基于剩磁各向异性方法对华北下三叠统红层磁倾角浅化效应的研究

本文报道利用岩石剩磁组构对华北下三叠统红层进行磁倾角浅化效应的进一步识别与校正研究结果.首先,采用45°等温剩磁各向异性方法,即通过沿与样品原始水平面(即层面)呈45°夹角方向施加磁化场获得等温剩磁,并进行逐步热退磁,获得平行于层面和垂直于层面的等温剩磁分量随外加磁场和热退磁温度的变化趋势,计算获得浅化因子f=0.70.其次,应用高场等温剩磁各向异性方法,结合峰值为100 mT的交变退磁和120℃热退磁处理,分离获得碎屑赤铁矿对剩磁各向异性的贡献;由直接测量获得的单颗粒碎屑赤铁矿的各向异性度(a=1.35),计算获得f=0.59.该结果与前人对刘家沟组红层进行E/I法磁倾角浅化校正的结果(f=0.60)具有很好的一致性;表明华北下三叠统刘家沟组红层磁倾角浅化效应显著,其浅化因子为f=0.59;高场等温剩磁各向异性方法是红层磁倾角浅化校正的最有效方法.同时,如果有足够的独立样品数,且特征剩磁来自于单一剖面或有证据表明多条采样剖面之间未发生显著的相对运动,E/I法对红层磁倾角浅化因子的估计也是可信的.

Reconnaissance on inclination shallowing effect of Lower Triassic red beds from North China Block by the anisotropy of remanence

A magnetic anisotropy study was performed on red beds of Early Triassic Liujiagou Formation from the Qinshui Basin,North China Block(NCB).First,a magnetic field was applied at 45°to the cylinder axis in a plane and then the samples were subjected to successively thermal demagnetization up to about 680℃.Measurements of isothermal remanent magnetizations along specimen′s axial and radial axes yielded an inclination shallowing(IF)factor of 0.70.Second,high-field isothermal remanent magnetization was used in conjunction with 100 mT alternating field and 120℃thermal demagnetization to isolate the contribution of hematite to the remanence anisotropy,and we finally obtained an IF factor of 0.59 in the light of a direct measurement of remanence anisotropy of single hematite grain(a=1.35).The IF factor isolated from the anisotropy of high-field isothermal remanent magnetization(hf-AIR)is more consistent with that calculated previously from E/I(elongation/inclination)method(f=0.60).This further indicates that the Early Triassic red beds in the NCB suffered from a conspicuous IF with a factor of 0.59.Noting that the hf-AIR measurements is an effective method for identifying and correcting possible IF in red beds,the E/I method is also available to estimate the IF factor of red beds given that enough individual remanence directions are obtained from single sampling section/locality or from multiple sections/localities without significant relative motions within them.