TECTONIC FORMS IN THE SOUTHWESTERN INA VALLEY IN NAGANO PREFECTURE, CENTRAL JAPAN

Abstract

The Ina Valley between the Kiso Mountains and the Ina Mountains, is one of the typical intermount basins in the Honshu Island. The valley is situated along the southeastern border of the Inner Zone of the Southwestern Japan, and its general strike runs almost parallel with the Median Dislacation Line which is one of the main geotectoniclines in the Japanese Islands. The valley (300-1, 000 meters in altitude) is 80 kilometers long from Tatsuno where the water of the Tenryu River gathers to form the main river to Hiraoka where it dissects the southwestern margin of the Ina Mountains. It is 15 kilometers wide from Takato to Tengu at the longest stretch in the valley. The maximum relative height between the Kiso Mountains and the valley is over 2, 300 meters, on the other hand that between the Ina Nountains and the valley is about 1, 500 meters. Many typical river terraces and fans are widely distribute in the northern half of this valley, while lots of gorges and the remains of the old basin-floor are well developed in the southern half. At the famous Tenryu-kyo, situated on the northern part in the investigated area of this study, the Tenryu River has taken its antecedent course for the first time, and the river runs finally through the Akaishi Mountains. Under what condition was this long valley formed? Many previous works on this problem carried out, but the writter has limitted his discussion to the tectonicforms, the fault topographies and the valley-floor inclined toward the upper reaches of the Tenryu River, in the southwestern area of this valley. Discussing the topics, in this paper, at first thet tpographical features in this area are described (_??_ig. 1); secondly the stratigraphical succession and the distribution of rocks and strata are expressed (_??_ig. 2); thirdly the distribution of the faults and that of the Tsukue-yama beds, the indicator of the displaced valley-floor, are mentioned (Fig. 2, 3 and 4); forthly the topographical significances of the faults (Fig. 5, 6 and 8) and the valley-floor inclined toward the upper reaches of the Tenryu River are discussed (Fig. 7); finally the geomonphological significances of the Southwestern Ina Valley are examined. The main subjects are summarized as follows: 1. Judging from the distribution of the faults in this area, the Southwestern Ina Valley is a ramp valley (or a graben), it cannot be, however, concluded to be a typical one. 2. There are several minor block basins and block hills in this ramp valley. The writter thinks that the Oshimojo, the Tomikusa, the Komaba, the Sankochi anc the Yonegawa Basin are the examples of the former, and the Miho hills are that of the latter. 3. It seems probable that the valley-floor of this area, in total, was elevated in comparision with the base-levels of the area since late Miocene time. The amonnt of its up lift was, however, naturally smaller than those of the Kiso and the Ina Mountains. 4. The fault movements which have affected the topographies in this area probably began in early Pliocene time and have been contenuous ever since. Strictly speaking, the movements in the early Pliocene time can be correlated with Kobayashi's Tobe phase, and those in Pleistocene time can be done Stilie's Pasadena phase and Kobayashi's Shobudani phase. 5. Judging from the distribution of the Tsukue-yama beds, the valley-floor of this area was inclined toward the northwest, the upper reaches of the Tenryu River, after late Pliocene or early Pleistocene time. To be exact, the amount of the vertical displacement in the distance (10.5 kilometers) between Naka-yama and Tsukue-yama at Ryuto (the left bank of the Tenryu River) ranges 200-300 meters (Fig. 7-B(2)), while that in the distance (7.6 kilomers) between the west base of Nihonmatsu-yama atRyuto and Usui-bara at Ryusei (the righe bank of the river) ranges 250-300 meters.