A Shifting Rift—Geophysical Insights Into the Evolution of Rio Grande Rift Margins and the Embudo Transfer Zone Near Taos, GEOSPHERE; V

Home , Amalia Tuff, Hinsdale Formation, Hondo Group, Jemez Lineament, Latir volcanic field, Los Pinos Formation

Research Paper

GEOSPHERE A shifting rift—Geophysical insights into the evolution of Rio Grande rift margins and the Embudo transfer zone near Taos, GEOSPHERE; v. 13, no. 3

doi:10.1130/GES01425.1 New Mexico V.J.S. Grauch1, Paul W. Bauer2,*, Benjamin J. Drenth1,*, and Keith I. Kelson3,* 16 figures; 3 tables 1U.S. Geological Survey, MS 964, Federal Center, Denver, Colorado 80225, USA 2New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA CORRESPONDENCE: tien@usgs.gov 3U.S. Army Corps of Engineers, South Pacific Division Dam Safety Production Center, 1325 J Street, Sacramento, California 95814, USA

CITATION: Grauch, V.J.S., Bauer, P.W., Drenth, B.J., and Kelson, K.I., 2017, A shifting rift—Geophysical insights into the evolution of Rio Grande rift margins ABSTRACT the end of Laramide time formed the first west-down master faults. The Em and the Embudo transfer zone near Taos, New Mex‑ budo fault may have initiated in early Miocene southwest of the Taos region. ico: Geosphere, v. 13, no. 3, p. 870–910, doi:10 .1130 We present a detailed example of how a subbasin develops adjacent to Normal-oblique slip on these early fault strands likely transitioned in space /GES01425.1. a transfer zone in the Rio Grande rift. The Embudo transfer zone in the Rio and time to dominantly left-lateral slip as the Embudo fault propagated to the Grande rift is considered one of the classic examples and has been used northeast. During and shortly after eruption of Servilleta Basalt, proto-Embudo Received 7 September 2016 Revision received 23 December 2016 as the inspiration for several theoretical models. Despite this attention, the fault strands were active along and parallel to the modern, NE-aligned Rio Accepted 17 February 2017 history of its development into a major rift structure is poorly known along Pueblo de Taos, ~4–7 km basinward of the modern, mapped Embudo fault Published online 7 April 2017 its northern extent near Taos, New Mexico. Geologic evidence for all but its zone. Faults along the northeastern subbasin margin had northwest strikes for young rift history is concealed under Quaternary cover. We focus on under most of the period of subbasin formation and were located ~5–7 km basinward standing the pre-Quaternary evidence that is in the subsurface by integrat of the modern Sangre de Cristo fault. The locus of fault activity shifted to more ing diverse pieces of geologic and geophysical information. As a result, we northerly striking faults within 2 km of the modern range front sometime after present a substantively new understanding of the tectonic configuration Servilleta volcanism had ceased. The northerly faults may have linked with and evolution of the northern extent of the Embudo fault and its adjacent the northeasterly proto-Embudo faults at this time, concurrent with the de subbasin. velopment of N-striking Los Cordovas normal faults within the interior of the We integrate geophysical, borehole, and geologic information to interpret subbasin. By middle Pleistocene(?) time, the Los Cordovas faults had become the subsurface configuration of the rift margins formed by the Embudo and inactive, and the linked Embudo–Sangre de Cristo fault system migrated to Sangre de Cristo faults and the geometry of the subbasin within the Taos em the south, to the modern range front. bayment. Key features interpreted include (1) an imperfect D-shaped subbasin that slopes to the east and southeast, with the deepest point ~2 km below the valley floor located northwest of Taos at ~36° 26′N latitude and 105° 37′W INTRODUCTION longitude; (2) a concealed Embudo fault system that extends as much as 7 km OLD G wider than is mapped at the surface, wherein fault strands disrupt or truncate The Neogene Rio Grande rift forms a series of north-south elongated struc- flows of Pliocene Servilleta Basalt and step down into the subbasin with a tural basins that extend from Mexico to northern Colorado (inset, Fig. 1). The minimum of 1.8 km of vertical displacement; and (3) a similar, wider than ex basins are characterized as one or more asymmetric half-grabens that gener- pected (5–7 km) zone of stepped, west-down normal faults associated with ally tilt toward northerly striking master faults, with the polarities of the tilts OPEN ACCESS the Sangre de Cristo range front fault. varying between basins of the rift (Chapin and Cather, 1994). In northern New From the geophysical interpretations and subsurface models, we infer Mexico, the rift basins make significant right steps. In addition, tilt directions relations between faulting and flows of Pliocene Servilleta Basalt and older, and associated master faults of the half-grabens vary from north to south, buried basaltic rocks that, combined with geologic mapping, suggest a re from east-tilted in the Albuquerque Basin, to west- to northwest-tilted in the vised rift history involving shifts in the locus of fault activity as the Taos sub Española Basin, and again east-tilted in the San Luis Basin. basin developed. We speculate that faults related to north-striking grabens at The northeast-striking, left-oblique Embudo and related faults have long This paper is published under the terms of the been recognized as the mechanism for accommodating transfer of strain be- CC‑BY license. *E-mail: bauer@nmbg.nmt.edu, bdrenth@usgs.gov, keith.i.kelson@usace.army.mil tween the oppositely tilted half-grabens of the Española and San Luis Basins

107°W106°W San Luis Basin 105°W San Juan 37°N COLORADO V.F. EXPLANATION Taos NEW MEXICO

Miocene and younger Tusas Mtns volcanic rocks Plateau SdC Latir V.F. Sedimentary fill in Rio Volcanic Study Grande Rift SdC Area Middle Tertiary volcanic Field rocks Paleozoic sedimentary Figure 1. Regional geologic setting of the Taos rocks study area. The Embudo fault system m Sangre de Cristo Mtns yysste transfers strain between the westerly dip Proterozoic rocks lt s ping Española Basin on the south to the

u t

aau t easterly dipping San Luis Basin on the l

Major rift fault, ball on o f l u ud u north. Inset shows the map area with re a b f down-thrown side spect to basins (yellow) of the Rio Grande m RIO GRANDE RIFT E Española s rift. Modified from Lipman (1983) and 36°N o c Keller and Baldridge (1999). Jemez lin t Jemez basin e eament from Aldrich (1986). PM—Picuris P San Luis - COLORADO s Basin Mountains; SdC—Sangre de Cristo fault;

V.F. i

r r

u u V.F.—volcanic field.

c i Santa P

Pajarito faultfaul

Española Basin Albuquerque Basin Albuquerque JEMEZ LINEAMENT NEW basin 02550 MEXICO

Kilometers TEXAS Albuquerque MEXICO 35°N

(Fig. 1; Muehlberger, 1979; Aldrich, 1986; Chapin and Cather, 1994; Faulds and where Miocene basin-fill units (Santa Fe Group) are well exposed (Manley, Varga, 1998). They link the two master faults of the Española and San Luis 1978; Muehlberger, 1979; Dungan et al., 1984; Aldrich, 1986; Ingersoll et al., Basins, the east-dipping Pajarito fault in the Española Basin and the west-dip- 1990; Aby and Koning, 2004; Bauer and Kelson, 2004a; Koning et al., 2004; ping Sangre de Cristo fault in the San Luis Basin into one active fault system Koning et al., 2013). Sedimentological evidence for the late Miocene uplift of (Kelson et al., 2004a). The linked fault system is considered an archetype of the flanking Picuris Mountains has been used to signal the onset of activity on an antithetic transfer zone, which links normal faults of opposing dips (Faulds the northern Embudo fault (Manley, 1978; Muehlberger, 1979; Dungan et al., and Varga, 1998; Goteti et al., 2013). Similar structures are observed in the East 1984; Ingersoll et al., 1990; McDonald and Nielsen, 2004). Others have sug- African Rift (Rosendahl, 1987; Morley et al., 1990), alternatively called conver- gested that the northern Embudo fault has been active since early Miocene, gent transfer zones. based on inferences that it had already formed a linked system between the Evidence for early Miocene initiation of the Embudo transfer zone comes Sangre de Cristo fault on the north or the Pajarito fault on the south (Bauer mainly from the southern segment of the zone in the northern Española Basin, and Kelson, 2004a; Koning et al., 2004). In any case, a common assumption

is that the northern Embudo fault has been active near its present location REGIONAL SETTING throughout its history. Direct evidence of the pre-Quaternary history of the northern Embudo fault The study area is located in the southeastern corner of the San Luis Basin, is buried beneath extensive alluvial cover. Subsurface data are required to in- surrounding the Town of Taos, New Mexico (Fig. 1). The San Luis Basin is vestigate the geometry and structure of the subbasin formed by the linked one of a series of north-south–elongated structural basins that compose the Embudo–Sangre de Cristo fault system. Fortunately, a series of hydrologic and Cenozoic Rio Grande rift, which extends from Mexico to Colorado (Fig. 1 hydrogeologic studies conducted and ongoing in the Taos area over the past inset). As the basins developed in response to rifting of the crust, they filled three decades have resulted in the collection of an unprecedented amount of mostly with clastic sediments and some lava flows. The poorly to moder- deep well information and geophysical data that can be used for indirect in- ately consolidated sediments, known as the Santa Fe Group, accumulated vestigation of subsurface structure (Reynolds, 1986, 1992; Bauer et al., 1999; in the San Luis Basin mainly during the period 26–2 million yr B.P. (Ma) Benson, 2004; Drakos et al., 2004; Grauch et al., 2004; Bauer et al., 2014; Bauer (Ingersoll et al., 1990; Brister and Gries, 1994; Bauer and Kelson, 2004a; et al., 2016; Johnson et al., 2016). Additional geophysical studies have focused Smith, 2004). on understanding the three-dimensional framework of the Rio Grande rift Proterozoic rocks, which form the basement of the San Luis Basin and much (Bauer et al., 2004; Grauch and Keller, 2004; Bankey et al., 2007). The Taos area of the surrounding mountains, are composed of a diversity of rock types that is also remarkable as the setting for geological and geophysical training of are a product of continental accretion, plutonism, and regional metamorphism astronauts (Dickerson, 2004; Muehlberger, 2004), whose data also contributed (Karlstrom et al., 2004). Both the Picuris-Pecos fault system and the Jemez to this study. lineament (Fig. 1) are considered to be major crustal boundaries that first de- Despite the numerous subsurface investigations, several geophysical ob- veloped in Proterozoic time, remaining episodically active ever since (Aldrich, servations have remained puzzling. First, gravity data suggest that the basin 1986; Bauer and Ralser, 1995). During the late Paleozoic, highlands located in margin associated with the Embudo fault zone has a more gradual slope and the Tusas and Sangre de Cristo Mountains (Fig. 1; Kluth, 1986) shed sediments greater structural relief than expected for common models of antithetic or into intervening basins that were occasionally covered by shallow seas (Baltz convergent transfer zones (Grauch and Keller, 2004). Second, both correlation and Myers, 1999). During the Late Cretaceous to Eocene Laramide orogeny, of basaltic rocks in wells and analysis of aeromagnetic data suggest that a the Tusas and Sangre de Cristo Mountains were elevated again, shedding sedi >200 m deep structural graben exists west of the Sangre de Cristo mountain ments into local basins east of the San Juan and Tusas Mountains (Manley, front (Drakos et al., 2004; Grauch et al., 2004); yet analysis of the broad fea- 1981; Brister and Gries, 1994). tures of gravity and aeromagnetic data suggest that no such graben exists at From late Eocene to late Oligocene time, voluminous lava flows and ash- the basin floor (Grauch and Keller, 2004; Grauch et al., 2004). Finally, several flows erupted primarily from the San Juan volcanic field (Fig. 1), blanketing aeromagnetically inferred structural domains of differing orientations, which most of the surrounding area (Southern Rocky Mountain volcanic field of converge north of the Town of Taos, are difficult to understand in light of the Lipman, 2007). The related Latir volcanic field, located just north of the study locations of the present-day rift margins (Grauch et al., 2004). area (Fig. 1), was active toward the end of this period with the eruption of the Thus, we were motivated to integrate all the subsurface information with 25.4 Ma Amalia Tuff and accompanying collapse of the Questa caldera (Lipman detailed geologic mapping to resolve the puzzling observations and develop a et al., 1986; Zimmerer and McIntosh, 2012). Oligocene volcanic rocks are ex- clearer understanding of the subsurface geology and Cenozoic evolution of the posed in the central Taos Plateau (Thompson et al., 1986), indicating that such northern Embudo and Sangre de Cristo faults. From this synthesis, we propose rocks may be widespread beneath the Taos Plateau volcanic field. several revisions to the fault history and development of the associated rift The transition from widespread volcanism to rift sedimentation is com- basin. Importantly, we suggest that fault activity has shifted to different loca- monly considered to be ca. 26–25 Ma, marked by eruption of the Amalia Tuff tions throughout rift development. We speculate that the shifts have only par- and the onset of regional basaltic volcanism (Lipman and Mehnert, 1975; tially followed a systematic migration away or toward the basin depocenter. Thompson et al., 1991). In reality, both tectonic styles overlap in age by several To develop the arguments for this and other hypotheses, we first present million years (Smith, 2004; Zimmerer and McIntosh, 2012), and some workers background information for all the evidence and data that were considered have suggested that basin subsidence did not initiate until 21 Ma (Ingersoll during the synthesis. The information comes from geologic mapping, and et al., 1990). The pre-rift stage of volcanism and plutonism persisted as late as gravity, aeromagnetic, borehole, and physical-property data. After describing ca. 23–22 Ma (Zimmerer and McIntosh, 2012), while erosion of volcanic high- the geophysical methods we used, we present key geophysical interpreta- lands was widespread (Ingersoll et al., 1990; Smith, 2004). tions of the subsurface geology and the reasoning behind the conclusions. We Basin subsidence and accumulation of Santa Fe Group sediments in discuss age implications for the geophysical interpretations and then, finally, the San Luis Basin was at its peak during middle Miocene time (Bauer and propose a revised history of the structural evolution of this corner of the rift, Kelson, 2004a; Smith, 2004). Basaltic lavas erupted locally near the Colorado– illustrated by a set of generalized paleogeographic maps. New Mexico border during 15–11 Ma (Miggins et al., 2002; Thompson et al.,

2007, 2015) and in the northern part of the Jemez volcanic field (Fig. 1) during Geologic Units 14–8 Ma (Goff and Gardner, 2004; Thompson et al., 2006; WoldeGabriel et al., 2013). Pliocene volcanism of the Taos Plateau volcanic field (Fig. 1) occurred Detailed geologic mapping recently compiled and updated by P.W. Bauer later in rift history (6–2 Ma) and was characterized by multiple eruptions of and K.I. Kelson from five 7.5 min quadrangles (Bauer et al., 1997; Kelson et al., basaltic- and intermediate-composition magma from scattered cones and 1998a; Bauer et al., 2000; Bauer and Kelson, 2001; Kelson and Bauer, 2003) plus shield volcanoes (Lipman and Mehnert, 1979; Dungan et al., 1984; Thompson regional mapping from a sixth quadrangle in the northwest corner of the study and Machette, 1989). The total volume of eruptive material was predominantly area (R.A. Thompson, U.S. Geological Survey (USGS), 2004, written commun.) olivine tholeiitic Servilleta Basalt (Lipman and Mehnert, 1979; Dungan et al., are generalized for the study area in Figure 3. A map representing a subset of 1984; Read et al., 2004). Eruptions of intermediate- to felsic-composition lavas the updated, detailed geologic mapping, focused on the Embudo fault zone, were isolated and not widespread; whereas Servilleta Basalt flows generally is presented in Bauer et al. (2016). The generalized units are described below, traveled much farther (Lipman and Mehnert, 1979; Dungan et al., 1984). During noting only aspects that are important to the Cenozoic geologic history and the waning stages of rifting, voluminous ash-flow eruptions accompanied the geophysical interpretation. formation of the Valles and related calderas in the Jemez volcanic field in the interval 1–2 Ma. Regional gravity data indicate that the southern San Luis Basin con- Pre-Rift Rocks tains a deep (>3 km), north-elongate graben generally located between the Sangre de Cristo Mountains on the east and the Rio Grande gorge on Proterozoic and Paleozoic rocks are exposed in the Sangre de Cristo and the west (Cordell, 1978; Keller et al., 1984; Grauch and Keller, 2004). West of Picuris Mountains (Figs. 2 and 3). Proterozoic rocks (Xu) include a diversity the graben, the structurally elevated exposures of Oligocene volcanic rocks of rock types. Paleoproterozoic granite and granitic gneiss are exposed in the within the central Taos Plateau, combined with evidence from geophysi- eastern Picuris Mountains and in the northeastern study area (Lipman and cal data and borehole data in southern Colorado, indicate a north-trending Reed, 1989; Bauer et al., 1997). Paleoproterozoic quartzite and schist of the intrarift horst underneath the Taos Plateau volcanic field that extends al- Hondo Group are exposed in the Picuris Mountains, where they are folded and most the entire length of the San Luis Basin (Kleinkopf et al., 1970; Lipman overlie the mixed felsic and mafic metavolcanic-metasedimentary sequence of and Mehnert, 1979; Keller et al., 1984; Thompson et al., 1986; Brister and the Paleoproterozoic Vadito Group (Bauer, 1993). Gries, 1994; Grauch and Keller, 2004; Johnson and Bauer, 2012). This horst Paleozoic rocks (Pzu) exposed east of the Town of Taos represent strata that remained structurally high during rifting. were deposited in a trough (Taos trough), which may be as much as 1800 m Basin tilting and uplift of surrounding rift borders continued into Qua- thick in the Sangre de Cristo Mountains (Baltz and Myers, 1999). The strata are ternary time, resulting in the accumulation of coalescing alluvial fans near mostly fine-grained siliciclastic rocks with minor limestone (Miller et al., 1963). the eastern mountain fronts and incision of the Rio Grande gorge (Dungan The Paleozoic rocks are fault-bounded on the south and north, where they ter- et al., 1984; Wells et al., 1987; Bauer and Kelson, 2004a). Quaternary fault minate against Proterozoic rocks (Fig. 3). How far they extend beneath the rift activity was concentrated along the Sangre de Cristo and Embudo fault basin west of the Sangre de Cristo fault is uncertain. Baltz and Myers (1999) zones, and in several swarms of north-striking faults cutting alluvial de- argued that they extend west of the Rio Grande and underlie the southern Taos posits within the Taos graben (Machette and Personius, 1984; Bauer and Plateau. Woodward et al. (1999) argued that the Taos trough was bounded to Kelson, 2004a). the west by an active Picuris-Pecos fault during the Pennsylvanian, thus limit- ing the western extent of deposition.

GEOLOGY Pre-Rift and Early Rift Deposits The study area is located within the Taos embayment, a structural and topographic embayment in the mountains surrounding the Town of Taos (Fig. 2). Although early rift volcanic rocks of the Latir volcanic field (Fig. 1) are not The embayment, as well as the Rio Grande rift margins, are defined by the exposed in the study area, they may exist on the basin floor. If so, their com- linked, Embudo fault zone and the Cañon and Hondo sections of the Sangre positions may be similar to the intermediate-composition lavas and Amalia de Cristo fault system. The major crustal boundary represented by the north- Tuff that are exposed in two small hills in the central Taos Plateau (Thompson south Picuris-Pecos fault system intersects and is cut by these faults where et al., 1986). they join. The Los Cordovas faults are interior to the Taos embayment and the The Picuris Tuff of Cabot (1938) is composed of distal volcaniclastic de- rift basin. The Taos Plateau, a physiographic feature associated with the Taos posits and proximal sediments (Rehder, 1986; Aby et al., 2004; McDonald and Plateau volcanic field, is west of the Rio Grande (Fig. 1). Nielsen, 2004); thus, Picuris Formation more appropriately reflects the varied

N 105°45′0″W 105°37′30″W 0 ″

Figure 12 n io t 36 ° 30 ′ c e

d Los Cordovas n Ho faults

Rio Lucero

A U T

Rio Grande Rio s E Tao de o T l eb A u P L Y M E N io R t A l u

o fa

i st r Figure 8 C Figure 2. Tectonic setting of the study area e d showing mapped faults compiled from e r geologic 7.5- min quadrangle maps (Bauer g et al., 1997; Kelson et al., 1998a; Bauer n S N

et al., 2000; Bauer and Kelson, 2001; Kel N

0 ″ Sa I RR f son and Bauer, 2003). The Sangre de Cristo i A ioo o T fault is divided into the Cañon and Hondo n GG Town N A O S P sections (Kelson et al., 1998b, 1998c). The rra o an n of Taos ti U 36 ° 22 ′3 d T d short-dashed red line marks the limits of e c e O

dd e the Town of Taos. Locations of Figures 8 e e M Rio Pueblo de Taos l s l and 13 are shown by the solid red outlines. R O S R E M B Rio Grande a a on A n n c ñ O ch T h T o o Ca S I

e zoneD U E D U R G Rio Grand D U N fault A S

U D D U D U Picuris-Pecos fault system Embudo PICURIS MOUNTAINS

05Mapped fault, dashed where approximately located, dotted where Stream concealed. Bar on down-thrown side of normal fault; arrows show KILOMETERS U strike-slip motion. Upthrown (U) and downthrown (D) sides of Rift flanks D high-angle fault; arrows show strike-slip motion.

N 105°45′0″W 105°37′30″W 0 ″

BBOORR--7 BIA-24 Tbs

36 ° 30 ′ C Bearear Stetew B QTl Qa Xu Limit of shallow Servilleta Basalt

ero BOBOR-R-55 BOBOR-R-6 Qa Tbs Taos AiAirprporort Luc Tbs QTl MW-1 CoCololoniniasas Rio MW-2 RWRWP-P-2 Tbs MW-4 Rio Grande Rio s ShSheepseepskikin Tao MW-3 de Pmag Tbs o Site bl Tbs ue ToTorrerreon P TPTPDRWDRW QTl io R Tbs K3K3 RGRG837383730 Tv Tbs Tbs QTl QTl Arroyo Park QTl ElElststonon Tbs BBIIAA--5 B′ Tbs Tbs

N RRPP32003200 QTl ToTownwn Yarard

0 ″ BOBOR-R-3 Figure 3. Geology of the study area, show Tbs Tbs R io ing profile locations and selected wells.

QTl G Town The geologic map is simplified from re ra n of Taos 36 ° 22 ′3 d cent compilation and revision of previous e

d 1:24,000-scale geologic mapping by Bauer A Rio Pueblo de TaosTbs e RRuucckekendondorrffeerr Qa l l

R and Kelson (see text). Label “Pmag site” Rio Grande TV-233 Tbs a TV-195 n along the Rio Grande is where Brown et al. BOBOR-R-11 c TV-149 h Pzu o (1993) analyzed paleomagnetic measure UNMUNM/T/Taoaos Tbs TV-115 TV-167 ments for a vertical section of Servilleta Basalt. Limit of shallow Servilleta Basalt TV-103 Tbs (dark-gray, long-dashed line) is interpreted C′ from aeromagnetic data. Tsf Tsf Tbs Tsf D TV-104 U Pzu Tsf D U Rio Grande Tsf D U Tsf 02 Alcon Xu Tp KM U D Tsf Tbs D TV-230 U D U Tp Tp Xu A′ Tp Qa

EXPLANATION Stream Surficial Deposits Pre-rift to Early Rift Deposits BBOOR-R-11 Selected well Tp—Tertiary Picuris Formation. Qa—Quaternary alluvium, stream terrace, colluvium, land- (Table 1) slide, and eolian deposits Pre-rift Rocks Synrift Deposits Pzu—Paleozoic sedimentary rocks, dominantly clastic QTl—Upper part of Santa Fe Group (Lama formation). Deposits interbedded with and overlying Servilleta Basalt Xu—Proterozoic rocks, undivided, including granitic rocks, felsic and mafic metamorphic rocks, and quartzite Tbs—Pliocene basalt, primarily Servilleta Basalt Structure Tv—Pliocene intermediate to felsic rocks of Taos Plateau Mapped fault, dashed where approximately located, dotted volcanic field where concealed. Bar on down-thrown side of normal fault; arrows show strike-slip motion. Upthrown (U) and U Tsf—Lower part of Santa Fe Group (Miocene), including downthrown (D) sides of high-angle fault; arrows show D Tesuque and Chamita Formations. strike-slip motion.

lithologies and is used here. The deposits range in age from greater than and Mehnert, 1979; Appelt, 1998; Read et al., 2004; Cosca et al., 2014; Kelson 34.5 Ma to less than 18.6 Ma (Aby et al., 2004), thus preserving an important et al., 2015). Where they are exposed in the Rio Grande gorge, basalts form record of the erosion of highlands during the transition from Oligocene vol a 150-m-thick section composed of individual flows that are discontinuous, canism to early rift basin subsidence. interfingered, and variable in thickness (Dungan et al., 1984). Intervening sedi Aby et al. (2004) divided the Picuris Formation into three informal mem- mentary intervals generally are 0.35–4.5 m thick (Leininger, 1982) but have bers. The lower member (>34.5–28.3 Ma) is composed of fine-grained material thicknesses as much as 50 m in water wells near Taos (Glorieta Geoscience, with interbedded coarse conglomerate and is 300–400 m thick just southwest Inc., 2007) and 70 m in river gorges northwest of the study area (Dungan et al., of the study area. Conglomerates contain Proterozoic, Paleozoic, and volcanic 1984; Read et al., 2004). clasts. The volcanic clasts must be older than the Latir volcanic field; so they Previous workers have relied on the division of the Servilleta Basalt into likely were transported southward from the San Juan volcanic field or de- Upper, Middle, and Lower units developed by Dungan et al. (1984) in the Rio rived from a local source. The middle tuffaceous sandstone member (28.3 to Grande gorge to correlate basalts across distances and between wells (e.g., <23 Ma) records the influx of debris from the Latir volcanic field to the north, Drakos et al., 2004). However, paleomagnetic studies of Servilleta Basalt at with progressively greater input from basement highlands through time. It is several sites within the gorge demonstrated that the magnetostratigraphy of <125 m thick in the study area (Aby et al., 2004). The upper member (<23 Ma flow packages do not correlate with these divisions (Brown et al., 1993). More- to <18.6 Ma) is composed of volcaniclastic pebble-conglomerate to mudstone over, recent age dating and chemical studies of flows in the Rio Grande gorge and is gradational into the overlying Santa Fe Group units. Thickness is ~200– reveal much greater discontinuity of flows and complexities due to paleo 250 m in the study area. Paleoflow measurements and clast compositions in- topography than previously thought (M.A. Cosca and R.A. Thompson, USGS, dicate that the main source was the Latir volcanic field to the north, with minor 2015, oral commun.). These results warn against adhering to a strict basalt contributions from Proterozoic basement and San Juan volcanic field lavas stratigraphy when correlating basaltic intervals between wells. (Rehder, 1986; Aby et al., 2004). Nomenclature for rift sediments that are contemporaneous with eruption of Servilleta Basalt is not consistent in the literature. In this study, we follow the criteria established by Kelson et al. (2015), who informally define the Lama Synrift Deposits formation (QTl) as Santa Fe Group sediments that are interbedded with flows of Servilleta Basalt and were deposited after the first Servilleta Basalt flow (ca. The Miocene section of the Santa Fe Group (Tsf on Fig. 3) is poorly ex- 5.5 Ma) and before incision of the Rio Grande (early[?] to middle[?] Pleistocene). posed in the study area but has been well characterized to the southwest in The Lama formation consists of poorly sorted sand, pebbles, and cobbles, com- the Española Basin (Fig. 1). From studies of exposures in the northern Picuris monly in a fine-grained matrix (Bauer and Kelson, 2004a; Kelson et al., 2015). Mountains and examination of cuttings from wells, workers generally accept The sediments are highly oxidized and weathered, suggesting they were satu- that correlative formations of Miocene age occur in the subsurface of the rated by a high water table before the water table lowered in association with study area, including the Tesuque and overlying Chamita Formations (Bauer regional stream incision (Bauer and Kelson, 2004a). Clast compositions and and Kelson, 2004a; Drakos et al., 2004; Bauer et al., 2016). The Tesuque and paleoflow indicators suggest sources came from Proterozoic, Paleozoic, older Chamita Formations are both composed of siliciclastic sediment, primar- volcanic, and Servilleta Basalt terranes from a variety of directions (Kelson ily sand (Koning et al., 2013). Only the lower part of the Tesuque Formation et al., 2015). Bordering the northeastern part of the study area, large thicknesses (Chama–El Rito Member) is exposed in the study area in the northern Picuris (>25 m) of Lama formation record the advancement of massive alluvial fans Mountains. A distinctive eolian sand unit (Ojo Caliente Sandstone Member), from the mountain front to the east (Kelson and Bauer, 2006). which overlies the Chama–El Rito Member, is exposed in the southwestern- most study area and is recognized as a marker unit in well logs (Drakos et al., 2004). By recognizing this marker unit, a minimum of 840 m of Miocene Santa Surficial Deposits Fe Group sediments are documented within the basin in the RP3200 well near the center of the study area (Fig. 3) (Glorieta Geoscience, Inc., 2007). Surficial deposits (Qa) cover most of the study area (Fig. 3) and generally Limited exposures of intermediate-composition volcanic rocks (Tv) of lie unconformably on the Lama formation and equivalent units. This uncon- the Taos Plateau volcanic field are present on the west side of the study area formity marks the beginning of regional stream incision (Kelson et al., 2015). (Fig. 3). Servilleta Basalt (Tbs) is exposed more extensively (Fig. 3) and is com- Although the deposits are not differentiated on Figure 3, detailed mapping of monly encountered in water wells drilled throughout the northwestern part of the surficial deposits has provided key information for understanding the tim- the study area (Benson, 2004; Drakos et al., 2004; Bauer et al., 2016). Servilleta ing and nature of exposed fault scarps (e.g., Kelson et al., 2004a, 2004b) and Basalt refers to multiple flows of olivine tholeiitic basalt with a distinctive dikty helps constrain the age of past activity of faults that are interpreted mainly taxitic texture, ranging in age from ca. 5.5 to 3 Ma in New Mexico (Lipman from geophysical evidence.

The mountain fronts are dominated by coalescing alluvial fans, ranging dominantly northwest-down normal slip as it approaches the Sangre de Cristo in age from early(?) Pleistocene to Holocene (Bauer and Kelson, 2004a; Bauer fault near the intersection with the Picuris-Pecos fault system (Fig. 2; Kelson et al., 2016). They interfinger with stream terrace deposits that flank the major et al., 2004a). streams (Kelson and Wells, 1989). Landslide deposits cover the deep gorge The exposed Embudo fault zone is several kilometers in width (Bauer walls in the Rio Grande and Rio Pueblo de Taos, and colluvium is prevalent on and Kelson, 2004a, 2004b; Kelson et al., 2004a). The complex patterns in the steep slopes at the base of the Sangre de Cristo Mountains. Eolian deposits styles of deformation reflect large variability in local displacements and net form a thin blanket over much of the southern Taos Plateau. slips along strike (Kelson et al., 2004a). Reverse thrust relations are well documented in the southwestern part of the study area where the Embudo fault zone makes a bend from northeast to east (Muehlberger, 1979; Machette and Structure Personius, 1984; Kelson et al., 2004a; Koning et al., 2004). Kelson et al. (2004a) demonstrated that these are local features associated with a flower structure Picuris-Pecos Fault System and that the overall sense of movement is left lateral. The timing and amount of lateral displacement along the fault zone is The north-south Picuris-Pecos fault system is a major crustal structure poorly known. On the basis of sedimentological evidence, previous workers that has been episodically active since Proterozoic time (Montgomery, 1953; inferred that the Embudo fault zone formed in concert with the uplift of the Miller et al., 1963; Bauer and Ralser, 1995). It consists of an 8-km-wide zone Picuris Mountains in late Miocene (Manley, 1978; Muehlberger, 1979; Dungan of north-striking faults that can be traced for ~84 km south of the study area et al., 1984; Ingersoll et al., 1990; McDonald and Nielsen, 2004) or early Mio- (Fig. 1). The major strands juxtapose Proterozoic metasedimentary rocks, cene (Bauer and Kelson, 2004a; Koning et al., 2013). Kinematic observations Proterozoic granite, Paleozoic sedimentary rocks, and Tertiary strata in vari- from areas southwest of the study area show that left-lateral slip has dom- ous combinations (Bauer and Kelson, 2004a). During the Late Cretaceous inated since ca. 12–11 Ma (Steinpress, 1981; Aby and Koning, 2004; Kelson Laramide orogeny, west-up, right-lateral displacement formed north-trending et al., 2004a). Within the study area, Muehlberger (1979) approximated 3 km of grabens that were later filled with sediments of the Picuris Formation (Bauer total vertical offset between Proterozoic rocks uplifted in the Picuris Mountains and Kelson, 2004a; McDonald and Nielsen, 2004). Kinematic evidence of west- and the deep basin to the north using preliminary gravity-data estimates for down and right-lateral slip involving the youngest part of the Picuris Formation basin thickness. Bauer and Kelson (2004b) estimated 105 m of vertical throw and increasing throw to the north suggests that early rift development included on a 3-Ma basalt just southwest of the study area. Combined with detailed dextral-oblique movement on north-south faults of the Picuris-Pecos fault sys- measurements of rake, they estimate 35 m per million years (m/m.y.) and 102 tem (Bauer and Kelson, 2004a; McDonald and Nielsen, 2004). Displacement of m/m.y. for the vertical and horizontal components of slip, respectively. The a 5.7-Ma basalt southwest of the study area indicates fault activity as young as youngest fault activity displaces fan deposits that are latest Pleistocene to late Miocene (Bauer et al., 2005). Holocene in age (Kelson et al., 2004a). Because the Picuris-Pecos fault system is a major crustal boundary that is The coincidence of the Embudo fault system with the Jemez lineament, a interrupted at the Embudo fault zone, it likely has a northward extension into more regional alignment of structural zones and Pliocene volcanism (Fig. 1), the southern San Luis Basin. Bauer and Kelson (2004a) speculated that the suggests that its development may have been controlled, at least in part, by a northward extension underlies and controls the much younger Los Cordovas preexisting crustal fabric (Aldrich, 1986; Koning et al., 2004). The Jemez linea- faults within the Taos embayment (Fig. 2). McDonald and Nielsen (2004) sug- ment is considered to be the manifestation of a crustal weakness along which gested that the northward extension parallels the present-day Hondo section tectonic and magmatic activity has concentrated since 15 Ma (Lipman, 1980; of the Sangre de Cristo fault (Fig. 2). Aldrich and Laughlin, 1984). The crustal weakness may be a suture of accreted crust that formed during the Proterozoic (Magnani et al., 2004).

Embudo Fault Zone Sangre de Cristo Fault System The northeast-striking Embudo fault zone is ~64 km long and links the west-dipping Sangre de Cristo fault system in the southern San Luis Basin The Sangre de Cristo fault system refers to a series of west-dipping normal to the southeast-dipping Pajarito fault in the northern Española Basin (Fig. 1). faults that form the ~200-km-long eastern rift border of the San Luis Basin in The southwest-to-northeast transition between the northeast-striking Embudo Colorado and New Mexico. It is divided into sections on the basis of differ- fault zone to the north-striking Sangre de Cristo fault system appears gradual ences in geomorphic expression, including the Hondo and Cañon sections in (Kelson, 1997; Bauer and Kelson, 2004a). Slip on the Embudo fault is mainly the study area (Fig. 2; Kelson et al., 1998b, 1998c). The Hondo section meets left-oblique on steep fault strands, with dominantly lateral slip transitioning to the Cañon section at the large bedrock salient in the range front where the

Rio Pueblo de Taos emerges from the mountains. The Hondo section extends water-rights settlements. Several hydrogeologic studies have been undertaken northwestward another 15 km, out of the study area. The 14-km-long Cañon to compile and synthesize the lithologic information from the deep wells, which section merges with the Embudo fault zone at the intersection of the Picuris- also provide a subsurface stratigraphic framework for the Taos area (Bauer Pecos fault system, where Rio Grande del Rancho emerges from the moun- et al., 1999; Benson, 2004; Drakos et al., 2004). Information from these and addi tains (Fig. 2; Bauer and Kelson, 2004a). tional water wells was compiled in recent hydrogeologic studies undertaken North of the bedrock salient at Rio Pueblo de Taos, fault strands in the by New Mexico Bureau of Geology and Mineral Resources (Bauer et al., 2014; Hondo section form branching patterns as much as 2 km wide that wrap Johnson et al., 2016). As part of these studies, well locations were verified on around a bedrock protrusion in the mountain front. The pattern diverges lo- the ground and lithologic information was reassessed from driller’s logs. cally from the overall strike of N 30° W of the Hondo section north of the study For the current study, we used well information from the previous studies, area (Fig. 1). The range-front morphology is steep and linear, with prominent supplemented by data provided courtesy of Glorieta Geoscience, Inc. (GGI). Lo- fault scarps developed on Pleistocene to Holocene fan material within the cations for a subset of these wells are shown on Figure 3 and listed in Table 1, study area (Machette and Personius, 1984; Menges, 1990; Kelson et al., 1998b). along with information about basalt intervals penetrated by the wells. During The Cañon section is also composed of a series of branching fault strands the course of the current study, lithologic picks for the Town Yard well were re- that extend as much as 2 km in width. Unlike the Hondo section, the Cañon vised from previous work, which reported the well had encountered Paleozoic section strikes about N 20° E. Prominent scarps developed on alluvial fan sur- bedrock at 219-m depth (e.g., Bauer and Kelson, 2004a; Drakos et al., 2004). faces suggest multiple events have occurred since late Pleistocene (Bauer and We concluded that the well did not reach bedrock, based on comparisons of Kelson, 2004a; Kelson et al., 2004b). Fault planes typically dip steeply (>60°) well logs and lithologic cuttings with those of the nearby BOR-3 well, analysis west to northwest. Slickenlines are dominantly dip slip with increasing left- of gravity data between these wells, and estimates of physical properties from oblique slip in the transition zone to the Embudo fault zone. borehole logs. Moreover, detailed examination of archived well cuttings by D. Koning and S. Aby (New Mexico Bureau of Geology and Mineral Resources [NMBGMR], 2013, written commun.) suggested that both the BOR-3 and Town Los Cordovas Faults Yard wells penetrated a gravel sequence within the Miocene Tesuque Forma- tion at the interval formerly considered as Paleozoic rocks. The Los Cordovas faults are generally north-striking, west-down normal faults that are exposed within a 5–6-km-wide zone interior to the Taos embayment (Fig. 2). A wider zone (5–9 km) extending farther east is suggested from Gravity Data photo reconnaissance and aeromagnetic data (Machette and Personius, 1984; Grauch et al., 2004; S. Personius, USGS, 2015, written commun.). Gravity data for the study area were compiled from several sources. Vin- Several of the mapped Los Cordovas faults form west-facing, eroded tage regional data, which are archived from data collected by various workers scarps 15–30 m high that juxtapose Servilleta Basalt against downthrown since the 1960s, were retrieved from the PACES online database (http://gis.utep Quaternary sedimentary units (Fig. 3; Bauer and Kelson, 2004a). Some faults .edu, accessed November 2006). In 1999 and 2000, NMBGMR collected a large likely have greater throw in the subsurface (Machette and Personius, 1984). number of gravity stations at close spacing along a network of profiles on Taos The most recent fault movement may be as old as early(?) Pleistocene and no Pueblo lands. Problems with the positioning information and data processing younger than middle Pleistocene (Machette and Personius, 1984; Bauer and of the data were later resolved (B. Drenth, USGS, 2011, proprietary data). Ad- Kelson, 2004a). Fault planes are poorly exposed, although fault dips of 89° ditional gravity data were collected to fill critical gaps in coverage within Taos and 45° have been reported (Lambert, 1966 and Bauer and Kelson, 2004a, re- Pueblo and the Town of Taos in 2010 and 2015 (Drenth, 2016). Some of the new spectively). stations reoccupied and ultimately replaced data from the vintage database. Standard Bouguer corrections (Blakely, 1995) were applied for each gravity station using a reduction density of 2670 kg per cubic meter (kg/m3). Terrain DATA SOURCES corrections were applied within a 167-km radius to the data at each station using digital terrain data with a resolution as fine as 10 m. The station data were Well Data then interpolated onto a grid at a 500-m interval. In order to focus on density variations within the upper crust, an isostatic regional field was removed from Lithologic information is available from a number of deep (>150 m) water the terrain-corrected Bouguer gravity grid using parameters established for wells east of the Rio Grande within the study area (Fig. 3; Table 1). In addition to New Mexico by Heywood (1992). The resulting “isostatic residual gravity map” wells drilled for domestic and municipal supply, many of the deep wells were (Fig. 4) generally isolates the gravity effects produced by sources within the drilled by Federal agencies to understand and monitor groundwater as part of upper 10 km of the crust (Simpson et al., 1986).

TABE 1. SEECTED WES FOR THE STDY AREA Surface Elevation at Elevation, top Easting Northing elevation Total depth total depth Basalt depth intervals of basalt Well identifier m m m m m m m

Wells that penetrate basalt Alcon 439,079 4,017,493 2363 3052058 256–2802106 Arroyo Par 443,690 4,028,645 2097 80 2017 20–36; 53–64; 77–TD2077 BIA‑5 442,420 4,028,390 2065 3051760 12–20; 32–47; 70–87; 116–157 2052 BOR‑3 445,792 4,026,044 2101 6431458 177–1891924 BOR‑5 447,346 4,036,115 2209 5581652 555–TD 1655 BOR‑6 444,797 4,036,008 2193 6211572 229–293; 320–329 1965 BOR‑7 444,278 4,039,056 2229 9571272 181–2612048 Elston 444,400 4,028,150 2064 4271637 30–46; 58–76; 107–131; 143–149 2034 K3 446,684 4,030,642 2120 5471572 87–105; 130–145; 192–229 2033 MW‑1 442,684 4,034,217 2161 97 2064 41–50; 56–62; 97–TD2120 MW‑2 442,324 4,033,924 2154 93 2061 62–73; 78–84 2092 MW‑3 442,937 4,033,282 2155 92 2063 45–55; 68–75 2110 MW‑4 442,580 4,033,633 2153 93 2060 69–79; 93–TD 2084 RG83730 442,608 4,030,290 2112 1221990 15–27; 46–70; 79–94; 101–TD2097 RP3200 440,344 4,026,206 2032 9841049 9–34; 58–73; 105–137 2023 RWP‑2 436,527 4,033,740 2131 1931939 1–1802130 Taos airport 439,430 4,034,965 2153 5241629 11–20; 55–137; 146–154 2142 Torreon 448,592 4,031,517 2124 4421682 372–3861753 Town Ya rd 446,929 4,026,830 2108 3111800 183–2011925 TV‑115 438,728 4,022,238 2098 1831915 58–642040 TV‑149 437,854 4,022,5112086 1751910 46–73; 101–105; 134–1432040 TV‑195 436,504 4,023,133 2042 1461895 18–58; 73–1372023 TV‑233 440,656 4,023,366 2099 1061994 60–78; 93–TD 2039 Wells that do not penetrate basalt Bear Stew 450,308 4,037,6112300 1032196 BIA‑24 447,454 4,038,551 2255 3051950 BOR1 442,074 4,022,809 2121 6111511 Colonias 445,531 4,034,943 2170 1522017 Rucendorfer 449,410 4,024,125 2216 1892027 Sheepsin 448,141 4,033,437 2160 1102050 TPDRW 450,877 4,032,385 2163 85 2077 TV‑103 439,575 4,020,899 2160 3051855 TV‑104 440,789 4,019,142 2268 3291939 TV‑167 442,917 4,022,252 2134 1401993 TV‑230 439,362 4,016,255 2484 4272058 NM/Taos 441,426 4,022,701 2124 3661758 Notes: Well data are from Bauer et al. 2016 and Johnson et al. 2016. All wells except Alcon, Rucendorfer, and TV‑230 penetrate only basin fill with or without interbedded basalt. Alcon reaches Proterozoic uartzite at total depth; Rucendorfer reaches Paleozoic shale at 180 m depth; TV‑230 is entirely within Proterozoic basement. Well identifiers are used to label well locations on figures. Eastings, northings, and elevation refer to niversal Transverse Mercator proection, zone 13, World Geodetic System 1984 WGS84. TD—total depth.

N 105°45′0″W 105°37′30″W 0 ″

BBOOR-R-7 BIA-24

36 ° 30 ′ C B BBeearar Stetew Limit of shallow Servilleta Basalt

BBOORR--55 BOBOR-R-6 Taos AAiirrppoorrt

MW-1 CoCololoniniasas Rio Lucero MW-2 RWRWP-P-2 MW-4 Rio Grande Rio ShSheepeepskiskin MW-3

ToTorrrreoeonn TPTPDRWDRW

KK33 RGRG837383730

Figure 4. Gravity map of the study Arroyo Park area after removal of the isostatic ElElssttoonn regional field of Heywood (1992), BBIIAA--5 showing station locations. Note the B′ nonlinear color display to enhance variations. Limit of shallow Servilleta

N RRPP32003200 Basalt (dark-gray, long-dashed line) is TToowwnn Yaarrd

0 ″ BOBOR-R-3 interpreted from aeromagnetic data R io (Fig. 5). A few artificial data points

G were constructed to constrain the 3D ra n

36 ° 22 ′3 modeling, which extended outside the d e study area (see text). Only one of these d A Rio Pueblo de Taos e RuRuckckendorendorffeer constraint points is located within the l

TV-233 R study area (southwest corner). a TV-195 BBOORR--11 nnc c TV-149 h UUNMNM//TTaaoos o TV-115 TV-167 TV-103 C′

D TV-104 U D U Rio Grande D U

Alcon

U D D TV-230 U D U A′

05 Milligals Gravity station Additional gravity –51 –9 KILOMETERS –48–46 –44–42 –40–36–32–26 –20–16 –14 constraint for modeling

Magnetic Data the problem of using extrapolated or interpolated data, RTP transformation was not applied, because this operation also relies on knowledge of data in a Magnetic data used for this study come from three aeromagnetic surveys, broader area to be accurate. a regional compilation of aeromagnetic data, and a ground magnetic traverse. High-resolution aeromagnetic data are available over the basin area of the PHYSICAL PROPERTIES AND GEOLOGIC UNITS study area (yellow dashed outline on Fig. 5). The eastern, western, and far northern portions of this area are covered by the Taos (Bankey et al., 2004), Physical properties of geologic units are important for geophysical interpre- Taos West (Bankey et al., 2007), and central San Luis (Bankey et al., 2005) aero- tation because they provide the tie between lithology and geophysical fields. magnetic surveys, respectively. The latter two surveys were flown by fixed- For gravity data, the applicable physical property is bulk density, which is the wing aircraft; the Taos survey was flown by helicopter. All three surveys were overall mass per unit volume of rocks, sediments, and their pore spaces. For flown along traverse lines oriented east-west, spaced 200 m apart, and nomi- magnetic data, the applicable physical property is total magnetization, which nally 150 m above ground. Orthogonal lines were flown north-south at 1000-m is determined by the quantity of naturally occurring magnetic minerals in rocks spacing. After flight-line data from the surveys were processed to remove and sediments and the nature of their permanent magnetizations. known Earth’s field variations and reduce ordinary data-acquisition errors, they Figure 6 shows a stratigraphic column of generalized geologic units ex- were interpolated onto a 50-m grid. The gridded data were analytically con- pected to lie in the subsurface of the study area in comparison to graphs of den- tinued from the variable observation surface to a surface consistently draped sity and magnetic properties of specific geologic units or rock types. The graphs 100 m above ground. To extend the aeromagnetic data coverage outside of the summarize physical properties measured or estimated from rocks or geologic area of high-resolution survey coverage and over the mountain flanks, gridded units within the San Luis or neighboring basins, tabulated in Tables A1 and A2 data from a compilation of regional aeromagnetic surveys (Kucks et al., 2001) in the Appendix. The graphical portrayal of the physical properties facilitates the for the State of New Mexico were merged. A noticeable reduction in resolution identification of the key geologic units for gravity and magnetic interpretation, of anomalies occurs outside the boundary of high-resolution survey coverage that is, those units to which each geophysical method is most sensitive. (Fig. 5). A standard reduction-to-pole (RTP) transformation was then applied to the Key Geologic Units for Gravity Interpretation gridded data using a declination of 10° and inclination of 64°, which is the general orientation of the Earth’s field in the Taos area. Reduction-to-pole transfor- The key geologic units for gravity interpretation are the combined sedi- mations correct for shifts of anomalies away from the centers of their magnetic mentary deposits within the basin, encompassing the Picuris Formation, Santa sources; these shifts are an effect of the oblique orientation of the measured Fe Group (including the Lama formation) and surficial deposits. Many previous magnetic field at high latitudes with respect to Earth’s surface (Blakely, 1995). workers have recognized that the low bulk densities of poorly consolidated This type of correction is useful for Oligocene and younger volcanic fields, sedimentary deposits that infill basins composed of moderate- to high-density where remanent magnetizations are expected to have orientations that are bedrock produce gravity lows with amplitudes that are generally proportional within ~25° of the Earth’s field direction (Bath, 1968). Figure 5 is a color shaded- to basin-fill thickness (e.g., Cordell, 1978, 1979; Birch, 1982; Keller et al., 1984; relief image of the final, RTP aeromagnetic data. Bott and Hinze, 1995). In addition to aeromagnetic data, a 6-km ground magnetic traverse was Logs from various boreholes within the Albuquerque and Española Basins acquired across the Embudo fault zone, where high-resolution aeromagnetic show that densities of the Santa Fe Group are systematically much lower (≤400 coverage was not available (Fig. 5). The data were acquired as continuous kg/m3 difference) than the older rock types that compose the basin floor and total-field measurements from a cesium-vapor magnetometer mounted on a rift flanks (Grauch and Connell, 2013). Because lithologic types appear fairly pole held at head height as the operator traversed the ground on foot. Mea- similar for the Santa Fe Group in the Albuquerque, Española, and San Luis surements were acquired continuously except for one interruption to cross a Basins, we use the simplified density-depth function derived by Grauch and major road (State Highway 68) that was bordered by berms and barbed-wire Connell (2013) to a depth of 2.25 km in the top portion of Figure 6B to compare fences. After collection, data spikes were removed and data were interpolated densities. Density information for the Picuris Formation is limited to estimates across the 110-m gap in measurements. The data were then upward continued from a sonic log in one well (Bauer, 2016). Using a common empirical relation by 100 m, gridded, and merged with the total-field aeromagnetic grid so that (Gardner et al., 1974), densities ranging from 2200–2300 kg/m3 were estimated a straight profile could be extracted for modeling. Some loss in data integrity for the Picuris Formation from the sonic log in the depth range of 100–366 m. is expected as a result of this procedure, because (1) accurate upward contin- No systematic variation of density with depth was observed. These densities uation relies on knowledge of data in a broader area than was sampled along and shallow depths are only slightly higher than the density-depth function for the ground (Blakely, 1995), and (2) bends in the ground traverse forced the data the Santa Fe Group (Fig. 6B). Thus, the Picuris Formation cannot be reliably extraction to pull data from extrapolated gridded data. To avoid compounding distinguished from the Santa Fe Group on the basis of density.

N 105°45′0″W 105°37′30″W 0 ″

BBOORR--7 BIA-24

36 ° 30 ′ C B Bearear Stetew Limit of shallow Servilleta Basalt BOBOR-R-55 BOBOR-R-6 Taos AiAirporport

MW-1 CoCololoniniaass Rio Lucero MW-2 RWRWP-P-2 MW-4

Rio Grande Rio s ShSheepseepskikin Tao MW-3 de Pmag o Site bl ue ToTorrerreon P TPTPDRDRW io R K3K3 Figure 5. Color shaded-relief image RGRG837383730 of merged aeromagnetic data after application of the reduction-to-pole transformation. Note the nonlinear Arroyo Park color display to enhance variations ElElststonon and the shading to enhance linear fea BIBIA-A-5 tures (illumination is from the east). B′ The white, long-dashed line delimits the inferred area of shallow Servilleta

N RPRP32003200 Basalt, indicated by the high-frequency ToTownwn Yarard 0 ″ BOBOR-R-3 aeromagnetic patterns. The dashed R io yellow line is the limit of high-resolu G Town tion aeromagnetic surveys. Location of ra n of Taos magnetic ground traverse is indicated 36 ° 22 ′3 d e by the white-outlined blue line along

d A Rio Pueblo de Taos e RuRuckckendorendorfefer profile A–A′. Label “Pmag site” along l

R Rio Grande TV-233 the Rio Grande is where Brown et al. a TV-195 BBOORR--11 n (1993) analyzed paleomagnetic mea c TV-149 h surements for a vertical section of Ser UNUNM/M/TaTaosos o Survey villeta Basalt. TV-115 TV-167 Boundary TV-103 C′ Ground traverse D TV-104 U D U Rio Grande D U

Alcon

U D D TV-230 U D U A′

05 Nanoteslas Deepest part of basin –838 154 KILOMETERS –600 –550 –500 –450 –400 –350 –300 –250

B DENSITY, kg/m3 C POTENTIAL RANGE OF A SUBSURFACE STRATIGRAPHY TOTAL MAGNETIZATION, A/m 2,200 2,400 2,6002,8003,000 0.00 0.01 0.10 1.00 10.00100.00 0 QUATERNARY Surficial Deposits (Qa) Servilleta Basalt with Quaternary Lama formation (QTl) Lama formation alluvium Pliocene 3 TPVF 3 2,450 kg/m Lama formation Servilleta 1 2,170 kg/m Basalt (Tbs) Intermediate to Servilleta Basalt felsic volcanic 1.25 rocks (Tv) Santa Fe Group Intermediate- composition Syn-rift Density-Depth Function Deposits volcanics of TPVF Santa Fe Figure 6. Densities and total magneti Group (Tsf) 2,300 kg/m3 zations for geologic units of the study Miocene Depth within the basin, km area. (A) Conceptual stratigraphic col Santa Fe Group IA RY 2 umn for geologic units and their time relations to rift formation. TPVF—Taos Older rift-related basalt Plateau volcanic field; SJVF—San Juan TE RT volcanic field. (B) Graphical represen tations of ranges of densities for spe 2,670 kg/m3 cific units or rock types from Table A1; Picuris Early-rift depicted as a density-depth function Formation (Tp) and for the synrift deposits. Values mark Pre-rift Picuris Picuris Formation median values, considered to be the Formation Deposits 2,250 kg/m3 best representation of the overall den Middle Tertiary sity of the unit. Densities are compared volcanic rocks to the average crustal density of 2670 Oligocene SJVF and Latir SJVF volcanic kg/m3 (red dashed line). (C) Graphical volcanic rocks rocks 2,450 kg/m3 representations of potential ranges of total magnetizations for specific units or rock types from Table A2. Note the log scale, which allows for better Paleozoic shale comparisons at low magnetizations Chiefly and sandstone but deemphasizes the large differ 2,540 kg/m3 Upper clastic Paleozoic rocks ences between strongly and weakly rocks (Pzu) Paleozoic magnetic units. Qualitative categories LEOZOIC of total magnetizations for aeromag limestone 3 PA 2,670 kg/m netic interpretation, from effectively Pre-rift non-magnetic to strongly magnetic Rocks for consideration, are from Bath and Metasedimentary Metasedimentary Jahren (1984). rocks rocks Meso- Quartzite & schist 2,680 kg/m3 (Xu) Granite and Granite and granitic gneiss granitic gneiss 2,600 kg/m3 Meta- Metasedimentary Metasedimentary and metavolcanic and metavolcanic morphic Granite 3 sequence 2,700 kg/m sequence Paleo- rocks Metavolcanic Metavolcanic PROTEROZOIC sequence sequence 2,910 kg/m3 Effectively Weakly Strongly non-magnetic magnetic magnetic Magnetic

2,200 2,400 2,6002,8003,000 0.00 0.01 0.10 1.00 10.00100.00

Due to the variability of thicknesses and extents of multiple flows of rocks in the study area are limited to the Servilleta Basalt (Brown et al., 1993), Servilleta Basalt, we decided to estimate density for the combined package requiring different approaches to estimating remanent components or total of Servilleta Basalt and intervening Lama formation rather than consider the magnetizations for other geologic units, as explained in Table A2. For example, effects of high-density (~2700 kg/m3) individual flows. To do so, we assumed remanent components for sedimentary units are considered negligible based that the variability of densities within the Servilleta-Lama package would be on rock type. For volcanic units, observed aeromagnetic magnetic-field data similar to that found within Guadalupe Mountain, a hill composed of trachy- over hills composed of the volcanic unit can be compared to data computed dacites and interbedded sediments at the east-central part of the Taos Plateau, from magnetic models of the same hills to estimate total magnetization di- northwest of our study area. An average density of 2450 kg/m3 for the hill (total rectly. Estimates of minimum and maximum total magnetizations (which we relief of ~1200 m) was estimated from geophysical analysis (Grauch et al., call the potential range because vector components were mostly unknown) 2015). Even if Guadalupe Mountain is not a realistic analog for the Servilleta- were then tabulated for particular geologic units in the study area (Table A2) Lama package, the range of depths of the package is fairly small compared and are summarized graphically on Figure 6C. to the overall thickness of the Santa Fe Group within the interior of the basin The key geologic units for magnetic interpretation are Tertiary volcanic (Fig. 6B). Thus, the gravity effects caused by any density errors are small com- rocks, which are considered to be magnetic to strongly magnetic for pur- pared to the gravity effects of the larger volume of sedimentary fill. poses of aeromagnetic interpretation (Fig. 6C). Of these, Servilleta Basalt is Figure 6B shows that median density values for pre-rift geologic units (ex- the most important, because it is the most widespread in the study area. cept for the pre-rift lower member of the Picuris Formation) have a significant Remanent components are more than an order of magnitude greater than density contrast with the Santa Fe Group (>150 kg/m3) at all depths. Although the induced components for Servilleta Basalt and Middle Tertiary volcanic the range of densities for the San Juan volcanic field rocks overlaps with Santa rocks and for some of the intermediate-composition volcanic rocks (Table Fe Group densities, the range was determined by considering typical densi- A2). Considering that remanent polarities of these rocks can be normal or ties for two dominant rock types within the volcanic field: ignimbrites (2200 reversed (Brown et al., 1993; M.R. Hudson, USGS, 2012, written commun.; kg/m3) and andesites (2500 kg/m3) (Table A1; Drenth et al., 2012). Because the Walker et al., 2012) they are expected to produce both positive and negative, andesites have much greater volume than the ignimbrites (Drenth et al., 2012), large-amplitude aeromagnetic anomalies in the study area. Aeromagnetic the density of the greatest volume of San Juan volcanic rocks that may be anomalies reflecting remanent polarities set at the time of cooling have been present within the study area likely fall within the higher end of the density recognized in the region previously (Grauch and Keller, 2004; Thompson range (>2450 kg/m3). et al., 2006; Drenth et al., 2011; Grauch et al., 2015; M.R. Hudson, USGS, 2015, written commun.). We have assumed that the induced component is the dominant component Key Geologic Units for Magnetic Interpretation for Proterozoic rocks (Table A2) because (1) the long time since their formation increases the likelihood that remanence has been reduced significantly due Total magnetization is the vector sum of two components: remanent and to heat in the crust (McElhinny, 1973), and (2) large-amplitude aeromagnetic induced (Blakely, 1995; Hansen et al., 2005). The induced component is a func- anomalies over them are mostly positive in the region (Cordell 1976; Grauch tion of the quantity of magnetic minerals (commonly magnetite) and is a vec- and Keller, 2004). Aeromagnetic anomalies associated with these rocks are tor that is always oriented parallel to the present-day Earth’s magnetic field. mostly regional and located outside the study area. For these reasons and The quantity of magnetic minerals is proportional to magnetic susceptibility, a from experience after aeromagnetic analysis during the course of this study, property that can be measured from hand samples or outcrops. Typical mag- we consider the effects of the most magnetic Proterozoic rocks to be only lo- netic susceptibilities for geologic units in the study area are tabulated in Table cally important within our study area. A2 of the Appendix. Induced components in Table A2 are obtained by multipli- Magnetic susceptibilities of the Santa Fe Group and Picuris Formation are cation (with proper unit conversion) of the magnetic susceptibilities with the at the high end of common ranges of magnetic susceptibilities for sediments amplitude of the Earth’s magnetic field in the study area (assumed to be 51,700 (Hudson et al., 2008; Grauch and Hudson, 2011), likely due to the large percent- nanoteslas). age of volcaniclastic content. Faults that juxtapose units of the Santa Fe Group The remanent component represents the vector sum of all permanent with differing magnetic susceptibilities are easily detected by high-resolution magnetizations held by the magnetic minerals, which have fixed directions aeromagnetic surveys where no volcanic rocks are present (Grauch and Hud- irrespective of the ambient magnetic field (McElhinny, 1973). Remanent com- son, 2007). However, aeromagnetic anomalies are not produced where there ponents that are generally aligned with or opposite to the present-day Earth’s is no contrast in magnetic properties between juxtaposed units, even if both field are considered to have normal or reversed polarity, respectively. The units have high magnetic susceptibilities. Because most of the synrift sedi- remanent component is determined from paleomagnetic laboratory mea- ments in the study area have high magnetic susceptibilities, it is likely that this surements of oriented samples. Measurements of remanent magnetization of latter scenario is common.

METHODS and multiple faults may appear as a sloping surface. Despite these problems, major basin boundaries are still readily identifiable, albeit somewhat ambigu- 3D Basin Model from Gravity Inversion ous regarding dip and presence of a single versus multiple faults. Modeled thicknesses are underestimated where Servilleta Basalt overlies the sedimentary fill. Gravity data are particularly effective in determining the subsurface config- Attempts to remove the gravity effects of the basalts before inversion suffered uration of structural basins in the Rio Grande rift, owing to the generally large from uncertainty in the extent and thickness of the Servilleta-Lama package contrasts between low-density sedimentary fill and surrounding high-density across the study area and appeared to introduce error rather than correct for it. crystalline bedrock (Cordell, 1979; Keller et al., 1984). We use a gravity inversion Instead, we qualitatively recognize from 2D modeling experiments that the 3D developed specifically for sedimentary basins by Jachens and Moring (1990) basin model surface can be as much as 150 m too shallow underneath a 200-m that takes advantage of these large density contrasts. The technique has been thickness of Servilleta Basalt. Because the thickest basalt layers occur mostly used effectively in several other basins of the Rio Grande rift (Grauch et al., 2009; where the basin is >1 km deep, this error is less than 15%—within the errors Drenth et al., 2013; Grauch and Connell, 2013). It is based on separation of ob- expected to arise from smoothing of normal faults at the basin margins. served data into regional and residual components, where the residual compo- The pre-rift age of the lower part of the Picuris Formation and its similar nent represents the gravity effects of the thickness of basin fill, and the regional density to that of rift-age basin fill introduce some error in the assertion that component represents the effects of density variations in bedrock underlying the 3D model represents a rift subbasin. Maximum estimates of the thickness and adjacent to the basin (Blakely, 1995). Important advantages of this technique of the lower Picuris Formation range from 300 to 400 m (Aby et al., 2004; Mc- are that it (1) incorporates variations of density with depth known from bore- Donald and Neilsen, 2004). If the lower Picuris Formation evenly blanketed the holes, (2) satisfies independent geological and geophysical constraints, (3) treats study area before 25 Ma, the 3D model may overestimate rift fill thickness by the rift basins separately from the complications of density variations in pre-rift as much as 400 m. However, well data from the northern flanks of the Picuris rocks as much as possible, and (4) accommodates 3D aspects of basin shape. Mountains suggest that some erosion of the deposits occurred at higher struc- The 3D gravity inversion of Jachens and Moring (1990) was applied to tural levels (Bauer et al., 2016). Combined with the limitations of modeling isostatic residual gravity data for the San Luis Basin using computer pro- Servilleta Basalt that underestimate basin-fill thickness, both errors may ap- gram DEPTH2BS, version 1.6.8 (B.A. Chuchel, U.S. Geological Survey). Input proximately cancel each other within the deep parts of the 3D model. included the density-depth function for basin fill that was developed for the Albuquerque Basin (Grauch and Connell, 2013; Fig. 6B), wells that reached Aeromagnetic Interpretation Methods below basin fill, and estimated basin thicknesses from seismic data (none of which are in the study area; Drenth et al., 2013). A few additional, empirical Interpretation of the aeromagnetic map centered on recognition of patterns constraints were applied in the extreme southwestern part of the study area to that correspond to the key geologic units, mainly Servilleta Basalt. Patterns of regulate how the method dealt with the large gravity and topographic gradi- strongly negative versus strongly positive magnetic-field values can be used ents in the area (green, circled X in the lower left corner of Fig. 4). to help distinguish basalt with reversed or normal polarity, respectively. Com- We constructed a structural elevation grid representing the basin floor parison of aeromagnetic patterns to topographic patterns also helps determine (Fig. 7) by subtracting thickness values resulting from the 3D inversion from the polarity of the total magnetization. Aeromagnetic patterns that have similar a smoothed digital terrain model. The terrain was smoothed so that artifacts shape to topographic features indicate normal polarity. Aeromagnetic patterns of topography were not introduced during the subtraction. In order to smooth that are similar to topographic features but are inverted (e.g., an aeromagnetic extreme topographic relief in the basin areas (e.g., the Rio Grande gorge) with- low is located over a topographic high) indicate reversed polarity. out overly smoothing the relief in the mountainous rift flanks, we separated the Patterns with high-frequency variations and large ranges in amplitude are digital topography grid into basin and non-basin areas. Lowpass filters were indicative of magnetic rocks that are shallow. In contrast, broad variations with applied to the separated grids, but the filter was more extreme for the basin small ranges in amplitude are indicative of magnetic rocks that are deep. Aero- area grid. The smoothed grids were then merged together before subtraction magnetic lineaments, or alignments of gradients or elongated anomalies, gen- of the thickness grid. erally correspond to steeply dipping, linear geologic contacts, such as faults. General limitations of the 3D gravity inversion approach are discussed in Computing the horizontal gradient magnitude (HGM) of the RTP aeromag- detail in Grauch and Connell (2013, their appendix E). The most significant lim- netic grid facilitated recognition of aeromagnetic lineaments. The method itations for the study area are that (1) abrupt thickness variations across nor- produces a map that forms ridges over steep gradients, intuitively similar to mal faults are extremely smoothed, and (2) thicknesses are somewhat under- finding the inflection point of a curve (Cordell and Grauch, 1985). However, estimated where high-density volcanic rocks are present. Smooth rather than interpretation of these maps is not straightforward for stacked magnetic layers abrupt thickness variations across normal faults arise from the reliance on grids (Grauch and Hudson, 2011), such as the multiple basalt flows represented by to model thickness. Small-offset faults may not be recognizable in the model, Servilleta Basalt, and requires an experienced interpreter.

N 105°45′0″W 105°37′30″W 0 ″

BBOORR--7 BIA-24

36 ° 30 ′ C B Bearear Stetew Limit of shallow Servilleta Basalt BBOORR--55 BOBOR-R-6 Taos Aiirrppoorrtt

MW-1 CCoolloonniiaass Rio Lucero MW-2 RRWWPP--2 MW-4

Rio Grande Rio s SShheeepepsskikin Tao MW-3 de o bl ue TToorrrreeoonn P TTPPDDRWRW io R KK33 RRGG883733730

Arroyo Park EEllssttoonn BBIIAA--5 B′ Figure 7. Structural elevation of the 3D gravity model of the Taos subbasin.

N RRPP33200200 The elevations represent the basin TToowwnn Yaarrd 0 ″ BBOORR--3 R floor and rift flanks with basin fill re io moved. The limit of shallow Servilleta

G Basalt is inferred from Figure 5. ra n 36 ° 22 ′3 d e

d A Rio Pueblo de Taos e RRuucckekenndordorffeer l

R Rio Grande TV-233 a TV-195 BBOORR--11 n c TV-149 h UUNMNM//TTaaoos o TV-115 TV-167 TV-103 C′

D TV-104 U D U Rio Grande D U

Alcon

U D D TV-230 U D U A′

05Elevation (meters above sea level) Deepest part of basin –238 3557 KILOMETERS 400600 8001000 1200 1600 2000 2400 2600 2800

2D Profile Models The matched-filtered regional field was not removed from the data for A–A′, because these data were not RTP, as discussed in the section on Geo- Approach physical Data. In this case, magnetic contrasts in the basement were included in the model. Three profile models were located across the study area (Figs. 3–5) to examine basin features. For each profile, data were extracted from the isostatic residual gravity and RTP aeromagnetic grids. Locations were chosen to op- Magnetic Depth Estimation timize constraints from wells and to demonstrate certain concepts. Geologic relations and geophysical representations were developed synergistically, us- Using the principle that shallow sources generate anomalies with steeper ing constraints from geologic mapping, well data, 3D gravity modeling, and gradients than those produced by similar deep sources, aeromagnetic data are magnetic modeling. Structure of the basin floor was constrained by gravity commonly analyzed to estimate depths to tops of magnetic sources. Grauch modeling, using the 3D basin model as a general guide. The only limited seis- et al. (2004) applied several different depth-estimate methods to gridded aero- mic data that were available provide constraints on B–B′. The configuration magnetic data for the Taos survey, providing an overall view of depths to mag- of shallow basalts is constrained by well data, with help from magnetic mod- netic sources that is consistent with depths to basalts found in wells. For this eling for A–A′. Numerous unknown variables on B–B′ and C–C′ led us to use study, we used the computer program PDEPTH on profile data (Phillips, 1997), magnetic modeling only as a general guide to configuring the shallow basalts, which incorporates a variety of methods so that results can be examined to- explained in the Modeling Limitations section below. gether. One of these, the multiple-source Werner method (Hansen and Sim- monds, 1993), gave the most robust results. The multiple-source Werner method analyzes profile data in a window that Regional Field Removal slides across the profile data. Window size can be varied by the user to focus on shallow or deep sources (small or large window sizes, respectively). The Following standard practice, a regional field was removed from the pro- solutions resulting from the analysis of each overlapping window of data clus- file data before 2D profile modeling. For consistency between profiles, the ter at the top of a source, giving a good estimate of its depth where the source regional fields were extracted from grids. For the gravity profiles, the com- is isolated and the data are free of noise. We tested solutions for both contact puted grid resulting from the final step of the gravity inversion was used as and sheet source assumptions. A contact source is a vertical step with infinite the regional field. This choice allows the modeling to focus on aspects of the depth extent. A magnetic sheet source has zero thickness and extends to in- basin shape. As a result, density contrasts within the basement or caused finity in three directions. From fundamental principles, contact sources are al- by the Paleozoic sedimentary section above the basement need not be in- ways estimated at shallower levels than are sheet sources. Because most mag- cluded in the modeling. For this reason, model bodies representing base- netic sources are better represented by a shape that is somewhere between ment or basement overlain by Paleozoic rocks were all assigned a density a contact and a sheet, a pragmatic approach is to consider that estimates us- of 2670 kg/m3. ing contact and sheet sources provide minimum and maximum estimates of In all profiles, except for A–A′, the regional field removed from the RTP depth, respectively (Hansen and Simmonds, 1993). aeromagnetic data was the longest wavelength output of a matched filtering The large variability caused by layered magnetic basalts that are stacked process (Syberg, 1972; Phillips, 2001). Matched filtering uses the frequency over each other in the study area presents a challenge for any magnetic depth spectrum of the data to design filters that match different parts of the spec- estimation method (Grauch and Hudson, 2007). Thus, the results of the multi- trum. In this case, the regional field was developed from matched filtering of source Werner method were used only as a general guide for modeling pur- the longest wavelengths from an RTP aeromagnetic compilation of surveys poses. Solutions resulting from a variety of window size and contact versus that cover the San Luis Basin from Taos, New Mexico, to Alamosa, Colorado. sheet source assumptions are shown. The sheet source assumption tended to The regional field is assumed to reflect broad variations in basement mag- provide more solutions than for the contact source assumption. netization; so its subtraction from the observed data serves to remove these effects. Thus, model bodies representing basement or basement overlain by Paleozoic rocks were all assigned a magnetic susceptibility of zero. On the Modeling Limitations other hand, some basement-related magnetic anomalies remain in the profile data in areas where the basement is fairly shallow because the wavelengths The 3D gravity model provides the fundamental building block of the sub- of the shallow anomalies are too short to be included in the long-wavelength surface interpretations, because it is a good representation of the overall ba- component of the data. This was the case for C–C′, where an intrabasement sin geometry. However, it cannot distinguish between steeply sloping versus magnetic body was required to fit the data at the mountain front. faulted basin margins. Therefore, the 3D model was used only as a general

guide for constructing basement faults on the 2D models. Faults on the 2D ded with sediments, where the shallowest flows are mostly within 100 m of the models were located using mapped faults or local gradients in the observed surface, or at elevations of 2023–2130 m above sea level (Table 1). The deep- data; then fault displacements were constrained by the fit of the calculated est basalt intervals in these wells generally extend downward to elevations of gravity curve to the observed curve. ~1900 m, giving an overall interval of Servilleta Basalt and intervening Lama Modeling of shallow Servilleta Basalt for the profiles proved difficult be- formation of ~200 m. cause of the high sensitivity of the magnetic modeling to multiple, unpredict- The patterns and commonly low aeromagnetic values within the area in- able unknowns: (1) variations in thickness and total magnetization of individual terpreted as shallow Servilleta Basalt indicate that total magnetizations of the layers, and (2) variations in thickness, number of layers, and total magnetiza- multiple layers are dominantly reversed polarity. This inference is supported tions between layers within a stack of multiple layers. Only small variations in by comparison of aeromagnetic patterns to topographic patterns and paleo- any one of these variables produce large differences in the calculated magnetic magnetic measurements from the Rio Grande gorge. Topographic relief de- curve. One could fit the magnetic curve by arbitrarily choosing to vary only veloped on Servilleta Basalt inversely correlates to the shapes of anomalies one of these variables, but the result may not be meaningful. Moreover, three- in several places, indicating that the total magnetizations of layered Servilleta dimensional lateral variations in the basalts are only poorly captured from a Basalt in those areas are dominated by strong, reversed-polarity remanence. profile across the aeromagnetic data; 2D modeling may not properly represent For example, the Rio Grande gorge, which is a topographic low incised into the lateral variations. Instead, we rely mostly on well data to constrain thickness layers of Servilleta Basalt, correlates inversely to a strong linear aeromagnetic and depth to basalts for the profile models. Magnetic depth estimates provided high across most of the gorge’s extent across the study area (Fig. 5). Paleo- some general guidance on depth to the bottom of stacked basalt layers and lat- magnetic measurements (Brown et al., 1993) from a 132-m vertical stack of eral terminations of the basalts. Gravity modeling provided little help because layers of Servilleta Basalt in the Rio Grande gorge (“Pmag site” on Fig. 5) show the data are fairly insensitive to variations in depth and thicknesses of the ba- polarities from the lower 38–132 m of the section are reversed and for the top salts in comparison to variations in thickness of the basin fill. 38 m are normal.

GEOPHYSICAL INTERPRETATIONS Southern Subbasin Margin and the Embudo Fault Zone

In this section, the geophysical data are integrated with geologic mapping A zone of northeasterly trends is evident in the aeromagnetic image be- and well data to build different aspects of the picture of the buried geometry tween the mapped Embudo fault system and the Rio Pueblo de Taos, an area of the Taos subbasin. We begin by describing the aeromagnetic expression of that is mostly covered by surficial deposits (L1 and L2 on Fig. 8). The north- Servilleta Basalt, because it is a prerequisite for understanding the geophysical easterly zone, and lineament L1 in particular, is remarkably straight for almost expression of faults at the subbasin margins. We then address the subsurface 11 km. Lineament L1 corresponds to the southern limit of the interpreted area configurations of subbasin margins associated with the major rift fault sys- of shallow Servilleta Basalt (long-dashed lines on Figs. 3, 5, and 7). The region tems and their relations to intrabasin basaltic layers. We end with a view of the of very smooth, relatively high magnetic values to the southeast of this line is overall subbasin geometry compared to shallow fault patterns. interpreted as the expression of the sedimentary section without the presence of Servilleta Basalt. This interpretation is supported by deep water wells in the area (cf. Fig. 8 and Table 1). Lineament L2 generally follows Rio Pueblo de Aeromagnetic Expression of Servilleta Basalt Taos (Fig. 8). Close inspection of the anomalies compared to the stream channel suggests the sources of the aeromagnetic highs are contained within the The highly varied, “busy” anomaly pattern observed in the aeromagnetic southern bank of the stream rather than a topographic effect (Fig. 8). Previous image in the central and northwestern region of the study area (Fig. 5) is inter- workers have speculated that a structure underlies this stream on the basis of preted as the expression of near-surface volcanic rocks related to the Taos Pla- its linearity (Muehlberger, 1979; Machette and Personius, 1984). teau volcanic field, primarily Servilleta Basalt. This interpretation is supported The orientation and linearity of anomalies within the northeasterly zone by (1) exposures of Servilleta Basalt scattered throughout the area and in the suggest that they are associated with the Embudo fault zone. The abrupt limit Rio Grande gorge (Fig. 3); (2) their strong magnetizations (Fig. 6C; Table A2 in of the aeromagnetic patterns associated with Servilleta Basalt at aeromagnetic the Appendix); and (3) presence and absence of basalt in shallow water wells lineament L1 suggests that the basalt terminates against a fault. This inter- that match the differences in the aeromagnetic patterns (Grauch et al., 2004). pretation is supported where the lineament coincides with a fault mapped for The eastern and southern limit of the area interpreted as shallow Servilleta a short distance across the basalt-rimmed, Hondo Canyon in the southwest- Basalt is outlined by a long-dashed line (Fig. 5). Within the interpreted area of ern corner of the study area (Fig. 8). The basalt is not present south of the shallow Servilleta Basalt, well data show evidence of multiple flows interbed- mapped fault.

02 Arroyo Park 02 Arroyo Park L2ElElststonon L2ElElststonon KM BIBIA-A-5 KM BIBIA-A-5

RPRP32003200 RPRP32003200

L1 L1

A Rio Pueblo de Taos A Rio Pueblo de Taos

Rio Grande TV-233 Rio Grande TV-233 TV-195 BOBOR-R-11 TV-195 BOBOR-R-11 TV-149 TV-149 UNUNM/M/TaTaosos UNMUNM/T/Taoaos TV-115 TV-167 TV-115 TV-167 TV-103 TV-103

Hondo Hondo L2 Canyon D L2 Canyon D TV-104 U TV-104 U

Rio Grande Rio Grande

L1 Alcon L1 Alcon

U D U D D TV-230 U D D TV-230 U D U A′ U A′

Figure 8. Southwestern portion of the study area (located on Fig. 2) and location of A–A′. (A) Geologic map and well locations extracted from Fig. 3. (B) Reduced-to-pole (RTP) aeromagnetic image extracted from Figure 5. Arrows point out aeromagnetic lineaments L1 and L2 that define the limits of a zone of northeast-trending aeromagnetic lineaments. The lineaments are inferred as faults. One short segment of a mapped fault in Hondo Canyon corresponds to lineament L1. The basalt at the rim of the canyon does not continue south of the fault, which supports the interpretation of the aeromagnetic lineament, depicted in A–A′ (Fig. 9).

To examine the interpretations of this zone of lineaments more closely, we termination of Servilleta Basalt. A number of other faults are inferred from constructed a 2D model along profile A–A′ (Fig. 9). This profile location utilizes the aeromagnetic data. Well data suggest that the number of flows and their the aeromagnetic data across the lineament zone on the north and the ground thicknesses increase to the north within the fault blocks. magnetic data across the mapped Embudo fault system on the south (Fig. 5). Second, constraints from the 3D basin model and geologic mapping sug- Lithologic picks from a number of water wells, which fall within 2 km of the gest that the Embudo fault zone is 7 km wide along profile A–A′, with signifi profile location (Fig. 8), were used to help construct the geologic cross section cant normal displacement (at least 1.8 km). The zone extends from mapped and geophysical model. Geologic relations for this section were also guided by faults 0.5 km north of well TV-230 to the Rio Pueblo de Taos (distance 8 km a number of additional parallel and crossing, geophysically constrained, geo- to 1 km, respectively, on Fig. 9). Almost 1 km of the normal displacement is logic cross sections (not shown), which were constructed south of Rio Pueblo accommodated between the L1 fault and the northernmost mapped strand of de Taos for a hydrogeology study (Bauer et al., 2016). the Embudo fault (between 3.5 and 6 km distance on Fig. 9). Section A–A′ (Fig. 9) shows several important features that are well con- Finally, prominent anomalies from the ground magnetic traverse (shown strained by the data and geologic mapping. First, wells and 2D magnetic after upward continuation on Fig. 9 between distance 5.5 and 7.5 km) are likely modeling support the interpretation of aeromagnetic L1 as the southern fault caused by basaltic rocks reported deep in the Alcon well. The best fit from

GEOSPHERE | Volume 13 | Number 3 Grauch et al. | Geophysical insights into the evolution of Rio Grande rift margins and the Embudo transfer zone Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/3/870/2316968/870.pdf 889 by guest on 01 October 2021 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/3/870/2316968/870.pdf Research Paper Figure 9. Geophysical model and geologic cross section along profile A–A The fault lines are wavy to indicate that they represent complex fault zones. Regional gravity field was removed before modeling. by total magnetization polarity and intensity in amperes per meter (A/m). Circled A and T indicate strike- slip movement away and toward the viewer, respectively. Elevation (meters) Elevation (meters) Gravity (mGal) Magnetics (nT) A 1,000 1,500 2,000 2,500 1,000 1,500 2,000 2,500 (Northwest ) –400 –300 –30 –20 –10 500 500 0 0 Ve Qa To Basin Gravity Dat a Geologic Section Geophysical Model rtical Exaggeration =2 Rio Pueblo Rio Pueblo tal-field Magnetic Dat Ts Ts Tp de de QT f b T l Ta Ta 1 TV-195 TV-195 os os A L2 Tp Reversed, 1.5 Reversed, 0.5 Normal, 0.5 Reversed, 5.5 T Aeromagnetic dat Ts f A Susceptibility= 0.001 SI Density= 2,170 kg/m Basin fill T × projected Tp 2 TV-149 projected TV-149 Ts A/ m A f A/ m A/ m A/ m a a T Tp projected TV-115 projected Ts TV-115 f GEOPHYSICAL 3 A 3 Older Basalt (Tbo ) Servilleta Basalt

L1 L1 L1 QT Xu Highway Highway l Distance (kilometers) Tp Ts 68 68 f Tbo? 4 ′ (located on Fig. 8). Geologic codes from Figure 3. Modeled basalt layers are color coded T MODE L EXPLANA projected TV-10 3 projected TV-10 3 A Upward-continued traverse dat Tp 5 Qa Xu Tb Ts Density= 2,670 kg/ Basement Rocks Susceptibility= 0.010 SI projecte d TV-103 f o

T Susceptibility= 0.001 SI Density= 2,300 kg/ Basin fill TION A Xu 6 Figure 8 We Base of 3D basin mode Physical-property boundary Tb Calculated Calculated Observed Observed Ts T ll from m o f 3

Tp A m Density= 2,670 kg/ Basement Rocks Susceptibility= Xu a T 3

able 1, located on Tb 7 o Alcon Alcon T Xu A Regional field 0 T l m 8 Qa 3 A

Xu TV-230 TV-230 A′ (Southeast)

–30 –20 –10 –400 –300 2,000 3,000 4,000 5,000 6,000 7,000 8,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 0

(feet) Elevation (feet) Elevation (mGal) Gravity (nT) Magnetics

GEOSPHERE | Volume 13 | Number 3 Grauch et al. | Geophysical insights into the evolution of Rio Grande rift margins and the Embudo transfer zone 890 Research Paper

magnetic modeling was a sheet-like source with strong, reversed-polarity total cept, but conflicts with the gravity constraints, which show a progressively magnetization, consistent with a volcanic layer at the elevation found in the westward-deepening basin floor. A concept of a low-angle normal fault with well. Total magnetization with reversed polarity is more indicative of a volcanic rotated blocks also assumes all basaltic intervals are Servilleta Basalt but uses rather than Proterozoic source, for reasons given in the section on magnetic significant back tilting on rotated blocks to explain the basaltic intervals at dif- properties. In its position near the base of the Picuris Formation, the basaltic ferent depths (Fig. 10B). This concept is contraindicated by seismic sections, rocks in the Alcon well must be older than 34.5 Ma (Aby et al., 2004). Total which show subhorizontal or slightly west-tilted horizons instead of east-tilted magnetization of 5.5 A/m, derived from magnetic modeling, is consistent with blocks within 4–5 km of the mountain front (Fig. 10B). Moreover, there is no magnetizations estimated for middle Tertiary volcanic rocks (Fig. 6C). Although geologic evidence of low-angle faulting at the mountain front farther east. The we have not tried to depict its presence deeper than 1200 m elevation on Fig- third concept (Fig. 10C) is a half-graben, which assumes that the basaltic in- ure 9, our discovery of likely late Eocene lava flows at the southern part of this tervals in the wells do not correlate from east to west. The eastern basaltic profile suggests that deep pre-rift volcanic rocks may exist elsewhere, deeper rocks are assumed to be older than Servilleta Basalt, implying that Servilleta within the subbasin. Basalt was never deposited on or was eroded off the structural block on the The details of the fault shown beneath Rio Pueblo de Taos in A–A’ are not east. Making this assumption allows for a half-graben geometry that fits all the well constrained. No fault is exposed, yet a fault is suggested by aeromagnetic independent evidence. Implications of this scenario are that the master fault lineament L2. L2 on A–A’ is represented by the prominent high-low anomaly of the half-graben was previously located between the two eastern wells and pair between 0.5 and 1.5 distance (Fig. 9) The high of the pair follows the south then later shifted to the modern location next to the range front. bank of the stream. We found it difficult to model the anomaly pair as faulted Note that all three conceptual models in Figure 10 depict listric faults that and displaced basalt layers while maintaining a semblance of correlation be- join a detachment at the brittle-ductile transition below the subbasin floor. tween basalts in wells along the Rio Pueblo de Taos off the profile (e.g., Bauer Such a detachment is required to provide a tectonic mechanism for develop- et al., 2016). Instead, we model the anomaly pair as an individual, normal- ment of the left-lateral slip observed for the Embudo fault zone (Morley et al., polarity flow restricted to the south bank of Rio Pueblo de Taos and extending 1990; Faulds and Varga, 1998; Goteti et al., 2013). lengthwise in and out of the plain of the cross section. To explain the linearity The utility of the third concept of a half-graben with different-age basaltic of the high along L2, we argue that this model represents a late-stage Servilleta rocks for satisfying all the known evidence is demonstrated by geophysical lava that flowed along a structurally controlled channel. Overall, correlations models for profiles B–B′ and C–C′ (Figs. 11 and 12). Profile B–B′ crosses the in the wells and magnetic modeling are difficult along the Rio Pueblo de Taos; subbasin near its deepest spot and utilizes several deep wells for constraints these difficulties allow for alternate models depicting the details of this area. (Fig. 7). The lowest basaltic interval penetrated in the K3 well on B–B′ is verti- Stratigraphic relations that are difficult to correlate between wells are consis- cally separated by 162 m (530 ft) from the interval intersected by the Torreon tent with growth faulting, which is commonly observed in the area (Bauer and well; yet the wells are less than 1.5 km apart. If the basalt in the Torreon well is Kelson, 2004a). older than those in the K3 well, following the concept in Figure 10C, the modeled faults fit constraints from gravity data and a seismic-reflection section while explaining the relations hypothesized for the basaltic rocks found in the Eastern Subbasin Margin and the Sangre de Cristo Fault System wells. A 6-km-wide zone of down-to-the-west basement faults, accommodating the cumulative displacement along the eastern subbasin margin, is mod- Previous workers have interpreted a deep (>200 m) graben along the east- eled west of the range front. ern subbasin margin (Drakos et al., 2004) by correlating basalts between wells. Similar relations among faulted basalts and subbasin geometry that de- For example, the shallowest basalts in BOR-5 and Torreon wells, which are veloped from modeling B–B′ also result from modeling profile C–C′ (Fig. 12). only 1–3 km east of the area interpreted as shallow Servilleta Basalt (Fig. 5), C–C′ crosses the subbasin margin just north of where the Embudo and Sangre are more than 300 m below the elevation of 1900 m expected for the lowest de Cristo fault systems join (Fig. 2) and ties to B–B′ at the Taos Airport well. As Servilleta Basalt. The shallowest basalts in the BOR-3, BOR-6, and Town Yard in B–B′, hypothesizing that basaltic rocks that are found deep (180 m) in the wells are only slightly higher than 1900 m elevation (Table 1). Magnetic depth BOR-3 and Town Yard wells are older than those found much shallower (30 m) estimates also delineate this zone of deep magnetic basalt east and southeast in the Elston well only 2 km to the northwest allows a model that is compatible of the interpreted area of shallow Servilleta Basalt (Grauch et al., 2004). with both the well data and the gravity constraints. Three different concepts to explain the deep basaltic intervals in wells on Both profile models B–B′ and C–C′ depict basement faults stepping down the east are illustrated schematically in Figure 10. Previous workers’ interpre- into the subbasin away from the range front and then transitioning to a gentler tation of a graben within a half-graben (Fig. 10A) is the simplest explanation incline on the basin floor that slopes up toward the Rio Grande. This basin-floor and assumes that the basaltic intervals are all Servilleta Basalt. A major, east- slope transition occurs underneath the area of shallow basalt in both models, just down normal fault between the two eastern wells is required to fit this con- west of well K3 in B–B′ and near the Rio Pueblo de Taos in C–C′ (Figs. 11 and 12).

A WEST Supporting or Permissive? Interpreted Seismic Horizons EAST ime

T 0.0 Ground surface GRABEN WITHIN A Basaltic rocks in wells ) BASALT BEDROCK Exposed geology avel BASALT

HALF-GRABEN Tr Gravity data 0.5 BASALT Aeromagnetic data (seconds (Disrupted) o-way

Seismic horizons Tw

Basaltic flows Sedimentary rift fill

Approximate base of fill from gravity modeling Pre-rift bedrock

Listric faults merge at brittle-ductile boundary Schematic Basin Geometry

B Figure 10. Schematic representations of WEST Supporting or Permissive? Interpreted Seismic Horizons EAST three different concepts to explain abrupt ime T 0.0 Ground surface LOW-ANGLE NORMAL Basaltic rocks in wells ) eastward deepening of basaltic intervals BASALT BEDROCK Exposed geology avel BASALT observed in wells relative to the Sangre FAULTS WITH ROTATED Tr BASALT Gravity data 0.5 de Cristo fault zone. A checklist indicates

BLOCKS (seconds (Disrupted) Aeromagnetic data whether independent evidence supports o-way

Seismic horizons Tw or permits (green check) or conflicts with (red X) each concept. Gravity data con straints are represented by the dotted Basaltic flows magenta line. An interpretation of hori Sedimentary rift fill zons constrained by seismic-reflection Approximate data (Reynolds, 1986) is shown for com base of fill from parison at the appropriate position at the gravity modeling range front. Small-displacement, west- Pre-rift bedrock down, Los Cordovas faults are not shown. Assuming basaltic rocks are different ages (C) is the only concept where all known evidence is satisfied. Listric faults merge at brittle-ductile boundary Schematic Basin Geometry C WEST Supporting or Permissive? Interpreted Seismic Horizons EAST me

Ti 0.0 Ground surface HALF-GRABEN WITH Basaltic rocks in wells ) BASALT BEDROCK Exposed geology avel BASALT

BASALTIC ROCKS OF Tr T Gravity data 0.5 BASAL

DIFFERENT AGES (seconds (Disrupted) Aeromagnetic data o-way

Seismic horizons Tw

Basaltic flows Sedimentary rift fill

Approximate base of fill from gravity modeling Pre-rift bedrock

Listric faults merge at brittle-ductile boundary Schematic Basin Geometry

B (Northwest) B′ (Southeast)

200 RTP Aeromagnetic Data –300 Regional field (nT) 100 –400 0 –500 –100 =Calculated –600 Good fit to observed data not attempted; too many unknown variables Regional field

Magnetic Data (nT) –200 =Observed –700 Regional field (mgal 0 Basin Gravity Data 0

–10 Regional field –10

Gravity (mGal) –20 –20 =Calculated =Observed ) –30 –30

2,500 Geophysical Model Rio Pueblo 8,000 Taos Seismic Line Rio Airport de Taos Torreon Grande MW-2 MW-4 MW-3 K3 (projected) 7,000 2,000

6,000 Elevation (feet)

oterozoic rocks ? 1,500 5,000 Basin Fill Density= 2,170 kg/m3 ? rocks overlying Pr Susceptibility= 0.001 SI 4,000 Elevation (meters) Series of ? Paleozoic 1,000 small-displacement Density boundary, not a geologic contact 3,000 Basin Fill Basement Rocks faults, Density= 2,300 kg/m3 Basement Rocks 3 no Density= 2,670 kg/m t modeled Susceptibility= 0.001 SI Density= 2,670 kg/m3 2,000 Susceptibility= 0 500 Fault blocks composed of Susceptibility= 0

1,000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Vertical Exaggeration =3× Distance (kilometers)

GEOPHYSICAL MODEL EXPLANATION Normal, 1.0 A/m Physical-property boundary Taos Magnetic Depth Estimates Airport Well from Table 1 and Sheet source, window=100 m Reversed, 1.0 A/m Servilleta Basalt Base of 3D basin model located on Figure 3 Contact source, window=100 m Reversed, 1.5 A/m Mapped normal fault, with Sheet source, window=2,100 m Reversed, 2.0 A/m Older Basalt sense of displacement Sheet source, window=4,300 m

Figure 11. Geophysical section along profile B–B′. Structure of the basin floor was constrained by gravity modeling, using the 3D basin model as a general guide, and seismic data near the range front (Reynolds, 1986). Faults are implied by steps and abrupt offsets of the modeled bodies. Modeled densities and magnetic susceptibilities are given except for basalt layers, which are color coded by total magnetization polarity and intensity in amperes per meter (A/m). Older basalt (dark blue) is conceptually required to reconcile geophysical and well data. The regional fields shown were removed from the data before modeling. Magnetic depth estimates are depicted as solutions from the multi-source Werner method (Hansen and Simmonds, 1993), where the circle shows the top of a contact or sheet, as indicated. RTP—reduced-to-pole.

C (Northwest) C′ (Southeast)

200 RTP Aeromagnetic Data –200 Regional field (nT) Regional field 100 –300 0 –400 –100 =Observed –500 Good fit to observed data not attempted; too many unknown variables =Calculated –600

Magnetic Data (nT) –200

0 Basin Gravity Data Regional field (mgal) Regional field –10 –10 –20 =Observed

Gravity (mGal) –20 =Calculated –30 –30

2,500 Geophysical Model Ruckendorfer 8,000 Taos Rio Pueblo (projected) Arroyo Airport RG83730 de Taos Park BOR3 Elston 7,000 2,000

6,000

Elevation (feet) ? s 1,500 ? 5,000 Basin Fill Density= 2,170 kg/m3 erozoic rock Susceptibility= 0.001 SI 4,000 Elevation (meters)

1,000 Ser ies of small-d Density boundary, not a geologic contact 3,000

3 isplacem 04 SI

Basin Fill 0 ent faults, Density= 2,300 kg/m3 sed of Paleozoic rocks overlying Prot ocks 2,000 Basement Rocks R 70 kg/m Basement Rocks not modeled Susceptibility= 0.001 SI 6 500 Density= 2,670 kg/m3 Density= 2,670 kg/m3

Susceptibility= 0 ity= 2, Susceptibility= 0 ceptibility= 0. 1,000

Basement

Dens Fault blocks compo Sus

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Vertical Exaggeration =3× Distance (kilometers)

GEOPHYSICAL MODEL EXPLANATION

Normal, 0.2 A/m Physical-property boundary Magnetic Depth Estimates Normal, 1.0 A/m Base of 3D basin model Sheet source, window=100 m Reversed, 0.5 A/m Servilleta Basalt Mapped normal fault, with Sheet source, window=2,100 m sense of displacement Reversed,1.0 A/m Sheet source, window=4,300 m Taos Reversed, 2.5 A/m Airport Well from Table 1 and Contact source, window=4,300 m located on Figure 3 Reversed, 2.0 A/m Older Basalt

Figure 12. Geophysical section along profile C–C′. Description of modeling, symbols, and annotations as in Figure 11. A magnetic, intrabasement contact is indicated by the cluster of magnetic depth estimates at about distance 19 km. The modeled body has narrow extent, and may represent an intrusion within the basement. Its top may represent the contact between magnetic crystalline rocks and overlying, much more weakly magnetic, Paleozoic sedimentary rocks. RTP—reduced-to-pole.

This overall depiction of the basin floor in both models is well constrained by is present at ~300 m depth in this location but is not present to the east. Cor- the gravity data. The 3D basin model shows a smoothed version of this overall relating this interpreted basalt layer to basalt evidence in the Torreon well to geometry (purple dotted line on Figs. 11 and 12). The profile modeling builds the west suggests only small displacements due to basement faulting. Finally, on the 3D model depiction, yielding a more geologically reasonable depiction subhorizontal older basaltic layers extending southeastward from BOR-3 well of basement faulting that provides a good model fit and is guided by mapped in C–C′ are supported by the aeromagnetic modeling, aeromagnetic map inter- faults and by breaks indicated by the aeromagnetic and seismic data (where pretation, and magnetic depth estimates. Magnetic depth estimates indicate a available). Although the details of offsets and dips of basement faults are still significant magnetic contrast occurs somewhere within the top 200 m of the poorly resolved, we have confidence in the general depiction of the basin floor. surface at about distance 17 km (Fig. 12), which is consistent with the trunca- Models for both B–B′ and C–C′ show that the deep, older basalt terminates tion of magnetic basaltic rocks against sedimentary basin fill. west of the range front and has little to no displacement overlying faulted basement to the west, where the older basalt is modeled underneath the younger basalt (between 16 and 18 km distance on Fig. 11 and 15–17 km distance on Northeastern Subbasin Margin Fig. 12). These relations derive from several independent constraints. First, the easternmost presence of modeled older basalt in B–B′ is constrained by Trends of contours in the 3D model show that the northeastern subbasin the proprietary seismic line that nearly coincides with the eastern end of B–B′ margin changes orientation going northward, from north to northwest. It is (Fig. 11). A strong reflection observed at the western end of the line is the only most obvious along contours that follow the locations of the Torreon, Sheep- strong reflection along the line with a travel time that places it well above the skin, BOR-5, and BOR-7 wells (Fig. 13B). Even though the north-striking part of gravity-constrained basement. This observation suggests that the older basalt the mapped Hondo section of the Sangre de Cristo fault deviates from the ori-

A L4 L5 B L4 L5 BIA-24 BIA-24 BBOOR-R-7 BBOOR-R-7

L3 Bearear StStewew L3 Bearear StStewew

BBOORR--55 BBOORR--55

BOBOR-R-6 BOBOR-R-6

MW-1 CoCololoniniaass Rio Lucero MW-1 CoCololoniniaass Rio Lucero MW-2 MW-2 MW-4 MW-4 SShheeepepsskikin ShSheepsepskikin MW-3 MW-3

TToorrrreeanan TTPPDDRRW TToorrrreeoonn TTPPDDRRW L3 L3 L5 L5 KK33 L4 K3K3 L4 RRGG83733730 RGRG837383730 02 02

KM KM

Figure 13. Maps of the northeastern portion of the study area (located on Fig. 2). (A) Reduced-to-pole (RTP) aeromagnetic image, mapped faults, and well locations extracted from Figure 5. (B) Map of 3D basin model elevation contours extracted from Figure 7. Arrows point out north-trending aeromagnetic lineaments L3 and L5 and northwesterly L4, all of which are inferred to be buried faults that terminate the lateral extent of different basalts. Short-dashed white lines are seismic line locations (Reynolds, 1986, 1992). Long-dashed white line is the limit of shallow Servilleta Basalt inferred from the aeromagnetic data (Fig. 5).

entation of the subbasin margin within the study area, the northwest trend of defined than the aeromagnetic expression of mapped Los Cordovas faults to the subbasin margin parallels the overall northwest strike of the Hondo section the west (cf. Figs. 5 and 12). We explain the difference by the absence of the to the north, outside of the study area (Fig. 1; Machette and Personius, 1984; youngest Servilleta Basalt flows on the east and interpret this area to be out- Kelson et al., 1998b; Bauer and Kelson, 2004a). A subtle gradient in the aero- side the limit of inferred shallow Servilleta Basalt (Fig. 13A). magnetic image (lineament L4) has a similar northwest trend (Fig. 13A). (Note that several high-frequency anomalies in this area are caused by buildings located along NE-SW roads). In fact, L4 is located where the slope of the sub- Overall Subbasin Geometry and Shallow Fault Patterns basin margin steepens somewhat (not easily apparent from the color contour maps). Thus, although the aeromagnetic expression of L4 is caused by basalt The geophysical models show an elongated and asymmetric subbasin that was emplaced late in rift history, its coincidence with the major subbasin within the Taos embayment (Fig. 7). The subbasin is crescent-shaped, sim- boundary suggests that it is associated with a fault system that has had a long ilar to the outline of the Taos embayment, and reaches a maximum depth history since early rift time. of ~2 km below the valley floor near the maximum curvature of the crescent An area of smooth aeromagnetic texture extends for ~4 km west of the (X on Fig. 7), or at ~36 °26′ N latitude and 105° 37′ W longitude. Closed con- Hondo section of the southern Sangre de Cristo fault (Fig. 13A). It is bounded tours on Figure 7 suggest an imperfect D shape that is elongated on the north on both east and west sides by northerly trending lineaments L3 and L5 and is and southwest. centered over the northeastern subbasin margin. We interpret the broad low These overall subbasin shapes are the result of cumulative fault activity as evidence that the deep older basaltic rocks modeled in B–B′ and C–C′ are at the rift margins since the inception of rifting, at ca. 25 Ma. In contrast, lin- widespread in this area. Wells where deep basaltic rocks are found are located eaments in the aeromagnetic data represent faults expressed by offsets in within the area of the low (wells BOR-5, BOR-6, and Torreon). Evidence for the shallow, magnetic volcanic rocks, mainly 5.5–3 Ma Servilleta Basalt within the 300–400-m-deep, strong subhorizontal reflections from the proprietary seis- interior part of the subbasin (Fig. 5). Thus, comparing the 3D model to faults mic data collected along lines near and across Rio Lucero also generally falls inferred from aeromagnetic data can provide insights into the structure and within the broad low (Fig. 13A; Reynolds, 1986, 1992). perhaps age of shallow versus deep parts of the subbasin. The lineaments L3 and L5 support the hypothesis that major normal faults To develop a fault map from the aeromagnetic data for the study area, we have been active on either side of the region of the broad low, where we in- computed the horizontal gradient magnitude (HGM) of the RTP aeromagnetic terpret deep basaltic rocks overlain by sediments. On the eastern side, L5 is data (Fig. 14A). Lines drawn where faults are interpreted from the HGM map a curvilinear, narrow high-low aeromagnetic anomaly pair that is located be- are combined with mapped faults in Fig. 14B. The overall pattern of faulting tween and generally parallel to the mapped range-front faults and the Rio Lu- shows (1) northeasterly trending faults paralleling Rio Pueblo de Taos that cero (Fig. 13). The narrow low consistently corresponds to a distinctive zone extend across the whole study area, (2) a northwesterly trend that intersects of incoherent reflections in the proprietary seismic data (Reynolds, 1986, 1992) the northeasterly pattern near the confluence of the Rios Pueblo de Taos and and to a zone where stratigraphic correlations break down between time- Lucero, (3) a zone of faults (both mapped and aeromagnetically inferred) that domain electromagnetic soundings within the top 300 m of the surface (L. Ball, follow the entire mountain front surrounding the Taos embayment, and (4) a USGS, 2015, proprietary data). Synthesis of these proprietary data with the wide zone of northerly trending faults throughout the interior of the subbasin aeromagnetic and gravity data suggests that the narrow aeromagnetic low that is bounded on the east by the northeasterly trend. coincides with (1) a major-displacement fault at the basin margin associated The fault patterns from geologic mapping and aeromagnetic interpretation with the Sangre de Cristo fault system and (2) a buried paleochannel incised are overlain on the 3D basin model from the gravity data on Figure 15. The at the fault zone that separates basalt on the west from basement rocks on the northeasterly and northwesterly trends in the fault patterns generally follow east (Bauer et al., 2014). the crescent-shaped eastern margin of the subbasin (compare inferred faults On the western side of the broad aeromagnetic low, L3 is a prominent to the zone of green colors that parallel the mountain front on Fig. 15), suggest- NNW, linear aeromagnetic high that lies near the eastern limit of the inter- ing that these younger, shallow faults followed the same general pattern as the preted area of shallow Servilleta Basalt (white long-dashed line on Fig. 13). older, deeper faults. The northeasterly trend is aligned with the more regional Two other similar linear aeromagnetic highs parallel lineament L3; one is 1 km Jemez lineament (Fig. 1), suggesting a more pervasive influence of this crustal and the other is 2 km farther west. The prominent linear magnetic highs can be boundary than previously thought. The shallow faults that ring the embay- explained by faults that disrupt layers of primarily reversely polarized basal- ment at the mountain front deviate from the overall subbasin margin only in tic layers, with negative anomalies over the layers and highs over the faulted the northeastern part of the study area, where the deep subbasin margin fol- edges. The linear anomalies parallel mapped Los Cordovas faults to the west lows the narrow zone of northwesterly striking shallow faults instead (e.g., L4 (Fig. 2), suggesting the linear anomalies mark the easternmost extent of Los on Fig. 15). The wide zone of northerly trending faults in the central part of the Cordovas–style faulting. These linear anomalies are more prominent and well study area occurs within the deep part of the basin. The easternmost of these

A 105°45′0″W 105°37′30″W B 105°45′0″W 105°37′30″W

L5 L4

Rio Lucero

Grande

o Ri

R i

0 ″N 0 ″N o

e Taos G d r an d e

36 ° 22 ′3 36 ° 22 ′3 Town io Pueblo d R e l

of Taos R

a n c h L2 L1 o

D D U U D U D U D U Rio Grande D U

U D U D D D U D U D U U

Nanoteslas/meter 05 Deepest part of basin 0.00 0.03 0.05 0.10 0.19 0.28 0.44 2.76 KILOMETERS

Figure 14. Aeromagnetically inferred faults. (A) Horizontal-gradient magnitude (HGM) map of the reduced-to-pole aeromagnetic data (HGM map) for the high-resolution survey area only. Inter ference from buildings is evident within the Town of Taos (white-dashed line). (B) Faults (magenta lines) interpreted along linear ridges and breaks in patterns in the HGM map and original aero magnetic image following methods discussed in text. Aeromagnetic lineaments L1–L5 are from Figures 8 and 13. Mapped faults are from Figure 3. Note the prominent zones of northeasterly and northerly patterns of lineaments that abruptly terminate or change course eastward of a “corner,” ~3 km east of the deepest part of the basin (x).

faults crosses the northeastern subbasin margin obliquely. The divergence of DISCUSSION strikes of the shallow faults at the northeastern subbasin margin suggests that northwest-striking margin faults represent an early phase of rift-margin devel- Details of the relations among faulting and basaltic layers resulting from opment, which later shifted to more northerly strikes. Alternatively, the north- the geophysical interpretations and modeling have important implications for west-trending rift margin could be the result of parallel, north-striking faults the evolution of the four major fault systems surrounding the Taos embay- that (a) systematically increase in strike length to the west, and (b) consistently ment. In this section, we discuss several of these implications and how well lose vertical throw toward their northern tips. they are constrained.

N 105°45′0″W 105°37′30″W 0 ″ 36 ° 30 ′ L5 L4

Rio Lucero e Rio Grand Rio s Tao de o bl ue P io R

Figure 15. Mapped (black) and inter preted (yellow) faults from Figure 14 overlain on the 3D model of Figure 7. Interpreted faults associated with N

aeromagnetic lineaments of Figures 0 ″

R 8 and 13 are labeled. Limit of shallow io Servilleta Basalt (white long-dashed G ra line) is interpreted from aeromagnetic n 36 ° 22 ′3 d e data.

d Rio Pueblo de Taos e l

Rio Grande R aan n c h L2 o L1 Survey Boundary

D U D U Rio Grande D U

U D D U D U

05Elevation (meters above sea level) Deepest part of basin –238 3557 KILOMETERS 400 600 800 1000 1200 1600 2000 2400 2600 2800

Ages of Deep Basalts vation for the top of the Picuris Mountains near profile A–A′, the total structural relief across the Embudo fault zone is ~2.6 km, somewhat less than the An important hypothesis resulting from the modeling done to reconcile 3.0 km originally estimated by Muehlberger (1979). Using this estimate, an geophysical and well data is the presence of faulted basaltic rock in the eastern average vertical-slip rate of 87 m per million years (m/m.y) is required since and southeastern subbasin margins that is older than Servilleta Basalt (Figs. rift initiation, at ca. 25 Ma. Alternatively, considering the 1.8 km thickness 9–12). The presence of older basaltic rock is compelling at A–A′ because it is of low-density fill indicated by gravity modeling as a minimum estimate of found in the Alcon well near the base of the Picuris Formation. The minimum vertical throw since rift initiation, an average vertical-slip rate greater than age of the lower member of the Picuris Formation is constrained by overlying 72 m/m.y. is required. Using a displaced Servilleta Basalt flow as a marker, ash dated at ca. 34.5 Ma (Aby et al., 2004). These authors speculated that clasts Bauer and Kelson (2004b) estimated average vertical and horizontal com- of Tertiary intermediate to mafic volcanic rocks found in the lower member ponents of slip of the Embudo fault zone since 3 Ma to be 35 m/m.y. and 96 along the northern flanks of the Picuris Mountains were derived from interme- m/m.y., respectively. Clearly, a vertical-slip rate of 35 m/m.y. cannot account diate-composition volcanic rocks from the San Juan volcanic field to the north, for the observed vertical relief over 25 m.y. Either vertical slip rates were or from a buried, more local source. considerably greater before 3 Ma, the Embudo fault has been active for much The hypothesis that deep basaltic rocks in other wells (BOR-3 and Town longer than 25 m.y., the estimates of vertical-slip rates since 3 Ma are too low, Yard, BOR-5, and Torreon) are older than Servilleta Basalt has some support or a combination of these. from preliminary microprobe examination of basalt samples from the Torreon, Aeromagnetic lineaments L1 and L2 (Fig. 8) are interpreted to represent Town Yard, and BOR-5 wells (N. Dunbar, NMBGMR, 2014, written commun.). earlier fault activity related to the modern Embudo fault zone. The continu- These basalts have dissimilar chemistry and texture to known samples of ity of these lineaments over ~11 km distance suggests that buried Servilleta Servilleta Basalt. Moreover, basalts in the Torreon and BOR-5 wells appear Basalt flows are disrupted at, or terminate against, segments of the Embudo similar in chemistry and texture to each other. Preliminary examination of fault. There are two plausible explanations for the terminated basalt, both of the Torreon well basalt by laser-induced breakdown spectroscopy (LIBS) also which require significant throw: (1) lava flows ponded against an existing fault suggests that this basalt is dissimilar from typical Servilleta Basalt, although scarp, or (2) lavas flowed across the fault and were later vertically displaced alteration of the sample may have affected the outcome (N. McMillan, New and eroded off the footwall. The first explanation suggests that a very long, Mexico State University, 2014, written commun.). Basalt that is penetrated by topographically prominent fault scarp or paleochannel existed along the fault the BOR-6 well is at an elevation of 1965 m (Table 1), close to the lowest ele- at the time of eruption. From relations at well TV-115 along profile A–A′ (Fig. 9), vation expected for Servilleta Basalt. Thus, these rocks may also be older than ~6 m thickness of lava would have accumulated against Santa Fe Group sedi Servilleta Basalt. Alternatively, it may represent the oldest, 5.5-Ma Servilleta ments (Table 1). Because 6 m seems unreasonably high for a topographic Basalt. Additional laboratory analyses and comparisons of basaltic samples scarp developed in Santa Fe Group sediments, this thickness of lava suggests from wells might provide clarification on the correlations of basalts in our that multiple flows might have erupted while the fault was active. The second cross sections. explanation suggests that faulting with at least 6 m of vertical displacement From relations developed during construction of geologic cross sections in must have occurred after the basalt was emplaced but became inactive prior the area of these two wells, basaltic rocks from the BOR-5 and Torreon wells to middle Pleistocene, the oldest age of the materials that cover it today (Bauer appear to be within the lower part of the Santa Fe Group, or middle Miocene et al., 1997, 2000, 2016). From inspection of the geologic and geophysical re- in age (Bauer et al., 2014). If so, the magmatism may be correlative with the lations observed for the model of profile A–A′, we prefer the second explana- episodic basaltic eruptions that occurred during 15–11 Ma in the central San tion. Thickening and increasing number of basalt layers north of TV-115 along Luis Basin (Miggins et al., 2002; Thompson et al., 2007, 2015) and 14–8 Ma in A–A′ suggest that faulting was successively progressing southward toward the the northern Jemez volcanic field (Goff and Gardner, 2004; Thompson et al., mountain front. 2006; WoldeGabriel et al., 2013). Aeromagnetic lineament L2 (Fig. 8) follows the banks of the Rio Pueblo de Taos. The linearity of the stream prompted previous workers to infer an underlying structural origin (Muehlberger, 1979; Machette and Personius, 1984). The Implications for the Embudo Fault Zone extensive length and northeast trend of L2 are generally parallel to lineament L1; so we similarly infer that it is caused by oblique-slip faulting that has dis- Northwest-down, dominantly left-lateral slip is well documented along rupted Servilleta Basalt. Multiple basalt flows are present in numerous shallow the Embudo fault zone in the southwestern corner of the study area (Bauer water wells on either side of the stream (Bauer et al., 2016). The wide varia- and Kelson, 2004b; Kelson et al., 2004a), yet a surprising amount of overall tions in thickness of basalt layers and intervening sediments in these wells vertical displacement is evident from the geophysical models and geologic make correlations difficult across distances as short as 1 km. We interpret this cross sections (Figs. 7 and 9; Bauer et al., 2016). Considering a 3.3 km ele- stratigraphic variability as an indication that faulting, volcanism, erosion, and

sedimentation were all episodically active in this area during the age range from the K3 well in B–B′ and the Elston well in C–C′ suggest that the older of the basalts (5.5–3 Ma). The extreme variability might also be explained by basaltic rocks must be below the total depths of these wells, if present at all. basalt that flowed along paleochannels of Rio Pueblo de Taos, as hypothesized Although we conclude that the older basaltic layers are thus displaced to in the geophysical model for section A–A′ (Fig. 9). much lower levels under these wells, it is also possible that the older lavas Because the aeromagnetic anomalies that make up the zone of north- did not flow into this area originally or were removed by erosion before easterly lineaments arise from Servilleta Basalt, these inferred faults must faulting. have been active during and/or shortly after the period of time when Ser- The patterns of shallow faults inferred from geologic mapping and aero- villeta Basalt flows were erupted (5.5–3 Ma). Their strong alignment with magnetic data compared to the overall shape of the northeastern subbasin the Jemez lineament (Fig. 1) suggests an underlying crustal control on margin (Fig. 15) also have implications for fault history. The northeast margin their initiation. The absence of fault scarps offsetting late Quaternary sedi- must have had a prolonged, early period of margin formation along north- ments within this zone (Fig. 3) suggests that the faults are older than middle west-striking faults that later shifted to activity along more northerly faults Pleistocene, the age of the oldest alluvium that covers the area (Bauer et al., near the present-day mountain front. This conclusion results from the north- 1997, 2000). west trend of the subbasin margin, which diverges from the northerly trends of faults near the range front, and is crossed obliquely by northerly trending faults in the interior of the basin. Moreover, as discussed earlier, we interpret Implications for the Sangre de Cristo Fault System the northwest-trending, aeromagnetic lineament L4 as a boundary between oldest, Pliocene Servilleta Basalt on the west and an extensive area of subhori Several relations from modeling profiles B–B′ and C–C′ have implications zontal, deep, possibly middle Miocene basaltic layers on the east. The deeper for the development of the Sangre de Cristo fault system and the eastern basaltic layers overlie the northwest-trending part of the subbasin margin but and northeastern subbasin margins. First, the models support a concept of show little displacement (e.g., Figs. 11 and 12), except at lineament L4. There- a half-graben with Servilleta Basalt on the west and older basaltic rocks on fore, we hypothesize that fault activity on the northeastern subbasin margin the east that are structurally higher, as illustrated in Figure 10C. Important shifted in time, from NW-striking faults east of L4 before eruption of the oldest implications are that the master fault of the half-graben was earlier located basaltic flows, to a focus at L4 during eruption of Servilleta Basalt, then east- 4–5 km west of the modern mountain front and then later shifted to the east. ward to a focus at north-striking aeromagnetic lineament L5, and finally even Moreover, Servilleta Basalt is absent on the footwall of the earlier half-graben, farther eastward to the modern range front. where it either was not deposited or was eroded soon after deposition. Second, flows of Servilleta Basalt abruptly terminate against some basement faults but are not obviously offset by others, and where the flows Relations between Picuris-Pecos and Los Cordovas Fault Strands terminate, the topmost flow(s) extend farther toward the range front than do the flows below (Figs. 11 and 12). These relations are supported by the Strands of the Picuris-Pecos fault system are cut by the younger Embudo profile magnetic modeling and aeromagnetic map interpretation. They fault zone but likely extend northward at the basin floor (Bauer and Kelson, were guided by the results from modeling of A–A′ across the Embudo fault 2004a). Using the patterns of mapped and aeromagnetically inferred faults zone (Fig. 9), where these relations are well constrained. In both cases, (Figs. 13 and 14), we can speculate on how the strands extend northward the relations observed suggest growth faulting and Servilleta volcanism and possibly influenced younger faulting. Within the wide zone of north- were coeval. erly trending faults associated with Los Cordovas faults, individual faults Third, the models show that the older basaltic layers do not extend as far appear to bend to the southeast near and across the northeasterly trending east as the range front and have little to no displacement overlying faulted zone of faults that includes L1 and L2 (Fig. 14B). The bends suggest there basement to the west, where the older basalt flows are modeled underneath may be local vertical-axis rotation of fault blocks adjacent to the Embudo the Servilleta Basalt (between 16 and 18 km distance on Fig. 11 and 15–17 km fault zone, consistent with structural variability observed in the field (Kelson distance on Fig. 12). These relations imply that, during or shortly after eruption et al., 2004a). South of lineament L1, the northerly faults are not well de- of the older flows, normal faulting was concentrated where the older basaltic fined aeromagnetically, because shallow Servilleta Basalt is generally ab- layers terminate. In addition, some normal faulting involving the basement to sent. Nevertheless, one can envision connections with fault strands of the the west of this point occurred before the eruption of the older lava, possibly Picuris-Pecos fault system in the mountain flanks (Fig. 15) that include pos- even prior to rift initiation. sible left-lateral displacements at the northeasterly zone. Thus, the patterns Finally, the depiction of large fault displacements on the older basaltic are compatible with previous hypotheses that the Los Cordovas faults are layers beneath Servilleta Basalt has implications for the ages of faulting but inherited from remnant strands of the Picuris-Pecos fault system north of the modeling uncertainties make these implications speculative. Constraints Embudo fault zone.

02 A KM 23–18 Ma (early Miocene)

BOR-5 Proto-Sangre de Cristo Fault Volcaniclastic Volcaniclastic Upper Picuris formation and possibly earliest Santa Fe Group deposition of detritus detritus volcaniclastic detritus from the north. ? Buried remnants of an early Eocene(?) lava flow, possibly from the

s ? northwest, that is now preserved in the Alcon well. Taos e

p Reorganization of Picuris-Pecos fault system from post-

e Laramide graben formation to west-down normal-oblique slip, k

c ? o increasing in throw in the central part of the study area, r

d ? e indicated by larger hachures on normal faults. Intrusion of b

e l plutons in Latir volcanic field (north of study area). Buried Eocene(?) lava b ? i ? s ? s Alcon o ? P ? ? ? ?

02 B 18–11 Ma (early and middle Miocene) KM Rapid Extension and Basin Formation BOR-5 s n Santa Fe Group deposition of volcaniclastic detritus from the north and

ai ? t lithoclastics from the emerging, low-relief Sangre de Cristo Mountains.

n Volcaniclastic u Volcaniclastic o ? Basaltic lava flow, possibly from a local source, that is now preserved in Figure 16 (on this and following page). detritus M and lithic wells (e.g., BOR-5) in the eastern part of the study area. o Generalized paleogeography of the study detritus Taos t s i r Paleozoic sedimentary rocks and Proterozoic basement composing area for six age intervals (A–F), based on

C ? low-relief mountains, with Picuris formation preserved in grabens. geologic evidence and geophysical inter e

d pretations. Size and orientation of arrows e r g Period of rapid extension, basin formation, and west-down indicate relative rate and direction of n a deposition. The Town of Taos and Alcon S activity on normal faults at the eastern basin margin with

g and BOR-5 wells are shown for reference. n i Sangre de Cristo Mountains beginning to emerge. On the south, lt g u Lithic r e activity on the Picuris-Pecos fault system is waning (dashed detritus m E lines). Oblique slip on the proto-Embudo fault develops by 11 Alcon Ma, with most activity southwest of the study area. Santa Fe Group includes widespread eolian deposits from ~13–11 Ma. Proto-Embudo fa

0 2 L4 C 11–6 Ma (middle and late Miocene) KM Embudo Fault Extends to Northeast

Volcaniclastic s BOR-5 detritus in

a Volcaniclastic t Santa Fe Group deposition of volcaniclastic detritus from the north and

and lithic u lithoclastics from the emergent Sangre de Cristo and Picuris Mountains. detritus o M

Paleozoic sedimentary rocks and Proterozoic basement composing the

o Taos t rising mountains, with Picuris formation preserved in grabens. s i r

e Rapid extension continues, with lesser activity on faults shown L2 d

e t r with dashed lines. The Embudo fault propagates to the north- g n a east, following the present-day trend of the Rio Pueblo de Taos S

t (lineament L2). Left-oblique slip dominates the southwestern n Embudo Faul e g r Embudo fault. Dip slip dominates on the northeast, focused on Lithic e m lineament L4. En echelon faults develop between the Embudo detritus E Alcon and the Sangre de Cristo fault systems, where multiple faults may be active and possibly linked. Older lava is covered by Emerging Picuris Mtns Santa Fe Group and displaced by normal faults.

02 L4 D 6–3 Ma (late Miocene and Pliocene) KM BOR-5 Eruptions of Taos Plateau Volcanic Field (TPVF)

L5 Alluvial fans of Lama formation deposited between and after multiple TPVF lavas eruptions of Servilleta Basalt

s Servilleta Basalt

a t Intermediate-composition lava flows from TPVF, less widespread than L2 n u Servilleta Basalt TPVF lavas ? o Taos M L1 Paleozoic sedimentary rocks and Proterozoic basement composing the o t t s i mountains, with Picuris formation preserved in grabens. r

e Multiple eruptions of Servilleta Basalt while normal faults at the d Embudo Faul e r eastern basin margin begin to focus along lineament L5. g n a Parallel, NE-striking, oblique faults likely flank the southeastern S basin margin (e.g., L1 and L2), causing gentle southeastward tilt Alcon of basalt layers. Hard links between oblique and normal fault Picuris Mtns strands are possible (queried). Basalt may have ponded against L4 (shown) or was later faulted at L4 and the footwall eroded.

0 2 Los Cordovas Faults E 3–0.8 Ma (Pliocene and early Pleistocene) KM BOR-5

L3 Los Cordovas Faults Pliocene alluvial fans of Lama formation and early Pleistocene alluvium and piedmont deposits

s Servilleta Basalt

Interbedded alluvial, n Paleozoic sedimentary rocks and Proterozoic basement composing the

u lacustrine(?) and Taos o mountains, with Picuris formation preserved in grabens. M eolian deposits Figure 16 (continued). o t s End of Taos Plateau volcanism. Continued deposition of Lama i r

C formation in the Pliocene, with large fans advancing across the t e L1 d northeastern part of the study area. Embudo and Sangre de Cristo e r g faults are linked, following lineament L1 and the present-day n Embudo Faul a S range front on the north. Servilleta Basalt is eroded from footwalls of normal faults at southeastern margin. N-S Los Alcon Cordovas faults develop in a wide zone west of L3, perhaps Picuris Mtns following old Picuris-Pecos fault strands within the basement.

0 2 F <0.8 Ma (middle Pleistocene to Present) KM Modern Linked Embudo-Sangre de Cristo Faults BOR-5 Surficial deposits associated with modern drainages and slopes. Eolian

ns i deposits on the Taos Plateau

ta n Servilleta Basalt u Taos o e

g r Paleozoic sedimentary rocks and Proterozoic basement composing the o o t s i G high-relief mountains, with Picuris formation preserved in grabens. r e C

d e n d a Los Cordovas and strands of Embudo and Sangre de Cristo r e r G g faults within the basin interior become inactive. The linked o i n a R S Embudo and Sangre de Cristo faults have shifted away from the

e li basin interior to the present location. The Sangre de Cristo and e r - h Picuris Mountains have high relief. Rio Grande and Rio Pueblo ig Alcon H de Taos have cut deep gorges in Servilleta Basalt layers. High-relief Picuris Mtns Surficial deposits cover most of the study area.

A PROPOSED RIFT HISTORY FOR THE TAOS AREA have formed a link at this time. The Picuris Mountains continued to emerge, with detritus shed to the north and northwest. Implications from the integrated geophysical interpretations and models During latest Miocene and early Pliocene time (6–3 Ma), Servilleta Basalt allow us to revise previous scenarios of the Cenozoic structural development was periodically erupting from multiple vents and spreading across the area of the Taos area (Bauer and Kelson, 2004a; Smith, 2004; Smith et al., 2004). (Fig. 16D). Between eruptive cycles, the clastic sediments of the Lama forma- Although some aspects of the history are well constrained, the complete pic- tion were being shed from the nearby mountains. Intermediate-composition ture through time is not. Nevertheless, we have developed a set of six paleo- volcanoes were erupting northwest and west of the study area, including geographic maps that depict the evolution of the Rio Grande rift within the Tres Orejas at the western edge of the study area. Deeper layers of possible study area (Fig. 16). These maps draw heavily on the previous work of many Servilleta Basalt as old as 5.5 Ma (or an earlier, unrecognized eruptive se- researchers. quence) may have flowed as far north as the north-central part of the study The first paleogeographic map (Fig. 16A) depicts early stages of rifting in area, which may lie at depth in well BOR-6 today. These lavas flowed into early Miocene time (23–18 Ma). Volcanism related to the San Juan volcanic the rapidly subsiding subbasin, where they ponded against fault scarps or field had ended. A lava flow related to the early volcanism, which is now pre- lapped over them and were subsequently faulted and eroded from the foot- served in the Alcon well (Fig. 9), had been buried by deposits of the lower walls shortly after deposition. The principal segment of the Embudo fault part of the Picuris Formation. During the 23–18 Ma interval, grabens along continued to occupy the area of the present-day Rio Pueblo de Taos (linea- high-angle, north-striking faults formed as the Picuris-Pecos fault system was ment L2), although other segments formed to the south, which were also transitioning from Laramide-style to rift-style kinematics. Abundant detritus aligned with the regional Jemez lineament. Slip was probably left-oblique, from the Latir volcanic field was being transported southward into the study with increasing dip slip along the north-central strands of the fault. The area and deposited as the upper part of the Picuris Formation, coeval with northwest-striking fault at the northeastern subbasin margin (lineament L4) deposition of earliest Santa Fe Group elsewhere. was still active, because the Servilleta Basalt was unable to cross it or was By early and middle Miocene (18–11 Ma), graben formation had transi- being eroded from its footwall. tioned to intense rift-basin formation and accumulation of the Santa Fe Group During late Pliocene to middle Pleistocene time (3–0.8 Ma), eruption of Ser- (Fig. 16B). Uplift of the Sangre de Cristo Mountains accompanied basin sub- villeta Basalt had ceased within the study area and the linked Embudo–Sangre sidence, providing additional detritus of Proterozoic and Paleozoic rocks to de Cristo fault had shifted closer to the present-day mountain front, following the mix of sediments shedding from the north. Eruption of basaltic rocks aeromagnetic lineament L1 (Fig. 16E). Slip on the fault was oblique, but with that are now deeply buried on the east side of the study area would have enough dip slip to allow for erosion of the basalt off the footwall. It is likely occurred sometime during the latter part of this time period. Possible sources that other normal and oblique-slip faults were active between the main fault are mafic lavas that erupted from the Jemez volcanic field to the southwest and the mountain front during this time. Southeast tilting of the basalt layers at ca. 13 Ma (Goff and Gardner, 2004), or, more likely, a local source contem- in concert with growth faulting developed toward the end of this time. The Los poraneous with the 15–11 Ma basaltic magmatism in the central San Luis Cordovas faults also became active, possibly controlled by buried strands of Basin. Oblique slip began to develop on a proto-Embudo fault aligned with the Picuris-Pecos fault system. the regional Jemez lineament, with activity mostly to the southwest of the From middle Pleistocene to present (<0.8 Ma), the Los Cordovas faults and study area. the northeast-trending faults within the interior of the basin became inactive From middle to late Miocene (11–6 Ma), the principal slip on the Embudo and were partially buried beneath middle Pleistocene and younger fan ma- fault was along the area of the modern Rio Pueblo de Taos (lineament L2), terial (Fig. 16F). Activity became concentrated at the present-day, high-relief with mostly normal slip at the southern subbasin margin within the study mountain front, along the still linked Embudo–Sangre de Cristo fault, which area (Fig. 16C). The dominance of normal slip at this time explains the large developed greater concave curvature surrounding the subbasin. The Rio amount of vertical displacement observed across the Embudo fault zone Grande and Rio Pueblo de Taos became incised into layers of Servilleta Basalt, today. Based on the strong northwest trend in the 3D model and displace- allowing throughgoing drainage from the San Luis Basin to the south. ments on the older basalts inferred from the cross-section models, we infer that most of the activity along the northeastern rift margin was concentrated on the northwest-striking fault (lineament L4) that is basinward of the SUMMARY present-day mountain front. Perhaps the northwest strike developed as the northward-propagating faults from the study area started to interact with the We present a detailed example of how a subbasin develops adjacent to southward-propagating faults from north of the study area, following con- a transfer zone in the Rio Grande rift. The Embudo transfer zone in the Rio cepts described by Rosendahl (1987). Likewise, the proto-Embudo and proto– Grande rift is considered one of the classic examples and has been used Sangre de Cristo fault systems were interacting with each other, and they may as the inspiration for several theoretical models. Despite this attention, the

history of its development into a major rift structure is poorly known along This hypothesis resolves apparent conflicts between previous struc- its northern extent, near Taos, New Mexico. Geologic evidence for all but tural models resulting from well correlations and constraints from its young rift history is buried under Quaternary cover. We focus on under gravity data. standing the pre-Quaternary evidence that is in the subsurface by integrat- 6. Overall patterns of inferred and mapped shallow faults show gen- ing diverse pieces of geologic and geophysical information. As a result, we eral correspondence compared to the overall shape of the deep sub- present a substantively new understanding of the tectonic configuration basin margins, as expressed by the 3D gravity model. Northeasterly and evolution of the northern extent of the Embudo fault and its adjacent trends align with the Jemez lineament, a northeasterly regional crustal subbasin. boundary. Exceptions to the correspondence are north of Taos, where The ~64-km-long, Embudo fault zone forms the link between the east-down north-striking faults related to the Hondo section of the Sangre de western border of the northern Española Basin and the west-down eastern Cristo fault diverge from the northwest-trending, northeastern sub border of the southern San Luis Basin. At the northern end of the fault zone, basin margin. the rift margins follow an eastward curving reentrant in the mountain front 7. North-striking, early to middle Pleistocene Los Cordovas faults in the known as the Taos embayment, which partially surrounds the Town of Taos, interior of the basin extend ~2–4 km farther east than mapped from sur- New Mexico, on the south and east. The NE- to E-striking, left-oblique Embudo face exposures. They also obliquely cross the northwest trend of the fault zone transitions to the N-striking, west-down, normal Sangre de Cristo northeastern subbasin margin. fault along the edges of the embayment, forming the structural margins of the Taos subbasin. Using these interpretations, we infer relations between faulting and flows To better understand the subsurface geology, we synthesize aeromagnetic of Pliocene Servilleta Basalt and older, buried basaltic rocks that reveal previ- and gravity data, borehole and physical-property information, and geologic ously unrecognized aspects of the history of faulting and subbasin formation. mapping. We rely heavily on a 3D basin model derived from gravity data, pat- Combined with evidence from geologic mapping, we improve the understand- terns on the aeromagnetic map, and 2D profile modeling, to develop the fol- ing of the Cenozoic evolution of the Taos subbasin and its faulted margins. lowing new interpretations about subsurface geology: The history involves shifts in the locus of fault activity at the rift margins as the Taos subbasin developed, as illustrated by a set of paleogeographic maps for 1. The subbasin within the Taos embayment has an imperfect D shape in the study area. map view, with the deepest point (2 km depth) at 36° 26′ N latitude and During 23–18 Ma (early Miocene), we speculate that early rift extension was 105° 37′ W longitude, northwest of Taos. The basin floor slopes gently concentrated on strands of the Picuris-Pecos fault system, a northerly trend- east and southeast toward this point. ing crustal boundary that today intersects the Embudo and Sangre de Cristo 2. Multiple Servilleta Basalt flows and intervening sediments, generally faults where they join. We suggest that some of the graben-forming faults that 200 m thick, lie at depths of 0–100 m over a limited area within the inte- were active within this fault system at the end of Laramide time formed the rior of the subbasin. first west-down master faults of the future subbasin (proto–Sangre de Cristo 3. The Embudo fault zone, along the southern subbasin margin, extends fault system). wider in the subsurface than its mapped width at the surface, encom- During 18–11 Ma (early and middle Miocene), rapid extension and ba- passing a zone as much as 9 km wide that extends northward from the sin formation increased activity at the eastern subbasin margin, while fault range front to the Rio Pueblo de Taos. Aeromagnetic lineaments indicate strands of the Picuris-Pecos fault system to the south in the Picuris Mountains that flows of Servilleta Basalt, concealed by middle Pleistocene(?) allu- became less active. Lava flows related to the basaltic rocks presently buried in vium, are disrupted or truncated by faults in this zone. wells in the eastern part of the study area might have erupted along the east- 4. The basin floor gradually steps down across the wide Embudo fault ern subbasin margin at this time. zone, with a vertical displacement of ~1.8 km and a total structural relief During 11–6 Ma (middle and late Miocene), proto-Embudo fault strands of ~2.6 km. Average vertical slip rates of 72–96 m/m.y. are required to ac- were likely aligned with the Jemez lineament and the modern, NE-aligned commodate these estimates of vertical displacement since rift initiation Rio Pueblo de Taos, as much as 7 km basinward of the modern Embudo at ca. 25 Ma. These rates are much higher than the average vertical slip fault zone. Left-oblique slip had developed at the southwest end of the fault of 35 m/m.y. previously estimated for the past 3 Ma. zone, out of the study area, but likely transitioned to mostly normal slip to 5. Along the eastern subbasin margin, a 5–7-km-wide zone of normal, the northeast, within the study area. Northwest-striking normal faults formed west-down, stepped faults extends basinward from the north-strik- the margin of the northeastern part of the subbasin, an orientation diver- ing Sangre de Cristo range front fault. We hypothesize that Servilleta gent from the modern north-striking Sangre de Cristo faults. The faults at all Basalt is absent within this zone and that basaltic rocks found deep the subbasin margins were likely interacting with each other and may have in wells within the zone are older, perhaps middle Miocene in age. started to link.

During 6–3 Ma (late Miocene and Pliocene), volcanism of the Taos Plateau the northeastern subbasin margin changed orientation and location to more volcanic field was well under way. Proto–Embudo fault strands at the southern northerly striking faults within 2 km of the current range front. The Sangre de subbasin margin remained active during eruption of Servilleta Basalt along Cristo fault and Embudo faults were likely linked at this time. and parallel to the modern, NE-aligned Rio Pueblo de Taos. Servilleta Basalt After middle Pleistocene(?) time (<0.8 Ma), the Los Cordovas faults and likely did not extend across the northeastern subbasin margin, where north- strands of the extended Embudo fault zone that were along and parallel west-striking faults were still active 1–4 km west of the modern, north-striking to the current Rio Pueblo de Taos became inactive. The locus of activity range front. along the linked Embudo–Sangre de Cristo fault system shifted southward During 3–0.8 Ma (Pliocene and early Pleistocene), volcanism of the Taos and eastward to the present-day mountain front. The modern landscape of Plateau was ending, and north-striking Los Cordovas normal faults became high-relief mountains surrounding the Taos embayment was well estab- active in the interior of the subbasin. Their locations may have been controlled lished, and the Rio Grande and Rio Pueblo de Taos incised deep gorges in by relict Picuris-Pecos fault strands located on the basin floor. Faulting at the basalt layers.

APPENDIX

TABE A1. DENSITIES BY GEOOGIC NIT Range of bul Estimated mean densities density Geologic unit Data type g/m3 g/m3Rationale and data source

Pliocene volcanic rocs Servilleta Basalt Tbs plus Geophysical 2450 Assume similar density as that estimated for the bul of volcanic rocs and interbedded sediments ama formation Tl estimate composing Guadalupe Mountain, east‑central Taos Plateau Grauch et al., 2015. Intermediate volcanic rocs Tv Geophysical 2450 Assume similar density as that estimated for the bul of volcanic rocs and interbedded sediments estimate composing Guadalupe Mountain, east‑central Taos Plateau Grauch et al., 2015. Sedimentary basin fill Santa Fe Group above 1.25 m From 2100–2250 2170 Following the density‑depth function developed from borehole density logs for Albuuerue Basin Grauch depth Tl, Tsf Albuuerue Basin and Connell, 2013. Santa Fe Group below 1.25 m From 2250–2400 2300 Following the density‑depth function developed from borehole density logs for Albuuerue Basin Grauch depth Tsf Albuuerue Basin and Connell, 2013. Picuris Formation Tp Sonic log 2200–2300 2250 Estimates from a sonic log of one well at depth range of 100–366 m Bauer, 2016. Rocs from San Juan volcanic field SJVF Ignimbrites and andesites Weighted estimate 2200–2500 2450 Weighted estimate based on ignimbrites density of 2200 and andesites density of 2500 composing 1/6 and 5/6 of the volume of the field, respectively, from Drenth et al. 2012. Paleozoic rocs Pzu Shale and sandstone Samples 2460–2660 2540 Average density from measurements of saturated samples from Taos area Grauch and Drenth, 2016. imestone Samples 2670–2680 2670 Average density from measurements of saturated samples from Taos area Grauch and Drenth, 2016. Proterozoic rocs u Granite and granitic gneiss Samples 2550–2680 2600 Bul density measurements from Tusas and Picuris Mountains Grauch and Drenth, 2016. Metasedimentary rocsSamples 2590–2860 2680 Bul density measurements of Hondo Group from Tusas and Picuris Mountains Grauch and Drenth, 2016. Metasedimentary and Samples 2590–3060 2700 Bul density measurements of Vadito Group from Tusas and Picuris Mountains Grauch and Drenth, 2016. metavolcanic seuence Metavolcanic seuence Samples 2770–3050 2910 Bul density measurements from Tusas Mountains Grauch and Drenth, 2016. Geologic unit codes explained on Figure 3. From Albuuerue Basin—Compilation of various density logs from deep boreholes in the Albuuerue Basin, described in Grauch and Connell 2013. Geophysical estimate—Estimated by finding the Bouguer reduction density that gives least correlation to terrain, described as Nettletons method Telford et al., 1990. Samples—Measurements taen on hand samples collected in the field from one or more outcrops. Sonic log—Density range and median value estimated using empirical relations for sedimentary basins Gardner et al., 1974 and inspection of borehole sonic log. Weighted estimate—Representative density of whole volcanic field estimated based on borehole density estimates and weighted by volume within the volcanic field, from Drenth et al. 2012. Density estimated from median values of sample measurements Grauch and Drenth, 2016 or as noted.

TABLE A2. MAGNETIC PROPERTIES BY GEOLOGIC UNIT Typical magnetic Estimated total susceptibility × 103 (SI)† Induced component§ (A/m) magnetization# (A/m) Polarity indicated Geologic unit* Minimum Maximum Minimum Maximum Minimum Maximum from anomalies

Surficial deposits (Qa) 0.41 0.67 0.02 0.03 [0.02] [0.03] Unknown

Servilleta Basalt (Tbs) 2.00 5.32 0.08 0.22 1.78 4.37 Normal and reversed Intermediate-composition volcanic rocks (mostly out of study area) Cerro de los Taoses andesite 12.1 14.1 0.50 0.58 10.0 12.0 Reversed Cerro Montoso andesite 3.05.0 Reversed Cerro Negro dacite 15.0 15.0 0.62 0.62 2.03.0 Reversed Tres Orejas dacite 1.42.0 Normal Sedimentary basin fill Santa Fe Group, Lama formation (QTl) 1.50 3.50 0.06 0.14 0.06 0.18 Unknown Santa Fe Group (Tsf) 1.20 3.69 0.05 0.15 0.05 0.19 Unknown Picuris Formation (Tp) 3.26 4.58 0.13 0.19 [0.13] [0.19] Unknown Middle Tertiary volcanic rocks (out of study area) Basalt of Hinsdale Formation (ca. 21 Ma), Tusas Mountains 4.39 22.0 0.18 0.91 1.02.0 Normal Basalt of Hinsdale Formation (ca. 26 Ma), San Luis Hills 3.04.5 Normal Andesite of upper Conejos Formation (ca. 30 Ma), San Luis Hills 2.510.0 Reversed

Paleozoic rocks (Pzu) 0.10 0.15 0.00 0.01 [0.00] [0.01] Unknown Proterozoic rocks (Xu) Granite and granitic gneiss 0.28 1.84 0.01 0.08 [0.01] [0.08] Unknown Metasedimentary rocks (Hondo Group) 0.00 1.00 0.00 0.04 [0.00] [0.04] Unknown Metasedimentary and metavolcanic sequence (Vadito Group) 0.02 0.50 0.00 0.02 [0.00] [0.02] Unknown Metavolcanic rocks from Tusas Mountains 1.00 6.42 0.04 0.26 [0.04] [0.26] Unknown *Geologic unit codes explained on Figure 3. Ages and locations of geologic units from Tusas Mountains and San Luis Hills are from Drenth et al. (2011) and Thompson et al. (2015), respectively. †Magnetic susceptibility values are from field and sample measurements by Grauch and Drenth (2016). Minimum and maximum median values from multiple sites are reported for all geologic units except for Cerro de los Taoses andesite (two samples from one site) and Cerro Negro dacite (one sample), where median values could not be computed. §Induced component computed by multiplying the typical minimum and maximum values of magnetic susceptibility by Earth’s magnetic field of 51,700 nanoteslas, following Hansen et al. (2005) to convert units to amperes per meter (A/m). #Total magnetizations estimated as follows. Values in square brackets assume negligible remanent magnetization based on rock type, resulting in total magnetizations that are essentially the same as the induced components. Minimum and maximum values of total magnetization for Servilleta Basalt were estimated from the range of natural remanent magnetizations (NRM) of 2.00 to 4.15 A/m measured by Brown et al. (1993) minus or plus the maximum induced component, respectively. Values for the ca. 21 Ma basalt of the Hinsdale Formation were estimated from geophysical modeling (Drenth et al., 2011). All other values for intermediate-composition volcanic rocks and Middle Tertiary volcanic rocks were estimated by qualitative assessment of observed aeromagnetic anomalies compared to magnetic terrain models of hills composed of the unit listed. A maximum 25% contribution of NRM to the total magnetization is assumed for Santa Fe Group units, following findings from Albuquerque Basin (Hudson et al., 2008).

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