Progressive orogenic deformation and metamorphism along the Combin Fault and Dent Blanche Basal Thrust in the Swiss-Italian Western Alps

Abstract

In the Western Alps of Switzerland and Italy, a stack of oceanic and continental units assembled during Late Cretaceous – Paleogene Alpine subduction, exhumation, and accretion. In the study area in the southern Valais and northern Aosta regions, oceanic units from the Piemont-Ligurian (South-Penninic) domain, the Combin zone and Zermatt-Saas zone, and continental units from the Adriatic continental margin, the Dent Blanche nappe, represent the structurally highest units of this nappe pile. These are separated from each other by two major Alpine tectonic contacts, the Combin Fault and the Dent Blanche Basal Thrust (DBBT). Structural and petrological analyses of shear zones along the two contacts were performed to deduce the evolution of the tectonic units and the sequence of deformation in the tectonic context of the Western Alps.<br /> Structural analyses at Lago di Cignana in the western Valtournenche of Italy suggest that early exhumation of the Zermatt-Saas zone after the peak of (U)HP metamorphism at ca. 43 Ma occurred during normal-sense top-E shearing under eclogite- to upper greenschist-facies conditions. Exhumation to mid-crustal levels and juxtaposition with the overlying Combin zone occurred along greenschist-facies top-SE shear zones. A phase of dominant pure shear deformation subsequently affected units in the footwall and hanging wall of the Combin Fault and obliterated earlier deformation. The Combin zone and Dent Blanche nappe at Lago di Cignana are characterized by greenschist-facies top-NW shearing related to syn- and post-exhumational deformation. <br /> High-pressure metamorphism and deformation within the Combin zone and the lowermost Dent Blanche nappe have been largely obliterated by subsequent greenschist-facies deformation and retrogression. However, prograde blueschist-facies assemblages are partly preserved within metasediments along the Combin/Dent Blanche boundary at Becca d’Aver in the western Valtournenche. Petrological, microstructural, and thermodynamic investigations suggest that prograde metamorphism was linked to progressive breakdown of lawsonite, associated water release, and fluid-mediated element transfer and mineral growth. Kinematic indicators suggest top-NW shearing during prograde metamorphism in a SE-dipping subduction zone.<br /> The overall sequence of greenschist-facies deformation in the study area has been deduced from analyses of shear zones along the Combin Fault and DBBT. The main stage of nappe stacking and ductile top-(N)W shearing between ca. 48 and 44 Ma (D1) was related to exhumation of the Dent Blanche nappe and Combin zone, formation of greenschist-facies mylonites along the DBBT, and a pervasive greenschist-facies overprint. This phase was followed by transpressional to orogen-parallel top-(S)W shearing between ca. 43 and 40 Ma (D2). Top-(S)W shearing especially affected the northwestern Combin Fault and occurred during ongoing exhumation of the Combin zone. The DBBT was probably characterized by continuing top-W shearing during early stages of this phase. Normal-sense top-SE shearing between ca. 39 and 37 Ma (D3) affected the northwestern Combin zone but only very locally the DBBT. Top-SE shear senses are subordinate in the southeastern realm and can locally be observed along the Combin Fault and at higher structural levels within the Combin zone. Top-SE shearing is held responsible for juxtaposition of the Combin zone and underlying (U)HP rocks of the Zermatt-Saas zone at crustal levels. It was followed by renewed top-NW shearing and folding from ca. 35 Ma onwards (D4) as a result of reactivation of the DBBT as out-of-sequence thrust. This phase was followed by brittle NW-SE extension after ca. 30 Ma (D5) due to updoming of the Vanzone antiform southeast of the study area.<br /> This study suggests that progressive orogenic deformation and metamorphism are recorded within shear zones along the Combin Fault and DBBT. However, early stages of the Alpine orogenic cycle have been largely obliterated by subsequent greenschist-facies deformation and retrogression.