Room-and low-temperature magnetic fabrics of fine-grained clastic rocks: A case study of the Cretaceous Huicheng basin, Qinling orogen

The magnetic fabrics of fine-grained sediments in the sedimentary basin can provide crucial structural information about basin evolution. However, it is an important issue that the characteristics of the magnetic fabrics of the fine-grained sediments in the inverted basins and its correlation with the basin formation and inversion still need to be explored. To address this issue, we selected the fine-grained sediments from the Cretaceous strata of the Huicheng Basin that is generally considered to be the boundary of the East and West Qingling for detailed studies. We conducted systematic rock magnetism, as well as the room-and low-temperature magnetic fabric studies on fine-grained sedimentary rocks in the basin, especially fine-grained sedimentary rocks near the synsedimentary faults (including growth strata). Based on field observation, there are a large number of NNE-striking normal faults with growth strata adjacent in the basin, which indicates the basin is an extensional basin. The rock magnetism experiments showed that the magnetic susceptibility values of the fine-grained sediments in the basin are generally low, which denotes they are mainly controlled by paramagnetic minerals despite few contributions from ferromagnetic minerals including magnetite and hematite. Both room-and low-temperature magnetic fabric results indicate that the magnetic lineation developed in the Cretaceous strata of Huicheng Basin is NWW-SEE oriented, and it intersects vertically or at high angles with the main synsedimentary normal faults. Moreover, the magnetic lineations, exhibiting characteristics of the initial deformation fabric, record the paleostress when the basin formed, and indicate a dominant NWW-SEE extension during the formation of Huicheng Basin. Combined with previous proposal that the Wenxian-Taibai fault bounding the Huicheng basin is a strike-slip fault, our results further indicate that it is a sinistral transtensional fault during the Cretaceous. In addition, compared with the room temperature magnetic fabric analysis, the low temperature magnetic fabric analysis significantly highlights the signal of paramagnetic fabric. Thus, low temperature magnetic fabric method could better improve the signal of preferred orientation of the magnetic fabric and petrofabrics. Although the Cretaceous strata of Huicheng Basin have undergone multiple phases of deformation, the initial deformation fabrics recorded in the strata had not discernibly reformed, providing crucial information for basin analysis.