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Faults and fluids: deep penetration of meteoric waters into the Alps during their exhumation

English title Faults and fluids: deep penetration of meteoric waters into the Alps during their exhumation
Applicant Vennemann Torsten
Number 124787
Funding scheme Project funding (Div. I-III)
Research institution Institut de Minéralogie et Géochimie Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geology
Start/End 01.05.2009 - 30.04.2010
Approved amount 80'124.00
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Keywords (9)

Alps; Structures; Deformation; Geochemistry; Isotopes; Faults; Fluids; Veins; Exhumation

Lay Summary (English)

Lead
Lay summary
The role of fluids during faulting and fracturing was examined in the case of late brittle deformations associated with the Miocene to recent exhumation of the central-western Swiss Alps. The post-metamorphic tectonic evolution of the Alps is well constrained in terms of deformation and cooling history, forming the basis for the research. In this project mineralizations associated with brittle deformation features are used as proxies for deep paleofluid circulation. Mineralizations from the latest generations of alpine faults, tension gashes and major regional joint families are examined for their geometric relationships with each other, for their mineralogy and geochemistry. A transect that runs from the Simplon fault westward to the Dent-Blanche nappe "depression", along an estimated Early Miocene paleo-crestline of the Alps has been investigated. The chosen transect covers zones of estimated paleo-mixing between a surficial hydrostatic, unconfined fluid regime and fluids derived from deeper metamorphic, highly confined and most probably over pressurized regimes within the ductiley deforming crust. The late brittle fault pattern has three preferred orientations: NNE-SSW, NW-SE and E-W. The majority of the measured fault planes are normal faults, even if in some cases there is evidence of some transform fault motion: in two distinct cases it is possible to see a set of quartz striae that are bent, indicating a transition between transform- to normal-behaviour. A late-stage vein mineralogy of quartz-hematite, quartz-chlorite and/or quartz-carbonate with occasional K-feldspar or mica is present along the fault planes. Stable isotope compositions of cogenetic minerals and quartz-chlorite thermometry indicate crystallization temperatures of between 150 to 200°C. Fluids calculated to be in equilibrium with these minerals appear to have been largely buffered by the wall rocks although a marked difference in oxygen isotope composition of about 3‰ between hanging wall and foot wall veins exists along the central parts of the Simplon fault. U-Th-He measurements of zircons contained within the veins indicate cooling ages of about 18 Ma, while the same zircons retain a U-Pb age of 270-280 Ma and have thus been inherited. Given closure temperatures for the U-Th-He system similar to those estimated from the vein mineral stable isotope fractionations, the U-Th-He cooling ages could be taken as ages of vein formation. A direct relationship between a change in deformation/stress regime and the onset of deep-circulating meteoric waters into the rising body of the Alps is not obvious from the results at this stage.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
109519 Faults and Fluids: deep penetration of meteoric waters into the Alps during their exhumation 01.10.2005 Project funding (Div. I-III)
111757 Controls on mineral-fluid oxygen isotope fractionations in geologic systems 01.04.2006 Project funding (Div. I-III)
105806 Origin of retrograde fluids, PTt paths and fluid- mineral equilibria in alpine fissures from the Central Alps 01.10.2004 Project funding (Div. I-III)
126973 Rheology and Permeability of Extensional Detachments 01.11.2009 Project funding (Div. I-III)
143885 Origin and growth of Alpine fissure minerals: fluid-mineral interaction and implications for the Alpine metamorphism 01.10.2012 Project funding (Div. I-III)
164032 CLUMPING LAUSANNE: A REQUEST FOR A REGIONAL FACILITY TO MEASURE CLUMPING IN CARBONATES 01.01.2016 R'EQUIP
140498 Chlorite as a key mineral for geothermometry: Empirical verifications of thermodynamic models for Fe-Mg aluminous chlorite and calibration of the oxygen isotope fractionation between chlorite, quartz and calcite 01.05.2012 Project funding (Div. I-III)

Abstract

The role of fluids during faulting and fracturing is examined in the case of late brittle deformations associated with the Miocene to recent exhumation of the central western Swiss Alps. The post-metamorphic tectonic evolution of the Alps is quite well constrained in terms of deformation and cooling history, forming the basis for our research. In the fourth and last year of this project, mineralizations associated with brittle deformation features are used as proxies for deep paleofluid circulation. Mineralizations from the latest generations of alpine faults, tension gashes and major regional joint families are examined for their geometric relationships with each other, for their mineralogy and geochemistry. A transect that runs from the Simplon fault westward to the Dent-Blanche nappe "depression", approximately along an estimated Early Miocene paleo-crest-line of the Alps is being investigated. Late alpine tilting and differential exhumation allows access to at least 10 km of a vertical section, with deeper portions of very young "middle" crust of up to amphibolite facies now exhumed at the eastern extremities. The chosen transect covers zones of estimated paleo-mixing between a surficial hydrostatic, unconfined fluid regime and fluids derived from deeper metamorphic, highly confined and most probably overpressurized regimes within the ductiley deforming crust.Results obtained during the first phase of this project indicate a late stage vein mineralogy of quartz-hematite, quartz chlorite and/or quartz-carbonate with occasional K-feldspar or mica. Stable isotope compositions of cogenetic minerals and quartz-chlorite thermometry indicate temperatures of between 150 to 200?C. Fluids calculated to be in equilibrium with these minerals appear to have been largely buffered by the wall rocks although a marked difference in oxygen isotope composition of about 3‰ between hanging wall and foot wall veins exists along the central parts of the Simplon fault. U-Th-He measurements of zircons contained within the veins indicate cooling ages of about 22 Ma, while the zircons retain a U-Pb age of about 280 Ma. Given closure temperatures for the U-Th-He system similar to those estimated from the vein mineral stable isotope fractionations, such ages could be taken as ages of vein formation, an aspect that is to be exploited further during the final phase of this project. In addition, further vein minerals as well as the same minerals from transects within the adjacent country rocks are to be analyzed for their chemical and isotopic composition in order to better constrain the timing of fluid-rock interactions during vein formation and the origin of the fluids. The completed and planned research will provide new constraints about the evolution of the brittle/ductile transition zone during exhumation of the Alps. A direct relationship between a change in deformation/stress regime - from compression/lateral extrusion to radial extension -and the onset of deep-circulating meteoric waters into the rising body of the Alps is not obvious from the results at this stage. If such a change in fluid regime were to be detected, this research could allow for the identification of (exhumed) paleo-seismic faults relative to their (creeping, "slow") counterparts on the basis of their structural/mineralogical criteria.
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