阿尔金断裂中段百年-千年尺度古地震事件的释光年代学研究

The Altyn Tagh fault is one of the most prominent active strike-slip faults in the India-Eurasia collision, and its history of motion and earthquake behavior bears significance for revealing the style of continential deformation (distributed or microplate-like), which is highly debated. The paleoseismic study is an effective approach to reconstruct such history, of which the absolute chronology on centenial-millennial earthquake history is the most crucial and most difficult part. Although the accuracy and precision of luminescence dating technique have been improved significantly in the past five years(giving it the resolution on centenial-millennial scale) it is still challenging to establish high resolution luminescence chronology for the trench sediments, due to their complex sedimentary facies, as well as the partial bleaching problem (the sediments are not zero age when deposited). In this study, we plan to carry out systematic luminescence dating on the earthquake related sediments of several trenches along the central Altyn Tagh fault, for which the paleoseismic events have already been well identified through the previous paleoseismic study. We will start with surveying the basic luminescence characteristics, such as luminescence sensitivity, signal components, bleachability, residual dose as well as thermal transfer for luminescence signals, which are critical for the reliability of luminescence ages. The effect of grain size, mineral species and measurement procedures will also be investigated. Based on the characteristics of the various luminescence signals, several measurement protocols will be optimized and applied (internal check). The Bayesian model will also be incorporated to improve the precision of the measured luminescence ages with the first order stratigraphic constraints, which will be helpful for establishing the paleoseismic events sequence on centenial-millennial scale for the central western part of Altyn Tagh fault.

阿尔金断裂作为印度-欧亚板块碰撞形成的主要走滑断裂之一，其运动学特征和构造活动历史对于揭示大陆变形模式具有重要意义。通过探槽建立精细的古地震序列是恢复其运动学特征和构造活动历史的理想手段，其关键点和难点均在于对探槽中不同期次的古地震进行时间分辨率达到千年，甚至是百年的绝对年代测定。虽然释光测年技术在近五年来的迅速发展使其已能达到千年-百年的分辨率，但是将其应用在沉积相较为复杂，可能受到沉积前释光信号不完全晒退影响的探槽古地震相关沉积物上仍存在巨大挑战。本研究拟对阿尔金断裂中段车尔臣河-清水河段若干已有古地震期次详细划分的探槽进行系统的释光测年研究。在对不同粒级，不同矿物，不同测量条件下释光信号灵敏度，组分特征，晒退速率，残留剂量，热转移强度等基本释光性质进行研究的基础上，利用不同方法进行年代测量(内检验)，并结合贝叶斯方法(外校正)建立阿尔金断裂中段西部百年-千年尺度的古地震序列。

阿尔金断裂带是青藏高原北边界的巨型走滑断裂带，吸收着印度-欧亚板块碰撞造成的陆内变形。虽然历史记录中鲜有阿尔金断裂带的强震事件描述，但是其中段若干古地震探槽揭示出该地区在过去数千年甚至千年以内发生了多次强震事件。若能建立不同地点的古地震年代序列，将有助于理解陆内走滑型强震的发生规律与陆内变形模式及其地球动力学过程。然而，或由于测年材料缺乏，或由于测年方法本身的不确定性，该地区的古地震年代数据非常有限，亟待突破千年-百年尺度地震事件年代测定这一难题。光释光技术由于其测年材料易得，因此在阿尔金断裂带中段这一有机质缺乏的地区具有极大应用潜力。本研究旨在尝试通过光释光测年方法为该地区的古地震探槽建立千年-百年尺度强震事件年代序列，并通过其他独立年代验证其可靠性。.本研究基于不同方法获得了阿尔金断裂带中段不同地点钾长石post-IR IRSL170信号非光敏残留剂量介于0.1–2 Gy，发现了钾长石颗粒post-IR IRSL170信号非光敏残留剂量与其对辐照剂量响应灵敏度呈负相关，提出将post-IR IRSL170释光信号灵敏度作为数据筛选标准，识别受非光敏残留剂量影响较小的颗粒；确认了应用最小年龄模型可识别出该地区洪积物中沉积前post-IR IRSL170信号经历了充分回零的钾长石颗粒。在上述方法学研究基础上，将钾长石单颗粒post-IR IRSL170测年方法应用于4个古地震探槽的沉积物测年，建立了中段东部铜矿探槽的地层年代序列，在1.6–7.4 ka的年代范围内，钾长石单颗粒post-IR IRSL170方法得到的地层沉积年代与14C方法约束的年代大致吻合；中段西部3个古地震探槽最近一次地震事件的发生时代在千年以内，但百年尺度的准确年代仍需进一步研究。本研究验证了钾长石单颗粒post-IR IRSL170测年方法可用于阿尔金断裂中段千年尺度古地震事件年代测定，为后续更多地点的古地震年代测定提供了参考，并揭示了阿尔金断裂带中段车尔臣河-清水河一线最近一次地震事件发生于千年以内，为评价该地区地震复发周期和潜在地震震级提供了基础数据。