Ramón Carbonell

Seismic wide-angle constraints on the crust of the southern Urals

A wide-angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide-angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6–15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0–6.0 km/s, which correlates with the surface geology; 5–7 km depths at which the velocities increase to 6.2–6.3 km/s; 10–30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30–35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N-S and one for the E-W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poissons ratio and anisotropy were calculated from the velocity models. The Poissons ratio increases in the lower crust at both sides of the root zone. A localized 2–3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50–56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.

Some attributes of wavefields scattered from Ivrea-type lower crust

Abstract The Ivrea Zone in northern Italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of Alpine lithospheric shortening. Based on two 1:25,000 geological maps from the central Ivrea Zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained, stochastic seismic models of Ivrea-type lower crust. A stochastic rather than a deterministic approach is applied due to the limited availability of. detailed geological maps and the high degree of small-scale structural and petrophysical complexity of the Ivrea Zone. The primary characteristic of the resulting model is its “layered” self-affine or fractal structure and its bimodal velocity distribution. While we are not suggesting that Ivrea-type structure is universal for extended lower continental crust, synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data: 1. (1) Ivrea-type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust; 2. (2) observed lower-crustal Q -factors can be largely explained by scattering losses and intrinsic Q -factors of reflective lower crust are likely to be larger than 1000, which is incompatible with the presence of free volatiles; 3. (3) in agreement with recent broad band studies of the lower crust, Ivrea-type lower crust does not create any tuning effects; and 4. (4) for Ivrea-type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures, which suggests that multiple scattering may be important in the reflective lower crust.

Transpressional collision tectonics and mantle plume dynamics: the Variscides of southwestern Iberia

Abstract In southwestern Iberia, three continental domains (the South Portuguese Zone (SPZ), Ossa-Morena Zone (OMZ) and Central Iberian Zone (CIZ) collided in Devonian-Carboniferous time. The collision was transpressional, with left-lateral kinematics, and was interrupted by extensional tectonics during the earliest Carboniferous, when bimodal magmatism (with associated mineral deposits) and basin development were the dominant orogenic features. Transpression was renewed in Visean time, and persisted until the end of the Carboniferous. The IBERSEIS deep seismic reflection profile helps to define the 3D geometry of transpressional structures: out-of-section displacements concentrate in bands, which bound wedges of upper crust; this crustal wedging strongly modifies the geometry of the sutures between continental blocks. A mid-crustal strongly reflective thick band (the Iberseis Reflective Body, IRB) is interpreted as a huge body of basic rocks. The IRB magma trapped in the middle crust was linked to the Early Carboniferous mantle-derived magmatism that crops out in the SPZ, OMZ and CIZ. Magmatism at the surface and trapped in the crust, high thermal gradients and basin development reflect a thermal anomaly in the underlying mantle, influencing both the thermal and the stress state of the orogen at that time. A mantle plume is inferred to have existed in the Early Carboniferous, the transpressional tectonic regime dominating again after its decay.

Sensitivity of the lateral correlation function in deep seismic reflection data

In the absence of multiple scattering the lateral correlation function of the reflected wavefield can be related to the lateral correlation function of the medium. We have analyzed the lateral correlation function of a series of synthetic seismograms from stochastic models of reflective lower crust with the structural and petrophysical statistics of the Ivrea Zone. Our results show that in the presence of Ivrea-type lower crustal heterogeneity the lateral correlation function of the backscattered wavefield is insensitive to the lateral correlation function of the scattering medium, which suggests that multiple scattering is important in the reflective lower crust. This interpretation is consistent with lower crustal reflections commonly being shorter than a Fresnel zone, the absence of clear diffractions and the poor performance of standard migration algorithms on deep seismic reflection data.

La estructura sísmica de la corteza de la Zona de Ossa Morena y su interpretación geológica

El experimento de sismica de reflexion profunda IBERSEIS ha proporcionado una imagen de la corteza del Orogeno Varisco en el sudoeste de Iberia. Este articulo se centra en la descripcion de la corteza de la Zona de Ossa Morena (OMZ), que esta claramente dividida en una corteza superior, con reflectividad de buzamiento al NE, y una corteza inferior de pobre reflectividad. Las estructuras geologicas cartografiadas en superficie se correlacionan bien con la reflectividad de la corteza superior, y en la imagen sismica se ven enraizar en la corteza media. Esta esta constituida por un cuerpo muy reflectivo, interpretado como una gran intrusion de rocas basicas. La imagen de las suturas que limitan la OMZ muestra el caracter fuertemente transpresivo de la colision orogenica varisca registrada en el sudoeste de Iberia. La Moho actual es plana y, en consecuencia, no se observa la raiz del orogeno.

3D Elastic Full Waveform Inversion - On Land Study case

Full Waveform Inversion is one of the most advanced processing methods that is recently reaching a mature state after years of solving theoretical and technical issues such as the non-uniqueness of the solution and harnessing the huge computational power required by realistic scenarios. In this work, we present the application of this method to a 3D on-land dataset acquired to characterize the shallow subsurface. The current study explores the possibility to apply elastic isotropic Full Waveform Inversion using only the vertical component of the recorded seismograms. One of the main challenges in this case study remains the costly 3D modeling that includes topography and free surface effects. Nevertheless, the resulting models provide a higher resolution of the subsurface structures than starting models, and show a good correlation with the available borehole measurements.