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Neogene anticlockwise rotation of central Anatolia (Turkey): preliminary palaeomagnetic and geochronological results
ELSEVIER
Tectonophysics 299 (1998) 175–189
Neogene anticlockwise rotation of central Anatolia (Turkey): preliminary palaeomagnetic and geochronological results E.S. Platzman a,Ł , C. Tapirdamaz b , M. Sanver b b
a Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK ¨ niversitesi, Maden Faku¨ltesi, Jeofizik Mu¨hendisligi Bo¨lu¨mu¨, Istanbul, Turkey Istanbul Teknik U
Received 19 October 1997; accepted 19 March 1998
Abstract A palaeomagnetic and geochronological study was carried out on fifty sites from the Neogene volcanic province of central Anatolia. Results from this analysis show that there has been a progressive anticlockwise rotation of this region over the period spanning at least the last 10–12 Ma. We interpret this rotation as having resulted from the collision of Arabia with Anatolia along the Bitlis suture. If we assume a constant rate of rotation we obtain a rate of 2.4º=Ma. This value is significantly higher than the present-day rotation rate obtained from GPS (1:3 š 0:1º=Ma) and also implies that these rocks have been rotating at their present rate since well before collision occurred along the Bitlis suture (¾12 Ma): a geologically unreasonable conclusion. If, however, we combine these data with data from other palaeomagnetic studies with good age control the resultant curve seems to show three distinct linear segments: (1) 0–5 Ma with a very slow rate of rotation (1.2º=Ma); (2) 5–12 Ma where we observe a rapid increase of rotation rate to 6.5º=Ma; and (3) >12 Ma, when the rotation rate is again essentially zero ( 0.041º=Ma). These results are generally consistent with a model calling for Middle Miocene collision along the Bitlis suture and Late Plio–Pleistocene initiation of motion along the North and East Anatolian fault zones. 1998 Elsevier Science B.V. All rights reserved. Keywords: Anatolia; Neogene; palaeomagnetism; geochronology; rotation
1. Introduction The Cenozoic tectonic development of the Eastern Mediterranean, as an integral part of the Alpine Himalayan collisional belt, has been dominated by the continued convergence of the African and Arabian plates with Eurasia (Fig. 1). In this context, present-day Anatolia is an amalgamation of small Ł Corresponding
author. Present address: Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK. Fax: C44 (171) 388-7614; E-mail: [email protected]
continental fragments that separated from the northern margin of Gondwana in the late Palaeozoic to early Mesozoic. The details of the developmental history of Anatolia are controversial (Sengor and Yilmaz, 1981; Robertson and Dixon, 1985; Dercourt et al., 1993; Robertson et al., 1996), but it is generally believed that these fragments drifted northward and opened new ocean basins in their wake which in turn began to close in the Late Cretaceous. Collision and amalgamation of the continental fragments began in the Eocene and probably achieved its present configuration in the Middle Miocene, with the collision of Arabia along the Bitlis suture. Convergence
0040-1951/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 2 0 4 - 2
176 E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189 Fig. 1. Simplified map of Central Anatolia showing the Tertiary volcanics and the location of the area studied. NAF D the North Anatolian fault zone; EAF D the East Anatolian fault zone; DST D the Dead Sea transform. Insert shows the regional plate tectonic setting. The arrows indicate the direction of motion relative to Eurasia.
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189
and thickening across Anatolia, however, continues today as Arabia moves northward with respect to Anatolia along the Dead Sea transform. Recent palaeomagnetic work throughout Anatolia has suggested that Anatolia may have rotated anticlockwise with respect to the North Pole (Platzman et al., 1994; Tartar et al., 1996; Gu¨rso¨y et al., 1997). Recent SLR and Global Positioning System (GPS) (Oral et al., 1992; Reilinger et al., 1997) studies have confirmed that the Anatolian plate is, at present, moving both westward and anticlockwise with respect to Eurasia. How far we can extrapolate this regime into the geologic past, how much total rotation has taken place and how this rotation has been accommodated is still a matter of debate. The aim of this study is to present new palaeomagnetic and K–Ar whole-rock ages from the Neogene volcanics of Central Anatolia and to use these preliminary results to place constraints on the timing and rate of the vertical axis rotations throughout Central Anatolia.
2. Geologic framework Central Anatolia, forms the western end of a high plateau (average elevation 1.5 km) that extends from Central Turkey through Armenia into NE Iran. This elevation reflects a thickened continental crust resulting from the collision of the Arabian and Anatolian plates. Continued plate convergence from the Miocene until the present resulted in an initial thickening of the continental crust throughout Anatolia followed by westward expulsion of what has come to be known as the Anatolian plate. At present the Anatolian plate is characterised by a region of relatively low seismicity west of the Karliova triple junction bounded by two major transcurrent faults: the dextral North Anatolian fault (NAF) to the north and the sinistral East Anatolian fault (EAF) to the south. Motion along these two boundary faults is presently accommodating westward expulsion of the Anatolian plate. The North Anatolian fault is a narrow, well-defined zone of dextral shear that extends E–W for about 1200 km along most of the southern margin of the Black Sea. Along this boundary Anatolia moves westward with respect to Eurasia at a rate of about 30 km=Ma (Jackson and McKenzie,
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1984; Barka and Kandinsky-Cade, 1988; Westaway, 1994; Reilinger et al., 1997). This zone is seismically active; between 1939 and 1961, the fault zone broke sequentially from west to east in a series of magnitude 6–7 earthquakes (Ketin, 1948; Ambraseys, 1970), and the last major earthquake occurred in March 1992 at Erzincan. Focal mechanism solutions for these earthquakes show that motion across this plate boundary, occurs predominantly by right-lateral slip on faults trending parallel to the main trace of the North Anatolian fault zone. The age of initiation of motion and the amount of cumulative offset along this fault zone are still controversial (Sengor, 1979). Estimated ages for the initiation of motion along this boundary range from Early Miocene to Pliocene (Sengor and Canitez, 1982), whereas estimates of offset range from 25 km (Saroglu, 1988) to 400 km (Pavoni, 1961) although most now agree on a figure of approximately 100 km. The left-lateral EAF has a more complicated morphology and pattern of seismicity. It is represented by a series of thrust and strike-slip faults extending westwards from the intersection with the NAF at the Karliova triple junction to its join with the Dead Sea transform (Fig. 1). Motion on this boundary is believed to be contemporaneous with the NAF but slip rates are significantly slower. Recent estimates of slip rates based on historic earthquakes, geologic observations or plate kinematic reconstructions vary from 4 to 30 km=Ma. GPS work points to a presentday slip rate of 15 š 3 mm=year (Reilinger et al., 1997), but this assumes that plate motion is accommodated on a single-fault trace and no allowance is made for anelastic deformation of the plates. At present the differential slip rates of the two bounding fault zones (NAF and EAF) almost certainly accommodate most of the anticlockwise rotation.
3. Central Anatolian Neogene volcanic province Neogene volcanism extends across the Central Anatolian plateau in a broad belt roughly following the inner border of the Taurus mountain chain (Fig. 1). These volcanics are typically of calc-alkaline composition and lie unconformably on pre-Neogene rocks varying from Palaeozoic metamorphic rocks to Eocene limestones. They are closely asso-
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ciated with a complex system of Miocene to recent extension and strike-slip faults. The origin of the Neogene volcanics has traditionally been attributed to upper plate extension that has occurred above a subduction zone between Africa and Eurasia (Innocenti et al., 1975; Batum, 1978). Early geochemical studies from this region (Innocenti et al., 1975) have noted, however, that while some of the calc-alkaline volcanics are undoubtedly typical subduction related, much of the early volcanism is characterised by high potassic calc-alkaline volcanics. This type of volcanism has much in common with the volcanics further to the east (Notsu et al., 1995; Pearce et al., 1990) in the collision zone and has been shown to be characteristic of lithospheric sources and may be associated with a process which removes the lower lithosphere. The youngest Quaternary volcanics in this region are characterised by more basic compositions and may well reflect the later stage extension and adiabatic melting. Previous isotopic work in Central Anatolia has shown that the volcanic rocks that crop out in this region range in age from Late Miocene (10:1 š 1:6 Ma) to Quaternary (Innocenti et al., 1975; Besang et al., 1977; Batum, 1978; Ercan et al., 1990; Notsu et al., 1995). The eruption of volcanic centres on the periphery of Mount Erciyes was recorded by Strabo in ancient times. These rocks therefore present an ideal opportunity for studying the timing and rate of anticlockwise rotation of the Anatolian block. Samples for this study have been obtained from three geographically distinct volcanic fields in central Anatolia: southwest of Konya, south of the cities of Aksaray and Kayseri (Cappadocia), and southeast of the city of Sivas. The least information is available from the region to the southwest of Konya. It has been discussed from a petrological point of view by Jung and Keller (1972), Innocenti et al. (1975), and Keller et al. (1977) who confirm that it is composed of Neogene calc-alkaline volcanics. Most of these authors have grouped these volcanics together with the region further east, south of the cities of Aksaray and Kayseri. The area south of the cities of Aksaray and Kayseri has been studied in great detail from a petrologic, geochemical and a structural perspective. Pasquare et al. (1988) divided its Neogene develop-
mental history into three separate stages, but more recent studies by Go¨ncu¨oglu¨ and Toprak (1992) have suggested that the volcanic edifices of this region probably result from one continuous evolutionary process. Both these studies agree, however, that early andesitic volcanism passes upwards into a thick ignimbrite sequence. The ignimbrites in this region cover an area of approximately 11,000 km2 and have been found at a distance of 100 km from their presumed source. These early sequences are often interbedded with lacustrine sediments and palaeosols. They form a volcaniclastic platform upon which a number of more recent volcanic complexes have developed. It is common for the volcanics to occur at the intersection of two fault sets which are prominent throughout the region (Toprak and Go¨ncu¨oglu¨, 1993), suggesting a close relationship between faulting and volcanism. The first system is the conjugate strike-slip Tuzgo¨lu¨–Ecemis fault system. The dextral Tuz Go¨lu¨ fault trending NW–SE while the sinistral Ecemis fault trends ENE–WSW (Dirik and Go¨ncu¨oglu, 1996). These faults and their related strands have probably been active since the latest Mesozoic–earliest Tertiary (Uygun, 1981; Yetis, 1984), although they are in places buried beneath very recent flows. The second set of faults are normal faults striking N60–70E that seem to have been active in the Mio–Pliocene but are buried under more recent deposits (Toprak and Go¨ncu¨oglu¨, 1993). The easternmost area sampled is the region surrounding the Sivas basin. Two phases of volcanic activity, Paleocene and Miocene have been previously documented (Go¨kten, 1983; Su¨mengen et al., 1990). This region lies wedged in the gap between the North Anatolian fault to the north and the East Anatolian fault to the south which are converging eastwards to the Karliova triple junction. The structural history of the Sivas area is complex and reflects the changing dynamics of the Anatolian plate (Cater et al., 1991). Final closure of the ocean basin which occupied this region occurred in pre-Eocene times, and is marked by ophiolite obduction (Poisson et al., 1996; Temiz, 1996; Guezou et al., 1996). Continued closure resulted in the formation of a narrow basin that became emergent in the Late Eocene as evidenced by the deposition of continental clastic and evaporitic sequences.
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The majority of the volcanics sampled for this study did not come from the Sivas basin proper but from the Kizilirmak basin to the north of the Sivas basin and the Kangal basin to the south of the Sivas basin (Guezou et al., 1996). The Kizilirmak basin, as defined by Guezou et al. (1996) separates the Sivas basin from the Kirsehir basement to the north. This basin consists of nearly undeformed Middle Miocene sandstone and conglomerates and Paleocene lacustrine limestones. The apparently flat-lying volcanics in this region are reputedly Early Miocene (Guezou et al., 1996), although no radiometric ages have been published. Further south, the Kangal basin lies above the thrust that places Taurus Limestone on top of Miocene continental deposits (Guezou et al., 1996). This basin forms a high plateau dominated by flat-lying continental deposits which are succeeded by what have been assumed to be Quaternary age basalts. In general, the Neogene volcanics of Central Anatolia are rarely interbedded with sedimentary deposits but more commonly lie either conformably or unconformably on top of earlier deposits. This, in conjunction with the inherent problem of original dips in volcanic rocks, sometimes made it difficult to determine the true tectonic dip of the beds and hence is a potential source of error in the results that we have obtained. Measurement and evaluation of the palaeomagnetic results was done at the palaeomagnetic laboratory at Oxford and K=Ar analysis was done at the NERC Isotope Facility at Keyworth.
4. Methods Samples for palaeomagnetic analysis were collected using a portable drill. Eight to sixteen cores were collected per site from as many volcanic flows as were available. The basaltic–andesitic lava flows that crop out in this region are generally 2–4 m thick, so in practice it was not possible to sample more than three flows at any given locality. The cores were oriented using a Brunton compass and an orienting platform which includes a sun compass for magnetically strong samples. Standard cylindrical cores with a diameter of 2.5 cm and a length of 10 cm were later drilled and cut into 2.25-cm-long samples in the laboratory.
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In the laboratory the samples were analysed to isolate the stable characteristic component of remanent magnetisation (ChRM). Stepwise thermal and alternating field (AF) demagnetisation procedures were used. During the thermal demagnetisation procedure each sample was demagnetised with a minimum of ten steps based on a scheme designed after a detailed pilot demagnetisation study. Natural remanent magnetisation (NRM) measurements were made on either a two-axis cryogenic magnetometer or a spinner magnetometer. Data obtained from the demagnetisation experiments were plotted on Zijderveld vector diagrams. Palaeomagnetic directions could then be determined from these plots by using linear regression techniques. On a set of representative samples, bulk susceptibility was measured between each thermal demagnetisation step to monitor possible changes in magnetic mineralogy that can occur as a result of heating and dehydration. K–Ar analyses were carried out on whole-rock samples from 31 of the palaeomagnetic sites. Potassium was analysed in duplicate by mixed-acid digestion followed by flame photometry using a known lithium standard. The quoted percent error is the difference between the mean and any individual result. Argon was analysed by an isotope dilution method. Samples were loaded into a high-vacuum extraction line and baked at 180º overnight to maximise the vacuum and to minimise the possible contamination from absorbed atmospheric gasses. Argon was then extracted by fusion, under vacuum, using an RF induction coil and then analysed in a VG-Micromass 1200 mass spectrometer. The errors quoted on the argon analysis were calculated by combining the errors on the isotope ratio measurements, an assumed error of 1% on the ‘spike’ calibration, and any magnification error incurred as a consequence of a correction for atmospheric argon contamination. Both the value of the Ar spike and the K are regularly calibrated against international standard and are maintained at an accuracy within the quoted precision limits. Additional samples from the same site taken from different pieces of core were run as often as possible to assess the repeatability of the analyses. Ages were then calculated using the decay constants recommended by Steiger and Jager (1977) and errors are quoted at the 95% confidence level.
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5. Results 5.1. Geochronology (Table 1) K–Ar whole-rock age determinations were obtained from 31 sites and the results are presented in Table 1. At six sites more than one sample was analysed to check the reproducibility of the results. Five sites showed excellent agreement between duplicate samples: the results for the two samples agreed
within the limits of analytical precision indicating that these dates are accurate and likely to represent a real isotopic event. In site, KA7 however, the date did not agree within the analytical precision. This could be due to sample inhomogeneity because both the K and Ar measurements are made on different samples and so may not have been representative of the entire sample. However, duplicate K measurements on separate aliquots of the sample indicated that the percent potassium was relatively consis-
Table 1 K–Ar whole-rock ages Sample
A1-1 A5-5 A7-3 A7-8 A9-8 A10-4 A11-7 A12-11 K2-3 K2-4 K3-8 K4-5 K6-6 K7-7 K7-4 K10-7 KA2-3 KA2-5 KA2-11 KA4-3 KA5-4 KA6-6 KA7-1 KA7-6 N3-1 N6-1 N6-2 S2-1 S3-7 S3-3 S4-2 S5-1 S5-2 S7-3 S9a-7 S10-5 S11-5
K (%) 1.152 š 1.0 1.909 š 1.0 1.840 š 1.0 1.924 š 1.0 2.390 š 1.0 2.709 š 1.0 1.560 š 1.0 0.490 š 1.0 2.000 š 1.0 2.025 š 1.0 2.108 š 1.0 2.322 š 1.0 1.864 š 1.0 1.584 š 1.0 1.589 š 1.0 1.670 š 1.0 0.578 š 1.0 0.550 š 1.0 0.569 š 1.0 0.887 š 1.0 1.449 š 1.0 0.393 š 1.0 1.450 š 1.0 1.418 š 1.0 0.927 š 1.0 1.592 š 1.0 1.510 š 1.0 1.000 š 1.0 0.810 š 1.0 0.806 š 1.0 1.698 š 1.0 1.294 š 1.0 1.330 š 1.0 1.967 š 1 0.580 š 1.0 0.515 š 1 0.566 š 1.0
Atmospheric Ar nl
%
0.71 1.46 0.48 0.92 0.50 0.48 1.37 0.93 0.84 0.31 0.79 0.83 0.70 0.43 2.8 0.38 0.77 1.36 2.10 9.36 1.87 0.73 0.94 1.11 0.31 15.96 12.52 1.43 6.95 0.53 1.15 1.66 0.26 1.23 1.90 1.12 0.55
97.81 94.25 73.44 86.49 52.21 30.14 98.98 95.29 59.45 41.28 66.16 68.76 68.54 80.55 90.06 47.15 77.8 96.81 92.76 93.39 91.40 92.42 60.16 64.38 94.58 95.26 95.90 86.17 89.80 47.87 76.83 93.15 52.66 43.96 85.11 80.49 53.56
Radiogenic Ar (nl=g š %)
Age š 2 s.d. (Ma)
Lat.=long.
0.0138 š 47.04 0.11 š 17.00 0.1751 š 3.06 0.2051 š 6.66 0.4118 š 1.57 0.4414 š 1.10 0.0115 š 112.4 0.0409 š 21.7 0.5369 š 1.0 0.5116 š 1.25 0.3968 š 2.2 0.4298 š 2.46 0.1298 š 2.44 0.1261 š 4.47 0.1352 š 9.42 0.390 š 1.79 0.5.6 š 3.71 0.0445 š 32.6 0.0479 š 13.27 0.1232 š 16.44 0.1309 š 10.95 0.0282 š 12.6 0.6048 š 1.85 0.4647 š 2.1 0.0029 š 19.55 0.6097 š 29.75 0.5325 š 32.9 0.2290 š 6.47 0.7609 š 9.51 0.4723 š 1.39 0.3453 š 3.5 0.2014 š 14.03 0.2272 š 1.6 1.406 š 1.28 0.3291 š 5.91 0.2636 š 4.34 0.3040 š 1.55
0.292 š 0.275 1.48 š 0.50 2.40 š 0.15 2.74 š 0.37 4.43 š 0.16 4.19 š 0.12 <0.2 2.15 š 0.93 6.69 š 0.281 6.49 š 0.31 4.68 š 0.23 4.76 š 0.27 1.79 š 0.09 2.05 š 0.19 2.19 š 0.41 5.98 š 0.25 2.25 š 0.17 2.08 š 1.35 2.16 š 0.58 3.57 š 1.17 2.32 š 0.51 1.84 š 0.47 10.7 š 0.4 8.41 š 0.39 0.08 š 0.03 9.83 š 5.83 9.05 š 5.96 5.88 š 0.77 24.0 š 4.6 15.0 š 0.5 5.27 š 0.38 4.0 š 1.12 4.39 š 0.17 18.3 š 0.6 14.5 š 1.7 13.1 š 1.2 13.8 š 0.5
38º14=34º01 38º13=34º26 38º05=34º25 38º05=34º25 38º03=34º39 38º03=34º40 38º21=34º38 38º26=34º35 37º39=31º58 37º39=31º58 37º48=33º45 37º47=33º44 37º22=33º11 37º22=33º11 37º22=33º11 37º49=33º53 38º21=35º29 38º21=35º29 38º21=35º29 38º25=35º45 38º29=35º42 38º34=35º12 38º32=35º05 38º32=35º05 38º44=34º36 38º38=34º57 38º38=34º57 39º10=37º05 39º11=36º53 39º11=36º53 38º52=36º51 38º45=37º04 38º45=37º04 39º01=37º42 39º16=37º58 39º25=36º12 39º26=36º18
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189 Fig. 2. Typical vector diagrams in geographic coordinates of samples undergoing thermal and AF demagnetisation [indicated as (AF)]. Horizontal projections are represented by closed squares and vertical projections are represented by open squares. Temperatures on the plots showing thermal demagnetisation are in ºC. AF demagnetisations were done routinely at: 0, 50, 100, 150, 200, 250, 300, 400, 500, 600, 800 and 950 mT. 181
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Table 2 Remanent magnetization parameters Central Anatolia Site
N
Aksaray volcanics A1 5 A4 5 A1-4 10 A6 5 A5 4 A7 7 A8 6 A5,7-8 18 A9 6 A10 6 A9-10 12 A11 8 A12 8 A11-12 16 Kaiseri volcanics KA1 4 KA2 a 3 KA3 6 KA4 8 KA5 7 KA6 b 4 KA7 4 KA1-6 5 Konya volcanics K1 8 K2 4 K1-2 12 K3 4 K4 5 K3-4 9 K5 5 K9 5 K5,9 10 K6 5 K7 7 K6-7 12 K10 5 Nevsehir volcanics N1 5 N2 5 5 N3 N5 5 N1-5 4 N6 5 Sivas volcanics S2 S3 S4 S5
5 8 6 8
Lat.=long.
Flow
38º14=34º01 38º10=34º04
1 1 2 1 1 1 1 3 1 1 2 1 2 3
38º05=34º02 38º13=34º26 38º05=34º25 38º10=34º31 38º03=34º39 38º03=34º40 38º21=34º38 38º26=34º35
38º28=35º31 38º21=35º29 38º19=35º25 38º25=35º45 38º29=35º42 38º34=35º12 38º32=35º05
1 3 3 1 1 1 1 7
37º41=32º02 37º39=31º58
1 1 2 1 1 2 1 1 2 1 1 2 1
37º48=33º45 37º47=33º44 37º51=33º56 37º49=33º56 37º22=33º11 37º22=33º11 37º49=33º53 38º34=34º43 38º36=34º42 38º44=34º36 38º36=34º35 38º38=34º57
2 2 1 1 6 1
39º10=37º05 39º11=36º53 38º52=36º51 38º45=37º04
1 1 2 1
K–Ar age (Ma) 0.292 š 0.27
Dec.
Inc.
Þ 95
k
334 349 341 352 193 171 170 175 336 337 336 5 173 358
66 71 68 47 61 51 72 61 54 60 57 55 33 45
8.3 8.2 5.4 11.4 11.7 4.6 4.6 5.4 6.6 7.4 14 5.3 4.8 7.4
86.3 87.4 81.3 46.2 62.8 173.2 216 41.7 125 104 318.4 109 140 27.4
6 158 344 162 0 164 307 356
64 39 56 55 49 60 34 58
4.6 28.5 3.5 4.6 7.7 6.6 10.4 8.6
402 53 370 173 70 137 77.3 81
174 358 355 7 177 4 348 350 349 5 205 196 152
30 57 39 43 63 54 63 59 61 44 48 47 45
8.0 6.3 9.0 5.1 6.1 7.8 6.1 9.6 9.0 5.8 5.4 5.3 4.6
48.4 212 24.0 326 183 45 169 64.6 775 176 125 68.2 275
9.05 š 6.0
175 180 2 1 180 159
64 70 39 39 53 40
8.1 7.6 5.3 4.7 19.0 5.8
91 102 207 266 24.4 176
5.88 š 0.77 24.0 š 4.6 5.27 š 0.38 4.0 š 1.2
151 95 1 181
55 73 51 56
10 2.9 8.3 3
61 78 66 344
<1.6 1.48 š 0.5 2.57 š 1.5
4.43 š 1.6 4.19 š 0.12 <0.2 2.15 š 0.93
2.08 š 1.4 3.57 š 1.17 2.32 š 0.51 1.84 š 0.5 10.7 š 0.4
6.9 š 0.3 4.7 š 0.2 4.76 š 0.27
1.79 š 0.09 2.1 š 0.2 5.98 š 0.25
0.08 š 0.03
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189
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Table 2 (continued) Site
N
Lat.=long.
Flow
K–Ar age (Ma)
Dec.
S7 S8 S7-8 S9a S9b S9a-b S10 S11 S10-11
7 5 12 7 5 12 6 7 13
39º01=37º42 39º01=37º42
1 1 2 1 2 3 2 2 4
18.3 š 0.6
336 348 344 151 143 152 131 150 144
39º16=37º58 39º16=37º58 39º25=36º12 39º26=36º18
14.5 š 1.7
13.1 š 1.2 13.8 š 0.50
Inc. 17 34 28 56 65 59 68 31 48
Þ 95
k
6.0 6.3 7.5 3.2 14.3 6.5 4.3 6.5 11.5
103 146 29 367 29 35 244 86 14.1
Note: N D number of samples per locality; lat.=long. D latitude and longitude; flow D number of flows sampled; Þ95 D the 95% confidence interval; k D the precision parameter; dec. D mean declination; inc. D mean inclination. Shaded lines indicate mean values for adjoining sites of equivalent age. To calculate values, N was used when the number of sites was 2 and the site means were used when the number of sites was ½2. a Indicates excluded sites (see text). b Site was corrected for dip of strata (178, 12W).
tent between separate aliquots analysed on separate days. K–Ar results from the Neogene Volcanic province of Central Anatolia provide dates that range from <0.2 Ma to 24:0 š 4:6 Ma and are in general agreement with previously obtained dates. The precision on quite a number of the sites is low due to the fact that many sites register a high atmospheric contamination. This atmospheric Ar may have come from a number of sources: (1) absorption onto the surface of mineral grains during sample preparation; (2) atmospheric contamination in the extraction line either in the crucibles where the fusion took place and in the line itself; (3) from atmospheric argon incorporated into the volcanic rocks during crystallisation. It is, however, a problem which is common in very young rocks analysed in this way. It is perhaps the largest source of error in these dates. When the large atmospheric content is subtracted from the total Ar to get the radiogenic content the uncertainty in the Ar radiogenic becomes quite large and in some cases swamps the measurement. In a few cases the volume of radiogenic argon was below the level of resolution of the mass spectrometer and these samples have been eliminated. 5.2. Palaeomagnetic results (Table 2) Of the 50 sites drilled for palaeomagnetic analysis, 43 gave interpretable results. These results are presented in Table 2. The other seven sites were
either magnetically unstable or had been completely remagnetised by lightning strikes. Samples from at least 17 sites had been subjected to lightning strikes. These samples characteristically have a NRM with a very strong intensity which ranges in magnitude from 150 to 200 A=m. In many cases the random overprint imparted by the lightning could be either completely or partially removed by either thermal of AF demagnetisation. Samples could then be analysed using linear regression analysis or great circle analysis. Other samples were completely overprinted with no hint of an original consistent ChRM. As a result six sites proved completely unusable. The remaining sites yielded well defined, interpretable directions. The intensity of the natural remanent magnetisation (NRM) ranged from 0.90 to 200.0 A=m. Detailed thermal and AF experiments revealed that the NRM of these samples is composed of one, two, or three components of magnetisation. Fig. 2 shows examples of demagnetisation data from the various regions. In a vast majority of cases the low-temperature components of magnetisation are unblocked below 400ºC. In some samples, however, overlap in the unblocking temperature spectra of the lower- and higher-temperature components leads to a curved segment of the Zijderveld plots and necessitated analysis using great circles. If the secondary components could be successfully removed, the magnetisation then decays in a straight-line vector to the origin. In most cases the characteristic
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Fig. 3. Map of area studied showing site locations and palaeodeclinations of site means with 95% confidence interval. Reversed polarity site means have been converted to normal polarity. Sites are located at the tail end of the arrows. Many of the arrows represent an average of several sites as grouped in Table 2.
component of remanent magnetisation, probably carried by magnetite, is unblocked in the temperature range of 350º to 580ºC. At some sites there is indication of a higher ultimate unblocking temperature and a probable haematite contribution to the NRM. Site-mean direction show inclinations with both normal and reverse polarities. Antipodal were from sequential flows indicating magnetic stability. Sitemean declinations show an overall tendency to be rotated anticlockwise (Fig. 3). This rotation increases with the age of the flows (Fig. 4). There is, however, a fair amount of scatter in this data which is probably related to the combined effects of tectonics and secular variation. The majority of these sites have been taken from young lava or pyroclastic flows where flow tops could be observed and little or no dip is evident. In some instances, however, it was difficult to obtain a credible structural correction because it was impossible to separate the imposed tectonic dip from originally dipping flows with undulating flow tops. In addition to potential error related to uncorrected tectonic dips, each site-mean determination is based on one to three cooling units and hence may not average out secular variation. According to the most recent models of secular variation (McFadden et al.,
1991), secular variation for the Miocene should be approximately 20º and hence should lead to a 20º scatter about the mean line. Therefore, the deviation of the declination from the expected direction results from the combined effects of secular variation and tectonics and is not a direct measure of the vertical axis rotation. These are common problems with palaeomagnetic work in volcanic terrains and can only be overcome by sampling enough flows to average out these effects.
6. Discussion The palaeomagnetic results obtained in this study confirm that much of Anatolia has rotated anticlockwise with respect to the North Pole. This result is in agreement with recent GPS work that demonstrates that the anticlockwise rotation of Anatolia is apparent in both the Eurasian and the Arabian fixed reference frames (Reilinger et al., 1997). GPS, however, estimates present-day motion whereas palaeomagnetic determinations estimate finite motion over geologic time. The declinations obtained in this study and presented in Fig. 4 show a general tendency to be rotated
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Fig. 4. Observed rotations plotted as a function of age of sites from this study.
anticlockwise, which increases as a function of age. The amounts of rotation for these data have been calculated relative to the present-day dipole field because during the time interval represented the predicted declination and inclination do not diverge significantly from the present direction. The rotation rate obtained in this straightforward manner is 2.4º=Ma. It is apparent, however, that the one data point dated at 25 Ma is heavily biasing this result. If we remove this point assuming that it has not been adequately corrected for the dip of the strata, we obtain a rotation rate of 1.7º=Ma. This value is still significantly higher than the rotation rate of 1:3 š 0:1º=Ma obtained from GPS. In addition, it implies that these rocks have been rotating at the rate presently observed for the last 25 Ma: since well before the collision occurred along the Bitlis suture at approximately 12 Ma. Although this scenario cannot be disproved, a rotation rate that remains unchanged in the wake of a continental collision is counterintuitive. If we combine these data with data from other palaeomagnetic studies from central and northern Anatolia with good age control (Platzman et al., 1994; Krijgsman et al., 1996), an alternative scenario emerges. The data chosen are collected from locations which lie east of 31º where the character of the Anatolian block changes. West of this meridian Anatolia begins to interact with the Aegean extensional domain. This is evidenced by the changing character of the NAF as well as by the complex
pattern of vertical axis rotations (Kissel et al., 1987) and residual velocities obtained by GPS (Reilinger et al., 1997). In Fig. 5 the selected data have again been plotted against time. Early Tertiary sites (Eocene) are calculated relative to a predicted Eocene declination of 8º. The resultant curve appears to be composed of three distinct linear segments: 0–5 Ma, 5–12 Ma and >12 Ma. Other recently published palaeomagnetic data from central Anatolia (Tartar et al., 1996; Gu¨rso¨y et al., 1997) do not include radiometric ages and so cannot be plotted in this manner. However, the results obtained in these studies are generally consistent with the results obtained in this study. Between 0 and 5 Ma we observe a very slow rate of rotation (1.2º=Ma) comparable to that obtained by GPS (1:3 š 0:1º=Ma). It is evident, however, that there is a significant amount of scatter on these data. As discussed earlier secular variation models predict š20º of variation which encompass most of the observed scatter. Because the rate of rotation does not appear to change significantly until about 5 Ma it suggests that contemporary deformation probably reflects those geologic processes that have characterised this region for the past 4–5 Ma. It therefore follows that since 5 Ma the Anatolian plate has been decoupled from Eurasia along the NAF. This is in agreement with recent studies (Platzman et al., 1994; Reilinger et al., 1997), which suggest that the NAF is Pliocene in age and that fault slip-rates have not changed significantly since the fault was initiated. If
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Fig. 5. Plot of observed rotation as a function of age for sites from this study combined with dated localities from Platzman et al. (1994) and Krijgsman et al. (1996). Error of 2¦ are shown for each age and the 95% confidence interval is shown for rotation data.
Anatolia has been decoupled from Eurasia along the NAF for the last 5 Ma, there should be no rotation associated with this period of deformation north of the NAF. These palaeomagnetic data and GPS estimates of rotation rate predict that the amount of rotation in this period, from 5 Ma to present, would be approximately 7º. We would, therefore, predict an average difference of approximately 7º in the total amount of rotation across the NAF. This is a hypothesis that will be tested in forthcoming studies. If 7º of rotation has occurred since the Pliocene this leaves the majority of the rotation to have occurred before 5 Ma, prior to the initiation of motion along the NAF. This is consistent with the observation that anticlockwise rotations extend north of the present trace of the NAF to the Black Sea coast (Platzman et al., 1994). Between about 5 and 12 Ma we observe a rapid increase of rotation rate to 6.5º=Ma. This interval encompasses the time period between collision along the Bitlis suture and initiation and localisation of motion along the NAF and EAF. It is a time when deformation was probably more distributed or continuous and rotation was probably accommodated on relatively small blocks as crust spread down a potential energy gradient
away from the area of high potential energy in the continental collision zone to the area of low potential energy in the Aegean. Pre-12 Ma the rotation rate again decreases to essentially zero ( 0.041º=Ma). This is the pre-collisional era and as a result blocks were not rotating. Therefore all rocks older than 12 Ma should reflect the total cumulative rotation. It is in these rocks that we should observe the differential rotation north and south of the NAF. Unfortunately, there are at present not enough geochronological data to perform this test with any degree of statistical validity. This is the preferred interpretation of the data although we also investigated the possibility that rotations might vary with distance from the collision zone. In Fig. 6 rotation and radiometric age are plotted as a function of latitude and longitude. A best-fit line has been calculated for each data set and the resulting equation is displayed at the top of each plot. The plots show that, at the 5% level, there is significant correlation of rotation with latitude but there is a poor correlation of rotation with longitude. However, it is also evident from Fig. 6a that the age of the volcanic unit is varying in a like manner with latitude. Therefore, it seems more likely that rotation
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7. Conclusions Data from Central Anatolia support the conclusion that the entire Anatolian block has rotated anticlockwise. If we assume a constant rate of rotation we obtain a rate of 2.4º=Ma. However, more careful analysis of the data suggests that rate of rotation may have been only 1.2º=Ma until approximately 5 Ma ago when rotation rates increased to around 7º=Ma. The rotation rate then appears to decrease to almost zero in the pre-collisional era at approximately 12 Ma. These results are generally consistent with a model calling for Middle Miocene collision along the Bitlis suture and Late Plio–Pleistocene initiation of motion along the North and East Anatolian fault zones.
Acknowledgements Special thanks to C.C. Rundle, J.P. Platt, C. Sengo¨r, N. Go¨ru¨r, and O. Tuysuz without whose help this project would not have been possible. The authors would also like to thank A. Poisson and A.H.F. Robertson for their constructive reviews of this paper. This research was supported by a grant from the Natural Environmental Research Council GR3=7666.
Fig. 6. Plot of observed rotation and age as a function of latitude (a) and longitude (b).
is a function of age rather than distance from any particular geologic boundary although some effect cannot be ruled out. Finally, it is interesting to note that both this study and earlier work seems to indicate that we do not see anticlockwise rotations in the region west of the Tuz Go¨lu¨ fault and east of the landward continuation of the Cyprean Arc. This might be due either to a buttressing effect of the advancing thrust edge of the Taurus mountains or to a decoupling along the boundary of the ancient Kirsehir block as defined by the active strand of the left-lateral Tuz Golu fault. To clarify these trends far more geochronological and palaeomagnetic data need to be collected throughout central Anatolia.
References Ambraseys, N.N., 1970. Some characteristic features of the Anatolian fault zone. Tectonophysics 9, 143–165. Barka, A.A., Kandinsky-Cade, K., 1988. Strike-slip fault geometry in Turkey and its influence on earthquake activity. Tectonics 7, 663–684. Batum, I., 1978. Geology and petrology of Acigo¨l and Go¨llu¨dag volcanics at SW of Nevsehir (Central Anatolia, Turkey) . Yerbilimlerli 4, 17–20. Besang, C., Eckhardt, F.J., Harre, W., Kreuzer, H., Mu¨ller, P., 1977. Radiometriche Alterbestimmungen an neogene Eruptivgesteinen der Turkei. Geol. Jahrb. B25, 3–36. Cater, J.M.L., Hanna, S.S., Ries, A.C., Turner, P., 1991. Tertiary evolution of the Sivas Basin, central Turkey. Tectonophysics 195, 29–46. Dercourt, J., Ricou, L.E., Vrielynk, B., 1993. Atlas Tethys Palaeoenviromental Maps. Beicip-Franlab. Dirik, K., Go¨ncu¨oglu, M.C., 1996. Neotectonic characteristics of Central Anatolia. Int. Geol. Rev. 38, 807–817. Ercan, T., Fujitani, T., Matsuda, J.I., Tokel, S., Notsu, K., Ui,
188
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189
¨ lmez, M., T., Can, B., Selvi, Y., Yildirim, T., Fisekci, A., O Akbasli, A., 1990. The origin and evolution of the Cenozoic volcanism of Hasandagi–Karacadag area (cental Anatolia). Geomorphol. Bull. 18, 39–54. Go¨kten, E., 1983. Stratigraphy and geologic evolution of southsoutheastern Sarkisla, Sivas. Bull. Geol. Soc. Turk. 26, 167– 176. Go¨ncu¨oglu¨, M.C., Toprak, V., 1992. Neogene and Quaternary volcanism of Central Anatolia: a volcano-structural evaluation. Bull. Soc. Volcanol. 26, 1–6. Guezou, J.-C., Temiz, H., Poisson, A., Gu¨rsoy, H., 1996. Tectonics of the Sivas Basin: the Neogene record of the Anatolian accretion along the inner Tauric suture. Int. Geol. Rev. 38, 901–925. Gu¨rso¨y, H., Piper, J.D.A., Tartar, O., Temiz, H., 1997. A palaeomagnetic study of the Sivas Basin, Central Turkey: crustal deformation during lateral extrusion of the Anatolian Block. Tectonophysics 271, 89–106. Innocenti, F., Mazzuoli, R., Pasquare, G., Radicati di Brozolo, F., Villari, L., 1975. The Neogene calcalkaline volcanism of Central Anatolia: geochronological data on Kayseri-Nigde area. Geol. Mag. 112, 349–360. Jackson, J., McKenzie, D.P., 1984. Active tectonics of the Alpine–Himalayan belt between western Turkey and Pakistan. Geophys. J. R. Astron. Soc. 77, 185–264. Jung, D., Keller, J., 1972. Die Jungen Vulkanite im Raum Zwischen Konya und Kayseri (Zentral Anatolien). Z. Dtsch. Geol. Ges. 123, 503–512. Keller, J., Jung, D., Burgath, K., Wolf, F., 1977. Geologie und Petrologie des Neogenen Kalkalcali-Vulkanismus von Konya (Erenler Dag-Alaca Dag Massif, Zentral Anatolien). Geol. Jahrb. 25, 37–117. Ketin, I., 1948. Uber die tektonisch–mechanischen Folgerungen aus den grossen anatolischen Erdbeben des letzten Dezenniums. Geol. Rundsch. 36, 77–83. Kissel, C., Laj, C., Sengor, A.M.C., Poisson, A., 1987. Paleomagnetic evidence for rotation in opposite senses of adjacent blocks in northeastern Aegea and western Anatolia. Geophys. Res. Lett. 14, 907–910. Krijgsman, W., Duermeijer, C., Langereis, C.G., De Bruijn, H., Sarac¸, G., Andriessen, P.A.M., 1996. Magnetic polarity stratigraphy of late Oligocene to middle Miocene mammal-bearing continental deposits in Central Anatolia (Turkey). Newsl. Stratigr. 34, 13–29. McFadden, P.L., Merril, R.T., McElhinny, M.W., Lee, S., 1991. Reversals of the Earth’s magnetic field and temporal variations of the dynamo families. J. Geophys. Res. 96, 3923–3933. Notsu, K., Fujitani, T., Ui, T., Matsuda, J., Ercan, T., 1995. Geochemical features of collision-related volcanic rocks in central and eastern Anatolia, Turkey. J. Volcanol. Geotherm. Res. 64, 171–192. Oral, M.B., Reilinger, R.E., Tokso¨z, M.N., 1992. Deformation of the Anatolian Block as deduced from GPS Measurements (abstract). Eos Trans. AGU 74, 120. Pasquare, G., Poli, S., Vezzoli, L., Zanchi, A., 1988. Continental arc volcanism and tectonic setting in Central Anatolia. Tectonophysics 146, 217–230.
Pavoni, N., 1961. Die nordanatolische Horizontalverschiebung. Geol. Rundsch. 51, 122–139. Pearce, J.A., Bender, J.F., De Long, S.E., Kidd, W.S.F., Low, P.J., Gu¨ner, Y., Saroglu, F., Yilmaz, Y., Moorbath, S., Mitchell, J.G., 1990. Genesis of collision volcanism in Eastern Anatolia, Turkey. J. Volcanol. Geotherm. Res. 44, 189–229. Platzman, E.S., Platt, J.P., Tapirdamaz, C., Sanver, M., Rundle, C.C., 1994. Why are there no clockwise rotations along the North Anatolian Fault Zone, Turkey. J. Geol. Res. 99, 21705– 21715. ¨ zturk, A., Inan, S., Temiz, H., Poisson, A., Guezou, J.-C., O ¨ zden, S., 1996. Tectonic setting Gu¨rso¨y, H., Kavak, K.S., O and Evolution of the Sivas Basin, Central Anatolia, Turkey. Int. Geol. Rev. 38, 838–853. Reilinger, R.E., McClusky, S.C., Oral, M.B., King, R.W., Toksoz, M.N., Barka, A.A., Kinik, I., Lenk, O., Sanli, I., 1997. Global Positioning System measurements of presentday crustal movements in the Arabia–Africa–Eurasia plate collision zone. J. Geophys. Res. 102, 9983–9999. Robertson, A.H.F., Dixon, J.E., 1985. Introduction: aspects of the geological evolution of the Eastern Mediterranean. In: Dixon, J.E., Robertson, A.H.F. (Eds.), The Geological Evolution of the Eastern Mediterranean, 17. Blackwell, Oxford, pp. 1–112. Robertson, A.H.F., Dixon, J.E., Brown, A., Collins, A., Morris, A., Pickett, E., Sharp, I., Ustao¨mer, T., 1996. Alternative tectonic models for the late Palaeozoic–early Tertiary development in the Eastern Mediterranean region. In: Morris, A., Tarling, D.H. (Eds.), Palaeomagnetism and Tectonics of the Mediterranean Region. Geol. Soc. London, Spec. Publ. 105, 239–263. Saroglu, F., 1988. Age and offset of the North Anatolian Fault. METU J. Pure Appl. Sci. 21, 65–79. Sengor, A.M.C., 1979. The North Anatolian transform fault: its age, offset and tectonic significance. J. Geol. Soc. London 136, 269–282. Sengor, A.M.C., Canitez, N., 1982. The North Anatolian Fault. In: Berckhemer, H., Hsu, K. (Eds.), Alpine–Mediterranean Geodynamics. American Geophysical Union, Washington, DC, pp. 205–216. Sengor, A.M.C., Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241. Steiger, R., Jager, E., 1977. Convention on use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359– 362. Su¨mengen, M., Unay, E., de Bruijn, H., Terlemez, I., Gu¨rbu¨z, M., 1990. New Neogene rodent assemblages from Anatolia (Turkey). In: Lindsey, E.H., Falbusch, V., Mein, P. (Eds.), European Neogene Mammal Chronology. Plenum Press, New York, pp. 61–72. Tartar, O., Piper, J.D.A., Gu¨rsoy, H., Temiz, H., 1996. Regional significance of neotectonic counterclockwise rotation in Central Anatolia. Int. Geol. Rev. 38, 692–700. Temiz, H., 1996. Tectonostratigraphy and thrust tectonics of the central and eastern parts of the Sivas Tertiary basin, Turkey. Int. Geol. Rev. 38, 957–971. Toprak, V., Go¨ncu¨oglu¨, M.C., 1993. Tectonic development of
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189 the Neogene–Quaternary Central Anatolian volcanic province, Turkey. Geol. J. 28, 357–369. Uygun, A., 1981. Geology, evaporite occurrences and hydrocarbon potential of Tuzgo¨lu¨ (Salt Lake) Basin. Central Anatolian Geology Symposium, Geological Society of Turkey Publications, Ankara, pp. 66–71. Westaway, R., 1994. Present-day kinematics of the Middle East
189
and eastern Mediterranean. J. Geophys. Res. 99, 12071– 12090. Yetis, C., 1984. New observations on the age of the Ecemis Fault. In: Tekeli, O., Go¨ncu¨oglu, M.C. (Eds.), Geology of the Taurus Belt. Mineral Research and Exploration Institute, Ankara, pp. 159–164.
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Neogene anticlockwise rotation of central Anatolia (Turkey): preliminary palaeomagnetic and geochronological results E.S. Platzman a,Ł , C. Tapirdamaz b , M. Sanver b b
a Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK ¨ niversitesi, Maden Faku¨ltesi, Jeofizik Mu¨hendisligi Bo¨lu¨mu¨, Istanbul, Turkey Istanbul Teknik U
Received 19 October 1997; accepted 19 March 1998
Abstract A palaeomagnetic and geochronological study was carried out on fifty sites from the Neogene volcanic province of central Anatolia. Results from this analysis show that there has been a progressive anticlockwise rotation of this region over the period spanning at least the last 10–12 Ma. We interpret this rotation as having resulted from the collision of Arabia with Anatolia along the Bitlis suture. If we assume a constant rate of rotation we obtain a rate of 2.4º=Ma. This value is significantly higher than the present-day rotation rate obtained from GPS (1:3 š 0:1º=Ma) and also implies that these rocks have been rotating at their present rate since well before collision occurred along the Bitlis suture (¾12 Ma): a geologically unreasonable conclusion. If, however, we combine these data with data from other palaeomagnetic studies with good age control the resultant curve seems to show three distinct linear segments: (1) 0–5 Ma with a very slow rate of rotation (1.2º=Ma); (2) 5–12 Ma where we observe a rapid increase of rotation rate to 6.5º=Ma; and (3) >12 Ma, when the rotation rate is again essentially zero ( 0.041º=Ma). These results are generally consistent with a model calling for Middle Miocene collision along the Bitlis suture and Late Plio–Pleistocene initiation of motion along the North and East Anatolian fault zones. 1998 Elsevier Science B.V. All rights reserved. Keywords: Anatolia; Neogene; palaeomagnetism; geochronology; rotation
1. Introduction The Cenozoic tectonic development of the Eastern Mediterranean, as an integral part of the Alpine Himalayan collisional belt, has been dominated by the continued convergence of the African and Arabian plates with Eurasia (Fig. 1). In this context, present-day Anatolia is an amalgamation of small Ł Corresponding
author. Present address: Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK. Fax: C44 (171) 388-7614; E-mail: [email protected]
continental fragments that separated from the northern margin of Gondwana in the late Palaeozoic to early Mesozoic. The details of the developmental history of Anatolia are controversial (Sengor and Yilmaz, 1981; Robertson and Dixon, 1985; Dercourt et al., 1993; Robertson et al., 1996), but it is generally believed that these fragments drifted northward and opened new ocean basins in their wake which in turn began to close in the Late Cretaceous. Collision and amalgamation of the continental fragments began in the Eocene and probably achieved its present configuration in the Middle Miocene, with the collision of Arabia along the Bitlis suture. Convergence
0040-1951/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 2 0 4 - 2
176 E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189 Fig. 1. Simplified map of Central Anatolia showing the Tertiary volcanics and the location of the area studied. NAF D the North Anatolian fault zone; EAF D the East Anatolian fault zone; DST D the Dead Sea transform. Insert shows the regional plate tectonic setting. The arrows indicate the direction of motion relative to Eurasia.
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and thickening across Anatolia, however, continues today as Arabia moves northward with respect to Anatolia along the Dead Sea transform. Recent palaeomagnetic work throughout Anatolia has suggested that Anatolia may have rotated anticlockwise with respect to the North Pole (Platzman et al., 1994; Tartar et al., 1996; Gu¨rso¨y et al., 1997). Recent SLR and Global Positioning System (GPS) (Oral et al., 1992; Reilinger et al., 1997) studies have confirmed that the Anatolian plate is, at present, moving both westward and anticlockwise with respect to Eurasia. How far we can extrapolate this regime into the geologic past, how much total rotation has taken place and how this rotation has been accommodated is still a matter of debate. The aim of this study is to present new palaeomagnetic and K–Ar whole-rock ages from the Neogene volcanics of Central Anatolia and to use these preliminary results to place constraints on the timing and rate of the vertical axis rotations throughout Central Anatolia.
2. Geologic framework Central Anatolia, forms the western end of a high plateau (average elevation 1.5 km) that extends from Central Turkey through Armenia into NE Iran. This elevation reflects a thickened continental crust resulting from the collision of the Arabian and Anatolian plates. Continued plate convergence from the Miocene until the present resulted in an initial thickening of the continental crust throughout Anatolia followed by westward expulsion of what has come to be known as the Anatolian plate. At present the Anatolian plate is characterised by a region of relatively low seismicity west of the Karliova triple junction bounded by two major transcurrent faults: the dextral North Anatolian fault (NAF) to the north and the sinistral East Anatolian fault (EAF) to the south. Motion along these two boundary faults is presently accommodating westward expulsion of the Anatolian plate. The North Anatolian fault is a narrow, well-defined zone of dextral shear that extends E–W for about 1200 km along most of the southern margin of the Black Sea. Along this boundary Anatolia moves westward with respect to Eurasia at a rate of about 30 km=Ma (Jackson and McKenzie,
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1984; Barka and Kandinsky-Cade, 1988; Westaway, 1994; Reilinger et al., 1997). This zone is seismically active; between 1939 and 1961, the fault zone broke sequentially from west to east in a series of magnitude 6–7 earthquakes (Ketin, 1948; Ambraseys, 1970), and the last major earthquake occurred in March 1992 at Erzincan. Focal mechanism solutions for these earthquakes show that motion across this plate boundary, occurs predominantly by right-lateral slip on faults trending parallel to the main trace of the North Anatolian fault zone. The age of initiation of motion and the amount of cumulative offset along this fault zone are still controversial (Sengor, 1979). Estimated ages for the initiation of motion along this boundary range from Early Miocene to Pliocene (Sengor and Canitez, 1982), whereas estimates of offset range from 25 km (Saroglu, 1988) to 400 km (Pavoni, 1961) although most now agree on a figure of approximately 100 km. The left-lateral EAF has a more complicated morphology and pattern of seismicity. It is represented by a series of thrust and strike-slip faults extending westwards from the intersection with the NAF at the Karliova triple junction to its join with the Dead Sea transform (Fig. 1). Motion on this boundary is believed to be contemporaneous with the NAF but slip rates are significantly slower. Recent estimates of slip rates based on historic earthquakes, geologic observations or plate kinematic reconstructions vary from 4 to 30 km=Ma. GPS work points to a presentday slip rate of 15 š 3 mm=year (Reilinger et al., 1997), but this assumes that plate motion is accommodated on a single-fault trace and no allowance is made for anelastic deformation of the plates. At present the differential slip rates of the two bounding fault zones (NAF and EAF) almost certainly accommodate most of the anticlockwise rotation.
3. Central Anatolian Neogene volcanic province Neogene volcanism extends across the Central Anatolian plateau in a broad belt roughly following the inner border of the Taurus mountain chain (Fig. 1). These volcanics are typically of calc-alkaline composition and lie unconformably on pre-Neogene rocks varying from Palaeozoic metamorphic rocks to Eocene limestones. They are closely asso-
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ciated with a complex system of Miocene to recent extension and strike-slip faults. The origin of the Neogene volcanics has traditionally been attributed to upper plate extension that has occurred above a subduction zone between Africa and Eurasia (Innocenti et al., 1975; Batum, 1978). Early geochemical studies from this region (Innocenti et al., 1975) have noted, however, that while some of the calc-alkaline volcanics are undoubtedly typical subduction related, much of the early volcanism is characterised by high potassic calc-alkaline volcanics. This type of volcanism has much in common with the volcanics further to the east (Notsu et al., 1995; Pearce et al., 1990) in the collision zone and has been shown to be characteristic of lithospheric sources and may be associated with a process which removes the lower lithosphere. The youngest Quaternary volcanics in this region are characterised by more basic compositions and may well reflect the later stage extension and adiabatic melting. Previous isotopic work in Central Anatolia has shown that the volcanic rocks that crop out in this region range in age from Late Miocene (10:1 š 1:6 Ma) to Quaternary (Innocenti et al., 1975; Besang et al., 1977; Batum, 1978; Ercan et al., 1990; Notsu et al., 1995). The eruption of volcanic centres on the periphery of Mount Erciyes was recorded by Strabo in ancient times. These rocks therefore present an ideal opportunity for studying the timing and rate of anticlockwise rotation of the Anatolian block. Samples for this study have been obtained from three geographically distinct volcanic fields in central Anatolia: southwest of Konya, south of the cities of Aksaray and Kayseri (Cappadocia), and southeast of the city of Sivas. The least information is available from the region to the southwest of Konya. It has been discussed from a petrological point of view by Jung and Keller (1972), Innocenti et al. (1975), and Keller et al. (1977) who confirm that it is composed of Neogene calc-alkaline volcanics. Most of these authors have grouped these volcanics together with the region further east, south of the cities of Aksaray and Kayseri. The area south of the cities of Aksaray and Kayseri has been studied in great detail from a petrologic, geochemical and a structural perspective. Pasquare et al. (1988) divided its Neogene develop-
mental history into three separate stages, but more recent studies by Go¨ncu¨oglu¨ and Toprak (1992) have suggested that the volcanic edifices of this region probably result from one continuous evolutionary process. Both these studies agree, however, that early andesitic volcanism passes upwards into a thick ignimbrite sequence. The ignimbrites in this region cover an area of approximately 11,000 km2 and have been found at a distance of 100 km from their presumed source. These early sequences are often interbedded with lacustrine sediments and palaeosols. They form a volcaniclastic platform upon which a number of more recent volcanic complexes have developed. It is common for the volcanics to occur at the intersection of two fault sets which are prominent throughout the region (Toprak and Go¨ncu¨oglu¨, 1993), suggesting a close relationship between faulting and volcanism. The first system is the conjugate strike-slip Tuzgo¨lu¨–Ecemis fault system. The dextral Tuz Go¨lu¨ fault trending NW–SE while the sinistral Ecemis fault trends ENE–WSW (Dirik and Go¨ncu¨oglu, 1996). These faults and their related strands have probably been active since the latest Mesozoic–earliest Tertiary (Uygun, 1981; Yetis, 1984), although they are in places buried beneath very recent flows. The second set of faults are normal faults striking N60–70E that seem to have been active in the Mio–Pliocene but are buried under more recent deposits (Toprak and Go¨ncu¨oglu¨, 1993). The easternmost area sampled is the region surrounding the Sivas basin. Two phases of volcanic activity, Paleocene and Miocene have been previously documented (Go¨kten, 1983; Su¨mengen et al., 1990). This region lies wedged in the gap between the North Anatolian fault to the north and the East Anatolian fault to the south which are converging eastwards to the Karliova triple junction. The structural history of the Sivas area is complex and reflects the changing dynamics of the Anatolian plate (Cater et al., 1991). Final closure of the ocean basin which occupied this region occurred in pre-Eocene times, and is marked by ophiolite obduction (Poisson et al., 1996; Temiz, 1996; Guezou et al., 1996). Continued closure resulted in the formation of a narrow basin that became emergent in the Late Eocene as evidenced by the deposition of continental clastic and evaporitic sequences.
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The majority of the volcanics sampled for this study did not come from the Sivas basin proper but from the Kizilirmak basin to the north of the Sivas basin and the Kangal basin to the south of the Sivas basin (Guezou et al., 1996). The Kizilirmak basin, as defined by Guezou et al. (1996) separates the Sivas basin from the Kirsehir basement to the north. This basin consists of nearly undeformed Middle Miocene sandstone and conglomerates and Paleocene lacustrine limestones. The apparently flat-lying volcanics in this region are reputedly Early Miocene (Guezou et al., 1996), although no radiometric ages have been published. Further south, the Kangal basin lies above the thrust that places Taurus Limestone on top of Miocene continental deposits (Guezou et al., 1996). This basin forms a high plateau dominated by flat-lying continental deposits which are succeeded by what have been assumed to be Quaternary age basalts. In general, the Neogene volcanics of Central Anatolia are rarely interbedded with sedimentary deposits but more commonly lie either conformably or unconformably on top of earlier deposits. This, in conjunction with the inherent problem of original dips in volcanic rocks, sometimes made it difficult to determine the true tectonic dip of the beds and hence is a potential source of error in the results that we have obtained. Measurement and evaluation of the palaeomagnetic results was done at the palaeomagnetic laboratory at Oxford and K=Ar analysis was done at the NERC Isotope Facility at Keyworth.
4. Methods Samples for palaeomagnetic analysis were collected using a portable drill. Eight to sixteen cores were collected per site from as many volcanic flows as were available. The basaltic–andesitic lava flows that crop out in this region are generally 2–4 m thick, so in practice it was not possible to sample more than three flows at any given locality. The cores were oriented using a Brunton compass and an orienting platform which includes a sun compass for magnetically strong samples. Standard cylindrical cores with a diameter of 2.5 cm and a length of 10 cm were later drilled and cut into 2.25-cm-long samples in the laboratory.
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In the laboratory the samples were analysed to isolate the stable characteristic component of remanent magnetisation (ChRM). Stepwise thermal and alternating field (AF) demagnetisation procedures were used. During the thermal demagnetisation procedure each sample was demagnetised with a minimum of ten steps based on a scheme designed after a detailed pilot demagnetisation study. Natural remanent magnetisation (NRM) measurements were made on either a two-axis cryogenic magnetometer or a spinner magnetometer. Data obtained from the demagnetisation experiments were plotted on Zijderveld vector diagrams. Palaeomagnetic directions could then be determined from these plots by using linear regression techniques. On a set of representative samples, bulk susceptibility was measured between each thermal demagnetisation step to monitor possible changes in magnetic mineralogy that can occur as a result of heating and dehydration. K–Ar analyses were carried out on whole-rock samples from 31 of the palaeomagnetic sites. Potassium was analysed in duplicate by mixed-acid digestion followed by flame photometry using a known lithium standard. The quoted percent error is the difference between the mean and any individual result. Argon was analysed by an isotope dilution method. Samples were loaded into a high-vacuum extraction line and baked at 180º overnight to maximise the vacuum and to minimise the possible contamination from absorbed atmospheric gasses. Argon was then extracted by fusion, under vacuum, using an RF induction coil and then analysed in a VG-Micromass 1200 mass spectrometer. The errors quoted on the argon analysis were calculated by combining the errors on the isotope ratio measurements, an assumed error of 1% on the ‘spike’ calibration, and any magnification error incurred as a consequence of a correction for atmospheric argon contamination. Both the value of the Ar spike and the K are regularly calibrated against international standard and are maintained at an accuracy within the quoted precision limits. Additional samples from the same site taken from different pieces of core were run as often as possible to assess the repeatability of the analyses. Ages were then calculated using the decay constants recommended by Steiger and Jager (1977) and errors are quoted at the 95% confidence level.
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5. Results 5.1. Geochronology (Table 1) K–Ar whole-rock age determinations were obtained from 31 sites and the results are presented in Table 1. At six sites more than one sample was analysed to check the reproducibility of the results. Five sites showed excellent agreement between duplicate samples: the results for the two samples agreed
within the limits of analytical precision indicating that these dates are accurate and likely to represent a real isotopic event. In site, KA7 however, the date did not agree within the analytical precision. This could be due to sample inhomogeneity because both the K and Ar measurements are made on different samples and so may not have been representative of the entire sample. However, duplicate K measurements on separate aliquots of the sample indicated that the percent potassium was relatively consis-
Table 1 K–Ar whole-rock ages Sample
A1-1 A5-5 A7-3 A7-8 A9-8 A10-4 A11-7 A12-11 K2-3 K2-4 K3-8 K4-5 K6-6 K7-7 K7-4 K10-7 KA2-3 KA2-5 KA2-11 KA4-3 KA5-4 KA6-6 KA7-1 KA7-6 N3-1 N6-1 N6-2 S2-1 S3-7 S3-3 S4-2 S5-1 S5-2 S7-3 S9a-7 S10-5 S11-5
K (%) 1.152 š 1.0 1.909 š 1.0 1.840 š 1.0 1.924 š 1.0 2.390 š 1.0 2.709 š 1.0 1.560 š 1.0 0.490 š 1.0 2.000 š 1.0 2.025 š 1.0 2.108 š 1.0 2.322 š 1.0 1.864 š 1.0 1.584 š 1.0 1.589 š 1.0 1.670 š 1.0 0.578 š 1.0 0.550 š 1.0 0.569 š 1.0 0.887 š 1.0 1.449 š 1.0 0.393 š 1.0 1.450 š 1.0 1.418 š 1.0 0.927 š 1.0 1.592 š 1.0 1.510 š 1.0 1.000 š 1.0 0.810 š 1.0 0.806 š 1.0 1.698 š 1.0 1.294 š 1.0 1.330 š 1.0 1.967 š 1 0.580 š 1.0 0.515 š 1 0.566 š 1.0
Atmospheric Ar nl
%
0.71 1.46 0.48 0.92 0.50 0.48 1.37 0.93 0.84 0.31 0.79 0.83 0.70 0.43 2.8 0.38 0.77 1.36 2.10 9.36 1.87 0.73 0.94 1.11 0.31 15.96 12.52 1.43 6.95 0.53 1.15 1.66 0.26 1.23 1.90 1.12 0.55
97.81 94.25 73.44 86.49 52.21 30.14 98.98 95.29 59.45 41.28 66.16 68.76 68.54 80.55 90.06 47.15 77.8 96.81 92.76 93.39 91.40 92.42 60.16 64.38 94.58 95.26 95.90 86.17 89.80 47.87 76.83 93.15 52.66 43.96 85.11 80.49 53.56
Radiogenic Ar (nl=g š %)
Age š 2 s.d. (Ma)
Lat.=long.
0.0138 š 47.04 0.11 š 17.00 0.1751 š 3.06 0.2051 š 6.66 0.4118 š 1.57 0.4414 š 1.10 0.0115 š 112.4 0.0409 š 21.7 0.5369 š 1.0 0.5116 š 1.25 0.3968 š 2.2 0.4298 š 2.46 0.1298 š 2.44 0.1261 š 4.47 0.1352 š 9.42 0.390 š 1.79 0.5.6 š 3.71 0.0445 š 32.6 0.0479 š 13.27 0.1232 š 16.44 0.1309 š 10.95 0.0282 š 12.6 0.6048 š 1.85 0.4647 š 2.1 0.0029 š 19.55 0.6097 š 29.75 0.5325 š 32.9 0.2290 š 6.47 0.7609 š 9.51 0.4723 š 1.39 0.3453 š 3.5 0.2014 š 14.03 0.2272 š 1.6 1.406 š 1.28 0.3291 š 5.91 0.2636 š 4.34 0.3040 š 1.55
0.292 š 0.275 1.48 š 0.50 2.40 š 0.15 2.74 š 0.37 4.43 š 0.16 4.19 š 0.12 <0.2 2.15 š 0.93 6.69 š 0.281 6.49 š 0.31 4.68 š 0.23 4.76 š 0.27 1.79 š 0.09 2.05 š 0.19 2.19 š 0.41 5.98 š 0.25 2.25 š 0.17 2.08 š 1.35 2.16 š 0.58 3.57 š 1.17 2.32 š 0.51 1.84 š 0.47 10.7 š 0.4 8.41 š 0.39 0.08 š 0.03 9.83 š 5.83 9.05 š 5.96 5.88 š 0.77 24.0 š 4.6 15.0 š 0.5 5.27 š 0.38 4.0 š 1.12 4.39 š 0.17 18.3 š 0.6 14.5 š 1.7 13.1 š 1.2 13.8 š 0.5
38º14=34º01 38º13=34º26 38º05=34º25 38º05=34º25 38º03=34º39 38º03=34º40 38º21=34º38 38º26=34º35 37º39=31º58 37º39=31º58 37º48=33º45 37º47=33º44 37º22=33º11 37º22=33º11 37º22=33º11 37º49=33º53 38º21=35º29 38º21=35º29 38º21=35º29 38º25=35º45 38º29=35º42 38º34=35º12 38º32=35º05 38º32=35º05 38º44=34º36 38º38=34º57 38º38=34º57 39º10=37º05 39º11=36º53 39º11=36º53 38º52=36º51 38º45=37º04 38º45=37º04 39º01=37º42 39º16=37º58 39º25=36º12 39º26=36º18
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189 Fig. 2. Typical vector diagrams in geographic coordinates of samples undergoing thermal and AF demagnetisation [indicated as (AF)]. Horizontal projections are represented by closed squares and vertical projections are represented by open squares. Temperatures on the plots showing thermal demagnetisation are in ºC. AF demagnetisations were done routinely at: 0, 50, 100, 150, 200, 250, 300, 400, 500, 600, 800 and 950 mT. 181
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Table 2 Remanent magnetization parameters Central Anatolia Site
N
Aksaray volcanics A1 5 A4 5 A1-4 10 A6 5 A5 4 A7 7 A8 6 A5,7-8 18 A9 6 A10 6 A9-10 12 A11 8 A12 8 A11-12 16 Kaiseri volcanics KA1 4 KA2 a 3 KA3 6 KA4 8 KA5 7 KA6 b 4 KA7 4 KA1-6 5 Konya volcanics K1 8 K2 4 K1-2 12 K3 4 K4 5 K3-4 9 K5 5 K9 5 K5,9 10 K6 5 K7 7 K6-7 12 K10 5 Nevsehir volcanics N1 5 N2 5 5 N3 N5 5 N1-5 4 N6 5 Sivas volcanics S2 S3 S4 S5
5 8 6 8
Lat.=long.
Flow
38º14=34º01 38º10=34º04
1 1 2 1 1 1 1 3 1 1 2 1 2 3
38º05=34º02 38º13=34º26 38º05=34º25 38º10=34º31 38º03=34º39 38º03=34º40 38º21=34º38 38º26=34º35
38º28=35º31 38º21=35º29 38º19=35º25 38º25=35º45 38º29=35º42 38º34=35º12 38º32=35º05
1 3 3 1 1 1 1 7
37º41=32º02 37º39=31º58
1 1 2 1 1 2 1 1 2 1 1 2 1
37º48=33º45 37º47=33º44 37º51=33º56 37º49=33º56 37º22=33º11 37º22=33º11 37º49=33º53 38º34=34º43 38º36=34º42 38º44=34º36 38º36=34º35 38º38=34º57
2 2 1 1 6 1
39º10=37º05 39º11=36º53 38º52=36º51 38º45=37º04
1 1 2 1
K–Ar age (Ma) 0.292 š 0.27
Dec.
Inc.
Þ 95
k
334 349 341 352 193 171 170 175 336 337 336 5 173 358
66 71 68 47 61 51 72 61 54 60 57 55 33 45
8.3 8.2 5.4 11.4 11.7 4.6 4.6 5.4 6.6 7.4 14 5.3 4.8 7.4
86.3 87.4 81.3 46.2 62.8 173.2 216 41.7 125 104 318.4 109 140 27.4
6 158 344 162 0 164 307 356
64 39 56 55 49 60 34 58
4.6 28.5 3.5 4.6 7.7 6.6 10.4 8.6
402 53 370 173 70 137 77.3 81
174 358 355 7 177 4 348 350 349 5 205 196 152
30 57 39 43 63 54 63 59 61 44 48 47 45
8.0 6.3 9.0 5.1 6.1 7.8 6.1 9.6 9.0 5.8 5.4 5.3 4.6
48.4 212 24.0 326 183 45 169 64.6 775 176 125 68.2 275
9.05 š 6.0
175 180 2 1 180 159
64 70 39 39 53 40
8.1 7.6 5.3 4.7 19.0 5.8
91 102 207 266 24.4 176
5.88 š 0.77 24.0 š 4.6 5.27 š 0.38 4.0 š 1.2
151 95 1 181
55 73 51 56
10 2.9 8.3 3
61 78 66 344
<1.6 1.48 š 0.5 2.57 š 1.5
4.43 š 1.6 4.19 š 0.12 <0.2 2.15 š 0.93
2.08 š 1.4 3.57 š 1.17 2.32 š 0.51 1.84 š 0.5 10.7 š 0.4
6.9 š 0.3 4.7 š 0.2 4.76 š 0.27
1.79 š 0.09 2.1 š 0.2 5.98 š 0.25
0.08 š 0.03
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189
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Table 2 (continued) Site
N
Lat.=long.
Flow
K–Ar age (Ma)
Dec.
S7 S8 S7-8 S9a S9b S9a-b S10 S11 S10-11
7 5 12 7 5 12 6 7 13
39º01=37º42 39º01=37º42
1 1 2 1 2 3 2 2 4
18.3 š 0.6
336 348 344 151 143 152 131 150 144
39º16=37º58 39º16=37º58 39º25=36º12 39º26=36º18
14.5 š 1.7
13.1 š 1.2 13.8 š 0.50
Inc. 17 34 28 56 65 59 68 31 48
Þ 95
k
6.0 6.3 7.5 3.2 14.3 6.5 4.3 6.5 11.5
103 146 29 367 29 35 244 86 14.1
Note: N D number of samples per locality; lat.=long. D latitude and longitude; flow D number of flows sampled; Þ95 D the 95% confidence interval; k D the precision parameter; dec. D mean declination; inc. D mean inclination. Shaded lines indicate mean values for adjoining sites of equivalent age. To calculate values, N was used when the number of sites was 2 and the site means were used when the number of sites was ½2. a Indicates excluded sites (see text). b Site was corrected for dip of strata (178, 12W).
tent between separate aliquots analysed on separate days. K–Ar results from the Neogene Volcanic province of Central Anatolia provide dates that range from <0.2 Ma to 24:0 š 4:6 Ma and are in general agreement with previously obtained dates. The precision on quite a number of the sites is low due to the fact that many sites register a high atmospheric contamination. This atmospheric Ar may have come from a number of sources: (1) absorption onto the surface of mineral grains during sample preparation; (2) atmospheric contamination in the extraction line either in the crucibles where the fusion took place and in the line itself; (3) from atmospheric argon incorporated into the volcanic rocks during crystallisation. It is, however, a problem which is common in very young rocks analysed in this way. It is perhaps the largest source of error in these dates. When the large atmospheric content is subtracted from the total Ar to get the radiogenic content the uncertainty in the Ar radiogenic becomes quite large and in some cases swamps the measurement. In a few cases the volume of radiogenic argon was below the level of resolution of the mass spectrometer and these samples have been eliminated. 5.2. Palaeomagnetic results (Table 2) Of the 50 sites drilled for palaeomagnetic analysis, 43 gave interpretable results. These results are presented in Table 2. The other seven sites were
either magnetically unstable or had been completely remagnetised by lightning strikes. Samples from at least 17 sites had been subjected to lightning strikes. These samples characteristically have a NRM with a very strong intensity which ranges in magnitude from 150 to 200 A=m. In many cases the random overprint imparted by the lightning could be either completely or partially removed by either thermal of AF demagnetisation. Samples could then be analysed using linear regression analysis or great circle analysis. Other samples were completely overprinted with no hint of an original consistent ChRM. As a result six sites proved completely unusable. The remaining sites yielded well defined, interpretable directions. The intensity of the natural remanent magnetisation (NRM) ranged from 0.90 to 200.0 A=m. Detailed thermal and AF experiments revealed that the NRM of these samples is composed of one, two, or three components of magnetisation. Fig. 2 shows examples of demagnetisation data from the various regions. In a vast majority of cases the low-temperature components of magnetisation are unblocked below 400ºC. In some samples, however, overlap in the unblocking temperature spectra of the lower- and higher-temperature components leads to a curved segment of the Zijderveld plots and necessitated analysis using great circles. If the secondary components could be successfully removed, the magnetisation then decays in a straight-line vector to the origin. In most cases the characteristic
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Fig. 3. Map of area studied showing site locations and palaeodeclinations of site means with 95% confidence interval. Reversed polarity site means have been converted to normal polarity. Sites are located at the tail end of the arrows. Many of the arrows represent an average of several sites as grouped in Table 2.
component of remanent magnetisation, probably carried by magnetite, is unblocked in the temperature range of 350º to 580ºC. At some sites there is indication of a higher ultimate unblocking temperature and a probable haematite contribution to the NRM. Site-mean direction show inclinations with both normal and reverse polarities. Antipodal were from sequential flows indicating magnetic stability. Sitemean declinations show an overall tendency to be rotated anticlockwise (Fig. 3). This rotation increases with the age of the flows (Fig. 4). There is, however, a fair amount of scatter in this data which is probably related to the combined effects of tectonics and secular variation. The majority of these sites have been taken from young lava or pyroclastic flows where flow tops could be observed and little or no dip is evident. In some instances, however, it was difficult to obtain a credible structural correction because it was impossible to separate the imposed tectonic dip from originally dipping flows with undulating flow tops. In addition to potential error related to uncorrected tectonic dips, each site-mean determination is based on one to three cooling units and hence may not average out secular variation. According to the most recent models of secular variation (McFadden et al.,
1991), secular variation for the Miocene should be approximately 20º and hence should lead to a 20º scatter about the mean line. Therefore, the deviation of the declination from the expected direction results from the combined effects of secular variation and tectonics and is not a direct measure of the vertical axis rotation. These are common problems with palaeomagnetic work in volcanic terrains and can only be overcome by sampling enough flows to average out these effects.
6. Discussion The palaeomagnetic results obtained in this study confirm that much of Anatolia has rotated anticlockwise with respect to the North Pole. This result is in agreement with recent GPS work that demonstrates that the anticlockwise rotation of Anatolia is apparent in both the Eurasian and the Arabian fixed reference frames (Reilinger et al., 1997). GPS, however, estimates present-day motion whereas palaeomagnetic determinations estimate finite motion over geologic time. The declinations obtained in this study and presented in Fig. 4 show a general tendency to be rotated
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Fig. 4. Observed rotations plotted as a function of age of sites from this study.
anticlockwise, which increases as a function of age. The amounts of rotation for these data have been calculated relative to the present-day dipole field because during the time interval represented the predicted declination and inclination do not diverge significantly from the present direction. The rotation rate obtained in this straightforward manner is 2.4º=Ma. It is apparent, however, that the one data point dated at 25 Ma is heavily biasing this result. If we remove this point assuming that it has not been adequately corrected for the dip of the strata, we obtain a rotation rate of 1.7º=Ma. This value is still significantly higher than the rotation rate of 1:3 š 0:1º=Ma obtained from GPS. In addition, it implies that these rocks have been rotating at the rate presently observed for the last 25 Ma: since well before the collision occurred along the Bitlis suture at approximately 12 Ma. Although this scenario cannot be disproved, a rotation rate that remains unchanged in the wake of a continental collision is counterintuitive. If we combine these data with data from other palaeomagnetic studies from central and northern Anatolia with good age control (Platzman et al., 1994; Krijgsman et al., 1996), an alternative scenario emerges. The data chosen are collected from locations which lie east of 31º where the character of the Anatolian block changes. West of this meridian Anatolia begins to interact with the Aegean extensional domain. This is evidenced by the changing character of the NAF as well as by the complex
pattern of vertical axis rotations (Kissel et al., 1987) and residual velocities obtained by GPS (Reilinger et al., 1997). In Fig. 5 the selected data have again been plotted against time. Early Tertiary sites (Eocene) are calculated relative to a predicted Eocene declination of 8º. The resultant curve appears to be composed of three distinct linear segments: 0–5 Ma, 5–12 Ma and >12 Ma. Other recently published palaeomagnetic data from central Anatolia (Tartar et al., 1996; Gu¨rso¨y et al., 1997) do not include radiometric ages and so cannot be plotted in this manner. However, the results obtained in these studies are generally consistent with the results obtained in this study. Between 0 and 5 Ma we observe a very slow rate of rotation (1.2º=Ma) comparable to that obtained by GPS (1:3 š 0:1º=Ma). It is evident, however, that there is a significant amount of scatter on these data. As discussed earlier secular variation models predict š20º of variation which encompass most of the observed scatter. Because the rate of rotation does not appear to change significantly until about 5 Ma it suggests that contemporary deformation probably reflects those geologic processes that have characterised this region for the past 4–5 Ma. It therefore follows that since 5 Ma the Anatolian plate has been decoupled from Eurasia along the NAF. This is in agreement with recent studies (Platzman et al., 1994; Reilinger et al., 1997), which suggest that the NAF is Pliocene in age and that fault slip-rates have not changed significantly since the fault was initiated. If
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Fig. 5. Plot of observed rotation as a function of age for sites from this study combined with dated localities from Platzman et al. (1994) and Krijgsman et al. (1996). Error of 2¦ are shown for each age and the 95% confidence interval is shown for rotation data.
Anatolia has been decoupled from Eurasia along the NAF for the last 5 Ma, there should be no rotation associated with this period of deformation north of the NAF. These palaeomagnetic data and GPS estimates of rotation rate predict that the amount of rotation in this period, from 5 Ma to present, would be approximately 7º. We would, therefore, predict an average difference of approximately 7º in the total amount of rotation across the NAF. This is a hypothesis that will be tested in forthcoming studies. If 7º of rotation has occurred since the Pliocene this leaves the majority of the rotation to have occurred before 5 Ma, prior to the initiation of motion along the NAF. This is consistent with the observation that anticlockwise rotations extend north of the present trace of the NAF to the Black Sea coast (Platzman et al., 1994). Between about 5 and 12 Ma we observe a rapid increase of rotation rate to 6.5º=Ma. This interval encompasses the time period between collision along the Bitlis suture and initiation and localisation of motion along the NAF and EAF. It is a time when deformation was probably more distributed or continuous and rotation was probably accommodated on relatively small blocks as crust spread down a potential energy gradient
away from the area of high potential energy in the continental collision zone to the area of low potential energy in the Aegean. Pre-12 Ma the rotation rate again decreases to essentially zero ( 0.041º=Ma). This is the pre-collisional era and as a result blocks were not rotating. Therefore all rocks older than 12 Ma should reflect the total cumulative rotation. It is in these rocks that we should observe the differential rotation north and south of the NAF. Unfortunately, there are at present not enough geochronological data to perform this test with any degree of statistical validity. This is the preferred interpretation of the data although we also investigated the possibility that rotations might vary with distance from the collision zone. In Fig. 6 rotation and radiometric age are plotted as a function of latitude and longitude. A best-fit line has been calculated for each data set and the resulting equation is displayed at the top of each plot. The plots show that, at the 5% level, there is significant correlation of rotation with latitude but there is a poor correlation of rotation with longitude. However, it is also evident from Fig. 6a that the age of the volcanic unit is varying in a like manner with latitude. Therefore, it seems more likely that rotation
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7. Conclusions Data from Central Anatolia support the conclusion that the entire Anatolian block has rotated anticlockwise. If we assume a constant rate of rotation we obtain a rate of 2.4º=Ma. However, more careful analysis of the data suggests that rate of rotation may have been only 1.2º=Ma until approximately 5 Ma ago when rotation rates increased to around 7º=Ma. The rotation rate then appears to decrease to almost zero in the pre-collisional era at approximately 12 Ma. These results are generally consistent with a model calling for Middle Miocene collision along the Bitlis suture and Late Plio–Pleistocene initiation of motion along the North and East Anatolian fault zones.
Acknowledgements Special thanks to C.C. Rundle, J.P. Platt, C. Sengo¨r, N. Go¨ru¨r, and O. Tuysuz without whose help this project would not have been possible. The authors would also like to thank A. Poisson and A.H.F. Robertson for their constructive reviews of this paper. This research was supported by a grant from the Natural Environmental Research Council GR3=7666.
Fig. 6. Plot of observed rotation and age as a function of latitude (a) and longitude (b).
is a function of age rather than distance from any particular geologic boundary although some effect cannot be ruled out. Finally, it is interesting to note that both this study and earlier work seems to indicate that we do not see anticlockwise rotations in the region west of the Tuz Go¨lu¨ fault and east of the landward continuation of the Cyprean Arc. This might be due either to a buttressing effect of the advancing thrust edge of the Taurus mountains or to a decoupling along the boundary of the ancient Kirsehir block as defined by the active strand of the left-lateral Tuz Golu fault. To clarify these trends far more geochronological and palaeomagnetic data need to be collected throughout central Anatolia.
References Ambraseys, N.N., 1970. Some characteristic features of the Anatolian fault zone. Tectonophysics 9, 143–165. Barka, A.A., Kandinsky-Cade, K., 1988. Strike-slip fault geometry in Turkey and its influence on earthquake activity. Tectonics 7, 663–684. Batum, I., 1978. Geology and petrology of Acigo¨l and Go¨llu¨dag volcanics at SW of Nevsehir (Central Anatolia, Turkey) . Yerbilimlerli 4, 17–20. Besang, C., Eckhardt, F.J., Harre, W., Kreuzer, H., Mu¨ller, P., 1977. Radiometriche Alterbestimmungen an neogene Eruptivgesteinen der Turkei. Geol. Jahrb. B25, 3–36. Cater, J.M.L., Hanna, S.S., Ries, A.C., Turner, P., 1991. Tertiary evolution of the Sivas Basin, central Turkey. Tectonophysics 195, 29–46. Dercourt, J., Ricou, L.E., Vrielynk, B., 1993. Atlas Tethys Palaeoenviromental Maps. Beicip-Franlab. Dirik, K., Go¨ncu¨oglu, M.C., 1996. Neotectonic characteristics of Central Anatolia. Int. Geol. Rev. 38, 807–817. Ercan, T., Fujitani, T., Matsuda, J.I., Tokel, S., Notsu, K., Ui,
188
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189
¨ lmez, M., T., Can, B., Selvi, Y., Yildirim, T., Fisekci, A., O Akbasli, A., 1990. The origin and evolution of the Cenozoic volcanism of Hasandagi–Karacadag area (cental Anatolia). Geomorphol. Bull. 18, 39–54. Go¨kten, E., 1983. Stratigraphy and geologic evolution of southsoutheastern Sarkisla, Sivas. Bull. Geol. Soc. Turk. 26, 167– 176. Go¨ncu¨oglu¨, M.C., Toprak, V., 1992. Neogene and Quaternary volcanism of Central Anatolia: a volcano-structural evaluation. Bull. Soc. Volcanol. 26, 1–6. Guezou, J.-C., Temiz, H., Poisson, A., Gu¨rsoy, H., 1996. Tectonics of the Sivas Basin: the Neogene record of the Anatolian accretion along the inner Tauric suture. Int. Geol. Rev. 38, 901–925. Gu¨rso¨y, H., Piper, J.D.A., Tartar, O., Temiz, H., 1997. A palaeomagnetic study of the Sivas Basin, Central Turkey: crustal deformation during lateral extrusion of the Anatolian Block. Tectonophysics 271, 89–106. Innocenti, F., Mazzuoli, R., Pasquare, G., Radicati di Brozolo, F., Villari, L., 1975. The Neogene calcalkaline volcanism of Central Anatolia: geochronological data on Kayseri-Nigde area. Geol. Mag. 112, 349–360. Jackson, J., McKenzie, D.P., 1984. Active tectonics of the Alpine–Himalayan belt between western Turkey and Pakistan. Geophys. J. R. Astron. Soc. 77, 185–264. Jung, D., Keller, J., 1972. Die Jungen Vulkanite im Raum Zwischen Konya und Kayseri (Zentral Anatolien). Z. Dtsch. Geol. Ges. 123, 503–512. Keller, J., Jung, D., Burgath, K., Wolf, F., 1977. Geologie und Petrologie des Neogenen Kalkalcali-Vulkanismus von Konya (Erenler Dag-Alaca Dag Massif, Zentral Anatolien). Geol. Jahrb. 25, 37–117. Ketin, I., 1948. Uber die tektonisch–mechanischen Folgerungen aus den grossen anatolischen Erdbeben des letzten Dezenniums. Geol. Rundsch. 36, 77–83. Kissel, C., Laj, C., Sengor, A.M.C., Poisson, A., 1987. Paleomagnetic evidence for rotation in opposite senses of adjacent blocks in northeastern Aegea and western Anatolia. Geophys. Res. Lett. 14, 907–910. Krijgsman, W., Duermeijer, C., Langereis, C.G., De Bruijn, H., Sarac¸, G., Andriessen, P.A.M., 1996. Magnetic polarity stratigraphy of late Oligocene to middle Miocene mammal-bearing continental deposits in Central Anatolia (Turkey). Newsl. Stratigr. 34, 13–29. McFadden, P.L., Merril, R.T., McElhinny, M.W., Lee, S., 1991. Reversals of the Earth’s magnetic field and temporal variations of the dynamo families. J. Geophys. Res. 96, 3923–3933. Notsu, K., Fujitani, T., Ui, T., Matsuda, J., Ercan, T., 1995. Geochemical features of collision-related volcanic rocks in central and eastern Anatolia, Turkey. J. Volcanol. Geotherm. Res. 64, 171–192. Oral, M.B., Reilinger, R.E., Tokso¨z, M.N., 1992. Deformation of the Anatolian Block as deduced from GPS Measurements (abstract). Eos Trans. AGU 74, 120. Pasquare, G., Poli, S., Vezzoli, L., Zanchi, A., 1988. Continental arc volcanism and tectonic setting in Central Anatolia. Tectonophysics 146, 217–230.
Pavoni, N., 1961. Die nordanatolische Horizontalverschiebung. Geol. Rundsch. 51, 122–139. Pearce, J.A., Bender, J.F., De Long, S.E., Kidd, W.S.F., Low, P.J., Gu¨ner, Y., Saroglu, F., Yilmaz, Y., Moorbath, S., Mitchell, J.G., 1990. Genesis of collision volcanism in Eastern Anatolia, Turkey. J. Volcanol. Geotherm. Res. 44, 189–229. Platzman, E.S., Platt, J.P., Tapirdamaz, C., Sanver, M., Rundle, C.C., 1994. Why are there no clockwise rotations along the North Anatolian Fault Zone, Turkey. J. Geol. Res. 99, 21705– 21715. ¨ zturk, A., Inan, S., Temiz, H., Poisson, A., Guezou, J.-C., O ¨ zden, S., 1996. Tectonic setting Gu¨rso¨y, H., Kavak, K.S., O and Evolution of the Sivas Basin, Central Anatolia, Turkey. Int. Geol. Rev. 38, 838–853. Reilinger, R.E., McClusky, S.C., Oral, M.B., King, R.W., Toksoz, M.N., Barka, A.A., Kinik, I., Lenk, O., Sanli, I., 1997. Global Positioning System measurements of presentday crustal movements in the Arabia–Africa–Eurasia plate collision zone. J. Geophys. Res. 102, 9983–9999. Robertson, A.H.F., Dixon, J.E., 1985. Introduction: aspects of the geological evolution of the Eastern Mediterranean. In: Dixon, J.E., Robertson, A.H.F. (Eds.), The Geological Evolution of the Eastern Mediterranean, 17. Blackwell, Oxford, pp. 1–112. Robertson, A.H.F., Dixon, J.E., Brown, A., Collins, A., Morris, A., Pickett, E., Sharp, I., Ustao¨mer, T., 1996. Alternative tectonic models for the late Palaeozoic–early Tertiary development in the Eastern Mediterranean region. In: Morris, A., Tarling, D.H. (Eds.), Palaeomagnetism and Tectonics of the Mediterranean Region. Geol. Soc. London, Spec. Publ. 105, 239–263. Saroglu, F., 1988. Age and offset of the North Anatolian Fault. METU J. Pure Appl. Sci. 21, 65–79. Sengor, A.M.C., 1979. The North Anatolian transform fault: its age, offset and tectonic significance. J. Geol. Soc. London 136, 269–282. Sengor, A.M.C., Canitez, N., 1982. The North Anatolian Fault. In: Berckhemer, H., Hsu, K. (Eds.), Alpine–Mediterranean Geodynamics. American Geophysical Union, Washington, DC, pp. 205–216. Sengor, A.M.C., Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241. Steiger, R., Jager, E., 1977. Convention on use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359– 362. Su¨mengen, M., Unay, E., de Bruijn, H., Terlemez, I., Gu¨rbu¨z, M., 1990. New Neogene rodent assemblages from Anatolia (Turkey). In: Lindsey, E.H., Falbusch, V., Mein, P. (Eds.), European Neogene Mammal Chronology. Plenum Press, New York, pp. 61–72. Tartar, O., Piper, J.D.A., Gu¨rsoy, H., Temiz, H., 1996. Regional significance of neotectonic counterclockwise rotation in Central Anatolia. Int. Geol. Rev. 38, 692–700. Temiz, H., 1996. Tectonostratigraphy and thrust tectonics of the central and eastern parts of the Sivas Tertiary basin, Turkey. Int. Geol. Rev. 38, 957–971. Toprak, V., Go¨ncu¨oglu¨, M.C., 1993. Tectonic development of
E.S. Platzman et al. / Tectonophysics 299 (1998) 175–189 the Neogene–Quaternary Central Anatolian volcanic province, Turkey. Geol. J. 28, 357–369. Uygun, A., 1981. Geology, evaporite occurrences and hydrocarbon potential of Tuzgo¨lu¨ (Salt Lake) Basin. Central Anatolian Geology Symposium, Geological Society of Turkey Publications, Ankara, pp. 66–71. Westaway, R., 1994. Present-day kinematics of the Middle East
189
and eastern Mediterranean. J. Geophys. Res. 99, 12071– 12090. Yetis, C., 1984. New observations on the age of the Ecemis Fault. In: Tekeli, O., Go¨ncu¨oglu, M.C. (Eds.), Geology of the Taurus Belt. Mineral Research and Exploration Institute, Ankara, pp. 159–164.