Active nappe with a high slip rate: Seismic and gravity profiling across the southern part of the Itoigawa-Shizuoka Tectonic Line, central Japan

Nappe structures at different scales have been reported from many orogenic belts in the world, but almost all the examples are old, inactive structures. Under conditions of low erosion rate and/or high tectonic strain rate, active nappes are expected to grow with time. We report in this paper an example of active nappe from central Japan. The Itoigawa-Shizuoka Tectonic Line (ISTL) in central Japan is a fault zone with a very high slip rate in Pliocene–Quaternary time. We carried out seismic reflection, refraction and gravity surveys along a 30-km line across the Shimotsuburai-Ichinose Fault (SIF), a main active strand of ISTL west of the Kofu Basin. Seismic and gravity data indicate that the fault is a west-dipping, low-angle (~15°) thrust, which separates horizontally-layered basin-fill sediments (total thickness ~1.5 km) on the east from highly deformed Miocene and older rocks on the west. On the hanging wall side of the fault, the seismic line is located in a deeply incised valley. We precisely determined the surface projection of the fault in this valley by analyzing closely spaced gravity data, and found that the fault emerges 1.35±0.10 km west of the mountain front. This indicates that the frontmost 1.35 km of the hanging wall lies horizontally over Pliocene–Quaternary basin fill, forming an active nappe. Fault-bend folding is associated with the nappe formation. A fluvial terrace, which is tephrochronologically dated at 50–60 ka, is involved in this deformation. The amount of slip during the past 50–60 kyr is estimated from tectonic landforms, yielding an average slip rate as high as 7.5–11 mm/yr. It is likely that the front of the nappe on SIF is nearly at a dynamic equilibrium between erosive and tectonic forces, because the present-day width (~1.35 km) of the nappe in the valley is only a fraction of the total amount of thrust-front advance. A comparison is made with the Miocene Main Central Thrust (MCT) in the central Himalaya, which evolved under climatic conditions similar to those in central Japan but at a slip rate twice as high as that on the ISTL. The High Himalayan nappe front, after the MCT became inactive, has been retreating due to head-ward erosion by small tributaries of Ganges; cooling ages of crystalline klippen in the Lesser Himalaya give an estimate of the retreating rate (~10 mm/yr). This value is about a half of the present-day convergence rate (~20 mm/yr) between India and Tibet. It is therefore likely that, in Early to Middle Miocene time when the MCT was active, the High Himalayan nappe had been growing laterally at a rate as high as 10 mm/yr.