Neotectonic (Miocene to recent) vertical movements in the Lao Cai Basin (Red River Fault Zone, Vietnam): An approach to seismic hazard assessment

In this contribution, we present a model of the neotectonic (middle Miocene to recent) development of the Lao Cai Basin located in the Vietnamese segment of the Red River Fault Zone. The model is constructed on the basis of the analysis of mixed-layered illite-smectite composition and vitrinite reflectance analysis of middle Miocene strata. The model is supported by new data on small-scale brittle tectonic structures cutting these strata and is supplemented with C14 dates of Quaternary river terraces. Furthermore, we present the first evidence for paleoseismicity predating the last hundred years in the Lao Cai Basin. Our results show that: (1) the Lao Cai Basin is a graben formed during the left-lateral shift along the Red River Fault Zone due to oblique slip on graben-bounding and graben-cutting strike-slip faults as well as dip slip on graben-cutting normal faults; (2) the exhumation of the middle Miocene strata filling the graben might have amounted up to 2.4 km and took place during the right-lateral shift along the Red River Fault Zone, possibly being affected by the reversal of slip on graben-bounding and graben-cutting faults and, (3) the Lao Cai Basin was affected by M5 + earthquakes, whereas the return period of such earthquakes is more than 370 years but less than ca. 2300 years. Moreover, we tentatively estimate the late Holocene surface uplift rate in the Lao Cai Basin at ca. 7 mm/year.