Didier Marquer

Fluid circulation, progressive deformation and mass-transfer processes in the upper crust: the example of basement-cover relationships in the External Crystalline Massifs, Switzerland

Abstract Basement and cover rocks of the external zone in the Swiss Alps are affected by Tertiary ductile deformation under greenschist facies conditions (300–450°C; 3–4.5 kbar). The analyses of stable isotope systematics of veins and shear zones, as well as variation profiles of chemical elements across major shear zones, distinguish two types of fluid-rock interactions. (i) Closed systems: most syntectonic veins within the Helvetic carbonate cover have δ 18 O compositions depending on the adjacent wall rock compositions and varying with respect to the initial chemical heterogeneity of each sedimentary layer. Within the granitic basement, chemical profiles across minor granite shear zones show equal volume gains and losses for each oxide Na 2 O or K 2 O. (ii) Open systems: variation profiles in major cover thrusts show a variable increase of 87 Sr/ 86 Sr ratios combined with a strong decrease in δ 18 O approaching an isotopic equilibrium with the basement rocks. Within the major basement shear zones, decrease in CaO and increase in MgO content are observed with progressive deformation increase. A two-step tectonic and geochemical model is proposed to explain the coexistence of open and closed systems. Fluid sources and transport mechanisms of chemical elements are discussed.

Shear zone patterns and strain distribution at the scale of a Penninic nappe: the Suretta nappe (Eastern Swiss Alps)

Abstract Shear zone patterns within the Suretta nappe of the Swiss Alps are studied in order to understand finite strain and bulk tectonic displacements. The Suretta nappe is chosen for two main reasons; the availability of late Variscan granites deformed during Alpine tectonics only, and because of continuous outcrop from bottom to top of the nappe. This Alpine nappe recorded two major ductile deformation phases, which developed under low-grade metamorphic conditions. The analysis of shear zone patterns in the Roffna granite reveals that the first deformation is well developed in the entire nappe; the second deformation, however, is more localized in the lower and upper parts. The bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other Penninic nappes in this area: first a stacking of the nappe towards the NW, followed by the top to the E shearing during post-thickening extensional tectonics. The shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale.

Basement-cover relationships in the Tambo nappe (Central Alps, Switzerland): geometry, structure and kinematics

In the Pennine zone of the Central Alps, the Tambo nappe forms a thin crystalline sliver embedded in the Mesozoic cover. Four Tertiary Alpine deformations are observed. The D1 ductile deformation is linked to the progressive Eocene stacking of the nappes towards the northwest. During D1, basement deformation is governed by imbricate tectonics whereas cover is thin-skinned and intensively folded. These different structures reflect the original strong rheological contrast between basement and cover. During the heterogeneous and ductile D2 deformation, the behaviour of the basement and cover became similar. The strong vertical D2 shortening, associated with a ‘top-to-the-east’ shear, led to the folding of the prior SE-dipping structures, developing SE-vergent folds with axes close to the E-W L2 stretching lineation. D2 corresponds to post-collisional crustal thinning following D2 crustal thickening. The D3 and D4 deformations occurred under retrograde conditions and can be correlated with uplift and late dextral movement on the Insubric line, respectively.

Dating the evolution of C–S microstructures: a combined 40Ar/39Ar step-heating and UV laserprobe analysis of the Alpine Roffna shear zone

Abstract A technique combining 40Ar/39Ar step-heating analyses, in situ UV laserprobe dating and electron microprobe chemical characterization has been developed to unravel the relationships between microstructures, metamorphism and K/Ar systematics. This technique has been applied to white micas of a discrete shear zone in the Roffna metarhyolite in the Suretta nappe (SE Switzerland). Neoformed phengites from the Roffna metarhyolite crystallised in shear zones during Tertiary Alpine deformation under blueschist- and greenschist-facies conditions. In situ 40Ar/39Ar UV laser dating of white micas defining the schistosity and C-planes yield systematically younger isotopic dates in the C-planes, which are also correlated with decreasing celadonite content (i.e., Si/Al ratio). These results suggest that the timing of the high-pressure deformation event is preserved in the schistosity; high-Si micas appear to have lost no Ar. Later deformation was localised preferentially in C-planes giving younger 40Ar/39Ar dates probably related to a later period of phengite growth. The age sequence is broadly consistent with the textural constraints and geological field relations.

Thermodynamic Modeling and Thermobarometry of Metasomatized Rocks

Determining the P-T conditions at which metasomatism occurs provides insight into the physical conditions at which fluid-rock interaction occurs in the crust. However, application of thermodynamic modeling to metasomatized rocks is not without pitfalls. As with “normal” metamorphic rocks, the main difficulty is to select mineral compositions that were in equilibrium during their crystallization. This essential task is particularly difficult in metasomatized rocks because it is often difficult to distinguish textures produced by changes in P-T conditions from those caused by fluid-rock interactions and associated changes in bulk composition. Furthermore, the selection of minerals in equilibrium in metasomatized rocks is made difficult by the great variability of scale of mass transfer (see Chaps. 4 and 5), and therefore equilibrium, which varies from micrometer- to hand-sample or larger scale, depending on the amount of fluid involved and the fluid transport mechanisms (e.g. pervasive or focused). Finally, another major limitation that is discussed in detail in Chap. 5, is that fluid composition coming in or out of the rock is unknown. Since fluid is a major phase component of the system, neglecting its impact on the phase relations might be problematic for thermobarometry. Despite these pitfalls, we describe in this contribution examples where thermobarometry has been apparently successfully applied. We emphasize that pseudosection thermobarometry is particularly suitable for metasomatized rocks because the effects of mass transfer can be explored through P-T-X phase diagrams. Application of thermodynamic modeling to metasomatized rocks requires (1) detailed mineralogical and textural investigation to select appropriate mineral compositions, (2) essential geochemical analyses to define the relative and absolute mass changes involved during the metasomatic event(s), and (3) forward modeling of the effects of mass transfer on phase relations.

Strain partitioning along the anatectic front in the Variscan Montagne Noire massif (southern French Massif Central)

We decipher late-orogenic crustal flow characterized by feedback relations between partial melting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strain pattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations (D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in the central and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontal shortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextral transpression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720 ± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-to-NE and top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transition zone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directed stretching. Depending on structural level, the metamorphic conditions associated with D3 deformation range from partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. We suggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioning on both sides of the anatectic front that may correspond to a major rheological boundary within the crust.

Early Cretaceous intra-oceanic rifting in the Proto-Indian Ocean recorded in the Masirah Ophiolite, Sultanate of Oman

Abstract The Masirah Ophiolite (Sultanate of Oman) was part of an oceanic basin (Proto-Indian Ocean) formed by the break-up of Gondwana in Late Jurassic times similar to the Somali basin. It was obducted onto the Arabian continental margin in the Early Paleocene, 100 Ma after its formation. Hence, it is possible to investigate the different tectonic and magmatic processes that have affected the oceanic lithosphere during these 100 Ma. Tithonian ridge magmatism, tectonism and hydrothermal alteration are responsible for the formation of the oceanic crust of the Masirah Ophiolite. In the Early Cretaceous (Hauterivian-Barremian), after 20 Ma of normal drift and subsidence, the oceanic lithosphere underwent extensional tectonics and renewed magmatism. Geometry, kinematics, intrusion mechanisms and related sedimentation during this intra-oceanic rifting are widely described and illustrated by field observations. Exhumation of deep-seated oceanic lithosphere, alkaline volcanism, intrusion of a hornblende gabbro-dolerite-granite suite and uplift of crustal blocks to sea level with the unconformable deposition of platform carbonates are the processes characterising this intra-oceanic rifting. The Hauterivian-Barremian age of oceanic rifting coincides with an important reorganisation of the motion of the Indian plate relative to Africa, Antarctica and Australia. We interpret the rifting recorded in the Masirah Ophiolite as the local response to the motion of the Indian plate due to the opening of the South Atlantic and the spreading in the Eastern Indian Ocean.

Impact Regimes and Post-formation Sequestration Processes: Implications for the Origin of Heavy Noble Gases in Terrestrial Planets

The difference between the measured atmospheric abundances of neon, argon, krypton, and xenon for Venus, Earth, and Mars is striking. Because these abundances drop by at least 2 orders of magnitude as one moves outward from Venus to Mars, the study of the origin of this discrepancy is a key issue that must be explained if we are to fully understand the different delivery mechanisms of the volatiles accreted by the terrestrial planets. In this work, we aim to investigate whether it is possible to quantitatively explain the variation of the heavy noble gas abundances measured on Venus, Earth, and Mars, assuming that cometary bombardment was the main delivery mechanism of these noble gases to the terrestrial planets. To do so, we use recent dynamical simulations that allow the study of the impact fluxes of comets upon the terrestrial planets during the course of their formation and evolution. Assuming that the mass of noble gases delivered by comets is proportional to the rate at which they collide with the terrestrial planets, we show that the krypton and xenon abundances in Venus and Earth can be explained in a manner consistent with the hypothesis of cometary bombardment. In order to explain the krypton and xenon abundance differences between Earth and Mars, we need to invoke the presence of large amounts of CO2-dominated clathrates in the Martian soil that would have efficiently sequestered these noble gases. Two different scenarios based on our model can also be used to explain the differences between the neon and argon abundances of the terrestrial planets. In the first scenario, cometary bombardment of these planets would have occurred at epochs contemporary with the existence of their primary atmospheres. Comets would have been the carriers of argon, krypton, and xenon, while neon would have been gravitationally captured by the terrestrial planets. In the second scenario, we consider impacting comets that contained significantly smaller amounts of argon, an idea supported by predictions of noble gas abundances in these bodies, provided that they formed from clathrates in the solar nebula. In this scenario, neon and argon would have been supplied to the terrestrial planets via the gravitational capture of their primary atmospheres whereas the bulk of their krypton and xenon would have been delivered by comets.

Stable isotope transfer in open and closed system across chemically contrasted boundaries: metacarbonate–granitoid contacts in the Quérigut magmatic complex (Eastern Pyrenees, France)

The processes of stable isotope transfer between chemically contrasted boundaries are examined for two metacarbonate–granitoid contacts in the Quérigut complex, Pyrénées, France. External contacts, between the sedimentary basement and granitic intrusions, behaved like a fluid-dominated open system whereas septa contacts, between carbonate septa and the host granitoids, behaved like a closed system with respect to external fluids. Along external contacts, skarns of several decimetres to several metres were developed. δ18O values of calcite, buffered at 13.5–14‰, suggest an advection of metamorphic aqueous fluid with a minor contribution of low δ13C CO2 during skarn formation. Internal contacts are characterized by a thin centimetre-sized skarn layer. δ13C and δ18O values of calcite follow an evolution explained by decarbonation processes alone. δ18O exchange profiles across the contact show a typical diffusion profile with their inflection points slightly displaced towards the metacarbonate side, interpreted as a limited influx of magmatic fluids. Moreover, the shape of septa profiles varies according to the thermal energy budget induced by the intrusive rock: more isotopic alteration appears where the intrusion size is larger.

Evolution of mass-transfer during progressive oblique under-thrusting of the Variscan foreland: eastern Bohemian Massif

AbstractFour ductile shear zones were sampled in the autochthonous Thaya basement and the Upper Bites nappe (Moravian unit) at the Eastern margin of the Bohemian massif. In both studied units, the tectono-metamorphic evolution and the chemical mass transfer are different. Two deformational events are recognised: the first deformation stage under amphibolite facies conditions is overprinted by a second event under greenschist facies conditions.The first deformation affected the western margin of the Thaya basement and the whole Bites nappe: microstructures are characterised by dynamic recrystallisation of feldspars and quartz, and occurrence of myrmekites and grain-boundary migration of quartz. None or weak chemical mass transfer is related to this medium to high temperature deformation. This deformation corresponds to the thrusting of Moldanubdian units on the Brunovistulian units (Moravian nappes and autochthonous Thaya basement).The second deformation generated shear zones in the until then preserved Th...