Multidetachment analogue models of fold reactivation in transpression : the NW Persian Gulf

Two deformation events have been documented in the NW Persian Gulf during the Late Cretaceous and the Late Cenozoic. The most distinctive feature in this part of the Persian Gulf is the reactivation of the Late Cretaceous NNE-SSW Arabian trending folds by NE-SW shortening during the Late Cenozoic Zagros orogeny. In general, if a set of folds with horizontal axes is shortened roughly parallel to its fold axis, a dome-basin fold interference pattern is produced. In the NW Persian Gulf, reactivation of some old folds occurred instead of a fold interference pattern. Reactivation may be influenced by the following factors: i) the presence of incompetent layers (i.e. evaporites), ii) a variable overburden, iii) basement faults, and iv) obliquity between the younger deformation shortening axis and fold axis. It is this last factor that we investigated by means of analogue modelling. The experimental apparatus is similar to that commonly used for experiments with brittle-ductile systems at the Laboratory of Experimental Tectonics of Géosciences Rennes. The model consisted of an alternation of ductile and brittle horizontal layers with a stratigraphy similar to the one found in the NW Persian Gulf. The model was deformed by two deformation events with an angle a between the two directions of shortening. After deformation, the resulting structure resembled a fold facing the static wall with internal thrust faults and detachment faults arranged into a geometry similar to a fish tail. In the second shortening event, the fold was reactivated without formation of an interference pattern. Moreover, the displacement on both the reactivated and newly formed faults varied between almost pure thrust faults for low a and oblique thrust faults with a strike-slip component for high a. The models suggest that the presence of incompetent layers plays an important role in fold reactivation and confirm that basement faults are not necessary

Minorca, an exotic Balearic island (western Mediterranean)

Despite forming part of the Balearic group of islands, Minorca differs stratigraphically and structurally from Majorca and Ibiza: i) Paleozoic rocks are abundant in Minorca but are very scarce in Majorca and are absent in Ibiza. Eocene-Oligocene sediments are virtually absent in Minorca but crop out extensively in Majorca, ii) Contractional structures in Minorca differ in direction (aligned SW-NE in Majorca and Ibiza and N-S in Minorca) and in age from those in Majorca and Ibiza. In addition, Paleozoic deposits of Minorca do not correlate with those of Sardinia, where in addition the Triassic sediments are not very abundant. Contractional deformation in Sardinia is in part older (late Eocene-early Miocene) than in Minorca (early Miocene?). Given its Neogene clockwise rotation, Minorca cannot be considered a small block dragged by the early Miocene counter clockwise rotation of the Corsica-Sardinia block. Furthermore, the Paleozoic and Mesozoic stratigraphy of Minorca (siliciclasticlate Paleozoic rocks, Triassic Germanic facies and Jurassic carbonates) has affinities with that of the southern part of the Catalan Coastal Ranges. Thus, of all the Balearic islands, Minorca seems to have traveled the farthest during the Valencia Trough rifting with the result that it resembles an exotic island forming part of the Balearic foreland

Volcanic Stratigraphy of Hannah Point, Livingston Island, South Shetland Islands, Antarctica

The Upper Cretaceous volcanic succession of Hannah Point is the best exposure of the Antarctic Peninsula Volcanic Group on Livingston Island. The aim of the present paper is to contribute to the characterisation of the stratigraphy and petrography of this little studied succession, and briefly discuss some aspects of the eruptive style of its volcanism. The succession is about 470 m thick and is here subdivided into five lithostratigraphic units (A to E from base to top). Unit A, approximately 120 m thick, is mainly composed of polymict clast-supported volcaniclastic breccias and also includes a dacitic lava laye r. Interstratified in the breccias of this unit, there is a thin laminated devitrified layer which shows some degree of welding. Unit B, approximately 70 m thick, is almost entirely composed of volcaniclastic breccias, and includes a volcaniclastic conglomerate layer. Breccias in this unit can be subdivided into two distinct types; polymict clast-supported breccias, and monomict matrix-supported breccias rich in juvenile components and displaying incipient welding. Unit C, about 65 m thick, is mainly composed of basaltic lavas, which are interlayered with minor volcaniclastic breccias. Unit D, approximately 65 m thick, is lithologically similar to unit B, composed of an alternation of polymict clasts upported breccias and matrix-supported breccias, and includes a volcaniclastic conglomerate layer. Unit E, about 150 m thick, is mainly formed of thick andesitic lava layers. Minor basaltic dykes and a few normal faults cut the succession, and the contact between units A and B can be interpreted both as an unconformity or a fault. The matrix-supported breccias included in the succession of Hannah Point have high contents of juvenile components and incipient welding, which suggest that part of the succession is the result of pyroclastic fragmentation and emplacement from pyroclastic flows. In contrast, the polymict clast-supported breccias suggest reworking of previous deposits and deposition from cool mass flows. The lavas indicate effusive volcanic eruptions, and the absence of features indicative of subaqueous volcanism suggests that at least these portions of the succession were emplaced in a subaerial environment

Mesozoic geology of Cape Shirreff, Livingston Island, South Shetland Islands, Antarctica

The stratigraphic succession at Cape Shirr e ff has a minimum thickness of 450 m and is mainly composed of lavas and a smaller amount of volcaniclastic breccias. Lavas are subalkaline olivine basalts and basaltic andesites which are locally well - bedded, but in most areas give a homogeneous, sometimes massive aspect to outcrops. Volcaniclastic massive breccias are found in the northern part of Cape Shirreff where they are interstratified with lavas in a few outcrops. Breccias are indurated, heterometric, grain - supported and consist of angular to subangular volcanic rock fragments, 5 to 80 cm in diameter. Petrographic evidence of magma supercooling (skeletal microlites of plagioclase, thermal - shocked phenocrysts of olivine) enables a subaqueous environment for part of the lavas to be deposited and suggests a possible hyaloclastic origin for some interstratified volcanic breccias. The whole succession is cut by subve rtical dykes (10 to 40 cm thick and a few meters to 400 m long), which show variable trends around a NW-SE maximum. These consist of commonly vesiculated basaltic and basaltic andesite porp hyritic rocks, commonly indistinguishable from the lava bodies. Lavas dip variably between 15 and 80º, more gently in the south than in the central and northern areas. Although significant variations in dipping attitude could be associated with depositional geometries, the general dipping pattern is clearly consistent with NW-SE oriented large-scale folds showing a gentle dome and basin structure. In the southern area, the NW-SE folds are not so well developed and the structure seems to be consistent with an interference of N-S and E-W oriented folds. The large scale NW-SE folds are consistent with NE-SW shortening. This shortening direction is parallel to the direction of extension deducible from dykes and, hence, folds and dykes are not consistent with the same deformational event

The Geology of Chile

6 páginas.-- Book review of "The Geology of Chile", by Teresa Moreno and Wes Gibbons (eds.) (2007). Geological Society. London (United Kingdom). 414 pages, 286 figures including maps, charts and pictures; 27, 5 x 21 cm, ISBN 978-1- 86239-219-9 (hardback) and ISBN 978-1-86239-220-5 (softback).Peer reviewe

Variscan and Alpine structure of the hills of Barcelona: geology in an urban area

Line 9 of the underground railway is currently being constructed in Barcelona. This undertaking necessitates tunnelling through a number of hills that are mainly made up of Paleozoic rocks, which exhibit a complex structure due to the superposition of Variscan, Mesozoic, Paleogene, and Neogene structures. We present a geological map of the hills of Barcelona originally compiled at 1:5000 scale. Unpublished field notes from surveys carried out in the 1940s and in the early 1970s were crucial for drawing up this detailed map, which together with subsurface data from public works and our study of the few remaining outcrops, enabled us to provide fresh insights into the structure of this area. We also discuss the age of the structures on the basis of cross-cutting relationships and regional considerations. Our conclusions highlight the ongoing need for a geological survey of cities given that our understanding of their geology depends on impermanent outcrops and on the recovery of lost subsurface data. These considerations call for a suitable management of the geological information in urban areas with a complex geology for planning and developing safe infrastructures

Book review

Teresa MORENO and Wes GIBBONS (eds.), The Geology of Chile. London: Geological Society, 2007

Tsunami hazards in the Catalan Coast, a low-intensity seismic activity area

The final publication is available at Springer via http://dx.doi.org/10.1007/s11069-017-2918-zThe potential impacts of tsunamis along the Catalan Coast (NW Mediterranean) are analysed using numerical modelling. The region is characterized by moderate to low seismic activity and by moderate- to low-magnitude earthquakes. However, the occurrence of historical strong earthquakes and the location of several active offshore faults in front of the coast suggest that the possibility of an earthquake-triggered tsunami is not negligible although of low probability. Up to five faults have been identified to generate tsunamis, being the highest associated possible seismic magnitudes of up to 7.6. Coastal flooding and port agitation are characterized using the Worst-case Credible Tsunami Scenario Analysis approach. The results show a multiple fault source contribution to tsunami hazard. The shelf dimensions and the existence of submerged canyons control the tsunami propagation. In wide shelves, waves travelling offshore may become trapped by refraction causing the wave energy to reach the coastline at some distance from the origin. The free surface water elevation increases at the head of the canyons due to the sharp depth gradients. The effects of potential tsunamis would be very harmful in low-lying coastal stretches, such as deltas, with a high population concentration, assets and infrastructures. The Ebro delta appears to be the most exposed coast, and about the 20% of the delta surface is prone to flooding due to its extremely low-lying nature. The activity at Barcelona port will be severely affected by inflow backflow current at the entrance of up to 2 m/s.Peer ReviewedPostprint (author's final draft

Constraints on the Neogene Mediterranean kinematic evolution along a 1000 km transect from Iberia to Africa

This paper is part of the special publication No.156, The Mediterranean basins: Tertiary extension within the Alpine Orogen. (eds B. Durand, L. Jolivet, F. Horvath and M. Seranne). We present in this paper a simple kinematic model to illustrate the evolution of the western Mediterranean since upper Oligocene times. The five-step mass balanced reconstructions of the region are constructed along a 1000 km long crustal-scale transect from Iberia to Africa across the Valencia and Algerian basins. The model uses constant divergence and convergence rates and fits the geological and geophysical data well along the transect. The amount of convergence and divergence along the studied transect can be resolved by the combination of two tectonic mechanisms: the northern motion of the African plate and the southwards retreat of a subducting Tethyan oceanic slab located between the pre-Neogene Iberian and African margins. The evolution of the western Mediterranean region during the last 30 Ma can be summarized as follows: (a) pre-Neogene (late Oligocene) Iberian and African margins separated by a north-dipping fragment of Tethyan oceanic crust; (b) widespread extension affecting the Iberian margin developing the Valencia trough and the Algerian basin from upper Oligocene to the top of Langhian; (c) end of the formation of the Valencia trough and the collision between the Kabylies and Africa domains at c. 13 Ma; (d) end of the Algerian basin opening and northern Kabylies and Tellian thrust systems coupled tectonic domains in the upper Tortonian (8 Ma); (e) shortening within the whole domain by the northwards motion of Africa after late Tortonian times. The propagation of stress within the western Mediterranean region seems to affect the whole region as far north as the northern side of the Pyrenean range located a distance of 1200 km from the south Atlas front, the southern limit of the system