Jorge Gallastegui

Analysis of kilometric-scale superposed folding in the Central Coal Basin (Cantabrian zone, NW Spain)

Abstract The Carboniferous paralic sequence of the Central Coal Basin (Cantabrian zone, Hercynian NW Spain) contains an outstanding example of kilometric-scale superposed folding that allows comparison with previous experimental models. First generation folds have a N-S trend and mainly constitute a fold train of fault propagation folds in which an increase of interlimb angle and an increase in the dip of the axial plane is observed from west to east. A second folding phase gave rise to two types of upright, roughly E-W oriented, superposed folds. The superposed folds generated in general by buckling, but their trend and situation are controlled by lateral ramps of the previous thrusts in many cases. Superposed folds of the first type are the most common and have developed on the limbs of previous folds without folding their axial surfaces. They occur in zones and usually have kilometre-scale hinge lines with syngenetic curved traces. These folds present wavelengths distinctly smaller than those of the previous folds. The second type of superposed folds affect both the limbs and axial planes of the previous folds. These second-type folds are, in some cases, smaller than previous folds, and have developed on closed early folds, but in other cases they are large wavelength folds related to the tightening of lateral ramps of the thrusts.

2-D seismic modeling of the Variscan foreland thrust and fold belt crust in NW Spain from ESCIN-1 deep seismic reflection data

Abstract The foreland thrust and fold belt (Cantabrian Zone) of the Variscan Belt in NW Spain and the transition to the hinterland (Westasturian–Leonese Zone) was the location of a seismic experiment in 1991. The seismic reflection profile (ESCIN-1) is 140 km long and runs in an E–W direction. The interpretation and seismic modeling of the main reflective interfaces in the profile were made integrating available geological and geophysical data including surface geology, deep seismic reflection data from ESCIN-1, transmission velocities from a borehole, refraction and laboratory data. The geological and velocity model of the crust was tested obtaining synthetic seismograms and can be correlated with surface geology. The velocity model images the main crustal structures interpreted from ESCIN-1. The basal detachment of the foreland thrust and fold belt dips gently from 12 km in the easternmost part of the profile to 16 km in the transition to the hinterland to the west. A new interpretation is proposed for the structure above the basal detachment in the eastern end of the profile, where the basal detachment, at a depth of 12 km, is duplicated at 6.5 km by a N-dipping Alpine thrust that also duplicates part of the basement. In the western part, two deep reflective bands dip westward and the most conspicuous one can be followed from 16–27 km depth. The two bands, previously interpreted as crustal ramps, join a reflective lower crust that is located between 25 and 29 km and fades westwards. A reflective Moho interpreted at the base of the reflective lower crust dips and fades in the same direction from 30–34 km.

The Fossil Accretionary Wedge of the Bay of Biscay: Critical Wedge Analysis on Depth-Migrated Seismic Sections and Geodynamical Implications

The accretionary wedge of the Bay of Biscay is an east-west compressive belt buried under recent sediments of the abyssal plain at the north Iberian margin. This structure formed through the partial closure of the previously extended Biscay basin during the Cenozoic north-south collision between Europe and Iberia, the same collision that produced the Cantabrian-Pyrenean range on land. Three north-south seismic sections have been prestack depth migrated, showing a narrow-tapered wedge (7°–8°) whose internal structure corresponds to a set of south-dipping thrusts converging toward a basal decollement. There are differences along strike within the wedge: thrust spacing, the dip of the basal thrust, and the thickness of the sediments at the trough augment toward the east, increasing its overall size. The two-dimensional velocity models obtained through migration analysis reflect values between 2000 km/s at the sea floor (4500 m) and 5000 km/s at 12-km depth. The syntectonic package thickness varies from 1.5 to 3 km, while the posttectonic cover attains 1.5–2 km. A simple analysis based on critical wedge theory approaches suggests that the Biscay wedge formed in similar conditions to active submarine wedges, the strength of the decollement being lower than the strength of the wedge itself. Further comparison with other examples indicates high basal stress, which could be an added factor in the convergence stopping at this margin. The eastward size increase is attributed to the provision of extra sediments by the coetaneous rising of the cordillera on land. This weight steepens the basal angle without affecting the overall taper. Surprisingly, the eastward change from an oceanic to a transitional basement does not seem to be crucial in its geometry.

The Pre-Andean Phases of Construction of the Southern Andes Basement in Neoproterozoic–Paleozoic Times

During the late Neoproterozoic and Paleozoic times, the southern Andes of Argentina and Chile (21o–55o S) formed part of the southwestern margin of Gondwana. During this period of time, a set of continental fragments of variable extent and allochtony was successively accreted to that margin, resulting in six Paleozoic orogenies of different temporal and spatial extension: Pampean (Ediacaran–early Cambrian), Famatinian (Middle Ordovician–Silurian), Ocloyic (Middle Ordovician–Devonian), Chanic (Middle Devonian–early Carboniferous), Gondwanan (Middle Devonian–middle Permian), and Tabarin (late Permian–Triassic). All these orogenies culminate with collisional events, with the exception of the Tabarin and a part of the Gondwanan orogenies that are subduction-related.