Upper-mantle Velocity Research Articles

The Erris and eastern Rockall Troughs: structural and sedimentological development

Abstract Wide-angle seismic reflection profiles across the western Irish shelf region have resolved the deep crustal structure of the shelf transition into the Rockall Trough. A three-layered sedimentary succession up to 5 km thick is determined within the Rockall Trough. The lowermost layer probably comprises a thin early Mesozoic syn-rift sediment package, while the upper two layers represent Cretaceous to Recent, mostly post-rift, sediments. The seismic response of the crust changes beneath the Erris Trough into the eastern Rockall Trough, as the crust thins from c. 30 km onshore in Ireland to between 5 and 6 km in the deep water region of the Rockall Trough. However, the thinning of seismic layers in response to stretching is not uniform. Differential stretching of the crust can explain seismic observations of both crustal and upper mantle structure. Low upper mantle velocities may be due to partial serpentinization of the upper mantle lithosphere, facilitated by rheological hardening during lithospheric stretching. The model predicts the occurrence of starved deep marine sediments during the late Mesozoic and Cenozoic.

The East African rift system in the light of KRISP 90

On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100–150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study.

Tomographic inversion for the seismic velocity structure beneath northern New England using seismic refraction data

The seismic velocity structure of the crust and the upper mantle beneath northern New England was investigated from the travel time data of the 1984 Maine Seismic Refraction Profile and the 1988 Ontario‐New York‐New England Seismic Refraction Profile seismic refraction experiments by using tomographic inversion techniques. Strong lateral seismic velocity variations were found in the upper 15 km of the crust, and these variations are correlated with regional geologic and tectonic structures. The Grenville basement in the Adirondack Mountains of eastern New York is characterized by an unusually high P wave velocity of 6.7 km/s in the upper 15 km of the crust. This high‐velocity basement extends eastward below western Vermont to a depth between 15 and 20 km from the surface. More typical upper crustal P wave velocities of 5.6 to 6.4 km/s were generally found in central and southern Maine and in New Hampshire, and the anisotropic upper crustal zone west of Penobscot Bay in Maine was imaged by locally slower velocities. The correlation of the inversion velocity structure with the tectonic features of northern New England shows that the Appalachian mountains in northern New England may extend, tectonically, to at least 20 km deep in the crust. The velocity distribution at 20 km depth shows three northeast elongate juxtaposed belts which correspond to the Connecticut Valley‐Gaspe Synclinorium, the Bronson Hill Anticlinorium and the Kearsarge‐Central Maine Synclinorium, respectively. The Connecticut Valley‐Gaspe and Kearsarge‐Central Maine Synclinoria have relatively high velocities from 6.94 to 7.12 km/s while the Bronson Hill Anticlinorium has relatively low velocities from 6.52 to 6.75 km/s in the lower crust. Another low‐velocity area of 6.52 km/s at 20 km depth is imaged beneath the White Mountain Batholith in central New Hampshire and may be a lower crustal expression of this batholith. Upper mantle velocity structures were obtained for the Grenville basement, the Appalachians in northern New England and the Avalon terrane in southeastern Maine from the long‐range seismic refraction data. Generally, the older Grenville lithosphere (about 1100 Ma) in Quebec has higher velocity in the upper mantle than does the younger Avalon terrane (no older than 650 Ma) in the southeastern part of northern New England. The data suggest that the velocity of the upper mantle beneath the old North American craton increases stepwise with observable velocity discontinuities in depth, while that beneath the Avalon terrane does not increase through the first 40 km of the upper mantle. The data analysis also shows that the upper mantle suture between the Grenville basement and the Avalon terrane occurs in a narrow belt beneath the Appalachian mountains of central Maine and New Hampshire. The did of the Moho discontinuity beneath northern New England is modeled as nearly flat as 0.7° in the N57°W direction and as steep as 1.2° beneath the Grenville basement.

Upper-mantle velocity structure beneath the Siberian platform

SUMMARY We present a new velocity model for the continental upper mantle beneath central Siberia based on observations of the 1982 'RIFT' Deep Seismic Sounding (DSS) profile. Three Peaceful Nuclear Explosions (PNE) were detonated to provide energy for the 2600 km long profile that extends from the Yamal Peninsula to the Mongolian border SE of Lake Baikal. In this paper, we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle-transition-zone discontinuities. The analysis combines forward-traveltime modelling and waveform matching using reflectivity synthetic seismograms. Our model for the lithosphere has velocities of 8.25-8.20 km s-' from the Moho to 117km depth. Between 117 and 123km depth, a strong velocity gradient (8.30 - 8.53 km s-') is required while a moderate gradient (8.53-8.55 km s-l) exists between 123 and 136km depth. A low-velocity zone from 136 to 210km depth terminates this phase arrival branch. The gradient again rises between 210 and 233 km and depth, culminating in a high-gradient zone (8.63-8.80 km s-') between 233 and 235 km depth. Below the high-gradient zone, more moderate gradient (8.80-8.85 km s-') is required from 235 to 253 km depth, terminating in a zone of lower velocity (8.62 - 8.64 km s-') from 253 to 400 km depth. The upper-mantle transition zone consists of two high-gradient zones separated by a more moderate gradient. The upper zone is best modelled as a 35 km thick velocity gradient (8.64-9.45 km s-') from 400 to 435 km depth. The existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance. A model with a first-order discontinuity predicts significant arrivals at ranges closer than 1580 km. Our observations contain no compelling evidence for a 520 km discontinuity, although a small discontinuity cannot be ruled out. The lower-transitionzone discontinuity is modelled as a 4 km thick gradient zone (10.25-10.62 km s-I) from 655 to 659 km depth.

Surface-wave group velocity tomography of East Asia

Abstract Group velocities of both Rayleigh and Love waves are used in a tomographic inversion to obtain group velocity maps of East Asia (60–140°E and 20–60°N). The period range studied is 30–70 s. The Tibetan plateau, a region undergoing intense north-south compression, appears as a prominent low-velocity (about −15% from the average) structure in this area; central Tibet appears as the area with the lowest velocities, and southern Tibet, south of the Zangbo suture zone and the Himalayas, is an area of high velocities. The Tibet low-velocity area extends northward to the southern Tarim basin. The North China-Korean platform, an area traditionally recognized as the oldest platform in China, does not have a consistent crust and upper-mantle velocity signature. The North China plain, a part of the platform where extensional tectonics dominates, is an area of high velocities, probably as a result of thin crust. The other parts of the platform have relatively low velocities. The south China block, the least tectonically active region of China, is generally an area of high velocity. For periods longer than 40 s, a high group velocity gradient clearly exists along longitude 105°E; the velocities are noticeably higher east of this longitude than west of it. This transition corresponds closely to that in the Bouguer gravity map. In general, processes in the crust and upper mantle owing to recent tectonic activities, rather than the age of the basement rocks, seem to have dominant control of crust and upper-mantle velocity structures.

The nature and cause of polarization anomalies of surface waves crossing northern and central Eurasia

SUMMARY Results of a pilot experiment preliminary to a systematic study of surface-wave polarization across Eurasia are presented. Long-period fundamental Rayleigh and Love waves recorded by the broad-band seismic stations KIV (Kislovodsk) and OBN (Obninsk) of the IRIS/IDA network deployed in the Former Soviet Union (FSU) were analysed from 14 events to the north-east, east and south-east of these stations in search of particle-motion anomalies using a technique called Frequency-Time Polarization Analysis (FTPAN). All anomalies indicate deviations of wave-propagation paths to the north relative to the great circle paths. Polarization anomalies at OBN are uniformly small (<5°). Significant frequency-dependent polarization anomalies (5°–20°) are found for Rayleigh and Love waves arriving at KIV from teleseismic events at a wide range of backazimuths (26°–103°). Polarization measurements are repeatable (both for a number of nearly degenerate events and for reciprocal paths) and vary smoothly and continuously as epicentral location is moved. Consequently, such measurements are robust and can provide useful structural information. The systematics of the frequency dependence of the polarization anomalies as epicentral position is moved from north to south display sensitivity to structures at a number of length-scales. Results from synthetic experiments using coupled normal-mode and Gaussian-beam synthetics reveal that: (1) recently constructed long-wavelength aspherical models produce polarization anomalies that are significantly smaller (<5° at KIV) but of the same sign (negative) as the observed anomalies, and (2) the frequency dependence of the observed anomalies must result from a combination of structures of differing wavelengths, with scale-lengths ranging from regional to global. Thus, polarization measurements provide new information about currently unmodelled structures. A model that fits the polarization anomalies observed at KIV includes a regional-scale low-velocity feature near to KIV in order to fit the large-magnitude, short-period polarization anomalies and a smaller magnitude, continent-scale increase in upper mantle velocities and/or the reduction of crustal thickness from south to north in central Eurasia in order to fit the broad-band, longer period anomalies. The small-scale, low-velocity feature is a model of the sedimentary basin of the sub-Caspian depression. We conclude that measurements of polarization anomalies can be obtained accurately, that they are reproducible, that they contain currently unmodelled information, and that they should prove to be useful in combination with velocity and amplitude information in future tomographic inversions, especially to help focus global-scale models to regional wavelengths.

Deep seismic structure of the Earth's crust along the Baltic Sea profile

Abstract In summer of 1989 the Institute of Oceanology of the Russian Academy of Sciences (Moscow) for the first time used a new large volume airgun for deep seismic sounding (DSS) in the Baltic Sea. The experiment was carried out during the 23-rd cruise of R/V Professor Shtockman. The wave-field analysis showed clear P and S wave arrivals, including Moho reflections. The Moho reflections were distinctly observed at distances exceeding 100 km. The P waves can be followed to a maximum distance of 368 km. Two-dimensional raytracing modelling of the data revealed a pronounced topography of the Moho along the BALTIC SEA profile: a depression in the middle of the profile at a depth of 45 km is bounded by 2–3 km steps and uplifts on both sides. Reflections off a sub-Moho boundary at a depth of 60 km have also been recorded. The upper mantle velocities vary between 7.8 km/s under the depression to 8.1 km/s to the side of it. The crustal P waves velocities define three layers: 5.8–6.3, 6.3–6.7 and 6.9–7.1 km/s. All crustal velocity boundaries bend over the Moho depression and reach their deepest points in the central part of the profile. The velocities in the crust also change laterally, reaching their minimum within the central depression. The crustal thickness along the profile varies notably between 38 and 45 km. A comparison with the FENNOLORA and the BABEL B profiles indicates that the Moho depression found along the BALTIC SEA profile correlates with similar depressions on the other profiles.

Crustal velocity structure of the Omineca Belt, southeastern Canadian Cordillera

Travel time inversion and amplitude modeling of a 350‐km Lithoprobe seismic refraction/wide‐angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the Omineca Belt of the Canadian Cordillera. The upper crust to 12–18 km depth has velocities from 5.6 to 6.2 km s−1, and two shear zones, the Monashee Décollement and Gwillim Creek Shear Zone, are imaged by the wide‐angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Décollement may be related to separate rock origins. Prominent reflections define the boundaries of a low‐velocity midcrustal layer from 10–15 km to 20–25 km depth with velocities less than 6.1 km s−1. The low velocities of the midcrust, associated with high electrical conductivities and high heat flow, may be considered as support for the hypothesis of fluids in the Cordilleran crust, though other possibilities, such as the effect of high temperatures on rock velocities are possible. In the lower crust velocities range from 6.4–6.5 km s−1 at the top of the lower crust to 6.6–6.8 km s−1 at its base. The Moho is very clearly defined by the refraction/wide‐angle reflection data and has a gentle southerly dip. Crustal thicknesses are 35–37 km. A thin crust‐mantle transition zone of 1–2 km thickness in which velocities vary between 7.6 and 7.7 km s−1 is consistent with coincident reflection data. Upper mantle velocities range from 7.9 to 8.1 km s−1 with indications from the data of upper mantle layering. In comparison with neighboring regions, the Omineca Belt has an anomalously thin crust, low crustal velocities, and a low‐velocity upper mantle, similar only to the Basin and Range province. The velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method. The characteristics of a thin crust and lithosphere, along with low velocities from midcrust to mantle suggests that both the Basin and Range and the southern Canadian Cordillera are currently being heated from a source within the mantle.

Shear wave velocity structures of the Arabian Peninsula

The shear velocity structures of the different tectonic provinces of the Arabian Peninsula has been studied using surface wave data recorded by the RYD (Riyadh) station. The inversion of Rayleigh wave group velocities indicates that the Arabian shield can be modeled by two layers, each of which is 20 km thick with a shear velocity of 3.61 km/s in the upper crust and 3.88 km/s in the lower crust. The underlying upper mantle velocity is 4.61 km/s. Inversion of both Love and Rayleigh waves group velocities shows that the Arabian platform upper and lower crusts are comparable in their thicknesses to those of the shield, but with shear velocities of 3.4 and 4 km/s, respectively. The upper mantle velocity beneath the platform is 4.4 km/s and the average total thickness of the crust is 45 km.

Seismic evidence for anomalous crustal structure beneath Mesozoic fracture zones in the Gambia Basin, eastern equatorial Atlantic

Five sonobuoy refraction profiles shot in the eastern equatorial Atlantic reveal the presence of thin and heterogenous crust in a zone at least 200 km wide near the southern margin of the Gambia Basin. Oceanic Layer 2 (V p ≈ 4.1–4.4 km/s) is detected on all profiles. On some sections it is underlain by crustal material with apparent seismic velocities between 5.7 km/s and 7.0 km/s; on others, Layer 2 directly overlies a zone with apparent velocities characteristic of normal upper mantle velocities ( ≈ 8 km/s). The formation of anomalous crust is attributed to a modification of accretion processes resulting from a close spacing of fracture zones during a period of slow spreading in the Early Cretaceous.

The use of P wave amplitude data in a joint inversion with travel times for upper mantle velocity structure

A joint inversion of P wave travel time and amplitude data is performed to test whether the amplitudes can be used to increase the resolution of inversions using travel times alone. Geometrical spreading amplitudes depend on the curvature of the slowness field and may thus help to resolve sharp gradients. Travel times and amplitudes of short‐period P waves observed at the Washington Regional Seismograph Network are jointly inverted for upper mantle velocity structure below the array. The processing of the observed P wave amplitudes is discussed in detail, and it is shown that it is important to correct the raw data for station statics, as these dominate the observed amplitude signal. A robust averaging procedure is used to identify and remove outliers from the data set, which is comprised of 8697 travel time and 4255 amplitude measurements. The travel time data alone are used to obtain a reference model for the amplitude data and in this way the nonlinear behavior of the amplitude equations is minimized. The results show that the amplitude data induce small, short‐scale slowness perturbations to the starting model, producing an amplitude misfit reduction to an arbitrary degree, depending on the regularization. Inversions with synthetic data are performed to explain this result. An idealized model of a subducting slab is used to generate synthetic data sets. The model resulting from a joint inversion produces an amplitude misfit reduction of 77%, while the travel time misfit is unaffected. In agreement with the results for the real data sets, this is achieved by making small adjustments throughout the model, which do not alter the overall slowness variations. The amplitude data therefore do not improve the resolution of the sharp gradients present in the synthetic slab model. These results can be explained by both the strong sensitivity of geometrical spreading amplitude to slowness perturbations along the ray path and to the distribution of the amplitude data set, which is not complete enough to induce more than incoherent changes to the starting model. The strong sensitivity is due to the combination of long teleseismic ray paths and small‐scale (about 30 km) slowness variations allowed by the model parameterization. With the present amplitude data set the curvature is changed on the scale of the node spacing in the model, whereas the overall slowness variations on a scale length of several times the node spacing are unaffected. The regularization has little effect on these small‐scale changes, as these changes increase the total model roughness by only 1%. It is concluded that with the amplitude data set available, amplitudes do not improve the resolving power of travel time inversions for upper mantle velocity structure. This is due to the number of data required to both decrease the observed variance in the raw amplitudes to the variance in travel time data and to obtain a good coverage of the model. The application of P wave amplitude data as a validation tool is suggested for models obtained with, for example, travel time inversion. Applications to both synthetic and observed data sets are shown. Anelastic damping is ignored in the inversions. Using linear relations between velocity, temperature, and Q, it is shown that the effect of anelastic attenuation is of the order of 10–15% of that of geometrical spreading through the upper mantle.

Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO

In this paper, regionalP-wave upper mantle structure is investigated using slant-stack velocity analysis of short-period earthquake data recorded at station MAJO (Matsushiro, Japan). Shallow earthquakes from 1980–1986 within 35° of MAJO are used to construct a common receiver gather. Processing of the wavefield data includes focal depth and static time corrections, as well as deterministic deconvolution, in order to equalize pulse shapes and align wavelets on the first arrivals. The processed wavefield data are slant stacked and interatively downward continued to obtain a regional upper mantle velocity model. The model includes a low velocity zone between 107 and 220 km. Beneath the LVZ, the velocity increases smoothly down to the discontinuity at 401 km. In the transition zone, the velocity model again increases linearly, although there is some suggestion of further complexity in the downward continued wavefield data. At the base of the transition zone, a second velocity discontinuity occurs at 660 km, with a linear velocity gradient below. In addition to slant-stack analysis, travel times and synthetic seismograms are computed and compared with the processed and unprocessed wavefield data.

Complete Pp-Waveform Modelling For Determining Crust and Upper Mantle Structure

SUMMARY We analyse the PP phase at upper mantle distances (25 to 60) to quantify its propagation characteristics and potential for determining crust and upper mantle velocity structure. Upper mantle distance PP waveforms are very complex, involving interference between a variety of arrivals traversing different depth ranges in the Earth. Even for laterally homogeneous structures, complete synthetic seismograms such as provided by reflectivity calculations are required to adequately model the full suite of arrivals that contribute to upper mantle PP phases. The complexity of PP waveforms can be exploited to extract crust and upper mantle structural information for relatively homogeneous paths. While PP-P differential traveltimes and PPlP amplitude ratios provide constraints on average upper mantle velocities above the transition zone, early PP coda, comprized of crustal reflections and conversions, and precursors to the PP phase, comprized of multiple Moho underside reflections, can provide information on crustal thickness and velocity gradients in the uppermost mantle. Positive velocity gradients below the crust generate large amplitude Moho underside reflections that are particularly visible for short-period and broad-band seismograms at distances from 25 to 38. We demonstrate the potential use of the PP phase for determining upper mantle structure by modelling long-period signals traversing relatively 'pure' paths across North America. A continuum of P-wave velocity models with varying lid structure is appropriate for the upper mantle beneath North America. The highest velocities are found beneath the northern Canadian shield, with the lid becoming progressively slower beneath the southern shield, continental platform and tectonically active regions, respectively. Most of the heterogeneity in P-velocity structure is concentrated above a depth of 250 km, although some data are consistent with models that are relatively slow at greater depths.

Moho and petrologic crust-mantle boundary coincide under southeastern Australia

New seismic data from the Lake Bullen Merri area of southeastern Australia indicate strongly that there are no large differences between the depth to the crust-mantle boundary defined by petrological and geochemical parameters and the depth to the Moho defined by seismic methods, contrary to previous seminal work on crustal and upper-mantle xenoliths. These data are in agreement with interpretations from extensional basins on other continents where similar information is available. Under the northern margin of the Otway basin in southeastern Australia, the Moho depth of 31 km and the upper-mantle velocity of 7.8 km/s interpreted from seismic data are in good agreement with laboratory measurements reported by other workers on spinel Iherzolites in xenoliths from this same area; the laboratory data give an average P -wave velocity of 7.88 km/s at 1 GPa and 850 °C, the physical conditions thought to apply in the uppermost mantle at 30 km depth.

Cause of low velocity anomaly along the South Atlantic hotspots

A recent upper‐mantle seismic velocity model shows a narrow strip of asthenospheric low velocity anomaly in the South Atlantic that connects four hotspots, Tristan da Cunha, St. Helena, Ascension and Cape Verde. This deep strip of slow velocity anomaly runs approximately along the seam where Africa and South America broke up, according to available plate reconstruction models. There is, thus, a possibility that the deep, slow velocity strip is preserving the shape of the boundary along which the break‐up occurred. If this is the case, it will be a useful constraint in the reconstruction of plates. A question arises, however, as to why there is a line of low velocity anomaly rather than localized low velocity anomalies near hotspots. We propose three possible causes for this observation, which are (1) upwelling along this line, (2) lithosphere‐controlled passive breakup and (3) plume controlled breakup with lateral channeled flow from plumes to the seam of breakup, when rifts were created between adjacent hotspots. We favor the last hypothesis since it naturally explains the local velocity minimums near hotspots as well as the connected low velocity anomaly and, furthermore, the same mechanism seems to be working under the Red Sea in the present Earth.

Correlation of P wave amplitudes and travel time residuals for teleseisms recorded on the Southern California Seismic Network

P waves from 13 teleseismic events, well distributed in azimuth and recorded on a large short‐period network in southern California, show a positive correlation between travel time residuals and relative amplitudes. Slower arrivals are consistently larger than faster arrivals. Although there is a large amount of scatter, the data show an amplitude increase of about a factor of 2 for a 1‐s increase in travel time. The correlation can be seen in various subsets of the data: individual earthquakes, single stations recording 10 or more earthquakes and data averaged over all recorded events at each station. Because the variations in amplitude and travel time can be associated with surface geology, much of the data recorded at non bedrock sites can be explained in terms of a near‐surface layer of varying velocity. The travel time residual would then be directly proportional to the velocity in the layer and the amplitude proportional to the square root of the velocity contrast at the base of the layer. However, some stations show azimuthally varying travel time residuals that are attributed to upper mantle velocity structures. We use a two‐dimensional finite difference calculation to show that velocity anomalies in the upper mantle can also contribute to the observed correlation of travel time and amplitude.

Upper mantle structure from long-period differential traveltimes and free oscillation data

SUMMARY We have performed an inversion to model upper mantle structure using SS-S differential traveltime residuals in conjunction with previously reported structure coefficient estimates obtained from free oscillation frequency and attenuation measurements. The model we obtain is consistent with both the differential traveltime and structure coefficient data sets and provides nearly the same level of fit as independent inversions of these data sets. We find that a relatively simple model parametrization appears to be adequate for the data used. Upper mantle velocity perturbations and lateral variations in the radii of boundaries at 400 and 670 km depth are parametrized with spherical harmonic expansions up to degree and order 8. The expansions are fixed at specific depth knots (separated by roughly 200 km) spanning the range 20-670 km. Radial variations in velocity are obtained by linear interpolation between these depth knots. Smoothness constraints were applied as part of the inversion procedure, as well as corrections for the thickness and velocity variations of the crust. The smoothness constraints were adjusted to require a smoother structure in the bottom portion (400 to 670 km depth) of the model, where the structure coefficients have the greatest sensitivity. The upper part of the model is primarily constrained by the differential traveltime data. The resulting misfit of the final model shows that, on average, the individual structure coefficients and SS-S measurements are fit to within approximately 1.5 standard deviations. We suggest that some of the misfit is due to lower mantle structure which is not included in the modelling. The power as a function of harmonic degree for the top portion of the model (20 to 220 km depth) does not fall off significantly at the higher harmonic degrees, suggesting that the differential traveltime data may require models which include structure for degrees greater than 8. Boundary perturbations at depths of 400 and 670 km trade off strongly with the velocity variations. Allowing variations of the order of f10 km in the radii of the discontinuities at 400 and 670 km provides a slight improvement to the data fit. Comparisons of several depths of our upper mantle velocity model to the geoid show the strongest correlation at degree 2 for structure near 670 km depth. The distribution of slab structures is highly correlated to the degree 2 components of the model for depths of 200 to 670 km. Examination of the model in the vicinity of hotspots shows no consistent hotspot related signal.

Lithospheric structure of northern California from teleseismic images of the upper mantle

Teleseismic P wave travel time residuals from 120 earthquakes recorded across the U.S. Geological Survey California seismic network were used to determine the lithosphere P wave velocity structure beneath northern California, a region characterized by complex interactions between the Pacific, North American, and Gorda plates. Lateral P wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals. Inversion results for the crust show strong correlations to volcanic features. The active volcanic fields, Shasta‐Medicine Lake, Lassen, and Clear Lake, are characterized by crustal low‐velocity anomalies that average approximately −6%, possibly identifying partially molten magma bodies. Cooled, solidified magma bodies beneath the extinct volcanic fields, Sonoma, southern Clear Lake, and Sutler Buttes, are denoted by relative velocity highs averaging +3%. The largest upper mantle velocity variations occur in the depth range 30–110 km, where velocities vary from −5.5% to +9.5%. These velocity variations reflect changes in the thickness and geometry of the Pacific, North American, and Gorda plates where they interact at the Mendocino Triple Junction. North of the Mendocino Triple Junction, the steep 70° east dipping portion of the Gorda plate is imaged as a +5% velocity high to depths near 270 km. A presumed segment of the Gorda plate, observed beneath the northern Great Valley and south of the inferred edge of the plate, is characterized by a +9% velocity high in the depth range 30–70 km. Beneath the northern Coast Ranges, shallow asthenosphere is imaged in the depth range 30–100 km as a pronounced southward tapering −4% low‐velocity zone, which we interpret as the slab window. Results from this study provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and development of the lithosphere beneath western North America.

The ocean-continent boundary off the western continental margin of Iberia-II. Crustal structure in the Tagus Abyssal Plain

SUMMARY An 80 km long reversed seismic refraction line (Line 5) was shot over the Tagus Abyssal Plain off Portugal. The main P-wave reflected and refracted phases were modelled both for traveltime and amplitude. The resulting P-wave velocity/depth model has the following features: (a) an extremely thin crust of about 2 km; (b) the absence of oceanic layer 3; and (c) very low upper mantle velocities between 7.6 and 7.9 km s-'. This very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at Atlantic fracture zones, and in particular to the structures found in profiles shot along the transform valley and near ridge-transform intersections. A magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of Line 5. As an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the Tagus Abyssal Plain is located at about 1130'W, in a symmetric position with respect to the ocean-continent transition in the conjugate South Newfoundland Basin, then magnetic anomalies can be modelled simply by assuming sea-floor spreading west of 11'45'W at 10 mm yr-' beginning at M11 time (133 Myr BP), and blocks of rifted continental crust to the east. The location of the proposed ocean-continent transition in the Tagus Abyssal Plain is marked by a well-defined N-S linear magnetic anomaly which is adjacent to the oldest sea-floor spreading block. East of the proposed ocean-continent transition there is an increase in the depth to basement similar to that found east of the ocean-continent transition in the Iberia Abyssal Plain and elsewhere. This model also allows us to explain why Purdy's (1975) seismic refraction line A-AR in the Tagus Abyssal Plain cannot be interpreted as a conventional reversed pair because most of Line A was shot over the ocean-continent transition zone and most of Line AR over thinned continental crust. Remarkably similar velocity/depth structures to that under Line 5 are found close to the ocean-continent transition zone off the whole of western Iberia, in areas which show no clear evidence of fracture zones. Therefore it appears more likely that the seismic structure of Line 5 is due to its proximity to the ocean-continent transition than to a local association with a fracture zone and further, that its structure is typical of this transition off the western margin of Iberia. We also suspect that the low upper mantle velocities associated with the ocean-continent transition indicate the widespread occurrence of serpentinized peridotite.

Crustal and uppermost mantle structure under southern California

The travel times of Pg and Pn arrivals are used together as a new approach to estimate the station delays, lateral velocity variations in the upper crust and upper mantle, crustal thickness variations, and Pn anisotropy in the upper mantle under southern California. The main advantage of this approach is that crustal effects are estimated directly from the data and therefore do not contaminate the results from Pn arrivals. Significant negative station delays are found in the Colorado desert, San Jacinto Mountains, San Gabriel Mountains, central Mojave desert and from the southern Ventura Basin to Santa Cruz Island in addition to the positive delays associated with well-known sedimentary basins. Lateral velocity variations in the upper crust exceed a few percent only in the Coso ranges. These variations show no clear association with surface faulting; they also show little resemblance to the upper mantle velocity variations, which have fast velocity under the Mojave and near the offshore area and slow velocity in most other areas. The southern California Moho shows large variations. Large crustal thinning occurs near the Salton Sea, northwest of the White Wolf fault, in the Coso Range near San Diego and offshore, while the San Bernardino Mountains, western Granite Mountains, and eastern Mojave (south of the Ludlow fault) are underlain by roots. A significant amount (3%) of Pn anisotropy is present in the uppermost mantle with a fast direction N 75° ± 4°W. This could be explained very well by averaging the Pn anisotropy estimates east and west of the San Andreas.