Synrift Structural Evolution And Crustal Extention In The Central Songliao Basin

Songliao Basin, the largest oil-produing basin and the largest rift basin in China, is the center of late Mesozonic rifting and lithospheric thinning on the eastern margin of Eurasian continent. However, synrift structural evolution of the basin is still poorly revealed due to a thick cover of thermal subsidence successions. The dynamics and its relationship with lithospheric thinning are still in controversy.The structural interpretation of new high-resolution seisimic data indicates that three sets of growth faults trending NNE, NNW and near NS developed during rifting, the major ones among which are mostly listric faults dipping to the east and control half grabens tilting to the west. The geometry and distribution of these faults and the related synrift subbasins are probably controlled by the EW trending Solonker Suture and the NNE trending Sunwu-Shuangliao Fault Zone. The Changling Synrift Subbasin (CSS) located in the central Songliao Basin is controlled by a NNE fault in the north and a NNW fault in the south, forming a unique〈-like fault pattern with NS faults in the center. This complicated fault pattern is probably resulted from the reactivation of various pre-existing basement weaknesses, like conjugate shear fractures and tensile fractures related to the prerift sinistral strike-slip faulting of NNE basement faults. Tectonophysical modelings on NNE sinistral strike-slip faulting yield fracturs that distribute and assemble similarly with synrift faults in the Songliao Basin, and thus verify this explanation.Due to this complicated fault pattern and the resultant uncertainty of extension direction, it is questionable to apply traditional balanced cross-sections restoration technique under 2D strain assumption. To solve this problem, this paper proposes a method of regional strain analysis based on multi-directional cross-section restoration under 3D strain assumption, that is, to construct and restore regional sections respectively perpendicular to each fault set and the extension ratio of such sections are assumed to represent regional linear strains, which can be used to find out the amounts and orientations of regional principal strains via Morh's strain diagram. This method is an expansion and combination of traditional strain analysis and cross-section restoration techniques and is a novel tool to estimate crustal extension strain and stress in complicated rift basins where few outcrops are available. Three groups of regional cross-sections othorgonal to NNE, NNW and neasr NS faults, respectively, in CSS are constructed and sequentially restored in this paper. Synrift structural evolution, amount, rate and principal strain of crustal extension are obtained following the above method. The results indicate that the overall Songliao Basin is controlled by WNW-ESE (117°) extension, or orthogonal rifting, with 23.7% or 50-60km crustal extension. However, rifting in the Songliao Basin is not a single event, but can be divided into two extension episods. The early episode lasted from Huoshiling age to Shahezi age (ca.157-130Ma). It is characterized by biaxial stretching and distributed faulting, but NNE-SSW extension was slightly dominant at the beginning of rifting in CSS and thus NNW faults were more active. Later, the WNW-ESE extension increased while NNE-SSW extension decreased with time and extension strain was localized on listric boundary faults. The late episode lasted from Yingcheng age to Denglouku age (ca.130-102Ma) and is characterized by WNW-ESE uniaxial stretching. The extension on NNE and near NS boundary listric faults in CSS continued, inducing minor faults on their hangingwalls, while dextral strike slip probably occurred on NNW faults. The extension strain is supported by structural analysis, sedimentary response in CSS and crustal strains revealed by stretching lineation related to metamorphic core complexes around the Songliao Basin, substantiating our method and results. The two extension episodes both initiated with a relatively rapid extension stage that was coeval with intense volcanism and was followed by a slow extension stage which corresponded to volcanic quiescency and structural trasition, suggesting the coupling between crustal faulting and volcanism.Besides, a quantitative relationship between horizontal crustal extension and vertical lithospheric thinning is estabilished under the assumptions of 3D strain and volume conservation of lithosphere. Accordingly, crustal extension in the central Songliao Basin is estimated to contribute to most of the crustal thinning but only part of the thinning of lithospheric mantle, implying the existance of other mechanisms besides tectonic stretching. It is suggested that the delamination of lithospheric root can more satisfiedly explain the biaxial extension and thermal doming at the beginning of rifting.Finally, a two-stage dynamic model is proposed to explain the synrift structural evolution, crustal extension and lithospheric thinning in the central Songliao Basin. It is emphasized that the prerift sinistral NNE strike-slip faulting and lithospheric thickening play important roles in rifting. The early extension episode was probably controlled by thermal doming and pure shear rifting attributed to the delamination of the thickened lithosphere root, while the late episode was probably dominated by regional crustal detaching and simple shear rifting ascribed to the subduction of Izanaqi plate.