The Importance of Low-Angle Normal Faults in the Rio Grande Rift of Central New Mexico

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Embryonic core complexes in the Rio Grande rift, central New Mexico Embryonic core complexes in narrow continental rifts: The importance of low-angle normal faults in the Rio Grande rift of central New Mexico

Jason W. Ricketts1, Karl E. Karlstrom1, and Shari A. Kelley2 1Department of Earth and Planetary Sciences, University of New Mexico, MSC03-2040, University of New Mexico, Albuquerque, New Mexico 87131, USA 2Earth and Environmental Sciences Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA

ABSTRACT away, and isostatically rotated low-angle have led to the development of two Geological normal faults. Although the degree of exten- Society of America (GSA) Special Papers: GSA The Rio Grande rift in central New Mex- sion was too low to juxtapose ductile footwall Special Paper 291, edited by Keller and Cather ico provides an excellent location to study the rocks against brittle hanging-wall rocks, if (1994), and GSA Special Paper 494, edited by interaction between high-angle and low-angle extension had progressed in the Albuquerque Hudson and Grauch (2013). (15°–35°) normal faults during crustal exten- basin, eventually a mature metamorphic core One aspect of rift formation that is relevant to sion. Here we evaluate the relative impor- complex would have formed, similar to those these issues is the relative importance of high- tance of low-angle normal faults (LANFs) preserved in the adjacent Basin and Range angle versus low-angle normal faults (LANFs) in the Albuquerque basin of central New Province. The Rio Grande rift, therefore, in accommodating crustal extension and how Mexico with goals of testing two confl icting provides a snapshot of the embryonic stages fault geometries may vary as extension pro- models of rift geometry and producing evo- of core complex formation, bridging the gap gresses. Low-angle normal faults (commonly lutionary models for the northern and south- between mature core complexes and incipi- referred to as “detachment faults”) in the Rio ern parts of the basin. Using physiographic ent extensional environments. Grande rift have been recognized for decades relationships, fi eld observations, structural (e.g., Black, 1964), where they typically exist data analysis, and thermal history modeling, INTRODUCTION as isolated faults or fault fragments within a we document two brittle LANF systems on predominantly high-angle normal fault environ- salients in adjacent opposite-polarity half- The Rio Grande rift, extending more than ment (Fig. 1) (Baldridge et al., 1984). However, grabens. These fault systems were both active 1000 km from Colorado to Mexico, is one of the because of the relative paucity of LANFs in the ca. 20–10 Ma and are locations of maximum world’s premier and best studied continental rift rift compared to their high-angle counterparts, fault slip as indicated by thickness of sedi- systems. Although research on understanding the recent models of basin formation in the central mentary fi ll in adjacent sub-basins and high- geology of the rift has been ongoing for decades, Rio Grande rift tend to emphasize high-angle est elevation rift fl anks. Average fault dip many questions still remain regarding the timing normal faults to accommodate crustal exten- increases basinward, and outbound faults and style of its structural development. Contin- sion (e.g., Connell, 2008; Grauch and Connell, were abandoned while intrabasinal faults ued interest in the rift is driven not only by the 2013), and models including LANFs are gen- cut Quaternary units, supporting an evolu- need to understand continental extensional pro- erally lacking. This is in contrast to LANFs in tionary model where master normal faults cesses but also because of a need to understand the adjacent Basin and Range Province, where initiated at a higher dip, were shallowed by and characterize potential seismic hazards (e.g., large-magnitude Eocene–late Miocene crustal isostatic footwall uplift in regions of high- Wong et al., 2004), the valuable water resources extension in western North America formed a est slip, and became inactive while younger of the basin-fi ll aquifers in Colorado and New continuous belt of detachment faults and meta- normal faults emerged basinward. These Mexico (e.g., Bartolino and Cole, 2002; Plum- morphic core complexes that extends from geometrical and kinematic observations are mer et al., 2004; Johnson et al., 2013), and poten- British Columbia, Canada, to Sonora, Mexico predicted by the rolling-hinge model for tial oil and gas production (Black, 2013). These (Fig. 1) (e.g., Coney, 1980; Axen et al., 1993; the formation of LANFs. This mechanism issues are especially important in the Albu- Dickinson, 2002, 2009). The timing and style has been widely applied to core complexes querque metropolitan area, the fastest growing of extension to form metamorphic core com- in highly extended terranes (e.g., Basin and region in New Mexico. To address important plexes in the Basin and Range has been exten- Range), regions of orogenic collapse, and earthquake, oil and gas, and water-related issues sively studied, and these structures are typically mid-ocean ridges, and it is shown here to also in New Mexico, it is necessary to have a fi rm thought to have formed via a rolling-hinge be applicable to narrow continental rifts of understanding of young deformation (e.g., Berg- mechanism, where isostatic rebound of footwall modest (~35%) extension. Similarities to core lund et al., 2012; Ricketts et al., 2014a) as well as rocks causes initially high-angle normal faults complexes include a physiographic expres- the longer term evolutionary history of rift faults to rotate to shallower angles (e.g., Spencer, sion of domal uplifts, evolution of a master and basin geometry. Continuing efforts to under- 1984; Buck, 1988; Hamilton, 1988; Wernicke detachment horizon that initiated as a break- stand these issues related to the Rio Grande rift and Axen, 1988; Axen and Bartley, 1997).

Geosphere; April 2015; v. 11; no. 2; p. 425–444; doi:10.1130/GES01109.1; 13 ﬁ gures; 3 tables. Received 24 July 2014 ♦ Revision received 12 December 2014 ♦ Accepted 25 January 2015 ♦ Published online 27 February 2015

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Figure 1. Regional tectonic and ally a second generation of faults will form and volcanic map of the Cordillera crosscut the fi rst generation of faults. Based on margin highlighting the loca- 55 Ma Migratory mechanical considerations, the amount of per- 20 Mavolcanic front tions of metamorphic core com- 50 Ma missible fault rotation in a stationary stress fi eld plexes in the highly extended Cordilleran is ~20°–45° (based on rock strength and depth Basin and Range Province and core complex in the crust) before a new set of faults must form 45 48–40 Ma Cascades Ma isolated low-angle normal faults Low-angle normal (Fig. 2A) (Nur et al., 1986). subduction zone in the Rio Grande rift. Times of faults of the In relatively hot crust, the brittle upper crust Rio Grande rift volcanic activity are shown in is mechanically decoupled from the lower duc- San 36–22 Ma red, and times of rapid exten- 40 Ma Rio Grande tile crust (e.g., Brun et al., 1994; Whitney et al., sion associated with metamor- 35 Ma rift 2013). Under these conditions, the crust extends Andreas phic core complexes are in blue. via a rolling-hinge mechanism, where strain 20 Ma M Arrows on metamorphic core SJ becomes localized along a single, large-offset UT CO complexes denote the transport AZ NM normal fault system, which often initiates at a direction of the upper plate. fault 20 Ma high angle and rotates to shallower dips due to 12–8 Ma 28–14 Ma M—Marysvale volcanic fi eld; Figure 3 isostatic rebound (Spencer, 1984; Buck, 1988; MD—Mogollon-Datil volcanic Oligocene MD Wernicke and Axen, 1988; Wdowinski and field; SJ—San Juan volcanic volcanic field Axen, 1992; Axen and Bartley, 1997). New field; SMO—Sierra Madre Basin and Range 25 Ma USA high-angle faults then emerge basinward, and if Baja Rio Grande rift Gulf Occidental; TP—Trans-Pecos 30 Ma Mexico this process continues, eventually lower-crustal, Colorado Plateau TP volcanic fi eld. UT-CO-AZ-NM Californiaof 35 Ma ductilely deformed rocks can be exhumed to the is the Utah-Colorado-Arizona– N California surface in the footwall of the LANF to form a New Mexico Four Corners 0 300 km SMO core complex (Fig. 2B). In this model, normal junction. Map is modifi ed from faults young toward the axis of the basin. The Dickinson (2009). Basin and Range Province (e.g., Coney and Harms, 1984; Buck, 1991; Axen et al., 1993), the Aegean Sea (e.g., Lister et al., 1984; Keay Within this general setting, this paper exam- extensional fault geometries and then summa- et al., 2001), and Tibet (e.g., Kapp et al., 2008) ines and evaluates the importance of several rize observations and constraints on fault geom- are several geologic examples of a rolling-hinge exposed LANFs in the central Rio Grande rift etry and fault age in the Albuquerque basin, cen- mechanism operating in hot crust. to test the hypothesis that they formed through tral New Mexico. Low-angle normal faults are observed in sand- a similar rolling-hinge mechanism as LANFs box experiments (e.g., McClay and Ellis, 1987; that are part of metamorphic core complexes BACKGROUND ON EXTENSIONAL Brun et al., 1994; Mourgues and Cobbold, 2006), in the Basin and Range Province. This study FAULT GEOMETRIES are predicted in numerical models (e.g., Buck, aims to help answer the question of the overall 1988; Rosenbaum et al., 2005; Tirel et al., 2008), importance of exposed LANFs in the modest- The expression of extension in both oceanic and are a near-ubiquitous feature of extensional extension (~17%–35%; Russell and Snelson, and continental settings appears to be strongly domains. However, LANFs in modest-exten- 1994) Albuquerque basin of the Rio Grande rift, dependent on the geotherm and the ability of the sion, narrow (<100-km-wide), upper-crustal central New Mexico. We apply a combination of deep crust to fl ow (Block and Royden, 1990; continental rifts are not as well documented. physiographic observations, fault orientations, Brun and van den Driessche, 1994; Lavier et al., This paper takes the view that understanding kinematic studies, and low-temperature thermal 2000; Rey et al., 2009a, 2009b; Whitney et al., faults in these settings may provide clues about history modeling to understand the evolution of 2013). Heat fl ow dictates, to a large extent, the early development of LANFs and core com- some long-recognized (Black, 1964), but poorly the degree to which the brittle upper crust and plexes in general. understood (e.g., Baldridge et al., 1984), LANF lower ductile crust are mechanically coupled. In segments. We examine whether these LANFs regions of cold to average continental heat fl ow, GEOLOGIC SETTING OF THE initiated and moved at low or high angle and there is a strong mechanical coupling between ALBUQUERQUE BASIN what can be learned in this moderate extension, the brittle upper crust and the lower ductile narrow, continental rift system to inform mod- crust, and two-dimensional models predict that The north-south–trending Rio Grande rift els for rift basin segmentation, rift geometry, the lower ductile crust will thin homogeneously is a classic, well-exposed, and well-studied and early stages of core complex development without responding to gravitational stresses example of a narrow and still-active continental in extensional domains. On a scale of the Rio (Whitney et al., 2013). As the lower crust thins, rift system (e.g., Baldridge et al., 1984; Chapin Grande rift, we explore the importance of fault numerous normal faults develop in the brittle and Cather, 1994; Russell and Snelson, 1994; rotation to form LANFs, as opposed to models upper crust to produce graben and half-graben Berglund et al., 2012; Ricketts et al., 2014a). involving only high-angle faults (e.g., Grauch geometry, as well as domino-style block rota- Early extension began ca. 36–37 Ma (Kelley and and Connell, 2013), to explain rift-fl ank geom- tion (Fig. 2A) (e.g., Proffett, 1977; Chamberlin, Chamberlin, 2012) and resulted in modest mag- etries and strain compatibility between upper- 1983; Nur et al., 1986; Stewart, 1998; Brady nitude extension that likely began the process of crustal and deeper-crustal extension. Within this et al., 2000). As extension progresses, the indi- separating the relatively undeformed Colorado context, and with the ultimate goal of producing vidual domino blocks and normal faults are Plateau to the west from the Great Plain province a robust fault evolutionary model for the Albu- systematically rotated, and faults become pro- to the east. However, the modern physiography querque basin, below we fi rst briefl y discuss gressively less suited for continued slip. Eventu- and N-S extent of the rift are the results of mainly

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A DOMINO-BLOCK ROTATION MODEL B ROLLING-HINGE MODEL Normal crust 1 Hot crust 1 2 3 Stage 1 Fault 1 emerges Multiple faults at a high angle. Stage 1 emerge at a high angle. Brittle-ductile transition Brittle-ductile Brittle-ductile shear zone transition Brittle-ductile shear zone 1 1 2 3 2 Stage 2 Stage 2

Faults and fault blocks on active As slip continues, fault 1 side of basin begin to rotates to shallower angles, and simultaneously rotate to fault 2 becomes active. shallower angles.

Initial upwarping due to isostatic rebound Stage 3 4 20–45° 3 2 4 5 Stage 3 1 5 6 1 2 fault rotation 3

Older faults become unfavorably oriented for continued slip after rotating ~20–45° (Nur et al., 1986), and new high-angle faults form. New faults emerge basinward during continued upwarping of fault plane. Ductily-deformed fault rocks Homogeneous thinning in lower crust are exhumed towards the surface.

Figure 2. Schematic evolution of low-angle normal faults. (A) Under normal crustal conditions, faults rotate via domino-style block rotation (after Fletcher and Spelz, 2009). In this model, multiple faults form at high angles over a distributed region and simultaneously begin to rotate to shallower angles (e.g., Chamberlin, 1983; McClay and Ellis, 1987; Whitney et al., 2013). Faults are eventually crosscut by a younger generation of faults after they have rotated ~20°–45° from their initial orientations (Nur et al., 1986). (B) Under hot crustal conditions, faults rotate via a rolling-hinge mechanism (Buck, 1988; Wernicke and Axen, 1988; Lavier et al., 1999). In this model, slip is concentrated along a single large-offset fault system. As extension progresses, isostatic uplift rotates the initial fault to shallower angles, and new faults emerge basinward as older faults are continually rotated.

Miocene extension, which was the time of accu- The Albuquerque basin, located in central that bounds the Ladron rift fl ank (2.7 km eleva- mulation of most of the E-W extensional strain New Mexico, is one of the largest basins of the tion; Fig. 3). (e.g., Kelley et al., 1992; Chapin and Cather, Rio Grande rift. Structurally it is bounded on Several additional LANFs are located within 1994). Normal fault geometries were infl uenced all sides by normal fault systems, and growth the Albuquerque basin, including the Carrizo to some degree by reactivated earlier faults of of the basin has resulted in the development of fault along the center of the western margin of Precambrian, Ancestral Rockies, and Laramide multiple sub-basins that are fi lled with the syn- the Albuquerque basin (Callender and Zilinski, age (e.g., Karlstrom et al., 1999; Marshak et al., extensional sedimentary basin fi ll of the Santa 1976) and LANFs along the southeastern edge 2000). The rift is composed of a series of NS- Fe Group (Fig. 3). The northern (Albuquerque) of the basin (Beck and Chapin, 1994; DeMoor trending basins that form an array of en echelon sub-basin is an east-tilted half-graben, where et al., 2005) (Fig. 3). These LANFs, however, grabens and half-grabens that alternate polarity Santa Fe Group sediments dip toward west- differ from LANFs in the Sandia and Ladron and are linked by NE-trending accommodation dipping normal fault systems (the high-angle uplifts because the total offset along these struc- zones (e.g., Chapin and Cather, 1994) that serve Rincon fault and low-angle Knife Edge detach- tures is several hundred meters at most. Thus to transfer the maximum displacement from one ment fault) along the Sandia uplift (>3 km these faults may have formed through alter- rift margin to the other (e.g., Russell and Snel- elevation; Fig. 3). The southern (Belen) sub- native processes that were not operative on a son, 1994; Faulds and Varga, 1998; Minor et al., basin encompasses multiple half-grabens that basin-wide scale, and they do not seem to play 2013). Within the Rio Grande rift, isolated expo- are fi lled with gently dipping Santa Fe Group an important role in overall basin evolution. sures of LANFs have been known for decades sediments. Steeper southwest-dipping beds of One of the goals of this paper is to evaluate (e.g., Baldridge et al., 1984), although they exist the Santa Fe Group are restricted to the south- two confl icting models for rift geometry and evo- within a predominantly high-angle normal fault western part of the basin (Grauch and Connell, lution of the Albuquerque basin of central New environment, and models for their formation are 2013) and dip into an east-dipping, low-angle Mexico. Russell and Snelson (1994) used seis- largely lacking. normal fault system (Jeter detachment fault) mic refl ection and borehole data to develop one

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0 107 0′W 106 30′W D Explanation Neogene basin fill SJVF B Ns A CO 35 30′N (Santa Fe Group) Nv * -2 Nv Neogene volcanic rocks ** -1 **** * MDVF B B A Paleozoic and Ns Pz-Mz ** C Mesozoic strata R * C a Precambrian granitic * NM R C′ Pc * ZSH FH and metamorphic rocks C FH Ortiz intrusions ** EB * A * EP TX Grauch and Nv -2 Structural elevation of the Connell (2013) Sandia -2 base of the Santa Fe Albuquerque -3 Mexico Pc Group (km above sea 1 sub-basin level) (Grauch and Russell and -1 Snelson (1994) Connell, 2013) ABQ -2 Pz-Mz eras lt Tij fau Subsurface fold axes (Grauch and 35 0′N Nv Connell, 2013) 0 -2 -1 Nv Low-angle normal fault 28 with dip direction and dip amount where measured 0 Other fault. Ball and bar are on hanging wall of select normal faults Pz-Mz -1 Pc Cross-section lines Jeter Belen Manzanos shown in Figure 4 1 fault -2 sub-basin Pz-Mz Ns Sample for apatite D FH fission-track (F), apatite

Ladrones -1

Sierra 0 (U-Th)/He (H), or both (FH).

Pc 34 30′N FH 1 (See Table 1). F H Los PinosRanchos B A C L S LP fault Topographic profiles (A–A′ and B–B′) and Ladron WJ 30 C 3 cross-section lines fault B′ RP C Rincon Knife (C–C′ and D–D′). fault 37 J Edge A′ N E D′ 25 fault 25–35 Pz- Mogollon-Datil A′ 36 2 Mz volcanic centers 30 Ns Outline of the Pc Socorro magma body

0 8 16 24 32 1 02 km Ns km

Figure 3. (A) General outline of the Rio Grande rift and study area in New Mexico. Red stars are locations of Oligocene–early Miocene calderas or igneous intrusions. SJVF—San Juan volcanic fi eld; MDVF—Mogollon-Datil volcanic fi eld. (B) Simplifi ed geologic map of the Albuquerque basin in central New Mexico. The directions toward the nearby Ortiz intrusions and the Mogollon-Datil volcanic fi eld are shown, as well as the general outline of the Socorro magma body, which underlies the southern portion of the Albuquerque basin. Topo- graphic profi les (A–A′ and B–B′) follow the topographically highest ridges along the eastern and western fl anks of the basin. Cross section lines (C–C′ and D–D′) extend from the rift fl ank toward the axis of the basin in the northern and southern parts of the basin. ABQ—Albu- querque; C—Cliff fault; CA—Coronado-Alameda strand, East Heights fault zone; EB—Eubank Blvd strand, East Heights fault zone; EJ—East Joyita fault; EP—East Paradise fault; LB—Loma Blanca fault; LP—Loma Pelada fault; R—Rincon fault; Ra—Ranchito fault; RP—Rio Puerco fault; SC—Silver Creek fault; WJ—West Joyita fault; ZSH—Zia and Star Heights fault zones. (C) Close-up of low-angle normal faults in the Sandia Mountains. Numbered locations are referred to in the text. (D) Explanation of map units.

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of the most widely cited models of the subsurface Paleozoic sediments in the hanging wall. The the bedrock facet above called the Shield, sug- geometry of the Albuquerque basin, which sug- rift-bounding structure along the eastern margin gesting that the “Knife Edge of the Shield” is an gested that the Albuquerque basin is composed of of the Ladron uplift is the Jeter detachment fault, exhumed upward continuation of this fault zone two individual half-grabens that alternate polar- the best known and most well studied LANF in as shown in Figure 6A. Riedel shears and slick- ity. In their subsurface interpretations, surface the Albuquerque basin (e.g., Black, 1964; Lewis enlines indicate normal-sense movement with faults are listric and sole into low-angle faults at and Baldridge, 1994; Read et al., 2007). This top-to-the-west movement direction (Fig. 6B). shallow (5–10 km) crustal depths. Alternatively, structure dips ~15°–30° east and contains Pre- Farther north, the Ranchos detachment fault Grauch and Connell (2013) utilized diverse geo- cambrian granitic and metamorphic rocks in the (location 3 in Fig. 3C) separates 15° east- dip- physical data sets from the Albuquerque basin to footwall (Fig. 5). The hanging wall adjacent to ping Santa Fe Group sediments from Mesozoic construct a model of the subsurface that indicates the Jeter detachment fault is an intensely brec- footwall rocks along a normal fault dipping ~30° a predominance of extension along high-angle ciated zone containing slivers of Paleozoic and (Fig. 6D; Kelley, 1975; May et al., 1994). Trun- faults and a more complex anticlinal accom- Mesozoic rocks, Neogene volcanic and vol- cation of hanging wall strata and a ~6-cm-thick modation zone between opposite polarity basin caniclastic rocks, and Santa Fe Group rift fi ll. gouge zone show this is not an unconformable segments. The differences in interpretations are Santa Fe Group sediments dip ~25°–35° west contact. At all locations, deformation associated depicted in Figure 4, which shows ~EW cross toward the fault plane (Read et al., 2007). The with these structures is entirely brittle. sections constructed across the northern Albu- fault consists of a 3- to 4.5-m-thick brecciated querque basin by Russell and Snelson (1994) zone that is composed of fi ne red clay surround- Central Albuquerque Basin and Grauch and Connell (2013) that highlight ing intensely fractured and pulverized angular Accommodation Zone their different interpretations of basin depth and Precambrian clasts (Black, 1964). Fault plane fault geometry. The most prominent difference measurements along the Jeter detachment fault Accommodation zones in the Rio Grande rift between these two interpretations is the impor- defi ne a shallowly dipping normal fault system, vary in width from >10 km to narrower trans- tance of low-angle normal faults (LANFs) in the and slickenline orientations trend toward the fer zones that are <3 km wide. They variably Russell-Snelson model and the lack of such fea- southeast (Fig. 5). In the hanging wall of the consist of single faults, multiple fault zones, tures in the Grauch-Connell model. Assuming no Jeter detachment fault, the Silver Creek detach- single folds, multiple folds, or any combination movement in or out of the sections and rigid body ment fault dips ~25°–35° east and is inferred to of these (Faulds and Varga, 1998; Smith et al., rotations and translations, palinspastic recon- sole into the Jeter master detachment at depth. 2001; Goteti et al., 2013; Koning et al., 2013). structions of these two cross sections along the The Silver Creek detachment largely separates The observation of opposing polarities of half- Great Unconformity at the top of Precambrian Paleozoic and Mesozoic rocks in the footwall grabens between the northern and southern parts rocks suggest that LANFs in the subsurface pro- from Santa Fe Group rift-fi ll sediments in the of the Albuquerque basin necessitates the pres- vide a better way to balance upper-crustal brittle hanging wall (Read et al., 2007). ence of an accommodation zone in the central extension with middle-crust ductile extension Albuquerque basin, but its geometry has been across shear zones, an observation that has been Northern Albuquerque Basin and controversial. Originally, this structure was supported by numerous authors (e.g., McKenzie, the Sandia Uplift hypothesized to be a narrow fault zone that was 1978; Wernicke, 1985). The Russell-Snelson a continuation of the Tijeras fault at the south- model is also supported by seismic refl ection The Sandia uplift is the highest elevation ern end of the Sandia uplift (Fig. 3) (Cather, data, which image near-horizontal detachments part of the eastern rift fl anks and is located 1992; Lewis and Baldridge, 1994; May et al., at depth (Russell and Snelson, 1990, 1994). In along the northeastern edge of the Albuquerque 1994; Russell and Snelson, 1994). However, addition, the Grauch and Connell (2013) cross basin. It is composed of Precambrian granitic aeromagnetic surveys of the Albuquerque basin section does not explicitly address strain com- and metamorphic rocks capped by east-dipping did not directly image such a structure (Grauch, patibility issues but presumably would rely on Paleozoic sedimentary rocks (Fig. 3C). The rift- 2001), calling into question its existence. More pervasive, pure shear-dominated middle-crustal bounding structures on the west side of the San- recently, utilizing diverse geophysical data fl ow (McKenzie, 1978). Low-angle normal faults dia uplift include the high-angle Rincon fault sets, Grauch and Connell (2013) suggested that exposed at the surface in the Albuquerque basin and disconnected LANFs at three separate loca- this accommodation zone instead consists of a (Fig. 3) also support the likelihood they are pres- tions (1, 2, and 3 in Fig. 3C). Location 1, at the series of NW-trending anticlinal structures and ent at depth as well and contribute to the overall base of the Sandia uplift in the Juan Tabo area, referred to it as an oblique anticlinal accommo- extension. preserves a small klippe of ~30° east-dipping dation zone, after Faulds and Varga (1998). Mesozoic rocks that are structurally underlain Southern Albuquerque Basin and by Precambrian granite (Fig. 6). Although the METHODS OF STUDY the Ladron Uplift fault is mostly concealed, magnetic surveys plus the geometry of the contact suggest this The goals of this study are to construct evo- The Ladron uplift is a high-elevation promon- is a west-dipping, low-angle fault contact (Van lutionary models of the northern and southern tory that protrudes east along the southwestern Hart, 1999). We refer to this low-angle fault as parts of the Albuquerque basin that explain edge of the Albuquerque basin, at the junction the Juan Tabo detachment fault. At location 2, the observed high- and low-angle normal fault between a relatively narrow Rio Grande rift to Read et al. (1999) mapped a low-angle fault networks. Here we synthesize relationships the north and more distributed extension to the zone that cuts Precambrian granitic rocks. They between basin geometry and fault networks, as south that is similar in style to the Basin and refer to this structure as the Knife Edge fault. well as timing of faulting for producing evo- Range Province. This uplift is bounded on its New mapping, along with fault plane and slick- lutionary (kinematic) models. We constructed western edge by the high-angle, reverse-sense enline measurements along this zone, suggest longitudinal topographic profi les from 10 m Ladron fault, which separates Precambrian gra- it dips ~25°–35° west with top-to-the-west slip digital elevation models to observe variation in nitic and metamorphic rocks in the footwall from (Fig. 6). This fault is aligned with the ridge of rift-fl ank elevation from north to south on each

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A Grauch and Connell (2013) 2 Qts 0 Mz Ts

–2 Pz

–4 Tis Pc Tel Elevation (km) –6

Brittle-ductile –8 transition –10

14% extension (our calculation)

B Russell and Snelson (1994) 2

–2 Ts

–4 Tis –6 Mz Elevation (km) Pz Pc –8 Brittle-ductile transition –10

23% extension (our calculation)

C Qts Pliocene-early Pleistocene synrift deposits (upper Santa Fe Group) Tel Paleogene sedimentary rocks Mz Mesozoic sedimentary rocks Late Oligocene-Miocene synrift deposits Ts (middle and lower Santa Fe Group) Pz Upper Paleozoic sedimentary rocks Pc Proterozoic crystalline rocks Tis Oligocene volcanic and volcaniclastic rocks

Figure 4. (A) East-west cross section across the northern Albuquerque basin (Grauch and Connell, 2013) and our palinspastic reconstruction at the top of the Precambrian rocks. (B) ~East-west cross section across the northern Albuquerque basin (Russell and Snelson, 1994) and a modiﬁ ed version of their palinspastic reconstruction. (C) Explanation of units.

rift fl ank (lines A–A′ and B–B′ in Fig. 3). These Fault geometries and ages were also investi- and geologic cross sections were constructed were compared to longitudinal profi les that were gated along transverse cross-section lines C–C′ from published geologic maps in the northern created along the base of the Santa Fe Group rift and D–D′ through the Sandia and Ladron uplifts (Connell, 2008) and southern (Machette, 1978; fi ll near the eastern and western edges of the rift (Fig. 3). These cross sections are parallel to the DeMoor et al., 2005; Connell and McCraw, which are based on structure contour maps by extension directions as inferred from slicken- 2007; Read et al., 2007) parts of the Albuquer- Grauch and Connell (2013). lines on fault planes. Fault dips were compiled, que basin, as well as new mapping of faults.

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A SW NE

Jeter fault

Precambrian granite

Santa Fe Group

B W E

Santa Fe Group

Precambrian granite

2 m

N S C D 2 WE

2 cm n = 45

Figure 5. (A) View looking northwest of the Ladron uplift, bounded by the low-angle Jeter fault. (B) Jeter fault separating Precam- brian granite in the footwall from Santa Fe Group rift ﬁ ll in the hanging wall. (C) Close-up view of reddish-purple foliated fault gouge that makes up the core of the Jeter fault. Gouge contains pulverized fragments of granite, with well-developed shear bands and sigma clasts (outlined), indicating normal-sense movement. (D) Lower-hemisphere equal-area stereographic projections of fault

planes (plotted as planes) and slickenlines (plotted as lines) for the Jeter fault. S1, S2, and S3 are the maximum, intermediate, and minimum kinematic axes, respectively, from a linked Bingham analysis (Marrett and Allmendinger, 1990; Allmendinger et al., 2012).

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A Pennsylvanian Madera Group

NW SE

Knife Edge fault (Location 2)

Precambrian granite Precambrian metamorphic rocks

Jurassic Morrison Fm (Location 1)

S N B C 2

W R shear E Ranchos fault n = 15

S1, S2, S3 (strain axes)

S1 S3 S Main slip surface 1

Knife Edge fault n = 33 S 2 S1, S2, S3 (strain axes)

D W E

Ranchos fault Santa Fe Group rift fill Cretaceous Mancos Shale

Figure 6. (A) View looking N-NE toward the Sandia uplift, where the Knife Edge fault cuts Precambrian granite in the background. In the foreground, a low-angle fault at Juan Tabo separates Mesozoic rocks from Precambrian rocks. (B) Close-up of the Knife Edge fault looking north. Synthetic R shears form at angles of ~15° from the main shear surface. (C) Lower-hemisphere equal-area stereographic projections of fault planes (plotted as planes) and slickenlines (plotted as lines) for the Knife

Edge fault (in red) and Ranchos fault (in black). S1, S2, and S3 are the maximum, intermediate, and minimum kinematic axes, respectively, from a linked Bingham analysis (Marrett and Allmendinger, 1990; Allmendinger et al., 2012). (D) View looking north of the Ranchos fault, separating Mesozoic rocks in the footwall from Santa Fe Group rift ﬁ ll in the hanging wall.

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We also report new data on timing constraints measured in order to apply an age correction along these structures is more diffi cult than esti- on fault activity in the Sandia and Ladron uplifts (Ft) (Farley et al., 1996). Apatite (U-Th)/He mating the timing of Quaternary faulting. For inferred from apatite fi ssion-track (AFT) and analysis was performed at the Arizona Radio- this study we use these available timing con- apatite (U-Th)/He (AHe) cooling ages. We genic Helium Dating Laboratory at the Univer- straints and report the times of most recent fault build on existing data that includes AFT ages sity of Arizona. Methods for AFT analysis are activity from the U.S. Geological Survey and for the Ladron uplift (Kelley et al., 1992) and described in Kelley et al. (1992). New Mexico Bureau of Geology and Mineral AFT and AHe data for the Sandia uplift (House In total, two existing AFT and AHe samples Resources (2006). et al., 2003) with the ultimate goal of produc- from the Sandia uplift (House et al., 2003), one ing continuous thermal history models of same existing AFT sample from the Ladron uplift MORPHOMETRIC ANALYSIS OF samples, or nearby samples, using both AFT and (Kelley et al., 1992), and three newly-acquired THE ALBUQUERQUE BASIN AHe constraints. The power of using both AFT samples from the Ladron uplift were targeted to and AHe on the same sample is that the tem- produce four new joint AFT and AHe thermal Basin Geometry and Rift-Flank perature sensitivities of AFT (120–60 °C) over- history models from four fault blocks with the Topography lap with those of AHe (90–30 °C), providing the purpose of identifying times of rapid exhuma- ability to jointly invert AFT and AHe data sets to tion associated with slip along rift-bounding Facing rift fl anks on opposite sides of the constrain the sample’s continuous cooling path normal faults. These structural blocks for which Albuquerque basin (Fig. 7A) show roughly through the temperature range ~120 to ~30 °C thermal history models were produced include a mirror image of the other, with the Sandia (e.g., Donelick et al., 2005; Ketcham, 2005; the footwall of the Jeter fault and the fault sliver (3.2 km) and Ladron (2.7 km) uplifts forming Flowers et al., 2009). bounded by the Jeter and Silver Creek detach- the highest rift fl anks in the northern and south- To expand upon the existing thermochrono- ments from the Ladron uplift and both the foot- ern Albuquerque basin, respectively. Geophysi- logic data set, three new samples were col- wall and hanging wall of the Knife Edge fault cal data indicate these uplifts are adjacent to lected from the Ladron uplift for AFT and in the Sandia uplift (Fig. 3). Thermal history the deepest depocenters located in the north- AHe. Apatite grains were picked from sample models were produced utilizing HeFTy version eastern and southwestern portions of the basin. 88LAD-6 in the footwall of the Jeter detach- 1.7.5 (Ketcham, 2005), which incorporates a The northern Albuquerque sub-basin depo- ment for AHe analysis. Two additional samples radiation damage accumulation and annealing center reaches minimum elevations of <–3 km, were collected from the hanging wall of the model (Flowers et al., 2009). In each simula- while the southern Belen sub-basin depocenter Jeter detachment (Fig. 3) (footwall of the Silver tion, 10,000 random paths were generated, with extends to <–2 km elevation (Fig. 3) (Grauch Creek fault). Sample 91LAD-43 provided an resulting envelopes that encompass all “good” and Connell, 2013). Using the elevation differ- AFT age, while JR10-6 yielded suffi cient apa- fi t paths (goodness of fi t >0.5) and all “accept- ence between rift fl anks and base of the Santa tite grains for AHe analysis (Tables 1–3). For able” fi t paths (goodness of fi t >0.05). Fe Group as a fi rst-order proxy for total throw AHe analysis, individual apatite crystals were Intrabasinal faults in the northern and south- along rift-bounding normal faults, we identify selected using a standard binocular micro- ern parts of the Albuquerque basin typically a close spatial association between regions of scope. Suitable grains are suffi ciently large, offset the thin Quaternary alluvium that covers maximum fault throw, highest rift fl anks, and have euhedral crystal shape, and have no vis- much of the Santa Fe Group basin fi ll. Thus, localities of LANFs, as it is in these two loca- ible inclusions. Individual grain lengths were estimating the long-term periods of faulting tions that LANFs are best developed (Fig. 7A).

TABLE 1. APATITE FISSION-TRACK (AFT) AND APATITE (U-Th)/He (AHe) SAMPLE LOCATIONS Elevation Location Sample no. Rock type Data used Latitude Longitude (m) Reference Footwall of Jeter fault 88LAD-6 Granite AFT and AHe 34°25.65′ 107°01.73′ 1829 Kelley et al. (1992); this study Hanging wall of Jeter fault JR10-6 Sandstone AHe 34°23.84′ 107°01.78′ 1750 This study Hanging wall of Jeter fault 91LAD-43 Sandstone AFT 34°23.88′ 107°02.12′ 1756 This study Footwall of Knife Edge fault 99ASC-4 Granite AFT and AHe 35°14.54′ 106°28.00′ 2633 House et al. (2003) Hanging wall of Knife Edge fault 99ASC-7 Granite AFT and AHe 35°14.64′ 106°28.45′ 2499 House et al. (2003)

TABLE 2. APATITE FISSION-TRACK DATA ρ 5 ρ 5 ρ 6 s × 10 i × 10 d × 10 Central age (Ma) Uranium content Mean track length (µm) Sample no. Grains counted (t/cm2) (t/cm2) (t/cm2) (±1 SE) (ppm) (±1 SE) 91LAD-43 9 0.79 (28) 0.88 (156) 1.91 (4600) 59.9 ± 12.4 7 11.2 ± 0.8 (21) ρ ρ ρ Note: s—spontaneous track density; i—induced track density; d—track density in muscovite detector; SE—standard error. Numbers in parentheses are total number of tracks counted. Zeta value is 351 ± 40 based on CN5 glass.

TABLE 3. APATITE (U-Th)/He DATA Radius Mass U Mass Th Mass Sm U Th Sm eU 4He Raw age Corrected age Mean age Sample no. (µm) (ng) (ng) (ng) (ppm) (ppm) (ppm) (ppm) (nmol/g) Ft (±1σ) (Ma) (±1σ) (Ma) (±1σ) (Ma) 88LAD-6 12.4 ± 0.3 a1 62 0.0393 0.0506 0.2665 4 5 27 5 0.267 0.77 9.5 ± 0.2 12.4 ± 0.3 JR10-6 9.6 ± 0.8 a1 62 0.0432 0.0323 0.2160 13 10 67 15 0.733 0.77 8.6 ± 0.4 11.2 ± 0.5 a2 46 0.1323 0.2151 0.3715 100 163 281 138 7.131 0.68 9.5 ± 0.2 13.9 ± 0.3 a3 57 0.0234 0.0730 0.1710 9 29 69 16 0.543 0.74 6.1 ± 0.3 8.3 ± 0.4 a4 49 0.0097 0.0250 0.0527 6 15 33 10 0.179 0.70 3.4 ± 0.3 4.9 ± 0.4

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3000 A Eastern Rift Flank B Sandia Peak A A′ Ladron Peak C–C′ displacement (m) 2600 6000 V.E. ~ 5 6000

Sandias Vertical 4000 Manzanos 4000 Los Pinos 2200 Knife Edge fault 2000 2000 D–D′

Elevation (m) 1800 0 0

Elevation (m) 1400 V.E. ~ 7.5 –2000 Albuquerque sub-basin Jeter fault depocenter –4000 90 B B′

Western Rift Flank displacement (m) 80 Rincon fault 6000 y = 31.47ln(x)r2 = .64 – 19.56 6000 70 Vertical 4000 Sierra Ladrones 4000 60 Knife Edge y = 14.87ln(x) + 20.24 V.E. ~ 5 fault 2000 2000 50 Jeter fault r 2 = 0.49 0 0 40 Ranchos-Juan Tabo

Elevation (m) 30

–2000 Fault dip (degrees) faults Belen sub-basin 20 depocenter Silver Creek fault –4000 10 0 20 40 60 80 100 120 140 0 5 10 15 20 25 30 Horizontal distance (km) 30 25 20 15 10 5 0 Horizontal distance (km)

Figure 7. (A) North-south topographic profi les (solid black lines) along the uplifted shoulders of rift margins. Black dashed line is base of the Santa Fe Group (Grauch and Connell, 2013). Low-angle normal faults (LANFs) are shown as thick red lines along each topographic profi le. Variations in vertical displacement along rift-bounding faults (blue lines) are estimated by subtracting the base of Santa Fe Group from topographic profi les. Locations of topographic profi les shown in Figure 3. (B) Plots of horizontal distance versus elevation (top) and fault dip (bottom), along C–C′ (in blue) and D–D′ (in black). In the bottom plot, the solid black and blue lines are the best-fi t logarithmic functions for each data set. Note that the blue logarithmic function (northern Albuquerque basin) is plotted with × = 0 on the right side of the graph. V.E.—vertical exaggeration.

Fault Geometry are present toward the axis of the basin (Figs. Jeter detachment suggest rapid exhumation 3 and 8A). from ca. 20 to 10 Ma. Long track lengths with In both the northern and southern parts of the In the southern parts of the basin, faults have unimodal histograms suggest that these rocks Albuquerque basin adjacent to the Ladron and somewhat different geometries. Low-angle nor- cooled rapidly at rates of ~12–20 °C/Ma during Sandia uplifts, average fault dip increases from mal fault segments are restricted to the Ladron this time (Kelley et al., 1992). Similarly, AFT the rift ﬂ ank toward the axis of the basin (Figs. uplift, and there is a progression from low to and AHe data in the footwall and hanging wall 7B and 8). In the northern Albuquerque basin, high angle toward the center of the basin. For of the Knife Edge detachment in the Sandia low-angle normal faults (faults at locations 1, example, the Jeter detachment fault dips ~15°– uplift suggest rapid cooling to near-surface 2, and 3 in Fig. 3) are restricted to the Sandia 30°. In the hanging wall of the Jeter detach- temperatures between ca. 22 and 14 Ma (House Mountain front. Based on total offset and rela- ment, the Silver Creek detachment fault has et al., 2003). Track length and age data indi- tion to the high-angle faults nearby, the Ranchos multiple strands, with recorded fault plane dip cate that this structural block cooled at rates of and Juan Tabo detachment faults are nearly measurements of 24° and 48° in the direction ~7–12 °C/Ma during the early Miocene (Kelley coplanar and may have once been part of the toward the axis of the basin (Figs. 7B and 8B; et al., 1992). same fault system but now exist as isolated rem- Read et al., 2007). All other intrabasinal faults in In the Ladron uplift, three new samples were nants that were cut by the high-angle Rincon this region dip >50°, and extension is primarily collected for AFT and AHe analysis (Tables fault (Fig. 3C). accomplished through slip along faults that are 1–3). In the footwall of the Jeter detachment, The Knife Edge detachment fault is most synthetic with the Jeter detachment (Fig. 8B). sample 88LAD-6, with an existing AFT age likely a separate structure from the other two of 14.1 ± 2 Ma (Kelley et al., 1992), yielded a because it separates Precambrian from Pre- Thermochronometric Constraints on single-grain AHe date of 12.4 ± 0.3 Ma (Table cambrian rocks without signiﬁ cant offset of Timing of Fault Slip 3). Two samples were collected from the fault the Great Unconformity. This is supported by sliver bounded by the Jeter detachment and the indistinguishable AFT cooling ages on either Constraints on the timing of main fault activ- Silver Creek fault. Sample 91LAD-43 yielded side of the fault (House et al., 2003). Neverthe- ity associated with the Sandia and Ladron an AFT age of 59.9 ± 12.4 Ma (Table 2), and less, the orientations of the maximum and mini- uplifts is important for producing evolution- sample JR10-6 yielded an average AHe age of mum strain axes calculated for the Knife Edge ary models for these fault systems within the 9.6 ± 0.8 Ma, an average of four single-grain fault are very similar to those calculated for the northern and southern parts of the Albuquerque analyses (Table 3). Ranchos fault to the north (Fig. 6C). All other basin. Existing AFT ages from both locations Joint AFT and AHe thermal history models faults along the cross-section line dip >50° show a roughly linear relationship with eleva- for the footwall of the Jeter detachment fault (Fig. 7B). In addition, much of the extension tion (Fig. 9A) (Kelley et al., 1992; House et al., suggest that it was rapidly exhumed to near- is accommodated through slip along synthetic 2003). In the Ladron uplift, AFT data and track surface temperatures during a major pulse faults, although small-offset antithetic faults length measurements from the footwall of the ~20–10 Ma (Fig. 9B). In contrast, the fault

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A Northern Albuquerque basin Juan Tabo and Ranchos faults Knife Edge fault (Projected) C Zia fault zone East Paradise Rincon fault C′ fault zone Star Heights 5 Coronado-Alameda strand, 5 fault zone Ranchito fault 4 East Heights fault zone 4 3 Eubank Blvd strand, 3 East Heights fault zone

2 2 ELEVATION(KM) 1 1 Tsf 16.5 0 0 –1 –1 –2 Tv –2 Mz Thermochron Sample –3 99ASC-4 –3 ELEVATION (KM) ELEVATION Mz –4 17.2 Thermochron sample used –4 Pz 16.2 –5 99ASC-7PC in HeFTy modeling –5 16.7 –6 17.6 99ASC-4 Sample name –6 15.2 16.4 –7 15.2 AFT age (Ma) –7 –8 Pc 10.6 AHe age (Ma) –8 –9 –9 0 5 10 15 20 25 30 35 40 Horizontal distance (km) Southern Albuquerque basin B D Jeter fault D′ Loma Blanca East Joyita Loma Pelada Rio Puerco 5 fault West Joyita fault 5 fault fault 4 Branches of fault 4 MZ Silver Creek fault 3 P Cliff fault 3

2 2 ELEVATION(KM) 13.4 1 1 Tsf 0 11.2 10.6 88LAD-6 26.8 0 Tv –1 14.1 Mz –1 10.0 JR10-6 Pr 12.4 Pn 24.2 –2 Ladron fault 12.0 9.6 –2 –3 91LAD-43 –3

ELEVATION (KM) ELEVATION –4 59.9 –4 –5 (hanging wall of Jeter fault –5 Jeter fault) –6 PC –6 –7 –7 –8 –8 –9 –9 0 5 10 15 20 25 30 35 40 Horizontal distance (km) C Mesozoic sedimentary Permian sedimentary Tsf Tertiary Santa Fe Group Mz Pr rift fill rocks rocks PC Precambrian granitic and Tertiary volcanic and Paleozoic sedimentary rocks Pennsylvanian sedimentary metamorphic rocks Tv Pz Pn volcaniclastic rocks (undifferentiated) rocks

Figure 8. (A) Cross sections depicting fault geometries in the (A) northern and (B) southern parts of the Albuquerque basin. Cross sections are drawn along lines C–C′ and D–D′ in Figure 3. The northern cross section is modifi ed from Connell (2008). The southern cross section was constructed from surface geology and available geophysical constraints on subsurface geometries from Russell and Snelson (1994) and Grauch and Connell (2013). The ramp–fl at-ramp geometry of the Jeter fault is suggested by the dip of the fault at the surface and the depth at which this fault soles to horizontal from Russell and Snelson (1994). Blue numbers are sample names (as in Table 1); black numbers are apatite fi ssion-track (AFT) ages in Ma; and red numbers are apatite (U-Th)/He (AHe) ages in Ma. Red circles are samples used in thermal history modeling. (C) Explanation of rock units.

sliver that is bounded by the Jeter and Silver The Ladron fault along the western ﬂ ank of (e.g., Callender and Zilinski, 1976; Lewis and Creek detachments (hanging wall of Jeter fault) the Ladron uplift shows reverse-sense move- Baldridge, 1994) suggests to us that the Lad- records two separate pulses of exhumation. Ini- ment (Fig. 8B), and although Lewis and Bal- ron fault was most likely developed during the tial ca. 80–60 Ma exhumation of this fault block dridge (1994) suggest that this structure could Laramide orogeny. If so, then the ca. 80–60 Ma was followed by relative stability for ~50 m.y. A have formed during extension of the Rio Grande period of cooling preserved in the hanging wall second period of uplift ca. 10–5 Ma brought this rift, a long belt of contractional faults along of the Jeter fault may have been accomplished block to near-surface temperatures (Fig. 9B). the western length of the Albuquerque basin through compressional deformation along

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A 3300 3300 likely not exhumed to shallow enough levels in 3100 Ladron uplift 3100 Sandia uplift the crust to have cooled through ~110 °C in the 2900 2900 Laramide. 2700 2700 99ASC-4 2500 2500 Spatial Progression of Fault Slip 2300 88LAD06 2300

Elevaon (m) Elevaon 99ASC-7 2100 91LAD-43 2100 Elevaon (m) Elevaon Figure 10 shows a plot of horizontal distance 1900 1900 along cross-section lines C–C′ and D–D′ versus 1700 1700 fault age. Although adequately constraining the 1500 JR10-6 1500 0204060 80 0 10 20 30 times when faults became active and inactive is Age (Ma) Age (Ma) challenging, here we use available constraints Sample used in AHe AFT on faults in the Albuquerque Basin from ther- thermal history modeling mal history models of different fault blocks as B 0 well as latest movements reported for Quater- 20 Footwall of Jeter fault 20 Footwall of Knife nary faults in the Albuquerque basin. In both Edge fault 40 40 the northern and southern Albuquerque basin, Sample 88LAD06 Sample 99ASC-4 60 60 10,000 paths 10,000 paths intrabasinal faults typically record Quaternary 80 80 932 acc 154 acc displacements (U.S. Geological Survey and 100 226 good 100 46 good 120 120 New Mexico Bureau of Geology and Mineral 140 140 Resources, 2006), including the Rincon fault, Temperature (°C) Temperature Temperature (°C) Temperature 160 160 with a probable latest rupture event younger 180 180 than ca. 5 ka (Connell, 1995). Combining avail- 200 200 0 0 able age constraints on different faults, the early- 20 Hanging wall of 20 Hanging wall of Knife formed faults seem restricted to the Sandia and Edge fault 40 Jeter fault 40 Ladron uplifts. These faults were subsequently Sample 99ASC-7 60 60 abandoned, and intrabasinal faults typically cut 80 80 10,000 paths 100 100 287 good Quaternary units (Fig. 10). This observation 120 Samples 91LAD-43 120 72 good is supported by the presence of long, narrow 140 and JR10-6 140 structural benches preserved along the fl anks Temperature (°C) Temperature 10,000 paths 160 160 41 acc of the basin forming an extensional imbricate 180 180 7 good zone (Chapin and Cather, 1994). An important 200 200 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 difference, however, is that in the Sandia region Time (Ma) Time (Ma) young faults tend to cut earlier-formed LANFs, whereas in the southern part of the basin there is Figure 9. (A) Apatite fi ssion-track (AFT) and apatite (U-Th)/He a more systematic progression of younger syn- (AHe) age-elevation plots. Apatite fi ssion-track data from the Ladron thetic faults forming basinward through time. uplift (except 91LAD-43) are from Kelley et al. (1992). All AFT and These observations are in good agreement AHe data from the Sandia uplift are from House et al. (2003). Note with previous studies in the southwest Basin and the different x-axis scales. (B) Apatite fi ssion-track and AHe joint Range that also document basinward migrations inversion thermal history models of samples within the Ladron and in faulting through time (e.g., Axen et al., 1999; Sandia rift-fl ank uplifts. Black boxes are initial time-temperature Spelz et al., 2008; Fletcher and Spelz, 2009). constraints; blue line is the weighted mean time-temperature path; These studies used a geomorphic approach of gray bands highlight the time interval from 20 to 10 Ma. intrabasinal Quaternary scarp-forming faults to help determine the relative ages of scarp-forming earthquakes. Although beyond the scope of the Ladron fault. The ca. 20–10 Ma period of face ca. 20–12 Ma (Fig. 9). This suggests that this paper, future work on Quaternary intraba- cooling preserved in the footwall of the Jeter movement along the Knife Edge detachment is sinal faults in the Albuquerque basin would be detachment most likely represents erosional relatively minor and that unroofi ng may have useful to further test the hypothesis that faulting exhumation during a time of rapid slip along the taken place on the larger-displacement Juan migrated basinward through time, which would Jeter fault. Finally, the most recent ca. 10–5 Ma Tabo–Ranchos detachment system during this support a rolling-hinge mechanism for forma- cooling in the hanging wall of the Jeter fault time. The Juan Tabo fault, which juxtaposes tion of fault networks. occurred after main slip along the Jeter fault and Triassic and Jurassic rocks against Precambrian is interpreted to record normal-sense slip along granite, has accumulated more than 3 km of DISCUSSION the Silver Creek detachment to further exhume stratigraphic displacement (Van Hart, 1999). this fault sliver to the surface. Low-temperature thermochronometers used Kinematic Evolution of Rift-Flank Uplifts In the northern Albuquerque basin, thermal in this study do not detect any Laramide-age in the Albuquerque Basin history models of the footwall and hanging cooling in the Sandia uplift, but previous work wall of the Knife Edge detachment fault both suggests that the Sandia uplift was affected by Synthesizing observations on topography, record similar cooling histories, where both Laramide compression (Karlstrom et al., 1999), basin depth, fault geometry, and fault ages, we fault blocks were exhumed to the near sur- such that Precambrian granitic rocks were most propose a model for LANFs in the Albuquerque

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25 C Ranchos-Juan Tabo faults C′ deformation associated with the Jeter fault. Northern Albuquerque basin 20–12 Ma 20 Zia fault zone However, microtextural analysis of rocks col- <0.75 Ma lected from this zone suggests that movement 15 Star Heights Eubank Blvd strand Knife Edge fault along this shear zone was reverse sense (Fig. fault zone East Paradise <0.75 Ma Pre 20–15 Ma 10 <0.75 Ma fault zone Coronado-Alameda strand 11). The Jeter detachment, therefore, is most <0.13 Ma <0.75 Ma likely a brittle fault that offsets rocks that have 5 Rincon fault Fault age (Ma) <5 ka undergone an older (Precambrian?) ductile 0 strain. In addition, Lewis and Baldridge (1994) 25 suggested that the Jeter fault had an original dip D Southern Albuquerque basin D′ 20 Silver Creek fault of ~48° based on the cutoff angle between the 10–5 Ma Jeter fault and Paleozoic beds along the western 15 Jeter fault Loma Blanca fault edge of the Ladron uplift. Pre 20–10 Ma <0.13 Ma 10 Cliff fault In our model, the basin-bounding Jeter fault Rio Puerco fault <0.13 Ma 5 <0.75 Ma (pre–ca. 20 Ma) initiated as a moderately high- Fault age (Ma) Loma Pelada fault angle (~48°) structure that inherited an earlier <0.13 Ma 0 fabric in a region that had experienced Laramide 051015 20 25 30 contraction (Figs. 12A–12C). Continued exten- Horizontal distance (km) sion caused the Jeter fault to rebound isostatically due to the lateral removal of hanging-wall Figure 10. Plots of horizontal distance versus fault age along C–C′ and D–D′ for the north- material. As discussed previously, the low-angle ern and southern Albuquerque basin. Dotted lines indicate possible continuation of faulting. Jeter fault lies along the southern fl ank of the For Quaternary faults, dotted lines are conservative estimates that these faults have been basin depocenter (Fig. 7). We interpret this to active since at least the late Miocene–Pliocene. Red lines highlight the inferred abandon- be a potential consequence of isostatic foot- ment of faulting in rift-fl ank uplifts and continuation of faulting toward the axis of the wall uplift, as this would elevate portions of basin. Quaternary fault youngest slip events are from U.S. Geological Survey and New the deep basin to shallower levels in regions Mexico Bureau of Geology and Mineral Resources (2006). where extension is greatest. After ca. 10 Ma, the Jeter detachment is interpreted to have become inactive because it was unfavorably oriented for basin that applies previous models developed ited from ca. 16 to 8 Ma. Thermal history models continued slip, and the new high-angle Silver for highly extended terranes, where isostatic suggest that the hanging wall of the Jeter fault, Creek fault formed basinward to facilitate con- uplift is a main driving force that rotates high- which preserves Laramide-age exhumation, tinued extension (Fig. 12D). The upwarping of angle faults to shallower angles via a rolling- was further exhumed to the surface ca. 10–5 Ma the abandoned Jeter detachment during progres- hinge mechanism (Spencer, 1984; Buck, 1988; (Fig. 9B), which we interpret to refl ect a main sive slip exposed the Precambrian-cored Ladron Wernicke and Axen, 1988; Wdowinski and period of slip along the Silver Creek fault. uplift as a rift salient, and continued slip began to Axen, 1992; Axen and Bartley, 1997). In the A several-meter-thick belt of mylonitic rocks rotate strands of the Silver Creek fault to lower Albuquerque basin, isostatic uplift of footwall is preserved in the Precambrian granitic rocks angles as younger intrabasinal faults emerged blocks has been proposed for the Sandia uplift immediately adjacent to the Jeter detachment (Fig. 12E). The observed abandonment of fault- (May et al., 1994; Roy et al., 1999) and the (Read et al., 2007) suggesting possible ductile ing in the rift-fl ank uplift and the continuation Ladron uplift (Wernicke and Axen, 1988; Lewis and Baldridge, 1994). Our model expands on these previous studies and expands on the recent WE synthesis by Grauch and Connell (2013) by Jeter fault attempting to integrate the observed high- and low-angle faults, geophysical data, and thermochronometric data into a robust kinematic model for the central Rio Grande rift.

Southern Albuquerque Basin Following initial uplift ca. 80–60 Ma during the Laramide orogeny, initial extension in the Long axis C of quartz grains Albuquerque basin began during the Oligocene C S (Chapin and Cather, 1994). Continued, rapid S S exhumation captured by AFT and AHe tech- C niques began prior to 20 Ma, with major slip 1 mm 1 mm concentrated along the Jeter fault. This main period of extension is supported by thermal history models (Fig. 9), as well as by thick syn- Figure 11. Microphotographs of Precambrian mylonitic rocks from the footwall of the Jeter tectonic rift-ﬁ ll sections (Fig 7A); Grauch and fault. The left photograph displays S-C fabrics, and the right photograph displays grain Connell (2013) estimate that more than half the shape preferred orientation, both of which indicate reverse shear sense when oriented with thickness of Santa Fe Group rift ﬁ ll was depos- respect to the Jeter fault.

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A B Laramide-age contraction of the upper crust along the Ladron fault 0 Mz 0 Mz 1 JR10-6 and Pr 1 Pr 91LAD-43 2 Pn 2 Pn 3 88LAD-6 Precambrian 3 59.9 Ma AFT PC PC 4 shear zone 4 kilometers 5 Pre ca. 80 Ma 5 Ladron fault 6 kilometers 6 7 ca. 80–40 Ma 8 C ~48° Jeter fault 0 Incipient Silver Creek fault 1 Tsf Tv 2 Mz 3 Pr Pre 18–10 Ma Pn 4

5 14.1 Ma AFT Oligocene volcanic rocks are deposited across PC 6 12.4 Ma AHe the region. Main slip along the Jeter fault occurs kilometers 7 Ladron fault at this time, which produces initial isostatic uplift 8 of the Ladron block. 9 Early Miocene brittle-ductile transition 10

D Jeter fault Silver Creek fault 0 Incipient intrabasinal fault 1 Tsf Tv 10–5 Ma 2 Mz 3 9.6 Ma Pr 4 AHe Pn Main slip is transferred basinward to the 5 Silver Creek fault. The Jeter fault continues 6 Ladron fault PC to rotate to shallower angles due to kilometers 7 isostatic rebound. 8 9 10 11 The Jeter and Silver Creek faults become inactive as E Jeter fault intrabasinal faults emerge to facilitate continued extension. 5–0 Ma Loma Pelada 5 5 fault Loma Blanca East Joyita Branches of West Joyita 4 fault Rio Puerco fault 4 Silver Creek fault fault fault 3 Cliff fault 3

2 2 ELE 1 1 Tsf VATION(KM) 0 0 PCpC Tv –1 Mz –1 Pr Pn –2 –2 Ladron fault –3 –3

ELEVATION (KM) ELEVATION –4 pC –4 –5 PC –5 –6 Jeter fault –6 –7 –7 Modern brittle-ductile transition –8 –8 –9 –9 0 5 10 15 20 25 30 35 40 Horizontal distance (km) Figure 12. Kinematic evolution of the southern part of the Albuquerque basin. The yellow dots are the approximate locations of apatite ﬁ ssion-track (AFT) and apatite (U-Th)/He (AHe) samples for which thermal history models were produced (Fig. 9). Blue numbers are sample names (as in Table 1); black numbers are AFT ages in Ma; red numbers are AHe ages in Ma. Rock units are as in Figure 8.

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of faulting within the axis of the basin (Fig. to shallower angles, and slip along them dimin- astheno spheric circulation (McMillan et al., 10) are characteristic features of an operative ished, new intrabasinal faults facilitated con- 2000). Additional minor magmatic centers are rolling-hinge mechanism in the southern part of tinued extension in the northern Albuquerque found east of the Sandia Mountains (Fig. 3), the Albuquerque basin and suggest that LANFs basin, including the Rincon fault, which dis- including the Ortiz belt, active from ca. 36 to have been important contributors to the overall sected the Ranchos–Juan Tabo detachment fault 27 Ma, which consists of at least 12 individual extension and are most likely preserved at depth system (Fig. 13E). As in the southern Albuquer- laccoliths (Maynard, 2005). as they are at the surface in the Ladron uplift. que basin, the abandonment of faulting in the By ca. 21 Ma, regional volcanism and mag- rift-fl ank uplift and the continuation of faulting matism had elevated the heat fl ow in much of Northern Albuquerque Basin within the axis of the basin suggest a rolling- New Mexico and southern Colorado (Kelley , Thermal history models generated for the hinge mechanism for the development of fault 2002). These magmatic centers also likely Sandia uplift, along with current fault geom- networks that accommodate extension. heated the crust for several million years after etry, differ slightly from the Ladron block to The klippe of Mesozoic rocks (location 1 in their emplacement. Continued Neogene heat the south. In the Sandia uplift, both the hanging Fig. 3C) has long been a conundrum in the evo- fl ow is exemplifi ed by modern-day heat-fl ow wall and footwall of the Knife Edge detachment lutionary history of the Sandia Mountains (e.g., measurements above the Socorro magma body fault were exhumed at about the same time, ca. Read et al., 1944; Kelley and Northrup, 1975; (Fig. 3), which are higher than the heat fl ow 20–12 Ma (Fig. 9). This suggests no signifi cant Van Hart, 1999). The proposed evolutionary estimated from a basaltic melt of its size (Reiter offset along that structure after this time (House history outlined here provides a viable expla- et al., 2010). The calculations by Reiter et al. et al., 2003) and that another major fault was nation of its development. Initially, the Meso- (2010) suggest that the measured heat fl ow is responsible for the exhumation of the Sandia zoic klippe formed the hanging wall of a rela- residual from now-solidifi ed magmatic intru- block toward the surface. We suggest that main tively high-angle normal fault that accumulated sions that were emplaced ca. 1–3 Ma. Similarly, exhumation was related to the Ranchos–Juan enough displacement to place it against Precam- activity in magmatic and volcanic centers in Tabo detachment system. In addition, published brian granitic rocks. As slip progressed along southwestern New Mexico ended ca. 21 Ma, yet cross sections of the northern Albuquerque basin this fault as well as others, the fault plane rotated residual heat fl ow likely elevated temperatures display steeper faults offsetting older, slightly and was eventually dissected by younger faults. in the crust for several million years after vol- shallower normal faults (e.g., Connell, 2008). A The preserved structures thus require an interac- canic and magmatic activity ceased. Based on new aspect of our model is that the Sandia block tion of both high- and low-angle fault segments. thermal history models of the Sandia and Lad- preserves at least three now-separate LANF seg- ron uplifts, rapid extension along early-formed ments, with segments of the once-continuous Elevated Heat Flow faults began prior to ca. 20 Ma in the northern LANF system preserved between younger high- and southern parts of the basin. These times of angle normal faults. Our evolutionary model for the central Rio rapid extension coincide with times of elevated These observations lead to a kinematic model Grande rift recognizes synchronous slip on heat fl ow in much of New Mexico. In the Sandia for the northern Albuquerque basin in which opposite mirrored active rift fl anks and devel- region, House et al. (2003) use AFT data, unit early-formed normal faults most likely emerged opment of a broad accommodation zone begin- thicknesses, and thermal conductivity values as relatively high-angle (~50°–70°) structures ning ca. 20 Ma. In addition, we perceive a link for different rock types to estimate geothermal (Fig. 13). Instead of being concentrated within between LANF development and morphometry gradients. Their calculations indicate that at the a single narrow zone, the strain was instead par- of both rift fl anks and basin depth. This section end of Oligocene magmatic and volcanic activ- titioned among several different structures (Fig. seeks to understand the evolution of LANF sys- ity, heat fl ow in this region was ~105 mW/m2, 13C). As extension continued, faults began to tems in the southern and northern parts of the corresponding to a geothermal gradient in the rotate to shallower angles. Fault rotation in this Albuquerque basin in terms of elevated heat upper crust of ~38 °C/km, compared to a cal- region was likely predominantly due to isostatic fl ow that existed at the end of the Oligocene. culated modern-day heat fl ow of ~80 mW/m2, rebound of the footwall block (Fig. 13D). Flex- Widespread intermediate to silicic compo- corresponding to a modern geothermal gradient ural modeling using fi nite-element code involv- sition eruptions affected much of the western of ~29 °C/km (House et al., 2003). Due to its ing joint-inversion of topography and gravity United States and Mexico during the Eocene– close proximity to the Mogollon-Datil volcanic suggests that ~18% extension of the northern Oligocene (e.g., Elston, 1984; Lipman, 1992; fi eld, it is possible that heat fl ow in the southern Albuquerque basin near the Sandia Mountains Farmer et al., 2008). In Colorado and New Rio Grande rift may have even been higher than would result in isostatic uplift that is suffi cient Mexico, volcanic and magmatic activity was in the Sandia uplift. to produce the modern rift-fl ank geometry and mainly centered in two major centers, the In the Ladron uplift, our model suggests the observed ~15° east dip of Paleozoic sedi- San Juan volcanic fi eld in southwestern Colo- that rapid slip along the Jeter fault led to iso- ments on the eastern side of the Sandias (Roy rado and the Mogollon-Datil volcanic fi eld in static uplift, rotation of early-formed faults et al., 1999). In addition, LANF segments are southwestern New Mexico (Figs. 1 and 3). The to shallower angle, and the emergence of new found above the northern fl ank of the deepest Mogollon-Datil volcanic fi eld, active from ca. 40 intrabasinal faults at a high angle through a roll- depocenter in the Albuquerque basin (Fig. 7), to 21 Ma, covers ~40,000 km2, produced more ing-hinge mechanism. Similar mechanisms of similar to relationships in the southern parts than ten calderas in southwestern New Mexico, formation for LANFs have been widely applied of the basin. We again suggest this may be a and is surrounded by a large apron of ignim- to explain the formation of core complexes in potential consequence of isostatic basin uplift in brites and lavas (Chapin et al., 2004a). Vol canic regions of large magnitude (>100%) extension, a region of maximum extension, which would and magmatic rocks in the Mogollon-Datil vol- such as the Basin and Range Province (e.g., uplift the basin depocenter to shallower levels canic fi eld are thought to have been derived Spencer, 1984; Wernicke and Axen, 1988; Axen as it rotated early-formed normal faults to shal- from lithospheric sources, although the ultimate and Bartley, 1997; Lavier et al., 1999; Fletcher low levels. As the early-formed Ranchos, Knife cause of melting may have been the foundering and Spelz, 2009), along mid-ocean ridges (e.g., Edge, and Juan Tabo detachment faults rotated of the underlying Farallon slab, which caused Ohara et al., 2001; Okino et al., 2004; Escartín

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A Laramide-age contraction of the upper crust B 0 0 Mz 1 Mz

kilometers 1

2 kilometers 2 Pz 3 Pz 3 99ASC-7 4 99ASC-4 4 5 PC Pre ca. 80 Ma PC ca. 80–40 Ma 5 6 6 Ranchos fault Juan Tabo fault Knife Edge fault C Pre 20–12 Ma 0 Tv Mz 1 Oligocene sedimentary and volcanic rocks fill 2 early-formed basin. Multiple high-angle normal Pz 3 kilometers faults emerge during initial development of 4 the northern Albuquerque basin. Geotherms are 5 still elevated enough to allow minor isostatic PC 6 uplift, which begins to rotate faults slightly. 7 Early Miocene brittle-ductile transition 8 Ranchos fault Juan Tabo fault D Incipient intrabasinal faults Knife Edge fault 0 ca. 12–0 Ma 1 Tsf 2 Tv kilometers Extension continues as 3 16.7 Ma AFT 4 geotherms return to normal, Mz 16.4 Ma AHe 5 and faults rotate primarily 17.6 Ma AFT Pz PC 6 by fault-block rotation 15.2 Ma AHe 7 8

The Knife Edge, Ranchos, and Juan Tabo faults Juan Tabo fault Present geometry are further rotated to their present orientations (Projected) Knife Edge fault Rincon fault E Zia fault zone Ranchito fault 5 Star Heights East Paradise Coronado-Alameda strand, 5 4 fault zone fault zone East Heights fault zone 4 3 Eubank Blvd strand, 3 2 East Heights fault zone 2

1 1 ELEVATION(KM) Tsf 0 0 –1 –1 Tv –2 Tv –2 –3 Mz –3 ELEVATION (KM) ELEVATION MzMz –4 –4 Pz –5 PC –5 Pc –6 –6 –7 –7 –8 –8 Modern brittle-ductile transition –9 –9 0 5 10 15 20 25 30 35 40 Horizontal distance (km)

Figure 13. Kinematic evolution of the northern part of the Albuquerque basin. The yellow dots are the approximate locations of apatite ﬁ ssion-track (AFT) and apatite (U-Th)/He (AHe) samples for which thermal history models were produced (Fig. 9). Blue numbers are sample names (as in Table 1); black numbers are AFT ages in Ma; red numbers are AHe ages in Ma. Rock units are as in Figure 8.

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et al., 2008; Smith et al., 2012), as well as in rolling-hinge mechanism suggests that LANFs 1992; Chapin and Cather, 1994; Landman and regions undergoing orogenic collapse such as exposed in other parts of the rift (Fig. 1) may Flowers, 2013; Ricketts et al., 2014b). These Tibet (Chen et al., 1990; Pan and Kidd, 1992; have also formed through a similar mechanism. observations on the timing of extension in the Harrison et al., 1995; Kapp et al., 2000, 2008). For example, Blanca Peak in southern Colorado narrow Rio Grande rift differ from the sweeps of As discussed previously, the development of forms a salient into the San Luis basin of the Rio extension and volcanism in the highly extended core complexes such as these requires hot Grande rift that is cored by Precambrian crys- Basin and Range Province, suggesting the pos- crustal conditions to expose ductilely-deformed talline rocks, similar to the Sandia and Ladron sibility of slightly different driving mechanisms rocks at the Earth’s surface. Deformation along uplifts of the Albuquerque basin. Blanca Peak is causing extension (e.g., Ricketts et al., 2014b). the Jeter detachment fault is entirely brittle, also bounded along its western edge by a system Nevertheless, despite these differences, it although fault ages and geometries are consis- of west-dipping LANFs (Jones, 1991; Benson, appears that in both domains LANFs are devel- tent with early stages of a core-complex evo- 1997), raising the possibility that fault networks oped through a rolling-hinge mechanism, and lutionary model. The high-elevation footwall, in this vicinity of the rift may also represent early if extension progressed in the Rio Grande rift, rift promontory aspect, and domal character stages in the development of a core complex. it would eventually result in the formation of of a footwall core are all similar to core com- Fully developed metamorphic core com- a metamorphic core complex similar to those plex geometries seen in high-extension regions plexes are found within a large belt of the highly observed in the Basin and Range Province. (e.g., Spencer, 1984). Thus we propose that the extended Basin and Range Province (Fig. 1), Ladron uplift is an example of an embryonic but the recognition of a rolling-hinge mecha- CONCLUSIONS core complex and that a combination of insuf- nism operating within the Rio Grande rift at a fi cient extensional strain and heat fl ow follow- lesser scale offers an opportunity to learn about Our work sheds light on early stages in the ing late Oligocene magmatic and volcanic activ- the early stages in the development of a core development of LANF systems in regions of ity inhibited the development of a mature core complex. In the Albuquerque basin, the Sandia elevated heat fl ow and leads to the following complex exposing ductilely-deformed rocks. and Ladron uplifts represent the beginnings of general conclusions. (1) The Albuquerque basin In the Sandia uplift, the style of extension a domal footwall, which, in mature core com- evolved into a narrow, low-extension (~17%– is very similar to that observed in the southern plexes, is generally thought to form through 35%) setting characterized by an abandonment Albuquerque basin, where older faults are rotated buoyancy forces due to removal of hanging-wall of faults within rift fl anks through a rolling-hinge to shallow angles as extension progresses. The material (e.g., Spencer, 1984; Buck, 1988; Wer- mechanism, 10-km-scale salients cored by base- Knife Edge fault accumulated relatively minor nicke and Axen, 1988). Low-angle normal faults ment uplifts that form the highest parts of the amounts of slip, and unroofi ng of the Sandia in the Albuquerque basin are also entirely brittle , rift fl anks, broad accommodation zones separat- block was most likely accomplished via slip in contrast to LANFs associated with mature ing deep sub-basins, and, at least in this case, along the Juan Tabo–Ranchos detachment fault core complexes, which can accumulate up to simultaneous slip on opposite sides of the rift. system. In this part of the basin, it is clear that 50 km of slip and expose a thick (0.1–3 km) (2) We support previous work suggesting that younger faults (i.e., Rincon fault) crosscut older ductile shear zone (e.g., Axen, 2004). Thus, at core complexes typically form in regions of hot LANFs, suggesting that perhaps fault rotation least in the case of the Albuquerque basin, initial crust where decoupling between the upper brit- may have been due to a combination of fault doming of the footwall and signifi cant rotation tle crust and lower ductile crust localizes strain. block rotation coupled with isostatic uplift. of early-formed normal faults occurred during (3) In the southern Albuquerque basin, elevated the narrow rift stage represented by the Albu- heat fl ow and extension magnitude were insuf- Regional Implications for Extension in the querque basin and before exposure of the brittle- fi cient to lead to a mature core complex where Rio Grande Rift and Basin and Range ductile transition at the surface. ductilely-deformed rocks are preserved in the As seen in Figure 1, earliest extension associ- footwalls of large-offset LANFs. (4) Low-angle The results of this study emphasize that an ated with core complexes in the Basin and Range normal faults in the northern Albuquerque basin interaction between high- and low-angle nor- occurred in the north and swept southward may have formed dominantly through isostatic mal faults resulted in the present-day geometry through time, while a similar sweep of extension footwall uplift. Thus, we show that fundamental of the Albuquerque basin. The hypothesis that began in northern Mexico and propagated north. processes that are required to produce core com- these structures formed through a rolling-hinge These trends closely followed similar sweeps plexes (both oceanic and continental) operated mechanism provides a framework for how to in volcanism, although extension was generally to moderate degrees within the Rio Grande rift interpret the current geometry of the Albuquer- postponed by several million years following from ca. 25 to 5 Ma. These regions, therefore, que basin (Grauch and Connell, 2013) within volcanism (e.g., Axen et al., 1993; Humphreys, where LANFs make up a small percentage of the context of its structural and sedimentologi- 1995, 2009; Dickinson, 2009). These space- the total exposed fault population, are important cal evolution from the mid-Miocene to the pres- time relationships are generally attributed to and under-utilized natural laboratories for docu- ent. We build on previous models emphasiz- removal of the underlying Farallon plate from menting the sequential stages in development of ing possible rotation of LANFs in the Ladron the base of North America in some fashion fol- highly extended core complexes and provide a and Sandia uplifts (e.g., Lewis and Baldridge, lowing the Laramide orogeny (e.g., Coney and new perspective on the importance of LANFs in 1994; May et al., 1994) and incorporate the Reynolds, 1977; Atwater, 1989; Chapin et al., narrow continental rifts. most detailed geophysical observations of the 2004b). In contrast, while earliest extension in ACKNOWLEDGMENTS subsurface (Grauch and Connell, 2013) to illus- the Rio Grande rift began ca. 36–37 Ma (Kelley trate that mechanisms that operate in the highly and Chamberlin, 2012), the ca. 20–10 Ma time Partial funding came from the New Mexico Geo- extended Basin and Range Province also oper- periods of rapid cooling in the Sandia and Lad- logical Society, New Mexico Statemap program, the Alfred P. Sloan Foundation, and the University of ated to a lesser degree in the Rio Grande rift. The ron uplifts are similar to previous studies of the New Mexico for JWR, and grants EAR-0607808, observation that several of these LANFs in the rift that document synchronous extension along 0838575, and 1119629 for KEK. We thank Dirk Van Albuquerque basin may have formed through a its length during the mid-Miocene (Kelley et al., Hart, Gary Axen, Peter Reiners, Tien Grauch, and an

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