At first, the term “neotectonics” is examined. The term seems to have been used for the first time by SCHULTZ (1939) and is defined by NIKOLAEV (1962) as a science of crustal movements that have given rise to the present relief of the earth's surface. On the other hand, ICHIKAWA (1958) defined it as the tectonics of Neogene and Quaternary periods, this definition showing the coincidence with the view of Nikolaev and his colleagues. The Neotectonics Commission is established in INQUA and works for neotectonic maps. In this Commission, the Pacific Regions Working Group was organized in 1965. The activity of this Working Group is described in vol. 6 (1967) no. 1 of this journal in English. The National Committee on Neotectonics corresponding to the Commission and to the Working Group was organized in Japan. Moreover, we have the research team for Quaternary tectonic map of Japan, the results being summarized in this issue. Besides, there are some research teams for studying neotectonics in the Geological Survey, the Earthquake Research Institute, and other organizations. Each of these “neotectonics” is one of or a combination of three different meanings: Neogene tectonics, Quaternary tectonics, and present-day tectonics (Lebende Tektonik). In this article, the author limits himself to discuss only the Quaternary tectonics. Secondly, methods and characteristics of the study on Quaternary tectonic movements are made clear by comparing recent tectonic movements detected by geodetic means (A), Quaternary tectonic movements (B), and past tectonic movements detected by geologic survey (C), with each other. Table 1 is for the comparison. The means to know vertical displacements are bench marks for leveling, ancient shore-line and geomorphologic surface, and marine sedimentary formation, respectively. For all these means, the sea-level is employed as a criterion. The sea-level oscillates with different periods however the term considered is short or long, the changes being caused by ocean current and atmospheric pressure, glacial control, and epirogenic movements, respectively. To get the amount of vertical tectonic movements (other than epirogenic ones), it is necessary to separate it from the amounts of those factors. Horizontal displacements associated with recent tectonic movements can be detected by triangulation, and displacements associated with past geologic time can be inferred from paleomagnetic data. For Quaternary there is no effective method, except geomorphologic means including a clue using horizontal offsets of river courses and other features. The adequacy of these various techniques for describing tectonic movements in space and time is quite different. The geodetic technique gives complete coverage in space but little or no information about time variations. The geological techniques give a fairly adequate description of the variations over time, but cannot give a complete picture for all areas. The geomorphological techniques are intermediate to the geodetic and geologic ones in both time and space coverage. The development of submarine geology and isotope chronology are expected for future studies to diminish these defects in (B). However, the basic knowledge for Quaternary tectonics is in geomorphology and Quaternary stratigraphy. Thirdly, emphasis is put on relationships between sea-level changes and tectonic movements. Fig. 1 is an example of consideration on the relationship between them, published before the glacial eustasy began to be favoured. Fig. 2 is an example that indicates the correlation between the regional distribution of the amount of vertical displacements since the time of the highest sealevel after the post-glacial transgression, and the one of vertical displacements at a major earthquake. Persistency in tectonic movements is expected from this relation.
Since the middle Jomonian cultural age (about 4, 500y. B. P.) when sea level in the Recent transgression attained the highest level, the tectonic patterns and the rates of the crustal movements in several areas in Japan are demonstrated to continue to the present time.
The problem discussed in this paper is whether the crustal movements in the Japanese Islands changed in nature or rate during the transition from the Tertiary to the Quaternary periods. The Late Tertiary sedimentary basins gradually migrated under the control of the orogenic movements which had begun in the Early Miocene. The Lower Pleistocene Series deposited in the areas succeeded the Pliocene sedimentary basins. In most parts of the Japanese Islands, a conspicuous break in sedimentation and physiographic development can be recognized between the Early and Middle Pleistocene. Since the Middle Pleistocene, sediments lap off the underlying sediments and form successive terraces, whereas the younger sediments covered the older during the Early Pleistocene. Flat lying Middle Pleistocene and younger terrace formations are striking in contrast to the folded and faulted Early Pleistocene formations, though the intensity of their deformation varies locally. The break between the Early and Middle Pleistocene mentioned above suggests progressive crustal movements since the Neogene and the relative rise of the whole Japanese Islands against sea level though whether diastrophic or eustatic is not yet certain. Concerning the rate of deformation of the Early and Middle Pleistocene formations, the absolute ages of the sediments must be determined and the mechanism of the deformation be elucidated. In Japan, knowledge on the absolute and relative chronology of the Late Cenozoic is now in progress and the interest of structural geologists with regard to the exact age of the tectonics is increasing. More detailed discussion on dated Quaternary crustal movements will become possible in the near future.
Neotectonic Researches in Japan have advanced as an applied geology. In 1891, a great earthquake occurred in the provinces Mino and Owari in Central Japan. After this earthquake, for the purpose of taking preventive measures against earthquake disasters, the Japanese Earthquake Investigation Committee was organized and geologists began to study geotectonic meaning of earthquakes and eruption of volcanoes. After the earthquake of Kwanto Areas in 1923, the Earthquake Research Institute was established in the University of Tokyo and studies of neotectonic structure of the Japanese Islands became active. In the recent years, neotectonic studies have advanced exceedingly for several purposes, such as co-operation with the International Upper Mantle Project, prediction of the region where risk of earthquake may be large, and dynamic and hydrographic survey of the ground of industrial areas. Character of the studies in the recent years has changed from qualitative to quantitative.
The geological investigation of the Neodani earthquake fault clearly came into view at the occasion of the Nobi earthquake in 1891 was the first contribution to the younger tectonics in Japan (KOTO 1893). Immediately after this earthquake, the Imperial Earthquake Investigation Committee was organized. The Kanto earthquake of September 1923, which severely attacked the hearty region of Japan including the cities of Tokyo and Yokohama, called forth the mind of Japanese geologists and geographers the importance of the stratigraphical and tectonic studies in the lowland areas. The Earthquake Research Institute of the University of Tokyo was thus established for the purpose of all scientific investigations on the earthquake problems including geotectonics. The development of the neotectonic researches in Japan since 1923 would be divided into the following three stages. The first stage (1923-1945)-The age of the “embryonal” researches on the neotectonics, or the age of the descriptive researches. Many researches especially on the apparent or locational relations between earthquakes, geomorphology and geology were discussed. and published. Among them nearly all of the living or recent problems in progress during later stages were found, though only fragmentally or not substantially. These studies are as follows: The conception of the Kanto structural basin (YABE 1925, YABE & AOKI 1927, AOKI & TAYAMA 1930); drowned valley and the latest land connection (YABE 1929); the conception of tilted blocks with active faults (YAMASAKI 1925, 1927); relation between geomorphology and tectonics (YAMASAKI & TADA 1927, TADA 1929); distribution of geomorphologically distinct faults (TSUJIMURA 1923, 1926); elevation of raised beaches and river terraces in connection with tectonics (IMAMURA 1928, 1932, 1933); Pleistocene stratigraphy and tectonics in Kinki district (NAKAMURA 1930, 1933, 1934, TAKEYAMA 1933, IKEBE 1933, UEJI 1936); geomorphic development and neotectonics based on the detailed stratigraphy (OTUKA 1931, 1933, 1939, 1948); deformation measured by precise levelling (Earthquake Research Institute 1927-); measurement of amount of strike-slip dislocation of the Tanna fault (KUNO 1936); Quaternary movement of the Median tectonic line of SW. Japan (KAWADA 1939, KOBAYASHI 1941). Among above-mentioned, there were many neotectonically important or suggestive conceptions or data to be expected of more detailed study. However, under the severe circumstance of the unfortunate war, many research works were obliged to interrupt. The second stage (1946-1955)-“A phoenix stage” or the age of new development after the war. After the demobilization, revival of academic works had immediately performed by the researchers of the preceeding stage. Moreover, younger researchers educated during or after the war, had taken the place of the above-mentioned workers in the neotectonic circle of science. The Nankaido earthquake in 1946 had served as a moment to the revival of geosciences in Japan (NAGATA 1947, WATANABE 1948). Several flood disasters caused by typhoons had stimulated the investigations on the geomorphology and geology of alluvial lowland areas, especially by using airphotographs (NAKANO 1954, MINATO et al 1950, 1953). SUGIMURA and NARUSE (1954, 1955) discussed the relation between the eustatic and crustal movements. SUGIMURA (1952), succeeding OTUKA's works, realized the parellelism of the active folding measured by the height distribution of river terraces to the structure of the basement Neogene formations. MACHIDA and OKURA (OTA) (MACHIDA 1948, 1951, OKURA 1953, 1958, OTA 1964) tried to realize the characteristics of neotectonic movement from the analytical studies of river and coast terrace deposits. Age of the Pleistocene glaciation in the central mountains in Japan were discussed by KOBAYASHI and SHIKAMA in 1949.
Geological researches revealed vertical crustal movements of several thousand meters and horizontal movements of tens of km in comparatively recent geological time such as the last 10-20 million years by faulting, bending, folding or overthrusting. Such changes as may now be in progress are considered to be detected and measured by geodetic means. However, the actual rates of changes per year are extremely small, and since it was doubtful that the conventional geodetic surveys may contribute much to clarify the tectonic movements in a reasonable period of years. Of geodetic surveys, precise levelling have, in principle, remarkable high precision (better than 1×10-6) and contributed till now to the detectians of features of crustal deformations accompanied by large earthquakes. However, we have yet only insufficient data to trace the progressing tectonic activities, because the time intervals between successive resurveys were too long (20-30 years). Recently, under the Project of Earthquake Prediction, the Geographical Survey Institute has started to repeat precise levelling along the first order levelling routes all over the land of Japan every five years. It is expected that such works may be able to reveal the details of tectonic deformations and accumulation of strain in the land of Japan. The precision of horizontal positionings by conventional precise triangulations is far inferior to that of precise levelling. It is estimated to be somewhat better than 1×10-5. If horizontal deformations are assumed to be of the order of 10-7 per year, this means that one century or more years are necessary to trace the movements. This might be the reason why reliable records were not obtained by triangulation except when sudden destructions were caused by large earthquakes as in Japan and California. Recently, the accuracy of distance measurement with an electrooptical instrument (such as Geodimeter) has attained to about 1×10-6, and we have now the back ground of detecting and tracing the horizontal deformations in a reasonably short period (say, in several tens of years). The direct evidence of the theory of continental drift may be provided by changes in astronomical positions, but it was unfortunate the suggested rapid changes are in longitude. Until very recently, the reliability of longitude observation was not sufficient owing to the inaccuracies of observing instruments and the inaccurate corrections to the time of propagation of wireless time signals. Now, the standard time signals (UTC) in the world are synchronized in the accuracy of better than 1ms (=1/1000sec), and high precision instruments (PZT and precise astrolabe) have also been developed. Relative movements of continents of the order of several cm per year may be detected in the near future. Furthermore, artificial satellites have become promising means of connecting geodetic nets in accurate and pure three dimentional geometrical ways. In conclusion, although conventional geodetic survey did not contribute so much until recently, we may expect in near future newly developed geodetic means will afford useful data which clarify the tectonic movements of the crust and reveal the internal activities of the earth.
The Pacific coast of Southwest Japan has been attacked by violent earthquakes accompanied by remarkable crustal deformation at intervals of 100 to 150 years in the historical period. At the most recent great earthquake in 1946, promontories protruding south into the Pacific Ocean were upheaved by about one meter, being tilted northwards, and inland mountainous regions were subsided. The mode of recent crustal deformation, including seismic one, of Shikoku has been revealed with precise levellings, as shown in Figs. 1-3. In the southern part of Shikoku runs a hinge line of the recent crustal deformation, which was subsided at the seismic time and upheaved in the inter-seismic periods. The coastal areas south of the hinge line were tilted southwards in the inter-seismic periods and remarkably northwards at the seismic time, while the mountainous region north of the line was quite reversely deformed. In the vicinity of Muroto Promontory, the southeastern tip of Shikoku, a characteristic process of post-seismic crustal deformation was clarified with precise levellings carried out seven times for six years after the great earthquake in 1946 (Okada et al., 1953). Immediately after the earthquake, Muroto Promontory was rapidly tilted southwards, and then the rate of southward tilting exponentially decreased to become as constant as in the pre-seismic period. The post-seismic crustal deformation (Fig. 3) is nearly reverse to the seismic one (Fig. 2) and is different in its mode from the pre-seismic one (Fig. 1), while similar minor features are found in their mode. It is, therefore, inferred that the post-seismic crustal deformation was chiefly caused by seismic after-effect and the pre-seismic crustal deformation resulted from secular tectonic movement and decelerated seismic after-effect. From this inference secular crustal deformation of Shikoku was tentatively estimated, by subtracting the post-seismic vertical displacement from the triplicated pre-seismic one. Although there is no definite reason for triplicating the latter, the estimated secular crustal deformation (Fig. 6) is nearly concordant with the patterns of geomorphological and geological structure of the mountains, and is negatively correlated with Bouguer's anomalies of gravity (Geogr. Surv. Inst., 1966). Resultant crustal deformation of Shikoku in the seismic and inter-seismic periods was also obtained from results of precise levellings (Fig. 7), assuming that great earthquakes accompanying crustal deformation of similar mode have occurred at intervals of about 120 years. Coastal terraces on the south coast descend northward and their heights have a positive correlation with the resultant deformation (Fig. 9). Topographic features of various types caused by subsidence are found along the hinge line, which is inferred to have been subsided as a result of the seismic and inter-seismic crustal deformation. Heights of the mountains north of the hinge line show positive correlations with the resultant deformation, except in the central part of the western Shikoku Mountains, and coefficients of regression are larger in higher mountains than in lower ones (Figs. 10 and 11). In the coastal areas south of the hinge line, however, correlation between heights of the mountains and the resultant deformation are negative (Figs. 9 and 11). This means that the mountains have been upheaved by tectonic movement of similar mode to the recent crustal deformation including seismic one, which probably dates from before the formation of coastal terraces and at least after the evolution of the lower mountains. A quite similar relation between the recent crustal deformation and the geomorphic features is also found in Kii Peninsula, east of Shikoku.
The Quaternary tectonic map here referred to consists of five maps. Three of these are contour maps showing amounts of Quaternatry uplift and subsidence and the other two show distribution of Quaternary faults and folds. The uplift and subsidence maps are the one given by the team of geomorphologists (Fig. 1), the one by the team of geologists (Fig. 2), and the one complied from the former two (Fig. 3). Fig, 1 shows the present heights of the erosion flat surfaces formed during the latest Tertiary or the earliest Quaternary age. It is assumed that the surfaces were formed not far from the sea level. Fig. 2 shows the present altitudes and the depths of the locations where upper Pliocene to lower Pleistocene marine sedimentary formations crop out or are buried. It is assumed also in this case that the sedimentation took place at shallow depths. Hence, the vertical displacement approximately represents the algebraic total since the beginning of the Quaternary up to the present. The errors derived from these assumptions seem to be smaller than those derived from the roughness of time estimation, which gives the Plio-Pleistocene age from 3 m. y. ago to 1 m. y. ago. In Fig. 1, the amount of uplift can be traced regionally but the amount of subsidence can not be known. In Fig. 2, both amounts of uplift and subsidence can be known but they can be determined only locally. Taking advantage of the two methods, the authors synthesized Fig. 3, which is one of the conclusions of this work. Both of the faults distribution map (Fig. 4) and the folds distribution map (Fig. 5) are drawn of those faultings and foldings that have deformed sedimentary beds and terrace surfaces since upper Pliocene age. Faults more than 1km in length and folds 500m to 30km in a wave-length are selected. Generally speaking, mountain districts have been uplifted, while lowlands have been subsided. The maximum uplift of about 1700m is found in the Central Ranges, and the maximum subsidence of about 1400m in the Kanto Plain (Fig. 3). It is concluded that the present reliefs of the Japanese Islands are largely due to the Quaternary tectonic movements. The regional trend of fold-axes and strike of faults shows a characteristic pattern (Figs. 4 and 5). The Japanese Islands may be divided into some tectonic provinces by the intensity and nature of the Quaternaty tectonic movements.
The term “active fault (Katsu-danso)” appeared in the 1920's in papers of some geomorphologists in Japan. It has been used for faults active in Quaternary or late Quaternary, although the usage of the word “active” and its age-limit are different by authors. At any rate, the active fault is significant in geology in that the evidence of fault movements is recorded in the topography on the fault trace which enables detail analysis of the faulting, and in that the faulting might be detectable by means of geophysical methods, and it might be closely related to the occurrence of an earthquake. Recent studies in Japan showed that an active fault moves under a regionally-stressed condition of the crust, which has persisted during recent geologic time. If so, an “active fault” could be defined as a fault which is active under the present-day stress system. Then, the origin, orientation, and variation in time and space, of the “present” stress system are to be an important and fruitful research subject on the Tertiary to Quaternary tectonic history of the Japanese Islands. Some of the recent investigation on the active fault began to focus on this line. Studies of active faults in Japan have started from two points: the geological studies of the earthquake faults by geologists and the studies of the fault topography by geomorphologists. The geological studies of the earthquake fault was commenced by Koto, B. (1894) who had observed the surface break of the Mino-Owari earthquake of 1891 along the Neo Valley fault. Since then, geologists have investigated and described more than ten earthquake faults from Japan and Formosa. Particularly through the experiences of 1927- and 1930-earthquakes, it became clear that the mode of displacement along the earthquake fault during the earthquake is the same in the sense of displacement as the mode of the long-term displacement through the geologic periods. For example, Kuno, H. (1936) found that the Tanna fault which moved a few meters left-laterally during 1930 earthquake, has accumulated left-lateral displacement ca. 1000m since the middle Pleistocene. Almost independently of the geologists'works, geomorphologists had investigated the fault topography of Japan. By these studies, it had become clear before the War II that there are many fault scarps and fault valleys in this country, which have been produced probably by Quaternary faulting. Tsujimura, T. (1932) published a distribution map of topographically-recognized faults, in which 413 fault systems were registered. Recently, the geological and geomorphological studies have been jointly performed and the movement-history of some active faults in Quaternary time are clarified quantitatively (Table 1). It is also shown that many active faults hitherto considered to have only vertical displacement are strike-slip faults accompanied by lesser dip-slip components. The Atera fault (Sugimura & Matsuda, 1965), the Atotsugawa fault (Matsuda, 1966), and the Median Tectonic line (Kaneko, S., 1966; Okada, A., 1968), which was described recently, are examples. Some features of the active strike-slip faults described from Japan may be summerized as follows: A number of active faults or fault zones are present particularly in central Japan (the Chubu, Kinki, and Shikoku districts). They are, however, less than one hundred kilometers in length and a few kilometers in total displacement of the basement rocks, except for the Median Tectonic line (and probably the Itoigawa-Shizuoka Tectonic line). There is a marked regularity in the fault systems of the central Japan between the trend of faults and the sense of the displacement: the NW-trending faults are left-lateral, whereas the NE-trending faults are right-lateral. This implies that the earth's crust of the region is under the same stress system having the maximum (compressional) principal axis of approximate east-west.
History of active fold study in Japan is critically reviewed and current problems in this branch are discussed with introduction of a case study of active fold along the River Shinano. The term active fold is here used for a fold (structure), the last deformation of which occurred in a very recent period (less than 105 years B. P.). Active fold, as reported first by Otuka (1941, 1942a) and Ikebe (1942), was discovered by deformed river terraces and vertical displacement of bench marks, both of them being concordant to underlying Neogene fold structures. Significance of this discovery lies in that the tectogenic stress which yielded the fold structure is in action at present and that we can observe the time mode of deformation of the structure. During the period 1943-1959, besides accumulation of examples found in the inner belt of Northeast Japan, a direct approach was tried by Miyamura (1943, 1949, 1956) for obtaining data of vertical displacement by bench marks newly set up for this particular purpose (Fig. 1). In the course of his study, the Futatsui Earthquake (1955, M. 6.2) happened to occur near the newly set levelling route. This event associated with possible pre-seismic deformation and coseismic progress of active fold, much stimulated general interest in the relationship between active fold and seismic activity. Since 1960, still more examples have been added, partly due to impact of UMP and earthquake prediction project. Based upon the accumulated results, active folding has been discussed with related crustal movement such as crustal unduration and active faulting. The following conclusions seem to be almost established by the recent work. 1. Active fold has been formed with the maximum time rate of folding (tilting) of the order of 10-6 per year during the past 104∼5 years. This rate is abnormally high compared with deformation in other continental parts of the globe. 2. Time rate of deformation is generally inversely correlated with wave length. The above maximum rate is attained by the fold with wave length of several kilometers or less. This high-rate, short-wave fold is characteristic of areas with thick semiconsolidated sediments such as the inner belt of Northeast Japan. This fact suggests that folding of this sort is genetically related to the presence of thick soft material. 3. Due to the high rate, displacement observed within ten years or less, is able to be correlated to tectogenic deformation. An active fold along the lower course of the River Shinano is described referring mainly to Ota (1969). The distribution of river terraces is shown in Fig. 3 along with a syncline and an anticline of terrace deformation. The axes of this active fold almost duplicate those of the fold in the underlying Plio-Pleistocene deposits. Exact correlation of terraces is a difficult but important problem in an area of high rate of deformation like this. Fig. 4 is a cross section of folded terraces. Fig. 5 presents the distribution of bench marks near Sekihara shown in Fig. 2 and its relation to fold axis of terrace plains. Fig. 6 shows a part of the bench mark net set up for the study of active fold, vertical displacement during the past nine years, and the distribution of the Geodimeter baseline. This area apparently experienced steady progress of folding with a rate of 10-6 during the past 70 years, without appreciable earthquakes. From the foregoing review and a case study, future problems as below were summarized and discussed: 1. Further accumulation of reliable examples. 2. Quantitative study of original form of terrace topography. 3. Age determination of topographic plain. 4. Nature and distribution of active fold and its dependence on the physical property of underlying rocks which is the historic result of geology of the area. 5. Mechanism of active fold, its dependence on depth and measurement of horizontal strain.
In the first half of the present paper, a brief review on recent field researches in Japan on fractures including joints and minor faults, are dealt with. For applying fracture analysis to the tectonic developments, it is at first necessary to recognize the succession and, if possible, the time of fracturing. From this point of view, an example of the historical changes of tectonic stress field in the Quaternary time, found in south Kanto district by the writer and his collaborators, is shown.
Part 1 of this paper is concerned with various types of deformation of volcanic cones as listed in Table 1. In regard to the crustal displacements before and after eruptions, the local upheaval and fracturing near crater remain as a permanent deformed topography, but the areal upheaval and subsidence before and after eruption are believed to be a reiterative movement caused by increase and decrease of the pressure in magma reservoir. Most of collapse calderas in Japan belong to Krakatoan type and some to Kilauean type, but none of resurgent cauldrons is known. These deformations of volcanic origin are generally circular in their horizontal shape and display larger magnitude and rate in vertical displacement than those of non-magmatic gravitational and tectonic origin. Non-magmatic, superficial, gravitational deformation of volcanic cones such as the settlement of cones and landslides of large scale are emphasized, because of their important role to deform volcanic cones on the weak foundations such as Cenozoic altered volcanics. It is also known that non-magmatic tectonic movements such as the active faulting connected with earthquakes, the active folding of the basement etc., deform volcanic cones. In part 2, firstly, horizontal arrangement of volcanoes is separated into three types of different extent. Two volcanic zones, East Japan and West Japan, are recognized. The two display remarkable cross-sectional asymmetry of both volume of volcanic products and nature of magma from oceanic side to continental side. This asymmetry is believed to be due primarily to the continental-ward inclination of the mantle earthquake zone where parental magma generates. Volcanic center alignments are in parallel with mountain ranges of about 50km in width and are related to the pre-existing faults which were formed during the mountain building. Volcanic vent alignments run in perpendicular or diagonal direction to the volcanic center alignments and are inferred to be related to tension cracks parallel to the late Quaternary local principal horizontal stress direction. Secondly, the vertical distribution of volcanoes of different ages in relation to the configurations of their basements is discussed (Figs. 1, 2, 3, 4). Older volcanoes which were formed during middle and early Quaternary usually rest on the crest of the rised or rising mountain ranges. On the contrary, younger volcanoes which are active during late Quaternary tend to occur on the relatively lower places than older volcanoes. The last chapter is devoted to deal with relationship between the distribution of Quaternary volcanoes of different ages and neotectonics in Japan. Fig. 5 shows a possible working hypothesis on the relationship.
Crustal movements deduced from the deformation of dissected fans are summarized taking those in the Tokai region, the Pacific Coast of Central Japan, as an example. Two Quaternary crustal movements are recognized in this region. One is undulations with axes of NE-SW direction, running en echelon, along the margin of the Akaishi sphenoid. The other is a diagonal upwarping with an axis of NW-SE direction, which is accompanied by the upheaval of the mountainous hinter-land. Vertical movements of the region calculated by the levelling survey during the latest 60 years have also close relation to the above-mentioned crustal movements. The rate of the vertical component of an undulation in the vicinity of Shizuoka in the south Fossa Magna amounts to 3.3mm/year, which corresponds nearly to those of upheaval indicated by the Holocene Numa coastal terrace, south Boso Peninsula, and the seismic upheaval of Oiso district, south Kanto region. The rate of the inclination in the lower courses of Tenryu and Oi rivers is estimated at 2° to 10°/106 years. The undulatory movement can be interpreted as a continuation of the Neogene crustal movement of this region with reference to the evolution of sedimentary basins, though anticlinal elevations and synclinal subsidences of the Quaternary are reciprocal to the underlying Neogene folds in the sense of the up-and down-warping.
The tectonic development of the Noobi sedimentary basin is discussed in relation to crustal movements of southwest Japan in the period of the Quaternary. The basin is an eastern part of the Second Setouchi sedimentary province formed in the inland area of southwest Japan during the Plio-Pleistocene. In the Setouchi province, two types of tectonic movements originated in crustal undulation are recognized: Type-1 is of long-wave undulation which has formed the main depressional zone with parallel axis to the trend of the Setouchi province. Type-2 is of short wave undulation crossing, in almost cases, the trend of the axis of the province, which has formed the alternating arrangement of basin and ridge in the depressional zone. From the tectonic point of view, the province is divided into three crustal blocks from east to west:-(1) Chubu, (2) Kinki and (3) Chugoku. The eastside of the Chubu block is marked off by the Fossa Magna, and westside by the Tsuruga-Ise bay line. The Noobi basin is situated in the western end of this block. A subsidence of the basin has been largely caused by tilting movements of the block active since the Pliocene, and less by the crustal undulation. This tilting block movement in large scale is a tectonic movement characteristic in the Setouchi province. And also in the Chubu block, the trend of axis of type-2 undulation changes into parallel direction to that of the Setouchi province. The rate of the tilting movement in the Noobi basin seems to increase in the latest periods. The mean rate of the tilting movement is estimated to be 7×10-8 per year during the latest 35, 000 years according to geological evidences. This figure is almost the same to the one of the recent crustal movements measured geodetically in Japan.
Many geologists are giving attentions to the zonal arrangement of geological and geophysical features of the Japan island arc system, in which two recently active belts along the Izu-Mariana and Ryukyu Arcs cut the Honsyu Arc almost at right angles. These two active belts are thought to be the late Cenozoic orogenic belts (Matsuda et al., 1967). Southwest Japan lies between them and have behaved rather cratonically during Cenozoic times unlike Northeast Japan. J. Makiyama (1956) described this region as “quasi-cratonic”. SW Japan is divisible into two zones, Inner Zone in the north and Outer Zone in the south, separated by the Median Tectonic Line. Three series of the Cenozoic sediments lie scattered on the basement rocks of Inner Zone; the First Setouti Series belonging to the middle Miocene, the Setouti Volcanic Series of the upper Miocene, and the Second Setouti Series which is the Plio-Pleistocene in age (Huzita, 1962). Setouti means “Inland Sea” area. 1) Fig. 1 shows the depressional zones which formed the First Setouti Series, and those of the lower part of the Second Setouti Series are also shown in Fig. 3. They have a similar tendency in the structural trends. Fig. 4 shows the horizontal directions of the maximum principal stress axes (compressive) which are inferred from the structural trends of the latter. 2) Fig. 5 shows the main structures controlling the distribution of the upper part of the Second Setouti Series. Fig. 6 is the stress directions inferred from above structures. A distinct change of the stress state in SW Japan seen between Figs. 4 and 5 may have occurred in the middle of Pleistocene, but it did not make distinct angular unconformity in the Second Setouti Series. 3) The faults affecting the Second Setouti Series may be classified into two systems as shown in Figs. 7 and 8. Fig. 7 is the thrust system appeared in the earlier stress field, and Fig. 8 is the fault pattern occurred in the later state of regional stress. Some of the older thrusts have been rejuvenated as strike-slip faults. 4) The later movements have produced various types of structures in the basement rock-bodies depending on the different behavior of each body for the same regional stress. For example, weak warping associated with strike-slip faults has occurred in the Paleozoic Tamba terrain, but comparatively intense deformation of folding type with thrusting has affected the granitic regions (Fig. 2). The trends of these structures are almost N-S and subparallel with the Izu-Mariana or Ryukyu Arcs. 5) The structures due to the later movements have been superimposed on the older structures parallel to the E-W Honsyu Arc. Both these structures control the present geomorphology of SW Japan. 6) In contrast to Inner Zone occupied by granitic batholithes and plutons, Outer Zone has older rocks showing distinct linear features aligned almost perpendicular to the trends of the younger structures. Rejuvenation of the Median Tectonic Line due to the later movements has resulted in strike-slip as shown in Fig. 8. 7) The later crustal movements are clearly expressed in the “Kinki Triangle” occupying the centre of SW Japan (Figs. 2, 6). The western edge of the Triangle is formed by the Rokko mountain range, so the term “Rokko Movements” is proposed to these movements (Ikebe & Huzita, 1966)