Juan Luis Alonso

Growth and propagation of buckle folds determined from syntectonic sediments (the Ubierna Fold Belt, Cantabrian Mountains, N Spain)

Abstract The geometry of syntectonic Tertiary sediments in the Ubierna Fold Belt can be explained using the buckling folding theory. The geometric features of the fold train in the pre-orogenic succession also agree with a buckling process. Each fold grew with progressive and coeval rotation of both limbs, although rotation was asymmetric and greater on the forelimb than the backlimb. The diachroneity of synrotational sequences (progressive unconformities) through different folds indicates that fold propagation migrated towards the foreland; however, the initial folds were tightened during a later deformation stage synchronous with the development of younger folds.

Sequences of thrusts and displacement transfer in the superposed duplexes of the Esla Nappe Region (cantabrian zone, nw spain)

Abstract In a cross-section through the southern arm of the Cantabrian Zone, several duplexes have been identified below the Esla Nappe, which is the uppermost and main thrust sheet of the area. The folds deforming the Esla Nappe are culmination walls linked to frontal and lateral ramps belonging to the lower thrust sheets. The thrust sequence can be established on the basis of quantitative analysis of displacement transfer and out of sequence thrusting. The primitive footwall ramps of the Esla Nappe Region were often subsequently broken by decollements developed in successively lower stratigraphic levels of these footwalls. The kinematics of the lowest duplex are more complicated than those of typical duplexes described elsewhere: some thrusts transfer only part of their displacement to the roof thrust, while the remaining part is accommodated along the higher thrusts of previously emplaced duplexes, cutting out of sequence one or more floor or roof thrusts. Cumulative displacement of the thrusts in this region is about 90 km, giving a present thickness 3 times that of the original pre-orogenic sequence, together with a translation of at least 60 km, for the synorogenic basin.

Forelimb deformation in some natural examples of fault-propagation folds

An analysis of two natural examples of fault-propagation folds from the Pyrenees and the Cantabrian zone (N Spain) is presented. These folds possess common features despite their different geological settings, i.e. related to thrust tectonics in the Pyrenean example and to strike-slip faulting in the Cantabrian zone. Transition from fault-bend to fault-propagation folding along individual thrusts is related to changes in lithology. The fault-propagation folds studied are tight and markedly asymmetric with respect to limb thickness and shape. The forelimbs are curvilinear and display progressive thinning of beds towards the fault surface, which can be interpreted in terms of a characteristic heterogeneous strain superimposed onto flexural shear, and restricted to them. This strain has been factorized by means of grid construction, and is found to be a combination of thrust-parallel heterogeneous simple shear and pure shear, attributed here to a shear zone with diverging walls and extrusion. In contrast with previous models, it is shown that thrust-parallel simple shear is not necessarily distributed above the entire fault surface but may instead be restricted to regions where motion was inhibited such as tip zones or ramps.

Stratigraphy and structure of the southeastern Garona Dome

AbstractNorth-facing recumbent folds in the Palaeozoic rocks of the Garona Dome have been recognized, and a modified pre-Silurian stratigraphic sequence is suggested. The gradual disappearance of part of this sequence may be interpreted as a result of truncation by an unconformable late Ordovician conglomeratic unit.

Hinterland-foreland zonation of the Variscan orogen in the Central Pyrenees: comparison with the northern part of the Iberian Variscan Massif

Abstract A new sequence of Variscan deformations is proposed for the Palaeozoic rocks of the Central Pyrenees. The non-metamorphic units include south-directed thrust systems and related folds with a poorly developed cleavage. In the metamorphic units north-verging, recumbent to inclined folds (D1), associated with a subhorizontal to south dipping cleavage, are refolded by south-verging, upright to inclined folds (D2), with a subvertical to north-dipping axial plane cleavage, and offset by south-directed thrusts approximately coeval with D2. The structural evolution of these units suggests a subdivision of the Variscan Central Pyrenees into two different regions consistent with the zones known for a long time in the core of the Ibero-Armorican or Asturian arc (northern part of the Iberian Variscan Massif). The structure of the Pyrenean non-metamorphic units has foreland affinities and is comparable to that of the Cantabrian Zone, whereas the deformation observed in the Pyrenean metamorphic units is characteristic of the hinterland and is consistent with the features of the West Asturian–Leonese Zone or Central–Iberian Zone. Since the Pyrenean non-metamorphic units are located southwards of the metamorphic ones and the Variscan thrusts are south-directed, we tentatively correlate the Variscan Pyrenees with the northern branch of the Ibero-Armorican or Asturian arc.

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen, northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.

Cambrian–Ordovician conodonts from slump deposits of the Argentine Precordillera: new insights into its passive margin development

Fil: Voldman, Gustavo Gabriel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Cordoba. Facultad de Ciencias Exactas Fisicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina

Silurian Conodonts from the Rinconada Formation, Argentine Precordillera

Abstract. At the eastern margin of the Precordillera and close to the boundary with Sierras Pampeanas, the Rinconada Formation displays extensional faults, boudinaged sequences and scarce slump folds for which age is not well constrained. It represents a stacking of “broken formations” intercalated with scarce debris flows. A calcareous iron-rich sample from the upper levels of the Rinconada Formation yielded 134 conodont elements, including Dapsilodus obliquicostatus, Decoriconus fragilis, Oulodus sp., Pseudooneotodus beckmanni, P. b. bicornis and Wurmiella excavata together with ozarkodinids of the “Oz.” bohemica—“Oz.” snajdri lineage. The record of “Oz.” aff. snajdri suggests a late Homerian—early Gorstian (late Wenlock—early Ludlow) minimum depositional age for the upper debris flows of the Rinconada Formation. The studied conodont fauna improves the biostratigraphic correlation of the Rinconada melange with other successions in the Precordillera and the world as well as provides clues on the tectonostratigraphic development of the SW Gondwanan foreland during Silurian times.