Some Physical Properties of Suevites from the Bosumtwi Impact Crater, Ghana

Suevite is a polymict breccia of clastic material derived predominantly from the crystalline basement. It is an impact-derived rock usually found at meteorite impact crater sites. In Ghana, suevites have been found at two locations at the Bosumtwi meteorite impact crater. The suevites are located in the Northern and Southern parts outside the crater rim. Due to the presence of different rock clasts of various sizes, the suevite exhibits physical properties that are quite different from those of other rocks such as granites, gneisses, etc. Suevites found in the North and South locations have some characteristic differences. In this paper, we report on the anisotropic behaviour of the compressional wave velocity Vp with pressure and azimuth for suevite samples collected from the North and South locations. The effect of pressure on Vp for the sample from the South is more pronounced than that from the North because of the high porosity of the sample at the South location. Also, the seismic velocity anisotropy is more pronounced in the samples from the South probably due to the distribution of rock inclusions in the matrix. Vp-minimum directions determined for some samples indicate that the Vp-minimum axes seemed to point toward the center of the crater. This supports the reasoning that after the impact, the ejected material on the ground might have assumed a preferred orientation with respect to the center of the crater. It was also found that suevite samples require higher saturation pressures (650 MPa and above) than solid rocks such as amphibolite which reaches velocity saturation at 100 MPa. Key words: suevites, impact crater, compressional wave velocity, anisotropy, velocity saturatio

Frequency dependence of Q for seismic body waves in the Earth's mantle

In this paper an attempt is made to determine the frequency dependence of Q in the Earth's mantle in the frequency range 0.03–1.5 Hz from the spectral ratio of teleseismic S- and P-waves. Digital broad-band data of 17 earthquakes at 40° < ∆ < 90° recorded at the Central Seismological Observatory of the Federal Republic of Germany at Erlangen were analysed. The method implies the following assumptions: frequency independence of the crustal transfer function, proportionality of Qp (f) and Qs (f), and proportionality of P- and S-source spectra. This last and most critical assumption was carefully investigated by kinematic and dynamic source models. The calculated Q-spectra for the individual events vary considerably but all have in common a general increasing trend with frequency which can best be represented by a power law Q ≈ f α with 0.25 < α < 0.6. A further increase in slope near 1 Hz suggests an absorption band corner with an upper cut-off relaxation time τm = 0.33 ± 0.18 s. The significance of the Q-spectra and their variability is estimated by manipulating semi-synthetic seismograms with different error-producing processes such as length and shape of the time window, superposition of noise, digital filter process and source spectra. It is concluded that none of these processes is able to destroy or to imitate the observed increasing trend of Q with frequency. The results are compared with those from other seismological investigations and from laboratory experiments on mantle rocks at high temperature and in the seismic frequency band.           ARK: https://n2t.net/ark:/88439/y081538 Permalink: https://geophysicsjournal.com/article/270 &nbsp

Frequency Dependent Rheology of Vesicular Rhyolite

Frequency dependent rheology of magmas may result from the presence of inclusions (bubbles, crystals) in the melt and/or from viscoelastic behavior of the melt itself. With the addition of deformable inclusions to a melt possessing viscoelastic properties one might expect changes in the relaxation spectrum of the shear stresses of the material (e.g., broadening of the relaxation spectrum) resulting from the viscously deformable geometry of the second phase. We have begun to investigate the effect of bubbles on the frequency dependent rheology of rhyolite melt. The present study deals with the rheology of bubble-free and vesicular rhyolite melts containing spherical voids of 10 and 30 vol %. We used a sinusoidal torsion deformation device. Vesicular rhyolite melts were generated by the melting (at 1 bar) of an Armenian obsidian (Dry Fountain, Erevan, Armenia) and Little Glass Mountain obsidian (California). The real and imaginary parts of shear viscosity and shear modulus have been determined in a frequency range of 0.005–10 Hz and temperature range of 600°–900°C. The relaxed shear viscosities of samples obtained at low frequencies and high temperatures compare well with data previously obtained by parallel plate viscometry. The relaxed shear viscosity of vesicular rhyolites decreases progressively with increasing bubble content. The relaxation spectrum for rhyolite melt without bubbles has an asymmetric form and fits an extended exponent relaxation. The presence of deformable bubbles results in an imaginary component of the shear modulus that becomes more symmetrical and extends into the low-frequency/high-temperature range. The internal friction Q −1 is unaffected in the high-frequency/low-temperature range by the presence of bubbles and depends on the bubble content in the high-temperature/low-frequency range. The present work, in combination with the previous study of Stein and Spera (1992), illustrates that magma viscosity can either increase or decrease with bubble content, depending upon the rate of style of strain during magmatic flow

Viscoelasticity of crystal- and bubble-bearing rhyolite melts

The effect of non-deformable inclusions on the frequency-dependent rheology of a rhyolite melt plus crystals has been investigated using a sinusoidal torsion deformation device for measurements of shear viscosity and modulus in the frequency range of 5 mHz to 20 Hz at temperatures of 750–1050°C. The relaxed shear viscosity and unrelaxed shear modulus of rhyolite magma (rhyolite melt plus crystals plus bubbles) decreases with increasing bubble content and increases with the addition of crystals. At a crystal concentration of about 45% a relaxed value of the shear viscosity is not attainable. The presence of rigid inclusions results in an imaginary component of the shear modulus that becomes more symmetrical and shifted to the low-frequency—high-temperature range with respect to that for a crystal-free melt. The slope of log(Q−1) (internal friction) as a function of the dimensionless variable log(ωτ), is unaffected in the low-temperature—high-frequency range of crystals, with Q−1 ≈ 1/(ωτ)0.5 (the same as for bubble- and crystal-free rhyolite). For the present type of suspension, the internal friction is practically constant and independent of log(ωτ) in the high-temperature—low-frequency limit (ωτ 1). The shape of the Cole-Cole diagram becomes symmetrical and can be described as a Caputo body with parameter γ ≈ 0.45, whereas for bubble-bearing and inclusion-free rhyolite melts the shape of diagram relates to the β-relaxation exponent with β ≈ 0.5. The present work demonstrates that magma may or may not follow a power-law rheology depending on the relative volume proportion between crystals and bubbles

Crustal structure of active deformation zones in Africa: Implications for global crustal processes

The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust sills, and intruded as sills and dikes at shallower depths. In the Main Ethiopian rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward-dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km-thick crust. In the 30Ma-Recent CVL, which lacks a hotspot age-progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hotspot towards the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology and contributing significantly to global carbon and water fluxes

Seismicity Associated With the Formation of a New Island in the Southern Red Sea

Volcanic eruptions at mid-ocean ridges are rarely witnessed due to their inaccessibility, and are therefore poorly understood. Shallow waters in the Red Sea allow the study of ocean ridge related volcanism observed close to sea level. On the 18th December 2011, Yemeni fishermen witnessed a volcanic eruption in the Southern Red Sea that led to the formation of Sholan Island. Previous research efforts to constrain the dynamics of the intrusion and subsequent eruption relied primarily on interferometric synthetic aperture radar (InSAR) methods, data for which were relatively sparse. Our study integrates InSAR analysis with seismic data from Eritrea, Yemen, and Saudi Arabia to provide additional insights into the transport of magma in the crust that fed the eruption. Twenty-three earthquakes of magnitude 2.1–3.9 were located using the Oct-tree sampling algorithm. The earthquakes propagated southeastward from near Sholan Island, mainly between December 12th and December 13th. The seismicity is interpreted as being induced by emplacement of a ∼12 km-long dike. Earthquake focal mechanisms are primarily normal faulting and suggest the seismicity was caused through a combination of dike propagation and inflation. We combine these observations with new deformation modeling to constrain the location and orientation of the dike. The best-fit dike orientation that satisfies both geodetic and seismic data is NNW-SSE, parallel to the overall strike of the Red Sea. Further, the timing of the seismicity suggests the volcanic activity began as a submarine eruption on the 13th December, which became a subaerial eruption on the 18th December when the island emerged from the beneath the sea. The new intrusion and eruption along the ridge suggests seafloor spreading is active in this region

Surface wave tomography across Afar, Ethiopia: crustal structure at a rift triple-junction zone

The Afar Depression in northeast Africa contains the rift triple-junction between the Nubia, Arabia and Somalia plates. We analyze Rayleigh wave group velocity from 250 regional earthquakes recorded by 40 broadband stations to study the crustal structure across Afar and adjacent plateau regions in northern Ethiopia. The dispersion velocities are inverted to obtain surface wave tomographic maps for periods between 5 and 25 seconds, sensitive to approximately the top 30 km of the lithosphere. The tomographic maps show a significant low dispersion velocity anomaly (>20%) within the upper crust, below the site of recent dyke intrusions (2005–present) in the Dabbahu and Manda-Hararo magmatic segments. Similar low velocity regions are imaged where magma intrusion in the Afar crust has been inferred over the last decade from seismicity or volcanic eruptions. We invert two group velocity curves to compare the S-wave velocity structure of the crust within an active magmatic segment with that of adjacent areas; the active region has a low velocity zone (Vs ∼ 3.2 km/s), between about 6–12 km, which we infer to be due to the presence of partial melt within the lower crust

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure