LINEAR DEPRESSIONS IN THE SOUTHWESTERN PART OF MT. HINOKIODAKE, THE KISO MOUNTAIN RANGE, CENTRAL JAPAN

Abstract

Linear depressions of NE-SW direction are dominant and widely distributed in the Kiso Mountain Range. They are densely distributed in the vicinity of the main-divide between Mt. Hinokiodake (2, 728m) and Mt. Kumazawadake (2, 778m) in the central part of the range (Fig. 1). A linear depression in the southwest slope of Mt. Hinokiodake has a length of about 200m and is the largest one in this region (Figs. 2 and 3). The origin and developmental environment of these linear depressions are studied in relation to the activity of the Hinokio Fault I (Figs. 4 and 5). The results are summarized as follows: 1. A linear depression to the west of the main-divide is surrounded by both a fault scarplet and periglacial smooth slope. The cross-section of this linear depression shows asymmetrical form, because the fault scarplet is steeper than periglacial smooth slope, maximum angle of which is 38° (Fig. 3). The fault scarplet is downslope-facing to the east of the main-divide and upslope-facing to the west of the main-divide. The steep fault scarplet with a uniform height of 11_??_14m appeared as a result of the dislocation of periglacial smooth slope by the activity of the Hinokio Fault I. 2. The Hinokio Fault I strikes N42_??_72°E and dips 62_??_68°S (Fig. 5) and it extends more than 500 m and occupies a crushed zone of more than 20 cm wide. The bearing and plunge of slickenside with streamline-shaped grooves (Fig. 6) indicate that the Hinokio Fault I is a typical example of translational faults as well as of left-normal-slip faults. 3. The faulting occurred during the period of Late Glacial to early Holocene. The maindivide area was vegetationless at that time. Debris-production and debris-transportation by frost action and solifluction were still active on the periglacial smooth slope, although massive joint-blocks of more than 1 m in diameter were scarcely produced. The Hinokio Fault I was formed under such environment. The net slip, bearing N85-E-'570-E with a plunge of 48_??_55°, attained to 17m at maximum. Besides, the northwestern part of the Geographical Review of Japan 55-4 258_??_273 1982 Hinokio Fault I (footwall) moved 5_??_7m (strike slip) southward relative to the southeastern part of this fault (hanging wall). 4. The relative height and steepness of the fault scarplet have been reduced, because this scarplet was differentially worn down by frost action. The fault scarplet to the east of the main-divide lost its continuity and its shape was changed into for-like rocks arranged in a straight line. While the fault scarplet to the west of the main-divide maintains its continuity, particularly between the main-divide and Ri (small ridge on periglacial smooth slope). Talus was developed at the foot of this scarplet between the main-divide and R1. The fault scarplet south of Rl was completely destructed (Fig. 2). The fault scarplet, for-like rocks, tales and periglacial smooth slope have been stabilized with the dominance of vegetation (pinus pumila) in the vicinity of the Hinokio Fault I. They are now fossil landforms. 5. It is concluded that the existence of linear depressions suggest the presence of normal fault. Most of these faults were formed during the time when both debris-production and debris-transportation were active. The estimation of the dimension such as length of fault line, vertical slip of fault and dip of fault plane based on topography of linear depressions is not always accurate. In the case of the Hinokio Fault I, the difference between these estimated values on the dimentions of the fault and the most probable value attains more than 88%, 18_??_43% and 8_??_35%, respectively. Moreover, it is difficult in general to determine the bearing of net slip only by linear depressions. Geological survey is indispensable to clarify the tectonic movement in high mountain regions.