Interrelated P-T-t-d Paths in the Variscan Erzgebirge Dome (Saxony,Germany): Constraints on the Rapid Exhumation of High-Pressure Rocks from the Root Zone of a Collisional Orogen |
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| Abstract: | The Erzgebirge dome consists of several superimposed composite tectonometamorphic units of medium- to high-grade metamorphic rocks from different crustal depths. These exhibit high pressure-high temperature and even ultrahigh-pressure imprints inherited from the root zone of a Variscan orogen and were exhumed almost immediately after attainment of maximum pressures at ～341 Ma. At present, the entire stack of tectonometamorphic units lies underneath an upper-crustal sequence of Paleozoic metasediments and tectonic slivers of pre-Carboniferous metamorphic rocks. Shear zones active at different times and at different depths are preserved, mainly recording two successive stages of the exhumation history between 340 and 330 Ma. Tectonic transport during exhumation was remarkably constant in an E-W direction, swinging to NW-SE in the eastern part of the Erzgebirge parallel to a ductile transtensional zone (Elbe zone) that was concomitantly active. The various tectonometamorphic units have characteristically correlated, convergent P-T-t-d paths (both “cooling during decompression” and “heating during decompression”) that can be deduced from the dominant quartzofeldspathic rocks. These paths indicate successive exhumation of hotter rocks from increasingly deeper structural positions and juxtaposition against cooler rocks in higher positions, concomitant with the excision of intermediate crustal levels. We interpret this type of successive vertical telescoping of the metamorphic profile to be the result of extension of the thickened tectonometamorphic stack. Extensional unroofing in the middle and upper crust was contemporaneous with and outlasted underthrusting and hence prograde metamorphism and deformation at deeper levels of the tectonometamorphic pile. Underthrusting is documented by a major inversion of the maximum pressure conditions in the lowermost units. However, structures related to compressional stacking now generally occur only as relics transposed by extensional deformation at lower pressure, or are restricted to rare small slivers with preserved prograde structures. Sedimentation of Lower Dinantian turbidites occurred along the flanks of the Erzgebirge dome during the exhumation process. The extrusion of high-pressure rocks is interpreted to have been driven mainly by a major regional buoyancy instability caused by the delamination of the lithospheric mantle underneath the neighboring Bohemian Massif, which represented overthickened crust at least from the Devonian to the early Visean. Major controlling factors were boundary forces exerted by the thickened crustal bulge on the neighboring thin crustal segments in the north and east, effecting lateral extension of this orogenic wedge and extrusion-i.e., convective upward flow of gravitationally unstable crustal material. |
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