Characteristics and Origin of the"Seesaw Type"Negative Inversion Structure in the Yabulai Basin of the Alxa Block
The Yabulai Basin is located in the Alxa Block and experienced intra-plate tectonic events since the Mesozoic Era.The Jurassic sediments are controlled by the NE-SW reverse faults,forming a compressional de-pressed basin with reverse fault patterns including imbricate fans,back-thrust,ramp structure,pop-up anti-clines,and faulted anticlines.This compressional stress originated from the subduction of the Okhotsk Ocean and the Paleo-Pacific Ocean beneath the Eurasian plate.Lower Cretaceous sedimentation is controlled by the Yabulai normal fault,forming an extensional faulted depression basin with northwest faulting and southeastern overlap.Fault assemblages such as stepped,domino,and conjugate styles developed,and stresses resulting from the rollback of Pacific plate subduction.The Jurassic compressional basin and the Cretaceous extensional basin superimposed in opposite directions to form a"seesaw type"negative inversion structure.Since the Ceno-zoic Era,the subduction and collision of the Indian Plate with the Eurasian continental plate has caused the Bei-dashan fault to thrust northward over the Jurassic basin,leading to the sinistral slip of the Yabulai piedmont fault.The results of analogue models show that the superposition of bidirectional compression and unidirectional extension is the main factor controlling the structural patterns in the Yabulai Basin.
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