Abstract: Raoyang sag is a Cenozoic halfgraben-like fault depression with faulting in the eastern part and overlapping in the western part, in which a series of extensional and strike-slip structures are developed, dominated by extensional structure. The statistics of 4th-grade faults strikes in each horizon show that they trend from near EW to NW-SE and to near SN in the Early to Late Paleogene by turn clockwise. Dextral strike-slip structure assumes such main fault plane steep-tilting styles as enechelon, brush, flower structures and main fault tail structures, etc., which are primarily formed in the third segment of Shahejie period and Dongying period. The ramp-flat patterns and extensional activities of master faults like Maxi, Hejian and Liulu faults result in regular distribution of the fault belt, major hollow groove belt, central buried hill belt, sub-swamp groove belt and slope from east to west direction. The main detachment plane with big depth in the north is basement-tilted area, the central part with medium depth is basement fault-bend area, and the southern part with small depth is cap rock-slip fault-fold area.

Key words: Bohai Bay basin, Raoyang sag, Extensional structure, Strike-slip structure, ramp-flat fault, detachment surface