Relation of thin-skinned thrusting of strata in southwestern to magmatism

OLIVIER R. MERLE;* 1 f Department of Geosciences, University of , Tucson, Arizona 85721 GEORGE H. DAVIS RICHARD P. NICKELSEN Department of , Bucknell University, Lewisburg, Pennsylvania 17837 PIERRE A. GOURLAY Compagnie Générale d'Informatique, 30 Rue du Château des Rentiers, 75640 Paris, France

ABSTRACT INTRODUCTION tures (for example, Dutton, 1880; Gregory and Moore, 1931; Gregory, 1951; Bowers, Structural studies in Upper Cretaceous and Conventional Picture of Regional Geology 1972; Hintze, 1988). The plateau itself is Eocene sedimentary rocks in the High bounded on the west and east by the Sevier of southwestern Utah attempted to establish the Sedimentary rocks within the Paunsaugunt and Paunsaugunt late Cenozoic normal faults, cause of layer-parallel shortening expressed in Plateau, in the southwestern part of the Col- which are considered to be the easternmost a regional-scale arcuate pattern of thin-skinned orado Plateau (Fig. 1), have long been de- manifestation of crustal extension in the Ba- thrusts and related structures. The thrusts sole scribed as almost everywhere flat lying and sin and Range province. The Paunsaugunt into of the Carmel Forma- lacking compressional deformational struc- Plateau was uplifted en bloc during the Late tion. Tectonic transport direction verges radi- ally from 125° to 215°. Stress analysis reveals a fan-like pattern of the principal compressional stress sweeping southeastward to south-south- westward through 90° along an arc extending 35 km across the . Re- gional compressional structures of Sevier and Laramide origin are older than the thin- skinned deformation, and regional extensional structures of Basin and Range origin are younger. The thrusting postdates deposition of the Claron Formation (Eocene) and probably took place in the interval 30 Ma to 20 Ma, that is, coeval with the formation of the Marysvale volcanic center located 40 to 60 km northwest of the thrust belt. The forces that created the thrusts could have formed through a combi- nation of processes related to the dynamics of formation of the Marysvale volcanic center, including gravity gliding and/or compressional push related to the emplacement of batholithic intrusions, and gravitational spreading and/or end-loading related to vertical loading of the column and the underlying décolle- ment by the weight of the thick Marysvale vol- canic pile.

*Present address: Laboratoire de Tectonophy- Figure 1. Index map to the of the High Plateaus of southwestern Utah. WP = Wilson sique, Institut de Géologie, Université de Rennes Peak; HC south of = Hillsdale Canyon; HC northeast of Tropic = Henderson Beaulieu, 35042 Rennes cedex, France. Canyon; PP = Powell Point; PHF = Pine Hills ; and RIF = Rubys Inn fault.

Geological Society of America Bulletin, v. 105, p. 387-398, 14 figs., 1 table, March 1993.


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2500 m




500 LUNDIN, 1989

Figure 2. Structure section showing large-scale /fault relationships and flattening of the thrust into the Jurassic Carmel evaporites (from Lundin, 1989).

Cretaceous/early Laramide Dip slip on the Rubys Inn was from the Claron Formation up into the Sevier (Hintze, 1988), and along the edge of the estimated to be 125 m (Lundin, 1989, p. 1044). River Formation. On Bower's map, however, Paunsaugunt Plateau there is clear expression In Johns Valley, located to the northeast of the Formation is shown cover- of Laramide basement-cored uplift in the the Pine Hill and Rubys Inn faults (Fig. 3), ing the Rubys Inn thrust and a strike-slip fault form of the East Kaibab (Davis, seismic data reveal another thin-skinned in the Claron Formation, suggesting that dep- 1978). West of the Paunsaugunt Plateau by 60 thrust fault, the Johns Valley thrust (Lundin, osition of the Sevier River Formation post- km is the front of the Sevier thin-skinned or- 1989), which trends northeast-southwest, dated thrusting. If the Sevier River Formation ogenic belt, which was active until the late dips northwest, verges southeast, and soles of the Bryce Canyon area does indeed cor- Campanian (75 Ma) (Averitt and Threet, into the . The thrust belt relate with the Sevier River as mapped to the 1973; Armstrong, 1974; Coney, 1976). Within displays an arcuate map pattern that swings north in (Fig. 1), it is older the Paunsaugunt Plateau there are the Rubys across the Paunsaugunt Plateau from an east- than 14 Ma (Anderson and Rowley, 1975, Inn and Pine Hills faults that were interpreted west trend in the west to a southwest-north- p. 41) and perhaps as old as 20 Ma (?). We by Gregory (1951) as steeply dipping normal east trend in the east (Fig. 3). believe that the Boat Formation is even faults forming an east-west-trending horst. older. Timing of Thrusting North of the study area, at Casto Bluff Discovery of Young Thrusts (Fig. 1), the Claron Formation is overlain un- Within the area of study, the youngest conformably by volcanics that are thought to Recently, the Rubys Inn fault was recog- rocks involved in the thrusting belong to the be equivalent to the late Oligocene and Mio- nized as a south-verging thrust fault by Davis Claron Formation, which is —220 m thick and cene Mt. Dutton Formation of the Markagunt and Krantz (1986) and Lundin and Davis of presumed Eocene age (Anderson and Plateau. The volcanics have been dated as 27 (1987). They demonstrated that this thrust Kurlich, 1989). The oldest undeformed rocks, to 21 Ma (Anderson and others, 1990b). We fault, which dips 30° to the north, places Cre- unconformably overlying the Claron Forma- have no clear evidence indicating whether the taceous over Eocene strata. In places, the tion, are conglomeratic sandstones and lime- volcanics are involved in the thrusting or not. thrust is associated with a large, upright to stones of either the Boat Mesa Formation or On the basis of our present knowledge, we overturned south-verging . Lundin the Sevier River Formation. Bowers (1990) conclude that the thrusting took place be- (1989, Figs. 2 and 10) provided the first com- regards the Boat Mesa Formation as Oli- tween latest Eocene and middle Miocene, plete structural analysis of this thrusting, gocene and the Sevier River Formation as and most likely in the interval 30 to 20 Ma. demonstrating that the structural style is Pliocene-, but these age interpre- "thin skinned." Seismic information made tations are uncertain. Our study of exposures Objectives available by Chevron to Davis and Lundin of Boat Mesa Formation did not reveal any (Lundin and Davis, 1987; Lundin, 1989) re- outcrop-scale deformational features related The recent discoveries and reinterpreta- veals that the Rubys Inn fault curves at depth to thrusting, even though both strike-slip tions of structures in the Paunsaugunt Plateau and roots horizontally into the evaporite-rich faults and thrusts were present at a number of gave focus to this investigation, the objectives layers of the Jurassic Carmel Formation, not places in the Claron Formation directly below of which included (1) defining the total re- offsetting the underlying stiff, thick Navajo the unconformity. Poor exposures of the con- gional extent of thrusting, (2) evaluating the Sandstone (Fig. 2). The seismic data indicate tact between the Sevier River Formation and tectonic significance of the regionally arcuate that the Pine Hill fault is an antithetic south- the underlying Claron Formation at the two pattern of thrust faults and related structures, dipping backthrust that terminates against the localities at the north boundary of Bryce Can- (3) establishing the age relationships between Rubys Inn fault at depth (Lundin, 1989, yon National Park (Bowers, 1990) precluded the thin-skinned thrusting and the Paunsau- Fig. 10, section B-B'). determining if deformational features extend gunt and Sevier Basin and Range normal

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Figure 3. Structural ge- ologic map of the thrust and strike-slip fault rela- UTAH tionships in the Paunsau- gunt Plateau.



faults, and (4) interpreting the ultimate cause termination of the south-verging Elbow thrust trace of the thrust trends east-northeast, and of the thrusting. (Fig. 3). Yet another strike-slip fault, tenta- its presence is marked by an elongate topo- tively identified as such by Lundin (1989), is graphic ridge. Exposures of Claron along the RESULTS OF MAPPING located at the eastern boundary of the Elbow ridge display a nearly horizontal zone of high thrust. The trace of this fault trends approx- shear strain, expressed in the form of conju- With benefit of mapping by Lundin (1989, imately north-south. A projection northward gate, slickenlined thrusts and backthrusts. Fig. 2) and Bowers (1990), we were able to from the segment closest to the eastern ter- Because the fault is essentially horizontal map in even greater detail certain critical ge- mination of the Elbow thrust "hits" the south where exposed, and because Claron Forma- ologic relationships, explore for additional end of the trace of yet another, identically tion makes up both hanging wall and footwall, structures, and track the fundamental defor- oriented, sinistral strike-slip fault (Fig. 3), we do not know the magnitude of displace- mational features to an even greater regional whose aerial photo expression is pronounced. ment of the Hunt Creek thrust. The fault sur- extent. Results of our mapping are shown in The relationship between the two fault seg- face itself may be close to horizontal. Figure 3. ments suggests the presence of a long, through-going strike-slip fault that marks the Thrust Geometry—Hillsdale Canyon Area Strike-Slip Faults southwestern termination of the Johns Valley thrust and the eastern termination of the El- Hillsdale Canyon provides the best expo- A number of strike-slip faults were identi- bow thrust. sure of the Rubys Inn thrust (Figs. 1 and 4). fied, and these segment the thrust belt (Fig. 3). The Rubys Inn thrust trends 110° as it ap- A north-northeast-trending sinistral strike- Hunt Creek Thrust proaches Hillsdale Canyon from east to west. slip fault occurs near the eastern end of the South-southeast of Wilson Peak the thrust Rubys Inn thrust and can be traced northward A newly discovered thrust, referred to here splits into two branches: a northern branch to where it marks the eastern termination of as the Hunt Creek thrust, was traced for >5 that extends for another kilometer west- the north-verging Pine Hill and the western km within the Claron Formation (Fig. 3). The northwest before dying out in a gentle mon-

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Figure 4. Structural geologic map of the Hillsdale Canyon area.



ocline and a southern, main branch that can directed toward 210°, and these are over- that of fault-propagation folds. As Suppe be traced westward for several kilometers be- printed by slickenlines trending 220°. (1985) emphasizes, such folds can lock, and fore being truncated and down-dropped by Structural relationships very similar to this when this happens, the fault may branch into the Sevier fault (Fig. 4). Displacement can be observed near the eastern end of the two surfaces that propagate along the syncli- steadily decreases from east to west along the Rubys Inn fault, 400 m southwest of Route 12 nal and anticlinal axial surfaces. Where only northern branch. The dominant structure (Figs. 1 and 6A), where vertical beds of one thrust fault is created, the fault propa- marking the north branch changes from an Claron Formation are sandwiched between gates along the anticlinal axial surface, leav- asymmetrical, south-verging, faulted anti- two thrust faults. Beneath the southern fault, ing a sharp in the footwall just be- cline (Fig. 5, sections A and B) to a gentle, a very sharp syncline rotates the bedding from neath the thrust plane. south-facing monocline (Fig. 5, section C). horizontal to vertical (Fig. 6B). Above this Only Cretaceous rocks are involved. Hang- thrust surface, beds of the Claron Formation DETAILED ANALYSIS OF OUTCROP- ing-wall rocks are everywhere flat lying. At are vertical or even slightly overturned, com- SCALE STRUCTURES higher elevations at the location of section A, posing the footwall for the northern fault, the south branch of the Rubys Inn thrust is which carries horizontal Cretaceous beds on Relationship of Deformation Mechanisms to marked by south-dipping Cretaceous beds top of the steeply dipping Claron. Northward, Lithology resting in thrust-fault contact on footwall the upper (northern) branch of the thrust de- Claron Formation. In fact, a remnant of the creases in dip and merges with the lower During layer-parallel compression of the Cretaceous/Eocene depositional contact is (southern) branch. Large fault grooves plung- Cretaceous and Eocene sedimentary rocks, exposed in the hanging wall (Fig. 5, section ing toward 320° down the fault surface (Lun- different sedimentary materials accommo- A). Further downdip along the thrust, mod- din, 1989, Fig. 5) are overprinted by finer dated shortening in different ways (Nickelsen erately dipping Cretaceous strata in the hang- slickenlines that plunge in the direction 355°. and Merle, 1991). Cretaceous behaved ing wall rest in thrust-fault contact on up- These indicators plus overall vergence dem- as brittle materials and failed in 1- to 2-mm- turned Eocene and Cretaceous beds. The onstrate hanging-wall transport toward azi- spaced extension joints ( cleats). The ex- vertical, locally overturned beds represent muth 140°, followed sequentially by transport tension joints are parallel to the bearing of the the north limb of a tight, south-verging chev- toward azimuth 175°. greatest horizontal principal stress direction ron syncline in the footwall (Fig. 5). Chevron's seismic data reveal a ramp-flat as inferred from structures such as faults and Horizontal shortening based on balanced fault geometry at depth, such that the overall rock cleavage in other rock types. Calcite- cross sections is estimated to be —700 m. The geometry of hanging-wall strata is marked by cemented Cretaceous sandstones failed along direction of transport, based on the trend of fault-bend folding (see Fig. 2). The fault and extension joints of similar regional orientation slickenlines on fault surfaces, averages 215°. fold geometries revealed in Hillsdale Canyon to the joints in coals, though with wider spac- Early-formed slickenlines indicate transport and along Highway 12, however, resemble ing. In addition, these sandstones deformed

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EOCENE (CLARON FORMATION) Figure 5. Geologic structure sections show- ing the geometry of folding and thrusting in the Hillsdale Canyon area. CRETACEOUS



100 m I I

2460 m

by intrabed conjugate strike-slip faults and by the sand shears and deformation bands are decrease to the south and southeast, finally thrust-backthrust faults that form wedges identical. The sand shears are matrix-rich disappearing 10 to 20 km in front of the Rubys (Cloos, 1961). Argillaceous limestones of the shear surfaces and shear zones formed by Inn thrust. Claron Formation failed along joints and con- cataclasis of sedimentary grains along dis- jugate fault systems but also shortened crete planes (Fig. 7). They lack surface orna- Fault Rocks through development of spaced dissolution mentation such as slickensides or slicken- cleavage not seen in other rock types. lines. Conjugate sets of thrust/back-thrust The top boundary of each major thrust is Friable, well-sorted Cretaceous sand- sand shears are acutely bisected by bedding usually very sharp and corresponds to a well- stones that do not appear to have been lithi- and may be either systematically or unsys- defined thrust plane (Fig. 8). Beneath the fied at the time of deformation failed along tematically oriented with respect to the inter- thrust plane there is a zone of fault rocks —0.5 conjugate arrays of structures that we have preted principal stress directions. m thick. The highly deformed zones of fault informally called "sand shears." They Clearly, the ways in which strain is accom- rocks stand in marked contrast to the condi- closely resemble "deformation bands" of modated in horizontal beds near the zone of tion of the surrounding rocks, which com- Aydin (1978) and Aydin and Johnson (1978), thrusting are dependent on lithology. The ex- monly appear only slightly strained or com- but we are not certain that the mechanisms of pressions and magnitude of strain are seen to pletely undeformed. The character of the fault

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A dominate and vertical where thrust faults EOCENE NN (CLARON FORMATION) dominate. C) Orientations of the axis of greatest princi- pal compressional stress reveal a striking fan- like pattern (Fig. 9), trending south-southwest (215°) in the west to southeast (125°) in the eastern part of the study area. The two ex- treme directions enclose an angle of about 90°. The fan-like pattern of compression is consistent with the arcuate trace of thrusts as seen on the structure map (Fig. 2). In addi- tion, steeply dipping spaced dissolution cleavage in the Claron Formation follows this arcuate pattern. Radial thrusting in a brittle environment could not have been readily achieved without strike-slip faults. Strike-slip faults functioned as relays between panels of different orientations and motions.

Geographic Extent of the Thin-Skinned Deformation

Knowledge of the outcrop-scale structures B associated with the thrusting was used as a guide to exploring for the record of layer- EOCENE CRETACEOUS (CLARON FORMATION) parallel compressional deformation to the west and east of the Paunsaugunt Plateau NW SE (Fig. 1). The Rubys Inn thrust cannot be traced westward from the Sevier fault through the Markagunt Plateau to the Hurri- cane fault zone. Along Route 14 (Fig. 1) to the west of the Sevier fault, however, there are excellent exposures of Claron and Creta- ceous rocks, which fortunately are not cov- 10 m ered by the voluminous Tertiary and Hol- A ocene volcanics of the Markagunt Plateau. Figure 6. (A) Structural geologic map of the eastern end of the Rubys Inn thrust. (B) Cross The Claron and Cretaceous rocks along section showing relationships between folding and faulting at location "A" on map. Route 14 display abundant outcrop-scale structures such as strike-slip faults, thrust faults, joints, dissolution surfaces, spaced rocks varies with rock type. For example, were located close to major thrust faults cleavages, slickensides, and slickolites. friable Cretaceous sands are deformed into where conjugate sets offaults with consistent, These structures have consistent orientations alternating bands of sand shears and well- systematic orientations were well developed that disclose north-south to north-northeast- sorted, undeformed country rock. The bands (Fig. 10). There are certain assumptions in the south-southwest compression. The suite of are 2 to 3 cm thick and are parallel to the methodology: (1) fault slip is in the maximum structures, identical to structures that occur thrust. shear direction, (2) a uniform stress field acts along the Rubys Inn thrust, can be traced Kinematic indicators within the fault rocks on all surfaces, and (3) faulting is simulta- west to the Hurricane fault zone and to within include (1) slickensides and slickenlines, (2) neous on all fault surfaces. Knowledge of the 7 km of the Sevier orogenic front. grain alignments in friable sands, (3) calcite- orientation of striations on fault planes per- Structures related to the thrusting event filled gash veins, and (4) offset rigid markers. mits a quantitative interpretation of the rela- can be observed east of the Paunsaugunt fault tive magnitudes of principal stresses. and monocline. In Henderson Canyon, STRESS ANALYSIS OF FRACTURES Twelve localities (Fig. 10; Table 1) yielded southwest of Powell Point on the Table Cliff satisfactory measurements for computing the Plateau (Fig. 1), close-spaced microjoints in Stress tensors were reconstructed on the orientations of the three principal stress di- coal strike 325°, consistent with the predicted basis of geometric/kinematic analysis of con- rections. The axis of greatest principal com- pattern of northwest-southeast compression jugate thrust faults and strike-slip faults, using pressional stress is everywhere horizontal (or that generated the thin-skinned deformation the methods of Etchecoparand others (1981). nearly so), whereas the axes of intermediate immediately to the northwest. Henderson Analysis requires a large number of meas- and least principal stress "swap" orientations Canyon marks the eastern limit of recognized urements for each station where a determi- in different three-dimensional stress fields deformation associated with the thin-skinned nation is being sought and well-defined stri- (Aydin and Reches, 1982). In general, Sigma thrusting event. ations along fault planes (Fig. 9). Stations III is horizontal where strike-slip faults Thus, structures of the thin-skinned defor-

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when the Paunsaugunt monocline is unfolded do the calculated stress tensors at station 6 line up with the regional pattern of horizontal axes of greatest principal compressive stress.

Depth of Thrusting

Estimates of the depth of thrusting of the Cretaceous rocks were based on the organic maturity of the coals. Reflectance measure- ments range from 0.43% to 0.45%, indicating subbituminous A coal (J. Levine, 1991, per- sonal commun.). On the basis of correlation between mean random vitrinite reflectance and temperature, this reflectance level indi- cates that the maximum temperature sus- tained during burial was about 60 °C (Barker and Pawlewicz, 1986). Assuming a geother- mal gradient of 30 °C/km at the time of de- formation, the depth of burial was about 2 km. The estimated thickness of the Cretaceous from its upper contact with the Claron to the base of the sequence in which we investigated structures is —570 m. The full thickness of the Claron Formation, as measured in the south- ern Sevier Plateau near Casto Bluff (Fig. 1), ranges from 530 m to 585 m. An additional 1.2 mation event have been found at localities cline, which is a northward extension of the km of post-Claron overburden may have been beyond the 35 km "discovery" arc on the Paunsaugunt fault (Fig. 3). At map station 6 present during deformation within the area of Paunsaugunt Plateau. This arcuate belt of de- (Figs. 9 and 10), outcrop-scale thrusts and a study (E. Sable, 1990, personal commun.). formation extends for at least 90 km, from strike-slip fault were passively rotated with Thus, the rocks observed in the field were Henderson Canyon on the east to the Hurri- the bedding as the monocline formed. Only deformed at a very superficial level under cane fault zone on the west (Fig. 1). This belt of compressional deformation is not very large when viewed at a regional scale, and this in itself imposes some constraints on the or- igin of the thin-skinned thrusting event. The type area for this thin-skinned thrusting will remain the Paunsaugunt Plateau, and for this reason, we suggest the name "Paunsaugunt Thrust Belt" as an appropriate name for this regional deformation.

Relative Age of the Thrusts to Basin and Range Faulting

The thin-skinned structures are demonstra- bly older than the Basin and Range faults. Although the Rubys Inn fault is cut off on the west by the Basin and Range Sevier fault (Fig. 3), the truncation itself cannot be ob- served because of poor exposure. Immedi- ately north of the intersection of the two faults, however, structures related to the thin- skinned deformation are present in Creta- ceous sandstone in the same outcrops where the sandstone is cut off by the Sevier fault. The thin-skinned structures can also be shown to be older than the Basin and Range Figure 8. The sharp upper surface of the Rubys Inn thrust as exposed in Eocene Claron Paunsaugunt fault. The critical relationships Formation north of Highway 12. One-half-meter-thick zone of fault rock underlies the thrust can be seen along the Paunsaugunt mono- plane. Authors Nickelsen (standing) and Merle for scale.

Geological Society of America Bulletin, March 1993 393

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sedimentary rocks over a weak basal layer and away from a structural high created by magmatic intrusion. The best example is the Bearpaw in Montana, where sed- imentary rocks exhibit annular décollement thrusting involving slip away from an uplifted Cenozoic laccolithic and volcanic center (Reeves, 1925; Hearn, 1976; Gucwa and Kehle, 1978). The structural style is that of décollement thin-skinned deformation along two main glide planes in bentonitic shale in the Upper Cretaceous Colorado Shale. If the gravity gliding interpretation of the Bearpaw geology is correct, the presence of a weak décollement horizon is essential, for centri- fuge modeling of laccolithic intrusions does not reveal lateral thrusting (Dixon and Simp- son, 1987), and the classic of the display only modest folds outward from the laccolithic centers (Gilbert, 1877; Johnson and Pollard, 1973; Pollard and Johnson, 1973; Hunt, 1983). The possibility of compressional push dur- ing batholithic or laccolithic emplacement is explicit in Smith's (1981) "fluid push" model, Figure 10. Map showing the fan-like pattern of the directions of greatest principal compressive which he proposes for the scale of the North stress (black arrows) during the thin-skinned thrusting. Numbers refer to the 12 data stations. American Cordillera. In this model, the ther- See Table 1. mally weakened zone of the volcanic arc be- haves mechanically as a liquid and exerts a ~ 1-2 km of overburden. The well-sorted, fri- along the Jurassic Carmel evaporite décolle- hydrostatic pressure upon the back end of the able, noncalcareous Cretaceous sandstones ment; (2) compressional push as a result of sedimentary wedge. The push from the rear could have been unlithified at the time of the horizontal growth of wedge-shaped intru- exerted by the magma provides compres- deformation. sions that "shouldered aside" the upper por- sional stresses large enough to initiate a thrust tions of the column; (3) fault along the base of the sedimentary wedge. gravitational spreading as a result of loading Indeed, field descriptions and experimental GEOLOGICAL INTERPRETATION OF of the sedimentary rock column by the thick results indicate that horizontal shortening and THE THRUSTING EVENT volcanic pile, causing a spreading of the vol- thrusting can occur at the front of viscous canic complex and the mechanically weak intrusions. Large-scale horizontal viscous in- The radial, thin-skinned thrusting episode Carmel evaporites at depth; and (4) end load- trusion of the deepest basement into shal- took place in a noncompressive tectonic in- ing by horizontal deviatoric stresses that lower levels has been experimentally studied terlude when the main activity was explosive arose during vertical loading of the sedimen- (Merle and Guillier, 1989) and described in the volcanism and granitic intrusion. In south- tary column directly beneath the volcanic Swiss Alps (Schmid and others, 1990). Shal- western Utah, the huge, explosive Oligocene/ center and adjacent to the relatively "unload- lower-level examples occur in the Gulf of Miocene eruptions produced volcanic centers ed" sedimentary column, where the thrusts Mexico, where a thrust zone in front of a salt that are among the most voluminous found broke to the present level of observation. Dy- sill intrusion has been described by Huber anywhere. The inferred timing of the thin- namic conditions favoring gravitational glid- (1989). Cao and others (1989) have developed skinned thrusting event and the strategic lo- ing and compressional push would arise dur- numerical models on faulting of sedimentary cation of the Marysvale volcanic center ing emplacement of batholithic bodies, which formations as a result of an intruding salt (Fig. 1) relative to the belt of thrusting cause gave rise to the volcanic pile. Dynamic con- sheet. us to consider a possible relationship between ditions favoring gravitational spreading and The combination of gravitational gliding the volcanic center and the horizontal, thrust- end loading would arise when the stresses and compressional push related to magmatic producing compression. created by the load of the volcanic pile emplacement might have been responsible, at We are considering four possible mecha- reached critical thresholds. least in part, for the development of the Paun- nisms, each related to the dynamics of for- saugunt thrust belt (Figs. 10 and 11). The mation of the Marysvale volcanic center, Forces Generated during Batholithic compressional stress trajectories converge each capable of operating independently or Emplacement northwestward toward the Marysvale vol- in combination with others: (1) gravity gliding canic center, which was formed from 30 to 20 as a result of vertical growth of wedge-shaped The literature contains examples in which Ma (Steven and others, 1984). Rocks of this intrusions that uplifted and tilted the sedimen- workers have proposed layer-parallel com- center crop out expansively in the southern tary rock column, permitting detachment pression resulting from gravitational gliding of , the northern Markagunt

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Locality and Type of data' Stress orientation R

geologic formation (TF or SSF) (

Hunt Creek Thrust Sigma I N200.8°E 1.1° .59 East Sigma2 N290.8°E 2.7° Ciaron Formation Sigma3 N88.2°E 87.0°

Hunt Creek Thrust 24 TF Sigma I N8.8°E 1.5° West 01 SSF Sigma2 N278.7°E 1.1° Ciaron Formation Sigma3 Nlíl.^E 88.1°

Johns Valley Thrnst Sigma 1 N314.9°E 89.0° Ciaron Formation Sigma2 N224.9°E .2° Sigma3 N122.8°E 89.0°

Pine Lake Sigma 1 N304.1°E 1.6° Ciaron Formation Sigma2 N214TE .1° Sigma3 N120.2°E 88.4°

Elbow Thrust Sigma 1 N322.1°E .2° Ciaron Formation Sigma2 N118.7°E ;9 .7° Sigma3 N232.TE .1°

Henderson Point 24 SSF Sigma I N332.2°E .0° Ciaron Formation 02 TF Sigma2 N183.2°E 90.0° Sigma3 N62.2°E .0°

Shakespear Point 21 SSF Sigma 1 NI51.1°E .0° Ciaron Formation 02 TF Sigma2 N60.9°E 90.0° 05 Joints Sigma3 N241.1°E .0°

Strand A and B 49 SSF Sigma 1 N348.8°E .1° Ciaron Formation 21 TF Sigma2 N78.8°E 2.6° Sigma3 N256.4°E 87.4°

Amphitheater 39 TF Sigma 1 N337.3°E .0° Ciaron Formation 04 SSF Sigma2 N67.3°E .0° Sigma3 N202.8°E 90.0°

Water Canyon 54 SSF Sigma 1 N348.7°E .5° Ciaron Formation 04 TF Sigma2 N244.4°E 88.0° Sigma3 N78.7°E 1.9°

Hillsdale Canyon Sigma 1 N35.9°E 6.8° Cretaceous Sigma2 N146.6°E 71.5° Sigma3 N303.8°E 17.2°

Hillsdale Canyon 41 TF Sigma 1 N200.8°E 18.2° Ciaron Formation 05 SSF Sigma2 N107.1°E 11.3° Sigma3 N346.8°E 68.4°

Note: results of analysis are presented in Figures 9 and 10. *No. - number of measurements in data set. Total is 509. fTF = thrust fault; SSF = strike-slip fault.

Plateau, and the southern Sevier Plateau son and others, 1990b), no more than 40 km (Anderson and Rowley, 1975, p. 24). The (Fig. 1) (Anderson and Rowley, 1975, p. 2, north to northwest of the Paunsaugunt thrust Spry intrusion has been dated at 26.1 ± 1.8 Fig. 1). The complex of small laccolithic belt. These intrusions the Claron For- Ma and is assumed to be a cupola on a deeper domes and underlying batholithic intrusions mation and the 26-25 Ma Buckskin Breccia batholithic intrusion defined by magnetic data that are the foundation of the Marysvale vol- and Bear Valley Formation, but they are sur- (Blank and Kucks, 1989; Anderson and oth- canic center created significant elevation. The rounded by nearly horizontal alluvial and vent ers, 1990b). It provides both the best example clearest relationship between intrusive activ- facies of the volcanic Mt. Dutton Formation, of doming and horizontal intrusion within the ity and geologic structure is provided by the 26 to 23 Ma in age. The topographic high laccolithic complex and the clearest evidence small intrusion and laccolithic dome in Sho- formed by the Spry intrusion dammed and of the timing of the deformation relative to walter (Fig. 1) (Anderson and oth- deflected Mt. Dutton volcanic flows, until it sedimentation and volcanic activity. ers, 1990a) and by the Spry intrusion (Ander- was eventually inundated by the volcanics Experiments were conducted with B. Vendeville at the Applied Dynamics Labo- ratory of the Bureau of Economic Geology Figure 11. Cartoon (The University of Texas at Austin) to see if showing general nature of thrusts can be produced by laccolithic intru- thin-skinned thrusting re- sion. In simulating the Rubys Inn thrust, sand lated to magmatic intru- was used as a rock analogue for modeling the sion. Thrusting is shown as brittle part of the sedimentary pile, whereas a response to gravity glid- silicone was used as an analogue of the evap- ing and a lateral compres- orite-rich layer of the Carmel Formation sional "push." (Fig. 12)(FaugereandBrun, 1984; Vendeville

396 Geological Society of America Bulletin, March 1993

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- - 74 cm - - Figure 12. Apparatus "I used in scale-model experi- SAND (1,5 cm) 1—• SILICONE (0,3 mm) (Pink) ments. Dark silicone simu- lating plastic/viscous magma is laterally compressed and forced to move upward into the silicone and sand layers, \ which simulate sedimentary SCREW SILICONE (Dark) JACK cover.

5 cm

Figure 13. Cross section of model (no. 107). Lateral spreading of the dark silicone laccolithic intrusion (black) at the back end of the brittle overburden. Silicone (stippled) at the base of the brittle overburden. Sand (white) represents Cretaceous and Eocene sedimentary cover.

and others, 1987). The hot magma was sim- that a weak substratum of clay-rich rocks, as outward from the summit of the volcano, 20 ulated with silicone putty of higher viscosity. well as the volcano itself, is spreading toward km outward from the base. Most experiments displayed the same basic the east and south. As a result, basal thrust In addition, Borgia and others (1990) report geometrical features (Fig. 13). At the back faults and fault-propagation folds are forming the presence of fault-propagation folds and end of the model, the rising silicone produces in an arcuate belt (Fig. 14). Estimated mini- thrusts exposed outward from the base of the a dome-shaped elevation inducing severe ex- mum thrusting is 1.2 km eastward and 0.5 km Central Costa Rica Volcanic Range. The tension in the overlying sand overburden. southward, which are on the same order of thrusts, folds, and associated structures crop Halfway through each experiment, the sili- magnitude as the Paunsaugunt thrust belt. out in a distinctive ridge that parallels the cone crops out at the surface because the sand The compressional belt at Etna lies 30 km range, lying ~ 15 km from the range axis. The has been completely removed by extensional faulting. Part of the ascending silicone hori- zontally intrudes the low-viscosity strata at the base of the brittle sand pile. The vertical extrusion and laccolithic intrusion of silicone both contribute to creating a basal and surface slope (about 5°) in the overlying strata. One or several thrusts develop ahead of the lacco- lithic intrusion. Thrusting occurs as a combi- nation of push from the rear and gravitational gliding. The thrusts dip 30° and root into the low-viscosity strata. According to model ra- tios used during experiments, thrust faults in nature would be located several tens of kilo- meters ahead of the magmatic intrusion.

Forces Generated by Volcanic Loading

Significant forces were generated by vol- canic loading associated with the develop- ment of the Marysvale volcanic center, and these forces could have been responsible for, or contributed to, the development of the Paunsaugunt thrust belt. The Marysvale vol- canic center covers 5,000 km2, and the thick- ness of the volcanics is 3,000+ m. Recent studies have called attention to the fact that certain contemporary volcanic centers, along with their substratum, are failing through gravitational spreading. Figure 14. Gravitational spreading model as presented by Borgia and others (1992) for Mount For example, Borgia and others (1992) re- Etna. Shows relation of Mount Etna to the break-out zone of thrusting, and details of the fold/ port gravitational spreading at Mt. Etna, such fault geometry. Adapted from Borgia and others, 1992, Figures 2B and 3B.

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thrust climbs from a depth of700 m at an angle thanks go to T. Johnson (Bryce Canyon Na- Dutton, C. E.. J880, Report on the geology of the High PJateaus of Utah: U.S. Geographical and Geological Survey, Rocky of 15° over a distance of a few kilometers, then tional Park) and C. Guillette ( National Mountain Region, 307 p. Etchecopar, A., Vasseur, G.. and Daignieres, M., 1981, An inverse it ramps up from a depth of 600 m along a 40° Forest in Panguich), who made aerial photo- problem in microtectonics for the determination of stress ten- angle and ends in a synclinal hinge 400 m graphs of the study area available, and to sors from fault striation analysis: Journal of Structural Geol- ogy, v. 3, p. 51-65. below the surface. Borgia and others (1992) E. Kormier, who served so well as a field Faugere, E., and Brun, J. P., 1984, Modélisation expérimentale de la distention continentale: Comptes Rendus des Séances de consider this structure to be a scaled version assistant. R. W. Krantz and J. E. Spencer are l'Académie des Sciences de la Terre, ser. II. v. 299, no. 7, of the deformation surrounding Olympus also thanked for critical reading of the man- p. 365-370. Gilbert,G. K., 1877, Report on the geology of the Henry Mountains: Mons on Mars. This giant shield volcano uscript. U.S. Geographic and Geological Survey, Rocky Mountain Region, 160 p. stands 26.4 km high, is 600 km in diameter, Gregory. H. E., 1951, The geology and geography of the Paunsau- and is surrounded by an annular ridge con- gunt region, Utah: U.S. Geological Survey Professional Paper 226, 116 p. sidered to be the product of compressional Gregory, H. £., and Moore. R. E., 1931, The Kaiparowits region, REFERENCES CITED a geographic and geologic reconnaissance of parts of Utah and deformation related to loading. Arizona: U.S. Geological Survey Professional Paper 164, Anderson. J. J.. and Kurlich, R. A., 1989, Post-Claron formation, 161 p. 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Utah: U.S. Geological Survey Miscellaneous In- tion of SEPM Foundation Annual Research Conference, 10th, ing for which there was no equivalent coun- vestigations Series Map 1-1999. scale 1:50,000. December 10, 1989, Program and Abstracts. Anderson. J. J., Rowley. P. D., Blackman, J. T.. Mehnert, H. H., Hunt.C. B., 1956, Cenozoic geology of the Colorado Plateau: U.S. terforce in Claron and Cretaceous rocks Grant, T. C., 1990b. Geologic map of the Circleville Canyon Geological Survey Professional Paper 279, 99 p. south of the Marysvale volcanic center. This area, southern Markagunt Plateau, Beaver, Garfield, Iron, Hunt, C. B., 1983, Development of the La Sal and other laccolithic and Piute Counties, Utah: U.S. Geological Survey Miscella- mountains on the Colorado Plateau, in Guidebook to North- too could have contributed to the layer-par- neous Investigations Series Map 1-2000, scale 1:50,000. ern and Uncompahgre Uplift: Grand Junction, Armstrong, R. L., 1974. Magmatism, orogenic timing, and orogenic Colorado, Grand Junction Geological Society, p. 29-32. allel shortening that is observed. diachronism in the Cordillera from Mexico to Canada: Na- Johnson, A. M., and Pollard. D. D., 1973, Mechanics of growth of ture. v. 247, p. 348-351. some laccoliths in the Henry Mountains. Utah—Part I. Field Averitt, P.. and Threet, R. L., 1973, Geologic map of the Cedar City observations. Gilbert's model, physical properties and flow of CONCLUSIONS quadrangle. Iron County, Utah: U.S. Geological Survey Geo- the magma: Tectonophysics, v. 18, p. 261-309. logic Quadrangle Map 1120. Kehle, R. O.. 1970, Analysis of gravity sliding and orogenic trans- Aydin, A., 1978, Small faults formed as deformation bands in sand- lation: Geological Society of America Bulletin, v. 81, stone: Pure and Applied , v. 116. p. 913-930. p. 1641-1644. We conclude that the Paunsaugunt thrust Aydin, A., and Johnson, A. M.. 1978. Development of faults as Lundin. E. R., 1989, Thrusting of the Claron Formation, the Bryce belt is not related to the regionally prominent zones of deformation bands and as slip surfaces in sandstone: Canyon region, Utah: Geological Society of America Bulletin, Pure and Applied Geophysics, v. 116, p. 931-942. v. 101, p. 1038-1050. Sevier, Laramide, and Basin and Range de- Aydin, A., and Reches, Z.. 1982, Number of orientations of fault sets Lundin, E. R., and Davis, G. H., 1987, Southeast vergent thrust in the field and in experiments: Geology, v. 10, p. 107-112. faulting and folding of the Eocene (?) Claron Formation, formations. The inferred 30 to 20 Ma age of Barker. C. E., and Pawlewicz, M. J., 1986, The correlation of vit- Bryce Canyon National Park, Utah: Geological Society of the compressional deformation did not coin- rinite reflectance with maximum temperature in humic or- America Abstracts with Programs, v. 19, no. 5, p. 317. ganic matter, in Buntebarth. G., and Stegena, L., eds., Pa- Merle. O., and Guillier, B., 1989, The building of the Central Swiss cide with a time of regional compressive tec- leogeothermics: Berlin, Germany, Springer-Verlag, p. 79-93. Alps, an experimental approach: Tectonophysics, v. 165, Blank, H. R., and Kucks, R. P., 1989. Preliminary aeromagnetic, p. 41-56. tonics. We believe that the compressive gravity, and generalized geologic maps of the USGS Basin and Nickelsen, R. P., and Merle, 0-, 1991, Structural evolution at the tip stresses that created the Paunsaugunt thrust Range—Colorado Plateau transition zone study area in south- line of a mid-Tertiary compressional event in southwestern western Utah, southeastern , and northwestern Ari- Utah: Geological Society of America Abstracts with Pro- belt were derived from forces related to the zona (the "BARCO" project): U.S. Geological Survey Open- grams, v. 23, no. 1. p. 109. File Report 89-432. 16 p.. 2 maps. Pollard. D. D., and Johnson, A. M., 1973, Mechanics of growth of batholithic emplacement history and/or ver- Borgia, A.. Burr, J., Montero. W., Morales, L. D., and Alvadado. some intrusions in the Henry Mountains, Utah—Part tical crustal loading history that marked the G. E., 1990, Fault-propagation folds induced by gravitational II. Bending and failure of overburden layers and sill formation: failure and slumping of the Central Costa Rica Volcanic Ridge: Tectonophysics, v. 18, p. 311-354. formation of the Marysvale volcanic center. Implications for large terrestrial and Martian volcanic edifices: Reeves, F., 1925, Shallow folding and faulting around the Bearpaw Journal of Geophysical Research, v. 95. no. B9. Mountains: American Journal of Science, ser. 5, v. 10, The southern part of the Marysvale volcanic p. 14,357-14,382. p. 187-200. center lies <40 km northwest of the thrust Borgia, A.. Ferrari, L., and Pasquara, G. P., 1992, Importance of Schmid, S. M., Ruck, Ph., and Schreurs, G., 1990, The significance gravitational spreading in the tectonic and volcanic evolution of the Schams nappes for the reconstruction of the paleotec- belt and coincides in location with the locus of of Mount Etna: Nature, v. 357, p. 231-235. tonic and orogenic evolution of the Penninic zone along the Bowers, W. E.. 1972, The Canaan Peak Hollow and Wasatch For- NFP-20 East traverse (Grisons, eastern Switzerland): Mém- convergence of horizontal, compressive mation in the Table Cliff region, Garfield County, Utah: U.S. oires de la Société Géologique de Suisse, no. 1, p. 263-287. greatest principal stress axes calculated on Geological Survey Bulletin 1331-B, 39 p. Smith, A. G., 1981. Subduction and coeval thrust belts, with Bowers, W. E., 1990, Geologic map of Bryce Canyon National Park particular reference to North America, in McClay, K. R., the basis of structures in the thrust belt. and vicinity, southwestern Utah: U.S. Geological Survey and Price, N. J., eds.. Thrust and nappe tectonics: Geologi- Miscellaneous investigations Series Map i-2108, scale cal Society of London Special PubJicaîion, v. 9, p. 111- 1:24.000. 124. Cao, S., Lerche, I., and O'Brien. J. J., 1989. Moving salt sheets and Steven, T. A., Rowley. P. D., and Cunningham, C. G., 1984, ACKNOWLEDGMENTS the deformation and faulting of sedimentary formations: Gulf Calderas of the , west central Utah: Coast Section of SEPM Foundation Annual Research Con- Journal of Geophysical Research, v. 89, p. 8765-8786. ference. 10th, December 10, 1990, Program and Abstracts. Suppe, J., 1985, Principles of structural geology: Englewood Cliffs, This work was made possible by the finan- CIoos, E.. 1961, Bedding slips, wedges and folding in layered se- New Jersey, Prentice-Hall, 537 p. quences: Geological Society of Finland Bulletin, v. 196, Vendeville, B., Cobbold, P. R., Davy, Ph., Brun, J. P., and Chou- cial support of the American Chemical Soci- p. 105-122. kroune, P., 1987, Physical models of extensional tectonics at Coney, P. J., 1976, Plate tectonics and the , in various scales, in Coward, M. P., Dewey, M. F., and Han- ety (Petroleum Research Fund). We thank Woodward. L. A., and Northrop, S. A., eds.. Tectonics and cock, P. L., eds., Continental extensional tectonics: Geolog- R. Reynolds, S. Calclazer, R. Vogel, and their mineral deposits of southwestern North America: New Mex- ical Society of London Special Publication 28, p. 95-107. ico Geological Society Special Publication Number 6. p. 5-10. Zhao, G., and Johnson, A. M-, 1992, Sequence of deformations staff at Bryce Canyon National Park for their Davis, G. H.. 1978. Monocline fold pattern of the Colorado Plateau. recorded in joints and faults, , Utah: in Matthews, V., ed.. Laramide folding associated with base- Journal of Structural Geology, v. 14, p. 225-236. logistical support during field studies. The au- ment block faulting in the western United States: Geological thors benefited from discussions with J. J. Society of America Memoir 151, p. 215-233. Davis, G. H., and Krantz, R. W.. 1986, Post-"Laramide" thrust Anderson, W. E. Bowers, W. R. Dickinson, faults in the Claron Formation. Bryce Canyon National Park. Utah: Geological Society of America Abstracts with Pro- M. P. Jackson, R. W. Krantz, J. R. Levine, grams. v. 18, no. 5, p. 98. F. Maldonado, P. D. Rowley, E. G. Sable, Dixon, J. M., and Simpson, D. G., 1987, Centrifuge modelling of MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 21, 1992 laccolith intrusion: Journal of Structural Geology, v. 9, REVISED MANUSCRIPT RECEIVED JULY 27, 1992 E. Snow, and B. C. Vendeville. Special p. 87-103. MANUSCRIPT ACCEPTED JULY 28, 1992

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