Jérôme Bascou

Plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites

Abstract We use an anisotropic viscoplastic self-consistent (VPSC) model to simulate the development of omphacite lattice preferred orientations (LPOs) in response to deformation by dislocation glide. In these simulations, we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside. Simulated LPOs reproduce very well the characteristic omphacite LPO pattern in naturally deformed eclogites: a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles sub-perpendicular to the foliation. These models reconcile the interpretation of omphacite LPOs in eclogites and TEM observations of naturally deformed omphacite, since they show that omphacite LPOs in naturally deformed eclogites may develop by dislocation glide on 1/2〈110〉{110}, [001]{110} and [001](100). We also investigate the effect of the strain regime on the omphacite LPO development. The simulations show that changes in deformation regime lead to second-order variations in omphacite LPO patterns similar to those observed in eclogites, such as an asymmetry of the LPO relative to the structural frame, a stronger concentration of the [001]-axes relative to the (010)-pole concentration, or a dispersion of [001]-axes in the foliation plane. This suggests that omphacite LPO patterns may carry information on the deformation regime (simple shear, transtension…) active during the high-pressure events.

EBSD-measured lattice-preferred orientations and seismic properties of eclogites

We investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities (Alps, Norway, Mali and eastern China) through the analysis of their microstructures and lattice-preferred orientations (LPO). These samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions. Omphacite and garnet LPO were determined from electron backscatter diffraction (EBSD) technique. Garnet appears to be almost randomly oriented whereas omphacite develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the (010)-poles concentrated sub-perpendicular to the foliation. In order to analyze the deformation mechanisms that produced such omphacite LPO, we compare our observations to LPO simulated by viscoplastic self-consistent numerical models. A good fit to the measured LPO is obtained for models in which the dominant slip systems are 1/2h110i{11 ¯ 0}, [001] {110} and [001] (100). Dominant activation of these slip systems is in agreement with TEM studies of naturally deformed omphacite. Seismic properties of eclogite are calculated by combining the measured LPO and the single crystal elastic constants of omphacite and garnet. Although eclogite seismic anisotropies are very weak (less than 3% for both P-and S-wave), they are generally characterized by a maximum P-wave velocity sub-parallel to the lineation and by a minimum velocity approximately normal to foliation. The mean P-and S-wave velocities are high (respectively, 8.6 and 4.9 km/s). The S-wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation. Calculated reflection coefficients show that an eclogite/crust interface is generally a good reflector (Rc > 0.1), whereas an eclogite body embedded in the upper mantle would be hardly detectable.

High-temperature deformation in the Neoproterozoic transpressional Ribeira belt, southeast Brazil

Abstract The Neoproterozoic Ribeira belt is subdivided in two domains with contrasting tectonic characteristics. The northern domain is dominated by shallowly dipping foliations and orogen-normal thrust tectonics. The southern domain is characterized by a 1000-km-long network of anastomosing transcurrent shear zones parallel to the belt. This contrast is interpreted as reflecting continent–continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain. The central, transitional, domain of the Ribeira belt displays the northern termination of the transcurrent shear zone network: the Alem Paraiba–Padua shear zone system (APPSS). The 250-km-long Alem Paraiba–Padua system involves granulites facies mylonites deformed through transpression. A detailed study of the microstructure and lattice preferred orientation (LPO) of the rock-forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust. Plagioclase crystals are plastically deformed; they display curved twins and cleavages, mechanical twins, and evidence of dynamic recrystallization. LPO of plagioclase is consistent with activation of the (010) [100] and (010) [001] slip systems. LPO of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the (100) [001] slip system. Quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration. The LPO of quartz is therefore the result of a static transformation of an initial, syn-kinematic LPO, and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization.

Titanohematite lattice-preferred orientation and magnetic anisotropy in high-temperature mylonites

Magnetic and crystallographic fabric studies were performed in mylonitic granulites from a km-wide strike-slip shear zone in the Ribeira Belt (southeastern Brazil). In these mylonites, a strong compositional layering underlines the tectonic foliation and the elongation of titanohematite, amphibole and orthopyroxene crystals defines a mineral stretching lineation. Magnetic fabric deduced from anisotropy of magnetic susceptibility (AMS) measurements and tectonic fabric compare favorably. Rock-magnetic studies show that both paramagnetic and ferromagnetic minerals can be carriers of AMS. The anisotropy of isothermal remanent magnetization, which is due to the shape-preferred orientation of magnetite grains, is coaxial with AMS. Lattice-preferred orientation (LPO) measurements using the electron backscattered diffraction technique show that orthopyroxene, amphibole, biotite and titanohematite have a strong LPO tightly related to the tectonic fabric. Among these four minerals, titanohematite is the only mineral present in relatively large proportions (s 1.5%) in all studied samples. Titanohematite LPO is characterized by a strong concentration of (0001) poles (c-axes) sub-perpendicular to the foliation and by a distribution of the poles of the (2110) and (1010) prism planes within the foliation with a maximum close to the lineation. This characteristic LPO is interpreted as resulting from dislocation creep during the mylonitization. Magnetic fabrics and titanohematite LPO fit well: the axis of minimum susceptibility is aligned with the c-axis maximum and the axis of maximum susceptibility coincides with the maximum concentration of poles of the prism planes. Titanohematite LPO may provide a valuable constraint for the kinematic interpretation of the magnetic fabrics. fl 2002 Elsevier Science B.V. All rights reserved.

Geology of the Terre Adélie Craton (135 – 146° E)

More than 15 years of field and laboratory investigations on samples from Terre Adélie to the western part of George V Land (135 to 146°E) during the GEOLETA program allow a reassessment of the Terre Adélie Craton (TAC) geology. The TAC represents the largest exposed fragment of the East Antarctic Shield preserved from both Grenville and Ross tectono-metamorphic events. Therefore it corresponds to a well-preserved continental segment that developed from the Neoarchean to the Paleoproterozoic. Together with the Gawler Craton in South Australia, the TAC is considered as part of the Mawson continent, i.e. a striking piece of the Rodinia Supercontinent. However, this craton represents one of the less studied parts of the East Antarctic Shield. The three maps presented here clearly point out the extent of two distinct domains within the Terre Adélie Craton and suggest that the TAC was built up through a polyphased evolution during the Neoarchean-Siderian (c.a. 2.5Ga) and the Statherian (c.a. 1.7Ga) periods. These data support a complete re-assessment of the TAC geology and represent a valuable base for the understanding of global geodynamics changes during Paleoproterozoic times. Citation: Ménot, R.P., G. Duclaux, J.J. Peucat, Y. Rolland, S. Guillot, M. Fanning, J. Bascou, D. Gapais, and A. Pêcher (2007), Geology of the Terre Adélie Craton (135 – 146 ̊ E), in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10 ISAES, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007-1047, Short Research Paper 048, 5 p.; doi:10.3133/of2007-1047.srp048

Tide‐induced microseismicity in the Mertz glacier grounding area, East Antarctica

The deployment of a seismic network along the Adelie and George V coasts in East Antarctica during the period 2009–2012 provides the opportunity to monitor cryoseismic activity and to obtain new insights on the relationship between tidal cycles and coastal glacier dynamics. Here we focus on records from a seismometer located on a rocky outcrop in the vicinity of the grounding line of the 35 km broad Mertz glacier, a major outflow of this region. We detect numerous icequakes (50,000 events within 10 months and up to 100 events/h) and demonstrate their clear tidal modulation. We suggest that they result from ice friction and fracturing around the rocky peak and from the glacier flexure in response to the falling and rising tides at its grounding area. We propose that such icequake monitoring could be used as a climate proxy since grounding lines are subject to migrate with sea level changes.

Using the anisotropy of magnetic susceptibility to infer flow-induced orientation of anisotropic particles: feasibility and sensitivity

In this paper, the use of anisotropy of magnetic susceptibility (AMS) measurements has been investigated in order to check the sensitivity of this technique versus the flow conditions. The orientation of anisotropic magnetic particles during the flow of a polystyrene

Crystal preferred orientations of garnet: comparison between numerical simulations and electron back-scattered diffraction (EBSD) measurements in naturally deformed eclogites

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An integrated study of microstructural, geochemical, and seismic properties of the lithospheric mantle above the Kerguelen plume (Indian Ocean)

magnetite blend through a capillary rheometer has been studied. Thanks to the magnetic properties of the magnetite, AMS measurements are possible. Different values of the filler concentration, viscosity, and shear rate were used. It is shown that the AMS technique is able to detect accurately particle orientation and that sensitivity decreases when increasing the concentration of magnetite grains. In addition, the rectangular-shaped sample imposed by the rheological device does not affect measures of AMS significantly. The results give reasons to consider that the measure of AMS is an accurate and sensitive method to access the mean local rheological behavior in filled or non-filled systems containing anisotropic tracers.