Fold-accommodation Faults: Models And Case Studies

Fold-accommodation faults are secondary structures pervasively presenting innatural fault-fold deformations and experimental models. Four main types offold-accommodation faults are commonly found, that are, into-anticline and out-ofsyncline thrusts, wedge thrusts, forlimb-backlimb thrusts and back thrusts. In thispaper, five models concerning the development mechanisms of fold-accommodationfaults namely, concentric folding, kink-band folding, buckle flexural-slip folding,chevron folding, and pure-shear detachment folding are systematically summarized.Diagnostic criteria and analytical methods are employed to distinguish and analyzedifferent genres of fold-accommodation faults. Geometric and kinematic modelsresting on differential shear are questioned and alternative models are put forward forsatisfactory explanation.Four accommodation mechanisms of out-of syncline, into-anticline and limbthrusts could be tightness change of folds, migration and limb rotation with respect tohinges, buckling flexural-slip folding, and high competency contrast.(1) Whencurvature radius are smaller than folded bedding thickness in the hinge zone,progressive concentric folding cannot continue. In this case, space problem wouldlead to faults.(2) Limb rotation with respect to active axial surfaces or/and fixed axialsurfaces without keeping bedding thickness and length could result in faults. Faultsoriginate from hinge zone and thrust out of the core along forelimb or/and backlimbcould form as a result of this mechanism.(3)Accumulated layer-parallel shearing inlimbs associated with buckling flexural-slip folding may give way to faults ofcut-up-section when layer-parallel movement was blocked.(4) Progressivecontraction deformation of vertically heterogeneous lithological package could beaccommodated by secondary folds and penetrative deformation in weak stratigraphy.However, in the competent layers, penetrative differential strain could contribute tofaulting. Sometimes escape of incompetent units from the core of fold following thepreviously intensive concentration would result in secondary faults.Hinge wedge thrusts have two forms, which include faults result from curvaturechange in hinge zone and detachment thrust fault from the limb. Comparing to the former type, wedge thrust accommodating faults detachment has larger displacementand axial surface in the hangingwall and footwall was displaced as well. Forelimbthrusts have a characteristically high cut-off angle, and they usually developed whenbedding rotated to a deep angle. In this case, flexural slip was locked-up whilenon-axial shearing operated as the main operative mechanism.There are five main diagnostic criteria for differentiating fold-accommodationfaults.(1) Secondary faults strike parallel to host-fold axes in map view and theyshow a more or less symmetric arrangement to the axial planes of folds.(2)Secondary faults are smaller than host-fold in scale and fault planes are isolated andlimited in extent; they occur at different stratigraphic levels, most across competentlayers and not connected with main thrust.(3) Fold-accommodation faults haveshallow cut-off angle in the limbs while have high cut-off angle in the hinge zone.(4)Fold-accommodation faults have a harmoniously geometric and kinematicrelationship to surrounding folds.(5) Most fold axes can be followed from footwallinto the hanging wall and thrust planes are commonly deformed more or lessharmoniously with associated folds; Although structures in the hangingwall andfootwall are different to some extent, the whole fold geometry would not changeapparently.Combining established models associated with fold-accommodation faults andnine sample structures in Proterzoic sequence in west and middle part of Yanshanintraplate orogenic belt further evidences the soundness of models mentioned above.Study of sample structures mainly center on characteristics and properties of differentfold-accommodation faults types, mechanisms, cut-off angle changes, map view andprofile view features. Also, sample structures are chosen for comparing typicalfold-accommodation faults and other types of fault structures. Methods such asdisplacement-distance diagrams and stereonet diagrams are used in both directinvestigation and comparative analysis of sample structures. These methods arehelpful for extracting information of fault geometric and kinematic evolution.