Hugues Raimbourg (University-ISTO), Florence Cagnard (BRGM-ISTO)
Background and rationale
The lithosphere is a complex system whose dynamics are under the control of numerous interdependent parameters such as lithology, pressure, temperature, deformation rates, and the chemistry of the fluids circulating therein. All these parameters interfere in a way still poorly understood in the processes of localization or non-localization and deformation, and therefore in plate kinematics. Furthermore, the modern continental lithosphere has a long history, which remains very heterogeneous, and this heritage plays an important role in localization. The localization of a lithospheric fault and a deep ductile shearing zone is a process which extends over geological durations and during which the various parameters mentioned above are involved at different stages. The tectonic heritage certainly plays a major role in the initiation of the processes, but the metamorphic reactions and changes in grain size rapidly become very significant and no doubt, fluids above all. ISTO’s geodynamics researchers have recognized experience in the field of lithospheric deformation, both observational and experimental, and the parameters that control it. The BRGM’s researchers have acquired vast experience in the 3D description of geological objects at different scales and the mineral resources linked to mineralizer fluids.
The project is focused on deformation localization processes at different scales. Whether it is a question of the formation of large shearing zones, lithospheric or crustal, which accommodate hundreds or even thousands of kilometers of movement and control the tectonics of the lithosphere or the interactions between ductile and brittle behavior in the genesis of earthquakes, it is a question above all of understanding what the parameters are that control localization and how they interact. Beyond the obvious role of intensive parameters (pressure and atmosphere), those played by the fluid contents and the nature of these fluids as well as their interactions with rocks still need to be defined.
Answering this question requires following a logical process with back-and-forth between field observations, laboratory experimentation and numerical modeling, all of which together will allow quantifying the physicochemical parameters. It is particularly a matter of:
1) Recognizing and defining crustal and lithospheric structures, identifying potential fluid conduits and
constraining their spatial-temporal evolution, from an approach in the field. This part of the work requires describing the deformation gradients at different scales, from the map scale (several tens or hundreds of km) to the thin-section scale to identify the zones of maximum movement and deformation. We will try to maximally quantify as precisely as possible the deformation in the field, by conventional methods of structural geology (mapping at different scales, analysis of deformation) or by measurement of the anisotropy of magnetic susceptibility when the lithology lends itself to it. Superimpositions of structures will be the subject of particular attention in order to separate the effects of possible successive deformation phases from a normal progressive deformation process. The main lithologies and the various states of deformation has been the subject of sampling for P-T quantifications, dating and experimentation. We have selected 1)geological objects in which we already have vast experience in the field (South Armorican shear, Hidaka shear zone or the Shimanto overlaps in Japan, the Red River shear zone, Vietnam, the Cycladic detachments, the compressive shear zones of the outer Alps, Oisans, Norwegian Caledonides), 2) emblematic objects such as the Basin and Range or the Himalayan overlaps where we must acquire this experience.
2) Estimating the P-T conditions of fluid circulation in these structures, as well as knowing the nature of the fluids and their isotopic geochemistry, in situ. This part of the project requires implementing modern thermobarometric methods such as multi-equilibrium methods and methods for in situ fluid analysis, for their major elements but also for their isotopic composition (oxygen isotopes) to trace the origin and temperature of the various fluid generations in the veins. The combined expertise of the BRGM and ISTO, and the use of the entire instrument inventory available on the site, has been essential in this field.
3) Estimating the formation rate and localization of large shear zones. To do this, it is vital to obtain numerous measurements of radiometric age with the two methods that appear the most suitable: 40Ar-39Ar and Rb/Sr. These two methods are complementary during the exhumation of rocks through the brittle-ductile transition, the Rb/Sr method taking over the low temperatures from the 40Ar-39Ar method on white mica. Our project has been first to set up a 40Ar-39Ar dating laboratory (for in situ dating and for single grain dating); the Rb/Sr method can be subcontracted to ETH of Zurich.
4) Measuring the fluid-rock interactions in rheological terms. Field observations allow showing the contrasts in rheological behavior among different rocks or variations in these behaviors over time during localization of deformation, interactions between deformation and magmatism and exhumation from the deep, but it is often difficult to quantify the various rheological parameters. The experimental study of large deformations in the presence of fluids requires the use of the Paterson and Griggs presses to conduct short-term and long-term experiments in the presence of fluids.
5) Numerical modeling the processes from the laboratory experiment scale to the lithosphere scale by setting up a team that is strong in this field, thanks to the LABEX. The collaboration with ETH (Taras Gerya) has enhanced this team’s dynamics.
Results of WP 1 (2011-2014)
In line with project objectives, studies have focused since 2011 on two main approaches: (i) field work in the Aegean and ensuing kinematic reconstructions showing the detailed time evolution of the subduction zone and its relationships with both magma production and related ore deposits and (ii) numerical modeling of the effects of the slab retreat and slab tearing on crustal deformation (Fig. 1)
Figure 1: 3D schematic views of the eastern Mediterranean subduction zone and associated mantle and crustal processes controlling the emplacement of (a) the Cu-rich metallogenic province in the late Cretaceous, (b) the Pb-Zn-rich metallogenic province in the latest Eocene-Oligocene and (3) the Au-rich metallogenic province in the Miocene (From Menant et al., 2018, Ore Geology Reviews 94, 118-135).
On the basis of the results recorded and acquired experience in 3D numerical modeling, new work started in 2014 on the understanding of the distribution of thermal anomalies from the scale of the mantle (subduction dynamics, Figure 2) to the scale of geothermal fields controlled by large-scale detachments in Western Turkey (Figure 3).
Figure 2: Global map with red box showing the location on the study area. (b) Simplified tectonic maps showing the main metamorphic core complexes and associated detachments faults in the Aegean region (modified from Jolivet et al., 2015). SiD: Simav Detachment, AD: Ala¸sehir Detachment, BD: Büyük Detachment, NAF: North Anatolian Fault. (c) Tomographic models from Piromallo and Morelli(2003)showing the Vp anomalies at the 100 km depth. White star shows the location of a slab tear below the Menderes Massif. (d) Simplified cross-sections highlighting slab retreat and formation of crustal detachments. CCcorresponds to the 2-D numerical model cross-section
Figure 3: (a) Geometry and mesh used for 2-D models of fluid circulation in the upper crust of the Menderes area, where MCCs and detachments are reproduced. This cross section would correspond, from left to right, to a N–S cross section, from Salihli to Salavatlı geothermal areas. Mesh is refined at the top surface and within detachments, where permeability is the highest. (b) Boundary conditions and range of values for permeability (note the depth-dependence of the host rock permeability). (c) Steady-state conductive regime, where isotherms in white are separated by 100◦C. Colors refer to permeability values affected at time t>0
Lastly, new research was also initiated in 2013 in the North Pyrenean Zone (NPZ), a major crustal-scale deformation zone where mineral deposits have long been recognized, mostly associated with the Cretaceous stage of rifting.
In the meantime, an entirely new 40Ar/39Ar laboratory with three spectrometers has been installed at ISTO. One of the spectrometers was acquired on LABEX funds. The laboratory is now operational, and the first irradiations and dating have started in March 2016. Altogether, this new Ar/Ar platform is one of the largest in Europe (Figure 4).
Figure 4. The Ar/Ar dating facility installed at ISTO, equipped of three new spectrometers Helix-SFT (Thermo Scientific), each allowing either spot-laser or bulk dating of minerals and glasses.
Its first goal will be to test whether strain gradients in shear zones do correlate with age gradients, thereby testing the widely held assumption of a single closure temperature usually made when interpreting argon data. An example of such microscale dating with high precision is shown on Figure 5 below.
Figure 5. Example of age countours (in Ma) in a single muscovite crystal from the Proterozoic Harney Peak leucogranite (muscovite provided by Peter Nabelek, Missouri University, USA): Datation by Jehiel Nterne-Mukonzo, PhD-ISTO)
Among the new tools installed at ISTO in the framework of the ERC RHEOLITH, the Equipex PLANEX and Labex VOLTAIRE, is an improved Griggs apparatus newly designed for high-temperature and high-pressure experiments of rocks deformation. The press has been installed at ISTO (Figure 6).
Figure 6. Left panel: Griggs press installed at ISTO. This apparatus allows deformation experiments to be performed up to 3 GPa and 1600°C, hence to explore upper mantle to lower crust conditions. Right panel: Shear band development during deformation experiment of a granite sample in the new generation Griggs-type apparatus (Précigout et al., 2018, Jove). A) Starting material: peraluminous granite from Carnac (Brittany, France) core drilled (8 mm diameter) and rectified at 15 mm long; B) Strain-stress curve during deformation at a pressure (P) of 1200 MPa, a temperature (T) of 650 °C and a strain rate () of 10-6 s-1. No water has been added. = Differential stress; C) Picture of the sample after deformation (top left inset) and back scattered electron image of the shear band area. D) Phase map around a local shear band revealed by X-ray energy diffraction spectroscopy. Qtz = Quartz; Bt = Biotite; Or = Orthoclase ; Ab = Albite.
One of our collaborators is Holger Stunitz, professor at Tromsö University in Norway, one of the best specialists of the Griggs apparatus. Holger Stünitz is now a part-time member of our team. Since 2015, His salary is shared by Tromsö and Orléans Universities with LABEX funds, until 2021. This operation has been crucial for the success of the development of experimental deformation at ISTO.
Results of WP 1 (2015-2018)
A large effort was made to describe the distribution of deformation in space, using case studies in the Hercynian orogenic belt (Montagne Noire, South Armorican Shear Zone), the Mediterranean domain (Cyclades, Turkey, Corsica) or Japan (Hidaka Shear Zone, Shimanto Belt) (e.g., Beaudoin et al., 2015; Ducoux et al., 2017; Laurent et al., 2016; Raimbourg et al., 2017). This WP gives the geodynamic framework of ore deposits (e.g., Ducoux et al., 2017, Tectonophysics) which are considered in greater details in WP2. A similar study of deformation gradients was carried at much smaller case, leading to unravel the interplay between fluid and deformation (Précigout et al., 2017, Nat Com).
The tectonic work, which produced deformation maps encompassing a large range of scales, provided the ground to study, with the Ar-Ar facilities at ISTO, how deformation localizes during the lifetime of crustal-scale shear zones. The combined Griggs press and Ar/Ar facilities are used to determine the role of deformation on closure temperature of micas. In the example shown below (Figure 7), an hercynian granite has been deformed at HP/HT and the age of its muscovite measured before and after deformation.
Figure 7. Age distribution of muscovite crystals (blue ) in an undeformed hercynian granite giving an average age of 303.3 ± 3.5 Ma) compared to the ages retrieved from the same micas after its host granite has been deformed with the Griggs Press (red) at 12 kbar and 650 °C (20% deformation). From Alexane (Master 2-ISTO, 2018)
If we are not able yet to decipher the role of deformation on Ar-Ar ages, preliminary results also show that beyond temperature (i.e. “closure” temperature of the isotopic system) Ar concentrations are also influenced by circulation of fluids (Laurent et al., 2017). Consequently, techniques of laser ablation and argon (and other noble gas) measurements may give in the near future precious clues regarding fluid circulations, using (1) the halogen signatures of the fluids and (2) the connection between fluids and microstructures enabled by in-situ analyses. Our results concerning the extent of the fluid circulation (“closed”, with local fluxes, vs. “open”, with exotic fluids) as a function of depth compartment (Raimbourg et al., 2018), will strongly benefit from these technical developments.
In parallel of these works on naturally deformed rocks and terranes, we have also carried out experimental deformation studies to analyze the factors controlling the deformation processes, in particular the interaction between fluids and solid state deformation. First based on the Paterson rig, these studies focused on a large range of material and crustal depths, from porous sedimentary rocks (Gadenne et al., 2014) to silicate melts with a large proportion of crystals (Laumonier et al., 2015). The high-pressure deformation rig (Griggs-type apparatus) acquired to cover the full range of deformation conditions in the crust and upper mantle, enables us to take into account the effect of pressure. This high-pressure rig was in particular used to show, in water-rich conditions, the contribution of grain-size sensitive processes to overall strain and the role of recrystallization to redistribute water (Palazzin et al., 2018; Précigout et al., 2017).
Figure 8. Effect of the application of P-T and deformation on the water content of Hyuga porphyroclasts, containing initially a large number of fluid inclusions, in experiments on initial mixtures of porphyroclasts+fine-grained matrix (both quartz). (a) Evolution of water concentration and absorption bands in Hyuga porphyroclasts ‘as-is’ (red), ‘hot-pressed’ (dark blue) and ‘deformed’ (blue) (all spectra are represented with the same scale). Deformation and recrystallization to a small grain size results in the expulsion of the largest fraction of water initially present. Note also the important sharpening in discrete absorption bands from the ‘as-is’ to the ‘hot pressed’ material. Most of initial discrete bands disappear with deformation while a new band is detected at 3595 cm−1. b) Comparison of water amount between a recrystallized Hyuga porphyroclast and the surrounding matrix. The two FTIR spectra show a similar absorbance, the same shape and the same discrete absorption bands, while the two material (matrix and porphyroclasts) had initially very different grain sizes and water contents. This convergence point to the evolution, with strain, towards equilibrium distribution and speciation of water in the grain aggregate (From Palazzin et al, Journal of Structural Geology 114 (2018) 95–110)
This set of deformation apparatuses will be complemented, in the near future, by semi-transparent HP-HT rigs, which enable in situ X-ray monitoring of mineral reactions (Equipex PLANEX). This set of equipment, combining deformation and reaction of minerals, will be used to study for example the thermodynamics of clay dehydration and its influence on mechanical properties and possibly on earthquake generation.