Thesouthern
NewMexicoRocky
Mountains
in west-central
Laramide
riftextension
structures
andtheirimpact
ontheRioGrande
by PasulCabuas,Paris,France
r120
1240
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Lowis and Clark
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OOMAI'{E CORDILLERAIN
'E,iolffi,l,,f,,
o
7
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FIGURE l-Structural map
of the westem United States
Cordillera (modified from
Auboin et al., 1986).
Legend:
1, transcurrent fault;
2, normal faulu
3, fault undifferentiated;
4, thrust fault;
5, strontium line
flSrfSr = 0.705);
6, Quatemary volcanism;
7, Tertiary volcanism;
8, main basins;
9, granites (large batholiths
inside and outside the
pluto.volcanic axis where
small batholiths are not
detailed).
10-12, Cordillera area:
10, autochthonous
a, deformed during
Laramian orogeny;
b, not deformed;
11, allochthonous;
12, main "metamorphic
core complexes."
13, pluto-volcanic area.
14-16, Coast Range:
14, Great Valley units, with
ophiolites;
15, Franciscanunits;
16, Salinian block.
A, Albuquerque; B,
Boise; C, Chihuahua; D,
Denver, EP, El Paso; H,
Helena; H, Hermosillo;
L, laramie, Ll\, Los
Angeles, LV, Ias Vegas;
4 Phoenix; S, Seattle; SF,
San Francisco;SLC, Salt
Lake City; T, Tucson; V,
Vancouver.
tt;\
DOMAINE PLUTO.VOLCANIOUE
[_rr,j,m
[i!iiti!t;t;l
t3
CHAiNE
cOT]ERE
l l,n=r5ffi,,
S.-i*'(Contentson next pgel
PascalCabezasparticipated in a long-term "student program" of
French doctoral students in the U.S. and Canada from a number
of French universities, initiated in 1978by Soci6t6Nationale Elf
Aquitaine. The company's American (EAP) and Canadian (ACC)
branchessupported this program both logistically and technically.
Mapping and field work in the West American Cordillera was
carried out during the summer months fuom7979through 1988by
1.2students. Lab work and thesis writing took place during wintertime at the French universities. Participantswere supervisedby
French and American thesis advisors. Students were able to maintain close contact with American universities, scientific organizations, and the USGS.Cooperation and good understanding betrveen
French and American scientistswere very pleasantbyproducts of
this "student program."
PascalCabezasparticipated as the ninth student in the program.
He carried out extensivefield work, including mapping in central
New Mefco. He was based in Socorro during three consecutive
summers (1985-1987)and was in closecontactwith the New Mexico
Bureau of Mines and Mineral Resources.
PascalCabezasreceived his doctoral degree in 1989 from the
University of Nice and Sophia Antipolis (France). He published
the main results of his dissertation in the Bulletin Centres de Recherches Exploration-Production Elf Aquitaine, v. 13, no. 2, pp.
229J45, Dec. 4, 1989,Boussens,France, from which the present
text has been translated.
Mathis A. Zimmermann
Denver, Colorado
February 25,7997
This paper is important because it presents a comprehensive
synthesis of the Cenozoic tectonic evolution of the Socorroregion
as seen through "new" eyes. Not everyone will agree with the
interpretations and some ideas may not stand the test of time, as
is generally true of all geologic investigations. But the ideas, so
well illustrated in the maps and crosssectionsfor which the French
are famous, should provoke much discussion and a fresh look at
the geology of central New Mexico.
PascalCabezasaccomplisheda remarkable amount of mapping
and synthesis in three short field seasons,in a strange country,
and largely on his own. I was privileged to accompanyPascaland
his French advisors during some of their field checks. I was especially impressedwith the beautiful sketchesthey made at nearly
every stop. I felt like an amateur as I stood there merely looking
at the geologic structures while they industriously sketched them
in their field notebooks. And I wondered if I could have accomplished so much had I been assigneda comparableareain a foreign
country with three field seasonsto get it done.
The student program initiated by Elf Aquitaine is an excellent
example of how company participation in the education process
can benefit an exploration project and also help assure a future
supply of well-trained geologists.Mathis Zimmermann was Resident Manager for Elf Aquitaine during Pascal'sproject and a member of Pascal'sdoctoral committee. I wish to acknowledge his
contributions here and to thank him for his help in translating the
paper. Thanks are also due to Dr. JacquesRenault of the Bu-reau
staff who did the original translation from the French. The maps
and cross sectionswere not relabeledin English becauseit is quite
easy to understand them in French; the figure captions in the original publication were printed in both French and English, so the
original English version has been used here. We also thank Elf
Aquitaine for permission to reprint the paper.
CharlesChaoin
Director and
StateGeologist
Alsoin thisissue
NMGS1990springmeetingabstracts
NewMexicoMineralSymposium
abstracts
Service/News
geologicmeetings
Upcoming
NMGS1991fallfieldconference
Staffnotes
p. 38
p. 39
p.42
p. 43
p. 43
p.44
INTRODUCTION
The region studied is located in New Mexico in the folded and
faulted foreland of the westem American Cordilleran system,
which extends for more than 4,800km from Alaska in the north
to Mexico in the south.
This folded and faulted foreland (Rocky Mountain foreland)
abuts in the east on the Great Plains, a vast region of little deformation, and is bordered on the west by an eastward vergent
overthrust belt emplaced during the Late Cretaceousand eartY
Tertiary. Underlainby the Precambrianmetamorphic and Sranitic
series of the American craton, the foreland includes two great
history.
The field area studied is located in the region of Socorro, between the Colorado Plateau to the west and the Great Plains to
the east (Fig. 2). It is borderedon the east by the Manzano-tos
Pinos Mountains and the Cerro del Viboro [also known as Loma
New AAexnc@
GEOLOGV
o SGienGe
andSeilice
tssN 0196-948X
Volume 13, No- 2, MaY 1991
Editor:CarolA Hjellming
Kathryn Campbell
Drafting Assistance;
Published quarterly by
New Mexico Bureau of Mines and
Mineral Rercuces
a division of New Mexico lnstitute of
Mining & Tshnology
BOARD OF REGENTS
Ex-Officio
Bruce King, Gnmor ol Nru Mexico
Alan Morgan, Superinlerdentof Public lnstruction
Appointed
Steve Tores, Pr6 , 1967-1997,Albuquqque
Carol A Rymer, MD , Pres-Desigrute,1989-1995,
Albu4urque
Lt Gen Lo Marqtez, SE lTtus , 7989-199'
Albuquoque
RobertO Ande6on,1 7-1993, Roswll
Charles Zimmerly, 199l-1997,Srono
New Mexico Institute of Mining & Technology
laurence H. Lattman
Praident
New Mexico Bureau of Mina & Mineral Resoures
Charles E Chapin
Dirrctor and StateG@logist
February, lv{a, Autust,
Sub{riptiore: lsued qwterlt
November; subsription price $6 Oo/calendaryear
Editorisl mttq: Articls submitted for publication
should be in the editor's hands a minimum of five
(5) months before date of publication (February,
Ma, August, or November) and should be no longer
than 20 typewritten, double-spacedpages All
scientific papers will be reviewed by at least two
people in the appropriate field of study Address
inquiriG to Carcf A Hyellrning, EAiw of Ntu Msia
Geotojy, New Mexico Bureau of Mines & Mineral
Resources, Scoro, NM 878n147
Publishedas public domin, thereforereprducible without
pemission Sourceqedit rcqusted
Circulation:1,6U
Pritrter:Univeruity of New Mexico Printing Seruies
May 1991 Nm Mexico Geology
.
Santa Fe
Albuquereue
as cruces
NEW MEXICO
coLoarxo".^rr ,
-d]
"
x\\\
20rm
EA\\\ _
FIGURE2-The main morpho-structural
ensembles
of the Socorro
region.
de las CafrarEd.l and on the west by the Lucero uplift and the
Ladron Mountains. In the middle, tha Albuquerqueind Socorro
Basins form part of the Rio Grande rift, and [o the southwest the
Datil-Mogollon volcanic field (Oligocene to Recent) masks the
earlier structures. This is a key
aree to studv the structural evo-Mountains
lution of the southern Rocky
and the effect of more
recent extension by the Rio Grande rift.
gional tectonic evolution is presented.
STRATIGRAPHY
The studied region_contains a very incomplete
stratigraphic
column ranging from Proterozoic to Recent.
Upper Paleozoic to Mesozoic
preorogenic deposits
Upper Paleozoic
Becausethe lower Paleozoicis missing, the oldest sediments
lying on the Precambrianbasementare locally preserved,masiive
gr.ay.epicontinentalcarbonatesof the Kelly Foimation (Mississippian) (Fig. 3) (maximum 30 m). The ovdrlying upper paleozoic
rocks comprise the Magdalena Group (Pennsylvanian to basal
Permian) and the Manzano Group (Permian). [Manzano Group
was a stratigraphic name used by Lee and Girty (1909);it was
abandonedby the U.S. GeologicalSurvey and now is obsolete.Ed.l
The Magdalena Group (Fig. 3) constitutes a complete cycle of
four sedimentary packages.1) The transgressiveSandia Formation is characterizedby coarse-grained,crossbeddedquartz sandstones and encrinites alternating with shales and platform
limestones. 2) The lower gray limestone member of the Madera
Formation is composed of very fossiliferous, massive gray carbonates, typical of platform sedimentation in an open marine
environment. 3) These are overlain by platform carbonatesand
terrigenous sediments of the arkosic limestone member of the
Madera Formation. 4) The Bursum Formation is characterizedby
the appearanceof red beds (continental?)alternating with marinL
carbonates indicating regression. This formation is transitional
between the marine carbonatesof the Madera Formation and the
continentalbasalrocks of the Manzano Group.
The average thickness of the Magdalena Group is from 600 to
800 m. One of the main characteristicsis the presenceof reduced
and condensedseries(0-140m), indicating the existenceof shallow or emergent zones in the Pennsylvanian (Kottlowski and
Stewart, 1970).
The Manzano Group (Fig. 3) (see note above) is from 520 to
680 m thick and includes from bottom to top: 1) the dark-red Abo
Formation is continental as indicated by the presence of mudcracks and imprints of raindrops and plants. 2) The Yeso Formation is essentially terrigenous; the sandstones of its Meseta
BlancaMember mark the beginning of the Permian transgression
toward the north that is completedby the carbonatesof the Torres
Member (McKee, 1962 Kottlowski, 1953).The top of the Yeso
Formation (top of the TorresMember and CafrasMember) is rich
in evaporitesindicating a lagoonalenvironment. 3) The gray to
yellow sandstones of the Glorieta Formation are composed of
well-sorted and rounded grains derived from ancientbeachsands
and enclose an evaporite horizon near Riley. 4) The carbonates
of the San Andres Formation were deposited at the maximum
extent of the Permian transgression.The presenceof evaporites
near Socorro indicates, however, a restricted marine environment.
The sea withdrew after deposition of the San Andres Formation. This withdrawal is documentedby a karst surfacecontaining
pockets filled with sand of the Upper Permian Bernal Formation
(Tonking, 1957;Smith and Budding,1959).
Mesozoic
Only Triassicand Upper Cretaceousstrata crop out in the Socorro region (Fig. 3).
The Triassicis representedby alternatingshales,sandstones,
and red conglomerates(average thickness: 250 m). They are of
continental origin as indicated by mudcracks,the red color of the
sediments, and the imprints of plants. The pelitic rocks represent
deposits of an alluvial plain, and the sandstonesand conglomeratesare channeldeposits.
After a long hiatus of erosion and peneplanation, New Mexico
was covered again by an epicontinental sea in the Late Cretaceous, resulting in deposition of 560 to 600 m of terrigenous
epicontinental deposits interstratified with more typically marine
horizons. Two sedimentary cycles are recognized (Hook, 1983;
Hook et al., 1983):1.)The Greenhorncycle(Cenomanianto middle
Turonian; Fig. 3) begins with sands of the Dakota Formation and
is followed by the lower part of the Mancos Shale (dark marine
pelites). The regressionis representedby the coastalsands of the
Atarque Member of the TresHermanos Formation and terminates
with deposition of continental deposits (lacustrineand fluvial) of
the CarthageMember (Landis et a1.,"1.973).2)
The Carlile rycle
(upper Turonian through Coniacian; Fig. 3) begins with coastal
sandstonesof the Fite Ranch Member. The sea transgressedtoward the east-southeastand attained its maximum extent during
Neu Mexico Galogy
May 1997
27
C o l o n n es t r a t i g r a p h u
i qe
Echelleam6ricaine
Fm. CrevasseCanvon(Kcc)
O - C r o s sT o n g u e o f M a n c o s S h a l e ( K m d )
E
F m .T r e s
H e r m a n o s( K t h )
Mbr. Fite Ranch
Mbr. Carthage (Kthc)
CRETACEOUS
Mancos Shale Lower Part (Kml)
Fm. Bernal (Pbe)
Mbr. Cafras(Pyc)
Mbr. Meseta
B l a n c a( P y m )
F m . B u r s u m( P b )
c
o
o
PENNSYLV
et granitique
Soclemdtamorphique
FIGURE 3-Precambrian to Mesozoic stratigraphic chart for the Socono region.
28
May 1991 Nm Mexico Geology
Ma
66
E c h e l l ee u r o p 6 e n n e
C o l o n n e s t r a t i g r a p hi q u e
l r v i n g t o n i an
F m .S i e r r a
Ladrones(Tsl)
Ma
Echelleam5ricaine
Blancan
-LEI5I UUENE
1,66
PLIOCENE
5
Hemphillian
Clarendonian
MIOCENE
MIOCENE
B a r s t o v i an
Rhyolite of Water
Hemingfordian
24
Arikareean
tar Tuff (Tl)
B a s al t i c - A n d e s i t e
(TbaI
OLIGOCENE
Whitneyan
Vicks PeakTuff (Tv)
Orellan
OLIGOCENE
of Granite Mountain
Fm. Spears(Ts)
Chadronian
36
Duchesnean
Uintan
EOCENE
EOCENE
Bridgerian
Wasatchian
Clarkforkian
55
Tattanian
Torrejonian
PALEOCENE
P u e r c an
Maastrichtian
66
CRETACEOUS
FIGURE 4-Cenozoic stratigraphic chart for the Socorroregion.
deposition of,marine shaledepositsof the D-CrossTongue.The
sandstonesof the Gallup Formltion mark the beginning jf regression. The CrevasseCanyon Formation,composedof ihales"and
sands grading into carbonaceousmaterial isedimentation in a
deltaicor lagoonalenvironment,Johansen,1983),ends the cvcle.
Middle Eoceneto Oligocene
uplift
of
the
Rocky Mountains and before opening of the
_.Aflur
Rio Grande rift, the following strata were emplaced.At tlie base,
the.BacaFormation,a post-tectonicmolasse,-caps
the structures
of the Rocky Mountains (Cathea 1983)with unconformity. At the
top is a thick volcanicand volcaniclasticassemblage
1orpile) comprising the Spears Formation and basaltic andesite flows and
interstratified tuff horizons.
Baca Formation
The BacaFormation is composed of coarse-grainedsands and
boulder conglomerates(35 to 230 m thick) characterizedby the
presenceof Precambriangranite boulders and by the absenceof
volcanic clasts.The formation lies with angular unconformity on
a peneplaned surfaceof Pennsylvanian,Permian, Triassic,or Upper Cretaceousstrata. It containsreworked clastsof all theserocks
indicating tectonic uplift subsequentto the Coniacian and prior
to its deposition. In west-centralNew Mefco, the BacaFormation
includes vertebrate fragments of middle to late Eoceneage (Snyder,1970;Schieboutand Schrodt,1.981;
Lucaset al., 1981.;
Lucas,
1982). It is covered by the volcaniclastic Spears Formation. Radiometric dates on andesite clasts taken from the basal Spears
New Mexico Geology May 1997
indicate I(/Ar agesof 39.6 + 1.5 Ma and 38.6 -+ 1.5 Ma (Osburn
and Chapin, 1983)giving a late Eoceneage as an upper limit to
the BacaFormation.
Late EoceneOligocene volcanism
Beginning in the late Eocenea large volcanic area, the Mogollon-Datil volcanic field, manifested itself in southwestern New
Mexico with the development of calderas,some as large as 30 km
in diameter.This volcanismpossesses
calc-alkalineaffinities(Chapin
and Seager,1975;Elstonand Bomhorst,1979;Morganet al., 1986).
These authors recognize two periods of activity. The first, from
40 Ma to 35 Ma (late Eocene),is characterizedby potassic calcalkaline volcanism with emplacement of lahars and andesitic flows
that form the base of the SpearsFormation (Chapin and Seager,
1975;Osburn and Chapin, 1983;Cather, 1986;Morgan et al., 1986;
Chapin et al., L987).The second,between 36 Ma and 30 Ma (early
Oligocene), is marked by the regional emplacement of the first
rhyolitic and rhyodacitic tuffs intercalated in volcaniclastichorizons at the top of the SpearsFormation (Chapin andSeager,1975;
Osburn and Chapin, 1983;Cather,1986;Morgan et al., 1986).
Oligocene to Recent
opening of the Rio Grande rift
In the region of Socorro,most authors place the opening of the
rift around 29 to 31,Ma (bracketed by late Oligocene to early
Miocene; Chapin and Seager, L975; Chapin, L979; Chamberlin,
1983; Chapin et al., L987;Morgan et al., 1986)and distinguish
two periods of deformation recorded in the sedimentation.
The first, from late Oligocene to early Miocene (Fig. 4), is recognized by epigene dikes and sills dated between 3L.3 and 24.3
Ma (late Oligocene;Aldrich et al., 1986)in the region of the Rio
Salado and by basalt flows and basalticandesitesdated between
30 and 24 Ma (Chapin and Seager,1975;Elston, 1984;Morgan et
al., L986)displaying calc-alkalineaffinities. This volcanic activity
is related to the initial extension of the late Oligocene to Miocene
basins of the rift, which are filled by as much as 2,500 m of
continental terrigenous deposits (PopotosaFormation) lying unconformably on older formations (Machette, 1978).
The secondperiod of deformation ranges from late Miocene to
early Pliocene(Fig. a) and was accompaniedby basalticvolcanism
with tholeiitic or alkaline affinities dated between 8.3 + 0.2 and
0.5 + 0.02Ma (lateMioceneto Holocene;Bachmanand Mehnert,
1978;Baldridgeet al., 1987).The modernbasinsof the rift opened
during this period and were filled with up to 900m of continental
deposits of the Sierra Ladrones Formation (Pliocene).
TECTONICS
One of the main featuresof the Socorroregion is the Rio Grande
rift, a great structural depression extending for more than 1,000
km from Leadville, Colorado, to El Paso,Texas.It is marked by
elongated north-south basinsseparatedby mountainous massifs,
which have undergone commonly more than a kilometer of erosion. In the study area, the rift is broken into asymmetric northsouth-trending basins (Albuquerque, Socorro, La Jencia,|ornada
del Muerto, and Mulligan Gulch Basins)separatedby tilted horst
blocks of the same orientation (Lemitar, Magdalena, Bear, and
GallinasMountains).
The rift and its fill masks, in large part, older structures that
tend to be overlooked. Before its opening, two periods of structural deformation are observed: one in the earlv Pennsvlvanian
to early Permian, then a major one from late Paleoceneto middle
Eocene that was connected with the uplift of the Rocky Mountains.
Late Paleozoic tectonism
Arguments in favor of a tectonicevent in the late Paleozoicare,
in this region, mainly stratigraphic. They depend on evidenceof
May L991 New MexicaGeolagy
FIGURE 5-The main structural ensemblesof the southem Rocky
Mountains. [1-4, cross section lines for Figs. 6 and 7.-Ed.]
a north-south paleotopographic high (Armstrong et al., 1979),
which is covered by a reduced and condensedPennsylvanian to
Lower Permian section and, in turn, oveilain by the Abo Formation (Lower Permian) whose basal contact is an erosion surface. Even though the tectonicuplift causingthis paleotopography
has not been documented structurally in the field, the absenceof
an angular unconformity at the base of the Abo Formation and
the notable differences in deformation style between the Pennsylvanian and the Permian arguein favor of extensionaltectonics.
This uplift ceasesto exist at the base of the Permian becausethe
Abo Formation (Lower Permian)buries the paleotopography; the
time of its inception remains to be determined in this area.
Compressive tectonism of the Rocky Mountains
(Laramide orogeny)
Field studies, complemented by analysis of COCORP (Consortium for Continental ReflectionProfiling) seismicprofiles, show
that compressive tectonism can be subdivided into two phases:
1) a major east-west compressionresulting in the imbrication of
north-south-oriented crustal slices with eastward vergence; 2)
transpressionof less importance marked by right-lateral displacements of north-south orientation on the eastern border of the
Colorado Plateau.
In the region of Socorro, the Baca Formation concealsmost
compressive structures of the Rocky Mountains. In the absence
of Upper Cretaceoussediments and Paleoceneor lower Eocene
sediments, the exactage of the beginning of compressivetectonic
movement could not be established.The work of Baltz (1967)in
northwestern New Mexico suggests a latest Paleoceneto early
Eoceneage for the compressivephaseand a middle to late Eocene
age for the transpressivephase.
Three major compressional regions (Fig. 5)
The westem border of the Great Plains-In the east of the
study area, the frontalmost compressional structures mark the
boundary with the Great Plains. This zone is characterizedby
Coupe
pli faille de
I'A del Tajo
/t'2-''
Loma de
las Canas
1800
LF1 60 0
1F
r
,\
j
1400
5000
4000
ft
rkm
)
Coupe 2
1200
m
FIGURE 6-Two geologic cross sectionsof the southern Rocky Mountains front (location on Fig. 5). 1, Precambrian;2, Sandia Fm.; 3, lower grey
limestone mbr.; 4, arkosic limestone mbr.; 5, Bursum Fm.; 6, Abo Fm.; 7, Meseta BlancaMbr.; 8, TorresMbr.; 9, Glorieta Fm.; 10, San Andres Fm.;
11, Sierra Ladrones Fm.
E
[
|
2000
1800
F1600
i. 1400
T
1200
[
Faille comanche
Faille Santa Fc
E
[
2000
1800
l1600
i1400
lL rzoo
Coupe
4
F a i l l eL a d r o n
FIGURE 7---Two geologic cross sectionsof the western boundary of the southern Rocky Mountains (location on Fig. 5; same legend as Fig. 6)
tight, overturned, north-south-striking, east-vergentfolds ahead
of the thrust front that pass eastwardinto broad, open, widely
spaced(>10 km) folds.The tight foldsdevelopedabovetwo levels
of detachment observed in the field. The shales of the Sandia
Formation (Lower Pennsylvanian) give rise to intense disharmonic folding (Arroyo de los Pinos and Arroyo del Tajo; Cabezas,
1989,p. 150).Another set of widespread spectacularfolds developed above evaporitesof the YesoFormation (Upper Permian)
that contain subhorizontal detachment surfaceswith local lateral
ramps marked by tectonicsolution brecciasup to 20m thick (Baker
Hill; Cabezas,1989,p. 158,fig. 100).The displacementon thesc
surfaces is accompaniedlocally by tectonic removal of the gypsiferous interval of the YesoFormation, of all or part of the GIo-
rieta Formation,and sometimesof part of the SanAndres Formation
(Fig. 6). These detachments, even if they are sPectacular,in no
case imply important displacements;the restored cross section
indicates a shortening of only 3To(Cabezas,7989).
The southern Rockv Mountains-The central zone, which is
masked in large part-by the basin fill of the Rio Grande rift,
correspondsto the southernRocky Mountains sensustrictu.
The overthrust front (Fig. 5) is located at the base of the Manzano-Los Pinos Mountains slice and extends for more than 50
km along the east flank of the Manzano Mountains southward
to Arroyo de las Cafras,where it disappearsagainsta more recent
normal fault. It can be divided into three segments:1) to the north,
Nau Mexico Gmlogy
|l'f.ay l99l
106"45
107"oo
t06to
3q'4s-
uucenoV:
uPLtFTri
Bassin d'Atbuquerque
r-rHES 1
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OUAI€NNARY. ?€RTIARY
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E
PR€CAilERIAN
VOLCATIIC ANO
SEOIMEN'AFY
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s€ 0D€N rlRY
CRYS'ALLINE
FOC|(S
ROCXS
FIGURE 8-Location map of the COCORPline (in Cape et al., 1983).
B a s s i nd A l b u q u e r q u e
Horst des
LadronMountains
s o u s - b a s s i no u e s t
s o u s - b a s s i nc e n t r a l
300
250
200
Horst des
JoyitaHitls
SlAli AP1
S o u s - b a s s i ne s t
Manzano
Los Pinos
Mountains
;y''
I
'[__| sf-l
z[n,Ti]4F',tSl
F.I9yR-E9-Synthetic cross section-of the Albuquerque Basin from COCORP data. (Heavy lines are Rocky Mountains thrust faults; thin lines are
rift faults') Legend: 1, Miocene and Pliocene;2,-upp-erEoceneand Oligocene;3, Paieozoit and Mesozoic;'4, Precambrian.
May 1991 New Mexico Geology
on the east flank of the Manzano and Los Pinos Mountains, it is
representedby the Montosaand Cerro Montoso faults, two eastvergent reverse basement structures oriented N15"E. The Paleozoic cover is deformed into large monoclinal east-facingfolds. 2)
Toward the south, in the area of Sierra de la Cruz and Arroyo
Tinajas, the reversebasementfaults are intersectedby two rightlateral transpressivefaults oriented N40"E, the Sierra de la Cruz
fault zone and the del Curto fault. The shorteningis taken up by
north-south-oriented,en echelonfolds. 3) Farther to the south,
the overriding front reappearsin the region of Arroyo del TajoArroyo de las Caflas,where the folds and overthrusts of the cover
are oriented N20'W to N20"E. A beautiful example is the faulted
fold of Arroyo del Tajo, where the detachmentof the cover occurs
in the shaly rocks of the Sandia Formation.
The boundary between the southern Rocky Mountains and
Colorado Plateau (Fig. 7!-The Comanche overthrust in the Lucero uplift and the Ladron overthrust on the western flank of the
Ladron Mountainsare the westernmostfracturesof RockyMountain style observedin the Socorroregion. They are consideredas
marking the border between the Colorado Plateauand the Rocky
Mountains. The north-south-trendingComancheoverthrust affects rocks of the sedimentarycover (Pennsylvanianand Permian), which are deformed into tight east-vergentfolds. The
overthrust is cut on the south by the MesaAparejo fault, a N45"E
transpressivedextral strike-slipfault.
To the south, on the west flank of the Ladron Mountains, the
Precambrian basement overrides the Pennsylvanian toward the
west along the Ladron overthrust to which are associatedmonoclinal west-vergent folds. Toward the north, the Ladron thrust
can be projected to the Mesa Aparejo fault by the presenceof
west-vergentmonoclinalfolds in the prolongationof the Ladron
overthrust.
A diagrammaticeast-west crosssectionacrossthe Albuquerque
Basincan be drawn by interpreting the combinedCOCORPseismic lines AP1 (Abo Pass1) and S1A(Socorro1A) (Fig. 8) to show
the deep geometry of structuresunder the rift. Two large basement slicesof Laramide origin can be observed(Fig. 9): to the
east,that of the Manzanoand Los PinosMountainsoverlyingthe
Montoso fault (dipping 20"to 40"westward),which is a basement
overthrust of easterlyvergence;to the west (concealedunder the
rift fill) that of the Ladron Mountains,underlainby an overthrust
dipping 40'to 50'westward. The arching,which expressesitself
in the sedimentarycover in the horst of the Ladron Mountains,
is associatedno doubt with compression.
North of the Ladron Mountains horst the COCORPprofile is
subparallelto the structuresmaking interpretationsinconclusive.
The Colorado Plateau-The western part of the study area
belongsto the ColoradoPlateau,which is a vast, little-deformed
zone characteizedby broad folds. In outcrop,thesefolds involve
the Triassicand the Upper Cretaceous,notably in the Riley area
north of Magdalena.
The dextral transpressivestructures
The north-south-trendingCharlieHill fault (Fig. 10)in the Lucero uplift is a dextral strike-slip fault confirmed by the presence
of very steeply plunging, Z-shapedfolds associatedwith nearly
horizontal striationsin the plane of the fault. This fault cuts folds
and overthrustsformed during the major compressivephase.In
addition, the Charlie Hill fault cannot be related to rift opening
that gavebirth to north-south-striking normal faults. Similar rightlateral wrench faults have been describedalong the easternborder
of the ColoradoPlateau.For instance,north of the Lucero uplift,
the Rio Puerco fault zone was described by Slack and Campbell
(1976)as the type pull-apart favored in rightJateral north-southstriking wrench faults. Likewise, Baltz (1957)envisageda rightlateral wrench fault in the Nacimiento uplift (Fig. 10) and dated
it middle to late Eocene.
This wrenching phaseis poorly displayed in the Socorroregion
becauseof the absenceof folds or overthrusts.
Extensional tectonics of the Rio Grande rift
The structuralhistory and magmaticevolution of the Rio Grande
rift have been the subject of many investigations (Chapin and
Seager, 1975;Chamberlin, L983;Aldrich et al., 1986;Morgan et
al., 1986).New observations in the Lemitar Mountains north of
Socorro have allowed the establishment of a tectonic evolution
in four stages that is consistent with what is known elsewhere.
The chronological evidence for each stage and its deformational
characteristicsare discussedbriefly in the following paragraphs.
The subsurfacegeometry of the rift basins is then examined with
special attention to the influence of previous deformations (notably the Laramide structures of the Rocky Mountains) on the
extensional geometries.
Tectonic history
Oligocene to early Miocene (Fig. 11-1)-The principal elements
that permit dating the beginning of the Rio Grande rift extension
in the Rio Salado area are: the emplacementof numerous uPPer
Oligocene(31.3to 24.3Ma, Aldrich et al., 1985)basalticandesite
sills and dikes; the angular unconformity of the Popotosa Formation (Miocene) on the Oligocene volcanic formations, especially the South Canyon Tuff, which is dated at 26.3Ma (Osburn
NArPeAr,
and-Chapin, 1983)[sincedated at 27.36 1- 0.07Ma by
Mclntosh et al., 190-Ed.l. The dikes of the Rio Saladozone are
emplaced along north-south-striking, steeply dipping, normal
faults with small displacements(severaltensof meters).The method
of dihedral angles of Angelier and Mechler (L977)to determine
stress orientati,on at the time of faulting was applied to measurements at four fault sites. The results show that the minimum
principal stress o, is subhorizontal and oriented from east-west
to N50'W. This direction does not correspond to the one (N70"E)
proposedby other authors (Aldrich et al., 1985).It is likely that
ihe results obtained in the Rio Salado zone have only local importance; therefore, it is important to extend thesemeasurements
io other areas to determine more reliable regional paleostress
orientation. This extension is accompanied by an initial blocktilting stage displayed in the angular unconformity (20'on av(r
erage) at the base of the PopotosaFormation.
, S\
'B€ren
thiN
25mr
-
I tr'L;dron
Mtns.
.S
FIGURE lO-Middle to late Eocenewrench faults along the eastern
border of the Colorado Plateau (after Baltz, 1978;Slack and Campbell,
7976;and Cabezas,L989).
Nao Mexico Geology
May 1991
Fm Sierra Ladrones
,+
f+
++ +
++
l+
Red Mountain
2
#-.-AU4{*;
++
+
!
t
f
++
La Jara Peak Basaltic Andesite (<)
Vicks Peak Tuff
Jencia Tuff
Iuff of GraniteMountain
Fm Spears
Fn Baca
Madera
FIGURE l1-Retrotectonic
cross sections of the Lemitar Mountains.
Miocene(?) (Fig. 11-2F-A further period of extension (intraMiocene) caused the associatedfaults to cut the Popotosa Formation. These faults are themselvescut by younger faults of the
third_period.A more preciseage for thesemoveinentsdepends
on a better understanding of the age of the PopotosaFormation
in the Lemitar Mountains. In this period falls the displacement
of near-surfacefaults (e.g., Red Mountain fault), which occurs
without further tilting of the fault blocks.
Late Miocene to early Pliocene (Fig. 11-3)-An unconformity
at the base of the Sierra Ladrones Formation (pliocene) on the
lopotosa Formation (Miocene)marks the beginning of MiocenePliocene extension. Eventually, extension continues into the pliocene as certain faults that affect one part of the Sierra Ladrones
Formation are concealed in turn by younger basalts dated by
Baldridgeet al. (1987)at3.7 + 0.4 Ma. A fault with sucha history
is the Santa Fe fault bounding the Lucero uplift.
May 1997 Nau Mexico Geology
This faulting period fixes the geometry of the modern rift basins
by normal faults that are either steeply dipping (e.9. Santa Fe
fault) or listric (Bustos Well fault east of Socorro), and whose
displacements can attain several thousand meters. Paleostress
determinationsmade by the author at sevensiteson the border
of the rift give consistent results: tr3 was subhorizontal and oriented east-west,a direction identical to that proposedby other
authors (Aldrich et al., 1985).The faulting causedrenewedtilting
of blocks, notably in the Lemitar Mountains where the angular
discordance between the Sierra Ladrones and Popotosa Formations attains30'.
Late Pliocene to the present (Fig. 11-4)-Extension during this
most recent time is the causeof faults affecting the upper Pliocene
and Quaternary and of recent active seismicity. Of north-south
orientation, the faults are located principally in the rift basins.
They are few in number, have small displacements, and have
been catalogedby Callender et al. (1983).
w
P L I O C E N ES U P E R I € U R - P L E I S T O C E N E
M I O C E N ES U P E R I E U R - P L I O C E NI N
EF E R I E U R
O L I G O C E N ES U P E R I E U R
M I O C E N EI N F E R I E U R
E O C E N EM O Y E N( ? ) - E O C E N ES U P E R I E U (R? )
F a i l l ed e C h a r l i eH i t l
P A L E O C E N ES U P E R I E U R _ E O C E NI N
EF E R I E U R
t"*i""''j
Vadlon
ilojnti,n"
€odt\\e
^otll'nt'noS/
"---"
PENNSYLVANIENINFERIEUR-PERMIENBASAL
\
Zuni Uplift
'
-V
\
\
Joyita Hills
,'
P e d e r n a lU p l i l t
FIGURE 12-Tectonic timing for the Socono region. (Discussionon p. 35.)
Nm
Mexico Geology
May 1991
The COCORP seismic profile
The interpretation of the COCORP profile (Fig. 9, the normal
faults of the rift are drawn in fine lines) has been discussedon
pages 32 and 33. From a morphostructural point of view, the
southern Albuquerque Basin is subdivided into three subbasins
(east,central,and west) separatedby two horsts (Ladron Mountains and joyita Hills). The west and centralsubbasinsare asymmetric and tilted toward the west and their Miocene-Pliocene
sedimentaryfill attains1.5 secondstwo-way travel time [approximately 3.5 km-Ed.l.
Geometry of the normal faults-At the west end of the profile,
the tectonic system is dominated by east-dippinglistric faults.
They affect the entire sedimentary sequenceand-often the Precambrian basement.The best exampleis the east-dippingfault
bordering the centralsubbasinon the west. BetweenVP 230and
VP 180,at the contactbetweenthe Precambrianbasementof the
Ladron Mountains horst and the Tertiary volcanics,the fault dips
steeply to the east.It flattens toward the eastto a nearly horizontal
attitude between VP 180and VP 150in the Paleozoicand Mesozoic
cover; east of VP 150 the fault disappears in the Precambrian
basement before merging with the Montosa fault.
Faults observedin the west subbasinalso seem to be listric and
appearto flattenat the basement-sediment
contact.It is important
to note that this part of the profile is not perpendiculai to the
directionsof faulting. The apparentdip is thus lessthan the true
dip.
Farther east on the profile, on the western flank of the Joyita
Hills horst and in the eastern subbasin, dip of the faults vaiies
from moderate to steep. Normal faults merge with the reverse
Montosa fault at depths from 2 to 4 secondstwo-way travel time
[approximately6 to 72 km-Ed.] without ever cutting it (de Voogd
et al., 1986).
Discussion of the COCORPprofile interpretation-The normal
faults on the east side of the rift, the faults of the ToyitaHills horst
and those of the eastern subbasin, do not see- io project very
deeply into the crust but abut on, or merge with, the reverse
basementfaults of Laramideage.This crucialobservationimplies
that these large crustal discontinuities, formed during the Rocky
Mountain compression, have been reactivated by the extension
with an opposite senseof displacement.Comparablerelationships have been observed in the Basin and Rangeprovince (Allmendinger et al., 1983).
The models of normal faulting associatedwith zones of detachment (a zone of weak resistanceto shearingsuch as a basement-sedimentary cover contact, a Iayer of salt, or a pre-existing
fault zone) show that structural evolution can be divided into two
stages (Faure and Seguret, 1988):1) displacement of the upper
block along the detachment createspotentially empty spacb; 2)
its collapse causesa rollover or a compensation graben with antithetic normal faults. Structures in the eastern part of the COCORP profile compare favorably with experimental models. In
this case,there is at once a compensationgrabenand a rollover.
In the western part of the Albuquerque Basin, extensional tectonics is dominated by east-dipping listric faults; their dip, approtmately 50onear the surface,is closeto horizontalat the base
of the sedimentary cover. This flattening can be explained by the
presenceof a horizon of weak resistanceto shearing, such as the
evaporites of the Yeso Formation or the shales of the Sandia
Formation.
In conclusion,it seemsthat reactivationof Laramidebasement
faults with an opposite senseof displacementplayed an important
role in the mechanismof extension.It explainsthe subsidinceof
the rift basins.The newly formed normal faults are either antithetic faults relatedto formation of a compensationgrabenor listric
faults flatteningin the sedimentarycover.
CONCTUSIONS:TECTONIC EVOLUTION (Fig.12)
Late Paleozoictectonics
(ancestralRocky Mountains)
In the Pennsylvanian and Early Permian, the North American
platform was broken into north-south-trendinghorsts and graMay 1991 New Merico Geology
bens mantled by the Lower Permian (Abo Formation). The uplifts
(horsts) are characterizedby a reduced and condensedsedimentary column.
Compressional tectonics of the Rocky Mountains
(Laramide orogeny)
From late Paleoceneto early Eocene, compression led to differentiation of the Colorado Plateau, the Rocky Mountains, and
the Great Plains. This is principally a compressional basement
tectonic style characterizedby crustal slicesof north-south strike
and eastvergencebeing imbricated along reversefaults of shallow
dip (Manzano-Los Pinos Mountains plate, Ladron Mountains).
The deformation of the sedimentary cover is controlled by the
basementblocks and by severallevels of detachment.Transverse
faults of N45'E orientation with dextral strike slip cut the basement slices (Tijeras-MesaAparejo fault, Santa Cruz fault).
By middle late Eocene, dextral north-south wrench faults developed on the easternborder of the Colorado Plateau.This transpressivephaseis poorly documentedin the Socorroregion because
of the absenceof folds and thrusts. These dextral wrench faults
may indicate a displacementof the Colorado Plateau toward the
north at the end of the compressiveepisode of the Rocky Mountains (Chapin and Cather, 1981).
Rio Grande rift
We have discussedabove the preponderance of reverse basement faults of Laramide age that represent zones of weaknessto
shearing reactivated in the opposite senseduring extension;this
reactivation is sufficient to explain subsidenceof the rift basins.
As for the recent faults. thev are antithetic and related to the
formation of a compensation graben or listric flattening in the
sedimentary cover.
In the late Oligocene and early Miocene, the beginning of extensional opening is well dated, notably in the area of the Rio
Salado northwest of Socorro,by the emplacementof dikes dated
as late Oligocene. Basinsopening in the course of this phase are
filled by the PopotosaFormation of Miocene age, which lies unconformably on Oligocenevolcanic and volcaniclasticrocks in the
Lemitar Mountains.
The shallow listric faults in the middle Miocene(?)of the Lemitar
Mountains are local and do not seem to be associatedwith this
breakup.
From late Miocene(?)to early Pliocene, the second episode of
extension resulted in formation of the present rift basins. It is
accompaniedby a second tilting, producing the angular unconformity of the Sierra Ladrones Formation (Pliocene)that covers
the Popotosa Formation (Miocene) and fills the newly opened
basins.
From late Plioceneto the present, continued extensionalfaulting has cut upper Pliocenerocks and Quaternary pediments. This
is accompanied by important seismic activity in the region of
Socorro and north to Albuquerque.
AcxNowrrocMENrs-I wish to thank Messieurs B. Plauchut
and M. Zimmermann and Soci6t6Nationale Elf Aquitaine (France)
and its American affiliate Elf Aquitaine Peholeum who have placed
at my disposition scientific support at the scaleof the proposed
study and the technical and financial means necessaryfor my
three periods of field work in New Mexico. I expressmy thanks
to my advisors R. Blanchet, C. E. Chapin, and M. Tardy for their
counsel and their support.
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