新疆南部皮山北环形构造——古陨石撞击的记录

摘要

Objectives: The results of the planetary exploration program since the 1970 s show that the impact is widespread in the solar system. Although the Earth have the protection of the atmosphere, the impact is not an accident. The process of asteroid impact plays an important role in both the evolution of the solar system and the transformation of the Earth's surface, and even the life system. Influenced by "positive flower"structure, the central uplift of ring structure have a characteristic of compression, while the margin is a depression that controlled by "negative flower " structure. We studied 3D seismic data, petrology structure, core data and geochemical analysis on debris to reveal the characteristic and forming mechanism of ring structure of northern Pishan County, southern Xinjiang.Method: Based on the analysis of the characteristic of structure—geomorphic and 3D and 2D seismic data, we studied the characteristic of ring structure. Core data from well PSBX1 disclosed that the formation of the top of ring structure is upper Cretaceous which is composed of breccia dolostone and limestone. Study shows that positive anomalies in iridium nearly exist in all hundreds of K—E ( contact of Cretaceous and Cenozoic) contacts around the world. Through the test analysis of samples that equidistantly acquired from core or rock debris in well PSBX1, we analyzed anomalies in iridium appear in entire upper strata of breccia.Results: Ring structure of northern Pishan County, southern Xinjiang, have a structure—geomorphic feature of uplift in the central and collapse in the margin that controlled by ring faults. Borehole data disclosed that the strata of ring structure of such area are intensely fractured and together with positive anomaly of iridium with maxium value of 0. 41μg/g. Breccia section developed below Aertashi Formation of Paleogene, upper part of Aertashi Formation was filled by filling that from other place, while lower part by in-situ broken rocks. Correspond with structural deformation, strata consecutiveness of central ring structure was getting worse and lower Paleozoic thickening but thinning in the margin. Upper Paleozoic is the opposite, the strata were thickening in the margin and thinning in the central. The contrast analysis of typical impact structural of the world shows that the ring structure of northern Pishan was induced by meteorite impact; and that the impact event had happened at the Late Cretaceous or before the Late Cretaceous can be inferred from the strata that positive anomaly of iridium developed in Southwest of Tarim basin.Conclusions: The ring structure in the northern Pishan area, southern Xinjiang, is meteorite impact related structure. This is more reasonable than the viewpoint of diapir structure in that dome style is the feature of diapir structure in which the collapse should influenced by normal faults, furthermore, the central area of collapse in diaper structure was a depression instead of an uplift. The explanation of caldera is also indefensible because the central area of caldera is igneous rock or hydrothermal transformation and volcano extrusive should exist rather than products of in-situ rock fragmentation, not to mention positive anomalies in iridium in stratum.%环形构造中央隆起受"正花状"构造控制, 具有挤压构造特征;边缘受"负花状"构造控制, 形成凹陷.为了揭示皮山北环形构造的特征及成因, 基于三维地震资料、岩石学结构、岩芯、岩屑地球化学分析, 识别出了皮山北环形构造是一个受环形断裂控制的中央凸起, 边缘凹陷构造.钻井资料揭示该地区环形构造地层岩石破碎强烈, 并伴有铱 (Ir) 元素正异常, 最高达0.41μg/g.角砾岩段发育于古近系阿尔塔什组之下, 上段有异地物质充填, 下段主要为原地岩石碎裂产物.与构造变形相对应, 环形构造中央核部地层连续性变差, 而且下古生界在中央增厚, 边缘减薄;上古生界则相反, 具有中央减薄, 边缘增厚的特征.与世界典型撞击构造对比分析, 皮山北环形构造应为陨石撞击所致, 从塔西南铱 (Ir) 元素正异常发育层位, 推断该次撞击事件应发生于白垩纪末期或更早.