Deep Lithospheric Structure Research Articles

Komatiite Magmas and Sulfide Nickel Deposits: A Comparison of Variably Endowed Archean Terranes

Komatiites are found in most Archean granite-greenstone terranes, but sulfide Ni mineralization associated with these rocks has a very biased distribution. The global endowment of sulfides Ni in Archean komatiites is overwhelmingly dominated by the Kalgoorlie terrane of the Eastern Goldfields Superterrane in the Yilgarn Craton. The question of whether the Kalgoorlie terrane komatiites possess any exceptional attributes which could explain this bias is addressed through an exhaustive compilation of geochemical data from this terrane and a number of others: the southeastern Youanmi terrane of the Yilgarn Craton; the eastern terranes of the Eastern Goldfields superterrane (Kurnalpi, Burtville and others); and the Abitibi greenstone belt of the Superior province. High MgO komatiite magmas, with MgO in the 25 to 30% range, are found in all the terranes sampled, but the proportion of these compositions appears to be higher in the Kalgoorlie terrane, and the abundance of strongly adcumulate olivine cumulates is much higher. Crustal contamination is apparently more extensive and advanced in the Kalgoorlie terrane than in all the others, but there is no systematic difference in the lithophile trace element (LILE) characteristics and degree of source depletion evident in the most primitive magmas from all the terranes. Consideration of variable-valency transition metals V and Cr indicates there are no systematic variations in oxidation state between the terranes and komatiites uniformly evolved close to the quartz-fayalite-magnetite buffer. Platinum-group element (PGE) variations imply that komatiites in all the terranes were emplaced sulfide undersaturated and were derived from sources with remarkably similar PGE contents. There is no evidence that the Kalgoorlie terrane magmas were in any way exceptional. The Forrestania and Lake Johnston Belts of the Youanmi terrane include the only known examples of Al-depleted komatiites hosting significant Ni sulfide resources, and they are also the only komatiites of this type that form adcumulate dunite bodies. The presence of adcumulate dunites, formed by high magma fluxes in central conduits, is the common feature between these belts and the Kalgoorlie terrane. Coupled with evidence for higher degrees of contamination in the Kalgoorlie terrane, and the availability of accessible crustal S sources, it appears that magma flux, rather than primitive magma composition, is the critical factor, and that craton-scale deep lithospheric structure is the ultimate control on the rate of magma supply between mantle source and crustal emplacement site. The Kalgoorlie terrane komatiites were emplaced at exceptionally high rates, giving rise to well-developed long-lived magma conduits, either lava tubes or subvolcanic channelized sills, which are interpreted to be the essential condition for forming large deposits.

Architecture of basin-mountain systems and their influences on gas distribution: A case study from the Sichuan basin, South China

The basin-mountain systems in the Sichuan basin exert major control on today’s oil/gas distribution, chiefly through their controlling influence on petroleum reservoirs preservation, the thickness of deposited terrestrial formations and degree of uplift and denudation. The composite basin-mountain system in the Sichuan basin and its peripheral mountains can be classified as two types – margin-plate systems and interior-plate systems. The margin-plate basin-mountain systems include the Sichuan basin and its surrounding Longmen Mountains, Micang Mountains and Daba Mountains, which are not only the western and northern marginal area of the Sichuan basin, but also represent the western margin of the Yangtze plate (South China Block). The margin-plate basin-mountain systems, with binary units of large-scale thrust belts and foreland basins, have different deep lithospheric structures, abrupt boundaries and great contrast in today’s geomorphology between the basin and the mountains. On the contrary, the interior-plate basin-mountain systems comprised by the Sichuan basin and its adjacent interior-plate Qiyue Mountains, Dalou Mountains and Daliang Mountains; represent the eastern and southern marginal area of the Sichuan basin located within the Yangtze plate. They have similar lithospheric structure and gradual boundaries between the basin and the adjacent mountains, lacking foreland basins. Today’s medium-large scale natural gas accumulations in the Sichuan basin are mainly distributed in the areas influenced by the margin-plate basin-mountain systems, and are located especially in the foreland basins.

Transverse Variations of Crustal Thickness and VP/VS Ratio Under the Stations in the Liaodong Anteclise‐Yanshan Belt‐Xingmeng Orogenic Belt and Their Tectonic Implications

AbstractWe obtained the crustal thickness and average crustal Vp/Vs ratio along the profiles across the Liaodong anteclise, Yanshan belt and Xingmeng orogenic belt using H‐к stacking of receiver functions. The result shows that the average crustal thicknesses of these three tectonic units are 32, 33 and 35 km, respectively, indicative of slight differences, and display distinct transverse variations in the crustal structure. The crust is thicker in the middle and thinner on each side in the Liaodong anteclise, whereas the thickness of the crust varies smoothly in the Yanshan belt. In the Xingmeng orogenic belt, the crust thickens sharply from southeast to northwest near the Solonker suture. The Vp/Vs ratio also fluctuates obviously in the study region. In particular, it varies laterally in the Yanshan belt and increases markedly nearby the boundary area between the Yanshan belt and the Xingmeng orogenic belt (near the north‐south gravity gradient zone). However, the Vp/Vs ratio is relatively low and has small change in the Xingmeng orogenic belt. The distinct crustal structures of the Yanshan belt and the Xingmeng orogenic belt are probably indicative of a more intense crustal modification in the Yanshan belt compared with the Xingmeng orogenic belt during Mesozoic‐Cenozoic time. Significant changes in the crustal structure roughly coincide with the north‐south gravity gradient zone aforementioned. This together with the concordant change in the deep lithospheric structure observed previously suggests that this gravity gradient zone may be a lithospheric‐scale intra‐continental tectonic boundary, the opposite sides of which may have evolved differently in the Phanerozoic. The thickening of the crust in the Xingmeng orogenic belt (near the Solonker suture) and the Liaodong anteclise appears to mirror the surface topography, and corresponds to relatively stable Vp/Vs ratios. This feature probably indicates relatively even Phanerozoic modifications of the crust in each region, resulting in minor lateral variations in the crustal structure and components at present.

Wave-Field Continuation and Decomposition for Passive Seismic Imaging Under Deep Unconsolidated Sediments

Abstract The coastal plains of the central and eastern United States contain deep sections of unconsolidated to poorly consolidated sediments. These sediments mask deeper crustal and upper-mantle converted phases in teleseismic receiver functions through large amplitude, near-surface reverberations. Thick sediments also amplify ambient noise levels to generally reduce data signal-to-noise ratios. Removing shallow-sediment wave-propagation effects is critical for imaging deep lithospheric structures. A propagator matrix formalism is used to downward-continue the wave field for teleseismic P waves into the midcrust and then to separate the upgoing S -wave field from the total teleseismic response of the P wave to expose deep Sp conversions. This method requires that the Earth model from the surface to the reference depth be known. Teleseismic P -wave data for the Memphis, Tennessee, station (MPH) are analyzed using a reference-station deconvolution technique to produce vertical and radial P -wave transfer functions. These transfer functions are modeled using a simple model parameterization for sediment structure through grid inversion. The inverted Earth model is incorporated into the wave-field continuation and decomposition technique to estimate the upgoing S -wave field at 10 km depth in the crust. Moho and possible deeper Ps conversions are identified with this process.

Direct evidence for upper mantle structure in the NW Pacific Plate: Microstructural analysis of a petit-spot peridotite xenolith

Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite, gabbro, and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine, orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere.

Upper mantle S-velocity structure and Moho depth variations across Zagros belt, Arabian–Eurasian plate boundary

The collision of the Arabian and Eurasian plates in the early Miocene, after the subduction of Neo-Tethys ocean beneath Eurasia, formed the Zagros belt, a seismically active continental-continental plate boundary in southwest Iran. The Zagros suture zone is an important seismotectonic boundary indicating an abrupt cutoff between the intense seismicity of the Zagros and the almost aseismic Central Iran plateau. Compared with other more evolved plate boundaries, little is known about the deep lithospheric structure of the Zagros belt, such as the fate of subducted Neo-Tethys plate and ambiguity in the presence of intermediate and deep earthquakes under the Zagros suture zone. In this study, we use the partitioned waveform inversion (PWI) method to image the upper mantle S-velocity structure and Moho depth variations across Zagros collisional zone. The resulting Moho depth along the profile shows the average Moho depth value of order of 40–45 km for most parts of the profile with abrupt crustal thickening in the middle of the profile up to about 65 km. As expected, the derived models show that the relatively old and cold Arabian plate has higher velocity at depth than the younger lithosphere farther north in Central Iran. A sharp and steep subcrustal boundary is found roughly coincident with the surficial expression of the Main Zagros Thrust (MZT), separating two different mantle domains. A high-velocity anomaly, possibly representing a fragment of subducted lithosphere, has been imaged beneath Central Iran at a depth between 350 and 600 km, which is quite similar to the case of Indo-Asian collision in Qinghai-Tibetan plateau. These observations as well as the sudden changes of shallow velocities along the cross-section with a sharp boundary under the Zagros suture zone support the idea that subducted oceanic lithosphere has broken-off under the region of maximum Moho depth. A relatively low velocity region beneath the Arabian plate is also imaged in our high resolution tomography model which is interpreted as an indication of lithospheric delamination within the Arabian lithosphere.

Conductivity structure and rheological property of lithosphere in Southern Tibet inferred from super-broadband magnetotelluric sounding

To understand deep lithosphere structure beneath the Qinghai-Tibet Plateau more comprehensively and objectively and to explore important scientific issues, such as characteristics of plateau lithospheric deformation, state of strain, thermal structure, plate (or terrane) movement, and crust-mantle rheology, it is necessary to research the variation of crust-mantle electrical structure in the east-west direction in every geological unit. For this purpose, six super-broadband magnetotelluric (MT) sounding profiles have been completed by INDEPTH-MT Project in the Himalayas-Southern Tibet. Based on the imaging results from the six profiles, three-dimensional electrical conductivity structure of the crust and upper mantle has been analyzed for the research area. The result shows that the high-conductivity layers in the middle and lower crust exist widely in Southern Tibet, which extend discontinuously for more than 1000 km in the east-west direction and become thinner, shallower and more resistive toward the big turning of the Yarlung Zangbo River. The discussion on the rheology of lithosphere in Southern Tibet suggests that the mid-lower crust there is of high electrical conductivity, implying the existence of “partial-melt” and “hot fluid” in the thick crust of Tibet, which make the medium hot, soft, and plastic, or even able to flow. Combining the experimental result of petrophysics and the MT data, we estimate the melting percentage of the crustal material to be up to 5%–14%, which would reduce the viscosity of aplite in the crust to meet the flow condition; but for granite, it is likely not enough to cause such a change in rheology.

Deep lithospheric structures along the southern central Chile margin from wide-angleP-wave modelling

P>Crustal- and upper-mantle structures of the subduction zone in south central Chile, between 42 degrees S and 46 degrees S, are determined from seismic wide-angle reflection and refraction data, using the seismic ray tracing method to calculate minimum parameter models. Three profiles along differently aged segments of the subducting Nazca Plate were analysed in order to study subduction zone structure dependencies related to the age, that is, thermal state, of the incoming plate. The age of the oceanic crust at the trench ranges from 3 Ma on the southernmost profile, immediately north of the Chile triple junction, to 6.5 Ma old about 100 km to the north, and to 14.5 Ma old another 200 km further north, off the Island of Chiloe. Remarkable similarities appear in the structures of both the incoming as well as the overriding plate. The oceanic Nazca Plate is around 5 km thick, with a slightly increasing thickness northward, reflecting temperature changes at the time of crustal generation. The trench basin is about 2 km thick except in the south where the Chile Ridge is close to the deformation front and only a small, 800-m-thick trench infill could develop. In south central Chile, typically three quarters (1.5 km) of the trench sediments subduct below the decollement in the subduction channel. To the north and south of the study area, only about one quarter to one third of the sediments subducts, the rest is accreted above. Similarities in the overriding plate are the width of the active accretionary prism, 35-50 km, and a strong lateral crustal velocity gradient zone about 75-80 km landward from the deformation front, where landward upper-crustal velocities of over 5.0-5.4 km s<SU-1</SU decrease seaward to around 4.5 km s<SU-1</SU within about 10 km, which possibly represents a palaeo-backstop. This zone is also accompanied by strong intraplate seismicity. Differences in the subduction zone structures exist in the outer rise region, where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise. This plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle. The southern profiles do not exhibit such a strong alteration of the lithosphere, although this effect may be counteracted by plate cooling effects, which are reflected in increasing rock velocities away from the spreading centre. Overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the M-w = 9.5 great Chile earthquake from 1960 ruptured through all these differing age segments. The rupture area, however, appears to coincide with a relatively thick subduction channel.

The deep lithospheric structure of the Namibian volcanic margin

A model of the thermal, compositional, density, and seismological structure of the lithospheric and sublithospheric mantle along a 500-km transect across the Namibian volcanic passive margin is presented. This margin juxtaposes old oceanic lithosphere and a Precambrian continental domain. The model combines within an internally consistent framework data from petrology, mineral physics, and geophysical observables. The calculated mantle temperature and density distributions down to a depth of 400 km are consistent with available xenolith-derived data, and fit simultaneously the observed free-air anomaly, geoid height, surface heat flow, and elevation. The model also explains the anomalously thick oceanic crust and the depletion of the lithospheric mantle in the ocean–continent transition and in the Proterozoic continental domain. Seismic velocities predicted by the present model are in good agreement with values obtained from wide angle reflection/refraction and tomography experiments. The thermal lithospheric thickness is ~ 100 km in the oceanic domain, increasing gradually to ~ 125 km across the ocean–continent transition and then more sharply to ~ 175 km in the continental domain. The density distribution in the mantle differs significantly from current purely thermal approaches where density is assumed to be only temperature-dependent. Non-negligible compositional density differences are encountered between the oceanic, transitional, and continental domains. Results show that non-thermal effects such as composition and phase changes cannot be neglected in models of the upper mantle.

Teleseismic traveltimes, topography and the lithospheric structure across central Mongolia

Teleseismic travel times recorded along a 1000 km‐long, ∼N‐S transect across central Mongolia are used together with topography and gravity data to constrain the deep lithospheric structure of this region. Time residuals appear positively correlated with the topography, suggesting that P‐wave velocity changes correspond to density variations which in turn cause an isostatic uplift of the topography. Using a simple local isostasy model and a Monte‐Carlo inversion for the crust and asthenosphere velocities and density/velocity conversion factors, we determine best‐fitting Moho and lithosphere‐asthenosphere boundary (LAB) geometries which satisfyingly reproduce the observed topography and gravity data. The model is validated using previously published 1D S‐wave velocity models obtained from receiver function analyses. Our results indicate that most of the long‐wavelength topography is supported by the asthenosphere buoyancy. The Hangai dome, in the southern half of the transect, appears mainly supported by a rather shallow (60–70 km) asthenospheric uplift, whereas the Siberian platform at the extreme north of the profile is underlain by a very thick (∼200 km) lithosphere. Misfits to the observed Bouguer gravity anomaly occur near major faults, probably because of lithospheric flexure.

ГЕОДИНАМІКА

In the article the seismicity and deep structure of lithosphere of Bukovyna and adjoining territories is analysed and the general geomechanical scheme of seismotectonic process in subregion is proposed, which adjusts with present tectonophysical, seismological, geological and geodesic data and explains observed here spatial-temporal, energetic and other peculiarities of local seismicity and modem geodynamics of lithosphere.

Deep structure of the Amur lithospheric Plate border zone

Based on the study of the deep Amur Plate border structure, seismogravimetric (density) and geoelectric sections of the plate lithosphere crossing geologically and seismically defined plate boundaries are compiled to construct a three-dimensional model of its lithosphere. The model demonstrates the almost ring structure of the plate with its inner part dipping for 50 km and uplifted fringing. The plate boundary zone consists of orogenic belts (Mongol-Okhotsk, Selenga-Stanovoi, Transbaikal, and Sikhote-Alin) and margins of the Siberian and North China platforms. The plate boundaries are described by mantle diapirism models with a largely bilateral inclined dip of its bordering belts.

Deep lithospheric structure and the Upper Silesian-Cracovian giant Zn-Pb ore deposits in Poland

Structural study of Palaeozoic folds has been followed by fractal and structural analyses of the fault network in the rock mass underlying the Upper Silesian-Cracovian giant Zn-Pb ore deposits of Mississippi Valley-type. Spatial distribution of the ore occurrences plotted against a background of the results of tectonic study points to determined basement fractures and related Palaeozoic secondary faults as to the zones that enable short-distance flow of mineralising solutions.

Crustal properties from S-wave and gravity data along a seismic refraction profile in Romania

VRANCEA'99 is a seismic refraction line that was carried out in 1999 to investigate the deep structure and physical properties of the upper lithosphere of the southeastern Carpathians and its foreland. It runs from the city of Bacau to the Danube River, traversing the Vrancea epicentral area of strong intermediate-depth seismicity and the city of Bucharest. Interpretation of P-wave arrivals led to a velocity model that displays a multi-layered crust with velocities increasing with depth. The range of P wave velocities in the sedimentary cover increases from N to S and a structuring of the autochthonous basement of the Moesian Platform is observed. The crystalline crust displays thickness variations, but at the same time the lateral velocity structure along the seismic line remains almost constant. An intra-crustal boundary separates an upper crust from the lower crust. Within the upper mantle a low velocity zone is detected at a depth of about 55-km. The interpretation of observable S-waves resulted in a velocity model that shows the same multi-layered crust, with S-velocities increasing similarly with depth as the P-waves. The corresponding Poisson's ratio is highly variable throughout the crust and ranges from 0.20–0.35 for the sedimentary cover to 0.22–0.25 for the crystalline crust. The interpretation of the V p, V s and Poisson's ratio in petrological terms suggests a large variety of rocks from sand and clay to sandstone, limestone and dolomite within the sedimentary cover. Within the crystalline crust the most probably rock types are granite, granodiorite, granite–gneiss and/or felsic amphibolite–gneiss in the upper part and gneiss and /or amphibolite in the lower part. Based on the 2-D seismic velocity model, a density model is developed. Density values are assigned to each layer in agreement with the P-wave velocity model and with values accepted for the geological units in the area. After several iterations a good fit between the computed and observed Bouguer anomalies was obtained along the seismic line.

Non-Reviewed Contribution: Ongoing passive seismic experiments unravel deep lithosphere structure of the Bohemian Massif

Research on high-resolution tomography, three-dimensional anisotropy modelled from shear-wave splitting and P-residual analysis, as well as receiver function interpretation, are in progress with the aim to image the crust and uppermost mantle velocities in the Bohemian Massif. Structure of the deep lithosphere and location of boundaries of mantle domains will be compared with tectonics of the crust and limits of terranes derived by geologic and palaeomagnetic methods.

Mapping the deep lithospheric structure beneath the eastern margin of the Tibetan Plateau from gravity anomalies

Various authors have investigated the mechanism of lateral support of the lithosphere in maintaining the uplift of the Tibetan Plateau, based on gravity data and elastic flexural modeling, in the south of the plateau across the Himalayas and in the north and the northwest across the Altyn Tagh and West Kunlun Shan. However, the degree of the regional compensation and lateral support to the east of the Tibetan Plateau has remained unknown. In this paper we present a lithospheric flexure model by interpreting gravity and topography to understand the first‐order problem of lateral support of the lithosphere at the eastern margins of the Tibetan Plateau. The flexural modeling constrains the geometry and strength variation of the lithosphere from the eastern margin of Tibet past the Longmen Shan ranges on to the Sichuan Basin. A lithosphere of intermediate strength (Te = 45 km) beneath the Sichuan Basin is weakened (Te = 36 km) by the thrusting underneath the eastern margin of the Tibetan Plateau. The strength variations indicate that about 10 km of the crustal material of the lithosphere has been ripped off and integrated into the thickened, upper crust of the Tibetan Plateau, which, however, has no appreciable strength. The stiffness of the lithosphere keeps the Tibetan Plateau relatively stable at its eastern margin and enables the weight of the plateau to be supported in the east by the Sichuan. Lithospheric deformation in the southern Longmen Shan area is more pronounced than in the northern region.

Structure of the crust and uppermost mantle beneath southern Finland revealed by analysis of local events registered by the SVEKALAPKO seismic array

In 1998–1999, a large-scale seismic array was deployed in Finland as a part of the EUROPROBE/SVEKALAPKO subproject, involving 14 European universities and research institutes. The objective of the project was to map the deep lithosphere structure and thickness beneath the Fennoscandian Shield by means of teleseismic events. In addition, about 580 local seismic events were registered during the data acquisition period. Among them, only eight local earthquakes were recorded, the rest being quarry blasts from mining sites in Russia, Finland, Estonia and Sweden. In this study, we present the analysis of the seismic wave field from the strongest local events registered by the majority of the stations of the SVEcofennian–KArelian–LAPland–KOla Transect (SVEKALAPKO) array with the aim of mapping the structure of the upper mantle beneath the array. For this purpose, we selected the events corresponding to a single source type and compared these recordings with those from wide-angle reflection and refraction experiments in the area to identify the regional phases. The record sections of selected events demonstrate strong reflections (PmP) from the Moho boundary. The refracted Pn phases can be seen as first arrivals at distances of about 200–400 km from the source. At offsets of about 400–800 km, phases reflected from inhomogeneities in the uppermost mantle (P1) and double reflections from the Moho boundary (PmPPmP) were recorded. Results from 2D forward ray trace modeling of reflected and refracted P-waves along four profile swathes from SVEKALAPKO stations demonstrate that the mantle reflections originate from two different groups of boundaries beneath the array: one group of phases arrive from subhorizontal and gently dipping reflectors below the Moho boundary at a depth of 70–90 km, while the other group are phases originating from a depth of 100 to 130 km. Based on the irregular character of the first group of reflections, their different spatial orientation and correlation with the Moho offsets, we interpret the boundaries of this group as relicts of ancient subduction and collision processes. The second group of reflections can be explained by a transition from mechanically strong to mechanically weak lithosphere.

Deep lithospheric structure and hypogene metallogeny at convergent plate margins

The correlation between hypogene mineral deposits and seismically active fracture zones, cutting the continental lithosphere of convergent plate margins in South America, Indonesia and Mexico, was investigated. It was found that all large mineral deposits and most important mining districts in Andean South America between 18-34° S, Sumatra and Java, and central and southern Mexico are situated in the outcrops of deep rooted seismically active fracture zones induced and/or activated by the process of subduction of the Nazca, Indo-Australian and Cocos plates. We suppose that active fracture zones, penetrating the whole thickness of the continental lithosphere, effectuate a channeling of ore-bearing solutions liberated from the subducting oceanic lithosphere. The repeated seismic activity stimulates the long-term permanent re-opening of import paths for ore-bearing solutions that causes the accumulation of enormous amounts of metals in restricted domains of the earth crust. It appears that the fracturing of the continental lithosphere remains preserved in time and space, being re-activated under favourable conditions by a sequence of subduction cycles of different age.

The role of deep lithospheric structure in the genesis of large and superlarge ore deposits in Uzbekistan: A review

The role of deep lithospheric structure in the genesis of large and superlarge ore deposits in Uzbekistan: A review

The role of deep lithospheric structure in the genesis and distribution of giant and supergiant concentrations of metals in the crust

The role of deep lithospheric structure in the genesis and distribution of giant and supergiant concentrations of metals in the crust