977, 1980]. The valley fill in the center of Rose Valley is over 1670 m thick, which indicates substantial downfaulting on the valley margins. The west side of Rose Valley is bounded by the northeast striking Sierra Nevada frontal fault zone, where Duffield and Smith [1978] report over 1200 m of vertical movement. To the east of Rose Valley there are several step-faulted, west tilted blocks of the Pliocene Coso Formation [Power, 1958]. The faulting in Rose Valley is high-angle normal slip and results in low to moderately tilted fault blocks. Coso Valley (Figure 1) is in the south central portion of the Coso Range. This graben valley is bounded on the west by the Airport Lake fault zone (Figures 1 and 2), which is a left stepping, en echelon, range-front fault zone. Two asymmetrical graben structures are within the zone of faulting, one of which is nearly 2 km wide (Figure 3). The Airport Lake fault strikes NlO ø to N20øE and dips from 50OE to vertical. Bounding the east side of Coso Valley is the highly step-faulted Wild Horse Mesa. Along this zone, thin sheets of 3.0-m.y.-old basalt and andesite lava [Duffield et al., 1980] are broken by high-angle normal faults with a sinuous, left stepping, en echelon pattern. The Coso Hot Springs fault (east of CHS in Figure 1) strikes N25OE with a dip of 45 to 55 SE and bounds Coso Valley to the north. It is actually part of the Airport Lake fault (Figure 1). The left stepping en echelon displacement along this zone is consistent with orientation of the maximum compressive stress at N15øE to N25OE and the minimum compressive stress at N65 This paper is not subject to U.S. copyright. Pub- to N75oW. These stresses are shown together lished in 1980 by the American Geophysical Union. with a detailed schematic drawing of faults in Paper number 9B / 80 / 09B ]. O0 2434

2 Roquemore: Coso Geothermal Area ø ø Fig. 1. Index and fault map of the Coso Range and adjacent areas: CF, Cactus Flat; MF, McCloud Flat; UCF, Upper Cactus Flat; CP, Cactus Peak; CHS, Coso Hot Springs and Coso Basin (Coso Hot Springs fault bounds the basin on the northwest); SLM, Sugarloaf Mountain; AP, Airport Lake; LL, Little Lake; and LLF, Little Lake fault. The shaded area marks the topography above 5000 feet. The hatched areas are rhyolite domes. the Coso Basin graben in Figure 2. The sense of stress is rotated about 25 ø clockwise from that obtained by Carr [1974] for the Nevada Test Site to the east. Strike Slip Faults Most of the normal faults in the Coso Range are northerly trending and have right slip displacement associated with them either as right oblique slip or as a left stepping en echelon pattern. Examples of right slip faults include the Airport Lake fault zone, Wild Horse Mesa, and the Little Lake faults. The Airport Lake fault zone is a prime example of an en echelon fault zone associated with right slip movement, as evidenced by right slip offset on a basalt flow and by the typical left stepping en echelon pattern in Coso Valley (Figure 1). On Wild Horse Mesa a pattern of sinuous left stepping fractures step down west to the Coso Basin (CHS Figure 1). The Little Lake fault (Figures 1 and 2) is perhaps the most spectaccular example in this part of the Basin and Range; it has many of the typical landforms characteristic of strike slip faults [Slemmons, 1977], including rhombic depressions, benches, side-hill ridges, linear troughs, and shutter ridges (Figure 4). Similar manifestations of transcurrent motion east of Walker Lane in the Basin and Range province re not described in the literature. White Hills Anticline The White Hills anticline, which strikes N65 ø to N75øW, provides a clear representation of the stress field. It is perpendicular to the direction of maximum compressive stress inferred by the faulting pattern (N15 ø to N25øE: Figures 1 and 2). The Wilson Canyon fault, running parallel to this fold and north of it, was active in pre-quaternary time. Zbur [1963] reported the sense of movement on the Wilson Canyon fault to be left slip; however, if that is the case it must be part of an older structural regime. If the fault is part of the present structural mechanism it would have to be a thrust fault. Airport Lake is structural depression with a crude rhombic pattern. The White Hills anticline is the south boundary of the rhombohedron. The Wilson Canyon fault has been located in this area by seismic exploration methods [Zbur, 1963], and therefore the sense of movement remains speculative.

3 2436 Roquemore: Coso Geothermal Area Alignment of Volcanoes akamura [1977] found that the orientation of average tectonic stress may be determined by utilizing dike patterns and the alignment of volcanic cones. With equidirectional tectonic stress the dikes extend radially from the source, but with differential horizontal stress they tend to be parallel to the direction of maximtm horizontal compresslye stress. Assuming a single source of the magma of the basaltic cones near Volcano Peak (Figure 2), the N10øE alignment of cinder cones (circled by dashed line on Figure 2) indicates the same general direction of maximum compressive stress as shown by the faults and the White Hills anticline. Evidence for the single magma source for these cones is (1) close spatial relation along a line (Figure 2) and (2) similar composition of the basalts [Duffield and Bacon, 1977]. Vertical Rate and Style of Vertical and Horizontal Displacement Displacements Vertical displacements are expressed in the Coso Range as horst and graben structures, tilted blocks, and step faults. Vertical tectonic rates are very rapid as two examples here show. First, a distinctive 2.5-m.y., capping rhyodacite flow [Duffield et al., 1980] above the Coso Formation is offset 1600 m at a site on the west flank of the Coso Range [Roquemore, 1977]. This determination of offset is based on the present position atop the Coso Range and the location of the assumed same flow buried in Rose Valley, identified by geophysical methods [Healy and Press, 1964]. The inferred rate of vertical movement based on this offset is 1.8 mm/yr. Second, on Wild Horse Mesa (Figure 1), another area of tectonic extension and associated step faults, Duffield and Bacon [1977] have dated a lava flow at about 3 m.y. The total offset of this flow has not been determined because the lowest downdropped block is buried in alluvium of the Coso Basin, but it is at least 600 m. This gives a minimum rate of vertical deformation on Wild Horse Mesa at 0.2 mm/yr. Horizontal Displacements Geomorphological evidence can be used to estimate horizontal rates of offset on the Little Lake and Airport Lake faults. The Little Lake fault extends northward from near the Garlock fault at a strike of N40oW. It is best exposed near the settlement of Little Lake, where a young lava flow is offset. The Little Lake fault is predominantly right slip and / ^ _Red " CT, y - Hill ' - - I -.k, COSO / / % / /x /. - / BASIN % '- I eeeee e ' X, r, ' -- '-- p _ ß F. w _ ' K 117'ø45 / Fig. 2. Schematic diagram of the southern Coso Range showing the local stress orientation; o3 is the direction of least compressive stress, and o] is the direction of maximum compressive stress. The star symbols are cinder cones. The cones enclosed by a dashed line are the same age and composition.

4 Roquemore: Coso Geothermal Area , :½ -:-' ::....,::: --,:?;:.....! :- :... ; :: '"" -;.- -?, ,::: --. :'; ': -' ' ".... ::a... ;" ½ :...:;- " i "... ß c:...-=.,:".: :..%... ' --"-'"" /'. :t'-:,** ' '.-a½-": -;...::.... ',...'-'", : : :½ (4 "-'-" -"' '-½ 'a% -:...?' '"',,.....;-' :W :.. :, -,..:.,:,:'..: '½, '.; : :... ½.:% a, ß...-+::.:;. ::*?½... :;::";.::. '-:/ :: -- ::. ; -: ; $ ,. ;.... ½..... :... :..:½.... ;:.,..-:-:...%....,...,.::., :...17;::.½:,½ ;'...'*.,.,, " '..., '.:;./'::---- :,..::..::::::::::::::::::::: "*'-'*"*** 3 : ; :" ;;,,: :,f;..':'"7:"?a ;,: '½ ::*. :*a 's':" " '-:', :;.'-a... -:;E?...,:::::,½;;;:½:';"'*.;:;. :, ;.:.. * ' -:%;:: :>-- :,,... ;;..., :,-...*',...: ; :%...::,::::..;; :*?*"':-'. ';' ½;4-.":...,:,:.a.;..'...,...,-a:::s(:.,::,::;': ;;:****,::;,½'=;: ;;' 4;;4'0':' *: * *:): '"...a.a?"' * ::... :;:::."..-'- :*-*-½;.: :, :s ;; --:::.--:*½.,½;...,:.s:...,:.-...:.,:6 : :,; a:.,:,;;. 3 qs s,... ::::'7;, " :'? fi;;:. :'... ½ :';,.'... _....-:-½;;:.-.;','!:;% ':... ': :.:7 :%:;::. --½," ½;= '" Fig. 3. Aerial view of a 2- -wide tension graben located along the south end of the right slip Airport Lake fault. offsets by 250 mm a basalt flow dated at 440,000 years [Duffield and Smith, 1978]. The displacement is calculated by reconstruction of the rhombic structures as well as a right slip offset in a basalt cliff. This indicates an average slip rate of 0.6 mm/yr. The lava flow is modified by stream erosion, so this is probably a minimum estimate. A shutter ridge along the fault is offset only 30 m, but wash channels that contain highly crushed landslide material from the Sierra Nevada are diverted. This landslide material must be younger than the 440,000-year basalt because it is not eroded by the ancient Owens River. An offset basalt flow that has not been dated by K-Ar provided a measured displacement of 125 m on this fault. On the basis of K-Ar dating [Duffield et al., 1980] this flow has an age of about 1.08 m.y. This date implies an offset of 0.1 m/yr on this fault. Horizontal deformation in the Coso Range is consistent with a transition between the San Andreas fault and Basin and Range extension. Fumerolic Activity Along Basin and Range Faults in the Coso Range Austin et al. [1971] noted radial faults projecting outward toward the circumference of a feature in the Coso Range that they identified as a ring structure. In this interpretation one would expect fumerolic activity to concentrate along these radial faults and that motion on them would be normal faulting, since they are tensional in his model. Among the dozens of known hot springs in the Coso Range, all but two (Sugarloaf Mountain shown on Figure 1, SLM, and Devils Kitchen 1 km east) are associated with faults. However, most of the faults in the Coso Range that are associated with hot springs have significant components of oblique slip (discussion on lateral faults above). As two examples, consider the Airport Lake fault zone with right slip offset of a basalt flow and the Coso Hot Springs fault with a left stepping en echelon pattern. Both of these faults contain the main concentration of hot springs in the region. These faults have a sense of strike slip displacement that is consistent with the regional tectonic stress pattern as seen in Figure 2. The strike slip character shown is not required by ring fault or calderalike features. Arcuate Faults in the Coso Range In the area around McCloud Flat, MF in Figure 1, a set of short, segmented, and slightly curved faults define a crude arch. These faults have been interpreted as a structure that resembles those associated with calderas [Austin et al., 1971; Koenig et al., 1972]. Chinnery [1966] has proposed that secondary faults forming at the ends of large-scale strike slip master faults are often arcuate (Figure 5). This is a

5 .. ß....=....: Roquemore: Coso Geothermal Area plausible concept for the arcuate faults in the Coso Range because the Coso Range is at the south end of a long zone of right slip-as evidenced by the Owens Valley fault zone. Also south of the Coso Range is a very diffezent tectonic style as evidenced by the east-west trending, left slip Garlock fault. There is no requirement for arcuate faults to be directly associated with volcanogenic origins. In the Coso Range some of the arcuate faults have strike slip striations on fault planes (P. St. Amand, personal communication, 1975) as predicted by Chinnery [1966]. Ring dikes or other volcanic features can not be linked with these arcuate faults. Seismicity Seismic studies [Walter and Weaver, 1980] show an abundance of magnitude 0.5 to 3.9 earth- quakes in the Coso Range. Focal mechanisms [see Walter and Weaver, 1980, Figure 7] from these events imply that even though most of the faults appear to have dip slip component, they are actually en echelon oblique slip faults, and the Coso Range is dominated by a right slip mechanism. Regional north-south compression is also consistent with the fault plane solutions. The seismicity defines conjugate strike slip patterns, and seismic gaps define crustal block faulting [Walter and Weaver, 1980]. The seismicity does not indicate a magma source or focal mechanisms consistent with normal slip faulting on ring faults associated with incipient caldera formation. Discussion Austin [1971], Koenig [1972], and Duffield [1975] have interpreted the arcuate faulting in SlE RRA N E' 'ADA SHu'TT'ER RIDGE ' TECTONICi PRESSURE RIDGE Fig. 4. Aerial view of the Little Lake fault showing the pressure ridges, rhombic depressions, and shutter ridge.

6 Roquemore: Coso Geothermal Area 2439 Fig. 5. (a) Diagram of the existing fault pattern and (b) the fault patterns predicted by Chinnery [1966] at the end of right slip master faults. the Coso Mountains in terms of recent subsidence within a calderalike structure. Thus under these interpretations, volcanism controls the structural features of the region. In this paper I have shown that the sense of faulting is consistent with the predominant principal stress directions of the Basin and Range, i.e., east-west extension and northeastsouthwest compression. The volcanic and fumerolic activities are clearly associated with structures that relate to these stresses and thus are manifestations of them rather than indicators of the dominant tectonic mechanism of the area. It follows then that, the overall assessment of the geothermal potential may be reduced by these factors [Bacon et al., 1980]. The pattern of regional stress developed in this paper is consistent with that expected in the southwest Basin and Range province: graben structures and normal faulting running N-S. This paper documents a significant component of right lateral strike slip motion that is consistent with San Andreas-Garlock tectonism to the west and south. Therefore the area fits Wright's [1976] classification of Basin and Range provinces (his Deformational Field II) as he predicted it would: The Coso Range area is a region of transition between San Andreas-Garlock and Basin and Range provinces, with recent folding and faulting showing characteristics of each of these provinces. Acknowledgments. This work was supported by the U. S. Navy and two Sigma Xi grants-in-aid, 1977 and The author is indebted to P. St. Amand for many hours of consultation. D.B. Slemmons and Wm. Peppin critically reviewed the manuscript and made several helpful comments. References Allen, C. R., P. St. Amand, C. F. Richter, and J. M. Nordquist, Relationship between seismic- ity and geologic structure in the southern California region, Bull. Seismol. Soc. Amer., 5.5, , Austin, C. F., W. H. Austin, Jr., and G. W. Leonard, Geothermal science and technology - A national program, Pap , 95 pp., U.S. Nav. Weapons Center Tech. Serv., China Lake, Calif, Bacon, C., W. A. Duffield, and K. Nakamura, Distribution of the quaternary rhyolite domes of the Coso Range, California: Implications for extent of the geothermal anomaly, J. Geophys. Res., 85, this issue, Carr, W. G., Summary of tectonic and structural evidence for stress orientations at the Nevada Test Site, Open File Rep , 53 pp., U.S. Geol. Surv., Menlo Park, Calif., Chinnery, M. A., Secondary faulting - Geological aspects, Can. J. Earth Sci.,, , Duffield, W. A., Late Cenozoic ring faulting and volcanism in the Coso Range of California, Geology,, , Duffield, W. A., and C. R. Bacon, Preliminary geologic map of the Coso volcanic field and adjacent areas, Inyo County, California, Open Field Map , U.S. Geol. Surv., Menlo Park, Calif., Duffield, W. A., and G.I. Smith, Pleistocene history of volcanism and the Owens River near Little Lake, California, J. Res. U.S. Geol. Surv.,, , Duffield, W. A., C. R. Bacon, and G. B. Dalrymple, Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California, J. Geo. phys. Res., 85, this issue, Goodwin, L. H., L. B. Haigler, R. B. Rioux, D. E. White, L. J.P. Muffler, and R. G. Wayland, Classification of public lands valuable for geothermal steam and associated geothermal resources, U.S. Geol. Surv. Circ., 647, 18 pp., 1971.

7 2440 Roquemore: Coso Geothermal Area Healy, J. H., and F. Press, Geophysical studies of basin structures along the eastern front of the Sierra Nevada, California, Geophysics(3), , Koenig, J. B., S. J. Garawrecki, and C. F. Austin, Remote sensing survey of the Coso geothermal area, Inyo County, California, Tech. Publ. 5233, Nav. Weapons Center, China Lake, Calif., Nakamura, K., Volcanoes as possible indicators of tectonic stress orientation, J. Volcanol. Geotherm. Res., 2, 1-16, Power, W. R., Jr., Preliminary report on the geology and uranium deposits of Haiwee Ridge, Inyo County, California, Rep. RME-2066, 37 pp., Washington, D.C., Roquemore, G. R., Cenozoic history of the Coso Mountains as determined by tuffaceous lacustrine deposits, M.A. thesis, 65 pp., Calif. State Univ., Fresno, Roquemore, G. R., Evidence for Basin and Range/ Sierra Nevada transitional zone structures in the Coso Mountains, California, Geol. Soc. Amer. Abstr. Programs, 10, 144, 1978a. Roquemore, G. R., Active faults and related seismicity of the Coso Mountains, Inyo County, California, Earthquake Notes, 49, 24, 1978b. Slemmons, D. B., State-of-the-art for assessing earthquake hazard in the United States, Report 6, Faults and earthquake magnitudes, Pap , U.S. Army Eng. Waterway Exp. Stat., Vicksburg, Miss., Walter, A. W., and C. S. Weaver, Seismicity of the Coso Range, California, J. Geophys. Res., 85, this issue, Wright, L. A., Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block, Geology,, , Zbur, R. T., A geophysical investigation of Indian Wells Valley, California, Tech. Publ. 2795, 98 pp., China Lake Nav. Ord. Test Sta., China Lake, Calif., (Received March 9, 1979; revised April 13, 1979; accepted December 3, 1979.)