Estimating compositions of the deep continental crust

2021 ◽  
Author(s):  
Laura Sammon ◽  
William McDonough ◽  
Walter Mooney
Keyword(s):  
Continental Crust ◽  
Seismic Velocity ◽  
Bulk Composition ◽  
Regional Scale ◽  
Geophysical Data ◽  
Seismic Velocities ◽  
Geochemical Model ◽  
Crustal Composition ◽  
Deep Crust ◽  
Multidisciplinary Model

<p>The deep continental crust's chemical makeup is central to the debate of crustal formation, evolution, strength, and bulk composition. The impenetrable depths and pressures of the deep (roughly > 10 km) crust force geoscientists to rely on indirect sampling methods, studying medium- to high-grade metamorphic terrains and xenoliths to ascertain the composition of the middle and lower continental crust. Analyzing the deep crust in situ requires geophysical data, such as seismic velocities: Vp, Vs, and the Vp/Vs ratio. Each method provides a different perspective on deep crustal composition, but alone, neither is definitive. </p><p>To address the nonuniqueness in crust composition modeling, we use thermodynamic modeling software (i.e. Perple_X) to relate observed seismic velocities to bulk compositions and mineralogies. We present a multidisciplinary model for the composition of Earth's deep crust, using geochemical and geophysical data. Through a Monte Carlo modeling approach, we determine the best-fit geochemical model for bulk middle and lower crustal compositions. For 12 different tectonic regimes, we quantify uncertainties in crustal composition, temperature, and seismic velocity while recognizing our own scientific biases. We present a global model of deep crustal composition conclude that regional scale geological variations benefit from a higher resolution model. Overall, our model forecasts 77% of the deepest continental crust has 45 to 55 wt.% SiO<sub>2</sub>; 15% 55 to 65 wt.% SiO<sub>2</sub>; 8% may have > 65 wt.% SiO<sub>2</sub>. Of perhaps equal or greater importance, however, we present a scalable, modular program that can be altered to incorporate additional petrological and geophysical constraints, allowing geoscientists to more easily compare different scenarios for the deep crust.</p>

Numerical Insights into the Formation and Stability of Cratons

2021 ◽  
Author(s):  
Charitra Jain ◽  
Antoine Rozel ◽  
Emily Chin ◽  
Jeroen van Hunen
Keyword(s):  
Continental Crust ◽  
Seismic Velocity ◽  
Potential Temperature ◽  
Crustal Material ◽  
Spinel Lherzolite ◽  
Seismic Velocities ◽  
Geophysical Research ◽  
Thermal Buoyancy ◽  
Depleted Mantle ◽  
Cratonic Mantle

<div>Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth's continents to the presence of strong and viscous cratons underlying the continental crust. The cratons are underlain by thick and cold mantle keels, which are composed of melt-depleted and low density peridotite residues [1]. Progressive melt extraction increases the magnesium number Mg# in the residual peridotite, thereby making the roots of cratons chemically buoyant [2, 3] and counteracting their negative thermal buoyancy. Recent global models have shown the production of Archean continental crust by two-step mantle differentiation, however this primordial crust gets recycled and no stable continents form [4]. This points to the missing ingredient of cratonic lithosphere in these models, which could act as a stable basement for the crustal material to accumulate on and may also help with the transition of global regime from "vertical tectonics'' to "horizontal tectonics''. Based on the bulk FeO and MgO content of the residual peridotites, it has been proposed that cratonic mantle formed by hot shallow melting with mantle potential temperature, which was higher by 200-300 °C than present-day [5]. We introduce Fe-Mg partitioning between mantle peridotite and melt to track the Mg# variation through melting, and parametrise craton formation using the corresponding P-T formation conditions. Using self-consistent global convection models, we show the dynamic formation of cratons as a result of naturally occurring lateral compression and thickening of the lithosphere, which has been suggested by geochemical and petrological data. To allow for the material to compact and thicken, but prevent it from collapsing under its own weight, a combination of lithospheric strength, plastic yielding, dehydration strengthening, and depletion-induced density reduction of the depleted mantle material is necessary.</div><div> </div><div> [1] Boyd, F. R. High-and low-temperature garnet peridotite xenoliths and their possible relation to the lithosphere- asthenosphere boundary beneath Africa. In Nixon, P. H. (ed.) <em>Mantle Xenolith</em>, 403–412 (John Wiley & Sons Ltd., 1987).</div><div>[2] Jordan, T. H. Mineralogies, densities and seismic velocities of garnet lherzolites and their geophysical implications. In <em>The Mantle Sample: Inclusion in Kimberlites and Other Volcanics</em>, 1–14 (American Geophysical Union, Washington, D. C., 1979).</div><div>[3] Schutt, D. L. & Lesher, C. E. Effects of melt depletion on the density and seismic velocity of garnet and spinel lherzolite. <em>Journal of Geophysical Research </em><strong>111</strong> (2006).</div><div>[4] Jain, C., Rozel, A. B., Tackley, P. J., Sanan, P. & Gerya, T. V. Growing primordial continental crust self-consistently in global mantle convection models. <em>Gondwana Research</em> <strong>73</strong>, 96–122 (2019).</div><div>[5] Lee, C.-T. A. & Chin, E. J. Calculating melting temperatures and pressures of peridotite protoliths: Implications for the origin of cratonic mantle. <em>Earth and Planetary Science Letters</em> <strong>403</strong>, 273–286 (2014)</div>

Using satellite data to decipher geodynamics of the northeast Atlantic

2021 ◽  
Author(s):  
Alexander Minakov ◽  
Carmen Gaina
Keyword(s):  
Upper Mantle ◽  
Seismic Velocity ◽  
Reference Model ◽  
Pure Shear ◽  
Bulk Composition ◽  
Regional Scale ◽  
Gravity Gradient ◽  
Atlantic Region ◽  
Study Region ◽  
Northeast Atlantic

<p>We explore the mantle density structure of the northeast Atlantic region by performing constrained linear inversion of the satellite gravity gradient tensor data using statistical prior information. The residual gravity gradient signal and the prior covariance matrix are obtained using a crustal model constrained by updated database of seismic reflection and refraction profiles. We construct a 3D reference density distribution in the upper mantle assuming a pure shear model for lithospheric rifting. The mantle reference density model is consistent with mineral phase equilibria assuming a pyrolitic bulk composition. The forward modeling of the gravity gradients in the 3D reference model is performed on a global scale using a spherical harmonics approach. The northeast Atlantic model is represented using a spherical shell covering the study region down the depth of 410 km. We use tesseroids as mass elements for solving the forward and inverse gravity problem at the regional scale. The relationship between the seismic velocity and density anomalies in the Iceland-Jan Mayen region and the low-density corridor across central Greenland are discussed for understanding the origin of heterogeneities in the upper mantle of the northeast Atlantic region and their possible connections with the Cenozoic Iceland plume activity.</p>

Regional seismic velocity changes following the 2019 Mw7.1 Ridgecrest California earthquake from autocorrelations and P/S converted waves

2021 ◽  
Author(s):  
Y Lu ◽  
Y Ben-Zion
Keyword(s):  
Temporal Resolution ◽  
Seismic Velocity ◽  
Time Windows ◽  
Regional Scale ◽  
Early Period ◽  
Temporal Changes ◽  
High Temporal Resolution ◽  
Seismic Velocities ◽  
Seismic Velocity Changes ◽  
Velocity Changes

Summary We examine regional transient changes of seismic velocities generated by the Mw 7.1 2019 Ridgecrest earthquake in California, using autocorrelations of moving time windows in continuous waveforms recorded at regional stations. We focus on travel time differences in a prominent phase generated by an interface around 2 km depth, associated with transmitted Pp waves and converted Ps waves from the ongoing microseismicity. Synthetic tests demonstrate the feasibility of the method for monitoring seismic velocity changes. Taking advantage of the numerous aftershocks in the early period following the mainshock, we obtain a temporal resolution of velocity changes up to 20 min in the early post-mainshock period. The results reveal regional coseismic velocity drops in the top 1–3 km with an average value of ∼2 per cent over distances up to 100 km from the Ridgecrest event. These average velocity drops are likely dominated by larger changes in the shallow materials, and are followed by rapid recoveries on timescales of days. Around the north end of the Ridgecrest rupture and the nearby Coso geothermal region, the observed coseismic velocity drops are up to ∼8 per cent. The method allows monitoring temporal changes of seismic velocities with high temporal resolution, fast computation, and precise spatial mapping of changes. The results suggest that significant temporal changes of seismic velocities of shallow materials are commonly generated on a regional scale by large events.

scholarly journals Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations

2021 ◽  
Vol 13 (14) ◽  
pp. 2684
Author(s):  
Eldert Fokker ◽  
Elmer Ruigrok ◽  
Rhys Hawkins ◽  
Jeannot Trampert
Keyword(s):  
Pore Pressure ◽  
Seismic Velocity ◽  
Pressure Head ◽  
Groundwater Table ◽  
Surface Velocity ◽  
Seismic Velocities ◽  
Stress Monitoring ◽  
Near Surface ◽  
Velocity Variations ◽  
Velocity Changes

Previous studies examining the relationship between the groundwater table and seismic velocities have been guided by empirical relationships only. Here, we develop a physics-based model relating fluctuations in groundwater table and pore pressure with seismic velocity variations through changes in effective stress. This model justifies the use of seismic velocity variations for monitoring of the pore pressure. Using a subset of the Groningen seismic network, near-surface velocity changes are estimated over a four-year period, using passive image interferometry. The same velocity changes are predicted by applying the newly derived theory to pressure-head recordings. It is demonstrated that the theory provides a close match of the observed seismic velocity changes.

scholarly journals Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden

2020 ◽  
Author(s):  
Felix Kästner ◽  
Simona Pierdominici ◽  
Judith Elger ◽  
Christian Berndt ◽  
Alba Zappone ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Seismic Velocity ◽  
Metamorphic Rocks ◽  
Seismic Velocities ◽  
Mafic Rocks ◽  
Seismic Properties ◽  
The Core ◽  
Nappe Complex ◽  
Thrust Zone ◽  
Mountain Belts

<p>Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply-eroded orogens like the Scandinavian Caledonides allow to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications for a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can help to constrain the origin of this reflectivity. In this study, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. The core and downhole velocities deviate by up to 2 km/s. However, velocities of mafic rocks are generally in close agreement. Seismic anisotropy increases from about 5 to 26 % at depth, indicating a transition from gneissic to schistose foliation. Differences in the core and downhole velocities are most likely the result of microcracks due to depressurization of the cores. Thus, seismic velocity can help to identify mafic rocks on different scales whereas the velocity signature of other lithologies is obscured in core-derived velocities. Metamorphic foliation on the other hand has a clear expression in seismic anisotropy. To further constrain the effects of mineral composition, microstructure and deformation on the measured seismic anisotropy, we conducted additional microscopic investigations on selected core samples. These analyses using electron-based microscopy and X-ray powder diffractometry indicate that the anisotropy is strongest for mica schists followed by amphibole-rich units. This also emphasizes that seismic velocity and anisotropy are of complementary importance to better distinguish the present lithological units. Our results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.</p>

Quaternary sodic and potassic intraplate volcanism in northeast China controlled by the underlying heterogeneous lithospheric structures

Geology ◽  
2021 ◽  
Author(s):  
Xingli Fan ◽  
Qi-Fu Chen ◽  
Yinshuang Ai ◽  
Ling Chen ◽  
Mingming Jiang ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Northeast China ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Subcontinental Lithospheric Mantle ◽  
Seismic Velocities ◽  
S Wave ◽  
Crust And Upper Mantle ◽  
Potassic Volcanism

The origin and mantle dynamics of the Quaternary intraplate sodic and potassic volcanism in northeast China have long been intensely debated. We present a high-resolution, three-dimensional (3-D) crust and upper-mantle S-wave velocity (Vs) model of northeast China by combining ambient noise and earthquake two-plane wave tomography based on unprecedented regional dense seismic arrays. Our seismic images highlight a strong correlation between the basalt geochemistry and upper-mantle seismic velocity structure: Sodic volcanoes are all characterized by prominent low seismic velocities in the uppermost mantle, while potassic volcanoes still possess a normal but thin upper-mantle “lid” depicted by high seismic velocities. Combined with previous petrological and geochemical research findings, we propose that the rarely erupted Quaternary potassic volcanism in northeast China results from the interaction between asthenospheric low-degree melts and the overlying subcontinental lithospheric mantle. In contrast, the more widespread Quaternary sodic volcanism in this region is predominantly sourced from the upwelling asthenosphere without significant overprinting from the subcontinental lithospheric mantle.

scholarly journals Seismic detection of rockslides at regional scale: examples from the Eastern Alps and feasibility of kurtosis-based event location

2018 ◽  
Vol 6 (4) ◽  
pp. 955-970 ◽  
Author(s):  
Florian Fuchs ◽  
Wolfgang Lenhardt ◽  
Götz Bokelmann ◽  
Keyword(s):  
Large Scale ◽  
Seismic Velocity ◽  
Regional Scale ◽  
Vertical Component ◽  
Mean Amplitude ◽  
Velocity Model ◽  
Automatic Processing ◽  
Eastern Alps ◽  
Seismic Records ◽  
Seismic Networks

Abstract. Seismic records can provide detailed insight into the mechanisms of gravitational mass movements. Catastrophic events that generate long-period seismic radiation have been studied in detail, and monitoring systems have been developed for applications on a very local scale. Here we demonstrate that similar techniques can also be applied to regional seismic networks, which show great potential for real-time and large-scale monitoring and analysis of rockslide activity. This paper studies 19 moderate-sized to large rockslides in the Eastern Alps that were recorded by regional seismic networks within distances of a few tens of kilometers to more than 200 km. We develop a simple and fully automatic processing chain that detects, locates, and classifies rockslides based on vertical-component seismic records. We show that a kurtosis-based onset picker is suitable to detect the very emergent onsets of rockslide signals and to locate the rockslides within a few kilometers from the true origin using a grid search and a 1-D seismic velocity model. Automatic discrimination between rockslides and local earthquakes is possible by a combination of characteristic parameters extracted from the seismic records, such as kurtosis or maximum-to-mean amplitude ratios. We attempt to relate the amplitude of the seismic records to the documented rockslide volume and reveal a potential power law in agreement with earlier studies. Since our approach is based on simplified methods we suggest and discuss how each step of the automatic processing could be expanded and improved to achieve more detailed results in the future.

scholarly journals Constraining crustal silica on ancient Earth

2020 ◽  
Vol 117 (35) ◽  
pp. 21101-21107 ◽  
Author(s):  
C. Brenhin Keller ◽  
T. Mark Harrison
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
Incompatible Element ◽  
Silica Content ◽  
Efficient Mechanism ◽  
Element Abundances ◽  
Terrigenous Sediments ◽  
Earth History ◽  
Crustal Composition ◽  
Earth’S Mantle

Accurately quantifying the composition of continental crust on Hadean and Archean Earth is critical to our understanding of the physiography, tectonics, and climate of our planet at the dawn of life. One longstanding paradigm involves the growth of a relatively mafic planetary crust over the first 1 to 2 billion years of Earth history, implying a lack of modern plate tectonics and a paucity of subaerial crust, and consequently lacking an efficient mechanism to regulate climate. Others have proposed a more uniformitarian view in which Archean and Hadean continents were only slightly more mafic than at present. Apart from complications in assessing early crustal composition introduced by crustal preservation and sampling biases, effects such as the secular cooling of Earth’s mantle and the biologically driven oxidation of Earth’s atmosphere have not been fully investigated. We find that the former complicates efforts to infer crustal silica from compatible or incompatible element abundances, while the latter undermines estimates of crustal silica content inferred from terrigenous sediments. Accounting for these complications, we find that the data are most parsimoniously explained by a model with nearly constant crustal silica since at least the early Archean.

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