Reflected and refracted waves from a linear transition layer     

Miss Wadhwa  Santosh Kumari 《Pure and Applied Geophysics》1971,89(1):45-66

Summary The problem of a transition layer lying between two homogeneous liquid media is discussed. After obtaining the formal solution for a periodic point source lying in the upper layer, the integrand in the expression for the displacement potential of the upper layer is expanded into series of negative powers of exponentials. Some of the terms in the Bromwich expansion are then evaluated along the Sommerfeld loops which give various reflected and refracted waves. The results for the refracted waves are discussed for the two extreme cases, when frequency is extremely low and extremely high. In both the cases it is found that the frequency dependence for refraction arrivals is the same as expected from a sharp boundary, viz., ^–1. And that for high frequency the travel-time of refraction arrivals is the same as expected from geometric ray theory. Both first- order and second-order discontinuities in density and bulk modulus are considered at the boundaries of the transition layer.  相似文献   

The disturbance due to a point source in a homogeneous liquid layer over a heterogeneous liquid half-space     

Miss Santosh Kumari Wadhwa 《Pure and Applied Geophysics》1971,89(1):67-83

Summary The problem of a periodic point source in a homogeneous liquid layer overlying a heterogeneous liquid half-space is discussed. After obtaining the formal solution, the path of integration for the displacement potential of the layer is transformed from the positive real axis to the positive imaginary axis and the Sommerfeld contour and the latter is further distorted to the modified Sommerfeld contour. The residues of the integrand at the poles contained within the Sommerfeld loop constitute the normal mode solution to the problem. The integrands in the expressions for the integrals along the imaginary axis are expanded in a series of negative powers of exponentials and then some of the terms in these expansions are evaluated approximately. This gives various waves reflected from the interface and the integral along the Sommerfeld loop vanishes. The frequency equation is obtained, also by the principle of constructive interference. An expression for the reflection coefficient at an interface of two liquid media, the upper medium being homogeneous and the lower one inhomogeneous, is obtained.  相似文献   

Early cretaceous igneous activities in the north flank of the North China Craton: the Shouwangfen complex example     

Pengsheng Dong  M. Santosh  Zhuanrong Sun  Huawei Li  Shushu Wang 《International Geology Review》2020,62(6):714-739

ABSTRACT

The Early Cretaceous igneous complexes in the north flank of the North China Craton (NCC) provide a window to investigate the Mesozoic magmatism in the NCC. Here we report the precise timing of Early Cretaceous magmatism and magma petrogenesis of the different rock types in the north flank of the NCC based on petrology, mineral chemistry, geochemistry, zircon geochronology and Sr-Nd isotopes. Zircon U-Pb dating reveals that the Shouwangfen complex was crystallized at 130–128 Ma. The rocks display enrichment in large ion lithophile elements (LILE) and LREE, and depletion in high field strength elements (HFSE) and HREE, with relatively high Sr/Y and La/Yb values, typical of subduction-related magmatic rocks. The mafic microgranular enclaves show typical igneous textures, acicular apatites, sieve-texture of plagioclase phenocrysts and overgrowth of amphibole around the pyroxene, corresponding to magma mixing and mingling and thermal exchange. The quartz monzonites display normal zoning of the plagioclase, low and homogeneous εNd(t) values and linear co-variations in two-component diagrams. The quartz monzodiorites are characterized by high and homogeneous εNd(t) values. Petrologic feature and geochemical data suggest that the quartz monzonites represent lower crustal magma with minor contribution of enriched melt of the sub-continental lithospheric mantle (SCLM), followed by fractional crystallization. The mafic microgranular enclaves were sourced from enriched SCLM, followed by mixing and mingling with the host quartz monzonitic magma. The quartz monzodiorites are mainly SCLM-derived and subsequently mixed with lower crust melts. The high water contents (≥3%) of these Early Cretaceous igneous complexes suggests a hydrous SCLM beneath the NCC. The Early Cretaceous igneous complexes formed within an extensional tectonic setting which were related to the retreat and dehydration of the paleo-Pacific slab. The hydration of the lithosphere induced extensive crust-mantle interaction and large-scale water-rich magmatism, leading to lithospheric thinning in the NCC during the Mesozoic.  相似文献   

First Report of Sapphirine+Quartz Assemblage from Southern India: Implications for Ultrahigh-temperature Metamorphism   总被引：1，自引：0，他引：1  

Keiko Tateishi  Toshiaki Tsunogae  M. Santosh  A.S. Janardhan 《Gondwana Research》2004,7(4):899-912

We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet, (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al₂O₃), and (iii) in symplectitic intergrowth with orthopyroxene (Al₂O₃= 5.9–6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (X_Mg = 0.79–0.80) and low in Si content (1.55–1.56 pfu) as compared with those associated with orthopyroxene and cordierite (X_Mg= 0.77–0.79, Si = 1.59–1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>1050 °C peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al₂O₃ content of up to 9.7 wt.%, suggesting T = 1040–1060°C and P = 8 kbar. FMAS reaction of sapphirine+quartz→garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950–1000°C) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite (±sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.  相似文献   

Dunite, Glimmerite and Spinellite in Achankovil Shear Zone, South India: Implications for Highly Potassic CO₂-rich Melt Influx Along an Intra-continental Shear Zone     

V.J. Rajesh  M. Arima  M. Santosh 《Gondwana Research》2004,7(4):961-974

In the Kakkaponnu area within the Achankovil Shear Zone (ACSZ), southern India, an undeformed ultramafic body occurs within intensely deformed granulite facies metamorphic rocks of Pan-African age. The Kakkaponnu ultramafic body is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinel-glimmerite, and phlogopite-graphite-spinellite. The spinel-dunite is a fine- to medium-grained rock composed mainly of olivine and aluminous spinel and is characterized by relatively high MgO (50.39–50.90 wt.%), (Mg/ (Mg+Fe) = 0.95), Al₂O₃ (7.8–8.98 wt.%), and low Ni (10–14 ppm). The phlogopite-dunite comprises serpentinized olivine, phlogopite and subordinate amounts of dolomite and is high in MgO (36.5 wt.%), Mg# [(Mg/(Mg+Fe) = 0.97], and K₂O (%%5.5 wt.%). Olivine in the spinel-dunite is marked by unusually high MgO (Mg# = 0.96) and extremely low NiO (<0.14 wt.%). Spinels in all rock variants are highly aluminous with low Cr# [Cr/(Al+Cr)] ratio (<0.01). Magnesian ilmenite [Mg# = 59], rutile, zirconolite and baddeleyite are main accessory phases. No significant compositional variation is noted between large grains and small inclusions for all minerals. Abundant graphite, magnesite, melt and ubiquitous CO₂ fluid inclusions are identified in the olivine and spinel grains. The data imply that the Kakkaponnu ultramafic body was formed by progressive crystallization of highly potassic CO₂-rich melts injected into lower crustal levels. K-Ar ages of 470.5±9.3 and 464.5±9.2 Ma are obtained for phlogopite separates from glimmerite and phlogopite-dunite respectively. These ages are comparable to the phlogopite K-Ar ages reported from lithospheric shear zones in southern Madagascar, which was once conjugated to the Southern Peninsular India prior to the Gondwana breakup. This implies widespread highly potassic CO₂-rich fluid/melt influx along shear zones in this part of East Gondwana continent.  相似文献   

Geochemistry and petrogenetic evolution of the diatexites of Central Kerala,India     

P K Rajan  M Santosh  K K Ramachandran 《Journal of Earth System Science》1984,93(1):57-69

The hornblende-biotite gneisses of Central Kerala which cover approximately 490km² exhibit schlieric and nebulitic structures, tending towards a homophanous nature and are classified here as diatexites. Mafic protoliths and restite biotite, each representing the refractory residuum of two independent partial melting episodes are widely present in the gneisses. The general mineral assemblage of the gneisses comprise quartz, K-feldspar, oligoclase, biotite and hornblende. Chemically, they are dominantly adamellitic and the behaviour of major and trace elements is consistent with a magmatic parentage. Based on petrochemical criteria, a two-stage evolution model is proposed here, which involves (i) partial melting of mafic granulites under high Archean geothermal gradients and generation of tonalite/trondhjemite through amphibole and plagioclase fractionation and (ii) partial melting and subsequent quartz-alkali feldspar fractionation of the tonalite/trondhjemite under amphibolite facies conditions with synchronous K-enrichment resulting in the diatectic adamellites.  相似文献   

Air pollution potential over South India     

D. V. Viswanadham  K. R. Santosh 《Boundary-Layer Meteorology》1989,48(3):299-313

The potential of the atmosphere to disperse and dilute pollutants emitted into it by myriad sources, depends upon various factors such as wind, vertical mixing, inversion of temperature in the vertical, etc. A study of such parameters is attempted in the present article over south India. The spatial distribution of isothermals, inversions, lapse conditions, mixing heights and ventilation coefficients over south India is studied for the months of January, April, July and October, which are typical of winter, premonsoon, monsoon and post monsoon seasons, respectively. Diurnal variations are also studied.  相似文献   

The Sino-Korean Craton and supercontinent history: Problems and perspectives   总被引：3，自引：0，他引：3  

John J.W. Rogers  M. Santosh   《Gondwana Research》2006,9(1-2):21

The Sino-Korean Craton (SKC) is an enigmatic block in the history of supercontinents older than Pangea. Its accretion to Eurasia and the effect of the broad region of Mesozoic–Cenozoic extension in northern Eurasia that crosses the eastern part of the SKC are among several problems that need to be resolved in understanding the configuration of SKC and the overprinting of earlier histories in this block. We present a synopsis of these problems and perspectives for future research.  相似文献   

Geochronology of the khondalite belt of Trivandrum Block, Southern India: Electron probe ages and implications for Gondwana tectonics   总被引：6，自引：6，他引：6  

M. Santosh  T. Morimoto  Y. Tsutsumi 《Gondwana Research》2006,9(3):261-278

In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma.The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities.The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.  相似文献   

Ultrahigh-temperature metamorphism in the Achankovil Zone: Implications for the correlation of crustal blocks in southern India   总被引：1，自引：3，他引：1  

Shunsuke Ishii  Toshiaki Tsunogae  M. Santosh 《Gondwana Research》2006,10(1-2):99

The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and > 100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite + garnet + quartz + plagioclase + spinel + ilmenite + magnetite ± orthopyroxene ± biotite ± K-feldspar ± sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene + garnet, (2) perthite and/or anti-perthite, (3) cordierite ± orthopyroxene corona around garnet, and (4) cordierite + quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P–T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720 ± 60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone.  相似文献