Displays

The lower mantle can be grouped into high, low, and average (i.e., ambient) seismic velocity domains at each depth, based on the amplitude and polarity of wavespeed perturbations (% δlnVs, % δlnVp). Many studies focus on elucidating the thermo-chemical and structural origins of fast and slow domains, in particular. Subducted slabs are associated with fast seismic anomalies throughout the mantle, and reconstructed palaeo-positions of Cenozoic to Mesozoic subduction zones agrees with seismically imaged deep slabs. Conversely, slow wavespeed domains account for the two antipodal LLSVPs in the lowermost mantle, which are potentially long-lived features, as well as rising hot mantle above the LLSVPs and discrete mantle plumes. However, low-amplitude wavespeeds (close to the reference velocity models) are often overlooked By comparing multiple P- and S-wave tomographic models individually, and through “vote maps”, we reveal the depth-dependent characteristics and the geometry of ambient structures, and compare them to numerical convection models. The ambient velocity domains may contain early refractory and bridgmantic mantle with elevated Si/(Mg+Fe) and Mg/Fe ratios (BEAMS; bridgmanite-enriched mantle structures). They could have formed by early basal magma ocean (BMO) fractionation during a period of core-BMO exchange of SiO₂ (from core to BMO) and FeO (from BMO to core), or represent cumulates of BMO crystallization with bridgmanite as the liquidus phase. The high viscosity of bridgmanitic material may promote its convective aggregation and stabilise the large-scale, degree-2 convection pattern. Despite its high viscosity, bridgmanitic material, representing a primitive and refractory reservoir for primordial-like He and Ne components, might be entrained in vigorous, deep-rooted plumes. The restriction of a weak seismic signal, ascribed to iron spin-pairing in ferropericlase, to the fast and slow domains, supports the notion that the ambient lower mantle domains are bridgmanitic.

How to cite: Shephard, G. E., Hernlund, J., Houser, C., Trønnes, R., and Crameri, F.: Ambient lower mantle structure and composition inferred from seismic tomography, convection models, and geochemistry., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11806, https://doi.org/10.5194/egusphere-egu2020-11806, 2020.

Numerical modeling facilitates the exploration of geodynamic mechanisms that are inaccessible to direct geological sampling. However, quantitative comparison of geochemical signatures predicted by models with real petrological analyses remains restricted. On one hand, efficient melting parameterizations are limited in the information that they provide, on the other, thermodynamic models are not optimized for 3D geodynamic codes. In the Eastern Atlantic, several archipelagos are located near the continental margin, e.g. the Canaries, Cape Verde, Cameroon Volcanic Line, but the origin of this volcanic activity remains poorly understood. Suggested origins range from mantle-plume activity (deep origin) to Edge-Driven Convection (EDC, shallow origin), or an interaction of both mechanisms. To test and constrain these models, we use a recently developed parameterization, which can constrain major-element geochemistry of primary magmas in the form of wt% as a function of the P-T path, together with regional numerical models of EDC with or without plumes. In this work, using the finite-element code CITCOM, we explore 3D models with a step of lithospheric thickness (or “edge”) and with variable distances between an imposed plume and the edge. We predict characteristic compositional trends that depend on model parameters, such as plume temperature or distance of the plume from the continental edge, and compare them with actual melt-inclusion data from the Canary Islands and Cape Verde. We find geochemical trends ranging from alkalic – for the models without thermal anomalies or with weak plumes – to more tholeiitic – for the cases with vigorous plumes. In turn, EDC alone cannot explain the volcanic fluxes observed at the Canary Islands or Cape Verde, with predicted melting rates well below 1 km³ Myr^-1. Comparison with melt inclusions points towards the importance of CO₂, but a thermal anomaly (plume) is also needed. We use the obtained major elements together with the melt volumes and the plume buoyancy flux to constrain the most likely set of mantle properties that originate the aforementioned islands. Our preferred model is a weak, relatively cold plume (ΔT < 150 ˚C), moderately rich in volatiles, that is affected by the nearby EDC pattern.

How to cite: Manjón-Cabeza Córdoba, A., Ballmer, M. D., Allison, C., and Gazel, E.: Testing geodynamic models with major elements geochemistry: implications for Edge-Driven Convection and Mantle plumes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19194, https://doi.org/10.5194/egusphere-egu2020-19194, 2020.

Most cratonic lithospheres are stable entities that have not been deformed since their formation in the Archean. In contrast, geological and geophysical inferences showed that North China and Wyoming Cratons have been deformed/destroyed under specific geodynamic circumstances (e.g metasomatization, slab dehydration). For instance, lithospheric roots are densified-destabilized and they may eventually sink into the mantle. Here, numerical experiments are used to investigate how high-density anomalies/eclogite in the lower crust that is varying in size, density and geometry may control the lithospheric removal process. Based on a large set of parametric numerical calculations, we first classified the lithospheric removal style (e.g localized, non-localized, high degree, and pierce through). In the case where the eclogite blocks attached to the lower crust, two different conditions develop; localized deformation and non-localized deformation occur due to the small-scale convection. Two new different removal mechanisms are evolved after the eclogite becomes detached from the lower crust; (i) pierce through mechanism subsequent to localized deformation and (ii) high-degree deformation following non-localized deformation. While the width of the eclogite block causes high-degree deformation, it is observed that with increasing thickness it leads to the formation of viscous drips. Experimental results indicate that eclogite block(s) under the cratons may still be there while creating small wavelength MOHO depth variations.

How to cite: Ballı, A., Göğüş, O., Artemieva, I., and Thybo, H.: Deformation Regimes in Cratons Caused by Gravitational Instabilities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19758, https://doi.org/10.5194/egusphere-egu2020-19758, 2020.

The thermal structure of the Tibetan plateau remains largely unknown. Numerous avenues, both geophysical and petrological, provide fragmentary pressure/temperature information, both at the present, and on the evolution of the thermal structure over the recent past. However, these individual constraints have proven hard to reconcile with each other. This study presents a series of models for the simple underthrusting of India beneath southern Tibet that are capable of matching all available constraints on its thermal structure, both at the present day and since the Miocene. Three consistent features to such models emerge: (i) present day geophysical observations require the presence of relatively cold underthrust Indian lithosphere beneath southern Tibet; (ii) geochemical constraints require the removal of Indian mantle from beneath southern Tibet at some point during the early Miocene, although the mechanism of this removal, and whether it includes the removal of any crustal material is not constrained by our models; and (iii) the combination of the southern extent of Miocene mantle-derived magmatism and the present-day geophysical structure and earthquake distribution of southern Tibet require that the time-averaged rate of underthrusting of India relative to central Tibet since the middle Miocene has been faster than it is at present.

How to cite: Craig, T., Kelemen, P., Hacker, B., and Copley, A.: Combining geophysical and petrological estimates of the thermal structure of southern Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4876, https://doi.org/10.5194/egusphere-egu2020-4876, 2020.

The relative motions of the tectonic plates show remarkable variation throughout Earth’s history. Major changes in relative motion between the tectonic plates are traditionally viewed as spatially and temporally isolated events linked to forces acting on plate boundaries (i.e., formation of same-dip double subduction zones, changes in the strength of the boundary), or thought to be associated with mantle dynamics. A Cretaceous global plate reorganization event has been postulated to have affected all major plates. The Cretaceous ‘swing’ in Africa-Eurasia relative plate motion provides an ideal test-bed for assessing the temporal and spatial evolution of both relative plate motions and surrounding geological markers. Here we show a novel plate kinematic model for the closure of the Tethys Ocean by implementing intra-Cretaceous Quiet Zone time markers and combine the results with the geological constraints found along the convergent plate boundary. Our results allow to assess the order, causes and consequences of geological events and unravel a chain of tectonic events that set off with the onset of horizontally-forced double subduction ~105 Myr ago, followed by a 40 Myr long period of acceleration of the Africa relative to Eurasia that peaked at 80 Myr ago (at rates four times as high as previously predicted). This acceleration, which was likely caused by the pull of two same-dip subduction zones was followed by a sharp decrease in plate velocity, when double subduction terminated with ophiolite obduction onto the African margin. These tectonic forces acted on the eastern half of the Africa-Eurasia plate boundary, which led to counterclockwise rotation of Africa and sparked new subduction zones in the western Mediterranean region. Our analysis identifies the Cretaceous double subduction episode between Africa and Eurasia as a link in the global plate tectonic chain reaction and provides a dynamic view on plate reorganizations.

How to cite: Gürer, D., Granot, R., and van Hinsbergen, D. J. J.: Plate tectonic chain reaction constrained from noise in the Cretaceous Quiet Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2777, https://doi.org/10.5194/egusphere-egu2020-2777, 2020.

Plate reorganization events are a characteristic of plate tectonics that punctuate the Phanerozoic. They fundamentally change the lithospheric plate-motion circuit, influencing the planet’s tectonic-mantle system and both ocean and atmospheric circulation through changes in bathymetry and topography. The development of full-plate reconstructions for deep time allows the geological record to be interrogated in a framework where plate kinematic reorganizations can be explored. Here, the geological record of the one of the most extensive tracts of Neoproterozoic crust on the planet (the Arabian-Nubian Shield) is interpreted to reflect a late Tonian plate reorganization at ca. 800-715 Ma that switched plate-convergence directions in the Mozambique Ocean, bringing Neoproterozoic India towards both the African cratons and Australia-Mawson, instigating the closure of the intervening ocean and the future amalgamation of central Gondwana ca. 200 million years later. This plate kinematic change is coeval with constraints on break-up of the core of Rodinia between Australia-Mawson and Laurentia and Kalahari and Congo.

How to cite: Collins, A., Blades, M., Merdith, A., and Foden, J.: A late Tonian plate reorganization event: Using a deep-time full-plate global model to unravel Neoproterozoic tectonic convulsions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3171, https://doi.org/10.5194/egusphere-egu2020-3171, 2020.

We investigate true polar wander (TPW) since 300Ma. We construct a hotspot reference frame using an updated list of active hotspots with improved criteria aimed at detecting their depth origin, a compilation of terrestrial volcanic data suspected to reveal hotspot activity, and a set of plate reconstructions, based initially on paleomagnetism corrected with respect to hotspots under the assumption of hotspot fixity. The polar motion curves (representing the motion of the mantle taken as a whole) during the periods t=[0 and 150-170] and [150-170 to 280Ma] roughly aligns along two great circles which poles  are both located close to the equator, with a  longitude differing  by some 50°, and positioned close to an axis passing through the Large Low Shear Velocity Provinces (LLSVPs), and close to the maximum degree 2 geoid high under Africa. The TPW rate is slowly decreasing with respect to time but remains close or below the observed 10cm/yr present value.

            We compare our TPW data with those obtained from a mantle density heterogeneities model which computes the temporal evolution of the Principal Inertia Axis (PIA).  The minimum PIA is shown to be in agreement with the two poles previously determined, while the maximum PIA  path (which represents the evolution of the geographic pole) displays strong similarities with the observed TPW (directions, cusps). The sudden changes of TPW direction (i.e., cusps) can be explained  by mass reorganizations within the mantle principally linked to changes in subductions, while the domes greatly stabilize the system.

How to cite: Besse, J., Greff, M., and Vicente de Gouveia, S.: Estimates of true Polar wander since 300Ma, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9789, https://doi.org/10.5194/egusphere-egu2020-9789, 2020.

The breakup of the southern edge of Gondwanaland resulted in the formation of the Scotia Plate and the opening of Drake Passage throughout the Cenozoic. During the same period, the Tasman Seaway opened, although the timing of this opening is much better constrained. Rapid cooling of the Antarctic continent followed the openings of Drake Passage and the Tasman Seaway. The opening of Drake Passage or the Tasman seaway allowed the onset of the Antarctic Circumpolar Current, which is held responsible for the late Miocene global cooling, but discussions about the most important opening are still ongoing.

The opening of Drake Passage and the development of the Scotia plate have been studied in multitude, but paleogeographic reconstructions show many differences and inconsistencies in both timing of opening Drake Passage as well as paleo-locations of crustal segments. The paleogeographic or tectonic reconstructions of the opening of Drake Passage and the formation of the Scotia plate are hard to compare, because differences in shapes of crustal segments, geographic projections and relative movements of segments chosen by previous authors make it difficult to observe similarities and differences between the different reconstructions.

We present a thorough analysis of the previously published paleogeographic reconstructions with the aim to identify agreements and inconsistencies between these reconstructions. We re-defined the crustal segments that formed after the break-up of Gondwanaland by re-interpreting the bathymetry and magnetic anomalies of the study area. We re-modelled and compared georeferenced reconstructions from earlier studies in GPlates plate reconstruction software using our own defined crustal segments.

This comparison shows that the different reconstructions agree quite well along the South Scotia Ridge, but that the North Scotia Ridge shows significant variations between different reconstructions or is not even considered in the reconstructions. Also, the nature and age of the crust of the Central Scotia Sea is heavily discussed, resulting in different opening scenarios. We argue that the tectonic evolution of the North Scotia Ridge and Central Scotia Sea is a crucial factor in identifying the timing of the development of an ocean gateway. We made a new tectonic reconstruction of the North Scotia Ridge crustal segments with less overlaps and gaps between the reconstructed crustal segments.

The next step would be to compare the global sea-level changes and paleo-bathymetry with the different opening scenarios. Because we standardized all scenarios with the same crustal segments, we will then be able to provide opening ages of Drake Passage for the different scenarios that can be compared in a quantitative way.

How to cite: Oldenhage, A., Beniest, A., and Schellart, W. P.: The Cenozoic tectonic evolution of the Scotia Sea area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-178, https://doi.org/10.5194/egusphere-egu2020-178, 2020.

The formation of Gondwana results from a complex history, which can be linked to many orogenic sutures. Those sutures have often been gathered in the literature under broad orogenies — in particular the Eastern and Western Pan-African Orogenies — although their ages may vary a lot within those wide belts.

The Panalesis model is a plate tectonic model, which aims at reconstructing 100% of the Earth’s surface, and proposes a geologically, geometrically, kinematically, and geodynamically coherent solution for the evolution of the Earth from 888 Ma to 444 Ma. Although the model confirms that the assembly of Gondwana can be considered complete after the Damara and Kuunga orogenies, it shows above all that the detachment and amalgamation of “terranes” is a roughly continuous process, which even persisted after the Early Cambrian.

By using the wealth of Plate Tectonics, the Panalesis model makes it possible to derive numerous additional data and maps, such as the age of the sea-floor everywhere on the planet at every time slices, for instance. The evolution of accretion rates at mid-oceanic ridges and subduction rates at trenches are shown here, and yields results consistent with previous estimates. Understanding the variation of the global tectonic activity of our planet through time is key to link plate tectonic modelling with other disciplines of Earth sciences.

How to cite: Vérard, C.: 888 – 444 Ma global plate tectonic reconstruction with emphasis on the formation of Gondwana, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3306, https://doi.org/10.5194/egusphere-egu2020-3306, 2020.

The region of Taiwan is undergoing active, oblique arc-continent colision between the Luzon Arc on the Philippine Sea Plate and the continental margin of Eurasia. The Fold-and-Thrust Belt (FTB) in Taiwan passes southwards into a submarine accretionary wedge at the Manila subduction zone. The aim of this contribution is to examine how an on land FTB changes into a marine accretionary prism in the context of an oblique arc-continent collision. The Miocene pre-orogenic sediments of the continental margin are widespread in the FTB ca. 23° latitude while the offshore wedge is built up dominantly by Pliocene to recent syn-orogenic sediments. In the transition area from the marine accretionary wedge ca. 21° latitude to the on land FTB, the thrust wedge is climbing up the slope of the Eurasian continental margin. The deformation front is at sea floor depth of ca. 4 km in the south to less than 1 km as it reaches the coast line. Here we use the island surface geology, marine reflection seismic profiles, and seismic tomography models to construct contour maps of the basal thrust and the depth to the Moho across a transition area from near 23° to near 21° latitude. In this zone, the deformation front draws a convex curvature as the wedge widens from ca. 50 in the north and south, to more than 130 km near 22° latitude. The basal thrust surface shows a scoop shape as its dip changes from southeast near the coast line to east southward. The basal thrust reaches over 7 km deep beneath the rear of the FTB before ramping into de basement and merging into the Chaochou fault at 10 km depth. Offshore, it shows a gentler dip from 7 km to c. 10 km depth before getting steeper towards the east below the Hengchung Ridge. The basal cuts laterally along-strike through the margin’s sedimentary cover to incorporate thicker Miocene pre-orogenic sediments onto its hanging wall as it passes from the offshore wedge to the on land FTB.

In the offshore area, the Moho (we use a Vp proxy of 7.5 km/s extracted from the seismic tomography) shallows southeastward, from near 25 km depth below the shelf slope break to less than 17 km depth below the offshore wedge near 21.5° latitude before it starts to deep east towards beneath the Taiwan coast. The Moho dips northeast from near 25 km depth below the coast near Kaohsiung, to near 40 depth below the rear of the FTB at 23.5°, latitude. This complex morphology of the Moho may be related to the changes in crustal thickness and the obliquity of the collision. Because of this, crustal thickening is less pronounced beneath southern Taiwan where the thinner part of the margin is colliding with the arc.

This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency from the Ministry of Science Innovation and Universities of Spain.

How to cite: Alvarez-Marrón, J., Brown, D., Alcalde, J., Marzán, I., and Kuo-Chen, H.: The crustal structure in the transition from the on land fold-and-thrust belt to the offshore accretionary prism in the Taiwan arc-continent collision , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2846, https://doi.org/10.5194/egusphere-egu2020-2846, 2020.

Siberian Craton is generally recognised as one of the building blocks of two supercontinents: Mesoproterozoic Nuna (Columbia) and Neoproterozoic Rodinia. Although the exact Siberian positions in Nuna and Rodinia are debated, most workers agree that the southern part of Siberia (hereafter in present day coordinates) has been located not too far from the northern margin of Laurentia (cratonic part of North America) between ca. 1600 Ma and ca. 700 Ma. New geochronological, paleomagnetic and geochemical data from the Siberian craton obtained in recent years improved our understanding of Siberian geological history comparing to previous reviews. The progress in global Precambrian paleogeography also contributed to a re-evaluation of the Siberian tectonic history. The compilation of Siberian paleomagnetic data suggests that after the final assembly of Siberian Craton and until Ediacaran time the craton mostly occupied the low- to moderate latitudes. Most of this time western, northern and eastern Siberian edges have been passive or active oceanic margins. The southern margin Siberian margin has been probably connected with some other continent. Using new geological and paleomagnetic data, in particular recent results of the detrital zircons distributions in Siberia, Laurentia and other ancient continents, we tested several paleogeographic reconstructions of this connection. We also propose a new model of the breakup of Siberia from the remnants of Rodinia and consequent opening of the Paleo-Asian Ocean.

How to cite: Pisarevsky, S., Donskaya, T., and Gladkochub, D.: New geological and paleomagnetic data from Siberian Craton and implications for the post 2 Ga global paleogeography , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12730, https://doi.org/10.5194/egusphere-egu2020-12730, 2020.

A key ingredient to reproduce plate-tectonics in numerical models is a viscoplastic rheology. Strongly temperature-dependent rheology generates a rigid lid at the surface, whereas plastic rheology allows for the formation of plate boundaries. The yield stress limiter  controls the strength of the lithosphere.

Depending on the value used for  different tectonics regimes can be observed: (i) dripping behaviour (low , (ii) plate-like behaviour (intermediate-low ), (iii) Episodic behaviour (intermediate-high ) and (iv) Stagnant lid behaviour (high ).

Each lid behaviour can be distinguished by comparing the evolution profile of several parameters: temperature, viscosity, surface Nusselt number and mobility (Tackley, 2000a.).

Despite the great importance of physical parameters, the outcome of geodynamical models is also affected by the grid resolution as it has been shown that the critical that separates each lid behavior is resolution dependent (Tosi et al., 2015).

Here we use the code StagYY (Tackley, 2008) in a 2D spherical annulus geometry (Hernlund & Tackley, 2008) to determine the resolution-dependent tectonic regime in a global-scale convection setting. We tested 12 grid resolutions (ranging from 128x32 to 1024x128 nodal points) and 9 different  (ranging from 10 to 90 MPa), keeping all the remaining physical parameters unchanged.

For these simplified models we assume isothermal free slip boundaries, constant radiogenic heating, no melting, endothermic (410) and exothermic (660) phase transitions. Each simulation was run for 15 Gyrs with a Rayleigh number of ≈8*10^7 to make sure that steady-state conditions were reached.

Our resolution tests show that the observed tectonic regime is affected by grid resolution as this parameter controls how well the lithosphere is resolved. Low radial resolutions favour weak lid regimes (dripping and plate-like) as the lithosphere is defined by few thick cells, that propagate basal stress to shallower depths. On the other hand low azimuthal resolutions favour strong lid regimes (episodic and stagnant) since plate boundaries remain unresolved. In conclusion, only at high grid resolutions (512x128 and higher) the numerical influence on the observed tectonic regime is low.

How to cite: Marzotto, E., Thielmann, M., and Golabek, G.: Effect of grid resolution on tectonic regimes in global-scale convection models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4483, https://doi.org/10.5194/egusphere-egu2020-4483, 2020.

The India-Asia collision is one of the most well-studied orogenic events on Earth; it recorded the terminal stages of the central Tethys ocean basins and offers invaluable insight into the geological processes associated with continental collision. In this study, we integrate bedrock datasets, observations of subducted slabs in the mantle, and plate kinematic constraints, to constrain models for the India-Asia collision and the central Tethys oceans.

Previously proposed models for the India-Asia collision differ in terms of subduction zone configurations and paleogeographic reconstructions of Greater India, which represents to northern passive margin of India prior to collision. Five distinct subduction zone configurations have been proposed previously, which differ in the number of active trenches (one or two trenches) in the central Neotethys Ocean and differ in the respective timing, duration, location and migration of those trenches. Three distinct paleogeographic reconstructions of Greater India have been proposed previously, which differ in size and structure. Here, we consider the validity of these subduction zone configurations and Greater India reconstructions with respect to the bedrock record, plate kinematics and the deep mantle structure of subducted slabs beneath the Indian hemisphere.

Following the assumption that slabs sink vertically through the mantle, the positions and geometries of subducted slabs determined from seismic tomography constrain the locations and kinematics of paleo-subduction zones. Integrating this with bedrock constraints allows us to constrain post-Triassic subduction zone configurations for the central Tethys oceans. Our analysis demonstrates that the Neotethys Ocean was consumed by at least two subduction zones since the Jurassic. At the onset of the India-Asia collision at 59±1 Ma, one subduction zone was active along the southern Asian continental margin at ~20°N. At that time, a second may have been active at subequatorial latitudes, but support for this from a bedrock perspective is lacking. This subduction zone configuration allows for three reconstructions for Greater India: The (1) minimum-area; (2) enlarged-area; and (3) Greater India Basin reconstructions. We integrate these reconstructions and subduction zone configurations in a plate kinematic framework to test their validity for the India-Asia collision.

Our findings show that no single model is entirely satisfactory and each invokes assumptions that challenge accepted concepts. These include our understanding of suture zones, subduction-erosion processes, and the limits of continental subduction. We explore these challenges and their implications for our understanding of the India-Asia collision and continental collisions in general.

How to cite: Parsons, A., Hosseini, K., Palin, R., and Sigloch, K.: Integration of bedrock, seismic tomographic and plate kinematic constraints to test models of the India-Asia collision, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4742, https://doi.org/10.5194/egusphere-egu2020-4742, 2020.