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We present 2D and 3D Fourier-domain modeling of gravity effects, including the gravity potential and its first- and second- order derivatives, generated by an arbitrary polyhedron with constant and exponential density distributions. Fourier-domain expressions are obtained using Gauss’s divergence theorem repeatedly to transform the volume integral first into surface integrals and then to line integrals in the wave number domain. Both the derivation and the final expressions are simpler and more compact than space-domain ones. The highly accurate and efficient Gauss-FFT algorithm is then applied to transform the Fourier-domain expressions back to space-domain gravity fields. Synthetic and real model tests show that the Fourier-domain algorithm presented can provide forward results almost identical to space-domain analytical or numerical solutions at places where the exact solution changes smoothly. However, high-frequency truncation errors do become noticeable in the near vicinity of the source body, where the exact solution changes abruptly, or even discontinuously. The Fourier-domain algorithm captures almost all frequency components of the exact solution that are lower than the Nyquist frequency, which is determined by the chosen grid intervals. The algorithm offers a more efficient solution for 2D and 3D modeling of gravity fields on large and densely sampled regular grids than classical space-domain solutions, at the cost of a small loss of accuracy.

Differential code biases (DCBs) account for the most significant systematical biases when sensing the earth’s ionosphere with GNSS observations and are also important correction parameters in GNSS applications of positioning, navigation and timing. With the continuous modernization of the American GPS and Russian GLONASS systems, and also the rapid developments of the European Galileo and Chinese BeiDou systems, there is a strong demand of precise satellite DCB products for multiple constellations and frequencies. This study proposes a new method for the precise determination of multi-GNSS triple-frequency DCBs, which can be divided into three steps. The first step is to precisely retrieve slant ionospheric delays and “additional code biases” based on a newly established full-rank triple-frequency precise point positioning (PPP) model with raw observations. Both the slant ionospheric delays and “additional code biases” containing the DCBs need to be estimated. Then, an enhanced IGGDCB (IGG stands for Institute of Geodesy and Geophysics) method is used to estimate the DCBs between the first and second frequency bands with the PPP-derived slant ionospheric delays. At last, the previously estimated DCBs between the first and second frequency bands are substituted into the “additional code biases” and DCBs between the first and third frequency bands are estimated. Multi-GNSS slant ionospheric delays from the triple-frequency PPP method are compared with those from the traditional dual-frequency carrier-to-code level (CCL) method, in terms of formal precision and zero-baseline experiment. Quad-system average formal precisions are 0.08 and 0.41 TECU, for PPP and CCL methods, respectively, indicating the obvious improvements of PPP over CCL. One month of data from 60 globally distributed multi-GNSS experiment stations are selected, and totally eight types of DCBs are estimated for GPS, GLONASS, Galileo and BeiDou. Multi-GNSS satellite DCBs generated with the proposed method are compared with the products from different agencies, including Center for Orbit Determination in Europe (CODE), Deutsches zentrum für Luft-und Raumfahrt (DLR) and Chinese Academy of Sciences (CAS). For GPS C1WC2 W DCBs, RMS values with respect to CODE products are 0.24, 0.07 and 0.09 ns for DLR, CAS and IGG (this study), respectively. RMS values are 0.31/0.25 and 0.19/0.15 ns, for GPS C1WC5X and C1WC5Q DCBs and with respect to DLR/CAS, respectively. For GLONASS C1PC2P DCBs, RMS values with respect to CODE are 0.68, 0.49 and 0.33 ns for DLR, CAS and IGG, respectively. For Galileo, RMS values are 0.16/0.20 and 0.13/0.14 ns, for C1XC5X and C1XC7X DCBs and with respect to DLR/CAS, respectively. For BeiDou, RMS values are 0.32/0.25 and 0.34/0.41, for C2IC7I and C2IC6I DCBs and with respect to DLR/CAS. These results show that the proposed method can provide multi-GNSS and multi-frequency satellite DCB estimation with high precision, processing efficiency and flexibility.

The emerging GNSSs make single-frequency (SF) RTK positioning possible. In this contribution two different types of low-cost (few hundred USDs) RTK receivers are analyzed, which can track L1 GPS, B1 BDS, E1 Galileo and L1 QZSS, or any combinations thereof, for a location in Dunedin, New Zealand. These SF RTK receivers can potentially give competitive ambiguity resolution and positioning performance to that of more expensive (thousands USDs) dual-frequency (DF) GPS receivers. A smartphone implementation of one of these SF receiver types is also evaluated. The least-squares variance component estimation (LS-VCE) procedure is first used to formulate a realistic stochastic model, which assures that our receivers at hand can achieve the best possible ambiguity resolution and RTK positioning performance. The best performing low-cost SF RTK receiver types are then assessed against DF GPS receivers and survey-grade antennas. Real data with ionospheric disturbances at low, medium and high levels are analyzed, while making use of the ionosphere-weighted model. It will be demonstrated that when the presence of the residual ionospheric delays increases, instantaneous RTK positioning is not possible for any of the receivers, and a multi-epoch model is necessary to use. It is finally shown that the low-cost SF RTK performance can remain competitive to that of more expensive DF GPS receivers even when the ionospheric disturbance level reaches a Kp-index of 7−, i.e. for a strong geomagnetic storm, for the baseline at hand.

We have measured the geometric deformations of the Onsala 20 m VLBI telescope utilizing a combination of laser scanner, laser tracker, and electronic distance meters. The data put geometric constraints on the electromagnetic raypath variations inside the telescope. The results show that the propagated distance of the electromagnetic signal inside the telescope differs from the telescope’s focal length variation, and that the deformations alias as a vertical or tropospheric component. We find that for geodetic purposes, structural deformations of the telescope are more important than optic properties, and that for geodetic modelling the variations in raypath centroid rather than focal length should be used. All variations that have been identified as significant in previous studies can be quantified. We derived coefficients to model the gravitational deformation effect on the path length and provide uncertainty intervals for this model. The path length variation due to gravitational deformation of the Onsala 20 m telescope is in the range of 7–11 mm, comparing elevation 0 \(^{\circ }\) and 90 \(^{\circ }\) , and can be modelled with an uncertainty of 0.3 mm.

The fast motion of low earth orbit (LEO) satellite contributes to the geometric diversity, allowing for rapid convergence of precise point positioning (PPP). In this contribution, we investigate the PPP performance of the LEO constellation-augmented full operational capability (FOC) multi-GNSS. We design six LEO constellations with different satellite numbers, orbit altitudes and orbit types, together with the FOC multi-GNSS constellations, and then simulate both the onboard LEO and ground-based observations. The multi-GNSS POD result shows much better orbit accuracy of 3.3, 2.7 and 2.6 cm in radial, along-track and cross-track components, respectively, compared with that of 10.3, 9.2 and 8.9 cm for GPS-only POD. Furthermore, the performance of LEO-augmented multi-GNSS PPP is evaluated. With the augmentation of 60-, 96-, 192- and 288-satellite LEO constellation, the multi-GNSS PPP convergence time can be shortened from 9.6 to 7.0, 3.2, 2.1 and 1.3 min, respectively, in midlatitude region. For LEO-augmented GPS- and BDS-only PPP, the improvement is more significant with the convergence time dramatically shortened by 90% from about 25 to within 3 min with 192- or 288-satellite constellation. The augmentation capability is also found to be associated with station latitude, and the higher latitude, the better performance. To enable about more than 70% significant reduction on convergence time, as well as considering the cost, the 192-satellite LEO constellation scheme is suggested. In terms of orbit altitude, the scheme of 1000 km presents better performance than that of 600 km. As for orbit type, the performances are comparable for polar and sun-synchronous orbits. Additionally, LEO-only PPP can be achieved with the convergence time of about 6.5 min.

The drift rate of relative gravimeters differs from time to time and from meter to meter. Furthermore, it is inefficient to estimate the drift rate by returning them frequently to the base station or stations with known gravity values during gravity survey campaigns for a large region. Unlike the conventional gravity adjustment procedure, which employs a linear drift model, we assumed that the variation of drift rate is a smooth function of lapsed time. Using this assumption, we proposed a new gravity data adjustment method by means of objective Bayesian statistical inference. Some hyper-parameters were used as trade-offs to balance the fitted residuals of gravity differences between station pairs and the smoothness of the temporal variation of the drift rate. We employed Akaike’s Bayesian information criterion (ABIC) to estimate these hyper-parameters. A comparison between results from applying the classical and the Bayesian adjustment methods to some simulated datasets showed that the new method is more robust and adaptive for solving problems caused by irregular nonlinear meter drift. The new adjustment method is capable of determining the time-varying drift rate function of any specific gravimeter and optimizing the weight constraints for every gravimeter used in a gravity survey. We also carried out an error analysis for the inverted gravity value at each station based on the marginal distribution. Finally, we used this approach to process actual gravity survey campaign data from an observation network in North China.

The Earth Orientation Center of the International Earth Rotation and Reference Systems Service (IERS) has the task to provide the scientific community with the international reference time series of Earth orientation parameters (EOP), referred to as IERS EOP C04 or C04. These series result from a combination of operational EOP series derived from VLBI, GNSS, SLR, and DORIS. The C04 series were updated to provide EOP series consistent with the set of station coordinates of the ITRF 2014. The new C04, referred to as IERS EOP 14C04, is aligned onto the most recent versions of the conventional reference frames (ITRF 2014 and ICRF2). Additionally, the combination algorithm was revised to include an improved weighting of the intra-technique solutions. Over the period 2010–2015, differences to the IVS combination exhibit standard deviations of 40  \(\upmu \) as for nutation and 10  \(\upmu \) s for UT1. Differences to the IGS combination reveal a standard deviation of 30  \(\upmu \) as for polar motion. The IERS EOP 14C04 was adopted by the IERS directing board as the IERS reference series by February 1, 2017.

The accurate computation of gravitational effects from topographic and atmospheric masses is one of the core issues in gravity field modeling. Using gravity forward modeling based on Newton’s integral, mass distributions are generally decomposed into regular mass bodies, which can be represented by rectangular prisms or polyhedral bodies in a rectangular coordinate system, or tesseroids in a spherical coordinate system. In this study, we prefer the latter representation because it can directly take the Earth’s curvature into account, which is particularly beneficial for regional and global applications. Since the volume integral cannot be solved analytically in the case of tesseroids, approximation solutions are applied. However, one well-recognized issue of these solutions is that the accuracy decreases as the computation point approaches the tesseroid. To overcome this problem, we develop a method that can precisely compute the gravitational potential \(\left( V\right) \) and vector \(\left( V_x, V_y, V_z\right) \) on the tesseroid surface. In addition to considering a constant density for the tesseroid, we further derive formulas for a linearly varying density. In the near zone (up to a spherical distance of 15 times the horizontal tesseroid dimension from the computation point), the gravitational effects of the tesseroids are computed by Gauss–Legendre quadrature using a two-dimensional adaptive subdivision technique to ensure high accuracy. The tesseroids outside this region are evaluated by means of expanding the integral kernel in a Taylor series up to the second order. The method is validated by synthetic tests of spherical shells with constant and linearly varying density, and the resulting approximation error is less than \(10^{-4}\,\hbox {m}^2\,\hbox {s}^{-2}\) for V, \(10^{-5}\,\hbox {mGal}\) for \(V_x\) , \(10^{-7}\,\hbox {mGal}\) for \(V_y\) , and \(10^{-4}\,\hbox {mGal}\) for \(V_z\) . Its practical applicability is then demonstrated through the computation of topographic reductions in the White Sands test area and of global atmospheric effects on the Earth’s surface using the US Standard Atmosphere 1976.

Local ties (LTs) at co-located sites are currently used to combine different single-technique solutions to determine global terrestrial reference frames (TRFs). We assess by simulations the impact of different LT standard deviations, biased LTs and a selection of LTs on the datum realization of global TRFs. The simulations are based on Global Positioning System (GPS), Satellite Laser Ranging, and Very Long Baseline Interferometry (VLBI) observations covering the time span 2008–2014. We find that LT standard deviations of 1 cm and better yield differences in the TRF-defining parameters below 1 mm. VLBI is most affected by altering the LT standard deviations, especially in the translations since VLBI is inherently not sensitive to the origin of the TRF. Altering the standard deviations of the LTs applied in ITRF2005, ITRF2008, ITRF2014 results in small differences reaching a maximum of 0.6 mm at the VLBI stations. Simulating technique-wise biased LT stations shows the largest differences in the TRF-defining parameters of more than 2 mm, if all GPS LT stations are biased by 1 cm, proving that GPS plays the major role in the connection of the three techniques. Simulating single biased LT stations by 1 cm in either the north, east, or height component indicates small differences of less than 0.8 mm in the TRF-defining parameters, the largest differences result at LT stations located on the southern hemisphere. The selection of LTs demonstrates that the southern hemisphere LTs are very important, especially for the realization of the scale.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Kan Li, Long Li, D. Graham Pearson, Thomas Stachel

A long-standing unresolved problem in understanding Earth's deep carbon cycle is whether crustal carbon is recycled beyond arc depths. While isotopic signatures of eclogitic diamonds and their inclusions suggest deep recycling of crustal material, the crustal carbon source remains controversial; seafloor sediment – the widely favored crustal carbon source – cannot explain the combined carbon and nitrogen isotopic characteristics of eclogitic diamonds. Here we examined the carbon and oxygen isotopic signatures of bulk-rock carbonate for 80 geographically diverse samples from altered mafic-ultramafic oceanic crust (AOC), which comprises 95 vol% of the crustal material in subducting slabs. The results show: (i) AOC contains carbonate with δ13C values as low as −24‰, indicating the presence of biogenic carbonate; (ii) carbonate in AOC was mainly formed during low-temperature (<100 °C) alteration processes. Modeling accounting for this newly recognized carbon source in the oceanic crust with formation temperatures <100 °C yields a global carbon influx of 1.5±0.3 × 1012 mol C/yr carried by subducting AOC into the trench, which is 50–90% of previous estimates, but still of the same order of the carbon influx carried by subducting sediments into the trench. The AOC can retain carbon better than sediment during subduction into the asthenosphere, transition zone and lower mantle. Mixing of asthenospheric and AOC fluids provides the first consistent explanation of the diverse record of carbon and nitrogen isotopes in diamonds, suggesting that AOC, instead of sediment, is the key carrier of crustal carbon into the deep mantle.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Giacomo Pozzi, Nicola De Paola, Robert E. Holdsworth, Leon Bowen, Stefan B. Nielsen, Edward D. Dempsey

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 104), which remain so far poorly investigated and understood.

We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 m s−1), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique.

Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients >0.6). Cataclasis, aided by twinning and crystal plasticity, operates forming an extremely comminuted shear band (mean grain size ∼200 nm). As the fault starts weakening, shear localises within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 μm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault re-strengthens, upon deceleration to arrest, the ultramylonite may be partially reworked by brittle deformation.

Our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow, adjacent deactivated layers, where mirror-like surfaces are located. This shows that EBSD techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and, potentially, natural faults.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Samuel M. Howell, Jean-Arthur Olive, Garrett Ito, Mark D. Behn, Javier Escartín, Boris Kaus

Oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid-ocean ridges, and is associated with dramatic changes in seafloor morphology. Detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes (OCCs), and often transition to multiple regularly-spaced normal faults that form abyssal hills parallel to the spreading axis. Previous studies have attributed these changes to along-axis gradients in lithospheric strength or magma supply. However, despite the recognition that magma supply can influence fault style and seafloor morphology, the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along-axis variations in magma supply remain poorly understood. This study investigates this issue using two complementary modeling approaches. The first consists of semi-analytical, two-dimensional (2-D) cross-axis models designed to address the fundamental mechanical controls on the longevity of normal faults. These 2-D model sections are juxtaposed in the along-axis direction to examine the response of the plan-view pattern of faults to along-axis variations in magmatic accretion in the absence of along-axis mechanical coupling. The second approach uses three-dimensional (3-D), time-dependent numerical models that simulate faulting and magma intrusion in a visco-elasto-plastic continuum. The primary variable studied through both approaches is the along-axis gradient in the fraction M of seafloor spreading that is accommodated by magmatism. The 2-D and 3-D results predict different abyssal hill spacing and orientation, however the plan-view geometry of self-emerging detachment faults predicted by the 3-D numerical models are well explained by the juxtaposed 2-D models. This indicates a first-order control by cross-axis effects of changing values of M. These models are also shown to explain the along-axis extent and plan-view curvature of the well-developed 13°20′N and Mt. Dent OCCs (Mid-Atlantic Ridge and Cayman Rise) in terms of quantifiable along-axis gradients in magma emplacement rates.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Kyle T. Ashley, Michael S. Ramsey

Our understanding of the Venusian surface composition is limited to the in-situ bulk rock chemical analyses collected by the Vega and Venera missions. However, these analyses have exceedingly large analytical uncertainties – up to 50% by weight (1σ) for certain oxide components. In this study, we use the Venera 14 lander data and apply a Monte Carlo approach to assess how significant uncertainties affect modeling of lava solidification. Thermodynamic modeling of mineral-melt equilibria is conducted over 1,000 iterations of simulated bulk composition within the Gaussian probabilistic bounds of the reported analytical uncertainty along a cooling path from 1350 °C to 950 °C, with a fixed pressure of 90 bars. The results are used to calculate melt and apparent viscosity (ηmelt and ηapp, respectively). Despite the significant analytical uncertainty of the lander data, lava viscosity is tightly constrained. Mean log ηmelt increases from 1.65 ± 0.46 Pa s at 1350 °C to 8.57 ± 1.42 Pa s at 950 °C (1σ). Log ηapp is less well-defined due to increased scatter in crystal fraction with solidification, increasing to 17.7 ± 1.3 Pa s at 950 °C (γ=10−6 s−1). A significant increase in ηapp occurs between 1240 °C and 1080 °C due to a rapid increase in crystal mass fraction (Φ increases from ∼0.05 at 1250 °C to 0.8 at ∼1100 °C). Slow cooling through this 150 °C window must occur so as to not drastically increase lava viscosity and impede flow. These results show that despite limited geochemical data and large analytical uncertainties, reasonable constraints for the physical and chemical evolution of lava solidification can be obtained. Most of the Venusian surface is composed of volcanic plains and rises, containing abundant landforms characteristic of fluid basaltic lava. Our results provide new insights into the crystallization processes in Venusian lava flows, which are fundamental for understanding Venusian igneous processes, geodynamics, and resurfacing. This work provides a necessary framework for future thermorheological flow models to determine flow volume and effusion rates.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Costanza Morino, Susan J. Conway, Þorsteinn Sæmundsson, Jón Kristinn Helgason, John Hillier, Frances E.G. Butcher, Matthew R. Balme, Colm Jordan, Tom Argles

Molards have been defined in the past as conical mounds of debris that can form part of a landslide's deposits. We present the first conclusive evidence that molards in permafrost terrains are cones of loose debris that result from thawing of frozen blocks of ice-rich sediments mobilised by a landslide, and hence propose a rigorous definition of this landform in permafrost environments. We show that molards can be used as an indicator of permafrost degradation, and that their morphometry and spatial distribution give valuable insights into landslide dynamics in permafrost environments. We demonstrate that molards are readily recognisable not only in the field, but also in remote sensing data; surveys of historic aerial imagery allow the recognition of relict molards, which can be used as an indicator of current and past permafrost conditions. The triggering of landslides as a result of permafrost degradation will arguably occur more often as global atmospheric temperatures increase, so molards should be added to our armoury for tracking climate change, as well as helping us to understand landslide-related hazards. Finally, we have also identified candidate molards on Mars, so molards can inform about landscape evolution on Earth and other planetary bodies.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Ruifeng Zhang, Rachel L. Kelly, Kathryn M. Kauffman, Amber K. Reid, Jonathan M. Lauderdale, Michael J. Follows, Seth G. John

Phytoplankton growth in many high-latitude and equatorial regions of the world ocean is limited by low iron (Fe), while regions of lower nutrient upwelling and higher dust supply such as the subtropical gyres are typically not considered Fe-limited. Recent work which showed that adding Saharan dust leachate promotes blooms of Vibrio bacteria in the North Atlantic Ocean challenges this simple paradigm, because Fe is one of the most prevalent micronutrients in the leachate. We also find that adding high quantities of dust leachate to the North Atlantic near Bermuda stimulates Vibrio growth. However, inorganic Fe alone did not stimulate the growth of Vibrio in any of eleven field experiments in the North Atlantic, central North Pacific, or the coastal Pacific, suggesting that some other growth factor besides Fe is partially or completely responsible for the Vibrio growth response with added dust leachate. We consider the possibility that organic carbon, phosphorous, or other trace-metals in dust might have stimulated Vibrio growth either directly or indirectly. Laboratory culture experiments demonstrate that dust leachate can stimulate the growth of Vibrio even when sufficient Fe is present in the medium. Global modeling suggests that dust fluxes high enough to stimulate Vibrio growth are rare.

Publication date: Available online 17 April 2019

Source: Earth and Planetary Science Letters

Author(s): C. Pederson, V. Mavromatis, M. Dietzel, C. Rollion-Bard, G. Nehrke, R. Neuser, N. Jöns, K.P. Jochum, A. Immenhauser

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): C. Garbelli, S.Z. Shen, A. Immenhauser, U. Brand, D. Buhl, W.Q. Wang, H. Zhang, G.R. Shi

Earth's transition from an icehouse to hothouse during the Late Paleozoic was characterized by a series of high paleolatitude glacial-interglacial events. However, the exact timing of these events remains unresolved. Here, we report on fifty-five calcitic shells of brachiopods and bivalves screened to obtain reliable 87Sr/86Sr ratios for Cisuralian (298.9-272.95 Ma) and Guadalupian (272.95-259.1 Ma) seawater. Specifically, we used well-preserved shells to build a 87Sr/86Sr seawater curve for the Sydney Basin, and then with the marine Look-up Table of the Permian calculated numerical ages of the stratigraphic succession. This allowed us to match seawater changes to the coeval P1 – P3 glacial-interglacial cycles recorded in the sedimentary successions of the Sydney Basin. Evaluation of the 87Sr/86Sr results revealed that onsets of the P2 and P3 glaciations in the Sydney Basin should be assigned to the early Artinskian and late Wordian, respectively. The range of 87Sr/86Sr values recorded by brachiopods from the Wandrawandian Siltstone coupled with recent geochronological dating of the Broughton Formation suggest that glacial phase P3 lasted about 2 Myr and was confined to the late Wordian – early Capitanian. Dating obtained using the Sr-isotope proxy from brachiopods agrees with the geochronologic ages, and they suggest that the glacial phases P1 to P3 became progressively shorter in duration and less intense. Conversely, the corresponding interglacials became progressively longer, and thus, documenting the gradual transition of the Permian icehouse to an ice-free greenhouse world. Our study confirms that strontium isotopes measured in screened brachiopod shell archives are sufficiently robust to date Paleozoic marine sedimentary successions, and are most valuable in sedimentary successions that otherwise lack geo-chronological dating.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Mathieu Rospabé, Mathieu Benoit, Georges Ceuleneer, Mary-Alix Kaczmarek, Florent Hodel

On Earth, most of the critical processes happen at the frontiers between envelopes and especially at the Moho between the mantle and the crust. Beneath oceanic spreading centers, the dunitic transition zone (DTZ) appears as a major interface between the upwelling and partially molten peridotitic mantle and the accreting gabbroic lower crust. Better constraints on the processes taking part in the DTZ allows improved understanding of the interactions between silicate melts and hydrated fluids, which act competitively to generate the petrological Moho. Here we combine mineral and whole rock major and trace element data with a structural approach along three cross-sections up to 300 m thick above the fossil Maqsad mantle diapir (Oman ophiolite) in order to understand the vertical organization of the DTZ with depth. Our results highlight that most of the faults or fractures cross-cutting the DTZ were ridge-related and active at an early, high temperature magmatic stage. Chemical variations along the cross-sections define trends with a characteristic vertical scale of few tens of meters. There is a clear correlation between the chemical variation pattern and the distribution of fault zones, not only for fluid-mobile elements but also for immobile elements such as REE and HFSE. Faults, despite displaying very limited displacements, enhanced both melt migration and extraction up to the crust and deep hydrothermal fluids introduction down to the Moho level. We propose that these faults are a vector for upwelling melt modification by hybridization, with hydrothermal fluids and/or silicic hydrous melts, and crystallization. Infiltration of these melts or fluids in the country rock governs part of the gradational evolutions recorded in composition of both the olivine matrix and interstitial phases away from faults. Finally, these faults likely control the thermal structure of the mantle-crust transition as evidenced by the spatial distribution of the crystallization products from percolating melts, organizing the transition zone into pure dunites to impregnated dunites horizons. In this context, the DTZ appears as a reactive interface that developed by the combination of three primary processes: tectonics, magmatism and deep, high temperature hydrothermal circulations. Accordingly, these features fundamentally contribute to the variable petrological and geochemical organization of the DTZ and possibly of the lower crust below oceanic spreading centers, and may be a clue to interpret part the heterogeneity observed in MORB signatures worldwide.

Publication date: 15 June 2019

Source: Earth and Planetary Science Letters, Volume 516

Author(s): Nadine Staudenmaier, Thessa Tormann, Benjamin Edwards, Arnaud Mignan, Stefan Wiemer

We analyse the frequency-size-distribution of non-volcanic tremors observed along the Parkfield section of the San Andreas Fault. We suggest that these non-volcanic tremors follow a power-law scaling typical of scale-invariant, stick slip tectonic earthquakes, but with an unusually high scaling exponent of more than 2.0 and a systematic depth-dependency. While each individual non-volcanic tremor releases only a minuscule amount of energy and slip, this is more than compensated by their sheer numbers. Consequently, the integrated contribution of this largely ‘invisible’ seismicity (non-volcanic tremors and nano-earthquakes) is non-negligible and could potentially account in selected patches along the San Andreas fault for up to 100% of the plate motion.