圆锥状褶皱的几何分析

长期以来,地质学家习惯于把不同类型和不同成因的褶皱的基本形态当作圆柱状几何性状处理,建立了褶曲构造的基本几何要素,并且以此作为划分构造均匀区段的几何准则。从最近十余年来关于区域变质造山带的大量观测研究证明,还有另一种褶皱,即圆锥状褶皱,广泛发育于褶皱复杂的变质岩系之中,在不同尺度上显示其控制作用和对区域构造格架的影响。

GEOMETRICAL ANALYSIS OF CONICAL FOLDING

It has recently been shown from a good deal of observations in orogenic regional metamorphic belts that the conical folds, as an important type which geometrically separates with and genetically related to the cylindrical ones, are extensively developed in metamorphic terranges of different ages, gave at all scales influence on the pre-existing background of rock structures and reveal, in accompanying with the associated cylindrical type, evolution of tectonic deformation within an area.In contrast to the cylindrical geometry, folds of conical type generally exhibit, on πS-diagrams, girdle fabrics to which small circles fit and from which the central axis of the cone (K), vertex angle (φ), axial plane of fold B-axis and the S-cone could be reconstructed in routine methods. Some excelllent examples have been provided from the Archean rock Anshan Groups of Anshan area, Liaoning province and Dengfeng Group of Songshan area, Henan province and the Protereozoic rock group of Xinyu area, Ji-angxi province to enunciate the analytical processes and geological significance of these results.Based on discussion of field examples in comection with some thenretical conside rations, condition for the development of conical folding can be summarized as follows: (1) It was formed mainly by the flexural-slip mechanism and initiated on a tilting background that made the strain Y-axis intersects the original form surface (S); (2) Conical domain may be resulted from superposition or interference of folds of contrasting ages: (3) In regional scale the folds may distribute along and within a conical envelope surface, thus forming the conical-enveloped folding bundle.