Palaeomagnetic study of Tertiary volcanic domains in Southern Turkey and Neogene anticlockwise rotation of the Arabian Plate

Tectonophysics 465 (2009) 114–127

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Palaeomagnetic study of Tertiary volcanic domains in Southern Turkey and Neogene anticlockwise rotation of the Arabian Plate H. Gürsoy a, O. Tatar a, J.D.A. Piper b,⁎, A. Heimann c, F. Koçbulut a, B.L. Mesci a a b c

Department of Geology, Cumhuriyet University, SİVAS 58140, Turkey Geomagnetism Laboratory, Department of Earth and Ocean Sciences, University of Liverpool, LIVERPOOL L69 7ZE, United Kingdom Geological Survey of Israel, 30 Malkhe Israel St., JERUSALEM 95501, Israel

a r t i c l e

i n f o

Article history: Received 12 March 2008 Received in revised form 15 October 2008 Accepted 3 November 2008 Available online 11 November 2008 Keywords: Palaeomagnetism Volcanic rocks Neotectonics Miocene Southern Turkey Arabian Plate Anatolides Red Sea Tectonic rotation

a b s t r a c t Following collision with Eurasia, the Arabian Shield indenter has continued to deform into the weak Anatolian collisional collage that resulted from subduction of the Neotethyan Ocean. Differential movements have involved rotation and continuing northwards translation, and have been accommodated mainly by slip along major transforms including the northward extension of the Dead Sea Fault Zone (DSFZ) and the East Anatolian Fault Zone (EAFZ). To aid in evaluating post-collisional motions the palaeomagnetism of extensive volcanic domains at the northern margin of the stable shield in southern Turkey is reported here together with the timing of emplacement as constrained by K–Ar study. The age dating results indicate that volcanic activity occurred mainly during mid-late Miocene times corresponding to the final stages of suturing. Volcanic fields in the east of the investigated region are younger and correspond to Neotectonic volcanism in Brunhes and Matuyama chrons. Thermal and alternating field demagnetization of 399 cores from 83 sites in basaltic lavas identifies 29 units of normal and 43 of reversed polarity with 11 sites having transitional or random directions. Volcanic fields west of the Euphrates (Kilis–Gaziantep region) with ages in the range 7.0– 20.3 Ma (average 14.9 Ma, SD = 4.3 Ma) have mean remanence D/I = 353/52° (38 sites, 5.3°). Lava fields east of the Euphrates (Urfa region) are dated 10.4–12.1 Ma and yield a comparable mean remanence D/I = 350/50° (17 sites, 6.0°). These collections are shown to have properly recorded palaeosecular variation with only minimal inclination shallowing and inferred anticlockwise rotations with respect to Eurasia since midMiocene times are 10.9 ± 4.3° and 14.0 ± 5.0° respectively. These contrast with clockwise rotation of 6.3 ± 4.3° derived from late Matuyama–Brunhes epoch volcanic rocks immediately to the north west of the Arabian margin where rotational impingement of the shield indenter into fault blocks within the Karasu Rift at the northern extension of the DSFZ interacts with left lateral motion at a rate of ~ 0.5 cm/year along this zone to produce small scale vertical axis rotations. Results from the volcanic suites sited on the stable shield are compared with other results from the Arabian Plate to conclude that it did not rotate significantly following closure of the Bitlis Suture until Late Miocene–Early Pliocene times since when it has rotated anticlockwise at a rate of ~ 1.0°/Myr. This is comparable to the present day rate of rotation deduced from GPS and correlates with crustal separation and sea floor spreading in the Red Sea. It also temporally links the rotation of the Arabian Plate to the initiation of the intracontinental transforms (North and East Anatolian fault zones) within Anatolia and to the subsequent extrusion of blocks within this accretionary domain by tectonic escape to the west. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The African, Arabian and Eurasian plates are presently interacting in the vicinity of southern Turkey in one of the most complex tectonic zones along the Alpine–Himalayan orogenic belt. This complexity results primarily from the separation of the Afro-Arabian and Eurasian plates by a weak accretionary collage, the Anatolides, which formed

⁎ Corresponding author. E-mail address: [email protected] (J.D.A. Piper). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.11.001

during closure of the Neotethyan Ocean prior to mid-Miocene times. It is further complicated by the ongoing motion of Arabia into the Anatolides in which continuing northward motion of the former is accompanied by anticlockwise rotation (McClusky et al., 2000); in the south this is accommodating the opening of the Red Sea. Thus Arabia comprises a large indenter primarily responsible for tectonic escape of Anatolian terranes to the west and south during the post-collisional phase of deformation (e.g. Barka and Reilinger, 1997). Associated differential motions and block rotations have in turn progressively expanded the curvature of the Tauride Arc to the west (Tatar et al., 2002; Piper et al., 2006). They are dominated by a strike slip regime

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that gives way to an extensional province in western Turkey driven by trench roll-back on the Hellenic Arc and terrane extrusion from central Anatolia (Gürsoy et al., 2003), although the relative contributions of push by tectonic escape and trench pull are as yet unresolved. The Anatolian region is not a plate sensu stricto but is instead undergoing comprehensive internal deformation with strike slip movements between fault blocks on a 10–100 km scale plus a component of anelastic deformation (Gürsoy et al.,1997, 1998; Tatar et al., 2002; Piper et al., 2006); inclusion of the latter is a possible explanation for the contrasting short term GPS and long term palaeomagnetic views of Anatolian deformation. Following establishment of the North and East Anatolian intracontinental transforms by Early Pliocene times (Westaway and Arger, 1996), tectonic escape has extruded these terranes to the west and south. Hence the Cenozoic tectonic history in Anatolia is loosely divided into palaeotectonic and neotectonic phases with the former dominated by collisional tectonics and the latter comprising the postcollisional deformation. The differential motion of the Arabian and African Plate is taken up primarily by strike slip along the Dead Sea Fault Zone (DSFZ). In most kinematic models this fault zone is considered to meet the East Anatolian Fault Zone (EAFZ) at an unstable FFF triple junction sited close to city of Kahramanmaraş in southern Turkey (Fig. 1(A)). Palaeomagnetic investigations have played an important part in unravelling the neotectonic deformation of the Anatolides and have proved effective for defining block rotations even on time scales as short as a few hundred thousand years (Piper et al., 2006). To extend the regional coverage of these investigations to the south east and compare deformation within Anatolia with the Arabian indenter to

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the south we report here palaeomagnetic studies from volcanic domains sited on the northern margin of the Arabian Shield in southern Turkey. 2. Regional geological framework Protracted geologic processes along convergent plate boundaries produce orogenic belts with complex rock assemblages during final stages of ocean closure, one of the most significant examples being the ~1500 km long India–Asia collision active since Eocene times (Molnar and Tapponnier 1975; Tapponnier et al., 1982, 1986). Comparable processes within the Anatolian peninsula and surroundings have resulted from N–S convergence between Eurasian and Arabian– African plates since the Late Cretaceous. The later part of this convergence comprised closure of the southern branch of Neotethyan Ocean between the collage of accreted terranes of Anatolia and Arabia. The ocean was consumed along a N-dipping subduction zone between Early Cretaceous and Middle Miocene times and obliterated by continental collision producing a collisional belt, the Bitlis Suture Zone (BSZ), emplaced towards the close of Miocene times (Şengör and Yılmaz, 1981; Robertson, 2000). The Arabian Plate has since continued to impinge into the collage to produce the crustal thickening and uplift of the Anatolian Plateau. The Tauride and SE Anatolian Fold Belts resulting from collision (Fig. 1A) were defined by Ketin (1966) in terms of morphological, morphotectonic and geological criteria. The BSZ juxtaposes contrasting rock assemblages (Yılmaz et al., 1991; Yılmaz, 1993). The Arabian platform on the southern side has a cover of shallow marine sediments of Early Palaeozoic to Miocene age on Precambrian basement (Pearce et al., 1990; Yılmaz et al., 1991;

Fig. 1. (A) Tectonic framework and location map of the Turkish sector of the Alpine–Himalayan orogenic belt after Piper et al. (2006). Large open arrows show directions of current relative plate motions and the smaller half arrows are directions of movement on major strike-slip faults. Tectonic lineaments are abbreviated: SLF, Salt Lake Fault Zone; LKF (LaçinKızılırmak Fault Zone; KEF (SEFZ), Kırıkkale-Erbaa Fault Zone (becoming Sungurlu-Ezinepazarı Fault Zone, SEFZ to the west); AF, Almus Fault Zone: EFZ (CAFZ), Ecemiş Fault Zone (Central Anatolian Fault Zone); CATB, Central Anatolian Thrust Belt and DSF, Dead Sea Fault Zone. The shaded area comprises deformed terranes within the Isparta Angle and was formed mainly during the Palaeotectonic era by interference of verging allochthonous units during final stages of Tethyan convergence. (B) The orogenic framework of Turkey and adjoining regions.

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Brew et al., 2001b) and was largely characterised by tectonic stability from Palaeozoic times to end Cretaceous times; Jurassic–Triassic structural trends are oriented WSW–ENE and subsequent Late Cretaceous to present day trends are aligned SW–NE suggesting anticlockwise rotation of this margin since the late Mesozoic (Coşkun, 2004). Sedimentation and deformation of rock units around the fold belt have been controlled by three tectonic factors (Yılmaz and Duran, 1997) namely: (1) epirogenic movements caused to sea-level changes during the Cambrian–Lower Cretaceous interval, (2) ophiolitic melange obduction caused by Upper Cretaceous compression and the closure of the southern branch of Neotethyan ocean and (3) ongoing Middle Miocene and later Anatolian collage-Arabian platform collision and related Neogene–Quaternary volcanism. The fold belt in southern Turkey and bordering areas of Syria and Iraq has been the subject of much recent hydrocarbon-related investigation (see Yılmaz and Duran, 1997; for Syria see Brew et al., 2001b). 3. The intracontinental transform boundaries The EAFZ is a narrow zone of sinistral transpression trending south west from Karlıova in east–central Turkey, where it joins the North Anatolian Fault Zone (NAFZ), to the Amanos Range at the northern extension of the DSFZ (Fig. 1) and has been tectonically active since Pliocene times. Estimated offsets on this fault zone are summarised by Westaway (2003) and include 3.5–13 km from offset of displaced Euphrates river channels (Hempton, 1987; Arpat and Şaroğlu, 1972), 15–27 km from displaced pre-Pliocene units (Arpat and Şaroğlu, 1972; Şaroğlu et al., 1992) to 35–40 km from analysis of the Gölbaşı basin and Hazar pull-apart basin (Westaway and Arger, 1996). Equivalent average slip rates range from 0.6 cm/year (Kiratzi, 1993; Westaway and Arger, 1996), ~ 0.9 cm/year (Kasapoğlu, 1987; Yürür and Chorowicz, 1998), 1.9 cm/year (Lyberis et al., 1992) and 2.9 cm/year (Taymaz et al., 1991); the preferred estimate for the contemporary slip rate is 0.40–0.46 cm/year (Westaway, 2003; Wdowinski et al., 2004). The DSFZ is predominantly a sinistral transform fault (Mart and Rabinowitz, 1986), with regional transtension responsible for rift depressions (Quennell, 1958, 1984; Freund et al., 1970), and connects the extensional plate boundary in the Red Sea in the south with the Tauride collisional zone in the north (Fig. 1(A)). It extends for more than 1000 km and has been a zone of active strike slip since at least Middle Miocene times (Garfunkel et al., 1981). A maximum offset on the DSFZ of 105–107 km since ~ 15 Ma is determined in the southern sector by matching a range of features between the Gulf of Aqaba and Mt. Hermon in Lebanon (Quennell, 1958, 1984, Freund et al., 1970). The age of the DSFZ has been estimated as 19–15 Ma because dykes of this age in the Sinai are offset by ~ 105 km from their equivalents in Jordan (Eyal et al., 1981), an observation yielding an average slip rate of 7 mm/ year since the mid-Miocene (Garfunkel, 1981). The DSFZ is offset to the east through the Palmyride Fold Belt in southern Lebanon and Syria and ultimately splays into an array of faults in northernmost Syria and southern Turkey (Fig. 2). Folding and thrusting within the Palmyrides appear to account for about 25 km of shortening (Chaimov et al., 1990; Searle, 1994; Badawy and Horvath, 1999) but some of this may predate the initiation of the DSFZ. The DSFZ becomes braided north of 36.5° latitude into three major fault segments comprising the Amanos Fault Zone (AFZ) in the west, the East Hatay Fault (EHF), and the Afrin Fault (Fig. 2). The cumulative left lateral slip across the first two faults has been estimated to be 70– 80 km from offset of the Hatay ophiolite (Freund et al., 1970; Dewey et al., 1986; Lyberis et al., 1992 but see Chaimov et al., 1990 for a contrary view). The key observation here is the presence of ophiolites in the Karasu Rift near Hassa (Figs. 1 and 2) considered to be separated by left lateral motion from ultrabasic rocks south east of Antakya in the BaerBassit Mountains of Syria. Westaway (2003) revises the 70–80 km estimate for offset downwards to ~ 45 km across the AFZ and ~10 km across the EHF. The contribution of strike slip across the Afrin Fault

(Fig. 2) remains unknown but subdued topographic expression suggests that slippage is much less than the other two faults. The total offset across this fault array could therefore be as much as 20– 30 km less than the offset at the southern end of the DSFZ with the difference being accommodated by folding across the Palmyride Belt (Zanchi et al., 2002). The AFZ bounds the eastern margin of the Amanos Range and is topographically the most important lineament in southern Turkey (Figs. 1 and 2). According to Adıyaman and Chorowicz (2002) it has a reverse throw component in addition to sinistral strike slip. These authors interpret the Amanos range as an anticlinal structure, with a Palaeozoic/Precambrian core and limbs of Mesozoic sedimentary rocks mantled by ophiolitic nappes, and the Karasu Rift to the east as a complementary syncline. Earlier transpressive deformation in this region is supported by striation analysis (Yürür and Chorowicz, 1998), positive flower structures identified on seismic profiles in the Gaziantep Basin (Coşkun and Coşkun, 2000), by reverse faulting observed in Miocene formations south of Iskenderun (Çapan et al., 1987) and by NW–SE compression related to rotation of the Arabian Plate (Zanchi et al., 2002). However, this regime now seems to be displaced at higher crustal levels because normal or transtensional faults are identified along the western border of the Karasu valley (Rojay et al., 2001) and striation analysis in the Antakya region identifies a stress regime change from transpressional to transtensional (Över et al., 2002; Boulton and Robertson, 2008). This tectonic transition appears to be responsible for the access of basaltic melts to the surface in this region, volcanism which has been concentrated within the last 1 Ma (Tatar et al., 2004). Transtension in the Syrian sector of the DSFZ is also identified from active pull-apart basins in the Missyaf region (Zanchi et al., 2002). As discussed by Westaway (2003), the average slip rate of ~ 7 mm/ year embraced by the 19–15 Ma interval of activity on the DSFZ is unlikely to be representative of this full time period. Recent work in the southern part of the DSFZ in the Arava Valley of Israel and Jordan indicates a deceleration in the rate of movement to ~3.5–4.5 mm/year during Plio-Quaternary times. Further north in Lebanon the main strand of the DSFZ (the Yammouneh Fault) forms the western margin of the Bekaa Valley and is oriented N30°E although at least two other left-lateral faults, including the Serghaya Fault, contribute to the overall motion with the latter dying out in distributed shortening across the Palmyra fold belt. Westaway (2003) discusses arguments that the Syrian segment of the DSFZ ceased to be active during the Mio-Pliocene (Butler et al., 1997) or only became active during the Pliocene (Brew et al., 2001a) and concludes that activity is ongoing and comparable at its northern and southern sectors. The conformity of the tectonically complex region where the Amanos Fault Zone (AFZ) and EAFZ meet to conventional Plate Tectonic paradigms remains unclear. The AFZ is considered to have formed the main strand of the Africa–Arabian plate boundary from mid Miocene until late Pliocene times (Westaway and Arger, 1996; Yurtmen et al., 2002) with sinistral motion between the plates stepping eastwards from the DSFZ in northern Syria onto the AFZ. From surface evidence that motion on the AFZ is too small to accommodate current relative motion between the Arabian and African plates, Yurtmen et al. (2002) suggest that most subsequent strike slip has occurred to the east of the Amanos Range and Karasu Rift, and sidesteps onto faults at the western margin of the Gaziantep Basin (Coşkun and Coşkun, 2000). Other earlier interpretations also considered the EAFZ to be a direct northward continuation of the DSFZ through fault sets along the eastern margin of the Karasu Rift and/or Gaziantep Basin, but have expressed contrasting views about contemporary kinematics (Arpat and Şaroğlu, 1972, Muehlberger and Gordon, 1987). More discrete models consider that the AFZ merges with the Anatolian/African Plate boundary in an unstable FFF triple junction at Türkoğlu close to Kahramanmaraş (Karig and Kozlu, 1990 and Fig. 1). The EAFZ, defining the plate boundary between

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Fig. 2. Outline of the geological map of southern Turkey with palaeomagnetic sampling sites of this study.

Anatolia and Arabia, is then interpreted to continue southwestwards from Türkoğlu across the Amanos Mountains (Düziçi Fault Zone), past the town of Osmaniye as the Karataş–Osmaniye Fault Zone (KOFZ in Fig. 2(A)), and offshore across the Mediterranean Sea to the Kyrenia (Girne) Range of northern Cyprus (e.g. Hempton, 1987; Westaway and Arger, 1996). This interpretation requires significant contemporary movement on this westward continuation of the EAFZ and is supported by palaeomagnetic evidence (Gürsoy et al., 2003). Neogene volcanism near this transform junction has been concentrated within the late Matuyama and Brunhes chrons and has permitted application of palaeomagnetic studies to help resolve relative motions. The Karasu rift bordering the Amanos range is partially infilled with lavas almost entirely younger than 1 Ma in age in which clockwise rotation of 8.8 ± 4.0° since the peak of magmatism at 0.66–0.35 Ma is accommodated by movements of fault blocks between the AFZ and EHF coupled to anticlockwise rotation of the Arabian indenter (Tatar et al., 2004); the estimated slip rate on the AFZ

is 0.46 cm/year and evidently accommodates most or all of the relative movement at this northern extension of the DSFZ; evidence for low slip rates from surface breaks on the AFZ (Yurtmen et al., 2002) would appear not to embrace the full seismogenic crust. However, motions are evidently also partitioned in this region because the EAFZ continues across the Amanos Range into the Karataş–Osmaniye Fault Zone bordering the northern margin of the Gulf of Iskenderun where block rotations indicate a maximum slip rate of 0.6 cm/year (Gürsoy et al., 2003). In each case the young volcanic activity appears to be concentrated at sphenocasms created by the block rotation which have permitted mantle melts to access the surface. The evolving character of tectono-magmatic activity in this region is presumably a reflection of the inherent instability of FFF plate junctions. Since the neotectonic history of this region is controlled by relative motions between the Anatolian collage and the Arabian indenter, resolution of the post-collisional rotation and translation of this latter block is a key facet of the neotectonism. Accordingly we have conducted

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Table 1 Summary of palaeomagnetic sampling sites and locations in volcanic rocks, northern margin of the Arabian Shield in Southern Turkey

Table 1 (continued)

Site no.

74 75 76 77 78 79 80 81 82 83

Locality

Site coordinates

Tilt

Lat (°N) / Long (°E) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Kilis region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Gaziantep region Pazarcık region Pazarcık region Pazarcık region Pazarcık region Pazarcık region Pazarcık region Pazarcık region Pazarcık region Yavuzeli region Yavuzeli region Yavuzeli region Yavuzeli region Yavuzeli region Birecik-Urfa region Birecik-Urfa region Birecik-Urfa region Birecik-Urfa region Birecik-Urfa region Suruç-Urfa region Suruç-Urfa region Suruç-Urfa region Urfa region Urfa region Urfa region Urfa region Urfa region Urfa region Urfa region Urfa region

36° 43′ / 37° 13′ 36° 44′ / 37° 14′ 36° 44′ / 37° 15′ 36° 44′ / 37° 14′ 36° 46′ / 37° 13′ 36° 46′ / 37° 14′ 36° 46′ / 37° 15′ 36° 49′ / 37° 19′ 36° 48′ / 37° 18′ 36° 44′ / 37° 07′ 36° 45′ / 37° 06′ 36° 50′ / 37° 14′ 36° 49′ / 37° 14′ 36° 49′ / 37° 14′ 36° 50′ / 37° 10′ 36° 51′ / 37° 08′ 36° 52′ / 37° 08′ 36° 54′ / 37° 09′ 36° 39′ / 37° 28′ 36° 42′ / 37° 14′ 36° 58′ / 37° 08′ 36° 59′ / 37° 10′ 36° 53′ / 36° 49′ 36° 53′ / 36° 49′ 36° 54′ / 36° 48′ 36° 54′ / 36° 48′ 36° 56′ / 36° 48′ 36° 52′ / 36° 51′ 36° 53′ / 36° 55′ 36° 55′ / 36° 59′ 37° 15′ / 37° 18′ 37° 15′ / 37° 18′ 37° 16′ / 37° 18′ 37° 16′ / 37° 18′ 37° 16′ / 37° 18′ 37° 21′ / 37° 14′ 37° 21′ / 37° 14′ 37° 11′ / 37° 17′ 37° 13′ / 37° 16′ 37° 16′ / 37° 17′ 37° 18′ / 37° 17′ 37° 18′ / 37° 18′ 37° 01′ / 37° 21′ 37° 01′ / 37° 21′ 37° 24′ / 37° 23′ 37° 24′ / 37° 24′ 37° 24′ / 37° 24′ 37° 24′ / 37° 24′ 37° 20′ / 37° 18′ 37° 26′ / 37° 18′ 37° 25′ / 37° 17′ 37° 25′ / 37° 17′ 37° 14′ / 37° 31′ 37° 14′ / 37° 32′ 37° 16′ / 37° 33′ 37° 20′ / 37° 33′ 37° 21′ / 37° 33′ 37° 04′ / 38° 08′ 37° 04′ / 38° 08′ 37° 03′ / 38° 08′ 37° 04′ / 38° 16′ 37° 05′ / 38° 17′ 37° 02′ / 38° 25′ 37° 02′ / 38° 27′ 37° 04′ / 38° 29′ 37° 09′ / 38° 32′ 37° 02′ / 38° 37′ 36° 58′ / 38° 43′ 36° 53′ / 38° 37′ 36° 51′ / 38° 40′ 36° 50′ / 38° 41′ 36° 49′ / 38° 44′ 37° 08′ / 39° 05′

H 18/144 16/59 18/144 H H 22/202 H 18/220 H H H H H H H H H 44/178 H H H H H H H H H H H 16/351 16/351 21/298 19/76 41/29 14/9 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H

Site no.

Locality

Site coordinates

Tilt

Lat (°N) / Long (°E) Urfa Urfa Urfa Urfa Urfa Urfa Urfa Urfa Urfa Urfa

region region region region region region region region region region

37° 37° 37° 37° 37° 37° 37° 37° 37° 37°

11′ / 38° 48′ 11′ / 38° 48′ 12′ / 38° 49′ 11′ / 38° 49′ 13′ / 38° 47′ 13′ / 38° 48′ 26′ / 38° 49′ 26′ / 38° 49′ 25′ / 38° 48′ 26′ / 38° 16′

H H H H H H H H H H

All sites are in basaltic lavas; H = horizontal or emplaced on primary slope.

a regional palaeomagnetic study of extensive late Tertiary volcanic fields that border this northern margin of the shield in southern Turkey and the results comprise the subject of this paper.

4. Regional geological framework, sampling and age dating results Closure of the southern branch of the Neotethyan ocean was followed by widespread collision-related volcanism from Miocene to Recent times both within the Anatolides, along the BSZ, and along the periphery of the Arabian platform (Leo et al., 1974; Innocenti et al., 1982; Yılmaz et al., 1987; Yoldemir, 1987; Yılmaz, 1990; Pearce et al., 1990, Ulu et al., 1991; Notsu et al., 1995; Aydar et al., 2003; Tatar et al., 2004; Bridgland et al., 2007; Demir et al., 2007). Eruptive centres of this volcanic activity appear to have been focussed on zones of local extension within regions of general N–S transpression (Güner and Table 2 Summary of age results (Ma) from volcanic rocks, northern margin of the Arabian Shield in Southern Turkey Site no.

Locality

Age results

8 9 10 11 12 15 16 17 18 19 32 33 34 36 37 38 39 40 41 42 45 46 48 49 50 51 52 58 59 60 66 67 68 69 81 82

Kilis Kilis North of Kilis North of Kilis Kilis-Polateli Kilis-Polateli North of Polateli North of Polateli North of Polateli South of Elbeyli North of Gaziantep North of Gaziantep North of Gaziantep North of Gaziantep North of Gaziantep NW of Gaziantep NW of Gaziantep NW of Gaziantep NW of Gaziantep NW of Gaziantep SE of Pazacık SE of Pazarcık SE of Pazarcık NW of Gaziantep South of Pazarcık South of Pazarcık South of Pazarcık East of Birecik East of Birecik East of Birecik West of Urfa West of Urfa SW of Urfa SW of Urfa North of Urfa North of Urfa

9.22 ± 0.2 16.30 ± 0.34 20.27 ± 0.05 18.33 ± 0.27 10.00 ± 0.47 10.26 ± 0.04 9.24 ± 0.19 9.77 ± 0.11 7.02 ± 0.07 16.16 ± 0.35 18.90 ± 0.74 16.32 ± 0.01 16.90 ± 0.33 18.39 ± 0.40 16.94 ± 0.39 Lavas postdate the Oligocene–Lower Miocene Fırat Formation. A single K–Ar age is 12.1 ± 0.4 (Yoldemir, 1987)

21.24 ± 2.04 16.53 ± 0.35, 13.92 ± 0.34 17.74 ± 0.40 Age evidence as for sites 38–42 above

K–Ar date from sites 58–60 is 10.4 ± 0.2 (Yoldemir, 1987) K–Ar age from these sites are 12.1 ± 0.4 and 10.6 ± 0.2 (Yoldemir, 1987) 7–8 Ma (Ulu et al., 1991) Western extension of the Karacadağ volcanic complex. Nearby lavas dated 0.94 ± 0.33 and 0.83 ± 0.88 (Pearce et al., 1990).

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Şaroğlu, 1987). Lava successions on the Arabian platform comprise thin cover on Tertiary and Mesozoic platformal sediments and are stepwise down faulted by the EHF and related faults at the western margin of the study region into the Karasu rift. Further to the east lavas comprise extensive cover in the Kilis–Gaziantep region and several outcrops to the east of the Euphrates River where a lava plain is exposed mainly within gullies leading into the major river course. Palaeomagnetic sites were sampled, typically by 7 distributed cores, using a portable drilling motor with cores oriented in situ using both magnetic and sun compasses. From field observation most units sampled were considered to be quasi horizontal or to have flowed down primary slopes. Exceptions are units 31 to 36 NW of Gaziantep which border the continuation of the Hatay Fault Zone and are closest to the putative DSFZ–EAFZ FFF triple junction; the ground here is dissected into fault blocks which are variably tilted and palaeomagnetic results are accordingly adjusted for dip. Most remaining sites are considered in situ. Site data and locations are summarised in Table 1 and shown in Fig. 2. Published evidence for the age of these volcanic events is sparse (Yoldemir, 1987; Ulu et al., 1991) and to improve this constraint we have conducted K–Ar dating on samples from a number of the palaeomagnetic sites; results are summarised in Table 2. It is apparent that the lavas in the Kilis and Gaziantep regions are mid-Miocene in age (Serravallian–Tortonian, average age = 14.9 Ma, SD = 1.9 Myr) but become somewhat older (Burdigalian) in age to the north of Gaziantep. There is some evidence for bimodality in these outcrops to the west of the Euphrates valley: sites 8–22 have a lower mean age of 12.3 Ma (SD = 4.4 Myr) whilst lavas sites 31–57 sited mostly further north in the Gaziantep region have a mean age of 17.3 Ma (SD = 1.0 Myr). To the east of the Euphrates valley evidence for the age of the lava fields west of Urfa is limited (Table 2) but at ~10.4– 12.1 Ma is apparently of comparable (late Seravallian–Tortonian) age to the Kilis volcanics. The Seravallian age is contemporaneous with

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initiation of the uplift of the Anatolian Plateau to the north (Dewey et al., 1986). The last sites of this study (80–83) are morphologically a southern extension of the Karacadağ volcanic complex a large shield volcano located on the southern side of the collisional boundary. Pearce et al. (1990) reported geochemical data indicating a derivation from mantle lithosphere enriched by small volumes of asthenospheric melts. K–Ar age data (Sanver, 1968; Pearce et al., 1990; Notsu et al., 1995) identify volcanic activity concentrated here in Late Pliocene and younger times with at least three eruptive periods at ca. 1.9, 1.5–1.0 and 0.3–0.1 Ma (Notsu et al., 1995 and Table 2); from 87Sr:86Sr ratios these latter authors attribute the Karacadag complex to a mantle source with little or no crustal assimilation. 5. Palaeomagnetic results Total Natural Remanent Magnetisations (NRMs) were measured using ‘Minispin’ magnetometers. Intensities of NRM vary by more than two orders of magnitude but are strong in all units, as is typical of young basalt lavas. Approximately 20% of the collection was thermally demagnetised using temperature steps of 100 °C reduced to 25 °C steps near the Curie point and applied in Magnetic Measurements MM2D demagnetisers; the remainder were demagnetised using a range of alternating fields (a.f.) to peak fields between 100 and 140 milliTesla (mT). With few exceptions sample magnetisations are highly stable to progressive demagnetisation and, often following removal of a viscous component, illustrate long linear trajectories converging to the origin of orthogonal projections (Figs. 3 and 4); sometimes samples with reversed magnetisations illustrate the removal of a normal component in the present field or more rarely normally magnetised samples have a reversed component removed first (Fig. 4). Characteristic Remanent Magnetisations (ChRMs) were resolved from the orthogonal projections and their equivalent directions calculated by Principal Component Analysis. Within-site

Fig. 3. Orthogonal projections of the magnetization vector during progressive thermal (samples 1–2 and 10–5) and alternating field (a.f.) demagnetisation of representative samples of the palaeomagnetic collection. The magnetization vector is projected onto the horizontal (closed circles) and vertical (open circles) planes. Units of magnetization are × 10− 5 A m2/kg and treatment fields are in milliTesla or temperatures in °C.

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H. Gürsoy et al. / Tectonophysics 465 (2009) 114–127

Fig. 4. Orthogonal projections of the magnetization vector during progressive alternating field (a.f.) and thermal demagnetisation. Symbols are as for Fig. 3.

groupings of components were then evaluated and means calculated using standard Fisher (1953) analysis. Results are summarised in Table 3 where the tight within-site dispersions and alpha 95 angles of no more than a few degrees achieved from grouping data from dispersed samples within sites indicate that strong local fields over strongly magnetised young lava (which may have the potential for deflecting the field from the local direction during cooling, see Valet and Soler, 1999) are not a significant problem here. The reference normal and reversed directions of the geomagnetic field in this region are D/I = 0/56.5° and D/I = 180/−56.5° respectively and cursory examination of the site mean data in Table 3 shows a mixture of normal and reversed polarities with the latter predominating (29 normal, 43 reversed); since the field was slightly biased towards normal polarity (54%) during the ~ 10–20 Ma interval embraced by the bulk of the volcanism represented here (Cande and Kent, 1995), it was evidently episodic rather than uniform. Although entirely postdating the collisional deformation, these volcanics are locally influenced by neotectonic faulting; although we are unable to rule out the influence of fault-related tilting on some of these directions we believe that our field assessment has accommodated subsequent tilting in large measure and the magnitude of residual effects is likely to be small. A few site mean directions are clearly anomalous and probably record transitions of the palaeofield between polarities; on the assumption that deviations from the ambient field by N45° are a clear indication of this, 10 sites belong to this group. Individual sites represent rapidly-chilled volcanic units that are expected to record palaeofield components of the secular variation of geomagnetic rather than have longer term palaeomagnetic significance; in this circumstance a number of site directions are always required to yield a time-averaged mean for tectonic analysis. This is satisfied here by the larger collections from the Kilis, Gaziantep and Urfa regions and additionally lavas of both polarities are found in each area confirming that volcanic activity spanned more than one polarity

chron in each case. The angle between the normal and reversed mean directions from the Kilis lavas is 2.7° yielding a Class C reversal test applying the McFadden and McElhinny (1990) reversal test (critical angle = 18.5°, P = 0.920). The angle between the normal and reversed means from the Gaziantep lavas is 9.3° yielding a Class C reversal test applying the same test (critical angle = 18.9°, P = 0.476). In contrast the angle between the normal and reversed means from the Urfa lavas is relatively large (13.6°) and yields a negative reversal test when the reversal test is applied (critical angle = 13.5°, P = 0.047). Scrutiny of the site mean directions shows that the bulk of the site mean directions are rotated anticlockwise with respect to the present field direction with the exception of sites within the northern extension of the Karasu Rift at the western extremity of the investigated region (sites 23–30) where rotations are clockwise. The mean of this latter group (D/I = 12.5/50.1°, α95 = 4.7°, 5 reversed and 2 normal sites) is rotated slightly clockwise with respect to the reference field direction predicted from Eurasia (Table 4). It compares closely with the mean direction from Brunhes chron volcanics from the Karasu Rift to the south (D/I = 8.8/54.7°, Tatar et al., 2004) and confirms that this northern extension is tectonically integral with the remainder of the rift; a comparable explanation in terms of block rotation between the AFZ and EHF motivated by indentation and clockwise rotation of the Arabian Plate (Tatar et al., 2004) is presumed to apply here. In contrast, the bulk of the rotations to the east of the EHF are distinctly anticlockwise (Table 3 and Fig. 5) with respect to the predicted field direction in mid-Miocene times (Table 4). Sites from the volcanic outcrops west of the Euphrates in the Kilis and Gaziantep region are considered collectively in Table 3 and Fig. 5. Reversely magnetised sites yield a mean D/I = 173.4/−53.0° (20 sites, α95 = 6.3°) which defines an axis not statistically different from the normal sites yielding a mean of D/I = 353.8/49.9° (18 sites, α95 = 9.6°). Hence both polarities may be considered collectively to define a mean direction of

H. Gürsoy et al. / Tectonophysics 465 (2009) 114–127 Table 3 Palaeomagnetic results from volcanic rocks, northern margin of the Arabian Shield in Southern Turkey Site no.

Polarity

(i) Kilis Region 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

D

I

N 354.7 48.5 N 33.8a 57.0a R 182.4a − 42.2a R 165.6a − 62.0a N 357.9 42.1 N 359.3 43.7 a N 334.5 57.4a I 109.9 − 72.1 60.5a N 14.7a N 331.0 69.0 I 156.2 − 13.5 N 302.3 73.6 R 170.9 − 24.5 R 174.7 − 75.2 R 183.5 − 71.1 I 77.9 − 32.2 R 186.7 − 62.1 No groupings recognised I 223.4a − 22.4a N 357.1 21.3 N 8.7 49.7 N 12.1 54.0

(ii) Western Kilis Region (north extension of the Karasu Rift) 23 R 193.9 − 52.7 24 R 202.2 − 56.5 25 R 202.2 − 49.5 26 R 189.9 − 48.4 27 N 6.5 41.8 28 R 194.0 − 55.1 29 N 0.4 44.9 30 No groupings recognised

N/n

R

α95

k

7/7 7/6 7/7 7/7 7/6 7/7 7/7 7/7 7/7 7/7 7/7 7/6 7/6 4/4 7/7 7/5 7/6

6.98 5.89 6.98 6.90 5.95 6.95 6.97 6.98 6.98 6.91 6.96 5.99 5.98 3.99 6.97 4.81 5.99

3.5 10.0 3.8 7.7 6.5 5.6 4.2 3.8 3.1 7.3 5.0 2.9 4.5 4.6 4.3 16.9 3.4

293.5 45.5 247.1 63.0 108.1 119.0 210.1 250.3 375.3 69.9 144.2 541.3 225.3 393.3 194.1 21.5 390.3

7/5 8/8 7/5 5/5

4.96 7.93 4.93 4.95

7.3 5.7 10.2 8.6

111.2 95.8 57.1 80.3

7/7 6/6 7/7 5/4 5/5 5/4 7/6

6.97 5.97 6.99 3.93 4.98 3.98 5.99

4.5 5.2 2.9 14.6 4.8 7.5 3.6

179.9 166.1 435.8 40.4 260.1 152.7 339.8

(iii) Gaziantep Region 31 32 33 34 35 36 37 38 39 40 41 42 43 44

N N R R R I I R N R I R R R

352.5a 354.6a 149.5a 178.2a 169.4a 317.5a 301.4 187.0 29.5 158.4 195.8 124.9 183.6 183.2

22.7a 21.6a − 51.3a − 64.0a − 52.4a − 13.7a − 8.9 − 45.8 78.9 − 49.8 44.1 − 67.3 − 44.4 − 44.7

7/7 7/7 7/7 6/6 7/7 6/6 7/7 7/7 7/5 7/6 7/5 7/6 6/6 5/5

6.99 6.95 6.99 5.96 6.98 5.95 6.90 6.97 4.98 5.97 4.62 5.97 5.97 4.96

2.9 5.7 2.2 6.0 3.8 6.8 7.7 4.1 5.9 5.5 24.9 4.9 4.8 7.7

447.7 114.7 733.8 127.2 251.5 97.3 62.1 220.5 170.9 146.8 10.4 191.0 196.9 100.8

(iv) Pazarcık Region 45 46 47 48 49 50 51 52

R R R I N N R R

177.4 188.7 159.9 304.7 21.0 342.1 176.4 181.5

− 50.0 − 61.1 − 49.0 10.0 54.4 33.1 − 56.9 − 39.8

7/7 7/5 7/5 6/6 7/7 7/7 7/7 7/7

6.99 4.89 4.90 5.79 6.95 6.95 6.98 6.99

2.5 12.6 12.0 14.0 5.3 5.6 3.2 2.5

567.4 37.8 41.6 23.7 128.4 118.5 357.1 590.7

(v) Yavuzeli Region 53 54 55 56 57 58 59 60 61 62 63 64 65 66

N 326.7 31.0 N 324.3 48.0 I 43.8 55.3 R 170.5 − 31.2 I 310.0 29.9 N 346.3 67.9 R 161.9 − 49.0 N 353.1 49.6 R 166.3 − 53.7 R 184.4 − 62.1 R 161.6 − 61.2 N 343.3 52.9 R 152.9 − 70.9 No groupings recognised

7/6 7/7 6/5 7/6 6/6 6/6 7/7 5/5 5/5 5/5 5/4 5/5 4/4

5.99 6.98 4.95 5.99 5.99 5.95 6.90 4.93 4.98 4.98 3.99 4.87 3.94

3.5 3.5 9.0 3.1 2.9 7.1 7.9 10.2 5.5 5.5 4.2 13.9 13.5

368.5 298.4 73.1 456.4 535.7 91.1 59.3 57.6 195.9 191.9 472.8 31.1 47.1

(continued on next page)

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Table 3 (continued) N/n

R

α95

k

R 166.2 −46.6 R 156.5 −32.9 No groupings recognised R 165.3 −36.1 R 131.2 −34.4 R 173.8 −40.5

7/7 7/5

6.98 4.89

3.7 12.7

265.5 37.1

7/4 7/7 7/6

3.93 6.98 5.98

14.6 3.1 4.7

40.8 392.0 200.5

(vii) Urfa Region, East 73 74 75 76 77 78 79

N N N R R R N

12.8 359.9 1.2 139.8 152.2 139.1 4.5

55.7 37.3 40.1 −25.1 −49.2 −30.0 40.5

7/7 7/6 5/5 6/6 6/5 4/4 7/7

6.98 5.97 4.95 5.89 4.59 3.96 6.97

3.4 4.8 8.5 10.2 25.9 10.1 4.4

321.7 195.4 81.8 43.8 9.6 83.4 189.4

(viii) Urfa Region, North 80 81 82 83

R R R R

189.4 197.1 188.7 160.5

−36.2 −41.2 −41.6 −54.8

7/7 7/7 8/8 7/7

6.97 6.99 7.98 6.94

4.5 2.4 3.0 6.2

177.2 515.9 353.2 94.4

6.95

5.6

118.8

10.48 5.71 16.19

10.7 16.7 8.2

19.3 17.1 19.7

6.42 13.63 20.00

19.7 6.3 7.3

10.4 35.4 19.9

36.10

5.4

19.5

5.88 12.46 19.16 16.56

10.6 9.0 7.0 6.0

41.0 22.08 22.5 36.7

6.91

7.4

66.9

Site no.

Polarity

(v) Yavuzeli Region 67 68 69 70 71 72

D

I

Group mean calculations: (i) North Karasu Rift Lavas (sites 23–30) (2N, 5R) 12.2 50.1 7 (ii) Kilis Lavas, Arabian Shield West of Euphrates Valley Normal sites 358.7 54.1 11 Reversed sites 176.7 −56.6 6 All sites 358.0 55.0 17 (iii) Gaziantep Lavas, Arabian Shield West of Euphrates Valley Normal sites 347.5 42.7 7 Reversed sites 172.1 −51.5 14 All sites 350.5 48.7 21 (iv) All Miocene Lavas, Arabian Shield West of Euphrates Valley 353.6 51.6 38 Palaeomagnetic pole: 265.5°E, 82.8°N (dp/dm = 5.0/7.4°) (v) Urfa District Lavas, Arabian Shield East of Euphrates Valley Normal sites 356.1 48.3 6 Reversed sites 156.2 −46.4 13 All sites 343.7 48.0 20 Sites excluding 71, 76 and 78 350.0 50.3 17 Palaeomagnetic pole: 275.5°E, 79.7°N (dp/dm = 5.4/8.0°) b (vi) Brunhes–Matuyama chron lavas, NE of Urfa 8.4 44.7 7

D/I is the mean direction derived from n samples out of a total site population of N measured samples yielding a resultant vector of length R. Alpha95 is the radius of the cone of 95% confidence about the mean direction and k is the Fisher precision parameter. Polarities are N = Normal, R = Reversed, I = Intermediate. a Directions adjusted for tilt; the remaining units are horizontal or considered to have been emplaced on primary slopes. b Sites 80–82 of this study combined with 4 sites from the same area listed by Sanver (1968).

D/I = 353.6/51.6°, α95 = 5.4°). Sites from outcrops in the Urfa region east of the Euphrates valley show a distinct difference in declination between the axis defined by the group of lavas with normal polarity Table 4 Predicted normal polarity palaeofield directions in the study region computed from African and Eurasian apparent polar wander paths Time window (Ma)

α95

D/I

171.6 160.8 151.9

2.6 2.0 2.7

3.0/53.9 4.0/54.1 5.1/54.4

172.0 162.5 154.8

2.6 2.0 2.7

3.2/53.7 4.5/53.6 6.4/53.4

Pole position °N

°E

(i) Africa 0–10 10–20 20–30

86.5 86.0 85.4

(ii) Eurasia 0–10 10–20 20–30

86.3 85.4 84.0

Reference poles are from Besse and Courtillot (2002). The site location used is the centre of the Kilis–Gaziantep region (37.0°E, 36.9°N).

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Fig. 5. Stereographic projects showing distributions of site mean directions from the three regional divisions of the palaeomagnetic collection discussed in the text. Open circles are upper hemisphere plots and closed circles are lower hemisphere plots. The stars are the direction of the present day field axis in this region and the crosses are the mean axis of magnetisation defined by the overall groupings of normal and reversely magnetised sites comprising each collection.

(D/I = 358.2/48.0°, α95 = 9.4°) and reversed polarity (D/I = 156.1/−46.4°, α95 = 9.1°) although the former population is relatively small and comprises only 7 units. The combined result is D/I = 343.6/48.0° (α95 = 7.0°) suggesting anticlockwise rotation by a somewhat larger amount than the sites to the west although the difference is within confidence limits and as discussed below, this result appears to be biased by three anomalous sites. 6. Discussion Tectonic rotation, R′, in deformed terranes is determined by comparison of the mean directions with the predicted palaeofield axis from the adjoining stable plates. Both observed and reference directions have confidence limits (ΔD and ΔDref respectively). Beck (1980) proposed a confidence limit on R′ defined by ΔR′ = √(ΔD2 + ΔD2ref). Demarest (1983) concluded that this value overestimates errors and showed that, provided α95 is small and preferably less than 10°, a standard correction factor is applicable. For n ≥ 6 this correction factor lies between 0.78 and 0.80; ΔR′ can then be derived from the equation ΔR′ = 0.8√(ΔD2 + ΔD2ref). Only if R′ N ΔR′ can a rotation be regarded as significant. Group mean directions are compared with reference palaeofield directions in this region calculated from the apparent polar wander path of plates bordering the Alpine Himalayan orogen (Besse and Courtillot, 2002). Eurasia is the appropriate comparator here because the Bitlis suture between Arabian and the Anatolian collage had fully closed by the time of the volcanic activity

and subsequent deformation is represented by intracontinental transform fault movement and block rotations within the collage whereas movements relative to Africa incorporate the opening of the Red Sea; nevertheless there is little contrast with palaeofield directions predicted from equivalent African data during the time interval in question (Table 4). We have no evidence for the age of sites 23–30 at the northern extension of the Karasu Rift other than morphologic observation for their youth but from comparison with lavas infilling the rift further to the south it is anticipated that they are young and were emplaced late in the Matuyama Chron and in the Brunhes Chron (Tatar et al., 2004). Comparison with the youngest palaeofield direction from Eurasia then yields a clockwise rotation of 12.5 ± 5.6° statistically identical to the clockwise block rotation observed within this rift further to the south by Tatar et al. (8.8 ± 4.4°). The mid-Miocene Kilis–Gaziantep volcanics west of the Euphrates Valley are compared with the 10–20 Ma mean field direction derived from Eurasia and identify anticlockwise rotation of 10.9 ± 4.3°. The volcanics of comparable age east of the Euphrates Valley yield an anticlockwise rotation of 20.9 ± 5.8°; thus it is possible that anticlockwise rotation increases in magnitude to the east and incorporates internal deformation at this margin of the platform. However, the latter result is biased by three sites (71, 76 and 78) with more easterly declinations and relatively shallow inclinations; although the lava outcrop is now fragmented by erosion (Fig. 2) it is possible that these sites with comparable directions of magnetisation record the same

H. Gürsoy et al. / Tectonophysics 465 (2009) 114–127

volcanic event. Furthermore sites 73–79 are sited in the Akçakale Graben south of Urfa, a N–S graben connected to a right-lateral NW– SE strike slip fault and interpreted as an impactogen on the Arabian platform by Şengör and Yılmaz (1981); it is therefore also possible that unaccommodated deformation has influenced these sites. When these three sites are excluded the mean is revised to D/I = 350.0/−50.3° (N = 17, α95 = 6.0°). This tighter grouping of site mean directions does not quite produce a positive reversal test (angle between normal and reversed axes = 12.1° compared with a critical angle of 11.7°) but yields a mean which is statistically identical to the volcanics of similar age from the west of the Euphrates and supports the view that this plate margin has been integral since mid-Miocene times. Anticlockwise rotation of Arabia since ~11.5 Ma deduced from the Urfa volcanics is then 14.0 ± 5.0°. The supposition that these three collections have recorded essentially the same mid-Miocene palaeofield and have been subject to a common later rotation will be valid if they have each averaged palaeosecular variation and any significant internal dispersion due to later deformation has been appropriately corrected for. This may be tested by comparing the dispersions of directions with predicted geomagnetic field dispersions at this latitude. The circular standard deviation (S ≈ δ63) for the distribution of virtual geomagnetic poles (VGPs) from the Kilis Lavas is 16.6° with a 95% confidence interval between 14.0° and 20.6° (Cox, 1969). This compares closely with a value of approximately 19° and lower and upper values of 17.7° and 20.5° at latitude 30° between 5 and 22 Ma for palaeosecular variation model G of McFadden et al. (1991, there are no data for the 30–40° latitudinal band) and implies that secular variation is appropriately represented in this dual polarity collection. The circular standard deviation for the distribution of Gaziantep VGPs is 18.4° with a 95% confidence interval between 15.4° and 22.8° which again compares with expected secular variation at latitude 30° between 5 and 22 Ma for the G model. The circular standard deviation of the Urfa VGPs is 17.6° with a 95% confidence interval between 15.4° and 20.6° and is also comparable to the Sλ of approximately 19° for palaeosecular variation model G. Thus in each case these dual polarity collections appear to have properly recorded secular variation in mid-Miocene times without obvious between-site tectonic disturbance. A previous result from just 20 samples in two lavas from the Gaziantep region (Kissel et al., 2003) yielded a mean D/I = 328/34° which is evidently unrealistic for evaluating tectonic rotation here although a result of these authors from Miocene marls near Antakya of D/I = 347/43° appears to yield an estimate representative of the true rotation at this margin of the Arabian Block (Table 5).

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We can draw few specific conclusions from the lavas at the southern margin of the Karacadağ volcano (southeast of the Bitlis suture zone) other than to note that these are young and sites 80–82 imply clockwise rotation; site 83 suggesting anticlockwise rotation is from the vicinity of the Atatürk Dam and removed to the west of the other three sites (Fig. 2). Sites 80–82 are a preliminary indication that indentation at the north eastern extension of the Arabian Plate is producing clockwise rotation of terranes by tectonic escape to the east in this region (Figs. 1 and 2). Currently few palaeomagnetic results have been reported from volcanic fields to the east and north of those studied here. Sanver (1968) recorded results from 4 Brunhes–Matuyama chron lavas (sites B18, BT22, 26 and 30) from the region NE of Urfa and close to sites 80–82. The collective group mean is D/I = 8.4/44.7° (Table 3) which, together with results reported from Sanver (1968) from 12 flows of comparable young age from the Van and Ararat regions further to the north east (mean D/I = 12.9/53.4°), supports the view that clockwise rotation has characterised the region on the east side of the Arabian syntaxis. Two of the major lava collections of this study (Kilis and Gaziantep) have mean magnetic inclinations very close to the ~20 Ma predicted field inclination of 54° (c.f. Tables 3 and 4) and in no case is the difference significant at the 95% confidence level (Kilis: I = 55.0°, α95 = 8.2°, Gaziantep: I = 51.6°, α95 = 5.4° and Urfa: I = 50.3°, α95 = 6.0°). Thus inclination shallowing is not clearly evident in this sector of Anatolia as was also found in the Karasu Rift by Tatar et al. (2004). This contrasts with results further to the west in Turkey and in the Aegean where this effect is larger (Beck et al., 2001; Piper et al., 2006). Furthermore there are no apparent differences in detectable shallowing between mid-Miocene and late Matuyama–Brunhes chron results indicating that post-mid-Miocene northward movement of the Arabian Shield is below the limits of palaeomagnetic detection. The phenomenon of inclination shallowing, widely observed elsewhere within the Alpine–Himalayan Orogen, is extensively discussed in the literature (see for example Cogné et al., 1999 and Beck et al., 2001) and specific possible causes of the phenomenon in igneous rocks has recently been discussed by Krijgsman and Tauxe (2004). When it has been observed the flattening effect in young lava successions is analogous, although lower in scale, to the inclination shallowing produced by the range of syn- and post-depositional effects in sediments. Tauxe and Kent (2004) provide a test for isolating this effect from departures from the geocentric axial dipole (GAD) source which they achieve by comparing an elongation parameter E defined by the ratio of eigenvectors of the directional distribution with the elongations predicted from palaeosecular variation models at

Table 5 Summary of 0–20 Ma palaeomagnetic results from the Arabian Plate and adjoining margin No.

Rock unit

Age

Lat./Long.

D/I

Pol:

α95

Ref:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Volcanics, Syriaa Basalts, Jordana Basalts, Saudi Arabiaa Basalts, Lebanon Volcanics, Israel Basalts, Syria Volcanics, Yemena Volcanics, Adena Hatrium Fm., Israel Intrusives, Sinai Volcanics, Gaziantepa Marls, Antakyaa Kilis Volcanicsa Gaziantep Volcanicsa Volcanics, E. Euphratesa Volcanics, NE Urfad

19.5b Pl-m,0–2 18.7c Plu,2–4 Pl,0–5 2–5 5–10c 5–10c 2–5c 15–34c 5–23 5–23 M, 7–20,(12.3) M,16–10,(17.3) M,10.4–12.1,(11.3) 0–2.5

33.3/36.3 31.4/35.8 21.9/39.3 34.0/36.0 33.5/35.5 35.0/36.0 13.0/45.0 12.8/45.0 31.5/35.3 28.5/33.9 36.9/37.2 36.3/36.1 ~ 37.4/36.8 ~ 37.3/37.5 ~ 38.7/36.7 ~ 37.5/39.0

359/23.1 353/31 1.2/33.7 1.6/46.2 3.5/38.3 350.1/36.2 353.9/20.2 353.0/24.0 8.1/49.1 3.0/39.0 328.0/34.0 347.0/43.0 356.8/51.5 350.5/48.7 350.0/50.3 8.4/44.7

M M M M M R M M M N M M M M M M

5.9 4.9 5.2 7.7 20.4 10.3 6.3 3.0 15.3 15.3 – 7.6 8.3 7.3 6.0 7.4

783 1189 2887 2961 2961 3391 3405 3638 8580 8580 9001 9003 This paper This paper This paper This paper

a b c d

Results from stable shield. Ar39–Ar40. K–Ar, other ages are stratigraphic; Pl = Pliocene. Polarities are N = normal, R =reversed, M =mixed. The reference numbers are assigned in the Global Palaeomagnetic Database (GPDB). Mean calculation includes sites from Sanver (1968). Mean ages shown in brackets for results 13–15 are those used in Fig. 7.

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different latitudes. The asymmetry of the data set is assumed to be caused by an “inclination error” f acting on the distribution according to tan(I) = (1/f) tan(I⁎) where I⁎ is the transformed inclination. The TK03 model predicts and elongation of 1.6 for the Miocene palaeolatitude of this region; elongations in the Kilia, Gaziantep and Urfa collections deteriorate from this when f values of less than 1–0.95 supporting the inference from their mean inclinations that they are reliable records of the palaeofield with declinations influenced only by vertical axis rotation (Fig. 5). Since the primary results of this study come from volcanics of midMiocene age we are unable to resolve from these data alone whether the rotation of the Arabian Shield since their emplacement has occurred uniformly or whether this rotation has been episodic. To help clarify this point we summarise in Table 5 other palaeomagnetic results from the Arabian Plate and immediately adjoining regions (Fig. 6). Results 1 and 3 are linked to single age dates and indicative of no rotation; they are evidently unrepresentative of the remaining data. Substantive palaeo-

magnetic studies 7, 8 and 13–15 from igneous suites all show small anticlockwise rotations between 3 and 10° with no apparent temporal trend through Middle and Upper Miocene times (Fig. 7). The implication is that the Arabian Plate did not begin to rotate following closure of the BSZ in mid-Miocene times until late in the Miocene and possible in early Pliocene times (i.e. after units 7 and 8 from the south western sector of Arabia). Subsequent anticlockwise rotation has therefore occurred at a minimum rate of ~1°/Myr. During this interval the rotation of the African Plate has been slightly clockwise (c.f. Table 4 and Besse and Courtillot, 2002) and the differential motion has been taken up by opening of the Red Sea and sinistral slip along the DSFZ. Since the rate of motion on the DSFZ has not been uniform (Section 2) and opening of Red Sea has been episodic (Gass, 1977), anticlockwise rotation of Arabia is unlikely to have been uniform in detail. Fission track data indicate that break-up of Arabia from Africa took place in two stages of uplift, erosion and extension beginning at ~34 Ma (Omar and Steckler, 1995). The plates were apparently not

Fig. 6. Outline tectonic map of the Arabian Block and adjoining regions with mean magnetic declinations of normal magnetisation plotted with 95% confidence limits (see Table 5). Note that for clarity only some results outside of the integral plate are plotted on this figure.

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Turkey shows that they are of mid-Miocene age and have been rotated anticlockwise by between 10 and 14° with respect to the Eurasian Plate since emplacement. Middle to Upper Miocene directions of magnetisations for igneous suites from Arabia all show small anticlockwise rotations with no temporal trends. It is therefore concluded that Arabia did not begin to rotate anticlockwise following the closure of the Bitlis Suture until after Late Miocene times. Subsequent rotation has occurred at a minimum rate of 1°/Myr which is comparable to the contemporaneous rate deduced from GPS (McClusky et al., 2000). Rotation of Arabia is therefore shown to correlate with the initiation of the NAFZ and EAFZ transforms within Anatolia and with the ensuing regime of tectonic escape to the west. Rotational transpression of Arabia into blocks at the western margin of the shield south of the FFF junction is producing clockwise rotation of blocks at the shield margin in the Karasu Rift at the northern extension of the DSFZ whilst the continuing northward motion of the shield is the motivator of tectonic escape of the Anatolian accretionary collage to the west along the north western margin of the indenter as expressed in differential anticlockwise block rotations (Piper et al., 2006). Along the margin of the north eastern promontory of the Arabian Plate rotations appear to be clockwise as terranes are expelled to the east on this side of the syntaxis; this contrast with extrusion of terranes towards the Mediterranean on the western side is currently based on few data and wider palaeomagnetic investigation of neotectonic volcanism in eastern Turkey is required to evaluate this further. Acknowledgements

Fig. 7. Magnetic declination as a function of time as shown by palaeomagnetic results from the Arabian Plate. The numbers refer to the listing of results in Table 5.

unzipped but opened as rigid bodies hinged to the south. Subsequent separation seems to have involved three phases (Gass, 1977; Sultan et al., 1993; Bosworth et al., 2002). The first was the formation of a half graben in early Miocene times with 22–24 Ma dykes and fault kinematics indicating a N55°E extension direction; this phase correlates with initiation of the DSFZ (Eyal et al., 1981). Development of a triple junction at the south end of the Gulf of Suez at 12–14 Ma led to a second extension direction of N15°E after 12–14 Ma and oblique rifting parallel to the Gulf of Aquaba (Bartov et al., 1980). Finally the axial trough has developed during late Miocene to Recent times (b4– 5 Ma) with attenuation of continental crust giving way to emplacement of oceanic crust; in the Gulf of Aden this has also accompanied differential rotation of the Horn of Africa. Thus the palaeomagnetically observed rotation of Arabia correlates with this last phase (Fig. 7) and also corresponds to the timing of initiation of the NAFZ and EAFZ intracontinental transforms within Anatolia and subsequent regime of tectonic escape (Piper et al., 2006). The pole of rotation of the Arabian Plate relative to Africa (6.7° anticlockwise about a pole at 18.1°E, 34.6°N) is derived from matching geological features across the Red Sea (Sultan et al., 1993). Although the proximity of this pole to the pole indicated for total motion along the DSFZ (Joffe and Garfunkel, 1987) would suggest that motion between Arabia and NE Africa has on average been parallel to total motion along the DSFZ, the rotation of Arabia after 7 Myr implies that accommodation along the DSFZ must be more complex in detail and involve transtension in the south and transpression in the north (Tatar et al., 2004). 7. Conclusions Palaeomagnetic investigation and age dating of volcanic fields emplaced along the northern rim of the Arabian Shield in southern

This study was carried out as part of a NATO supported investigation of neotectonics in central-southern Turkey and we are grateful to NATO (Grant No: CLG-EST-977055) and TUBITAK (Grant YDABAG 101Y023 and ÇAYDAG 104Y262) for supporting the field and laboratory studies. The British Council is thanked for supporting academic links between the Geomagnetism Laboratory of the Department of Earth and Ocean Sciences at Liverpool and the Department of Geology, Cumhuriyet University, Sivas. We are grateful to Mike Sandiford and an anonymous reviewer whose comments helped to materially improve the manuscript. References Adıyaman, Ö., Chorowicz, J., 2002. Late Cenozoic tectonics and volcanism in the northwestern corner of the Arabian Plate: a consequence of the strike slip Dead Sea fault zone and the lateral escape of Anatolia. J. Volcanol. Geotherm. Res., V. 117, 327–345. Arpat, A., Şaroğlu, F., 1972. The East Anatolian fault system: thoughts on its development. Bull. Miner. Res. Expl. Inst. Turk. 79, 33–39. Aydar, E., Gourgaud, A., Ulusoy, İ., Digonnet, F., Labazuy, P., Sen, E., Bayhan, H., Kurttas, T., Tolluoğlu, A.Ü., 2003. Morphological analysis of active Mount Nemrut stratovolcano, eastern Turkey: evidences and possible impact areas of future eruption. J. Volcanol. Geotherm. Res. 123, 301–312. Badawy, A., Horvath, F., 1999. The Sinai subplate and tectonic evolution of the northern Red Sea region. J. Geodyn. 27, 433–450. Barka, A., Reilinger, R., 1997. Active tectonics of the Eastern Mediterranean region: deduced from GPS, neotectonic and seismicity data. Annali Di Geofisica 40, 587–610. Bartov, Y., Steinitz, G., Eyal, M., Eyal, Y., 1980. Sinistral movement along the Gulf of Aqaba— its age and relation to the opening of the Red Sea. Nature 285, 220–222. Beck, M.E., 1980. Palaeomagnetic record of plate margin processes along the western edge of North America. J. Geophys. Res. 87, 7115–7131. Beck, M.E., Burmester, R.F., Kondopoulou, P., Atzemoglou, A., 2001. The palaeomagnetism of Lesbos, NE Aegean, and the eastern Mediterranean inclination anomaly. Geophys. J. Int. 145, 233–245. Besse, J., Courtillot, V., 2002. Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr. J. Geophys. Res. 107 (B11), 2300. doi:10.1029/20000JB000050. Boulton, S.J. and Robertson, A.H.F., 2008. The Neogene-Recent Hatay Graben, South Central Turkey: graben formation in a setting of oblique extension (transtension) related to post-collisional tectonic escape. Geol. Mag. Published online by Cambridge University Press 11 Jun 2008. doi:10.1017/S0016756808005013. Bosworth, W., Smith, D.A., Carlson, K.W., Barnard, J.J., Raslan, M.F., 2002. Opening history and structural evolution of the Northern Red Sea based on integration of

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