Aeromagnetic study of the Mexicali- Cerro Prieto geothermal area
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Authors Evans, Kenneth Robert, 1947-
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/566522 AEROMAGNETIC STUDY OF THE MEXICALI-CERRO PRIETO
GEOTHERMAL AREA
by
Kenneth Robert Evans
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 7 2 STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of re quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg ment the proposed use of the material is in the interests of scholarship, In all other instances, however, permission must be obtained from the author.
SIGNED:
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below: ACKNOWLEDG MENTS
For his never-ending assistance throughout the completion of this thesis, I am most indebted and grateful to Dr. John S. Sumner, who suggested the problem to me. Without his services as advisor, pilot, consultant, and critic, it is doubtful that the survey would have ever been completed.
Drs. Kenneth L. Zonge and Larry K. Lepley have also offered helpful criticisms and suggestions at various times during the comple tion of this work.
The survey was flown in cooperation with the Comision Federal
Electricidad de Mexico, and financial assistance from the Comision covered most of the costs incurred during the study. Ing. Bernardo
Dominquez and Ing. Alfredo Manon of the Comision were helpful in pro viding me with a better understanding of the nature of the Cerro Prieto steam fields. Ing. Mauricio de la Fuente, my fellow student, helped by offering his services as translator on many occasions.
Finally, I would like to thank my wife, Karen, for being tolerant and understanding at difficult times during the completion of the thesis.
iii TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... v
ABSTRACT ...... v ii
GEOTHERMAL FIELDS ...... 1
Aeromagnetics in Geothermal Exploration ...... 4 Cerro Prieto Steam Field ...... 6
REGIONAL SETTING AND GEOLOGY OF THE MEXICALI-CERRO PRIETO AREA ...... 11
THE MEXICALI-CERRO PRIETO AEROMAGNETIC S U R V E Y ...... 20
Data C ollection ...... 20 M a g n etiza tio n s ...... 26 Brief D escrip tio n of the A erom agnetic M ap...... 28 Brief Trend Analysis ...... 31 Qualitative Interpretation ...... 33 Quantitative Interpretation and M odeling ...... - 35
OTHER GEOPHYSICAL D A T A ...... 41
C O N C LU SIO N ...... 47
REFERENCES...... 49
iv LIST OF ILLUSTRATIONS
Figure Page
1. Diagram of an Idealized Geothermal System ...... 3
2. Location of the Mexicali-Cerro Prieto A erom agnetic Survey A r e a ...... 5
3. Temperature Log from Drill Hole M-3, Illustrating Quaternary S trata...... 8
4. Temperature Logs from Drill Holes in the Cerro Prieto Geothermal F ie ld ...... 9
5. Locations of the Productive Geothermal Wells Shown on Figure 4 ...... 10
6. Salton-Mexicali Trough Showing Locations of the Major Strike-slip Faults ...... 13
7. Idealized System of Spreading Centers and Compensating Right-lateral Strike-slip Faults in the Colorado River Delta Region ...... 14
8. Generalized Stratigraphic Column of the Colorado River Delta Region ...... 16
9. Reconnaissance Geologic Map of the Mexicali Trough . . 17
10. Location of Epicenter Locations in the Salton- M exicali Trough Occurring During April and M ay 1969 ...... 19
11. Proton Precession Magnetometer and Interfacing Unit Used in the Survey ...... 22
12 . Location Camera Mounted on the Floor in the Tail Section of the Airplane . . . . ; ...... 23
13. Diurnal Variation for July 10, 1971 ...... 24
14. International Geomagnetic Reference Field Which Was Removed from the Original Total Magnetic Intensity Data...... 25
v vi
LIST OF ILLUSTRATIONS— C ontinued Figure Page
15. Flow Chart of Procedure Followed in Constructing the Residual Total Intensity Aeromagnetic Map o f the M ex ica li-C erro Prieto A r e a ...... 27
16. Residual Aeromagnetic Map of Mexicali-Cerro Prieto G eotherm al A r e a ...... in p ock et
17. Rosette Diagram Illustrating the Results of a Trend Direction Compilation from the Aero magnetic Map of the Mexicali-Cerro Prieto G eotherm al A r e a ...... 32
18. Residual Aeromagnetic Map of Mexicali-Cerro Prieto Geothermal Area Showing Major Strike- slip Faults and Magnetic Intensity Profile L o c a t i o n s ...... in p ock et
19. Magnetic Intensity Profiles Taken from the Residual Total Intensity Aeromagnetic Map (Fig. 18) 36
20. The Observed Total Intensity Profile and Two Basement Configuration Models and Their Resulting Profiles...... 39
21. Gravity and Magnetic Data of the Salton Sea Geothermal A rea ...... 42
22. Aeromagnetic Map of the Salton Sea Geothermal Area . . . 43
23. Bouguer Gravity. Map of the Mexicali Valley...... 45
24. Bouguer Gravity of the Mexicali-Cerro Prieto Geo thermal Area Using a Contour Interval of 1 mgal. . in pocket • ABSTRACT
This study was undertaken to determine the magnetic character and the nature of the heat source for the Cerro Prieto geothermal area and to seek out favorable areas for further geothermal exploration. The work was carried out in cooperation with the Government of Mexico through the Comision Federal de Electricidad de M exico.
The aeromagnetic survey of the Mexicali-Cerro Prieto geother mal areas was flown during July 1971 with a flight-line spacing of one km and a constant barometric altitude of 1,000 feet above mean sea level. The residual total intensity aeromagnetic map displays a close correlation with previous geophysical data in the Salton-M exicali trough.
Present subsurface knowledge of the area does not allow a determination of whether the heat source for the Cerro Prieto geothermal fields is a cooling magma at depth or a combination of crustal thinning and faulting. However, the aeromagnetic survey does indicate that future exploration should be concentrated along strike-slip faults, particularly in areas where linear magnetic lows are coincident with the faults. These lows are probably produced by hydrothermal alteration in the fault zones along which there have been upwelling geothermal fluids.
vii GEOTHERMAL FIELDS
For centuries man has observed and wondered about the pres ence of geothermal locations widely distributed over the earth. It was not, however, until the early 1900's that the great energy capacities of steam fields were actually realized. Only since the 1950's has explora tion been conducted for geothermal areas for the generation of electricity.
Increased use of steam for the generation of electricity has be come increasingly desirable due to the ever-increasing use of electricity, the growing shortage of fossil fuels, and the anti-pollution movement.
Geothermal areas appear to offer a clean, virtually inexhaustible source of electricity. Technical advances have made it economically feasible to extract salts and minerals from the hot water as well as producing electrical energy and fresh water. With the advancement of this rela tively new source of energy, methods of locating geothermal areas must be developed.
Until very recently, geothermal exploration has consisted of sinking a drill hole in areas of obvious geothermal activity. This is analogous to the day when mineral exploration consisted of sinking a shaft where there were visible signs of mineralization at the surface.
Just as mineral exploration has progressed due to a better un derstanding of the processes of mineralization, geothermal exploration will progress with a better knowledge of geothermal system s.
Geothermal systems require a set of conditions which will per mit the heating up and convective movement of fluid and the restriction 1 2 of the hot fluid to the system . The optimal environment for a geothermal field is generally agreed to require (1) a potent heat source, such as a cooling magma chamber at a depth of between 2 and 5 miles to provide enough insulation to maintain a high water pressure and temperature and to insure effective heat transfer to circulating fluids; (2) a reservoir of adequate volume, porosity, and permeability to allow convective heat transfer and recharging of the fluids; and (3) a cap rock of low perme ability inhibiting convective heat flow, thus maintaining the high tem peratures at shallow depths. Figure 1 is an idealized diagram of a geothermal system.
It should be noted that the nature of the heat source, reservoir, and cap rock is dependent on the local geology of the particular area.
Reservoirs are located in highly fractured volcanic rocks in some areas and in unconsolidated alluvium in others.
It is readily observed that known areas of geothermal phenom ena correlate well with areas of late Tertiary or Quaternary vulcanism.
This could be interpreted to indicate that geothermal heat sources are intrusive bodies cooling at depth. Areas of recent vulcanism also cor relate well with areas of recent seismic or tectonic activity, particularly areas of crustal thinning or rhombochasms (J. R. Sumner, 1971). This introduces the possibility that the heat source for geothermal systems may be a result of crustal thinning with faulting facilitating the deep circulation of hot fluids. Indeed, it is possible that a geothermal heat source results from a combination of these factors. 3
WATER INTO FISSURE NATURAL VENT GEOTHERMAL STEAM WELLS
~ SEMI
**6 * 'i . » „ 0 *
CONVECTING MAGMA
Figure 1. Diagram of an Idealized Geothermal System
Heat derived from the molten rock is distributed throughout the porous rock by convection of thermal fluids. The fluids and the heat are retained in the system by the relatively impermeable cap rock.— After Barnea (1972). 4
Aeromagnetics in Geothermal Exploration
The Mexicali-Cerro Prieto aeromagnetic survey was undertaken to obtain more information about geothermal heat sources. The goal of the survey was to produce a total field residual magnetic map which might be compared with other geophysical data to indicate the nature and location of the heat source associated with the Cerro Prieto geother mal system. Hopefully, this would provide a better understanding of geothermal phenomena and delineate favorable areas for further investi gation. A magnetic map of this type might also reveal basement struc ture and topography possibly related to the geothermal system. Figure 2 shows the location of the survey area.
If the heat necessary for the geothermal system was derived from a cooling magma, then the presence of this igneous material could become apparent by a comparison of magnetic and gravity data. Gravity highs produced by the denser intrusion should align with either magnetic highs resulting from the shallower occurrence of the higher susceptibility material or magnetic lows caused by extreme temperatures in the magma chamber, ferromagnetic susceptibility and remanence being removed when rock is heated above 550°C. If, however, the heat source is a combination of crustal thinning and faulting, permitting convective heat transfer to the geothermal reservoir, different results might become ap p aren t.
Crustal thinning could be indicated in the aeromagnetic map by either a broad regional high resulting from the shallower depth of the higher susceptibility subcrustal material or as a broad low resulting from the shallower depth to the Curie point isotherm. These anomalies should 114°
CAL ZONA
MEXICALI T###
BAJA CAL F O R N I A SONORA
Figure 2. Location of the Mexicali-Cerro Prieto Aeromagnetic Survey Area 6 correlate with broad gravity highs related to the denser subcrustal ma terial occurring closer to the surface. Faulting could be indicated in the aeromagnetic data by offset anomalies, gradients, or even conceivably as linear magnetic lows. If faults are acting as conduits for convecting fluids, it is possible that hydrothermal alteration has taken place along the faults replacing magnetite with pyrite. The fracturing of rock along faults would decrease the overall density and should therefore be re flected in the gravity data as negative anomalies.
A third possible heat source could be a combination of the two processes mentioned above. If the heat source is a combination of two processes, there might be a rather complex set of data, but hopefully one of the processes would be dominant and lend itself to interpretation.
In interpreting the geophysical data of this area it was neces
sary to keep in mind that the various anomalies mentioned above were
superimposed on each other. An example of this is the nature of gravity
data in the area. Broad, regional positive anomalies resulting produced
by the basins and by fracturing. Fortunately, the scale and shape of the
various anomalies allowed interpretation of the possible causes.
Cerro Prieto Steam Field
The Cerro Prieto steam field, about which the survey was cen
tered, is one of the largest of all known geothermal fields. The current
ly productive area is located in the Salton-M exicali trough approximately
34 km south-southeast of M exicali, Baja California, and is roughly 12
sq km in area. At the present, more than twenty deep wells have been
drilled to depths of between 914 and 1,524 meters, with the deepest 7 well, M -3, encountering granitic basement rock at 2,532 meters. Tem peratures in the productive well range between 230°C and 390°C, and well head pressures at some of the wells are in excess of 1,000 pounds per square inch. Figures 3 and 4 show temperature logs from nine of the wells drilled in the Cerro Prieto area. The Comision Federal de Electrici- dad de Mexico is presently installing two 27,500 kilowatt generators which will make use of steam from 15 w ells, which average 1,370 meters in depth and discharge an average of 130,000 pounds of separated steam per hour per w ell. Figure 5 shows the location of the production w ells.
By making use of the temperature logs of the drill holes (Figs.
3 and 4), it can be seen that in several holes decreasing temperatures were noted after penetrating the reservoir. This indicates that convec tion in the geothermal system is not restricted to vertical movement and the heat source is not necessarily located directly under the hot aquifer.
The cap for the geothermal system, as indicated by drill cores,
consists of a sequence of impermeable clays up to 600 meters thick.
The reservoir is dominantly sand with interbedded clays, and there ap
pears to be no bottom depth to the porous permeable sediments short of
the crystalline basement. The bottom of the reservoir appears to be
limited instead by the circulation of the hot brine. utray Strata Quaternary DEPTH IN METERS 1000 200 300 400 500 700 600 800 900 0° 40F 500°F 400°F 300°F
iue Tmeaue o fo Drl Hoe 3, lusrtng stratin Illu , -3 M ole H rill D from Log Temperature . 3 Figure 1 1 CLAY 86 7c 86 CLAY 7c 14 SAND LY 317c CLAY 7c 69 SAND CLAY 7 7, 7 CLAY 1 ______I
8 DEPTH IN METERS 1400 1200 1300 1100 1000 iue Tmeaue os rm il es n h Cro reo ohr l ield F al eotherm G Prieto Cerro the in s le o H rill D from Logs Temperature . 4 Figure 600° F 600° CD 10
M-26
Figure 5. Locations of the Productive Geothermal W ells Shown on Figure 4 REGIONAL SETTING AND GEOLOGY OF THE
MEXICALI-CERRO PRIETO AREA
The Salton-M exicali trough extends from Coachella Valley north of the Salton Sea through the Imperial and M exicali Valleys to the Gulf of
California. The trough is bounded on the north by the Transverse Ranges, on the west by the Peninsular Ranges, on the east by the Colorado and
Sonoran D eserts, and on the south it opens up into the Gulf of California.
Geophysical investigations have indicated that the trough is character ized by high heat flow closely related to crustal thinning, by great thicknesses of upper Tertiary and Quaternary sediments, and by major northwest-trending strike-slip faults of the San Andreas fault system.
Apparently the faults act as lateral boundaries for the trough (Rex, 1966;
Biehler, Kovach, and Allen, 1964; and Kovach, Allen, and Press, 1962).
Geologically, the Imperial-Mexicali valley system consists of a broad, flat, structural depression filled with lacustrine and deltaic sands and shales of upper Tertiary age overlain by Quaternary alluvium and deltaic material from the Colorado River. The depression is a graben, apparently related to the San Andreas fault system, which is bordered by block-faulted mountains of granitic rocks and metasedimentary rocks, stratigraphically underlying the thick sequence of Tertiary sediments.
The entire region is highly faulted and structurally complex.
In addition to the pre-upper Tertiary block faulting, it is marked by en echelon adjustments of the Banning-Mission Creek, Imperial, San
11 12
Jacinto, and Elsinore or Laguna Salada faults. Figure 6 shows the trough and numerous dissecting faults.
The region is obviously integrally related to the Gulf of Califor nia, and it is apparent that spreading centers similar to those present in the gulf (J. R. Sumner, 1971) are undoubtedly hidden beneath the thick deltaic sediments of the Colorado River. The strike-slip faults of the
San Andreas fault system most likely result from extensional compensa tion caused by the spreading. Figure 7 shows proposed locations of spreading centers (Lomnitz and others, 1970) and the resulting exten sional compensation.
Whether these spreading centers are the result of the introduc tion of subcrustal material by processes of classical sea-floor spreading and transform faulting or the result of the Pacific Plate moving north relative to the North American Plate is not pertinent to this th esis. What is important is that the spreading centers and the accompanying strike- slip faults mark the center of a broad zone of crustal thinning and tec tonic activity. The local geology of the Mexicali-Cerro Prieto area is a direct result of the tectonic activity characteristic of this zone.
Sierra de los Cucapas, immediately southwest of the survey area, represents the oldest rock cropping out in the area. These block- faulted mountains consist of many separate mid-Cretaceous batholithic i plutonic rocks, ranging from granite to gabbro, which apparently intruded the overlying, strongly folded upper Paleozoic (?) metasediments (Dibblee,
1 9 5 4 ). .
Some have felt that the mid-Cretaceous plutons were emplaced simultaneously with the rifting of the gulf (Rusnak and Fisher, 1964). 13
Figure 6. Salton-M exicali Trough Showing Locations of the Major Strike-slip Faults.—After Kovach, Allen, and Press (1962) 14
Son Andreas fault
Solton Buttes
Imperial fault ______
fcerro Prieto
San Jacinto fault
Consog Rock cWogner bosin
Figure 7. Idealized System of Spreading Centers and Compen sating Right-lateral Strike-slip faults in the Colorado River Delta Region. —After Lomnitz and others (1970) 15
It is generally agreed, however, that appreciable opening of the gulf be gan as recently as four million years ago (Larson, Menard, and Smith,
1968; Moore and Buffington, 1968). Normal faulting took place in the early mid-Tertiary, creating the relief in the crystalline basement be tween the Sierra de los Cucapas and the M exicali Valley.
The upper Tertiary and Quaternary sediments which fill the
M exicali Valley consist of both alluvium and Colorado River delta sedi ment. These sediments are probably thicker than 20,000 feet in some areas of the M exicali Valley (Kovach and others, 1962). The alluvium derived from the upthrown Cucapas constitutes the basin fill in the west part of the valley and interfingers with the Colorado River delta sediment to the east. This lateral change in sediment character is probably impor tant to the geothermal system of this area and will be discussed further in a later section. Figure 8 is a generalized stratigraphic column of the Colorado delta region, and Figure 9 is a geologic map of the area.
At some time after the block faulting which produced the major relief in the Mexicali Valley and is continuing at the present, the valley has been dissected by strike-slip faults of the San Andreas fault system.
This event probably occurred simultaneously with rifting of the Gulf of
California four million years ago, particularly if the gulf is to be con
sidered a result of sea-floor spreading. These faults, generally trend
ing N. 50° W ., correlate well with the overall structural trend of the region. The San Jacinto fault, undoubtedly the most active fault in the
M exicali Valley, extends from the Cerro Prieto cone southeasterly to the
Gulf of California. Seismic activity recorded fro April and May 1969
(Lomnitz and others, 1970) indicated movement along the San Jacinto 16
Alluvium. Tcrrcccs. Lako Coohilo bedg RECENT Ocotillo conc.lomerote (0-20001) and Brov/ley formation *! •* V (0-2000* sandstone, lacustrine cloystone) • • ••••• — • U w mm w •••••* •••«
Borreoo formotioni 0-6000' lacustrine cloystone. sparse sandstone 1 w ~ w w w
Palm Borina formation* 0-7000'sandstone, siltstone, 1 cloystone: brackish marine in part 1 Concbroko conqlomerotc: 0-90001 fanqlamerate
Imoeriol formation* 0-3900' marine sandstone, siltstone. cloystone PLIOCENE ? PLEISTOCENE m Alvcrson andesite: 0-7001 andesite,breccia, tuff
i ,Z>35- L --X - . ! ...... l Solit Mountain formation* 0-2700' fonolomerote. gypsum, orenite; marine in upper part MIOCENE M Granitic intrusives, schist, gneiss • PRE- TERTIARY
Figure 8. Generalized Stratigraphic Column of the Colorado River Delta Region.—After Kovach and others (1962) L:SLX& \V 3LC&kO
EXPLANATION POST-BATKOUTHIC SEDIMENTARY ROCKS BATHOLITHIC ROCKS .
QUATERNARY cd odometlile ond gronit* A) ad gd granodiorite qd ol olluvium; Qd dunes gd
Qm Of Om morine; Ol lluviol t tonolile gr gtomtfc • •••■ • Ol Ol locuslflne f f -
Jpm PLIOCENE . PRE-BATHOUTHIC ROCKS Tpf Tpm morine; Tpl lluviol pbs melo sedimenlorys pbc Poleoiolc sequent# POST-BATHOUTHIC VOLCANIC ROCKS si slole om ompbibolile sch schist gn gneiss Qb Ouolernory, Tpb Pliocene pbq qvorliite Tmb Miocene: bosohs ond pb undillef entloled I t .up mimed metomotpbic plutomC botolfic ondesiles
Tmv Tpv Pliocene, Tmv Miocene wokonic rocks: A ondcsite £ f Hiyolile ond docife
Figure 9. Reconnaissance Geologic Map of the M exicali Trough.—After Gastil, Allison, and Phillips (1971) 18 fault 50 km southeast of Gerro Prieto and 14 km northeast of Cerro Prieto along the Imperial fault. Figure 10 shows the results of this study. Both motions were right lateral, possibly indicating the presence of a spread ing center around Cerro Prieto (Fig. 7). Further support for the presence of a spreading center is the observation that recent movement along the
San Jacinto fault ceases north of Cerro Prieto and along the Imperial fault southeast of a point opposite Cerro Prieto.
Cerro Prieto itself is a rhyodacite cinder cone of Quaternary(?) age. This evidence of recent surface vulcanism has been interpreted by some to be indicative of a possible cooling magma chamber at depth which is the geothermal heat source. However, it should be noted that
Cerro Prieto and Laguna Volcano, and area of mud volcanoes and blow holes just west of the steam field and obviously related to them, are both located along the San Jacinto fault, so it is quite possible that there is no magma-chamber heat source but that the fault has facilitated the convection of fluids, both volcanic and geothermal. 19
El Centro C l ' US.A_ Yuma ------'V '* " " MEXICO . <- Moxicoti \ 'S.
Ensenada
Gu// of Californio
\wVSFP \San Felipe
Figure 10. Location of Epicenter Locations in the Salton- M exicali Trough Occurring During April and May 1969 .—After Lomnitz and others (1970) THE MEXICALI-CERRO PRIETO
AEROMAGNETIC SURVEY
Data Collection
The Mexicali-Cerro Prieto aeromagnetic survey was flown July
10 to July 1.2, 1971. Since the depths of geologic features related to the geothermal system were unknown, it was decided to use a flight-line spacing and elevation, which are interdependent, that would enable us to record relatively shallow (500 meters) magnetic anomalies. The "half width" rule of thumb was used to relate depth of anomaly to flight-line spacing. The survey was made with a flight-line spacing of one kilo meter and a constant barometric elevation of 1,000 feet (0.305 km).
These dimensions implied that any anomalous magnetic body deeper than
345 meters would be reflected in the data and that surface "noise" from shallower depths would be selectively filtered from the data. Additional lines were flown with a 0.5-km spacing over the 240 sq km area sur rounding the Cerro Prieto steam fields. The closer spacing resulted in better control and also provided data capable of reflecting even shal lower anomalies related to the known geothermal system.
The survey was flown using a Geo Metrics portable proton pre cession magnetometer. Model G-806, mounted in a Cessna 180. The sensor was towed on a cable 30 meters behind the plane. The magne tometer had a sensitivity of + 1 gamma and a sampling rate of two sec onds . Attached to the magnetometer was an interfacing unit which contained an internal digital clock and provided simultaneous output for 20 21 analog (Rustrak) and digital (BCD) recording. At intervals of 32 magne tometer readings (approximately every minute), the interfacing unit com manded that the time be punched on the digital output and simultaneously triggered the positioning camera. Figure 11 shows the magnetometer and the interfacing unit. The location camera, which was shock-mounted on the floor in the tail section of the airplane, was a Beattie-Coleman
Variatron Model D 9-68 equipped with a data recording chamber. The data recording chamber contained a clock and frame counter, and the time and frame number were recorded on the side of each photograph.
Figure 12 shows the positioning camera.
The survey flight lines were positioned by means of the aerial photographs taken by the camera along the flight paths. Exact locations of magnetic contours may have discrepancies as large as 0.5 km due to data gaps between flight lines and interpolation between positioning photographs.
The original total field data were corrected for diurnal variation as observed at Imperial, California, 52 km north of the center of the sur veying area. Total field magnetic intensity readings were taken every half hour during the flight period using an Elsec proton precession mag netometer, Type J, with a sensitivity o f+ 0.5 gammas. Diurnal data were plotted (Fig. 13) and removed from the raw aeromagnetic data using a
Fortran IV computer program.
The regional magnetic field shown on Figure 14 was removed using a Fortran computer program derived from a Goddard Space Flight
Center (NASA) report X-611-64-316 by Cain and others (1964). The
spherical harmonic coefficients and their derivatives used with the 22
Figure 11. Proton Precession Magnetometer and Interfacing Unit Used in the Survey
The interfacing unit contains a digital clock, produces analog output and digital output, and triggers the location camera. 23
Figure 12. Location Camera Mounted on the Floor in the Tail Section of the Airplane The data recording chamber, on the left, contains a clock and frame counter which are photographed with the location picture. The film chamber is in the center, and the advance mechanism and an external frame counter are on the right. A ...... DATA RECORDED BY TUCSON J!»C a GS MAGNETIC OBSERVATORY, LESS 776 GAMMAS © ------DATA RECORDED BY IMPERIAL, CALIFORNIA INTENSITY (GAMMAS)
4 9 7 7 5
A——
4 9 7 5 0
.----- O
4 9 7 2 5
4 9 7 0 0 1------'------X------1------1------'------1------'------1------'------5------'------1------'------'------'— ■ 0500 0530 0600 0630 0700 0730 0800 0330 0300 0930 1000 1030 1100 1130 1200 1230 1300
Figure 13. Diurnal Variation for July 10, 1971.
Recording stations were located at Imperial, California, and Tucson, Arizona
26 program to represent the regional magnetic field were taken from Cain and Cain (1968).
East-west tie lines were flown to provide the additional correc tion of bringing all flight lines to the same base level and to facilitate contouring. Finally, a bias of 500 gammas was added to the corrected total field data to produce all positive magnetic values.
Figure 15 illustrates the procedure followed in collecting and reducing the data to produce the residual total intensity aeromagnetic map (Fig. 16, in pocket).
Magnetizations
To reduce the considerable ambiguity present in this kind of regional magnetic survey, it is desirable to know the magnetization of all rock units within the survey area, but determination of these mag netizations posed a problem. Stratigraphic control of bedrock in the
M exicali Valley is limited to one drill hole (hole M-3) which intersected
a dioritic basement at 2,532 meters. Because of this limited control, it is necessary to assume basement rock types as well as their suscepti bility. It is reasonable to assume that the crystalline basement beneath the M exicali Valley composed of pre-Tertiary rocks is similar to that
exposed in the Sierra de los Cucapas. This does not really provide a
solution to the problem, because, as previously mentioned, these moun
tains are composed of metasediments intruded by plutons ranging from
granite to gabbro. To make the best interpretation of the aeromagnetic
data, it would ideally be necessary to sample all major rock units found
in the Cucapas. Due to the absence of roads of any type, the rugged Raw total field magnetic data Diurnal variation recorded at Location photographs taken every collected on BCD paper tape Imperial, California with Elsec 32 magnetometer readings (approxi in blocks of 32 readings, total intensity magnetometer at mately every 60 seconds or 6.5 km) I i BCD paper tape converted to Diurnal variation removed from Positions recovered from air photo punch cards aeromagnetic data by FORTRAN graphs IV computer program DIVAR
Data deck containing up to 16 readings per card, with card containing latitude, longitude, and time separating blocks of 32 magnetic readings
IGRF removed by aeromagnetic data and bias of 500 gammas added by FORTRAN IV com puter program REMOVES
Flight lines plotted on 1:50,000 scale with labeled tie marks at 10-gamma contour crossings J Isogamma contour values manual ly connected to produce residual total intensity magnetic map
Figure 15. Flow Chart of Procedure Followed in Constructing the Residual Total Intensity Aeromagnetic Map of the Mexicali-Cerro Prieto Area 28 nature of these mountains, and the finite time limitation placed on data gathering, samples were collected only from accessible areas on the east flank of the range. The areas of sampling, as well as the Sierra de los
Cucapas in general, appeared to be deeply weathered. This weathering causes difficulties in that none of the samples, ranging from granite to diorite and including a schist float, possessed a detectable magnetic susceptibility. This observation is also true for rhyodacite samples collected from Cerro Prieto. Although magnetite sometimes remains stable under weathering conditions, it is invariably unstable in hydro- thermal alteration environments with free sulfur present. Because of the inability to determine magnetization of rock types within the survey area, it was decided to use magnetization values of corresponding lithologic units in adjacent areas (J. R. Sumner, 1971; Griscom and Muffler,
1 9 7 1 ).
This thesis is concerned with the interpretation of the crystal line basement structure and possible cooling intrusions related to geo thermal activity, so remanent magnetizations were assumed to have the characteristically low Koenigsberger ratio of plutonic rocks, and on the basis of this remanence effects have been ignored. Table 1 shows the range of magnetization values used in the quantitative interpretation
(J. R. Sumner, 1971) of aeromagnetic data in the surveyed area.
Brief Description of the Aeromagnetic Map
The aeromagnetic map of the area (Fig. 16) shows a residual
magnetic intensity range of 700 gammas, from a high of 920 to a low of
210 gammas. Both the high and a corresponding low of 220 gammas are 29
Table 1. Induced and Remanent Magnetization for Different Major Rock U nits.—Modified from J. R. Sumner (1971)
Induced Magnetization 3 (x 10-6 emu/cc) Remanent Magnetizationb Rock U nit Range Average (x 10~6 em u /cc) granitic and 20 - 260 175 175 gneissic rocks
Miocene (?) extru- 0 - 490 250 100 - 150 sives (rhyodacite)
Pliocene (?) basalts 200 - 650 1000
Metamorphic rocks 20 - 1000 (c) sm all
Pinacate basalt 600 - 1000 800 2700 - 4000
Sedim en ts 2 2 .3 sm all (one sample)
a. In direction of present earth's field—0.5 oersted inducing fie ld .
b. Assumed parallel to present earth's field.
c. Too few samples to give any average.
i 30 associated with the Cerro Prieto volcanic cone at lat 32°25l N ., long
115°18' W. Referring to Figure 16, it should be noted that a 500-gamma bias has been added to the residual values to produce positive values.
If one discounts the anomaly produced by the extrusive volcanic feature, partially due to topographic relief as well as a magnetization contrast, the magnetic intensity range becomes 330 gammas. The broad low anomaly at lat 32o30' N ., long 115°10' W. is interpreted to be the result of basement topography, reflecting the deep part of the M exicali
Valley believed to be more than 20,000 feet deep. This is in agreement with the relatively broad contour spacing present to north and northeast of the low. The large positive anomalies at lat 32027' N ., long 115023l
W ., and lat 32022' N ., 115o10' W. are probably the result of topo graphic relief, in agreement with gravity interpretations, but are also related to lateral changes in susceptibility. The anomalies could con
ceivably have been produced by a more magnetic cooling intrusive body,
possibly the heat source for geothermal activity in the area.
It can be observed that anomalies near the base of the Sierra
de los Cucapas, which form the southwest boundary of the survey area,
are directly related to topography. It should also be noted that the
breadth of anomalies, as well as the contour spacing, increases to the
northeast. This is indicative of the great relief of the crystalline base
ment in this area and correlates with seismic and gravity data indicating
greater depths to crystalline basement in the northeast portion of the
survey area. 31 Brief Trend Analysis
The dominant trends of the magnetic anomalies in the survey area were studied with the assumption that trends of magnetic anomalies are a reflection of the structural and lithologic trends in the crystalline basement. From merely looking at Figure 16, the dominant northwest trend becomes apparent and a weaker northeast rend is somewhat less obvious. As mentioned in a previous section, this area has been re peatedly dissected by northwest-trending dip-slip faults, and possible spreading centers provide a northeast-southwest offset of major strike- slip faults. Therefore, the magnetic anomaly trends apparently follow the structural grain of the area.
As a result of this initial observation it was decided to do a standard linear-element trend compilation for the aeromagnetic map to provide possible further insight into the structural framework of the area and to facilitate interpretation.
The methods used in the trend analysis were taken from Affleck
(1963) and Gay (1972). The rosette diagram shown in Figure 17 was con structed using primary anomaly trends, represented by closed highs and > lows, and gradient lineations. As pointed out by Affleck (1963), data control is a deciding factor in determining trends, that is, a trend must be at least as long as the flight-line spacing for it to be compiled.
Forty-five anomaly trends were plotted on the aeromagnetic map. The : trends were then compiled into 5-degree increments. The length of each
5-degree segment was determined by adding the lengths of all trends within the azimuth increment, dividing by the total of all trend lengths on 32
NORTH r I57<
Figure 17. Rosette Diagram Illustrating the Results of a Trend Direction Compilation from the Aeromagnetic Map of the Mexicali-Cerro Prieto Geothermal Area 33 the map, and multiplying the resulting value by 100 percent so that each
5-degree segment is an expression of a percentage of the total trend.
By examinaing the anomaly-trend rosette diagram (Fig. 17), it becomes apparent that there are four major trend directions in the area.
The dominant trend direction between N. 60° W. and N. 65° W. has been interpreted to be a reflection of the Tertiary "Basin and Range" type block faulting which is responsible for the topographic relief in the
area. The cluster of trends between N. 25° W. and N. 60° W. may be related to strike-slip displacement along faults of the San Andreas fault
system. The San Jacinto fault, which is represented as an offset in the
large positive anomaly in the southeast portion of the aeromagnetic map
(Figure 18, in pocket) illustrates this with a trend of N. 40° W. The
other two major trends shown on the rosette diagram, due north and N.
35° E ., have been interpreted as a result of the en echelon offset pres
ent in the dip-slip and strike-slip faulting, respectively.
Qualitative Interpretation
A general qualitative interpretation of the aeromagnetic map has
been presented in the brief description and trend analysis sections above
However, this thesis is concerned with geothermal activity and geother
mal heat sources in particular. It is therefore essential to take a closer
look at anomalies and trends possibly related to geothermal activity.
Special attention is invited to the large positive anomaly in the
southeast portion of Figure 18 (in pocket) near lat 32022' N ., long 115°
10' W. This anomaly may have been offset along the dominant northwest
trend in a right-lateral strike-slip movement. This displacement is to be 34 expected because the offset coincides with the right-lateral strike-slip
San Jacinto fault. The interesting aspect of the offset is the anomalous low superimposed upon it which is interpreted to be a result of hydro- thermal alteration replacing the magnetite by pyrite in the crystalline b a sem en t.
This interpretation suggests that the fault provided a conduit for ascending thermal fluids of a major convection cell. As a result, bedrock adjacent to the fault has been hydrothermally altered and the magnetite was destroyed. This proposal assumes that the alteration postdates the altered rock and the original movement along the fault.
However, the dissected anomaly could conceivably been produced by a recent intrusion, since the anomaly shows only a 2-km offset, and the intrusion is acting as the heat source for geothermal activity during cooling. If this assumption is correct, the intrusive body has been offset by strike-slip movement, and ascending fluids from the cooling heat source have altered the outer, already cooled regions of the intru
sion. The fact that this anomalous high occurs along a northwest trend which includes Cerro Prieto and Laguna Volcano may indicate that there was a deep zone of weakness in this area and perhaps the cooling pluton was simultaneously emplaced with the eruption of Cerro Prieto.
Another possible interpretation of the positive anomaly is that
it was primarily produced by topographic relief and has been amplified
by the more magnetic metamorphic rocks. The right-lateral offset has
resulted from recent strike-slip faulting, which is younger than mid-
Tertiary, and hydrothermal alteration produced the anomalous low coinci
dent with the offset. This interpretation necessarily assumes that the 35
Mexicali-Cerro Prieto steam fields is a combination of crustal thinning and faulting. So aeromagnetic data alone provide insufficient evidence for determining the nature of the heat source for the geothermal activity in this area.
Whatever the nature of the geothermal heat source in the area, it is evident from a study of the regional structure and the linear- element trend analysis and from the recent tectonic activity in the area, that geothermal activity in the Mexicali-Cerro Prieto area is integrally related to the strike-slip faulting of the San Andreas fault system. The
San Jacinto fault either provided a zone of weakness for an intruding pluton, or it permitted convection of fluids heated by subcrustal m aterial.
Quantitative Interpretation and Modeling
To substantiate the qualitative interpretation presented above, it became desirable to perform a quantitative interpretation by attempting to model the magnetic anomalies. Model configurations corresponding to each of the interpretations made were used to find the one which would come closest to reproducing the observed anomaly.
First order depth approximations were made by applying Peters'
(1949) method to the three profiles shown in Figure 19. The locations of these profiles in the survey area are shown on Figure 18 (in pocket).
Depth approximation for the large positive anomalies in the center of the map are 170 meters at lat 32°27' N ., long 115°23' W. (profile A-A') and 1,500 meters at lat 32022' N ., long 115°10' W. (profile B-B').
These depth estimates correlate well with depths previously derived 36
A'
500
4 5 0
4 0 0
350
3 0 0 B'
Figure 19. Magnetic Intensity Profiles Taken from the Residual Total Intensity Aeromagnetic Map (Fig. 18) 37 from gravity data interpreted by Velasco H. and Martinez B. (1970).
Depth estimates for the broad negative anomaly in the northeast part of the mapped area were made using profile C -C (Fig. 19) and indicate depths in excess of 6,000 meters for this deep area in the Mexicali
Valley. This is also in agreement with previous interpretations (Kovach
and others, 1962; and Velasco H. and Martinez B., 1970).
A two-dimensional model was first devised, using a constant
magnetic susceptibility with the basement rock topography derived from
the gravity data. Models using uniform basement topography and large
susceptibility contrasts within the crystalline basement were also con
structed. The models were analyzed by means of a Fortran IV computer
program, which produced a calculated total magnetic intensity profile
that was compared with the observed profile. The magnetic suscepti
bilities used in modeling were those from Griscom and Muffler (1971)
and J. R. Sumner (1971).
After several models were evaluated, it appeared that a granitic
basement of uniform susceptibility of 0.0001 and 0.0003, which are
acceptable values for the susceptibility of granite in the area, would
not produce the desired anomaly profile. To approximate the observed
profile, the basement would require more relief than would be logical
from the gravity data. Similarly, lateral changes in magnetic suscepti
bility alone would require a high susceptibility contrast than is charac
teristic of intrusive rock in the area. So, it appears that these anomalies
result from a combination of relief and lateral susceptibility changes in
the crystalline basement. 38
As a result, it was'decided to modify the "relief model," that is, the model of uniform susceptibility, by incorporating higher suscepti bility metamorphic rocks, such as those present in the Sierra de los
Cucapas. The "intrusive model," which makes use of susceptibility contrast, was modified by merely giving relief to the higher susceptibil ity intrusive body. By these modifications, the basement topography and susceptibilities were kept within limits acceptable to previous data.
Figure 20 shows an observed total magnetic intensity profile,
B-B', over the large positive anomaly in the southeast portion of the mapped area at lat 32022l N ., 115o10' W. with the calculated profiles from the "relief model" (B) and the "intrusive model" (A) shown at the bottom of the diagram. The depth to the basement along the profile ap pears to range between 1,500 meters and 8,000 meters for both models.
For the "relief model," the crystalline rock on either side of the positive anomaly appears to have been downfaulted in a horst and graben manner.
The grabens formed the deeper portion of the M exicali Valley visible in the gravity data. The "intrusive model" indicates that the pluton intruded above and was upthrown or remained above the surrounding crystalline 1 basement and that block faulting "produced the graben to the northeast with a depth of between 7,000 and 8,000 meters.
Both models make use of an altered zone approximately 4.5 km wide. This zone coincides with the San Jacinto fault (Fig. 18, in pocket) and results from hydrothermal alteration along the fault. It should be kept in mind that these are only two of an infinite number of basement configurations which produce the desired magnetic intensity profile; however, these two do agree very well with earlier geological and 39
CALCULATED FROM A-v
OBSERVED
CALCULATED FROM B
l I I I 1 l KM.
FLIGHT PATH SEA LEVEL SEDIMENTS J=0
ALTERED INTRUSIVE ZONE GRANODIORITEt?) J = .0008
GRANITE GRANITE J=.OOOI J= .0001 A FLIGHT PATH SEA LEVEL SEDIMENTS I - METAMORPHICS J = 0 j=.ooo8/rh— METAMORPHICS ALTERED J = .0008 ZONE
_ GRANITE J = .OOOI
B
Figure 20. The Observed Total Intensity Profile and Two Base ment Configuration Models and Their Resulting Profiles
A. The "intrusive model"; B. the "relief model." 40
geophysical work done in and around the Salton-M exicali trough. Minor modification of the basement configuration could improve the curve fit of the two models, but it was decided that since further modifications would be minor they did not warrant the time required.
The models indicate that either of the interpretations presented could be modeled within the limits set by previous information, so, on the basis of current knowledge, both are acceptable interpretations. OTHER GEOPHYSICAL DATA
Due to the inherent ambiguity in the interpretation of potential field data, it was decided to examine other available geophysical infor mation from the Salton-M exicali trough. A combination of geophysical methods should provide a better evaluation of the nature of the geother mal heat source.
Heat flow measurement have detected areas of abnormally high heat flow in the vicinity of Cerro Prieto and Obsidian Buttes, just south of the Salton Sea, but there are no areas of abnormal heat flow between these two places (Koenig, 1967) which may indicate that these "hotter
spots" are caused by cooling plutons . •
Figure 7, showing the proposed locations of spreading centers,
may provide another credible explanation. If spreading centers are
located beneath the two anomalous areas, it is possible that the crustal
thinning permits convective heat transfer to a greater degree. When the
tremendous difference in efficiency between conductive and convective
heat transfer is considered, it becomes apparent that the "hot spots"
could easily be explained by the presence of spreading centers.
Quaternary volcanics in the Salton-M exicali trough may also in
dicate recent intrusive activity. Gravity and magnetic surveys by Kelly
and Soske (1936) have shown a strong correlation of gravity and magnetic
high with outcrops of Quaternary volcanics (Figs. 21 and 22). This cor
relation has been interpreted by Biehler et al. (1964) and McNitt (1965)
as evidence of a pluton which is providing heat necessary for geothermal
41 42
■■■ ■ 968 —— Dewgutr «»o»ity, mgei* (4-1000)
Wogntiic inltniily, goftmei
Down dry el CO^ ('Old
Seel# in m.lso
Solton Sea
Figure 21. Gravity and Magnetic Data of the Salton Sea Geo thermal Area
Magnetic data is after Kelly and Soske (1936), and the gravity data is after Kovach and others (1962). Figure 22. Aeromagnetic Map of the Salton Sea Geothermal Area.--After Griscom and Muffler (19 71) 44 activity. I maintain that this is not necessarily the case. The gravity and magnetic anomalies are probably a reflection of igneous rock, but there is no evidence that it is providing the necessary heat for the geo thermal activity. Quaternary igneous activity is no doubt related to the geothermal systems of the Salton-M exicali trough, but the relation ship is uncertain. Crustal thinning and faulting may provide the neces sary conditions for the movement of both magmatic and geothermal fluids.
Gravity surveys in the M exicali Valley have provided additional information. Figure 23 shows the results of gravity surveys by Kovach and others (1962) and Figure 24 (in pocket) shows the results of gravity surveys by Velasco H. and Martinez B. (1970). The correlation of the linear negative anomalies with the San Jacinto fault may indicate highly fractured rock within the fault zone. The decreasing values to the south east along the fault could be interpreted as resulting from an increasing distance to crystalline basement in that direction. This, in turn, could be interpreted as a greater amount of crustal thinning and higher temper atures to the southeast. An interesting aspect of this negative gravity anomaly is that it correlates extremely well with a positive temperature anomaly shown in data taken from shallow wells (B. Dominquez, oral communication, 1972). An area of greater crustal thinning might be ex pected to be an area of greater heat flow and, therefore, an area of anomalously high temperatures.
Other geophysical data apparently further substantiates the interpretation that the Cerro Prieto geothermal systems are fault located, and the data may imply that the geothermal heat results from crustal thinning rather than from a plutonic body. Figure 23. Bouguer Gravity Map of the Mexicali Valley.—After Kovach, Allen, and Press (1962) Cn 46
From the information presented, the need for further geophysical exploration is apparent. Probably the best geophysical methods to apply at this stage in the exploration of the Mexicali Valley are resistivity and heat flow measurements. Resistivity lines normal to the San Jacinto fault
should delineate zones of upwelling of geothermal fluids. Since resis
tivity is inversely proportional to temperature, the zones of upwelling
should appear as resistivity lows. Similarly, heat flow measurements
may detect the presence of thermal convection cells.
There is one formidable problem in performing a resistivity sur
vey. As mentioned in the section on geology, the Mexicali Valley west
of the Cerro Prieto extrusion is capped by impermeable clays weathering
from the granitic rock of Sierra de los Cucapas. East of Cerro Prieto the
sediment consists of more permeable deltaic deposits. As a result, re
charging of the geothermal fluid occurs to the east but not to the w est.
This means that the fluids in the geothermal system east of Cerro Prieto
Are a mixture of relatively fresh, meteoric water and water which has been
through the geothermal cycle. Geothermal fluids to the west are restricted
to fluids which have been heated at least once. This is reflected in the t relative salinities of water in the valley. Salinity decreases to the east.
Since resistivity is inversely proportional to salinity as well as tempera
ture, it will be necessary to remove the effect of the salt content. Per
haps the easiest method of doing this would be to construct a contour
map of isosaline lines based on present drill-hole and water-well data.
Resistivity lines could then be laid out parallel to lines of equal salinity,
and resistivity values could be normalized to remove the effect of the
change in salinity. CONCLUSION
The northwesterly right-lateral strike-slip faults of the San
Andreas fault system which dissect the Salton-M exicali trough are prob ably zones of extensional compensation produced by spreading centers located beneath the thick sediments in the trough. These fault zones appear to be the controlling factor in the location of the M exicali-
Cerro Prieto geothermal fields.
The temperature logs shown in Figures 3 and 4 can be used to interpret the depth to the top and bottom of the geothermal reservoir from the surface location of the drill hole. By comparing the location of the drill holes (Fig. 5) with the temperature logs, it be comes apparent that the bottom of the reservoir gets progressively deeper to the north and east in a direction normal to the San Jacinto fault. This may indicate that the fault is responsible for the location of the zones of upwelling of goethermal fluids as well as the location of the heat source for the Mexicali-Cerro Prieto geothermal system.
The greatest value of aeromagnetic surveying is that it pro vides a method by which estimates can be made of crystalline basement configuration beneath great thicknesses of nonmagnetic sediments.
Since the location of goethermal systems is apparently dominantly con trolled by basement structure, aeromagnetic surveys are an obvious geothermal exploration method. As shown on Figure 16 (in pocket), the
aeromagnetic data reveal the locations of faults of both strike-slip and
dip-slip displacement. This ability to reveal the fault zones which 47 48 determine the location of geothermal systems illustrates the apparent success of the aeromagnetic technique applied to geothermal prospecting.
As discussed, it is possible that hydrothermal alteration asso ciated with the convection of geothermal fluids has destroyed magnetite along the fault zones which act as conduits for the fluids. This may be reflected in the aeromagnetic data as magnetic lows coincident with the fault zones.
The most obvious areas in which to concentrate further, more . detailed geophysical studies are along the strike-slip faults of the San
Andreas fault system. If the linear magnetic lows coincident with the
fault zone have been accurately interpreted, it would be advisable to
center further surveys in the M exicali Valley around these magnetic
lows, which represent zones of alteration and probably zones of upwell-
ing of geothermal fluid. This means that the geothermal fluid in the
areas of alteration is at a higher temperature than in other areas of the
geothermal system and would therefore allow the most efficient geother
mal exploitation.
The results of this study indicate that aeromagnetic surveys
are capable of providing useful information for geothermal exploration
without necessarily determining the nature of the heat source. Aero
magnetic surveys appear to be most useful in delineating favorable
areas which can be explored by methods which provide better detail but
at a much greater expense, such as resistivity, shallow temperature log
ging, or microseismics. REFERENCES
Affleck, J ., 1963, Magnetic anomaly trend and spacing patterns: Geophysics, v. 28, no. 3, p. 379-395.
Barnea, Joseph, 1972, Geothermal power: Scientific American, v. 226, no. 1, p. 70-77.
Biehler, Shawn, Kovach, R. L., and Allen, C. R ., 1964, Geophysical framework of northern end of Gulf of California structural prov ince: Am. Assoc. Petroleum Geologists Mem. 3, p. 126-143.
Cain, J. C ., and Cain, S . J ., 1968, Derivation of the International Geomagnetic Reference Field. A report to IAGA Committee II, Working Group 4: Goddard Space Flight Center preprint X-612-86-501.
Cain, J. C ., Hendricks, S., Daniels, W. E ., and Jensen, D. C ., 1964, Computation of the main geomagnetic field from spherical harmonic expansions: Goddard Space Flight Center, preprint X—611-64-316.
Dibblee, T. W ., 1954, Geology of the Imperial Valley region, Califor nia, in Geology of Southern California: California Dept. Nat. Res., Division of Mines Bull. 170, p. 21-28.
Gas til, G. R ., Allison, E. C ., and Phillips, R. P ., 1971, Reconnais sance geologic map of the State of Baja California: prepared by students and staff of the Universidad Autonoma de Baja Califor nia and San Diego State College.
Gay, S. P ., 1972, Fundamental aeromagnetic lineaments, their geologic significance, and their significance to geology: Denver, Colorado, American Stereo Map C o., 94 p.
Griscom, Andrew, and Muffler, L. J. P ., 1971, Aeromagnetic map and interpretation of the Salton Sea geothermal area, California: U.S. Geological Survey
Kelly, V.' C ., and Soske, J. L ., 1936, Origin of the Salton volcanic domes, Salton Sea, California: Jour. Geology, v. 44, p. 496- 5 0 9 .
Koenig, J. B ., 1967, The Salton-Mexical geothermal province: Califor nia Division Mines and Geology, Mineral Information Service, v. 20, no. 7, p. 75-81.
49 50
Kovach, R. L ., Allen, C. R ., and Press, F., 1962, Geophysical Inves tigations in the Colorado Delta region: Jour. Geophys. R es., v. 67, p. 2845-2871.
Larson, R. L ., Menard, H. W ., and Smith, S. M ., 1968, Gulf of California: A result of oceanfloor spreading and transform fault ing: Science, v. 161, p. 781-784.
Lomnitz, C ., Mooser, F ., Allen, C. R., Brune, J. N ., and Thatcher, W ., 1970, Seismicity and tectonics of the northern Gulf of California region, M exico. Preliminary results: Geofisica Internacional, v. 10, no. 2, p. 37-48.
McNitt, J. R ., 1964, Rev. 1965, Exploration and development of geo thermal power in California: California Division of Mines and Geology Special Report 75, 45 p.
Moore, D. G ., and Buffington, E. C ., 1968, Transform faulting and growth of the Gulf of California since the late Pliocene: Science, v. 161, p. 1238-1241.
Peters, L. J., 1949, The direct approach to magnetic interpretation and its practical application: Geophysics, v. 14, no. 3, p. 290-320.
Rex, R. W ., 1966, Heat flow in the Imperial Valley of California: Am. Geophys. Union Trans., v. 47, no. 1, p. 181.
Rusnak, G. A., and Fisher, R. L ., 1964, Structural history and evolu tion of Gulf of California: Am. Assoc. Petroleum Geologists Mem. 3, p. 144-156.
Sumner, J. R ., 1971, Tectonic significance of geophysical investiga tions in southwestern Arizona and northwestern Sonora, M exico, at the head of the Gulf of California: unpublished Ph.D. disser tation, Stanford University, 90 p.
Velasco Hernandez, J., and Martinez Bermudez, J. J ., 1970, Leranta- mlento gravimetrico zona geotermica de M exicali, Baja Califor nia: Consejo de Recursos Naturales No Renovables Bull. 74, 20 p . '
i!
*
40 £?79l i Figure 24. Bouguer Gravity Map of the Mexicali - Cerro Prieto geothermal area, using a contour interval of I mgal. After Velasco H. and Martinez B. 1970. K. R. EVANS DEPT. OF GEOSCIENCES THESIS 1972 £ 9 7 9 / / 972- 3 ^ 8 115o30' .liN 'ttC bi , , e , Mt s' j :. " i t - 1 > < : 1 4 #-»< w - -~—• - — —— ■*'* " '"-^Z Vt** 'tir*<'' . W- X • se*B4 X - 4 -:% -__ v ' v. ^.4 T -- -■ V -'-^r — r " __A j^eeuep” ” * ^ 1 5. * '**> (*> iW i x i F iMueRiAL courx •,” liAN0S 7. J IT 11/ y;5 > Vw ^ 350 a !$_ \*** ^ - . A r r * ** > » a ie x ic o 300 .... las ^nis t \ 22 ^ Cofem ^ do Bravo ■? i t i if < 1 C A L U J1 r iaspii •' v [Off / <4~\ X aX ^ •■WT »| v. \ Ubrie de u ^ o i j u- V x\ / , V- ' i^. j andtip j/j i Xw v v xy jfP i jLgw X W 5 7 xEWV \i Colowa^-^wajif V k i j l i # ;«s‘\ tTT--- T^Hl to ■ v — *v \ \ r- C SZ jr Jr;:V8. ':- «*»«?/• V*C. V f* k .. . f "i 'Eltanton' 3fX j E r t ut% Carjg'o Anaya # T fk' - :u d o ■ - k i r ->tX" ' N'Xie;->5 ------L iscupauIslas vt < Cayjq 4 j k r p 5 ^nchfian M.^ ^ y n i .'r :') ■* 1 " > w W? r«' 1 h *a^, ~** x 7 "" ' ^ £ q £ ^ o r b o s 'X "— ‘—st—T— ^ —>v'— — ,--.»r . X - s H JX U E |\ dQ jkU ^T ft \ \ ) \ i , h j L ■ > i K.7 . E jid p , \ 3 50; rr ,/is P The original total field data were corrected for diurnal * 0 , variation as observed at Imperial, California during the flight period. The regional magnetic field, as shown on the r - . accompanying index map, was removed, using a Fortran # n / V computer program derived from a Goddard Space Flight Cen Subest ter (NASA) report X-611- 64-316 by Cain r l a l . , Oct. 1964. vi-mer, The spherical harmonic coefficients and their first and S V second derivatives used with the program to represent the C ob regional magnetic field are from Cain and Cain, 1968, Ji]— , C o fq n ia “ Derivation of the International Geomagnetic Reference n X Jlj MChtamilte^eci Field. A Report to 1AGA, £ommittee H, Working Group 4 ” : v — - ■Lr*1’" — —>— CALIFORNIA N A - Goddard Space Flight Center preprint X-612-86-501. The residual values shown on this map are uniformly biased such that the plus 500-gamma contour represents a zero residual. . c : •> The Mexicali-Cerro Prieto aeromagnetic survey was Ro h' REGIONAL MAGNETIC FIELD w y > ——4 7 7 r / \ 1 MEXICALI-CERRO PRIETO AREA A w n . /. y / % 115°15' K. R. EVANS Figure 16. Residual Aeromagnetic Map of Mexicali-Cerro Prieto Geothermal Area DEPT. OF GEOSCIENCES THESIS 1972 £ 97 ^ 1 / 9 7 Z _ . • ■3b8 11503QI 115° 15' i \ * r x - • v ? w - 'J 1 ' j>'y 5 W f y 4 . d - - - r ------~ KititCANV- -M*r V**y^ I — ------' ♦■- 1 V V' ,JL \ .vt9 ' t i - . 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J \ iu R V'AcjrfCt^ ‘ c^ Js, x j I Escueia Ru( \ "^Oo- r;\ ^ * Xf^VOUl H e c h X fte r X-/ i redera> X < ^ X l V X30 0 \ 'X x>\ S-Letarla de A p#U u\ t a n XV 'X 4 Esteban \: y X L . • V i T — T r r x c 1^ ..^C v T \ S w ^ ' VJ.U3 yiQRROfe ------/\%% V x , ^ ^ A 1 ■ X-- y X -x J 5 o . & 2 / x » X ‘ V i x PV" E :'id O '/t U6'XZ > v n x < r 4d o Sf#«—4- /'K VV € i i d g _ . -3 5 °,1 Z ' /> k- Litis x /• 'JtquiljX 32*30' x x X V X x/kX>X4' v / X o' , X , x ) f x \ \ ~ \ X - j - - v ' '" X x x 7 ? V ' .j;V ■4 50- hH w V'K $ -»lu l r x x xX ing for the area very near the Cerro Prieto geothermal fields. , . L > \ \ \ ^"T T V Z vv X - z / ' - * The survey instrument was a Geometries digital record Z v / - ing proton precession magnetometer provided by R. J. Wold L . V A T |Q\ OF AEROMAGNETIC STUD> AREA d 1 o 1 The University of Wisconsin-Milwaukee. jC * ^ s A vfD v: H .T & ^ Las Pa Flight lines were positioned from aerial photographs /9 X t-r- taken along the flight paths. Exact locations of magnetic / contours may have discrepancies as large as 0.5 km due to , L t > ----V — - •'C.j C iR Z x X data gaps between flight lines and interpolation of locations 7 V- tvciqn V ;r.ton between positioning photographs. The data used in compil X x ing this map are available. L j L U t ^ 1 K A x , Z 32*15 E-, % Z > < / / ? V- y q r - x - ■ v S V X. 'XT d i V X REGIONAL MAGNETIC FIELD ft MEXICAL I - CERRO PRIETO AREA H5°I5' Figure 18. Residual Aeromagnetic Map of Mexicali-Cerro Prieto Geothermal Area (showing major strike-slip faults and magnetic intensity profile locations) K R. EVANS DEPT. OF GEOSCIENCES THESIS 1972 £ 979/ 3b& f