Yukari Kido

Hot fingers in the mantle wedge : New insights into magma genesis in subduction zones

Abstract Quaternary volcanoes in the Northeast Japan arc can be grouped into 10 volcanic clusters striking transverse to the arc; these have an average width of 50 km, and are separated by parallel gaps 30–75 km wide. This clustering of volcanic centres, topographic profiles, low-velocity regions in the mantle wedge and local negative Bouguer gravity anomalies along the Japan Sea side of the volcanic arc are closely correlated. All these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined, 50 km wide fingers. Each of the 10 fingers recognised extends from deep mantle (>150 km) below the back-arc region towards the shallower mantle (∼50 km) beneath the volcanic front. Quaternary volcanoes are built immediately above the hot mantle fingers. The volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust, accompanied by Quaternary volcanic activity at the surface. Although volcanic activity is rare along the Japan Sea coast, tomographic results show that hot, low-velocity mantle fingers exist within the mantle wedge. The negative Bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers; these have not yet been erupted, but have accumulated at the Moho discontinuity.

Tectonic features of the Japan Trench convergent margin off Sanriku, northeastern Japan, revealed by multichannel seismic reflection data

Near the Japan Trench convergent plate margin the seaward edge of the continental plate is deformed by subduction of the oceanic plate. We report the results of a multichannel seismic survey in the northern Japan Trench in which this deformed zone is demarcated from the rigid continental framework by a pronounced landward dipping reflector. The oceanic plate also undergoes deformation as the two plates interact in the subduction processes, resulting in a progressive deformation or destruction of a horst structure along the top of the subducting oceanic crust. This may eventually lead to the formation of a smooth plate boundary at the greater depth. More than 45 km landward from the trench axis, a smooth reflector suggesting a stable slip plane is visible along and above the oceanic crust. Our data indicate that the deformed zone pinches out landward ∼60 km from the axis at 13 km depth and the slip plane becomes less obvious there. Seismicity of interplate earthquakes rapidly increases landward from this location, leading us to speculate that this is where coupling at the plate boundary becomes strong enough for earthquakes to occur. We conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characterized by the tectonic feature of a pinchout of the deformed sediments which overlie the subducting oceanic crust.

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Stress State in the Largest Displacement Area of the 2011 Tohoku-Oki Earthquake

Stressed Out Large seismic events such as the 2011 magnitude 9.0 Tohoku-Oki earthquake can have profound effects not just on the severity of ground motion and tsunami generation, but also on the overall state of the crust in the surrounding regions. Lin et al. (p. 687) analyzed the stress 1 year after the Tohoku-Oki earthquake and compared it with the estimated stress state before the earthquake. In situ resistivity images were analyzed from three boreholes drilled into the crust across the plate interface where the earthquake occurred. Stress values indicate a nearly complete drop in stress following the earthquake such that the type of faulting above the plate boundary has changed substantially. These findings are consistent with observations that the sea floor moved nearly 50 meters during the earthquake. Borehole stress measurements indicate a nearly total stress drop in the region of largest slip. The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced a maximum coseismic slip of more than 50 meters near the Japan trench, which could result in a completely reduced stress state in the region. We tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the Integrated Ocean Drilling Program approximately 1 year after the earthquake and by inferring the pre-earthquake stress state. On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis, in situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate interface, which is consistent with coseismic fault weakening and a nearly total stress drop.

Heat flow estimated from BSR and IODP borehole data: Implication of recent uplift and erosion of the imbricate thrust zone in the Nankai Trough off Kumano

Heat flow values were estimated in the Nankai Trough fore-arc slope region off Kii Peninsula from the Bottom-Simulating Reflector (BSR) and other data obtained during IODP Expeditions 315 and 316. The heat flow has an uncertainty of ∼20% mainly due to ambiguities in the temperature estimate at the BSR and uncertainties on the thermal conductivity. BSRs occur intermittently in the Imbricate Thrust Zone (ITZ). They are significantly shallower below anticlines, designated “Anticlinal High Zone” (AHZ), than in the adjacent synclines (“Basal Low Zone” (BLZ)). The heat flow in the BLZ (55–65 mW/m2) is consistent with the regional heat flow trend. The shallow BSR in the AHZ produces an apparent high heat flow anomaly of 70–90 mW/m2, and discontinuities are observed across thrust faults. The most likely cause is the transient effect of thrust faulting followed by erosion on the hanging wall side. A one-dimensional time-dependent numerical model, with latent heat for hydrate and gas transition considered, indicates that the relaxation time for the BSR to reequilibrate after such a disturbance is ∼10 kyr. Although the possibility that the shallow BSR is a remnant base of hydrate stability zone cannot be ruled out, if the shortening has occurred in the recent 10 kyr the convergent rate at the ITZ would be ∼30 m/kyr, which is much larger than the recent horizontal shortening across the megasplay fault, and could even exceed the portion of the plate convergence than that accommodated at the frontal thrust.

Re-evaluation of temperature at the updip limit of locked portion of Nankai megasplay inferred from IODP Site C0002 temperature observatory

In 2010, the first long-term borehole monitoring system was deployed at approximately 900 m below the sea floor (mbsf) and was assumed to be situated above the updip limit of the seismogenic zone in the Nankai Trough off Kumano (Site C0002). Four temperature records show that the effect of drilling diminished in less than 2 years. Based on in situ temperatures and thermal conductivities measured on core samples, the temperature measurements and heat flow at 900 mbsf are estimated to be 37.9°C and 56 ± 1 mW/m2, respectively. This heat flow value is in excellent agreement with that from the shallow borehole temperature corrected for rapid sedimentation in the Kumano Basin. We use these values in the present study to extrapolate the temperature below 900 mbsf for a megasplay fault at approximately 5,200 mbsf and a plate boundary fault at approximately 7,000 mbsf. To extrapolate the temperature downward, we use logging-while-drilling (LWD) bit resistivity data as a proxy for porosity and estimate thermal conductivity from this porosity using a geometrical mean model. The one-dimensional (1-D) thermal conduction model used for the extrapolation includes radioactive heat and frictional heat production at the plate boundary fault. The estimated temperature at the megasplay ranges from 132°C to 149°C, depending on the assumed thermal conductivity and radioactive heat production values. These values are significantly higher, by up to 40°C, than some of previous two-dimensional (2-D) numerical model predictions that can account for the high heat flow seaward of the deformation front, including a hydrothermal circulation within the subducted igneous oceanic crust. However, our results are in good agreement with those of the 2-D model, which does not include the advection cooling effect. The results imply that 2-D geometrical effects as well as the influence of the advective cooling may be critical and should be evaluated more quantitatively. Revision of 2-D simulation by introducing our new boundary conditions (37.9°C of in situ temperature at 900 mbsf and approximately 56 mW/m2 heat flow) will be essential. Ultimately, in situ temperature measurements at the megasplay fault are required to understand seismogenesis in the Nankai subduction zone.

Gravity and magnetic constraints on the crustal structure and evolution of the Horeki seamount in the Izu-Ogasawara (Bonin) arc

Data on the crustal structure, bulk composition, and eruption ages of an arc seamount were obtained in an investigation aimed at studying the spatial and temporal variations in the magma composition of an intra-oceanic arc. We conducted gravity and magnetic surveys of the Horeki seamount and evaluated the density and magnetization structure using inversion and forward modeling. The seamount is located on the back-arc side of the Izu-Ogasawara arc. This seamount has an elliptical shape, a flat summit, and satellite ridges and cones on its northern and southern flanks. The flanks are consistent with a higher density anomaly, with an estimated mean density corresponding to dense basaltic rocks. A low-density anomaly is distributed in the seamount top, indicating that the top likely consists of porous basalts or differentiated rocks. The prominent circular low Bouguer gravity anomaly, which appeared in the northern part of the flat-topped summit, indicates that a light-density material fills the summit. The main body of seamount is normally magnetized. Combined with the age of the rocks, the volcanism constructing the main body may be most robust in the Gauss chron. The deeper part of seamount may consist of intrusive rocks, with induced magnetization over remanence. The eastern part of the northern ridges is reversely magnetized, while the western part is normally magnetized. These features and the ages of the sampled rocks suggest that these ridges were constructed in the Matuyama and Brunhes chrons, respectively. The southern part of the seamount flanks shows weak normal magnetization, probably caused by the small cones with different polarities of remanent magnetization.

Polysaccharide hydrolase of the hadal zone amphipods Hirondellea gigas

Abstract Hirondellea species are common inhabitants in the hadal region deeper than 7,000 m. We found that Hirondellea gigas thrived in the Challenger Deep possessed polysaccharide hydrolases as digestive enzymes. To obtain various enzymes of other H. gigas, we captured amphipods from the Japan Trench, and Izu-Ogasawara (Bonin) Trench. A phylogenetic analysis based on the cytochrome oxidase I gene showed close relationships among amphipods, despite the geographic distance between the localities. However, several differences in enzymatic properties were observed in these H. gigas specimens. We also carried out RNA sequencing of H. gigas from the Izu-Ogasawara Trench. The cellulase gene of H. gigas was highly homologous to cellobiohydrolase of Glucosyl Hydrolase family 7 (GH7). On the other hand, enzymatic properties of H. gigas’s cellulase were different from those of typical GH7 cellobiohydrolase. Thus, these results indicate that hadal-zone amphipod can be good candidates as the new enzyme resource. The deepest sea amphipod, Hirondellea gigas digests plant debris with GH7 cellobiohydrolase, GH9 ß-1. 4 glucanase, and amylase to obtain nutrients in oligotrophic sea bottom.

Faulting and Bending of Oceanic Crust Around Japan Trench

This study was made as one of oceanic crust dynamics researches which Japan Marine Science and Technology Center (JAMSTEC) currently conducts.

Visualization of attenuation structure and faults in incoming oceanic crust of the Nankai Trough using seismic attenuation profiling

Seismic attenuation properties were tested as indicators of lateral variation in geological structures and detection of faults within poorly reflective oceanic crust, on a seismic survey line along the Nankai Trough. We can specify both sedimentary structures by configuration of reflections and faults by offsetting of reflections on seismic reflection profiles. This procedure is often applied to analyze geological structures and existence of faults within sedimentary layers; however, it is almost impossible to analyze them within igneous oceanic crust because seismic reflections are inherently invisible there. Therefore, we applied seismic attenuation profiling to visualize geological structures and faults within poorly reflective oceanic crust. As a result, oceanic crust altered by late-coming volcanisms as well as damaged by intraplate earthquakes was imaged as extremely high-attenuation property, which was clearly distinguished from normal oceanic crust. Many faults were observed in the sedimentary unit on the seismic reflection profile, whereas possible lower segments of the faults were imaged as high-attenuation stripes in the oceanic crust on the seismic attenuation profile. Thus, the effectiveness of seismic attenuation profiling to structural and fault imaging within poorly reflective oceanic crust was successfully demonstrated.