Geodynamics of the Alps 2

Claudio L. Rosenberg, ISTeP, Sorbonne Université, France. Nicolas Bellahsen, ISTeP, Sorbonne Université, France.

Introduction xi
Nicolas BELLAHSEN and Claudio L. ROSENBERG

Chapter 1. Paleozoic Evolution and Variscan Inheritance in the Alps 1
Jean-Baptiste JACOB, Stéphane GUILLOT, Denis THIÉBLEMONT, Jonas VANARDOIS, Pierre TRAP, Michel FAURE and Didier MARQUER

1.1. Introduction 2

1.2. The Paleozoic setting in Europe 3

1.2.1. General setting: a crustal basement structured by Paleozoic orogenies 3

1.2.2. Pre-Variscan history of the European basement 5

1.2.3. The Variscan orogeny in Europe 6

1.3. The basement outcrops in the Alps 10

1.4. Paleozoic evolution in the Alpine basement 13

1.4.1. Late Proterozoic evolution: an active margin setting along northern Gondwana 13

1.4.2. Cambrian-Ordovician extension and opening of eastern Rheic Ocean 14

1.4.3. Ordovician to Silurian crustal extension 18

1.4.4. Devonian-Carboniferous convergence and Variscan collision 20

1.4.5. Crustal extension and post-orogenic collapse 23

1.5. Discussion 25

1.5.1. Place of the Alpine domain in the Variscan puzzle 25

1.5.2. Pre-Variscan position of the Alpine basement units 28

1.5.3. Significance of Variscan high-pressure metamorphism in the Alps 29

1.6. Conclusion 31

1.7. Acknowledgments 32

1.8. References 32

Chapter 2. Paleogeography and Architecture of the Alpine Tethys Margins 45
Yves LAGABRIELLE

2.1. Introduction: conjugate passive margins 45

2.2. The contours and the internal division of the Adria plate 48

2.2.1. Situation in the Triassic (250-200 Ma) 48

2.2.2. Opening of the Piemont-Liguria Ocean (200-145 Ma) 51

2.2.3. Fragmentation of the Adria plate 54

2.3. The Alpine passive paleomargin of Europe 54

2.3.1. General information 54

2.3.2. The Delphino-Helvetic domain: the external (or proximal) margin 58

2.3.3. The Briançonnais domain: the internal margin 67

2.3.4. The internal crystalline massifs and their detached covers (Briançonnais domain in the broad sense) 79

2.3.5. The Piedmont zone: from the continental margin to the ocean 85

2.3.6. The Briançonnais and pre-Piedmont of the Chablais-Romande Prealps and of the Valais. 89

2.3.7. The Valais domain 94

2.3.8. The Maritime and Ligurian Alps 101

2.4. The Alpine passive paleo-margin of the Adria plate 102

2.4.1. Introduction 102

2.4.2. The Central Alps 104

2.4.3. The Eastern Alps 108

2.4.4. The Southern Alps 115

2.5. Architecture of the Alpine paleomargins: from the model of tilted blocks to the concept of crustal hyper-thinning 121

2.6. By way of conclusion, the contours of the Piedmont-Ligurian oceanic paleo-domain: how many passive continental margins have been inverted in the Alpine belt? 134

2.7. Acknowledgments 137

2.8. References 138

Chapter 3. One View on the Petrology and Geochemistry of the Mantle and Oceanic Crustal Remnants in the Alps, Reconciling Rifts and Ultra-slow Seafloor Spreading 157
Othmar MÜNTENER and Anders MCCARTHY

3.1. Introduction 157

3.2. Geological constraints on the Alpine Tethys seafloor 158

3.3. Alpine-Apennine peridotites: fingerprints of (ancient) melting and syn-rift re-fertilization 161

3.3.1. Three main groups of peridotites 161

3.3.2. Bulk rock chemistry: fertile versus re-fertilized 163

3.3.3. Mineral chemistry: inherited versus re-fertilized 165

3.4. Gabbroic rocks: small volumes of crystallization - differentiation, mostly in the mantle 171

3.5. Basalts: records of low-degree melting of garnet-bearing sources 175

3.6. Timing of magmatism of Alpine-Apennine ophiolites: a short-lived event 181

3.7. A brief overview of isotopic (dis)equilibrium between magmatic and mantle rocks 186

3.8. Conclusion and outlook 189

3.9. Acknowledgments 191

3.10. References 192

Chapter 4. Oceanic Subduction in the Alps: Lessons from the Rock Record 205
Philippe AGARD

4.1. Introduction: how important is "oceanic" subduction in the Alps? 205

4.2. Petrology and geodynamics 206

4.2.1. Remnants of subducted oceanic lithosphere 207

4.2.2. Geodynamic setting and boundary conditions for oceanic subduction 210

4.2.3. Material types: ultramafic, mafic and sedimentary 211

4.3. Subducted oceanic fragments scattered throughout the Alps 214

4.3.1. Metasedimentary-dominated (S) units from the Western Alps 214

4.3.2. Mafic/ultramafic dominated (MUM units from the Western Alps) 215

4.3.3. Subducted fragments from the Central and Eastern Alps 218

4.3.4. Comparing restorations at ~60 Ma 219

4.4. Discussion: subduction dynamics through space and time 223

4.4.1. Contrasting recovery through space and time 223

4.4.2. Four distinct sectors: paleogeographic contrasts in subduction dynamics 225

4.4.3. A fragmentary yet faithful (rock) record of subduction 228

4.4.4. What the Alpine example typifies 230

4.5. Conclusion 232

4.6. Acknowledgments 233

4.7. References 234

Chapter 5. Continental Subduction in the Alps: From Field Data to Kinematic Models 255
Paola MANZOTTI and Michel BALLÈVRE

5.1. Historical background and main questions 255

5.2. A reference P-T space 258

5.3. A reference map and cross-section of the Alps 260

5.3.1. Large-scale overview of the Alpine belt 260

5.3.2. Large-scale structure of the Western Alps 263

5.4. Subduction of the Mesozoic cover 266

5.4.1. Petrological evidence for subduction metamorphism in the Mesozoic covers 266

5.4.2. Structural considerations 267

5.5. Subduction of the Paleozoic basement 268

5.5.1. The pre-Alpine crust of the Adriatic plate 269

5.5.2. The pre-Alpine crust of the Briançonnais domain 277

5.5.3. Tectonic implications for building the Alpine stack 277

5.6. Deformation history of the subducted crust in the Alps 280

5.6.1. Evidence for heterogeneous deformation during subduction 280

5.6.2. Rheological behavior of the subducted continental crust 285

5.7. Metamorphic history of the subducted crust: P-T paths and timing 290

5.7.1. Incompleteness of the rock record 290

5.7.2. Types of P-T paths of the subducted crust in the Alps 292

5.7.3. Metamorphic gradient at the scale of the Alpine belt 296

5.7.4. Timing of the continental subduction in the Alps 300

5.7.5. The exhumation record in syn- to post-orogenic sediments 303

5.8. A dynamic view of continental subduction in the Western Alps 306

5.8.1. Kinematics of the subduction and collision 306

5.8.2. Exhumation mechanisms of UHP-HP rocks 310

5.9. Prospects for the future 312

5.10. Acknowledgments 313

5.11. References 313

List of Authors 341

Index 343

Summaries of other volumes 345