Viscous mantle flow under moving lithospheric plates and under subduction zones

Abstract At subduction zones, most diamonds form by carbon saturation in hydrous fluids released from lithospheric plates on equilibration with mantle rocks. Although organic molecules are predicted among dissolved species which are the source for carbon in diamonds, their occurrence is not demonstrated in nature, and the physical model for crustal diamond formation is debated. Here, using Raman microspectroscopy, I determine the structure of carbon-based phases inside fluid inclusions in diamond-bearing rocks from the Alps. The results provide direct evidence that diamond surfaces are coated by sp2-, and sp3-bonded amorphous carbon and functional groups of carboxylic acids (e.g., carboxyl, carboxylate, methyl, and methylene), indicating the geosynthesis of organic compounds in deep hydrous fluids. Moreover, this study suggests diamond nucleation via metastable molecular precursors. As a possible scenario, with carbon saturation by reduction of carboxylate groups, I consider tetrahedral H-terminated C groups as templates for the growth of sp3-structured carbon.

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On the geometry of mantle flow beneath drifting lithospheric plates

Geophysical Journal International ◽

10.1046/j.0956-540x.2000.01325.x ◽

2001 ◽

Vol 144 (2) ◽

pp. 356-372 ◽

Cited By ~ 23

Author(s):

Gabriele Marquart

Keyword(s):

Mantle Flow ◽

Lithospheric Plates

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Structural framework and regional significance of the Northandean Cretaceous subduction cycle

10.5194/egusphere-egu2020-18206 ◽

2020 ◽

Author(s):

Andreas Kammer ◽

Michael Avila

Keyword(s):

Subduction Zones ◽

Normal Fault ◽

Large Igneous Province ◽

Mantle Flow ◽

Spreading Ridge ◽

Forearc Basin ◽

Plate Convergence ◽

Continental Plate ◽

Igneous Province ◽

Cretaceous Subduction

The Northandean plate margin underwent a fundamental change in its structural configuration during a Cretaceous subduction cycle, as evidenced by the formation and accretion of a province of basic igneous arc rocks that gave rise to the basement of an Northandean Western Cordillera. Further north, this igneous terrane links to the Caribbean Large Igneous Province and has been associated, with respect to its origin, to an actively spreading ridge of the Farallon plate, implying a far-travelled origin with respect to Southamerica and calling for the existence of giant strike-slip faults. We challenge this allochthonous scenario by an alternative option of a forearc origin, invoking the possibility of a forearc opening by the forcing of a toroidal mantle flow at the northern end of the Andean trench, which would have introduced mantle material from the Pacific into the Andean realm through a Central American gap. Support for such an opening mode of a forearc basin comes from extensional tectonics, that accompanied the emplacement of the basic arc units and a concomitant subduction of the extrusive basic units at the inner border of this postulated forearc basin. This intraplate subduction comprises a distinct three-partite evolution: (I) Convergence first became manifest by the reactivation of a normal fault located within the supposed forearc basin and inboard of an inherited Triassic-Jurassic suture, but still failed at a crustal level. (II) A succeeding contractional stage involved the reactivation of the inherited Triassic-Jurassic suture and the tectonic erosion of a frontal compartment of the continental margin. After an incipient underplating, slivers of this continental compartment returned within a time span of about 20 Ma. (III) A final Late Cretaceous subduction stage evolved under the conditions of an oblique SW-NE oriented plate convergence and is characterized by extensional pulses, as may be concluded from the structural setting of the giant Antioquia batholith. In the Campanian subduction definitely locked, as evidenced by the regional buckling of the forearc realm and a rebound of the upper continental plate. Both onset and shutoff of this subduction cycle may be linked to deformation phases and are dated by syntectonic, fault-guided intrusions. This scenario of a forearc origin of the basic igneous province calls for the existence of two paired subduction zones: on its outer margin the subducting Farallon slab imposed a trench-parallel mantle flow and constrained an expansion of the forarc basin by slab rollback. On its inner margin, a secondary subduction compensated a surplus expansion of the actively forming forearc basin.

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An analytical model concerning the possible initiation of subduction between the India and Africa plates, caused by the Morondova plume

10.5194/egusphere-egu2020-6050 ◽

2020 ◽

Author(s):

Bernhard Steinberger ◽

Douwe van Hinsbergen

Keyword(s):

Subduction Zones ◽

Plate Tectonics ◽

Eastern Mediterranean ◽

Plate Boundary ◽

Plate Motion ◽

Mantle Flow ◽

Plate Motions ◽

Subduction Initiation ◽

Euler Pole ◽

Geodynamic Processes

Identifying the geodynamic processes that trigger the formation of new subduction zones is key to understand what keeps the plate tectonic cycle going, and how plate tectonics once started. Here we discuss the possibility of plume-induced subduction initiation. Previously, our numerical modeling revealed that mantle upwelling and radial push induced by plume rise may trigger plate motion change, and plate divergence as much as 15-20 My prior to LIP eruption. Here we show that, depending on the geometry of plates, the distribution of cratonic keels and where the plume rises, it may also cause a plate rotation around a pole that is located close to the same plate boundary where the plume head impinges: If that occurs near one end of the plate boundary, an Euler pole of the rotation may form along that plate boundary, with extension on one side, and convergence on the other.&#160; This concept is applied to the India-Africa plate boundary and the Morondova plume, which erupted around 90 Ma, but may have influenced plate motions as early as 105-110 Ma. If there is negligible friction, i.e. there is a pre-existing weak plate boundary, we estimate that the total amount of convergence generated in the northern part of the India-Africa plate boundary can exceed 100 km, which is widely thought to be sufficient to initiate forced, self-sustaining subduction. This may especially occur if the India continental craton acts like an &#8220;anchor&#8221; causing a comparatively southern location of the rotation pole of the India plate. Geology and paleomagnetism-based reconstructions of subduction initiation below ophiolites from Pakistan, through Oman, to the eastern Mediterranean reveal that E-W convergence around 105 Ma caused forced subduction initiation, and we tentatively postulate that this is triggered by Morondova plume head rise. Whether the timing of this convergence is appropriate to match observations on subduction initiation as early as 105 Ma depends on the timing of plume head arrival, which may predate eruption of the earliest volcanics. It also depends on whether a plume head already can exert substantial torque on the plate while it is still rising &#8211; for example, if the plate is coupled to the induced mantle flow by a thick craton.

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A physical model for the volume and composition of melt produced by hydrous fluxing above subduction zones

Philosophical Transactions of the Royal Society of London Series A Physical and Engineering Sciences ◽

10.1098/rsta.1991.0051 ◽

1991 ◽

Vol 335 (1638) ◽

pp. 355-364 ◽

Cited By ~ 81

Keyword(s):

Water Content ◽

Physical Model ◽

Subduction Zones ◽

Source Region ◽

Water Flux ◽

Fluid Phase ◽

Vertical Movement ◽

Mantle Flow ◽

Hydrous Minerals ◽

Water Contents

Thermal models of subduction zones, restrict the melt source region to a domain at sufficiently high temperature with water present (either as a free phase or in hydrous minerals). Water, released into the mantle by slab dehydration, traverses the wedge horizontally by a combination of (i) vertical movement as a fluid phase and (ii) fixed in amphiboles carried by the induced mantle flow; only in mantle hotter than amphibole stability can melts escape upwards. We develop a one-dimensional model for the source region fluxed with water. The induced mantle flow advects heat laterally to balance the latent heat of melting, in a column where the liquidus of the melt is depressed by its water content. Melt flux, fraction, temperature and water content are calculated assuming steady state. Melt compositions are predicted from the melt fraction distribution as a function of depth, constrained by the experimental data of Green. On investigating a range of plausible models, we find that the average degrees of melting predicted vary from ca . 2 to 8% . The predicted primary magmas are mafic high magnesium basalts with water contents ranging from 1.6 to 6 wt% , and temperatures from 1160 to 1290 °C. Models with shallower depths of segregation have higher degrees of melting and lower water contents. The volumes predicted by the physical model are a strong function of the water flux assumed to enter the source region. Previous estimates of are growth would suggest either low water fluxes or that not all the melt reaches the arc crust.

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Role of the Adria plate structural heterogeneities on the dynamics of the Central-Western Mediterranean region

10.5194/egusphere-egu21-9978 ◽

2021 ◽

Author(s):

Rosalia Lo Bue ◽

Manuele Faccenda ◽

Jianfeng Yang

Keyword(s):

Steady State ◽

Continental Margin ◽

Mediterranean Region ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Western Mediterranean ◽

Mantle Flow ◽

Crystal Preferred Orientation ◽

The Mediterranean ◽

Structural Heterogeneities

In the geodynamic context of the slow Africa-Europe plates convergence, the Central-Western Mediterranean region has been involved in a complex subduction process, which in the last 30 Myr was characterized by the rapid retreat of the Ionian slab, the opening of back-arc extensional basins (i.e., Liguro-Proven&#231;al, Algerian, Alboran, and Tyrrhenian basins) and episodes of slab lateral tearing, segmentation and break-off. &#160;A proper modelling of 3-D mantle flow evolution beneath the Mediterranean could provide important clarifications about the complex mantle dynamics of this region and help us understanding the interaction between surface tectono-magmatic processes and mantle convection patterns.&#160;The mantle flow and its relations with plate horizontal and vertical motions can be determined by measuring seismic anisotropy generated by strain-induced lattice/crystal preferred orientation (LPO/CPO) of intrinsically anisotropic minerals. Seismic anisotropy is widespread in the Mediterranean and it shows an intricate pattern that likely has some relations with the recent (20-30 Myr) behavior of subducting slabs. The extrapolation of the mantle flow from seismic anisotropy is neither simple nor always warranted, especially at subduction zones where complex and non-steady-state 3D flow patterns may establish.&#160; A promising approach, which helps reducing the number of plausible models that can explain a given anisotropy dataset, is to compare seismic measurements with predictions of numerical and experimental flow models (Long et al.,2007). Recently, Faccenda and Capitanio (2013) and Faccenda (2014) have extended this methodology to account for the non-steady state evolution typical of many subduction zones, yielding mantle fabrics that are physically consistent with the deformation history.In this study, we apply a similar modelling approach to the complex Central-Western Mediterranean convergent margin. We use the wealth of observations from the Mediterranean region available in the literature to design and calibrate 3D thermo-mechanical subduction modelling. We test different initial configurations defined at 30 Ma according to the paleogeographic and tectonic reconstructions derived from (Lucente and Speranza, 2001; Carminati et al., 2012; van Hinsbergen et al., 2014) in order to optimize the fit between predicted and observed slabs position and obtain a final model configuration resembling the present-day surface and deeper structures.In particular, here we want to evaluate the influence on rollback rates, trench shape and the occurrence and timing of slab tears (Mason et al., 2010) of structural heterogeneities within the Adria plate as proposed by (Lucente and Speranza, 2001). In all models, subduction migrates south-eastward driven by the subducting oceanic lithosphere, and slab lateral tearing or break-off occurs when a continental margin enters the trench. More importantly, we show that the presence of a stiffer continental promontory in central Adria together with a thinned continental margin in the Umbria-Marche region plays a fundamental role on (i) the development of a slab window below the Central Apennines, (ii) the retreat of the Northern Apenninic trench till the Adriatic Sea, and (iii) the retreat of the Ionian slab till the present-day position.

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Centrifuge Experiments of the Initiation of Self-Sustaining Subduction

10.5194/egusphere-egu21-433 ◽

2021 ◽

Author(s):

Yossi Mart ◽

Liran Goren ◽

Einat Aharonov

Keyword(s):

Subduction Zones ◽

Lateral Force ◽

Cardinal Process ◽

Lithospheric Plates ◽

Weakness Zone ◽

Subducting Plate ◽

Break Up ◽

Back Arc ◽

Analog Models ◽

Analog Experiments

The post-Triassic age of all oceanic lithospheres indicates the efficiency and the sustainability of lithospheric subduction, which consumes the basaltic seafloor and recirculates it in the upper mantle. Since at present the initiation of subduction is very rare, comprehension of this cardinal process should be carried through modeling &#8211; numeric or analog. While deciphering processes through numeric modeling is commonly comprehensive, the analog models can determine major factor that constrain a tectonic procedure. Analog centrifuge experiments were applied to initiate self-sustained modelled subduction, trying to determine the critical factors that trigger its early stages.Analytically we presumed that where densities of two lithospheric plates, juxtaposed across a weakness zone, exceed a critical value, then the denser lithosphere eventually will drive underneath the lighter one, provided the friction across the interface is not too high.&#160;Consequently, analog experiments were carried out in a centrifuge at acceleration of ca. 1000 g., deforming miniaturized models of three layers representing the asthenosphere, the ductile and the brittle lithosphere. The lithospheres were modeled to include lighter and denser components, juxtaposed along a slightly lubricated contact plane, where the density difference between these components was ca. 200 kg/m3. No mechanism of lateral force was applied in the experiment (even though such a vector exists in nature due to the seafloor spreading at the oceanic ridges), to test the possibility of subduction in domains where such a force is minor or non-existent.The analog experiments showed that the penetration of the denser modeled lithosphere under the lighter one led to extension and subsequent break-up of the over-riding plate. That break-up generated seawards trench rollback, normal faulting, rifting, and formed proto-back-arc basins. Lateral differential reduction of the friction between the juxtaposed plates led to the development of arcuate subduction zones. The experimental miniaturization, and subsequent numerical and analytical modeling, suggest that the observed deformation in the analog models could be meaningful to the planet as well.Constraints of the analog experimentation setting did not enable the modeling of the subduction beyond the initial stages, but there is ground to presume that at depths of 40-50 km, metamorphic processes of the generation of eclogites would change the initial mineralogy on the subducting plate. Reactions with water would convert basalts into metamorphic serpentinites and schists. Higher temperatures and pressures would melt parts of the subducted slab to generate felsic magmas, which would ascend towards the surface diapirically due to their lighter density. Alternately, low availability of H2O would gradually alter the oceanic basalt and gabbro into eclogite, which would sink into the mantle due to its increased density.

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Subduction zone seismo-dynamics: how to bridge the gap between long-term subduction dynamics and megathrust seismicity?

10.5194/egusphere-egu21-2322 ◽

2021 ◽

Author(s):

Adam Beall ◽

Fabio A. Capitanio ◽

Ake Fagereng ◽

Ylona van Dinther

Keyword(s):

Stress State ◽

Subduction Zone ◽

Subduction Zones ◽

Large Scale ◽

Large Earthquake ◽

Force Balance ◽

Mantle Flow ◽

Plate Interface ◽

Plate Motions

The largest and most devastating earthquakes on Earth occur along subduction zones. Here, long-term plate motions are accommodated in cycles of strain accumulation and release. Episodic strain release occurs by mechanisms ranging from rapid earthquakes to slow-slip and quasi-static creep along the plate interface. Slip styles can vary between and within subduction zones, though it is unclear what controls margin-scale variability. Current approaches to seismo-tectonics primarily relate the stress state and seismogenesis at subduction margins to interface material properties and plate kinematics, constrained by recorded seismic slip, GPS motions and integrated strain. At larger spatio-temporal scales, significant progress has been made towards the understanding of subduction dynamics and emerging self-consistent plate motions, tectonics and stress coupling at plate margins. The margin stress state is ultimately linked to the force balance arising from interactions between the slab, mantle flow and upper plate. These mantle and lithosphere dynamics are thus expected to govern the tectonic regimes under which seismicity occurs. It remains unclear how these longer- and shorter-term perspectives can be reconciled. We review the aspects of large-scale subduction dynamics that control tectonic loading at plate margins, discuss possible influences on the stress state of the plate interface, and summarise recent advances in integrating the earthquake cycle and large-scale dynamics. It is plausible that variations in large-scale subduction dynamics could systematically influence seismicity, though it remains unclear to what degree this interplay occurs directly through the plate interface stress state and/or indirectly, corresponding to variation of other subduction zone characteristics. While further constraints of the geodynamic controls on the nature of the plate interface and their incorporation into probabilistic earthquake models is required, their ongoing development holds promise for an improved understanding of the global variation of large earthquake occurrence and their associated risk.

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Computing LPO for Geodynamic Models in ASPECT

10.5194/egusphere-egu2020-12579 ◽

2020 ◽

Author(s):

Magali Billen ◽

Menno Fraters

Keyword(s):

Water Content ◽

Flow Pattern ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Flow Patterns ◽

Mantle Flow ◽

Lattice Preferred Orientation ◽

Axis Alignment ◽

Geodynamic Models ◽

Initial Results

When modeling subduction processes, the results are usually constrained by looking at the geological surface expressions, geochemistry and geophysical observations such as tomography and seismic anisotropy. Of these observations, seismic anisotropy is the only type of observation that can potentially be directly linked to the spatial flow pattern in the mantle. Seismic anisotropy in the mantle is due to lattice-preferred orientation (LPO) of olivine minerals. In subduction environments, which can have complex and changing flow patterns, it is not expected that the LPO necessarily aligns with the flow pattern. This is partly due to the fact that it takes time to realign the LPO and partly because the olivine fast axis alignment depends on the water content and the magnitude of stress. To overcome this problem, the LPO must be computed for realistic and end member subduction zones in order to be able to relate seismic anisotropy to mantle flow and thereby slab dynamics.There are many ways to compute LPO. For this study we have used DREX (Kaminski et al., 2004), because the underlying method is accurate and fast enough for use in geodynamic models. To achieve a good and native integration with ASPECT (Kronbichler et al., 2012; Heister et al., 2017; Bangerth et al,. 2019), we have rewritten DREX in CPP as a plugin for ASPECT. In this presentation we will show how it was implemented and what the limitations and possibilities are. Furthermore, we will show initial results from 3D subduction models to study the link between seismic anisotropy and mantle flow.

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