BELT, - University of New Mexicoeps et al...shear zone, but the seismic lines do not extend across the belt, so direct correlation is not possible. Other events on this data set are

GRAVITY PROFILES ACROSS THE CHEYENNE BELT, A PRECAMBRIAN CRUSTAL SUTURE IN SOUTHEASTERN WYOMING

R. A. JOHNSON', K. E. KARLSTROM', S. B. SMITHSON', AND R. S. HOUSTON1

' Program for Crustal Studies. Departmen1 of Geology a ~ d Geophysics, University of Wyoming, Laramie, WY 82071, U.S.A.

Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, U S A .

(Accepted August 24, 1984)

ABSTRACT

Johnson, R. A., Karlstrom, K. E.. Smithson, S. B. and Houston, R. S., 1984. Gravity profiles across the Cheyenne Belt, a Precambrian crustal suture in southeastern Wyoming. Journal of Geodvnamics, 1: 445472.

Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to inter- pretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50-100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48-54 km) in Colorado than in Wyoming (37-41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm3. Differences in crustal thickness may have originated 1700 Ma ago because: I ) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with l0dal Laramide uplifts and sub-perpendicular to regional long- wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity "signature" of the Cheyenne Belt is different fram "S"-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which, suggests fundamental differences between continent-continent and island arc-condnent collisional processes.

INTRODUCTION

Studies of Archaean and early Proterozoic rocks in southern Wyoming have defined a regional geologic, geochronologic and geophysical discontinuity, a probable Precambrian suture, which coincides with a major shear zone (Fig. 1). This boundary forms the southern margin of the Archaean

0264-3707/84/%3.00 F 1984 Geophysical Press Ltd.

t. PUBLISHED REFLECTlOW PRQFILES I

Wyoming Province and is called the Cheyenne Bdt $ w d n et al, 1979). Geologic models (Hills and Houston, 1979; Karlstr~m, gt & w)b suggest that rocks adjacent to the Cheyenne Belt r~ca&,My Pg@jjqwoi~ pifting (23062000 Ma ago), followed by"i$apd.: ~ ~ ~ t q g k d .qqwaesis (I 8dGVXK.lMa ago). %I.&, '&f @?rerlpo~~$~& p&bably a Itt.r~termoic sahld ~epatating the 'Xkhapfh Wj( ' ~ , , w a t o n to the north f m maomePed , RoWzuic '&%&l i r d ' tb$&dv-~ pa 'sbuth (Hills and Houstan, 1979; K a f W W ~ e t dl:, 1381; %'ar&tioni d al., 1983; Karlstrom qnd Houston, 1984). . .

This p q k t ~ l & ' ~ < , ~ i ~ v i t y signature of the ~ h ~ ~ G ' ~ d t i0.m effort to understand jtke f&&e-w&sibnal structure of a ~recamkxiaq gp@a&s@re. We will show that the ~ h e y h n e Belt separates crustal' section8 of diBrent

!#q;%M;;: ::,.$...:.;.;-...'L;-:;a; - ., :y . , - . . - 7 . . . .

? . ? . ' .

- . GRAVITY PROFILES

rocks in the southern terrains were derived from the mantle and accreted to North America 1800-1600 Ma ago (De Paolo, 198 1; Condie, 1982; Stacey and Hedlund, 1983). i -: Proterozoic terrains south of the shear zone include a highly deformed k- ainphibolite-grade volcanogenic gneiss complex containing deformed pillow basalts and subaerial tuffs and agglomerates of probable island arc origin. A variety of syntectonic intrusive rocks (1800-1600 Ma old) invade the volcanogenic gneisses, and these are also believed to be island arc-related. Post-kinematic intrusions of granite and anorthosite-syenite are the youngest Precambrian rocks in the area and are about 1400 Ma old (Hills and Houston, 1979).

Juxtaposition of the two contrasting geologic and geochronologic terrains across the shear zone probably took place by thrusting of the southern terrain (island arc) over the northern terrain (craton), possibly as a consequence of south-dipping subduction. The main collision took place probably in the interval 1800-1 600 Ma ago prior to the intrusion of 1650 Ma old granites which crosscut the shear zone.

The Precambrian geologic record in the Laramie Mountains is less complete. North of the inferred position of the Cheyenne Belt are an Archaean quartzo-feldspathic gneiss terrain and the Elmers Rock Greenstone Belt. Although age relationships between the two are not well known, the Elmers Rock Greenstone Belt may, in part, be correlative with supracrustal rocks below the Phantom Lake Metamorphic Suite to the west. There are no preserved remnants of the Snowy Pass Supergroup in the Laramie Mountains.

More detailed discussions of the Precambrian geologic setting in southeastern Wyoming can be found in Houston et al., (1968), Houston and Karlstrom (1980), Karlstrom et al., (1981), Graff et al., (1982), and Snyder (1 980).

CONSTRAINTS ON CRUSTAL STRUCTURE FROM SEISMIC DATA

.r - & Seismic information in southern Wyoming and adjacent Colorado (Fig. 1)

is limited essentially to 1) U.S. Geological Survey refraction studies of crust and upper mantle structure conducted in the early 1960's (Pakiser, 1963; Jackson et al., 1963; Jackson and Pakiser, 1965; Roller and Jackson, 1966; Wilden, 1965; Prodehl, 1970; Braile et al., 1974; Prodehl and Pakiser; 1980); 2) University of Wyoming seismic reflection studies of the Laramie Anorthosite-Syenite Complex conducted in 1974 and 1975 (Smithson et al., 1977; Taylor, 1978; Iltis, 1983); 3) Consortium for Continental Reflection Profiling (COCORP) seismic reflection studies across the Laramie Mountains conducted in 1979 (Brewer et al., 1982; Allmendinger et al., 1982).

I, q, U.S. Geological Survey refraction data were recorded along unreversed ' C .-

450 JOHNSON, KARLSTROM, SMITHSON AND HOUSTON

profiles at recording stations too widely spaced to prevent spatial aliasing of later arriving phases. This wide spacing of stations precludes reliable interpretation of other than direct-arriving phases and limits detailed interpretation of the crust which is both laterally and vertically heterogeneous p ix ; 1965; Smithson et al., 1977). U.S.G.S. refraction results suggest that crustal thickness in the southern part of the Wyoming Province (Fig. 1) is about 37-41 km whereas the crust thickens to 48-54 km beneath the Front Range of Colora'do. Jaekson and Pakiser (1965) suggested that an abrupt northward thinning of the crust occtrr"s east of the Uinta Mountains which approximately corresponds to the position of the Cheyenne Belt and the southern boundary of the Wyoming Prwince.

Mean crustal velocities (based on crustal velocity irrterpretations of Jackson et al., 1963; Jackson and Pakiser, 1965; Wilden, 1965; and Prodehl and Pakiser, 1980) are not significantly different north and south of the Cheyenne Belt. However, due to the lack of detail of the refraction data and uncertain relationship between P-wave velocity and density (Ludwig et al., 1970), significant density differences across the boundary, although not indicated by existing data, are possible.

University of Wyoming reflection data, obtained using a dynamite source which provided higher frequencies than COCORP vibroseis techniques (Taylor, 1978; Schilt et al., 1979; Iltis, 1983), exhibit better resolution of crustal structure although the survey was of limited areal extent. Reflections from the base of the Laramie Anorthosite-Syenite Complex indicate a thickness of 4-5 km. Deeper events may be mflections from the Cheyenne Belt shear zone, but the seismic lines do not extend across the belt, so direct correlation is not possible. Other events on this data set are interpreted to be from the base of the Sherman Granite below the anorthosite body and upper and middle crust intrusions or structural variations (Smithson et al., 1977).

COCORP reflection profiles from the Laramie Mountains exhibit a multicyclic event from south of the Cheyenne Belt at a depth of about 48 km. This event has been interpreted by Allmendinger et al. (1982) to be a Moho reflection since it occurs at the same depth as the Moho in northern Colorado (based on refraction kterpretations of Jackson et al., 1965 and Prodehl and Pakiser, 1980). Discontinuous events at approximately 27-39 km depth on COCORP Lines 4, 5 and 6 are also interpreted to be Moho reflections, and a 10 km difference in crustal thickness a m 5 s the shear zone is suggested (Allmendinger et al., 1982). If this interpretation is correct, the north-to-south increase in crustal thickness occurs within 20 km of the Cheyenne Belt. Assuming reasonable densities for lower crust and upper mantle, an abrupt Moho step of this magnitude would cause a larger, steeper gravity gradient than is observed. Our gravity madels suggest that the change in crustal thickness probably takes place more gradually in the Laramie Mountains.

GRAVITY PROFILES 45 1

Furthermore, flat-lying events interpreted to be Moho reflections on the COCORP data are observable only below sedimentary rocks of the Denver- Julesberg and Laramie Basins, and similar events occur in the data above and below these "Moho reflections." This suggests that the events result from multiple reflections or out-of-the-plane seismic energy rather than reflections from the Moho.

COCORP reflection data may provide three-dimensional information on the zone of contact between Archaean crustal rocks of the Wyoming Province and Proterozoic crustal rocks in Colorado. Allmendinger et a1 (1982) interpreted events on the Laramie Mountains seismic data to be reflections from the Cheyenne Belt. These events reportedly have a true dip of about 55O SE. The attitude of the Cheyenne Belt shear zone, based on surface geology, is nearly vertical to steeply southeastdipping, but the dip may indeed become less steep with depth. The base of the Laramie Anorthosite is interpreted to dip gently southeastward at a depth of 1.5-4 km (Allmendinger et al., 1982). A depth of about 4 km is consistent with University of Wyoming seismic data.

GRAVITY DATA

The Bouguer gravity map of southeastern Wyoming (Fig. 3) was compiled from data from several sources: the University of Wyoming (UW), the Department of Defense (DOD) Mapping Agency, and the Bouguer Gravity Map of Colorado (Behrendt and Bajwa, 1974). UW and DOD data for the State of Wyoming have been adjusted to the International Gravity Standar- dization Net of 197 1 (International Association of Geodesy, 197 1). Bouguer anomaly values from Behrendt and Bajwa (1974) were adjusted by -1 5 mgal to approximately tie to the UW and DOD data.

Wyoming gravity measurements were reduced to Bouguer anomaly values using a theoretical gravity determination based on the Geodetic Reference System formula of 1967 (International Association of Geodesy, 197 1) and a Bouguer slab density of 2.67 g/cm3. Terrain corrections were not made to the Wyoming data set because we are concerned mainly with large gravity variations, and terrain effects in southern Wyoming are generally less than a few milligals except near mountain fronts. A Bouguer reductipn density of 2.67 g/cm3 was used on the published Colorado gravity map, and computer- generated terrain corrections were made to a radius of 166.7 kilometres.

The Bouguer gravity map of the Cheyenne Belt region (Fig. 3) exhibits anomaly values which range from about -100 to -3lOmgal; the more

I negative values generally correspond to sedimentary basins or large areas of outcropping granite. A noticeable correlation exists between relatively high

I I

gravity values and Precambrian-cored, Laramide uplifts (stippled pattern in

JOHNSON, KARLSTROM, SMITHSON AND HOUSTON

Fig. 3. Gravitp map of southeastern Wyoming (same area as Fig. 2). Precambrian uplifts are stippled. A - A ', B - Bf, and C - C are locations &f gravity profiles.

GRAVITY PROFILES 453

Figure 3). Localized high-frequency gravity highs and lows within the Precam- brian uplifts are associated with outcropping mafic and felsic bodies.

The most important feature of the gravity data shown in Figure 3, and one which is not explained by high-frequency anomalies associated with near surface bodies, is the strong, southward gradient within the Precambrian-cored uplifts. This gradient is particularly noticeable in the Laramie Moantains- Front Range where values vary from -150 mgal in the north to -250 mgal in the south. A similar gradient is also seen in the Sierra Madre-North Park Range areas. Although probably influenced by Laramide structure and localized density changes, this regional decrease in gravity is on the order of 50-100 mgal. Such a large, low-frequency variation in gravity (over hundreds of kilometres) must, at least in part, reflect deep crustal structure, a principal topic of this study.

Gravity Profiles and Modelling Considerations . .

In order to reduce near-surface Lararnide structural effekts as much as possible, gravity profiles and geologic cross sections have been constructed across the Cheyenne Belt approximately along the axes of the Precambrian- cored ranges of southern Wyoming and northern Colorado (Fig. 3). These gravity profiles, which are roughly perpendicular to the trend of the Cheyenne Belt, are much less affected by changes in sedimentary rock thickness and Laramide structure in the basins. The geologic cross sections have been used to model the gravity profiles (Figs. 4 to 7) and have, in turn, been modified to fit the constraints of the gravity data. A two-dimensional gravity modeling program similar to that discussed by Talwani et al. (1959) was used. Although two-dimensional gravity modelling is inadequate to represent accurately the three-dimensional nature of outcropping bodies along the profiles, deep crustal structure associated with the Cheyenne Belt is believed to be roughly two-dimensional. Approximating near-surface bodies in two- dimensions is justified only to indicate that these bodies contribute a high- frequency addition to the much lower frequency gradient associated with deep crustal structure that is of prime interest to this study.

Upper-crustal density, used as the background value for determining anomalous density, was assumed to be 2.70 g/cm3; this estimate is based on reasonable values for the Archaean gneisses exposed in the Laramie Mountains (Hodge et al., 1973) and mqy be consistent with the observation that mafic and felsic lithologies occur in sub-equal abundance in terrains south of the shear zone (Fig. 2). The area is heterogeneous and more precise estimates of surface density are not available. Lower-crust and mantle densities were assumed to be 2.95 and 3.30 g/cm3 respectively.

Isostatic effects should be considered when modelling crustal sections.


I

CHEYENNE BELT

DISTANCE FROM CHEYENNE BELT ( km 1

Fig. 4. Alternative explanations for regional gradient in Bouguer values across the Cheyenne Belt. 4A: Dashed and dotted curve (bottom) is observed gravity from the Sierra Madre profile A - A f , dashed curve is observed gravity from the Medicine Bow Mountains profile B - B', and solid curve (top) is observed gravity from the Laramie Mountains profile C - C'. Change in observed giavity from profile C - C' to profiles A - A 1 and B - B' reflect general westward wustal thickening. Shaded area is generalized gravity envelope. Mode/4B: Variable crustal thickmss across Cheyenne Belt with constant crustal density. Calculated values are +'s. Model 4C: Variable mid-crustal density across Cheyenne Belt. Some crustal thickening is required to match southward gradient. Calculated values are 0's. Both models fit gravity data to & 1 mgal. These models approach "end member solutions" to gravity field when g&ological constraints are m~~sldtyed.

Along the profiles we consider in this paper, topography is high (2000-4000 m), and elevation generally increases southward. When approx- imate average elevations in the northern and southern portions of the profiles are considered, profile models A -A1, B - B', and C - C' (Figs. 5, 6 and 7) roughly approach isostatic compensation without increased mean crustal density where modelled crust is thicker (Wellman, 1978). Isostatic compen- satbn, which involves both crust and mantle (Dorman and Lewis, 1972), is accomplished regionally and not below a single profile or mountain range (Woollard, 1959, 1972). Thus, further efforts to rigorously "balance" the cross sections would not have significant regional importance.

1 , , * , , , * , ' , , , , , * 40 : MI @I IX) I40 1 ~ ~ 1 ; F . -.

same as in Figure 2. . , ., 1-:-


CHEYENNE BELT

GRAVITY PROFILES

RESULTS OF LARGE-SCALE MODELS

Gravity profiles from the Laramie Mountains-Front Range, Medicine Bow Mountains, and Sierra Madre-North Park Range show that gravity values decrease toward the south by about 50 to 100 mgal over distances of up to 350 km. These observed profiles, plotted as a function of distance from the Cheyenne Belt shear zone (Fig. 4), show a similarity in gradient across the zone that strongly suggests a direct association with the Cheyenne Belt. The Laramie Mountains gravity profile is consistently less negative than the other two which probably reflects a westward component of Laramide (?) crustal thickening (Malahoff and Moberly, 1968, and Fig. 8). Superimposed on the gradients are the effects of relatively near-surface structures and intrusive bodies. However, the large amplitudes and broad wavelengths of the regional gradients suggests that the Cheyenne Belt is a discontinuity of crustal proportions. The gradients observed across the shear zone can be explained in three ways: 1) the crust becomes thicker towards the south; 2) the crust (or a significant portion of the crust) is less dense to the south; or 3) crustal thickness changes and density differences combine in such a way that a relative mass deficiency exists in the southern part of the area.

Generalized models for possible crustal structure across the Cheyenne Belt based on (1) and (3) (Figs. 4B and 4C) have equivalent responses (f 1 mgal) and adequately match the broad gradients observed (Fig. 4A). Possibility (2) seems excessively difficult to reconcile with seismic refraction data and will not be pursued further. In the first model (Fig. 4B) crustal thickening occurs below, and somewhat south of, the surface position of the Cheyenne Belt, and gradual thickening continues to the southern end of the model. Relatively small ramp or step increases in thickness of the modelled crust can occur (as between 145 and 180 km south of the shear zone in Figure 4B) and, perhaps, are present in the earth's crust below major Precambrian shear zones in Colorado which may be related to Proterozoic continental accretion. Continuous (though non-linear) southward crustal thickening to the end of the model is necessary to match the slope of the gravity profiles (especially the Laramie Mountains profile) at their southern end.

The specific nature of the zone in which relatively abrupt southward crustal

Fig. 6. Gravity profile B - B' from the Medicine Bow Mountains. Top: Solid line is observed Bouguer anomaly, points are calculated anomaly values. Middle: Crustal model, density contrasts are in g/cm3 assuming average upper crustal densities of 2.7 g/cm3 and average upper mantle densities of 3.3 g/cm3. Bottom: Geologic cross-section along B - B'. Lake Owen complex is shown in the hinge of a large north-verging, but refolded nappe. Arlington Thrust of Laramide age places Precambrian rocks over Phanerozoic sediments along the north and east sides of the Medicine Bow Mountains.


I N F E R R E D NORTHERNMOST POSITION OF C H E Y E N N E BELT

.02 1 2

unun

krn 30- 40- 50 - 60-

i -, !;-b 3 , -,,, 1 ; >?!@*fl! !L! 4 A so 6 i0 , i 0 69 )?qt tWrfi4O I@ :.-mj 3t9@ WS iE9;D 260 aB@ 300 320 343

Fig. 7. Gravity prcrfile G- C' from the Laramie Range-Front Range. Top: Solid line is observed Bouguw anomaly, points are calculated anomaly values. Middle: Crustal model, density contrasts are in g/cm3 assuming average upper crustal densities of 2.7 g/cm3 and average upper mantle densities of 3.3 g/cm3. Note that two ramps in the Moho are needed to reproduce the regional gradient. The more northerly ramp is uhder the Laramie anorthosite but substantially south of the inferred trace of the Cheyenne Belt; the more southerly ramp approximately coincides with mapped shear tones southwest of Boulder (Fig. 2). Bottom: Geologic cross-section along C- C'. Thickness of anorthosite (4 km) is constrained by seismic reflection data.

GRAVITY PROFILES

. WYO. !

Fig. 8. Bouguer gravity field filtered to remove anomalies with wavelengths. less than 250 km. (After Hildenbrand and others, 1982). Coincidence of low-frequency anomaly with elevated topography of Rocky Mountain-Colorado Plateau region indicates Cenozoic crustal thickening of very large area. Values on contours in milligals. Bold lines indicate locations of gravity profiles.

thickening may occur cannot be resolved from the gravity models (Fig. 4B). A vertical "step" in crustal thickness at 25 km south of the shear zone would have essentially the same gravity response at the surface as the ramp shown in the model (between 20 and 30 km south of the shear zone). Other details of the generalized crustal thickening model also could be modified somewhat and still give an adequate fit to the observed profiles. However, the gravity gradient generated by the model is fairly sensitive to the position of the initial increase in crustal thickness. This m m s that moving the ramp (or step) in crustal thickness in the model too far north or south would create a poor fit between observed and calculated gravity profiles. For instance, placing the change in crustal thickness directly below the shear zone does not give an acceptable fit and thus implies that the shear zone dips toward the south if the crustal thickening and shear zone are related. This model of crustal thickening across the Cheyenne Belt (Fig. 4B) is consistent with refraction measurements that show crustal thickening to the south and with models of crustal thickening during plate collision (Dewey and Burke, 1973) and is therefore compatible with existing geologic models for Cheyenne Belt evolution.

The second generalized model (Fig. 4C) is comprised primarily of lateral density change in the crust, but also includes a component of southward crustal thickening which is necessary (as in the previous model) to match the strong southward slope of the observed gravity profiles. In this model the

. -

. . 2;;s 460 JOHNSON, KARLSTROM, SMITHSON AND HOUSTON

position of the increase in crustal thickness is somewhat arbitrary (due to variations possible with two bodies), and the change in crustal thickness is not abrupt. A mid-crustal position for the density contrast was chosen because average near-surface densities north and south of the Cheyenne Belt may be similar, but the exact location and thickness are also arbitrary. A very pronounced slope was necessary at the boundary of the mid-crustal density variation to broaden and smooth the resultant gravity profile. The implication of this construction is that the dip of the shear zone is fairly steep in the uppe7crust and lessens in the mid-crust, or that the density contrast lies at greater depth, as in 4B. Thus, mid-crustal density variation, accompanied by some southward crustal thickening, is a possible explanation of the gravity data and is consistent with refraction measurements of crustal thickness alfhough the very gradual crustal thickening in the southern half is more dificult to reconcile with Jackson and Pakiser's (1965) interpretation. Furthermore, crustal density differences could be expected across suture zones

- -wk& 4xxn-rn- of xbpamteage mcEoriQin Tf so, B e ~mplication of this model is that the Archaean crust of the Wyoming Province may be slightly more dense on average than accreted Proterozoic crust.

Ambiguities of gravity interpretation are well known, and no unique crustal structure can be inferred from models such as Figures 4B and 4C. In fact, numerous other combinations of crustal thickness and mid-crustal density changes could be devised that would have essentially the same gravity response as the models shown in Figure4. These combinations could even include situations where crust was substantially thicker and more dense south of the Cheyenne Belt, but this seems unlikely in light of refraction crustal thickness measurements. Nevertheless, gravity models which combine geological and geophysical constraints essentially rule out models utilizing only crustal density change. Crustal thickening south of the shear zone is required and can be modeled in various ways with Figures 4B and 4C approaching "end-member" solutions. The important conclusion to draw is that either model shows the Cheyenne Belt to be a major crustal discontinuity separating terrains of different crustal structure.

RESULTS OF SPECIFIC MODELS

Detailed crustal models (Figs. 5, 6 and 7) were constructed with different crustal thicknesses ~ ~ ~ l o n g - w a v e l e n & gr&ient*Tkepeinh+tlme - - -

-------

specific models is to demonstrate that the profiles (A - A 1 , B - B' and C - C' of Fig. 3) can each be matched by near-surface anomalies superimposed on long-wavelength, crustally derived gradients. More detailed discussion of specific models is presented in Johnson (1984).

GRAVITY PROFILES

' . ~ l i < Sierra Madre-Nmrk,P&~k Riz~pe , . .

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The Medicine Bow Mountains gravity profile (Fig. 3, B - B') exhibits a - wide range of gravity values (from -180 to -235 mgal) but is dominated by

two large positive anomalies. The northern gravity high is situated approximately over Archaean supracrustal rocks, and the large southern gravity high is associated with two Proterozoic mafic complexes in the central part of the range (Fig. 6).

Low-angle Laramide thrusting on the Arlington Fault occurs along the eastern and northeastern margins of the Medicine Bow Mountains (Houston et ial., 1968; Blackstone, 1983) and causes a strong northeasterly gravity gradient bat the nothern end of Profile B - B' (Figs. 3 and 6). In this area, relatively


lowdensity Phanerozoic rocks underlie Precambrian volcanic r.ocks. A large gravity high associated with the Precambrian volcanic rocks is displaced substantially "inboard" of the mountain front, presumably to where Precam- brian rock thickness increases. Thus, in the gravity model and cross section (Fig. 6), sedimentary rocks (density = 2.50 g/cm3, Jndd and Shakoor, 198 1) are shown to extend several kilornetres underneath Precambrian rocks of the Phantom Lake Metamorphic Suite. This suggests oil and gas potential in Palaeozoic and Mesozoic rocks beneath the crystalline thrust sheet.

Due to the short profile, effects of Laramide structure and the large gravity anomaly over the m f i c compl.exes, a broad, crustally-derived gravity gradient is not obvious. However, as shown in Figure 4 and by more detailed gravity modeling (Fig. 6), this profile is consistent with discontinuity associated with the Cheyenne Belt.

Laramie Mountains-Front Range

The Laramie Mountains-Front Range gravity profile (Fig. 7) is 350 km long. Bouguer gravity values range from about -120 to -260 mgal, but the regional decrease within Precambrian rocks (Fig. 4) is closer to 100 mgal. Important near-surface bodies which perturb the regional field. along profile C - C' (Fig. 3) include the Elrners Rock Greenstone Belt, Laramie Anorthosite-Syenite Complex, Sherman Granite and other Proterozoic granites. Decrease in gravity values at the northern end of the profile (Fig. 7) is due to low-density sedimentary rocks of the Powder River Basin.

The Laramie Anorthosite-Syenite Complex has been modelled in detail by Hodge et al. (1973), and the present interpretation is essentially the same. Norite associated with syenites along the northern margin of the compkex are modelled as flooring the syenite, but mafic rocks of the Elmrs Rock Greenstone Belt, extending south below the syenite, also could accqunt for the gravity high. Placement of the Cheyenne Belt below the Laramie Anorthosite- Syenite Complex is inferred by projection from the Medicine Bow Mountains to highly sheared rocks of the Richeau Hills east d the Laramie Mountains (Fig. 2).

PRECAMBRIAN VERSUS LARAMIDE ORIGIN FOR DIFFERENCES IN CRUSTAL r4 STRUCTURE

The Bouguer gravity field over many parts of the western United States has been interpreted to reflect Cenozoic tectonic events (Malahoff and Moberly, 1968; Behrendt et al., 1969; Woollard, 1972; Eaton et al, 1978). Separation of Laramide effects upon the gravity data used in this study is accomplished to some extent by the placement of gravity profiles along Precambrian-cored

. .. _ - GRAVITY PROFILES

uplifts. However, recognition of remaining Cenozoic tectonic contributions to the gravity fields (especially regional contributions) is essential for understanding the importance of the gravity field associated with the Cheyenne Belt.

Western U. S. Bouguer gravity values which have been filtered to remove anomalies with wavelengths less than 250 km (Hildenbrand et al., 1982) exhibit a large gravity low in the Rocky Mountain-CoIorado Plateau region. The southwesterly gradient associated with this very low-frequency anomaly contributes a component to the gravity profiles presented in this study (Fig. 8). However, the gradients observed in our profiles are at high angles to the Laramide gradient shown in Figure 8, and Profile A -A ' essentially is unaf- fected by the gradient, Furthermore, the coincidence of this filtered anomaly with elevated topography within the Rocky Mountains-Colorado Plateau physiographic provinces indicates that the anomaly reflects topography associated with Cenozoic crustal thickening or upper mantle (lithospheric) structure over the entire region. In this context differences in crustal structure across the Cheyenne Belt are "higher frequency," and the large change in gravity values (even if a low-frequency regional gradient is removed) argues for a "local" origin.

Although it might be argued that crustal thickness differences across the Cheyenne Belt result from isostatic compensation of higher average elevations in Colorado, crustal thickness in northeastern Colorado where average elevation is relatively low is essentially the same (48-50 km) as beneath the -

adjacent Front Range (52km) (Jackson et al., 1963; Roller and Jackson, 1966; Prodehl and Pakiser, 1980). Furthermore, stratigraphic evidence (R.M.A.G., 1972) indicates that northern Colorado has been topographically high with respect to much of Wyoming periodically in the past (e.g. the ancestral Rockies). Thus, it may be reasonable to conclude that differences in crustal thickness across the Cheyenne Belt first developed in the early Proterozoic.

Differences in crustal thickness which developed during Proterozoic orogeny might seem unlikely to persist because isostatic compensation of

- ' adjacent crustal blocks and consequent erosion of over-compensated material . should tend to eliminate differences in crustal thickness, and because creep in

J r

, the lower crust and upper mantle over long time periods may tend to remove - Moho offsets. However, these arguments for relatively rapid re-equilibration of

crustal thickness under orogenic zones are difficult to reconcile with observed thickness variations under Palaeozoic orogens, e.g. the northern and southern

, Appalachians (Sheridan and Drake, 1968). Furthermore, gravity and some - seismic data suggest that there are differences in crustal thickness between

adjacent Precambrian tectonic provinces within the Canadian Shield (Gibb - and Thomas, 1976) and the Australian Shield (Wellman, 1978) which must


have originated in Precambrian time. Soviet DSS seismic data also support the existence of Moho offsets in Precambrian shields of eastern Europe (Sollogub et al., 1975). Together these data indicate that proximal differences in crustal thickness can persist for billions of years in continental areas as long as isostatic constraints are fulfilled on a regional basis. This is the approach taken by Thomas and Gibb (1977), Wellman (1978) and in Figure 4.

The most compelling evidence that the differences in crustal thickness across the Cheyenne Belt originated in the Proterozoic is that the north-south Bouguer gravity gradient in southern Wyoming and northern Colorado corresponds in position to the trace of the Cheyenne Belt (Fig. 4) which is an early Proterozoic tectonic feature (Hills and Houston, 1979). Several Laramide structural features in Colorado may also be related to Proterozoic tectonic features. Mathews and Work (1978) have documented changes in styles of Laramide deformation going south in the Front Range from the Wyoming border to the Denver, Colorado area. These changes may coincide with Precambrian shear zones and changes in crustal thickness (see Fig. 7) which formed in the Proterozoic. Similarly, northeast-trending shear zones in Colorado (e.g. Colorado Mineral Belt) probably represent reactivated Proterozoic shear zones (Tweto and Sims, 1968; Weimer, 1980).

DISCUSSION OF GRAVITY "SIGNATURES" OF ANCIENT SUTURES

Gibb and Thomas (1976), working on Precambrian province boundaries of the Canadian Shield, have suggested that many of the boundaries have a characteristic "signature," and they suggest that the signature reflects a common deep crustal structure developed during plate collision at convergent margins. This idealized "signature" of ancient plate sutures is shown in Figure 9A. It is characterized in map view by linear, paired anomalies which are parallel to the province boundary. In profile, the anomaly has an asymmetric "S" shape with a gravity low over the older craton, decreasing to a minimum near the province boundary. A sharp high (high frequency anomaly) over the younger province is interpreted to result from both higher density crust and increased crustal thickness under the younger province, often accen- tuated by high-density mafic rocks such as basalts and ultrarnafics in the younger province (Thomas and Gibb, 1977; Kearey, 1976).

Thomas (1983), in an analysis of the southern Appalachians, has extended this concept of gravity "signatures" of ancient "sutures" to Phanerozoic orogens. Gravity anomaly profiles extending from the Valley and Ridge Province to the coastal plain in the southern and central Appalachians have the same asymmetric shape as profiles across the Precambrian province boundaries of Gibb and Thomas (1976). Thomas (1983) suggests that this "signature" can be used in general to identify the position of ancient sutures,

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even under concealing cover. He concludes that the suture is located directly under the steep gradient of the anomaly (Fig. 9). Furthermore, Thomas suggests that the direction of palaeosubduction can be inferred from the anomaly, with the dip of the palaeosubduction zone toward (i,e, under) the younger province. However, seismic data indicate that crust under the younger terrain (i.e. the Carolina Slate Belt and east) may be thinner, not thicker, than crust under the older terrain (i.e. under the inner Piedmont and Blue Ridge) (Lee, 1980; Carts and Bollinger, 1981). Alth~ugh the observed gravity anomaly in the Appalachians is similar to Proterozoic examples, the simple crustal model proposed by Gibb and Thomas (1976) which involves two crustal blocks (the younger of which is thicker and denser) may not explain present crustal structure of the southern Appalachians without contravening seismic data. Perhaps a better case for the model could be made in the Newfoundland Appalachians (Weaver, 1967; Karlstrom, 1983) where an anomaly having the same shape is present and where the accieted crust does appear from seismic data to be thicker than the older crust (Sheridan and Drake, 1968).

In spite of the common occurrence of "S"-shaped gravity anomalies above apparent suture zones, we have reservations regarding extrapolation of similar crustal structure to suture zones from gravity data. Even where the "typicat

.

signature" of sutures is observed, there is more than one possible explanation of deep crustal structure. In the southern Appalachian example, the "usual" model of thicker crust under the accreted terrain (Fig. 9A) may not apply. '

This problem is emphasized by relationships across the Ivrea-Verbano Zone of Italy (Kaminski and Menzel, 1968; Fountain and Salisbury, 1981) where the characteristic "S"-shaped anomaly can be- explained by complex crustal structure, suggested by seismic data, that involves emplacement of lower crustal material near the surface. Karner and Watts (1983) suggest other gravity models for the southern Appalachians and Alps that involve lithospheric flexure associated with the obduction of crustal blocks or flakes during orogeny.

A more general reservation, based on geologic considerations, is that it is unrealistic to lump all suture zones as a single phenomenon with similar crustal thickness and density relationships. The process of plate collision may indeed have certain invariant characteristics (e.g. subduction), but large variations in the thickness and mean density of colliding blocks would be expected, depending upon whether the collision is continent-continent, arc-arc or arc-continent. Further, the geometry of the collision zone would be expected to produce different suture zone configurations and, in old orogens, post-orogenic activity such as rifting and uplift history would modify the original crustal structure. Perhaps crustal thickening in the accreted terrain during plate collision is the normal situation which might explain the


CONCLUSIONS

1. Bouguer gravity profiles in the Precambrian-cored uplifts of southeastern Wyoming and northern Colorado show a low-frequency southward gradient with gravity values decreasing as much as 100 mgal to the south.

2. Reflection data from the Laramie Mountains provide some details of crustal structure but do not constrain crustal thickness. Attitude of the Cheyenne Belt shear zone, interpreted from surface geology, is nearly vertical to steeply southeastdipping. Gravity models suggest that the boundary between crustal blocks may become less steep with depth. If crustal thickening accounts for the gravity gradients, the average dip of the suture would range from about 45O to 75O. Location. of the zone of thickening in the models is not unique, but cabnot be moved north or south more than about 10 km without causing a poor fit to the data. Less- steep dip (on the order of 35") may characterize the zone at depth if crustal density differences are an important cause of the gravity gradients.

3. Models of differences in crustal thickness and/or density can explain the observed low-frequency gravity gradient. Implications are that Proterozoic crust south of the Cheyenne Belt is thicker and/or more felsic than Archaean crust to the north. From the gravity data alone these models cannot be differentiated, and constraints from seismic refraction data are not sufficient to exclude either possibility. However, some component of crustal thickening is strongly suggested by the combined gravity and seismic data.

4. Specific gravity models and structural cross-sections across the Cheyenne Belt demonstrate that high-frequency anomalies generally can be matched by upper crustal and near-surface density variations. Long-wavelength gradients seen across the Cheyenne Belt reflect middle or deep crustal lateral density differences.

5. Crustal thickness variations across the Cheyenne Belt probably originated during the Proterozoic and have persisted for 1700 Ma. This is indicated by the coincidence of the gravity gradient with the Cheyenne Belt, a Proterozoic structural feature, and by the observation that regional low- frequency gravity gradients (which probably reflect Cenozoic crustal thickening) cannot fully explain the observed gradients in southern Wyoming.

6. Discontinuities in lithology, radiometric dates and metamorphic grade as well as gravity and seismic refraction data show the Cheyenne Belt to be a major crustal feature. The combined geological and geophysical data can be explained by models involving collision and accretion of island arcs onto the Archaean Wyoming Province about 1700 Ma ago. In these

ACKNOWLEDGEMENTS

We wish to thank Mark Bronston, Charles Hurich, Robert Kukowski and Steve Stresky for their assistance in collecting gravity data in Wyoming. The Department of Defense Mapping Agency provided a considerable amount of gravity data for both Wyoming and Colorado. Processing of gravity data was performed on the University of Wyoming, Program for Crustal Studies DISCO Vax 111780. Financial support was provided by the National Science Foundation under grants EAR-7815192 and EAR-8026580. Thanks are due to D. M. Fountain for useful comments on the manuscript and to R. A. Gibb and M. D. Thomas for thoughtful reviews of this ~ a ~ e r .

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GRAVITY PROFILES 469

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Documents

BELT, - University of New Mexicoeps et al...shear zone, but the seismic lines do not extend across the belt, so direct correlation is not possible. Other events on this data set are