Hidden Tectonics: Finding faults in (seemingly) climate controlled landscapes

Central Asia’s arid landscape provides a key natural laboratory to study the effects of slow deformation in continental interiors. Far-field stresses of the India-Eurasia collision have created major transpressive fault systems across the continent since the Cenozoic. In the 20^th century the northward progression of this deformation resulted in four major earthquakes in Mongolia, among which was the 1957 M_w 8.1 Gobi Altai earthquake in southern Mongolia. Palaeoseismic research following this event has allowed for quantification of deformation rates since the Late Pleistocene. Yet, the application of classic palaeoseismological methods disregards the possibility of more dispersed deformation, as was suggested in other continental interiors.

The 1957 earthquake ruptured ~350 km of the Bogd fault in southern Mongolia, along the mountain front of a series of Gobi Altai restraining bends just south of the Valley of (Gobi) Lakes basin. The high restraining bends are bound by small, steep alluvial fans that reflect a ~100 kyr climate cyclicity, whereas the low relief Valley of Gobi Lakes is characterized by endorheic lakes and sparsely dated large, gentle fans. To determine whether deformation during the 1957 earthquake was representative of regional deformation, we expanded the active tectonic record by increasing the spatial and temporal scales of our studies. Along the highest restraining bend, Ikh Bogd Mountain (~4,000 m asl), we confirmed vertical slip rates of <0.3 mm/yr along single fault strands. We also observed cumulative deformation and increased steepness of older alluvial fan levels, which could suggest progressive tilting by reverse faults along the mountain front. If this tilting is merely tectonically induced, uplift rates of Ikh Bogd could reach 0.9-1 mm/yr. Morphometric analyses indicate that faults in the restraining bend’s interior still affect river steepness. This could imply that multiple sub-parallel faults are active simultaneously, accumulating to the higher uplift rate suggested by fan tilting.

The basin north of Ikh Bogd comprises the endorheic Orog Nuur (lake) which is mostly fed by the Tuyn Gol (river) that drains the Hangay Mountains in central Mongolia. Its large alluvial fans are cross-cut by four tectonic lineaments that can each accommodate M~7 earthquakes and that have a cumulative vertical slip rate that is similar to the Bogd fault. This suggests that they are significant components of the regional structure, yet they were previously overlooked because the recurrence intervals of surface-rupturing events are slower than climatic rates. In the Orog Nuur Basin itself, reflection seismics indicate that Jurassic-Cretaceous extension structures were reactivated by Miocene-Present transpression. The effect these structures have on the Basin’s modern geomorphology indicates that they may still be active, although lacustrine and fluvial sediments do not reflect any tectonic activity since MIS 5 (~120 ka).

By expanding spatial and temporal scales of active tectonic studies in southern Mongolia, we show that variability in the interplay between climate, tectonics, and geomorphology can mask the complexity of a tectonic structure. By adapting methods and incorporating the different processes that affect landscapes, such studies contribute to more complete seismic hazard assessments in slowly deforming continental interiors.