Structural investigations of the Western Swiss molasse basin : from 2D seismic interpretation to a 3D geological model
DOKPE
- Gruber, Marius University of Fribourg
- Mosar, Jon (Degree supervisor) ORCID University of Fribourg
- 2017 : Département de géosciences, sciences de la terre, Université de Fribourg ; Fribourg (Suisse)
1 ressource en ligne (190 pages) ; 1 fichier pdf
PhD: Université de Fribourg (Suisse), 2017
English
The Swiss Molasse Basin is located between the Jura Mountains and the Alps and extends from Lake Geneva in the SW to Lake Constance in the NE. The Swiss Molasse Basin is an asymmetric foreland basin filled with detrital erosional material of Oligocene to Miocene age from the adjacent Alpine orogeny. During the Late Miocene, the western Swiss Molasse Basin (WSMB) evolved from a flexural foreland basin to a wedge-top basin due to the detachment of the Mesozoic and Cenozoic sedimentary cover above the mechanically rigid basement during the formation of the Jura Fold-and-Thrust-Belt.
The present thesis’ object is to characterize the structural and kinematic configuration and the tectonic evolution of the WSMB in the area of Canton Fribourg, Canton Bern and Canton Vaud. Based on the interpretation of more than 200 seismic reflection profiles (from oil industry, total length: 2890 km) calibrated with more than 30 wells and combined with surface data, a new 3D geological model is elaborated. This model includes eight 3D depth grids (representing the stratigraphic boundaries of the near Base Cenozoic, near Top Upper Malm, near Top Lower Malm, near Top Dogger, near Top Liassic, near Top Keuper, near Top Muschelkalk and near Base Mesozoic) and 3D fault surfaces in the pre-Mesozoic basement, in the Mesozoic and in the Cenozoic stratigraphic units. This new 3D geological model gives insight into the structural setting of the WSMB down to a depth of approximately 5000 m below sea level.
Twenty enclosures illustrate the structural setting of the studied area: a seismic location map (Encl. 01), a cross-section from the Jura Fold-and-Thrust- Belt across the Molasse Basin into the Préalpes Klippen (Encl. 02), a tectonic map at surface (Encl. 03), two-way travel time maps (Encl. 04), seismic velocity maps (Encl. 05), depth maps of stratigraphic boundaries (Encl. 06 – 13), seismic transects (or seismic cross-sections, Encl. 14 – 19) and vertical thickness maps of stratigraphic units (Encl. 20).
The present study in the WSMB reveals different structural elements which developed during several extensional phases since the Late Paleozoic:
- The formation of Permo-Carboniferous basins during a Late Paleozoic extension event was associated with faulting in the pre-Mesozoic basement. Seismically reflective zones in the pre-Mesozoic basement suggest the existence of Permo-Carboniferous sediments which is locally proven by wells.
- Thickness variations of the Dogger unit across individual fault zone (being most pronounced across the northern part of the N-S trending Fribourg Fault Zone) constrain a period of mild extension and synsedimentary normal faulting during the Middle Jurassic which correlates with a major extension phase in central Europe.
- A NE-SW oriented basin parallel normal fault system can be associated with a flexural extension phase due to the down-bending of the European lithosphere during Late Eocene to Oligocene times.
During the Late Miocene main compression event, the Mesozoic and Cenozoic sedimentary cover of the WSMB was detached along a basal décollement zone located in Triassic evaporite-rich units and transported towards the NW. Favourably oriented inherited normal faults in the detached Mesozoic and Cenozoic sedimentary cover were reactivated as strike-slip faults (e.g. the Fribourg Fault Zone) and possibly also as reverse faults in an overall NW-SE oriented compressional stress field. The deformational style in the sedimentary cover is characterized by frontal and lateral ramps as well as pop-up structures (e.g. Hermrigen and Tschugg Fault Zones), reverse faults and low amplitude folds (e.g. Courtion Fault Zone), en-echelon type strike-slip faults (e.g. Fribourg Fault Zone), a conjugated strike-slip fault system (near the city of Yverdon-les-Bains) and kilometric scale strikeslip faults running from the Jura Fold-and-Thrust-Belt into the Molasse Basin (e.g. La Lance, La Sarraz and Pontarlier Fault Zones). These structural elements are confined to the detached Mesozoic and Cenozoic sedimentary cover and faults most likely root in the basal décollement zone.
The basal décollement zone is characterized by vertical and lateral partitioning of deformation. Deformation related to the basal décollement zone (thrusting, duplexing, halokinetic movements and lateral migration of evaporite-rich units) is dominantly localized in the Muschelkalk unit (Zeglingen Formation) E of a line Vevey - Moudon – Estavayer-le-Lac and in the Keuper unit (Klettgau and Bänkerjoch Formations) W of this line.
The formation of a shallower décollement zone in the lowermost Cenozoic unit led to the development of a thrust fault system in the Subalpine Molasse reflecting successive incorporation of Molasse sediments into the Alpine wedge. The Folded Molasse, a belt of thrust related, asymmetric folds located NW of the Subalpine Molasse Frontal Thrust, suggests a north-westward propagation of the Subalpine Molasse décollement zone towards the Plateau Molasse. Back-thrusting in the Folded Molasse leads to the formation of broad triangle zone in the Linden – Thun area.
Thick-skinned basement involved reverse faults can be seen along the Savigny – Châtel-Saint-Denis – Bulle Fault Zone (E of the city of Lausanne below the Subalpine Molasse imbricates) and are less pronounced near the cities of Thun and Yverdon-les- Bains. Reverse faulting in the mechanical basement occurs along inherited Late Paleozoic normal faults and may be associated with the development of new crustal imbricates, in a recent attempt of the Alpine wedge to regain stability. Ongoing tectonic activity in the WSMB, documented based on instrumentally recorded earthquakes, is related to an overall NWSE oriented maximum horizontal stress. Earthquake clusters along the Fribourg Fault Zone reveal ongoing strike-slip faulting in the detached Mesozoic and Cenozoic sedimentary cover.
Overall it was possible to describe a much more complex structural pattern than hitherto known. Strain distribution is strongly heterogeneous in a vertical profile, but also laterally, leading to a variety of structural patterns. The development of these structures in a wedge-shaped detached foreland close to criticality is rendered even more complex due to the reactivation of numerous inherited faults.
The present thesis’ object is to characterize the structural and kinematic configuration and the tectonic evolution of the WSMB in the area of Canton Fribourg, Canton Bern and Canton Vaud. Based on the interpretation of more than 200 seismic reflection profiles (from oil industry, total length: 2890 km) calibrated with more than 30 wells and combined with surface data, a new 3D geological model is elaborated. This model includes eight 3D depth grids (representing the stratigraphic boundaries of the near Base Cenozoic, near Top Upper Malm, near Top Lower Malm, near Top Dogger, near Top Liassic, near Top Keuper, near Top Muschelkalk and near Base Mesozoic) and 3D fault surfaces in the pre-Mesozoic basement, in the Mesozoic and in the Cenozoic stratigraphic units. This new 3D geological model gives insight into the structural setting of the WSMB down to a depth of approximately 5000 m below sea level.
Twenty enclosures illustrate the structural setting of the studied area: a seismic location map (Encl. 01), a cross-section from the Jura Fold-and-Thrust- Belt across the Molasse Basin into the Préalpes Klippen (Encl. 02), a tectonic map at surface (Encl. 03), two-way travel time maps (Encl. 04), seismic velocity maps (Encl. 05), depth maps of stratigraphic boundaries (Encl. 06 – 13), seismic transects (or seismic cross-sections, Encl. 14 – 19) and vertical thickness maps of stratigraphic units (Encl. 20).
The present study in the WSMB reveals different structural elements which developed during several extensional phases since the Late Paleozoic:
- The formation of Permo-Carboniferous basins during a Late Paleozoic extension event was associated with faulting in the pre-Mesozoic basement. Seismically reflective zones in the pre-Mesozoic basement suggest the existence of Permo-Carboniferous sediments which is locally proven by wells.
- Thickness variations of the Dogger unit across individual fault zone (being most pronounced across the northern part of the N-S trending Fribourg Fault Zone) constrain a period of mild extension and synsedimentary normal faulting during the Middle Jurassic which correlates with a major extension phase in central Europe.
- A NE-SW oriented basin parallel normal fault system can be associated with a flexural extension phase due to the down-bending of the European lithosphere during Late Eocene to Oligocene times.
During the Late Miocene main compression event, the Mesozoic and Cenozoic sedimentary cover of the WSMB was detached along a basal décollement zone located in Triassic evaporite-rich units and transported towards the NW. Favourably oriented inherited normal faults in the detached Mesozoic and Cenozoic sedimentary cover were reactivated as strike-slip faults (e.g. the Fribourg Fault Zone) and possibly also as reverse faults in an overall NW-SE oriented compressional stress field. The deformational style in the sedimentary cover is characterized by frontal and lateral ramps as well as pop-up structures (e.g. Hermrigen and Tschugg Fault Zones), reverse faults and low amplitude folds (e.g. Courtion Fault Zone), en-echelon type strike-slip faults (e.g. Fribourg Fault Zone), a conjugated strike-slip fault system (near the city of Yverdon-les-Bains) and kilometric scale strikeslip faults running from the Jura Fold-and-Thrust-Belt into the Molasse Basin (e.g. La Lance, La Sarraz and Pontarlier Fault Zones). These structural elements are confined to the detached Mesozoic and Cenozoic sedimentary cover and faults most likely root in the basal décollement zone.
The basal décollement zone is characterized by vertical and lateral partitioning of deformation. Deformation related to the basal décollement zone (thrusting, duplexing, halokinetic movements and lateral migration of evaporite-rich units) is dominantly localized in the Muschelkalk unit (Zeglingen Formation) E of a line Vevey - Moudon – Estavayer-le-Lac and in the Keuper unit (Klettgau and Bänkerjoch Formations) W of this line.
The formation of a shallower décollement zone in the lowermost Cenozoic unit led to the development of a thrust fault system in the Subalpine Molasse reflecting successive incorporation of Molasse sediments into the Alpine wedge. The Folded Molasse, a belt of thrust related, asymmetric folds located NW of the Subalpine Molasse Frontal Thrust, suggests a north-westward propagation of the Subalpine Molasse décollement zone towards the Plateau Molasse. Back-thrusting in the Folded Molasse leads to the formation of broad triangle zone in the Linden – Thun area.
Thick-skinned basement involved reverse faults can be seen along the Savigny – Châtel-Saint-Denis – Bulle Fault Zone (E of the city of Lausanne below the Subalpine Molasse imbricates) and are less pronounced near the cities of Thun and Yverdon-les- Bains. Reverse faulting in the mechanical basement occurs along inherited Late Paleozoic normal faults and may be associated with the development of new crustal imbricates, in a recent attempt of the Alpine wedge to regain stability. Ongoing tectonic activity in the WSMB, documented based on instrumentally recorded earthquakes, is related to an overall NWSE oriented maximum horizontal stress. Earthquake clusters along the Fribourg Fault Zone reveal ongoing strike-slip faulting in the detached Mesozoic and Cenozoic sedimentary cover.
Overall it was possible to describe a much more complex structural pattern than hitherto known. Strain distribution is strongly heterogeneous in a vertical profile, but also laterally, leading to a variety of structural patterns. The development of these structures in a wedge-shaped detached foreland close to criticality is rendered even more complex due to the reactivation of numerous inherited faults.
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- Faculté des sciences et de médecine
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- Département des Géosciences
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- English
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- Earth sciences
- Series statement
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- GeoFocus ; 41
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CC BY
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- https://folia.unifr.ch/unifr/documents/332637
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