Download Regional geophysical setting of the Rio Grande rift

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Regional geophysical setting of
the Rio Grande rift
LiNDRITH CORDELL
U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225
ABSTRACT
The Rio Grande rift encompasses uplifts
of the southern Rocky Mountains and their
southern extension as well as axial fault
blocks. The rift widens irregularly southward from a narrow horst in Colorado into
j broad collapsed vault, characterized by
grabens, in southern New Mexico. Whether
manifested by horsts or grabens, primarily
extensional strain is involved which increases in magnitude southward. Extensional faulting along the rift occurred in
N'eogene to Quaternary time, but the rift
follows an axis of Laramide, Pennsylvanian, and possibly earlier uplifts. Gravity
gradients due to the low density of graben
fill delineate major faults of the rift system,
which show a gridded or en echelon pattern
over distances of tens of kilometres.
Aeromagnetic data show these faults to be
aligned with basement structural grain.
Zigzags hundreds of kilometres long in the
trend of the rift may also be related to
basement grain. Basement trends in the
Colorado Plateau to the west seem to differ
in direction from those in the High Plains to
the east. Seismic data also show that the rift
occurs in an area of transition between
anomalous crustal and upper mantle structure typical of the Cordillera and crustal
structure typical of the High Plains. Deep
seismic data are sparse within the rift, but
high heat flow, high elevation, high electrical conductivity, and both residual positive
(shallow source) and negative (deep source)
gravity anomalies suggest the presence of
symmetrical anomalous crustal and
upper-mantle structure along the axis of the
rift. In the Socorro area, where the rift has
been studied intensively, available data indicate relatively low compressional velocity, rapid Holocene uplift, and the presence
of magma within the crust. In view of
geomorphic evidence for widespread
Holocene faulting, the seismicity of the rift
is surprisingly low.
INTRODUCTION AND GEOLOGICAL
BACKGROUND
This paper provides a review of results of
geophysical studies in the Rio Grande rift.
The rift trends northward across New
Mexico and Colorado, the location of its
sides and endpoints depending somewhat
on one's definition of a rift. Mixed usage of
the terms "rift" and "graben" is useful in
this regard: a nearly continuous "Rio
Grande graben" extends along the axis of
the southern Rocky Mountains uplift from
Leadville, Colorado, to Santa Fe, New
Mexico, and continues southward in New
Mexico to Socorro (Fig. 1). The graben is
about 25 to 50 km wide. In contradistinction to this, the term "Rio Grande rift" entails genetic concepts that extend the area of
consideration to include en echelon and
bifurcating uplifts far to the north of Leadville and south of Socorro, and into the
Basin and Range1 country of southern New
Mexico, west Texas, and the Republic of
Mexico. As used here, the term "rift" also
encompasses flanking uplifted borders and
previously uplifted borders that have subsequently been dropped along zones of antithetic faults. The width of the "rift" is several hundred kilometres.
Hammond (1964) subdivided the western United States into four major physiographic types: plains, tablelands, mountains, and mountains-in-plains. These
physiographic subdivisions have characteristic geophysical signatures and reflect an
evolutionary sequence that is strikingly developed in the Rio Grande rift. Major faults
of the rift system are shown with a locality
index and physiographic province map in
Figure 1. Figure 2 shows generalized elevation contours.
As is apparent in Figures 1 and 2, the rift
follows an axis of high mountains. The
southern Rocky Mountain system here, as
in the earliest geological study of the rift
(Lee, 1907), is considered to extend into
Geological Society of America Bulletin, v. 89, p. 1073-1090, 15 figs., July 1978, Doc. no. 80710.
1073
southernmost New Mexico (Fig. 2). The
Colorado Plateau (the large "tablelands"
tract near the center of Fig. 1) is rimmed by
mountains, and tableland country similar to
the plateau extends east of the southern
Rocky Mountains. En echelon offsets
characterize the rift throughout New
Mexico and Colorado. In New Mexico the
graben borders are offset, but the axis of the
graben is essentially continuous. In Colorado the graben and related fault structure
north of Leadville comprise separate elements en echelon in a right-lateral sense.
The most recent episode of uplift and extensional faulting along the rift occurred in
Neogene and Quaternary time, although
prior episodes of tectonism have occurred
along this same trend. Geophysical discontinuities along the axis of the rift suggest inherited Precambrian structural grain.
Linear uplift, arkosic sedimentation, and
high-angle faulting occurred along the rift
in Pennsylvanian-Permian and Late Cretaceous-middle Eocene (Laramide) time,
separated by episodes of planation during
. late Mesozoic time (compare Cordell,
1976a, Fig. 4) and again during the late
Eocene (Epis and Chapin, 1975). The approximate coincidence of uplifts in time
(Chapin and Seager, 1975; Tweto, 1975) is
apparent in New Mexico and especially in
Colorado, even to the superposition of
Holocene and Pennsylvanian alluvial fans
along the mountain front (Howard, 1966).
An extensive surface of generally low relief, developed during late Eocene time
(Epis and Chapin, 1975; Seager, 1975),
provides a structural datum separating the
rift from earlier structural regimes. Voluminous andesitic, calc-alkalic magmatism
(Elston, 1976) was initiated about 37 m.y.
B.P. A chain of these primarily Oligocene
andesites occurs within the rift, and is restricted to the rift, between the Davis
Plateau in Texas (300 km south of El Paso;
Fig. 1) and North Park in Colorado.
Extensional faulting began 25 to 29 m.y.
300 KILOMETERS
' ^W""-:>--^
Figure 1. Locality index, major physiographic provinces (modified from Hammond, 1964), and faults of Rio Grande rift. Heavy lines show princ
faults of Rio Grande graben, determined primarily from gravity data. Odier faults (from Woodward, and others, 1975; Dane and Bachman, 1965;
other sources) are shown schematically with lighter lines. Base map showing Utah, Colorado, Arizona, and New Mexico modified from U.S. Geolog
Survey shaded relief maps, scale 1:500,000. NP = North Park, D = Denver, L = Leadville, SVF = San Juan volcanic field, JVF = Jemez volcanic field,
= Mount Taylor, SF = Santa Fe, A = Alburquerque, JL = Jemez lineament (with arrows), S = Socorro, DMVF = Daril-Mogollon volcanic field, LC =
Cruces, EP = El Paso, SM = Sacramento Mountains, GM = Guadalupe Mountains.
300 KILOMETERS
J
Figure 2. Generalized elevation, in metres, relative to sea level, of Rio Grande rift.
Contour interval is 500 m. Heavy lines show major faults of axial graben, located
primarily by gravity data; light lines show schematically other faults of rift and possibly
related structure.
1076
ago, according to Chapin and Seager (1975)
and Seager (1975), who described the
Neogene tectonic development in central
and southern New Mexico in terms of initial extension during latest Oligocene and
early Miocene time, characterized by broad
warping and transition from calc-alkalic to
"basaltic andesite" volcanism. Uplift, extensive block faulting, and bimodal basaltrhyolite volcanism ("fundamentally basalt
volcanism" in the sense of Christiansen and
Lipman, 1972, and Lipman and others,
1972) occurred during latest Miocene and
Pliocene time. Similarly, Scott (1975) and
Taylor (1975) described accelerated uplift
during Pliocene time in Colorado.
Tholeiitic basalt occurs along the axis of
the rift in southern Colorado and northern
New Mexico, according to Lipman (1969)
and Lipman and Mehnert (1975), who
showed that the tholeiite fractionated at
shallow depth within an inferred mantle
bulge. Kudo and others (1971; Kudo, 1976)
have pointed out, however, that in central
New Mexico tholeiites also occur west of
the axial graben, in the vicinity of Mt.
Taylor. Laughlin and others (1972) have
also emphasized that a chain of Quaternary
basalts trends northeastward from the Mt.
Taylor area obliquely across the graben
along the Jemez lineament (Fig. 1; Lambert,
1966). This is true, but, as discussed below,
these basalts occur within the rift as broadly
defined.
The distribution of basalts along the
Jemez lineament may reflect the intersection
of the rift with a transverse structural discontinuity within the Precambrian basement. Basement structure has been assumed
to have played an important role in the
Cenozoic development of the rift, but the
concept has remained conjectural because
of the difficulty of mapping in three dimensions.
T
L. CORDELL
berg and Smithson (1975), Cordell and
others (1973), and Cordell (1972, 1976a,
1976b), forms the basis for drawing the
principal graben border faults as shown in
Figures 1 through 7. The faults display a
northwest-northeast gridded pattern in
southern New Mexico (Ramberg and
Smithson, 1975). A similar gridded pattern
is observed in the western part of the graben
in New Mexico north of Socorro. Here,
however, the eastern margin of the graben
comprises north-trending en echelon faults.
Gravity data can also be used to estimate
structural relief across the graben faults,
although the estimates are subject to errors
in assigned densities and other factors
(Joesting and others, 1961). Estimated
structural relief is typically 2 to 3 km and,
exceptionally, as much as 5 km. Larger
structural relief reported for the San Luis
basin in southern Colorado may be in error,
due to suspected error in elevation control
(W. F. Isherwood and G. R. Keller, 1976,
oral commun.).
Apart from the shallow-source gravity
anomalies caused by low-density sedimentary rocks, two broad, deep-source anomalies are observed. A residual broad positive
gravity anomaly attributed to the penetration of mantle material into the crust occurs
along the rift axis (Cordell, 1976a). A very
broad negative anomaly is associated with
the regional uplift. These show up best on
profiles and will be discussed in connection
with Figures 9 through 15.
A Bouguer gravity anomaly map of the
Four Corners states is shown in Figure 3.
Sparse seismic data (Fig. 5) indicate regional variation in seismic velocity of the
upper mantle, suggesting that the broad features in the gravity map are related to density variation within the upper mantle as
well as variation in crustal thickness. The
broad topographic and gravity features
(Figs. 2, 3) coincide, reflecting rapid
GRAVITY DATA
buoyant response (Bridwell, 1976) to regional density changes in the crust or manGravity data facilitate mapping the gra- tle related to Neogene tectonics. Like the
ben border faults, basement structural uplifts, the associated broad gravity lows
grain, and deep-crustal and upper-mantle are Neogene in age.
Gravity anomalies associated with essenstructure related to Neogene tectonics,
although the gravity effects of all of these tially static intracrustal density variation
are superimposed. Gravity gradients related include those associated with the lithology
to low-density graben fill delineate major of the sedimentary cover, such as the graben
faults of the rift system having displace- fill anomaly; Paleogene batholiths in
ments of several kilometres each. These southwestern Colorado, southwestern New
faults commonly do not follow the trend of Mexico, and perhaps elsewhere (Plouff and
smaller faults mapped at the surface. The Pakiser, 1972; Steven, 1975; Elston and
use of gravity data in mapping the graben others, 1973, 1976); and intrabasement
border faults is discussed in detail elsewhere density variation in eastern New Mexico
(Cordell, 1970, 1976a). This interpretation, and elsewhere. The long northeast-trending
plus reference to gravity maps of Sanford gravity gradient crossing northeastern
(1968), Kleinkopf and others (1970), Ram- Arizona and the extreme northwest corner
of New Mexico (Fig. 3), not apparent eithc
in the topography (Fig. 2) or in the structur
of the sedimentary cover, is associated VVJ.L
an aeromagnetic lineament and probabl
delineates a northeast-trending Precarn
brian discontinuity. A parallel northeast
trending gravity gradient may be present in
New Mexico, intersecting the rift obliqUe|y
between Santa Fe and Socorro, but this
trend, if it exists at all, is obscured by grav.
ity trends related directly to the rift.
AEROMAGNETIC DATA
Aeromagnetic coverage of Colorado js
available at a flight-line spacing of l.g t0
8.0 km (Zietz and Kirby, 1972), and coverage of Arizona is available at a flight-line
spacing of 5 km (Sauck and Sumner, 197Q)
In New Mexico, aeromagnetic surveys were
flown at 1.6-km flight-line spacing,
although coverge does not yet extend far
east nor west of the rift; all the available
data are shown in Figure 4. The New
Mexico aeromagnetic data are available as
U.S. Geological Survey Open-File maps at a
scale of 1:125,000 and a contour interval of
20y. Generalized trends are shown at a contour interval of 100 y in Figure 4.
Rocks of the sedimentary cover are effectively nonmagnetic. Except over areas of
extensive volcanic or shallow intrusive
rocks, the magnetic anomaly gradients delineate structural or lithologic boundaries in
the Precambrian basement. In a preliminary
analysis, I have shown that aeromagnetic
gradients and inferred basement structures
are aligned with graben border faults in
central New Mexico (Cordell, 1976a). The
alignment can be seen' in various places in
Figure 4. On the basis of the observed parallelism t>f graben faults with magnetic
anomaly trends, it seems likely that inherited Precambrian basement grain has
influenced, in a passive way, the trend of
Cenozoic graben faults at a scale of tens of
kilometres.
Principal regional trends in the magnetic
anomaly gradients which are thought to be
representative of basement structural grain
are indicated on the gravity map (Fig. 3) by
dotted lines. In this preliminary identification, the selection of trends and the evaluation of the effects of volcanic cover and
basement relief are subjective. Together
with trends in the graben border faults,
which are in part basement controlled, the
aeromagnetic trends seem to indicate a
change in basement grain across the rift.
Trends are more or less north-south, northeast, or random in the High Plains east of
the rift. To the west the trends are strongly
northwest or northeast, and the rift seems
REGIONAL GEOPHYSICAL SETTING OF THE RIO GRANDE RIFT
't either
rructu rc
ed with
robably
'recarn.
'theast-sent in
'liquely
ut this
v grav-
to zigzag along these conjugate directions.
An aeromagnetic gradient (Sauck and
Sumner, 1970) is associated with the pronounced northeast-trending gravity gradient in northeastern Arizona, and both are
attributed to a source in the Precambrian
basement. A parallel magnetic gradient to
the west, however, is not associated with
any obvious gravity trend. A major
northeast-trending age-province boundary
also occurs across eastern Arizona and
northwestern New Mexico (L. T. Silver,
1077
unpub. data). At the latitude of Albuquerque, tholeiitic basalt occurs west of the
axial graben along the northeast-trending
Jemez lineament (Fig. 1). From this general
area southward, the rift widens and changes
character.
ado is
1.6 to
coverht-line
1970).
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Figure 3. Bouguer gravity anomaly contours and trends of aeromagnetic gradients (dotted
lines) thought to be related to structure in Precambrian basement. Base map showing generalized
trends of rift faults is from Figure 1. Gravity map is generalized from map prepared by Defense
Mapping Agency, Aerospace Center, St. Louis Air Force Station, St. Louis, Missouri, for publication with U.S. National Atlas.
104°
Gravity and aeromagnetic data considered together suggest that the Colorado
Plateau is characterized and delimited by
elements of conjugate northwest-northeast
basement grain. Lack of parallelism with
trends in the craton east of the plateau
could indicate that the plateau was appended onto the craton during Precambrian
time, forming a basement suture along the
site of the future rift. Such a feature could
account for the observed angulation and en
echelon pattern of the rift as well as the recurrence of tectonism along these trends. At
present, evidence for a suture is circumstantial. Completion of the aeromagnetic
coverage in eastern and northwestern New
Mexico should establish whether parallelism exists in the basement grain east and
west of the rift. Preliminary interpretation
of the deep seismic reflection data (Oliver
and Kaufman, 1976; Kaufman and others,
1977) suggests a fundamental crustal discontinuity within the rift, as discussed below.
SEISMIC DATA
AND SEISMIOTY
Noninstrumental historical records since
1849 indicate a concentration of seismicity
along the rift which probably can be attributed only in pan to concentration of
population in the Rio Grande Valley (Sanford and others, 1972; Northrop, 1976). In
discussing more recent instrumental data,
Sanford and others (1972, 1976), Olsen
and others (1976), and Smith and Sbar
(1974) described belts of seismicity along
the Rio Grande graben, the northeasttrending Jemez lineament, and the arcuate
fault zone across northern New Mexico,
southwestern Colorado, and eastern Utah
35°
(Fig. 1). All of these are included within the
rift as broadly defined here. The highest
level of seismicity occurs within the rift between Socorro and Albuquerque. It should
be noted, however, that the seismicity of the
rift, at least as determined instrumentally
since 1960, is much lower than that of the
intermountain seismic belt (compare Smith
and Sbar, 1974, Fig. 2) and the East African
(Fairhead and Girdler, 1971; Wohlenberg,
34° i
1975), Baikal (Solonenko, 1968), and
perhaps other Neogene-Quaternary rifts.
The comparatively low level of anomalous seismicity associated with the Rio
Grande rift seemingly is not an artifact of
insufficient data. Noting the abundance of
figure 4. Total magnetic intensity map over Rio Grande graben. Contour interval is 100 y,
generalized from large-scale maps cited in text. Contour labels are removed because flight levels and Holocene fault scarps in the Socorro area,
Sanford and others (1972) estimated longreduction datum are not uniform.
REGIONAL GEOPHYSICAL SETTING OF THE RIO GRANDE RIFT
seismicity using King and Knopoff s
) equation relating fault length, offset,
J associated earthquake magnitude. They
nc [ u ded that the historical record showrelatively modest seismicity along the
rift is probably representative of longer
term trends, although their analysis of
paleoseismicity was admittedly subject to
considerable uncertainty. More recently,
soil studies (Machette, 1976) and geologic
1079
mapping in progress by Machette, G. O.
Bachman, C. E. Chapin, and others (1977,
oral communs.) have brought to light more
large fault scarps, some of which can be
dated. Because of the importance, in terms
%$M$&&3m
"
-
-
-
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ZOO
KILOMETERS
Figure 5. Seismic data, showing dq>th in kilometres below sea level to M discontinuity and, in parentheses, compressional wave velocity of upper
mantle in kilometres per second. Stars denote shot points. Arrows at stars indicate reversed profiles; arrows at dots indicate nonreversed profiles. Location
of first vibroseis profile in New Mexico by Consortium on Continental Reflection Profiling (Oliver and Kaufman, 1976) is shown by label COCORP.
Recently (Kaufman and others, 1977), these profiles have been extended across western border of graben. Dashed lines show pans of western border of
Colorado Plateau. Sources: a, Hill and Pakiser (1967); b, Ryall and Stuart (1963); c, Keller and others (1975) and Braile and others (1974); d, Roller
(1965); e, L. Pakiser (1977, personal commun.); f, Jackson and Pakiser (1965); g, Jackson and others (1963); h, Warren (1969); i, Toppozada and Sanford
(1976) and Toppozada (1974); j, crustal "low-rigidity" zone of Sanford and others (1973); k, Stewart and Pakiser (1962).
L. CORDELL
1080
of tectonics and seismic risk, of determining
the long-term seismicity of the rift, it would
be worthwhile to update the paleoseismicity
analysis.
During the early 1960s several long
seismic refraction lines in the western
United States (Pakiser, 1963} were obtained
by the U.S. Geological Survey; these data
have established important distinctions
among the tectonic provinces (Fig. 5). In
Figure 5, depth to the M discontinuity is
relative to sea-level datum and rounded off
to whole kilometres. In round numbers, the
crust is about 30 km thick in the Basin and
Range province, 40 km thick in the Colorado Plateau, and 50 km thick in the western Great Plains. Low seismic velocities in
the upper mantle (?„ <8.0) are observed in
the Basin and Range province and the Colorado Plateau.
Closest to the Rio Grande rift are the following refraction lines: (1) in the Colorado
Plateau, Hanksville-Chinle (Roller, 1965)
and Gasbuggy (Toppozada and Sanford,
1976; compare Warren and Jackson,
1968); (2) in the southern Rocky Mountains, Climax (Jackson and Pakiser, 1965)
and unpublished data of C. Prodehl and
L. C. Pakiser; and (3) in the High Plains,
Lamar (Jackson and others, 1963) and
Gnome (Stewart and Pakiser, 1962). Taken
at face value, these indicate asymmetric
crustal and upper-mantle structure across
the rift between the High Plains and the
Colorado Plateau. It is emphasized, however, that except for the Gasbuggy interpretation (i in Fig. 5), there are no refraction
data over the rift itself.
Interpretation of natural teleseismic
events is essentially in agreement with the
results from explosion seismology. Phinney
(1964) identified by means of spectral
analysis of long-period P waves intracrustal
and M discontinuities at Albuquerque,
although his range of depths to Moho (35
to 40 km) and range of ?„ velocity (7.4 to
8.4 km/s) are too large to resolve essential
questions about deep structure within the
rift. Bucher and Smith (1971) obtained
depths to Moho of 32 km in the eastern
Basin and Range province and 39 km in the
western Colorado Plateau. They did not
indicate an upper-mantle lid in the plateau
(compare Archambeau and others, 1969),
and they showed the low-velocity zone in
the upper mantle to be about 80 km thick in
both provinces. Layering within the upper
mantle in the vicinity of the Rio Grande rift
has not been described by induced-energy
seismic studies, although Jackson and
Pakiser (19t>5\ Warren (1969), Decker and
others (1975), Bridwell (1976), and others along the lat 40°N parallel, is shown in Fig.
have suggested configurations of the low- ure 6. Porath (1971) measured high electrivelocity zone determined from gravity data cal conductivity and inferred high temperaand isostatic considerations.
ture within the upper mantle along this
Most of the earlier seismic studies iden- same trend (as discussed below). Detailed
tified a Conrad or other discontinuities refraction studies in Arizona by Warren
within the crust (not shown in Fig. 5). Pro- (1969) show Basin and Range crust to be
dehl (1970) reinterpreted these and other anomalously thin on the southwest edge of
data in the western United States according the Colorado Plateau as well (Fig. 5).
to a specialized inversion procedure and
Detailed refraction data are not available
suggested a low-velocity zone within the for the Rio Grande rift; however, prelimicrust rather than a Conrad discontinuity. nary results of a study of surface-wave disProdehl also depicted the M discontinuity persion data (Keller and others, 197fib;
to be a zone 3 to 5 km thick in the Basin and G. R. Keller, 1977, oral commun.) indicate
Range and as much as 10 km thick in the that the average crustal thickness within the
Colorado Plateau. The crustal low-velocity rift is about 35 km. Anomalously low
zones may be related to zones of low rigid- seismic velocities {Toppozada and Sanford,
ity having possible tectonic significance in 1976) and other similarities between the rift
areas of extensional faulting (Shurbet and and the Wasatch front, as discussed below,
Cebull, 1971; compare Landisman and suggest that Figure 6, with the M disconothers, 1971).
tinuity shifted down about 10 km, may
Detailed seismic refraction data are provide a predictive model for the rift.
available at the Basin and Range-Colorado
Within the Rio Grande graben, recent
Plateau boundary along the Wasatch front seismic and other investigations are focused
in Utah and in central Arizona (Figs. 1, 5). on the possible occurrence of magma at
Although these areas are distant from the shallow depth in the Socorro area. Sanford
Rio Grande rift, their tectonic styles are and Long (1965) and Sanford and others
similar, and it is worthwhile to consider the (1977) identified on microearthquake seisUtah and Arizona data in the light of a pos- mograms .S-phase to P-phase and S-phase to
sible analogy with the rift. Keller and others S-phase reflections from a discontinuity
(1975, 1976a) showed a mantle bulge (M within the crust. The ratio of the observed
24 km below sea level) and extremely low amplitudes of these phases, corrected for
seismic compressional velocity (Pn = 7.4 to angles of incidence, were compared with
7.5 km/s) along the Wasatch front in Utah. theoretical ratios for several types of disA crustal profile across the western border continuities. The best match was obtained
of the plateau at the Wasatch front, deter- for a discontinuity having solid rock undermined by projecting the seismic refraction lain by material of very low rigidity. By inmodels (Fig. 5) along strike onto a profile ference, this material could be considered to
Wasatch
Front
WEST
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Or
EAST
110"
10
20
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6.6 .
6.7
7.9
6.5
7.5
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7.5
6.8
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100 KILOMETERS
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Figure 6. Interpretive profile along lat 40°N across mantle bulge and low-velocity anomaly in both
lower crust and upper mantle west of Wasatch front in Utah. Based on refraction data of Keller and
others (1975) and other refraction studies shown in Figure 5 projected onto profile line. Numbers
show compressional-wave velocities in crust and upper mantle; dashed line shows projected position
of M discontinuity.
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REGIONAL GEOPHYSICAL SETTING OF THE RIO GRANDE RIFT
molten. Cornpressional velocities indithat this interface is not the M discon" J t v The zone of low rigidity is estimated
!l
, e ' a bout 18 km deep at Socorro. It is
''I tifiable as far as 60 km north of
" -orro, where it reaches a depth of about
30 km. The latter depth is tentative because
wide-angle reflections were used, and these
depend critically on an accurate knowledge
of S-phase velocity (A. R. Sanford, 1976,
oral commun).
Repeat leveling data of Reilinger and
Oliver (1976) indicate relative uplift of the
1081
Socorro area in the vicinity of Sanford and
others' (1977) proposed shallow magma
body. They reported 20 cm of vertical uplift
in 40 yr, indicating an exceptionally high
average relative velocity of 0.5 cm/yr.
A network of deep seismic reflection
"vibroseis" profiles (165 km) has been
300 KILOMETERS
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Figure 7. Heat flow. Data in New Mexico and Colorado are from Reiter and others (1975), published with permission of M. Reiter and Geological
Society of America. Data in Arizona and Utah are from Diment and others (1975). Solid circle = >2.5 hfu; large dot = 2.0 to 2.5 hfu; small dot = 1.5 to
-•0 hfu; open circle = <1.5 hfu.
1082
L. CORDELL
completed recently in the Socorro area by
the Consortium on Continental Reflection
Profiling (COCORP). Preliminary results
(Oliver and Kaufman, 1976; Kaufman and
others, 1977) indicate reflections from as
deep as 50 to 60 km, a zone of sparse
reflections near the graben border, and differences in record character at the east bor-
38°
Figure 8. Electrical conductivity zonation,
probably within upper mantle, determined from
geomagnetic variation studies. Dots show locations of recording arrays. Profile along lat 38°N is
shown in Figure 9. Conductivity map after
Gough (1974) and published with his permission
and that of Journal of Geoelectricity and
Geomagnetism.
der suggestive of a major lateral disco
tinuity. A strong reflection above an anotn
lously homogeneous zone, observed Ove
large areas within the graben, correlat
well with the inferred magma body of San
ford and others (1973, 1977).
HEAT FLOW, ELECTRICAL
CONDUCTIVITY, AND DEEP
TEMPERATURE STRUCTURE
Temperature within the lower crust and
.ntie is panicuiany
upper mantle
particularly aitnc
difficult to
measure.
iure. Near-surface heat flow would
to provide the most direct information
*,.. temperature at depth, although the
heat-flow data are subject to relatively
San Juan
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Figure 9. Elevation profile and conductivity model (Porath, 1971, copyrighted by American Geophysical Union and published with permission) at Ui '
38°N. Vertical exaggeration of elevation profile is about 150:1. Dashed line in lower part of figure shows top of a zone of anomalously high electrical
conductivity. Although conductivity model is not unique, it does indicate high electrical conductivity and inferred high temperature (probably within upper |
mantle) beneath Rio Grande rift and Wasatdi-Hurricane front. Heavy dashed line in top part of figure shows accordance of nonvolcanic summits with
broad features of elevation profile.
1083
REGIONAL GEOPHYSICAL SETTING OF THE RIO GRANDE RIFT
FiiJ'^)'.
How-source contamination from radio• heat production and convective heat
1-1
isfer by moving ground water. Available
i,r. it-flow data are shown in Figure 7. Most
he data are from the remarkably com•hensive work of Reiter and others
' ig"'?). In general, very high heat flow is as, uted with the Rio Grande rift. Reduced
J.,t-tlow values suggest that the heat-flow
, , nli ilv along the rift is probably of magr itic and subcrustal origin, rather than
,'jjiogenic heat produced within the upper
' r l i s t (Decker and Smithson, 1975; M. Rei.. r unpub. reduced heat-flow data). The
i, -jt-tlow anomaly is very likely associated
•oth a corresponding high-temperature
,,,omaly at depth that is probably at least
pliocene in age, considering the thermal difm-ivity, other thermal parameters, and the
•cologic history. The existence of transverse
• rends in the heat-flow data — as, for
c\ nnple, along the Jemez lineament — is
debatable.
rlectrical conductivity measurements
provide an indirect means of mapping deep
•emperature structure because conductivity
increases with increase in temperature.
I mie variations in the Earth's magnetic
Held induce electrical currents in conductive
regions, which in turn generate secondary
;,ii.jl magnetic fields. Variations in the
Fjrth's magnetic field measured with arrays
,if recording magnetometers provide local
magnetic field component data from which
conductivity and, by inference, deep temperature structure can be derived. In fact,
g e o m a g n e t i c - v a r i a t i o n s t u d i e s were
pioneered in this country in the southern
Rio Grande rift by Schmucker (1964, 1970;
x.r Weidelt, 1974, for an inversion of
Schmucker's data in terms of highvonductivity distribution within the crust
jnd upper mantle). The technique offers
much promise, in conjunction with the
heat-flow data, as a means of mapping deep
Temperature structure within the rift. In this
case it is the present temperature that is interred, inasmuch as the electromagnetic re>ponse is e f f e c t i v e l y i n s t a n t a n e o u s .
Vhmucker's discovery of very high electri>M| conductivity associated with the rift in
Miuthernmost New Mexico has been conHrmed in preliminary detailed studies by
lowle and Fitterman (1975), Pedersen and
Hermance (1976), and D. Bennett and
M. Landisman (unpub. data).
The results of reconnaissance geomagnetic-variation studies in the southwestern
L'nited States are shown in Figure 8 (taken
trom Porath, 1971, and Gough, 1974).
Generally high conductivity is associated
with the Basin and Range province. Belts of
very high conductivity follow the southern
Rocky Mountains-Rio Grande rift and
Wasatch-Hurricane front along the east
edge of the Great Basin. Station spacing
(about 100 km) is too wide to determine
source depths. The Wasatch front and
Rocky Mountain anomalies seem to die out
at about lat 43°N.
A conductivity distribution in the upper
160 km compatible with the data (after
Porath, 1971) is shown along lat 38°N in
Figure 9 (lower pan). The solution is not
unique, and a conductivity interface could
be shifted downward to considerable depth.
Porath (1971) and Gough (1974) discussed
independent temporal, geoelectrical, and
tectonic considerations indicating that the
highly conductive sources occur in the
upper mantle, as shown. They suggested
that the seismic and conductivity data,
therefore, be considered together to reflect
partial melting within the low-velocity zone
in the upper mantle. Very shallow-source
effects related to zones of hydrothermal
alteration could make a significant contribution locally to the conductivity anomalies; this has not been taken into account
(W. D. Stanley, unpub. data). It is unlikely,
however, that the regional data could be
satisfied by means of reasonable conductivity anomalies within the upper crust or the
sedimentary cover alone. It seems more
likely that a zone of high electrical conductivity, and, by inference, anomalously high
temperature, having considerable width
and depth extent exists within the lower
crust and upper mantle under both the Rio
Grande rift and the Wasatch front.
| t ! <fe
ir',!?.««
If,
-100 ,—
«• -ZOO
i,
-300
4000 -
360O -
2400 -
2000
800
III'
110*
103'
108*
107'
106*
105'
104'
103'
102'
Figure 10. Gravity and elevation profiles along lat 37°N (Colorado-^Jew Mexico border). Dashed
segment of gravity profile shows, schematically, residual broad positive anomaly upon which gravity
low associated with graben fill is superimposed.
1084
L. CORDELL
TOPOGRAPHIC AND GRAVITY
PROFILES ACROSS THE RIFT
ben (compare Fig. 2) is a narrow cleft in the velocities in the crust and upper mantle
vault of the southern Rocky Mountains up- (Fig. 6), uplift, high seismicity, relatively
lift. The vault in the Great Basin seemingly high heat flow, high electrical conductivity
Elevation provides a crude structural has collapsed. In terms of the four major and an anomalously thin crust. Conductivdatum, and its broader components repre- physiographic subdivisions discussed in ity data of Schmucker (1970) and prelimisent a systematic response to integrated connection with Figure 1, the plains sub- nary results from studies now in progress
density and rheological effects in the crust division represents the stable craton; the (W. Isherwood, 1976, oral commun.) indiand upper mantle. East-west profiles across mountains with flanking dissected table- cate similarly very anomalous conditions
the Rio Grande rift at intervals of 1° of lands represent incipient rifting as seen in along the Sierran front. Thus, the principal
latitude (about 115 km) are shown in Fig- the Rocky Mountains in the northern pan geophysical and topographic anomalies of
ures 9 through 15. In Figure 9 the profile at of the Rio Grande rift and at the edges of the Great Basin occur at its edges. If the
lat 38°N is extended westward across the the Great Basin; and the mountains-in Basin and Range (mountains-in-plains)
Great Basin to the Sierran front so that the plains subdivision represents the climax physiography grew outward from a onceGreat Basin and the Rio Grande rift can be state as seen within the Great Basin itself.
narrower and geophysically anomalous
compared.
Interestingly, the mountains, rather than axial uplift, such as is now seen in the
All ramifications of the Atwater (1970) the mountains-in-plains, are the more southern Rocky Mountains, the anomalous
hypothesis require large Cenozoic extension anomalous geophysically. Geophysical data conditions at the center may have been rewithin the North American Cordillera. are most complete along the Hurricane- lieved, in part, by collapse and extension of
Hundreds of kilometres of extension were Wasatch front, where we observe, relative the vault itself.
established beforehand by Hamilton and to both the Great Basin on the west and the
Reference to "collapse" is misleading beMyers in 1966. Hamilton (1969) described Colorado Plateau on the east, the following cause of the extreme vertical exaggeration
a system in which everything west of the characteristics: anomalously low seismic on the profile. Although both horizontal
southern Rocky Mountain front moved
under inertial body forces northwestward
relative to the craton. In this system rifts,
such as the Rio Grande, represent secondary effects formed where coherent blocks,
such as the High Plains and the Colorado
Plateau, "pulled apart."
Alternatively, mechanisms such as back- 1 -20°
arc spreading (Scholz and others, 1971) involve vertically directed magmatic forces,
rather than horizontal body forces, as a first
cause. Eaton and others (1976) "showed the
Great Basin to be bilaterally symmetrical
about a north-trending axis (Fig. 9) and
-300
suggested that the causal mechanism is
rooted to this axis at depth. Their suggestion of symmetric "spreading" east and
west from the center of the Great Basin
4400 i—
raises a question as to which direction the
Colorado Plateau was "pulled."
4000
In the profile across the Great Basin and
Dolhorl
Rio Grande rifts shown in Figure 9, neither
3600
Ierosion nor constructional volcanic features
obscure the general accordancy of summits
along the simple double bulge shown by the
dashed line. Eaton and others' (1976) axis
2800
of symmetry within^the Great Basin trends
roughly normal to the profile at about long
115° to 116°W. From this perspective it becomes difficult to visualize the Rio Grande > 2OOO
rift as a "pull apart" foundered in the wake uj_j
of the westward-drifting Colorado Plateau. uj
1600
Rather, the plateau looks like a "pushed
aside," lying between two great rifts. The
1200
100
200 KILOMETERS::
Rio Grande and the Great Basin are parallel, consanguineous, yet separate structures
800
showing different stages of the extensional
J_
J_
J_
J_
J_
J_
_L
_L
Ml'
110*
109'
108'
107*
106'
105'
104'
103'
102'
process developed over about the same time
span. At this latitude the Rio Grande graFigure 11. Gravity and elevation profiles along lat 36°N (Jemez Mountains).
_L
1085
REGIONAL GEOPHYSICAL SETTING OF THE RIO GRANDE RIFT
'
rtcal movement contributed to the
physiography, and
tal movement is the greater by an
"'i'.Vof magnitude, there is no structural
" l - n c c showing which occurred first.
'U|L limatic and pollen data of Axelrod
' g-V) and Axelrod and Bailey (1976) indiibout 1.200 m of epeiric uplift of the
'.''.'I Grande rift after 15 m.y. B.P. — that is,
V-r the initiation of extensional faulting,
' -fit-ably before the culmination of exi's'ional faulting during Pliocene time, but
f<: probably contemporaneous with it.
•i-n/ir pollen data seem to indicate Oligo",,f uplift in southern New Mexico,
in this case they attribute highflora to constructional volcanic tov rather than to uplift.
h«ure 9 does not establish that the
,nihern Rocky Mountains-Rio Grande
,.([ ind the Great Basin represent different
•-! ices in the rift process." This idea is
. IM.-J on l^e observation that the Rio
, , r i n J e rift progresses southward from
..,,untains into mountains-in-plains, which
, illustrated with a series of profiles in Fig.r^ 10 through 15. The progression may
v niore in the domain of intensity than in
me. as there is at present no evidence of
1^1- progression along the rift.
The profile along the Colorado-New
Mexico state border at lat 37°N (Fig. 10) is
•^fntially free of constructional volcanic
• ( ,pography and characterizes the southern
Rocky Mountain section of the rift more
ipily than Figure 9. The principal mass flux
* .is upward, comprising a horst 100 to 150
v m wide having at least 3 km of structural
-i-licf (on the Precambrian basement) superinposeJ on a sinusoidal bulge several hunircJs of kilometres wide. Part of the struc•ural relief on the Precambrian basement is
1 jramide in age (Woodward, 1976). Topographic relief, by contrast, was developed
m the late Eocene peneplane and indicates
•oughly the minimum amount of postI aramide, primarily Neogene, uplift. Struc•ural relief in the narrow Rio Grande grax-n, lying near the center of the axial valley,
> .ilso about 3 to 4 km on its eastern side. If
ie faults dip uniformly 63°, the component
i horizontal extension would be equal to
•he sum of the structural relief of all the
'ursts and grabens, say, about 6 to 8 km.
ihis represents a minimum amount of ex'cnsion, because some of the fault blocks
•K covered and the fault planes probably
'atten with depth (Moore, 1960; Woodward and DuChene, 1975; Woodward,
. Thus, in the southernmost Colorado
n of the rift, horizontal extension is
-loo ,—
n
1200
100
800
III'
110*
109*
108'
200
KILOMETERS
i
i
t
_L
107'
106*
105'
!04«
103'
102'
Figure 12. Gravity and elevation profiles along lat 3S°N (Albuquerque).
-100
i -200
o
03
-300
Magdaleno Mts
I
Rio Grande
(Socorro)
3200
Mogollon Rim
San Agustin
Plains
Los Prnos
Mts
Pecos River
t
2800
s
z2000
p 1600
§
UJ
d |2°o
800
j_
J_
III
MO*
109*
108*
107*
106*
_L
105*
i
104*
Figure 13. Gravity and elevation profiles along lat 34°N (Socorro).
103*
I02«
1086
greater, but probably not an order of magnitude greater, than vertical uplift. A similar
140-km-wide basement-cored horst containing an axial graben is also observed in
the lat 36°N profile (Fig. 11).
First stages in major extension and collapse of the uplifted vault are apparent in
Figure 12. The western shoulder of the rift
occurs as far west as long 108°W along the
northeast-trending Jemez lineament (Fig.
1). The Rio Puerco antithetic fault zone
probably represents a 100-km-wide zone of
late Neogene extension. Mesozoic rocks are
not eroded in the Rio Puerco zone nor
within the graben at Albuquerque (Black
and Hiss, 1974), as they would be had the
area been uplifted. To judge from regional
tectonic and magmatic patterns, however,
the Rio Puerco zone is obviously part of the
rift. (Evidence for a possible Laramide ancestory is discussed by Slack, 1975.)
Mesozoic rocks may have been preserved in
this area because accelerated horizontal extension didn't allow much vertical uplift to
develop.
North of lat 35°N (Fig. 12), the rift crosses a suspected northeast-trending discontinuity in the Precambrian basement (Jemez
lineament) or perhaps a series of parallel
northeast-trending discontinuities. It is in
this general area that the rift changes trend
and character and horizontal extension increases significantly (Fig. 12). Profiles at lat
34° 33° and 32°N are shown in Figures 13
through 15. West of about long 108°W the
profiles cross the structural grain at small
angles (Fig. 2) and are probably misleading.
The eastern border of the rift, however, is
fairly well depicted by the profiles. The
eastern border extends along the Sacramento-Guadalupe Mountains (Fig. 1),
across Trans-Pecos Texas, and into the Republic of Mexico. The location of the western and southern borders is arbitrary. Figures 13 through 15 show a general progression into the "mountains-in-plains"
physiography typical of the Great Basin
(compare Figs. 9 and 15). Even though the
exact number of horsts and grabens is not
known, it is likely that by lat 32°N (Fig. 15),
horizontal extension is more than an order
of magnitude greater than uplift.
The topographic profiles show the rift
zone to be essentially either a horst or a
broad uplift, which from Colorado southward sags in the center as it increases in
width. Bouguer gravity profiles (Figs. 10
through 15) show a reciprocal but essentially similar north-to-south progression.
L. CORDELL
Neglecting the narrow gravity low associated with graben fill, one observes a residual positive gravity anomaly over the
axial graben (dashed line in Figs. 10
through 12), and a broad residual negative
anomaly over the regional uplift. Amplitudes of both residual anomalies would be
significantly increased if near-surface density variations were taken into account
(compare Decker and others, 1975).
Gravity profiles across the Rio Grande
rift resemble profiles across the Gregory rift
in Kenya (Searle, 1970), which Girdler
(1975; Girdler and others, 1969) interpreted in terms of thinning of the lithosphere over an asthenospheric bulge. Similarly, from gravity analysis with seismic and
heat flow constraints, Bridwell (1976)
suggested a 60-km-thick "intrusion" of asthenosphere beneath the Rio Grande rift in
northern New Mexico, and Decker and
Smithson (1975) suggested thinning of the
crust by 75% to 100% over a 30- to 60km-thick sill of "low-density mantle." To
the extent that within interpretational
latitude the low-density mantle extends
nearly to the base of the lithosphere, these
two interpretations for the Rio Grande rift
are somewhat the same. They have in common with each other and with Girdler and
others' (1969) model for Kenya a very great
thickness of inferred low-density,
asthenosphere-like upper mantle. In pan,
the resulting very great thickness of anomalous upper mantle is built into these
analyses because only steady-state, conductive heat transfer is considered. More recently, Decker and Smithson (1977) have
considered transient heat sources (analogous to convective, magmatic heat transport) leading to a significantly thinner
anomalous zone.
-100
o
CD
-300
Rio Grande
3200i—
110"
109°
I08«
I07«
106°
105°
I04«
103*
Figure 14. Gravity and elevation profiles along lat,33°N (Jornada syndine).
102°
REGIONAL GEOPHYSICAL SETTING OF THE RIO GRANDE RIFT
The broad gravity low may be related to
s hape of the low-velocity zone in the
' p,.r mantle, but it may be more critically
|jted to density changes due to thermal
x pansion and mass transport within the
pner mantle (see Fig. 6) and to both inhernt jnd intrusion-related lateral density var,j ( ion within the crust. High heat flow and
high electrical conductivity indicate that the
, w.velocity zone is hot, but its shape, in•crnjl density structure, and convective
.,,otion are unknown. One can guess at the
i c nsity and invert the gravity data in terms
,,i the shape of the low-velocity zone, but as
^.jre seismic and electrical data become
jvjilable it might become possible to const'jin the shape and invert the gravity data in
•erms of the density distribution within the
low-velocity zone.
The residual positive gravity anomaly aswK-iaced with the axial graben is apparent in
Kifures 10 and 11 in the vicinity of long
;i)fT\V and is very obvious in Figure 12. I
hjve attempted to illustrate the anomaly by
interpolating the gravity field over the
jbrupt local gravity low associated with
;,iw-density graben fill, as shown by dashed
mes in Figures 10, 11, and 12. An axial
positive gravity anomaly within a broad
gravity low is also characteristic of the
drcgory rift and other continental rifts.
Model studies (Searle, 1970; Girdler, 1975;
lordell, 1976a; Bridwell, 1976) indicate
,
the source of the axial positive anomaly to
be narrow and steep sided. This shape
seems inconsistent with freeboard topography on a buoyantly supported asthenolith. It seems more likely that the axial positive gravity anomaly indicates mantle
material decoupled from the underlying
anomalous upper mantle and intruded by
dynamic means into the crust.
DISCUSSION
The southern Rocky Mountain uplift (the
Neogene-Quaternary uplift) and the Rio
Grande graben cannot be considered separately. They are parts of a system of uplift,
magmatism, and extension that makes up
the Rio Grande rift. Among the continental
rifts, a unique feature of the Rio Grande rift
is its vergence, providing from south to
north a son of down-structure view of the
rift process initiated as mountains and
culminating as mountains-in-plains.
The geophysical data show that heat and
mass have been added to the system, as is
the case with spreading ridges. On the other
hand, the relatively low level of seismicity
would indicate that the rift is not very active
at present. The low seismicity may not be
representative of long-term trends, however, to judge from the evidence provided
by seismology and electrical conductivity
data for existing major deep structure. De-
1087
tailed seismic data are sparse within the rift
itself, except in the Socorro area. The rift
seems to occur in the general area of a lateral transition in crustal and upper-mantle
structure between the High Plains and the
Colorado Plateau. As Figure 9 suggests,
however, the plateau may have no particularly characteristic crustal structure in its
own right, the more significant Neogene
structure possibly being symmetric to the
axis of the rift.
Zigzags in the fault blocks on the order of
tens of kilometres, as well as zigzags in the
trend of the rift itself on the order of hundreds of kilometres, are aligned with basement structural grain, as traced under sedimentary cover by means of the aeromagnetic data. Inherited structural grain has
undoubtedly influenced both the rift and
earlier uplifts along this same trend.
Whether the rift has sought out a fundamental basement suture or has simply taken
advantage of ubiquitous basement cracks is
uncertain.
The tectonic work has been accomplished
by some combination of the effects of heat,
density, chemical phase change, and mass
transport, all of which are interrelated.
Similarly, the geophysical data — gravity,
heat flow, seismic velocity, electrical conductivity, and elevation — are functionally
interrelated. The system is overdetermined
and, in principle, could be solved uniquely.
To do so realistically would require more
constraints on the geometry of deep structure within the rift itself; that is, physicalproperty i n f o r m a t i o n derived from
potential-field data is reasonably complete,
but sounding-type geophysical data that
could provide constraints on shape and
depth of critical interfaces are needed.
When that information becomes available,
the study of the rift will enter an exciting
new phase in which the geophysical and
petrological data and the geological history
can be quantitatively combined.
ACKNOWLEDGMENTS
110'
109*
108'
• 107'
106'
105*
104'
Figure IS. Gravity and elevation profiles along lat 32°N (El Paso).
103'
102*
For access to unpublished data and their
interpretation I am grateful to G. O.
Bachman, C. E. Chapin, G. P. Eaton, W.
Hamilton, W. F. Isherwood, G. R. Keller,
M. Landisman, M. N. Machette, J. E.
Oliver, L. C. Pakiser, M. Reiter, A. R. Sanford, L. T. Silver, and W. D. Stanley. For
criticism of the first draft of the manuscript
I thank R. J. Bridwell, C. E. Chapin, G. P.
Eaton, W. Hamilton, G. R. Keller, A. R.
1088
Sanford, and W. D. Stanley. I am grateful to
many colleagues for their long-standing assistance and good will.
REFERENCES CITED
Archambeau, C. B., Flinn, E. A., and Lambert,
D. G., 1969, Fine structure of the upper
mantle: Journal of Geophysical Research,
v. 74, p. 5825-5865.
Atwater, Tanya, 1970, Implications of plate tectonics for the Cenozoic tectonic evolution
of western North America: Geological Society of America Bulletin, v. 81, p. 35133536.
Axelrod, D. I., 1975, Tertiary floras from the Rio
Grande rift: New Mexico Geological Society Guidebook, 26th Field Conference, Las
Cruces Country, p. 85-88.
Axelrod, D. I., and Bailey, H. P., 1976, Tertiary
vegetation, climate, and altitude of the Rio
Grande depression, New Mexico-Colorado: Paleobiology, v. 2, p. 235-254.
Black, B. A., and Hiss, W. L, 1974, Structure
and stratigraphy in the vicinity of the Shell
Oil Co. Santa Fe Pacific No. 1 test well,
southern Sandoval County, New Mexico:
New Mexico Geological Society Guidebook, 25th Field Conference, Ghost Ranch,
central-northern New Mexico, p. 365 —
370.
Braile, L. W., Smith, R. B., Keller, G. R., Welch,
R. M., and Meyer, R. P., 1974, Crustal
structure across the Wasatch front from detailed seismic refraction studies: Journal of
Geophysical Research, v. 79, p. 26692677.
Bridwell, R. J., 1976, Lithospheric thinning and
the late Cenozoic thermal and tectonic regime of the northern Rio Grande rift: New
Mexico Geological Society Guidebook,
27th Field Conference, Vermejo Park,
p. 283-292.
Bucher, R. L, and Smith, R. B., 1971, Crustal
structure of the eastern Basin and Range
province and the northern Colorado
Plateau from phase velocities of Rayleigh
waves, in Heacock, J. G., ed., The structure
and physical properties of the earth's crust:
American Geophysical Union Geophysical
Monograph 14, p. 57-70.
Chapin, C. E., and Seager, W. R., 1975, Evolution of the Rio Grande rift in the Socorro
and Las Cruces areas: New Mexico Geological Society Guidebook, 26th Field Conference, Las Cruces Country, p. 297—321.
Christiansen, R. L., and Lipman, P. W., 1972,
Cenozoic volcanism and-.plate tectonic
evolution of western United States. II. Late
Cenozoic: Royal Society of London Philosophical Transactions, ser. A., v. 272, p.
249-284.
Cordell, Lindrith, 1970, Gravity and aeromagnetic investigations of Rio Grande depression in northern New Mexico [abs.]: New
Mexico Geological Society Guidebook, 21st
Field Conference, p. 158.
1972, Complete Bouguer anomaly gravity
map of the Jemez area, New Mexico: U.S.
Geological Survey Open-File Map, scale
1:250,000.
L. CORDELL
1976a, Aeromagnetic and gravity studies of
the Rio Grande graben in New Mexico between Belen and Pilar, in Woodward, L. A.,
and Northrop, S.A., eds., Tectonics and
mineral resources of southwestern North
America — A volume honoring Vincent C.
Kelley: New Mexico Geological Society
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MANUSCRIPT RECEIVED BY THE SOCIETY
JANUARY 4, 1977
REVISED MANUSCRIPT RECEIVED JULY 6, 1977
MANUSCRIPT ACCEPTED AUGUST 9, 1977
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