Paleomagnetism of Tertiary intrusive and volcaniclastic rocks | RESEARCH Paleomagnetism of Tertiary intrusive and volcaniclastic rocks of the Cerrillos Hills and surrounding region, Española Basin, New Mexico, U.S.A.: Assessment and implications of vertical-axis rotations associated with extension of the Rio Grande rift Stephen S. Harlan1,* and John W. Geissman2 1DEPARTMENT OF ATMOSPHERIC, OCEANIC AND EARTH SCIENCES, GEORGE MASON UNIVERSITY, MS 5F2, FAIRFAX, VIRGINIA 22030, USA 2DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MSC03 2041, 1 UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, NEW MEXICO 87131, USA ABSTRACT Paleomagnetic data from parts of the northern Rio Grande rift provide evidence for clockwise (CW) and counterclockwise (CCW) vertical-axis block rotations associated with strike-slip deformation along basin-bounding faults during rift evolution. Despite the spatial consistency of the results, the quality and statistical signifi cance of data sets are diffi cult to evaluate because of small sample size and potential failure to average secular variation. To understand the extent, importance, and origin of such rotations, we report paleomagnetic data from Tertiary intrusive and volcaniclastic rocks in the Cerrillos Hills and surrounding areas in the Española Basin. Paleomagnetic data from in situ Ter- tiary intrusions and tilt-corrected volcaniclastic strata of the Oligocene Espinaso Formation sampled at four localities yield a grand mean of α declination = 342.9° and inclination = 58.3° ( 95 = 3.5°; N = 32 sites of normal polarity/21 reverse). Correction for minor postemplacement tilt α of the Cerrillos Hills and La Cienega data sets yields a grand mean (declination = 349.5°, inclination = 55.3°, 95 = 3.4°) that is indistinguishable from the 30 Ma reference direction for the study area, and there is no evidence of rotation (R = 1.8° ± 6.4°). However, if an alternative refer- ence direction is used, minor CCW rotation (–6.6° ± 5.8°) is possible. Our data suggest that the magnitude of rotation in the Española Basin is signifi cantly less than previously estimated and may be negligible. Regardless, paleomagnetic data from elsewhere in the basin suggest that CCW rotations may be an important component of recent rift extension and deformation. LITHOSPHERE; v. 1; no. 3; p. 155–173. doi: 10.1130/L53.1 INTRODUCTION from ca. 30 Ma to ca. 20 Ma was characterized Paleomagnetism can be a powerful tool in by a broad zone of distributed extension along identifying and understanding the mechanisms of The Rio Grande rift is a major intracratonic low-angle faults, crustal doming, deposition distributed crustal deformation in orogenic belts rift that separates the thick and relatively unde- of sediments, and widespread magmatic activ- and extensional settings (Hillhouse, 1989; Beck, formed crust of the Colorado Plateau on the west ity (Chapin, 1988; Prodehl and Lipman, 1989). 1989) such as the Rio Grande rift. The paleo- from the Great Plains of the stable craton to the Formation of growth structures in sedimentary magnetic method is well suited to identifying east (Kelley, 1977; Chapin and Cather, 1994). basin fi ll indicates that extension and deposition crustal block rotations where the remanent mag- It extends from near the vicinity of Leadville, were coeval (Minor et al., 2006). Deformation netization of a series of rocks, acquired during Colorado, southward to where it merges with during this period may have resulted from upper a suffi ciently long interval (i.e., generally 105 to the eastern Basin and Range Province of east- mantle asthenospheric upwelling and thermal 106 a) in order to suffi ciently average the effects ern Arizona, New Mexico, and northern Mex- lithosphere erosion (Seager and Morgan, 1979; of paleosecular variation, can be compared with ico (Fig. 1A). It is characterized by high heat Morgan et al., 1986; Olsen et al., 1987; Wilson time-equivalent reference directions from unde- fl ow, recent crustal deformation, and Cenozoic et al., 2005). Between ca. 20 and 10 Ma, the formed rocks (typically based on paleomagnetic to Holocene mafi c magmatism. The amount of rate of extension accelerated, but rifting was poles derived from rocks of the stable craton) to extension increases southward, and the relative restricted to a narrower zone and characterized identify the sense and magnitude of rotations. elevation of fault-bounded basins decreases. In by high-angle normal faulting and the onset With adequate sampling, the size and geometry northern New Mexico, the structural manifes- of alkalic mafi c magmatism (Golombek et al., of rotational domains may be identifi ed. Such tation of the rift is a series of right-stepping, 1983; Prodehl and Lipman, 1989; Baldridge studies provide important insight into mecha- asymmetric fault-bounded en echelon basins et al., 1991). The change in structural style has nisms responsible for crustal rotations in contrac- (Kelley, 1982). From south to north, these been inferred to refl ect a major change in the tional orogenic belts, as well as extensional and are the Albuquerque, Española, and San Luis regional stress fi eld in response to a change in transtensional/transpressional regimes. Basins, respectively (Fig. 1A). Rift deformation North American–Pacifi c plate boundary condi- In northern New Mexico, several paleo- tions (Golombek et al., 1983; Prodehl and Lip- magnetic studies (Brown and Golombek, *[email protected] man, 1989; Atwater and Stock, 1998). 1985; Brown and Golombek, 1986; Salyards For permission to copy, contact [email protected] | © 2009 Geological Society of America 155 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/1/3/155/3049918/i1941-8264-1-3-155.pdf by guest on 29 September 2021 HARLAN and GEISSMAN KANSAS 108°W 106°W 104°W 106°W Taos Plateau RIO GRANDE QTv A RIFT B UTAH 38°N San Luis Basin Alamosa COLORADO EFEF OKLAHOMA QTv Albuquerque Taos Basin Españ ola 36°N Basin Santa Fe JemezJemez QTv MMountainsountains Albuquerque 36°N ARIZONA NEW MEXICO Socorro 34°N F P Southern PFZPFZ - Rio Grande P-PFP Basin QTv Cerros del Rio Las Cruces QTv 32°N El Paso Area of Fig. 2 TEXAS MEXICO LBFLBF CerrillosCerrillos Santa Ana QTv 5.5°N Mesa HillsHills 3 0 100 200 miles 6060 6161 50 100 kilometers SFFSFF 56,5956,59 1 5858 5757 Explanation RFRF OrtizOrtiz MMountainsountains Quaternary-Tertiary volcanic rocks F QTv -C undifferentiated T-CFT Tertiary intrusions ABQ SFSF 5°N Tertiary Espinaso Formation 3 0 10 20 30 kilometers Paleomagnetic site 106°W 105.5°W Figure 1. Tectonic map of part of northern New Mexico showing the location of the Cerrillos Hills and the Española block. Circles show the location of paleo- magnetic sites sampled in this study. T-CF—Tijeras-Cañoncito fault zone; PFZ—Pajarito fault zone; EF—Embudo fault zone; P-PF—Picuris-Pecos fault zone; LBF—Lobato Mesa fault; SFF—San Felipe fault; SF—Sandia fault; ABQ—city of Albuquerque. Location map modifi ed from http://cires.colorado.edu/science/ groups/sheehan/projects/riogrande/images/faq2.jpg. Faults on tectonic map are from the U.S. Geological Survey Quaternary fault and fold database (http://gldims.cr.usgs.gov); geologic units are from Green and Jones (1997). et al., 1994) have reported data supporting the (Muehl berger, 1979; Brown and Golombek, that some reported rotations could be artifacts presence of statistically signifi cant vertical- 1986) (Fig. 1B). Outside of the Española Basin, of small sample size or may arise from other axis block rotations associated with extension other studies have shown either negligible or sta- complications within individual data sets. The accompanying development of the Rio Grande tistically signifi cant clockwise (CW) rotations, purpose of this study is to better assess possible rift. In particular, paleomagnetic studies of intru- thus documenting a complex image of crustal block rotations in the southern Española Basin sive rocks of the Ortiz porphyry belt within the block rotation accompanying rift extension. and thus contribute to a better understanding Española Basin (Fig. 1B) yielded an apparent Many paleomagnetic rotation estimates from of the kinematics of Rio Grande rift formation. counterclockwise (CCW) rotation of –17.8° ± the rift and surrounding areas are based on small Accordingly, we conducted a paleomagnetic 11.4° (Brown and Golombek, 1986). The CCW sample populations, and it is unclear whether study of intrusive and volcaniclastic rocks within rotations were postulated to result from left- results from individual studies adequately sam- the Cerrillos Hills and surrounding areas of the lateral slip along major faults bounding the rift pled paleosecular variation. Hence, it is possible Española Basin (Figs. 1B and 2). 156 www.gsapubs.org | Volume 1 | Number 3 | LITHOSPHERE Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/1/3/155/3049918/i1941-8264-1-3-155.pdf by guest on 29 September 2021 Paleomagnetism of Tertiary intrusive and volcaniclastic rocks | RESEARCH PREVIOUS PALEOMAGNETIC TABLE 1. SELECTED PALEOMAGNETIC POLES FOR NORTH AMERICA AND CORRESPONDING INVESTIGATIONS IN NORTHERN EXPECTED DIRECTIONS P lat. P long. A K Age (Ma) Exp. Exp. α Source* NEW MEXICO 95 95 (°N) (°E) (°) declination inclination (°) (°) (°) Paleomagnetic studies documenting the 90.0 180.0 – – 0 360.0 55.0 – a 86.5 144.7 3.9 N.A. <10 356.0 53.7 3.3 b apparent CW and CCW tectonic rotations in 85.9 151.1 3.6 N.A. 20 355.1 54.0 3.1 c the northern Rio Grande rift have been con- 79.6 187.9 5.4 63.2 30 347.7 58.3 4.2 d ducted by Brown and Golombek (1985, 1986), 79.3 180.4 4.6 92.4 35 347.0 57.1 3.7 d 77.3 167.7 7.3 62.4 40 344.4 57.4 5.8 d Salyards et al. (1994), and Minor et al. (2006). 86.0 154.0 3.0 144 30 355.2 54.1 2.6 e In addition, paleomagnetic data presented in 84.0 168.0 4.0 143 40 352.6 55.1 3.3 e 85.4 127.1 6.2 N.A.