Paleomagnetism of the Quaternary Bandelier Tuff: Implications for the Tectonic Evolution of the Española Basin, Rio Grande Rift

Home , Bandelier Tuff, Pajarito Plateau, Valles Caldera

Paleomagnetism of the Quaternary Bandelier Tuff: Implications for the tectonic evolution of the Española Basin, Rio Grande rift

Aviva J. Sussman1,2, Claudia J. Lewis1,*, Stephanie N. Mason2, John W. Geissman2, Emily Schultz-Fellenz1, Belen Oliva-Urcia3, and Jamie Gardner1,† 1EARTH AND ENVIRONMENTAL SCIENCES DIVISION, LOS ALAMOS NATIONAL LABORATORY, LOS ALAMOS, NEW MEXICO 87545, USA 2DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MSC 03 2040, 1, UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, NEW MEXICO 87131, USA 3DEPARTMENTO DE GEODINAMICA, UNIVERSIDAD DE ZARAGOZA, ZARAGOZA 50009, SPAIN

ABSTRACT

We present newly acquired paleomagnetic data from Bandelier Tuff exposures in the Jemez Mountains (New Mexico) that show no statistically signifi cant tectonic rotation over Quaternary time. Cooling units of the tuff were mapped in detail and correlated using new geochemical data, allowing us to confi dently sample isochronous units for paleomagnetic remanence directions. In total, 410 specimens were α ≤ subjected to step-wise thermal and alternating fi eld demagnetization. Of the 40 accepted site means, 30 have 95 values 5°. Analysis of the geographic distribution of the site-mean declinations of the data set reveals no statistically signifi cant tectonic rotation either across (northwest/southeast) the northeast-striking Jemez fault or across (east/west) the north-striking Pajarito fault zone. Similarly, our data do not record any measurable relative change in declination difference (−1.1° ± 1.6°) that could be interpreted as a rotation over the ~0.36 m.y. time duration between deposition of the two principal stratigraphic members of the Bandelier Tuff. The step-over discussed in this paper is an area of exceptional structural complexity and, as such, meets the defi nition of “accommodation zone.” We propose the name “Jemez- Embudo accommodation zone” for this composite of structural and volcanic features in recognition of its regional importance in the evolution of the Rio Grande rift. In this part of the rift, where Proterozoic- and Laramide-age faults have preconditioned the crust, idealized relay ramps, prevalent locally, do not occur at the regional scale. Instead, transfer fault zones have developed between half grabens dominated by preexisting faults. The pattern of faulting and accommodation of strain in the right-relayed step-over of the rift has been more or less invariant since the onset of rifting. From a global perspective, the difference between areas of modest crustal extension dominated by distributed deformation and those regions that develop transfer fault zones may ultimately be diagnostic of crustal conditioning and fault strength, such that weak fault systems focus strain within narrow zones.

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INTRODUCTION Acocella et al., 2005). These studies at the scale of ductile strain between normal faults (e.g., of the mechanical model and the outcrop, while Peacock et al., 2000; Acocella et al., 2005), Over a wide range of scales and tectonic set- providing valuable information on the geom- whereas a transfer fault is a discrete, subverti- tings, en echelon normal fault systems defi ne etry and kinematics of en echelon fault systems, cal fault oblique to a rift that transfers displace- areas of complex structure that hold critical are not straightforward to apply to the regional ment between two adjacent basins undergoing clues to understanding the initiation and growth scale, where continental crust is precondi- differential extension (Gibbs, 1984). Transfer of continental rifts. Structural analysis of the tioned by previous deformational phases. At the faults may be confi ned to the upper plate of a step-overs between faults in these systems can regional (or rift) scale, the role of preexisting detachment or transect the detachment faults, provide information on the strength (or weak- faults in concentrating deformation is thought if not the entire crust (e.g., Lister et al., 1986). ness) of the crustal volume between the bound- to be signifi cant (e.g., Acocella et al., 2005). It The term “accommodation zone” was originally ing fault systems and the processes by which is our purpose to examine an excellent example suggested by Bosworth (1985) to denote trans- faults propagate and link together to form con- of a regional-scale step-over with known faults fer fault systems of great structural complexity nected rift basins. In the last 15 yr, many inves- of long ancestry in the Rio Grande rift (New that separate half grabens of opposing detach- tigations of en echelon fault systems have been Mexico), and determine the way in which the ment polarity (e.g., Gregory rift, Africa). The pursued, both in the fi eld (e.g., Trudgill and step-over accommodated strain in Quaternary term was later expanded to include areas of Cartwright, 1994) and in the laboratory (e.g., time and in earlier periods of the rift’s evolution. differential extension but similar detachment Deformation of the crustal volume between polarity (e.g., Gulf extensional province; Stock en echelon faults, or fault systems, has been and Hodges, 1989). Accommodation zones are *Current address: LewisLithoLogic, 40 Camino An- con, Santa Fe, New Mexico 87506, USA. described as two end members that defi ne a now considered to be a fundamental element of †Current address: Gardner Geoscience, 14170 Hwy 4, continuum of possibilities: the relay ramp and rift architecture (Rosendahl, 1987) and are pres- Jemez Springs, New Mexico 87025, USA. the transfer fault. A relay ramp is a broad area ent at all scales of rifting.

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Conceptual models of relay ramp evolu- 108°W 106°W tion predict that the crustal volume between Colorado Springs the bounding fault systems should show evidence for vertical-axis rotation (e.g., Ferrill and Morris, 2001). For example, strata deposited between two normal faults of the San Marcos fault system in northeast Mexico record ~30° Pueblo of clockwise vertical-axis rotation that occurred during deposition, with rotation magnitude 38°N increasing with lithologic age (González-Nara- njo et al., 2008). Evidence for vertical-axis rota- SJVF tion is an important test of relay ramp formation. San Luis Basin Therefore, the primary objective of this N investigation was to determine the temporal and spatial distribution of the rotational component (about a vertical axis) of deformation along the Colorado western boundary of the Española Basin, which New Mexico lies in the step-over between the Albuquerque Raton and San Luis extensional basins (Fig. 1). This complex zone of faulting is commonly con- 0 50 100 km sidered to be a relay ramp (e.g., Kelley, 1982; Acocella et al., 2005). However, it has features that typify transfer fault zones (e.g., Gulf of Colorado Plateau Aden and northern Red Sea [Tamsett, 1984] JVF and northern Gulf extensional province [Stock, 36°N 2000]), including important strike-slip faults at VC Española Basin a high angle to the rift (Embudo, Jemez, and UTAH COLORADO Tijeras fault zones; e.g., Muehlberger, 1979; Aldrich and Dethier, 1990; Kelley et al., 2003), a major locus of synextensional volcanism (the

Jemez volcanic ﬁ eld; e.g., Smith et al., 1970), ARIZONA NEW and an aborted, early rift basin decoupled from MEXICO 35°N ABQ the main rift basins that have been active since Study Area ca. 7 Ma (Baldridge et al., 1994). For the mod- RIO GRANDE est amount of extension that occurred in the Albuquerque RIFT Rio Grande rift (~10%; Golombek et al., 1983), Basin Miocene to recent volcanic rocks mechanical models predict formation of relay ramps rather than transfer fault zones (Acocella Oligocene to Miocene volcanic rocks et al., 2005). However, Acocella et al. (2005) also pointed out that in extensional settings Tertiary intrusive rocks Socorro where extension is <21%, reactivation of pre- 34°N existing structures, subparallel to the extension Rio Grande rift basin direction and along rift margins, may encourage the formation of transfer faults. Topographic caldera rim

To investigate the rotational component of RGR normal fault with barbs on downthrown block deformation, we studied the regionally extensive MDVF Bandelier Tuff. East of the Valles caldera, on the Rio Grande Rift Pajarito Plateau, the Bandelier Tuff cooling unit Jemez Lineament stratigraphy has been mapped in detail and correlated with geochemical data (e.g., Broxton and Figure 1. Map of the Rio Grande rift (RGR) in New Mexico and Colorado (after Gardner and Goff, Reneau, 1995; Goff, 1995). Beyond the Pajarito 1984). Abbreviations: ABQ—Albuquerque, JVF—Jemez volcanic fi eld, MDVF—Mogollon-Datil vol- Plateau, it is challenging to map the Bandelier canic fi eld, SJVF—San Juan volcanic fi eld, and VC—Valles caldera. Tuff based on cooling unit stratigraphy alone because welding and vapor-phase alteration vary greatly with radial distance from the Valles graphic and geochemical data for the Bandelier cal-axis rotations, we also tested for sequential caldera, the source of the tuff, as well as with Tuff with the paleomagnetism of its different variability in rotation by using more than one paleotopography. Geochemical correlation is ignimbrite sheets across the Jemez Mountains isochronous unit (discrete ignimbrite sheet) in an important means of distinguishing among in order to test the possibility of geographic our paleomagnetic analysis. Our study reveals the cooling units and is an essential component variability of vertical-axis rotation. In addition no statistically signifi cant magnitude of relative of this investigation. We integrated new strati- to determining geographic variability of verti- rotation over the area covered by outfl ow facies

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of the Bandelier Tuff and has direct implications for understanding temporal and spatial patterns Taos 0 10 20 30 of deformation and evolution of the western Plateau margin of the Rio Grande rift. Speciﬁ cally, our kilometers work demonstrates that the complex structure of the Jemez Mountains area does not constitute a regional relay ramp, but rather it is a major transfer fault zone, or accommodation zone. EF Our work highlights the signiﬁ cance of preexist- CF ing structures and fault strength on concentrating deformation within narrow zones. NC Jemez GEOLOGIC SETTING AND OVERALL Mountains APPROACH 36°N

P-PF The Rio Grande rift, ~1000 km long, consists of a series of right-stepping en echelon basins (Kelley, 1982; Chapin and Cather, 1994), PF including the Albuquerque, Española, and San JF Luis Basins (Fig. 1). The Jemez lineament, a broad zone where volcanism and localized Cerros deformation have affected all levels of the shal- del Rio low crust (e.g., Aldrich et al., 1986; Lutter et al., 1995; Lewis et al., 2009), crosses the Rio Grande rift in the Jemez Mountains and corresponds CCF with a right step-over between the Albuquerque LBF Cerrillos and San Luis Basins. The Valles caldera, source Santa Ana

Hills 35.5°N of the Bandelier Tuff, is located at the junction Mesa of the Rio Grande rift and the Jemez lineament. In our study area, a local manifestation of this SH SFF lineament (Fig. 2) is the Jemez fault, a broad Ortiz southwest-northeast–trending zone of locally Mountains concentrated, shallow crustal deformation (e.g., Rhyolites Lutter et al., 1995). Quaternary BandelierTuff Previous paleomagnetic investigations in T-CF sedimentary and igneous rocks of the Espa- Tertiary volcanic rocks ñola Basin have established vertical-axis rota- SF Tertiary intrusions tions since middle Miocene time (Brown and ABQ Golombek, 1985, 1986; Salyards et al., 1994; Tertiary Espinaso Fm

Hudson et al., 2004). A recently published 35°N 106°W 105.5°W paper questioned whether some of these data sets adequately averaged paleosecular variation Figure 2. Detailed map of the Española tectonic block showing fault zones (modifi ed from Har- of the geomagnetic fi eld and whether they could lan and Geissman, 2009). Fault subsystems include: CCF—Cañada de Cochiti, CF—Cañones, EF— be used to provide an accurate, absolute esti- Embudo, JF—Jemez, LBF—La Bajada, NC—Nacimiento, PF—Pajarito, P-PF—Picuris-Pecos, SFF— mate of rotations (Harlan and Geissman, 2009). San Felipe, SH—Sand Hill, SF—Sandia, and T-CF—Tijeras-Cañoncito fault systems. Faults are from Those investigations in which secular variation the U.S. Geological Survey Quaternary fault and fold database (http://gldims.cr.usgs.gov); geologic map units are from Green and Jones (1997). Abbreviations: ABQ—Albuquerque. has been adequately averaged document modest (~7°–9°) counterclockwise vertical-axis rotation in Pliocene basalt fl ows (Hudson et al., 2004) and no to modest counterclockwise cooling unit members of the Bandelier Tuff for the geologic stability of their magnetic vertical-axis rotation in Oligocene intrusive (dates from Izett and Obradovich, 1994; Phil- remanence (e.g., Grommé et al., 1972; Reyn- and volcaniclastic rocks (Harlan and Geiss- lips, 2004; Phillips et al., 2007; Gardner et al., olds, 1977; Geissman et al., 1980; Weiss and man, 2009), providing equivocal support for 2010) form a semicontinuous cover from the Noble, 1989). studies done in sedimentary rocks (Brown and eastern edge of the Colorado Plateau (eastern Paleomagnetic data from ash-fl ow tuffs, in Golombek, 1985, 1986; Salyards et al., 1994). fl ank of the Sierra Nacimiento), across the particular, from outfl ow facies, where accurate Until our study, no one had examined the exten- western margin of the Rio Grande rift and reference of the remanence to the paleohorizon- sive ignimbrite sheets of the Bandelier Tuff for toward the axis of the rift at the latitude of tal can be accomplished, have been successfully vertical-axis rotations. the Española Basin. Unaltered ignimbrite or used both to correlate such pyroclastic depos- The Quaternary ignimbrite sheets that ash-fl ow tuff sheets (including welded and its (e.g., McIntosh, 1983; Best et al., 1995; comprise the ca. 1.85, 1.61 Ma, and 1.25 Ma partly welded to nonwelded units) are noted Maughan et al., 2002) and to quantify rotations

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about vertical axes (e.g., Hudson and Geissman, that the resulting thermoremnant magnetism is BANDELIER TUFF STRATIGRAPHY AND 1987; Wells and Hillhouse, 1989; Lewis and not a high-fi delity recorder of the geomagnetic GEOCHEMISTRY Stock, 1998b; Petronis et al., 2009). Intracaldera fi eld (e.g., Rochette, 1987). After accounting facies of ash-fl ow tuffs often present complexi- for these issues, deviations in declination from Previous Work ties in that compaction fabrics are not always locality to locality, as recorded in tuffs, are likely a faithful representation of the paleohorizontal to refl ect vertical-axis rotations (e.g., Wells and The Bandelier Tuff consists of three members or because intercaldera hydrothermal alteration Hillhouse, 1989; Petronis et al., 2009). erupted as a series of ash fl ows at ca. 1.85 Ma, has affected the paleomagnetic signal. Because As a possible contributor to crustal defor- 1.61 Ma, and 1.25 Ma (e.g., Izett and Obra- ash-fl ow tuffs typically cool at a rate that is con- mation, vertical-axis rotation of parts of the dovich, 1994; Spell et al., 1996; Phillips, 2004; siderably faster (e.g., Riehle, 1973; Riehle et al., Earth’s crust can be quantifi ed by comparing Phillips et al., 2007; Gardner et al., 2010). While 1995) than fi eld directional changes refl ecting the directions (specifi cally declination) of rema- the eruption mechanism of the oldest member secular variation of the geomagnetic fi eld, they nent magnetizations of rocks to the geomagnetic of the Bandelier Tuff, the La Cueva Member, are usually high-fi delity recorders of the geo- fi eld direction at the time the rocks acquired is currently unknown (Gardner et al., 2010), magnetic fi eld over short periods of time. Their their remanence. This comparison is done using both the older Otowi Member and the overlying paleomagnetic directions typically provide sin- one of two approaches. In the fi rst, if the col- Tshirege Member of the Bandelier Tuff erupted gle virtual geomagnetic poles (VGPs), and they lective data from numerous temporally distinct during enormous, caldera-forming events and cannot be a priori assumed to refl ect any form rock units in a part of the crust suspected of rota- are extensively exposed throughout the Jemez of long-term averaging of the geomagnetic fi eld tion provide an adequate long-term (±106 yr) Mountains (Smith et al., 1970). The Otowi and (e.g., Reynolds, 1977; Geissman et al., 2010). average of the geomagnetic fi eld, then the mean Tshirege Members, the stratigraphic foci of this Barring potential complexities associated of such data can be compared with an expected investigation, each consist of a sequence of ash- with emplacement on irregular topography, the fi eld direction for the appropriate time that is rich ignimbrites containing crystals, pumice, inclination of the magnetization characteristic determined from the paleomagnetic apparent and lithic fragments (Fig. 3). The commonly of the tuff should be fi xed with respect to the polar wander path from an adjacent (stable) cra- accepted age of the Otowi Member is 1.613 paleohorizontal (Byrd et al., 1995) as identifi ed ton. In the second, directional data from a single ± 0.011 Ma (2σ weighted mean of four sam- through fi eld relationships, regardless of the isochronous unit (e.g., lava fl ow or ash-fl ow tuff ples), obtained by the 40Ar/39Ar method from magnitude of tilting or vertical-axis rotation. cooling unit) at a locality suspected of rotation pumice lumps of the Guaje pumice bed (Izett In addition, compaction/welding processes can can be compared with data from the identical and Obradovich, 1994; their sample 91G35, result in signifi cant inclination fl attening, as has single unit at a locality outside of the suspected close to this study’s site AN1). been observed in very thick ash-fl ow tuff depos- deformation zone to estimate a magnitude of For the Tshirege Member, Phillips et al. its (e.g., Rosenbaum, 1986), or the development relative rotation (e.g., Wells and Hillhouse, (2007) used single crystal laser fusion on four of such a strongly anisotropic magnetic fabric 1989; Petronis et al., 2009). samples to obtain high-resolution 40Ar/39Ar

A Intercalated Surge Beds Qbt4

Surge Beds

Tshirege Member Surge Beds & Pumice Swarms

Slope-forming tuffs 1.25 Ma Vapor-phase Notch Colonnade tuff

Qbt1 Qbt2 Qbt3 B Qbt2 Tsankawi Pumice Bed

Cerro Toledo Qct Tephras & Volcaniclastic Sediments Qbt1

Otowi Member Qbo Qbo

1.61 Ma

Guaje Pumice Bed

Figure 3. (A) Stratigraphic columnar summary of the Bandelier Tuff pyroclastic deposits (after Lewis et al., 2009). See text for details. (B) Field photograph, looking to the southwest, of the Bandelier Tuff in Eagle Canyon. Scale for the cliff is ~150 m.

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dates. An outfl ow-facies ignimbrite from the and may depend on several factors, including generally below detection limits and therefore Pajarito Plateau yielded their preferred date of temperature at time of deposition or input of are not reported. 1.256 ± 0.010 Ma (Phillips et al., 2007; sample meteoric water during cooling (Lipman and EP-44), as the sample had no anomalously aged Friedman, 1975), major- and trace-element Unit Descriptions crystals and gave a more precise 40Ar/39Ar result data are often used to provide defi nitive iden- than their other three samples. This date is sta- tifi cation of eruptive units, and we take this Otowi Member of the Bandelier Tuff (Qbo) tistically indistinguishable from published ages approach in this study. The Tshirege Member Ash fl ows of the Otowi Member were of Izett and Obradovich (1994) and Spell et al. is stratigraphically zoned from high-silica rhy- erupted and emplaced during the volcanic and

(1996) that have been recalculated using the olite (76–77 wt % SiO2) at its base to low-silica structural events that formed the Toledo caldera

same sanidine fl ux monitor age. rhyolite (72 wt% SiO2) at its top (Smith and (Smith and Bailey, 1966). On the Pajarito Pla- The dates of individual eruptive units of Bailey, 1966), consistent with a progressively teau, the Otowi Member is a mostly nonwelded the Bandelier Tuff are indistinguishable within deeper source within a zoned magma body ignimbrite, up to 125 m thick, with an 8-m-thick uncertainties from one another, such that erup- during eruption (e.g., Smith, 1979). Major- moderately welded zone near the top (borehole tion of the entire Tshirege Member is thought to and trace-element concentrations vary in a pre- SHB-3; Gardner et al., 1993). West of the cal- have occurred on a time scale shorter than a few dictable manner in the Tshirege Member (see dera, however, much thicker sections of densely thousand years (e.g., Phillips et al., 2007). On following), and eruptive units can be readily welded Otowi Member are common, especially the other hand, data based on predictive model- distinguished (e.g., Caress, 1995; Broxton et in the Seven Springs area (Kelley and Kelley, ing studies from the Bishop Tuff of California al., 1996). 2004). Other than the general lack of welding indicate an eruptive cycle lasting approximately in the Otowi Member on the Pajarito Plateau, 3 yr, with periods of quiescence between erup- Methods even in thick sequences relatively close to the tive units in various sections of 5–800 d (Sheri- caldera source, it is petrographically similar to dan and Wang, 2005). Given that no evidence We collected samples of Bandelier Tuff for the Tshirege Member. Quartz, commonly in for unconformities has been found within the whole-rock X-ray fl uorescence (XRF) analysis bipyramidal form, and sanidine are the most Tshirege Member and that the estimated erup- to aid in unit identifi cation and facilitate strati- conspicuous crystal phases in the Otowi Mem- tive volume of the Bishop Tuff is 600 km3 (Wil- graphic correlations (e.g., Krier et al. 1998; ber. In contrast to the Tshirege Member, the son, 2008), as compared to a Tshirege volume Gardner et al., 1999, 2001). We sampled sec- Otowi tends to contain more lithic fragments, of 250 km3, it is likely that the Tshirege was tions (measured using a Suunto hand level) at with signifi cant contributions from Precam- erupted over a very short time interval. Cat Mesa, Eagle Canyon, San Juan Mesa, Saw- brian lithologies (Eichelberger and Koch, 1979; Ignimbrite sequences are typically sub- yer Mesa, and Seven Springs, as well as individ- Gardner et al., 1993). divided into “cooling units” (Smith, 1960), ual cooling units at San Antonio and Vallecitos which are unique to these volcanic rocks and (Fig. 4). At all these sites, we took samples from Tshirege Member of the Bandelier Tuff represent single or multiple pyroclastic fl ows an outcrop depth of 5–10 cm to avoid weather- (Qbt) in which the juvenile material (pumice and ing features. The Bandelier Tuff stratigraphy at Ash fl ows of the Tshirege Member were shards) was welded during compaction and Pajarito Mesa and Mortandad Canyon (Fig. 4) erupted and emplaced during the volcanic and cooling. The Otowi Member is a single cooling is well described from detailed geologic studies structural events that formed the Valles cal- unit, whereas the Tshirege Member is a com- (e.g., Lavine et al., 2003; Gardner et al., 2008; dera, which is the modern physiographic cal- pound cooling unit, showing at least four dis- Lewis et al., 2009); no additional geochemical dera (Smith and Bailey, 1966; Gardner et al., tinct welding reversals attributed to intervals of analysis was performed at those localities. 2010). Tshirege Member thicknesses range cooling between emplacement of successive Major and trace elements were analyzed in up to ~1000 m on the Pajarito Plateau, but the packages of pyroclastic fl ows. 55 bulk samples using an automated Rigaku typical thickness of outfl ow facies of Qbt is Cooling units in the Tshirege Member, wavelength-dispersive XRF spectrometer. Sam- ~300 m. The highest volume and most later- which may or may not coincide with contacts ples were crushed and homogenized in 15–20 g ally extensive cooling units are Qbt1 and Qbt2. between eruptive units (e.g., Gardner et al., parts in a tungsten-carbide shatterbox. Sample In the following paragraphs, we describe those 2001), have been mapped based on primary and splits were dehydrated at 110 °C for 4 h and then cooling units from which we collected data for secondary features, including welding, devitri- allowed to equilibrate with the ambient atmo- our geochemical and paleomagnetic investiga- fi cation, vapor-phase alteration, oxidation, and sphere for 12 h. One-gram splits were fused at tions; these units are: Qbt1, Qbt2, Qbt3, and relative content of crystals, pumice, and lithic 1100 °C with 9 g of lithium tetraborate fl ux to Qbt4. Due to its absence in most of the sam- fragments. The cooling unit stratigraphy on obtain fusion disks. Additional one-gram splits pled sections, data from Qbt3t (Gardner et al., the Pajarito Plateau has been well documented were heated at 1000 °C to obtain the loss-on- 1999) were not collected. (e.g., Vaniman and Wohletz, 1990; Broxton ignition (LOI) measurements. Elemental con- Qbt1. Qbt1 is the basal ignimbrite of the and Reneau, 1995; Gardner et al., 1999, 2001; centrations were calculated by comparing X-ray Tshirege Member. Qbt1 ranges from entirely Dethier et al., 2007). In other parts of the intensities for the samples to those for 21 stan- vitric to entirely vapor-phase altered, with a Jemez Mountains beyond the Pajarito Plateau, dards of known composition, after correcting near continuity of variations in between. On the the cooling unit stratigraphy is less well deter- for absorption. The XRF method we employed Pajarito Plateau, Qbt1 consists of a sequence mined, and that was a primary motivation for calculates the concentrations of ten major oxides of variably welded and altered subunits, Qbt1g

the stratigraphic and related geochemical stud- (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, (glassy), Qbt1v-c (vapor-phase altered, colon-

ies reported here. Na2O, K2O, and P2O5), ten trace elements (V, Cr, nade), and Qbt1v-u (upper vapor-phase altered). Because several features in ash-ﬂ ow tuffs, Ni, Zn, Rb, Sr, Y, Zr, Nb, and Ba), and weight Generally, Qbt1 contains 15%–30% pumice, including degree of welding, devitriﬁ cation, loss on ignition (Table 1). Elemental concentra- 10%–20% quartz and sanidine phenocrysts, and and vapor-phase alteration, can vary laterally tions of V, Cr, and Ni in the Bandelier Tuff are 1%–5% lithic fragments. Crystals are primarily

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Sampling Localities

EF This study, with site initials 36.00°N MacDonald and Palmer (1990) Doell et al. (1968) SE N 10 km Los Valles Alamos AN SC Caldera MC PM LC 35.85°N

PJF SA CM VA

Jemez Springs DR JZF SJ YR PC 35.70°N EC

Santa Fe

106.85°N 106.50°N 106.15°N Figure 4. Map showing sampling localities within the Valles caldera region. Sampling localities include: LC—Los Alamos Canyon, CM—Cat Mesa, DR— Dome Road, EC—Eagle Canyon, MC—Mortandad Canyon, PC—Peralta Canyon, PM—Pajarito Mesa, AN—San Antonio Canyon, SE—Seven Springs, SA—Sawyer Mesa, SJ—San Juan Mesa, SC—Upper Sandia Canyon, VA—Vallecitos, and YR—Young’s Ranch. Tectonic features include: JF—Jemez fault, EF—Embudo fault, and PF—Pajarito fault.

quartz and sanidine in subequal proportions, amounts. A distinctive feature of the pheno- are relatively large (up to 2 cm in diameter) and with trace amounts of clinopyroxene, horn- cryst population of Qbt3 is the relatively coarse consist of intergrown feldspar (anorthoclase) blende, and fayalite. Qbt1 is widespread across crystal size—most crystals reach 4–6 mm in and quartz with minor amphibole and pyrox- the Pajarito Plateau. diameter. Crystals are also commonly euhedral, ene. Thin sections show that the anorthoclase in Qbt2. This cooling unit is also widespread as single crystals or single-crystal fragments. these agglomerations is commonly cored with across the Pajarito Plateau. The greatest thick- The top of Qbt3 is locally characterized by gas plagioclase (Gardner et al., 2001). This subunit nesses and highest degree of welding in Qbt2 escape pipes up to 10 cm across (Crowe et al., has been referred to as “the anorthoclase unit” occur in the west, closer to the source, whereas 1978) and funnel-shaped fossil fumaroles asso- (e.g., Doell et al., 1968). eastern outcrops are thin and poorly welded. ciated with small-displacement faults and ﬁ s- The base of Qbt2 is commonly marked by a sures (Gardner et al., 2008). Geochemical Results pumice swarm (typically <1 m thick) containing Qbt4. This unit includes two distinct cool- ~30% pumice lapilli up to 15 cm in diameter. ing units. The lower subunit consists of poorly A compilation of XRF data on bulk rock Otherwise, Qbt2 contains ~10%–15% pumice, to moderately welded, crystal-poor ignimbrite samples from the Otowi and Tshirege Members 15%–25% quartz and sanidine phenocrysts, and (e.g., Gardner et al., 1998). This basal part of is given in Table 1. Variations in major-element

<1%–2% accidental lithic fragments; no maﬁ c Qbt4 is relatively poor in pumice (less than 5%) oxides SiO2 and TiO2 and trace elements Rb accessory minerals were observed. The top of and crystals (10%–15% phenocrysts of mostly and Nb show distinct trends in both members Qbt2 is marked by a gradational decrease in quartz and sanidine to 2–3 mm across in diam- (Fig. 5). The earliest erupted ash ﬂ ows in both welding (over ~1 m) from moderately welded eter) with rare accidental lithic fragments. Fer- Otowi and Tshirege are higher in silica and are Qbt2 to nonwelded Qbt3. romagnesian silicate crystals are extensively more radiogenic than subsequent deposits (e.g., Qbt3. Qbt3 is rich in pumice (~30%) com- altered and in many localities are yellow in color Stimac et al., 1996).

pared to the underlying Qbt2 and contains rela- and surrounded by distinct white alteration The SiO2 content in sampled Otowi units

tively abundant (locally up to 5%) accidental halos (Lewis et al., 2002). The upper subunit of ranges from 73.4 to 78.1 wt%. TiO2 ranges from lithic fragments, most of which are around 5 cm Qbt4 (including the geochemically and miner- 0.08 to 0.19 wt%. These trends are similar to in diameter. Qbt3 is enriched in phenocrysts alogically deﬁ ned Qbt5 of Warren et al., 2007) samples recovered from borehole SHB-3 on the compared to other units, with at least 30% consists of nonwelded to densely welded tuff Pajarito Plateau (Gardner et al., 1993), although crystals. Quartz and sanidine occur in subequal with conspicuous crystal agglomerations, which the ranges determined in this study are wider

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TABLE 1A. X-RAY FLUORESCENCE (XRF) MAJOR- AND TRACE-ELEMENT DATA FROM TSHIREGE MEMBER (NORMALIZED) Unit XRF site Major oxide (wt%) Trace element (ppm) (Pmag site) SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2OP2O5 Total Zn Rb Sr Y Zr Nb Ba Cat Mesa Qbt2 QR-7-05 77.4 0.09 11.97 1.58 0.04 0.06 0.16 4.22 4.50 ND 100.0 104 138 16 46 212 73 80 Qbt2 QR-8-05 77.6 0.09 12.01 1.53 0.04 0.07 0.13 4.01 4.51 ND 100.0 86 138 9 65 231 71 96 Qbt2 QR-9-05 CM1 77.9 0.09 12.11 1.49 0.05 0.06 0.17 3.68 4.45 ND 100.0 118 105 ND ND 199 64 65 Qbt2 QR-10-05 77.6 0.09 12.12 1.45 0.04 ND 0.13 4.04 4.57 ND 100.0 ND 187 36 86 201 99 134 Qbt2 QR-11-05 77.2 0.11 12.20 1.58 0.05 0.07 0.19 4.00 4.61 ND 100.0 ND 94 25 64 272 54 180 Qbt3 QR-12-05 77.5 0.11 12.00 1.43 0.04 0.05 0.20 4.10 4.55 ND 100.0 91 101 58 46 307 54 279 Eagle Canyon Qbt1 QR-39-06 75.2 0.06 11.69 1.51 0.08 0.05 0.37 4.18 4.23 2.62 100.0 ND 247 11 96 237 119 34 Qbt2 QR-40-06 76.5 0.09 11.43 1.47 0.06 0.09 0.50 4.05 4.15 1.67 100.0 106 103 27 47 249 53 143 San Antonio Qbt1 or QR-5-08 AN1 77.6 0.09 12.00 1.52 0.06 ND ND 4.15 4.53 ND 100.0 113 128 20 63 220 58 93 Qbt2 San Juan Mesa Qbt1 QR-24-06 75.6 0.05 11.21 1.47 0.08 0.06 0.34 4.21 4.26 2.75 100.0 ND 247 41 98 241 146 35 Qbt1 QR-25-06 75.2 0.06 11.56 1.53 0.08 0.05 0.24 3.64 4.64 2.96 100.0 ND 225 35 107 235 123 39 Qbt1 QR-26-06 75.9 0.06 11.67 1.50 0.08 0.08 0.29 3.99 4.37 2.04 100.0 ND 246 14 116 250 130 56 Qbt1 QR-27-06 75.7 0.06 11.51 1.49 0.08 0.09 0.69 3.91 4.25 2.27 100.0 ND 233 29 103 234 122 62 Qbt1 QR-28-06 76.3 0.06 11.31 1.46 0.07 0.05 0.28 3.94 4.05 2.53 100.0 ND 210 25 93 216 109 68 Qbt1 QR-29-06 76.3 0.07 11.63 1.50 0.08 0.06 0.25 3.86 4.37 1.90 100.0 ND 235 19 91 223 120 50 Qbt2 QR-30-06 76.3 0.09 12.01 1.43 0.06 ND 0.20 3.78 4.51 1.65 100.0 ND 136 18 73 221 71 96 Qbt2 QR-23-06 76.7 0.09 11.99 1.45 0.06 0.00 0.22 3.78 4.55 1.17 100.0 93 156 10 81 214 86 76 Qbt1 QR-32-06 SJ1u 77.9 0.06 11.65 1.41 0.06 0.08 0.29 3.97 4.59 ND 100.0 128 196 21 77 223 126 97 Qbt2 QR-33-06 SJ2u 77.8 0.09 11.88 1.43 0.05 0.07 0.20 4.13 4.39 ND 100.0 ND 137 16 39 235 58 88 Qbt2 QR-34-06 SJ2u 77.5 0.10 11.84 1.52 0.06 0.07 0.35 3.89 4.63 ND 100.0 89 153 17 59 220 90 89 Qbt2 QR-35-06 SJ3u 77.7 0.09 12.00 1.54 0.06 ND 0.11 4.05 4.48 ND 100.0 ND 127 18 32 228 57 142 Sawyer Mesa Qbt2 QR-56-05 SA1 78.1 0.08 11.68 1.39 0.05 ND 0.17 3.88 4.35 0.33 100.0 100 133 12 55 227 73 71 Qbt3 QR-57-05 SA3 76.7 0.13 12.32 1.57 0.05 ND 0.20 4.00 4.55 0.44 100.0 93 84 61 39 320 50 299 Seven Springs Qbt1 KC1-4 76.9 0.06 12.50 1.68 0.10 ND ND 4.19 4.54 0.02 100.0 134 272 ND 89 277 146 76 Qbt1 KC1-5 77.0 0.06 12.36 1.66 0.09 ND ND 4.28 4.55 ND 100.0 135 288 ND 46 279 141 ND Qbt1 KC1-6 76.9 0.06 12.31 1.74 0.09 ND ND 4.32 4.56 ND 100.0 129 278 ND 67 274 146 ND Qbt1 KC1-7 77.0 0.06 12.34 1.62 0.10 ND ND 4.32 4.54 ND 100.0 128 287 ND 63 273 147 ND Qbt1 KC1-8 77.1 0.06 12.13 1.58 0.07 ND 0.13 4.36 4.55 ND 100.0 122 273 9 66 269 144 ND Qbt1 KC1-9 77.3 0.06 12.13 1.60 0.09 ND ND 4.24 4.58 ND 100.0 122 269 ND 64 264 124 86 Qbt1 KC1-10 77.3 0.06 12.10 1.65 0.08 ND ND 4.22 4.57 ND 100.0 124 262 8 68 267 145 ND Qbt1 KC1-11 77.2 0.06 12.24 1.67 0.08 ND ND 4.15 4.60 ND 100.0 122 250 ND 94 260 150 ND Qbt1 KC1-12 77.0 0.07 12.43 1.57 0.09 ND ND 4.28 4.56 ND 100.0 123 248 10 83 249 132 ND Qbt1 KC1-13 77.0 0.08 12.16 1.62 0.09 ND ND 4.34 4.72 ND 100.0 216 230 20 91 240 119 57 Qbt1 KC1-14 76.9 0.08 12.17 1.52 0.07 ND 0.36 4.31 4.64 ND 100.0 92 202 26 106 223 107 72 Qbt1 KC1-15 77.5 0.08 12.10 1.56 0.07 ND ND 4.11 4.54 ND 100.0 95 124 22 77 220 71 93 Qbt1 QR-55-05 SE7 76.9 0.08 12.52 1.64 0.07 0.00 0.10 3.99 4.66 0.00 100.0 125 206 9 91 253 119 48 Qbt1 QR-1-08 SE8 77.8 0.06 11.98 1.59 0.08 ND ND 4.01 4.49 ND 100.0 171 237 ND 51 250 140 ND Qbt1 QR-2-08 SE9 77.6 0.08 12.11 1.60 0.07 ND ND 4.01 4.57 ND 100.0 131 199 10 91 234 114 ND Qbt2 QR-3-08 SE10 77.1 0.08 12.39 1.61 0.10 ND ND 4.07 4.61 0.01 100.0 137 171 ND 131 242 88 57

Note: ND—nondetectable. Two-sigma analytical uncertainties (same units as analyses): SiO2—0.9; TiO2—0.01; Al2O3—0.3; Fe2O3—0.05; MnO—0.01; MgO—0.04; CaO—0.1; Na2O—0.1; K2O—0.1; P2O5—0.01; Zn—17; Rb—15; Sr—8; Y—12; Zr—15; Nb—15; Ba—25.

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TABLE 1B. X-RAY FLUORESCENCE (XRF) MAJOR- AND TRACE-ELEMENT DATA FROM OTOWI MEMBER (NORMALIZED) XRF site Pmag site Major oxide (wt%) Trace element (ppm)

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2OP2O5 Total Zn Rb Sr Y Zr Nb Ba Cat Mesa QR-6-08 VA1 78.0 0.10 11.94 1.38 0.04 ND 0.17 3.90 4.48 ND 100.0 67 105 ND ND 202 53 63 QR-8-08 CM3 78.1 0.10 11.91 1.29 0.02 ND 0.14 3.95 4.49 ND 100.0 118 105 ND ND 199 64 65 Eagle Canyon QR-36-06 76.8 0.10 12.43 1.64 0.08 0.10 0.42 4.07 4.39 ND 100.0 163 317 27 97 241 172 102 QR-37-06 76.7 0.13 12.51 1.57 0.05 0.28 0.61 3.82 4.30 0.02 100.0 88 140 50 51 187 72 134 QR-38-06 78.1 0.10 11.76 1.30 0.05 0.09 0.39 3.62 4.57 ND 100.0 66 139 22 54 191 73 107 Seven Springs QR-50-05 SE1 77.6 0.08 12.28 1.37 0.05 ND 0.14 4.19 4.31 ND 100.0 83 196 14 53 181 101 76 QR-51-05 SE2 77.5 0.08 12.16 1.37 0.05 ND 0.28 4.20 4.33 ND 100.0 96 197 14 54 181 89 52 QR-52-05 SE3 76.3 0.15 12.63 1.74 0.08 0.18 0.47 4.16 4.21 0.04 100.0 108 165 59 49 190 83 181 QR-53-05 SE4 76.1 0.13 12.59 1.61 0.07 0.11 0.69 4.30 4.40 0.02 100.0 91 163 50 53 183 87 126 QR-54-05 SE5 75.8 0.17 12.61 1.83 0.07 0.24 0.65 4.19 4.36 0.05 100.0 87 162 74 49 190 86 190 KC1-1 73.4 0.19 12.78 1.88 0.06 0.55 0.70 5.68 4.67 0.12 100.0 79 146 96 43 228 71 157 KC1-2 75.9 0.15 13.19 1.72 0.06 0.15 0.46 4.00 4.37 0.03 100.0 69 130 57 43 201 67 192

A 0.20 350

300 0.16 250

0.12 200 2 Rb (ppm) Legend: (wt%) TiO 150 0.08 Qbo Qbt1 100 Qbt2 Qbo Qbo Qbt3 0.04 50 Cat Mesa 72 74 76 78 80 0 50 100 150 200 Eagle Canyon SiO (wt%) Nb (ppm) San Antonio 2 San Juan Mesa Sawyer Mesa B 0.20 350 Seven Springs 300 0.16 250

0.12 200 2 Rb (ppm)

TiO (wt%) TiO 150 0.08 100 Qbt Qbt 0.04 50 72 74 76 78 80 0 50 100 150 200

SiO2 (wt%) Nb (ppm)

Figure 5. Geochemical variation in the (A) Otowi Member and (B) Tshirege Member. SiO2 versus TiO2 (left) and Rb versus Nb (right). Data for Qbo in A are from Gardner et al. (1999). Data for Qbt1, Qbt2, and Qbt3 in B are from Broxton et al. (1995, 1996); Stimac et al. (1996); and Lavine et al. (2003).

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(Fig. 5). Rb concentration ranges from 105 to brites with partial cooling breaks between them, Paleomagnetic investigations in the Ban- 317 ppm, and Nb ranges from 53 to 172 ppm, compared to the densely welded Qbt2 typical of delier Tuff have been previously conducted by showing a trend of increasing depletion of the Pajarito Plateau (e.g., Lavine et al., 2003). In Doell et al. (1968) and MacDonald and Palmer incompatible elements from older to younger other studies of Qbt1 and Qbt2, X-ray diffrac- (1990). In their classic investigation of Quater- cooling units. Rb and Nb trends ﬁ t with SHB-3 tion data were used for correlation purposes due nary volcanic rocks in the Jemez Mountains to trends as well. This is consistent with system- to the similarity in the mineralogy of these units elucidate details of the geomagnetic polarity atic depletion of incompatible elements upward (e.g., Broxton et al., 1996; Broxton and Reneau, time scale, Doell et al. (1968) collected samples in the ignimbrite (Stimac et al., 1996). 1995; Lavine et al., 2003). The mineralogical from six sites in the Otowi Member (Qbo) at two

The SiO2 content in the Tshirege units we variations among deposits on the Pajarito Pla- geographic localities and six sites in the Tshirege sampled ranges from 75.2 to 78.1 wt%, and teau result from the distinctive pattern of vapor- Member (Qbt) at two different locations. In their

TiO2 ranges from 0.05 to 0.13 wt%. SiO2 and phase alteration, which is not conﬁ ned to a study of the anisotropy of magnetic susceptibil-

TiO2 contents are inversely correlated, with particular stratigraphic level (e.g., Stimac et al., ity of the Bandelier Tuff with the goal of infer-

SiO2 decreasing up section and TiO2 increasing 1996). Qbt3 is exposed locally west and south of ring pyroclastic transport directions, MacDonald up section. This is the typical pattern observed the caldera, but it does not have the lateral con- and Palmer (1990) concentrated on the Tshirege in the Tshirege Member on the Pajarito Pla- tinuity that is common on the Pajarito Plateau. Member and reported data from 18 sites around teau (e.g., Gardner et al., 1999). Rb concentra- Nonetheless, outcrop characteristics that distin- the Valles Caldera, collected at 14 distinct locali- tion (84–228 ppm range) and Nb concentration guish these units on the Pajarito Plateau (e.g., ties. The paleomagnetic data from these studies (50–150 ppm range) both decrease up section, high crystal content in Qbt3) are also observed are discussed later herein. but there is some overlap between cooling units. in these other areas. Based on cooling unit characteristics (as Methods described here) and geochemical data (Table 1; PALEOMAGNETISM OF THE BANDELIER Fig. 5), we correlated our measured sections TUFF Using a gas-powered, water-cooled drill, from areas in the Jemez Mountains with respect we obtained 8–20 independent samples within to well-characterized Pajarito Plateau trends. Previous Work the fi ve cooling units of the Bandelier Tuff at a The Cat Mesa section spans Qbo through Qbt3. total of 40 sites. Standard oriented cores were The uppermost site collected is well within Numerous paleomagnetic investigations of drilled from adequately welded intervals, with the Qbt3 compositional fi eld described by the ash-fl ow tuffs have demonstrated that these vol- oriented block samples collected at seven sites Pajarito Plateau data (Figs. 4 and 5). The low- canic deposits can provide faithful, isochronous where only nonwelded layers were present. Field ermost sample from Eagle Canyon is Qbo, and records of the geomagnetic fi eld acquired at the cores were oriented using both magnetic and the upper sample may be either Qbt1 or Qbt2; time of emplacement, cooling, and compaction sun compasses, with differences in orientation its Rb/Nb signature is in the transitional zone (e.g., Riehle, 1973; Reynolds, 1977; Byrd et al., that were less than 2° between determinations. between the two units. The sample from San 1995; Geissman et al., 2010). The characteris- After preparation, all specimens were stored Antonio is either Qbt1 or Qbt2; on both the Rb/ tic magnetization (ChRM) of most ash-fl ow in a magnetically shielded room with aver-

Nb and SiO2/TiO2 graphs, it plots in a zone of tuffs is typically a thermoremanent magnetiza- age fi eld intensities below 200 nT. Specimens overlap between Qbt1 and Qbt2 data. The San tion, carried by a low-titanium magnetite and/ were demagnetized using both AF and thermal Juan Mesa samples fall into two groups, those or maghemite. Isolation of the ChRM is usually methods. Demagnetization procedures were that match with Qbt1 (the lower part of the sec- accomplished with progressive alternating-fi eld performed in low-magnetic-fi eld environments tion) and those that fi t with Qbt2 (the upper (AF) demagnetization to peak fi elds of 120–160 in paleomagnetic laboratories at the University part). The two geochemical samples from Saw- mT. In some cases, because of a relatively high of Michigan and the University of New Mexico yer Mesa are in Qbt2 and Qbt3, well within concentration of high-coercivity, single-domain using 2-G Enterprises cryogenic magnetometers. those fi elds on the geochemical variation plots. magnetite, a combination of AF followed by In total, 410 specimens were demagnetized; of Even though the lower sample could be in Qbt1 progressive thermal demagnetization has been these, ~20% were thermally treated, and ~80% based on geochemistry, its stratigraphic position used to fully isolate the ChRM. Maximum labo- were subjected to AF demagnetization. Thermal is more consistent with Qbt2. All of the Seven ratory unblocking temperature ranges between demagnetization employed 15–23 temperature Springs section is in Qbo and Qbt1, except for ~610 °C and 630 °C have been interpreted to steps ranging from 70 °C to 680 ° C using a fur- the topmost sample, a small mesa-top remnant, suggest that a low-titanium maghemite is an nace with magnetic fi elds less than 10 nT in the which could be Qbt2. important contributor to the remanence. Because sample region. Alternating fi eld demagnetization an ash-fl ow tuff cooling unit cools over a range employed 14–25 steps ranging from 2 to 120 mT. Summary of Stratigraphic Data of temperatures typical of those associated with Average intensities of natural remanent mag- thermoremnant magnetism blocking, a single, netization (NRM) were ~0.95 A/m. Illustrative East and southeast of the caldera (Sawyer discrete ash-fl ow tuff unit should acquire a uni- vector component diagrams of thermal and alter- Mesa and Eagle Canyon), the Bandelier Tuff form ChRM direction, with an inclination that nating fi eld demagnetization response (Fig. 6) stratigraphic section is basically the same as on is fi xed relative to the paleohorizontal unless show peak laboratory unblocking temperatures the Pajarito Plateau. West and southwest of the affected by differential fl attening due to extreme between 570 °C and 620 °C and progressive ran- caldera, Tshirege subunits Qbt1 through Qbt3 compaction and/or welding. Thus, data from the domization of the NRM over a range of alternat- are present. However, the thickness, degree of same outfl ow facies cooling unit collected from ing peak fi elds, indicating that a relatively mod- welding, and vapor-phase alteration vary con- different geographic locations allow for the erate-coercivity mineral (possibly both magnetite siderably from relations on the Pajarito Plateau. examination of differential, or relative, rotation and maghemite) is the remanence carrier. Qbt1 at San Juan Mesa is almost entirely vitric. expressed as statistically signifi cant differences The ChRM direction for each specimen was Qbt2 on Cat Mesa is a sequence of thin ignim- among paleomagnetic declinations. computed using principal component analysis

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AB C W, Up 300 °C W, Up NRM SE07-Ac SE07-Fd 20 mT NRM Site SE7 Qbt2 500 °C NRM Dec = 178.0° 40 mT Inc = –33.2° α95 = 3.4° N = 31 specimens 300 °C Each division 5*10-4 S N 40 mT 500 °C 580 °C 20 mT 590 °C NRM S N E, Down Each division 5*10-4 E, Down NRM W, Up SE07-Fb 300 °C W, Up SE07-Fg 20 mT NRM 500 °C NRM 580 °C S 40 mT

-4 590 °C S N NRM Each division 5*10 40 S N 20 mT 5 Each division 5*10-4 mT 300 °C 00

°C E, Down E, Down Figure 6. Examples of representative progressive demagnetization results from the Bandelier Tuff. (A) Orthogonal demagnetization diagrams showing response to progressive alternating-ﬁ eld demagnetization. The magnetization vector is plotted onto the horizontal (circles) and vertical plane (squares). Peak demagnetizing ﬁ elds are plotted along the projection data points. (B) Orthogonal demagnetization diagrams showing response to progressive thermal demagnetization. Temperatures are plotted along the projection data points. (C) Equal-area projection of the site mean calculated from specimen directions at a representative site (SE07) from the Seven Springs locality. Open circles are projections on the upper hemisphere (reverse polarity).

(Kirschvink, 1980). Following determination of of the paleomagnetic data obtained in this study to defi ne a single, instantaneous fi eld direction, sample ChRM directions (Table 2), site-mean are given in geographic (in situ) coordinates. and our collection of data from multiple sites in characteristic directions were calculated using Tectonic corrections were not applied at any of the Bandelier Tuff provides a robust approach Fisher (1953) statistics, due to the straightfor- the sampling localities. Throughout most of the with which to evaluate directional variations ward nature of Fisher’s approach to calculating Jemez Mountains where the Bandelier Tuff is within isochronous cooling units. confi dence limits and applying signifi cance tests exposed, orientations of eutaxitic structures and/ (Butler, 1992). In order to assess data quality, or contacts between cooling units are essentially Paleomagnetic Results α we chose to use 95 values (as opposed to the horizontal. In a few cases, we measured eutaxitic precision parameter, k) because it provides a structures and/or contacts between cooling units Our 40 paleomagnetic sites were distributed tool for visualization, data comparison, and with dips up to 5°, yet it is diffi cult to assess in the fi ve distinct cooling units of the Bande- calculation of any vertical-axis rotations. For whether such inclined compaction fabrics are lier Tuff (described previously) at 14 localities our investigation, only site means with a 95% tectonic, and thus should be corrected for, or if around the Valles caldera (Fig. 4). Mean direc- α ≤ confi dence limit ( 95) 10° were considered they are primary features associated with initial tions were considered in temporal and spatial acceptable. Two site means were eliminated emplacement and compaction and/or welding. contexts to look for any systematic distribution from further study (EC5 and SJ2l), and of the Although a single ash-fl ow tuff cooling unit of paleomagnetic declinations that could be α remaining 40 site means, 30 had 95 values cools over too short an interval to average secular interpreted as a record of vertical-axis rotations. ≤5°. Site mean ChRM directions are listed in variation of the geomagnetic fi eld, it is possible First, we combined site mean directions into Table 2, and examples of paleomagnetic data at that subtle, small-magnitude directional changes stratigraphically ordered data sets, to test for any the site level (Fig. 6) show typical distributions are recorded within a single cooling unit, or systematic temporal variation in directions. This of sample directions used to estimate site mean among cooling units that are closely emplaced was accomplished by correlating paleomagnetic directions. After calculating the mean direction over time; a good example of this is the Miocene sites with measured sections and geochemical for each site, we corrected for local magnetic Peach Springs Tuff (Wells and Hillhouse, 1989). sample locations to be certain directional data declination by adding 10° to the site mean; all Concentrated sampling at specifi c stratigraphic were from the same cooling unit. We compared site mean directions are of reverse polarity. All levels within a single ash-fl ow tuff offers a way results from units within the Tshirege Member

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and between the Tshirege and Otowi Members TABLE 2. SITE-MEAN DIRECTIONS OF CHARACTERISTIC REMANENT MAGNETIZATION from sites where a vertical section was exposed. α κ Unit Site XRF site Dec Inc 95 N/No Lat Long Second, we evaluated the possibility for spatial (°) (°) (°) (°N) (°W)

variability of rotation. This was achieved by This study combining site mean directions from specifi c geographic localities and grouping locality Los Alamos Canyon means with respect to major tectonic structures Qbt2 BR1 171.0 –34.0 3.5 197.6 9/10 35.862 106.250 (e.g., the Jemez and Pajarito faults). Cat Mesa Qbt2 CM1 QR-9-05 186.1 –32.8 4.0 148.7 9/19 35.746 106.679 Temporal and Spatial Relationships Qbt2 CM2 170.9 –50.9 6.7 36.5 13/14 35.801 106.632 We collected data from each of the iso- Qbo CM3 168.5 –52.3 3.4 118.7 16/16 35.808 106.624 chronous cooling units in the Bandelier Tuff, Dome Road including Qbo, the Otowi Member, and Qbt1, Qbt1 DR1 168.4 –29.7 5.0 124.4 7/9 35.712 106.385 Qbt2, Qbt3, and Qbt4 in the Tshirege Mem- Qbt2 DR3 173.8 –33.6 3.6 489.1 4/10 35.712 106.386 ber, to determine whether relative vertical-axis Qbt2 DR4 182.6 –34.4 5.2 137.0 6/10 35.715 106.394 rotations have affected the western margin of Eagle Canyon the Española Basin since ca. 1.2 Ma, as well Qbt2 EC1 173.0 –39.0 8.0 52.0 8/9 35.699 106.372 as over the time interval from ca. 1.6 to 1.2 Ma Qbt2 EC2 170.0 –28.0 5.0 148.0 8/9 35.700 106.372 between major eruptions. By correlating our Qbt2 EC3 174.0 –40.0 9.0 12.0 8/8 35.720 106.370 paleomagnetic sites with measured sections and Qbo EC4 184.0 –46.0 8.0 52.0 8/8 35.703 106.372 Qbo EC5* 192.0 –43.0 18.0 9.0 7/7 35.703 106.371 geochemical data (Tables 1 and 2; Fig. 4), we identifi ed each specifi c Bandelier Tuff cooling Mortandad Canyon unit from which all paleomagnetic data were Qbt3 MC1 181.0 –40.0 7.0 117.0 6/6 35.859 106.259 Qbt2 MC6 190.1 –36.1 3.1 179.8 5/5 35.859 106.259 sampled (with the exception of site AN-1, which may be either Qbt1 or Qbt2). Where possible, Pajarito Mesa we incorporated previously published paleo- Qbt4 PM2 175.2 –29.6 3.2 133.1 16/19 35.862 106.302 magnetic data from Doell et al. (1968) and Qbt4 PM3 185.0 –40.1 3.8 93.1 16/16 35.862 106.302 Qbt4 PM4 180.3 –21.9 2.7 283.7 11/12 35.861 106.301 MacDonald and Palmer (1990) to increase the Qbt4 PM5 177.3 –27.1 2.9 211.4 12/14 35.861 106.301 overall confi dence for each estimate of a spe- Qbt3 PM6 176.1 –52.8 3.5 304.8 6/6 35.861 106.302 cifi c cooling unit mean direction. Peralta Canyon Our paleomagnetic data were obtained from Qbt1 PC1 172.4 –41.6 2.6 520.9 7/7 35.692 106.448 measured sections of multiple cooling units at Cat Mesa, Eagle Canyon, San Juan Mesa, and San Antonio Seven Springs, as well as from individual cool- Qbt1 or AN1 QR-5-08 187.4 –42.7 2.8 171.6 16/16 35.882 106.721 Qbt2† ing units at Los Alamos Canyon, Dome Road, Mortandad Canyon, Pajarito Mesa, Peralta San Juan Mesa Canyon, San Antonio Canyon, Sawyer Mesa, Qbt1 SJ1l 181.0 –34.0 5.0 112.0 7/9 35.712 106.644 upper Sandia Canyon, Vallecitos, and Young Qbt1 SJ1u 180.0 –30.0 7.0 50.0 8/8 35.710 106.640 Qbt1 SJ21* 159.0 –29.0 23.0 6.0 6/6 35.720 106.640 Ranch (Fig. 4). Qbt1 SJ2u QR-33-06 172.0 –38.0 3.0 311.1 7/8 35.720 106.640 First, we consider the four cooling units of QR-34-06 the Tshirege; overall unit mean directions from Qbt2 SJ3l 173.0 –34.0 4.0 153.1 8/8 35.742 106.641 selected localities where we were able to gather Qbt2 SJ3u QR-35-06 175.0 –34.0 3.0 363.0 6/8 35.740 106.640 data in vertical and lateral sections are shown in Sandia Canyon, upper Figure 7. Vertical section data were collected at Qbt2 SC1 174.3 –29.1 5.1 80.1 10/11 35.863 106.312 San Juan Mesa and Sawyer Mesa, and data com- Sawyer Mesa prising a lateral section were collected at Paja- Qbt2 SA1 QR-56-05 175.0 –36.0 2.0 775.0 10/10 34.803 106.312 rito Mesa. For San Juan Mesa (Fig. 7A), three Qbt3 SA2 172.0 –35.0 2.0 460.0 10/10 35.803 106.362 sites (28 samples) were located in Qbt1, yielding Qbt3 SA3 QR-57-05 173.0 –41.0 2.0 890.0 10/10 35.804 106.363 α mean values of D = 177.8°, I = –34.1°, and 95 = Seven Springs 8.7°; two sites were located in Qbt2 (14 samples), Qbo SE1 QR-50-05 178.0 –41.0 2.0 731.0 9/10 35.938 106.705 yielding mean values of D = 174°, I = –34.0°, Qbo SE2 QR-51-05 180.0 –43.0 3.0 294.0 10/10 35.940 106.705 α Qbo SE3 QR-52-05 179.0 –40.0 2.0 430.0 10/10 35.950 106.710 and 95 = 3.6°. For Sawyer Mesa (Fig. 7B), one site was located in Qbt2 (10 samples), and it gave Qbo SE4 QR-53-05 174.0 –44.0 3.0 286.0 10/10 35.950 106.720 a mean of D = 175.0°, I = –36.0°, and α = 2.0°, Qbt2 SE7 QR-55-05 178.0 –33.2 3.4 171.0 31/33 35.959 106.713 95 QR-2-08 and two sites were located in Obt3 (20 samples), Qbt1 SE8 QR-1-08 174.6 –43.4 7.2 88.3 6/6 35.951 106.712 α giving a mean of D = 172.5°, I = –38.0°, and 95 Qbt1 SE10 QR-3-08 180.2 –19.7 2.7 197.2 14/17 35.951 106.712 = 13.2°. We included one Doell et al. (1968) site Vallecitos (D12) in Qbt4 from Sawyer Mesa because it was Qbo VA1 QR-6-08 189.5 –34.9 3.5 36.9 15/18 35.791 106.598 less than 3 km away, and it allowed us to com- pare another unit within the Tshirege. The mean (continued)

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TABLE 2. SITE-MEAN DIRECTIONS OF CHARACTERISTIC REMANENT MAGNETIZATION data show no statistically signifi cant declination (continued ) change across (northwest/southeast) the Jemez α κ Unit Site XRF site Dec Inc 95 N/No Lat Long fault (4.0° ± 2.7°) or across (east/west) the Paja- (°) (°) (°) (°N) (°W) rito fault zone (1.9° ± 1.7°). Similar results were obtained for the Otowi Member from a total of This study 62 samples distributed among 14 sites (Fig. 9). Young’s Ranch The whole member mean yields the follow- α Qbt1 YR1 184.0 –38.0 2.7 183.7 8/8 35.714 106.472 ing orientation: D = 176.7°, I = –45.0°, 95 = Qbt1 YR2 179.0 –36.0 5.2 52.1 9/9 35.726 106.461 3.0°, with no statistically signifi cant declination Qbt1 YR3 185.0 –29.0 4.8 81.8 8/8 35.737 106.389 change across (northwest/southeast) the Jemez fault (−3.9° ± 2.6°) or across (east/west) the Macdonald and Palmer (1990) Pajarito fault (7.9° ± 2.3°). Ancho Canyon We also consider whether any localities Qbt2 MP-16 154.0 –52.0 5.5 64.8 10/10 35.79 106.25 show a systematic shift in declination between Qbt2 MP-17 162.0 –47.0 2.4 283.6 12/12 35.79 106.25 the Otowi and Tshirege Members that could Sandia Canyon, lower indicate a phase of rotation between ca. 1.6 and Qbt2 MP-15 173.0 –37.0 2.2 337.5 12/12 35.85 106.21 1.2 Ma. The overall Otowi and Tshirege Mem- Virgin Mesa ber means differ by ~9° in inclination yet less Qbt3 MP-11 174.0 –35.0 4.0 175.0 7/7 35.78 106.71 than 2° in declination (–1.1° ± 1.6°). Our data Qbt3 MP-12 175.0 –37.0 4.2 185.2 6/7 35.78 106.71 therefore do not show evidence for vertical-axis Qbt3 MP-13 170.0 –39.0 3.3 180.0 10/10 35.78 106.71 rotation between the time of Qbo and Qbt erup- Qbt3 MP-14 169.0 –34.0 3.5 177.8 9/9 35.78 106.71 tions. This is a simplifi ed assessment, however, as the difference in inclination is signifi cant, and Doell et al. (1968) part of it probably refl ects paleosecular varia- Fish Hatchery tion of the fi eld over the ~0.36 m.y. time interval Qbo D-1 178 –45 3.4 282.6 6/6 35.93 106.71 between major eruptive events, as previously Qbo D-2 176 –47 6.0 90.7 6/6 35.93 106.71 noted by Doell et al. (1968). Qbo D-3 171 –45 5.0 156.8 5/6 35.93 106.71 Qbo D-4 175 –45 4.0 204.2 6/6 35.93 106.71 Other Potential Causes of Data Dispersion Holiday Mesa Our data (Figs. 7–9) show dispersion that Qbt3 D-8 175 –34 2.8 625.0 4/6 35.77 106.74 cannot be explained within a tectonic frame- Qbt3 D-9 177 –33 2.9 388.4 6/6 35.77 106.74 work. Other parameters that may contribute to Qbt3 D-10 173 –35 6.2 85.0 6/6 35.77 106.74 data outliers in ash-fl ow tuffs include: variable Qbt3 D-11 169 –27 7.9 52.3 6/6 35.77 106.74 compaction and welding, paleosecular variation San Diego Canyon recorded during extended cooling, vapor-phase Qbo D-5 171 –49 3.6 252.1 6/6 35.84 106.65 alteration or fumarolic activity, and identifi cation Qbo D-6 173 –45 5.6 125.0 5/6 35.84 106.65 and/or sampling errors. Palmer et al. (1996) and SR4 Qbt4u D-7 174 –36 2.6 414.2 7/7 35.83 106.34 McIntosh (1991) reported that the Bishop Tuff and the Mogollon-Datil ignimbrites, respec- Switchbacks tively, show little correlation between welding Qbt4l D-12 173 –38 4.4 144.6 7/7 35.83 106.37 and inclination and that thermoremnant magne- Note: Sites are listed in alphabetical order by sampling location. Dec—declination; Inc—inclination; tism directions are generally laterally and ver- α κ 95—95% confi dence interval; —precision parameter; N—number of samples used to calculate mean; No—number of independently oriented samples collected in fi eld; Lat—latitude of sampling site; Long— tically consistent within individual ignimbrite longitude of sampling site XRF—X-ray fl ourescence. outfl ow sheets, showing no evidence of syncool- *Site mean with α ≥10° and excluded from further study. 95 ing secular variation or subblocking temperature †Site AN1 is excluded from the calculation of Qbt1 and Qbt2 means but is included in the Qbt mean calculation. fl owage. For the Bishop Tuff, the chief source of within-unit variation in site-mean directions is uncertainty in structural corrections, particularly for the Sawyer Mesa Doell et al. (1968) site is: within acceptability criteria as indicated by the in areas of strong tectonic extension. This is also α α D = 173.0°, I = –38.0°, 95 = 4.4° (7 samples). low 95 value (Butler, 1992), the dispersion in the case in northeastern Baja California, where For the lateral section shown in Figure 7C, D = inclination and declination visually represented tilting caused dispersion in inclinations (Lewis α 179.3°, I = –29.7°, and 95 = 9.6°. This is based in Figure 8A is undeniable. Since a temporal and Stock, 1998a, 1998b). In terms of our data, on four site means (61 samples). change spanning the four cooling units of the six sites out of 59 (10%) can be considered outli- Given the consistency of site means among Tshirege is not the source of the dispersion ers; these are MP16, MP17, PM6, PM4, PM5, the Tshirege cooling units (Qbt1–Qbt4) from the (Fig. 7), we next look at the infl uence of faults and SE10 (Table 2; Fig. 4). MP16 and MP17 are vertical and lateral sections described here, we in causing spatial variation in declinations. the only two sites from a previous investigation then considered the Tshirege as a whole mem- Here, we present our Tshirege data in terms (MacDonald and Palmer, 1990) for which we ber (Fig. 8A). In total, 321 samples from 45 sites of locality means with respect to the two major could not assign a member, Otowi or Tshirege. provided a Qbt member mean of D = 175.6°, I tectonic features in the study area: the Jemez Since proper stratigraphic correlation is para- α = –35.7°, 95 = 2.2°. While data quality is well and Pajarito faults (Figs. 4, 8B, and 8C). Our mount when determining a member mean, we

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strike slip (Sanders et al., 2006). Throughout its Cenozoic history, strike-slip displacement A San Juan Mesa: Vertical Qbt1 Dec: 177.8° has been closely focused within the Tijeras fault Inc: –34.1° zone, suggesting strain partitioning in this part of α95: 8.7° N: 3 site means the rift. These interpretations contrast with structural models of the Rio Grande rift in north-cen- Qbt2 tral New Mexico, which suggest that regional- Dec: 174.0° scale (approximately tens of kilometers) block Inc: –34.0° α95: 3.6° rotations accommodated an important compo- N: 2 site means nent of strain (e.g., Brown and Golombek, 1985, 1986; Salyards et al., 1994; Hudson, et al., 2004). For example, the block bounded by the Embudo, S Pajarito, Tijeras- Cañoncito, and Pecos-Picuris Figure 7. Equal-area projections faults is thought to have rotated counterclock- Qbt2 of locality means comprising wise (Brown and Golombek, 1985). We suggest Dec: 175.0° vertical and lateral sections. (A) that well-documented vertical-axis rotations in Sawyer Mesa: Vertical Inc: –36.0° B α95: 2.0° A vertical section from San Juan this region represent localized effects, rather than N: 1 site mean Mesa. Cooling units represented a fundamental component of the regional defor- include Qbt1 and Qbt2. (B) A ver- Qbt3 tical section from Sawyer Mesa. mation (Fig. 10); work by Goteti (2009) supports Dec: 172.5° Cooling units represented include this conclusion. Inc: –38.0° Qbt2, Qbt3, and Qbt4. (C) A lateral α95: 13.2° Ingersoll et al. (1990) noted that the Plio- N: 2 site means section within Qbt4 from Pajarito cene–Quaternary Rio Grande rift is distinctly Mesa. Given the consistency of different from its precursor that evolved in early site means among the Tshirege Qbt4 Miocene time, especially in structural style. In Dec: 173.0° cooling units shown here, we pro- Inc: –38.0° ceed to consider the Tshirege as many regards, this conclusion is robust. How- α 95: 4.4° a whole member. Circles indicate ever, the pattern of faulting and accommoda- N: 1 site mean S our confi dence cone, Dec.—decli- tion of strain in the right-relayed step-over of nation; Inc.—inclination. the rift has been more or less invariant since the onset of rifting. In the Rio Grande rift, where Late Cretaceous to early Tertiary Laramide style C Pajarito Mesa: Lateral structures, and perhaps much earlier phases of Qbt4 crustal shortening of late Paleozoic and even Dec: 179.3° Proterozoic age, conditioned everything that Inc: –29.7° α95: 9.6° followed (e.g., Kelley et al., 1992; Bauer and N: 4 site means Ralser, 1995; Pazzaglia and Kelley, 1998; Ingersoll, 2001; Magnani et al., 2004; Cather et al., 2006; Sanders et al., 2006; Koning et al., 2007), idealized relay ramps do not occur at the regional scale, although they are prevalent locally (e.g., Kelley, 1979; Goteti, 2009; Lewis S et al., 2009). Instead, transfer zones (e.g., Mor- ley et al., 1990) between half grabens are zones of distributed faulting dominated by north- and were forced to remove these two data points from DISCUSSION: TECTONIC IMPLICATIONS northeast-striking faults inherited from previous our analysis. Whereas PM6 has a high inclina- deformational events. Subvertical faults such as tion, PM4 and PM5 have low inclinations; none The data presented here from isochronous the Tijeras fault, and probably the Jemez and of these sites is any more or less welded than the cooling units in the Quaternary Bandelier Tuff Embudo faults, strongly infl uenced the orien- other sites at the Pajarito Mesa locality. How- reveal no temporal or spatial variations in dec- tations and locations of transfer zones during ever, the presence of fumaroles (Gardner et al., lination. We conclude that, at the scale of this extension (cf. Ingersoll et al., 1990). 2008) suggests that protracted cooling, resulting study, no block rotations about a vertical axis In the northern part of the Rio Grande in paleosecular variation, could account for devi- occurred near the major faults (Jemez and Paja- rift, three half-graben basins (Albuquerque, ations in inclination from other sites. Given that rito) that transect the area (Fig. 10). Similarly, Española, and San Luis) developed begin- PM6 was a block sample, we removed this data data from the Oligocene Espinaso Formation ning in the middle Miocene (Ingersoll et al., point in the event of a sampling error. Finally, show no signifi cant vertical-axis rotation and 1990; House et al., 2003). Exposures of early SE10 had a low inclination, which we are unable indicate that footwall rocks of the La Bajada rift-fi ll deposits (Santa Fe Group, including to explain, as none of the factors associated with fault did not rotate throughout the entire period basalts dated between 16.5 and 13.0 Ma; e.g., data dispersions is relevant for this site; as such, of rifting (Harlan and Geissman, 2009). This is Hulen et al., 1991; Gardner and Goff, 1984) we kept data from SE10 in our analysis. In the especially notable given the close proximity of form a northeast-trending belt that marks the end, of the six sites deemed outliers, we removed the La Bajada fault to the Tijeras fault, a major local boundary of the rift during the Miocene. three and kept three. northeast-striking fault with a long history of In addition, basalts were erupted along the

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All Qbt

Dec: 175.6° A Inc: –35.7° α95: 2.2° N: 45 site means

North of the East of the Jemez fault S Pajarito fault

Dec: 179.9° Dec: 177.4° Inc: –34.8° Inc: –34.4° α95: 13.3° α95: 3.3° N: 4 site means N: 17 site means BC South of the West of the Jemez fault Pajarito fault

Dec: 175.9° Dec: 175.5° Inc: –34.5° Inc: –34.7° α95: 1.8° α95: 2.3° N: 38 site means N: 25 site means

S S Figure 8. Equal-area projections of locality mean directions in the Tshirege Member. (A) All localities. (B) Localities north and south of the Jemez fault. (C) Localities east and west of the Pajarito fault. See Figure 4 for locations, Dec.—declination; Inc.—inclination.

All Qbo A Dec: 176.7° Inc: –45.0° α95: 3.0° N: 14 site means

North of the East of the Jemez fault S Pajarito fault

Dec: 175.6° Dec: 184.0° Inc: –44.4° Inc: –46.0° α95: 2.1° α95: 8.0° N: 10 site means N: 1 site mean BC

South of the West of the Jemez fault Pajarito fault

Dec: 179.5° Dec: 176.1° Inc: –46.4° Inc: –44.9° α95: 12.0° α95: 3.1° N: 4 site means N: 13 site means

S S Figure 9. Equal-area projections of locality mean directions in the Otowi Member. (A) All localities. (B) Localities north and south of the Jemez fault. (C) Localities east and west of the Pajarito fault. See Figure 4 for locations, Dec.—declination; Inc.—inclination.

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Group rocks in and near the Valles caldera (e.g., 106°W 105.5°W Hulen et al., 1991). Variation through time in fault rakes presented by Minor et al. (2006) may reﬂ ect a changing balance of strain partitioning between normal and strike-slip faults. Clearly, more work is needed to understand strain partitioning and the role of fault ancestry in this CF complex step-over. EF The step-over we discuss in this paper is a fundamental element of the Rio Grande rift and an area of exceptional structural complexity, dominated by wrench tectonics. The location of d ancestral faults determined the shape of the rift at this latitude and the boundaries of individual

36°N c P-PF basins. We propose the name “Jemez-Embudo No observed Española e accommodation zone” (JEAZ) for this compos- rotations ite of structural and volcanic features in recogni- PF tion of its regional importance in the evolution NC of the Rio Grande rift. JF On a regional scale, a further implication of b the paleomagnetic results we report in this study Santa Fe concerns interpretations involving large magni- a tudes of dextral or sinistral shear between the Colorado Plateau and Rio Grande rift. Some authors have argued for a time period during

CCF which signiﬁ cant sinistral offset characterized LBF rift-age structures in northern New Mexico 35.5°N SFF N (e.g., Kelley, 1982; Chapin and Cather, 1994), whereas others have argued that dextral shear, 0 10 20 30 related to plate-boundary deformation, was the dominant shear-sense component during rift for- SH kilometers mation (Wawrzyniec et al., 2002). In addition to our results, paleomagnetic data from Oligocene tuffs that crop out across the Colorado Plateau– Timing of fault activity Rio Grande rift margin in southern Colorado T-CF >70-40 Ma Initial show no vertical-axis rotations since the time the tuffs were deposited (Mason, 2011). Thus, SF motion ~26 -10 Ma any dextral or sinistral shear within this region may have been accommodated by oblique slip Albuquerque ~10 - 0 Ma

35°N Reactivation on faults without vertical-axis block rotations and should be factored into any models of the Figure 10. Evolution of faulting in the Española Basin region. White dots indicate sites from Harlan evolution of the Rio Grande rift. and Geissman (2009) showing little to no rotations. White dots with letters indicate sites from Finally, to contribute to the understanding other studies in which rotations were documented: (a) Hudson et al. (2004); (b) Salyards et al. of large-scale rift systems globally, we note that (1994); (c) Brown and Golombek (1986); (d) Brown and Golombek (1986); and (e) Salyards et al. in contrast to our study, vertical-axis rotations (1994). Yellow shading indicates regions where locally generated rotations can be predicted by modeling results from Goteti (2009). CCF—Cañada de Cochiti, CF—Cañones, EF—Embudo, JF— have been documented in the northern Gulf Jemez, LBF—La Bajada, NC—Nacimiento, PF—Pajarito, P-PF—Picuris-Pecos, SFF—San Felipe, SH— extensional province of Baja California, Mexico Sand Hill, SF—Sandia, and T-CF—Tijeras-Cañoncito fault systems. (Lewis and Stock, 1998b). Distributed deformation appears to be characteristic of that region and perhaps more widespread than previously northeast-striking faults that were active during placement along the northeast-striking Embudo considered (Seiler et al., 2010). The pronounced that period (Gardner and Goff, 1984). Between fault changed from dominantly dip slip to dom- difference in deformation style between the two 10 and ca. 7 Ma, the rift boundary evolved as inantly strike slip (Aldrich and Dethier, 1990). regions may ultimately be diagnostic of fault a swath of north-striking normal fault zones Our paleomagnetic results strongly suggest strength. For example, whereas the relative isot- (Gardner and Goff, 1984; Baldridge et al., that all these events occurred during one prin- ropy of the Cretaceous batholithic rocks in north- 1994) (Fig. 2). By ca. 4 Ma, the primary locus cipal phase of transtensional shear deformation ern Baja may have favored distributed deforma- of extension in the Española Basin had shifted inﬂ uenced by ancestral faults. Ancient faults tion, the crustal anisotropy in north-central New eastward to the Pajarito fault system and associ- that strike SW-NE may have facilitated early Mexico favored concentrated deformation along ated structures, directly linking the Albuquer- development of pull-apart basins of the same preexisting faults. The Rio Grande rift, then, may que and San Luis Basins. During this time, dis- trend, accounting for thick sections of Santa Fe be analogous to the San Andreas and Altyn Tagh

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fault systems, where weak faults focus strain Stephen Self and two anonymous reviewers central New Mexico: Implications for Mesoproterozoic to late Cenozoic tectonism: Geosphere, v. 2, p. 299– (Zoback, 2000; Dupont-Nivet et al., 2004). greatly improved this manuscript. We appreciate 323, doi:10.1130/GES00045.1. fi eld assistance from: Ruth Soto, Rajesh Goteti, Chapin, C.E., and Cather, S.M., 1994, Tectonic setting of the CONCLUSIONS Troy Held, Scott Broome, and the 2008 axial basins of the northern and central Rio Grande rift, in Keller, G.R., and Cather, S.M., eds., Basins of the Rio Earthwatch Institute Student Challenge Award Grande Rift—Structure, Stratigraphy, and Tectonic Set- In this contribution, we concentrate on the Program participants. We are grateful for ting: Geological Society of America Special Paper 291, rotational component of the deformation fi eld funding from the Institute for Geophysics and p. 5–25. 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We sug- p. 790–802, doi:10.1029/JB090iB01p00790. Gardner, J.N., Lavine, A., WoldeGabriel, G., Krier, D.J., gest that modest-magnitude counterclockwise Brown, L.L., and Golombek, M.P., 1986, Block rotations in Vaniman, D., and McCalpin, J., 1998, Evolution of the the Rio Grande rift, New Mexico: Tectonics, v. 5, no. 3, western boundary of the Rio Grande rift, northern vertical-axis rotations since middle Miocene p. 423–438, doi:10.1029/TC005i003p00423. New Mexico: Eos (Transactions, American Geophysi- time, as documented in some previous studies, Broxton, D.E., and Reneau, S.L., 1995, Stratigraphic Nomen- cal Union), v. 79, no. 45, p. F614. represent highly localized phenomena. The pat- clature of the Bandelier Tuff for the Environmental Res- Gardner, J.N., Lavine, A., WoldeGabriel, G., Krier, D., Vani- toration Project at Los Alamos National Laboratory: man, D., Caporuscio, F., Lewis, C.J., Reneau, P., Kluk, E., tern of faulting and accommodation of strain in Los Alamos National Laboratory Report LA-13010-MS, and Snow, M.J., 1999, Structural Geology of the North- the right-relayed step-over of the rift has been 21 p. western Portion of Los Alamos National Laboratory, Broxton, D.E., Ryti, R.T., Carlos, D., Warren, R.G., Kluk, E., Rio Grande Rift, New Mexico: Implications for Seismic more or less invariant since the onset of rifting. and Chipera, S., 1996, Natural Background Geochemis- Surface Rupture Potential from TA-3 to TA-55: Los Ala- The difference between areas of modest crustal try of the Bandelier Tuff at MDA P, Los Alamos National mos National Laboratory Report LA-13589-MS, 112 p. extension dominated by distributed deformation Laboratory: Los Alamos National Laboratory Report Gardner, J.N., Reneau, S.L., Lewis, C.J., Lavine, A., Krier, D., LA-UR-96-1151, 42 p. 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