风成红黏土序列磁化率各向异性特征对区域应力变化的响应

对六盘山以西132 m水洛红黏土剖面磁化率各向异性特征的研究表明,水洛红黏土剖面记录的14.8~8.7 Ma沉积序列受到了同沉积时期不同程度的应力作用,磁化率各向异性变化特征对应于应力作用强度较弱的初期和应力作用强度有所加强的中期。进一步分析发现,14.8~11.0 Ma之间显示出3次应力增强和后续应力减弱交替变化的特点,这一应力作用事件可能与青藏高原东北缘在此阶段强烈构造活动的区域应力传递相关,而11.0~8.7 Ma期间存在一个相对较强的应力作用事件,可能是对局部地区应力增强事件的响应。

Anisotropy of Magnetic Susceptibility Response to the Regional Stress Variation in Aeolian Red Clay Sequence

The Aeolian sediment sequences from the Chinese Loess Plateau record abundant paleoclimatic and paleoenvironmental information, while little is known about whether there were any stress effect during the sedimentary period. In terms of tracking stress variation, anisotropy of magnetic susceptibility(AMS) has some particular advantages in comparison with other traditional methods. Systematic magnetostratigraphy, rock magnetism and anisotropy of magnetic susceptibility analysis were carried out on Shuiluo Aeolian red clay profile, which is located at the western Liupan Mountain. In the field, there are not any obvious tectonic traces that can be observed, which indicates the primitive deposit, but AMS results have many distinct differences between Shuiluo aeolian red clay profile and other typical aeolian sediment sequences(loess/paleosol, red clay). These differences mainly manifest from the following aspects. Firstly, abundant magnetic ellipsoid is located in the prolate area; Secondly, the stereonet projections of the κ_max and κ_min principle axes of AMS ellipsoids show that the κ_max axes are well grouped in an approximately east-west direction, while the κ_min shows a slight girdle in an approximately north-south direction; and in addition, the Pj-T diagram shows that the magnetic ellipsoid is being transition from oblate to prolate magnetic ellipsoid. These characteristics listed above suggested that Shuiluo red clay profile had gone through a weakly stress effect, and comparing with previous research we infer that these AMS characteristics be correspondent to the incipient deformation with a weaker stress intensity and the moderate deformation with a stronger stress intensity. After further classification, comparison and using a variety of analysis methods, we find that from 14.8 Ma to 11.0 Ma, the stress state is accompanied by three stress increase and decrease alternately, and these stress effects may correspond to the stress from the northwest Tibetan plateau. In about 11.0~8.7 Ma, AMS shows a strong stress effect, and almost all AMS ellipsoid is located in the prolate magnetic ellipsoid area. At the same time, the κ_min axes shows a strong girdle in an approximately north-south direction and it should be in response to the stress increasing in the local area.