Study on the Gravity Field and Deep-Seated Crustal Structure at the North Margin of the Qinghai-Tibet Plateau

Based upon the 1:500,000 gravity profiles, surveyed along both the Lao Mangnai--Argan line across the Altun Mts. and the Golmud--Ejin Qi line across the Qinan Mts. and Qaidam basin, with the regional gravity field and the corresponding geological data of the Altun Mis. and the Qilian Mts. taken into cnosideration, a study has been made on the gravity features of the Qinghai-Tibet Plateau's north made. This area has been divided into the following five geo-tectonioc units: the Qaidam basin, the Altun fault system, the mid-south Qi- lian terrane, the northern Qilian fold belt and the southern margin of the Tarim basin. The Airy isostatic anomaly in this area has been calculated and analyzed. The occurrence of the negative isostatic anomaly is probably related to the relative depression of the inner zone and the thick sediments resulting from the thrusting in the upper crust, as well as to the thickening of the lower crust brought about by the subduction of the Tarim plate; while the occurrence of the low positive isostatic anomaly is originated from the tectonic emplacement of the frontal typhonic rocks, resulting from the thrust spreading of the upper crust The Altun fault zone, the Hexi Zoulang's south margin and the North Qilian fault zone are considered to be the dynamic boundary of the northern margin of the Qinghai-Tibet plateau.