Ground Magnetic Anomalies in the Tanna Fault and Their Implications.

Abstract

The Tanna fault is a left lateral fault extending in the middle of the Izu Peninsula. We carried out a detailed ground magnetic survey at a grid spacing of 50 to 150m along the Tanna fault and constructed a magnetic model based on the ground survey with the help of drilling data in the Tanna basin and measurement of rock magnetism.Total intensity distribution obtained by the ground magnetic survey represents a distinguished contrast forming relative magnetic highs in the west to the fault and lows in the east. The Tanna fault is a remarkable border terminating magnetic highs.Drillings revealed that formations filling the Tanna basin are mainly the middle Pleistocene Taga volcanics which possess high susceptibility as a whole. A recent work suggests that the Taga volcanics were produced in normal polarity and had strong remanent magnetization. Therefore, remanent magnetization is inferred to be dominant in the total magnetization.The magnetic contrast forms almost a north-south trend throughout the study area. The magnetic modeling using E-W profiles delineated strong normally magnetized sources of 6A/m confined to the west to the fault. Three drillings indicated that the east formations to the fault were wholly disintegrated and the west formations, by contrast, included fresh lavas. These evidences suggest that the major cause of magnetic contrast is not the difference of geologic unit but instead the weakened remanent magnetization in the east adjacent area to the fault.Seismic reflection and shallow drillings indicate the possibility of existence of shallow wrench fault. Bending usuallly appears along a strike slip and actually several basins such as Tanna basin and the Tashiro basin exist along the fault, and some magnetic lows are supposed to be caused by sinking of volcanic layer or pull-apart movements. The striking magnetic discontinuity, on the other hand, implies that the fault surface been displaced ever since Pleistocene has always been the known Tanna fault.In conclusion, the cause of relative magnetic lows in the east to the fault is responsible for effects of, either remanent magnetization reduced, and sinking and truncating of magnetic sources deformed, by the fault activities.