High-velocity Lower Crust Research Articles

Crustal Architecture of the Inverted Central Lapland Rift Along the HUKKA 2007 Profile

We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km.

Crustal velocity structure beneath Saurashtra, NW India, through waveform modeling: Implications for magmatic underplating

The Saurashtra peninsula in the northwestern segment of the Deccan volcanic province of India, is characterized by several high gravity and magnetic anomalies, which correspond to mafic crustal intrusions. This study attempts to quantify the alterations to the crust caused by the Deccan volcanism, by estimating the crustal Poisson’s ratio, shear wave velocity (Vs) structure and the shear velocity contrast across the Moho, through teleseismic waveform modeling. The P receiver functions (RFs) for six broadband seismic stations were constructed using about 575 high quality (S/N≥2.5) teleseismic waveforms of earthquakes (M≥5.5) recorded during the period 2004–2010. The moveout corrected RF summation stacks were inverted using the Neighborhood algorithm (NA) to estimate the shear velocity structure beneath each station. The crustal thickness is estimated to range from 38km in western Saurashtra to 33km close to the southern extension of the Cambay rift. A low velocity zone possibly corresponding to sub-basaltic sediments is detected beneath all the stations. The average crustal Vs and Poisson’s ratio are estimated to be 3.68km/s and 0.276 respectively. The crustal Poisson’s ratio indicates an intermediate to mafic composition for the crust. Modeling reveals a relatively high velocity lower crust with an average Vs≈3.88±0.1km/s which is consistent with the high Vp ≈ 7.1km/s reported for this region through seismic refraction and wide angle reflection studies, overlying a relatively low velocity (Vs ≈ 4.4±0.1km/s) upper mantle. The shear velocity contrasts across the Moho, derived from the amplitudes of the P-to-s (Pms) conversions from the Moho range from 0.08 to 0.17 which are much smaller than those (>0.20) observed across the Indian shield, implying a gradational Moho due to underplating. This study reveals that the crust beneath Saurashtra is distinctly different from that of the Archean Indian shield largely due to significant alterations by the Deccan volcanism.

Seismic velocity model of the crust and upper mantle along profile PANCAKE across the Carpathians between the Pannonian Basin and the East European Craton

Results are presented of a seismic wide-angle reflection/refraction survey along a profile between the Pannonian Basin (PB) and the East European Craton (EEC) called PANCAKE. The P- and S-wave velocity model derived can be divided into three sectors: the PB; the Carpathians, including the Transcarpathian Depression and the Carpathian Foredeep; and the south-western part of the EEC, including the Trans European Suture Zone (TESZ). Seismic data support a robust model of the V p velocity structure of the crust. In the PB, the 22–23 km thick crust consists of a 2–5 km thick sedimentary layer (V p = 2.4–3.7 km/s), 17–20 km thick upper crystalline crust (5.9–6.3 km/s) and an up to 3 km thick lower crustal layer (V p = 6.4 km/s). In the central part of the Carpathians, a 10–24 km thick uppermost part of the crust with V p ≤ 6.0 km/s may correspond to sedimentary rocks of different ages; several high velocity bodies (V p = 5.35, 5.95 and 6.05 km/s) within the sedimentary flysch sequences may represent volcanic sequences. The Moho depth changes from 25 km to 45 km over ca. 100 km distance beneath the Carpathians, west of TESZ. The cratonic crust has a typical three layer structure with a pronounced thickening of the lower crust towards the Ukrainian Shield, where a high velocity lower crust (V p > 7.2 km/s) is observed. Two low-velocity lenses in the upper crust of the EEC are interpreted beneath major sedimentary troughs (Lviv and Volyn-Podolsk). Mantle reflectors are observed at depths of ~ 45 km and ~ 75 km below the PB and 10–20 km below the Moho in the EEC. Sub-Moho (Pn) velocities increase from 8.0 km/s beneath the PB to 8.1 km/s beneath the Carpathians and to ~ 8.3 km/s beneath the EEC. S-waves of acceptable quality are recorded in the EEC; their signal-to-noise ratios increase towards the Ukrainian Shield. • The WARR survey was carried out in Ukraine and Hungary crossing Carpathians. • P- and S-wave velocity model was constructed using ray-tracing forward modelling. • The thickness of the crust along the profile varies from 20 to 48 km. • A significant change in Moho topography is observed beneath the Carpathians. • Full waveform synthetic sections were calculated to support the velocity model.

The Davis Strait crust—a transform margin between two oceanic basins

The Davis Strait is located between Canada and Greenland and connects the Labrador Sea and the Baffin Bay basins. Both basins formed in Cretaceous to Eocene time and were connected by a transform fault system in the Davis Strait. Whether the crust in the central Davis Strait is oceanic or continental has been disputed. This information is needed to understand the evolution of this transform margin during the separation of the North American plate and Greenland. We here present a 315-km-long east–west-oriented profile that crosses the Davis Strait and two major transform fault systems—the Ungava Fault Complex and the Hudson Fracture Zone. By forward modelling of data from 12 ocean bottom seismographs, we develop a P-wave velocity model.We compare thismodel with a density model from ship-borne gravity data. Seismic reflection and magnetic anomaly data support and complement the interpretation. Most of the crust is covered by basalt flows that indicate extensive volcanism in the Davis Strait. While the upper crust is uniform, the middle and lower crust are characterized by higher P-wave velocities and densities at the location of the Ungava Fault Complex. Here, P-wave velocities of the middle crust are 6.6 km s−1 and of the lower crust are 7.1 km s−1 compared to 6.3 and 6.8 km s−1 outside this area; densities are 2850 and 3050 kg m−3 compared to 2800 and 2900 kg m−3. We here interpret a 45-km-long section as stretched and intruded crust or as new igneous crust that correlates with oceanic crust in the southern Davis Strait. A high-velocity lower crust (6.9–7.3 km s−1) indicates a high content of mafic material. This mantle-derived material gradually intruded the lower crust of the adjacent continental crust and can be related to the Iceland mantle plume. With plate kinematic modelling, we can demonstrate the importance of two transform fault systems in the Davis Strait: the Ungava Fault Complex with transpression and the Hudson Fracture Zone with pure strike-slip motion. We show that with recent poles of rotation, most of the relative motion between the North American plate and Greenland took place along the Hudson Fracture Zone.

The continental margin of Uruguay: Crustal architecture and segmentation

The Uruguayan continental margin comprises three sedimentary basins: the Punta del Este, Pelotas and Oriental del Plata basins, the genesis of which is related to the break-up of Gondwana and the opening of the Atlantic Ocean. Herein the continental margin of Uruguay is studied on the basis of 2D multichannel reflection seismic data, as well as gravity and magnetic surveys. As is typical of South Atlantic margins, the Uruguayan continental margin is of the volcanic rifted type. Large wedges of seaward-dipping reflectors (SDRs) are clearly recognizable in seismic sections. SDRs, flat-lying basalt flows, and a high-velocity lower crust (HVLC) form part of the transitional crust. The SDR sequence (subdivided into two wedges) has a maximum width of 85 km and is not continuous parallel to the margin, but is interrupted at the central portion of the Uruguayan margin. The oceanic crust is highly dissected by faults, which affect post-rift sediments. A depocenter over oceanic crust is reported (deepwater Pelotas Basin), and volcanic cones are observed in a few sections. The structure of continental crust-SDRs-flat flows-oceanic crust is reflected in the magnetic anomaly map. The positive free-air gravity anomaly is related to the shelf-break, while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the SDR sequence. Given the attenuation, interruption and/or sinistral displacement of several features (most notably SDR sequence, magnetic anomalies and depocenters), we recognize a system of NW–SE trending transfer faults, here named Río de la Plata Transfer System (RPTS). Two tectono-structural segments separated by the RPTS can therefore be recognized in the Uruguayan continental margin: Segment I to the south and Segment II to the north.

Deep seismic structure of the Indian shield, western Himalaya, Ladakh and Tibet

P and S receiver functions from seismograph stations in the Indian shield, Western Himalaya, Ladakh and Tibet are processed with a method which provides estimates of the P and S velocities and their ratio as a function of depth. The time difference between the P660s and P410s phases in the north of the Indian shield and the Lesser Himalaya is 1.0–1.5 s larger than the normal 24 s. This is an effect of a low temperature with implication that the consumed material of the Indian shield has reached the transition zone. The waveforms of the P410s and S410p phases at some stations in the Indian shield are indicative of a thin (a few tens of kilometers) low S velocity layer atop the 410-km discontinuity, which is usually related to mantle upwelling. The mantle S velocity under the Indian shield at depths less than 180 km is 4.4–4.5 km/s, much lower than the 4.7 km/s, typical for Precambrian shields. We explain this low S velocity mainly by a recent (Tertiary?) metasomatic alteration of the high-velocity mantle keel. Beneath the western Himalaya, Ladakh and western Tibet (but not eastern Tibet) the S velocity in the mantle at depths less than 100–150 km is around 4.7 km/s, Vp/Vs is anomalously low, and we argue that this high-velocity layer is a remnant of the mantle lithosphere of the northern Greater India. At most locations in the Indian shield high S velocities (3.5 km/s and more) are dominant in the middle and lower crusts, and the elevated S velocity is accompanied by an increased Vp/Vs ratio (1.8–2.1 versus the standard 1.73). In the foothills of the Himalaya, the crust is 50–55 km thick and consists almost entirely of a high-S-velocity (3.7 km/s and more) rock with the increased Vp/Vs ratio in the middle and the standard Vp/Vs ratio in the lower crust. This observation suggests that the upper crust of the Indian plate is scraped off in the collision zone, whereas the high-velocity lower crust is subducted jointly with the mantle lithosphere. The high velocities are responsible for the P-wave teleseismic travel time anomaly of ~ 1 s relative to Ladakh. Under the Himalaya the Vp/Vs ratio in the crust is normal, which suggests a change in composition relative to the crust of the Indian shield. Under Ladakh and Tibet the anomalously high Vp/Vs ratio in the crust is observed again. Beneath Tibet our analysis reveals a low-velocity crustal zone of partial melt between the 20-km and 45-km depths. Previously, the 45-km discontinuity was interpreted as the effect of eclogitization.

Crust and lithosphere structure of the northwestern U.S. with ambient noise tomography: Terrane accretion and Cascade arc development

To address the tectonic and magmatic modifications of the Pacific Northwest lithosphere, including transformation of the Farallon oceanic terrane “Siletzia” into continent, we study the crust and uppermost mantle of the Pacific Northwest with fundamental-mode Rayleigh-wave ambient noise tomography using periods 6–40 s, resolving isotropic shear-wave velocity structure from the surface to 70 km depth (3 crustal layers and 2 upper mantle layers). We optimize this estimate with the aid of a neighborhood search algorithm, which we also use with receiver functions to estimate Moho depth. Horizontal node spacing is 0.25°. The EarthScope Transportable Array, the Wallowa array, a portion of the High Lava Plains array, and seven permanent stations are joined to achieve high resolution. Very slow western Columbia Basin upper crust above very fast lower crust expresses the large Eocene sedimentary basins above a magmatically underplated crust of extended Siletzia lithosphere. High-velocity lower crust in adjacent areas to the east and south represents Siletzia thrust under the pre-accretion North America forearc. This interpretation is supported by an anomalous absence of post-accretion magmatism in these areas, implying an absence of slab removal. The southeast termination of the fast lower crust is especially strong and sharp about 35 km southeast of the Klamath-Blue Mountains gravity lineament, suggesting the Farallon slab to the southeast was torn away. The Columbia River Flood Basalts erupted at ~ 16 Ma, apparently creating a hole of diameter ~ 150 km in the edge of the underthrust Siletzia lithosphere. The magmatically active Oregon Cascade arc is slow at all depths, and the much less active Washington Cascades tend to have a volcano-centered structure that is slow in the lower crust but fast in the upper crust and upper mantle. This structure suggests that magmatic intrusion has increased upper crustal velocity, but that the higher temperatures beneath the active Oregon Cascades have a dominating effect and create low velocities.

Coupling of basaltic magma evolution and lithospheric seismic structure in the Emeishan Large Igneous Province: MELTS modeling constraints

Seismic tomography and deep sounding reveal high-velocity upper mantle (HVUM), high-velocity lower crust (HVLC) and crustal strips beneath the Emeishan Large Igneous Province (ELIP). Such a lithospheric structure is similar to that of typical large igneous provinces (LIPs) elsewhere, such as Ontong Java oceanic plateau and Columbia River LIP (Furumoto et al., 1976; Catchings and Mooney, 1988). This study models the formation mechanism of the lithospheric structure and its relationship with the magmatic evolution of the ELIP. Petrological modeling package MELTS is used and the simulation results show that crystal fractionation can account for the main magmatic processes related to the Emeishan mantle plume. It is suggested that the partial melting of the plume head generated the picritic magma, which then finally evolved into high-Ti basalts after undergoing a two-stage differentiation at Moho and in the upper crust. The Sub-continental Lithosphere Mantle (SCLM)-derived magmas experienced fractionation at the Moho boundary and their residual melt fits well with the low-Ti basalts both in oxides composition and evolutionary trends. Calculated fractionated mineral assemblages at Moho match well with the HVLC both in P-wave velocity (Vp) and in inferred mineral phases (olivine pyroxenite with Vp of 7.53–7.84 km s − 1 and clinopyroxenite with Vp of 7.32–7.55 km s − 1 ). Some residual melts may have also been emplaced in the crustal weak layers and formed the high-Vp gabbroic cumulates. Moreover, the HVUM was probably crystallized from the upwelling deep komatiitic magma as the plume's thermal anomaly gradually decayed. The differentiation mode of Emeishan basaltic magmas couples well with the lithospheric seismic structure and may shed lights on the understanding of mafic–ultramafic intrusions and associated ore deposits in the ELIP area.

Structure and composition of the crust and upper mantle of the Archean-Proterozoic boundary in the Fennoscandian shield obtained by joint inversion of receiver function and surface wave phase velocity of recording of the SVEKALAPKO array

SUMMARY We present a 3-D model of absolute values of S-wave velocities and VP/VS ratios in the crust and upper mantle beneath the SVEKALAPKO temporary seismic array that covered the transition zone between Archean and Proterozoic domains in the Precambrian Fennoscandian Shield. The model was obtained using joint inversion of P-wave receiver functions, Rayleigh phase velocities and traveltimes of waves converted from the 410 km discontinuity. P-wave receiver functions and traveltimes of Ps waves converted from the 410 km boundary were estimated for 30 broad-band stations of the SVEKALAPKO array and short-period, smallaperture RUKSA array in Russian Karelia. The phase velocities of Rayleigh waves were taken from previous surface wave studies (Bruneton et al. 2004a,b). For each station, the different data sets were merged and inverted by simulated annealing method. After that, a 3-D S-wave velocity model and distribution of VP/VS ratio was obtained from 1-D velocity models, using special interpolation technique. The new 3-D seismic model demonstrates pronounced lateral variations of values of VS and VP/VS ratio in the crust and uppermost mantle. The depth to the Moho boundary varies from 51 to 63 km in our model, which agrees with the results of previous controlled-source seismic studies in the region. The Moho boundary is overlain by a high-velocity lower crust (HVLC), with high VS, which is non-uniform in composition and origin. Our study showed no systematic correlation between the lithosphere structure and tectonothermal age of Archean and Proterozoic crustal terrains in the study area. The exposed Archean–Proterozoic suture (so-called Ladoga-Bothnian Bay Zone) is not observed as a mega-scale structure in the crust and upper mantle. Generally, the Archean–Proterozoic transition occupies a larger area in the lithosphere than it was thought earlier. It is marked by a Moho depression stretching to the North and by a zone of high VP and high VP/VS in the mantle. Our results supports the theory that the Fennoscandian Shield was assembled as a result of extensive collisional accretion of island arcs and microcontinental blocks and shows that these processes were working both in Archean and Proterozoic.

Delamination of eclogitized lower crust: Control on the crust–mantle boundary in the central Fennoscandian shield

The crustal structure of the central part of the Fennoscandian shield is dominated by a high velocity lower crust (HVLC) ( V P 7.3–7.5 km/s, at the depth of 40–63 km in an area of ca. 400 km × 600 km) formed in the Palaeoproterozoic Svecofennian orogen at ∼ 1.91–1.87 Ga. In the area of the HVLC the Moho is deeper than 50 km and attains depths of up to 63 km in eastern Finland at the Archaean–Proterozoic boundary zone. We suggest that the present Moho dates back to late Svecofennian time and was determined by delamination of high-density eclogitized lower crust after tectonic collisional thickening of crust. Densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle. New geological evidence from kimberlite-hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets. In addition, small clinopyroxene–garnet mineral aggregates (microxenoliths) show omphacitic clinopyroxene compositions. Therefore, the presence of eclogites in the high velocity lower crust is implied, although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present HVLC. With results on near-incidence and wide-angle seismic studies and geological and geochemical data on heat producing elements we modeled the Svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model. The constructed thermal model simulates the genesis of Svecofennian synkinematic (at 1.89–1.88 Ga) and postkinematic (1.88–1.87 Ga) granitoids, as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths. Moreover, the narrow distribution of the ages of the Svecofennian mafic–ultramafic magmatism (ca. 1.89–1.87 Ga) dispersed around the HVLC area is explained by the homogeneous warming of the lithosphere. The conditions of lower crustal delamination were estimated with a Rayleigh–Taylor convective instability model constrained with lower crustal and upper mantle rheological data. Delamination of a lower crustal eclogite layer 5–30 km thick takes place in about 7–50 Ma if the rock dynamic viscosity is ≤ 1·10 21 Pa s. It requires temperatures of ≥ 1000 °C in the lower crust and upper mantle. In the Svecofennian crust, such temperatures were attained at about 1860–1850 Ma, i.e. after the orogenic peak. We conclude that the Moho of the HVLC area was controlled by delamination of a ca. 5–10 km thick layer of high-density lowermost crust into the mantle. Such a density-controlled process may have controlled the Moho depth also in other Proterozoic shield areas.

Wide-angle seismic profiling across the middle Izu-Ogasawara arc for understanding boundary structure between the mature and juvenile arcs - KY0609 cruise -

We carried out a deep wide-angle seismic experiment using a large airgun array and total 95 ocean bottom seismo- graphs (OBSs) in the middle Izu-Ogasawara arc area, which was conducted by R/V Kaiyo of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from August 3, 2006 to September 1, 2006 (UTC) named as KY06-09 cruise. The seis- mic line runs from the Izu-Ogasawara trench to the eastern foot of the Kinan Seamount Chain in the Shikoku basin through the northern tip of the Ogasawara trough, the volcanic front located off south of the Sofugan and the Miocene Nishi-shichito ridge. Most important objective of this cruise is to construct a velocity structure across this area and clarify commonalities and dif fer- ences by comparison of velocity structures obtained with previous studies in the whole of Izu-Ogasawara arc regions. The key structures are the continuity of the crustal layers of the Ogasawara ridge and trough, and the current active arc obtained by p re- vious KY05-02 and KY05-11 cruises. And the high velocity lower crust with P-wave velocity of over 7.0 km/s is also one of the key structures because it was discovered at the arc-backarc transition zones in the western Izu-Ogasawara arc and the Mariana arc. The seismic signals propagated from an airgun array with a total capacity of 12,000 cu.in. were recorded by not only OBSs but also a 12-channnel streamer towed from stern. In this paper, we introduce the seismic experiment, especially the schedule and examples of OBS and reflection data.

Crustal structure below the Polish Basin: Is it composed of proximal terranes derived from Baltica?

The large-scale seismic refraction and wide-angle reflection experiment POLONAISE'97 together with LT-7 and TTZ profiles carried out with the most modern techniques gave a high resolution of crustal structure of the Trans-European Suture Zone (TESZ) in NW and central Poland. The results of seismic investigations show the presence of relatively low velocity rocks ( V p < 6.1 km/s) down to a depth of 20 km beneath the Polish Basin (PB), and a high velocity lower crust ( V p = 6.8–7.3 km/s). The crustal thickness in the TESZ is intermediate between that of the East European Craton (EEC) to the northeast (40–45 km) and that of the Variscan crust (VB) to the southwest (∼ 30 km). Velocities in the uppermost mantle are relatively high ( V p = 8.25–8.45 km/s). The crust is three-layered with substantial differences in the velocities and thickness of individual layers. The area of the TESZ in NW and central Poland can be divided into at least two crustal blocks (terranes), called here Pomeranian Unit (PU, in the northwest) and Kuiavian Unit (KU, in the southeast). The postulated boundary between KU and PU is rather sharp at particular levels of the crust. Velocity distribution in the middle and lower crystalline crust in the TESZ area resemble values recognized in the EEC area, the fundamental difference being the much smaller thickness of both these layers. Our hypothesis/speculation is that the attenuated lower and middle crust of the TESZ belong to proximal terranes built of the EEC crust detached in the southeast and re-accreted to the EEC due to the process of anti-clockwise rotation of the Baltica paleocontinent during the Ordovician–Early Silurian.

Crustal structure of the Vøring Margin, NE Atlantic: a review of geological implications based on recent OBS data

Modelling of extensive seismic datasets recorded on Ocean Bottom Seismographs (OBS) on the outer Vøring Margin, NE Atlantic, has provided significant new insights into deeper sedimentary structures, distribution of sill-intrusions in the sedimentary section, top of the crystalline crust, the lower crust and Moho. Primarily based on the modelling of S-waves, it is concluded that the high-velocity lower crust most likely consists of a mixture of plume-related Late Cretaceous/Early Tertiary mafic intrusions mixed with older continental blocks. Northeastwards in the Vøring Basin, the landward limit of the lower crustal high-velocity layer steps gradually seawards, closely related to five crustal scale lineaments. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Iceland plume, and the passive component is illustrated by the fact that all resolved crustal lineaments to a certain degree acted as barriers to magma emplacement. A lithospheric delamination model is invoked to explain the observed variations in crustal velocities and thickness. The location of six Tertiary domal structures in the Vøring Basin is between, or in the vicinity of, pre-breakup high-velocity structures, which may act as rigid blocks during compression. It is proposed that the existence and trend of these high-velocity structures, subject to mild NW–SE compression, is the most important factor controlling the formation, spatial distribution and trend of the domes. Structures in the high-velocity lower crust may be the single most important element in controlling the formation of the domes; all modelled highs in the lower crustal Early Tertiary intrusive layer seem to be related to the formation of domes.

Highly magnesian olivines and green-core clinopyroxenes in ultrapotassic lavas from western Yunnan, China: evidence for a complex hybrid origin

Oligocene ultrapotassic rocks from western Yunnan have abundant olivines of varying size and green-core clinopyroxenes. Olivine macrocrysts (1–5 mm) have very high forsterite contents (Fo 92–94 ), and high CaO (0.15–0.30 wt%) and Cr 2 O 3 (up to 0.23 wt%) contents. They are believed to be magmatic rather than disintegrated crystals from the upper mantle. These magnesian olivines are not in equilibrium with melts corresponding to the whole-rock compositions of host lavas and, as such, may represent xenocrysts entrained by ultrapotassic magmas during their ascent to the surface. While komaiites, boninites and olivine lamproites are potential sources of these xenocrysts, a genetic link to the late Permian Emeishan large igneous province (LIP) is favoured, because the Emeishan flood basalt is the only known geologic event in the studied area that could have produced highly magnesian olivines. The olivine xenocrysts may have been entrained from the high velocity lower crust (V p = 7.1–7.8 km/s), which is believed to have formed from high Mg basalts (picrites?) that underplated and intruded the crust during the Emeishan flood basalt episode. Given the presence of olivine xenocrysts and reverse-zoned clinopyroxenes, it is apparent that some magnesian ultrapotassic lavas from Yunnan have a complex hybrid origin. While the former point to ultramafic/primary melts the latter indicate interaction between evolved melts and relatively primary melts thus indicating complex magma mixing processes.

Spatial relationship between recent compressional structures and older high-velocity crustal structures; examples from the Vøring Margin, NE Atlantic, and Northern Honshu, Japan

The geological evolution of the volcanic passive margin off mid-Norway, NE Atlantic, and the active volcanic arc in northern Honshu, related to the subduction of the Pacific plate beneath Japan, are similar in the sense that long periods of extensional deformation has shifted to recent compression with formation of domal structures. Seismic wide-angle data have been acquired across six contractional domes in the Vøring Basin, mid-Norwegian Margin, and over the Ou Backbone Range ‘pop-up’ structure, northern Honshu. The models derived from the seismic data reveal that the location of all investigated structures in the Vøring Basin is between, or in the vicinity of pre-Eocene high-velocity structures, which may act as rigid blocks during compression. It is proposed that the existence and trend of these structures, subject to mild NW–SE compression, is the most important factor controlling the formation, spatial distribution and trend of the domes. Structures in the high-velocity lower crust may be the single most important element in controling the domal formation; all modelled highs in the lower crustal early Tertiary intrusive layer seem to be related to the formation of domes in the NW–SE direction. The Ou Backbone Range pop-up structure is localised at a low rigidity zone mapped as low upper crustal seismic velocities. The structure coincides with the present day volcanic front, which may be interpreted as the main reason for the existence of low velocities and the main factor controling the localization of compressional deformation. The Ou Backbone Range is squeezed between a high velocity upper crustal block to the east and a high velocity lower crustal block to the west, and it is speculated that the corresponding lateral and vertical variations in crustal rigidity may have contributed to the actual localization of the present day volcanic front.

An integrated study of the NE German Basin

The NE German Basin is part of the Southern Permian Basin south of the TransEuropean Suture Zone (TESZ). Here we report an attempt to integrate a variety of geological and geophysical data in order to reveal the present day deep crustal structure of the NE German Basin. Special focus is taken on detailed geological information, available reflection seismic data, wide angle refraction seismic data and gravity data.Based on this integrative approach, a concise crustal model is developed which can be subject to further evaluation. Furthermore, it is shown that the NE German Basin is an outstanding feature quite different from classical basin types. As the Moho is flat below the basin, neither a simple-shear-, nor a pure-shear-extension model can be applied. Instead, a thick high-velocity lower crustal layer is present below the basin centre. This high-velocity lower crust is also observed in the western part of the basin and can be taken as characteristic for East Avalonia. Results of gravity modelling indicate the presence of a high-density lower crust, thus supporting the wide-angle data. Furthermore, changes in crustal structure can be derived from changes in reflectivity pattern and from the gravimetric signature along the Elbe Fault System, indicating the presence of different crustal domains north and south of the fault system.An elastic plate model, where the forces applied are the sediment load, a vertical load at the southern margin accounting for the Harz Moutains and a NNE–SSW-directed compression, indicates a buckling of the crust during Late Cretaceous to Tertiary inversion.However, the correlation between the observed crustal structure and the tectonic events that have affected the area remains a subject of discussion. A Permian thermal event affected the upper mantle and the lower crust and, therefore, may have modified the ‘crustal memory’ with regard to Caledonian and Variscan events. Additional tectonic events affected the area during the Mesozoic. In conclusion, it remains an open question whether the high-velocity and high density lower crustal structure represents a remnant of East Avalonia, whether it is related to the formation of the Permo-Triassic basin, or is the cumulative result of a series of events during the Palaeozoic and Mesozoic.

Geophysical studies of the structure of the Appalachian orogen in the Atlantic borderlands of Canada

Results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the Appalachian orogen in eastern Canada. Wide-angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins. High-velocity lower crust, attributed to underplating, is found below the former Laurentian continental margin in Newfoundland and below the Magdalen basin. Potential field data are used to trace the surface zones of the orogen from the northeast Newfoundland shelf to Cape Breton, but extrapolation to New Brunswick and Quebec is unclear because of late Paleozoic basin development. The central mobile belt of the orogen is only a few tens of kilometres wide in southwest Newfoundland and Cape Breton, but broadens substantially to around 200 km elsewhere. Reflection images show a strong deep-crustal fabric that runs along the orogen, with a margin that crosses into the Avalon zone in southern Newfoundland but coincides with the Avalon-Gander zone boundary elsewhere. The fabric formed during mid-Silurian continental collision and (or) during postorogenic collapse. Variation in fabric pattern and metamorphic grade, tightening of structures towards southwest Newfoundland and Cape Breton, and voluminous plutonism in southern Newfoundland are all in accord with maximal crustal thickening followed by erosion and isostatic readjustment in southwest Newfoundland and Cape Breton, and relatively little in northeast Newfoundland and its adjacent shelf. Reflection fabrics in the upper crust appear to be detached from those in the deeper crust; this is attributed to rheological contrast across the base of a quartz-rich upper crust.

Seismic investigation of the East Greenland volcanic rifted margin

The SIGMA project (Seismic Investigation of the Greenland MArgin) was designed to make accurate measurements of crustal thickness, velocity structure and seismic reflectivity along the hotspot-influenced volcanic rifted margin (VRM) off South-East Greenland (Fig. 1). SIGMA is a joint project between researchers at Woods Hole Oceanographic Institution (Woods Hole, Mass., USA) and the Danish Lithosphere Centre (DLC), and data was acquired on a cruise with R/V Maurice Ewing in August–October 1996. VRMs are characterised by a prism of igneous rocks that occupies the continent–ocean transition zone in an 80 to 150 km wide belt, several times thicker than normal oceanic crust, and which extends in some regions for more than 1500 km along strike. This thick igneous crust has two characteristics on seismic data: a seawarddipping reflector sequence (SDRS) interpreted as subaerially erupted basalt flows and intercalated volcanoclastics, and a high-velocity lower crust with P-wave velocities (7.2–7.6 km/s) suggestive of mafic to ultramafic intrusive rocks (Hinz, 1981; Mutter et al., 1982, 1984, 1988; Larsen &amp; Jakobsdóttir, 1988; White &amp; McKenzie, 1989; Holbrook &amp; Kelemen, 1993). Several models for the thermal and mechanical processes involved in the formation of VRMs have been proposed, including: decompression melting during passive upwelling near a mantle plume (White &amp; McKenzie, 1989); actively upwelling plume heads impinging on the base of the lithosphere (Richards et al., 1989; Duncan &amp; Richards, 1991; Griffiths &amp; Campbell, 1991); enhanced upper mantle convection driven by steep, cold lithospheric edges adjacent to the rift (Mutter et al., 1988) and hot upper mantle due to non-plume ‘hot cells’ or insulation by supercontinents (Gurnis, 1988). SIGMA consists of four transects systematically sampling the structure of the South-East Greenland margin and the continent–ocean transition at increasing distance from the Iceland hotspot track, in order to investigate the South-East Greenland VRM with respect to the following questions:1) What is the structure of the transition from continental to thick igneous crust, and thence to normal oceanic crust? Is the transition abrupt or gradual? To what extent does faulting play a role? Does the abruptness of the continent–ocean boundary change with distance from the Iceland plume? 2) What was the total volume of magmatism during continental breakup on the South-East Greenland margin and its conjugates, and how does it vary in space and time? How does this magmatism relate to distance from the Iceland plume and to its temporal magmatic budget? What is the proportion of plutonic to volcanic rocks, and how does this vary with distance from the hotspot track and with total crustal thickness? 3) Does high velocity lower crust exist beneath the margin, and if so, is there any evidence that its composition, thickness, and distribution change along strike? How might such changes relate to variations in melting conditions (temperature and degree of melting) with distance from the plume? 4) Is the structure of the South-East Greenland margin symmetrical with its conjugate margins on the Hatton–Rockall Bank and Iceland–Faeroes Ridge? What combinations of pure shear and simple shear processes might explain the conjugate structures?

The Otway Continental Margin Transect: Crustal Architecture From Wide-angle Seismic Profiling

Wide-angle seismic profiling along the Otway Continental Margin Transect defines the velocity structure from the continental crust onshore to the deep Southern Ocean across an area of extended and faulted basement to the Otway Basin. A significant section of Otway Basin sequences (up to 5 km) lies offshore on the continental slope overlying uppermost Palaeozoic basement (P-wave velocity 5.5 km/s to 5.7 km/s). A deeper basement velocity of 6.15 km/s to 6.35 km/s is prominent. The whole basement section thins from 16 km thickness onshore to 3.5 km at 120 km from the coast.The lower crustal velocity of 6.4 km/s to 6.8 km/s extends down to a 1 km to 2 km thick upper mantle transition zone (6.9 km/s to 7.8 km/s) overlying the Moho at 30 km depth onshore, shallowing to 12 km in the deep ocean basin. The upper mantle velocity is 8.05 km/s, increasing to 8.3 km/s at 40 km depth.There is no evidence of a high-velocity lower crust (>7 km/s) that would indicate that this is a ‘volcanic’ margin as defined on some North Atlantic margins. Analogues for the Otway margin include the ‘non-volcanic’ Galicia Bank off the Iberian Peninsula and the Nova Scotia margin off eastern Canada.

Seismic studies of the crust under the Williston Basin

The seismic refraction method was used in 1981 to study the crust under the northern half of the Williston Basin, in Saskatchewan. A new method of spatial seismic recording, based on a triangular arrangement of receivers, was used for the first time to obtain three-dimensional structure and velocity information. The broadside seismic refraction and wide-angle reflection data obtained by the technique were of particular value in defining several faulted blocks. These blocks are also characterized by aeromagnetic anomalies trending in a northerly direction. The crustal thickness in the southern part of the western provinces shows large variation ranging from 35 to 50 km. Much of the area is also notable for the presence of one or more low-velocity layers and a high-velocity lower crust. There is good evidence for significant lateral heterogeneity, and detailed deep seismic reflection and refraction studies would likely yield information on dips and strikes of beds and faults around the basin as well as define the properties of the various terranes of the Hudsonian mobile belt.