Structural Analysis of the Multi-layer Detachment Folding in Eastern Sichuan Province

A serial of＂comb-like and trough-like＂folds developed in eastern Sichuan,controlled by the multi-layer detachment folding,is different from the classical Jura-type structure in their development.The key factor resulting in the development of these structures is the occurrence of detachment layers in different parts of Neoprotozoic to Mesozoic stratigraphy of study area,which, from the bottom to the top,are the lower part of Banxi Group,Lower Cambrian（Niutitang Formation）,Lower Silurian（Longmaxi Formation and Luoreping Formation）,Upper Permian （Wujiaping Formation） and Lower Triassic（Daye Formation）.On the basis of field survey combined with sand-box modeling,this study argued that the detachment layer of the lower part of Banxi Group controlled the development of the＂comb-like＂folds,and the lower part of Cambrian detachment layer controlled the development of＂trough-like＂folds.Because of several detachment layers occurring in the study area,the development of duplex structures different scales is an important deformation mechanism,and the duplexes are the important structures distinguished from the typical detachment folding structures.Due to these duplexes,the surface structures and structural highs may not be the structural highs in the depth.Meanwhile,the detachment layers are good channels for oil/ gas migration benefiting the understanding of accumulation and migration of oil and gas.

A geometric model of faulted detachment folding with pure shear and its application in the Tarim Basin, NW China

We present an improved geometric model of faulted detachment folding with pure shear that is characterized by core thickening and a ramp-discordant backlimb. The model includes a two-stage evolution： 1） detachment folding involving pure shear with fixed hinges, and 2） faulted detachment folding, in which the core of anticline thrusts above a break-through fault in forelimb by limb rotation. The growth strata patterns of the model are also discussed with respect to factors such as limb rotation, tectonic uplift rate, and sedimentation rate. A thrust-related fold, called a TBE thrust fold, in the Tarim Basin in NW China, is analyzed as an example of the theoretical model. The result indicates that the TBE thrust fold has undergone a two-stage evolution with shortening of a few hundred meters. Both the theoretical model and the actual example indicate that the shortening in the detachment folding stage takes up a large proportion of the total shortening. The structural restoration of the TBE thrust fold also provides new evidence that the formation of a series of thin-skinned structures in the SE Tarim Basin initiated in the Late Ordovician. The model may be applicable to low-amplitude faulted detachment folds.

The Maximum Effective Moment Criterion (MEMC) and Its Implications in Structural Geology

The MohroCoulomb criterion has been widely used to explain formation of fractures. However, it fails to explain large strain deformation that widely occurs in nature. There is presently a new theory, the MEMC, which is mathematically expressed as Meff = （（σ1-σ3） L.sin 2α sin α）/2, where σ1-σ3 represents the yield strength of the related rock, L is a unit length and a is the angle between σ1 and deformation bands. This criterion demonstrates that the maximum value appears at angles of ±54.7° to σ1 and there is a slight difference in the moment in the range of 55°±10°. The range covers the whole observations available from nature and experiments. Its major implications include： （1） it can be used to determine the stress state when the related deformation features formed; （2） it provides a new approach to determine the Wk of the related ductile shear zone if only the ratio of the vorticity and strain rate remains fixed; （3） It can be used to explain （a） the obtuse angle in the contraction direction of conjugate kink-bands and extensional crenulation cleavages, （b） formation of low-angle normal faults and high-angle reverse faults, （c） lozenge ductile shear zones in basement terranes, （d） some crocodile structures in seismic profiles and （e） detachment folds in foreland basins.