Articles

Séminaire de Mélanie Gérault (post-doctorante au MIT, CAMBRIDGE, Etats-Unis)

Net rotation of the lithosphere in mantle convection models with self-consistent plate generation

Lateral variations in the Earth’s viscosity structure give rise to a global net rotation between the lithosphere and the mantle. Plate motion reconstructions, mantle flow computations, and inferences from seismic anisotropy all indicate some amount of net rotation based on various mantle reference frames. For the present-day, while the direction of rotation is somewhat consistent across studies, the predicted amplitudes range from ~0.1 deg/Myr to ~0.3 deg/Myr and more. Further back in time, the discrepancies are even greater. Such a lack of contraints is a major impediment to a deeper understanding of fundamental topics that rely on absolute surface kinematic data. Besides, the dynamics that govern the net rotation, its fluctuations in amplitude and direction, remain largely unidentified.
In this presentation, I will show the first time-dependent assessment of the net rotation in 3-D spherical mantle convection models with self-consistent plate generation. We run the computations for billions of years of numerical integration. The mantle convection problem is solved with the finite volume code StagYY using a visco-pseudo-plastic rheology [Tackley, 2008]. We look into how sensitive the net rotation is to heterogeneities in the upper boundary layer, such as the presence of continents of variable thickness. We also explore the links between net rotation and the initiation, development and cessation of subduction.
In all models, large fluctuations in net rotation occur over a few tenth of millions of years. The amplitudes vary from nearly zero to over 0.3 deg/Myr, with time averages toward the low end of the present-day estimates. These variations are generally associated with the initiation or cessation of subduction. The results suggest that the net rotation is closely related to the tectonic make-up of the surface, evolving with the nature of plate boundaries and the physical arrangement of the plates. A rapid increase (decrease) in net rotation is sometimes associated with the acceleration (deceleration) of one or two plates. Yet, the fastest mean surface velocity does not always correlate with the fastest lithospheric net rotation. The results highlight some of the interplays between deep mantle and surface dynamics over time.

Tackley, P. J. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Phys. Earth Planet. Inter., 171(1-4), 7-18.

19 décembre 2018

Séminaire de Jean-Claude Ringenbach (TOTAL SA, Pau)

Rifted margins structural styles, forcing parameters and implications for hydrocarbons

Résumé : Le modèle le plus abouti sur les marges, celui de la paire Ibérie – Terre Neuve, repose sur une recherche et des acquisitions de données continues (sismique et legs ODP) depuis le milieu des années 80 (groupe Boillot, Manatschal, Reston). Appuyé par le superbe analogue de terrain des Alpes Suisses (Froitzheim, Manatschal), un modèle d’évolution très abouti, du rifting initial à l’océanisation a pu être proposé. Comme pour tout beau modèle, le risque est dans la généralisation de son application. Depuis environ 2006, une grande quantité de lignes sismiques industrielles 2D et 3D, imageant la croûte jusqu’au Moho, a permis de mieux comprendre la géométrie des marges continentales riftées et leur variabilité. L’exposé en montre des exemples, propose un classification des marges et des paramètres importants de leur évolution, tout en mettant en perspective les aspects pétroliers.

Suivi d’une courte présentation sur: Salt Tectonics in the Sivas Basin (Turkey) »

La seconde partie du séminaire présentera des analogies stupéfiantes de tectonique salifère à l’affleurement en Turquie et les comparera avec des lignes sismiques (en introduction, voir « Sivas + Salt Tectonics » sur Youtube – lien: https://www.youtube.com/watch?v=ok2QAY5j0q8)

17 décembre 2018

Séminaire de Nicola Piana Agostinetti de (Université de Vienne)

Passive seismics as a tool for crustal exploration:
performances, key-achievements and future developments.

Exploration of the Earth’s crust is often based on active seismic data, but in complex geology settings active seismic imaging can face serious problems. Alternative exploration techniques can help to reduce risks for building accurate Earth’s models. In the last decade, the analysis of passive seismic data, i.e. natural seismicity and seismic ambient noise (collectively called Passive Seismic Techniques, PST), have been widely used to investigate the subsurface. In particular, the propagation of seismic waves generated by teleseismic earthquakes, i.e. earthquakes occurred more than 3000 km from the study area, have been exploited to constrain the fine details of the crustal structure and elasticity. Receiver Function (RF) analysis is a PST adopted in academia that makes use of teleseismic waves to image the structure of the crust. RF analysis has been applied for both deep and shallow crustal imaging, e.g. from Moho to basin mapping. Moreover, RF analysis can be used to locate anisotropic materials at depth, a tricky target for active seismics.

In this talk, I will briefly introduce RF analysis, measuring the performance of this PST against the results obtained by active seismic survey and borehole lithostratigraphies. I will show key-applications of RF analysis for crustal exploration, like: (a) how joint interpretation of passive and active seismic data can be useful to improve our knowledge of the subsurface at different depth-levels; (b) how RF analysis can be used to explore the presence of anisotropic materials in geothermal fields, indicating anomalous PT conditions; and (c) how teleseismic waves can used to improve sedimentary basin investigations. Finally I will discuss the cons of the PSTs and the future development of the research in this area.

30 novembre 2018

Séminaire de Gurumurthy, Birbal Sahni Inst. of Paleosciences, India

Silicate weathering in humid tropics and coastal biogeochemical processes affecting the elemental budgets to Oceans

The Western Ghats form a major mountain belt, next to the Himalayas, in controlling the flux of chemical elements and sediments to the northern Indian Ocean. Numerous small mountain streams flow towards the Arabian Sea draining the western slopes of Western Ghats because of the prevailing orographicity. The weathering process and the associated atmospheric CO2 drawdown due to silicate weathering have been deduced using the dissolved and particulate chemical composition in river basins having similar geomorpho-climatic settings but differing lithology (granite-gneisses, charnockites and basalts). The speaker discusses the silicates weathering process in the small mountain streams of Western Ghats, the rate controlling parameters and their role in geological carbon sequestration.

Further, the geochemical redistribution of redox sensitive elements during the weathering and fluvial transport process with special reference to molybdenum geochemistry in the riverine and estuarine environments will be discussed. The role of oxidative scavenging of dissolved molybdenum on to particulate phase in river, and its subsequent release/scavenging of metal elements in the coastal region will be discussed in detail to demonstrate the role of coastal/interface biogeochemical processes in the source-sink pathways of chemical elements, and the extent these biogeochemical processes affect the elemental budgets.

Email: gurumurthygp@bsip.res.in

27 novembre 2018

Séminaire de Kenni PEDERSEN (Univ. Aarhus, Danemark)

Models of plate tectonic processes and their interaction with the convecting mantle: magmatism, vertical movements and thermo-chemical structure

During this talk, I will present different results from 2D high-resolution numerical modeling of lithosphere/mantle-scale processes involving plate tectonics and convection in the Earth’s mantle at different scale. The talk will be subdivided into three themes that all employ this modeling technique.

1. Thermal subsidence of extensional basins in the presence of sublithospheric convection. Here, I present models of lithosphere extension above an asthenosphere mantle that is convecting with sufficient vigor to explain e.g. surface heat flow of old (‘steady state’) oceanic lithosphere. The models indicate that thermal subsidence in the presence of such convection can differ substantially from the well-known McKenzie model that only implicitly accounts for convective heat flow. For example, thermal subsidence can become strongly protracted, because the thermal boundary layer may become advected upwards during thinning and therefore transport more heat through the cooling lithosphere than in the case, where the thermal boundary layer is assumed at a constant depth (e.g. the McKenzie model). Furthermore, the presence of sublithospheric convection beneath a sedimentary basin also results in dynamic vertical movements in the order of 10-100 m at a time scale of 2-20 Myr. Such movements are a possible cause of stratigraphic cycles that are otherwise commonly attributed to glacial eustasy.

2. LIP formation as a consequence of rift-induced mantle overturn. Here, I present modeling results that provide yet another mechanism for anomalous magmatism such as that associated with LIP formation in the embryonic North Atlantic during the Paleocene/Eocene as well the protracted high degree of activity that followed and continues in Iceland today. Using thermomechanical modeling we show that the following line of events are physically plausible: Rifting along an old Caledonian suture with a mafic lower crust partly in eclogite facies may lead to rapid delamination of the mantle lithosphere. This again leads to a rapid, but not particularly voluminous phase of magmatism. The detached mantle lithosphere rapidly sinks into the lower mantle and induces a return flow due off lower mantle material into the upper mantle. If the lower mantle has elevated potential temperature relative to the upper mantle, the return flow is amplified by thermal buoyancy, and a partial overturn of the mantle is initiated. Within 6 Myr from onset of lithosphere delamination, hot lower mantle material rises to the base of the rifts and starts melting. This melt phase leads to fast production of large volumes of melt consistent with North atlantic LIP formation at 55 Ma. After this, lower mantle upwelling and melt productivity decreases, but still continues in the following time until the present, consistent with the long-lived melt anomaly of the North Atlantic and present day Iceland.

3. Syn-convergent extension and UHP exhumation in the D’Entrecasteaux Islands of the Woodlark Basin. In this part of the talk, I will briefly present a thermomechanical model for the exhumation of Ultra-High-Pressure (UHP) rocks in the young core complexes of the D’Entrecasteaux Islands that are situated just west of the oceanic Woodlark Basin. Plate kinematic constraints of the latter indicate that UHP exhumation was associated with North/South extension of at least 100 km since 4 Ma. This had led some authors to propose a mechanism of reverse subduction (eduction), where the Northern margin of the Australian continental plate was first subducted and then exhumed by normal motion along the former subduction plane. In the other hand, structural fabrics of the exhumed UHP units imply diapiric exhumation which however is at odds with the amount of extension required from plate kinematics. The model I here present shows that exhumation by reverse subduction led to ductile extrusion and diapir-like fabric, even though exhumation was largely driven by >100 km extension. The model further shows that despite continued convergence between the Australian and the Pacific plates, coeval extension due to the opening of the Woodlark Basin could have been accommodated by subduction further North in the New Britain trench, as subduction of the Australian margin was reversed.

References

Petersen, K., Armitage, J., Nielsen, S. & Thybo, H. Mantle temperature as a control on the time scale of thermal evolution of extensional basins. Earth and Planetary Science Letters 409, 61-70, (2015).

Petersen, K. D., Nielsen, S. B., Clausen, O. R., Stephenson, R. & Gerya, T. Small-Scale Mantle Convection Produces Stratigraphic Sequences in Sedimentary Basins. Science 329, 827-830, (2010).

Petersen, K. D., Schiffer, C. & Nagel, T. LIP formation and protracted lower mantle upwelling induced by rifting and delamination. Scientific Reports 8, 16578, (2018).

Petersen, K. D. & Buck, W. R. Eduction, extension, and exhumation of ultrahigh‐pressure rocks in metamorphic core complexes due to subduction initiation. Geochemistry, Geophysics, Geosystems 16, 2564-2581, (2015).

23 novembre 2018