长江口地区地面沉降的深部动力学机制分析

ANALYSIS OF DEEP DYNAMICS ON LAND SUBSIDENCE IN YANGTZE RIVER ESTUARY,EASTERN CHINA

地表变形、活动断裂和地球物理的综合分析表明,菲律宾洋壳向欧亚大陆的俯冲导致的地幔对流是控制中国东部沿海地区晚新生代以来构造作用的主导因素,是长江口地区地面沉降的主要深部动力学机制。由于地幔对流和青藏高原挤压共同作用导致的地壳热流值的差异则是长江口地区西部隆升、东部沉降且向东沉降速率增大的直接驱动力。预测未来长江口地区的基岩沉降范围将以>10cm/a的速率向西扩大,沉降速率将呈明显加速趋势,40000a之内上海市可能被海水淹没,但板块构造演化的"渐变"特征决定其对当地未来的人类活动不会造成显著影响。根据"地壳均衡理论",建议在长江口南西部(浙江省北东部)的丘陵山区加大重力载荷如加快城市化进程或人工造山以减小和控制上海地区的沉降。

The observed GPS velocity vectors relative to the Eurasian Plate in the East Asia display that the coastal area of East China constitutes a transitional region in crustal horizontal deformation, expanding to its east and west. Combined with the reverse GPS velocity vectors in the Philippines Ocean characterized by increased values northwardly, it is suggested that the East China continental crust is easterly rotated negatively relative to Eurasia, under the co-influence of westerly Philippines Ocean crust subduction and easterly Tibetan Plateau extrusion. The generated mantle convection stretched the continental crust and controlled the neotectonic evolution in the coastal area of East China. It also constitutes the major deep dynamic mechanism of land subsidence in the Yangtze River Estuary. This model is well supported by the attribution and spatial and temporal distribution of the active faults, as well as the geophysical features in the Yangtze River Estuary. It is consistent with the regional tectonic setting characterized by asthenosphere upwelling and lithosphere extension, and the major tectonic mechanism of the modern tectonic subsidence in East China Sea basin. The analysis on geophysics further suggests that the differentiation of heat flow in the crust, caused by the co- influence of mantle convection and extrusion from the Tibetan Plateau, drove uplift in the west of the Yangtze River Estuary and subsidence in its east, with an increasing velocity easterly. The driving force is similar to that of the Tuscan Basin in Italy, and in line with the Theory of Isostasy. Therefore, for the purpose of reduce and control the land subsidence in Shanghai area, it is proposed here to increase the loading in the upland of northeastern Zhejiang Province, southwest of Yangtze River Estuary, e.g., accelerate the urbanization process or build up mountains artificially. The westerly subduction of the Philippines Ocean crust, together with the easterly extrusion from the Tibetan Plateau, would cause the mantle upwelling area beneath the Yangtze River Estuary move westerly with a velocity of 〉 10cm/a, according to the supraposition of the GPS velocity vectors in the Philippines Ocean and the Yangtze River Estuary. It implies that, based on the newly suggested mechanism, the subsided area in the Yangtze River Estuary would be expanded westerly with a velocity of 〉 10cm/a, the uplifting area to the west would be evolved to be subsided area, the continental crust under Yangtze River Estuary would be tensed and thinned gradually and become a mature back-arc basin like Okinawa Trough. Owing to the crust-thinning under a tensional background, its capability resistant to the mantle convection could decrease remarkably, and the subsidence velocity of local bedrock would be accelerated obviously in the future. According to the thickness and age of local Late Cenozoic strata, combined with the local average elevation, it is calculated that the Shanghai area would be subsided under sea level in 40000a, if no process is taken.