Subduction of continental lithosphere, changes in negative buoyancy, and slab-plume interaction: consequences for slab breakoff

The time evolution of buoyancy of oceanic and continental lithosphere during subduction is estimated on the basis of a kinematic model with both constant and time-varying convergence rates. The negative buoyancy before the arrival of continental lithosphere at the subduction zone increases with increasing depth of penetration of the slab at a rate proportional to the convergence rate. The time required for the negative buoyancy to be reduced to zero by the subduction of positively buoyant continental material depends on the depth reached by the oceanic slab and the convergence rate, but is typically of the order of a few tens Ma. If a rising plume impinges from below on a near-stationary slab in the upper mantle, 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. A plausible example of this situation may be provided by the Alpine slab subduction beneath the Adria plate at Paleocene time. Here, the Tertiary convergence between Europe and Africa plates was characterized by the consumption of both oceanic and continental European crust. Some million years later (45-30 Ma ago), two contrasting magmatic suites developed in the south-eastern sector of the Alps, partially overlapping in time: a) a calc-alkaline, subduction-related suite, and b) an alkaline, plume-related volcanic suite. On the basis of geological, geophysical and geochemical data we supposed that both the magmatic suites originated by a common and primary deep mantle plume the root of which was located beneath the Cape Verde-Madeira-Canary Islands region, while the head of which was swerved and frayed by the Eurasian plate that from this latitude moved northeastwards. In the Alpine region, the plume head material might have interacted with the Alpine subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the so formed plate window is supposed to be the responsible for either the partial melting of the lithospheric mantle wedge and for the partial melting of the plume material itself

Slab detachment and mantle plume upwelling in subduction zones: An example from the Italian south-eastern Alps volcanism

Abstract The geochemical properties of the South-Eastern Alps volcanics (SEAV, Eocene age) call for a within-plate origin of the most primitive basalts, in contrast to the widespread calc-alkaline magmatism which developed some million years later northwestwards along the Periadriatic Lineament. The two contrasting magmatic suites that coexist in the Alpine area define binary mixing relationships in the Sr–Nd and Sr–Pb isotopic space, the end members of which being a crustal component (e.g. lower continental crust) and a HIMU-DMM component (e.g. the SEAV). The occurrence of a HIMU (high μ= high 238 U/ 204 Pb) component, which normally traces mantle plumes of deep mantle origin, in a tectonic regime dominated by collision tectonics (the tertiary convergence of European and Adriatic plates) can be explained by slab detachment and ensuing upwelling of mantle material through the lithospheric gap. We combine geochemical data and geophysical modelling to unravel the evolution of the Alpine slab after interaction with plume material and the genesis of the Alpine magmatism. 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 variations of magmatism in the overriding lithosphere above a subduction zone. The thermal evolution of a subducting slab is modified by contact with the plume material which decreases significantly the total strength of the slab and favours slab detachment. Interactions between the HIMU component and the shallower depleted mantle can account for the geochemical characteristics of the SEAV. Counterflows of plume material towards the top of the subducting slab may also increase heating and partial melting of the overriding mantle wedge, giving rise to the calc-alkaline suite outcropping in the proximity of the Periadriatic Lineament. © 2007 Elsevier Ltd. All rights reserved

On the relation between lithospheric strength and ridge push transmission in the Nazca plate

The ridge push force and the total lithospheric strength of the Nazca plate are compared along an East-West transect from the East Pacific Rise to the Peru-Chile trench at latitude 12°S. The thermal structure of the plate is estimated from the plate cooling model and constrained by heat flow, bathymetry, and geoid height data. The best fitting thermal model has a basal temperature of ~1600K and an asymptotic plate thickness (not reached because of the relatively young age of the plate at the trench) of ~101km. The ridge push force, also determined from the plate cooling model, is of the order of 1.5TNm-1 at the trench. The total lithospheric strength as a function of age is estimated for a possible range of conditions (compressional/extensional intraplate tectonic regime, wet/dry rheology). A comparison of ridge push force with lithospheric strength, extended beyond the Nazca plate by considering different spreading rates and ages, shows that oceanic plates with dry rheology have strengths higher than the ridge push force at any age if the tectonic regime is compressional, and comparable if the regime is extensional. On the other hand, oceanic plates with wet rheology have strengths lower than the ridge push force, especially if the tectonic regime is extensional. Therefore, if the rheology is wet and mantle drag at the base of the plate is sufficiently strong, the ridge push force may result in intraplate deformation and be partly dissipated within the plate

HIMU-OIB magmatism in subduction zones: An example from the Italian south-eastern Alps

The geochemical features of the SE Alps volcanics (SEAV, Tertiary age) are comparable to the numerous volcanic eruptions of Tertiary-Quaternary age from the western Mediterranean area for which a plume-related origin has been assessed, and contrast to the widespread calcalkaline magmatism which developed northwestwards along the Periadriatic Lineament. The occurrence of a HIMU component, which is the hallmark of hotspot basalts, in a collision environment (the Tertiary convergence of Europe and Africa plates) is here explained in terms of slab breakoff. Evidence for the European slab breakoff comes from seismic tomography which shows that the present-day fast velocity material, interpreted as the European slab subducted below the Alpine chain, is shorter by about 300 km than the total length of the subducted slab estimated by paleotectonic reconstructions. Other piece of evidence comes from a kinematical model consisting in evaluating the time evolution of buoyancy of oceanic and continental lithosphere during subduction with both constant and time-varying convergence rates. If the subducted slab intercepts a rising plume from below 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. On the basis of geological, geophysical and geochemical data we conclude that both magmatic suites originated from a common and primary deep mantle plume the root of which was located beneath the Cape Verde-Madeira-Canary Islands region, while the head was dragged and frayed by the northeastward motion of the Eurasian plate

Integrated field, satellite and petrological observations of the November 2010 eruption of Erta Ale

Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ?0.006 km3. The mineralogy of the 2010 lava is plagioclase?+?clinopyroxene?+?olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H2O contents ?1,300 ppm and CO2 contents of ?200 ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7–42 MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ?1,150 °C, consistent with previously published field measurements