Some geomorphic indices in the North Central Vietnam

Abstract

We have applied geomorphology to investigate the tectonic fault in the North Central Vietnam. At the local scale, the active tectonic possibility of fault zones in the study area is geomorphologically assessed for the first time. The results show the geomorphic indices of drainage basin asymmetry (AF = 0–34), valley floor width-to-height ratio (Vf = 0.21–1.71) applied to 50 drainage basins and mountain-front sinuosity (Smf = 1.07–1.58) calculated along 17 tectonic fault zones. The geomorphic analysis indicates that the modern activity of faults in the study area is moderate-weak. Only a few segments of the Truong Son and Dakrong-Hue faults are assessed as strongly active. The tectonic activity of this area is assessed with an average uplifting rate of about 0.2 mm/year according to the Smf and Vf analyses. In order to prove the existence of faults as well as their active possibility, the field survey has also been carried out by structural geological and geomorphological methods. The survey was conducted at 369 outcrops and approximately 12,000 fractures were measured. Based on the data on displacement and the processing of data on slip observed at the outcrops, we have identified the strike-slip tectonic stress state of Pliocene-Quaternary period with the sub-longitudinal compressional direction and the slip mechanism of fault zones in this period. It is a new finding of the contemporary crustal stress orientation of North Central Vietnam. The results of this study provided the geomorphic evidence to evaluate the potential seismic hazards of the North Central Vietnam.