Reflective Lower Crust Research Articles

Deep seismic reflection profiling revealing the complex crustal structure of the Tongling ore district

The deep seismic profiling across the Tongling ore district reveals a complex crustal structure. Strong contrasting dipping layered reflections (4—11 s, TWT)，which dominate the lower crust of the northern part of the profile, are interpreted as the underplating of the basalt related to extensional tectonism. In the south of the profile, the Yangtze craton is characterized by strong reflections in the middle crust, showing a distinct two-layer crustal structure. Over the region of the Tongling uplift, there appear the complex arc shape reflections, suggesting folded, faulted and intruded structures, and the transparent zone below them reveals the existence of batholith. The south dipping strong reflections between the upper crust and lower crust (4—7 s, TWT) suggest a detachment between them. The detachment provided space for the magma intrusion, and caused the formation of the batholith. The Yangtze craton has a clear Moho reflection, while the Tongling uplift has a weak Moho, whereas below the reflective lower crust in the northern part of the profile, there are sub-Moho reflections. The abrupt variation of the Moho characteristics within a short distance indicates the complexity of magmatic activity.

Deep reflection surveying in central Tibet: lower-crustal layering and crustal flow

SUMMARY In the summer of 1998, a series of multichannel, deep seismic reflection tests were conducted in central Tibet around 89 ◦ E as part of the third phase of Project INDEPTH, a multidisciplinary study of the structure of the core of the Tibetan Plateau. Short seismic reflection sections from four locations spanning the Jurassic Banggong‐Nujiang Suture show several features: thin (<2 km), sedimentary cover sequences; an unreflective upper crust down to approximately 25 km depth; strongly reflective lower crust down to 65 km depth (22 s traveltime) and a distinct change in seismic character from layered reflectivity to unreflective that we interpret as marking the reflection Moho. These profiles are unlike those in parts of the Yadong‐Gulu Rift to the south where few deep reflections exist and reflection profiles are dominated by highamplitude reflection bright-spots at 6 s which have been interpreted as fluid bodies, either water or magma. Analyses of seismic amplitude decay on individual shot records confirm sourcegenerated energy at traveltimes beyond 22 s, thus indicating that the change in reflection character at the Moho is not the result of a loss of signal penetration. The depths to the top and bottom of the lower-crustal reflectivity match both a mid-crustal velocity increase and the Moho determined for this area from refraction profiling. Lower-crustal reflectivity is consistent with current ductile deformation as suggested by models for middle and lower-crustal flow in Tibet. However, Cretaceous-age magmatic intrusions may explain at least part of the observed reflectivity.

Seismic structure of 15 Ma oceanic crust formed at an ultrafast spreading East Pacific Rise: Evidence for kilometer‐scale fracturing from dipping reflectors

A grid of seismic reflection data from 15 Ma Cocos Plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading (∼200 mm/yr, full rate) East Pacific Rise (EPR). The data contain an unexpected quantity of bright reflectors throughout the crust, generally &lt;5 km in length. Significant reflection dip angles (∼10–30°) are seen only on the isochron profiles, and several reflectors have a confirmed isochron dip orientation. Certain upper crustal reflectors (UCRs) appear to form a single network with isochron dip. The most prominent example (apparent reflection coefficient ∼0.2) projects toward a ∼70 m disruption in the igneous seafloor, and waveform analysis demonstrates that the reflector is likely the product of a low‐velocity zone on the order of tens of meters thick. Our observations suggest that the isochron‐dipping UCRs represent small‐slip, kilometer‐scale faults/fractures, and none are primarily related to the seismic 2/3 boundary or to the sheeted dike/gabbro transition. Lower crustal reflectors (LCRs) have no preferred orientation or regular pattern on the 30 km scale of this survey. Certain LCRs may be similar to the UCRs in origin, while others may originate from near‐axis ductile flow, lithologic banding, off‐axis magmatism, or some combination thereof. Overall, reflectors in this area are abundant and complex, consistent with previous off‐axis studies, but at odds with observations of young crust (&lt;5 Ma) near the EPR. This dichotomy may be resolved by a delay in reflector formation, through some combination of postaccretionary tectonics, magmatism and hydrothermal alteration.

Moho topography and lower crustal wide-angle reflectivity around the TESZ in southern Scandinavia and northeastern Europe

The Moho topography is strongly undulating in southern Scandinavia and northeastern Europe. A map of the depth to Moho shows similarities between the areas of the Teisseyre–Tornquist Zone (TTZ) in Poland and the Fennoscandian Border Zone (FBZ), which is partly coinciding with the Sorgenfrei–Tornquist Zone (STZ) in Denmark. The Moho is steeply dipping at these zones from a crustal thickness of approximately 32 km in the young Palaeozoic Platform and basin areas to approximately 45 km in the old Precambrian Platform and Baltic Shield. The Moho reflectivity (P MP waveform) in the POLONAISE'97 refraction/wide-angle seismic data from Poland and Lithuania is variable, ranging from ‘sharp’ to strongly reverberating signals of up to 2 s duration. There is little or no lower crustal wide-angle reflectivity in the thick Precambrian Platform, whereas lower crustal reflectivity in the thin Palaeozoic Platform is strongly reverberating, suggesting that the reflective lower crust and upper mantle is a young phenomena. From stochastic reflectivity modelling, we conclude that alternating high- and low-velocity layers with average thicknesses of 50–300 m and P-wave velocity variations of ±3–4% of the background velocity can explain the lower crustal reflectivity. Sedimentary layering affects the reflectivity of deeper layers significantly and must be considered in reflectivity studies, although the reverberations from the deeper crust cannot be explained by the sedimentary layering only. The reflective lower crust and upper mantle may correspond to a zone that has been intruded by mafic melts from the mantle during crustal extension and volcanism.

CDP and DSS data along the Uchta–Kem profile (the Baltic Shield)

Detailed Common Depth Point (CDP) and Deep Seismic Sounding (DSS) observations were carried out along the W–E trending Kem–Uchta profile between the White Sea and the Russian–Finnish border. The cross-sections obtained from wide-angle and near-vertical reflections show different features. In the CDP cross-section, several inclined boundaries are traced from the surface down to 25–30 km depth. They correlate with the well-known fault zone between the Belomorian Mobile Belt and the Korelian Protocraton. The DSS data also show these inclined reflectors as well as a near horizontal boundary, K1 (velocity of 6.3 versus 6.4 km/s, depth of 10–15 km), under a low velocity zone. The boundary generates high amplitude wide-angle reflections, which cross the CDP inclined reflections. A lower crustal boundary, K2 (velocity of 6.7 versus 6.8 km/s, depth of 30 km), has no clear expression in the CDP reflectivity pattern as well. Strong reflections from Moho (PmP) coincide with the boundary between reflective lower crust and transparent upper mantle on the CDP section (depth 40 km). This suggests that the near-horizontal crustal boundaries and the Moho are transition zones with high velocity gradients and not sharp discontinuities. The data analysis suggests the crustal layering to be a result of rock transformation (metamorphism, change in mechanical properties) after consolidation of the crust. Tectonic features of orogenic periods were overprinted by the metamorphic processes in the lower crust. In the middle crust, a destruction (low velocity) zone and the K1 boundary were created, but the initial tectonic structure of the upper crust was preserved. The Moho is a young feature that was formed during the platform stage of the crustal evolution.

Crustal architecture of the southern Uralides from true amplitude processing of the Urals Seismic Experiment and Integrated Studies (URSEIS) vibroseis profile

True amplitude processing of the Urals Seismic Experiment and Integrated Studies (URSEIS) vibroseis deep reflection seismic data acquired by the URSEIS consortium shows the southern Uralide crust to be composed of four major blocks with distinctive reflection characteristics. These blocks are juxtaposed along crustal‐scale boundaries. The foreland thrust and fold belt, developed from the East European craton crust, is imaged as subhorizontal to east dipping reflectivity that can be related to its Paleozoic and older tectonic history. The Moho beneath the foreland thrust and fold belt is not imaged in the vibroseis data set. The Main Uralian fault (the major arc‐continent suture) is unreflective, but its subsurface location can be inferred by the truncation of the reflection pattern of the East European craton and its contrast with that of the Magnitogorsk arc. The Magnitogorsk arc reflectivity is characterized by patchy, noncoherent to coherent reflections in the upper ∼10 – 15 km that are interpreted to be related to the arc volcanic rocks. Below this, reflectivity is diffuse, or the arc crust is transparent, and the Moho is not imaged. The East Magnitogorsk fault zone, which juxtaposes the arc against the East Uralian zone, is not imaged by the data. The upper 5 to 6 km of the East Uralian zone, corresponding to the Dzhabyk granite, is transparent. Below the granite the crust is characterised by east dipping patches of moderately coherent, high‐amplitude reflections that in the east become shallowly west‐dipping. A ∼10 km thick, west dipping band of coherent, high amplitude reflections between 12 and 35 km depth, corresponding to the Kartaly Reflection Sequence, extends beneath almost the entire East Uralian zone. The crust beneath the easternmost East Uralian zone reaches 53 km in thickness. The upper and middle crust of the Trans‐Uralian zone is characterized by a series of east and west dipping, concave upward, moderately coherent, high‐amplitude reflections. The lowermost middle and lower crust displays thin bands of west dipping, coherent, low‐amplitude reflectivity. The Moho is imaged as a sharp transition from reflective lower crust to transparent upper mantle at ∼49 km depth, and the lower crustal reflections appear to merge with it. The URSEIS vibroseis data are integrated with results from the explosion source reflection data, the wide‐angle reflection data, and the surface geology to place constraints on the crustal architecture of the orogen and on the timing of its assemblage.

Refraction/wide-angle reflection investigation of the Cadomian crust between northern Brittany and the Channel Islands

A seismic refraction experiment has been carried out off the coast of northern Brittany (NW France) to investigate the deep structure of the North Armorican block. The seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations. We present 2D velocity models computed from two crossing seismic refraction profiles (labelled P1 and P2 and each about 135km long) oriented NE–SW and NNW–SSE, i.e. parallel and perpendicular, respectively, to the regional basement structures. Wide-angle reflection profiles were also performed to extend the 2D structures to a 3D crustal interpretation. Observed and calculated P-wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models. The model which best fit the data consists of: (1) high basement velocities at the top of the crust, except in an area with shallow seismic velocity which can be as low as 3.0km/s at the surface and possibly related to the existence of shallow Paleozoic sedimentary basins; (2) the mean crustal velocity ranges from 5.5 to 6.8km/s at a depth of 15km depth; (3) lower crustal velocities range from 7.0 to 7.5km/s at a depth of 35–38km and then from 7.5 to 8.1km/s at the crust–mantle boundary located between 35 and 38km depth. Comparisons with the crustal section imaged by the coincident seismic vertical reflection profile SWAT 10 show a good correlation between velocity gradients and lower crustal reflectivity. When compared to surrounding basement areas, the North Armorican Cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this Upper Proterozoic block from later Paleozoic and Mesozoic tectonic episodes. The crustal structure of the North Armorican block thus appears to be an intercalated feature between the surrounding Paleozoic areas and older shield or platform domains.

Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic

The island of Tasmania in southeast Australia consists of a number of stratotectonic elements. The relationships between these elements are largely obscured by younger cover of the Tasmania Basin, which contains extensive dolerite sills that limit the ability of potential field techniques to map basement. Therefore the development of a robust tectonic model for Tasmania has been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around the entire island was designed to map the large-scale structures of Tasmania at depth. The airgun seismic energy was also recorded at a number of seismographs deployed across the island, allowing low resolution 3D tomographic imaging. Short reflection profiles were recorded onshore across structures which could not be imaged by the offshore profiling. This paper focuses on eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large rotated blocks with weakly reflective boundary faults, indicating extension of the Tyennan Element by block faulting. The deposition of the sedimentary succession of the Adamsfield–Jubilee Element was related to this extensional event. In the northeast, a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust and possibly fragments of oceanic crust. The Early Palaeozoic sedimentary succession of the Northeast Tasmania Element formed across the inverted margin. The apparently complex geology of eastern Tasmania therefore fits into an extensional model where continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east. The extension was probably related to Late Neoproterozoic extension recorded elsewhere in Australia. The region was subsequently shortened, probably in a northeast–southwest direction, with most shortening accommodated in the seismically reflective, probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust.

Results of the Basin and Range Geoscientific Experiment (BARGE): A marine‐style seismic reflection survey across the eastern boundary of the central Basin and Range Province

Approximately 120 km of marine‐style deep seismic reflection data were shot during a survey on the waters of Lake Mead in southeastern Nevada. The survey extends from near the abrupt eastern edge of the Basin and Range Province (BRP) to a point ∼80 km into the extended domain. Data quality throughout the survey ranged from fair to poor; the recorded data include significant towing noise and occasionally problematic diffractions and sideswipe from canyon walls. The upper 2–4 s of the data shows well‐defined reflections from sedimentary fill, but below that point, reflectivity is weak. Lower crustal reflectivity is generally absent under the eastern part of the survey, with a slight increase in reflectivity to the west. The reflection Moho appears as a series of weakly defined, discontinuous reflections, most of which occur at 10–11 s. A particularly interesting feature of the data set is the relative lack of reflectivity from the lower crust, which is a region of strong reflectivity on other seismic reflection data sets from the BRP.

Seismic wide‐angle constraints on the crust of the southern Urals

A wide‐angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide‐angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6–15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0–6.0 km/s, which correlates with the surface geology; 5–7 km depths at which the velocities increase to 6.2–6.3 km/s; 10–30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30–35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N‐S and one for the E‐W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poisson's ratio and anisotropy were calculated from the velocity models. The Poisson's ratio increases in the lower crust at both sides of the root zone. A localized 2–3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50–56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.

Seismic images of Caledonian, lithosphere-scale collision structures in the southeastern North Sea along Mona Lisa Profile 2

The unexposed suture between Baltica and Eastern Avalonia is imaged by coincident normal-incidence reflection and wide-angle reflection/refraction seismic data of the MONA LISA project. We present new results of the upper lithospheric, seismic structure from the N–S-striking profile 2 across the Caledonian Deformation Front, which represents the crustal collision suture between Baltica and Eastern Avalonia that formed after closure of the Tornquist Sea during the Caledonian orogeny in Late Ordovician times. Three different crustal types are identified with great similarities to the nearby profile 1: (1) a three-layered crust typical of shields to the north; (2) a transitional crust (suture zone) in the central part; and (3) a two-layered crust of Caledonian origin to the south characterized by very low velocities throughout the crust. The crustal thickness varies from 38–35 km under the northern margin of the Ringkøbing-Fyn High (Baltica crust) to 28–27 km beneath the North German Basin in the Caledonian crust to the south. The suture zone is imaged by S-dipping crustal reflections from 1.9 to 10.6 s two-way travel time (twt) over a horizontal distance of ∼70 km within the transitional crust. The reflection suture zone terminates in a ∼60 km wide reflective lens with velocities of 6.6–6.8 km/s in the lowermost crust. The reflective lens may be interpreted as a remnant of oceanic or island-arc crust that was accreted to the leading edge of Baltica during closure of the Tornquist Sea. Alternatively, the reflective lens may represent an indentor of Baltica crust into the Avalonian terrane. The change in lower crustal reflectivity and the abrupt transition from a three- to a two-layered crust further south suggest that the Trans-European Fault and the Elbe Lineament continue into the North Sea. The upper mantle reflectivity is dominated by bands of strong-amplitude, S-dipping reflections from 13.5 to 21.8 s twt, and a weaker band of N-dipping reflections from 12 to 16 s twt. Reversed wide-angle refractions and reflections indicate the presence of a S-dipping high-velocity layer (8.65–8.8 km/s) in the uppermost mantle. The sub-Moho high-velocity layer partially coincides with the strong S-dipping normal-incidence mantle reflections. We propose that the S-dipping mantle structure may represent a late-Caledonian or Late Carboniferous to Early Permian mantle shear zone.

Seismogenic layer, reflective lower crust, surface heat flow and large inland earthquakes

Regional variations in the cutoff depth of seismicity in southwest Japan are derived from more than 60,000 well-determined earthquakes. The thickness of the seismogenic layer is closely related to the strength of the crust and accordingly to the occurrence of large inland earthquakes, since the seismic–aseismic boundary is thought to be related to the brittle–ductile boundary in the crust. Large inland earthquakes are likely to occur in the area where the cutoff depth of seismicity changes abruptly. It is therefore important to survey the regional variations in this depth to evaluate the potential of large inland earthquakes. The cutoff depth of seismicity roughly coincides with a temperature range of 300–400°C in the crust. In addition, distinct S-wave reflectors as well as a reflective lower crust have generally been observed in some areas in Japan. The depths of the reflector and the top of the reflective lower crust lie several kilometers below the cutoff depth of seismicity and they seem to coincide with a temperature range of about 300–450°C. Therefore, in those areas where no earthquakes occur but a reflective layer exists, the cutoff depth can be mapped from a survey of reflectivity in the crust. The heterogeneity of the crust is very important for understanding the nature of the crust where large earthquakes are frequent. This paper proposes a model of crust inhomogeneity based on observation results.

Structure and composition of the Aleutian island arc and implications for continental crustal growth

We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a continental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two.

Seismic refraction data in the Gulf of Saint Lawrence: implications for the lower-crustal blocks

A refined model for the wide-angle reflection-refraction profile 88-3 that crosses the foreland basin of the Appalachian orogen is presented. The two major layers of the cratonic crust have velocities of 6.2 and 6.7 km·s-1 with low gradients. Near the exposed North American craton, the 6.7 km·s-1 velocity appears at the shallow depth of 13 km and the M discontinuity at greater than 40 km depth. Towards the orogen, a southeastward-dipping ramp is developed at mid-crustal levels at the top of the 6.7 km·s-1 layer. Near the Appalachian Front lower-crustal velocities typical of the Grenville are replaced with a velocity of 7.2 km·s-1. The collinear reflection profile confirms the southward-dipping ramp, the shallowing M discontinuity, and the lateral changes in the lower crust. Refraction profiles combined with reflection and gravity data are used to distinguish the Grenville lower-crustal block. The Grenville lower-crustal block is defined as a pattern of lower-crustal and upper-mantle reflectivity, with associated velocities, and a more negative gravity anomaly than lower-crustal blocks of the Appalachian terranes. The Grenville block terminates northwest of the coast of Newfoundland near the Appalachian Front. Thus, this lower-crustal province does not underlie the Humber tectono-stratigraphic zone. This is different than the relationship of the edge of the Grenville lower-crustal block to surface zones in New England. A thick-skinned tectonic style with basement-involved deformation appears to be typical of the Newfoundland Appalachians; in contrast, a thin-skinned structural style is observed in New England. The change in structural styles occurs in the bend of the Appalachian orogen across the Gulf of St. Lawrence.

Seismic-reflection evidence that the Hayward fault extends into the lower crust of the San Francisco Bay area, California

Abstract This article presents deep seismic-reflection data from an experiment across San Francisco Peninsula in 1995 using large (125 to 500 kg) explosive sources. Shot gathers show a mostly nonreflective upper crust in both the Franciscan and Salinian terranes (juxtaposed across the San Andreas fault), an onset of weak lower-crustal reflectivity beginning at about 6-sec two-way travel time (TWTT) and bright southwest-dipping reflections between 11 and 13 sec TWTT. Previous studies have shown that the Moho in this area is no deeper than 25 km (∼8 to 9 sec TWTT). Three-dimensional reflection travel-time modeling of the 11 to 13 sec events from the shot gathers indicates that the bright events may be explained by reflectors 15 to 20 km into the upper mantle, northeast of the San Andreas fault. However, upper mantle reflections from these depths were not observed on marine-reflection profiles collected in San Francisco Bay, nor were they reported from a refraction prifile on San Francisco Peninsula. The most consistent interpretation of these events from 2D raytracing and 3D travel-time modeling is that they are out-of-plane reflections from a high-angle (dipping ∼70° to the southwest) impedance contrast in the lower crust that corresponds with the surface trace of the Hayward fault. These results suggest that the Hayward fault truncates the horizontal detachment fault suggested to be active beneath San Francisco Bay.

Crustal geometry of the Abitibi Subprovince, in light of three-dimensional seismic reflector orientations

We provide precise estimates of reflector orientations beneath the Archean Abitibi Subprovince, using two distinct approaches based on Lithoprobe seismic reflection data. In the first, we use the dip of reflections observed on intersecting profiles to establish the three-dimensional orientation of reflective structures. In the second, the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth, dip, depth, and medium velocity. Mid-crustal reflectors define two areas with distinctive geometry: reflectors beneath the southern Abitibi belt are oppositely dipping, and convergent at depth, providing a V-shape aspect to the greenstone rocks; other reflectors beneath the northern Abitibi belt are, in general, subparallel, dipping at an average of 30° toward the north. These north-dipping reflectors are partly disrupted by a low-reflectivity zone, which is attributed to rocks of the Opatica Subprovince, located underneath the northern Abitibi belt. Lower-crustal reflectors have a similar, shallowly north-dipping orientation throughout the Abitibi Subprovince. The geometry of the reflectors recovered is consistent with the different tectonic histories proposed for the southern and northern Abitibi assemblages, until common deformation during a north-south shortening event. Attitudes recovered in the northern Abitibi belt are consistent with tectonic scenarios involving underthrusting of Abitibi middle and lower crustal terranes beneath the Opatica belt, whereas the oppositely dipping reflectors recovered in the middle crust beneath the southern Abitibi belt could be representative of a rifted volcanic arc environment.

Some seismic signatures in the Romanian crust

Some new seismic reflection data in the W of the Moesian platform, Focşani depression and Transylvania depression show seismic patterns which reflect the local structure and evolution of the each area. These local areas were affected by subsidence which over-printed the changes on the old pattern. The old and more or less reflective lower crust was fractured and some rigid blocks have preserved the reflectivity up to the present. The Ba˛ileşti section illustrates a transparent or diffractive upper crust and a well-marked lower crust by an alternation of reflective and non-reflective zones. The crystalline crust of the Râmnicu Sa˛rat section has two patterns: one relatively reflective, near the bottom of the depression (the W side), whilst the other, on the E flank of depression, is transparent. The E flank seems to be more fractured due to some tensile fractures originating in the bending stress during subsidence. The Târgu Mureşsection has a specific crustal pattern. A near-transparent crystalline crust overlies a very large crust-mantle transition zone which is fragmented in alternated reflective and transparent blocks.

Seismic reflection fabrics of continental collision and post-orogenic extension in the Middle Urals, central Russia

Deep seismic reflection surveys in the Middle Urals, central Russia, provide the basis for a revised structural and tectonic interpretation of late Paleozoic continental collision and early Mesozoic post-orogenic extension in this orogen. These profiles (ESRU '93 and ESRU '95) form a transect from the former East European platform margin in the west to accreted island-arc and oceanic terranes in the east, crossing the Main Uralian fault (MUF), the principal suture zone in the Urals. The foreland of the Middle Urals is characterized by a fold-and-thrust belt developed above a gently E-dipping detachment at ∼10 km depth, and a deeper, presumably older, and previously unrecognized detachment system of probable Late Proterozoic (Baikalian) age. A large (∼5 km) frontal ramp marks the axis of the regional Kvarkush anticlinorium which dominates the foreland structure. Reflectivity in the middle and lower crust of the East European platform is inferred to be primarily a Precambrian fabric. Based on correlation of the seismic data with surface geology, the MUF is redefined spatially, seismically and structurally as the top of an ∼40°E-dipping reflective package, placing it (1) at the boundary between upper greenschist to amphibolite facies rocks of continental affinity to the west and unmetamorphosed to lower greenschist facies rocks of the Tagil island arc to the east, (2) at the primary topographic break in the Middle Urals, and (3) ∼10 km further east at this latitude than recent interpretations, but consistent with previous Russian maps. At depth, the MUF appears to merge in a listric fashion with a zone of subhorizontal mid-crustal reflectivity (∼15–20 km depth) that truncates a series of large-scale antiforms and associated W-dipping (E-vergent?) shear zones of inferred Paleozoic age across the Uralian hinterland. These relationships suggest that the MUF was reactivated as a normal fault during late- to post-orogenic evolution of the orogen. Based on interpretation of the Moho as the base of lower-crustal reflectivity, crustal thickness varies from ∼48 to 51 km (∼14–15 s) across the ESRU '93 profile, in concert with existing Russian studies showing a thickened crust beneath the Urals. To the east, on the ESRU '95 profile (beneath the Tagil island arc), a strongly W-dipping reflective package in the lower crust suggests abrupt thinning of the crust to ∼45 km (∼13 s), consistent with nearby wide-angle reflection data. At the surface, this crustal thinning coincides with the Triassic Imenovskii graben. We interpret these collective observations as evidence for post-orogenic extension of the Middle Urals in early Mesozoic time, in association with reactivation of the MUF as a breakaway normal fault at the westernmost boundary of the West Siberian basin extensional province. In contrast, results from the URSEIS '95 experiment indicate that the Southern Urals are largely unextended, implying that post-orogenic evolution varied along strike within the orogen.

Structure and tectonics of the Proterozoic Aravalli-Delhi Fold Belt in northwestern India from deep seismic reflection studies

Seismic imaging of the crust along a 400-km-long deep seismic reflection profile across the Palaeo/Mesoproterozoic Aravalli-Delhi Fold Belt, in the northwestern Indian Shield, brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt. The reflectivity pattern varies considerably for different crustal units along the profile.The deep-crustal reflection data image two sets of oppositely dipping strong reflection bands, from upper- to lower-crustal levels. These are identified as the signatures of the collision corresponding to Aravalli and Delhi orogeny. The data also exhibit a clear Moho and strong lower-crustal reflections near the collision boundaries. A stack of dipping reflections from the top of the Moho to the surface is identified as a major thrust fault indicating that the Proterozoic collision and deformation were primarily thick-skinned in nature

Seismicity pattern and style of seismic faulting at the eastern borderfault of the southern Rhine Graben

Seismicity recorded on high-gain, digital networks in the southern Rhine Graben reveals that the area around the eastern borderfault system shows the highest level of seismic activity. Across this fault system remarkable differences of maximum focal depths exist: 12–13 km are characteristic within the graben, and 18–20 km beneath the western foothills of the southern Black Forest, located approximately between the eastern borderfault of the graben and the river Rhine (inner borderfault). Farther to the east, in the southern Black Forest proper, maximum depths around 20–22 km are found. At particular sites a few earthquakes with depths of up to 26 km, close to the Moho, were detected. The deep cut-off of seismic activity in the southern Black Forest and its western foothills is persistent for about 100 km distance between the Freiburg focal area in the north and the Swiss Jura in the south. Seismic reflection profiling indicates that the deep foci lie well within a reflective lower crust (southern Black Forst, Swiss Jura) or just above a highly reflective top of the lower crust (western foothills of the southern Black Forest, Dinkelberg block). Within the graben proper, seismic activity is restricted to the upper crust and ceases about 5 km above the top of the lower crust.Low seismic activity is found in areas of low P-wave velocities as in the upper crust of the central Black Forest, north of the Freiburg focal area, and at midcrustal levels in the southern Black Forest (at depths of about ±10 km).Well constrained focal mechanisms are presented for 47 events. Most of them occurred adjacent to the eastern borderfault. Nearly all mechanisms show an oblique component of slip. Strike-slip and normal-faulting mechanisms prevail. Only a few events are of thrust type. Dominance of a certain faulting style with depth was not found. Focal mechanisms of events which occur within 10 km of the borderfault show variations of faulting style, indicating local deviations from the average regional stress tensor and/or from the frictional parameters in the proximity of the particular rupture planes. Although the nodal planes correlate with the strike directions of surface trends of major faults in general, it is difficult to recognize their link to individual faults.