Fault structures of the eastern boundary fault zone of the Niigata plain, central Japan, revealed by gravity survey and analysis

Abstract:

The eastern boundary fault zone of the Niigata plain (EBFZNP) is about 60 km-long and is distributed in the NNE-SSW direction along a boundary between the Niigata plain and the Echigo-Iide Mts., central Japan. The plain is a very thick sedimentary basin of which the deepest is over 6000 m (JNGA and JOPD, 1992) and the Cretaceous basement rocks are exposed at the Mts. side. This region has been involved in compressive deformation (ca. 3 Ma ~), developing the fold structures, such as Niitsu hills located in the western part of the EBFZNP, in sediment layers and the EBFZNP as a bedding-slip fault along the unconformity between the Miocene and the basement rocks (e.g., Ikeda et al., 2002). In order to clarify more detailed subsurface structures and characteristics of the EBFZNP, we have conducted a gravity survey and analysis around this region.

A gravity survey was carried out in September 2014 and March 2015. We set four gravity survey lines, one of which corresponds to a seismic survey line (Kato et al., 2013), across the EBFZNP. The number of measurement points is 216. We compiled gravity data measured in this survey and published (Yamamoto et al., 2011; Geological Survey of Japan, AIST, 2013; Geographical Survey Institute of Japan, 2015), and calculated Bouguer anomalies. In order to illustrate discontinuities of the subsurface structures, filtering processes of a horizontal and vertical first-order differential operation were applied to the Bouguer anomalies. We also analyzed density structures along the survey lines using the 2D Talwani’s method (Talwani et al., 1959).

The Bouguer anomalies are relatively low in the plain side and 40 mGal higher in the Mts. side. Both of the steep horizontal gradients and the zero isolines of the vertical differentiation, which indicate tectonic discontinuities of the subsurface structure, are continuous and clearly extend along the EBFZNP. These features suggest that the EBFZNP forms a continuous fault structure below the surface. From the density structure analysis, we recognize that the EBFZNP is wholly a west-dipping fault structure and that the Niitsu hills have an anticlinorium structure with a basement high in the south part of the hills and an anticline structure formed in sediment layers in the north part, respectively.

This study is supported by JSPS KAKENHI Grant Number 26400450.