ABSTRACT The Scandinavian Caledonides have a complex latest Proterozoic–Early Devonian history, but they were finally assembled during the Silurian–Devonian (Scandian orogeny) collision between Baltica and Laurentia. Their dominant structural components are the Lower (Baltican margin), Middle (Baltican and farther outboard), Upper (Iapetan arcs), and Uppermost (Laurentian margin) Allochthons. This study examined the Blåhø Nappe, a complex unit of metamorphosed, intensely deformed igneous and sedimentary rocks assigned to the Middle Allochthon. Metamorphic grades are regionally amphibolite facies, but granulite- and eclogite-facies rocks are locally found. Although most metamorphic ages span a range from Middle Ordovician to Devonian, Blåhø eclogite and other high-pressure rock ages are exclusively Scandian. We analyzed 95 samples of Blåhø Nappe metamorphosed igneous rocks, which were mostly mafic rocks, composed of a minor arc-derived set and a major set transitional between arc and depleted to enriched mid-ocean-ridge basalt (MORB), a range characteristic of back-arc basins. Historically, the Blåhø Nappe has been assigned to the Seve Nappe Complex, the upper part of the Middle Allochthon as mapped in western Sweden and easternmost Norway. In contrast to the Blåhø Nappe, eclogites and other high-pressure rocks in the Seve Nappe Complex have yielded exclusively pre–Scandian orogeny Cambrian and Ordovician ages. Additionally, post–mid-Proterozoic igneous rocks of the Seve Nappe Complex are overwhelmingly dike swarms that were emplaced during the latest Proterozoic breakup of Rodinia, which have rift and MORB-type chemical signatures rather than arc and back-arc signatures, as has the Blåhø Nappe. We hypothesize that the Blåhø Nappe precursors formed on the upper plate, above a west-directed, late Cambrian to Ordovician subduction zone off the Baltican margin. Subduction of the Baltican margin, and possibly rifted fragments on the lower plate, produced the older Seve Nappe Complex eclogites and thrust the Blåhø and Seve Nappe Complex materials onto Baltica. This left the Blåhø Nappe and Seve Nappe Complex precursors on the lower plate during Scandian subduction and collision with Laurentia, allowing exclusively Scandian eclogite formation in the Blåhø Nappe. The Blåhø Nappe and Seve Nappe Complex thus seem to have distinct origins and should not be correlated with one another.

Abstract The study of complex orogenic belts commonly begins in the frontal regions with well-defined 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.