Andréa Tommasi - Academia.edu (original) (raw)
Papers by Andréa Tommasi
Torsion experiments were performed in polycrystalline ice at high temperature (0.97 T m) to repro... more Torsion experiments were performed in polycrystalline ice at high temperature (0.97 T m) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.
Geochemistry Geophysics Geosystems, 2014
How reliable are shear wave splitting measurements as a means of determining mantle flow directio... more How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strainhistory-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab's descent from the near surface to the bottom of the mantle transition zone-which we simulate in our models-have not yet been overprinted by subsequent continuous steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trenchparallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle.
ABSTRACT In order to investigate upper mantle structure and flow beneath french Polynesia, we dep... more ABSTRACT In order to investigate upper mantle structure and flow beneath french Polynesia, we deploy a broad-band seismic network in november 2001 for a two years period. After completion, this project called PLUME (Polynesian Lithosphere and Upper Mantle Experiment) should improve the body and surface wave tomographic imaging of the upper mantle beneath the french Polynesia. We present preliminary results of SKS and SKKS shear wave splitting obtained at PLUME temporary stations, at the permanent LDG/CEA stations in the Tuamotu (TPT), Gambier (RKT) and Austral (TBI) archipelagos and at the CEA/Geoscope station PPT in Tahiti. In such oceanic environment, the generally low signal-to-noise ratios render the shear wave splitting measurements difficult. Despite the presence of few good events, the first few months of recording at the temporary PLUME stations do not show clear individual splitting. We observe instead several unsplit SKS phases of good quality arriving from backazimuth of N70°W. We find clear splitting at RKT with fast split shear waves oriented roughly N60°W, i.e., parallel to the Absolute Plate Motion (APM) of the Pacific plate, and delay times between fast and slow waves around 1.5 s. We do not observe clear individual splitting at TPT and TBI stations, but interestingly, the stacking methods of Wolfe and Silver (1998) indicates at these stations an anisotropy trending parallel to the plate motion (respectively N70°W and N80°W) and rather small delay times (respectively of 0.7 and 1.0 s). We find no detectable anisotropy at PPT. This upper mantle apparent isotropy may be explained by short-scale heterogeneities, vertically oriented mantle flow, or simply by the fact that the mantle plume may have thermally erased the preexisting upper mantle fabric.
Geophysical Journal International, Oct 29, 2020
We investigate the effect of solute drag on the grain growth (GG) kinetics in olivine-rich rocks ... more We investigate the effect of solute drag on the grain growth (GG) kinetics in olivine-rich rocks through full field and mean field modelling. Considering a drag force exerted by impurities on grain boundary migration allows reconciling laboratory and natural constraints on olivine GG kinetics. Solute drag is implemented in a full field level-set framework and in a mean field model that explicitly accounts for a grain size distribution. After calibration of the mean field model on full field results, both models are able to both reproduce laboratory GG kinetics and predict grain sizes consistent with observations in peridotite xenoliths from different geological contexts.
Geophysical Research Letters, Aug 2, 2023
Amphibole‐rich rocks constitute significant components of the mid‐ to lower continental crust, pa... more Amphibole‐rich rocks constitute significant components of the mid‐ to lower continental crust, particularly in active orogens characterized with thick and hot crusts. Nevertheless, experimental data on their viscosity remain scarce. We conducted axial compression deformation experiments on synthetic amphibolites under temperature and pressure conditions resembling deep sections of overthickened crust. A novel flow law for a calcic‐amphibole‐rich rock (80% amphibole +20% garnet) in the dislocation creep regime is derived from these experiments. Contrary to common assumptions, our results reveal that calcic‐amphibolite is 1‐2 orders of magnitude weaker than plagioclase‐rich amphibolite, granulite, or gabbro. A calcic‐amphibole‐rich, low viscosity deep crust may not only support the “channel flow” model proposed for the Tibetan Plateau but also explain the observed high crustal seismic anisotropy in the region.
Elements
The interior of the Earth remains our last terra incognita, inaccessible to direct observations. ... more The interior of the Earth remains our last terra incognita, inaccessible to direct observations. Our understanding of the deformation of the mantle, which shapes our planet through convection and plate tectonics, is based on analysis of: (1) rare mantle rocks carried to the Earth’s surface by volcanic or tectonic processes, (2) the consequences of this deformation on the planet’s surface, and (3) geophysical data. These observables combined with laboratory experiments and numerical modeling imply that olivine deforms via the motion of defects within its crystalline structure and along grain boundaries. Ductile deformation by these crystal-scale processes results in anisotropic propagation of seismic waves, which allows us to probe upper-mantle deformation at scales of tens to hundreds of kilometers.
Frontiers in Earth Science, 2015
Earth and Planetary Science Letters, 2015
Rheology of mantle rocks at lithospheric temperatures remains poorly constrained, since most expe... more Rheology of mantle rocks at lithospheric temperatures remains poorly constrained, since most experimental studies on creep mechanisms of olivine single crystals ((MgFe)2SiO4, Pbnm) and polycrystalline olivine aggregates were performed at high-temperatures (T >> 1200oC). In this study, we have performed deformation experiments on oriented single crystals of San Carlos olivine and polycrystalline olivine aggregate at temperatures relevant of the uppermost mantle (ranging from 800o to 1090oC) in tri-axial compression. The experiments were carried out at a confining pressure of 300 MPa in a high-resolution gas-medium mechanical testing apparatus at various constant strain rates (from 7 × 10-6 s-1 to 1 × 10-4 s-1). Mechanical tests yield differential stresses ranging from 88 to 1076 MPa. All samples were deformed at constant displacement rate and for finite strains ranging from 4 to 23 %, to provide insight into possible effects of hardening, softening or stick-and-slip. The single...
The mechanical behaviour of the Earth's upper mantle and many physical properties such as... more The mechanical behaviour of the Earth's upper mantle and many physical properties such as seismic anisotropy and viscosity are strongly coupled to the rheological properties of olivine, the dominant phase in this region. Until recently, experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks lead to the conclusion that olivine deforms essentially by dislocation
ABSTRACT In order to investigate upper mantle structure, dynamics and flow beneath French Polynes... more ABSTRACT In order to investigate upper mantle structure, dynamics and flow beneath French Polynesia, we deploy a broadband seismic network in November 2001 for a two years period [Barruol et al., 2002]. The network is deployed over the five polynesian archipelagos, over two major fracture zones (Marquesas and Australs) and samples oceanic lithosphere between 30 and 100 My old. This multidisciplinary project called PLUME (Polynesian Lithosphere and Upper Mantle Experiment) aims to improve the body and surface wave tomographic imaging of the upper mantle beneath the french Polynesia, and particularly the geometry at depth of the mantle plumes related to the surface hotspots (at least four). PLUME should also bring constraints on the upper mantle flow and the interaction between hotspots and the oceanic lithosphere. To illustrate this last point, we present preliminary observation of teleseismic shear wave splitting of PKS, SKS, SKKS, and deep S phases. Data are recorded at the 10 PLUME temporary stations (between 8 and 12 months of data), the 4 permanent LDG/CEA broadband stations, the 4 permanent IRIS broadband stations, and the permanent GEOSCOPE station. The splitting parameters are calculated using the method of Silver and Chan [1991] and the alternative multi-events method of Wolfe and Silver [1998]. Despite the presence of rather high seismic noise on the Pacific islands, we find some evidence of individual splitting at the temporary PLUME stations with a NW-SE fast split direction, i.e., rather close from the plate motion vector. Delay times are slightly above 1.0 s. Since most of the teleseismic events arrive from the western Pacific subductions zones with a NW backazimuth, we observe numerous unsplit SKS phases, some of them of very good quality. Two of the four LDG stations exhibit an anisotropy trending N070°W and with a delay time around 1.5s. In addition to the teleseismic shear wave splitting that sample the upper mantle along the vertical direction beneath a given station, we examine the P-wave polarization that may give complementary and independent constraints on upper mantle anisotropy. In an anisotropic medium, the quasi-P wave polarization is indeed slightly deviated from the propagation direction. Statistical analysis of this deviation as a function of the event backazimuth can be used to get some information on upper mantle anisotropy. Such technique has the main advantage to increase the azimuthal coverage but also to sample the upper mantle anisotropy beneath a station with some lateral offset since the incidence angle are much larger than the SKS phases.
Spinel peridotite mantle xenoliths from Persani Mountains, Romania, derived from about a 20 km de... more Spinel peridotite mantle xenoliths from Persani Mountains, Romania, derived from about a 20 km depth-transect of the shallow subcontinental lithosphere display deformation by dislocation creep accompanied by dynamic recrystallization at continuously varying temperature, strain rate and stress conditions. The continuous variation in deformation conditions allowed us to characterize the effect of dynamic recrystallization on olivine fabric strength, which is the main parameter controlling the anisotropy of physical properties, such as electrical conductivity, seismic wave propagation and thermal diffusivity in the upper mantle. We have defined a dimensionless dispersion factor, which describes the randomization of crystallographic orientations of recrystallized grains with respect to the parental porphyroclasts. This dispersion factor is largely independent of the number of grains analyzed, of the recrystallized grain size, which directly relates to the deviatoric stress, and of the o...
ABSTRACT Our understanding of subduction zone processes is tightly connected to our knowledge of ... more ABSTRACT Our understanding of subduction zone processes is tightly connected to our knowledge of the cycling of volatiles in the Earth, namely the loci of devolatilization reactions and the fluid migration mechanism. The exact nature of fluid pathways at high-pressure conditions is poorly known and still highly speculative. Studies metamorphic terrains that record main dehydration reaction are, thus, an invaluable tool to decipher the mechanism for fluid expulsion. Among other dehydration reactions in subduction zones, the antigorite (Atg) breakdown is rather discontinuous, releases the largest amount of fluids (ca. 9 wt. %) and is considered to have important seismological implications. The antigorite dehydration front in the Cerro del Almirez (Betic Cordillera, Spain) offers, thus, an unique opportunity to investigate the dynamics of fluid expulsion through the study of micro- and macrotextures recorded in the prograde assemblage (chlorite harzburgite). Granoblastic texture are interspersed in decameter-sized domains with spinifex-like chl-harzburgite and were formed under similar P-T conditions (~1.6-1.9 GPa and 680-710°C). We ascribe these textures to shifts of the growth rate due to temporal and spatial fluctuations of the affinity of the Atg-breakdown reaction. These fluctuations are driven by cyclic variations of the excess fluid pressure which are ultimately controlled by the hydrodynamics of deserpentinization fluid expulsion. Crystallization at a low affinity of the reaction, correspondig to the granoblastic texture, may be attained if fluids are slowly drained out from the dehydration front. During the advancement of the dehydration front, overpressured domains are left behind preserving highly metastable Atg-serpentinite domains. Brittle failure results in a sudden drop of the fluid pressure, and a displacement of Atg equilibrium towards the prograde products that crystallizes at a high affinity of the reaction (spinifex-like texture). Evidences of brittle failure are found along grain-size reduction zones (GSRZ), a few mm to meters wide, which form roughly planar conjugate structures and crosscut the metamorphic texture. GSRZ are characterized by (1) sharp, irregular shapes and abrupt terminations contacts with undeformed metaperidotite, (2) an important reduction of the olivine grain size (60-250 µm), and (3) decrease in the opx modal amount. Analysis of olivine crystal-preferred orientations in GSRZ shows similar patterns, but a higher dispersion than in neighboring metaperidotite. These structures are interpreted as due to hydrofracturing allowing for the formation of high permeability channelways for overpressured fluids. This textural bimodality (granofels and Spinifex-like) and the record of brittle failure hence witnesses a unique example of the feedbacks between the cyclic dynamic of metamorphic fluid expulsion, the reaction rate and crystallisation of the Atg-dehydrating system.
In the past few years, most people have concluded that seismic anisotropy measured with teleseism... more In the past few years, most people have concluded that seismic anisotropy measured with teleseismic shear wave splitting is linked to the lattice preferred orientation of olivine minerals in the upper mantle and, so, to flow directions in the mantle, either lithospheric shearing or asthenospheric flow or a combination of both. The mantle beneath New Zealand has been extensively investigated with shear wave splitting and the results obtained so far from SKS phases show, roughly, a NE/SW fast direction, which is parallel to the Alpine Fault in the South Island and to the strike of the Hikurangi subduction zone in the North Island. However, some zones are not constrained and need to be investigated further, such as the western part of the North Island and the southernmost part of New Zealand. We therefore examined shear wave splitting from teleseisms recorded on the new New Zealand GeoNet broadband stations deployed in 2001. The results obtained for the central South Island and the eastern part of the North Island confirm the earlier pattern and therefore extend the limits of the anisotropic region. However, the southernmost station, Whether Hill (WHZ), yields N/S fast direction and big delay time (2.4 +/-0.3 s). This suggests that shear strain parallel to the Alpine Fault decreases in the mantle in southernmost New Zealand but the strong delay measured may indicate another layer of anisotropy. Also, in the north of the North Island, two stations show different results from different back azimuths. This may also be linked to the presence of two or more anisotropic layers, therefore deserving further investigations. Petrophysical analysis of mantle xenoliths from the Raglan (south of Auckland), Dunedin (South Island) and Chatham regions are used to constrain the lithospheric contribution to the observed seismic anisotropy. This preliminary work will continue with further studies on mantle samples, further measurements on the new stations and with the study of the data collected by a deployment of new permanent and portable stations in the Northland region.
Torsion experiments were performed in polycrystalline ice at high temperature (0.97 T m) to repro... more Torsion experiments were performed in polycrystalline ice at high temperature (0.97 T m) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.
Geochemistry Geophysics Geosystems, 2014
How reliable are shear wave splitting measurements as a means of determining mantle flow directio... more How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strainhistory-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab's descent from the near surface to the bottom of the mantle transition zone-which we simulate in our models-have not yet been overprinted by subsequent continuous steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trenchparallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle.
ABSTRACT In order to investigate upper mantle structure and flow beneath french Polynesia, we dep... more ABSTRACT In order to investigate upper mantle structure and flow beneath french Polynesia, we deploy a broad-band seismic network in november 2001 for a two years period. After completion, this project called PLUME (Polynesian Lithosphere and Upper Mantle Experiment) should improve the body and surface wave tomographic imaging of the upper mantle beneath the french Polynesia. We present preliminary results of SKS and SKKS shear wave splitting obtained at PLUME temporary stations, at the permanent LDG/CEA stations in the Tuamotu (TPT), Gambier (RKT) and Austral (TBI) archipelagos and at the CEA/Geoscope station PPT in Tahiti. In such oceanic environment, the generally low signal-to-noise ratios render the shear wave splitting measurements difficult. Despite the presence of few good events, the first few months of recording at the temporary PLUME stations do not show clear individual splitting. We observe instead several unsplit SKS phases of good quality arriving from backazimuth of N70°W. We find clear splitting at RKT with fast split shear waves oriented roughly N60°W, i.e., parallel to the Absolute Plate Motion (APM) of the Pacific plate, and delay times between fast and slow waves around 1.5 s. We do not observe clear individual splitting at TPT and TBI stations, but interestingly, the stacking methods of Wolfe and Silver (1998) indicates at these stations an anisotropy trending parallel to the plate motion (respectively N70°W and N80°W) and rather small delay times (respectively of 0.7 and 1.0 s). We find no detectable anisotropy at PPT. This upper mantle apparent isotropy may be explained by short-scale heterogeneities, vertically oriented mantle flow, or simply by the fact that the mantle plume may have thermally erased the preexisting upper mantle fabric.
Geophysical Journal International, Oct 29, 2020
We investigate the effect of solute drag on the grain growth (GG) kinetics in olivine-rich rocks ... more We investigate the effect of solute drag on the grain growth (GG) kinetics in olivine-rich rocks through full field and mean field modelling. Considering a drag force exerted by impurities on grain boundary migration allows reconciling laboratory and natural constraints on olivine GG kinetics. Solute drag is implemented in a full field level-set framework and in a mean field model that explicitly accounts for a grain size distribution. After calibration of the mean field model on full field results, both models are able to both reproduce laboratory GG kinetics and predict grain sizes consistent with observations in peridotite xenoliths from different geological contexts.
Geophysical Research Letters, Aug 2, 2023
Amphibole‐rich rocks constitute significant components of the mid‐ to lower continental crust, pa... more Amphibole‐rich rocks constitute significant components of the mid‐ to lower continental crust, particularly in active orogens characterized with thick and hot crusts. Nevertheless, experimental data on their viscosity remain scarce. We conducted axial compression deformation experiments on synthetic amphibolites under temperature and pressure conditions resembling deep sections of overthickened crust. A novel flow law for a calcic‐amphibole‐rich rock (80% amphibole +20% garnet) in the dislocation creep regime is derived from these experiments. Contrary to common assumptions, our results reveal that calcic‐amphibolite is 1‐2 orders of magnitude weaker than plagioclase‐rich amphibolite, granulite, or gabbro. A calcic‐amphibole‐rich, low viscosity deep crust may not only support the “channel flow” model proposed for the Tibetan Plateau but also explain the observed high crustal seismic anisotropy in the region.
Elements
The interior of the Earth remains our last terra incognita, inaccessible to direct observations. ... more The interior of the Earth remains our last terra incognita, inaccessible to direct observations. Our understanding of the deformation of the mantle, which shapes our planet through convection and plate tectonics, is based on analysis of: (1) rare mantle rocks carried to the Earth’s surface by volcanic or tectonic processes, (2) the consequences of this deformation on the planet’s surface, and (3) geophysical data. These observables combined with laboratory experiments and numerical modeling imply that olivine deforms via the motion of defects within its crystalline structure and along grain boundaries. Ductile deformation by these crystal-scale processes results in anisotropic propagation of seismic waves, which allows us to probe upper-mantle deformation at scales of tens to hundreds of kilometers.
Frontiers in Earth Science, 2015
Earth and Planetary Science Letters, 2015
Rheology of mantle rocks at lithospheric temperatures remains poorly constrained, since most expe... more Rheology of mantle rocks at lithospheric temperatures remains poorly constrained, since most experimental studies on creep mechanisms of olivine single crystals ((MgFe)2SiO4, Pbnm) and polycrystalline olivine aggregates were performed at high-temperatures (T >> 1200oC). In this study, we have performed deformation experiments on oriented single crystals of San Carlos olivine and polycrystalline olivine aggregate at temperatures relevant of the uppermost mantle (ranging from 800o to 1090oC) in tri-axial compression. The experiments were carried out at a confining pressure of 300 MPa in a high-resolution gas-medium mechanical testing apparatus at various constant strain rates (from 7 × 10-6 s-1 to 1 × 10-4 s-1). Mechanical tests yield differential stresses ranging from 88 to 1076 MPa. All samples were deformed at constant displacement rate and for finite strains ranging from 4 to 23 %, to provide insight into possible effects of hardening, softening or stick-and-slip. The single...
The mechanical behaviour of the Earth's upper mantle and many physical properties such as... more The mechanical behaviour of the Earth's upper mantle and many physical properties such as seismic anisotropy and viscosity are strongly coupled to the rheological properties of olivine, the dominant phase in this region. Until recently, experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks lead to the conclusion that olivine deforms essentially by dislocation
ABSTRACT In order to investigate upper mantle structure, dynamics and flow beneath French Polynes... more ABSTRACT In order to investigate upper mantle structure, dynamics and flow beneath French Polynesia, we deploy a broadband seismic network in November 2001 for a two years period [Barruol et al., 2002]. The network is deployed over the five polynesian archipelagos, over two major fracture zones (Marquesas and Australs) and samples oceanic lithosphere between 30 and 100 My old. This multidisciplinary project called PLUME (Polynesian Lithosphere and Upper Mantle Experiment) aims to improve the body and surface wave tomographic imaging of the upper mantle beneath the french Polynesia, and particularly the geometry at depth of the mantle plumes related to the surface hotspots (at least four). PLUME should also bring constraints on the upper mantle flow and the interaction between hotspots and the oceanic lithosphere. To illustrate this last point, we present preliminary observation of teleseismic shear wave splitting of PKS, SKS, SKKS, and deep S phases. Data are recorded at the 10 PLUME temporary stations (between 8 and 12 months of data), the 4 permanent LDG/CEA broadband stations, the 4 permanent IRIS broadband stations, and the permanent GEOSCOPE station. The splitting parameters are calculated using the method of Silver and Chan [1991] and the alternative multi-events method of Wolfe and Silver [1998]. Despite the presence of rather high seismic noise on the Pacific islands, we find some evidence of individual splitting at the temporary PLUME stations with a NW-SE fast split direction, i.e., rather close from the plate motion vector. Delay times are slightly above 1.0 s. Since most of the teleseismic events arrive from the western Pacific subductions zones with a NW backazimuth, we observe numerous unsplit SKS phases, some of them of very good quality. Two of the four LDG stations exhibit an anisotropy trending N070°W and with a delay time around 1.5s. In addition to the teleseismic shear wave splitting that sample the upper mantle along the vertical direction beneath a given station, we examine the P-wave polarization that may give complementary and independent constraints on upper mantle anisotropy. In an anisotropic medium, the quasi-P wave polarization is indeed slightly deviated from the propagation direction. Statistical analysis of this deviation as a function of the event backazimuth can be used to get some information on upper mantle anisotropy. Such technique has the main advantage to increase the azimuthal coverage but also to sample the upper mantle anisotropy beneath a station with some lateral offset since the incidence angle are much larger than the SKS phases.
Spinel peridotite mantle xenoliths from Persani Mountains, Romania, derived from about a 20 km de... more Spinel peridotite mantle xenoliths from Persani Mountains, Romania, derived from about a 20 km depth-transect of the shallow subcontinental lithosphere display deformation by dislocation creep accompanied by dynamic recrystallization at continuously varying temperature, strain rate and stress conditions. The continuous variation in deformation conditions allowed us to characterize the effect of dynamic recrystallization on olivine fabric strength, which is the main parameter controlling the anisotropy of physical properties, such as electrical conductivity, seismic wave propagation and thermal diffusivity in the upper mantle. We have defined a dimensionless dispersion factor, which describes the randomization of crystallographic orientations of recrystallized grains with respect to the parental porphyroclasts. This dispersion factor is largely independent of the number of grains analyzed, of the recrystallized grain size, which directly relates to the deviatoric stress, and of the o...
ABSTRACT Our understanding of subduction zone processes is tightly connected to our knowledge of ... more ABSTRACT Our understanding of subduction zone processes is tightly connected to our knowledge of the cycling of volatiles in the Earth, namely the loci of devolatilization reactions and the fluid migration mechanism. The exact nature of fluid pathways at high-pressure conditions is poorly known and still highly speculative. Studies metamorphic terrains that record main dehydration reaction are, thus, an invaluable tool to decipher the mechanism for fluid expulsion. Among other dehydration reactions in subduction zones, the antigorite (Atg) breakdown is rather discontinuous, releases the largest amount of fluids (ca. 9 wt. %) and is considered to have important seismological implications. The antigorite dehydration front in the Cerro del Almirez (Betic Cordillera, Spain) offers, thus, an unique opportunity to investigate the dynamics of fluid expulsion through the study of micro- and macrotextures recorded in the prograde assemblage (chlorite harzburgite). Granoblastic texture are interspersed in decameter-sized domains with spinifex-like chl-harzburgite and were formed under similar P-T conditions (~1.6-1.9 GPa and 680-710°C). We ascribe these textures to shifts of the growth rate due to temporal and spatial fluctuations of the affinity of the Atg-breakdown reaction. These fluctuations are driven by cyclic variations of the excess fluid pressure which are ultimately controlled by the hydrodynamics of deserpentinization fluid expulsion. Crystallization at a low affinity of the reaction, correspondig to the granoblastic texture, may be attained if fluids are slowly drained out from the dehydration front. During the advancement of the dehydration front, overpressured domains are left behind preserving highly metastable Atg-serpentinite domains. Brittle failure results in a sudden drop of the fluid pressure, and a displacement of Atg equilibrium towards the prograde products that crystallizes at a high affinity of the reaction (spinifex-like texture). Evidences of brittle failure are found along grain-size reduction zones (GSRZ), a few mm to meters wide, which form roughly planar conjugate structures and crosscut the metamorphic texture. GSRZ are characterized by (1) sharp, irregular shapes and abrupt terminations contacts with undeformed metaperidotite, (2) an important reduction of the olivine grain size (60-250 µm), and (3) decrease in the opx modal amount. Analysis of olivine crystal-preferred orientations in GSRZ shows similar patterns, but a higher dispersion than in neighboring metaperidotite. These structures are interpreted as due to hydrofracturing allowing for the formation of high permeability channelways for overpressured fluids. This textural bimodality (granofels and Spinifex-like) and the record of brittle failure hence witnesses a unique example of the feedbacks between the cyclic dynamic of metamorphic fluid expulsion, the reaction rate and crystallisation of the Atg-dehydrating system.
In the past few years, most people have concluded that seismic anisotropy measured with teleseism... more In the past few years, most people have concluded that seismic anisotropy measured with teleseismic shear wave splitting is linked to the lattice preferred orientation of olivine minerals in the upper mantle and, so, to flow directions in the mantle, either lithospheric shearing or asthenospheric flow or a combination of both. The mantle beneath New Zealand has been extensively investigated with shear wave splitting and the results obtained so far from SKS phases show, roughly, a NE/SW fast direction, which is parallel to the Alpine Fault in the South Island and to the strike of the Hikurangi subduction zone in the North Island. However, some zones are not constrained and need to be investigated further, such as the western part of the North Island and the southernmost part of New Zealand. We therefore examined shear wave splitting from teleseisms recorded on the new New Zealand GeoNet broadband stations deployed in 2001. The results obtained for the central South Island and the eastern part of the North Island confirm the earlier pattern and therefore extend the limits of the anisotropic region. However, the southernmost station, Whether Hill (WHZ), yields N/S fast direction and big delay time (2.4 +/-0.3 s). This suggests that shear strain parallel to the Alpine Fault decreases in the mantle in southernmost New Zealand but the strong delay measured may indicate another layer of anisotropy. Also, in the north of the North Island, two stations show different results from different back azimuths. This may also be linked to the presence of two or more anisotropic layers, therefore deserving further investigations. Petrophysical analysis of mantle xenoliths from the Raglan (south of Auckland), Dunedin (South Island) and Chatham regions are used to constrain the lithospheric contribution to the observed seismic anisotropy. This preliminary work will continue with further studies on mantle samples, further measurements on the new stations and with the study of the data collected by a deployment of new permanent and portable stations in the Northland region.