Reply to Comment by W. Kurz on “Tectonic map and overall architecture of the Alpine orogen”

Abstract

99We very much welcome the comment by Kurz (2005), whichraises some important questions and allows us to further clari-fy some concepts behind our correlations between Westernand Eastern Alps (Schmid et al. 2004). As a matter of fact, theprincipal question raised by Kurz (2005), namely the possiblecontinuation of the Northpenninic (=Valais) ophiolites andBundnerschiefer, originally defined in the Western Alps(Trumpy 1955, 1960), towards the east and into the area of theTauern window (see Fig. 1 and plate 1 in Schmid et al. 2004)does not concern “details” (Kurz 2005), but represents a firstorder problem in any attempt to propose possible correlationsof tectonic units and paleogeographical domains along theAlpine chain between Nice and Vienna. As correctly stated by Kurz (2005), the Glockner nappe(referred to as “Upper Schieferhulle Unit” by Schmid et al.2004), comprises remnants of an oceanic basement in the senseof a partly incomplete ophiolitic sequence, of course besidesthe volumetrically dominating calcschists referred to as “Bund-nerschiefer” or “schistes lustres” in the Western Alps. Indeedcalcschists and ophiolitic remnants are not diagnostic for theLower Penninic nappes derived from the Valaisan paleogeo-graphical domain, and it is true that they also occur in UpperPenninic ophiolitic units of the Western Alps, derived from thePiedmont-Liguria ocean (such as in the Zermatt-Saas ophio-lites of Western Switzerland and the Avers Bundnerschieferof Eastern Switzerland). However, we do not understand whythe fact that the Matrei zone at the southern rim of the Tauernwindow is occasionally missing, the Glockner nappe oftenbeing in direct contact with the Austroalpine lid, “wouldfavour a Southpenninic (= Piedmont-Liguria) origin of theGlockner nappe” (Kurz 2005). The Matrei zone, which ofcourse also contains calcschists (besides metapelites), is actu-ally defined by the presence of tectonic slivers (or olistolithsaccording to the interpretation of Frisch et al. 1987) of Aus-troalpine derivation (Kurz et al. 1998). This clearly makes theMatrei zone an analogue of the Upper Penninic Arosa zone ofEastern Switzerland (Manatschal et al. 2003), also character-ized by material derived from the Austroalpine domain (asparts of a tectonic melange according to Ludin 1987). In Eastern Switzerland, as well as in the Tauern Window,these Upper Penninic units are considered as an integral partof the Cretaceous-age top-WNW nappe edifice by all workers.However, we differ from many Alpine geologists in that wefollow Froitzheim et al. (1994, 1996) and consider top-WNWthrusting during Cretaceous orogeny as being very distinctfrom top-N thrusting during Tertiary orogeny, the two oroge-nies being separated by a Late Cretaceous (syn-Gosau) exten-sional event. It is Tertiary orogeny that led to the nappe stack-ing of the Lower Penninic (e.g. Glockner nappe), and the Sub-penninic units (e.g. the “Zentralgneise” nappes, see Kurz et al.1998), which constitute the deeper and major part of theTauern window. Hence, contrary to the “classical interpreta-tion” of most Austrian geologists (e.g. Frisch et al. 1987, Kurzet al. 1998), that envisage collision between the Zentralgneiseand the Austroalpine margin to have occurred during Creta-ceous orogeny, we interpret the contact between Glocknernappe and Matrei zone to mark a tectonic contact between twonappe edifices that formed during two distinct orogenies (Cretaceous vs. Tertiary). Thereby we are led by the stronganalogies between the architecture of the Engadine window,which demonstrably was not closed before Tertiary times(Froitzheim et al. 1994), and the Tauern window. This stronganalogy is additionally supported by the results of along-strikereflection seismic data (Pfiffner and Hitz 1997, their line E2)which demonstrate that the Lower Penninic Bundnerschieferof the Engadine window are directly underlain by basement