Abstract:

The occurrence of anticlinal structures in coincidence with synsedimentary thrusts have been usually seen in Qaidam basin.The author thus proposeda new type of contemporaneous structure—normal drag structure.The normal drag structures appear to be similar to the reverse dragstructures(rolling anticlines)in shape,but the scale of the former is muchbigger than the latter,the character of faults and their dynamical mechanismare also completely different to reverse drag structures.The faults,distributing in bands,may usually reach to and is controlled by the basement rock,therefore,the fault plane is smooth,its dip-angle usually exceeds 45°.In comparison with the downthrown sides,the strata on the upthrown sides is thinner and itsgrain-size is coarser,and besides the fault displacement increases with depth.The normal drag anticlines,in general,are developed on upthrownsides,being confined in between two thrusts in a shape of reverse chinese character “丷”.The anticlines are characterized by thinner strata and coar-ser grain-size on thier tops,steep dip angle in the deeper part and gentledip angle in the shallower,as well as the vertical congruence of antictinal heightsat v arious depths with the basement rises.There are some post-depositional anticlines,with thicker strata on itstop and steeper dip-angle in upper beds,superim posing on the earlier con-tamporaneous normal drag anticlines.This fact shows that in different stages of the anticlinal development,we must analyse and distinguish themthroughly.Normal drag structure is a contemporaneous anticline controlled by thesynsedimentary fault in basement rock under continuous horizontal compres-sion of block-fault,crust-wave and folding movements during deposition in Meso-Cenozoic periods.According to the data of Qaidam basin,the normaldrag structure may be mainly divided into two types:normal drag anticlineand wedge-like anticline.Experience has demonstrated that normal drag structures are favour able to oil and gas accumulation and usually constitute some oil and gas accu-mulation zones.There are various types of oil and gas pools in normal dragstructures.Exploring for normal drag structure along synsedimentary thrustsis always a effective approach for oil and gas exploration.The author believes that more and more normal drag structures wouldbe continually discovered in the oil and gas-bearing basins which formedin the compressive zones in western China.