| 1. |
- Simancas, J.F., et al.
(författare)
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The seismic crustal structure of the Ossa-Morena Zone and its geological interpretation
- 2004
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Ingår i: Journal of Iberian Geology. - 1698-6180 .- 1886-7995. ; 30, s. 133-142
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Tidskriftsartikel (refereegranskat)abstract
- The IBERSEIS deep reflection seismic experiment has provided a crustal image of the Variscan orogen of southwest Iberia. A brief presentation of the entire seismic profile is given, and then the Ossa-Morena Zone (OMZ) and its boundaries are considered. The crust of the OMZ is shown to be divided into an upper crust, characterized by dominantly NE-dipping reflectivity, and a poorly reflective lower crust. The reflectivity of the upper crust has good correlation with the geological cross-section constructed from surface mapping. In the seismic image, the upper crustal geological structures are seen to merge in the middle crust. Nevertheless, the OMZ middle crust is not a mere detachment level, as it shows very unusual features: it appears as a band of strong reflectivity and irregular thickness (the Iberian Reflective Body, IRB) that we interpret as a great sill-like intrusion of basic rocks. The boundaries of the OMZ are considered sutures of the orogen, and their geometrical features, as deduced from geological mapping and the seismic image, are in accordance with the transpressional character of the Variscan collision recorded in SW Iberia. The present Moho is flat, obliterating the root of the orogen.
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| 2. |
- Ayarza, P, et al.
(författare)
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Contrasting tectonic history of the are-continent suture in the Southern and Middle Urals: implications for the evolution of the orogen
- 2000
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 157, s. 1065-1076
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Tidskriftsartikel (refereegranskat)abstract
- The Main Uralian Fault has been considered the original arc–continent suture for 2000 km along the Uralide orogen. The symmetry of the tectonic units across it suggested a consistent east-dipping polarity for the palaeosubduction zone, which, together with its topographic and aeromagnetic signature, supported the idea of a single suture. However, several characteristics vary at different latitudes. In the Middle Urals, it is a strike-slip fault zone with moderately deformed and metamorphosed volcanic arc fragments in its hanging wall, and low-grade metamorphic rocks of the East European Craton in its footwall. Here, it has a prominent NNW-trending magnetic signature which cross-cuts north-trending anomalies in its hanging wall, and a pronounced reflection seismic signature that can be traced to the top of the middle crust at c. 5 s. TWT. In the Southern Urals, it is a serpentinite mélange zone of ambiguous kinematics, with a weakly deformed and metamorphosed volcanic arc in its hanging wall, and moderately metamorphosed to high pressure rocks of the East European Craton in its footwall. In this part of the orogen, it has a weak reflection seismic character, and a magnetic signature that parallels that of its hanging wall. On the basis of an integrated analysis of these different data sets, we suggest that the Main Uralian Fault, as it is currently defined, is not a single entity, but rather the original arc–continent suture in the south, and the western strand of a strike-slip fault system that reworked the original suture in the Middle Urals.
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| 3. |
- Ayarza, P., et al.
(författare)
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Geophysical constraints on the structure of a limited ocean-continent subduction zone at the north Iberian margin
- 2004
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Ingår i: Tectonics. ; 23, s. 1010-
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Tidskriftsartikel (refereegranskat)abstract
- Late Cretaceous to Cenozoic convergence between Iberia and Europe led to the partial closure of the Bay of Biscay with limited southward subduction of oceanic crust below the North Iberian Margin. Inclined sub-Moho reflections and diffractions observed in deep seismic reflection profiles shot across the margin are especially well represented in two reflection profiles: ESCIN-3.2 and ESCIN-3.3. These two profiles have been chosen to test if the sub-Moho reflections correspond to true primary deep events and, provided that they are reflecting off the subduction zone, to investigate its deep structure. Spectral analysis together with travel time estimation and migration allow us to characterize a number of these sub-Moho events as deep-source, low-frequency (∼19 Hz), reflections and diffractions. Synthetic seismograms were generated by three-dimensional seismic modeling of a sub-Moho southward dipping surface, interpreted to correspond to the top of subducted oceanic crust. Comparison between the real and synthetic data show that inclined, low-frequency sub-Moho reflections in both, ESCIN-3.2 and ESCIN-3.3 profiles may correspond to reflections from southward subducted Bay of Biscay oceanic crust. Geoid, free-air gravity, and absolute topography modeling provides additional constraints on the lithospheric-scale structure of this limited ocean-continent subduction zone beneath the North Iberian Margin.
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| 4. |
- Ayarza, P, et al.
(författare)
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Integrated geological and geophysical studies in the SG4 borehole area, Tagil Volcanic Arc, Middle Urals: Location of seismic reflectors and source of the reflectivity
- 2000
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Ingår i: Journal of Geophysical Research. - : AMER GEOPHYSICAL UNION. - 0148-0227 .- 2156-2202. ; 105:B9, s. 21333-21352
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Tidskriftsartikel (refereegranskat)abstract
- Near-vertical incidence reflection seismic data acquired in the Tagil Volcanic Arc (Middle Urals) show the upper crust to be highly reflective. Two intersecting seismic lines located near the ongoing ∼5400 m deep SG4 borehole show that the main reflectivity strikes approximately N-S and dips ∼35°–55° to the east. Prominent reflections intercept the borehole at ∼1000, ∼1500, 2800–2900, ∼3400, and between ∼4000 and 5400 m, which correspond to intervals of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. Multioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data. Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and suggests that the P to S conversions are best explained by low-velocity porous intervals rather than higher-velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E-W seismic line. This discrepancy between the VSP and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector. Surface and borehole data suggest that the low-velocity/low-density/low-resistivity intervals are the most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids. Tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and/or intrusions. Some of them might also represent high-porosity lava flows or pyroclastic units, common in island arc environments.
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| 5. |
- Brown D., Juhlin C., Tryggvason A., Steer D., Ayarza P., Beckholmen M., Rybalka A. and Bliznetsov M.
(författare)
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The Crustal Architecture of the Southern and Middle Urals from the URSEIS, ESRU, and Alapaev Reflection Seismic Profiles, in Orogenic Processes in the Paleozoic Uralides
- 2002
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Ingår i: Geophysical Monograph Series, AGU. ; , s. 33-48
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Tidskriftsartikel (refereegranskat)
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| 6. |
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| 7. |
- Carbonell, R., et al.
(författare)
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Geophysical Evidence of a Mantle Plume Derived Intrusion Complex
- 2004
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 31, s. L11601-
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Deep seismic reflection data acquired as part of the SW-Iberia EUROPROBE project across the transpressional Variscan orogen sample three tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone, and the Central Iberian Zone. The seismic data reveal the existence of a mid-crustal reflective body 140 km long and of variable thickness (up to 5 km), the Iberian Reflective body. The conductivity image provided by coincident MT soundings, the amplitude characteristics of the seismics, mineralization studies related to magmatic ore deposits, and the surface geology suggest that the IRB is a mantle-derived mafic intrusion. The geophysical, geological and petrological data suggest that the IRB is most probably an Early Carboniferous (approximately at 350–340 Ma) mantle-derived intrusion possibly linked to plume activity that took place in Europe in the Carboniferous and Permian.
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| 9. |
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| 11. |
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| 12. |
- Simancas, J. F., et al.
(författare)
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Crustal structure of the transpressional Variscan orogen of SW Iberia : SW Iberia deep seismic reflection profile (IBERSEIS)
- 2003
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Ingår i: Tectonics. - : AGU. - 0278-7407 .- 1944-9194. ; 22, s. 1062-
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Tidskriftsartikel (refereegranskat)abstract
- IBERSEIS, a 303 km long (20 s) deep seismic reflection profile, was acquired across the Variscan belt in SW Iberian Peninsula. The acquisition parameters were designed to obtain a high-resolution crustal-scale image of this orogen. The seismic profile samples three major tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone, and the Central Iberian Zone, which were accreted in Late Paleozoic times. These terranes show a distinctive seismic signature, as do the sutures separating them. Late strike-slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures. The upper crust appears to be decoupled from the lower crust all along the seismic line, but some deformation has been accommodated at deeper levels. A sill-like structure is imaged in the middle crust as a 1–2 s thick and 175 km long high-amplitude conspicuous reflective band. It is interpreted as a great intrusion of mafic magma in a midcrustal decollement. Taking into account surface geological data and the revealed crustal architecture, a tectonic evolution is proposed for SW Iberia which includes transpressional collision interacting during Early Carboniferous with a mantle plume. The Moho can be identified along the entire transect as subhorizontal and located at 10 to 11 s, indicating a 30–35 km average crustal thickness. Its seismic signature changes laterally, being very reflective beneath the South Portuguese Zone and the Central Iberian Zone, but discontinuous and diffuse below the Ossa Morena Zone.
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