コルディエラ・ブランカ (ペルー) 西麓の活断層

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The Cordillera Blanca of the Peruvian Andes is a glaciated mountain range, the highest peak of which is Nevados Huascaran, 6768 meters high above sea level. The Cordillera Blanca is composed mainly of a batholith dating from 3 to 12 million years ago. The southwest slopes of the Cordillera are steep fault scarps developed in the Quaternary (WILSON et al., 1967 DALMAYRAC, 1974). These fault scarps are developed along what is here termed the Cordillera Blanca Fault. To the southwest, the Cordillera Blanca is bounded by the fault and the valley of Rio Santa, while to the northeast of the Cordillera, no notable active fault appears in the Mesozoic fold belt (Figs. 1 and 2). The Cordillera Blanca Fault is easily recognized even on the LANDSAT images (Photo. 1). The total length of the fault is about 180-200 km and the relative height of fault scarps is over 4000 meters in muximum near Nevados Huascaran. The topographic features of the fault change from north to south. In the northern part, the fault scarp is high and steep and the fault line is simple. In the central part, the fault line waves and a cusp is formed to the northeast of Huaraz city. In the southern part, the relative heights of fault scarps become smaller and many fault scarplets run discontinuously in échelon (Figs. 3 and 5). Two regions in the southern part of the Cordillera Blanca Fault were investigated in November, 1970. The amount of fault displacements at the Queroccocha Valley and the Tuco Valley were measured using correlative moraine ridges in the valley sides and surfaces of fluvial terraces in the valley floors (Figs. 5-12). Unfortunately, data on the absolute ages of these landforms was unobtainable. They were estimated by comparing the succession of moraines with the age-known moraines formed from the last glacial expansion to the present in the Chilean Andes (for example, MERCER, 1976). Our investigation demonstrates that the Cordillera Blanca Fault is an active normal fault and that faulting has occurred progressively in the late Quaternary. The main results obtained follow : 1) Faulting has occurred progressively at least during the last 20, 000 years. The outermost morain ridge estimated about 20, 000 years ago (M1) has a vertical offset of 57 meters at the Tuco Valley. The second ridge estimated about 13, 000 years ago (M2) has a vertical offset of 25 meters at the Queroccocha Valley. 2) The average rate of vertical displacement along the fault is estimated to be 2 meters at the Queroccocha Valley and 3 meters at the Tuco Valley per one thousand years. 3) The values of vertical offsets which occurred during the different ages suggest that the amount of vertical displacements at one event of the faulting may be 2 and 3 meters at the Queroccocha and Tuco Valleys, respectively. 4) These results imply that the Cordillera Blanca Fault has a recurrence interval of about one thousand years. 5) If the faulting has continued at the same rate as in the late Quaternary, the amount of vertical displacement ranges from 2, 000 or 3, 000 meters for one million years. These values are of the same order as the relative heights of fault scarps at the western slope of the Cordillera Blanca. This means that the Cordillera Blanca Fault has been playing an important role in relief forming of the Cordillera Blanca and the Santa Valley.

コルディエラ・ブランカ (ペルー) 西麓の活断層

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