Lithospheric Rocks Research Articles

On the co-evolution of dislocations and grains in deforming rocks

Deformation at tectonic plate boundaries is governed by the mechanical properties of crustal and lithospheric rocks, which evolve through changes in the microstructure of their constituent minerals. Geological observations of microstructure can, therefore, provide estimates of the rock's deformation conditions through various piezometers (e.g., the relationship between stress and grain size or dislocation density). However, grain size and dislocation density do not evolve independently, and thus their associated piezometers are unlikely to be decoupled. We present a new theoretical model coupling the evolution of grain boundaries and dislocations in a deforming rock and make a novel prediction that the equilibrium dislocation density at a given stress (the dislocation piezometer) can be non-unique, with co-existing deformation states or piezometric branches. One stable piezometric branch exists at high stress and large grain-size, for which the steady state dislocation density is large and dictated by stress, in agreement with experimental observations and earlier theoretical models. However, another stable branch exists at low stress and small grain-size, in which case dislocation density is small, weakly stress-dependent, and is governed by the balance between dislocation sinks and sources at grain boundaries. The contrast in dislocation density between grains of different sizes induces grain boundary migration from smaller grains into larger grains. Dislocation-induced grain boundary forces thus act in the opposite direction to those dictated by surface energy, for which grain boundaries migrate from larger grains into smaller ones. When forces due to surface and dislocation energies are balanced, the system reaches a stable microstructural equilibrium, thereby halting grain growth and helping preserve plate boundary weakness. When the forces are not in balance, unstable and oscillatory behavior can ensue, with oscillation periods on the order of decades. Finally, the new theory provides a microphysical model for transient rheological behavior, which is particularly relevant to tectonic activity in which deformation is relatively rapid and where the steady state flow laws are a poor approximation, such as during postseismic relaxation or postglacial rebound.

Nonlinear convective motion of the asthenosphere and the lithosphere melting: a model for the birth of a volcano

The processes of heat transfer occurring between the Earth’s asthenosphere and lithosphere are responsible for partial melting of rocks, leading to the magma generation and its migration and segregation in the crust and, possibly, to volcanoes generation at the surface. Convection is the dominant mechanism regulating the heat transfer from the asthenosphere to the lithosphere, although many aspects of the whole process are not yet clear. Therefore, the knowledge of the physical processes leading to the melting of the lithospheric rocks has important consequences in understanding the interior Earth dynamics, the surface volcanic dynamics, and its related hazards. Rock melting occurs when the temperature gradient meets the rock solidus. Here, we propose a nonlinear convective 1D analytical model (representing an approximation of more 3D complex models). The steady-state solution of our equation is in good agreement with the estimated geotherms of the asthenosphere. A perturbative approach leads to a heat swelling at the boundary between asthenosphere and lithosphere able to determine its melting and the birth of a volcano.

Evolution and demise of passive margins through grain mixing and damage

How subduction-the sinking of cold lithospheric plates into the mantle-is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.

>2.7 Ga metamorphic peridotites from southeast Greenland record the oxygen isotope composition of Archean seawater

The δ18O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ18O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ18O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ18O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ18O of Archean seawater from triple oxygen isotope (δ17O, δ18O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ18O values for peridotites (in olivine, 1.7‰≤δ18O≤4.6‰, relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450°C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ18O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ18O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ18O of seawater had reached its current steady state value by 2.7 Ga ago.

Hydrogen and carbon speciation in nominaly anhydrous minerals of lithospheric mantle rocks: a comprehensive FTIR and STA + QMS study

Hydrogen and carbon speciation in nominaly anhydrous minerals of lithospheric mantle rocks: a comprehensive FTIR and STA + QMS study

The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

AbstractThe physics of rock deformation in the lithosphere governs the formation of tectonic plates, which are characterized by strong, broad plate interiors, separated by weak, localized plate boundaries. The size of mineral grains in particular controls rock strength and grain reduction can lead to shear localization and weakening in the strong ductile portion of the lithosphere. Grain damage theory describes the competition between grain growth and grain size reduction as a result of deformation, and the effect of grain size evolution on the rheology of lithospheric rocks. The self‐weakening feedback predicted by grain damage theory can explain the formation of mylonites, typically found in deep ductile lithospheric shear zones, which are characteristic of localized tectonic plate boundaries. The amplification of damage is most effective when minerallic phases, like olivine and pyroxene, are well mixed on the grain scale. Grain mixing theory predicts two coexisting deformation states of unmixed materials undergoing slow strain rate, and well‐mixed materials with large strain rate; this is in agreement with recent laboratory experiments, and is analogous to Earth's plate‐like state. A new theory for the role of dislocations in grain size evolution resolves the rapid timescale of dynamic recrystallization. In particular, a toy model for the competition between normal grain growth and dynamic recrystallization predicts oscillations in grain size with periods comparable to earthquake cycles and postseismic recovery, thus connecting plate boundary formation processes to the human timescale.

Spontaneous generation of ductile shear zones by thermal softening: Localization criterion, 1D to 3D modelling and application to the lithosphere

The generation of ductile shear zones is essential for the formation of tectonic plate boundaries, such as subduction or strike-slip zones. However, the primary mechanism of ductile strain localization is still contentious. We study here the spontaneous generation of ductile shear zones by thermal softening using thermo-mechanical numerical simulations for linear and power-law viscous flow in one-dimension (1D), 2D and 3D. All models are velocity-driven. The 1D model exhibits bulk simple shear whereas the 2D and 3D models exhibit bulk pure shear. The initial conditions include a small temperature perturbation in otherwise homogeneous material. We use a series of 1D simulations to determine a new analytical formula which predicts the temperature evolution inside the shear zone. This temperature prediction requires knowledge of only the boundary velocity, flow law and thermal parameters, but no a priori information about the shear zone itself, such as thickness, stress and strain rate. The prediction is valid for 1D, 2D and 3D shear zones in bulk pure and simple shear. The results show that shear heating dominates over conductive cooling if the relative temperature increase is >50°C. The temperature variation induced by the shear zone is nearly one order of magnitude wider than the corresponding finite strain variation so that no significant temperature variation occurs between shear zone and wall rock. Applying typical flow laws for lithospheric rocks shows that shear zone generation by thermal softening occurs for typical plate tectonic velocities of few cm.yr−1 or strain rates between 10−16 and 10−14 s−1. Shear stresses larger than 200 MPa can already cause strain localization. The results indicate that thermal softening is a feasible mechanism for spontaneous ductile shear zone generation in the lithosphere and may be one of the primary mechanisms of lithospheric strain localization.

3-D Pn tomography reveals continental subduction at the boundaries of the Adriatic microplate in the absence of a precursor oceanic slab

Slab pull generated by subducting oceanic lithosphere is generally considered as a major trigger for the onset of continental subduction. However, this may be in conflict with the occurrence of UHP terranes bearing no evidence of oceanic lithospheric rocks involved in the exhumation cycle. Here, we image the uppermost mantle P velocity structure beneath the Central Mediterranean, suggesting the possibility that the initiation of continental subduction may not require a precursor oceanic slab. We combine (i) a three-step inverted 3-D Pn tomography model of the Adriatic microplate with (ii) available geologic constraints and palinspastic reconstructions of the Africa–Eurasia plate-boundary zone. Our Pn tomography model reveals elongated regions with Vp < 7.6 km/s around the Adriatic microplate, clearly connected with the slab structure inferred from teleseismic P wave tomography and supportive of continental subduction along the Dinaric, Alpine and Apenninic subduction zones. Contrasting styles of subduction are observed on the opposite sides of the Adriatic microplate: a laterally variable SW-dipping subduction is documented beneath the Apennines, continental to the north and oceanic to the south, where rollback is faster; a laterally continuous NE-dipping continental subduction is documented under the Dinarides. The lack of a precursor oceanic slab under the Dinarides demonstrates that the onset of continental subduction, in complex plate-boundary zones, can be controlled by plate-tectonic processes far away from the subduction initiation site, and may take place without the contribution of the negative buoyancy of an old oceanic lithosphere.

Serpentines as the indicators of mesozoic peridotites metamorphic and geodynamic transformations in the Internal Ukrainian Carpathians

Formulation of the problem. Peridotites of ophiolite complexes,being the fragments of the oceanic upper mantle that have undergone several stages of partial melting, brought to the surface by tectonic movements, also have undergone metamorphic transformations almost immediately after its formation. Because of serpentinization, the mineral composition of the rocks became more complicated. The analysis of the final structure and composition of apoperidotites allows obtaining data for geodynamic reconstructions regarding the stage of their formation. Review of previous publications. It has been determined that serpentines are the most common secondary minerals of peridotites of the Uholskyi complex in the Ukrainian Carpathians, and the processes of serpentinization took place at a depth of 40–50 to 100 km (?) at T = 450–600 °C and P = 13–16 kbar (Stupka O., 2013). The study of serpentinites of the Main Ural Fault (Panas'yan L. et al., 2014) revealed that high alumina and high chromium serpentinites have ultrabasic protoliths formed in the mantle, and medium alumina and low chromium varieties – the protoliths of the basic composition which were born in the conditions of the crust. Based on the study of serpentinites in the orogenic Qinling belt (China), researchers (Wu K. et al., 2018) determined their mantle origin: magnetite-enriched antigorite serpentinites were formed as a result of the interaction of serpentinized apoperidotites of mantle protoliths with molten rock in the subduction channel. Purpose. We have investigated secondary serpentines in order to reconstruct the geodynamic conditions of the formation and transformations of the peridotites (Uholskyi complex), localized in the Marmarosh rocky zone in the Internal Ukrainian Carpathians, and are most widely spread in the interfluve of Velyka and Mala Uholka-rivers. Methods. The work is based on the results of geological observations of the Uholskyi complex rocks in natural outcrops, as well as petrographic, mineralogical (including X-ray diffraction, thermal and microprobe analysis), and geochemical studies. Results. The paper presents the study results of serpentinized apoperidotites of the ophiolite Uholskyi complex in the Internal Ukrainian Carpathians. Serpentinized apoperidotites (T2–K1?) form olistoliths in the Soimulska olistostrome-conglomerate strata of the Lower Cretaceous age. The investigated serpentines are the rock-forming minerals of lizardite and antigorite serpentinites. Lizardite serpentinites are characterized by lenticular-looped textures formed by α-lizardite and non-altered chrome-spinellids. Antigorite serpentinites, recognized by striped-shale textures, contain antigorite, β-lizardite and magnetite. Lizardite serpentinites are characteristic of the regressive metamorphism of the greenschist facies upper part, and antigorite serpentinites are a typical formation of the progressive metamorphism of the lower greenschist – upperlower epidote-amphibolite facies. Regressive metamorphism occurred under geodynamic conditions of spreading and the progressive ones – under subduction conditions between the terrains of Dacia and Tisza, which led to the closure of the Transylvanian-Mureş Paleocean. It has been concluded, that the protoliths of lizardite serpentinites were the primary mantle rocks of the ultrabasic (restitic) composition, and the protoliths of the antigorite serpentinites were the lithospheric rocks of the basic composition. Conclusions. The complex study of serpentinized apoperidotites of the Uholskyi complex makes it possible to reconstruct the metamorphic transformations of the primary mantle protoliths and to determine the stages of lithosphere formation within the fold-nappe structures. The obtained results can be used for prediction of serpentinite mineralization.

Relation between mean stress, thermodynamic, and lithostatic pressure

AbstractPressure is one of the most important parameters to be quantified in geological problems. However, in metamorphic systems the pressure is usually calculated with two different approaches. One pressure calculation is based on petrological phase equilibria and this pressure is often termed thermodynamic pressure. The other calculation is based on continuum mechanics, which provides a mean stress that is commonly used to estimate the thermodynamic pressure. Both thermodynamic pressure calculations can be justified by the accuracy and applicability of the results. Here, we consider systems with low‐differential stress (&lt;1 kbar) and no irreversible volumetric deformation, and refer to them as conventional systems. We investigate the relationship between mean stress and thermodynamic pressure. We discuss the meaning of thermodynamic pressure and its calculation for irreversible processes such as viscous deformation and heat conduction, which exhibit entropy production. Moreover, it is demonstrated that the mean stress for incompressible viscous deformation is essentially equal to the mean stress for the corresponding viscous deformation with elastic compressibility, if the characteristic time of deformation is five times longer than the Maxwell viscoelastic relaxation time that is equal to the ratio of shear viscosity to bulk modulus. For typical lithospheric rocks, this Maxwell time is smaller thanc. 10,000 years. Therefore, numerical simulations of long‐term (&gt;10 kyr) geodynamic processes, employing incompressible deformation, provide mean stress values that are close to the mean‐stress value associated with elastic compressibility. Finally, we show that for conventional systems the mean stress is essentially equal to the thermodynamic pressure. However, mean stress and, hence, thermodynamic pressure can be significantly different from the lithostatic pressure.

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhon area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial 187Os/188Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.

The behaviour of zirconium and hafnium during water-rock interaction

Zr and Hf are two elements with same ionic charge and similar ionic size at a given coordination number. Despite the Zr/Hf ratio is quite constant in meteorites and lithospheric rocks, in natural waters can be either higher or lower compared to values of interacting minerals and rocks. Here, we reanalyze very recent published and present new data on continental and brine waters indicating that the Zr and Hf behavior is dependent on the properties of the authigenic phases formed during the water-rock interaction process. Our results show that water pH in the range between 1 and 9 and water ionic strength in the range between 0.001 and 4 mol kg−1 are responsible for the change of the Zr/Hf ratio. However, analyzing the colloidal fraction comprising Fe-oxyhydroxides, carbonate and clay minerals (occasionally gypsum and halite), obtained by ultrafiltration at the dimension between 10 kDa and 0.45 μm diameter pore, we found that the Zr/Hf molar ratio is very low and variable between 5 and 35. We propose that the dissolved Zr/Hf ratio is dependent on the morphological and electrical properties of authigenic mineral surfaces and emphasize the lack of a quantitative evaluation of the ion speciation of Zr and Hf in natural waters and brines limiting the transport prediction of Zr and Hf during the complex water-rock interaction process.

Insights into the mantle geochemistry of scandium from a meta-analysis of garnet data

The meta-analysis of about 13,000 analyses of scandium content in garnet grains shows that, below the spinel–garnet transition, this phase carries about three-quarters of the Sc budget of the mantle, indicating its control on Sc mobility. The Sc content of garnets in mafic rocks is low, due to a dilution effect resulting from their high modal content in garnet. Garnets from ultramafic rocks exhibit a wider range of Sc concentrations. We assess the relative influence of thermobarometry, crystal chemistry and fluid-related events on the distribution of Sc in garnet from such rocks to improve the tracking of geochemical processes in the mantle. Pressure and temperature of equilibration in the mantle are second-order factors influencing the Sc content of garnet, while crystal chemistry, in particular CrCr+Al and CaCa+Mg, is the main parameter controlling the compatibility of Sc. Scandium is incorporated in both X and Y sites of Cr-Ca-rich garnets, resulting in a behaviour intermediate between rare-earth elements, incorporated in the X site, and trivalent transition elements, occupying the Y site. This affinity for both sites results in a mild compatibility of Sc in the garnet stability field of the mantle; hence Sc concentration in garnet increases with melt extraction and can be reduced by silicate-melt metasomatism. In contrast, metasomatism by volatile-rich fluids increases the Sc concentration in garnet. The control of garnet on the compatibility of Sc in deep lithospheric rocks demonstrates the potential of using Sc to track the conditions of formation of magmas and their residual rocks, as well as the origin and nature of metasomatic fluids.

Thermodynamic modelling of metamorphic processes: state of the art in pseudosection approach

Understanding global-scale orogenic processes related to supercontinents, and their relationship to the secular evolution of the Earth’s lithosphere, represent important challenges for Earth scientists today. The record of these processes is preserved in the microstructures, mineral assemblages and mineral compositions of lithospheric rocks exhumed to the Earth’s surface. Given a well-characterized microstructural evolution, thermodynamic modelling is the key to quantifying changes in pressure and temperature, with the rate of these changes being provided by rock-forming and accessory mineral-based geochronology. Thus, metamorphic rocks provide Pressure–Temperature–time–deformation (P–T–t–d) data that help to parameterize orogenic processes.

Thermal subsidence and sedimentary processes in the South China Sea Basin

We coupled analysis of regional reflection seismic data with core data from International Ocean Discovery Program (IODP) Expedition 349 and found differences in both depositional features and calculated subsidence between the East and Southwest Subbasins. We calculated the original (decompacted) thicknesses of sediment for different time slices. The depositional features of the basin show that the sedimentation is controlled by both climate and tectonics. Decoupling of sedimentation rates and fluxes is often observed between the continental margin and deep basin. An abrupt increase in sedimentation rates since the Pliocene suggests that glacial–interglacial climate variability impacted erosion rates. The sedimentation rate peaked during the Pliocene in the Southwest Subbasin, but peaked during the Quaternary in the East Subbasin, showing strong influence of surrounding tectonic events, such as the buildup of Taiwan Orogen, on erosion rates. We examine thermal subsidence of the two subbasins by fitting the isostatically balanced basement depths from the seismic profiles with the half space cooling model. The estimated thermal diffusivity of the lithosphere in the Southwest Subbasin is much higher than in the East Subbasin. These contrasting observations suggest that lithospheric rock compositions may contribute to different tectonic subsidence curves between the East and the Southwest Subbasins.

The role of hydrothermal cooling of the oceanic lithosphere for ocean floor bathymetry and heat flow

AbstractWe investigate the influence of hydrothermal circulation on cooling of oceanic lithosphere. We include hydrothermal convection in a 1‐D lithosphere cooling model by using a scaling law which relates the Rayleigh to the Nusselt number. This law allows calculating an effective thermal conductivity accounting for the extra heat transport. Based on a global data set for bathymetry and surface heat flow for ocean floor ages between 0.7 and 160 Ma, we perform a joint inversion based on a downhill simplex algorithm to constrain characteristic parameters for hydrothermal cooling (beside the classical parameter mantle temperature, thickness of the lithosphere, and thermal expansivity of lithosphere rocks). Hydrothermal cooling parameters are crack aspect ratio which controls the hydrothermal penetrations depth, characteristic cementation time at which cracks become closed and hydrothermal circulation ceases, and sealing time when enough sedimentary cover on top of the lithosphere has accumulated to prevent hydrothermal fluid escaping into the ocean. Best fitting parameter sets predict mantle temperatures between 1350 and 1450°C and lithosphere thickness between 70 and 90 km and further suggest that (1) the fit to the data is strongly improved if hydrothermal cooling effects are considered, (2) hydrothermal cooling is important for up to 10 Ma old lithosphere and leads to a significant deviation from the 1/square root—law with an exponent closer to 1/3, (3) sedimentary sealing is completed for ~1.5 Ma old lithosphere, and (4) fitting of the data is improved for an apparently 1.4 times higher value of thermal expansivity which may account for effects of melt solidification.

Zr- Hf Fractionation During Water-Rock Interaction

Zr and Hf are two elements having same ionic charge and similar ionic size at a given coordination number. Despite the Zr/Hf ratio is considered to be quite constant in meteorites and lithospheric rocks, seawaters collected from the surface down to varying depths of several Pacific Ocean stations reveal that the Zr/Hf ratio increases by one order of magnitude. Very recent studies have shown that, in both ground waters and lake waters, the Zr/Hf ratio is either higher or lower compared to the interacting minerals displaying a large variability in the distribution of these twin elements. In this communication the possible processes responsible for such a large fractionation are discussed but further work is needed to test the validity of these interpretations. This basic problem of scientific significance needs more attention from the water-rock interaction community.

To the development of the mechanism of interaction of galactic comets with the terrestrial planets

We formulate an approach to the study of the physics of interaction of galactic comets with the terrestrial planets and the Moon, based on idealized hydrodynamic models of “elastic” and “inelastic” impact proposed by M A Lavrentiev. According to these models, a large crater arises on the solid surface of planet in case of “elastic” collision, whereas the narrowly focused shock wave is formed in result of “inelastic” impact. This shock wave penetrates deep into the lithosphere rocks causing their heating. Using the factual data for Earth, Mars and Moon, we came to the conclusion that the interaction of galactic comets with planets goes on with the participation of both the physical processes at once. At that, with a decrease of power of gaseous envelope of planet, the action of “inelastic” collision mechanism is reduced, “elastic” —amplified.

Geological structures created by falls of galactic comets

With the use of the author's theoretical model are discussed geological structures which can be created by fallings of galactic comets on terrestrial planets: Mercury, Mars, Earth, Venus and the Moon. The model predicts that depending on combination of a number of conditions galactic comets may form on these planets following types of structures: craters, diatremes, lava sheets, volcanoes, dome-shaped surface uplift, as well as coronae and montes (on Venus). The main factors that influence on origin of these structures on planets are (i) density of gas shell, (ii) thickness of planetary lithosphere, (iii) composition and degree heating of lithosphere rocks, (iv) frequency of fallings galactic comets. We are discussing specific features of these structures on the Moon, Mars, Earth and Venus.

Geophysical investigation and reconstruction of lithospheric structure and its control on geology, structure, and mineralization in the Cordillera of northern Canada and eastern Alaska

A reconstruction of the Tintina fault is applied to regional geophysical and topographic data, facilitating the definition of west trending lineaments within the lower crust and/or mantle lithosphere, oblique to the NW trending structure of the Cordilleran terranes. The lineaments, which exhibit a range of geophysical and geological signatures, are interpreted to be related to the Liard transfer zone, continuous to the Denali fault, that divided lower and upper plates during late Proterozoic-Cambrian rifting of the Laurentian margin. Three-dimensional gravity models show a density increase in the lower crust and mantle lithosphere to the north. The transfer zone also divides bimodal mantle xenolith suites to the south from unimodal suites to the north. These conclusions suggest that extended North American basement, related to Laurentian margin rifting that would have brought mantle lithosphere rocks to a shallow depth, continuously underlies a thin carapace of accreted terranes in western Yukon and eastern Alaska. The interpreted continuity of North American basement reaffirms that if oroclinal bending of the Intermontane terranes occurred, then it was prior to its emplacement upon the rifted basement. Examination of the spatial relationships between mineral occurrences and postaccretionary, Cretaceous lithospheric lineaments, from their manifestation in geophysical, geological, and topographic data, suggest that the late Proterozoic lineaments influenced Mesozoic mineralization through influence on the development of the shallow crustal structure, intrusion, and exhumation and erosion.