Continental Collisions and the Creation of Ultrahigh-Pressure Terranes: Petrology and Thermochronology of Nappes in the Central Scandinavian Caledonides
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Continental collisions and the creation of ultrahigh-pressure terranes: Petrology and thermochronology of nappes in the central Scandinavian Caledonides Bradley R. Hacker† Philip B. Gans‡ Department of Geological Sciences, University of California, Santa Barbara, California 93106-9630, USA ABSTRACT what extent is this paradigm correct? Conti- review the petrology and geochronology of nent-collision orogens form through a series of the major thrust sheets and then present new The formation of the vast Devonian stages involving (1) early arc ophiolite emplace- thermobarometry and thermochronology from ultrahigh-pressure terrane in the Western ment and continental passive margin contraction a key section inboard of the UHP terrane. We Gneiss Region of Norway was investigated by (typifi ed by the modern Australia-Banda arc conclude that the largest Norwegian UHP ter- determining the relationship between these collision) and subsequent relaxation (typifi ed by rane formed in the end stages of the continental ultrahigh-pressure rocks and the structur- Oman), (2) emplacement of oceanic sediments collision, following ophiolite emplacement and ally overlying oceanic and continental Köli and telescoping of the ophiolite-on-passive- passive margin subduction. Throughout, we use and Seve Nappes in the Trondelag-Jämtland margin assemblage, (3) emplacement of the the time scales of Tucker and McKerrow (1995) region. Thermobarometry and thermochro- upper-plate continent with its Andean-style arc, and Tucker et al. (1998). nology reveal that the oceanic Köli Nappes and (4) plateau formation and intracontinental reached peak conditions of 9–10 kbar and shortening (e.g., Tibet–Pamir). Can UHP ter- THE SCANDINAVIAN CALEDONIDES 550–650 °C prior to muscovite closure to ranes form during all of these stages, as implied Ar beginning at ca. 425 Ma. The continental by Searle et al. (2001)? The Scandinavian Caledonides are conven- Seve Nappes attained slightly higher pres- This question can be profi tably addressed in tionally subdivided into a number of structurally sures and temperatures (~11–12 kbar and the Scandinavian Caledonides, an archetype oro- defi ned units: the autochthon, Lower Alloch- 700–725 °C) and closed to Ar loss in musco- genic belt composed of thin, laterally extensive, thon, Middle Allochthon, Upper Allochthon, vite by 415 Ma in the east and by 400 Ma in far-traveled nappes or thrust sheets (Törnebohm, and Uppermost Allochthon (Roberts and Gee, the west. In contrast, the ultrahigh-pressure 1888) and three or four HP to UHP provinces of 1985) (Fig. 1). The autochthon consists of Pre- rocks were still deep in the mantle at eclogite- different ages (Brueckner and Roermund, 2004). cambrian Baltica basement overlain by Vendian facies pressures at 410–400 Ma. These data, We focus on the UHP province formed during through Upper Silurian sedimentary rocks. The in combination with structural, petrological, the 430–390 Ma Scandian orogeny—either the Lower Allochthon is composed of metasedi- and thermochronological data from else- largest or second largest on Earth (Ernst, 2001). mentary and crystalline rocks, compositionally where in the orogen, show that the ultrahigh- These UHP rocks might be a result of early arc similar to the autochthon, that have been thrust pressure metamorphism occurred in the ophiolite emplacement (case 1 above), but the east-southeastward over the autochthon. In the late stages of continental collision, after the latest ophiolite emplacement onto Baltica also more deformed and metamorphosed core of earlier stages of ophiolite emplacement and happened 10–20 m.y. before ultrahigh pressures the orogen, the Lower Allochthon contains the passive-margin subduction. were attained. They might be a result of passive UHP signature of the Scandian orogeny. These margin subduction during the initial stages of UHP rocks recrystallized at pressures as high Keywords: ultrahigh-pressure, Barrovian, continental collision (case 2 above), but sedi- as 3.6 GPa at ca. 410–400 Ma (Cuthbert et al., continental collision, argon geochronology, mentological (Soper et al., 1992) and paleomag- 2000; Terry et al., 2000a; Terry et al., 2000b; Scandinavian Caledonides. netic (Torsvik et al., 1996) studies suggest that Carswell, 2001; Krogh et al., 2003; Root et the collision between Baltica (Norway–Swe- al., 2004). The Middle Allochthon consists of INTRODUCTION den) and Laurentia (Greenland–eastern North crystalline and sedimentary rocks also inter- America) began as much as 20–35 m.y. before preted to have been derived from Baltica, but Ultrahigh-pressure (UHP) terranes, charac- the UHP metamorphism. Perhaps they formed from farther outboard than the autochthon. The terized by the presence of regional metamorphic as a result of intracontinental subduction, like Upper Allochthon consists of continental rocks coesite (pressures ≥27 kbar), are widely equated the Hindu Kush (case 4 above?). thought to represent the outermost margin of with the collisions between continents—but to The purpose of this paper is to examine pos- Baltica, plus ophiolitic rocks interpreted to sible cause-and-effect relationships between represent chiefl y Iapetus Ocean lithosphere. Scandian UHP tectonism and the emplace- The Upper Allochthon has been subdivided into †E-mail: [email protected]. ment of oceanic and continental thrust sheets many nappes; for the purposes of this study we ‡E-mail: [email protected]. onto the Baltica continental margin. We group them into two simplifi ed units: the Köli GSA Bulletin; January/February 2005; v. 117; no. 1/2; p. 117–134; doi: 10.1130/B25549.1; 9 fi gures; 3 tables; Data Repository item 2005024. For permission to copy, contact [email protected] © 2005 Geological Society of America 117 HACKER and GANS Broken Formation Devonian–Carboniferous(?) sedimentary basins * Västerbotten Uppermost Allochthon Gäddede Helgeland Nappe, Smøla terrane Limingen Group * Upper Allochthon Nord- Käli: Meråker, Støren, Tännfors and Gula Nappes Trøndelag Seve, Blåhø, Surna, and Skjøtingen Nappes Jämtland Snåsa Vestranden Åre Middle Allochthon Risbergbet, Tännäs, Dalsfjord, Jotun, Tømmerås Tännfors Särv, Sætra, Offerdal, Valdres, and Kvitvola Nappes Inndalen Handöl Lower Allochthon Meråker Rondane, Åmotsdal, Bergsdalen, and Osen–Røa Nappes/Western Gneiss Complex Fongen– Baltica basement Hyllingen Selbusjøen Trondheim Røros plutonic rock Sør- Trøndelag Holonda Figure 2 Høgstegia Hitra Folldal Smøla Otta Dombås Vågamo ophiolite W intrusive/ e depositional s t contact e r fault n Jotun eclogite G nappe n e i s s ultrahigh- pressure R areas e g i o Hornelen area n Nordfjord– Sogn detachment zone Figure 1. Geologic map of southwestern Scandinavian Solund– Atløy Caledonides, highlighting the Western Gneiss Region Stavfjord ophiolite and nappes. Emplacement of the Uppermost, Upper, and N Solund Middle oceanic and continental-margin Allochthons is related to the ultrahigh-pressure metamorphism in the 100km Bergen Lindås Nappe core of the orogen. 118 Geological Society of America Bulletin, January/February 2005 CONTINENTAL COLLISIONS AND ULTRAHIGH-PRESSURE TERRANES and Seve Nappes ( Stephens and Gee, 1985). the east; and (3) the Lower Köli Nappe locally rocks in the Gula Nappe are similar to the Upper The Uppermost Allochthon is lithologically grades lithologically and structurally downward Köli Nappe (Krutfjellet Nappe) in Västerbotten distinct from Baltica and is considered to be a into the Seve Nappes (Stephens, 1980; Stephens and Nordland (Stephens and Gee, 1985) and the fragment of Laurentia. This study focuses on the and Gee, 1985). Støren Nappe (Grenne et al., 1999). The Tänn- tectonic histories of the better-known nappes in Outcrops south of the study area near Otta fors Nappe (Fig. 2) has been correlated with the the Trondheim region (Fig. 2) but draws on rela- (Fig. 1) reveal that older parts of the Köli Nappes Lower Köli Nappe (Beckholmen, 1978). tionships across the Western Gneiss Region. were emplaced onto the Baltica margin prior to The youngest volcanoplutonic sections of the late Arenig (Sturt and Roberts, 1991) before the Köli Nappes are marginal-basin ophiolites NAPPE TECTONOSTRATIGRAPHY, younger parts of the Köli Nappes had even such as the Solund–Stavfjord (443 ± 3 Ma; PLUTONISM, DEFORMATION, AND formed. There, the MORB-affi nity Vågåmo Fig. 1) and Sulitjelma (437 ± 2 Ma; north of METAMORPHISM ophiolite lies in fault contact on psammites and Fig. 1) (Boyle, 1980; Dunning and Pedersen, crystalline rocks interpreted as part of Baltica 1988; Furnes et al., 1990; Pedersen et al., 1991). Uppermost Allochthon and is unconformably overlain by the Otta Con- Formation of these ophiolites was accompanied glomerate (Sturt and Roberts, 1991) that has a by the intrusion of widespread ca. 445–432 Ma The Uppermost Allochthon is considered late Arenig–early Llanvirn (485–464 Ma) fauna gabbroic to granitic, plutonic–hypabyssal bod- to be a fragment of Laurentia, based on C and of mixed Baltican–Laurentian affi nity (Bruton ies in the Upper Köli Nappe (Gee and Wilson, Sr isotopic chemostratigraphy (Melezhik et al., and Harper, 1981). This ophiolite-emplacement 1974; Senior and Andriessen, 1990; Pedersen 2002; Roberts et al., 2002a), early NW-directed event caused the appearance of detrital chromite et al., 1991; Stephens et al., 1993; Mørk et al., thrust faults (Roberts et al., 2001), and sedi- in upper Caradoc shales and limestones on the 1997), Middle Köli Nappe (Claesson et al., mentary successions that are distinctly different craton in the Oslo area