东非海岸Davie构造带的构造演化特征及其成因机制

以东非海岸Davie构造带为研究对象,通过东非海岸地形、自由空气重力及磁异常等资料的综合分析,确定了Davie构造带的南北展布范围;通过对地震资料的精细解释,明确了Davie构造带不同部位的构造特征;利用层拉平、生长指数及平衡剖面等方法,结合区域动力学分析,对Davie构造带不同时期的构造特征进行了详细的解剖。结果表明:①在平面上,Davie构造带南北展布范围为4°S~22°S;剖面上,构造特征具有南北分段的特点,南段可见构造脊,伴随典型的底劈构造,北段可见正反转构造。②受东非海岸地区板块差异活动的影响,Davie构造带在不同时期具有不同特点,在晚石炭世—早侏罗世,主要受拉张应力影响,在层拉平后的地震剖面中可见典型的地堑结构;在中侏罗世—早白垩世,受马达加斯加向南漂移影响转变为走滑性质;晚白垩世以来,进入被动大陆边缘阶段,Davie东断层基本停止活动,而Davie西断层仍持续活动。③Davie构造带发育演化具有继承性,其南北构造差异是由于热传导与板块应力转变共同作用产生的结果,由此建立了“南段构造脊持续活动、北段扩张后局部挤压”的构造变形模式。该研究成果为Davie构造带油气地质研究奠定了理论基础。

Tectonic evolution and formation mechanism of Davie Fracture Zone in East Africa coast

The north-south-trend length of Davie Fracture Zone along the East African coast was comprehensive analyzed based on East African coast topography,free air gravity anomalies and magnetic anomalies.The structural characteristics of different parts of Davie Fracture Zone were clarified through fine interpretation of seismic data.The structural characteristics of the Davie Fracture Zone in different periods were analyzed by means of layer flattening,growth index and balanced profile,combined with regional dynamic analysis.The results show that the horizontal distribution of Davie Fracture Zone is between 4°S and 22°S from north to south.The vertical struture has different characteristics of north-south segmentation,and structural ridges can be seen in the southern section,accompanied by typical undercut structures,and in the northern section,positive inverted structures can be seen.Affected by the differential activities of the plates in the East African coast,the Davie Fracture Zone has different characteristics in different periods.In Late Carboniferous to Early Jurassic,the Davie Fracture Zone was mainly affected by tensile stress,and typical graben structures can be seen in the flattened seismic section.During the Middle Jurassic-Early Cretaceous,it turned into strike-slip property due to the southward drift of Madagascar.Since the Late Cretaceous,it entered the passive continental margin stage,and the Davie East Fault basically ceased its activity,while the Davie West Fault continued to be active.The development and evolution of the Davie Fracture Zone is inherited,and its north-south structural difference is the result of the combined effect of heat conduction and plate stress transition.Thus,a tectonic deformation model of"continuous activity of the southern structural ridge and local compression after the northern expansion"was established.The above conclusions could make a contribution for the study of oil and gas geology of the Davie Fracture Zone.