河南西南部典型白垩纪剖面的岩石磁组构特征及其构造意义

Characteristic of Anisotropy of Magnetic Susceptibility (AMS) and its tectonic significance in typical Cretaceous section from Xixia basin,Central China

对河南西南部西峡盆地阳城-丹水上白垩统岩石磁学、磁组构的研究表明,该陆相沉积地层磁性矿物以及特征剩磁载体以赤铁矿为主,并含少量磁铁矿,且磁铁矿与赤铁矿相对含量比例随剖面由老到新显示出逐渐增加的趋势。岩石磁组构主要受顺磁性矿物以及反铁磁性矿物的控制。全剖面磁化率椭球结果显示在原生沉积组构基础上叠加了最早期阶段的弱变形组构;同时,全剖面以及各岩石地层单元磁化率椭球体主轴方向的变化揭示了两个不同方向及大小的构造挤压作用导致磁化率最大轴从NNW-SSE向转至NW-SE向,后又到NNW-SSE向的变化模式,一方面是由于华南、华北板块NE-SW向的构造挤压导致;另一方面是由两板块构造挤压而引起的桐柏-大别造山带上地壳物质相对下地壳发生东向构造逃逸和挤出过程中与东部块体物质间发生的挤压作用引起,这两种不同的挤压应力作用在盆地发展的不同阶段各占主要地位。

Through preliminary rock magnetic investigation on samples from the Yangcheng-Danshui section of the Xixia basin in Henan Province,Central China,hematite and subordinate magnetite were asserted to be the main magnetic carriers of remanence in this terrestrially depositional section. The results showed also a general gradually-increasing trend of the relative proportion of magnetite to hematite complying with height from the base to the top of the studied section. On the other hand,relatively low magnetic susceptibility values of the samples from the whole section indicate that the observed magnetic fabric is mainly dominated by antiferromagnetic hematite and some paramagnetic minerals,such as silicate. Though the tightly grouped principal minimum (k3) axes are nearly perpendicular to the bedding plane,indicating a predominant sedimentary fabric for the studied section,the relatively low corrected anisotropy values and the tightly grouped principal maximum (k1) axes suggest that the studied section was subject to an embryonic deformation. Variations of principal maximum (k1) axis' preferred orientation for the whole section and each rock unit unravel that preferred stress orientation in the studied strata has experienced a transformation from NEE-SWW to NE-SW,and then back again to NEE-SWW direction. This systematic change of preferred compressive orientation could be interpreted as reflecting one of the compression by persistent NE-directed indentation of the South China Block into the North China Block until the Late Triassic and the other triggered by extruded part of the upper crust from the lower crust colliding with substance to the east of the Tongbaishan-Dabie orogenic belt,these two different kinds of compaction may alternatively dominate the tectonic setting in different evolutionary stages of the basin.