Geodynamic Models Research Articles

Will Earth's next supercontinent assemble through the closure of the Pacific Ocean?

Earth's known supercontinents are believed to have formed in vastly different ways, with two endmembers being introversion and extroversion. The former involves the closure of the internal oceans formed during the break-up of the previous supercontinent, whereas the latter involves the closure of the previous external superocean. However, it is unclear what caused such diverging behavior of supercontinent cycles that involved first-order interaction between subducting tectonic plates and the mantle. Here we address this question through 4D geodynamic modeling using realistic tectonic set-ups. Our results show that the strength of the oceanic lithosphere plays a critical role in determining the assembly path of a supercontinent. We found that high oceanic lithospheric strength leads to introversion assembly, whereas lower strength leads to extroversion assembly. A theoretically estimated reduction in oceanic crustal thickness, and thus its strength, during Earth's secular cooling indicates that introversion was only possible for the Precambrian time when the oceanic lithosphere was stronger, thus predicting the assembling of the next supercontinent Amasia through the closure of the Pacific Ocean instead of the Indian-Atlantic oceans. Our work provides a new understanding of the secular evolution of plate tectonics and geodynamics as the Earth cooled.

Petrography and Geochemistry for Proposal of Geodynamic Model For The Irbiben Granite in Tagragra d’Akka inlier, (western Anti-Atlas, Morocco)

This study aims to contribute to improve the knowledge on the setting of the Irbiben granites, located south of the gold deposit of this locality (Tagragra d'Akka buttonhole, Anti-Atlas, Morocco). The petrographic characterization showed leucocratic porphyry rocks, with a mineralogy dominated by quartz and phenocrysts of plagioclase, alkali feldspars of sometimes centimetric size as well as very small sulphides of metallic luster. Two generations of quartz have been identified: a QIquartz with undulating extinction phenocrysts testifying to an episode of deformation orchestrated in this inlier, and a QII quartz with more rounded and limpid minerals indicating an intense silicification. Plagioclase and alkali feldspars are deeply altered to sericite and epidote. Geochemical characterization classifies these rocks as calc-alkaline series granites, rich in potassium, with a peraluminous character indicating their crustal origin. Their arc geochemical signature, Ba enrichment, and negative Nb, Ti, and P anomalies are characteristic of a subduction zone. This subduction could be associated with an episode of convergence between an oceanic lithosphere located in the north and the West African craton in the south, as shown by the proposed geodynamic model.

Theoretical predictions of melting behaviors of hcp iron up to 4000 GPa

The high-pressure melting diagram of iron is a vital ingredient for the geodynamic modeling of planetary interiors. Nonetheless, available data for molten iron show an alarming discrepancy. Herein, we propose an efficient one-phase approach to capture the solid-liquid transition of iron under extreme conditions. Our basic idea is to extend the statistical moment method to determine the density of iron in the TPa region. On that basis, we adapt the work-heat equivalence principle to appropriately link equation-of-state parameters with melting properties. This strategy allows explaining cutting-edge experimental and ab initio results without massive computational workloads. Our theoretical calculations would be helpful to constrain the chemical composition, internal dynamics, and thermal evolution of the Earth and super-Earths.

Location, extent, and magnitude of dynamic topography in the Late Cretaceous Cordilleran Foreland Basin, USA: New insights from 3D flexural backstripping

AbstractMantle‐induced dynamic topography (i.e., subsidence and uplift) has been increasingly recognized as an important process in foreland basin development. However, characterizing and distinguishing the effects (i.e., location, extent and magnitude) of dynamic topography in ancient foreland basins remains challenging because the spatio‐temporal footprint of dynamic topography and flexural topography (i.e., generated by topographic loading) can overlap. This study employs 3D flexural backstripping of Upper Cretaceous strata in the central part of the North American Cordilleran foreland basin (CFB) to better quantify the effects of dynamic topography. The extensive stratigraphic database and good age control of the CFB permit the regional application of 3D flexural backstripping in this basin for the first time. Dynamic topography started to influence the development of the CFB during the late Turonian to middle Campanian (90.2–80.2 Ma) and became the dominant subsidence mechanism during the middle to late Campanian (80.2–74.6 Ma). The area influenced by >100 m dynamic subsidence is approximately 400 by 500 km, within which significant (>200 m) dynamic subsidence occurs in an irregular‐shaped (i.e., lunate) subregion. The maximum magnitude of dynamic subsidence is 300 ± 100 m based on the 80.2–74.6 Ma tectonic subsidence maps. With the maximum magnitude of dynamic uplift being constrained to be 200–300 m, the gross amount of dynamic topography in the Late Cretaceous CFB is 500–600 m. Although the location of dynamic subsidence revealed by tectonic subsidence maps is generally consistent with isopach map trends, tectonic subsidence maps developed through 3D flexural backstripping provide more accurate constraints of the areal extent, magnitude and rate of dynamic topography (as well as flexural topography) in the CFB through the Late Cretaceous. This improved understanding of dynamic topography in the CFB is critical for refining current geodynamic models of foreland basins and understanding the surface expression of mantle processes.

Crustal deformation and dynamics of the south-eastern tibetan plateau from stress fields and geodesy

South-eastern Tibet rotates clockwise around the eastern Himalayan syntaxis due to the eastward extrusion of the eastern Tibetan lithosphere and it attracts much attention as a window for studying the dynamics of continental collision between India and Eurasia. Competing geodynamic models to describe the deformation of the eastern Tibet have been the debating subject and the dynamic processes responsible for plateau evolution remain poorly understood, partly because the mechanical state at depth and its relationship with surficial deformation are unclear. In this context, a continuous east-west oriented tensional zone was identified at the south-eastern edge of the Tibetan Plateau based on seismogenic stress field and global positioning system data. The profiles of surficial velocity field reveal a relatively speedy motion belt parallel to the Xianshuihe fault extending from north to south in the rotating region east of the tensional zone. Segmented features of the profiles around the extensional zone indicate its close relation with flow channels delineated from magnetotelluric images. Therefore, the deformation at the south-eastern edge of the Tibetan Plateau is mechanically coupled within the crustal depth. We propose a complex regional kinematics with a localised speedy belt and a continuously extensional zone, where material coupling and deformation compatibility were revealed within the crustal depth.

Earth’s gradients as the engine of plate tectonics and earthquakes

The processes occurring on the Earth are controlled by several gradients. The surface of the Planet is featured by complex geological patterns produced by both endogenous and exogenous phenomena. The lack of direct investigations still makes Earth interior poorly understood and prevents complete clarification of the mechanisms ruling geodynamics and tectonics. Nowadays, slab-pull is considered the force with the greatest impact on plate motions, but also ridge-push, trench suction and physico-chemical heterogeneities are thought to play an important role. However, several counterarguments suggest that these mechanisms are insufficient to explain plate tectonics. While large part of the scientific community agreed that either bottom-up or top-down driven mantle convection is the cause of lithospheric displacements, geodetic observations and geodynamic models also support an astronomical contribution to plate motions. Moreover, several evidences indicate that tectonic plates follow a mainstream and how the lithosphere has a roughly westerly drift with respect to the asthenospheric mantle. An even more wide-open debate rises for the occurrence of earthquakes, which should be framed within the different tectonic setting, which affects the spatial and temporal properties of seismicity. In extensional regions, the dominant source of energy is given by gravitational potential, whereas in strike-slip faults and thrusts, earthquakes mainly dissipate elastic potential energy indeed. In the present article, a review is given of the most significant results of the last years in the field of geodynamics and earthquake geology following the common thread of gradients, which ultimately shape our planet.

Vastly Different Heights of LLVPs Caused by Different Strengths of Historical Slab Push

AbstractTwo large low velocity provinces (LLVPs) are observed in Earth's lower mantle, beneath Africa and the Pacific Ocean, respectively. The maximum height of the African LLVP is ∼1,000 km larger than that of the Pacific LLVP, but what causes this height difference remains unclear. LLVPs are often interpreted as thermochemical piles whose morphology is greatly controlled by the surrounding mantle flow. Seismic observations have revealed that while some subducted slabs are laterally deflected at ∼660–1,200 km, other slabs penetrate into the lowermost mantle. Here, through geodynamic modeling experiments, we show that rapid sinking of stagnant slabs to the lowermost mantle can cause significant height increases of nearby thermochemical piles. Our results suggest that the African LLVP may have been pushed more strongly and longer by surrounding mantle flows to reach a much shallower depth than the Pacific LLVP, perhaps since the Tethys slabs sank to the lowermost mantle.

Magnetotelluric evidence for the crustal deformation beneath the region around the Lijiang-Xiaojinhe fault, SE margin of the Tibetan Plateau

• Crustal electrical structures reveal the deformation in the SE Tibetan Plateau. • High conductor C3 results in the geothermal activities in the Lugu Lake region. • Ninglang fault has a high potential for large earthquakes. • Ductile flow has been moving from SW to NE at the Lijiang-Xiaojinhe fault. We used data from 62 broadband magnetotelluric (MT) stations along two NNW - SSE profiles cross sections around the Lijiang - Xiaojinhe fault (LXF) using 3-D inversion. This is a crucial area where the long-wavelength topography can be used to identify motions of the ductile flow beneath the eastern Tibetan Plateau to the Yangtze platform. The results indicate that there are two high conductors (1–10 Ωm) beneath the LXF zone. One of the conductors is at a depth of approximately 15–35 km with a length of 40 km in the southwest profile, whereas the other is at a depth of approximately 10–20 km with a length of 20 km in the northeast profile. These results indicate that the conductors along the LXF zone become shallower and smaller from the SW to NE. Based on the framework of the material escape to the east of the Tibetan Plateau and the strike-slip polarity of the active faults, we propose a new geodynamic model for the study area. The high conductors probably originated northwest of the Lijiang area. After crossing the LXF, part of the mid-crustal ductile material moved to the NE between the LXF and Ninglang fault. Therefore, the LXF presents significant left-lateral strike-slip deformation, and the Ninglang fault presents right-lateral strike-slip deformation. In addition, with the thermal springs exposed in the subsurface of the Lugu Lake region, we suggest that a potential geothermal field could be developed underneath the LXF and NW sides of the Lugu Lake region.

Structural characterization and K–Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

Abstract. We studied the Zuccale Fault (ZF) on Elba, part of the Northern Apennines, to unravel the complex deformation history that is responsible for the remarkable architectural complexity of the fault. The ZF is characterized by a patchwork of at least six distinct, now tightly juxtaposed brittle structural facies (BSF), i.e. volumes of deformed rock characterized by a given fault rock type, texture, colour, composition, and age of formation. ZF fault rocks vary from massive cataclasite to foliated ultracataclasite, from clay-rich gouge to highly sheared talc phyllonite. Understanding the current spatial juxtaposition of these BSFs requires tight constraints on their age of formation during the ZF lifespan to integrate current fault geometries and characteristics over the time dimension of faulting. We present new K–Ar gouge dates obtained from three samples from two different BSFs. Two top-to-the-east foliated gouge and talc phyllonite samples document faulting in the Aquitanian (ca. 22 Ma), constraining east-vergent shearing along the ZF already in the earliest Miocene. A third sample constrains later faulting along the exclusively brittle, flat-lying principal slip surface to < ca. 5 Ma. The new structural and geochronological results reveal an unexpectedly long faulting history spanning a ca. 20 Myr time interval in the framework of the evolution of the Northern Apennines. The current fault architecture is highly heterogeneous as it formed at very different times under different conditions during this prolonged history. We propose that the ZF started as an Aquitanian thrust that then became selectively reactivated by early Pliocene out-of-sequence thrusting during the progressive structuring of the Northern Apennine wedge. These results require the critical analysis of existing geodynamic models and call for alternative scenarios of continuous convergence between the late Oligocene and the early Pliocene with a major intervening phase of extension in the middle Miocene allowing for the isostatic re-equilibration of the Northern Apennine wedge. Extension started again in the Pliocene and is still active in the innermost portion of the Northern Apennines. In general terms, long-lived, mature faults can be very architecturally complex. Their unravelling, including understanding the dynamic evolution of their mechanical properties, requires a multidisciplinary approach combining detailed structural analyses with dating the deformation events recorded by the complex internal architecture, which is a phenomenal archive of faulting and faulting conditions through time and space.

New SKS splitting anisotropy measurements point to tearing beneath Central Italy

In the middle of the Mediterranean, the partly still active Apennines subduction system has been usually defined using tomographic images and available shear wave splitting measurements. In this paper we describe the new seismic anisotropy dataset for Central Italy, the region where the transition between Northern and Southern Apennines occurs. The new measurements show NW-SE fast polarisation directions beneath the belt, due to the retreat of the slab, NNE-SSW orientations from proper Adriatic mantle sources, and E-W directed anisotropy, attributed to mantle convection flow at the Thyrranian side. Additionally, the new data suggest the presence of a toroidal mantle flow through a tear in the Apenninic slab, from the Adria to the Tyrrhenian side. However, mantle circulation and flows, identified by the pattern of shear wave splitting results, seem different from what was proposed in previous geodynamic models. Indeed, our results support the presence of a vertical slab tear with limited dimension. In the geodynamic model we propose, the tear acts to accommodate a differential slab retreat. The slab partitioning results in a different pattern and strength of seismic anisotropy traced from the Central Apennines with respect to the adjacent Northern and Southern Apennines.

Seismic anisotropy across Adria plate, from the Apennines to the Dinarides

The Adria microplate has the particular feature to be involved in two subduction systems with slab dipping in opposite directions, one toward west beneath the Apennines and the other to the east beneath the Dinarides. The deep structure of Adria and the shape and characteristics of the slabs have mainly been studied through seismic tomography. However, the uncertainty about the presence and dimensions of tear and windows along the Apennines and the Dinarides slabs is still large. An instrument that can be used to draw mantle flows and to support the possible presence of slab windows or tears is the detection of seismic anisotropy, in particular core phases shear wave splitting. In this paper, to give more light to the structure of Adria slabs and possible mantle circulation beneath this microplate, we benefit from data recorded by seismic stations located along a profile running across the central Adriatic from the Apennines to the edge of the Panonnian basin. The new measurements, together with previous findings, show an evident change of the anisotropic properties when moving along the profile. The distribution of SKS-splitting measurements in the Apennines strongly agree with previous measurements that already described the toroidal flow generated by the slab rollback of the Calabrian arc. In addition, the N-S and NE-SW directions found beneath the Apulia are in agreement with those attributed previously in the outer northern Apennines, to a proper typical pattern of the mantle beneath Adria, which is undeformed by the slab retreat. The pattern of the anisotropy in the Dinarides region shows lateral and vertical variations that together with recent tomographic images that better define the slab window allow us to speculate as follows: the new SKS measurements, interpreted in terms of mantle deformation and flows, agree with the geodynamic model that justifies the mantle circulation beneath Adria with the presence of slab windows in both the Apennines and Dinarides slabs.

Decoding the Emeishan Permian mantle plume in the southeastern margin of Tibet from the seismic signature of the local lithosphere

SUMMARY The Emeishan large igneous province (ELIP), which is located along the southeastern margin of Tibet, is widely regarded as the erosional remnants of voluminous Permian mafic volcanic successions that were associated with mantle plume activity. In addition, evidence for the eruption of the mantle plume and its signature of lithospheric modification should have been retained in the lithosphere after cooling of the plume. Here we use calculated compressional- and shear wave receiver functions (PRFs and SRFs, respectively) from >750 broad-band stations, together with heat flow and Bouguer gravity anomaly data, to investigate the seismic signature within the lithosphere beneath the southeastern margin of Tibet. Our results from the inner zone (INZ) of the ELIP indicate that the ∼60 km depth of the Moho interpreted from depth-migrated PRF images is consistent with that inferred from SRFs and is deeper than that in the region surrounding the INZ, which shows a continuous positive converted phase at ∼35–40 km depth. The crustal geometry and physical properties show a 15–20-km-thick mafic underplated layer that extends for 150–200 km in the E–W direction and ∼400 km in the N–S direction along the base of the crust in the northeastern INZ and its surrounding region. We interpret the underplated layer as the concealed Emeishan plume hotspot track and suggest that the head of the postulated mantle plume is centred approximately beneath Panzhihua City and its surroundings. We propose a new geodynamic model for the formation of the ELIP, in which the lower–middle crust has been strengthened by voluminous mafic intrusions and underplating, whereas the Yangtze Craton in the plume region has been destroyed by removing the base of the lithosphere.

New lithostructural map of the Doropo region, northeast Côte d'Ivoire: Insight from structural and aeromagnetic data

This study, performed in the Doropo region (northeastern Côte d'Ivoire) in the Paleoproterozoic domain (southern part of the WAC), aims to produce a detailed lithostructural map of this area. The geology of this region is less known due to the thick lateritic overburden and the scarcity of outcrops. Recent airborne geophysical data (aeromagnetic) integrated with field structural and lithological observations allowed to distinguish several lithologies including biotite granite, gneissic and migmatitic granite, granodiorite, tonalite, gabbro, amphibolite, rhyolite and dolerite. Filtering techniques of derivatives, upward continuation, tilt, analytical signal, spectral analysis and 3D Euler deconvolution showed the presence of faults, shear zones and intrusions. The structural analysis revealed four deformation events (D1-D4): (i) D1 is a N–S compression marked by E-W foliation (S1), conjugate NE-SW and NW-SE sinistral and dextral shears, respectively, as well as F1 folds with E-W axial planes; (ii) D2 is a major transpressive phase characterized by an intense NE-SW foliation (S2), E-W dextral shear zones, a weakly dipping stretching L2 lineation plunging N-E, and NE-SW faults; (iii) D3 is a NE-SW compressional phase generated NW-SE foliation (S3) with ∼E-W shears, and NW-SE axial plane F3 folds; (iv) D4 is a late, rather brittle phase characterized by brittle structures (faults, fractures, etc.) and the emplacement of NE-SW and NW-SE dolerite dykes. This tectonic evolution demonstrates that the Doropo region was subject to ductile and then brittle deformations. Furthermore, we suggest that the geodynamic model which could have prevailed in the Doropo region occurred in an oceanic arc collisional tectonic context.

Cenozoic deposits of western Kotel’nyi Island (New Siberian Islands): key insights into the tectonic evolution of the Laptev Sea

ABSTRACT The Arctic sedimentary basins are still poorly studied in comparison with other regions. The lack of deep wells across the eastern Russian Arctic has resulted in numerous contrasting geodynamic models for the geological evolution and age of sedimentary successions within this frontier region, where a modern mid-ocean ridge breaks through the continental crust in the Laptev Sea. The only onshore evidence of rifting processes is a number of small graben-like depressions exposed on the New Siberian Islands and along the Laptev Sea coast. We present U-Pb detrital zircon provenance and palynology study results of the Cenozoic sedimentary rocks filling graben-like depressions across western Kotel’nyi Island. Palynological data indicate that these sedimentary rocks are Early Eocene to Pleistocene in age. Based on U-Pb detrital zircon dating, Early Eocene and Late Oligocene clastic sediments were sourced from underlying deformed Palaeozoic rocks as well as by reworking of Upper Mesozoic rocks outcropping elsewhere on Kotel’nyi Island, which bear Siberian signature. Plio-Pleistocene clastic sediments were not derived from the erosion of deformed Palaeozoic rocks, suggesting the cessation of active uplift by this time and the development of a regional peneplain. Therefore, by extrapolating our onshore observations to the neighbouring offshore, we propose that graben structures imaged by seismic profiles along the eastern flank of the Laptev Rift System are likely to host Eocene and Oligocene sediments. Thus, it implies the Cenozoic extension led to formation of grabens on- and offshore in the eastern portion of Laptev Sea as early as Eocene. Further studies are necessary to evaluate the age of graben-related basins in the central and western part of the Laptev Sea.

Late‐Syn‐ to Post‐Rift Salt Tectonics on Wide Rifted Margins—Insights From Geodynamic Modeling

AbstractRifted margins are often associated with widespread and thick evaporite (salt) deposits and pronounced salt tectonics. The largest salt basins formed during the latest stages of rifting, immediately prior to continental breakup. Salt tectonics along these rifted margins commonly exhibit structural domains characterized by gravity‐driven updip extension, translation, downdip shortening, and salt nappe advance. The precise spatial and temporal links between these structural processes, their relative contributions and dynamics are still a topic of debate on many margins. We use 2D thermo‐mechanically coupled finite‐element modeling of lithospheric extension to investigate the evolution of salt tectonics along wide rifted margins and the interplay between rifting and post‐rift deformation. The models integrate lithospheric extension with post‐rift salt tectonics using a geodynamically self‐consistent approach where the geometries of the lithosphere and salt basins are not prescribed. They confirm that wide salt‐bearing rifted margins are characterized by gravity‐driven updip extension and downdip shortening, but also that syn‐depositional salt flow and salt stretching occurs in their distal portions prior to and during continental breakup. This produces widening of the basin and emplacement of a salt nappe over newly formed oceanic crust. Post‐rift updip extension is mostly balanced by downdip diapir shortening, all related to Couette flow. The salt nappe initiated by late syn‐rift stretching advances further by post‐rift pressure‐driven Poiseuille salt flow so that its final width is a product of both processes. The results can be directly compared to examples from various salt‐bearing rifted margins and improve our understanding of their enigmatic genesis and evolution.

Earthquake Rupture on Multiple Splay Faults and Its Effect on Tsunamis

AbstractDetailed imaging of accretionary wedges reveals splay fault networks that could pose a significant tsunami hazard. However, the dynamics of multiple splay fault activation during megathrust earthquakes and the consequent effects on tsunami generation are not well understood. We use a 2‐D dynamic rupture model with complex topo‐bathymetry and six curved splay fault geometries constrained from realistic tectonic loading modeled by a geodynamic seismic cycle model with consistent initial stress and strength conditions. We find that all splay faults rupture coseismically. While the largest splay fault slips due to a complex rupture branching process from the megathrust, all other splay faults are activated either top down or bottom up by dynamic stress transfer induced by trapped seismic waves. We ascribe these differences to local non‐optimal fault orientations and variable along‐dip strength excess. Generally, rupture on splay faults is facilitated by their favorable stress orientations and low strength excess as a result of high pore‐fluid pressures. The ensuing tsunami modeled with non‐linear 1‐D shallow water equations consists of one high‐amplitude crest related to rupture on the longest splay fault and a second broader wave packet resulting from slip on the other faults. This results in two episodes of flooding and a larger run‐up distance than the single long‐wavelength (300 km) tsunami sourced by the megathrust‐only rupture. Since splay fault activation is determined by both variable stress and strength conditions and dynamic activation, considering both tectonic and earthquake processes is relevant for understanding tsunamigenesis.

Constraining Upper Mantle Viscosity Using Temperature and Water Content Inferred From Seismic and Magnetotelluric Data

AbstractMantle viscosity controls a variety of geodynamic processes such as glacial isostatic adjustment (GIA), but it is poorly constrained because it cannot be measured directly from geophysical measurements. Here we develop a method that calculates viscosity using empirical viscosity flow laws coupled with mantle parameters (temperature and water content) inferred from seismic and magnetotelluric (MT) observations. We find that combining geophysical constraints allows us to place significantly tighter bounds on viscosity estimates compared to using seismic or MT observations alone. In particular, electrical conductivity inferred from MT data can determine whether upper mantle minerals are hydrated, which is important for viscosity reduction. Additionally, we show that rock composition should be considered when estimating viscosity from geophysical data because composition directly affects seismic velocity and electrical conductivity. Therefore, unknown composition increases uncertainty in temperature and water content, and makes viscosity more uncertain. Furthermore, calculations that assume pure thermal control of seismic velocity may misinterpret compositional variations as temperature, producing erroneous interpretations of mantle temperature and viscosity. Stress and grain size also affect the viscosity and its associated uncertainty, particularly via their controls on deformation regime. Dislocation creep is associated with larger viscosity uncertainties than diffusion creep. Overall, mantle viscosity can be estimated best when both seismic and MT data are available and the mantle composition, grain size and stress can be estimated. Collecting additional MT data probably offers the greatest opportunity to improve geodynamic or GIA models that rely on viscosity estimates.

The Role of a Detachment Fault in the Spatial Distribution of Ore-Bearing Paleofluid Flows in the Central Kolyma Region: A Nonconventional Approach to Predictive Metallogenic Modeling

—The Central Kolyma region is the main gold-bearing part of the Verkhoyansk–Kolyma fold-and-thrust belt. Analysis of the developed geodynamic models of fold and thrust belt formation mechanisms, the Verkhoyansk–Kolyma belt in particular, suggests the leading role of subhorizontal movements on the detachment zone (decollement) at the base of an orogen as the “sole,” on which nappes detached at an early stage and with which major reverse strike-slip listric faults were directly associated at the collisional stage. In our opinion, the role of a detachment fault, the most important regional structure, is obviously underestimated in predictive metallogenic models. The detachment fault zone is complicated by transverse NE-trending faults, where its thickness and the fluid permeability can occur. The paper proposes a variant that links previously discovered gold deposits and occurrences in five gold mineralization strips along the inferred paleofluid flow routes. Here, the paleofluid flow route is the horizontal projection of the most probable migration pathway of released fluids from their generation zone to the ore deposition zone, which is drawn across the largest ore accumulations.

Petrogenesis of Eocene high-silica granites in the Maliaoshan area, northern Tibet: Implications for the Eocene magmatic flare-up in the Northern Qiangtang Block

The Eocene magmatic flare-up in the Northern Qiangtiang Block is a key event that was genetically related to the uplift of the Tibetan Plateau. This uplift has been considered to have an important impact on the global cooling and oceanic isotopic compositions. Here, we report zircon ages and bulk rock elemental and isotopic compositions for high-silica porphyritic granites in the Maliaoshan area of the Northern Qiangtang Block. Zircon U–Pb dating shows that the porphyritic granites were generated at ca. 41.02 Ma. The porphyritic granites are characterized by uniformly high SiO2 contents (74.3–75.0 wt%), high contents of total alkalis, metaluminous compositions, and low contents of TiO2, Fe2O3T, MgO, CaO, and P2O5. Moreover, the Maliaoshan porphyritic granites have Nd–Hf isotopic compositions similar to those of coeval metaluminous magmatic rocks in the Northern Qiangtang Block. These data together with geochemical modeling results suggest that the Maliaoshan porphyritic granites were formed by the fractional crystallization of 43.06% major minerals and 0.76% accessory minerals from the assumed starting composition (average Eocene metaluminous igneous rock). Based on our new data and Eocene magmatic flare-up event in the Northern Qiangtiang Block, we propose a new geodynamic model to explain such a magmatic flare-up event. The stress produced by the collision of India and Eurasia gradually transferred northward, prompting the Songpan–Garzê Block to subduct southward beneath the Northern Qiangtang Block at ca. 55 Ma, accompanied by minor magmatic activity. Subsequently, the break-off of the Songpan–Garzê Block at ca. 46 Ma resulted in the Eocene magmatic flare-up.

An entropy method for geodynamic modelling of phase transitions: capturing sharp and broad transitions in a multiphase assemblage

SUMMARY Phase transitions play an important role for the style of mantle convection. While observations and theory agree that a substantial fraction of subducted slabs and rising plumes can move through the whole mantle at present day conditions, this behaviour may have been different throughout Earth’s history. Higher temperatures, such as in the early Earth, cause different phase transitions to be dominant, and also reduce mantle viscosity, favouring a more layered style of convection induced by phase transitions. A period of layered mantle convection in Earth’s past would have significant implications for the secular evolution of the mantle temperature and the mixing of mantle heterogeneities. The transition from layered to whole mantle convection could lead to a period of mantle avalanches associated with a dramatic increase in magmatic activity. Consequently, it is important to accurately model the influence of phase transitions on mantle convection. However, existing numerical methods generally preclude modelling phase transitions that are only present in a particular range of pressures, temperatures or compositions, and they impose an artificial lower limit on the thickness of phase transitions. To overcome these limitations, we have developed a new numerical method that solves the energy equation for entropy instead of temperature. This technique allows for robust coupling between thermodynamic and geodynamic models and makes it possible to model realistically sharp phase transitions with a wide range of properties and dynamic effects on mantle processes. We demonstrate the utility of our method by applying it in regional and global convection models, investigating the effect of individual phase transitions in the Earth’s mantle with regard to their potential for layering flow. We find that the thickness of the phase transition has a bigger influence on the style of convection than previously thought: with all other parameters being the same, a thin phase transition can induce fully layered convection where a broad phase transition would lead to whole-mantle convection. Our application of the method to convection in the early Earth illustrates that endothermic phase transitions may have induced layering for higher mantle temperatures in the Earth’s past.