西藏安多-错那湖地堑的第四纪地质、断裂活动及其运动学特征分析

地表调查和初步的沉积物年代测试结果表明,晚第四纪期间,在安多-错那地堑中主要发育了分别形成于44.2kaB.P.和9～7kaB.P.左右的两套湖泊沉积物和约42kaB.P.以来的5套冲、洪积物。在安多-错那地堑的边界主要发育了包括安多南缘断裂、北缘断裂、错那湖东缘断裂和西缘断裂共4条第四纪正断层。其中活动强度最大的为安多北缘断裂,其第四纪最小垂直活动速率为0.24±0.02mm/a;其次为安多南缘断裂和错那湖东、西两侧边界断裂,它们的最小垂直速率分别为0.19mm/a,0.12～0.16mm/a和0.10～0.12mm/a。晚第四纪以来的断裂活动主要集中在平均垂直活动速率为0.41±0.22mm/a的安多北缘断裂带的西段。安多及邻区现今的地表构造格局及断裂带的几何学和运动学特征符合近南北向地壳缩短背景下由于近东西向伸展变形而引发的菱形断块发育模式。根据断层的活动速率估算结果,晚第四纪期间安多-错那地堑的平均伸展速率为0.25±0.15mm/a,而整个羌塘块体总的东西向伸展变形速率可能达到11±8mm/a。

ACTIVE FAULTS AND THEIR KINEMATIC FEATURE AT THE AMDO-TSONA GRABEN, CENTRAL XIZANG

There is a “sickle-shaped” basin termed the Amdo-Tsona graben in central Xizang. It contains a NE trending graben known the Amdo basin to the north and a NNE trending graben called the Tsona lake basin to the south （Fig.1）. The graben, about 70km long and 5～12km wide, cuts the east trending thrust-folds belt which had been formed before Late Cenozoic. The basin floor is covered by Late Quaternary deposits containing two sets of lacustrine and 5 sets of alluvial deposits formed since about 44.2kaB.P. and 42kaB.P.,respectively. The graben is symmetric （Fig.3）. There are four main boundary normal fault zones containing Amdo-Tsona north-margin faults, striking N30°～70°E and dipping 50°～70° to the south-east, Amdo south-margin faults, striking N50°～60°E and dipping 50°～70° to the north-west, Tsona-lake east-margin faults, striking N10°～20°E and dipping 50°～70° to the west, and Tsona-lake west-margin faults, striking N10°～20°E and dipping 50°～70° to the east. The main boundary faulting has concentrated on the Tsona lake northern margin fault zone since the Late Quaternary. There are prominent N30°E trending, south-east facing triangular facets and fault scarps along the footwall of the Tsona-lake north-bounding fault （（Fig.2c～f）.） The results of in-situ measurement with level instrument and tape shows that there are 4 fault scarps which represent vertical displacement about 1.4±0.1m, 6.5±1.1m, 10.1±1.9m and 22.2±3.5m offsetting alluvial second to fifth set fans or terrace Ⅱ, Ⅲ,Ⅳ and V from young to old, respectively （Fig.4）. The dating of U-series, Thermo-luminescence （TL） and ～14C of alluvial fans and terraces provides constrains to the vertical slip rate of the faults. The offsets of 1.4±0.1m, 6.5±1.1m, 10.1±1.9m and 22.2±3.5m represent the faults throws since 6.8～4.2kaB.P., 27～12kaB.P., 35.5ka B.P. and 46～41kaB.P. respectively. The dating results suggest that the vertical slip rate is 0.20～0.63（mm/a） at north side of the Tsona lake graben. The height of triangular facets and minimum thickness of Quaternary sediment taken from drill hole in the Amdo basin implied that the minimum cumulative throws on faults are about 670±50m, 540m, 390±50m and 310±25m across the Amdo-Tsona north-margin faults, Amdo south-margin faults, Tsona-lake east-margin faults, and Tsona-lake west-margin faults since about 2.8Ma, and the minimum vertical slip rates are about 0.24±0.02mm/a, 0.19mm/a, 0.12～0.16mm/a and 0.10～0.12mm/a,respectively. The Amdo-Tsona, Dong Co and Peng Co basin apparently constitute an about 120km long, （N40°±）（5°E） trending and left-lateral echelon array rift system （Fig.5）. The pattern of faults of Amdo-Tsona and its adjacent area is similar to “rhombohedral block tectonic” advocated by Rothery et al. （1984） （Fig.5）. The slip rates of faults correspond to a “spreading” rate of 0.25±0.15mm/a along the extensional direction of （105°±）5°cross the Amdo graben. Based on the slip magnitude of normal faulting, the total “spreading” rate along a 110-km-long area ESE traverse across Amdo area is about 1.1±0.8mm/a （Fig.6）, so the cumulative extensional rate perhaps reaches 11±8mm/a cross the central Tibet block during Late Quaternary.