Crustal structure of the Indian subcontinent

Studies carried out by various investigators up to 1971 to delineate the Indian crustal structure using body wave travel times, surface wave dispersion and gravity methods are summarised and reviewed. The average crustal thickness is found to be 35–40 km in the Indian peninsular shield, 30–35 km in the Indo Gangetic plains and 60–80 km in the Himalayas and the Tibetan plateau region. The limitations of the various methods used and the errors in the estimation of crustal thickness by them are discussed. As no deep refraction work for crustal studies has been carried out so far in India, this topic is not covered in this study.     

Tectonic implications of the earthquakes in the Indian subcontinent

The Bhuj earthquake (26 January 2001) in India and the Ghori earthquake (8 October 2005) in Pakistan, both occurred close to the Indian-Iranian plate boundary related to the activity along the intercontinental Chaman transform fault. It is suggested that the seismic activity along NNW — NNE trending weak zones or faults is more intense in the sub-continent than along the WNW trending zones. Since the stress along the former is less compressive but more of the shear or translational type. The devastative Koyna (1967) and Latur (1993) earthquakes both occurred along faults or weak zones that were close to the meridional rather than the equatorial trend. The Indian plate is moving to the north or NNE or NNW, along a rotational trajectory and hence the force tends to be more compressive along the equatorial weak zones. In contrast, it tends to be less compressive and more of the shear or translational along the weak zones that are close to meridional trend. The seismic activity is therefore more intense along the weak zones with NNW to NNE trend than along the ENE to EW trending zones.     

Distribution of heavy metals in the core sediments of a tropical wetland system

Five sediment cores from the fresh water region of the Vembanad wetland system were studied for the trace element contents The average concentration of iron, manganese, nickel, copper, zinc, cadmium, lead, mercury and chromium were determined. The core samples were collected using gravity type corer, digested with a mixture of nitric acid and perchloric acid and analyzed by atomic absorption spectrophotometry. Heavy metals such as iron, copper, nickel and zinc reported enrichment towards the surface of the core sediment sample collected from the centre of the lake. Lead, cadmium and mercury showed uniform distribution through out the core. Quality of the sediments were evaluated based on sediment quality guidelines, pollution load index, sum of toxic units and with effect range low/effect range median and threshold effect level/probable effect level values of Environmental Protection Agency guidelines. The degree of contamination for each station was determined. The concentration of different heavy metals has been compared with the world average concentration of shale values. Results of the analysis showed that Vembanad lake is facing serious metal pollution with increased rate of deposition.     

Hydrochemical and isotopic investigation of groundwater of Al-Batin alluvial fan aquifer,Southern Iraq

Major ions and important trace elements in addition to δ¹⁸O and δ²H were analysed for 43 groundwater samples sampled from the Al-Batin alluvial fan aquifer, South Iraq. The most dominant ions (with respect to molarity) were: Na⁺ > Cl^? > SO₄ ^2? > Ca²⁺ > Mg²⁺ > NO₃ ^? > HCO₃ ^?, with total dissolved solids (TDS) averaging 7855 mg/L. High concentrations were found for the trace elements U, Mo, V, B, Sr, and Cr. This study suggests a hydraulic connection exists near the fan apex between the uppermost part of the Al-Batin aquifer and the underlying Dammam aquifer by means of the Abu-Jir fault system. Except for the effects of extensive irrigation, fertilizer use, and poorly maintained sewers, the groundwater chemistry is mainly controlled by geological processes such as dissolution of evaporites and the enrichment of dissolved ions as a result of the high evaporation and low recharge rate. Furthermore, it is shown that the Kuwaiti fuel–oil burning during Gulf War in 1991 contributed to the enrichment of V and Mo in the studied aquifer. The spatial distribution of most ions appears to generally increase from the south-west towards the north-east, in the direction of groundwater flow. The stable isotopes show heavier values in groundwater with a gradually increasing trend in the direction of groundwater flow due to the decreasing depth to groundwater and thus increasing of evaporation from both groundwater or irrigation return water. Additionally, the stable isotope signature suggests that rainfall from sources in the Arabian Gulf and the Arabian Sea is the major source of recharge for the Al-Batin aquifer. Except for two samples of groundwater, all samples were not suitable for potable use according to the WHO standards. Most of the groundwater is suitable for some agricultural purpose and for livestock water supply. Apart from the high salinity, boron represents the most critical element in the groundwater with respect to agricultural purposes.     

Hydrochemical and isotopic investigation of a sandstone aquifer groundwater in a semi arid region,El Ma El Abiod,Algeria

This work present results of the hydrochemical and isotopic studies on groundwater samples from the study area. Chemical and environmental isotope data are presented and discussed in terms of the origin of dissolved species and of groundwater. All of the investigated groundwater are categorized into two chemical types: low and relatively high mineralized waters type. Interpretation of chemical data, based on both thermodynamic calculations and stability diagrams, suggests that the chemical evolution of groundwater is primarily controlled by water-rock interactions. Interpretation of ¹⁸O and ²H suggests that the recharge of the investigated groundwater may result from differents mechanisms     

白洋淀湿地地下水系统水化学变化特征及演化模式

在气候变化和人类活动影响下探究区域地下水系统水化学特征及演化规律,对于揭示地下水循环变化、水资源安全与管理、区域生态环境保护等具有重要作用.本研究以白洋淀湿地地区为研究对象,通过对比分析地表水、地下水水化学特征及空间分布规律,研究白洋淀地区地下水系统水化学演化作用及分区模式.研究结果表明白洋淀地区地表水、地下水水化学具...     

Lower Miocene (Aquitanien-Burdigalian) palaeobiogeography of the Indian subcontinent

The paper analytically discusses certain palaeoecological criteria in the reconstruction of Lower Miocene (Aquitanian-Burdigalian) palaeobiogeography of the Indian subcontinent. The period is characterised by major marine transgressions and a prolific marine invertebrate and terrestrial vertebrate faunas. Faunal affinities of molluscs from western (Sind-Baluchistan) and eastern (Garo Hills) sectors are distinct and different. Molluscs of Garo Hills show affinity to Burmese species, whereas Sind-Baluchistan species shows similarity to that of the Mediterranean region. Marine molluscs resemble with the present-day taxa of the Arabian Sea and Bay of Bengal, where more than 35% species are common. This similarity is also evidenced by marine elasmobranchs, where approximately 33% species and 86% genera are similar to that of the Recent taxa. Terrestrial mammals show an intermingling of southern U.S.S.R. and African forms along with the presence of probably some groups belonging to the Indian subcontinent.Angiosperms are the dominant group. The Lower Miocene floras of the eastern and the western sectors are entirely different. The palm,Sabal major andPinus are reported from various localities. The climate in the western sector and South India was tropical, whereas in the eastern sector, it was variable.The Lower Miocene period is also marked by the contraction of Sindhu Sea, Bangla Sea and the Burmese Gulf southwards resulting in the final disappearance of the sea by the Pliocene.

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Zusammenfassung Die Paläobiogeographie des unteren Miocän (Aquitan-Burdigal) läßt sich auf dem indischen Subkontinent aus den palökologischen Daten rekonstruieren. Zu dieser Zeit gab es viele marine Invertebraten und terrestrische Vertebraten-Faunen, die große marine Transgression anzeigen. Die Mollusken des Westens (Find-Baluchistan) und Ostens (Garo-Gebirge) sind unterschiedlich. Die Mollusken des Garo-Gebirges sind mit den Spezies von Burma verwandt. Dagegen sind die Faunen von Sind-Baluchistan für die mediterrane Faunenprovinz typisch. Die marinen Mollusken sind verwandt mit den Faunen des Arabischen Meeres und der Bucht von Bengalen; etwa 35% der Spezies sind gleich. Von den marinen Elasmobranchien sind ungefähr 33 % Spezies und 86% Genera gleich wie man sie in den rezenten Faunen findet. Terrestrische Sänger zeigen eine Mischung von südlicher U.S.S.R. und Afrika mit einigen Teilen des Indischen Subkontinentes.In den Pflanzen-Gruppen herrschen die Angiospermen vor. Die Untermiocän-Flora der östlichen und westlichen Gebiete ist ganz unterschiedlich. Der Palme, dieSabal Major undPinus sind aus verschiedenen Lokalitäten bekannt. Das Klima im Westen und im Süden Indiens war tropisch, im Osten dagegen war es wechselfeucht.Das Untermiocän-Meer im Gebiet Sind, Bangladesh und Burma verschwindet vor dem Pliozän.

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Résumé Cet article analyse certains critères paléoécologiques dans la reconstitution de la paléogéographie du Miocène inférieur (Aquitanien-Burdigalien) de l'Inde subcontinentale. La période est caractérisée par de grandes transgressions marines et par des faunes abondantes d'invertébrés marins et de vertébrés terrestres. Les affinités fauniques des mollusques sont différentes dans les secteurs occidentaux (Sind-Baluchistan) et orientaux (Garo Hills). Les mollusques des Garo Hills montrent des affinités avec les espèces de Burma, tandis que les espèces de Sind-Béloutchistan montrent des similitudes avec celles de la région méditerranéenne. Les mollusques marins ont des ressemblances avec les taxons actuels de la Mer d'Arabie et du Golfe du Bengal, où plus de 35% des espèces sont les mÊmes. Cette similarité vaut aussi pour les élasmobranches marin, dont 33% des espèces et 86% des genres ressemblent aux taxons récents. Les mammifères terrestres montrent un mélange de formes du sud de l'U.S.S.R. et de formes africaines, allant de pair avec la présence de certains groupes probables appartenant au sous-continent indien.Les angiospermes forment le groupe dominant. Les formes du Miocène inférieur des secteurs orientaux et occidentaux sont entièrement différentes. Le palmier, le grand Sabal et le pin, sont connus dans différentes localités. Le climat dans le secteur occidental et dans l'Inde méridionale était tropical, tandis que dans le secteur oriental, il était variable.La période du Miocène inférieur est également marquée par le rétrécissement de la Mer de Sindhu, de la Mer de Bangla et du Golfe de Burma vers le sud, avec comme résultat la disparition finale de la mer au Pliocène.

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(Aquitanian Nurdigalian) . . (Sind- Beluchistan) (Garo Hills) . , . : 35% . Elasmobranchs 33 % 86 % . , . . , Sabal major . , , , , , — . Sindhu, .

     

Hydrochemical and stable isotopic investigation of groundwater quality and its sustainability for irrigation in the Hammamet-Nabeul basin,northeastern Tunisia

The major ion hydrochemistry, sodium absorption ratio (SAR), sodium percentage, and isotopic signatures of Hammamet-Nabeul groundwaters were used to identify the processes that control the mineralization, irrigation suitability, and origin of different water bodies. This investigation highlights that groundwater mineralization is mainly influenced by water-rock interaction and pollution by the return flow of irrigation water. The comparison of groundwater quality with irrigation suitability standards proves that most parts of groundwater are unacceptable for irrigation and this long-term practice may result in a significant increase of the salinity and alkalinity in the soils. Based on isotopic signatures, the shallow aquifer groundwater samples were classified into (i) waters with depleted δ¹⁸O and δ²H contents, highlighting recharge by modern precipitation, and (ii) waters with enriched stable isotope contents, reflecting the significance of recharge by contaminated water derived from the return flow of evaporated irrigation waters. The deep-aquifer groundwater samples were also classified into (i) waters with relatively enriched isotope contents derived from modern recharge and mixed with shallow-aquifer groundwater and (ii) waters with depleted stable isotope contents reflecting a paleoclimatic origin. Tritium data permit to identify three origins of recharge, i.e., contemporaneous, post-nuclear, and pre-nuclear. Carbon-14 activities demonstrate the existence of old paleoclimatic recharge related to the Holocene and Late Pleistocene humid periods.     

High resolution numerical weather prediction over the Indian subcontinent

In this study, the Florida State University Global Spectral Model (FSUGSM), in association with a high-resolution nested regional spectral model (FSUNRSM), is used for short-range weather forecasts over the Indian domain. Three-day forecasts for each day of August 1998 were performed using different versions of the FSUGSM and FSUNRSM and were compared with the observed fields (analysis) obtained from the European Center for Medium Range Weather Forecasts (ECMWF). The impact of physical initialization (a procedure that assimilates observed rain rates into the model atmosphere through a set of reverse algorithms) on rainfall forecasts was examined in detail. A very high nowcasting skill for precipitation is obtained through the use of high-resolution physical initialization applied at the regional model level. Higher skills in wind and precipitation forecasts over the Indian summer monsoon region are achieved using this version of the regional model with physical initialization. A relatively new concept, called the ‘multimodel/multianalysis superensemble’ is described in this paper and is applied for the wind and precipitation forecasts over the Indian subcontinent. Large improvement in forecast skills of wind at 850 hPa level over the Indian subcontinent is shown possible through the use of the multimodel superensemble. The multianalysis superensemble approach that uses the latest satellite data from the Tropical Rainfall Measuring Mission (TRMM) and the Defense Meteorological Satellite Program (DMSP) has shown significant improvement in the skills of precipitation forecasts over the Indian monsoon region.     

Hydrochemical and isotopic study of groundwater in the Yinchuan plain, China

The Yinchuan plain is located in the arid climate zone of NW China. The western margin of the plain is the Helan mountain connecting a series of normal slip faults. The eastern margin of the plain connects with the Yellow River and adjacents with the Ordos platform. The south of the plain is bordered by the EN fault of the Niushou mountain. The bottom of the plain is the Carboniferous, Permian, or Ordovician rocks. Based on the analysis of groundwater hydrochemical and isotopic indicators, this study aims to identify the groundwater recharge and discharge in the Yinchuan plain, China. The hydrochemical types of the groundwater are HCO₃–SO₄ in the west, HCO₃–Cl in the middle, and Cl–SO₄ in the east. The hydrochemical types are HCO₃–SO₄ in the south, HCO₃–Cl and SO₄–HCO₃ in the middle. The hydrochemical types are complex in the north, mainly SO₄–HCO₃ and Cl–SO₄. Deuterium, ¹⁸O, and tritium values of groundwater indicate that groundwater recharge sources include precipitation, bedrock fissure water, and irrigation return water. Groundwater discharges include evaporation, abstraction, and discharge to surface water. According to the EW isotopic profile, the groundwater flow system (GFS) in the Yinchuan plain can be divided into local flow systems (LFS) and regional flow systems (RFS). Groundwater has lower TDS and higher tritium in the southern Yellow River alluvial plain and groundwater age ranges from 6 to 25 years. The range of groundwater renewal rates is from 11 to 15 % a^?1. The depth of the water cycle is small, and groundwater circulates fast and has high renewal rates. Groundwater has higher TDS and lower tritium in the northern Yellow River alluvial plain. The range of groundwater age is from 45 to 57 years, and renewal rate is from 6 to 0.1 % a^?1. The depth of the water cycle is larger. Groundwater circulates slowly and has low renewal rates.     

北京市泉水的水化学、同位素特征及其指示作用

对北京市泉水的水温、溶解氧、电导率、氧化还原电位、pH值、总溶解固体等进行了现场检测,对泉水中的K+、Na+、Ca2+、Mg2+、Cl-、SO42-、HCO32-、NO3-离子,D、18O组成和Sr、87Sr/86Sr比值等进行了实验室检测,对泉水按成因进行了分类,并与30年前的泉水普查资料作了对比。调查表明,曾以泉多著称的北京市,1980s初存在的几乎全部二类泉和60%的一类泉如今已消失,仅存的13眼泉的出水量也明显减小,这与降水量的减少和大规模开采地下水有关。水质分析表明,现在绝大部分泉水水质良好,物理和水化学指标与30年前相比变化甚微,明显没有受人类活动的污染;泉水仅在浅部循环,在地下滞留时间较短,更新能力强;泉水接受大气降水补给,与浅层地下水联系密切,构成了统一的地下水系统;泉水中的Sr为碳酸盐岩风化来源。     

北京市泉水的水化学、同位素特征及其指示作用

对北京市泉水的水温、溶解氧、电导率、氧化还原电位、pH值、总溶解固体等进行了现场检测,对泉水中的K+、Na+、Ca2+、Mg2+、Cl-、SO42-、HCO32-、NO3-离子,D、18O组成和Sr、87Sr/86Sr比值等进行了实验室检测,对泉水按成因进行了分类,并与30年前的泉水普查资料作了对比。调查表明,曾以泉多著称的北京市,1980s初存在的几乎全部二类泉和60%的一类泉如今已消失,仅存的13眼泉的出水量也明显减小,这与降水量的减少和大规模开采地下水有关。水质分析表明,现在绝大部分泉水水质良好,物理和水化学指标与30年前相比变化甚微,明显没有受人类活动的污染;泉水仅在浅部循环,在地下滞留时间较短,更新能力强;泉水接受大气降水补给,与浅层地下水联系密切,构成了统一的地下水系统;泉水中的Sr为碳酸盐岩风化来源。     

Numerical simulation of present day tectonic stress across the Indian subcontinent

International Journal of Earth Sciences - In situ measurements of maximum horizontal stress (SHmax) in the Indian subcontinent are limited and do not present regional trends of intraplate stress...     

西藏日喀则市查孜地热田水化学及同位素特征研究

查孜地热田位于青藏高原西南部。通过野外地质调查及地热钻孔揭露,发现该地热田具有较好的地热资源开发潜力。对该地热田地下热水的水文地球化学及同位素特征开展研究,发现地下热水为HCO₃-Na型; 热水与冷水的离子浓度存在差异,显示二者具有不同的物质来源,但又具有一定的水力联系。热水中的δD和δ¹⁸O同位素特征表明: 该地热田地下热水的主要补给来源为大气降水和冰雪融水,补给海拔为5 652 m以上; 大气降水和冰雪融水下渗并与沿断裂破碎带向上运移的地热流体混合后形成地下热水。断裂破碎带不仅是温泉的主要通道,也是地热流体的储集场所,地热田热水在地下运移滞留至少41 a。据SiO₂地热温标估算得出,该区地下热储温度为148.18 ~153.49 ℃,天然放热量为2 264.33×10¹² J/a。     

Hydrochemical and isotopic characteristics of Chazi geothermal field in Shigatse in Tibet

Chazi geothermal field is located in Southwestern Tibetan Plateau. The geothermal potential has been ascertained by field survey and geothermal drilling. The hydrogeochemical characteristics and isotopic composition of this geothermal field show that the underground water belongs to HCO₃-Na. The difference of ion concentration between hot water and cold water shows that they have different material sources and certain hydraulic relations. The isotope analysis of δD and δ¹⁸O determines that the major source of the geothermal water in this area is meteoric water and water melt from the mountains snow and ice with the height above 5 652 m. The geothermal water was the result of the mixture of deep infiltrated meteoric water and deep-source fluid when they move along the fracture zone. The fracture zone is the main channel of hot spring and the reservoir of geothermal fluid. The migration retention time of the geothermal water in this geothermal field was at least 41 years. According to the calculated temperature of SiO₂ geothermometer, the geothermal temperature of the underground heat reservoir is about 148.18~153.49 ℃, and natural heat discharge is 2 264.33×10¹² J/a.     

Hydrochemical and isotopic characteristics of groundwater in the Souss Upstream Basin, southwestern Morocco

Heterogeneous shallow Plio-Quaternary formations of the Souss Plain represent the most important aquifer in southern High Atlas Mountains in Morocco. The present work was conducted in the Souss Upstream Basin to identify the chemical characteristics and the origin of groundwater in an aquifer under semi-arid climate. Isotopic and hydrochemical compositions combined with geological and hydrogeological data were used for this purpose. The total dissolved solids vary from 239 to 997 mg l⁻¹, and the following groundwater types are recognized: Ca²⁺–Mg²⁺–HCO₃⁻, Ca²⁺–Mg²⁺–SO₄²⁻ and Ca²⁺–Mg²⁺–Cl⁻. The groundwater is saturated and slightly supersaturated with respect to carbonate minerals and undersaturated with respect to evaporite minerals, which means that the groundwater composition is largely controlled by the dissolution of carbonate rocks known in the basin. The isotopic contents of groundwaters ranged from −8‰ to −5.2‰ for δ¹⁸O, from −52‰ to −34‰ for δD, and from 0 to 5.5 TU for tritium. The hydrogen (δD) and oxygen (δ¹⁸O) isotope signatures reveal a significant infiltration before evaporation takes place, indicating a major recharge directly from fractures in the crystalline and limestone formations of Atlas Mountains (above 800 m a.s.l.) and infiltration of surface water in the alluvial cones at the border of the Atlas basins. The very low tritium values suggest that the groundwater recharge follows a long flow path and a mixing between old and modern water is shown. However, a slight evaporation effect is noted in the southern part of the basin close to the Anti-Atlas Mountains.     

Hydrochemical and isotopic characteristics of groundwater in the northeastern Tennger Desert,northern China

Groundwater is typically the only water source in arid regions, and its circulation processes should be better understood for rational resource exploitation. Stable isotopes and major ions were investigated in the northeastern Tengger Desert, northern China, to gain insights into groundwater recharge and evolution. In the northern mountains, Quaternary unconsolidated sediments, exposed only in valleys between hills, form the main aquifer, which is mainly made of aeolian sand and gravel. Most of the mountain groundwater samples plot along the local meteoric water line (LMWL), with a more depleted signature compared to summer precipitation, suggesting that mountain groundwater was recharged by local precipitation during winter. Most of the groundwater was fresh, with total dissolved solids less than 1 g/L; dominant ions are Na⁺, SO₄ ^2? and Cl^?, and all mineral saturation indices are less than zero. Evaporation, dissolution and cation exchange are the major hydrogeochemical processes. In the southern plains, however, the main aquifers are sandstone. The linear regression line of δD and δ ¹⁸O of groundwater parallels the LMWL but the intercept is lower, indicating that groundwater in the plains has been recharged by ancient precipitation rather than modern. Both calcite and dolomite phases in the plains groundwater are close to saturation, while gypsum and halite can still be dissolved into the groundwater. Different recharge mechanisms occur in the northern mountains and the southern plains, and the hydraulic connection between them is weak. Because of the limited recharge, groundwater exploitation should be limited as much as possible.     

Geochemical and isotopic investigation of the aquifer system in the Djerid-Nefzaoua basin, southern Tunisia

In the Djerid-Nefzaoua region, southern Tunisia, about 80% of agricultural and domestic water supply is provided by the complex terminal (CT) aquifer. However, 20% of this demand is provided by other hydraulically connected aquifers, namely the continental intercalaire (CI) and the Plio-Quaternary (PQ). Overexploitation of the CT aquifer for agricultural practices has contributed to the loss of the artesian condition and the decline of groundwater level which largely increased the downward leakage from the shallow PQ aquifer. Excess irrigation water concentrates at different rates in the irrigation channels and in the PQ aquifer itself. Then, it returns to the CT aquifer and mixes with water from the regional flow system, which contributes to the salinization of the CT groundwater. A geochemical and isotopic study had been undertaken over a 2-years period in order to investigate the origin of waters pumped from the CT aquifer with an emphasis on its hydraulic relationships with the underlying and the overlying CI and PQ aquifers. Geochemistry indicates that groundwater samples collected from different wells show an evolution of the water types from Na-Cl to Ca-SO₄-Cl. Dissolution of halite, gypsum and anhydrite-bearing rocks is the main mechanism that leads to the salinization of the groundwater. Isotopic data indicate the old origin of all groundwater in the aquifer system. Mixing and evaporation effects characterizing the CT and the PQ aquifers were identified using δ²H and δ¹⁸O relationship and confirmed by the conjunction of δ²H with chloride concentration.     

印度陆块新生代两次仰冲事件及其构造驱动机制:论印度洋、特提斯和欧亚板块相互作用

印度陆块与欧亚大陆的碰撞是印度洋扩张和特提斯洋闭合综合作用的结果。本文通过综合分析和研究提出这3个板块的相互作用致使印度陆块发生过2次向北的仰冲:早期(古新世末-始新世初,~57Ma)仰冲受其超高速运动(140mm/yr)的驱动,与特提斯之间产生的速度差致使两者间的边界发生破裂,密度小的印度陆块沿印度洋东经90°海岭和马尔代夫岛链向北仰冲到特提斯洋壳之上,两者的叠加导致印度陆块北缘——特提斯喜马拉雅地壳增厚(~70km)并且沉积了一套造山磨拉石——柳曲砾岩;晚期(渐新世-中新世之交,~25Ma)仰冲发生在碰撞后,由于高喜马拉雅结晶岩系沿主中央冲断带和藏南拆离断裂发生的垂向挤出,位于上盘的特提斯喜马拉雅沉积盖层同时发生重力垮塌,沿大喜马拉雅反冲断裂仰冲到冈底斯岩浆岩带之上并且造成后者的隆升和前陆下陷,其北缘充填了一套造山磨拉石沉积——大竹卡砾岩。这两次构造事件均受印度陆块的快速运动驱动。此外,在印度陆块超高速运动的挤压下,特提斯洋可能在早白垩世之后就停止了扩张,而老的洋壳不是俯冲消减了就是被仰冲的印度陆块掩盖了,这解释了为什么雅鲁藏布江缝合带只存早白垩世蛇绿岩。印度洋内东经90°海岭和马尔代夫岛链构成印度陆块的南东和南西边界,前者呈右行走滑,后者呈左行走滑,两者勾画出印度陆块向北漂移的轨迹。