Andrés Pérez-Estaún

Seismic image of the Cantabrian Mountains in the western extension of the Pyrenees from integrated ESCIN reflection and refraction data

Abstract Integrated analysis of normal-incidence and large-aperture seismic reflection data collected in 1992 and 1993 within the Spanish ESCIN and complementary projects provide a first complete NS crustal transect across the Northern Iberian Peninsula and continental margin. Images of the crustal structure of the Cantabrian Mountains and their transition to the Duero basin and to the Cantabrian margin are obtained from: (a) a 65-km-long vertical reflection profile ESCIN-2 on land; (b) a 200-km-long reversed refraction profile; and (c) wide-angle recordings of the marine ESCIN-4 profile. Consistent results between reflectivity pattern and velocity-depth distribution reveal important lateral variations in the deep structure. The reflective crust imaged in the ESCIN-2 profile changes its attitude from sub-horizontal beneath the Duero basin to north-dipping beyond the Mountain front. Basement thrusts are observed in the upper crust merging into a detachment at 6 s (TWT) and may have triggered the Alpine uplift of the range. The Moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile, about 35 km inland. Modelling of the refraction data laterally extends the seismic image and provides evidence for Variscan crustal features beneath the Duero basin. Northwards, the velocity in the lower crust decreases and the Moho, constrained by the wide-angle data from profile ESCIN-2, deepens to about 60 km ending abruptly at the shoreline. The velocity-depth model is constrained along the Asturian platform up to the continental slope, where the crust-mantle boundary is located at 24 km depth. This ‘margin Moho’ shows a progressive deepening southwards, and extends to the coast where it is found at 30 km depth. The present seismic data support an important Alpine reworking and thickening of the crust under the Cantabrian Mountains. The onshore/offshore transition is marked by an imbrication of two crusts of very different thicknesses. This signature offers a strong parallelism with the one previously observed further east across the Pyrenees in the ECORS seismic profile.

CRUSTAL-SCALE STRUCTURE AND EVOLUTION OF AN ARC-CONTINENT COLLISION ZONE IN THE SOUTHERN URALS, RUSSIA

The outcropping geology of the southern Urals contains a well-preserved accretionary complex related to the Paleozoic collision that took place between the Magnitogorsk arc and the former East European Craton. The crustal-scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles. The reflection profiles show the accretionary complex to be highly reflective, allowing direct comparison of many reflections with surface geological features. We interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material, syncollisional sediments derived from the arc, deeply subducted high-pressure gneisses that are intercalated with eclogites and blueschist, and, at the highest structural level, ophiolite complexes. It is bound at the base by a thrust and at the rear by a highly deformed zone (the Main Uralian fault) adjacent to the backstop (the Magnitogorsk arc). Deposition of the Late Devonian volcaniclastic sediments of the Zilair Formation appears to be related to collision, uplift, and erosion of the arc, possibly following the arrival of the full thickness of the East European Craton continental crust at the subduction zone. With the arrival of the continental crust at the subduction zone, offscraping and underplating of Paleozoic slope and platform material took place at the base of the accretionary complex. Uplift of the arc was followed by its collapse and the unconformable deposition of Lower Carboniferous shallow water carbonates on top of it. A time lag of 10 – 15 Myr occurred between the high-pressure metamorphism and the subsequent arrival of the East European Craton at the subduction zone.

Geometric and kinematic evolution of the foreland thrust and fold belt in the southern Urals

The first balanced and restored cross section through the foreland thrust and fold belt of the southern Urals along the Belaya River of Bashkortostan is presented. This cross section is used to discuss the Paleozoic structure and kinematic evolution of the area and to provide quantitative estimates of fault displacement and shortening. The Belaya River thrust stack can be divided into a late imbricate thrust system (Nygush thrust system), the Burzyan thrust sheet, and an early duplex system (Timirovo thrust system). Overprinting relationships indicate two deformation events during which the foreland thrust and fold belt evolved by a forward propagating thrust sequence. The first deformation event involved emplacement of the low metamorphic grade Timirovo thrust system and the overlying Zilair Nappe during accretion of the Magnitogorsk volcanic arc with the East European Craton at the end of the Devonian. A deformational hiatus followed until the end of the Carboniferous, at which time a second deformation event related to final closure of the Uralian ocean resulted in development of the Nygush thrust system. The calculated shortening in the Belaya River area (16.7 km or 17%) is significantly less than that in other foreland thrust and fold belts. The basal thrust in the Belaya River section is located within the basement and cuts continuously up section with a staircase-like geometry, instead of following an easy glide horizon.

SEISMIC STRUCTURE OF THE NORTHERN CONTINENTAL MARGIN OF SPAIN FROM ESCIN DEEP SEISMIC PROFILES

Abstract By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary. A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESCIN-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows ‘layered’ reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.

The structural architecture of the footwall to the Main Uralian Fault, southern Urals

Abstract The Uralide orogen is a linear collisional belt formed during the Upper Paleozoic as a result of convergence between the East European Craton and outboard terranes, and accretion of intervening island arcs and obduction of oceanic crust onto the East European Craton. This paper presents a new structural subdivision of the southern Urals, to the west of the main suture zone, that puts this part of the mountain belt in a context similar to that of other collisional orogenic belts. This subdivision consists of a west-vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin, and a parautochthonous foreland thrust and fold belt, flanked to the east by a metamorphic thrust stack. The latter units are structurally overlain by allochthonous rocks of an accretionary complex, slope sediments, oceanic crust and high grade, eclogite-blueschist-bearing, gneiss. Where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults.

Remagnetizations and postfolding oroclinal rotations in the Cantabrian/Asturian arc, northern Spain

Devonian carbonates in the Cantabrian Arc reveal characteristic magnetizations with coherent, shallow, upward inclinations. The magnetizations appear to be carried by magnetite. Within-site directions are very well grouped, but site-mean declinations range from easterly to south–southwesterly in in situ as well as tilt-corrected coordinates, as has also been observed in previous studies of other formations in the arc. The widely varying declinations of all studies roughly correlate with the overall structural trends of the arc and suggest that the sites underwent rotations in a process that involved folding about vertical axes and tightening of arc. Upon tilt correction the inclinations of our study, on the other hand, become scattered, and it is concluded that the magnetizations were acquired after Late Carboniferous folding about horizontal axes. The oroclinal rotations therefore also must have occurred after the Late Carboniferous folding phase. The previous paleomagnetic results had been interpreted mostly as primary magnetizations residing in hematite. However, inclination only fold-tilt tests applied to these results suggest that many, if not all, of the directions were acquired during the earlier stages of the Late Carboniferous folding. Thus all paleomagnetic results from the Cantabrian Arc appear to be remagnetizations, but the ages of the remagnetizations vary from pre-to synfolding for the mostly hematitic formations to postfolding for the Devonian carbonates. The reversed-polarity inclinations of the hematite-bearing formations have mean values ranging from +25° to +5°, whereas the carbonates have a mean inclination of −8°. On the basis of inclinations predicted for the area from results from stable Europe, the Pyrenees, and the Iberian Meseta, the ages of these remagnetizations can be inferred to range from about 320 Ma to 260 Ma. Because all the remagnetizations reveal rotated declination patterns, the oroclinal rotations occurred well after the main phase of Hercynian deformation (320–280 Ma). While the timing of the rotations is unconstrained at the younger end, they must have occurred during or after the Permian (best estimate is less than 260 Ma), which is much later than anticipated from other geological considerations.

Structure and evolution of the Magnitogorsk forearc basin: Identifying upper crustal processes during arc‐continent collision in the southern Urals

The southern Urals of Russia contain a well-preserved example of a Paleozoic arc-continent collision in which the intraoceanic Magnitogorsk volcanic arc and its forearc basin sediments accreted to the East European Craton during the Devonian. The Magnitogorsk arc records the evolution from incipient intraoceanic subduction to a mature arc, and by comparing its surface geological features with those in active arc-continent collision settings it is possible to identify upper crustal processes that were active in the southern Urals. The arc edifice can be divided into western and eastern volcanic fronts that were active during different stages of arc evolution and for which two distinct phases of forearc basin development can be recognized. The late Lower to Middle Devonian Aktau Formation represents a remnant of the intraoceanic to collisional forearc basin to the Irendyk volcanic front, whereas the Middle Devonian to Lower Carboniferous Ulutau, Koltubanian, and Zilair Formations were deposited in a suture forearc basin to the east Magnitogorsk volcanic front. It was not until the Late Devonian that these two basins were joined. Structural mapping, combined with reflection seismic profiling, shows these basins to be affected by open, nonlinear, volcanic basement-cored synsedimentary folds. The Karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture forearc basin. The forearc region is affected by minor thrusting that involves the volcanic basement, although it is not clear if these thrusts reactivate preexisting trench-parallel faults. Synsedimentary deformation, slumping, and olistostrome development were common throughout the suture forearc basin history but were especially widespread during the Late Devonian, when the full thickness of the continental crust is interpreted as having arrived at the subduction zone.

Two-dimensional geostatistical modeling and prediction of the fracture system in the Albala Granitic Pluton, SW Iberian Massif, Spain

Abstract We use a fracture index distribution method of geostatistical modelling and prediction to characterize quantitatively the fracture system in two-dimensions (2D) in the Mina Ratones area, located in the Albala Granitic Pluton (SW Iberian Massif). The fracture index (FI) is a quantitative estimate of the fracture density in discrete domains. To validate the results of geostatistical modeling a detailed structural map of the area was also made on a scale of 1:1000. The resulting grids, expressed as pixel-maps, describe the continuous value of the FI in 2D for the whole Mina Ratones area. Based on the modelled distribution of the FI and their correlation with mapped faults, we distinguish two structural domains in the studied area: elongated bands of fracture zones with high FI values and romboidal blocks located between them with low FI values. The separation between both domains is gradual. Though a threshold value of the FI that separate both structural domains is not clearly defined, the fracture zones generally present FI>1 and the individualized blocks FI

BASEMENT INFLUENCE ON FORELAND THRUST AND FOLD BELT DEVELOPMENT : AN EXAMPLE FROM THE SOUTHERN URALS

Abstract The reactivation of two sets of pre-existing basement faults provided strong control on the geometric and kinematic evolution of the Paleozoic south Urals foreland thrust and fold belt. Pre-existing basement faults oriented subparallel to the Uralide structural grain were related to several stages of Precambrian deformation. Reactivation focused the deformation along these earlier faults, resulting in the subsequent incorporation of crystalline basement thrust sheets in the thrust belt at an early stage of its evolution. Pre-existing basement faults oriented subperpendicular to the developing thrust belt structural grain were related to aulacogens in the Archean crystalline basement that were filled by variable thickness of Riphean-aged sediments. These influenced the localisation and development of numerous structures in the thrust belt that caused along-strike structural changes related to lateral ramps. Recognition of these features and their influence on the geometric and kinematic evolution of the south Urals foreland thrust and fold belt are important for understanding seismic reflection fabrics in the south Urals thrust belt, and for potential field, 3-D geological, and kinematic modelling.

Along-strike structural variations in the foreland thrust and fold belt of the southern Urals

Abstract The foreland thrust and fold belt of the southern Urals has a number of along-strike structural changes in which regional-scale structures terminate abruptly. One important along-strike structural change is the southern termination of the Precambrian-cored Bashkirian Anticlinorium, which plunges southward beneath the synformal Zilair Nappe. Balanced and restored dip cross-sections, a N-S-oriented strike section, together with a geological interpretation of a CDP reflection seismic profile have been used to determine the 3D structure and the amount of shortening in the foreland thrust and fold belt in the area of the Bashkirian Anticlinorium and the Zilair Nappe. The geometry of the foreland thrust and fold belt is that of a west-vergent, basement-involved thrust stack with shortening on the order of 13% to 17% (approximately 17 km). The small amount of shortening allows a correlation to be made between the along-strike structural changes and pre-existing basement structures. In particular, the southern termination of the Bashkirian Anticlinorium coincides with a Precambrian-age aulacogen, the Sernovodsk-Abdulino Aulacogen, developed in the basement. An E-W-oriented tear fault, the Belaya tear fault, appears to have developed in the basement, along the northern margin of the Sernovodsk-Abdulino Aulacogen. The Belaya tear fault was active during much of the history of the foreland thrust and fold belt, and exerted strong control on its structural and tectonosedimentary development. Because of the small amount of shortening involved, the foreland thrust and fold belt of the southern Urals provides an important example for understanding the effects of pre-existing basement structures on the development of foreland thrust and fold belts.