• 

Recognition on Space 

Photographs of Structural 

Elements of Baja California 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 718 

Work done in cooperation with the National 

Aeronautics and Space Administration 

Recognition on Space 

Photographs of Structural 

Elements of Baja California 

By WARREN HAMIL TON 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 718 

Work done in cooperation with the National 

Aeronautics and Space Ad;ninistration 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1971 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director 

Library of Congress catalog-card No. 74-181492 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 45 cents (paper cover) 

Stock Number 2401-1174 
CONTENTS 

Page 

Abstract·------------------------------------------·--··-----------·-········-··--·--·--···------- 1 

Introduction·-----·--------------··--·---·-··--·---·--···--····-·-------··-·--·-··--·---·---·-- 1 

Space photographs ..... ·-·--·---·---··-·---·-------·--·--··--·--·-·-··--·-------·- 1 

Baja California .. --------------·--·-·····--·--··--···---··--··---·-··-·······--····- 2 

Strike-slip faults in California ................ ---···········-----··---·-- 2 

Northern Baja California ................. ----···-···--·-····--·---·--·-··--·-·---·- 4 

Agua Blanca fault .............. ------·-··-···-···-------···-······-····----·····- 5 

Main fault block of northern Baja California................. 7 

Sierra San Pedro Martir ... ------------·-····-·-·-·--·--·--··--·---·-···--··- 7 

Comparison with Sierra Nevada........................................ 8 

Northern Baja California- Continued 

Valle San Felipe and Valle Chico...................................... 8 

Transform mechanics of Agua Blanca fault ... -----·--·------- 13 

Sierra Juarez and Laguna Salada ..... -----·--·--··---------·--------- 16 

Sierra Cucapas __________ .. ·--·---··--··--··-··-....... ·-·---·-·----·--.. ·-----···--- 18 

Colorado River delta faults ................. ----·--····--·--·-···--·--·-·--- 18 

Volcanic rocks--------·-·--·--·--·-----·-·-----------·----·--·-···--·---·-·----·----·- 18 

Central Baja California ....... ----------·····-···---·--·---·-·--·------··-·---------·- 20 

Southern Baja California .... -----·-----------·-···------------·······-·--··-··---·-- 22 

References cited ..... ------------······--·-----------·---·-···--·-·--------················· 25 

ILLUSTRATIONS 

Page 

FIGURE 1. Index n1ap ............................................... -------·--······--·-----··-------·-·-··-··--··--·--··---·-··----·---·---·····---·----·--··-------··----········----------------·---·-·· 3 

2. Oblique photograph northward over the coast of southern California .... -------··--···----------------------------·-------·------·------·---·-- 4 

3. Sketch of area shown in figure 2 .... ---------·---------------------------------------------·--·----------·----··-------------------·············--·------------·-·-------·- 5 

4. Photograph northeastward over northeastern~aja California .................. ---------·-···------------------···-·--·-------·------·······--·--·--·- 6 

5, 6. Photographs steeply downward at the Salton depression ... ------·--------·---------···-······------------·---------·-······--·-------------··-------··--8, 9 

7, 8, 9, 10. Vertical space photographs of northern Baja California, mounted for stereoscopic viewing .................... 10, 11, 12, 13 

11. Map of part of Baja California, showing Quaternary structures visible on space photographs .............................. 14 

12. Map showing vectors of relative motion of the crustal plate south of the Agua Blanca fault .................................. 15 

13. Oblique photographs of the southern part of northern Baja California .. -----···-······--·-·--·--···-··-···-----·---·---·---·--·--··--··--·---- 17 

14. Oblique photograph southward over the delta of the Colorado River ...... -----··-··------------------------------·----------------·-----·---·--- 19 

15. Oblique photograph of the mouth of the Colorado River ............ ---······-·-·····-----·······--·-------···-----················--···--···-···········-· 20 

16. Photograph steeply downward over Rosario-Santa Maria sector of Baja California ... -----······--·-···-····-·----·-··-------------- 21 

17. Oblique photograph southeastward down the Gulf of California .............. ----------·--···········------·--·--·-·----------·-·-······--·-···--·-- 22 

18. Oblique photograph northwestward over Isla Tiburon to Baja California .............. --···----····---··---------------------·-···--·--······ 23 

19. Map showing possible strike-slip faults of central and southern Baja California ......... -----·······--·--·---·-----·-·---·--·-----·---- 24 

20. Photograph steeply downward to La Paz region .................. -----·--·-···--···-······-········-··----·--···------·--·--·--·--·--------------··--------····· 25 

III 

RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS OF 

BAJA CALIFORNIA 

By WARREN HAMILTON 

ABSTRACT 

Photographs of Baja California taken from space- by 

astronauts using hand-held cameras on various Gemini and 

Apollo missions and by automatic camera on the Apollo 6 

flight- provide many illustrations of known structural features of the peninsula and also reveal some structures that 

had not been recognized previously. Although the space photographs provide no data that could not be obtained, and in 

much more detail, by ground mapping and by the interpretation of conventional aerial photographs, it is obvious that 

space photography will provide a valuable reconnaissance tool 

when pictures made from space with mapping cameras eventually become available. 

An apparent transform relationship between strike-slip 

and normal faulting is illustrated by the overlapping vertical 

photographs from Apollo 6 of northern Baja California. The 

active Agua Blanca right-lateral strike-slip fault, known to 

have a Quaternary displacement of at least 5 kilometers, 

trends east-southeastward to end at the north end of the 

deeply alluviated north-northwest-trending half graben of 

Valle San Felipe and Valle Chico. The crustal plate bounded 

on the north by the strike-slip fault is bounded on the east 

by the east-facing normal-fault scarp of the Sierra San Pedro 

Martir; the normal fault has a vertical offset of about 5 

kilometers and requires at least 4 kilometers of horizontal 

extension. The Agua Blanca fault appears to be a transform 

to that crustal extension. 

The uplands of the high Sierra San Pedro Martir are 

seen on the photographs to be a low-relief surface deformed 

by young faults, monoclines, and warps, which mostly produce west-facing steps and slopes. The topography is basically structural, not erosional. The west-facing topographic 

steps of the older and more eroded west slope of the Sierra 

Nevada of California may have a similar structural origin. 

The Sierra Cucapas of northeasternmost Baja California 

and the Colorado River delta of northwesternmost Sonora are 

seen to be broken by northwest-trending strike-slip faults, 

which belong to the same family as the Elsinore and San 

Jacinto fault systems of southwestern California. 

Space-photograph coverage of central and southern Baja 

California is poor. No strike-slip faults were recognized in 

the central region. In the southern region, however, a strikeslip fault is inferred to trend northward obliquely across the 

tip of the peninsula from near Cabo San Lucas to La Paz, 

thence offshore along the east coast until it comes ashore 

again as the Bahia Concepcion strike-slip fault. 

INTRODUCTION 

The internal structure of Baja California, MeYico, is of significance because the peninsula now belongs to the great tectonic plate of the Pacific Ocean 

floor west of the crest of the East Pacific Rise rather 

than to the North American plate. The Pacific plate 

is now moving about 6 em (centimeters) per ye~n· 

relatively northwestward past the North American 

plate, the boundary between the two plates being tr~ 

strike-slip San Andreas fault system in California 

and the oblique-extension Gulf of California rift 

through the continental mass in Mexico (!sacks and 

others, 1968 ; Larson and others, 1968 ; Le Pichon, 

1968; Morgan, 1968). 

Photographs of Baja California taken fr01n 

space - by astronauts using hand-held cameras during several Gemini and Apollo flights and by an 

automatic camera during the unmanned Apollo 6 

mission - show structural features important fc--:-

an understanding of the Cenozoic development C'f 

the peninsula. These features, and particularly 

strike-slip faults and related structures, are discusse~ 

in this report. Most of the structures described have 

been identified by other geologists by surface geologic mapping and by interpretation of conventional 

aerial photographs, but some have not been recobnized previously. 

SPACE PHOTOGRAPHS 

The space photographs of Baja California vary 

widely in quality and utility. Photography was a 

major purpose only during the Apollo 6 mission, 

when an automatic camera on an unmanned vehicle 

took overlapping vertical photographs in a single 

strip across Baja California and the southwester:"! 

United States; the pictures can be viewed stereoscopically, and they provide far more information 

than do those of any other Apollo or Gemini mirsions. Some astronauts on the manned flights pro1 
2 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

duced excellent pictures, but some missions produced 

almost no geologically useful photographs. Even the 

best of the hand-held pictures are taken at random 

horizontal and vertical angles. The cameras - 70-

mm (millimeter) Hasselblads, modified to operate 

almost like box cameras- used on the Gemini flights 

were described by Nathan (1969) and Underwood 

(1968). The automatic camera on the unmanned 

Apollo 6 flight was a 76-mm Navy airplane-wing 

"gun" camera (a Maurer 220G, modified to use 70-

mm film). No mapping camera has yet been used in 

nonmilitary space flights. Many space photographs 

have been published in popular books and articles: 

see, for example, Cortright (1968), Lowman (1966, 

1968), and National Aeronautics and Space Administration (1967). 

The transparencies and prints generally available for scientific study are two or three copy-steps 

removed from the original transparencies. Much 

degradation of both color quality and resolution is 

necessarily introduced by these steps. That the original transparencies hold far more information than 

do the copies is made clear by the reproductions in 

the volume by Lowman (1968): these 26 X 26-cm 

color plates were published from prints made with 

great care directly from the original transparencies, 

and even though degraded by the publication process 

contain more detail than do most prints and transparencies currently available for scientific study. 

The overlapping vertical photographs from 

Apollo 6 vividly demonstrate, despite their rather 

low resolution, the potential value of space photography for tectonic interpretation. When vertical space 

photography with mapping cameras becomes available, we will learn much that is new about tectonics, 

particularly of the arid regions of the world. The 

hand-held through-the-window space snapshots from 

the manned flights, however, are useful primarily as 

illustrations, not as serious scientific tools. 

Figure 1 shows the localities covered by the 

space photographs in this report. The vertical photographs of the Apollo 6 strip across Baja California 

are reproduced in black and white in figures 7, 8, 9, 

and 10, and the best of the hand-held Gemini and 

Apollo pictures are presented as black-and-white figures. The useful hand-held photographs are mostly 

those taken steeply downward. The many low-angle 

photographs of the peninsula mostly convey little 

geologic information and will not be discussed in 

this report. 

BAJA CALIFORNIA 

The term "Baja California" as used here refers 

to the entire peninsula, including the Mex!~an portions of the Peninsular Ranges and of tb~ desert 

west of the Colorado River. The geology of Baja 

California has been summarized by Beal (1£\48), and 

in less detail but with incorporation of newer information by Allison (1964). 

A field and aerial-photograph reconnaissance of 

the north half of the peninsula has been made in the 

last decade by R. G. Gastil, the late E. C. Allison, 

and their associates and students, mostly at the San 

Diego State College. Little of their work ha~ yet been 

published, but brief summaries include tho~e by Allison (1964), Gastil (1968), Gastil, Allison, Phillips, 

and Obregon (1969), and Gastil, Allison, and Phillips (1969a). Gastil generously gave me an unpublished 1: 500,000 geologic map of the peninsula north 

of the 28th parallel that was compiled by himself, 

E. C. Allison, and others; generalized contacts, but 

not structure, from this map appear on the new geologic map of Mexico (Herm:lndez-S.-M. and Salas, 

1968). 

I have had the opportunity during two brief 

tours to see in the field many of the structural features in northern Baja California that are referred 

to here. Field impressions have, of course, influenced 

my interpretations of the space photograph;;. I have 

not visited the rest of Baja California. 

STRIKE-SLIP FAULTS IN CALIFORNIA 

Inasmuch as southern California contains known 

major strike-slip faults (see geologic maps by Dibblee, 1967; Jennings, 1967; Rogers, 1966; ancl Strand, 

1962), it is appropriate to look at the space photographs of these faults to determine the criteria by 

which such faults can be recognized on pictures of 

the quality now available. The maximurr latitude 

reached by orbits of Gemini satellites was about 34 o 

- the latitude of Los Angeles - and moqt of the 

space photographs are low-angle obliques looking 

into California from points considerably south of 

this limit. One of the best of these oblique photographs is reproduced as figure 2 (fig. 3 sl''1WS geographical and geologic features in fig. 2), and one 

of several obliques that provide a view northeastward over the Salton Sea is given as figure 4. Handheld space photographs taken steeply downward at 

the region of the Salton depression of southeastern 

California (figs. 5, 6) provide better detail. 
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INTRODUCTION 

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FIGURE 1. - Baja California and southern California, showing localities covered by space photographs in this report. 

3 
4 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

FIGURE 2.- View northward over the coast of southern California, showing the appearance from a distance of the major 

right-lateral northwest-trending fault zones. The San Andreas fault zone is visible along much of its course west-northwest of Cajon Pass as the trough between linear mountain fronts and low, narrow uplifts, but it cannot be identified 

southeast of the pass. The Elsinore and San Jacinto fault zones are conspicuous as alined topographic elements and as 

lines of color change. The strong northwest-trending topographic alinements of the Mojave Desert beyond the San Bernardino Mountains may be due to middle or late Tertiary strike-slip faults, now inactive. The position of the left-lateral, 

northeast-trending Garlock fault is marked in figure 3, but the structure is not conspicuous on the photograph. About 

300 km of coastline appear in the picture. Apollo 7 photograph, AS7- ll- 2021, 1968. (Frames 2020 and 2022 are similar.) 

The comparison of structures visible on the 

space photographs with geologically mapped strikeslip faults indicates that in pictures of comparable 

resolution in desert regions elsewhere, strike-slip 

faults are likely to be recognizable where they produce straight mountain fronts, alined canyons, and 

narrow, straight uplifts, and they are unlikely to be 

visible where they cross alluvial terranes of low 

relief. 

NORTHERN BAJA CALIFORNIA 

Many young structural features (fig. 11) of the 

northern part of Baja California are visible on the 

space photographs. Most of these structures are 

known from surface geologic mapping, but some 

have not been recognized previously. The most useful of the photographs of this region are the overlapping vertical pictures taken by unmanned satellite Apollo 6 (figs. 7, 8, 9, 10). The photographs were 

made on 70-mm positive color film; paper prints can 

be studied stereoscopically. Much of the same area 

is depicted also on hand-held photographs taken 

steeply downward during several manned Gemini 

and Apollo flights on orbits that crossed the peninsula on almost the same track as that of Apollo 6. 

The best of the hand-held pictures have good resolution, color, and sun angles; but because they cannot 

be viewed stereoscopically, they are less useful than 

the overlapping vertical photographs. 

Comparison of the Apollo and Gemini photographs with the unpublished 1: 500,000 geologic map 

of northern Baja California by R. G. Gastil, E. C. 

Allison, and others demonstrates that in the desert 

region, extending east from the crest of the Sierra 

San Pedro Martir to the Gulf of California, many 

contacts between metamorphic rocks, granitic rocks, 
NORTHERN BAJA CALIFORNIA 5 

MOJAVE DESERT 

Cajon Pass 

/ 

San Gabriel Mts San Bernardino Mts 

Santa Cruz 

Island 

LOS ANGELES 

Cat~ lsl;~~a~ 

~ SAN DIEGO 

San Clemente 

Island 

FIGURE 3. - Geologic and geographic features of area shown in figure 2. 

and Tertiary volcanic rocks are easily recognized on 

the pictures. On the scrub-covered west slope, from 

the range crest to the coast, some areas of granitic 

rocks can be recognized on the photographs, but 

contacts with pregranitic metamorphic rocks, postgranitic basalts, and other units cannot generally be 

identified. 

AGUA BLANCA FAULT 

The Agua Blanca right-lateral strike-slip fault 

(figs. 11, 12) trends west-northwestward across Baja 

California from Valle San Felipe to the Pacific coast 

south of Ensenada. Allen, Silver, and Stehli (1960) 

found Holocene stream offsets, typically of about 

50 m (meters), to be shown along most of the fault 

westward from central Vaile Trinidad and displacements of older surficial features, not dated within 

the Pleistocene, to reach at least 270 m for stream 

valleys and 5 km (kilometers) for alluvial fans. The 

fault may have broken at the surface during the 

very severe earthquake of 1892 (Shor and Roberts, 

1958). Total displacement is not established; Allen, 

Silver, and Stehli suggested a possible offset of 

11 km between granitic rocks in a sliver in the fault 

and those outside the fault, and a less likely offset 

of 22 km between rocks of another sliver and wallrocks, but no rocks have yet been matched across the 

fault. The unpublished 1 : 500,000 reconnaissance geologic map by R. G. Gastil, E. C. Allison, and others 

does not suggest that large displacement is probable; 

one possible indicator, the narrow west coastal belt 

of Upper Cretaceous (postbatholith) strata, shows 

an apparent offset of only 10 or 15 km. The fault 

may be entirely of Quaternary age. 

The Agua Blanca fault throughout its length 

lies within a topographic depression (figs. 7, 8). Such 

depressions are characteristic of strike-slip faults 

across mountainous terrain, and probably are due as 

much to lateral transport of varying topography as 

to erosion along fault zones. The depression is narrow except near its ends. In the west, Bahia de Todos 

Santos and its small coastal plain are bounded on 

the south by the fault. Near the east end of the fault, 

the plain of Valle Trinidad extends south from the 

fault to an irregular boundary against faulted highlands. No Holocene faulting was recognized along 
6 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

FIGURE 4.- Photograph northeastward over northeastern Baja California and southeastern California to Arizona and 

beyond. The position of the San Andreas fault is marked by arrows; the fault can be detected on the picture only as the 

contact between light alluvium and dark mountains (mostly upper Cenozoic sediments) at the left edge of the photograph; the fault is not visible in the alluvium along the northeast side of the Salton Sea. Several faults of the San 

Jacinto and Elsinore zones of right-lateral faults produce linear mountain fronts and other ali ned topographic features in the lower left part of the picture, but like the San Andreas they cannot be identified away from mountain fronts 

or linear fault valleys. The alined longitudinal canyons of the Sierra Cucapas probably mark strike-slip faults. The low 

mountain ranges between the San Andreas fault and the northeast limit of known strike-slip faults may be outlined 

mostly by strike-slip faults - east-trending left-lateral faults and northwest-trending right-lateral ones- but no direct 

evidence for this interpretation is apparent on the photograph. Gemini V photograph, magazine 1, frame 13 (NASA 

numbers S-65-45382 for black and white, S-65-45747 for color), taken 22 August 1965 by L. G. Cooper and Charles 

Conrad, Jr. 
NORTHERN BAJA CALIFORNIA 7 

the fault by Allen, Silver, and Stehli (1960) in the 

eastern part of that plain or in the pass through 

which the fault projects into Valle San Felipe. 

The trace of the Agua Blanca fault- as distinguished from the conspicuous narrow valley which 

it typically occupies- is visible on the space photographs primarily in the 50-km sector centered on 

Santo Tomas (fig. 7). The active trace of the fault 

in this sector leaves the valley floor in several places 

to trend along the lower wall of first one side of the 

valley and then the other, and to cut across low hills. 

Wherever the fault occurs thus on the sidewalls, the 

canyons draining into the valley are offset, and the 

drainage pattern above the fault trace does not correspond to that below. The resolution of the photographs is adequate to show clearly the truncation 

and dissimilarity of the now-juxtaposed drainages 

but is not quite good enough to allow picking of precise matches of canyons which should once have been 

continuous: the right-latera l character of the fault, 

obvious on the ground, cannot be read with confidence from the photographs. The fault trace crosses 

parts of the plain of Vaile Trinidad but cannot be 

certainly identified across the flat areas, although its 

position is apparent from the alinement of the faulttrace valleys at opposite ends of the plain and from 

the straight mountain fronts along parts of the Valle 

Trinidad on the line projected between these valleys 

(figs. 7, 8). 

MAIN FAU LT BLOCK OF NORTHERN BAJA CALIFORNIA 

The western two-thirds of the northern part of 

Baja California consists of a westward-tilted fault 

block. The high eastern part of the block south of 

the Agua Blanca fault is the Sierra San Pedro 

Martir, which faces the Gulf of California as an 

imposing scarp. The crest is lower and more diffuse 

north of the Agua Blanca fault, but still farther 

north the Sierra Juarez rises and in its turn gains 

a high east-facing boundary scarp. 

The dominant rocks are granitic and yield Late 

Cretaceous and late Early Cretaceous radiometric 

age determinations, insofar as they have been dated 

(Estavillo and Rogers, 1970; Krummenacher and 

Gastil, 1970; Silver and others, 1969). Contacts between granitic, metamorphic, and young volcanic 

rocks on the western slope are not, in general, apparent on the space photographs, although the larger 

areas of exposed granitic rocks can generally be 

recognized by their joint-controlled rectangular or 

arcuate drainage patterns. 

SIERRA SAN PEDRO MARTIR 

The surface altitude of the main fault block 

south of the Agua Blanca fault rises irregularly 

eastward to a level of 1,000-1,500 mat the west side 

of the Sierra San Pedro Martir proper. Faults and 

warps in that area step the surface upward. The 

crest of the range, at the top of the bounding eastfacing scarp, has altitudes of 2,000-3,100 m and includes the highest peaks in Baja California. 

The late Cenozoic internal structure of the Sierra San Pedro Martir as depicted on the map (fig. 

11) is interpreted from stereoscopic study of the 

space photographs (fig. 8). The Sierran uplands are 

so little dissected that they cannot long have had 

their present high altitude and bounding scarps; the 

topography probably is primarily due to Quaternary 

and ( ?) very late Tertiary uplift and deformation 

of a low-relief surface. The aspect of the surface is 

that of structural, not erosional, topography. West 

of the range crest, steep scarps give way along strike 

to gentler slopes: normal faults apparently grade 

into monoclines. A dome gives way along strike to a 

convex break in slope, apparently as an anticlinal 

axis grades into the flexure at the top of a monocline, 

and a broad topographic low gives way to a concave 

break in slope as a syncline becomes a flexure at the 

bottom of a monocline. 

The low-relief erosion surface was once widely 

covered by thin Eocene and Paleocene deposits (Gastil and others, 1969a) and, hence, dates from the 

Late Cretaceous or earliest Tertiary. 

The east face of the Sierra San Pedro Martir 

is one of the most imposing fault scarps in North 

America. The 2,000- to 3,000-m crest of the range is 

along the top of the scarp, which falls steeply to the 

alluvial fans at an altitude of 500 m. The crest is 

only 10 km from the fans 1.5 to 2.5 km below. High 

cliffs and extremely rugged canyons characterize the 

upper and most-eroded part of the scarp. The lower 

part of the scarp maintains valley-facing slopes of 

30° for altitude gains of 1 km in many places. The 

base of the scarp appears on the space photographs 

to mark a single fault trace. 

Incised canyons high on the scarp are alined 

subparallel to the range crest, and altitudes of the 

east rims of these canyons appear on the space photographs (fig. 8) to be anomalously high. The interpretation is made in figure 11 that these canyons 

mark a west-facing normal fault on the east-facing 

scarp; but the ali ned canyons might instead be products of erosion of the upper, longest exposed part of 

the main fault scarp. 
8 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

FIGURE 5. - Photograph looking steeply downward at the Salton depression, southern California and northern Baja California. 

Some of the known right-lateral faults of the San Jacinto and Elsinore fault zones are apparent as range-front alinements trending northwestward (toward lower left), but the faults are not visible in surficial deposits. Apollo 9 photograph AS9- 22-3437, 9 March 1969. 

COMPARISON WITH SIERRA NEVADA 

The main fault block of northern Baja California is comparable to the SierraN evada of California, 

although it is not quite so wide or high. It is less 

dissected than the Sierra Nevada, and its drainage 

is much less integrated; so presumably the Baja 

California uplift is largely younger than the California one, although climatic differences must account for some of this contrast. 

The west slope of the Sierra Nevada displays 

several west-facing topographic steps, eroded back 

in irregular patterns. Bateman and Wahrhaftig 

(1966, p. 149-154) summarized the controversy in 

the literature regarding these steps, presented new 

data, and concluded that the steps are due to weathering and erosion of an initially more uniform slope. 

The much younger, uneroded steps of the Sierra San 

Pedro Martir appear to be structural, formed by 

monoclinal warping and normal faulting. 

VALLE SAN FELIPE AND VALLE CHICO 

The fault trough of Valle San Felipe and Valle 

Chico extends along the east base of the Sierra San 

Pedro Martir. The trough is mostly 10-20 km wide, 

its single-fault west boundary contrasting markedly 

with the topographic complexity of its irregular east 

boundary, which is defined by small mountain masses 

faulted against and buried beneath it (fig. 9). 

The valley is filled with upper Cenozoic marine and 

nonmarine sedimentary and volcanic rocks, which 

gravity and magnetic data suggest to be about 2.5 

km thick in the center of the trough (Slyker, 1969). 
NORTHERN BAJA CALIFORNIA 9 

FIGURE 6.- Photograph looking steeply downward at the Salton depression, southern California and northern Baja California. 

The alined canyons of the Sierra Cucapas (lower center) mark strike-slip faults. The high normal-fault scarp of the 

Sierra Juarez is obvious at lower left. The San Andreas fault trends northwestward (toward upper left, across upper 

third of picture) but is not apparent on the photograph. The ranges northeast (above and right) of the San Andreas may 

be outlined primarily by strike-slip faults, but these cannot be identified in the picture. Apollo 9 photograph AS9-23- 3558, 

11 March 1969. 

The total structural relief on the normal-fault system is thus about 5 km, from Sierra crest to basement floor of trough. 

The great normal fault bounding the valley on 

the west requires crustal extension. The amount depends upon the configuration of the normal fault at 

depth. A conventional assumption of a 50° dip on 

the fault requires about 4 km of horizontal extension 

to account for the 5 km of vertical offset. The alternative assumption, which I prefer, that the fault 
10 RECOGNITION ON SP ACE PHOTOGRAPHS OF STRUCTURAL ELEME NTS, BAJA CALIFORNIA 

FIGURE 7. - Vertical space photographs of northern Baja California, mounted for ster eoscopic viewing. Overlaps on right 

with figure 8. Apollo 6 phot ographs AS6-2-1432 (left) and 1433. 

flattens downward, requires more extension, perhaps 1970), however, emphasizes that no major fault has 

as much as 10 km. yet been proved by fieldwork to be present in the 

The Agua Blanca fault apparently enters the pass through which the fault projects from Valle 

north end of the Valle San Felipe from the west- Trinidad to Valle San Felipe.) It is apparent from 

northwest (fig. 8). (R. G. Gastil (oral commun., field study (Slyker, 1969), from interpretation of 
NORTHERN BAJA CALIFORNIA 11 

FIGURE 8.- Vertical space photographs of northern Baja California, mounted for stereoscopic viewing. Overlaps on left with 

figure 7 and on right with figure 9. Apollo 6 photographs AS6- 2-1433 (left) and 1434. 

conventional aerial photographs (R. G. Gastil and The low ranges of granitic, metamorphic, and 

others, unpub. map), and from interpretation of the volcanic rocks east and north of the Valle San Fespace photographs that the fault does not cross the lipe- Valle Chico trough display many conspicuous 

low ranges on strike to the east-southeast on the fault scarps. The interpretation of these faults in 

east side of Valle San Felipe (figs. 8, 9). figure 11 is conservative, as can be seen from the 
12 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

FIGURE 9.- Vertical space photographs of northern Baja California, mounted for stereoscopic viewing. Overlaps on left with 

figure 8 and on right with figure 10. Apollo 6 photographs AS6-2- 1434 (left) and 1435. 

space photographs of figure 9; these are little-eroded 

fault scarps, 100-800 m high. Many other faults, 

some smaller and others older and more subdued, 

could be inferred also. The dominant trend of the 

young normal faults is northeastward. The topographic relationships of several of these northeasttrending structures are such that the faults might 

be largely left-lateral rather than normal. 
NORTHERN BAJA CALIFORNIA 13 

Sand dunes 

FIGURE 10. - Vertical space photographs of northern Baja Cali1fornia, mounted for stereoscopic viewing. Overlaps on left with 

figure 9. Apollo 6 photographs AS6-2-1435 (left) and 1436. 

TRANSFORM MECHANICS OF AGUA BLANCA FAULT 

All strike-slip faults must be transform faults, 

in the broad sense of the term proposed by Wilson 

(1965), for all must give way at both ends to ten428-893 0 - 71 - 2 

sion or compression by which the strike-slip motion 

is absorbed. In Wilson's original use of the term, a 

transform fault shifts extension (in the specific form 

of a spreading midocean ridge) from one side of the 
14 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

117° 116. 

32° 

31. 

0 

-

0 

EX PLANATION 

Contact 

Fault 

Dotted w he,-e co'rU:t!t1led, Arrows s how 

relCttive offsel un slrike-slip f ault; bar 

and ball on downthrown side of normal 

fault 

Lower axis U pper axis 

Monocline 

Armwhead marks sleeper limb 

Anticline Syncline 

25 

Cabo 

San Quintin 

50 MILES 

FIGURE 11. - Part of Baja, California, showing Quaternary structures visible on space photographs. Areas of Quaternary 

sediments, mostly continental, are stippled; Colorado River delta is shaded. 

fault at one end to the opposite side at the other end. 

Other variants are possible. A transform in a compressive system can shift shortening from one side 

at one end to the opposite side at the other end. Or, 

shortening and extension can occur at opposite ends 

of the same side of a transform. Compression and 

tension can be either abrupt or distributed. 

The Agua Blanca fault has a right-lateral displacement of at least 5 km. It does not cut the hills 

east of Valle San Felipe, so the displacement must 

be absorbed in extension or compression somewhere 

west of those hills. (R. G. Gastil (written commun., 

1969) has arrived at conclusions similar to those 

presented here.) The fault was vigorously active in 

late Quaternary time, so hypothetical cross structures cannot be called upon to displace it out of 

sight. Transform relationships must be sought. 

Three normal faults trend southward from the 

plain of Vaile Trinidad. The extension represented 

by these faults presumably has absorbed some of the 
NORTHERN BAJA CALIFORNIA 15 

116° 

32° 

The region north of the Agua Blanca fault and 

east of Valle San Felipe and Valle Chico is assumed to be stable in position and shape for 

the purpose of drawing vectors of relative 

motion of the plate south and west of these 

bounding structures 

31° 

--

0 

30° 

Extension to form 

Valle Chico 

Margin of stable 

plate 

0 

FIGURE 12.- Vectors of relative motion of the crustal plate south of the Agua Blanca fault needed to account for strike-slip 

faulting on the structure and for extension producing the Valle San Felipe and the Valle Chico. Map and vector scales 

are equal. Compare with figure 11. 

strike-slip displacement on the Agua Blanca fault. 

Holocene displacement decreases eastward in the 

same sector of the Agua Blanca (Allen and others, 

1960), so the youngest strike-slip motion may be 

taken up by the extension represented by this normal 

faulting. These normal faults are, however, too small 

to account for all of the Quaternary displacement on 

the Agua Blanca fault. 

The crustal plate bounded on the north by the 

Agua Blanca fault is bounded on the east by the 

great normal-fault scarp of the Sierra San Pedro 

Martir and the deep half graben of Valle San FePpe 

and Valle Chico. The Agua Blanca has a right-lateral 

Quaternary displacement of at least 5 km, and the 

normal fault represents an extension of at least. 4 

km. Apparently the Agua Blanca fault is the trans-
16 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

form to the crustal extension to which the trough of 

Valle San Felipe and Valle Chico owes its normal 

faulting and deep subsidence. The plate bounded by 

strike-slip and normal faults has moved relatively 

west-northwestward away from the terrain east of 

the fault trough as well as past the terrain north of 

the Agua Blanca fault itself (figs. 11, 12). The extensional normal faulting has occurred in the lee of 

the drifting plate. 

This transform relationship accounts for the 

proved displacement on the Agua Blanca fault. If, 

however, the actual total displacement is greater 

than perhaps 10 km, then some of that amount must 

be transformed by structures other than the large 

normal-fault trough. 

A transform relationship of subsidiary structures within the trough of Valle San Felipe and 

Valle Chico is suggested by the geometry of the 

trough. The constriction of the trough separating it 

into the halves bearing the separate names is defined 

by abrupt deflections of both east and west margins 

westward from the general north-northwestward 

trends. A strike-slip fault striking west-northwestward, parallel to the Agua Blanca fault, beneath the 

constriction would account for this geometry (fig. 

12). Valle San Felipe and Valle Chico may be separate grabens, the extensional motion being transformed 8-10 km between them on this strike-slip 

fault. 

No strike-slip fault leaving Valle Chico at the 

south is apparent on the space photographs (figs. 

9, 13). Because the set of extensional troughs and 

transform strike-slip faults does not appear to be 

complete, counterclockwise shear or rotation of the 

Sierra San Pedro Martir away from Valle San Felipe and Vaile Chico may be indicated. The internal 

deformation of the Sierra, the height of its frontal 

fault scarp, and the width of Valle Chico all decrease 

southward from the central part of the range, in 

accord with the inference that the amount of extension decreases in the same sector. This concept, 

illustrated by figure 12, if correct requires that internal deformation of the plates bounded by the 

Agua Blanca fault operates to prevent rotation of 

the southern plate away from the northern plate 

west of Vaile Trinidad to Bahia de Todos Santos. 

The young normal or left-lateral faults east of 

the Valle San Felipe-Valle Chico trough trend dominantly northeastward, perpendicular to the regional 

strike of the basement rocks and approximately 45° 

from the extensional axes of the Valle San Felipe 

and Valle Chico troughs. Presumably these faults 

represent jostling and oblique extension of the plate 

east of the main zone of extension. 

SIERRA JUAREZ AND LAGUNA SALADA 

The eastern uplands north of the Agua Blanca 

fault rise northward to form the Sierra Juarez. The 

south end of this mountain mass is within tht: Apollo 

6 vertical-photographs strip (figs. 7, 8), ~.nd the 

northeastern part is shown by several oblique Gemini 

and Apollo photographs (for example, fig. 6). 

The south end of the Sierra Juarez has a diffuse 

crestal region and gives way to the low desert to the 

east via irregular small mountain masses. Tl'o. crest 

of the range within the vertical-photograpl's strip 

(figs. 7, 8) consists of a low-relief surface, developed 

primarily on granitic rocks, from which rise irregular erosional remnants of sheets of Tertiary volcanic 

rocks : stripping of the volcanics has exhuned the 

older and more resistant erosion surface. Thr~e postvolcanism normal faults, trending northwestward 

oblique to the north-northwest trend of the range, 

are seen on the photographs to break the eastern 

slope of the range and to step and tilt the volcanic 

sheets and the underlying erosion surface irrt:gularly 

downward. 

The northern part of the Sierra Juarez is similar to the Sierra San Pedro Martir: it consists of 

broad upland surfaces, generally sloping gently westward and deformed by steps which appear to include 

both monoclines and normal faults, and its high east 

normal-fault scarp faces the deeply alluviated valley 

of the Laguna Salada. The top of the frontd scarp 

has an altitude of 1,500-1,800 m, and the bcttom is 

at 50-250 m; the central part of the valley floor is 

below sea level. 

The crestal upland and east scarp of the Sierra 

Juarez are in the lower right part of figure 5. The 

dark terrain across the base of the photograph is the 

wooded upland, and the slightly lighter terrain between that and the alluvial basins is the arid east 

scarp. The base of the scarp is shown by the photographs to be a less sharp line than that of the Sierra 

San Pedro Martir: the Sierra Juarez fault apparently was much less active during late Quaternary 

time than was the boundary structure of tht: Sierra 

San Pedro Martir. 

The geometric similarity of the Sierra San Pedro 

Martir-Valle San Felipe and the Sierra JuarezLaguna Salada structural pairs suggests common 

mechanisms for both. A transform mechanism is incHeated for the former, and a similar mechanism 

also may account for the latter. The right-lateral 

Elsinore fault zone trends southeastward to the 

northwest corner of the Laguna Salada baFin, and 

perhaps it is a transform fault related to oblique 

extension which produced the basin and the normal 

fault bounding the Sierra Juarez. The Elsinore fault, 
NORTHERN BAJA CALIFORNIA 17 

however, projects into the faults of the Sierra Cucapas, discussed subsequently; so apparently only part 

of its displacement is absorbed by extension of the 

Laguna Salada. 

Three earthquakes in 1956 were the result of 

right-lateral slippage of less than 1 m along a 20-km 

sector of a fault trending N. 60° W. in the Valle 

Rodeo, north of Vaile Trinidad and west of the 

southern Sierra Juarez (Shor and Roberts, 1958). 

The Valle Rodeo, like the Alamo Plain adjoining it, 

is a broad low-relief area of deeply weathered bedrock and minor alluvium. The activated fault segment is not visible on the space photographs. The 

photographs do show, however, that no important 

strike-slip fault offsets the volcanic mesas of the 

Sierra Juarez crest on strike a few kilometers southeast of the end of the 1956 fault trace, and so the 

fault presumably is a minor one although it may, as 

Shor and Roberts concluded, continue northwestward 

far beyond the 1956 break. 

Several lineaments of alined minor streams and 

scarps trend N. 50°-60° W. from the Alamo Plain 

northwestward. These lineaments can be recognized 

on the photographs (figs. 7, 8), although the topographically most conspicuous segments are farther 

FIGURE 13.- Overlapping oblique photographs of the southern part of northern Baja California. A. Southwestward 

(toward upper left) over Valle Chico (YC) and the southern Sierra San Pedro Martir. (Compare with figs. 7, 8.) 

No strike-slip fault is apparent leaving the south end of 

the valley toward the Gulf of California. Part of Apollo 7 

photograph AS7-3-1528, 1968. B. Southeastward (toward 

bottom) along the peninsula. Apollo 7 photograph AS7-5-

1629, 1968. 
18 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

northwest, beyond the area covered by the space 

photographs, and perhaps mark minor strike-slip 

faults. These structures are not shown in figure 11 

because the photographs do not provide clear evidence as to their character. None of the lineaments 

is visible as far southeastward as the Agua Blanca 

fault or the Sierra Juarez high country. 

SIERRA CUCAPAS 

Strikingly linear narrow valleys lace the Sierra 

Cucapas, east of the Laguna Salada, and are continuous throughout the range, crossing drainage divides 

and convergences (figs. 6, 14). These valleys have the 

topographic signature of strike-slip faults. The 

troughs arc in a gently anastomosing subparallel 

array, from west-northwestward trends in the southeast to northwestward ones in the northwest. In the 

eastern part of the range, massive granitic rocks, 

and the migmatites which bound them on the east, 

are cut at high angles by the troughs. My brief field 

examination suggested right-lateral displacements 

of the granitic and migmatitic rocks of about 5 km 

along each of two of the troughs. The range has been 

mapped by Barnard (1968) . 

The effect of these faults is, in transform terms, 

to widen southward the Colorado River delta sector 

of the Gulf of California depression. 

COLORADO RIVER DELTA FAULTS 

Several of the Apollo and Gemini photographs, 

including those in figures 10 and 14, show a conspicuous straight fault in the delta sediments of 

northwesternmost Sonora. The picture best illustrating the structure is reproduced as figure 15. (The 

fault was also recognized on space photographs by 

Lowman, 1968, pl. 3, who presented an excellent 

color reproduction of the same photograph.) Ponding along the southwest side of the fault shows the 

northeast side to be relatively uplifted by at least a 

few meters. 

The modern delta ends on the northeast against 

a scarp eroded by distributaries into older Quaternary river and delta materials (fig. 15; Merriam, 

1965). This scarp may have been eroded back from 

a fault scarp formed by repeated offsets along the 

active fault trace visible in the photographs, or it 

may follow another fault strand. 

Several faults were recognized on aerial photographs and in the field by Merriam (1965) near the 

coast in the area shown in the lower right quarter 

of figure 15. These are strike-slip faults with a vertical component that is generally up on the northeast 

side; the faults lie northeast of and parallel to the 

projection of the delta fault. Delta sediments have 

been raised at least 150 m by the vertical component 

of late Quaternary motion, and of this at least 15 m 

-the altitude of a barnacle-encrusted Indian artifact- is late Holocene (Merriam, 1965). Several of 

these faults that were identified by Merriam can be 

recognized on the space photographs, and the positions of two of them are marked by arrows in fig. 10. 

The Imperial Valley and Colorado delta region 

are laced by active right-lateral northwest-trending 

faults which have produced many severe earthquakes. 

The unconsolidated sediments of the delta could not 

long maintain a strike-slip fault trace as a conspicuous surface feature. The delta fault probably 

broke during the earthquake of December 31, 1934 

(Biehler and others, 1964, fig. 1; Kovach and others, 

1962). There was a very severe shock in 1903 "south 

of the Imperial Valley" (Ulrich, 1941, table 1) and 

a major earthquake in the delta region in 1915 

(Biehler and others, 1964), either or both of which 

might also have resulted from surface breakage on 

the delta fault. Kovach, Allen, and Press (1962) suggested that this fault is the southeastern projection 

of the San Jacinto fault of southern California. The 

Sierra Cucapas and delta faults converge at a low 

angle (fig. 14), so the Elsinore-Cucapas and San 

Jacinto faults may join in the delta. 

VOLCANIC ROCKS 

Middle and upper Cenozoic basaltic, andesitic, 

and rhyolitic rocks are widespread in northern Baja 

California (Barnard, 1968; Gastil and others, 1969; 

Gastil and Krummenacher, 1970; James and LaBorde, 1969; Sommer and Garcia, 1970). Within the 

area of the vertical space photographs (figs. 7, 8, 9, 

10), volcanic rocks are abundant in the crestal region of the Sierra Juarez and in the desert mountains to the east of it, particularly the Sierra Pintas. 

The volcanic rocks are conspicuous, because of their 

bright or dark colors, on color photographs from 

space, although the distinction between volcanic 

rocks younger than the granitic rocks and metamorphic rocks older than the granitic rocks cannot 

everywhere be made unambiguously. 

Within the northern part of the photographed 

crestal area of the Sierra Juarez, the volcanic rocks 

form erosional remnants of extensive reddish and 

brownish volcanic sheets, presumably silicic welded 

tuffs, that stand as buttes and mesas rising above 

the exhumed prevolcanism erosion surface on the 

dominantly granitic rocks of the basement complex. 

Similar sheets cap normal-fault blocks and ramps 

stepping down the east side of the range, although 

the broad structure of the east slope of the range 

here, as defined by the attitude of the base of the 

volcanic rocks, appears to be monoclinal rather than 
NORTHERN BAJA CALIFORNIA 19 

FIGURE 14.- View southward over the delta of the Colorado River. The inferred strike-slip faults (arrows) trending northwestward along the northeast side of the delta and through the Sierra Cucapas (sc) may connect in the irrigated region 

(dark). VSF, Valle San Felipe ; vc, Valle Chico ; SSPM, Sierra San Pedro Martir; LS, Laguna Salada; SJ, Sierra Juarez; SP, 

Sierra Pintas. Apollo 9 photog raph AS9- 21-3288, 9 March 1969. 

faulted. The volcanic rocks are likely of early or 

middle Miocene age (Gastil and others, 1969b) and 

appear to wholly antedate the faulting and tilting. 

The southern part of the photographed crestal 

area of the Sierra Juarez exposes thicker complexes 

of bright-colored volcanic rocks, which lack the conspicuous layering of the sheet volcanics to the north 

and may have been eroded from andesitic or dacitic 

stratovolcanoes, although no constructional topography is apparent. Bright-colored volcanic rocks 
20 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

FIGURE 15. - Near-vertical photograph of the mouth of the 

Colorado River and the head of the Gulf of California. The 

desert ranges and alluvial fans of northeastern Baja California are on the left, and the sandy Altar Desert of northwestern Sonora is on the right; between them is the delta 

of the Colorado. The straight line between the arrows is 

presumably a very active strike-slip fault. Part of Gemini 

IV photograph, magazine 8, frame 3 (NASA numbers S- 65-

32910 for black and white, S-65-34673 for color), taken 

5 June 1965 by E. H. White. 

form irregular desert mountains, notably the Sierra 

Pintas, east of the Sierra Juarez; these rocks also 

lack the even layering of some of the crestal rocks 

and appeared on brief ground inspection to be deeply 

eroded stratovolcanoes of dominantly intermediate 

rocks. 

CENTRAL BAJA CALIFORNIA 

The northern part of central Baja California is 

shown with fair detail on several moderate-oblique 

hand-held space photographs, of which the best are 

reproduced as figures 13, 16, and 17. The southern 

portion of the central part of the peninsula is represented primarily by low-angle oblique photographs, 

most of which show relatively little detail; figure 18 

is an exception and combines fair detail with coverage of a large region. The geology is complicated -

a basement complex of granitic and metamorphic 

rocks is overlain by Tertiary sedimentary rocks and 

by upper Cenozoic volcanic rocks and is disrupted 

by irregular basins surfaced by Quaternary sediments (Hermindez-S.-M. and Salas, 1968). The Quaternary basins are obvious on the pictures, and the 

sheetlike character of the resistant volcanic rocks 

permits their recognition in many places (as in fig. 

17). Most of the peninsula is desert at this latitude: 

the west slope largely lacks the dense scrub present 

farther north. It will obviously be possible to much 

improve upon available geologic maps by interpretation of stereoscopic space photographs when such 

are available here, but I have not attempted to make 

a map from the random oblique photographs. 

The pictures show clearly that no narrow topographic trough trends northwestward across the 

peninsula in this sector: there are no apparent crosscutting strike-slip faults analogous to the Agua 

Blanca fault or to the strike-slip structures of the 

Peninsular Ranges of California. (Crosscutting 

strike-slip faults have been hypothesized in this sector- for example, the Sal si Puedes and Santa Rosalia faults of Rusnak, Fisher, and Shepard (1964, 

fig. 4) .) 

In the northern part of central Baja California, 

there is an alinement of structural features along 

the axis of the peninsula (figs. 13A, 13B, and 16). 

'The prints I have st11died lack resolution adequate to 

define satisfactorily the nature of this alinement, 

whose most conspicuous element is narrow dark hills. 

That it is not a major strike-slip fault seems indicated by the lack of its continuation in the mountainous regions to the northwest and southeast along 

its trend or, alternatively, by the lack of basins of 

possible transform character at either end. 

The islands - Isla Angel de la Guarda and Isla 

San Lorenzo - east of the central part of the peninsula are elongate parallel to the general trend of the 

Gulf coast of Baja California but are markedly disjunct to structures within it (figs. 17, 18). The 

islands rise from a narrow, straight submarine high 

that is separated from the peninsula by a straight 

and narrow trench 800-1,500 m deep (Chase, 1968). 

On the peninsula, a conspicuous alluviated normalfault trough, bounded on the west side by a high 

scarp, trends northward obliquely to the coast south 

of Isla Angel de la Guarda (fig. 17) . The submarine 

trench must mark a major fault, and in the regional 

context it could well be a right-lateral strike-slip 

fault, as Rusnak, Fisher, and Shepard (1964, p. 70) 

concluded. 

The northeastern half of the peninsula between 

t he parallels of 26° and 30° N. is marked by northtrending topographic elements, oblique to the northwest trend of the peninsula. A number of these 

elements are within the view of figure 18. One 

mountain front - to the left of the lower half of 
CENTRAL BAJA CALIFORNIA 21 

FIGURE 16.- View steeply downward over the Rosario-Santa Maria sector of Baja California. North is toward top. The 

small basins in the lower right quarter appear to be outlined in part by normal faults. No strike-slip faults are apparent. 

Apollo 9 photograph AS9-21-3262, 8 March 1969. 

Isla .Angel de la Guarda in figure 18-in this system 

of northerly elements is obviously a young normal 

fault, and it is marked by a nearly straight scarp 

about 1,500 m high. (See also fig. 17.) When space 

photographs providing detailed coverage of this sector are available, it should be possible to tell much 

about the extent of similar structures elsewhere 

among the north-trending elements. The structures 

could of course also be studied on conventional aerial 

photographs. 
22 RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

FIGURE 17.- View southeastward (toward upper left) down the Gulf of California. The mainland is on the left, and Baja 

California is on the right. Eroded sheets of dark volcanic rocks are present in upper right quarter. Normal faulting is 

indicated by various scarps in the lower right part of the picture. Apollo 7 photograph AS7-5- 1630, 1968. 

SOUTHERN BAJA CALIFORNIA 

The southern half of Baja California is but 

poorly covered by available space photographs. Several small parts of it are shown by near-vertical photographs, but most appear only on low-angle oblique 

pictures which convey relatively little information. 

A possible major strike-slip fault is inferred in 

figure 19 to cross northward through the tip of the 

peninsula and thence to arc northwestward along the 

northeast coast, and to have a total length of at least 

600 km. Part of the evidence for this fault comes 

from the space photographs and part from other, 

published sources. The map is annotated to summarize the evidence. 

A marked lineament of narrow vegetational and 

topographic elements crosses northward obliquely 
SOUTHERN BAJA CALIFORNIA 23 

' ,; 

·•~· ~A 

""' -...... ..... ~ 

FIGURE 18.- Oblique view northwestward over Isla Tiburon to central Baja California. Alluvia ted basins in the upper half 

of the picture have generally northward trends, oblique to the peninsula. A conspicuous dome appears in the lower left 

corner. Gemini V photograph, magazine 3, frame 31 (NASA numbers S- 65- 45460 for black and white, S-65-45702 for 

color), taken 21 August 1965 by L. G. Cooper and Charles Conrad, Jr. 

through the south end of the peninsula to La Paz 

(figs. 19, 20). The north part of this line is followed 

by drainage toward Bahia de la Paz, but the rest of 

the lineament cuts directly across westward-flowing 

streams, and so the line is inferred to mark a strikeslip fault. 

The southward projection of this lineament is 

shown by continuous-seismic submarine profiling to 

mark a fault down the continental slope, the east 

side being relatively downdropped (Normark and 

Curray, 1968). 
24 

28° 

26° 

24° 

RECOGNITION ON SPACE PHOTOGRAPHS OF STRUCTURAL ELEMENTS, BAJA CALIFORNIA 

Isla Cedros 

0 

f\ Bahia V Sebastian 

<;;:) Vizcwtrw 

0 

0 

100 

100 

Straight line on photographs crosses 

drainage; strike-slip fa u lt(?) 

Possible offset of upper, { 

continenta l slope ' 

Acoustic profiling shows east-side-down--{ 

faulting along trough down continental 

slope. Little i f any lateral offset of 

base of s lope (Normark and Curray, 

1968) 

200 MILES 

200 KILOMETERS 

FIGURE 19.- Possible strike-slip faults in the region of central and southern Baja California. 
REFERENCES CITED 25 

FIGURE 20.- View steeply downward to La Paz region of 

southern Baja California. The arrow points along a vegetational and topographic lineament which cuts obliquely 

across the peninsula and directly across much of the drainage pattern, and which may mark a strike-slip fault. The 

length of the peninsula within the picture is about 200 km. 

Gemini V photograph, magazine 2, frame 39 (NASA numbers S-65-45585 for black and white, S-65- 45530 for color), 

taken 27 August 1965 by L. G. Cooper and Charles Conrad, 

Jr. 

McFall (1968) mapped the area about Bahia 

Concepcion and found that bay to be developed along 

a strike-slip fault that trends northwestward along 

the Gulf coast. McFall deduced a post-early Miocene 

displacement of about 30 km along the fault, although control over the amount of movement is poor. 

The fault is expressed by a straight topographic 

trough- the onshore continuation of the bay depression - where it is exposed southeast of the bay 

(McFall, 1968, fig. 3). A space photograph shows 

this depression to be continuous southeastward to 

the coast west of Isla Carmen. 

Continuity of faulting near the coast between 

La Paz and Bahia Concepcion faults is suggested in 

figure 19, because the peninsula is bounded by the 

southeastward projection of the Bahia Concepcion 

structure. Carmen, Agua Verde, San Jose, and Espiritu Santo islands are all inferred to stand east of 

the Bahia Concepcion-La Paz fault. Rusnak, Fisher, 

and Shepard (1964, fig. 4 and p. 71) made the same 

interpretation. The Bahia Concepcion-La Paz fault 

- if its character and continuity have been correctly 

inferred- may represent motion between the North 

American and western Pacific tectonic plates before 

the mouth of the present Gulf of California began to 

open by separation of the Cabo San Lucas terrain 

from mainland Mexico. (See Larson and others, 

1968; McKenzie and Morgan, 1969.) 

REFERENCES CITED 

Allen, C. R., Silver, L. T., and Stehli, F. G., 1960, Agua 

Blanca fault- A major transverse structure of northern 

Baja California, Mexico: Geol. Soc. America Bull., v. 71, 

no. 4, p. 457-482. 

Allison, E . C., 1964, Geology of areas bordering Gulf of California, in Marine geology of the Gulf of CaliforniaA symposium: Am. Assoc. Petroleum Geologists Mem. 3, 

p. 3-29. 

Barnard, F. L., 1968, Structural geology of the Sierra de los 

Cucapas, northeastern Baja California, Mexico, and Imperial County, California : Colorado U niv. Ph. D. thesis, 

189 p.; abs. in Dissert. Abs., Sec. B, Sci. and Eng., v. 29, 

no. 9, p. 3360B, 1969. 

Bateman, P. C., and Wahrhaftig, Clyde, 1966, Geology of the 

Sierra Nevada, in Geology of northern California: California Div. Mines and Geology Bull. 190, p. 107-172. 

Beal, C. H., 1948, Reconnaissance of the geology and oil possibilities of Baja California, Mexico: Geol. Soc. America 

Mem. 31, 138 p. 

Biehler, Shawn, Kovach, R. L., and Allen, C. R., 1964, Geophysical framework of northern end of Gulf of California 

structural province, in Marine geology of the Gulf of 

California- A symposium: Am. Assoc. Petroleum Geologists Mem. 3, p. 126- 143. 

Chase, T. E., 1968, Sea flcor topography of the central eastern Pacific Ocean: U.S. Bur. Commercial Fisheries Circ. 

291, 33 p. 

Cartwright, E. M., compiler and ed., 1968, Exploring space 

with a camera: Nat!. Aeronautics and Space Adm. Spec. 

Pub. 168, 214 p. 

Dibblee, T. W., Jr., 1967, Areal geology of the western Mojave 

Desert, California: U.S. Geol. Survey Prof. Paper 522, 

153 p. 

Estavillo, William, and Rogers, Mark, 1970, Potassium-argon 

dating of metamorphic and plutonic igneous rocks from 

San Matias Pass, Baja California, Mexico: Geol. Soc. 

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