太行山中南段中生代以来构造演化的低温热年代学制约

The Taihang Mountain occupies the central part of the Greater Khingan-Taihang Mountain-Wuling Mountain gravity anomaly belt, East China. Studies on the tectonic evolution of the Taihang Mountain play an important role in analyzing the destruction of the North China Craton. Previous studies have shown that the Taihang Mountain had experienced remarkable tectonic transition from compression to extension since the Mesozoic. However, precious geochronological data is insufficient to constrain this process as the Jurassic to Paleocene sedimentary strata being widely absent in this area, especially for the south-central part of the Taihang Mountain. Results of our preliminary work in the south-central part of the Taihang Mountain indicate that the apatite fission track of the Late Paleozoic-Triassic sandstones had experienced totally annealing process before the Cenozoic. Moreover, the Late Paleozoic to Triassic sandstones and coalbed are widely exposed in the western flank of the south-central part of the Taihang Mountain. We therefore plans to analysis this tectonic reversion using vitrinite reflectance of the Late Paleozoic coalbed, zircon and apatite fission tracks of the Late Paleozoic-Triassic sandstones, respectively. Combination with analyses of low-temperature thermochronology data and field section measurements, it can be probably predicted that kinematic and geometric characteristics of related structural elements could be obtained. In addition, based on the results, we can define time-sequences for these tectonothermal events. We therefore could provide evidence for the tectonic evolution of the North China Craton since the Mesozoic and constraints for reasonable tectonic model.

太行山位于中国东部大兴安岭-太行山-武陵山重力梯度带的中部。对其中生代以来构造演化的解析对研究华北克拉通破坏的动力学机制具有重要意义。前人研究揭示：太行山中生代以来经历了由挤压到拉张的构造转换。然而，由于该地区大面积缺失侏罗纪到古近纪的沉积地层，对该期构造转换过程缺乏精确的年代学制约。申请人对太行山中南段晚古生代-三叠纪砂岩的初步工作显示砂岩中的磷灰石裂变径迹样品在新生代之前已经历过完全退火。据此，申请人根据太行山中南段西缘广泛出露晚古生代到三叠纪煤层及砂岩地层的地质事实，拟利用镜质体反射率Ro、锆石及磷灰石裂变径迹等低温热年代学的方法对该期构造转换进行解析。可以预见根据这些低温热年代学结果结合野外剖面分析可以定量化部分构造要素的运动学与几何学特征，厘定该地区构造热事件发生的时间序列，从而为华北克拉通中东部中生代以来的构造演化提供线索，为区域构造演化提供合理的模型制约。

太行山中南段广泛缺失中生代-古新世地层，从而导致缺少记录中生代晚期以来区域构造变形的有效载体，严重制约了对该地区中生代以来构造演化的认识。本项目借助太行山中南段晚古生代砂岩地层中锆石与磷灰石裂变径迹等手段，结合煤系地层中镜质体反射率Ro，对该区中生代以来的构造热演化史进行了恢复。结果显示：锆石裂变径迹年龄普遍相当或老与沉积地层年龄，表明太行山中南段晚古生代砂岩地层中的锆石裂变径迹未曾经历过完全退火过程，也即现今露头区所经历过的最高古地温可能小于其封闭温度180~350℃。这与该区煤系地层的镜质体反射率（约1.7~2.1）所反映的古地温基本吻合（小于180℃）。磷灰石裂变径迹年龄则全部晚于晚白垩世，远小于沉积地层年龄，表明磷灰石裂变径迹后期经历过完全退火过程。磷灰石裂变径迹热史反演结果显示：太行山中南段白垩纪以来经历三期快速冷却事件。第一期反映了晚白垩世末期的区域性快速挤压隆升过程，其动力学机制可能与该时期欧亚大陆东侧某块体的碰撞挤压有关；第二期则与古新世末期开始的区域性弧后拉张有关，该期太行山中南段经历了拉张机制下的被动快速剥露；第三期则记录了新近纪以来，与印度-欧亚碰撞有关的区域性张性裂陷背景下的快速剥露。此外，根据上述不同温标的地温估计，假设地温完全由埋深控制，则可以合理推测太行山中南段中生代以来垂直剥露厚度可能超过5千米（以地表温度20℃，地温梯度30℃/km计）。根据该区断裂带内样品的磷灰石裂变径迹年龄与高程关系分析，建立了早期挤压隆升、后期伸展裂陷地区的裂变径迹年龄-高程脱藕模型，并用其成功地解释了断层活动所导致的数据差异。结合区域地质背景，太行山地区白垩纪-古新世早期可能长期处于挤压剥露状态；自古新世晚期开始转换为弧后扩张背景下的被动剥露，与其东侧的渤海湾盆地形成拉张机制下的盆山耦合关系；至中新世以来，受控于印度-欧亚板块碰撞的远程效应，太行山西侧发育汾渭地堑等张性裂谷，区域上再次经历快速剥露。