Crustal Rheology Research Articles

Haasttse‐baad Tessera Ring Complex: A Valhalla‐Type Impact Structure on Venus?

AbstractVenus preserves ∼1,000 impact craters, yet to date no impact basins larger than 300 km in diameter—common in the oldest terrains on Mercury, Mars and the Moon—are recognized on Venus. The tessera terrain is Venus' oldest recognized terrain. We describe a ∼1,500 km‐diameter concentric ring‐graben complex preserved on Haasttse‐baad Tessera, Venus that we identify as the Haasttse‐baad Tessera Ring Complex (HTRC). Based on geologic relations and numerical modeling, we propose that the HTRC may represent a Valhalla‐type multiring impact basin formed late during the evolution of its host ribbon‐tessera terrain (rtt). Formation of Valhalla‐type impact basins could involve a unique three‐layer target rheology with a thin elastic layer above a low viscosity layer above a deep strong layer. This multi‐layer rheological sandwich is consistent with crustal rheology previously proposed for the formation of Venus' rtt. If the HTRC is a Valhalla‐type impact basin, it would be Venus' oldest, and currently largest, impact structure, providing a rare window into Venus' ancient past and with implications for early crustal processes on Venus.

Timing and characteristics of co-seismic surface ruptures in the Yadong rift, southern Tibet

The Yadong-Gulu rift (YGR) is the most prominent and seismically active of the seven main ∼ NS-trending rifts in southern Tibet. Although the morphology of the southern YGR clearly indicates it has witnessed large earthquakes in the past, and despite its significant late Quaternary throw rates of ∼1 mm/yr, no large historical or instrumental earthquakes have been reported, including in the southernmost Pagri half-graben, in contrast to the northern part of the rift which is highly seismically active. Here, geomorphic characteristics helped us constrain the timing of a paleoearthquake that produced surface ruptures along the Pagri half-graben, used to document its past activity and evaluate its seismic hazard. We demonstrate that the co-seismic surface ruptures extend for ∼65 km along the Yadong normal fault, with a maximum vertical displacement ranging from 2 to 4.0 ± 0.1 m. Based on empirical relationships between magnitude, surface rupture length, and fault displacement, we suggest that this event may correspond to a Mw6.9–7.2 earthquake. Combined with previous studies, our radiocarbon (14C) and Optically Stimulated Luminescence (OSL) ages from three pits within the earthquake wedge across the surface ruptures constrain the paleoearthquake timing at 3470-2056 years BP. We suggest that the southern YGR currently has a high regional seismic hazard for a Mw6.8–7.1 earthquake, considering the significant throw rates and long timespan since the last strong event. Furthermore, we suggest that such different seismic activity and throw/extension rates between the southern and northern YGR may be explained by different upper crustal rheology behavior and mid-crustal structure.

Geodynamic modeling on continental collision in Qilian orogenic belt

The Qilian orogenic belt (QOB) located in the northeast margin of the Tibetan Plateau is featured by remarkable crustal thrusting and shortening, providing a key natural example to understand the lithospheric deformation of the Tibetan Plateau. Two types of continental collision are observed in the QOB: lithosphere subduction beneath Southern Qilian and crust underthrusting of Alxa terrain along the North Border Thrust (NBT). Deep seismic reflection profiles reveal complex stress field evolution, including compressional deformation in the lower crust, extensional deformation in the upper crust, and detachment deformation in the middle crust. In this study, we use 2D numerical modeling to investigate the dynamics of these two different collision types and the evolution of Qilian uplift. Model results suggest three patterns of continental collision, i.e., crust underthrusting follows lithosphere subduction, lithosphere subduction and the failed underthrusting/subduction. The key factors that may influence model evolution, including crustal rheology, convergence direction and rate, are systematically investigated. Our model results are further compared to observations, suggesting that lower convergence rate and crust underthrusting along NBT likely control the uplift and crust stress stratification of the QOB.

The Role of Lower Crustal Rheology on Surface Deformation During Oblique Extension: Insights From Sandbox Modeling

AbstractExtensive researches have been conducted on the relationship between surface deformation and the properties of upper crust. However, the link between surface deformation and lower crustal rheology, especially in a three‐dimensional context, remains unclear. In this study, we utilize sandbox modeling to investigate the impact of lower crustal rheology on surface deformation during oblique extension. Under the same conditions, six models with different lower crustal viscosities, both with and without syn‐kinematic deposits, are conducted. The results indicate that a decrease in lower crustal viscosity may contribute to an increase in: (a) graben width, (b) graben length, (c) graben spacing, (d) the number of isolated rifts and (e) topographic relief of oblique extensional systems, while also leading to a reduction in the total number of grabens. Notably, there exists a negative linear correlation between graben spacing and lower crustal viscosity. In map view, the angle between fault strike and the direction of pre‐existing discontinuities increases as the viscosity of the lower crust decreases. Furthermore, the frequency of large rakes (>50°) decreases with decreasing lower crustal viscosity. These findings align with natural examples such as the East African Rift System, Weihe Graben and the South Tibetan Rift in terms of geomorphology, tectonics, and crustal rheology. Through a comprehensive comparison of the graben width, spacing, and the angle between fault strike and the direction of pre‐existing discontinuities, our study provides valuable insights into the rheology of the lower crust in natural settings.

Effects of crustal rheology and fault strength on the formation of the Qaidam Basin: Results from 2-D mechanical modeling

The Cenozoic Qaidam Basin on the northern Tibetan Plateau is bounded by the foreland thrust belts of the East Kunlun Mountains to the south and Qilian Mountains to the north. In this basin, the maximum thickness of the accumulated sedimentary strata is ∼17 km near the geographic center of the basin. Such a basin architecture challenges the classic foreland-basin deformation model, wherein substantial sediments should be localized near the basin margins owing to thrust loads on a bending plate. Based on the available geological and geophysical constraints, we built a two-dimensional viscoelastoplastic finite element model to investigate whether the unique pattern of the Qaidam Basin was mechanically related to the rheological structure of the crust and frictional strength of the basin-bounding thrust fault. By testing reasonable bounds on the viscosity of the ductile crust and effective frictional coefficient of the thrust faults, we modeled a basin framework that fits well with that of Qaidam Basin. The model parameters include a layered rheological structure, with the viscosity of the ductile crust of the basin being two orders of magnitude greater than that of the adjacent mountain belts and an effective frictional coefficient of >0.2. Furthermore, the initial width of the basin significantly affected its deformation style. Therefore, the layered rheological structure of the crust and frictional strength of the basin-bounding thrust faults played important roles in the formation of the Qaidam Basin. Our findings suggest that the inelastic behavior of the crust affects foreland deformation, highlighting that the classic foreland basin model must be revised for some tectonic settings.

Integrated Investigation on Heterogeneous Lower Crust Rheology in Kyushu and Afterslip Behavior Following the 2016 Mw7.1 Kumamoto Earthquake

AbstractThe viscoelastic lower crust beneath Kyushu Island, influenced by the volcanic arc, interplays with active crustal faults in this region and helps to shape local tectonics. In this study, we employed a three‐dimensional viscoelastic finite element model to gain insights into the lithospheric rheology and crustal faulting kinematics, through modeling the postseismic deformation processes of the 2016 Mw 7.1 Kumamoto earthquake. Our model reveals a viscosity of 2 × 1020 Pa s for the lower crust and 2 × 1019 Pa s for the upper mantle. A reduced lower crust viscosity of 2 × 1019 Pa s in the volcanic arc area is required for better reproducing the Global Positioning System data. The stress‐driven afterslip decays rapidly over time and is up to 0.3 m within 5 years after the earthquake. We propose additional normal‐component afterslip to better explain the complex postseismic deformation in the near field, which may be due to the interaction between the fault and volcano Aso.

Dominant slip systems of quartz under lower amphibolite-facies conditions identified from microstructures and CPOs in quartz phenocrysts

Identifying the dominant slip systems in naturally deformed quartz is crucial for understanding continental crust rheology. Although the basal <a> slip system is considered dominant in quartz under upper to middle crustal conditions, its activity is controversial because an oriented nucleation and growth model has been proposed for the c-axis distribution near the minimum strain axis. This study examined the crystallographic orientation and shape of quartz phenocrysts in a deformed granitic porphyry in the Ryoke belt, SW Japan, to clarify the dominant slip systems in naturally deformed quartz at temperature conditions of ∼400–500 °C utilizing optical and electron backscatter diffraction (EBSD) observations. Identified active slip systems include prism <a>, basal <a>, prism [c], and rhomb <a> through misorientation analyses. The aspect ratios of phenocrysts with dominant prism <a> and basal <a> slip systems are higher than prism [c] and rhomb <a> slip systems, indicating similar strengths between prism <a> and basal <a> slip systems, which are weaker than prism [c] and rhomb <a> slip systems under lower amphibolite-facies conditions. The c-axis of phenocrysts with dominant basal <a> distribute at pole figures peripheries, indicating basal <a> activation over the proposed oriented nucleation and growth model.

Rift Propagation Interacting With Pre‐Existing Microcontinental Blocks

AbstractRift propagation is a 3D thermo‐mechanical process that often precedes continental breakup. Pre‐existing microcontinental blocks and the associated lithospheric strength heterogeneities influence the style of rift propagation. Interestingly, some rifts propagate into pre‐existing blocks and eventually cut through them (e.g., the Zhongsha Block and the Reed Bank), while others bypass these microcontinental blocks forming distinct overlapping rift branches (e.g., the East African Rift System). In this study, we use 3D numerical models to investigate the interaction between microcontinental blocks and rift propagation under different far‐field extension rates. In doing so, we assess the impact of mantle lithospheric thicknesses and lower crustal rheology on the style of rift propagation. Our models reproduce the two types of rift propagation, characterized by propagating rifts that either split or bypass the pre‐existing microcontinental blocks. We find that lithospheric thickness exerts dominant control, while lower crustal rheology of microcontinental blocks and the extension rate have less effect on rift propagation. Our model results can explain rift propagation patterns, block rotation and strong lithospheric thinning in the South China Sea, the East African Rift System, and the Woodlark Basin.

3D Crustal Architecture and Along‐Strike Variation in the Mid‐Northern South China Sea Rifted Margin

AbstractRifted margins often show apparent along‐strike variations at the continent‐ocean transition (COT) region, typically occurring gradually over hundreds‐to‐thousands of km between the “magma‐rich” and “magma‐poor” endmembers. In contrast, comparatively smaller‐scale lateral variabilities of rifting structure and faulting style in hyper‐thinned continental crust are not yet well described. The South China Sea (SCS) was recently defined as the archetype of a new “intermediate‐type” margin with neither excess magmatism nor mantle exhumation. However, along‐strike crustal variations along this new type of margin are still little explored. Based on previously unpublished 2D grid of seismic reflection lines and overlapping 3D seismic volumes, we have investigated the crustal structure, faulting style, and the lower crustal reflectivity along ∼400 km of the mid‐northern SCS margin. Our new maps of crystalline‐crust thickness and top basement show that the SCS extension developed in two segments with contrasting tectonic styles separated by a narrow boundary. The eastern domain, resembling a wide‐rift model, displays low‐angle detachment faults and high‐velocity lower crust mainly associated with syn‐rift magmatism indicating weak crustal rheology. The neighboring western domain, resembling a narrow rift model, displays steep crustal‐scale faults indicating comparatively strong rheology. The change from wide to narrow rift—or weak to strong rheology—occurs abruptly, which appears at odds with the current conventional wisdom of mostly gradual transitions over a long distance between rift models. We speculate that the inherited heterogeneity in mantle composition and 3D melt focusing effect may drive this abrupt along‐strike variability in the mid‐northern SCS.

On the Potential Role of Viscoelasticity in Fluid‐Induced Seismicity

AbstractFluid‐induced earthquakes adversely affect industrial operations like hydraulic fracturing (e.g., 4.6 Mw in Alberta, Canada) and enhanced geothermal systems (e.g., 5.5 Mw in Pohang, South Korea). Identifying all underlying physical processes contributing to fluid‐induced seismicity presents an open challenge. Recent work reports signatures of event‐event triggering or aftershocks—common for tectonic settings—within the context of fluid‐induced seismicity. Here, we investigate the underlying potential cause of these field observations from a modeling perspective. We extend a novel conceptual model to simulate the characteristics of crustal rheology and stress interactions in a porous medium by combining viscoelastic effects with fluid diffusion and invasion percolation associated with a point source. Our model successfully reproduces realistic aftershock behavior and statistical properties similar to those resulting from tectonic loading indicating that the statistical properties of aftershocks are unaffected by the fluid injection rate. At the same time, the Gutenberg‐Richter relation, the spatial footprint of fluid‐induced events and their dependence on the permeability field are largely unaltered by the viscoelasticity of the medium and the aftershocks it causes. Furthermore, we investigate the impact of varying fluid injection rates on detecting aftershocks and event‐event triggering sequences during viscoelastic stress redistribution. We find that when the injection rate is sufficiently high, aftershock detection and recovery of their statistical properties are only feasible when the underlying internal stress redistribution is directly accessible. This could explain why aftershocks have not been reported in some field studies of fluid‐induced seismicity.

Lateral changes of crustal extension and passive margin type along the Brazilian southeastern margin

For the last half century, studies based on an increasing, diverse data set, have focused on passive continental margin evolution, as a result of a sequence of tectonic processes that occur in crustal scale and in the geological time. Where extensive subsurface data exists, two distinct endmembers of continental margin architectures were first described along specific margins of the North Atlantic: 1) wide continent-ocean transitions or hyper-extended; 2) narrow continent-ocean transitions or hypo-extended. The lateral transition between these endmember margins, which can occur laterally in short distances, is still not fully understood. The same two endmember margin types are observed across passive margins around the world and notably along the South Atlantic margin. In an area known for controversial interpretations about the crustal nature and the limits of thinned continental crust, this investigation integrates crustal thickness, seismic interpretation, and facies analyses across ∼150,000 km of seismic data along the Brazilian southeastern margin. This study has implications for those investigating the crustal geometry variations for basin analysis and results impact hydrocarbon assessments for the southeastern margin of Brazil. We resume continental margin analyses, in a critical moment as hydrocarbon exploration advances from the continental slope to ultra-deep waters. Results indicate a marked change from a narrow, hypo-extended, sub-aerial, Iceland-like, plume-related volcanic crust in the Pelotas Basin, to a hyper-extended, Iberia-like, magma-rich crust in the Santos Basin, which is separated by a pronounced oceanic transform-fault zone. Dextral movement along this NW-oriented fault zone accommodated differential extensional rates between the hypo- and the hyper-extended margins. Lateral variations in magmatic content within these margin types are interpreted as result from the interaction with mantellic plumes. Margin architecture can locally be affected by pre-existing fault zones and be later modified by oceanic transform faults. The interactions between crustal extensional rates, crustal rheology, mantle underplating, and volcanic material exhumed through mantle-derived plumes, are the key controls for the evolution of continental margins. Tectonic framework classification proposed in this study presents an alternative, original model for continental passive margins evolution.

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest−southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast−southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Lower Crustal Rheology Controls Strain Partitioning and Mode of Intracontinental Deformation

AbstractThe factors that control strain partitioning along plate boundaries and within continental interiors remains poorly resolved. Plate convergence may be accommodated via distributed crustal shortening or discrete crustal‐scale strike‐slip faulting, but what controls these differing modes of deformation is debated. Here we address this question by examining the actively deforming regions that surround the Tarim Basin in central Asia, where deformation is uniquely partitioned into predominately strike‐slip faults in the east and distributed fold‐thrust belts in the west to accommodate Cenozoic India‐Asia plate convergence. We present integrated geological and geophysical observations to elucidate patterns in crustal deformation and compositional structure in and around the Tarim Basin. The thrust‐dominated western Tarim Basin correlates with a strongly‐magnetic lower crust, whereas strike‐slip faulting along the eastern margins of the Tarim Basin lack such magnetic signals. We suggest that the lower crust of the western Tarim is more mafic and stronger than in the east, which impacts intra‐plate strain partitioning. A stronger lower crust results in vertical decoupling to drive mid‐crust horizontal detachments and facilitate thrust faulting, whereas a more homogenized crust favored vertical transcrustal strike‐slip faulting. These rheological differences likely originated from the impingement of the Permian Tarim plume focused in the west. A comparison with the Longmen Shan of eastern Tibetan Plateau reveals remarkably similar strain partitioning that correlates with variations in foreland rheology. Our results highlight how variations in lower‐crust viscosity impact strain partitioning in an intra‐plate setting and how plume processes exert a strong control on later continental tectonic processes.

The Impact of Climate‐Driven Lake Level Changes on Mantle Melting in Continental Rifts

AbstractClimate cycles fundamentally control surface processes that affect the distribution of water and sediment, and their associated loads, across the Earth's surface. Here, we use a geodynamic model to examine how water loading can affect mantle melt generation in continental rift settings covered by deep lakes. Our modeling results suggest that lake level fluctuations can modulate the timing and rate of mantle melting. A rapid lake level drop of 800 m has the potential to increase mantle melt volumes by enhancing mantle upwelling beneath the rift, whereas a lake level rise can lead to a reduction of mantle melting. The volume of melt produced driven by lake level fluctuations is also dependent on crustal rheology, extension rate, mantle potential temperature, and lithosphere thickness. Our study identifies the importance of water loading for controlling rift processes, while also demonstrating critical links between changing climate, rift evolution and mantle melting.

Numerical Modeling of Earthquake Cycles Based On Navier‐Stokes Equations With Viscoelastic‐Plasticity Rheology

AbstractVisco‐elastic‐plastic modeling approaches for long‐term tectonic deformation assume that co‐seismic fault displacement can be integrated over 1000s–10,000s years (tens of earthquake cycles) with the appropriate failure law, and that short‐timescale fluctuations in the stress field due to individual earthquakes have no effect on long‐term behavior. Models of the earthquake rupture process generally assume that the tectonic (long‐range) stress field or kinematic boundary conditions are steady over the course of multiple earthquake cycles. This study is aimed to fill the gap between long‐term and short‐term deformations by modeling earthquake cycles with the rate‐and‐state frictional (RSF) relationship in Navier‐Stokes equations. We reproduce benchmarks at the earthquake timescale to demonstrate the effectiveness of our approach. We then discuss how these high‐resolution models degrade if the time‐step cannot capture the rupture process accurately and, from this, infer when it is important to consider coupling of the two timescales and the level of accuracy required. To build upon these benchmarks, we undertake a generic study of a thrust fault in the crust with a prescribed geometry. It is found that lower crustal rheology affects the periodic time of characteristic earthquake cycles and the inter‐seismic, free‐surface deformation rate. In particular, the relaxation of the surface of a cratonic region (with a relatively strong lower crust) has a characteristic double‐peaked uplift profile that persists for thousands of years after a major slip event. This pattern might be diagnostic of active faults in cratonic regions.

The effect of in-situ metasomatism on the electrical resistivity of the lower crust

Improved estimates of composition and temperature in the lower crust are vital in characterising crustal rheology and better understanding tectonic processes. Fluids introduced into the lower crust during collisional events leave behind large domains of metasomatised crust which are long-lived and should be observable via magnetotelluric sounding. With this in mind, the electrical conductivity of a subduction-related eclogite and its anhydrous, anorthositic granulite protolith were experimentally determined at atmospheric pressure and lower crustal temperatures (773–1123 K). Complex impedance spectra were collected between 20 Hz and 2 MHz while oxygen fugacity was controlled about the quartz-fayalite-magnetite (QFM) buffer using a CO:CO2 gas mix. At <1000 K, both samples are characterised by impurity conduction with low observed activation enthalpies of 0.48 ± 0.03 eV and 0.39 ± 0.02 eV for the granulite and eclogite respectively. High temperature data from the granulite (>1000 K) indicates a switch to small polarons with an activation enthalpy of 1.02 ± 0.02 eV. Results indicate that conversion of Fe-poor mafic granulite to eclogite does not enhance conductivity at lower crustal temperatures. Comparisons with previous experiments reveal a much stronger relationship between conductivity and FeOT than previously anticipated. Modelling indicates that an increase in FeOT of 2 wt% is equivalent to a temperature increase of 100 K and reasonable variations in FeOT in the lower crust may account for ∼4 orders of magnitude variation in conductivity. Elevated FeOT up to 20 wt% may enhance conductivity to ∼100 S m−1 at a moderate 873 K. A system of linear equations is derived to estimate iron content, electrical conductivity or temperature within dry, mafic lower crust. For the temperature range 773–973 K, conductivity may be predicted to within 0.16 log units using log σ(T,FeOT) = 0.006 T + 0.3FeOT − 11.35. Under the same conditions, FeOT may be predicted to within 0.51 wt% using FeOT (T,σ) = 37–0.0193 T + 3.24 log σ. Finally, lower crustal temperatures may be estimated to within 24 K using T(FeOT,σ) = 1728 − 42FeOT + 139 log σ.

Mechanism of Structure Variations at Rifted Margins in the Central Segment of South Atlantic: Insights from Numerical Modeling

AbstractRifted margins in the central South Atlantic portray spatial variability in terms of preserved width and thickness, which relates to complex rift‐related fault activities. However, there is still a lack of systematic and quantitative explanations for the causes of the variations that are observed along the paired rifts. To elucidate this issue, 2D viscous‐plastic thermomechanical numerical models are applied to capture the behavior of deformation, in which we investigate the effects of extensional rate, crustal strength and thickness on crust‐mantle coupling, and timing of transition from rifting to breakup. Our numerical experiments demonstrate that crust‐mantle decoupling accounts for crustal hyperextension, and that incorporating moderate‐intensity rheology into lower crust may yield insights into the hyper‐extended crust and asymmetric architecture observed in the central South Atlantic. The results also suggest that undulations in lithospheric basement cause asymmetric mantle upwelling. The lower crust of fold belts takes priority to be thermally weakened over craton and induces rift migration simultaneously. A new mechanism for the formation of failed rift is described, where the mechanical decoupling derived from thermally weakened lower crust gives access to dual rift migration. These results reinforce the interpretation on how crustal rheology shapes margins architectures and highlight the first‐order effects of crust‐mantle coupling.

Focused Mid‐Crustal Magma Intrusion During Continental Break‐Up in Ethiopia

AbstractSignificant volumes of magma can be intruded into the crust during continental break‐up, influencing rift evolution by altering the thermo‐mechanical structure of the crust and its response to extensional stresses. Rift magmas additionally feed surface volcanic activity and can be globally significant sources of tectonic CO2 emissions. Understanding how magmatism may affect rift development requires knowledge on magma intrusion depths in the crust. Here, using data from olivine‐hosted melt inclusions, we investigate magma dynamics for basaltic intrusions in the Main Ethiopian Rift (MER). We find evidence for a spatially focused zone of magma intrusion at the MER upper‐lower crustal boundary (10–15 km depth), consistent with geophysical datasets. We propose that ascending melts in the MER are intruded over this depth range as discrete sills, likely creating a mechanically weak mid‐crustal layer. Our results have important implications for how magma addition can influence crustal rheology in a maturing continental rift.

First-principles investigation on diffusion in stishovite and CaCl2-type silica: Implication for MORB viscosity in the lower mantle

Subducted oceanic crust is thought to play a key role in generating the compositional heterogeneities in the lower mantle. Its rheological properties are essential for understanding the segregation of oceanic crust from the slab and the further interaction with the surrounding mantle. Here, we report first-principles results for Si and O diffusions in stishovite and CaCl2-type silica and evaluate the viscosity of the oceanic crust under lower-mantle conditions. Our results show that the Si diffusion in CaCl2-type silica is enhanced by pressure, with a negative activation volume of ∼−0.2 cm3/mol along the 〈111〉 direction. We find that the Si diffusivity of CaCl2-type silica is two orders of magnitude faster than that of bridgmanite in the deep lower mantle, and therefore, it becomes a rheological weaker phase compared to bridgmanite. In addition, CaCl2-type silica could substantially reduce the crustal viscosity and facilitate the segregation of upper oceanic crust from the lower lithosphere with increasing depths. The delaminated crustal fragments may be sufficiently stirred and stretched by the vigorous mantle convection and account for the ubiquitous small-scale heterogeneities in the lower mantle. Our results provide new constraints on the crustal rheology data for future geodynamic simulations to better understand the crustal recycling in the lower mantle.

The tectonics and volcanism of Venus: New modes facilitated by realistic crustal rheology and intrusive magmatism

To explain Venus’ young surface age and lack of plate tectonics, Venus’ tectonic regime has often been proposed to be either an episodic-lid regime with global lithospheric overturns, or an equilibrium resurfacing regime with numerous volcanic and tectonic activities. Stratigraphic analysis suggests that Venus’ surface tectonics could be a combination of these two end-member models with a global resurfacing event that created most of the crust followed by tectonic and volcanic activities until now. Recent analyses of Venus’ satellite images also suggest widespread lithospheric mobility in the lowland basins that does not fit the episodic-lid tectonic regime. Here, we use global 2-D thermochemical convection models with realistic parameters, including rheology (dislocation creep, diffusion creep, and plastic yielding), an experiment-based plagioclase (An75) crustal rheology, and intrusive magmatism, to investigate the tectonics and mantle evolution of Venus. We find that surface tectonics is strongly affected by crustal rheology. With a “weak” plagioclase-rheology crust, models exhibit episodic overturns but with continuously high surface mobility and high distributed surface strain rates between overturns, leading to a new tectonic regime that we name “deformable episodic lid”. On the other hand, olivine-crustal-rheology models exhibit either standard episodic-lid tectonics, i.e. with mobility that is high during overturns and near zero between overturns, or stagnant-lid tectonics, i.e. with near-zero mobility over the entire model time. In models with plagioclase-rheology crust, magmatic resurfacings are short-lived and randomly located, leading to a rather uniform resurfacing rate. Also, a combination of plagioclase crustal rheology and dislocation creep can weaken the lithosphere sufficiently to facilitate lithospheric overturns without applying plastic yielding. Internally, the composition-dependent density profile results in a “basalt barrier” at the mantle transition zone, which strongly affects Venus’ mantle evolution. Only strong plumes can penetrate this basalt barrier and cause global lithospheric overturns. This basalt barrier also causes global internal episodic overturns that generate global volcanic resurfacing in stagnant-lid models, which suggests a new resurfacing mechanism that does not involve lithospheric overturns. We name this regime “stagnant episodic-volcanic-resurfacing”. Results in models with plagioclase crustal rheology and low eruption efficiencies best fit current estimates of crustal thickness, surface age, and tectonic structures based on observations of Venus. The crustal thickness in models with an olivine-rheology crust is limited by the basalt–eclogite phase transition and is higher than observation-based estimates. Higher intrusion rates better fit these estimates, as models with high eruption efficiency predict very low surface age (< 100 Myr).