the yavapai-mazatzal crustal boundary in the …eps karlstrom... · the yavapai-mazatzal crustal...

The Yavapai-Mazatzal crustal boundary in the Southern Rocky Mountains Col in A. Shaw and Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.

ABSTRACT

A major geologic boundary has been proposed in the Southern Rocky Mountains separating Proterozoic crustal provinces with different ages and tectonic histories. These provinces probably correlate with the Yavapai (1.8-1.7 Ga) and Mazatzal (1.7-1.6 Ga) provinces of Arizona. Geologic, geochemical, geochronologic, and xenolith data suggest that the boundary lies within a ~ 3 0 0 km-wide zone that trends northeastward through southern Colorado and northern New Mexico. This zone also seems to have focused later tectonic and thermal effects. However, no major shear zone that might represent a discrete tectonic suture has been identified in the area, and there is no agreement on precisely where the boundary is or what tectonic significance it may have.

We present a review of evidence supporting extrapolation of the Yavapai-Mazatzal boundary through the Southern Rocky Mountains. Limitations in the precision, quantity, and interpretation of available data probably contribute to disagreement over the location of the boundary. However, the disparity in boundaries defined by different data sets may partly reflect a complex or gradational transition between crustal domains. We propose a speculative model for the boundary based on a preliminary structural analysis. lkctonic fabrics appear to be consistent with the initial juxtaposition of arc terranes of the Yavapai and Mazatzal provinces on a low- angle thrust system with later modification and steepening of the boundary during continued crustal shortening. This model explains the diffuse isotopic boundary as a manifestation of a vertically heterogeneous crustal column that might promote isotopic mixing. The cryptic structural expression of the suture may result from a layer-parallel style of suturing and complex post-accretionary tectonic overprinting. KEY WORDS: province boundary, continental accretion, Proterozoic, island arcs, isotopic provinces, Colorado, Southern Rocky Mountains, sutures, shear zones.

INTRODUCTION Plate tectonic theory postulates that new conti-

nental lithosphere is formed by differentiation of basaltic material in magmatic arcs and assembly of the buoyant arc terranes at convergent margins (Hamilton, 1969, 1981, 1988). This process should produce continents comprising discrete terranes separated by tectonic boundaries (sutures) representing the closure of oceans or back-arc basins. Post-accretionary shuffling of tectonic blocks may also produce structural boundaries or reactivate primary accretionary structures (Jones et al., 1983; Hamilton, 1988). A complete model of continental accretion and its effects on lithospheric architec-

ture requires an understanding of the boundaries between terranes and the processes that knit sepa- rate arc terranes together into a coherent continent.

In the southwestern United States, a > 1000 km wide zone of juvenile crust was accreted to the rifted southern margin of the Wyoming craton between about 1.8 and 1.6 Ga (Karlstrom and Bowring, 1988, 1993). Rocks of this orogen are exposed in Laramide uplifts along a 700 km-long transect in the South- ern Rocky Mountains. This is an important natural laboratory for the study of accretionary boundaries and processes because a variety of possible boundaries is exposed and because mid-crustal levels that

Rocky Mountain Geology, v. 34, no. 1, p . 37-52, 4 figs., March, 1999 37

C. A. SHAW AND K. E. KARLSTROM

are inaccessible in younger accretionary orogens are now at the surface.

This paper reviews evidence supporting a proposed arc/arc-accretionary boundary in central Colorado. This boundary is one of several potential eastward extensions of an orogenic and isotopic boundary first recognized in Arizona that separates the ca. 1.8-1.7-Ga Yavapai province from the ca. 1.7- 1.6-Ga Mazatzal province (Silver, 1965, 1968; Karlstrom and Bowring, 1988, 1993). The province boundary has been recognized by previous workers on the basis of geochronologic, isotopic, petrologic, and structural discontinuities and/or gradients (e.g., Condie, 1982,1986; Bennett and DePaolo, 1987; Wooden and DeWitt, 1991; Karlstrom and Bowring, 1993; Karlstrom and Daniel, 1993). However, in the Rocky Mountains, no discrete structural suture has been identified, and different data sets identify different locations for the boundary (Fig. 1). In an ef- fort to stimulate and focus future work on the boundary, we have tried to integrate published isotopic, geochronologic, and xenolithic results with compiled metamorphic and structural data to produce a speculative model for the juxtaposition of two or more arc terranes along initially low-angle structures. This model is an attempt to reconcile conflicting proposed locations for the boundary, to explain the lack of an obvious structural suture, and to provide a testable cross section for further geological and geophysical studies.

REGIONAL TECTONIC SETTING

The best documented example of a fundamental accretionary boundary exposed along the Rocky Mountain transect is the suture between Archean and Proterozoic crust, the Cheyenne belt of southern Wyoming (CB in Fig. 1). Steeply-dipping shear zones of the Cheyenne belt mark the juxtaposition of the rifted Archean basement (> 2.5 Ga) of the Wyoming craton and its Paleoproterozoic passive

margin cover ( > 2.0 Ga) with a succession of 1.8- 1.7-Ga arc terranes (Hills and Houston, 1979; Condie, 1982; Karlstrom and Houston, 1984; Duebendorfer and Houston, 1986, 1987; Hoffman, 1989; Houston et al., 1989; Van Schmus et al., 1993a). Sharp N d and Pb isotopic discontinues between the two provinces coincide with the tectonic suture (Zartman, 1974; Bennett and DePaolo, 1987; Ball and Farmer, 1991).

Within the Proterozoic provinces of Colorado and New Mexico, it has been more difficult to identify terranes and sutures. By analogy with modern arcs, individual terranes should rarely exceed several hundred kilometers in width suggesting that the southwestern Proterozoic provinces should be composed of a number of distinct terranes separated by tectonic suture zones. Numerous shear zones in the Southern Rocky Mountains divide the Protero- zoic crust into tectonic blocks (Fig. l), but these zones record a long history of 1.8-1.4-Ga tectonism and it has been difficult to distinguish the more fundamental paleo-subduction boundaries from later accommodation structures. Indeed, many early accretionary structures may have been reac- tivated or overprinted during progressive shortening after accretion, especially at the middle crustal levels now exposed (Karlstrom and Williams, 1998).

Silver (1965) first proposed a northeast-trending geochronologic boundary between an older 1.8- 1.7-Ga block to the north and a younger 1.7-1.6-Ga block to the south based on work in central Ari- zona. The boundary was imprecisely located, but extended from Arizona through the Four Corners region and was postulated to continue through central Colorado (Fig. 1). Variations of this boundary have appeared in regional compilations ever since (e.g., Silver et al., 1977; Van Schmus and Bickford, 1981; Nelson and DePaolo, 1985; Bennett and DePaolo, 1987; Van Schmus et al., 1987, 199313; Karlstrom and Bowring, 1993). At least six other versions of this boundary have been proposed and,

Figure 1, facing page. Proterozoic provinces of the southwestern United States (modified from Karlstrom and Bowring, 1993). Our preferred orogenic province boundaries are shown by heavy dotted lines; CB = Cheyenne belt (see also Introduction, this issue). Stipple shows transition zones between provinces. Proposed province boundaries: MF = Mazatzal deformation front (Karlstrom and Bowring, 1993; Karlstrom and Daniel, 1993), Nd = Nd model age boundary between T,, = 2.0-1.8-Ga and T,, = 1.8-1.6-Ga crust (from Bennett and DePaolo, 1987), Pb = Pb isotope boundary (from Wooden and DeWitt, 1991), C1 = Condie’s (1981; 1982) boundary between 1.80-1.72-Ga and 1.72- 1.65-Ga supracrustal age provinces; C2 = Condie’s (1987) boundary between 1.70-1.68-Ga supracrustal rocks and 1.80-1.76-Ga supracrustal rocks, K&D = extrapolation of the southern limit of Yavapai-age crust showing Laramide strike-slip offset of boundary (Karlstrom and Daniel, 1993). NVF = Navajo volcanic field. Open circles represent diatremes with hydrous and deformed xenoliths including eclogites (northwest group). Filled circles represent diatremes (southeast group) with anhydrous granulite xenoliths (Selverstone et al., in press). CMB = Colorado mineral belt, GM = Green Mountain magmatic arc, CBA = composite back-arc, SG = Salida Gunnison magmatic complex (Reed et al., 1987).

38 Rocky Mountain Geology, v. 34, no. I , p . 37-52, 4 figs., March, 1999

YAVAPAI-MAZATZAL CRUSTAL BOUNDARY

Rockg Mountain Geology, v. 34, no. I , p . 37-52, 4figs., March, 1999 39


taken together, the proposed boundaries define a 300 km-wide transition zone between the Yavapai and Mazatzal provinces (Fig. 1). Both the northern and southern limits of the province correspond roughly to a geophysical lineament. The northern limit follows the Holbrook lineament in western Arizona and the southern limit coincides with the Jemez lineament in northern New Mexico (Fig. 1; Karlstrom and Conway, 1986). The coincidence of these geophysical features with the proposed geochemical and geochronologic boundaries suggests that at least some of these boundaries may have important deep-crustal significance.

In evaluating tectonic boundaries within orogenic belts it is important to apply a multi- disciplinary approach to clearly define provinces and boundaries. Different techniques may “see” different aspects or levels of a complex transition between tectonic blocks. The following sections review geochronologic, isotopic, xenolithic, and structural evidence for the proposed Yavapai-Mazatzal boundary where it intersects the Southern Rocky Moun- tains. We also present compiled metamorphic and structural data from general geologic studies in the area of the proposed boundary, focusing on the northern part of the transition in central and southern Colorado (see Williams et al., this issue, for overview of northern New Mexico). No geophysical studies have specifically targeted the proposed Yavapai-Mazatzal boundaries in the Rocky Moun- tains, but seismic lines are planned as part of the upcoming Continental Dynamics Rocky Mountain transect study (see Introduction to this issue).

GEOCHRONOLOGIC AND ISOTOPIC STUDIES

Geochronologic and isotopic studies provide the primary line of evidence for defining the Yavapai- Mazatzal boundary in the Southwest (Silver, 1965; Condie, 1981, 1982; Bennett and DePaolo, 1987). Proposed isotopic boundaries in the Southern Rockies are broadly consistent at a regional scale but differ significantly in detail (Fig. 1; DePaolo, 1981; Condie, 1982,1987; Nelson and DePaolo, 1985; Bennett and DePaolo, 1987; Reed et al., 1987; Wooden and DeWitt, 1991; Aleinikoff et al., 1993).

Geochronology Following the pioneering work of Silver (1965,

1968), Condie (1981, 1982) identified two Protero- zoic crustal provinces that stretch from Arizona to the Rocky Mountain region. The northern province

is characterized by supracrustal volcanic rocks with U-Pb zircon and Rb-Sr isochron ages of 1.80 to 1.72 Ga and the southern province by ages of 1.72 to 1.65 Ga (Condie, 1982). Condie placed the boundary between these provinces in northern New Mexico south of the Sante Fe Range (C1 in Fig. 1). In a later modification, Condie (1987) proposed that a northern arc province of 1.80-1.76-Ga crust extended as far south as the Manzano and Pedernal mountains of New Mexico. It was overlapped by a 1.70-1.68-Ga continental arc or back-arc volcanic province that reached from the Grenville front to central Colo- rado near Salida (C2 in Fig. 1) creating a composite transition zone in southern Colorado and northern New Mexico. Condie also identified smaller arc and back-arc terranes inset into the larger provinces in New Mexico (1.72 and 1.65 Ga) and the Salida-Gunnison area of central Colorado (1.74-1.73 Ga).

Reed et al. (1987) subdivided the Yavapai (Colo- rado) province into three subprovinces on the basis of lithologic contrasts (Tweto, 1987) and generally southward-decreasing U-Pb ages (Fig. 1). The three subprovinces are: (1) the Green Moun- tain arc characterized by syntectonic pluton ages greater than about 1.75 Ga; (2) an intervening composite back-arc basin with deformed plutons older than ca. 1.70 Ga [except for the ca. 1.67-Ga Kroenke pluton near the southern edge of the subprovince, see Premo and Fanning (1997) for revised age of Boulder Creek quartz monzonite]; and (3) the Salida-Gunnison magmatic-arc complex in central and southern Colorado with pluton and metavolcanic ages from 1.76-1.60 Ga (Reed et al., 1987).

Common Lead Wooden and DeWitt (1991) proposed a common

Pb boundary in Arizona (W&D in Fig. 1) that roughly coincides with Condie’s (1 982) geochronologic boundary. Rocks (mainly plutonic) from southeast Arizona (Mazatzal province) have lower z08Pb/204Pb for a given value of z06Pb/204Pb than rocks from central Arizona (Yavapai province). The data implies a lower time-integrated Th/U for the source of Yavapai rocks ( ~ 2 ) than for the Mazatzal source ( ~ 4 ) . There is no corresponding difference in 207Pb/ 204Pb vs. 206Pb/204Pb (Wooden and DeWitt, 1991). The data of Stacey and Hedlund (1983) and Wooden and Aleinikoff (Wooden and Aleinikoff, 1987) suggest that the differences in common lead persist into New Mexico and Colorado with the Pb province boundary passing through northern New Mexico

40 &cky Mountain Geology, v. 34, no. I , p . 37-52, 4figs., March, 1999


(Fig. 1; Wooden et al., 1988; Wooden and DeWitt,

Aleinikoff et al. (1993) interpreted differences in Pb isotopic ratios between rocks of southern Colo- rado and northern New Mexico and rocks of northern Colorado as evidence for a Pb transition zone in central Colorado supporting Reed’s boundary between a composite back-arc domain (northern Colorado) and the Salida-Gunnison magmatic arc (Fig. 2). Inferred Th/U of the source calculated from z08Pb/204Pb and z06Pb/z04Pb ratios are scattered from 1 to 9 in the northern back-arc area and more tightly clustered between 1 and 3 in the Salida-Gunnison area and northern New Mexico. However, the mean values for the two populations are similar. The tighter clustering of inferred Th/U in the southern samples might partly reflect the fact that about 50 percent of the southern samples came from a relatively small area in the n o s Range (Aleinikoff et al., 1993, figs. 2 and 6, table 1). The zo7Pb/z04Pb vs. z0sPb/z04Pb are similar in the northern and southern sample populations. Any real differences in the common lead signatures of the back arc domain in northern Colorado and the Salida-Gunnison area could reflect different source rock lithologies (Aleinikoff et al., 1993) or variable uranium loss due to different thermal and metamorphic conditions (cf. Wooden et al., 1988; Aleinikoff et al., 1993).

It is important to note that the Pb transition zone proposed by Aleinikoff et al. (1993) does not correspond to the boundary drawn by Wooden and DeWitt (1991). The latter boundary separates a domain with calculated Th/U values of N2 in the north (Yavapai) from a domain with values of N4 in the south (Mazatzal), whereas the former marks a per- ceived transition between variable Th/U (north) and more typical Yavapai province ratios in the Salida-Gunnison area and northern New Mexico. If a statistically significant difference between the common Pb signatures of rocks in southern Colo- rado and those in northern Colorado does, in fact, exist, the Pb-transition zone of Aleinikoff et al. (1993) may mark an internal lithotectonic boundary within the Yavapai province as proposed by Reed et al. (1987).

1991).

Nd Model Ages

Nd model ages (TDM) are interpreted to date the time of crustal differentiation from a depleted mantle source (DePaolo, 1981). Bennett and DePaolo (1 987) proposed two Proterozoic “crustal formation” provinces on the basis of Nd data (Fig. 1). Nd provinces are broadly consistent, at a regional

scale, with crystallization age provinces (Condie, 1981,1982,1986) and Pb isotopic provinces (Nelson and DePaolo, 1985; Bennett and DePaolo, 1987; Wooden et al., 1988). A northern province (Ndprov- ince 2) with model ages (TDM) of 2.0-1.8 Ga corresponds roughly to the 1.8-1.7-Ga supracrustal age province (Yavapai) and a southern province (Nd province 3) with model ages of 1.8 Ga, corresponds to the 1.7-1.6-Ga age province (Mazatzal). However, in the Southern Rockies the location of the Nd boundary is only approximately known because it is delimited by only three sample localities, separated by about 150 km (Fig. 2; DePaolo, 1981; Bennett and DePaolo, 1987). The limited number of samples in this area makes it difficult to assess whether the Nd-boundary is gradational or discon- tinuous and how well it coincides with other isoto- pically defined boundaries in the area.

The interpretation of Nd model ages as “crust- formation ages” has been questioned by Arndt and Goldstein (1 987). Nonetheless, the regionally consistent pattern of Nd ages, rarity of documented mantle additions to the crust, and the general lack of significantly older potential source material in the Southwest are cited as justifications for using Nd isotopes to define crustal provinces in this region (e.g., Farmer and DePaolo, 1983; Nelson and DePaolo, 1985; Bennett and DePaolo, 1987). The Nd crust-formation provinces of Bennett and DePaolo (1987) were constructed using Nd model ages of crustally derived granitoids ranging in age from Paleoproterozoic to early Tertiary. The consistency of results from this diverse suite of rocks suggests that they may be derived from long-lived, stable, lower-crustal sources with distinct Sm-Nd systematics (Livaccari and Perry, 1993).

Synthesis of Isotopic and Geochronologic Studies

Despite the discordance of proposed isotopic boundaries based on different data sets, the balance of isotopic evidence from geochronology, common Pb, and Nd model ages seems to support the con- clusion that a regionally significant crustal boundary passes through southern Colorado or northern New Mexico. The three most probable explanations for disparity in the boundaries determined by different isotopic systems and methods are: (1) experimental and statistical uncertainties limit the precision of isotopic boundaries to worse than about f 100 km; (2) the grouping of samples into populations with arbitrary age or compositional limits may not be consistent between studies employing

Rocky Mountain Geology, v. 34, no. I , p . 37-52, 4figs., March, 1999 41


Figure 2. Map of the northern part of the proposed Yavapai-Mazatzal (Y-M) transition area showing generalized lithologies of Precambrian rocks, metamorphic grade, and the locations of younger magmatic centers. EL = 1.4-Ga Electra Lake gabbro, PP = 1.1-Ga Pikes Peak batholith, Q = Questa caldera (Jemez trend), SJ = Tertiary San Juan volcanic field including: SC = Silverton, LC = Lake City, CR = Creede calderas. Stippled area labeled LVM represents low-velocity mantle (see Lerner-Lam et al., 1998). Pb %ansition Zone (horizontal hatch) is from Aleinikoff (1993). Three alternative proposed locations for the Y-M boundary in southern and central Colorado are shown: MF = Mazatzal front showing approximate northern limit of Mazatzal-age (ca. 1.70-1.65 Ga) deformation (Karlstrom and Bowring, 1993), geochronologic boundary separating 1.80-1.76-Ga and 1.70-1.68-Ga provinces (Condie, 1986), and Nd = boundary separating Nd TDM ages of 2.0-1.8 Ga to the north from T,, 1.8-1.7 Ga to the south. Nd sample locations that constrain the position of the proposed Nd boundary in this area are also shown (DePaolo, 1981; Bennett and DePaolo, 1987). Metamorphic data compiled from: Logan, 1966; Normand, 1972; Boardman, 1976; Bailey, 1980; Afifi, 1981; Grambling, 1988; Lanzirotti, 1988; Hannula, 1989; Earley, 1991; Hetherington, 1991; Pedrick, 1995.

42 Rockg Mountain Geology, v. 34, no. I , p. 37-52, 4 figs., March, 1999


different geochemical tools; or (3) different isotopic systems and methods may be sensitive to different aspects of a complex, heterogeneous transition between isotopic provinces. The first two possibili- ties explain the range of proposed boundaries as an artifact of sampling, experimental, and statistical methods. If this is the case, with continued detailed work, the profusion of boundaries should converge on a single discrete province boundary representing the interface between crustal blocks with distinct geochronologic and isotopic properties. The third possibility is a suggestion that the range of proposed isotopic boundaries may, in fact, reflect real complexities of the boundary zone. More detailed isotopic work is needed to better understand the nature of the isotopic changes in the boundary zone and refine the positions of the various proposed boundaries. However, we suggest that a complex or gradational boundary zone should be considered as a viable hypothesis to explain discrep- ancies in proposed isotopic province boundaries.

XENOLITH STUDIES

Consistent differences in xenolith populations from different parts of the Navajo Volcanic Field (NVF in Fig. 1) in the Four Corners region have recently been interpreted as evidence of a lower- crustal petrologic boundary coinciding with the northern limit of the Yavapai-Mazatzal boundary zone (Selverstone et al., 1997b, in press). Xenoliths from diatremes in the northwest part of the volcanic field show evidence of hydration and deformation and include a wide variety of rock types including metasedimentary gneisses and eclogites. Some xenoliths from the northwestern diatremes preserve evidence of counterclockwise P-T paths (Tmax before P,,,) reaching maximum temperatures of N 85OOC and maximum pressures of N 10 kbar. The southeastern diatremes contain only undeformed granulite xenoliths with no evidence of hydrous alteration or penetrative deformation. A few xenoliths from the southeast preserve fragmentary evidence for clockwise P-T evolution. Selverstone et al. (in press) interpret the differences between the xenolith populations as evidence of a transition between lower-crustal blocks with significantly different Proterozoic tectonic histories (Xe in Fig. 1). Moreover, they postulate that the observed range of rock types, hydration, deformation, and P-T history of the northwestern xenoliths suggest a north-dipping paleo-subduction zone in the Four

Corners area. A north-dipping Proterozoic subduction zone is consistent with xenolith and other evidence, but this interpretation is not unique given the inherently fragmentary nature of the xenolith record, the lack of reliable age constraints, and the unknown geometry of deep-crustal structures.

METAMORPHISM AND STRUCTURE

Karlstrom and Bowring (1988) defined the Yavapai and Mazatzal orogenic provinces based on contrasting deformational and metamorphic histories of fault-bounded tectonic blocks exposed in Arizona. The Yavapai orogenic province comprises 1.8-1.7-Ga juvenile crust deformed before about 1.7 Ga. The Mazatzal province is characterized by 1.7- 1.6-Ga crust deformed ca. 1.66-1.60 Ga (Karlstrom and Bowing, 1988,1993). These orogenic provinces correspond roughly to Silver’s (1965,1968) Yavapai and Mazatzal age provinces. The boundary between the provinces is marked by steep shear zones separating tectonic blocks, but does not necessarily coincide exactly with isotopic province boundaries (Karlstrom and Bowring, 1993; Karlstrom and Daniel, 1993). Instead, the Mazatzal deformation front extends beyond the 1.7-1.6-Ga supracrustal rocks into 1.8-1.7-Ga rocks more characteristic of the Yavapai province indicating that deformation was transmitted into older rocks of the foreland during continental assembly (Karlstrom and Daniel, 1993).

The Southern Rocky Mountains lie along the trend of the Proterozoic accretionary orogens from the Precambrian exposures of Arizona and may be correlative (Fig. 1; Silver, 1965; Karlstrom and Bowring, 1993; Karlstrom and Daniel, 1993). How- ever, the boundary zone in the Rockies appears much more diffuse and lacks the steep foliation and sub-vertical shear zones typical of Precambrian exposures in Arizona. In the Southern Rockies, steep shear zones are concentrated in the northeast-trending Colorado mineral belt (CMB in Fig. 1; Weto and Sims, 1963; Warner, 1978), which lies north of the inferred isotopic and geochronologic boundaries. The relationship between the isotopic boundaries of central and southern Colorado and the Colorado mineral belt is uncertain. The following sections and accompanying figures present compilations of metamorphic and structural data from the region of the boundary. These data are essential to unrav- eling the thermal and kinematic history of accretion and modification that may be responsible for the cryptic nature of the boundary zone.

Rocky Mountain Geology, v. 34, no. I, p . 37-52, 4 figs., March, 1999 43

C . A. SHAW AND K. E. KARLSTROM

Metamorphism A generalized compilation of metamorphic

grade in the area of the boundary zone shows the inferred “peak metamorphic conditions (maximum temperature) reported by a number of authors (Fig. 2). In central Colorado, no-one has yet inves- tigated possible composite effects of superimposed metamorphic events, as has recently been documented to the north (Selverstone et al., 1997a; Shaw et al., in press) and south (Pedrick et al., 1998). The limited available data seem to reflect a predomi- nance of amphibolite-grade conditions, typical of much of Colorado and New Mexico (e.g., ’Ihreto, 1987; Grambling, 1988; Reed et al., 1993), although anomalous granulite-grade rocks occur in the Wet Mountains (Fig. 2; Brock and Singewald, 1968; Ttveto, 1987) and northern Thos Range (Grambling et al., 1989; Pedrick, 1995; Pedrick et al., 1998).

Steep gradients in the conditions of peak metamorphism from greenschist and amphibolite to granulite grade in these areas are consistent with similar observed field gradients and inferred pluton-enhanced metamorphism elsewhere in the Southwest (Williams and Karlstrom, 1996). There is no obvious pattern to help define a tectonic boundary. However, the occurrence of granulites within an otherwise medium-grade metamorphic domain is consistent with differential uplift and tectonic juxtaposition of different crustal levels on large-scale structures (Reed et al., 1987). In the Wet Mountains, the Ilse fault zone (IF in Fig. 2), a subvertical, north-trending structure with Precam- brian ancestry separates granulites on the west from amphibolite grade rocks on the east (Singewald, 1966; Tweto, 1987). It is possible that the Ilse fault played a role in juxtaposing granulite grade and amphibolite grade rocks, but the tectonic significance of the Ilse fault remains unclear. The paucity of quantitative thermobarometric data, systematic analyses of metamorphic overprinting, and the re- lation of metamorphism to deformation is a seri- ous impediment to interpreting the metamorphic history of the proposed boundary zone. Further work in this area is needed.

Structure The Mazatzal-age deformation front identified

in Arizona may be extrapolated into the Rocky Mountain region (Fig. 1; Karlstrom and Daniel, 1993). A line representing the Mazatzal-age (ca. 1700-1650 Ma) orogenic front can be drawn between 1690-1650-Ma plutons that suffered significant synemplacement deformation and those that are relatively undeformed (Fig. 3). South of the pro-

posed deformation front, we observe penetratively deformed plutons and pluton margins. The Garrell Peak pluton (1663 f 4 Ma) is strongly, but variably deformed and the 1672 f 5 Ma llout Creek pluton north of Salida is highly deformed only at its southern margin (Fig. 3; Boardman, 1976; Bickford et al., 1989; Shaw and Karlstrom, 1997). In the Needle Mountains, the ca. 1690-Ma Tenmile Creek and Bak- ers Bridge plutons crosscut early fabrics, but were deformed by post-1690-Ma tectonism that also affected the unconformably overlying Uncompahgre group (Gibson, 1987, 1990; Gibson and Simpson, 1988; Harris, 1990). This event may coincide with the 1.67-1.65-Ga ductile thrusting and imbrication of the Hondo group of New Mexico (Williams et al., this issue) and syncontractional emplacement of 1.65-Ga plutons in southern New Mexico (Silver, 1965; Karlstrom and Bowring, 1988,1993; Bauer and Williams, 1994). Tectonism of this age has been cor- related to the Mazatzal orogeny of Arizona (e.g., Karlstrom and Bowring, 1993; Karlstrom and Daniel, 1993). In contrast 1.65-Ga deformation does not seem to have strongly affected the area north of the proposed deformation front. For example, the 1676 f 5 Ma Cochetopa granite (Fig. 3) is virtually undeformed (Bickford et al., 1989; Shaw and Karlstrom, 1997), and the primary phase of deformation farther north in Colorado probably occurred before 1.70 Ga (e.g., Barovich, 1986; Premo and Fan- ning, 1997). The northern limit of Mazatzal-age deformation is still poorly defined in the Southern Rockies, but this orogenic front may represent an important structural discontinuity.

Our preliminary analysis of foliation trends in southern Colorado and northern New Mexico suggest the regional development of an initially gently-dipping, west- to northwest-striking tectonic foliation (tentatively designated S,) usually parallel to primary compositional layering (So). These early fabrics are variably overprinted by a steeper tectonic fabric (S,) in much of the area south of the Mazatzal front (Fig. 3; Shaw and Karlstrom, 1997). In less deformed areas S, is well preserved. For example, foliations near Salida and in the northern Wet Mountains dip shallowly north (Fig. 3). Steeply dipping northeast- and northwest-striking fabrics are developed in some areas (e.g., south and northeast of Gunnison, Fig. 3), but seem to be the result of folding of S, (Afifi, 1981). Early top-to-the-south shear-sense has been identified in the mas, Cimarron (Pedrick et al., 1998), and Rincon ranges (Read et al., this issue) of New Mexico, but we have not yet identified sufficient kinematic indicators to confirm this vergence in central Colorado. In other areas steep northeast-striking S, fabrics pre-


YAVAPAI -MAZATZAL CRUSTAL BOUNDARY

Figure 3. Maps showing generalized structural trends and interpretations in the vicinity of the northern Yavapai- Mazatzal transition. A, Foliation map and equal area plots. Lithologies are highly simplified to emphasize structure. Fine lines show general trend of the dominant tectonic or magmatic foliation. Foliations shown are not necessarily correlative between different areas. Equal area (Schmidt) plots show normals to foliation in key areas. Contoured data represents So (primary compositional layering) or composite So-S, foliation otherwise noted (C.I. = 1-3%/1% area). MF = Mazatzal front, IF = Ilse fault. Small squares on Salida area plot represent normals to S,. n o s Range plots show average orientation (great circles and normals) to orientation data in several structural domains. B, Detail of central Colorado showing ca. 1.67-Ga plutons used to support the location of Mazatzal deformation front and sche- matic representation of structural interpretation for several key areas including Gunnison area (Iris quadrangle), Pout Creek pluton aureole (north of Salida), central Wet Mountains (Mt. qndall quadrangle), and northern Needle Mountains. Structural data compiled from: Logan, 1966; Wobus, 1966; Hansen, 1971; Normand, 1972; Hedlund and Olsen, 1973; mylor et al., 1975a, 197513, and 1975c; Boardman, 1976; Olsen, 1976a and 197613; Bailey, 1980; Afifi, 1981; Tkwksbury, 1981; Plummer, 1986; Noblett, 1987; Noblett et al., 1987; Lanzirotti, 1988; Thacker, 1988; Hannula, 1989; Earley, 1991; Hetherington, 1991; Gibson and Harris, 1992; Reed et al., 1993; and Pedrick, 1995.

dominate. Steeply dipping, northeast-striking S, foliations and related upright folds are developed in the central Wet Mountains, the northern Sangre de Cristo Mountains, Gunnison area, and Needles Mountains. The F, folds and S, foliation are interpreted to reflect a dominantly pure-shear northwest-southeast shortening of the region (D,) subsequent to S , development. This pattern of shallow fabrics overprinted by steeper ones is similar to the pattern developed near the Yavapai-Mazatzal boundary in central Arizona (Karlstrom, 1989).

The superposition of these two dominant regional fabrics, as well as other locally important fabrics has created intricate composite structural patterns. A comparison of two areas in central Colo- rado illustrates the inferred regional deformation sequence and the southeast to northwest decreas-

ing D, deformation gradient (Fig. 3). In the central Wet Mountains (near the granulite exposures) complex interference patterns indicate at least three generations of superimposed folds (Fig. 3; Brock and Singewald, 1968; Lanzirotti, 1988). F, is the dominant generation of fold with shallow to moderately northeast-plunging axes and steep northeast-striking axial planes consistent with northwest-southeast shortening. A steep S, axial-planar cleavage is variably developed. Both northwest-trending F, fold axes and axial planes are tightly folded by F, folds. The closed figures formed by the traces of form-sur- faces apparent in the map patterns of some folds are consistent with a hybrid type 2 (mushroom)- type 3 (convergent-divergent) interference pattern (Ramsay, 1967; Ramsay and Huber, 1987). This in- dicates that the axial planes of F, folds were prob-

Rockg Mountain Geology, v. 34, no. I , p . 37-52, 4 figs., March, 1999 45


ably at a high angle to the subvertical flow vector for F, folding (Ramsay and Huber, 1987) consistent with an initial low-angle axial plane for F, folds and a 90” change in the direction of maximum com- pression between D, and D, (Lanzirotti, 1988). Origi- nally shallowly northwest plunging F, fold axes were rotated about northeast-trending axes into steep northwest and southeast plunging orientations (Fig. 3). The northeast fabrics of the central Wet Moun- tains are abruptly truncated to the east by the Ilse fault.

Near the proposed Mazatzal deformation front immediately north of Salida shallowly dipping, east-west-striking metavolcanic and associated rocks are relatively undeformed and preserve primary volcaniclastic and intrusive textures (Fig. 3; Boardman, 1976, 1980, 1986). However, as the margin of the 1672 f 5 Ma Wout Creek pluton is ap- proached the shallow fabrics in these rocks (S,) are progressively transposed into a sub-vertical, northeast-trending fabric parallel to magmatic and solid state fabrics within the pluton (S,). The deformation is clearly enhanced by proximity to the pluton, and we interpret the age of deformation to be about 1670 Ma.

We tentatively correlate the F, folds and S, foliation in the central Wet Mountains with the trans- position of shallow (So and S,) fabrics into steep, northeast-trending fabrics north of Salida on the basis of trend and structural style. Both of these structures seem to be manifestations of the regionally extensive D, deformation. If this correlation holds up it implies that D, deformation occurred ca. 1670 Ma, coeval with the Mazatzal orogeny in New Mexico and Arizona, and also that a deformation gradient (or discontinuity) exists between the penetrative deformation of the central Wet Moun- tains and the limited, pluton-enhanced deformation north of Salida.

Our regional synthesis of structural data sup- ports the following tentative kinematic model. (1) Initial thrust-sense deformation took place on a low- angle, layer parallel S, foliation. If any high-strain shear zones developed during this phase they might be difficult to identify because they cut compositional and tectonic layering at a low angle and may have been pre-peak metamorphism. (2) S, foliation was variably overprinted by a steep, northeast-striking S, fabric and folded by F, upright folds during a pure-shear phase of deformation, perhaps related to large-scale northwest-southeast crustal shortening after ca. 1690 Ma. The D, deformation was largely limited to the area south of the Salida- Gunnison area (Mazatzal Front (MF) in Fig. 3).

YOUNGER PLUTONISM AND TECTONISM

In addition to the isotopic, xenolith, and structural data there are indications that the proposed boundary zone in southern Colorado has been an important tectonic and magmatic boundary at various times since the accretion of the continent (Karlstrom and Humphreys, 1998). For example, ca. 1.4 Ga, when regional “anorogenic” magmatism was dominantly granitic at exposed crustal levels, mafic plutons reached the middle crust in only two places-the Laramie anorthosite of the Cheyenne belt area and the Electra Lake gabbro of the Needle Mountains, within the proposed Yavapai-Mazatzal boundary zone, (Fig. 2). We speculate that older zones of weakness, related to the accretionary boundaries, facilitated emplacement of mafic rocks at middle crustal levels at these localities. Likewise at about 1.1 Ga (Grenville age), most of the magmatic activity was concentrated to the south in Texas and Arizona (Smith et al., 1997), yet the volumi- nous Pikes Peak pluton was emplaced at the northern margin of the proposed boundary zone (Figs. 2 and 3). Again in the Tertiary, calderas of the San Juan volcanic field penetrated the crust along this line (Fig. 2). Although the location and nature of Proterozoic structures remains uncertain, we speculate that at different times segments of the boundary acted both as zones of enhanced fertility for magmas and as conduits for magma emplacement, potentially because of early accretionary structures. A major center of anomalously low velocity mantle underlying the boundary zone (Fig. 2) may also be influenced by lithospheric architecture inherited from the assembly of the continent (Lerner-Lam et al., 1998).

Present physiography and mantle structure also suggest that accretionary structures may have a persistent influence. A transition from the closely spaced uplifts of the Colorado Rockies to the nar- row and widely spaced ranges of New Mexico broadly coincides with the Yavapai-Mazatzal boundary zone. This transition echoes the change in style between the broad basins of the Wyoming Rockies and the denser Colorado Rockies that occurs across the Cheyenne belt (see Pazzaglia and Kelley, 1998, fig. 1). Recent work has also identified important gradients in denudation history across the zone (Pazzaglia and Kelley, 1998), and the highest peaks in the Southern Rocky Mountains occur at the in- tersection of the Rio Grande rift and the Colorado Mineral belt immediately north of the Yavapai- Mazatzal boundary zone (Reed, 1996). All of these

46 Rocky Mountain Geology, v. 34, no. 1 , p . 37-52, 4figs., March, 1999

YAVAPAI -MAZATZAL CRUSTAL BOUNDARY

observations suggest that Proterozoic accretionary structures have had a persistent influence on later tectonism, although the mechanisms remain unclear.

SPECULATIVE MODEL FOR THE BOUNDARY

Despite the obvious need for more data, the evidence for a province boundary in southern Colo- rado or northern New Mexico seems moderately strong at the regional scale. Assuming that a boundary does exist, any model must either dismiss the disparity of proposed geochronologic, isotopic, and structural boundaries as an artifact of the limited data set and inherent limits of precision of the methods used, or explain it in terms of crustal geometry and geologic processes. We think it is most constructive to propose a testable working hypothesis that is consistent with the available data, ex- pecting that the model will be modified or rejected as new information is discovered.

We suggest that the best explanation for differences in proposed province boundaries is a gradational or complex transition zone. This transition could be a laterally heterogeneous zone composed of a tectonic mosaic of small terranes permitting isotopic mixing at a scale below the resolution of isotopic methods, or a gradational change in crustal properties produced by a continuous process with- out distinct terranes or sutures. The lack of major shear zones associated with the isotopic boundaries argues against the mosaic model, which predicts abundant sutures. Furthermore, most plate tectonic models as well as geophysical and geologic observations suggest that large scale ( > 1000 km) continental growth usually proceeds by the discon- tinuous accretion of discrete tectonostratigraphic terranes rather than by continuous processes (e.g., Hamilton, 1981, 1988; Jones et al., 1983; Hoffman, 1989).

Another possible model for a gradational transition is a vertically heterogeneous crustal section with crustal blocks juxtaposed on low-angle sutures and modified by later tectonism. As a working hypothesis, we tentatively propose that, in the South- ern Rockies, terranes of the Mazatzal and Yavapai provinces were juxtaposed along a low-angle boundary that evolved from a north dipping subduction zone (Fig. 4). During collision, overthrusting of Mazatzal terranes may have been facilitated by mid- crustal detachments in thermally weakened arc lithosphere (Fig. 4C). In this model the transition zone defined by the range of isotopic boundaries (Fig. 1) broadly coincides with a region of overlap

between the Yavapai and Mazatzal provinces which constitute the hanging wall and footwall, respec- tively, of a crustal-scale thrust system (Fig. 4E). In our model, the surface expression of the suture lies in northern New Mexico coincident with Condie’s (1982) supracrustal boundary. The postulated fos- sil subduction zone underlying the northern limit of the boundary zone in the Four Corners area might represent oceanic crust preserved at the intersec- tion of the suture with the moho (Fig. 1; Fig. 4E; Selverstone et al., 1997b, in press). Moreover, we suggest the possibility that a mantle suture separating Yavapai and Mazatzal mantle lithosphere may continue dipping NW under Colorado (cf. Calvert et al., 1995). This mantle suture, and perhaps a similar southeast-dipping structure related to the Chey- enne belt subduction zone, may have influenced the development of Proterozoic shear zones and Laramide mineralization in the Colorado mineral belt (CMB, Fig. 1; Fig. 4E).

This model explains the disparity in proposed isotopic boundaries as a manifestation of vertical crustal heterogeneity (Fig. 4E). Different isotopic and geochronologic techniques sample different portions of the crustal column. The Nd boundary (Bennett and DePaolo, 1987) defined using plutonic rocks derived largely from the lower crust lies significantly farther north than the province boundary defined by the ages of supracrustal rocks exposed at the surface (Condie, 1981, 1982). Common Pb systematics are strongly influenced by the integrated crustal column through which the host magma has passed. Hence, either Yavapai, Mazatzal, or intermediate Pb signatures might be expected in the boundary zone.

The lack of an obvious structural suture at the surface may be the result of initial near-layer-parallel thrusting creating a cryptic boundary between crustal blocks. Subsequent folding and/or imbrication of the boundary in response to continued crustal shortening might have locally steepened the boundary, compositional layering, and early tectonic foliations. The integrated effects of early low- angle thrusting and later pure-shear shortening may have produced a geometrically complex tectonic interfingering of Yavapai and Mazatzal blocks in the mid-crust. Anomalously high-grade metamorphic rocks, as observed in the Wet Mountains and Thos Range, could be parts of the higher-grade hanging wall preserved as klippen above a mid-crustal detachment surface (Fig. 4E). Depending on the initial geometry of ramps and flats in the subduction zone, and the nature of post-accretion deformation in the middle crust, different rheological layers may have acted as detachments, and the suture zone may

Rocky Mountain Geology, v. 34, no. 1. p . 37-52, 4 figs., March, 1999 47


48 Rocky Mountain Geology, v. 34, no. I , p . 37-52, 4figs., March, 1999

YAVAPAI - MAZATZAL CRUSTAL BOUNDARY

step down through the lithosphere. If so, reactiva- tion of different segments of the suture in the mantle, lower crust, or middle crust at different times could have focused magmatism or deformation in different parts of the transition zone (Fig. 4E).

Although our model is highly speculative it has the following advantages. First, it provides a coherent explanation for the discrepancy in province boundaries inferred from different methods. Sec- ondly, it is consistent with structural observations. Thirdly, it proposes a framework for understanding Proterozoic accretionary tectonics that is consistent with modern, low-angle convergent margin structures (Hamilton, 1981, 1988) and seismic images of other accretionary boundaries (e.g., Calvert et al., 1995). Finally, it presents a testable model cross-section for future geophysical and geologic investigations.

IMPLICATIONS FOR STYLE OF SUTURING

A comparison of the cryptic Yavapai-Mazatzal boundary in the Southern Rocky Mountains and the more obvious Archean-Proterozoic suture at the Cheyenne belt reveals two fundamentally different structural styles of suturing. It may also be that the nature of the Yavapai-Mazatzal boundary varies significantly along trend, with fabrics and bounding structures changing from steep (in Arizona) to shallow (in Colorado). These observations demonstrate that tectonic sutures within continental crust are both complex and diverse. In addition, the differences between the Yavapai-Mazatzal and Cheyenne belt accretionary boundaries suggest that the lithospheric properties of the colliding blocks may partly control the structural style of accretionary orogens. During collision, the rifted cratonic footwall of the Cheyenne belt may have acted as a tectonic back- stop dictating the geometry of thrusting and pro-

moting the development of a discrete suture zone characterized by steep fabrics. In contrast, for the Yavapai-Mazatzal boundary, the lack of a rigid cratonic footwall and high heat flow related to arc magmatism may have permitted a low-angle suture to develop. Overthrusting of one arc terrane onto another and penetrative pure shear shortening of the boundary could have been facilitated by a thermally weakened middle crust.

If our preliminary model of the Rocky Moun- tain segment of the Yavapai-Mazatzal boundary as a folded, initially low-angle suture that is detached in the mid-crust is correct, it would imply that the strength of the mid-crust is an important control on tectonic style and that sutures in arc-arc collision zones may be wide zones of tectonic and chemi- cal mixing of adjacent terranes. This would have important implications for modeling the geodynamic processes of accretion, for understanding suspect terranes, and for evaluating the effects that accretionary structures may have on later tectonism and plutonism.

ACKNOWLEDGMENTS Research for this paper was supported by NSF

Continental Dynamics Program grant number EAR- 9614787 (Karlstrom) and a Kelly-Silver graduate research fellowship from the Caswell Silver Foundation (Shaw). Helpful reviews by Christine Siddoway and John Geissman greatly improved the manuscript.

REFERENCES CITED Afifi, A. M., 1981, Precambrian geology of the Iris area, Gunnison

and Saguache counties, Colorado [M.S. thesis]: Golden, Colo- rado School of Mines, 197 p.

Aleinikoff, J. N., Reed, J. C., Jr., and Wooden, J. L., 1993, Lead isotopic evidence for the origin of Paleo- and Mesoproterozoic rocks of the Colorado province, U.S.A.: Precambrian Research, v. 63, p. 97-122.

figure 4, facing page. Cartoons depicting speculative model for the development and geometry of the Southern Rocky Mountain segment of the Y-M boundary. A, Arc terranes belonging to the Yavapai and Mazatzal provinces collide at north-dipping subduction zone. B, Subduction zone evolves into a low-angle thrust system as older Yavapai crust overrides Mazatzal arc. C, Thrusting is facilitated by detachment system in the thermally weakened middle crust. Gently-dipping S , simple-shear foliation develops in mid-crustal rocks. Fold-and-thrust deformation may have extended well into the foreland of the Yavapai crustal province. D, Continued converg2ence coaxially shortens crust and folds suture and older foliation and producing steep S, foliation in Mazatzal terrane and in southern part of Yavapai province. Mazatzal front (MF) marks the limit of penetrative deformation associated with this late conver- gence. Mazatzal front is the northern limit of this deformation. E, Magmas migrate vertically across boundary lead- ing to emplacement of plutons derived from Mazatzal lower crust in Yavapai upper crustal country rocks. Finally, erosion exposes mid-crustal levels and complex metamorphic and structural mosaic resulting from folded low-angle boundary. Different segments of the suture may have focused magmatism and fluid flow through geologic time to create the Colorado mineral belt (CMB, 1700-50 Ma), Pikes Peak batholith ( ~1100 Ma), San Juan volcanic field ( < 40 Ma), Jemez lineament volcanics ( < 10 Ma), and present mantle low-velocity anomaly.

Rockg Mountain Geology, v. 34, no. 1, p. 37-52, 4 figs., March, 1999 49


Arndt, N. T., and Goldstein, S. L., 1987, Use and abuse of crust- formation ages: Geology, v. 15, p. 893-895.

Bailey, M. H., 1980, Bedrock geology of western mylor Park, Gunnison County, Colorado [M.S. thesis]: Corvallis, Oregon State University, 123 p.

Ball, T. T., and Farmer, G. L., 1991, Identification of 2.0 to 2.4 Ga Nd model age crustal material in the Cheyenne belt, southeastern Wyoming; implications for Proterozoic accretionary tectonics at the southern margin of the Wyoming craton: Geology, v. 19, p. 360-363.

Barovich, K. M., 1986, Age constraints on Early Proterozoic deformation in the northern Front Range, Colorado [M.S. thesis]: Boulder, University of Colorado, 49 p.

Bauer, P. W., and Williams, M. L., 1994, The age of Proterozoic orogenesis in New Mexico, U.S.A.: Precambrian Research,

Bennett, V. C., and DePaolo, D. J., 1987, Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping: Geological Society of America Bulletin, v. 99, p. 674-685.

Bickford, M. E., Shuster, R. D., and Boardman, S. J., 1989, U-Pb geochronology of the Proterozoic volcano-plutonic terrane in the Gunnison and Salida areas, Colorado, in Grambling, J. A., and lkwksbury, B. J., eds., Proterozoic geology of the Southern Rocky Mountains: Geological Society of America Special Paper 235, p. 33-49.

Boardman, S. J., 1976, Geology of the Precambrian metamorphic rocks of the Salida area, Chaffee County, Colorado: The Mountain Geologist, v. 13, p. 89-100.

- 1980, Evidence for two stages of Precambrian metamorphism in the Salida area, central Colorado: Geological So- ciety of America Abstracts with Programs, v. 12, no. 6, p. 267.

- 1986, Early Proterozoic bimodal volcanic rocks in central Colorado, U.S.A.; Part I, Petrography, stratigraphy and depo- sitional history: Precambrian Research, v. 34, p. 1-36.

Brock, M. R., and Singewald, Q. D., 1968, Geologic map of the Mount l3mdall quadrangle, Custer County, Colorado: U.S. Geological Survey Geologic Quadrangle GQ-596, scale 1:24,000, 2 sheets.

Calvert, A. J., Sawyer, E. W., Davis, W. J., and Ludden, J. N., 1995, Archean subduction inferred from seismic images of a mantle suture in the Superior province: Nature, v. 375, p.

Condie, K. C., 1981, Precambrian rocks of the southwestern United States and adjacent areas of Mexico: New Mexico Bureau of Mines b Mineral Resources Resource Map 13, scale 1:1,500,000, 2 sheets.

- 1982, Plate-tectonics model for Proterozoic continental accretion in the southwestern United States: Geology, v. 10, p.

- 1986, Geochemistry and tectonic setting of Early Protero- zoic supracrustal rocks in the southwestern United States: Journal of Geology, v. 94, p. 845-864.

- 1987, Early Proterozoic volcanic regimes in southwestern North America, in Pharaoh, T. C., Beckinsale, R. D., and Rickard, D., eds., Geochemistry and mineralization of Pro- terozoic volcanic suites: Geological Society [London] Spe- cial Publication no. 33, p. 211-218.

DePaolo, D. J., 1981, Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic: Na- ture, v. 291, p. 193-196.

V. 67, p. 349-356.

670-674.

37-42.

Duebendorfer, E. M., and Houston, R. S., 1986, Kinematic history of the Cheyenne belt, Medicine Bow Mountains, southeastern Wyoming: Geology, v. 14, p. 171-174.

- 1987, Proterozoic accretionary tectonics at the south margin of the Archean Wyoming craton: Geological Society of America Bulletin, v. 98, p. 554-568.

Earley, D., 111, 1991, Metamorphic petrology of the Gold Brick District, west-central Colorado, and implications for the evolution of a Proterozoic accretionary terrain [M.S. thesis]: Minneapolis, University of Minnesota, 345 p.

Farmer, G. L., and DePaolo, D. J., 1983, Origin of Mesozoic and Tertiary granite in the western United States and implications for pre-Mesozoic crustal structure; 1, Nd and Sr isotopic studies in the geocline of the northern Great Basin: Journal of Geophysical Research, v. 88, p. 3379-3401.

Gibson, R. G., 111, 1987, Structural studies in a Proterozoic gneiss complex and adjacent cover rocks, West Needle Mountains, Colorado [Ph.D. dissert.]: Blacksburg, Virginia Polytechnic Institute and State University, 186 p.

- 1990, Proterozoic contact relationships between gneissic basement and metasedimentary cover in the Needle Moun- tains, Colorado, U.S.A.: Journal of Structural Geology, v. 12, p. 99-111.

Gibson, R. G., and Harris, C. W., 1992, Geologic map of Protero- zoic rocks in the northwestern Needle Mountains, Colorado: Boulder, Colorado, Geological Society of America Map and Chart Series MCH075, scale 1:31,680, 1 sheet.

Gibson, R. G., and Simpson, C., 1988, Proterozoic poly- deformation in basement rocks of the Needle Mountains, Colorado: Geological Society of America Bulletin, v. 100, p.

Grambling, J. A., 1988, A summary of Proterozoic metamorphism in northern New Mexico: The regional development of 520°C, 4 kbar rocks, in Ernst, W. G., ed., Metamorphism and crustal evolution of the western United States: Englewood Cliffs, New Jersey, Prentice Hall, Rubey Volume VII, p. 447- 465.

Grambling, J. A., Williams, M., Smith, R., and Mawer, C., 1989, The role of crustal extension in the metamorphism of Pro- terozoic rocks in northern New Mexico, in Grambling, J. A., and lkwksbury, B. A., eds., Proterozoic geology of the Southern Rocky Mountains: Geological Society of America Special Paper 235, p. 87-110.

Hamilton, W. B., 1969, Mesozoic California and the underflow of Pacific mantle: Geological Society of America Bulletin, v.

- 1981, Crustal evolution by arc magmatism: Royal Society of London Philosophical Tkansactions, ser. A, v. 301, p. 279- 291.

- 1988, Plate tectonics and island arcs: Geological Society of America Bulletin, v. 100, p. 1503-1527.

Hannula, K. A., 1989, Proterozoic metamorphic conditions of central Colorado, in Woodard, H. H., ed., Second Keck Re- search Symposium in Geology (Abstracts volume): Colo- rado Springs, Colorado, April 14-16,1989, The Keck Geology Consortium, v. 2, p. 62-65.

Hansen, W. R, 1971, Geologic map of the Black Canyon of the Gunnison River and vicinity, western Colorado: U.S. Geo- logical Survey Miscellaneous Investigations Map 1-584, scale 1:31,680, 2 sheets.

Harris, C. W., 1990, Polyphase suprastructure deformation in metasedimentary rocks of the Uncompahgre Group; rem-

1957-1970.

80, p. 2409-2430.



nant of an early Proterozoic fold belt in southwest Colo- rado: Geological Society of America Bulletin, v. 102, p. 664- 678.

Hedlund, D. C., and Olsen, J. C., 1973, Geologic map of the Iris NW quadrangle, Gunnison and Saguache counties, Colo- rado: U.S. Geological Survey Geologic Quadrangle GQ-1134, scale 1:24,000, 1 sheet.

Hetherington, E. D., 1991, Structural analysis of Proterozoic rocks in Gunnison County, Colorado [Ph.D. dissert.]: Minneapo- lis, University of Minnesota, 298 p.

Hills, F. A., and Houston, R. S., 1979, Early Proterozoic tectonics of the central Rocky Mountains, North America: Contribu- tions to Geology, University of Wyoming, v. 17, p. 89-109.

Hoffman, P. F., 1989, Precambrian geology and tectonic history of North America, in Bally, A. W., and Palmer, A. R, eds., The geology of North America-An overview: Boulder, Colo- rado, Geological Society of America, The Geology of North America, v. 1, p. 447-512.

Houston, R S., Duebendorfer, E. M., Karlstrom, K. E., and Premo, W. R, 1989, A review of the geology and structure of the Cheyenne belt and Proterozoic rocks of southern Wyoming, in Grambling, J. A., and Tewksbury, B. J., eds., Proterozoic geology of the Southern Rocky Mountains: Geological Soci- ety of America Special Paper 235, p. 1-12.

Jones, D. L., Howell, D. G., Coney, P. J., and Monger, J.W.H., 1983, Recognition, character, and analysis of tectonostratigraphic terranes in western North America, in Hashimoto, M., and Uyeda, S., eds., Accretionary tectonics in the Circum-Pacific regions: Tbkyo, Terra Science Publishing Company, p. 21- 35.

Karlstrom, K. E., 1989, Early recumbent folding during Protero- zoic orogeny in central Arizona, in Grambling, J. A., and Tewksbury, B. J., eds., Proterozoic geology of the Southern Rocky Mountains: Geological Society of America Special Paper 235, p. 155-171.

Karlstrom, K. E., and Bowring, S. A,, 1988, Early Proterozoic assembly of tectonostratigraphic terranes in southwestern North America: Journal of Geology, v. 96, p. 561-576.

- 1993, Proterozoic orogenic history of Arizona, in Van Schmus, W. R., Bickford, M. E., and 23 others, 'kansconti- nental Proterozoic provinces, in Reed, J. C., Jr., and 6 others, eds., Precambrian: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. C-2, p. 188-211.

Karlstrom, K. E., and Conway, C. M., 1986, Early Proterozoic geology of Arizona: Geology, v. 14, p. 625-626.

Karlstrom, K. E., and Daniel, C. G., 1993, Restoration of Laramide right-lateral strike slip in northern New Mexico by using Proterozoic piercing points; tectonic implications from the Proterozoic to the Cenozoic: Geology, v. 21, p. 1139-1142.

Karlstrom, K. E., and Houston, R S., 1984, The Cheyenne belt: analysis of a Proterozoic suture in southern Wyoming: Pre- cambrian Research, v. 25, p. 415-446.

Karlstrom, K. E., and Humphreys, E. D., 1998, Persistent influence of Proterozoic accretionary boundaries in the tectonic evolution of southwestern North America: Interaction of cratonic grain and mantle modification events: Rocky Mountain Geology, v. 33, p. 161-179.

Karlstrom, K. E., and Williams, M. L., 1998, Heterogeneity ofthe middle crust: Implications for strength of continental lithosphere: Geology, v. 26, p. 815-818.

Lanzirotti, A., 1988, Geology and geochemistry of a Proterozoic supracrustal and intrusive sequence in the central Wet

Mountains, Colorado [M.S. thesis]: Socorro, New Mexico Institute of Mining and Technology, 164 p.

Lerner-Lam, A. L., Sheehan, A., Grand, S., Humphreys, E., Dueker, K., Hessler, E., Guo, H., Lee, D.-K., and Savage, M., 1998, Deep structure beneath the southern Rocky Mountains from the Rocky Mountain Front Broadband Seismic experiment: Rocky Mountain Geology, v. 33, p. 199-216.

Livaccari, R. F., and Perry, F. V., 1993, Isotopic evidence for pres- ervation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States: Geology,

Logan, J. M., 1966, Structure and petrology of the eastern margin of the Wet Mountains, Colorado [Ph.D. dissert.]: Norman, University of Oklahoma, 283 p.

Nelson, B., and DePaolo, D. J., 1985, Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continent: Geo- logical Society of America Bulletin, v. 96, p. 746-754.

Noblett, J. B., 1987, Geology of the Precambrian metamorphic rocks along South Hardscrabble Creek, Wet Mountains, Colo- rado: The Mountain Geologist, v. 24, p. 67-76.

Noblett, J. B., Cullers, R L., and Bickford, M. E., 1987, Protero- zoic crystalline rocks in the Wet Mountains and vicinity, central Colorado: New Mexico Geological Society, 38th Field Conference, Guidebook, p. 73-82.

Normand, D. E., 1972, Precambrian geology of the northern Sangre de Cristo Range, Chaffee, Fremont, and Saguache counties, Colorado [Ph.D. dissert.]: Lubbock, Texas Tech University, 88 p.

Olsen, J. C., 1976a, Geologic and structural maps and sections of the Marshall Pass mining district, Saguache, Gunnison and Chaffee counties, Colorado: U.S. Geological Survey Miscel- laneous Investigations Map 1-1425, scale 1:24,000, 1 sheet.

- 1976b, Geologic map of the Iris quadrangle, Gunnison and Saguache counties, Colorado: U.S. Geological Survey Geo- logic Quadrangle GQ-1071, scale 1:24,000, 1 sheet.

Pedrick, J., 1995, Polyphase tectonometamorphic history of the 'ILlos Range, northern New Mexico [Ph.D. dissert.]: Albuquer- que, University of New Mexico, 146 p.

Pedrick, J. N., Karlstrom, K. E., and Bowring, S. A., 1998, Recon- ciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico: Journal of Metamorphic Petrol-

Plummer, D. A., 1986, The petrology and structure of Protero- zoic rocks northeast of Salida, Colorado [M.S. thesis]: Albu- querque, University of New Mexico, 88 p.

Premo, W. R., and Fanning, M. C., 1997, U-Pb zircon ages for the Big Creek Gneiss, Wyoming and the Boulder Creekbatholith, Colorado: Implications for the timing of early Proterozoic accretion of the northern Colorado province: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. 407.

Ramsay, J. G., 1967, Folding and fracturing of rocks: New York, McGraw-Hill, 568 p.

Ramsay, J. G., and Huber, M. I., 1987, Modern structural geology, volume 2: Folds and fractures: London, Academic Press, 700 P.

Reed, J. C., Jr., 1996, Proterozoic underpinnings of the Southern Rocky Mountains in Colorado: Geological Society of America Abstracts with Programs, v. 28, no. 7, p. 270.

Reed, J. C., Jr., Bickford, M. E., Premo, W. R, Aleinikoff, J. N., and Pallister, J. S., 1987, Evolution of the Early Proterozoic

V. 21, p. 719-722.

ogy, V. 16, p. 687-707.

Rocky Mountain Geology, v. 34, no. I , p . 37-52, 4 figs., March, 1999 51


Colorado province: Constraints from U-Pb geochronology: Geology, v. 15, p. 861-865.

Reed, J. C., Jr., Bickford, M. E., and Weto, O., 1993, Proterozoic accretionary terranes of Colorado and southern Wyoming, in Reed, J. C., Jr., and 6 others, eds., Precambrian: Conter- minous US.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. C-2, p. 110-121.

Selverstone, J., Hodgins, M., Shaw, C., Aleinikoff, J. N., and Fan- ning, C. M., 1997a, Proterozoic tectonics of the northern Colorado Front Range, in Bolyard, D. W., and Sonnenberg, S. A., eds., Geologic history of the Colorado Front Range: Denver, Rocky Mountain Association of Geologists, p. 9-18.

Selverstone, J., Pun, A., and Condie, K. C., 1997b, Xenolithic evidence for Proterozoic subduction polarity beneath the Colo- rado Plateau: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. 467.

- 1999, Xenolithic evidence for Proterozoic crustal evolution beneath the Colorado Plateau: Geological Society of America Bulletin, v. 110 (in press).

Shaw, C. A., and Karlstrom, K. E., 1997, Heterogeneity of 1.7-1.68 Ga deformation in southern Colorado, Implications for orogenic province boundaries: Geological Society of America Abstracts with Programs, v. 29, no. 6, p. 467.

Shaw, C. A., Snee, L., W., Selverstone, J., and Reed, J. C., Jr., 1999, 40Ar/39Ar thermochronology of Mesoproterozoic metamorphism in the Colorado Front Range: Journal of Geology (in press).

Silver, L. T., 1965, Mazatzal orogeny and tectonic episodicity [abstract]: Geological Society of America Special Paper 82, p.

- 1968, U-Pb isotope relations and their historical implications in Precambrian zircons from Bagdad, Arizona [abstract]: Geological Society of America Special Paper 101, p. 420.

Silver, L. T., Bickford, M. E., Van Schmus, W. R, Anderson, J. L., Anderson, T. H., and Medaris, L. G., Jr., 1977, The 1.4-1.5 b.y. transcontinental anorogenic plutonic perforation of North America: Geological Society of America Abstracts with Programs, v. 9, no. 7, p. 1176-1177.

Singewald, Q. D., 1966, Description and relocation of part of the Ilse fault zone,Wet Mountains, Colorado: U.S. Geological Sur- vey Professional Paper 5504 , p. C20-C24.

Smith, D. R, Barnes, C., Shannon, W., Roback, R, and James, E., 1997, Petrogenesis of mid-Proterozoic granitic magmas; examples from central and west 'Exas: Precambrian Re- search, v. 85, p. 53-79.

Stacey, J. S., and Hedlund, D. C., 1983, Lead-isotopic composi- tions of diverse igneous rocks and ore deposits from southwestern New Mexico and their implications for Early Proterozoic crustal evolution in the western United States Geological Society of America Bulletin, v. 94, p. 43-57.

Thylor, R B., Scott, G. R, and Wobus, R A., 1975a, Reconnais- sance geologic map of the Howard quadrangle, central Colo- rado: U.S. Geological Survey Miscellaneous Investigations Series Map 1-892, scale 1:62,500, 1 sheet.

Thylor, R B., Scott, G. R, Wobus, R A., and Epis, R C., 1975b, Reconnaissance geologic map of the Cotopaxi 15-minute quadrangle, Fremont and Custer counties: U.S. Geological Survey Miscellaneous Investigations Series Map 1-900, scale 1:62,500, 1 sheet.

- 1975c, Reconnaissance geologic map of the Royal Gorge quadrangle, Fremont and Custer counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map 1-869, scale 1:62,500, 1 sheet.

Wksbury, B. J., 1981, Polyphase deformation and contact relationships of the Precambrian Uncompahgre Formation,

185-186.

Needle Mountains, southwestern Colorado [Ph.D. dissert.]: Boulder, University of Colorado, 475 p.

Thacker, M. S., 1988, Geology and geochemistry of early-Prot- erozoic supracrustal rocks from the northern Sangre de Cristo Mountains and adjacent area, central Colorado [M.S. thesis]: Socorro, New Mexico Institute of Mining and lkch- nology, 129 p.

'Itveto, 0. L., 1987, Rock units of the Precambrian basement in Colorado: U.S. Geological Survey Professional Paper 1321- A, 54 p.

'Itveto, 0. L., and Sims, P. K., 1963, Precambrian ancestry of the Colorado mineral belt: Geological Society of America Bul- letin, v. 74, p. 991-1014.

Van Schmus, W. R, and Bickford, M. E., 1981, Proterozoic chro- nology and evolution of the midcontinent region, North America, in Kroner, A., ed., Precambrian plate tectonics: Amsterdam, Elsevier, Developments in Precambrian Geol-

Van Schmus, W. R, Bickford, M. E., and 23 others, 1993, Pans- continental Proterozoic provinces, in Reed, J. C., Jr., and 6 others, eds., Precambrian: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. (3-2, p. 171-334.

Van Schmus, W. R, Bickford, M. E., and Condie, K. C., 1993, Early Proterozoic crustal evolution, in Reed, J. C., Jr., and 6 others, eds., Precambrian: Conterminous US.: Boulder, Colo- rado, Geological Society of America, The Geology of North America, v. C-2, p. 270-281.

Van Schmus, W. R, Bickford, M. E., and Zietz, I., 1987, Early and Middle Proterozoic provinces in the central United States, in Kroner, A., Proterozoic lithospheric evolution: Washing- ton, D.C., American Geophysical Union, Geodynamics Se- ries, v. 17, p. 43-68.

Warner, L. A., 1978, The Colorado Lineament: A middle Precam- brian wrench fault system: Geological Society of America Bulletin, v. 89, p. 161-171.

Williams, M. L., and Karlstrom, K. E., 1996, Looping P-T paths and high-T, low-P middle crustal metamorphism; Protero- zoic evolution of the southwestern United States: Geology,

Wobus, R A., 1966, Petrology and structure of Precambrian rocks of the Puma Hills, southern Front Range, Colorado [Ph.D. dissert.]: Palo Alto, California, Stanford University, 146 p.

Wooden, J. L., and Aleinikoff, J. N., 1987, Pb isotopic evidence for Early Proterozoic crustal evolution in the southwestern US.: Geological Society of America Abstracts with Programs, v. 19, no. 7, p. 897.

Wooden, J. L., and DeWitt, E., 1991, Pb isotopic evidence for the boundary between the early Proterozoic Mojave and central Arizona crustal provinces in western Arizona: Arizona Geological Society Digest, v. 19, p. 27-50.

Wooden, J. L., Stacey, J. S., and Howard, K. A., 1988, Pb isotopic evidence for the formation of Proterozoic crust in the southwestern United States, in Ernst, W. G., ed., Metamorphism and crustal evolution of the western United States: Englewood Cliffs, New Jersey, Prentice Hall, Rubey Volume

Zartman, R E., 1974, Lead isotope provinces in the Cordillera of the western United States and their geologic significance: Economic Geology, v. 69, p. 792-805.

O ~ Y , p. 261-296.

V. 24, p. 1119-1122.

VII, p. 69-86.

MANUSCRIPT SUBMITIED FEBRUARY 9, 1998 REVISED MANUSCRIPT SUBMIITED MARCH 13, 1998 MANUSCRIPT ACCEPTED APRJL 22, 1998


the yavapai-mazatzal crustal boundary in the …eps karlstrom... · the yavapai-mazatzal crustal...

Documents