7 research outputs found
Natural and anthropogenic radionuclides in rocks and beach sands from Ezine region (Canakkale), Western Anatolia, Turkey
This paper represents the first reports on the natural and anthropogenic radionuclides in Kestanbol granitic pluton and surrounding rocks, and coastal region of the Ezine town. To assess the radiological hazard of the natural radioactivity, the radium equivalent activity, the absorbed dose rate and the external hazard index were calculated, and in situ gamma dose rates were measured. The high-activity concentrations were measured in the pluton and sands, which was originated mainly from the pluton, due to the presence of zircon, allanite, monazite, thorite, uranothorite and apatite. The average activity concentrations of U-238, Th-232 and K-40 are 174.78, 204.69 and 1171.95 Bq kg(-1) for pluton, and 290.36, 532.04 and 1160.75 Bq kg(-1) for sands, respectively. Cs-137 in Ezine region ranged from 0-6.57 Bq kg(-1). The average absorbed dose rate for the granitic and sand samples were calculated to be 251.6 and 527.92 nGy h(-1), respectively. The maximum contribution to the total absorbed gamma dose rate in air was due to the Th-232 (52.3 % for pl uton and 67.1 % for sands). The Raeq activities of the pluton and sands are higher than the recommended maximum value of 370 Bq kg(-1) criterion limit of Raeq activity for building materials. (C) 2006 Elsevier Ltd. All rights reserved
A Preliminary Framework for Magmatism in Modern Continental BackâArc Basins and Its Application to the TriassicâJurassic Tectonic Evolution of the Caucasus
Abstract Extension within a continental backâarc basin initiates within continental rather than oceanic lithosphere, and the geochemical characteristics of magmatic rocks within continental backâarcs are poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five exemplar modern continental backâarc basinsâthe Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatoliaâto establish a geochemical framework for continental backâarc magmatism. This analysis shows that continental backâarcs yield geochemical signatures more similar to arc magmatism than intraoceanic backâarcs do. We apply this framework to published data for TriassicâJurassic magmatic rocks from the Caucasus arc system, which includes a relict continental backâarc, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains debated. Our analysis of 40Ar/39Ar and UâPb ages indicates PermianâTriassic arc magmatism from âŒ260 to 220 Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (âŒ220â210 Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc and opening of the Caucasus Basin. This activity was followed by Jurassic arc magmatism in the Lesser Caucasus from âŒ180 to 140 Ma and backâarc spreading in the Caucasus Basin from âŒ180 to 160 Ma. Trace element and SrâNd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental backâarcs and that the source to the Lesser Caucasus arc became more enriched at âŒ160 Ma, likely from the cessation of backâarc spreading
Neotethyan closure history of western Anatolia: a geodynamic discussion
International audienceThis paper addresses the lithosphere-scale subductionâcollision history of the eastern termination of the Aegean retreating subduction system, i.e. western Anatolia. Although there is some general consensus on the protracted subduction evolution of the Aegean since the early Cenozoic at least, correlation with western Anatolia has been widely debated for more than several decades. In western Anatolia, three main tectonic configurations have been envisaged in the past years to reconstruct slab dynamics during the closure of the Neotethyan oceanic realm since the Late Cretaceous. Some authors have suggested an Aegean-type scenario, with the continuous subduction of a single lithospheric slab, punctuated by episodic slab roll-back and trench retreat, whereas others assumed a discontinuous subduction history marked by intermittent slab break-off during either the Campanian (ca. 75 Ma) or the Early Eocene (ca. 55â50 Ma). The third view implies three partly contemporaneous subduction zones. Our review of these models points to key debated aspects that can be re-evaluated in the light of multidisciplinary constraints from the literature. Our discussion leads us to address the timing of subduction initiation, the existence of hypothetical ocean basins, the number of intervening subduction zones between the Taurides and the Pontides, the palaeogeographic origin of tectonic units and the possibility for slab break-off during either the Campanian or the Early Eocene. Thence, we put forward a favoured tectonic scenario featuring two successive phases of subduction of a single lithospheric slab and episodic accretion of two continental domains separated by a continental trough, representing the eastern end of the Cycladic Ocean of the Aegean. The lack of univocal evidence for slab break-off in western Anatolia and southward-younging HP/LT metamorphism in continental tectonic units (from ~85, 70 to 50 Ma) in the Late CretaceousâPalaeogene period suggests continuous subduction since ~110 Ma, marked by roll-back episodes in the Palaeocene and the Oligo-Miocene, and slab tearing below western Anatolia during the Miocene