Séminaire de Wang Xin (invité au LOG) Geosciences School, Zhejiang University, Hangzhou, China intitulé :
Large-scale thrusting along the northern margin of the Tibetan Plateau and the Southwestern Tarim Basin : 230 km long active Hotien thrust sheet
Date : 14 octobre 2016 à 14h00
Lieu : salle Pruvost SN5 à Villeneuve d’Ascq
We present the geometry, kinematics and mechanics of large-scale active thrusting in the western Kunlun Range and the Southwestern Tarim Basin, which accounts for 130-165km total shortening of Tarim crust at the northern margin of Tibet Plateau. The great frontal structure is the 230 km long bedding-parallel Hotien thrust sheet, which is one of the longest active intact thrust sheets in the world, composed off lat-lying strata of the Tarim Basin sliding northward on a regional gypsum detachment at the base of the Cenozoic sequence. The toe of the Hotien thrust ramps to the surface two thirds of the way across the Tarim Basin, forming the Selibuya-Mazhatag hills in the Taklamakan sand desert. At the southern edge of the Tarim Basin in the Kunlun Range foothills, a set of high-amplitude anticlines are growing by complex break-forward ramping and wedging in the Hotien thrust sheet as it steps up to the Cenozoic gypsum detachment from a regional Cambrian evaporate detachment that extends under Tibet. More interior structures such as the Tiklik thrust bring older strata and Proterozoic basement to the surface, together with their Cenozoic Tarim cover in the Buya Basin. The Cambrian detachment also extends northward under the Tarim Basin with minor hanging-wall deformation that locally warps the overlying Hotien thrust sheet, producing a complete syntectonic record in seismically imaged growth strata of its northward motion over these warps. Seismic profiles in the Southwest Tarim foothill belt also reveal widespread growth strata that record much of the structural history beginning in the Neogene Atushi Formation. Ages of seismic reflectors are calibrated to a surface magnetostratigraphic sequence (Zheng et al., 2015). The beginning of thrusting and folding in the Southwestern Tarim Basin north of the Tiklik thrust is dated at 22-24Ma with shortening >28km and a progressive northward propagation toward the Selibuya- Mazhatag hills. The overall shortening rate is 1.2mm/yr. The gypsum detachment of the 240km long Hotien thrust sheet has an effective friction of <0.02-0.03 from its tapered wedge geometry (Suppe, 2007). In the last 1Ma it has moved at 13mm/y, suggesting an effective linear viscosity for the 60-100m thick gypsum detachment of <0.7-4.0x10-18 Pas, which is consistent with a creeping detachment as opposed to frictional sliding.