研究生知识库

撞击盆地一般是指月球上直径大于200千米以上的撞击构造，是月球上最显著的地貌单元之一，是深入理解大型撞击作用过程及结果的重要窗口，是推断岩浆活动规模及时间的重要研究区域，也是研究月球早期演化和现今状态的重要纽带。在月球上的撞击盆地中，超过半数都具有质量瘤的特征，而目前主流的观点均认为质量瘤是撞击盆地的“副产物”，且与月海玄武岩的充填具有密切关系，是月球早期演化过程的记录和佐证。但是，月球上撞击盆地并非都具有质量瘤的特征，且月海玄武岩充填的盆地也并非都具有质量瘤特征，这说明对质量瘤的形成机制及演化的认识上还存在不足。本文利用近年来获得的多源数据，结合前人的研究成果，从地质演化研究的角度出发，通过综合对比分析不同区域、不同类型撞击盆地的区域地质演化的差异性，推断不同撞击盆地所经历的地质作用过程，探讨质量瘤的形成机制及演化，其开展的研究内容及取得新认识如下： （1）参考前人的对月球撞击盆地的识别和年龄研究的成果，排除争议较大的盆地，本次研究列出了直径大于200 km的74个撞击盆地（见附录2），并确定了盆地的形成的先后顺序（即相对年龄）。从月球正面和背面的撞击盆地分布来看，其在月表分布是不均的：1）月球背面的撞击盆地数量多于月球正面的数量，但月球正面的撞击盆地规模相对较大；2）在PKT和SPAT地区的撞击盆地数量基本相当，但PKT中的撞击盆地规模相对较大。（2）确定了质量瘤的特征指标并建立了质量瘤数据库。本次研究识别了46个质量瘤，超过六成的撞击盆地具有质量瘤特征，在月球表面分布是不均的，说明撞击盆地所处位置的地质背景对质量瘤的形成具有重要的影响。对质量瘤的布格重力异常特征与盆地规模的关系的分析发现，布格重力异常最大值和最小值的差值与撞击盆地规模呈一定比例增大，且质量瘤的大小与撞击盆地的规模呈一定比例增大，表明质量瘤的布格重力异常的差值和质量瘤大小的主控因素是撞击盆地的规模。另外，根据布格重力异常向上延拓发生衰减的速度来看，质量瘤规模越大其重力异常场源深度越大，反之越小；对于规模相当的质量瘤，异常场源深度由小到大依次为FHT、SPAT、月球正面和月球背面交界区域、最后是PKT。在此基础，建立了月球质量瘤数据库，分别从名称、位置、布格重力异常最大值、最小值、异常差值、质量瘤半径、撞击盆地半径等七大参数来定性描述质量瘤。（3）基于质量瘤和月海玄武岩充填两个重要特征，将月球上撞击盆地划分为了两个一级类型，即质量瘤盆地和非质量瘤盆地，四个二级类型，分别为月海质量瘤盆地、非月海质量瘤盆地、月海非质量瘤盆地和非月海非质量瘤盆地，代表经历不同地质演化过程的四类盆地。这一分类体系能够体现撞击盆地所经历的地质作用，结合地质年代信息，可以综合对比不同类型撞击盆地的区域地质演化差异性。不同类型盆地及其年龄、规模和分布位置的统计分析表明，月球上撞击盆地主要形成于艾肯纪和酒海纪，经历了较长时间的地质演化过程，且具有多种类型的撞击盆地，指示各类型盆地的区域地质演化存在差异性，而雨海纪的盆地数量比较少且均为质量瘤盆地，说明越年轻的撞击盆地越易保留质量瘤的特征。（4）通过综合对比不同类型撞击盆地的演化差异，结合月球早期地质演化历史，认为不同类型撞击盆地差异演化的主控因素：1）撞击盆地的形成年代，越老的盆地被破坏和改造的越严重；2）撞击盆地所处的区域地质特征的差异，如生热元素含量和月壳厚度的差异；3）后期岩浆作用的改造；4）月幔中熔融物质的侧向运移（蠕变）。 （5）根据不同类型撞击盆地的差异性演化，探讨了质量瘤的形成机制及演化。大型撞击事件发生时，挖掘出巨量物质而质量亏损导致重力非均衡，在短期时间内，为了重新达到重力均衡的状态，月球岩石圈表现为弹性响应，引起月幔的隆起，形成质量瘤。然而，在撞击盆地形成以后的改造阶段中，由于早期月球内部温度高，物质流动性强，内部物质在横向压力的作用，隆起的月幔随着演化时间发生粘性的松弛 (Melosh等,2013；Kamata等,2015)，与此同时，部分熔融的物质也将重新发生结晶分异使月壳厚度也相应的增厚，因而撞击盆地早期形成的质量瘤特征逐渐消失，在重力异常并没有明显靶心状特征。随着地质演化的推移，月球内部的热量减少、温度降低，且岩石圈具有足够的强度来支撑质量瘤，隆起的月幔在后期变形程度很小而保留了质量瘤的特征，而质量瘤盆地的年龄相对比较年轻也印证这一过程。后期的大规模月海玄武岩充填，使大量高密度的物质覆盖在浅表层，导致该类盆地的布格重力异常值增大，但是月海玄武岩的厚度与壳幔界面的沉降深度相差不大（例如危海），说明月海玄武岩从地幔深部到达月表的同时，导致岩浆房内空虚而发生沉降，而在盆地的重力异常差值（重力异常最大值和最小值之差）与盆地规模的线性比例关系上并没有明显的改变。

Impact basins, generally larger than 200 km, are the most significant geomorphic units on the Moon that formed by impact and some of them filled by later mare basalt. The investigation of impact basins is one of the most important aspects of lunar science that can provide lots of clues and evidence to further understanding the impact process and magmatic or volcanic activities, even early thermal state of the Moon. Among the impact basins, more than half are mascons, which are the “by-product" of impact events and also have a close relationship with filled mare basalt, records and evidence for early evolution process of the Moon. However, there is a fundamental question of why all basins on the Moon are not mascon basin, even filled by mare basalt, indicating that there are still have confusion about the formation mechanism of mascon. In this study, combining the previous study results and using the multi-sources of obtained data, from the perspective of geological evolution, we try to comprehensively compare the evolution of different types basin that from different regions to infer the geologic processes that they went through and to discuss the formation mechanism and subsequent evolution. The study contents and results include:(1) Compare the previous study result about the impact basin, we proposed that there are 74 impact basins that larger than 200 km on the Moon when excluding the controversial basins. The distribution of impact basins on the Moon is different: 1) there are more impact basins on the farside, but their diameter mostly smaller than the impact basin located at the nearside, 2) The impact basins within PKT are generally larger than SPAT’s, but the amounts of impact basin are very close.(2) In this study, we recognized 46 mascons on the Moon and the distribution is different, indicating that the geologic setting of the target region has an important influence on the formation of mascon. We extracted several parameters about Bouguer gravity anomaly (BGA) of each mascon basin, including BGA maximum, BGA minimum, BGA difference, mascon radius, to relate to mascon basins’ diameter. The results show that the Bouguer difference value and mascon radius are increased with the diameter in linear growth with a relatively constant slope, indicating that the basin scale is the main control factor to mascon. And according to the attenuation rate of upward continuation of Bouguer gravity anomaly, the larger mascon size and the deeper of gravity anomaly field source. For mascons with similar size, the depth of gravity anomaly field source, from shallow to deep, is FHT, SPAT, PKT_out, and PKT, respectively. At last, we proposed several characteristics to describe the mascons on the Moon and constructed a database of mascon.(3) Based on the characteristic of mascon or not, we divided impact basins into two first-level types, mascon basin and non-mascon basin, and divided into four second-level types based on filled mare basalt or not, mare mascon basin, non-mare mascon basin, mare non-mascon basin and non-mare non-mascon basin, respectively, representing four types basins that experienced different geological processes. Combining the geologic history, we can compare their different evolution. From the statistics of ages, size and location among different types basins, we found that most of the basins formed during Aitkenian and Nectarian with all four types basins, indicating the differential evolution with the geologic time. However, the basins formed during Imbrian, they are all mascon basin, indicating that the younger basin, the easier to reserve the mascon feature.(4) Comparison of different types of basins and combining the geological history of the Moon, the main controlling factors for the differential evolution of different types of impact basins are as follows: 1) the older the impact basin is, the more serious the damage and reconstruction will be; 2) differences in regional geological characteristics of the impact basin, such as the content of thermogenic elements and the thickness of the lunar crust; 3) the destruction by post-impact magmatic activities; 4) Lateral migration of molten matter in lunar mantle (creep).(5) According to the differential evolution of different types of impact basins, we proposed that the formation mechanism and evolution of mascon as follows. The huge amount of material was excavated when impact event occurred, resulted in gravity disequilibrium. In order to reach the state of gravity equilibrium, the lithosphere of the Moon exhibited an elastic response, which caused the uplift of lunar mantle and formed mascon. At early stage of the Moon, the interior with high temperature will prompt the viscoelastic lateral flow and the relaxation of uplifted mantle will occur with a relative long geological time and the crust will thicken because of differential of partial melt (Melosh et al., 2013; Kamata et al., 2015), the Bull’s-eye of Bouguer gravity anomaly will disappear gradually. With the passage of geological evolution, the heat and temperature inside the moon decreased, and the lithosphere was strong enough to support the mascon and the impact basin retained the characteristics of mascon with a small degree of deformation in the later stage, which was also confirmed by the relatively young age of the mascon basin. The subsequent high-density mare basalt filled the basins (mare basin) that result in the regional gravity anomaly higher than the basins without mare basalt. However, the thickness of mare basalt is close to the depth of subsidence of crust-mantle interface, indicating that the magma chamber under the basins will empty and subsidence when mare basalt erupted to the surface, explaining that there is little change of the linear relationship between Bouguer difference value and basin size.