Published in Geology 30, issue 10, 875-878, 2001, 1 which should be used for any reference to this work Glarus overthrust: A major pathway for the escape of fluids out of the Alpine orogen Nicolas P. Badertscher Institut de GeÂologie, Universite de NeuchaÃtel, Emile-Argand 11, 2007 NeuchaÃtel, Switzerland Georges Beaudoin MEDEF, DeÂpartement de GeÂologie et de GeÂnie GeÂologique, UniversiteÂLaval, QueÂbec G1K 7P4, ReneÂTherrien Canada Martin Burkhard* Institut de GeÂologie, UniversiteÂde NeuchaÃtel, Emile-Argand 11, 2007 NeuchaÃtel, Switzerland ABSTRACT element model of ¯uid ¯ow coupled with the advective-dispersive Thrust-related ¯uid ¯ow coupled with isotopic exchange be- transport of oxygen isotopes and associated exchange between ¯uids tween ¯uid and rock is simulated in a three-dimensional ®nite- and rock matrix. Numerical simulations are designed to reproduce pat- element model of the Glarus nappe, eastern Swiss Alps. Numerical terns of 18O depletion observed along and across the Glarus thrust. simulations are matched against well-established oxygen isotope Matching the results of such simulations with observed oxygen isotope gradients on the kilometer scale along the thrust. At internal south- patterns provides strong constraints on the various parameters that gov- ern locations, strongly channelized thrust-parallel ¯uid ¯ow re- erned ¯uid ¯ow. quires a high permeability contrast of .100:1 between the mylo- nite zone and country rocks and a high hydraulic head in the GLARUS OVERTHRUST hinterland and footwall. In contrast, isotopic patterns ;5±10 km During Oligocene±Miocene continent-continent collision, passive- farther north indicate a predominantly vertical, upward drainage margin sediments of the southern European shelf were involved in of ¯uids. We propose a situation in which the Glarus thrust evolved deformation to form the Helvetic foreland fold-and-thrust belt. In the northward across the boundary between the lithostatic and hydro- Glarus Alps (Fig. 1), Permian Verrucano red beds (d18O 5 10½) were static ¯uid-pressure regimesÐthe ``impermeable cap.'' A cyclic be- thrust northward over Mesozoic carbonates (d18O 5 25½) and Tertiary havior of ¯uid-pressure buildup, fracturing, channelized ¯uid es- ¯ysch series (d18O 5 19½). The thrust fault is continuously lined by cape, and sealing explains structural and geochemical observations an enigmatic 1±5-m-thick layer of Lochseitenkalk mylonite (Schmid, and the best-®t three-dimensional ¯uid-¯ow model parameters. 1975). Metamorphism ranges from anchizone in the north and in the footwall ¯ysch to lower greenschist facies in the south and in the Ver- Keywords: Glarus Alps, thrust faults, ¯uids, stable isotopes, ®nite- rucano hanging wall (Rahn et al., 1995). The anchizone-epizone bound- element analysis. ary (300 6 30 8C isograd) is offset along the Glarus thrust by ;2km to the north (Rahn et al., 1995) as the result of post±peak metamorphic INTRODUCTION thrusting between 25 and 20 Ma (Hunziker et al., 1986). The Glarus thrust of the eastern Swiss Alps (Fig. 1A) is an ex- tremely sharp and well-de®ned thrust contact (Schmid, 1975). Given OXYGEN ISOPLETHS ALONG THE THRUST PLANE strongly contrasted oxygen isotope compositions in footwall and hanging- Lochseitenkalk d18O (calcite) values are massively depleted with wall rocks (Burkhard et al., 1992), this structure is particularly well respect to the presumed Helvetic carbonate protolith of ;25½ (Burk- suited for the study of syndeformation interactions between ¯uid and hard et al., 1992). Several generations of veins in different states of rock. Despite abundant evidence for an intimate relationship between ductile overprinting provide a strong argument for the in®ltration of faulting and ¯uid advection (McCaig, 1989; Kirschner et al., 1999), ¯uids contemporaneous with thrusting (Badertscher and Burkhard, documentation and quanti®cation of ¯uid ¯ow remain dif®cult tasks 2000). Smooth regional trends in mean Lochseitenkalk d18O have been (McCaig et al., 1995). Transport theory (Bickle and McKenzie, 1987; established (Fig. 1B): a steep northward increase in d18O from 11½ Baumgartner and Rumble, 1988; Baker and Spiegelman, 1995) predicts to 19½ over ;4 km exists in the southern part, where Verrucano is the displacement of a sharp isotope front in a rock system by advective thrust over carbonates; farther north, above ¯ysch, Lochseitenkalk d18O ¯uid transport in the direction of ¯ow and broadening of the front by values are level at ;19½ (Badertscher, 2001). The asymmetric south- diffusion and hydrodynamic dispersion. Distension of sharp fronts also north gradient has been interpreted as an isotope-exchange front that occurs if the rate of ¯uid-rock reaction is slow compared to ¯uid- is due to the advection of 18O-depleted ¯uids expelled along the thrust in®ltration velocities (Lassey and Blattner, 1988; Bowman et al., 1994; fault (Burkhard et al., 1992; Bowman et al., 1994). Potential ¯uid Abart and Pozzorini, 2000). Inversely, the position and shape of an sources are evolved formation brines and metamorphic waters with an observed isotope front provide information about the direction of ¯uid estimated d18O composition of 4½±7½, in equilibrium with basement ¯ow, time-integrated ¯uid ¯uxes, the mechanism of ¯uid transport, and rocks (Marquer and Burkhard, 1992). Vertical d18O pro®les across the the provenance of in®ltrating ¯uids. thrust suggest that there has been an important component of ¯ow from Stable isotope systematics of the Glarus thrust and surrounding the footwall ¯ysch upward across the Lochseitenkalk and into the Ver- rock masses have been established in great detail (Badertscher, 2001). rucano hanging wall in northern parts of the thrust (Fig. 1C). Here the A regional-scale gradient of northward-decreasing 18O depletion is well dominant ¯uids (d18O 5 13½±15½, temperature, T 5 300 8C) must de®ned by more than 800 isotopic measurements covering a 15 3 20 have been derived from compaction and dewatering of ¯ysch series km area of thrust surface (Fig. 1B). Sharp asymmetric vertical gradients (calcite d18O 5 19½). on the meter scale are very well documented for many locations along the thrust (Fig. 1, C and D). These data provide strong evidence for SIMULATION OF FLUID FLOW COUPLED WITH highly channelized ¯uid ¯ow along the thrust in the south and vertical ISOTOPIC EXCHANGE upward ¯ow across the thrust in the north. Fluid ¯ow is modeled by using the three-dimensional ®nite- Here we present the application of a three-dimensional ®nite- element model, developed by Therrien and Sudicky (1996), that sim- ulates Darcy ¯ow and advective-dispersive transport coupled with iso- *Corresponding author: E-mail: [email protected]. topic exchange between ¯uids and rock matrix. The rock matrix is 2 Figure 1. A: Cross section of Glarus Alps in eastern Switzerland. B: Contour map of mean d18O composition (in per mil relative to Vienna standard mean ocean water [V- SMOW]) of Lochseiten calc-mylonite horizon obtained by kriging between sample sites (white dots). Irregular contours indicate map pattern of thrust contract. Coordinates in ki- lometers are according to Swiss grid. White dashed line represents boundary between Mesozoic carbonates and Tertiary ¯ysch in footwall of thrust. C: Observed calcite d18O composition in vertical pro®le of Verrucano hanging wall overlying ¯ysch at Lochseiten locality. D: Observed calcite d18O composi- tion in vertical pro®le at Vorab Pign locality. All isotopic data are from Badertscher (2001). discretized in three dimensions with the inclusion of two-dimensional balance considerations are used to calculate the evolution of the iso- planes of high permeability to represent fault zones (Beaudoin and topic composition of all rock cells and ¯uids with time. Resulting pat- Therrien, 1999). Kinetic oxygen isotope exchange between ¯uid and terns of isotopic composition are illustrated by using a color code (Fig. calcite is computed at each time step by using a ®rst-order reaction- 2). rate law. The Glarus thrust evolved at temperatures estimated as 260± Four representative simulations are shown in Figure 2 (B±F). A 330 8C (Burkhard et al., 1992; Rahn et al., 1995). An updip 70 8C more complete set of 15 additional simulations is available1. Boundary temperature gradient could account for a downstream, down-tempera- conditions for the ¯uid-¯ow simulations consist of prescribed hydraulic ture isotopic shift of no more than 2½ in d18O calcite, insuf®cient to heads at in¯ow boundaries, a lower hydraulic head equal to 0 m at explain the large isotopic shifts of .10½ observed. Isothermal con- out¯ow boundaries, and impermeable boundaries elsewhere (Fig. 2, B± ditions are assumed for simplicity. F). For the transport simulations, the isotopic composition of ¯uids is The domain considered for the numerical simulation has horizon- imposed at in¯ow boundaries, and a zero-dispersive ¯ux is assigned at tal dimensions of 21 km east to west, 18 km north to south, and a out¯ow boundaries. Two types of incoming ¯uids are distinguished. At vertical thickness of 2 km (Fig. 2A). Lithological units are discretized the southern in¯ow boundary (blue arrows in Fig. 2, B±F), a basement- by using three-dimensional rectangular prisms with horizontal dimen- derived ¯uid X has a d18O of 4.5½. A ¯ysch-derived ¯uid Y with an sions of 250 m 3 250 m. The vertical dimension of prisms is 250 m initial d18O of 14½ is in®ltrating at the bottom in the northern part of except in the vicinity of the thrust fault, where we used a layer of 10 the model domain (green arrows in Fig. 2, B±F). In a ®rst simulation cells of 1 m vertical dimension above and below the discrete thrust (Fig. 2B), a hydraulic head of 1500 m is speci®ed for ¯uid X, while a fault. Another 24 cells with 10 m vertical dimension populate the ®rst lesser head of 500 m is speci®ed for ¯uid Y. Out¯ow boundaries are 250 m both in the footwall and hanging wall adjacent to the thrust.