Bouguer gravity anomaly and isostasy at western Sichuan Basin revealed by new gravity surveys

A dense relative gravity and Global Positioning System observation network with 302 stations was measured in the western Sichuan Basin (SB) to study gravity and isostasy. Bouguer Gravity Anomalies are negative in the study area, and change gradually from about -110 mGal (10(-5)m s(-2)) in the southeast to -220 mGal in the northwest. The new data reveal that the Moho beneath the western SB changes gradually from 39.5 km in the southeast to 43.7 km in the northwest and is inclined slightly to the northwest beneath the Chengdu Plain. The isostatic crustal thickness calculated by Airy isostatic theory varies between 39.5 and 42.0 km. The Longquan Shan (LQS) marks a clear boundary in the state of isostastic compensation in the study area. The Moho depth is almost identical to the Airy isostatic crustal thickness east of the LQS, where the crust is in isostatic balance. However, on the Chengdu Plain west of the LQS, differences between the Moho depth and Airy isostatic crustal thickness became larger from east to west, where the crust is out of isostatic balance. This indicates that the load of the Longmen Shan (LMS) on the west part of the crust of SB is mainly borne by the crust beneath the Chengdu Plain, and does little influence the crust east of the LQS. As a result we deduce that the LQS fault zone is a deeply rooted high angle fault zone that was broken by the load of the LMS.