佳木斯地块与松嫩地块俯冲碰撞的深反射地震剖面证据

佳木斯地块和松嫩地块是东北地区两个十分重要的地质构造单元,由于二者之间发育一套含有蓝片岩的俯冲增生杂岩-黑龙江杂岩(原称黑龙江群),其地质构造意义长期为人们所关注.巴彦—桦南深反射地震剖面揭示,佳木斯地块与松嫩地块之间存在明显向西俯冲的深反射信息,以壳内和幔内向西倾伏的楔状反射区为特征.壳内楔状反射区东与浅表层出露的黑龙江杂岩相连,向西倾伏延深至莫霍面,是俯冲增生杂岩在地壳深部的反映;幔内楔状反射区东起小兴安岭之下的莫霍面,向西倾伏延深至松辽盆地东缘,尖灭深度约78km,与多种方法得出的该区现今的岩石圈厚度(75~80km)基本一致.这一证据充分说明佳木斯地块的岩石圈地幔向西俯冲到松嫩地块岩石圈地幔之下.

The evidence from the deep seismic reflection profile on the subduction and collision of the Jiamusi and Songnen Massifs in the northeastern China

The Jiamusi and Songnen massifs are two major tectonic units in the east part of the northeastern China. The blueschist-containing Heilongjiang complex as a subducted accretional complex is distributed between the two massifs in a NS-trending. The Bayan-Huanan deep seismic reflection profile provides strong evidence that the Jiamusi massif had been westwards subducted beneath the Songnen massif in lithosphere scale, characterized by the west-dipping wedge-shaped reflectors in the crust and mantle respectively. The intracrustal reflector can be eastwards traced to the Heilongjiang complex exposed at the surface, and westwards deepens to the Moho beneath the Xiao-Xinganling mountains, which is inferred to be an hidden subducted accretional complex. The intramantle reflector appears under the Moho beneath the Xiao-Xinganling mountains, and westwards deepens and wedges out at a depth of about 78 km beneath the east margin of the Songliao basin, indicating the mantle of the Jiamusi massif had been subducted westwards beneath that of the Songnen massif. The depth of 78 km maybe represents the present lithosphere thickness of the area. An overlapped Moho anomaly area appears in the crust-mantle boundary beneath the Xiao-Xinganling mountains and separates the intra-crust and -mantle reflectors, where the largest crustal thickness up to 39km is preserved in the profile. The intracrustal reflectors can devide the profile into three types of the crustal structures: the Songnen massif in west, subducted accretional complex in middle and the Jiamusi massif in east. The Songnen massif is characterized by the layered reflectors: the upper layer with horizontal reflector is about 0~6 km in thick, corresponding to the Mesozoic-Cenozoic sedimentary rocks of the Songliao basin; the middle layer is 7~9 km in thick and composed mostly of the Upper Paleozoic sedimentary rocks and the early Mesozoic granites; the lower layer is characterized by a series of eastward gentle-dipping and some short, discontinuous arcuate reflections,which is inferred to be the metamorphosed crystalline basement.#br#The thinnest crust occurs in the Bayan area in the east margin of the Songliao basin, where the Moho is buried at only 28 km, indicating the Moho uplift does not show a mirror-image relation with the Songliao basin.The subducted accretional complex is situated between the Jiayi and Mudanjiang faults and is characterized by the west-dipping layered reflectors. The upper layer is about 0~6 km in thick, correspongding to the Cenozoic faulted-basin; the middle layer is marked by the west-dipping wedge-shaped reflector with a thickness about 13~18 km, maybe representing the subducted Heilongjiang complex; the lower layer is the westwards subducted lower crust of the Jiamusi massif. The Jiamusi massif is characterized by relative simple reflectors and stable crustal thickness of 36km. A gentle west-dipping intracrustal reflection in the depth about 18~20 km divides the crust of the Jiamusi massif into the upper and lower parts. The upper part consists mainly of amphibolite-facies metamorphic rocks, and the lower part is composed mostly of the granulite-facies metamorphic rocks, which are the predominant rock types in the massif and widely exposed along the profile.