IMEDL 2004: Remnants of ancient margins in the Alps, part II FIELD GUIDE Additional field trips (not visited during IMEDL) but equally important for the understanding of ocean-continent transitions in the Eastern Central Alps Remnants of the ancient Tethys margins preserved in the Davos (Totalp) and Margna-Malenco units in SE Switzerland and N- Italy (Central Alps) 1 IMEDL 2004: Remnants of ancient margins in the Alps, part II Fig. 1 Stratigraphy of an ocean-continent-transition zone as determined from field observations from the south Pennine-Austroalpine boundary zone in Grisons (SE Switzerland). From Manatschal and Nievergelt, 1997. Introduction The oceanic sediments overlying the ophicalcites include: (1) shales and breccias; (2) The close assemblage of serpentinites, basalts cm-bedded ribbon cherts (Radiolarite Formation, and radiolarites, first described by Gustav Middle to Upper Jurassic, Bill et al. (2001)); (3) Steinmann (1905) from the Arosa zone and the gray, thin-bedded limestones with greenish, shaly Platta nappe, is distinctly different from typical interbeds (Calpionella or Aptychus Limestone, fast-spreading ridge associations and can be Berriasian); (4) black shales with dm-bedded fine- compared to the situation described from ocean- grained limestones (Palombini Formation, approx. continent-transitions (OCT) of present-day magma- Valanginian to Barremian) (Weissert and Bernoulli, poor rifted continental margins (Manatschal and 1985). The red shales occur only locally; they are Bernoulli, 1999). This association includes associated with debris-flow deposits, serpentinised peridotites, which were exhumed unfossiliferous and may be interpreted as along detachment faults and which are locally hydrothermal metalliferous sediments drived from overlain by extensional allochthons of continental fault rocks associated with mantle exhumation. The crust; in addition, minor gabbroic intrusions, radiolarites and the Calpionella Limestone contain tholeiitic flows, pillow lavas and pillow breccias breccias and graded sandstones including clasts of and a succession of oceanic sediments occur (Fig. both serpentinite and continental basement rocks 1). indicating local submarine relief at the time of The serpentinised peridotites are fragments of deposition. The facies evolution of the pelagic sub-continental mantle and record deformation sequence is determined by subsidence and under decreasing temperatures during extension and paleoceanographic changes, in particular by a exhumation leading to their final exposure at the pronounced drop of the calcite compensation depth Jurassic sea-floor. at the Jurassic-Cretaceous boundary accompanying Tectono-sedimentary breccias, the so-called the replacement of radiolarian-dominated planktic ophicalcites, occur along the surface of the assemblages by coccolith- and nannoconid- exhumed mantle rocks and are overlain by other, dominated ones. sedimentary breccias and/or pelagic sediments or basalts. The igneous rocks consist of gabbros intruding Sites visited the already serpentised mantle rocks at shallow In eastern Switzerland, ophiolite fragments and depth 161 Ma ago and basaltic flows and pillow associated oceanic sediments are preserved in the lavas stratigraphically overlying the exhumed sub- south-Pennine Malenco-Forno unit, the Platta continental mantle rocks and the tectono- nappe and in the Arosa zone (Fig. 2). These units sedimentary breccias (Desmurs et al., 2001) (Fig. are sandwiched between the middle (Briançonnais) 1). Gabbros and basalts are derived from an and north Pennine units in the footwall and the asthenospheric MORB-type source and appear to Austroalpine nappes in the hanging wall, which are document the onset of sea-floor spreading across an derived from the northwestern and the southeastern exhumed subcontinental mantle during the earliest margin respectively of the Alpine segment of the evolutionary stages of a slow-spreading ridge Tethys ocean. (Schaltegger et al., 2002). 2 IMEDL 2004: Remnants of ancient margins in the Alps, part II Fig. 2 Tectonic sketch map of the South-Pennine- Austroalpine boundary zone in Grisons. D:Davos, B: Bivio, SM: San Murezzan Modified after Froitzheim et al., 1994. In the Arosa zone, the Totalp imbricate near Davos shows the clearest relationship between the basement of the ocean-continent transition and the overlying oceanic sediments (1st day). In the Malenco-Forno unit, a Permian crust-mantle boundary is preserved, which allows to study lower crustal and upper mantle rocks as well as their exhumation history during rifting (2th day). Aims of the excursion Our excursion will focus on the tectonic evolution of the Alpine segment of the Tethys ocean (Liguria-Piemonte ocean). More particularly, we focus during the four days of excursion on: (1) the interplay between tectonic, sedimentary and diagenetic processes during the exhumation of mantle rocks at the seafloor (first day: Totalp), and (2) characteristics of subcontinental mantle and its relation to the pre-rift-lower continental crust (second day: Malenco). Mantle exhumation processes: Totalp area (Davos) Daniel Bernoulli Geologisches Institut der Universität Basel, Bernoullistrasse 32, CH-4056 Basel Gianreto Manatschal CGS - EOST, CNRS-Université Louis Pasteur, F-67084 Strasbourg, e-mail : [email protected] Othmar Müntener Geology Institute, University of Neuchâtel, Rue Emile Argand 11, CH-2007 Neuchâtel, e-mail : [email protected] 3 IMEDL 2004: Remnants of ancient margins in the Alps, part II Excursion will focus on: Locally, peridotite mylonites indicate deformation at mantle depth which is possibly related to a pre- • Characteristics of subcontinental mantle Mesozoic event. Ar/Ar ages on phlogopites derived • Interplay between tectonic, sedimentary and from a pyroxenite of the Totalp serpentinite yielded diagenetic processes associated with the an age of 160±8 Ma (Peters and Stettler, 1987), exhumation of subcontinental mantle indicating that the mantle rocks were uplifted to • Ophicalcite-genesis about 10 km depth in the late Middle Jurassic. Initial serpentinisation accompanied exhumation Geological overview and deformation under decreasing temperatures The Arosa zone in southeastern Switzerland, to during Jurassic rifting. This evolution is which the Totalp serpentinite belongs, is a highly documented by serpentinite mylonites, deformed complex zone comprising imbricates derived from under greenschist-facies conditions, overprinted by an ancient ocean-continent transition and from the cataclasis and a post-deformational calcification adjacent distal continental margin (Fig. 2). The under still lower temperatures. Differences in Arosa zone is sandwiched between the middle hydrogen isotope ratios between serpentinites and Pennine (Briançonnais) and Austroalpine nappes ophicalcites suggest that serpentinisation was which are derived from the northwestern and the initially restricted to deformation zones and southeastern margin of the Alpine segment of the continued during and after ophicalcite formation Tethys ocean (Fig. 3). Along the upper and lower and Alpine orogeny (Früh-Green et al., 1990). boundaries of the Arosa zone, as well as within the zone, tectonic melanges are widespread (Lüdin, “Ophicalcites” 1987). Within the Arosa zone, the Totalp imbricate Locally, the serpentinites are crossed by a near Davos shows the clearest relationship between system of calcite veins. According to Peters (1963), the basement of the ocean-continent transition and these veins are restricted to the stratigraphically the overlying oceanic sediments, which are the upper part of the serpentinites and to the theme of the excursion. In the Totalp imbricate, neighbourhood of the ophicalcites. Rocks generally Mesozoic igneous rocks are not observed. Alpine described as ophicalcites are breccias with metamorphism is of pumpellyite-prehnite grade unsorted, millimetre- to metre-sized clasts of fresh (Peters, 1963) which is also reflected in the or oxidised serpentinite, set in a calcitic matrix equilibration with Alpine metamorphic fluids of the stained red to pink by hematite or greenish-gray by oxygen isotopes in the calcite of the ophicalcites chlorite minerals and/or cemented by sparry calcite. and the overlying pelagic limestones (Früh-Green et The fabric varies considerably, from serpentinite al., 1990). host rock filled by red-stained limestones or white calcite with a typical cement fabric to clast- supported breccias in which the serpentinites were fragmented in situ (ophicalcite type I of Lemoine et al., 1987). The breccias are of polyphase origin, as shown by the different phases of fragmentation and generations of cementation and sediment infill (Bernoulli and Weissert, 1985). Clast- and matrix-supported breccias, locally containing clasts of Triassic dolomites and of Fig. 3 Schematic cross-section across the Totalp continental basement rocks, indicate lateral imbricate, D. Bernoulli and G. sediment transport, probably from nearby Manatschal (unpublished). extensional allochthons, by debris flow. This type of breccia includes the ophicalcites type II of Mantle rocks Lemoine et al. (1987). Macroscopically, the contact between The major part of the Totalp serpentinite is a serpentinite host rock and carbonate matrix or heterogeneous, variably serpentinised, amphibole- cement is often sharp, but, in places, it is obscured bearing spinel lherzolite with (garnet) pyroxenite by the replacement of serpentine and peridotite layers and locally phlogopite-hornblendite