Morphotectonics and paleoseismology of the North Darhad fault (SW Baikal Rift, Mongolia)

Abstract

The North Darhad Fault, a major N-S fault at the SW Baikal Rift, demonstrates weak present-day activity, but several morphotectonic features indicate the occurrence of strong surface-rupturing earthquakes in the past. The seismic potential of the North Darhad Fault has not been accurately assessed, leaving a gap in the assessment of the seismic hazard. In this paper we present remote and field morphotectonic and paleoseismological studies carried out along this fault system. A detailed topographic mapping of the fault scarp in alluvial surface and trenching across the fault revealed two paleoseismic events responsible for the scarp formation. Dating of organic remnants found in the cross-section allow estimating a Holocene age for the paleoearthquakes and bracket the vertical slip rates along the North Darhad Fault between 0.3 ± 0.06 mm/yr (min) and 0.6 ± 0.12 mm/yr (max) over the last ∼ 8.4 ka. The obtained morphotectonic and paleoseismological data suggest that the North Darhad Fault has produced two magnitude Mw7 earthquakes separated by mean recurrence period of ∼ 3.5 ka. We have demonstrated that within the SW Baikal Rift the ∼ M7-7.5 earthquakes likely occur during temporal clusters along the faults separated in space by up to 250 km. The slip rates along the North Darhad Fault allowed us to compute the age of the Darhad Basin at 6.5–3.3 Ma, suggesting a synchronous formation of fault-controlled horst-graben systems throughout the SW Baikal Rift. The opening of the Darhad Basin fits well into the pull-apart model of deformation between the two largest strike-slip fault systems - Bulnay in the south and Mondy-Tunka-Sayan in the north.