Tectonic reconstruction and sediment provenance of a far-travelled oceanic nappe, Helgeland Nappe Complex, west-central Norway

Autor: Tore Prestvik, Øystein Nordgulen, Kelsey McArthur, Calvin G. Barnes, Carol D. Frost
Rok vydání: 2013
Předmět:
Zdroj: Geological Society, London, Special Publications. 390:583-602
ISSN: 2041-4927
0305-8719
Popis: The Helgeland Nappe Complex (HNC), part of the Uppermost Allochthon of the northcentral Norwegian Caledonides, originated near the Laurentian margin and was transferred to Baltica during the closure of Iapetus in Late Silurian–Early Devonian time. The islands of Rodoy, Bolvaer and Leka, located in the Sauren–Torghatten (S–T) nappe of the HNC, are composed of ultramafic and mafic basement rocks unconformably overlain by metaconglomerates and fine-grained metasedimentary rocks. Geochemical and isotopic characteristics of the basement rocks are consistent with formation in a supra-subduction zone setting. Overlying metasedimentary rocks record an increasing proportion of continental detritus supplied to the basins through time. Precambrian cratonic source regions supplied cobbles and other detritus. This source area may have been located in modern SE Greenland/Labrador or in the Lower Nappe of the HNC. The second alternative best accounts for the short transport distances required by the coarsegrained conglomerates. The maximum age of deposition is constrained by the age of the youngest zircon grain dated at 471 + 8 Ma. Final sedimentation, nappe thrusting and nappe stacking occurred in rapid succession during c. 480–475 Ma. Supplementary material: Geochemical analyses and Nd isotopic data are available at http:// www.geolsoc.org.uk/SUP18654 The Scandinavian Caledonides record an important part of the Early Palaeozoic Caledonide–Appalachian orogeny related to the closure of the Iapetus Ocean (McKerrow et al. 2000). The Middle–Late Ordovician Taconian phase (c. 470–450 Ma) is interpreted as the product of an arc–continent collision along the Laurentian margin (Stanley & Ratcliffe 1985; Yoshinobu et al. 2002; Roberts 2003). In the Silurian–Early Devonian Scandian phase, oblique plate convergence culminated in the closure of the Iapetus Ocean and the collision of Baltica and Laurentia. It was during this phase at c. 420–400 Ma that the Uppermost Allochthon, the structurally highest of four major allochthons that compose the Scandinavian Caledonides, was uprooted from Laurentia and transported on to the Baltic margin (e.g. Roberts & Gee 1985; Stephens et al. 1985; Roberts 2003). Early tectonic models for closure of Iapetus showing orthogonal collision (Wilson 1966) are now understood to be too simplistic. Recent tectonic reconstructions indicate that sinistral orogen-parallel strike-slip motion accompanied Scandian contraction and accretion (Roberts et al. 2007). The Uppermost Allochthon contains oceanic assemblages and continental rocks containing rare fauna of Laurentian affinity (Roberts et al. 2007). The constituent nappe complexes of the Uppermost Allochthon record a complex pre-Scandian periLaurentian history of formation and assembly from Neoproterozoic to Early Ordovician time (Barnes et al. 2007); amalgamation, metamorphism and magmatism of these nappes has been ascribed to Taconian tectonism by Roberts et al. (2002) and Yoshinobu et al. (2002). In north-central Norway, the Helgeland Nappe Complex (HNC) is the structurally highest part of the Uppermost Allochthon (Fig. 1). Five nappes have been recognized in the southern HNC, which can be divided into two groups. One group is composed of the Upper, Lower and Horta Nappes and consists of migmatitic gneiss, calc-silicate rocks and marble, with no exposed depositional basement (Thorsnes & Loseth 1991; Barnes et al. 2007). The From: Corfu, F., Gasser, D. & Chew, D. M. (eds) New Perspectives on the Caledonides of Scandinavia and Related Areas. Geological Society, London, Special Publications, 390, http://dx.doi.org/10.1144/SP390.3 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics second group, comprising the Middle and Sauren– Torghatten (S–T) Nappes, is composed of metaconglomerates, marble, calc-silicate schist and pelitic schist deposited on discontinuously exposed ultramafic and mafic meta-igneous rocks (Thorsnes & Loseth 1991; Heldal 2001). The latter group of 65°00' 12°00' 66°00' 14°00' 65°00' 14°00' 65°30' 14°00' Atlantic Ocean To se nfj or d Bindsfjord Vikna Up pe r A llo ch th on Rodingsfjallet Nappe Complex (UmA) lfjo rd 66°00' 12°00' 65 00' 1400' ~465-440 Ma Granitoid rocks > 465 Ma Granitoid rocks Serpentinized peridotite & ophiolitic rocks Lower Nappe (pelitic schist, migmatite/marble) Sauren-Torghatten Nappe Central Norway Basement Window Undifferentiated metamorphic rocks Horta Nappe (carbonate & gneiss/migmatite) Middle Nappe Upper Nappe Helgeland Nappe Complex (Uppermost Allochthon) Undifferentiated metamorphic rocks Rodingsfjallet Nappe Complex
Databáze: OpenAIRE
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