Date

2015-10-09

Author

Aydın, Harun
Hılton, Davıd Raymond
Mutlu, Halim
Güleç, Nilgün Türkan

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Chemical and isotopic compositions of fluids (liquid or gas) that discharge from Earth’s surface vary over time due to several dynamic processes such as volcanic and seismic activities and water-rock interaction arising from the heterogeneous structure of the crust. In addition to hydrochemical and environmental isotopes, helium isotope ratios with well defined end-members (mantle, crust and atmosphere) greatly contribute to the understanding of such processes. The Çaldıran fault which was formed as a result of active tectonism in eastern Anatolia comprises one of the major discontinuity planes in the region. The 1966 Varto (Mw: 6.7), 1976 Çaldıran (Mw: 7.3) earthquakes and, most recently, 2011 Van earthquake (Mw: 7.2) and aftershocks are the latest manifestations of tectonic unrest in eastern Anatolia. In addition to tectonism, recent volcanism also played an important role in the development of a lowenthalpy geothermal potential in eastern Anatolia (Çaldıran, Erciş, Diyadin, Çukur and Tutak geothermal fields). Results of helium isotope studies on previously sampled fluids (1988-2009 period) and approximately one month after the 2011 Van earthquake have given us an opportunity to investigate hydrogeological structures of the region. 84 For this purpose, helium isotope compositions measured at 7 geothermal sites in eastern Anatolia and estimated mantle-He contributions are evaluated on the basis of seismic activities (Mw: > 3.5) during the 1988-2011 period. R/RA ratios of the samples are 1.62-4.02 in 1988, 0.93-6.36 in 2009 and 0.84-6.37 in 2011 (Table 1) which are noticeably greater than the crustal value (0.05 RA). Fluids with higher R/RA values are generally found around historically active volcanoes (e.g. Nemrut crater) or main tectonic lines (e.g. Çaldıran fault). Based on a three end-member mixing model, mantle-He contributions from Nemrut crater, Çaldıran fault zone and Diyadin geothermal area are 45.5-80.9%, 5.8-53.3% and 1.5-23.2%, respectively. Although He isotope compositions and mantle-He contributions of springs in the vicinity of Nemrut crater did not change during periods of seismic unrest, those of Çaldıran and Diyadin geothermal areas varied significantly. This anomaly is closely related to seismic activity that occurred during the 1983-2011 period. Changes in He-isotope systematics and hydrogeological character of the region after the 2011 Van earthquake were recorded in an area at a distance of about 60 km to the epicenter. This finding is also supported by previous geophysical modeling works for the region. Movement of the northerly block during the earthquake (thrust fault) has caused permeability and porosity to decrease in areas to the south of epicenter due to compression and to increase in areas at the north due to extension (Figure 1). Throughout the region, dynamic processes have resulted in opening of present fracture-crack systems and the development of new ones which have facilitated the release of mantle and crustal volatiles to the surface.

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https://hdl.handle.net/11511/74695
https://cdniys.tarimorman.gov.tr/api/File/GetFile/425/KonuIcerik/745/1093/DosyaGaleri/4-ulusal-hidrolojide-izotop-teknikleri-sempozyumu-kitabi.pdf

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Department of Geological Engineering, Conference / Seminar