CHARACTERISTICS AND GENETIC MECHANISM OF “BROOM-LIKE SYN-DEPOSITIONAL FAULTS” IN WEST SEGMENT OF CENTRAL UPLIFT BELT IN DONGYING DEPRESSION

The development characteristic of the "broom like faults" in the west segment of the central uplift zone in the Dongying Depression was discussed in this paper by calculating the growth coefficient of the faults. Our results show that the main faults and the subsidiaries of the "Broom like faults" are not synchronous according to their growth index. The subsidiary faults have their highest growth index in the 3rd Member of the Shahejie Formation of mid Eocene, while the main faults have that in the 2st Member of Shahejie Formation of late Eocene. There found no sand bodies formed in the fracture angle between the main faults and the subsidiary faults. The depocenter is not stable but migrate from the hanging wall of the subsidiary faults during mid Eocene to the hanging wall of the main faults during late Eocene. These characters are different from those of the real "broom like faults". In the case of the real "broom-like faults", main faults are formed synchronously with the subsidiaries, and the depocenter is rather stable and located in the fracture angle between the main fault and the subsidiary fault. Therefore, we call this kind of faults the "quasi-broom like faults". The genetic mechanism of the "quasi-broom like faults" and the influence of the activity of "quasi-broom like faults" on sedimentary patterns are probed in this paper. It is concluded that the "quasi-broom like faults" were the result of the joint action of the independent extensional tectonic system and the SN strike and strike-slip system in NE direction.
The stage of Cenozoic faulting in the Jiyang Depression can be divided into two episodes according to the differences in tectonic stress fields. In the first episode during the early to mid Eocene, the Dongying Sag was under a N-S extensional tectonic stress field which caused the formation of E-W trending growth faults, while in the second episode during the late Eocene, the Dongying Sag was under the control of a NW-SE trending extensional tectonic stress field, which caused the formation of the NE-SW trending growth faults. The proper spatial combination of the E-W trending growth faults and the NE-SW trending growth faults due to the sinistral movement of the Tanlu Fault during the Eocene caused the "quasi-broom like faults" to take shape. The influences of the "quasi-broom like faults" on deposition pattern are far different from those of real broom like faults, so it is a must to pay much attention to the influence of "quasi-broom like faults" on sedimentation, in highly explorated basins, such as the Bohai Bay Basin.