Structure and petrology of a Tertiary gneiss complex in northwestern Utah

Abstract

A gneiss complex in the Grouse Creek Mountains, northwestern Utah, consists of 2.5-b.y.-old adamellite that was remobilized and intruded younger Precambrian and Paleozoic cover rocks 25 m.y. ago during an extended period of synkinematic metamorphism. One or more structural domes formed in the region during metamorphism. The remains of a middle Tertiary culmination in the central Grouse Creek Mountains is an asymmetrical welt of schistose Precambrian adamellite that protrudes into greatly attenuated autochthonous and allochthonous cover rocks. Fine-to medium-grained gneiss and schist of the upper 200 m of the welt, or dome, grade upward to retrograde phengite-quartz-albite schist that intruded the lowermost beds of upper Precambrian(?) quartzite. The age of the schistose Precambrian adamellite and its geographic continuity with less-metamorphosed Precambrian adamellite lying unconformably beneath the quartzite indicate that remobilization of the former was post-Precambrian. Simultaneously, cover rocks were thinned to one-fifth their original thickness by metamorphic flattening, extension, and low-angle faulting.A 25-m.y.-old pluton underlies the area of remobilization. Its outer shell bears metamorphic folds related to a second deformation. These folds also predominate in a metasomatic aureole in the Precambrian adamellite surrounding the Tertiary pluton and in the upper, schistose part of the gneiss dome. Thus, development of the dome was closely related to the second deformation and to the intrusion 25 m.y. ago.Late-metamorphic folds and low-angle faults chiefly affected allochthonous cover rocks, and postmetamorphic movements carried parts of these rocks over 1 l-m.y.-old sedimentary rocks in the adjoining basin. Isotopic data on autochthonous rocks suggest that metamorphic temperatures persisted into Miocene time and provided the potential for continuing uplift and basinward shedding of sheets which ended after 11 m.y. ago. on July 19, 2015 memoirs.gsapubs.org Downloaded from 350 K. A. HOWARD Cambrian and Paleozoic cover rocks about 25 m.y. ago. It is part of a Tertiary metamorphic and igneous complex in the Raft River and Grouse Creek Mountains which has been described by Compton and others (1977) (Fig. 1). Age relations between metamorphic minerals and superimposed fold sets in the gneiss complex and surrounding rocks indicate that its rise was related to intrusion of adamellite (rock names according to the usage of Williams and others, 1954, p. 121) magma during regional synkinematic metamorphism in early Miocene time.

Processes of regional Tertiary extension in the western Cordillera: Insights from the metamorphic core complexes

“…In the Grouse Creek and Raft River MCC of northwestern Utah, Eocene to Miocene ductile flattening and east-west extension are documented within plutons as young as 24 Ma (Todd, 1980;Compton, 1980;Compton and others, 1977). Gans and Miller (1983) documented a similar history in the Snake Range (1) and (2), detachment is formed by dikelike pluton emplacement.…”

Stratigraphic variation and low-angle faulting in the North Hansel Mountains and Samaria Mountain, southern Idaho

“…on July 4, 2015 memoirs.gsapubs.org Downloaded from others, 1977; Davis and Coney, 1979;Crittenden, 1980;Miller, 1980;Todd, 1980). The relations between metamorphic complex and foreland thrust belt have been a subject of debate for many years (e.g., Crittenden, 1979;Compton and Todd, 1979), and part of the reason for that debate has been the lack of information on the structural evolution of the intervening region.…”

“…Widespread, Tertiary, low-angle normal faulting has been well documented both to the west and south (Compton et al, 1977;Todd, 1980;McDonald, 1976;Davis and Coney, 1979). Though age data are lacking for the upper allochthon in the North Hansel Mountains, the crosscutting relations and microstructure analysis indicate that it may have been emplaced during the regional Tertiary extension.…”

Structural geometry and sequence, Bovine Mountain, northwestern Utah

“…Data presented in this paper suggest that rocks exposed at Bovine Mountain may be transitional between those typical of the metamorphic terrane and essentially unmetamorphosed neighboring regions of the eastern Basin and Range. The structural geology of Bovine Mountain will be described and compared to the complex deformation history of the Grouse Creek, Raft River, and Albion Mountains previously described by Compton and Todd (Compton and others, 1977;Compton and Todd, 1979;Todd, 1980) and Miller (1980). Bovine Mountain forms the southern end of the Grouse Creek Mountain in northwestern Utah (Fig.…”

“…The metamorphic core complex to the north, as reported by Compton and others (1977), Compton and Todd (1979), and Todd (1980), is characterized by widespread metamorphism and generally thinned stratigraphie units. Thinning has been accomplished by both ductile flow during metamorphism and by omission of section along bedding plane faults.…”