Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80–100 km long transcrustal ramps, the southward dipping ‘Sub-Tauern-Ramp’ and the northward-dipping ‘Sub-Dolomites-Ramp’. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The ‘crocodile’-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic–African continents.     

Formation of diapiric structure in the deformation zone,central Indian Ocean: A model from gravity and seismic reflection data

Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises and high-angle reverse faults. The structure (basement trough) has a length of about 150 km and deepens by up to 1 km from its regional trend (northward dipping). The basement trough includes a rise at its center with a height of about 1.5km. The rise is about 10 km wide with rounded upper surface and bounded by vertical faults. A broad freeair gravity low of about 20 mGal and a local high of 8 mGal in its center are associated with the identified basement trough and rise structure respectively. Seismic results reveal that the horizontal crustal compression prevailing in the diffusive plate boundary might have formed the basement trough possibly in early Pliocene time. Differential loading stresses have been generated from unequal crust/sediment thickness on lower crustal and upper mantle rocks. A thin semi-ductile serpentinite layer existing near the base of the crust that is interpreted to have been formed at mid-ocean ridge and become part of the lithosphere, may have responded to the downward loading stresses generated by the sediments and crustal rocks to inject the serpentinites into the overlying strata to form a classic diapiric structure.     

The crustal structure of the Guayana Shield, Venezuela, from seismic refraction and gravity data

We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.     

Basin-scale structure control of Carlin-style gold deposits in central Southwestern Guizhou,China: Insights from seismic reflection profiles and gravity data

Carlin-style gold deposits are widely distributed in Southwestern Guizhou, China. Research has dominantly focused on deposit-scale geology and structural control of mineralization. The relationship between the gold deposits and regional structures, in particular their control on the formation and distribution of those deposits is less well understood. Here we use seismic survey and gravity data to reveal the basin-scale structure control of the gold deposits in central Southwestern Guizhou. The seismic reflection profiles show strong reflection horizons including Upper Permian coal measures, and Dongwu and Guangxi unconformities. The seismic data provide important evidence of the basinal structural style and the relationship between fault-related folding and the Carlin-style gold deposits. Regional gravity data also revealed several paleo-uplifts, which appear to control the distribution of Carlin-style gold deposits.The Carlin-style gold deposits are spatially and temporally associated with fault-related folds. Based on their location, we classify the gold deposits into two types, (1) low-angle thrust-controlled deposits; and (2) high-angle reverse fault-related deposits. In the low-angle thrust-controlled deposits, main thrusts are listric, strike NW-SE and dip southwest. These thrusts detach into coal measures, tuffs and volcanic rocks and are bedding-parallel in the vicinity of the Dongwu unconformity. Gold ore bodies are found in the thrusts and characterized by cutting-layer veins near the surface and bedding-parallel veins at depth. The high-angle reverse fault-related deposits are located in the hinge of northeast-southwest-striking fault-related folds, with high-angle reverse faults near the hinge of folds crosscutting the Upper Permian coal measures, volcanic rocks and Dongwu unconformity, and then into the limestone of Middle Permian Maokou Formation. Ore bodies are bedding-parallel in anticline culminations where interlayer fracture zones in Upper Permian coal measures and volcanic rocks are generated by high-angle reverse faulting.We consider that the formation of the gold deposits is part of the tectonic evolution in the area. The two sets of NW-SE- and NE-SW-striking fault-related folds were successively formed during the closure of eastern Tethys Ocean in Late Triassic, and major gold deposits formed in the fault-related folds. The Dongwu unconformity and faults provided conduits for ore fluid flow, and the coal measures and the tuffs of Dachang Member acted as detachment layers and hosts for ore deposition.     

East Avalonia,the third partner in the Caledonian collisions: evidence from deep seismic reflection data

Further evidence for the existence of the terrane East Avalonia (Cadomia) in north-west Europe, its boundaries and its role in the Caledonian collisional processes comes from studies of deep seismic reflection data at sea and on land. Various sutures are found in the north-east and the north-west, and a generally poor reflectivity dominates in the major part. Further details of reflectivity patterns support the idea that a huge terrane which split from the northern rim of Gondwana moved northward and collided with the merging plates Laurentia-Baltica. Its docking features are analysed.     

Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data

A crustal depth section was obtained from Deep Seismic Soundings (DSS) along the Alampur-Koniki-Ganapeshwaram profile, cutting across the northern part of the Proterozoic Cuddapah basin, India, running just south of latitude 16° N and between longitude 78° E and 81°E. The existence of a low-angle thrust fault at the eastern margin of the Cuddapah basin (Kaila et al., 1979) was confirmed along a second profile. Another low-angle thrust, along which charnockites with the granitic basement are upthrust against the Dharwars was delineated further east. The contact of the khondalites (lower Precambrian) with quaternary sediments near the east coast of India seems to be a fault boundary, which may be responsible for the thick sedimentary accumulation in the adjoining offshore region.The basement in the western part of the Cuddapah basin is very shallow and is gently downdipping eastward, to a depth of 1.7 km about 20 km west of Atmakur. It attains a depth of about 4.5 km in the deepest part of the Kurnool sub-basin, around Atmakur. Under the Nallamalai ranges its depth varies between 3.5 and 6.5 km, with an easterly dip. In the region north of the Iswarkuppam dome, the basement is at a depth of about 5.0 km, to about 6.8 km in the eastern part of the Cuddapah basin. Outside the eastern margin of the basin, the depth of the basement is about 1.8 km and further eastwards it is exposed. A fault at the contact of the khondalites with quaternary sediments near the east coast brings the basement down to a depth of approximately 1.3 km.In the Kurnool sub-basin the depth to the Moho discontinuity varies from 35 km under Atmakur to 39 km under the Nallamalai hills. In the region of the Iswarkuppam dome it is at a depth of about 36 km, deepening to about 39 km before rising to 37 km towards the east. Two-dimensional velocity modelling using ray-tracing techniques tends to confirm these results.Gravity modelling of the crustal structure, utilizing a four-layer crustal model in most parts along this profile, conforms to the observed gravity values. A weak zone in the eastern part of the profile where high-density material (density 3.05 g/cm³) has been found seems to be responsible for the gravity high in that part.     

The major features of the crustal structure in north-eastern Venezuela from deep wide-angle seismic observations and gravity modelling

Venezuela is located on the plate boundary zone between the South American continent and the Caribbean plate. A relative movement of 2 cm/year is accommodated by a system of strike–slip faults running from the Andes to the Gulf of Paria. The Interior Range, a moderate-height mountain range, separates the Oriental Basin from the Caribbean. To the south, predominantly Precambrian rocks are outcropping in the Guayana Shield south of the Orinoco River. Results of deep wide-angle seismic measurements for the region were obtained during field campaigns in 1998 (ECOGUAY) for the Guayana Shield and in 2001 (ECCO) for the Oriental Basin. The total crustal thickness decreases from 45 km beneath the Guayana Shield, to 39 km at the Orinoco River, and 36 km close to El Tigre, in the center of the Oriental Basin. The average crustal velocity decreases in the same sense from 6.5 to 5.95 km/s. Detailed information was obtained on the velocity distribution within the Oriental Basin. Velocities are as low as 2.2 km/s for the uppermost 2 km, 4.5 km/s down to 4 km in depth, and a maximum depth of 13 km was derived for material with seismic velocities up to 5.9 km/s, interpreted as the base of the sedimentary basin. A gravimetric model confirms the structures derived from the seismic data. Discrete increases in sedimentary thickness along the basin may be associated to extension processes during the passive margin phase in the Cretaceous, or during earlier extension phases.     

Crustal structure of the southern Dabie ultrahigh-pressure orogen and Yangtze foreland from deep seismic reflection profiling

A new 140‐km‐long seismic reflection profile provides a high‐resolution crustal‐scale image of the southern Dabieshan high‐pressure (HP) metamorphic belt and the Yangtze foreland fold‐and‐thrust belt. The seismic image of the stacked section shows that the southern Dabieshan metamorphic terrane and Yangtze foreland belt are separated by a large north‐dipping fault. In the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid‐Triassic; it was reworked by crustal extension resulting in the formation of a late Jurassic and Cretaceous red‐bed basin. The southern Dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt. The Moho reflectors at 10–11 s (～30–33 km) are seismically prominent and segmented by a number of south‐verging thrusts that were probably developed by foreland‐directed thrusting of the deeply subducted continental crust during exhumation. The seismic reflection profile suggests that structures related to the Triassic–Jurassic subduction and exhumation of the Yangtze plate are preserved despite the severe crustal extension superimposed during the late Mesozoic and Cenozoic.     

Reconstructing the Wolfe Creek meteorite impact: deep structure of the crater and effects on target rock

The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed?<?300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July?–?August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of?>?12.0 m, a crater formation time of 3.34 s, and an energy of impact of ～0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.     

Some interpretational aspects of time cross-sections based on seismic reflection data from the Sea of Okhotsk

The results of investigations in the Sea of Okhotsk by the seismic reflection method are considered with special attention paid to the acoustic anomalies. Two types of such anomalies are definable in the basin: local (“gas channel”) and areal (“turbid sequences”). It is shown that the acoustic anomalies distinguishable in the single channel seismic profiling (SCSP) records are characteristic of sediments with partial gas filling. Many local anomalies documented on the Sakhalin slope are spatially connected with zones of faulting. The simultaneous presence of acoustic anomalies and BSR in the upper part of the section serves as a reliable diagnostic feature of gas hydrates. The dispersion of the velocities in the wave packet characterizing the acoustic anomaly associated with the lower part of the sedimentary cover in the Kurile Basin points to the significant thickness of the gas-bearing sediments in the latter.     

地球深部探测的先锋: 深地震反射方法的发展与应用

深地震反射剖面早已被国际上证实是一种地球深部探测的先锋技术,它利用比石油地震勘探更长的接收排列、更大的激发能量,探测上至地表,下达上地幔的精细结构和构造特征,现已被国内外越来越多地应用于大陆及海洋地壳与岩石圈上地幔探测上。深地震反射剖面技术在揭示地球深部动力学过程,论述大地构造演化,确定盆山耦合关系,推测成矿成藏条件,分析地震灾害等方面取得了丰硕成果。系统地梳理和概括了国内外陆地地区深地震反射剖面技术的研究现状以及典型应用案例。在此基础上,从深地震反射野外数据采集、数据资料处理、剖面地质解译以及多方法联合探测4个方面,对地震反射剖面技术未来发展方向进行了展望。研究认为：在野外采集方面,研发不同激发、接收组合类型的采集技术、提升深地震反射数据采集质量,减少环境破坏,降低经济成本;在数据处理中,继续探究提高深地震反射剖面的信噪比与分辨率,定量监控深地震反射数据处理过程中振幅保真度的相对变化;在资料解译时,将深入挖掘深地震反射剖面的叠前、叠后潜在信息,降低单纯依靠深地震反射振幅解译的非唯一性;在综合研究上,从多学科、多角度、多尺度、多方法上相互补充印证,降低剖面的多解性,提高成果的准确性和可靠度。

     

The structure of the Kohistan-Arc terrane in northern Pakistan as inferred from gravity data

Modelling of gravity data taken across the Kohistan Island-Arc terrane in northern Pakistan can be used to constrain the shape and thickness of the Arc.Over 600 new gravity measurements were made across the Kohistan Island-Arc terrane in northern Pakistan. These data were taken along traverses normal to the structures bounding the Arc and were reduced to terrain-corrected Bouguer values. The reduced data were then modelled using standard two-dimensional modelling techniques.The southern margin of the Arc, the Main Mantle Thrust (MMT), dips to the north at approximately 45° and gradually flattens out at a depth of 7–9 km. The northern margin of the Arc, the Main Karkoram Thrust (MKT), also dips towards the north, but at a shallower initial angle (15°). From the models, the Arc terrane now appears to be around 7–9 km thick with the thicker sections occurring closer to the southern margin.The proposed model, in particular the angle of the MMT and the MKT, may have been significantly affected by the recent and rapid uplift that is occurring along the Nanga Parbat-Haramosh Massif.     

A new tectonic model for Abu-Dabbab seismogenic zone (Eastern Desert,Egypt): evidence from field-structural,EMR and seismic data

Abu-Dabbab area is the most active seismic zone in the central Eastern Desert of Egypt, where seismic activities are daily recorded. The reported earthquakes are microearthquakes of local magnitudes (ML < 2.0). A spatial distribution of these microearthquakes shows that the earthquakes of the area follow an ENE–WSW trending pattern, which is nearly perpendicular to the Red Sea Rift. Focal mechanisms of different fault styles were recognized with dominant normal faulting (with a strike-slip component) events characterized by focal depths greater than 7 km and reverse ones of shallower focal depths. Several lines of evidence indicating that the brittle-ductile transition zone underlies the Abu-Dabbab area occurs at a relatively shallow depth (10–12 km) and it is acting as a low-angle normal shear zone (LANF). Field-structural, EMR and seismic data (this study) reveal that the maximum compressive stress (σ1) in the area is perturbed from the regional NW–SE direction to ENE–WSW orientation. This stress rotation is evidently akin to the reactivation of the crustal scale Najd Fault System (NFS), where such reactivation is attributed to the ongoing activity/opening of the Red Sea. Our tectonic model proposes that the continuous activity on the brittle-ductile transition zone including the LANF led to stress localization, which triggering a brittle deformation in the upper crustal-levels and associated shallow dipping thrusts. Such bimodal tectonic model suggests that the deep earthquakes are owing to the tectonic movement on the LANF (transtension), whereas the shallow earthquakes are related to a brittle deformation inside the fault blocks of the upper crust (transpression). Deformation creep along this zone didn’t permit continuous accumulation of strain and hence reduce the possible occurrence of large earthquakes.     

The 1992 drill core from the Kalkkop impact crater, Eastern Cape Province, South Africa: stratigraphy, petrography, geochemistry and age

New drill core data are provided which support earlier interpretations that the Kalkkop structure, a 600–630 m wide, near-circular feature south-southwest of Graaff-Reinet in the Eastern Cape Province of South Africa, is a meteorite impact crater. Shock metamorphosed clasts in suevitic crater fill and Re---Os isotope data of this breccia indicate the presence of a minor (0.05%) meteoritic component in the suevite. The new data come from a 1992 borehole, which transected the complete crater fill and extended from about 160 to 380 m depth into the sedimentary basement belonging to the Koonap Formation of the Beaufort Group (Karoo Supergroup). Dyke breccias were found in the otherwise coherent Beaufort Group sediments forming the floor to the Kalkkop Crater. Mostly narrow zones of different breccia types, including injections of lithic impact breccia, a possible pseudotachylite veinlet and cataclasite occur predominantly in an approximately 65 m wide zone below the crater floor, with a few other cataclasite occurrences found lower down in the basement. Stratigraphical crater constraints provide information for the depth-diameter scaling and breccia volumes associated with such small, bowl-shaped impact craters formed in sedimentary targets.U---Th series dating of limestone samples from near the top and the bottom of the crater sediment fill constraints the age of the Kalkkop impact event to about 250 ± 50 ka, similar to the age of the Pretoria Saltpan impact crater, also located in South Africa. The variety of different breccia types (polymict and monomict impact breccias; local formations of pseudotachylitic and cataclastic breccias) observed in the crater fill of the Kalkkop Crater indicates the need to carefully distinguish different breccia types in order to assess the respective importance of each formation.     

The structure of the lithosphere beneath the Eastern rift, East Africa, deduced from gravity studies

A compilation of all published and unpublished gravity data for the Eastern rift between latitudes 1°N and 5°S is presented. The Bouguer anomaly map reveals that the shape of the negative regional anomaly associated with the rift is approximately two-dimensional, striking east of north, of width 350 ± 50 km and amplitude500 ± 100 g.u. relative to the background value of−1300 ± 100 g.u. to the west. The regional anomaly is interpreted in terms of an upward thinning of the lithosphere and replacement by low-density asthenosphere. This model is different from previous interpretations in that major lithospheric thinning is restricted to the region of the Eastern rift affected by the domal uplift and does not extend beneath the Lake Victoria region to the west. The gravity and seismic models are compatible if the anomalous upper mantle (asthenospheric part), beneath the rift, is in a state of partial melt. A consequence of the revised regional anomaly is that it reduces previous amplitude estimates of the axial positive residual anomaly within the rift by at least 50% and generates negative anomalies over the rift shoulders in areas covered by Cenozoic volcanics. These negative anomalies are considered to be caused by the low density of the surface volcanics. Within the rift, elongated negative anomalies of amplitude 100–350 g.u. are associated with sedimentary basins and are attributed to low-density sediments up to 3 km thick. The positive residual anomaly along the axis of the rift can be interpreted in terms of either a dyke injection zone less than 15 km wide or by a dense infill body about 2.5 km thick. The positive anomaly is shown to be confined to the volcanic province of the Eastern rift and has its southern termination in the Magadi—Natron area, just north of where the Kenya rift valley changes to block faulting in N. Tanzania. This termination coincides with a change in the spatial distribution of the seismic and geothermal activity.     

Crustal structure and sedimentation history over the Alleppey platform,southwest continental margin of India:Constraints from multichannel seismic and gravity data

The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that:(1) the Alleppey Platform is associated with a basement high in the west of its present-day geometry(as observed in the time-structure map of the Trap Top(K/T boundary)),(2) the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and,(3) both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns(as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region). The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region.The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension,in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.     

Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey

A ～400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.     

三门峡断陷盆地及邻区深部结构及成因——来自深地震反射与大地电磁测深的证据

近年来,围绕三门峡断陷盆地中的油气、地热资源做了大量的工作,成因机制研究较少,严重制约了矿产资源的勘探开发。本文在前人研究工作基础上,结合野外地质调查并利用高精度深反射地震剖面、大地电磁（MT）、重磁等地球物理探测技术,对三门峡盆地进行综合研究。发现三门峡盆地主要由东、西2个负花状构造构成,西花状构造体大于东花状构造体;盆地东部边缘以观音堂隆起与洛阳凹陷相邻,观音堂隆起发育有壳内透镜状低速体,其东、西两侧均发育有规模较大的隐伏逆断层。研究区内莫霍面为大约5 km厚度滑脱层,在深反射地震剖面上表现为蚯蚓状反射特征,指示滑脱层为西向运动。莫霍面滑脱层上部与下部新发现多条弧形断层。地质与地球物理资料综合研究表明,莫霍面滑脱层的解耦作用是三门峡断陷盆地花状构造形成的主因;在不同时空构造力系作用下,形成研究区新生代全地壳旋转花状构造盆地。     

Structural interpretation of the Marmara region, NW Turkey, from aeromagnetic, seismic and gravity data

The seismically active Marmara region, located in NW Turkey, lies on the westward end of the North Anatolian Fault (NAF). The NAF is well defined on land. Previous investigations of its extension in the Marmara Sea include marine bathymetry, seismological activity and seismic profiles. In this study, faults and their configurations identified inland are extended into the Marmara Sea by means of aeromagnetic anomalies, as well as seismic and gravity profiles. The deep structure was resolved by constructing a map of the Tertiary bottom. Shallow Curie isotherm was determined by spectral analysis, indicating a thinner crust in the northern Marmara depression area with respect to the continental crust. A combination of the geophysical data allows us to propose the existence of subsidence and isostatic equilibrium in the northern Marmara Sea. A less-active zone identified in the central high zone dividing the Marmara Sea into two parts may also be deduced from the seismic data. This structural arrangement may play a key role in earthquakes that will affect the surrounding regions.     

Tectonic setting of post-orogenic granites within SW Fennoscandia based on deep seismic and gravity data

Within the Svecononvegian Province of SW Scandinavia granite intrusions are abundant. Prominent ones are the Bohus-Iddefjord and the Flå granites, the so- called Bohus-Flå Granite belt. The age of these granites, consistent at ≅ 900 Ma, coincides with the late extensional stage of the Svecononvegian-Grenvillian orogen. Gravity observations and deep seismic profiling lines are presented that also cover the Skagerrak Sea. The geophysical data suggest that the Bohus granite continues seaward for at least 80 km and its thickness, offshore, is estimated to be 2–4 km. Where the seismic profiling lines intersect the inferred seaward extension of the Bohus granite, a distinct reflection pattern is observed at ≅ 1.6 s. TWT. This coincides with a gravity modelled thickness of ≅ 4 km. The seismic profiles also show a large Moho offset beneath the modelled granite. It is proposed that this offset is related to Svecononvegian crustal underthrusting and that the granite melt could have formed by anatexis of mid-crustal rocks downthrusted to greater depths in the vicinity of the seismically observed Moho offset.