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Kinetic versus mechanical control of petrological pressure record of the Monte Rosa nappe: a combined geomechanical and petrological approach

English title Kinetic versus mechanical control of petrological pressure record of the Monte Rosa nappe: a combined geomechanical and petrological approach
Applicant Schmalholz Stefan Markus
Number 165756
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 01.04.2016 - 31.08.2020
Approved amount 446'797.00
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All Disciplines (3)

Discipline
Geology
Geochemistry
Mineralogy

Keywords (7)

peak pressure ; kinetics; stress variations; Monte Rosa nappe; heterogeneity; petrology; tectonics

Lay Summary (German)

Lead
Die derzeitigen petrologischen Abschätzungen für den Alpinen Maximaldruck in der Monte Rosa Decke liegen sehr weit auseinander (12 bis 26 kilobar). Die entsprechenden maximalen Subduktionstiefen der Monte Rosa Gesteine variieren deshalb zwischen 40 und 90 km. Aufgrund dieser Unsicherheit in der Druck- und Tiefenabschätzung gibt es zur Zeit kein verlässliches Modell für die tektonische Entwicklung der Monte Rosa Decke und der Westschweizer Alpen.
Lay summary

Unser übergeordnetes Ziel ist, die Ursachen für die unterschiedlichen Druckabschätzungen zu erklären. Drei mögliche Ursachen werden untersucht: (i) der Druck in der Monte Rosa Decke war homogen aber unterschiedliche Gesteine haben unterschiedliche petrologische Drucke aufgrund von kinetischen Effekten aufgezeichnet, (ii) die Druckunterschiede lassen sich auf unterschiedliche petrologische Methoden und Datenbanken zurückführen, und (iii) die Gesteine der Monte Rosa Decke waren mechanisch heterogen (festere und weichere Einheiten) und deshalb war auch der Druck heterogen. Mit Hilfe von petrologischen und strukturellen Feldkartierungen werden die Gradienten in den metamorphen Bedingungen kartiert und quantifiziert. Mit petrologischen Methoden wird untersucht ob unterschiedliche metamorphe Bedingungen mit kinetischen Effekten (z.B. Wasseraktivität) erklärt werden können. Mit geomechanischen numerischen Modellen werden mögliche Druckunterschiede aufgrund Unterschieden in der mechanischen Härte untersucht.    

Unsere Arbeit wird neue und wichtige Informationen über den Alpinen Maximaldruck und die Druckverteilung in der Monte Rosa Decke generieren. Die Ergebnisse werden ein genaueres und verlässlicheres tektonisches Modell für die Westschweizer Alpen ermöglichen.

Direct link to Lay Summary Last update: 29.03.2016

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
172513 Chlorine, fluorine, and water in contact metamorphic environments 01.04.2017 Project funding (Div. I-III)

Abstract

The tectonic and petrologic evolution of the Monte Rosa nappe, a major basement nappe in the Swiss/Italian Alps, is still incompletely understood. A main reason for this lack of understanding is that estimates of the Alpine peak pressure for the Monte Rosa nappe vary significantly, and published values range between 1.2 and 2.7 GPa. If these peak pressures are used to estimate the maximum burial depth of the Monte Rosa nappe, by assuming a lithostatic pressure, then the maximal burial depth ranges between approximately 40 and 90 km. Burial and exhumation from 40 km depth can be explained with orogenic wedge models, but for the exhumation from 90 km contrasting models are currently discussed. The peak pressures for the Monte Rosa nappe have been determined for different rock types, such as metamafics, pelites or whiteschists, and with different methods, such as geo-thermo-barometry or pseudosection modelling. There are three end-member possibilities to explain the peak pressure differences: (1) the Monte Rosa nappe experienced homogeneous metamorphic conditions, but different rock types recorded different thermodynamic pressures due to kinetic effects, for example, some rocks were so dry that certain prograde reactions did not take place. (2) The differences are due to different methods used to evaluate pressure and fluid conditions, or due to differences in thermodynamic data bases. (3) The Monte Rosa nappe was mechanically heterogeneous and experienced a heterogeneous stress so that the different peak pressure estimates are due to local stress deviations from the lithostatic pressure. The main aims of this project are (1) to map and quantify regional gradients of metamorphic conditions in the Monte Rosa nappe, (2) to investigate, with petrologic and petrographic tools, whether metamorphic gradients are due to a variation in chemical kinetic response, (3) to quantify potential stress variations in a mechanically heterogeneous Monte Rosa nappe with two-dimensional thermo-mechanical numerical simulations, (4) and finally, to compare petrologically-observed with mechanically-calculated pressure variations, and to provide a peak pressure estimate which has a regional significance providing a robust estimate for the maximum burial of Monte Rosa nappe. Chemical kinetic effects are studied by investigating equilibrium domains in mineral assemblages observed in the granite within individual thin sections. Water activity gradients will be assessed by H-concentration in biotite and phengite using secondary ion microprobe analytics. Strength contrast estimations for the Monte Rosa nappe will be based on a fold shape analysis of dikes which crosscut the Monte Rosa granite and the surrounding Schieferhülle. The dikes are folded only in the Schieferhülle but not in the granite indicating a significantly higher strength of the granite. The simulations will be performed with an existing finite element algorithm tailored to model lithospheric deformation for compressible and incompressible material, with viscoelastoplastic rheologies, gravity and thermo-mechanical coupling. Four mechanical model domains of the Monte Rosa nappe will be considered: strong granites / dikes and weak Schieferhülle / whiteschists lenses. Small lenses and dikes can be numerically resolved with the applied unstructured triangular finite element mesh. Three loading scenarios will be applied: loading with constant rate of shortening, loading with constant force and loading due to gravity only. We request three years funding for one PhD student to work on the thermo-mechanical modelling, and we request two and a half years funding for a PhD student who already started working on the petrological part of the project. The project will provide key constraints for the peak pressure in the Monte Rosa nappe which are essential for reconstructing the tectonic evolution of this nappe and the Alps in general.
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