贺兰山构造带及邻区中-新生代构造事件:来自不整合面和裂变径迹的约束

贺兰山构造带及邻区的构造属性长期以来存在争议,确定该地区中新生代的构造事件及隆升过程是了解这一重要陆内变形带动力学机制的关键所在。本文采用不整合面分析法和低温热年代学方法,综合分析探讨了贺兰山构造带及邻区中新生代的构造事件及其构造演化过程。通过对该地区的野外地质调查,本次在中-新生代地层中由底到顶识别出6个不同类型的不整合面,它们分别是:(1) T_(2-3)/P平行不整合面;(2) J/AnJ角度不整合面-微角度不整合面;(3) K_1/AnK_1高角度不整合面;(4) E_3q/AnE_3;(5) N_1/AnN_1;(6) Q/An Q。在T_3d~3、J_2y和K_1变形前锋,可见与逆冲-褶皱造山带相关的同构造沉积生长地层,其在形态上表现为超覆、削截,在黄草滩等地局部与倒转背斜相伴生。这些不整合和生长地层是构造活动的直接证据。本次研究对采自该地区的12件样品分别进行了磷灰石、锆石裂变径迹测年及热史模拟分析。结果表明,裂变径迹年龄主要分布在4个区间,对应地质时代分别为中侏罗世-晚侏罗世(168～159Ma)、早白垩世末(139～91Ma)、晚白垩世末(79～66Ma)、始新世(59～50Ma),反映出该地区在这4个时期发生了明显的冷却抬升事件,且这4期构造事件与野外观察到的地质特征有很好的地质响应。同时,热史模拟表明该地区整体上经历了晚侏罗世、早白垩世、晚白垩世末-始新世3期快速隆升事件。综合研究表明,该地区主体逆冲褶皱的时间是从中侏罗世开始,早白垩世末构造运动最强烈,新生代又有所活动。

Meso-Cenozoic tectonic events in the Helanshan Tectonic Belt and its adjacent areas: Constraints from unconformity and fission track data

The tectonic attribute of the Helanshan Tectonic Belt and its adjacent areas has been in dispute for a long time, while it is the key point for the understanding of the dynamic mechanisms of this important intracontinental deformation zone to determine the Meso-Cenozoic tectonic events and uplifting processes in this area. Based on unconformity analysis and low temperature thermochronology study, the Meso-Cenozoic tectonic events and tectonic evolutions of the Helanshan tectonic belt and its adjacent area are analyzed comprehensively. Through detailed field geological survey, 6 different types of unconformities are identified in the Meso-Cenozoic strata of this area, and from the bottom to the top, namely:(1) T_2-3/P parallel unconformity; (2) J/AnJ angular unconformity-microangular unconformity; (3) K₁/AnK₁ high-angle unconformity; (4) E₃q/AnE₃; (5) N₁/AnN₁; (6) Q/AnQ. In the J₂y and K₁ deformation fronts, accompanied by inverted anticline in the Huangcaotan and other places, the syn-tectonic sedimentary growth strata related to the thrust-fold orogenic belt can be observed, which is super-covered and truncated in morphology. In addition, the growing strata can be identified on the seismic survey line. Unconformity and growth strata are direct evidence of tectonic activity. In this study, 12 samples were tested by means of zircon and apatite fission track dating, and thermal history simulation analysis. The results show that the fission track ages are mainly distributed in 4 intervals, corresponding to the geological times of the Middle-Late Jurassic (168~159Ma), the end of the Early Cretaceous (139~91Ma), the end of the Late Cretaceous (79~66Ma) and the Eocene (59~50Ma) respectively, reflecting four obvious cooling and uplift events in this region, which are also well correspondent with the geological features observed in field surveys. At the same time, the thermal history simulation shows that this region has experienced 3 periods of rapid uplifts, which occurred in turns in the Late Jurassic, the Early Cretaceous, and the Late Cretaceous-Eocene. After comprehensive analyses, the initiation of the main thrust fold in this study area was limited to the Middle Jurassic, the strongest tectonic movement occurred in the end of the Early Cretaceous which was still in activity in the Cenozoic.