Shear Heating in Extensional Detachments: Implications for the Thermal History of the Devonian Basins of W Norway

Tectonophysics 608 (2013) 1073–1085 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Shear heating in extensional detachments: Implications for the thermal history of the Devonian basins of W Norway Alban Souche ⁎, Sergei Medvedev 1, Torgeir B. Andersen 1, Marcin Dabrowski 2 Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway article info abstract Article history: The supra-detachment basins in western Norway were formed during Lower to Middle Devonian in the Received 9 July 2012 hangingwall of the extensional system of detachments known as the Nordfjord-Sogn Detachment Zone Received in revised form 27 May 2013 (NSDZ). The basins experienced elevated peak temperatures (up to 345 °C adjacent to the main detachment Accepted 4 July 2013 fault) during the terminal stages of the extension. In this study, we show that the heat generated by the de- Available online 12 July 2013 formation of the rocks within the shear zone of the detachment, also known as shear heating, played a role in the thermal evolution of the entire region during the extension and could have induced the elevated peak Keywords: fl Shear heating temperatures of the supra-detachment basins. We numerically analyse the in uence of the rheology and Supra-detachment basins the deformation style within the shear zone, the rate of exhumation of the footwall, and the thickness of Numerical modelling the sedimentary accumulation on the top of the system. The model reproduces the elevated temperatures Post-Caledonian extension recorded in the supra-detachment basins where locally 100 °C and 25% of the total heat budget can be attrib- uted to shear heating. We show that this additional heat source may increase the peak temperatures of the hangingwall as far as 5 km away from the detachment. © 2013 Elsevier B.V. All rights reserved. 1. Introduction 1) The footwall records higher-grade synkinematic metamorphism and younger metamorphic cooling ages than the hangingwall. The Large-scale extensional detachments, resulting from gravitational footwall is often referred to as a metamorphic core complex, and collapse of over-thickened crust, or extension driven by plate motions, in extreme cases the entire crust may be stretched off to expose have been recognized and documented in a number of places and pro- mantle rocks (Manatschal, 2004; Wernicke, 1985). vide a major mechanism to exhume large segments of the lower crust 2) The shear zone is usually characterized by the ductile deforma- and mantle (Dewey, 1988; Lister et al., 1986; Manatschal, 2004; Wernicke, 1985). The main geological feature of such crustal-scale exci- tion with normal-sense kinematic indicators in the mylonites, sion is the juxtaposition of lower and upper crustal rocks across rela- which are commonly capped by brittle fault rocks. In the case of tively narrow (100 m to a few kilometre thick) damage and shear the Nordfjord-Sogn Detachment Zone, the mylonites may be sever- zones (e.g. Andersen and Jamtveit, 1990; Norton, 1986). Based on geo- al kilometers thick and associated with displacement in the order of logical observations, the ‘simple shear model’ or modified versions of it, 100 km (Andersen and Jamtveit, 1990; Hacker et al., 2003). explains the excision of the lithosphere along low angle normal faults 3) In the upper stratigraphic level of the hangingwall, syntectonic and shear zones. However, despite the general acceptance of this supra-detachment basins develop along active faults (e.g. low- model, several important questions remain regarding the dynamics angle-normal faults, listric normal faults or strike-slip transfer faults) and the overall thermal evolution of crustal-scale detachments. and are commonly found in tectonic contact with the shear zone A detachment consists of a footwall (“lower plate”), a shear zone, (Andersen, 1998; Osmundsen et al., 1998, 2000; Seguret et al., 1989). and a hangingwall (“upper plate”). The thermal evolution of a crustal-scale detachment has to be considered with two distinct aspects: fast exhumation and highly- localized deformation. Pressure-temperature-time (PT-t) paths of the ⁎ Corresponding author at: Institute for Energy Technology (IFE), Instituttveien 18, footwall metamorphic core complexes typically indicate fast decom- NO-2007 Kjeller, Norway. Tel.: +47 22 85 60 48. pression at near-isothermal conditions during exhumation from the E-mail address: [email protected] (A. Souche). lowertotheuppercrustlevel(Labrousse et al., 2004). The time window 1 Present address: Centre for Earth Evolution and Dynamics (CEED), University of bracketing such isothermal exhumation is only a few million years Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway. 2 fi Present address: Computational Geology Laboratory, Polish Geological Institute – (Hacker et al., 2010; Johnston et al., 2007) and is signi cantly smaller National Research Institute, Wrołcaw 53-122, Poland. than the time required for thermally equilibrating the crust by diffusion 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.07.005 1074 A. Souche et al. / Tectonophysics 608 (2013) 1073–1085 (several tenths of Myr, e.g., Turcotte and Schubert, 2002). Therefore, the The other characteristic feature controlling the thermal evolution thermal structure of the crust after fast exhumation of a high- to of the detachment zone results from the relative movement between ultra-high-pressure [(U)HP] terrane might be strongly perturbed by in- the foot- and the hangingwall. Large strain can accumulate inside the teraction between a “hot” footwall and a “cold” hangingwall. shear zone of the detachment, and this may have a significant effect a 5 E 6 E Bremangerlandet Hornelen Frøya Gjegnalunds- Svelgen Sandane breen Kalvåg Hornelen basin Ålfobreen Hyen Hovden Florø 61.6 N Håsteinen basin Askrova Trondheim Førde Kvamshesten basin Norway Bygstad Dale Bergen Oslo Fure Værlandet Fjaler Sørbøvåg Hyllestad 61.2 N Solund basin Leirvik Devonian basins (hangingwall) Sula Lavik Hardbakke Major detachment faults Ytre Sula Caledonian nappes (hangingwall / shear zone) 10 km Western Gneiss Region (footwall) b Erosion W E Devonian basins Burial depth basins >10 km Exhumation of the footwall? Shear heating? Fig. 1. a) Geological map of the Nordfjord-Sogn Detachment Zone in western Norway. b) Schematic cross section (E-W) across the detachment. Heat flux introduced at the base of the Devonian basins can come from the exhumation of the footwall or from shear heating in the shear zone. A. Souche et al. / Tectonophysics 608 (2013) 1073–1085 1075 on the heat budget of the system in form of shear heating (Brun and Hacker et al., 2003 (peak) Hacker et al., 2003 (retro) Cobbold, 1980; Burg and Gerya, 2005; Campani et al., 2010; Hartz and 2.5 Johnston et al., 2007 (peak) Podladchikov, 2008; Leloup et al., 1999; Schmalholz et al., 2009; HP Johnston et al., 2007 (retro) (WGR) Scholz, 1980). Shear heating rate depends on the non-elastic strain Average of error ellipses (1σ) rate and the deviatoric stress experienced by rocks during deforma- 2 HP tion. The amount of shear heating produced in a geological system (Caledonian nappes) is difficult to measure and modelling provides a viable tool for ~60 km assessing the role of shear heating. 1.5 We base our study on the geological observations of the “Isothermal” decompression Nordfjord-Sogn Detachment Zone (NSDZ) of western Norway. In the ~40 km peak conditions top stratigraphic level of the detachment, several supra-detachment 1 basins formed (Fig. 1) as response to repetitive tectonic motions dur- HP ing the exhumation of the footwall (Osmundsen et al., 1998; Seguret Pressure (lithostatic) [GPa] (retrograde overprints) ~20 km et al., 1989; Seranne et al., 1989; Steel et al., 1977). These supra- 0.5 detachments basins are commonly named the Devonian basins due to the presence of Devonian plant fossils in the sediments (Høegh, 0 1945; Kolderup, 1916, 1921, 1927). The special feature of the Devoni- 400 500 600 700 800 an basins is the documented low-grade metamorphism (Seranne and Temperature [oC] Seguret, 1987; Souche et al., 2012; Svensen et al., 2001; Torsvik et al., 1988). Souche et al. (2012) showed that the temperature conditions Fig. 2. Peak and retrograde Caledonian metamorphism from the WGR and HP Caledo- at the burial depth of 9.1 km was as high as 345 °C which can be trans- nian nappes exposed beneath the Solund and Hornelen basins. Thermobarometry data lated to a geothermal gradient of 38 °C/km. Additionally, the peak from Hacker et al. (2003) and Johnston et al. (2007). temperatures in the Devonian basins increase with proximity to the detachment contact and thus suggest that the lateral variation of and Dallmeyer, 1992; Cuthbert, 1991; Fossen and Dallmeyer, 1998; maximum temperatures was controlled by a dynamically evolving Young et al., 2011). system, most likely linked to the detachment. Was shear heating con- The mylonites of the NSDZ have a maximum thickness of 5.5 km tributing to the heat budget in the system? Could it have an impact on north of the Hornelen basin, but the precise original thicknesses cannot the lateral variation of the peak temperatures within the supra- be accurately determined because of excision by later faults. Marques detachment basins? et al. (2007) estimated the shear strain partitioning in the NSDZ and In this study, we analyse the thermal evolution of the detachment showed that the lower parts of the mylonites experienced considerable zone and estimate the contribution of shear heating on the heat bud- flattening across the foliation in addition to simple shear (ratio of sim- get of the area. We first present a compilation of available geological ple to pure shear ~1). The upper ~2 km of the mylonites are, however, data from the NSDZ to build a well-founded geological model that can characterized mainly by simple shear, with a shear strain of approxi- be studied numerically.

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