Quaternary neotectonics of the Somogy Hills, Hungary (part I): Evidence from field observations

Quaternary and directly underlying Late Miocene (Pannonian) outcrops were analysed by structural, tectono-morphologic and sedimentologic methods to describe the main fault directions, to separate mass movements from faulting and folding and to separate earthquake-induced sediment deformations from other (e.g. periglacial) effects in the Somogy Hills. This is a gentle hilly area elevated at 200–300 m above sea level, located immediately south of Lake Balaton, Hungary.

Quaternary outcrops showed several consistent directions of faulting, and co-depositional seismic activity. Three different Mohr-sets of faults/joints could be differentiated in Quaternary sediments. The three sets are considered Late Quaternary since all cut young loess sections and have morphological expressions.

On the basis of the microtectonic measurements and morphotectonic investigations, the following sequence of Quaternary events can be proposed:

1. A (W)NW–(E)SE compression and perpendicular extension would create E–W to WNW–ESE oriented right lateral, NNW–SSE to N–S oriented left lateral shear zones, and NW–SE striking normal faults. Some of these can be evidenced in morphology and among the individual fault measurements. Some reactivated faults might suggest that this field is a relatively older one, but fresh topographic elements suggest that this stress field might be operational sub-recently.

2. A second stress field with NNW–SSE extensional and ENE–WSW oriented compressional directions could be separated. This stress field could create NNE–SSW and NW–SE oriented shear fractures and ENE–WSW oriented conjugate normal faults. Flat thrusts giving ENE directed shear may also be active under this field.

3. A third stress field might be proposed with N–S compression and perpendicular extension directions. This would create NE–SW and NW–SE oriented shear fractures, which are observed in the measured fault data. It is remarkable that the NE–SW faults are all steep, subvertical, and give a very well defined fault set. Based on the fresh topographic expression, this stress field is also sub-recent.

The different sub-recent stress fields and related fault patterns might succeed each other or might alternate through time. The first and third deformations have fresh topographic expressions and cannot play synchronously. The observed features suggest a compressionally active neotectonics of the study area.     

Late Quaternary dynamics of the Tisza River: Evidence of climatic and tectonic controls

The focus of this study is to understand a dramatic avulsion event on the Tisza River. During the Late Pleistocene the river course switched by about 80 km from the east to west of the Great Hungarian Plain (GHP) through the Záhony bend to its present meander belt. The aim of this study is to date this Záhony avulsion: based upon radiocarbon and pollen samples from six cores in the Polgár study area, situated in the west of the GHP at the middle course of the Tisza River. In addition, a grain size composition and heavy mineral analysis has been performed. The results of these analyses reveal a sequence of paleochannels and have been plotted on a high-resolution digital elevation model, illustrating the paleochannel form and age relationships.

The study suggests that the age of this major avulsion event is significantly older than it was previously supposed. Instead of 10 to 11 ka it is, according to our new data, 16–18 ka, and definitely predates the Last Glacial Maximum (LGM). The river cut into the previous, contiguous surface and formed at least one climatic terrace, then drifted gradually westward as a response to a tilt or the differential subsidence of tectonic origin.     

Tertiary paleomagnetic results and structural analysis from the Transdanubian Range (Hungary): rotational disintegration of the Alcapa unit

We carried out an integrated paleomagnetic and structural study in the Transdanubian Range, western and central Hungary. As a result, the Tertiary tectonic history of this area can be characterized by three events of counterclockwise (CCW) rotation and four or five phases of brittle deformation. The change of the orientation of stress axes between phases is mainly apparent and reflects the rotation of the faults predating a particular rotation event. The first two rotation events (R1 and R2) were probably governed by the rollback mechanism of the subducting European plate. We suggest that these rotations were taking place from 18–17 and 16–14.5 Ma, respectively, i.e. simultaneously with the rotations of the North Hungarian Paleogene Basin and the main part of the Western Carpathians. However, the angle of both rotations was less in the Transdanubian Range due to increasing distance from the subduction front. The differential rotation was accommodated by extensional faulting by formation of a graben system. On the other hand, the youngest rotation event R3 seems to be connected to the renewed rotation of the Adriatic plate around 5 Ma. Our combined data set strongly supports earlier conclusions, namely, that the different subunits of the Eastern Alpine–Western Carpathian–Northern Pannonian unit (Alcapa) did not form a rigid unit, although they moved in similar manner.     

Neotectonics of the Marikina Valley fault system (MVFS) and tectonic framework of structures in northern and central Luzon, Philippines

Recognition of neotectonic features along the Marikina Valley fault system (MVFS) in central Luzon, Philippines indicates a dominantly dextral strike-slip motion during its most recent activity believed to be Late Pleistocene to Holocene in age. Variations in the ratios of vertical to horizontal displacements for the segments imply a dominantly dextral motion of the West Marikina Valley fault (WMVF) and oblique dextral motion for the East Marikina Valley fault (EMVF). The displacement data further suggest that rupturing along the EMVF involved multiple segments and occurred separately from the events along the WMVF segments. Estimated earthquake magnitudes for the WMVF and EMVF based on single-event offsets fall within the range M 7.3–7.7. The vertical slip component in the northern part of the Marikina Valley is associated with the development of a basin between the EMVF and WMVF while the large vertical component in the southernmost segment of the EMVF (Talim) is attributed to volcanism-related extension. Lateral advection of the block bounded by the MVFS and the Philippine fault zone (PFZ), rather than pure shear resulting from an assumed east–west compression, best explains the observed kinematics of the MVFS. This is the result of compression during the westward drift of the Philippine Sea Plate and northern Luzon and occurs through slip along the WMVF and EMVF at rates of 5–7 mm/yr.     

Neotectonics of East Anatolian Plateau (Turkey) and Lesser Caucasus: implication for transition from thrusting to strike-slip faulting

Abstract

The east Anatolian plateau and the Lesser Caucasus are characterised and shaped by three major structures: (1) NW- and NE-trending dextral to sinistral active strike-slip faults, (2) N-S to NNW-trending fissures and /or Plio-Quatemary volcanoes, and (3) a 5-km thick, undeformed Plio-Quatemary continental volcanosedimentary sequence accumulated in various strike-slip basins. In contrast to the situation in the east Anatolian plateau and the Lesser Caucasus, the Transcaucasus and the Great Caucasus are characterised by WNW-trending active thrust to reverse faults, folds, and 6-km thick, undeformed (except for the fault-bounded basin margins) continuous Oligocene-Quaternary molassic sequence accumulated in actively developing ramp basins. Hence, the neotectonic regime in the Great Caucasus and the Transcaucasus is compressional-contractional, and Oligocene-Quaternary in age; whereas it is compressional-extensional, and Plio-Quatemary in age in the east Anatolian plateau and the Lesser Caucasus.

Middle and Upper Miocene volcano-sedimentary sequences are folded and thrust-to-reverse-faulted as a result of compressional- contractional tectonic regime accompanied by mostly calc-alkaline volcanic activity, whereas Middle Pliocene-Quaternary sequences, which rest with angular unconformity on the pre-Middle Pliocene rocks, are nearly flat-lying and dominated by strike-slip faulting accompanied by mostly alkali volcanic activity implying an inversion in tectonic regime. The strike-slip faults cut and displace dykes, reverse to thrust faults and fold axes of Late Miocene age up to maximum 7 km: hence these faults are younger than Late Miocene, i.e., these formed after Late Miocene. Therefore, the time period between late Serravalian (~ 12 Ma) continent-continent collision of Arabian and Eurasian plates and the late Early Pliocene inversion in both the tectonic regime, basin type and deformation pattern (from folding and thrusting to strike-slip faulting) is here termed as the Transitional period.

Orientation patterns of various neotectonic structures and focal mechanism solutions of recent earthquakes that occurred in the east Anatolian plateau and the Caucasus fit well with the N-S directed intracontinental convergence between the Arabian plate in the south and the Eurasian plate in the north lasting since Late Miocene or Early Pliocene in places. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Active crustal deformation in two seismogenic zones of the Pannonian region — GPS versus seismological observations

The seismicity and the associated seismic hazard in the central part of the Pannonian region is moderate, however the vulnerability is high, as three capital cities are located near the most active seismic zones. In our analysis two seismically active areas, the Central Pannonian and Mur-Mürz zones, have been considered in order to assess the style and rate of crustal deformation using Global Positioning System (GPS) and earthquake data.We processed data of continuous and campaign GPS measurements obtained during the years 1991–2007. Velocities relative to the stable Eurasia have been computed at HGRN, CEGRN and EPN GPS sites in and around the Pannonian basin. Uniform strain rates and relative displacements were calculated for the investigated regions. GPS data confirm the mostly left lateral strike slip character of the Mur-Mürz–Vienna basin fault system and suggest a contraction between the eastward moving Alpine-North Pannonian unit and the Carpathians.The computation of the seismic strain rate was based on the Kostrov summation. The averaged unit norm seismic moment tensor, which describes the characteristic style of deformation, has been obtained from the available focal mechanism solutions, whereas the annual seismic moment release showing the rate of the deformation was estimated using the catalogues of historical and recent earthquakes.Our analysis reveals that in the Central Pannonian zone the geodetic strain rate is significantly larger than the seismic strain rate. Based on the weakness of the lithosphere, the stress magnitudes and the regional features of seismicity, we suggest that the low value of the seismic/geodetic strain rate ratio can be attributed to the aseismic release of the prevailing compressive stress and not to an overdue major earthquake. In the Mur-Mürz zone, although the uncertainty of the seismic/geodetic strain rate ratio is high, the seismic part of the deformation seems to be notably larger than in the case of the Central Pannonian zone. These results reflect the different deformation mechanism, rheology and tectonic style of the investigated zones.     

Evidence for coupled reverse and normal active faulting in W Iberia: The Vidigueira–Moura and Alqueva faults (SE Portugal)

The Vidigueira–Moura fault (VMF) is a 65 km long, E–W trending, N dipping reverse left-lateral late Variscan structure located in SE Portugal (W Iberia), which has been reactivated during the Cenozoic with reverse right-lateral slip. It is intersected by, and interferes with the NE–SW trending Alentejo–Plasencia fault. East of this intersection, for a length of 40 km the VMF borders an intracratonic tectonic basin on its northern side, thrusting Paleozoic schists, meta-volcanics and granites, on the north, over Cenozoic continental sediments preserved in the basin, on the south. West of the faults intersection, evidence of Cenozoic reactivation is scarce. In the eastern sector, Plio-Quaternary VMF reactivation is indicated by geomorphologic, stratigraphic, and structural data, showing reverse movement with a right-lateral strike-slip component, in response to a NW–SE trending compressive stress. An average vertical displacement rate of 0.06 to 0.08 mm/yr since late Pliocene (roughly the last 2.5 Ma) is estimated. The Alqueva fault (AF) is a subparallel, northward dipping, 7.5 km long anastomosing fault zone that affects Palaeozoic basement rocks, and is located 2.5 km north and on the hanging block of the VMF. The AF is also a reverse left-lateral late Variscan structure, which has been reactivated during the Tertiary with reverse right-lateral slip; however, Plio-Quaternary reactivation was normal left-lateral, as shown by abundant kinematical criteria (slickensides) and geomorphic evidence. It shows an average displacement rate of 0.02 mm/yr for the vertical component of movement in the approximately last 2.5 Ma. It is proposed that the normal displacements on the AF result from tangential longitudinal strain on the upthrown block of the VMF above a convex ramp of this main reverse structure. According to this model of faults interaction, the AF is interpreted to work as a bending-moment fault sited above the VMF thrust ramp. Consequently, it is expected that the displacements on the AF increase towards the topographic surface with the increase in the imposed extension, declining downwards until they vanish above or at the VMF ramp. In order to constrain the proposed scheme, numerical modeling was performed, aiming at the reproduction of the present topography across the faults using different geodynamic models and fault geometries and displacements.     

Soft-sediment deformation structures in Late Miocene–Pleistocene sediments on the pediment of the Mátra Hills (Visonta, Atkár, Verseg): Cryoturbation, load structures or seismites?

The studied area, built up by silty clayey and partly sandy sediments and paleosols, lies on the tectonically active Northern margins of the Pannonian Basin. Wavy, sagging load casts can be observed in the upper part of the Late Miocene alluvial complex and larger scale sagging load casts, flame structures, drops and pillows detected in its Quaternary cover were studied in detail, in order to understand the origins of soft sediment deformation which characterized this young sedimentary suite. Sedimentological, paleopedological and mineralogical observations suggest that:

1. One of the reasons for the soft-sediment deformation might have been the relatively low cohesive strength of the predominantly smectitic sediment covering a gentle slope similar to the actual landscape.

2. On such a surface, the down-slope gravitational component of the mud-blanket might easily have been sufficient to overcome its cohesive strength.

3. Frost action traceable in the studied formations might also have contributed to the observed deformation, particularly along the eroded top of the Late Miocene sediments.

Combined evidence from field observations and laboratory analyses support the idea that liquefaction–fluidization was of prime importance in bringing about the observed structures. In conclusion two alternative Quaternary/Holocene scenarios are proposed, which might have resulted in the unusual behaviour of the sediments/paleosols. One is a seismic event, the other is the combined effect of freeze–thaw cycles and of the sloping foothill position, which might have resulted in episodic downslope transport and the associated deformation of the eroded soil material when its water content surpassed a certain threshold. We accept that the anomalous abundance of soft-sediment deformation in this marginal position may be causally related to paleo-earthquakes, but the obvious complexity of the phenomenon requires caution. In case the proposed scenarios would not have been alternatives but acted simultaneously, the analysed phenomena were to be interpreted as the joint results of tectonics and climate change.     

Tectonic evolution of the Qumran Basin from high-resolution 3.5-kHz seismic profiles and its implication for the evolution of the northern Dead Sea Basin

The Dead Sea Basin is a morphotectonic depression along the Dead Sea Transform. Its structure can be described as a deep rhomb-graben (pull-apart) flanked by two block-faulted marginal zones. We have studied the recent tectonic structure of the northwestern margin of the Dead Sea Basin in the area where the northern strike-slip master fault enters the basin and approaches the western marginal zone (Western Boundary Fault). For this purpose, we have analyzed 3.5-kHz seismic reflection profiles obtained from the northwestern corner of the Dead Sea. The seismic profiles give insight into the recent tectonic deformation of the northwestern margin of the Dead Sea Basin. A series of 11 seismic profiles are presented and described. Although several deformation features can be explained in terms of gravity tectonics, it is suggested that the occurrence of strike-slip in this part of the Dead Sea Basin is most likely. Seismic sections reveal a narrow zone of intensely deformed strata. This zone gradually merges into a zone marked by a newly discovered tectonic depression, the Qumran Basin. It is speculated that both structural zones originate from strike-slip along right-bending faults that splay-off from the Jordan Fault, the strike-slip master fault that delimits the active Dead Sea rhomb-graben on the west. Fault interaction between the strike-slip master fault and the normal faults bounding the transform valley seems the most plausible explanation for the origin of the right-bending splays. We suggest that the observed southward widening of the Dead Sea Basin possibly results from the successive formation of secondary right-bending splays to the north, as the active depocenter of the Dead Sea Basin migrates northward with time.     

Middle to late Pleistocene uplift rate of the Hungarian Mountain Range at the Danube Bend, (Pannonian Basin) using in situ produced He

Topography of the terraced Danube Bend area indicates fast incision of the Danube River, which was followed by its tributaries dissecting deeply the former terrace levels. These surfaces are vertically bended along the river course, indicating antecedent incision of the Danube into the SW–NE trending Hungarian Mountain Range (HMR). Timing and rate of the incision of the Danube into the HMR and consequently, the rate of vertical motions have remained unresolved so far. This study aims at quantifying the landscape evolution and neotectonic deformation of the central part of the HMR. We used terrace levels along the antecedent section of the Danube River to constrain its incision rate, which is a measure for the uplift rate of the HMR.

Here we use ³He, a terrestrial in situ produced cosmogenic nuclide (TCN), to date uplifted geomorphologic levels along in the Danube Bend gorge. This method, first applied in the Carpathian–Pannonian system in the framework of present study, proved to be suitable for the quantification of landscape evolution in this area. Our ³He exposure age data suggest a maximum incision rate of 2.7 ± 0.1 mm/y for the last 170 ky. Considering likely effect of erosion a more conservative value of 1.6 mm/y for the last 270 ky, was obtained. Both rates are significantly higher than the incision rate of 0.41 mm/y of the Danube derived from previous geologic and geomorphic data for the last 360 ky. The formation of the terrace levels in the Danube Bend probably occurred during the last two glacial cycles (OIS 1–8). According to the exposure age data, there is no direct relationship between the terrace formation and climate in the Danube Bend. Incision of the Danube appears to be connected to the uplift of the HMR, obtained incision rate values can be taken as valid approximations of the uplift rate in the Danube Bend area.     

Structural evolution of the Teletsk graben (Russian Altai)

Lake Teletskoye in the northeastern part of the Altai mountain range has attracted the attention of geo-scientists for a long time, because it fills an impressive tectonic depression. The lake is 77 km long and 4 km wide, and it has a maximum water depth of 325 m. The vertical offset of the basement surface is up to 3000 m. A multidisciplinary study of the Teletsk graben was carried out during the last few years, including satellite image and air photo analysis, bathymetric-, structural- and geomorphological mapping, high-resolution seismic profiling and seismic refraction. The structural study revealed that reactivation of preexisting weak basement zones is important in controlling the basin formation. These zones separate different tectonic terranes at the contact of which the Teletsk graben developed.This study identifies the significance of the basin in the regional neotectonic context. It shows that the major vertical movements are restricted to the basin itself, but do not characterize the whole region. Outside of the basin, recent tectonic structures have the same pattern as adjacent areas of Northeast Altai and West-Sayan. Quaternary glaciations have had no major influence on the basin formation.Two stages of faulting are identified. First, transpressive movements restricted to discrete (reactivated) fault zones controlled the opening of the basin. In the second stage, normal faulting is dominant and is responsible for the modern basin outline.An echo-sounding survey led to the recognition of several morphological characteristics of the lake bottom. In the southern part, the uppermost sediments seem slightly disturbed, whereas further north, transverse ridges and slope breaks are increasingly common. The deepest part of the lake is located in a highly disturbed zone of normal fault-bounded blocks. The structural difference between the southern and northern subbasins is supported by the interpretation of a deep seismic refraction profile which indicates a substantial increase of basement isochores in the area where the reactivated Teletsk (Paleozoic) shear zone crosses the lake.Correlation of high-resolution seismic profiles suggests that the Teletsk graben started to evolve during the Pleistocene, and that its present shape was formed in two stages. The first stage was responsible for the opening of the southern basin. It probably started in the Middle Pleistocene. A second kinematic stage induced by a sinistral reactivation of the NE striking West-Sayan fault initiated the opening of the different segments of the northern subbasin due to opposite movements between the reactivated Teletsk and West-Sayan faults. This second stage was active after the end of Late Pleistocene glaciations and during the Holocene. The recent lateral extension and the related N–S-trending normal faults result from a change in tectonic regime, with related extensional movements along the main reactivated fault zones. These recent movements result in the lateral escape of the lake borders and the collapse of the area between them.     

Oxygen isotope ratios of phenocrysts from alkali basalts of the Pannonian basin: Evidence for an O-isotopically homogeneous upper mantle beneath a subduction-influenced area

Oxygen isotope compositions of olivine and pyroxene phenocrysts and pyroxene and amphibole megacrysts from Neogene alkali basalts of the Pannonian basin (0.5–11 Ma) have been determined by laser fluorination. Measured δ¹⁸O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20‰ and from 5.07 to 5.34%., respectively, with cpx-ol fractionations Δ¹⁸O(cpx-ol) ranging from + 0.04 to + 0.29‰. These δ¹⁸O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the ¹⁸O/¹⁶O ratios of the groundmass of host rocks, even in fresh looking samples, whereas their phenocrysts have retained original oxygen isotope compositions. The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source. Variations in radiogenic isotope compositions in the basalts have been explained by the interaction of subduction-related fluids with the mantle source of the basalts. If this is the case, then the fluids which caused significant changes in the Sr and Pb isotope ratios of the mantle source clearly did not noticeably modify its oxygen isotope composition. These data support the opinion that the upper mantle is more homogeneous with respect to its oxygen isotope composition than it was previously considered.     

On the mechanics of basin formation in the Pannonian basin: Inferences from analogue and numerical modelling

We present the results of a thrust fault reactivation study that has been carried out using analogue (sandbox) and numerical modelling techniques. The basement of the Pannonian basin is built up of Cretaceous nappe piles. Reactivation of these compressional structures and connected weakness zones is one of the prime agents governing Miocene formation and Quaternary deformation of the basin system. However, reactivation on thrust fault planes (average dip of ca. 30°) in normal or transtensional stress regimes is a problematic process in terms of rock mechanics. The aim of the investigation was to analyse how the different stress regimes (extension or strike-slip), and the geometrical as well as the mechanical parameters (dip and strike of the faults, frictional coefficients) effect the reactivation potential of pre-existing faults.

Results of analogue modelling predict that thrust fault reactivation under pure extension is possible for fault dip angle larger than 45° with normal friction value (sand on sand) of the fault plane. By making the fault plane weaker, reactivation is possible down to 35° dip angle. These values are confirmed by the results of numerical modelling. Reactivation in transtensional manner can occur in a broad range of fault dip angle (from 35° to 20°) and strike angle (from 30° to 5° with respect to the direction of compression) when keeping the maximum horizontal stress magnitude approximately three times bigger than the vertical or the minimum horizontal stress values.

Our research focussed on two selected study areas in the Pannonian basin system: the Danube basin and the Derecske trough in its western and eastern part, respectively. Their Miocene tectonic evolution and their fault reactivation pattern show considerable differences. The dominance of pure extension in the Danube basin vs. strike-slip faulting (transtension) in the Derecske trough is interpreted as a consequence of their different geodynamic position in the evolving Pannonian basin system. In addition, orientation of the pre-existing thrust fault systems with respect to the Early to Middle Miocene paleostress fields had a major influence on reactivation kinematics.

As part of the collapsing east Alpine orogen, the area of the Danube basin was characterised by elevated topography and increased crustal thickness during the onset of rifting in the Pannonian basin. Consequently, an excess of gravitational potential energy resulted in extension (σ_v > σ_H) during Early Miocene basin formation. By the time topography and related crustal thickness variation relaxed (Middle Miocene), the stress field had rotated and the minimum horizontal stress axes (σ_h) became perpendicular to the main strike of the thrusts. The high topography and the rotation of σ_h could induce nearly pure extension (dip-slip faulting) along the pre-existing low-angle thrusts. On the contrary, the Derecske trough was situated near the Carpathian subduction belt, with lower crustal thickness and no pronounced topography. This resulted in much lower σ_v value than in the Danube basin. Moreover, the proximity of the retreating subduction slab provided low values of σ_h and the oblique orientation of the paleostress fields with respect to the master faults of the trough. This led to the dominance of strike-slip faulting in combination with extension and basin subsidence (transtension).     

Low-frequency 3-D seismic survey of upper crustal magmatic intrusions in the northeastern Pannonian basin of Hungary

A specially designed 700-km² grid survey, deploying 1000 regularly distributed low-frequency seismic recording systems, successfully investigated one of the most complex geologic environments of the Pannonian basin. The wide-angle signals penetrated through over 1000 m of multi-phase igneous lithology and recognized, for the first time, the underlying enigmatic Permian to Early Triassic basement rocks. Tomographic inversion of the first arrival grid data resulted in determination of an accurate three-dimensional (3-D) velocity field, to a depth of 4 km. The anomalous changes of the spatial velocity data outline the regional extent of the Late Miocene magmatic intrusions, which are covered by over 2000 m of Mid-Miocene to Pleistocene clastics. Complex relationship was found between the surface potential data and the intrusive bodies. This multi-faceted geophysical data analysis established a functional technique for mapping a subsurface with intricate acoustic and structural complexity.     

华北盆地地震剖面地质解释及其构造演化

从地震剖面显示华北盆地发育为手风琴式演化史,大体上形成有三层“断-坳”结构：（l）中新元古界的“断”古生界—三叠系的“坳”;（2）侏罗系的“断”上白垩统的“坳”;（3）古近系的“断”新近系的“坳”。断陷与坳陷分别由拉张与挤压应力场所致,这种应力场的变化是由相邻洋壳板块俯冲倾角由小到大的变化所引起的。由于地壳的多旋回运动,多次发生构造沉积演变,构成了多套生储盖组合,多领域、多种圈闭类型的油气藏,从中新元古界—古生界—中生界—新生界,各断陷-坳陷结构的盆地都有可能形成油气藏的地质条件。     

Crustal structure in the Carpatho-Pannonian region: insights from three-dimensional gravity modelling and their geodynamic significance

A three-dimensional gravity modelling of the Carpatho-Pannonian region was carried out to get a better image of the Moho boundary and the most prominent intra-crustal density heterogeneities. At first, only the major density boundaries were considered: the bottom of the Tertiary basin fill, the Moho discontinuity and the lithosphere to asthenosphere boundary. Density contrasts were represented by relative densities. The improved density model shows a transitional unit of high density at the base of the crust along the Teisseyre-Tornquist Zone. In the Western Carpathians, an extensive, relatively low-density unit was inferred in mid-crustal levels. The border zone between the Southern Carpathians and the Transylvanian basin is characterized by a sharp, step-like contact of the two crustal units. The Moho configuration reveals important information on the tectonic evolution of the region. Zones of continental collision are represented by thick Moho roots (Eastern Alps, Eastern Carpathians). Transpressional orogenic segments, however, are different: in the Western Carpathians, the Moho is a flat surface; in the Dinarides, a medium Moho root is observed; the Southern Carpathians are characterized by a thick crustal root. The differences are explained with the presence or absence of “subductible” oceanic crust along the Carpathians during the extrusion of Pannonian blocks.     

Crustal structure of the ancestral northwestern California forearc region from seismic reflection imaging: implications for convergent margin tectonics

Analysis of a suite of 2-D seismic reflection profiles reveals that the northwestern Sacramento Valley and eastern Coast Range foothills, northern California, are underlain by a system of blind, west-dipping thrust faults. Homoclinally east-dipping and folded Mesozoic marine forearc strata exposed along the western valley margin define the forelimbs of northeast-vergent fault-propagation folds developed in the hanging walls of the thrusts. Exhumed coherent blueschists of the accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern Coast Ranges are in the hanging wall of the blind thrust system, and have been displaced from their roots in the footwall. Deep, east-dipping magnetic reflectors in the footwall of the thrust system may be fragments of sheared, serpentinized and attenuated ophiolitic basement. Restoration of slip on the thrusts suggests that the Coast Range fault, which is the exposed structural contact between the coherent blueschists and attenuated ophiolite, originally dipped east and is associated with the east-dipping magnetic reflectors in the footwall. This interpretation of the reflection data is consistent with previous inferences about the deep structure in this region, and supports a two-stage model for blueschist exposure in the eastern Coast Ranges: (1) blueschist exhumation relative to the forearc basin by attenuation of the ophiolitic basement along the east-dipping Coast Range fault system in late Cretaceous; (2) blueschists, attenuated ophiolite, and forearc strata all were subsequently uplifted and folded in the hanging wall of the blind thrust system beginning in latest Cretaceous–early Tertiary. The blind thrust system probably rooted in, and was antithetic to, the east-dipping subduction zone beneath the forearc region. Active transpressional plate motion in western California is locally accommodated, in part, by reactivation of blind thrust faults that originally developed during the convergent regime.     

The North Maladeta Fault (Spanish Central Pyrenees) as the Vielha 1923 earthquake seismic source: Recent activity revealed by geomorphological and geophysical research

The Spanish Central Pyrenees have been the scenario of at least two damaging earthquakes in the last 800 years. Analysis of macroseismic data of the most recent one, the Vielha earthquake (19 November 1923), has led to the identification of the North Maladeta Fault (NMF) as the seismic source of the event. This E–W trending fault defines the northern boundary of the Maladeta Batholith and corresponds to a segment of the Alpine Gavarnie thrust fault. Our study shows that the NMF offsets a reference Neogene peneplain. The maximum observed vertical displacement is  730 m, with the northern downthrown sector slightly tilting towards the South. This offset provides evidence of normal faulting and together with the presence of tectonic faceted spurs allowed us to geomorphically identify a fault trace of 17.5 km. This length suggests that a maximum earthquake of M_w = 6.5 ± 0.66 could occur in the area. The geomorphological study was improved with a resistivity model obtained at Prüedo, where a unique detritic Late Miocene sequence crops out adjacent to the NMF. The section is made up of 13 audiomagnetotelluric soundings along a 1.5 km transect perpendicular to the fault trace at Prüedo and reveals the structure in depth, allowing us to interpret the Late Miocene deposits as tectonically trapped basin deposits associated with normal faulting of the NMF. The indirect age of these deposits has been constrained between 11.1 and 8.7 Ma, which represents a minimum age for the elevated Pyrenean peneplain in this part of the Pyrenees. Therefore, we propose the maximum vertical dip-slip rate for the NMF to be between 0.06 and 0.08 mm/a. Normal faulting in this area is attributed to the vertical lithospheric stress associated with the thickened Pyrenean crust.     

Tectonic inversion events in the western San Jorge Gulf Basin from seismic,borehole and field data

The San Jorge Gulf Basin, located in Central Patagonia, has been interpreted as a Jurassic-Cretaceous rift basin that was later inverted mainly in its western sector. Consequently, the Bernardides System formed as a set of foreland contractional structures that constitute the core of the Patagonian broken foreland, exhuming continental deposits of the Cretaceous Chubut Group, 500 km away from the Pacific trench. In spite of the intense research done in the San Jorge Gulf Basin many aspects remain under discussion, particularly those regarding the age of uplift of the Bernárdides System. In order to unravel the tectonic evolution of the western San Jorge Gulf Basin (Río Mayo Sub-Basin), we analyzed subsurface information (2D and 3D seismic lines and oil wells) located in the western area of the basin and compared this with surface data of the southern Bernárdides System. Based on our interpretation, the western part of the basin could have been uplifted in a series of deformational events that began as early as late Early Cretaceous, related to the initial uplift of the Patagonian broken foreland, during the early stages of South Atlantic opening. Subsequent stages of tectonic reactivation identified in this system have selectively inverted previous extensional structures according to the variable direction of the greatest horizontal stress (σ1) acting at each time.     

巴颜喀拉沉积盆地基底为扬子地台西缘的一部分：来自花岗岩的证据

侵入于巴颜喀拉沉积盆地中的扎加岩体，主要由高钾钙碱性的黑云母花岗岩组成，并出现有具岩浆结构的暗色微粒包体。花岗岩富集轻稀土（LREE）及大离子亲石元素（LILE），相对亏损重稀土（HREE）和高场强元素（HFSE），具Eu负异常（0．5～0．7）、较低的εNd（t）值（-3．5～-6．2）和中等的（^87Sr／^86Sr）初始比值（0．7088～0．7090），它们的亏损地幔Nd模式年龄tDM在1．1～1．3Ga之间。与寄主花岗岩相比，闪长质暗色微粒包体具有较低的SiO2，更低的Eu负异常（0．2～0．4）和εNd（t）值（-4．7～-6．1），更高的（^87Sr／^86Sr）初始比（0．7084～0．7124），以及稍老的tDM（1．4Ga）。岩石的地球化学资料表明，扎加花岗岩是在碰撞后构造环境下幔源岩浆上涌诱发下地壳岩石部分熔融的产物，其物源可能是苦海杂岩和万宝沟岩群的混合物。巴颜喀拉沉积盆地下面存在中元古代的基底，属于扬子地台西缘的一部分