A major synmetamorphic Early Devonian thrust and extensional fault system in the Mid Norway Caledonides: relevance to exhumation of HP and UHP rocks

Abstract

The study of complex orogenic belts commonly begins in the frontal regions with welldefined tectonostratigraphy, and relatively simple structure and metamorphism, and proceeds into the progressively more complex hinterland, which nevertheless may contain the best geochronological record of the most intense orogenic events. The northern part of the Western Gneiss Region in the hinterland of the Scandian orogen contains a robust U-Pb zircon geochronological framework on rocks subjected to high-pressure (HP) and ultra-high-pressure (UHP) metamorphism that has implications for tectonic development both there and towards the foreland. HP and UHP eclogite crystallization occurred at 415 -410 Ma (Early Devonian, Lochkovian to Pragian), followed by pegmatite crystallization at c. 395 Ma (Late Emsian) during exhumation and return to amphibolite-facies conditions, thus limiting the process to 15-20 myr. The nature and sequence of events are much more complex than in the foreland, causing difficulty in correlation, yet the combined tectonics in the two regions provides the necessary context to explain, for example, how rocks were subjected to deep-seated, high-temperature metamorphism and then exhumed to shallower levels. Here, we suggest how a recently recognized extensional detachment fault and a recently recognized out-of-sequence thrust might be linked to the timing of HP metamorphism and later exhumation. The postulated Agdenes extensional detachment in its footwall has basement gneisses containing Mesoproterozoic igneous titanite fully reset at 395 Ma, as well as Devonian pegmatites, and in the hanging wall Ordovician to Early Silurian granitoids of the Støren Nappe containing igneous titanite barely influenced by Devonian recrystallization and no evidence of post-Ordovician melts. This implies removal of a significant crustal section on a large-scale detachment. Rocks both above and below are overprinted by the same late, subhorizontal, sinistral ductile extensional fabric, obscuring any fabrics produced during development of the detachment itself. Eastern Trollheimen escaped the late, strong, subhorizontal overprint, and shows: (1) early emplacement of thrust nappes of Lower and Middle Allochthons over Baltican basement and its Late Neoproterozoic quartzite cover; (2) major, SE-directed, recumbent folding of the entire thrust-imbricated sequence; and (3) major, out-of-sequence, SE-directed thrusting (Storli Thrust), for an 80 km minimum transport across-strike, of the recumbent-folded sequence over deeper, less deformed, lower basement gneisses and unconformable Neoproterozoic quartzite cover. The upper basement contains boudins of eclogite and garnet-corona gabbro lacking in the lower basement. Similar basement imbrications occurred in the Tømmerås window, the Skarddøra Antiform, the Mullfjället Antiform and the Grong-Olden Culmination, up to 240 km NE of Trollheimen, as well as in the Reksdalshesten antiform 100 km west, all within the postulated minimum 400 ×180 km area of the Agdenes detachment.

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

“…The second phase, COSC-2, involves a second 2.5 km deep borehole that will start in the Lower Allochthon and aims to penetrate the Jämtlandian décolle-ment as well as at least one of the underlying enigmatic basement reflectors. The focus of COSC-2 lies in understanding the thin-skinned thrusting over this detachment horizon, the character of the deformation in the underlying crystalline Fennoscandian basement, and how this foreland deformation relates to the partial subduction of the Baltica margin in the hinterland (e.g., the Western Gneiss Region of southwestern Norway) in the early Devonian (Robinson et al, 2014).…”

“…CDP 3300-3500, there is evidence of substantial shortening, with a quartzite-dominated thrust stack in Oviksfjällen and much internal basement deformation in Olden. The Olden Antiform is of particular interest because it contains an upper part of allochthonous basement (Gee, 1980;Robinson et al, 2014) thrust over the CambroSilurian sedimentary rocks of the Jämtland Supergroup. The extent to which sedimentary rocks of the Lower Allochthon might be represented at deeper structural levels than those exposed in the Olden and Oviksfjällen antiforms is, at present, impossible to say; MT methods have difficulty in detecting any features below a strong conductor like the alum shales that is so well defined in the overlying décollement levels.…”

New perspectives on the Caledonides of Scandinavia and related areas: introduction

“…Similarly, Corfu & Heim (2013) present U-Pb TIMS data from an anorthositic complex within the Espedalen nappe of southern Norway, revealing a Telemarkian affinity that subsequently underwent Sveconorwegian and Caledonian overprints. Robinson et al (2013) describe the geometry and complex superposition of Caledonian thrusting and extension, which led to highly attenuated nappes in the northern part of the Western Gneiss Region. Finally, Rice (2013) presents a detailed review of the structural architecture of the Caledonides in Finnmark and, based on branch line restoration, discusses the pre-Caledonian basin architecture along the northern Baltican margin (Fig.…”

Metamorphosed cumulate gabbros from the Støren Group of the Upper Allochthon, northern Western Gneiss Region, Norway: petrology and metamorphic record

“…Scandian metamorphism in the Upper Allochthon was generally greenschist facies, but up to amphibolite facies particularly near contacts with the underlying Middle Allochthon. Robinson et al (2014), working in central Norway, interpreted this pattern to indicate that the highest metamorphic grades developed in the Upper Allochthon during detachment faulting toward the end of Scandian orogenesis. Our goals in this study were: (1) to characterize the metamorphic history of the rocks using mineral compo sitions, textures, and thermodynamic modeling, and (2) to place the timing of metamorphism into the tectonic framework generally worked out for the Upper Allochthon.…”

“…The cross-hatched parts of the Støren Nappe show the Lower and Upper Hovin groups and related units. The Agdenes detachment (Robinson et al, 2014) …”