INTRODUCTION

Overthrust belts are generally continuous zones of deformation that are first-order structural features of the earth's crust (Bucher, 1951, p. 514). They grow out of geosynclines, and the more continuous and regionally systematic structures generally coincide with miogeosynclines. An obvious coincidence in space of miogeosynclines and overthrust belts younger than Precambrian is demonstrated in North America (Fig. 1). Development in sequence, as well as space coincidence, suggests a genetic relation between the two geotectonic features: folds and thrusts develop during mountain building that destroys the geosyncline.

One of the most striking features of overthrust belts is their sinuosity. Salients and reentrants alternate along trend (salients are defined as those arcs that are convex cratonward). Net horizontal movement during overthrusting and folding is toward the craton, and there is apparent diverging movement across salients. In many local areas field relations indicate episodic development of structure that is grossly progressive toward the craton. The greatest concentration of structural features appears to be in the vicinity of the boundary between former miogeosyncline and shelf. In this zone syntectonic sediments commonly are involved in the latest phases of deformation.

Crystalline basement ordinarily is not structurally concordant with faults and folds developed in the sedimentary layers (e.g., Gwinn, 1964; Bally et al., 1966). The passive nature of the basement in overthrust belts contrasts with the direct involvement of the crystalline basement in folds and thrusts at the border of block-uplift mountains (Prucha et al., 1965, p. 984).

During destruction of the miogeosyncline, a linear or semilenticular basin characteristically develops at the edge of the craton, particularly in front of salients, and is progressively filled with debris shed from the rising tectonic lands. The subsiding surface has been termed a "marginal basin" by European geologists (e.g., Beloussov, 1962, p. 314) and in America commonly is labeled an "exogeosyncline" (Kay, 1951, p. 17) or "foredeep basin" (Eardley, 1967, p. 39).

The detrital sediments include much coarse

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syntectonic material. Aggregate thickness of sedimentary rock ranges from a few thousand feet to more than 5 mi (8 km). Subsiding foredeep basins are complimentary to the tectonic lands rising from the miogeosyncline; thus, adjacent areas are moving vertically, but in opposite sense. Net horizontal movement during overthrusting and folding is from lands rising to areas subsiding, and occurs during the time of most pronounced vertical movement. Much of the foredeep sedimentary rocks eventually may be overlapped and included in the youngest compressional structures.

This paper investigates movements during deformation in one salient: the western Wyoming salient of the Cordilleran overthrust belt. Particular emphasis is placed on vertical movements and their relation to thrusting and folding.

WESTERN WYOMING SALIENT

GROSS STRUCTURE

The western Wyoming salient rises from below the Snake River lava plain in Teton and Madison Counties, Idaho, and trends southeastward into Wyoming; the frontal edge intersects the state line at the south end of the Teton Mountains (Fig. 2). The cratonward edge of the belt bends relatively abruptly in the vicinity of the Hoback River and strikes toward La Barge, Wyoming, in the vicinity of which the easternmost structures are developed. Southward the belt swings toward the west end of the Uinta Mountains in Utah. The salient is partly overlapped on the south end by sedimentary rocks younger than the overthrusts; however, well data indicate that the frontal edge probably enters Utah about 20 mi (32 km) east of the southwest corner of Wyoming. Structures at the frontal edge probably die out in the subsurface before intersecting the Uinta Mountains; however, folds genetically related to the salient are well developed near Coalville, Utah.

The north-south exposed length of the salient is more than 200 mi (322 km) and the width is about 60 mi (97 km). Major overthrusts, from west to east, in apparent age sequence, are the Paris, Meade, Crawford, Absaroka, Darby, Prospect, and Hogsback, with the oldest faults on the west (Armstrong and Oriel, 1965, p. 1860). Individual thrust plates have a maximum thickness of strata of 20,000-25,000 ft (6,100-7,630 m); however, a sedimentary section with an aggregate thickness of approximately 75,000 ft (22,880 m) is involved in thrusting (Rubey and Hubbert, 1959, p. 190).

Amplitude of folds ranges from those in single outcrops to thousands of feet; displacements on major overthrust faults are in the order of miles or tens of miles (Eardley, 1967, p. 37). Net transport was from hinterland to foreland (west

Fig. 1. A. Phanerozoic miogeosynclines of North America (after Kay, 1951). B. Overthrust belts of North America younger than Precambrian.

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Fig. 2. Tectonic map of western Wyoming salient. Structural contours east of overthrust belt drawn with datum on top of Mesaverde Formation (Cretaceous).

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to east). Asymmetrical folds face the craton. The structural motif ranges from locally intense imbrication and close folding to open folding.

Overthrust faults probably dip a few degrees west, southwest, or northwest, although they commonly intersect the surface at higher angles (Crosby, 1967). The existence of low dips may be inferred across extensive areas because of the common occurrence of parallelism of fault and stratigraphic surfaces. Only in a few local areas, however, is there strong evidence for a subhorizontal attitude of fault surfaces. Examples are the Absaroka thrust in the Bedford quadrangle (Rubey, 1958) and the Meade thrust in the Montpelier area (Armstrong and Cressman, 1963).

Crystalline basement appears to have been passive during deformation inasmuch as it is not exposed at the base of any thrust plate. Basement rocks were brought to the surface after overthrusting by movements along the Wasatch high-angle fault near Ogden, Utah. The nearest crystalline rocks exposed behind the overthrust belt in Idaho are 80 mi (130 km) west in the Albion Range. Precambrian Belt-like sedimentary rocks are exposed along high-angle faults in the vicinity of Pocatello, Idaho, but these are mechanically not unlike the overlying Paleozoic section.

Deformed rocks range in age from Cambrian to early Eocene. Oldest and youngest strata are exposed along the eastern edge of the salient; however, only early Paleozoic rocks crop out at the western edge. A large area in Bannock, Oneida, and Franklin Counties, Idaho, just west of the overthrust belt, is underlain exclusively by Cambrian and Precambrian Belt-like sedimentary rocks.

MOVEMENT PICTURE

The arcuate map pattern of the western Wyoming salient results from the plotting of axial plane traces of folds and the traces of overthrust faults. The general trend of individual faults and folds is that of the overthrust belt as a whole; i.e., the local structures parallel the regional structural grain. A measure of the arcuateness of the salient is the difference of approximately 75° in the average azimuths of axial trends between the north and south ends (Fig. 2).

Maximum principal stress generally is considered to be perpendicular to fold axes and fault traces that develop during deformation. This symmetrical relation is axiomatic where the symmetry of deformation is monoclinic (Turner and Weiss, 1963, p. 399). Insofar as deformation was monoclinic, and insofar as movements and stress can be tied to the gross geometry of structure in the western Wyoming salient, the directions of maximum principal stress, and hence movements, were seemingly radial around the salient.

Detailed structural geometry has been investigated throughout the salient (Crosby, in press) by measuring orientations of small-scale structures in 22 local areas. Detailed geometry was determined by statistical treatment of large numbers of data for each locality. Movement and stress directions were inferred from geometry, and a movement picture constructed by integrating the local movements.

The direction and sense of horizontal components of movement during folding and overthrusting were determined to be those shown in Figure 3. By this analysis the pattern of movements is radial and divergent across the salient. Moreover, the statistical point of divergence for the horizontal movement vectors is in northern Oneida County, Idaho (Crosby and Link, 1966, p. 45). This was determined by projecting the horizontal movement vectors westward and contouring the density of points of intersections. It should be stressed, however, that defining a statistical point of divergence does not imply that all thrust plates originated at a point.

CRETACEOUS FOREDEEP BASIN

Uplift that began in Late Jurassic time (Peterson, 1957, p. 433) shifted eastward across southeast Idaho and apparently increased in elevation during the Cretaceous. Concomitant with uplift on the west a narrow foredeep basin developed on the east at the edge of the foreland, and was filled by more than 20,000 ft (6,100 m) of sediments, largely syntectonic at the western edge (Fig. 4). Distribution and types of sediments suggest that the uplift was episodic in its rise and eastward shift. Subsidence and depositional rate increased from Early to Late Cretaceous (Rubey and Hubbert, 1959, p. 193), a fact that probably reflects the increasing intensity of uplift on the west.

The bases for the assertion that the uplift shifted eastward are the facts that (1) the westward

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limit of Cretaceous rocks is progressively farther east from oldest to youngest strata of the system, and (2) the axis of the trough shifted eastward (Armstrong and Oriel, 1965, p. 1855).

The basin is semilenticular with the steep side adjacent to the uplift. It exhibits the same arcuate pattern as the subsequently developed salient of the overthrust belt. The curvature, however, may be in part the result of fault displacements which have moved rocks in the trough a few tens of miles eastward. The thickest sediments are along the axis just west of the nose of the salient and the section thins toward reentrants in the overthrust belt in southwestern Montana and at the western end of the Uinta Mountains in Utah.

UPLIFT OF FOREDEEP BASIN

The Cretaceous System in western Wyoming contains marine beds throughout its great thickness. The Adaville, the highest wholly Cretaceous

Fig. 3. Horizontal movement vectors illustrating directions and sense of movement during folding and overthrusting at 22 localities in western Wyoming salient (modified from Crosby, in press). P is statistically defined point of diverging movement.

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formation, has marine beds containing both marine and brackish-water fossils (Schultz, 1914, p. 67). Abundant oyster-bearing sandstone bodies apparently were deposited near the shore. Intercalated with the marine sedimentary rocks are numerous coal and conglomerate units. These sedimentary rocks indicate deposition in a low-lying area which was subsiding and prograding to produce a fluctuating shoreline, and which occasionally received coarse clastic materials that reflect episodic uplift of the nearby mountains on the west.

The Cretaceous-Tertiary boundary is at the top of the Hams Fork Member in the Evanston Formation which overlies the Adaville. The position of the boundary within the Evanston is variable because of differential pre-Wasatch erosion. Although marine fossils have not been identified in the Evanston, it contains much coal, a few freshwater mollusks, and dinosaurian bone fragments (Rubey et al., 1961, p. 154). Apparently the Cretaceous strata were near sea level at the time of final deposition.

It is apparent from present-day elevations that the region has risen considerably above sea level. There is no datum within the overthrust belt that truly expresses the amount of primary vertical uplift because all beds which are Cretaceous and older have been deformed in horizontal compression. The predominantly horizontal movements during folding and overthrusting could have had significant vertical components. Just east of the frontal thrusts, however, Cretaceous and older strata are nearly planar, and at most dip a few degrees regionally eastward to the low side of the Green River basin. The beds in this zone are not involved in overthrusting; therefore, their present elevation is a measure of primary uplift from sea level following emplacement of the easternmost thrust plates, ne r the close of the Cretaceous Period. However, this assumption ignores the uncertain relation between Cretaceous and present sea levels.

The foreland edge of the overthrust belt generally is overlapped a short distance by Tertiary strata in Wyoming except at its intersections with

Fig. 4. Total Cretaceous isopach map depicting foredeep basin (compiled from Eardley, 1960; Weimer, 1961; Haun and Barlow, 1962). CI = 1,000 ft.

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the Gros Ventre and Teton uplifts in southwest Teton County, Wyoming. Undisturbed Cretaceous rocks are exposed in very few places. As a result of drilling activity in the La Barge complex, many well data are available from the nose of the salient. Drilling density is moderately great from the La Barge area north to Mickelson Creek, a distance of about 40 mi (64 km). A few wildcat wells have penetrated the section just in front of the overthrusts in the other areas.

Structural contours just east of the overthrust belt, shown in Figure 2, were drawn with datum on top of the Mesaverde Formation using data from Petroleum Information, Inc., completion cards. Although the Mesaverde is not the top of the system, it is identified readily in the subsurface and can be traced to outcrops in the Rock Springs area. The top of the Cretaceous usually is not reported. The Mesaverde is approximately equivalent to the Adaville in the overthrust belt, although the boundaries of the two formations doubtlessly transgress time lines. The top of the Cretaceous ranges from several hundred to several thousand feet above the surface represented by the structural contours.

The top of the Mesaverde exhibits a regional surface with a gentle westward rise. The highest structure is at the nose of the salient in the La Barge area. If the Mesaverde surface were projected westward into the deformed terrane, toward the axial part of the preexisting Cretaceous foredeep basin (Fig. 4), the surface would attain great heights. The question that needs to be answered is: where would the regional Mesaverde-Adaville surface be if there were some way of removing all vertical components of movement contributed by thrusting and folding? The Adaville Formation is exposed at elevations of about 2 mi (3.2 km) in front of the Absaroka thrust in the upper Fontenelle basin about 15 mi (24 km) west-southwest of La Barge, but these rocks presumably are above the Hogsback thrust.

The Mesaverde surface (Fig. 2) is more than 6,000 ft (1,830 m) above sea level in the La Barge area. Well control was selected from areas not affected by known local high-angle faults near Calpet, a few miles west of La Barge. The top of the Lance (uppermost Cretaceous) is reported in a few wells in the Green River basin. Its upper surface averages approximately 400 ft (122 m) above the Mesaverde in wells east of La Barge, but reaches a maximum of 650 ft (198 m) above the Mesaverde in the California Co. No. 2 well in Sec. 22, T26N, R110W. The top of the Cretaceous System, therefore, is more than 1 mi above sea level in the La Barge platform area.

The structural contours plunge below sea level in front of the overthrust belt approximately 62 mi (100 km) both north and south of the nose of the salient. More than anything else, these negative areas relative to the top of the Cretaceous reflect the inadequacies of using the difference in elevation between the top of the Mesaverde and present sea level as a measure of the amount of uplift. The Mesaverde surface was used as a datum for contours because it is a widespread unit on which structural data are available. This surface was itself depressed below sea level, and, therefore, its present elevation above the sea level records only partly the amount of uplift.

This discrepancy becomes particularly acute both north and south of the nose of the salient. Northward, for example, subsidence continued into early Tertiary time so that approximately 12,600 ft (3,850 m) of sediments of the Hoback and Pass Peak Formations accumulated during Paleocene and early Eocene times (Dorr, 1958, p. 1238). Furthermore, the elevations of the surface of sedimentation during those times were much lower than at present (p. 1239), and this implies that the top of the Cretaceous was depressed about 12,600 ft (3,660 m) below sea level. It is significant that overthrusting continued into the Paleocene and early Eocene at the west edge of this thick accumulation.

Within the overthrust belt, southwest from La Barge, a similar thickening occurs. This is demonstrated by the thickening of the Evanston-Almy-Knight section.

Use of the structural contours, with datum on the top of the Mesaverde Formation, as a measure of uplift requires application of two corrections: (1) projection upward to the top of the Cretaceous System somewhere in the Evanston Formation and (2) an adjustment that expresses the amount of the depression below sea level of the top of the Cretaceous during early Tertiary. Adequate data are not available to make satisfactory corrections in either category. The latter correction, however, is known to increase both north and south from the nose of the salient.

There is direct evidence of uplift of between

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6,000 and 7,000 ft (1,830-2,135 m) in the La Barge area. Application of the two correction factors suggests that the uplift may exceed 2 mi (3.2 km), particularly for a projection of the uplift a short distance westward into the overthrust belt.

HISTORY OF VERTICAL MOVEMENTS

The miogeosyncline had a long history of dominant subsidence and deposition from the Cambrian to the Late Jurassic. A local basin within the miogeosyncline in southeastern Idaho sank somewhat deeper than adjoining parts of the trough and received a maximum of about 50,000 ft (15,210 m) of sediment at a depocenter in southern Power County. The depocenter, it should be stressed, is defined by the aggregate thickness of Paleozoic and Mesozoic sedimentary rocks, and does not indicate the position of a depocenter for any particular system. The Wyoming shelf remained relatively positive during this long period. Three independent Paleozoic-Mesozoic isopach maps suggest this gross picture for the pattern of sedimentation prior to development of the overthrust belt (Blackstone, 1963, p. 167; H rberg, et al., 1949, p. 192; Crosby, in press).

Beginning in Late Jurassic time, the miogeosyncline began to break up, and by the end of the Mesozoic Era the former area occupied by the local basin had attained considerable height. Cambrian clasts in Cretaceous conglomerates (Armstrong, 1968, personal commun.) suggest that the western mountains were eroded deeply within a short period. The pre-Paleozoic surface that had subsided nearly 50,000 ft (15,220 m) rebounded beyond its former position to become a marked positive feature. Coincident with uplift in southeast Idaho, a marginal trough in western Wyoming subsided rapidly and filled with more than 20,000 ft (6,100 m) of sediment. This is a relative vertical movement of more than 70,000 ft (21,360 m) between the two adjacent areas.

A culmination in the uplift of the former miogeosyncline developed in southeast Idaho, approximately in the area of the preexisting depocenter. The evidence for the culmination is a broad area in Bannock, Oneida, and Franklin Counties wherein only Cambrian and Precambrian Belt-like sedimentary rocks crop out (Eardley, 1968). Younger rocks crop out in all directions from this region.

The point of diverging movement, deduced in the movement picture above, is on the west edge of the culmination. These tectonic elements are illustrated in the summary diagram (Fig. 5).

Overthrusting and folding occurred during uplift on the west and subsidence on the east. At some time after overthrusting, the vertical movements reversed again with a definite trend being established in the Oligocene or possibly as early as the late Eocene. The former Cretaceous foredeep basin rebounded to bring youngest Cretaceous marine rocks to their present elevations of more than 1 mi above sea level. At the same time the western positive area collapsed along a system of high-angle faults.

The elevations attained in the uplift in southeast Idaho and the amount of subsequent subsidence are unknown because of the lack of rocks of diagnostic age. That these vertical movements did occur, however, is suggested by the fact that

Fig. 5. Location map of tectonic elements that are important indicators of vertical movements. Diagonally lined area is Cambrian and Precambrian outcrops (after Eardley, 1968, Fig. 1). Dashed lines are total Paleozoic-Mesozoic isopach with depocenter (D), and point of diverging movement during overthrusting (P). Contour values are in thousands of feet. CI = 5,000 ft.

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erosion is much deeper on the west (the Precambrian is exposed) than on the east, although present surface elevations in Wyoming average 1,600 ft (488 m) higher than in southeast Idaho. Block faulting in Idaho and the rest of the Basin-Range province is considered by some (e.g., Mackin, 1960, p. 1921) to be an expression of subsidence. The point that is germane to the present problem, however, is the relative movements between the adjacent areas of southeast Idaho and western Wyoming.

ISOSTASY AND VERTICAL MOVEMENTS

On the premise that the principle of isostasy is one factor in vertical movements of this region, and that the present is a page in the long history of uplift and subsidence, the condition of isostasy has been investigated for a possible indication of continuing movements of the latest cycle. Earthquakes in this area suggest that crustal adjustments are occurring. Gravity data used in the following analysis of isostasy are reduced to the Bouguer anomaly.

SIGNIFICANCE OF BOUGUER ANOMALY

The Bouguer gravity anomaly is determined by the relationship

[EQUATION]

The elevation correction includes the free-air correction which adjusts observed gravity to mean sea level and the mass correction that accounts for the mass of the rocks between the station and mean sea level. No cognizance is made of possible lateral variations in density below sea level. The Bouguer anomalies are almost everywhere negative over continents, particularly in mountainous areas, and positive over ocean basins. This relation is readily explained by horizontal changes in mass below sea level, usually considered to be at the base of the crust.

The principle of isostasy holds that the crust is in a state of floating equilibrium. An additional load placed on the crust, such as a mountain range, should be compensated by low-density material at depth. Explosion seismology supports the idea that the mass deficiency at depth is in the form of low-density crustal material extending deeper than normal into relatively high-density mantle material beneath the excess mass of the mountains. Inasmuch as the excess mass at the surface is removed in the Bouguer anomaly reductions, the mass deficiency at depth remains and is reflected in the reduced gravity data. Hence, as the elevation becomes higher, the Bouguer anomaly for areas in isostatic balance becomes more negative.

An additional reduction, the isostatic correction, adjusts for the mass deficiency at depth on the basis of the average elevation surrounding the gravity station. This correction involves additional assumptions about mass distribution at depth and the depth of compensation. These added assumptions are avoided in a method that utilizes a relation developed by Woollard (1959). By plotting worldwide Bouguer anomaly values against their corresponding elevations, Woollard statistically determined the curve that fits the data (Woollard, 1959, Fig. 2, p. 1523). Data from any particular area may be compared with this mean world curve as an order of approximation test for isostatic equilibrium. Though the method is empirical, it is based on the rationale that most areas of the world are near e uilibrium, and that the world as a whole is isostatically balanced.

GRAVITY DATA

Published gravity data cover the southeastern Idaho region. Detailed coverage is available for a few local areas. Bonini (1963) prepared a Bouguer anomaly map of the state. La Fehr and Pakiser (1962) applied gravity data to an investigation of the structure of the Snake River plain. A detailed gravity survey was carried out in Gem Valley, Caribou County, by Mabey and Armstrong (1962). Cook et al. (1964) overlapped a short distance into Idaho in a gravity survey of the northern Great Salt Lake Desert. The gravity field in maps accompanying each of these reports is represented with Bouguer anomaly values.

The U.S. Coast and Geodetic Survey has published principal facts on pendulum stations in both Idaho and Wyoming (Duerksen, 1949). The 1381st Geodetic Survey Squadron, USAF, has established several stations in western Wyoming (C. T. Whalen, written commun.). The Bouguer anomaly map of Idaho overlaps a short distance into Wyoming; however, principal facts for stations outside Idaho are not published.

Additional gravity measurements were undertaken as part of this study because of inadequate gravity coverage in western Wyoming. Measurements

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were carried out using a Worden meter having a sensitivity of 0.0877 mgal per dial division. The average of three readings at each station was corrected for instrument drift. Base stations were read at intervals of 4 hours or less. In the main, elevations were taken at bench marks, or at points tied to nearby bench marks by hand leveling. Spot elevations at road intersections, and elevations from railroad surveys also were used.

Terrane corrections were not applied. However, an effort was made to select station locations at which the relief through zone E of the Hammer (1939) terrane correction chart, a distance of 1,280 ft (390 m), was similar to that at the base to which the station was tied. The station believed to be associated with the largest terrane effect was determined to have a 2.73-mgal terrane-correction difference between the station and its base. Average error from all causes is less than 2 mgal, but maximum error may exceed 5 mgal for two stations with questionable elevations (see Table I).

All gravity measurements were tied to the USAF gravity base at Cokeville, Wyoming, at which observed gravity is reported as 979772.008 mgal. Two supplementary base stations were established by the looping method of Nettleton (1940, p. 38). Data were reduced to the Bouguer anomaly using standard procedures. Principal facts for these stations are tabulated in Table I.

RELATION TO OVERTHRUST BELT

Gravity data were separated for analysis into areas apparently experiencing opposed vertical movements, as discussed in a preceding section. Totals of 113 and 53 stations are used, respectively, in southeast Idaho and the area of the foredeep basin, mostly in Wyoming. Station locations are shown in Figure 6.

The group of stations in southeast Idaho is within the concavity of the western Wyoming salient. The data are those of Bonini (1963) bounded by lat. 42°-43° and long. 111°30^prime-113°. Inasmuch as Bonini's measurements provide adequate coverage, and the principal facts are published, the analysis of this area is restricted to his

Table I. BOUGUER ANOMALY VALUES IN WESTERN WYOMING OVERTHRUST BELT

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data. Moreover, using data obtained in a single survey improved the internal consistency.

The elongate grouping of stations on the east (Fig. 6) coincides with the position of the former foredeep basin, and is within the overthrust belt or just east of it. Stations are concentrated particularly toward the frontal edge where stratigraphic information on vertical movements is available. No single survey gives adequate coverage in this area. U.S. Coast and Geodetic Survey stations 1054-1059, 1063-1067, and 1070-1075 (Duerksen, 1949), Bonini's (1963) stations 932, 956, 1336, and 1343, the Air Force's Afton, Cokeville, and Jackson stations, and 29 stations by the writer are combined.

The average Bouguer-anomaly value in the region behind the overthrust belt in southeast Idaho is 171.4 mgal. By contrast, the average in the overthrust belt is 228.9. The difference of 57.5 mgal, more negative on the east, indicates that an additional slab 1,700 ft thick (516 m), with a density of 2.67 g/cm³, can be supported isostatically in the overthrust belt, if it is assumed that southeast Idaho is in isostatic balance.

The average elevation of all gravity stations in the Idaho area is 5,098 ft (1,551 m), and in Wyoming 6,701 ft (2,044 m). Station elevations are not representative, however, because gravity measurements are taken mainly in the valleys. Because this is true for both areas, the value for the elevation contrast probably would not change significantly, if the representative values were determined. Insofar as the station elevations differ from the average by a like amount, the small discrepancy in the actual thickness of the additional crust above sea level in Wyoming, about 100 ft (30 m), suggests that (1) the overthrust belt is slightly overcompensated, (2) southeast Idaho is slightly undercompensated, or (3) both. The third alternative would imply a slight tendency for the overthrust be t to rise and the area behind it to subside. The discrepancy is small, however, and possibly could disappear with a more rigorous analysis of elevations in the two areas.

Perhaps a more meaningful approach is to plot

Fig. 6. Location of gravity stations used in determination of condition of isostasy.

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Bouguer anomaly values against corresponding elevations for both areas, and to compare the distributions with Woollard's mean world curve (Fig. 7). Most of the gravity data from the overthrust belt plot on the negative side of the mean world curve, whereas the reverse is true for the area of the former uplift behind the overthrust belt. The average departure is 17.5 mgal negative for data in the overthrust belt, and 6 mgal positive for the Idaho area. The statistical departures are small, but the fact that they are on opposite sides of the mean world curve appears to be significant. It should be stressed, however, that gravity data from a particular area may depart markedly from the mean world curve and the area still be in isostatic balance, if locally there are anomalous densities i the crust or upper mantle. Should this be true, a geologic correction is required.

Such a correction should, in fact, be applied to the Idaho data. Gravity measurements were reduced to mean sea level by use of the commonly accepted value of 2.67 g/cm³ for crustal density (Bonini, 1963, p. 5). If gravity data are used to investigate isostasy this practice introduces error where density differs from this value. Southeast Idaho, together with other parts of the Basin-Range province, is characterized by a series of mountain ranges, composed of Paleozoic bedrock, alternating with intermontane valleys filled with low-density Cenozoic clastic rocks.

In gravity interpretations elsewhere in the province, density contrast between the two materials generally has been assigned a value of 0.4 g/cm³ (e.g., Cook et al., 1966, p. 69; Johnson and Cook, 1957, p. 53) or 0.5 g/cm³ (e.g., Stewart, 1958, p. 1155; Thompson, 1959, p. 221). Depth of valley fill commonly is more than 6,000 ft (1,829 m) (Cook et al., 1964, p. 738), and may reach 12,000 ft (3,660 m) or more (Cook et al., 1966, p. 69).

In the two local areas in Idaho which are covered by detailed gravity surveys, Gem Valley in Caribou County and Curlew Valley in southeast Oneida County, the maximum depth of the valley fill is interpreted to be 9,000 ft (2,743 m) (Mabey and Armstrong, 1962, p. D74) and 6,000 ft (1,829 m) (Cook et al., 1964, Fig. 4), respectively. Maximum amplitude of gravity anomalies in the Basin-Range province is as much as 60 mgal between valleys and adjacent ranges (Mabey, 1960, p. B283). The "half amplitude" of the gravity anomaly in Gem Valley is 35 mgal,

Fig. 7. Graph of Bouguer anomaly values versus corresponding station elevations. Solid circles are data from southeast Idaho; open circles are data from overthrust belt.

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which suggests that a total amplitude of more than 35 mgal will be observed if gravity control is extended into the adjacent mountains with the same detail as that across the valley.

Thompson (1959, p. 221) concluded that more than half of the material above sea level in the area between Hazen and Austin, Nevada, is low-density valley fill. About half bedrock and half valley fill above sea level also is estimated for this part of the Basin-Range province. On this assumption it would appear that half the stations should be reduced to sea level by use of a density value smaller than 2.67 g/cm³, but the number of stations requiring this geologic correction is even greater. In the reconnaissance survey of Bonini, measurements were made along roads, which are in valleys parallel with ranges. The roads cross the mountains in short stretches through low drainage divides. Fully 85 percent of the measurements were made across the debris-filled grabens--in the ar as of gravity lows.

Conservatively taking the density contrast value of 0.4 g/cm³ and ignoring the possible presence of Cenozoic deposits below sea level, the geologic correction is

[EQUATION]

where 5,098 is the average elevation of all stations, and G is the universal gravitational constant in English units. Adding the 22.1-mgal correction to the 6-mgal average departure from the mean world curve, there would be 28.1 mgal on the positive side of the theoretical equilibrium.

A smaller geologic correction must be applied to the average Bouguer anomaly for the overthrust belt. Block faulting decreases in magnitude from southeast Idaho into the overthrust belt, whereas post-thrusting, high-angle faults persist to the eastern edge of the fold and thrust belt. The easternmost grabens that are filled with significant amounts of Cenozoic deposits are the Cokeville and Star-Grand Valleys near the Idaho-Wyoming border. In addition to five stations in these valleys, five stations are on a significantly thick section of undeformed Tertiary Hoback and Pass Peak Formations north of La Barge and just east of the overthrust belt. By applying a similar treatment to these 10 stations, the geologic correction is 6.5 mgal. This value added to the average of -17.5 mgal for t e overthrust belt gives a corrected average value of 11 mgal negative with respect to the mean world curve. Thus, the difference in the averages for the two areas is 39.1 mgal.

The large contrast of about 40 mgal between the overthrust belt and the area behind it, and the fact that one is negative and the other positive relative to the mean world curve, appear significant in terms of isostasy.

THEORETICAL CONSIDERATIONS

On the basis of this analysis, it is concluded that the overthrust belt is rising and the southeast Idaho area is subsiding relative to each other. The combined gravity and geologic data suggest that the overthrust belt, at least, is rising absolutely with reference to sea level. There are uncertainties inherent in these assertions because the distribution of density values in the deep crust and upper mantle is unknown. This unknown quantity has prompted some (e.g., Mabey, 1966, p. 110) to suggest that isostasy should be investigated with a mean curve developed on a region-to-region basis rather than with one based on worldwide data. The attempt to correct for density variations above sea level, included in this treatment, should improve the basis for use of the mean world curve. More ver, contrast between the two areas is of more importance here than absolute values.

The interpretation presented here appears to be valid concerning certain points: (1) near-surface variations in mass affect local gravity values more than deep-seated compensating masses; (2) statistical departures from the mean world curve between the two areas are relatively large; (3) the signs are reversed; and (4) the interpretation is consistent with geologic evidence for most recent vertical movements.

Inasmuch as the isostatic mechanism cannot drive vertical movements until some other mechanism disturbs the condition of balance, it may seem surprising that adjustments are continuing so long after orogenesis in the overthrust belt. Emplacement of the final thrust mass occurred about 60 million years ago. Crittenden (1963, p. 5526) estimated the relaxation time for the substrate beneath parts of Utah, Nevada, and Idaho to be approximately 4,000 years, based on rebound inferred from shoreline elevations of former Lake Bonneville. The relaxation time is defined

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as the time in years required to reduce the isostatic imbalance to 1/e of its initial value, where e is the base of natural logarithms.

If, during mountain building that produced the fold and thrust belt, a state of unbalance was created, equilibrium long since would have been restored, and even the relatively minor gravity effect observed would not have persisted. Furthermore, imbalance in the degree of approximately 40 mgal cannot be explained by adjustments lagging erosion. The rate of adjustment is proportional to the degree of imbalance, and is related directly to the physical properties of the substrate. With a denudation rate of a few inches per thousand years, and an equivalent viscosity of the mantle of 10²¹ poises (Crittenden, 1963, p. 5526), the rates of erosion and isostatic adjustment are equal if the imbalance is the mass of a slab approximately 4 ft (1.2 m) thick. The gravity effect is about .14 mgal.

Whatever the process that produced the vertical movements of the surface in this region, whether it be phase changes at the crust-mantle boundary (Kennedy, 1959; Thompson, 1960), horizontal transfer of deep crustal material (Gilluly, 1963, p. 157), migration of partial melts (Scheidegger and O'Keefe, 1967), subsidence due to ascent of magmas to the surface (Mackin, 1960, p. 1921), massive basaltic intrusions (Eardley, 1963, p. 217), mantle diapirs (Maxwell, 1968, p. 40), or some unknown process, it must be slow and progressive through time.

The present situation suggests, applying the principle of uniformitarianism, that the process or processes causing post-thrusting movement have, through long duration, operated to maintain slight isostatic anomalies. Alternatively, the process(es) may have been active periodically, producing small anomalies which later disappeared in compensatory adjustments. The history of block faulting in this area since the Eocene (Armstrong and Oriel, 1965, p. 1862) apparently supports this concept.

SUMMARY AND CONCLUSIONS

Throughout the Paleozoic and most of the Mesozoic Era the miogeosyncline subsided and was filled to a maximum thickness of about 10 mi (16 km) in a local depocenter within the miogeosynclinal trough in southeast Idaho. During this long period of slow subsidence the Wyoming shelf remained relatively positive (Fig. 8A). Beginning in Late Jurassic time the vertical movements reversed, with orogeny culminating during the earliest Tertiary. An area centered in Oneida and Bannock Counties, Idaho, rose an amount equal to previous subsidence plus an additional mile or more. The axis of uplift apparently shifted eastward. During the period of uplift a foredeep basin developed nearby in western Wyoming with particularly rapid subsidence during the Late Cretaceous

Fig. 8. Diagrammatic restored west-east sections through southeast Idaho and western Wyoming. A. early Mesozoic. B. Late Cretaceous. C. Holocene. Shaded layer is Precambrian Belt-like sedimentary rocks.

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(Fig. 8B). Sinking amounted to about 4 mi (6.5 km). Overthrust-belt structures developed, from west to east, while the western area was rising and the eastern area was subsiding. Net horizontal movement was from west to east. Since the time of latest overthrusting, the opposed vertical movements have reversed once more with southeast Idaho subsiding an unknown amount and the eastern edge of the western Wyoming salient rising approximately 2 mi (Fig. 8C). Isostatic evidence is in agreement with the final complementary vertical movements.

During the two periods in which the western area was subsiding and the eastern area rising, vertical movements and depositional rates were slow, and there was no marked development of surficial compressional structures. During the intervening time, when the western area was rising and the eastern area subsiding, vertical movements and deposition rates were relatively rapid. Folds and thrusts developed in the sedimentary rocks at this time. This rate variation appears to be reconciled more easily to a mechanism involving vertical movements accommodated by net horizontal flow in the deep crust or mantle than to phase changes at the crust-mantle boundary.

The history of vertical movements has important mechanical implications for the origin of overthrusts in the western Wyoming salient. These implications stem from the following points: (1) gross vertical movements of the two adjacent areas always have been opposed since the Precambrian; (2) the time of overthrusting corresponds to vertical movements of greatest magnitude; (3) overthrusting occurred approximately during the time of most rapid uplift and subsidence; (4) overthrusting was away from the rising lands and toward the subsiding area; (5) net surficial movements during folding and overthrusting were approximately parallel with the dip vector on the Precambrian surface, and, most significantly, (6) post-thrusting vertical movements have tended to reverse the dips on overthrust aults from east to west.

These factors strongly suggest that folding and thrusting are secondary responses to vertical movements. Inasmuch as there appears, to the writer, to be nothing in the areal geology of southeast Idaho to suggest the existence of a former boundary force to provide a rearward push for the overthrust masses, the origin of thrust plates in the western Wyoming salient are relegated here to a gravitational gliding mechanism.

Overthrusts in the western Wyoming salient have been investigated in terms of their time and space relations (Armstrong and Oriel, 1965), detailed mechanics (Rubey and Hubbert, 1959), and the gravitational gliding mechanism (Eardley, 1967). An attempt has been made in this investigation to shed some light on geological evidences for vertical movements that are related to overthrusting, and possibly furnish minor corroborating evidence for latest movements from isostatic considerations. Overthrusting by body forces seems inherently more appropriate than boundary forces if compressional deformation is viewed with reference to vertical movements that preceded, accompanied, and followed overthrusting. Evidence against the gravitational gliding hypothesis is that present-day dips on overth ust faults seemingly indicate that the thrust plates moved uphill. The latest opposed vertical movements in western Wyoming and southeastern Idaho would overcome this objection in that such movements have tended to reverse, or have reversed, the azimuth of the dip vector on the gliding surfaces from east to west.