A structural transect across the Mongolian Western Altai: Active transpressional mountain building in central Asia

We present results from the first detailed geological transect across the Mongolian Western Altai using modern methods of structural geology and fault kinematic analysis. Our purpose was to document the structures responsible for Cenozoic uplift of the range in order to better understand processes of intracontinental mountain building. Historical right-lateral strike-slip and oblique-slip earthquakes have previously been documented from the Western Altai, and many mountain fronts are marked by active fault scarps indicating current tectonic activity and uplift. The dominant structures in the range are long (>200 km) NNW trending right-lateral strike-slip faults. Our transect can be divided into three separate domains that contain active, right-lateral strike-slip master faults and thrust faults with opposing vergence. The current deformation regime is thus transpressional. Each domain has an asymmetric flower structure cross-sectional geometry, and the transect as a whole is interpreted as three separate large flower structures. The mechanism of uplift along the transect appears to be horizontal and vertical growth of flower structures rooted into the dominant right-lateral strike-slip faults. The major Bulgan Fault forms the southern structural boundary to the range and is a 3.5-km-wide brittle-ductile zone that has accommodated reverse and left-lateral strike-slip displacements. It appears to be linked to the North Gobi Fault Zone to the east and Irtysh Fault zone to the west and thus may be over 900 km in length. Two major ductile left-lateral extensional shear zones were identified in the interior of the range that appear to be preserved structures related to a regional Paleozoic or Mesozoic extensional event. Basement rocks along the transect are dominantly metavolcanic, metasedimentary, or intrusive units probably representing a Paleozoic accretionary prism and arc complex. The extent to which Cenozoic uplift has been accommodated by reactivation of older structures and inversion of older basins is unknown and will require further study. As previously suggested by others, Cenozoic uplift of the Altai is interpreted to be due to NE-SW directed compressional stress resulting from the Indo-Eurasian collision 2500 km to the south.