Geomorphological and sedimentological processes beneath modern glaciers and ice sheet s have not been observed directly and are poorly understood. On the contrary, abundant glacial landscapes can be observed, which provide us with evidence about processes underway at the beds of the past ice sheets. Consequently, careful studies of glacial landforms and sediments provide a wealth of information of these processes. During the last decade, there have been various debates regarding subglacial landforms and their formation processes: drumlins is a major issue, and no satisfactorye xplanation of their mode of formation has yet been obtained. By overviewing recent research on the drumlin problem, this article attempts to draw attention to the major concepts and controversies behind the formation of subglacial landform, together with new developments in understanding the subglacial environment. The most recent explanations for drumlin formation have been examined in the light of our knowledge of the subglacial environment. In particular, J. Shaw and his co-workers draw attention to the significance and the implication of subglacial meltwater processes. They suggested that large-scale meltwater floods were responsible for the formation of some drumlins. Later, erosional drumlins, bedrock erosional marks, tunnel channels, and Rogen moraine were added to the forms resulting from catastrophic floods. Conversely, G.S. Boulton developed a semi-quantitative flow model for the deformation of rapidly deforming soft sediments (A-horizon) on the basis of field observations. The drumlin problem stands as a conspicuous instance of how much there is still to understand about the interplay of glacier motion, sediments, topography, and subglacial environmental conditions. It is thus emphasized that accumrate explanations of the complexities of subglacial environments are necessary to understand subglacial landform development, sediment deposition, and other geomorphic processes at the ice/bed interface, together with extraglacial effects of ice sheet dynamics on fluvial systems, marine sedimentation, ocean currents, and climate.
The Yamizo Mountains consist of four mountain blocks: the Yamizo, Torinoko, Keisoku and Tsukuba Mountain Blocks, from north to south. The northern three mountain blocks are composed mainly of Mesozoic sedimentary rocks such as chert, shale, and sandstone. The sedimentary complex in the Keisoku Mountain Block is subdivided into the following four lithostratigraphic units: the Kasama, Kunimiyama, Takatori, and Ayuta Units based on the lithology and geologic age. The Kasama Unit is composed mainly of alternations of sandstone and shale with a small amount of chert and tuff layers. The Kunimiyama Unit consists of massive sandstone and alternations of sandstone and shale. The Takatori Unit is characterized by the chert-clastic sequence comprising “Toishi-type shale, ” bedded chert, siliceous shale, and clastic rocks in ascending order. The Ayuta Unit is composed mainly of alternations of sandstone and shale. These units are in fault contact with each other. Bedded chert of the Takatori Unit yields Middle Triassic to Early Jurassic radiolarians belonging to the Triassocampe deweueri, Triassocampe noua, Canoptum triassicum, Parahsuum simplum, and Hsuum hisuikyoense Assemblages. Middle Jurassic radiolarians, which are comparable to the Tricolocapsa plicarum and Tricolocapsa conexa Zones, occur from siliceous shale of the Takatori Unit. Late Jurassic radiolarians belonging to the Tricolocapsa yaoi, Mirifusus baileyi and Pseudodictyomitra primitiva Assemblages are obtained from clastic rocks of the Keisoku Mountain Block. Furthermore, late Early Triassic conodont fauna referable to the Neospathodus homeri Zone was discriminated from the “Toi shi-type shale” of the Takatori Unit. Based on the lithological and chronological characteristics, Mesozoic sedimentary complex of the Keisoku Mountain Block is correlated to those of the Tamba, Mino and Kiso areas in southwest Japan. Mesozoic strata distributed in the Keisoku Mountain Block is regarded as an accretionary wedges formed mainly by of fscrape-accretion during Late Jurassic time.
This paper proposes a method to identify the directivity of rupture propagation based on the branching features of active fault traces. Direction of ruptre propagation is closely related to strong ground motions and resulting earthquake damage. Therefore, predicting rupture directivity is crucial in predicting strong motions to mitigate earthquake damage. However, the directions of fault ruptures were ascertained only after earthquakes from the observed seismological records and not before the earthquakes. We found an interdependent correlation between the branching direction of the surface ruptures and the direction of their propagation as shown in Fig. 1, from an investigation of recent earthquake fault ruptures such as the 1995 Northern Sakhalin earthquake, the 1995 Hyogoken-nambu earthquake, the 1992 Landers earthquake, the 1990 Luzon earthequake, the 1979 Imperial Valley earthequake, and the 1930 Kita-Izu earthequake. The branching of faults during rupture propagation is regarded as an effective energy dissipation process and could result in final rupture termination. Because patterns of surface traces of active faults are the results of repeated earthquake faulting, the branching of active faults leads us to suggest that the direction of rupture propagation is also predictable before the active faults generate earthquakes in the future. Several active faults with well-defined branching such as the active faults of the strike -sliptype in the Kobe-Osaka area, those in California, and the active fault sysytem in the northern Luzon, Philippines are examined. Branching of the reverse faults in the foot -hills of Darjeeling Himalaya is also shown as an example of active faults of the dip -slip type. This test clearly shows that the direction of rupture propagation, and in some cases the epicenter location, can be deduced from the branching features on the basis of our proposed method.