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 7.9 M 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 40 Myr ago and became an external boundary of a northeasterly directed extrusion‐style plateau growth, predominantly through crustal thrusting and thickening.