Hugues Raimbourg

Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes?

Understanding 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 report the finding of eclogite breccias, formed at ∼80 km depth during subduction, in an almost intact 10-km-scale fragment of exhumed oceanic lithosphere (Monviso ophiolite, Western Alps). These eclogite breccias correspond to meter-sized blocks made of 1–10 cm fragments of eclogite mylonite cemented by interclast omphacite, lawsonite, and garnet, and were later embedded in serpentinite in a 30–150-m-wide eclogite facies shear zone. At the mineral scale, omphacite crack-seal veins and garnet zoning patterns also show evidence for polyphased fracturing-healing events. Our observations suggest that a possible seismic brecciation occurred in the middle part of the oceanic crust, accompanied by the input of externally derived fluids. We also conclude that these eclogite breccias likely mark the locus of an ancient fault zone associated with intraslab, intermediate-depth earthquakes at ∼80 km depth.

Interpretation of porosity and LWD resistivity from the Nankai accretionary wedge in light of clay physicochemical properties: Evidence for erosion and local overpressuring

In this study, we used porosity to assess the compaction state of the Nankai accretionary wedge sediments and any implications for stress and pore pressure. However, hydrous minerals affect porosity measurements, and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state. The water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale. We used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals. The results indicate that several apparent porosity anomalies are significantly reduced by this correction, implying that they are in part artifacts from hydrous minerals. The correction also improves the fit of porosity estimated from logging-while-drilling (LWD) resistivity data to porosity measured on cores. Low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion, and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity-effective stress relationships of the sediments to a reference curve. Additionally, a comparison of LWD data with core data (resistivity and P wave velocity) obtained at Site C0001 landward of the mega-splay fault area, suggested a contribution from the fracture porosity to in situ properties on the formation.

Initiation of crustal‐scale thrusts triggered by metamorphic reactions at depth: Insights from a comparison between the Himalayas and Scandinavian Caledonides

Active eclogitization has recently been inferred at depth beneath the Himalaya from geophysical observations, and the mechanical consequences of eclogitization can be observed in the field in the eroded and extended nappe stack of the Scandinavian Caledonides. There, Proterozoic metastable granulites and igneous protoliths underwent partial eclogitization during the collision of Baltica with Laurentia. The reaction began in pseudotachylites and veins and eventually formed a connected network of eclogite-facies shear zones that localized deformation and weakened the lower crust of Baltica during the collision with Laurentia. All these features can be compared with the seismic activity of the Indian Lower Crust, its strength loss beneath the Himalayan ranges, and its delayed density increase regarding its penetration in the eclogite facies. The Caledonian Bergen Arc eclogites and the Himalayan Ama Drime eclogites are both derived from continental crust. In both cases, these eclogites were formed contemporaneously with the activation of the main thrusts responsible for the construction of the orogenic wedges, the Main Central Thrust in the Himalayas, and the main thrust below the Jotun Nappe Complex in the Caledonides. The similarities in these two orogens, which compare both in size and structure, highlight the importance of eclogitization at depth as a mechanism for weakening of the lower crust and for decoupling of the crust and lithospheric mantle in collision zones.

Acoustic and mechanical properties of Nankai accretionary prism core samples

We studied undeformed sediment and accreted strata recently recovered by Ocean Drilling Program/Integrated Ocean Drilling Program (ODP/IODP) drilling in Nankai Trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation. We have derived acoustic (Vp) and mechanical (uniaxial poroelastic compliance, compaction amplitude) properties of samples from various drill sites along the Muroto (ODP 1173) and Kii transects (IODP C0001, C0002, C0006, and C0007) from isotropic loading tests where confining and pore pressure were independently applied. We quantified the dependence of Vp on both effective (Peff) and confining (Pc) pressure, which can be used to correct atmospheric pressure measurements of Vp. Experimental Vp obtained on core samples extrapolated to in situ conditions are slightly higher than logging-derived velocities, which can be attributed either to velocity dispersion or to the effect of large-scale faults and weak zones on waves with longer wavelength. In the high-porosity (30%–60%) tested sediments, velocities are controlled at first order by porosity and not by lithology, which is in agreement with our static measurements of drained framework incompressibility, much smaller than fluid incompressibility. Rather than framework incompressibility, shear modulus is probably the second-order control on Vp, accounting for most of the difference between actual Vp and the prediction by Woods (1941) suspension model. We also quantified the mechanical state of Nankai samples in terms of anisotropy, diagenesis, and consolidation. Both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions, attesting to the very low anisotropy of the tested material. When considering the porous samples of the Upper Shikoku Basin sediments (Site 1173) as examples of diagenetically cemented material, several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation. We also detected incipient cementation in samples from IODP Site C0001 (accretionary prism unit). In terms of consolidation, we distinguished two classes of material response (shallow, deformable samples and deep, hardly deformable ones) based on the amount of compaction upon application of a Peff large with respect to the inferred in situ value, with a transition that might be related to a critical porosity.

Long‐term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan

The Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic melange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a “simple” giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional.

Kinematics of syneclogite deformation in the Bergen Arcs, Norway: implications for exhumation mechanisms

Abstract The northwestern part of Holsnøy island, in the Bergen Arcs, Norway, consists of a granulite-facies protolith partially transformed at depth in eclogite (700 °C, > 19 kbars) and amphibolite (650 °C, 8–10 kbars) facies during the Caledonian orogenesis. Eclogitized zones are mainly planar objects (fractures with parallel reaction bands and cm-to-100 m-scale shear zones). Eclogitic zones are distributed in two sets of orientations and the associated deformation can be described as ‘bookshelf tectonics’. The major shear zones strike around N120 and dip to the North, and show consistent top-to-the-NE shear sense throughout the area. In the large-scale kinematic frame of Caledonian NW-dipping slab, eclogitic shear zones are interpreted as the way to detach crustal units from the subducting slab and to prevent their further sinking. As the retrograde amphibolitic deformation pattern is similar to the eclogitic one, the detached crustal units started their way up along these eclogitic shear zones. Radiometric ages of eclogitic and amphibolitic metamorphism and their comparison with the chronology of Caledonian orogenesis show that the deformation recorded on Holsnøy occurred in a convergent context. The mechanism we propose can thus account for the first steps of exhumation during collision.

Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique

High-resolution structure analyses using electron beam techniques have been performed for the investigation of subgrain boundaries (SGBs) in deformed orthopyroxene (Opx) in mylonite from Hidaka Metamorphic Belt, Hokkaido, Japan, to understand ductile deformation mechanism of silicate minerals in shear zones. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis of Opx porphyroclasts in the mylonitic rock indicated that the crystal orientation inside the Opx crystals gradually changes by rotation about the b-axis by SGBs and crystal folding. In order to observe the SGBs along the b-axis by transmission electron microscopy (TEM) or scanning TEM (STEM), the following sample preparation protocol was adopted. First, petrographic thin sections were slightly etched with hydrofluoric acid to identify SGBs in SEM. The Opx crystals whose b-axes were oriented close to the normal of the surface were identified by EBSD, and the areas containing SGBs were picked and thinned for (S) TEM analysis with a focused ion beam instrument with micro-sampling system. High-resolution TEM imaging of the SGBs in Opx revealed various boundary structures from a periodic array of dissociated (100) [001] edge dislocations to partially or completely incoherent crystals, depending on the misorientation angle. Atomic-resolution STEM imaging clearly confirmed the formation of clinopyroxene (Cpx) structure between the dissociated partial dislocations. Moreover, X-ray microanalysis in STEM revealed that the Cpx contains a considerable amount of calcium replacing iron. Such chemical inhomogeneity may limit glide motion of the dislocation and eventually the plastic deformation of the Opx porphyroclasts at a low temperature. Chemical profiles across the high-angle incoherent SGB also showed an enrichment of the latter in calcium at the boundary, suggesting that SGBs are an efficient diffusion pathway of calcium out of host Opx grain during cooling.

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

We used a field analysis of rock deformation microstructures and mesostructures to reconstruct the long-term orientation of stresses around two major active fault systems in Japan, the Median Tectonic Line and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, active since the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching in the Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinity during each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more fault normal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting an extremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of this long-term stress perturbation include a nearly total release of shear stress during earthquakes, a low static friction coefficient, or lowelastic properties of the fault zones comparedwith the country rock. Independently of the preferred interpretation, the nearly fault normal orientation of the direction of maximum compression suggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluid pressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than in preearthquake fluid overpressures.

Mechanical properties and processes of deformation in shallow sedimentary rocks from subduction zones: An experimental study

To better constrain the mechanical behavior of sediments accreted to accretionary prism, we conducted triaxial mechanical tests on natural samples from the Miura-Boso paleo-accretionary prism (Japan) in drained conditions with confining pressures up to 200 MPa as well as postexperiments P-wave velocity (Vp) measurements. During experiments, deformation is principally noncoaxial and accommodated by two successive modes of deformation, both associated with strain-hardening and velocity-strengthening behavior: (1) compaction-assisted shearing, distributed in a several mm-wide shear zone and (2) faulting, localized within a few tens of μm-wide, dilatant fault zone. Deformation is also associated with (1) a decrease in Youngs modulus all over the tests, (2) anomalously low Vp in the deformed samples compared to their porosity and (3) an increase in sensitivity of Vp to effective pressure. We interpret this evolution of the poroelastic properties of the material as reflecting the progressive breakage of intergrain cement and the formation of microcracks along with macroscopic deformation. When applied to natural conditions, these results suggest that the deformation style (localized versus distributed) of shallow (z < a few km) sediments is mainly controlled by the variations in stress/strain rate during the seismic cycle and is therefore independent of the porosity of sediments. Finally, we show that the effect of strain, through cement breakage and microcracks formation, may lower Vp for effective pressure up to 40 MPa. As a consequence, the low Vp anomalies observed in Nankai accretionary prisms by seismic imaging between 2 and 4 km depth could reflect sediment deformation rather than porosity anomalies.

Tertiary evolution of the Shimanto belt (Japan): a large-scale collision in Early Miocene

To decipher the Miocene evolution of the Shimanto belt of southwestern Japan, structural and paleo-thermal studies we carried out in the western area of Shikoku Island. All units constituting the belt, both in its Cretaceous and Tertiary domains, are in average strongly dipping to the NW or SE, while shortening directions deduced from fault kinematics are consistently orientated NNW-SSE. Peak paleo-temperatures estimated with Raman spectra of organic matter increase strongly across the southern, Tertiary portion of the belt, in tandem with the development of a steeply-dipping metamorphic cleavage. Near the southern tip of Ashizuri Peninsula, the unconformity between accreted strata and fore-arc basin, present along the whole belt, corresponds to a large paleo-temperature gap, supporting the occurrence of a major collision in Early Miocene. This tectonic event occurred before the magmatic event that affected the whole belt at ~15 Ma. The associated shortening was accommodated in two opposite modes, either localized on regional-scale faults such as the Nobeoka Tectonic Line in Kyushu, or distributed through the whole belt as in Shikoku. The reappraisal of this collision leads to reinterpret large-scale seismic refraction profiles of the margins, where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the Shimanto belt. When integrated into reconstructions of Philippine Sea Plate motion, the collision corresponds to the oblique collision of a paleo Izu-Bonin-Mariana Arc with Japan in Early Miocene.