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SUMMARY From the S-wave data collected along a 270-km-long profile spanning the Kunlun mountains in NE Tibet, 14595 Sg phase arrivals and 21 SmS phase arrivals were utilized to derive a whole-crustal S velocity model and, together with a previously derived P velocity model, a Poisson's ratio (σ) model beneath the profile. The final tomogram for the upper 10–15 km of the crust reveals the lower velocities associated with the predominantly Neogene-Quaternary sediments of the Qaidam basin to the north and the higher velocities associated with the predominantly Palaeozoic and Mesozoic upper crustal sequences of the Songpan-Ganzi terrane and Kunlun mountains to the south. This study finds no evidence that the Kunlun mountains are involved in large-scale northward overriding of the Qaidam basin along a shallow south-dipping thrust. The σ in the upper 10–15 km of the crust are often lower than 0.25, indicating a preponderance of quartz-rich rocks in the upper crust beneath the profile. Below 10–15 km depth, the remainder of the crust down to the Moho has an average σ of 0.24 beneath the Songpan-Ganzi terrane and Kunlun mountains and 0.25 below the Qaidam basin. These low σ are similar to other low σ found along other profiles in the northeastern part of the plateau. Assuming an isotropic situation and no significant variation in σ between 10–15 km depth and the Moho, then the lower crust between 25–30 km depth below sea level and the Moho with P velocities varying from 6.6 km s−1 at the top to around 6.9 km s−1 at the base and σ of 0.24–0.25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part. As the pre-Cenozoic Qaidam basin crust has probably not lost any of its lower crust during the present Himalayan orogenic cycle in the Cenozoic and only has a σ of 0.245–0.25, then it appears that the pre-Cenozoic Qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average σ of 0.265–0.27 and the Tarim and Sichuan basin crusts. This situation then probably facilitates the collision and promotes the formation of new high plateau crust at the NE margin of Tibet. South of the Qaidam basin, the crust of the Songpan-Ganzi terrane and Kunlun mountains has an even lower average crustal σ of 0.23–0.24 and is thus presumably even weaker and more easily deformable than the crust beneath the Qaidam basin. This then supports the hypothesis of Karplus et al. that ‘the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust'.

We present a model that may explain deep crustal earthquakes observed, in particular, in several areas of highly reflective (laminated) lower continental crust. We combine observations from earthquake seismology, crustal reflection seismics and tectonic-rheological concepts. The study concentrates on parts of the northern Alpine foreland where many earthquakes occur inside the laminated lower crust, which is generally considered to be warm and weak. Thin mafic/ultramafic, sill-like intrusions and invisible dykes are assumed to form a corset-like network with high strength. This model can explain the observed strong and multiple reflections and the occurrence of rupture inside a stable structure within a weak lower crust. Tectonic stress transfer (from the Alpine collision zone or/and the Upper Rhine Graben) and its release may follow classical friction concepts. In addition, the heterogeneity of the laminated lower crust may also favour various viscous instabilities.

Signals from 11 shots and 8 earthquakes, and numerous teleseismic events were recorded along the 400-km seismic line INDEPTH III in central Tibet and interpreted together with previous seismic and tectonic data. The abnormal behavior of various mantle phases reveals a complex Moho-transition zone, especially in the northern part of the line, in the Changtang Block, where the lower crust and the mantle show unusually low velocities, a shingled appearance of Pn and no low-velocity layer in the upper crust. The strong east-west anisotropy in the Changtang Block is related to an easterly escape movement of the whole lithosphere, facilitated by the warm and weak layers in the lower crust and the upper mantle, bounded apparently by two prominent west-east running fault zones.

��������� � �� ���������� SUMMARY Seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere. We discuss the origin of anisotropy at various depths, and relate it to tectonic stress, geotherms and rheology. The anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine, and may result from ductile deformation, dynamic recrystallization or annealing. Anisotropy beneath young orogens has been measured for the seismic phase Pn that propagates in the uppermost mantle. This anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event, and thus provides information on the process of mountain building. Whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts, Pn anisotropy is aligned parallel to the structural axis. We interpret this to indicate mountainparallel ductile (i.e. creeping) deformation in the uppermost mantle that is a consequence of mountain-perpendicular compressive stresses. The preferred orientation of the fast axes of some anisotropic minerals, such as olivine, is known to be in the creep direction, a consequence of the anisotropy of strength and viscosity of orientated minerals. In order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal ‘escape’ (or ‘extrusion’) tectonics to the uppermost mantle. We present rheological model calculations to support this hypothesis. Mountain-perpendicular horizontal stress (determined in the upper crust) and mountain-parallel seismic anisotropy (in the uppermost mantle) require a zone of ductile decoupling in the middle or lower crust of young mountain belts. Examples for stress and mountain-parallel Pn anisotropy are given for Tibet, the Alpine chains, and young mountain ranges in the Americas. Finally, we suggest a simple model for initiating mountain parallel creep.

The main goal of a joint project undertaken by the geophysical and hydrological research units of Kiel University is to study preferential flow in a large open-air, full-scale model, looking in particular at near-surface penetration and flow of water through the unsaturated vadose zone. An artificial irrigation device is installed in place of natural rain, and a homogeneous sand body is used instead of natural soil. This provides a reference model for future field experiments. Inside the sand body there are a large number of geophysical and hydrological sensors to measure DC resistivity (using various electrode configurations), water content and water potential (using TDR and tensiometer instruments, respectively). A ground-penetrating radar (GPR) system is installed at the surface, whereas at the bottom several containers and a thin gravel layer are embedded to measure the flow arrival and the discharge of water. Irrigation is varied in intensity, time, area, and salt content (tracer). Results of the first six experiments show that the percolation of intruding water can be followed by all techniques and percolation is finally controlled by the discharge measurements. These display some undulations and variations of the water ‘front’ and agree with the measurements of all other sensors. The redundancy achieved by the use of multiple methods was intended to enable an assessment of the reliability of the techniques used. The true values of electrical resistivity before and after irrigation reflect the distribution of water saturation within the sand body. A numerical 3-D inversion of the apparent resistivity provides information regarding future field experiments, in which it will be possible to install only some of the sensors in order to preserve the natural structure of the soil.

An attempt is made to compare the tectonic units and their evolution in central Europe with the deep seismic velocity structure and patterns of reflectivity. Caledonian and Variscan terrane accretion and orogenic collapse dominate the tectonic development in central and western Europe and have left their marks in a distinct velocity structure and crustal thickness as well as in the various reflectivity patterns. Whereas the memory of old collisional structures is still preserved in the rigid upper crust, collapse processes have formed and modified the lower crust. They have generally created rejuvenated, thin crusts with shallow Mohos. In the Variscan internides, the center of collision and post-orogenic heat pulses, the lower crust developed strong and thick seismic lamellae, the (cooler) externides show a thrust and shear pattern in the whole crust, and the North German Basin experienced large mafic intrusions in the lower crust and developed a high-velocity structure with only very thin lamellae on top of the Moho. The various kinds of reflectivity patterns in the lithosphere can be explained by a thermo-rheological model from terrane collision, with crustal thickening to collapse in a hot, post-orogenic setting.

We discuss three interconnected processes that occur during continental compression and extension: delamination of the lower crust and sub-crustal lithosphere, escape tectonics (i.e., lateral crustal flow), and crustal uplift. We combine calculations of lithospheric viscosity–depth curves with geologic observations and seismic images of the deep crust to infer the mechanisms controlling these processes. The basic driving force for delamination is the negative buoyancy (in some regions) of the continental lower crust and sub-crustal lithosphere with respect to the warm, mobile asthenosphere. A phase transformation in the lower crust from mafic granulite facies to eclogite may be important for providing negative buoyancy. Where negative buoyancy exists, the onset of delamination is mainly a question of the presence of a suitable decoupling zone between the denser lithosphere and the lighter upper and middle crust. We estimate the depth to potential decoupling zones by calculating lithospheric viscosity–depth curves based on reasonable geotherms and models of lithospheric composition. Low-viscosity zones occur at three depths: (1) at the base of the felsic (upper) crust; (2) within the lower crust; and (3) several tens of kilometers below the Moho. The commonly observed absence of a high-velocity (>6.8 km/s) lower crustal layer beneath extended crust may be explained by delamination wherein decoupling occurs at the top of the lower crust. In addition to being zones of potential decoupling, crustal low-viscosity zones are avenues for lateral crustal flow, a process that is often referred to as crustal escape (e.g., eastern Tibetan Plateau). The third process addressed here, crustal uplift, is mainly found in compressional environments and can be related to mature (i.e., complete or nearly complete) delamination and/or a thick low-viscosity lower crust. Mature delamination generates crustal uplift as the sinking, dense lithosphere is replaced by the mobilized hot asthenosphere. A very different mechanism of uplift is associated with some continental high plateaus, where a high convergence rate and the lateral intrusion of cold, rigid shield crust into warm, low-viscosity orogenic crust acts like a solid piston moving into hydraulic fluid. The displacement of the low-viscosity crustal `fluid' generates broad plateau uplifts. Modern examples are the intrusion of the Indian shield into the Tibetan Plateau and the Brazilian shield into the Andes. All of these processes, delamination, tectonic escape, and uplift are interconnected and are related to weakness in the lower crust during continental compression and extension.

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

After reviewing the microscopic and macroscopic texture of fault zones, the localisation of rupture and creep is described, and the reduced strength of fault zones is investigated. Simple strength and viscosity models for the whole lithosphere play a major role for the geometry of fault zones. Compressional faults (thrusts) show ramp- and flat structures, often soling in a (weak) detachment zone, extensional faults prefer a listric shape, often turning subhorizontal and being invisible in the seismic lamellae of the ductile lower crust which according to the models is also a broad zone of detachment and decoupling. Faults in the lower crust are rare and restricted to strain hardened fault rocks or intrinsically rigid rocks like oceanic mafic-ultramafic rocks of suture zones. Transient faults in the lower crust are created by rupture processes of large earthquakes. Some deep faults, moderately dipping or flat, are again observed in the (rather rigid) uppermost mantle. The observation of folding inside the crust seems to be connected with an intermediate viscosity range and with a broad (vertical) transition zone where rigid reactions and ductile processes are mixed. Various methods of detecting faults by seismic studies are critically reviewed, and some sequence of reliability is suggested. Impedance contrasts, polarity, dip, thickness, and multiple fault strings have to be deciphered by wavefront modelling. Origin and survival of faults are considered to be a consequence of tectonic and thermal evolution under specific stress systems.

The continuation of the basement of the southwestern Baltic Shield toward the SW is of major interest for the Caledonian evolution in the southwestern Baltic Sea and northern Germany. A reinterpretation of various marine seismic data suggests a reconsideration of the Caledonian collision pattern. The Sorgenfrei Tornquist Zone forming the northwestern extension of the Tornquist Zone is regarded as the northeasterly boundary fault and the Elbe Lineament is regarded as the southwestern limit of an area which suffered from the docking of East Avalonia to Baltica. It shows many northwestsoutheast striking structures, anomalies, and fault patterns. From our studies, two alternative models emerge: one which considers the Caledonian Deformation Front as the northeastern limit of East Avalonia and a zone of major transpression, collision, and subduction of the Tornquist Sea. Another model considers the Elbe Lineament in Schleswig-Holstein (NW Germany) as a major (dextral) strike-slip fault which is interpreted as the northeasterly limit of East Avalonia and the southwest boundary of the Baltic Shield. Along the Elbe Lineament the Tornquist Sea plus rotating Avalonia were displaced to the northwest. A large part of the area to be studied might have developed into a passive margin toward the Upper Ordovician. A decision which of the two models is to be prefered has to be the major goal of future geophysical experiments.

SUMMARY From the BABEL near-vertical record sections a sporadically strong sub-Moho event, NE of Bornholm dipping 20"-30" to the NE (along line), was observed over a distance of about 70km, 4 to 5s two-way traveltime (TWT) below the Moho. The same reflection from the uppermost mantle (Rum) was observed in the piggy-back wide-angle records of station 10 (Bornholm), about 2 to 3 s TWT later than the Moho event P,P (slant ray paths) at offsets of 100 to 150 km. A nearby station (station 11) recorded similar, although much weaker, Rum signals. Our preferred model, inferred from ample computations of synthetic seismograms, is a N- to NE-dipping reflector of limited thickness below a low velocity zone. Studies of similar, slightly dipping events around Great Britain and elsewherealthough mostly not confirmed by wide-angle studies-indicate the presence of faults or shear zones in a rigid uppermost mantle. Listric faults in the upper crust, merging with the zone of unusually high reflectivity of the lower crust, which acted as a transfer zone, seem to be connected with still deeper faulting. The wide-angle information of Rum provides additional evidence for faulting processes in the uppermost mantle, most probably connected with the Alpine orogeny.

At the southern boundary between India and Tibet, the Indian tectonic plate subducts northwards beneath the Tibetan Plateau. Seismic imaging shows that at the northern boundary of Tibet, the Asian plate is also subducting southwards beneath the plateau, and the Tibetan lithosphere is separate. The southern boundary between India and the Tibetan Plateau represents a classical case of continental subduction, where the Indian continental lithosphere is subducted northwards beneath the Tibetan Plateau1,2,3,4,5,6. At the northern boundary, southward subduction of Asian lithosphere beneath the Tibetan Plateau has also been proposed7, but imaging has been hampered by inadequate data quality. Here we analyse the plate tectonic structure of the northern boundary between Tibet and Asia using the S receiver function technique. Our passive source seismic data build on, and extend further northwards, the existing geophysical data from the International Deep Profiling of Tibet and the Himalaya project8,9,10. We detect, beneath central and northern Tibet, a relatively thin, but separate, Tibetan lithosphere overriding the flat, southward subducting Asian lithosphere. We suggest that this overriding Tibetan lithosphere helps to accommodate the convergence between India and Asia in central and northern Tibet. We conclude that the Tibetan–Himalayan system is composed of three major parts: the Indian, Asian and Tibetan lithospheres. In the south, the Indian lithosphere underthrusts Tibet. In central and northern Tibet a separate, thin Tibetan lithosphere exists, which is underthrust by the Asian lithosphere from the north.

Deep seismic reflection profiles in Europe and elsewhere cover a range of different tectonic units. Specifically, in western and central Europe they cross structures relating to the Alpine, Variscan and Caledonian orogens with considerable crustal shortening, delamination, and interfingering. The Variscan mountain belts in France and Germany show collapsed structures from various collision events between 300 and 350 Ma ago. Still further north in middle England and the southwest Baltic Sea traces of the Caledonian collision around 400 Ma and associated collapse structures are visible. Here, the terrain East Avalonia (Cadomia) docked to the colliding continents of Baltica and Laurentia in a complex pattern with closing oceans and compressional boundaries, which can still be seen in today's seismic sections in Britain and the SW Baltic Sea. All the processes of crustal shortening, interfingering and delamination were certainly active during the compressional stages of these earlier orogens and have left their marks, which are still recognizable in today's seismic image of the crust. Crustal roots and high elevations have disappeared in the extensional collapse phase, thermal events have intruded, ‘underplated’ or otherwise modified the stretched lower crust. In the Variscan internides massif granite production started, and the lower crust assumed an especially strong and thick sheared, laminated structure with a plane Moho. The various tectonic stages are illuminated by a gross analysis of reflectivity patterns. We postulate that the fate of these patterns from their origin to their death is imbedded in thermally and rheologically varying creep processes, which always accompany the brittle and ductile deformation in the Earth's crust.

SUMMARY The terrane concept is understood as an important extension of plate tectonics and is based on the recognition of allochthonous, mobile geological units. The concept is successfully applied to the Variscides with their wide range of collisional belts. It is mainly supported by the dense deep-seismic network of DEKORP, which reveals certain reflectivity patterns and succeeds in mapping old and new deep fault zones between the terranes. Variscan terranes are rooted in the ductile lower crust and seem to consist of continental crust only, partly exclusively of rigid upper crust. Oceanic terranes, on the other hand, are always rooted in the asthenosphere. The development of continental terranes and their boundaries depends strongly on their thermal and rheological history. In the case of post-orogenic collapse with heating and extension of the lower crust, seismic lamellae develop and often truncate former thrust faults.

The velocity and attenuation of compressional([Formula: see text], [Formula: see text] respectively) and shear waves ([Formula: see text], [Formula: see text], respectively), determined with the Pulse Transmission technique at a frequency of about 100 kHz, are compared with the grain size, shape, porosity, density, and static frame compressibility of dry and water‐saturated sands. Except for [Formula: see text], all the quantities [Formula: see text], [Formula: see text] and [Formula: see text] are dependent on grain size and are higher in coarser grains than in finer grains. [Formula: see text] decreases significantly with increasing differential pressure in coarse‐grained sediments, but the same sediments show an anomalous increase with differential pressure in [Formula: see text] at low pressures. We have also modeled the [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] of these samples to understand the mechanisms governing the observed changes. The Contact Radius model with surface force effects predicts both [Formula: see text] and [Formula: see text] to be dependent on grain size. Frictional losses in unconsolidated coarse‐grained sands must also be considered at small strains [Formula: see text]. Velocity and losses measured in saturated sands are higher than those predicted by the Biot model, which does not account for any grain size dependence of the seismic qualities.

Some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes. The role of fluids in enhancing or destroying such structures is studied. Seismicity is strongly dependent on the different pore pressure of fluids. In intraplate settings, in strike slip or extensional areas hydrostatic pore pressure seems to be approached, in accordance with standard rheological models and standard seismicity-depth distributions. In suture zones or zones of developing loads, possibly also along zones of near-horizontal normal faults, extremely high pore pressure develops which approaches lithostatic values. They are accompanied by a number of weakening processes with enhanced shear and reduced viscosity (η). These values correlate with low seismic velocities V p in accordance with an empirical V p -η relationship.

The dense network of deep seismic reflection lines in western and central Europe makes a systematic comparative study possible. The reflectivity of the continental crust is not the same but can change significantly from one seismic line to the next. Different seismic reflectivity patterns can be observed and correlations to specific tectonic units are found.

Summary Within the DEKORP project (DEKORP: Deutsches Kontinentales Reflexionsseismisches Programm) a joint deep seismic reflection venture with the BELCORP (Belgian Continental Reflection Seismic Programme) group of the Belgian Geological Survey was carried out in 1987 across the Rhenish Massif, a part of the mid-European Variscides. This orogenic belt developed in the Upper Devonian/Carboniferous. Mostly Devonian rocks crop out at the surface. The Rhenish Massif is bordered by two sedimentary troughs: the sub-Variscan Foredeep in the north and the Permo-Carboniferous Saar-Nahe Basin in the south. In the east-west direction it is subdivided by the axial depression of the Eifel Nord-Sud Zone. The aim of the survey which totals almost 220 km of seismic profiling, was to investigate the crustal structure of the western part of the Rhenish Massif and to compare it with the line DEKORP 2-N which crosses the eastern portion of the massif. The results indicate the presence of NW-vergent tectonics of various styles that can often be traced down to deep parts of the crust. Horizontal Variscan compression plays a dominant role in the northern part while post-Variscan extension seems to dominate in the Saar-Nahe Basin, although even there traces of Variscan compression seem to be preserved in the middle crust. Common characteristics of the pre-Palaeozoic basement, differences between the western and eastern parts of the Rhenish Massif, and the deep extension of the Aachen Thrust (Faille du Midi) have been clearly observed. This prominent thrust in the north with its characteristic ramp and flat structure has been followed over 100 km length down

A complete set of two-dimensional equations for conservation of mass, momentum and energy of a two-phase system of melt in a deformable matrix is used to derive analytically a model for the vertical distribution of the melt fraction. Also, a set of asymptotic relations for parameters in the velocity field of the matrix and the melt in the neighbourhood of the symmetry axis of sea-floor spreading is obtained. These relations have been used to derive solutions appropriate to the melt extraction from the partially molten regions beneath mid-ocean ridges and the conditions for their existence. The results show the convergence of the melt streamlines towards the spreading center. It is caused by: (1) a horizontal gradient of the piezometric pressure associated with the deformations of the matrix, and (2) an upward increase of the porosity due to melting. For accepted values of the relevant parameters, a focusing effect of melt towards spreading centers exists, thereby providing an explanation for the mid-ocean “paradox” of a narrow region of ridge axis volcanism overlying a broad melt production zone.

A systematic comparative study of the reflectivity of the continental crust is undertaken ― made possible by the expanded data basis of deep seismic reflection profiles in Europe. The reflectivity is not at all the same but can change dramatically from one seismic line to the next. Different seismic reflectivity patterns ― independent of applied techniques ― are observed. Correlations between these seismic patterns and specific tectonic units have been found

Processes, active during continental collisions and orogenic collapse, create distinct structures which are imaged by reflection seismic profiling. Examples are shown and discussed

Deep reflection seismic data collected in Germany during the last three decades allow an insight into the structure of the Variscan crust in Germany. Although the crustal thickness is relatively constant, reflection data revealed important internal structures. Most profiles show a poorly reflective upper crust (disregarding fault zones) while its lower part is characterized by many flat-lying reflections. But also dipping reflections in the upper and lower crust have been observed on DEKORP reflection lines. Beneath the geothermal anomaly of Urach an updoming of the reflective lower crust coinciding with a pronounced low-velocity body can be observed. Near the suture zones between Moldanubian, Saxothuringian and Rhenohercynian abundant diffractions appear in the middle and lower crust. One exception is found in the west where post-Variscan developments created the deep Saar-Nahe-trough. The North Variscan Deformation Front exhibits thin-skinned tectonics west of the Rhine while farther east more steeply dipping fault zones are observed. In the North German Basin, just north of the Variscides, an industrial north-south profile shows strong bands of reflections from both midcrustal and Moho levels. The distance between these bands becomes smaller to the north. An industrial west-east line of 90 km length in Schleswig-Holstein, south of Kiel, shows a thinning of the crust of about 6 km towards the North Sea, although the Moho is not faulted. The sediments reach down to at least 15 km.

The Seismic reflection profiles of DEKORP (DEutsches KOntinentales ReflexionsSeismisches Programm) in the Federal Republic of Germany to date have been limited to areas of the Variscan orogeny. Nevertheless, the character of their reflections differs considerably and may be correlated to certain Variscan and post-Variscan developments. Lower crust lamellae develop in areas of high heat flow, mostly associated with post-Variscan extensional processes; “crocodile” and nappe tectonics are best preserved in the cores and at the flanks of older massifs which were incorporated into the Variscan orogeny. So far poor reflectivity has been observed only in the area of the London-Brabant Massif which was not involved in any of the Phanerozoic orogenies.

A deep Seismic reflection profile collected by DEKORP and BELCORP in the western Rhenish Massif was supplemented by wide-angle measurements. Signals from a vibrator source were successfully recorded to a distance of 60 km. A passive recording array was operated that recorded all shots along the profile. The wide-angle and near-vertical data were used to construct a velocity model for the profile. Most of the wide-angle reflections coincide with strong near-vertical reflections or bands of high reflectivity. The North Variscan Deformation Front, seen as a prominent shallow reflection on many profiles in this region, separates an upper crust with rather nigh velocities from a layer with lower velocities underneath. At a depth of 20–22 km a thin (2–3 km thick) layer of high velocities is found. The Moho is not reflective either in the near-vertical or in the wide-angle data, suggesting the presence of a thick crust-mantle transition zone.

Based on observations of reflectivity and lengths of reflecting boundaries in various geological units a consistent picture for explaining the different reflectivity patterns is derived. Whereas in the upper, brittle part of old crusts reflectivity concentrates on fault zones of former or present compressional zones it is absent in warm extensional areas, where in addition, granitic plutons may disturb any upper crustal “layering”. The lower crust of these young and warm areas with its reflecting lamellae is dominated by strain and shear processes i.e. by an ordering process in a low-viscosity environment. Any change of sialic or low-grade to more mafic or high-grade layers at mid-crustal levels (Conrad discontinuity) is not related to the beginning of the lamellae but may be responsible for a finger-type appearance of lower crustal reflectivity.