松辽盆地成因演化与软流圈对流模式

松辽盆地位于中国东北,是晚中生代在活动大陆边缘上发育的裂谷-坳陷盆地。松辽盆地有两个特点:一是裂谷期前火山岩分布以盆地西部的大兴安岭厚度大、面积大,盆地东部靠近俯冲边缘火山岩分布厚度、面积变小;二是裂谷期主要发育东倾控坳断层。由此推测在板块俯冲牵引作用下,在楔形区产生单向环流。单向环流在大兴安岭一带上升,在地表形成强烈的火山作用,然后沿岩石圈底部向东运动,并逐渐转变为下降流,火山作用也逐渐减弱。单向环流由上升流逐渐转入近平流后,对岩石圈底面施加单向剪切牵引作用,岩石圈伸展在脆性上地壳形成主要东倾控坳断层。单向环流可以源源不断地从深部将热能和动能带到浅部,满足岩石圈减薄和伸展的需要。而且用单向环流解释活动大陆边缘和弧后区火山岩的成分极性可能更趋于实际。

THE ORIGIN AND EVOLUTION OF SONGLIAO BASIN AND ITS RELATION WITH ASTHENOSPHERE CONVECTION MODEL

Songliao basin located at Northeastern China, is a rift\|depression along the active continental margin of late Mesozoic (J 3\|K), associated with subduction of Paleo\|Pacific plate underneath Asian continent. The basin could be divided into three tectonic layers,among which the lowermost is Pre\|rifting volcanics (PRV) ,the intermediate is brittle half\|graben, and the top is post\| rifting depresssion tectonic level. PRV mainly occurs along the western part of Songliao basin with greater distribution area and thickness, to the interior of the basin near the subductive margin, its thickness and distribution area decrease rapidly. Moreover,the rifting\|fault mainly dip to east. It is suggest that unidirectional convection cells(UCCs) are generated by drag associated with plate subduction. UCCs rise underneath the Daxinganling, characterized by intense volcanism, then migrate to the east along the bottom of lithosphere , gradually change to subsidence. The volcanism also became weaker to the east. The lateral migration of UCCs might exert an unidirectional shearing to the bottom of lithosphere ,resulting in formation of east\|dipping fault\|controlled depression in the upper crustal level. UCCs transit thermal and dynamic energy to the shallow level, leading to the extension and thinning of lithosphere. There fore, UCCs provide a very useful model to explain chemical polarities of volcanics in the setting of active continental margin and back\|arc.