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Gravity Modeling of the Colorado Mineral Belt

Annie M. McCoy1*, Mousumi Roy1, Leandro Trevino2, and G. Randy Keller2

1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico

2Department of Geological Sciences, University of Texas, El Paso, Texas * presently at John Shomaker & Associates, Inc., Albuquerque, New Mexico

The Colorado Mineral Belt (CMB) is a northeast-trending belt of Laramide and older magmatism that coincides with a profound negative Bouguer gravity anomaly that is among the largest gravity anomalies in North America. Additionally, the CMB coincides with a region of low seismic velocities in the uppermost mantle indicated by travel-time tomography based on teleseismic arrivals. The subsurface structure of the intrusive bodies that comprise the CMB remains uncertain, and the relation of these intrusives to the present-day density structure of the underlying mantle is unclear. This study explores simple distributions of sub-surface mass deficits that can exp lain the CMB negative Bouguer gravity anomaly, constrained by geologic estimates of the extent of crustal plutonic bodies and by seismically- inferred crustal and upper mantle density anomalies. We discuss the viability of our first-order forward models and their consistency with seismic observations, and suggest refinements that can improve our understanding of the structure of the CMB.

1.0 INTRODUCTION The Colorado Mineral Belt (CMB) is a northeast-trending belt of Laramide and older magmatism (Mutschler et al., 1987) that extends more than 200 kilometers (km) across central Colorado and coincides with one of the most negative Bouguer gravity anomalies in the United States (Case, 1965; Isaacson and Smithson, 1976). The Laramide (Late Cretaceous to Oligocene) magmatism is typically expressed at the surface as relatively small shallow intrusive bodies associated with major gold, silver, lead, and molybdenum deposits (Tweto and Sims, 1963). Proterozoic rocks show evidence for magmatic episodes and shear zone movements around 1.7 billion years ago (Ga), 1.4 Ga, and 1.1 Ga focused along the CMB (McCoy et al., this volume; Shaw et al., 2001; Nyman et al., 1994; Barker et al., 1975).

A Bouguer gravity map of Colorado constructed using data of Oshetski and Kucks (2000) shows two major negative anomalies that are among the most prominent features in North America gravity (Figure 1). The northern negative anomaly is centered on the CMB and the southern anomaly is centered on the middle to late Tertiary (mainly Oligocene) magmatic centers of the San Juan Volcanic Field (Figure 1). Negative Bouguer gravity anomalies reach values of -340 milliGals in the CMB and –337 milliGals in the San Juans.

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Crustal thickness estimates based on teleseismic receiver functions ind icate little crustal thickening from the Colorado Great Plains (49.9 km average thickness) to the Colorado Rocky Mountains (50.1 km average thickness), suggesting that the high topography of the Colorado Rocky Mountains is probably not simply compensated by an Airy-type crustal root (Sheehan et al., 1995). Recent seismic studies in the southern Rocky Mountains have confirmed these observations, suggesting that the high topography of the Colorado Rocky Mountains must be supported by some combination of Airy and Pratt isostasy (e.g., Li et al., 2002; Snelson et al., this volume).

In this study, we explore Pratt and Airy-type forward models along northwest-trending

profiles across the CMB in order to explain the observed negative Bouguer gravity anomaly. Medium to short-wavelength (100 km) negative gravity anomalies can be adequately matched with inferred low-density structures in the crust, consistent with geologic mapping that suggests the presence of crustal intrusions in the CMB. The longer wavelength (400 to 500 km) gravity low across the CMB is adequately explained by low-density structures in either the uppermost mantle or the lower crust (as there is no geologic evidence for an extensive, 500-km scale, low-density body in the upper crust). In our modeling we explore both possibilities and discuss the non-uniqueness of our results.

We recognize that shallower, crustal features that explain the long-wavelength gravity

low across the CMB are consistent with surface-wave tomographic studies in the region that suggest low crustal densities below the Sawatch Mountains region in the central CMB (Li et al., 2002). However, to explain the 500 km scale gravity low surrounding the CMB, we find that a low density body in the lower crust would need to be regionally extensive, extending well beyond the CMB to the northwest and southeast. Current seismic interpretations of crustal structure in this region preclude such a low-density structure in the lower crust (Rumpel et al., this volume; Snelson et al., this volume).

On the other hand, low densities in the uppermost mantle are consistent with travel-time

tomographic results that show low seismic velocities in the upper mantle beneath the CMB region (Deuker et al., 2001). These results suggest that a low-velocity upper mantle anomaly is centered on the CMB, with up to 2 percent slower P-wave velocities than expected at depths of 50 to 250 km (Deuker et al., 2001). This low-velocity body is interpreted as a lithospheric anomaly derived from partial melt content due to melting of olivine-poor lithologies. We highlight below the issue of whether the low-density bodies occur in the lower crust or upper mantle in the density models of this paper and suggest that important future advances can be made by more detailed studies of the CMB combining seismic and gravity modeling.

2.0 METHODS The observed Bouguer gravity values were extracted from a regional Colorado Bouguer gravity dataset (Oshetski and Kucks, 2000) along northwest-trending profiles across the CMB (Profiles 1, 2, 3, and 4 in Table 1 and Figures 1 and 2). Gravity data within a 4-km wide swath were projected orthogonally onto these profiles using Generic Mapping Tools (GMT; Wessel and Smith, 1991). The gravity data were also gridded using the GMT grdtrack function, a

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bicubic interpolation of neighboring values onto the central profile line. Data extracted along each profile using both the swath-width method and the gridded data are plotted in Figure 2. A two-and-a-half dimensional modeling approach was used to determine the geometries and densities of subsurface bodies. We used forward-modeling software based on the Talwani method of calculating predicted gravity from bodies of arbitrary geometry (our code was written by D. Roberts, and is a rewrite of tal.25dgrav by S.F. Lai; Talwani et al., 1959, and Cady, 1980). Matlab pre- and post-processors allowed for efficient organization and alteration of input files and graphical output. Input parameters included (a) the geometries of the modeled bodies defined by vertices with x,y coordinates in the two-dimensional planes of the profiles, (b) the densities of the modeled bodies defined with respect to an upper crustal density of 2,750 kilograms per cubic meter (kg/m3), and (c) the distances to which the bodies are to be projected into and out of the plane of the profile (hence the two-and-a-half dimensionality of the model).

3.0 OBSERVED GRAVITY ANOMALIES The observed Bouguer gravity anomalies along the profiles are composites of several anomalies with varying wavelengths (Figures 2a through 2d). First, a regional trend in the Bouguer gravity is present, with generally more negative values to the northwest (Figures 2a and 2b). This regional trend is likely a reflection of gradually thickening crust from southeast to northwest, or a gradual reduction in average subsurface densities, or a combination of both. If we attribute this trend completely to a thickening of the crust, the amount of thickening required to explain the regional trend is no more than 4 or 5 km over a distance exceeding 500 km, which is well within errors of seismic Moho determinations. We do not attempt to remove this longest-wavelength regional trend in the current study; instead, we maintain a crust of constant thickness and focus on modeling the 500 km or shorter wavelength anomalies that are described below.

The longest wavelength anomaly we model is the 400 to 500-km wide anomaly with a shape that resembles a shallow dish with steep shoulders centered on the CMB. This anomaly probably reflects a low-density body present in the lower crust or upper mantle. Additionally, a shorter-wavelength (200 km) anomaly that coincides with the boundaries of the CMB as defined by Tweto and Sims (1963) appears to represent a shallower low-density body in the crust. Finally, the very short-wavelength (several km) Bouguer gravity variations probably represent density variations associated with lithologic changes in the upper and middle crust.

4.0 FITTING A MODEL TO THE OBSERVED ANOMALIES Unlike the Himalayas and the central Andes (Zhao et al., 1993; Zandt et al., 1994), the negative Bouguer gravity anomaly of the CMB cannot be explained by Airy-type isostasy. Figure 3 shows the predicted gravity anomaly if high topography were supported by an Airy-type crustal root along Profile 3. The most negative predicted gravity value coincides with the most negative observed Bouguer gravity value and the highest topography along the profile. However, the predicted gravity anomaly has more negative values along the sides of the CMB than observed, and less negative values at the center of the CMB. This is consistent with seismic

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studies that find a lack of a thick crustal root beneath the Colorado Rocky Mountains, suggesting the importance of Pratt isostasy (Sheehan et al., 1995; Li et al., 2002).

The gravity models presented in this paper are non-unique solutions with built- in assumptions about the densities of the Earth’s crust and mantle. The assumptions used in this study include an upper crust with a density of 2,750 kg/m3, a lower crust with a density of 2,950 kg/m3, a mantle with a density of 3,280 kg/m3, and a body in the crust with a density of 2,600 kg/m3. The upper crust is 20-km thick, in accordance with the Conrad discontinuity indicated by seismic refraction studies (Snelson et al., this volume; Rumpel et al., this volume), and the lower crust is 25-km thick, giving a total crustal thickness of 45 km consistent with seismic observations. The assumed upper crustal density is similar to that determined for Precambrian rocks in the Sawatch Range and Elk Mountains of central Colorado (2,710 kg/m3, Isaacson and Smithson, 1976; 2,760 kg/m3, Tweto and Case, 1972). The assumed density of a body in the crust is similar to that determined for Tertiary granitic rocks in the Sawatch Range and Elk Mountains of central Colorado (2,630 kg/m3, Isaacson and Smithson, 1976; 2,620 kg/m3, Tweto and Case, 1972). The density contrast between the upper crust and the body in the upper crust is 150 kg/m3, which is within the typical range of 50 to 180 kg/m3 measured in samples of granites and the country rocks in which they are emplaced (Bott and Smithson, 1967). The increase in background density in the lower crust is an attempt to reflect the depth-variation of densities for continental rocks, although we recognize that future refinements of this work will need to incorporate realistic density-depth relations for crystalline rocks (e.g. Christensen and Mooney, 1995).

In Figures 4a and 4b, we explore models with low-density mantle bodies that can explain the relatively long-wavelength (400 to 500 km) dish-shaped observed Bouguer anomaly across the CMB. We attempt to model this anomaly along Profile 3 with a body in the mantle that is thick and has a small density contrast of 20 kg/m3 (Figure 4a). Then, we attempt to model the same anomaly with a thinner body with a larger density contrast of 100 kg/m3 (Figure 4b). It is easily observed that the wavelength produced by the thinner body with the higher density contrast is a better fit to the wavelength defined by the shoulders of the shallow dish-shaped observed anomaly. The wavelength defined by these shoulders also places some constraints on the horizontal width of the model body. As the horizontal width is decreased (Figure 4c), the modeled anomaly develops a shorter wavelength. We also attempt to model the shallow dish-shaped observed anomaly with a body in the lower crust (Figure 5a) or a Moho with varying elevation (Figure 5b). A body in the lower crust with a density of 2,800 kg/m3 and a thickness of about 10 km provides a good fit of the observed anomaly. Rayleigh-wave tomography (Li et al., 2002) and seismic refraction observations (Snelson et al., this volume; Rumpel et al., this volume) show a range in Moho depth of about 10 km, from depths of 40 to 50 km, and this Moho topography may explain part of the shallow dish-shaped anomaly. Figures 5a and 5b show that both a low-density body in the lower crust and Moho topography provide good fits of the observed gravity and illustrate the fundamental non-uniqueness of gravity modeling, suggesting that further seismic work is required to determine the relative importance of low densities (if present) in the lower crust, low densities in the uppermost mantle, and/or crustal thickness variations. In this paper, we confine our presentation to models

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that incorporate low densities in the mantle, consistent with recent tomographic findings (Dueker et al., 2001; Li et al., 2002; Lee and Grand, 1996). The shoulders of the shallow dish-shaped anomaly represent steep gradients in gravity values and are asymmetric; the northwestern shoulder has more negative Bouguer gravity values. A model body that is thicker at the northwestern end provides a good fit of the anomaly’s asymmetric geometry. This geometry may also represent lateral variations in density contrasts between the body and the mantle. Below, we present models that explore the best geometries for subsurface density bodies that explain the observed Bouguer gravity along Profiles 1 through 4.

5.0 RESULTS OF FORWARD MODELING The model bodies that fit the observed gravity along the four profiles have some common features. The low-density body in the crust extends from shallow crustal depths (several km below land surface) to about 20-km depths in each profile. The crustal body is about 150 to 200-km wide near the surface, but narrows to 10 to 20-km wide in the middle crust.

The low-density body in the upper mantle is asymmetric and is thicker at its northwestern endpoint than in its central portion. In the southern CMB, along Profiles 3 and 4, the low-density body in the upper mantle is thicker at both endpoints than in its central portion. The body in the mantle is typically 10 to 20-km thick in its central portion, and is 400 to 500-km wide. Although we recognize the trade-off between the depth-extent of the low-density body in the upper mantle and the density contrast between this body and the surrounding mantle (Figure 4), the wavelength and shape of the observed anomaly around the CMB leads us to favor a shallow upper mantle body with greater density contrast.

5.1 Profiles 1 through 3 Along Profiles 1 through 3 (Profile 1 is the northeasternmost profile; Figure 1), the Moho

separates a lower crust with a density of 2,950 kg/m3 from a mantle with a density of 3,280 kg/m3 (Figures 6, 7, and 8). The regional dish-shaped anomaly is modeled with a body below the Moho with a density of 3,180 kg/m3, which represents a density contrast of 100 kg/m3. In all cases, this body is more than 400-km wide and asymmetric (Table 2). In Profile 3, the required body is slightly wider than in Profiles 1 and 2. In all profiles, the northwestern end of the body is thicker than the central portion of the body. In Profiles 3 and 4, in the southern part of the CMB, both ends of the body are much thicker than the central portion of the body (Table 2). In contrast, Profile 2 requires an upper mantle low-density body that is almost uniformly thick (20 km) across the entire region (Table 2).

The shorter, 200-km wavelength anomaly was modeled in Profiles 1 through 3 with a

body in the crust with a density of 2,600 kg/m3 (Table 2; Figures 6, 7, and 8). In all three cases, the body is 10-20 km wide in the middle crust and 165-200 km wide near the surface. It is about 20-km thick in its central portion and has ‘wings’ that taper and pinch out at shallow depths.

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5.2 Profile 4 Along Profile 4 (southwesternmost profile; Figure 1), the results for the upper mantle

body are similar to Profiles 1 and 3 above, in that the body is 480-km wide and asymmetric. The body is 35-km thick at its southeastern end and is 45-km thick at its northwestern end. As in Profile 3, the body’s endpoints are more than twice as thick as the central portion of the body.

The short-wavelength anomaly was modeled with a body in the crust with a density of

2,600 kg/m3 (Figure 9). The body is 35-km wide in the middle crust and 142-km wide near the surface. It is 17-km thick in its central portion, and about 5-km thick at its endpoints. Although this body has the winged geometry seen in crustal bodies in Profiles 1, 2, and 3, there is a general thickening of the crustal body in Profiles 3 and 4, in a southwestern direction along the CMB.

A short-wavelength positive anomaly located southeast of the CMB was modeled with a

relatively high-density body in the upper crust. This body is 50-km wide and 7-km thick and has a density of 2,950 kg/m3. This high-density upper crustal body is one possible interpretation of this gravity feature. The gravity feature coincides closely with outcrop of the Lower Cretaceous-age Dakota Sandstone and Purgatoire Formation (Tweto, 1979), sedimentary rocks that are not likely to generate positive density contrasts in the upper crust. We suggest that further work needs to be done to investigate other (possibly deeper) density anomalies that might explain this gravity feature.

6.0 DISCUSSION The negative Bouguer gravity anomaly of the CMB is similar to that of other parts of the U.S. Cordillera (e.g., the southern Sierra Nevada, Wernicke et al., 1996), in that it cannot be explained by simple Airy-type isostasy. In the southern Sierra Nevada, for example, Wernicke et al. (1996) explained the negative Bouguer gravity anomaly through a combination of lateral changes in crustal and mantle densities and proposed asthenospheric upwelling beneath the high mountains. Previous work on the CMB Bouguer gravity anomaly in the vicinity of the Sawatch Range and the Elk Mountains attributed the anomaly to a granitic batholith (Isaacson and Smithson, 1976). Isaacson and Smithson (1976) determined a maximum depth to the top of the low-density body to be 12 km and thus concluded that the negative anomaly is in the upper crust. Tweto and Case (1972) showed similar findings for the Sawatch Range near Leadville, Colorado. Isaacson and Smithson (1976) note the close correlation between the negative Bouguer gravity anomaly and outcrops of Cretaceous and Tertiary granitic stocks.

In the current study, our models define two major contributions to the regional gravity anomaly across the CMB. First, a low-density ‘batholith- like’ body in the crust, that is wide and sheet- like in the upper crust, with a narrower root extending into the middle crust. We find that this crustal body thickens to the southwest, and is thickest in Profiles 3 and 4 in the southwestern part of the CMB. Additionally, along each profile, we model a regional dish-shaped anomaly (over 500-km wide) that may be explained by a low-density body in the uppermost mantle. This body is over 400-km wide in all parts of the CMB, and generally thicker at the endpoints than in

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the central portion (although we note that in Profile 2 this body may be almost uniformly thick across the ent ire region). In all cases, the best-fit model has an asymmetric body in the upper mantle, thicker at the northwestern edge of our study area. We recognize that the changes in thickness in the modeled upper mantle body may equivalently be represented by lateral changes in density, rather than thickness.

In this work we have ignored upper-crustal density variations important to resolving the

short-wavelength gravity features across the study area. To refine our understanding of the size and shape of a large, crustal-scale batholith beneath the CMB and a regional-scale upper mantle low density body, we need further work to combine seismic and gravity observations. For example, geologic maps and upper-crustal seismic velocity models combined with velocity-density scaling relations (e.g., Christensen and Mooney, 1995) will be useful to determine the short-wavelength (10 km) variations in density and gravity. These will in turn highlight the nature of the more regional (over 200-km scale) gravity features. Furthermore, better resolution in seismic tomography models might be able to help resolve the size and shape of the zone of low densities in the uppermost mantle (e.g., resolve whether the body in the uppermost mantle does change appreciably in thickness and/or density across the region).

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TABLES

Table 1. Gravity profiles used for modeling the Colorado Mineral Belt Bouguer gravity anomaly

Profile southeastern end point

northwestern end point

1 103?26’02”, 37?00’00”

107?04’48”, 41?00’00”

2 103?30’00”, 37?00’00”

108?30’00”, 41?00’00”

3 104?00’00”, 37?00’00”

109?00’00”, 41?00’00”

4 104?18’00”, 37?00’00”

108?54’36”, 40?37’48”

Table 2. Properties of model bodies for Profiles 1, 2, 3, and 4, Colorado Mineral Belt

Profile

Thickness of crust body at

endpoints, km

Thickness of crust body in

central portion,

km

Width of crust

body near surface,

km

Thickness of mantle body at

endpoints, km

Thickness of mantle body

in central portion, km

Width of mantle body, km

Profile 1 0 19 200 3 in SE,

27 in NW 9 420

Profile 2

2 to 6 19 165 21 in SE, 25 in NW

21 410

Profile 3 3 19 192 25 in SE,

37 in NW 12 530

Profile 4

4 to 6 17 142 35 in SE, 45 in NW

15 480

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FIGURE CAPTIONS Figure 1. Bouguer gravity data of Oshetski and Kucks (2000) draped over 1-kilometer digital

elevation model topography, and profiles used in gravity modeling of the Colorado Mineral Belt. Tweto and Sims’s (1963) boundary of the Colorado Mineral Belt is outlined in black. Estimated P-wave velocity variation contours from Dueker et al. (2001) are in red (-1.25%) and orange (-1%). Figures 1 and 2 are made using the public domain Generic Mapping Tools software (Wessel and Smith, 1991).

Figure 2. Observed gravity and elevation along Profiles 1 (a), 2 (b), 3 (c), and 4 (d). See text for

details on the gridded gravity data and the swath widths used in this and subsequent figures.

Figure 3. Observed and predicted gravity along Profile 3 based on Airy-type isostasy. Figure 4. Attempts to match observed gravity along Profile 3 with (a) a thick body in the upper

mantle with a density contrast of 20 kg/m3 between the body and mantle, and (b) a thinner body in the upper mantle with a higher density contrast of 100 kg/m3 between the body and the mantle. In (c), the width of the thinner body is reduced. The predicted gravity for the body shown in (b) is the best fit of the observed gravity.

Figure 5. Attempts to match observed gravity along Profile 3 with (a) a body in the lower crust

that has a density of 2,800 kg/m3, and (b) topography on the Moho. Figure 6. Observed and predicted gravity along Profile 1 based on a body with a density of

2,600 kg/m3 in the upper crust (2,750 kg/m3) and a body with a density of 3,180 kg/m3 in the mantle (3,280 kg/m3).

Figure 7. Observed and predicted gravity along Profile 2 (densities as in Figure 6). Figure 8. Observed and predicted gravity along Profile 3 (densities as in Figure 6). Figure 9. Observed and predicted gravity along Profile 4 (densities as in Figure 6).

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REFERENCES Barker, F., D.R. Wones, W.N. Sharp, and G.A. Desborough, The Pikes Peak batholith, Colorado Front Range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite, Precamb. Res., 2, 97-160, 1975. Bott, M.H.P., and S. Smithson, Gravity investigations of subsurface shape and mass distributions of granite batholiths, GSA Bull., 78 (7), 859-877, 1967. Cady, J.W., Calculation of gravity and magnetic anomalies of finite-length right polygonal prisms, Geophys., 45 (10), 1507-1512, 1980. Case, J.E., Gravitational evidence for a batholithic mass of low density along a segment of the Colorado Mineral Belt, GSA Spec. Pap., 82, 26, 1965. Christensen, M.I., and W.D. Mooney, Seismic Velocity Structure and Composition of the Continental-Crust - a Global View, Journal of Geophysical Research Solid Earth, 10 (100), 9761-9788, 1995. Dueker, K., H. Yuan, and B. Zurek, Thick-structured Proterozoic lithosphere of the Rocky Mountain region, GSA Today, 11 (12), 4-9, 2001. Isaacson, L.B., and S.B. Smithson, Gravity anomalies and granite emplacement in west-central Colorado, GSA Bull., 87, 22-28, 1976. Li, A.B.; D.W. Forsyth, and K.M. Fischer, Evidence for shallow isostatic compensation of the southern Rocky Mountains from Rayleigh wave tomography, Geology; 30 (8), 683-686, 2002. Mutschler, F.E., E.E. Larson, R.M. Bruce, Laramide and younger magmatism in Colorado: new petrologic and tectonic variations on old themes, in Cenozoic volcanism in the Southern Rocky Mountains, edited by J.W. Drexler and E.E. Larson, pp. 1-47, Colo. School of Mines Quart., 82 (4), Golden, 1987. Nyman, M.W., K.E. Karlstrom, E. Kirby, and C.M. Graubard, Mesoproterozoic contractional orogeny in western North America: Evidence from ca. 1.4 Ga plutons, Geology, 22, 901-904, 1994. Oshetski, K.C. and R.P. Kucks, Colorado aeromagnetic and gravity maps and data: a web site for distribution of data: USGS Open-File Rpt., 00-0042, web only at http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-00-0042/colorado.html, 2000. Shaw, C. A., K.E. Karlstrom, M.L. Williams, M.J. Jercinovic, and A.M. McCoy, Electron-microprobe monazite dating of ca. 1.71-1.63 Ga and ca. 1.45-1.38 Ga deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent itnracontinental tectonic zone, Geology, 29 (8), 739-742, 2001. Talwani, M., J.L. Worzel, and M.G. Landisman, Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone [Pacific Ocean], J. Geophys. Res., 64 (1), 49-59, 1959. Tweto, O.L., and J.E. Case, Gravity and magnetic features as related to geology in the Leadville 30-minute quadrangle, Colorado, USGS Prof. Pap., 726-C, pp. 1-31, 1972. Tweto, O.L., and P.K. Sims, Precambrian ancestry of the Colorado Mineral Belt, GSA Bull., 74, 991-1014, 1963. Tweto, O.L., Geologic Map of Colorado, USGS , scale 1:500,000, 1979. Wernicke, B., R. Clayton, M. Ducea, C.H. Jones, S. Park, S. Ruppert, J. Saleeby, J.K. Snow, L. Squires, M. Fliedner, G. Jiracek, G.R. Keller, S. Klemperer, J. Luetgert, P. Malin, K. Miller, W. Mooney, H. Oliver, R. Phinney, Origin of High Mountains in the Continents: The Southern Sierra Nevada, Science, 271, 190-193, 1996.

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Wessel, P., and W.H.F. Smith, Free software helps map and display data, AGU EOS Trans., 72, 441, 1991. Zandt, G., A.A. Velasco, and S.L. Beck, Composition and thickness of the southern Altiplano crust, Bolivia, Geology, 22 (11), 1003-1006, 1994. Zhao, W., K.D. Nelson, J. Che, J. Guo, D. Lu, C. Wu, X. Liu, L.D. Brown, M.L. Hauck, J.T. Kuo, S. Klemperer, and Y. Makovsky, Deep seismic reflection evidence for continental underthrusting beneath southern Tibet, Nature, 366 (6455), 557-559, 1993.

elevation

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Figure 2. Observed gravity and elevation along Profiles 1 (a), 2 (b), 3 (c), and 4 (d).

5

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Figure 3. Observed and predicted gravity along Profile 3 based on Airy-type isostasy.

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mantle body 3,260 kg/m3

mantle body 3,180 kg/m3

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mantle body 3,180 kg/m3

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(c) Profile 3 density model

Figure 4. Attempts to match observed gravity along Profile 3 with (a) a thick body in the upper mantle with a density contrast of 20 kg/m between the body and the mantle, and (b) a thinner body in the upper mantle with a higher density contrast of 100 kg/m between the body and the mantle. In (c), the width of the thinner body is reduced. The predicted gravity for the body shown in (b) is the best fit of the observed gravity.

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predicted gravity for mantle body shown in (a)

observed gravity(4 km swath)

observed gravity(gridded)

predicted gravity for mantle body shown in (b)predicted gravity for mantle body shown in (c)

SE NW

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lower crust body 2,800 kg/m3

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lower crust 2,950 kg/m3

mantle 3,280 kg/m3

upper crust 2,750 kg/m3

predicted gravity for lower crust body shown in (a)

observed gravity(4 km swath)

observed gravity(gridded)

Figure 5. Attempts to match observed gravity along Profile 3 with (a) a body in the lower crust that has a density of 2,800 kg/m, and (b) topography on he Moho.3

predicted gravity for moho topography shown in (b)

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ilom

eter

s

Profile 1 density model

0 100 200 300 400 500 600

distance, kilometers

upper crust 2,750 kg/m

lower crust 2,950 kg/m3

3

mantle 3,280 kg/m3

mantle body 3,180 kg/m3

crust body 2,600 kg/m 3

Figure 6. Observed and predicted gravity along Profile 1 based on a body with a density of 2,600 kg/m in the upper crust (2,750 kg/m ) and a body with a density of 3,180 kg/m in the mantle (3,280 kg/m ).3 3

3

3

SE NW

-150

-200

-250

-300

-350

grav

ity, m

illig

als

predicted gravity

observed gravity(4 km swath)

observed gravity(gridded)

Profile 2 observed and predicted gravity

0

20

40

60

80

dept

h, k

ilom

eter

s

Profile 2 density model

0 100 200 300 400 500 600

distance, kilometers

Figure 7. Observed and predicted gravity along Profile 2 based on a body with a density of 2,600 kg/m in the upper crust (2,750 kg/m ) and a body with a density of 3,180 kg/m in the mantle (3,280 kg/m ).

upper crust 2,750 kg/m

lower crust 2,950 kg/m3

3

mantle 3,280 kg/m3

mantle body 3,180 kg/m3

crust body 2,600 kg/m 3

3

3 33

SE NW

predicted gravity

observed gravity(4 km swath)

observed gravity(gridded)

-150

-200

-250

-300

-350

grav

ity, m

illig

als

Profile 3 observed and predicted gravity

Profile 3 density model

dept

h, k

ilom

eter

s

upper crust 2,750 kg/m

lower crust 2,950 kg/m3

3

mantle 3,280 kg/m3

mantle body 3,180 kg/m3

crust body 2,600 kg/m30

20

40

60

80

100

Figure 8. Observed and predicted gravity along Profile 3 based on a body with a density of 2,600 kg/m in the upper crust (2,750 kg/m ) and a body with a density of 3,180 kg/m in the mantle (3,280 kg/m ).3 3

3

3

SE NW

-200

-250

-300

-350

grav

ity, m

illig

als

Profile 4 observed and predicted gravity

predicted gravity

observed gravity(4 km swath)

observed gravity(gridded)

0

20

40

60

80

100

dept

h, k

ilom

eter

s

Profile 1 density model

0 100 200 300 400 500 600 distance, kilometers

upper crust 2,750 kg/m

lower crust 2,950 kg/m3

3

mantle 3,280 kg/m3

mantle body 3,180 kg/m3

crust body 2,600 kg/m 3

Figure 9. Observed and predicted gravity along Profile 4 based on bodies with densities of 2,600 kg/m and 2,950 kg/m in the upper crust (2,750 kg/m ) and a body with a density of 3,180 kg/m in the mantle (3,280 kg/m ).

crust body 2,950 kg/m 3

3

3

3

3 3

SE NW