Crustal Density Structure, Lithosphere Flexure Mechanism, and Isostatic State Throughout the Qinling Orogen Revealed by In Situ Dense Gravity Observations

Abstract

AbstractGravity and GPS hybrid measurements were conducted at 385 stations throughout the Qinling Orogen, China, to update Bouguer gravity anomalies and free‐air gravity anomalies of the area. The crustal density structure, lithosphere flexure mechanism, and isostatic state of the Qinglin Orogen were studied in detail via these in situ observations. Bouguer gravity anomalies in the study area are mainly negative, ranging from −410 mGal in the Tibetan Plateau to −100 mGal in the eastern Qinglin Orogen. The estimated crustal thickness ranges from 56 to 35 km and thins eastward along the Qinling Orogen. The optimal effective elastic thickness (Te) of the study area is 14 km, and the loading comes from the Earth's surface, the interface between the upper and lower crust, and the Moho. Vertical tectonic stress borne by the lithosphere varies observably in the study area. Downward stress reaches a peak of −14 MPa in the Liupanshan Mountains, whereas the highest value of upward stress (8 MPa) is attained in the middle Qinling Orogen. Considering distributions of crustal density structures and vertical tectonic stress of the lithosphere, in addition to the distinct loading ratios of the eastern and western Qinling Orogen, a piecewise combination model was developed to interpret the uplift of the Qinling Orogen. This model shows that the eastern part of the Orogen is thickened by the upward migration of mantle materials under the thrusting of the South and North China Blocks, whereas uplift of the western part results from lower crustal flow derived from the Tibetan Plateau.