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-stratigraphic 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 such as the Wetterau trough and consists mainly of gneisses and mica schists subordinate the Saar-Nahe-Basin. Thus the earliest occurrence of to quarzite, marble and amphibolite (Hirschmann and specific detrital minerals and rock fragments in the se- Okrusch 1988; Okrusch 1990, in press). The adjacent dimentary record of the Permo-Carboniferous basins Odenwald can be divided into two units with different provides a minimum age for surface exposure of tectono-metamorphic histories. Its western part, the MGCR and southern Rhenohercynian zone. Bergstr/~sser Odenwald, is dominated by granitic to gra- The present day crustal structure at the suture be- nod±or±tic rocks, whereas metasediments, amphibolites tween Saxothuringian and Rhenohercynian zones was and orthogneisses are rare (Willner et al. 1991; Krohe revealed by deep seismic line DEKORP 2 S. The inter- 1992). Modeling focuses on its southern portion, so pretation by Behr and Heinrichs (1987) shows several strictly speaking modeling results are valid only for major thrust zones with large-scale ramp structures, unit III of the Bergstr/~sser Odenwald (according to typically dipping at 15-20 ° , locally at up to 30% Willner et al. 1991). The Bergstrfisser Odenwald is sep- arated by the Otzberg Zone from the B611steiner Oden- wald, which consists mainly of orthogneisses with minor Calibration data: geological constraints for thermal intercalations of paragneisses and amphibolites (Alten- modeling berger et al. 1990; Krohe 1992). Thermal modeling concentrates on the late Variscan Information on the prograde metamorphic evolution evolution of five tectonic complexes. The appropriate of the western MGCR is sparse. Therefore thermal 581 modeling had to be confined to the final stage of exhu- Quantitative model: calculation of transient mation following the main Variscan metamorphic temperature distribution event. Earlier exhumation from deeper burial is not considered. Thermo-chronometric data from various One-dimensional thermal models for the analysis of minerals usually provides several calibration points for P-T-t paths from orogenic settings were presented by the temperature range between 300 and 600 °C. No England and Thompson (1984 1986). Their models de- data is available for the final Variscan cooling stage be- scribe crustal thickening either by homogeneous strain low about 270 ° C. Apatite fission track ages from the or by instantaneous nappe emplacement duplicating Bergstr~isser and B611steiner Odenwald record only a part of the crust. In the latter case, the resulting geo- Late Cretaceous exhumation event (Wagner and Storz- therm shows an initial 'sawtooth' shape, which disap- er 1975), presumably related to inversion of the Alpine pears with time as heat is conducted from the upper foreland. Apatite fission track data from the Spessart plate downward into the lower plate. If the time be- crystalline complex and southern Rhenohercynian zone tween thrusting and exhumation is sufficiently long, yield ages of 90 Ma and between 110 and 150 Ma, re- both units undergo subsequent heating as the geotherm spectively (Hurford and Carter 1994). relaxes towards the steady-state geotherm. The one-di- The footwall evolution of the Rhenohercynian zone mensional approach of England and Thompson (1984) is documented by the Northern Phyllite Zone (NPZ) assumes conductive transfer of heat in vertical direction and Taunuskamm Unit (TKU), located directly north only. The fundamental equation is of the suture. The NPZ forms a narrow belt of green- schist-grade sediments and volcanics. According to An- OT 02T OT A -- = - u(z) +-- derle et al. (1990), this tectonic slice of Ordovician to Ot Oz ~z pC Emsian rocks most likely represents more metamor- phosed, deeper parts of the Rhenohercynian basin. The (Carslaw and Jaeger 1959), where T is temperature, t is NPZ was thrusted onto the TKU to the north, which is time, K is thermal diffusivity (=thermal conductivity/ composed of metasediments ranging from Early De- pC), z is depth, A is rate of heat production per unit vonian to Vis6an in age (Oncken 1988; Anderle et al. volume, p is density and C is heat capacity. The vertical 1990). velocity function u(z) describes the rate of exhumation The burial and exhumation histories of the NPZ and with respect to the surface. Exhumation is the com- TKU are constrained by several K/Ar ages on white bined effect of rigid block exhumation and erosion as mica. In contrast with older interpretations (e.g. Ah- well as extensional deformation along low-angle nor- rendt et al. 1983), work by KliJgel and Oncken (1994) mal faults and ductile thinning of the crust (Hames et suggests that temperatures in the southern Rhenoher- al. 1989). In the case of erosion only, u(z) is a constant cynian zone always remained below the white mica clo- equal to the negative of the erosion rate at the land sur- sure temperature. Therefore, syn-kinematicaUy grown face. In the case of crustal extension, u(z) is a function white micas sampled from various shear zones provide of depth and strain rate. Thus exhumation rates derived crystallization ages related to deformation events rath- from geothermal modeling can be explained either by er than cooling ages. Temperatures in the NPZ never erosion only or by extensional strain only or any combi- exceeded 330°C (Anderle et al. 1990), although this nation of the two. Based on England and Thompson unit was buried at about 18 km depth for 15 Ma. This (1984), Haugerud (1986 1989) designed the computer contrasts strongly with the Spessart crystalline complex, program 1DT, which was also used for one-dimensional where at a similar depth temperatures of about 600°C thermal modeling in this study. were achieved. Obviously, a one-dimensional thermal modeling ap- At the end of the Carboniferous the study area was proach cannot describe lateral heat transport mecha- at or very close to the erosion surface. The oldest Per- nisms and heat transfer during thrusting. These proc- too-Carboniferous sediments of the Wetterau trough esses are included in two-dimensional thermal models situated between Rhenohercynian zone and MGCR developed by Shi and Wang (1987) and Germann (Kowalczyk 1983) as well as of the Sprendlinger Horst (1990). Their work shows that for thrust emplacement area north of the Odenwald (Marell 1989) contain de- at rates of up to several cm/a hardly any temperature tritus from the adjacent Variscan massifs. These sedi- inversion occurs. Sawtooth-shaped geotherms, the typ- ments are older than a volcanic phase, which is most ical features of one-dimensional thermal models result- likely time-equivalent to a similar sequence in the near- ing from the instantaneous thrusting assumption, occur by Saar-Nahe-Basin. In the latter, volcanism is confined only at unreasonable plate convergence rates of about to a short time span between 293 and 288 Ma (Lippolt 2 m/a. The study presented in this paper uses a two- and Hess 1989; Henk 1993). Thus, it can be expected dimensional thermal-kinematic modeling approach to that exhumation of the five tectono-metamorphic com- calculate the crustal temperature field during crustal plexes was essentially completed at about 295 Ma. thickening by thrusting as well as crustal thinning by erosion and extension. Transfer of heat is described by two-dimensional heat conduction and heat advection. Modeling is based on the finite element code ANSYS 582

(registered trademark of Ansys Inc., Houston, Tx., uplift to shallower crustal levels, they equilibrated at USA). Coupled-field elements with thermal-structural amphibolite facies conditions before Late Vis6an-Early field capabilities are used to compute the thermal evo- Namurian exhumation. Thus in the case of Spessart and lution of a single thrust sheet moving over a ramp struc- B/511steiner Odenwald sufficient time for an almost ture onto the footwall. The fault plane is modeled with complete thermal equilibration was available. Intrusion contact elements. Following Shi and Wang (1987), the of granitic magmas in the Bergstrfisser Odenwald most kinematic model is based on a velocity vector in the likely modified the temperature field in the middle hanging wall which is always parallel to the fault line crust. Consequently, the assumption of an equilibrated directly beneath it. Additionally, the horizontal compo- initial geotherm may be a simplification resulting in an nent of plate convergence is assumed to be constant overestimation of the true surface heat flow. However, within the thrust sheet. Local Airy isostasy is main- in the case of the other four complexes the values cal- tained throughout the simulation. culated for the initial surface heat flows provide reason- Comparison between calculated P-T-t paths and pe- able estimates for the thermal state of the crust during trological, thermo-chronometric and tectonic calibra- late Variscan metamorphism and immediately before tion data is based on a forward modeling approach. For exhumation. a set of lithospheric parameters (see later), the burial Thermal modeling of the western MGCR is confined and exhumation history was varied by trial and error to the exhumation stage as information on the prograde until a good fit between the observed and modeled data metamorphic evolution is too sparse. Regarding the da- was achieved. Modeling results provide conservative tabase presently available, a one-dimensional modeling estimates for exhumation rates as the approach was to approach was considered adequate to simulate the late maximize the duration of the rapid uplift stages. orogenic exhumation histories of Spessart and Oden- wald. In case of the southern Rhenohercynian zone, lateral heat transfer and heat transfer during thrusting was Input data important, particularly because the time between thrusting and exhumation was short and the rocks had The transient temperature distribution in the one- and no time to equilibrate thermally. Consequently, a two- two-dimensional models is computed for constant val- dimensional modeling approach including the burial ues of k, A, p and C listed in Table 2. Internal heat gen- stage was used to compute the temporal evolution of eration from the decay of radioactive elements is as- the temperature field. sumed to be confined to the upper 18 km of the crust. Below this level, the heat flow from the mantle into the lower crust was kept constant. Depth was converted to Results of thermal modeling pressure assuming an average crustal density of 2.78 x 103 kg m -3. The modeled P-T, P-t and T-t paths for Spessart, Od- Thermal modeling starts with an initial geotherm in enwald and the southern Rhenohercynian zone are equilibrium with mantle heat flow and crustal radiogen- summarized in Figs. 3 and 7. The figures include the ic heat production. The heat contribution from the calibration data with margins of error in metamorphic mantle was varied to meet temperature-depth condi- conditions, age determinations and mineral closure tions of the main Variscan metamorphic event in each temperatures. Exhumation rates are compiled in Fig. 8. of the five tectonic complexes. The assumption of an Late Variscan exhumation of the MGCR and southern equilibrated initial geotherm requires further discus- Rhenohercynian zone was characterized by non-linear, sion. It is justified in the southern Rhenohercynian ba- mainly two-stage unroofing histories. Cooling and de- sin because of its longstanding passive margin setting. It compression could have been also modeled by multi- is also considered an adequate description of the ther- stage exhumation, but more than two or three stages do mal state of the crust in Spessart and Odenwald in spite not seem justified regarding the calibration data pres- of their position near an active margin and the contin- ently available. uous convergence since the Late Devonian. During prograde evolution, these crystalline complexes went through an early stage of deep burial (?Late Devonian; Spessart Altenberger 1992; Dombrowski et al. 1994). Following Pressure-temperature conditions of the main Variscan metamorphic event in the Spessart, about 600°C at Table 2 Constants used for thermal modeling 21 km depth, suggest an initial surface heat flow of Symbol Parameter Value 88 mW m 2. Regional metamorphism has not been reli- ably dated so far and can only be delimited by the be- k Thermal conductivity 2.5 W m-1 K-1 ginning of plate convergence during Frasnian times and A Radiogenicheat production 2.7 x 10 -6 W m -3 the earliest cooling ages. However, as hornblende clo- C Heat capacity 9.0 x 102 J K-1 kg -1 p Density 2.78 N 10 3 kg m -3 sure temperatures are close to peak metamorphic temperatures and subsequent cooling was very rapid, no 583

Spessart B611steiner Odenwald Bergstr~isser Odenwald Temperature (°C) Temperature (°C) Temperature (°C) 100 200 300 400 500 600 700 800 0 10(3 200 300 400 500 600 700 800 0 1 O0 200 300 400 500 600 700 800 0 , iIi F=t~rqPd q ~,, i I, I ,t I i I i I i I i I { I r 50 ~ f •

10 10- i i g 15 15o

20 a 20- 20 25 25- 251 30 30- 3O

800 8O0 B00 -

700 700 700 . o~ 600 o~ 600 o~ 600. Hbl I' Hbl Hbl 500 500 o~ 500. 400 400 ~ 400 - Msk Msk '3OO 300 300. Bi Bi I'- 200 P= 200 F- 200~

100 su~ace 100 surface , , re 100 sudace exposure , , ure, , , 0 0 , , , , Q 360 340 320 300 280 3150 340 320 300 280 360 340 3213 3(30 280 S, Time (Mabp) b Time (Mabp) Time (Mabp)

Fig. 3a---e Modeled pressure-temperature and temperature-time sart crystalline complex had to be related to exhuma- paths for the western Mid-German Crystalline Rise. Boxes indi- tion. However, the data presently available can also be cate the uncertainties in metamorphic conditions, age determination and mineral closure temperature, a Spessart (petrological explained by cooling due to thrusting over colder foot- data from Okrusch 1990; thermo-chronometric data from Lippolt wall rocks (see later). Additional P-T information on 1986 and Nasir et al. 1991). b BOllsteiner Odenwald (petrological the retrograde evolution is urgently required to distin- data from Willner et al. 1991; thermo-chronometric data from guish between the two competing interpretations. Lippolt 1986). c Bergstr~isser Odenwald (petrological data from Willner et al. 1991; thermo-chronometric data from Kreuzer and Harre 1975 recalculated by Hess and Schmidt 1989) BOllsteiner Odenwald significant exhumation of the Spessart before the Na- Thermal modeling suggests a surface heat flow in the murian is likely. B011steiner Odenwald area of 116 mW m -2 to meet me- The observation that the thermo-chronometric data tamorphic conditions before late Variscan exhumation. are essentially independent of mineral type and closure Although not synchronously, exhumation of the B611- temperature points to a very rapid cooling history. In- steiner Odenwald occurred at rates very similar to the terpretation of the cooling rates of up to 70 ° C/Ma by Spessart crystalline complex. Initial cooling rates were exhumation requires average decompression velocities lower, about 40-60 ° C/Ma. A rapid unroofing phase be- of 3.2 mm/a between 326 and 321 Ma, the Early Namu- tween 335 and 332 Ma is required to achieve the iso- rian. Owing to the relative inefficiency of crustal con- thermal decompression documented by the P-T data. ductive heat transport, the surface heat flow may have Exhumation rates during that stage averaged at 3 ram/ increased to more than 130roW m-; as rocks ap- a. The subsequent exhumation until about 295 Ma is proached the surface during this rapid exhumation characterized by strongly reduced exhumation rates of stage. However, this value may overestimate the true 0.2 mm/a. surface heat flow because convective heat transport mechanisms are not considered. After 321 Ma exhumation slowed down and was on average only 0.2 mm/a Bergstr~isser Odenwald for the following 26 Ma. The rapid initial unroofing stage was very effective in cooling the Spessart crystal- The assumption of an equilibrated geotherm results in line complex as the temperature decreased quickly a very high initial surface heat flow of 143 mW m -2. from 600 ° C at 326 Ma to 195 ° C at 315 Ma. The result- This most likely is an overestimation of the true surface ing P-T path is characterized by relatively isothermal heat flow because the thermal effects of the granitic in- decompression followed by more nearly isobaric cool- trusions are not taken into account. Exhumation of the ing. It should be emphasized that only Variscan peak southern Bergstr~isser Odenwald occurred almost con- metamorphic conditions are known so far. Consequent- temporaneously with exhumation of the B011steiner ly, by analogy with the Odenwald, cooling of the Spes- Odenwald. An initial phase of rapid unroofing started 584

NW SE age

E 90 1 initial surface heat flow 9o ~E 70 r-" -- ...... --~70 0 1 ,, Rhenohercynian100 Zone ~ I~ Mid-German. Crystalline, Rise 0 200 300 .... ~ ~ l 10 2010 t , ,, ,, 400 500 ~ ,, 30 ,,,mr 600 700 ' 20 _~E "' . 30 b prior to thrusting

Rhenohercynian Zone • T Mid-German Crystalline Rise 0 ~ ' 0 10~ 100 200- 300 ~ ~ -- I 20 ¢o "~-~-'~" N_.?------' ' lo 30 ~..,,.--~"~..~---~500 ~ -- 6,,JO- __ __ , 700 --~200 e 1.8 Ma after initiation of thrusting

Rhenohercynian Zone Mid-German Crystalline Rise 200 ~-"~--- ~~_..._. ~ ~ 0 lO 300 ~ 10 2o°l ~---T ~ -- 40o---N-.N_T_CJ___ ~_~.~_ ~ - ~ 2 t 20 d 5.7 Ma after initiation of thrusting

Rhenohercynian Zone Mid-German Crystalline Rise O~1oo 2oo~ ~ = ~1 ° lo| -3on------'- ~------~-~• ~ .,---- ~ -4 up

30 , ~ • ~ ,%" ~u~ 50u/5Uv 6O0 ~ 700 ...I-'- "' ~ 30 e 10 Ma after initiation of thrusting

Rhenohercynian Zone T..N aS Mid-German Crystalline Rise 0 1 1QO ~ I 0 10] 200 300-- -- 400 f 10 20 -- 500 20 30 30 f after final exhumation

Fig. 4 Temperature distribution (in° C) in the southern Rheno- face heat flow of 88 mW m -z assuming an equilibrated hercynian zone and western Mid-German Crystalline Rise during Late Vis~an to Stephanian times (see text for further explanation; geotherm after thrusting and thickening of the radio- T=Taunuskamm Unit, N=Northern Phyllite Zone, S =Spes- genic layer, respectively. Thus before thrusting a re- sart) duced surface heat flow can be expected. Thermal-kinematic modeling starts in Late Vis6an times, when rocks of the future NPZ and TKU were at about 338 Ma. It lasted for about 5 Ma and achieved still at or close to the surface (Fig. 4). The initial tem- exhumation at velocities of 1.3 mm/a. After 333 Ma perature field represented by the steady-state solution exhumation was reduced to rates of on average is shown in Fig. 4b. The initial surface heat flow is as- 0.2 mm/a. sumed to increase from 73 to 88 mW m -2. The temperature increase towards the south reflects the transition from the foreland (previous passive margin of the Rhe- Northern Phyllite Zone and Taunuskamm Unit nohercynian shelf) to the forearc setting north of a dim- inishing subduction zone. Movement of the MGCR The two-dimensional thermal-kinematic model de- onto the Rhenohercynian zone results in a disturbed scribes burial of the southern Rhenohercynian Zone by temperature field. The exact temperature distribution thrusts emplacing 18 km of crust over the NPZ and in the overthrust structure depends mainly on thrusting 11 km over the TKU, respectively. The thermal state of velocity and thrust ramp angle. The relationship be- the crust in the southern Rhenohercynian basin is con- tween the horizontal convergence rate and temperature strained by P-T data from Anderle et al. (1990) and at the bend between the thrust ramp and flat, being the Hell (1994). Their data from various stratigraphic units minimum temperature at 18 km depth, is shown in ranges between 220 ° C at 1.8 kbar ('Siegener Normalfa- Fig. 5. The lower the ramp angle, the faster the thrust zies') and 290 ° C at 3 kbar (TKU). Such an temperature sheet has to move to keep temperatures in the footwall distribution with depth would be compatible with a sur- below a certain limit. For ramp angles of 18 ° as indi- 585

7°°t 4ooI .-- ...... _8m_m_/~_ 1

600 l""" "" o j-...2d..4.8o -. ¥ a00 4 '-~.~ "K.. "-. 4.5~ -.. ~ "-. 300

"'"'-"-?- -.. r-,, E ® .... 22--. "--.

300- "...... z:~

200- ' ~' ' ' ...... i 200 0 10 00 0 5 10 15 Thrusting velocity (mm/a) Time after initiation of thrusting (Ma)

Fig. 5 Thrusting velocity versus minimum temperature at 18 km Fig. 6 Minimumtemperature at 18 km depth versus time after in- depth for various thrust ramp angles itiation of thrusting for various thrusting velocities (ramp angle 18°). Thermal equilibration at the base of the hanging wall is pre- vented by continuous thrusting over colder footwall rocks cated by the interpretation of DEKORP 2 S (Behr and Heinrichs 1987), initial thrusting velocities of at least 30 mm/a are required to maintain temperatures at difficult because uplift occurred isothermally. Depend- 18 km depth below the critical 330 ° C. Any slower con- ing on the assumed duration of this phase, exhumation vergence would result in excessive heating of the rates between 1 and 3 mm/a can be expected. The sub- NPZ. sequent model path, which most closely approximates Assuming a ramp angle of 18 ° and 30 mm/a horizon- the common exhumations of TKU and NPZ, suggests tal convergence velocity, burial to 18 km depth was unroofing at velocities of about 0.75 mm/a between achieved within less than two Ma (Fig. 4c). As indi- 310 Ma and the final surface exposure at about 295 Ma cated by the pressure data, the NPZ stayed at this (Fig. 4 0 . depth until about 310 Ma. If the NPZ would have remained in the footwall of the thrust for approximately 15 Ma, temperatures would have increased to more interpretation of exhumation rates: erosion versus than 550 ° C. To prevent thermal equilibration and to extension keep temperatures below 330 ° C, the rocks had to be cooled without exhumation. An efficient process is Exhumation rates derived from thermal modeling pro- cooling from beneath by thrusting over colder footwall vide estimates for either maximum erosion rates or rocks. Thus shortly after the initial burial stage the maximum extensional strain rates. In reality, erosion NPZ had to change its tectonic position from the foot- and tectonic denudation will act concomitantly, the net wall to the hanging wall. Subsequently, it moved, i.e. as result of both processes being the modeled exhumation a small tectonic slice attached to the base of the over- rates (England and Molnar 1990; Jamieson 1991). If riding crustal slab, with the hanging wall and was con- erosion operated exclusively, the modeled exhumation tinuously cooled from beneath (Fig. 4d, 4e). The mini- rates would equal the erosion rates. In contrast, if exhu- mum thrusting velocity required to keep temperatures mation was entirely related to tectonic denudation below 330°C is 12 mm/a (Fig. 6). Any slower conver- through normal faulting and penetrative ductile strain, gence would again result in excessive heating of the the modeled exhumation rates have to be interpreted in NPZ. The narrow temperature interval of 270-300°C terms of strain rates only. Between these extremes, the indicated by the mineral assemblage and fabric of the erosion rate can be decreased (or increased) without shear zone rocks can be achieved with thrusting veloci- changing the model path if balanced by an increase (or ties of 20 mm/a. decrease) in the strain rate. To evaluate the relative The TKU achieved maximum burial at about contribution of both processes to the late Variscan ex- 320 Ma (Fig. 4d). Thrusting velocities were slower than humation of the study area, the modeled rates are com- in the NPZ and an almost complete thermal equilibra- pared with published data from other orogenic set- tion with the steady-state geotherm was achieved after tings. maximum burial as the TKU remained in the thrust's A compilation of erosion and exhumation rates from footwall at about 11km depth (Fig. 7). A phase of a variety of geodynamic settings is given by Leeder strike-slip deformation at about 310 Ma positioned the (1991). Exhumation rates of thickened crust for several NPZ next to the TKU. Quantification of the related ex- orogens usually vary between 0.1 and 3 mm/a and occa- humation velocities by means of thermal modeling is sionally reach 6 mm/a. In contrast, long-term regional 586

Fig. 7a, b Modeled pressure-time and tempera- NorthernPhyllite Zone Taunuskamm Unit ture-time paths for the south- Time (Mabp) Time (Mabp) ern Rhenohercynian zone. seo 340 320 300 280 360 340 320 300 280 Boxes indicate the uncertain- o i I ~ I I I 0 r r r i I t I ~ I I ] i i I ' ' ' L/_J ' ties in metamorphic condi- sedimentation sedimentation /' ~su~ace exposure tions, age determination and 5 - su'/~.osg~/ 5- mineral closure temperature, a Northern Phyllite Zone (pe- lO- 10 trological data from Anderle thrusting strike-slfp et al. 1990; thermo-chronomet- ~ 15 15- ric data from Kltigel and r~ Oncken 1994). b Taunuskamm ~ 20 E 20- Unit (petrological data from thrusting strike-slip Anderle et al. 1990; thermo- 25 - 25- chronometric data from Klti- gel and Oncken 1994) 3o 30- 800 800

700 - 700 -

600- 600 -

o - ¢ 500- 500

400 400 " thru~ 0 thrusting strike-slip ~ 300 300 - # 200- 200 -

1 O0 - I ~ surface 100 " se im " surface ,, sedimen~,,, r__~o .... 0 360 34-0 320 300 280 360 340 320 300 280 a Time (Mabp) Time [Mabp) erosion rates are an order of magnitude lower and gen- ratio of possible erosion rates to modeled exhumation erally range between 0.01 and 0.5 mm/a depending on velocities suggests that temporarily more than 75% of the mean drainage basin elevation. A detailed investi- the exhumation was related to tectonic denudation. gation on the controls of present day erosion rates in Thus upper crustal extension at strain rates of up to major world drainage basins was published by Summer- 10 -14 S -1 may have occurred during the initial rapid un- field and Hulton (1994). Their work shows that erosion roofing stages. and chemical denudation are most strongly related to A possible geodynamic process that could provide basin relief, annual runoff and basin elevation. Basin sufficient extensional strain to account for such high ex- area, runoff variability and mean temperature, howev- humation rates in an orogenic setting is gravitational er, are only weakly associated with rates of erosion. collapse. Orogen collapse is controlled by the gravita- The maximum erosion rate documented by this study is tional instability of overthickened crust. It can be en- 0.7 mm/a (Brahmaputra drainage basin). Crustal-scale hanced by tensional plate boundary forces, delamina- strain rates in late orogenic settings are generally in the tion of the mantle lithosphere and convective erosion range 10 -16 to 10 -~4 s -a (Hames et al. 1989). For exam- of the thermal boundary conduction layer, respectively ple, Sonder et al. (1987) consider the Cenozoic exten- (Dewey 1988). The combination of these processes re- sional strain rates in the Basin and Range Province a sults, among others, in rapid exhumation, an increase in 'few times 10 d6 s -1'. In the Aegean region, continental geothermal gradient and extensional basin formation - crust is extending at 3 x 10 -~5 s -1 (Jackson and McKen- characteristics which also describe the late Variscan his- zie 1988), whereas extensional strain rates for the Tibe- tory of Central Europe. Consequently, several workers, tan Plateau are about an order of magnitude lower e.g. Lorenz and Nicholls (1984), Dewey (1988) and (Molnar and Lyon-Caen 1989). Menard and Molnar (1988), have suggested gravitational collapse as the main process controlling the late orogenic evolution of the Variscides and have drawn pa- Discussion rallels to the Basin and Range Province and the Tibe- tan Plateau. However, sedimentation in the Saxothurin- Comparison of modeling results with published data gian and Rhenohercynian basins continued throughout strongly suggests that exhumation velocities of up to the Tournaisian and parts of the VisEan (Ziegler 1990). 3.2 mm/a as documented in the Spessart, BOllsteiner The marine environment suggests that in these areas no Odenwald and possibly the NPZ (Fig. 8) are much too significantly thickened crust existed during this time fast to be the result of erosion alone. Consequently, ex- span. Only the MGCR had sufficient crustal thickness tensional strain has to be considered the major process to rise above sea level and shed detritus into the adja- responsible for such high decompression velocities. The cent basins. Crustal thickening in the southern Rheno- rnm/a Bergstr sser Odenwald hercynian zone did not start before the Late Vis6an. The rapid exhumation stages of Odenwald and Spessart occurred contemporaneously with convergence in the 1.3 Saxothuringian basin to the south as well as the Rheno- 1-1I hercynian zone further to the north (see Fig. 2). There- o-[I 0.2 fore a general extensional collapse affecting the entire .- ~0 330 320 310 300 290 Mabp crust in the Variscan Internides is considered as an un- _~ likely explanation for the rapid exhumation rates docu- z ~ mented in the study area. Exhumation of metamorphic rocks in a convergent geodynamic setting can be explained in terms of the mm/a B611steiner Odenwald mechanical behavior of an accretionary wedge (e.g. 4- Davis et al. 1983). The geometry of such an orogenic 3- 3.0 wedge is the result of a dynamic balance between the gravitational forces arising from the slope of the wedge, ~. 2- the push from the rear and the traction exerted on its I base by the overridden plate. Underplating of sediment 0.2 or crustal slices during thrusting thickens the wedge o I I from beneath and increases its surface slope. To regain "-~ ~o 330 320 3 0 300 290 Mabp x~- a stable geometry, the upper part of the wedge will re- -, spond by horizontal extension, even though conver- m-o gence is continuing (Fig. 9a; Platt 1986, 1987). Tectonic denudation is confined to the upper crustal levels and mmla Spessart accomplished by conjugate normal faulting and ductile 4- thinning, both parallel and perpendicular (lateral extru- 3.2 sion) to the convergence direction. Instability of the or- ~ 3- ogenic wedge and, therefore, extensional strain can also ~ 2- result from the topography of the basal d6collement. The preferential sites for upper crustal extension dur- 1 ing thrusting are thrust ramps, i.e. the basement ramp o 0.2 at an inherited rifted margin (Fig. 9b; Stockmal et al. .~ 300 290 Mabp 1986; Jamieson and Beaumont 1989). x~ c The suture between the Rhenohercynian zone and ~-o the MGCR is characterized by major thrust zones with large-scale ramp structures as indicated by deep seismic mm/a line DEKORP 2 S (Behr and Heinrichs 1987; their Northern Phyllite Zone 4- fig. 9). Additionally, thermal-kinematic modeling of the NPZ provides evidence for crustal underplating 3- during late orogenic convergence. Thus the prerequi- 1-3 sites are given to explain the rapid exhumation of the -~ 2- study area by the general convergence between the 1 Saxothuringian and Rhenohercynian zones. During the Late Visdan and Namurian, collision of both units was 0 310 300 290 Mabp apparently characterized by underplating of Rhenoher- m I _o~- cynian rocks beneath the MGCR as well as thrusting z I I along faults with ramp-flat topography. Exhumation of thrusting metamorphic rocks was related mainly to upper crustal 18 km tectonic denudation and occurred contemporaneously mm/a Taunuskarnm Unit with continuous compression in the lower crust. The re- 4- sulting pronounced strain partioning would explain 3- why rocks in the MGCR experienced exhumation and extension at a time when other parts of the Saxothurin- "~ 2- gian and Rhenohercynian zone were still under corn- " pression. 1 0.75 o .'_ 330 Mabp n¢- "-~ I m Fig. 8 Compilation of late orogenic exhumation rates in Oden- thrusting wald, Spessart and southern Rhenohercynian zone 11 km 588

Extension Late Variscan exhumation of the study area started in the Odenwald. Between about 338 and 332 Ma, southern Bergstrfisser and BOllsteiner Odenwald ex- [ Buttress perienced a phase of rapid, but differential, exhumation. Differential movements were related to the Otz- berg Zone (see Fig. 1) - a NNE-SSW trending shear Extension zone with westward dip separating Bergstr~isser and B011steiner Odenwald. Normal fault movements during upper crustal extension as described by Krohe (1992) would provide a mechanism to explain the variable exhumation rates. The BOllsteiner Odenwald formed the footwall relative to the Otzberg Zone and, therefore, experienced faster exhumation than the Bergstrfisser Odenwald in the hanging wall. As no deep seismic data is available it can only be speculated that underplating Inherited rifted margin and thrusting along a large-scale ramp similar to the Spessart was responsible for the high exhumation rates Fig. 9a, b Conceptual models for rapid exhumation by tectonic of up to 3 mm/a. Such rapid exhumation cannot be ex- denudation during regional convergence. Upper crustal extension plained by erosion only, but requires a substantial con- can be related to underplating (a; modified after Platt 1987) and tribution of extensional strain. Undeformed lampro- thrust ramps (b; modified after Jamieson and Beaumont 1987) phyres intruding the shear zone provide evidence that movement along the Otzberg Zone ended before 328 Ma (Hess and Schmidt 1989). This is in agreement Synthesis: Late Variscan convergence and exhumation with thermal the modeling results. They suggest a com- history mon exhumation history of the Bergstr~isser and B611- steiner Odenwald after the initial differential exhuma- The tectono-metamorphic history of the western tion stage ended at about 332 Ma. The subsequent joint MGCR and southern Rhenohercynian zone outlined exhumation occurred at average velocities of 0.2 mm/a. here tries to integrate the thermal modeling results with Such a rate can be explained completely either by ero- geodynamic models proposed by Behr and Heinrichs sion or tectonic denudation. Consequently, no esti- (1987), Oncken (1988), Franke and Oncken (1990), An- mates on the relative contribution of the two processes derle et al. (1990), Behrmann et al. (1991), Franke to the final exhumation history of the western MGCR (1992) and Krohe (1992). is possible. The spatial distribution of the initial surface heat Between 332 and 326 Ma tectonic activity migrated flows immediately before late Variscan exhumation of further to the north. During the Late Visdan-Early Na- the western MGCR and thrusting in the southern Rhe- murian the southernmost parts of the Rhenohercynian nohercynian zone portrays the geodynamic setting of Basin were accreted to the active margin at the north- the study area during the Vis6an. The lowest values of ern rim of the Saxothuringian zone and transported 73-88 mW m -2 are recorded in the northern part of the over the foreland. Starting at about 326 Ma, uplift of study area. They reflect the relatively 'cold' passive the Spessart crystalline complex was contemporaneous margin setting of the southern Rhenohercynian zone with thrusting and deep burial of the NPZ. The high and the forearc position of the Spessart crystalline com- exhumation velocities of 3.2 mm/a documented in the plex. The modeled heat flows depict typical values for Spessart were mainly the result of upper crustal exten- collisional fold belt and ocean trench foreland settings sion in relation to underplating and uplift at thrust (Allen and Allen 1990). Variable peak metamorphic ramps. Two-dimensional thermal-kinematic modeling temperatures recorded at similar pressures in Spessart suggests convergence rates between the MGCR and and the NPZ are no indication of different surface heat Rhenohercynian zone of at least 30 mm/a during the in- flows. They are the result of specific tectono-meta- itial burial stage of the NPZ. Subsequently, relative morphic histories with highly different time lags be- movement between the two units either slowed down tween thrusting and exhumation as well as continuous or was taken up by several thrust zones. The P-T-t cooling due to thrusting over colder footwall rocks. In data suggest reduced convergence at rates of 20 mm/a contrast with the Spessart, rocks of the NPZ had no between about 324 and 310 Ma. These convergence vel- time to equilibrate thermally. Further to the south, sur- ocities derived from thermal modeling are in broad face heat flows are significantly higher, even if the esti- agreement with independent estimates of 1-2 cm/a mates for the Bergstr~isser Odenwald may be too ex- based on flysch and deformation front migration in the treme. Surface heat flows of 115 mW m -2 and more Rhenohercynian foreland (Oncken personal communi- suggest a 'hot' crust, which results from the active mar- cation). As the NPZ was accreted to the base of the gin setting and related arc magmatism in this part of the advancing MGCR, its initial location must have been MGCR. much further south-east of its present position. An esti- 589

mate for this distance is difficult because of the error in suggest that crustal underplating as well as strain par- age determination. However, the modeled convergence tioning between the lower and upper crust played a sig- rate for this stage is equivalent to 20 km displacement nificant part during late Variscan evolution. for every Ma of thrust movement. A conservative esti- Although the modeling approach and available data mate of 10 Ma duration between thrusting and uplift cannot resolve all details of the late orogenic burial and would imply 200 km north-westward movement of the exhumation, they provide a geodynamic history of the NPZ relative to its initial position. study area which is consistent with thermal require- During a phase of strike-slip deformation culminat- ments. More data, particularly on the prograde evolu- ing at about 310 Ma, the NPZ was exhumed to approx- tion of the MGCR, are needed to quantify the com- imately 11 km depth. It was positioned next to the plete tectono-thermal history of the study area and to TKU, which had remained at this depth since its burial place further constraints on the modeling results. at about 320 Ma. After 310 Ma, about Westphalian C, joint exhumation of both units proceeded at average Acknowledgements This paper benefited greatly from discus- velocities of 0.75 mm/a. This is in contrast with the sions with O. Oncken and T. KHigel. Critical comments by P. Blg- MGCR, which depicts rather uniform exhumation rates reel, M. Okrusch, O. Oncken and an anonymous reviewer on an earlier version of the manuscript are gratefully acknowledged. of 0.2 mm/a during this time. The difference was proba- This work was kindly funded by the Deutsche Forschungsgemein- bly related to extensional reactivation of fault zones schaft as part of the priority programme 'Orogenic processes - along which the Saxothuringian was previously their simulation and quantification using the Variscides as exam- thrusted onto the NPZ. Footwall uplift due to normal ple'. faulting could be the main reason for the higher exhumation rates in the southern Rhenohercynian zone in contrast with adjacent areas. It is worth noting that the References final exhumation rates of 0.2 mm/a as documented in Ahrendt H, Hunziker JC, Weber K (1978) K/Ar-Altersbestim- the Spessart and Odenwald crystalline complexes are mungen an schwach-rnetamorphen Gesteinen des Rheinisch- erosion rates typical for an area at about 1500 m mean en Schiefergebirges. Z Dtsch Geol Ges 129:229-247 drainage basin elevation (Leeder 1991). This would be Allen PA, Allen JR (1990) Basin analysis. Blackwell Scientific, the surface elevation of 40-42 km thick, isostatically Oxford, 451 pp Altenberger U (1992) The B011stein Odenwald - evidence for compensated crust. pre- to early Variscan plate convergence in the Central Euro- Regarding the present crustal thickness in the south- pean Variscides. Frankfurter Geowiss Arb A 11 : 194-197 ern Rhenish Massif of about 30 km, the modeled exhu- Altenberger U, Besch T, Mocek B, Zaipeng Y, Yong S (1990) mation rate of 0.75 mm/a for the final uplift stage is Geochemie und Geodynamik des BOllsteiner Odenwaldes. Mainz Geowiss Mitt 19 : 183-200 equivalent to a crustal-scale strain rate of 7.7 x 10 -16 s 1. Anderle H-J, Massone H-J, Meisl S, Oncken O, Weber K (1990) Similar strain rates resulted in the formation of the Southern Taunus Mountains. Field Guide 'Mid-German Crys- Saar-Nahe Basin in the western part of the study area. talline Rise & Rheinisches Schiefergebirge'. In: Int Conf Pal- Cross-section balancing and subsidence analysis suggest eozoic Orogens in Central Europe, G6ttingen-Giessen, Au- that between the Westphalian and Lower Permian, the gust-September 1990, pp 125-148 Behr HJ, Heinrichs T (1987) Geological interpretation of DE- crust in the area of the basin was extended at average KORP 2-S. A deep reflection profile across the Saxothurin- strain rates of 4.1 x 10 -16 s -1 (Henk 1993). The modeled gian and possible implications for the Late Variscan structural exhumation rates in the southern Rhenohercynian zone evolution of Central Europe. Tectonophysics 142:173-202 were certainly not entirely due to extension, but also Behrmann J, Drozdzewski G, Heinrichs T, Huch M, Meyer W, Oncken, O (1991) Crustal-scale balanced cross sections include erosional effects. Thus crustal strain rates in the through the Variscan fold belt, Germany: the central EGT- southern Rhenohercynian and Saar-Nahe Basin be- segment. Tectonophysics 196:1-21 come almost identical. Carslaw HS, Jaeger JC (1959) Conduction of Heat in Solids. 2nd edn. Clarendon Press, Oxford, 529 pp Davis D, Suppe J, Dahlen FA (1983) Mechanics of fold-and- thrust belts and accretionary wedges. J Geophys Res Conclusions 88:1153-1172 Dewey JF (1988) Extensional collapse of orogens. Tectonics One- and two-dimensional thermal modeling tech- 7:1123-1139 niques constrained by thermo-chronometric, petrologi- Dombrowski A, Okrusch M, Henjes-Kunst F (1994) Geother- mometry and geochronology on mineral assemblages of or- cal and tectonic data were applied to documented non- thogneisses and related metapelites of the Spessart Crystalline linear unroofing histories of the western MGCR and Complex, NW Bavaria, Germany. Chem Erde 54:85-101 southern Rhenohercynian zone. This approach pro- England PC, Molnar P (1990) Surface uplift, uplift of rocks, and vides quantitative estimates for exhumation rates dur- exhumation of rocks. 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