Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

Geosphere ◽

10.1130/ges02334.1 ◽

2021 ◽

Author(s):

Yong-Fei Zheng ◽

Ren-Xu Chen

Keyword(s):

Subduction Zones ◽

Plate Tectonics ◽

The Other ◽

Ultrahigh Pressure ◽

Plate Boundaries ◽

Lower Grade ◽

Thermal Gradients ◽

Mountain Building ◽

Convergent Plate Boundaries ◽

Metamorphic Facies

Crustal metamorphism under extreme pressure-temperature conditions produces characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) mineral assemblages at convergent plate boundaries. The formation and evolution of these assemblages have important implications, not only for the generation and differentiation of continental crust through the operation of plate tectonics, but also for mountain building along both converging and con- verged plate boundaries. In principle, extreme metamorphic products can be linked to their lower-grade counterparts in the same metamorphic facies series. They range from UHP through high-pressure (HP) eclogite facies to blueschist facies at low thermal gradients and from UHT through high-temperature (HT) granulite facies to amphibolite facies at high thermal gradients. The former is produced by low-temperature/pressure (T/P ) Alpine-type metamorphism during compressional heating in active subduction zones, whereas the latter is generated by high-T/P Buchan-type metamorphism during extensional heating in rifting zones. The thermal gradient of crustal metamorphism at convergent plate boundaries changes in both time and space, with low-T/P ratios in the compressional regime during subduction but high-T/P ratios in the extensional regime during rifting. In particular, bimodal metamorphism, one colder and the other hotter, would develop one after the other at convergent plate boundaries. The first is caused by lithospheric subduction at lower thermal gradients and thus proceeds in the compressional stage of convergent plate boundaries; the second is caused by lithospheric rifting at higher thermal gradients and thus proceeds in the extensional stage of convergent plate boundaries. In this regard, bimodal metamorphism is primarily dictated by changes in both the thermal state and the dynamic regime along plate boundaries. As a consequence, supercontinent assembly is associated with compressional metamorphism during continental collision, whereas supercontinent breakup is associated with extensional metamorphism during active rifting. Nevertheless, aborted rifts are common at convergent plate boundaries, indicating thinning of the previously thickened lithosphere during the attempted breakup of supercontinents in the history of Earth. Therefore, extreme metamorphism has great bearing not only on reworking of accretionary and collisional orogens for mountain building in continental interiors, but also on supercontinent dynamics in the Wilson cycle.

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Emergence of plate tectonic during the Archean: insights from 3D numerical modelling.

10.5194/egusphere-egu21-8809 ◽

2021 ◽

Author(s):

Andrea Piccolo ◽

Boris Kaus ◽

Richard White ◽

Nicolas Arndt ◽

Nicolas Riel

Keyword(s):

Phase Transitions ◽

Subduction Zones ◽

Plate Tectonics ◽

Large Scale ◽

Potential Temperature ◽

Secular Change ◽

Plate Boundaries ◽

Plate Tectonic ◽

Subduction Initiation ◽

The Earth

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 (Tp) that are thought to have existed around that time, because Tp 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.The Archean Eon was characterized by a high Tp[2], 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 Tp, 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.Here, we investigate the effects of the composition and Tp 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;or assemble them .&#160;[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.[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.[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.

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Evidence of deformation control on the P-T record in compositionally heterogeneous shear zone during subduction-exhumation cycle

10.5194/egusphere-egu2020-6637 ◽

2020 ◽

Author(s):

Matteo Maino ◽

Leonardo Casini ◽

Stefania Corvò ◽

Antonio Langone ◽

Filippo Schenker ◽

...

Keyword(s):

High Pressure ◽

High Temperature ◽

Subduction Zones ◽

Equilibrium Temperature ◽

Tectonic Stress ◽

Heat Diffusion ◽

Ultrahigh Pressure ◽

Lithostatic Pressure ◽

Lower Grade ◽

Thermal Anomalies

Pressure-temperature paths are a major tool for tectonic reconstruction as proxies of the burial and exhumation history of the rocks during subduction-exhumation phases. The mineral assemblages are commonly considered to reflect lithostatic pressure and near-equilibrium regional geothermal gradients. These axioms ground on the assumptions that the rock cannot support high differential stress in one place, and that heat diffusion in rocks is fast enough to defocus localized thermal anomalies, respectively.The rare but systematic occurrence, in actual mountain ranges, of ultrahigh-pressure and/or high-temperature rocks within lower grade metamorphic rocks rise a major challenge for developing a consistent geodynamic model for exhumation of such deep seated rocks. Subduction zones are, in fact, efficient player driving material from the surface down into the Earth's mantle. However, the mechanisms to exhume part of this material (and particularly the denser oceanic rocks) back to the shallow crust are still highly debated.In this contribution, we present new structural, petrological and thermochronometric data from an exhumed subduction zone - the Cima di Gagnone in the Central Alps&#8211; where small ultramafic inclusions (peridotite) preserving high temperature and high pressure record are enveloped within amphibolite-facies gneisses, defining a classical inclusion-in-matrix system. We found evidence of heterogeneous metamorphic and temperature records in both peridotite and felsic rocks, being the gneisses generally characterized by much lower pressure. However, we detect also in the matrix gneiss close to peridotite inclusions high-pressure and high-temperature remnants, which are structurally and temporally associated with those of ultramafic bodies.The coexistence, at the outcrop scale, of such different conditions implies either extreme mechanical decoupling or extremely variable metamorphic equilibrium during Alpine subduction and exhumation. A possible alternative explanation is to consider part of the metamorphic record as due to mechanical deviations from lithostatic pressure and equilibrium temperature. We compare the observed metamorphic pattern with the outcome of numerical simulations obtained from elasto-visco-plastic 2D Finite Difference models. The evolution of rocks strength and viscosity is furthermore monitored to control the effectiveness of physical conditions simulated with the analytical dataset. Finally, we discuss a possible positive feedback of tectonic stress on the development of apparently incompatible metamorphic patterns.

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Melting processes at convergent plate boundaries: from melt segregation, extraction to the formation of crustal magmatic systems

10.5194/egusphere-egu2020-6912 ◽

2020 ◽

Author(s):

Harro Schmeling

Keyword(s):

Continental Crust ◽

Subduction Zones ◽

Two Phase Flow ◽

Solidus Temperature ◽

Plate Boundaries ◽

Phase Flow ◽

Two Phase ◽

Convergent Plate Boundaries ◽

Melt Generation ◽

Open Question

Melting at convergent plate boundariesAt divergent plate boundaries hot mantle upwelling is associated with abundant melt generation and volcanism. At convergent plate boundaries such as subduction zones and continental collision zones thick and cold plates feed mantle downwellings. Yet these "cold" regions also show abundant volcanic activity with mean volcanic output rates of almost similar order of magnitudes (White et al., 2006, G-cubed). Responsible melt generation mechanisms are addressed including a) volatile driven decrease of the solidus temperature, b) decompressional melting in the mantle wedge or in shallow asthenosphere associated with delamination, or c) increased radiogenic heating within thickened continental crust.&#160;&#160;&#160; &#160;Melt transport mechanismsThe above processes form partially molten regions. By which mechanism(s) does the melt segregate out of the melt source region and rise through the mantle or crust. The basic mechanism is two-phase flow, i.e. a liquid phase percolates through a solid, viscously deforming matrix. The corresponding equations and related issues such as compaction or effective matrix rheology are addressed. Beside simple Darcy flow, special solutions of the equations are addressed such as solitary porosity waves. Depending on the bulk to shear viscosity ratio of the matrix and the non-dimensional size of these waves, they show a variety of features: they may transport melt over large distances, or they show transitions from rising porosity waves to diapiric rise or to fingering. Other solutions of the equations lead to channeling, either mechanically or chemically driven. One open question is how do such channels transform into dykes which have the potential of rising through sub-solidus overburden. A recent hypothesis addresses the possibility that rapid melt percolation may reach the thermal non-equilibrium regime, i.e. the local temperature of matrix and melt may evolve differently. &#160;Once dykes have been formed they may propagate upwards driven by melt buoyancy and controlled by the ambient stress field. As another magma ascent mechanism diapirism is addressed. &#160;Modelling magmatic systems in thickened continental crust Once basaltic melts rise from subducting slabs, they may underplate continental crust and generate silicic melts. Early dynamic models (Bittner and Schmeling, 1995, Geophys. J. Int.) showed that such silicic magma bodies may rise to mid-crustal depth by diapirism. More recent approaches (e.g. Blundy and Annan, 2016, Elements) emplace sill intrusions into the crust at various levels and calculate the thermal and melting effects responsible for the formation of mush zones. Recently Schmeling et al. (2019, Geophys. J. Int.) self-consistently modelled the formation of crustal magmatic systems, mush zones and magma bodies by including two-phase flow, melting/solidification and effective power-law rheology. In these models melt is found to rise to mid-crustal depths by a combination of compaction/decompaction assisted two-phase flow, sometimes including solitary porosity waves, and diapirism. An open question in these models is whether or how dykes may self-consistently form to transport the melts to shallower depth. First models which combine the two-phase flow crustal models with elastic dyke-propagations models (Maccaferri et al., 2019, G-cubed) are promising.&#160;&#160;&#160; &#160;&#160;&#160;&#160;&#160; &#160;&#160;

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Plate Tectonics by Creeps

10.1093/oso/9780198717867.003.0006 ◽

2018 ◽

Author(s):

Roy Livermore

Keyword(s):

Turbine Blade ◽

Solid Rock ◽

Plate Tectonics ◽

The Other ◽

Plate Boundaries ◽

Living World ◽

The Earth ◽

Natural Home ◽

Aero Engine ◽

Form Part

Earthquakes are caused primarily by the movement of plates. Plates move over the mantle, and the mantle is solid rock. So why aren’t there earthquakes beneath the plates? If all plates are moving (and they are), there should be earthquakes just about everywhere, at a depth corresponding to the base of the plates. The answer is that, while plate boundaries are the natural home of jerks, the mantle beneath the plates is the preserve of creeps. Except where cold slabs penetrate to depths of 650 km or more, the mantle moves only very gradually by creep, a phenomenon that occurs in many solids, including metals, when subjected to stress. Creep can be very inconvenient, particularly where a metal happens to form part of, say, a turbine blade in an aero engine. On the other hand, creep is the process that transformed the Earth into a living world.

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Abyssal Serpentinites: Transporting Halogens from Earth’s Surface to the Deep Mantle

Minerals ◽

10.3390/min9010061 ◽

2019 ◽

Vol 9 (1) ◽

pp. 61 ◽

Cited By ~ 1

Author(s):

Lilianne Pagé ◽

Keiko Hattori

Keyword(s):

Mass Balance ◽

Subduction Zones ◽

Hydrothermal Fluids ◽

Igneous Rocks ◽

The Other ◽

Ultrahigh Pressure ◽

Conservative Estimate ◽

Mantle Lithosphere ◽

Deep Mantle ◽

Subducting Slab

Serpentinized oceanic mantle lithosphere is considered an important carrier of water and fluid-mobile elements, including halogens, into subduction zones. Seafloor serpentinite compositions indicate Cl, Br and I are sourced from seawater and sedimentary pore fluids, while F may be derived from hydrothermal fluids. Overall, the heavy halogens are expelled from serpentinites during the lizardite–antigorite transition. Fluorine, on the other hand, appears to be retained or may be introduced from dehydrating sediments and/or igneous rocks during early subduction. Mass balance calculations indicate nearly all subducted F is kept in the subducting slab to ultrahigh-pressure conditions. Despite a loss of Cl, Br and I from serpentinites (and other lithologies) during early subduction, up to 15% of these elements are also retained in the deep slab. Based on a conservative estimate for serpentinite thickness of the metamorphosed slab (500 m), antigorite serpentinites comprise 37% of this residual Cl, 56% of Br and 50% of I, therefore making an important contribution to the transport of these elements to the deep mantle.

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The thermal–mechanical evolution of crustal orogenic belts at convergent plate boundaries: A reappraisal of the orogenic cycle

Journal of Geodynamics ◽

10.1016/j.jog.2011.10.004 ◽

2012 ◽

Vol 56-57 ◽

pp. 124-145 ◽

Cited By ~ 77

Author(s):

Olivier Vanderhaeghe

Keyword(s):

Plate Boundaries ◽

Mechanical Evolution ◽

Convergent Plate Boundaries ◽

Orogenic Cycle ◽

Orogenic Belts

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Synconvergent and coherent (ultra)high-pressure crustal rock exhumation

10.5194/egusphere-egu21-11119 ◽

2021 ◽

Author(s):

Lorenzo G. Candioti ◽

Joshua D. Vaughan-Hammon ◽

Thibault Duretz ◽

Stefan M. Schmalholz

Keyword(s):

Numerical Models ◽

A Priori ◽

Western Alps ◽

Ultrahigh Pressure ◽

Plate Motion ◽

Crustal Rock ◽

Plate Boundaries ◽

Crustal Rocks ◽

Natural Data ◽

The Alps

Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.&#160;Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (>80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called m&#233;langes. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.&#160;Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.&#160;The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.

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