Imaging of subsurface lineaments in the southwestern part of the Thrace Basin from gravity data

Linear anomalies, as an indicator of the structural features of some geological bodies, are very important for the interpretation of gravity and magnetic data. In this study, an image processing technique known as the Hough transform (HT) algorithm is described for determining invisible boundaries and extensions in gravity anomaly maps. The Hough function implements the Hough transform used to extract straight lines or circles within two-dimensional potential field images. It is defined as image and Hough space. In the Hough domain, this function transforms each nonzero point in the parameter domain to a sinusoid. In the image space, each point in the Hough space is transformed to a straight line or circle. Lineaments are depicted from these straight lines which are transformed in the image domain. An application of the Hough transform to the Bouguer anomaly map of the southwestern part of the Thrace Basin, NW Turkey, shows the effectiveness of the proposed approach. Based on geological data and gravity data, the structural features in the southwestern part of the Thrace Basin are investigated by applying the proposed approach and the Blakely and Simpson method. Lineaments identified by these approaches are generally in good accordance with previously-mapped surface faults

Turkey

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the I I different gas fields along the NW-SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH4, thermogenic C2+ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH4 is bacteriogenic (Calculated delta(13)C(C1-C) = -61.48%.; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH4. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (delta(13)C(C1-C) = -35.80 parts per thousand; Hamitabat Field) and Group-3 (delta(13)C(1-C) = -49.10 parts per thousand; Degirmenkoy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C2+ gases. The Group-2 C2+ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (delta(13)C = -21 parts per thousand) present in the Eocene deltaic Hamitabat shales. The Group-3 C2+ gases, produced from one field, were generated from isotopically light marine kerogen (delta(13)C = -29 parts per thousand). Lower Oligocene Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C2+ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (delta(13)C = -21 parts per thousand; -29 parts per thousand) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R-eq = 0.55-0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin. (c) 2005 Elsevier Ltd. All rights reserved.C1 Univ Pamukkale, Geol Engn Dept, TR-20070 Denizli, Turkey.Univ Oklahoma, Norman, OK 73019 USA.High Bank House, Tenbury Wells WR15 8JJ, Worcs, England.Turkish Petr Corp, Explorat Grp, Ankara, Turkey

The geometry of the North Anatolian transform fault in the Sea of Marmara and its temporal evolution: implications for the development of intracontinental transform faults

International audienceThe North Anatolian Fault is a 1200 km long strike-slip fault system connecting the East Anatolian convergent area with the Hellenic subduction zone and, as such, represents an intracontinental transform fault. It began forming some 13-11 Ma ago within a keirogen, called the North Anatolian Shear Zone, which becomes wider from east to west. Its width is maximum at the latitude of the Sea of Marmara, where it is 100 km. The Marmara Basin is unique in containing part of an active strike-slip fault system in a submarine environment in which there has been active sedimentation in a Paratethyan context where stratigraphic resolution is higher than elsewhere in the Mediterranean. It is also surrounded by a long-civilised rim where historical records reach well into the second half of the first millennium BCE (before common era). In this study, we have used 210 multichannel seismic reflexion profiles, adding up to 6210 km profile length and high-resolution bathymetry and chirp profiles reported in the literature to map all the faults that are younger than the Oligocene. Within these faults, we have distinguished those that cut the surface and those that do not. Among the ones that do not cut the surface, we have further created a timetable of fault generation based on seismic sequence recognition. The results are surprising in that faults of all orientations contain subsets that are active and others that are inactive. This suggests that as the shear zone evolves, faults of all orientations become activated and deactivated in a manner that now seems almost haphazard, but a tendency is noticed to confine the overall movement to a zone that becomes narrower with time since the inception of the shear zone, i.e., the whole keirogen, at its full width. In basins, basin margins move outward with time, whereas highs maintain their faults free of sediment cover, making their dating difficult, but small perched basins on top of them in places make relative dating possible. In addition, these basins permit comparison of geological history of the highs with those of the neighbouring basins. The two westerly deeps within the Sea of Marmara seem inherited structures from the earlier Rhodope-Pontide fragment/Sakarya continent collision, but were much accentuated by the rise of the intervening highs during the shear evolution. When it is assumed that below 10 km depth the faults that now constitute the Marmara fault family might have widths approaching 4 km, the resulting picture resembles a large version of an amphibolite-grade shear zone fabric, an inference in agreement with the scale-independent structure of shear zones. We think that the North Anatolian Fault at depth has such a fabric not only on a meso, but also on a macro scale. Detection of such broad, vertical shear zones in Precambrian terrains may be one way to get a handle on relative plate motion directions during those remote times.La faille nord-anatolienne est constituée d’un système de failles décrochantes d’une longueur de 1200 km qui relie le secteur convergent anatolien à la zone de subduction hellénique elle représente ainsi, une faille transformante intracontinentale. Elle a commencé à se former il y a environ 13 à 11 Ma dans une ceinture déformée dominée par des déplacements de décrochement (« keirogène »), nommé la zone de cisaillement nord-anatolienne; elle s’élargit d’est en ouest. Elle atteint sa largeur maximale de 100 km à la latitude de la mer de Marmara. Le bassin de Marmara est unique en ce qu’il contient une partie d’un système actif de failles de décrochement dans un environnement sous-marin où il y a eu de la sédimentation active dans un contexte de mer Paratéthys et où la résolution stratigraphique est plus élevée qu’ailleurs en Méditerranée. La faille est aussi entourée par une bordure où, grâce aux civilisations de longue date, il existe des documents historiques datant de la deuxième moitié du premier millénaire avant notre ère. Dans la présente étude, nous avons utilisé les données obtenues par 210 profils de réflexion sismique multicanale, ce qui donne un total de 6210 km de profils; nous avons aussi utilisé de la bathymétrie à haute résolution et des profils de fluctuation de longueur d’onde disponibles dans la littérature pour cartographier toutes les failles plus récentes que l’Oligocène. Pour ces failles, nous distinguons celles qui recoupent la surface et celles qui ne la recoupent pas. Pour ces dernières, nous avons créé un tableau temporel de génération de failles basée sur la reconnaissance de la séquence sismique. Les résultats sont surprenants; en effet, peu importe l’orientation des failles, elles contiennent toutes des sous-ensembles actifs et des sous-ensembles non actifs. Cela suggère qu’à mesure de l’évolution de la zone de cisaillement, les failles de toutes directions sont activées et désactivées d’une manière qui semble presque aléatoire. Cependant, nous notons une tendance à restreindre le mouvement général à une zone qui se rétrécit avec le temps depuis de début de la zone de cisaillement, c.-à-d. tout le « keirogène », à sa pleine largeur. Dans les bassins, avec le temps, les bordures se déplacent vers l’extérieur alors que les zones surélevées gardent leurs failles libres de couverture sédimentaire, ce qui complique la datation. Toutefois, de petits bassins à leur sommet permettent d’obtenir une datation relative. De plus, ces bassins permettent de comparer l’histoire géologique des zones surélevées à celle des bassins avoisinants. Deux creux, situés à l’ouest dans la mer de Marmara, semblent être des structures héritées de la collision entre le fragment Rhodope–Pontide et le continent Sakarya. Cependant, ils ont été grandement accentués par le soulèvement des zones surélevées durant l’évolution du cisaillement. Lorsque nous partons de l’hypothèse qu’à une profondeur supérieure à 10 km, les failles qui forment actuellement la famille de failles de Marmara pourraient avoir des largeurs approchant les 4 km, l’image qui en ressort ressemble à une version agrandie d’une fabrique de zone de failles au faciès des amphibolites. Cette inférence concorde avec une structure de zones de cisaillement, indépendante de l’échelle. Nous croyons que la faille nord-anatolienne constitue un système qui a une telle fabrique en profondeur, non seulement à échelle moyenne mais aussi à grande échelle. La détection de telles larges zones verticales de cisaillement dans des terrains précambriens pourrait être une manière de comprendre les directions relatives des plaques à ces temps anciens

Map view restoration of Aegean–West Anatolian accretion and extension since the Eocene

The Aegean region (Greece, western Turkey) is one of the best studied continental extensional provinces. Here, we provide the first detailed kinematic restoration of the Aegean region since 35 Ma. The region consists of stacked upper crustal slices (nappes) that reflect a complex paleogeography. These were decoupled from the subducting African-Adriatic lithospheric slab. Especially since !25 Ma, extensional detachments cut the nappe stack and exhumed its metamorphosed portions in metamorphic core complexes. We reconstruct up to 400 km of trench-perpendicular (NE-SW) extension in two stages. From 25 to 15 Ma, the Aegean forearc rotated clockwise relative to the Moesian platform around Euler poles in northern Greece, accommodated by extensional detachments in the north and an inferred transfer fault SE of the Menderes massif. The majority of extension occurred after 15 Ma (up to 290 km) by opposite rotations of the western and eastern parts of the region. Simultaneously, the Aegean region underwent up to 650 km of post-25 Ma trench-parallel extension leading to dramatic crustal thinning on Crete. We restore a detachment configuration with the Mid-Cycladic Lineament representing a detachment that accommodated trench-parallel extension in the central Aegean region. Finally, we demonstrate that the Sakarya zone and Cretaceous ophiolites of Turkey cannot be traced far into the Aegean region and are likely bounded by a pre-35 Ma N-S fault zone. This fault became reactivated since 25 Ma as an extensional detachment located west of Lesbos Island. The paleogeographic units south of the Izmir-Ankara-Sava suture, however, can be correlated from Greece to Turkey