COMPRESSION AND EXTENSION IN THE LUNPOLA BASIN, CENTRAL TIBET FROM REPROCESSING OF 3D SEISMIC DATA FROM THE CHINA NATIONAL STAR PETROLEUM COMPANY

Reprocessing and interpretation of 3D seismic reflection data provided by the Chinese National Star Petroleum Company from the Lunpola Basin, Central Tibet details Cenozoic north-south shortening on reverse faults at the basin margins and pervasive east-west extension on high angle normal faults within the basin. The post-Eocene Dingqing Formation is thickest in the western center of basin and thins outwards in all direction (130m to1470m). However, the Eocene-Paleocene Niubao Formation is thickest in the west and thins to the east and southeast (1670m to 4320m). The depth to Cretaceous and Jurassic basement varies from 2760m at the southeast corner to 5000m at the western center of the basin. Indications of a channel system on a 3D time-slice near the base of the basin confirm a non-marine deposition environment at the initial stages of basin development. We observe very little evidence of extension in the earlier stages of basin sedimentation, arguing against a rift origin. However, east-west extension is substantial in the later basin history: seismic stratigraphic analysis suggests that numerous normal faults developed some time after 34 Ma (estimated age of deposition of the post-Eocene Dingqing Formation). These north-south trending normal faults are terminated by the east-west trending thrust faults at the northern flank of the basin. This relationship suggests that thrust faulting may have been active until relative recently. Moreover, the geometry of the thrust faults bounding the northern flank of the basin deduced from the 3D seismic data suggests that they are less steep than previously reported from sparser 2D data, implying significantly greater shortening. The data are most consistent with the Lunpola basin originating in a compressional regime (foreland basin), possible due to Cenozoic reactivation of the Jurassic Bangong-Nujiang Suture Zone along its northern flank. The tectonic environment subsequent became dominated by transpressional (flanks) extensional tectonics (center) consistent with the strike-slip tectonics that characterizes current kinematics in central Tibet. This transition is most logically related to the onset of crustal collapse due to plateau uplift.