Epeirogeny in the Southern Rocky Mountains region ... › 282f › 6a26a9c87... · Mountains of Colorado and New Mexico, i.e., 1140 mountains with elevations >3048 m, ~200 with elevations

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Epeirogeny in the Southern Rocky Mountains region: Evidence and origin

Gordon P. EatonTexas A&M University, 9505 Northpointe Boulevard, Suite 1002, Spring, Texas 77379, USA

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Geosphere; October 2008; v. 4; no. 5; p. 764–784; doi: 10.1130/GES00149.1; 15 fi gures; 1 table.

ABSTRACT

The mountain peaks of the present-day Southern Rocky Mountains are the highest peaks in the Rocky Mountain system. They represent a second generation of moun-tains, one that originated from a different tectonic mechanism from that of the prede-cessor Laramide Rockies. Epeirogeny lifted the Laramide ranges in Colorado and New Mexico after their Late Cretaceous–early Cenozoic orogenic creation. The area was lifted tectonically from 1300 m to perhaps as much as 2000 m, the result of heating of the lithosphere stemming from its thin-ning, as well as infl ation of the crust by the intrusion of extensive, relatively low density batholiths and plutons of middle Tertiary age. This uplift produced an elongate north-striking crustal swell that cuts across major structural features in the crust, including the northeast-trending fundamental sutures that resulted from assembly of the North Ameri-can plate, the northwest-striking trends of the Ancestral Rocky Mountains in Colorado, and the northeast-trending Colorado Min-eral Belt. The contemporary Southern Rock-ies are unique in that their eastern piedmont slope is quite unlike that of other prominent orogenic mountain ranges around the globe owing to the presence of this supporting swell, or epeirogen. The lithosphere beneath the epeirogen’s summit is characterized by a coincident geoid anomaly, diminished seismic velocities in the upper mantle, and a north-trending, elevated Curie isothermal surface in the lower crust, all suggestive of elevated temperatures. Surface heat fl ow on the summit is complex, revealing both shallow crustal heat sources and a much deeper, more profound source that strikes north. Uplift resulting from these factors was initiated in post–middle Eocene time. At the wavelength of topographic smoothing employed here, the epeirogen’s regional topography makes it the highest general feature on the North American plate, individual mountain peak elevations, here and elsewhere, excepted. A

fi rst-order, re-leveled survey line in southern Colorado suggests that the epeirogen is still rising today.

INTRODUCTION

This paper revisits observations made more than two decades ago (Eaton, 1986, 1987) in light of new data and new paradigms acquired in the past decade and a half. Some of the interpre-tations originally made are substantiated, some were incorrect and have required discarding, and some are new.

Morgan and Swanberg (1985) and Morgan (2003) investigated the subject of uplift of the Colorado Plateau and southern Rocky Moun-tains, utilizing some of the same observations and interpretations that are referenced here. They proposed three possible mechanisms as the cause of uplift: (1) phase changes in the lithosphere, (2) physical thickening of the crust, and (3) thermal expansion and thinning of the lithosphere. In examining each of these in rig-orous detail, Morgan (2003) chose uplift asso-ciated with phase changes in the lithosphere as the preferred model. In this paper I emphasize lithospheric thinning and thermal expansion as the preferred mechanism, though some contri-bution from phase changes in the upper mantle cannot be ruled out entirely. There is broad agreement between us on certain of the issues involved here, but differences on others, and the interested reader is urged to consult Morgan’s (2003) paper in conjunction with this one.

Topography and Elevation

Individual peaks in the Southern Rocky Mountains of Colorado and New Mexico, i.e., 1140 mountains with elevations >3048 m, ~200 with elevations >3960 m, and 54 with elevations >4270 m, are the highest mountains along the crest of the Rocky Mountain system, a complex chain of differing age and deformational style that extends from northern British Columbia to New Mexico. Figure 1 shows the topography of the southern Rocky Mountains region in shaded relief. It consists of two additive components:

(1) the topography of the mountains, and (2) outward from the mountains, the topography of a giant swell that lifted the mountains and upon which they sit today.

In the Middle Rocky Mountains and the southeastern part of the Northern Rocky Moun-tains (the physiographic province names are those of Fenneman, 1931), there are fewer than 24 Rocky Mountain peaks that exceed 3660 m and only a few that exceed 3960 m; most, if not all of the latter, are associated with thermotec-tonic uplift related to the subsurface presence of the Yellowstone hotspot of Pierce and Mor-gan (1992). North of the Yellowstone hotspot, however, all the way to northernmost British Columbia, there are no peaks that achieve ele-vations of 3960 m.

There are two other regions on the North American plate where individual mountain peaks have elevations >4270 m. In California, principally in the Sierra Nevada and White Mountains, there are 12 peaks higher than 4270 m; the elevation of the highest (Mount Whitney) exceeds that of the highest peak in the southern Rockies (Mount Elbert, Colorado) by only 72 m. Even in Alaska, with its many high mountain ranges created by repeated docking of exotic terranes, there are only 14 peaks with elevations >4270 m. Of the latter, however, 7 exceed 4572 m, including Mount McKinley (Denali), with an elevation of 6194 m.

The number of peaks having elevations >4270 m is thus much larger in the southern Rocky Mountains than in the other two regions (54 versus12 versus14), but there are other con-trasts between these mountain regions, includ-ing differences in surface heat fl ow, the number of thermal springs, and the regional elevation.

The Alaska Range and Wrangell Mountains and much of their surrounding terrain appear to have uniform surface heat fl ow values of 80–85 mW/m2 (Blackwell and Richards, 2004; see http://www.smu.edu/geothermal/2004NAMap/2004NAMap.htm.) However, observations are sparse and very widely separated; hence, lateral extrapolations of heat fl ow values are tenuous at best. Surface heat fl ow in the Sierras ranges from 30 to 50 mW/m2 and in the Southern

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Rocky Mountain Epeirogeny

Geosphere, October 2008 765

Rocky Mountains ranges from 70 to as much as 150 mW/m2 (Decker et al., 1988). Differences in the number and geographic density of ther-mal springs, evidence of a relatively warm and shallow, fractured crust, are as follows. Alaska has 108 known thermal springs, but they are scattered all over the state (see National Geo-physical Data Center listings for Alaska); those in the California region of the Sierra Nevada

and White Mountains number ~20 (Waring, 1965), but they occur largely at the east base of these mountains where they adjoin the Basin and Range Province, not on the summits of the ranges; and there are 84 in the Southern Rocky Mountains, where thermal springs occur largely within the range. There are also differences in the elevation of the regional topography from which the peaks rise, as demonstrated here. All

these differences set the Southern Rocky Moun-tains region apart as anomalous.

GENERAL OBSERVATIONS OF THE SOUTHERN ROCKY MOUNTAINS REGION

The Laramide Southern Rocky Mountains underwent severe erosion both during and

Figure 1. Topography of the Southern Rocky Mountains region (G.R. Keller, 2008, personal commun.). Shaded relief map of the mountains is superimposed on that of the Southern Rocky Mountains epeirogen (see text for explanation).


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766 Geosphere, October 2008

following the period in which they were cre-ated orogenically. The entire Paleozoic and pre-Laramide Mesozoic stratigraphic section, ~1.5–3 km thick, and a not-insignifi cant part of the uppermost Precambrian basement, was eroded away from the arched basement block uplifts. The resulting high-standing Rocky Mountain erosion surface (Epis and Chapin, 1975; Coleman, 1985; Chapin and Kelley, 1997; Steven et al., 1997; Kelley and Chapin, 2004) has been described and mapped in regional detail. It is overlain locally by volcanic rocks of latest Eocene and Oligocene age. Rounded hills and low rounded mountains are present. Kelley and Chapin (2004) argued that Pikes Peak is a Laramide monadnock on this surface.

Steven et al. (1997) subdivided the erosion surface geographically and described different parts of it in terms of their history of Eocene and Miocene incisional stream drainage. One of the subdivisions drained eastward and south-eastward in late Eocene time, the other drained eastward and northeastward in Miocene time. This evidence is signifi cant. It indicates that the epeirogen was tectonically active. Kelley and Chapin (2004) utilized apatite fi ssion track data and identifi cation of partial annealing zones to determine the degree of denudation of the Lar-amide southern Rockies. They found that depths of denudation ranged from 1.4 km to as much as 3.85 km. The removal of slabs of rock of these

thicknesses would clearly result in epeirogenic uplift, but not as much as today.

The western Great Plains immediately east of the Southern Rockies in southern Wyoming and all of eastern Colorado and New Mexico stand higher at the foot of the mountains than they do both to the north and south. In this part of the high plains, a postorogenic sedimentary cover (the late Eocene–early Oligocene White River; late Oligocene–early Miocene Arikaree; and middle Miocene–early Pliocene Ogallala Formation) contributes locally to higher plains elevations. The geographic distribution of these formations can be viewed on the maps of King and Beikman (1974), Tweto (1979a), Love and Christiansen (1985), and the New Mexico Bureau of Geology and Mineral Resources (2003).

Anomalous Eastern Piedmont Slope of the Southern Rocky Mountains

Figure 2 provides a comparison of the modern eastern piedmont slope of the Southern Rocky Mountains (curve A) with the piedmont slopes of other global mountain ranges (curves B–I). The Southern Rocky Mountains profi le was drawn along the interfl uve between the Arkan-sas and Canadian Rivers in southern Colorado. In contrast with an earlier approach (Eaton, 1986, 1987), the profi les here were drawn from the foot of the mountains, rather than from their

crest, out to a distance of 1000 km. Elevations were sampled at 25, 50, 100, 200, 300, 400, 500, 750, and 1000 km from the mountain base of each range. The profi les were arbitrarily assigned an elevation value of zero at a distance of 1000 km in order to provide interrange com-parison. Actual elevations at the bases of indi-vidual ranges are listed in Table 1, along with information regarding the geographic end points of each source profi le, the full lengths of which have not been included in Figure 2. They fall into two separate elevation classes, those with mountain base elevations of <600 m and those with mountain base elevations ≥1000 m.

The piedmont slopes of all the ranges illus-trated in Figure 2 are concave upward and each has its own age of initial uplift and plate tectonic origin. Every elevated mountain range under-goes the fate of general erosion and, especially, erosional incision, though the rates and degree of uplift of its peaks vary from range to range. What differentiates the Southern Rocky Moun-tains profi le from the others is the much higher elevation at its mountain base. As its elevation declines to the east, this profi le has much greater height than that of the others, to a distance of 600–700 km from the mountain front. The dif-ference in mountain base elevation between the Southern Rocky Mountains and the Atlas Moun-tains, a range whose piedmont slope extends nearly as far from the mountains as does that of the Southern Rocky Mountains, is 1260 m, or nearly 1300 m.

In comparing these profi les it is apparent that profi le A is not typical of the piedmont slope of an orogenic mountain range. It is the eastern limb of a mammoth swell or epeirogen of tec-tonic origin. The focus of this paper is on this epeirogen, uplift of which took place chiefl y in post–middle Eocene time and later.

I (Eaton, 1986) referred to this feature as the “Alvarado Ridge” because the topography was essentially identical to that of a spreading ocean ridge. It is here renamed the Southern Rocky Mountain epeirogen because of its gross three-dimensional morphology. The epeirogen is slightly >2750 m high at its highest point and ~2000 km wide at its base. The Southern Rocky Mountains thus have less than one-tenth the width of the epeirogen. The highest mountain peaks, as measured from the top of the epeiro-gen, have elevations that are lower than the sum-mit of the epeirogen (and its eroded Laramide mountain core) relative to sea level. I (Eaton, 1987) proposed that the epeirogen was created by lithospheric thinning, and Roy et al. (1999) modeled the effect of high, but laterally dimin-ishing, temperatures at the base of the crust. These interpretations have been modeled quan-titatively (Cordell et al., 1991; Roy et al., 1999,

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Figure 2. Profi les of piedmont slopes of major mountain ranges of the world. Each profi le was anchored at the base of the mountains on the left side of the illustration and was run out to a distance of 1000 km, where the elevation of each has been reduced arbitrarily to zero. Many slopes terminate at much shorter distances. Profi le labels are as follows: A— Southern Rocky Mountains; B—Alps; C—Atlas; D—Andes; E—Pyrenees; F—Himalayas; G— Caucasus; H—Wrangell Mountains; I—Alaska Range. Their full lengths, including the topography of their adjoining mountains, are not shown, only their piedmont slopes.




Fig. 3 therein). In addition to their proposed basal crustal temperature distribution, Roy et al. (1999) also considered, but rejected, for ana-lytical reasons, two other mechanisms: (1) long-wavelength fl exural uplift owing to extension in the Rio Grande rift, and (2) thermal buoyancy and passive asthenospheric upwelling driven by extension along the Rio Grande rift.

Roy et al. (2004) suggested the possibility of a wholly different interpretation: the uplift resulted from signifi cant mantle dedensifi cation and thermal expansion during a massive mid-Tertiary magmatic invasion of the crust. These mechanisms may very well have made a signifi -cant contribution to the high regional elevation.

A variety of different kinds of observations led to a confi rmation of earlier interpretations, extending back more than a half century, to the effect that the Southern Rocky Mountains underwent epeirogenic uplift following the Laramide orogeny (e.g., see Eardley, 1951). This is a geologic certainty. Many investigators have addressed various aspects of this uplift: Cordell (1978), Morgan and Swanberg (1985),

Eaton (1987), Roy et al. (1999), McMillan et al. (2002), Sahagian et al. (2002), Morgan (2003), and Roy et al. (2004). Using paleoaltimetry, Sahagian et al. (2002) and Sahagian and Prous-sevitch (2007) documented what they referred to as “the rise of the Colorado Plateau” during the past 23 m.y., but a number of their collect-ing sites immediately adjoin or are actually located within the Southern Rocky Mountains, and some are even located in the Rio Grande rift in the interior of the range; hence, both of these elements were lifted, not the Colorado Plateau alone. If their interpretation is valid (discussed here later), it is a key piece of evidence that there was uplift of the epeirogen in Miocene time.

There is no latitudinally continuous, crust-penetrating, active young fault system separat-ing and, therefore, mechanically isolating, the Colorado Plateau from today’s Southern Rocky Mountains. As there is likewise no through-going, active, young regional fault system that mechanically separates today’s Great Plains from the Southern Rocky Mountains, it can be argued that regional uplift has affected all three

TABLE 1. LOCATIONS OF SOURCE PROFILES IN FIGURE 1

Profile Range Source profile Actual elevation at base of mountains

(m)

Longitude Latitude

105.62W 37.10 N A SRM to 2260 89.28 W 36.07 N 10.66 E 46.76 N B Alps to 1000 9.27 E 57.01 N 5.92 E 31.85 N C Atlas to 1200 0.67 E 21.46 N 78.39 W 1.42 S D1 Northern

Andesto 490

63.69 W 2.63 S 66.61 W 17.28 S D2 Southern

Andesto 1000

60.18 W 10.14 S 0.27 W 42.77 N E Pyrenees to 550 0.62 W 49.80 N 79.65 E 36.06 N F Himalayas to 440 82.79 E 42.25 N 46.74 E 36.06 N G Caucasus to 410 49.67 E 52.32 N 140.64 W 59.73 N H Wrangell to 1525 140.00 W 62.81 N 150.34 W 60.47 I Alaska to 1000 152.26 W 67.09 Note: SRM—Southern Rocky Mountains.

provinces simultaneously. As fi rst applied to the Colorado Plateau (Morgan and Swanberg, 1985), it is here applied to the Great Plains.

Prior to the Laramide orogeny, all three of these provinces were part of the North American craton and constituted an essentially laterally uninterrupted lithospheric continuum close to sea level. The crust extended from Utah across both what is now the Colorado Plateau and the Southern Rocky Mountains and continued east-ward beneath the Great Plains and Central Low-land. One cannot postulate post-Laramide uplift of one of these provinces without addressing simultaneous uplift of the other two.

The great eastern escarpment of today’s Southern Rocky Mountains is both a Laramide structural escarpment and a gigantic east-facing monocline with an east-verging Laramide thrust fault zone at depth, but it is also a later erosional escarpment, where post-Laramide continen-tal sedimentary rocks that unconformably lay across the older, upturned rocks of the structural escarpment have since been stripped away. As steep and abrupt as the eastern escarpment of the southern Rocky Mountains is almost every-where, it is not a giant fault scarp in the usual sense (Chapin and Cather, 1994; Leonard and Langford, 1994; Kelley and Chapin, 2004).

The younger continental postorogenic Ceno-zoic sedimentary cover on the Great Plains onlaps the Precambrian basement in south-ern Wyoming at an elevation of ~2300 m and is preserved within the mountains of central Colorado as scattered remnants overlying the late Eocene erosion surface at an elevation of ~2800 m. A fundamental question is, do differ-ences in elevation of parts of these sedimentary rocks represent deformation of the Ogallala Formation, or do they simply represent an ero-sionally interrupted, but once continuous, depo-sitional surface on which post-Laramide sedi-ments draped themselves over a then-existing topography? I lean ambivalently toward both answers. The Sahagian et al. (2002) and Saha-gian and Proussevitch (2007) data clearly point toward deformation.

Signifi cance of the Evidence of Simultaneous Uplift of the Colorado Plateau and Rocky Mountains

From as far back in North American geologic thought as the work of Dutton (1882), much has been made of the high elevation of the Colorado Plateau relative to sea level and to the general elevation the Basin and Range Province, but little, if anything, seems to have been made of the uniquely high elevation of the western Great Plains, at the foot of today’s Southern Rocky Mountains opposite the Colorado Plateau.


Eaton


The average thickness of the Colorado Pla-teau crust (40–45 km), even given its variabil-ity (Parsons et al., 1996), more or less matches that of much of the rest of the North American craton both east and northwest of the Southern Rocky Mountains. Crustal thickness is gener-ally similar in eastern Washington and Oregon, Idaho, North Dakota, South Dakota, Nebraska, Kansas, Iowa, and Missouri, plus most of Min-nesota (Braile et al., 1989, Fig. 4 therein).

Gao et al. (2004) argued that the thickness of the crust along a northwest-trending, passive seismic line (the LA RISTRA line; Colorado Plateau–Rio Grande rift–Great Plains Seismic Transect) that sampled half of the Colorado Pla-teau in two dimensions only (x and z), is 45.6 km, making it thicker than the crust of the Great Plains, which they determined to be 44.1 km thick along their transect. Gao et al. (2004) suggested that the extra degree of isostatic buoyancy that would follow from the somewhat greater plateau crustal thickness should result in ~300 m of additional elevation of the plateau, leading to some of the observed asymmetry of the epeirogen. They fur-ther determined that the crustal thickness of the Rio Grande Rift is notably less than that of both the plateau and plains, i.e., 35 km.

Gao et al. (2004) identifi ed a low-velocity zone with fairly sharply defi ned lateral boundar-ies in the upper mantle. It extends vertically to a depth of 200–250 km. Its Pn and Sn velocities are 5% and 8% slower, respectively, than those of its surroundings. They inferred lower densi-ties in this part of the mantle, attributing them to elevated temperature. Cordell et al. (1991) cal-culated the thickness of the lithosphere beneath the Colorado Plateau to vary from 75 km near its edges to a little more than 125 km at its cen-ter. In contrast, the lithosphere beneath the Great Plains was determined to be >200 km. This would also lead to asymmetry of the epeirogen.

Assuming that Cordell et al.’s (1991) interpre-tations are valid, the thickness of the lithosphere is notably less beneath the Colorado Plateau than beneath the Great Plains. This should lead to greater buoyancy of the plateau and, hence, to its generally higher elevation. Morgan and Swanberg (1985) noted that low upper mantle seismic velocities beneath the plateau suggest that the upper mantle is anomalously hot and that thermal expansion is to be expected. A signifi -cant question is what mechanism is responsible for the signifi cant difference in contemporary lithosphere thickness between the Colorado Pla-teau and the Great Plains? They are separated by the Southern Rocky Mountains, which are only 200 km wide, a fraction of the east-west width of both the Colorado Plateau and the Great Plains.

My sense is that the crust of the Colorado Plateau was not appreciably thickened during

the Laramide orogeny. It was simply lifted and compressionally buckled somewhat as a result of low-angle plate subduction and lithospheric drag by the Farallon plate beneath the continen-tal lithosphere. It was elevated epeirogenically in post–middle Eocene time. Could the mantle portion of the lithosphere of the Colorado Pla-teau been thinned by passage of the mechani-cally coupled Farallon plate beneath it, a result of the shearing away of 75 km of its base?

The Colorado Plateau does not require appre-ciable thickening in order to account for its elevation relative to that of the Great Basin. The extension and collapse of the Great Basin crust appear to account for a major part of the differ-ence in elevation between these two provinces.

I (Eaton, 1987, Fig. 4) and Roy et al. (1999, Fig. 3, profi les 2, 3, 4, and 7) demonstrated the regular westward decrease in elevation from the crest of the Southern Rocky Mountains out toward the center of the Colorado Plateau, the laterally limited western piedmont slope of the Southern Rockies. Topographic profi les drawn west for 250 km from the mountain crest are similar to, but less steep than, the eastern slope of the upper Great Plains. Although the plains extend many hundreds of kilometers east of the mountains, the plateau profi les are interrupted west of the center of the plateau by the presence of the erosional stumps of Early Cretaceous mountains at the western edge of the Colorado

Plateau, a product of the Sevier orogeny of Arm-strong (1968), by Tertiary volcanism at the pla-teau’s edge, as well as by rift-fl ank uplift of the Wasatch Mountains at the eastern boundary of the highly extended terrain of the Great Basin. It is thus not expected that the western piedmont slope of the southern Rocky Mountain epeiro-gen should match the eastern slope precisely or in its entirety.

SUMMIT REGION OF THE SOUTHERN ROCKY MOUNTAIN EPEIROGEN

Figure 3 presents two topographic profi les and two smoothed topographic maps of the summit region of the epeirogen. A substantial part of the cross-sectional bulk of what is identi-fi ed here as the epeirogen is, of course, that of the residual Laramide Southern Rocky Moun-tains, which had not been eroded to sea level or even to levels of a few hundred meters to 1 km above it. The profi les are based on topography along lines of latitude, at 36°N and 33°N. The fi rst one (Fig. 3A) crosses the epeirogen 1° south of the Colorado–New Mexico line. The second one (Fig. 3B) crosses it 1° north of the New Mexico–Mexico line. Dashed arches have been fi tted to these profi les by eye, profi les that graze the present-day topographic surface of the Great Plains and eastern part of the Colo-rado Plateau (as well as the southern Basin and

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Figure 3 (continued on following pages). Two regional topographic profi les across the South-ern Rocky Mountains region at lat 36°N and lat 33°N (A and B, respectively), plus two topographic maps of the smoothed topography of the summit region of the Southern Rocky Mountain epeirogen and its surroundings (C and D, on next pages). The contour inter-val is 250 m. The map in C is based on Zoback et al. (1990). The map in D is based on a wavelength-fi ltered gravity map (Hildenbrand et al., 1982) inverted by utilizing the observed relationship between gravity and elevation (illustrated in Eaton, 1987). The dashed black lines between the surface-grazing, solid red lines in A and B are drawn as subjective sketches illustrating the general confi guration of the epeirogen. The map in C replaces these in all the fi gures that follow and in discussions in the body of the text.




Range Province in the west half of Fig. 3B). A generalized topographic map could be made from a series of such profi les, but to eliminate the inherent subjectivity involved in drawing them, particularly the western slope, a digital elevation model was borrowed from Zoback et al. (1990). The result is shown in Figure 3C. Because all the topography in the region was included in the smoothing process, the eleva-tions of the mountains made a contribution, even as that contribution was diminished by the process of upward projection. For this reason, a

second map was prepared in a different manner as a basis for comparison.

The regional topographic map in Figure 3D was derived by inverting a Bouguer gravity map subjected to a 250 km low-pass fi lter (Hilden-brand et al., 1982), thus eliminating the effects of shallow gravity anomaly sources. Elevations on the epeirogen were determined by utilizing the regional empirical relationship between elevation and Bouguer gravity (illustrated in Eaton, 1987, Fig. 10 therein). The similarity between major features seen on these two maps

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is obvious, though the derived elevations dif-fer somewhat. Either of these maps could be employed in the series of arguments that fol-low, but the map in Figure 3C was chosen arbi-trarily, simply because the calculations involved in making it were extended to the whole of the North American plate.

Each time the contours from the map in Figure 3C appear on the fi gures that follow, the reader is asked to keep in mind the following: topographic interference from a lesser regional high at the western edge of the Colorado Pla-teau obscures part of the western slope of the epeirogen’s summit (subjective dashed lines show what the full summit might look like were it not for this contributory interference).

Relationship of the Southern Rocky Mountains, the Rio Grande Rift, and the Southern Rocky Mountain Epeirogen

Figure 4A shows the location of the South-ern Rockies and the Rio Grande rift with a superimposed topographic map of the Southern Rocky Mountain epeirogen. The mountains are bordered by deep structural basins fi lled with post-Laramide Cretaceous and Paleogene sedi-mentary rocks on both sides. As noted above, the deformed rocks in these basins are uncon-formably overlain by Oligocene and Miocene continental sedimentary rocks near the moun-tain front. The total structural relief on the Precambrian surface as measured on a cross section from the deepest part of the Piceance Basin eastward to the crest of Mount Elbert, Colorado, and down to the deepest part of the Denver Basin is >6500 m; see the map of Bump (2003, Fig. 1 therein).

The solid and dashed red line in the southern part of this fi gure marks the northern boundary of the southern Basin and Range Province. South of this line many mountain ranges strike approx-imately N55W, but in the southern Rio Grande rift, which continues south of the dashed red line, they strike north-south (Fig. 4B). The southern-most Rio Grande rift appears to be younger than the southern Basin and Range Province, which it adjoins, but because of their general physi-ological and even structural similarity, Fen-neman (1931) identifi ed the ranges within the Rio Grande rift as part of his Basin and Range Province. Although I agree that the southern Rio Grande rift displays basin and range structure, I hesitate to agree with Fenneman’s (1931) phys-iographic province “annexation.” Their creating stress fi elds set the two apart. Only the struc-ture is similar. The spreading Rio Grande rift is a by-product of development of the Southern Rocky Mountain epeirogen, which, in turn, is a product of lithospheric thinning. The profi le in


Eaton


Figure 3B shows that bowing of the epeirogen’s crust at that latitude (33°N) includes that of the southern Basin and Range Province, which, in turn, permits the interpretation that the epeiro-gen is younger than the province.

Figure 4B is a map of normal faults in the area where these two geologic provinces meet. The normal faults are displayed in contrasting colors in order to separate them as to origin. The pattern they display is not dissimilar to that of normal faults near the intersection of the Ethio-pian rift system and the Afar rift. Note the fl aring or widening of both the Rio Grande rift on the left, and the Ethiopian rift on the right, as they approach areas of transverse rifting. The fl ared version of the southern Rio Grande rift is shown in Figure 4A by dashed blue lines. It coincides

more or less precisely with the southward fl ar-ing of high surface heat fl ow values (as demon-strated in a following fi gure). The area limited solely to north-trending normal faults is defi ned by solid blue lines, a more traditional defi nition of the Rio Grande rift.

A sampling of the uppermost Great Plains ele-vations at the foot of the east face of the southern Rockies yields an average elevation of 1925 m, but topographic variations result from the fact that the east face of the mountains is an irregular erosional edge. The main point here is that the base of the mountains proper is fairly well cir-cumscribed by, or just a little below, the 2000 m contour on the epeirogen, thus this contour can be used as a reference guide defi ning the base of the mountains in all the fi gures that follow.

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Figure 3 (continued).

With respect to the Rio Grande rift in Colo-rado, Tweto (1979b) made a distinction between what he called “the Rio Grande rift proper” and the “Rio Grande rift system.” The former term refers to the continuous graben of Neogene age that begins at the head of the upper Arkansas River and broadens southward. The latter term includes “the…more extensive belt of Neogene block faulting, including not only the rift proper, but neighboring and north-continuing faults that were tectonically active concurrently with the rift proper” (Tweto, 1979b, p. 34). Although these normal faults continue to the Wyoming state line, they do not represent signifi cant lithospheric rifting, only crustal extension. As demonstrated here, the northern Rio Grande rift system, as defi ned by Tweto (1979b) for Colo-rado, and the southern Rio Grande rift in New Mexico, are more or less coincident with (1) an elongate geoid anomaly, (2) an elongate, long-wavelength, deep crustal magnetic anomaly, and (3) a surface heat fl ow anomaly. These anoma-lies refl ect the existence of the southern Rocky Mountain epeirogen in northern Colorado, and the faults seen there are related to the epeirogen, not to a perceived northward extension of the Rio Grande rift. These geophysical anomalies continue southward to southern New Mexico coincident with the southern Rio Grande rift as defi ned by Seager and Morgan (1979) and rede-fi ned by Baldridge et al. (1984).

The Rio Grande rift proper is the one shown in Figure 4A (in blue). The proposed northward extension of Tweto’s Rio Grande rift system is shown in green. The rift proper opens and broadens southward from the headwaters area of the Arkansas River. The greater width of rifted terrain shown at the southern end of the rift, near the Mexican border, has traditionally been shown to be narrower, essentially restricted to a belt of north-trending normal faults fl ank-ing the Rio Grande River. To the west, however, contemporaneous lateral spreading includes the northeast-trending Neogene normal faults border-ing the Plains of San Agustin graben southwest of Datil, New Mexico, and the San Francisco, Brushy, and Saliz Mountains near Reserve, New Mexico. To the southeast, it includes the north-northwest–trending normal faults bordering the Brokeoff and Guadalupe Mountains and the Salt Basin near El Paso Gap, New Mexico. The full pattern of these splaying normal faults separat-ing mountain ranges and intervening valleys and basins is best seen on the map of Baldridge et al. (1984), where the distribution of middle to late Cenozoic valley fi lls defi nes both the broad southern end of the rift and the northern bound-ary of the Basin and Range Province.

The northern starting point of the rift proper sits at the highest point on the epeirogen and




broadens steadily to southern New Mexico, gen-erally following the axis of the epeirogen south-ward to an elevation of 2000 m. There this axis divides. The division owes to the rather abruptly increased breadth of the rift and consequent wider separation of its high fl anking ridges. The 1500 and 1750 m contours defi ne the southern termi-nus of the epeirogen, crossing the wide southern end of the Rio Grande rift, thereby including part of the Basin and Range Province.

Fenneman (1931) did not recognize the exis-tence or signifi cance of the Rio Grande rift, which was not identifi ed until after 1931. The physiographic province boundary between his Southern Rocky Mountains and Basin and Range Province crosses the rift at the south end of the Sierra Nacimiento and Jemez Mountains on the west and the Sangre de Cristo Mountains on the east. The actual surface topography, as well as the smoothed regional topography, undergoes a signifi cant drop in elevation on the south side of this boundary. The aforementioned three ranges end abruptly at the boundary, and are succeeded southward by the northern end of the laterally offset Sandia Mountains. This east-west phys-iographic province boundary is a major tectonic boundary of some sort, yet no single fault or dis-tributed fault system is evidenced at the surface. It appears that the southern 30% of the Southern Rocky Mountain epeirogen has either partially collapsed or, more likely, has not been lifted like the area to the north. North-trending Laramide thrust faults and folds continue south and south-east of this boundary well into Mexico, thus the Laramide orogeny clearly deformed the region

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Figure 4. (A) Map of Fenneman’s (1931) Southern Rocky Mountains (black dashed lines with black horizontal ruling), the Rio Grande rift (dashed blue lines with blue vertical ruling) and Tweto’s (1979b) north-ern extension of his rift system (green dot-ted lines), with the smoothed topography of the Southern Rocky Mountain epeirogen superimposed. The solid blue lines defi ne the borders of that part of the southern rift marked by north-trending normal faults, the traditional view of the boundaries of the southern Rio Grande rift. The south end of the rift merges into the southern Basin and Range Province, but is not of the same origin or age. The diagonal red line marks the northern edge of the Basin and Range Province. (B) Map comparing the splayed pattern of rift-related normal faults of the Rio Grande rift and Ethiopian rift as they approach major transverse rift systems (Stewart, 1978; WoldeGabriel et al., 2004).


Eaton


south of this abrupt boundary. The northern 70% may be a fairly good geologic analog of the Great Basin prior to its collapse.

RELATIONSHIP OF THE SOUTHERN ROCKY MOUNTAIN EPEIROGEN TO OLDER TECTONIC FEATURES

Figure 5 shows the topography of the South-ern Rocky Mountain epeirogen in relation to (1) present-day exposures of Precambrian base-ment rocks (black), (2) crustal sutures and major faults in the Precambrian basement (maroon) (Karlstrom et al., 2005), (3) outlines of the Late Pennsylvanian ancestral Rocky Mountains (dashed blue lines) (Miller et al., 1992), and (4) the Colorado Mineral Belt (solid green). Although the Colorado Mineral Belt contains mineral deposits of Laramide age, there are more abundant deposits of middle Cenozoic age within it. The second and fourth of these features generally trend northeast; the third trends north-west. The Southern Rocky Mountain epeirogen trends due north, cutting diagonally across the general trend of the North American Cordillera (as illustrated in Eaton, 1986, Fig. 10 therein). It lifted a belt of older, west-northwest– to north-northwest–trending Laramide ranges. In Wyo-ming and Colorado, the most prominent of these ranges are the southern Front Range, the Gore Range, the Medicine Bow Mountains, the San-gre de Cristo Mountains, the southern San Juan Mountains, the Sawatch Range, the Uncompah-gre Plateau, and the Wet Mountains. Their average strike is N40W; the range of strikes is N36W to N48W. Such mountain ranges are few in New Mexico: the Chuska Mountains, the Gallinas Mountains, and the Pinos Altos Range. Elsewhere the trends of most New Mexico mountain ranges is approximately north-south, dominated by east-west crustal spreading asso-ciated with the southward widening Rio Grande rift. The orientation of the northwest-southeast

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Figure 5. Map showing the relationship of Southern Rocky Mountain epeirogen (SRMe) topography to fundamental litho-spheric sutures and thrust faults in the Precambrian basement (heavy maroon lines), basement outcrops of rocks from the Laramide Southern Rockies (black areas), outlines of the Ancestral Rocky Mountains (dashed blue lines), and the Colorado Min-eral Belt (green-shaded area). The axis of the SRMe strikes north across these ear-lier tectonic elements. It does so because it originates at the lithosphere-asthenosphere boundary, not within the lithosphere.




ranges is due to Pennsylvanian and Laramide orogeny, but these mountains were later broadly uplifted with the emergence of the north- trending Southern Rocky Mountain epeirogen, a feature uninfl uenced by major, fi rst-order geologic structures in the crust created before it. The simplest explanation for this azimuthal discordance is that epeirogenic uplift originated at the lithosphere-asthenosphere boundary, well below the crust and much of the lithosphere.

SMOOTHED TOPOGRAPHY OF THE UNITED STATES

Figure 6A shows the results of extending the smoothed regional topography of the epeirogen’s summit to the whole of the contiguous United States and immediately adjoining parts of Can-ada and Mexico. Figure 6B is a similar map for a large part of Alaska. The latter map includes the regional settings of the Alaska Range (let-ter A) and the Wrangell–St. Elias Mountains (letter W). Given the wavelength employed in smoothing these maps, they demonstrate that the Southern Rocky Mountain epeirogen has the highest regional elevation on the upper surface of the entire North American plate. The Sierra Nevada, with its 12 peaks exceeding 4270 m, and the 14 high peaks in Alaska that have eleva-tions >4270, both display regional topography

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Figure 6. (A) Smoothed topography of the contiguous United States and adjoining parts of Canada and Mexico (modifi ed from Zoback et al., 1990). Both the contour intervals and color intervals are 250 m. Uncolored regions are below 1000 m in eleva-tion. (B) Identical map for most of the state of Alaska. The Southern Rocky Mountain epeirogen (SRMe) displays the highest topography (areas shown by orange, red, and pink) on the whole of the North American plate.


Eaton


that is 500 m lower than that of the Southern Rocky Mountains region.

There are other regions in western North Amer-ica that exhibit regional elevations >1750 m, for example, central Idaho, central Wyoming, east-ern Arizona, and northwestern New Mexico. They are shown by pale green and yellow color intervals. The whole of North America west of the Great Plains and Prairie Provinces in Canada is higher than the rest of the plate.

There are only two regions with smoothed elevations exceeding 2250 m on the maps in Figure 6A. Both are shown by the orange color interval, but one of them, the Southern Rocky Mountain epeirogen, rises higher than the other, to slightly more than 2750 m. It is shown by red and pink color intervals. The fi rst is associated with the Yellowstone hotspot, the second with the region of the Southern Rocky Mountains. Both are fi rst-order anomalous plate-scale features.

RELATIONSHIP OF THE SOUTHERN ROCKY MOUNTAIN EPEIROGEN TO THE GEOID

The geoid represents an equipotential sur-face of the Earth approximately coinciding with sea level. Theoretically, a geoid anomaly in a continental region should not correlate well with topography because of subsurface density contrasts associated with isostatic compensation. Part of the National Oceanic and Atmospheric Administration’s GEOID 99 map (www.ngs.noaa.gov/GEOID/GEOID99) is shown in Figure 7. The smoothed topography of the epeirogen (white contours) is superim-posed on that of the geoid (black contours with colored intervals). In the northwestern corner of this fi gure (marked by the –10 m contour label) is the southernmost portion of a gigantic geoid anomaly associated with the Yellowstone hotspot (Pierce and Morgan, 1992). The Yellow-stone hotspot geoid anomaly is by far the largest observed in the contiguous U.S. It displays an elevation of −8 m to −10 m at its summit and extends east to both north-central North Dakota and west-central Wyoming.

Immediately east of the southeast corner of the main Yellowstone hotspot anomaly, and essen-tially fi lling the rest of the illustration in Figure 7, is a narrower, south-trending, geoid anomaly associated with the Southern Rocky Mountain epeirogen and Rio Grande rift. Its elevation has a maximum value of −11 m at the Wyoming-Colorado border and it declines in elevation in a regular manner down the axis of the epeirogen to southern New Mexico, where it terminates. The 1500 m white elevation contour of the epeirogen locally follows the −22 m black contour of the geoid on the east. The 2750+ m high point on

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the epeirogen is south of the elevation maximum on the geoid at a level of −13 m. It appears that the regular southward decrease in elevation of the epeirogen’s geoid anomaly is infl uenced by the existence of the regionally much larger Yel-lowstone hotspot anomaly to its north. Whether or not this is so, isostatic compensation is not yet complete in the Southern Rocky Mountains region and, hence, the assertion to the contrary (Eaton, 1987) was incorrect.

Because deviations of the geoid surface from sea level result from density variations in the subsurface, either lateral density variations asso-ciated with contrasts in rock composition or con-trasts in physical properties, such as elevated tem-perature, could be the cause. Such deviations can also result from dynamic and/or fl exural support of topography, as occurs at subduction zones.

We turn now to evidence supporting what I believe to be the driver of the epeirogen,

Figure 7. Map of GEOID99 topography, shown by black contours (from Richard Saltus, 2004, personal commun.). Contour interval for the geoid is 1 m, with contour labels shown every 5 m. The smoothed topography of the Southern Rocky Mountain epeirogen (SRMe), shown by white contours and labels, is superimposed. Note their spatial coincidence, sug-gesting a lack of full isostatic support of the SRMe. The dashed blue line in the southwest corner is the trace of part of the LA RISTRA seismic line (Gao et al., 2004; see text).




lithospheric thinning and resultant elevated temperatures in the lithosphere. This interpreta-tion differs from the preferred interpretation of Morgan (2003), who, after considering several contrasting mechanisms to account for post-Laramide uplift of the Southern Rocky Moun-tains and Colorado Plateau, chose changes in phase in the upper mantle and lower crust as the most likely explanation, as noted earlier.

TEMPERATURES IN THE LITHOSPHERE

Seismic evidence for what is probably ele-vated upper mantle temperatures in the region was offered in three publications from the LA RISTRA passive seismic experiment: Gao et al. (2004); Wilson et al. (2005a, 2005b); and La stowka and Sheehan, Snelson et al., and Li et al., cited in Karlstrom and Keller (2005).

Among the several conclusions the LA RIS-TRA authors drew from their data is that sig-nifi cant velocity anomalies are present in the upper mantle beneath parts of the LA RISTRA line, where notably low upper mantle veloci-ties extend to a depth of nearly 250 km. They concluded that this zone refl ects higher solid state temperatures than its surroundings, though the presence of a small amount of partial melt or hydration of the lithosphere (both of which represent phase changes of the sort proposed by Morgan, 2003), were not ruled out entirely as possible causes for depression of the seismic velocities. A partial illustration of LA RISTRA results, along with a profi le of the smoothed Southern Rocky Mountain epeirogen topogra-phy shown above it, is presented in Figure 8. Note that one of the upper mantle’s two separated low-velocity zones (the wider one) is broader than the surface expression of the Rio Grande rift, the object of their study, but, together, the two have a total span narrower than the summit of the Southern Rocky Mountain epeirogen.

Morgan (1983) presented curves of surface uplift values resulting from varying amounts of lithospheric thinning. From these curves, I deduce that a 200-km-thick lithosphere beneath the Great Plains (Cordell et al., 1991), thinned to 50 km, should result in an uplift of 2.0–2.75 km as the outcome of slow and rapid thinning rates. However, not all of the bulk of the South-ern Rocky Mountain epeirogen, as defi ned by the topography of its upper surface, is due to epeirogeny, as emphasized earlier. A lithosphere of 125 km thickness beneath the Colorado Plateau, thinned to 50 km, should result in an approximate uplift of 1.35 km at most.

Figure 9 shows the distribution of surface heat fl ow values in the region (Blackwell and Rich-ards, 2004) plus an outline of the Rio Grande

GAO ET AL.: CONVECTION BENEATH THE RIO GRANDE RIFTGAO ET AL.: CONVECTION BENEATH THE RIO GRANDE RIFT

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Figure 8. Profi le of P velocities to a depth of 250 km beneath the Southern Rocky Moun-tain epeirogen (modifi ed from Gao et al., 2004), with the smoothed topography of the epeirogen shown above it. The red lines in the upper fi gure identify the areas of anoma-lously low P velocities. RGR—Rio Grande rift.

rift shown by a pale blue dashed line. Surface heat fl ow values range from <50 mW/m2 to >150 mW/m2 in the map area (Decker et al., 1988). The mean surface heat fl ow for conti-nents worldwide is 65 mW/m2, based on 10337 measurements (Stein, 1995). Mean heat fl ow for regions of exposure of Proterozoic basement rocks, as in the Southern Rocky Mountains, is 58.3 mW/m2.

The correlations between areas of high sur-face heat fl ow values, high elevation, and expo-sures of the eroded Precambrian basement are geographically variable. In the Front Range of north-central Colorado, the area of the largest continuous exposure of Precambrian basement rocks, the correlation is positive and strong, as it is across the Uncompahgre Plateau in southwest-ern Colorado. Elsewhere, elevated surface heat fl ow is seen above exposures of middle Ceno-zoic volcanic fi elds, especially the San Juan fi eld of Colorado and part of the Mogollon-Datil fi eld of New Mexico. The spatial correlation between high surface heat fl ow and mid-Tertiary igneous rocks is just as striking.

There is high surface heat fl ow beneath the summit of the Southern Rocky Mountain epeirogen owing to the presence of these post-Laramide rocks at shallow depths, but there is also high reduced heat fl ow. As an aside, 95% of the thermal springs in Colorado and New Mex-ico illustrated by Waring (1965) occur in areas where the surface heat fl ow shown by Blackwell and Richards (2004) is 80 mW/m2 and higher.

The approximate surface heat fl ow values at locations A, C, F, and G in Figure 9 are illustra-tive here. They are immediately above the deep-

est parts of four large Laramide structural basins that fl ank the Southern Rockies, two on the east side of the range and two on the west. In the basins labeled A, C, and F (the Piceance, Den-ver, and San Juan Basins, respectively), the Pre-cambrian basement surface is at depths of more than −2000 m. In basin G (the Raton Basin), the basement surface is at a depth of a little more than –1000 m (Bump, 2003). Approximate sur-face heat fl ow values above the center of these four basins are as follow: A, 65–70 mW/m2; C, 75–80 mW/m2; F, 65 mW/m2; G, 85 mW/m2. All equal or exceed the mean surface value of continental regions. At the sites labeled B, D, E, and H, the basement surface, though buried, is much shallower than the basin fl oors. Because Precambrian basement rocks underlie the entire area of Figure 9, there clearly is no relationship between basement surface elevation and sur-face heat fl ow.

In a hypothetical model of horizontally lay-ered sedimentary rocks underlain by fl at Pre-cambrian basement granites that display radio-genic heat production, vertical offset of the basement surface along a fault will not produce a contrast in surface or reduced heat fl ow across the fault; hence, the difference in surface heat fl ow values observed here between mountains and fl anking basins is due to some other fac-tor. One possibility is a difference in the depth of erosion of the basement, it being greater in the mountains. Deeper levels of the basement rocks are exposed there, rocks locally display-ing penetrative ductile fl ow. It is possible that the composition of these Precambrian rocks is somewhat different from those underlying the


Eaton


fl anking basins, and perhaps they are rocks with higher radiogenic heat production.

In sampling igneous and metamorphic rocks in the Southern Rocky Mountains in order to ascertain levels of radiogenic heat production for calculations of reduced heat fl ow, Decker et al. (1988, Table 3 therein) found especially high levels in igneous rocks of post-Laramide middle and later Cenozoic age, the preponder-ance of their samples. These rocks are abun-dant in the summit area of the Southern Rocky Mountain epeirogen. Their spatial distribution at the surface is shown in a later fi gure. High values of surface heat fl ow in the Southern Rockies thus seem to owe signifi cantly, but not exclusively, to heat production in post-Laramide rocks younger than middle Eocene in age, some of them exposed at the surface

and some occurring as relatively low density plutons in the shallow crust.

In their concluding remarks, Decker et al. (1988) stated that their study demonstrated unequivocally that the Front Range of the Southern Rocky Mountains in Colorado is characterized by a high reduced heat fl ow of 54–58 mW/m2 substantially exceeding that of part of its surroundings. Blackwell et al. (1991) agreed, suggesting that the Southern Rocky Mountains and Rio Grande rift have higher heat fl ow from beneath the radiogenic crustal layer. There is thus a deep source of heat in the area, as well as a shallow one.

(Reiter, 2008) also addressed the topic of heat fl ow, as well as Bouguer gravity anoma-lies and teleseismic velocity anomalies in the region, and concluded that heat sources, as well

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Figure 9. Map of surface heat fl ow (Blackwell and Richards, 2004) with topography of the Southern Rocky Mountain epeirogen summit superimposed. The dashed pale blue lines defi ne the approximate borders of the Rio Grande rift proper. The white dashed line in the southwest defi nes the northern border of the Basin and Range Province. Nor-mal, continental, surface heat fl ow values, worldwide, are 50–60 mW/m2, the boundary between the yellow and green contour intervals on this map.

as gravity anomaly sources, occur in both the upper mantle and the crust. There is thus a clear consensus on the existence of high heat fl ow from beneath the crust.

In discussing the constraints on rift thermal processes, Morgan (1981) offered that the lack of broad regional heat fl ow anomalies outside the fl oors of rift zones suggests that thermal anomalies originating in the mantle have not yet reached the surface by conduction. He noted that because the lithosphere thins at a rate faster than a conducted thermal disturbance can prop-agate through it, it would be only after a very long period of time that the effect of lithospheric thinning would appear as a thermal perturbation at the surface. Morgan (1981) presented a curve that makes possible the determination of the length of time the effect of a 200-km-thick litho-sphere thinning to 50 km would take to appear at the surface. In this instance, that period of time is 32 m.y. In the last section herein, I focus on the timing of epeirogenic uplift and show that the initial uplift during the second episode of epeirogenesis in the region probably took place earlier than 32 m.y. ago, thus allowing for the effect of a thermal perturbation resulting from lithospheric thinning to appear at the surface.

In other major international rift zones, where there is an absence of broad heat fl ow anomalies straddling the rift, as is seen in the northern two-thirds of the Southern Rocky Mountain epeiro-gen, high heat fl ow is geographically restricted to the fl oors of rifts. This would seem to be the case for heat fl ow anomalies in the southern third of the epeirogen. The more restricted width of the anomaly there and the spatial coincidence of surface heat fl ow values ranging from 80 to >100 mW/m2 within the rift seem to suggest that heat from the base of the lithosphere has not yet reached the surface or, perhaps, is just now beginning to reach it. It is interesting to note in the data of Sahagian et al. (2002) that the col-lecting sites they occupied that have been lifted by the least amount are all in the southern third of the epeirogen, where uplift has taken place only in the past 4.7 m.y.

Signifi cance of Pre-Laramide Igneous Activity along the Southern Rocky Mountain Epeirogen Summit

Figure 10 shows the locations of igneous rocks of several different ages emplaced in the summit region of the Southern Rocky Mountain epeirogen prior to Laramide time: (1) granite plutons of Proterozoic age extending the entire length of the southern Rocky Mountain epeiro-gen, (2) diatremes of the period 800–500 Ma in southern Wyoming and northern Colo-rado clustered with fossil-bearing Devonian




diatremes from the period 404–326 Ma (Lester et al., 2001; Smith et al., 1979), and (3) calcic- carbonatites and ferro-carbonatites, syenites, alkali-feldspar granites, and mafi c igneous rocks of Cambrian–Ordovician age, their ages over-lapping that of the youngest diatremes. These rocks span the period of 574–427 Ma in Colo-rado and 664–457 Ma in New Mexico. Their composition implies extensional stresses in the

crust in early Paleozoic time (McMillan and McLemore, 2004). The chemical compositions of these rocks are consistent with partial melt-ing of upwelling asthenosphere in an exten-sional environment. Apparently, the lithosphere underwent aborted prerifting here long before development of the Ancestral Rocky Moun-tains, the Laramide Southern Rockies, the Southern Rocky Mountain epeirogen, and the

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Figure 10. Map of pre-Laramide igneous rocks in the Southern Rocky Mountains region. Symbols: X—Proterozoic and Devonian diatremes; fi lled circles—Proterozoic (ca. 1.4 Ga) plutons; squares—Cambrian–Ordovician extension-related plutons; red line—a high- temperature zone in the Proterozoic basement. Modifi ed from Lester et al. (2001), Shaw et al. (2005), and McMillan and McLemore (2004).

Rio Grande rift. The episode left an indelible, and what I believe to be a highly signifi cant, north-trending heterogeneity in the lithosphere, one that was tectonically “remembered” in post–middle Eocene time. Because the base of the lithosphere is an isotherm, a period of cool-ing following this episode might have resulted in recovery of its pre-event thickness, but not its pre-event composition and physical proper-ties. The same is true for the Proterozoic period of granite emplacement. In discussing the evidence and cause of intracontinental uplifts in Africa, Bailey (1972) suggested that dur-ing translations and rotations of a continental plate, structures in the crust and irregularities along the base of the plate give rise to irregular stress distributions.

Figure 10 also shows a Proterozoic high-temperature zone (enclosed by a red line) that Shaw et al. (2005) interpreted as an area where Proterozoic temperatures reached as high as 500–550 °C (roughly equivalent to the Bar-rovian biotite zone). They postulated that the rocks of this zone may represent a deeper level of exposure of the basement than elsewhere in the region. If so, a simple corollary is that these rocks represent the area of greatest basement uplift. As noted earlier, they display penetrative ductile deformation.

As discussed later, magmatic invasions of the crust also took place in both late Eocene– Oligocene and late Miocene time. This is not to suggest that elevated temperatures resulting from any of these magmatic episodes lingered until late Miocene time or to the present, they simply demonstrate that the summit area of this epeirogen has undergone a very long and long-punctuated period of igneous intrusion and vol-canic activity, some of it implying aborted rift-ing, which left its imprint on the lithosphere. A signifi cant heterogeneity was created in both the crust and the upper mantle.

Temperature-Related Seismic Velocities in the Mantle

It was discussed earlier that anomalously low values of Pn and Sn occur along the LA RIS-TRA line (Gao, 2004). Figure 11 is a map of Pn velocity anomalies at 100 km depth showing their horizontal subsurface distribution. This fi g-ure was modifi ed from a digital map (Dueker et al., 2001; Dueker, 2004, personal commun.) by contouring it as an analog map. Elevation con-tours on the summit area of the Southern Rocky Mountain epeirogen are superimposed. It can be seen that, in general, most areas of low Pn velocities occur within the epeirogen’s enclos-ing 1750 m contour. A single exception occurs in northeastern Colorado.


Eaton


The area of lowest Pn values is fl anked east and west by areas of higher values shown in blue. The same is seen on the cross section of the LA RISTRA line. Given the known cor-relation between compressional wave veloci-ties and densities, it is mildly puzzling that the fairly abrupt boundaries between depressed and elevated Pn velocities as deep as 100 km are not marked by coincident gravity gradients at the surface. This is especially true for the outer boundaries of the two (blue) zones that straddle the Southern Rocky Mountains.

Dueker (2004) and Dueker et al. (2001) focused on individual Pn anomalies in their interpretation of this map, but the focus here is on the entire cluster of low Pn anomalies because it is spatially coincident with the sum-mit region of the epeirogen.

Elevated Crustal Temperatures Suggested by the Regional Magnetic Field

Figure 12 presents an upward-projected magnetic anomaly map of the region (Mayhew, 1982; Taylor et al., 1983). The contour interval is 5 nT. This map displays a long-wavelength regional magnetic low coincident with the full length of the epeirogen (southern Wyoming to southernmost New Mexico). Areas with val-ues lower than −20 nT are shaded. Taylor et al. (1983) suggested that such an anomaly could arise from any one of several different causes: (1) lateral, intracrustal contrasts in magnetiza-tion (lateral rock compositional variations at depth), (2) variations in crustal thickness, or (3) an anomalously shallow depth of the Curie iso-thermal surface (local diminution in the thick-

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Figure 11. Map of Pn velocities at 100 km depth beneath the Southern Rocky Mountain epeirogen (modifi ed from Dueker, 2004). White areas mark the absence of data. Veloci-ties are lowest beneath the summit region of the epeirogen, but equally low values are also observed on the upper east fl ank in northeastern Colorado.

ness of the magnetized crustal layer). Mayhew (1982) associated the anomaly with the Rio Grande rift and elevated crustal temperatures.

Magnetic minerals lose their ability to main-tain magnetization at temperatures above their Curie temperature, which, for magnetite, the most commonly occurring magnetic mineral in nature, is ~575–580 °C, depending on varia-tions in its composition (Carmichael, 1989). Saturation magnetization decays to zero at the Curie temperature.

Although the crust thins laterally across the southern part of the epeirogen, modern maps of crustal thickness (e.g., Keller et al., 1998), especially thickness variations from north to south, reveal a limited to no relationship to the long-wavelength magnetic map. The regional magnetic low in Figure 12 originates at a deep level, probably in the lower crust, owing to a north-trending, arched, and elevated Curie geo-therm, as Mayhew (1982) proposed, although rock compositional variations cannot be ruled out entirely, given the distribution of igneous rocks at the surface and, by inference, within the crust.

TILTING ON THE EAST FLANK OF THE SOUTHERN ROCKY MOUNTAIN EPEIROGEN

The dips of postorogenic sediments on the east fl ank of the Southern Rocky Mountain epeirogen are composite, displaying both pri-mary (depositional) and younger tectonic dips. The Ogallala Formation consists of both fl uvial and eolian sediments that once constituted an enormous, gently east dipping, bajada-like sedi-mentary apron that blanketed the east face of the Southern Rocky Mountains. I believe that it has been tilted since the end of its accumulation, which occurred ~6–5 m.y. ago (Eaton, 1986; McMillan et al., 2002). Its upper surface now defi nes the eastern surface of the southern Rocky Mountain epeirogen. It is not, however, the only tilted surface of signifi cance in the region.

In studying the denudation history of the Colorado Front Range and Wet Mountains, House et al. (2003) discovered a middle Ceno-zoic partial annealing zone (PAZ) tilted to the east. It passes eastward from a point above the crest of the Sandia Mountains near Albuquer-que, New Mexico, through the present-day topographic surface at Santa Rosa, New Mex-ico, and to the east it is observed at depth in wells in Oklahoma. In the stretch of its profi le illustrated in Figure 10 of House et al. (2003) and in Roy et al. (2004), uplift of the west end of this PAZ surface relative to the east end is ~3250 m. Roy et al. (2004) noted that sur-face exposures of Triassic sandstone between




Santa Rosa and the edge of the Rio Grande rift, nearly 200 km to the west, yield AFT ages of 30–25 Ma; therefore, tilting has occurred since that time. Chronologically distributed volca-nic age data described in the following reveal an igneous event-count maximum between 29 and 25 Ma (Chapin et al., 2004, their Fig. 4). The wholesale invasion of the crust by magmas of this age probably marked the initiation of

signifi cant epeirogenic uplift, i.e., the birth of the Southern Rocky Mountain epeirogen from pluton emplacement buoyancy, crustal thick-ening, and thermal expansion. This episode lasted from that time until a notable hiatus in igneous activity in middle Miocene time. The data of McMillan et al. (2002) and Leonard (2002), plus a set of additional new data pro-vided here, suggest that uplift continued after

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Figure 12. Long-wavelength, satellite magnetic anomaly associated with the summit of the Southern Rocky Mountain epeirogen. Contour interval is 5 nT. The anomaly extends from northern Colorado south nearly to the Mexican border, as does the epeirogen. Modifi ed from Mayhew (1982) and Taylor et al. (1983).

initial deposition of the Ogallala Formation in middle Miocene time. Another example of east-ward tilting of the east fl ank of the mountains is based on the identifi cation of one or more sloping erosion surfaces and the orientation of incised paleo-stream valleys, as discussed ear-lier. Steven et al. (1997) postulated that general eastward tilting took place in two episodes, one in middle Miocene time and the other in latest Miocene to Quaternary time.

Figure 13 presents the results of fi rst-order re-leveling on the east fl ank of the epeirogen in Colorado. The data were drawn from fi les of the National Oceanic and Atmospheric Administra-tion. The leveled and re-leveled line was arbi-trarily anchored east of the town of Las Animas, Colorado, on the Great Plains, 150 km from the mountain base. It was continued west through the city of Pueblo and up into the Southern Rocky Mountains to the town of Salida. This is the route of U.S. Highway 50, which follows the course of the Arkansas River. Although the latter is a modern erosional incisor of the epeirogen today, and though some very small part of the elevation change in the middle 1950s along this line hypothetically could be the result of erosion by the Arkansas River, the lapsed time between leveling runs seems far too short to account for signifi cant erosion, and therefore much, if any, eastward tilting.

Save for two, sharp, single-benchmark down-turns on the profi le in Figure 13 (both are most likely associated with groundwater withdrawal and respondent local subsidence), the change in elevation along this line increases steadily from east to west. It represents still another tilted surface, one that indicates continued uplift of the epeirogen today. The re-leveling results displayed in Figure 13 represent two separate spans of historic time. The eastern segment was re-leveled in 1985 following an original fi rst-order leveling survey in 1934.The western seg-ment was re-leveled in 1985 along a portion of a line originally run in 1953. The two segments are joined at a shared benchmark (circled) at the bottom of the sharp downturn immediately west of the city of Pueblo.

The resurveys represent two different lengths of historic time, both of them relatively short; the eastern one, a span of 51 yr, the western one, only 32 yr. The general uplift rate suggested by Figure 13, which has a tectonic cause, is a full order of magnitude lower than the rate of isostatic uplift of 1700 m of the Fennoscandian shield following the melting (removal of the load) of its ice cap during the past 10–15 k.y. (Aubrey and Emery, 1993), and thus it is not an entirely unreasonable rate regarding the response of the lithosphere to application of positive or negative crustal loads.


Eaton


Timing of Epeirogenic Uplift

Given the geographic relationship between broad epeirogenic uplift, lithospheric thinning, elevated mantle temperatures, and crustal rift-ing, we examine surface manifestations of Cenozoic elevated crustal temperatures, i.e., the presence of volcanic and plutonic rocks exposed at the surface today, in order to gain some understanding of when non-erosion–related epeirogenic uplift began.

Figure 14A is a sketch map of widespread post-Laramide igneous rock distribution in the summit region of the epeirogen. Most of these rocks are shown in black, but two are shown in orange. Older igneous rocks, shown in other colors, are present in the same area. Those in green represent the diatremes, those in red, the 1.4 Ga plutons, and those in purple, the extension-related Cambrian–Ordovician plutons. Outcrops of these older rocks that are within the black areas of post-Laramide rock outcrops are not shown.

Magmatic and volcanic rocks ranging in age from 42 Ma to younger than 1 Ma and in com-position from andesite to basalt (and bimodal basalt rhyolite) are included among the rocks shown in black. Despite the age and composi-tional differences among these rocks and the changing plate tectonic regimes that created such compositional differences, it is seen that

the crust in the region has been intruded repeat-edly (in Proterozoic, Devonian, Ordovician, and middle Cenozoic time), and subjected to past high-temperature melts that repeatedly brought advective heat and buoyancy to the crust. Some of this activity led to emplacement of simple dikes and some to the development of large vol-canic fi elds, remnants of which are still present in the area, as well as some to large, relatively low density plutons in the crust.

With regard to the timing of epeirogeny, it is necessary to ask what the distribution of these rocks was in time. Figure 14B shows two super-imposed step plots of the ages of these rocks in 3 m.y. intervals. The fi rst data set (represented by the heavier line), was assembled from the com-pilations of Luedke (1993a, 1993b) and Luedke and Smith (1978a, 1978b). These ages are from igneous rocks in Colorado, New Mexico, and adjoining parts of eastern Utah and eastern Ari-zona. Those represented by the lighter line were taken from Chapin et al. (2004) and recast from 1 to 3 m.y. intervals. They are limited to New Mexico outcrops only.

A great many rock ages are shared between the two data sets; therefore, it is scarcely surpris-ing that the occurrence of the two major igneous event peaks and the trough between them essen-tially coincide. In the New Mexico data set, the great bulk of magmatic/volcanic events of the fi rst long igneous episode took place in the time

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Figure 13. Profi le of elevation changes along U.S. Highway 50 from an arbitrary starting point east of the town of Las Animas, Colorado, on the Great Plains, as determined from fi rst-order leveling and re-leveling. The line continues westward to Salida, Colorado, in the mountains. Towns: S—Salida; P—Pueblo; LA—Las Animas.

interval 37–22 Ma, peaking between 28 Ma and 25 Ma. It appears that by adding the ages of rocks in Colorado to those in New Mexico, the Eocene–Miocene event peak in the light line plot broadens to a somewhat wider time span, suggesting, but scarcely demanding, that initia-tion of intense igneous events began somewhat earlier in the north and lasted somewhat longer.

As seen, both plots display a pair of maxima, the one just described and the other between 16 and 0 Ma, and peaking between 4 and 1 Ma. Chapin et al. (2004) suggested that roll-back of the low-dip, subducted Farallon plate took place between 36 and 24 Ma. I offer that it actually may have begun as early as 38 Ma. The fi rst evidence of Cenozoic crustal stretch-ing in the region occurred ca. 36–35 Ma, late Eocene time (Cather, 1990; Chapin and Cather, 1994). Crustal extension thus began within 1 m.y. before or after the beginning of sharply increased magmatism. It was well under way when the peak of ignimbrite explosions was reached at 28 Ma. It was followed by an abrupt increase in the number of igneous events in the epeirogen’s future summit region, including profound ignimbrite eruptions. De Silva (2008) proposed that ignimbrite fl are-ups represent a high magmatic fl ux driven by an elevated rate of energy transfer from the mantle. In the case of the Southern Rocky Mountains region, the ini-tiation of such a fl are-up likely represented an advanced stage of lithospheric thinning, as well as the rise of hot asthenosphere resulting from rollback of the Farallon plate. The uppermost crust was weakened and the brittle-ductile tran-sition moved upward, enabling listric normal faulting in the shallow crust. One of the ignim-brites, the Wall Mountain Tuff, is geographically widespread, extending eastward onto the upper Great Plains. It has an age of 36.7 Ma (McIntosh and Chapin, 2004), and its distribution suggests an east-dipping slope or later eastward tilting. If the subducted Farallon plate was sinking and rolling back a little before this time, the oppor-tunity for hot asthenospheric material to fl ow in and replace it had to be substantial (Chapin et al., 2004). It doubtless led to heating of the lower lithosphere. It is interesting that the area of the Southern Rocky Mountain epeirogen and its igneous rocks of all ages occupy a signifi cant part of what has been dubbed the “Laramide magma gap,” where no igneous rocks of Lar-amide age exist, save for those in the Colorado Mineral Belt. It would seem that magmatism was delayed here from Late Cretaceous to late Eocene time.

The younger of the two episodes of post-Laramide igneous activity began immediately fol-lowing the relative hiatus from 20 to 16 Ma. This younger magmatic event-frequency maximum




appears far more pronounced in the New Mexico data set owing in part to more abundant recent sampling and radiometric dating there, much of it along the northeast-trending Jemez volcanic zone, but basalts of this period are widespread across southern New Mexico and southeastern Arizona. Similarly, the highest event-frequency maximum in the heavy-line step plot between 28 and 25 Ma is probably the result of oversam-pling in the San Juan and Mogollon-Datil volca-nic fi elds of Colorado and New Mexico.

The basaltic rocks of the second event maxi-mum mark a different tectonic regime, as does the development of the Jemez volcanic zone that follows the Precambrian suture between the Yavapai and Mazatzal basement provinces. It is probable that decompression melting of upwell-ing asthenosphere began to generate basalts, as suggested by Chapin et al. (2004). The Jemez zone saw surface leaking of small fl oods of basalt, but rocks 16 Ma and younger are sparse along the full length of the Rio Grande rift. Inas-much as the rift underwent further, but slower, extension at this time, one might have antici-pated volcanism of the sort commonly seen in and along other major rift zones. Wilson et al. (2005a, 2005b) suggested that a lack of highly focused mantle upwelling may explain the defi -cit of rift-related volcanism.

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Figure 14. (A) Map of post-Laramide and pre-Laramide igneous rocks in the sum-mit region of the epeirogen, from various source maps. Those shown in black are of post-Laramide age. The letter c identifi es clusters of post-Laramide outcrops that are too small to be shown individually. The thick dashed orange line in the lower right marks the eastern boundary of Cenozoic igneous rocks exposed at the surface. It coincides closely with the 1250 m contour on the epeirogen. Localities identifi ed by the symbol X are from Figure 9 and are color keyed as follows: green—Proterozoic and Devonian diatremes; purple—extension-related Ordovician plutons; red—1.4 Ga plutons. (B) Two versions of an igneous event frequency count and the chronology of the igneous rocks shown in black in A. The thin line step plot was modifi ed from Chapin et al. (2004) and represents igneous rocks in New Mexico only. The heavier line step plot includes the ages of rocks in all of Colorado and easternmost Utah and Arizona, as well as those in New Mexico. The data are from Luedke (1993a, 1993b) and Luedke and Smith (1978a, 1978b).


Eaton


The fi ndings of Sahagian et al. (2002) and Sahagian and Proussevitch (2007) suggest more or less continuous epeirogenic uplift of the surface between 23 and 1.98 Ma. The tech-nique they employed was discussed in Sahagian (1985), Sahagian et al. (1989), Sahagian and Maus (1994), and Sahagian and Proussev-itch (2007). It was validated and calibrated on Hawaiian basalt fl ows taken from a large range of elevations (45 m to 4361 m) on Mauna Loa volcano. The maximum uplift they determined for the Southern Rockies region is 2170 m. It took place at 20 Ma. This episode of uplift began to tilt the oldest Ogallala sediments (17–12 Ma; Eaton, 1986) eastward as they were deposited and seemingly has continued to do so. Figure 15 shows the location of the greatest elevation changes determined by Sahagian et al. (2002) plotted on the topography of the South-ern Rocky Mountain epeirogen. Data from their sites at the west edge of the Colorado Plateau, that they plotted separately, are not included here, but they fi t the model offered here inas-much as they occur below the summit of the epeirogen, well down its western slope.

Those collecting sites with the highest ele-vations are those of the oldest lavas in their collection suite. Those that are youngest are seen at the lowest elevations in Figure 14, hav-ing undergone the least amount of uplift. The data permit the interpretation that Miocene and younger uplift began fi rst in the region of the northernmost sites and continued there as it propagated southward. The Miocene cli-matic optimum, a warm, dry period, began ca. 17.5 Ma, and was followed by signifi cant cooling and a moister climate from 15 Ma to a little after 12.5 Ma. The oldest Ogallala sedi-ments began to be deposited ca. 17.0 Ma, while the oldest lavas in the Sahagian et al. (2002) suite were erupted at 23 Ma.

To reiterate, a massive magmatic invasion of the crust took place following erosional planing and incision of the mountains at the end of the Laramide orogeny. It was initiated between 40 and 37 Ma and peaked between 37 and 22 Ma (late Eocene–early Miocene time). This episode brought abundant, relatively low density magma and advected heat energy to the Laramide-thickened crust, resulting in greater lithospheric buoyancy. With what can be assumed to be a hot asthenosphere beneath it, the result of roll-back of the Farallon plate, the crust thickened still more than it had from orogenic compres-sion and the lithosphere expanded thermally, bulging upward to an elevation similar to that of the 34 Ma Florissant fossil beds (Colorado), ~2600 m (Gregory and Chase, 1992).

The period of deposition of the Ogallala Formation was followed by its eastward tilt-

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Figure 15. Distribution of Miocene and younger measurement sites from the paleoeleva-tion determinations of Sahagian et al. (2002) and Sahagian and Proussevitch (2007) plot-ted on the topography of the Southern Rocky Mountain epeirogen. The ages of the tested lavas at these sites and the measures of the greatest elevation change they found at each are plotted here. They show that the oldest and greatest epeirogenic uplift occurred in Fen-neman’s (1931) Southern Rocky Mountains province. South of these mountains, at lower elevations, where evidence of Laramide thrusting and folding is nevertheless present, the amount of interpreted uplift is notably less and took place later.

ing (McMillan et al., 2002). The re-leveling data illustrated in Figure 13 show that tilting is continuing today. Given the post-Laramide chronologic igneous history of the region, it is concluded that the Southern Rocky Mountain epeirogen is partially, if not wholly, an uncol-lapsed geologic analog of the Great Basin. The crust of the region that was to become the Great Basin was signifi cantly thickened during the Nevadan and Sevier orogenies and had a high elevation. It later collapsed as a result of both its potential energy and a signifi cant epi-sode of extension that thinned the crust.

CONCLUSIONS

The site of the present-day Southern Rocky Mountains is one of regional uplift in postoro-genic time. A major fraction of the elevation of the highest peaks in today’s mountain range results in part from the development of epeiro-genic uplift, created in part by lithospheric thin-ning and crustal heating and in part from stream incision following a Miocene climate change in the region. There thus have been two somewhat different, but colocated, Southern Rocky Moun-tain ranges in this region, one of Laramide age,




the other of post–middle Eocene and younger age. The highest elevations supporting today’s Southern Rocky Mountains make the range, on average, the highest region on the North Ameri-can plate. Conducted and advected heat energy and a crust infl ated by middle Tertiary plutons and batholiths are responsible for the elevation of the Southern Rocky Mountains region today.

ACKNOWLEDGMENTS

This paper benefi ted substantially from insightful and challenging reviews by C.E. Chapin and Paul Morgan. Both persuaded me to revisit and alter some of my original interpretations and to tighten or reject others. These two earth scientists, along with S.A. Kel-ley and S.M. Cather, all of them long students of the region discussed here, provided penetrative questions and patient instruction that were of considerable help. I am also indebted to Richard Saltus for providing a copy of the GEOID 99 map he prepared from the fi les of the National Oceanic and Atmospheric Administra-tion (NOAA), Renee Shields of NOAA, who directed me to the fi rst-order, level and re-level data on the Great Plains and eastern Southern Rocky Mountains, and Kris Verdin of the EROS (Earth Resources Obser-vation and Science) Data Center, who was technically supportive in a number of ways.

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Epeirogeny in the Southern Rocky Mountains region ... › 282f › 6a26a9c87... · Mountains of Colorado and New Mexico, i.e., 1140 mountains with elevations >3048 m, ~200 with elevations