拉萨地块保吉地区晚侏罗世——早白垩世地层磁组构特征

拉萨和羌塘地块拼合形成了青藏高原的核心,但迄今对两者的具体拼合时间仍存在激烈的争论。为进一步寻找约束两者碰撞时限的地质证据,对拉萨地块晚侏罗世—早白垩世地层的磁组构特征进行研究。结果显示:晚侏罗世地层磁组构特征显示其遭遇过较强的构造应力,吐卡日组地层磁化率主轴k_1方位与地层面斜交,但经地层校正后,k_1方位与区域褶皱方向一致,表明应力方位为北北东—南南西向;萨波直不勒组地层k_1方位在地层校正前平行于层面,指示了垂直于主压应力的方向,推断晚侏罗世地层磁组构记录了同一期应力,应力方向均为北北东—南南西向。早白垩世多尼组地层磁组构显示其后期遭受的构造应力场强度弱,且与晚侏罗世应力并非同一期。因此,通过对比晚侏罗世与早白垩世地层的磁组构特征,认为保吉地区晚侏罗世地层磁组构记录了北北东—南南西向较强的构造应力,推断该期应力来源于拉萨-羌塘地块的碰撞拼合事件,而多尼组地层并未受该期应力场的影响,仅记录了区域褶皱隆起时的应力场。

Magnetic fabric study of the Late Jurassic-Early Cretaceous strata in Baoji area of Lhasa block

The Lhasa and Qiangtang blocks are at the heart of the Tibetan Plateau. However, the time for the specific combination of the two blocks is still in heated controversy. To further find geological evidence that can constrain the time limit between the two blocks, the authors studied the characteristics of the magnetic fabric of the Late Jurassic-Early Cretaceous strata in Lhasa block. According to the results obtained, the magnetic structure of Late Jurassic strata encountered strong tectonic stress, and the principal axis k₁ of the magnetic susceptibility of the Tukari Formation was oblique to the ground plane. After the stratigraphic correction, the k₁ azimuth and regional fold direction consensus indicate that the stress azimuth was NNE-SSW; the k₁ orientation of the Sabozhibule Formation was parallel to the plane (NNE-SSW) before the stratigraphic correction, indicating the direction perpendicular to the principal compressive stress and inferring the late tilting. The Late Jurassic formation magnetic structure recorded the same period of stress, and the stress direction was NNE-SSW. The Early Cretaceous Duoni Formation magnetic structure shows that the subsequent tectonic stress field intensity was weak, and was not in the same period as the Late Jurassic stress. Therefore, by comparing the magnetic fabric characteristics of the Late Jurassic and Early Cretaceous strata, the authors concluded that the magnetic fabric of the Late Jurassic in Baoji area recorded a strong tectonic stress in NNE-SSW, and that the stress in this period originated from the LhasaQiangtang block collision event, while the Duoni Formation was not affected by the stress field, only the stress field of the regional fold uplift was recorded.