Geologic structure of the northern margin of the Chihuahua trough 43 BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA D GEOL DA Ó VOLUMEN 60, NÚM. 1, 2008, P. 43-69 E G I I C C

O A

S 1904

M 2004 . C EX . ICANA A C i e n A ñ o s Geologic structure of the northern margin of the Chihuahua trough: Evidence for controlled deformation during Laramide Orogeny

Dana Carciumaru1,*, Roberto Ortega2

1 Orbis Consultores en Geología y Geofísica, Mexico, D.F, Mexico. 2 Centro de Investigación Científi ca y de Educación Superior de Ensenada (CICESE) Unidad La Paz, Mirafl ores 334, Fracc.Bella Vista, La Paz, BCS, 23050, Mexico. *[email protected]

Abstract

In this article we studied the northern part of the Laramide foreland of the Chihuahua Trough. The purpose of this work is twofold; fi rst we studied whether the deformation involves or not the basement along crustal faults (thin- or thick- skinned deformation), and second, we studied the nature of the principal shortening directions in the Chihuahua Trough. In this region, style of deformation changes from motion on moderate to low angle thrust and reverse faults within the interior of the basin to basement involved reverse faulting on the adjacent platform. Shortening directions estimated from the geometry of folds and faults and inversion of fault slip data indicate that both basement involved structures and faults within the basin record a similar Laramide deformation style. Map scale relationships indicate that motion on high angle basement involved thrusts post dates low angle thrusting. This is consistent with the two sets of faults forming during a single progressive deformation with in - sequence - thrusting migrating out of the basin onto the platform. We found that the style of deformation in the Chihuahua trough is variable. In places such as the East Potrillo Mountains and Indio Mountains is typical of the thin- skinned style, associated with the Cordilleran thrust belt, while in other places, the thick – skinned deformation present is typical of the Laramide orogeny in the southern Rocky Mountains. The Franklin Mountains record the transition from thick- to thin – skinned deformation. We notice that this difference in the style of deformation is related to the thickness of the Cretaceous section within the Chihuahua trough. On the other hand, the orientation of the shortening direction can be explained based on the geometry of the trough and especially the strike of its eastern margin. Along strike variations in shortening direction and kinematics are controlled by the curved northeast margin of the trough and refl ect stress reorientation along the weak interface between the strong platform and weak basin interior. These processes were wide spread affecting the 300 km long eastern margin of the Chihuahua trough between El Paso and the Big Bend region of west Texas.

Key words: Chihuahua trough, kinematic analysis, Laramide orogeny, thin-skinned deformation, thick-skinned deformation, stress inversion.

Resumen

En este artículo estudiamos la parte norte del frente de arco de la Cuenca de Chihuahua. Éste trabajo tiene dos objetivos: el primero es estudiar si la deformación involucra o no el basamento a lo largo de fallas en la corteza (deformación de piel delgada o gruesa) y el segundo es estudiar la naturaleza de las direcciones de acortamiento en la cuenca de Chihuahua. En esta región los estilos de deformación de fallamiento inverso cambian de moderado a bajo ángulo dentro de la cuenca, incluyendo el fallamiento inverso en la plataforma adyacente. Las direcciones de acortamiento estimadas de los pliegues y fallas, y la inversión de estrias de fallas indican que las estructuras de la cuenca y del basamento registran similar estilo de deformación a las deformaciones de las estructuras Laramide. Las relaciones que se observan a escala cartográfi ca indican que el movimiento de alto ángulo en el basamento incluyen 44 Carciumaru y Ortega fallas inversas posteriores de bajo ángulo. Este estilo es consistente con las dos series de fallas que se formaron en una deformación progresiva migrando de la cuenca a la plataforma. En este trabajo encontramos que el estilo de deformación de la cuenca de Chihuahua es variable. En regiones como East Potrillo Mountains y las Indio Mountains es típica la deformación estilo ‘piel delgada’, mientras que en el sur de las Rocky Mountains el estilo es de ‘piel gruesa’. Las Franklin Mountains registran transición de delgada a gruesa. Por otro lado la dirección de acortamiento se puede explicar basada en la geometría de la cuenca, especialmente en el margen occidental. A lo largo de la cuenca, las variaciones de acortamiento y la cinemática del movimiento de las fallas están controladas por la curvatura del margen noroccidental y refl ejan la reorientación de esfuerzos sobre una débil interface entre la plataforma y el interior de la cuenca. Éste proceso se extiende a lo largo de los 300 km que cubre el margen occidental de la cuenca de Chihuahua entre El Paso y la región de Big Bend en el oeste de Texas.

Palabras clave: Orogenia Laramide, Norte de Mexico, Cuenca de Chihuahua.

1. Introduction within the Laramide foreland and greatly infl uenced the style of deformation in southern New Mexico, west Texas The Chihuahua trough is a major sedimentary basin of and northwestern Chihuahua (Seager and Mack, 1985). We Mesozoic age located in the southern part of the North show that the variation in structural style between base- American craton that lies along the U.S.A - Mexico border ment involved faulting in southern and southwestern New (Figure 1). Laramide crustal shortening strongly deformed Mexico and thin skinned thrusting within the Chihuahua the basin. This basin was intensely deformed by contrac- trough is caused by changes in thickness of the Mesozoic tional structures associated with the Laramide orogeny cover thus helping to explain contrasting views of this part and extensional structures related to the Rio Grande rift. of the Laramide orogen (Corbitt and Woodward, 1973; This paper focuses on Laramide structures. In general, the Drewes, 1982; Seager and Mack, 1985). Laramide foreland is composed of a relatively thin marine sedimentary wedge where the thrust system fl attens into a subhorizontal detachment separating the deformed rocks 2. Geological Setting from the basement below, namely thin - skinned deforma- tion. However in southern New Mexico, west Texas and The Laramide orogen of western North America is a northern Chihuahua, the style of deformation is usually chain of generally north trending block uplifts stretching thick-skinned. The nature of this thick - skinned style of from Montana to Mexico (Coney, 1972). These uplifts deformation has been under debate for many years. This are generally cored by Precambrian rocks and commonly debate has centered on whether the main style of deforma- have high angle reverse faults on one or both margins of tion is the thin- or thick - skinned model. the uplifts which root into subhorizontal detachments at Only a few descriptive models (Haenggi, 2001) have depth (Erslev and Rogers, 1993). Laramide deformation been presented to explain the tectonic evolution of the is generally bracketed to have occurred between the late trough. In order to investigate the basin, we reconstructed Cretaceous (~85 Ma) and the Eocene (~36 Ma) (Bird, the tectonic history by means of dynamic, kinematic 1998). The earliest Laramide deformation overlaps in time and geometric analysis. This study is based on structural with the end of thrusting within the Cordilleran orogenic geology of the faults and folds, detailed mapping of East belt (Bird, 1988). Several models have been proposed to Potrillo, Franklin Mountains and Cerro de Cristo Rey and drive Laramide orogenesis including, low angle subduc- compilation of the large amount of geological data now tion (Bird, 1988; Dickinson and Snyder, 1978), detachment available for this region (Brown and Clemons, 1983; and delamination within the crust (Erslev, 1993; Oldow Cather, 2001; Corbitt and Woodward, 1973; Drewes and et al., 1990), collisional orogenesis (Maxson and Tikoff, Dyer, 1993; Harbour, 1972; Kelley and Matheny, 1983; 1996), extension within the Sevier hinterland (Livaccari, Lovejoy, 1976 and 1980; Seager, 1975; Seager and Mack, 1991), rotation of the Colorado Plateau (Cather, 1999) and 1994; Rohrbaugh, 2001; Wu, 2002, Zeller, 1970). lithospheric buckling (Tikoff and Maxson, 2001). All of One of the striking features of the Chihuahua trough these mechanisms attempt to explain the transmission of and much of the Laramide foreland in New Mexico and stress well into the plate interior causing deformation of Colorado is that deposition of marine clastic successions the foreland. continued until Maastrichtian time (Jones et al., 1998; The kinematics of the Laramide orogeny has been contro- Seager and Mack, 1985) just prior to the onset of Laramide versial. For instance many models for Laramide orogenesis shortening. Thus, much of the Laramide foreland is com- emphasize contractional strains with little to no transcur- posed of a relatively thin marine sedimentary wedge rent component (Hamilton, 1988; Woodward et al., 1997). (Coney, 1978; Jones et al., 1998). The Chihuahua trough In contrast, other models have emphasized the importance is one of the thickest sections of lower Cretaceous rocks of right handed transcurrent displacement during Laramide Geologic structure of the northern margin of the Chihuahua trough 45

F F T B L R R U O A E R R 107° 105° S N R ID H O K E A N. MEX. R L L M M I . N H A U T H 32 N P S U A A L . M S I E T F E. POTRILLO MTS C M T T C S O H T TEXAS . S S E . w M . T R T M I S C EP . T A O S S S. PALOMAS S A . P B JU IG AR L H E E A Z S S . TC . DIABLO PLATFORM H C DI E H T IN R A M O Ìo N. MEX. T S . S B B S . R L S ra A v io O M o G STUDY AREA A r L an A d . Y e U C A

I

B D . A L V U IS C M E T R S. O

V. A H

C T H EX

I

H A S

S S U . A O R A A N S D H . O 0 O M S

U 30 R O

A J I A N A A LD AM A P L S A . N T ID FO O R OJ M S . G R A N D Chihuahua trough E LA S B O A T B Jurassic basin I IA M A A IC Lineament L MER ALD A A NE H RA RT TER NO ED South edge RET ACC

Jurassic evaporites A A U L I 0 50 100 km H Ranges A U U H H A I O H C C Figure 1. Geological features of the Chihuahua trough. The current boundaries are the Aldama platform and Alamitos lineament at the south and the Diablo Platform at the north. Villa Ahumada (V.AH), Ojinaga (OJ), Aldama (ALD), Van Horn (VH and El Paso (EP) are indicated (after Haenggi, 2001). orogenesis in Colorado and New Mexico along the eastern proposed by Gries (1970), which describes it as a Jura edge of the Colorado Plateau (Cather, 1999; Chapin and - Cretaceous basin with several thousand meters of ma- Cather, 1981; Karlstrom and Daniel, 1993). A key part of rine and siliciclastic rocks built on a base of Jurassic resolving this controversy is deciphering the kinematic evaporates. DeFord (1964) fi rst used the term “Chihuahua history of Laramide fault zones (Erslev, 2001) and under- trough” to name the depositional basin that contained standing how strain is transferred to the north and south of the thick Mesozoic sediments and are exposed today in the proposed region of transcurrent displacement. the Laramide Chihuahua tectonic belt. The extent of the Within the study area, in northern Chihuahua, southern Chihuahua trough as defi ned by DeFord was northeast- New Mexico and west Texas, the Cordilleran orogen and ern Chihuahua and adjacent parts of Texas, New Mexico the Laramide orogen overlap. Corbitt and Woodward and Sonora that form a deep Mesozoic basin. An arbitrary (1973) and Drewes (1988) concluded that although the southern boundary (Figure 1) is placed at the edge of the structures in this region were roughly contemporaneous North American Craton (Marathon fold and thrust belt). with those in the Laramide Rockies, they had a distinct Muehlberger (1980) selected two arbitrary limits: a north- deformation style and represented the southern continu- western limit is selected along the 109o W meridian and ation of the Cordilleran fold and thrust belt. In contrast, a southeastern limit is along the Alamitos lineament. The Seager and Mack (1985) concluded that uplifts and basins basin extends beyond these boundaries to the south into in this region were structurally similar to those in classic Coahuila and to the west into Arizona and Sonora. In its Laramide ranges. They further concluded that differences present confi guration, the Chihuahua trough is a NW - SE in deformation style were attributed to deformation within elongated elliptical basin bounded by the Aldama Platform thick sequences of Mesozoic strata and locally developed to the southwest and by the Diablo Platform to the north- transpressive structures. east. The late Paleozoic Pedregosa basin is, in effect, the In this work we used the defi nition of Chihuahua Trough proto - Chihuahua trough (Haenggi, 2001). 46 Carciumaru y Ortega The Chihuahua trough was subsequently shortened in ing unconformably on Precambrian basement (Figure 2 Laramide time leading to intense folding and imbrica- and 3). Cretaceous rocks only form a thin veneer cover- tion of the Jura - Cretaceous section. (Gries, 1970; Seager ing Permian rocks on the southeast margin of the Diablo and Mack, 1985). Jurassic evaporates at the base of the Platform (Figure 3 and 4). section formed a décollement during Laramide shorten- Triassic and Jurassic rocks generally are absent over ing (Gries, 1970). The fold and thrust belt built within the most of southern New Mexico, although marine Jurassic Chihuahua trough is referred to as the Chihuahua tectonic rocks are known from a deep oil test southwest of Las belt and trends parallel the trough’s northeast margin. Its Cruces (Seager and Mack, 1985). These Jurassic carbon- northeast margin of the trough is defi ned by an abrupt ates thicken southward into the Chihuahua trough where transition from the thick Mesozoic section onto the Diablo Jurassic evaporites are diapiric and are responsible for Platform dominated by Permian sedimentary rocks rest- Laramide décollement and thin-skinned folding in the

107°00' 106°00' Tv K San Andres Mountains 33°00' K Caballo Mountains Pcb Pz Tlr Palomas Basin Tv Qt Pcb Pcb Tv Tlr Tv Pz

Tv Pz Jornada Pcb Tv Basin Tv Explanation (Qt) Quaternary Undivided Tularosa Basin (Qbf) Quaternary basalt flows Qt (Tv) Tertiary volcanic rocks Pz (Tir)Tertiary intrusive rocks (Tlr)Tertiary Love Ranch Formation Organ Mountains (K) Cretaceous sedimentary rocks Tv Tv Pz Tir (Pz) Paleozoic sedimentary rocks Tv Pcb (Pcb) Precambrian rocks Tv Tv Tir Tir Tir Normal faults Thrust faults Pz Mesi Tv Syncline axis lla Valley Fault Pz Pcb SierraTv de las Uvas Pz Pz Tv Tv

do Fault Pz Tv Qbf 3 Tv Roble Pz Qt Pz Pz Pz Franklin Mountains Mesilla Hueco Mountains Pz Basin West Qbf Pz Potrillo Pz Pz New Mexico 32°00' Volcanic Fitzgerald Fault Qt Texas Tir Field Tir Pz 1 Margin of Chihuahua trough Pz Pcb Pz Pz Hueco Tv East Qbf Pz Basin K Potrillo Tir Pz Pz Pz Mountains New Mexico

Pz Mexico Texas N Pz Mexico 2 Sierra Juarez K

0 50 100 km

Figure 2. Geological features of southeast New Mexico and adjacent regions. The major faults and geological units are indicated. The two important ranges in the southern Mesilla basin are the East Potrillo Mountains (insets 1 and 2) and Franklin Mountains (inset 3) (modifi ed after Ruiz, 2004). Geologic structure of the northern margin of the Chihuahua trough 47

F N r a n

k

l in A Mn ppro

t x s imate e El Paso D astern bounda ia blo Ciudad Pla Juarez tf or Sierra de ry m Juarez A of thrust belt pp rox im ateE T C ex dg h a eofT ih ua r hu ou a gh

R io R Gra io n d e

S i erra d e S a Van nI E g a Horn n Q g a u le c i M io tma t ns nM

I tns n d i o M t n V s a nH M t o n rn

Explanation Quaternary and High angle reverse faults Tertiary, Undivided Thrust faults Tertiary Volcanic Rocks Eocene to Miocene Syncline Intrusive Rocks Anticline Mesozoic Rocks Overturned anticline Permian Rocks Overturned syncline Pennsylvanian and Monocline Older Rocks 01020 Precambrian Rocks Km

Figure 3. Tectonic map of northeastern margin of the Chihuahua trough southeast of El Paso, Texas showing location of the Indio Mountains (see box). Note position of trough margin with respect to edge of fold and thrust belt (modifi ed after Rohrbaugh, 2001).

Chihuahua tectonic belt, including the Sierra de Juarez of 1388 m (Seager and Mack, 1994). Going northwest to- (Drewes and Dyer, 1993; Gries and Haenggi, 1970; Gries, wards the basin margin at Cerro de Cristo Rey Cretaceous 1970; Seager and Mack, 1994). The age of these evapo- rocks have an exposed (minimum) thickness of 495 m rites are controversial, Haenggi (2002) proposed that these (Lovejoy, 1976). Farther to the northeast, in the foothills evaporites are deposited above a basal Jurassic clastic of the Franklin Mountains the Cretaceous section is less section, therefore, they belong to a post-Jurassic deposi- than 100 m thick (Lovejoy, 1975). tion (probably Neocomian). Cretaceous strata thin dra- This dramatic thinning of section on northeastern margin matically across the study area, from the interior of the of the trough is controlled by a series of large displacement, trough onto the Diablo Platform. For instance, in the Indio down to the west, normal faults of Jurassic age (Uphoff, Mountains along the Texas - Mexico border Reaser (1982) 1978). The margin roughly parallels the Rio Grande and reports a thickness of 5652 m for Cretaceous rocks in a encroaches up to 25 km onto the Texas side of the bor- deep oil test. Immediately to the east Underwood (1962) der, from El Paso to the southwestern edge of Big Bend reports a thickness of 1549 m in measured sections of National Park (Figure 1). Muehlberger (1980) also postu- the Cretaceous. Similarly, in the El Paso area the thick- lated a reentrant into the eastern part of the Big Bend area est section of Cretaceous rocks is found in the Sierra de because Laramide structures are similar to those along the Juarez where exposed Cretaceous rocks have a thickness margin of the trough. The normal faults were subsequently 48 Carciumaru y Ortega

Explanation a) 325 b) 346 Precambrian Outcrops Wrench fault zone Evaporites of Chaplin and Cather B- Basement involved uplift n=45 T- Thin skinned uplift Pecos isopach line Buckles 1000 (meters) Strike slip fault Anticline Thrust faults B

0 Burr 2000 oU pl 4000 ift 6000 0 New Mexico 10000 T Texas

T

Arizona New Mexico Chihuahua

T 10000

T

0 2000 4000 6000 8000 10000 10000 4000

Figure 4. Isopach map for the base of the lower Cretaceous rocks ( Neocomian - Aptian) in and around the Chihuahua trough, after LeMone et al., (1983). Faults are adapted from Seager and Mack, (1985). a) Rose diagram showing trends of folds shown in fi gure 2.3 north of line marked “Rose Line”. b) Rose diagram showing trends of folds shown in Figure 2.3 south of line marked “Rose Line”. See text for discussion.

buried by a thick Cretaceous sequence, but Uphoff (1978) ometries. Principal Laramide structural geometries will be interpreted well data to show the existence of one of these described and interpreted in the following sections. faults beneath the Hueco Bolson and Mack (2004) also de- In order to estimate the regional shortening direction, bed- scribes one from the Bisbee group in southeast Arizona. ding plane orientations were measured in order to obtain These normal faults are important because they determine fold axes. Assuming a cylindrical geometry of the folds, the geometry of the sedimentary basin and helped control a best fi t circle can be fi t to the poles to bedding planes, the location of Laramide structures. and the pole to this great circle represents the fold axis (Ramsay and Huber, 1983). In most cases the shorten- ing direction is taken to be perpendicular to the fold axis. 3. Methods Slaty cleavage was also used in order to better constrain the shortening direction. Poles to cleavage generally ap- In order to determine the kinematic and strain history of proximate the local shortening direction (Ramsay and the Chihuahua trough during Laramide deformation geo- Huber, 1983). For faults, the orientation of slickensides metric data on the orientations of faults, folds, and frac- were measured and assumed to represent the direction of tures were collected in the fi eld. Locations of data obser- maximum shear stress in the fault plane. Shear sense in vations (~10 m accuracy) were obtained using handheld brittle faults was inferred from the geometry of calcite GPS units. For the paleostress analysis fault orientation veins adjacent to faults, steps in slickensides on the fault and slip direction were inverted for the orientations of the planes, geometry of Reidel shears and conjugate fault sets three principal stresses (Angelier, 1984). These analyses (Petite, 1987). Calcite and quartz fi lled vein arrays were were performed in the East Potrillo Mountains, Cerro de also measured and used to infer the orientation of the least Cristo Rey, and the northern Franklin Mountains. This data compression direction. is compared with preexisiting data sets from the southern Paleostress reconstruction is based on fault slip inversion. Franklin Mountains and Indio Mountains. The dataset in The inverse method assumes that faults slip in the direc- these areas was split into domains based on structural ge- tion of shear stress acting on the fault plane. Suppose that Geologic structure of the northern margin of the Chihuahua trough 49 we have N measurements and all the faults slipped under clasts of early Cretaceous carbonates (Broderick, 1984) the stress σ. The shear traction at the αth fault, sα, is defi ned and are therefore younger than early Cretaceous. as: Thrust faults and folds are well exposed in the East Potrillo sα = σnα - [(nα)Tσnα]nα (1) Mountains (Figure 5, 6 and 7). These contractional struc- tures are the major deformation features in the East Potrillo where nα is the unit vector normal to the αth fault, and T Mountains. Most of the studied thrust faults are located means the transpose of the matrix. The slip direction mea- in the northern part of the range. They are parallel to the sured at the fi eld is expected to be in the direction -sα . The general alignment of the range with faults dip 30° – 60° stress tensor is determined by minimizing the sum of the towards the southwest. Slickensides on fault surfaces and angular misfi ts of sα. Basic assumptions behind all stress the strongly asymmetric nature of fault related folds in- inversions are that each fault is caused by a homogeneous dicate that the hanging walls of the major faults moved stress fi eld, the fault has not been reoriented, and that the slip towards the northeast relative to the footwalls. direction in the fault represents the direction of maximum The strongly asymmetric nature of the folds (Figure 7) and resolved shear stress (Ramsay and Lisle, 1997). Since only presence of thrust faults cutting up through the overturned angles are measured, we obtain only the orientations of the limbs of the folds is consistent with the folds being the three principal stresses and the ratio of the three principal result of fault propagation, similar to examples cited by stress differences φ. Three methods were used for stress Erslev and Rogers (1993). Fault propagation folds form at inversion in all the different areas, in all the cases the num- the tip of a growing fault as fault displacement increases. ber of data was greater than 30, however in some cases we This is because the shortening strain taken up by the fault is divided the studied area in subregions. The fi rst method is absorbed by folding at the tip of the fault. A consequence the moment tensor summation as coded by Allmendinger of this process is that the folds tighten as the fault propa- (2001). The second is the inverse method of Michael gates until folding becomes ineffi cient and a fault breaks (1984) which allows the use of fold measurements. The thorough the limb of the fold. Fault propagation folding third is the inversion method of Marrett and Allmendinger produces a triangular shear zone that widens away from (1991). The results are obtained in terms of a 1) reduced the fault tip, with the highest strain being near the fault tip stress tensor, consisting of the components of the principal and strain decreasing away from the fault tip (Suppe and stresses and 2) the ratio of principal stress differences. The Medwedeff, 1990). Strain within the triangular shear zone stereoplots were constructed using Stereowin, a Stereonet at the fault tip can be accumulated by fl exural slip fold- program for Windows ( Allmendinger, 2002 - 2003). ing, cataclastic fl ow, rigid body rotation, and plastic fl ow of incompetent materials. To evaluate the roles of these mechanisms, the folds and associated faults were carefully 4. Results studied. On the other hand, there is a problem in directly relating the inferred stresses to folding since the stresses The region studied in this work is depicted in Figures 2 at any stage of the deformation may have no simple geo- and 3 indicating the ranges around the Rio Grande rift. In metrical relationship to the folding process. Parallelism of the following sections we discuss the fi ve regions studied: principal stresses and strains relationship may hold true for East Potrillo Mountains, Franklin Mountains, Anthony coaxial deformation, but would not hold true if the bulk Gap and Tom Mays Park, Cerro de Cristo Rey and Indio deformation is signifi cantly non-coaxial. In anyway, fold- Mountains. ing analysis was performed only for completeness. In most cases, bedding is generally well preserved with obvious primary sedimentary structures. This generally 4.1. East Potrillo Mountains rules out pervasive cataclastic or plastic fl ow as being the dominant folding mechanisms, with an important excep- The East Potrillo Mountains are located 20 - 22 miles south tion discussed below. Bedding planes in sandstones and of Las Cruces, New Mexico and are part of a north - north- limestones are frequently marked by slickensides, sugges- west trending mountain chain that crosses the New Mexico tive of fl exural slip. Plots of poles to bedding show that - Mexico border (Figure 1, 5 and 6). They form an isolated fold axis plunge 2 o to 350o (Figure 8). outcrop of Permian and Cretaceous rocks along the north- Plots of poles to thrust fault planes are shown in Figure ern margin of the Chihuahua trough. In the East Potrillo 8. As can be seen, most faults dip toward the southwest Mountains, Seager and Mack (1994) measured 1,900 ft of at moderate angles, however a large number of faults dip marine clastic and carbonate shelf deposits above a basal northeast. Field observations show that these faults gen- conglomerate, all of which thin southward along the range. erally occur parallel to or at low angles to northeast dip- These somewhat arkosic clastics and limestone contain ping bedding planes. Fitting a great circle to poles to fault an Albian - Aptian fauna and correlate with the Hell-to- planes yields a beta axis of 6o to 337o, essentially parallel Finish and U-Bar formations of southwestern New Mexico to the fold axis determined from the bedding planes. (Seager and Mack, 1994). Conglomerate beds contain This close geometric relationship shows that faulting and 50 Carciumaru y Ortega

107°00'

N

Qpu 12 12

Kud 14 P 23 8

53 8 Qpo EXPLANATION 11 Kud Qpu Younger piedmont slope deposits 5 35 20 22 Qpo Older piedmont slope deposits P 13 18 Khb Kub 15 18 Qcp Santa Fe Group. Camp Rice Formation 9 35 9 Qpa Undifferentiated piedmont deposits 45 35 13 6 Kha A Kua Kua P B' Kue U-Bar Formation, massive limestone 15 46 Khb 12 A' 30 Khb Kud U-Bar Formation, silstone-limestone 53 Kub 15 Kuc U-Bar Formation, rudistic limestone P 24 Kha 25 Kub Qpo 29 Kub U-Bar Formation, sandstone 45 43 47 Kud 15 Kua Kua U-Bar Formation, lower limestone

34 Kuc 46 25 Kua Khb Hell-to-Finish Formation, mottled silstone 44 Kub30 C' B 10 D' Kha Hell-to-Finish, conglomerate 20 49 Kub P Yeso-San Andres Formation 15 Kud Khb 3 Thrust faults Normal faults Bedding planes P Kua 35 Kha Kuc 21 Qpo Khb 13 15 15 12 15 5 P C 13 15 5 21 Khb P Kub 29 13 15 10 Kha 10 20 19 Qpo Kud 45 45 25 D Kha 25 30 30 30 8 Kha P 25-30 8 25 25-30 P 9 28 P 45 35 Kha Kub 37 0 1 km 60 35 Kud 28 28 Khb Kua 30 30 31° 52' 30'' Figure 5. Geologic map of the northern part of the East Potrillo Mountains from Seager and Mack (1994) and fi eld reconnaissance.

folding are cylindrical with respect to each other. To sum- mottled siltstone member considerable evidence is present marize, cataclastic fl ow and pervasive plastic fl ow are un- for the operation of pressure solution during folding. For likely to have been important deformation mechanisms in instance, a tectonic cleavage defi ned by a millimeter scale sandstone, conglomerate and limestone during folding in parting is developed. In places the cleavage is as strong the East Potrillo Mountains. Presence of slickensides on as or stronger than bedding. This cleavage is highly vari- fault planes combined with the close geometric relation- able in its degree of development ranging from completely ships between the faults and folds shows that fl exural slip absent to being present as widely separated cleavage do- was the dominant folding mechanism in these lithologies. mains, to a pervasive pencil cleavage, to a fully devel- An important exception to the above is in the mottled oped slaty cleavage. It is diffi cult to assess what factors siltstone member of the Hell-to- Finish formation. In the controlled the intensity of cleavage development, but in Geologic structure of the northern margin of the Chihuahua trough 51

107°00' 31°52' 30" Khb 38 Kud 25 40 25 Kud 44 20 40 Khb Kuc Kub 30 10 30 20 35 0 1 km 35 15 Kuc 25 10 N 25 30 20 33 12 25 Khb 12 20 30 18 28 13 30 10 P 12 15 25 Kua 23 16 15 22 P Qpo 15 25 20 Khb Kub 30 30 Kud Kud Qpo 20 Kuc Kub 26 23 20 Kud 23 20 23 Qpo Kub P Kud Kub 23 15 Kue Khb Kud 14 Qpo 25 EXPLANATION 25 29 30 Kue 30 18 Qpu Younger piedmont slope deposits P Qpo Older piedmont slope deposits Kud 23

Qcp Santa Fe Group. Camp Rice Formation Qpo Kue 18 18 Qpa Undifferentiated piedmont deposits Kue 13 13 18 Kue U-Bar Formation, massive limestone 14 23 13 22 Kud U-Bar Formation, silstone-limestone 23 Kud 33 Kuc U-Bar Formation, rudistic limestone 13 25 Kub U-Bar Formation, sandstone 35 20 Kua U-Bar Formation, lower limestone 39 Kud

Khb Hell-to-Finish Formation, mottled silstone Kue Kue 20 20 26 Kha Hell-to-Finish, conglomerate Qpo Khb 26 P Yeso-San Andres Formation Kub 20 35 Kuc 25 Bedding planes Normal faults 25 26 Kud 20

Figure 6. Geologic map of the southern part of the East Potrillo Mountains from Seager and Mack (1994) and fi eld reconnaissance. general the most intensely cleaved rocks are found on the the development of pits on pebbles in thin conglomerate overturned limbs of folds. Evidence for pressure solution layers within the mottled siltstone. Taken together these during cleavage development comes from several observa- observations suggest that pressure solution was the defor- tions. First, cleaved rocks are often associated with calcite mation mechanism associated with folding in the mottled and quartz vein networks. Second, many of the cleaved siltstone. Further evidence for ductile strain within the rocks contain millimeter or larger crystals of pyrite and mottled siltstone member is the development of boudinage are bleached white or light gray, suggesting reduction where more competent sand beds are found interlayered of hematite which forms cement in these rocks. Third is with the siltstone (Figure 9). 52 Carciumaru y Ortega

SW NE

A feet Kub A' Kua 5000 Kha Kud P Khb Kuc 5000 Qpo Kub Kua QTc Khb

4000 P Kha 4000 P

n=20 n=17

Fold axis Fold axis Poles to planes Poles to planes B B' Kuc Kud Khb Kub 5000 Kua Kub Qpo Kuc 5000 Qpo P Kua QTc Khb 4000 P Kha 4000 P

n=49

Fold axis Poles to planes C Kha Khb C' Kud 5000 P Kuc Qpo 5000 Qcp Kua QTc P Kub 4000 Khb P 4000 Kha

n=79

Fold axis Poles to planes

D Kud D' 5000 P Khb Kuc 5000 Qpo KuaKub Kua Kha QTc Khb 4000 4000

Figure 7. Cross sections in the East Potrillo Mountains (after Seager and Mack, 1994). The stereographic projections of poles to bedding and the fold axis were added for all the regions; the shortening direction is indicated. The stress inversion results are also presented; σ1, σ2 and σ3 are the principal stress axis of the stress tensor (σ1>σ2>σ3) represented with fi ve, four and three star points respectively.

Geologic structure of the northern margin of the Chihuahua trough 53

M

a) b) e a

n

t

h

r

u best fit s

grade t f

a

u l t le gird t fit Bes

pole to bedding pole to thrust fault

c) d)

slip direction greatest compression axis least compression axis Figure 8. Stereonets of summarizing structural data in the northern Potrillo Mountains (area of fi gure 2.5). a) Poles to bedding with best fi t great circle shown. Star indicates orientation of inferred fold axis. b) Poles to thrust faults. Star shows pole to best fi t great circle. c) Faults plotted as great circles with slip direction inferred from slickensides shown. d) Greatest compression and least compression axes determined for faults in c) using the inversion method of Marrett and Allmendinger (1991). Contours for compression and tension axis are also shown. Data are consistent with northeast - southwest horizontal compression and vertical extension.

In order to evaluate if the pressure solution cleavage propagation folding. It is also possible that the stress fi eld formed during folding or another phase of deformation, was heterogeneous at the scale of the study area. Field re- poles to cleavage were plotted on stereographic projection. lationships however, show no evidence of large scale cross A great circle fi t to the poles to cleavage yields a beta axis - cutting relationships making it likely that contractional of 11 to 343, sub parallel to the fold axis calculated from deformation resulted form a single progressive deforma- poles bedding (Figure 10). This again suggests that the tion, not superposed deformation, consistent with the in- cleavage in the mottled siltstone member of the Hell-to- terpretations of Seager and Mack (1994). Finish formation formed synchronously with fault propa- To further evaluate the possibility of a heterogenous stress gation folding. fi eld and possible limitations of inversion method, stress To evaluate the orientation of the stresses responsible for inversion was also preformed on selected locations us- folding, fault slip inversion was performed. A total of 56 ing the inversion method of Michael (1984) and the direct thrust faults were measured in the northern part of the East inversion method of Allemendinger (2001). Three loca- Potrillo Mountains (Figure 8). Of those 41 faults had slick- tions were selected in the northern part of the East Potrillo ensides which could be used to infer the slip direction and Mountains and results are shown in Figure 11 and 12. As the sense of shear. Of these faults, 37 gave solutions for the can be seen each of these regions gives stress orientations orientations of the greatest compression axis and least com- which are generally consistent with northeast - south- pression axis which are shown in Figure 8 (d), using the west shortening and subvertical extension. However the method of Marret and Allmendinger (1991). Distribution direct inversion method (Allmendinger, 2001) gives re- of compression and tension axes is consistent with a thrust sults which diverge the most from the other two inversion strain fi eld of northeast - southwest horizontal compres- methods. However, the fault data when combined with the sion and vertical extension (Figure 8, d). There is a larger orientations of folds at all three localities (Figure 7) are variation in the inferred shortening direction than the in- consistent with northeast - southwest horizontal shorten- ferred extension direction suggesting either superposed ing across the region. deformation, rotation of faults during progressive strain Intensity of deformation in the East Potrillo Mountains or a heterogeneous stress fi eld. Of these possibilities, it is varies form north to south. In the southern part of the range most likely some of the faults were reoriented during fault a nearly complete stratigraphic section from the base of 54 Carciumaru y Ortega

W E

Boudine neck Pencils Slaty Cleavage

Figure 9. Boudine neck and slaty cleavage provide evidence for ductile deformation of the mottled siltstone in the East Potrillo Mountains. The cleavage is related to asymmetric folds developed during Laramide deformation. the Cretaceous to the upper siltstone member of the U-bar that are located along the west side of the range with the formation is exposed and is uninterrupted by thrust faults largest outcrop occurring at Cerro de Cristo Rey just to the (Figure 6). The fi rst major thrust fault and associated folds southwest of the Franklin Mountains (Figure 2 and 13). An occurs in the central part of the range in the hanging wall of important feature of the range is the presence of the thin a major (younger) low angle normal fault (Figure 5). North - skinned style thrusting in Pennsylvanian and younger of the fault complexity and intensity of the contractional rocks on the western side of the range. In contrast, high structures increase. Greatest shortening in the range occurs angle reverse faults and basement involved faults are re- nears its northern end. In this area Permian rocks are thrust stricted to the western boundary fault zone and stretch over the siltstone - limestone member of the U-Bar forma- from the southern tip of the range to Tom Mays Park in the tion implying ~ 427 meters of stratigraphic repetition. central Franklin Mountains (Lovejoy, 1975; Wu, 2002). Prior work in the Franklin Mountains identifi ed Laramide structures as being a large syncline and anticline pair ex- 4.2. Franklin Mountains posed southwest of Anthony Gap (Figure 13), a west - trending monocline north of the Webb Gap and a southwest The Franklin Mountains are a key exposure of the - trending monocline southeast of North Anthony’s Nose Chihuahua tectonic belt because they record the transition (Kelley and Matheny, 1983). Because these folds have not from thick - skinned basement involved thrusting (typical been precisely dated their assignment to Laramide tecto- Laramide style) to the thin - skinned thrusting of the belts nism has been based on their similarity to Laramide struc- interior (Wu, 2002). The range is a west tilted fault block on tures regionally (Chapin and Seager, 1975). the eastern margin of the Mesilla Basin and is composed of Paleozoic and Precambrian rocks which dip homoclinally towards the west at ~30o (Wu, 2002). Laramide structures 4.3. Anthony Gap and Tom Mays Park are best developed in rocks that are Pennsylvanian and younger found along the western side of the range. The The area from Anthony Gap extending south, including youngest of these structures belong to Cretaceous rocks low hills west of the range at Tom Mays Park, exposes a Geologic structure of the northern margin of the Chihuahua trough 55

Poles to bedding Poles to cleavage a) b) Pole to the best fit circle Pole to the best fit circle

c)

Figure 10. Poles to the bedding in rocks with clevage (a), poles to the clevage planes (b) and the best fi t circle to these poles in the East Potrillo Mountains. In c) both bedding and cleavage are plotted together showing that cleavage and bedding are coaxial with folds implying folding and cleavage development occurred synchronously.

series of northwest - southeast striking folds with shallow plunges (Figure 14). This map scale fold axis lies parallel plunges (Figure 13). Prior workers in the region have to a series of small scale folds, particularly well developed concluded that the large syncline - anticline pair west of in shale and evaporate beds of the Panther Seep formation the Franklin Mountains at Anthony Gap was north - south (Figure 15). These folds are open to tight, generally verge trending (Lovejoy, 1975; Kelly and Matheny, 1983). This to the northeast and have shallowly northwest - southeast was based largely on the topographic expression of the folds plunging hinges. Both the map scale folds and outcrop which have a long north - south striking west dipping limb scale folds are interpreted to result from the fi rst phase of that stretches from Anthony Gap to Tom Mays Park. This folding. The structures of the western side of the Franklin gives the impression of a large south plunging syncline. Mountains are the result of either reverse faulting along However, careful examination of the map pattern and the western boundary fault zone or it could be the result of bedding orientations shows that these folds are the result tilting of the range during the formation of the Rio Grande of at least two phases of deformation (Figure 13 and 14). rift. No exposures of the western boundary fault zone are Bedding measurements from the Hueco group and Panther found north of Tom Mays Park suggesting the fault zone Seep formation at Anthony Gap, when plotted on stereo- terminates at this point (Seager, 1981). However, recently graphic projection, defi ne a great circle girdle which in- reinterpreted seismic data from Web Gap suggests that the dicates northwest - southeast trending folds with gentle western boundary fault system extends in the subsurface 56 Carciumaru y Ortega

107°00'' N

12

12 12

14 23 8

53 8 11 5 35 20 22 13 18 15 18 9 35 9 45 35 13 6

46 12 30 53 24 25 29 45 43 47 15 34 46 25 44 30 10

20 49 15 3

21

13 15 15 12 15 5 13 15 5 21

29 13 15 10 10 20 19 45 45 25

25 30 30 30 8 25-30 8 25 25-30 9 28 0 1000m 45 35 37 60 35 28 28 30 30 31°52' 30"

Figure 11. Results of stress inversion using the method of Michael (1984) for three selected locations in the northern Potrillo Mountains. Data are consistent with northeast-southwest horizontal shortening and subvertical extension. at least as far north as Web Gap in the northern Franklin Anthony Gap, occurs in the same rocks and therefore most

Mountains (Ruiz, 2004). likely resulted from the F1 folding event. Wu (2002) showed that beds adjacent to the Franklin Several models have been proposed for the formation Mountains steepen dramatically as the reverse faults of of the folds between Tom Mays Park and Anthony Gap. the western boundary fault zone are approached. This sug- Perhaps the most popular and most controversial is that gests that a large north - trending monoclinal fold associat- of Lovejoy (1975). He argued that faults and folds in the ed with western boundary fault zone trends along much of Franklins resulted from large scale landsliding off of the the western fl ank of the Franklin Mountains. We therefore west tilted Franklin mountains. This conclusion was based think it is likely that the monocline in the Hueco forma- on the extension of similar relationships found to the south tion is a result of reverse faulting along the western bound- at Crazy Cat Mountain (southern tip of Franklins) where ary fault zone. However, it is likely that tilting and normal Paleozoic rocks failed in a catastrophic landslide (Lovejoy, faulting amplifi ed this monocline during uplift of the mod- 1975). ern Franklin Mountains. Going south from Anthony Gap However, several features preclude a landsliding origin to Tom Mays Park folds change from northeast vergent for these folds. First, the vergence of folds at Anthony Gap to southwest vergent (compare Figure 16a and 16b). This is northeast vergent, opposite what is predicted by the land is most obvious in the kilometer scale asymmetric fold sliding model. Second, faults exposed in gypsum beds of found just to the west of Tom Mays Park (Figure 16, a). the Panther Seep formation expose a number of features However, the fold axis in this area parallels folds found at which indicate northeast directed transport during thrust- Geologic structure of the northern margin of the Chihuahua trough 57 ing, again opposite to that predicted by the land sliding thrusting in the hanging wall of a décollement developed model. These include the development of C/S structures in the upper part of the Panther Seep formation. This inter- in gypsum beds, slickensides with steps indicating north- pretation provides an explanation for why rocks older than east transport, and chocolate tablet boudinage of shale the Panther Seep are not tightly folded since they would beds which record northeast - southwest directed horizon- have been mechanically detached from the hanging wall tal shortening. Taken together these features indicate that during thrusting. The change to southwest vergence at the folds at Anthony Gap resulted form northeast directed Tom Mays Park and further south is interpreted to result

2 a) -0.52 -0.16 -0.19 σ = -0.16 -0.77 -0.48 -0.19 -0.48 1.29

N=15

2

b) -0.71 -0.50 -0.26 σ = -0.50 -0.77 0.24 -0.26 0.24 1.49

N=12

c) -0.79 -4.62 -2.80 σ = -4.62 0.73 3.42 -2.80 3.42 0.06

N=6 Figure 12. Examples of kinematic analysis via direct and inverse fault - slip stress determination in the East Potrillo Mountains. A, B and C correspond to the northern, central and southern localities shown in Figure 2.11. As can be seen from the data direct and inverse methods give similar results at the three localities shown in fi gure 2.11. The largest difference comes in the southern location, where the direct inversion method gives a greatest compres- sion axis indicative of west - northwest horizontal compression. This is likely due to the presence of faults with west - northwest plunging slickenisdes at this locality. σ1, σ2 and σ3 are the principal stress axis of the stress tensor (σ1>σ2>σ3) represented with fi ve, four and three star points respectively. The fi rst column is the summation moment tensor and the average plane solution is represented as a dihedral equal area projection. The elongation region plotted as dark area. 58 Carciumaru y Ortega

47 39

35 N Explanation 27

41 Phl (Qal) Quaternary Undivided

65

33 36 10 25 35 (K) Cretaceous Undivided 35

40 Phm 15 35 Phu (Phu) Upper Hueco Limestone 50 20

35 50 30

60 (Phm) Middle Hueco Limestone 34 17 46 48 57 38 60 Pml 67 17 51 20 45 31 69 (Phl) Lower Hueco Limestone 17 38 Dpc42 Phu 26 33 50 Mn 63 Pmb 35 20 Pmu 21 45 33 35 (Pmu) Panther Seep Formation 31 38 46 31 20 15 14 40 F’

70 49 50 60 55 20 Phl 34 33 (Pmbc) Bishops Cap Formation 28 41 51 70 70 18 Pmbc 58 26 42 40 33 Pmbc 30 12 18 46 Sf 43 (Pmb) Berino Formation

17 43

Pmbc 42 (Pml) La Tuna Formation 30 42 51 52 44 40 47 32

31 80 10 3 49 50 54 39 30 47 42 52 57 45 (Mh) Helms Formation 45 45 3 53 47 52 10 11 49 41 pebr Pml 39 50

Mh 39 pebr (Mri) Mississippian Rancheria and F 58 40 43 42 Las Cruces Formation

35 (Dpc) Devonian Percha Shale and Pmbc pebr 68 Canutillo Formation Mn 37 43 ANTHONY GAP AND (Sf) Silurian Fusselman Dolomite TOM MAYS PARK

9 35 79 35 48 Dpc (Om) Montoya Group 7

50 Om

85 65 pebr 10 88 (Oe) El Paso Group

3 70 44 65 40 40 6 68 49 65 39 25 35

27 8 1 62 E’ 28 (Oeb) Bliss Sandstone 20 36 35 35 25 18 50 30 62 42 16 45 36 Pmbc 21 8 Sf 19 Pmb 20 30 25 37 65 69 35 35 18 18 64 57 44 20 60 50 28 41 (peb) Basaltic Dikes and Intrusions F Anticline 15 16 1 21 32 24 14 64 36 15 16 40 pebr pelu35 71 40 pebr 13 28 34 40 41 27 41 pelm (per) Pegmatite 38 19 40 F Syncline 18 40 1 60 pebr 70 39 85 46 37 53 40 69 44 36 66 48 29 75 55 (pebr) Red Bluff Granite 43 E 64 63 51

50 F2 Syncline 59 pebr 33 29

31° 54' 31 35 (pet) Thunderbird Group 34

28 F2 Monocline 80 34 30 pebr (pelu) Upper Lanoria Quartzite 76 28

Pml Proterozoic Ordovician Pennsylvanian Permian 42 40

30 5 20 35 40 40 (pelm) Middle Lanoria Quartzite Normal fault 6 pelu 36 25 35 30 50 80 85 81 30 25 38 60 38 34 (peg ) Lower Lanoria Quartzite Thrust fault 56 80 peg pet pelm 46 (pec) Castner Marble and 17 Strike and dip of bedding Pmbc 16 Mundy Breccia 22 35 30 15 peg pebr

15 15 47 24 30 71 55 pelm 32 76 46 30 23 35 30pet 17 25 28 -106° 30' 45

Figure 13. Geologic map of the northern Franklin Mountains from Harbour (1972) and fi eld reconnaissance. Geologic structure of the northern margin of the Chihuahua trough 59

a) Poles to beddings b) Fold axis

c) d)

Figure 14. Fold axis SW of Anthony Gap, based on bedding measurements. Panther Seep Formation (a), Bishop Cap Formation (b), Hueco limestone (c). All the poles to bedding were plotted together for a better fi t (d). from back thrusting along the same detachement fault as Mountains (Figure 17). shortening progressed. Wu (2002) found a similar strain fi eld along the west- In contrast to the East Potrillo Mountains, few thrust ern boundary fault in the Franklin Mountains, where faults are exposed in the Franklin Mountains. This is due Precambrian and lower Paleozoic rocks form the hanging partly to Tertiary and Quaternary cover along much of wall of high angle reverse faults. Additionally, if north the western Franklin Mountains and a general decrease in trending folds adjacent to the western boundary fault are the intensity of deformation when compared to the East rotated to correct for extension related tilting of the range, Potrillo Mountains. The best exposures of thrust faults then the folds are subparallel to those found at Tom Mays occur in a series of small quarries south of Anthony Gap, Park and Anthony Gap (Wu, 2002). This result suggests and outcrops in the mouth of Avispa Canyon at the north that faults associated with low angle thrust and high angle end of Tom Mays Park (Figure 13). At Anthony Gap the reverse faults in the Franklin Mountains resulted from the faults are best developed in the evaporate beds of the same progressive deformation event. Panther Seep formation, which are strongly sheared. These rocks contain a series of imbricate fl at and ramp structures which show clear northeast vergence. Slickensides are 4.4. Cerro de Cristo Rey defi ned by fi bers of gypsum developed in shear zones which are generally parallel to bedding. In a number of To the southwest of the Franklin Mountains, the Cerro de locations the gypsum rich beds contain a foliation which Cristo Rey exposes the 45 Ma Muleros Andesite which is oblique to bedding. The obliquity of this fabric is also intrudes Cretaceous sedimentary rocks (Lovejoy, 1976). consistent with northeast directed transport. Inversion of The rocks around the intrusion are strongly deformed by fault slip data in the northern Franklin Mountains shows a series of faults and folds that both pre - date and formed that the faults formed in response to northeast - southwest synchronously with intrusion of the pluton (Lovejoy, directed shortening, nearly identical to the East Potrillo 1976). These rocks provide an important tie as they expose 60 Carciumaru y Ortega NE SW

NE SW

Figure 15. Examples of Laramide deformation at Anthony Gap, Northern Franklin Mountains. a) Northeast vergent fold in gypsum beds in quarry to southwest of Anthony gap. b) Small scale back thrust in gypsum bed shale beds of Panther Seep Formation. faults and folds of similar style and orientation to both the the Franklin Mountains or East Potrillo Mountains. This Franklin and East Potrillo Mountains demonstrating that is likely due to modifi cation of the folds during intrusion structures throughout the region record the same deforma- of the Muleros Andesite, which largely post dated fold- tion event. ing (Lovejoy, 1976). Inversion of fault slip data at Cristo Folds, like the Powerline syncline at Cristo Rey (Figure Rey also shows the same northeast - southwest directed 18) are northwest trending with shallow plunges, similar shortening direction and subvertical extension (Figure 19), to folds found in both the Franklin Mountains and the East confi rming that structures across the Laramide structures Potrillo Mountains. The largest difference in the structures across the Mesilla Basin record the same contractional at Cristo Rey is that folds trend more westerly than in either strain fi eld. Geologic structure of the northern margin of the Chihuahua trough 61

Minor Folds a) Fold axis based on bedding orientation SW NE

Fold axis Poles to axial plane

Mrl pc Phl D Sf Pps

Phu Phm Phl Sf l mb c Pps P Mrl p 600m Pm D Mh Phu Pps Phl E E' Phm Mrl Pmb Mh 0 600m Pml Sf Dpc

b) Minor Folds Poles to bedding

Fold Axis Fold axis based on Poles to Axial Planes bedding orientation SW NE/W E

Mh Dpc Qal l Qal rl Qal Pm M f l Sf S rl Om m Oe bc m Oe Pg O Phm b P M Phl Pm m Phm Pmu P Ogb Pmu Phl Ogb Dpc

Mh Mrl Dpc Sf F 0 1 2 km F Pmu Phm Pmbc Oe

Pmb Ogb Phl Pg

Om Pml Qal

Figure 16. Cross sections of the Northern Franklin Mountains. a) Structure of Tom Mays Park area. b) Structure of Anthony Gap area. Stereographic projections of the fold axes and poles to bedding are lower hemisphere, equal area projections in both a) and b). The shortening direction is indicated.

4.5. Indio Mountains the Indio Mountains and Rohrbaugh (2001) estimated a minimum of six kilometers of displacement along thrusts The Indio Mountains are located to the southwest of Van in the Indio Mountains. Horn, Texas and form a small range along the U.S - Mexico In comparison with Laramide structures found around border. The structural geology of the Indio Mountains has the Mesilla basin (northern of the trough margin), sever- been described by Underwood (1962) and Rohrbaugh al important differences are observed. First, folds in the (2001), so only the details pertinent to the present study Indio Mountains and adjacent areas trend more northerly will be reviewed. Thrust faults in the Indio Mountains (Figure 20). Second, there is abundant evidence for left strike north - northwest and result in stratigraphic repeti- lateral transpression in the Indio Mountains (Rohrbaugh, tion of the Cretaceous section. Underwood (1962) esti- 2001), including east - southeast directed transport along mates a minimum of 8000 ft of stratigraphic repetition in thrust faults, clockwise rotation of fold hinges, and local- 62 Carciumaru y Ortega

Figure 17. Example of kinematic analysis in the North Franklin Mountains. σ1, σ2 and σ3 are the stress axis of the stress tensor (σ1>σ2>σ3) represented with fi ve, four and three star points respectively. Data is consistent with NW-SE shortening.

ly developed small scale strike slip shear zones with left the Franklin Mountains expose the transition from thick handed sense of displacement. Additionally, in an inver- - skinned thrusting typical of the Laramide orogen, where- sion of fault slip data Rohrbaugh (2001) showed that the as the East Potrillo and Indio Mountains expose a thin - greatest compression direction was east - southeast (Figure skinned deformation style more typical of the Cordilleran 20). This contrasts substantially with northeast directed orogen. These fundamental differences in deformation are shortening observed to the north in the Franklin and East closely related to the geometry of the Chihuahua trough, Potrillo Mountains. particularly the strike of its eastern margin and the thick- These differences could be due to the Indio Mountains ness of the sedimentary section within the trough. recording a distinct phase of deformation not observed Figure 4 shows an isopach map for the base of the lower to the north. However, the Laramide structures are Cretaceous. Also shown are Rose diagrams which show developed in Albian - Aptian rocks, similar to the East the orientations of map scale folds in the northern portion Potrillo Mountains and Cerro de Cristo Rey and occur of the trough and the eastern margin of the trough near the below an unconformity with ~36 Ma welded tuffs, Indio Mountains. These Rose diagrams were constructed suggesting deformation occurred in the late Cretaceous from the map in Figure 3, by measuring the trend of map and early Tertiary, similar to the Franklin and East Potrillo scale folds in 5 kilometer increments. Folds that curved Mountains. Finally, the Indio Mountains occur in nearly more than fi ve degrees (the size of the rose petals) were the same structural position with respect to the margin not plotted. Note that in the northern part of the Chihuahua of the Chihuahua trough as Cerro de Cristo Rey. Taken trough folds trend to 325, whereas in along the trough’s together, the data suggest that differences in deformation southeast side, the folds trend to 346. These differences style between the northern margin of trough and the Indio in fold trend coincide with changes in the trends of the Mountains are the result of heterogeneous strain during isopachs, which are generally northwest trending in the Laramide shortening. This possibility is investigated north and north - northwest trending in the south (compare further below. Figure 3 and 4). This geometric relationship indicates the geometry of the trough’s eastern margin imposed a pro- found infl uence on the development of the folds and re- 5. Discussion lated faults. Another feature which is apparent from the isopach map Fundamental differences in intensity of deformation is that the regions which have a thin - skinned structural and deformation style exist in different regions of the style are restricted to areas where the Cretaceous section is Chihuahua trough. Additionally, the shortening direction thicker than 300 m, whereas regions which have basement estimated for the Franklin and East Potrillo Mountains is involved faulting, the Cretaceous section is or absent. northeast - southwest, whereas the shortening direction This is true not only for the regions described here, but for the Indio Mountains is east - southeast. Furthermore, also regionally. Seager and Mack (1985) showed that the Geologic structure of the northern margin of the Chihuahua trough 63 Normal fault Strike and dip of bedding Strike and dip of overturned bedding Reverse fault Strike slip fault Syncline Anticline 45 40 Explanation Quaternary undivided Muleros Andesite Loma Blanca Felsite Boquillas Formation Buda Formation Del Río Formation Anapra Sandstone Shale Mesilla Valley Muleros Formation Formation Smelter Town Del Norte Formation Finlay Limestone (Qa) (Kma) (Klb) (Kbo) (Kbu) (Kdr) (Kan) (KMe) (KMu) (Ksm) (Kdn) (Kfn) 31° 46'

Te x a s 20 ihuahua Ch Te xas 30 New 25 15

20 Me xico U.S.A. 20 N Mexico 10 20 30 15 Kan 20 Kdn 20 25 15 ed from Lovejoy, (1980). ed from Lovejoy, Ksm, 25 fi 25 15 25 15 5 40 15 35 10 Syncline 15 10 20 20 15 Powerline 25 20 25 KMe 30 20 40 40 40 30 25 15 KMu 5 Kan 10 15 15 25 Qa 20 25 Kdr 30 70 20 60 45 A 40 10 70 15 KMu 20 65 50 Kdr 60 Kan s 70 40

co 55 80

xa 75 45 35 Te 40

!Mexi 70

w 20 45 80

Ne 45 35 25 Kbu 80 70 30 25 20 80 50 20 KMe 20 Kdr 35 Kan Kdr 25 25 55

co 30 35 Kbo

xi 15 35

Me ihuahua 40 40 35 60 45

40 Ch Kdr

New Figure 18. Geologic Map of Cerro de Cristo Rey modi 25 45 Kbo 20 25 85 30 75 30 30 70 30 30 55 50 Klb Ksm 80 30 35 80 40 75 40 30 65 40 Kma 45 KMu 25 KMe 40 25 35 40

30 co 60

40 i 1 km

.S.A.

U Mex 30 50

45

Klb acks 40 35 40 A 50

40

35

ad Tr ad 40

o 55 R

35

75 Rail 45 45 Qa 25 25 40 -106° 33' 64 Carciumaru y Ortega

A A' (Qa) (Kma) Kmu Qa Kma (Klb) Klb Klb Kmu Kfn 300 m Ksm (Kbo) SW NE (Kbu) 0 300 m All Data Power line syncline (Kdr) (Kan) (KMe) (KMu) (Ksm) Pole to Bedding (Kdn) Thrust faults and folds at Cristo Rey record σ1 estimated from fault slip data 1m (Kfn) northeast-soutwest shortening σ 3 estimated from fault slip data

Figure 19. Geological cross - section of Cerro de Cristo Rey uplift based on map of Lovejoy (1980). Stereonets showing poles to all bedding planes consistent with northwest plunging map scale folds. Estimates of the orientations of greatest compression and least compression directions based on inversion of fault slip data, consistent with northeast-southwest directed compression and subvertical extension. Sketch shows geometry of folds in the Mesilla Valley shale on southwest limb of Powerline syncline adjacent to the Muleros Andesite showing intense deformation of shale.

Burro uplift of southwestern New Mexico contains clear folded during shortening. Additionally, our observations in evidence of basement involved thrusting. This region was the East Potrillo and Franklin Mountains show that ductile a high during the Cretaceous and lies outside of the region deformation is focused within shales and evaporates sug- of Cretaceous sedimentary rocks (Figure 4). Directly north gesting lithological controls on deformation partitioning. of the Franklin Mountains, the Bear Peak fault zone places Shortening directions inferred from fold trends and fault Proterozoic basement over Paleozoic rocks and is therefore slip inversions are nearly perpendicular to the troughs mar- thick - skinned, and again lower Cretaceous sedimentary gin along its length between the East Potrillo Mountains rocks are absent. These relationships indicate that the thin and Indio Mountains. This is despite there being a nearly - skinned style of deformation is restricted to areas where 30o difference in the orientation of the margin of the trough the Cretaceous sedimentary section is present. The one ex- between the north and south. This high angle relationship ception to this is along the western fl ank of the Franklin and change from strain dominated by northeast - southwest Mountains where thin - skinned detachment style folds are directed horizontal shortening (Franklin and East Potrillo found in Permian and Pennsylvanian rocks. However, as Mountains, Cerro de Cristo Rey) to left lateral transpres- shown above, these folds are developed above thin bedded sion (Indio Mountains) indicate along strike variations in shales and evaporate beds of the Panther Seep formation, the strain fi eld that coincide with changes in the geometry which are mechanically similar to the Cretaceous section. of the trough’s margin. Taken together, the data indicate that the preexisting ge- Harland (1971) presented a simple kinematic model for ometry of the Chihuahua trough placed a profound control variation in kinematics around curved deformation zones on the intensity, style and orientation of Laramide struc- (Figure 22, a). According to this model, transpression oc- tures along the margin of the Chihuahua trough. A simple curs along margins which are oblique to the shortening di- model that compares the thin- and thick- skinned relations rection whereas convergence occurs in regions that are at with sedimentary thickness is presented in Figure 21. a high angle to the transport direction. Assuming an east The strong geometric relationships between the geometry - northeast shortening direction for the Laramide oroge- of the trough and the style of Laramide deformation sug- ny (Bird, 1998) and applying Harland’s geometric model gests that there may be a fundamental mechanical control predicts that the region around El Paso should record left on deformation imposed by the anisotropy inherited from lateral transpression and the region near the Indio moun- the trough. The sediments found within the trough are tains should be contractional or undergoing right lateral generally interbedded sandstones, shales, and limestones, transpression. Instead, the observed deformation patterns with shales accounting for 1/3 to 1/2 of the exposed sec- show dominantly contractional strains near El Paso and tion. These sequences of Cretaceous rock are fl oored by left lateral transpression in the Indio Mountains. Clearly, Jurassic evaporates throughout much of the Chihuahua the model of Harland (1971) does not fi t the observed pat- trough (Gries, 1970). These evaporates have acted as de- tern of deformation in the Chihuahua trough. tachment surfaces above which the Cretaceous section has It is a well known that weak interfaces focus deformation Geologic structure of the northern margin of the Chihuahua trough 65

a) b)

n=212 Incremental stretching axis Poles to bedding Incremental shortening axis

Figure 20. Stereonets summarizing structural data from the Indio Mountains, modifi ed from Rohrbaugh, (2001). a) Compression and tension axis estimated from fault slip data using the inversion method of Marrett and Allmendinger (1991). Average greatest shortening direction is indicated by star marked “z”, and average least shortening direction is indicated by star marked “x”. b) Poles to bedding with best fi t great circle shown, star indicates inferred fold axis. Both sets of data are consistent with west-northwest, east - southeast directed shortening and subvertical extension.

and lead to reorientation of stress fi elds such that the short- more apparent where basin margins are curved and may ening direction is at a high angle or perpendicular to the lead to the development of oroclines in fold belts. weak interface (Mount and Suppe, 1987). We infer that An additional feature of the trough which appears to be the occurrence of evaporates and shales along the trough’s controlled by the geometry of the basin is the presence of margin lead to focusing of deformation into the trough younger on older thrusting along the margin of the trough. which was weak relative to the adjacent platform and the It is clear from the map pattern in Figure 3 that from the in- along strike differences of shortening direction. The short- terior of the Chihuahua trough Cretaceous rocks have been ening direction was forced to be nearly perpendicular to telescoped over Permian and older Paleozoic rocks. This is the margin because of the weak interface. This process somewhat counter to normal thrust situations where older provides a ready explanation for the along strike variations rocks are typically thrust over sequences of younger rocks in shortening direction and kinematics as well as a mecha- (Suppe, 1985). However, in the case of the Chihuahua nism to focus deformation within the weak interior of the trough, the thick sequences of Cretaceous rocks adjacent trough. We suspect that this process may be common in the Diablo Platform produced a situation where out of ba- many foreland basin settings where weak sedimentary sec- sin thrusting resulted in younger rocks piling up on older tions abut stronger platforms. This process is likely to be sequences of rocks. Thus, the unusual situation where

Thickness Thickness

Basement

Basement

a) b)

Figure 21. Thick- and thick- skinned models. a) Thick skinned deformation (basement involved) is controlled by the relative small thick- ness of the Cretaceous sedimentary layer and b) thin skinned occurs when the Cretaceous sedimentary layer is thicker than 300 m. 66 Carciumaru y Ortega

a) b)

Diablo platform 1 Chihuahua tect 1

onic belt

Harland’s Observed predictitive displacement model pattern

Figure 22. a) Diagram showing displacement pattern predicted from application of Harland’s (1971) kinematic model for curved displacement belts. b) Observed kinematic pattern along margin of Chihuahua trough. Large arrows show displacement direction inferred for Laramide deformation (Bird, 1998). younger rocks are thrust over older rocks is also explained variation also infl uenced intensity of deformation with the by the presence of a thick Cretaceous section adjacent to most strongly deformed rocks being found in the basin an older platform sequence. interior. Additionally, the geometry of the basin margin, A fi nal observation is that the geometry of the Chihuahua where weak sedimentary rocks abut a stronger platform trough was largely controlled by the geometry of Jurassic caused the shortening direction to maintain a high angle to through Cretaceous normal faults which led to the initial the basin margin independent of far fi eld stresses. These subsidence of the trough (Uphoff, 1978). This initial ge- effects are likely to be common in deformed foreland ba- ometry of the basin then strongly infl uenced the develop- sin settings and may be an underling cause of deformation ment of Laramide structures. We suspect that much of the patterns in foreland basins. complexity observed in deformed foreland basins may re- For the Laramide compressional event, the results show a sult from the initial geometry of the basins. NE-SW orientation of the shortening direction along the The results of this study are summarized in Table 1. The northern margin of the Chihuahua trough (East Potrillo Laramide deformation and magmatic activity have been Mounatins, Franklin Mountains and Cerro de Cristo Rey) attribuited to a change in the convergence rate between and a change to E-SE, going to the south along the margin the Farallon and the North American plates. Moreover, the of the trough (Indio Mountains). These differences in the most accepted cause of this major deformation has been orientation of the shortening direction can be explained attributed to fl attening of the subduction of the Farallon based on the geometry of the trough and especially the plate. strike of its eastern margin. Another factor which has to be taken into account is the rock mechanics. The sedi- ments which compose Chihuahua trough are Cretaceous 6. Conclusions interbedding sandstone, shales, and limestone fl oored by Jurassic evaporates. These relatively weak sediments en- The Chihuahua trough is a major Mesozoic basin along ter in contact with the stronger Precambrian basement and the U.S - Mexico border between El Paso and Big Bend, Paleozoic carbonate rocks of the Diablo Platform. The Texas. The initial geometry of the basin strongly infl u- contact in between these two interfaces generates a princi- enced the tectonic development of the Chihuahua trough pal plane of stress and the shortening direction was forced during Laramide shortening. Thickness variations lead to to be at high angles or almost perpendicular to this princi- thin - skinned deformation being focused within the inte- pal plane. Therefore, the chane in strike along the margin rior of the basin, whereas basement involved deformation of the trough will result in a change of orientation of the typical of the Laramide orogen was restricted to regions shortening direction along the rim of the Chihuahua trough found where the Cretaceous section is thin or absent. This when approached from the north to the south. Geologic structure of the northern margin of the Chihuahua trough 67

Table 1. Summary of the main tectonic events in the studied region. Period Epoch Ma Main tectonic events Stress field FH Quater Holocene 0.01 nary Pleistocene Second 1.8 Pliocene extension 5.3 event Neogene Miocene 23.8

Oligocene 33.7 Cenozoic Tertiary Ignimbrite flare-up Consolidation of Rio Grande Rift formation Paleogene Eocene

Pre RGR 54.8 process * Uplift IM Paleocene 65.0 * EPM Late * FM * TM * Laramide deformation 99.0 CR Thrust faults and folds formation

Early Mesozoic Cretaceous Uplift Deposits of Chihuahua trough 144

IM- Indio Mountains. EPM - East Potrillo Mountains, FM - Franklin Mountains. TM - Tom Mays Park, CR - Cristo Rey.

Acknowledgments Bibliographic references

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