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Continental Collision and the Dynamic and Thermal Evolution of the Variscan Orogenic Crustal Root Ð Numerical Models

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Continental Collision and the Dynamic and Thermal Evolution of the Variscan Orogenic Crustal Root Ð Numerical Models Journal of Geodynamics 31 (2001) 273±291 www.elsevier.nl/locate/jgeodyn Continental collision and the dynamic and thermal evolution of the Variscan orogenic crustal root Ð numerical models J. Arnold a,*, W.R. Jacoby a, H. Schmeling b, B. Schott c aInstitut fuÈr Geowissenschaften, Johannes Gutenberg-UniversitaÈt Mainz, Saarstr. 21, D-55099 Mainz, Germany bInstitut fuÈr Meteorologie und Geophysik, Johann Wolfgang Goethe-UniversitaÈt Frankfurt, Feldbergstr. 47, D-60323 Frankfurt/M, Germany cFaculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Received 1 February 2000; received in revised form 5 September 2000; accepted 5 September 2000 Abstract Orogeny is modelled numerically by treating continental collision within full convection solutions, in order to better understand some aspects of the Variscan structures and processes. Three dierent approaches are taken: (1) collision where one `continental plate' is `pushed' against another across a zone of weakness; (2) gravitational instability of a lithospheric mantle root leading to delamination, slab break-o and crustal root reduction; (3) melting in the lower part of a crustal orogenic root. The ®rst approach demonstrates that thick (but in the models: cool) roots can accumulate, in which upper crustal rocks are carried to great depth and mantle material may be carried towards upper crustal levels. The second approach shows that lithospheric root break-o can lead to rapid crustal uplift and thinning of the lower crust if its viscosity is suciently low. The third approach suggests that internal heating in a thickened crust may lead to melting and granit formation, however, only after a long geological time (in the order of 100 Ma), while delami- nation and asthenospheric heat advection may achieve this in shorter time periods (in the order of 10 Ma). The dierent models tested all demonstrate that crustal root formation and destruction by uplift and exhumation can be achieved in geologically short time periods (1±10 Ma). # 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction One of the motivations for this work is to understand why and how the Variscan belt has lost its crustal root (Meissner and Tanner, 1993; Enderle et al., 1997). While other Palaeozoic oro- gens, such as the Urals, have preserved a thick crustal root accompanying moderate topography * Corresponding author. E-mail address: [email protected] (J. Arnold). 0264-3707/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0264-3707(00)00023-5 274 J. Arnold et al. / Journal of Geodynamics 31 (2001) 273±291 (Stadtlander et al., 1999), the present Variscan crust, with similarly moderate topography, is generally rather ``thin'', around 30 km (Blundell, 1992). There is no doubt that some of the exposed continental Variscan rocks had experienced high pressure metamorphism that docu- ments a former sizeable crustal root (Masonne, 1999). We approach the problem via numerical experiments of continental collision. We do not attempt to simulate details, but hope to approach `reality' with meaningful approximations. The European Variscan belt represents a multi-terrane collage assembled in late Paleozoic time, culminating between 350 and 300 Ma. A general feature seems to be extensive high-pressure (HP) metamorphism during the Lower Carboniferous in several parts of the orogen, followed by rapid uplift and exhumation of metamorphic core complexes. During this time, HT/LP metamorphism occurred with increasing volcanic activity indicating high heat ¯ow and extension in the upper crust which has been removed by erosion and denudation. The study is guided by recent observations (Franke, 2000) of a now better known region: the SE±NW section from the Moldanubian, crossing the Erzgebirge to the foreland in the NW, or the Saxothuringian belt including the mid-German Crystalline High (Fig. 1). Deep burial and sub- sequent uplift with extension, volcanism, high heat ¯ow and crustal melting occurred at 340 Ma within an extremely short interval of de®nitively <10 Ma, perhaps 2 Ma (Willner, 1998; O'Brien, 1999). Marine sedimentation occurred on the Moldanubian basement (Tepla-Barandian) con- temporaneous with deep convergence and shallow extension (Zulauf et al., 1998). HP granulite Fig. 1. Sketch of the Variscan belt in Central Europe showing the section by which the presented models are guided (modi®ed after Franke, 1992). J. Arnold et al. / Journal of Geodynamics 31 (2001) 273±291 275 facies metamorphism is dated, both in the Saxonian granulites (Reinhardt et al., 1998) and in the Erzgebirge (Masonne, 1999), at 340 Ma and time-equivalent sedimentation and volcanism is documented in the upper crust overlying the HP rocks. The apparent con¯ict (Franke, 1998) may be resolved by this study. Convergence, collision and extension in the upper crust during very short spatial and temporal scales may have a modern analogy in the region forming the Alboran sea and surrounding mountain ranges. Occasional deep earthquakes (Tandon et al., 1998) indicate ongoing subduction of a piece of delaminated and detached mantle lithosphere while mountain ranges are presently formed and uplifted and a marine basin forms in-between; Docherty and Banda (1995) point out that here adjacent coeval compression and extension have occurred from Miocene to Recent. They envision the development of the Alboran sea basin through a south-easterly migration of a delaminating continental lithosphere to explain the younger extension in the eastern basin. Evi- dence for the migration comes from tectonic inversion in the eastern Alboran sea basin north of Al-Mansour Seamount. This is an interesting similarity to the marine sedimentation on the Moldanubian basement (Tepla-Barandian) contemporaneous with deep convergence and shallow extension (see above). Both regions share similar features and hopefully modelling sheds light on both situations. In a ®rst attempt, 1D conductive and advective heat transfer (Haugerud, 1989) has been modelled for the Erzgebirge evolution by E. Sebazungu (see Willner et al., 1997, where regional infor- mation is given; Willner et al., 2000) to understand especially the metamorphic structure as a consequence of thrusting, uplift and lateral extension and denudation of the upper crustal block within <10 Ma. The petrological data on the `observed' pTt-paths of sandwiched metamorphic units (Willner et al., 1997: a HT/HP unit between two lower-pT units in the Erzgebirge) can be reproduced with crustal uplift rates for the thrust-up block, from top to bottom, of 2.2, 3.4 and 8.9 mm/a, and 1.3 mm/a for the low-grade ramp. Thermal anomalies in the middle crust can be modelled by sandwiching hot material between cooler units. A ®nal HT/LP overprint of the low- grade ramp occurred during the last stage of exhumation. The model is one of tectonic and ero- sional denudation during ongoing convergence in deeper levels. The crustal root disappears when the gravitationally unstable mantle lithosphere delaminates and breaks o during ongoing con- tinental convergence. This will be discussed in Section 2 mostly based on results by Schott and Schmeling (1998), after the build-up of the crustal orogenic root, modelled mostly by J. Arnold and presented here. To test tectonic models and ideas, 2-d numerical FD-calculations are carried out. The approach is that of convection modelling in a dynamically, rheologically and thermally consistent way, by solving on a grid of points by ®nite dierences the equations of mass, momentum and energy conservation. Two regimes are investigated: `compressive' and `extensional' (Schott et al., 2000b). In the ®rst case compressional forces, e.g. ``ridge push'', are required to push one plate down (`collision') and initiate delamination when it gains its own instability. The compressive regime with dense lithospheric mantle sinking causes crustal thickening and surface uplift, and the uplift causes the upper crust to spread. This is implied by the observation that uplift rates become greater with increasing structural depth indicating lateral extension. The PTt paths are clockwise; the retrograde branch shows isothermal decompression even with some heating (Willner et al., 1997; Nega et al., 1999). The second case is for crustal root destruction, and lithospheric extension may prevail while it is dominated by the cold and dense mantle lithosphere 276 J. Arnold et al. / Journal of Geodynamics 31 (2001) 273±291 sinking that causes delamination. The extension is generated by the obliquely descending heavy slab `pulled' down by gravity and somewhat `guided' by the `immobile' surrounding mantle. An important aspect is that around 340 Ma the entire central portion of the Variscan orogen was aected by a HT/HP metmorphism closely postdated by large masses of granitoids between 330 and 320 Ma. Melting in the orogenic crustal root may be due to radiogenic heating of the stacked material (Gerdes et al., 1997) and/or due to asthenospheric rise after delamination. Sev- eral authors (Kalt et al., 1998; O'Brien, 1999) object to the ®rst suggestion and think that an external heat source is required. H. Schmeling has recently completed the numerical realisation of partial melting and melt segregation in a convecting mantle (Schmeling, 2000) and has applied it to the present study in an attempt to resolve this question. 2. Method The aim of numerical modelling of collisional processes is to catch some essential features and to test geodynamic hypotheses to ®nd out how crust and mantle may behave in nature in accor- dance with the laws of physics, i.e. the conservation laws for mass, momentum and energy and especially the rheological properties of the various crust and mantle units. This is a ¯uid dynamic convection approach. The governing conservation equations are solved on an equidistant grid in a rectangular box (®nite dierences) with the routine FDCON (Schmeling, 1989). The biharmonic idealized Navier±Stokes equation for the stream function is solved on 61Â61 or 61Â121 grids (and larger ones) by using the Cholesky decomposition.
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