The Tectonic Characteristics And Evolution Of The Middle Part Of The Central Depression In Songliao Basin During The Early Cretaceous

The study area lies in the central part of the SongLiao Basin , including Gulong faulted subsidence, the middle part of the faulted-uplift and Xujiaweizi faulted subsidence . Based on the geologic interpretations of the seismic reflection data, the growth rate and the evolution of the master faults forming basin boundaries are investigated with the implementation of the theory on the relationship of displacement - length among normal faults and the approaches for growth rate analysis. A series of balanced cross sections were constructed in order to understand the variations in basin formation mechanism both in timeand space. The major conclusions and findings of this research are summarized as follows:1 , Many little faults connected and formed the Xuxi fault and Songxi fault from the Shahezi period to the Yingcheng Period. The average growth rate of the Xuxi fault is about 35m/Ma in the Shahezi Period and 18 m/Ma in the Yicheng Period. The average growth rate of the Songxi fault is 71m/Ma in the Shahezi Period and 85 m/Ma in the Yicheng Period. The growth rate of Songxi fault is much more higher than the Xuxi fault in the Yingcheng Period.2 , It was made clear that a soft linkage exsisted between the Xuxi fault and the Songxi fault. The evolution and characters of the Xujiaweizi faulted subsidence was controlled to some extent by the soft linkage of these two faults. These two faults grew independently to each other in the Shahezi Period and two isolated faulted subsidence were controlled by them respectively. A soft-linkage developed between them during Yingcheng Period with the formation of a transformation slope. As the result of this development a unified faulted subsidence with one sedimentation centre was formed. The thickest sequence of sedimentation was accumulated in the linking part between the two faults instead of the middle part of each single fault, then kept the phenomena up to the present through some part having displaces after the Yincheng Period.3 , The faults in the study areas were divided into three zones from the west to east in terms of the linkage style .The faults in east zones are characterized by soft-linkage. The faults in the center zone are dominated by the end of linkage , while hard-linkage in the west zone.4, The basin-bounding faults experienced 6-stage evolution: the nucleation of the fault in the Huoshiling Period; the connect along strike between growth fault in the Shahezi Period; the short break in the later-Shahezi Period; the persistent Hard-linkage stage of growth fault in the Yingcheng Period; the break in the later-Yingcheng Period and the re-activation of the fault after the Yingcheng Period. The main activity time of fault is the Shahezi period. The average growth rate of fault is more than 60 m/Ma.5 , The faulted basin underwent a four-stage evolution in Early Cretaceous. They are the pregnancy of the faulted subsidence in Huoshanling period, the extensive faulting subsidence in the Shahezi Period, the weak-contraction period at the end of Shahezi period as well as the transition period from faulted subsidence to depression Yingcheng Period— early Denglouku Period.6, The extension and contraction rate of the faulted basin in Early Cretaceous have been estimated. The extension rate is estimated to be about 1.2—2.5 m/Ma, the volume ofextension is around 8—20km in the Shahezi period, which takes about 60% of the volume of total extension. Little contraction was achieved in the later-Shahezi Period. The extension rate is inferred to be about 0.36—0.66 m/Ma and the volume of extension is around 2-5 km during the Shahezi Period, which takes about 20% of the total extension of the faulted basin.