The Southern Urals. Decoupled Evolution of the Thrust Belt and Its Foreland: a Consequence of Metamorphism and Lithospheric Weakening
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Tectonophysics 320 (2000) 271–310 www.elsevier.com/locate/tecto The Southern Urals. Decoupled evolution of the thrust belt and its foreland: a consequence of metamorphism and lithospheric weakening Eugene V. Artyushkov a,*, Michael A. Baer a, Peter A. Chekhovich b, Nils-Axel Mo¨rner c a Institute of Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya 10, 123810, Moscow, Russia b Institute of the Lithosphere of Marginal Seas, Russian Academy of Sciences, 109180, Moscow, Russia c Stockholm University, Institute of Paleogeophysics and Geodynamics, Kra¨feriket 24, S-10691, Stockholm, Sweden Abstract An analysis is presented of the mechanisms of tectonic evolution of the southern part of the Urals between 48N and 60N in the Carboniferous–Triassic. A low tectonic activity was typical of the area in the Early Carboniferous — after closure of the Uralian ocean in the Late Devonian. A nappe, ≥10–15 km thick, overrode a shallow-water shelf on the margin of the East European platform in the early Late Carboniferous. It is commonly supposed that strong shortening and thickening of continental crust result in mountain building. However, no high mountains were formed, and the nappe surface reached the altitude of only ≤0.5 km. No high topography was formed after another collisional events at the end of the Late Carboniferous, in the second half of the Early Permian, and at the start of the Middle Triassic. A low magnitude of the crustal uplift in the regions of collision indicates a synchronous density increase from rapid metamorphism in mafic rocks in the lower crust. This required infiltration of volatiles from the asthenosphere as a catalyst. A layer of dense mafic rocks, ~20 km thick, still exists at the base of the Uralian crust. It maintains the crust, up to ~60 km thick, at a mean altitude ~0.5 km. The mountains, ~1.5 km high, were formed in the Late Permian and Early Triassic when there was no collision. Their moderate height precluded asthenospheric upwelling to the base of the crust, which at that time was ~65–70 km thick. The mountains could be formed due to delamination of the lower part of mantle root with blocks of dense eclogite and/or retrogression in a presence of fluids of eclogites in the lower crust into less dense facies. The formation of foreland basins is commonly attributed to deflection of the elastic lithosphere under surface and subsurface loads in thrust belts. Most of tectonic subsidence on the Uralian foreland occurred in a form of short impulses, a few million years long each. They took place at the beginning and at the end of the Late Carboniferous, and in the Late Permian. Rapid crustal subsidence occurred when there was no collision in the Urals. Furthermore, the basin deepened away from thrust belt. These features preclude deflection of the elastic lithosphere as a subsidence mechanism. To ensure the subsidence, a rapid density increase was necessary. It took place due to metamorphism in the lower crust under infiltration of volatiles. The absence of flexural reaction on the Uralian foreland on collision in thrust belt together with narrow-wavelength basement deformations under the nappe indicate a high degree of weakening of the lithosphere. Such deformations took also place on the Uralian foreland at the epochs of rapid subsidences when there was no collision in thrust belt. * Corresponding author. Fax: +7-095-255-60-40. E-mail address: [email protected] (E.V. Artyushkov) 0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0040-1951(00)00044-5 272 E.V. Artyushkov et al. / Tectonophysics 320 (2000) 271–310 Nomenclature 1 c volume per cent of gabbroic intrusions d thickness of lithosphere hc thickness of crust 0 hc initial thickness of crust 1 hc thickness of crust after collision or stretching her thickness of eroded rocks hde thickness of delaminated eclogites he thickness of eclogites hgi total thickness of gabbroic intrusions hgg thickness of garnet granulites hml thickness of mantle lithosphere hn thickness of nappe hor thickness of sediments overridden by nappe hpg thickness of pyroxene granulites hs depth of sedimentary basin th hs depth of sedimentary basin formed by thermal relaxation of lithosphere hw depth of water L0 minimum width of deflection of elastic lithospheric P pressure T temperature ° Ta temperature of asthenosphere (1300 C) V Te e ective elastic thickness of lithosphere TM temperature at Moho VP P-wave velocity gb VP P-wave velocity in gabbro pr VP P-wave velocity in peridotites a thermal expansivity (3×10−5 K−1) b intensity of stretching Dhc increase in thickness of crust due to collision Dhlc thinning of lower crust by stretching Dhml decrease in thickness of mantle lithosphere due to asthenospheric upwelling Dhs thickness of sediments pushed out by nappe Df uplift of shortened crust Dfde uplift of crust due to delamination of eclogites Dfgi uplift of crust due to gabbroic intrusions e Dfrg uplift of crust due to retrogression of eclogites to garnet granulites gg Dfrg uplift of crust due to retrogression of garnet granulites to pyroxene granulites Dfuw uplift of crust due to asthenospheric upwelling e intensity of compaction of sediments overridden by nappe f altitude of crustal surface 0 fn minimum altitude of nappe in a case of no density changes in lithosphere j tectonic subsidence in shortened region due to density increase in lithosphere −3 ra density of asthenosphere (3220 kg m ) −3 rc density of crust (2830 kg m ) re density of eclogite rer density of eroded rocks −3 rgb density of gabbro (2930 kg m ) rgg density of garnet granulites rlc density of lower crust −3 rm density of mantle (3350 kg m ) rn density of nappe rpg density of pyroxene granulites rs density of sediments −3 rw density of water (1030 kg m ) 1 In the nomenclature, symbols without assigned values have been assumed to be variable. E.V. Artyushkov et al. / Tectonophysics 320 (2000) 271–310 273 Weakening of the lithosphere can be explained by infiltration of volatiles into this layer from the asthenosphere and rapid metamorphism in the mafic lower crust. Lithospheric weakening allowed the formation of the Uralian thrust belt under convergent motions of the plates which were separated by weak areas. © 2000 Elsevier Science B.V. All rights reserved. Keywords: collision; crustal subsidence; eclogitization; lithospheric weakening; mountain building; Urals 1. Introduction question as to whether or not vertical crustal movements of this type also occur in fold belts. The formation of foreland basins is commonly It has been shown that the Neogene foredeep of considered as a result of plate collision and flexing the East Carpathians was formed without signifi- of the elastic lithosphere towards convergent plate cant lithospheric stretching and at the epochs when boundaries without strong density changes in the no collision took place in the adjacent thrust belt lithosphere (Quinlan and Beaumont, 1984; (Artyushkov et al., 1996). At times of strong colli- Malinverno and Ryan, 1986; Royden, 1993; sion, the crustal surface in the thrust belt remained Stewart and Watts, 1997). Mountain building in at a low altitude. The Carpathian mountains began thrust belts is explained by a synchronous isostatic to grow 8 m.y. after the end of the collision. These response to thickening of the crust from plate vertical crustal movements required density changes collision (Molnar and Tapponier, 1975; Miyashiro in the lithosphere. No flexural reaction occurred on et al., 1982; Zonenshain et al., 1990) with its the Carpathian foreland at the epochs of collision. possible lowering by slab pull (Royden, 1993). In This indicates a drastic weakening of the lithosphere this scheme, vertical crustal movements in fold that ensured a strong crustal subsidence under the belts result from horizontal plate motions. It is nappe without a synchronous subsidence on the also commonly believed that the formation of adjacent foreland. thrust belts is associated with no significant In this paper, we analyse the tectonic develop- changes in the density of the lithosphere and the ment of the southern part of the Urals after closure of the Uralian ocean — since the start of the thickness of its elastic part T , which can be e Carboniferous. This area, 1300 km long, includes strongly reduced only due to steep bending or the Southern and Middle Urals (Fig. 1). For the strong heating of this layer in some places (Kusznir sake of simplicity, it will be called ‘the Southern et al., 1991; Burov and Diament, 1995). Urals’. In the preceding publications, attention has The crustal subsidence and uplift, however, been focused on a description and timing of colli- widely occurred in plate interiors without any sional events in the area (e.g. Ruzhentsev, 1976; significant lithospheric stretching and far from Zonenshain et al., 1984; Ivanov et al., 1986; Brown convergent boundaries. They have taken place, for et al., 1997; Puchkov, 1997, 1999). Here, we con- example, during the subsidence in the West sider another problem: what vertical crustal move- Siberian, Peri-Caspian, Volga–Urals, Timan– ments took place near to collisional boundaries in Pechora and Vilyuy basins (Artyushkov and Baer, the Southern Urals, and could these movements 1986a; Artyushkov, 1993). In the Neogene, many result from plate motions, or were they caused by mountain ranges and plateaus were formed by a deep seated processes? The main questions to be strong crustal uplift without significant compres- answered are: sive deformations in East Siberia, north-eastern 1. What were the modes of crustal subsidence on Asia and Africa (Nikolaew and Neumark, 1977; the Uralian foreland and their role in the basin Partridge and Maud, 1987; Makarov, 1990; Ollier, formation? 1991; Summerfield, 1991).