Properties Of Lithosphere Research Articles

Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

Kinking is an important strain-accommodating process during crystal plastic deformation under relatively large stresses and may influence the mechanical properties of the Earth's lithosphere and planetary cryosphere. To better understand the origins, mechanisms, and microstructural effects of kinking, we present detailed microstructural analyses of coarse-grained (∼1300 μm) ice samples deformed under uniaxial compression at -30°C. Deformed samples show elongated (aspect ratio ≥ 4) kink domains developing within or at the tips of remnant original grains (≥300 μm, aspect ratio < 4). Small, equiaxed subgrains also develop along the margins of remnant grains. Moreover, many remnant grains are surrounded by mantles of small, recrystallized grains (<300 μm, aspect ratio < 4). Together, these observations indicate that grain nucleation is facilitated by both kinking and dynamic recrystallization. Low- (<10°) and high-angle (mostly >10°, many >20°) kink bands within remnant grains have misorientation axes that lie predominantly within the basal plane. The c-axes of most kink domains are oriented sub-perpendicular to the sample compression axis, indicating that kinking may produce or modify a crystallographic preferred orientation. Kink band densities are highest within remnant grains that have basal planes sub-parallel to the compression axis—these data are inconsistent with models suggesting that (if kinking is the only strain-accommodating process) there should be higher kink band densities within grains that have basal planes oblique to the compression axis. One way to rationalize this inconsistency between kink models and experimental observations is that kinking and dynamic recrystallization are both active during deformation, but their relative activities depend on the crystallographic orientations of individual grains. For grains with basal planes sub-parallel to the compression axis, strain-induced grain boundary migration (GBM) is inhibited, and large strain incompatibilities can be relaxed via kinking when other processes such as subgrain rotation (SGR) recrystallization are insufficient. For grains with basal planes oblique to the compression axis, strain-induced GBM might be efficient enough to relax the strain incompatibility via selective growth of these grains, and kinking is therefore less important. For grains with basal planes sub-perpendicular to the compression axis, kink bands are seldom observed—for these grains, the minimum shear stress required for kinking exceeds the applied compressive stress, such that kinks cannot nucleate.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

A Global Survey of Lithospheric Flexure at Steep‐Sided Domical Volcanoes on Venus Reveals Intermediate Elastic Thicknesses

AbstractTopographic flexure in response to vertical loads reveals key lithospheric properties, including elastic thickness and the heat flow from the interior. Flexural stresses that depend on elastic thickness may in turn control volcano morphology. One previous study predicted that steep‐sided domes on Venus usually form where the elastic thickness is ∼15–40 km. Coronae and large volcanoes may typically form at regions with lower and higher elastic thickness, respectively. We surveyed flexural signatures around steep‐sided domes and confirmed this hypothesis. Specifically, we extracted radial profiles of topography from Magellan altimetry data and a new elevation model derived from stereo images. Nearly 20% of the identifiable domes had topographic profiles that were amenable to flexural interpretations. We determined elastic thicknesses using a curve‐fitting algorithm and plate bending models that treat each volcano as either a Cartesian line load or an axisymmetric disc load. We used a yield strength envelope featuring brittle failure and ductile flow at the top and bottom of the lithosphere, respectively, to convert elastic thickness into mechanical thickness and surface heat flow. The average elastic thickness for domes not near coronae is ∼30 km, corresponding to a heat flow of ∼60 mW/m2. Coronae on Venus are typically associated with elastic thicknesses of &lt;10–15 km. Domes near coronae yielded elastic thicknesses in that range, and higher heat flows than domes not near coronae. Ultimately, flexural signatures are probably abundant around volcanic and tectonic features and would be further revealed in higher‐resolution topographic data.

Survey of mid-sized Venusian volcanoes using stereo-derived topography

We have examined the morphology and morphometry of 21 mid-sized Venusian volcanoes (flow apron diameter 50–300 km) in the area covered by Magellan stereo imaging, excluding the already well-studied steep-sided domes. Volcanoes in this size range have received little previous attention because they are not adequately sampled by Magellan altimetry, but we have been able to discern their basic shapes with stereo-derived topography from multiple-look radar imagery. We grouped the volcanoes into five general categories based on morphology and slope: stellate fracture centers, shield-like, fissure-derived, concave-sided, and volcano/corona hybrids. Statistically, these constructs have slopes significantly lower than those of terrestrial and Martian volcanoes of similar diameter, as has been noted for large Venusian volcanoes, though some of the steepest constructs in this work have slopes similar to the shallowest terrestrial and Martian volcanoes; portions of the flanks of Ne Ngam Mons and an edifice that formed in the caldera of Toci Tholus exceed 9° and 20°, respectively. These 21 volcanoes formed in regions of middling crustal thicknesses and generally formed away from tessera terrains; they are over neither major mantle downwellings nor upwellings (with the possible exception of fissure-derived constructs). Similar constructs can be found at significantly larger diameters spanning over an order of magnitude in diameter, unlike other features such as steep-sided domes. The presence of these constructs over small mantle temperature anomalies and the existence of counterparts for each class at larger diameters suggest that a mantle heat source must be available to drive volcanism, which would explain why these volcanoes do not occur over areas of inferred downwelling. The volcanoes are not in areas of thickened crust, perhaps because it hinders magma ascent. The strength of the upwelling seems to have a significant effect on the size of the feature, with only large volcanoes over the highest negative mantle density anomalies. The varying morphologies reflect the local and regional stress regime and other crustal and lithospheric properties. We identified a correlation between mantle dynamics and base elevation: volcanoes in regions of inferred mantle upwelling tend to have higher base elevations than those in regions of inferred mantle downwelling, suggesting that modern base elevations are partly controlled by mantle dynamics. These mid-sized volcanoes are generally located high in the stratigraphic column, indicating that the majority of volcano forming events occurred late in the active portion of Venus' geologic history which is in agreement with previous works. Two volcanic constructs, a hybrid and a concave-sided volcano, appear to predate all but the earliest stratigraphic markers, which implies that, as a family, these volcanoes are not limited to some particular period in the observable surface history. We identified multiple volcano/corona hybrids, features with morphologies characteristic of both coronae and volcanoes. Often, these have topography incongruous from that inferred from radar images alone; for example, a topographic depression on the flank of a volcano with purely radial lava flows. This incongruous topography indicates significant changes in volcanic processes that may be useful in unraveling the time dependent evolution of the Venusian surface. The existence of these hybrids with flow diameters less than 100 km and their inferred deformation suggest that the elastic lithosphere must have been near negligible thickness during or following their formation. These hybrids form near regions of extension, which supports the hypothesis that a relatively thin elastic lithosphere is necessary to form them.

Thermo‐Mechanical State of Ultraslow‐Spreading Ridges With a Transient Magma Supply

AbstractThe thermal structure of mid‐ocean ridge axes is a critical control on the mechanical properties of young lithosphere and modulates the faulting styles that shape the Earth's seafloor. Models that balance a steady input of magmatic heat with a vigorous hydrothermal output are generally successful at explaining the thermal state of magmatically robust mid‐ocean ridges. However, the magma supply of ultraslow spreading ridges is often subdued and highly episodic, and the depth‐extent and vigor of hydrothermal circulation in these settings remain poorly known, requiring modifications to the standard magmatic‐hydrothermal paradigm. We develop models that couple repeated magmatic intrusions with hydrothermal convection in a permeable domain whose boundaries are controlled by the extent of grain‐scale thermal cracking. Our simulations reveal decreasing trends of venting temperatures with increasing intrusion periodicity, as well as with increasing permeability. Our model explains the seismically inferred depth (∼10 km) to the brittle‐ductile transition and the depth of MORB crystallization at the Mid‐Cayman Spreading Center by invoking repeated intrusion of sills every 20–50 kyrs, depending on their size. Doubling this periodicity predicts a thicker (∼15 km) seismogenic layer and lower‐temperature venting, as observed at the Southwest Indian Ridge (SWIR) at 64˚E. However, if fluid convection is not possible below ∼6 km, our model requires that high‐temperature venting at ultraslow ridges (e.g., Longqi at SWIR 49.7˚E) be fueled by shallow episodic magma intrusions into the long‐term convective region, which maintain high‐output circulation for at most a few tens of kyrs.

Understanding earthquakes using the geological record: an introduction

Understanding earthquakes using the geological record: an introduction

The interaction between mantle plumes and lithosphere and its surface expressions: 3-D numerical modelling

SUMMARY The rise of mantle plumes to the base of the lithosphere leads to observable surface expressions, which provide important information about the deep mantle structure. However, the process of plume–lithosphere interaction and its surface expressions remain not well understood. In this study, we perform 3-D spherical numerical simulations to investigate the relationship between surface observables induced by plume–lithosphere interaction (including dynamic topography, geoid anomaly and melt production rate) and the physical properties of plume and lithosphere (including plume size, plume excess temperature, plume viscosity, and lithosphere viscosity and thickness). We find that the plume-induced surface expressions have strong spatial and temporal variations. Before reaching the base of the lithosphere, the rise of a plume head in the deep mantle causes positive and rapid increase of dynamic topography and geoid anomaly at the surface but no melt production. The subsequent impinging of a plume head at the base of the lithosphere leads to further increase of dynamic topography and geoid anomaly and causes rapid increase of melt production. After reaching maximum values, these plume-induced observables become relatively stable and are more affected by the plume conduit. In addition, whereas the geoid anomaly and dynamic topography decrease from regions above the plume centre to regions above the plume edge, the melt production always concentrates at the centre part of the plume. We also find that the surface expressions have different sensitivities to plume and lithosphere properties. The dynamic topography significantly increases with the plume size, plume excess temperature and plume viscosity. The geoid anomaly also increases with the size and excess temperature of the plume but is less sensitive to plume viscosity. Compared to the influence of plume properties, the dynamic topography and geoid anomaly are less affected by lithosphere viscosity and thickness. The melt production significantly increases with plume size, plume excess temperature and plume viscosity, but decreases with lithosphere viscosity and thickness.

Fluids in Submarine Mid-Ocean Ridge Hydrothermal Settings

Seawater interaction with the oceanic lithosphere crucially impacts on global geochemical cycles, controls ocean chemistry over geologic time, changes the petrophysical properties of the oceanic lithosphere, and regulates the global heat budget. Extensive seawater circulation is expressed near oceanic ridges by the venting of hydrothermal fluids through chimney structures. These vent fluids vary greatly in chemistry, from the metal-rich, acidic fluids that emanate from “black smokers” at temperatures up to 400 °C to the metal-poor, highly alkaline and reducing fluids that issue from the carbonate–brucite chimneys of ultramafic-hosted systems at temperatures below 110 °C. Mid-ocean ridge hydrothermal systems not only generate signifi-cant metal resources but also host unique life forms that may be similar to those of early Earth.

Composition and Seismic Properties of the Oceanic Lithosphere: A Synthesis of Ophiolites and Core Samples of the IODP

AbstractOur knowledge of the oceanic lithosphere largely comes from analogy with ophiolite complexes and the direct scientific drilling of the present‐day oceanic crust (e.g., Christensen and Salisbury, 1975, 1989; Smith and Vine, 1989; Dilek and Furnes, 2011, 2014). In this study, we summarized previous experimental results on seismic properties of oceanic lower crust and upper mantle according to different tectonic settings. The results are used to highlight the compositional heterogeneity and the nature of the oceanic Moho. Observation in different ophiolites reveal an ideal oceanic lithosphere profile with ideal petrologic units and seismic units (Dilek and Furnes, 2011, 2014). The lithospheric mantle beneath ocean basins is composed of tectonized peridotites, which include layered lherzolites and harzburgites and lenses of dunites with chromitites and nearly correspond to the seismic Layer 4. The overlying layered gabbros and mafic sheeted dike complex equal to the seismic Layer 3, as a result of crystallization from a magma chamber. The transitional unit between the former two petrologic units consists of layered ultramafic and mafic rocks, corresponds to the petrological Moho. The seismic Layer 2 and 1 are well defined by pillow lavas and massive flows, and the overlying abyssal sediments, respectively. Compared these results with the refraction seismic profiles, the oceanic crust and upper mantle show different composition and structure. The P‐wave velocities of the Layer 3 gabbros varies from 6.7 to 7.0 km s‐1 and have low velocity gradients of &lt;0.1 km s‐1. Although the gradual increase of P‐ and S‐wave velocities with depth can be attributed to the increasing proportion of mafic minerals from the top to the bottom, prehnite‐pumpellyite facies alteration of basalts, greenschist‐faces metamorphism to epidote‐amphibolite facies metamorphism of gabbros will decrease the velocities of the Layer 2 and Layer 3 (Christensen and Salisbury, 1975, 1989), because the P‐wave velocities of chlorite and hornblende are 6.00 and 7.00 km s‐1, respectively, lower than those of plagioclase and pyroxene, respectively (Carlson, 2004). In addition, local velocity anomalies near the petrologic Moho can be related serpentinization of ultramafic rocks (Salisbury and Christensen, 1978;Carlson et al., 2009). In the Layer 4, the characteristic P‐wave velocities of the upper mantle should fall in the range of 7.8 to 8.2 km s‐1. Poisson's ratios of chrysotile and lizardite, which are stable in oceanic crustal environments according to the phase diagram, is 0.267 and 0.359, respectively, higher than those of olivine and pyroxene (Wang et al., 2013). Serpentinization will significantly decreased velocities and densities of peridotites and is the main reason for the variation of the Moho reflectivity beneath oceans.

Discovery of flat seismic reflections in the mantle beneath the young Juan de Fuca Plate

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. Using a novel wide-angle seismic imaging technique, here we show the presence of two sub-horizontal reflections at ∼11 and ∼14.5 km below the seafloor over the 0.51–2.67 Ma old Juan de Fuca Plate. We find that the observed reflectors originate from 300–600-m-thick layers, with an ∼7–8% drop in P-wave velocity. They could be explained either by the presence of partially molten sills or frozen gabbroic sills. If partially molten, the shallower sill would define the base of a thin lithosphere with the constant thickness (11 km), requiring the presence of a mantle thermal anomaly extending up to 2.67 Ma. In contrast, if these reflections were frozen melt sills, they would imply the presence of thick young oceanic lithosphere (20–25 km), and extremely heterogeneous upper mantle.

What can Olympus Mons tell us about the Martian lithosphere?

Under gravitational loading, a volcanic edifice deforms, and the underlying lithosphere downflexes. This has been observed on Earth, but is equally true on other planets. We use finite element models to simulate this gravity-driven deformation at Olympus Mons on Mars. Eleven model parameters, including the geometry and material properties of the edifice, lithosphere and underlying asthenosphere, are varied to establish which parameters have the greatest effect on deformation. Values for parameters that affect deformation at Olympus Mons, Mars, are constrained by minimising misfit between modelled and observed measurements of edifice height, edifice radius, and flexural moat width. Our inferred value for the Young's modulus of the Martian lithosphere, 17.8 GPa, is significantly lower than values used previously, suggesting that the Martian lithosphere is more porous than generally assumed. The best-fitting values for other parameters: edifice density (2111 – 2389 kg.m–3) and lithosphere thickness (83.3 km) are within ranges proposed hitherto. The best-fitting values of model parameters are interdependent; a decrease in lithosphere Young's modulus must be accompanied by a decrease in edifice density and/or an increase in lithosphere thickness. Our results identify the parameters that should be considered within all models of gravity-driven volcano deformation; highlight the importance of the often-overlooked Young's modulus; and provide further constraints on the properties of the Martian lithosphere, namely its porosity, which have implications for the transport and storage of fluid throughout Mars' history.

Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project

AbstractSerpentinization plays a key role on the evolution of the physicochemical properties of the mantle lithosphere. The rate of serpentinization reactions is controlled by the transport of fluid, which itself depends on volume change during reaction. Element transfer can strongly modify the magnitude and sign of volume change. Here, we measure solid volume change and element transport perpendicular to a serpentine vein in a serpentinized dunite collected at depth during the Oman Drilling Project. The sample is extensively replaced (extent of reaction &gt; 80%) by a serpentine/brucite mixture parallel to a main serpentine vein network. The Mg content of serpentine and brucite indicates reaction with a small amount of fluid at temperatures below 100°C. Concentrations of fluid‐mobile trace elements (Na, Ca, Sr, Rb, and Ba) decrease perpendicular to the main vein. Primary olivine contains parallel platelets of a clinopyroxene/magnetite symplectite. Tomography at the nanoscale reveals that these inclusions do not react during serpentinization but are cracked and displaced. We use these inert markers to measure a 59% to 74% positive volume change that is close to the 52% expected for reaction in a closed system. Chemical data indicate no change in major element composition during reaction except for the addition of water. The initial olivine zoning in Al, Ti, V, Sc, and Cr is still preserved in serpentine and brucite. Serpentinization can thus be a local replacement process during which the solid volume homogeneously increases at the micrometer scale and the transport of aqueous species is limited.

The 2018 Mw 7.5 Papua New Guinea Earthquake: A Possible Complex Multiple Faults Failure Event With Deep‐Seated Reverse Faulting

AbstractAlthough the Papuan fold and thrust belt (PFTB) is thought to accommodate convergence between the Pacific and Australian plates, the possible strain partitioning and its kinematic controls in the region are poorly understood. On 25 February 2018, a devastating Mw 7.5 earthquake jolted the southern area of the PFTB, central Papua New Guinea. The detailed imaging of the source parameters of this large earthquake is an urgent demand for a better understanding of the tectonism in the region. Here we used spaceborne interferometric synthetic aperture radar data to carefully determine the possible ruptured faults and associated fault geometries and further estimated the detailed slip of the 2018 Mw 7.5 earthquake. We find that the earthquake involved multiple faults failure, and a four‐segment fault model can reasonably produce the complex coseismic deformation, especially the observed bimodal displacements. The earthquake was dominated by reverse fault motion, and significant slip of up to ~0.7 m was documented on faults extending at depths great than 25 km. An integrated analysis of the fracture complexities suggests that possible strain partitioning in the PFTB could be tightly coupled with regional fault geometries and lithospheric properties. Our findings on the deep brittle failures suggest that crustal shortening across the PFTB could primarily control the growth of fold and thrust belt in the central Papua New Guinea.

Thermo-rheological structure of the northern margin of the South China Sea: Structural and geodynamic implications

Rheological properties of continental lithosphere are key controls on the behavior of continental deformation. Using thermal structure, constrained by surface heat flow data and measured thermal properties of rocks, the present study calculates different thermo-rheological structure scenarios for the ocean–continent transition (OCT) at the northern margin of the South China Sea, using two different models: a conventional model, taking into account frictional sliding and power-law creep, and a model that additionally includes a high-pressure brittle-fracture mechanism. Two compositions of the lower part of the lithosphere are considered: a soft case with felsic granulite lower crust and wet peridotite lithospheric mantle, and a hard case with mafic granulite lower crust and dry peridotite lithospheric mantle. The former scenario shows a major rheological change from a “jelly sandwich” to a “Christmas tree” type of rheology from north to south along the margin. This complex rheological structure explains lateral changes in earthquake distribution and geometries of extensional faults of the OCT at the northern margin of the South China Sea. Further, our analyses indicate that the initial lithospheric rheology profile probably has only one ductile layer in the lower part of upper crust. Such an initial lithospheric rheology model predicts focused extension to form asymmetric margins, which is the case for the SCS.

Perspectives on Continental Rifting Processes From Spatiotemporal Patterns of Faulting and Magmatism in the Rio Grande Rift, USA

AbstractAnalysis of spatiotemporal patterns of faulting and magmatism in the Rio Grande rift (RGR) in New Mexico and Colorado, USA, yields insights into continental rift processes, extension accommodation mechanisms, and rift evolution models. We combine new apatite (U‐Th‐Sm)/He and zircon (U‐Th)/He thermochronometric data with previously published thermochronometric data to assess the timing of fault initiation, magnitudes of fault exhumation, and growth and linkage patterns of rift faults. Thermal history modeling of these data reveals contemporaneous rift initiation at ca. 25 Ma in both the northern and southern RGR with continued fault initiation, growth, and linkage progressing from ca. 25 to ca. 15 Ma. The central RGR, however, shows no evidence of Cenozoic fault‐related exhumation as observed with thermochronometry and instead reveals extension accommodated through Late Cenozoic magmatic injection. Furthermore, faulting in the northern and southern RGR occurs along an approximately north‐south strike, whereas magmatism in the central RGR occurs along the northeast to southwest trending Jemez lineament. Differences in deformation orientation and rift accommodation along strike appear to be related to crustal and lithospheric properties, suggesting that rift structure and geometry are at least partly controlled by inherited lithospheric‐scale architecture. We propose an evolutionary model for the RGR that involves initiation of fault‐accommodated extension by oblique strain followed by block rotation of the Colorado Plateau, where extension in the RGR is accommodated by faulting (southern and northern RGR) and magmatism (central RGR). This study highlights different processes related to initiation, geometry, extension accommodation, and overall development of continental rifts.

Correlation of Elastic Moduli and Serpentine Content in Ultramafic Rocks

Understanding the physical properties of ultramafic rocks is important for evaluating a wide variety of petrologic models of the oceanic lithosphere, particularly upper mantle and lower crust. Hydration of oceanic peridotites results in increasing serpentine content, which affects lithospheric physical properties and the global bio/geochemical cycles of various elements. In understanding tectonic, magmatic, and metamorphic history of the oceanic crust, interpreting seismic velocities, rock composition, and elastic moduli are of fundamental importance. In this study, we show that as serpentine content increases, density decreases linearly with a slope of 7.85. Porosity of the samples does not show any systematic correlation with serpentine content, as it is more strongly affected by local weathering and erosional processes. We also correlate increase in serpentine content with a linear decline in shear, bulk, and Young’s moduli with slopes of 0.48, 0.77, and 0.45, respectively. Our results show that increase in serpentine content of mantle wedge and forearc mantle contributes to their brittle behavior and result in break-offs, obduction, and overthrusting. Therefore, serpentine content strongly affects tectonic processes at subduction zones, particularly serpentinization may be responsible for formation of weak fault zones. Also, serpentinization of fresh oceanic peridotite in slow and ultra-slow spreading ridges may be responsible for observed discontinuities in thin crust.

New insights into seismic absorption imaging

In recent years, attenuation has been used as a marker for source and dynamic Earth processes due to its higher sensitivity to small variations of lithospheric properties compared to seismic velocity. From seismic hazard analysis to oil and gas exploration and rock physics, many fields need a better reconstruction of energy absorption, a constituent of seismic attenuation generally considered a reliable marker of fluid saturation in space. Here, we propose absorption tomography (AT), a technique grounded on the principles of scattering tomography and Multiple Lapse Time Window Analysis. We benchmark its efficiency to image absorption in space by comparing its results with those obtained using two of the most common mapping strategies, regionalization and kernel-based inversion for a diffusive regime. We applied these methodologies to three datasets, each characterized by a different tectonic setting and quality of the dataset: the Pollino fault area (Southern Italy), Mount St. Helens Volcano (USA) and Vrancea (Romania). AT overcomes the assignment of a single coda quality factor value between each source-receiver pair by modelling and inverting for the spatial distribution of energy as a function of different lapse times. It can then reconstruct node-dependent envelopes using analytic and computational sensitivity kernels and solve for coda attenuation with a grid-search approach. AT allows for a better reconstruction of the localized absorption anomalies than standard methodologies, even if the efficiency of the technique depends on the quality of the dataset, and identifies outliers in the data that could alter the final result and its interpretation. The implementation of analytical diffusive kernels in AT allows for a fast and highly-resolved imaging of well-sampled seismic faults structures. While slightly reducing resolution on faults and being computationally more expensive, implementing multiple-scattering lapse-time-dependent kernels still provides satisfactory results as well as a more physically-correct forward model.

Inherited terrane properties explain enigmatic post-collisional Himalayan-Tibetan evolution

AbstractObservations highlight the complex tectonic, magmatic, and geodynamic phases of the Cenozoic post-collisional evolution of the Himalayan-Tibetan orogen and show that these phases migrate erratically among terranes accreted to Asia prior to the Indian collision. This behavior contrasts sharply with the expected evolution of large, hot orogens formed by collision of lithospheres with laterally uniform properties. Motivated by this problem, we use two-dimensional numerical geodynamical model experiments to show that the enigmatic behavior of the Himalayan-Tibetan orogeny can result from crust-mantle decoupling, transport of crust relative to the mantle lithosphere, and diverse styles of lithospheric mantle delamination, which emerge self-consistently as phases in the evolution of the system. These model styles are explained by contrasting inherited mantle lithosphere properties of the Asian upper-plate accreted terranes. Deformation and lithospheric delamination preferentially localize in terranes with the most dense and weak mantle lithosphere, first in the Qiangtang and then in the Lhasa mantle lithospheres. The model results are shown to be consistent with 11 observed complexities in the evolution of the Himalayan-Tibetan orogen. The broad implication is that all large orogens containing previously accreted terranes are expected to have an idiosyncratic evolution determined by the properties of these terranes, and will be shown to deviate from predictions of uniform lithosphere models.

Edge-driven convection and thinning of craton lithosphere: Two-dimensional thermal-mechanical modeling

There are usually abrupt changes in lithospheric thickness at the boundaries between ancient cratons and adjacent young mobile belts. Lateral variations in temperature and density between the two can trigger small-scale mantle convection (edge-driven convection, EDC). Here, we use two-dimensional thermal-mechanical simulations to explore the EDC caused by the lithospheric step between a craton and a mobile belt, and its role in the thinning of the craton lithosphere. The results show that the impact of EDC on a craton depends on the properties of the craton lithosphere and on their contrasts with the adjacent mobile belt, given the same initial condition. When the craton lithospheric density is relatively large, a high-strength craton has strong ability to resist EDC, and craton lithospheric thinning is limited to the edge. In contrast, the ability of a low-strength craton to resist EDC is weak, and the craton lithosphere is gradually eroded by the downward flow, eventually leading to large-scale thinning of the cratonic lithosphere. When the craton lithospheric density is relatively small, regardless of the strength of the cratonic lithosphere, the craton can well withstand the impacts of EDC. In this case, upwelling flow drives cratonic lithosphere materials to the base of the mobile belt, and lithospheric thinning only occurs at the edge of the craton lithosphere. The Archean North China Craton (NCC) was surrounded by Phanerozoic mobile belts, and its lithospheric thinning first occurred on the northern and eastern margins, a process that lasted for a long time. We suggest that EDC has played an important role in lithospheric thinning of the NCC, in particular the initiation of lithospheric thinning, but we cannot rule out the additional contributions from other mechanisms.

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

AbstractAlthough seismic velocity and electrical conductivity are both sensitive to temperature, thermal lithosphere properties are derived almost exclusively from seismic data because conductivity is often too strongly affected by minor highly conductive phases to be a reliable indicator of temperature. However, in certain circumstances, electrical observations can provide strong constraints on mantle temperatures. In the southeastern United States (SEUS), magnetotelluric (MT) data require high resistivity values (&gt;300 Ωm) to at least 200‐km depth. As dry mantle mineral conduction laws provide an upper bound on temperature for an observed resistivity value, the only interpretation is that lithospheric temperatures (&lt;1330 °C) persist to 200 km. However, seismic tomography shows that velocities in this region are generally slightly slow with respect to references models; this observation has led to a view of relatively thin (&lt;150 km), eroded thermal lithosphere beneath the SEUS. We show that MT and seismic (tomography, attenuation, receiver function) results are consistent with thick (~200 km), coherent thermal lithosphere in this region. Reduced seismic velocities (relative to reference models) can be explained by considering the effect of finite grain size (anelasticity). Calculated velocity as a function of temperature is overall slower when including anelastic effects, even at reasonable grain sizes of 1 mm to 1 cm; this permits mantle temperatures that are colder than would typically be inferred. We argue for a geodynamic scenario in which the present thermal lithosphere in the SEUS formed in association with the Central Atlantic Magmatic Province and has subsequently survived intact for ~200 Ma.