Joachim R. R. Ritter

A mantle plume below the Eifel volcanic fields, Germany

Abstract We present seismic images of the upper mantle below the Quaternary Eifel volcanic fields, Germany, determined by teleseismic travel time tomography. The data were measured at a dedicated network of more than 200 stations. Our results show a columnar low P-velocity anomaly in the upper mantle with a lateral contrast of up to 2%. The 100 km wide structure extends to at least 400 km depth and is equivalent to about 150–200 K excess temperature. This clear evidence for a plume below a region of comparatively minor volcanism suggests that deep mantle plumes could be more numerous than commonly assumed. They may often be associated with small volcanic fields or may have no volcanic surface expression at all.

3D shear-wave velocity structure of the Eifel plume, Germany

Abstract The Quaternary Eifel volcanic fields, situated on the Rhenish Massif in Germany, are the focus of a major interdisciplinary project. The aim is a detailed study of the crustal and mantle structure of the intraplate volcanic fields and their deep origin. Recent results from a teleseismic P-wave tomography study reveal a deep low-velocity structure which we infer to be a plume in the upper mantle underneath the volcanic area [J.R.R. Ritter et al., Earth Planet. Sci. Lett. 186 (2001) 7–14]. Here we present a travel-time investigation of 5038 teleseismic shear-wave arrivals in the same region. First, the transverse (T) and radial (R) component travel-time residuals are treated separately to identify possible effects of seismic anisotropy. A comparison of 2044 T- and 2994 R-component residuals demonstrates that anisotropy does not cause any first-order travel-time effects. The data sets reveal a deep-seated low-velocity anomaly beneath the volcanic region, causing a delay for teleseismic shear waves of about 3 s. Using 3773 combined R- and T-component residuals, an isotropic non-linear inversion is calculated. The tomographic images reveal a prominent S-wave velocity reduction in the upper mantle underneath the Eifel region. The anomaly extends down to at least 400 km depth. The velocity contrast to the surrounding mantle is depth-dependent (from −5% at 31–100 km depth to at least −1% at 400 km depth). At about 170–240 km depth the anomaly is nearly absent. The resolution of the data is sufficient to recover the described features, however the anomaly in the lower asthenosphere is underestimated due to smearing and damping. The main anomaly is similar to the P-wave model except the latter lacks the ‘hole’ near 200 km depth, and both are consistent with an upper mantle plume structure. For plausible anhydrous plume material in the uppermost 100 km of the mantle, an excess temperature as great as 200–300 K is estimated from the seismic anomaly. However, 1% partial melt reduces the required temperature anomaly to about 100 K. The temperature anomaly associated with the deeper part of the plume (250 to about 450 km depth) is at least 70 K. However, this estimate is quite uncertain, because the amplitude of the shear-wave anomaly may be larger than the modelled one. Another possibility is water in the upwelling material. The gap at 170–240 km depth could arise from an increase of the shear modulus caused by dehydration processes which would not affect P-wave velocities as much. An interaction of temperature and compositional variations, including melt and possibly water, makes it difficult to differentiate quantitatively between the causes of the deep-seated low-velocity anomaly.

The KRISP 94 lithospheric investigation of southern Kenya — the experiments and their main results

Abstract Following two previous experiments in 1985 (KRISP 85) and 1989–1990 (KRISP 90), a series of geophysical experiments was undertaken in 1993–1995 (KRISP 94) to study the lithospheric structure of the southern Kenya rift down to depths of greater than 100 km, with special emphasis on the Chyulu Hills, a complex of volcanic centres on the eastern flank of the rift. KRISP 94 involved a teleseismic tomography experiment of the Chyulu Hills area in July and August 1993, a seismic refraction-wide-angle reflection survey across southern Kenya from Lake Victoria to the Indian Ocean in February 1994, seismicity studies of southern Kenya from 1993 to 1995, a special seismicity study of the Lake Magadi area in February 1994, a gravity study along the seismic-refraction lines before and after the seismic-refraction study, and a magnetotelluric study of southern Kenya in February 1995. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the southern Kenya rift and its flanks for several 100 km to the west and to the east and their evolution, to study the relationship between deep crustal and uppermost mantle structure, to learn more on the development of sedimentary basins and volcanic features on the flanks and its relation to the Kenya rift, to obtain information on the temperature conditions underneath the rift and its flanks, to perform a particular integrated and calibrative study of seismological and petrological data in the Chyulu Hills, and to understand the processes which are producing extension, uplift, and extensive magmatism. This report is an introduction to a series of subsequent papers. It focuses on the technical description of the main seismic surveys of the KRISP 94 effort and summarizes the key results. During the teleseismic survey an array of 31 seismographs was deployed to record teleseismic, regional and local events for a period of about 3 months from June to August 1993. The elliptical array covered an area about 150 km (N-S) × 100 km (E-W) and spanned the central portion of the Chyulu Hills and its surroundings, with an average station spacing of 10–30 km. The seismic refraction-wide-angle reflection survey was carried out in a 2-week period in February 1994. It consisted of two profiles: one extending from Lake Victoria across the western flank and the southernmost Kenya rift at Lake Magadi, the other extending from Athi River near Nairobi across the eastern flank of the rift, traversing the Chyulu Hills and terminating at the Indian Ocean near Mombasa. A total of 204 mobile seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to 730 km. Key results are as follows: (1) The crust reaches a maximum thickness of up to 44 km under the Chyulu Hills. (2) Only a minor upwarping of the crust-mantle boundary is seen under the rift proper in the Lake Magadi area. (3) To the west the crust shallows to about 34 km near Lake Victoria, in contrast to the thickening of the crust further north from the central part of the rift near Lake Baringo towards the west. (4) There is a steep rise of the Moho east of the Chyulu Hills towards the Indian Ocean. (5) P-wave velocities in the uppermost mantle are above 8 km/s except under the rift proper and under the Neogene volcanic centre of the Chyulu Hills, where the velocity is 7.9–8.0 km/s. (6) Under the Chyulu Hills, the Moho is replaced by a gradual crust-mantle transition, and the low velocities near the crust-mantle boundary extend to greater depths as evidenced by teleseismic tomography which indicates a velocity decrease of 3–5%, i.e. from 8.1–8.2 km/s to at least 7.9 km/s. Both effects are probably caused by the local recent volcanic activity, but cannot be interpreted as due to plume activity which is assumed to be present under the Nyanza craton further west. (7) Gravity modelling and first preliminary results of the magnetotelluric measurements support the seismic-refraction and tomographic results. Furthermore, under the western flank the magnetotelluric and gravity data indicate increased conductivity and decreased density in the uppermost mantle below 60–80 km depth.

A tomography study of the Chyulu Hills, Kenya

Abstract The Chyulu Hills, a Quaternary volcanic field on the eastern shoulder of the Kenya rift in East Africa, have been the target of a multidisciplinary research project. In this contribution teleseismic travel-time residuals from a passive seismic experiment are imaged into P-velocity perturbations in the lithosphere underneath the Chyulu Hills. The images reveal lateral velocity contrasts of about 5% with a prominent low-velocity zone which is located directly beneath the volcanic range down to 70 km depth. Due to the small size and amplitude of the anomaly this zone of decreased seismic velocity is interpreted as evidence for small magma chambers with elevated temperatures in the lower crust and uppermost mantle. A major upwelling of the asthenosphere can be excluded, because the seismic anomaly is too limited in size. However, the increased temperatures below the Chyulu Hills are interpreted as a sign for the still active magmatic processes related to the rifting in East Africa.

An integrated model for the deep structure of the Chyulu Hills volcanic field, Kenya

The Chyulu Hills, a 1.4 Ma B.P. to Holocene volcanic field located about 150 km to the east of the Kenya rift, is one of the few locations on Earth for which detailed geochemical (volcanic rocks), thermobarometric (xenoliths), seismological and gravity data are available. This paper combines these data to achieve an integrated seismic-petrological model for the deep structure of this volcanic field. Results of a wide-angle reflection and refraction experiment reveal an average crustal thickness of 40 km and a thickness of 20 km for the lower crust. Beneath the volcanic field, the crust thickens to about 44 km. In this region a low-velocity body (LVZ) is modelled which extends from about 30 ± 5 km depth to the Moho. The LVZ is characterised by an increased vP/vS-ratio ranging from 1.81 to 1.93 depending on the possible extents of this body. This is in contrast to the surrounding crust where a ratio of only about 1.76 is observed. In the same area, the results of a teleseismic tomography study show a P-wave low-velocity anomaly of −3%. The seismic data can be explained by either an anorthositic body directly above the Moho in the region of the Chyulu Hills or by the presence of partial melt. Directly beneath the Chyulu Hills, a P-wave velocity of 7.9 km/s is determined for the uppermost mantle; this velocity is 0.2–0.3 km/s lower than that of the surrounding mantle region. The teleseismic tomography model suggests a P-wave low-velocity anomaly of −2.5 to −3.5% in the uppermost mantle (<70 km depth). Widespread garnet-bearing pyroxenitic and lherzolitic mantle xenoliths are mostly well equilibrated and suggest an apparent lithospheric thickness of about 105 km. Most garnet-free spinel harzburgitic xenoliths and some garnet pyroxenitic xenoliths were significantly heated before they were sampled and erupted by the host magmas. Heating events lasted for less than 210 ka as indicated by chemical diffusion profiles observed in orthopyroxene grains. It is suggested that heating was caused by stagnating magmas in the uppermost lithospheric mantle. At the same depth P-wave velocity perturbations of the tomographic model show a low-velocity zone directly underneath the youngest part (SE) of the volcanic field. At depths greater than about 70 km, this low-velocity zone is shifted towards the east, away from the volcanic field.

SCATTERING OF TELESEISMIC WAVES IN THE LOWER CRUST. OBSERVATIONS IN THE MASSIF CENTRAL, FRANCE

Abstract High-frequency coda signals consistently recorded by a temporary seismic network of 29 short-period stations during a 6-month survey in the French Massif Central are concordant with independent evidence for a heterogeneous lower crust obtained from wide-angle and near-vertical reflection experiments in the same region. The teleseismic recordings of 22 events have been analysed in record sections rather than as single station seismograms. Following the low-frequency (LF; 0.5–1.5 Hz) first P-arrival a high-frequency (HF) coda (2–4 Hz dominant frequency) extends over several seconds duration. The HF signals become clearly visible after bandpass filtering, but can also be identified in the original seismograms. In event sections the HF coda forms a consistent pattern of reverberations which is characterized by the following properties: (1) the duration is typically longer on the radial (4–15 s) than on the vertical (3–11 s) component; (2) the beginning of the HF coda, referred to the picked LF first P-arrivals varies between 0–2.5 s, with a dominance around 1.5–2 s; (3) the amplitude of the HF coda is about 3%–10% of the primary LF P-phase amplitude; (4) the signals are incoherent between neighbouring stations and over the network; (5) the HF coda arrives dominantly around the plane of incidence of the teleseismic wave in the first 2 s to maximum 4 s; afterwards a widening of the particle motion to an elliptical shape is observed, indicating arrivals off the sagittal plane. The data parameters (1) to (5) point to the conclusion that the HF signals are generated by a scattering process. Wide-angle and CDP-reflection experiments in the same region measured a similar reverberating coda pattern and located the origin also in the lower crust. These observations and the data parameters of the HF coda suggest that the HF teleseismic waves are scattered at heterogeneities in the lower crust in the Massif Central. In this contribution we mainly concentrate on the presentation of the teleseismic recordings and conclude with a preliminary structural model which contains randomly distributed scatterers in the lower crust.

The Seismic Signature of the Eifel Plume

In 1997/98 the seismological Eifel Plume experiment was conducted in Belgium, France, Germany and Luxembourg to record a comprehensive dataset for several analysis techniques. Here we summarise the main points about the field experiment, the 3D tomography studies using compressional (P) and shear (S) waves and the analysis of the P-coda for scattering characteristics. Our seismic models are compared with results from surface wave dispersion studies, teleseismic shear-wave splitting and receiver functions. The models contain as a common feature a low-velocity anomaly in the upper mantle. This anomaly starts at about 50–60 km depth and reaches down to at least 410 km depth. The velocity reduction reaches up to 2% for compressional waves and 5% for shear waves in its upper part (50–100 km depth). In the lower part of the upper mantle the reduction is about 1% for both wave types. The diameter of this anomaly is about 100–120 km. As cause for the velocity reduction we infer increased ambient temperatures of about 100–150 °C. At 50–100 km depth there may be also about 1% of partial melt. We interprete this seismic anomaly as small upper-mantle plume that buoyantly upwells from the transition zone and starts to melt just below the continental lithosphere and possibly also at greater depth. At the surface the plume activity is documented by the Quaternary Eifel volcanic fields. Solidified magma chambers and dykes in the lithosphere are inferred to cause strong scattering of seismic waves as discovered by coda analyses.

Crustal tomography of the central kenya rift

Abstract During the Kenya Rift International Seismic Project 1990 (KRISP90) experiment the shots of the seismic refraction programme were recorded by a two-dimensional teleseismic network. This was installed in the central part of the Kenya Rift and on its shoulders, from Lake Baringo in the north to Lake Naivasha in the south P-wave first arrivals were used to determine lateral crustal velocity heterogeneities with a three-dimensional inversion programme. Mainly high-velocity zones were found which can be correlated with other geophysical anomalies and geological features. This comparison shows that the high-velocity bodies found within the crust are likely indicators for mafic cumulates or magma chambers. These evolved during the major magmatic episodes which are typical indicators for an active rift.

Deep structure of Medicine Lake volcano, California

Medicine Lake volcano (MLV) in northeastern California is the largest-volume volcano in the Cascade Range. The upper-crustal structure of this Quaternary shield volcano is well known from previous geological and geophysical investigations. In 1981, the U.S. Geological Survey conducted a teleseismic tomography experiment on MLV to explore its deeper structure. The images we present, calculated using a modern form of the ACH-inversion method, reveal that there is presently no hint of a large (> 100 km3), hot magma reservoir in the crust. The compressional-wave velocity perturbations show that directly beneath MLVs caldera there is a zone of increased seismic velocity. The perturbation amplitude is +10% in the upper crust, +5% in the lower crust, and +3% in the lithospheric mantle. This positive seismic velocity anomaly presumably is caused by mostly subsolidus gabbroic intrusive rocks in the crust. Heat and melt removal are suggested as the cause in the upper mantle beneath MLV, inferred from petro-physical modeling. The increased seismic velocity appears to be nearly continuous to 120 km depth and is a hint that the original melts come at least partly from the lower lithospheric mantle. Our second major finding is that the upper mantle southeast of MLV is characterized by relatively slow seismic velocities (−1%) compared to the northwest side. This anomaly is interpreted to result from the elevated temperatures under the northwest Basin and Range Province.

Seismic images illustrate the deep roots of the Chyulu Hills Volcanic Area, Kenya

A recent delay time tomography experiment has revealed the still ongoing processes that formed the East African Rift, the worlds largest continental rift system. These new data have been used to produce tomographic images of the crust and upper mantle beneath the Chyulu Range (Figure 1), a Quaternary volcanic field on the rifts eastern shoulder. The images show a prominent low-velocity zone where the youngest volcanism occurred directly beneath the Chyulu Range. Velocity contrasts as large as 4% have been found in 40–90 km depth (Figure 1). The reduced seismic velocity beneath the volcanic range probably suggests the presence of small active magma chambers in the crust and upper mantle. A large-scale asthenospheric upwelling underneath the rift shoulder is excluded as cause of the volcanism because the anomaly is too small in size as well as in percentage of velocity reduction.