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Titel
The Helvetic thrust sheets of Vorarlberg and Upper Allgäu : thermo-tectonic evolution of an external fold-and-thrust belt at the northern alpine rim / Michael Zerlauth
VerfasserZerlauth, Michael
Begutachter / BegutachterinHerwegh, Marco ; Sachsenhofer, Reinhard
Betreuer / BetreuerinOrtner, Hugo ; Fügenschuh, Bernhard
Erschienen2014
Umfang132 S. : zahlr. Ill., graph. Darst., Kt.
HochschulschriftInnsbruck, Univ., Diss., 2014
Anmerkung
Enth. u.a. 3 Veröff. d. Verf. aus den Jahren 2014
Datum der AbgabeSeptember 2014
SpracheEnglisch
Bibl. ReferenzOeBB
DokumenttypDissertation
Schlagwörter (DE)Austria / Germany / nappe complex / Jurassic basin / Cretaceous facies variations / seismic and well data / very low-grade metamorphism / vitrinite reflectance / apatite fission track dating / thermal modeling / Helveti: units / Penninic Rhenodanubian Flysch units / Subalpine Molasse units / brittle-ductile deformation / porosity / cementation
Schlagwörter (EN)Austria / Germany / nappe complex / Jurassic basin / Cretaceous facies variations / seismic and well data / very low-grade metamorphism / vitrinite reflectance / apatite fission track dating / thermal modeling / Helveti: units / Penninic Rhenodanubian Flysch units / Subalpine Molasse units / brittle-ductile deformation / porosity / cementation
Schlagwörter (GND)Vorarlberg / Oberallgäu / Gefaltete Molasse / Helvetikum / Flysch
URNurn:nbn:at:at-ubi:1-1839 Persistent Identifier (URN)
Zugriffsbeschränkung
 Das Werk ist frei verfügbar
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The Helvetic thrust sheets of Vorarlberg and Upper Allgäu [21.29 mb]
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Zusammenfassung (Deutsch)

The aim of this thesis was to develop a comprehensive tectonic model of the Helvetic zone of Vorarlberg and Upper Allgäu on the one hand, and on the other hand to document the thermo-tectonic evolution of this external Alpine unit in order to provide a basis for eventual future deep hydro-geothermal projects.

We present a refined tectonic model of the Helvetic thrust sheets east of the Rhine River, considering already existing propositions and by combining them with additional surface, well, and seismic data. On the basis of this dataset, the Helvetic zone of Vorarlberg and Upper Allgäu is proposed to consist of two distinct nappes: the Hohenems nappe and the overlying Vorarlberg Säntis nappe. Both encompass Early Jurassic to Eocene shelf sediments detached from the former European margin during Neoalpine orogenesis. Deformation of the Helvetic sequence was controlled by thickness variations and facies differences caused by synsedimentary faulting. Approximately 3 km of stacked Lower to Middle Jurassic mud- and siltstones, preserved within the Vorarlberg Säntis nappe below the Jurassic Kanisfluh anticlinorium, are interpreted to represent the infill of an inverted Jurassic basin, which accounts for arcuate fold axes at map-scale. Assumed transfer faults running along the Rhine and Iller valleys, image the lateral extension of the scraped out basin sediments, as well as of the Hohenems nappe in the footwall. Across these faults, radical changes in deformation style occur. To the east of the Rhine Valley, the Jurassic and the Cretaceous stockwerks remained in one tectonic unit that was detached in the (Lower to) Middle Jurassic basin sediments. To the west of the Rhine Valley, the Helvetic basal thrust runs in the Permian Glarus Verrucano Group. Besides, the Cretaceous units (Swiss Säntis nappe) are tectonically separated from their Jurassic substratum (Gonzen-Walenstadt slices) along the bedding-parallel Säntis thrust. The boundary between these two domains is an interpreted transfer fault in the subsurface of the Rhine Valley. This way, the Helvetic nappes of Vorarlberg/Upper Allgäu can be parallelized in terms of their tectonic position and facies pattern with those of eastern Switzerland, although exhibiting different detachment levels: the Mürtschen nappe below the Gonzen-Walenstadt imbricates is equivalent to the Hohenems nappe. The Swiss Säntis nappe together with the Gonzen-Walenstadt imbricates are in continuity with the Vorarlberg Säntis nappe.

Thermodynamic 1D modeling of Penninic Rhenodanubian Flysch, Helvetic, and Subalpine Molasse units is based on VR and AFT data, and revealed that all of them were exposed to temperatures higher than the upper limit of the APAZ (approximately 120C) during deepest burial, but remained within the (higher) diagenetic stage according to VR values and not or only partially annealed zircon grains. The Penninic Rhenodanubian Flysch units attained peak conditions during the Paleocene. Deepest burial of Helvetic units occured from the Late Eocene to Early Oligocene and of Subalpine Molasse units during the Late Miocene. For the Helvetic zone of Vorarlberg and Upper Allgäu, where most VR analyses were done, it has been shown that maturation increases with the age of the individual stratigraphic members, as well as towards the hinterland. Data from drill core samples yielded varied VR values for the same stratigraphic member exposed in different depth levels. At the same time, depth trends are disturbed along faults. Hence, maturation of the Helvetic units is proposed to be syn- to posttectonic. Helvetic burial models argue for approximately 4.7 km of overburden in the north to approximately 6.7 km in the south, whereas Northalpine Foreland Basin sediments contributed to this load in both areas. From the Middle Miocene onwards, the entire nappe stack was eroded, resulting in a late-stage exhumation of approximately 1.5-2 km from the Miocene/Pliocene on.

Microtectonic structures documenting the brittle-ductile to fully brittle deformation of nappe-scale faults, namely the Jaghausen thrust fault and the Ostergunten transfer fault system, exhibit a clear dominance of a NNW-SSE to N-S oriented compressive stress field, which is related to tectonic transport of the Helvetic nappes of Vorarlberg and Upper Allgäu. Orientation of subordinate stress fields is clearly controlled by folding, documented by an almost constant geometrical relation between microtectonic structures and fold axes. The two most abundant fracture sets are perpendicular or parallel to the respective fold axial surface trace. Furthermore, there are frequently conjugate fractures at high as well as at low angle to the latter. These are probably shear fractures, whereas the former are interpreted to represent (ex)tension joints. No correlation to different structural domains of folds (limbs or hinge zones) or fold geometry is given, which might argue for fracture formation at an early stage of buckle-folding. Subsequent faulting is responsible for local variations of the fracture pattern. In homogeneously deformed areas far from faults statistics have shown that fracture frequency, normal spacing, and height can be controlled by layer thickness of the well-bedded Helvetic limestone-marl sequences. Rock texture plays a minor part. Faulting can increase the degree of fracturing at least threefold. Fracture porosity estimations, based on fracture area and fracture aperture, reveal low values of around 3 %.

Stabile O-C isotope analyses of fracture cements and host rock showed that they are similar, indicating local pressure solution-precipitation as the main process controlling cementation. Scattered differences are interpreted to result from mixing with an external fluid from the footwall. Th values obtained from liquid-rich two-phase FI in calcite and quartz cements are in agreement with results of burial models with VR and AFT data as referencing parameters. Attention has to be paid due to the observed stretching of FI and a necessary pressure correction, which could not be applied. However, the majority of stable O-C isotope and FI data suggests an early onset of fracture cementation before deepest burial of the Helvetic units.

Zusammenfassung (Englisch)

The aim of this thesis was to develop a comprehensive tectonic model of the Helvetic zone of Vorarlberg and Upper Allgäu on the one hand, and on the other hand to document the thermo-tectonic evolution of this external Alpine unit in order to provide a basis for eventual future deep hydro-geothermal projects.

We present a refined tectonic model of the Helvetic thrust sheets east of the Rhine River, considering already existing propositions and by combining them with additional surface, well, and seismic data. On the basis of this dataset, the Helvetic zone of Vorarlberg and Upper Allgäu is proposed to consist of two distinct nappes: the Hohenems nappe and the overlying Vorarlberg Säntis nappe. Both encompass Early Jurassic to Eocene shelf sediments detached from the former European margin during Neoalpine orogenesis. Deformation of the Helvetic sequence was controlled by thickness variations and facies differences caused by synsedimentary faulting. Approximately 3 km of stacked Lower to Middle Jurassic mud- and siltstones, preserved within the Vorarlberg Säntis nappe below the Jurassic Kanisfluh anticlinorium, are interpreted to represent the infill of an inverted Jurassic basin, which accounts for arcuate fold axes at map-scale. Assumed transfer faults running along the Rhine and Iller valleys, image the lateral extension of the scraped out basin sediments, as well as of the Hohenems nappe in the footwall. Across these faults, radical changes in deformation style occur. To the east of the Rhine Valley, the Jurassic and the Cretaceous stockwerks remained in one tectonic unit that was detached in the (Lower to) Middle Jurassic basin sediments. To the west of the Rhine Valley, the Helvetic basal thrust runs in the Permian Glarus Verrucano Group. Besides, the Cretaceous units (Swiss Säntis nappe) are tectonically separated from their Jurassic substratum (Gonzen-Walenstadt slices) along the bedding-parallel Säntis thrust. The boundary between these two domains is an interpreted transfer fault in the subsurface of the Rhine Valley. This way, the Helvetic nappes of Vorarlberg/Upper Allgäu can be parallelized in terms of their tectonic position and facies pattern with those of eastern Switzerland, although exhibiting different detachment levels: the Mürtschen nappe below the Gonzen-Walenstadt imbricates is equivalent to the Hohenems nappe. The Swiss Säntis nappe together with the Gonzen-Walenstadt imbricates are in continuity with the Vorarlberg Säntis nappe.

Thermodynamic 1D modeling of Penninic Rhenodanubian Flysch, Helvetic, and Subalpine Molasse units is based on VR and AFT data, and revealed that all of them were exposed to temperatures higher than the upper limit of the APAZ (approximately 120C) during deepest burial, but remained within the (higher) diagenetic stage according to VR values and not or only partially annealed zircon grains. The Penninic Rhenodanubian Flysch units attained peak conditions during the Paleocene. Deepest burial of Helvetic units occured from the Late Eocene to Early Oligocene and of Subalpine Molasse units during the Late Miocene. For the Helvetic zone of Vorarlberg and Upper Allgäu, where most VR analyses were done, it has been shown that maturation increases with the age of the individual stratigraphic members, as well as towards the hinterland. Data from drill core samples yielded varied VR values for the same stratigraphic member exposed in different depth levels. At the same time, depth trends are disturbed along faults. Hence, maturation of the Helvetic units is proposed to be syn- to posttectonic. Helvetic burial models argue for approximately 4.7 km of overburden in the north to approximately 6.7 km in the south, whereas Northalpine Foreland Basin sediments contributed to this load in both areas. From the Middle Miocene onwards, the entire nappe stack was eroded, resulting in a late-stage exhumation of approximately 1.5-2 km from the Miocene/Pliocene on.

Microtectonic structures documenting the brittle-ductile to fully brittle deformation of nappe-scale faults, namely the Jaghausen thrust fault and the Ostergunten transfer fault system, exhibit a clear dominance of a NNW-SSE to N-S oriented compressive stress field, which is related to tectonic transport of the Helvetic nappes of Vorarlberg and Upper Allgäu. Orientation of subordinate stress fields is clearly controlled by folding, documented by an almost constant geometrical relation between microtectonic structures and fold axes. The two most abundant fracture sets are perpendicular or parallel to the respective fold axial surface trace. Furthermore, there are frequently conjugate fractures at high as well as at low angle to the latter. These are probably shear fractures, whereas the former are interpreted to represent (ex)tension joints. No correlation to different structural domains of folds (limbs or hinge zones) or fold geometry is given, which might argue for fracture formation at an early stage of buckle-folding. Subsequent faulting is responsible for local variations of the fracture pattern. In homogeneously deformed areas far from faults statistics have shown that fracture frequency, normal spacing, and height can be controlled by layer thickness of the well-bedded Helvetic limestone-marl sequences. Rock texture plays a minor part. Faulting can increase the degree of fracturing at least threefold. Fracture porosity estimations, based on fracture area and fracture aperture, reveal low values of around 3 %.

Stabile O-C isotope analyses of fracture cements and host rock showed that they are similar, indicating local pressure solution-precipitation as the main process controlling cementation. Scattered differences are interpreted to result from mixing with an external fluid from the footwall. Th values obtained from liquid-rich two-phase FI in calcite and quartz cements are in agreement with results of burial models with VR and AFT data as referencing parameters. Attention has to be paid due to the observed stretching of FI and a necessary pressure correction, which could not be applied. However, the majority of stable O-C isotope and FI data suggests an early onset of fracture cementation before deepest burial of the Helvetic units.