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Research Paper

GEOSPHERE Pliocene–Holocene deformation in the southern Rio Grande rift GEOSPHERE; v. 14, no. 4 as inferred from topography and uplifted terraces of the Franklin https://doi.org/10.1130/GES01572.1 Mountains, southern New Mexico and western Texas 6 figures L.K. Armour, Richard P. Langford, and Jason W. Ricketts CORRESPONDENCE: jwricketts8@gmail.com Department of Geological Sciences, The University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, USA

CITATION: Armour, L.K., Langford, R.P., and Rick etts, J.W., 2018, Pliocene–Holocene deformation in the southern Rio Grande rift as inferred from topog raphy and uplifted terraces of the Franklin Mountains, ABSTRACT INTRODUCTION southern New Mexico and western Texas: Geo sphere, v. 14, no. 4 p. 1677–1689, https://doi.org/10 .1130/GES01572.1. In most extensional terrains such as the Rio Grande rift, alluvial fans and Normal fault systems develop through incremental slip events (earth- bajadas cover faults and terraces as extension progresses, thus limiting the quakes) that, over time, result in the final displacement profile of the fault. -Ex Science Editor: Shanaka de Silva faults and terraces as useful records of uplift. However, in the Franklin Moun- tended terranes are also characterized by multiple normal fault systems that Associate Editor: Graham D.M. Andrews tains of western Texas and southern New Mexico (USA), rapid aggradation overlap to form relay ramps or intersect as extension progresses and individual of basin floors by extensive playa lakes and floodplain deposits of the Rio faults grow in length (e.g., Peacock and Sanderson, 1991; Childs et al., 1995; Received 13 June 2017 Grande during the Pliocene buried the irregular mountain-front fans, thus Peacock, 2002; Nicol et al., 2010). These characteristics are true in cases of sim- Revision received 14 February 2018 creating a low-gradient surface. This originally planar surface was subse ple lithology and sandbox experiments (e.g., McClay and Ellis, 1987; McClay, Accepted 18 April 2018 Published online 17 May 2018 quently uplifted and deformed during faulting, providing a record of the 1990; Childs et al., 1995) but are also applicable to regions of diverse rock types Pliocene–Holocene extensional deformation in the southern Rio Grande rift. and heterogeneous crustal features (e.g., Nicol et al., 2005). Although these re- Deformation and uplift of the Franklin Mountains in the southern Rio Grande lationships appear to be generally true for a wide range of extensional faults rift was estimated by measuring the elevation of late Pliocene terraces that regardless of fault size, amount of offset, or lithology (e.g., Schlische et al., 1996), are adjacent to range-bounding faults. The uplifted terraces are exposed geologic evidence documenting sequential fault growth is commonly lacking. along both sides of the Franklin Mountains, and lie as much as 130 m above In models of idealized, isolated normal faults, displacement profiles should their original elevation. Together the uplifted terraces form an anticlinal arch preserve maximum displacement at the center of the fault, and displacement that mimics the profile of the range crest of the mountains. Three important should decrease to zero at the fault tip lines (Barnett et al., 1987; Walsh and conclusions can be drawn from the similarity of profiles among the terraces Watterson, 1987). However, displacement patterns are almost always seg- and mountain crest. First, the observation that the terraces mimic the range mented and irregular, and are also commonly asymmetrical, where maximum crest implies that the present-day topography of the mountains is likely tec- fault throw is centered closer to one fault tip than the other (e.g., Walsh and OLD G tonic in origin. Second, the east-side terraces are higher than the west-side Watterson, 1987; Childs et al., 1995; Mansfield and Cartwright, 1996; Schlagen- terraces, suggesting rotation of the mountains during deformation. Esti- hauf et al., 2008). As slip accumulates, fault tips typically propagate in two mated rotation since the Pliocene is ~5% of the total rotation. Third, fault directions and fault length increases. This process results in a power-law rela- throw rate calculations indicate differential slip along the length of the east- tionship, where fault length scales linearly with fault displacement (e.g., Pick- OPEN ACCESS ern boundary fault zone. The fault profile and throw rate calculations along ering et al., 1995; Schlische et al., 1996). As fault length increases, adjacent the eastern margin of the range are skewed to the south, suggesting that faults eventually overlap and link, which has been shown to be an important the southern segment of the Franklin Mountains has accumulated a majority fault growth mechanism in extensional settings (e.g., Cartwright et al., 1995; of the slip during this time frame. These observations, coupled with geo- Finch and Gawthorpe, 2017; Whipp et al., 2017). Alternatively, some fault sys- physical data highlighting buried faults beneath the El Paso (Texas)–Juárez tems develop through a constant-fault-length model, where they approach (Mexico) metropolitan region, suggest that normal faults related to uplift of their maximum length early and fault tips become fixed in space as fault dis- This paper is published under the terms of the the Franklin Mountains have been growing in length toward the south over placement continues to accumulate (e.g., Nicol et al., 2005; Amos et al., 2010; CC‑BY-NC license. the last several million years. Mouslopoulou et al., 2012; Curry et al., 2016).

GEOSPHERE | Volume 14 | Number 4 Armour et al. | Pliocene–Holocene deformation in the southern Rio Grande rift Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/4/1677/4265540/1677.pdf 1677 by guest on 29 September 2021 Research Paper

The Rio Grande rift (southwestern North America) is a region of litho- multiple studies have been undertaken to decipher fault displacement profiles spheric extension along a narrow north-south zone from northern Colorado along the Pajarito fault system (Carter and Winter, 1995; Lewis et al., 2009; to southern New Mexico, western Texas, and northern Mexico (Fig. 1A). It re- Goteti et al., 2013). Carter and Winter (1995) suggested that individual fault mains a remarkable natural setting to investigate styles of faulting, evolution segments are generally bell shaped in the direction of dip, although they are of normal faults, and fault linkage in a modest-extension environment. Differ- asymmetric and displacement abruptly decreases at fault tips. Lewis et al. ential slip along the length of normal faults, as described above, is common (2009) built upon these previous results and used the 1.25 Ma (Phillips, 2004) in extensional terranes such as the Rio Grande rift. Previous studies indicate Tshirege Member of the Bandelier Tuff as a datum to construct a fault throw that fault propagation and linkage may be an important process governing rift profile for the Pajarito fault system. Their results indicate that the Pajarito fault development. For example, in the Española Basin in northern New Mexico, system is composed of multiple shorter segments. Along some of these fault

109°W 108°W 107°W 106°W 105°W 106°40′W 106°30′W N A B Fillmore A' 39°N 32°10 ′ Arkansas Basin Explanation Gap Terrace elevations (m) 1257–1280 DEM elevations Rio Grande 1280–1300 High : 2739 m 38°N 1300–1325 1325–1350 Low : 1079 m LMS San Luis 1350–1382 Pc-Mz Colorado Basin LMS (1245–1256 m) Edge of

37°N Figure 1. (A) Geometry and extent of the Rio New Mexico Quaternary fault terrace Grande rift in Colorado, New Mexico, west- Webb ern Texas, and northern Mexico. Shaded Española Basin Gap region is rift basin fill. Study location is Hitt highlighted by the red box. RM—Robledo 36°N N Anthony Canyon Mountains. (B) Study site in and surround- Gap ing the Franklin Mountains. Precambrian- N Rio Albuquerque New Mexico through Mesozoic-aged (Pc–Mz) rocks Basin exposed in the Franklin uplift are shown in

32°00 ′ Fig. 1C 35°N Grande Texas the stippled pattern. Locations where ter- Socorro Basin La Mesa Vinton race elevations were measured with GPS surface Canyon Trench site are shown as circles, and measurements estimated from topographic maps and digi tal elevation models (DEMs) are shown as 34°N 050 100 Hueco Avispa squares. Background DEM shows elevation Palomas km Canyon Basin in shades of tan. Line A-A′ shows the trace Basin of the profile shown in Figure 5. EBFZ— Tularosa Transmountain Jornada Mesilla eastern boundary fault zone; WBFZ—west-

33°N Basin Road Basin ern boundary fault zone; LMS—La Mesa (Loop 375) Fig. 1B Basin surface. (C) Close-up of terraces along the Mimbres RM Fusselman western margin of the Franklin Mountains, Basin Hueco Basin Canyon highlighting the dissected nature of these

32°N deposits by modern arroyos. Blue dashed EBFZ Texas WBFZ lines are intermittent drainages that have Mexico N Mesilla Basin carved into the terrace surface. McKelligon 31°50 ′ C 00.5 1 Canyon km N New Mexico Scenic Drive Chihuahua, Mexico A Texas Downtown 052.5 10 El Paso Downtown km Ciudad Juárez

segments, the maximum fault throw is near the center of the fault trace, while 35 Ma Red Bluff Granite suite, resulting in amphibolite-grade contact metamor- in others it is shifted to one side, producing an asymmetrical fault throw pro- phism (Thomann, 1981; Shannon et al., 1997). These rocks are in turn overlain file. In addition, each of these shorter fault segments merges and overlaps with by rhyolitic ignimbrites, porphyritic trachytes, and volcaniclastic sediments of other segments in a series of monoclinal folds and distributed small-offset the Thunderbird Group, which are interpreted to be the eruptive equivalent faults (Lewis et al., 2009). These observations are consistent with the results of the Red Bluff Granite suite (Thomann, 1981). A 2717-m-thick succession of of three-dimensional finite-element modeling of the evolution of the Pajarito Paleozoic sedimentary rocks are separated from the Precambrian rocks by a fault system, which predicts the development of overlapping fault segments major nonconformity (LeMone, 1988). Uplift of the Franklin Mountains was and relay ramps (Goteti et al., 2013). primarily accomplished through slip along the eastern boundary fault zone To build upon these previous studies documenting sequential growth of (EBFZ), which marks the eastern edge of the range (Fig. 1B). Trenching studies normal faults through time in the Rio Grande rift, this paper focuses on uplifted of the EBFZ, shown in Figure 1B, identified multiple rupture events over the Pliocene terraces preserved along the flanks of the Franklin Mountains, an ex- last 16,400 yr, with an average slip rate of 0.175 mm/yr (Keaton and Barnes, tensional rift-flank uplift in the southern Rio Grande rift of far west Texas and 1995; McCalpin, 2006). south-central New Mexico. As described below, uplift of these terraces since The Franklin Mountains uplifted block separates the Mesilla Basin to the deposition records recent movement along main rift-related normal faults. west from the Hueco Basin to the east (Fig. 1). The Mesilla Basin is ~35 km Their current elevations are therefore a valuable record of the recent evolution wide by 100 km long. Mack et al. (1997) also considered the Mimbres Basin of these fault networks. These observations are used to compare results from west of the Mesilla basin to be part of the Rio Grande rift, and combined, the the southern rift to fault profiles that have been studied to the north (Carter and total width of the Mesilla and Mimbres Basins spans ~100 km in southern Winter, 1995; Lewis et al., 2009) and to discuss fault propagation and linkage New Mexico. of these faults through time during continued development of the southern Beginning in the Miocene, sediment carried by the Rio Grande from the Rio Grande rift. mountains of Colorado and northern New Mexico modified the landscape in ways important to this study. The southern part of the rift was the depositional center of this extended axial stream system; the preexisting topography was REGIONAL SETTING AND GEOLOGICAL DESCRIPTIONS largely buried under rapidly aggrading fluvial and lacustrine sediments (Mack et al., 2006). The first evidence of the Rio Grande in the southern rift near the The Rio Grande rift exists as a narrow continental rift extending from Franklin Mountains has been dated at 5 Ma by using paleomagnetic reversals northern Colorado to Socorro, New Mexico. South of Socorro, the rift abruptly and dating of volcanic rocks interbedded within the sediments (Mack et al., widens and continues to at least southern New Mexico and western Texas, 1998b; 2006). Fluvial deposition had shifted to the east side of the Franklin and probably into northern Mexico (e.g., Chapin and Cather, 1994) (Fig. 1A). Mountains by 2 Ma when the ancestral Rio Grande spilled through Fillmore In southern New Mexico, the rift is a wide feature and is physiographically Gap between the Organ and Franklin Mountains (Seager et al., 1984; Mack more similar than the northern segments of the rift to the highly extended et al., 2006) (Fig. 2). By the early Quaternary (2 Ma), the ancestral Rio Grande Basin and Range province to the west. Changing patterns in sedimentation, had bifurcated north of the Franklin Mountains near Fillmore Gap and had magmatism, and style of deformation all suggest that by 30–32 Ma, the area begun filling both the Hueco and Mesilla Basins with river sediments (Haw- was subjected to early extension (Morgan et al., 1986), although the main ley et al., 1969; Gustavson, 1991; Hawley and Kennedy, 2004). The terraces phase of extension likely did not begin until ca. 25 Ma (Kelley et al., 1992; mapped in this study are buried by these sediments in Fillmore Gap, indicating Chapin and Cather, 1994; Kelley and Chapin, 1997; Ricketts et al., 2016). This a terrace age significantly older than 2 Ma. period of rapid extensional deformation lasted until ca. 10 Ma and resulted in Deposition on the basin floors ended ~700,000 yr ago (Vanderhill, 1986; a series of north-south–trending, linked half-grabens that alternate polarity Mack et al., 1993, 1998a, 1998b; Hawley et al., 1969; Lucas et al., 1999; Gile (e.g., Mack and Seager, 1990; Chapin and Cather, 1994). Although extension et al., 2007). The 640 ka Lava Creek B ash is found in the oldest inset sediments, has slowed since 10 Ma, paleoseismic investigations and GPS studies indicate demonstrating that initial Mesilla Valley downcutting occurred before 640 ka the rift is still active today (McCalpin, 2005; McCalpin et al., 2011; Olig et al., (Izett and Wilcox, 1982; Seager et al., 1984; Gile et al., 1981). The final deposi- 2011; Berglund et al., 2012). tional surface forms the lower La Mesa surface (LMS) (Figs. 1, 2) (Gile et al., The Franklin Mountains are a north-south–trending range extending from 1981). The LMS of today is a broad flat slope that dips southward at ~0.001. The El Paso, Texas, into southern New Mexico (Fig. 1) (Harbour, 1972). Uplift and LMS has not been heavily deformed and is therefore thought to preserve the westward tilting of the range has exposed a rich assemblage of igneous, meta- southerly gradient of the paleo–Rio Grande. The LMS is offset by north-south– morphic, and sedimentary rocks, which document a complex history. The old- trending Quaternary faults and accumulation of eolian dunes. However, over- est rocks include Mesoproterozoic carbonates, siliciclastic sediments, and vol- all southerly gradients of 0.001–0.002 are preserved on the lower LMS flanking canic rocks and tuffs. These are preserved as roof pendants within the 1120 ± the Rio Grande, with higher slopes in the northern part of the basin and lower

Rio Grande valley and adjacent bolsons South-central New Mexico Trans-Pecos Texas Age Age unit s

cycles River valley Hueco and Red Candelaria Glacial Chrons Epochs Climate Bolsons Border slopes Flood plain Light bolsons area 1 ka arroyo arroyo 1 ka 2 alluvium alluvium s 2 3 ) 3 4 4 5 5 s 6 IG e 6 Fillmore alluvium 7 Holocene 7 8 8 Fillmor deposit 9 9 Organ playa

10 ounger valley-fill 10 Y alluvium (fluvial facies 15 Isaacs Inner river-valley fill 15 alluvium incision) Ranch (lake “Tahoka” Fort Seldon 20 pluvial (arroyo valley E deposits) 20 25 Leasburg alluvium 25

30 30 40 40 Rio Grande and tributary arroyo deposit Lat e e 50 ) 50 Wis- 60 PG 60 70 consin Balluco 70 80 gravel 80 Brunhes

90 IG Sanga- Hill surfac 90 Ramey monian Alluvium of Gold

100 Picacho alluvium 100

(fluvial facies Figure 2. Stratigraphic column show-

gravel 125 125 ing Miocene to Quaternary deposits and Jornada II older valley-fill alluvium Gillis events in south-central New Mexico and far 150 PG Illinoisian E 150 western Texas (adapted from Hawley [1975, 175 gravel 175 his table 2] and Hawley and others [1969, 0.2 Ma Tortugas Maden 0.2 Ma their figure 5]). Note the scale changes in

Yar- E s IG alluvium gravel the geologic time scale. Glacial cycle abbre- 0.3 mouthian initial river Place surfac e 0.3 Alluvium of Kern viations: PG—full glacial; IG—interglacial. Jornada valley I Miser Pleistocene E—valley incision and/or major erosion. 0.4 PG Kansan surface gravel 0.4 incision ace Middle I Jornada 0.5 La Mesa surf 0.5 IG Altonian surface Nebra- 0.6 PG 0.6 skan 0.7 0.7

0.8 High river-terrace deposit 0.8

0.9 0.9

1.0 1.0 Calabrian 1.5 1.5 Camp Rice Formation Camp Rice Formation Matuyama 2.0 Upper Santa Fe Group 2.0 River (fluvial-deltaic) facies Bolson fill Piedmont-slope (alluvial) facies Piedmont-slope (alluvial) facies sian Gela-

2.5 Basin-floor (lake and playa) facies 2.5 Upper Santa Fe Group Central lake Fort Hancock 3.0 3.0 depositional environments plains Formation

3.5 3.5 Gaus s 4.0 4.0 Lower Fort Hancock Lower Fort 4.5 Pliocene 4.5 Formaton Hancock 5.0 Formaton 5.0 5.5 5.5 Gilber t Mio- 6.0 cene 6.0

slopes to the south. Dunes and erosion result in high-frequency noise in eleva- two paleoearthquakes exhibited displacement of ~3 m. A mean slip rate of tion profiles, but the surfaces exhibit well-defined gradients. These gradients 0.175 mm/yr was estimated as a minimum slip rate from the last 16,400 yr from are similar to those of the modern Rio Grande, which has a modern gradient the total displacement of 11.2 m. of 0.001 through the Mesilla Basin. Western Boundary Fault Zone Eastern Boundary Fault Zone The western boundary fault zone (WBFZ) is a poorly exposed structure with The Franklin Mountains are separated from the Hueco Basin to the east by uncertain history. This fault may extend along the entire length of the west- the eastern boundary fault zone (EBFZ) (Fig. 1). The EBFZ is an east-dipping ern margin of the Franklin Mountains and possibly as far north as the Organ, structure that trends north-south along the entire length of the Franklin Moun- San Augustin, and San Andres ranges (Lovejoy, 1975). However, it is partially tains for a total distance of 53 km (Machette et al., 1998). Although in some covered by possible landslides, especially adjacent to the Franklin Mountains. locations the surface expression of the fault has been concealed or destroyed Alternatively, Harbour (1972) saw no evidence of this fault north of Avispa by urban development, the fault appears to be a single structure along most Canyon (Fig. 1). The WBFZ cuts a major Laramide thrust fault and numerous of its length. The fault can be mapped south to the southern edge of the Frank- smaller faults, suggesting that latest movement occurred during the Neogene lin Mountains, where it becomes concealed beneath the urbanized regions of (Scharman, 2006). The WBFZ appears to be inactive today because nowhere El Paso (Texas) and Ciudad Juárez (Mexico). Rocks exposed in the footwall does it disrupt Pliocene terraces (Scharman, 2006). Instead, the active fault of the EBFZ include the Precambrian Red Bluff granitic suite and associated along the western side of the Franklin Mountains is the Mesilla Valley fault, Thunderbird rhyolite, which are overlain by Paleozoic strata. The hanging wall which lies along the Rio Grande corridor and west of the terraces of this study is composed of late Oligocene Santa Fe Group rift fill capped by Camp Rice (Henry et al., 1985). Formation (Gile et al., 1981; Collins and Raney, 1991; Hawley et al., 2009). Basin subsidence in this region occurred from ca. 10 to 2 Ma when the majority of fill was deposited (Hawley et al., 2009), suggesting that main slip along the EBFZ SEDIMENTARY RECORD also occurred at this time. Recent activity along the EBFZ and associated normal faults within the The sedimentary record of the southern Rio Grande rift system includes Hueco Basin is expressed through numerous fault scarps that cut the surface Miocene strata and the Plio-Pleistocene Fort Hancock and Camp Rice Forma- of the basin (Collins and Raney, 1991). Ramberg et al. (1978) documented en tions of the Santa Fe Group (Hawley, 1975; Mack et al., 1998a, 1998b; Hawley echelon faults along the east side of the Franklin–Organ–San Augustin–San and Kennedy, 2004; Hawley et al., 2009) (Fig. 2). Along most of the east flank of Andres chain of mountains. They estimated 9.6 m of Holocene to late Quater- the Franklin Mountains, Fort Hancock sediments consist of coarse gravels de- nary vertical slip based on a 35-m-long fault scarp on the east side of the north rived from the Franklin Mountains that intertongue with basin-floor playa mud- Franklin Mountains, and about 7 m of slip near the south end of the Franklin stones. These units are typically exposed within a few meters above bedrock. Mountains. Collins et al. (1996) estimated an average slip rate of 0.1 mm/yr and Along the western side of the mountain, fluvial sediments of the Camp Rice noted that Quaternary scarps were up to 60 m high. Formation containing exotic clasts not derived from the Mesilla Basin form The EBFZ is the more active of the boundary fault zones. Studies near the upper 750 m of the basin fill. These sediments are intermittently exposed White Sands Missile Range, New Mexico, to the north of the EBFZ indicate within a few tens of meters horizontally from the western front of the Franklin that the last movements on the network of faults extending from the Organ Mountains, and have been interpreted as deposits of the Rio Grande derived Mountains to the east Franklin Mountains occurred within the last 4000–5000 from upstream sources (Chapin and Seager, 1975; Collins and Raney, 1991). yr (Seager, 1980). The date was determined by comparing soil development Dating of the basin fill of the Hueco Basin is problematic due to lack of suit- on the oldest unfaulted fan to the soil on the youngest faulted fan (Seager, able material and difficulty in correlation with other basins. However, nearby 1983). Movement along the Artillery Range fault between the north Franklin basins began to fill ca. 12–15 Ma, and the majority of the basin fill is probably and Organ Mountains may have blocked the ancestral Rio Grande’s course Miocene and Pliocene (Cather et al., 1994; Langford et al., 1999). Hawley et al. into the Hueco Basin at least 650,000 yr ago (Mack et al., 2006). (2009) noted a wedge of upper Santa Fe Group sediments that expands hori A study of the EBFZ by McCalpin (2006) focused on a trench at the mouth zontally toward the EBFZ that they tentatively correlated with Pliocene and of Hitt Canyon, 3 km south of the Texas–New Mexico state line (Fig. 1). Mc- early Quaternary sediments derived from the Rio Grande. Calpin (2006) found five normal faults, though there was uncertainty associ- The fill of the Mesilla Basin has been characterized by several authors ated with the three oldest faults. He was able to date three slip episodes using (Hawley and Lozinsky, 1992; Sellepack, 2003; Hawley and Kennedy, 2004), and 14C and compared them with infrared-stimulated luminescence done on four is summarized in Figure 2. The Camp Rice and Fort Hancock Formations, which fine inorganic silt samples. The mean throw was measured at 3.5 m, although form the upper Santa Fe Group, represent the influx of Rio Grande sediments

into the region. Mack and Seager (1990) placed the arrival of the Rio Grande well as the LMS. The elevation data were generally more accurate than ±30 cm in the Mesilla Basin at 5 Ma. However, far-traveled quartzite clasts first arrived and commonly better than ±15 cm of true elevation above mean sea level. At in the Mesilla Basin by 3.58 Ma based on the proximity of the Camp Rice many locations, alluvial fan deposits blanket the terraces within close proxim- Formation to the Gilbert-Gauss geomagnetic polarity boundary (Mack et al., ity to the mountain front. Therefore, to estimate the true elevation of these ter- 1993; Repasch et al., 2017), suggesting that the Rio Grande had reached the races, elevations were measured every ~21 m in an east-west direction across Mesilla Basin by this time. In the Hueco Basin, Albritton and Smith (1968) and 22 of the terraces to establish profiles along the crest of each terrace from the Gustavson (1991) dated the appearance of the Rio Grande sometime before alluvial fan to the toe of the terrace (Fig. 3). From these data, the elevations 2.06 Ma based on the Huckleberry Ridge ash that is interbedded with Camp of the terrace toes are interpreted to be the true elevation at each location. In Rice sediments. However, Hawley et al. (2009) suggested an earlier appear- areas where the terraces were inaccessible such as the firing range at Fort Bliss ance of the Rio Grande in the Hueco Basin, generally coincident with its ap- military base and the LMS north of 32°N latitude, elevations were estimated pearance in the Mesilla Basin. from U.S. Geological Survey 7½-minute topographic maps or digital elevation models with accuracy of ±1.5 m on 14 terraces. This resulted in a total of 59 elevation measurements along the entire eastern and western flanks of the DEFORMED TERRACES OF THE FRANKLIN MOUNTAINS Franklin Mountains that were used to estimate deformation (Fig. 1).

Unlike most of the uplifts associated with the Rio Grande rift, the Franklin Mountains preserve a set of uplifted terraces that flank both sides of the range RESULTS (Lovejoy 1971, 1975). The terraces overlie Rio Grande channel fills on the west flank of the range and therefore are younger than 5 Ma (Mack et al., 2006). Terrace Morphology and Stratigraphy The ancestral Rio Grande left cross-bedded and horizontally laminated pebbly sandstones and mudstones (Mack et al., 1998b) that formed the surface across A typical uplifted terrace in the study area is a relatively low-gradient penin which the terraces were initially deposited. The terraces are older than 2 Ma sula of higher topography extending from the mountains toward the surround- based on K-Ar dating of volcanic deposits (Chapin and Seager, 1975; Seager ing basins (Fig. 4). Uplifted terraces are typically 85–150 m across and extend et al., 1984), vertebrate fauna (Strain, 1969; Metcalf, 1969), and studies of soil 140–350 m from the steeper alluvial fans to the toes (Fig. 4B). Crests along development on the upper beds of the Camp Rice Formation (Seager, 1980; the length of the uplifted terraces are low gradient with slopes of 5% to 10% Gile et al., 1981). When compared to the undeformed early Pleistocene LMS from the mountain front. The flanks of the uplifted terraces are steeper, with (Gile et al. 1981), these terraces are likely middle Pliocene in age. slopes of 35% to 50% into the surrounding arroyos. Many of the larger up- The terraces at the same elevation along the eastern side of the Franklin lifted terraces are associated with canyons in the mountains that carve into Mountains are underlain by interbedded playa mudstones and alluvial fan terrace surfaces. The canyons debauch into arroyos that are diverted along the gravels of the Fort Hancock Formation along the southern portion of the range, mountain front for a short distance before continuing between the uplifted ter- and Camp Rice Formation fluvial sandstones along the northern portion. The race remnants to the basins. This suggests that early deposition of alluvial fan terraces are capped by thick alluvial fan gravels. During deposition of the Fort gravel on the upper terraces protected them from erosion. The more exposed Hancock and Camp Rice Formations, uplift of the mountain block began to lift areas flanking the canyons have thinner gravels and were more vulnerable to parts of this low-relief surface above the aggrading basin floors. This caused incision. deposition to cease and defined the uplifted terrace surfaces that are the sub- Two processes modified the terraces after deposition and altered data ac- ject of this study. Continued uplift of the Franklin Mountains also uplifted these quisition. First, the margins of the terraces were eroded. The distal ends of the terraces within the footwall of the EBFZ (Harbour. 1972; Lovejoy, 1971) so that terraces were reduced in elevation and the terraces were incised by arroyos the terraces on both flanks of the range record overall deformation of the exiting the Franklin Mountains, which cut the surface into a series of fingers range after their deposition. extending from the mountain (Figs. 1C, 4A). Additionally, younger alluvial fans prograded over many of the uplifted terraces. These deposits have steeper slopes than the terraces (Figs. 3, 4). An oblique aerial view commonly gives METHODS some indication of the extent to which alluvial fans prograde over the uplifted terraces. The surfaces of these alluvial fans are composed of coarse, angular The terraces flank large portions of both sides of the Franklin Mountains, al- clasts derived from the mountain slopes. The transition from uplifted terrace though they have been dissected by erosion (Figs. 1B, 1C). To constrain defor to alluvial fan is gradual and can be difficult to identify in the field. At the toe mation of these surfaces, a Trimble Geo Xh 2005 Series Pocket PC was used to of the uplifted terrace, the surface cover ranges from thin desert soil with a acquire 109 GPS-located points along both flanks of the Franklin Mountains as scattering of medium to coarse gravel to a well-cemented cobble surface. The

Figure 3. (A) View looking north of Fort Hancock Formation sediments in depositional con- A tact with Paleozoic units. Contact shown as dashed black line. Note the gradual change in slope. The road in the foreground is Scenic Drive (see Fig. 1B). Photo location is 31.789297 °N, WE 106.474260 °W. (B) View looking south of Fort Hancock sediments in depositional contact with Precambrian basement rocks. The dashed white line is the approximate contact. Photo taken from 31.835847 °N, 106.461728 °W. (C) Plot of GPS-measured elevations from the alluvial fan to the terrace toe shown in B. Although difficult to see in B., the alluvial fan has a higher slope than the terrace toe due to downward erosion of material from upslope. Therefore, only the distal ends of terrace toes were used in uplift calculations. X axis refers to meters east within zone 13R using a UTM coordinate system.

varying thickness of alluvial fan gravels and the transition to terrace toes create noise in the interpreted terrace elevations (Fig. 3). Slope angles up the crests of terraces range from 0° to 10°. The internal structure and stratigraphy of the uplifted terraces are usually concealed by indurated Pleistocene or Holocene calcic soils, except where 2 m exposed by arroyo incision or city development. Although investigating the internal stratigraphy of terraces in the Franklin Mountains is difficult in many B places, terrace remnants are preserved in the Robledo Mountains 60 km to the E W northwest of the study area and 15 km north of Las Cruces, New Mexico (Fig. 1A), and are a useful analogue. In the Robledo Mountains, uplifted terraces are incised through their length, and the internal structure of terraces is bet- Precambrian basement ter exposed than in the Franklin Mountains. These terraces consist of fine- to Terrace toe Alluvial fan medium-grained sands of the Camp Rice Formation that were measured with clinometers to dip 2°–4° into the Robledo Mountains block. This tilting is similar to, but at lower angles than, the overall tilt of the range and is ascribed to tilting of the hanging-wall block during extensional deformation. Similar strata are exposed along the eastern and western flanks of the Franklin Mountains, but the limited exposure precludes estimates of the dip of the bedding, although dips are likely similar to bedding dips measured in the Robledo Mountains terraces. In the incised terraces flanking the Robledo Mountains, the basin-floor strata are overlain by layers of coarse gravel that dip gently basinward. These beds 1 m lap onto the basin-floor sediments and are truncated at the top, and are inferred to represent the progradation of alluvial fan gravels across the Camp Rice sedi ments prior to erosion that occurred during uplift of the hanging wall. The ter- 1330 race surface overlying these inclined strata is much flatter and appears to be C an almost horizontal finger extending from the range front toward the basin. 1325 y = –0.1005x + 37666 R2 = 0.9968 1320 Along-Strike Geometry of Terraces 1315 Alluvial fan Beginning ca. 5 Ma, deposition of the Camp Rice and Fort Hancock For- y = –0.0414x + 16284 2 mations began to form an initially planar surface that filled the Hueco and 1310 R = 0.8253 Elevation (m ) Mesilla Basins (Strain, 1980; Gile et al., 1981; Mack et al., 2006). Subsequent Terrace toe faulting related to continued uplift of the range deformed the originally planar 1305 surface, providing a datum from which to estimate displacement along the 1300 length of main normal faults. To estimate fault throw, we used the nearly hori 361900 361850 361800 361750 361700 361650361600 zontal LMS as an approximation of the original geometry of terrace deposits. Easting (m)

Figure 4. (A) Google Earth view looking east of the northern Franklin Moun- tains in southern New Mexico. The terraces can be seen extending from the mountain front and are dissected by intermittent streams to produce terrace toes. Image is from approximately 32.085061 °N, 106.579488 °W. (B) View looking north of the west side of the Franklin Mountains. Flat terraces can be seen extending from the Franklin uplift. Note the steeper alluvial fan at the top of the terraces. Photo location is approximately 31.905679 °N, 106.553396 °W. (C) Highest-elevation terraces along the east-central part of the range can be seen in the mid- ground in front of Precambrian and Paleozoic rocks that make up the sky- line. Note the level surface that can be projected across the dissected ter- alluvial fan Precambrian and flat terraces race. Photo is taken from 31.868014 °N, B C Paleozoic rocks 106.457494 °W. (D) Exposure of a terrace projected level interior revealed by construction on terrace surface the eastern flank of the Franklin Moun- tains. The strata visible are largely playa and shallow lacustrine siltstones and shales. The uppermost white layer is a calcic soil developed in the terrace gravel that buries the lacustrine sediments. Photo location is approximately 31.839617 °N, 106.455837 °W. (E) Ter- race from the southeast part of the range (31.789425 °N, 106.473652 °W) 1 m showing the contact of the lacustrine sediments with the Paleozoic rocks of the Franklin Mountains. Note the steep contact formed by the burial of the D calcic soil lacustrine E range by aggrading Plio-Pleistocene Paleozoic rocks terrace sediment and the flat upper surface of sediment the terrace.

10 m

The LMS forms a low-gradient surface with a 0.06% slope and preserves the southernmost terrace along the eastern profile is ~79 m above the LMS, southerly gradient of the paleo–Rio Grande. Calculated fault throw along while the southernmost terrace along the western profile is ~7 m above the the length of the faults, described below, is assumed to be minimum because LMS (Fig. 5). the Camp Rice and Fort Hancock deposits in the Mesilla and Hueco Basins continued to aggrade during uplift of the range until 570,000 yr ago (Mack Uplift Rates et al., 2006). The uplifted terraces on both sides of the range are at different elevations Minimum and maximum uplift rates can be calculated using reasonable above the modern LMS (Fig. 5). On the eastern side of the range, the highest estimates of the total time over which uplift occurred and the total uplift magni- terrace elevations are 1370 m, ~118 m higher than the LMS. These terraces tude. The maximum age of the terraces is 5 Ma, based on the date of the entry are ~4 km south of South Franklin Mountain. At the north end of the range, of the Rio Grande into the Mesilla Basin (Mack et al., 1998b, 2006). Terraces the terrace elevations are much lower and only ~22 m above the LMS. The are older than the 700,000-yr-old upper LMS, west of Las Cruces, New Mex- terrace surface is underlain by fluvial channel sands of the Camp Rice For- ico (Vanderhill, 1986; Mack et al., 1993). A reasonable minimum age would be mation (Hawley and Kottlowski, 1969). Lava Creek B ash, which erupted at 3 Ma, due to the magnitude of uplift relative to the LMS. Using these estimates, 0.64 Ma (Lanphere et al., 2002), is preserved in the uppermost strata west of fault throw rates were calculated for each terrace location along the eastern Fillmore Gap, suggesting that uplift of terraces in this region occurred within and western lengths of the Franklin Mountains (Fig. 6). These throw rates were the last 0.64 m.y. The Fillmore Gap segment is also not deformed like the up- calculated using assumed times of deposition of 5 Ma, 4 Ma, and 3 Ma. Mini- lifted terraces to the south. This leads to the inference that the Fillmore Gap mum throw rates of ~0.001–0.002 mm/yr are found along the western profile is a younger terrace that formed on sediment that buried the older deformed at the southern tip of the Franklin Mountains where they intersect with the terrace. In this area, deposition in the Mesilla and Hueco Basins was faster than city of El Paso, Texas. Maximum throw rates of ~0.02–0.04 mm/yr are located the rate of uplift of the terraces (Mack et al., 2006). along the eastern margin south of Fusselman Canyon and north of McKelligon The terrace elevations, when plotted along strike of the Franklin Moun- Canyon. These values are an order of magnitude lower than those of McCalpin tains, show a broad 40-km-long asymmetrical anticlinal arch (Fig. 5). At (2006), who estimated Quaternary fault slip rates of ~0.18 mm/yr at a trench the northern end of the uplift, terraces are preserved at a height of ~30 m site along the EBFZ near the Texas–New Mexico border (Fig. 1). One possible above the LMS. The height increases dramatically to the south, reaching explanation for this discrepancy is that throw rates calculated in this study are maximum heights of ~118 m near South Franklin Mountain along the east- averaged over a much longer interval of time than the throw rates calculated ern profile. Height above the LMS surface then decreases to the south. The by McCalpin (2006).

2250 A south Fusselman Canyon north A′ 2000 Scenic Drive 1750 Anthony Webb Gap 1500 Gap Fillmore Gap 1250 Figure 5. Elevation profiles of uplifted ter- VE ≈ 6.5x 140 races (green and pink) in relation to the un-

opographic profile (m) 1000

T deformed La Mesa surface. The elevation 1375 East-side terrace elevations 120 along the crest of the Franklin Mountains West-side terrace elevations (bold black line) is also included; terrace el- La Mesa surface elevations 1350 100 evations mimic the topographic profile of Third-order polynomial fits the Franklin Mountains. Square symbols 80 with error bars are locations where ele- 1325 vations were estimated from topographic 60 maps and digital elevation models. Fault 1300 throw axis is the vertical difference be- 40 Fault throw (m) tween terrace elevations and the La Mesa

rrace elevation (m) surface. Location of profile A-A′ is shown in Te 1275 20 Figure 1. VE—vertical exaggeration. VE ≈ 11.5x La Mesa surface (780 ka) 1250 0

0510 15 20 25 30 35 40 45 Horizontal distance (km)

.05 West Franklin Mountains

.04 S 3 Ma terrace N 4 Ma terrace .03 5 Ma terrace Figure 6. Fault throw rates calculated from .02 south to north for the eastern boundary fault zone and western boundary fault

Throw rate (mm/yr) .01 zone. Throw was calculated relative to the elevation of the modern La Mesa surface, 0 and throw rates were calculated using .05 estimated maximum (5 Ma) and minimum East Franklin Mountains (3 Ma) ages for the terraces. Errors were .04 S N propagated using terrace elevation uncer- 3 Ma terrace tainties of ±30 cm for locations measured 4 Ma terrace with GPS and ±1.5 m for locations mea- .03 5 Ma terrace sured with topographic maps and digital elevation models. UTM—Universal Trans- .02 verse Mercator, zone 13R.

Throw rate (mm/yr) .01

0 3515 3520 3525 3530 3535 3540 3545 3550 3555 3560 UTM northing (x1000) (m)

DISCUSSION uplift at the northern tip of the range (Fig. 5) suggests that over the last 5 m.y., the EBFZ has extended to the north, by either tip line migration, linkage of ini- Terraces along the western edge of the Franklin Mountains have been up- tially isolated faults, or some combination of the two. The 79 m of fault throw lifted by various amounts relative to the LMS since deposition, and show an adjacent to the EBFZ at the southern tip of the Franklin Mountains suggests anticlinal arch similar to, but at lower elevations than, that of the eastern flank that this tip line has also been migrating south during the last 5 m.y. This in- of the range (Fig. 5). Minimum uplift of ~7–8 m is at the southernmost terrace terpretation is supported by gravity data that suggest the presence of multiple location, and maximum uplift of ~90 m is located near Fusselman Canyon. As faults that have been obscured by city development; these faults trend north- discussed previously, the WBFZ is likely inactive because it is not observed to south and extend into Ciudad Juárez (Marrufo, 2011; Avila et al., 2015). In par- offset Pliocene terraces (Scharman, 2006). Therefore, uplift of these terraces ticular, while the EBFZ is difficult to trace south of the Franklin Mountains, Avila was likely controlled by the EBFZ along the eastern flank of the range. The et al. (2015) noted a steep north-south–trending gradient in gravity data and observed uplift of these terraces suggests that, in addition to rotation of the infer that the EBFZ extends for as much as 30 km south of its mapped southern Franklin Mountains during slip movement along the EBFZ, the entire range extent, bisecting the downtown El Paso and Ciudad Juárez populated areas was also uplifted during continued extension. This deformation resulted in the and terminating along the eastern edge of the Sierra de Juárez uplift in Mexico. observed terrace profile in Figure 5, and accounts for the tilting and rotation of This is also supported by water well information for the area (Keaton et al., the range first documented by Lovejoy (1971). 1995; Hawley et al., 2009). The presence of buried faults beneath El Paso and The geometry of the fault throw profile along the length of the EBFZ delin- Juárez (Marrufo, 2011; Avila et al., 2015) and the observation that maximum eated by the uplifted terraces suggests that displacement has been concen- uplift and throw rates are concentrated to the south all suggest that the EBFZ trated to the south during the last 5 m.y., with maximum fault throw estimates is growing in this direction, either by tip line migration or fault linkage. As a of ~118 m south of Fusselman Canyon. In contrast, the northern segment of whole, these data are consistent with a model in which the EBFZ has been the fault has accumulated a total of ~30 m of throw during the same time (Fig. growing to the north and to the south during the last 5 m.y. 5). At the southern tip of the range, the EBFZ has been uplifted ~79 m above The asymmetrical nature of the fault throw profile can also give some in- the LMS, while the southernmost location along the western margin has only dication of the fault’s past and possible future behavior. Skewed displacement been uplifted ~7 m and extends farther south (Fig. 5). profiles, where maximum displacement is not centered along the fault, sug- Based on the geometry of uplifted terraces, the EBFZ appears to conform gest elastic interactions with neighboring faults (e.g., Peacock and Sanderson, with normal fault growth models described above. To the north, the 30 m of 1991; Peacock, 2002). In these situations, faults can be hard linked (intersect)

or soft linked (faults overlap and form relay ramps, but do not intersect). Fault REFERENCES CITED interaction strongly affects the final displacement profile and is characterized Albritton, C.C., Jr., and Smith, J.F., Jr., 1968, Geology of the Sierra Blanca area, Hudspeth County, by steep displacement gradients and asymmetrical profiles (Peacock, 2002), Texas: U.S. Geological Survey Professional Paper 479, 131 p. Amos, C.B., Burbank, D.W., and Read, S.A.L., 2010, Along-strike growth of the Ostler fault, similar to what is observed along the southern edge of the displacement pro- New Zealand: Consequences for drainage deflection above active thrusts: Tectonics, v. 29, file for the EBFZ. 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