中下部地壳拆离断层带演化中的褶皱作用:以辽南变质核杂岩为例

Folding during the evolution of mid-lower crustal detachment fault zone:a case study of the Liaonan metamorphic core complex

作为变质核杂岩构造的重要组成部分,拆离断层带内广泛发育的褶皱构造与其寄主岩石一样记录了中下地壳拆离作用过程。选取辽南变质核杂岩金州拆离断层带内褶皱构造作为研究对象,基于叶理与褶皱构造关系分析,划分了褶皱期次与阶段性;通过形态组构分析、结晶学组构分析及石英古温度计等技术方法的应用,初步分析了拆离断层内褶皱的形成机制,为辽南地区拆离作用过程提供约束。根据褶皱形成与拆离作用的时间关系,将拆离带内褶皱分为拆离前褶皱、拆离同期褶皱和拆离后褶皱;拆离作用同期的褶皱按时间早晚分为早期(a1)阶段、中期(a2)阶段、晚期(a3)阶段。不同阶段褶皱的野外形态、叶理与褶皱关系等方面的差异,以及形态组构与结晶学组构的特征,为判断和恢复褶皱的形成机制提供了佐证,揭示出拆离断层带褶皱是在纵弯-压扁和顺层流变的共同作用下递进剪切变形的产物。在拆离作用过程中,a1阶段和a2阶段褶皱以纵弯、压扁褶皱作用为主,a3阶段褶皱以弯滑作用为主。褶皱作用记录了拆离断层一定温度范围内(主要集中在380～500℃)的变形特征,拆离作用从早期到晚期的演化整体处于相对稳定的应变状态下。对金州拆离断层带而言,在区域NW-SE向伸展过程中,还伴随着NE-SW向微弱的收缩。

Many fold structures are well-preserved in mid-lower crustal detachment fault zones owing to progressive shearing along the fault zone during exhumation of metamorphic core complexes.These fold structures,like host rocks,bear important information on the evolution of the detachment fault zone.In this paper,we studied the various fold structures from the Jinzhou detached fault zone of the Liaonan metamorphic core complex.Based on the relationship between mylonitic foliation and fold,the folds are subdivided into pre-,syn-,and post-shearing folds.The syn-shearing folds can be further grouped into early(a1),mid(a2)and late(a3)syn-shearing folds.The transition from early to late syn-shearing is ascribed to progressive shearing during the detachment faulting.By means of morphological and crystallographic fabric analysis and quartz paleothermometry,we performed preliminary examination on the formation mechanism of folds along the detachment fault to provide constraints to the detachment faulting process in southern Liaoning.Our study shows that differences in macroscopic characteristics or relationship between foliation and folds of different stages,as well as morphological and crystallographic fabric characteristics of quartz grains in the folded rocks,can provide corroborative evidence for determining and restoring formation mechanism of folds.We suggest that the folds from the detachment fault zone result from progressive shearing under the combined effects of buckling and rheological properties of folded layers.During progressive shearing,a1 and a2 stage folds are dominated by buckling,while a3 stage fold is attributed primarily to flexural slip.Folding mainly occurred at a temperature range of 500 to 380℃.The detachment fault zone evolution maintained a relatively steady strain state overall from the early to late stage,while a weak contraction occurred in the NE-SW direction during the regional NW-SE extension in the Jinzhou detachment fault zone.