Juan José Dañobeitia

Lateral variation of the crust in the Iberian peninsula: New evidence from the Betic Cordillera

Abstract New results from a seismic refraction/wide-angle reflection survey carried out in the Betic Cordillera in autumn 1989, contribute to a better picture of its deep structure. One NW-SE profile cuts across the Iberian Massif, the external and internal Betics. The structure of the crust in the Iberian Massif shows characteristics similar to those found in previous experiments. The lower crust is found as a distinct layer, 12 km thick, with an average velocity of 6.8 km · s−1; the Moho is found at about 35 km depth. This structure extends southeastward until a 3–4 km upwelling of the Moho, about 30 km north of the present-day surface boundary between the external and internal Betic units. Further southeast the Moho deepens to 38 km and the lower crust is no longer seismically detected. The absence of differentiated lower crust beneath part of the external Betics may be related with the Mesozoic rifting of the South-Iberian passive margin. Instead, this absence under the internal Betics may be caused by rifting in conjunction with the collisional evolution of the orogen. A WNW-ESE profile lying in the internal Betics shows the presence of a prominent reflector at 10–12 km depth. This seems to be a widespread feature in the internal Betics and may be interpreted as a detachment surface. The Moho is found at 38 km depth rising strongly in the easternmost Betics. Seismic data suggest a thin crust in the offshore area southwest of Malaga, probably containing a massive zone of high-velocity rocks which is also supported by available geophysical and geological data.

Mapping active faults offshore Portugal (36°N–38°N): Implications for seismic hazard assessment along the southwest Iberian margin

Swath-bathymetry and acoustic-backscatter data from the southwest Iberian margin, which hosts the present-day boundary between the European and African plates, reveal the surficial expression of several fault structures <100 km offshore of Portugal. High-resolution and multichannel seismic reflection profiles collected perpendicular to these structures show folding and reverse faulting of the Quaternary units, suggesting present-day tectonic activity. Successive submarine-landslide deposits at the base of the scarps provide evidence of cyclic fault activity. The location and dimension of these newly identified structures agree with the modeled source suggested for the A.D. 1755 Lisbon earthquake and tsunami, possibly the most destructive event in western Europe during historical time. These fault escarpments and deformed seafloor sediments associated with a cluster of shallow seismicity suggest that these thrusts are active and may pose a significant earthquake and tsunami hazard to the coasts of Portugal, Spain, and Morocco.

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The S-transform is becoming popular for time-frequency analysis and data-adaptive filtering thanks to its simplicity. While this transform works well in the continuous domain, its discrete version may fail to achieve accurate results. This paper compares and contrasts this transform with the better known continuous wavelet transform, and defines a relation between both. This connection allows a better understanding of the S-transform, and makes it possible to employ the wavelet reconstruction formula as a new inverse S-transform and to propose several methods to solve some of the main limitations of the discrete S-transform, such as its restriction to linear frequency sampling.

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We present the results of a multidisciplinary geophysical study, conducted to investigate the lithospheric structure of the Costa Rican Isthmus. The physical properties of the lithosphere are resolved by three-dimensional (3-D) simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2-D forward modeling of onshore and offshore seismic refraction and gravity data. According to our results, the northern half of the Costa Rican Isthmus is constituted by a ∼40-km-thick crust, with a 6- to 7-km-thick oceanic crust subducting under it. The uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities, in good agreement with those described for flood basalts. The midlevel shows velocities and densities representative of oceanic crust. The bottommost level (20-40 km) shows high velocities and densities, typical of mafic rocks, and the upper mantle displays anomalously low densities and velocities. Intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies. These results are consistent with a basement beneath the Costa Rican Isthmus being part of the Caribbean plateau, originated at 85-90 Ma with the onset of the Galapagos hotspot. The upper level corresponds to the flood basalts extruded during this phase, and it includes most of the marginal wedge. The second level represents the preexisting oceanic crust. The mafic lower crust, intracrustal heterogeneities, and anomalous upper mantle are interpreted to be built up by underplating, intrusion, and crystallization of basaltic melts, formed under the influence of subducting lithosphere dehydration.

-Transform From a Wavelet Point of View

Abstract The eastern and southeastern margins of Iberia were affected during Neogene time by a rifting tectonic process superimposed on the Alpine structures. The Valencia Trough, situated off the northeastern coast of Iberia, has been defined as a rift system that began its activity in late Oligocene-Early Miocene time. The opening of the Valencia Trough produced a series of tectonic grabens created by fractures which strike parallel to the coastline. As a consequence of the rifting, subsidence occurred and depositional sequences of Miocene age were subsequently deposited over the Triassic and Mesozoic basement. Some of the Mesozoic faults were reactivated and affected the Tertiary deposits. The depositional history was controlled mainly by extensional faulting during the late Paleocene-early Miocene, and by rapid subsidence. During the late Pliocene a second extensional period occurred in the Iberian Chain resulting in the present configuration of the continental margin in the study area. The Pliocene-Quaternary evolution of the margin was largely controlled by fluctuation of the Ebro River sediment supply and sea-level oscillations.

Lithospheric structure of the Costa Rican Isthmus: Effects of subduction zone magmatism on an oceanic plateau

The seismic structure of the crust of the NW Iberian Peninsula, along the Cantabrian Mountains and their southern foreland, the Duero basin, is investigated from the first data set available from refraction and wide-angle reflection profiles. The velocity-depth distributions obtained along E-W and N-S transects evidence lateral variations in crustal structure that can be associated to Variscan and Alpine tectonic episodes. The western part of the range, toward the Variscan hinterland zones, as well as the Duero basin below the sediments, show a crustal structure similar to that of the Variscan belt elsewere in Europe. The total crustal thickness is 30–32 km, and three main layers (upper, middle, and lower crust with average velocities 6.0, 6.25, and 6.8 km/s) are resolved. Some Paleozoic structures such as the basal detachment of the Cantabrian Zone are expressed in the seismic models. The crust appears to vary eastward in the external zones. The most outstanding feature is a crustal root beneath the highest Cantabrian summits where the Moho is found at 47 km depth. A prominent Alpine reworking of the crust in the eastern part of the Cantabrian Zone is thus revealed, and the N-S seismic transects delineate a geometry similar to the one found across the central Pyrenees in the Etude Continentale et Oceanique par Reflexion et Refraction Sismique (ECORS) profile, revealing the importance of the Alpine deformations in the northern part of the Iberian plate.

Geological framework of the Ebro continental margin and surrounding areas

Abstract New constraints on the crustal structure of the Valencia trough and its Iberian and Balearic borders have been obtained from a seismic experiment carried out in autumn 1989. A total of 290 sea shots were fired along three refraction/wide-angle reflection profiles recorded by 10 Ocean Bottom Seismometers (OBSs) in the Valencia trough and 110 land stations distributed on both flanks. The huge amount of recorded data in each profile has been integrated and jointly interpreted to obtain a single 2-D velocity-depth model along each line. The geometry and deep crustal structure of the basin are fairly well defined in terms of laterally inhomogeneous layered media. The sedimentary cover shows a layer of consolidated sediments, overlain by Neogene and Quaternary sediments, with a velocity of 5.1 km/s interpreted as Mesozoic carbonates at the northwestern side of the Valencia trough. A fairly homogeneous crystalline basement is found on all profiles at a mean depth of 4 km below sea-level, with a velocity of 6.0–6.1 km/s. The observed critical distances and arrival times of the Moho-reflected waves are firm evidence supporting a thinning of the crust towards the centre of the basin. The Moho rises up to 13 km in the axis of the trough, in the NW-SE and E-W profiles, and up to 15 km in the NE-SW profile. Moreover, the analysis of the well defined PMP phase shows differences between the deep structure of the flanks and the trough, respectively. A sharp velocity contrast at the Moho from a fairly uniform lower crust of 6.4 km/s velocity is proposed in both flanks, in contrast to a high velocity gradient within the lower crust interpreted near the axis of the trough. An anomalous upper-mantle velocity of 7.8 km/s fits the data throughout the Valencia trough and Mallorca, with slightly higher values of 7.9 km/s required towards the continent. These velocity values do not favour an interpretation in terms of underplating. The geometry of the Moho discontinuity shows a gradual deepening towards the Iberian Peninsula, in contrast to a more abrupt slope towards Mallorca. Depth values of 30–32 km, typical of a continental crust, are inferred about 40 km inland on the Iberian Peninsula, in agreement with the crustal thicknesses already reported for the Iberian range and the Ebro basin. The present crustal thinning resulting from rift episodes in the Valencia trough and the Iberian border is thus confirmed and constrained from our seismic results of the deep crust and upper mantle.

Seismic signature of Variscan and Alpine tectonics in NW Iberia: Crustal structure of the Cantabrian Mountains and Duero basin

We report the results of a multichannel seismic study on eastern Central Atlantic oceanic crust. Unmigrated and migrated sections have been used to identify the main seismic boundaries within the crust. The upper crust generally is seismically transparent except for a 20-km-long continuous reflection, which we interpret as associated with a long-lived normal fault in the basement and with hydrothermal circulation. Short and discontinuous reflections in the upper crust have also been observed. The lower crust is either transparent or variably reflective in form of subhorizontal layered reflection packages and/or dipping reflections. The reflectivity patterns are interpreted as magmatic in origin except for the dipping reflections that affect the whole crust, which are interpreted as expressions of extensional faults. The reflections dipping ridgeward are thought to have acted as detachment surfaces where plate separation took place. The Moho has been identified along most of the length of the seismic profiles either as the base of the reflective lower crust or as a sharp single reflection. The igneous crust shows large variations in thickness sometimes associated with fracture zones in the Cretaceous crust. Fracture zones in the Jurassic crust do not show, in general, a crustal thinning. This may be due to different accretionary processes during Jurassic and Cretaceous times.

Deep crustal configuration of the Valencia trough and its Iberian and Balearic borders from extensive refraction and wide-angle reflection seismic profiling

An offshore vertical incidence reflection seismic study with simultaneous on-land wide-angle recording has been conducted, as part of the Estudio Sismico de la Corteza Iberica Norte (ESCIN) Project ...

Seismic boundaries of the eastern Central Atlantic Mesozoic crust from multichannel seismic data

Abstract The velocity crustal structure and the Moho discontinuity have been determined fromwide-angle and seismic refraction data across the Nicoya Peninsula and the neovolcanic zone ofnorthern Costa Rica. The data analyzed here are from 8 terrestrial shots recorded at 120 closelyspaced landstations. These onshore recordings complete earlier offshore investigations and resultsto perform a seismic velocity model along a transect of 250 km, from the outer rise to theback-arc basins. The oceanic Moho (M1) reaches 10 km depth W of the Middle AmericanTrench (MAT), deepening progressively towards the isthmus to reach an angle of subduction of35° down to 40 km depth. The obtained onshore velocity crustal structure is characterized bysignificant lateral changes in the surface velocity (from 2.0 km/s to 5.0 km/s) which areconsistent with most outstanding geological units (ophiolitic outcrops, sedimentary basins, etc.),an upper crust of 4 km with velocities ranging 5.3–5.7 km/s, a mid crust with a mean thicknessof 13 km and velocity variations of 6.2–6.5 km/s and a thick lower crust up to 20 km withvelocities between 6.9 and 7.3 km/s. The Moho (M2) is attained at 38–40 km. We discuss hereinthe significance of this crustal structure as Caribbean crust over-thickened or essentially built byGalapagos mantle plume-derived material.