UNDERSTANDING RECENT TOPOGRAPHIC GROWTH HISTORY OF NORTHERN VIETNAM

Northward motion of India relative to Eurasia has been accommodated within the brittle upper crust, by clockwise rotation of blocks bounded by left-lateral faults. The considerable regional stresses over that period caused the SE extrusion of the Red River Fault (RRF) most of which took place between 35 and 17 Ma. Apatite fission track (AFT) data from across northern Vietnam record the associated rapid rock uplift and exhumation and yielded cooling ages between 35–25 Ma. However regional rock uplift did not end in the Miocene. GPS data from across the region show c. 7–10 mm/yr of N-S shortening is taking place at the surface but major structures such as the Red River Fault do not dominate the present-day kinematics of the area (current slip rates of <1mm/yr are low compared to tens of centimeters during 35-17 Ma). Although GPS data record recoverable elastic strain build-up, not permanent deformation it is likely that some of this shortening may have contributed to the recent surface uplift that is evident in the longitudinal profiles for tributaries of the Red River that cut deeply into a pre-uplift low relief landscape. The circa 1000-1500 m of river incision points to kilometer-scale surface uplift across the region but when this initiated is unclear as is the nature of the deformation: There is no evidence for widespread recent thrusting or reverse faulting to support ongoing shortening in the RRF region. To try and better understand the regional deformation history we collected samples from across northern Vietnam and the RRF to compliment previous work. Combining both AFT and apatite U-Th-He analyses we find that Paleogene cooling dominates the exhumation record and that the modern relief development has not led to widespread exhumation of rock from sufficient crustal depths to pinpoint when the most recent phase of surface uplift began.