Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Crustal and mantle reflectors from Palaeoproterozoic orogens and their relation to arc-continent collisions D. B. SNYDER t, S. B. LUCAS 2 & J. H. McBRIDE 1 1 BIRPS, Bullard Laboratories, Madingley Road, Cambridge CB30EZ, UK 2 Geological Survey Canada, 601 Booth Street, Ottawa, Ontario KIA OE8, Canada Abstract: Two prominent geological features characterize the Palaeoproterozoic orogenic belts of Laurentia and Baltica: (1) Archaean cratons form the stable footwall during early stages of convergence and throughout crustal collision; and (2) juvenile, predominantly arc-derived crust was accreted to Archaean cratons through arc-continent collision. Seismic reflection profiling over the Svecofennian (Baltic Shield), Lewisian (British & Irish Isles) and Trans-Hudson (Canadian Shield) segments of an arguably once continuous orogenic belt has provided geometries of reflectors throughout both Palaeoproterozoic and Archaean crustal blocks as well as laterally coherent mantle reflectors. Two BABEL deep seismic reflection profiles within the Baltic Shield revealed structures along an irregularly shaped boundary between the juvenile 2.0-1.8 Ga Svecofennian domain and an Archaean craton (Karelia Province). An important result of the survey is the approximately 100 km horizontal offset between the inferred mantle suture, or palaeo-subduction boundary, and the geochemically-mapped crustal suture between juvenile Proterozoic crust and the Archaean craton. On the other side of the Atlantic, LITHOPROBE reflection profiles across the Trans-Hudson Orogen in central Canada reveal that 1.92-1.83 Ga juvenile arc and oceanic terranes of the Reindeer Zone form an allochthonous carapace about an Archaean basement block. Unexpectedly, the juvenile allochthons dip beneath the bounding Superior and Hearne cratons, defining a crustal-scale culmination in the core of the Orogen. The British and Irish Isles form an important bridge between the Palaeoproterozoic orogens of Laurentia and those of Baltica. Twenty deep reflection profiles on the continental shelf north and west of Scotland have traced a mantle reflector for over 800 km along its reconstructed strike, which is at a high angle to younger structures. This feature dips away from the reconstructed cratonic nucleus of Laurentia and may trace the Palaeoproterozoic (1.7-1.9 Ga) suture between the Archaean Lewisian block and accreted juvenile crust represented by the c. 1.8 Ga Rhinns complex. A common theme that emerges from the integration of all these geological and seismic results is the role played by the Archaean lithosphere during arc-continent collision. In all cases studied, juvenile 1.9-1.8 Ga lithosphere was delaminated and its crustal flakes overrode the Archaean margins. This consistency reflects the relative strength and durability of Archaean crust/lithosphere, and suggests that large parts of the lithosphere underlying detached and flaked Palaeoproterozoic juvenile terranes, such as the Svecofennian or Reindeer Zone terranes, may be Archaean in age but modified during Proterozoic tectonism. Over the past decade, deep seismic reflection marine profiles (McGeary & Warner 1985; Flack profiles of Precambrian orogenic belts (e.g. Green et aL 1990; Lie & Husebye 1994; Calvert et aL et al. 1988; BABEL Working Group 1993; Nelson 1995). Many of these observations consist of single et al. 1993; Lewry et al. 1994) have revealed crust features recorded on a lone profile (e.g. Flack et al. and mantle reflectors that are geometrically similar 1990; Best 1991). However, a majority of the to structures at modem plate convergence zones mantle features were substantiated by their con- (e.g. Choukroune & ECORS Team 1989; Pfiffner sistent appearance on local grids or neighbouring et al. 1990). Whereas the pattem of cmstal reflec- parallel profiles (Flack & Warner 1990; BABEL tivity generally complements collisional archi- Working Group 1993; Baird et al. 1995). These tecture inferred from geologic studies (e.g. Ga~il & deep mantle reflectors occur predominantly Gorbatschev 1987; Lucas 1989; Lewry et al. 1990), beneath outcrops of Proterozoic or Archaean the mantle reflectors provide particularly important continental crust (e.g. Calvert et al. 1995), clues to the processes of Precambrian plate excluding those reflectors clearly associated with convergence and collision. Wadati-Benioff zones and active subduction (e.g. For over a decade unequivocal reflections from Davey & Stem 1990). Several of the best studied the uppermost mantle have been observed on deep mantle reflectors lie along the c. 1.7-2.0 Ga seismic reflection profiles, most commonly on boundaries between Archaean cratons and From Brewer, T. S. (ed.), 1996, Precambrian Crustal Evolution in the North Atlantic Region, Geological Society Special Publication No. 112, pp. 1-23. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 D.B. SNYDER ET AL. Palaeoproterozic terrains that occur around the data is that the crustal portions of Palaeoproterzoic present North Atlantic (Fig. 1). Palaeoproterozoic juvenile terranes appear to have been largely ages for both crust and mantle events in these areas detached from their lithospheric mantle during the are inferred from the continuity of reflectors with process of accretion to, or juxtaposition between, dated crustal structures (Lucas et al. 1994), the lack relatively rigid Archaean cratons. This result under- of younger major thermo-tectonic events (BABEL lines the importance of delamination and flake Working Group 1993; Lewry et al. 1994), or super- tectonics in crustal growth and recycling, via the position relationships in which reflectors associated mechanisms of accretion of juvenile crust and the with dated crustal deformations cut other, pre- subduction of mantle and possibly lower crust. sumably older, reflectors (Snyder & Flack 1990). Largely independent geological, geochrono- Terrain links and correlations logical and geochemical studies in the Baltic Shield (Ga~il & Gorbatschev 1987; Ohlander et al. 1993), Recent compilations of age and structural data have the British Isles (Muir et al. 1992), Greenland led to a reconstruction of Proterozoic Laurentia, (Kalsbeek et aL 1987, Bridgewater et aL 1990), and reuniting Archaean blocks and Proterozoic belts the Canadian Shield (Hoffman 1988; Bickford et al. observed in Labrador and Greenland (Van 1990; Lewry et al. 1990; St-Onge et al. 1992; Van Kranendonk et al. 1993). This reconstruction Kranendonk et al. 1993; Lucas et al. 1996; Stern requires only small translations to produce reason- et al. 1995a, b) have established that the interval able terrane fits. Attempts to make reconstructions 1.9-1.8 Ga was characterized by the generation of that include the Precambrian blocks of the British juvenile crust and the accretion of juvenile terranes Isles and the Baltic are substantially more difficult onto Archaean cratonic margins throughout and involve much larger translations that developed Laurentia and Baltica (Fig. 2). These Palaeo- over longer periods of time. At present, no general proterozoic belts formed an orogenic system on the agreement exists as to the fit between Laurentia scale of the Tethyan Alpine-Himalayan Orogen, and Baltica following the 1.8-1.7 Ga accretion/ and preserve remnants of the destroyed oceanic collision events (compare Wardle et al. 1986; realm in the form of ophiolites (Kontinen 1987; Gower 1990; Kalsbeek et al. 1993; Park 1994). Scott et al. 1991; St-Onge et al. 1992; Stern et al. Recent advances in quantifying the palaeo- 1995b), oceanic island arcs (Syme 1990; Stern location of Baltica up to 600 Ma ago using palaeo- et al. 1995a) and accretionary collages (Park 1991; magnetic data have already produced some Lucas et al. 1996). As with the modern Alpine- surprising implications concerning Vendian Himalayan Orogen, the individual components of continental assemblages (Torsvik et al. 1992; the Precambrian Orogen are better understood than Soper 1994). The implied drift history for Baltica their linkage and interrelationships along-strike, indicates that an anticlockwise rotation of nearly although recent syntheses made significant 180 ~ occurred between 525-425 Ma ago, coeval advances (e.g. Patchett & Arndt 1986; Hoffman with a more general northward drift throughout the 1988; Park 1991; Muir et al. 1992; Van Palaeozoic (Torsvik et al. 1992; Personen et al. Kranendonk et al. 1993). 1991). This orientation enables the juxtaposition of Seismic reflector geometries along segments of a combined northern Scotland--eastern Greenland this proposed Proterozoic Orogen provide import- block and the Tornquist Zone region of central ant constraints on former convergence and collision Europe across a major rift zone in order to explain processes. To date, no attempt has been made extensive rift sequence sediments in these areas to compare the structure of Palaeoproterozoic (Soper 1994). orogenic segments as interpreted from deep seismic Reconstructions showing Laurentia and Baltica reflection data, or to examine similarities and at 1.8-1.7 Ga must be extrapolated from the differences in accretion and collision events Vendian (600 Ma) configuration using much between segments. In this paper, the principal inferior palaeomagnetic constraints. Most recon- results of seismic reflection profiling of the structions have therefore attempted to link regions Svecofennian Orogen, the Laxfordian deformation of Grenvillian and Sveco-norwegian