南乍得盆地Doseo坳陷背形负花状构造成因分析

Genetic Mechanism of Antiformal Negative Flower Structure in the Doseo Depression,Southern Chad Basin

南乍得盆地Doseo坳陷是中非剪切带重要的含油气盆地,其北东东—南西西向走滑断裂带上发育一种特殊的由正断层组成的花状构造,背形负花状构造。断层在平面上呈现雁列式排布,在地震剖面上兼具正花状和负花状构造的特点,表现为下部“向形”、上部“背形”的花状构造。根据断裂的发育特征以及区域应力演化特征,结合二维和三维地震资料解释,并通过平衡剖面恢复技术,将Doseo坳陷划分为5个演化期次。结合走滑位移量结果,对坳陷内背形负花状构造进行成因机制分析,认为其是坳陷中部北东东—南西西向的基底正断层在早白垩世张扭期负花状构造形成阶段和古近纪压扭反转期背形构造形成阶段叠加的结果,且早期张扭强度远大于后期压扭改造强度,二者走滑位移分量相差10~30倍,强度的差异使其主要呈现正断层组成的负花状构造样式,并叠加反转期形成的背形样式。

The Doseo depression in the South Chad Basin is an important petroliferous basin in the Central African Shear Zone.A new kind of flower structure composed of normal faults(termed antiformal negative flower structure),is developed on the ENE-WSW-trending strike-slip fault zone.The faults there exhibit an en echelon arrangement in plane view,and have characteristics of positive flower-like and negative flower-like structures in the seismic profile,which are shown as a flower structure of lower"synform"and upper"antiform"shape.According to the development characteristics of the faults and the regional stress evolution,combined with 2D and 3D seismic data interpretation,the Doseo depression is divided into five evolution stages by the balanced cross-section reconstruction technique.Based on the strike-slip displacement,we analyzed the genetic mechanism of the antiformal negative flower structure in the depression.We considered that it is the result of superposition of the ENE-WSW-trending basement normal fault in the middle part of the depression in the formation stage of negative flower structure in the Early Cretaceous transtension period,and the antiform formation stage in the Paleogene transpression reversal.The early transtension strength is much greater than the later transpression reversal strength,and the difference between the strike-slip displacement components of the two is 10 to 30 times.The difference in strength makes it mainly present the negative flower structural style composed of normal faults,and only the antiform style formed during the inversion period is superimposed.