9:45 AM - 10:00 AM
[SSS28-08] Geologic structure across the central part of the western marginal faults of the Kitakami Lowland
Keywords:the western marginal faults of the Kitakami Lowland, gravity anomaly, active fault
The gravity survey was conducted across the faults with a Sintrex gravity meter CG-5 along an E-W survey line, 12 km long. The typical interval of observation sites is 200 m. The elevation of the sites was surveyed with RTK-GPS. Acquired gravity data was processed to obtain Bouguer anomaly mostly according to the methodology of Geological Survey of Japan, AIST (2004). We assumed that the density for Bouguer and terrain corrections were 2.2 g/cm3.
Obtained Bouguer anomaly after trend correction shows lower value around the western marginal faults, and higher value in the eastern and western areas. The difference between these values is about 20 mgal. We assume three layers in our model, which have densities of 2.1 g/cm3 (layer 1), 2.5 g/cm3 (layer 2) and 2.7 g/cm3 (layer 3), respectively. The interpretation of the model is as follows. Layer 1 is correlated to the surface covers, Pliocene and upper Miocene sedimentary rocks, layer 2 middle Miocene sedimentary rocks, and layer 3 basement rocks.
In the model, two half-grabens, filled with middle Miocene, develop below the western sides of the western marginal faults and the Tsunatori fault, both of which constitute listric boundary faults of the half-grabens. And they show thrusting displacement. We will discuss the development of this inversion structure in detail.
Geological Survey of Japan, AIST, 2004, Gravity CD-ROM of Japan.
Kato et al., 2006, Journal of Structural Geology, 28, 2011-2022.