Lithospheric structure beneath the Kenya Dome

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.     

Moho-offset beneath northern Scotland

Summary. Independent sets of LISPB data are presented, having as a common feature a local lateral variation in the Moho beneath northern Britain.
The evidence for this step-like feature on the Moho is taken from P and S travel times and from PS reflection times and phase velocities.
Another important observation is that a high-frequency Moho refraction is generated north of the step, whilst the refraction from the step region and south of it contains only low frequencies. This leads to the assumption that a change in the Moho structure exists in the same region as where the step has been deduced from travel-time interpretation. A numerical model is presented which generates a high-frequency refracted wave from an input signal which contains low- and high-frequency spectral energy.
The significance of this offset on the Moho, in particular its relationship to surface tectonics, is critically discussed.     

Deep crustal geoelectric structure beneath the Northumberland Basin

Summary. In terms of lateral variations in conductivity structure, the southern Southern Uplands and Northumberland Basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone. Five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor. The response data are used to investigate the deep geoelectric crustal structure of the region. Three appropriate sets of response data have been subjected to two construction algorithms for 1-D inversion. The geoelectric profiles recovered identify a deep crustal conducting zone underlying the Northumberland Basin. The zone, modelled as a layered structure, dips steeply from mid-crustal depths underneath the Northumberland Basin to lower crustal depths to the NW. The structure thus correlates, in location and geometry, with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile.     

Structure of the lower crust beneath the Gulf of Maine

Summary. The variability of deep crustal reflections in USGS line 1A across the offshore New England Appalachians shows the differing influence of Paleozoic and Mesozoic tectonic events. Mesozoic extension has not significantly modified Paleozoic thrust faults penetrating the lower crust in the northern Gulf of Maine. Mesozoic extension or Paleozoic crustal melting could explain the lower crustal character in the central Gulf.     

Upper mantle heterogeneities beneath Central Europe

Summary. An inversion of ISC travel-time data from selected earthquakes in the distance range 30°-90° to 53 stations in Central Europe has been used to model velocity down to 600 km depth. The model explains 0.1–0.2s of the residuals, as for other array studies, leaving 0.5 s unexplained as noise. The uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology. This may be due to deep-seated thermal anomalies or, in some areas, to delays introduced by passage of the rays through sedimentary cover. The deeper anomalies are smaller and unrelated to those in the lithosphere, which suggests that the asthenosphere is decoupled from the rigid lithosphere. The structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity. The models show some suggestion of a high velocity slab trending from east to west beneath the Alps.     

Stress and Flow beneath Island Arcs

Summary. The flow pattern, stress distribution, topography, and gravity anomalies were computed from numerical models having density and viscosity distributions resemblant to the Aleutian arc. The results were compatible with the hypothesis that the excess density of the slab drives its descent and that hydrodynamic forces are responsible for topographic and gravity highs over the outer rise seaward of the trench and the frontal arc and lows over the trench. In models with simple distributions of rheological parameters, the force from the slab was transmitted directly upward producing a negative gravity anomaly over the arc. Material with low resistance to flow was needed along the fault plane above the slab or within the crust of the frontal arc and within the wedge of asthenosphere above the slab to reduce that force and to allow the horizontal lithosphere to move with the slab. Models with the resistance to flow thus reduced had outer rises, deep trenches, horizontal tension seaward of the trench, horizontal compression under the trench, and downdip tension in the slab. Free air gravity anomalies, which are the sum of between deflections of the free surface due to hydrodynamic forces and direct attractions from the masses driving the flow, were not fit excellently by any of the models, in part because the coarse grid used precluded accurate representation of the fault zone above the slab and the frontal arc. An alternate to the hypothesis that about 5 kb of stress on the fault plane is needed to produce an outer rise is offered by these models. Shear stress between the slab and the island arc was always below 700 bars in the more successful models if the density distribution was scaled to match the topography of the trench. This is much less than the 2000 bars stresses needed if frictional heating causes island arc volcanism.     

Imaging crustal discontinuities and the downgoing slab beneath western Crete

The migration of teleseismic receiver functions yields high-resolution images of the crustal structure of western Crete. Data were collected during two field campaigns in 1996 and 1997 by networks of six and 47 short-period three-component seismic stations, respectively. A total of 1288 seismograms from 97 teleseismic events were restituted to true ground displacement within a period range from 0.5 to 7 s. The application of a noise-adaptive deconvolution filter and a new polarization analysis technique helped to overcome problems with local coda and noise conditions. The computation and migration of receiver functions results in images of local crustal structures with unprecedented spatial resolution for this region. The crust under Crete consists of a continental top layer of 15–20 km thickness above a 20–30 km thick subducted fossil accretionary wedge with a characteristic en echelon fault sequence. The downgoing oceanic Moho lies at a depth of 40–60 km and shows a topography or undulation with an amplitude of several kilometres. As a consequence of slab depth and distribution of local seismicity, the Mediterranean Ridge is interpreted as the recent accretionary wedge.     

The shear-wave velocity structure in the upper mantle beneath Eurasia

We develop an approach that allows us to invert for the mantle velocity structure within a finely parametrized region as a perturbation with respect to a low-resolution, global tomographic model. We implement this technique to investigate the upper-mantle structure beneath Eurasia and present a new model of shear wave velocity, parametrized laterally using spherical splines with ∼2.9° spacing in Eurasia and ∼11.5° spacing elsewhere. The model is obtained from a combined data set of surface wave phase velocities, long-period waveforms and body-wave traveltimes. We identify many features as narrow as few hundred kilometres in diameter, such as subducting slabs in eastern Eurasia and slow-velocity anomalies beneath tectonically active regions. In contrast to regional studies in which these features have been identified, our model encompasses the structure of the entire Eurasian continent. Furthermore, including mantle- and body-wave waveforms helped us constrain structures at depths larger than 250 km, which are poorly resolved in earlier models. We find that up to +9 per cent faster-than-average anomalies within the uppermost ∼200 km of the mantle beneath cratons and some orogenic regions are separated by a sharp gradient zone from deeper, +1 to +2 per cent anomalies. We speculate that this gradient zone may represent a boundary separating the lithosphere from the continental root, which might be compositionally distinct from the overlying lithosphere and remain stable either due to its compositional buoyancy or due to higher viscosity compared with the suboceanic mantle. Our regional model of anisotropy is not significantly different from the global one.     

The lithosphere–asthenosphere boundary beneath the western United States

S receiver functions from 67 broad-band seismic stations in the western United States clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpret as the lithosphere–asthenosphere boundary (LAB). The average depth of the LAB is ∼70 km. The boundary is relatively sharp with an overall sharpness of less than 20 km. The boundary is more prominent south of the Mendocino Triple Junction, where the Farallon Plate has completely subducted. This may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window. A double low velocity zone is observed at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ∼100 km, well correlated with previous studies of lithospheric delamination in the area.