Systematic rare earth and minor element concentration variations in basalt recently erupted along the post-glacial Reykjanes Ridge Axis and its northward extension over Iceland reflect the existence, spatial influence, and primordial nature of the Iceland hot mantle plume.

Iceland Mantle Plume: Geochemical Study of Reykjanes Ridge

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source-equalizd horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat-lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high-velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust-mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zonemore » between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.« less

Seismic evidence for an ancient rift beneath the Cumberland Plateau, Tennessee: A detailed analysis of broadband teleseismic P waveforms

Summary

The dispersion of Love and Rayleigh waves in the period range 17–167 s is used to detect the change in the structure of the upper mantle as the age of the sea-floor increases away from the mid-ocean ridge Using the single station method, the group and phase velocities of Rayleigh waves were measured for 78 paths in the east Pacific In order to describe the observed Rayleigh wave dispersion, both a systematic increase in velocities with the age of the sea-floor and anisotropy of propagation are required The maximum change in velocity with age is about 5 per cent, with the contrast between age zones decreasing with increasing period The greatest change occurs in the first few million years, due to the rapid cooling and solidification of the upper part of the lithosphere In the 0–5 My age zone, the average thickness of the lithosphere can be no greater than 30 km, including the water and crustal layers Within 10 My after formation, the lithosphere reaches a thickness of about 60 km As the mantle continues to cool, the shear velocity within the lithosphere increases Within the area of this study, no change occurs in the upper mantle deeper than about 80 km



Rayleigh waves travel fastest in the direction of spreading The degree of anisotropy in Rayleigh wave propagation is frequency-dependent, reaching a maximum of 2–0±0–2 per cent at a period of about 70 s Several models are constructed which can reproduce this frequency-dependent anisotropy



The regional phase velocities of the fundamental and first higher Love modes have been simultaneously measured using a new technique The Love wave data is inconsistent with the Rayleigh wave data unless SH velocity is higher than SV velocity within the uppermost 125 km of the mantle Anisotropy deeper than 250 km is suggested, but not required, by the data

The Early Structural Evolution and Anisotropy of the Oceanic Upper Mantle

The Lesser Antilles volcanic chain : a study in arc magmatism.

The broad topographic domes, or plateaus, of East Africa and Afar are characterized by long-wavelength negative Bouguer gravity anomalies and volcanically active rift valleys. Gravity and topography data from the East African and Afar plateaus and data from the stable cratonic regions to the west were subdivided into 17 smaller regions to study the variation of elastic plate thickness within part of the African continent, its relation to rifting processes within these intracontinental plateau regions, and compensation mechanisms for the broad uplifts. Assuming that loads at the surface, within, and beneath the base of a thin elastic plate contribute to the observed Bouguer gravity anomalies, the wavelength dependence of the coherence between gravity and topography was used to determine the effective elastic plate thickness in each of the subregions. Estimates of elastic plate thickness were found to be 64–90+ km in the stable cratonic areas, provided that surface and subsurface loads are uncorrelated. Lower estimates (43–49 km) were obtained in the largely unfaulted regions encompassing the broad uplifted plateaus and the narrower Darfur dome to the west of the Afar plateau. Estimates of 21–36 km correspond to regions that include the severely faulted and commonly volcanically active Kenya, Western, and Ethiopian rift valleys as well as unfaulted regions adjacent to the rift valleys. We attribute the smallest estimates of elastic plate thickness (21–36 km) to averaging unfaulted topography with mechanically weakened topography within the severely faulted Kenya, Western, and Ethiopian rifts. The linear transfer function between gravity and topography within the uplifted East African plateau region at wavelengths longer than 1000 km can be explained by a dynamic uplift mechanism and associated heating of the thermal lithosphere above a convecting region within the asthenosphere. These isostatic and dynamical compensation mechanisms are consistent with existing geological and geophysical data and with constraints on the timing of volcanism and uplift within the East African plateau region.

Effective elastic plate thickness beneath the East African and Afar plateaus and dynamic compensation of the uplifts

Summary. A seismic refraction experiment along am across the Lesser Antilles island arc in the eastern Caribbean has shown that the crust beneath the arc contains two refractors with average seismic velocities of 6.2 and 6.9 km/s. The upper, 6.2 km/s refractor varies in depth and shows large fluctuations in its seismic velocity. The layer betweeen the refractors has an average thickness of about 10 km, and varies between 2 and 20 km. The total thickness of the crust of the arc is about 30 km beneath Martinique and about 35 km beneath St Vincent. The approximate depths of the Moho beneath the centres of Grenada and Tobago Troughs either side of the arc are 21 and 25 km respectively. Both these troughs, although of different origin, are underlain by crust of modified oceanic type.

A seismic refraction investigation of crustal structure beneath the Lesser Antilles island arc

Summary Durham University’s seismic array at Kaptagat was set up on the western flank of the Kenya Rift. Analysis of the kst arrivals of local and regional earthquakes recorded at the array has provided a crustal model across the Rift margin. It is concluded that a steep structural boundary separates normal shield crust and sub-Moho material beneath the western flank of the Gregory Rift from a mantle derived crustal intrusion beneath the Rift axis. A detailed model of the shield crust is presented and an indication is given of the complex structure which probably exists beneath the Rift itself.

/pdf/the-structure-on-the-western-flank-of-the-gregory-rift-kenya-2549rq02a4.pdf

The Structure on the Western Flank of the Gregory Rift (Kenya). Part I. The Crust

Summary. The existence of an anomalously low-velocity, low-density zone within the upper mantle beneath the Kenya Dome has been deduced on the basis of previous gravity and seismic studies. This paper describes an experiment to measure teleseismic delay times across the Gregory Rift near the equator and along a SE radius of the Kenya Dome. The delay times have been determined with good relative accuracy and provide further independent evidence for the existence of the anomalous zone. The pattern of delay times along the two profiles and at other stations indicates that the zone thins rapidly to the SE away from the rift axis, mirroring the attenuation observed, from Kaptagat, for the same zone to the NW. The trend is for the thinning to become very much less rapid with distance, but there is also clear evidence for localized thickening of the zone under the Kilimanjaro–Chyulu volcanic area.



Significantly smaller delay times are measured at the centre of the rift than at the edges. This is shown to indicate that the anomalous zone penetrates the crust to form an intrusion of relatively high-velocity material along the rift axis. The clear correlation of the delay time low with the axial Bouguer high indicates that they are both manifestations of the same underlying structure. Thus the delay time results provide independent confirmation of the existence of the axial intrusion previously inferred from gravity data. The width of this intrusion at the normal base of the crust is well defined by the data as 30 km.

Lithospheric structure beneath the Kenya Dome

Summary Slowness and delay time data for teleseismic P-wave arrivals recorded at the temporary seismic array station at Kaptagat on the western flank of the Gregory Rift are presented. The data are used to elucidate the shape and structure of the low velocity zone in the mantle shown from surface-wave dispersion data to underlie the East African Rift. The data suggest that a lateral variation in velocity associated with a steeply dipping upper surface for the zone is the principal cause of the large slowness anomalies observed at Kaptagat. This upper surface is mapped from the slowness data to show a broad ellipsoidal structure lying at some 150 km depth on which is superimposed at steep-sided structure connecting it to the crustal intrusion along the Rift axis. The zone appears to be attenuated not only westwards away from the Rift axis but also northwards to correspond to the dying out of the Rift in Northern Kenya.

The Structure of the Western Flank of the Gregory Rift. Part II. The Mantle

Summary An evaluation is made of relative teleseismic P-wave delays between stations in Iceland and EKA (Scotland), KIR (Sweden) and KTG (Greenland). A detailed analysis of possible sources of error including the use of array data to detect the onset of events, shows there is a delay of at least 1 s associated with the upper mantle beneath Iceland. The delay does not appear to vary with distance or azimuth, and shows little variations over Iceland itself. Large teleseismic P-wave delays have been observed in Iceland both as residuals from travel-time determinations (Clearly & Hales 1966; Herrin & Taggart 1968; Lilwall & Douglas 1970) and by Tryggvason’s special determination at Reykjavik (Tryggvason 1964). The Icelandic crust has an anomalously high velocity (Bath 1960) so this delay has been associated with an anomalous upper mantle beneath Iceland. Its determination is therefore important in the study of the mantle beneath Iceland in particular and the mid-Atlantic ridge in general. Tryggvason (1964) has compared delays and structures at Reykjavik and Kiruna (Sweden), and concluded that the Icelandic delay was consistent with a vertical extension of the anomalous 7.4 layer observed by the refraction work of Bath (1960), to a depth of about 240 km. Three possible sources of error in these studies may, however, be cited: (a) Most earthquakes recorded at teleseismic distances are from a single seismic region (the island areas of the North Pacific) so that delays at source might give an apparent station delay. (b) Errors in travel-time tables can give apparent delays. Tryggvason’s estimate is relative to J & B tables which are known to be in error over the region of interest (Cleary & Hales 1966; Herrin et al. 1968). (c) It has been suggested (Stefansson 1967) that the Icelandic delay arises from a consistently late reading of the records-the onsets being lost in the high background noise of Iceland. The present paper seeks to establish the presence of a delay due to anomalous mantle beneath Iceland, using an improved analysis of the data from a wider range of stations, with improved travel-time tables (Herrin et al. 1968), and incorporating array processing to improve the observation of onsets. The method employed here attempts to separate from the gross travel-time residual the relative delay between pairs of stations caused by the different crust and upper mantle beneath each station. Relative delays are measured between Akureyi (AKU) and a temporary array station at Hveravellir in Iceland, Eskdalemuir

/pdf/teleseismic-p-wave-delay-time-in-iceland-2y3o4q94e3.pdf

Teleseismic P-wave Delay Time in Iceland

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