Along the Upper Susa Valley transect (Cottian Alps), basement, cover and oceanic units usually referred to the Briançonnais, Piedmont and Ligurian palaeogeographic domains are exposed. On the basis of stratigraphic data various tectono-stratigraphic units (TSUs sensu DELA PIERRE et alii, 1997) of different palaeogeographic origin have been distinguished: -"ligurian" oceanic TSUs (Lago Nero, Aigle and Vin Vert) -continental margin TSUs (Chaberton-Grand Hoche, Vallonetto and Ambin Massif cover), characterized by Mesozoic sequences of Briançonnais and Piedmont affinity -continental crust TSU (Ambin Massif) Units of comparable origin are piled up at different structural levels in the nappe stack and underwent indipendent tectono-metamorphic evolution. The analysis of the Alpine metamorphic evolution in each TSU led to recognize three different metamorphic peaks (lawsonite-blueschists, epidote-blueschists and anchimetamorphic conditions). The distribution of the metamorphism in the nappe stack is not consistent with a regular geometric setting; this means that TSUs from the same palaeogeographic domain may have followed indipendent PTt paths during the Alpine orogenesis. According to metamorphic and structural features, the recognized TSUs can be grouped in three different tectono-metamorphic complexes (TMCs): -TMC1 (including Re Magi and Chaberton-Grand Hoche TSUs), marked by a very low grade metamorphic imprint and by the lack of HP/LT metamorphism; -TMC2 (Valfredda, Vin Vert and Lago Nero TSUs), characterized by an earlier lawsonite-blueschists facies event followed by a later greenschists facies event; -TMC3 (Ambin, Vallonetto and Aigle TSUs), which underwent an earlier event developed under epidote-blueschists facies conditions followed by a greenschists facies re-equilibration. TMCs are juxtaposed by west-dipping thrust planes, locally cross-cut by later high-angle strike-slip or normal faults. TCM1 is the upper structural element, and it is tectonically overthrusted onto TMC2; TMC3 is the lower element of the nappe stack exposed in the area. Also the boundaries between different TSUs consist of west-dipping thrust planes. Furthermore, each TSU preserves relics of a polyphasic ductile evolution of Alpine age - not correlatable through the various TSUs - which shows an E-vergence since the earliest tectonic phases. It suggests a common tectonic transport direction towards East at any crustal level during the Alpine orogenesis. The seismic image of the Alpine chain (CROP-ECORS-NFP10 programs) shows a triangular-shaped geometric framework, defined by the development of thrust planes verging towards the internal part of the chain in opposition to the deeper internal-dipping suture. On the surface, these planes correspond to structures which are usually considered in the literature as late back-folding of earlier structures. On the basis of the data here discussed, the internal verging trajectories are interpreted as primary transport directions in the orogenic wedge instead of late back-folding of the nappe stack. Therefore, the peculiar structural setting observed in the Upper Susa Valley transect, which corresponds to the axial sector of the belt, may be related to the exhumation trajectories within the orogenic wedge.
Lungo la sezione dell'Alta Valle di Susa affiorano unità tettono-stratigrafiche (UTS) di basamento, di copertura ed ofiolitiche classicamente riferite ai domini paleogeografici brianzonese, prepiemontese e ligure-piemontese. Le unità paleogeograficamente affini non hanno tuttavia seguito gli stessi percorsi all'interno del prisma orogenico durante l'evoluzione alpina. Esse risultano infatti attualmente impilate a diversi livelli strutturali e mostrano evoluzioni tettono-metamorfiche indipendenti. Le diverse UTS possono essere raggruppate in tre complessi tettono-metamorfici (CTM1, CTM2 e CTM3), caratterizzati da condizioni metamorfiche di picco che ricadono all'interno dei campi degli scisti blu ad epidoto, degli scisti blu a lawsonite e dell'anchimetamorfismo e giustapposti generalmente per mezzo di sovrascorrimenti immergenti verso ovest, localmente intersecati da piani ad alto angolo. Anche le singole UTS sono separate tra loro da contatti di natura tettonica immergenti verso ovest, ed al loro interno conservano tracce di una evoluzione duttile polifasica alpina che fin dalle prime fasi ha evidenza di vergenza verso l'interno della catena. Ciò suggerisce una comune direzione di trasporto tettonico vergente verso l'interno durante tutta l'evoluzione orogenica alpina. Questa particolare strutturazione può essere interpretata come dovuta alle traiettorie di risalita all'interno del prisma orogenico.
Approccio multidisciplinare per la ricostruzione dell'assetto tettono-stratigrafico e dell'evoluzione metamorfico-strutturale di un settore di catena orogenica: l'esempio dell'alta Valle di Susa (Alpi occidentali).
Mosca P;Dela Pierre F;
2002
Abstract
Along the Upper Susa Valley transect (Cottian Alps), basement, cover and oceanic units usually referred to the Briançonnais, Piedmont and Ligurian palaeogeographic domains are exposed. On the basis of stratigraphic data various tectono-stratigraphic units (TSUs sensu DELA PIERRE et alii, 1997) of different palaeogeographic origin have been distinguished: -"ligurian" oceanic TSUs (Lago Nero, Aigle and Vin Vert) -continental margin TSUs (Chaberton-Grand Hoche, Vallonetto and Ambin Massif cover), characterized by Mesozoic sequences of Briançonnais and Piedmont affinity -continental crust TSU (Ambin Massif) Units of comparable origin are piled up at different structural levels in the nappe stack and underwent indipendent tectono-metamorphic evolution. The analysis of the Alpine metamorphic evolution in each TSU led to recognize three different metamorphic peaks (lawsonite-blueschists, epidote-blueschists and anchimetamorphic conditions). The distribution of the metamorphism in the nappe stack is not consistent with a regular geometric setting; this means that TSUs from the same palaeogeographic domain may have followed indipendent PTt paths during the Alpine orogenesis. According to metamorphic and structural features, the recognized TSUs can be grouped in three different tectono-metamorphic complexes (TMCs): -TMC1 (including Re Magi and Chaberton-Grand Hoche TSUs), marked by a very low grade metamorphic imprint and by the lack of HP/LT metamorphism; -TMC2 (Valfredda, Vin Vert and Lago Nero TSUs), characterized by an earlier lawsonite-blueschists facies event followed by a later greenschists facies event; -TMC3 (Ambin, Vallonetto and Aigle TSUs), which underwent an earlier event developed under epidote-blueschists facies conditions followed by a greenschists facies re-equilibration. TMCs are juxtaposed by west-dipping thrust planes, locally cross-cut by later high-angle strike-slip or normal faults. TCM1 is the upper structural element, and it is tectonically overthrusted onto TMC2; TMC3 is the lower element of the nappe stack exposed in the area. Also the boundaries between different TSUs consist of west-dipping thrust planes. Furthermore, each TSU preserves relics of a polyphasic ductile evolution of Alpine age - not correlatable through the various TSUs - which shows an E-vergence since the earliest tectonic phases. It suggests a common tectonic transport direction towards East at any crustal level during the Alpine orogenesis. The seismic image of the Alpine chain (CROP-ECORS-NFP10 programs) shows a triangular-shaped geometric framework, defined by the development of thrust planes verging towards the internal part of the chain in opposition to the deeper internal-dipping suture. On the surface, these planes correspond to structures which are usually considered in the literature as late back-folding of earlier structures. On the basis of the data here discussed, the internal verging trajectories are interpreted as primary transport directions in the orogenic wedge instead of late back-folding of the nappe stack. Therefore, the peculiar structural setting observed in the Upper Susa Valley transect, which corresponds to the axial sector of the belt, may be related to the exhumation trajectories within the orogenic wedge.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.