On buoyancy and diapirism as drivers for exhumation of the basement infrastructure in the Western Gneiss Region, southern Scandinavian Caledonides

Abstract

The Western Gneiss Region (WGR) is dominated by orthogneisses and bounded by normal-sense shear zones against overlying allochthons. This vast mass of granitoid rocks underwent subduction and re-emergence from the throat of the subduction channel, possibly rupturing the overlying orogenic wedge to open a tectonic window in the orogenic hinterland [2]. In this contribution I will explore available information regarding the role of buoyancy in driving tectonics during formation of this huge tectonic window (e.g. [5]) as an additional factor to permissive uprise within an externally-imposed kinematic system (e.g. [1], [8]).The WGR is characterised by foliation domes (culminations) in which orthogneisses emerge from below the Scandian allochthons or UHP domains emerge from below HP rocks [4], [5] [8]. Some are metamorphic core complexes (MCC’s) with solid ductile cores [8] but others, cored by migmatite, resemble gneiss domes [7] such as the eastern part of the WGR, a classic area for the study of gravity tectonics [5]. The domes, ovoidal in plan form, are wrapped by the allochthons; the gneiss cores also over-ride the allochthons to form basement-cored fold-nappes. Ramberg’s analogue models of rising gneiss diapirs generated a similar architecture. A key factor is that the gneisses are initially overlain by a denser lid, which creates gravitational instability; this was possibly represented by the ophiolites and arc rocks of the Trondheim Nappe Complex. The density inversion is enhanced by partial melting in the gneisses. The Oppdal domes area have also been interpreted as giant sheath-folds in a simple-shear field [6]. This may be consistent with a scenario where lateral channel flow is combined with diapiric action [7] where breaching of the lid forms an “aneurism”. MCC’s and gneiss domes are important mechanisms for heat dissipation in orogens; in the eastern WGR metamorphic grade in the nappes flanking the domes increases towards the gneisses and with depth in infolded synformal “keels” [3], [4] suggesting transfer of heat advected by the gneiss into the cover. Inverted metamorphic gradients may be generated where domes over-ride the cover.Understanding the relative roles of buoyancy as a direct driver of exhumation tectonics in the WGR versus permissive uprise controlled by the shear-zone framework will require more detailed mapping-out of Caledonian-age partial melting and metamorphic patterns in the orthogneisses, and new studies of kinematics of the eastern and northern dome systems of the WGR.Financial support from the National Science Centre, Poland (grant 2014/14/E/ST10/00321) and from AGH UST, Krakow, Poland.[1] Bottrill et al. (2014) Geochem. Geophys.Geosyst. doi:10.1002/2014GC005253[2] Brueckner & Cuthbert (2013) Lithosphere doi:10.1130/L256.1[3] Goldschmidt (1915) Skrift. Vid.-Selksk. Kristiana I. Mat.-Naturvid. Klasse, 6: 1-38[4] Krill (1985) In: Gee & Sturt The Caledonide orogen: Scandinavia and Related Areas, pp. 475-483. J. Wiley & Sons Ltd., Chichester.[5] Ramberg (1966) Bull. geol. Instn. Uppsala 43: 72pp.[6] Vollmer (1988) Journal of Structural Geology 10, 735-743[7] Whitney et al. (2004) Geol. Soc. America Special Paper 380: 1-19.[8] Wiest et al. (2020) Journal of the Geological Society, London doi:10.1144/jgs2020-199