J.D.A. Piper

Tectonics and Magmatism in Turkey and the Surrounding Area

This volume contains 23 papers from a range of international contributors, describing recent research into the tectonics and magmatism of Turkey and its surroundings. This region is sited at the collision zone between Eurasia and Afro-Arabia and, as such, provides an extraordinarily complete and well-exposed record of the staged tectonic evolution of this sector of the Alpine-Himalayan orogen. The geological history of this area involves separation of continental fragments from the margin of Gondwana, their migration across the Tethyan oceans, the subsequent closure of these oceans and, finally, the development of the neotectonic regime, which continues to evolve to the present day. Such a comprehensive record is relevant to the understanding of collisional zones worldwide. The volume is divided into five sections: Tethyan evolution, Neotethyan ophiolites, post-Tethyan basin evolution, neotectonics and igneous activity. The first two sections deal with Tethyan oceans, whose growth and subsequent closure dominated the geodynamic framework in the Mesozoic and Cenozoic. The subsequent sections deal with more recent geological developments from the Balkan Peninsula in the west to the Transcaucasus in the east that followed consumption of the Tethyan oceans. There is a broad mix of papers throughout the volume: wide-ranging review papers on ocean development and extensional tectonics are followed by detailed descriptions of petrology and geochemistry and geographically focused studies on basin evolution, specific aspects of extensional and strike-slip tectonics and discussions of the relationship of magmatic activity to the tectonic development of the area. Tectonics and Magmatism in Turkey and the Surrounding Area presents up-to-date results and ideas from a large number of international contributors on a wide range of current research activity in this region. It is essential reading for all geoscientists with an interest in both academic and applied aspects of eastern Mediterranean geology.

The Precambrian palaeomagnetic record: the case for the Proterozoic Supercontinent

The case for a single supercontinent in Proterozoic times is examined in the context of a modified reconstruction with the Precambrian shields collectively comprising a primary lens-shaped body of crust. The palaeomagnetic data base is now sufficient to provide a definitive test of this model and it is shown that palaeopoles from the major shields conform to a single narrow path from ca. 2600 to 570 Ma. The agreement is especially compelling because the model requires no stringent data selection although it is itself highly rigid with adjustments only necessary at 1100 Ma and then applicable only to peripheral shields. There is close correspondence of the predominant field polarities from the constituent shields giving added confidence that the same polarities are being correlated, and the model supports the view that the global palaeoradius has been approximately constant since 2600 Ma. The pre-2200 Ma palaeopoles from Laurentia define “track 6” which is shown to be precisely applicable to the contemporaneous record from the African, the Australian, and possibly the Indian, Shields. The post-2200 Ma palaeopoles record higher rates of apparent polar wander (a.p.w.) movement although the magnitude of these movements is identified within one shield (the Laurentian) alone, and the data from other shields continue to conform to the unique reconstruction. The a.p.w, path is best constrained by the data from anorogenic igneous units over the intervals 2600–1800 and 1500–1000 Ma with second rank data from sedimentary successions in all of the major shields supporting the path defined by the igneous data.The a.p.w, paths over the intervals 1800–1500 and 1000–850 Ma are represented predominantly by uplift magnetisations in metamorphic terrains of the Fennoscandian and Laurentian Shields, although they are matched by magnetisations in supracrustal successions in Africa, South America and Australia, and in Africa and Siberia, respectively. The weakest part of the analysis covers the interval 800–570 Ma and the solution cannot yet be regarded as unique because it relies on magnetisations from sedimentary rocks; on the model developed here these data conform to a single a.p.w, path which specifically subdivides at the base of the Cambrian as the supercontinent was dismembered. Many Precambrian tectonic lineaments are brought into parallelism on this model and confirm the validity of the palaeomagnetic reconstruction. They include the later greenstones (2900–2200 Ma) formed in a permobile environment, and the straight belts and mobile belts formed in a coherent ensialic environment; there appears to have been a long continuity in the prevailing stress system in the crust linked to evolving small-scale to large- scaleaesthenosphere systems. This analysis confirms that the continental crust has been a highly coherent unit since the beginning of Proterozoic times and restricts the tectonic models applicable to these times. The possibility of multi-continent plate tectonics is no longer an issue: it is not possible that the shields could have been repeatedly separated and returned to the same unique configuration necessary to satisfy the single a.p.w, path. Models of lithosphere evolution require that the continental crust stabilised towards the end of the Archaean retained a much higher relative strength than the oceanic lithosphere until late Proterozoic times.

The Neoproterozoic Supercontinent: Rodinia or Palaeopangaea?

Abstract The Rodinia reconstruction of the Neoproterozoic Supercontinent has dominated discussion of the late Precambrian Earth for the past decade and originated from correlation of sedimentary successions between western North America and eastern Australia. Subsequent developments have sited other blocks according to a distribution of ∼1100 Ma orogenic belts with break-up involving a putative breakout of Laurentia and rapid reassembly of continent crust to produce Gondwana by early Phanerozoic times. The Rodinia reconstruction poses several serious difficulties, including: (a) absence of palaeomagnetic correlation after ∼730 Ma which requires early fragmentation of continental crust although geological evidence for this event is concentrated more than 150 Ma later near the Cambrian boundary, and (b) the familiar reconstruction of Gondwana is only achieved by exceptional continental motions largely unsupported by evidence for ocean consumption. Since the geological evidence used to derive Rodinia is non-unique, palaeomagnetic data must be used to evaluate its geometrical predictions. Data for the interval ∼1150–500 Ma are used here to test the Rodinia model and compare it with an alternative model yielding a symmetrical crescent-shaped analogue of Pangaea (Palaeopangaea). Rodinia critically fails the test by requiring Antarctica to occupy the location of a quasi-integral Africa, whilst Australia and South America were much closer to their Gondwana configurations around Africa than implied by Rodinia. Palaeopangaea appears to satisfy palaeomagnetic constraints whilst surmounting geological difficulties posed by Rodinia. The relative motions needed to produce Gondwana are then relatively small, achieved largely by sinistral transpression, and consistent with features of Pan-African orogenesis; continental dispersal did not occur until the Neoproterozoic–Cambrian boundary. Analogies between Palaeopangaea and (Neo)pangaea imply that supercontinents are not chaotic agglomerations of continental crust but form by episodic coupling of upper and lower mantle convection leading to conformity with the geoid.

Assessment of heavy metal pollution from a Fe-smelting plant in urban river sediments using environmental magnetic and geochemical methods

Environmental magnetic proxies provide a rapid means of assessing the degree of industrial heavy metal pollution in soils and sediments. To test the efficiency of magnetic methods for detecting contaminates from a Fe-smelting plant in Loudi City, Hunan Province (China) we investigated river sediments from Lianshui River. Both magnetic and non-magnetic (microscopic, chemical and statistical) methods were used to characterize these sediments. Anthropogenic heavy metals coexist with coarse-grained magnetic spherules. It can be demonstrated that the Pollution Load Index of industrial heavy metals (Fe, V, Cr, Mo, Zn, Pb, Cd, Cu) and the logarithm of saturation isothermal remanent magnetization, a proxy for magnetic concentration, are significantly correlated. The distribution heavy metal pollution in the Lianshui River is controlled by surface water transport and deposition. Our findings demonstrate that magnetic methods have a useful and practical application for detecting and mapping pollution in and around modern industrial cities.

Palaeomagnetic study of Neoproterozoic glacial rocks of the Yangzi Block: palaeolatitude and configuration of South China in the late Proterozoic Supercontinent

Abstract Regional glaciation is recorded at two stratigraphic levels in the Neoproterozoic Sinian succession of the Yangzi (South China) Block. A palaeomagnetic study is reported here of the upper sequence (∼ 730-670 Ma) which contains red beds. An ‘A1’ component ( D I = 291 −57° , α95 = 5.0°, Palaeopole 151.2°E, 0.2°N) is resolved at high unblocking temperatures and resides in specular haematite. A study of contemporaneous deformation structures suggests that this component was acquired at an early stage of lithification and yields a representative palaeolatitude (37°) for the glaciation. Lower blocking temperature magnetizations referred to as ‘A2’ ( D I = 297 −19° , α95 = 7.6°, Palaeopole 171.8°E, 19.2°S) and ‘A3’ ( D I = 319 −1° , α95 = 3.2°, Palaeopole 169.1°E, 41.8°S) are later overprints correlating with probable Late Precambrian or Cambrian palaeofields. In other regions, the ‘A’ magnetizations were found to be partially or completely overprinted by a Recent field and/or a ‘B’ remanence ( D I = 4 13° ); the latter correlates with a remagnetization widely recorded elsewhere in South China and is attributed to the Mesozoic Indosinian Orogeny. An ∼90° apparent polar wander (APW) swathe is identified from the Yangzi Block between ∼730-670 Ma and 540 Ma. It is not yet clear whether poles from older Sinian rocks record earlier magnetizations or were overprinted during this APW motion. Comparison of the Sinian-Cambrian palaeomagnetic record from South China with the contemporaneous record from Australia implies that the former block was located immediately to the north-west of Australia during these times. This reconstruction is compatible with Early Palaeozoic biogeographic data. Continuity of the ∼900 Ma Cathaysian-Yangzi suture with the ∼ 1000 Ma Albany-Fraser belt in Australia suggests an extension of this reconstruction back to the earlier part of Neoproterozoic times. A proposed location of South China adjacent to western North America (Li et al., 1995) does not conform with the palaeomagnetic data. No support is also found for the Rodinia reconstruction during the critical 750-550 Ma interval required by the stratigraphic correlation from which the reconstruction was originally derived (Jefferson, 1980). We conclude that the large-scale three-fold sedimentary cycle, which formed the basis for this reconstruction, resulted from global eustatic changes and cannot be used to infer close spatial proximity.

Palaeomagnetic study of block rotations in the Niksar overlap region of the North Anatolian Fault Zone, central Turkey

Abstract This palaeomagnetic study investigates crustal deformation within, and adjacent to, the Niksar overlap area of the North Anatolian Fault Zone (NAFZ) in central-east Turkey. The studied rock formations comprise: (1) red limestones of Late Cretaceous age (3 sites); (2) mafic lavas of Eocene age on the north side (13 sites) and south side (9 sites) of the NAFZ; and (3) volcanic rocks of Pliocene-Quaternary age from the Niksar pull-apart basin within the NAFZ (8 sites). Comparisons with reference palaeofield directions computed from apparent polar wander paths of the Eurasian and Afro-Arabian plates identify two scales of regional and local tectonic rotation: 1. (1) A pre-tilting remanence in the Eocene volcanic rocks south of the NAFZ ( D I = 144.1 −47.5° , β95 = 7.6°) is interpreted to reflect counterclockwise rotation by 30–40° from the reference palaeofields. Contemporaneous volcanic rocks from the north side of the NAFZ have the same reverse polarity recorded in pre-tilting magnetisations. The remanence is also rotated counterclockwise ( D I = 152.4 −42.5° , α95 = 11.3°), but by about 8° less than the volcanics on the south side of the NAFZ. Hence similar amounts of rotation are observed on both sides of the NAFZ and are interpreted to reflect motions during the pre-Middle Miocene collisional history in this sector of the Pontides. No distributed clockwise rotations anticipated from subsequent dextral motion along the NAFZ intracontinental transform are observed. The slightly larger anticlockwise rotation found on the south side of the NAFZ probably records relative rotation of en-echelon wedges by continental escape during post-Middle Miocene strike slip along the transform. 2. (2) Within the narrow zone of intense deformation along the NAFZ, Cretaceous limestones appear to be rotated clockwise by dextral strike-slip motion whilst Plio-Quaternary lavas within a fault-bounded block in the overlap region associated with the Niksar pull-apart basin, have magnetisations consistently directed 240–270°E. Magnetic inclinations are not diagnostic of polarity but both polarity solutions identify rapid clockwise rotation at rates in excess of 50°/m.y. A normal polarity solution is favoured and implies that a block (ca. 5 km across) has undergone a strike-slip displacement of around 12 km within the NAFZ during the last polarity chron. Cretaceous-Eocene palaeolatitudes are closer to those predicted from Eurasia than Afro-Arabia, but a study of older rocks is required to resolve affinities of this sector of the Anatolian block. Theoretical models of crustal deformation across intracontinental transforms obeying power-law behaviour and treating the lithosphere as a viscous medium predict that distributed clockwise rotations should be observed about as broad across the NAFZ. These rotations are not observed. Instead intense clockwise rotation is confined to a narrow zone close to the major fault break. The concentration of historic seismic activity here also implies that the bulk of the dextral motion between the Eurasian Plate and the Anatolian block is accommodated by slip along faults close and parallel to the main trace of the NAFZ.

Palaeomagnetism of the Sveconorwegian mobile belt of the Fennoscandian Shield

Abstract The Sveconorwegian belt is a broad terrain in the south western sector of the Fennoscandian Shield subjected to thermal activation, widespread intrusion and limited tectonic activity between ca. 1100 and 950 Ma, and magnetised during slow uplift and cooling following these mobile events. This study comprises a palaeomagnetic investigation of four areas spanning the zone: the anorthosite, farsundite and related intrusive complexes of southwest Norway, the gabbro-norite intrusions of the Bamble sector bordering the Oslo Graben, the gabbro-anorthosite complex at Bratton and Algon in west Sweden, and dolerite intrusions near the eastern tectonic front to the zone in Sweden. Site mean directions and demagnetisation trends in the Rogaland intrusive complex define an apparent polar wander (a.p.w.) swathe of just over 30° of arc; poles fall along this swathe according to their position in the complex and their blocking temperatures, and the resultant path is interpreted as a migration of this zone away from the palaeopole at a rate of 1.3–3.0° Ma−1. The total swathe represents 10–20° of a.p.w. movement at 900–940 Ma; an isolated later (‘Y’) magnetisation may represent a later extension of the path at ca. 840 Ma. Two sets of magnetisations, a NW negative, predominantly hematite-held, and SSW negative, predominantly magnetite-held, are recognised in the gabbro-norite intrusions of the Bamble sector and are linked to radiometric events at 1090 and 1010 Ma. In the Bratton and Algon norite-anorthosite bodies of west Sweden, demagnetisation trends and site mean directions define a short swathe of westerly negative directions linked to radiometric ages averaging ∼ 1000 Ma. Minor dolerite (hyperite) bodies intruded near the eastern margin of the Sveconorwegian belt possess predominantly steep positive and negative directions of magnetisation comparable to results from the Scania region at the southern margin of the belt. The collective palaeomagnetic data from the Sveconorwegian belt and its borders define a flattened anticlockwise a.p.w. loop with an amplitude of ∼ 70° and executed over the approximate interval 1050-840 Ma with the apex, or hairpin, defined by the Rogaland data at ca. 940 Ma. This loop correlates precisely with a loop defined by the uplift and cooling magnetisations from the Grenville terrain of the Laurentian Shield and shows that the Fennoscandian and Laurentian Shields were in juxtaposition between 1050 and 850 Ma with the two linear frontal zones disposed at a high angle to one another. These fronts were probably initiated in alignment, however, because they are disposed in this way on a primitive reconstruction derived from the pre-1190 Ma palaeomagnetic data. Break up and relative movements of the two shields occupied a short period between 1190 and 1050 Ma, following the Grenville-Sveconorwegian tectonism. It was associated with widespread rifting, dolerite intrusion and alkaline magmatism into the already stabilised parts of the shields.

Palaeomagnetic study of the Karaman and Karapinar volcanic complexes, central Turkey: neotectonic rotation in the south-central sector of the Anatolian Block

Abstract In the Anatolian sector of the Afro–Eurasian collision zone a palaeotectonic collisional phase (Paleocene to Miocene) responsible for emplacement of the Pontide and Tauride orogens has been replaced by a neotectonic phase of continental deformation (Late Miocene/Early Pliocene to Recent). The latter phase appears to have been accommodated mainly by crustal thickening during Late Miocene and Pliocene times, but was succeeded by complex differential rotations of fault blocks during crustal extrusion in Late Pliocene and Quaternary times. In this study we have investigated palaeomagnetism of Miocene–Recent volcanic rocks comprising the western extension of the Central Anatolian Volcanic Province located in the south-central part of the Anatolian Block with the aim of resolving deformations near to the border with the Tauride orogen. Rock magnetic investigations identify low-Ti magnetite assemblages of primary cooling-related origin. These have predominant multidomain structures but significant fractions of single domains are always present; low-temperature alteration is largely absent. The Karaman Volcanic Complex (Late Pliocene) shows a net rotation of −5.7±6.9° not significantly different from the regional field axis during Recent times. The Karapinar Volcanic Field (Brunhes epoch) identifies a larger net rotation of −23.1±12.0° in a restricted sample. The adjoining Karacadag Volcanic Complex (Late Miocene–Pliocene) and Middle Miocene lavas beneath the Hasandag Complex define net rotations of −8.1±5.9° and −16.4±8.9° respectively. Analysis of palaeomagnetic results from Late Cretaceous–Recent rock units emplaced in Anatolia during the palaeotectonic and neotectonic regimes shows that rates of rotation have accelerated in post-Pliocene times as crustal thickening has given way to tectonic escape. A near-uniform anticlockwise rotation of 25–35° has characterised much of this block during the most recent phase of deformation and appears to have occurred in common with the Eurasian Plate to the north of the North Anatolian Fault Zone. Whilst this rotation appears to extend south eastwards across the Ecemis Fault Zone towards the East Anatolian Fault, the present study shows that smaller differential anticlockwise rotations have characterised the south-central region of the block where it has interacted at its southwestern margin with oroclinal bending focussed on the Isparta angle.

Deformational behaviour of continental lithosphere deduced from block rotations across the North Anatolian fault zone in Turkey

Abstract Theoretical considerations of lithosphere deformation across transform plate boundaries predict an expression in terms of 3istributed deformation. The magnitude of rotation is expected to diminish away from the fault zone in a way which depends on the length of the fault, the amount of displacement, and the ductility of the lithosphere. Palaeomagnetic studies across the North Anatolian transform fault zone, which separates the Eurasian Plate and Anatolian Block in northern Turkey, show that clockwise rotations predicted from the sense of dextral motion are indeed present and have attained finite rotations of up to 270° during the ∼ 5 Ma history of Neotectonic deformation. Such rotations are, however, confined to narrow ( ∼ 10 km wide) zones between system-bounding faults and appear to have resulted from rotation in ball-bearing fashion of equidimensional blocks a few kilometres in size. Outside of this zone only anticlockwise rotations are observed; these are unrelated to deformation across the fault zone and record regional anticlockwise rotation of Turkey which is complementing clockwise rotation of Greece and accompanying Neogene opening of the Aegean Sea. The observed behaviour of continental lithosphere satisfies no plausible value of power law behaviour. We therefore conclude that relative motion across this transform boundary occurs as a discrete zone of intense deformation within a brittle layer comprising the seismogenic upper crust. This is presumed to be detached from a continuum deformation response to shearing in the lower crust and mantle beneath.

A PALAEOMAGNETIC STUDY OF THE SIVAS BASIN, CENTRAL TURKEY : CRUSTAL DEFORMATION DURING LATERAL EXTRUSION OF THE ANATOLIAN BLOCK

Abstract The Sivas Basin is a complex collage of Eocene and younger rocks located within the wedge-shaped eastern margin of the Anatolian Block between the (dextral) North Anatolian Fault Zone and the (sinistral) Eastern Anatolian Fault Zone. It has been subject to ongoing deformation by movement of the Arabian Block into Eurasia and concomittant sideways expulsion of the Anatolian Block. Post-collisional deformation since mid-Miocene times has been dominated by NS to NWSE compression expressed by thrusting and strike-slip faulting. Cretaceous and Eocene rocks were magnetically overprinted to variable degrees during the collisional phase although these overprints have since been rotated mostly anticlockwise. Rocks emplaced during the neotectonic history are high-fidelity palaeomagnetic recorders of subsequent block movements. Regional anticlockwise rotation is recognised across the basin with differential rotation of fault and thrust-bounded blocks. An absence of perceptible differences between group mean rotations identified from Miocene, Pliocene and Quaternary units shows that most regional rotation has been concentrated within the latest phase of the neotectonic history during Quaternary times at an average rate of ∼ 10°/Ma. Commencement of this rotation postdates initiation of the North Anatolian Fault Zone implying that compression following collision was accomodated initially by crustal thickening during Late Miocene and Pliocene times. Subsequent anticlockwise rotations have resulted from sideways expulsion of blocks to the south of the Central Anatolian Thrust along major NESW sinistral faults to achieve the crustal shortening resulting from NS compression. These fault orientations and their sense of motion are explained by a Prandtl model involving deformation of a triangular plastic terrane (the Anatolian Block) between two rigid plates (Eurasia and Afro-Arabia). The variations in regional rotation identified by palaeomagnetism show that average contemporary anticlockwise rotation of Anatolia revealed by GPS data (∼ 1.2°/Ma) is achieved by variable, and locally large, block rotations between major thrusts and strike-slip faults.