Structure and kinematics of the northern Simano Nappe 63 ABSTRACT New structural data allow the internal structure and kinematics of the Lower Penninic Simano Nappe to be established, together with the relationship of this unit to the underlying Leventina Gneisses and the also underlying Maggia Nappe. Three clearly distinguishable Alpine deformation phases (D1, D2, D3) are recognized within the Simano Nappe. D1 developed in mid-crustal levels under metamorphic conditions of ca. 1 GPa and 500°C and produced kilometre-scale, recumbent, north-closing anticlinal folds, called nappes. During D2 these nappes were intensely refolded on the same scale and with identical style to form the present day recumbent fold pattern. The axial planar foliation S2 is the dominant planar fabric throughout most of the Central Alps. The subsequent phase D3 overprints the entire tectonostratigraphy of the Central Alps, with broader and more upright D3 folds trending oblique to the orogen. Regionally, these broad D3 folds have a major influence on the overall foliation and nappe outcrop pattern of the Central Alps. During D2 and D3, peak metamorphic conditions reached temperatures of up to 650°C, under pressures of 0.7-0.8 GPa. An important consequence of this deformation history is that lithological units separated only by D2-synforms formed part of the same tectonostratigraphic level after D1 nappe-stacking and therefore should not be interpreted as different nappes. This principle can be directly applied to the relationship between the Simano and the two adjacent units. The underlying Leventina Gneisses are separated from the Simano rocks by an originally intrusive contact that has been strongly reactivated during both D1 and D2. Therefore, it is taken to represent a true nappe contact, although locally it still retains some original intrusive relationships and has also been strongly reactivated during D2. The Maggia and Simano units represent alternate limbs of a D2 synform-antiform pair, the Mogno synform and the Larecc antiform, or more generally the Verzasca-Larecc-Ganna antiform. If the Maggia unit is simply the continuation of the Simano unit around the D2 Mogno synform, then post-D1 they represented a single Simano-Maggia Nappe. The Maggia "Nappe" is therefore also part of the European margin and cannot be separately assigned to the Briançonnais paleogeographic domain, as has been proposed. ZUSAMMENFASSUNGNeue strukturelle Erkenntnisse erlauben es, nicht nur die interne Struktur und Kinematik der unterpenninischen Simano-Decke zu definieren, sondern auch die Beziehungen dieser Einheit mit den darunterliegenden Einheiten der Leventina-Gneise und der Maggia-Decke zu klären. Drei klar unterscheidbare alpine Deformationsphasen (D1, D2, D3) können innerhalb der SimanoDecke beobachtet werden. D1 entwickelte sich in mittelkrustalem Niveau unter Metamorphosebedingungen von etwa 1 GPa und 500 °C. Dabei wurden kilometergrosse, isoklinal nord-schliessende Antiklinalen gebildet, für welche sich die Bezeichnung "Decken" eingebürgert hat. Wä...
the Leventina Nappe represents one of the lowermost exposed units in the Alpine nappe stack and corresponds to a slice of the European margin that was entrained into the Alpine continental accretionary prism during the tertiary tectonic event. this study yields details regarding the tectonic and metamorphic history of the Leventina Nappe, through detailed analysis of structures and shear zone patterns, and the examination of the si-content of white mica along a north-south profile. the Leventina Nappe underwent three phases of ductile deformation. Foliation s1 is mostly sub-parallel to the regionally dominant structural fabric (the s2 foliation). s2 foliation is penetratively developed in the structurally higher portions of the Leventina Nappe toward the simano Nappe, while it is only weakly developed in the core of the Leventina Nappe. A 50 to 200 m wide mylonite zone, with a D2 top-to-NW sense of shear marks the boundary to the simano Nappe. throughout the Leventina Nappe only small-scale D2 shear bands (mm to cm wide) are observed, showing a top-to-NW sense of shear. Deformation phase D3 locally generated a vertical axial plane foliation (s3) associated with the large-scale D3 Leventina antiform.Microtextural evidence and phengite geobarometry were used to constrain the temperature and pressure conditions of equilibration of the Leventina Gneisses. Highest si (pfu) values are preserved in the core of phengitic micas and reflect pressure and temperature conditions of around 8 kbar at 550 °c and 10 kbar at 650 °c in the northern and southern parts of the Leventina Nappe, respectively. Lower si (pfu) values from the rims of white micas correspond to a metamorphic pressure of ca. 5 kbar during the exhumation of the unit. these metamorphic conditions are related to the underthrusting of the thinned European margin into the continental accretionary prism during late Eocene time. these new data allow us to propose a kinematic model for the Leventina Nappe during the tertiary Alpine tectonics. ZusAMMENFAssuNGDie Leventina-Decke stellt eine der tiefsten Einheiten dar, welche im alpinen Deckenstapel aufgeschlossen ist. sie entspricht einem teil des europäischen Kontinentalrandes, welcher während der alpinen Kontinentalkollision in den Akkretionskeil einbezogen wurde.Die detaillierte Analyse der strukturelemente und der scherzonenverteilung sowie die untersuchung von si-Gehalten in metamorphen Hellglimmern entlang eines Nord-süd-Profiles ergeben neue Erkenntnisse zur tektonischen und metamorphen Geschichte der Leventina-Decke. Drei Deformationsphasen werden in der Leventina-Decke beobachtet. Die schieferung s1 ist mehrheitlich sub-parallel zur regional vorherrschenden schieferung (s2). s2 ist in den höheren teilen der Leventina-Decke zur simano-Decke hin penetrativ ausgebildet, wogegen sie im Kern der Leventina-Decke nur schwach ausgeprägt ist. Ein 50 bis 200 m mächtiger Mylonit-Horizont mit einem NWgerichteten D2-schersinn stellt im Hangenden die Grenze zur simano-Decke dar. In der ganzen Leventina-Decke werde...
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