RESEARCH ON IMPACT TECTONICS AND IMPACTITES:STATUS AND PROSPECTS
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摘要: 当陨石撞击地球表面时, 它所引起的瞬时(1秒至1微秒)动态高压(数十至数百个吉帕)和高温(大于1500℃), 必然导致陨石撞击坑(astroblems)的生成并使周围岩石发生变质作用, 这便是撞击变质作用(impact metamorphism)。目前关于撞击构造(impact tectonics)作用研究的主要进展涵盖如下四个方面:1.撞击坑的构造形貌及形成机制; 2.撞击变质作用及撞击变质岩; 3.撞击作用的数值模拟和实验研究; 4.撞击作用的经济地质意义。陨击作用研究不仅具有学术意义, 还具有实际的经济地质意义。南非的Vredefort-Witwatersrand是最老的撞击构造, 它以丰富的金铀矿床而闻名; 加拿大的肖德贝里(Sudbury)以同撞击作用期的大型铜镍矿床著称; 如今北美、中美不少地区正在探索与陨击期后构造有关的油气储集。此外, 许多陨击变质岩可以供作建筑材料。不少撞击坑已经辟为旅游胜地, 如德国的Nordlingen Ries撞击坑。我国撞击构造的研究已有不少进展。20世纪90年代初发现了海南白沙陨击坑, 此外内蒙古的多伦、江苏的太湖和辽宁的岫岩罗圈里等地也发现有陨击构造的证迹, 不过, 都尚待进一步的工作予以证实。全球撞击构造研究经验可以借鉴的是, 在长期构造稳定区较易于发现陨击坑; 不少撞击构造被隐伏在第四纪或更老的沉积物之下, 所以采用人造卫星发回资料解译地球表面的照片, 加上地球物理勘探, 可以有效地确定研究靶区, 在此基础上再加强岩石学、构造学研究予以确认。Abstract: Transient (within 1 second)dynamic high pressures (ca tens of to up to one hundred GPa) and high temperatures (>1500℃)caused by the impact process will eventually result in the development of an astrobleme when a large meteorite impacts the earth's surface, and the surrounding rocks must undergo a special kind of metamorphism called impact metamorphism. The studies of impact tectonics include the following four topics:1. structure and morphology of the impact crater and its formation mechanism; 2. impact metamorphism and impact metamorphic rocks; 3. numerical simulation and experimental calculation of impact tectonics; and 4. economic benefits of impact tectonics. The studies of impact tectonics not only have academc significance but also have actual economic benefits. Vredefort-Witwatersrand in South Africa is one of the oldest impact tectonics and is also famous for its gold-uranium deposits. The Sudbury impact structure in Canada is famous for occurrence of a large syn-impact Cu-Ni deposit. Now oil and gas reservoirs related to post-impact tectonics are being explored for in a number of areas of North America and Central America. In addition, many impactites are used as building materials and many astroblemes become famous resort places, such as the Ries crater in Nordlingen, Germany. Much progress has been made in impact studies in China since the end of the 20th century. The Baisha crater found in Hainan in the early 1990s is known to the world and there are reports on possible craters such as Taihu in Jiangsu, Duolun in Inner Mongolia and Luoquanli near Xiuyan in Liaoning. However these structures remain to be further confirmed. The studies of global impact tectonics show that long-term stable cratons are favorable to discovering impact craters and many craters are hidden beneath Quaternary and older sediments. So satellite-borne photography and geophysical exploration will be effective in determining targets, and on that basis, through strengthening petrological and structural studies, impact craters may be confirmed.
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图 5 瑞芬地盾和地台区陨石撞击坑的分布剖面图(据Puura and Plado, 2005)[9]
①~ ③为大陆区撞击坑, ④~ ⑤为沉没区水下撞击坑; ①撞击发育于结晶岩区; ②和④陨石撞击于沉积岩区; ③和⑤陨石撞击进入沉积岩和结晶岩区
Figure 5. Typical impact geological settings in the Svecofennian domain (after Puura and Plado, 2005)[9]
①~ ③Impact craters in the continent; ④~ ⑤ Subaqueous impact craters in sunken areas.① Impact occurred in the crystalline basement; ② and ④ Meteorites impacted sedimentary cover; ③ and ⑤ Meteorite impacts occurred in sedimentary and crystalline terrains
图D 德国Ries陨击坑的陨击角砾岩, 除了长石、石英之外还有鲕状灰岩的残片, 视域直径3.8mm(R梅森教授提供)
图D. Suevite from the Ries Crater, Besides feldspar and quartz there are fragments of oolitic limestone. Field width 3. 8mm (on courtesy of Prof.R Mason)
图E 德国Ries陨击坑陨击花岗岩中的陨击熔融体, 视域直径3.8mm (R梅森教授提供)
图E. Impact melt in impact granite, Ries crater. FW = 3. 8mm (on courtesy of Prof.R Mason)
表 1 瑞芬地区陨石撞击坑保存程度分级(据Puura and Plado, 2005)[9]
Table 1. Preservation level of impact craters in the Svecofennian crustal domain (after Puura and Plado, 2005)[9]
附录1 世界著名含金属矿床的陨击构造
附录1. Impact structures in world-famous metal deposits
附录2 世界著名与碳氢资源有关的陨击构造
附录2. World-famous impact structures related to hydrocarbon resources
附录3 世界著名具非金属矿产资源及其他经济效益的陨击构造
附录3. World-famous impact structures where there are nonmetallic mineral resouces and other economic benefits
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