Slow slip in subduction zones: Reconciling deformation fabrics with instrumental observations and laboratory results

Cataclasites are a characteristic rock type found in drill cores from active faults as well as in exposed fossil subduction faults. Here, cataclasites are commonly associated with evidence for pervasive pressure solution and abundant hydro­ fracturing. They host the principal slip of regular earthquakes and the family of so­called slow earthquakes (episodic slip and tremor, low to very low frequency earthquakes, etc.). Slip velocities associated with the formation of the different types of cataclasites and conditions controlling slip are poorly constrained both from direct observations in nature as well as from experimental research. In this study, we explore exposed sections of subduction faults and their dominant microstructures. We use recently proposed constitutive laws to estimate deformation rates, and we compare predicted rates with instrumental observations from subduction zones. By identifying the maximum strain rates using fault scaling relations to constrain the fault core thickness, we find that the instrumental shear strain rates identified for the family of “slow earthquakes” features range from 10−3s−1 to 10−5s−1. These values agree with estimated rates for stress corrosion creep or brittle creep possibly controlling cataclastic deformation rates near the failure threshold. Typically, pore­fluid pressures are suggested to be high in subduction zones triggering brittle deformation and fault slip. However, seismic slip events causing local dilatancy may reduce fluid pressures promoting pressure­solution creep (yielding rates of <10−8 to 10−12s−1) during the interseismic period in agreement with dominant fabrics in plate interface zones. Our observations suggest that cataclasis is controlled by stress corrosion creep and driven by fluid pressure fluctuations at near­lithostatic effective pressure and shear stresses close to failure. We posit that cataclastic flow is the dominant physical mechanism governing transient creep episodes such as slow slip events (SSEs), accelerating preparatory slip before seismic events, and early afterslip in the seismogenic zone

Diagnosis Of The Chronic Lymphocytic Leukemia (CLL) Using A Raman-Based Scanner Optimized For Blood Smear Analysis (M3s Project)

Introduction/ Background In hematology, actual diagnosis of B chronic lymphocyte-leukemia (CLL) is based on the microscopic analysis of cell morphology from patient blood smear. However, new photonic technologies appear promising to facilitate and improve the early diagnosis, prognostic and monitoring of personalized therapy. The development of automated diagnostic approaches could assist clinicians in improving the efficiency and quality of health services, but also reduce medical costs. Aims The M3S project aims at improving the diagnosis and prognosis of the CLL pathology by developing a multimodal microscopy platform, including Raman spectrometry, dedicated to the automatic analysis of lymphocytes. Methods Blood smears were prepared on glass slides commonly used in pathology laboratories for microscopy. Two types of sample per patient were prepared: a conventional blood smear and a deposit of “pure” lymphocyte subtypes (i.e. normal B, CLL B, T and NK), sorted out in flow cytometry by using the negative double labeling technique. The second sample is used for the construction of a database of spectral markers specific of these different cell types. The preparations were analyzed with the multimodal machine which combines i) a Raman micro-spectrometer, equipped with a 532nm diode laser excitation source; ii) a microscope equipped with 40x and 150x lenses and a high precision xyz motorized stage for scanning the blood smear, and localizing x-y coordinates of representative series (~100 for each patient) of lymphocyte cells before registering three Raman spectra; these cells of interest being previously localized by an original method based on the morphology analysis. After the Raman acquisitions, the conventional blood smears were submitted to immunolabelling using specific antibodies. For the establishment of the Raman classifiers, this post-acquisition treatment was used as reference to distinguish the different lymphocyte sub-populations. Raman data were then analyzed using chemometric processing and supervised statistical classifiers in order to construct a spectral library of markers highly specific of the lymphocyte type and status (normal or pathological). Results Currently, a total of 60 patients (CLL and healthy) were included in the study. Various classification methods such as LDA (Linear Discriminant Analysis), PLS-DA (Partial Least Square Discriminant Analysis), RF (Random Forest) and SVM (Support Vector Machine), were tested in the purpose to distinguish tumoral B lymphocytes from other cell types. These classification algorithms were combined with feature selection approaches. The best performances were around 70% of correct identification when a three-class model (B-CLL vs B-normal vs T and NK lymphocytes) was considered, and 80% in case of a two-class model (B-CLL vs B-normal lymphocytes). These encouraging results demonstrate the potential of Raman micro-spectroscopy coupled to supervised classification algorithms for leukemic cell classification. The approach can find interest more generally in the field of cyto-hematology. Further developments will concern the integration of additional modality such as Quantitative Phase Imaging on one hand to speed the exploration process of cells of interest to be probed, and on the other hand to extract additional characteristics likely to be informative for CLL diagnosis. In addition, the identification of prognostic markers will be investigated by confronting the photonic data to clinical patient information.

Breaking the Crust: Intermediate-depth Subduction Earthquakes Recorded by Eclogite Breccias

International audienceUnderstanding processes acting along the subduction interface is crucial to assess lithospheric scale coupling between tectonic plates and mechanisms causing intermediate-depth seismicity. Despite a wealth of geophysical studies aimed at better characterizing the subduction interface, we still lack critical data constraining processes responsible for seismicity within oceanic subduction zones. We herein present new field data from an almost intact, km-scale fragment of oceanic lithosphere (Monviso ophiolite, W. Alps) crosscut by 30 to 150 m wide eclogite-facies shear zones formed at ~80 km depth during subduction. One of them shows spectacular m-sized blocks of eclogite facies breccias made of 2-10 cm fragments of eclogite mylonite cemented by omphacite, lawsonite and garnet, and later embedded in serpentinite. At the mineral-scale, omphacite crack-seal veins and garnet zoning patterns also show evidence for polyphased fracturing-healing events. Our observations suggest that brecciation was seismic, accompanied by the input of externally-derived fluids and occurred in the middle part of the oceanic crust. We conclude that these eclogite breccias mark the locus of an ancient fault zone associated with intraslab, intermediate-depth earthquakes at ~80 km depth

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Highlights • Subduction plate interface constitution, thickness, geometry and viscosity • Rocks recovered are reliable probes of subduction processes and thermal regimes • Long-term mechanical coupling controls rock recovery from subduction zones • Subduction cooling induces a long-term trend in rock detachment from the slab • Rock recovery is a proxy for long- and short-term subduction dynamics and coupling Abstract Short- and long-term processes at or close to the subduction plate interface (e.g.,mineral transformations, fluid release, seismicity and more generally deformation) might be more closely related than previously thought. Increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle. This contribution uses rocks recovered (episodically) from subduction zones, together with insights from thermomechanical modelling, to provide a new dynamic vision of the nature, structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones (e.g.,coupling inferred from geophysical monitoring) and fossil ones (e.g.,coupling required to detach and recover subducted slab fragments). Based on critical observations and an exhaustive compilation of worldwide subducted oceanic units (for which the presence near the plate interface, rock types, pressure, temperature, T/P gradients, thickness and timing of detachment can be assessed), the present study demonstrates how long-term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery. Critical assessment of rock T/P characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to ~2.8 GPa. Rock clusters are identified at depths of 30, 55–60 and 80 km, with some differences between rock types. Data also reveal a first-order evolution with subduction cooling (in the first ~5 Myr), which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling, after which subduction is lubricated and mostly inhibits rock recovery. This contribution places bounds on the plate interface constitution, regular thickness (<300 m; i.e. where/when there is no detachment), changing geometry and effective viscosity. The concept of ‘coupled thickness' is used here to capture subduction interface dynamics, notably during episodes of strong mechanical coupling, and to link long- and short-term deformation. Mechanical coupling depends on mantle wedge rheology, viscosity contrasts and initial structures (e.g.,heterogeneous lithosphere, existence of décollement horizons, extent of hydration, asperities) but also on boundary conditions (convergence rates, kinematics), and therefore differs for warm and cold subduction settings. Although most present-day subduction zone segments (both along strike and downdip) are likely below the detachment threshold, we propose that the most favorable location for detachment corresponds to the spatial transition between coupled and decoupled areas. Effective strain localization involves dissolution-precipitation and dislocation creep but also possibly brittle fractures and earthquakes, even at intermediate depths

Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps)

International audienceThe Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents an almost intact fragment of Tethyan oceanic lithosphere metamorphosed at 80 km depth ( 2.6GPa-550 °C) during the Alpine subduction. We focus our study on a major shear zone cutting across this slab fragment at low angle (the Lower Shear Zone; LSZ). Here, in its talc and tremolite-rich serpentinite matrix, are embedded (together with metasedimentary lenses) variously brecciated Fe-Ti and Mg-Al metagabbro blocks. The latter were either interpreted as eclogitic breccias resulting from intermediate-depth rupture or as inherited, overprinted oceanic core complex features. Our new field, structural and petrographic data testify the genesis of this metagabbro breccia blocks at eclogite-facies conditions. Three types of eclogitic blocks can be distinguished, with non-random distribution (and decreasing size from top to base) throughout the 200-m-thick and 15 km-long LSZ: (1) Fe-Ti-metagabbros, brecciated and scattered in the upper to intermediate levels of the LSZ; (2) meter-size blocks and decameter-scale slivers of intact Mg-Al metagabbros, locally brecciated; (3) dm- to m-scale blocks of intact Fe-Ti metagabbros without breccia fabrics. Brecciation at eclogite facies conditions (at 80 km depth) is documented by: i) the eclogitic foliation of intact Mg-Al-rich metagabbros (composed of omphacite + clinozoisite ± rutile and locally garnet) cut by breccia planes (cemented by omphacite + garnet ± lawsonite) and ii) the occurrence in breccia clasts of minerals that are fractured and offset along peak P-T omphacite-bearing planes. Rupture preferentially affected the Fe-Ti metagabbros, suggesting that rheological contrasts controlled the locus of brecciation. The occurrence of a first omphacite-rich matrix (M1, 2.7GPa - 580 °C) crosscut by omphacite + garnet-bearing matrix M2 ( 2.4GPa - 560 °C), witnesses multiple brittle rupture events, prior to a stage of eclogite facies fluid ingression marked by massive lawsonite recrystallization (matrix M3)

No large earthquakes in fully exposed subducted seamount

Bathymetric highs on the ocean floor ultimately sink into highly seismic subduction zones, raising vigorous debates on their potential to trigger or arrest large earthquakes (Mw > 7.5). Many geophysical and seismological studies addressing this problem meet penetration and/or resolution issues and deal with only the most recent earthquakes. We herein present the missing piece of the puzzle with the time-integrated field and petrographic record of a unique, almost intact subducted seamount cropping out along a fossil subduction interface. We document seamount buildup and subduction down to ∼30 km, and we show that this seamount did not behave as a large earthquake asperity and may have acted as a barrier

Scales of fluid-rock interaction and carbon mobility in the deeply underplated and HP-Metamorphosed Schistes Lustrés, Western Alps

International audienceThe Schistes Lustrés metasedimentary complex, exposed in the Italian and French Alps, is a regionally extensive unit of blueschist to eclogite facies marble, calc-schist and metapelite providing a record of sediment underplating and fluid-rock interaction at 40-65 km of a paleo-subduction zone. Limestones, carbonate-rich mudstones, and black shales in SE France and central Switzerland, herein used as a proxy for the unmetamorphosed Schistes Lustrés, bear a striking resemblance to sediments currently subducting into the E. Sunda margin. The Schistes Lustrés, with estimated P-T conditions ranging from ∼1 GPa and 350 °C to ∼2.3 GPa and 550 °C, have on a regional scale had their average carbonate δ18OVSMOW lowered to +20.4‰, relative to the average of +28.5‰ of their likely protoliths. This decrease in δ18O can be most easily explained by varying combinations of: (1) closed-system equilibration of carbonate O with O in silicate minerals in the same rocks (i.e., at the μm to cm scale); and (2) interaction between carbonates and H2O-rich fluids released during prograde metamorphic devolatilization of the metapelitic rocks in the section (at up to km scales). Regional-scale decrease in carbonate δ18O in the Schistes Lustrés could have occurred without the need for infiltration by fluids generated outside of the Schistes Lustrés complex. More dramatic reductions in δ18O (as low as +8‰; average = +15.5‰) at/near major tectonic contacts, inferred to be fossil transient subduction interfaces, likely reflect deformation-enhanced infiltration by fluids derived in mafic or ultramafic rocks at greater depths. Pressure solution and concomitant cleavage formation in the Schistes Lustrés presumably were associated with some carbonate removal, and higher-grade rocks show some evidence for decarbonation. However, within the Schistes Lustrés, C is deposited as carbonate in locally abundant veins and it has proven difficult to identify clear evidence of wholesale carbonate removal from the complex (i.e., at scales of kms to 10s of kms). This study demonstrates that large fractions of subducted sedimentary carbonate sections could persist to depths approaching those beneath volcanic fronts, if they are not accreted or underplated and depending on the extents to which they are infiltrated by H2O-rich fluids capable of driving both decarbonation reactions and carbonate dissolution. Sediments such as these could supply C for additions to arc source regions or, with further subduction, convey C into the deeper mantle. The C removal and deposition reported by others for sites of particularly high fluid flux, along large fault systems, in zones of high fracture/vein density, and in metasomatized contacts with mafic/ultramafic rocks, contrast with the relative retention and lower degrees of mobilization of C observed within km-scale packets of Schistes Lustrés away from zones of enhanced deformation and behaving as relatively closed systems. The fragmented and potentially lithologically biased nature of HP/UHP metamorphic suites greatly complicates attempts to derive fluid and C fluxes scalable to modern subduction margins

High-resolution numerical modeling of tectonic underplating in circum-Pacific subduction zones: toward a better understanding of deformation in the episodic tremor and slip region?

International audienceStudy of now-exhumed ancient subduction systems have evidenced km-scale tectonic units of marine sediments and oceanic crust that have been tectonically underplated (i.e. basally accreted) from the downgoing plate to the overriding plate at more than 30-km depth. Such huge mass transfers must have a major impact, both in term of long-term topographic variations and seismic/aseismic deformation in subduction zones. However, the quantification of such responses to the underplating process remains poorly constrained. Using high-resolution visco-elasto-plastic thermo-mechanical models, we present with unprecedented details the dynamics of formation and destruction of underplated complexes in subductions zones. Initial conditions in our experiments are defined in order to fit different subduction systems of the circum-Pacific region where underplating process is strongly suspected (e.g. the Cascadia, SW-Japan, New Zealand, and Chilean subduction zones). It appears that whatever the subduction system considered, underplating of sediments and oceanic crust always occur episodically forming a coherent nappe stacking at depths comprised between 10 and 50 km. At higher depth, a tectonic mélange with a serpentinized mantle wedge matrix developed along the plates interface. The size of these underplated complexes changes according to the subduction system considered. For instance, a 15-km thick nappe stacking is obtained for the N-Chilean subduction zone after a series of underplating events. Such an episodic event lasts 4-5 Myrs and can be responsible of a 2-km high uplift in the forearc region. Subsequent basal erosion of these underplated complexes results in their only partial preservation at crustal and mantle depth, suggesting that, after exhumation, only a tiny section of the overall underplated material can be observed nowadays in ancient subduction systems. Finally, tectonic underplating in our numerical models is systematically associated with (1) an increasing thickness of the high-strained subduction channel and (2) an accumulation of fluid-rich materials that serve as an environment for episodic tremor and slip events assisted by tectonic shearing and fluid release and percolation