The initiation and linkage of surface fractures above a buried strike-slip fault: An experimental approach
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Keywords:
Echelon formation
Overburden
Wrench
Echelon formation
Transtension
Transpression
Transform fault
Extensional fault
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Wrench
Echelon formation
Outcrop
Fault trace
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Aim To find out those faults that are favorable for the formation of petroleum traps by studying the structural characteristics of low grade faults in Wubao fault zone of Gaoyou sag in Subei Basin.Methods Classification of faults was done by the application of Riedel single shear model theory on the basis of active periods,scale and cause of formation of faults,etc.Results Faults in Wubao fault zone can be classified into four levels: first order faults,second order faults(second order strike slip faults,second order transform faults),third order faults(R fractures and T fractures),fourth order faults(secondary normal faults formed by tensile genesis).Conclusion It is concluded that,as buried fault zone with strike slip extensional property,Wubao fault zone whose current tectonic framework was formed after several phrases and long term activities,was formed by the influence of Tanlu strike slip fault zone.The junction of the third order Strike-slip faults and feather faults induced by them and bending points of low grade en echelon faults formed by strike slip are stress free areas where usually are favorable for petroleum migration.
Echelon formation
Transform fault
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The orientation of fractures with transpressional and transtensional wrenches in pre-existing faults has not been quantitatively determined. Based on Coulomb failure criterion and Byerlee’s frictional sliding criterion, this paper has indicated quantitative geometric relationships between the pre-existing fault and the local induced principal stress axes caused by the rejuvenation of the pre-existing fault. For a hidden pre-existing fault with some cohesion, the angles between the local induced principal stress axes and the pre-existing fault quantitatively vary with the applied stress and the cohesion coefficient, the ratio of the thickness of the cover layer to the thickness of the whole wrench body, whether transpressional or transtensional wrenches occur. For a surface pre-existing fault with zero cohesion, the angles between the pre-existing fault and the local induced principal stress axes are related to the rock inner frictional angle regardless of both the applied stress and the cohesion coefficient where transpressional wrenches occur, and the local induced maximum principal stress axis is identical with the applied maximum principal stress axis where transtensional wrenches occur. Therefore, the geometric relationships between the pre-existing faults and their related fractures are defined, because the local induced principal stress axes determine the directions of the related fractures. The results can be applied to pre-existing weak fabrics. They can help to understand and analyze wrench structures in outcrops or subsurface areas. They are of significance in petroleum exploration.
Wrench
Principal stress
Cohesion (chemistry)
Echelon formation
Principal axis theorem
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Transtension
Echelon formation
Neogene
Half-graben
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Clockwise
Transpression
Shearing (physics)
Echelon formation
Orogeny
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The identification of wrench faults from subsurface structural data is problematical and requires detection of combinations of profile and map characteristics, namely: (1) basement offset, (2) steeply dipping dislocation at depth, (3) upward-spreading splays (negative and positive flower structures), (4) changes in direction of fault dip and upthrown side, (5) changes of separation sense and merged splays with different separations, (6) a narrow, log, straight, throughgoing, solitary zone of deformation and/or master fault, and (7) coeval, en echeolon flanking structures. The criteria that are present must be supported with evidence that refutes alternative styles. Wrench interpretations are negated mainly by a zigzag, discontinuous fault trace and absence of basement offset. The oblique (en echelon) or parallel structures that flank wrench faults are also essential elements of other styles. The cross occurrences of features and the broad range of structural types - extensional, contractional, or both - have resulted in numerous misidentifications of terrane as wrench faulted or wrench associated. Furthermore, convergent or divergent wrench zones with single fault strands can resemble reverse or normal basement-involved faults, or certain faults with inverted slip. Other profile geometries - narrow contractional horsts or narrow extensional grabens, step faults or splay structures, various anticlinesmore » with crestal faults, two-sided orogenes, narrow rift basins, minor disruptions in reflection continuity, and various structures with complex or equivocal relationships - have been misinterpreted as flower structures.« less
Wrench
Echelon formation
Horst and graben
Extensional fault
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Brittleness
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Half-graben
Echelon formation
Outcrop
Horst and graben
Rift zone
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