Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?

Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He, 40Ar/39Ar hornblende, and U-Pb zircon ages from the granites of Tinos Island in the Aegean Sea, Greece, suggest, together with published ZFT data, that there are three extensional detachments on Tinos. The Tinos granites crosscut the Tinos detachment. Cooling of the granites was controlled by the Livadi detachment, which occurs structurally above the Tinos detachment. Our U-Pb zircon age is 14.6 ± 0.2 Ma and two 40Ar/39Ar hornblende ages are 14.4 ± 0.4 and 13.7 ± 0.4 Ma. ZFT and AFT ages go from 14.4 ± 1.2 to 12.2 ± 1.0 Ma and 12.8 ± 2.4 to 11.9 ± 2.0 Ma. (U-Th)/He ages are from 10.4 ± 0.2 to 9.9 ± 0.2 Ma (zircon) and 11.9 ± 0.5 to 10.0 ± 0.3 Ma (apatite). All ages decrease northeastward in the direction of hanging wall transport on the Livadi detachment and age-distance relationships yield a slip rate of 2.6 (+3.3 / −1.0) km Ma−1. This rate is smaller than a published slip rate of 6.5 km Ma−1 for the Vari detachment, which is another detachment structurally above the Tinos detachment. Because of the different rates and because published ZFT ages from the footwall of the Vari detachment are ∼10 Ma, we propose that the Vari detachment has to be distinguished from the older Livadi detachment. We discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the Vari detachment cut down into the footwall of the Livadi detachment successively exhuming deeper structural units. The thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation

Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: New constraints from Palaeogene sedimentary units and low-temperature thermochronometry

Mountain building in the Al-Hajar Mountains (NE Oman) occurred during two major shortening stages, related to the convergence between Africa-Arabia and Eurasia, separated by nearly 30 Ma of tectonic quiescence. Most of the shortening was accommodated during the Late Cretaceous, when northward subduction of the Neo-Tethys Ocean was followed by the ophiolites obduction on top of the former Mesozoic margin. This shortening event lasted until the latest Santonian - early Campanian. Maastrichtian to Eocene carbonates unconformably overlie the eroded nappes and seal the Cretaceous foredeep. These neo-autochthonous post-nappe sedimentary rocks were deformed, along with the underlying Cretaceous tectonic pile, during the second shortening event, itself including two main exhumation stages. In this study we combine remotely sensed structural data, seismic interpretation, field-based structural investigations and apatite (U-Th)/He (AHe) cooling ages to obtain new insights into the Cenozoic deformation stage. Seismic interpretation indicates the occurrence of a late Eocene flexural basin, later deformed by an Oligocene thrusting event, during which the post-nappe succession and the underlying Cretaceous nappes of the internal foredeep were uplifted. This stage was followed by folding of the post-nappe succession during the Miocene. AHe data from detrital siliciclastic deposits in the frontal area of the mountain chain provide cooling ages spanning from 17.3 to 42 Ma, consistent with available data for the structural culminations of Oman. Our work points out how renewal of flexural subsidence in the foredeep and uplift of the mountain belt were coeval processes, followed by layer-parallel shortening preceding final fold amplification

Middle Miocene vertebrates from the Amazonian Madre de Dios Subandean Zone, Perú

A new middle Miocene vertebrate fauna from Peruvian Amazonia is described. It yields the marsupials Sipalocyon sp. (Hathliacynidae) and Marmosa (Micoureus) cf. laventica (Didelphidae), as well as an unidentified glyptodontine xenarthran and the rodents Guiomys sp. (Caviidae), “Scleromys” sp., cf. quadrangulatus-schurmanni-colombianus (Dinomyidae), an unidentified acaremyid, and cf. Microsteiromys sp. (Erethizontidae). Apatite Fission Track provides a detrital age (17.1 +- 2.4 Ma) for the locality, slightly older than its inferred biochronological age (Colloncuran-early Laventan South American Land Mammal Ages: ~15.6e13.0 Ma). Put together, both the mammalian assemblage and lithology of the fossil-bearing level point to a mixture of tropical rainforest environment and more open habitats under a monsoonal-like tropical climate. The fully fluvial origin of the concerned sedimentary sequence suggests that the Amazonian Madre de Dios Subandean Zone was not part of the Pebas mega-wetland System by middle Miocene times. This new assemblage seems to reveal a previously undocumented “spatiotemporal transition” between the late early Miocene assemblages from high latitudes (Patagonia and Southern Chile) and the late middle Miocene faunas of low latitudes (Colombia, Perú, Venezuela, and ?Brazil)

Constraining the long-term evolution of the slip rate for a major extensional fault system in the central Aegean, Greece, using thermochronology

The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 to 8.2 ± 1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4 ± 0.4 to 9.2 ± 0.3 Ma and 10.7 ± 1.0 to 8.9 ± 0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1 ± 1.4 Ma to 11.1 ± 1.0 Ma and 12.7 ± 2.8 Ma to 10.5 ± 2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3 ± 0.4 Ma and 9.8 ± 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of 6–8 km Myr− 1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of 50 km between 16 and 8 Ma

Intracontinental subduction: a possible mechanism for the Early Palaeozoic Orogen of SE China

International audienceThe Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite-granite domes, high-temperature metamorphism (M2) and granitic plutons related to post-orogenic crustal melting. We date here the syn-M1 ductile shearing at 453 +/- 7 Ma by U-Th/Pb method on monazite. Previous ages and our ne

Constraining the tectonic evolution of extensional fault systems in the Cyclades (Greece) using low-temperature thermochronology

The Miocene Hellenic subduction zone in the Aegean retreated with time to the south implying that accreted high-pressure rocks shifted from a fore-arc position to a back-arc position. The Cycladic islands in the central Aegean became part of the magmatic arc in the Late Miocene and are now in a back-arc position. They are famous for their blueschists and extensional detachments. It is widely assumed that exhumation of the blueschist unit from depths of 50-60 km was accomplished by detachment faulting in a back-arc setting. On Crete, the exhumation of the Miocene high-pressure rocks was achieved by normal faulting in a fore-arc position. This raises the question as to whether exhumation of the Cycladic blueschists was mostly accomplished when these rocks were still in a fore-arc setting. To answer this question we need to constrain the timing of detachments and arc related magmatism, define how many detachments exist, determine their slip rates and estimate the amount of displacement. Only with this type of data it will be possible to assess the extent to which these faults contributed to the exhumation of the blueschists. Using consistent apatite and zircon fission-track and apatite (U-Th)/He ages from sample transects parallel to the tectonic transport direction of major extensional detachments on eight Cycladic islands (Samos, Ikaria, Tinos, Mykonos, Naxos, Paros, Serifos and Ios), I have been able to estimate the timing, rate and extent of slip movement for each detachment. The time constraints on shear zones (ductile part of the extensional fault systems) indicate that the Selçuk shearing on Samos started to operate >21 Ma followed by the Tinos and Naxos/Paros shear zones at ~21-20 Ma while at about 15-10 Ma, when the granites intruded, a number of detachments started to operate (Kerketas on Samos, Messaria/Kallithea on Ikaria/Samos, Mykonos, Serifos and Ios) or remained active (Tinos and Naxos/Paros extensional fault system). This intimate relationship between arc-related magmatism and extensional detachments (especially for the Messaria/Kallithea detachments on Ikaria/Samos, as well as Mykonos, Serifos, and Ios detachments) was aided by relatively high thermal gradients and extensional stresses caused by an extensional boundary condition related to the subduction-zone retreat. This induced rapid cooling of the footwalls at ~75-25°C/Myr and fast slip rates of ~8-7 km/Myr. No specific pattern of the extension timing and/or slip rate have been recognized in the Aegean, linked to location of the islands within the arc. This study has also shown that the Naxos extensional fault system is unique in the Aegean. The Naxos detachment exhibits a slightly faster minimum slip rate and cooling rate at ~9-8 km/Myr and ~108°C/Myr related to the higher temperature condition during the formation of the Naxos extensional fault system. Furthermore, the slip rate increases across the brittle/ductile transition from ~6 km/Myr to ~9-8 km/Myr owing to the intrusion of a huge granodiorite close to the brittle fault zone. On Ikaria, the slip rate on the Messaria extensional fault system is constant at ~8 km/Myr across the brittle/ductile transition because the Ikaria granodiorite intrusion was synchronous with onset of shearing. Generally, it is assumed that the extensional shear zones rooted at the brittle/ductile transition while the Naxos extensional shear was rooted in the lower crust. The data also show that the detachments accomplished a minimum offset from ~53 km (Ikaria) to ~12 km (Tinos) related to rock exhumation of less than 10 km on Tinos, Ikaria and Mykonos. Therefore, Miocene normal faulting in the Aegean did not cause much exhumation of the blueschists. However, the fast-slipping Miocene normal faults were the primary agents for opening of the Aegean Sea. Although there are differences between the studied detachments with regard to timing, slip rate and depth of formation, there are important similarities among the Cycladic detachments especially for the intra-arc period between ~15-5 Ma.La zone de subduction Hellénique dans l'Egée, est un des meilleurs exemples au monde de retrait d'une zone de subduction. En raison de ce retrait vers le sud durant le Miocène, les roches de haute pression sont accrétées successivement en position d'avant arc vers une position d'arrière arc. Actuellement en position d'arrière arc, les îles Cycladiques, dans le centre de l'Egée, faisaient partie de l'arc volcanique au Miocène supérieur. Elles sont surtout célèbres pour leurs schistes bleus ainsi que leurs failles de détachement. Il est communément admis que l'exhumation des schistes bleus depuis des profondeurs de l'ordre de 60-50 km a été principalement accomplie par des failles de détachement. Cependant, en Crète, il a été démontré que l'exhumation des roches Miocène de haute pression a été accommodée par le jeu normal de grandes failles quand ces roches étaient en position d'avant arc. La question se pose donc à savoir si l'exhumation des schistes bleus Cycladiques fut ou non principalement accomplie quand les roches étaient encore en position d'avant arc. Pour répondre à cette question, il est indispensable de déterminer: 1) à quel moment ces détachements étaient actifs ainsi que le volcanisme d'arc associé; 2) quelles étaient les vitesses de glissement afin d'estimer le déplacement relatif de chacun de ces détachements; 3) leur contribution dans l'exhumation des schistes bleus. En utilisant les âges cohérents obtenus par les méthodes traces de fission sur apatite et zircon et (U-Th)/He sur apatite sur des échantillons prélevés selon des profils parallèles à la direction de transport tectonique des principaux détachements de huit îles Cycladiques (Samos, Ikaria, Tinos, Mykonos, Naxos, Paros, Serifos et Ios), j'ai pu estimer la période d'activité, la vitesse de glissement et la quantité de déplacement relatif à chaque détachement étudié. Les contraintes de temps apportées sur les zones de cisaillement indiquent que le cisaillement ductile de Selçuk sur Samos était le premier actif avant 21 Ma. Vers ~21-20 Ma, les zones de cisaillement de Tinos et de Naxos/Paros se sont développées tandis qu'entre 15 Ma et 10 Ma, quand la plupart des granites intrudent l'unité des schistes bleus Cycladiques, la majorité des détachements exposés commencent à fonctionner (les détachements de Kerketas sur Samos, de Messaria/Kallithea sur Ikaria/Samos, de Mykonos, de Serifos et de Ios) ou restent actifs (systèmes de failles extensives de Tinos et Naxos/Paros qui deviennent actives dans le cassant). Cette étroite relation des évènements entre magmatisme d'arc et détachements extensifs (spécialement pour les détachements Messaria/Kallithea de Ikaria/Samos, de Mykonos, de Serifos et de Ios) a été favorisé par l'existence de forts gradients thermiques et des contraintes extensives provoquées par le retrait de la zone de subduction. Les données thermochronologiques indiquent un refroidissement rapide des murs de faille compris entre ~75°C/Ma et ~25°C/Ma et des vitesses de glissement élevées voisines de 8-7 km/Ma. Aucune organisation particulière des âges des détachements et des vitesses associées n'a été reconnue selon la répartition spatiale des îles dans l'arc égéen. Cette étude a également mis en évidence que le système de faille extensive exposé sur Naxos est unique dans l'arc Egéen. En effet, le détachement de Naxos présente des vitesses minimum de glissement et de refroidissement légèrement supérieures à ~9-8 km/Ma et ~108°C/Ma, corrélées à des conditions de température élevée pendant la formation du système de faille. La vitesse de glissement semble augmenter au passage de la transition ductile/cassante de ~6 km/Ma à ~9-8 km/Ma. L'intrusion d'une granodiorite massive au voisinage de la zone de faille de Naxos, postérieurement à la formation de la zone de cisaillement ductile augmenterait la vitesse de glissement. Par contre sur Ikaria, la vitesse de glissement sur le système de faille extensive Messaria est constante du ductile au cassant parce que l'intrusion de la granodiorite semble être synchrone de la formation de la zone ductile de cisaillement. De plus, contrairement aux zones de cisaillement des autres îles qui s'enracinent aux environs de la transition ductile/cassante, la zone de cisaillement de Naxos s'enracinerait plutôt dans la croûte inférieure. Nos données montrent également que les détachements accomplissent des déplacements minimum de l'ordre de ~53 km sur Ikaria à 12 km sur Tinos, impliquant une exhumation des schistes bleus d'une profondeur inférieure à 10 km. Par conséquent, les failles normales Miocène des îles Cycladiques ne sont pas responsables d'une exhumation importante des schistes bleus. Ces failles normales à fortes vitesses de glissement ont accommodé l'ouverture de la mer Egée. Finalement, bien que des différences existent dans les âges, les vitesses ou bien la profondeur d'enracinement de ces systèmes de faille extensives, d'importantes similarités apparaissent pour la période où les Cyclades étaient en position d'intra arc entre 15 Ma et 5 Ma

UCL

Logic-based programming languages are increasingly applied as program query languages which allow developers to reason about the structure and behaviour of programs. To achieve this, the queried programs are reified as logic values such that logic quantification and unification can be used effectively. However, in many cases, standard logic unification is inappropriate for program entities, forcing developers to resort to overly complex queries. In this paper, we argue that such incidental complexity can be reduced significantly by customizing the unification algorithm. We present a practical implementation approach through inter-language reflection and open unification. These techniques are at the core of the logic program query language SOUL, through which we demonstrate custom unification schemes for reasoning over Smalltalk and Java programs. Queries written in this tailored version of SOUL can exploit advanced program matching strategies without increasing the incidental complexity of the queries. 1