scholarly journals Evolution of the Iberian Massif as deduced from its crustal thickness and geometry of a mid-crustal (Conrad) discontinuity

Solid Earth ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 1515-1547
Author(s):  
Puy Ayarza ◽  
José Ramón Martínez Catalán ◽  
Ana Martínez García ◽  
Juan Alcalde ◽  
Juvenal Andrés ◽  
...  
Keyword(s):  
Lower Crust ◽  
Normal Incidence ◽  
Crustal Thickness ◽  
P Wave ◽  
Iberian Massif ◽  
Velocity Increase ◽  
P Wave Velocity ◽  
Asymptotic Geometry ◽  
Surface Geology ◽  
European Variscides

Abstract. Normal incidence seismic data provide the best images of the crust and lithosphere. When properly designed and continuous, these sections greatly contribute to understanding the geometry of orogens and, along with surface geology, unraveling their evolution. In this paper we present the most complete transect, to date, of the Iberian Massif, the westernmost exposure of the European Variscides. Despite the heterogeneity of the dataset, acquired during the last 30 years, the images resulting from reprocessing the data with a homogeneous workflow allow us to clearly define the crustal thickness and its internal architecture. The Iberian Massif crust, formed by the amalgamation of continental pieces belonging to Gondwana and Laurussia (Avalonian margin), is well structured in the upper and lower crust. A conspicuous mid-crustal discontinuity is clearly defined by the top of the reflective lower crust and by the asymptotic geometry of reflections that merge into it, suggesting that it has often acted as a detachment. The geometry and position of this discontinuity can give us insights into the evolution of the orogen (i.e., of the magnitude of compression and the effects and extent of later-Variscan gravitational collapse). Moreover, the limited thickness of the lower crust below, in central and northwestern Iberia, might have constrained the response of the Iberian microplate to Alpine shortening. Here, this discontinuity, featuring a Vp (P-wave velocity) increase, is observed as an orogen-scale boundary with characteristics compatible with those of the globally debated Conrad discontinuity.

scholarly journals Evolution of the Iberian Massif as deduced from its crustal thickness and geometry of a mid-crustal (Conrad) discontinuity

2020 ◽  
Author(s):  
Puy Ayarza ◽  
José Ramón Martínez Catalán ◽  
Ana Martínez García ◽  
Juan Alcalde ◽  
Juvenal Andrés ◽  
...  
Keyword(s):  
Seismic Data ◽  
Lower Crust ◽  
Normal Incidence ◽  
Crustal Thickness ◽  
Iberian Massif ◽  
Nw Iberia ◽  
Asymptotic Geometry ◽  
Surface Geology ◽  
Internal Architecture ◽  
European Variscides

Abstract. Normal incidence seismic data provide the best images of the crust and lithosphere. When properly designed and continuous, these sections greatly contribute to understanding the geometry of orogens and, together with surface geology, to unravel their evolution. In this paper we present an almost complete transect of the Iberian Massif, the westernmost exposure of the European Variscides. Despite the heterogeneity of the dataset, acquired during the last 30 years, the images resulting from reprocessing with a homogeneous workflow allow us to clearly define the crustal thickness and its internal architecture. The Iberian Massif crust, formed by the amalgamation of continental pieces belonging to Gondwana and Laurussia (Avalonian margin) is well structured in upper and lower crust. A conspicuous mid-crustal discontinuity is clearly defined by the top of the reflective lower crust and by the asymptotic geometry of reflections that merge into it, suggesting that it has often acted as a detachment. The geometry and position of this discontinuity can give us insights on the evolution of the orogen, i.e. of the effects and extent of the late Variscan gravitational collapse. Also, its position and the limited thickness of the lower crust in central and NW Iberia constraints the response of the Iberian microplate to Alpine shortening. This discontinuity is here observed as an orogeny-scale feature with characteristics compatible with those of the worldwide, Conrad discontinuity.

Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

1984 ◽  
Vol 74 (4) ◽  
pp. 1263-1274
Author(s):  
Lawrence H. Jaksha ◽  
David H. Evans
Keyword(s):  
New Mexico ◽  
Wave Velocity ◽  
Lower Crust ◽  
Seismic Waves ◽  
Seismic Refraction ◽  
Velocity Model ◽  
P Wave ◽  
Uppermost Mantle ◽  
P Wave Velocity ◽  
Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

scholarly journals Lithospheric structure of the Iberian Central System (Iberian Massif) imaged by the wide-angle seismic reflection/refraction CIMDEF experiment 

2021 ◽  
Author(s):  
Irene DeFelipe ◽  
Puy Ayarza ◽  
Imma Palomeras ◽  
Juvenal Andrés ◽  
Mario Ruiz ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Seismic Reflection ◽  
P Wave ◽  
Lithospheric Structure ◽  
Mountain Range ◽  
Wide Angle ◽  
Topographic Feature ◽  
Iberian Massif ◽  
Central System ◽  
P Wave Velocity

<p>The Iberian Central System represents an outstanding topographic feature in the central Iberian Peninsula. It is an intraplate mountain range formed by igneous and metasedimentary rocks of the Variscan Iberian Massif that has been exhumed since the Eocene in the context of the Alpine orogeny. The Iberian Central System has been conventionally interpreted as a thick-skinned pop-up mountain range thrust over the Duero and Tajo foreland basins. However, its lithospheric structure and the P-wave velocity distribution are not yet fully resolved. In order to place geophysical constraints on this relevant topographic feature, to identify lithospheric discontinuities, and to unravel the crustal deformation mechanisms, a wide-angle seismic reflection and refraction experiment, CIMDEF (Central Iberian Mechanism of DEFormation), was acquired in 2017 and 2019. It is a NNW-SSE oriented 360-km long profile that runs through the Duero basin, the Iberian Central System and the Tajo basin. First results based on forward modeling by raytracing show an irregularly layered lithosphere and allow to infer the depth extent of the northern Iberian Central System batholith. The crust is ~ 31 km thick under the Duero and Tajo basins and thickens to ~ 39 km under the Iberian Central System. A conspicuous thinning of the lower crust towards the south of the Iberian Central System is also modeled. Along this transect, a continuous and high amplitude upper mantle feature is observed and modeled as the reflection of an interface dipping from 58 to 62 km depth featuring a P-wave velocity contrast of 8.2 to 8.3 km/s. Our preliminary results complement previous models based on global-phase seismic and noise interferometry and gravity data, provide new constraints to validate the accuracy of passive seismic methods at lithospheric scale, and contribute with a resolute P-wave velocity model of the study area to unravel the effect of the Alpine reactivation on the central Iberian Massif.<br>This project has been funded by the EIT-RawMaterials 17024 (SIT4ME) and the MINECO projects: CGL2016-81964-REDE, CGL2014-56548-P.</p>

scholarly journals Determination of rock densities in the Carpathian-Pannonian Basin lithosphere: based on the CELEBRATION 2000 experiment

2016 ◽  
Vol 46 (4) ◽  
pp. 269-287 ◽  
Author(s):  
Barbora Šimonová ◽  
Miroslav Bielik
Keyword(s):  
Lower Crust ◽  
Pannonian Basin ◽  
Average Density ◽  
East European Craton ◽  
P Wave ◽  
Seismic Velocities ◽  
Upper Crust ◽  
East European ◽  
P Wave Velocity

Abstract The international seismic project CELEBRATION 2000 brought very good information about the P-wave velocity distribution in the Carpathian-Pannonian Basin litosphere. In this paper seismic data were used for transformations of in situ P-wave velocities to in situ densities along all profiles running across the Western Carpathians and the Pannonian Basin: CEL01, CEL04, CEL05, CEL06, CEL09, CEL11 and CEL12. The calculation of rock densities in the crust and lower lithosphere was done by the transformation of seismic velocities to densities using the formulae of Sobolev-Babeyko, Christensen-Mooney and in the lower lithosphere also by Lachenbruch-Morgan’s formula. The density of the upper crust changes significantly in the vertical and horizontal directions, while the interval ranges of the calculated lower crust densities narrow down prominently. The lower lithosphere is the most homogeneous - the intervals of the calculated densities for this layer are already very narrow. The average density of the upper crust (ρ̅ = 2.60 g · cm−3) is the lowest in the Carpathian Foredeep region. On the contrary, the highest density of this layer (ρ̅ = 2.77 g · cm−3) is located in the Bohemian Massif. The average densities ρ̅ of the lower crust vary between 2.90 and 2.98 g · cm−3. The Palaeozoic Platform and the East European Craton have the highest density (ρ̅ = 2.98 g · cm−3 and ρ̅ = 2.97 g · cm−3, respectively). The lower crust density is the lowest (ρ̅ = 2.90 g · cm−3) in the Pannonian Basin. The range of calculated average densities ρ̅ for the lower lithosphere is changed in the interval from 3.35 to 3.40 g · cm−3. The heaviest lower lithosphere can be observed in the East European Craton (ρ̅ = 3.40 g · cm−3). The lower lithosphere of the Transdanubian Range and the Palaeozoic Platform is characterized by the lowest density ρ̅ = 3.35 g · cm−3.

Constraining crustal structure in the presence of sediment: a multiple converted wave approach

2019 ◽  
Vol 219 (1) ◽  
pp. 313-327 ◽  
Author(s):  
Erin Cunningham ◽  
Vedran Lekic
Keyword(s):  
Wave Velocity ◽  
Seismic Velocity ◽  
Seismic Station ◽  
Sedimentary Basins ◽  
Crustal Thickness ◽  
Receiver Functions ◽  
P Wave ◽  
Converted Wave ◽  
P Wave Velocity ◽  
Multiple Data Sets

SUMMARY Receiver functions are sensitive to sharp seismic velocity variations with depth and are commonly used to constrain crustal thickness. The H–κ stacking method of Zhu & Kanamori is often used to constrain both the crustal thickness (H) and ${V_P}$/${V_S}$ ratio ($\kappa $) beneath a seismic station using P-to-s converted waves (Ps). However, traditional H–κ stacks require an assumption of average crustal velocity (usually ${V_P}$). Additionally, large amplitude reverberations from low velocity shallow layers, such as sedimentary basins, can overprint sought-after crustal signals, rendering traditional H–$\ \kappa $ stacking uninterpretable. We overcome these difficulties in two ways. When S-wave reverberations from sediment are present, they are removed by applying a resonance removal filter allowing crustal signals to be clarified and interpreted. We also combine complementary Ps receiver functions, Sp receiver functions, and the post-critical P-wave reflection from the Moho (SPmp) to remove the dependence on an assumed average crustal ${V_P}$. By correcting for sediment and combining multiple data sets, the crustal thickness, average crustal P-wave velocity and crustal ${V_P}$/${V_S}$ ratio is constrained in geological regions where traditional H–$\ \kappa $ stacking fails, without making an initial P-wave velocity assumption or suffering from contamination by sedimentary reverberations.

Crustal structure across the Nain – Makkovik boundary on the continental shelf off Labrador from seismic refraction data

1996 ◽  
Vol 33 (3) ◽  
pp. 460-471 ◽  
Author(s):  
Ian Reid
Keyword(s):  
Continental Shelf ◽  
Wave Velocity ◽  
Crustal Structure ◽  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
P Wave ◽  
Air Gun ◽  
P Wave Velocity

A detailed seismic refraction profile was shot along the continental shelf off Labrador, across the boundary between the Archean Nain Province to the north and the Proterozoic Makkovik orogenic zone to the south. A large air-gun source was used, with five ocean-bottom seismometers as receivers. The data were analysed by forward modelling of traveltimes and amplitudes and provided a well-determined seismic velocity structure of the crust along the profile. Within the Nain province, thin postrift sediments are underlain by crust with a P-wave velocity of 6.1 km/s, which increases with depth and reaches 6.6 km/s at about 8 km. Moho is at around 28 km, and there is no evidence for a high-velocity (>7 km/s) lower crust. The P- and S-wave velocity structure is consistent with a gneissic composition for the Archean upper crust, and with granulites becoming gradually more mafic with depth for the intermediate and lower crust. In the Makkovik zone, the sediments are thicker, and a basement layer of P-wave velocity 5.5–5.7 km/s is present, probably due to reworking of the crust and the presence of Early Proterozoic volcanics and metasediments. Upper crustal velocities are lower than in the Nain Province. The crustal thickness, at 23 km, is less, possibly due in part to greater crustal stretching during the Mesozoic rifting of the Labrador Sea. The crustal structure across the Nain–Makkovik boundary is similar to that across the corresponding Archean–Ketilidian boundary off southwest Greenland.

scholarly journals No mafic layer in 80 km thick Tibetan crust

2021 ◽  
Author(s):  
Gaochun Wang ◽  
Thybo Hans ◽  
Irina M. Artemieva
Keyword(s):  
Wave Velocity ◽  
Lower Crust ◽  
Bulk Composition ◽  
P Wave ◽  
Indian Plate ◽  
Velocity Versus ◽  
Low Velocity ◽  
Crustal Earthquakes ◽  
Temperature Regimes ◽  
P Wave Velocity

<div> <p>All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. It has been suggested that the lower crust does not need to be basaltic, but until now all seismic observations show high P-wave velocity, which requires that the bulk composition of the lower crust must include at least 20-40% of mafic rocks. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (V<sub>p</sub>>7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (V<sub>p</sub><6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. The calculated typical velocity versus depth curves for different crustal lithologies and temperature regimes imply the composition of the lower crust is felsic. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.</p> </div><p> </p>

scholarly journals The Conrad discontinuity and the P wave velocity of the lower crust in Shikoku and its surrounding regions.

1986 ◽  
Vol 34 (6) ◽  
pp. 531-542 ◽  
Author(s):  
Kennosuke OKANO ◽  
Shozo KIMURA ◽  
Tadashi KONOMI
Keyword(s):  
Wave Velocity ◽  
Lower Crust ◽  
P Wave ◽  
P Wave Velocity
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