Inversion styles in the Songliao basin (northeast China) and estimation of the degree of inversion

Abstract

Two inversion patterns, fault-reactivated and cover-folded inversions, are recognized in the Songliao basin of northeastern China. Mild inversion with a reactivated fault showing normal separation in all synrift beds, moderate partial inversion, and total inversion are the common styles of fault-reactivated inversion. The magnitude of extension by rifting, as well as orientation of the synrift faults in relation to the compressional direction during inversion, significantly controlled the inversion structures. The same amount of shortening may produce a moderate inversion style where the normal slip during the rifting was large but a total inversion in which the initial normal slip was small. The cover-folded inversions in the Songliao basin have either no normal faults or have one or more normal faults that occurred prior to inversion and remained dormant during the inversion. The geometry of cover-folded inversion may be controlled by deep faults in the basement. A graben-like basement fault assemblage can result in an antiformal geometry, while a horst-like basement fault assemblage may control a synformal cover-folded inversion. An inversion with antiformal geometry of the sedimentary sequences is the major style in the Songliao basin. A new method of estimating the degree of fault inversion is discussed. The degree of fault inversion is the ratio between reverse slip during inversion and normal slip during extension, both measured along the fault. It can be estimated in most fault-reactivated inversions where the top of the basement or the floor of the synrift deposits does not fold during the inversion. The horizontal inversion ratio represents the relative amount of shortening and extension of a structure or basin. It can be used to analyse the degree of inversion of complex structures or basins. However, both fault inversion and horizontal inversion ratios are primarily influenced by the amount of extension. If undergoing same amount of shortening, a structure with a small amount of extension may have a much greater inversion ratio than one with a large amount of extension.