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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 west­down 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 Normal­oblique 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 left­lateral 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, NE­aligned 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 pre­Quaternary 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 proto­Embudo faults at this time, concurrent with the de­ subbasin. velopment of N­striking 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 D­shaped 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, west­down 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 north­striking 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 © 2017 The Authors 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 870 by guest on 01 October 2021 Research Paper 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 study area. The Embudo fault system rocks m Sangre de Cristo Mtns yysste transfers strain between the westerly dip­ Proterozoic rocks lt s ping Española Basin on the south to the PM 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 Fe 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 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 871 by guest on 01 October 2021 Research Paper 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.

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