月球背面加加林地区地质及演化特征

Geological and evolutionary characteristics of the Gagarin Region on the far side of the Moon

以月球背面加加林地区为研究对象,采用多源遥感数据解译,通过地质要素定量统计、分布特征与成因分析等研究方法,揭示了月球背面典型区域的地质特征,进而探讨了加加林地区区域地质演化历史,初步建立了该区地质特征与月球演化过程之间的联系。研究结果表明:区内发育的全月最长深部断裂的形成为南极艾肯撞击事件与月球热膨胀等内外应力共同作用的结果;自南极-艾肯盆地向外延伸的多处月壳厚度的线性异常延伸至月球正面各大盆地,说明在月球深部,月球背面南极-艾肯盆地的影响范围可以辐射到月球正面单元;月球背面加加林地区撞击坑数量与面积的变化规律与月球所处的地月系及太阳系演化进程有关;撞击坑定量统计结果及玄武岩单元定年结果表明,艾肯纪至雨海纪是加加林地区外动力地质作用的活跃时期,雨海纪是其内动力地质作用的活跃时期。

[Objective]This study focuses on the Gagarin region on the far side of the Moon,aiming to reveal the geological characteristics,distribution features,and genesis of typical areas on the lunar far side.Additionally,it seeks to explore the regional geological evolution history of the Gagarin region.[Methods]The study primarily employs methods such as multi-source remote sensing data interpretation,regional geological mapping,and quantitative analysis of geological elements'quantity and distribution characteristics.[Results](1)656 impact craters were discovered in the study area,of which 552 have diameters greater than 20 kilometers.Approximately 71.5%of the Gagarin region is covered by ancient basins and their ejecta from the Aitken period.Based on comprehensive area and diameter data,the Aitken period is identified as the geological era with the largest proportion of large impact craters(diameter greater than 70 kilometers)and the largest average diameter.From the Aitken period to the Copernican period,the total area of impact craters in each geological era shows a decreasing trend from old to new.(2)In the study area,six parallel lunar grabens,62 lobate scarps,one sinuous rille,50 crater floor fractures,and 70 shallow faults were discovered.It also includes parts of the two longest inferred deep faults on the Moon,originating from the South Pole–Aitken tectonic zone and almost spanning the entire highland tectonic zone.According to Bouguer gravity anomalies and crustal thickness data,linear crustal thickness anomalies extending outward from the South Pole–Aitken basin reach the major basins on the near side of the Moon.(3)The Gagarin region is primarily located in the anorthositic highlands on the far side of the Moon.The rocks mainly consist of ferroan anorthosite(fa)suites,with some crater floors showing magnesium anorthosite(ma)suites.In the central and southern parts of the Gagarin region,low-titanium(TiO_(2)>1.5 and<4.5)and very low-titanium(TiO_(2)<1.5)basalts are sparsely distributed on the floors of certain impact craters and basins.(4)For this study,we selected impact craters such as Aitken and Van der Graaf,with diameters ranging from 350 to 1400 m,for dating analysis.The results of crater sizefrequency distribution dating indicate ages of 3.47 GA and 3.32 GA,respectively.(5)The quantitative statistics of impact craters and the dating results of basalt units indicate that the Aitkenian to Imbrian periods were active periods of external dynamic geological processes in the Gagarin region,while the Imbrian period was an active period of internal dynamic geological processes.[Conclusion](1)The region’s longest and deepest faults are the result of the combined effects of the South Pole–Aitken impact event and internal and external stresses,including lunar thermal expansion.(2)The variations in the number and size of impact craters in the Gagarin region on the far side of the Moon are related to the evolution of the Earth–Moon system and the solar system.(3)Based on the quantitative statistical results of impact craters and the dating results of basalt units,this study elucidates the regional geological evolution history,and different stages of the geological processes in the Gagarin region were divided according to the active periods and stage characteristics of internal and external dynamic geological processes.[Significance]The study revealed the geological features of key areas on the far side of the moon,delving into the geological history of the Gagarin region and tentatively establishing a correlation between its geological traits and the lunar evolutionary history.