Relation of thin-skinned thrusting of Colorado Plateau strata in southwestern Utah to Cenozoic magmatism OLIVIER R. MERLE;* 1 f Department of Geosciences, University of Arizona, Tucson, Arizona 85721 GEORGE H. DAVIS RICHARD P. NICKELSEN Department of Geology, 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 Plateaus 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 Jurassic evaporites 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 Paunsaugunt Plateau. 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 evaporite décolle- ment by the weight of the thick Marysvale vol- canic pile. *Present address: Laboratoire de Tectonophy- Figure 1. Index map to the geography of the High Plateaus of southwestern Utah. WP = Wilson sique, Institut de Géologie, Université de Rennes Peak; HC south of Wilson Peak = Hillsdale Canyon; HC northeast of Tropic = Henderson Beaulieu, 35042 Rennes cedex, France. Canyon; PP = Powell Point; PHF = Pine Hills fault; and RIF = Rubys Inn fault. Geological Society of America Bulletin, v. 105, p. 387-398, 14 figs., 1 table, March 1993. 387 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/3/387/3381714/i0016-7606-105-3-387.pdf by guest on 28 September 2021 MERLE AND OTHERS 2500 m 2000 1500 1000 500 LUNDIN, 1989 Figure 2. Structure section showing large-scale fold/fault relationships and flattening of the thrust into the Jurassic Carmel evaporites (from Lundin, 1989). Cretaceous/early Tertiary Laramide orogeny Dip slip on the Rubys Inn thrust fault 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 Sevier River 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 monocline (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 Carmel Formation. 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 Markagunt Plateau (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 Mesa 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 anticline. 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-Pleistocene, 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 388 Geological Society of America Bulletin, March 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/3/387/3381714/i0016-7606-105-3-387.pdf by guest on 28 September 2021 THRUSTING OF COLORADO PLATEAU STRATA, UTAH PANGUÏTCH Figure 3. Structural ge- ologic map of the thrust and strike-slip fault rela- UTAH tionships in the Paunsau- gunt Plateau. EOCENE QUATERNARY BASALT THRUST FAULT STRIKE SUP FAULT MID-TERTIARY -j- HORIZONTAL ROAD CRETACEOUS NORMAL FAULT VOLCANIC ROCKS BEDDING ATTITUDE 5 VERTICAL 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.