Solid Earth Discussions

Published by Copernicus Publications
Online ISSN: 1869-9537
The South Atlantic rift basin evolved as branch of a large Jurassic-Cretaceous intraplate rift zone between the African and South American plates during the final breakup of western Gondwana. By quantitatively accounting for crustal deformation in the Central and West African rift zone, we indirectly construct the kinematic history of the pre-breakup evolution of the conjugate West African-Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic Pre-salt sag basin and the S\~ao Paulo High. We model an initial E-W directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities until Upper Hauterivian times ($\approx$126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial $\approx$17 Myr-long stretching episode the Pre-salt basin width on the conjugate Brazilian and West African margins is generated. An intermediate stage between 126.57 Ma and Base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE-SW. Final breakup between South America and Africa occurred in the conjugate Santos--Benguela margin segment at around 113 Ma and in the Equatorial Atlantic domain between the Ghanaian Ridge and the Piau\'i-Cear\'a margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of this conjugate passive margins systems and can explain the first order tectonic structures along the South Atlantic and possibly other passive margins.
(a) Measurement data were averaged for eruptive and non-eruptive periods. The BrO/SO 2 mean is plotted as function of time. Eruptive data are presented as red columns and non eruptive data as blue columns. Although this is only a small statistical approach, it demonstrate that during eruptions lower BrO/SO 2 ratios have been measured. (b) Additionally to (a) mean BrO/SO 2 values of the third month before the eruption (violet columns) and mean BrO/SO 2 values of the month just before the eruption (magenta columns) have been compared for the four eruptive events described in the text. Clear enhanced mean BrO/SO 2 values are determined for the third month before the eruption and smaller mean BrO/SO 2 values the month before the eruption see also Table 1.
Sketch of the empirical model, for detailed information see text. (a) In contrast to chlorine and fluorine, we assume that bromine is less soluble in the magmatic melt than sulphur. The mean value of non-eruptive periods during 2006-2009 is around 2 × 10 −4. (b) Assuming bromine is less soluble in magmatic melt than sulphur, bromine starts to be released earlier in comparison to sulphur when fresh magma is rising up − > maxima BrO/SO 2 ratios rise over 3 × 10 −4. The mean value is around 2.3 × 10 −4. in month three before the 2006 and 2008 eruptions as well as in month three and two before the two violent lava fountaining events in 2007. (c) When magma is rising further, sulphur starts to be released and might overtake bromine; BrO/SO 2 ratios decrease (SO 2 fluxes should increase). The mean BrO/SO 2 value of the last month before the observed eruption is 1.8 × 10 −4. (d) Further decrease-no further magma rise from below, melt already depleted in bromine, sulphur out-gasing is determining the ratio. The lowest BrO/SO 2 value during eruptions observed was 0.47 × 10 −4. The mean BrO/SO 2 ratio was calculated to 1.3 × 10 −4. 505
Summary of mean BrO/SO 2 values during the various time periods as described in the text. The table also displays the number of data points available for the various time frames.
Over a three year period, from 2006 to 2009, frequent scattered sun light DOAS measurements were conducted at Mt. Etna in a distance of around six kilometres downwind from the summit craters. During the same period and in addition to these measurements, volcanic observations were made by regularly visiting various parts of Mt. Etna. Results from these measurements and observations are presented and their relation is discussed. The focus of the investigation is the bromine monoxide/sulphur dioxide (BrO/SO<sub>2</sub>) ratio, and its variability in relation to volcanic processes. That the halogen/sulphur ratio can serve as a precursor or indicator for the onset of eruptive activity was already proposed by earlier works (e.g. Noguchi and Kamiya, 1963; Menyailov, 1975; Pennisi and Cloarec, 1998; Aiuppa, 2002). However, there is still a limited understanding today because of the complexity with which halogens are released, depending on magma composition and degassing conditions. Our understanding of these processes is far from complete, for example of the rate and mechanism of bubble nucleation, growth and ascent in silicate melts (Carroll and Holloway, 1994), the halogen vapour-melt partitioning and the volatile diffusivity in the melt (Aiuppa et al., 2009). With this study we aim to add one more piece to the puzzle of what halogen/sulphur ratios might tell about volcanic activities. Our data set shows an increase of the BrO/SO<sub>2</sub> ratio several weeks prior to an eruption, followed by a decline before and during the initial phase of eruptive activities. Towards the end of activity or short afterwards, the ratio increases to baseline values again and remains more or less constant during quiet phases. To explain the observed evolution of the BrO/SO<sub>2</sub> ratio, a first empirical model is proposed. This model suggests that bromine, unlike chlorine and fluorine, is less soluble in the magmatic melt than sulphur. By using the DOAS method to determine SO<sub>2</sub>, we actually observe most of the emitted sulphur of Mt. Etna. Regarding bromine however, we are aware that by determining only the bromine monoxide (BrO) radical we might just observe a small or even a variable fraction of the total emitted bromine. Therefore we present first studies to justify the assumption that despite the disadvantage just mentioned, the BrO/SO<sub>2</sub> ratio can nevertheless serve as a new parameter to indicate the state of a volcano, when measurements are conducted under certain, but rather convenient conditions.
After 1.5 ms extreme velocity increment causing the 28 th March 2005 earthquake, the Sumatra region had been quaked as the resonance on 13.66 (1/2 of Moon's sidereal period) LOD variations with 2 days delay in all three occurring earthquakes. Earthquakes M>5.8 are marked by triangle.
In contrast to unsuccessful searching (lasting over 150 years) for correlation of earthquakes with biweekly tides, the author found correlation of earthquakes with sidereal 13.66 days Earth's rotation variations expressed as length of a day (LOD) measured daily by International Earth's Rotation Service. After short mention about earthquakes M 8.8 Denali Fault Alaska 3 November 2002 triggered on LOD maximum and M 9.1 Great Sumatra earthquake 26 December 2004 triggered on LOD minimum and the full Moon, the main object of this paper are earthquakes of period 2010-June 2011: M 7.0 Haiti (12 January 2010 on LOD minimum, M 8.8 Maule Chile 12 February 2010 on LOD maximum, map constructed on the Indian plate revealing 6 earthquakes from 7 on LOD minimum in Sumatra and Andaman Sea region, M 7.1 New Zealand Christchurch 9 September 2010 on LOD minimum and M 6.3 Christchurch 21 February 2011 on LOD maximum, and M 9.1 Japan near coast of Honshu 11 March 2011 on LOD minimum. It was found that LOD minimums coincide with full or new Moon only twice in a year in solstices. To prove that determined coincidences of earthquakes and LOD extremes stated above are not accidental events, histograms were constructed of earthquake occurrences and their position on LOD graph deeply in the past, in some cases from the time the IERS (International Earth's Rotation Service) started to measure the Earth's rotation variations in 1962. Evaluations of histograms and the Schuster's test have proven that majority of earthquakes are triggered in both Earth's rotation deceleration and acceleration. Because during these coincidences evident movements of lithosphere occur, among others measured by GPS, it is concluded that Earth's rotation variations effectively contribute to the lithospheric plates movement. Retrospective overview of past earthquakes revealed that the Great Sumatra earthquake 26 December 2004 had its equivalent in the shape of LOD graph, full Moon position, and character of aftershocks 19 years earlier in difference only one day to 27 December 1985 earthquake, proving that not only sidereal 13.66 days variations but also that the 19 years Metons cycle is the period of the earthquakes occurrence. Histograms show the regular change of earthquake positions on branches of LOD graph and also the shape of histogram and number of earthquakes on LOD branches from the mid-ocean ridge can show which side of the ridge moves quicker.
High resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent low-velocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future.
This is the discussion manuscript. For the final peer-reviewed and published version, please see here: . ... *** The eruption that started off the south coast of El Hierro, Canary Islands, in October 2011 has emitted intriguing eruption products found floating in the sea. These specimens appeared as floating volcanic "bombs" that have in the meantime been termed "restingolites" (after the close-by village of La Restinga) and exhibit cores of white and porous pumice-like material. Currently the nature and origin of these "floating stones" is vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have collected and analysed the structure and composition of samples and compared the results to previous work on similar rocks found in the archipelago. Based on their high silica content, the lack of igneous trace element signatures, and the presence of remnant quartz crystals, jasper fragments and carbonate relicts, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary rocks that were picked up and heated by the ascending magma causing them to partially melt and vesiculate. They hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies in the Canary Islands as well as in similar Atlantic islands that rest on sediment/covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of these "restingolites" does therefore not indicate the presence of an explosive high-silica magma that is involved in the ongoing eruption.
a) Overview Map. The white rectangle encloses the extent of the 3D structural 2 model. (b) Superimposed Bathymetry/topography (IBCAO 3.0; Jakobsson et al., 2012) 3
Stratigraphic megasequences resolved in the 3D structural model in relation 2 to regional tectonic events. Oldest sedimentes are preserved in the northern Kara Sea 3 Stratigraphic megasequences resolved in the 3-D structural model in relation to regional tectonic events. Oldest sediments are preserved in the northern Kara Sea whereas Cenozoic deposits are present only in the southwesternmost Barents Sea and in the oceanic domain.
Depth to the four modelled megasequence boundaries: (a) Earliest Eocene (b) MidCretaceous (c) Mid-Jurassic (d) Mid-Permian. The grey lines delineate structural features in the study area (for legend see Fig. 1b). The Earliest Eocene surface equals the Top Crystalline Basement in the oceanic domain. See Table A1 for database.
Thickness distribution of the megasequences and the depth to the top 3 crystalline crust: (a) Earliest Eocene-Present; (b)Mid-Cretaceous-Paleocene; (c) Mid4 Jurassic-Mid-Cretaceous; (d) Mid-Permian-Mid-Jurassic; (e) Pre-mid-Permian; (f) 5 Depth Top Crystalline Basement. The grey lines delineate structural features in the 6 study area (for legend see Figure 1b). See Appendix A for database. 7 8 Thickness distribution of the megasequences and the depth to the top crystalline crust: (a) Earliest Eocene-Present; (b) Mid-Cretaceous-Paleocene; (c) Mid-Jurassic-MidCretaceous; (d) Mid-Permian-Mid-Jurassic; (e) Pre-mid-Permian; (f) Depth Top Crystalline Basement. The grey lines delineate structural features in the study area (for legend see Fig. 1b). See Table A1 for database.
Structure of the deeper crust and the upper mantle: (a) depth to the Moho; (b) thickness of the crystalline crust; (c) depth to the lithosphere-asthenosphere boundary (LAB); (d) thickness of the lithospheric mantle.
The Barents Sea and Kara Sea region is located in the European Arctic. Due to its hydrocarbon potential this region is in focus of an increasing number of scientific and commercial studies. Although these studies mostly range from reservoir to sub-basin scale and thus have contributed to the knowledge about the local and specific tectonic and sedimentary histories of the corresponding subregions, there have been limited attempts to understand the region as a whole. We integrate multi-disciplinary data including well information, interpreted refraction and reflection seismic data and previously published 3D- models into a lithosphere-scale 3D-structural model. This model resolves the following surfaces: topography and seafloor, base Cenozoic, mid-Cretaceous, base Cretaceous, intra Permian, intra Carboniferous, top crystalline basement, Moho and lithosphere-asthenosphere boundary. Each corresponding layer is characterised by rock physical properties such as porosity and density. To analyse the density configuration of the subsedimentary lithosphere, in particular the distribution of high-density bodies, we perform 3D gravity modelling. The presented model provides an ideal base for further studies on the present-day thermal field or the geodynamic evolution of this region.
The Upper Pliensbachian-Toarcian (Jurassic) sedimentological, palaeontological and geochemical (belemnite 87Sr/86Sr, δ13C and δ18O) record is examined in two Eastern Tethyan (Bulgarian) locations. This interval contains the well-known Early Toarcian ocean anoxic event (T-OAE) and its manifestation and temporal context is examined in Bulgaria. Many of the features characteristic for the SW European sections were identified: collapse of carbonate platform productivity at the Pliensbachian/Toarcian boundary, the T-OAE (a short pulse of anoxic deposition in the Falciferum ammonite Zone), an Early Toarcian rapid warming event seen in the belemnite δ18O record that peaked around the Falciferum/Bifrons ammonite zonal boundary. The long-recognized positive δ13C excursion in the late Falciferum ammonite Zone is also seen but a precursor, sharp δ13C negative excursion seen around the Tenuicostatum/Falciferum ammonite zonal boundary in many organic carbon records is not evident in the belemnite data, a curious absence noted from other belemnite records. Subsequent fluctuations of the 87Sr/86Sr, δ13C and δ18O suggest there may be a further perturbation of the global isotopic systems. On the other hand, belemnite Sr isotope values from Bulgaria are in accord with those seen in Western Europe and hence its value for chronostratigraphy.
" Pole-2 " instrument setup at observatory proof ground.  
Leveled surfaces of electric field intensity within the area of sensor installation.
Plot of field pattern during lightning discharge on 2 October 2008: (a) electric field potential gradient in the near ground air; (b) air electroconductivity caused by 1-negative ions and 2-positive ions with factor (−1); (c) conduction current density.
Acoustic emission plots in seven frequency ranges and potential gradient of electric field intensity in the near ground air (in the bottom) during the lightning discharge on 2 October 2008.
The effect of a single lightning discharge on electric field intensity in the near ground atmosphere was investigated. The effect appeared as a sharp fall of electric field potential gradient from 80 V m-1 up to -21 V m-1. The process of intensity recovery is described by flat capacitor model with characteristic time of recovery of 17 c. Simultaneously with electric field, the acoustic emission response in the near surface rocks on lightning discharge was registered in the frequency range of 6.5-11 kHz.
The island of Seram, which lies in the northern part of the 180°-curved Banda Arc, has previously been interpreted as a fold-and-thrust belt formed during arc-continent collision, which incorporates ophiolites intruded by granites thought to have been produced by anatexis within a metamorphic "sole". However, new geological mapping and a re-examination of the field relations cause us to question this model. We instead propose that there is evidence for recent N-S extension that has caused the high-temperature exhumation of hot mantle peridotites, granites, and granulites (the "Kobipoto Complex") beneath low-angle lithospheric detachment faults. Greenschist- to lower-amphibolite facies metapelites and amphibolites of the Tehoru Formation, which comprise the hanging wall above the detachment faults, were overprinted by sillimanite-grade metamorphism, migmatisation and limited localised diatexis to form the Taunusa Complex. Highly aluminous metapelitic garnet + cordierite + sillimanite + spinel + corundum + quartz granulites exposed in the Kobipoto Mountains (central Seram) are intimately associated with the peridotites. Spinel + quartz inclusions in garnet, which indicate that peak metamorphic temperatures for the granulites likely approached 900 °C, confirm that peridotite was juxtaposed against the crust at typical lithospheric mantle temperatures and could not have been part of a cooled ophiolite. Some granulites experienced slight metatexis, but the majority underwent more advanced in situ anatexis to produce widespread granitic diatexites characterised by abundant cordierite and garnet xenocrysts and numerous restitic sillimanite + spinel "clots". These Mio-Pliocene "cordierite granites", which are present throughout Ambon, western Seram, and the Kobipoto Mountains in direct association with peridotites, demonstrate that the extreme extension required to have driven Kobipoto Complex exhumation must have occurred along much of the northern Banda Arc. In central Seram, smeared lenses of peridotites are incorporated with a major left-lateral strike-slip shear zone (the "Kawa Shear Zone"), demonstrating that strike-slip motions likely initiated shortly after the mantle had been partly exhumed by detachment faulting and that the main strike-slip faults may themselves be reactivated and steepened low-angle detachments. The Kobipoto Mountains represent a left-lateral pop-up structure that has facilitated the final stages of exhumation of the high-grade Kobipoto Complex through overlying Mesozoic sedimentary rocks. On Ambon, Quaternary "ambonites" (cordierite + garnet dacites) are evidently the volcanic equivalent of the cordierite granites as they also contain granulite-inherited xenoliths and xenocrysts. The geodynamic driver for mantle exhumation along the detachment faults and strike-slip faulting in central Seram is very likely the same - we interpret the extreme extension to be the result of eastward slab rollback into the Banda Embayment as outlined by the latest plate reconstructions for Banda Arc evolution.
New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large dataset enabled us to accurately estimate the Moho of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was clearly seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30-43 km) with a maximum thickness of about 48 km beneath the Peloponnesus Peninsula, whereas a thinner crust of about 27-30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean (Moho depth 23-27 km). Moreover, P receiver functions significantly imaged the subducted African Moho as a strong converted phase down to a depth of 180 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting reduced dehydration and nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along 8 profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocenter locations of temporary networks within the Aegean. P receiver function profiles significantly revealed in good agreement with the seismicity a low dip angle slab segment down to 200 km depth in the west. Even though, the African slab seems to be steeper in the eastern Aegean and can be followed down to 300 km depth implying lower temperatures and delayed dehydration towards larger depths in the eastern slab segment. Our results showed that the transition between the western and eastern slab segments is located beneath the southeastern Aegean crossing eastern Crete and the Karpathos basin. High resolution P receiver functions also clearly resolved the top of a strong low velocity zone (LVZ) at about 60 km depth. This LVZ is interpreted as asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus the Aegean mantle lithosphere seems to be 30-40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.
Five crises affecting ammonite evolution occurred during the Late Pliensbachian to Late Toarcian stages (Early Jurassic). The first two (Gibbosus and end-Spinatum zones) occurred during highly cold and regressive conditions which were followed by a global anoxic event generated during a supergreenhouse warm event (Levisoni subzone) concomitant with a worldwide transgressive event. The last two (Late Variabilis and Late Insigne zones) are related to regressive events. We present new carbon isotope data from Southern Peru that demonstrate that most of the major Toarcian crises affecting ammonite evolution can be correlated with the variations affecting the δ13C during that unstable period. The morphogenetic reactions of the ammonites during the five different upper Liassic critical episodes are analysed, showing how this group can be used as stress indicators.
Data: direction of dip for (a) the Sherwood Sandstone group (90 data) and (b) the Windermere Supergroup (572 data); orientations (doubled from the original range of [0, π ]) for (c) Bangladesh anticline axial planes (32 data) and (d) Bangladesh Landsat-derived lineaments (40 data). Note that segments of the rose diagrams are proportional to relative frequency within each data set separately, so are not comparable between data sets with respect to numbers of observations. If all data appeared within a single bin of the rose diagram then the corresponding segment would be equal in length to the radius of the circle. 
Selection of a mixture of von Mises (MVM) distributions.
Density of projected normal distributions fitted to combined Bangladesh data and wrapped around the circle. (a) Model 1, with all parameters pooled for the combined data set; (b) Model 2, with µ modelled separately for anticline axial planes and Landsat lineaments; (c) Model 3, with all parameters separate for anticline axial planes and Landsat lineaments. 
Comparison of a mixture of von Mises (MVM) with g components and projected normal (PN) distributions.
Angular data are commonly encountered in the earth sciences and statistical descriptions and inferences about such data are necessary in structural geology. In this paper we compare two statistical distributions appropriate for complex angular data sets: the mixture of von Mises and the projected normal distribution. We show how the number of components in a mixture of von Mises distribution may be chosen, and how one may chose between the projected normal distribution and mixture of von Mises for a particular data set. We illustrate these methods with some structural geological data, showing how the fitted models can complement geological interpretation and permit statistical inference. One of our data sets suggests a special case of the projected normal distribution which we discuss briefly.
The difficulties of seismic imaging beneath high velocity structures are widely recognised. In this setting, theoretical analysis of synthetic wide-angle seismic reflection data indicates that velocity models are not well constrained. A two-dimensional velocity model was built to simulate a simplified structural geometry given by a basaltic wedge placed within a sedimentary sequence. This model reproduces the geological setting in areas of special interest for the oil industry as the Faroe-Shetland Basin. A wide-angle synthetic dataset was calculated on this model using an elastic finite difference scheme. This dataset provided travel times for tomographic inversions. Results show that the original model can not be completely resolved without considering additional information. The resolution of nonlinear inversions lacks a functional mathematical relationship, therefore, statistical approaches are required. Stochastical tests based on Metropolis techniques support the need of additional information to properly resolve subbasalt structures.
Three-dimensional X-ray microtomographic rendering of natural sample EFJ080510b; a plagioclase crystal (indicated by the arrow), which is rare in products of this eruption that had less than 2 % crystallinity (Sigmundsson et al., 2010), can clearly be seen in the sample. The long dimension of this sample is 1.06 cm. 
We studied three-dimensional (3-D) vesicle size distributions by X-ray microtomography in scoria collected during the relatively quiescent Phase II of the 2010 eruption at Eyjafjallajökull volcano, Iceland. Our goal was to compare the vesicle size distributions (VSDs) measured in these samples with those found in Stromboli volcano, Italy. Stromboli was chosen because its VSDs are well-characterized and show a correlation with eruption intensity: typical Strombolian activity produces VSDs with power-law exponents near 1, whereas larger and more energetic Vulcanian-type explosions and Plinian eruptions produce VSDs with power-law exponents near 1.5. The hypothesis to be tested was whether or not the samples studied in this work would contain VSDs similar to normal Strombolian products, display higher power-law exponents, or be described by exponential functions. Before making this comparison we tested the hypothesis that the phreatomagmatic nature of the Eyjafjallajökull eruption might have a significant effect on the VSDs. We performed 1 atm bubble-growth experiments in which the samples were inundated with water and compared them to similar, control, experiments without water inundation. No significant differences between the VSDs of the two sets of experiments were found, and the hypothesis is not supported by the experimental evidence; therefore, VSDs of magmatic and phreatomagmatic eruptions can be directly compared. The Phase II Eyjafjallajökull VSDs are described by power law exponents of ~ 0.8, typical of normal Strombolian eruptions. The comparable VSDs and behavior of Phase II of the Eyjafjallajökull 2010 eruption to Stromboli are interpreted to be a reflection of similar conduit systems in both volcanoes that are being constantly fed by the ascent of deep magma that mixes with resident magma at shallow depths. Such behavior implies that continued activity during Phase II of the Eyjafjallajökull eruption could be expected and would have been predicted, had our VSDs been measured in real time during the eruption. However, the products studied show no peculiar feature that could herald renewed eruption intensity observed in the following Phase III of the eruption.
The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70-80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered midlithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).
Geographic distribution of heat flow measurements, as per the updated data base reported by Vieira and Hamza (2010).
Heat flow corrected for contributions of radiogenic heat at the base of sedimentary layers (upper panel), base of upper crust (middle panel) and base of lower crust (lower panel). Note that near surface heat flow (upper panel) provides very little clues as to the contrasts in the deep thermal structures of continental and oceanic regions. As the contributions of radiogenic heat of upper and lower crustal layers are progressively removed the differences in deep heat flux between these regions becomes evident (middle and lower panels).
New global maps of the depth to the boundary between the lithosphere and the asthenosphere are presented. The maps are based on updated global databases for heat flow and crustal structure. For continental regions the estimates of lithospheric thickness are based on determinations of subcrustal heat flow, after corrections for contributions of radiogenic heat in crustal layers. For oceanic regions the estimates of lithospheric thickness are based on the newly proposed finite half-space (FHS) model. Unlike the half-space cooling (HSC) and the Plate models the FHS model takes into account effects of buffered solidification at the lower boundary of the lithosphere and assumes that vertical domain for downward growth of boundary layer have an asymptotic limit. Results of numerical simulations reveal that theoretical values derived from FHS model provide vastly improved fits to observational data for heat flow and bathymetry than can be achieved with HSC and Plate models. Also, the data fits are valid for the entire age range of the oceanic lithosphere. Hence estimates of depths to lithosphere - asthenosphere boundary (LAB) based on FHS model, are believed to provide more reliable estimates than those reported in previous thermal models. The global maps of depths to LAB derived in the present work reveal several features in regional variations of lithosphere thicknesses that have not been identified in earlier studies. For example, regions of ocean floor with ages less than 55 Ma are characterized by relatively rapid thickening of the lithosphere. Also there is better resolution in mapping the transition from oceanic to continental lithosphere, as most of the latter ones are characterized by lithospheric thickness greater than 150 km. As expected the plate spreading centers in oceanic regions as well as areas of recent magmatic activity in continental regions are characterized by relatively thin lithosphere, with LAB depths of less than 50 km. On the other hand, the areas of continental collisions and Precambrian cratonic blocks and are found to have lithosphere thicknesses in excess of 250 km. Regional variations of lithosphere thickness in the interiors of continents are found to depend on the magnitude of subcrustal heat flux as well as the tectonic age of crustal blocks.
The problem of the asthenosphere for old Precambrian cratons, including East European Craton and its part - the Baltic Shield, is still discussed. To study the seismic lithosphere-asthenosphere boundary (LAB) beneath the Baltic Shield we used records of 9 local events with magnitudes in the range 2.7-5.9. The relatively big number of seismic stations in the Baltic Shield with a station spacing of 30-100 km permits for relatively dense recordings, and is sufficient in lithospheric scale. For modelling of the lower lithosphere and asthenosphere, the original data were corrected for topography and the Moho depth for each event and each station location, using a reference model with a 46 km thick crust. Observed P and S arrivals are significantly earlier than those predicted by the iasp91 model, which clearly indicates that lithospheric P and S velocities beneath the Baltic Shield are higher than in the global iasp91 model. For two northern events at Spitsbergen and Novaya Zemlya we observe a low velocity layer, 60-70 km thick asthenosphere, and the LAB beneath Barents Sea was found at depth of about 200 km. Sections for other events show continous first arrivals of P waves with no evidence for "shadow zone" in the whole range of registration, which could be interpreted as absence of asthenosphere beneath the central part of the Baltic Shield, or that LAB in this area occurs deeper (>200 km). The relatively thin low velocity layer found beneath southern Sweden, 15 km below the Moho, could be interpreted as small scale lithospheric inhomogeneities, rather than asthenosphere. Differentiation of the lid velocity beneath the Baltic Shield could be interpreted as regional inhomogeneity. It could also be interpreted as anisotropy of the Baltic Shield lithosphere, with fast velocity close to the east-west direction, and slow velocity close to the south-north direction.
The optimal use of conventional and unconventional hydrocarbon reservoirs depends, amongst others, on the local tectonic stress field. For example, wellbore stability, orientation of hydraulically induced fractures and - especially in fractured reservoirs - permeability anisotropies are controlled by the recent in situ stresses. Faults and lithological changes can lead to stress perturbations and produce local stresses that can significantly deviate from the regional stress field. Geomechanical reservoir models aim for a robust, ideally "pre-drilling" prediction of the local variations in stress magnitude and orientation. This requires a~numerical modelling approach that is capable to incorporate the specific geometry and mechanical properties of the subsurface reservoir. The workflow presented in this paper can be used to build 3-D geomechanical models based on the Finite Element Method (FEM) and ranging from field-scale models to smaller, detailed submodels of individual fault blocks. The approach is successfully applied to an intensively faulted gas reservoir in the North German Basin. The in situ stresses predicted by the geomechanical FE model were calibrated against stress data actually observed, e.g. borehole breakouts and extended leak-off tests. Such a validated model can provide insights into the stress perturbations in the inter-well space and undrilled parts of the reservoir. In addition, the tendency of the existing fault network to slip or dilate in the present-day stress regime can be addressed.
The discovery nearly two decades ago of a 90 km-diameter impact crater below the lower Chesapeake Bay has gone unnoted by the general public because to date all published literature on the subject has described it as "buried". To the contrary, evidence is presented here that the so-called "upland deposits" that blanket ∼5000 km<sup>2</sup> of the U.S. Middle-Atlantic Coastal Plain (M-ACP) display morphologic, lithologic, and stratigraphic features consistent with their being ejecta from the 35.4 Ma Chesapeake Bay Impact Structure (CBIS) and absolutely inconsistent with the prevailing belief that they are of fluvial origin. Specifically supporting impact origin are the facts that (i) a 95 %-pure iron ore endemic to the upland deposits of southern Maryland, eastern Virginia, and the District of Columbia has previously been proven to be impactoclastic in origin, (ii) this iron ore welds together a small percentage of well-rounded quartzite pebbles and cobbles of the upland deposits into brittle sheets interpretable as "spall plates" created in the interference-zone of the CBIS impact, (iii) the predominantly non-welded upland gravels have long ago been shown to be size sorted with an extreme crater-centric gradient far too large to have been the work of rivers, but well explained as atmospheric size-sorted interference-zone ejecta, (iv) new evidence is provided here that ~60 % of the non-welded quartzite pebbles and cobbles of the (lower lying) gravel member of the upland deposits display planar fractures attributable to interference-zone tensile waves, (v) the (overlying) loam member of the upland deposits is attributable to base-surge-type deposition, (vi) several exotic clasts found in a debris flow topographically below the upland deposits can only be explained as jetting-phase crater ejecta, and (vii) an allogenic granite boulder found among the upland deposits is deduced to have been launched into space and sculpted by hypervelocity air friction during reentry. An idealized calculation of the CBIS ejecta-blanket elevation profile minutes after the impact was carried out founded on well established rules for explosion and impact-generated craters. This profile is shown here to match the volume of the upland deposits ≥170 km from the crater center. Closer to the crater, much of the "postdicted" ejecta blanket has clearly been removed by erosion. Nevertheless the Shirley and fossil-free Bacons Castle Formations, located between the upland deposits and the CBIS interior and veneering the present day surface with units ∼10–20 m deep, are respectively identified as curtain- and excavation-phase ejecta. The neritic-fossil-bearing Calvert Formation external to the crater is deduced to be of Eocene age (as opposed to early Miocene as currently believed), preserved by the armoring effects of the overlying CBIS ejecta composed of the (distal) upland deposits and the (proximal) Bacons Castle Formation. The lithofacies of the in-crater Calvert Formation can only have resulted from inward mass wasting of the postdicted ejecta blanket, vestiges of which (i.e. the Bacons Castle and Shirley Formations) still overlap the crater rim and sag into its interior, consistent with this expectation. Because there appear to be a total of ∼10 000 km<sup>2</sup> of CBIS ejecta lying on the present-day surface, future research should center the stratigraphic, lithologic, and petrologic properties of these ejecta versus both radial distance from the crater center (to identify ejecta from different ejection stages) and circumferentially at fixed radial distances (to detect possible anisotropies relating the impact angle and direction of approach of the impactor). The geological units described here may comprise the best preserved, and certainly the most accessible, ejecta blanket of a major crater on the Earth's surface and therefore promise to be a boon to the field of impact geology. As a corollary, a major revision of the current stratigraphic column of the M-ACP will be necessary.
Titinoclinohumite-bearing dunites from Fujiwara, the Sanbagawa metamorphic belt of high-pressure type, Japan, were described to examine the possibility of Ti mobility during metasomatism within the mantle wedge. The Fujiwara dunite body and surrounding high-pressure Sanbagawa schists possibly form a subduction complex, and the dunites are a good analogue to the mantle wedge overlying the slab. The Fujiwara dunites are of deserpentinization origin; the deserpentinized olivine is high in Fo (up to 96) and low in NiO (0.2 to 0.3 wt %), and contains magnetite inclusions. Titanoclinohumites are associated with the deserpentinized olivine, as lamellar intergrowth or veinlets, up to 1 cm in width. Other metamorphic minerals include antigorite, brucite, chlorite, ilmenite, perovskite, Ti-rich ludwigite, and carbonates. The protolith of the Fujiwara dunite was partially serpentinized cumulative dunites from intra-plate magma, containing relatively low-Fo (85 to 86) olivines and TiO2-rich (up to 3 wt %) chromian spinels. The metamorphic olivines and titanoclinohumites contain micro-inclusions of methane (CH4) with or without serpentine and brucite. The source of Ti for titanoclinohumite was possibly the Ti-rich chromian spinel, but Ti was mobile through hydrocarbon-rich fluids, which were activated during the metamorphism. The hydrocarbons, of which remnants are carbonates and methane micro-inclusions, were derived from carbonaceous materials or bitumen, possibly incorporated in the precursory serpentinized and brecciated peridotite (= the protolith for the Fujiwara dunites) before subduction. Ti can be mobile in the mantle wedge if hydrocarbons are available from the subducted slab.
Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB). They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derived basalt occurring in the other suites of the Eastern Ghats Belt. These two groups of xenoliths in the Paleoproterozoic Eastern Ghats Province have quite distinct Nd-model ages- 1.9 Ga and 2.5 Ga respectively, which may be interpreted as their crustal residence ages. Strong positive Nb anomalies, indicating subducted oceanic crust in the source, LREE enrichment and strongly fractionated REE pattern are key geochemical signatures attesting to their origin as OIB-type magma. Also low Yb and Sc contents and high (La / Yb)N ratios can be attributed to melting in the presence of residual garnet and hence at great depths (> 80 km). The variable enrichment in radiogenic 87Sr, between 0.70052 and 0.71092 at 1.9 Ga and less radiogenic 143Nd between ɛ-1.54 and 7.46 are similar to those of the OIBs compared to MORBs. As OIBs commonly contain some recycled oceanic crust in their sources, we suggest that the residue of the oceanic crust from a previous melting event (~ 2.5 Ga) that produced the Arc-derived basalts (protoliths of hornblende-mafic granulite xenoliths) could have subducted to great depths and mechanically mixed with the mantle peridotite. A subsequent re-melting event of this mixed source might have occurred at ca. 1.9 Ga as testified by the crustal residence ages of the biotite-mafic granulite xenoliths of the Chilka Lake granulite suite.
Type curves of the dimensionless well-pressure (a), its logarithmic derivative (b), and of the normalized well-pressure (c) as functions of dimensionless time. Note, (c) is actually a restricted zoom of the data presented in (a) and (b). For example, the clockwise bending of the curve for T D = 100 prominent in (a) is outside of the chosen scale. The red line in (c) represents the normalized curve p ∞ wD /2.45τ 1/4 where p 
Snap-shots of the normalized pressure field N p for fractures with dimensionless 705 fracture conductivity of D 0.314 T = (top) and D 5 T = (bottom). 706 
Dimensionless distances iD x and iD y of isobars of normalized pressure N p along 708 the D x-and D y-axis as function of dimensionless time for a) D 0.314 T = , N 0.01 p = , b) 709 D 0.314 T = , N 0.05 p = , c) D 5 T = , N 0.01 p = , and d) D 5 T = , N 0.05 p = . 710 Fig. 4. Dimensionless distances x iD and y iD of isobars of normalized pressure p N along the x D-and y D-axis as function of dimensionless time for (a) T D = 0.314, p N = 0.01, (b) T D = 0.314, p N = 0.05, (c) T D = 5, p N = 0.01, and (d) T D = 5, p N = 0.05. 
Type curves of the normalized well pressure for indicated values of dimensionless fracture conductivity T D. The dashed grey lines are the deviation lines representing the end time using the transition criterion (Eq. 18), the solid grey lines are the deviation lines representing the reflection criterion (Eq. 19). 
Bilinear flow occurs when fluid is drained from a permeable matrix by producing it through an enclosed fracture of finite conductivity intersecting a well along its axis. The terminology reflects the combination of two approximately linear flow regimes, one in the matrix with flow essentially perpendicular to the fracture and one along the fracture itself associated with the non-negligible pressure drop in it. We investigated the characteristics, in particular the termination, of bilinear flow by numerical modeling allowing an examination of the entire flow field without prescribing the flow geometry in the matrix. Fracture storage capacity was neglected relying on previous findings that bilinear flow is associated with a quasi-steady flow in the fracture. Numerical results were generalized by dimensionless presentation. Definition of a dimensionless time that other than in previous approaches does not use geometrical parameters of the fracture permitted identifying the dimensionless well pressure for the infinitely long fracture as the master curve for type curves of all fractures with finite length from the beginning of bilinear flow up to fully developed radial flow. In log-log-scale the master curve's logarithmic derivative initially follows a 1/4-slope-straight line (characteristic for bilinear flow) and gradually bends into a horizontal line (characteristic for radial flow) for long times. During the bilinear flow period, isobars normalized to well pressure propagate with fourth and second root of time in fracture and matrix, respectively. The width-to-length ratio of the pressure field increases proportional to the fourth root of time during the bilinear period and starts to deviate from this relation close to the deviation of well pressure and its derivative from their fourth-root-of-time relations. At this time, isobars are already significantly inclined with respect to the fracture. The type curves of finite fractures all deviate counterclockwise from the master curve instead of clockwise or counterclockwise from the 1/4-slope-straight line as previously proposed. The counterclockwise deviation from the master curve was identified as the arrival of a normalized isobar reflected at the fracture tip sixteen times earlier. Nevertheless, two distinct regimes were found regarding pressure at the fracture tip when bilinear flow ends. For dimensionless fracture conductivities TD < 1, a significant pressure increase is not observed at the fracture tip until bilinear flow is succeeded by radial flow at a fixed dimensionless time. For TD > 10, the pressure at the fracture tip has reached substantial fractions of the associated change in well pressure when the flow field transforms towards intermittent formation linear flow at times that scale inversely with the fourth power of dimensionless fracture conductivity. Our results suggest that semi-log plots of normalized well pressure provide a means for the determination of hydraulic parameters of fracture and matrix after shorter test duration than for conventional analysis.
Morphological features and general properties of soils. 
Soils affected by forest wildfires in 2010 in Russia were studied on postfire and mature plots near the Togljatty city, Samara region. Soil biological properties and ash composition dynamics were investigated under the forest fire affect: a place of local forest fire, riding forest fire and unaffected site by fire-control (mature) during 3 yr of restoration. Soil samples were collected at 0–15 cm. Soil biological properties was measured by the fumigation method. The analytical data obtained shows that wildfires lead to serious changes in a soil profile and soil chemistry of upper horizons. Wildfires change a chemical composition of soil horizons and increase their ash-content. Fires lead to accumulation of biogenic elements' content (P and K) in the solum fine earth. Calcium content is increased as a result of fires that leads to an alkaline pH of the solum. The values of nutrients decreased as a result of leaching out with an atmospheric precipitation during the second year of restoration. Thus, when the upper horizons are burning the ash arriving on a soil surface enrich it with nutrients. The mature (unaffected by fire) soils is characterized by the greatest values of soil microbial biomass in the top horizon and, respectively, the bigger values of basal respiration whereas declining of the both parameters was revealed on postfire soils. Nevertheless this influence does not extend on depth more than 10 cm. Thus, fire affect on the soil were recognized in decreasing of microbiological activity.
The Cantabrian Transitional Area (CTA) is located in the eastern portion of the Cantabrian Mountain Range of the northern Spain. It represents the most important internal boundary within the Upper Cretaceous to Cenozoic E–W elongated Pyrenean Orogen. In the south-verging portion of this orogen, the CTA divides the western thick-skinned Cantabrian Domain, which accommodated for a limited portion of the total N–S oriented orogenic shortening, from the Pyrenean realm to the east, where the south-verging frontal structures are characterised by a marked thiN–Skin style of deformation, and significantly contributed to accommodate the total shortening. In the Cantabrian Transitional Area, Cenozoic syn-orogenic left-lateral, right-lateral and reverse dip-slip movements have occurred along different directions, postdating early-orogenic extensional structures. The latter indicate that the southern portion of the study area formed the eastern termination of the northward concave roughly E–W oriented proto Duero Foreland Basin. This basin was flanked to the north by the thick-skinned proto Cantabrian Belt, which included in its easternmost part the northern portion of the Cantabrian Transitional Area. Onset of right-lateral strike-slip tectonics along the WNW-ESE striking Ubiernal-Venatniella Fault System, which locates to the SW of the CTA and crosses the entire Cantabrian Belt and its formerly southern foreland basin, caused the dislocation of the belt-foredeep system. Contextually, thiN–Skinned structures belonging to the eastern domain of the Pyrenean Orogen laterally propagated and incorporated the eastern part of the proto Duero Foreland Basin. Coexistence of right-lateral and reverse movements to the west and to the east, respectively, determined the onset of an intrabelt compression at the boundary between the Cantabrian and Pyrenean domains, which was the ultimate act of the fusion of the two domains into a single orogen. Paradoxically, this fusion has basically occurred due to the penetration of the NW-SE-striking intraplate right-lateral transpressive system of the Iberian Chain into the Cantabrian Domain of the Pyrenean Orogen. Cenozoic right-lateral reactivation of the Ubierna Fault System, in fact, is part of a NW-SE striking intraplate strike-slip transpressive system, which to the south-east includes the Iberian Chain until the Mediterranean Sea and that, in the western termination of the Ubierna Fault System, branches off into three main splay faults, which are the Ventaniella and Leon faults, and the Duero frontal thrust. Taking into account the role of this Cenozoic transpressive system allows to drastically reduce the gap between plate kinematic reconstructions and geological evidences. This implies that, despite the limited amount of displacement, the Iberian Chain and the Ubierna-Ventaniella systems must be elevated to the rank of microplate boundary, which divided two sectors of the Iberian Plate. Accordingly, the intersection between this system and the Pyrenean Orogen, which occurs in the CTA, must be regarded as a triple junction zone.
We examine the microfabric development in high-pressure, low-temperature metamorphic serpentinite mylonites exposed in the Erro-Tobbio Unit (Voltri Massif, Italy) using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites are derived from mantle peridotites, were serpentinized at the ocean floor and underwent high pressure metamorphism during Alpine subduction. They contain diopside and olivine porphyroclasts embedded in a fine-grained matrix essentially consisting of antigorite. The porphyroclasts record brittle and crystal-plastic deformation of the original peridotites in the upper mantle at stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution-precipitation creep resulting in a foliation with flattened olivine grains at phase boundaries with antigorite, crenulation cleavages and olivine and antigorite aggregates in strain shadows next to porphyroclasts. It is suggested that the fluid was provided by dehydration reactions of antigorite forming olivine and enstatite during subduction and prograde metamorphism. At sites of stress concentration around porphyroclasts antigorite reveals an associated SPO and CPO, characteristically varying grain sizes and sutured grain boundaries, indicating deformation by dislocation creep. Stresses were probably below a few tens of MPa in the serpentinites, which was not sufficiently high to allow for crystal-plastic deformation of olivine at conditions at which antigorite is stable. Accordingly, any intragranular deformation features of the newly precipitated olivine in strain shadows are absent. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation in the upper mantle of the oceanic lithosphere probably related to rifting processes, whereas the antigorite matrix records deformation at low stresses during subduction and exhumation.
Study area (top), monthly rainfall (red bars) and average groundwater nitrate concentration from 2006 to 2008 (green line). 
Experimental design. 532 
Soil nitrate content in vineyard plot (average of upper, middle and lower position), buffer strips and nonbuffer strips below the V. sativa cover crop (black line), the Triticum durum cover crop (grey line), and conventional tillage (broken line). Horizontal lines represent statistical difference according to repeated measure statistical analysis. 
Atom % 15 N excess in soil over the time in the buffer strips and nonbuffer strips. Black, grey, and white histograms represent 3, 6, and 9 m distances, respectively, from the 15 N application zone. 
When soil nitrate levels are inadequate, plants suffer nitrogen deficiency but when the levels are excessive, nitrates (NO3-N) can pollute surface and subsurface waters. Strategies to reduce the nitrate pollution are necessary to reach a sustainable use of resources such as soil, water and plant. Buffer strips and cover crops can contribute to the management of soil nitrates, but little is known of their effectiveness in semiarid vineyards plantations. The experimental site, a 10 m wide and 80 m long area at the bottom of a vineyard was selected in Sicily. The soil between vine rows and upslope of the buffer strip (seeded with Lolium perenne) and non-buffer strips (control) was managed conventionally and with one of two cover crops (Triticum durum and Vicia sativa cover crop). Soil nitrate was measured monthly and nitrate movement was monitored by application of a 15N tracer to a narrow strip between the bottom of vineyard and the buffer and non-buffer strips. L. perenne biomass yield in the buffer strips and its isotopic nitrogen content were monitored. V. sativa cover crop management contribute with an excess of nitrogen, and the soil management determined the nitrogen content at the buffer areas. A 6 m buffer strip reduce the nitrate by 42% with and by 46% with a 9 m buffer strip.
, the observed
Mathematics and observations suggest that the energy of the geological activities resulting from plate tectonics is equal to the latent heat of melting, calculated at mantle's pressure, of the new ocean crust created at midocean ridges following sea floor spreading. This energy varies with the temperature of ocean floor, which is correlated with surface temperature. The objective of this manuscript is to calculate the force that drives plate tectonics, estimate the energy released, verify the calculations based on experiments and observations, and project the increase of geological activities with surface temperature rise caused by climate change.
The goal of the paper was to verify triggering of earthquakes by the length of day variations, i.e. the sidereal 13.66 days Earth's rotation variations, in contrast with tidal biweekly 14.76 days variations (full and new Moon), which for hundred years of investigation give negative results. Earthquake triggering governed by sidereal variations caused by variable Moon's declination accelerates and decelerates the Earth's rotation. Profound Schuster's test proved that earthquakes are triggered both in Earth's deceleration and acceleration. For this investigation the most prominent earthquakes from 2010-2011 were used from Mid-Atlantic Ridge, Southeast Indian Ridge, Sumatra and Andaman Sea, Chile trench, Haiti and Honshu region including important older earthquakes of Sumatra 26 December 2004 and Denali Fault 3 November 2002. Dominant number of earthquake occurring in extremes of length of day variations initiated the calculation of forces acting in these time intervals. Calculated forces of tidal force acting on Earth's flattening and the westward drift are strong enough to trigger earthquakes and the movement of plates follows from GPS performed immediately after earthquakes on continents and from increased number of earthquakes of the side of the mid-ocean ridge belonging to the moving plate. Generally the Northern Hemisphere moves quicker westward than the Southern one. Earthquakes are repeated in 19 yr Metonic cycle. Repetitions caused by tidal force acting on Earth's fattening are exact in date. Repetitions caused by westward drift are delayed for several months.
We investigate the influence on mantle convection of the negative Clapeyron slope ringwoodite to perovskite and ferro-periclase mantle phase transition, which is correlated with the seismic discontinuity at 660 km depth. In particular, we focus on understanding the influence of the magnitude of the Clapeyron slope (as measured by the Phase Buoyancy parameter, P) and the vigour of convection (as measured by the Rayleigh number, Ra) on mantle convection. We have undertaken 76 simulations of isoviscous mantle convection in spherical geometry varying Ra and P. Three domains of behaviour were found: layered convection for high Ra and more negative P, whole mantle convection for low Ra and less negative P and transitional behaviour in an intervening domain. The boundary between the layered and transitional domain was fit by a curve P = αRaβ where α = -1.05, and β = -0.1, and the fit for the boundary between the transitional and whole mantle convection domain was α = -4.8, and β = -0.25. These two curves converge at Ra≈2.5×104 and P≈-0.38. Extrapolating to high Ra, which is likely earlier in Earth history, this work suggests a large transitional domain. It is therefore likely that convection in the Archean would have been influenced by this phase change, with Earth being at least in the transitional domain, if not the layered domain.
Dynamic models of subduction and continental collision are used to predict dynamic topography changes on the overriding plate. The modelling results show a distinct evolution of topography on the overriding plate, during subduction, continental collision and slab break-off. A prominent topographic feature is a temporary (few Myrs) deepening in the area of the back arc-basin after initial collision. This collisional mantle dynamic basin (CMDB) is caused by slab steepening drawing material away from the base of the overriding plate. Also during this initial collision phase, surface uplift is predicted on the overriding plate between the suture zone and the CMDB, due to the subduction of buoyant continental material and its isostatic compensation. After slab detachment, redistribution of stresses and underplating of the overriding plate causes the uplift to spread further into the overriding plate. This topographic evolution fits the stratigraphy found on the overriding plate of the Arabia-Eurasia collision zone in Iran and south east Turkey. The sedimentary record from the overriding plate contains Upper Oligocene-Lower Miocene marine carbonates deposited between terrestrial clastic sedimentary rocks, in units such as the Qom Formation and its lateral equivalents. This stratigraphy shows that during the Late Oligocene-Early Miocene the surface of the overriding plate sank below sea level before rising back above sea level, without major compressional deformation recorded in the same area. This uplift and subsidence pattern correlates well with our modelled topography changes.
Continental collision is an intrinsic feature of plate tectonics. The closure of an oceanic basin leads to the onset of subduction of buoyant continental material, which slows down and eventually stops the subduction process. We perform a parametric study of the geometrical and rheological influence on subduction dynamics during the subduction of continental lithosphere. In 2-D numerical models of a free subduction system with temperature and stress-dependent rheology, the trench and the overriding plate move self-consistently as a function of the dynamics of the system (i.e. no external forces are imposed). This setup enables to study how continental subduction influences the trench migration. We found that in all models the trench starts to advance once the continent enters the subduction zone and continues to migrate until few million years after the ultimate slab detachment. Our results support the idea that the trench advancing is favoured and, in part provided by, the intrinsic force balance of continental collision. We suggest that the trench advance is first induced by the locking of the subduction zone and the subsequent steepening of the slab, and next by the sinking of the deepest oceanic part of the slab, during stretching and break-off of the slab. The amount of trench advancing ranges from 40 to 220 km and depends on the dip angle of the slab before the onset of collision.
Subduction modelling in regional model domains, in 2-D or 3-D, is commonly done using closed, vertical boundaries. In this paper we investigate the merits of using open boundaries for 2-D modelling of lithosphere subduction but with implication for 3-D modelling. Open sidewalls allow for lateral in- and outflow consistent with the internal dynamics of the model and may simulate the real-mantle environment of subduction much better than closed boundaries, which induce return flows. Our experiments are focused on using open and closed (free-slip) sidewalls while comparing results for two model aspect ratios of 3:1 and 6:1. Slab buoyancy driven subduction with open boundaries immediately develops into strong rollback with high trench retreat velocities. Mantle asthenosphere flow forced by rollback is predominantly laminar and facilitated by the open boundaries. In contrast, free-slip sidewalls proof restrictive on subduction rollback evolution unless the lithosphere plates are allowed to move away from the sidewalls. This, however, initiates return flows pushing both plates toward the subduction zone speeding up subduction. Increasing the aspect ratio to 6:1 does not change the overall flow pattern when using open sidewalls. Again, in contrast, for free-slip boundaries, the slab evolution does change with respect to the 3:1 aspect ratio and does not resemble the 6:1 evolution obtained with open boundaries. We notice a general drop in the amplitude of mantle flow when changing to the 6:1 aspect ratio, which is caused by the increasing shear friction between mantle and lithosphere while the driving slab buoyancy is the same. Based on energy-dissipation arguments we applied a flow speed scaling to convert between flow fields of different model aspect ratios. This proved succesful for the open boundary model. We have also investigated the effect of far-field stress conditions in our open boundary models. Applying realistic normal stress conditions to the strong part of the overriding plate we show that "intra-plate" stresses control subduction dynamics resulting in slab roll-back, stationary or advancing subduction. We conclude that open boundaries are to be preferred for modelling subduction evolution (rollback, stationary or advancing). The relative independence of model aspect ratio avoids the need to place sidewalls at large distance and allows to focus all computational resources on a smaller modelling domain. Open boundaries simulate the natural subduction environment better and avoid the adverse effects (e.g. forced return flows) of free-slip boundaries.
Inset: generalized Fennoscandian Shield geology. Main map: geological overview of the Skellefte district, as loosely defined by the occurrence of the Skellefte Group metavolcanic rocks, and their immediate vicinity. DNSZ = Deppis-N ¨ asliden shear zone; VRSS = VidselR ¨ ojnöretojn¨ojnöret shear system. Intrusions: Vi = Viterliden, Re = RengårdReng˚Rengård, Ka = KarsträskKarstr¨Karsträsk, Si = SikträskSiktr¨Sikträsk, Bj = BjörklidenBj¨Björkliden, GI, GII, GIII, GIV = J ¨ orn type intrusions, phases I-IV. Ore deposits: H = HornträskvikenHorntr¨Hornträskviken, Kh = Kimheden, Kr = Kristineberg, R = R ¨ avliden, Rm = R ¨ avlidmyran, R ¨ o = R ¨ okåok˚okå. Geology after Kathol et al. (2005) and Bergman Weihed (2001). 
Structures in the A4 open pit, see Fig. 2 for location. (a) Geological overview of the A4 open pit. (b) Plunging inclined F2 folds in the sericite-altered stratigraphic hanging-wall to the A4 ore lense. Vertical section, view towards W. Width of view approximately 2 m. (c) Plunging upright F2 folds in the stockwork-system mineralization in the stratigraphic footwall to the A4 ore lense. View up-plunge along the fold axes, towards NE. Width of view approximately 80 cm. (d) A semi-brittle WNW-dipping high-strain zone constraining the western extent of the A4 ore lense. Vertical section, view towards NNE. 
Structural mapping and 3-D-modelling with constraints from magnetotelluric (MT) and reflection seismic investigations have been used to provide a geological synthesis of the geometrically complex Kristineberg area in the western part of the Palaeoproterozoic Skellefte district. The results indicate that, like the south-eastern parts of the Skellefte district, the area was subjected to SSE-NNW transpressional deformation at around 1.87 Ga. The contrasting structural geometries between the Kristineberg and the central Skellefte district areas may be attributed to the termination and splaying of a major ESE-WNW-striking high-strain zone into several branches in the northern part of the Kristineberg area. The transpressional structural signature was preferentially developed within the southern of the two antiformal structures of the area, "the Southern antiform", which exposes the deepest cut through the crust and hosts all the economic volcanogenic massive sulphides (VMS) deposits of the area. Partitioning of the SSE-NNW transpression into N-S and E-W components led to formation of a characteristic "flat-steep-flat" geometry defining a highly non-cylindrical hinge of for the Southern antiform. Recognition of the transpressional structural signatures including the "flat-steep-flat" geometry and the distinct pattern of sub-horizontal E-W trending to moderately SW-plunging mineral lineations in the deeper crustal parts of the Kristineberg area is of significance for VMS exploration in both near mine and regional scales. The 3-D-model illustrating the outcomes of this study is available as a 3-D-PDF document through the publication website.
We performed hydrothermal annealing experiments on quartzite at temperatures of 392 to 568 °C and fluid pressures of 63 to 399 MPa for up to 120 h during which hydrothermal grooves developed on the free surfaces of the samples. Analysis of surface topology and groove characteristics with an atomic force microscope revealed a range of surface features associated with the simultaneous and successive operation of several processes partly depending on crystal orientation during the various stages of an experiment. Initially, dissolution at the quartzite-sample surface occurs to saturate the fluid in the capsule with SiO2. Subsequently, grooving controlled by diffusion processes takes place parallel to dissolution and precipitation due to local differences in solubility. Finally, quench products develop on grain surfaces during the termination of experiments. Average groove-root angle amounts to about 80° and slightly depends on temperature, run duration, and misorientation between neighboring grains. The grooving is thermally activated, i.e., groove depth ranging from 5 nm to several micrometers for the entire suite of experiments generally increases with temperature and/or run time. We use Mullins' classical theories to constrain kinetics parameters for the transport processes controlling the grooving. In the light of previous measurements of various diffusion coefficients in the system SiO2-H2O, interface diffusion of Si is identified as the most plausible rate-controlling process. Grooving could potentially proceed faster if the fluid were not convecting in the capsule. Characteristic times of healing of microfractures in hydrous environments constrained from these kinetics parameters are consistent with the order of magnitude of time scales over which healing occurs in-situ according to geophysical surveys and of recurrence intervals of earthquakes.
Stress redistributions around large underground excavations such as coal mines may lead to failure of the surrounding rock mass. Some of these failure processes were recorded as seismic events. In this paper the different failure processes such as rock mass failure or the reactivation of faults are delineated from the seismic records. These are substantiated by rock mechanical analyses including laboratory strength tests on coal measure rocks obtained from underground drilling. Additionally, shear tests on discontinuities in coal measure rocks (slickensides in shale and rough sandstone joints) were conducted to grasp the possible variation of strength properties of faults. Numerical modeling was employed to evaluate the state of stress at the locations where seismic events did occur.
To investigate the crystallization of pyroxene in spinifex-textured komatiites and in chondrites we undertook a series of experiments in which compositions in the CMAS system were cooling rapidly in a thermal gradient. Cooling rates were generally between 5 to 10 °C h-1 but some runs were made at 100-200 °C h-1; thermal gradients were between 10 and 20 °C cm-1. These conditions reproduced those at various levels in the crust of komatiitic lava flow. The starting composition was chosen to have pigeonite on the liquidus and a majority of the experiments crystallized zoned pigeonite-diopside crystals like those in komatiite lavas. A~conspicuous aspect of the experimental results was their lack of reproduceability. Some experiments crystallized forsterite whereas others that were run under similar conditions crystallized two pyroxenes and no forsterite; some experiments were totally glassy but others totally crystallized to pyroxene. The degree of supercooling at the onset of pyroxene crystallization was variable, from less than 25 °C to more than 110 °C. We attribute these results to the difficulty of nucleation of pyroxene. In some cases forsterite crystallized metastably and modified the liquid composition to inhibit pyroxene crystallization; in others no nucleation took place until a large degree of supercooling was achieved, then pyroxene crystallized rapidly. Pigeonite crystallized under a wide range of conditions, at cooling rates from 3 to 100 °C h-1. The notion that this mineral only forms at low cooling rates is not correct.
Stress and strain rate changes during the seismic cycle, visualized by hypothetical crustal strength profiles (not to scale). Hypocentre is marked by asterisk. Around the lower termination of the downward propagating rupture plane of a large earthquake, i.e. at the boundary between schizosphere and plastosphere sensu Scholz (2002), quasi-instantaneous coseismic loading to high differential stress causes deformation of quartz in the low-temperature plasticity regime ("kick"). During subsequent stress relaxation and restoration of long-term state, quartz deforms by dislocation creep at decaying strain rate ("creep"), each stage of the earthquakerelated stress cycle leaving a characteristic microstructural imprint. In the present study, modification of microstructures acquired during the "kick" stage in the course of subsequent "creep" deformation is investigated in laboratory experiments.
(a) Selected force-time curve and (b) corresponding stressstrain curve for low-temperature, high-stress "kick" experiment CR1-3. (c) Selected force-time curves for high-temperature, lowstress ("creep") stage in the experiments CR1-13 (stress kept approximately constant by moving the piston into the sample assembly) and CR1-17 (stress relaxation, no active movement of piston).
Quartz microfabric after "kick" experiment CR-1-3 performed at 400 • C, 2 GPa confining pressure, and strain rate of 10 −4 s −1. The shortening direction is vertical in all images, indicated by white arrows. (a) Polarized light micrograph showing slightly misoriented domains parallel (0001), containing arrays of microcracks and lamellae parallel to {r} and {z} rhombohedral planes. Location of EBSD map displayed in (b) is indicated by yellow rectangle. (b) EBSD map showing domains with misorientation relative to reference point (red cross); colours indicate misorientation, displayed as overlay on band contrast image in grey shades; non-coloured areas are not indexed. (c) Orientation contrast image of area shown in (a) and pole figure (lower hemisphere, equal angle) displaying crystallographic orientation. Location of the EBSD map shown in (b) is indicated by yellow rectangle.
Quartz microfabric after "kick and creep" experiment CR1-16, initially deformed at 400 • C, 2 GPa confining pressure, strain rate 10 −4 s −1 , and afterwards at 900 • C, 2.5 GPa confining pressure, undergoing stress relaxation. The shortening direction is indicated by white arrows. (a, b) Polarized light micrographs showing bands of recrystallized grains and pronounced SWUE. (c) EBSD map showing misorientation relative to reference point (red cross); colours indicate relative misorientation, displayed as overlay on band contrast image in grey shades; non-coloured areas are not indexed. White lines indicate location of misorientation profiles shown in (f). (d) EBSD map showing Schmid factor for basal <a>glide. White rectangle indicates location of polarized light micrograph in (b). (e) Pole figures showing crystallographic orientation of host grain and recrystallized grains, respectively. (f) Misorientation relative to first points (A and C) along white lines displayed in (b).
Quartz microfabric after "kick and creep" experiment CR1-17, initially deformed at 400 • C, 2 GPa confining pressure, 10 −4 s −1 strain rate, and afterwards at 900 • C, 2.5 GPa confining pressure and approximately constant stress. The shortening direction is indicated by white arrows. (a) Polarized light micrograph showing strings of recrystallized grains, deformation bands and pervasive SWUE. Location of FIB cut TEM foil is marked by yellow rectangle. (b) EBSD map showing misorientation relative to reference point (red cross). Black line indicates location of misorientation profile displayed in (d). (c) Pole figures showing crystallographic orientation of host grain. Crystallographic planes, which are traced in (b), are indicated by great circles. (d) Misorientation relative to point A along yellow line displayed in (b).
Deformation experiments are carried out on natural vein quartz in a modified Griggs-type solid medium apparatus to explore the preservation potential of microfabrics created by crystal-plastic deformation at high stress, overprinted during subsequent creep at lower stress. a corresponding stress history is expected for the upper plastosphere, where fault slip during an earthquake causes quasi-instantaneous loading to high stress, followed by stress relaxation. The question is whether evidence of crystal-plastic deformation at high stress, hence an indicator of past seismic activity, can still be identified in the microstructure after overprint by creep at lower stresses. Firstly, quartz samples are deformed at a temperature of 400 °C and constant strain rate of 10−4 s−1 ("kick"), and then held at 900 to 1000 °C at residual stress ("creep"). In quartz exclusively subject to high-stress deformation, lamellar domains of slightly differing crystallographic orientation (misorientation angle
Principal model of the polarisation of a micro-crack associated with electromagnetic emissions. During the formation and/or growth of a micro-crack, oscillating dipoles are generated that are orientated perpendicular to the micro-crack walls (Rabinovitch et al., 2007). Therefore the measured magnetic component (B, blue) of the resulting electromagnetic field propagates parallel to the orientation of the micro-crack.
(A) Maximum signal strength of horizontal measurements (cf. Fig. 3) as a function of distance to DHO38. The strength of the defining energy (red, blue, and green) is a function of the distance to DHO38, while the maximum number of single pulses in each measurement (black) is mostly constant and often coincides with the transmitting frequency of DHO38. (B) Device settings of the Cerescope for the measurements displayed in (A).
In recent years, the ElectroMagnetic Radiation (EMR) method has been used to detect faults and to determine main horizontal stress directions from variations in intensities and directional properties of electromagnetic emissions, which are assumed to be generated during micro-cracking. Based on a large data set taken from an area of about 250 000 km2 in Northern Germany, Denmark, and Southern Sweden with repeated measurements at one location during a time span of about 1.5 yr, the method was systematically tested. Reproducible observations of temporary changes in the signal patterns, as well as a strongly concentric spatial pattern of the main directions of the magnetic component of the EMR point to VLF transmitters as the main source and hence raise serious concerns about the applicability of the method to determine recent crustal stresses. We conclude that the EMR method, at its current stage of development, does not allow determination of the main horizontal stress directions.
Quantifying the precise thermal structure of subduction zones is essential for understanding the nature of metamorphic dehydration reactions, arc volcanism, and intermediate depth seismicity. High resolution two-dimensional (2-D) models have shown that the rheology of the mantle wedge plays a critical role and establishes strong temperature gradients in the slab. The influence of three-dimensional (3-D) subduction zone geometry on thermal structure is however not yet well characterized. A common assumption for 2-D models is that the cross-section is taken normal to the strike of the trench with a corresponding velocity reduction in the case of oblique subduction, rather than taken parallel to velocity. A comparison between a full 3-D Cartesian model with oblique subduction and selected 2-D cross-sections demonstrates that the trench-normal cross-section provides a better reproduction of the slab thermal structure than the velocity-parallel cross-section. An exception is found in the case of strongly curved subduction, such as in the Marianas, where strong 3-D flow in the mantle wedge is generated. In this case it is shown that the full 3-D model should be evaluated for an accurate prediction of the slab thermal structure.
Experiments comprising sequences of deformation (at 300 or 600 °C) and annealing at varying temperature (700 to 1100 &degC), time (up to 144 h) and stress (up to 1.5 GPa) were carried out in a Griggs-type apparatus on natural olivine-rich peridotite samples to simulate deformation and recrystallization processes in deep shear zones that reach mantle depth as continuations of seismically active faults. The resulting olivine microfabrics were analysed by polarization and electron microscopy. Core-and-mantle like microstructures are the predominant result of our experiments simulating rapid stress relaxation (without or with minor creep) after a high-stress deformation event: porphyroclasts (> 100 μm) are surrounded by defect-poor recrystallized grains with a wide range in size (2 to 40 μm). Areas with smaller recrystallized grains (> 10 μm) trace former high-strain zones generated during initial high-stress deformation even after annealing at a temperature of 1100 °C for 70 h. A weak crystallographic preferred orientation (CPO) of recrystallized olivine grains is related to the orientation of the host crystals but appears unrelated to the strain field. Based on these findings, we propose that olivine microstructures in natural shear-zone peridotites with a large range in recrystallized grain size, localized fine-grained zones, and a weak CPO not related to the strain field are diagnostic for a sequence of high-stress deformation followed by recrystallization at low stresses, as to be expected in areas of seismic activity. We extended the classic Avrami-kinetics equation by accounting for time-dependent growth kinetics and constrained the involved parameters relying on our results and previously reported kinetics parameters. Extrapolation to natural conditions suggests that the observed characteristic microstructure may develop within as little as tens of years and less than ten thousands of years. These recrystallization microstructures have a great diagnostic potential for past seismic activity because they are expected to be stable over geological time scales, since driving forces for further modification are not sufficient to erase the characteristic heterogeneities.
The cause of intermediate-depth (> 40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, that include dehydration embrittlement, shear instabilities, and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The well-instrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new high resolution finite elements models that predict the dynamics and thermal structure of the Japan subduction system with a high precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths > 80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models, that take into account the seismically imaged modified upper mantle structure in this region, fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by time-dependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity.
Combined seismological, space-geodetic and numerical studies have shown that the seismicity at subduction zones may be modulated by tides and glacier fluctuations on timescales of 1-100 a, because these changes in loads on Earth's surface are able to alter the stress field in the upper plate and along the plate interface. Here we use a two-dimensional finite-element model of a subduction zone to investigate how glacial-interglacial sea-level changes affect the forearc region and the plate interface. The model results show that a sea-level fall by 125 m over 100 ka causes up to 0.7 m of vertical displacement, with the maximum uplift occurring between the trench and the coast. The uplift signal induced by the sea-level fall decreases to zero ~20 km landward of the coastline. A subsequent sea-level rise by 125 m over 20 ka causes subsidence, which is again most pronounced in the submarine part of the forearc. The sea-level changes cause horizontal displacements of up to 0.12 m, which are directed seaward during sea-level fall and landward during sea-level rise. With respect to the stress field, the sea-level changes lead to variations in the vertical stress and the shear stress of up to 1.23 MPa and 0.4 MPa, respectively. The shear stress variations are highest beneath the coast, i.e. in the area where the sea-level changes cause the strongest flexure. The resulting Coulomb stress changes on the plate interface are of the order of 0.2-0.5 MPa and indicate that earthquakes are promoted during sea-level fall and delayed during sea-level rise. Our findings imply that eustatic sea-level changes during glacial-interglacial periods may have induced displacements and stress changes that were large enough to affect the seismic cycle of subduction thrusts.
The properties of the subduction interplate domain are likely to affect not only the seismogenic potential of the subduction area but also the overall subduction process, as it influences its viability. Numerical simulations are performed to model the long-term equilibrium state of the subduction interplate when the diving lithosphere interacts with both the overriding plate and the surrounding convective mantle. The thermomechanical model combines a non-Newtonian viscous rheology and a pseudo-brittle rheology. Rock strength here depends on depth, temperature and stress, for both oceanic crust and mantle rocks. I study the evolution through time of, on one hand, the kinematic decoupling depth, zdec and, on the other hand, of the brittle-ductile transition (BDT) depth, zBDT, simulated along the subduction interplate. The results reveal that zBDT mainly depends on the friction coefficient characterising the interplate channel and on the viscosity at the lithosphere-asthenosphere boundary. The influence of the weak material activation energy is of second order but not negligible. zBDT becomes dependent on the ductile strength increase with depth (activation volume) if the BDT occurs at the interplate deocupling depth. Regarding the interplate decoupling depth, it is basically a function of (1) mantle viscosity at asthenospheric wedge tip, (2) difference in mantle and interplate activation anergy, and (3) activation volume. Specific conditions yielding zBDT = zdec are discussed. I then present how the subducting lithosphere age affects the brittle-ductile transition depth and the kinematic decoupling depth in this model. Simulations show that a rheological model in which the respective activation energies of mantle and interplate material are too close impedes strain localization during incipient subduction of a young (20 Myr old) and soft lithosphere under a thick upper plate. Finally, both the BDT depth and the decoupling depth are a function of the subducting plate age, but are not influenced in the same fashion: cool and old subducting plates deepen the BDT but shallow the interplate decoupling depth. Even if BDT and kinematic decoupling are instrinsically related to different mechanisms of deformation, this work shows that they are able to interact closely.
The exhumation of high and ultra-high pressure rocks is ubiquitous in Phanerozoic orogens created during continental collisions, and is common in many ocean-ocean and ocean-continent subduction zone environments. Three different tectonic environments have previously been reported, which exhume deeply buried material by different mechanisms and at different rates. However it is becoming increasingly clear that no single mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. In order for buoyant continental crust to subduct, it must remain attached to a stronger and denser substrate, but in order to exhume, it must detach (and therefore at least locally weaken) and be initially buoyant. Denser oceanic crust subducts more readily than more buoyant continental crust but exhumation must be assisted by entrainment within more buoyant and weak material such as serpentinite or driven by the exhumation of structurally lower continental crustal material. Weakening mechanisms responsible for the detachment of crust at depth include strain, hydration, melting, grain size reduction and the development of foliation. These may act locally or may act on the bulk of the subducted material. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Subduction zones change in style both in time and space, and exhumation mechanisms change to reflect the tectonic style and overall force regime within the subduction zone. Exhumation events may be transient and occur only once in a particular subduction zone or orogen, or may be more continuous or occur multiple times.
New temperature measurements from eight boreholes in the West African Craton (WAC) reveal superficial perturbations down to 100 meters below the alteration zone. These perturbations are both related to a recent increase of the surface air temperature (SAT) and to the site effects caused by fluids circulations and/or the lower conduction in the alterites. The ground surface temperature (GST) inverted from the boreholes temperatures is stable in the past (1700-1940) and then dramatically increases in the most recent years (1.5 °C since 1950). This is consistent with the increase of the SAT recorded at two nearby meteorological stations (Tambacounda and Kedougou), and more generally in the Sahel with a coeval rainfall decrease. Site effects are superimposed to the climatic effect and interpreted by advective (circulation of fluids) or conductive (lower conductivity of laterite and of high-porosity sand) perturbations. We used a 1-D finite differences thermal model and a Monte-Carlo procedure to find the best estimates of these sites perturbations: all the eight boreholes temperatures logs can be interpreted with the same basal heat-flow and the same surface temperature history, but with some realistic changes of thermal conductivity and/or fluid velocity. The GST trend observed in Senegal can be confirmed by two previous boreholes measurements made in 1983 in other locations of West Africa, the first one in an arid zone of northern Mali and the second one in a subhumid zone in southern Mali. Finally, the background heat-flow is low (30 ± 1 m Wm-2), which makes this part of the WAC more similar with the observations in the southern part (33 ± 8 m Wm-2) rather than with those in the northern part and in the PanAfrican domains where the surface heat-flow is 15-20 m Wm-2 higher.
The accuracy, reliability and best practices of Ti-in-quartz thermobarometry ("TitaniQ") in greenschist facies rocks have not been established. To address these issues we measured Ti concentrations in rutile-bearing samples of moderately deformed, partially recrystallized quartzite and vein quartz from Taiwan's Hsüehshan range. The spread of Ti concentrations of recrystallized grains in quartzite correlates with recrystallized grain size. Recrystallized quartz (grain size ~300 μm) that formed during early deformation within the biotite stability field shows a marked increase in intermediate Ti-concentration grains (~1-10 ppm) relative to detrital porphyroclasts (Ti ~0.1-200 ppm). Fine recrystallized quartz (~5% of the samples by area, grain size ~10-20 μm) has a further restricted Ti concentration peaking at 0.8-2 ppm. This trend suggests equilibration of Ti in recrystallized quartz with a matrix phase during deformation and cooling. Vein emplacement and quartzite recrystallization are independently shown to have occurred at 250-350 °C and 300-410 °C respectively, lithostatic pressure ~5 kbar, and hydrostatic fluid pressure. Estimates of the accuracy of TitaniQ at these conditions depend on whether lithostatic or fluid pressure is used in the TitaniQ calibration. Using lithostatic pressure, Ti concentrations predicted by the Thomas et al. (2010) TitaniQ calibration are within error of Ti concentrations measured by SIMS. If fluid pressure is used, predicted temperatures are ~30-40 °C too low. TitaniQ has potential to yield accurate PT information for vein emplacement and dynamic recrystallization of quartz at temperatures as low as ~250 °C, however clarification of the relevant pressure term and further tests in rutile-present rocks are warranted.
Precise weekly positions of 403 Global Positioning System (GPS) stations located worldwide are obtained by reprocessing GPS data of these stations at the time span from 4 January 1998 until 29 December 2007. The used processing algorithm and models as well as the solution and results obtained are presented. Vertical velocities of GPS stations having tracking history longer than 2.5 yr are computed and compared with the estimates from the colocated tide gauges and other GPS solutions. Examples of typical behavior of station height changes are given and interpreted. The derived time series and vertical motions of continuous GPS at tide gauges stations can be used for correcting tide gauge estimates of regional and global sea level changes.
Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modelling shows that long compositional trends for the ilmenites are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of picroilmenite's parental magmas was estimated for the three distinct phases of kimberlite activity from Yubileynaya and closely located Aprelskaya pipes showing heating and increase of Fe of mantle peridotites minerals from stage to stage and splitting of the magmatic system in the final stages. High pressure (5.5-7.0 GPa) Cr-bearing Mg-rich ilmenites (Group 1) reflect the conditions of high temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1-10/C1 and have flattened, spoon-like or S- or W-shaped REE patterns with Pb > 1. These result from melting and crystallization in melt - feeding channels in the base of the lithosphere, where high temperature dunite - harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group2) trace the high temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during the melt migration within the channels. Group 2 ilmenites reveal inclined REE enriched patterns (10-100)/C1 with La/Ybn 10-25 similar to those derived from kimberlites, and HFSE peaks (typical megacrysts). A series of similar patterns results from polybaric AFC crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high Cr content (Group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr- rich ilmenites are typical for veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. The highest in TRE ilmenites 1000/C1 have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet - spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn -Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst-forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
It is generally believed that subduction of lithospheric slabs is a major contribution to thermal heterogeneity in Earth's entire mantle and provides a main driving force for mantle flow. Mantle structure can, on the one hand, be inferred from plate tectonic models of subduction history and geodynamic models of mantle flow. On the other hand, seismic tomography models provide important information on mantle heterogeneity. Yet, the two kinds of models are only similar on the largest (1000s of km) scales and are quite different in their detailed structure. Here, we provide a quantitative assessment how good a fit can be currently achieved with a simple viscous flow geodynamic model. The discrepancy between geodynamic and tomography models can indicate where further model refinement could possibly yield an improved fit. Our geodynamical model is based on 300 Myr of subduction history inferred from a global plate reconstruction. Density anomalies are inserted into the upper mantle beneath subduction zones, and flow and advection of these anomalies is calculated with a spherical harmonic code for a radial viscosity structure constrained by mineral physics and surface observations. Model viscosities in the upper mantle beneath the lithosphere are ~1020 Pas, and viscosity increases to ~1023 Pas in the lower mantle above D". Comparison with tomography models is assessed in terms of correlation, both overall and as a function of depth and spherical harmonic degree. We find that, compared to previous geodynamic and tomography models, correlation is improved significantly, presumably because of improvements in both plate reconstructions and mantle flow computation. However, high correlation is still limited to lowest spherical harmonic degrees. An important ingredient to achieve high correlation - in particular at spherical harmonic degree two - is a basal chemical layer. Subduction shapes this layer into two rather stable hot but chemically dense "piles", corresponding to the Pacific and African Large Low Shear Velocity Provinces. Visual comparison along cross sections indicates that sinking speeds in the geodynamic model are somewhat too fast, and should be 2±0.8 cm yr-1 to achieve a better fit.
With the study and technical development introduced here, we combine analogue sandbox simulation techniques with seismic physical modelling of sandbox models. For that purpose, we designed and developed a new mini-seismic facility for laboratory use, comprising a seismic tank, a PC-driven control unit, a positioning system, and piezo-electric transducers used here the first time in an array mode. To assess the possibilities and limits of seismic imaging of small-scale structures in sandbox models, different geometry setups were tested in the first experiments that also tested the proper functioning of the device and studied the seismo-elastic properties of the granular media used. Simple two-layer models of different materials and layer thicknesses as well as a more complex model comprising channels and shear zones were tested using different acquisition geometries and signal properties. We suggest using well sorted and well rounded grains with little surface roughness (glass beads). Source receiver-offsets less than 14 cm for imaging structures as small as 2.0-1.5 mm size have proven feasible. This is the best compromise between wide beam and high energy output, and being applicable with a consistent waveform. Resolution of the interfaces of layers of granular materials depends on the interface preparation rather than on the material itself. Flat grading of interfaces and powder coverage yields the clearest interface reflections. Finally, sandbox seismic sections provide images of very good quality showing constant thickness layers as well as predefined channel structures and fault traces from shear zones. Since these can be regarded in sandbox models as zones of decompaction, they behave as reflectors and can be imaged. The multiple-offset surveying introduced here improves the quality with respect to S/N-ratio and source signature even more; the maximum depth penetration in glass bead layers thereby amounts to 5 cm. Thus, the presented mini-seismic device is already able to resolve structures within simple models of saturated porous media, so that multiple-offset seismic imaging of shallow sandbox models, that are structurally evolving, is generally feasible.
Top-cited authors
Shenglei Fu
  • Henan University
Huanping Lu
  • Chinese Academy of Sciences
G. Gascó
  • Universidad Politécnica de Madrid
Ana Méndez
  • Universidad Politécnica de Madrid
Sabin Zahirovic
  • The University of Sydney