- UW Department of Geology and Geophysics
A four-dimensional model for the evolution of the late-Paleoproterozoic Cheyenne belt arc-continent suture is presented based on available geologic mapping, structural analysis, geophysical constraints, geobarometry, geochronology, and isotopic data. All of the data are consistent with a southeast-dipping suture and deformation that lasted from 1.78 Ga to at least 1.76 Ga, and possibly as late as 1.74 Ga. Archean crustal components and/or detritus were subducted at least 30 to 70 km south of the trace of the suture. There is considerable variation in crustal structure and tectonic evolution along strike of the Cheyenne belt. Thick-skinned, intracratonic uplift along the Laramie Peak shear zone and synorogenic emplacement of the Horse Creek anorthosite complex occurred in the east, whereas low-grade metamorphism and late cataclastic thrust faulting occurred in the west. Some of this lateral variation may reflect the influence of preexisting crustal features such as high-angle normal faults and crustal heterogeneities related to ca. 2.0-Ga rifting, whereas other differences may reflect variations in collision geometry. The lithospheric architecture of this arc-continent suture created compositional and structural anisotropies that influenced later deformation and magmatism, such as the generation and emplacement of the 1.43-Ga Laramie anorthosite complex and location of Paleozoic diamond-bearing diatremes.
In this paper, the term “Medicine Bow orogeny” is proposed to describe the ca. 1.78-Ga arc-continent collision that formed the Cheyenne belt suture, and the subsequent structural evolution of the orogenic zone, which may have continued to ca. 1.74 Ga. The orogenic belt trends from southeastern Wyoming to northeastern Nevada, a distance of ∼1900 km. This term is invoked to differentiate the tectonic history along the Cheyenne belt from both the Yavapai orogeny to the south, and Central Plains orogeny to the east, because these orogenies include younger rocks and younger deformation. The arc terrane involved in the Medicine Bow orogeny is probably 50 to 100 km wide, and it makes up the basement of northern Colorado and continues at least as far south as the Soda Creek-Fish Creek shear zone in north-central Colorado. Accretion during the Medicine Bow orogeny represents a substantial addition to the North American continent.
- Medicine Bow orogeny
- plate tectonics
- Wyoming province
- Cheyenne belt
- crustal structure
- continent-arc collision
The Cheyenne belt in southeastern Wyoming is a major crustal-scale discontinuity with Archean basement to the north and ca. 1.8 Ga and younger Proterozoic basement to the south (Fig. 1). Geologic mapping and structural analysis by R. S. Houston and coworkers of rocks exposed in the Sierra Madre and Medicine Bow Mountains (Houston et al., 1968; 1989; 1993 and references within; Karlstrom et al., 1983; Duebendorfer and Houston, 1986; 1987; 1990) have led to tectonic models for the Cheyenne belt as a southeast-dipping, late-Paleoproterozoic, continent-island arc suture (Hills and Houston, 1979; Karlstrom and Houston, 1984; Houston et al., 1989; Houston, 1993). This suture is well exposed in southeastern Wyoming and is projected to extend from eastern Wyoming to northeastern Nevada. It is one of the major structures associated with the rapid assembly and growth of the North American continent during the Proterozoic by lateral accretion and crustal processes that are analogous to modern plate tectonics (e.g., Condie, 1982; Reed et al., 1987; Hoffman, 1988).
The purpose of this paper is to present a dynamic, four-dimensional view of the tectonic evolution of the suture and resulting crustal architecture. The model is based on available geologic mapping, and structural, geochronologic, isotopic, geobarometric, and seismic-reflection data. It is argued that the crustal architecture that was formed during rifting and collision controlled the tectonic expression of younger events. The paper also predicts a possible crustal cross-section beneath the Sierra Madre, which is the focus of an upcoming geophysical transect.
Rocks exposed north of the Cheyenne belt in southeastern Wyoming include supracrustal rocks and granitic gneiss intruded by Archean and early-Paleoproterozoic granites (Fig. 1). In the Medicine Bow Mountains and Sierra Madre, this older section is overlain structurally and stratigraphically by an early-to middle-Paleoproterozoic (∼2.4 to 2.0 Ga) passive margin sequence of conglomerate, quartzite, dolomite, and schist (Karlstrom et al., 1983; Houston et al., 1992; Houston and Karlstrom, 1992; Houston and Graff, 1995). In the Laramie Mountains to the east, the Paleoproterozoic section is largely absent, and the rocks exposed north of the Cheyenne belt consist dominantly of high-grade Archean granites and granitic gneiss (Fig. 1). In all three ranges, the Archean and early- to middle-Paleoproterozoic sections are intruded by ca. 2.1-to 2.0-Ga mafic sills and dikes that have been interpreted to be related to rifting (e.g., Hills and Houston, 1979; Karlstrom and Houston, 1984). Mafic intrusions are particularly abundant in the Laramie Mountains where they occur as 20 percent or more of the exposed rock and include both large, dismembered peridotite bodies and numerous tholeiitic dikes (Snyder, 1984, 1986; Snyder et al., 1989, 1990).
Structures associated with the Cheyenne belt are exposed in the Medicine Bow Mountains and Sierra Madre as a series of mylonitic, amphibolite-facies shear zones that dip steeply to the southeast and contain subvertical stretching lineations (Duebendorfer and Houston, 1986, 1987). In the Sierra Madre the ductile shear zone has been largely overridden, and the arc-continent contact has been displaced north by a late cataclastic thrust fault (Duebendorfer and Houston, 1990). In both ranges, greenschist-facies subhorizontal lineations and subvertical foliations overprint the earlier mylonites and have been interpreted to reflect late dextral, strike-slip movement along Cheyenne-belt structures (Duebendorfer and Houston, 1986).
In the Laramie Mountains, the projected trace of the Cheyenne belt is obscured by younger, 1.43-Ga intrusions of the Laramie anorthosite complex and Sherman batholith (Fig. 1). Graff et al. (1982) and Mueller (1982) placed the Cheyenne belt through the Richeau Hills east of the Laramie Mountains based on the presence of mylonitic rocks there. This is supported by recent geochronologic work that has established a ca. 1.78-Ga terrane south of the mylonitic rocks in the Richeau Hills (Harper, 1997; Harper and Chamberlain, 1998). The southernmost exposure of Archean rocks on the western flank of the Laramie Mountains, at Morton Pass, is also consistent with this projected trace of the Cheyenne belt.
GEOCHRONOLOGICAL CONSTRAINTS ON CHEYENNE-BELT EVOLUTION AND THE MEDICINE BOW OROGENY
Available geochronologic data related to the development and tectonic evolution of the Cheyenne belt are summarized in Table 1 and briefly described below. Rifting and development of a passive margin is constrained to ca. 2.1–2.0 Ga based on U-Pb zircon ages of gabbro dikes in the Sierra Madre and Laramie Mountains (Premo and Van Schmus, 1989; Cox et al., 1995). Detailed discussions of the evolution of the basin and sedimentary successions can be found in Karlstrom et al. (1983), Karlstrom and Houston (1984), Houston et al. (1989), and Houston (1993). Early deformation along the Cheyenne belt is constrained to ca. 1.78 Ga based on the ages of pre- and syndeformation plutons from the Medicine Bow Mountains and Sierra Madre (Loucks et al., 1988; Premo and Van Schmus, 1989) and metamorphic zircon from the Richeau Hills (Harper, 1997; Harper and Chamberlain, 1998). Deformation continued to at least 1.76 Ga and had ceased by 1.74 Ga (Premo and Van Schmus, 1989). Uplift along the Laramie Peak shear zone (Chamberlain et al., 1993) and emplacement of the Horse Creek anorthosite complex in the Laramie Mountains (Fig. 1) were synchronous at 1.76 Ga (Resor et al., 1996a; Scoates and Chamberlain, 1997) and may have overlapped in age with late cataclastic thrust faulting in the Sierra Madre (Premo and Van Schmus, 1989; Duebendorfer and Houston, 1990). K-Ar hornblende and U-Pb apatite ages from the high-grade Archean block between the Laramie Peak shear zone and the trace of the Cheyenne belt in the Laramie Mountains have been interpreted to record rapid uplift of the block during Cheyenne-belt deformation (Chamberlain et al., 1993; Resor et al., 1996a; Patel et al., 1998). Reset Rb-Sr and K-Ar biotite ages (1.55 to 1.33 Ga, Table 1) from Archean rocks that are exposed in a 150-km-wide swath north of the Cheyenne belt have been interpreted to record the limit of tectonic burial of the craton during orogeny (Karlstrom and Houston, 1984) and final unroofing and cooling of the southern margin of the craton during the Mesoproterozoic (Peterman and Hildreth, 1978; Hills and Armstrong, 1974). Alternatively, these ages may simply reflect differential response of the craton to a Mesoproterozoic thermal pulse, as the northern limit of these reset ages corresponds roughly to the northern limit of high concentration of 2.0-Ga mafic dikes in the Laramie Mountains. Thus this 150-km swath of crust may represent thin, transitional crust produced by extension and mafic plutonism during 2.0-Ga rifting and was thereby more susceptible to uplift and unroofing during the Mesoproterozoic (ca. 1.5 Ga) than the thicker craton to the north.
The 1.78- to 1.74-Ga orogeny associated with the Cheyenne belt has never been formally named, and the term “Medicine Bow orogeny” is adopted in this paper. Early papers on the timing of collision limited the deformation to a broad 30 to 40 m.y. period (e.g., Premo and Van Schmus, 1989; Houston et al., 1989). However, more recent studies have identified specific events within that 40 m.y. period in addition to the formation of the Cheyenne-belt suture, and describe a protracted tectonic evolution for this orogeny (e.g., Chamberlain et al., 1993; Resor et al., 1996a; Scoates and Chamberlain, 1997). In Sims (1995) the term “Central Plains orogeny” was extended to the west to include the Cheyenne belt, and in various publications by K. E. Karlstrom, the term “Yavapai orogeny” has been extended north from Arizona (e.g., Karlstrom and Bowring, 1993), but both of these names are inadequate and misleading for several reasons. First, the orogenic belt associated with the Cheyenne belt is exposed in the Rocky Mountains, not the Central Plains. Second, it is defined by mappable units and structures that have yielded data on the style of deformation, orientation of the suture, and direct geochronological constraints on the timing of collision between the Archean Wyoming province and Proterozoic arc rocks to the south. In contrast, the Central Plains orogeny as originally defined (Sims and Peterman, 1986) referred to the suture between the southern Superior province and Proterozoic terranes presently buried by Phanerozoic cover, and was based on drill core samples and geophysical patterns. Consequently, there are few direct constraints on the kinematics of the collision in the Central Plains, orientation of the suture, or the timing of orogeny. Third, and most important, the geochronologic data from both the Yavapai orogeny and Central Plains include younger rocks and have been interpreted to suggest younger deformation [1.68 to 1.70 Ga, Karlstrom and Bowring (1993); and ≤1.70 Ga, Marvin (1988); Sims (1990), respectively] than in the Cheyenne belt (1.78 to 1.74 Ga). Because there is a definite need to differentiate between the three orogenies in reconstructing Proterozoic evolution, I propose the term Medicine Bow orogeny for the 1.78 to 1.74-Ga orogeny associated with the Cheyenne belt that sutured the Archean Wyoming province and the northern portion of the Proterozoic Colorado province of Bickford et al. (1986). As such it defines a major east-west orogenic belt that extends from the intersection with the Dakota segment of the Trans-Hudson orogen in eastern Wyoming to at least eastern Utah, and possibly northern Nevada (Lush et al., 1988; Wright and Snoke, 1993), a distance of ∼1900 km.
ISOTOPIC CONSTRAINTS ON CRUSTAL ARCHITECTURE
A southeast-dipping suture has been postulated on the basis of the dip of shear zones at the surface and lack of Proterozoic arc magmatism north of the Cheyenne belt (e.g., Hills and Houston, 1979). Pb and Nd isotopic data support this geometry and can be interpreted to suggest that older crustal material existed at depth at least 30 km and possibly as much as 70 km southeast of the surface trace of the suture.
Pb isotopic compositions of feldspar from the 1.76-Ga Horse Creek anorthosite complex in the Laramie Mountains are reported in Table 2 and displayed in Figure 2. These data plot with elevated 207Pb/204Pb relative to Pb data from feldspars from the Proterozoic Colorado province to the south (Aleinikoff et al., 1993), and on a 1.76-Ga secondary isochron that starts at a mixing line between the 1.76-Ga Pb isotopic compositions of the mantle (M) and of Archean rocks farther north in the Laramie Mountains. These Pb isotopic data require the incorporation of Pb from older evolved sources during either the generation or emplacement of the Horse Creek anorthosite complex. Although the ages of the country rocks in the immediate vicinity of the complex are unknown, the rocks are similar to ca. 1.78-Ga Proterozoic rocks to the south and the incorporation of older material into the anorthosite complex is interpreted to have occurred at depth. The anorthosite complex crops out approximately 30 km southeast of the trace of the Cheyenne belt (Fig. 1), thereby, the data are consistent with a southeast-dipping suture. The source of the older material could be either a lower plate containing Archean crust, or subducted Archean sediments.
Nd isotopic data from 1.79- to 1.76-Ga igneous rocks immediately south of the Cheyenne belt are also consistent with a southeast-dipping suture. ϵNd values from the literature calculated for 1.76 Ga are listed in Table 3 and plotted in Figure 3 as a function of distance from the Cheyenne belt trace. The data from both the Medicine Bow and Laramie Mountains have low ϵNd values near the suture and higher values away from the suture, approaching values typical of the Colorado province between 20 and 70 km away. These trends require that older crustal material was incorporated to some extent in all of these igneous rocks and that the proportion of older material decreases with distance from the Cheyenne belt, but may still have been present as far as 70 km from the suture. In the Laramie Mountains, this crustal architecture persisted to at least 1.43 Ga as Nd data from rocks of the Laramie anorthosite complex also display an increase in proportion of Archean input from south to north (Mitchell et al., 1996; Scoates and Frost, 1996).
In contrast, the Nd data from ca. 1.78-Ga samples from the Sierra Madre region do not support the simple southeast-dipping suture modeled for the Laramie and Medicine Bow mountains, but require either a nearly vertical subduction zone, later docking of the island-arc terrane, or more post-subduction modification of the suture than in the other two ranges. The data from the Sierra Madre come from the Encampment River granodiorite and volcanic rocks of the Green Mountain Formation, which crop out less than 10 km from the present Archean-Proterozoic contact and have been dated at 1779 ± 5 and 1792 ± 15 Ma, respectively (Premo and Van Schmus, 1989). The Nd data overlap the range of values typical of the Colorado province from farther south and are interpreted to suggest that little to no older material was incorporated in the sources of these rocks. They formed either prior to the subduction of Archean crust and sediments, or outboard of the zone where Archean material was being actively subducted. Duebendorfer and Houston (1990) invoked a late thrust fault to explain the cataclastic nature of the Proterozoic-Archean contact in the western Sierra Madre and the change in orientation of the Cheyenne belt from northeast–southwest in the Medicine Bow Mountains to east-west in the Sierra Madre. A late thrust fault could also explain the Nd data as it would produce more substantial horizontal translation of arc terrane rocks in the Sierra Madre portion of the orogen than farther east in the portions exposed in the Medicine Bow and Laramie mountains. Alternatively, the 1.78-Ga rocks exposed in the Sierra Madre could predate collision in this portion of the orogen and may indicate a progression from older docking in the east to younger docking in the west. At present, both interpretations are possible; the results of a proposed geophysical transect may differentiate between the two, as a late thrust fault would have an upper to mid-crustal décollement that could be imaged seismically.
SEISMIC-REFLECTION CONSTRAINTS ON CRUSTAL ARCHITECTURE
Available geophysical data are presented in more detail elsewhere (Smithson and Boyd, this issue). However, the results of several seismic reflection studies are particularly pertinent to the reconstructions in this article and bear summarizing here. A COCORP study (Allmendinger et al., 1982) covered a 20-km swath across the Laramie anorthosite complex in the central Laramie Mountains, south of the exposures of Archean rocks. In this study, a reflector that dips southeast at about 55° was interpreted as the Archean-Proterozoic suture. This interpretation is consistent with the exposed geology, as projection of the plane to the surface places the trace of the Cheyenne belt slightly south of the southernmost outcrop of Archean rocks at Morton Pass. Other significant conclusions from the COCORP study are that the Laramie anorthosite complex is currently 4 km thick, consistent with earlier reflection data (Smithson et al., 1977) and gravity studies (Hodge et al., 1973), and that Moho is presently at 48 km depth in this portion of the Laramie Mountains. Farther south, a survey across the Horse Creek anorthosite complex concluded that this southern complex is also approximately 4 km thick (Speece et al., 1994). A reflection profile from the Medicine Bow Mountains south of the craton margin was interpreted to indicate that the Archean-Proterozoic suture dips 60° to the southeast (Templeton and Smithson, 1994). Two of the more dominant reflectors in this survey project to exposed shear zones within the Cheyenne-belt deformation zone.
GEOBAROMETRIC CONSTRAINTS ON PRESENT LEVEL OF EXPOSURE
Available geobarometric determinations are presented in Figure 4 along with the ages to which the pressures apply, if known. These data establish the present depth of exposure, as well as place limits on the metamorphic and unroofing histories of southeastern Wyoming.
In the northern Laramie Mountains, north of the Laramie Peak shear zone, the presence of andalusite in Archean metapelitic rocks is interpreted to imply that metamorphic pressures were less than 4 kbar (Snyder 1993; Patel, 1992; Chamberlain et al., 1993; Patel et al., 1998). Although the timing of this metamorphism has not been dated directly, Patel et al. (1998) argue that 4 kbar was a maximum pressure during the Proterozoic in this location.
Farther south, within the block between the Laramie Peak shear zone and the trace of the Cheyenne belt, higher grade rocks are exposed, including garnet amphibolites and kyanite-bearing metapelitic rocks (Snyder et al., 1995; Patel, 1992). Quantitative geothermobarometry, based on the TWQ program of Berman (1991), and textural evidence for decompression have been interpreted to indicate peak pressures in excess of 8 kbar with nearly isothermal decompression to 2.5 kbar (Patel, 1992; Patel et al., 1998). High-grade metamorphism due to thrust loading may have begun as early as 1.78 Ga based on metamorphic zircon growth in the Richeau Hills (Harper, 1997; Harper and Chamberlain, 1998) and regional constraints on deformation from the Medicine Bow Mountains and Sierra Madre (Table 1). Decompression and unroofing have been tightly constrained to the period from ca. 1.76–1.74 Ga based on U-Pb ages of syndeformational sphene growth (Resor et al., 1996a) and U-Pb cooling ages of apatite (Chamberlain et al., 1993; Patel et al., 1998). Uplift of the high-grade block was accommodated by south-side up reverse motion along the Laramie Peak shear zone (Resor et al., 1996a,b) and a vertical displacement of at least 10 km based on the difference in pressures exposed across the shear zone (Chamberlain et al., 1993; Patel et al., 1998).
Continuing south in the Laramie Mountains, several geobarometric estimates have been determined that are related to the emplacement of the Laramie anorthosite complex at 1.43 Ga (Scoates and Chamberlain, 1995). Contact metamorphic and magmatic assemblages of the satellite plutons of the Laramie anorthosite complex have been interpreted to indicate pressures of 2.5 to 3 kbar in the north (Furman et al., 1988; Grant and Frost, 1990) and slightly higher pressures of 4 kbar in the south (Kolker and Lindsley, 1989). The similarities in decompression pressures at 1.74 Ga in the central Laramie Mountains and the pressures recorded during emplacement of the Laramie anorthosite complex suggest that there was little to no unroofing for at least 300 m.y. after the Medicine Bow orogeny and that isostatically stable crust was produced during continent-arc suturing, as also seems to be true to the south (Bowring and Karlstrom, 1990; Williams and Karlstrom, 1996).
From the Medicine Bow Mountains and Sierra Madre there is only one geobarometric estimate. Metamorphic assemblages exposed along the northern shear zone of the Cheyenne belt in the Medicine Bow Mountains have been interpreted to indicate peak pressures of 3.5–4 kbar during the Medicine Bow orogeny, ca. 1.76 Ga (Duebendorfer 1988). Farther north, the early to middle-Paleoproterozoic supracrustal rocks have been metamorphosed only to greenschist facies (Duebendorfer and Houston, 1986, 1987), so it is unlikely that they were buried very deeply ca. 1.76 Ga. In the Sierra Madre, early to middle-Paleoproterozoic supracrustal rocks north of the Cheyenne belt have been metamorphosed to only lower-greenschist facies (Graff, 1978), so there may be a trend in the depth of exposure along the Cheyenne belt, with deeper rocks to the east and shallower rocks to the west.
FOUR-DIMENSIONAL RECONSTRUCTION OF THE CHEYENNE-BELT CONTINENT-ARC SUTURE
Syntheses of the constraints described above are presented in Figures 5 and 6 to demonstrate schematically the crustal architecture produced by continent-arc collision during the Medicine Bow orogeny. Cross-section lines are indicated on Figure 1; horizontal and vertical axes are at the same scale. The details of the architecture may change as more knowledge is attained; however, these diagrams present all of the constraints that are well known and which must be included in any viable reconstructions.
Figure 5 illustrates two stages in the tectonic evolution of the Laramie Mountains ca. 1.78–1.76 Ga. The solid line is the present-day exposure. In the upper diagram (5A), Archean basement of the Wyoming province is shown to be buried tectonically by thrust stacks, producing Barrovian-style metamorphism in the block presently exposed between the Laramie Peak shear zone and the trace of the Cheyenne belt. The thickness of the thrust stack is unknown as none of this material is presently preserved; however, the combined thickness of thrust stack plus Archean crust over the presentday exposures must have been 10–15 km, north of the Laramie Peak shear zone (thick enough to reset biotite ages, but 4 kbar or less), and at least 24 to 27 km thick south of the shear zone (based on geobarometry). The thickness of Archean basement is based on an assumed crustal thickness of 35 km prior to collision. This value is comparable to thicknesses of present-day rifts and extended crust (Christensen and Mooney, 1995) and is consistent with the shallower depth to present-day Moho in the southern Wyoming province relative to the center of the province (Smithson and Boyd, this issue; Keller et al., this issue; Snelson et al., this issue).
The southern Wyoming province margin is interpreted to be thin, transitional crust produced during rifting in the middle Paleoproterozoic based on abundant mafic dikes and plutons ca. 2.1 to 2.0 Ga in age. Although no attempt has been made in Figures 1 or 5 to illustrate accurately the abundance of mafic dikes, there are a thousand or more dikes exposed along a 150-km transect between the northern Laramie Mountains and the trace of the Cheyenne belt. They have average widths of 10–30 m, represent 15–20 percent of the basement, and produce a strong crustal anisotropy [cf. detailed quadrangle maps of Snyder (1984), (1986), (1992), (1993), and Snyder et al. (1995) and (1997)]. Normal faults, typical of rifting, have not yet been identified, but their existence may be inferred from orientation of younger, high-angle, convergent, high-strain zones, such as the Laramie Peak shear zone, which may have reactivated high-angle structures.
Figure 5B depicts uplift of the basement along the Laramie Peak shear zone, ca. 1.76 Ga, to juxtapose different crustal levels in the present-day exposures. Stretching lineations that plunge about 70° to the southwest (Resor, 1996) indicate that there is a sinistral, transpressive component to the shear zone, although the dominant motion is vertical (minimum of 10 km vertical uplift and 3 km horizontal). There is significant internal strain within the block south of the Laramie Peak shear zone, so bulk shortening and thickening may have occurred in addition to block uplift (Resor et al., 1996b). The slight differences in peak metamorphic pressures from north to south within the block (Patel et al., 1998) may reflect either some of this internal deformation, or a slight rotation during uplift, as shown. In general, however, the overall consistency in peak pressures and final decompression pressures throughout the block is interpreted to indicate uplift of a nearly horizontal block along a crustal-scale structure with offset of the crust-mantle boundary and a conjugate north-dipping high-angle shear zone to accommodate bulk shortening (Chamberlain et al., 1993). This geometry could account for the Bouguer gravity high that exists over the high-grade block (Johnson et al., 1984), and the apparent correlations of steep gravity gradients with the Laramie Peak shear zone (Chamberlain et al., 1993) and the trace of the Cheyenne belt (Johnson et al., 1984). A step in the Moho near the Cheyenne belt has been proposed to model gravity and refraction data from the Laramie Mountains (Johnson et al., 1984; Smithson and Boyd, this issue). The north-dipping fault, which is proposed here as a conjugate to the Laramie Peak shear zone, has not yet been mapped in Precambrian rocks; however, the Laramide (Cretaceous) Wheatland-Whalen fault system is a possible candidate. The Wheatland-Whalen fault system is subparallel to the Cheyenne belt, dips steeply to the north (66–73°), has been interpreted as a system of planar rather than listric faults (Blackstone, 1996) and may reflect reactivation of a Proterozoic feature. The Wheatland-Whalen fault was also active in the Tertiary and is one of the only faults in southern Wyoming with post-Laramide movement, further strengthening the interpretation that it is a major crustal structure. An alternative model for uplift along the Laramie Peak shear zone involving a listric thrust system that soles in the lower crust or at the Moho would not require a conjugate north-dipping fault. It would, however, result in more rotation of the upper plate than is observed based on the available geobarometric and gravity data (Chamberlain et al., 1993) and therefore it is not favored here. Unroofing followed uplift fairly rapidly, as the present-day level of exposure cooled from ∼650 ° C to less than 450 ° C in 20 m.y., based on U-Pb sphene and apatite ages (Table 1).
South of the trace of the Cheyenne belt in Figure 5B, seismic-reflection data are interpreted to constrain the dip of the suture to 55° to the southeast (34° apparent dip in this north-south cross-section) and depth to present-day Moho of 48 km. Overburden estimate of 13 km, at 1.76 Ga, above the present-day exposures, may be a minimum as it is based on barometry of the 1.43-Ga Laramie anorthosite complex. However, the Laramie anorthosite complex and the older 1.76-Ga Horse Creek anorthosite complex were emplaced into the same crustal level, and it is assumed for this reconstruction that there was no significant uplift during the 300 m.y. between them. If the present-day Moho was formed during the Medicine Bow orogeny, then the crust would have been at least 60 km thick. Conversely, the present-day depth to Moho could represent subsequent underplating, as wide-angle seismic data have been interpreted to indicate interlayered mafic and intermediate rocks in the lower crust (Gohl and Smithson, 1994; Smithson and Boyd, this issue). If the present-day lower crust was formed during 1.43-Ga anorthositic and granitic magmatism (Frost and Frost, 1997) or by even younger processes, then the crustal thickness at 1.76 Ga could have been somewhat less than the 60 km shown in Figure 5B.
A zone of transtension is indicated in Figure 5B, south of the trace of the Cheyenne belt, to account for the generation and emplacement of the Horse Creek anorthosite complex at 1.76 Ga (Chamberlain et al., 1994; Scoates and Chamberlain, 1997). The petrogenesis of anorthositic magmas involves the ponding of mafic melts at 10–12 kbar, fractionation, flotation of intermediate composition plagioclase, and ascent of diapirs with varying crystal-liquid proportions (Ashwal 1993; Emslie et al., 1994; Mitchell et al., 1995; Scoates and Frost, 1996). An extensional tectonic setting is commonly invoked to accommodate the petrogenetic and emplacement characteristics of anorthosite complexes (e.g., Emslie, 1978) even during overall convergent tectonism (Corrigan and Hanmer, 1997). The apparent synchroneity, at about 1.76 Ga, of emplacement of the Horse Creek anorthosite complex and uplift along the Laramie Peak shear zone suggests contrasting stress regimes in the middle crust within the orogen. These two events could represent either progressive change in stress over as much as 10 m.y. or simultaneous zones of transtension and transpression, perhaps produced by late-stage structural readjustments within the orogenic belt (e.g., Scoates and Chamberlain, 1997).
Medicine Bow Mountains
Reconstructions of the crustal architecture of the Medicine Bow Mountains and Sierra Madre regions are shown in Figure 6. Although the surface geology has been well mapped, there are very few constraints on the subsurface geology other than the isotopic data summarized above, and one seismic reflection transect in the Medicine Bow Mountains. Consequently, the cross-sections in Figure 6 are composite drawings that include present-day as well as Proterozoic features, rather than specific age sections as were developed for the Laramie Mountains.
In the Medicine Bow Mountains, the Cheyenne belt is approximately 7 km wide with four distinct shear zones, each 100–200 m wide, that separate disparate tectonostratigraphic blocks. Two of these shear zones were imaged seismically (Templeton and Smithson, 1994) dipping 60° to the southeast. The seismic record includes a weak reflector below 35 km that may be related to underplating, as shown in Figure 6, but no clear Moho was imaged, and the crustal thickness indicated is taken from the Laramie Mountain analysis described above. The overburden thickness of 15 km is based on metamorphic pressures produced during thrusting at 1.76 Ga. Uplift and unroofing may have preceded underplating, so it is unlikely that the crust was ever as thick as shown in Figure 6.
The contacts within the early- to middle-Paleoproterozoic margin sequence, north of the Cheyenne belt, are vertical at the surface and are depicted to sole listrically into contact with underlying gneiss. Neither the thickness of these supracrustal rocks nor their relationship with the Archean gneiss is well known, so this portion of the cross-section is poorly constrained. The contacts have been modeled to reflect thrust faults that telescoped the stratigraphic section and were rotated to high angle by continued convergence during the Medicine Bow orogeny (Karlstrom et al., 1983; Karlstrom and Houston, 1984; Houston, 1993). Yet at least some of the contacts may have formed as normal faults during rifting, since they are intruded by mafic sills, and breccia zones within them contain 2.0-Ga metamorphic sphene (Harper, 1997). It is unclear whether there was significant horizontal translation of these sediments during the Medicine Bow orogeny or whether the contact with the Archean gneiss is simply a faulted unconformity with little displacement. In the few places where the contact is exposed, it is a moderately dipping fault, only 1.2 km below the highest exposure of early- to middle-Paleoproterozoic sedimentary rocks, so the thickness of the upper plate may be much less than shown.
The cross-section of the Sierra Madre is less well-constrained than those of either the Laramie or Medicine Bow mountains, so many of the features shown in Figure 6 are taken from the other two cross-sections. There are no direct constraints on either the dip of the suture or the thickness of the crust. The Green Mountain block is depicted as overriding the ductile arc-continent suture after the model of Duebendorfer and Houston (1990). This is compatible with the Nd data discussed above and the cataclastic nature of the contact between continental and island-arc rocks. The timing of this thrust is unknown; it could be late in the evolution of the Medicine Bow orogeny, ca. 1.76 Ga, or as young as 1.43 Ga. The depth to the décollement is unconstrained and the location of the arc-continent suture is based on projection of the trend from the Laramie and Medicine Bow mountains.
There appears to be significant lateral variation in the crustal architecture that formed during the Medicine Bow orogeny. The Precambrian rocks exposed in the Laramie Mountains, Medicine Bow Mountains, and Sierra Madre indicate distinctive crustal sections along strike of the Cheyenne belt. In each range, there is abundant evidence for a southeast-dipping suture between the middle-Paleoproterozoic (ca. 2.0 Ga) rifted margin of the Archean Wyoming province and ca. 1.8-Ga island-arc terranes of the Colorado province, but the similarities end there. In the Laramie Mountains, thick-skinned, synorogenic deformation is documented by ca. 1.76-Ga shortening and intracratonic uplift along the Laramie Peak shear zone. The generation and emplacement of the ca. 1.76-Ga Horse Creek anorthosite complex is interpreted to indicate a local zone of transtension within the orogen, and the later obliteration of the Cheyenne belt suture by emplacement of the 1.43-Ga Laramie anorthosite complex may reflect a unique crustal geometry produced in the Laramie Mountains during the Medicine Bow orogeny. In the Medicine Bow Mountains, remnants of an early- to middle-Paleoproterozoic (ca. 2.4 to 2.0 Ga) passive-margin sequence are still preserved. In this range, deformation of the margin appears to be thin-skinned, with no evidence yet for thick-skinned, synorogenic, 1.78 to 1.74-Ga structures. The Paleoproterozoic history of the Sierra Madre is dominated by the lack of Archean input to the late-Paleoproterozoic rocks within and immediately south of the Cheyenne belt. This observation suggests either a steeper suture in the Sierra Madre, or more horizontal translation of the island-arc rocks in this range. The late cataclastic thrust fault, which brought the Green Mountain magmatic block north, is unique to the Sierra Madre and may represent either such a horizontal translation late in the Medicine Bow orogeny or younger modification of the suture. Some of the variation between the three ranges may reflect the influence of preexisting crustal features such as high-angle normal faults and crustal anisotrophies related to ca. 2.0-Ga rifting, whereas other differences may reflect variations in collision geometry.
One example of the possible influence of older structures on subsequent tectonic evolution is the occurrence of the Horse Creek and Laramie anorthosite complexes, 300 m.y. different in age, in the same tectonic region. Given that anorthositic rocks imply a set of rare, and perhaps still enigmatic, conditions for their generation and emplacement, the occurrence of two, temporally distinct complexes implies a geometrical influence of the crustal structure that can be reactivated multiple times and may not exist in the other two ranges. The zone of local transtension, invoked in this paper for the 1.76-Ga Horse Creek anorthosite complex, may represent either renewed extension of 2.0-Ga rift-related structures in the lower subducted crust, or an internal strike-slip jostling of the blocks within the Cheyenne belt shear zone, or both. The splay of shear zones in the Medicine Bow Mountains may continue into the Laramie Mountains, and there may be numerous slip planes that could be reactivated during late stages of deformation. At 1.43 Ga, the same structural geometry probably came into play again, leading to the generation and emplacement of the Laramie anorthosite complex.
Aspects of the Proterozoic crustal architecture presented here may help to explain the occurrence of the State Line diamond district that straddles the Wyoming-Colorado border. Diamond-bearing kimberlites are largely restricted to Archean crust and this occurrence in late-Paleoproterozoic juvenile crust is somewhat anomalous (Clifford, 1966). The generation of diamonds requires high pressures, corresponding to depths of 150 km or more, and relatively cool temperatures, less than 1200 ° C (Haggerty, 1986). The correlation of diamondiferous kimberlites with Archean lithosphere has been interpreted to imply that either these conditions were more prevalent in the Archean, or the survival of diamondiferous mantle roots is more likely beneath Archean terranes (e.g., Helmstaedt and Gurney, 1995). Projection of the plane of the Cheyenne belt suture into the mantle places Archean mantle lithosphere beneath the State Line district, so it is possible that the diamonds are Archean, if this mantle lithosphere was cold and thick enough. Eggler et al. (1988) and Shaver (1988) invoked this geometry to explain the State Line district and suggested an Archean origin for the diamonds, although the compositions of associated xenoliths are more typical of Proterozoic lithosphere (Coopersmith and Schultze, 1996). One problem with an Archean origin for the diamonds in the State Line district is that at least a fragment of the diamondiferous mantle root would have to survive ca. 2.0-Ga rifting and 1.78-Ga collision, processes that are interpreted by Helmstaedt and Gurney (1994) to be mantle-root destructors. As discussed here, the southern margin of the Archean Wyoming province was thinned during 2.0-Ga rifting and the lithosphere that was subducted ca. 1.78 Ga was transitional to oceanic in composition (Fig. 5a), so it is unlikely that any cold, deep, Archean mantle roots would survive south of the margin of the craton. For these reasons, it seems unlikely that the diamonds are Archean in age. However, conditions may have been appropriate for the formation of diamonds during the late-Paleoproterozoic Medicine Bow orogeny. One model for the formation of cool, depleted lithospheric root involves low-angle sub-duction and imbrication of oceanic lithosphere (Helmstaedt and Schulze, 1989) to bring eclogitized oceanic crust into the diamond stability field (Helmstaedt and Gurney, 1995). This mechanism could have operated during the Medicine Bow orogeny, ca. 1.78 Ga, and produced diamonds in the subducted remnants of middle-Paleoproterozoic (ca. 2.0 Ga) oceanic lithosphere. Imbrication of the subducted oceanic lithosphere beneath Colorado may have only occurred near the buoyant Wyoming craton, during the final stages of closure, leading to a concentration of diamondiferous kimberlites 100–200 km southeast of the Cheyenne belt trace, but none farther south.
A final observation, pertinent to the development of the Proterozoic lithosphere of the western United States, is that the arc terrane that docked to the southern Wyoming province during the Medicine Bow orogeny was at least 50 to 100 km wide and was distinct from the terranes that make up the Yavapai province exposed in Arizona. The basement of northern Colorado includes ca. 1.77-Ga granites and gabbros (Pallister and Aleinikoff, 1987) similar in age to those immediately south of the Cheyenne belt in southern Wyoming. This terrane has been named the Green Mountain magmatic arc by Condie (1982). The southern boundary of this block may be the Soda Creek-Fish Creek shear zone in north-central Colorado ∼60 km south of the Cheyenne belt (Snyder 1980a, b, c; Foster et al., next issue), although other shear zones farther south in Colorado are also candidates. Rocks of similar age occur in the Salida-Gunnison area ∼300 km south of the Cheyenne belt (Bickford et al., 1989; Reed et al., 1987) but are absent in the type region of the Yavapai province in central Arizona (Karlstrom and Bowring, 1988; Bowring et al., 1991; Karlstrom and Bowring, 1993). A further discriminator between the Green Mountain magmatic arc and Yavapai province is that the Green Mountain terrane had docked, and deformation had ceased along the Cheyenne belt by 1.74 Ga, well before the majority of the rocks in the Yavapai province had formed (e.g., Jones et al., 1996). The trend in the literature to lump the basement of northern Colorado into the Yavapai province is misleading, because it misrepresents the rich geologic history exposed along the Cheyenne belt.
Based on available geophysical, geologic, isotopic, and structural data, the Cheyenne belt in southeastern Wyoming is modeled as a continent-island arc suture that dips to the southeast at about 60°, was produced during the Medicine Bow orogeny, ca. 1.78–1.74 Ga, and involved the subduction of Archean crustal components and/or detritus, at least 30 to 70 km south of the trace of the suture. The arc terrane involved in the collision is interpreted to be at least 50 km wide and is distinctly older than Yavapai terranes exposed in Arizona. The southern margin of the Wyoming province is interpreted to be thinned, transitional lithosphere, injected by mafic magmas during rifting ca. 2.0 Ga. High-angle, crustal-scale features such as dikes and normal faults produced during rifting may have played an important role in evolution of the orogenic belt, facilitating the development of the Laramie Peak shear zone and the generation and emplacement of the Horse Creek anorthosite complex in the Laramie Mountains. The considerable lateral variation in the lithospheric architecture that formed during the Medicine Bow orogeny probably reflects a combination of preexisting crustal features and variations in collision geometry. The structural geometry of this rift/suture created compositional and structural anisotrophies that influenced later deformation and magmatism, such as the generation and emplacement of the 1.43-Ga Laramie anorthosite complex and the location of Paleozoic diamond-bearing diatremes.
The ideas expressed in this paper are the sole responsibility of the author, although they matured through discussions with many people, including: Robert Houston, James Scoates, Phil Resor, Bob Bauer, Art Snoke, Ron Frost, Carol Frost, and Kathleen Harper. Salary support for the author during the development of these ideas came from EAR-9205825, EAR-9418586, EAR-9628331, and EAR-9706296. The manuscript benefited from reviews by Ernie Duebendorfer, Karl Karlstrom, Randy Van Schmus, and Carol Frost.
- Received October 11, 1997.
- Revision received March 23, 1998.
- Accepted April 19, 1998.