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Late Variscan Exhumation Histories of the Southern Rhenohercynian Zone and Western Mid-German Crystalline Rise: Results from Thermal Modeling

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Late Variscan Exhumation Histories of the Southern Rhenohercynian Zone and Western Mid-German Crystalline Rise: Results from Thermal Modeling Geol Rundsch (1995) 84:578-590 © Springer-Verlag 1995 ORIGINAL PAPER A. Henk Late Variscan exhumation histories of the southern Rhenohercynian Zone and western Mid-German Crystalline Rise: results from thermal modeling Received: 10 June 1994 / Accepted: 27 March 1995 Abstract Thermal modeling techniques constrained by Introduction published petrological and thermo-chronometric data were applied to examine late orogenic burial and exhu- The Variscan orogeny in Central Europe was charac- mation at a Variscan suture zone in Central Europe. terized by the Devonian to Carboniferous closure of The suture separates the southern Rhenohercynian narrow oceans and subsequent collision of continental zone from the Mid-German Crystalline Rise and traces units (see Franke 1992 for review of the tectono-strati- the former site of a small oceanic basin. Closure of this graphic evolution). Exhumation of the resulting collage basin during Variscan subduction and subsequent colli- of crustal blocks started during the final convergence sion of continental units were responsible for different stage and continued during late orogenic extension. tectono-metamorphic evolutions in the suture's foot- The case study presented here concentrates on the late wall and hanging wall. Relative convergence rates be- Variscan evolution of five tectonic complexes (Fig. 1) tween the southern Rhenohercynian zone and western in relation to the final closure of one of these small Mid-German Crystalline Rise can be inferred from the oceanic basins, the Rhenohercynian or Lizard-Giessen- pressure-temperature-time evolution of the Northern Harz Ocean (Franke and Oncken 1990). Its former site Phyllite Zone. During Late Vis6an-Early Namurian is marked by a prominent suture zone, which separates times, horizontal thrusting velocities were at least two of the main tectono-stratigraphic units of the Varis- 20 mm/a. Thermal modeling suggests that exhumation can fold belt, the Rhenohercynian and Saxothuringian of the Mid-German Crystalline Rise occurred tempo- zones. It also forms the boundary between the external rarily at rates of more than 3 mm/a. Such rapid exhu- fold and thrust belt and the crystalline complexes of the mation cannot be produced by erosion only, but re- orogen's interior. The suture traces a major fault zone quires a substantial contribution of extensional strain. along which the Saxothuringian was thrusted on the Exhumation by upper crustal extension occurred con- Rhenohercynian to the north. Consequently, rocks of temporaneously with convergence and is explained by the northern Saxothuringian zone, the so-called Mid- continuous underplating of crustal slices and thrusting German Crystalline Rise (MGCR), and the southern along faults with ramp-flat geometry. Finally, implica- Rhenohercynian zone went through very different tec- tions for the tectono-metamorphic history of the study tono-thermal histories during the final stage of the Var- area and the thermal state of the crust during late Var- iscan orogeny. Their different evolutions are still pre- iscan exhumation are discussed. served in variable pressure-temperature-time (P-T-t) paths. Deciphering of these P-T-t records by thermal Key words Thermal modeling • Late Variscan • modeling techniques can therefore provide not only Exhumation • Rhenohercynian Zone • Mid-German quantitative estimates for the late orogenic exhumation Crystalline Rise velocities and the thermal state of the late Variscan crust, but also for the convergence rates between the Rhenohercynian zone and MGCR. Modeling approach Andreas Henk Institut far Geologic, Pleicherwall 1, 97070 Wt~rzburg, Germany Tel.: +49-931-31569 Thermal modeling is based on a geodynamic interpre- Fax.: +49-931-57705 tation of the late Variscan evolution of Central Europe 579 Tournaisian (355 Ma) Mid-German ').C Crystalline Rise Moldanubian Rhenoh~ a RhenohercynianOcean Upper Visean (325 Ma) Namurian/Weslphalian (315 Ma) ® I@. .~.~" c Carboniferous/Permian (290 Ma) ')io' "£ ?i "2~( [ -'._ - . 1 d Saar-Nahe Basin Active faults ".',~v.,- Molasse ~ " Abandonedfaults ~ Early Pataeozoic Lithological boundaries ~ Lower Devonian Fig. t Simplifiedgeological map showing the main tectono-strati- graphic units of the Variscan fold belt in Central Europe and Var- @ Intrusions (undifferentiated) ~ Mid Devonian iscan massifs of the study area Flysch ~] Mid Devonian- Lower Carboniferous Fig, 2a-d Geodynamic evolution of the study area during the Carboniferous and Early Permian (simplified after Franke and by Franke and Oncken (1990). The aim of this paper is Oncken 1990; see text for further explanation) to extend this qualitative model so that it is compatible with heat transport mechanisms. The study uses one- and two-dimensional thermal modeling techniques to the Late Devonian, as indicated by Tournaisian flysch compare calculated P-T-t paths with published petro- sedimentation in the autochthonous foreland (Fig. 2a). logical, thermo-chronometric and tectonic data. Plate convergence continued throughout the Early Car- boniferous and was accompanied by the intrusion of voluminous granites and granodiorites into parts of the Qualitative model: late Variscan geodynamic MGCR (Fig. 2b). During the Early Carboniferous, evolution rocks of the MGCR experienced a regional low to me- dium pressure type of amphibolite facies metamor- The variable tectono-metamorphic evolution within the phism (see Okrusch 1995 in press for review of the me- study area dates from the Silurian. At this time, crustal tamorphic evolution). Late orogenic exhumation of the extension started in the southern parts of the future MGCR started during Late Visdan and Early Namu- Rhenohercynian and northern parts of the Saxothurin- rian times. It was contemporaneous with the onset of gian zone, respectively (Franke 1989, 1992; Franke and convergence in the southern Rhenish Massif and con- Oncken 1990). Continued extension formed the Rhe- tinuous compression in the Saxothuringian basin fur- nohercynian or Lizard-Giessen-Harz Ocean, a narrow ther south (Fig. 2c). Relative movement between the ocean, presumably only a few hundred kilometers wide. Rhenohercynian zone and MGCR was not confined to The earliest indication for the closure of this small thrusting, but also involved strike-slip displacement, oceanic basin is given by Frasnian flysch sediments particularly in the final collision stage (Oncken 1988; (Franke and Oncken 1990). Thus at least from the Mid- Krohe 1992; Klagel and Oncken 1994). During the dle Devonian onwards, the beginning of convergence Westphalian, compression in the external parts of the between the Rhenohercynian and Saxothuringian had fold and thrust belt was concomitant with extension changed the geodynamic position of the MGCR from a and basin subsidence in the orogen's interior. Thrusts passive to an active margin setting related to a SE di- at the suture zone between the Rhenohercynian and rected subduction zone. Consumption of oceanic crust Saxothuringian were reactivated as major normal faults was short-lived and essentially completed at the end of (Fig. 2d). At least from the uppermost Carboniferous 580 Table 1 Available P-T-t data (after Kreuzer and Harre P (kbar) T (° C) t (Ma) Description 1975; Kowalczyk 1983; Lippolt 1986; Hess and Schmidt 1989; Bergstrfisser Odenwald (Unit III of Willner et al. 1991) Marell 1989; Anderle et al. 3.2-+ 0.5 645 ± 30 Variscan metamorphic event 1990; Okrusch 1990; Nasir et 1.6-+ 0.5 450±50 al. 1991; Willner et al. 1991; 530 ±40 335± 2 K-Ar hornblende cooling age Kl~igel and Oncken 1994; mi- 300 ±40 327+ 2 K-Ar biotite cooling age neral closure temperatures af- Surface exposure 295+ 5 Detritus in Wetterau/Saar-Nahe Basin ter Geyh and Schleicher 1990) BOllsteiner Odenwald 4.4± 0.5 650±20 Variscan metamorphic event 3.1± 0.5 540-+20 530-+40 328-+15 K-Ar hornblende cooling age 350-+ 40 325 ± 8 K-Ar muscovite cooling age 30O-+4O 319± 9 K-Ar biotite cooling age Surface exposure 295± 5 Detritus in Wetterau Basin Spessart 5-6.5 570-620 Variscan metamorphic event 530 ± 40 321 ± 4 K-Ar hornblende cooling age 350-+ 40 321 ± 4 K-Ar muscovite cooling age 300-+40 321± 3 K-Ar biotite cooling age Surface exposure 295± 5 Detritus in Wetterau Basin Northern Phyllite Zone (NPZ) Sedimentation < ?330 Inferred from stratigraphic and facial similarities to TKU 5-6 300-330 325 ± 5 Phengite barometry, statistic recrystallization of quartz, absence of biotite, K-Ar white mica formation age 5 ± 1 270-300 310 ± 5 Phengite barometry, static recrystallization of quartz, K-Ar white mica formation age 3-+1 270-300 310+ 5 Phengite barometry, static recrystallization of quartz, K-Ar white mica forming age, no subsequent differential movement betwen NPZ and TKU Surface exposure 295± 5 Detritus in Wetterau Basin Taunuskamm Unit (TKU) Sedimentation < 330 Vis6an 2 in Hintertaunus Unit 3 + 1 270-300 320 ± 5 Phengite barometry, static recrystallization of quartz, K-Ar white mica formation age 3+1 270-300 310± 5 Phengite barometry, static recrystallization of quartz, K-Ar white mica formation age Surface exposure 295± 5 Detritus in Wetterau Basin (< 295 Ma) onwards, the western MGCR and southern P-T-t data sets are summarized in Table 1. The MGCR Rhenohercynian zone were in a position relative to or hanging wall with respect to the main suture is rep- each other similar to the present situation. Both areas resented by the crystalline complexes of Spessart and were subject to erosion and provided detritus to fill in- Odenwald (Fig. 1). The Spessart crystalline complex tervening depressions
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