The Laccolith-Stock Controversy: New Results from the Southern Henry Mountains, Utah

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The laccolith-stock controversy: New results from the southern Henry Mountains, Utah

MARIE D. JACKSON* Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 DAVID D. POLLARD Departments of Applied Earth Sciences and Geology, Stanford University, Stanford, California 94305

ABSTRACT rule out the possibility of a stock at depth. At Mesa, Fig. 1). Gilbert inferred that the central Mount Hillers, paleomagnetic vectors indi- intrusions underlying the large domes are Domes of sedimentary strata at Mount cate that tongue-shaped sills and thin lacco- floored, mushroom-shaped laccoliths (Fig. 3). Holmes, Mount Ellsworth, and Mount Hillers liths overlying the central intrusion were More recently, C. B. Hunt (1953) inferred that in the southern Henry Mountains record suc- emplaced horizontally and were rotated dur- the central intrusions in the Henry Mountains cessive stages in the growth of shallow (3 to 4 ing doming through about 80° of dip. This are cylindrical stocks, surrounded by zones of km deep) magma chambers. Whether the in- sequence of events is not consistent with the shattered host rock. He postulated a process in trusions under these domes are laccoliths or emplacement of a stock and subsequent or which a narrow stock is injected vertically up- stocks has been the subject of controversy. contemporaneous lateral growth of sills and ward and then pushes aside and domes the sed- According to G. K. Gilbert, the central intru- minor laccoliths. Growth in diameter of a imentary strata as it grows in diameter. After the sions are direct analogues of much smaller, stock from about 300 m at Mount Holmes to stock is emplaced, tongue-shaped sills and lacco- floored intrusions, exposed on the flanks of nearly 3 km at Mount Hillers, as Hunt sug- liths are injected radially from the discordant the domes, that grew from sills by lifting and gested, should have been accompanied by sides of the stock (Fig. 4). Mount Holmes, bending of a largely concordant overburden. considerable radial shortening of the sedi- Mount Ellsworth, and Mount Hillers are taken According to C. B. Hunt, the central intru- mentary strata and a style of folding which is to represent successive stages in the development sions are cylindrical stocks, sheathed with a not observed. Geologic and geophysical data of this process. Although the inference that the zone of shattered sedimentary rocks, and the and mechanical models support a laccolithic central intrusions in the Henry Mountains are small flanking sills and laccoliths grew later- origin for the central magma chambers under- stocks has found acceptance in some textbooks ally as tongue-shaped masses from the dis- lying the domes. (Shelton, 1966, p. 16-17), the nature of these cordant sides of these stocks. New geologic bodies is recognized as problematic in others mapping demonstrates that the sedimentary INTRODUCTION (Billings, 1972, p. 341). Indeed, the present overburden, now partially eroded from the erosional level in the Henry Mountains makes domes, was uplifted about 1.2 km at Mount Toward the end of the last century, G. K. it difficult to distinguish Hunt's stocks from Holmes, 1.8 km at Mount Ellsworth, and at Gilbert (1877) originated the concept of a lacco- Gilbert's laccoliths as the principal constituents least 2.5 km at Mount Hillers. The radii of the lith, on the basis of his observations in the Henry of the large domes, on the basis of surficial domes are similar, between 5 and 7 km. The Mountains of Utah. He postulated a process in mapping alone. strata over the domes have a doubly hinged which magma rises vertically in a dike or nar- To clarify the differences between laccoliths shape, consisting of a concave-upward lower row pipe-like conduit until it spreads between and stocks as illustrated in Figures 3 and 4, the hinge and a concave-downward upper hinge. horizontal strata to form a sill. The sill propa- following points are emphasized. A limb of approximately constant dip joins gates laterally until it lifts the overburden and (1) Laccoliths may be low in height relative these two hinges and dips 20° at Mount causes the sedimentary strata to bend concor- to their horizontal dimensions, and they range Holmes, 50° to 55° at Mount Ellsworth, and dantly over the thickening laccolith. In the from circular to tongue-shaped in plan form. 75° to 85° at Mount Hillers. The distal por- southern Henry Mountains (Fig. 1), sedimentary Stocks have a great height relative to a roughly tion of each dome is composed of a gently domes forming Mount Holmes, Mount Ells- constant diameter, and they approximate a tall, dipping peripheral limb 3 to 4 km long, pre- worth, and Mount Hillers (Fig. 2) are approxi- upright, circular cylinder. sumably underlain by sills and minor lacco- mately equidimensional in plan form and have (2) Laccoliths have a local feeder, such as a liths. Although geologic cross sections and radii of 5 to 7 km. Near the summits of Mount dike or stock, which has a very different size and regional aeromagnetic data for the three Ellsworth and Mount Hillers, large masses of mechanism of formation from those of the lacco- domes are consistent with floored, laccolithic diorite porphyry are exposed and are interpreted lith. Stocks do not have a local feeder; they are central intrusions, these data alone do not as the tops of central intrusions. Numerous dikes continuous to great depth, perhaps extending to and sills of diorite porphyry crop out in and a deep magma reservoir. adjacent to the domes, as do small laccoliths that •Present address: U.S. Geological Survey, Hawaii (3) Stocks grow upward, perhaps by stoping, National Park, Hawaii 96718. have well-exposed floors (for example, Black zone melting, and/or diapiric piercement as dis-

Geological Society of America Bulletin, v. 100, p. 117-139, 19 figs., 2 tables, January 1988.

117

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Figure 1. Simplified geologic map of the southern Henry Mountains (after Hunt, 1953, and this study). Younger rocks crop out pro- gressively from east to west, indicating a gentle 1° to 2° dip to the west-southwest. Open triangles indicate mountain summits. Out- lined areas indicate maps shown in Figures 7, 8, and 9.

cussed by Daly (1933) and Marsh (1982), and so they are not floored and they may be largely discordant. Laccoliths grow from a thin sill that thickens into a floored body, and so they are largely concordant. Distinguishing between laccoliths and stocks is made more difficult because laccoliths can attain greater height by peripheral faulting. This produces bysmaliths, bodies that have discordant sides but are floored. The purpose of this paper is to introduce new evidence that bears on the laccolith-stock controversy and on the origins of epizonal magma chambers. We describe the structure of the three southern domes in the Henry Mountains on the basis of new geologic mapping. We give detailed descriptions of the host-rock flexures, using cross sections through each dome. Models of the shapes and volumes of the central intrusions based on regional aeromagnetic surveys are summarized, and the sequence of intrusion is interpreted using paleomagnetic data from sills and thin laccoliths at Mount Hillers. Finally, some of the mechanical differences that might characterize laccoliths and stocks are reviewed. From this work, a sequential progression of magmatic and deformational events is inferred and compared with the previous hypotheses of Gilbert (1877) and Hunt (1953).

GEOLOGIC SETTING

The Henry Mountains, in southeastern Utah (Fig. 1), are one of more than ten mountain ranges on the central and eastern Colorado Pla- teau composed of sedimentary domes with igneous cores, surrounded by dikes, sills, and minor laccoliths. The ages of these ranges cluster 110°45' 110° about the Cretaceous-Tertiary boundary and the mid-Tertiary (Armstrong, 1969; Cunningham and others, 1977). The Henry Mountains form a range that is about 60 km long, trends roughly 48 Ma. Sullivan (1987), however, has found The mountains are situated on the gently dip- north-south, and is composed of 5 peaks sepa- younger ages for these rocks, 20 to 29 Ma, ping (about 2° west) eastern limb of a north- rated by low passes. From north to south, the using fission-track methods. In the southern south-trending structural basin that is bounded peaks are Mount Ellen, Mount Pennell, Mount Henry Mountains, all of the intrusive rock on the west by the Waterpocket monocline Hillers, Mount Holmes, and Mount Ellsworth. is diorite or quartz diorite porphyry. The (Hunt, 1953, Plate 1). A stratigraphic section K-Ar dating of the diorite porphyry from flank- petrology of these rocks is described by Engel (Fig. 5) compiled from data of Hunt (1953), ing laccoliths at Mount Ellen and Mount Hillers (1959), and Kilinc (1979) has studied their Hintze (1963), Peterson and others (1980), and by Armstrong (1969) gives Eocene ages, 44 to crystallization temperatures. Stokes (1980) shows estimated thicknesses of

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the formations. About 2.7 km of strata, from the p. 51) gives a maximum of 1 km of lower Ter- about 3.7 km of sedimentary rock overlay the base of the Permian Cutler Formation to the tiary sediments presently overlying the Mesa Permian Cutler Formation during the early to Cretaceous (Campanian) Mesa Verde Forma- Verde Formation about 100 km west of the mid-Tertiary, when the intrusions were em- tion, is exposed in the Henry Mountains. A strat- Henry Mountains in western Garfield County. placed. Stratigraphie sections compiled by igraphie section compiled by Doelling (1975, According to these estimates, a maximum of Hintze (1963, p. 154-155) and Doelling (1975, p. 17) show between 1.3 and 1.9 km of stratified rocks between the base of the Permian Cutler Formation and crystalline basement rocks.

CONSTRUCTION OF GEOLOGIC MAPS AND CROSS SECTIONS

Mount Holmes, Mount Ellsworth, the southern flank of Mount Hillers, and the gently dipping sedimentary rocks surrounding these domes (Fig. 1) were mapped during the course of this

Figure 2. (a) Aerial photograph of the south flank of Mount Hillers. Interleaved sills, thin laccoliths, and sedimentary host rocks form concentric rings around the dome, (b) Cross-sectional view of the south flank of Mount Hillers. A long peripheral limb extends several kilometres beyond steeply dipping sedimentary rocks within the central limb of the flexure.

Mount Hillers

Mount Pennel

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sea level

Figure 3. G. K. Gilbert's concept of laccoliths in the Henry Moun- tains (after Gilbert, 1877). (a) Geologic cross section of Mount Hillers, striking N35°W. Diorite is in black, (b) Gilbert's interpretation of the subsurface structure of Mount Hillers. (c) Idealized laccolithic intrusion with a narrow feeder at its base.

study (Jackson, 1987). Relevant parts of this angulation net, because topographic base maps 1:24,000 topographic base for Mount Holmes mapping are reproduced herein (Figs. 6, 7, 8, were not available in the 1930s. To draw com- and Mount Ellsworth and to a 1:12,000 base for and 9). These maps provide more accurate rep- plete cross sections through the domes, we used Mount Hillers, using a PG-2 stereoplotter. Ele- resentations of the structures and geology than new aerial photography, topographic base maps, vations and positions of contacts, faults, and were possible in the past. Gilbert (1877) made and a PG-2 stereoplotter in our mapping. other geologic features were located within sketch maps, which give the general geology of Throughout most of the study area, the geol- about ±10 m. To obtain greater accuracy, the the domes. Hunt (1953) mapped at a scale of ogy was mapped in the field on aerial photo- strikes and dips of the gently dipping sedimen- 1:31,680, using an alidade, plane table, and tri- graphs. This information was transferred to a tary rocks surrounding the domes were mea-

Mount Shattered . Mount Mount Hillers Ellsworth zone, Stewart .Ridge Buckhorn , Shattered stock 10000ft stock stock ,Ridge laccolith I (3048 m) Jgc A

Figure 4. Structure contour maps and cross sections of Mounts Holmes, Ellsworth, and Hillers, illustrating C. B. Hunt's concept of the relationship between the stocks and uplift of the beds (after Hunt, 1953). Black = igneous rocks. Stipples = shattered zone. K = Cretaceous rocks. Jsr = San Rafael group rocks. Jgc = Glen Canyon group rocks. U = Triassic rocks. P = Permian rocks.

È sured using surveying techniques and projections < from the geologic maps. H Eocene .• Intrusive Rocks: Radially oriented cross sections (Fig. 10) ec -1000m Tertiary Diorite Ui were constructed through sectors of the domes h* Paleocene (Doelling,1975) Porphyry; Td that contain no faults having map-scale dis- Upper Cretaceous } placements and no large intrusions which dis- Mesa Verde rupt the curvature of the beds. We used a modified Busk method (Ragan, 1973) to recon- Masuk CO struct the flexure of the unexposed and eroded =} Emery host rocks. At the peripheries of the domes, the O LU 400meters formation thicknesses are based on regionally O< measured thicknesses (Fig. 5). Within radii of H about 4 km, where curvature of the beds is sig- LU Blue Gate OC nificant, the formations at Mount Ellsworth and O Mount Hillers were thinned to the deformed Ferron thicknesses observed on the mountain flanks. Tununk;Kmt The host rock flexures were constructed with Dakota; Kd continuously changing curvatures to match the ' Brushy Basin; Jmb smoothly arched beds over the domes. At Sawtooth Ridge Mount Holmes and Mount Ellsworth, the arc Salt Wash;Jms •Laccolith Black Mesa lengths of the upper layers are essentially equal O Summerville; Js 'Laccolith w to those of the lower layers. Because of the large curvature at Mount Hillers, however, the Busk Vi< Entrada; Je OC Jo construction gives upper layers that are about 3 Carmel; Jca 10% longer than the lower layers. The upper Navajo; J-Trn contacts of the central intrusions are drawn to be concordant with the constructed flexures. This is o° O Kayenta;Trk consistent with the exposed structural relations V) at Mount Ellsworth and Mount Hillers. V)< Wingate;Trw Chinle;Trc deepest rocks Moenkopi;Trm _ "exposed at GEOLOGIC DESCRIPTIONS Mount Holmes OF THE DOMES z White Rim; Pew < Organ Rock; Peor _Mount Ellsworth Mount Holmes S roof rocks 0C Cedar Mesa; Pccm x ^ Ui _Mount Hillers Mount Holmes (Gilbert, 1877, p. 27-28; O. 5 roof rocks Elephant Canyon Hunt, 1953, p. 134-137) is composed of two SEA LEVEL domes, capped by Jurassic sedimentary rocks that dip symmetrically outward. We focus on z Honaker Trail UJ.,..L,LJ. < TV'',' i .' i the larger dome (Figs. 6, 7), which forms the z mountain summit. There, three 20- to 30-m- thick, radially striking, vertical diorite dikes crop Ë out and extend down the flanks of the mountain. > Certain of these dikes crop out in discrete seg- zV) Figure 5. Stratigraphie sec- z Paradox tion for the Henry Mountains ments. The edges of these segments plunge Ui basin at the time of intrusive ac- steeply, suggesting that the dikes propagated a. TXT tivity. Stratigrahic units from the upward along steeply inclined trajectories (Pol- "co" Cretaceous Dakota Formation lard and others, 1975). Within the more steeply CO Red wall dipping (about 20°) central limb of the flexure, /rrrj to the Permian Cutler Forma- tion are exposed in the map porphyritic sills and small laccoliths (Fig. 7, locations 1, 2) intrude the sedimentary layers and U> i Ouray-Elbert area. Abbreviations of strati- o graphic units refer to labels used also dip about 20°. In plan view, some of these intrusions (for example, the Buckhorn Ridge z Lynch I? I Dolomite in cross sections (Fig. 10). < laccolith and the sill at location 2) have the tt V771 cc Unamed shale tongue-like shape described by Hunt (1953, m Muav S p. 90-91). Johnson and Pollard (1973, Bright Angel p. 270-272) described the cross-sectional form z Tapeats of the Buckhorn Ridge laccolith. < No large central igneous body crops out at the OC + + + + + top of the dome, and the location of an intrusion . 00 Crystalline basement LUS that might core the mountain can only be inferred. The dip directions of beds, measured CLSFO<

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MAP EXPLANATION a few metres of the contacts of dikes, sills, Figure 7. Geologic map of Mount Holmes. contact and small laccoliths. See Figure 6 for explanation. See Figure 1 for Hunt (1953, Plate 15) mapped a shatter zone location. inferred contact of Navajo Sandstone south of the large dike that • covered contact trends south from the summit of Mount Holmes .> fault, showing downthrown block (Fig. 7, location 4). There, the sandstone is y jointed and has been intruded by several pod- J geologic structure described in text like intrusions that may be the fingers of a gently along the radial cross section, dips in the Salt dipping laccolith. The dip of the beds is consist- Wash Member of the Morrison Formation are ently about 15° southeast in this area, and 4° to 6° northwest (Fig. 7, location 5). This Map Units there is no pervasive brecciation, metamor- could be due, in part, to the regional dip. At a phism, or invasion of magma into the sand- radial distance of nearly 6 km, the strata reverse Quaternary deposits stone. their dip direction and begin to turn gently up- Tdiorite porphyry Figure 10a shows a radially oriented cross sec- ward against the flank of Mount Hillers. There- B tion through the northwest quadrant of Mount fore, the radius of the dome is between 5 and K. Dakota sandstone Holmes. The flexure includes two broad open 6 km, as Hunt (1953, p. 134, Plates 15 and 16) BB J. Brushy Basin shale hinges and two limbs; a long peripheral limb is also noted. joined to a concave-upward lower hinge fol- J. Salt Wash sandstone sw lowed by a central limb and concave-downward Mount Ellsworth J.Summerville Fm. upper hinge. The beds steepen gradually up the mountain flank from 4° to 6° dips in the Jurassic This study adds to the descriptions of Mount J. Entrada sandstone Salt Wash Member of the Morrison Formation Ellsworth by Gilbert (1877, p. 22-27), Hunt J.Carmel Fm. to maximum dips of 23° in the Triassic Kayenta (1953, p. 137-139), and Koch (1981). Over the Formation. In the outcrops of Navajo Sandstone entire dome, the dip directions of beds and for- J-Tr. Navajo sandstone N that lie closest to the summit, the dips decrease mational contacts are radially symmetric from a Tr. Kayenta Fm. to about 3°. point that is about 400 m northeast of the Within the central limb of the flexure, mountain summit (Figs. 6, 8). The sedimentary Tr. Wi ngate sandstone bedding-plane fault zones crop out at some for- formations describe a nearly circular outcrop Tr Chinle Fm. mational contacts and along certain truncating pattern that is slightly elongate to the northeast. surfaces of large sandstone dunes. The locations Diorite porphyry, locally altered by hydrother- Tr. Moenkopi Fm. of these fault zones (Fig. 10a) indicate a spacing mal activity to a crumbly white mass, crops out 2 P White Rim sandstone within the central limb of about 150 to 200 m. near the summit over an area of about 1.5 km The fault zones are <1 m thick and consist (Fig. 8, location 1). These rocks form the upper P Organ Rock shale OR of thin deformation bands (Aydin, 1978) that part of a central intrusion or intrusive complex underlying the dome. Breccias composed of P Cedar Mesa sandstone form a complicated network (Fig. 11). Minor bedding-plane faults, composed of a single diorite and/or sedimentary clasts from the deformation band, occur between the fault White Rim sandstone member of the Permian contour interval 800 ft (243m) zones. Because the fault zones are oriented paral- Cutler Formation and from the Triassic Moen- lel to the beds, the shear displacements of the kopi Formation also crop out near the summit. Figure 6. Explanation for geologic maps layers are not known. Slickenline data, however, About 0.5 km from the summit, the White shown in Figures 7, 8, and 9. indicate that the layers slipped over one another Rim sandstone is exposed in the concave- in more-or-less radial directions. downward upper hinge of the flexure. The The curvature of the strata in the upper por- Organ Rock shale member of the Permian tion of the cross section was computed using Cutler Formation (Fig. 8, location 2) is the old- mostly on the truncated surfaces of large sand- finite differences in elevation measured at 300-m est sedimentary unit exposed on the mountain. stone dunes, however, are radially symmetric increments along the base of the Navajo Sand- These Permian rocks abut against diorite por- about the western 180° of the mountain (Fig. 7). stone (Fig. 12a). The curvature has a small nega- phyry of the central intrusion. Field relations These data and the structure contour map of tive value (about -4.5 x 10 4 m_1) in the upper suggest that this contact is a fault, along which Hunt (Fig. 4, Mount Holmes) suggest that at hinge, it is negligible within the central limb, and the Permian rocks were uplifted over the center least the western part of the intrusion or intru- it has a small positive value (about 4.0 x 10~4 of the dome. Near the center of the dome, blocks sive complex underlying the dome is approxi- m-1) within the lower hinge. At a radial distance of the Triassic Wingate, Chinle, and Moenkopi mately circular in plan. The deepest sedimentary of about 4 km from the center of the dome, the Formations, ranging from 100 to 300 m on a unit exposed on the mountain is the Triassic curvature approaches zero and the beds dip 7° side, are underlain by diorite and are displaced Chinle Formation (Fig. 7, location 3). The Ju- to 8° northwest. along steeply dipping map-scale faults (Fig. 8, rassic Navajo Sandstone crops out near the The regional dip must be removed from the location 3). Diorite dikes occupy most of these summit, and, aside from being cut by dikes and northwest end of the cross section (Fig. 10a) to faults, which are too numerous to be shown on locally eroded, this unit is continuous over the establish the perimeter of the dome. Between the map. Displacements across each fault uplift dome. At the center of the dome, the vertical Mount Holmes and Lake Powell, a regional dip blocks of host rock by as much as 100 to 200 m deflection of the Navajo Sandstone is about of slightly greater than 1° tilts the Triassic beds in a stepwise fashion over the central intrusion. 1.2 km. The host rock is not altered or northwestward (Hackman and Wyant, 1973). Johnson and Pollard (1973, p. 272-274) de- metamorphosed except for minor baking within About 5 km from the summit of Mount Holmes scribed similar structures, on a minor scale, at

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/1/117/3379075/i0016-7606-100-1-117.pdf by guest on 24 September 2021 Figure 8. Geologic map of Mount Ellsworth. See Figure 6 for explanation. See Figure 1 for location.

the upper contact of Sawtooth Ridge laccolith. Much of the region of faulted rock at Mount Ellsworth corresponds to the shatter zone that Hunt (1953, Plate 15) mapped near the center of the mountain. Figure 10b shows a radially oriented cross section through the northwest quadrant of Mount Ellsworth. Near the center of the dome, the vertical deflection of the White Rim sandstone is about 1.8 km. Beds within larger outcrops near the summit dip gently, between 8° and 3° northwest. Within the central limb of the flexure, the Triassic section of sandstones and shales, from the Moenkopi to the Kayenta For- mation, dips steeply, between 50° and 56° northwest. Numerous diorite sills and thin laccoliths are interleaved with these rocks. Bedding- plane fault zones are exposed at formational Figure 9. Geologic map contacts and within sandstone units of the cen- of Mount Hillers. See tral limb and have a typical spacing of about 75 Figure 6 for explanation. to 100 m. Outward from the central limb, the See Figure 1 for location. Jurassic Navajo, Carmel, and Entrada Forma- tions are bent into the open lower hinge (Fig. 10b). From the lower hinge outward, the strata dip gently to the northwest, forming a 3-km- long peripheral limb. We attribute the very gentle dips (about 1° to 2°) of Jurassic beds at radii >6 km to a regional dip (Hackman and Wyant, 1973). Although it is difficult to detect exactly where the peripheral limb of the flexure termi- nates, the radius of the dome is in the range of 6 to 7 km. From the center of the dome through its steeply dipping central limb, sills and thin laccoliths ranging from 5 to 20 m thick are interleaved with Triassic sedimentary rocks (Fig. 8, location 4). The most distal intrusions crop out in the shales of the Triassic Chinle Formation. These intrusions are concordant with the sedimentary beds, and their shape is concentric around the mountain, emphasizing the circular symmetry of the intrusive complex. The porphyritic diorite dikes that crop out on the flanks of the dome cut these sills and laccoliths (Fig. 8, locations 5,6), indicating that the dikes postdate sill emplacement. Most of the dikes strike radially, although some strike tangentially to the dome. The sedimentary rocks are baked within 1 or 2 m of these minor intrusions, and, on the northwest flank of the mountain, the deeper Permian White Rim sandstone is indurated and slightly metamorphosed (Fig. 8, location 2). Curvatures of the strata (Fig. 12b), measured on the base of the Navajo Sandstone, are signifi- cantly greater than those at Mount Holmes, reaching negative values of about -6.0 x 10~4 m-1 within the upper hinge. In the lower hinge, the curvature decreases from a positive maximum of about 7.5 x 10-4 m_1 at a 2.5-km

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"6k Zone of ÏW and he

Peripheral Limb Lower Central Upper Hinge Hinge Limb

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M 500 .g

1000 Zone of peripheral intrusions and faults

0 Kilometers N c

Peripheral Limb Lower Central Upper Hinge Limb Hinge

Figure 10. Interpretative radially oriented cross sections through the domes, based on geologic maps shown in Figures 7,8, and 9. Labels for stratigraphic units are shown in Figure 5. See text for methods used in construction of the sections. Short heavy lines = bedding-plane fault zones (bedding-plane faults at formational contacts are not shown at Mount HiUers). The sedimentary rocks have a gentle regional dip (10 to 2°) along the strike of the sections. Figure 10a shows the northwest (316°)-striking cross section through Mount Holmes. The roof-rock contact is drawn at the base of the Triassic section, the deepest rocks exposed on the mountain. Figure 10b. Northwest(316°)-striking cross section through Mount Ellsworth. Figure 10c. North-south(2°)-striking cross section through Mount HiUers. A, B, C, D, and E are sills and thin laccoliths sampled for paleomagnetic analysis. Cross section C-C extends 4.35 km into flat-lying sedimentary rocks that crop out beyond the perimeter of the geologic map shown in Figure 9.

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concave. upwards^

concave / downwards

concave upwards

concave / Figure 11. Bedding-plane fault zone com- downwards posed of a network of deformation bands, Navajo Sandstone, Mount Holmes. Pencil is 16 cm in length.

radial distance to nearly zero at a radial distance of about 4 km.

Mount Hillers

New mapping and field observations of the present study were restricted to the summit and southern flank of Mount Hillers (Fig. 9). Gilbert (1877, p. 30-35) and Hunt (1953, p. 121-134) have described this and other parts of the mountain. Dip directions of beds on the southern flank of the mountain are symmetric about a point that lies about 1 km south of the topographic summit. We infer that this point is the center of the dome (Fig. 9, location 1). Fresh diorite porphyry forms the higher, central portions of the mountain, cropping out over an area of 3.5 km2. We interpret this body to be the partially eroded remnant of a central intrusion, one of the largest in the Henry Mountains. The upward deflection of the strata was at least 2.5 km over the inferred center of the dome and could be as much as 3 km at the mountain summit. This difference suggests that the deflection of the overburden may be asymmetric. Figure 12. Curvature of the base of the Navajo Sandstone, computed from finite differences Figure 10c shows a radially oriented cross in elevation measured at 300-m increments along the radial cross sections. Stipples show section through the southern quadrant of Mount regions of very small curvature. Errors, as determined by repeated measurement, range to as Hillers. At the base of the mountain, the Cre- much as ±0.0002 units, (a) Mount Holmes, (b) Mount Ellsworth, (c) Mount Hillers.

taceous Dakota Formation dips 9° south. About Formation extends circumferentially around the tances of 3 and 7.5 km. To the east, however, 100 m to the north, the beds of the Jurassic dome more than 1.3 km or through 30° of the Ferron Sandstone member of the Cretaceous Morrison Formation dip 75° south. The lower change in strike of the host rock. The other sills Mancos Formation dips from 5° to 3° southeast. hinge of the flexure lies between these two out- and laccoliths are exposed over lesser distances. At radii between 6 and 7 km from the inferred crops, unfortunately covered by pediment grav- Some of these are segmented into fingers of dio- center of the dome, beds in the Cretaceous rocks els. Farther to the north, beds of the Jurassic and rite porphyry that plunge down the dip of the dip about 1° south and continue at this regional Triassic section dip 75° to 85° south and strike enclosing strata. This suggests (Pollard and oth- dip for 20 km southward (Hackman and about the mountain in regular concentric rings ers, 1975) that the apparent propagation direc- Wyant, 1973). We therefore suggest that the (Figs. 2, 9). The Permian Organ Rock shale is tion of those intrusions was parallel to the dip radius of the dome is between 6 and 7 km. exposed in the dome's upper hinge. Beds within direction. Radially striking and vertically dip- The curvature (Fig. 12c) of the upper portion lower exposures of this unit dip 70° south, ping dikes are abundant and crosscut the steeply of the cross section, measured on the extrapo- whereas those at higher exposures, along with dipping sills and thin laccoliths (Fig. 9, locations lated base of the Navajo Sandstone, reaches a beds within the Cedar Mesa sandstone, dip only 4, 5), and so these dikes are younger. large negative value (about -1.0 x 10~3 nr1) 45° south. The Cedar Mesa sandstone has been Within the steeply dipping limb of the flexure, within the upper hinge and decreases toward uplifted about 2.3 km at its uppermost exposure the sedimentary rocks show no metamorphic ef- zero through the central limb. From the extrapo- where it is in contact with the diorite (Fig. 9, fects, with the exception of baked zones at the lation of the Navajo Sandstone through the location 2). Much of the upper hinge of the contacts of minor intrusions. Closer to the con- outer hinge, we infer that this unit has a great flexure at Mount Hillers has been eroded. tact with the central intrusion, the White Rim positive curvature (about 1.2 x 10~3 m-1), Just north of the summit (Fig. 9, location 3), and Cedar Mesa sandstones are metamorphosed which decreases to zero at a radial distance of the metamorphosed Organ Rock shale dips to quartzite and the Organ Rock shale is meta- about 3.0 to 3.5 km. Numerous steeply dipping about 13° north and is in contact with the dio- morphosed to green hornfels. In addition, bedding-plane faults crop out within the central rite porphyry. A continuous section of metamor- fractures with nonsystematic attitudes have limb at Mount Hillers; however, the spacing of phosed Permian and Triassic rocks crops out on broken the metamorphic rocks into angular these faults is not well known. the ridge that extends northward from the sum- blocks, many no larger than 10 cm on a side. mit. The White Rim sandstone dips 24° north; Despite this fracturing, individual rock units are Similarities and Differences among the Moenkopi Formation dips about 40° north; identifiable, and bedding attitudes vary continu- Map-Scale Features of the Southern and farther to the north, more steeply dipping ously from outcrop to outcrop. If the contact of Henry Mountains members of the Chinle Formation crop out. the central intrusion follows the form of the These units help to define the northern portion upper hinge as depicted in the cross section (Fig. Mount Holmes, Mount Ellsworth, and Mount of the dome and suggest that the Permian rocks 10c), then the contact metamorphic zone is Hillers record successively greater amplitudes of in contact with the central intrusion were once about 150 m thick, extending to the base of the doming; the deflection of the beds at the center continuous over the top of the dome. White Rim sandstone. This is a greater degree of the domes increases from 1.2 to 1.8 to at least At least 10 porphyritic sills and thin lacco- and extent of metamorphism than is observed at 2.5 km. The mountains also expose, respec- liths, which range from 5 to 20 m thick, crop out Mount Ellsworth. Hunt (1953) included the sec- tively, deeper erosional levels of the central in- on the southern flank of Mount Hillers (Fig. 9). tion of Triassic and Permian sedimentary rock trusions or intrusive complexes. At Mount They account for at least 200 m of the 1,800-m- and interleaved sills extending from the Moen- Holmes, Jurassic sedimentary rocks are contin- thick section exposed in the central limb of the kopi Formation to the Cedar Mesa sandstone uously exposed over the center of the dome. flexure (Fig. 10c). One of the outermost intru- within his map unit called the "shatter zone." They are cut by radially striking vertical dikes at sions (Fig. 9, location A) in the Jurassic Entrada He did not, therefore, show the upper hinge of the summit. Gently dipping, tongue-shaped dior- the flexure in his fence diagram of the mountain ite sills and small laccoliths extend down the (1953, Plate 13). The new mapping has located flanks of the mountain. The summit of Mount this hinge, differentiated the stratigraphic units, Ellsworth exposes the top of a central intrusion, and established their continuity throughout which is hydrothermally altered, and a thin ve- this zone. neer of metamorphosed host rocks. More steeply Along the cross section (Fig. 10c), pediment dipping diorite sills and thin laccoliths are inter- gravels cover the bedrock between radial dis- leaved with Triassic sedimentary rocks within the central limb and upper hinge of the flexure. At Mount Hillers, more than 3.5 km2 of fresh porphyritic diorite from the eroded central intrusion crops out near the center of the dome. The Figure 13. Aeromagnetic map of the south- contact between the diorite and a thicker veneer ern Henry Mountains (after Affleck and of metamorphosed Permian host rocks is ex- Hunt, 1980). Flight height = 3,658 m. Dark- posed on the south flank of the mountain. ened triangles = mountain summits. Heavy Farther outward, the nearly vertical central lines = traverses interpreted by the modeling. limb of the flexure is composed of Triassic and Contour interval = 10 gammas. East-west Jurassic sedimentary rocks that are interleaved flight lines interpreted from the lower eleva- with at least ten diorite sills and thin laccoliths, tion (2,590 m) survey lie 750 m south of the some of which are crosscut by radially striking upper elevation (3,658 m) survey at Mount diorite dikes. Holmes and nearly coincide with the upper The distal portion of each dome is composed survey at Mount Ellsworth. of a gently dipping limb that is 3 to 4 km long.

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a.MOUNT HILLERS 500

400 Figure 14. Profiles of residual aeromag- 300 netic anomalies and cross sections through magnetic models, based on three-dimensional l/> 0 200 prisms. Regional gradients of 11 gammas/km b F to the west at Mount Holmes, 12 gam- 0 100 mas/km to the west at Mount Ellsworth, and O 0 9 gammas/km to the north at Mount Hillers were removed from the contoured data to 100 form the residual profiles. Solid lines = measured magnetic anomaly. Dashed lines = 0- -3658m flight I ine— computed laccolithic anomaly. Dotted lines = computed stock anomaly, K = susceptibili- 1- ty. (a) Mount Hillers, cross section A-A'. (b) & ID 2> Mount Ellsworth, cross section B-B'. (c) O K=0.001 K=0.0024 -K= 0.001 Mount Holmes, cross section C-C'. (d) Plan F o J- views of prisms. sea level 4-

18 17 16 15 14 13 12 11 10 9 l N S A Kilometers A'

h MOUNT ELLSWORTH « 150 2 loo £ 50- a o C.MOUNT HOLMES O 100-1 50' 0 300' -50 250' S 200- | 150' | 100^ O 50- OH

0- -3658m flight line- -3658m flight line

1- 2590m flight line- -2590m flight line 2 K= 0.001 £ £ 3- œ o> E J 4' sea level _o sea level I ¡2 5' 'vaJ 10 11 10 11 12 E E Kilometers W Kilometers w B B C C

Inward from this peripheral limb, the flexure Mount Holmes; it dips 50° to 55° at Mount the flexures are gently rounded near the centers consists of a lower concave-upward hinge and Ellsworth and 75° to 85° at Mount Hillers. The of the domes. On the south flank of Mount an upper concave-downward one, with a limb domes do not have the broad flat tops reported Hillers, beds exposed within the upper hinge dip of approximately constant dip between them. for some small laccoliths (Koch and others, 45°, whereas they dip <13° just north of the This central limb is longest and dips 20° at 1981). At Mount Holmes and Mount Ellsworth, summit. The radial distance from the center of

MOU NT HILLERS prisms (Telford and others, 1976, p. 189; (Fig. 10b). For Mount Holmes (Figs. 14c, 14d), d. J. Hendricks, 1986, personal commun.). Two the broad, flat form of the measured anomaly 3- regional aeromagnetic surveys, flown at 2,590 m suggests that the underlying intrusive complex .001^ = 12- elevation (Case and Joesting, 1972) and at may be deeper than that at Mount Ellsworth. 3,568 m elevation (Affleck and Hunt, 1980), The floor of the model intrusion (Fig. 14c) lies

.0024 respectively, provide the data for this study at 250 m below sea level, suggesting that the 10- .001 (Fig. 14). The sedimentary host rocks are essen- roof rock of the central intrusion could be the tially nonmagnetic relative to the diorite. Mea- Pennsylvanian Honaker Trail Formation (Fig. •001 .0024^ sured susceptibilities of the diorite are 0.0024 5). The maximum thickness of the more strongly V c.g.s. units at Mount Hillers (Avakian, 1970) magnetized portion of the model is 1.3 km, 001 and range from 0.0006 to 0.0023 c.g.s. units at compared to 1.2 km estimated geologically (Fig. 15 M 13 12 11 10 9 8 7 km Mount Holmes and Mount Ellsworth. The cen- 10a). The large lateral extent of the model intru- Al tral portions of the intrusive complexes were as- sion may reflect the presence of two intrusive signed susceptibility contrasts of 0.0024 c.g.s. complexes which form the two structural domes MOUNT ELLSWORTH units at Mount Hillers and 0.0020 c.g.s. units at at Mount Holmes (Gilbert, 1877, p. 27-28). Mount Holmes and Mount Ellsworth. Adjacent 001 Affleck and Hunt (1980, p. 112) interpreted .001 to the central intrusions at Mount Hillers and f the aeromagnetic anomalies as caused by more Mount Ellsworth, and over the top of the in- .002 or less vertical, cylindrical stocks and ruled out ferred central intrusion at Mount Holmes, the B—3- 4 the possibility that the domes are underlain by f host rock is modeled as consisting of minor dior- large mushroom-shaped laccoliths. We believe v 2 .001 001 ite intrusions (sills, dikes, and small laccoliths) that the narrow 400-km-wide stock plus minor 1 1— 5 6 7 km interspersed with sedimentary rock. An average intrusions suggested by Hunt (1953, Plate 16; susceptibility contrast of 0.0010 c.g.s. units was Fig. 4a) cannot account for the form of the MOUNT HOLMES chosen to approximate these mixed rocks. At magnetic anomaly at Mount Holmes. Because Mount Hillers, the diorite forming the central aeromagnetic anomalies are largely derived intrusion is magnetized in a direction nearly par- from near-surface features, however, it is not .001 allel to the expected mid-Tertiary declination possible to rule out the presence of a stock at and inclination (Table 1, central intrusion); we depth in association with the much wider, •002 presume that this is the case at Mount Holmes floored intrusion indicated in Figure 14c. Also, .002 E 5 and Mount Ellsworth. The ratio of induced to .002 if the central prisms of the magnetic models for .001 remnant magnetizations is about 2.5, indicating C-4 Mount Hillers and Mount Ellsworth are ex- that remnant magnetization gives a relatively too/ tended to great depth to represent a stock-like small contribution to the total anomaly. form (dashed lines; Figs. 14a, 14b), the com- .001 At all three mountains, floored laccolithic puted magnetic profiles (dotted lines; Figs. 14a, 14b) are not distinctly different from the mea- 34 5 6 7 8 9 10 11 km models (solid rectangular prisms, Fig. 14) consistent with the geologic maps and cross sections sured profiles. Therefore, although the aeromagnetic data cannot be used to rule out the Figure 14. (Continued). give good agreements between the computed magnetic profiles (dashed lines; Figs. 14a, 14b, presence of stocks at these mountains, 14c) and measured magnetic profiles (solid lines; our analysis does show that the magnetic anomalies are adequately explained by floored the dome to the point at which the curvature of Figs. 14a, 14b, 14c). For Mount Hillers (Figs. laccolithic models. the strata becomes very small is about 4 km at 14a, 14d), the floor of the model intrusion lies Mount Holmes and at Mount Ellsworth and 3.5 about 200 m above sea level, at the inferred km at Mount Hillers. The radial extent of strata elevation of the undisturbed Cedar Mesa sand- Volumes of Igneous Rocks uplifted by the intrusions is between 5 and 7 km. stone. This unit is in contact with the central Hunt (1953, Plates 12, 13, 15, and 16 and Fig. intrusion at the upper hinge of the flexure (Fig. The aeromagnetic models help constrain the 65) noted similar radii for the domes. 10c). The maximum thickness of the model in- total volume of igneous rock in each dome, intrusion is 2.75 km, whereas a range of 2.5 to 3.0 cluding the peripheral intrusions but excluding GEOPHYSICAL ASPECTS OF km was estimated from the geologic mapping. possible stocks or dikes below the floors of the THE DOMES The midpoint of the more strongly magnetized model intrusions (Fig. 14). These volumes portion of the model lies about 1.5 km south of (Table 2) are computed by summing the volume Interpretation of Aeromagnetic Data the summit, near the intrusive center as inferred of the prisms representing the diorite and half from the geologic map (Fig. 9). For Mount the volume of the prisms representing the in- Aeromagnetic highs are associated with intru- Ellsworth (Figs. 14b, 14d), the maximum thick- terspersed sedimentary and igneous rock. Two sive rocks of the three southern Henry Moun- ness of the model intrusion is 1.65 km, com- apparent contradictions in these results must be tains (Figs. 1, 13). To help constrain the pared to 1.8 km estimated geologically. The addressed. Mount Holmes has a greater volume subsurface forms and volumes of the intrusions, floor lies 250 m above sea level, at the inferred than does Mount Ellsworth, even though the profiles through the anomalies were interpreted elevation of the undisturbed Organ Rock shale, amplitude of the dome is smaller in cross section using models produced by three-dimensional the deepest rock unit exposed on the mountain (Figs. 10a, 10b). We suggest that this is largely

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TABLE 1. PALEOMAGNETIC DATA FROM DIORITE SILLS, MOUNT HILLERS Geological Survey Paleomagnetic Laboratory, Flagstaff, Arizona. Locality Bedding orientation Component of magnetization Temperature range of line fit The behavior of a characteristic sample (strike, dip) (dec. inc. (°C) during the demagnetization process is shown in (lat 38.1°, lnsitu Tilt long 248.9°) corrected Figure 15. The high-temperature component of magnetization, measured between the 400 and Sill A 110 87S 12 -10 335 64 400-origin 8 -10 332 64 300-origin 600 °C demagnetization steps, has a declination 18 -10 342 69 400-origin of 12° and an inclination of -10°, that is, a 20 -9 344 71 300-origin 1 2 306 56 300-origin paleomagnetic remnance vector of 12°, -10°. 357 -3 313 52 300-origin 10 -2 316 64 400-origin This is quite different from the expected Eocene field direction (351°, 59°) of Harrison and Sill B 86 85 S 181 40 182 -45 550-635 177 14 178 -71 450-635 Lindh (1982) or the Oligocene-Miocene direc- 166 21 155 -63 570-635 178 19 181 -66 520-570 tion (351°, 56°) of Diehl and others (1983). 184 6 211 -76 520 origin When the measured vector is rotated about the SilIC 90 80 S 359 -14 357 66 400-origin strike of the adjacent beds through their dip 10 -8 29 69 450-origin 27 -14 48 54 400-origin angle, however, the components are 335°, 64°, 357 -40 357 40 400-origin close to the expected mid-Tertiary field direc- Sill D 91 75 S 9 4 38 77 450-origin tion. This suggests that the sill was emplaced 0 -12 360 63 500-590 3 0 8 75 400-origin horizontally and was rotated to its present posi- 350 -3 327 69 500-origin tion after cooling below 400 °C. Sill E 90 70 S 0 -2 0 74 400-origin 356 2 344 77 450-origin The high-temperature components of magnet- 10 3 40 75 400-origin ization for 26 samples from the 5 intrusions are 333 -20 322 46 400-origin 345 -4 319 66 450-origin recorded in Table 1 and plotted in Figure 16.

Main 348 56 NRM-600 When the in situ paleomagnetic directions are intrusion rotated about the strike and through the dip angle of the adjacent beds, most of the vectors cluster around the normally polarized Eocene direction (solid triangle, Fig. 16). The average because Mount Holmes is composed of two approximately equal to that for the dome at tilt-corrected direction (350°, 65°) plunges structural domes, whereas Mount Ellsworth is Mount Ellsworth. Although the western Mount about 5° more steeply than does the expected composed of only one. Mount Hillers has a vol- Holmes dome has a smaller amplitude than does Eocene direction. This suggests that the beds ume more than twice that of Mount Ellsworth the Mount Ellsworth dome, it is greater in may have been slightly tilted to the south when even though the diameters of the domes are sim- diameter (Figs. 10a, 10b). the measured sills cooled through their high ilar and the amplitudes differ by less than a fac- As compared to the estimated volumes of the blocking-temperature spectra. The five tilt- tor of two. We suggest that the peripheral central magma chambers (about 20 to 35 km3), corrected vectors from sill B plunge to the south intrusions north and east of Mount Hillers con- the volumes of individual dikes and sills are on and cluster about the antithetical, reversely po- tribute to this greater volume. In contrast, the order of 0.01 km3, and the volumes of peri- larized mid-Tertiary direction (171°, -59°; Mount Ellsworth is unique in the southern pheral laccoliths are in the range 0.1 to 0.5 km3 open triangle, Fig. 16). The complex behavior Henry Mountains in the symmetry of its dome (Table 2). Apparently, these peripheral intru- of sill B during the demagnetization process and the paucity of peripheral intrusions. We sions represent very small magma pulses, com- is indicative of self-reversing components therefore conclude that only for Mount Ells- pared to the surges that fed the central of magnetization that were activated at high worth is the aeromagnetic volume estimate rep- intrusions. to intermediate temperatures. Apparently, resentative of a single magma chamber. sill B was emplaced during the same polarity epoch as were the other sills. Volumes of the central magma chambers may Paleomagnetism of the Diorite Sills be estimated independently of the aeromagnetic The low blocking temperature and low data by using the circular symmetry of the At least ten diorite sills and thin laccoliths are coersivity components of magnetization of the domes as shown on the geologic maps (Figs. 7, concordantly interleaved with steeply dipping, samples, measured through a progressive series 8, 9). The volumes (Table 2) are calculated by arcuate beds of sedimentary host rock exposed of low-temperature thermal and alternating field integrating to find the area under each flexure as on the south flank of Mount Hillers (Figs. 2, 9). demagnetization steps, give in situ directions that extrapolated from the cross sections (Fig. 10) A paleomagnetic analysis was undertaken to de- are close to the expected mid-Tertiary field. Our and then integrating around the axis of circular termine if these intrusions were emplaced before interpretation of this result is that the sills and symmetry. Note that the estimate for Mount or after the beds were tilted by doming. Five thin laccoliths were emplaced in subhorizontal Holmes includes only the western dome, and, oriented samples were collected from a 2- to orientations, and then the dome grew and tilted for Mount Hillers, the estimate ignores the signif- 3-m volume of diorite within the centers of each all of the concordant intrusions. Resetting of the icant volume of intrusive rocks to the north and of 5 concordant intrusions. Cored specimens initial low-temperature component occurred as east of the dome. The intrusive volume com- were thermally demagnetized and measured the tilted sills were reheated by the central puted for the western dome at Mount Holmes is using a cryogenic magnetometer at the U.S. intrusion.

TABLE 2. ESTIMATES OF INTRUSIVE VOLUMES plate that has the same resistance to bending as tance, the beds dip more and more gently, attain- does the stack of sedimentary layers over the ing regional 1° to 2° dips at radii of about 6 km. Holmes Ellsworth Hillers (km3) (km3) (km3) dome. Field observations of small, well-exposed lac- Normalizing w by wmax, the maximum coliths suggest possible explanations for the Geologic 23 23 34 origin of the peripheral limb. Exposures at Saw- cross sections deflection found at the center of the plate (r = 0), gives w/w = [1 - (r/a)2]2. This theoretical tooth Ridge laccolith east of Mount Hillers Floored aero- 34 21 49 mix magnetic models form is distinctly different from the curve pro- (Fig. 1) indicate that the laccolith thickened by Radial dike 0.02 duced by normalizing the vertical deflection of lifting a plug of overburden along a vertical per- Buckhorn Ridge 0.12 the flexure at Mount Holmes out to a radius of ipheral fault, and sills intruded radially outward laccolith 5.5 km (Fig. 17). The theoretical shape lacks the from this discordant contact (Johnson and Pol- Black Mesa 0.45 laccolith peripheral limb that characterizes the igneous lard, 1973, Fig. 11). Exposures at Buckhorn domes (Figs. 10, 12). We hypothesize that this Ridge laccolith on Mount Holmes (Fig. 7) indi- Note: estimates are for within the domes, exclusive of possible stocks or other cate that a peripheral dike cut through the lower underlying intrusions. peripheral limb is underlain by sills and thin laccoliths that are not directly related to the hinge of the host-rock flexure and turned over main episode of doming (the geological support into a radially directed sill (Johnson and Pol- for this hypothesis is given below). To analyze lard, 1973, Fig. 10). In the Highwood Moun- only the strongly flexed portion of the dome, we tains, Montana, multiple injections of magma The paleomagnetic data demonstrate that the subtract the maximum deflection of the peri- directed radially outward from the peripheries of sills were emplaced subhorizontally and cooled pheral limb (about 100 m) and compare the the laccoliths produced complex zones of sills through their high blocking temperatures in this flexure, out to a radial distance of about 4 km, to that are interleaved with the sedimentary host orientation. The sills were rotated with their host the theoretical form (Fig. 17). Their similarity rock (Hurlbut and Griggs, 1939; Pollard and rock on the flanks of the growing dome through corroborates Gilbert's concept (1877, p. 19) that others, 1975). By analogy, the peripheral limbs 75° to 80° of dip. This result, combined with the bending of the overburden was important, at on the major domes could be underlain by downdip plunge of some of the sill segments, least in the early stages of doming, and is con- tongue-shaped sills and thin laccoliths that were indicates that certain sills propagated radially sistent with the source of deformation being a injected radially from near the base of the cen- outward from feeders located near the present major laccolith with radius of less than ~4 km. tral intrusions or by concordant intrusions fed by center of the dome. radial dikes. The examples cited above demonstrate that the intrusion feeding these sills need Peripheral Limb of the Flexure not be a stock. MECHANICAL ASPECTS OF THE GROWTH OF THE DOMES The distal portion of each of the three domes Abundant satellitic sills and small laccoliths is composed of a long, nearly straight limb 3 to 4 are found within Cretaceous, Jurassic, and Tri- Elastic Bending Theory and Flexure Shape km in length, which dips gently away from the assic rocks on the flanks of the mountains (Hunt, center of the mountain (Fig. 10). Beds exposed 1953, Plates 12, 15). The paleomagnetic results The geologic cross sections (Fig. 10) illustrate at the upper end of this peripheral limb dip demonstrate that some of these minor intrusions the forms of the flexures and the locations of about 9° outward. With increasing radial dis- predate the growth of the central intrusions. The hinges at the crests and peripheries of the domes. It is instructive to compare the form of the flexure at Mount Holmes (Fig. 10a), which repre- sents an early stage in the doming, to the theoretical cross-sectional form (Pollard and W UP Johnson, 1973, p. 312-318) of a circular elastic INCLINATION plate, of radius a, and thickness T, that is clamped around its edge where r = a. The plate is pushed upward by a uniform driving pressure of magnitude P - pgT, where P is magma pres- HORIZ. sure, p is host-rock density, and g is acceleration 500° C of gravity. The vertical deflection, h>, of the 400°C middle surface of the plate is

DECLINATION 300°C "Q— — 3(P-pgT)(l-p2) NRM 150°C w 3 [ (t-r*? (1) 16 ET, DOWN

(Timoshenko and Woinowsky-Krieger, 1959, p. Figure 15. Zijderveldt plot showing the progressive demagnetization of sample A, sill A (first 55-56), where Te is the effective mechanical sample, Table 1). In situ coordinates. Axis = 2.33 e.m.u./gr. From the NRM to 400 °C thickness of the plate, E is Young's modulus, demagnetization steps, the declination is 15° and the inclination is 29°. From 400° to 600°, the and v is Poisson's ratio. The effective mechani- component is 358°, -17°. When corrected for the strike and dip of the adjacent beds, this cal thickness is the thickness of a single elastic component is 335°, 64°. Circle = declination. X = inclination.

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cumulative effect of such older intrusions would be to gently incline the overburden, thus con- tributing to the deflection of the peripheral limb.

The Sill-Laccolith Transition

In Gilbert's concept of laccolith formation (1877, p. 89), the first event is the emplacement of a thin horizontal sill. To understand sill emplacement, it is instructive to consider the theoretical dilation (Pollard and Johnson, 1973, p. 337-342) of a circular crack of radius a pushed open by a uniform driving pressure of magnitude P - pgT. The upward (and downward) displacement v of the crack walls is

7TE Figure 16. Equal-area stereographic plot of the high-temperature components of magnetization from 26 samples from the 5 sills, (a) In situ paleomagnetic remnance vectors, (b) Tilt- (Sneddon, 1946, p. 242). Equation 2 suggests corrected remnance vectors. The average orientation of the normally polarized vectors is 340°, that vertical displacement over a sill increases 68°. Cross = normally polarized vector (plots on lower surface of sphere). Open circle = only in proportion to the radius a, whereas the reversely polarized vector (plots on upper surface of sphere). Darkened triangle = normally vertical deflection over a laccolith (equation 1) polarized mid-Tertiary field direction. Open triangle = reversely polarized mid-Tertiary field increases as the fourth power of the radius. direction. X = direction of present-day magnetization. Pollard and Johnson (1973, p. 347-352, Fig. 25) identified the transition radius at which symmetric displacements above and below a sill would give way to significant flexure of the

overburden and a pronounced thickening of the Te > 560 m. At Mount Holmes, the typical continued well into the bending stage of laccolith. If we equate equations 1 and 2, the spacing between observed bedding-plane faults deformation. l/3 transition radius is atrans = 4Te/(3ir) = 2Te. is 150 to 200 m. If the mechanical layers were At Mount Holmes, sills and thin laccoliths are With further lateral growth of the laccolith, the all 200 m thick through 4 km of sedimentary exposed (Fig. 7, locations 1, 2, and Buckhorn flexure concentrates stresses over the periphery, rock, there were 20 layers and Te > 540 m. In Ridge) that have radial dimensions ranging to as leading to the development of faults, dikes, or both cases, the lower boundary for the transition much as 2.5 km. We hypothesize that these in- monoclinal flexures (Koch and others, 1981). radius is about 1.1 km. trusions did not precipitate significant doming Note that the sill-laccolith transition depends To estimate an upper boundary for the transi- because, as Hunt (1953) suggested, they are only on the effective mechanical thickness of the tion radius, we use the radial distance at which tongue shaped, not circular. At Mount Hillers, overburden. the curvature of the strata approaches zero on the largest sills and thin laccoliths overlying the At Mount Holmes, the bedding-plane faults, the Mount Holmes cross sections (Figs. 10a, central intrusion extend for only 30° out of found within the sandstone units and at forma- 12a). The curvature at the base of the Navajo 360°. We suggest that the restricted circumferen- tional contacts (Figs. 10, 11), indicate that the Sandstone approaches zero at a radial distance of tial extent of these older conformable intrusions overburden behaved as a stack of thin mechani- about 4 km. Taking this distance as an estimate prevented their development into a major dome. cal layers able, at least in some places, to slip of a trans> 'he corresponding upper boundary for over one another. These faults reduced the effec- effective thickness is about Te< 2.1 km. Bending and Stretching of the Overburden tive thickness to a value less than the total thick- For Black Mesa laccolith (Fig. 1), which is ness of overburden. For the extreme case of n about 850 m in radius and lies about 1.9 km Both bending and stretching of the over- 3 freely slipping layers, Te = (/j + t^ + • • • + higher in the stratigraphic section than does the burden contributed to the total strain as doming ;„3)1/3, where the are layer thicknesses (Pol- inferred base of the central intrusion at Mount progressed, and these strains can be estimated lard and Johnson, 1973, p. 348). Friction on Holmes (Fig. 5), Pollard and Johnson (1973, from structural and stratigraphic data. We hypo-

bedding-plane faults prevents free slip, but this p. 350) estimated Te ~ 300 m and ¿ztrans = 500 thesize that bending strains were predominant expression establishes a lower boundary for m. These values support Gilbert's concept that during the initiation of each dome, when the effective thickness and transition radius. If the lesser overburden should correspond to a overburden behaved as a single mechanical geologic formations (Fig. 5) represent the smaller radius for laccoliths. The estimated tran- layer. The bending strain is proportional to the mechanical layers, from the top of the mid- sition radius at Black Mesa is only about 60% of curvature of the flexed layer, C, and to the dis- Tertiary deposits (Doelling, 1975, p. 17) to the the radius at which a peripheral fault formed tance from the middle surface of the layer Permian and Triassic rocks over the central in- and uplift of a plug of overburden began. This (Johnson, 1970, p. 56). The greatest bending trusions, there were approximately 26 layers and implies that lateral propagation of the laccolith strain thus occurs at the top and bottom of the

a single mechanical layer 3 to 4 km thick at the a. initiation of doming, very large bending strains would have developed. The development of bedding-plane faults relieved these large strains by subdividing the overburden into thin mechanical layers that slipped over one another. Slip on bedding-plane faults also reduced the effective thickness and resistance to further bending. As the amplitude of the doming increased at Mount Ellsworth and Mount Hillers, radial and circumferential extension of the strata became (0 appreciable, eventually exceeding the bending E -5" strains. Over the entire dome, a layer is stretched from its original length, l0, to a final length, If, so that the average radial extension is es = (/f - l0)/l0. On the basis of the arc length of layers extrapolated over the radial cross section at

Mount Holmes (Fig. 10a), l0 = 5.3 km and /f = 5.5 km. These values give an extension of <4% for the final amplitude of doming. Radial extension at Mount Ellsworth is estimated from the arc length of the base of the Navajo Sandstone as it is extrapolated through the cross section

(Fig. 10b). Values of l0 = 5.7 km and lf = 6.4 km give a radial extension of about 12%. The data are insufficient to make this calculation for Mount Hillers, but the greater amplitude of doming suggests even greater extension.

Radial Expansion of Stocks

Hunt (1953, p. 139-141) proposed that the wmax" 1-2 km Mt. Holmes flexure central intrusions underlying the domes represent progressive stages in the growth of cylindrical stocks that pierced the strata as thin pipes and then increased in diameter from 300-400 m at Mount Holmes to 700-800 m at Mount Ells- a = 4.3 km worth to nearly 3 km at Mount Hillers (Fig. 4). a = 5.5 km- Hunt reasoned (1953, p. 135) that the beds adjacent to the stock would have been radially Figure 17. (a) Comparison of the deflection of the Navajo Sandstone at Mount Holmes with contracted and circumferentially extended as the the theoretical deflection of a thin circular elastic plate. Lower solid curve = normalized stock grew in diameter. If these strains were ac- deflection of bent plate. Upper solid curve = normalized deflection of bent plate if about 10% of commodated within the observed radii of the the deflection results from uniform uplift (for example, over a stack of thin sills and laccoliths). domes, about 6 km, the radial contraction Lower dashed curve = deflection of layer at Mount Holmes expressed over a radius of 5.5 km. would be about 50% at Mount Hillers. Near the Upper dashed curve = deflection of layer at Mount Holmes expressed over a radial distance of contact of such an expanding cylindrical stock, 4.3 km, the radius over which bending is important, (b) Shape of the flexure at Mount Holmes. this radial contraction might be accompanied by upward drag as the intrusion grew in height and diameter. To understand the mechanics of this process, 4 1 layer where eb = ±0.5 Ct and t is the layer is about -4.5 x 10" m" within the upper hinge we consider the deflection of a confined layer thickness. and about 4.0 x 10"4 nr1 within the lower subjected to horizontal compression and a lesser At Mount Holmes, the spacing of bedding- hinge. Using these curvatures and a mechanical vertical shear force (Johnson, 1970, p. 107). For plane fault zones suggests an average mechanical layer thickness of 150-200 m, we calculate a comparison, the theoretical form of such a layer layer thickness of about 150-200 m. From Fig- range of bending strains from ±3% to ±6% (Fig. 18a) is scaled to the vertical deflection ure 12a, the curvature of the Navajo Sandstone within the hinges. If the overburden behaved as (2.3 km) of the strata at the uppermost contact

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Figure 18. (a) Illustrations of shear and axial loads implicit in Hunt's model of stocks, (b) Solid curve = deflection of a layer under a shear force and a large axial load (after Johnson, 1970, p. 107) scaled to the vertical deflection of the Cedar Mesa sandstone at the contact with the central intrusion at Mount Hillers (about 2.3 km). Dashed line = observed deflection of the strata at Mount Hillers.

Figure 19. Vertical cross sections showing stages in the growth of the central intrusions and domes in the Henry Mountains, (a) Emplace- ment of a stack of tongue-shaped sills and thin laccoliths fed by vertical dikes. Insert = plan view of early-formed intrusions, showing their tongue-like shape. The incipient major laccolith (stipple pattern) has a circular plan shape, (b) Thickening of the major laccolith induces bedding-plane faulting, and the overlying intrusions are tilted. Peripheral dikes and faults form as lateral growth of the laccolith stops. Some sills and thin laccoliths intrude laterally under the peripheral limb of the dome, (c) The major laccolith, now probably formed of multiple intrusions producing a composite body, continues to thicken as the dome grows in amplitude. Beds steepen and stretch on the flanks of the dome, numerous faults lift the roof rock, the zone of peripheral intrusions enlarges, and radial dikes cut upward through the overburden. P, H, J, K, T = Permian, Triassic, Jurassic, Cretaceous, and Tertiary sedimentary host rocks.

Figure 19. (Continued).

of the intrusion at Mount Hillers (Fig. 18b). The A syncline, having a downward deflection trenches, that are formed by downward-directed layer would be bent steeply upward by drag that is a significant fraction of the upward deflec- shear forces (Turcotte and Schubert, 1982, near the contact, and the horizontal compression tion at the contact, should occur between 2 and p. 124-131). The absence of any synclinal would buckle the layer into a series of waves 3 km from the intrusive contact. This syncline is structure peripheral to the domes suggests that with amplitudes that decrease exponentially analogous to the anticlinal arches, observed upward drag of the host rock was not a signifi- away from the applied load. around oceanic volcanoes and next to ocean cant part of the deformation. Also, the absence

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of any structures indicative of buckling at Mount Hunt (1953, p. 121-124) emphasized the im- formation observed at Mount Holmes. The Ellsworth and Mount Hillers argues strongly portance of a shatter zone between the central overburden developed an effective thickness of against a radial contraction as great as 50%. diorite intrusions and their sedimentary host between 500 m and 2 km as layers slipped over In addition, piercing of the strata by a stock rocks and de-emphasized the rounded upper one another on bedding-plane faults. In addi- is inconsistent with the concordant relationships hinge at each of the three flexures (Fig. 10). Our tion, the layers of host rock were slightly that we observe between the host and igneous detailed mapping indicates that Hunt's shatter stretched over the dome as the amplitude of the rocks and with the concave-downward hinges zone includes many different rock types, such as deflection increased. We hypothesize that with found at Mount Ellsworth and Mount Hillers a veneer of brecciated and metamorphosed host increased doming, lateral propagation of the lac- (Fig. 10a, 10b). rock directly in contact with the inferred central colith continued to a radius of almost 4 km. intrusion, jointed and metamorphosed sedimen- Peripheral faults may have developed near the DISCUSSION tary rocks at some distance from this contact, floor of the laccolith, and some dikes and sills and sedimentary rocks that are neither meta- extended laterally beyond the discordant sides of The Laccolith-Stock Controversy morphosed nor strongly jointed but are cut by the central intrusion, forming a growing peri- numerous dikes, sills, and minor faults. The pheral limb. Gilbert (1877) and Hunt (1953) presented breccias consist of clasts of the adjacent sedi- As the central intrusion grew to a volume of conflicting hypotheses for the subsurface struc- mentary host rock and of the diorite and do not 20 to 35 km3 (Fig. 19c), the hinges of the over- ture of the domes, for the mechanisms of their contain rocks from deeper stratigraphic units. lying flexure tightened and the central limb emplacement and growth, and for the origins of Within the entire metamorphic zone, the strati- steepened to almost vertical. Erosion of surficial the peripheral sills and thin laccoliths. Because graphic units are readily identified and are con- rocks over the top of the dome may have further only the upper portions of the domes are ex- tinuous enough to be mapped at a scale of reduced the effective thickness of the over- posed, it is difficult to identify their structure at 1:24,000. The shatter zone unit of Hunt was a burden. Radial and circumferential stretching of depth unambiguously; however, the exposed useful aid to mapping the more complex parts of the overlying layers became more important, overburden (Fig. 10) is arched into doubly the domes during his studies of the region. This relative to bending. Radial dikes crosscut steeply hinged flexures, as Gilbert described (Fig. 3). zone contains little evidence for the crushing or dipping sills and thin laccoliths on the flanks of This shape is consistent with the bending of a buckling of the strata implied by Hunt's model the dome as circumferential stretching increased. circular plate driven upward by a thickening of radially expanding stocks, however. In addi- Inward from the upper hinge of the flexure, laccolith (Fig. 17). Hunt (1953, p. 121-124, tion, we have found no evidence for melting or large blocks of the sedimentary rock were dis- 148-149) inferred that the central intrusions are stoping on a scale that would make these placed in a stepwise fashion over the roof of the tall, pipe-like stocks that both domed the sedi- mechanisms viable for the emplacement of intrusion. At the edge of the dome, the cumula- mentary rock and pushed them aside to create a stock. tive effect of continued intrusion of satellitic sills room for the magma. Cross sections of the strata and laccoliths was to incline the overburden forming the domes are inconsistent with buck- Stages in the Growth of the Domes over the length of the long peripheral limb. Dur- ling of a plate that is loaded by a radially ing the last stage of the intrusive process, intense expanding stock (compare with Figs. 10, 18). The first stage (Fig. 19) in the development of fracturing and metamorphism of the host rock Hunt hypothesized (1953, p. 141-142) that a the igneous domes in the southern Henry Moun- was limited to a thin zone, perhaps 50 to 100 m radiating pattern of tongue-shaped sills and lac- tains involved the intrusion of numerous hori- thick at the immediate contact with the central coliths intruded laterally from the discordant zontal diorite sills, with volumes from 0.01 to intrusion. sides of a central stock after the stock began to 0.1 km3, presumably fed by dikes. Some of the dome the host rock. Such tongue-shaped intru- sills grew to become small laccoliths (Fig. 19a) ACKNOWLEDGMENTS sions are common, but the sequence of em- with volumes of 0.1 to 0.5 km3. Many of these placement is not necessarily consistent with concordant intrusions were tongue shaped with We thank S. Thliveris and G. Baer for assist- Hunt's hypothesis. For example, rotated paleo- long axes forming a radiating pattern around the ance in the field. C. Pillmore, J. Hendricks, magnetic vectors from the sills and thin lacco- incipient dome. Apparently, the restricted cir- and D. Elston provided laboratory and com- liths at Mount Hillers (Fig. 16) indicate that cumferential dimensions of the thin laccoliths puter facilities for the PG-2 stereoplotter, these intrusions cooled while horizontal and prevented them from growing into major, aeromagnetic modeling, and paleomagnetic were then tilted by the growth of an underlying dome-forming intrusions. We hypothesize that analysis, respectively. The paper benefited from intrusion. Also, where exposed, we found large- one sill grew near the base of the Permian Cutler reviews by D. Champion, C. Corry, D. De Paor, ly concordant relationships between the central Formation with a circular plan to a sufficient C. B. Hunt, Z. Reches, M. Ryan, I. J. Witkind, intrusion and its host rock. Aeromagnetic radius (>1 km and <3 km) to begin thickening and S. Wojtal. anomalies associated with the igneous rock in by bending the entire 3 to 4 km of overburden. the domes are adequately explained by a floored The growth of this laccolith was enhanced by its model, but these data do not rule out the pres- circular plan shape and perhaps by local heating ence of a vertical stock at depth. In summary, of the host rock by numerous older intrusions. REFERENCES CITED the new mapping, geophysical data, and me- Affleck, J., and Hunt, C. B., 1980, Magnetic anomalies and structural geology As the central laccolithic intrusion inflated of stocks and laccoliths in the Henry Mountains, Utah, in Picard, M. D., chanical arguments are consistent with Gilbert's ed., Henry Mountains symposium". 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