IMEDL2004:Regional tectonic framework
Regional tectonic framework and reconstruction of the ancient Adriatic margin in Grisons
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Field guide: Birth and early evolution of Alpine ocean basins
Fig. 5 Reconstruction of the ancient Adriatic margins in Grisons: (a) Tectonic sketch map of the south Pennine-Austroalpine boundary zone in northern Italy and SE Switzerland. (b) Profile across the south Pennine-Austroalpine units in Grisons (modified after Froitzheim et al. (1994) and Manatschal and Nievergelt (1997)). (c) The top section represents portions of the margin, now in different tectonic imbricates which preserve rift-related structures and depositional contacts with sediments. The corresponding lithostratigraphic columns document measured sections. For further reference to the stratigraphic columns, see Eberli (1988) and Manatschal and Nievergelt (1997). Abbreviations are as follows: LC: Lower Cretaceous; UJ: Upper Jurassic; uMJ: upper Middle Jurassic; MJ: Middle Jurassic; LJ: Lower Jurassic; uLJ: upper Lower Jurassic; lLJ: lower Lower Jurassic; P-T: Permian-Triassic. (d) Interpreted architecture of the former northern Adriatic margin. Deep structure of the proximal margin refers to the observations in the southern Alps (Bertotti 1991). Modified after Manatschal and Bernoulli (1999).
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Field guide: Birth and early evolution of Alpine ocean basins
Reconstruction of ancient rift-structures in the Alps
Understanding of the tectonic evolution of the Alps is a pre-requisite to reconstruct the stratigraphic evolution and architecture of the ancient rifted margins preserved within them. However, explaining this evolution would go beyond the aim of this document and, we refer, therefore, to the papers of Froitzheim et al. (1996) and Schmid et al. (1996) for a modern interpretation of the Alps.
Parts of the Adriatic margin including its OCT can be reconstructed along an east-west transect in central Grisons (Fig. 5). The former Adriatic margin has been telescoped by strike-slip dominated thrusting during the Late Cretaceous; however, the OCT has never been subducted to great depth and metamorphism did not exceed lower greenschist facies along the central transect (Ferreiro Mählmann 1995, and references therein). West directed shortening was of the order of about 200 - 300% (about 100 - 150 km). In Grisons, the resulting nappe edifice includes (from top to bottom) the upper Austroalpine (Ötztal, Silvretta-Sesvenna and Campo-Ortler), the lower Austroalpine (Bernina and Err nappes) and the south Pennine nappes (Arosa zone, Platta nappe and Forno-Malenco complex) (Fig. 5). Late Cretaceous nappe stacking was followed by extension which occurred still during the Late Cretaceous (Froitzheim et al., 1994, 1996). The amount of extension, however, was small compared with the amount of previous shortening. Compression and extension were parallel to previous extension during Jurassic rifting and most of the Jurassic basins were inverted along rift-related faults. During this process, higher crustal levels including sediments and shallow basement were detached from their original basement; however, the coaxial Cretaceous phases of deformation allowed us to visualize a relatively straightforward kinematic inversion of the margin (for details, see Froitzheim et al. (1994) and Manatschal and Nievergelt (1997)). According to this kinematic reconstruction, the higher nappe units are derived from the proximal margin (upper Austroalpine), whereas the lower nappes represent the distal margin (lower Austroalpine, Err and Bernina nappes) and the transitional crust (Platta nappe) (Fig. 5). Because of the detachment of the uppermost crust and its sedimentary cover during Alpine convergene, the deep structures of the margin are not preserved anymore in our transect. Shallow crustal structures of the proximal margin (Ortler-Silvretta) (Eberli 1988; Froitzheim 1988; Conti et al. 1994), the distal margin (Err) (Handy et al. 1993; Manatschal and Nievergelt 1997) and of the ocean floor sequence (Platta) (Dietrich 1969 and 1970 and Desmurs et al. 2001) are well preserved along the transect in Grisons (Fig. 5). Younger, N-S directed shortening and associated transcurrent faulting during Tertiary time are of subordinate importance within the east-west transect (Froitzheim et al. 1994) and are not considered in the kinematic inversion and palinspastic reconstruction.
Sites visited on the three field trips
On our three field trips we will visit some of the localities where the geometry of the ancient Alpine Tethys margin can be studied directly in the field and can be compared with the geometry of present-day magma-poor rifted margins. The three localities are within in the Penninic and Austroalpine nappe stack in eastern Switzerland (Fig. 3b), which preserves remnants of the former proximal and distal Adriatic margin and distal European/Briançonnais margin (Fig. 3c). The Austroalpine nappes (Field trips I and II) are derived from the northern continental margin of Adria, an African promontory or independent micro plate of the western Tethys (Fig. 2). The south-Pennine ocean (Piemonte Liguria ocean) to the north of Adria opened during the mid-Jurassic and was involved in subduction from the Late Cretaceous onwards (Fig. 2b). It was bounded to the north or west by a continental fragment, the Briançonnais ridge, from which the middle Pennine nappes are derived (Fig. 1a). From this margin we will see only the preserved ocean-continent transition in the Tasna nappe (Field trip III); of the Piemont-Liguria ocean we will study remnants of the ocean-continent transition formed by exhumed mantle rocks, volcanics and overlying sediments exposed today in the Platta nappe (Field trip III (Falotta)).
On our three field trips we shall see rocks belonging to different nappes of the Pennine and Austoalpine units of the Alps. Because in detail the tectonic relationship between the different visited areas are very complex (for a detailed description see Froitzheim et al. (1994) and Schmid et al. (1996)), we will concentrate on the sectors where we can observe the original geometrical and stratigraphical relation-ships of the former margin, without going into the details of the complex Alpine geology. An itinerary of the three excursions as well as a schematic block diagram of the visited area showing the 3-D geometry of the major Alpine tectonic units is presented in figures 6 and 7.
Aims of the excursion
Our field trips will focus on the tectonic evolution of the Alpine segment of the Alpine Tethys ocean (Liguria-Piemonte ocean). More particularly, we focus on: (1) the basin architecture and stratigraphic evolution of the proximal Adriatic margin (Field Trip I: Il Motto); (2) deformation processes associated with continental breakup and architecture of detachment faults in the distal margin (Err nappe) and ocean-continent transition (Platta nappe) (Field Trip II: Err/Platta); and (3) the interplay between tectonic, magmatic, sedimentary and diagenetic processes during the exhumation of mantle rocks at the seafloor (Field Trip III: Tasna). Other field trips focussing on the sedimentary record on top of the exhumed mantle (Davos) and on the geometry of the pre-rift crust-mantle boundary (Malenco) are found in an additional field guide.
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Field guide: Birth and early evolution of Alpine ocean basins
Fig. 6 Itinerary of the IMEDL field excursions.
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Fig. 7 Schematic block diagram of SE Grisons and adjacent Italy (modified after Froitzheim et al. 1994).
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