Structural transect of the southern Chihuahua Fold Belt between

TECTONICS,
Structural
transect
VOL. 13, NO. 6, PAGES 1445-1460, DECEMBER 1994
of the southern
Chihuahua
Fold Belt
between Ojinaga and A!dama, Chihuahua, Mexico
PeterH. Hennings
Departmentof GeologicalSciences,The Universityof Texasat Austin
Abstract.
A restorable
structural
transect
has been
structed across the southern end of the Chihuahua
Fold
conBelt
segment of the Cordilleran foreland fold and thrust belt of
northeastern Chihuahua, Mexico, and west Texas.
The 160-
km transect begins near Ojinaga and ends near Aldama,
Chihuahua.
It has been found that the Mesozoic
Chihuahua
Trough was inverted by Laramide tectonism and regionally
shortenedapproximately20 km (about 9%) along the transect
to form the ChihuahuaFold Belt. Along the transectthe belt
consistsof two allochthonsof opposingvergence directions.
The d6collementof the eastern,northeastvergent allochthon
shallowsfrom within the Jurassicevaporites,along the center
of the belt, and surfacesin Upper Cretaceousclastic rocks at
the eastern thrust front south of Ojinaga, Chihuahua. The
eastern allochthonwas shortenedapproximately 6 km and
may be divided, from west to east, into main ranges and an
eastern frontal zone. Structures of the main ranges are
detachedin Jurassicevaporitesand were greatly influencedby
syntectonicflowage, resultingin salt-coredanticlines and salt
withdrawal synclines. To the east the d6collement shallows
over the Tascotal
basement block which formed
a buttress to
northeastwardtransport forming a frontal zone containing
ramp-relatedanticlinesand emergentthrusts. The westernallochthonis southwestvergent. The ddcollementcuts up section from the center of the belt, where Precambrian basement
is detachedin the PlomosasUplift, to Lower Cretaceouscarbonates of the western frontal
zone.
The western
allochthon
was shortenedapproximately20 km at the Paleozoic level
and approximately 11 km at the Mesozoic level. Local
basementinvolvementin the PlomosasUplift may result from
strike-slip deformationalong a northwesttrending basement
fault.
Introduction
for the Chihuahua segmentof the Cordilleran fold and thrust
belt. This paper presentsa 160-km long cross section (Plate
1) constructedto studythe style of deformationof the southern
end of the belt and the resultsare used to proposea tectonic
model for Laramideevolutionof the region. Traversemapping
was conducted along the transect and integrated with well
data, previous geologic maps, space shuttle and Landsat
photography,and published geologic data to construct a
restorablecrosssection. This traverseis possiblebecausethe
Conchos
River
has removed
the closed basin fill
that buries
Laramide structuresto a higher level elsewhere in the fold
belt.
It has been found that the senseof vergenceof the belt was
bidirectional, northeast on the eastern side and southwest on
the western
side.
Therefore
the belt
can be divided
into
eastern and western allochthons. Each allochthon may be
subdivided into main ranges and frontal zone tectonic
domains. NE transportof the easternallochthonhalted as
allochthonous Mesozoic rocks were stacked against the
Diablo Platform in Texas and eastern Chihuahua, which acted
as a buttressthat restrained Laramide shortening. The eastern
main ranges overlie regions of Jurassicor Early Cretaceous
evaporites,which were mobile duringthrusting. Folding and
flowage of ductile material into upright to asymmetric folds
accommodated more horizontal shortening than did thrust
faulting. As is typical in fold and thrust belts with a thick
evaporited6collement,the senseof vergenceof structuresof
the eastern main ranges is bidirectional in the plane of
transport [Davis and Engelder, 1985], however, northeast
vergent structures predominate. East of the eastern main
ranges, the regional detachment shallows to the level of
Lower Cretaceouscarbonatesand parallels stratigraphyfor 30
km, forming the eastern frontal zone. Deformation of the
upper plate of the easternfrontal zone is characterizedby
ramp anticlineswith less than 2 km of structuralrelief. The
thrust surfaces on the eastern side of the belt near the Rio
The Chihuahua Fold Belt of the Sierra Madre
the southern continuation
of the North American
Oriental
is
Grande, 5 km south of Presidio.
Cordilleran
Precambrian and Paleozoic rocks deformed by Laramide
fold and thrustbelt into Mexico. It is a NW trending belt that
extendsthroughoutmost of easternChihuahua,Mexico, and
adjacentpartsof Trans-Pecos
Texas (Figures1 and 2). Until
this study,no regionalcrosssectionshave been constructed
1Nowat MobilExploration
andProducing
Technical
Center,Mobil
Researchand DevelopmentCorporation,Dallas, Texas.
Copyright 1994 by the AmericanGeophysicalUnion.
Paper number94TC00800.
0278-7407/94/94TC-00800510.00
1445
tectonism
occur at the surface in the Chihuahua
Fold
Belt
only in the Placerde Guadalupe/Carrizalillostructuralmassif,
herein called the Plomosas Uplift (Figures 1 and 2). The
Plomosas Uplift is the dominant tectonic element of the
westernallochthon. It appearsto havebeenhigh duringearly
ChihuahuaTroughtime and remainedfree of evaporites. The
PlomosasUplift is herein proposedto have resultedfrom a
combinationof regionalhorizontalshorteningand left-lateral
wrench faulting along a NW trendingstrike-slip fault, herein
nmnedthe Plo•nosasbasementshearzone. The uplift consists
of two thrustswhich place Precambrianthrough Cretaceous
rocksover Lower Cretaceousrocksforming the la[geststruc-
1446
HENNINGS: STRUCTURALTRANSECTOF SOUTHERNCHIHUAHUA FOLD BELT
105 ø
NEW
MEXICO
IIIIIIII
•IIIIIIIII
IIIIIIIIIIiiig
,ELPASO
ß
TEXAS
IIIIII
Figure 3
OHIHUAHUA
i:/-?':-?::,:}!:
Plomosas
Uplift
:.'.!!!
i-:-.'d.!
(outcrops
ofPzrocks) -•:.•:
•___•.•.o
• evaporites
(Jur, base Chi.
C,.••,•
trough)
"•
30 ø
•'...............
•...........
•; Chihuahua
;:•.•...........
.•: trough(Mz)
platform (Mz)
Chihuahua
Belt
Tectonic
COAHUILA
ALDAMA,
3'Laramide
Thrusts
Plate 1
0
i
Frontal
6o
i
120
i
150
i
km
Figure 1.
1987).
Structural
elements
of theChihuahua
FoldBelt[afterGriesandHaenggi,1970;Handschy
et al.,
turein the belt. The regionaldetachment
parallelsbedding
Mesozoicand CenozoicStratigraphyand Tectonics
westof the PlomosasUplift forming a westernfrontal zone
containingemergentthrustsand ramp anticlines,structures
that are similar in geometryto those of the easternfrontal
DevelopingMesozoic basinsin northernMexico formed a
more or less continuous feature from the Gulf of Mexico.
zone.
Basindevelopment
wasprobablylinkedintimatelywith the
Phanerozoic
GeologicHistoryof the Region
openingof the Gulf of Mexicoin the Jurassic
[Salvadorand
Green,1980;Salvador,1987] and probablyreflectsextensionalor transtensional
processesassociatedwith the move-
PaleozoicStratigraphy and Tectonics
A shelfenvironment
existedthroughsoutheastern
Arizona,
mentof SouthAmericaawayfromNorthAmericaduringthe
early Mesozoic[Dickinson,1981; Andersonand Schmidt,
1983].
southwestern
New Mexico, and centralChihuahua
during
A lateJurassic
through
middleCretaceous
marinetransgresCambrian
to Mississippian
timealongthe Paleozoicpassive sionresulted
in thedeposition
of 3650-6400
m of sedimentary
margin of North America. In eastern central Chihuahua and
rocksin theChihuahua
Troughwhichrestunconformably
on
westTexasthe passivemarginevolvedinto the Pedregosa rocksof thePedregosa
Basin[GriesandHaenggi,1970]. Late
foreland basin, as tectonic activity associatedwith the
Jurassic
deposition
of evaporites
marksthe beginning
of maOuachitaorogenyincreased
to the southandeast[ Greenwood rinedeposition
in theChihuahua
Trough
[DeFord
andHaenggi,
etal., 1977](seeFigure3). Thedominant
sediment
typesdur- 1970;Cordoba, 1970] (Figure 3). One of the Petroleos
ingthePennsylvanian
werelimestone
andshale,withlarge Mexicanos
(PEMEX)CuchilloParadowells(Plate1, well 5),
quantifiesof coarseterrigenousclasticdebriserodedfrom surlocatedon the crestof an evaporite-cored
anticline,drilled
roundinguplifts. Armin [1987] conducteda detailed examina-
2,300 m of rock consistingof 80% halite and 20% clasticma-
tionof Wolfcampian
conglomerates
in the Pedregosa
Basin
andconcluded
thatthesouthern
partof thebasin,including
theregionof thePlomosas
Uplift,subsided
slowlyduringmost
of Paleozoic
time but rapidlyevolvedinto a deepforeland
basinduring early or middle Wolfcampianas a result of
terial[Ramœrez
andAcevedo,1957]whichhasbeenpalynologicallydatedas Kimmeridgian
[Salvador,
1987]. Hennings
[1991]estimates
thatthe evaporites
hada depositional
thicknessof approximately
1000m. In theChihuahua
Troughthe
Cretaceous
sequence
beginswith a basalconglomerate,
fol-
Ouachitatectonics. The lack of any deformationin the
Plomosas
Uplift whichcan be directlyattributedto Ouachita
tectonicsimplies that the PedregosaBasin resided in the
forelandnorthwest
of the Ouachitaorogenic
front[Bridges,
1964;Flawn et al., 1961].
lowed by interbeddedquartzosesandstoneand shale of
Neocomian
age [DeFordandHaenggi,1970](Figure3). The
clastic-dominated
sectiongradesupward into interbedded
shaleand limestonewith minor amountsof evaporitein
Aptinn time. Platformal carbonate formations dominate the
HENNINGS:
STRUCTURAL TRANSECT OF SOUTHERN CHIHUAHUA
FOLD BELT
1447
Figure 2. NASA spaceshuttlephotograph(frame 61A-47-025, oblique) of the southeastern
ChihuahuaFold
Belt. The boundariesof the frame are shown as the dashedpolygon on Figure 1. The locationof the transect
and Highway 16 are shown. Abbreviationsare A, Aldama, Chihuahua;C, Coyame; O, Ojinaga; P, Presidio,
Texas; PU, PlomosasUplift; RC, Rio Conchos;RG, Rio Grande; S, Sierrita; SCP, Sierra Cuchillo Parado; SG,
Sierra Grande; SGz, Sierra Gomez; SL, Sierra La Santa Cruz; SM, Sierra Morrion; SS, Sobaco Syncline; and
SSo, Sierra Soldado. Scale is variable; the transectline is 160 km long.
Albian throughearly Cenomaniansection and constitute the
rigid structuralmember in the deformed Mesozoic sequence.
Most
anticlinal
structures
in the fold
belt
are eroded
to the
level of the Albian carbonates(Figure 4).
The structurallyhigh Diablo Platform formed a steep-sided
easternbasinmargin for the ChihuahuaTrough. The dramatic
westward thickening of the Las Vigas Formation and to a
lesserextent, the Cuchillo Formationand Aurora Group, are
interpretedto be the result of extensional growth faulting
along the western margin of the Diablo Plateau Lehman,
1986;Hennings,1991] (Figure3). The patternof stratigraphic
thickeningof the Lower Cretaceoussectionacrossthe trough
suggeststhat only the northeast side of the basin is fault
bounded. The southwesternboundary of the basin is the
1448
HENNINGS'
STRUCTURAL
TRANSECT
OFSOUTHERN
CHIHUAHUA
FOLDBELT
C_onchos
gravels
lavaflows(unnamed)
El Picacho Fm
San Carlos Fm
70 km
Ojinaga Fm
ChihauhuaTrough
-
Buda Fm
Del Rio
Quitman
Mtns
-
DiabloPlatform
VanHornMtns
WylieMtns
Loma Plata Fm
Benevides
Fmj
Finlay Fm
='
O
n..
O
L;•grima/CoxFm
Benigno
Fm
Cuchillo
major
,'
unconformity--¾,
Fm
Las Vigas Fm
_.._._
,
Navarrete
(abundant
evaporites)
'
,'
LaCasitaFm ,,'
presumed
normalfaultsforming
Plomosas
Fm
eastern margin of Chihuahua
Troughandcontrolling
thickness
of Mesozoic strata
Pastor Limestone
•
0
- Monillas
Fm
SolisLimestone
Sostenes Fm
N
•E
Figure
3. Diagrammatic
trough-to-shelf
transect
(see
Figure
1forlocation)
[after
Amsbury
and
Reaser,
1988]
andgeneralized
stratigraphic
column
ofPaleozoic
[after
Bridges,
1964]
through
Mesozoic
rocks
[after
Gries,
1970]in theeastern
Chihuahua
Trough.
AldamaPlatformwhichremained
stableduringChihuahua UpperCretaceous
sediments,
whicharewell-preserved
in the
Troughsubsidence
[Handschy
andDyer, 1987].
OjinagaBasinwestof Ojinaga(Plate1), are tectonostratiSubsidence
confined
to theChihuahua
Troughceasedin
graphically
correlative
with the pretectonic
to syntectonic
Cenomanian
timepriorto thedeposition
of UpperCretaceous Cretaceous
clasticwedgesfoundin otherforelandareasof the
clasticrocks. The transitionfrom marineto continentalfacies
NorthAmerican
Cordillera.
By analogy
thisimplies
thatthe
occurredin Santoniantime [Lehman, 1986, 1991]. These
ChihuahuaFold Belt was distal to the interior of the
HENNINGS: STRUCTURAL TRANSECT OF SOUTHERN CHIHUAHUA FOLD BELT
1449
Andres Mountains of southern New Mexico [Drewes, 1988];
undeformedvolcanic rocks (47 Ma) in Sierra Samalayuca in
northern Chihuahua [Drewes, 1991]; and undeformed
ignimbrites (44 Ma) resting over thrust faulted ignimbrites (53
Ma) [ Goodell et al., 1988; P. C. Goodell, oral communication,
1990].
On the basis of these dates and stratigraphic
relationships,Cordilleran deformation in eastern Chihuahua
rangedbetween74 and 44 Ma.
Widespread rhyolitic volcanism blanketed the region in
mid-Tertiary time. Along the transect between Cuchillo
Parado and Aldama are remnants of thin but formerly
extensive rhyolite lava flows, which unconformably overlie
Cretaceousrocks and generally dip at lower angles [King and
Adkins, 1946].
Late Cenozoicnormal faulting, possiblyassociatedwith the
southerncontinuationof the Rio Grande Rift, has over printed
many of the preexistingMesozoic structuresin the Chihuahua
Fold Belt [Gries, 1979]. The general trend of Late Cenozoic
faulting is parallel to the Laramide structuralgrain and has
accentuated the elongate range/intermontane
basin
physiography
which is characteristic
of the region.
Intermontane basins (bolsons) in the southern segment
Chihuahua Fold Belt generally contain up to 100 m of
Quaternaryage coarsegravel interbeddedwith clay named the
Conchosgravels [Burrows, 1910].
Structural
Figure 4. View east of the Rio Conchoscanyon through
Sierra Grande showing exposuresof the CretaceousAurora
Group (see Plate 1 for location). Canyon is approximately
300 m deep. Abbreviationsare L, Loma Plata Formation;B,
BenevidesFormation;and F, top of Finlay Formation.
Cordilleran orogeny during Late Cretaceous time, but the
ensuingdepositionof coarser clastics during Santonian time
implies that the deformationfront was progressingeastward.
Laramide tectonism produced a north-northwest trending
fold and thrust belt with an arcuate eastern front that roughly
parallels the Texas-Chihuahuaborder between E1 Paso and
Presidio (Figure 1). The exposedportion of the Chihuahua
Fold Belt, east of the Tertiary ignimbrite cover of the Sierra
Madre Occidental,is approximately200 km wide and extends
400 km southfrom northwestof E1 Pasoto an abruptsouthern
termination
south of Presidio.
The structure
and evolution
of
the southern segment of the Chihuahua Fold Belt is the
subjectof this paper.
Precisedating of Cordilleran deformationin the Chihuahua
Fold Belt is difficult becausethere are few preservedsyntectonic deposits. However, timing bracketsmay be placed on
the deformationby combining the following data: dikes (65
Ma) with preferredorientationin the Sierra Del Cuervo area;
deformed Cenomanian rocks (97-91.5 Ma) cut by an undeformed andesite dike (47 Ma) in the Jfiarez Mountains of
northern Chihuahua[Handschy and Dyer, 1987]; syntectonic
debris (74-58 Ma) of Chihuahua Trough provenance in the
Tornillo Basin of west Texas [Lehman, 1986, 1991]; transition
from syntectonicto post-tectonicdepositionof the Paleocene
to Eocene Love Ranch Formation (57-44 Ma) in the San
Transect
of the Southern
Chihuahua
Fold Belt
The formation
of the southern Chihuahua
Fold Belt will
be
summarizedby describing a transect between Ojinaga and
Aldama, Chihuahua (Plate 1). The location of the transect
has been chosenso as to crossall significant structuresof the
southernChihuahuaFold Belt and depict the overall structural
geometry of the region. The eastern half of the transect is
constructedfrom new field mapping at 1:25,000 scale augmented by data from five PEMEX wells. The western half
was traverse-mappedat 1:50,000 scale and augmented by
previousstudiesin the region of the PlomosasUplift.
cross section is shown in deformed
The
and restored states. Where
possible, the methods of Suppe [1983], Jamison-.[1987], and
Mitra [1990, 1992] were used to aid in projecting surface
geometryto depth.
Eastern
Frontal
Structures
Zone
of the eastern frontal
zone of the Chihuahua
Fold
Belt are exposed throughout the Ojinaga Basin (Plate 1)
These
fault-related
folds
are
detached
within
the
Lower
Cretaceouscarbonatesection. The thrust front is exposedon
the Mexican side of the Rio Grande in segmentsbetween 12
km southand 37 km northwestof Presidio. Along the transect
the frontal
structure is called Sierra La Santa Cruz which
is an
east vergent, fault-cored anticline exposedat the level of the
Loma Plata Formation (Figure 5 and Plate 1). Its structural
relief is approximately1.6 km and topographicrelief is 300 m.
The controlling thrust is west dipping and places Loma Plata
Formation over Upper Cretaceousclastic rocks. Modeling
Sierra La Santa Cruz as a fault propagationfold predictsthat
the controlling thrust ramps up section from the Benevides
Formation. Detachment at this level along the easternside of
1450
HENNINGS:
STRUCTURAL
TRANSECT
OF SOUTHERN
CHIHUAHUA
FOLD BELT
Figure 5. West view acrossthe Rio Grande(RG) of Sierrala SantaCruz (seePlate I for location). The range
is a fault propagationfold in the hangingwall of the easternthrustfront of the ChihuahuaFold Belt. The fold
plungesinto the subsurface
at either sideof the view. The exposureis at the level of the Loma Plata Formation
(L); the vergencedirectionis towardviewer. Topographicrelief is approximately250 m.
the eastern frontal zone is corroborated by data from the
PEMEX Ojinaga I (Plate 1, well 1), located 18 km to the
the oldest stratigraphicinterval to be depositedacrossthe
ChihuahuaTrough and Tascotalblock with relatively constant
southwest of Sierra La Santa Cruz, where the Benevides and
thickness.
Loma Plata Formations are repeated by a thrust fault. The
western normal fault of the Presidio Bolson (graben) cuts the
be
eastern limb of Sierra La Santa Cruz.
The Bolson
contains
the Tascotal
continuation
of
block
the
can be considered
Diablo
Platform
to
into
Chihuahuaduring Mesozoic time.
at
least 800 m of late Cenozoic fill [Groat, 1970].
The level of regional d•,collement deepens to the west
under a fault-bendfold nmned Sierrita. The PEMEX Chapo 2
(Plate 1, well 3), along strike of Sierrita, drilled a thrust fault
that places Benigno over Del Rio Formation. The southwest
limb of Sierrita dips more steeply than the northeast limb
becausethere is a west vergent back thrust in the core of the
structure.
Therefore
a southern
Eastern Main Ranges
Sierra
Grande
is the easternmost
structure
of the main
rangestectonicdomain. In the regionof the transectmap it is
a broadanticlinewith a slightlysteepereasternlimb (Figure
6). As a topographicfeature, it extends250 km to the north
into the Eagle Mountainsof west Texas. In the map area,
Sierra Grande has 1 km of topographicrelief and 2 km of
structuralrelief. At depth the structureis interpretedas an
eastdirectedstep-upin the level of d•,collementfrom Jurassic
evaporites to the Aurora Group carbonates,over an older
normal fault that was inverted during Laramide thrusting.
The easternfrontal zone contains the largest area of preservedUpper Cretaceousrocksin the region, herein called the
Ojinaga Basin. This present-daystructuraland topographic
low coincideswith a Bouguergravity high called the Tascotal
Gries and Haenggi [1970] and Gries [1980] believe Sierra
Uplift [Handschyet al., 1987|, which was topographicallyhigh
during sedimentationin the Chihuahua Trough. A relatively
Grandemarksthe easterndepositionallimit of Kimmeridgian
thin Las Vigas and lower Aurora section is present on the
evaporites.I agreeand believethis is a normalfault boundary
that forms the westernedgeof the Diablo Platform. Presumed
Tascotal Uplift (block), and there are no Jurassicevaporites
down-to-the-westmotion on the normal fault(s) formed the
[Cantu et al., 1985]. In addition, the PEMEX Ojinaga 1 well
encounteredno Paleozoic units younger than Devonian and
margin of the Jurassic evaporite basin (early Chihuahua
drilled into basementconsistingof quartz-monzonite,yielding
Trough)and provideda mechanismfor westwardthickeningof
a Rb/Sr age of 977+78 Ma [Lopez, 1988]. These data, in
the Las Vigas through Aurora section (Figure 3) [Lehman,
addition to the observations of Handschy et al. [1987],
1986; Hennings,
1991]. Similar tectonostratigraphic
relationshipsare proposedfor the Diablo Plateau/Chihuahua
strengthenthe postulatethat the Tascotal block was uplifted
and eroded as an "ancestral Rocky Mountains" block [ Kluth
Trough boundary as far north as the Eagle and Quitman
and Coney, 1981] in the foreland of the Ouachita orogenic Mountains segment of the Chihuahua Fold Belt [Hennings,
1991]. Reversal of motion along the normal fault during
front. Although covered by all formations of the Lower
Cretaceous system, the Tascotal block remained relatively
Laramide time is herein invoked to explain the structural
elevation of the region between Sierra Grande and the
high until Albian depositionof the Finlay Formationwhich is
HENNINGS:
STRUCTURAL
TRANSECT OF SOUTHERN CHIHUAHUA
FOLD BELT
1451
$6
Figure 6. Southview of SierraGrande(SG) from Chihuahua
Highway16 (seePlate 1 for location). This
broad,asymmetricanticlineformsthe easternstructureof the main rangesand marksthe easternlimit of involvement
of evaporites
in Laramidethrusting.The topographic
reliefis 1 km, andtheexposure
is at thelevel
of the Loma Plata Formation.
RC is Rio Conchos.
Cuchillo Parado anticline comparedwith that of the Ojinaga
Basin.
Structureswest of and includingSierra Grande are detached
at the level of Jurassicevaporites. The anticlines are broad,
and many have been breachedto .the level of the evaporites,
leaving distinct hogbacksof Las Vigas and younger rocks.
Outcropsof evaporitesare scarcebecausethey are typically
covered with a thin veneer of alluvium.
The Cuchillo
Parado
anticline,with 3.5 km of structuralrelief, is coredby evapor-
o
ites and is the largeststructurein the easternmain ranges
(Figure7). Palinspasticrecontructions[Hennings, 1991] indicatethat the sourceof the salt was the Sobacosynclineto
the westwherecompletesaltwithdrawalresultsin a presumed
disconformable
"salt weld" betweenthe Las Vigas Formation
and Paleozoic
rocks.
Characteristicallyassociatedwith breached anticlines in
the eastern main ranges of the Chihuahua Fold Belt,
includingthe CuchilloParadoanticline,are overturnedpanels
sop
Figure 7. View southwest
of theeasternflankof CuchilloParadoanticline(SCP)fromChihuahua
Highway16
(seePlate 1 for location). Abbreviationsare O, overturnedbedsof Loma Plata Formation;V, verticalbeds;and
RC, Rio Conchos.
1452
HENNINGS:
STRUCTURAL
TRANSECT
OF SOUTHERN
CHIHUAHUA
FOLD BELT
Figure 8. Aerial view southof a "pop-up"anticline(P) located5 to 10 km northof the Rio Conchos(RC) and
ChihuahuaHighway 16. It is adjacentto an overturnedpanelof Loma Plata Formation(O) on the easternflank
of SierraGrande. Pop-upis approximately500 m wide. See text for explanation.
of upperAuroraGrouplimestonewhichform recumbentfolds
after delaminating along shale sequencesin the Cox or
BenevidesFormations(Figure 8). Adjacentto the overturned
panels,it is typical to find a narrow symmetricanticline
(Plate 1, "pop-up")with lengthequalto that of the overturned
panels. The wavelengthof these folds and the dip of their
limbs
indicate
detachment
is at the base of the Loma
Plata
Formation implying considerableflowage in the underlying
Benevides shale section.
The pop-ups form as
accommodation
structures
for material
that moved
out of the
Figure 9. Kink folds in Cox Formationon easternlimb of Cuchillo Parado anticline (see Plate 1 for location)
The outcropis approximately50 m high.
Plate I. Structural transect of the southern Ch/bualma Fold Belt
105o15 '
29 ø15'
105o30 '
105c45 '
t05•15 '
105 ø
104 ø45'
29"30'
'•,.\']' /
CHIHUAHUA
'' • TEXAS
•.
Cuchillo
Parade
betweenOjinagaandAldama,Chihuahua.(top) Stripgeologic
mapof thetransect.(middle)Transecttopography
andapparent
dips. (bottom)Present-day
structural
geometry,
transect
restored
to pre-Laramide
configuration.
Pz is Paleozoic
section;
Mz is
Mesozoic
THANSECT GEOLOGIC MAP
•
Sierra de Gomez
Quaternary
I
Placer de
Presidio
Guadalupe
•
•
•
Sierra Monii(as
I ..
CROSS SECTION
__
• ß
section.
.
[&Tertiary
e,,uvium
&
cofiuvium
X.border
Tertiary
volcanics
I Ilot3er
•, paved
highway
J(
•
or all-weather
LINE
SierraSo)is
EE2ZIZ2
/r,•,,•, It' (Cuohi,o• thrust
faett
[_.,_L,_E] •'•'•'•'•
Picachos
.•
r• through
Lama /
carbonales
Plata
Fins)-{ normal
fault
1.::<:.
rv
{N•,varmte
I":,/' :' -I/,.,,,,•
'1•"•'•;
• throughLa..q
j_
. ,.
Figure12 --
"•
'.?:':. Jtlrasslc
ßS[erraMortion
S salt-cored
anticline
I:• •'.q evaporites
Piemesas
•
....
•
und,vided
IX'x.'X'Fa.•eozolc
•
•
lineofproiection
• we•l
number
'..•'/proiected
well
I,',',". Precambrian ...x•...-distance
(kin)
arid
diredion
I.'.•.,."/.
". igneous
and
J•.',,?/•'.melamorphic
Sierra
Z •J• toactual
we•l
site
basement
topographicc.i, = 250 m
apparentdip
,,• of projecled
TOPOGRAPHY
AND
data
APPARENT DIPS
'
Western
Frontal
Zone
Sierra
do
Gomez ..........
,P,Plomosas
Uplift
- ALLOCHTHON/ALLOCHTHON
WESTERN
T EASTERN
.......
}
.
•
,.
I
•.
....
'
-"x
......
•c e
MONILLASTHRUST•
/ C I' C •' I• I' IN,.>'•.. •.•ts•errataeninas
Estacion
P'resentEros'o• n Suna
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;,,1
,'I';I"•'-•
'..•------•_•
•/' Picac,
hOs
]
SOLIS
THRUST •,,•'•
•
II
•mo deChilicole/
Eastern
Ma•n
Ranges
CUCHILLO
PARADe
ANTICLINE
.-'•..-•
S!erra
Cuch/{o
Parado
-- SYNCLINE
/
•
' ;'•'
iI
•
•/ be
beds
known
lo
overturned
Eastern
Frontal
Zone
•'
OJ/NAGA
BASIN
FRONTAL
THRUST
....
•h bc::•is
rotated
by laulting
Sierra
LaSanta
Cruz •io_
fault. dashed
where interred
multiple
episoOos
at
fautt
arrow
near motion,
fault is earlier
noverticalexaggeration
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sho•ening
ofwestern
altechthen:
Pzlever
=20km,
Mzlevel
=11km
pre-Laramide
fau•t(notrestored)
' loose lines: dashed
forwestern
allochthon,
solidforeasternallochthon
/•
11
••
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RESTORATION
TO
PRE-LARAMIOE
TIME. reslored
fault
, , , ,,,
,,,,,,,
, ,,
shortening
ofeastern
ailochthon:
Pzlevel
=O,Mzlevel
=6km
' •
II
'
,,,,
-2
-4
-6
m,',',',',',',',',',',',',',',',',',',',',','-',',',','>,•,•,:,•,•,•,•,.
, , , • , , ,, , , , ,,,,,,,,,,,,,
0 km
,,,
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total
sho•enmg
=20kmor9•
-8
, ,,,
,
-10
HENNINGS' STRUCTURAL TRANSECT OF SOUTHERN CHIHUAHUA FOLD BELT
1455
adjacentrecumbentfolds. The limbs of the evaporite-cored PlomosasUplift
foldsare typicallycut by numerous
tear faults, which separate
The transition between the eastern and western allochthons
overturnedpanelsfrom thosewhichremainedupright.
Three scales of folds are associated with the Cuchillo
occursacrossa broadbolsonin which thereare no outcropsof
Paradoanticline. The uprightto overturnedlimb of Cuchillo
pre-Neogenerocks. The bolson is interpretedto be a half
Parado anticline, the first-order fold, has been greatly
grabenwith a homoclinalnortheastdip. Sixteen kilometersto
thickenedby second-order
kink folds (Figure9). Third-order
folds are found in outcropsof highly contorted Navarette
the southwest, the transect crosses the northeast limb of the
Formation on the east side of the core of the anticline (Figure
10). Thesethird-orderfolds are interpretedto directlyoverly
evaporitesand thereforeprovide a hint to their structural
complexity.The threeordersof foldshave a paralleltrend,
suggesting
contemporaneous
formation.
The southwestlimb of Sobaco syncline is truncated by a
down-to-the-west normal fault, forming the northeastern
boundary of the Llano de Chilicote, which is, in turn,
truncatedon its southwestern
sideby anotherdown-to-the-west
normal fault. The Llano de Chilicote is mainly flat lying
along the transect,but to the southit is folded into broad
northwest trending anticlines and synclines, forming the
westernmost
exposure
of theeasternallochthon.Southwest
of
the Llano de Chilicote, the effects of syntectonicevaporite
flowage are absent.
PlomosasUplift, which is a complexly deformed domal structure exposing rocks with ages that range from Precambrian
throughTertiary (Figure 11). First interpretedby King and
Adkins [1946] as an anticlinorium with several east vergent
thrusts,the area was reinterpretedby Bridges [1964] to be a
southwestvergent Laramide structure with recumbent anticlines and thrusts. Ward [1977] found evidence of three folding events which locally involve Paleozoic and Mesozoic
rocks,but he was unableto place them conclusivelyin an orogenic framework.He suggeststhat a Laramidephaseof northeast directed shorteningpredateseast and southeastdirected
shortening. On the basisof thesepreviousstudies,analysisof
existing 1:50,000 scale lithologic maps and 1:250,000 scale
geologicmaps,and first-handfield observations,
I proposethat
the Plomosas Uplift may be geometrically modeled in the
region of the transectas a southwestvetgent fault bend fold,
placingPrecambrianthroughLower Cretaceousstratain thrust
contact with Lower Cretaceous strata. This thrust (herein
called Solis thrust) continues to the southwest, in the local
directionof transport,as a detachmentwithin the lower Aurora
Group. The level of detachmenthas been deduced from the
•;..::.:x:•:;:•
::•:•:•:•::•::'•.--•'•....•::::•:'
.....
thickness
...•:..
•
.•:.
•..•
......
":'•-.-M;
•.,•
....
,•;...•:.•:•'•:.:•
•.•:::..."
' •g•.•
.....
...•:•;F•:;•:•:•
ß
:.:.•:;.••s.-• •:• :5.:::.y.- •.•,-:•.:•f:'•
..........
:::•..:
ß.,•
....•;•4•}
.'.'
>-'.-.
.......
?.
.:•:7'• ..;:.-•:;•:
':
..<'.
•'•
..:
section
at Sierra
Morrion
and co-
tween 30ø and 50 ø and constrain the western limb (forelimb)
of the fault bend fold. The Monillas thrust splaysoff the Solis
thrust and places erosionally resistant conglomeratesof the
.... .
Plomosas
-.::.
....
ß
:L•.
ß
i....
of the thrusted
incides with a 100 m (maximum) gypsum bed within the
Cuchillo Formation. Displacement along the Solis thrust at
the Precambrian/Paleozoiccontact, in the Plomosas Uplift,
exceeds8 km. Structuralrelief is approximately6 km. The
Solis thrust produces a north-northwesttrending anticline at
the surface,with exposuresof clasticrocks assignedto the La
Casita and PlomosasFormations. A completesection of La
Casita through Buda Formation is exposed between Sierra
Solis and Sierra Soldado. Theserocksdip to the southwestbe-
'.'•::.::.'•
....
•"
....
?..
Formation
over
La
Casita
Formation.
At
Sierra
Monillas, beds of the La Casita Formation in the footwall of
the Monillas thrust have been locally overturnedby southwestard displacementof the overlying Monillas thrust sheet. The
Monillas thrusthas one hangingwall splay that duplicatespart
of the Plomosas Formation. The restoration of the transect, as
discussedbelow, suggeststhat the Monillas thrust sheet is, in
part, a wedge that was inserted above the Solis thrust sheet
and below the overlying Mesozoic section.
Modeling the PlomosasUplift in the vicinity of Sierra Solis
as a fault bend fold suggeststhat the Solis and Monillas
::i:.:
?:••:-:'•r
•'
• ::•:•
thrusts detach within
Precambrian
rocks.
A sliver
of meta-
morphic Precambrian basement [Quintero, 1984] has been
.............. -•...........
....
•::•
.....
:::•:•.
.:.:...
:.....•:. .....:.•:.:::...::•:•.:...•:•:•:•:::::•::•:•.•:gL•:•:..•:•.•::•:.?:.:...;•....:•*•.:.:•.:..*•
:::::::::::::::::::::::::::::
..........................
:.:..•x
...............
:•
.>?.::.:..:
......
Figure 10o Highly contortedNavarrete Formationat the
southern end of Cuchillo Parado anticline (see Plate 1 for
location). Thesebedsoverlieevaporitesand attestto their
complexdeformation.Outcropis 3 m high.
thrust
to
the
west
over
Plomosas
Formation
at
Cerro
Carrizalillo, 15 km to the southof Plomosas[Dyer and Reyes,
1987], indicating that detachment within Precambrian rocks
occursin the region.
Jurassicunits contain no significant evaporite beds in the
vicinity of the PlomosasUplift, although thin, discontinuous
1456
HENNINGS: STRUCTURAL TRANSECT OF SOUTHERN CHIHUAHUA FOLD BELT
.... -•"
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:
Figure 11. Southwestview of Plomosasuplift from ChihuahuaHighway 16 (seePlate 1 for location). P is
Permianconglomerates
in hangingwall of the Monillasthrust(seetext); W is westdippingpanelsof west side
of Plomosasuplift.
gypsumbeds do occur in both the La Casita and Cuchillo
Formations. Thus the westernlimit of Jurassicevaporitesthat
form the regional detachmentfor the easternallochthonmust
lie between
the Llano
de Chilicote
and the eastern limb
of the
PlomosasUplift. The transitionfrom an evaporiteto clasticdominatedJurassicsection is schematicallyshown in Plate 1.
Occidental. West of Sierra del Cuervo, the cover of Tertiary
volcanicrockspreventsobservationof underlyingstructures.
PrecambrianthroughCretaceousrocks crop out in Sierra del
Cuervo, but Laramide deformationwas minor [Handschyand
Dyer, 1987]. Therefore I believe that Sierra de Gomez forms
the westernboundaryof the ChihuahuaFold Belt.
Discussion
Western
Frontal
Zone
Structuresto the southwestof the PlomosasUplift lie in the
western frontal
zone of the western allochthon.
These struc-
tures result from detachment, back thrusting, and folding
above the Solis thrust which I interpret to be continuousfrom
the PlomosasUplift to the western edge of the Chihuahua
Fold Belt. Sierra Mortion is a northeast vergent, emergent
back thrust off the Solis thrust.
The back thrust is on the
northeastside of the range and places carbonatesof the
AuroraGroupagainstUpperCretaceous
clastics. The hanging
wall of the back thrustdips to the southwestin the northern
partsof therangebut is foldedinto an anticlineto the south.
The westernmoststructuresalong the transectat the southern end of Sierrade Gomez are interpretedto be a pair of west
vergent,fault propagation
folds,eachwith approximately1.6
A restorationof the transecthas been performed to check
the viability of the structuralinterpretationand to determine
the magnitudeof shorteningacrossthe fold belt (Plate 1).
Becauseof the opposingvergencedirectionsof the eastern
and westernallochthons,the pin lines for the restorationare
the east and west ends of the transect. The loose line for both
sidesof the transecthas been placed near the center of the
belt on the east side of the PlomosasUplift. Tertiary age
normalfaultswere removedfirst by elevatingthe downthrown
sideto the level of the upthrownblock. Contractionalfolding
and faulting were then removedby layer-parallelshear. The
salt was restoredby areabalance. Gaps and overlapsalong
the loose lines for the two allochthons were minimized.
The
entirebelt was shortened
approximately
20 km or 9%.
The Mesozoic
section of the eastern allochthon was short-
a singleanticlinalstructure,formingthe main body of Sierra
enedapproximately
6 km. The detachedmassis a northeasterly taperingwedgewhich variesin thicknessfrom 5 km on
de Gomez. There are numerous west vergent contractional
the southwest side to 2 km at the eastern thrust front.
structuresalong Highway 16 road cuts through Sierra de
Gomez (Figure 12). These, along with the asymmetric
geometryof the first-orderSierrade Gomezfolds,attestto the
ened other than minor inversionalong the preexistingnormal
km of structural relief.
These folds coalesce to the north into
overall west vergenceof the structure.
Sierradel Cuervo(seeFigure 14) lies 23 km west of Sierra
The autochthonous block of Paleozoic and older strata was not shortfault under Sierra Grande.
The Paleozoic section of the western allochthon was short-
enedapproximately
20 km by displacement
andfoldingalong
de Gomez at the transition between the Chihuahua Fold Belt
the Solis and Monillas
thrust faults that form the Plomosas
and the Tertiary volcanic cover of the Sierra Madre
Uplift. However,theMesozoicsectionwasshortened
only 11
HENNINGS: STRUCTURAL TRANSECT OF SOUTHERN CHIHUAHUA FOLD BELT
1457
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Figure 12. South view of one of numerous,minor, west directed contractionalstructuresin Loma Plata
FormationalongHighway 16 roadcutsthroughSierrade Gomez(personis 1.6 m high, seePlate 1 for location).
Thesecontractionalfeaturesprovideadditionalevidenceindicatingwest vergenceof the parentstructure.
km, including 4 km that were transmitted to the western
frontal zone along the Solis thrust, forming the structuresof
Sierra Morrion
and Sierra de Gomez.
The western
allochthon
tapersfrom 9 to 3 km alongits length.
There are no significantgapsor overlapsin the Phanerozoic
section of the restoration because the excess length of the
Mesozoic
section
of
the
eastern
allochthon
relative
to its
autochthonous
block has approximatelythe samelengthas the
excess length of the Paleozoic section of the western
allochthonrelative to its Mesozoic section. During shortening
the Monillas thrust sheet was wedged under Mesozoic rocks
along the eastern flank of the Plomosas Uplift to preserve
structuralbalance acrossthe belt (Figure 13 or Plate 1). This
requires the existence of a detachmentat the base of the
Jurassicsectionalong the easternflank of the PlomosasUplift.
This has not been observed in the field, although PaleozoicJurassiccontacts along the eastern flank of the uplift are
isolated, small, and poorly exposed. The isolated domal
outcropsof Albian carbonatesat EstacionPicachos[Bridges
and Deford, 1961], probably belonging to the Benigno
Formation,are in thrustcontactwith underlyingLas Vigas and
Cuchillo Formations [King and Adkins, 1946]. Several minor
thrustsplaysat the baseof the Picachossuggestthey traveled
northeastrelative to their footwall. King and Adkins [1946]
believed these blocks to be klippe representingthe farthest
outlier of a major thrust,which guidedtheir interpretationthat
the PlomosasUplift is a northeastvergent structural massif.
Alternatively,I suggestthat they result from a splay off the
upperdetachmentof the Monillas thrustwedge.
The isolated involvement
of the entire Phanerozoic
section
andthe deeplevel of detachmentin the PlomosasUplift sug-
gest that the mechanismof origin is different than that of the
rest of the fold belt. There is evidencein the central part of
the transect suggesting that left-lateral wrench faulting
occurredin conjunctionwith horizontal shorteningto form the
PlomosasUplift. A prominent ESE trending discontinuity,
evident on Landsat images (Figure 14), extends east from the
Sierra
Madre
Belt.
It crosses the transect
Occidental
the Plomosas Uplift.
Estacion
Picachos
across the southern
between
Chihuahua
Estacion
Picachos
Fold
and
I suggest that the syncline NW of
and the folds in the Llano
de Chilicote
owe
their anomalous NW trends, compared the typical NNW
trends of the southern Chihuahua Fold Belt, to left-lateral
shear displacementalong a deep-seatedstrike-slip fault zone.
In addition, I suggest that the isolated involvement of the
entire Phanerozoic section in the Plomosas Uplift was the
result of contractionaldeformation at a right step along the
strike-slip zone. The strongestevidencein supportof this is
the limited lateral extent of the structureswithin the uplift,
comparedto its deep detachmentlevel. This implies that the
driving mechanismthat incited deep detachmentwas also of
limited lateral extent. Several NW trending shear zones have
been postulatedto exist in northernMexico and west Texas,
including the Mojave-Sonora Megashear [Anderson and
Schmidt, 1983], the San Marcos Fault [see McKee et al.,
1990], the La Babia Fault [Charleston, 1981], and the Texas
Lineament [Muehlberger, 1980]. The most significant of
these is the Mojave-Sonora Megashear which is thought to
have accommodated800 km of left-lateral offset during the
Late Jurassic. The Texas Lineament is thought to have
accommodated left-lateral strike-slip displacement during
Laramide
deformation.
1458
HENNINGS:
STRUCTURAL
TRANSECT
OF SOUTHERN
CHIHUAHUA
FOLD
BELT
WESTERN
ALLOCHTHON
EASTERN
ALLOCHTHO
Ranges
'T'
Zone ..•
WesternFrontal
Zone
.._1_.Plomosas
'-J-'
Eastern
Main
Uplift
._1- Eastern
Frontal
To preserve structuralbalance across the be/t, a
wedge of Paleozoic rocks was thrust(Monillas
thrust)under the Mesozoicsectionalong the
eastern flank of the P/omosas Uplift.
SW
NE
The Plomosas Uplift
owes its deep
detachment
level to
wrenchingalong a leftlateral strike-slip
basement
UpperK
._L•
shear zone.
6
clastics&
"•1 carbonates'4
topography
•----•'='F T
I'•
/A
\
km
-2
Lower K red beds
•...
[:1..
I.....
I.,
.I...
I_
.I'=.
-2
%
Paleozoic
undivided
'
/
/
/
/
j/l•Ul
i iu/JUl
I
I
km
20
i
/
/
/
/
/
i'
/
%
/
%
/
%
/
I
40
%
/
%
/
%
/
%
/
%
/
%
/
%
/
,
/
i?
I
I
60
80
vertical exaggeration= 5
Figure 13. Schematiccrosssectionof the ChihuahuaFold Belt depictingpresent-day
geometry. SeePlate 1
for explanationof geologicsymbols.
Upper Cretaceous clastic rocks at the thrust front of the
Summary
eastern frontal zone.
The main goal of this work has been to characterize the
generalmorphologyof the southernChihuahuaFold Belt so
that a first
tectonic
model
could
be devised.
It
has been
shown here that a model invoking Laramid½basin inversion,
horizontal shortening, and minor basement wrenching can
fully account for the observed surface structures in the
southern
end
of
the
fold
belt.
The
most
critical
issue
remainingin unravelingthe tectonic history of the region is a
more thoroughunderstandingof the structure and kinematics
of the PlomosasUplift and how it interactedwith the rest of
the fold belt.
Eastern
Aliochthon
The eastern allochthon extends the entire length of the
ChihuahuaFold Belt and may be divided into two tectonicdomains. The eastern frontal zone is characterized by a relatively thin Mesozoic section, no Jurassic evaporites, an
emergentthrust front, and fault-related folds which are detached within the Aurora Group. The easternmain rangesare
characterizedby a relatively thick Mesozoic sectionwhich is
folded into asymmetric evaporite-cored anticlines and salt
withdrawal synclineswhich are detachedin Jurassicevaporites. The regional d•collement for the eastern allochthon
shallows from Jurassicevaporites under the main ranges to
The allochthon
was shortened 6 km.
The
easternfrontal zone rests on the Tascotal Uplift which was
high during Late Paleozoic and pre-Albian Mesozoic time.
The westernmargin of the Tascotal Uplift is modeled as a
normal fault which
formed
the eastern facies limit
of Jurassic
evaporites, controlled the thickness of the Las Vigas and
Aurora
section,
and formed
a buttress
that
restricted
northeastward:notion of the eastern allochthon,resulting in
the formation
Western
of Sierra Grande and structures to the west.
Allochthon
The PlomosasUplift, a fault bend fold with severalhanging
wall thrustsplays,formsthe centralstructureof the westernallochthon. Its deeply rooted detachmentlevel is the result of
left-lateral wrenching along a NW trending strike-slip fault
zone in basement. The regional d6collement shallows from
Precambrianrocks under the Plomosas Uplift to within the
Cuchillo Formation, forming the fold and thrust structuresof
the western frontal
zone.
The western
ened 20 km at the Paleozoic
allochthon
was short-
level and 11 km at the Mesozoic
level.
Acknowledgments. This paperrepresentsa portion of the author's
dissertation research conducted at The University of Texas. From
conceptionthrough completionthis study has greatly benefited from
HENNINGS:
STRUCTURAL
TRANSECT
OF SOUTHERN
CHIHUAHUA
FOLD
BELT
1459
o
•.....
........:.:.....:,..
%
* •,
.,.'•.•..•?.
.......
•:'
......... •--'.:
::•½.
?'"':....•r•
• •
.....
•:•' 71.',.,
.:,..,
•.•-.•
"'½•W•?•%
•'"
..-,.',...½:.<:,•;•.;:•/'
• • •
....
ß• •
.....
.. .
•, .....;$. ß
.
-•.
o
.•:
...•
,, ........
";•........
...
1460
HENNINGS:
STRUCTURAL TRANSECT OF SOUTHERN CHIHUAHUA
the ideas and enthusiasmof Bill Muehlberger. Careful reviews by
Kevin Burke, Harold Drewes, Walt Haenggi, Bill Muehlberger, and
Dave Wiltschkogreatlyimprovedthe manuscript.Fundinghasbeen
providedby a TexacoInc. graduatefellowship,The Universityof
Texas Geology Foundation,and studentgrantsfrom the American
FOLD BELT
Association of Petroleum Geologists, the Geological Society of
America, and Sigma Xi. I thank Lynn Glover, Fred Read, and the
Departmentof Geological Sciencesof Virginia PolytechnicInstitute
and State University and Mobil Research and Development
Corporationfor use of facilities.
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