Subvertical Boundaries Research Articles

Location and polarity of Variscan sutures based on petrological and seismological data from the Bohemian Massif and the implications for the European Variscides

The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Teplá-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle lithosphere domain beneath the Teplá-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Teplá-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (∼340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Teplá-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their granulite-amphibolite facies thermal overprint.

From mountain summits to roots: Crustal structure of the Eastern Alps and Bohemian Massif along longitude 13.3°E

The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearance, is clearly identified along most of the swath. The Variscan lithospheric blocks beneath the Bohemian Massif are imaged with sub-vertical boundaries. Beneath the Eastern Alps, the shape of the Moho is consistent with bi-vergent orogenic thickening, with a steeper and deeper-reaching Adriatic plate plunging northwards beneath the European plate in the north. At the junction of these plates at depth, around the previously proposed Moho gap, the root of the Eastern Alps is a broad trough characterized by a zone of low velocity-gradient that is up to 20 km thick, transitioning between crust and mantle. Our receiver-function results corroborate earlier lithosphere-upper mantle seismic tomography images, and highlight the Adriatic affinity of the Eastern Alpine slab. The zigzag deployment pattern of stations in the EASI experiment also allows distinction of short-wavelength variations perpendicular to the profile, both within the shallow and the deep crust. This underlines the importance of applying 3D imaging in complex geodynamic systems.

High-resolution velocity modeling by seismic-airborne TEM joint inversion: A new perspective for near-surface characterization

We discuss the use of helicopter-borne transient electromagnetic (HTEM) for a high-resolution spatial and vertical characterization of the near surface in a structure-controlled wadi in central Saudi Arabia. In this area, seismic data quality is poor, and seismic imaging suffers from a combination of scattering effects — due to swarms of faults reaching the surface — and large velocity variations occurring along subvertical boundaries between the wadi sediment infill and the surrounding carbonate plateaus. HTEM was selected from a suite of nonseismic methods for multiparameter velocity-model building to enhance the velocity estimation for the wadi and surrounding areas. HTEM data were modeled by performing spatially constrained 1D resistivity inversion to obtain a high-resolution image of the near surface with sensitivity to a depth of 400–500 m from the surface. Sharp boundaries of the wadi and fine vertical layering, obtained from the HTEM inversion, provide detailed information about the parameter variations in the near surface. A seismic-HTEM joint-inversion approach is developed using a cross-gradient structural operator to constrain the velocity inversion with the higher resolution HTEM data. Joint-inversion results provide sharp velocity reconstruction across the wadi boundaries and increase the dynamic range of the velocity variations when compared to a single-domain tomographic approach in which the HTEM contribution is ignored. Superior imaging results in both time and depth are derived from velocities estimated by the seismic-TEM joint-inversion approach.

Volcaniclastic dykes tell on fracturing, explosive eruption and lateral collapse at Stromboli volcano (Italy)

In the upper part of the Stromboli volcano, in the Le Croci and Bastimento areas, two dyke-like bodies of volcanic breccia up to two-metre thick crosscut and intrude the products of Vancori and Neostromboli volcanoes. We describe the lithofacies association of these unusual volcaniclastic dykes, interpret the setting of dyke-forming fractures and the emplacement mechanism of internal deposits, and discuss their probable relationships with the explosive eruption and major lateral collapse events that occurred at the end of the Neostromboli period.The dyke volcaniclastic deposits contain juvenile magmatic fragments (pyroclasts) suggesting a primary volcanic origin. Their petrographic characteristics are coincident with the Neostromboli products. The architecture of the infilling deposits comprises symmetrically-nested volcaniclastic units, separated by sub-vertical boundaries, which are parallel to the dyke margins. The volcanic units are composed of distinctive lithofacies. The more external facies is composed of fine and coarse ash showing sub-vertical laminations, parallel to the contact wall. The central facies comprises stratified, lithic-rich breccia and lapilli-tuff, whose stratification is sub-horizontal and convolute, discordant to the dyke margins. Only at Le Croci dyke, the final unit shows a massive tuff-breccia facies.The volcaniclastic dykes experienced a polyphasic geological evolution comprising three stages. The first phase consisted in fracturing, explosive intrusion related to magma rising and upward injection of magmatic fluids and pyroclasts. The second phase recorded the dilation of fractures and their role as pyroclastic conduits in an explosive eruption possibly coeval with the lateral collapse of the Neostromboli lava cone. Finally, in the third phase, the immediately post-eruption mass-flow remobilization of pyroclastic deposits took place on the volcano slopes.

Quality-factor and reflection-coefficient estimation using surface-wave ghost reflections from subvertical structures

Seismic interferometry can retrieve the Green's function between receivers from the cross-correlation and summation of recordings from a boundary of surrounding sources. Having the sources only along a boundary is sufficient if the medium is lossless. If the medium is dissipative, the retrieved result using cross-correlation contains non-physical (ghost) arrivals. When using receivers at the surface and transient sources in the subsurface for the retrieval of the reflection response in a dissipative medium, it has been shown that the retrieved ghost reflections are characteristic of the quality factor of the subsurface. The ghost reflections are caused by internal reflections inside subsurface layers. It has been shown with numerical examples for recordings in a borehole from a surface source that a ghost reflection can be discriminated from physical reflections and tied to a specific subsurface layer. After connecting the ghost reflection to a specific layer, the quality factor of the medium above this layer and the reflection coefficient at the layer interface can be estimated. In this article, we show how the above principles can be adapted and applied for surface waves. Due to intrinsic losses in the medium, surface-wave ghost reflections are retrieved from internal scattering between subvertical boundaries. We demonstrate the method on an ultrasonic dataset recorded on a sample composed of a PVC block and an aluminum block. The aluminum block has a groove parallel to the PVC/aluminum interface. Using a surface-wave ghost reflection between the groove and the PVC/aluminum interface, we estimate the quality factor of the PVC and the reflection coefficient at the PVC/aluminum interface. We also show that the ghost reflection can be identified and tied to the layer between the groove and the PVC/aluminum interface, thus confirming previous numerical findings.

Subvertical boundary between serpentinite sheet and Cretaceous sedimentary rocks in the Shimanto Belt, Yaseo area, Minami-Kyushu City, the southern Satsuma Peninsula, southern Kyushu, Japan

Subvertical boundary between serpentinite sheet and Cretaceous sedimentary rocks in the Shimanto Belt, Yaseo area, Minami-Kyushu City, the southern Satsuma Peninsula, southern Kyushu, Japan

The subvertical boundary between serpentinite sheet and Cretaceous sedimentary rocks in the Shimanto Complex, Yaseo area, Minami-Kyushu City, southern Satsuma Peninsula

The subvertical boundary between serpentinite sheet and Cretaceous sedimentary rocks in the Shimanto Complex, Yaseo area, Minami-Kyushu City, southern Satsuma Peninsula

Seismic migration by Gaussian beams summation

Searching for composite traps of hydrocarbons is an actual problem of oil- and gas-geology. The development of fast and at the same time accurate methods for migration of surface-recorded seismic data for construction of an image of the geological boundaries in the subsurface remains one of the major directions of development in modern exploration geophysics. The evolution of computers has enabled practical application of various expensive migration algorithms. For example, in seismic prospecting, pre-stack Kirchhoff migration is nowadays widely used. Unfortunately, in the construction of seismic images of complex geology containing vertical and/or subvertical boundaries, and also erosional surfaces and layers that have pinch-outs, numerous caustics arise, and most migration methods face difficulties in these cases.

Imaging the Dead Sea Transform with scattered seismic waves

SUMMARY With controlled seismic sources and specifically designed receiver arrays, we image a subvertical boundary between two lithological blocks at the Arava Fault (AF) in the Middle East. The AF is the main strike-slip fault of the Dead Sea Transform (DST) in the segment between the Dead Sea and the Red Sea. Our imaging (migration) method is based on array beamforming and coherence analysis of P to P scattered seismic phases. We use a 1-D background velocity model and the direct P arrival as a reference phase. Careful resolution testing is necessary, because the target volume is irregularly sampled by rays. A spread function describing energy dispersion at localized point scatterers and synthetic calculations for large planar structures provides estimates of the resolution of the images. We resolve a 7 km long steeply dipping reflector offset roughly 1 km from the surface trace of the AF. The reflector can be imaged from about 1 km down to 4 km depth. Previous and ongoing studies in this region have shown a strong contrast across the fault: low seismic velocities and electrical resistivities to the west and high velocities and resistivities to the east of it. We therefore suggest that the imaged reflector marks the contrast between young sedimentary fill in the west and Precambrian rocks in the east. If correct, the boundary between the two blocks is offset about 1 km east of the current surface trace of the AF.

Seismic activity and neotectonic evolution of theWestern Carpathians (Slovakia)

Seismic activity in the Western Carpathians is closely related to the crustal rheology and combined struc- tural pattern within the brittle upper crust. The structural pattern is a combination of three structural levels. The deep- est level is represented by the Paleo-Alpine suture zones dissected by neo-Alpine fault structures, above all strike- slip fault zones (second level). The recent tectonic regimes are controlled by the contemporary stress field and vertical movements (third level). Following the analyses of structures and various geophysical data, the principal seismo-tectonic zones of the Western Carpathians are defined. The most remarkable and important first-order tectonic structure in the Western Carpathians is the zone of the Pieniny Klippen Belt, which represents the contact of the Western Carpathian internides and the stable European Plat- form. The Mur-Mfault zone in the area of the Vienna Basin represents the contact of the Eastern Alps with the block of Western Carpathian internides. Both these tectonic lines represent subvertical boundaries with Tertiary tectonic activity. The ˇ Certovica fault zone is a surface projection of the thrust plane of the Veporic thick-skinned sheet over the Tatric unit. Based on geological and geophysical data it is as- sumed that the ˇ Certovica zone is recently active due to its extensional reactivation. Earthquake events are released here mostly along the Hron fault system of the ENE-WSW direc- tion.

A teleseismic study of the Altiplano and the eastern Cordillera in northern Bolivia: New constraints on a lithospheric model

The Altiplano, one of the world's great plateaus, is the most significant geomorphological unit of the Central Andes. Knowledge of its lithospheric structure is essential to the understanding of the mountain building in this region. In order to study this, we performed a teleseismic field experiment in northern Bolivia. During a 4‐month period, 34 short‐period seismic stations were installed along a 320‐km‐long profile, crossing the Altiplano and the Eastern Cordillera, from the volcanic arc zone to the sub‐Andean zone, perpendicular to the main structural direction. A careful study of relative travel time residuals led us to propose a qualitative model related to the structural patterns; the maximum amplitude of these residuals along the profile reaches 3 s for P phases and have a strong azimuthal dependence. The residuals show a noticeable sudden increase, clearly associated with the fault system bordering the Eastern Cordillera to the west, the Cordillera Real fault system. The high quality of the data allows us to perform a velocity inversion and to calculate at what depth lateral variations occur. The origin of the perturbations in the upper crust is reasonably accounted for by the depth variations of the sedimentary fill, with the maximum thickness found under the Altiplano basin. The velocity perturbations in the lower crust are interpreted as variations of the Moho depth which decreases from about 60 km below the Altiplano to 50 km below the Eastern Cordillera. In the upper mantle, a high‐velocity zone is observed below the Eastern Cordillera. The major feature of this model is a subvertical boundary, dipping slightly to the southwest, which separates two strongly contrasting velocity units throughout the model, from the surface down to 140 km. This boundary coincides with the Cordillera Real fault system and is interpreted as an old suture. As the Altiplano is characterized by low velocities in the crust, the high velocities beneath the Eastern Cordillera down to 120 km may correspond to the Brazilian craton; in the region studied, the western limit of the underthrusting of the craton corresponds to the western border of the Eastern Cordillera.

A teleseismic delay time study across the Central African Shear Zone in the Adamawa Region of Cameroon, West Africa

Summary. P-wave relative teleseismic residuals were measured for a network of seismological stations along a 300 km profile across the Adamawa Plateau and the Central African Shear Zone of central Cameroon, to determine the variation in crust and upper mantle velocity associated with these structures. A plot of the mean relative residuals for the stations shows a long wavelength (> 300 km) variation of amplitude 0.45 s. the slowest arrivals are located over and just to the north, of the faulted northern margin of the Adamawa Plateau. the residuals do not correlate with topography, surface geology or the previously determined crustal structure, in any simple way. The Aki inversion technique has been used to invert the relative residuals into a 3-D model of velocity perturbations from a mean earth model. the results show the region is divided roughly into three blocks by two subvertical boundaries, striking ENE and traversing both the crust and upper mantle down to depths greater than 190km. the central block, which is 2 per cent slower than the adjacent blocks, roughly corresponds to the Central African Shear Zone. the Adamawa Plateau, as an individual uplifted area, is explained by the interaction of a regional anomalous upper mantle associated with the West African Rift System, and the Central African Shear Zone, which provided a conduit for heat flow to the surface.

A theoretical analysis of groundwater flow in small drainage basins

Theoretically, three types of flow systems may occur in a small basin: local, intermediate, and regional. The local systems are separated by subvertical boundaries, and the systems of different order are separated by subhorizontal boundaries. The higher the topographic relief, the greater is the importance of the local systems. The flow lines of large unconfined flow systems do not cross major topographic features. Stagnant bodies of groundwater occur at points where flow systems meet or branch. Recharge and discharge areas alternate; thus only part of the basin will contribute to the baseflow of its main stream. Motion of groundwater is sluggish or nil under extended flat areas, with little chance of the water being freshened. Water level fluctuations decrease with depth, and only a small percentage of the total volume of the groundwater in the basin participates in the hydrologic cycle.