Abstract:

Analysis of transform structures is important to study regional structural setting and hydrocarbon occurrence in extensional fault depressions. Based on previous studies and recent explanation of extensive seismic reflection survey in Bohai Bay Basin, some controversial and interesting problems of the transform structures are discussed in this paper. （1） We redefine the technical terms of the transform structures used in previous studies, such as segmentation structrure,transform structure，transfer zone,accommondation zone and transform zone,and illustrate its fundamental concepts. （2）According to different modes of the transform structures, which links two  relay master faults and related fault depressions,a new scheme of classification is presented.First,the transform structures can be classified as three major types：transfer zone,accommondation zone and transform zone,and than,considering its special criteria,each type of the tranform structures is subdivided further into some subtypes. （3）Formation mechanism of transfer zone and accommondation zone is closely related with regular change of displacement along fault strike. The maximum displacement occurs in center segment of fault and gradually reduced to minimum toward fault termination,at which the transfer zone and accommondation zone are developed.However,because of difference in some controlling factors,such as overlapping degree of relay faults,keeping or not keeping constant displacement,attitude of relay faults dip and transverse distance between the ends of relay faults in plane view,either transfer zone or accommondation zone and a wide variety of its related structure styles are formed.Differing from the transfer zone and accommondation zone,the transform zone is characterized by transform fault which links different segments of extensional structural domains.It is suggested that the transform zone is formed due to difference in crustal extensional rate.

Key words: transform structure, transfer zone, accommondation zone, transform zone