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1.
洋中脊及邻区洋盆的洋壳厚度能很好地反映区域岩浆补给特征,对于研究洋中脊内部及周缘岩浆活动和构造演化过程具有很好的指示意义.西北印度洋中脊作为典型的慢速扩张洋中脊,其扩张过程与周缘构造活动具有很强的时空关系.本文利用剩余地幔布格重力异常反演了西北印度洋洋壳厚度,由此分析区域内洋壳厚度分布和岩浆补给特征.研究发现,西北印度洋洋壳平均厚度为7.8 km,受区域构造背景影响厚度变化较大.根据洋壳厚度的统计学分布特征,将区域内洋壳分为三种类型:薄洋壳(小于4.5 km)、正常洋壳(4.5~6.5 km)和厚洋壳(大于6.5 km),根据西北印度洋中脊周缘(~40 Ma内)洋壳厚度变化特征可将洋中脊划分为5段,发现洋中脊洋壳厚度受区域构造活动和地幔温度所控制,其中薄洋壳主要受转换断层影响造成区域洋壳厚度减薄,而厚洋壳主要受地幔温度和地幔柱作用影响,并在S4洋中脊段显示出较强的热点与洋中脊相互作用,同时微陆块的裂解和漂移也可能是导致洋壳厚度差异的原因之一. 相似文献
2.
普里兹湾位于南极洲东部大陆边缘,其深部地壳结构特征对认识白垩纪冈瓦纳古陆裂解和新生代大陆边缘形成具有重要意义.本文利用重磁、多道反射地震、声纳浮标折射地震和ODP钻井数据对普里兹湾海域的深部地壳结构进行了研究.研究结果显示,普里兹凹陷表现为典型的盆地负重力异常特征,其沉积基底较深,而在四夫人浅滩为高幅重力正异常,其沉积基底普遍抬升.在大陆架中部存在SW-NE向条带状基底的抬升,且呈朝NE向逐渐变深的趋势.在中大陆架外侧,均衡残余重力异常呈V字形负异常条带状分布,其两翼分别与四夫人浅滩和弗拉姆浅滩外的大陆坡相连.该异常带在大陆架中部向陆的偏移可能是由于古大陆架边缘的地形影响,推测其与普里兹冲积扇同属于洋陆过渡带向陆的部分,在重力模拟剖面表现为地壳向海逐渐减薄.普里兹冲积扇的地壳厚度较薄,平均为6 km,最薄处可达4.6 km,并且根据洋陆过渡带向海端的位置,推测可能属于接近洋壳厚度的过渡壳.重力异常分区的走向与兰伯特地堑在普里兹湾的构造走向基本一致,可能主要反映了二叠纪-三叠纪超级地幔柱对普里兹湾的裂谷作用的影响.该区域的自由空间重力异常和均衡残余异常均表现为超过100×10-5m/s2的高幅正异常特征,可能由位于大陆架边缘的巨厚沉积体负载在高强度岩石圈之上的区域挠曲均衡作用所导致,可能与该区域第二期裂谷期之后的沉积间断以及快速进积加厚的演化过程有关.普里兹湾磁力异常的走向与重力异常明显不同,大致可分为东北高幅正异常区和西南低幅异常区.重磁异常在走向上的差异反映高磁异常主要来源于岩浆作用形成的铁镁质火成岩的影响,并且岩浆作用的时代不同于基底隆升的时代,而可能形成于前寒武纪或者南极洲和印度板块裂谷期间(白垩纪). 相似文献
3.
利用南海海盆及周边最新的重力,经过海底地形、沉积层的重力效应改正,并采用岩石圈减薄模型的温度场公式,校正了从张裂边缘到扩张海盆的热扰动重力效应.通过研究区的地震剖面和少量声呐数据得到的莫霍面深度点作为约束,采用基于"起伏界面初始模型"的深度修正量反演迭代公式,反演、计算了研究区的莫霍面深度及地壳厚度.结果表明,海盆区莫霍面深度在8~14 km之间,地壳厚度在3~9 km之间;东部海盆和西南海盆残留扩张中心沿NNE向展布向西南延伸至112°E,莫霍面深度超过12 km,地壳厚度在6 km以上,而西北海盆没有明显的增厚扩张中心;在西南海盆北缘的中沙地块南侧,存在一个近EW向地壳减薄带,地壳厚度在9~10 km;莫霍面深度14 km的等深线和地壳厚度9 km的等值线可指示洋陆边界位置. 相似文献
4.
本文首先论述了板块学说提出的过程和存在的一些不足与疑问,特别是该学说将Holmes（1948）的地幔热对流说作为驱使岩石圈板块运动的动力机制.而后又以青藏高原及邻区为例,根据区域地质、蛇绿岩和地质构造研究的成果,特别是地震测深研究的成果,详细地论证了本区不存在有大洋中脊扩张成为大洋盆地的新大洋和大洋板块简单的B型俯冲模式,但存在有海底扩张的陆间海和海洋地壳板片（蛇绿岩构造岩片）的仰冲以及大陆岩石圈板片复杂的A型俯冲新模式.新模式不是以地幔对流运动,而是以扩张分离A型俯冲的大陆岩石圈板片与软流圈之间的水平剪切相对运动机制作为它的躯动力. 相似文献
5.
洋中脊速度结构是揭示大洋岩石圈演化过程的重要约束.为探讨不同扩张速率下洋中脊的洋壳速度结构特征,挑选了全球152处快速(全扩张速率 90mm·a-1)、慢速(全扩张速率20～50mm·a-1)和超慢速(全扩张速率20mm·a-1)扩张洋中脊和非洋中脊的洋壳1-D地震波速度结构剖面,通过筛选统计、求取平均值等方法对分类的洋壳1-D速度结构进行对比研究,获得了不同扩张速率下洋中脊洋壳速度结构差异以及洋中脊与非洋中脊洋壳速度结构差异的新认识:(1)快速、慢速和超慢速扩张洋中脊的平均正常洋壳厚度分别为6.4km、7.2km和5.3km,其中洋壳层2的厚度基本相似,洋壳厚度差异主要源自洋壳层3;其洋壳厚度变化范围分别为4.9～8.1km、4.6～8.7km和4.2～10.2km,随着洋中脊扩张速率减小,洋壳厚度的变化范围逐渐增大;(2)快速扩张洋中脊的洋壳速度大于慢速和超慢速,可能与快速扩张脊洋壳生成过程中深部高密度岩浆上涌比较充足有关;(3)非洋中脊(10Ma)的洋壳比洋中脊(10Ma)的洋壳厚～0.3km,表明洋壳厚度与洋壳年龄有一定的正相关性. 相似文献
6.
南海地区岩石圈资料稀少,阻碍了其形成演化过程的研究.为此,本次研究结合大地热流、空间重力异常、高程、大地水准面和地震数据,在南海西南次海盆反演了两条2.5维岩石圈剖面.本次计算基于三种假设：岩石圈地幔的密度取决于岩石温度;研究区岩石圈处于热稳定状态;研究区处于重力均衡状态.在剖面A-E中,岩石圈底界面从珠江口盆地的105 km迅速抬升到西沙海槽处的50 km,在西沙海槽、西沙-中沙群岛和西南次海盆变化不大,为50~60 km.在剖面F-I中,岩石圈底界面从西沙群岛-中建地块处的88 km向海盆逐渐抬升,在西南次海盆处为46~50 km,到郑和隆起再逐渐变深至64 km.我们比较了西南次海盆岩石圈的冷却模型和热稳定模型,根据冷却模型由水深和热流数据所推断的西南次海盆年龄比实际年龄差很多,说明冷却模型不适用于西南次海盆.通过对比剖面A-E和剖面F-I,说明了剖面A-E经历了更长时间的拉伸,证明南海西南次海盆在形成演化过程中是从北东向南西逐步打开的渐进式扩张.最后,我们综合分析西南次海盆及其大陆边缘的岩石圈结构、减薄陆壳区范围、碳酸盐台地的分布、下地壳韧性流动、流变结构和沉积层特征等多方面资料,认为西南次海盆在形成演化过程中岩石圈地幔首先破裂而地壳后破裂,属于type Ⅱ型非火山型大陆边缘. 相似文献
7.
本文在前人研究大陆岩石圈板块有效弹性厚度的基础上,建立研究海洋岩石圈板块有效弹性厚度的理论模型,推导出与大陆岩石圈不同的海洋岩石圈板块响应函数 Z(k,Te) 理论计算公式.并分析海洋岩石圈板块响应函数 Z(k,Te) 的特点.文中对实际的海洋测量数据的响应函数 Z(k,Te) 进行计算和分析,估算我国南海南沙海域和南海中央海盆岩石圈板块有效弹性厚度分别约为10 km和6~7 km. 相似文献
8.
广角地震测线（OBS973-2）位于南海南部陆缘,其地壳深部构造是研究南海共轭扩张及形成演化的直接证据之一.本文采用2D射线追踪技术,结合与之重合的多道地震测线（NH973-2）时深转换结果,对OBS973-2测线重新进行了正、反演研究,得到了礼乐滩及邻近海区的精细地壳结构.与前人结果相比,本文基于正反演速度模型,把测线分为陆壳区（0~200 km）、洋陆过渡区（200~280 km）和洋盆区（280~370 km）.地壳结构在不同区域差异明显,陆壳区沉积层厚度横向差异大,且速度横向不均匀,地壳整体厚度大（约20 km）,有横向速度差;洋陆过渡区速度和厚度横向均匀,地壳减薄（约8 km）;洋壳区地壳厚度减薄至6 km.与以往研究相比,新的认识集中在两个方面,（1）在方法上,综合广角地震和多道地震数据,借助正演和反演方法,能够得到更多更可靠的地壳结构信息.（2）在地壳结构上,结合广角地震与多道地震,得到洋陆过渡区莫霍面向海减薄的形态及其埋深（约12~18 km,海平面为0 km）;进一步验证礼乐滩区域在洋陆过渡区没有明显的高速层,为非火山型陆缘,其共轭扩张点为中沙地块;陆壳区上地壳强烈的拉张作用在速度模型表现出横向速度异常和低速区,在多道地震剖面上表现为大量10~20 km的正断层. 相似文献
9.
Global scale patterns of continental fragmentation: Wilson's cycles as a constraint for long-term sea-level changes 总被引：1，自引：0，他引：1
We propose to characterize land–ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian. 相似文献
10.
南海经历了中生代主动大陆边缘到新生代被动大陆边缘的转换,其岩石层地球物理场具有明显的块、带特征.本文通过综合分析南海地区深地震探测、面波层析成像、重磁异常以及地热与岩石层流变学等各种地质地球物理资料,对南海地壳及岩石层的综合地球物理特征进行了深入总结,发现深地震探测剖面所确定的洋、陆壳转换位置与空间重力异常梯级带位置较为一致,据此拟定了南海洋、陆壳的转换边界;依据多条地壳结构剖面中拉张减薄的程度确定了正常减薄陆壳、洋陆壳过渡带及洋壳等属性特征,并初步圈定了南海下地壳高速层的分布范围.对比分析了南、北陆缘地壳结构及其拉张减薄的变化特征,从综合地球物理特征的相似性上推测了北部陆缘的中西沙陆块与南部陆缘的南沙礼乐滩陆块具有共轭对称性.依据S波速度梯度变化确定了南海岩石层厚度分布情况,揭示出南海北部陆缘存在一条岩石层厚度的减薄带,且该减薄带与高热流带具有较好的一致性.在综合分析的基础上,以深地震探测剖面与重、磁异常变化的对应性为基础,划定了南海边界构造的位置. 相似文献
11.
Christopher G.A. Harrison 《Earth and Planetary Science Letters》1973,19(2):242-245
Analysis of published data on sea floor spreading for the different oceans demonstrates a close correlation between interruptions of spreading at sea and compressive periods on land and between periods of spreading activity and periods of “no compression” of the orogenic regions. The evolution of both orogenic and oceanic areas appears to be rhythmic. The model is generalized to a dynamic model for the Earth's crust in which periods of global compression and extension follow alternately. Such a model fits better the geological evidences from orogenic regions than the present model for sea floor spreading which postulates an expansion in the mid ocean ridges and a compression along the continental margins underthrusted by oceanic crust. 相似文献
12.
重-磁-震联合反演是获取地壳结构的重要方法.此次研究,我们主要基于全球最新的水深、重磁异常、沉积物厚度等数据,结合实测地震数据和前人研究成果,分析了中国海-西太平洋地区的莫霍面展布特征,并利用重磁震联合反演方法获得了跨越中国海-西太平洋典型剖面的地壳结构和异常体分布,揭示了陆壳到洋壳的典型变化规律.结果表明,从浙江地区到马里亚纳俯冲带,地壳结构大致呈现由厚到薄、由老到新、由复杂到简单的特征.浙江地区（扬子块体和华夏块体）地壳结构复杂,三层结构明显,地壳内断裂带发育,并伴有广泛的岩浆侵入;东海地区莫霍面起伏剧烈,地壳厚度变化较大,冲绳海槽地壳明显减薄,是其过渡壳性质的体现;西菲律宾海盆、九州-帕劳海脊、帕里西维拉海盆、马里亚纳俯冲带等构造单元地壳结构相对简单,二层结构明显.其中,西菲律宾海盆和帕里西维拉海盆地壳内部磁异常变化较为剧烈,海盆扩张过程中形成的磁异常体分布广泛,地壳厚度（5~8 km）明显小于陆壳;九州-帕劳海脊地壳厚度可达~20 km,缺失中地壳,表现为岛弧地壳结构;同源的西马里亚纳岛弧和东马里亚纳火山弧地壳结构相似,浅层磁异常体分布广泛,西马里亚纳岛弧地壳厚度（~17 km）略小于东马里亚纳火山弧（~20 km）,体现了裂离的不对称性;马里亚纳海槽具有正常的洋壳结构（~7 km）,但扩张中心未发生明显破裂.对比各构造单元地壳结构的异同点,我们进一步认识到,陆壳与洋壳之间不是孤立的,陆壳可能会演化出洋壳的结构或组分,板块的演化总是处于动态循环过程中.此研究加深了我们对中国海-西太平洋深部构造特征的整体理解,促进了我们对大陆边缘演化与板块相互作用的认识,深化了我国管辖海域及邻近地区的基础地质调查. 相似文献
13.
西南印度洋中脊为典型的超慢速扩张洋中脊,其岩浆补给具有不均匀分布的特征.洋壳厚度是洋中脊和热点岩浆补给的综合反映,因此反演洋壳厚度是研究大尺度洋中脊和洋盆岩浆补给过程的一种有效方法.本文通过对全球公开的自由空气重力异常、水深、沉积物厚度和洋壳年龄数据处理得到剩余地幔布格重力异常,并反演西南印度洋地区洋壳厚度,定量地分析了西南印度洋的洋壳厚度分布及其岩浆补给特征.研究发现,西南印度洋洋壳平均厚度7.5 km,但变化较大,标准差可达3.5 km,洋壳厚度的频率分布具有双峰式的混合偏态分布特征.通过分离双峰统计的结果,将西南印度洋洋壳厚度分为0~4.8 km的薄洋壳、4.8~9.8 km的正常洋壳和9.8~24 km的厚洋壳三种类型,洋中脊地区按洋壳厚度变化特征可划分为7个洋脊段.西南印度洋地区薄洋壳受转换断层控制明显,转换断层位移量越大,引起的洋壳减薄厚度越大,减薄范围与转换断层位移量不存在明显相关性.厚洋壳主要受控于该区众多的热点活动,其中布维热点、马里昂热点和克洛泽热点的影响范围分别约340 km,550 km和900 km.Andrew Bain转换断层北部外角形成厚的洋壳,具有与快速扩张洋中脊相似的转换断层厚洋壳特征. 相似文献
14.
R.A. Edwards T.A. Minshull E.R. Flueh C. Kopp 《Earth and Planetary Science Letters》2008,272(1-2):437-445
The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere. 相似文献
15.
Thomas K. Nielsen Hans Christian Larsen John R. Hopper 《Earth and Planetary Science Letters》2002,200(3-4):271-286
We present new and reprocessed seismic reflection data from the area where the southeast and southwest Greenland margins intersected to form a triple junction south of Greenland in the early Tertiary. During breakup at 56 Ma, thick igneous crust was accreted along the entire 1300-km-long southeast Greenland margin from the Greenland Iceland Ridge to, and possibly 100 km beyond, the triple junction into the Labrador Sea. However, highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest Greenland margin at the same time. Thus, a transition from a volcanic to a non-volcanic margin over only 100–200 km is observed. Magmatism related to the impact of the Iceland plume below the North Atlantic around 61 Ma is known from central-west and southeast Greenland. The new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction. The extent of initial plume-related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere. Plume mantle is channelled to great distances provided that significant melting does not take place. Melting causes cooling and dehydration of the plume mantle. The associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift. We further suggest that thick Archaean lithosphere blocked direct flow of plume material into the magma-starved southwest Greenland margin while the plume was free to flow into the central west and east Greenland margins. The model is consistent with a plume layer that is only moderately hotter, 100–200°C, than ambient mantle temperature, and has a thickness comparable to lithospheric thickness variations, 50–100 km. Lithospheric architecture, the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism. 相似文献
16.
M.J. Bickle 《Earth and Planetary Science Letters》1986,80(3-4):314-324
The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures. 相似文献
17.
Variation of crustal thickness in the Philippine Sea deduced from three-dimensional gravity modeling
Abstract Crustal thickness of the northern to central Philippine Sea was gravimetrically determined on the simple assumption of four layers: seawater, sediments, crust and lithospheric mantle, with densities of 1030, 2300, 2800 and 3300 kg/m3 , respectively. As for the correction of the regional gravity variation, a 15 km difference of the lithospheric thickness with a density difference of 50 kg/m3 against the asthenosphere below between both sides of the Kyushu-Palau Ridge was taken into consideration. Mantle Bouguer anomalies were calculated on the assumption of constant crustal thickness of 6 km, and then the crustal thickness was obtained by three-dimensional gravity inversion method. The results show occurrence of thin crust areas with a thickness of approximately 5 km in the southern part and at the western margin of the Shikoku Basin and also of thick crust areas in the northwestern and northeastern parts of the Parece Vela Basin. We suggest that these are because of the variation of magma supply at the time of sea floor spreading in the Shikoku and Parece Vela Basins, which is possibly related to the variation of spreading rate and enhanced magmatism near the past arc volcanic fronts. The results further show the occurrence of crust thinner than 5 km in the northeastern part of the West Philippine Basin, of crust thicker than 15 km in the Amami Plateau, the Daito and Oki-Daito Ridges, and also in the northern part of Kyushu-Palau Ridge, whereas the southern part of the Kyushu-Palau Ridge the crust is thicker than 10 km. It was also inferred that small basins in the Daito Ridge province have the thinnest oceanic crust of less than 5 km in the Kita-Daito Basin. 相似文献
18.
汕头－吕宋岛岩石圈速度结构与震源构造──台湾浅滩7．3级地震构造之一 总被引：1，自引：0，他引：1
汕头－吕宋岛岩石圈速度结构剖面，划分出华南陆缘古生代陆壳、陆架区晚古生代－中生代陆壳、陆坡带中生代－早第三纪过渡壳、新生代南海海盆洋壳及吕宋岛中生代－新生代岛弧陆壳与东吕宋海槽洋壳等地壳构造组分，并确定了上述地壳构造之间的边界断裂构造及其性质。结合地震震源分布及机制，初步确定了华南陆架盆岭构造带北、南两侧地震构造的控震构造与发震构造性质及其震源力学特征；１）指出１９９４年９月１６日台湾浅滩７．３级地震属于板缘壳幔地震及造成一千公里有感范围的原因；２）马尼拉海沟的海底地堑构造与南海海盆岩石圈地幔上隆是马尼拉海沟俯冲带震源显示正断层性质的原因，且为被动的或转换俯冲带；３）东吕宋海槽仍属于菲律宾海俯冲带性质；吕宋岛东西两侧俯冲带岩石圈板片震源深度的准三层分布，可能表明俯冲带岩石圈板片存在相应的低速滑移层。 相似文献
19.
华南陆缘是我国重要的矿产、地热资源区.晚中生代以来,在太平洋板块西向俯冲,地幔热对流活动共同作用下,该区出现多期岩浆-热事件和大规模爆发式成矿作用.在前人研究基础上,本文利用地表热流观测资料、地震剪切波资料、重力位球谐系数,计算了壳-幔温度结构,分析了动力学背景.计算结果表明：华南陆缘东南沿海地带,地壳10 km以浅温度达200℃以上,居里点温度475℃,莫霍面平均温度550℃.地壳浅层较热,花岗岩中放射性元素衰变放热是地壳浅层地下水热活动的重要热源,但地壳总体温度不高,为"冷壳热幔"型热结构.地幔中,90 km深度,温度950~1250℃;120 km深度,温度1050~1400℃;150 km深度,温度1200~1450℃;220 km深度,温度1500~1700℃."热"岩石圈底界深度在110~150 km之间,西深东浅.岩石圈内,地幔应力场为挤压-伸展相间格局;岩石圈之下,地幔应力场为一个以南昌为中心、长轴NE-SW向的椭圆.分析认为,晚中生代以来,太平洋板块的西向俯冲,导致华南陆缘在区域性SE向地幔对流背景上叠加局域性不稳定热扰动,在175~85Ma期间,上地幔物质向上流动,形成不同的岩浆活动高峰期.同时,岩石圈地幔受俯冲洋壳流体的影响,含水量高,黏度小,在地幔流切向应力场作用下,岩石圈底界由西向东"波浪"状减薄.现今岩石圈之下仍具备地幔小尺度热对流温度条件,但除地表浅层外,地壳整体温度不高,岩石圈构造稳定. 相似文献
20.
The Southern Granulite Terrain of India, formed through an ancient continental collision and uplift of the earth’s surface,
was accompanied by thickening of the crust. Once the active tectonism ceased, the buoyancy of these deep crustal roots must
have supported the Nilgiri and Palani-Cardamom hills. Here, the gravity field has been utilized to provide new constraints
on how the force of buoyancy maintains the state of isostasy in the Southern Granulite Terrain. Isostatic calculations show
that the seismically derived crustal thickness of 43–44 km in the Southern Granulite Terrain is on average 7–8 km more than
that required to isostatically balance the present-day topography. This difference cannot be solely explained applying a constant
shift in the mean sea level crustal thickness of 32 km. The isostatic analysis thus indicates that the current topography
of the Southern Granulite Terrain is overcompensated, and about 1.0 km of the topographic load must have been eroded from
this region without any isostatic readjustment. The observed gravity anomaly, an order of magnitude lower than that expected
(−125 mGal), however, shows that there is no such overcompensation. Thermal perturbations up to Pan-African, present-day high
mantle heat flow and low Te together negate the possible resistance of the lithosphere to rebound in response to erosional
unloading. To isostatically compensate the crustal root, compatible to seismic Moho, a band of high density (2,930 kg m−3) in the lower crust and low density (3,210 kg m−3) in the lithospheric mantle below the Southern Granulite Terrain is needed. A relatively denser crust due to two distinct
episodes of metamorphic phase transitions at 2.5 Ga and 550 Ma and highly mobilized upper mantle during Pan-African thermal
perturbation reduced significantly the root buoyancy that kept the crust pulled downward in response to the eroded topography. 相似文献