Restraining Bends, Flower Structure Coalescence, and Transpressional Duplexing in the Mongolian Altai
Dickson Cunningham
Orogenic Processes Group
University of Leicester
wdc2@le.ac.uk
The Mongolian Altai is a tectonically active intra-continental mountain range dominated by dextral transpressional deformation. Late Cenozoic uplift of the Altai is believed to be a distant strain response to the Indo-Eurasia collision more than 2000km to the south. The modern range is essentially a reactivated Palaeozoic orogen containing basement terranes which represent arc, back-arc and subduction complex assemblages which lie between the more rigid Junggar and Hangay blocks which have resisted Cenozoic contractional deformation. The structural architecture of the Mongolian Altai is particularly interesting because the range is dominated by discrete block uplifts that occur along an array of NW-striking, regional strike-slip faults, and at their termination zones. Ranges can be categorized as single transpressional ridges, terminal restraining bends, partial restraining bends, flower-structure pop-ups, and triangular tilt blocks. Regional drainage patterns and active fault orientations indicate that most ranges are topographically asymmetric because they are structurally asymmetric flower structures or single thrust-bounded tilt blocks. The highest peaks in the region (Sutai and Tsambagarav Uul - 4000m+) are restraining bends containing oblique-slip duplexes with positive flower structure cross-sections. These ranges are likely to have experienced vertical axis rotations leading to along-strike and across-strike growth of the range and progressively changing fault kinematics. Outward growth of individual ranges is by simple overthrusting or growth of forebergs within oblique-slip duplexes in bounding alluvial basins. Reactivation of the predominantly NW-striking, NE-dipping basement fabrics and older faults is an important first-order influence on the orientation of major Cenozoic faults. Coalescence of transpressional ranges in the eastern Altai is causing progressive closure of intramontane basins (ramp basins) and development of a topographically continuous mountain belt. Anticlockwise rotation of the northern and southern Mongolian Altai regions is suggested by the regional oroclines at both ends, but only proven palaeomagnetically for the northwest end. If regional anticlockwise rotation has also affected the Late Cenozoic evolution of the southeastern Altai, then it is likely that strike-slip faults and whole mountain ranges have progressively reoriented to directions more favourable for oblique-slip and contractional deformation obscuring the older strike-slip history. The geometric relationship between rigid block boundaries, Palaeozoic basement structural grain and the dominantly northeast maximum horizontal stress has dictated the kinematics of Late Cenozoic deformation in the Mongolian Altai. Intra-plate transpressional mountain building processes as observed in the Altai probably occur in other tectonically active intra-continental regions such as Yunnan, western Tien Shan, Hindu Kush, and the Stanovoy Range, and are likely to have operated in ancient continental interior orogens driven by distant continental collisions.