Recent geophysical investigations provide the high-resolution seismic velocity structure of the crust and uppermost mantle of island arcs. This paper summarizes lithological investigations of the Northeast Honshu Arc and the Izu Arc by comparing the seismic velocity structures with high-pressure and high-temperature laboratory ultrasonic measurements of elastic wave velocity for rocks (e.g. Ichinomegata xenoliths) that are derived from the deep crust and upper mantle. The velocity structure of the lower crust of the Northeast Honshu Arc is well correlated with the tectonic history of the arc. The lower crust of the Northeast Honshu Arc is inferred to be composed mainly of pyroxene hornblende gabbro (Miocene rift basin), hornblende gabbro (Ou Backbone Range, Kitakami Lowland), and quartz-bearing rock (Kitakami Belt). Anomalously low-Vp/Vs uppermost mantle in the Kitakami Belt is inferred to be composed of orthopyroxenite, which can form by a reaction between slab-derived silicic melts and mantle peridotite. The results indicate that the lower crust and the uppermost mantle of the Izu Arc and the Northeast Honshu Arc are more heterogeneous than previously thought.
Rheological structures for continental and oceanic plates were calculated using the laboratory-based frictional and flow laws. Our results show that Peierls creep becomes the dominant mechanism for plastic deformation at low temperatures and high stresses under both dry and wet conditions. When Peierls creep, rather than dislocation-accommodated power-law creep, is the dominant mechanism, there is a lower maximum mechanical strength, and a shallower depth to the brittle-plastic transition in our model. The rheology of continental lithosphere is complex, but may feature a weak lower crust sandwiched between a strong upper crust and mantle. Extensive deformation of this weak zone may explain crustal duplication in continental collision zone. The thick continental lithosphere beneath craton is stable for billions of years. This is partly because partial melting has depleted these regions of water, causing an increase in the mechanical strength of the continental lithosphere. The depth of brittle-plastic transition at island arcs, which we inferred from the maximum depth of seismicity, suggests that the fore-arc regions are enriched in water, but that the back-arc regions are depleted. The rheological structure of oceanic plate is dependent on the age of the plate, and the thickness of oceanic lithosphere is largely consistent with a dry rheology. A relatively thin elastic thickness found for the oceanic lithosphere in the Mariana and Bonin arcs may be explained by local weakening as a result of hydration along outer-rise faults.
We show that the structure and chemistry of grain boundaries in natural rocks are comparable to those in advanced ceramic materials. As a result of grain boundary migration, the Zener relationship (log dol/dpx against log fpx; where dol, dpx and fpx are the grain sizes of olivine and pyroxene, and the fraction of pyroxene, respectively) measured in natural mylonite is remarkably similar to that observed in synthetic fine-grained aggregates of olivine and pyroxene. Superplasticity of the fine-grained synthetic mineral aggregate was demonstrated with observations of the same phase aggregation, dynamic grain growth and development of crystallographic preferred orientation, all of which were consequences of grain boundary sliding. We identify such processes in the natural rocks. We infer that similarities between natural and synthetic rocks are a result of the common behavior of grain boundaries in both materials. We conclude that synthetic fine-grained mineral aggregates are good analogues for polycrystalline rocks, and should be used to investigate grain boundary processes in the interior of the Earth.
Stress tensor inversion techniques are widely used in various fields of solid Earth sciences to elucidate (paleo) stress states in the Earth’s crust. In recent decades, these techniques have been developed and utilized for disaster prevention and assessment of nuclear power plant safety, as well as for scientific purposes. The data sources used in such studies include the orientations of mesoscale faults observed in outcrops, seismic focal mechanisms, dilatant fractures such as dykes and mineral veins, and mechanical twins in calcite grains. Inversion techniques for fault-slip data, including geological faults and seismic focal mechanisms, have been enhanced to detect multiple stress conditions from heterogeneous data and to examine spatiotemporal changes in stress. In addition, methods for analyzing dilatant fractures have been enhanced to determine stress regimes and stress ratios, with implications for fluid pressure levels. Recent refinements in calcite twinning analysis have aided estimates of differential stress magnitudes.
The Himalayan range was formed and uplifted in association with the southward migration of plate boundary thrusts that separate the Himalaya into four belts. Initially, during 50-35 Ma, the Tibetan marginal mountain range was uplifted after slab break-off of the Tethyan oceanic plate, which was subducted under the Asian continent to depths of up to 100 km. During the second stage at ~35-25 Ma, the Mesoproterozoic sediments deposited on the northern passive margin of the Greater India were subducted and underwent moderate-pressure metamorphism at depths of ~40 km. Subsequent to metamorphism, metamorphosed continental crust was separated from the underlying mantle by delamination, and its rapid exhumation and associated amphibolite facies metamorphism occurred during 22-16 Ma. Partially melted metamorphic rocks generated granitic melt that intruded both metamorphic rocks and Tibetan Tethys sediments during the Miocene. Exhumation of the metamorphic belt continued after its exposure at ~15 Ma, forming an extensive metamorphic nappe covering the Lesser Himalayan sediments. After ceasing movement at 11-10 Ma, the Indian plate started to subduct along the Main Boundary Thrust (MBT), which was newly formed in front of the southern margin of the metamorphic nappe. At the same time, the nappe and weakly metamorphosed underlying Lesser Himalayan sediments started to cool laterally from the southern front to the root zone at a rate of ~10 km/Myr. At 3-2.5 Ma, the plate boundary fault shifted to the Main Frontal Thrust (MFT) to the south of the MBT, causing rapid uplift of the marginal range of the Lesser Himalaya and the Siwalik Hills. Today, the Indian Plate is converging with the Asian Continent at the rate of 58 mm/yr, and half of this convergence is consumed by uplift of the Siwalik Hills along the MFT.
The Ohno Volcanic Rocks in eastern Kyusyu occur at the western tip of the Setouchi Volcanic Rocks. Previously reported K-Ar ages of the Ohno Volcanic Rocks range from 10.5 to 15.9 Ma. Here we present the results of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb analyses of three felsic rocks from the Ohno Volcanic Rocks. We obtained mean 238U-206Pb ages as follows: 14.71±0.20 Ma (2σ) for the Shiraiwasan pyroclastic flow deposit at the lowermost horizon within the sequence, 15.14±0.43 Ma for the Koguraki Tuff, and 14.58 ± 0.21 Ma for the Miyakeyama pyroclastic flow deposit at the uppermost horizon within the sequence. Our results indicate that the Ohno Volcanic Rocks, except for the high-Mg andesite found only as dikes, were formed over a shorter period than previously considered (ca. 1 m.y.).
We describe the characteristics of a Rhinocerotidae femur and tibia found within sea-floor sediments in the Bisan-Seto region, western Japan, a region for which the mammal fauna has been assigned the Middle Pleistocene age. Based on morphological and metrical comparisons between the studied specimens and those from Pleistocene rhinoceroses from Eurasia (Stephanorhinus, Coelodonta, and Rhinoceros), the studied specimens can be referred to as Stephanorhinus sp., although their more specific identification is not possible given a lack of further skeletal elements. Nevertheless, our study confirms the presence of Stephanorhinus during the Middle Pleistocene in Japan and supports similar finds elsewhere in western Japan (e.g., Isa in Yamaguchi Prefecture).