Search Results (9)

Mylonitic mantle peridotites exposed at the Tosa Megamullion in the Shikoku Basin, Philippine Sea, provide direct evidence for amagmatic ductile shear deformation of the upper mantle beneath a back-arc spreading center. Oceanic core complexes (OCCs), or megamullions, are dome-shaped structures formed by detachment faulting and occur locally along slow-spreading mid-ocean ridges and back-arc basins, where they expose fault rocks derived from ductile shear zones in the lower crust and upper mantle. The Shikoku Basin hosts several OCCs, including the Tosa Megamullion, which formed during the early stage of back-arc spreading. In this study, nine ultramafic rocks were collected from the Tosa Megamullion using the submersible Shinkai6500 during cruise YK23-05S. Although all samples were highly serpentinized, several preserved primary peridotitic textures were composed mainly of olivine, orthopyroxene, with subordinate clinopyroxene, plagioclase, and spinel. Seven samples exhibit well-developed foliation and porphyroclastic textures dominated by orthopyroxene porphyroclasts, ranging from rounded to strongly elongated forms, commonly showing microkinks and undulose extinction. Crystallographic preferred orientations (CPOs) of three representative samples, analyzed using SEM-EBSD, reveal E-type-dominant olivine fabrics characterized by the (001)[100] slip system, with a subordinate contribution from C-type (100)[001] slip. These CPOs suggest deformation under non-dry conditions involving moderate hydration and/or elevated differential stress. These results indicate that the ultramafic rocks from the Tosa Megamullion represent mantle-derived mylonitic peridotites formed by ductile shear beneath the spreading axis and subsequently exhumed under strongly magma-poor, amagmatic conditions. The Tosa Megamullion thus represents an amagmatic end-member of the OCC formation in back-arc basins, dominated by tectonic strain localization rather than by magmatic accretion. Full article

Olivine, the most abundant mineral in the upper mantle, exhibits elastic anisotropy. Understanding the seismic anisotropy and flow patterns in the upper mantle hinges on the crystallographic preferred orientation (CPO) of olivine. Similarly, hydrous minerals, which also display elastic anisotropy, play a crucial role in explaining seismic anisotropy in numerous subduction zones. High-temperature and -pressure simple shear experiments reveal that the CPO of amphibole can lead to significant seismic anisotropy. In this study, peridotite samples originating from the southern end of the Mariana Trench, commonly containing amphibole, were analyzed. The microdeformation fabric and seismic anisotropy were examined. The results indicate a weak fabric strength in olivine, yet identifiable deformation fabrics of A/D, D, and AG were observed. Various dislocation structures suggest that olivine experiences complex deformation across various temperatures. Not only can the original slip system transform, but the melt/fluid resulting from melting also has a substantial impact on the peridotite. Deformation precedes the melt/rock interaction, resulting in a strong melt/rock reaction under near-static conditions. Furthermore, the modal content of amphibole significantly alters the seismic anisotropy of peridotite. An increase in amphibole content (types I, III, and IV) enhances seismic anisotropy, particularly for type I amphibole. Notably, the presence of type I fabric amphibole promotes the Vs1 polarization direction parallel to the trench in subduction zones, a phenomenon observed in other subduction zones. Therefore, when considering mantle peridotite regions rich in amphibole, the impact of amphibole on seismic anisotropy must be accounted for. Full article

The microstructural relationship between olivine and clinopyroxene is significant in recovering the mantle evolution under clinopyroxene-saturated melting conditions. This study focuses on olivine/clinopyroxene-related ultramafic rocks (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) in the Ells Stream Complex of the Red Hills Massif. (Olivine) clinopyroxenites have an A/D-type olivine crystallographic preferred orientation (CPO) whereas peridotites have various olivine CPO types. B-type olivine CPO was newly discovered, which may have been generated under hydrous conditions. The discovery of B-type CPO means that all six olivine CPO types could exist in a single research area. Clinopyroxene CPOs also vary and have weaker deformation characteristics (e.g., lower M index and weaker intracrystalline deformation) than olivine; thus, they probably melted and the clinopyroxene-rich ultramafic bands existed as melt veins. Irregular clinopyroxene shapes in the peridotites and incoherent olivine and clinopyroxene CPOs ([100]_OL and [001]_CPX are not parallel) also indicate a melted state. The dominant orthorhombic and LS-type CPOs in olivine and clinopyroxene imply that simple shear was the main deformation mechanism. Such complicated microstructural characteristics result from the overprinted simple shear under high temperatures (>1000 °C) and hydrous melting environments until the melt-frozen period. This case study is helpful to better understand the olivine and clinopyroxene relationship. Full article

The podiform chromitite found within the Luobusha ophiolite comprises characteristic nodules and massive chromitites. However, the exact origin of these formations remains a topic of ongoing debate. In this study, the microstructures of olivine and chromite are investigated to unravel their formation processes and shed light on the associated geodynamic mechanisms. EBSD analysis provides insights into chromitite and host peridotite deformation mechanisms. Olivine grains in the host dunite and nodular chromite exhibit crystallographic preferred orientations (CPOs) with D-type fabrics, which show a girdle distribution in the [010] and [001] axes, normal to the foliation plane of the sample. The massive and disseminated chromitite displays B-type and C-type olivine fabric, with a concentration of [001] axes parallel to the lineation of the sample. Crystal plastic deformation can be observed in the Luobusha chromite grains, highlighting intercrystalline deformation processes. Small grains lacking misorientation observed in the massive chromitite are likely attributed to heterogeneous nucleation. Chromite nodules are found to be a patchwork of subgrains with various orientations and high-angle boundary misorientation. The formation of Luobusha chromitite involves deep-seated crystallization, followed by amalgamation, and subsequent deformation within the mantle peridotite. These findings distinguish Luobusha chromitite from other ophiolitic chromite deposits, offering valuable insights into the deformation history and formation processes. Full article

Combined observations from natural and experimental deformation microstructures are often used to constrain the rheological properties of the upper mantle. However, relating natural and experimental deformation processes typically requires orders of magnitude extrapolation in strain rate due to vastly different time scales between nature and the lab. We examined a sheared peridotite xenolith that was deformed under strain rates comparable to laboratory shearing time scales. Microstructure analysis using an optical microscope and electron backscatter diffraction (EBSD) was done to characterize the bulk crystallographic preferred orientation (CPO), intragrain misorientations, subgrain boundaries, and spatial distribution of grains. We found that the microstructure varied between monophase (olivine) and multiphase (i.e., olivine, pyroxene, and garnet) bands. Olivine grains in the monophase bands had stronger CPO, larger grain size, and higher internal misorientations compared with olivine grains in the multiphase bands. The bulk olivine CPO suggests a dominant (010)[100] and secondary activated (001)[100] that are consistent with the experimentally observed transition of the A to E-types. The bulk CPO and intragrain misorientations of olivine and orthopyroxene suggest that a coarser-grained initial fabric was deformed by dislocation creep coeval with the reduction of grain size due to dynamic recrystallization. Comparing the deformation mechanisms inferred from the microstructure with experimental flow laws indicates that the reduction of grain size in orthopyroxene promotes activation of diffusion creep and suggests a high activation volume for wet orthopyroxene dislocation creep. Full article

Shear deformation of a solid-fluid, two-phase material induces a fluid segregation process that produces fluid-enriched bands and fluid-depleted regions, and a crystallographic preferred orientation (CPO) characterized by girdles of [100] and [001] axes sub-parallel to the shear plane and a cluster of [010] axes sub-normal to the shear plane, namely the AG-type fabric. Based on experiments of two-phase aggregates of olivine + basalt, a two-phase flow theory and a CPO formation model were established to explain these microstructures. Here, we investigate the microstructure in a two-phase aggregate with supercritical CO₂ as the fluid phase and examine the theory and model, to evaluate differences in rheological properties due to the presence of CO₂ or basaltic melt. We conducted high-temperature and high-pressure shear deformed experiments at 1 GPa and 1100 °C in a Griggs-type apparatus on samples made of olivine + dolomite, which decomposed into carbonate melt and CO₂ at experimental conditions. After deformation, CO₂ segregation and an AG-type fabric were observed in these CO₂-bearing samples, similar to basaltic melt-bearing samples. An SPO-induce CPO model was used to explain to the formation of the fabric. Our results suggest that the influences of CO₂ as a fluid phase on the microstructure of a two-phase olivine aggregate is similar to that of basaltic melt and can be explained by the CPO formation model for the solid-fluid system. Full article

We investigated the microstructures of five ice core samples from the Styx Glacier, northern Victoria Land, Antarctica. Evidence of dynamic recrystallization was found in all samples: those at 50 m mainly by polygonization, and those at 170 m, largely by grain boundary migration. Crystallographic preferred orientations of all analyzed samples (view from the surface) typically showed a single cluster of c-axes normal to the surface. A girdle intersecting the single cluster occurs at 140–170 m with a tight cluster of a-axes normal to the girdle. We interpret the change of crystallographic preferred orientations (CPOs) at <140 m as relating to a combination of vertical compression, and shear on a horizontal plane, and the girdle CPOs at depths >140 m, as the result of horizontal extension. Based on the data obtained from the ground penetrating radar, the underlying bedrock topography of a nunatak could have generated the extensional stress regime in the study area. The results imply changeable stress regimes that may occur during burial as a result of external kinematic controls, such as an appearance of a small peak in the bedrock. Full article

Eight samples of limestones and marbles were studied by neutron diffraction to collect quantitative texture (i.e., crystallographic preferred orientations or CPO) of calcite deforming at different depths in the crust. We studied the different Texture patterns developed in shear zones at different depth and their influence on seismic anisotropies. Samples were collected in the French and Italian Alps, Apennines, and Paleozoic Sardinian basement. They are characterized by isotropic to highly anisotropic (e.g., mylonite shear zone) fabrics. Mylonite limestones occur as shear zone horizons within the Cenozoic Southern Domain in Alpine thrust-and-fold belts (Italy), the Briançonnais domain of the Western Alps (Italy-France border), the Sardinian Paleozoic back-thrusts, or in the Austroalpine intermediate units. The analyzed marbles were collected in the Carrara Marble, in the Austroalpine Units in the Central (Mortirolo) and Western Alps (Valpelline). The temperature and depth of development of fabrics vary from <100 ${}^{\circ }$ C, to 800 ${}^{\circ }$ C and depth from <10 km to about 30 km, corresponding from upper to lower crust conditions. Quantitative Texture Analysis shows different types of patterns for calcite: random to strongly textured. Textured types may be further separated in orthorhombic and monoclinic (Types A and B), based on the angle defined with the mesoscopic fabrics. Seismic anisotropies were calculated by homogenizing the single-crystal elastic tensor, using the Orientation Distribution Function calculated by Quantitative Texture Analysis. The resulting P- and S-wave anisotropies show a wide variability due to the textural types, temperature and pressure conditions, and dip of the shear planes. Full article

Principal slip zones (PSZs) are narrow (<10 cm) bands of localized shear deformation that occur in the cores of upper-crustal fault zones where they accommodate the bulk of fault displacement. Natural and experimentally-formed PSZs consistently show the presence of nanocrystallites in the <100 nm size range. Despite the presumed importance of such nanocrystalline (NC) fault rock in controlling fault mechanical behavior, their prevalence and potential role in controlling natural earthquake cycles remains insufficiently investigated. In this contribution, we summarize the physical properties of NC materials that may have a profound effect on fault rheology, and we review the structural characteristics of NC PSZs observed in natural faults and in experiments. Numerous literature reports show that such zones form in a wide range of faulted rock types, under a wide range of conditions pertaining to seismic and a-seismic upper-crustal fault slip, and frequently show an internal crystallographic preferred orientation (CPO) and partial amorphization, as well as forming glossy or “mirror-like” slip surfaces. Given the widespread occurrence of NC PSZs in upper-crustal faults, we suggest that they are of general significance. Specifically, the generally high rates of (diffusion) creep in NC fault rock may play a key role in controlling the depth limits to the seismogenic zone. Full article