DI53A-1674
Coseismic subduction zone strain-release as a constraint for slab dynamics
Slab rheology and the lateral viscosity variations in the ambient mantle in subduction zones are key controls on upper mantle dynamics but still poorly constrained. Mantle flow modeling puts seismic strain-rates into the context of regional subduction dynamics and global mantle circulation, and our goal is to arrive at a globally consistent model of plate motion and slab deformation. We analyze the co-seismic strain-rates that are recorded by the global centroid moment tensor (gCMT) catalog in conjunction with the temperature anomaly distribution and slab morphology that can be inferred from seismic tomography. Our analysis of gCMTs confirms the global patterns that were established by Isacks and Molnar (1971) where intermediate depth in- slab extension merges into deep in-slab compression in some, but not all subduction zones. The large increase of seismic data over the last tens of years allows spatial refinements of Kostrov summations. Those show ubiquitous, second-order complexity on regional scales, perhaps due to effects such as slab tears. We find that no simple kinematic model can provide an adequate description of this diversity in strain patterns. Our fluid dynamical modeling builds on the 2-D analysis of Vassiliou & Hager (1988) and regional 3-D studies by Billen & Gurnis (2003); we strive to explore both regional and global fluid-dynamical effects such as slab deflection and folding at 660~km, effects of a higher viscosity lower mantle and weak mantle wedge, and the role of depth-dependent slab strength. A better understanding of the interaction of the large-scale flow with subducting slabs as imaged by seismicity motivates improved 3-D models. Those can provide background information for what slab deformation states are predicted based on global plate dynamics, and what component of strain-release may be indicative of regional, possibly transient tectonic effects.
DI53A-1675
Evaluating Subduction Initiation Potential on the Present-Day Earth
Subduction, the process by which oceanic lithosphere is recycled into the Earth's mantle, is a central component of plate tectonic theory. However, the process by which new subduction zones initiate is not well understood. Several different models have been proposed, such as passive margin collapse aided by a mechanism for lithospheric weakening, polarity reversal at an active subduction zone, and gravitational collapse at a fracture zone or other preexisting zone of lithospheric weakness. In this study, we focus on the third type of mechanism, which has been explored through numerical models by Hall et al. (2003) and Gurnis et al. (2004). These models describe three conditions necessary for subduction initiation: presence of a fracture zone, offset in the age of the ocean floor along the fracture zone, and significant compression normal to the fracture zone. We evaluate the predictions of the Gurnis et al. (2004) model on the present-day Earth. With digital seafloor age data and global stress models we calculate a parameter predicting the location of relatively likely subduction initiation on the ocean floor. We compile a database of oceanic fracture zones with associated age offsets from seafloor age maps and evaluate the state of stress on each fault segment using the global stress models of Lithgow-Bertelloni and Guynn (2004) and Ghosh (2008). For each fault segment we calculate a "subduction initiation likelihood parameter" as the normalized magnitude of the stress normal to the fault strike. In this way, we identify fault segments that may undergo compressive strain as they have significant compressive stress normal to the fault strike. For many fault segments, our parameter depends heavily on which stress model is employed. We predict regions of relatively likely subduction initiation, including regions in the northern Pacific and Indian oceans. The evaluation of predictions of models for subduction initiation on the present-day Earth should lead to a better understanding of processes associated with the initiation and early evolution of subduction zones.
DI53A-1676
Regional Body-wave Tomography Reveals a Linear Deep-seated Low-velocity Zone Beneath the Cameroon Volcanic Line
Linear volcanic chains without age progression present challenges to current models of mantle dynamics. This study presents results from a seismic experiment to investigate the origin of the Cameroon Volcanic Line (CVL). The CVL is a 1600km feature traversing both continental Cameroon in west Africa and the offshore islands of Bioko (part of Equatorial Guinea), Sao Tome and Principe, and Annobon (also part of Equatorial Guinea). The CVL is a fairly linear feature, suggestive of the movement of the African plate over a stationary hotspot, but the volcanic rock ages of the CVL range from 42Ma to the present (with present volcanism occurring in the center of the line at Mt. Cameroon), contrary to what would be expected from a stationary hot spot. Several hypotheses have been proposed for the formation of the CVL such as multiple plumes, lateral flow from the Afar depression, and edge-flow convection initiated by the temperature differences between the mantle and the nearby Congo Craton. The Cameroon Seismic Experiment was deployed in Cameroon from January 2005 to January 2007, with 8 stations active the first year and an additional 24 stations installed in January 2006. The data from the 32 broadband seismometers has been used for a body-wave tomography study to study upper mantle structure. Results from P- and S-wave travel time tomography show a steep-sided linear low-velocity anomaly directly beneath the CVL that extends from shallow mantle depths to at least 350km. Preliminary 1D results from the stacking of receiver functions to image mantle transition zone discontinuities suggest that this anomaly continues to greater than 500km depth. This finding suggests that the anomaly is not caused by regional flow patterns associated with the Congo craton or a single plume but instead might require a model invoking a linear sheet-like thermal upwelling in the mantle.
DI53A-1677
Island Freeboard Reconstructions for the Cape Verde Archipelago Using the Geological Record
Ocean island freeboard of the Cape Verde Archipelago off the west coast of Northern Africa is contemporaneous with volcanism as determined by uplift of paleo sea-level markers over the 25 Ma history of hotspot activity. Whereas the growth and decay of ocean island volcanoes is accompanied by buoyancy- driven vertical movements such as: a) aging of the underlying lithosphere; b) lithospheric flexural response to surface and sub-surface loading and volume redistribution by mass-wasting; and c) volume changes from hotspot dynamics, tracking island freeboard through geological time potentially provides constraints on the relative importance of these different processes for lithospheric behavior and island building in response to hotspot activity. The Cape Verde Archipelago is unique among major hotspots due to its semi-stationary position and because it sits on the largest bathymetric anomaly in the oceans - the Cape Verde rise. Paleo sea-level markers include shallow marine carbonate terraces, beach deposits, submarine volcanic units, and lava deltas and marine abrasion surfaces. Using high-precision laser step heating Ar-Ar and U-Th disequilibrium geochronological methods, we dated a set of raised submarine volcanic flows, lava-deltas and terraces from the islands. By tracking the relative sea-level changes within the islands (using the age and the present elevation of the markers), and comparing them with the eustatic sea-level curve, we found evidence for substantial vertical movements accompanying volcanism, at rates ranging from 35-100m/Ma over the last ~6 Ma. The maximum vertical displacement is +450 m for Santiago Island. We deduce that island freeboard is contemporaneous with volcanism, and are currently evaluating parameters that account for total freeboard and differential uplift observed among the islands.
DI53A-1678
What do Satellite Gravity Data Suggest About Intraplate Volcanism in the Pacific?
The origin of intraplate volcanism is still debated and we need to image the Earth's interior structure in order to improve our understanding about the geodynamical processes involved. Plume dynamics are driven by density contrasts inside the Earth's mantle. Density contrasts are associated with gravity anomalies which can be measured, from space or at the Earth's surface. Thus, jointly analyzed with topographic data, gravity can provide important constraints on the flow models within the Earth's interior. With the advent of satellite gravimetry, GRACE (2002) and soon GOCE, new high-quality datasets are available. They lead to a valuable improvement of our knowledge of the gravity field both at long and medium wavelengths and can help to better comprehend the geodynamical processes producing intraplate volcanism. In this study, we focus on the example of the Pacific area, marked by numerous volcanic chains. We review the possible volcanism mechanisms and their associated gravity signatures, and we compare the expected gravity signals with observations from GRACE satellite gravity. We evidence large scale gravity anomalies over Hawaii and the Line Islands with very different characteristics, and that are consequently not consistent with a unique mechanism explaining the volcanism in the Pacific.
DI53A-1679
An Evaluation of the Fixed Hotspot Hypothesis for the Pacific Plate
Using geometry and ages from 12 Pacific seamount chains, we recently constructed two new Pacific absolute plate motion models that extend our self-consistent and high-resolution models back to 145 Ma. The WK08-A model maps the full uncertainty in the age progressions into uncertainties in rotation opening angles, yielding a relatively smooth plate motion model. The WK08-G model relaxes the mapping of age uncertainties in order to better isolate secondary geometry changes seen along many co-registered chains. Both models have been used to assess the viability of the fixed hotspot hypothesis in the Pacific. In constructing these models, we found that only a small group of age samples had to be discarded on the grounds that they were discordant with the dominant trends. We were able to connect plate motions for pre- and post-Emperor age intervals by including the Ratak-Gilbert-Ellice, Liliuokalani and Musicians trails in our analysis. However, as no active hotspot locations exist for the older chains their inclusion adds additional model parameters. Both age and geometry misfits increase with age, reflecting the observed increase in age uncertainties and the broader and less distinct nature of the older trails. Paleomagnetic observations from the Emperor seamount chain have been interpreted to suggest that these seamounts must have formed at latitudes significantly more northerly than the present location of the Hawaii hotspot, implying a drifting mantle plume. At the same time, new estimates of the age of the Hawaii- Emperor bend places bend formation at a time of global plate reorganization. We will present a complete analysis of inter-chain distances between coeval radiometric samples from Pacific chains and compare these distances to the inter-hotspot distances at the present time. Significant departures from the current hotspot separations would be direct and unequivocal evidence of motion between the Pacific hotspot reference frame and the spin axis and as such complement the paleomagnetic analysis. Preliminary results suggest the Emperor segment data may indicate an increase in hotspot separation.
DI53A-1680
Updated test of fixity of the Iceland hotspot relative to Pacific hotspots
Hotspot fixity has long been challenged on the basis of plate-circuit reconstructions. If the motion of one plate over its hotspots is known, and if these hotspots are assumed to be fixed with respect to those under the second plate, reconstructions of the past positions of the two plates can be used to compare the predicted positions of hotspots under the second plate with its known trace. If the plates are rigid, if all ancient plate boundaries in the global plate circuit have been recognized and incorporated, and if hotspots are fixed relative to one another, the predicted track should coincide with the known trace (within uncertainties). Inconsistencies, on the other hand, give an estimate of the relative motion between hotspots. Here we present updated predictions for the track of the Iceland hotspot assuming it to be fixed relative to a Pacific hotspot reference frame. We build on a new method for objectively estimating plate-hotspot rotations and their uncertainties (Andrews et al., 2006). Besides the uncertainties in plate-hotspot rotations, uncertainties in relative plate motions are accumulated through the plate circuit to obtain the final uncertainty in the predicted positions. Predictions are made for ages of 11 Ma (chron 5), 20 Ma (chron 6), 33 Ma (chron 13), 39 Ma (chron 18), 48 Ma (chron 21), 56 Ma (chron 25) and 68 Ma (chron 31). The current position of the Iceland hotspot is taken to be at 64°N, 344°E. The observed and predicted Iceland hotspot tracks are not significantly different over the past 48 Myr. The 68 Ma reconstructed point, however, lies ~1100 km from the oldest lavas on the West Coast of Greenland dated as about 65 Ma (Larsen et al., 1992). If this misfit occurred entirely between 68 Ma and 48 Ma, it gives an early Tertiary rate of motion between the Iceland and Pacific hotspots of about 55 ± 15 mm yr-1. Thus, we show that the Iceland hotspot has had no significant motion relative to a Pacific hotspot frame since 48 Ma. Prior to 48 Ma, however, the apparent inter-hotspot drift rates increase to about 55 ± 15 mm yr-1. A possible cause for the pre-48 Ma apparent motion is a systematic error in the global plate circuits used to make the predictions. Potential candidate for the error is pre-48 Ma motion across Antarctica.
DI53A-1681
New Insights on Cenozoic Plate Evolution and Mantle Dynamics in the Indo-Atlantic Hotspot Reference Frame
We present a new model of Cenozoic plate motions in the Indo-Atlantic hotspot reference frame. This reconstruction is very different from the previous model established in the global (Pacific) hotspot reference frame by Gordon and Jurdy (1986), mainly because the latter model was based on three main postulates which have recently been placed into considerable doubt: the fixity of the Pacific hotspots, a change in direction of the Pacific plate used to explain the Hawaiian Emperor Bend and the introduction of the Chatham Rise plate. We present a number of new results. Firstly, we found a present-day slow northward drift in the Indo-Atlantic frame which differs markedly from the fast westward drift obtained in the Pacific frame. Secondly, this new plate history does not show a major Eocene plate reorganisation, nor an abrupt change in direction of the Pacific plate which has been previously invoked as an explanation for the Hawaiian Emperor Bend. Thirdly, we found that the net rotation of the lithosphere is very slow during the last 60 Myr, which suggests that the HS and NNR reference frames are quite similar.
DI53A-1682
Probing the Causes of Epeirogenic Uplift Beneath NW Europe
During Cenozoic times, permanent uplift was generated by magmatic underplating when the North Atlantic ocean opened over the Iceland plume. Geochemical and petrological arguments show that much of this melt crystallised at Moho depths. However beneath the British Isles, the existence of magmatic underplating is debated and its spatial extent is poorly known. Evidence also exists for transient uplift at the present day. Both P- and S-wave tomography reveal low velocities beneath the lithospheric plate. These low velocities may relate to hot buoyant mantle, which is consistent with the dynamic support inferred from long-wavelength gravity anomalies. In collaboration with the British Geological Survey, 26 broadband seismometers have been installed across the British Isles. Teleseismic arrivals recorded at each station have been used to calculate receiver functions which yield the velocity structure. This method should enable the thickness and lateral extent of underplating to be determined. Initial results yield Moho depths of ~32 km with up to four intracrustal interfaces. Within the mantle, conversions at the 410 and 660 km discontinuities are observed and the Lehmann and 520 km discontinuities are patchily detected. The data have allowed a transect of the mantle from western Ireland to Belgium to be imaged, showing the depths and lateral extents of the various phase boundaries. A thinner mantle transition zone is visible beneath southern England and the 660 km discontinuity possibly exhibits splitting to the west of Ireland. The mantle data are currently being interpreted in terms of temperature and chemical variations using mineral physical models.
DI53A-1683
The Effects of Mantle Flow and Plate Rheology on the Lithospheric Stress Field
The relationship between surface deformation and mantle-lithosphere coupling depends strongly on the viscosity structure of both the lithosphere and convecting mantle. Lateral variations in the thickness and viscosity of the lithosphere affect both the flow pattern and shear stress magnitude at the base of deeply penetrating continental roots. We examine how lateral variations in mantle shear influence the deformation pattern in a simplified elastic lithosphere. We use CitcomS to compute the instantaneous mantle flow driven by both tomographically-inferred density heterogeneity (mantle) and surface plate motions (plate), and the corresponding basal shear tractions, for both layered and laterally-varying viscosity structures. We use ABAQUS to solve for the response of an elastic lithosphere to the applied net horizontal shear tractions (mantle + plate) for a 3-D spherical shell. While the viscosity structure of the lithosphere has only a minor influence on the orientations of the net basal tractions acting on the lithosphere, the magnitudes of these tractions vary strongly as a function of the lithospheric viscosity structure. As a result, the presence of thick cratonic roots enhances coupling at the base of the lithosphere leading to higher traction magnitudes in and around these regions, which affects patterns of elastic stresses within the lithosphere. Interestingly, the presence of thick viscous roots also leads to lower traction magnitudes in regions adjacent to some of the roots. The magnitude of the elastic stress field produced by basal tractions reflects both the gradients and absolute value of the tractions, so peaks in the elastic stress field magnitude often occur both within and adjacent to thick lithospheric roots. In general, changes in the magnitude of elastic stresses are smaller than those in the net horizontal tractions. In addition to the effects of laterally varying viscosity, we also examine the roles of plate boundary strength and magnitude scaling of the plate-driven basal tractions in determining regional traction and elastic stress patterns. Although an improvement to previous models, our results so far do not account for variable lithospheric rheology between the base of the lithosphere and the base of the elastic lithosphere. As the rheological structure of the lithosphere varies significantly as a function of tectonic province and age, future global flow models with regional mesh refinement should help to determine how sharp variations in the strength of lithosphere influence the relationship between large-scale mantle flow and surface deformation.
DI53A-1684
Numerical Mantle Convection Modeling: The Effect of Plates on the Surface Topography
Until today it is unclear in which way mantle processes find their expressions in surface signatures such as heat flow, gravity and topography. On Earth, plates may shield the surface from far below processes. Further, the situation is complicated by the interaction of internal dynamics and the motion of surface plates. In order to better understand this relationship and the possible imprints of internal dynamics on the surface, we have employed a numerical model of mantle convection with a complex rheology. The equations of mantle convection have been solved using the multigrid method in a finite volume formulation. In particular, we applied a viscosity structure which strongly depends on temperature, pressure and stress, thus allowing for surface plates to form naturally and as an integral part of the convective system. The experiments were carried out in a Cartesian box with stress-free, impermeable boundaries, reflecting sidewalls and isothermal temperatures at the top (T = 0) and at the bottom (T = 1). In this configuration we have performed several numerical experiments to understand the surface expression of rising mantle plumes. While it is common understanding that plumes elevate the surface topography, our results show that the existence of a mantle plume can also correlate with a depression in the topography rather than always leading to an elevation. One reason for this phenomenon is that the overriding plate sinks deeper into the mantle at the plume location and overcompensates the effect of the buoyancy-driven plume.
DI53A-1685
Coupling mantle convection and 3D elasto-visco-plastic surface layer including lithosphere and asthenosphere
The processes in the deep Earth are linked to the surface through the lithosphere and less viscous asthenosphere in the uppermost 200-300 km. This layer is the most heterogeneous and rheologically complex part of the Earth. The complexity of this layer is usually ignored or strongly simplified in geodynamic models, although it affects the interaction between the convecting mantle below and tectonic plates within this layer. In this project we use a newly developed 3D thermomechanical finite element numerical technique (Popov and Sobolev, PEPI in Press) to model a 200 km thick upper layer coupled with the convecting mantle. The present day temperature distribution and crustal structure within the layer are taken from existing models: continental temperature from Artemieva (2006), ocean temperature based on ocean age, and the crustal structure from the Crust2 model. We assume that the upper layer has non-linear temperature and stress dependent visco-elastic rheology, corresponding to the dry olivine (mantle) or naturally wet plagioclase (crust), combined with Mohr-Coulomb plasticity with an effective friction coefficient of 0.6. Plate boundaries are represented by the narrow zones of elasto-visco-plastic rheology with much lower friction than within the plates. The mantle below the 200 km depth is modeled using Hager and O'Connell's mantle flow spectral modeling technique with present day density and viscosity distribution from Steinberger and Calderwood (2006). The upper layer and mantle modeling domains are coupled through tractions and velocities at 200 km depth. Here we will show modeling results for the present day Earth structure focusing on the effect of the Earth potential temperature and frictional strength at plate boundaries on the plate velocities, dynamic topography and stress distribution in the crust.
DI53A-1686
Testing Mantle Circulation Models
Over the past decade, a new family of mantle convection models have been developed, which are conditioned by recent plate motion history (e.g. Bunge et al., 1997). They are commonly known as 'mantle circulation models' and allow for comparisons between present-day model predictions and ever improving seismic tomography images (e.g. Li et al. 2008). In this work, we present results from systematic investigations into the influence of various model parameters upon final model prediction/tomography correlations, to obtain a better understanding of their relative importance. These include a range of material properties, such as the radial viscosity structure, the Clapeyron slope of mineral phase transitions and compressibility; in addition to other aspects, such as the initial condition for the simulation. For our comparisons, we focus in particular on two large robust mid-mantle seismic anomalies, which others have related to the subduction of the Farallon and Tethys plates (e.g. Romanowicz, 1980). While these features are recovered with some fidelity in most simulations, the match can vary greatly. We find that there is a great deal of information in this mismatch, which includes information on the plate motion history.
DI53A-1687
Effect of lateral viscosity variations on mantle flow and the geoid
We address the long-standing question of how lateral viscosity variations in the mantle, such as due to presence of stiff slabs, affect the geoid. The long wavelength geoid is sensitive to the radial viscosity distribution within the Earth and an increase in viscosity, usually placed at the upper-lower mantle boundary at 660 km, is required to explain the long wavelength geoid. However, effects of lateral viscosity variations on the geoid are still not clearly understood. We are motivated by the findings of Zhong & Davies (1999) who found that introducing stiff slabs in the lower mantle degrades the fit to the Earth's long wavelength geoid compared to a model with only radial viscosity variations. This would indicate that slabs in the lower mantle are of the same strength as the ambient mantle, which is somewhat contrary to expectations. Moucha et al. (2007), however, recently argued that lateral viscosity variations inferred from seismic tomography have a minor effect on the geoid. We re-investigate the problem by computing the geoid in the presence of rheological complexity using the high resolution finite element mantle convection code, CitcomS. We use different models of slab and density anomalies in the mantle, and vary slab strength, temperature-dependent viscosity, and background layer viscosity profiles in a consistent way. We test different slab viscosities and compute the correlation with the observed geoid, striving to test which description of slab dynamics, and mantle rheology, is consistent with a range of constraints from the geoid and other inferences, such as from slab bending studies.
DI53A-1688
Can the longwavelength geoid be used to map postperovskite in the D" layer?
The recent discovery of the postperovskite phase due to mineral physics can have important consequences for understanding the structure of and processes in the lowermost mantle. However, geophysical observables that can confirm the presence and the very existence of this phase in the lower mantle are rare. So far, the search for postperovskite regions has been mainly based on analysing seismic records. In the present study, we investigate whether the analysis of other type of data, namely of the long-wavelength non-hydrostatic geoid, can bring some information about the distribution of postperovskite in the D" layer. We assume that the perovskite-to-postperovskite phase transition is accompanied by a decrease of viscosity, and, for a synthetic model of a subducting slab, we investigate the influence of such viscosity variation on the mantle flow which in turn affects the non-hydrostatic geoid associated with the slab itself. The results of our numerical tests suggest that the geoid is strongly sensitive to the presence of localized low-viscosity regions in the D" layer, especially if their lateral dimension is large (more than 1000 km) and if they are located close to the bottom end of the subducted slab. We also test the robustness of the geoid inversion using synthetic data and demonstrate that the resolution of the inversion can be significantly improved if a priori seismic constraints on geometry of the postperovskite domain are taken into account.
DI53A-1689
Determination of the electromagnetic and topographic core-mantle coupling torques based on geomagnetic observations: Influence of the electric conductivity of the Earth's mantle
The determination of the electromagnetic (EM) and topographic (TOP) core-mantle coupling torques is based on the observed geomagnetic field at the Earth's surface and additional assumptions like the electric conductivity profile of the Earth's mantle and the topography of the core-mantle boundary (CMB). For the calculation of the EM torques, we need the poloidal and the toroidal geomagnetic field at the CMB, because the EM torques are described as surface integrals over the CMB, on which this torques act. Therefore, we have to determine the poloidal geomagnetic field at the CMB by a non-harmonic downward continuation (NHDC), which considers the electrically conducting mantle. For the toroidal geomagnetic field, the related initial boundary value problem is formulated and solved numerically. The boundary values at the CMB are dependent on the poloidal geomagnetic field and the surface flow velocities of the fluid outer core at the CMB. This velocity field is determined by fluid flow inversion, based on the frozen-flux approximation and the additional constraint of tangential geostrophy (Wardinski 2004). This surface velocity fields are also consistently used for the calculation of the TOP torques. In addition, we need a model for the topography of the CMB. In our investigation we use different conductivity profiles and CMB topography models to calculate the EM and TOP coupling torques, which are compared with excitation function derived from observed polar motion and length of day variations (taken from the IERS).
DI53A-1690
Thermal Convection Heated both Volumetrically and from Below: Implications for Predictions of Planetary Evolution
Thermal convection with both volumetric heating and heating from below is considered as a model for the internal dynamics of planetary mantles. A series of 2D and 3D numerical experiments is described where a bottom heat flux is prescribed as well as a constant fraction of volumetric heating with either free-slip or no- slip conditions for the two horizontal boundaries. The assumption that hot plumes rising in the interior of the layer act as volumetric heat sources lead to a first order scaling for the structures of thermal boundary layers. Cases with a no-slip condition are shown to agree well with this simple scaling while cases with free- slip systematically deviates from this prediction: a decrease of approximately 20-30 % is observed for the temperature difference across the boundary layer when the fraction of heating from below is increased from 0 to 1. Earlier numerical results with a prescribed bottom temperature also follow the same deviation. We show that these variations are attributable to varying velocity structures near the boundary with the fraction of volumetric heating.
DI53A-1691
Effect of majorite on convection pattern and thermal evolution of mantle
Majorite is one of the MgSiO3 polymorphic minerals, occupying the highest temperature stability region (above 1900 kelvins) in a depth range of 500-670 km. Majorite has a wedge-shaped stability region in a@peridotite system, and is cut at high-P side by a polymorphic reaction of majorite to perovskite with a positive-steep Clapeyron slope and a large volume change. On the other hand, the low-P side of the stability region of majorite is limit by reactions of ringwoodite = majorite + wustite and wadsleyite = majorite + wustite, depending on the temperature. The low-P side of the stability region of majorite has a gentle negative slope. Rinwoodite, perovskite, and majorite make a triple point at the pressure of 23 GPa and the temperature of 1900 kelvins, which is slightly hotter than the geotherm. The effect of the presence of majorite stability region would be unique since it is asymmetric to convection: upwelling hot plume can pass through the stability region of majorite while downwelling cold plume can not. In this study we performed 3D numerical simulation on thermal convection of mantle taking into account majorite to discuss the effect of majorite not only on the convection patter of mantle but also thermal evolution of the earth. According to our numerical results, upwelling hot plumes lose their thermal energy at the phase boundary from perovskite to majorite because of endothermic reaction. On the other hand, downwelling cold plumes are not affected since they do not pass through the phase boundary. As a result upwelling hot plumes lose buoyant force and are decelerated. Such an asymmetric diversity becomes more obvious for higher temperature at the base of the mantle. Thus the presence of majorite is the key factor to investigate the thermal evolution of mantle in the past back to the Archean.
DI53A-1692
Time Dependent Layering in Earth's Mantle: Mantle Avalanches and Thermal Pulses
The possibility of time dependent layering, partial or otherwise, in Earth's mantle provides a mechanism,
which could introduce periodicity into Earth's thermal evolution. The implications for deep Earth science
would be significant. In the 3D spherical geometry modelling work presented here, the 660 km phase
boundary was manipulated to induce layering to varying degrees. The time evolution of models that pass
through various stages of full layering, partial layering and whole mantle convection is presented. Significant
periodic time dependence emerges in the surface heat flux as model cases move into a partially layered
regime. Previous work demonstrates cold upper mantle 'avalanches' into the lower mantle (Tackley et al.
1994), we confirm the existence of these avalanches and find evidence for the opposite process, a pulse of
hot material entering the upper mantle from below. Such a process has been predicted for a partially layered
mantle using parameterized models (Davies 1995). As far as we know, this process has not previously been
demonstrated in 3D spherical geometry modelling. Should conditions within Earth's mantle be suitable for
such processes to occur, the thermal evolution of the planet could show significant variation beyond secular
and radioactive cooling trends. The resulting moderation of heat flux over time could resolve problems with
projecting mantle temperature back in time with parameterized models (Butler and Peltier 2002). Additionally,
periodic heat pulses into the upper mantle could promote increased melt extraction at certain times in Earth
history. This provides a mechanism that could produce the phases of increased melting observed in some
geochemical studies.
Butler, S. L., and Peltier, W. R., 2002, Thermal evolution of Earth: Models with
time-dependent layering of mantle convection which satisfy the Urey ratio constraint: J. Geophys. Res., v.
107, p. doi: 10.1029/20000JB000018. Davies, G. F., 1995, Punctuated tectonic evolution of the
Earth: Earth Planet. Sci. Lett., v. 136, p. 363-379. Tackley, P. J., Stevenson, D. J., Glatzmaier, G. A.,
and Schubert, G., 1994, Effects of multiple phase transitions in a three dimensional spherical model of
convection in Earth's mantle: J. Geophys. Res., v. 99, p. 15877-15901.
DI53A-1693
Displacement of Lower Mantle Material Into the Upper Mantle and its Potential Effect on Mantle and Surface Redox Relations
Understanding the mantle and processes within it are key to understanding the Earth as a whole.
Fundamental to this is understanding the oxidation state of the mantle, speciļ¬cally iron valence states, as
this can have dramatic effects on mineral chemistry. It has long been known that in the upper mantle
Fe2+ is the dominant valence state, with only minimal amounts of Fe3+. Conversely in the lower
mantle a number of authors have observed that MgSi perovskite, the most abundant mineral in the Earth,
can incorporate as much as 50% Fe3+. This study investigated the redox reaction which takes place
when MgSi perovskite is displaced rapidly from the lower mantle into the upper mantle. (Mg, Fe)(Al, Si)
perovskite was synthesised in the multi anvil press and then analysed using Mössbauer spectroscopy,
which showed 48% Fe2+ and 52% Fe3+. The perovskite was later annealed in a piston cylinder at
2 GPa and 1200°C for 24 hours and then analysed with both Mössbauer and XRD, which showed
that the dominant mineral phase to be pyroxene with no perovskite present. The Mössbauer data gave
the pyroxene composition to be 90% Fe2+ and 8% Fe3+. Using the values obtained from
Mössbauer spectroscopy an equation can be written for the reaction of one mole of perovskite containing
8 wt% Fe oxide:
0.038FeOpv+ 0.021Fe2O3pv=0.072FeOpx
+0.003Fe2O3px +0.01O2 [1]
(The superscripts pv and px denote perovskite
and pyroxene, respectively.) Therefore, for every mole of perosvkite that is displaced into the upper mantle
0.01 moles of oxygen will be generated. Dynamic processes which displace material from the lower to the
upper mantle over a relatively short time scale (e.g. mantle avalanches or superplumes) could therefore
change the oxygen fugacity of the upper mantle over specific spatial and temporal scales. Such dynamic
processes have been linked to a rise in atmospheric oxygen levels through effects such as changes in ocean
productivity, photosynthesis and continental weathering. Our work shows that a further effect due to the
enhanced upper mantle oxygen fugacity over local scales which changes the chemistry of volcanic gas
emissions may play an important role in the link between Earth's deep interior and the surface.
DI53A-1694
A Late Cretaceous Contamination Episode of the EuropeanMediterranean Mantle
One of the most debated issues about the Tertiary-Quaternary alkaline magmatism of the Euro- Mediterranean region is the assessment of both the nature of its mantle source and the mechanism responsible for the common HIMU-like (High μ=high 238U/204Pb) character of erupted lavas, enduring over about 100 million years in diverse tectonic environments. We reconcile here geochemistry, timing and locations of the main Na-rich alkaline volcanic centers, seismic tomography and plate kinematics. We propose that the common component of the Euro-Mediterranean mantle derives from a contamination episode triggered by the rise of the Central Atlantic Plume (CAP) head. Highly incompatible element ratios and Sr-Nd-Pb isotope compositions indicate a common source for Na-rich alkali basalts of NE Atlantic, Europe and North Africa. Plate reconstruction shows that at Late Cretaceous-Paleocene time the oldest magmatic centers of the Euro-Mediterranean region were shifted more than 2000 km SW of their present day position, close to the CAP hot spot location, where seismic tomography detects a broad low seismic velocity region in the lower mantle. Thus, a possible common source for the Cenozoic Euro-Mediterranean volcanism could refer to this geographical area, representing both its Cretaceous paleo-position and geochemical endmember. The north-eastward migration of the Eurasian and African plates involved also the CAP contaminated mantle, which moved in the same direction coupled to the lithospheric plate, explaining the presence of geochemically-uniform material in the sub-lithospheric mantle. During the Tertiary, regional-scale convection and related processes such as rifting, back-arc spreading, slab detachment/windows, may have favored upwelling and partial melting of the frayed plume head material via adiabatic decompression, shaping the discontinuous spatial and temporal distribution of HIMU-like volcanics. The growing supply of subducted lithosphere may explain as well the increase of crustal isotopic signatures of alkaline magmas with time.
DI53A-1695
Generation of Plate Tectonics From Spherical, Visco-plastic Convection Models
The formation of plate tectonics from mantle convection is a major, unresolved problem in geophysics. Visco- plastic models have shown success and initial spherical results were discussed by van Heck & Tackley [submitted] for a restricted parameter space. Here, we present visco-plastic, mainly internally heated, 3-D spherical convection results with temperature-dependent viscosity, and strive to explore the Rayleigh number (Ra)--yield stress (σy) phase space using the finite element code CitcomS by Zhong, Tan, Moresi, and Gurnis. We also examine the effects of composite σy, core heating, and an asthenosphere. We find that with increasing Ra, the range of σy values that produce plate-like behavior widens: At high Ra, values close to those expected from experimental results (~1,000~MPa) can be used. We also find that convective planform and toroidal-poloidal velocity field ratio (TPR) are affected by near-surface viscosity variations. The TPR increases with Ra (~0.35 to 0.45), because stronger downwellings form, thereby increasing lateral viscosity variations. However, with a depth-dependent σy, TPR is low (~0.3) because the "plates" are everywhere weaker. All TPR values are lower than Earth's for the last 120 Ma (~0.55). The spatial wavelength of the dominant convective structure is generally large; most models favor spherical harmonic degree ℓ=1 convection [cf~Zhong et al., 2007]. Models with high Ra and low σy form ℓ=2 patterns, closer to the mantle according to seismic tomography. Furthermore, depth-dependent σy favors ℓ=2 convection for most Ra, indicating that a weaker surface allows shorter wavelength structures to form. According to TPR and convective planform, high Ra, constant σy models are most Earth-like. However, these models do not appear plate-like in terms of strain-rates, often showing diffuse deformation zones and areas with no coherent motion. This is improved by adding an asthenospheric viscosity reduction, after Tackley~[2000], which forms sharper deformation zones. Bottom heating has a strong effect on plate formation, as strong plumes tend to destroy surface plates. The plate regime requires sufficient internal heating, both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer. This points to a connection between the heating mode of the mantle and the low asthenospheric viscosity which is important for plate formation.
DI53A-1696
Cratons, the mantle, and time
Cratons contain the deepest records of time on the Earth and have potentially experienced the greatest variation in mantle dynamics. Yet, they have remained relatively unchanged and undeformed since their origin. Is this strictly due to their intrinsic characteristics or does the longevity of cratons imply anything about the evolution of the Earth's interior? The stability and longevity of cratons depends on their ability to resist deforming forces induced by the flowing and evolving mantle. Previous studies suggest that the combination of buoyancy, viscosity and finite strength provide cratons with sufficient stability to maintain a minimum lithospheric thickness atop a convecting mantle. However, geochemical observations also suggest that cratonic xenoliths originate from depths no greater than 250 km, which implies that a maximum craton thickness exists. What determines the maximum thickness that cratons? Does this thickness vary with time or mantle dynamics? We employ an analytical approach to relate the viscosity structure of the craton to flow within the underlying asthenosphere. In doing so, we show that as the net thickness of the chemical and underlying thermal boundary layers increases, the mantle-induced tractions on the combined structure will increase exponentially for non-Newtonian rheology. Thus, overly-thick lithosphere will be subjected to large stresses that will tend to weaken the thermal boundary layer, and diminish its role as a buffer between the flowing mantle and the chemically-distinct craton. This negative feedback prevents the cratonic lithosphere from exceeding some maximum value that depends on the viscosity structure of the thermal boundary layer. We present the initial estimates of the maximum thickness of the thermal buffer zone, which in turn controls the maximum thickness of cratonic chemical lithosphere and is required to maintain craton stability in the face of destabilizing mantle flow. In addition, we introduce preliminary scaling that suggests a non-dependence of maximum craton thickness on large-scale mantle dynamics.
DI53A-1697
Rotational bulge and True Polar Wander: influence of an elastic crust
The influence of a very viscous or elastic layer in the upper part of the lithosphere on the shift of the rotational axis is investigated. If this crust is elastic, there is a remnant rotational bulge perpendicular to the initial axis of rotation which tends to stabilize the polar motion. We discuss this stabilizing effect dependent on the order of magnitude of the inertia tensor perturbations due to the remnant rotational bulge and the ones due to the mantle density heterogeneities, for two cases: first a synthetic case with a blob sinking vertically within the mantle, and second a more realistic case taking into account simultaneously the subducted plates sinking within the mantle and the two upwelling superswells. Finally, the visco-elastic stresses are computed and the possible fracturation of this upper layer is discussed.
DI53A-1698
Thermally-Driven Mantle Plumes Reconcile Hot-spot Observations
Hot-spots are anomalous regions of magmatism that cannot be directly associated with plate tectonic processes (e.g. Morgan, 1972). They are widely regarded as the surface expression of upwelling mantle plumes. Hot-spots exhibit variable life-spans, magmatic productivity and fixity (e.g. Ito and van Keken, 2007). This suggests that a wide-range of upwelling structures coexist within Earth's mantle, a view supported by geochemical and seismic evidence, but, thus far, not reproduced by numerical models. Here, results from a new, global, 3-D spherical, mantle convection model are presented, which better reconcile hot-spot observations, the key modification from previous models being increased convective vigor. Model upwellings show broad-ranging dynamics; some drift slowly, while others are more mobile, displaying variable life-spans, intensities and migration velocities. Such behavior is consistent with hot-spot observations, indicating that the mantle must be simulated at the correct vigor and in the appropriate geometry to reproduce Earth-like dynamics. Thermally-driven mantle plumes can explain the principal features of hot-spot volcanism on Earth.
DI53A-1699
Quantitative Restoration of the Evolution of Mantle Structures Using Data Assimilation
Rapid progress in imaging deep Earth structures and in studies of physical and chemical properties of mantle rocks facilitates research in assimilation of data related to mantle dynamics. We present a quantitative approach to assimilation of geophysical and geodetic data, which allows for incorporating observations and unknown initial conditions for mantle temperature and flow into a three-dimensional dynamic model in order to determine the initial conditions in the geological past. Once the conditions are determined the evolution of mantle structures can be restore backward in time. We apply data assimilation techniques to model the evolution of mantle plumes and lithospheric slabs. We show that the geometry of the mantle structures changes with time diminishing the degree of surface curvature of the structures, because the heat conduction smoothes the complex thermal surfaces of mantle bodies with time. Present seismic tomography images of mantle structures do not allow definition of the sharp shapes of these structures. Assimilation of mantle temperature and flow to the geological past instead provides a quantitative tool to restore thermal shapes of prominent structures in the past from their diffusive shapes at present.
DI53A-1700
A Hypothesis Explaining the Dynamics of Plate Tectonics: A Dual-Inner Core Earth (DICE) System
Since the introduction of the plate tectonics theory, one of the unsolved fundamental problems has been the understanding of how and when plate tectonics began, as well as the initial dynamics of plate tectonics although several mechanisms have been suggested (i.e. the hypothesis that asthenospheric convections drive plates, which is not supported by modern geodynamic theories). The kinematics behind the subduction zones and sea- floor spreading have been well understood and well described by the theory of plate tectonics. However, there is no consensus on the main driving mechanism for the plate tectonics and how it began. We are proposing a hypothesis that attempts to explain several fundamental questions: (1) Why is the planet Earth unique among the silicate planets of the solar system in terms of plate tectonics? (2) How did plate tectonics start? (3) What did drive the lithospheric plates in the first place? (4) Why do mantle plumes form? and What kinds of forces are responsible for the plumes? (5) Why is the outer core fluid? (6) Why do "ultra-low seismic velocity zones" exist at the base of the mantle? and (7) Why does the magnetic field of Earth change? The other explanations and examples to be presented include three-dimensional animations of the Paleoproterozoic and Neoproterozoic Earth, and the kinematics and dynamics of the tectonic plates such as the Atlantic and Pacific plates.