Mitt. Osterr. Geol. Ges. ISSN 0251-7493 94 (2001) 99-122 Wien, August 2003 Adula Penninic nappes Subduction Exhumation Structural evolution of Adula nappe, Misox zone, and Tambo nappe in the San Bernardino area: Constraints for the exhumation of the Adula eclogites JAN PLEUGER1, RENE HUNDENBORN1, KERSTIN KREMER1, SLAVICA BABINKA1, WALTER KURZ1, EKKEHARD JANSEN2 & NIKOLAUS FROITZHEIM1 13 Figures and 1 Table Content Abstract 99 1. Introduction 100 2. Overview of the regional geology 101 3. Structural analysis of the Adula nappe 106 3.1 Overview 106 3.2 Dl/Sorreda Phase 106 3.3 D2/Trescolmen Phase 106 3.4 D3/Zapport Phase 109 3.5 D4/Leis phase 111 3.6 D5/Carassino phase 112 4. Structural analysis of the Misox zone and the Tambo nappe 113 4.1 Overview 113 4.2 F1 /Ferrera phase • 114 4.3 F2/Niemet-Beverin phase 115 4.4 F3/Domleschg phase 115 4.5 F4 115 5. Tectonic evolution, time constraints, and metamorphism 116 6. A kinematic scenario for eclogite exhumation in the Adula nappe 117 7. Acknowledgements 119 References 120 Abstract The Misox zone in the Central Alps is formed by metasediments and metavolcanics from the Valais ocean basin. It separates the underlying Adula nappe, derived from the distal European continental margin, dominated by pre-Mesozoic basement, and including Tertiary-age eclogites, from the overlying, eclogite-free Tambo basement nappe of Brianconnais origin. These three units were empiaced on each other during pervasive, top-north directed mylonitic shearing, termed Zapport-phase in the Adula nappe and Ferrera phase in the Misox zone and Tambo nappe. In the Adula nappe, the Zapport phase deformation occurred at ca. 35 Ma and was preceded by (1) subduction-related imbrication of basement and sediments and (2) eciogite-facies mylonitisation. Neutron diffractometry of five samples of eclogite mylonite from the Adula nappe yielded omphacite textures indicative of constrictional strain. The Zapport phase is postdated by top-east shearing during the Niemet-Beverin phase (35 to 30 Ma), which in the Adula nappe is very weak and only detectable from quartz textures, but becomes stronger towards the upper part of the Misox zone and in the Tambo nappe. The main exhumation of the Adula eclogites, from ca. 20 kbar to ca. 7 kbar, occurred before and during the Zapport phase. The southward-increasing pressure gap existing between the Adula nappe and the Tambo nappe is explained by a Briangonnais lower crustal and mantle wedge originally situated between the Adula nappe below and the Tambo nappe above. When this wedge sank off into the asthenosphere, the Adula nappe was exhumed from below into the top-north Zapport shear zone. Address of the authors 1 Jan PLEUGER, Rene HUNDENBORN, Kerstin KREMER, Slavica BABINKA, Walter KURZ and Nikolaus FROITZHEIM, Geologisches Institut, Universität Bonn, Nussallee 8, D-53115 Bonn, Germany 2 Ekkehard JANSEN, Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany KREMER, S. BABINKA, W. KURZ, E. JANSEN & N. FROITZHEIM <D 1. Introduction CO O ja si Tertiary-age, high-pressure CD xa 9-co and ultra-high-pressure meta- JZ CD d CO morphic rocks including eclog- '£ 9 e CD" = Q. ites, metapelites, and garnet CD Q- O) CO peridotites are found in the Adu- C CD la nappe in the eastern Central to m E Alps (Fig. 1). The peak meta- 5 a morphic conditions of these in­ B x crease from the northern end of !l the nappe (12 kbar/500°C) to C CO CD — the southern end (32 kbar/ E CO CD g 840°C), in accordance with the C CO O CO CG O assumed southward subduction co <C direction (HEINRICH 1986, BE­ CD CO E < CKER 1993, NIMIS and TROMMS- DORFF 2001, SCHMID et al. 1996). < — . Q_ The Adula nappe is overlain by CD . the Tambo nappe across the CO 'co Sil CO CD metasedimentary Misox zone, o, co o and underlain by the Simano SrT — ro "<O nappe. Neither Tambo nor Si­ mano include Alpine eclogite- m facies rocks. Thus, the Adula CO ^ cu E • nappe represents a high- to ul­ Q. CO CO Q. CO CD tra-high pressure sheet em- <3 _£? cö C CO ^. placed between two lower-pres­ »>-§- sure sheets. Such emplace­ Q..Ö 1 g -| CD ment, according to current mod­ els (CHEMENDA et al. 1995, ?<£ «2 CO ERNST 1999), would imply con­ CD <£ S X3 E temporaneous activity of a 5! CD _C0 c/) <|1 thrust at the base and a south- CD CO 3m o *- C .E Q- dipping normal fault at the top of r (B -J - 8 t CD the Adula nappe. To test the lat­ CD" <!> d co B R-« GL CO ter prediction, we conducted co 2 t O o c o field and microstructural work in CO X5 E c the middle Adula nappe and the Es« ~ CO -C C units overlying it, the Misox zone CJ CD £ and the Tambo nappe (Central tr co Alps in eastern Switzerland, o _ C CO < Figs. 1, 2). Our study corrobo­ ro C7> j= co -3; g CO D E g» rated earlier results (MEYRE and O ^ O O cö co o S PUSCHNIG 1993, PARTZSCH 1998) 92 —i cö that this zone is dominated by top-north shearing instead of the expected top-south normal Q.C0 < w faulting. The detailed structural E O SJ analysis finally leads to a tecton­ o ic scenario that explains high- CD J= pressure exhumation in the Pi framework of top-north directed shearing. ¥#1 co O During field work the area - co d. • - from San Bernardino pass in |ö g. the north to about 3 km south > CD S of the village San Bernardino was mapped (PLEUGER 2001, HUNDENBORN 2001, KREMER Sgl* 2001, BABINKA 2001). It com­ CD o prises the northern section of £ E ^ CL CO the Misox zone and adjacent co E "S LL CO E co CD parts of the underlying Adula Structural evolution of Adula nappe, Misox zone, and Tambo nappe in the San Bernardino area: Constraints for... 101 Fig. 2. Tectonic overview of the eastern Lepontine area. Aa; Austroalpine units; Ar: Areua slice; CL: Cima Lunga unit; CO: Chiavenna ophiolite; F: Flysch; FL: Forcola Line, Go: Gotthard "massif"; IL: Insubric Line; Kn: Knorren melange; Lo: Lostallo window; MC: Mesozoic cover of the Suretta and Tambo nappes; MG: Mesozoic cover of the Gotthard "massif"; NB: Axial trace of the Niemet-Beverin fold; TM: Turba mylonite zone; ZB: Bellinzona-Dascio zone. AC: Alp de Confin; AIA: Alpe Arami; AT: Alp de Trescolmen; CG: Cima di Gagnone; Eh: Einshom; MD: Monte Duria; Me: Mesocco; PF: Passo della Forcola; SB: San Bernardino. Rectangle: Study area. nappe and the overlying Tambo nappe (Figs. 3, 4). The strong top-north shearing which is dominant throughout structures of all these units were investigated in order to the whole area and led to nappe stacking, and second, a compare and, if possible, correlate different stages of de­ top-east shearing which is distinctly discernible in the formation which have been defined by previous authors Tambo nappe and upper parts of the Misox zone but con­ who mainly focussed either upon the Adula nappe or the tinuously fades towards the Adula nappe where it could overlying units (Low 1987, MARQUER 1991, MEYRE and only be detected by means of textural studies of quartz. In PUSCHNIG 1993, SCHREURS 1993, MAYERAT DEMARNE 1994, combination with this top-east shearing, south-vergent PARTZSCH 1998). backfolds of all scales developed in front of the Tambo As a result, four phases of ductile deformation were rec­ nappe. ognized within each unit; a first deformation phase of the Adula nappe can only be inferred mediately (see below). They are referred to as D1 to D5 for the Adula nappe and, to avoid confusion, F1 to F4 in the case of the overlying 2. Overview of the regional geology units. All of them, except D1 and D2 of the Adula nappe, are ascribed to postcollisional nappe stacking and further The described units are part of the eastern flank of the shortening whereas features related to the subduction Lepontine structural dome and therefore dipping towards stage are hardly preserved. Two processes mainly control­ the east with average angles around 30°. Hence, in the field led the development of the present nappe stack: First, a the sequence of Eastern Swiss Penninic basement nappes, 102 J. PLEUGER, R. HUNDENBORN, K. KREMER, S. BABINKA.W. KURZ, E. JANSEN & N. FROITZHEIM Pescion zone Structural evolution of Adula nappe, Misox zone, and lambo nappe in the San Bernardino area: Constraints for . comprising from bottom to top the Lucomagno-Leventina, 1937) are intercalated with these Bündnerschiefer and sep­ Simano, Adula, Tambo and Suretta nappes, is exposed arate the lower from the upper Uccello zone. According to from west to east. To the north they are enveloped by North STEINMANN (1994), the lower Uccello zone continues north­ Penninic Bündnerschiefer which are rooted to the south in ward into the Aul Schuppenzone, and the Upper Uccello the Misox zone and, in the case of the Tambo and Suretta zone becomes the Tomül nappe. The Grava nappe, the nappes, by the Schams nappes. To the south, these Pen­ most voluminous of the Bündnerschiefer nappes farther ninic nappes are limited by a mylonitic steep belt associat­ north, wedges out into the Gadriolzug towards our study ed with the Insubric line and to the southeast by the Bergell area. In addition, three smaller units accompany the front of intrusion (Fig. 2). the Tambo nappe: The Areua slice, probably originating Following the generally accepted statement that during from the Briangonnais basement; the Schams nappes, also Tertiary Alpine orogeny the Penninic nappes were subduct­ of Briangonnais origin, which occupy larger areas northeast ed in southward direction, the reconstruction of the situation of the study area and envelop the front of the Suretta and before the onset of continental convergence yields the se­ Tambo nappes; and a melange of unknown origin, the Knor­ quence of the present nappe stack, seen from bottom to ren melange (GANSSER 1937, MAYERAT DEMARNE 1994; top, arranged from external (northern) to internal (southern) Figs.