P-wave Residuals Research Articles

A wavefield-separation-based elastic least-squares reverse time migration

Elastic least-squares reverse time migration (ELSRTM) has the potential to provide improved subsurface reflectivity estimation. Compared with elastic RTM (ERTM), ELSRTM can produce images with higher spatial resolution, more balanced amplitudes, and fewer artifacts. However, the crosstalk between P- and S-waves can significantly degrade the imaging quality of ELSRTM. We have developed an ELSRTM method to suppress the crosstalk artifacts. This method includes three crucial points. The first is that the forward and backward wavefields are extrapolated based on the separated elastic velocity-stress equation of P- and S-waves. The second is that the separated vector P- and S-wave residuals are migrated to form reflectivity images of Lamé constants [Formula: see text] and [Formula: see text] independently. The third is that the reflectivity images of [Formula: see text] and [Formula: see text] are obtained by the vector P-wave wavefields achieved in the backward extrapolation of the separated vector P-wave residuals and the vector S-wave wavefields achieved in the backward extrapolation of the separated vector S-wave residuals, respectively. Numerical tests with synthetic data demonstrate that our ELSRTM method can produce images free of crosstalk artifacts. Compared with ELSRTM based on the coupled wavefields, our ELSRTM method has better convergence and higher accuracy.

Middle Miocene near trench volcanism in northern Colombia: A record of slab tearing due to the simultaneous subduction of the Caribbean Plate under South and Central America?

Field, geochemical, geochronological, biostratigraphical and sedimentary provenance results of basaltic and associated sediments northern Colombia reveal the existence of Middle Miocene (13–14 Ma) mafic volcanism within a continental margin setting usually considered as amagmatic. This basaltic volcanism is characterized by relatively high Al2O3 and Na2O values (>15%), a High-K calc-alkaline affinity, large ion lithophile enrichment and associated Nb, Ta and Ti negative anomalies which resemble High Al basalts formed by low degree of asthenospheric melting at shallow depths mixed with some additional slab input. The presence of pre-Cretaceous detrital zircons, tourmaline and rutile as well as biostratigraphic results suggest that the host sedimentary rocks were deposited in a platform setting within the South American margin. New results of P-wave residuals from northern Colombia reinforce the view of a Caribbean slab subducting under the South American margin.The absence of a mantle wedge, the upper plate setting, and proximity of this magmatism to the trench, together with geodynamic constraints suggest that the subducted Caribbean oceanic plate was fractured and a slab tear was formed within the oceanic plate. Oceanic plate fracturing is related to the splitting of the subducting Caribbean Plate due to simultaneous subduction under the Panama-Choco block and northwestern South America, and the fast overthrusting of the later onto the Caribbean oceanic plate.

P-wave velocity structure of the uppermost mantle beneath Hawaii from traveltime tomography

SUMMARY We examine the P-wave velocity structure beneath the island of Hawaii using P-wave residuals from teleseismic earthquakes recorded by the Hawaiian Volcano Observatory seismic network. The station geometry and distribution of events makes it possible to image the velocity structure between y40 and 100 km depth with a lateral resolution of y15 km and a vertical resolution of y30 km. For depths between 40 and 80 km, P-wave velocities are up to 5 per cent slower in a broad elongated region trending SE‐NW that underlies the island between the two lines defined by the volcanic loci. No direct correlation between the magnitude of the lithospheric anomaly and the current level of volcanic activity is apparent, but the slow region is broadened at y19.8uN and narrow beneath Kilauea. In the case of the oceanic lithosphere beneath Hawaii, slow seismic velocities are likely to be related to magma transport from the top of the melting zone at the base of the lithosphere to the surface. Thermal modelling shows that the broad elongated low-velocity zone cannot be explained in terms of conductive heating by one primary conduit per volcano but that more complicated melt pathways must exist.

Seismic anisotropy - a method for studying the fabric of deep continental lithosphere

Independent seismological results, obtained from shear-wave splitting, azimuthal variation of Pn velocities, P-residual spheres and dispersion of surface waves, have demonstrated to observed anisotropies originates in preferred orientation of olivine crystals in the mantle lithosphere. In comparison with oceanic regions, the large-scale fabric of the continental lithosphere is more complex. While the surface-wave data provide an integral property of a number of tectonic units, results on splitting of sub-vertically incident teleseismic shear waves show consistent orientations of the fast S waves within individual lithospheric blocks. In many provinces, the mean azimuth of the fast S polarization parallels the regional geological fabric at the surface (e.g. the strike of a mountain belt). The azimuth-incidence angle dependent terms of teleseismic P-wave residuals, cleared of the effects of source regions and deep mantle paths, show a systematic directional dependence of the relative residuals over distances of several hundred kilometres. Abrupt changes in orientations of the high- and low-velocity directions are related to important tectonic boundaries (deep-seated suture zones). The continental lithosphere related anisotropy scanned by P waves requires a tectonic block structure with dipping high- and low-velocity directions within the lithosphere. Present-day models and interpretations of individual observations of seismic anisotropy depend on the wavelength of different types of seismic waves, as well as on directions of the propagation (both as to azimuths and dips from the horizontal). A joint analysis of body waves - both anisotropic P-residual spheres and shear-wave splitting - allows detecting large-scale anisotropic structures approximated either by hexagonal or orthorhombic symmetries with inclined symmetry axes.

Teleseismic imaging of subaxial flow at mid-ocean ridges: traveltime effects of anisotropic mineral texture in the mantle

SUMMARY Deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy. Traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region, thus providing a signature of the deformation field at depth. In this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle. The finite-strain fields of the passive upwelling model versus the buoyancy-enhanced upwelling model are quite different. This suggests that mineral aggregates deform differently in the two models, thus developing seismic signatures that are distinguishable. Numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems. The activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress. The computed grain deformation reflects a balance between stress equilibrium, for the aggregate as a whole, and strain continuity between neighbouring grains within the aggregate. This approach enables a direct link to be made between the model flow fields and the resulting texture development. Given these mineral orientation distributions, elastic parameters are calculated and wavefronts are propagated through the anisotropic structure. Traveltimes for teleseismic body waves are computed using ray theory, and amplitudes are estimated for an across-axis profile extending 100 km from the ridge axis. Relative P-wave residuals of up to 1 s are predicted for the buoyant model with on-axis arrivals being earliest, since near-vertical velocities are fastest beneath the axis. On-axis P-wave arrivals for the passive model are half a second earlier than arrivals 60 km off-axis, and relative delays continue to increase slowly as distance from the ridge increases. S-wave splitting of almost a second is predicted for the buoyant model, whereas less than a half-second of splitting is determined for the passive model.

Anisotropy andP-wave tomography: a new approach for inverting teleseismic data from a dense array of stations

SUMMARY A new formulation is given for inverting teleseismic P-wave residuals in terms of both 3-D variations of seismic velocity and anisotropy of the subcrustal lithosphere. Two restrictive assumptions are made on the anisotropic elastic tensor to reduce the number of model parameters. First, the upper-mantle anisotropy is modelled by a preferred orientation of olivine crystals with b- and c-axes randomly distributed around the a-axis, and second, the elastic properties of polycrystalline aggregates are computed according to the Voigt averaging rule. The latter assumption allows us to link linearly the observed P-wave residuals to the proportion of oriented crystals in a realistic polycrystalline assemblage. In the inversion process, the orientation of the symmetry axis is fixed, either by a trial procedure or by using a priori hypotheses from independent data. A linear inversion scheme is then applied to the data to retrieve the 3-D variations of two scalar parameters giving the isotropic velocity perturbation and the amount of anisotropy. The inversion algorithm is tested for different source-receiver configurations. Using several idealistic ray geometries, it is concluded that the teleseismic P-wave residuals could potentially carry enough information for retrieving both the 3-D isotropic velocity variations and the amount of anisotropy. However, from a more practical point of view, by looking at the distributions of rays for two recent tomographic experiments in the southern Rhine Graben area and in the Pyrenees, it is concluded that the distributions of rays in these experiments do not fulfil the necessary requirements for retrieving reliable information about the lithospheric anisotropy. Additional data are thus needed to constrain the anisotropic model. If the symmetry axis is set as horizontal, the inversion of the P-wave residual of the Pyrenees leads to an east-west fast P-wave velocity orientation. The resulting anisotropy is compatible with SKS observations made in the Pyrenees. Whatever the anisotropy is, the numerical tests performed with the actual ray configurations show the robustness of the isotropic components in the inverted model. A long-wavelength anisotropy in the lithosphere, if present, should not drastically alter the isotropic 3-D velocity distributions that are obtained from classical methods of teleseismic tomography.

Large teleseismic P-wave residuals observed at the Alban Hills volcano, Central Italy

We collected teleseismic waveforms from a digital microseismic network deployed by the Istituto Nazionale di Geofisica (ING) in collaboration with the U.S. Geological Survey (USGS), on the Alban Hills Quaternary volcano during the 1989-1990 seismic swann. About 50 events were recorded by the network, 30 of them by at least 4 stations. We analysed the data in order to image crustal heterogeneities beneath the volcano. The results show large delay time residuals up to - 1 second for stations located on the volcano with respect to station CP9 of the National Seismic Network located about 20 km to the east, on the Apennines. This suggests that the whole area overlies a broad low-velocity region. Although the ray coverage is not very dense, we model the gross seismic structure beneath the volcano by inverting the teleseismic relative residuals with the ACH technique. The main features detected by tbc inversion are a low-velocity zone beneath the southwestern fiank of tbc volcano, and a high-velocity region beneath the center. The depth extension of these anomalous zones ranges between 5 and 16 km. The correspondence between the low-velocity region and the most recent activity of the volcano (- 0.027 Ma) leads us to infer the presence of a still hot magmatic body in the crust beneath the southwestern side of the volcano, whereas the central part overlies the older and colder high-velocity volcanic roots related to the previous central activity (0.7 to 0.3 Ma).

Lithospheric thickness and velocity anisotropy—seismological and geothermal aspects

We compare inferences of regional variations in the lithospheric thickness in central Europe based on three types of observations. Seismologically determined thicknesses derived from representative P-wave residuals agree in most regions both with the magnetotelluric inferences of depths of the top of a layer of increased electrical conductivity and with the geothermally based calculations that allow inferences of the lithospheric thickness from surface heat-flow data. The seismological definition of the lithosphere as a high-velocity layer overlying the low-velocity zone in the upper mantle seems to be close to the geothermal concept in which the lithospheric base is defined by temperatures near partial melting, decreasing the seismic velocities and increasing the electrical conductivity. Interpretations of seismic data used for constraining geothermal models of the deep lithosphere assume velocity variations to reflect mainly changes in the mineral composition. However, recent observations of large-scale seismic anisotropy show clearly that, in the subcrustal lithosphere, velocity variations due to anisotropy caused by preferred orientations of olivine crystals can be much larger than the velocity variations caused by temperature or compositional changes.

High-velocity anomaly under the Deccan Volcanic Province

Tomographic modelling of P-wave residuals at 22 vertical-component seismograph stations operated in the Deccan Volcanic Province (DVP) in west-central India and the Dharwar Craton point to the existence of a large, deep (at least 300 km), high-velocity anomaly (1–3% faster) directly beneath most of the DVP and Dharwar Craton. There is also marginal evidence that the westernmost part of the DVP may be characterised by low velocity (up to −1.5%) to a depth of about 200 km. This coherent high-velocity anomaly under the DVP and its extension into the south Indian shield is viewed as a lithospheric root, an architecture characteristic of Precambrian shields.

Teleseismic P-residual study in the Iberian Peninsula

Teleseismic P-wave residuals, cleared of the effects of various source regions and deep mantle paths, reflect the composition and structure of the lithosphere and asthenosphere beneath seismological stations. We use a method with which we compute two components of the relative residuals for each station, namely a directionally independent average and azimuth-incidence angle dependent terms, and also the ACH 3-D inversion method. The representative averages show consistent variations over the Iberian Peninsula with a relatively steep gradient in some areas—the Iberian Massif and the Betic Cordillera. These changes attain values around 1 s over a distance of about 200 km. The lateral variations of these residual averages are transformed into a tentative model of the lithosphere thickness. A thick lithosphere is observed beneath the Iberian Massif, the Pyrenees, and beneath part of the Betic Cordillera. Waves from events at different teleseismic distances and various azimuths provide consistent residual patterns and a systematic directional dependence of the relative residuals especially at stations in the Betic Cordillera.

The lithosphere in central Europe—seismological and petrological aspects

The lithosphere thickness in the Variscan belt of central Europe varies between about 60 and 150 km with typical values of 100–120 km. Our estimates, derived from directionally independent representative P-wave residuals, are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle. The large-scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative P residuals vary between 6.5 and 15.2% for P velocities; the S-wave anisotropies determined from SKS polarizations vary between 2.2 and 6.7%. These values are in reasonable agreement with the anisotropies of peridotites determined in laboratory. Systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere. The residual patterns at most stations in the Saxothuringicum, Rhenohercynicum and in the Massif Central imply northwesterly orientated dips of the anisotropic structures while stations in the Moldanubicum, the Alpine Foredeep and most of the Alps north of the Insubric line, suggest southeasterly orientated dips. In our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere. The Variscides of central Europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the Moldanubicum and the Saxothuringicum.

P‐wave residuals at stations in nepal: Evidence for a high velocity region beneath the Karakorum

P‐wave residuals recorded at stations in Nepal from events to the northwest and closer than about 20° are consistently earlier than those from other directions by about 2.5 s. These early arrivals are associated with paths confined to the upper 300 km of the Earth and suggest that cold material occupies the uppermost mantle beneath the Karakorum, northwest Himalaya, and western Kunlun. Thus, these data suggest that convective downwelling occurs more vigorously in this region than beneath the rest of the Himalaya, Tibet, and their surroundings.

Three-dimensional velocity inversion of teleseismic P-wave residuals from the telemetric network of the G.D.R.

Abstract Residuals of teleseismic P-wave arrival times at the CIPE network were used to estimate the three-dimensional velocity structure of the upper mantle by means of the inversion procedure described by Aki et al. (1977). Two models with different depth ranges (233 and 500 km, respectively) have been determined. They refer to the upper mantle beneath the Central European part of the Variscian fold-belt and its transition to the East European platform north of it. Both models reveal a lateral velocity increase within the upper mantle. It is directed to the northeast within the lithosphere and at least a part of the asthenosphere, and in a more northerly direction at greater depth. Furthermore, there seems to exist a local inhomogeneity west of Prague at a depth of some 200 km.

Determination of the three-dimensional seismic structure of the crust and upper mantle in the Central Midlands of England

Summary. Teleseismic P-wave residuals relative to CWF, a permanent shortperiod seismic station on Charnwood Forest in the Central Midlands of England, have been determined for two small aperture arrays deployed over the Precambrian block of Charnwood and its surrounding Phanerozoic sediments. The data have been inverted to produce a block model of the P-wave velocity variations in the crust and upper mantle beneath the study region. The results are consistent with significant variations penetrating to a depth of at least 50 km. Low velocities are associated with two upper crustal intrusive bodies, the Caledonian Mountsorrel granodiorite and the South Leicestershire diorites. A longer-wavelength variation at lower crustal/upper mantle depths could arise from the Moho dipping to the south-west beneath the study region, and whose strike sub-parallels the dominant Charnian trend of the major basement structures in this part of Central England.

P-wave travel-time residuals in the Newfoundland Appalachians

Teleseisms and quarry blasts were recorded at 24 temporary seismograph sites along a line across the northern Dunnage Zone of Newfoundland. Azimuthally dependent functions were also determined for teleseismic P-wave travel-time residuals at the permanent St. John's (STJ) and Corner Brook (CBK) seismographs. In order to minimize the effects of reading errors and structure far from the receivers, absolute residuals at the field sites were then obtained by correlating the records with those from STJ and CBK and using the residual functions for these two stations. The P-wave residuals in north-central Newfoundland tend to be up to half a second later than at STJ and CBK, with the more positive values towards the east of the region. The direction of fastest approach for rays varies from southeast at STJ to southwest towards the western side of the Dunnage Zone. The travel-time residuals are generally similar to variations observed elsewhere in the Appalachians.The residuals and gravity data indicate that the crust is about 40 km thick over most of the Dunnage Zone, possibly increasing by several kilometres under the Gander Zone and western Avalon Platform, which could be consistent with a former subduction zone in the region. The low crustal velocity of 6.2 km s−1 may support an allochthonous model for central Newfoundland, with average crustal thickening of about 10 km compared with adjacent platform areas.

Upwarp of anomalous asthenosphere beneath the Rio Grande rift

Continental rifts are possible analogues of mid-ocean ridges, although major plate tectonic features are less clearly observed1. Current thermal models of mid-ocean ridges2–4 consist of solid lithospheric plates overlying the hotter, less viscous asthenosphere, with plate thickness increasing away from the ridge axis. The lithospheric lower boundary lies at or near the melting point isotherm, so that at greater depths higher temperatures account for lower viscosity, lower seismic velocities and possibly partial melting. Upwarp of this boundary at the ridge axis concentrates heat there, thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium. At slow spreading ridges, a major central graben forms owing to the mechanics of magma injection into the crust5. Topography, heat flow, gravity and seismic studies support these models. On the continents, a low-velocity channel has been observed, although it is poorly developed beneath ancient cratons6–9. Plate tectonic models have been applied to continental basins and margins10–12, but further similarities to the oceanic models remain elusive. Topographic uplift is often ascribed to Airy type isostatic compensation caused by crustal thickening, rather than thermal compensation in the asthenosphere. Here we discuss the Rio Grande rift, in southwestern United States. Teleseismic P-wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening.

Effects of subducting lithosphere on p-wave amplitudes and residuals

In studying the structure of the lithosphere and asthenosphere using the kinematic and dynamic parameters of seismic waves, in any area, the tectonics of focal regions should also be considered. Subduction zones represent large-scale inhomogeneities which affect the propagation of seismic waves both at small and teleseismic epicentral distances. A study on the magnitude corrections of seismic stations in Central Europe and Scandinavia revealed that even in the case of close seismic stations the observed differences of the amplitudes of teleseismic P waves depend on the strike and dip of the sinking plates in the Northwestern Pacific. The smaller the angle between a seismic ray and the bottom of the lithospheric plate, the larger the numerical decrease of the magnitude corrections. A similar dependence was found for the P-wave residuals; the rays propagating more along the slab are accelerated more. Besides that, we have observed systematic differences in the P-wave residuals of about 2 s for foci with different positions within the subducting lithosphere of the Kurile arc in investigating the dependence of the residuals on the epicentral distance in the direction of subduction. The differences in the residuals disappear in this region at an epicentral distance of about 52°.

Structure of the upper mantle in the Northeastern Atlantic close to the azores-gibraltar ridge from surface-wave and body-wave observations

Phase velocities of teleseismic Rayleigh waves have been measured in the central North Atlantic on both sides of the Azores-Gibraltar Ridge (AGR) by means of a specially designed long-period station network. The dispersion data obtained were regionalized and then subjected to a “hedgehog” inversion, which gives a set of upper mantle models compatible with the observational data within specified error bounds. Reasonable model solutions were selected by using regional body-wave observations, such as P n - and S n -wave velocities determined from earthquakes along the AGR. The S (itn) velocities measured indicate that the shear-wave velocity in the mantle part of the lithosphere is much higher on the northern side of the AGR. Strongly negative P-wave residuals in this area indicate faster seismic propagation than implied by the Jeffreys-Bullen travel-time tables, while propagation is much slower in the Gulf of Cadiz area. Furthermore the residuals show a clear difference for paths through oceanic and continental domains and suggest that the transition between these two domains extends much further into the ocean on the southern side of the AGR than on the northern side. The proposed model for the structure of the upper mantle in that region shows that there exists a pronounced velocity contrast across the AGR. Thickening of the lithospheric plate with increasing plate age is indicated to the south of the ridge. The greatest thickness is reached close to the continental margin within a zone about 500 km wide, whose velocity close to the Canary Islands and Madeira is significantly lower, probably due to the well-known volcanic activity there. These observations together with the travel time residuals reveal that this zone seems to be of a transitional nature somewhere between a continental and oceanic structure.

Crust and upper-mantle velocity structure in the Appalachian orogenic belt: Implications for tectonic evolution

Research Article| April 01, 1982 Crust and upper-mantle velocity structure in the Appalachian orogenic belt: Implications for tectonic evolution STEVEN R. TAYLOR; STEVEN R. TAYLOR 1Department of Earth and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar M. NAFI TOKSÖZ M. NAFI TOKSÖZ 1Department of Earth and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Author and Article Information STEVEN R. TAYLOR 1Department of Earth and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 M. NAFI TOKSÖZ 1Department of Earth and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1982) 93 (4): 315–329. https://doi.org/10.1130/0016-7606(1982)93<315:CAUVSI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEVEN R. TAYLOR, M. NAFI TOKSÖZ; Crust and upper-mantle velocity structure in the Appalachian orogenic belt: Implications for tectonic evolution. GSA Bulletin 1982;; 93 (4): 315–329. doi: https://doi.org/10.1130/0016-7606(1982)93<315:CAUVSI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The crust and upper mantle structure of the northeastern United States (NEUS) was studied by combining teleseismic and regional body-wave observations with surface-wave dispersion measurements. The velocity models suggest that structures as much as 200 km deep and even deeper can be correlated with surficial geologic and tectonic features and that the Grenville and Appalachian orogenic belts show marked differences in crustal structure. This has the important implication that major orogenic belts have effects that reach well into the lithosphere and are stable for extended periods of time, perhaps as long as 1 b.y.Regional travel times recorded across the NEUS seismic network indicate that the northern Appalachians are characterized by a well-defined two-layer crust with a relatively high velocity lower layer. The crust of the Grenville Province in New York State is slightly thinner than that of the New England Appalachians, and it is vertically homogeneous, with nearly constant P and S velocities.Lateral variations in structure were studied using time-term analysis and teleseismic P-wave residuals. The suture between the Grenville and Appalachian Provinces in the northeastern United States probably occurs along a north-northeast-trending belt extending from northwestern Vermont to southwestern Connecticut. High crustal velocities and/or crustal thinning, a linear gravity high, a serpentinite belt, Precambrian uplifts, and the Taconic thrusts are found along much of this belt.Comparison of seismic velocities with resistivity measurements suggests that the lower crust of the Grenville Province may be composed of rocks with hydrous mineral phases, resulting in lowered velocities and a higher Poisson's ratio. Alternatively, the rocks of the lower crust beneath the Grenville Province may be similar to those found on the surface, while higher velocity mafic mineralogies are prevalent in the lower crust of the Appalachians. This is consistent with the hypothesis that the Grenville crust underwent substantial reactivation, thickened, and became vertically uniform during the Grenville orogeny. In contrast, the rocks of the Appalachian belt probably were associated with a cycle of oceanic opening and closure, which suggests an ensimatic origin of the lower crust in this region.Recent COCORP results indicate that the orogenic belt of the southern Appalachians is allochthonous and is underlain by a basal decollement that extends eastward to the coastal plain. However, contrasts in structural style and tectonic evolution between the northern and southern Appalachians suggest that it may not be possible to extrapolate the COCORP results to the northern Appalachians (north of about 41° latitude) and that the suture between the Grenville Province and the Appalachian orogenic belt in New England is located in the vicinity of, or just east of, the serpentinite belt. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The crust and upper mantle structure beneath southern Italy by earthquakes and DSS data

This paper describes a travel—time analysis performed for the Italian seismic stations, in particular those operating in southern Italy, in order to study the crust and upper mantle properties in the region. Average P-wave residuals of teleseisms in the distance range 30°–95° with respect to Jeffreys-Bullen tables, at thirteen permanent and temporary stations of southern Italy, are coherent with a high velocity zone beneath Calabria and northern Sicily and low velocity material in the mantle beneath the Eolian Islands. Travel—time residuals from Tyrrhenian intermediate earthquakes show a high velocity structure which extends in a NW direction from a depth of at least 200 km down to 450 km. A damped least-squares inversion applied to DSS data confirms the existence of low velocity zones in the crust beneath the Eolian Islands, at 8–12 km depth, that agrees with previous results and with the lack of S waves from local earthquakes.