Crustal Mass Research Articles

Regional crustal trends in South Africa from the spectral analysis of topographic and gravity data

The 2-D spectral analysis of gravity and topography data has been used to identify structural trends of recognized geological entities within the tectonic provinces of South Africa. The mean amplitude of the spectra within very thin rings, taken to have the same mean wavenumber, is subtracted from the total spectrum to give the non-isotropic component. This component of the spectrum reflects more of the heterogeneity of the data and is exploited to produce trends. These results are consistent with known structural trends, but also reveal new trends. Some trends, identified on the Kaapvaal craton using gravity data, are at variance with those obtained from the topography. This suggests that in this area crustal mass distribution is not correlated with surface relief. No dominant structural trend was identified in the Witwatersrand basin. it can be concluded from these findings that this technique can be successfully applied to various data sets, which reflect different physical properties and hence assist in geological interpretation, regardless of the scale.

Electromagnetic studies in the Fennoscandian Shield—electrical conductivity of Precambrian crust

Electromagnetic (EM) investigations of the 1980s in the Fennoscandian (Baltic) Shield produced an unique and unified EM data set. Studies include regional investigations by the magnetovariational (MV) method with large lateral sampling distance, investigations of anomalous conductivity structures by magnetotelluric (MT) soundings and other (EM) and electrical methods (audio MT soundings, d.c. dipole-dipole and VLF resistivity profilings) with shorter sampling distance, and studies of the near-surface conductivity by airborne EM surveys. The variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures. The Precambrian of the Fennoscandian Shield is characterized by roughly NW-SE-directed elongated belts of conductors which separate more resistive crustal blocks. The latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including (1) the Kittilä-Vetrenny Poyas conductor, (2) the Lapland Granulite Belt and Inari-Pechenga-Imandra-Varzuga conductors, (3) the Archaean-Proterozoic boundary conductor and (4) the Southern Finland Conductor. The conductive belts—orogenic conductors—indicate places where crustal masses collided and were finally sealed together. Enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite- and sulphide-bearing metasedimentary rocks. Estimations of the lower-crustal conductivity indicate a laterally heterogeneous lower crust in the Fennoscandian Shield. Archaean lower crust seems to be in general more resistive than the Early Proterozoic lower crust of the Karelian and Svecofennian Domains. The lower crust in the southwestern part of the Svecofennian Domain and in the Sveconorwegian Domain seems to be more resistive than in the central part of the Svecofennian Domain.

The composition of peridotite tectonites from the Ivrea Complex, northern Italy: Residues from melt extraction

A total of forty-three samples from the Balmuccia, Baldissero, and Finero peridotite tectonites have been analyzed for major elements, forty minor elements, mineral composition, and a subset of samples have been investigated for δD, δ 18O, δ 34S, and 87Sr/ 86Sr ratios. The spinel Iherzolites from Balmuccia and Baldissero contain on average 56% olivine, 28% orthopyroxene, 14% clinopyroxene, and 1.5% spinel. They have a neodymium and strontium isotopic signature similar to MORB. These peridotites were moderately depleted in Al and Ca and highly depleted in incompatible elements by the separation of 4.5% P-MORB. The MORB in excess to the formation and subduction of the oceanic crust fractionated into a tonalitic continental crust and a pyroxenitic residue which was recycled into the mantle. The crustal mass fraction of the upper mantle-crust system is 2.8%. A primitive mantle composition can be calculated from 97.2% Balmuccia model peridotite plus 2.8% bulk crust. The new set of primitive mantle concentrations is in accordance, within about 10%, with the data from fertile peridotite xenoliths and primitive meteorites for the elements Li, Mg, Si, Ca, Sc, V, Mn, Fe, Co, Ni, Zn, Ga, Y, Zr, La, Ce, Nd, Eu, Tb, Dy, Yb, Lu, and Hf. Larger differences occur for the elements F, Na, P, Ti, Cr, and Cu. The bulk crust concentrations of the highly incompatible and mobile elements K, Rb, Ba, Th, and U exceed those being supplied from a primitive upper mantle reservoir of 660 km depth. An additional lower mantle source is required for about half of the crustal accumulation of these elements. Moderately depleted peridotite bodies with a cross section of 3 km can be sufficiently well homogenized in compatible and moderately incompatible elements to represent MORB producing upper mantle. Concentrations of Na, Sc, Ti, V, Ga, Y, Zr, and Yb are well correlated with the Al abundance so that primitive mantle values can also be estimated using the cosmically derived primordial Al concentration. The spinel harzburgites from Finero contain on average 74% olivine, 16% orthopyroxene, 3% clinopyroxene, 5% amphibole, 1% phlogopite, and 1% spinel. These peridotites were depleted by a separation of about 18% MORB. Subsequently they have undergone metasomatic alteration probably from dehydration of a subducted lithospheric slab. They received two pulses of water-rich fluids at temperatures of about 800°C to form amphibole low in incompatible elements or amphibole plus phlogopite high in incompatible elements without complete reequilibration of the preexisting rocks. The accumulation of elements is correlated to their incompatibility with the exception of a depletion of U, Th, Nb, and Ti. Isotopic ratios of H, O, S, Sr, Nd, and Pb, chlorine in phlogopite and a relative deficit of Nb and Ti are consistent with a crustal imprint in the metasomatic fluids.

Neutron star crusts.

We calculate properties of neutron star matter at subnuclear densities using an improved nuclear Hamiltonian. Nuclei disappear and the matter becomes uniform at a density of about 0.6${\mathit{n}}_{\mathit{s}}$, where ${\mathit{n}}_{\mathit{s}}$\ensuremath{\approxeq}0.16 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}3}$ is the saturation density of nuclear matter. As a consequence, the mass of matter in the crusts of neutron stars is only about half as large as previously estimated. In about half of that crustal mass, nuclear matter occurs in shapes very different from the roughly spherical nuclei familiar at lower densities. The thinner crust and the unusual nuclear shapes have important consequences for theories of the rotational and thermal evolution of neutron stars, especially theories of glitches.

Kinematics, topography, shortening, and extrusion in the India‐Eurasia collision

We examine the problem of partitioning between shortening and extrusion in the India‐Asia collision since 45 Ma. We compute the amount of shortening expected from the kinematics of India's motion with respect to Eurasia, using the reconstruction at collision time to put bounds on the possible amounts of surface loss within Greater India and within Eurasia. We then compute the amounts of surface loss corresponding to the thickened crust of Tibet and of the Himalayas, assuming conservation of continental crust. The spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it. This distribution indicates an important eastward crustal mass transfer. However, the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis. The deficit may be accounted either by loss of lower crust into the mantle, for example through massive eclogitization, or by lateral extrusion of nonthickened crust. A mass budget of the crust of the Himalayas indicates that lower crust has not been conserved there, but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit. Alternatively, following Zhao and Morgan [1985], lower crust may have been transferred below the Tibetan crust. We conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between India and Asia since 45 Ma. This conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m.y. history of the collision.

Geochemical constraints on formation fluid ages, hydrothermal heat flux, and crustal mass transport mechanisms at Cajon Pass

He, Ne, and helium isotopic measurements have been made on borehole fluids collected during drill stem tests at the Cajon Pass scientific drill hole. These fluids represent a dominant component (greater than 90%) of the formation fluids as determined from fluoroscein tracer measurements. The results indicate a crustally derived radiogenic helium source with excess He concentrations up to 100x air‐saturated water conditions. The gas content and the chemical composition of the borehole waters reflect a variation in the formation waters entering the hole from several discrete, identified, and poorly interconnected fracture systems. These conditions indicate a local source for the excess 4He which allows 4He model ages of 33,000 to 5×106 years to be placed on the formation fluids at this depth. This constraint suggests an upper limit of <7% of the total heat flow can be removed by hydrothermal processes. These data thus provide geochemical support for a low stress fault. These model ages further indicate an effective vertical component of the fluid transport velocity of the order of 0.04–6 cm yr−1. Pressure head and in situ permeability measurements indicate flow velocities of 10−5 cm yr−1. To satisfy both of these extant conditions, fluid exchange must have occurred (as least once) in the last ∼33,000 to 5×106 years under flow conditions considerably faster than presently operable. Fluid flow in this region is therefore discontinuous with long periods of slow flow interrupted by periods of very rapid transport. Such a conclusion is consistent with both local and general observations of metamorphic geology and suggests a deep crustal fluid transport mechanism that is mechanically and/or tectonically controlled.

AGE AND TECTONIC SIGNIFICANCE OF HIGH-PRESSURE METAMORPHIC ROCKS OF CUBA

A combination of geological and isotopic-geochronological (K/Ar, 40Ar/39Ar, U-Pb-zircon methods) studies indicates that high-pressure-low-(in part, intermediate) temperature metamorphic rocks of both coherent complexes and inclusions in serpentinite melange were formed during Cretaceous and possibly to a small extent during Paleogene time. The original rocks are mostly of Jurassic and Cretaceous age. Pre-Jurassic metamorphic rocks have not been found within the complexes investigated. The metamorphic histories of many Cuban eclogites and garnet amphibolites were complex, but nevertheless reflect only a single Laramide tectono-metamorphic cycle. The discovery that the high-pressure meta-morphic rocks of Cuba are young confirms that major lateral displacement of crustal masses played a very important role in the structural development of the island.

Tectonics‐climate interaction studied

A growing number of Earth scientists are studying tectonic‐climate interaction, which is reflected in a constantly evolving mountain landscape. Robert S. Anderson, University of California, Santa Cruz, and Michael A. Ellis, Center for Earthquake Research and Information, Memphis State University, describe the field as “one of the most exciting interdisciplinary fields of Earth science.” By studying the landscape, researchers are gaining new insight into the kinematics and dynamics of crustal deformation.Several important feedbacks may exist at the large time and length scales involved in building mountains. Tectonic uplift produces mountains that influence both regional and global climate. At the same time, the evolution of mountain belts is influenced by climatically driven erosion, which transfers crustal mass away from the mountain belt. The distribution and intensity of the erosional process is strongly affected by the uplift, and the transfer of mass affects the stress system in the crust and mantle. It has also been suggested that large regions of uplift (plateaus) may trigger global climate changes through increased weathering rates at high elevations,

The continental crustal age distribution: Methods of determining mantle separation ages from Sm‐Nd isotopic data and application to the southwestern United States

Samarium‐neodymium isotope systematics provide a means of determining the age of the continental crust, where ”age“ refers to the amount of time the crustal rock material has been isolated from the convecting mantle. This age is referred to as the Sm‐Nd model age or the mantle separation age. Systematic analysis of rocks from the continents can be used to determine the age distribution by mass in the continents; this is a unique record of the thermal and chemical evolution of Earth over almost 4 b.y. The establishment of mantle separation ages for crustal rock reservoirs requires precise geochronological information typically provided by U‐Pb isotopic measurements of zircon, plus models for (1) Nd isotopic evolution in the mantle source of continental crust, (2) Nd isotopic evolution of crustal reservoirs subsequent to formation, (3) petrogenesis of certain types of granitic rocks in the crust, (4) the mechanisms of formation of hybrid crust (mixtures of material newly differentiated from the mantle with material derived from older crust), and (5) the evolution of the lower parts of the continental crust during continental margin magmatism and intracontinental extension. Methods are presented for treating isotopic data on continental rock materials to obtain meaningful mantle separation ages. The greatest uncertainties come from a lack of adequate information on lower crustal processes and composition. The methods are applied to the Precambrian and Mesozoic rocks of the southwestern United States to produce a mass‐age distribution for the region, which represents 1% of the global continental mass. The results suggest episodic crustal growth, with short growth periods at circa 2.8, 1.8, and 0.1 Ga. About 90% of the crust was formed by 1.8 Ga, the remaining 10% was added in the Phanerozoic. The mean age of this section of continent is determined to be 1.84 Ga. Almost all of the Phanerozoic additions occur in areas where there was no preexisting continental basement; Mesozoic magmatic additions account for only about 2% of the crustal mass in the area underlain by Precambrian basement. In the region of post‐Precambrian continental growth, the regions farthest from the Precambrian continental edge are composed of mostly (>85%) of material newly differentiated from the mantle. Adjacent to the Precambrian continental edge the new crust is typically composed about 50% of material newly differentiated from the mantle and 50% of old crustal material. There is no correlation between crustal thickness and the crust formation age in the area studied; the age‐area and age‐mass curves are nearly identical.

Late paleozoic orogeny in the northwestern Gondwana continental margin, western Argentina and Chile

During Early Carboniferous-Early Permian time, the segment of the northwestern Gondwana continental margin (western Argentina and Chile) between present-day latitudes 27°S and 50°S was structurally organized into a marginal orogen and a foreland rising into cratonic highlands. Orogenesis and magmatism were governed by subduction of an ancestral Pacific plate under Gondwana. During the Late Devonian-Early Carboniferous, former slope-rise deposits were thickened to form the orogen and were thrust slightly onto the foreland; parts of the orogen emerged above sea level. Following this early diastrophic phase, tectonic uplift in the orogen was regionally checked by erosion until the late Early Permian when uplift rates overcame erosion. The development of this orogen is peculiar in that throughout much of this time: a) the orogenic bett was partly occupied by marine areas; b) there was no reversal of provenance, so that foreland basins were continuously fed from the interior highlands; and c) the orogenic belt did not advance further onto the foreland. Thus the orogen stabilized in position and relief. Magmatism was intense during the period of orogenic stability and was dominated by plutonism; there are no remnants of a well developed volcanic arc. Reconstructions involving Late Devonian-Early Carboniferous collision of crustal masses against Gondwana and a seaward jump of the subduction trench are not entirely substantiated by available data. An alternative suggests that from the Late Devonian to the Early Permian the trench maintained approximately the same position seaward of present-day coastal exposures.

The deep structure of the central Alps inferred from both geophysical and geological data

ABSTRACTGravity surveys of the past century established that mountains have roots, seismic refraction lines shot in the second half of this century confirmed the downbulge of the Moho under the Alps, and recent reflection traverses provided new details on the behaviour of crustal layers in the deep part of the Alps. However, geophysical data are ambiguous geologically. For models of the root in terms of rock distribution to be tectonophysically acceptable, they must be the retrodeformable result of kinematic sequence that fits the geological surface data. For a cross‐section through the Swiss Alps based on refraction data and somewhat modified by the recent reflection traverses, a kinematic model compatible with large‐scale geological data may be obtained by the superposition of three Neogene phases with alternating vergence. Although Alpine collision is largely dextrally compressive in the central Alps, the N‐S component may be discussed in a cross‐section. Particularly puzzling geophysical features include a high‐velocity body in the middle crust and the disappearance of the layered foreland crust in the root. In order to account for these phenomena, it is proposed that the crustal root is interpreted as the result of complex reshuffling of middle and lower crustal masses as well as large‐scale phase transformations. The mid‐crustal highvelocity body is interpreted as a delaminated section of the lower crust of the Adria plate that was wedged into the middle crust of the Alps in the middle Miocene. The disappearance of the foreland lower crust is attributed to eclogitization attendant on the subduction of continental crust. Material balance estimates suggest that during Alpine collision large volumes of continental crust have disappeared through subduction.

On terrane analysis

Many recent papers on displaced crustal masses are organized around ‘terranes’ given only geographic names, and are bewildering for other than local experts; self-explanatory descriptive or genetic terms should be incorporated in most designations. Plate-tectonic interpretations of aggregated terranes that incorporate awareness of how modern arc systems vary and evolve contain implicit and testable predictions, but too many interpretations are based instead on invalid assumptions. As actualistic models are applied to analyses of orogenic belts, much more mobilistic interpretations than those generally now visualized will probably emerge. An example is made of the Carpathian region.

Crustal reworking in southern Africa: constraints from Sr-Nd isotope studies in Archaean to Pan-African terrains

Isotopic data for crustal rocks from southern Africa indicate that significant crustal growth occurred prior to 2.5 Ga and crustal growth since 1.0 Ga was negligible. Orogenic events since about 2.0 Ga appear to have reworked progressively larger proportions of pre-existing crustal material and the last major orogenic event, the Damara of Namibia, resulted in no detectable increase in crustal mass. Model age ratios ( dm nd t dm sr ) refect intracrustal fractionation of Rb/Sr relative to Sm/Nd and so constrain models for intracrustal reworking. Data from sedimentary rocks suggest that Rb/Sr ratios are increased at average rates of between 1.2 and 2.2 per billion years by processes of erosion and sedimentation, metamorphism and intracrustal melting in the continuously reworked upper crust. These inferred rates of Rb/Sr fractionation can account for the present day 87Sr 86 Sr ratios in modern river particulates and the restricted range of depositional 87Sr 86 Sr ratios in upper Proterozoic sediments.

Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga)

It is postulated that a mantle superswell (convective upwelling thousands of kilometres in diameter) developed beneath a stationary supercontinent aggregated in the late Early Proterozoic, giving rise to Middle Proterozoic anorogenic magmatism across Laurentia. Magmatism is attributed to invasion and ponding of mantle melts in the crust, causing uplift and anhydrous partial melting of the lower crust. Products include vast sheets of rhyolite, syenogranite, anorthosite, and gabbro, extensive mafic dike swarms, and rifts containing up to 20 km of basalt. Rifting occurred as a consequence rather than the cause of mantle upwelling. Thermal dissipation of lithospheric mantle may have provided rheological conditions favorable for whole-crustal imbrication observed in the parautochthonous Grenville orogen. The Middle Proterozoic supercontinent engendered more pronounced anorogenic magmatism than subsequent supercontinents because of secular cooling of the mantle. Before the Middle Proterozoic, crustal mass may have been insufficient to produce supercontinents large enough to effectively insulate the mantle and promote superswells. The theory of supercontinental episodicity makes testable predictions concerning paleolatitudes, relative sea levels, and globally synchronous episodes of Proterozoic orogenic and anorogenic activity.

Deep-reaching geodynamic processes in the Alps

Summary The Alps are a deep-reaching crust-mantle structure in Europe situated at the northernmost tip of the Adriatic promontory of the African plate. In that region the contact against the Eurasian plate is presently characterized by a NW-SE oriented compressional stress regime. A detailed regional analysis of seismic surface-wave dispersion and of P-wave travel time residuals (‘seismic tomography’) as well as long-range seismic refraction measurements and the interpretation of ‘stripped’ residual gravity anomalies have revealed a rather anomalous structure within the uppermost mantle under the Alpine arc. As a result of the massive continent-continent collision, parts of the lithosphere have been delaminated — a process which has led to ‘flaking’ in the upper crust, combined with a thickening of the entire crust and the formation of a pronounced relatively cold, dense and slowly subsiding ‘lithospheric root’ beneath the mountain chain. In this still continuing plate collision the lower parts of the lithosphere apparently have penetrated into the asthenosphere to a depth of 130 to 200 km in a steep zone of ‘subfluence’ (‘Verschluckung’). On this scale the presently measurable uplift of the Alpine chain (about 0.1 cm per year) is a secondary effect due to isostatic rebound of less dense crustal masses which previously had been forced to greater depths. A configuration of this type generates regionally a compressive dynamics of its own on which — within a wider framework — rotational processes may be superimposed.

Tectonics and paleomagnetism of structural arcs of the Pamir-Punjab syntaxis

The Pamir-Punjab syntaxis consists of two structural arcs with the crests facing northward—the Pamir and Hindu-Kush-Karakorum arcs. These arcs are mutually disharmonious, and the exterior (Pamir) arc is more tight, as compared to the inner one. Paleomagnetic study of the Pamir arc has shown that the structures of the future Pamirs had a northeastern strike in the Paleogene and Cretaceous, and they occurred on the eastern limb of the Darvaz-Kopetdag structural arc, whose crest faces south. The Pamir arc originated after the Paleogene during the process of the formation of the Pamir-Punjab syntaxis. Knowledge of the kinematics of the Pamir arc, combined with data on the geometry of the syntaxis and the character of its boundaries, enable us to choose a model of the development of the syntaxis. The process of “plastic flow” of crustal masses around the underthrusting segment of the Indian plate was likely of paramount importance.

187Os/186Os in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium

During their formation, marine manganese (or more properly ferromanganese oxide) nodules include and commonly concentrate elements dissolved in seawater. This attribute makes them especially valuable recorders of the isotopic composition of elements supplied to the oceans having radiogenic isotopes, for these can be used in exploring crustal composition and evolution1–5. From measurements of 187Os/186Os ratios in marine manganese nodules and comparison with the 143Nd/144Nd ratios of the same nodules, we have calculated the Os isotopic composition and the osmium and rhenium concentrations of the weatherable portion of the continental crust, together with the proportions of ‘ultra-mafic’ and ‘granitic’ rock that constitute this crustal mass.

CRUSTAL EVOLUTION IN THE EARLY PRECAMBRIAN

Recently published models present highly conflicting views of continental evolution. At one extreme, it is envisaged that continental crust was fully developed by ∼ 4000 Myr ago and quantitatively recycled through the mantle throughout geological time without net change of crustal mass (continental steady-state hypothesis). A fundamentally contrasted view is that the earliest continental crust (∼ 3700 Myr) occurred in small, localised patches, and that once juvenile continental crust is cratonised it cannot be returned to the mantle in bulk. This corresponds to the concept of irreversible chemical differentiation of the mantle with accompanying growth of continental crust through geological time (continental growth hypothesis). In between these extremes, it has been suggested that some relatively minor continental constituents may be selectively recycled into the mantle. This does not involve wholesale recycling of continental crust, and still allows continental growth. Whilst isotopic and trace element modelling imposes important constraints it is not absolutely definitive in deciding between contrasted evolutionary models. Additional criteria for continental evolution must be sought from assorted geological, geophysical, geochemical and geochronological data. As one example, it is important to determine the age and regional extent of exposed early Precambrian continental crust, and also whether exposed crust is underlain by significantly older crust at depth.

Crustal evolution in the early precambrian

Recently published models present highly conflicting views of continental evolution. At one extreme, it is envisaged that continental crust was fully developed by ∼ 4000 Myr ago and quantitatively recycled through the mantle throughout geological time without net change of crustal mass (continental steady-state hypothesis). A fundamentally contrasted view is that the earliest continental crust (∼ 3700 Myr) occurred in small, localised patches, and that once juvenile continental crust is cratonised it cannot be returned to the mantle in bulk. This corresponds to the concept of irreversible chemical differentiation of the mantle with accompanying growth of continental crust through geological time (continental growth hypothesis). In between these extremes, it has been suggested that some relatively minor continental constituents may be selectively recycled into the mantle. This does not involve wholesale recycling of continental crust, and still allows continental growth. Whilst isotopic and trace element modelling imposes important constraints it is not absolutely definitive in deciding between contrasted evolutionary models. Additional criteria for continental evolution must be sought from assorted geological, geophysical, geochemical and geochronological data. As one example, it is important to determine the age and regional extent of exposed early Precambrian continental crust, and also whether exposed crust is underlain by significantly older crust at depth. The use of radiogenic isotopes provides one of the most powerful tools for elucidating the evolution of any given sector of continental crust through geological time, and constraining crustal residence time. This approach, combined with several others, strongly favours the continental growth hypothesis.

Island arc processes relevant to crustal and mantle evolution

Recycling of continental crust occurs at convergent plate margins. Recycling must be investigated at several structural levels and time scales: shallow, intermediate, and deep. Surface recycling: sedimentary, igneous, and structural redistribution of crustal mass, is not included in this hierarchy, although it controls the composition of the recycled crust. Crustal growth, structural and geochemical evolution is directly tied to the larger, less complex mantle. Crustal recycling at the deepest structural level must be understood better in order to choose between alternative interpretations of the isotopic evolution of the mantle. Crustal recycling at intermediate structural levels by subduction of crust and its subsequent reappearance as a component of volcanic arc magmas has important implications for the question of long-term crustal growth. Crustal recycling at shallow structural levels is important in understanding the development of forearc regions. The mass flux at present day convergent plate margins has been diagramed in order to systematize future investigations and to clarify the distinction between the recycling at different levels, and the observable quantities in the present day earth that are relevant to recycling disputes.