Many Archean terranes are interpreted to have a tectonic and metamorphic evolution that indicates intra-crustal reorganization driven by lithospheric-scale gravitational instabilities. These processes are associated with the production of a signi cant amount of felsic and ma c crust, and are widely regarded to be a consequence of plume-lithosphere interactions. The juvenile Archean felsic crust is made predominantly of rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which are the result of partial melting of hydrous metabasalts. The geodynamic processes that have assisted the production of juvenile felsic crust, are still not well understood. Here, we perform 2D and 3D numerical simulations coupled with the state-of-the-art of petrological thermodynamical modelling to study the tectonic evolution of a primitive Archean oceanic plateau with particular regard on the condition of extraction of felsic melts. In our numerical simulations, the continuous emplacement of new, dry ma c intrusions and the extraction of the felsic melts, generate an unstable lower crust which drips into the mantle soon after the plume arrival. The subsequent tectonic evolution depends on the asthenosphere . If the is high enough (≥ 1500 • C) the entire oceanic crust is recycled within 2 Myrs. By contrast at low , the thin oceanic plateau slowly propagates generating plate-boundary like features.
<p>The Attic-Cycladic Massif (ACM) preserves the entire evolution of a NE dipping subduction zone.&#160; This includes the intra-oceanic subduction initiation associated with ophiolite obduction and formation of a metamorphic sole, to subduction-termination associated with burial and exhumation of the Cycladic continental margin to eclogite-blueschist facies conditions. The Tsiknias Ophiolite represents a piece of ca. 162 Ma Tethyan oceanic lower crust and mantle that was thrust towards the SW onto the ACM during a subduction initiation/ophiolite obduction event during the initial stages of oceanic closure. Beneath the Tsiknias Ophiolite lies a ~250 m thick sequence of amphibolites which represent the lower plate. These record an inverted metamorphic gradient at ca. 8.5 kbar reaching > 750&#176;C at the top (associated with small-scale partial melting) and 600&#176;C at the base and formed during high-grade metamorphism of ca. 190 Ma oceanic crust along the subduction zone interface beneath a major thrust fault (Tsiknias Thrust) under geothermal gradients of 30&#176;C/km. U-Pb zircon dating of leucodioritic melt veins constrains the timing of metamorphism to ca. 74 Ma, which may correlate with the switch in the motion of the Nubian plate from transcurrent to convergent with respect to Eurasia. Highly deformed greenschist facies pelagic metasediments underlie the amphibolites suggesting an inverted lithological sequence. This can be explained by the zone of active thrusting propagating down structural level with ongoing subduction, such that underplated material became accreted to the base of the ophiolite. A Miocene aged greenschist-facies shear zone truncates the metamorphic sole rocks and metasediments, placing them directly against the Cycladic Blueschist Unit (CBU) associated with burial of the Cycladic continental margin down the same NE-dipping subduction zone some 25 Myr later. Lawsonite bearing eclogite and blueschist-facies rocks crop-out < 1 km structurally beneath the metamorphic sole and record <em>P-T</em> conditions of 23 kbar and 550&#176;C at ca. 53-46 Ma. These rocks experienced variable retrogression through blueschist and then greenschist facies conditions. This retrogression was largely due to differential growth of lawsonite depending on bulk rock composition during prograde and peak metamorphic conditions causing some rocks to hold large quantities of water at peak conditions. Subsequent exhumation caused lawsonite to break down, hydrating the adjacent rocks and facilitating growth of secondary amphibole and epidote. These <em>P-T-t</em> conditions imply the CBU experienced geotherms of 6-7&#176;C/km during peak metamorphism, which suggests the subduction zone cooled at an average rate of ca. 1.5&#176;C/km/Myr between ca. 74 and ca. 53-46 Ma. This decrease in cooling rate raises two questions: &#160;(1) is this cooling rate a result of thermal conduction due to the burial of cold old oceanic lithosphere following subduction initiation?, or (2) are the hot apparent geothermal gradients recorded in the metamorphic sole due to processes other than conduction from the overriding lithospheric mantle?. Our thermometry data from the Tsiknias metamorphic sole suggest that: (1) the maximum temperatures increase structurally upwards towards the Tsiknias Thrust, (2) peak metamorphic temperatures are superimposed on the structure, and (3) the length scale of heating is inconsistent with thermal conduction alone.</p>
<p>In the plate tectonic convection regime, the external lid is subdivided into discrete plates that move independently. Although it is known that the system of plates is mainly dominated by slab-pull forces, it is not yet clear how, when and why plate tectonics became the dominant geodynamic process in our planet. It could have started during the Meso-Archean (3.0-2.9 Ga). However, it is difficult to conceive a subduction driven system at the high mantle potential temperatures (<strong>Tp</strong>) that are thought to have existed around that time, because <strong>Tp</strong> controls the thickness and the strength of the compositional lithosphere making subduction unlikely. In recent years, however, a credible solution to the problem of subduction initiation during the Archean has been advanced, invoking a plume-induced subduction mechanism[1] that seems able to generate plate-tectonic like behaviour to first order. However, it has not yet been demonstrated how these tectonic processes interact with each other, and whether they are able to eventually propagate to larger scale subduction zones.</p><p>The Archean Eon was characterized by a high <strong>Tp</strong>[2]<strong>, </strong>which generates weaker plates, and a thick and chemically buoyant lithosphere. In these conditions, slab pull forces are inefficient, and most likely unable to be transmitted within the plate. Therefore, plume-related proto-plate tectonic cells may not have been able to interact with each other or showed a different interaction as a function of mantle potential temperature and composition of the lithosphere. Moreover, due to secular change of <strong>Tp, </strong>the dynamics may change with time. In order to understand the complex interaction between these tectonic seeds it is necessary to undertake large scale 3D numerical simulations, incorporating the most relevant phase transitions and able to handle complex constitutive rheological model.</p><p>Here, we investigate the effects of the composition and <strong>Tp </strong>independently to understand the potential implications of the interaction of plume-induced subduction initiation. We employ a finite difference visco-elasto-plastic thermal petrological code using a large-scale domain (10000 x 10000 x 1000 km along x, y and z directions) and incorporating the most relevant petrological phase transitions. We prescribed two oceanic plateaus bounded by subduction zones and we let the negative buoyancy and plume-push forces evolve spontaneously. The paramount question that we aim to answer is whether these configurations allow the generation of stable plate boundaries. The models will also investigate whether the presence of continental terrain helps to generate plate-like features and whether the processes are strong enough to generate new continental terrains&#160;<span>or assemble them </span></p><p>.</p><p>&#160;</p><p>[1]&#160;&#160;&#160;&#160;&#160;&#160; T. V. Gerya, R. J. Stern, M. Baes, S. V. Sobolev, and S. A. Whattam, &#8220;Plate tectonics on the Earth triggered by plume-induced subduction initiation,&#8221; Nature, vol. 527, no. 7577, pp. 221&#8211;225, 2015.</p><p>[2]&#160;&#160;&#160;&#160;&#160;&#160; C. T. Herzberg, K. C. Condie, and J. Korenaga, &#8220;Thermal history of the Earth and its petrological expression,&#8221; Earth Planet. Sci. Lett., vol. 292, no. 1&#8211;2, pp. 79&#8211;88, 2010.</p><p>[3]&#160;&#160;&#160;&#160;&#160;&#160; R. M. Palin, M. Santosh, W. Cao, S.-S. Li, D. Hern&#225;ndez-Uribe, and A. Parsons, &#8220;Secular metamorphic change and the onset of plate tectonics,&#8221; Earth-Science Rev., p. 103172, 2020.</p>
<p>Slab-pull forces are considered the major driving forces of the present-day plate tectonic. Their efficiency relies on the buoyancy contrast between asthenosphere and subducting plate and on the strength of the latter. Subduction is not only pivotal for understanding the dynamics of plates but also represents the only modern geodynamic setting that produces significant amount of juvenile continental crust and allows exchange between the mantle, lithosphere and atmosphere.</p><p>One of the most important unsolved questions is related to the onset of plate tectonics, which is inherently linked to feasibility of the subduction during the early in Earth history. During the Archean, the mantle potential temperature was higher than nowadays, which promoted extensive mantle melting and possibly a weaker lithosphere. The intense magmatism associated with the high mantle potential temperature generated highly residual lithospheric mantle that was more buoyant than the underlying asthenosphere. Altogether these factors may have inhibited the dynamic effect of slab pull and prevented modern style tectonic during the Archean. However, the Archean mantle potential temperature is still not well constrained, and many of these theoretical considerations have not been fully tested by integrating petrological forward modelling into 3D numerical geodynamic modelling.</p><p>In our contribution, we focus on the feasibility of modern style plate tectonic as a function of the mantle potential temperature and the composition and structure of the lithosphere. We compute representative phase diagrams that represents the composition of mantle lithospheric and its complementary crust as a function of the mantle potential temperature and integrate them into large-scale 3D numerical experiments. The numerical setup is constructed assuming the existence of a set plates interacting with each other. We prescribe the principal plate boundaries and allow the model to spontaneously evolve as function of the thermal ages of the prescribed plate, testing the effect of continental terrains and oceanic plateau on overall geodynamic evolution. The overall goal is to understand the feasibility of plate tectonics at high mantle potential temperature and to estimate the amount of fluid released by the subduction processes, which provide useful insights on the formation of continental crust.</p>
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