纵向伸展断背斜类型及成因

Types and Causes of Longitudinal Extensional Faulted Anticline

纵向伸展断背斜是伸展断陷中重要的油气勘探领域。按照断背斜的几何形态将纵向伸展断背斜划分为双断型断背斜、单断型断背斜和叠合y型断背斜。根据渤海湾盆地新生代地质资料,讨论这3类纵向伸展断背斜的成因:(1)双断型断背斜发育于裂谷期,受对向双断断陷控制,主要是由于早期伸展过程中岩层局部缩短作用、晚期差异沉降和断块翘倾作用形成,其形成机制不同于前陆盆地的区域挤压作用产生的背斜;(2)单断型断背斜(或称逆牵引背斜)发育于裂谷期,受单断断陷控制,是由于控陷断层下降盘岩层下滑过程中岩层以逆牵引方式回倾、差异沉降和岩层弹性挠曲作用造成的,但岩层长度局部缩短作用仍是一个重要因素;(3)叠合y型断背斜发育于裂谷期后,受对称断陷和其中y形断洼控制,其形成与以纯剪切方式伸展运动和多期次断裂活动有关。它与走滑-伸展断背斜成因存在根本性差别,后者实质上不属于纵向伸展断背斜的范畴。

Longitudinal extensional faulted anticline in extensional fault-bounded depression is an important oil and gas exploration field. According to faulted anticline geometry, these anticlines can be divided into double-faulted(a pair of normal faults) anticline, single-faulted anticline, and superimposed y-type faulted anticline. Based on Cenozoic geological data of the Bohai Bay Basin, origin of these three kinds of faulted anticlines is suggested as follow:(1) Double-faulted anticline is developed in rift valley stage, and is controlled by a pair of normal faults with face-to-face dipping mode. Its fault-bounded depression is mainly caused by local rock shortening during the early-stage regional extensional, differential subsidence and fault block tilting occurred in the late stage;(2) Single-faulted anticline is developed in the rift valley stage, and is controlled by single extensional fault-bounded depression. It is likely caused by the reverse traction, differential subsidence, and elastic flexure of rock formation during the descending process of the fault-bounded depression. Nonetheless local rock formation shortening is still a major factor;(3) Superimposed y-type faulted anticline is developed in post-rift stage and controlled by symmetric fault-bounded depression and the y-type faulted sag(i.e.fault-bounded sub-depression) among them. It is fundamentally different from the cause of strike-slip and extensional superimposed y-type faulted anticline, which does not actually belong to the longitudinal extensional faulted anticline category.