Mémoires de Géologie (Lausanne) No. 15, 1993 - I - The Underground of the Western Alps by Robin Marchant IMPRIMERIE CHABLOZ S.A., TOLOCHENAZ ISSN 1015-3578 - II - This eloctronic version is identical to the printed edition, except for some figures which are in color here but in black and white in the printed edition. - I - Université de Lausanne Institut de Géologie Faculté des Sciences et de Paléontologie The Underground of the Western Alps Thèse de doctorat présentée à la Faculté des Sciences de l’Université de Lausanne par Robin Marchant géologue diplômé Jury de thèse: Prof. Gérard Stampfli (Directeur) Prof. Raymond Olivier (Lausanne) Dr. Riccardo Polino (Torino) Prof. Albrecht Steck (Lausanne) Prof. François Thouvenot (Grenoble) Mémoires de Géologie (Lausanne) No. 15, 1993 - II - Cover image (after a photo from the book "CERVIN, une montagne de PUB"; by courtesy of Yvan Hostet- tler): The Matterhorn, the most outstanding mountain of the Western Alps, has puzzled many generations of geologists, of which Emile Argand (see drawing above) was one of the most eminent. This peak is made of gneisses (the series of Arolla and Valpelline) originating in the Austroalpine terrain; they overlie the Piemont accretionary prism (the “Schistes Lustrés” of the Tsaté nappe) and remnants of the Piemont oceanic crust (the ophiolites of Zermatt-Saas). A few km down in the Matter valley, a large backfold affects the Siviez-Mischabel nappe (corresponding to Argand’s “Série de Casanna”), which originates in the Briançonnais exotic terrain. In order to puzzle out the deep structures in this region, the PNR/NFP-20 research programme shot two deep seismic lines at the foot of the Matterhorn: the W3 line runs up the Matter valley, passing through the village of Zermatt, up to Zmutt, where it is relayed by a short perpendicular profile, the W4 line. - III - Table of contents Abstract .................................................................... VI 5.1 General remarks .............................................. 19 5.1.1 Introduction ............................................ 19 Résumé ...................................................................... VI 5.1.2 Previous studies ...................................... 19 5.1.3 Geological setting ................................... 19 PART I - GENERAL REMARKS 5.1.3.1 Introduction .................................... 19 5.1.3.2 Deformation history ....................... 22 1. Introduction ............................................................. 1 5.1.3.3 Description of the main structural 1.1 The present study .............................................. 1 units ............................................... 22 1.2 The PNR/NFP-20 research programme ............ 1 5.1.4 Acquisition and processing .................... 24 1.3 The Ecors and Crop alpine research 5.2 Modelling and interpretation of the W2 programmes ...................................................... 1 profile ............................................................. 24 1.4 The Western Alps: nomenclature ..................... 3 5.2.1 The Vibroseis seismic sections .............. 24 5.2.2 Preliminary interpretation ...................... 27 2. Methodology ........................................................... 5 5.2.3 The “Initial” model ................................ 27 2.1 The modelling concept ..................................... 5 5.2.4 Velocity modelling ................................. 30 2.2 Surface geology data ........................................ 5 5.2.5 The “Final” model .................................. 33 2.3 Seismic tools ..................................................... 6 5.2.6 Interpretation of the W2 line .................. 36 2.4 Geodynamics .................................................... 7 5.3 Interpretation of the W3 profile ...................... 37 5.3.1 Introduction ............................................ 37 3. Computer programs ............................................... 7 5.3.2 Interpretation .......................................... 37 3.1 Introduction ....................................................... 7 5.4 Interpretation of the W4 profile ...................... 41 3.2 Reflection seismic processing programs ........... 7 5.4.1 Introduction ............................................ 41 3.2.1 GEOVECTEUR (CGG™) .................................... 7 5.4.2 Interpretation .......................................... 41 3.2.2 CHARISMA (GECO™) ................................ 7 5.5 The 3-D structural geometry ........................... 44 3.2.3 CIGOGNE ................................................... 10 5.6 A synthetic section of the Penninic nappes .... 56 3.2.3.1 Description ..................................... 10 5.6.1 Construction of the section .................... 47 3.2.3.2 An example with the PNR/NFP- 5.6.2 Nappe correlation ................................... 49 20 W4 line ..................................... 10 5.6.3 Backfolding of the External Crystalline 3.2.3.1 General remarks on migration ........ 12 massifs ................................................... 49 3.3 Seismic and gravity modelling programs ........ 13 5.7 Discussion ...................................................... 50 3.3.1 SIERRA ...................................................... 13 3.3.2 DEUDIM .................................................... 14 6. Modelling and interpretation along the 3.3.3 2MOD (LCT™).......................................... 14 southern half of the E1 profile ............................ 52 6.1 Introduction .................................................... 52 4. Physical properties of rocks from the Western 6.1.1 Previous studies ..................................... 52 Alps........................................................................ 15 6.1.2 Geological setting .................................. 52 4.1 Introduction ..................................................... 15 6.1.3 Acquisition and processing .................... 54 4.2 Densities and P-wave velocities of Western 6.1.4 Preliminary interpretations .................... 55 Alps rocks ............................................................ 15 6.2 3-D seismic models ......................................... 55 4.3 Specific density and velocity problems .......... 16 6.2.1 Introduction ............................................ 55 4.3.1 Mylonitization ........................................ 16 6.2.2 The “Initial” model ................................ 55 4.3.2 Marmorization ........................................ 17 6.2.3 The “South” model ................................ 60 4.3.3 Granulitization/eclogitization ................. 17 6.2.4 The “Final” model ................................. 61 4.3.4 Serpentinization ...................................... 18 6.3 2-D models ..................................................... 64 6.3.1 The front of the Tambo nappe ............... 64 6.3.2 The “Chiavenna” 2-D gravity model .... 67 PART II - MODELLING AND INTERPRETATION 6.4 Interpretation of the southern half of the E1 OF THE NAPPE SYSTEM profile ............................................................. 70 6.4.1. The migrated sections ........................... 70 5. Modelling and interpretation along the W2, W3 6.4.2 Interpretation .......................................... 72 and W4 profiles .................................................... 19 6.5 Discussion ...................................................... 73 - IV - PART III - CRUSTAL- AND LITHOSPHERIC- 8. Lithospheric-scale interpretation ........................ 99 SCALE INTERPRETATION 8.1 Introduction .................................................... 99 8.2. The lithosphere-asthenosphere transition ...... 99 7. Crustal-scale interpretation ................................ 74 8.3 The lithospheric cross-sections ..................... 101 7.1 Introduction .................................................... 74 8.4. Gravity modelling ........................................ 104 7.2 The Ecors-Crop Alp traverse ......................... 75 8.4.1 Introduction .......................................... 104 7.2.1 Introduction ............................................ 75 8.4.2 The gravity models ............................... 104 7.2.2 Interpretation .......................................... 76 8.5 Discussion ..................................................... 111 7.3. The PNR/NFP-20 Western traverse .............. 79 7.3.1 Introduction ............................................ 79 9. Palinspastic reconstructions .............................. 113 7.3.2 General concept ..................................... 79 9.1 Introduction .................................................. 113 7.3.3 The W1-W2 profile ............................... 80 9.2. From rifting to passive-margin stage ........... 113 7.3.4 The W3 profile ...................................... 82 9.3 Oceanic-subduction stage ............................. 115 7.3.5 The W4 profile ...................................... 85 9.4 Continental-collision stage ........................... 115 7.3.6 Synthetic section of the Western 9.4.1 Introduction .......................................... 115 traverse .................................................. 88 9.4.2 The Algero-Provençal phase (30 to 7.4 The PNR/NFP-20 Central traverse ................ 88 15 Ma) ................................................. 117 7.4.1 Introduction ........................................... 88 9.4.3 The Tyrrhenian phase (15 to 0 Ma) ..... 122 7.4.2 Interpretation ........................................