四川盆地早白垩世红层磁倾角偏低研究

Inclination shallowing of the Early Cretaceous red beds in the Sichuan Basin

对四川盆地东北部巴中地区和西部雅安地区早白垩世红层分别采集9块手标本,进行了详细的古地磁学研究.系统退磁获得两地的特征剩磁,其中巴中地区平均方向为,倾斜校正之前:D_g/I_g=25.3°/19.0°,k=18.6,α_(95)=8.8°;倾斜校正之后:D_s/I_s=25.8°/18.9°,k=24.3,α_(95)=7.6°.雅安地区平均方向为,倾斜校正之前:D_g/I_g=24.5°/45.0°,k=15.7,α_(95)=9.0°;倾斜校正之后:D_s/I_s=356.7°/35.6°,k=28.5,α_(95)=6.6°.磁化率各向异性实验结果显示两地均未受到显著的构造应力影响.等温剩磁各向异性(AIR)实验结果,巴中地区IRMz/IRMLx平均值为0.8194,表明18%的压实率;雅安地区IRMz/IRMx平均值为0.8909,表明11%的压实率,计算得到巴中和雅安地区校正后的磁倾角分别为22.7°和38.8°.根据等温剩磁各向异性(AIR)实验和Tauxe and Kent(2004)提出的EI校正法得到的结果表明,四川盆地早白垩世陆相碎屑岩层中存在沉积压实作用造成的磁倾角偏低现象,而且川东北巴中地区的偏低程度强于川西雅安地区.

Totally eighteen orientated block samples of the Early Cretaceous red beds were collected from Bazhong and Ya＇an areas in northeast and west Sichuan Basin, respectively. Detailed paleomagnetic studies were carried out. Stepwise thermal demagnetization was used to isolate the characteristic higher temperature component （HTC） from the Bazhong （Ds/Is=25.8/18.9°, k=24.3, α95=7.6°） and Ya＇an （Ds/Is=356.7/35.6°, k=28.5, α95=6.6°） areas. The anisotropy of magnetic susceptibility （AMS） results of the two areas both are intermediate fabric and reveal the presence of sedimentary origin fabric. Results obtained from the anisotropy of the isothermal remanence （AIR） method indicate that distinct inclination flattening occurred in our samples. The mean values of IRMz/IRMx, which are 0.8194 and 0.8909 for samples from Bazhong and Ya＇an area, suggest 18% and 11% compaction rate, respectively. The results obtained from both anisotropy of the isothermal remanence （AIR） method and corrected results of the elongation/inclination model of Tauxe and Kent （2004） indicate the presence of obvious inclination shallowing of the Early Cretaceous red beds in Sichuan Basin, which is caused by sedimentation and/or compaction, and the compaction intensity in Bazhong area （northeastern Sichuan Basin） is much higher than that of Ya＇an area （western Sichuan Basin）.