The Yavapai-Mazatzal crustal boundary in the Southern

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The Yavapai-Mazatzal crustal boundary in the
Southern Rocky M o u n t a i n s
C o l i n 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 km-wide
0
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 lowangle 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 continental 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-
Rocky Mountain Geology, v. 34, no. 1, p . 37-52, 4 figs., March, 1999
ture requires an understanding of the boundaries
between terranes and the processes that knit separate 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 Southern 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
37
C.A. SHAW AND K. E. KARLSTROM
are inaccessible i n 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 a n orogenic and isotopic
boundary first recognized in Arizona that separates
the ca. 1.8-1.7-Ga Yavapai province from the ca. 1.71.6-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, i n the
Rocky Mountains, n o discrete structural suture has
been identified, and different data sets identify different locations for the boundary (Fig. 1). In a n effort to stimulate a n d 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 a n 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 1.81.7-Ga arc terranes (Hills a n d Houston, 1979;
Condie, 1982; Karlstrom a n d 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 Proterozoic 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 reactivated 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 1.81.7-Ga block to the north and a younger 1.7-1.6-Ga
block to the south based on work in central Arizona. 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 a n d DePaolo, 1985; Bennett a n d
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 1.721.65-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
C. A. SHAW AND K. E. KARLSTROM
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 multidisciplinary 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 Mountains. 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 Mountains, 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
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 Colorado near Salida (C2 in Fig. 1) creating a composite transition zone i n 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 (Colorado) province into three subprovinces on the basis of lithologic contrasts (Tweto, 1987) and
generally southward-decreasingU-Pb ages (Fig. 1).
The three subprovinces are: (1) the Green Mountain 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
Geochronologic and isotopic studies provide the
primary line of evidence for defining the YavapaiMazatzal 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).
Wooden and DeWitt (1991) proposed a common
Pb boundary in Arizona (W&D in Fig. 1) that
roughly coincides with Condie’s (1982) 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 ( ~ 2than
)
for the Mazatzal source
( ~ 4 )There
.
is no corresponding difference in 207Pb/
204Pb
vs. 206Pb/204Pb
(Wooden and DeWitt, 1991).The
Geochronology
data of Stacey and Hedlund (1983) and Wooden and
Following the pioneering work of Silver (1965, Aleinikoff (Wooden and Aleinikoff, 1987) suggest
1968), Condie (1981, 1982) identified two Protero- that the differences in common lead persist into
zoic crustal provinces that stretch from Arizona to New Mexico and Colorado with the Pb province
the Rocky Mountain region. The northern province boundary passing through northern New Mexico
40
&cky Mountain Geology, v. 34, no. I , p . 37-52, 4figs., March, 1999
YAVAPAI-MAZATZAL CRUSTAL BOUNDARY
(Fig. 1; Wooden et al., 1988; Wooden and DeWitt,
1991).
Aleinikoff et al. (1993) interpreted differences
in Pb isotopic ratios between rocks of southern Colorado 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 differencesin 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 T h / U values of N2 in the north
(Yavapai) from a domain with values of N4 in the
south (Mazatzal), whereas the former marks a perceived 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 Colorado 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).
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 (Ndprovince 2) with model ages (TDM)
of 2.0-1.8 G a 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 discontinuous and how well it coincides with other isotopically defined boundaries in the area.
The interpretation of Nd model ages as “crustformation ages”has been questioned by Arndt and
Goldstein (1987). 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 conclusion that a regionally significant crustal boundary passes through southern Colorado or northern
New Mexico. The three most probable explanations
Nd Model Ages
for disparity in the boundaries determined by difNd model ages (TDM)
are interpreted to date the ferent isotopic systems and methods are: (1) experitime of crustal differentiation from a depleted mental and statistical uncertainties limit the
mantle source (DePaolo, 1981). Bennett and precision of isotopic boundaries to worse than about
DePaolo (1987) proposed two Proterozoic “crustal f 100 km; (2) the grouping of samples into populaformation” provinces on the basis of Nd data (Fig. tions with arbitrary age or compositional limits
1). Nd provinces are broadly consistent, at a regional may not be consistent between studies employing
Rocky Mountain Geology, v. 34, no. I , p . 37-52,4figs., March, 1999
41
C. A. SHAW AND K. E. KARLSTROM
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), geochronologicboundary 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
YAVAPAI-MAZATZAL CRUSTAL BOUNDARY
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 possibilities 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 discrepancies 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 lowercrustal 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 85OOC and maximum pressures of 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.
Selverstoneet al. (inpress) interpret the differences
between the xenolith populations as evidence of a
transition between lower-crustalblocks 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
N
N
Rocky Mountain Geology, v. 34, no. I, p . 37-52, 4 figs., March, 1999
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 1.71.6-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). However, 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 unraveling the thermal and kinematic history of accretion and modification that may be responsible for
the cryptic nature of the boundary zone.
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 investigated 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 predominance 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 i n 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 Precambrian ancestry separatesgranulites 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 relation of metamorphism to deformation is a serious 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 pro44
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-MaTenmile Creek and Bakers 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 correlated to the Mazatzal orogeny of Arizona (e.g.,
Karlstrom and Bowring, 1993; Karlstrom and
Daniel, 1993).In contrast 1.65-Gadeformation 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 Fanning, 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 i n 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 preRocky Mountain Geology, v. 34, no. I , p . 37-52, 4 figs., March, 1999
YAVAPAI -MAZATZAL CRUSTAL BOUNDARY
Figure 3. Maps showing generalized structural trends and interpretations in the vicinity of the northern YavapaiMazatzal 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 schematic representation of structural interpretation for several key areas including Gunnison area (Iris quadrangle),
Pout Creek pluton aureole (north of Salida), central Wet Mountains (Mt. q n d a l l 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 Colorado illustrates the inferred regional deformation
sequence and the southeast to northwest decreasRockg Mountain Geology, v. 34, no. I , p . 37-52, 4 figs., March, 1999
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-surfaces 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 indicates that the axial planes of F, folds were prob45
C. A. SHAW AND K.E. KARLSTROM
ably at a high angle to the subvertical flow vector
for F, folding (Ramsay and Huber, 1987) consistent
with an initial low-angleaxial plane for F, folds and
a 90” change in the direction of maximum compression between D, and D, (Lanzirotti, 1988).Originally 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 Mountains 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 approached 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 transposition 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 Mountains and the limited, pluton-enhanced deformation
north of Salida.
Our regional synthesis of structural data supports the following tentative kinematic model. (1)
Initial thrust-sense deformation took place on a lowangle, 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 SalidaGunnison area (Mazatzal Front (MF) in Fig. 3).
46
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 voluminous 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 narrow and widely spaced ranges of New Mexico
broadly coincideswith 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 intersection of the Rio Grande rift and the Colorado
Mineral belt immediately north of the YavapaiMazatzal boundary zone (Reed, 1996). All of these
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 Colorado 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, expecting 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 without 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 discontinuous 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 Southern Rockies, terranes of the Mazatzal and Yavapai
provinces were juxtaposed along a low-angleboundary that evolved from a north dipping subduction
zone (Fig. 4). During collision, overthrusting of
Mazatzal terranes may have been facilitated by midcrustal 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
Rocky Mountain Geology, v. 34, no. 1. p . 37-52, 4 figs., March, 1999
between the Yavapai and Mazatzal provinces which
constitute the hanging wall and footwall, respectively, 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 fossil subduction zone underlying the northern limit
of the boundary zone in the Four Corners area might
represent oceanic crust preserved at the intersection 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 Cheyenne 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 lowangle 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
47
C.A. SHAW AND K. E. KARLSTROM
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, reactivation 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. Secondly, 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 backstop 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 Mountain 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 a n important control
on tectonic style and that sutures in arc-arc collision zones may be wide zones of tectonic and chemical mixing of adjacent terranes. This would have
i m p o r t a n t i m p l i c a t i o n s for m o d e l i n g t h e
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 EAR9614787 (Karlstrom) and a Kelly-Silver graduate res e a r c h fellowship f r o m t h e Caswell Silver
Foundation (Shaw). Helpful reviews by Christine
Siddoway and John Geissman greatly improved the
manuscript.
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