Negative Flower Structures Research Articles

Integrated application of a high-resolution LIDAR outcrop survey of an unconventional Niobrara Reservoir, Denver Basin, Colorado

The Niobrara Shale is a major reservoir in the Denver Basin. To understand the importance of natural fractures in this reservoir a dataset comprised of fracture plane orientations and fracture intensity variations using a ground-based LIDAR (Light Detection and Ranging) and photogrammetry was acquired. The purpose was to use an outcrop as an analog to the subsurface reservoir for improved reservoir characterization. Listric faults associated with negative flower structures show increased fracture intensity near the faults. Fracture sets remain consistent throughout the interbedded chalks, marls, and limestones. However, there is an apparent variability of fracture spacing associated with changes in lithology. Mapping the fracture orientation and intensity into the subsurface using the outcrop analog should give rise to better geomechanical and fluid flow modelling.

Tan-Lu fault system and its significance in oil accumulation in the central Liaodong Bay subbasin, Bohai Bay Basin, China

The Tan-Lu fault system in the central Liaodong Bay subbasin, Bohai Bay Basin, has complex structural characteristics, and its tectonism during the Cenozoic is an important factor in oil accumulation. Three-dimensional seismic data were used to document the structural features and evolution of the system. Variations in source rock occurrence, oil catchment area, and faulting intensity were comprehensively evaluated to discuss the heterogeneity of oil occurrence along the system. Detailed analysis of the seismic data indicates a right-lateral slip for the Tan-Lu fault system. Transpression and transtension occur on two branches of the Tan-Lu fault with different orientations, indicating a likely slip azimuth that is between the two orientations, i.e., 30°–35°. The strike-slip began at the middle stage of deposition of the Shahejie Formation and reached its climax during deposition of the first and second members of the Dongying Formation. To date, the slip is still continuing. In the transpressional system, the lower strata on opposite sides of the strike-slip fault were shortened and folded, whereby the upper strata were stretched and normal faulted. As a result, a negative flower structure developed immediately above a positive flower structure. Strain distribution in this system is similar to that in a classic fold. Moderate tectonic deformation could enlarge the oil catchment area, benefiting oil accumulation. Weak or intense deformation is unfavorable for commercial oil accumulation because of a small oil catchment area and poor oil preservation, respectively.

Imaging Rock Density Distribution beneath Liwa Fracture Zone in the Southern Part of the Great Sumatran Fault System, Indonesia

We have imaged rock density distribution beneath Liwa fracture zone in the southern part of the the Sumatran Fault Zone by modelling and inverting Bouguer gravity data in two-and three-dimensional environments, respectively. The purpose of this study is aimed to figure out the subsurface distribution of rock densities associated with subsurface basement structure representing the evidence of trans-tensional tectonic product in the SF. In the gravity modeling, to eliminate distortions to the measured gravity values before modelling and inverting the data, Bouguer anomalies obtained in field measurements are reduced to the horizontal plane of z = +800 m as a representation of the average elevation in Liwa. For the inversion, we used algorithm implementing depth-and minimum volume weighting parameters in order to obtain a smooth model with better vertical resolution. The two-dimensional models show clearly surface topography of the basement rocks and the presence of normal faults. The reduced Bouguer anomaly of +800 m elevation shows the presence of structural lineaments extending primarily in a northwest-southeast direction, parallel to Sumatran Fault Zone and older graben faults showing a negative flower structure. From the three-dimensional inversion, the model illustrates an increase of density contrast, lower values being found near the surface and higher values in the deeper parts. The lower density contrast of 0.15 to 0.3 g/cm3 found in the rock groups at depths of 2 km and less is characteristic of relatively homogeneous and poorly compacted rocks. Rocks with moderate to high density contrast (>1.0 g/cm3) are recognized at depths of over 2 km. This model suggests a change of basement morphology as a function of depth, and delineates structural lineaments extending in northwest-southeast direction. This study supports the previous thought that Liwa area is underlain by graben structures, formed by trans-tensional tectonic events. Higher-density Tertiary volcanic breccia and lower-density Quaternary volcanic products of the Ranau Formation form the basement rocks and the overlying younger sediments, respectively.

Tectonic history and structural development of the Zallah-Dur al Abd Sub-basin, western Sirt Basin, Libya

The Zallah-Dur al Abd Sub-basin area lies in the western part of the Sirt Basin of Libya. 2D seismic data covering an area of about 32,000 km² were studied along with the formation tops from 240 wells. We mapped a complex network of normal and probable strike-slip faults, generally striking NNW-SSE that control the asymmetry of the basin. Subordinate NE–SW structures acted as transverse faults controlling local depocentres that segment the Zallah-Dur al Abd Sub-basin.A number of active faults in the intra-basin area have been identified in seismic sections with generally moderate to high dip angles, and displaying evidence for positive and negative flower structures. The bordering extensional fault (the Gedari fault) passes at depth into a moderately SW-dipping structure crossing most of the Upper Mesozoic to Cenozoic stratigraphic section. Thickness variations adjacent to other major faults suggest also an original extensional system where inherited high-angle faults were reactivated throughout this time.A detailed analysis of the available seismic reflection and drill hole data shows that an obliquely rifted, multi-cyclic, NNW-SSE trending basin developed during the complex Upper Mesozoic Cenozoic rearrangement of Mediterranean tectonics. Multiple phases of rifting can be observed in the study area affecting a number of different horizons from Upper Cretaceous to Eocene.In the study area, the basin was initiated as a result of a Tethyan oblique extensional rift system that began in the Early Cretaceous and peaked in the Late Cretaceous. The basin reached its rift maturation phase during the Upper Cretaceous as a result of the continuing extensional tectonics on the marginal bounding NNW-SSE trending normal growth faults. During the Alpine-related tectonic pulses of Middle–Late Eocene the Sirt Basin underwent compression resulted in northward tilting of the basin, causing abrupt subsidence in the north and uplift on the basin southern shoulders, possibly driving a late stage of minor regional subsidence.

Late Cenozoic transtensional fault belt discovered on the boundary of the Awati Sag in the northwestern Tarim Basin

Late Cenozoic transtensional fault belt was discovered on Shajingzi fault belt, NW boundary of the Awati Sag in the northwestern Tarim Basin. And numerous Quaternary normal faults were discovered on Aqia and Tumuxiuke fault belts, SW boundary of Awati. This discovery reveals Quaternary normal fault activity in the Tarim Basin for the first time. It is also a new discovery in the southern flank of Tianshan Mountains. Shajingzi transtensional fault belt is made up of numerous, small normal faults. Horizontally, the normal faults are arranged in right-step, en echelon patterns along the preexisting Shajingzi basement fault, forming a sinistral transtensional normal fault belt. In profile, they cut through the Paleozoic to the mid-Quaternary and combine to form negative flower structures. The Late Cenozoic normal faults on the SW boundary of Awati Sag were distributed mainly in the uplift side of the preexisting Aqia and Tumuxiuke basement-involved faults, and combined to form small horst and graben structures in profile. Based on the intensive seismic interpretation, careful fault mapping, and growth index analysis, we conclude that the normal fault activity of Shajingzi transtensional fault belt began from Late Pliocene and ceased in Late Pleistocene (mid-Quaternary). And the normal faulting on the SW boundary of Awati Sag began from the very beginning of Quaternary and ceased in Pleistocene. The normal faulting on Awati’s SW boundary began a little later than those on the NW boundary. The origin of Shajingzi transtensional normal fault belt was due to the left-lateral strike-slip occurred in the southern flank of Tianshan, and then, due to the eastward escape of the Awati block, a tensional stress developed the normal faults on its SW boundary.

Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator?

For the first time, a deep seismic data set acquired in the frame of the Algerian–French SPIRAL program provides new insights regarding the origin of the westernmost Algerian margin and basin. We performed a tomographic inversion of traveltimes along a 100-km-long wide-angle seismic profile shot over 40 ocean bottom seismometers offshore Mostaganem (Northwestern Algeria). The resulting velocity model and multichannel seismic reflection profiles show a thin (3–4 km thick) oceanic crust. The narrow ocean–continent transition (less than 10 km wide) is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with Miocene to Quaternary sediments. This fault system, as well as the faults organized in a negative-flower structure on the continent side, marks a major strike-slip fault system. The extremely sharp variation of the Moho depth (up to 45 ± 3°) beneath the continental border underscores the absence of continental extension in this area. All these features support the hypothesis that this part of the margin from Oran to Tenes, trending N65–N70°E, is a fossil subduction-transform edge propagator fault, vestige of the propagation of the edge of the Gibraltar subduction zone during the westward migration of the Alboran domain.

New discovery and geological significance of Late Silurian–Carboniferous extensional structures in Tarim Basin

Late Silurian–Carboniferous extensional structures have been discovered after careful interpretation of seismic reflection data in western Manjiaer Sag, Central Tarim Basin in central Asia. The extensional structures comprise numerous small normal faults in nearly N–S strike direction. Groups of normal faults in profile show features suggestive of negative flower structures and small horst-graben structures. Based on growth index calculation, these extensional structures formed in the Late Silurian period, continued activity in the Devonian and Carboniferous and then ceased at the end of Carboniferous. The peak-stage of normal fault activity occurred in Late Silurian. Late Silurian–Carboniferous normal faults also developed in the Tazhong and Tabei areas, which implies that Tarim Basin were under regional extensional tectonic setting during that periods. The extensional structure in southern Tarim resulted from the post-orogeny stress relaxation of the Kunlun Early Paleozoic orogenic belt, and those in northern Tarim resulted from the Paleozoic back-arc rifting which led to the opening of South Tianshan ocean.

Detailed fault structure of the Tarutung Pull-Apart Basin in Sumatra, Indonesia, derived from local earthquake data

The Tarutung Basin is located at a right step-over in the northern central segment of the dextral strike-slip Sumatran Fault System (SFS). Details of the fault structure along the Tarutung Basin are derived from the relocations of seismicity as well as from focal mechanism and structural geology. The seismicity distribution derived by a 3D inversion for hypocenter relocation is clustered according to a fault-like seismicity distribution. The seismicity is relocated with a double-difference technique (HYPODD) involving the waveform cross-correlations. We used 46,904 and 3191 arrival differences obtained from catalogue data and cross-correlation analysis, respectively. Focal mechanisms of events were analyzed by applying a grid search method (HASH code). Although there is no significant shift of the hypocenters (10.8m in average) and centroids (167m in average), the application of the double difference relocation sharpens the earthquake distribution. The earthquake lineation reflects the fault system, the extensional duplex fault system, and the negative flower structure within the Tarutung Basin. The focal mechanisms of events at the edge of the basin are dominantly of strike-slip type representing the dextral strike-slip Sumatran Fault System. The almost north–south striking normal fault events along extensional zones beneath the basin correlate with the maximum principal stress direction which is the direction of the Indo-Australian plate motion. The extensional zones form an en-echelon pattern indicated by the presence of strike-slip faults striking NE–SW to NW–SE events. The detailed characteristics of the fault system derived from the seismological study are also corroborated by structural geology at the surface.

Structural controls on a geothermal system in the Tarutung Basin, north central Sumatra

The Sumatra Fault System provides a unique geologic setting to evaluate the influence of structural controls on geothermal activity. Whereas most of the geothermal systems in Indonesia are controlled by volcanic activity, geothermal systems at the Sumatra Fault System might be controlled by faults and fractures. Exploration strategies for these geothermal systems need to be verified because the typical pattern of heat source and alteration clays are missing so that conventional exploration with magnetotelluric surveys might not provide sufficient data to delineate favorable settings for drilling. We present field geological, structural and geomorphological evidence combined with mapping of geothermal manifestations to allow constraints between fault dynamics and geothermal activity in the Tarutung Basin in north central Sumatra. Our results indicate that the fault pattern in the Tarutung Basin is generated by a compressional stress direction acting at a high angle to the right-lateral Sumatra Fault System. NW–SE striking normal faults possibly related to negative flower structures and NNW-SSE to NNE-SSW oriented dilative Riedel shears are preferential fluid pathways whereas ENE–WSW striking faults act as barriers in this system. The dominant of geothermal manifestations at the eastern part of the basin indicates local extension due to clockwise block rotation in the Sumatra Fault System. Our results support the effort to integrate detailed field geological surveys to refined exploration strategies even in tropical areas where outcrops are limited.

Recent geodynamic and geomorphological analyses of the Diendorf–Čebín Tectonic Zone, Czech Republic

Abstract The Diendorf–Cebin Tectonic Zone (DCTZ) located within the south-eastern margin of the Bohemian Massif is a complex Paleozoic structure reactivated during several phases of unequal thrusting of the Alpine–Carpathian orogenic blocks over the European Platform in the Neogene and Quaternary periods. Recent movement tendencies in its area have been studied through analyses of and correlations between regional geophysical fields, geomorphological phenomena and geodetic monitoring processed on the current geological background. The gravity and magnetic information complemented by data from deep seismic sections along with current GPS velocities documented a dominant transcurrent character of this zone with well-developed extensive upward branching negative flower-structure. The geomorphological data, including information on topolineament framework, fluvial system, distribution of flat surfaces and topographic depressions, pattern of repeated similar landforms, and other related landscape features, identified in addition to the major topographically prominent Diendorf–Cebin NE–SW to NNE–SSW striking lineament an occurrence of complex system consisting of numerous presumably active transversal morphotectonic zones trending NW–SE to WNW–ESE. A concordance was revealed between distribution and properties of gravity and magnetic anomalies and location of several morphotectonic zones. The geodetic data indicated a significant departure of “Brunnovistulian terrain” from the Moldanubian domain towards E and particularly the Western Carpathian Flysch Belt from the entire Bohemian Massif towards NE at the regional scale of the Morava network. At local scale, considerable vertical activity, confirmed by leveling measurement, and minor present-day strike-slip movements of dominant dextral component were detected along a trace of the DCTZ.

The Wan-An fault and its relationship with the Wan-An Basin, western South China Sea

The Wan An Basin, a major basin located in the southwestern part of the South China Sea, is suggested to be closely related to the extrusion of the Indo-China Block. But the published documentation about the Wan An Basin is very sparse. High-resolution seismic reflection profiles obtained by the petroleum industry along the southwestern margin of the South China Sea provide us a chance to study the characteristics of the Wan-An fault and its relationship to the Wan-An Basin. The Wan-An Fault staggered strata in the Wan-An Basin and formed negative flower structures. The deformation related to the Wan-An fault is localized along the fault, while the strata away from the strike-slip fault were not affected. The characteristics of strike-slip extensional basin are not obvious in the Wan-An Basin. However, this basin has obvious characteristics of rift basin. Initial analysis shows that the Wan-An Basin should be a continental margin rift basin.

STRUCTURAL STYLES AND THEIR DISTRIBUTION PATTERN IN THE SOUTHERN EAST CHINA SEA SHELF BASIN

Based on regional seismic data,we studied in this paper the structural characteristics of the southern East China Sea Shelf Basin.Our results show:(1) There are three structural styles,namely extensional,compressional,and complex structures,in the study area according to their dynamic mechanism.(2)The extensional structural style can be further subdivided into the half graben,composite graben,domino faulted graben,tilting block and magmatic structures.The compressional structural style can be subdivided into compressional anticline,faulted anticline and imbricated faults.The complex structural styles can be subdivide into positive inversion tectonics and negative flower structures.

Two-phased evolution of the Suşehri Basin on the North Anatolian Fault Zone, Turkey

This study has aimed to evaluate the current tectonic structure of the Suşehri Basin located on the eastern part of the North Anatolian Fault Zone (NAFZ), one of the most important active faults in Turkey. The work extends earlier investigations of offset and seismicity on the NAFZ and tests a range of evolutionary models. In this study, buried faults have been determined from Ground penetrating radar and magnetic anomalies and possible discontinuities identified by interpolating these data in a region between Gölova and Suşehri. The discontinuities are shown to be linked to negative flower structures formed within the strike-slip fault zone. Quickbird satellite images have been used to map faults and produce kinematic analyses which show that the active stress regime is dominantly strike-slip. However, normal faults and oblique-slip faults are also observed in the basin together with strike-slip faults and the stress regime creating the strike-slip faults is shown to have formed under NW-SE directed transtension. In addition, oblique faults formed under an extensional regime with NNE-SSW direction also occur in the Suşehri Basin as subsets formed under the constraining strike-slip regime. We conclude that the Suşehri Basin started to grow as a fault wedge basin following which it transformed into a pull-apart basin by a south splay on the NAFZ so it is now dominantly a transtensional pull-apart feature.

Geophysical investigations on the gravity and aeromagnetic anomalies of the region between Sapanca and Duzce, along the North Anatolian Fault, Turkey

In this paper, it is aimed to model subsurface structures to the east of the Gulf of Izmit through Duzce by using the gravity and aeromagnetic anomaly data. 1/500.000 scaled gravity anomaly map of the area was taken from the General Directorate of Mineral Research and Exploration (MTA) and it was digitized. The aeromagnetic anomaly data were obtained in the digital form. 3D and 2D models were constructed to reveal the subsurface structure in two different inset regions in the study area including most important negative and positive gravity anomalies. Seismic velocities obtained from the deep seismic recordings were converted to densities. In addition, density information from a previous research was also taken. These densities were used for construction of 3D and 2D gravity models where it was shown that there are narrow and long sedimentary basins and depressions with 0.5–3km depths. These sedimentary basins with the shape of negative flower structures indicating pull-apart basins are controlled by the active fault segments of the North Anatolian Fault (NAF). Earthquake epicenter data were also correlated with the constructed models from the gravity anomalies. Positive gravity anomalies are also caused by very shallow (about 2km) masses that are accepted as the crustal origin intrusions into the fractures of the NAF and, ophiolites and gabbro outcropping on the surface of the studied regions. These intrusives and remnants of the Tethys Ocean are located between the fault segments where the fault bifurcates and they also constitute barriers for straight extension of the NAF. Analytic signal method was applied to the aeromagnetic anomaly data to determine the locations and boundaries of the causative bodies. Those bodies are observed around Duzce, and to the E-SE of it, to the NW of Golyaka and a large mass between Adapazari and Sapanca. Shallow settlement of these magmatics was confirmed by the second vertical derivative of the aeromagnetic data. An anti-clockwise block rotation is evident in the northeast of the study area from the polarity changes of the magnetic anomalies.

3D geometry and kinematics of the Lassee flower structure: Implications for segmentation and seismotectonics of the Vienna Basin strike–slip fault, Austria

The active Vienna Basin strike–slip fault consists of several segments, which differ both in their kinematic and seismotectonic properties. Mapping of industrial 2D seismic, geomorphological data and Quaternary basin analysis proves that active deformation uses Miocene faults. This is shown in detail for the negative flower structure of the Lassee Fault Segment, which developed during the Miocene and was reactivated in the Quaternary. The flower structure consists of Riedel-type splay faults, which merge in a major branch line at the top of the principle displacement zone (PDZ) in approximately 3.5–5.5 km depth. Mapping the PDZ of the Vienna Basin strike - slip fault in the continuations of the Lassee Segment reveals several fault segments which differ by the orientation of the PDZ. NE- and NNE-striking segments with releasing bend geometries are associated with Quaternary basins. These segments are delimited by fault-bends where the strike of the PDZ changes by angles of 20°–35°, and are connected by ENE-striking segments orientated parallel to the displacement vector. Among the releasing bends, the Lassee Segment is subject to increased extension due to the high angle (ca. 35°) between the general slip vector and the orientation of the segment. Resulting extension seems to be accommodated by both, the negative flower structure and normal faults, which branch off from the PDZ south of the Lassee releasing bend. The significant fault bends of 20°–35° delimiting the geometrical fault segments are regarded to act as impediments during dynamic rupture propagation. Hence, fault segment dimensions can be used for constraining the maximum fault surfaces, which can break during single earthquakes. The data on active kinematics and fault segmentation of the Vienna Basin strike–slip fault system therefore may serve as a basic input for future assessments of maximum credible earthquake estimates.

Seismicity and source parameters for small-scale earthquakes along the splays of the North Anatolian Fault (NAF) in the Marmara Sea

Major discrepancy between present-day fault models in the Marmara Sea is closely related to structural development of depression areas beneath the sea. Therefore, in this study, the Marmara Sea and especially its major basins are investigated by means of recent seismological observations consisting of microseismicity (1.2 ≤ Md ≤ 3.6; 2002 May–2006 May), focal mechanisms (2.4 ≤Md ≤ 5.0; 1999–2006) and stress analysis based on the focal mechanisms. In the Çınarcık Basin where either pull-apart model or negative flower structure is proposed, northeastern margin is represented by strike-slip sources having a NW alignment, probably corresponding to a cross-basin strike-slip fault developing in a matured pull-apart basin. This suggestion is directly relevant to experiments of sand box analogue models. These experiments indicate that such a cross-basin strike-slip fault is produced after the pull-apart basins attain their mature stage, eventually resulting in less vertical displacements attributed to normal faults bounding the basin. This experimental decline may be corroborated by the paucity of normal mechanisms in the basin. Extensional motion in this region is more influential outside of the basin, especially to the area where an onshore seismic activity is monitored on the land. On the other hand, another strike-slip activity in the Çınarcık Basin is distributed on a possible strand, parallel to the northern coastline of the Marmara Sea in the Armutlu Peninsula. This may indicate that one or more short strands of the northern splay extend to the west by bounding the basin from the south. In the Central Basin, the existence of a right-lateral strike-slip faulting on an east—west trending fault is supported by similar strike-slip focal mechanisms located at both extremities of the basin. To the west, dominant style of deformation gradually shifts from strike-slip faulting to transpression along the western coastline of the Marmara Sea where the northern splay joins with the offshore continuation of the Ganos Fault. Finally, focal mechanisms in the Marmara Sea are inverted for the best fitting stress tensor, characterized by a subvertical σ2-axis and two subhorizontal stress axes of σ1 and σ3, oriented NW—SE and NE—SW, respectively. Conclusively, this study infers that the current tectonics of the Marmara Sea is mainly governed by a strike-slip regime, ascribed to both submarine splays of the NAF, and partly dominated by extensional and compressional features indicating shallow processes at local scales.

The Lassee segment of the Vienna Basin fault system as a potential source of the earthquake of Carnuntum in the fourth century a.d.

The Vienna Basin fault system is a slow moving (1–2 mm/y) active sinistral fault extending from the Alps through the Vienna Basin into the Carpathians. It comprises an array of NE-striking sinistral strike-slip segments, which differ both by their kinematic and seismologic properties. Among these, the Lassee segment 30 km east of Vienna is of particular interest for seismic hazard assessment as it shows a significant seismic slip deficit. The segment is located about 8 km from the Roman city of Carnuntum, for which archaeological data indicate a destructive earthquake in the fourth century a.d. (local intensity about 9 EMS-98). Mapping of the Lassee segment using 2D seismic, GPR, tectonic geomorphology and Pleistocene basin analysis shows a negative flower structure at a releasing bend of the Vienna Basin fault. The hanging wall of the flower structure includes a Quaternary basin filled with up to 100-m thick Pleistocene growth strata. Faults root in the basal detachment of the Alpine-Carpathian floor thrust at about 8 km depth. The active faults east of the flower structure offset a Middle Pleistocene terrace of the Danube River forming an up to 20-m high composite fault scarp. High-resolution GPR (40, 500 MHz) mapped at least four distinct surface-breaking faults along this scarp including three faults, which are covered by about 2 m of post-tectonic strata. The youngest fault offsets these strata and coincides with a 0.5-m high scarp. This scarp may be interpreted as the product of a single surface-breaking earthquake, provided that the mapped fault offset formed during coseismic surface rupture. Data indicate that the Lassee segment may well be regarded the source of the fourth century earthquake. The interpretation is in line with local attenuation relations indicating a source close to the damaged site, observed fault dimensions and the fault offsets recorded by GPR and morphology.

Tectonics related to the North Anatolian Fault in the Sea of Marmara: evidence from seismic reflection data

The North Anatolian Fault crosses the Sea of Marmara from east to west. Tectonic features of the Sea of Marmara were studied using multi-channel deep seismic reflection data. The northern branch of the North Anatolian Fault is active as a right lateral strike-slip fault zone and indicates both negative and positive flower structures. The North Anatolian Fault splays into two faults at the Sea of Marmara as a northern branch and north segment of the southern branch. The northern branch named the Main Marmara Fault extends in a complicated manner from the north of the Kapidagi Peninsula to westward in the Sea of Marmara. The north segment of southern branch extends between the Gemlik and Bandirma gulfs in the south of the Sea of Marmara. In addition, uplift areas arose by compression and a push-up style in between the Kapidagi Peninsula and the Main Marmara Fault. The North Anatolian Fault is characterized by a negative flower structure in basins and push-up style in uplift areas in the Sea of Marmara. An uplift area arose between the north segment of the southern branch and the northern branch of the North Anatolian Fault. The north segment of the southern branch of the North Anatolian Fault is a strike-slip fault and displays a pull-apart style in the seismic reflection data.

Seismological Evidences of the Crustal Structures and Tectonics in the Yinchuan Downfaulted Basin

AbstractA high resolution deep seismic reflection profile of 68.9 km long across the Yinchuan faulted basin has been accomplished which for the first time yields the fine crustal structures, characteristics of deep fault system (Yellow River fault, Yinchuan fault and Eastern piedmont fault of Helanshan) of faulted basin in graben style, and the relationship between shallow and deep structures in Yinchuan basin. The results show that the upper crust is the region above a reflector with 8 s two‐way traveling time (about 20 km deep), there are many strata in the upper part of the upper crust where the continuity of different segment of stratum is good, and there is no obvious layered feature in the lower part of upper crust, where the geological structure is simple. The reflection energy is weak in the lower crust (8~13 s), where reflection events are not obvious. The crust‐mantle transitional zone (around 13 s) below the lower crust consists of a group of reflection sequences that have stronger energy and longer duration (1.5 s), and the thickness is about 4.5 km. Luhuatai fault and Yinchuan fault merge into the Eastern piedmont fault of Helanshan in the depth of 12~12.5 km and 18~19 km, respectively, the Eastern piedmont fault of Helanshan merges into Yellow River fault in the depth of 28~29 km, and Yellow River fault is a deep fault cutting the Moho. Yinchuan graben is a negative flower structure that is assembled mainly by Yellow River fault and secondarily by other faults. Based on the relationship between the Eastern piedmont fault of Helanshan and Yinchuan fault, it is thought that the Eastern piedmont fault of Helanshan played a controlling role in 1739 M=8 Pingluo‐Yinchuan earthquake.

Submarine structures in the Gulf of İzmit, based on multichannel seismic reflection and multibeam bathymetry

Multichannel seismic reflection and multi-beam bathymetry data were used to study the active tectonic and syn-tectonic stratigraphic setting of the Gulf of Izmit in the Marmara Sea (Turkey). The gulf and its near surroundings are deformed by the northern strand of the dextral North Anatolian Fault. Three connected basins of the gulf, the western (Darica), central (Karamursel) and eastern (Golcuk) basins are formed by active faults, as observed in the stacked and migrated seismic sections, as well as the bathymetry map. The main branch and its surrounding sedimentary strata are confined by normal faults to the north and south. These normal faults converge at depth towards the main fault, forming a negative flower structure in the gulf. The average maximum sedimentation rate is 0.4 mm/year according to the three most recent seismo-stratigraphic units that are located to the south of the main fault branch within the central basin. A 20° south-dipping major discontinuity along the northern shoreline of the gulf represents the top of Paleozoic basement.