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

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

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

Investigation of the Lithosphere beneath the Vogelsberg volcanic complex with P-wave travel time residuals.

With the aim of investigating the P-wave velocity structure below the Tertiary volcano Vogelsberg.a network of 10 mobile short period seismograph stations was installed in May 1987 for a period of 20 months.Published20-293.3. Geodinamica e struttura dell'interno della Terrareserve

Investigation of the Lithosphere beneath the Vogelsberg volcanic complex with P-wave travel time residuals.

With the aim of investigating the P-wave velocity structure below the Tertiary volcano Vogelsberg.a network of 10 mobile short period seismograph stations was installed in May 1987 for a period of 20 months

Crustal structure at the western end of the North Anatolian Fault Zone from deep seismic sounding

The first deep seismic sounding experiment in Northwestern Anatolia was carried out in October 1991 as part of the "German - Turkish Project on Earthquake Prediction Research" in the Mudurnu area of the North Anatolian Fault Zone. The experiment was a joint enterprise by the Institute of Meteorology and Geophysics of Frankfurt University, the Earthquake Research Institute (ERI) in Ankara, and the Turkish Oil Company (TPAO). Two orthogonal profiles, each 120 km in length with a crossing point near Akyazi, were covered in succession by 30 short period tape recording seismograph stations with 2 km station spacing. 12 shots, with charge sizes between 100 and 250 kg, were fired and 342 seismograms out of 360 were used for evaluation. By coincidence an M b = 4.5 earthquake located below Imroz Island was also recorded and provided additional information on Moho and the sub-Moho velocity. A ray tracing method orginally developed by Weber (1986) was used for travel time inversion. From a compilation of all data two generalized crustal models were derived, one with velocity gradients within the layers and one with constant layer velocities. The latter consists of a sediment cover of about 2 km with V p » 3.6 km/s, an upper crystalline crust down to 13 km with V p » 5.9 km/s, a middle crust down to 25 km depth with V p » 6.5 km/s, a lower crust down to 39 km Moho depth with V p » 7.0 km/s and V p » 8.05 km/s below the Moho. The structure of the individual profiles differs slightly. The thickest sediment cover is reached in the Izmit-Sapanca-trough and in the Akyazi basin. Of particular interest is a step of about 4 km in the lower crust near Lake Sapanca and probably an even larger one in the Moho (derived from the Imroz earthquake data). After the catastrophic earthquake of Izmit on 17 August 1999 this significant heterogeneity in crustal structure appears in a new light with regard to the possible cause of the Izmit earthquake. Heterogeneities in structure are frequently also heterogeneities in strength and stress that impede or even lock rupture. The Izmit earthquake is discussed in relation to a large stepover or jog at the North Anatolian Fault