Comminution and fluidization of granular fault materials: implications for fault slip behavior     

Nobuaki Monzawa  Kenshiro Otsuki   《Tectonophysics》2003,367(1-2):127-143

Whilst faulting in the shallow crust is inevitably associated with comminution of rocks, the mechanical properties of the comminuted granular materials themselves affect the slip behavior of faults. Therefore, the mechanical behavior of any fault progresses along an evolutionary path. We analyzed granular fault rocks from four faults, and deduced an evolutionary trend of fractal size frequency. Comminution of fault rocks starts at a fractal dimension close to 1.5 (2-D measurement), at which a given grain is supported by the maximum number of grains attainable and hence is at its strongest. As comminution proceeds, the fractal dimension increases, and hence comminution itself is a slip weakening mechanism. Under the appropriate conditions, comminuted granular materials may be fluidized during seismic slip events. In this paper, we develop a new method to identify the granular fault rocks that have experienced fluidization, where the detection probability of fragmented counterparts is a key parameter. This method was applied to four fault rock samples and a successful result was obtained. Knowledge from powder technology teaches us that the volume fraction of grains normalized by maximum volume fraction attainable is the most important parameter for dynamic properties of granular materials, and once granular fault materials are fluidized, the fault plane becomes nearly frictionless. A small decrease in the normalized volume fraction of grains from 1 is a necessary condition for the phase transition to fluidization from the deformation mechanism governed by grain friction and crushing by contact stresses. This condition can be realized only when shearing proceeds under unconstrained conditions, and this demands that the gap between fault walls is widened. Normal interface vibration proposed by Brune et al. [Tectonophysics 218 (1993) 59] appears to be the most appropriate cause of this, and we presented two lines of field evidence that support this mechanism to work in nature.  相似文献   

Middle Miocene to Pleistocene magneto-biostratigraphy of ODP Sites 1150 and 1151, northwest Pacific: Sedimentation rate and updated regional geological timescale     

Isao Motoyama  Nobuaki Niitsuma  Toshiaki Maruyama  Hiroki Hayashi  Shin-ichi Kamikuri  Masamichi Shiono  Toshiya Kanamatsu  Kaori Aoki  Chikako Morishita  Kyoko Hagino  Hiroshi Nishi  Motoyoshi Oda 《Island Arc》2004,13(1):289-305

Abstract Shipboard and shore‐based investigation on siliceous and calcareous microfossil biostratigraphy, magneto‐stratigraphy and tephrostratigraphy identified numerous datum events from the sedimentary sequences of Sites 1150 and 1151 drilled on the forearc basin of northern Japan by the Ocean Drilling Program Leg 186. Some 83 datum events were selected to construct new age–depth models for the sites. Based on the reliable magneto‐stratigraphy from the Pleistocene to the Upper Miocene, which were correlated to the standard geomagnetic polarity timescale, and on excellent records of diatom and radiolarian biostratigraphy throughout the sequences, the shipboard age model was revised. Major revisions referred to stratigraphic position of the Miocene–Pliocene boundary that has been shifted more than 200 m downward in each sequence. The age–depth relations of the forearc sites represent drastic changes in the sedimentation rate—extremely high (40 cm/k.y. on average) in the Early Pliocene and low (less than 2 cm/k.y. on average) in the Middle Miocene—and several hiatuses exist throughout the sequence. The drastic changes can be related mostly to changes in diatom sedimentation and the tectonics of the Japanese Island Arc. Local ages for some foraminiferal, calcareous nannofossil and radiolarian bioevents are estimated from the age–depth models at each site. These newly calibrated bioevents and biozones as well as established diatom biostratigraphy are incorporated into the updated magneto‐biochronologic timescale, which will contribute to an improvement in biochronologic accuracy of Neogene sediments in northern Japan and adjacent areas.  相似文献   

Oblique subduction, collision of microcontinents and subduction of oceanic ridge: Their implications on the Cretaceous tectonics of Japan   总被引：2，自引：1，他引：2  

Kenshiro  Otsuki 《Island Arc》1992,1(1):51-63

Abstract The Izanagi plate subducted rapidly and obliquely under the accretionary terrane of Japan in the Cretaceous before 85 Ma. A chain of microcontinents collided with it at about 140 Ma. In southwest Japan the major part of it subducted thereafter, but in northeast Japan it accreted and the trench jumped oceanward, resulting in a curved volcanic front. The oblique subduction and the underplated microcon-tinent caused uplifting of high-pressure (high-P) metamorphic rocks and large scale crustal shortening in southwest Japan. The oblique subduction caused left-lateral faulting and ductile shearing in northeast Japan. The arc sliver crossed over the high-temperature (high-T) zone of arc magmatism, resulting in a wide high-T metamorphosed belt. At about 85 Ma, the subduction mode changed from oblique to normal and the tectonic mode changed drastically. Just after this the Kula/Pacific ridge subducted and the subduction rate of the Pacific plate decreased gradually, causing the intrusion of huge amounts of granite magma and the eruption of acidic volcanics from large cauldrons. The oblique subduction of the Pacific plate resumed at 53 Ma and the left-lateral faults were reactivated.  相似文献   

Westward migration of the Izu-Bonin Trench, northward motion of the Philippine Sea Plate, and their relationships to the Cenozoic tectonics of Japanese island arcs     

Kenshiro Otsuki 《Tectonophysics》1990,180(2-4):351-367

The essential elements in the understanding of the Cenozoic island arc tectonics of the Japanese Islands came from reconstructing the paleo-position of the plate boundaries and estimating the change in the Philippine Sea Plate motion. By using the 2nd law of the convergence rate of plates, it was estimated that the Izu-Bonin Trench wandered around 400 km east from its present position during the Paleogene and migrated westward thereafter. Island-arc tectonism is related to the convergence rate of plates (1st law of the convergence rate of plates), hence the changes in the Philippine Sea Plate motion was examined by compiling paleomagnetic data. As a result, the main events of the Cenozoic tectonics of Japan were well explained by the change in the position of the plate boundaries and the change in motion of the Philippine Sea Plate.  相似文献   

Photosynthetic pigment system of picophytoplankton of cyanophytes isolated from subsurface water in the Kuroshio area     

Tohru Ikeya  Kaori Ohki  Masayuki Takahashi  Yoshihiko Fujita 《Journal of Oceanography》1991,47(1):1-6

Photosynthetic pigment system of picophytoplankton of cyanophytes was examined with five strains isolated from the Kuroshio water at the depth of 70 m. Examination was made for the absorption spectra of intact cells of each strain. Analysis of pigment composition was also made withSynechococcus NIBB 1059 and 1071, which were isolated from surface waters of the Gulf Stream and Kuroshio area, respectively. Results indicated that (1) all strains contain phycoerythrin with a very high concentration, and (2) the phycoerythrin in these strains contains two chromophores, phycoerythrobilin and phycourobilin, and (3) a large abundance of phycoerythrin and phycourobilin in the phycoerythrin enablesSynechococcus picophytoplankton to absorb effectively the light in the blue-green region at the subsurface depth. These characteristics suggest that cyanophytes in the subsurface water can collectt the blue-green light and perform actively photosynthesis even at the bottom of euphotic layer.  相似文献   

Daily Variations in Nutrient Concentrations of Seawater at 321 m Depth in Toyama Bay, Japan Sea     

Masami Watanabe  Jun Ohtsu  Akira Otsuki 《Journal of Oceanography》2000,56(5):553-558

Nutrient concentrations (nitrate + nitrite, phosphate and silicate) in deep seawater (321 m depth) of Toyama Bay, Japan, were measured from August 1996 to July 1997 to determine the magnitude of daily variations. Significant daily variations were observed; concentrations ranged from 4.42 to 22.4 µM for nitrate + nitrite, from 0.86 to 1.98 µM for phosphate, and from 9.91 to 47.7 µM for silicate, respectively. However, there were not significant relationships between nutrient concentrations and water temperature, or between nutrients and salinity. Since temperature and salinity in the >300 m depth layer were constant through the year, the results suggest that there may be water masses with different nutrient concentrations in the deep layers (at about 320 m depth) of the bay, and a horizontal advection of these water masses may be responsible for the observed daily variations.  相似文献