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Progressive Orogenic Deformation and Metamorphism Along the Combin Fault and Dent Blanche Basal Thrust in the Swiss-Italian Western Alps

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Progressive Orogenic Deformation and Metamorphism Along the Combin Fault and Dent Blanche Basal Thrust in the Swiss-Italian Western Alps Progressive orogenic deformation and metamorphism along the Combin Fault and Dent Blanche Basal Thrust in the Swiss-Italian Western Alps Dissertation zur Erlangung des Doktorgrades (Dr. rer. nat.) der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn vorgelegt von FREDERIK KIRST aus Bonn Bonn 2014 Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn. 1. Gutachter: PD Dr. Thorsten Nagel 2. Gutachter: Prof. Dr. Thorsten Geisler-Wierwille Tag der Promotion: 06.02.2015 Erscheinungsjahr: 2015 „I will not live under a shadow of fear, never be crippled by chaos and doubt, fall prey to your madness. I am not shattered, out of the ashes I rise knowing that nothing is stronger than faith, finding hope in our hopeless lives.“ - John Petrucci - 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 structural evolution of the tectonic units and the sequence of deformation in the tectonic context of the Western Alps. At Lago di Cignana in the western Valtournenche of Italy, (U)HP rocks of the upper Zermatt-Saas zone are exposed and overlain by greenschist-facies rocks of the Combin zone and the lower Dent Blanche nappe. Structural analyses revealed a progressive structural evolution from (U)HP to greenschist-facies conditions related to Paleogene syn- to post-exhumational deformation. 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. Strain was mainly localized along the uppermost Zermatt-Saas zone and top-SE structures along the Combin Fault have been partly obliterated by subsequent deformation. Top-SE shearing was followed by a phase of pure shear deformation affecting units in the footwall and hanging wall of the Combin Fault. This phase was followed by top-NW shearing associated with renewed thrusting along the DBBT which continued until low-grade greenschist-facies conditions. A phase of dominant pure shear deformation after juxtaposition of the Combin and Zermatt-Saas zones may have led to the development of conjugate top-SE and top-NW shear zones on a regional scale within the Combin zone. This offers an explanation for contradicting views regarding the nature of the Combin Fault and the Combin zone, i.e. whether they represent thrust-related or normal-sense shear zones. Whereas deformation under (U)HP conditions can be well studied within rocks of the Zermatt-Saas zone, the overlying Combin zone and lower Dent Blanche nappe show a strong greenschist-facies overprint which largely obliterated early HP-related deformation and metamorphism. The Becca d’Aver continental sliver (BACS) in the western Valtournenche of Italy represents a continental fragment from the former Piemont-Ligurian/Adriatic ocean-continent transition and is structurally located at the Combin/Dent Blanche boundary. Prograde blueschist-facies assemblages are partly preserved within metasediments of the BACS and yield peak conditions around 1.7 GPa and 500° C. 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 and SE-directed subduction. Deformation structures within the BACS and the underlying Combin zone suggest a common structural evolution and complex kinematics on the retrograde path related to greenschist-facies top-(W)NW and top-(E)SE shearing. The last ductile deformation was related to top-NW shearing which evolved into brittle NW-SE extension. Most of the deformation observed in the study area occurred under greenschist-facies conditions and variably affected the northwestern and southeastern realms. The overall sequence of deformation and structural evolution of the tectonic units has been deduced from analyses of shear zones along the Combin Fault and DBBT and partly within the Combin zone. 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. 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. The proposed structural evolution suggests that the Combin zone and Dent Blanche nappe experienced a common retrograde evolution and penetrative top-(N)W shearing after an early blueschist-facies imprint and subduction-related top-NW shearing. Subsequent exhumation of the Zermatt-Saas zone from (U)HP depths occurred during normal-sense top-E shearing. Juxtaposition with the overlying Combin zone at crustal levels occurred along greenschist-facies top-SE shear zones. The relative chronology between greenschist-facies top-NW and top-SE shearing is often difficult to establish. Structural observations suggest that top-NW and top-SE shear zones may have partly overlapped in space and time due to a large pure shear component during greenschist-facies deformation, especially around 36 Ma during the transition from normal-sense shearing to renewed thrusting. A late phase of top-NW shearing due to out-of-sequence reactivation of the DBBT affected units in the study area before the onset of brittle deformation. Contents - CHAPTER 1 - Introduction ……………………………………………………………………… 1 1.1 Progressive deformation and metamorphism in Alpine-type orogens …… 1 1.2 The study area in the Swiss-Italian Western Alps ………………………… 3 1.3 Research question, methods, and organization of the thesis ……………… 7 - CHAPTER 2 - Structural evolution of the Penninic units at Lago di Cignana (Western Alps, Italy): constraining progressive orogenic deformation from (ultra)high-pressure to greenschist-facies conditions ………………………………………………… 11 2.1 Abstract ……………………………………..……………………………. 11 2.2 Introduction ………………………….…………………………………… 12 2.3 Geological setting ………………………………………………………… 13 2.4 Field relations …………………………………………………………….. 16 2.5 Deformation structures ……………………………………………………. 17 2.5.1 Zermatt-Saas zone ………………………………………………… 18 2.5.2 Combin zone ………………………………………………………. 23 2.5.3 Dent Blanche nappe ………………………………………………. 25 2.6 Discussion ………………………………………………………………… 27 2.6.1 Kinematics and geometry of deformation ………………………… 27 2.6.2 Previously published structural works ……………………………. 29 2.6.3 Structural and tectonic evolution …………………………………. 31 2.7 Conclusions ………………………………………………………………. 34 - CHAPTER 3 - Tectonometamorphic evolution of the Becca d’Aver continental sliver in the Western Alps (Valtournenche, Italy) …………………………………………… 36 3.1 Abstract ……………………………………………………………………. 36 3.2 Introduction ………………………………………………………………. 36 3.3 Geological setting …………………………………………………………. 37 3.3.1 Regional geology …………………………………………………. 37 3.3.2 Field relations ……………………………………………………... 40 3.4 Sample description ………………………………………………………… 42 3.4.1 Petrology and mineralogy …………………………………………. 42 3.4.2 Inclusions patterns …………………………………………………. 45 3.4.3 Microstructures and kinematics …………………………………… 46 3.5 Thermodynamic modelling and PT conditions ……………………………. 49 3.6 Discussion …………………………………………………………………. 52 3.7 Conclusions ………………………………………………………………. .. 57 - CHAPTER 4 - Kinematics and geometry of shear zones along the Combin Fault and Dent Blanche
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