Long-term Petrological Evolution of the Wenchuan and Beichuan Fault Zones (Longmen Shan, Sichuan)

The epicentre of the May 12 2008 (M7.9) Sichuan Earthquake seems to be near the junction between two main faults, the Wenchuan fault to the west and the Beichuan fault to the east. The rupture mainly propagated along the Beichuan fault over more than 250 km, with thrusting toward the East in the southern part becoming more dextral slip in the northern part. Displacement on the Wenchuan fault can not been excluded nevertheless. The structures of this area result from a polyphased evolution dating back from Triassic times. Particularly, intense deformation is concentrated around the Wenchuan shear zone, and was reactivated during Cenozoic times as a far effect of the Indian Asia collision. Our study was focussed on several sections across the Wenchuan and Beichuan fault zones to document their long term tectono- metamorphic evolution. Two cross sections across the Wenchuan fault were sampled, a southern one and northern one. The deformation to the south reflects southwest verging tranpressional movement, whereas strike slip movement dominated along the northern profile. The southern profile cross cut several slices in which a metamorphic gradient increasing from West to East. The largest and westermost slice consists of metabasic rocks, including metagrabbros, metabasalts and oceanic metasediments. Chemical analyses of the basic rocks show an adakitic signature. The PT estimations reveal that high pressure condition was reached for these rocks (~ 14 kbar). This slice may represented a former back arc unit having been subducted at great depth (45 km), and later exhumed along the Pengguan crystalline massif. Such gradient is not observed on the northern part of the Wenchuan fault where the temperature increases at constant depth. The age of the metamorphism is so far unknown. Hence, the metamorphic evolution of the northern Wenchuan is compatible with long term strike slip movement with little exhumation. The southern part exhumation from greater depth is evidenced, that is consistent with the transpressional structures observed in this area. For the Beichuan fault, preliminary petrological study (along a southern transect) indicates temperature conditions of the hanging wall (<350°C) lower than those from the Wenchuan fault, but with relatively high pressure (10 kbar). Temperature in the footwall reached 350°C. Preliminary petrological results suggest that the long term evolution of the Beichuan fault (at least in the south) was dominated by westward thrusting. Therefore, the complexity of the recent rupture shown by radar interferometry and field observations, is probably due to pre-existing geological structures.