In this paper, we show the seismicity of the past 20 years that occurred in Italy and surrounding regions. Hypocentral locations have been obtained by using P- and S-wave arrival times from the INGV national and several regional permanent seismic networks. More than 48,000 events, selected from an original data set of about 99,780, are used to reconstruct the most complete seismic picture of the Italian region so far. The seismicity distribution allows inference on seismotectonics of this complex region of subduction versus continental collision. Our results clearly reveal the geometry of the Adria and the Ionian subduction and a continuous normal fault belt in the upper crust, following the Apennines mountain range. The depth of the seismogenic layer is computed from the cut-off of seismicity at depth and shows large variations along and across the seismic active regions. Earthquakes are generated by the different velocity of slab retreat and the subsequent asthenospheric upwelling. 相似文献
We developed a simple, single-layer urban canopy model, and comparedit to both multi-layer and slab models. Our single-layer model has thefollowing features: (a) It is a column model of energy and momentumexchange between an urban surface and the atmosphere, (b) it includesthe influence of street canyons, which are parameterized to representthe urban geometry, (c) it includes shadowing from buildings andreflection of radiation, and (d) it estimates both the surfacetemperatures of, and heat fluxes from, three surface types: roof, wall,and road. In the simulation of the single-layer model, the roof washottest during the daytime, but coolest from midnight to early morning.This is consistent with output from the multi-layer model and fieldobservations at a residential area on a clear, summer day. The diurnalvariation of the energy budget from the single-layer model agrees wellwith that from the multi-layer model. Our single-layer model'sperformance is nearly that of a multi-layer model for studyingmesoscale heat islands. Nevertheless, it is simply parameterized,and thus easily included in larger-scale atmospheric models. The slabmodel has the largest nighttime cooling rate of the three models. Toovercome this, it needs more adjustments than for the canopy models. 相似文献
The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone.A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes. 相似文献
This study has reviewed the distribution and pedogenesis of late Mesozoic A-Type granitoids in SE China. These A-Type granitoids belong to four belts ( S Jiangxi-SW Fujian belt, Xiang-Gui-Yue belt, Can-Hang belt and coastal belt) due to their temporal-spatial distribution. Based on the comparative analysis of chronology, geochemistry and magmatic association, this study has discussed the formation of A-Type granitoids and the subduction and slab rollback process of paleo-Pacific plate beneath SE China. 相似文献
In the southernmost Dom Feliciano Belt of Uruguay, highly fractionated calc-alkaline granites, mildly alkaline granites, shoshonitic volcanics, and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones. Four representative magmatic unites of this diverse association were petrographic and geochemically investigated: the Solís de Mataojo Complex, a medium to high K2O calc-alkaline granite with signature typical of mature continental arcs and post-collisional settings; the Maldonado granite, highly fractionated calc-alkaline to alkaline, with characteristics that are transitional between both types of series; the Pan de Azúcar Pluton, with characteristics typical of post-collisional alkaline granites and the Las Flores shoshonitic basalts.
Geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high-K calc-alkaline magmas (Solís de Mataojo and Maldonado) shortly after collision. Extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity (Las Flores) 15and finally a shift to magmas with alkaline signatures (Pan de Azúcar) simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination. This evolution took place between 615 and 575 Ma, according to available data. Contrary to previous proposals, which considered this magmatism to represent the root of a continental magmatic arc, a post-collisional environment, transitional from orogenic to anorogenic, during transcurrent deformation is proposed. 相似文献
We report here, for the first time, on the new finding of extrusive calciocarbonatite (alvikite) rocks from the Pleistocene Mt. Vulture volcano (southern Italy). These volcanic rocks, which represent an outstanding occurrence in the wider scenario of the Italian potassic magmatism, form lavas, pyroclastic deposits, and feeder dikes exposed on the northern slope of the volcano. The petrography, mineralogy and whole-rock chemistry attest the genuine carbonatitic nature of these rocks, that are characterized by high to very high contents of Sr, Ba, U, LREE, Nb, P, F, Th, high Nb/Ta and LREE/HREE ratios, and low contents of Ti, Zr, K, Rb, Na and Cs. The O–C isotope compositions are close to the “primary igneous carbonatite” field and, thus, are compatible with an ultimate mantle origin for these rocks. The Sr–Nd–Pb–B isotope compositions, measured both in the alvikites and in the silicate volcanic rocks, indicate a close genetic relationship between the alvikites and the associated melilitite/nephelinite rocks. Furthermore, these latter products are geochemically distinct from the main foiditic-phonolitic association of Mt. Vulture. We propose a petrogenetic/geodynamic interpretation which has important implications for understanding the relationships between carbonatites and orogenic activity. In particular, we propose that the studied alvikites are generated through liquid unmixing at crustal levels, starting from nephelinitic or melilititic parent liquids. These latter were produced in a hybrid mantle resulting from the interaction through a vertical slab window, between a metasomatized mantle wedge, moving eastward from the Tyrrhenian/Campanian region, and the local Adriatic mantle. The occurrence of carbonatite rocks at Mt. Vulture, that lies on the leading edge of the Southern Apennines accretionary prism, is taken as an evidence for the carbonatation of the mantle sources of this volcano. We speculate that mantle carbonatation is related to the introduction of sedimentary carbon from the Adriatic lithosphere during Tertiary subduction. 相似文献
Recent geochemical studies of volcanic rocks forming part of the ophiolites within the Zagros and Naien-Baft orogen indicate that most of them were developed as supra-subduction ophiolites in intra-oceanic island arc environments. Intra-oceanic island arcs and ophiolites now forming the Naien-Baft zone were emplaced southwestward onto the northeastern margin of the South Sanandaj–Sirjan Zone, while those now in the High Zagros were emplaced southwestward onto the northern margin of Arabia. Thereafter, subduction continued on opposite sides of the remnant oceans. The floor of Neo-Tethys Ocean was subducted at a low angle beneath the entire Sanandaj–Sirjan Zone, and the floor of the Naien-Baft Ocean was subducted beneath the Central Iranian Micro-continent. The Naien-Baft Ocean extended into North-West Iran only temporarily. This failed ocean arm (between the Urumieh-Dokhtar Magmatic Assemblage and the main Zagros Thrust) was filled by thick Upper Triassic–Upper Jurassic sediments. The Naien-Baft Ocean finally closed in the Paleocene and Neo-Tethys closed in the Early to Middle Eocene. After Arabia was sutured to Iran, the Urumieh-Dokhtar Magmatic Assemblage recorded slab break-off in the Middle Eocene. 相似文献
The Mojanda–Fuya Fuya Volcanic Complex consists of two nearby volcanoes, Mojanda and Fuya Fuya. The older one, Mojanda volcano
(0.6 to 0.2 Ma), was first constructed by andesites and high-silica andesites forming a large stratovolcano (Lower Mojanda).
This edifice was capped by a basaltic andesite and andesitic cone (Upper Mojanda), which collapsed later to form a 3-km-wide
summit caldera, after large phreatomagmatic eruptions. The Lower Fuya Fuya edifice was constructed by the extrusion of viscous
Si-rich andesitic lavas and dacitic domes, and the emission of a thick sequence of pyroclastic-flow and fallout deposits which
include two voluminous rhyolitic layers. An intermediate construction phase at Fuya Fuya is represented by a mainly effusive
cone, andesitic in composition (San Bartolo edifice), the construction of which was interrupted by a major sector collapse
in the Late Pleistocene. Finally, a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre,
forming the Upper Fuya Fuya volcanic centre. This paper shows that the general evolution from an effusive to an explosive
eruptive style is related to a progressive adakitic contribution to the magma source. Although all the rocks of the complex
are included in the medium-K field of continental arcs, the Fuya Fuya suite (61–75 wt.% SiO2) shows depletion in Y and HREE and high Sr/Y and La/Yb values, compared to the less silicic Mojanda suite (55–66.5 wt.% SiO2). The Mojanda calc-alkaline suite was generated by partial melting of an adakite-metasomatised mantle source that left a
residue with 2% garnet, followed by fractional crystallization of dominant plagioclase + pyroxene + olivine at shallow, intra-crustal
depths. For Fuya Fuya, geochemical and mineralogical data suggest either (1) partial melting of a similar metasomatised mantle
with more garnet in the residue (4%), followed by fractional crystallization involving plagioclase, amphibole and pyroxene,
or (2) mixing of mafic mantle-derived magma from the Mojanda suite and slab melts, followed by the same fractional crystallization
process. 相似文献