A Gravity Study of the Longmenshan Fault Zone: New Insights Into the Nature and Evolution of the Fault Zone and Extrusion-Style Growth of the Tibetan Plateau Since 40 Ma

摘要

The Longmenshan Fault Zone (LFZ) is a tectonic boundary between the Tibetan Plateau and the Sichuan Basin of the South China Block. Knowledge of the nature and history of the LFZ is important for understanding the growth of continental plateaus and mechanisms for major earthquakes along their margins, as exemplified by the magnitude 7.9M Wenchuan earthquake of 12 May 2008. Flexural modeling of new and existing gravity survey data along three transects, combined with published seismic reflection profiles and earthquake focal mechanism data, indicates that the central-northern LFZ is a lithospheric-scale fault zone that has low elastic strength but with strong episodic dextral transpressional motions. In contrast, the southern LFZ is a crustal-scale thrust zone dominated by shallow-angle thrust motion of the Tibetan Plateau over a moderately stiff South China Block. In conjunction with the record of Cenozoic basin erosion in the western Sichuan Basin and a regional kinematic analysis, we suggest that the lithospheric-scale LFZ started 40Myr ago and became an external boundary of a northeasterly directed extrusion-style plateau growth, predominantly through crustal thrusting and thickening.Plain Language Summary The occurrence of magnitude 7.9M Wenchuan earthquake on 12 May 2008 at the eastern foothill of the Tibetan Plateau surprised many in the earthquake community. More intriguingly, the large number of aftershocks related to that earthquake almost exclusively occurred northeast of the main shock along the Longmenshan Fault Zone (LFZ), a phenomena current models have trouble explaining. Here we combine gravity modeling with a synthesis of an array of geological and geophysical observations to establish a regional hypothesis featuring (1) contrasting behaviors between the southern and central-northern LFZ, with the southern LFZ being a crustal-scale thrust zone, whereas the central-northern LFZ being a lithospheric-scale fault zone with strong episodic dextral transpressional motions; (2) such contrasting behaviors are driven by differential motions of two crustal blocks from the western highland (the Tibetan Plateau), pushed by India's northward motion; (3) as a consequence, the LFZ features a twisted fault plane with the Wenchuan earthquake being located at the junction of the LFZ and the bounding fault between the two crustal blocks to the west, and (4) the LFZ has been behaving like this since around 40Myr ago, and it became an external boundary for a northeasterly directed extrusion-style growth of the Tibetan Plateau.