- UW Department of Geology and Geophysics
Recent detailed work in key regions along two north–south transects in northern New Mexico highlights continued controversy about Proterozoic tectonic evolution. Ductile deformation features (folds, ductile thrusts, and associated foliations and lineations) are grouped into three deformation generations. D1 includes crytic bedding-parallel foliation and fold nappes. D2 involves north-verging, km-scale inclined folds, the main shortening foliation, and D2, structures that further attenuate or reactivate F2 folds. D3 involves east–west-trending open folds and domes and associated crenulation cleavage. Although others can dominate locally, S2 is the dominant regional foliation that could possibly be imaged seismically. Map relationships around ca. 1.65- and ca. 1.42-Ga plutons and porphyroblast-matrix studies of dated minerals show that D3 occurred at ca. 1.42. The age of D2 is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic studies also indicate multiple metamorphic events, M1–M3, that may relate to the deformational events. New geochronology indicates that most metamorphic minerals grew (or were reset) at ca. 1.47–1.35 Ga. U-Pb dates on metamorphic zircon, monazite, titanite, staurolite, garnet, and tourmaline suggest regional metamorphism to 550–700° C at 1.47–1.42 Ga. Metamorphic aureoles are present around plutons, but the highest grades of metamorphism are in areas with no exposed 1.42-Ga plutons. Metamorphism is interpreted to record a regional mantle-driven thermal event, the latter parts of which correspond to a time of pluton emplacement. 40Ar/39Ar dates record post–1.42-Ga cooling: the highest grade rocks yield the youngest cooling ages, indicating slow cooling and gradual unroofing of the 1.42-Ga thermal profile following 1.42-Ga metamorphism. Our preferred model is that macroscopic geometries (D1–D2) were established by 1.65 Ga, and that regional amphibolite-grade metamorphism and associated D3 deformation at 1.47–1.42 Ga produced localized high-strain domains and fabric reactivation at exposed levels. At deeper levels, structures and assemblages may increasingly record 1.42-Ga reactivation.
Seismic images, even detailed reflection lines such as those of the Rocky Mountain Continental Dynamics (CD) experiment (Karlstrom, this issue), are a snapshot of the coarse-scale anisotropy structure of today's crust. The most useful interpretations depend on linking observed reflectors to mapped surface structures. Traditional surveys preferentially image shallow-dipping reflectors, so it is important in designing the experiment, and in correlating imaged structures to surface geology, to understand the potential importance of shallowly dipping versus steeply dipping domains of foliation and rock contacts. Even more importantly, structures of widely different age and kinematic significance are superimposed, and form “double exposures,” on the snapshot images. These can only be resolved through correlation with surface structures of known age.
Exposed Proterozoic rocks throughout the southwestern U.S. were at depths corresponding to the middle crust (10–20 km) at the time of assembly of the continental lithosphere (Williams and Karlstrom, 1996). Presently exposed strucures and boundaries are the product of a complex sequence of middle-crustal ductile deformation and metamorphic events. A major goal of our recent field and laboratory research has been to propose models for the geometry and evolution of exposed (paleomiddle crustal) boundaries that can be tested and evaluated against new seismic images of today's crust. The combined data set (surficial geologic data and crustal images) should shed light on the geometry of suture zones in the ductile middle and lower crust, and possibly on the role that deep crustal boundaries have played in controlling younger tectonic features such as magmatic belts and faults.
This paper presents a summary of recent studies along two cross sections in northern New Mexico (Fig. 1). The studies combine regional mapping, microstructural analysis, metamorphic P–T studies, and U-Pb and 40Ar/39Ar chronologic studies. Our goal has been to construct detailed P–T–t–D loops that characterize Proterozoic tectonic evolution of different crustal blocks or domains (Williams and Karlstrom, 1996; Karlstrom and Williams, 1998). Peak metamorphic grades, structural geomeries, and local microstructural relationships vary from study area to study area, so interpretations about the style and timing of tectonism have also varied widely. Details of the individual studies are reported elsewhere (Read et al., this issue; Pedrick et al., 1998; Wingsted, 1997; Bishop, 1997; Lombardi, 1997). This paper attempts to summarize the available data and highlight ongoing problems in order to converge on a more robust model for the Proterozoic structures that will be imaged seismically.
Proterozoic rocks of northern New Mexico are part of a 1000-km-wide orogenic belt along which juvenile crust was accreted to the margin of Laurentia between 1.8 Ga and 1.6 Ga (Hoffman, 1998; Bowring and Karlstrom, 1990). Karlstrom and Bowring (1991) suggested that a diverse suite of island arcs, perhaps similar to those of modern-day Indonesia, was assembled, accreted, and stabilized as lithosphere. The belt has been subdivided into three provinces: Mojave, Yavapai, and Mazatzal (see Karlstrom and Humphries, this issue; Karlstrom and Bowring, 1988). The Mojave province contains isotopic evidence for earlier crustal components; the Yavapai province is dominated by 1.75–1.71 Ga juvenile crustal materials; the Mazatzal province is characterized by metamorphosed 1.7-Ga sedimentary and volcanic rocks. Deformation and metamorphism took place between 1.72 and 1.65 Ga in what may have been a progressive continental assembly event. Province boundaries were defined in central arizona, but Proterozoic rocks of northern New Mexico probably occur in the transitional zone between the Yavapai and Mazatzal provinces.
Proterozoic rocks in northern New Mexico have been divided into three main lithotectonic sequences (Bauer and Williams, 1989). The oldest group (ca. 1.76–1.72 Ga) consists of metamorphosed mafic volcanic rocks with a diverse suite of metaplutonic rocks. These include the ca. 1.755-Ga Moppin complex (Tusas Mountains), the ca. 1.765-Ga Gold Hill complex (Taos Range), and the 1.720-Ga Pecos complex of the southern Sangre de Cristo Mountains (Fig. 1; Bauer and Williams, 1989). Metasedimentary and felsic metavolcanic rocks of the Vadito Group (1.72–1.69 Ga) make up the next sequence. These overlie the mafic sequences and locally contain strongly to moderately deformed granitoids ranging in age from 1.7 to 1.65 Ga. The youngest sequence, the Hondo Group (1.70–1.69 Ga), consists of quartzites and pelitic schists that conformably overly the Vadito Group. Mildly deformed 1.5–1.4-Ga granitoids, parts of the “anorogenic granite suite” of Anderson (1989), are exposed in several ranges. Pegmatites in this age range are also widely exposed.
Except for the ca. 1.4-Ga granitoids, all of the Proterozoic rocks have experienced multiple deformation events; three or four deformation phases are interpreted in most areas. The style and intensity of deformation fabrics vary across the region, and one major problem for building a regional tectonic model concerns correlation of the fabrics, fold generations, and deformational events from study area to study area. The northern New Mexico Proterozoic is well known for its widespread amphibolitefacies (Al-silicate triple point) metamorphism (Grambling, 1986; Grambling et al., 1989). However, in detail, most regions preserve evidence for more than one metamorphic pulse or loop (Williams and Karlstrom, 1996).
The most critical and controversial aspect of the tectonic history of northern New Mexico concerns the age of tectonism. Most studies in the last ten years have concluded that the regional deformation and metamorphism occurred primarily during the 1.65-Ga Mazatzal orogeny (Williams, 1990; Bauer and Williams, 1994). Locally, the oldest units are believed to have experienced 1.7-Ga, Yavapai-age deformation (Robertson et al., 1993). More recent data underscore the intensity of 1.4-Ga metamorphism and deformation (Bauer et al., 1993; Bishop, 1997; Wingsted, 1997), to the point that some workers have suggested that the majority of exposed structures and assemblages may reflect 1.4-Ga tectonism (Grambling and Dallmeyer, 1993). Understanding the nature and relative intensity of the 1.65 versus 1.4 Ga event in northern New Mexico remains a major uncertainty in models of the evolution of the lithosphere in the region.
TWO PROTEROZOIC TRANSECTS IN NORTHERN NEW MEXICO
This paper includes discussions of two transects of Proterozoic rocks in northern New Mexico (Fig. 1). The western transect includes the Tusas and Picuris Mountains. These uplifts are on the flanks of the Rio Grande rift and were likely to have been nearly adjacent before Laramide–Tertiary faulting (Karlstrom and Daniel, 1993). Uplifts on the eastern transect include the Taos, Cimarron, and Rincon ranges of the Sangre de Cristo Mountains. They define the eastern front of the Rocky Mountains in nothern New Mexico.
For this paper, deformations are termed D1, D2, D3, with associated fold and foliation generations denoted F1, F2, F3 and S1, S2, S3 respectively. These are relative chronologies that are developed in local areas based on overprinting relationships. However, we also propose a regional “correlation” of structural generations. Such “correlations” are inherently suspect because of diachronous development of structures and heterogeneous deformation that allow fabric generations to vary in intensity, style, and orientation from place to place. In several areas, a relatively local fold or foliation generation is given a special denotation (i.e., D2,) to fit into its place in the local chronology while maintaining the regional correlation of the later structures. Metamorphic events, M1, M2, M3, are labeled according to the associated deformation event (M2 synchronous with D2, etc.). This system is not meant to suggest that the various labeled stages were necessarily distinct metamorphic events, but instead, to show which minerals grew (or were recrystallized) during a particular fabric-forming deformation. Our goal is to establish the progressive evolution of metamorphic conditions with deformation (i.e., the P–T–t–D path). Although timing of metamorphism relative to deformation cannot always be determined in the field, sufficient microstructural data are available from the study areas described here to make local correlations and to propose regional ones.
Proterozoic geology along the Tusas–Picuris transect (Fig. 1) has been summarized and locally remapped by Williams (1991) and Bauer (1993). Based on these investigations, three detailed M.S. studies, several field-camp mapping projects (K. E. Karlstrom, unpublished data), and U-Pb dating (A. Lanzirotti, unpublished data) have been carried out in three key areas to better characterize the deformational and metamorphic history (specifically the P–T–t–D path). Each of these areas exposes the same basic lithologic package, the 1.7-Ga Vadito Group, but the style and geometry of structures and the character of metamorphism differ dramatically among the three. These differences reveal important clues to the regional character of 1.65- and 1.4-Ga tectonism in northern New Mexico.
La Madera/Las Tablas Area, Tusas Mountains
Proterozoic rocks exposed in the La Madera/Las Tablas area preserve a superb record of interfering folds and fabrics at only moderate (upper greenschist to lower amphibolite) metamorphic grade (Fig. 2; Williams, 1991; Lombardi, 1997). Three generations of folds have been recognized (F2, F2, F3), and evidence for an earlier generation is locally present in the form of local bedding-parallel foliation (S1) and stratigraphic discontinuities (thrusts?). F2 folds are the dominant map-scale structures, with amplitudes on the order of hundreds of meters to kilometers. They are tight to isoclinal, reclined folds with a northwest-striking, southwestdipping axial plane cleavage (S2). F2, folds are similar in orientation and style to F2, folds, but they are smaller, ranging from centimeters to tens of meters in amplitude. Their axial planes cut S2 at angles on the order of 5–20°. D3 structures include weakly developed upright, open folds and an eaststriking, subvertical crenulation cleavage (S3).
Metamorphic assemblages document several metamorphic field gradients. The lowest temperature assemblages come from the central area, near Las Tablas, and contain garnet, staurolite, chlorite, muscovite, biotite, and kyanite. Peak conditions in this area were on the order of 450–500° C, 4 kbar (Lombardi, 1997). Rocks to the north and south contain garnet, staurolite, biotite, and both kyanite and sillimanite, indicating slightly hotter peak conditions (perhaps 500–550° C). Except for kyanite and some garnet cores, the metamorphic minerals are texturally late. They clearly overgrow the S2, crenulation cleavage. S3 is included in, but also wraps around, garnet and staurolite porphyroblasts, suggesting that these minerals were synchronous with S3. The peak metamorphism is thus termed M3. In quartzites, mats of lineated kyanite crystals are folded by F2 folds, suggesting that kyanite was present during the formation of these folds and perhaps earlier. M1 assemblages are probably restricted to muscovite, chlorite, plagioclase, and possibly kyanite.
Coarse pegmatites of the Petaca district uniformly cut F2 and F2, folds. The local intensification of S3 against some pegmatites suggests that they are roughly synchronous with F3 folding. Based on correlations with dated pegmatites to the south (Bishop, 1997), the pegmatites of the La Madera area and the associated D3 deformation and M3 metamorphism are interpreted to be ca. 1.4 Ga in age.
Cerro Colorado Area, Southern Tusas Mountains
Vadito Group rocks of the Cerro Colorado area, unlike those of the La Madera/Las Tablas area, are dominated by a strong composite foliation and by higher grade metamorphic assemblages with complex porphyroblast-matrix relationships (Williams, 1991; 1994; Bishop, 1997). The strong matrix foliation (S2) probably includes elements of S1, S2, and S2, from the La Madera area to the north. Crenulation cleavages in low-strain domains and porphyroblast inclusion geometries all document the existence of one or more earlier foliations (Bishop, 1997; Williams, 1991). S2 is characterized everywhere by a strong north-trending lineation (L2) that is interpreted to be a composite of D2 mineral lineations and older intersection lineations. The S2 foliation and L2 lineation were subsequently folded by a single broad upright antiformal fold (Fig. 3) with a weak and heterogenously developed axial planar cleavage (S3). Numerous pegmatites are oriented roughly parallel to the S3 foliation and show evidence of S3 intensification at their margins. U-Pb dates of ca. 1.42–1.41 Ga have been obtained for several of these pegmatites (A. Lanzirotti, unpublished data).
Porphyroblast timing relations (Fig. 3) show no evidence for porphyroblast growth earlier than the D2 event (i.e., no M1 porphyroblasts have been recognized). Garnet, Plagioclase, kyanite, and possibly staurolite porphyroblasts show evidence for growth during D2, suggesting that M2 metamorphism was in the upper greenschist or lower amphibolite facies. The peak of metamorphism occurred during D3. The presence of andalusite after kyanite, folded S2 inclusion trails in unstrained staurolite and cordierite, late fibrolite after andalusite and kyanite, and fibrolite aligned with S3, all allude to amphibolite-facies metamorphism (M3) during the final folding event. M3 metamorphism also involved the growth of inclusionfree rims on granet and plagioclase. The observed mineral parageneses indicate two small, looping P–T–t–D paths (see Williams and Karlstrom, 1996), with peak conditions on the order of 650° C and 4 kbar (Bishop, 1997).
Monazite inclusions extracted from the late M3 staurolite give concordant U-Pb ages of 1.47 Ga. U-Pb ages on titanite and xenotime from pegmatites indicate that these crystallized and cooled approximately 1.42–1.41 Ga. These data constrain M3 metamorphism and D3 deformation to between 1.47 and 1.41 Ga. There are no solid constraints on the age of the D2 and M2. They may represent an early stage in the ca. 1.4-Ga event or they may reflect the 1.65-Ga Mazatal orogeny (Bishop, 1997).
Southern Picuris Mountains
The Vadito Group rocks of the southern Picuris Range are dominated by a south-dipping, northeast-striking foliation, here correlated with S3 in the Tusas Mountains. Porphyroblasts and local low-strain domains contain evidence for an older, northwest-striking foliation (S2), that may cut an even older foliation (S1). Metamorphic assemblages include cordierite, andalusite, garnet, staurolite, biotite, and plagioclase (sillimanite is present in the rocks adjacent to the 1.436-Ga Peñasco pluton). The porphyroblasts can be divided into four groups (Fig. 4). Group 1 porphyroblasts (Mn-rich Grt, Pl) contain very fine-grained, northwest-striking, straight inclusion trails that are not continuous (i.e., through the porphyroblast rims) with the matrix foliation. Group 2 porphyroblasts (And, Sta, Grt) contain straight to sigmoidal inclusion trails that curve within the crystals into parallelism with the matrix foliation. Group 3 porphyroblasts (Crd, Bt) contain an internal fabric that is very similar in texture and orientation to that in the matrix; however, S3 also slightly wraps around the porphyroblasts. Group 4 porphyroblasts (And, Sta, Grt) contain S3 inclusion trails and do not show any evidence of deformation subsequent to their growth. All four assemblages are interpreted to have formed during a single low-P, high-T heating event (M3) that corresponds to the D3 transposition of a northweststriking S2 foliation into the dominant east–northeast-striking S3 foliation (Fig. 4; Wingsted, 1997).
Several lines of evidence indicate that the D3 deformation and M3 metamorphism occurred at ca. 1.5–1.4 Ga. Older plutons (1.7 to 1.6 Ga) are strongly deformed and show no metamorphic aureoles. The 1.436-Ga Peñasco quartz monzite (A. Lanzirotti, unpublished data) has a magmatic foliation that is parallel to a weak solid-state foliation which, in turn, is parallel to the regional matrix S3. The nature and parallelism of these foliations is interpreted to reflect pluton emplacement during D3 deformation (Bauer, 1993; Wingsted, 1997). Furthermore, monazite grains extracted from Group 2, 3, and 4 porphyroblasts yielded U-Pb ages of 1.45 Ga. Unless this monazite could have grown or been reset inside the porphyroblasts, these dates indicate that Group 2–4 porphyroblast growth all took place in one metamorphic event at ca. 1.45 Ga. U-Pb ages from staurolite and garnet in the northern Picuris Mountains also indicate metamorphism at 1.45 Ga (Lanzirotti and Hansen, 1997). It is important to note that the 1.45-Ga metamorphic porphyroblasts are 10–30 m.y. older than the Peñasco pluton, and they are not spatially associated in the field. Other 1.5–1.4-Ga plutons may be present at depth or, more likely, the 1.4-Ga heating event was more long-lived and widespread than the plutons.
All three study areas can be characterized in terms of three major fabric-forming phases of deformation: an early bedding-parallel foliation, a second, well-developed foliation that is axial planar to map-scale folds, and a third upright, east-striking foliation of variable intensity. In all three areas, the third event has been interpreted (and constrained to be) ca. 1.45 Ga. The La Madera/Las Tablas area has an extra fabric (S2), but because of similarities in style and orientation, it is linked with S2. If the correlations are valid, then the intensity of D3-related fabrics (and probably of the D3 event itself) is extremely heterogeneous on a regional scale. In the La Madera/Las Tablas area, it is characterized by a weak and locally developed crenulation cleavage. In the Cerro Colorado area, it is a moderately well-developed spaced cleavage. In the southern Picuris Mountains, it is the dominant cleavage in the field, representing an almost complete transposition of earlier fabrics.
The S2 foliation ranges from a strong crenulation cleavage to a penetrative schistosity in all three areas. However, in the La Madera/Las Tablas and Picuris areas it was northwest-striking and moderately to steeply southwest-dipping; in the Cerro Colorado area, it may have been nearly horizontal before F3 folding. This variation probably represents D2 strain heterogeneity rather than modification during later events.
All three areas show evidence for multiple pulses or stages of porphyroblast growth. The first (M2) was synchronous with D2 and involved wide-spread growth of garnet and kyanite, possibly with local staurolite. The second (M3) was synchronous with D3 and involved low-P high-T assemblages with andalusite, sillimanite, garnet, staurolite, and cordierite. The Al-silicate triple point assemblages characteristic of northern New Mexico probably developed during this stage, although some kyanite may be a relict phase of the earlier metamorphism. All attempts to directly constrain the age of M3 metamorphism consistently yield an age of ca. 1.45 Ga. Evidence includes U-Pb ages of monazite inclusions (Wingsted, 1997; Bishop, 1997), U-Pb ages of staurolite, tourmaline, rutile, and other metamorphic phases (Bishop, 1997), and 40Ar/39Ar ages of hornblende and mica (Karlstrom et al., 1997). No direct constraints exist on the older D2–M2 phase of tectonism, but S2 is present in 1.65-Ga plutons and is cross cut by 1.42-Ga pegmatites. It is important to note that the 1.45 Ga age of the D3–M3 event is significantly older than the age of the one exposed granitoid, the 1.436-Ga Peñasco pluton in the south Picuris area.
The Taos, Cimarron, and Rincon ranges (Fig. 1) host the highest grade Proterozoic rocks in New Mexico (650–700° C), and show pronounced metamorphic field gradients. Following the discovery of high-grade rocks containing sillimanite, K-feldspar, and hercynite, Grambling proposed a model for an extensional shear zone that juxtaposes medium-grade (4 kbar) upper plate rocks above high-grade (6–8 kbar), lower-plate rocks (Grambling et al., 1989; Grambling, 1990; Grambling and Dallmeyer, 1993). Recent work by Pedrick et al. (1998) and Read et al. (this issue) has not supported the extensional shear zone model and has reinterpreted the apparent field gradients as a complex mixture of pluton-related thermal gradients and disequilibrium assemblages that record a polyphase 1.7-Ga to 1.4-Ga metamorphic history.
The Taos Range can be subdivided into two blocks. The southern Taos Range was a lower-temperature block (perhaps at a slightly shallower depth) containing volcanogenic rocks of pre-1.7-Ga, Yavapai province arc basement. The northern Taos Range was a higher-temperture domain of quartz-rich supracrustal rocks interleaved with 1.65-Ga gneissic granitoids. Tectonic slices of quartzite and pelite (likely of the Hondo Group) are present in both northern and southern blocks and can be used to compare metamorphic grade. Pedrick et al. (1998) concluded that, while metamorphic field gradients are present, both the northern and southern Taos Range blocks record a similar pressure history (6 kbar decompressing to 4 kbar) and a similar range of temperatures (550–650° C). They found no evidence for an extensional shear zone that juxtaposes rocks with different P–T histories.
Structures in the Taos Range involve multiple fold generations. In the northern Taos Range, supracrustal rocks form thrust slices that are sandwiched between 1.68–1.64 Ga gneissic granitoids (Fig. 5). The main foliation (S2) is axial planar to F2 intrafolial, isoclinal microscopic and mesoscopic folds of an earlier S1 layering defined by kyanite. S2 dips moderately to the west, L2 lineations plunge northwest, and shear sense in S2 is top-to-the-east (thrust-sense). S2 is folded into open folds with a generally weak crenulation cleavage that creates broad north–south- trending F3 antiforms and synforms (Fig. 5).
Porphyroblast textures and quantitative thermobarometry suggest that a polyphase metamorphic history was associated with the polyphase deformational history. Kyanite is the earliest mineral (M1); peak M2 minerals include sillimanite and K-feldspar; M3 minerals include Mn-andalusite, chloritoid, and sillimanite. The Al-silicate sequence (Ky to Sil to And), combined with P–T work on cores (M2) and rims (M3) of garnet, indicate a clockwise looping P–T path where rocks were decompressed from 700° C, 6 kbar to 550° C, 4 kbar.
Timing of deformation is constrained by the plutonic rocks. All granitoid plutons (1.68–1.64 Ga) are strongly deformed and have S2 gneissic fabrics. Cross-cutting pegmatite dikes yield U-Pb zircon and monazite ages of 1.42 Ga. Thus, the major fabric-forming event took place between 1.64 and 1.42 Ga. The pegmatites occur as weakly folded dikes and as melt pods that accumulated during top-to-the-east shearing. Thus, the 1.42-Ga tectonism involved partial melting, reactivation of older S2 fabrics, and east-directed thrusting. No 1.65-Ga metamorphic ages have been obtained, but the highest-temperature rocks are confined to quartzite and pelitic schist bounded above and below by 1.643- and 1.678-Ga orthogneisses, respectively. This sandwich geometry suggests that heat from the plutons may have elevated local temperatures and created the observed thermal gradients. However, metamorphic minerals record ca. 1.4 Ga ages; zircon (U-Pb) in amphibolite is 1.42; titanite (U-Pb) is 1.4–1.38; hornblende (40Ar/39Ar) is 1.4–1.3. These dates indicate that rocks reached temperatures in excess of 600° C at 1.4 Ga. The 1.4-Ga minerals are aligned in S2, suggesting movement on S2 at 1.42 Ga.
The Cimarron Mountains also show large thermal gradients. Greenschist-grade rocks east of the steeply dipping Fowler Pass fault are juxtaposed across the fault with high-grade quartzites and granites that are similar to those in the northern Taos Range. The low-T block preserves a slightly disturbed 1.68-Ga hornblende age (see Karlstrom et al., 1997), indicating that temperatures were below 500° C since 1.68 Ga. In contrast, 40Ar/39Ar mineral ages suggest that the western Cimarron Mountains were hotter than 600° C at ca. 1.4 Ga, like the northern Taos Range. Thus, exposed rocks record thermal gradients of greater than 100 °C associated with 1.4-Ga metamorphism, and differential exhumation of blocks at some time after 1.4 Ga.
Deformational features in the high-grade areas of the Cimarron Mountains are similar to those in the northern Taos range. Quartzites contain inverted cross-bedding and a bedding-parallel foliation S1 that is parallel to the generally subhorizontal contact with the underlying granite (Fig. 5). The contact is a several hundred-meter-wide zone of transition grading from quartzite to quartz-rich gneiss to granitic gneiss. Pods of quartzite within gneiss are interpreted to be xenoliths in granite, indicating the intrusive nature of the contact (Pedrick et al., 1998). This contact was previously interpreted to be an extensional shear zone (Grambling and Dallmeyer, 1993). Both quartzite and granite contain S1, but in most areas the dominant fabric is an S2 foliation that is axial planar to northwest-trending F2 folds. S2 may be broadly warped by the Fowler Pass shear zone. Quartzite mylonites show multiple movements along S2. Top-to-the-east (normal-sense) shearing took place at amphibolite grade, as documented by oxide fish. Later, top-to-the-west (thrust-sense) shearing at greenschist grade is documented by lower-temperature quartz microstructures.
Metamorphism in felsic gneisses was near granulite-grade, as shown by hercynite–sillimanite–K-feldspar assemblages. Quartzite xenoliths contain a strong S1 foliation defined by sillimanite. The xenoliths are, in turn, enclosed in granite that is has a weak S2 foliaiton. This suggests that an early foliation (S1) and sillimanite-grade metamorphism occurred at 1.7 Ga. Sillimanite–chloritoid assemblages in quartzite away from the granite suggest a thermal gradient associated with the pluton, but the chloritoid is texturally late. As in the Taos Range, radiometric ages indicate that temperatures in excess of 600° C were reached at ca. 1.4 Ga. Monazite (U-Pb) in granitic gneisses is 1.43 Ga; titanite (U-Pb) is 1.4 Ga; and hornblende (40Ar/39Ar) is 1.4–1.39 Ga.
The Rincon Range shows a similar record of polyphase tectonism, as described by Read et al. (this issue). It also contains a transition in structural style and metamorphic grade from north to south. The northern Rincon Range contains steeply dipping, thrust-imbricated rocks with peak meta-morphism near the Al-silicate triple point. The southern Rincon range contains shallowly dipping, high-grade screens of supracrustal rocks interleaved with the intrusive, 1.68-Ga, Guadalupita granitic gneiss (Fig. 5).
The main deformation fabric in both regions is S2. Evidence for an earlier generation (F1–S1) includes the local kyanite-defined S1 foliation, inclusion trails in porphyroblasts, and the regionally overturned nature of the quartzite (interpreted as one limb of an F1 recumbent nappe). Over a north-to-south distance of several kilometers, S2 grades from a steeply dipping fabric into progressively shallower orientations. S1 inclusion trails in porphyroblasts remain shallowly dipping throughout the transition. The shallowing of S2 dips takes place near the diffuse contact with the 1.68-Ga Guadalupita pluton. The pluton can be seen to cross-cut an S1 fabric; it is pervasively foliated by S2. The shallow S2 fabric is folded into open F3 domes and locally crenulated forming a steeply dipping north-northeast–striking S3 cleavage.
Metamorphic data from the Rincon Range combined with data from the Rio Mora area (Grambling et al., 1989) show a series of shallowly dipping isograds defined by M2 minerals. Assemblages with kyanite and Mn-andalusite give way southward (and structurally downward?) to kyanite–sillimanite assemblages, then only sillimanite, and then a zone of sillimanite–K-feldspar–(hercynite) assemblages near the 1.68-Ga Guadalupita granite. The spatial association of the high-T assemblages near the pluton, as in the Taos Range, suggests that the metamorphism may be synchronous with pluton emplacement (i.e., 1.68 Ga). However, as in the Cimarron and northern Taos areas, all dated metamorphic minerals are ca. 1.4 Ga; monazite aligned in S2 gives U-Pb ages of 1.42 Ga, and hornblende 40Ar/39Ar ages are 1.4 Ga. These ages, and the presence of late-stage migmatites, indicate metamorphic temperatures in excess of 600° C at ca. 1.4 Ga.
This transect shows a complex geometry involving domains of steep and shallow Proterozoic foliations. Domains with shallowly dipping S2 are associated with areas of high-grade metamorphism and with 1.65–1.7-Ga plutons. This suggests that both the steep-to-shallow fabric transitions and early (M2) metamorphic gradients may have been established at 1.7–1.65 Ga. However, there are no unequivocal syntectonic granite relations and no 1.65 Ga dated metamorphic minerals (except in the low-grade block in the Cimarron Range), possibly because of subsequent annealing. Early top-to-the-southeast shearing in the northern part of the transect is over-printed by top-to-the-northwest shearing in the central and southern part of the transect. Intense 1.4-Ga metamorphism has set or reset all dated minerals to ca. 1.4 Ga. The ca. 1.4-Ga minerals are aligned in S2 and are thus “M2” minerals. However, because of the geometric association of 1.65-Ga plutons with M2 thermal gradients and transitions in fabric orientation, we suggest that these 1.4-Ga minerals record reactivation (new movement) on S2 rather than the initial development of the fabric (see Read et al., this issue). If we are correct, the intense 1.4-Ga metamorphism was also associated with significant 1.4-Ga reactivation of S2 and with all D3 structures such that the present geometry of the cross section must be considered to reflect this 1.4-Ga event, even though the major folds and thrusts are considered to have formed at 1.7–1.65 Ga. It should be noted that if the spatial association of the highest grade rocks and shallow S2 with the 1.68-Ga plutons is a coincidence, and not temporally related, then much more of the observed D2 strain could have occurred at ca. 1.4 Ga.
The field, structural, and metamorphic studies show a similar sequence of deformation and metamorphic events, and a similar set of problems, in both the Tusas–Picuris and Taos–Rincon transects, and convince us that bothe belts share a common tectonic history. S1 foliation (commonly cryptic) was related to early recumbent folds and thrusts and is also preserved as fine-grained porphyroblast inclusion trails. M1 assemblages were relatively low-grade, involving kyanite, oxides, muscovite, and locally hornblende. D1 is interpreted to have involved burial of rocks during thrusting, probably before 1.65 Ga. The vergence of thrusts is not well known, although the earliest shearing in the Taos–Cimarron area is south-verging whereas large-scale F1 folds in the Rincon Range are north-verging.
The dominant regional foliation (S2) is axial-planar to large-scale northeast-verging F2 folds in the Tusas–Picuris transect. The Taos–Rincon transect (and Cerro Colorado area) contains dominantly shallowly dipping S2, but there are also transitions to steep northwest-verging folds and fabrics in the southern Taos and northern Rincon ranges that resemble the Tusas transect. We infer that steep and shallow foliation domains were deforming simultaneously during D2 and that the later D3 event did not reorient the S2 cleavage on a regional scale. Both transects show that steep and shallow S2 developed after (or during) 1.65-Ga plutonism and before ca. 1.42-Ga pegmatite dike emplacement (i.e., dike swarms have regionally consistent orientations). Our interpretation is that D2 deformation involved penetrative northwest- to north-directed shortening of the orogen during assembly of lithosphere at 1.65 Ga, with an unknown, but significant, amount of renewed shortening at 1.47–1.42 Ga.
D3 involved significant, but perhaps relatively weaker, deformation. In both transects, the dominant S2 foliation can be traced into antiforms and synforms forming broad foliation domes with a weak S3 crenulation cleavage. These F3 folds trend east–west in the Tusas–Picuris transect and north-northeast in the Taos–Rincon transect, but are similar in style. In both transects, F3 crenulations are interpreted to be synchronous with 1.45-Ga staurolite, suggesting that theycan be correlated as a single D3 event. We suggest that this event variably caused doming of shallow foliations, tightening of upright northwest- to northeast-trending F2 folds, and reactivation of S2 during northwest-directed shortening. Deformation was locally intense during D3, in the south Picuris Mountains for example. The extent and kinematics of 1.4-Ga reactivation of S2 remains poorly known mainly because the M3 thermal event outlasted D3 deformation and caused widespread annealing of all earlier fabrics.
One of our major conclusions is that M3 was an intense and significant regional metamorphism. All dated metamorphic minerals in New Mexico are 1.47–1.38 Ga, except the single older hornblende age in the Cimarron Mountains. This spread of ages overlaps with ages of plutons (1.43–1.42 Ga), but suggests that metamorphism generally predated pluton emplacement, and that minerals closed to daughter diffusion during slow cooling after metamorphism. The presence of metamorphic monazite, titanite, and hornblende in both transects indicates that ca. 1.4 Ga temperatures were greater than 550° C. Migmatites in the highest grade areas (northern Taos, Cimarron, and Rincon ranges) may indicate temperatures locally greater than 650° C. Granoblastic fabrics and annealed micas around F3 folds are evidence for a major thermal overprint during or after S3. Thus, the 1.4-Ga metamorphic event was a regional event that heated rocks to 550–650° C, even in areas where there are no exposed 1.4-Ga plutons and few 1.4-Ga pegmatites. The long duration of metamorphism, indicated by ages of metamorphic minerals (1.47–1.35 Ga), seems enigmatic in contrast to the shorter duration indicated by pluton and dike ages. Their age range is 1.44–1.42 Ga in New Mexico, although plutons that range in age from 1.48–1.35 Ga can be found elsewhere in the Southwest. Finally, one important cause of continued uncertainty in correlating older fabrics and minerals from study area to study are is the extent and pervasiveness of annealing and coarsening associated with the ca. 1.4-Ga event.
The ultimate cause of the D3–M3 event in the Southwest is equally enigmatic. The extremely large extent (across and along orogenic strike) of the metamorphism and plutonism points away from models directly involving plate-boundary processes and toward processes involving heating from below, perhaps by mantle melting and underplating. Although the thermal perturbation may be unrelated to the active plate margin (well to the southeast at this time), deformation of the thermally weakened lithosphere could certainly reflect far-field stresses associated with the boundary (see also Nyman et al., 1994).
Perhaps the major single impediment to an integrated tectonic model for the New Mexico Proterozoic concerns the age and grade of the D2–M2 event. At present, there is controversy about whether M2 minerals (and S2 fabrics) formed at 1.6 Ga, 1.47–1.42 Ga, or both. One view is that all amphibolite and higher grade metamorphism and deformation in New Mexico occurred at ca. 1.45–1.42 Ga. Another is that the ca. 1.4-Ga amphibolite-grade event over-printed, and is difficult to distinguish from, a similar grade of 1.65-Ga metamorphism and deformation. If large-scale D2 structures formed during the early stages of the 1.4-Ga event, then we are left with the uncomfortable conclusion that very few structures or assemblages associated with the 1.7–1.65 arc accretion and assembly (Mazatzal orogeny) exist in northern New Mexico.
PREFERRED MODEL: 1.65-GA CRUSTAL GEOMETRY AND 1.4-GA THERMAL STRUCTURE
Despite continued uncertainty about the absolute timing and intensity of metamorphic and deformational events, we now recognize similar geologic histories across New Mexico, involving superposed tectonism. We predict that seismic images will show a large-scale geometry similar to that portrayed on our composite cross sections (Figs. 2 and 5). We interpret this geometry to have been dominantly established at 1.65 Ga, with important local modification and reactivation at ca. 1.4 Ga. This type of model has been suggested in local areas (Williams, 1990; Bauer et al., 1993), but it can now be generalized to the regional transect scale. The steep-to-shallow transition in S2 orientation may represent important ramp-flat geometries in a segmented middle crust; we expect that these may sole on a regional scale into overall shallow reflective domains in the (paleo) lower crust. If so, major shear-zone boundaries at the exposed crustal level, such as the Pecos thrust, may be offset from lower crustal or mantle expressions of the same tectonic boundaries.
Deeper crustal levels may show increasing dominance of subhorizontal structures (e.g., Karlstrom and Williams, 1998). This assumes that lower-crustal rocks at both 1.65 and 1.4 Ga were hotter than middle crustal rocks and that weakening of rocks at temperatures greater than 550° C must induce pervasive subhorizontal refabrication due to shearing. As a working hypothesis, we expect to see the present crustal geometry of mixed steep and shallow structural domains give way downward, perhaps within several km, to domains dominated by lower-crustal subhorizontal reflectivity. Given the present uncertainty in understanding the timing of the steep to shallow foliation transition in the Rincon transect, we might be unable to distinguish a 1.65-Ga subhorizontal fabric from a 1.4-Ga subhorizontal fabric. However, this may be resolved in the regional transect by images of blocks that were colder at 1.65 Ga, such as the southern Taos and eastern Cimarron ranges, and areas dominated at the surface by 1.4-Ga batholiths, such as the Hermits Peak Granite.
Seismic studies will also image the deeper crustal structure of the region. The recognition of the extent and intensity of ca. 1.4-Ga metamorphism suggests that there may be a large 1.4-Ga mafic underplate, which should produce a high-velocity layer in the paleo-lower crust and which may signify modification and possible mobility of the Moho during 1.4-Ga tectonism. Such a structure should change character across the transect, perhaps mimicking changes in 1.4-Ga metamorphic grade. Thus, the mainly older structural geometries established during accretion are expected to be increasingly overprinted at depth by the 1.45-Ga thermal reactivation. The currently exposed middle-crustal structural level may represent a critical transition between the 1.7–1.65 Ga upper-crustal signature and the 1.45-Ga lower-crustal signature. A continuing challenge will be to distinguish and deconvolute the two events in surface exposures, so that they can be projected to and interpreted on cross sections and seismic images.
Detailed and careful reviews by Paul Bauer and Bob Bauer are sincerely appreciated. Support for field and laboratory research and for M.S. thesis research (Bishop, Lombardi, Read, Wingsted) was provided by NSF Grant EAR-9507984 to Williams and Karlstrom. This work benefited greatly from collaboration and interaction with geologists throughout the Southwest, particularly Paul Bauer, Steve Ralser, and Laurel Goodwin from the New Mexico Bureau of Mines and Mineral Resources and New Mexico Tech. Their logistical and scientific support for all of the M.S. thesis research projects is particularly appreciated. The authors sincerely thank Sheila Seaman and Christopher Kopf for final reviews of the manuscript.
- Received December 20, 1997.
- Revision received February 27, 1998.
- Accepted April 30, 1998.