Tectonic evolution of the Yangsan Fault, SE-Korea, by gravity field interpretation

Abstract

SUMMARY To estimate the tectonic displacement history of the Yangsan Fault (YSF) in the Cretaceous Gyeongsang Basin (GB), SE-Korea, gravity field measurements were implemented and interpreted. A total of 8000 ground-measured gravity data were compiled and deployed to calculate the Bouguer and residual gravity field. The residual field shows that the mean value in the southern segment of the YSF is very similar to that of the northern segment, meanwhile that of the central segmentation is significantly lower than those of the other segmentations. This implies that the crustal structure and petrophysical properties of the central segment differ significantly from those of the other segments. Applying dip curvature analysis to enhance the image of the residual gravity field, we find that (1) the Gaum fault system (GF) in the western and middle part of the GB extends up to the YSF, where A-type granite outcrops are observed. (2) The middle- and eastern segments of the GF that formed before tectonic motion of the YSF are revealed in turn in areas east of the YSF, which were now covered by the younger sedimentary basin. (3) The middle segment of the GF which occupies between the YSF and the Yeonil tectonic line (YTL) has been displaced approximately 25 km southward from the main GF trace. (4) The easternmost segment is revealed buried in the eastern side of the YTL, which is displaced southward about 18 km from the main GF trace. Comparing all the available geoscientific data sets, we conclude that the right-lateral tectonic movement of the YSF, and the deeply buried middle segment of the GF remained stationary, while the most eastern segment of the GF is reversely moved to the north through left-lateral strike movement of the YTL.