PRELIMINARY STUDY OF THE QUATERNARY FAULTING ON THE QILIAN MOUNTAIN NORTHERN MARGINAL FAULT ZONE

The NEN-SWS conpressive stress field in the Qilian mountain region is understood basing on the surveys of the crustal deformation and the faulting in the Quaternary period. Since lata Quaternary, the Qilian mountain marginal fault zone exhibited transcureut thrusing with left lateral slip. The geological and georaorphic surveys show that the npfolding and upthrusting are piominent in the period between Q1 and Q2 and between Q2 and Q3, the plastic flow of the upper crust material and lateral displasement of the fault are piincipal. In this time, the eastern segment of the fault (e. g. Mao Mao mountain fault) reactivated as a left lateral shear zone in right-step pattern. The results of the investigation indicate that the average slip rate of the fault is 2-5 mm/y. But in the western segment of the fault, vertical displacement is principal (2 mm/y.), and at it's eastern segment, there is the prominent horizontal displacement (≥5 mm/y.). Basing on the data revealed by the trenching and the computation of the average seismic slip rate, we know that the recurrence intervals of M≥7 earthquakes in the seismic area is 2000~4000 years.