JournalTOCs

Developments in Geotectonics

[3 followers] Follow
Full-text available via subscription

Subscription journal
ISSN (Print) 0419-0254
Published by Elsevier

[2564 journals]

List of Authors
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  PubDate: 2012-12-15T09:31:26Z
Geodetic Control of Neotectonics in Venezuela
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  Henneberg, H.G., 1983. Geodetic control of neotectonics in Venezuela. In: P. Vyskočil, A.M. Wassef and R. Green (Editors), Recent Crustal Movements, 1982. Tectonophysics, 97: 1–15. Net installations for measurements of movement started in 1973 along the Bocono Fault, which is the main fault in the Venezuelan Andes. Investigations of horizontal movements are made on three locations along this fault: Mucubaji at 3700 m height, Mitisus at 1700 m and Yacambu at 700 m. At Mucubaji there are two nets with 8 stations each and at Mitisus we have three nets represented by 29 stations. Each station has a concrete pillar with a forced centering device on top and has a 1–4 m3 concrete plaform. These nets were measured several times, giving resulting difference vectors of a range of several cm. One of these nets is also a control net of a hydroelectrical Power Dam, installed near the fault trace. At Yacambu we have a 26 km extended tunnel net which is crossing the fault. This net was measured twice with difference vectors up to 6 cm. In the Maracaibo Basin Area the vertical component of subsidence due to oil extraction reaches 5 m as a result of continuous movements since 1925. Gravimetric research is also carried out over all the mentioned locations. In cooperation with the Universities of Hannover and Stuttgart and the German Geodetic Research Institute a programme was started to determine the geoid and deflections of the vertical in the high Andes mountains, especially around the Mucubaji fault site. Parallel to and in combination with these geodetic investigations seismological studies are carried out by a geophysical group from Merida University, Venezuela.
  
  PubDate: 2012-12-15T09:31:26Z
Further Titles in this Series
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  PubDate: 2012-12-15T09:31:26Z
Front Matter
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  PubDate: 2012-12-15T09:31:26Z
Copyright page
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  PubDate: 2012-12-15T09:31:26Z
Preface
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  PubDate: 2012-12-15T09:31:26Z
Author index to volume 48
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  PubDate: 2012-12-15T09:31:26Z
Subject index
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  PubDate: 2012-12-15T09:31:26Z
Evidence for Archean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of δ18O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower δ18O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early, Archean which is not known to be represented among the ecologite xenolith population
  
  PubDate: 2012-12-15T09:31:26Z
Growth of subcontinental lithosphere: evidence from repeated dike injections in the balmuccia lherzolite massif, Italian Alps
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hormblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein suite. Phlogopite (phl) veinlets that, formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean-island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid-ocean ridge basalt (MORB) asthenosphere. Lead isotopic compositions for all of the samples fall in the present-day MORB field on the 208Pb/204Pb vs. 206Pb/ 204Pb diagram but are displaced above this field on the 207Pb/204Pb vs. 206Pb/204Pb diagram. There is overlap in the data between the Cr-diopside suite and the Al-augite and hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other samples. In Pb-Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt (OIB)-like lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of emplacement, assimilation of crustal materials.
  
  PubDate: 2012-12-15T09:31:26Z
Trace element compositions of minerals in garnet and spinel periodotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet-spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel periodotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridoties, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser-ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.
  
  PubDate: 2012-12-15T09:31:26Z
Erosion of lithospheric mantle beneath the East African Rift system: geochemical evidence from the Kivu volcanic province
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  This study presents new major and trace element and Sr-Nd isotopic results for a suite of Miocene-Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably < 1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of 87Sr/86Sr, (La/Sm) n , Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of 87Sr/86Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest 87Sr/86Sr (0.70314) and highest εNd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr-Nd-Pb isotopic values among western rift lavas. Among Kivu lavas, variations in 143Nd/144Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints-also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies.
  
  PubDate: 2012-12-15T09:31:26Z
Evidence from mantle xenoliths for relatively thin (<100 km) continental lithosphere below the Phanerozoic crust of southernmost South America
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Garnet peridotite xenoliths in the Quaternary Pali-Aike alkali olivine basalts of southernmost South America are samples of the deeper portion of continental lithosphere formed by accretion along the western margin of Gondwanaland during the Phanerozoic. Core compositions of minerals in garnet peridotites indicate temperatures of 970 to 1160°C between 1.9 and 2.4 GPa, constraining a geothermal gradient which suggests a lithospheric thickness of approximately 100 km below this region. Previously, this lithosphere may have been heated and thinned to 175°C, and thickening, by about 20 km, of the lithosphere is reflected in low-temperature (970°C) peridotites by textural evidence for the transformation of spinel to garnet. A recent heating event, which probably occurred in conjunction with modal metasomatism related to the genesis of the Pali-Aike alkali olivine basalts, has again thinned the lithosphere to 970°C) peridotites as chemical zonation of pyroxenes involving a rimward increase in Ca, and by kelyphitic rims around garnet. The majority of moderate- and high-temperature (>970°C) xenoliths are petrochemically similar to the asthenospheric source of mid-oceanic ridge basalts: fertile (>20% modal clinopyroxene and garnet), Fe-rich garnet lherzolite with major element composition similar to estimates of primitive mantle, but large-ion lithophile and light-rare-earth element depletion relative to heavy-rare-earth elements, and with Sr, Nd, Pb, Os, and O isotopic compositions similar to MORB. In contrast, infertile, Mg-rich spinel harzburgite is predominant among low-temperature (
  PubDate: 2012-12-15T09:31:26Z
Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet-peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet-pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m2. Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (∼40 mW/m2) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.
  
  PubDate: 2012-12-15T09:31:26Z
The age of continental roots
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Determination of the age of the mantle part of continental roots is essential to our understnading of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signature document a history of melt- and fluid-rock interaction within the lithospheric mantle. In some suites of cratonic racks, such as eclogites, Nd and Pb isotopes have been able to trace probale formation ages. The Re—Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proteozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re-Os isotopes, where correlations between Os isotopic composition and Sor Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr−Nd−Pb−Hf−Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.
  
  PubDate: 2012-12-15T09:31:26Z
The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layering in a relatively short time of about 50 Ma. Due to secular cooling mantle differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two different parameterizations of the melting phase-diagram are used in the models. The results indicate that during the Archaean melting occurred on a significant scale in the deep regions of the upper mantle, at pressures in excess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram parameterization used in the model. The results reveal a strong correlation between lateral variations of temperature and the thickness of the continental root. This shows that cold regions in cratons are stabilized by a thick depleted root.
  
  PubDate: 2012-12-15T09:31:26Z
The continental tectosphere and Earth's long-wavelength gravity field
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  To estimate the average density contrast associated with the continental tectosphere, we separately project the degree 2–36 non-hydrostatic geoid and free-air gravity anomalies onto several tectonic regionalizations. Because both the regionalizations and the geoid have distinctly red spectra, we do not use conventional statistical analysis, which is based on the assumption of white spectra. Rather, we utilize, a Monte Carlo approach that incorporates the spectral properties of these fields. These simulations reveal that the undulations of Earth's geoid correlate with surface tectonics no better than they would were it randomly oriented with respect to the surface. However, our simulations indicate that free-air gravity anomalies correlate with surface tectonics better, than almost 98% of our trials in which the free-air gravity anomalies were randomly oriented with respect to Earth's surface. The average geoid anomaly, and free-air gravity anomaly over platforms and shields are significant at slightly better than the one-standard-deviation level: −11±8 m and −4±3 mgal, respectively. After removing from the geoid estimated contributions associated with (1) a simple model of the continental crust and oceanic lithosphere, (2) the lower mantle, (3) subducted slabs, and, (4) remnant glacial isostatic disequilibrium, we estimate a platform and shield signal of −8±4 m. We conclude that there is little contribution of platforms and shields to the gravity field, consistent with their, keels having small density contrasts. Using this estimate of the platform and shield signal, and previous estimates of upper-mantle shear-wave travel-time perturbations, we find that the average value of ϖln ρ/ϖln νs within the 140–440 km depth range is 0.04±0.02. A continental tectosphere with an isopycnic (equal-density) structure (ϖln ρ/ϖln νs=0) enforced by compositional variations is consistent with this result at the 2.0σ level. Without compositional buoyancy, the continental tectosphere would have an average ϖlnρ/ϖlnνs≈0.25, exceeding our estimate by 10σ.
  
  PubDate: 2012-12-15T09:31:26Z
Stability and dynamics of the continental tectosphere
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Continental cratons overlie thick, high viscosity, thermal and chemical boundary layers, where the chemical boundary layers are less dense than they would be due to thermal effects alone, perhaps because they are depleted in basaltic constituents. If the continental tectosphere is the same age as the overlying Archaean curst, then the continental tectosphere must be able to survive for several billion years without undergoing a convective instability, despite being both cold and thick. Sine platforms and shields correlate only weakly with Earth's gravity and geoid anomalies, acceptable models of the continental tectosphere must also satisfy this gravity constraint. We investigate the long-term stability of the continental tectosphere by carrying out a number of numerical convection experiments within a two-dimensional Cartesian domain. We initiate our experiments with a tectosphere (thermal and chemical boundary layers) immersed in a region of uniform composition, temperature, and viscosity, and consider the effects on the stability of the tectosphere of (1) activation energy (used to define the temperature dependence of viscosity), (2) compositional buoyancy, and (3) linear or non-linear rheology. The large lateral thermal gradients required to match oceanic and tectosphere structures initiate the dominant instability, a “drip” which develps at the side of the tectosphere and moves to beneath its center. High activation energies and high background viscosities restrict the amount and rate of entrainment. Compositional buoyancy does not significantly change the flow pattern. Rather, compositional buoyancy slows the destruction process somewhat and reduces the stress within the tectosphere. With a non-Newtonian rheology, this reduction in stress helps to stiffen the tectosphere. In these experiments, dynamical systems that adequately model the present ocean-contient structures have activation energy E *≥180 kJ mole−1 —a value about one third the estimate of activation energy for olivine, E *≈520 kJ mole−1. Although for E *≈520 kJ mole−1, compositional buoyancy is not required for the tectosphere to survive, the joint application of longevity and gravity constraints allows us to reject all models not containing compositional buoyancy, and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity.
  
  PubDate: 2012-12-15T09:31:26Z
The thermal structure and thickness of continental roots
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  We compare heat flow data from the Precambrian shields in North America and in South Africa. We also review data available in other less well-sampled Shield regions. Variations in crustal heat production account for most of the variability of the heat flow. Because of this variability, it is difficult to define a single average crustal model representative of a whole tectonic province. The average heat flow values of different Archean provinces in Canada, South Africa, Australia and India differ by significant amounts. This is also true for Proterozoic provinces. For example, the heat flow is significantly higher in the Proterozoic Namaqua-Natal Belt of South Africa than in the Grenville Province of the Canadian Shield (61 vs. 41 mW m−2 on average). These observations indicate that it is not possible to define single value of the average heat flow for all provinces of the same crustal age. Large amplitude short wavelength variations of the heat flow suggest that most of the difference between Proterozoic and Archean heat flow is of crustal origin. In eastern Canada, there is no good correlation between the local values of heat flow and heat production. In the Archean, Proterozoic and Paleozoic provinces of eastern Canada, heat flow values through rocks with the same heat production are not significantly different. There is therefore no evidence for variations of the mantle heat flow beneath these different provinces. After removing the local crustal heat production from the surface heat flow, the mantle (Moho) heat flow was estimated to be between 10–15 mW m−2 in the Archean, Proterozoic and Paleozoic provinces of eastern Canada. Estimates of the mantle heat flow in the Kaapvaal craton of South Africa may be slightly higher (≈17 mW m−2). Large-scale variations of bulk crustal heat production are well-documented in Canada and imply significant differences of deep lithospheric thermal structure. In thick lithosphere, surficial heart flow measurements record a time average of heat production in the lithospheric mantle and are not in equilibrium with the instantaneous heat production. The low mantle heat flow and current estimates of heat production in the lithospheric mantle do not support a mechanical (conductive) lithosphere thinner than 200 km and thicker than 330 km. Temperature anomalies with surrounding oceanic mantle extended to the convective boundary layer belong the conductive layer, and hence to depths greater than these estimates. Mechanical and thermal stability of the lithosphere require the mantle part of the lithosphere to be chemically buoyant and depleted in radiogenic elements. Both characteristics are achieved simultaneously by partial melting and melt extraction.
  
  PubDate: 2012-12-15T09:31:26Z
Heat flow and the structure of Precambrian lithosphere
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Studies of heat flow from Precambrian terrians have demonstrated three empirical relationships; a temporal relationship between heat flow and tectonic age, a spatial pattern between heat flow and the proximity of Archean cratons, and a temporal relationship between heat flow and the age of lithosphere stabilization. In the first relationship, heat flow is inversely related to tectonic age. The second pattern is characterized by low heat flow from Archean cratons and Proterozoic terrains adjacent to cratonic margins (pericratonic terrains), and higher heat flow from Proterozoic terrains that are more than a few hundred kilometers from a craton. In the third pattern, heat flow decreases as the age of stabilization of the lithosphere increases. A number of interpretations of Precambrian heat flow have been offered to explain one or more of these relationships. The simple cooling of a thermal boundary layer predicts essentially no change in heat flow in terrains older than ≈1.5 Ga, and therefore does not likely provide a comprehensive framework for the interpretation of Precambrian heat flow. By contrast, two other interpretations, (1) thicker lithosphere beneath Archean terrains than beneath Proterozoic terrains, and (2) greater heat production in Proterozoic crust than in Archean crust, when combined with the special structural configuration of sutures, can both contribute to the spatial and temporal heat flow distributions. Xenolith thermobarometry constraints on lithospheric temperatures, however, limit the contribution of age-dependent crustal heat production, and therefore at least part of the heat flow distributions derive from variations in lithosphere thickness.
  
  PubDate: 2012-12-15T09:31:26Z
Velocity structure of the continental upper mantle: evidence from southern Africa
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  The velocity model for southern Africa of Qiu et al. [Qiu, X., Priestley, K., McKenzie, D., 1996. Average lithospheric structure of southern Africa. Geophys. J. Int. 127, 563–587] is revised so as to satisfy both the regional seismic waveform data and the fundamental mode Rayleigh wave phase velocity data for the region. The revised S-wave model is similar to the original model of Qiu et al. except that the high velocity, upper mantle lid extends to 160 km depth in the revised model rather than to 120 km in the original model. Sensitivity tests of the regional seismic data show that the minimum velocity in the S-wave low velocity zone can be as high as 4.45 km s−1 compared to 4.32 km s−1 in the Qiu et al. model. The vertical S-wave travel time for the revised south African model is compared with the vertical S-wave travel times for the global tomographic models S12WM13 and S16B30, and they are found to be similar
  
  PubDate: 2012-12-15T09:31:26Z
Imaging the continental upper mantle using electromagnetic methods
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  The internal structure of the continental lithosphere holds the key to its creation and development, and this internal structure can be determined using appropriate seismic and electromagnetic methods. These two are complementary in that the seismic parameters usually represent bulk properties of the rock, whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix, such as the presence of a conducting mineral phase, e.g. carbon in graphite form, or of a fluid phase, e.g. partial melt or volatiles. In particular, conductivity is especially sensitive to the top of the asthenosphere, generally considered to be a region of interconnected partial melt. Knowledge of the geometry of the lithosphere/asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create, modify, and destroy the lithosphere. Accordingly, collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own. This paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural-source magnetotelluric technique, and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep-probing electromagnetic surveying. The resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust. Examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere-asthenosphere boundary. Also, the observations from laboratory measurements on candidate mineralogies representative of the mantle, such as olivine, are presented.
  
  PubDate: 2012-12-15T09:31:26Z
The deep structure of the Australian continent from surface wave tomography
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  We present a new model of 3-D variations of shear wave speed in the Australian upper mantle, obtained from the dispersion of fundamental and higher-mode surface waves. We used nearly 1600 Rayleigh wave data from the portable arrays of the Skippy project and from permanent stations (from Agso, Iris and Geoscope). AgSo data have not been used before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we improved the vertical parameterization, the weighting scheme that accounts for variations in data quality and reduced the influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained. Global compilations of geological and seismological data (using regionalization based on tectonic behavior or crustal age) suggest a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the tectonic history of crustal elements vary on wavelengths, much smaller than have been resolved with global seismological studies. Using our regional upper mantle model we investigate how the seismic signature of tectonic units changes with increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in the Archean cratons. The high wave speeds associated with Precambrian shields extend beyond the Tasman Line, which marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into Papua New Guinea and beneath the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals that some tectonic subregions of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and, surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian cratons that lack a thick high wave speed “keel” are located near passive margins, suggesting that convective processes associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep lithospheric structure varies as much within domains of similar-crustal age as between units of different ages, which hampers attempts to find a unifying relationship between seismic signature and lithospheric age.
  
  PubDate: 2012-12-15T09:31:26Z
Seismic imaging of lithospheric discontinuities and continental evolution
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  Discontinuities in physical properties within the continental lithosphere reflect a range of processes that have contributed to craton stabilization and evolution. A survey of recent seismological studies concerning lithospheric discontinuities is made in an attempt to document their essential characteristics. Results from long-period seismology are inconsistent with the presence of continuous, laterally invariant, isotropic boundaries within the upper mantle at the global scale. At regional scales, two well-defined interfaces termed H (≈60 km depth) and L (≈ 200 km depth) of continental affinity are identified, with the latter boundary generally exhibiting an anisotropic character. Long-range refraction profiles- are frequently characterized by subcontinental mantle that exhibits a complex stratification within the top 200 km. The shallow layering of this package can behave as an imperfect waveguide giving rise to the so-called teleseismic Pn phase, while the L-discontinuity may define its lower base as the culminations of a low velocity zone. High-resolution, seismic reflection profiling provides sufficient detail in a number of cases to document the merging of mantle-interfaces into lower continental crust below former collisional sutures and magmatic arcs, thus unambiguously identifying some lithospheric discontinuities with thrust faults and subgducted oceanic lithosphere. Collectively, there and other seismic observation point to a continental lithosphere whose internal structure is dominated by a laterally variable, subhorizontal layering. This stratigraphy appears to be more pronounced at shallower lithopheric levels, includes dense, anisotropic layers of order 10 km in thickness, and exhibits horizontal correlation lengths comparable to the lateral dimensions of overlying crustal blocks. A model of craton evolution which relies on shallow subduction as a principal agent of craton stabilization is shown to be broadly compatible with these characteristics.
  
  PubDate: 2012-12-15T09:31:26Z
Preface Composition, deep structure and evolution of continents
- Abstract: 1999
  Publication year: 1999
  Source:Developments in Geotectonics, Volume 24
  
  PubDate: 2012-12-15T09:31:26Z
Subject Index
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 14 Continental rifting: A final perspective
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 13 West and central african rift system
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 12 The southern oklahoma aulacogen
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 11 Rifted passive margins
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 10 The midcontinent rift system, u.s.a.: A major proterozoic continental rift
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 9 The oslo rift
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 8 The baikal rift system
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 7 The basin and range province
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 6 The Rio Grande Rift
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 3D Heat flow in rifts
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 3e Practical magnetotellurics in a continental rift environment
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 4 The european cenozoic rift system
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 5 The east african rift system
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 3C Potential field methods
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 3B Seismic techniques
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 1 Introduction: Progress in understanding continental rifts
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 2 Mechanisms of rifting: Geodynamic modeling of continental rift systems
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Chapter 3A Petrology, geochemistry, isotopes
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
Preface
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
The CREST international research group
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
List of collaborating authors
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
List of peer reviewers
- Abstract: 2006
  Publication year: 2006
  Source:Developments in Geotectonics, Volume 25
  
  PubDate: 2012-12-15T09:31:26Z
The Geodynamic Polygon of Santiago De Cuba: A Scientific and Socio-Economic Perspective
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  Since 1971 the Cuban Institute of Geodesy and Cartography and the Institutes of Geography corresponding to the Academy of Sciences from Cuba and the U.S.S.R. have been investigating recent geodynamic processes in Cuban territory, applying geodesy, geologic and geomorphologic methods. In the eastern region the correlation of morpho-structural characteristics with the velocities of recent vertical crustal movements, the results of seismologic observations and the socio-economic importance of the region Santiago de Cuba, led to the necessity for the establishment of a geodynamic polygon, for the study in complex and integral form of the relation between recent vertical movements and seismicity, to assist in the prediction of earthquakes. This paper shows the results in the preliminary profiles corresponding to relative velocities of vertical crustal movements for the most important first and second order nivelation routed in the studied area, the territorial distribution of the seismological events in the southeastern region, as well as judgments in the density of epicentral distribution; it explains the proposed altimetric observation net for a geodynamic polygon and the complex system of geoscientific investigations which must be carried out.
  
  PubDate: 2012-12-15T09:31:26Z
Releveling Evidence for Crustal Deformation in the United States
- Abstract: 1983
  Publication year: 1983
  Source:Developments in Geotectonics, Volume 20
  
  We have reduced and analyzed over 90% of existing U.S. National Geodetic Survey releveling observations, in order to evaluate the implications of these measurements for contemporary crustal movements. The data are displayed both in profile form (movement versus distance along route) and, where adequate coverage is available, as maps of apparent elevation change. Our analysis involves: (1) evaluating the reliability of the leveling measurements using modified geodetic checks as well as through comparison with independent measurements of crustal movement (e.g., tide and lake level measurements, seismological evidence, tilt and strain observations); (2) interpreting what appear to be real movements in light of other geological and geophysical information on crustal dynamics. Empirical analysis of the U.S. releveling data base indicates that apparent tilts of less than 3 microradians are pervasive and must be regarded as typical signal levels and/or as typical levels of systematic error. Systematic leveling errors are suggested in some cases by close correlations between apparent elevation change and topography, disagreements between leveling and tide gauge measurements, and internal inconsistencies in leveling circuits. Apparent movements which correlate with topography are observed in many areas of the U.S. Such apparent movements often reach 35 mm per 100 m of relief. Disagreements between leveling and tide gauge measurements along the east and west coasts and internal inconsistencies in leveling circuits suggest errors reaching 1 mm/km which can accumulate monotonically for distances exceeding 1000 km. Regional maps of apparent elevation change for the Eastern U.S. show serious inconsistencies with similar maps for adjacent portions of Canada suggesting that such maps may be more indicative of systematic leveling errors than real crustal movements. The largest signals (apparent tilts greater than 4.5 microradians) are for the most part associated with: (1) coseismic and postseismic movements of major (M > 6) dip-slip earthquakes, (2) regions of contemporary magmatic activity, (3) subsidence due to fluid withdrawal, and (4) topography-correlated apparent movements. Coseismic movements reported for eleven U.S. earthquakes are roughly consistent with simple models of elastic rebound. At least some postseismic movements appear consistent with after-slip on the fault or an extension of the fault that ruptured during the earthquake, although other explanations have been proposed (e.g., viscoelastic relaxation of the lithosphere-asthenosphere). Deformations possibly associated with contemporary magmatic processes are not restricted to volcanically active areas (e.g. Hawaii) having been observed in the Rio Grande rift and in Yellowstone National Park as well. Subsidence due to fluid withdrawal, well known in areas of intense pumping, is considerably more widespread than previously reported occuring throughout much of the Atlantic and Gulf coastal plains, along the Mississippi Valley and within many smaller sedimentary basins which have been exploited for groundwater. The association of many of the largest signals with real surface movements further demonstrates the ability of historic leveling to detect relatively subtle deformation.
  
  PubDate: 2012-12-15T09:31:26Z