Mesozoic structure of the Newfoundland Mountains, Utah: Horizontal shortening and subsequent extension in the hinterland of the Sevier belt RICHARD W. ALLMENDINGER | Department of Geological Sciences, Cornell University, Ithaca, New York 14853 TERESA E. JORDAN ABSTRACT ented at a low angle to bedding. Microfabric analysis of recrystallized The Newfoundland Mountains of northwestern Utah preserve an rocks in the contact aureole and unrecrystallized country rocks shows unusually complete record of Mesozoic deformation in the hinterland that most of the intracrystalline strain is pre-latest Jurassic in age and of the Sevier orogenic belt. Structural relations of Paleozoic miogeo- is characterized by layer-parallel shortening. Post-latest Jurassic in- clinal strata with the Newfoundland stock, which has biotite and tracrystalline strain is minor and records shortening originally ori- hornblende K/Ar ages of 153.2 ± 4.6 and 147.7 ± 4.4 m.y., respec- ented nearly vertical. The map geometries, geochronology, and tively, and dated quartz monzonite dikes (with K/Ar biotite ages of microfabric analysis suggest the following structural history (from 150.7 ± 4.5 and 143.7 ± 4.3 m.y. and a muscovite age of 149.1 ± 4.5 oldest to youngest): (1) pre-latest Jurassic regional shortening and m.y.) provide the basis for dating Mesozoic deformation in the range. thrusting and (2) syn- or post-latest Jurassic horizontal extension Geologic mapping at 1:24,000 has identified four geometric classes of during the Mesozoic. In the Cretaceous and early Tertiary, whereas faults with known or probable Mesozoic ages: (1) older-over-younger strata in the Idaho-Wyoming-northern Utah thrust belt to the east thrust faults (including the Desert Peak thrust, named herein); were shortened by 140-150 km, the rocks in the Newfoundland (2) inward-facing folds in the contact aureole of the stock; (3) low- Mountains experienced little internal strain or were being extended, angle normal faults at a high angle to bedding, intruded by even though they were also probably being translated eastward above undeformed Mesozoic dikes; and (4) younger-over-older faults ori- the deep-seated westward extension of the thrust belt décollement. Figure 1. Generalized tec- tonic map of northwestern Utah showing the location of the study area in the Newfoundland Mountains. Abbreviation» : RRM = Raft River Mountains; GC = Grouse Creek Mountains; BM = Bovine Mountain; HTM = Hogup-Terrace Mountains; GM = Grassy Mountains; CI = Cra- ter Island; SI = Silver Island; PR = Pilot Range. Patterns: dashes = Archean basement; hori- zontal lines = upper plate of the Willard thrust; stipple = alloch- thonous Pennsylvanian-Permian Oquirrh Group rocks; asterisks = Mesozoic plutons. Geological Society of America Bulletin, v. 95, p. 1280-1292, 9 figs., November 1984. 1280 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/11/1280/3419254/i0016-7606-95-11-1280.pdf by guest on 28 September 2021 MESOZOIC STRUCTURE, NEWFOUNDLAND MOUNTAINS, UTAH 1281 INTRODUCTION significant tectonism. On top of the Devonian strata, a major unconfor- mity, described in greater detail below, omits most, if not all, of the Deformation of rocks in northwestern Utah is the result of tectonic Mississippian and Pennsylvanian Systems. A thick sequence of Permian events that range in age from late Paleozoic to Holocene. The Basin and sandstone and carbonate with a thin basal conglomerate overlies the Range morphology is an expression of the most recent tectonism, the unconformity. ongoing extension that probably began in the middle Cenozoic (Arm- Although Mesozoic plutons and metamorphism are well known in strong, 1972,1982; Compton and others, 1977; Zoback and others, 1981; the hinterland (Armstrong and Suppe, 1973; Armstrong, 1976; Carroon, Compton, 1983). The region also lies within the hinterland of the Meso- 1977; Miller and others, 1982; Miller, 1983b; Moore and McKee, 1983), zoic-early Cenozoic Sevier thrust belt (Armstrong, 1968; Allmendinger it has been particularly difficult to define corresponding Mesozoic struc- and Jordan, 1981), and it contains several major unconformities that tures in southern Idaho, northwest Utah, and northeastern Nevada. suggest significant Paleozoic tectonic activity as well (Jordan and Doug- Mesozoic low-angle faults with both older-over-younger and younger- lass, 1980; Dickinson and others, 1983). This paper describes the timing over-older geometries recently have been described in the Albion Range, and kinematic style of Mesozoic deformation in one part of that region. Blackpine Mountain, and the Pilot Range (Fig. 1) (Smith, 1982; Miller We focus on the Mesozoic structural history, because it provides an impor- and others, 1983; Jordan and others, 1983). As shown below, the New- tant constraint on the mechanics of foreland thrusting to the east and on foundland Mountains contain the oldest documented Mesozoic thrusts in how that thrusting is related to events and processes within the Cordillera all of northwest Utah and southern Idaho. of Nevada and California. In contrast to the hinterland, foreland deformation to the east in the The Newfoundland Mountains are located in northwestern Utah in Idaho-Wyoming-northern Utah thrust belt is much better known, in the Great Salt Lake Desert, completely isolated from other exposures of terms of both geometry and age (Armstrong and Oriel, 1965; Royse and pre-Quaternary rock (Figs. 1,2 A). The mountains lie east and southeast of others, 1975; Dixon, 1982). At the latitude of the Newfoundland Moun- the multiply deformed and metamorphosed rocks of the Pilot Range, tains, the oldest and structurally highest commonly recognized foreland Grouse Creek Mountains, and Raft River Mountains (the last two have thrust is the Willard thrust (Fig. 1) (Crittenden, 1972). If correlations with been called "metamorphic core complexes") and west of the well-known the Paris fault are correct, the Willard thrust is probably of latest Jurassic Idaho-Wyoming-northern Utah foreland thrust belt (Armstrong and and(or) earliest Cretaceous age (Armstrong and Oriel, 1965; Royse and Oriel, 1965; Royse and others, 1975). Although the Newfoundland Moun- others, 1975; compare with Dover, 1983). A structurally higher alloch- tains have not escaped the profound regional Cenozoic extension, their thon of upper Paleozoic rocks (the "Hansel allochthon") has been recog- excellent exposure of a thick, little-deformed Cambrian to Permian mio- nized (Allmendinger and Piatt, 1983), and, although a mid-Jurassic age geoclinal section and, more importantly, the presence of a Late Jurassic was tentatively proposed (Allmendinger and Jordan, 1981), it can only be stock at their northern end make them well suited to a study of Mesozoic shown to be pre-Late Cretaceous in age (Jordan and others, 1983). structural styles. In addition to our own detailed mapping (scale 1:24,000) Obscuring many of the Mesozoic features of the hinterland are the of the northern half of the range and reconnaissance mapping in the rest of profound regional extension and local metamorphism that probably com- the range, we have relied strongly on the excellent mapping of Paddock menced in the Oligocene and are continuing at present (Armstrong, 1972; (1956), work on the Newfoundland Stock by Carroon (1977), and geo- Compton and others, 1977; Wernicke, 1981; Allmendinger and others, chronology by V. Todd (cited in Carroon, 1977), W. Hoggatt-Hillhouse, 1983). During this episode, the features now recognized as the metamor- and J. Nakata (1983, personal commun.), all of the U.S. Geological phic core complexes were formed (articles in Crittenden and others, 1980). Survey. The core complexes apparently represent isolated exposures of Cenozoic On the basis of field relations, microscopic structural analysis, and extensional features that underlie much of the eastern Basin and Range. geochronology, we recognize both pre-latest Jurassic thrust faults and Cenozoic structures are difficult to recognize in the Newfoundland pre-earliest Cretaceous normal faults. The principal period of shortening Mountains, because rocks younger than Jurassic and older than Quater- in the Newfoundland Mountains predates any known thrusting in the nary are not present. Of particular importance is the question of when and Idaho-Wyoming-northern Utah thrust belt farther east. During the subse- how the rocks in the range acquired their present westward dip. As shown quent Cretaceous and early Cenozoic foreland deformation, the rocks in by the angular unconformity, a very gentle component of southward tilt the Newfoundland Mountains either experienced little internal deforma- occurred prior to the Permian. Some open folding may have taken place in tion, or they were extended parallel to bedding. Palinspastic reconstruc- the Mesozoic prior to intrusion of the Newfoundland Stock, but major tions by Royse and others (1975) suggest that, regardless of the kinematic folds with steep or overturned limbs are notably absent. The consistent east setting of rocks in the Newfoundland Mountains, they probably were dip (50° to 80°) of dike swarms suggests that 10° to 40° of westward being translated eastward above a deep-seated décollement during the tilting postdates intrusion, assuming that the dikes were initially vertical. Cretaceous and early Cenozoic. Rotation of the mountain block by that amount could well have occurred due to Cenozoic normal faulting, perhaps above a shallowly dipping ex- GEOLOGIC SETTING tensional detachment. The Newfoundland