Geological and Experimental Studies on Mechanism of Block Faulting.
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概要
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Several block-faulted areas in Japan were surveyed and the mechanisms of block faulting were discussed on the basis of geological and experimental studies. The faults that bound individual fault blocks include normal, high-angle reverse, and strike-slip faults. The Abukuma range records four different periods of block faulting, assigned to Middle Cretaceous, Early Miocene, Late Miocene and Late Pleistocene. The first block faulting was of strike-slip faulting. By this faulting, the N-trending fractures, such as the Futaba and Hatagawa tectonic lines and the Idosawa fault, and the WNW-trending fractures, such as the Futatsuya, Akai, Yunotake faults, were formed as left-lateral and right-lateral strike-slip faults respectively. The second and later block faultings occurred as dip-slip faulting by utilizing essentially the same faults which had formed during the first block faulting. The structural analysis of the Abukuma range was carried out in the Soma, Hirono and Yotsukura areas. In the Soma area, the Wariyama horst composed of basement rocks rises within the fault zone along the Futaba tectonic line, and the Neogene sedimentary covers are distributed on both sides of it. The following structural features are revealed in this area : the Wariyama horst consists of five rock bodies that are separated by faults from each other ; most of the faults have vertical fault surfaces. In the Hirono area, the Futaba tectonic line is expressed by the combined structure of a flexure and a reverse fault that developed in the thick sedimentary cover. A reverse fault occurred after the deposition of Miocene to Lower Pliocene strata. These deformations are concluded as having resulted from the vertical differential movement of the underlying basement along a vertical fault. The Yotsukura area was subjected to block faulting in the Late Miocene time as well as in the Middle Cretaceous time. It is presumed that the faults related to the block faulting in the Late Miocene time were dip-slip vertical faults in the basement and nomal faults in the sedimentary cover. The faults and associated minor faults are divided into two sets of faults trending WNW and ENE respectively. Each set of faults shows a regularity with respect to the sense of slip and the direction of tilt of the intervening blocks, and produces a set of antithetic faults. In the southern area of the Suzuka range, dip-slip faulting occurred repeatedly during the Late Cenozoic time, so that rocks of the area were displaced into numerous fault blocks. These faults were grouped into several fault systems on the basis of the periods of activity, the strikes and the types of displacements. Some of the fault systems are sets of antithetic faults. The dips of faults become steeper as their ages of activity become older, whether the faults are normal or reverse. Therefore, the faults in the area are presumed to have been formed originally as those faults whose fault surfaces become steeper downward and ultimately vertial. In the central area of Gifu Prefecture, strike-slip faulting has actively prevailed in the Quaternary time. The faults are divided into two groups : faults that were newly formed in the Quaternary time and faults that had been formed before and were reactivated during the Quaternary time. These strike-slip faults have developed as a conjugate fault system during the Quaternary time and have divided the area into fault blocks with a size of 15-25 km across. Since the activity of these faults is regarded as having been steady through the Quaternary time, the size of individual fault blocks is considered to represent a minimum unit of block faulting. The relation between the occurrence of present-day earthquakes and the faults was examined. In the Matsushiro area, the formative process was studied of the Matsushiro earthquake fault that was newly formed in association with the Matsushiro swarm earthquake of 1965-66. The fault was about to divide a single fault block known as the Central Belt of Uplift into two fault blocks. The surface trace of the fault is recognized as an assemblage of minute ground cracks arranged in a double echelon and by a number of deformed man-made structures. These surficial expressions of faulting occurred within the zone of 500 m width. Instrumental observations on displacements of the fault and seismological observations on the earthquakes related to the faulting suggest that the Matsushiro earthquake fault is made up of a group of en-echelon arranged shear planes at depth as well as on the surface. In the Misakubo area, the deep-seated structures of the Komyo fault, a large left-lateral strike-slip fault, were studied. Many minor faults with striations on their fault surfaces were systematically measured in the vicinity of the Komyo fault. The minor faults occur mostly within a zone 2000 m wide adjacent to the main fault, and their frequency clearly increases toward the main fault. Left-lateral minor faults have a strike of N20°W in the neighborhood of the main fault, whereas they are nearly parallel to the main fault with a trend of N-S in the areas distant from it. This deficiency of parallelism was interpreted by the counterclockwise rotational movement of strata due to the left-lateral strike-slip faulting of the Komyo fault. In Sec. 7 the results were shown of the model experiments concerning the deformational manner of the sedimentary cover and the underlying basement at block faulting. The dip of the basement fault in the apparatus is variable from a normal fault dipping 60° through a vertical fault to a reverse fault dipping 60°. Sand or sand mixed with clay was used as model material. The experimental results show that the manner of deformation of the sedimentary cover is seriously dependent on the dip and the sense of displacement of the basement fault. When the basement is displaced along a high-angle reverse fault plane dipping 80°, only one reverse fault in addition to an intense flexure develops in the sedimentary cover. When the basement is displaced along a vertical fault plane, a set of normal and reverse faults as well as an intense flexure develops in the sedimentary cover. When the basement is displaced along a high-angle normal fault plane dipping 80°, a normal fault plane distinctly develops, and a reverse fault and a flexure become less clear in the sedimentary cover. When the basement is displaced along a normal fault plane dipping 70°, only a normal fault develops, and no intense flexure and no reverse fault develop in the sedimentary cover. In Sec. 8 were discussed several common features related to block faulting on the basis of geological and experimental studies. First, as for the manner of fracturing of rocks due to faulting, mylonitization, formation of minor faults with clear striations and brecciation of rocks were considered to occur successively with decreasing confining pressure. Secondly, the fault-forming process was discussed on the basis of the studies of the Matsushiro and Misakubo areas. The process is divided into four stages ; the first stage in which only flexuring occurs, the second stage in which minor faults begin to occur, the third stage in which a main fault occurs and grows with minor faults, and the fourth stage in which formation of minor faults declines and displacements along the main fault proceeds with flexuring. Thirdly, the repetition of block faulting was discussed. Block faulting occurred four times by using essentially the same faults in the Abukuma range, whereas it occurred repeatedly by forming successively new faults in the southern area of the Suzuka range. Fourthly, the manner of deformation of the sedimentary cover due to faulting in the basement was examined. As shown in the Hirono and Yotsukura areas, even if the basement underwent dip-slip displacements along the virtually vertical fault planes, different types of faults developed in the sedimentary cover : a reverse fault occurred in the former area and a normal fault occurred in the latter area. This difference was explained by supposing an additional horizontal stress exerted on the sedimentary cover. Fifthly, the dip of boundary faults was examined on the basis of several lines of field evidence, and a hypothesis that the dips of boundary faults are essentially vertical was presented. A model in which a brittle crustal layer overlies a ductile crustal layer was introduced in order to explain the mechanism of block faulting. The verticality of boundary faults is deduced from the model by considering energy needed for fracturing. In addition, the size of fault blocks as a minimum unit is deduced from the model. The minimum size is comparable to the thickness of the brittle layer.
- 1978-08-15
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