Paleomagnetic Dating of the Cerro Prieto Volcano

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i d .. /, .-- LBL-9547 CERRO PRIETO-06 COkIF-7L?/0/66--/0 MEXICAN-AMERICAN COOPERATIVE PROGRAM AT THE CERRO PRIETO GEOTHERMAL FIELD

PALEOMAGNETISM OF THE QUATERNARY CERRO PRIETO, CRATER ELEGANTE, AND SALTON BUTTES VOLCANIC DOMES IN THE NORTHERN PART OF THE GULF OF CALIFORNIA RHOMBOCHASM

DE ELECTRICIDA DEPARTMENT OF ENERGY Division of Geothermal Energy United States of Am

Lawrence Berkeley e Cerro Prieto Earth Sciences Division Apdo. Postal No. 3-636 University of California Mexicali, Bja. Cfa. Berkeley, California 94720 and P. 0. Box 248 Operating for the U.S. Department of Energy under Contract W-7405-ENG-48 q1WWTIOH OF THIS COCUMENT fs UHUIfTfg DISCLAIMER

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LEGAL NOTICE I This book was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Govern- ment nor any agency thereof, nor any of their employees, makes any warranty, express or im- plied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of MY information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commerd product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessaray constitute or imply its endorsement, recommendation, or favor- ing by the United States Government or any agency thereof. The views and opinions of authors ex- pressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. .-

Submitted to the Proceedings of the Second Symposium on the Cerro Prieto Geothermal Field, Baja California, Mexico, October 17-19, 1979

PALEOMAGNETISM OF THE QUATERNARY CERRO PRIETO, CRATER ELEGANTE, AND SALTON BUTTES VOLCANIC DOMES IN THE NORTHERN PART OF THE GULF OF CALIFORNIA RHOMBOCHASM

Dr. Jelle de Boer

Wesleyan University Middletown, Connecticut _.( PALEOMAGNETISM OF TXE QUATERNARY CEBBO WIETO, CRATER ELEGANTE, AND SALTON BUTTES VOLCANIC IXMES IN THE NORTKERN PART OF THE CULF OF CALIFORNIA RXOMBOCXASM

c Dr. Jelle de Boer

I Wesleyan University Middletown, Connecticut , USA

-, ABSTRACT Saiton Buttes

Deviating thermomagnetic directiops in vol- Four of the five Salton Buttes (Obsidian canics representing the second and fifth or sizth Butte, Rock Hill, Red Xill, and Red Island) pulse of volcanism suggest that the Cerro Prieto occur on a northeast-trending lineament which volcano originated about 110,000 years B.P. and coincides vith the long axis of a major magnetic continued to be active intermittently until about anomaly CGriscom and Muffler 1971). The fifth 10,000 years ago. (Mullet Island) .is located further north in the same magnetically anomalous area. The volcanics are low calcium, alkali rhyolites containing I QUATERNARY VOLCANIC CENTERS tholeiitic and granitic xenoliths (Robinson et ale, 1976). The domes are locally modified by wave-cut The tectonic entity comprising the Salton benches carved during various stands of prehistoric Sea, Imperial Valley , Mexicali Yalley , and Gulf Lake Cahuilla (Robinson et al., 1976). A single of California contains several volcanic com- KIAr age (16,000 to 55,000 B.P.) was obtained.from plexes that are considered to be of Quaternary Obsidian Butte (Muffler and White, 1969). Paleo- age. The largest complex (Sierra Pinacate) is magnetic samples were collected from Obsidian located in the Sonora desert (Fig. 1). Smaller, Butte along a northwest-southeast traverse across single domes or groups of domes occur along the the center, and from Red Bill in a quarry in its southern shore of the Salton Sea (Salton Buttes), southwestern flank. in the Mexicali Valley (Cerro Prieto). and in the Gulf of-California (Consag Rock, Isla San Luis, fsla Tortuga). To enable relative dating of Cerro Crater Elenante Prieto, samples were also collected from radio- metrically dated domes and craters in the Salton The Pinacate volcanic field is located in Sea and Pinacate regions. northwest Sonora, near the northern end of the Gulf of California. The field is dominated by the Sierra Pinacate, a large, broad, composite volcanic . pile (maximum elevation 1206 m) which contains eight 'collapse features. Crater Elegante is a caldera, about 1.6 km in diameter, 244 m deep, located on the northeast flank of the Sierra Pinacate (Gutmann, 1976). Paleomagnetic samples were collected from two flows and a dike exposed In the eastern wall of the depression. K/Ar data provided by Lynch (1979) gave ages of 0.465 f 0.065 m.y. for the oldest flow, and 0.461i 0.036 m.y. for the dike.

Cerro Prieto .- c The Cerro Prieto volcano consists of two slightly overlapping volcanic centers which devel- oped on a fracture zone trending N. 38O E. (Puente Cruz and de la Peiia Le, 1979). The Centers rise , 260 m above the floor of the Imperial Valley and have diameters of approximately 1000 m. The northeastern cone contains a small crater 200 m in diameter and 60 P deep. Both cones consist of rhyodacitlc fntrusives and flows (Reed,;1974). A stratigraphic analysis of a section along the southeastern flank of the northern complex revealed at least five eruDtive phases. At the base of the section is a laye; of ahcosic sands, probably of aeolian.origin. Overlying this unit is 30 cm of Figure 1. Location of sampl cas in the Salton fluvial sands. Secondary sorting and the presence Trough of small volcanic fragments indicate a phreatic

1 origin. This unit is overlain by 30 cm of gray pyroclastics which include several volcanic bread bombs. An additional four units can be recognized, each consisting of phreatic sediments capped by pyroclastics. The youngest unit le composed of 8 to 10 m of silts and fine sands with isolated clay clasts, leached pyroclastic fragments, and bread bombs (up to 100 cm in diameter) overlain by a 150 m thick accumulation of reddish gray brecciated rhyodacite flows. The flovs vere probably fed by magmas ascending along a northeast-trending fracture which is exposed in the caldera. The feeder dike can be distinguished from the flovs by subvertical northeast foliation and by magnetic intensity values an order of magnitude lover than those for the flovs.

The age of the domes is in question. Steam escapes along the northeast flank of the northern dome, suggesting a young volcanic mass. Examina- tion of cores by Reed (1979, written commun.) revealed the presence of apparently fresh crystal vitric tuffs at a depth of 191 P in well M-21, 6.5 km southwest of Cerro Prieto. Reed suggested that this material may have come from the Salton Buttes. There is, however, no evidence there for explosive activity that could have sent tuffs more than 100 km southward. The H-21 tuff therefore probably represents the eruption which breached the northern dome. A subrounded clast of Cerro Prieto rhyodacite, 6 cm in diameter, vas recovered in a core from vel1 H-26 at 1275-111depth (Reed, 1979. written commun.). Sedimentation rates in the Imperial Valley are high. An average rate of subsidence of 3 mm/yr was calculated for the last Figure 2. Location of sampling sites on Cerro 2 m.y. (Lofgren, 1974). Assuming this rate, the Prieto. tuff could be 60,000 years old and the clast 425,000 years old- Since Cerro Prieto is located onhthe Colorado delta, rates are probably higher (sediments at 2500-1~ depth contain upper Pleistocene ostracodes; U. Reed, per.. couim.). At paleomagnetism for dating purposes, detailed know- a rate of 10 mm/yr, the tuffs could be as young as ledge of the, location, and polarity of paleo- 19,000, the pebble 120,000 years old. In viaw of poles is required. The polar path for North America the location of Cerro Rieto on the Colorado delta, from Cambr+ to the present is shown in Figure 3A the latter numbers appear more probable. (Van Alstine and de Boer 1978). Hsgnetic polarity differences enable 6ubdivision of this path. In Paleomagnetic samples were collected from four the last 70 m-y., for instance, reversal frequency geologic units: has been high (estimates range from 0.2 to 1.0 re- versal per million years). The last reversal oc- 1. A northeast-trending dike-like feature (Fig. curred 0.69 m.y. ego. It marked the end of the 2, sites 1 and 2). This unit appears to Uatuyama period of predominant reversed polarity, represent the youngest volcanic event in the and initiated the Brunhes period of predominant complex. normal (north-seeking) magnetism. Uaafor excur- rions (sed-reversals) enable further aubdivision 2. Flows on the flanks of the northern and on of the Bnmhes epoch (Fig. 4). Such periods, how- the crest of the southern dome (sites 3 ever rhort, are of great value for the purpose of to 6). dating rocks, if the events can be proven to be truly global and if control by radiometric ages is 3. Steeply inclined silts and clays, probably reliable previous caldera deposits (site 7). Reliable historic records for variations of 4. Horizontal pyroclastics, cemented by partial the geomagnetic field cover roughly 400 years. velding and caliche (site 8). These volcanics Archaeomagaetism provided a record for secular vari- represent €he recond phase of explosive 001- ations of the earth's field Over the past 10,000 canism and are among the oldest of the complex. years. In the southwestern United States, chrono- logical control for the archaeomagnetic data is PALEOMAGNETISM provided by 14C dates on Indian fireplaces and dendrology. The polar path extending from A.D. 600 The magnetic field's behavior is characterized to the present shows that the magnetic pole circled by excursions and reversals. To enable use of the geographic pole in counterclock fashion. Three

2 Data from sediment cores collected in oceans and lakes and young volcanics suggest that major excursions or incomplete reversals may have OC- curred 12,000, 18,000, 30,000, and 110,000 years B.P. Excursions in the period from 8,000 to 19,000 years B.P. have been grouped together and\referred to as the Laschamp event (Bonhommet and Zahringer 1969; Noel and Tarling 1975). Confusion exists, however, since researchers were impressed by the radiometric ages but failed to compare the actual polar paths for the data obtained. The Starne path, 12,077 to 12,103 150 years B.P. (typical Laschamp according to Noel and Tarling, 1975) resembles that of the Gothenburg excursion (12,350 to 13,750 years B.P.) of Morner et al. (1971) and Morner and Lanser (19751, but is very different from the polar path of the Laschamp rocks (Fig. 4). The meridional path of the latter is almost Figure 3. Apparent polar wandering paths with orthogonal to that of the former two. Evidence for respect to North America: (A) 600 m.y. a major excursion, approximately 18,000 years B.P., to present (Van Alstine, and de Boer comes from Nakajima et ale (1973). Freed and Bealy 1978); (B) 900 to 1500 B. Pa @ubOis, (1974). and Noltimier and Colvinaux (1976). The 1974) pole again moved meridionally (136q.) and reached . -_- I equatorial latitudes. Interestingly enough , this I event (Lake Biwa II is compared with the Lake Mung01 event. Polar paths are indeed close, but ages differ significantly. smaller loops, suggesting elockwise motion Over periods of 250 years ; overprint the counterclock Evidence for a major excursion approximately loop. Polar motion in the period from A.D. 900 to 30,000 years ago has been supplied by Ninkovitch 1500 was mostly longitudinal. The pole moved back et al. (1966) , Bucha (1970), Barbetti and McElhinny . and forth in undulatory fashion across the geo- (1972) , and Freed and Healy (1974). The only good graphic pole, as shown in Figure W (Dubols, 1974). polar path and age determinations (14C) come from the Lake Hungo archaeological site (Barbetti and McElhinny, 1972). It reptesents'a meridional path at approximately 127OW. (Fig'. 4). The Lake Mungo and Laschamp paths virtually overlap , suggesting that they represent one and the same-excursion. Evi- dence in support of this hypothesis was obtained recently by Hall and York-(1978) who redated Las- champ-Olby materials and obtained ages ranging from 21,500 to 61,500 years with a weighted average at 45;400 f 2;500 pears B.P.

e event was first pr seotcd by Hinkovitch'ct al. (1966). Smith and Foster (1969) proved this event to be truly global, and estimated an-age range from 108,000 to 119,000 years B.P. Support deuce was provided by Wollin et al. (1971 a and Koci (1972).

port their existence. Thus, a major data gap

Brunhes periodr (1) Gothenburg, (2) Starno, (3) Lake Mungo, (4) Laschamp, (5) Lake Biwa, (6) and (7) Blake.

Virtual geomagnetic pole8 for Cerro I The distribution &f Brunhes poles Prieto (CPA, CPB, CPC, CPD), .Salton America generally shows tight clustering around Buttes (SB), and Crater Elegante {CE). the geographic' pole (Fig; 5) .- Some poles trend (CPD represents pole dike of site 2 at toward lower latitudes and may indicate excur- Cerro Prieto with a pole in the southe sions. If the Quaternary volcanics of the hemisphere; + south *o Salton/Imperial Trough region are truly Brunhes , northseeking field. gnetic poles sho

3 4 I

Figure 5. Quaternary virtual geomagnetic poles for western United States (Irving et al., 1976).

fall within these clusters. Barnard (1968) and Reed (1976) have stated that the Cerro Prieto volcanic eruption occurred during the Brunhes epoch. To be able to determine this more accu- rately, first it is necessary to establish what is known about the Brunhes magnetic field pole position in the region. Although information on paleopoles is scarce in this region, two Important studies exist. Measurements of the magnetization in recent sediments of the Gulf of California and San Francisco Bay provide virtually identical directions (Irving et ala, 1976; Graham, 1974). The vectorial distribution of the bay muds is shown in Figure 6B and provides a reliable measure for the present direction of ragnetization in northera Mexico and California. Paleomagnetic research of Quaternary lava flows from the Hedicine Lake Highlands of northern California has shown that the virtual geomagnetic pole (VGP) for these volcanics occur in far-sited positions with respect to the present pole. Brown and Hertanan (1979) compared these data XBL 801-6732 with other poles for the western United States and concluded that the observed lncllnation anomalies that cause this deviaton aay be due to Figure 6. Equal area projection (Schmidt net) a large-scale regional field variation. Such of paleomagnetic data for: (A) Certo a field variation should also affect the Quatar- Rieto volcanics, (B) San Francisco Bay nary volcanics of southern California and muds, (C) Salton Buttes volcanics and northern Hexico. @) Crater Elegante volcanics. (See Table 1 and Figure 4.) The paleomagnetic results are shown in Figure 6. With the uception of hm concentra- tions for Cerro Prieto, all data appear to wer- lap. This overlap, hovever, does not indicate similar age since the pole positions computed near-sited, as are the poles for the Baja from these data indicate clear differences. California and San Francisco Bay muds. This This Is shown in Figure 4, depicting the pole suggests that the regional field variations of positions computed from averages of directions Brovn and Hertzman (1975)'could only have been given on Figure 6. The VGPs for the.Salton temporary. Buttes and Cerro Prieto CPB group are clearly far-sited with regard to the present pole. The The following conclusions can be drawn from Crater Elegante and Cerro Prieto CPA group are the data:

4 TABLE 1. PALEOMAGNETIC DATA Sample group

" Cerro Prieto (CPA) (Dikes, Bites 1 and 2)

a

Cerro Prieto (CPC) (Pyroclastics, site 8) 15 269.6 57.8 3.3 9.9 N 177.8 W Cerro Prieto (CPD) (Dike, site 2) . 4 129.9 -19.9 4.7 38.9 N 129.5 E

San Francisco (tidal muds) 6 14.3 60.3 3.7 78.3 N 55.3 w Baja Cali (recent muds). 16 19.0 57. 7.5 79.0 N 50.0 W New Mexico (Valles Caldera) 48.0 5.0 83.0 N 83.0 E California (Lousetown Lavas) 45.3 8.4 67.3 N 3.2 W California (Wilson Creek Form. 49.3 4.5 82.2 N 59.1 E Mexico

(Iztaccihuatl YolcI_ .') 34.4 8.2 88.8 N 34.3 E

1. Paleomagnetic pole positions foi 'cerro favorably with the hypothetical age of the Prieto deviate significantly from those of the rhyodacitic pebble found in well H-26 at 1275- Salton Buttes and Crater Elegante. The data thus depth . suggest that the Cerro Prieto volcano may'not have been active in the period from 16,000 to 50,000 4. The paleomagnetic direction of a segment and from 46I,OOO to 465,000 years B. P. The former of the rhyodacitic dike differs significantly from age, however, is questionable. that of the flows. The northeast dike, which is exposed presently, therefore does not appear 2. The paleoma to represent the original feeder dike for the Cerro Prieto duster in three distinct groups. flows. Multiple injection, however, is likely

These groups provide different poles , suggesting ~ to have occurred%andthe feeder dike of the

differences In age between the partially Welded I I flows,may be covered by its related volcanics. pyroclastic (old event), rhyodacitic flows both Although difficult to prove, injection of the c cones), and rhyodacite dike (young event). dike may have been responsible for the formation of the small explosion crater in the northern 3. The data for the Cerro Prieto pyroclastics dome. Four samples collected along the dike's provide a tight cluster, indicating that tempera- southwest contact show consistent deviations and 4 tures were above the Curie point during depo- reversed magnetization. Since susceptibility sition. "be magnetic declinations deviate by and intensity values do not differ appreciably, as much as 90° from that of the igneous rocks this effect cannot have been caused by light- in the same complex. Since these deposits are ning Impact. It may, therefore, represent horizontal and undisturbed tectonically, t another excursion. The pole computed for these deviation must have been caused by a major data is located near the polar paths of the excursion of the pole. Comparison of dif Stan8 and Gothenburg events (12,000 to excursion paths (Fig. 4) euggests that the 13,750 years B.P.). Such an age would again pyroclastics were emplaced during the Blake * compare favorably with that obtained for vitric excursion, which occurred In the period from tuffs in well M-21 at 191 Q, which could be as 119 to 108,000 years B.P. This age compares young as 19,000-years.

5 ROCK MAGNETIC DATA magnetic poles. With the exception of a sample group CPD (site 21, the directions were normal and In addition to providing information on the most poles cluster around the geOgr8phiC pole, possible age of the volcanics , paleomagnetic data suggesting a Brunhes age. The Salton Buttes, can also be useful for analyzing magnetic anoma- Crater Elegante, and Cerro Prieto poles differ lies. Magnetic anomalies can only be modeled enough to conclude that volcanic activity occurred accurately if rock magnetic properties such as at different times. susceptibility (X) and the Koenigsberger ratio (Q) are known for the area. The first provides a Sample group CPD provides a pole located measure of intensity contrast, the second of close to the polar path for the Gothenburg and intensity variation of the magnetic vectors. Starno events (i12,000 years B.P.), and CPC Natural remanent magnetization (KRM), X, and Q are a pole near the excursion path of the Blake event shown in Figure 7 for samples from Cerro Prieto, (110,000 years B.P.). These directions cannot be Crater Elegante, and Cucapa Range. Q values for explained by statistical error, lightning impact, the Cerro Prieto rhyodacites and Crater Elegante incomplete tectonic correction, or other common hawaiites persistently have Q of 2 and greater, errors, and must be considered reliable magnetic indicating that the thermoremanent magnetization deviations. A high probability therefore exists (TRH) direction predominates. Q values for the that the Cerro Prieto volcanism was initiated about Cucapa Range granodiorites are low (i 0.2). 110,000 years B.P. and continued intermittently suggesting that the induced magnetization predom- until about 10,000 years ago. inates. Because the volcanics are young, they have a magnetization more or less parallel to the The twin domes and their hydrothermal zones present field and can thus be wdeled using present provide an ideal model for the Cerro Prieto field. magnetic flux lines. For most granodiorites, the As shown in Figure 8, the Laguna Volcano, a zone magnetic intensity is one or two orders lover than that of.the volcanics. This implies that different anomalies can be expected and that spectral analyses of the data would be useful. Several small anomalies occur southeast of Cerro Prieto. One or two of these magnetic anomalies, such as the Ode over the present geothermal field, does not coincide with a gravity anomaly. It appears possible that we are dealing here vith 8 basement L* I complex which is pervasively intruded by rhyolitic or rhyodacitic dikes (and sills). In such an area, magnetic contrast would be sufficient for anomalies, but density contrasts are minimal and no gravity anomalies would be expected.

The volcanic materials from the Salton Buttes, Crater Elegante, and Ccrro Prieto provided excellent directions of magnetization and paleo-

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Q Maqnetic anomalies r' Fault P 0 VPlcanie and hyrathermol areas XBL 802-6813 0.2 2 Pigure 8. Volcanotectonic map of the Cerro Prieto- Laguna Volcano area with approximate lo- cation of magnetic anomalies (based on Mognetic intensity emukc this work and Evans, 1972), eelf-potential anomaly (Corwin et al., 1979), major fault Figure 7. Q (Koenigsberger) plot of magnetic and fracture zones (based on Vonder Haar susceptibility versus intensity of KRM. and Howard, 1980, and de la Peaa, pers. Dots are Quaternary volcanics, crosses comun.), drill holes and volcanic and are Cucapa Range granodiorites. hydrothermal zones.

6 of sedimentary ridges made up mostly.of phreatic Gulf of Mexico sed deposits, is 1ocated:over a dnor magnetic anoma 104 at the intersection of.the northwest Cerro Prieto Grahgm, S., 1974, and northeast Pdtzcuaro faults. Tbis anomaly can tidal flat sed be explained by the presence of thyodacitic dike . California: Geology, V. 2, no. 5, p. 223-226. material 1n'the.basement complex below the sedi- Griscom, A., and Muffler, L. J. P., 1971, Aero- ments. Such a dike or dikes would trend northeast magnetic map aud retation of the SaltQn and either intrude or parallel the Pdtzcuaro' Sea geothermal lifornia: U. S. fault. As is the case in the Cerro Prieto volcano, ysical Investigations, hydrothermal activityxls concent Hap GP-754t northeastern segments of the fau Gutmann, J*'T., 1976, Lology of Crater Elegante, vertical ascent of the steam is Sonora, Mexico: Bull* Geol. Soc. Am., V. 87, dike (8). The Laguna Volcano ma pa 1718-1729. initial stage in the developmen Hall, C. &,-and Pork, D.;'1978, K-Ar and 4oAr/ 0 tYP 39Ar 'age of -the Las geomagnetic polarity reversal: -Nature. p. 462-464.. * Irving, E., Tancryk,,E., and Hastie. J., 1976, Cat- alogue of paleomagnetic directions and poles. Cenozoic results: Ottawa, Canada, Department of Energy, Mines , and Resources, Geomagnetic Sciences Division of LBL for reviewing vhe manu- series t10. script. Kawal, N., Pasakawa, K., and Nakajima, T., 1972, Oscillating geomagnetic field with a recurring Work perfornred for the U. S. Department of reversal discovered from Lake Biwa: Japan Energy, Division of Geothermal Energy, *der Acad. Proc., V. 48, no. 3, p. 186-190. contract W-7405-ENG-48. fikla, G. J., and Koci , A. , 1972, The end of the last interglacial and Loess record: Quater- nary Research, V. 2, no. 3, p. 374-383. REFERENCES CITED Lofgren, B. E., 1974, Measuring ground movement in geothermal areas of Imperial Valley, Barbetti , M. , and HcElhinny , M. California, &I Proc., Research for Development of a geomagnetic excursion 30,000 years B.P.: of Geothermal Energy Resources, California: Nature, v. 239, Q. 327-330. Q. 128-133. Barnard, F. L., 1968, Structural geology of the Lynch, D. J., 1979, Trachytes and alkali basalts Sierra de 10s Cucapas, northeastern Baja Cali- of the Pinacate volcanic field of northwest fornia, Mexico, and Imperial County California Sonorat Abstract, Geological Society of (Ph. D. dissertation): Univ. of Colorad America Cordilleran Section. 157 pe Henabe, Ken-Ichi, 1977, Reversed magnetozone in Bonhommet , N. , and Zahringer , JI, 1969, Pale the Late Pleistocene sediments from the netism and potassium argon age determinations Pacific coast of Odaka, northeast Japan: of the Laschamp geomagnetic polarity event: Quat. Res., v. 7, pp. 372-379. Earth Plan. Sci. Lett. v- 6, PO 43-46. Morner , N. A*, Lanser, J. P., and Hospers, J. , Brow, L., and Mertzman, S.A., 1979, Negative 1971, Late Weichselian paleomagnetic inclination anomalies from the Medicine Lake reversal: Nature Phys. Sci., V. 234, highland lavas, northern California: p. 173-174- Plan. Sci. Lett., v. 42, p. 121-126. Horner, 1. A., and Lanser, J.. 1975, Paleo- Bucha, V., 1970, Geomagnetic revsrsals fn magnetiem in deep-sea core A179-15: EPSL 26, nary revealed from a paleomagnetic investiga- p. 121-124. tion of sedimentary rocks: J. Geoma Muffler, L. J. Pa, and White, D. .E., 1969, Active Geoelectr., v. 22, p. 253-271- metamorphism of upper Cenozoic sediments in Corvin, R. F., Morrison, 8. F., Dkz C., S., and the Salton Sea geothermal field and Salton , J-, 1979, Self potential Trough, SE California: Bull. Geol. Soc. s at the Cerro Prieto geothermal field, V. 80, pa 157-182. ceedings, First Symposium on the Cerro Nakajima, T., Yaskawa, K., Natsuhara, N., Kawai, geothermal field , $aJa Calif ornia, N., and Horic, S., 1973, Very short period , September 1978: Berkeley, Lawrence geomagnetic excursion about 18,000 yr. B. P.: ey Laboratory, LBL-7098, p. 204-210. Nature Phys. Sci., v. 244, pp4 6-10. ., and Cor, A*, 1971, Evidencp that Ninkovich. De, @dyke, B., Beezen, B. C., and schamp polarity event did not occur Foster, J. E., 1966, Paleomagnetic stratig- - 30,000 years ago: kFSL 13, raphy, rates of deposition, and tephrachron- ology in North Pacific deep-sea sediments: Dubois, R. L., 1974, Secular variation in routh- EPSL 1, pp. 476-492. west United Stctes as suggested by archeomag- ,M., and Tarling, D. He, 1975, The Laschamp netic studies. Proc. Takest Nogata Conf., geomagnetic went: Nature, V. 253, p. 705- Pittsburg, pp. 133-144. 706. ans, K. R., 1972, Aeromagnetic study of the Noltimier, H. C., and Colvinaux, P. A., 1976, Mexicali-Cerro Prieto geothermal area: Geomagnetic excursion from Imuruk Lake, (Master's thesis): Wiv. of Arizona. Alaska: Nature, v. 259, pp. 197-200. Freed, W. K., and Qaly, N., 1974, Excursions of Puente C., I., and de la Peiia, A., 1979, Geo- the Pleistocene geomagnetic field recorded in logfa del campo geothrmico de Cerro 7 7

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Prieto, & Proceedings, First Symposium on Van Alrtine, D. R., 1978, Apparent polar wander- the Cerro Prieto geothermal field, Baja ing with respect to North America (Ph.D. California, Mexico , September 1978: Berkeley, dissertation): Pasadena, California Institute Lawrence Berkeley Laboratory, LBL-7098, p. of Technology 17-37. Van Alstine, D. R., and de Boer, J., 1978, A new technique for constructing apparent polar Reed, H. J., 1976, Geology and hydrothermal wander paths: Geology, v. 6, pp. 137-139. metamorphism in the Cerro Prieto geothermal Vonder Haar, S., and Howard, J. H., 1980, Inter- field, Mexico, Proceedings, Second United g secting faults and sandstone stratigraphy P Nations Symposium on the Development and Use at the Cerro Prieto geothermal field (this of Geothermal Resources, San Francisco, May volume) 1975: Washington, D.C., U. S. Government Weaver, 1. F., 1967, Magnetic clues help date the Printing Office, V. 1, p. 539-547. past: National Geographic Magazine, V. 131, I Robinson, P. T., Elders, W. A., and Muffler, no. 5, p. 696-701. L. J. P., 1976, Quaternary volcanism in the Wolling, G., Ericson, D. B., Ryan, W. B. F., and Salton Sea geothermal field, Imperial Valley, Foster, J. H., 1971, Magnetism of the Earth California: Bull. Geol. SOC. Am., V. 87, and climatic changes: EPSL 12, pp. 175-183. pp. 347-360. Yashawa, K., Nakajima, T., Kawai, N., Torii, H., Smith, J. D., and Foster, J. H., 1969, Magnetic Natsuhara, N., and Horie, S., 1973, Paleo- reversal in Brunhes normal polarity epoch: magnetism of a core from Lake Biwa: Journal Science, v. 163, p. 565-567. Geomag. and Geoelectr., v. 25, p. 447-474.

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