Wide-angle Seismic Studies Research Articles

Moho depth map of northern Venezuela based on wide-angle seismic studies

As part of the lithosphere, the crust represents Earth's rigid outer layer. Some of the tools to study the crust and its thickness are wide-angle seismic studies. To date, a series of seismic studies have been carried out in Venezuela to determine in detail the crustal thickness in the southern Caribbean, in the region of the Caribbean Mountain System in northern Venezuela, as well as along the Mérida Andes and surrounding regions. In this study, a review of the wide-angle seismic data is given, incorporating new data from the GIAME project for western Venezuela, resulting in a map of Moho depth north of the Orinoco River, which serves as the basis for future integrated models. Differences in Moho depths from seismic data and receiver function analysis are discussed. From the Caribbean plate, Moho depth increases from 20 to 25 km in the Venezuela Basin to about 35 km along the coast (except for the Falcón area where a thinning to less than 30 km is observed) and 40–45 km in Barinas - Apure and Guárico Basins, and Guayana Shield, respectively. Values of more than 50 km are observed in the Maturín Basin and in the southern part of the Mérida Andes.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 1. seismic data

For studying the structure of the lithosphere in southern Ukraine, wide-angle seismic studies that recorded the reflected and refracted waves were carried out under the DOBRE-4 project. The field works were conducted in October 2009. Thirteen chemical shot points spaced 35–50 km apart from each other were implemented with a charge weight varying from 600 to 1000 kg. Overall 230 recording stations with an interval of 2.5 km between them were used. The high quality of the obtained data allowed us to model the velocity section along the profile for P- and S-waves. Seismic modeling was carried out by two methods. Initially, trial-and-error ray tracing using the arrival times of the main reflected and refracted P- and S-phases was conducted. Next, the amplitudes of the recorded phases were analyzed by the finite-difference full waveform method. The resulting velocity model demonstrates a fairly homogeneous structure from the middle to lower crust both in the vertical and horizontal directions. A drastically different situation is observed in the upper crust, where the V p velocities decrease upwards along the section from 6.35 km/s at a depth of 15–20 km to 5.9–5.8 km/s on the surface of the crystalline basement; in the Neoproterozoic and Paleozoic deposits, it diminishes from 5.15 to 3.80 km/s, and in the Mesozoic layers, it decreases from 2.70 to 2.30 km/s. The subcrustal V p gradually increases downwards from 6.50 to 6.7–6.8 km/s at the crustal base, which complicates the problem of separating the middle and lower crust. The V p velocities above 6.80 km/s have not been revealed even in the lowermost part of the crust, in contrast to the similar profiles in the East European Platform. The Moho is clearly delineated by the velocity contrast of 1.3–1.7 km/s. The alternating pattern of the changes in the Moho depths corresponding to Moho undulations with a wavelength of about 150 km and the amplitude reaching 8 to 17 km is a peculiarity of the velocity model.

Delamination of eclogitized lower crust: Control on the crust–mantle boundary in the central Fennoscandian shield

The crustal structure of the central part of the Fennoscandian shield is dominated by a high velocity lower crust (HVLC) ( V P 7.3–7.5 km/s, at the depth of 40–63 km in an area of ca. 400 km × 600 km) formed in the Palaeoproterozoic Svecofennian orogen at ∼ 1.91–1.87 Ga. In the area of the HVLC the Moho is deeper than 50 km and attains depths of up to 63 km in eastern Finland at the Archaean–Proterozoic boundary zone. We suggest that the present Moho dates back to late Svecofennian time and was determined by delamination of high-density eclogitized lower crust after tectonic collisional thickening of crust. Densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle. New geological evidence from kimberlite-hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets. In addition, small clinopyroxene–garnet mineral aggregates (microxenoliths) show omphacitic clinopyroxene compositions. Therefore, the presence of eclogites in the high velocity lower crust is implied, although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present HVLC. With results on near-incidence and wide-angle seismic studies and geological and geochemical data on heat producing elements we modeled the Svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model. The constructed thermal model simulates the genesis of Svecofennian synkinematic (at 1.89–1.88 Ga) and postkinematic (1.88–1.87 Ga) granitoids, as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths. Moreover, the narrow distribution of the ages of the Svecofennian mafic–ultramafic magmatism (ca. 1.89–1.87 Ga) dispersed around the HVLC area is explained by the homogeneous warming of the lithosphere. The conditions of lower crustal delamination were estimated with a Rayleigh–Taylor convective instability model constrained with lower crustal and upper mantle rheological data. Delamination of a lower crustal eclogite layer 5–30 km thick takes place in about 7–50 Ma if the rock dynamic viscosity is ≤ 1·10 21 Pa s. It requires temperatures of ≥ 1000 °C in the lower crust and upper mantle. In the Svecofennian crust, such temperatures were attained at about 1860–1850 Ma, i.e. after the orogenic peak. We conclude that the Moho of the HVLC area was controlled by delamination of a ca. 5–10 km thick layer of high-density lowermost crust into the mantle. Such a density-controlled process may have controlled the Moho depth also in other Proterozoic shield areas.

Frozen magma lenses below the oceanic crust

The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust. Thermal modelling, tomography, compliance and wide-angle seismic studies, supported by geological evidence, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of approximately 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

Crustal structure of the Peruvian continental margin from wide-angle seismic studies

Durch Modellierung von seismischen Weitwinkeldaten konnten die Struktur, sowie die P-Wellen Geschwindigkeiten des Untergrundes der subduzierenden ozeanischen Nazca Platte und der kontinentalen Sud-Amerikanischen Platte von 8S bis 15S am peruanischen Kontinentalrand verifiziert werden. Drei detaillierte Profile entlang der Subduktionszone und ein Querprofil ueber das Lima Becken werden in dieser Arbeit vorgestellt. Die ozeanische Kruste der Nazca Platte ist mit einer duennen pelagischen Sedimentschicht bedeckt, die von 0 bis 200 m variiert. Die ozeanische Kruste im Messgebiet hat durchschnittlich eine Maechtigkeit von 6.4 km, die im Bereich des Trujillo Trogs bei 8S, einer Grabenstruktur, auf 4 km ausduennt. Die abtauchende Platte kann anhand der Daten, bis in eine Tiefe von 25 km beobachtet werden. Ein Subduktionskanal von 50 m Maechtigkeit, und der P-Wellen Geschwindigkeit von 4.5 km/s, wird mit der Methode der Wellenfeld-Modellierung nachgewiesen. Die Geschwindigkteis-Modelle zeigen, dass sich ein Akkretionskeil ausgebildet hat, welcher mit dem steilen unteren Kontinentalhang in Verbindung gebracht wird. Terrigene Sedimente werden dort abgelagert, welche durch niedrige P-Wellen Geschwindigkeiten charakterisiert sind. Diese lagern an eine landeinwaerts liegende Backstop Struktur, die aus kontinentalem Basement aufgebaut ist, und mit seismischen P-Wellen Geschwindigkeiten von 3.5 bis 6.0 km/s charakterisiert ist. Die Subduktionsprozesse vor Peru werden von tektonischer und basaler Erosion dominiert mit Perioden erhoehter Erosion durch die Nazca-Ruecken-Subduktion. Anzeichen dafuer sind der grosse Winkel zwischen Kontinentalhang und subduzierender Platte, lateral variierende Neigungswinkel entlang des Kontinentalhanges, variierende Materialeigenschaften der kontinentalen Platte und unterschiedliche Maechtigkeiten der Forearc Becken entlang des Kontinentalrandes.

Moho topography in the central Andes and its geodynamic implications

P-to-S converted waves at the continental Moho together with waves multiply reflected between the Earth’s surface and the Moho have been used to estimate the Moho depth and average crustal Vp/Vs variations in the central Andes. Our analysis confirms and significantly complements the Moho depth estimates previously obtained from wide-angle seismic studies and receiver functions. The resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins (30 km) further to the east in the Chaco plains. Beneath the Andean plateau, the Moho is deeper in the north (Altiplano) and shallower in the south (Puna), where the plateau attains its maximum elevation. A non-linear relation exists between crustal thickness and elevation (and Bouguer gravity), suggesting that the crust shallower than 50–55 km is predominately felsic in contrast to a predominately mafic crust below. Such a relation also implies a 100 km thick thermal lithosphere beneath the Altiplano and with a lithospheric thinning of a few tens of kilometers beneath the Puna. Absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau. In the Subandean ranges at 19–20°S, the relation between altitude and crustal thickness indicates a thick lithosphere (up to 130–150 km) and lithospheric flexure. Beneath a relative topographic low at the Salar de Atacama, a thick crust (67 km) suggests that the lithosphere in this region is abnormally cold and dynamically subsided, possibly due to coupling with the subducting plate. This may be related to the strongest (Ms=8.0) known intra-slab earthquake in the central Andes that happened very close to this region in 1950. The average crustal Vp/Vs ratio is about 1.77 for the Altiplano–Puna and it reaches the highest values (1.80–1.85) beneath the volcanic arc, indicating high ambient crustal temperatures and wide-spread intra-crustal melting.

Combined vertical-incidence and wide-angle seismic study of deepwater bright-spot reflections

Ocean island systems such as Hawaii and the Canaries have been built by large volumes of magma intrusions and by lava extrusions based on intraplate volcanism. The Canary Islands, off the Northwest African continental margin (Figure 1), were formed by hot-spot volcanism where subaerial volcanic activity started on Fuerteventura about 20 million years ago.

Electrical resistivity structure of the Valu Fa Ridge, Lau Basin, from marine controlled-source electromagnetic sounding

SUMMARY In December 1995 we carried out a comprehensive controlled-source electromagnetic survey of the Valu Fa Ridge at 22u25k Si n the Lau Basin. The Valu Fa Ridge is ab ack-arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity. Seismic studies have imaged a melt lens at an average depth of 3.2 km below the seafloor, surrounded by a zone of lowered seismic velocity, interpreted as a region of partial melt in the crust. The electromagnetic experiment was part of a multidisciplinary study which included wide-angle and reflection seismics, bathymetry and potential field measurements. Electromagnetic signals at frequencies between 0.25 and 40 Hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea-bottom receivers at ranges of up to 20 km from the source. Over 80 hr of data, consisting of the magnitude of the horizontal electric field at the seafloor, were collected. These data have extremely low scatter compared to similar data from previous surveys. The data were interpreted using a combination of 1- and 2-D forward modelling and inversion. The vertical resistivity gradient in the upper crust at the Valu Fa Ridge is abnormally low, with resistivities of less than 10 V m observed throughout layer 2 of the crust to a depth of 3 km. This is significantly more conductive at depth than the axis of the slow-spreading Reykjanes Ridge at 57u45kN, and the fastspreading East Pacific Rise at 13uN, where similar data sets have been collected in the past. Although the structure of layer 2 is well constrained by the electromagnetic data, its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it, and hence the data are not sensitive to the structure in layer 3, in particular the structure of the melt lens or surrounding low-velocity zone. The seismic velocity structure of the Valu Fa Ridge, determined from the coincident wide-angle seismic study, is similar to that observed at other mid-ocean ridges, with a steep seismic velocity gradient through layer 2 (although overall velocities are slightly lower). The seismic velocity anomaly calculated relative to an average off-axis structure is also small. This suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity. However, hot and/or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity. Since the conductive region extends unbroken from 3 km depth to the seafloor, it is probable that these fluids circulate to (or close to) the magma chamber itself.

Broadside wide-angle seismic studies and three-dimensional structure of the crust in the southeast Canadian Cordillera

Broadside, or fan, recordings of a Lithoprobe seismic refraction – wide-angle reflection experiment in the southeastern Canadian Cordillera show several features further illuminating the crustal structure beyond that previously derived from SCoRE '90 (Southern Cordillera Refraction Experiment of 1990) in-line data. Analysis of a nearly in-line profile centred on Castlegar, British Columbia, shows lower velocities in the upper crust associated with the Purcell Anticlinorium as well as velocity variations that may have some association with the Purcell fault zone. The depth to Moho is almost 38 km, somewhat deeper and on trend with the structure that has been established farther north. The broadside records show high signal-to-noise ratio PmP arrivals (i.e., reflections from the bottom of the crust). These PmP fan picks were analysed in regions away from in-line profiles, providing further measurements of the depth to Moho in the southeastern Cordillera. The analysis of the broadside records combined with the earlier in-line interpretations as well as older crustal seismic measurements make up a relatively high resolution database, compared with most other regions in Canada, from which we have generated maps of depth to Moho and average crustal velocity in the southeastern Cordillera of Canada. The maps show thin, low-velocity crust over much of the region and indicate a high degree of correlation between current crustal seismic properties and regional isotherms.