Ar-Ar dating of late Cenozoic basaltic volcanism in northern Syria: Implications for the history of incision by the River Euphrates and uplift of the northern Arabian Platform

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Ar-Ar dating of late Cenozoic basaltic volcanism in northern Syria: Implications for the history of incision by the River Euphrates and uplift of the northern Arabian Platform
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  Ar-Ar dating of late Cenozoic basaltic volcanism in northern Syria:Implications for the history of incision by the River Euphrates anduplift of the northern Arabian Platform Tuncer Demir, 1 Rob Westaway, 2,3 David Bridgland, 4 Malcolm Pringle, 5,6 Sema Yurtmen, 7,8 Anthony Beck, 9,10 and George Rowbotham 11 Received 27 February 2006; revised 1 December 2006; accepted 19 February 2007; published 14 June 2007. [ 1 ] Ar-Ar dating of basalt flows capping terracedeposits of the River Euphrates in northern Syria has provided a new quantitative chronology for the lateCenozoic evolution of this important river system andfor the associated history of surface uplift of thenorthern Arabian Platform through which it flows, aregion of relatively strong crust that has experiencedonly slow deformation. Notably, fluvial deposits  65 m above the Euphrates are overlain by basalt dated to 2717 ± 20 ka, those  45 m above the river areoverlain by basalt dated to 2116 ± 39 ka, and those 8– 9 m above the river are overlain by basalt dated to402 ± 11 ka. These new dates require the previousdating scheme, based on Paleolithic archaeology, to berevised; the Euphrates terrace deposits and theassociated incised valley are much older than was previously thought. Rates of incision by the Euphrates, providing a proxy for regional surface uplift that isinferred to be the isostatic response to regionalerosion, have varied significantly over the past   3 Ma, with indications that between   1.2 and  0.9 Ma, there was regional subsidence, which gaverise to fluvial aggradation. This unusual pattern,involving reversals in the sense of vertical crustalmotions, is interpreted to be a consequence of arelatively cold and thin mobile lower crustal layer, nomore than  5 km thick, evidently due to the presenceof a much thicker underlying layer of maficunderplating at the base of the crust. This study thusindicates previously unsuspected complexity in theisostatic response to regional erosion in an area of high crustal stability.  Citation:  Demir, T., R. Westaway,D.Bridgland,M.Pringle,S.Yurtmen,A.Beck,andG.Rowbotham(2007), Ar-Ar dating of late Cenozoic basaltic volcanism innorthern Syria: Implications for the history of incision by the River Euphrates and uplift of the northern Arabian Platform,  Tectonics , 26  , TC3012, doi:10.1029/2006TC001959. 1. Introduction [ 2 ] This paper reports the first isotopic dating of the basaltic volcanism in northern Syria, at sites along the River Euphrates upstream of Deir ez-Zor (Figure 1). With a lengthof    2800 km, the Euphrates (F ƒ rat in Turkey; Furat inSyria) is the longest river in SWAsia. From its source in NETurkey it flows initially west and south, then SE across theArabian Platform (in SE Turkey, northern Syria and westernIraq; Figure 1) to the Persian Gulf, >1000 km downstreamfrom the study region. The Arabian Platform was a marinedepocenter within the Tethyan Sea (linking the Mediterra-nean Sea and Indian Ocean) until the middle late Miocene[e.g.,  Lovelock  , 1984], although early middle Miocenefluvial sands and gravels are widespread near its northernmargin around Kahramanmaras¸ [e.g.,  Derman , 1999](Figure 1). Their diverse clast content, including metamor- phic lithologies not found in situ in the Arabian Platform,indicates that a major river, draining at least part of themodern upper Euphrates catchment, debouched into theTethyan Sea here at this time (stage 1 in Figure 1).[ 3 ] Across much of the northern Arabian Platform, thetransition from a marine depocenter to a subaerial landscape postdated the deposition of early Miocene marine limestone.Around Kahramanmaras¸, it is marked by basaltic andesitevolcanism, for which early middle Miocene K-Ar dates (all±2 s  ) of 18.6 ± 0.8 Ma, 17.1 ± 0.8 Ma, 16.5 ± 0.6 Ma [  Arger et al. , 2000], 19.1 ± 1.3 Ma and 17.0 ± 0.7 Ma [ Tatar et al. ,2004] have been obtained. At some localities, notably(again) around Kahramanmaras¸, fluvial gravels that weredeposited at about this time interdigitate with shallowmarine deposits and with lava flows [e.g.,  Karig and Kozlu ,1990;  Derman , 1999], indicating the effect of cyclicchanges in global sea level during emergence of the landsurface. However, the crests of some anticlines are capped TECTONICS, VOL. 26, TC3012, doi:10.1029/2006TC001959, 2007 1 Department of Geography, Harran University, S¸anl ƒ urfa, Turkey. 2 Faculty of Mathematics and Computing, Open University, Newcastle-upon-Tyne, UK. 3 Also at School of Civil Engineering and Geosciences, NewcastleUniversity, Newcastle-upon-Tyne, UK. 4 Department of Geography, Durham University, Durham, UK. 5 Scottish Universities’ Environmental Research Centre, Glasgow, UK. 6  Now at Laboratory for Noble Gas Geochronology, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. 7 Department of Geology, C¸ukurova University, Adana, Turkey. 8  Now at Newcastle-under-Lyme, UK. 9 Department of Geography, Durham University, Durham, UK. 10  Now at School of Computing, University of Leeds, Leeds, UK. 11 School of Earth Sciences and Geography, Keele University,Keele, UK.Copyright 2007 by the American Geophysical Union.0278-7407/07/2006TC001959 TC3012  1 of 30   by reef deposits of Oligocene or early Miocene age,implying local emergence above sea level earlier than for the landscape as a whole [e.g.,  Rigo de Righi and Cortesini ,1964]. Some of the largest of these mid-Cenozoic anticlines,overlying blind reverse faults, form the Mardin Dag˘ uplandsin SE Turkey, the Jebel Bishri and Jebel Abd al-Azizuplands in northern Syria, the Jebel Sinjar uplands in NWIraq, and the Jebel at’Tadmuriyeh uplands or ‘‘PalmyraFoldbelt’’ in central Syria (Figure 1). Many other basalt flows that likewise postdate the regional emergence of northern Syria have been mapped [e.g.,  Ponikarov and  Mikhailov  , 1964], although none has previously been dated.[ 4 ] Previous studies have revealed extensive Euphratesterrace deposits between Birecik in SE Turkey and the Syria-Iraq border [e.g.,  Besanc¸on and Sanlaville , 1981;  Minzoni- Deroche and Sanlaville , 1988;  Besanc¸on and Geyer  , 2003],as well as farther downstream in Iraq [ Tyra´cˇek  , 1987]. InSE Turkey, the oldest and highest of these exceed 200 mabove present river level. No direct age control evidencehas been provided previously for any locality in this reach,although ages have been suggested for Euphrates terracesin Syria and southern Turkey as part of a regionalcorrelation scheme based partly on Paleolithic artifact assemblages from the gravels. In western Iraq,  Tyra´cˇek  [1987] suggested that the observed change from a low-relief landscape to an increasingly entrenched river valley,as revealed by the geometry and disposition of the preserved terraces, marked the start of the middle Pleisto- Figure 1.  Regional map showing the location of the study area in relation to the River Euphrates andthe active left-lateral strike-slip faults (simplified from  Westaway  [2004a]) bounding the Arabian (AR),African (AF), and Turkish (TR) plates. Boxed names are basalt sample localities. Mountain rangesforming as a result of transpression along the DSFZ are labeled as follows: L.M., Lebanon Mountains;C.R., Syrian Coastal Range (Jebel Nusayriyah); A.M., Amanos Mountains. K.V. denotes the KarasuValley; crosses mark the suture of the Neotethys Ocean. 1, 2, 3, and 4 indicate the Euphrates coursesinferred in the middle Miocene, late Miocene, Pliocene, and Pleistocene. In the legend, 1 indicatesoutcrop of the Upper Fars Formation; 2 indicates concentrations of stacked fluvial sand and gravel, fromthe early middle Miocene and (?) late Miocene. TC3012  DEMIR ET AL.: ARABIAN PLATFORM UPLIFT2 of 30 TC3012  cene. This followed a similar interpretation by  Kukla [1978] of a comparable landscape change that is evident from terrace records in central Europe.[ 5 ] The disposition of the Euphrates terrace depositsevidently reflects changes in hydrology on Milankovitchor longer timescales, superimposed onto effects of regionaluplift. By analogy with other fluvial sequences in the region[e.g.,  Collier et al. , 2000;  Demir et al. , 2004;  Westaway et al. , 2006b], it is considered that aggradation of thesedeposits occurred primarily at times of cold climate, whenthe reduction of vegetation cover in upstream parts of thecatchment would have brought about increased influxes of sediment into the system and relatively low ratios of discharge to sediment load, as the rainfall during such coldstages is believed to have been relatively low. Conversely,during temperate climatic optima, increased vegetationcover would have restricted mass movement into the fluvialsystem and the associated higher rainfall would haveresulted in a high ratio of discharge to sediment load. Suchconditions would have enabled the river to incise, inresponse to the regional uplift, by which means progres-sively lower valley floors (terraces) would have formed.This mechanism for terrace formation, by cyclic climatictriggering in response to background uplift, has been widelyadvocated in recent years in areas rich in paleoenvironmen-tal and dating evidence, such as NW Europe [e.g.,  Maddy ,1997;  Bridgland  , 2000;  Westaway , 2002b;  Bridgland and Westaway , 2007]. Marine base level control is seen asunimportant, excepting very close to coastlines [e.g.,  Roseet al. , 1999;  Macklin et al. , 2002; cf.  Kiden and To¨rnqvist  ,1998].[ 6 ] The aim of this paper is to present results of Ar-Ar dating of basalts interbedded with the fluvial sequence,thereby constraining the ages of the Euphrates terracesand, assuming the terraces to provide a record of fluvialincision, to use this evidence to deduce the history of vertical crustal motion in this part of northern Syria. Inaddition, geochemical analysis is used to classify thesampled basalts and to address the thermal state of theunderlying crust and mantle lithosphere, knowledge of which is needed for the numerical modeling of the verticalcrustal motions that then follows. The work has implicationsfor the wider consideration of fluvial evolution in responseto crustal deformation and climatic fluctuation, as well asthe understanding of late Cenozoic landscape development in regions of relatively high crustal stability. Since theEuphrates deposits are repositories for Paleolithic artifacts[e.g.,  Besanc¸on and Sanlaville , 1981;  Copeland  , 2004; Sanlaville , 2004], the work will also have implications for the understanding of human occupation in the Levant,which lies on the probable migration route between Africaand Eurasia. 2. Regional Background 2.1. Regional Tectonics [ 7 ] Eastern Turkey forms the modern boundary zone between the African, Arabian, Eurasian, and Turkish plates.Westward motion of the Turkish plate relative to Eurasiaand Arabia is being accommodated by right-lateral slip onthe North Anatolian Fault Zone (NAFZ) and conjugate left-lateral slip on the East Anatolian Fault Zone (EAFZ;Figure 1). A promontory of the African plate persistsnorthward through western Syria into central southernTurkey (reaching the vicinity of Kahramanmaras¸; Figure 1), bounded relative to the Arabian plate by the Dead SeaFault Zone (DSFZ) and relative to the Turkish plate by other left-lateral faults. The kinematics of these strike-slip fault zones are now well constrained by both GPS satellitegeodesy and geological evidence [e.g.,  McClusky et al. ,2000;  Westaway , 2003, 2004a;  Westaway et al. , 2006a]. For much of their length these zones involve transform faulting,which requires no vertical component of crustal motion, but locally they delineate transpressive step overs, wherelocal crustal shortening requires localized surface uplift inorder to balance crustal volume [e.g.,  Westaway , 2003,2004a;  Gomez et al. , 2006] (Figure 1). This moderngeometry of strike-slip faulting is thought to date from  4 Ma [e.g.,  Westaway , 2003, 2004a;  Westaway et al. ,2006a]; it is thought to have superseded an earlier, but similar, geometry of strike-slip faulting, which existed between   7 and 4 Ma [e.g.,  Westaway and Arger  , 2001].Prior to this, this region accommodated northward (or northwestward) convergence of the African and Arabian plates relative to Eurasia.[ 8 ] The NAFZ and EAFZ converge at the Karl ƒ ova‘‘triple junction’’ in NE Turkey (  41   East). East of this point, Arabia-Eurasia convergence is accommodated directly, by active reverse faulting. Before the modern strike-slipfault systems developed, in the late Miocene and earlyPliocene, active reverse faulting (and active folding of anticlines over blind reverse faults) was much more wide-spread in eastern Turkey and northern Syria. It is thought that many such structures became active in the late Eocene,at the start of the continental collision between Africa/ Arabia and Eurasia; beforehand, the convergence betweenAfrica and Eurasia was instead accommodated by north-ward subduction of the Southern Neotethys Ocean beneathAnatolia [e.g.,  Aktas¸ and Robertson , 1984]. The suture of this ocean (Figure 1; sometimes known as the Bitlis-Zagrossuture), separating the Arabian Platform to the south fromthe Anatolian metamorphic terrane to the north, is the principal geological boundary in this region.[ 9 ] Superimposed onto these effects of plate motions has been a component of regional uplift. As  Arger et al.  [2000]first noted, since the middle Miocene, when much of theArabian Platform emerged above sea level, this hasamounted to   1000 m near the Neotethys suture and  500 m or more in parts of the Arabian Platform interior near the Turkey-Syria border. Recent studies [e.g.,  Arger et al. , 2000;  Demir et al. , 2004, 2007] indicate that thiscomponent of uplift has had nothing directly to do with plate motions, and seems instead to be the local manifesta-tion of the systematic increase in relief that has developedwithin the continents during the late Cenozoic, apparentlyas a consequence of coupling between surface processes(such as erosion) and induced flow in the lower continentalcrust [e.g.,  Westaway et al. , 2003;  Bridgland and Westaway , TC3012  DEMIR ET AL.: ARABIAN PLATFORM UPLIFT3 of 30 TC3012  2007] (see sections 3.4, 5.3, and 5.4). Regional uplift on thesame timescale has been even more dramatic within easternAnatolia, where early Miocene marine sediments are nowfound   2 km or more above sea level [e.g.,  Westaway and  Arger  , 2001]. It is evident that several factors must beconsidered when reconstructing the evolution of theEuphrates: the local effects of faulting and folding, lateralvariations in regional uplift, and the need to maintain adownstream channel gradient.[ 10 ] As illustrated in Figure 1, the modern left-lateralstrike-slip fault systems are located well to the west andnorth of the present study region. Before this moderngeometry of strike-slip faulting developed, at    4 Ma(3.73 ± 0.05 Ma according to  Westaway et al.  [2006a]),the relative motion between the Arabian and African platesseems to have been accommodated instead by a different system of faults located east of the modern DSFZ, some passing through the Gaziantep area of SE Turkey (Figure 1)and adjoining parts of Syria [e.g.,  Chaimov et al. , 1990; Cos¸kun and Cos¸kun , 2000;  Westaway , 2003]. As will bediscussed in more detail in section 5.3, the localized tiltingof some of the older Euphrates deposits (notably aroundHalabiyeh; Figure 1) and the localized faulting of some of the underlying bedrock (notably, at Shireen; Figure 1) isinferred to date from this time. 2.2. Cenozoic Evolution of the Euphrates Catchment [ 11 ] Most bedrock outcrops along the Euphrates in thevicinity of the Turkey-Syria border (Figure 1) consist of Cenozoic (upper Eocene to Oligocene) marine carbonates[  Ponikarov et al. , 1967], during deposition of which thenorthern Arabian Platform was submerged beneath theTethyan seaway linking the Mediterranean Sea and IndianOcean. Unconformable Miocene marine limestone in north-ern Syria was assigned by  Ponikarov et al.  [1967] to theJeribe Formation (equivalent to the F ƒ rat Formation in SETurkey [cf.  Terlemez et al. , 1997]). This is a stratigraphicunit of regional significance, extending from Syria to Iran,dated to the early middle Miocene (Langhian [  James and Wynd  , 1965;  Alsharhan and Nairn , 1995]). Diverse carbon-ate facies are evident, suggesting rapidly changing shallowmarine conditions. This limestone is overlain by depositsassigned by  Ponikarov et al.  [1967] to the Lower FarsFormation, another regionally significant unit, of late mid-dle Miocene (Burdigalian) age [  James and Wynd  , 1965;  Alsharhan and Nairn , 1995]. Around Shireen (Figure 1) thisconsists of interbedded green calcareous clay and marl, redmudstone and white limestone with occasional gypsumlayers, and is no more than a few tens of meters thick.Farther north, near the Turkish border, conglomerate andcross-bedded sandstone assigned to this unit indicate mar-ginal marine conditions with input of clastic sediment fromthe north. By this time a land bridge connected Arabia withTurkey (between Aleppo and Gaziantep; Figure 1), isolatingthe Mediterranean Sea from the Mesopotamian Basin of Iran, Iraq and NE Syria.[ 12 ] No younger stacked sediments are found in this area.However, east of Raqqa (Figure 1) there is extensiveoutcrop of the Upper Fars Formation, consisting of up to  250 m of clay, siltstone, fine sandstone and gypsum.These deposits record a marine to lagoonal or lacustrinetransition in the late Miocene [e.g.,  James and Wynd  , 1965;  Alsharhan and Nairn , 1995]. Younger stacked terrestrialsediments, assigned to the Pliocene and correlated with theBakhtiari Formation of Iraq, also crop out widely in easternSyria, notably around Deir ez-Zor. These consist of sandand gravel, with a general upward coarsening trend, withcalcareous clay and gypsum interbeds [  Ponikarov et al. ,1967], indicating a lacustrine depocenter with fluvial input.The gravel clasts include igneous (e.g., granite, peridotite)and metamorphic (e.g., quartzite, calc-schist, gneiss) rocksderived from Anatolia, indicating the existence at that timeof an ancestral Euphrates (stage 3 in Figure 1).[ 13 ] As with the other principal rivers of Syria, theOrontes (Aassi) and Kebir (Figure 1), the Euphrates se-quence has been extensively researched by French workers principally interested in the Paleolithic archaeologycontained in the deposits [e.g.,  Besanc¸on and Geyer  ,2003;  Copeland  , 2004;  Sanlaville , 2004]. A uniform terraceclassification and dating scheme was applied throughout,established first in the Orontes and Kebir and then extendedto the Euphrates by  Besanc¸on and Sanlaville  [1981]. Thisscheme recognizes five terraces, from QfV, the oldest, toQfI, the youngest (the Q standing for ‘‘Quaternary’’ and thef for ‘‘fluvial’’). These have been assumed to be of the sameage wherever recognized; typical heights above present river level of the tops of each of these terraces, as srcinallydefined, are listed in Table 1. In addition, the most recent (latest Pleistocene and Holocene) deposits are designatedQf0, including the modern floodplain and older deposits,forming very low terraces, some of which aggraded duringhistorical time. In contrast with the five terraces recognizedin Syria by  Besanc¸on and Geyer   [2003] and  Sanlaville [2004],  Tyra´cˇek   [1987] identified 10 terraces above theEuphrates floodplain in Iraq.[ 14 ] A degree of age constraint is provided by the relativedisposition of the various terrace deposits of the Kebir, in NW Syria, in relation to marine (raised beach) sedimentsinland of Latakia (Figure 1). Furthermore, temperate stagedeposits interbedded with cold stage gravels of terraceQfIII,  60 m above the modern level of the middle Orontesat Latamneh, have yielded a mammal fauna that is sugges-tive of an age around 0.5 Ma [cf.  Bridgland et al. , 2003].Correlation between different valleys and between different reaches of the same valley has relied upon altitude above present river level, reinforced by artifact assemblages fromthe various terrace deposits. In the most recent appraisal of the Euphrates sequence,  Sanlaville  [2004] proposed that terrace QfV formed in MIS 18 and earlier, QfIV formed inMIS 16, QfIII in MIS 14 and/or 12, QfII in MIS 8, and QfIformed in MIS 6 and/or 4 (Table 1). However, the main basis of this scheme, the assumption that rates of verticalcrustal motion do not vary laterally throughout Syria, hassince been refuted. Notably, surface uplift in the parts of western Syria that have hitherto yielded the age controlevidence is now known to relate in part to transpressionalong the DSFZ [e.g.,  Westaway , 2003, 2004a;  Gomez et al. ,2006] (Figure 1). Thus, although Paleolithic artifacts, like TC3012  DEMIR ET AL.: ARABIAN PLATFORM UPLIFT4 of 30 TC3012  Table 1.  Existing Terrace Scheme for the Raqqa–Deir ez-Zor Area a  Terrace Height,  b m MIS c Archaeology Classification c Artifact Types c Qf0   1 2–1 Neolithic/historical Neolithic and later QfI 10–15 6 and/or 4 middle Paleolithic d Levallois or ‘‘Levallois-like’’ material d QfII 20–25 8 Upper Acheulian d As QfIII but with hand axes; deep patina absent  d QfIII 50–60 14 or 12 middle Acheulian d flakes, cores, ‘‘choppers,’’ ‘‘picks’’;hand axes absent; deep brown patina d QfIV 80 16 none e none e QfV >100 18 or earlier none none a  This is a summary of the existing correlation and dating scheme for the Euphrates terraces in the Raqqa–Deir ez-Zor area, advocated by previousworkers [e.g.,  Besanc¸on and Sanlaville , 1981;  Sanlaville , 2004].  b Typical altitude above present river level of the upper surface of the terrace deposits. c After   Sanlaville  [2004] and/or   Copeland   [2004]. d In the upstream reaches referred to in note 3, assemblages with hand axes are reported from deposits attributed (on the basis of height) to QfIII andassemblages with Levallois or Levallois-like material are reported from QfII [cf.  Copeland  , 2004]. The miscorrelation suggested by the height-driveninterpretation on which this terrace scheme is based can be explained if the surface uplift has tapered southward, toward the interior of the Arabian Platform,as others [e.g.,  Arger et al. , 2000] have previously inferred, and is also indicated by the analysis in the present study. e In the Manbij/Birecik region farther upstream (toward and beyond the Turkish border), assemblages of flakes and cores (Lower Acheulian/Khattabian)occur in the terrace designated QfIV [e.g.,  Copeland  , 2004]. Figure 2a.  Landsat ETM+ image covering the Halabiyeh and Zalabiyeh plateaus (Figures 1 and 3b).This image has been projected into the Universal Transverse Mercator (UTM) coordinate system usingthe WGS84 reference frame. Adjoining the Zalabiyeh Plateau to the east is the larger (  15 by  20 km) basalt-capped Kasra Plateau. The local basalt appears to have emerged from a neck, Jebel as-Samman,reaching 419 m asl, shown in Figure 1. TC3012  DEMIR ET AL.: ARABIAN PLATFORM UPLIFT5 of 30 TC3012
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