New gravity and magnetic anomaly maps of Turkey

The General Directorate of the Mining Research and Exploration Company of Turkey (MTA) recently completed gravity and aeromagnetic surveying over the whole of Turkey. The gridded data (see below) have been released to the University of Ankara, but the original data remain confidential and the property of MTA. We present the information in the form of contour maps which reveal the potential field anomalies of the entire country. There was an active geomagnetic observatory in Istanbul from the late 1880s; geophysical investigations in the form of local surveys for prospecting have been undertaken in Turkey since the 1940s, e.g. Yungul (1956). The earliest large-scale geophysical investigations of Turkey were the aeromagnetic surveys for metalliferous minerals described by Hutchison et al. (1962). Ozelci (1973) investigated gravity anomalies in the eastern Mediterranean Sea and Turkey. Small-scale gravity and magnetic anomaly maps based on limited data sets have been presented by Akdogan (1995) and Aydin & Karat (1995). MTA has made 62000 gravity observations in Turkey at station intervals of 2–5km. The locations and elevations of the gravity stations were obtained by the Geodesy Department of MTA from 1:25000 scale topographic maps. Gravity values were tied to MTA and General Command of Mapping base stations related to the Potsdam 981260.00mGal (1Gal = 1cms−2) absolute gravity value accepted by the International Union of Geodesy and Geophysics in 1971. The gravity values are thus related to the International Gravity Standardization Network (Erden 1979). The following corrections were applied by the MTA to the observed gravity values: (1) latitude corrections using the Gravity Formula of 1967; (2) Bouguer corrections assuming a density of 2.40Mgm−3; (3) topographic corrections completed to a distance of 167km assuming a terrain density of 2.40Mgm−3; (4) free-air and tidal corrections. It is estimated that the error in the gravity anomalies is of the order of 5 per cent. The data were provided by MTA in the form of a 10km grid, the original data remaining confidential. The Bouguer gravity anomalies supplied were contoured at 10mGal intervals (Fig. 1). A version of this map, which incorrectly uses a Bouguer correction density of 2.67Mgm−3 in conjunction with a terrain correction density of 2.40Mgm−3, is available at the URL http://www.mta.gov.tr/tymdm/harita/bouger2.gif. Bouguer gravity anomaly map of Turkey. ontour interval=10mGal. Aeromagnetic data were collected along flight lines spaced at 1–5km intervals at an elevation of 600m above ground level, which was controlled by radar altimetry. The data were provided by MTA in the form of a 10km grid. We subsequently removed the International Geomagnetic Reference Field (IGRF) for the 1982.5 epoch from the aeromagnetic anomalies using geomagnetic field coefficients contained in a computer program developed by the International Union of Geodesy and Geophysics Association of Geomagnetism and Aeronomy (Baldwin & Langel 1993). Digital topographic height data, supplied by MTA in the form of a 10km grid, were used for the IGRF computation. The resulting aeromagnetic anomalies were contoured at 200nT intervals (Fig. 2). A version of this map is available at the URL http://www.mta.gov.tr/tymdm/harita/manyetik.jpg. Although entitled ‘Total Magnetic Intensity’, this appears to be a magnetic anomaly map, but the nature of the regional field removed from the observations is not stated. Aeromagnetic anomaly map of Turkey. Flight height=600m above ground surface. Contour interval=200nT. The gravity anomaly map of Turkey (Fig. 1) in general shows broadly E–W-trending contours. Most of the land area is represented by negative anomalies, reflecting the isostatically thickened continental crust which increases in thickness towards the east. There are strong gradients along the northern and southern coastlines, with gravity anomalies increasing over the quasi-oceanic crust of the Black and Aegean seas. The western Aegean coastline is characterized by a less steep, coast-parallel gradient, indicating a more gently sloping Moho. E–W-trending anomalies in the SW of Turkey (28.4°E, 37.3°N) correlate with graben in the West Anatolian Extensional Province (Ates et al. 1996). Surprisingly, the graben of the Sea of Marmara region in the NW of the country does not appear to be reflected in the gravity anomalies. To the east of the West Anatolian Extensional Province an extensive residual gravity anomaly closure centred on (32.0°E, 38.5°N) correlates with the largely sedimentary units of the western Anatolides. The Kirsehir Block in the centre of Turkey (33.9°E, 39.0°N) appears to correlate approximately with a zone of positive residual anomalies, reflecting its cratonic character. A complex suite of positive and subsidiary negative residual anomalies in NE Turkey correlates with the largely Tertiary volcanic rocks of the East Anatolian Contractional Province and NE Anatolian Block, reflecting the high density of these rocks and indicating that they have significant vertical extent. The aeromagnetic anomaly map (Fig. 2) shows that the country is divided into a southern region of subdued magnetic anomalies, where sources are either weakly magnetic or at considerable depth, and a northern region of high-amplitude, high-wavenumber anomalies. The sinuous boundary between these two regions approximately follows the northern margin of the Anatolides. The intense anomalies in NE Turkey, which are aligned E–W, correlate with the extensive Tertiary and Mesozoic volcanic and ultramafic rocks of this region. The intense anomalies in NW Turkey may owe their origin to the metamorphic basement rocks found in this region, although there might also be a contribution from ultramafic lithologies. The Kirsehir Block is a centre of relatively high-amplitude anomalies. A very striking feature of the magnetic anomaly map is the linear anomaly oriented NW–SE along the SW margin of Tuz Lake (33.7°E, 38.4°N). Surface lithologies appear to be insufficiently magnetic to cause a feature of such magnitude. It approximately coincides with a weak gravity anomaly gradient (Fig. 1), indicating that it may reflect the edge of a major crustal block, although the coincidence is not exact. This anomaly runs parallel to a right-lateral strike-slip fault coinciding with the NE shore of Tuz Lake, but there is insufficient information to suggest a causative link between these features. AA gratefully acknowledges the support of TUBITAK, the Turkish Scientific Research Council, who funded a visit to the University of Bristol under the European Scientific Exchange Program during which the maps were prepared. We thank the Geophysical Surveying Department of the General Directorate of Mining Research and Exploration Company of Turkey for the provision of gridded potential field and topographic data and Aydin Buyuksarac for assistance with the preparation of the maps.