Academic literature on the topic 'Seamount volcano'

AbstractFollowing more than 25 years of exploration and research since the last regional appraisal, the number of known subaerially exposed volcanoes in the northern Antarctic Peninsula region has more than trebled, from less than 15 to more than 50, and that total must be increased at least three-fold if seamounts in Bransfield Strait are included. Several volcanoes remain unvisited and there are relatively few detailed studies. The region includes Deception Island, the most prolific active volcano in Antarctica, and Mount Haddington, the largest volcano in Antarctica. The tectonic environment of the volcanism is more variable than elsewhere in Antarctica. Most of the volcanism is related to subduction. It includes very young ensialic marginal basin volcanism (Bransfield Strait), back-arc alkaline volcanism (James Ross Island Volcanic Group) and slab-window-related volcanism (seamount offshore of Anvers Island), as well as volcanism of uncertain origin (Anvers and Brabant islands; small volcanic centres on Livingston and Greenwich islands). Only ‘normal’ arc volcanism is not clearly represented, possibly because active subduction virtually ceased atc.4 Ma. The eruptive environment for the volcanism varied between subglacial, marine and subaerial but a subglacial setting is prominent, particularly in the James Ross Island Volcanic Group.

Abstract The Cook-Austral volcanic lineament extends from Macdonald Seamount (east) to Aitutaki Island (west) in the South Pacific Ocean and consists of hotspot-related volcanic islands, seamounts, and atolls. The Cook-Austral volcanic lineament has been characterized as multiple overlapping, age-progressive hotspot tracks generated by at least two mantle plumes, including the Arago and Macdonald plumes, which have fed volcano construction for ∼20 m.y. The Arago and Macdonald hotspot tracks are argued to have been active for at least 70 m.y. and to extend northwest of the Cook-Austral volcanic lineament into the Cretaceous-aged Tuvalu-Gilbert and Tokelau Island chains, respectively. Large gaps in sampling exist along the predicted hotspot tracks, complicating efforts seeking to show that the Arago and Macdonald hotspots have been continuous, long-lived sources of hotspot volcanism back into the Cretaceous. We present new major- and trace-element concentrations and radiogenic isotopes for three seamounts (Moki, Malulu, Dino) and one atoll (Rose), and new clinopyroxene 40Ar/39Ar ages for Rose (24.81 ± 1.02 Ma) and Moki (44.53 ± 10.05 Ma). All volcanoes are located in the poorly sampled region between the younger Cook-Austral and the older, Cretaceous portions of the Arago and Macdonald hotspot tracks. Absolute plate motion modeling indicates that the Rose and Moki volcanoes lie on or near the reconstructed traces of the Arago and Macdonald hotspots, respectively, and the 40Ar/39Ar ages for Rose and Moki align with the predicted age progression for the Arago (Rose) and Macdonald (Moki) hotspots, thereby linking the younger Cook-Austral and older Cretaceous portions of the long-lived (>70 m.y.) Arago and Macdonald hotspot tracks.

Ferromanganese crusts from four different areas of the North-West Pacific Ocean—the Detroit (northern part of the Imperial Ridge) guyot, the Zubov (Marshall Islands) guyot, the “Gummi Bear” seamount (an intraplate volcano near the Krusenstern FZ), and Belyaevsky volcano (the Sea of Japan)—were studied. Samples from the Detroit and Zubov guyots and the “Gummi Bear” seamount have similar chemical and mineral compositions of hydrogenetic cobalt-rich ferromanganese crusts. Crust from the Sea of Japan seems to reflect a hydrothermal influence. The gold content in most samples from the Detroit guyot was 68 ppb and from the Zubov guyot varied from 180 to 1390 ppb, which is higher than the average for the Pacific crusts (55 ppb). Gold content in two other samples was less than 10 ppb. Based on the electron microscopic studies, aggregation of gold particles with a size of 680 μm were identified in the Detroit guyot crust. The sizes of the Au particles are up to 10–15 μm, which has not been previously noted. Gold particles similar in morphology and size were also found in the Zubov guyot crust, which is located far from the Detroit guyot. The largest particle of gold (≈60 μm), represented by electrum, was found in the clay substrate from the “Gummi Bear” seamount. The lamellar, rudaceous morphology of the gold particles from the Detroit and Zubov guyots reflects their in situ formation, in contrast to the agglutinated, rounded with traces of dragging gold grain found in the substrate of the sample from the “Gummi Bear” seamount. Three-component (Ag-Au-Cu) gold particles were found in the hydrothermal crust from the Belyaevsky underwater volcano. Grains similar in composition were also found in Co-rich crust. The research results show that the gold was probably added to by hydrothermal fluid in the already-formed hydrogenetic ferromanganese crusts during rejuvenated volcanic stages. Biogeochemical processes may have played a major role in the formation of submicron solid-phase gold particles.

The rifting zone of Bransfield Strait, Antarctica, is tectonically and geologically unique. It is a back-arc basin that was opened by extensional forces associated to roll-back subduction after cessation of spreading activity of the Phoenix Ridge, and the transtension of the westward ending of Scotia-Antarctica Plate boundary. The Bransfield Rift/Ridge is still active generating volcanism or magma rise to force hydrothermal activity. During the ANT-XXV/4 cruise onboard R/V “Polarstern”, standard CTD and beam transmission measurements were done to determine temperature anomaly and turbidity. Water sampling was performed to determine δ3He and to find thermophilic microorganisms to examine the Orca seamount hydrothermal activity. A temperature anomaly of ~0.08 °C, a pick of turbidity, and high value of δ3He (>10%) were found inside Orca seamount. Results are consistent with a hydrothermal flux coming from the seamount. The report of the first observation of thermophilic and hyperthermophilic microorganisms in cold deep Antarctic waters is part of this study. Inside Orca seamount these microorganisms were found at three different depth levels close to the bottom. We suggest that the fluid migration from the volcano resulted from recent magmatic activity and provided the required elemental nutrients for microbial growth. Besides some thermophiles were found outside the seamount in a small quantity close to the seafloor. These would probably be related to subsidiary structures of the Orca seamount, or were transported by currents from other active volcanic sites as Deception Island. The finding of these thermophilic and hyperthermophilic microorganisms raise questions about the dispersal and their resistance in these extreme environments.

At the northern part of Emperor Seamount Chain we discovered and classified the previously unknown rounded volcano-tectonic 42-km deep-sea morphostructure as calderoida (authors terminology). It lies at -2900-2200 m depth and represents very complex structure being broken by faults into many blocks of different sizes and shapes. It is situated at the northern portion of the Detroit Seamount Rise (NW Pacific Ocean) which is a Pliocene horst. The calderoida was formed in the Early Pleistocene at depths from -3000 to -2000 m. Its analogues have not been found on Earth yet.

Abstract Magmatic systems are composed of melt accumulations and crystal mush that evolve with melt transport, contributing to igneous processes, volcano dynamics, and eruption triggering. Geophysical studies of active volcanoes have revealed details of shallow-level melt reservoirs, but little is known about fine-scale melt distribution at deeper levels dominated by crystal mush. Here, we present new seismic reflection images from Axial Seamount, northeastern Pacific Ocean, revealing a 3–5-km-wide conduit of vertically stacked melt lenses, with near-regular spacing of 300–450 m extending into the inferred mush zone of the mid-to-lower crust. This column of lenses underlies the shallowest melt-rich portion of the upper-crustal magma reservoir, where three dike intrusion and eruption events initiated. The pipe-like zone is similar in geometry and depth extent to the volcano inflation source modeled from geodetic records, and we infer that melt ascent by porous flow focused within the melt lens conduit led to the inflation-triggered eruptions. The multiple near-horizontal lenses are interpreted as melt-rich layers formed via mush compaction, an interpretation supported by one-dimensional numerical models of porous flow in a viscoelastic matrix.

This work studied the contribution of the geogenic sources volcanoes and fumaroles to the aerosol in marine atmosphere in the central Mediterranean basin. For this purpose, in the framework of the Med-Oceanor measurement program, we carried out a cruise campaign in the summer of 2017 to investigate the impact to the aerosol of the most important Mediterranean volcanoes (Mount Etna, Stromboli Island, and Marsili Seamount) and solfatara areas (Phlegraean Fields complex, Volcano Islands, Ischia Island, and Panarea submarine fumarole). We collected PM10 and PM2.5 samples in 12 sites and performed chemical characterization to gather information about the concentration of major and trace elements, elemental carbon (EC), organic carbon (OC), and ionic species. The use of triangular plots and the calculation of enrichment factors confirmed the interception of volcanic plume. We integrated the outcomes from chemical characterization with the use of factor analysis and SEM/EDX analysis for the source apportionment. Anthropogenic and natural sources including shipping emissions, volcanic and fumarolic load, as well as sea spray were identified as the main factors affecting aerosol levels in the study area. Furthermore, we performed pattern recognition analysis by stepwise linear discriminant analysis to seek differences in the composition of PM10 and PM2.5 samples according to their volcanic or solfatara origin.

AbstractThe Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume–ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (>80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.

The use of geophysical techniques to image the interiors of active volcanoes can provide a better understanding of their structure and plumbing. The need for such information is especially critical for undersea volcanoes, whose environment makes them difficult to investigate. Because undersea volcanoes are made up of highly magnetic basaltic rock, it is possible to use variations in the magnetic field to explore the internal structure of such edifices. This study combines magnetic survey data from 12 research cruises to make a magnetic anomaly map of volcanically active Loihi, located in the Hawaiian Island chain. NRM intensities and susceptibility measurements were measured from recovered rock samples and suggest that magnetic properties of Loihi are widely varied (NRM intensities range from 1-157 A/m and susceptibilities from 1.26 x 10-3 to 3.62 x 10-2 S.I.). The average NRM intensity is 26 A/m. The size and strength of magnetic source bodies were determined by using various modeling techniques. A strongly magnetized shield can explain most of the anomaly with a large nonmagnetic zone inside, beneath the summit. Prominent magnetic highs are located along Loihi's north and south rift zone dikes and modeling solutions suggest strongly magnetized source bodies in these areas as well as a thin, magnetic layer atop the nonmagnetic zone. The strong magnetic anomalies found along the volcano's rift zones cannot be readily attributed to recent lava flows at the surface. Instead, the source bodies must continue several kilometers in depth to give reasonable magnetization values and are interpreted as dike intrusions. Nonmagnetic anomalies at the summit and south of the summit suggest the presence of a magma system. The model solution suggests Loihi is an inhomogeneously magnetized seamount with highly magnetic dike intrusions along the rift zones with a nonmagnetic body at its center overlain with a magnetic layer.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2005.
Includes bibliographical references.
The 6000-km long, age-progressive linear Hawaii-Emperor Chain is one of the best defined hotspot tracks. This hotspot track plays an important role in the plume hypothesis. In this research, geochemical data on the Hawaiian-Emperor lavas are used to evaluate the plume hypothesis. There are important geochemical similarities among lavas from the Hawaii-Emperor Chain, such as high Ba/Th (>100), high ³He/⁴He (>10 R/R-A) and enrichment in incompatible elements. These distinctive geochemical characteristics are not present in mid-ocean ridge basalt (MORB). Consequently, it is inferred that a common mantle source has been contributing to the Hawaiian-Emperor volcanism for over 80 My, which provides the strongest geochemical argument supporting the plume hypothesis. The distinctive geochemical characteristics of Makapuu-stage Koolau lavas, such as high SiO₂ content, ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb*/²⁰⁶Pb*, [delta]¹⁸0, ¹⁸⁷Os/¹⁸⁸Os, La/Nb, Sr/Nb and low CaO content, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, Th/La, require that recycled ancient oceanic crust including sediments is present in the Hawaiian plume. Some of these distinctive geochemical characteristics are also present in other Hawaiian shields. For example, Kahoolawe lavas also range to low ²⁰⁶Pb/²⁰⁴Pb.
(cont.) However, Kahoolawe lavas are not characterized by relatively high SiO₂ content, Sr/Nb and La/Nb and low CaO content that are characteristic of Makapuu-stage Koolau lavas, and they are offset from other Hawaiian shield lavas to high ⁸⁷Sr/⁸⁶Sr at a given ¹⁴³Nd/¹⁴⁴Nd. Since Hawaiian shield lavas have similar Sr/Nd (variation less than a factor of three), I speculate that the ⁸⁷Sr/⁸⁶Sr offset is a result of varying roles of recycled plagioclase-rich gabbro in Hawaiian shield lavas. Modem Hawaiian volcanoes form two offset trends, i.e., the Loa and Kea trends. Lavas from Loa and Kea trends have important geochemical differences. Specifically, Loa trend lavas have relatively higher ²⁰⁸Pb/²⁰⁴Pb at a given ²⁰⁶Pb/²⁰⁴Pb than Kea trend lavas, that is, Loa trend lavas have higher ²⁰⁸Pb*/²⁰⁶Pb than Kea trend lavas. Loa and Kea trend lavas form different trends in plots of ²⁰⁸Pb*/²⁰⁶Pb vs Sr-Nd-Hf-He isotopic ratios, and the Loihi component (high ³He/⁴He) is a common source component for Loa and Kea trend lavas. The Loa-Kea geochemical differences are inferred to reflect source differences; consequently, different models of plume structure have been proposed. However, I propose an alternative model in which Loa and Kea trend volcanoes sample a common geochemically heterogeneous source.
(cont.) The observed Loa-Kea geochemical differences is inferred as a result of temperature difference between Loa and Kea trend volcanoes which reflects their distances from the center of the hot plume core.
by Shichun Huang.
Ph.D.

2013/2014
La domanda alla base di questa ricerca è stata se il metodo della gravimetria satellitare possa essere utilizzato per seguire le unità geologiche anche in luoghi difficilmente accessibili. L’obiettivo di questa ricerca è di verificare se le missioni satellitari di nuova generazione permettano di identificare la più grande delle province ignee della Terra (Bertrand et al., 2013), nota come CAMP (Central Atlantic Magmatic Province) in Africa nord-occidentale. Oltre alle motivazioni scientifiche, una possibile applicazione è l’esplorazione di risorse minerarie e lo sfruttamento di energia geotermica. Tale provincia ignea è una LIP (Large Igneous Province) che si estende in Nord e Sud America, Atlantico, Europa ed Africa (istituita in Marzoli et al. 1999). Essa si è sviluppata a seguito della frammentazione del super-continente Pangea al limite Triassico-Giurassico, ca. 200 Ma fa. A causa probabilmente del riscaldamento globale del mantello e/o dalla convezione dello stesso innescata da dislivelli di blocchi litosferici, dai dicchi-sorgente si produssero i cosiddetti basalti da flusso e si verificò un intenso vulcanismo con imponenti colate laviche tali da suggerire a taluni ricercatori che gli elementi volatili presenti nel magma abbiano contribuito ad aumentare i gas serra con conseguenze nel clima globale e nelle estinzioni di massa. I depositi in esame sono costituiti da lave, tholeiti continentali, doleriti, basalti e gabbri. Ciò che rimane di questa attività vulcanica sono dicchi singoli o in sciami, batoliti, sill, colate laviche e plateau basaltici (nei fondali oceanici). L’Africa nord-occidentale è costituita principalmente da un cratone composto da rocce molto antiche dell’Archeano (3000-2500 Ma). Esso emerge a nord nello scudo Reguibat e, a sud, la dorsale dell’Uomo o del Leone (Lucazeau et al., 1991). Il cratone è circondato dalle zone di geosutura (greenstone e cinture mobili) associate al cosiddetto evento termo-tettonico Pan-Africano, verificatosi ca. 650 Ma fa con l’assemblaggio del continente africano da blocchi crostali più piccoli. Nelle Mauritanidi affiora il basamento ercinico (ca. 350 Ma) mentre negli Atlas e nelle Magrebidi prevalgono rocce più recenti connesse all’orogenesi alpina (0-150 Ma). Al centro del cratone si trova il bacino paleozoico Taoudenni che riempie una vasta area depressa. Tutto il territorio in esame è caratterizzato da una forte presenza di rocce metamorfiche dense e magmatiche di ogni età, con presenza di più di un “punto caldo” che potrebbe essere definito un terreno igneo (Bryan et al. (2008) con più LIP spesso sovrapposte o limitrofe. Mediamente, tutte queste rocce magmatiche e metamorfiche hanno una densità di 3000 kg/m3 (Kröner, 1977 ), maggiore di quella della crosta standard e dei sedimenti. Particolare attenzione è stata dedicata ad un lineamento tettonico noto come Pelusium Megashare System (PMS) che attraversa tutta l’Africa nord-occidentale (Neev et al., 1982) chiaramente visibile in tutte le immagini satellitari di Google Earth ma che è riportato solo in pochissime pubblicazioni. Per la prima volta in questa tesi si ipotizza un collegamento tra la CAMP e PMS. La gravimetria satellitare consente di rilevare variazioni di densità nella crosta terrestre. Ove vi sono rocce più dense, il segnale rilevato (detto anomalia gravimetrica) è positivo e viceversa. La gravimetria da satellite si è rivelata un valido strumento per identificare le aree con surplus di massa. La risposta all’interrogativo iniziale è dunque affermativa anche se, da quanto esposto, risulta difficile o impossibile associare ad un certo segnale positivo una data LIP. L’elaborazione dei segnali è avvenuta partendo dai dati del satellite GOCE (ultima generazione, a un’orbita di 250 km ma già ammarato) e GRACE (obsoleto ma tuttora in orbita a ca. 450 km). I dati utilizzati sono dei modelli del campo di gravità terrestre che contengono i coefficienti di Stokes per lo sviluppo in armoniche sferiche del potenziale. I modelli utilizzati sono l’EGM2008 (comprendente anche dati di terra, con risoluzione massima 10 km se sviluppato al massimo ordine di 2159) e GOCO TIM R4 (con una risoluzione massima di 80 km, la migliore mai ottenuta da dati satellitari globali). Il modello EGM2008 è stato sviluppato fino all’ordine e grado 720 per eliminare dati spuri (Pavlis, 2012) e, in tal modo, ha permesso di raggiungere una risoluzione di ca. 27 km se si considera metà lunghezza d’onda. Dopo il controllo della qualità dei dati, essi sono stati elaborati nel seguente modo, come esposto nei capitoli 2, 3 e 4. Ai dati grezzi sono state applicate tre riduzioni per sottrarre gli effetti di gravità indesiderati che mascherano il segnale cercato più debole. È stato sottratto l’effetto di gravità della topografia, dei sedimenti e dell’interfaccia crosta-mantello (ICM). Partendo dall’anomalia “in aria libera” (FA), è stata quindi ottenuta l’anomalia di Bouguer (BA) e la BA corretta per i sedimenti. Poi, calcolata la Moho (ICM) isostatica, si è prodotto il residuo isostatico corretto per i sedimenti. I campi elaborati sono la gravità gz (espressa in milli Gal, mGal) ed il gradiente Tzz (misurato in Eötvös, E). Sono state usate le risoluzioni di 0.5° e di 0.05°, computati ad una quota di 4000 m s.l. m perché maggiore del più altro rilievo montuoso dell’area. Dopo aver modellato dei casi a geometria semplice (cap. 6) si è passati alla modellizzazione di tre casi reali. I tre siti scelti per l’approfondimento sono: Tindouf (Algeria), Taoudenni (Mali), Timbuktu (Mali). Nel bacino di Tindouf un sill doleritico CAMP è annesso al suo probabile dicco-sorgente reso evidente dalla gravimetria che identifica bene anche una vicina miniera di Ferro. Nel bacino Taoudenni, le due anomalie principali suggeriscono la presenza di cumuliti magmatici spessi una dozzina di chilometri e connessi con la superficie attraverso dicchi obliqui. Il sito presso Timbuktu è trattato nel dettaglio perché al di sotto del vicino lago Faguibine è stata rivelata un’intrusione magmatica lunga ca. 250 km. In superficie vi sono evidenze di magmatismo (per es. fumarole) tali da preoccupare le popolazioni locali (El Abbass et al., 1993). Tra i risultati inaspettati, si ricorda il forte segnale gravimetrico generato dalle peridotiti in Marocco ed un’importante anomalia (80 mGal) nel Grand Erg Occidental (Algeria) al di sotto del Sahara che sembrerebbe essere causata da un corpo denso lungo ca. 600 km.
The question behind this research was whether the method of satellite gravimetry can be used to follow the geological units even in inaccessible places. The goal of this research is to verify if the new generation of satellite missions serve to identify the largest of the Earth's igneous provinces (Bertrand et al., 2013), known as CAMP (Central Atlantic Magmatic Province) in Northwest Africa . Besides the scientific reasons, a possible application is the exploration of mineral resources and the exploitation of geothermal energy. This is an igneous province (LIP Large Igneous Provinces) that extends throughout North and South America, the Atlantic Ocean, Europe and Africa (established in Marzoli et al. 1999). It developed as a result of the fragmentation of the super-continent Pangea at Triassic-Jurassic limit, ca. 200 My ago. Probably because of mantel global warming and/or its convection triggered by differences in thickness of lithospheric blocks, from source-dikes were produced the so-called continental flow basalts (CFB) and there was an intense volcanism with massive lava flows that this would suggest to certain researchers volatile elements present in the magma have contributed to increasing greenhouse gases with consequences in the global climate and mass extinctions. The deposits in question consist of lavas, tholeites continental dolerites, basalts and gabbros. What remains of this volcanic activity are individual dykes or in swarms, batholiths, sills, lava flows and basaltic plateau (in the ocean). The north-western Africa consists mainly of a craton made of most ancient rocks dell'Archean (3000-2500 Ma). It emerges in the shield Reguibat north and on the south, the Man or the Lion shield (Lucazeau et al., 1991). The craton is surrounded by areas of geosutura (greenstone belts and mobile belts) associated with the so-called Pan-African thermo-tectonic event, occurred ca. 650 Ma ago with the assembly of the African continent by smaller crustal blocks. Mauritanides emerges in the Hercynian basement (ca. 350 Ma) while in the Atlas and Magrebides prevail younger rocks (Alpine orogenesis, 0-150 Ma). At the center of the craton is the Paleozoic basin Taoudenni that fills a large area depressed. All the territory concerned is characterized by a strong presence of dense magmatic and metamorphic rocks of all ages, with the presence of more than a "hot spot" that could be called “igneous terrane” (Bryan et al. (2008) with more LIPs overlapping or adjacent. On average, these igneous and metamorphic rocks have a density of 3000 kg / m3 (Kröner, 1977), greater than that of the standard crust and the sediments. Particular attention was dedicated to a tectonic lineament known as Pelusium Megashare System (PMS) that runs through the north-western Africa (Neev et al., 1982) clearly visible in all the satellite images of Google Earth but is reported only in very few publications. For the first time this thesis suggests a link between CAMP and PMS. Satellite gravimetry can detect density variations in the Earth's crust. Where there are rocks denser, the detected signal (called gravity anomaly) is positive and vice versa. The gravimetry by satellite has proved a valuable tool to identify areas with surplus Mass. The initial response to the question is therefore affirmative although, from the above, it is difficult or impossible to associate a positive signal a date LIP. Signals processing occurred from the data of the GOCE satellite (last generation, in an orbit of 250 km, mission already finished) and GRACE (obsolete but still in orbit at ca. 450 km). The data used are the models of the Earth's gravity field containing the coefficients of Stokes for the development of potential in spherical harmonics. The models used are the EGM2008 (also including land data, with a maximum resolution 10 km if developed to the maximum order of 2159) and TIM GOCO R4 (with a maximum resolution of 80 km, the best ever obtained by global satellite data). The model EGM2008 has been developed up to the order and degree 720 to remove spurious data (Pavlis, 2012) and, thus, allowed to reach a resolution of ca. 27 km considering half wavelength. After quality control of the data, they were processed in the following flowchart, as discussed in Chapters 2, 3 and 4. Raw data were processed applying three reductions to subtract the effects of gravity that mask the signal. It was reduced by the effect of gravity of the topography, sediment and crust-mantle interface (CMI). Starting by the anomaly "free air" (FA), was thus obtained the Bouguer anomaly (BA) and BA correct for sediment. Then, once calculated the isostatic Moho (CMI), has produced the sediment-corrected-isostatic residual. The fields processed are gravity gz (in milli Gal, mGal) and gradient tzz (measured in Eötvös, E). It has been used the resolutions of 0.5 ° and 0.05 °, computed at an altitude of 4000 m a.s.l., higher of mountains in the area. After mng cases with simple geometry (ch. 6) we moved to the modeling of three real case histories. The three sites chosen for the study are: Tindouf (Algeria), Taoudenni (Mali), Timbuktu (Mali). In the basin of Tindouf a CAMP doleritic sill is attached to its likely source source-dyke evident by gravimetry that identifies well a nearby mine of Iron. In the Taoudenni basin, the two main anomalies suggest the presence of magmatic cumulites a dozen kilometers thick and connected with the surface through oblique dikes. The site at Timbuktu is discussed in detail because in the nearby lake Faguibine was revealed a magmatic intrusion long ca. 250 km. On the surface there is evidence of magmatism (e.g. Fumaroles) such that worry local populations (El Abbass et al., 1993) .Among the unexpected results, please note the strong signal generated by gravimetric peridotites in Morocco and a major anomaly (80 mGal) in the Grand Erg Occidental (Algeria) below the Sahara that would seem to be caused by a dense body ca. 600 km long.
XXVI Ciclo
1972

ABSTRACT The genesis of the forearc magmatism in southwest Japan at 14 Ma was studied using geologic and seismic observations. Before the magmatism, the Shimanto accretionary complexes were uplifted by 1000–3000 m between 21 and 17 Ma during the opening of the Japan Sea and the Shikoku Basin. Opening of the Japan Sea and the Shikoku Basin terminated at 15 Ma, when the Kinan Seamount Chain on the Shikoku Basin activated. The magmatic products are distributed at segment boundaries and in aseismic areas of the subducting Philippine Sea plate. The segment boundaries are located on syncline and anticline axes of the subducting slab at Kyushu. The magmatic products at Shikoku are distributed at places where olistostromes uplifted between 21 and 17 Ma. Beneath the Kumano volcanic rocks at Kii, a significant discontinuity in the locations of deep earthquakes is observed. These observations indicate that rupturing of the subducting slab by the load of the overriding plate occurred at around 14 Ma. The slab rupturing would have enabled subslab asthenosphere and/or magma to be injected into the plate interface through the tear and cause the forearc magmatism. Since the oceanic plate has a number of preexisting weaknesses, such as fracture zones, slab rupturing could occur more commonly than previously considered. The forearc magmatism caused by slab rupture is an important process associated with the growth of continental crust in subduction zones.