鲁西地区沿寒武系/太古宇不整合面滑脱构造的主要特征及形成机制

郯庐断裂带以西的鲁西地区,寒武系/太古宇不整合面改造成为构造滑脱面,滑脱面上发育碎裂岩和动力变质岩。滑脱面上盘的寒武系底部产生强烈的构造变形,褶皱和破碎带发育;滑脱面之下的太古宇构造变形以劈理和裂缝为主,岩石破碎。构造滑动使露头区寒武系底部地层有不同程度的缺失,济阳坳陷南缘则出现地层重复现象。从地层缺失到褶皱产生再到地层重复是滑脱构造逐渐演化的结果。⁴⁰Ar-³⁹Ar、K-Ar、锆石和磷灰石裂变径迹数据分析表明:滑动的主要时期在早白垩世中-晚期(距今130~110Ma)和中始新世(距今43~37Ma)。各种构造指向标志表明滑动的方向以向北、北北西和北北东为主。滑脱构造是低角度的伸展构造,其机制与华北东部晚中生代的构造变革和新生代以来印度—欧亚板块碰撞、太平洋板块俯冲方向和速度的变化密切相关。

MAIN CHARACTERISTICS OF THE DECOLLEMENT STRUCTURES ALONG THE CAMBRIAN/ARCHEAN UNCONFORMITY SURFACE IN WESTERN SHANDONG

The field survey in the West Shandong Rise and geophysical exploration in the Jiyang Depression indicated that the unconformity between the Cambrian and Archean was reformed and became a structural décollement surface, along which developed the cataclastic rocks and dynamo-metamorphic rocks. There were different structural deformational styles developed near the décollement surface, above it(in the lowermost part of the Cambrian) occurred intensively structure deformation such as folding and interjacent-fractured zone, while cleavages and fractures were main types of structural deformation in the underlying Archean metamorphic rocks. The décollement structuring made stratigraphic absence of the lowermost Cambrian. In southern part of the Jiyang Depression, the seismic profiles and drilling wells revealed that the strata of the lowermost Cambrian might be repeated and thickened. Namely, the strata absence, strata folding and the strata repeatation should result from a progressive evolution of the décollement structures. The first generation of décollement structures occurred in mid-late period of the Early Cretaceous(130～110 Ma ago), and the second one in middle Eocene(43～37 Ma ago) according to the datings of the ⁴⁰Ar-³⁹Ar, K-Ar and the apatite/zircon fission track data. The main décollement direction oriented to NW, NNW and NNE indicated by the structural indicators. The décollement was an important lower-angle extensional structure. The mechanism of the décollement structures might be the transition of tectonic regime in eastern North China during Late Mesozoic, the Cenozoic India—Eurasian collision and the direction change of the Pacific Plate subducted the Asian Plate.