Boundaries Of Lithospheric Plates Research Articles

Effect of Grain-Size and Textural Weakening in Polyphase Crustal and Mantle Lithospheric Shear Zones

Strain localization to form narrow plate boundary shear zones in Earth’s lithosphere requires a significant amount of strain weakening. Here we investigate the relative contributions of grain-size-sensitive weakening versus textural weakening in polyphase shear zones in granitoid and peridotitic rocks through two-dimensional centimetre-scale bulk simple shear numerical models. The models deploy both constant grain size (only textural weakening) and dynamic grain-size evolution based on the paleowattmeter. Our results reveal that for granitoid rocks, textural weakening dominates, especially at temperatures around 550 °C, while grain-size-sensitive weakening plays a secondary yet significant role. For peridotitic rocks, intense weakening is evident below temperatures of ~1000 °C due to grain-size reduction, while textural weakening has a minor effect on weakening for experiments above 1000 °C. Two-dimensional experiments are compared to one-dimensional, single-phase models to reveal the effect of geometrical complexities in stress and grain-size evolution. These results are discussed in the context of natural lithospheric shear zones and are compared with established piezometers for individual mineral phases. Our findings underscore the vital role of grain-size-sensitive rheologies, particularly in the mantle lithosphere, for the initial weakening of ductile shear zones. These insights offer quantitative constraints that advance our understanding of the long-term strength of lithospheric plate boundaries.

Earthquakes in Turkey and Syria in 2023 and geodynamics of the Caucasus-Anatolian region

Background. The intensification of natural disasters all over the world requires the development of new approaches to the study of geological processes. This particularly concerns the areas at the boundaries of lithospheric plates, which are characterized by earthquakes, increased seismicity, volcanism, intensive heat flows, geothermal manifestations, landslide processes, tsunamis, and other dangerous natural processes and hazards. The Caucasus-Anatolian-Arabian region is a complex highly-stressed geodynamic structure, characterized by an increased heat flow, seismicity, magmatism, and volcanism. The geodynamics of this region is determined by the collision of the Eurasian and Arabian lithosphere plates, as well as by the evolution of the Alpine-Himalayan belt and surrounding areas. A 7.8 magnitude earthquake hit the South-East Turkey and North-West Syria on February 6, 2023. This was the largest earthquake in Turkey since the 1939 Erzincan earthquake, and the second-strongest since the 1668 North Anatolia earthquake. More than 52,800 deaths were confirmed, with about 46,100 in Turkey and 6,700 in Syria. It is the deadliest natural disaster in Turkey’s modern history with the estimated damage of over $100 billion. Aim. To construct geodynamic models for the deep structure of natural hazard regions, which can contribute to the study of active continental margins. This information is necessary for earthquake forecasting and prognosis, as well as for assessing geoecological risks and preparing the population in the event of natural disasters and catastrophes. Materials and methods. The formation and evolution processes of geological structures in complex geodynamic settings, as well as the forecasting and prognosis of natural hazards, required an analysis of all available geological and geophysical data. The methods of mechanical and mathematical modeling were used to formulate and solve the research problems. Results. Geodynamic models of the regions of hazardous natural processes were constructed with the purpose of forecasting and preventing natural disasters and catastrophes. An algorithm for creating monitoring systems was proposed. Conclusion. In several years (1–10 years), another earthquake near Istanbul can be expected. In the case that no large earthquake occurs in the East Anatolian fault in 1–2 years, this event is likely to occur in 100 years only.

Tectonic and geological setting influence hot spring microbiology.

Hydrothermal systems form at divergent and convergent boundaries of lithospheric plates and within plates due to weakened crust and mantle plumes, playing host to diverse microbial ecosystems. Little is known of how differences in tectonic setting influence the geochemical and microbial compositions of these hydrothermal ecosystems. Here, coordinated geochemical and microbial community analyses were conducted on 87 high-temperature (>65°C) water and sediment samples from hot springs in Yellowstone National Park, Wyoming, USA (n = 41; mantle plume setting), Iceland (n = 41, divergent boundary), and Japan (n = 5; convergent boundary). Region-specific variation in geochemistry and sediment-associated 16S rRNA gene amplicon sequence variant (ASV) composition was observed, with 16S rRNA gene assemblages being nearly completely distinguished by region and pH being the most explanatory parameter within regions. Several low abundance ASVs exhibited cosmopolitan distributions across regions, while most high-abundance ASVs were only identified in specific regions. The presence of some cosmopolitan ASVs across regions argues against dispersal limitation primarily shaping the distribution of taxa among regions. Rather, the results point to local tectonic and geologic characteristics shaping the geochemistry of continental hydrothermal systems that then select for distinct microbial assemblages. These results provide new insights into the co-evolution of hydrothermal systems and their microbial communities.

Geodynamics of the Caucasus – Anatolian-Arabian region and Turkey-Syria Earthquakes 2023

The activation of natural disasters in the world requires the development of new approaches to the study of geological processes, in particular, at the boundaries of lithospheric plates, characterized by earthquakes, increased seismicity, volcanism, landslide processes, tsunamis and other dangerous natural processes and hazards. Earthquake of M 7.8 struck south -eastern Turkey and north - western Syria on 6 February 2023. The M 7.8 earthquake is the largest in Turkey since the 1939 Erzincan earthquake, and the second-strongest recorded in the country, after the 1668 North Anatolia earthquake. More than 52,800 deaths were confirmed: more than 46,100 in Turkey, and more than 6,700 in Syria. It is the deadliest natural disaster in Turkey's modern history. The earthquakes caused over US$100 billion in damages. The geodynamic models construction for the deep structure of natural hazards regions is an important contribution to the study of active continental margins, which is necessary for the earthquake forecast, prediction and prognosis, assessing geoecological risks and preparing population actions in the event of natural disasters and catastrophes. The Caucasus - Arabian region is a complex highly-stressed geodynamic structure, characterized by increased heat flow, high seismicity, magmatism and volcanism. The geodynamics of the Caucasus - Arabian region is determined by the collision of the Eurasian and Arabian lithosphere plates, as well as the complex history of the development of the Alpine-Himalayan belt and surrounding territories. The problem solution of geological structures formation and evolution in various complex geodynamic settings and natural hazards forecast and prognosis requires an analysis of all available geological - geophysical data, as well as the formulation and solution of problems of mechanical and mathematical modeling. Slow lithospheric deformations are simulated by models of viscous flow in multi-layered, incompressible, high-viscosity Newtonian fluid, using Navier-Stocks equation and discontinuity equation. The solution of the inverse problem of geodynamics by the direct method is developed. The first inverse problem of geodynamics was solved - the restoration of the velocity fields, pressures and stresses at the depth of the lithosphere according to the available data on the velocities on the surface. The second inverse problem of geodynamics has been posed and solved - the determination of the movement of boundaries at the depth of the lithosphere based on the given movements of the surface. The solutions obtained can be used to analyze deep geodynamic problems, and together with geothermal modeling, geological and geophysical methods and seismic tomography can serve as a reliable apparatus for studying deep geodynamics due to the formation and evolution of geological structures and the lithosphere stress-strain state researches. The solution of the problem is analyzed on the example of the Caucasus - Arabian region geodynamics. The Geodynamic concept of geoenvironment has been developed. Geodynamic models of the regions of hazardous natural processes in order to predict and prevent natural disasters and catastrophes are constructed. An algorithm for creating monitoring systems is suggested.

STATISTICAL ANALYSIS OF GEOLOGICAL AND GEOPHYSICAL PARAMETERS OF THE MONGOLIAN-BAIKAL REGION

The origins and formation mechanisms of neotectonic structures in a part of the Mongolian-Siberian region were identified by geodynamic zoning based on multidimensional statistical analysis of numerical arrays describing geological-geophysical and geological-geomorphological processes. These processes in the regional lithosphere were described by a set of 11 geological and geophysical parameters, divided into three main groups using the hierarchical method of cluster analysis. The first group includes the seismic moment, the density of active faults, the rates of current horizontal deformations, and the magnitude of the deep heat flow. The second group involves the thicknesses of the earth’s crust and exogenous active layer, the velocities of current horizontal movements, and the amplitudes of vertical neotectonic movements. The third group includes gravity anomalies and the lithosphere thickness. The spatial grouping of parameters by cluster analysis (K-means method) yields seven clusters, whose spatial position and composition are determined by the geological history, geological structure, geodynamic evolution of the region, and the current deformation rates. Some clusters characterize large rigid lithospheric blocks, while other clusters describe large active fault systems in the given area. The search for latent factors that make the greatest contribution to the dispersion of the geological and geophysical parameter values was carried out using the principal component method, which makes it possible to minimize the number of factors. Four main factors were identified for areas that differ in the morphology and origin of neotectonic structures: (i) higher horizontal compressive and tensile strains, (ii) dynamic effect of mantle anomalies, manifested in uplifts and arching, (iii) activation of thinned lithosphere within the boundaries of lithospheric plates and large blocks, and (iv) active shear deformation of the earth’s crust. The results of clustering and factor analysis of numerical arrays describing the geological-geophysical and geological-geomorphological processes within the Mongolian-Siberian region can be interpreted in the framework of physical mesomechanics.

Корреляция геологических комплексов горного массива Хан-Тенгри в пограничных районах Кыргызского, Казахского и Китайского Тянь-Шаня

Researchers of the border regions in Kyrgyzstan, Kazachstan, and China discuss two two alternative models for the relationships between the main structural units of the Tianshan mountains. Most researchers believe that the Kyrgyz Middle Tianshan wedges out to the east along the Atbashi-Inylchek-Nalati marginal fault. According to another hypothesis, the Middle Tianshan structures continue within the range Nalati where they are described as Chinese Central Tianshan. Comparing the characteristics of the Paleozoic and Proterozoic sedimentary, volcanogenic, intrusive, and metamorphic formations of these regions leads us to the conclusion that the structural units of the Kyrgyz Middle and most of the Northern Tianshan, including the superimposed Middle-Late Paleozoic troughs, are not continued into China, but are successively cut along the echelon system of conjugated strike-slip faults, united by us into the Frontal Tianshan Dextral Strike-slip (FTDS). And only the northern segment of the Issyk-Kul terrane can be considered as an analogue of the Chinese Central Tianshan, displaced along the FTDS to the northwest for a distance over 80 km. Therefore, adjacent geological complexes are eroded along the FTDS similar to the oblique boundaries of convergent lithospheric plates affected by tectonic erosion.

ТЕПЛОВОЙ ПОТОК И РЕОЛОГИЧЕСКОЕ РАЗУПЛОТНЕНИЕ КОРЫ И ВЕРХНЕЙ МАНТИИ В РИФТОГЕННЫХ И ПЛЮМОВЫХ СТРУКТУРАХ НА ВОСТОЧНОЙ ОКРАИНЕ АЗИИ

Results of the correlation analysis of heat f low anomalies (Q) with distributions of density contrast ( -parameter) in rifting and plume structures on eastern margin of Asia are considered, which prove selective inverse correlation of these parameters in depth intervals corresponding to the position of the subcrustal viscous layer and asthenosphere. Rifting structures are characterized by ruptures in rigid layers displayed by maxima of the  -параметра, while plume-type central structures are characterized by concentrically zonal distribution of density contrast. The main contribution to the thermal anomalies in rifts is made by the subcrustal viscous layer. Thermal flow from the asthenosphere at the Ceinozoic boundaries of lithospheric plates is screened by subducting slabs. The identical correlation of Q and  -parameter in distant regions testifies to the same deep structure and universality of tectonic regimes on the eastern margin of Asia, where subduction, rifting and plume processes are coupled in space and time.

Diamond in the Oceanic Lithosphere. Volcanic Diamonds and Diamonds in Ophiolites

Diamonds were lately identified in chromitites from ophiolites and in volcanic rocks. Although the tectonic settings of diamonds found in these rocks are different, the diamonds are identical in small size, cuboctahedral habit, sets of minor admixture elements, and isotopic characteristics. A model is suggested for their formation during various stages of a single evolutionary cycle of the oceanic lithosphere, in relation to the geochemical and dynamic features of an ascending flow of mantle material, which produces the oceanic lithosphere at mid-oceanic ridges. In contrast to the continental lithosphere, in which mantle diamonds are usually related to kimberlite and lamproite magmatism in the presence of abundant CO2-rich fluid, diamonds in the oceanic lithosphere crystallize in environments poor in fluid and containing carbon mostly in its reduced forms. In the asthenospheric part of the ascending flow, carbon may occur in the form of nanometer-sized diamonds. In the upper parts of the oceanic lithosphere, the diamonds are overgrown and become microdiamonds (0.2–0.7 mm) within chromitites. After basaltic magma is derived from pyrolite, the residual harzburgites with lenses of diamondiferous chromitites are brought (at spreading) to the convergent boundaries of oceanic lithospheric plates, where the following two processes can proceed. If the oceanic lithosphere collides with a continental plate, the obducted material of the oceanic lithosphere is transferred to the surface of the continental margin and forms ophiolite massifs hosting diamond-bearing chromitites. If the oceanic lithosphere is subducted, the residual peridotite already enriched in volatiles is remelted. The arc magmas thus derived host diamond microcrystals, which have been formed in the chromitites and are sometimes found in volcanic lavas and ashes.

ГЕОДИНАМИКА И МАГМАТИЗМ ТРАНСФОРМНЫХ ОКРАИН ТИХООКЕАНСКОГО ТИПА: ОСНОВНЫЕ ТЕОРЕТИЧЕСКИЕ АСПЕКТЫ И ДИСКРИМИНАНТНЫЕ ДИАГРАММЫ

Transform margins represent lithospheric plate boundaries with horizontal sliding of oceanic plate, which in time and space replaced the subduction related convergent margins. This happened due to: spreading ridge–trench intersection (California; Queen Charlotte–Northern Cordilleran, West of the Antarctic Peninsula, and probably the Late Miocene–Pleistocene Southernmost South America) or ridge death along continental margin (Baja California); change in the direction of oceanic plate movement (Western Aleutian–Komandorsk; Southernmost tip of the Andes); and island arc-continent collision (New Guinea Island). Post-subduction magmatism is related to a slab window that resulted either from the spreading ridge collision (subduction) with a continental margin or slab tear formation, or slab break-off after subduction cessation due to other reasons. Igneous magmatic series formed in consequence of these events show diversity of tholeiitic (sub-alkaline), alkaline or calc-alkaline, high-alumina and adakitic rocks. The comprehensive geochemical dataset (more than 2400 analyses) on igneous rocks of the model transform and convergent geodynamic settings allowed to substantiate the most informative triple diagrams for the petrogenic oxides TiO2 × 10 – Fe2O3Tot – MgO and trace elements Nb – La– Yb. Mostly approved for the rock compositions with SiO2 < 63 wt. %, the new plots are capable of distinguishing igneous rocks formed above zones of subduction at an island arc and continental margin (related to convergent margins), from those formed in the tectonic setting of transform margins along continents or island arcs.

Pattern of Global Crustal Stresses of the Earth

The results of tectonophysical inversion of the global tectonic stress field are considered from the position of explaining the active forces that cause the lithospheric plate movements. The analysis of regularities of the stress state of the main structural elements of Earth’s lithosphere—ocean spreading, transformations, active continental margins, and intracontinental orogeny—is performed. It is shown that the stresses in most of the ocean lithosphere, including subduction zones, correspond to active traction forces from the submerged and weighted part of the lithosphere. A low level of deviatoric stresses in subduction zones and the presence of dynamo pair with sharply different mode of stress state were found. Horizontal compression is observed in the crust of the continental slope, and horizontal tension is observed in the ocean lithosphere, so the boundaries of lithospheric plates should not be considered as a source of increased horizontal compressive stress in the crust of continents. The formation of a horizontal compression regime in the crust of intracontinental orogens, as well as in the crust of active continental margins orogens, is associated with residual stresses of rocks that have been exhumed from the depths of the crust. A generalizing scheme for the formation of the global crustal stress field of the Earth is proposed, which corresponds to modern data on the movements of lithospheric plates and the deep structure of the upper mantle.

Якутия

. The paper presents the results of monitoring seismicity in Yakutia in 2013 based on the data from 24 digital seismic stations. A total of 5197 seismic events with Кp=6–15 are detected. A map of earthquake epicenters is compiled showing their distribution in different areas. Two major clusters of seismic events are recognized. The first one corresponds to the Arctic-Asian seismic belt (AASB) extending across the Laptev Sea shelf and the north eastern Asian continent towards the Sea of Okhotsk. The second Baikal–Stanovoy belt in the south of the region is traced from Lake Baikal through the Stanovoy Highlands and the Stanovoy Ridge to the Sea of Okhotsk. In South Yakutia, the Olekma–Stanovoy zone (OSZ), which makes the eastern part of the belt, is considered. Both belts form boundaries of major lithospheric plates. The first belt separates the Eurasian and North American plates, while the other divides the Eurasian plate from the Amurian one. The minimum seismic level is recorded within the OCZ, with no more than 0.04 % of the total seismic energy re-leased during 2013. The maximum seismicity is noted in the AASB with released energy making up 99.9 % of the total amount. That abrupt change in seismicity is due to the occurrence here of three strong earth-quakes, Ulakhan-Chistay, Ilin-Tas (Abyi), and Tas-Khayakhtakh with the intensities VII-IX in the epicenters (MS=4.9–6.9). The epicentral zone of the Tas-Khayakhtakh event (MS=4.9) that occurred on May, 2013 is considered in more detail. The motion in the source of this event indicate thrusting along the Dogdo fault. All the shakes took place within the Chersky Range, in the zone of influence of major faults of the Kolyma–Indigirka system (Ulakhan, Dogdo, Ilin–Tas) under conditions of NE compression (regime of interplate collision and transpression ). Seismotectonic relations, macroseismic data, and earthquake focal mechanism pa-rameters are discussed.

Dynamics of the Zones of Strong Earthquake Epicenters in the Arctic–Asian Seismic Belt

Our comprehensive study of the Russian Arctic region aims to clarify the features and types of seismotectonic deformation of the crust in the Arctic–Asian Seismic Belt, specifically in the zones of strong earthquakes in the Laptev Sea Segment, the Kharaulakh Segment, and the Chersky Seismotectonic Zone. We have analyzed modern tectonic structures and active fault systems, as well as tectonic stress fields reconstructed by tectonophysical analysis of the Late Cenozoic faults and folds. The investigated neotectonic structures are ranked with respect to the regional classification principles. Changes in the crustal stress–strain state in the lithospheric plate boundaries between the Eurasian, North American, and Okhotsk Sea Plates are analyzed, and regularities of such changes are discovered. A set of models has been constructed for the studied segments of plate boundaries with account of the dynamics of the regional geological structures. The models can give a framework for the assessment of potential seismic risks of seismogenerating structures in the Russian Arctic region.

Early Jurassic Magmatism of the Kobyumé Graben System (Verkhoyansk Framing of the Siberian Craton) and Its Geodynamic Nature

Data on the Early Jurassic volcanic rocks of the Kobyume graben system, which formed at the Verkhoyansk passive margin of the Siberian Craton, are presented. The volcanic complex contains products of basaltic eruptions that occurred under conditions of a shallow sea. The basalts have elevated TiO2 contents (>1.8 wt%), and variations of their composition were controlled by fractionation processes. By their geochemical characteristics, the volcanic rocks are transitional between ocean island basalts and enriched basalts of mid-oceanic ridges. Taking into account the remote location of the Kobyume graben region from the convergent boundaries of lithospheric plates and from the nearest areas of contemporaneous magmatism, the data obtained support the conclusion that the Kobyume tectono-magmatic areal is a result of a small mantle plume effect onto the passive margin of the Siberian continent.

A postglacial relative sea-level database for the Russian Arctic coast

We present the first quality-controlled relative sea-level (RSL) database for the Russian Arctic coast from the Barents Sea in the west to Laptev Sea in the east (29–152oE and 63 to 81oN). The database consists of 385 sea-level index points and 249 limiting dates and spans 24 ka to present. Sea-level indicators are derived from multiple proxies, including isolation basins, raised beaches, glacial erratics, marine terraces, laidas (salt marshes), and deltaic salt marshes. Here, we calculate the indicative meanings for all indicators and evaluated possible elevation errors. We have estimated the ages and uncertainties of index points and limiting dates using the most recent calibration datasets.In the western Russian Arctic (Barents and White Seas), RSL was driven by glacial isostatic adjustment (GIA) due to deglaciation of the Eurasian ice sheet complex. For example, within the Baltic crystalline shield, RSL fell rapidly from 80 to 100 m at 11–12 ka to 15–25 m at ∼4–5 ka. In the Arctic Islands of Franz-Joseph Land and Novaya Zemlya, RSL gradually fell from 25 to 35 m at 9 ka to 5–10 m at 3 ka. The Timan coast and the Kara Sea shelf are characterized by constant RSL rise due to proglacial forebulge collapse; Yamal and the Gydan Peninsula and Novaya Zemlya are all marked by a high LGM position of RSL, followed by a lowstand and consequent rise to a late Holocene highstand of several meters. Data from the Laptev Sea coasts and shelf and the New Siberian Islands demonstrate post-LGM RSL rise with a Holocene highstand of up to 5–10 m, with scatter caused by differential tectonic movements along a diffuse lithospheric plate boundary. The collected database allowed to estimate and discuss the reasons of both spatial and temporal variability of RSL histories in different parts of the Russian Arctic.

Evaluation of present-day deformations in the Amurian Plate and its surroundings, based on GPS data

We analyze recent crustal movements, based on data of GPS measurements on the Amurian Plate and its margins. Most of the velocities were borrowed from scientific publications. Also, our new and updated data of periodic and permanent GPS observations are presented. A continuous field of strain rates has been constructed using the combined field of movement velocities. Dilatation rates and the directions and values of the main strain axes have been calculated. We have made a quantitative estimation of the maximum shear rates and their directions. Zones of background deformations in the inner part of the Amurian Plate have been revealed along with high-strain zones. Zones with intense tectonic activity, corresponding to areas of reduced lithosphere, often show manifestations of Late Cenozoic volcanism. Part of them can be interpreted as the present-day boundaries of intracontinental geoblocks and small lithospheric plates.

Zoned magmatic areas and anorogenic batholith formation in the Central Asian Orogenic Belt (by the example of the Late Paleozoic Khangai magmatic area)

In the Late Paleozoic-Early Mesozoic, the Central Asian Rift System formed in the southern framing of the Siberian Craton. It has a specific structure owing to zoned magmatic areas comprising the largest world’s batholiths (Angara-Vitim, Khangai, and Khentei) in the core and coeval rift zones on the periphery. By the example of the Khangai batholith, which is the core of the Khangai zoned magmatic area, we have demonstrated that the batholith granites are a specific class of anorogenic granitoids. Each batholith is composed of nearly coeval rocks of widely varying compositions, from low-alkali to subalkalic and alkaline and from mafic to ultrafelsic. The geochemical and isotopic characteristics of rocks of the Khangai batholith are intermediate between the host continental crust and the juvenile mantle source. The melts from this mantle source formed lava sequences on the rift periphery of the batholith and were also manifested in its core as synplutonic mafic intrusions. The isotope-geochemical characteristics of the batholith granitoids evidence that mantle magmas took part in the formation of granite melts. With regard to the volume of the batholith rocks (~1 mln km3), the Khangai zoned magmatic area can be considered a silicic large igneous province, whose formation was controlled by a localized source of material and heat, i.e., mantle plume. We propose a model relating the composition, structure, and geologic position of batholiths to the impact of mantle plumes on the lithosphere of a folded area. The model assumes that mantle melts intruded into the base of crust by the underplating mechanism or formed underlayers, caused crustal anatexis, and were mixed with anatectic melts. The geochemical and isotopic characteristics of the resulted magmatic products were intermediate between the parameters typical of the convergent boundaries of lithospheric plates and those characteristic for intraplate environments.

Long-term prediction for seismic hazard for radioactive waste disposal

We consider possible approaches to the long-term prediction for seismic hazard in relation to the practical need for the safety of geological disposal of long-lived radioactive waste. The required period of prediction significantly exceeds the one reflected in the set of maps of General Seismic Zoning of the territory of the Russian Federation (GSZ-97). The first geological repository in Russia is planned to be set up in the Nizhnii Kan granite massif in the Krasnoyarsk Krai. This region is an intraplate territory with a relatively high seismic activity. We summarize the analysis of the known empirical generalizations and theoretical principles underlying the seismic hazard prediction. Real seismic events constantly violate forward-looking statements even for relatively short periods of time. These and other arguments suggest that the hypothesis of stationarity of the seismic regime, which is the basis of long-term prediction today, has limited and uncertain applicability in time. Intraplate earthquake prediction is especially uncertain because of the uncertainty in the factor responsible for generating tectonic stresses in these regions. The short horizon of the prediction, based on statistical methods, can be attributed to the nonlinearity of seismic geodynamic processes. Fundamental laws of tectonic processes should be used as the scientific basis for long-term predictions for seismic hazard at the sites chosen for geological disposal of long-lived radioactive waste. These processes can be reflected in models for the migration of the seismically active boundaries of lithospheric plates and the occurrence of seismic activity in intraplate regions.

Geomagnetic conjugacy of modern tectonic structures

An earthquake is an element of the global electric circuit (GEC) – this new idea suggested in the space age is tested in our study. In the frame of the GEC concept, one may expect that tectonic structures of the northern and southern hemispheres may be magnetically conjugated. It is found that the midocean ridges of the southern hemisphere, located along the boundary of the Antarctic lithosphere plate, are magnetically conjugated with the areas of the junction of continental orogens and platforms in the northern hemisphere. The closest geomagnetic conjugacy exists between the southern boundary of Nazca lithospheric plate and the northern boundaries of Cocos and Caribbean lithospheric plates.

High-resolution imaging of the deep anisotropic structure of the San Andreas Fault system beneath southern California

SUMMARY Geological and geodetic records show that motion is distributed over a broad and complex system of faults in southern California, with many large earthquakes occurring off the main San Andreas fault. This unusually complex plate boundary geometry is imposed by the big bend of the San Andreas Fault and the dynamics of the underlying lithosphere, but lithosphere rheology and distribution of applied forces are still poorly constrained. Geodynamic modelling is thus very uncertain, and distribution of lithospheric deformations highly controversial. Here, exploiting the property that deformation orients olivine crystals, we show that deep lithospheric deformations can be mapped directly with a new imaging approach of seismic anisotropy, with a resolution fine enough for detailed geodynamic interpretation. This method relies entirely on finite-frequency effects in the splitting of SKS waves. We build an extensive data set of more than 3400 SKS splitting measurements from the analysis of SKS waves recorded by all the broad-band stations in southern California. Resolution tests demonstrate that, using this data set, we are able to resolve anisotropic structures smaller than the size of the first Fresnel zone of SKS waves. The good vertical resolution in our tomographic images results from the short inter-station spacing in southern California. In our 3-D anisotropy model, we find no evidence for localized lithospheric shear deformation beneath the San Andreas Fault. Instead, the lithospheric plate boundary is localized along a broad shear zone beneath the East California Shear Zone. Therefore, surface and deep deformation patterns are poorly correlated and most likely decoupled. Active N–S convergence in the Transverse Ranges results in strongly coherent E–W alignment of olivine fast axes in the shallow lithosphere. At the top of the asthenosphere, the observed low level of anisotropy suggests weak lithospheric basal tractions and thus a strong decoupling between the lithosphere and the asthenosphere.

GEOPHYSICAL MODELLING OF THE MOLOPO FARMS COMPLEX IN SOUTHERN BOTSWANA; IMPLICATIONS FOR ITS EMPLACEMENT WITHIN THE 2 GA LARGE IGNEOUS PROVINCES OF SOUTHERN AND CENTRAL AFRICA

The Molopo Farms Complex is an extremely poorly exposed, major, ultramafic-mafic layered intrusion straddling the southern border of Botswana with South Africa. It lies within the south-western part of the ~2.0 Ga large igneous province of southern and central Africa that includes the better known Bushveld Complex. Integrated interpretation of regional gravity data and new high-resolution airborne magnetic data have constrained the geometry of the Molopo Farms Complex in southern Botswana as a strongly faulted, polyphase intrusion compartmentalised by regional ductile shear zones. Previous models showing that the Complex was emplaced in at least two discrete stages are supported. Ultramafic rocks were initially emplaced as a semi-coherent lopolithic sheet up to about 4 km in thickness cutting across Transvaal Supergroup strata that had already been folded into open eastwest trending dome and basin structures with wavelengths of about 4 km. Steeply dipping, dyke-like ultramafic bodies adjacent to, and within major shears are inferred to be solidified feeders to the main lopolithic part of the MFC. It is likely that the initial ultramafic sheet was emplaced at a high crustal level (<3 km depth) into an attenuated Transvaal Supergroup sequence. This lack of a thick hanging wall sequence is thought to be significant for the emplacement of the succeeding mafic sheets. The ultramafic sheet thermally altered its wall rock and also created a complex fracture system in its hanging wall rocks. Differentiation within the ultramafic sheet produced basal harzburgites overlain by bronzites and possibly mafic sheets. Later mafic/basic sheets and dykes, again fed along shear-controlled, steeply dipping zones, spread into the fracture network created by the initial emplacement of the ultramafic lopolith to form a distinctive spider’s-web pattern on high-resolution airborne magnetic maps. It is proposed that either post-emplacement regional folding or gravitational collapse of the basal ultramafic lopolith produced a major basin with a ~40 km eastwest diameter, north of the Jwaneng-Makopong Shear Zone and smaller basin to the southeast. The newly postulated, steeply dipping ultramafic/mafic feeders, as well as the ultramafic lopolith and areas with anomalous nickel values in soils are considered to be prospective for PGE-bearing magmatic nickel-copper sulphide mineralisation. Magmatic rocks dated at about 2.0 Ga are a common feature, not only of the Kaapvaal Craton, but of all the African cratonic blocks south of the Equator. Reactivated intracratonic faults and shears appear to control emplacement of individual magmatic complexes although a sub-continental thermal anomaly unconfined by lithospheric plate boundaries is a likely driving force for the widespread magmatism.