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Towards a self-consistent dynamic orogenic wedge model for the Western Alps

English title Towards a self-consistent dynamic orogenic wedge model for the Western Alps
Applicant Schmalholz Stefan Markus
Number 163169
Funding scheme Project funding (Div. I-III)
Research institution Institut des sciences de la Terre Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geology
Start/End 01.11.2015 - 31.10.2020
Approved amount 301'810.00
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All Disciplines (2)

Discipline
Geology
Geophysics

Keywords (6)

Orogenic wedge; Alpine orogeny; Numerical modeling; Tectonic nappes; Mountain building processes; Western Alps

Lay Summary (German)

Lead
Die Europäischen und Adriatischen Bereiche waren vor der Alpenbildung womöglich nicht durch eine ozeanische Platte getrennt, aber es gab zwischen beiden Bereichen eine extrem ausgedünnte kontinentale Platte. Das Ziel ist, ein thermo-mechanisches Modell der Alpenbildung für dieses Szenario zu entwickeln, welches vom derzeitigen Modell abweicht, aber die vorhanden Daten besser erklären könnte.
Lay summary
Es gibt zwei vor-Alpine Szenarien: (1) Die Europäischen und Adriatische Platten waren durch eine neu-gebildete ozeanische Platte getrennt. Die treibende Kraft der Gebirgsbildung war deshalb der Zug der schweren ozeanischen Platte, die in den Erdmantel sank. (2) Die Europäischen und Adriatischen Bereiche waren nicht durch eine ozeanische Platte getrennt, aber es gab zwischen beiden Bereichen eine extrem ausgedünnte, weil gestreckte, Platte. Durch die Streckung wurde die kontinentale Kruste teilweise komplett entfernt und der kontinentale Mantle kam direkt an den Meeresgrund. Die treibende Kraft der Gebirgsbildung war dann eine horizontale kompressive Kraft. Wir untersuchen die Bildung eines Deckengebirges für Szenario (2) mit Computersimulationen. Wir quantifizieren die mechanischen Spannungen, welche bei der Gebirgsbildung wirkten, und die damit verbundenen Temperaturerhöhungen, die durch Reibungsenergie erzeugt wurden. Die Resultate werden mit der Deckengeometrie und den maximalen Druck- und Temperaturwerten, welche mit petrologischen Methoden bestimmt wurden, verglichen. Wir wollen ein alpen-ähnliches Deckengebirge erzeugen, dass durch die Verkürzung eines Meeresbeckens erzeugt wird, welches Szenario (2) entspricht.     

Dieses Projekt kann ein Modell der Alpenbildung erzeugen, welches stark vom derzeitig angenommenen Modell abweicht, aber die vorhanden Daten besser erklärt.

Direct link to Lay Summary Last update: 01.10.2015

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
149380 Towards understanding the dynamics of tectonic nappe stacking and overthrusting in the Helvetic nappe system 01.11.2013 Project funding (Div. I-III)
144250 Towards understanding the tectonic evolution from magma-poor rifted margins to the Alpine orogen: Insights from 2D and 3D numerical modeling 01.11.2012 Project funding (Div. I-III)

Abstract

The understanding of the geodynamic evolution of the Western Alps and of the pre-Alpine paleogeography is still incomplete. There are two end-member paleogeographic scenarios: (1) The European and Adriatic continental plates have been separated by oceanic lithospheres which were newly created during Mesozoic sea floor spreading (the classical scenario). For this scenario, the main driving force of the Alpine orogeny is slab pull and rollback due to subduction of the oceanic lithospheres. (2) The European and Adriatic domains have been separated by hyper-extended margins and exhumed sub-continental mantle, and no significant oceanic lithosphere was created (the new scenario). The main driving force for the Alpine orogeny is hence an external compressive tectonic force. The tectono-metamorphic evolution of the Western Alps has been frequently described with the concept of an orogenic wedge. However, the current orogenic wedge models assume that stresses are close to the lithostatic pressure and hence these wedge models cannot explain Alpine nappes having peak-pressure estimates larger than about 1.5 GPa (corresponds to a “lithostatic” depth of approximately 55 km). We propose to perform two-dimensional (2-D) numerical simulations to study the formation of an orogenic wedge in a self-consistent way. Self-consistent means that the shear zone above which the orogenic wedge can form is not a priori defined in the model configuration but develops self-consistently due to thermal softening caused by shear heating. The initial model configuration will mimic the paleogeographic scenario (2) of hyper-extended margins with no oceanic lithosphere in order to test whether models starting from this new scenario are able to generate an orogenic wedge similar to the Alpine orogenic wedge. The applied numerical algorithm considers a viscoelastoplastic rheology, gravity, a free surface, heat transfer, erosion and thermo-mechanical coupling by shear heating and temperature dependent viscosities. In a first part of the project the necessary thermo-mechanical conditions for the formation of an orogenic wedge will be determined. In this part also the impact of mechanical strengthening due to metamorphic reactions will be analysed. Strengthening will be modelled in a simplified way by increasing the effective viscosities of numerical cells when their temperature and pressure exceeds certain values mimicking a prograde reaction. In the second part the impact of crustal heterogeneities on orogenic wedge formation will be quantified. These mechanical heterogeneities mimic different sedimentary layers or granitic intrusions. Geometrical variations of the layer interfaces mimic pre-Alpine half-graben structures. A particular aim is to understand the impact of such heterogeneities on the nappe formation and underplating at the base of the wedge. In the last part, the numerical results will be compared with available geological, petrological and geophysical data (e.g. geological cross sections, gravity data). Particularly, the spatial and temporal evolution of dynamic stresses and temperature will be quantified and compared with pressure and temperature estimates from metamorphic rocks. A main aim is to perform a numerical simulation which starts from a hyper-extended margin scenario, generates self-consistently an orogenic wedge, and explains the first-order tectono-metamorphic features of the Western Alps. We request one year of funding for one student to finish an ongoing PhD thesis, and we request another two years for a new student to start a new PhD thesis on the self-consistent modelling of dynamic orogenic wedges applied to the Western Alps. The results of this project will improve our understanding of mountain building processes in general and of the geodynamic evolution of the Western Alps in particular.
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