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1.
Marius Hills is a volcanic plateau on the nearside of the Moon.It is of great interest for its high concentration of volcanic features,including domes,cones,ridges,and rilles.However,the morphological and chronological characteristics of this plateau were not well studied due to the low resolution of early mission data.This study describes the detailed morphology of the volcanic features using the latest high spatial resolution images of the Terrain Camera(TC) onboard Selene-1(10 m/pix) and Narrow Angle Cam... 相似文献
2.
火山活动是月球最主要的内动力地质作用之一,是研究月球地质历史和热演化的重要窗口,也是月球科学及探测的重点目标.本文概要总结了月球火山作用的基本原理,并重点介绍了"岩墙扩展"模型.基于此模型,列举了由于岩墙在月壳内部上升程度的不同,导致的不同形式的喷发活动,并在月表产生了一系列火山地貌特征:① 当岩墙仅扩展到浅月表、未能穿透月壳并引起喷发活动时,可能会在月表产生坑链构造、地堑或底部断裂型撞击坑;② 当岩墙穿透了整个月壳并引起爆裂式喷发活动时,会在月表产生小型火山锥、区域性火山碎屑堆积物、全月分布的微小火山玻璃、暗晕凹陷构造及环形火山碎屑堆积物;③ 当岩墙穿透了整个月壳并引起溢流式喷发活动时,随着岩浆喷发通量的逐步增高,会在月表产生小型熔岩流、月海穹窿、复合熔岩流、蜿蜒型月溪、巨型熔岩流及火山高原复合体.本文也简要介绍了在月表观测到的若干非典型火山地貌特征,包括不规则月海斑块、环形凹陷穹丘及非月海富硅质穹窿.近年来新的探月数据加深了对这些特殊火山地貌特征的认识,但是更多的地质特征及成因模型细节仍有待未来月球研究及探测去解决. 相似文献
3.
Extensional and compressional structures are globally abundant on Mars. Distribution of these structures and their ages constrain the crustal stress state and tectonic evolution of the planet. Here in this paper, we report on our investigation over the distribution of the tectonic structures and timings of the associated stress fields from the Noachis-Sabaea region. Thereafter, we hypothesize possible origins in relation to the internal/external processes through detailed morphostructural mapping. In doing so, we have extracted the absolute model ages of these linear tectonic structures using crater size-frequency distribution measurements, buffered crater counting in particular. The estimated ages indicate that the tectonic structures are younger than the mega impacts events(especially Hellas) and instead they reveal two dominant phases of interior dynamics prevailing on the southern highlands, firstly the extensional phase terminating around3.8 Ga forming grabens and then compressional phase around 3.5-3.6 Ga producing wrinkle ridges and lobate scarps. These derived absolute model ages of the grabens exhibit the age ca. 100 Ma younger than the previously documented end of the global extensional phase. The following compressional activity corresponds to the peak of global contraction period in Early Hesperian. Therefore, we conclude that the planet wide heat loss mechanism, involving crustal stretching coupled with gravitationally driven relaxation(i.e.,lithospheric mobility) resulted in the extensional structures around Late Noachian(around 3.8 Ga). Lately cooling related global contraction generated compressional stress ensuing shortening of the upper crust of the southern highlands at the Early Hesperian period(around 3.5-3.6 Ga). 相似文献
4.
Rare felsic volcanic rocks of dacitic to rhyolitic composition occur in the central part of the Jack Hills metasedimentary belt in the Narryer Terrane of Western Australia, interleaved with clastic sedimentary rocks and amphibolite. Representative samples of the four identified felsic volcanic units reveal a similar complex pattern of zircon age distribution, with all samples containing zircon populations at ∼3.3–3.4, ∼3.0–3.1, ∼2.6 and ∼1.8–1.9 Ga. The ∼3.3–3.4 Ga zircons show well-developed oscillatory zoning in cathodoluminescence (CL) images and are interpreted as inherited igneous zircon derived from granitic precursors, similar to the ∼3.3 Ga trondhjemitic granitoids currently exposed along the northern and southern margins of the belt. The ∼3.0–3.1 Ga zircons also reveal well-developed oscillatory zoning in CL and are most likely derived from granitoid and/or volcanic rocks of this age, as recorded in the Murchison domain to the south and possibly also present in the Narryer Terrane. The ∼2.6 Ga population matches the age of nearby late Archean granitoids intruding the Jack Hills belt and their oscillatory zoning and U–Th chemistry is consistent with their origin from such a source. The youngest discrete group of zircon grains, with ages ranging from ∼1970 to ∼1775 Ma, show strong oscillatory zoning and average Th/U ratios of 0.76, features consistent with an igneous origin. These younger zircons are therefore interpreted as defining the age of crystallisation of the volcanic rocks. These results establish that the Jack Hills metasedimentary belt contains significant post-Archean components. Taken together with similar results obtained from zircon occurring as detrital grains in clastic sedimentary rocks at Jack Hills, these results overturn the generally-accepted view that the belt is entirely Archean in age and that sedimentation was completed around 3.0 Ga ago. Instead, there is a distinct possibility that much of the material currently exposed in the Jack Hills belt formed in the Proterozoic. A further implication of this study is that the metamorphism affecting these rocks also occurred in the Proterozoic and consequently the rocks should not be considered as forming an Archean greenstone or metasedimentary belt. The paucity of zircons >4 Ga in the known Proterozoic sedimentary rocks and their total absence in the felsic volcanic rocks suggests that such ancient source rocks were no longer present in the area. 相似文献
5.
月海盆地作为月球表面重要的地貌单元,分析其玄武岩喷发历史和构造作用,对于了解月球演化有着重要的意义。文中以澄海和静海两个相连通盆地为研究区,通过对LRO的DEM数据进行处理,获得两个月海的地形特征。基于Clementine多光谱数据处理,提取TiO2、FeO含量和成熟度分布图。经过对嫦娥一号CCD影像数据并结合LRO和LO全色波段影像的解译,提取了研究区126条月岭和114条月溪,并对比Cle-mentine提取的重力分布图,对其展布形式进行研究。综合分析结果表明,两个盆地虽然相邻连通,但岩性和构造分布有着明显的差别,玄武岩喷发不同期次界限明显,且澄海玄武岩年龄普遍晚于静海,相通处玄武岩与静海北部玄武岩同源。澄海中的线状构造展布形式与静海中的明显不同,呈现出一定规律,与质量瘤的有无及重力展布形式有关。 相似文献
6.
北京西山髫髻山组火山岩的地球化学特征与岩浆起源 总被引：45，自引：1，他引：45
对北京西山髫髻山组粗安岩斜长石Ar-Ar年龄测定获得全坪年龄和等时线年龄分别为148.91Ma±2.98Ma和146.60Ma±2.93Ma(相关系数r=0.9997),本文将其时代划为晚侏罗世.元素-同位素地球化学显示,西山髫髻山组火山岩主要为高钾钙碱性系列,低Ti\K,高Al\Ca和Na,轻稀土元素富集,Eu异常不明显,大离子亲石元素Rb\Sr\Ba等相对富集,高场强元素Nb\Ta\Ti\Zr\Hf相对亏损,Nd\Sr和Pb同位素比值较低,反映出火山岩形成于岛弧或活动大陆边缘,其源岩为富集地幔.据此认为髫髻山组火山岩形成于陆内挤压环境,由中生代玄武质岩浆底侵于下地壳-壳幔过渡带诱发古老(太古宙)的玄武质岩石部分熔融形成的. 相似文献
7.
月岭作为月海中一种广泛分布的线性构造,多表现为褶皱逆冲推覆为主的构造样式,且具雁列式展布。因此,月岭对月表区域乃至全月的构造应力分析有着非常重要的指示作用,但其成因机制不清。虹湾是中国嫦娥绕月探测工程研究的重点区域,区域内最明显的线性构造是虹湾东缘呈雁列式展布的月岭构造带,对这一月岭构造带的研究将有助于了解虹湾地区的区域应力特征和构造演化过程。文中通过对虹湾东缘雁列式月岭构造带的高分辨率影像进行解译并与其他区域对比分析,结合虹湾地区已有地质资料和前人提出的相关构造演化模式及目前最新的研究成果,认为虹湾东缘雁列式月岭构造带受虹湾撞击坑坑缘基底和表面玄武岩性影响,虹湾撞击盆地沉降引起的径向挤压应力和表面熔岩流导致的挤压应力由于方向与大小的差异,共同导致该月岭构造带呈类似走滑剪切中右阶左行的形貌特点。在此基础上讨论了此月岭构造带的演化过程,并根据月岭与撞击坑的交切关系,发现虹湾东缘月岭构造带在哥白尼纪仍在继续活动。 相似文献
8.
雨海盆地是月球上研究程度最高的多环结构盆地,月球上古老的和年轻的玄武岩在盆地中均有分布,因此雨海是研究月海玄武岩岩浆活动的理想区域。为了更合理的厘定雨海地区的玄武质岩浆演化历史,本文主要结合岩石学、年代学等工作对本区玄武岩的充填期次进行重新划分。利用嫦娥一号IIM光谱数据进行岩石类型分布图编制,初步划分了5类不同钛含量的月海玄武岩;基于高分辨率100m LRO宽视角影像数据通过撞击坑尺寸-频率定年法(CSFD)对本区玄武岩单元模式年龄进行厘定,共划分35个玄武岩单元,发现本区在3.49~2.23Ga均有玄武质岩浆充填活动,具有多期次性。在建立不同类别玄武岩、形貌特征与模式年龄的对应关系基础上,将玄武岩充填划分为4个期次:极低钛玄武岩(3.49~3.20Ga)、低钛玄武岩(3.29~2.83Ga)、中钛玄武岩(3.13~2.52Ga)、(极)高钛玄武岩(2.92~2.23Ga)。本区地形地貌高程特征与不同表面年龄的玄武岩单元之间总体上呈现出一定的负相关性。因此在本区玄武质岩浆期次划分考虑上,不仅要考虑玄武岩的成分特征,更要考虑结合与玄武岩演化密切相关的年代学及形貌学特征,利用形貌、成分数据和年代学信息来共同约束玄武质岩浆期次划分及演化历史。 相似文献
9.
撞击作用是行星形成和表面重塑的重要地质过程,记录和揭示了行星的演化历史.撞击作用形成的撞击坑可用于研究天体表面地质单元形成的时间.依据内太阳系天体表面的撞击历史,总结了通过对撞击坑的直径和频率分布进行统计,计算天体表面模式年龄的原理和方法.在此基础上,利用美国“月球勘测轨道器(LRO)”广角相机获得的图像,对月球虹湾地区的撞击坑进行了直径-频率分布统计研究,获得其3个主要地质单元的绝对模式年龄分别为3.33 Ga、3.21 Ga和2.60 Ga,有效限定了本区主要地质事件发生的时间. 相似文献
10.
Lunar topography and landform, resulting from endogenous and exogenous geophysical processes of various spatial and temporal scales, carry information of these processes and target properties. Geoscientists use morphometric analysis at different scales to study lunar topography, which is one of the four scientific objectives of China's lunar exploration project. This article first reviewed the lunar topographic types from different researchers, analyzed classifying method and progress, discussed geological mapping method of 1∶ 5 000 000 complied by United States Geological Survey in the 1970s. In consideration of the present situation of the lunar surface morphological characteristics, the pattern of macroscopic forcing, morphologic variation and combination characteristics and function way, etc., a matrix combining multi-stage classification method was put forward based on the characteristics of the topography and geologic age, which included 7 geologic ages and 14 morphologic classes. Geological ages can be divided into Copernican System (C), Copernican-Eartosthenian System (CE), Eartosthenian System (E), Eartosthenian-Imbrian System (EI), Imbrian System (I), Imbrian-PreImbrian System (IpI) and Pre-Imbrian System (pI). As to topographic types, the first class can be divided into lunar mare, lunar basin, lunar terra and lunar crater. As to their second class according to morphological differences, the lunar basin can be divided into basin plain and circum-basin, and lunar mare can be divided into mare plain and mare dome; lunar terra can be divided into terra plain, plateau and hill, and craters can be divided into main sequence crater, crater plain, secondary crater, crater chains and clusters, rayed craters, irregular crater and undivided crater. Thus, 46 subclasses including geologic and morphologic features were obtained in this classification system. The test mapping method was addressed in Sheet H010, which shows the combination classification method is reasonable. 相似文献
11.
月球表面环形构造主要有撞击坑、火山口和月海穹窿3种,其中撞击坑分布最广泛,是研究月表环形构造的主要内容。由于月表撞击坑数量大、种类多及其形成伴随着整个月球地质的演化过程,因此这种月表地形地貌比较完整地记录了月球表面地貌随时间的改造过程以及改造类型。文中通过研究撞击坑遥感影像及形貌特征,总结归纳为简单型、碗型、平底型、中央隆起型、同心环型、复杂型及月海残留型7种撞击坑类型,用来描述月表典型区域撞击坑的形态特征。从结构和物质两方面进行了月表典型区域撞击坑的形态地貌参数提取,综合利用嫦娥一号CCD 影像数据、LROC数据,得到了该区域撞击坑形态数据(坑底、坑唇、坑壁、坑缘、溅射物覆盖层、中央峰)和形态测量数据(直径、深度、地理位置)。研究发现,LQ 4地区的撞击坑分布可分为月陆区和月海区,月陆区的撞击坑多以中小型撞击坑为主,其分布密度极高,形成年代较早,月海区撞击坑多为年轻的撞击坑,分化程度较低,分布密度也较低。 相似文献
12.
Julie Brigham-Grette Lyn M. Gualtieri Olga Yu. Glushkova Thomas D. Hamilton David Mostoller Anatoly Kotov 《Quaternary Research》2003,59(3):386-398
The Pekulney Mountains and adjacent Tanyurer River valley are key regions for examining the nature of glaciation across much of northeast Russia. Twelve new cosmogenic isotope ages and 14 new radiocarbon ages in concert with morphometric analyses and terrace stratigraphy constrain the timing of glaciation in this region of central Chukotka. The Sartan Glaciation (Last Glacial Maximum) was limited in extent in the Pekulney Mountains and dates to 20,000 yr ago. Cosmogenic isotope ages > 30,000 yr as well as non-finite radiocarbon ages imply an estimated age no younger than the Zyryan Glaciation (early Wisconsinan) for large sets of moraines found in the central Tanyurer Valley. Slope angles on these loess-mantled ridges are less than a few degrees and crest widths are an order of magnitude greater than those found on the younger Sartan moraines. The most extensive moraines in the lower Tanyurer Valley are most subdued implying an even older, probable middle Pleistocene age. This research provides direct field evidence against Grosswald’s Beringian ice-sheet hypothesis. 相似文献
13.
Antarctic lunar meteorites Yamato-793169, Asuka-881757, MIL 05035, and MET 01210 (YAMM): Launch pairing and possible cryptomare origin 总被引：1，自引：0，他引：1
Tomoko Arai B. Ray Hawke Keiji Misawa Masamichi Miyamoto 《Geochimica et cosmochimica acta》2010,74(7):2231-3852
The Antarctic lunar meteorite Meteorite Hills (MET) 01210 is a polymict regolith breccia, dominantly composed of mare basalt components. One relatively large (2.7 × 4.7 mm) basalt clast in MET 01210 (MET basalt) shows remarkable mineralogical similarities to the lunar-meteorite crystalline mare basalts Yamato (Y)-793169, Asuka (A)-881757, and Miller Range (MIL) 05035. All four basalts have similar rock texture, mineral assemblage, mineral composition, pyroxene crystallization trend, and pyroxene exsolution lamellae. The estimated TiO2 contents (∼2.0 wt%) of the MET basalt and MIL 05035 are close to the bulk-rock TiO2 contents of Y-793169 and A-881757. These similarities suggest that Y-793169, A-881757, MIL 05035, and the MET basalt came from the same basalt flow, which we designate the YAMM basalt. The source-basalt pairing of the YAMM is also supported by their similar REE abundances, crystallization ages (approx. 3.8-3.9 Ga), and isotopic compositions (low U/Pb, low Rb/Sr, and high Sm/Nd). The pyroxene exsolution lamellae, which are unusually coarse (up to a few microns) by mare standards, imply a relatively slow cooling in an unusually thick lava and/or subsequent annealing within a cryptomare. Reported noble gas and CRE data with close launch ages (∼1 Ma) and ejection depths (deeper than several meters) among the four meteorites further indicate their simultaneous ejection from the moon. Despite the marginally close terrestrial ages, pairing in the conventional Earth-entry sense seems unlikely because of the remote recovery sites among the YAMM meteorites.The high abundance (68%) of mare components in MET 01210 estimated from a two-component mixing model calculation could have resulted from either lateral mixing at a mare-highland boundary or vertical mixing in a cryptomare. The proportion of mare materials in MET 01210 is greater than in Apollo core samples at the mare-highland boundary. The burial depth (>several meters deep) inferred from the lack of surface irradiation of MET 01210 exceeds the typical mare regolith thickness (a few meters). Thus, the source of the YAMM meteorites is likely a terrain of locally high mare-highland mixing within a cryptomare. We searched for a possible source crater of the YAMM meteorites within the well-defined cryptomare, based on the multiple constraints obtained from this study and published data. An unnamed 1.4 km-diameter crater (53°W, 44.5°S) on the floor of the Schickard crater is the most suitable source for the YAMM meteorites.The 238U/204Pb (μ) value of the YAMM basalts is extremely low, relative to those of the Apollo mare basalts, but comparable to those of the Luna 24 very low-Ti basalts. The low-μ source indicates a derivation from a less differentiated mantle with a lack of KREEP components. Although the chemical sources of materials and heat source of melting might be independent, the heat source that generated the source magma of the YAMM and Luna 24 basalts may not be related to KREEP, unlike the case of the Apollo basalts. The distinct chemical and isotopic compositions of mantle sources between the Apollo basalts and the YAMM/Lunar 24 basalts imply differences in mantle composition and thermal evolution between the Procellarum KREEP Terrane (PKT) and non-PKT regions of the nearside. 相似文献
14.
冀北张-宣地区后城组、张家口组火山岩锆石U-Pb年龄和Hf同位素 总被引：23，自引：0，他引：23
冀北张家口-宣化地区(张-宣地区)分布着中生代后城组和张家口组火山岩.锆石的LA-ICPMS U-Pb年龄和LA-MC-ICPMS Hf同位素分析结果表明, 后城组英安岩的结晶年龄为(130±1)Ma, 为早白垩世, 而不是前人所认为的晚侏罗世.张家口组流纹岩的结晶年龄为(126±1)Ma, 也为早白垩世.后城组英安岩中锆石Hf同位素组成为εHf(t)=-18.8~-25.5, Hf平均地壳模式年龄为TDMC=2.78~2.37 Ga, 平均2.54 Ga, 与冀北分布的基底岩石Nd、Hf模式年龄相同, 考虑到华北克拉通东部地壳生长的主要时期为晚太古代, 我们初步认为后城组粗安岩可能主要来源于晚太古代地壳物质的重熔作用; 而张家口组流纹岩中锆石Hf同位素组成为εHf(t)=-15.1~-18.5, Hf平均地壳模式年龄为TDMC=2.34~2.13 Ga, 明显比后城组火山岩年轻.张家口火山岩来源于太古代的地壳物质和部分幔源物质混合.这些年代学数据和锆石Hf同位素表明张-宣地区大面积的后城组和张家口组火山岩是华北克拉通东部晚中生代岩石圈强烈减薄作用的结果. 相似文献
15.
Fractures and faults in volcanic rocks (Campi Flegrei,southern Italy): insight into volcano-tectonic processes 总被引：1，自引：0，他引：1
The present study was focused to analyze fractures and faults in the Campi Flegrei calderas presently hosting several volcanic edifices, such as lava domes, scoria, and tuff cones. A complex network of fractures and faults affects the volcanic rocks, mostly as planar with highly variable density. Frequently faults appearing as conjugate structures showing normal kinematics often associated with ductile deformation such as drag folds and deflexed layers, suggesting a syn-eruption deformation. However, the most of faults, mainly hosted along the caldera/crater rims, are very steep with dominant normal and secondary reverse movements. The fracture pattern indicates a slight prevalence of NE–SW and NW–SE directions, but N–S and E–W trends also occur. Fractures and faults found in rocks older than 15 ka (Neapolitan Yellow Tuff included), measured in western and eastern sectors of the study area, indicate a rotation of ca. 30° of the main directions among these two sectors. For the faults occurring along the caldera/crater rims, we suggest a kinematic evolution characterized by the reactivation of tensile fractures previously formed in response to both regional extension and locale resurgent dome. Finally, normal faults located in the central sector of caldera, between La Starza and Accademia localities, cutting the youngest volcanic deposits, indicate a constant NNE–SSW extension probably related to the caldera resurgence. 相似文献
16.
月海玄武岩是月幔部分熔融喷出月表而形成的,其厚度可以反映月海玄武岩源区的深度。研究月海玄武岩厚度,对进一步认识月球区域岩浆作用或火山作用的演化历史具有不可替代的作用,也能够为整个月球的热演化和岩浆演化提供基本的约束条件。同时,玄武岩厚度可以用以推测月球内部产生玄武岩岩浆的体积,对月球火山作用的岩浆喷发总量以及月球内部的热状态具有指示作用。本文基于多源遥感数据,综合利用撞击坑的形貌特征与月坑挖掘深度法对南海地区撞击坑内(crater)和撞击坑间(intercrater)两类玄武岩地层的厚度进行了估算,并对玄武岩的面积、体积、年龄及岩浆活动做了简单分析。研究结果表明：南海地区撞击坑内的玄武岩厚度变化范围为0.11~4.75 km,平均值约为1.32 km,玄武岩的出露面积和出露体积分别为57.06~10 791.66 km2和10.25~51 260.38 km3;撞击坑间的玄武岩厚度变化范围为0.01~2.18 km,平均值约为0.34 km,玄武岩的出露面积和出露体积分别为6 487.89~33 170.55 km2和2 711.97~11 609.69 km3。因此,南海地区玄武岩厚度的变化范围分布在0.01~4.75 km,平均厚度约为600 m,出露的玄武岩总面积约为2.12×105 km2,总体积约为2.71×105 km3。通过分析南海地区的玄武岩年龄及分布特征,发现南海地区内的岩浆喷发活动主要集中发生在雨海纪至爱拉托逊纪时期,且其局部区域存在多次岩浆喷发及充填过程,但由于晚期玄武岩岩浆的喷发总量不足以覆盖早期已形成的玄武岩,导致晚期玄武岩与早期玄武岩同时存在于同一个玄武岩单元内。南海地区独特的玄武岩分布特征也与地形有关。 相似文献
17.
《Gondwana Research》2013,24(4):1484-1490
Evidence for the earliest known terrestrial crust comes predominantly from Jack Hills in Western Australia, where hafnium isotopic results from > 3.8 Ga detrital zircons indicate crustal precursors as old as ~ 4.4–4.5 Ga. We present evidence from magmatic cores in > 3.9 Ga xenocrystic zircons from a felsic volcanic rock in the North Qinling Orogenic Belt, China, of similar Hf crustal model ages up to 4.45 Ga. These lie on the same Lu/Hf trajectory as the least disturbed Jack Hills and Apollo 14 zircons, therefore providing only the second example of the earliest known generation of continental crust on Earth. In addition, the rims of two zircon grains record later growth at 3.7 Ga and, when combined with the fact that the grains are incorporated in Paleozoic volcanic rocks, imply long-lived crustal residence within the basement of the North China Craton. These results therefore establish the wider distribution and survival of the most ancient crustal material on the Earth and highlight the possibility for the further discovery of ancient crustal remnants. 相似文献
18.
赣东北婺源-德兴地区新元古代浅变质火山岩的地球化学和锆石U-Pb年龄 总被引：3，自引：1，他引：2
赣东北婺源-德兴地区新元古代地层中浅变质火山岩主要由变质玄武岩、英安岩和流纹岩组成.全岩地球化学分析表明浅变质玄武岩具有拉斑玄武岩的地球化学特征,起源于尖晶石辉橄岩低度部分熔融,英安岩岩浆起源于壳源杂砂岩部分熔融,流纹岩可能为英安质岩浆结晶分异的产物.LA-ICP-MS锆石U-Pb同位素定年揭示婺源浅变质英安岩形成于861±8Ma,德兴张村西浅变质流纹岩形成于860±3Ma,铜厂铜矿矿区凝灰质板岩形成于860±6Ma,均为早新元古代Tonian期.同时这些定年样品中保存了2.8 ～2.5Ga、2.0～1.7Ga和～1.0Ga的继承或捕获锆石记录.结合浅变质玄武岩和英安质火山岩的地球化学特征和成因,这套岩石最有可能形成于新元古代早期安第斯型活动大陆边缘弧后盆地构造背景. 相似文献
19.
《地学前缘(英文版)》2020,11(3):855-870
The global tectonics of Mercury is dominated by contractional features mainly represented by lobate scarps,high relief ridges,and wrinkle ridges.These structures are the expression of thrust faults and are linear or arcuate features widely distributed on Mercury.Locally,these structures are arranged in long systems characterized by a preferential orientation and non-random spatial distribution.In this work we identified five thrust systems,generally longer than 1000 km.They were named after the main structure or crater encompassed by the system as:Thakur,Victoria,Villa Lobos,Al-Hamadhani,and Enterprise.In order to gain clues about their formation,we dated them using the buffered crater counting technique,which can be applied to derive the ages of linear landforms such as faults,ridges and channels.To estimate the absolute age for the end of the thrust system's activity,we applied both Le Feuvre and Wieczorek Production Function and Neukum Production Functions.Moreover,to further confirm the results obtained with the buffered crater counting method,the classic stratigraphic approach has been adopted,in which a faulted and an unfaulted craters were dated for each system.The results gave consistent ages and suggested that the most movements along major structures all over Mercury most likely ended at about 3.6-3.8 Ga.This gives new clues to better understand the tectonics of the planet and,therefore,its thermal evolution.Indeed,the early occurrence of tectonic activity in the planet's history,well before than predicted by the thermophysical models,coupled with the orientation and spatial distribution of the thrust systems,suggests that other processes beside global contraction,like mantle downwelling or tidal despinning,could have contributed to the first stage of the planet's history. 相似文献
20.
Richard J. Walker G. N. Hanson J. J. Papike J. R. O''Neil 《Geochimica et cosmochimica acta》1986,50(12):2833-2846
The Nd, O and Sr isotopic characteristics of Precambrian metasedimentary, metavolcanic and granitic rocks from the Black Hills of South Dakota are examined. Two late-Archean granites (2.5-2.6 Ga) have Tdm ages of 3.05 and 3.30 Ga, suggesting that at least one of the granites was derived through the melting of significantly older crust. Early-Proterozoic metasedimentary rocks have Tdm ages that range from 2.32 to 2.45 Ga. These model ages, in conjunction with probable stratigraphic ages ranging from 1.9 to 2.2 Ga, indicate that mantle-derived material was added to the continental crust of this region during the early-Proterozoic. Previous studies of the Harney Peak Granite complex have reported U-Pb and Rb-Sr ages of about 1.71 Ga and most granite samples examined in this study have Sr isotopic compositions consistent with that age. Two granite samples taken from the same sill, however, give two-point Rb-Sr and Sm-Nd ages of 2.08 ±0.08 and 2.20 ±0.20 Ga (∑2200Nd = −15.5), respectively. In addition, whole-rock and apatite samples of the spatially associated Tin Mountain pegmatite give a Sm-Nd isochron age of 2000 ±100 Ma (∑2200Nd = −5.8 ±1.8).
The Sm-Nd, O and Rb-Sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post-crystallization processes, and the age of the pegmatite and parts of the Harney Peak Granite complex remain uncertain. Processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material, assimilation, mixing of REE between granite and country rock during crystallization via a fluid phase and post-crystallization mobility of Sr. The Nd isotopic compositions of the pegmatite and the Harney Peak Granite indicate that they were not derived primarily from the exposed metasedimentary rocks. 相似文献