Two Cenozoic tectonic events of N–S and E–W extension in the Lhasa Terrane: Evidence from geology and geochronology

Abstract

Cenozoic active structures in the Tibetan Plateau are mainly regional N–S trending extensional faults and grabens, and E–W trending extensional tracks that are related to the transition from syn- to post-collision between India and Asia. E–W trending tracks are parallel to the direction of Neo-Tethyan oceanic convergence and consist of extensional volcanic–sedimentary basins and magmatic dykes in the southern Lhasa Terrane, Tibet. N–S trending tracks comprise faults and grabens, which are widely developed in Tibet. It remains unknown how and when the geodynamic transition from E–W to N–S trending tectonic tracks occurred. This study describes both E–W and N–S trending tectonic tracks identified at Dazi area of southern Lhasa Terrane, where E–W trending mafic dykes intruded a granitoid and late-stage N–S trending felsic dykes cut across E–W trending mafic dykes. Zircons from four granitoid samples yield consistent crystallization ages of ca. 60Ma and positive εHf(t) values (~+9). An altered dioritic vein, which cuts the mafic dykes, yields an age of ca. 53Ma. These new dating results indicate that E–W trending dykes, which formed due to regional N–S extension, were emplaced between 60 and 53Ma. In addition, two N–S trending monzonitic porphyritic dykes, which cut the mafic dykes, yield U–Pb zircon ages of ca. 17Ma with moderate positive εHf(t) values (+3 to +9.6), as well as a NNE–SSW trending quartz monzonitic dyke, which cuts all other types of dykes, yields U–Pb ages of ca. 13Ma. This suggests that E–W extension took place between 17 and 13Ma. These results, in combination with existing age data for Gangdese granitoids and mafic magmatism, indicate the occurrence of two major extensional events at 60–53Ma and 17–13Ma. In turn, this implies that the transition from E–W to N–S trending tectonic and the onset of E–W extension occurred at ca. 17Ma or slightly earlier. Paleocene granitoids have geochemical characteristics that are indicative of both subduction and collision. Miocene felsic dykes show adakitic affinities, which hints the transition from syn-collision to post-collision setting. Break-off of the Neo-Tethyan slab is assumed to have been responsible for the formation of E–W trending dykes and volcanic–sedimentary basins. The N–S trending felsic dykes probably formed in response to the tearing or delamination of lithosphere subducted beneath southern Tibet.