169,843 research outputs found
Plume-related magmatism in collision zones: examples from the Tertiary-Quaternary magmatism in Italy.
No full textFrom late Cretaceous to present time, an extensive magmatic activity developed both in Europe and northern Africa, showing a progressive transition with time from calc-alkaline to Na-rich alkaline features in areas tightly connected with subduction systems (Morocco, Algeria, Tunisia, Spain, Italy), while Na-rich basalts and basanites, with minor tholeiitic volcanics, occur at extensional tectonic settings, both associated or not to orogenic dynamics (Valencia trough, Pannonian, Alboran, Tyrrhenian, and Aegean basins, Pantelleria-Etna-Iblean area, Veneto Province, Cenozoic Rift System). The widespread alkaline magmatism in the European and circum Mediterranean area shows a uniform HIMU-DMM type signature which has been recently ascribed to a mantle contamination episode triggered by the rise of the Central Atlantic Plume (CAP) head since Cretaceous time (Piromallo et al., 2008).
The occurrence of a HIMU-DMM component within lavas originated in collision environments such as the Veneto Volcanic Province in NE Alps (related to the Tertiary convergence of Europe and Africa plates) or the Roman Magmatic Province (related to the Cenozoic Adria subduction) might be explained in terms of slab breakoff (Macera et al., 2008; Gasperini et al., 2002). Evidence for slab breakoff in these areas comes from seismic tomography (Piromallo and Faccenna, 2004) and geophysical modeling consisting in evaluating the time evolution of buoyancy of oceanic and continental lithosphere during subduction with both constant and time-varying convergence rates (Macera et al., 2008). If the subducted slab intercepts a hot mantle diapir rising from the frayed plume head, the corresponding part of the slab is heated and therefore softened. The softening effect is enhanced if the slab includes continental material. The combination of changes in negative buoyancy caused by continental subduction, and softening of a part of the slab caused by slab-plume interaction, may act as a regulator for the time of slab breakoff and consequently for the time and type variations of magmatism in the overriding lithosphere above a subduction zone. In the Alpine region, we assume that the plume material interacted with the subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the resulting plate window is supposed to be the responsible for partial melting in the lithospheric mantle wedge and/or decompression melting of the ascending plume material.
Plume-related volcanism in subduction zones is possible either before the subducted slab intercepts plume head material , or after slab breakoff. The above considerations provide a framework for the occurrence of plume-related and calc-alkaline magmatism in the southeastern Alps and probably in other similar tectonic settings such as the Central Europe Magmatic Province. The heat released by the hot mantle component to the overriding mantle wedge contributed to its partial melting and production of calc-alkaline mafic magmas. The latter additionally favored the genesis of calc-alkaline felsic magmas through partial melting of the lower crust by underplating. A quantitative approach based on mixing calculations on Sr-Nd and Pb isotopes support these hypotheses
Surface imprint of toroidal flow at retreating slab edges: The first geodetic evidence in the Calabrian subduction system
No full textDense GPS observations can help Earth scientists to capture the surface imprint of mantle toroidal flow at slab edges. We document this process in the Calabrian subduction system, where the Ionian slab rollback took place during the past 30Ma, following a stepwise process driven by migration of lithospheric tearing. We found rotation rates of similar to 1.29 degrees/Ma (counterclockwise) and similar to 1.74 degrees/Ma (clockwise), for poles located close to the northern and southern slab edges, respectively. These small-scale, opposite rotations occur along complex sets of active faults representing the present-day lithospheric expression of the tearing processes affecting the southeastward retreating Ionian slab at both edges. The observed rotations are likely still young and the process more immature at the northern tear, where it is unable to reorient mantle fabric and therefore is unseen by SKS splitting
Recent hot spot volcanism in the European and Mediterranean area
No full textFrom late Cretaceous to present time, an extensive magmatic activity developed both in Europe and northern Africa, showing a progressive transition with time from calc-alkaline to Na-rich alkaline features in areas tightly connected with subduction systems (Morocco, Algeria, Tunisia, Spain, western Italy), while Na-rich basalts and basanites, with minor tholeiitic volcanics, occur at extensional tectonic settings, both associated or not to orogenic dynamics (Valencia trough, Pannonian, Alboran, Tyrrhenian, and Aegean basins, Pantelleria-Etna-Iblean area, Veneto Province, Cenozoic Rift System). The widespread alkaline magmatism in the European and circum Mediterranean area shows a uniform OIB-HIMU-type signature which has been previously ascribed to a plume-related european astenospheric reservoir (EAR) standing at less than 400 km depth in the upper mantle (Wilson and Downes, 1991). In the same area a large low seismic velocity zone below 900 km depth is evident by tomographic images from whole mantle models. The mantle transition zone under central-western Europe and Mediterranean is instead characterized by a high velocity anomaly, while the top most mantle is again dominated by a low-velocity zone on a large scale (Goes et al., 1999; Piromallo and Morelli, 2003). The origin of this mantle geochemical/geophysical anomaly has been related to the Mesozoic super-plume activity centered at Madeira-Canary-Cape Verde (MCCV), where the HIMU component appears to have its purest fingerprint (Hoernle et al., 1995; Wilson, 1997; Oyarzun et al., 1997; Gasperini et al., 2003).Achievable interpretations of any relationship between present seismic anomalies and alkaline magmatism in this wide area are restricted to recent time (Hoernle et al., 1995; Goes et al., 1999), since older geochemical and geophysical features depend on the paleo-position of each magmatic center toward the south-west. Assuming that a common and primary mantle plume has been upwelling beneath the present MCCV since at least 65-70 Ma, using a hot spot reference frame, we projected that in the early Cenozoic the theoretical positions of the European magmatic centers might outline the old plume head size. The shallower large-scale swell of this plume had been subsequently dragged by motion of Eurasia towards the northeast and trapped above the transition zone, favoring plume related volcanism if and where local extensional tectonics and slab rupture occur (Macera et al., 2003)
Where is the tail of the European plume volcanism?
No full textGeochemical and isotopic (Sr, Nd, and Pb) features of representative alkaline basalts from the eastern Atlantic (Serra de Monchique-Mt. Ormonde, Madeira, Canary and Cape Verde Islands), northern Africa (Morocco, Algeria, Tunisia), central Europe (French Massif Central, Rhon-Eifel, Austria, Poland, Czech Republic, Hungary, Italy), and also from the Tertiary British Province and Iceland, are characterized by a common mantle component with a typical OIB-HIMU signature. This mantle source appears to have its purest fingerprint in the eastern Atlantic (Cape Verde, Madeira, and Canary Islands), while it becomes more diluted both eastwards and northwards. In an eastwards direction, the dilution is caused by a crustal component (progressively increasing from Morocco-Algeria to Tunisia, and from the French Massif to Poland). In a northward direction, a DM component becomes important (Tertiary British Province and Iceland), likely related to the Mid-Atlantic Ridge.
Tomographic images from whole mantle models below the eastern Atlantic, northern Africa and central Europe show a widespread low sesmic velocity zone below 900 km depth (Goes et al., 1999). While the transition zone under central-western Europe and Mediterranean is characterized by a wide, recumbent high velocity feature (Goes et al., 1999), the top most mantle, above 400 km depth, appears again dominated by a low-velocity zone on a large scale (Hoernle et al., 1995; Piromallo and Morelli, in press).
The geophysical and geochemical data can be accounted for in terms of a hypothesis postulating a common and primary deep mantle plume material, upwelling beneath the Cape Verde-Madeira-Canary Islands region. It is possible to suggest that the head of this large-scale swell has been frayed by Eurasia, that moved at this latitude towards the northeast, and trapped in the shallower mantle (above the transition zone) by subduction episodes. At different times local extensional tectonics and slab rupture may favor plume related volcanism, also in areas dominated by plate convergence and subduction (Gasperini et al., 2002)
How deep can we find the traces of Alpine subduction?
No full textSlab-like seismic velocity heterogeneities below the
Alpine chain, interpreted as subducted lithosphere, are
imaged by tomographic studies down to only about 300 km
depth. A non-negligible discrepancy therefore exists
between tomographic and geological data, the latter
indicating at least 500 km of Tertiary convergence at
trench. Yet a recently published tomographic study detects a
pronounced high velocity anomaly at the bottom of the
upper mantle right below the Alpine area. Combining
tomographic images of the mantle, geological findings and
plate system kinematics, we investigate how the presence of
this feature in the transition zone below the Alps can be
traced back to the Tertiary Alpine subduction and possibly
explain the observed discrepancy. We propose that a part of
the fast velocity body now residing just above the 660 km
discontinuity once belonged to the Alpine slab, torn off by
an event occurred at about 30–35 Ma.PublishedL066053.3. Geodinamica e struttura dell'interno della TerraJCR Journalreserve
Subduction age and stress state control on seismicity in the NW Pacific subducting plate
No full textIntermediate depth (70–300 km) and deep (> 300 km) earthquakes have always been puzzling Earth scientists: their occurrence is a paradox, since the ductile behavior of rocks and the high confining pressure with increasing depths would theoretically preclude brittle failure and frictional sliding. The mechanisms proposed to explain deep earthquakes, mainly depending on the subducting plate age and stress state, are generally expressed by single parameters, unsuitable to comprehensively account for differences among distinct subduction zones or within the same slab. We analyze the Kurile and Izu–Bonin intraslab seismicity and detail the Gutenberg–Richter b-value along the subducted planes, interpreting its variation in terms of stress state, analogously to what usually done for shallow earthquakes. We demonstrate that, despite the slabs different properties (e.g., lithospheric age, stress state, dehydration rate), in both cases deep earthquakes are restricted to depths characterized by equal age from subduction initiation and are driven by stress regimes affected by the persistence of the metastable olivine wedge
Relative sea level variations caused by subduction
No full textBy means of a stratified Earth model with viscoelastic rheology, we have studied the long-term global fluctuations of Relative Sea Level (RSL) induced by subducting slabs. We have computed RSL variations for both a single subduction and a realistic distribution of slabs by a numerical simulation based on a simplified model of the subduction process. RSL is determined by the offset between the geoid and the dynamic topography; our analysis demonstrates that the latter provides the prevailing contribution. We have studied, in addition, the effects of rheological stratification upon the amplitude and time-evolution of these two quantities and, consequently, of RSL fluctuations. According to our results, an upper bound for the rate of RSL associated with subduction is of the order of 0.1 mm/yr, in agreement with previous studies. This rate of sea level variation is comparable with that attributed to changes in the tectonic regime on a large scale. This preliminary result corroborates the suggestion by other authors to include subduction in the list of geophysical mechanisms which contribute to long-term RSL fluctuations
ome remarks on the geodynamic of the Italian region
No full textIn this paper we present geological
and geophysical data which constrain the Tertiary
and Quaternary evolution of the Italian region,
relevant to the interpretation of the genesis of
magmatism in the frame of the geodynamic
processes. GPS results show that, at the longitude of
Sicily, the African approaches the Eurasian plate at a
velocity of about 5 mm/yr in a NW direction.
Furthermore, data show that the Adriatic foreland
presently moves independently from the African
plate. Geodetic and seismic data show that NW-SE
oriented extension is the main active tectonic
process in the Apennine chain.
Relative and absolute motions of the Africa and
the Eurasia plates indicate several hundred
kilometers of convergence since the early Tertiary,
in the central Mediterranean. This convergence has
been achieved by northwestward dipping subduction
of the African plate. The main present-day
geodynamic feature of the Italian region is
represented by seismicity on a well defined Benioff
zone, which reveals a still active process of
subduction from the Ionian foreland below the
Calabrian Arc and Tyrrhenian Sea. This slab is the
result of a NW directed long running subduction
process active since the Tertiary, which consumed
the Ligure oceanic basin first, then a small fragment
of the Apulian continental lithosphere, and finally
most of the present-day Ionian lithosphere, whose
subduction is still ongoing.
We also suggest that the lateral break-off of the
Ionian subducting lithosphere could allow lateral
astenospheric flow above the subducted plate either
from the Apulian plate and from the Sicily Channel
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