Mantle Reflections Research Articles

Lithospheric structure north of Scothland--I. P-wave modelling deep reflection profiles and gravity

The W-reflector and the Flannan reflector are two enigmatic planar structures in the lithospheric mantle north of Scotland, mapped in three dimensions using deep reflection profiling. The Flannan reflector dips eastwards from the Moho to a depth of 80 km at an angle of ∼30°, whilst the W-reflector is subhorizontal, lying 10–20 km beneath the Moho and terminating westwards against the Flannan. Both reflectors have a strong impedance contrast and are laterally coherent over tens of kilometres. Wide-angle seismic reflection and refraction data are used here to constrain the P-wave velocity structure of the lithosphere in this region and to investigate the physical properties and geometry of these reflective structures. We report the results from an integrated seismic survey in which a series of explosive shots fired at sea were recorded by ocean bottom seismographs, land stations and a streamer towed by a second ship in an expanding spread configuration. A well resolved P-wave velocity structure for the lithosphere above the mantle reflectors is derived by inverting the seismic traveltimes to optimize the fit of the model to the data, and by generating synthetic seismograms from the model to match the relative amplitudes of the observed seismic phases. A high-amplitude post-critical reflection indicates the existence of a discrete reflecting interface dipping gently westwards between 40 and 50 km depth in the lithospheric mantle. The P-wave velocity model is converted to two-way reflection time and correlated with normal-incidence reflection data: the Moho structure agrees well, and the reflecting interface defined by the wide-angle data is coincident with the subhorizontal W-reflector. Less distinct later arrivals observed in the data are consistent with modelled reflections from the Flannan reflector in the mantle and from the extrapolated position of the Outer Isles Fault in the lower crust. Modelling of the relative amplitudes and critical distances of the wide-angle mantle reflection shows that the reflecting layer must be at least 3 km thick and that it has considerably higher velocity (8.5 km s−1) and density (3.5 g cm−3) than normal mantle. Further modelling tests the vertical structure within the layer, and constrains the sharpness of the upper interface and whether the reflection amplitude could have been enhanced by internal layering. Gravity data measured over the region are consistent with the velocity model and with the high-density layer in the mantle, which is constrained by the gravity modelling to be less than 10 km thick. Published laboratory measurements of the physical properties of mantle rocks indicate that mafic eclogite with high velocity and density satisfies all the observations from the mantle layer. Our preferred hypothesis for the origin for such a layer is that it is a fragment of oceanic crust that has been subducted and metamorphosed to eclogite facies.

Lithospheric structure north of Scotland--II. Poisson's ratios and waveform modelling

SUMMARY A 200 by 50 km grid of near-normal-incidence seismic reflection data and a 350 km wide-angle seismic line have been acquired across the Flannan and W-mantle reflectors north of Scotland. A number of geological models have been published to explain the origin of these reflections; we examine these models by considering the physical properties of the lithosphere in this region. We present P- and S-wave velocity models for the lithosphere obtained using ray-based inversion schemes. We show full-wavefield synthetic P-wave data generated using a 2-D finite difference code to confirm that our velocity model can reproduce the amplitude behaviour of the observed mantle arrivals. P- and S-wave velocities beneath the Moho give a Poisson’s ratio of 0.25t0.01, which, together with evidence from local mantle xenoliths, suggests a normal peridotitic upper mantle. We observe both normal-incidence and wide-angle reflections from the mantle reflectors. Modelling and inversion of these reflection events show that the Flannan and W-reflectors represent at least a 20 per cent increase in acoustic impedance within the mantle, and that the region of elevated acoustic impedance has a thickness of at least 3 km. A mafic eclogite is the only common mantle rock type that can have an acoustic impedance significantly higher than peridotite. The physical properties of the upper mantle north of Scotland thus suggest that the Flannan and W-reflectors represent reflection events from the tops of slabs of eclogite. A weak event is interpreted as a wideangle reflection from the base of one of these slabs. A slab thickness of 6.0t1.0 km with a Poisson’s ratio of 0.29t0.02 was used to model this phase. The high Poisson’s ratio within the slab gives further support to the interpretation that these slabs are eclogitic. The lateral extent of these mantle reflection events and the thickness of the slab are most readily explained as oceanic crust that was metamorphosed to eclogite when subducted beneath the existing continent. We conclude that the eclogitic slabs are relicts of oceanic crust that remained within the lithosphere when subduction halted.

Lithosphere rheology during intraplate basin extension and inversion: Inferences from automated modeling of four basins in western Europe

Forward reconstructions of the (mainly) Mesozoic and younger rheological evolution have been made for four basins (Broad Fourteens Basin, Sole Pit Basin, Brittany Basin and the Iberian Basin) in a very consistent way by backstripping and automatic forward modeling of subsidence data, including potentially important effects of heat production, sediment infill and sedimentary blanketing. For default compositional and thermal parameters, the modeling results show strengthening in all basins, and in particular during inversion, with strength increases up to about 2 TN m −1 (20%) relative to their initial values. Given predominantly relatively constant intraplate stresses in continental lithosphere, this is in disagreement with repeated localization of basin deformation. In a thorough sensitivity analysis we explore the possibilities that permissible variations in tectonic history, compositional, rheological and thermal parameters can, in a particular combination, result in slight weakening of the basin, in agreement with reactivation during inversion. However, such a combination reflects an extreme scenario, which is not founded by geological evidence and, statistically, is very unlikely to apply for all basins. A far more likely explanation for relative and permanent weakening of the basins is the presence of pre-existing weak zones, deviating from standard rheological assumptions. At (upper) crustal levels, weakening can be attributed to pre-existing marked faults by a reduced friction angle. This weakening has a pronounced influence on lithospheric strength provided that the reduction in friction angle of pre-existing faults can be extrapolated to the upper mantle. Alternatively, weakening of the upper mantle can be attributed to (1) ductile localization mechanisms, as reflected by the occurrence of upper mantle shear zones, or (2) the occurrence of rheologically weak material, as indicated by upper mantle reflectors.

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.

Crustal structure and neotectonics of the Puysegur oblique subduction zone, New Zealand

Eocene–Oligocene rifting, followed by Miocene–Quaternary strike-slip and obliquely convergent tectonics, are responsible for crustal thinning in the Solander Basin and subsequent development of the Puysegur oblique subduction zone, southwest New Zealand. Highly oblique convergence at a rate of 35 mm/yr is being partitioned between oblique subduction at the Puysegur trench, thrusting within the trench slope, strike-slip faulting on the Puysegur Bank, and minor shortening within the Solander Basin. Subduction is young and a volcanic arc is not yet well-developed. We present seismic reflection data recorded across the Puysegur subduction zone and Solander Basin. Three strike-slip fault zones are recognised on the basis of seismic and bathymetric data in the Puysegur Bank region. North of the profile, some displacement on the eastern fault is transferred eastward to the Hauroko Fault, but most strike-slip motion is inferred to be west of Fiordland. South of the profile, strike-slip faults merge with faults of the Puysegur Ridge. Active thrusting dominates the trench slope, where a thick sedimentary section is evident and bottom simulating reflectors are common. The relatively steep average slope angle of ∼6° suggests the sediments are moderately well cemented, possibly similar to sediments found in the Solander Basin. The small amount of sediment found on the subducting plate is consistent with little or no accretion at the margin. Patches of reflectivity we interpret as sediment at the top of the subducted plate suggest that the slab has a moderate dip of 10–20° to 20 km depth. In the Solander Basin, seismic reflection data clearly image the sedimentary section, and show well developed crustal and upper mantle reflections to depths of 30–35 km (12–13 s TWT). On the basis of lower crustal reflectivity and gravity modelling, the base of the crust beneath the adjacent Stewart Island shelf is at ∼30 km (∼9 s TWT), and rises to a depth of ∼20 km (∼8 s TWT) beneath the Solander Basin. Prominent dipping reflections which we interpret as an active fault can be traced to ∼30 km depth (∼12 s TWT), where they merge into a zone of strong reflectivity in the upper mantle.

Imaging the lithospheric mantle in northwestern Canada with seismic wide‐angle reflections

Normal‐moveout‐corrected and stacked sections of wide‐angle reflection data collected in the Archean Slave province and Paleoproterozoic Wopmay orogen in northwestern Canada image the crust‐mantle boundary as a subhorizontal discontinuity at about 32–36 km depth along a 450 km lateral extent, and, for the first time from such data, layering within the lithospheric mantle at depths between 75 and 90 km over a lateral distance of 750 km. These images correspond to Moho and mantle reflections observed on a colocated multichannel seismic (MCS) reflection profile. Structural features and/or the basalt‐eclogite phase change, associated with subduction of oceanic and/or delaminated lower crust, are possible sources of the deep reflectivity. One mantle reflection group lies west of the MCS data and east of the Cordilleran deformation front. It could have similar sources or result from the Mesoproterozoic Racklan orogen or Meso‐Neoproterozoic rifting of the ancient margin.

Proterozoic and Cenozoic subduction complexes: A comparison of geometric features

Mantle reflections along the Lithoprobe Slave Northern Cordillera Lithospheric Evolution (SNORCLE) deep seismic reflection transect in northwestern Canada are interpreted as a relict subduction zone. Geometric features of the data in the vicinity of the interpreted subduction zone are compared with those of the Vancouver Island profile, which images a modern convergent margin. Comparisons are made at both a regional scale, as in the case of the location of the magmatic arc with respect to the subduction zone, as well as a local scale, such as the geometry of structures within the accretionary wedge. The comparison provides a test for the paleosubduction hypothesis and aids the interpretation of the deep reflection profile where it is difficult to trace individual features to outcrop. The similarity of geometry and scales of the structures gives support to the interpretation that the lithospheric structures in the Wopmay orogen are the products of accretionary processes at ∼1.88–1.84 Ga and that these accretionary processes are similar to those observed today.

Structure of an island‐arc: Wide‐angle seismic studies in the eastern Aleutian Islands, Alaska

New seismic wide‐angle data from the eastern Aleutian Islands show a mafic composition and a 30‐km‐thick island‐arc crust. Traveltimes of P and S refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern Aleutian Arc between the islands of Atka and Unimak using a three‐dimensional finite difference modeling and tomography code. We interpret the highest crustal P wave velocities of 7.2–7.4 km/s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded. Apart from the pieces of oceanic crust, the velocities suggest an overall mafic composition for the arc, composed mainly of metabasalts, diorite and diabase in the upper crust, and garnet‐granulite or amphibolite‐to‐hornblendite in the lower crust. Reflected arrivals from the subducting Pacific plate at depths of 45–55 km beneath the fore‐arc, together with Pn, show a mantle wedge with P wave velocities as low as 7.4 km/s, increasing with depth to about 8.1 km/s with an average of about 7.7 km/s. A mantle composition that grades from mainly pyroxenite (probably ultramafic cumulates) near the Moho to dunite at greater depths best explains these observed velocities.

Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian Shield

Lithoprobe deep seismic reflection data from the northwestern Canadian Shield provide images of structures to the base of the lithosphere between the Archean Slave Province on the east and the Cordillera on the west. Mantle reflections dip eastward from the lower crust to about 100 km depth beneath the ∼1.88–1.84 Ga Great Bear magmatic arc and almost certainly represent a subduction surface associated with arc development. Where the mantle reflections flatten into the lower crust, they merge with prominent west dipping crustal reflections thus delineating a lithospheric‐scale wedge that formed as a result of Proterozoic plate convergence between the Slave craton on the east and the ∼1.84 Ga Fort Simpson terrane on the west. The crust throughout the 725 km survey is highly reflective, and the Moho remains at a constant level between about 11.0 and 12.0 s beneath both Archean craton and Proterozoic accreted rocks; the most significant Moho deflection occurs beneath the ∼1.8–0.7 Ga Fort Simpson basin where it appears to rise to about 9.0 s (about 27 km). Within the crust of the Slave Province, east dipping, low‐angle reflections near the surface beneath the Yellowknife basin may be shallow detachments, and lower crustal geometry is consistent with low‐angle imbrication during Late Archean (∼2.65 Ga) tectonism associated with development of the Yellowknife basin volcanic rocks. West of the Slave craton, accreted Proterozoic crust is characterized by gently folded upper crustal layers overlying apparent thrust duplexes above detachment surfaces that flatten near the Moho. These lower crustal rocks were likely inserted as a tectonic wedge above the Moho and beneath the Slave Province during the contractional phase (∼1.90–1.88 Ga Calderian orogeny) of the Wopmay orogen. On the western end of the profile, the Fort Simpson extensional basin has up to 20–25 km of Proterozoic (meta) sedimentary rocks and sills(?) that are younger than 1.84 Ga.

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an “acoustic window” into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse. The appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west–oriented profiles, is characteristic of the East Shetland platform. However, a north-south–oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated appearance. Regional seismic velocity models derived from refraction data suggest that this reflectivity correlates with a continuous lower crustal layer that has an intermediate seismic velocity. The lower crustal reflectivity is determined to be older than Mesozoic age by the bending down and truncation of the two reflectivity levels at the western margin of the North Sea Viking graben by a major mantle reflector inferred to be associated with Mesozoic rifting. The results of this study are thus in contrast with orthodox interpretations of the reflective layered lower crust as being caused by mantle-derived igneous intrusion or by deformation fabrics associated with stretching in response to continental rifting.

Crustal seismic velocity and density structure of the Intermontane and Coast belts, southwestern Cordillera

Seismic refraction - wide-angle reflection data were recorded along a 450 km profile across the Intermontane, Coast, and Insular belts of the Canadian Cordillera. Crust and upper mantle structure was interpreted from traveltime inversion and forward-amplitude modelling, and the resultant seismic velocities were used to constrain modelling of the Bouguer gravity data along the profile. A high-velocity, high-density block in the upper 8 km of crust was interpreted as the subsurface extension of Harrison terrane; the Harrison fault at its eastern boundary may extend to at least 8 km depth and perhaps 20 km. Throughout the crust, both seismic velocities and densities are in general high beneath the Insular belt, low beneath the Coast and western Intermontane belts, and intermediate beneath the eastern Intermontane belt. However, densities are unusually low in the lower crust beneath the Coast belt (2800 kg/m3), relative to velocities (6.6-6.8 km/s). This indicates that Coast belt plutonic material is present throughout the crust; strong upper mantle reflectivity, previously interpreted on a Lithoprobe reflection line beneath the western Coast belt, may be high-density residue associated with the unusually low density plutonic material. Based on gravity data, Wrangellia must terminate sharply against the western edge of the Coast belt. In the lower crust, the lowest seismic velocities are found vertically beneath the surface trace of the Fraser fault, where velocities just above the Moho only reach 6.5 km/s, in contrast with 6.8 km/s beneath the western Coast belt and eastern Intermontane belt. This provides support for a subvertical geometry for the Fraser fault, perhaps with a broad zone of diffuse shearing in the lower crust. At this location, the Fraser fault does not appear to vertically offset the Moho, which is well-constrained at a uniform depth of km east of the Harrison fault.

An improved velocity model for the crust and upper mantle along the central mobile belt of the Newfoundland Appalachian orogen and its offshore extension

New modelling of wide-angle reflection-refraction data of the Canadian Lithoprobe East profile 91-1 along the central mobile belt of the Newfoundland Appalachian orogen reveals new features of the upper mantle, and establishes links in the crust and upper mantle between existing land and marine wide-angle data sets by combining onshore-offshore recordings. The revised model provides detailed velocity structure in the 30-34 km thick crust and the top 30 km of upper mantle. The lower crust is characterized by a velocity of 6.6-6.8 km/s onshore, increasing by 0.2 km/s to the northeast offshore beneath the sedimentary basins. This seaward increase in velocity may be caused by intrusion of about 4 km of basic rocks into the lower crust during the extension that formed the overlying Carboniferous basins. The Moho is found at 34 km depth onshore, rising to 30 km offshore to the northeast with a local minimum of 27 km. The data confirm the absence of deep crustal roots under the central mobile belt of Newfoundland. Our long-range (up to 450 km offset) wide-angle data define a bulk velocity of 8.1-8.3 km/s within the upper 20 km of mantle. The data also contain strong reflective phases that can be correlated with two prominent mantle reflectors. The upper reflector is found at 50 km depth under central Newfoundland, rising abruptly towards the northeast where it reaches a minimum depth of 36 km. This reflector is associated with a thin layer (1-2 km) unlikely to coincide with a discontinuity with a large cross-boundary change in velocity. The lower reflector at 55-65 km depths is much stronger, and may have similar origins to reflections observed below the Appalachians in the Canadian Maritimes which are associated with a velocity increase to 8.5 km/s. Our data are insufficient for discriminating among various interpretations for the origins of these mantle reflectors.

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.

Deep structure of the continental lithosphere in an unextended orogen: An explosive‐source seismic reflection profile in the Urals (Urals Seismic Experiment and Integrated Studies (URSEIS 1995))

Explosive‐source near‐vertical seismic reflection data from the Urals Seismic Experiment and Integrated Studies (URSEIS) profile displays an image of orogenic lithosphere to depths of 225 km. The reflective crustal section is characterized by dipping, crustal‐scale shear zones, and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late Paleozoic collision. Toward the east, the Uralian crustal root is composed of a partially subducted volcanic arc, while to the west it has affinity with the East European platform. The reflection character of the Moho varies across tectonic strike and implies that the Moho (1) acted as a structural detachment beneath the Trans‐Uralian Zone, (2) forms a tectonic boundary between terranes beneath the East Uralian Zone, (3) consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root, (4) is a 3–9 km thick reflective zone beneath the fold and thrust belt, and (5) is a boundary <200 m thick in the west. Mantle reflections at 80–100 km depth (22–24 s) may represent a continuation of the Main Uralian fault into the subcrustal lithosphere. Alternatively, these reflectors may signify Paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle. Deeper mantle reflections at 140–160 km (35–45 s) and 225 km (55 s) may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle.

Super-deep reflection profiling: exploring the continental mantle lid

Reflections from the continental mantle lithosphere have now been reported from common mid-point (CMP) profiles at numerous sites around the world. These reflections include: (1) distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle; (2) kilometer scale, discontinuous, diffusely distributed, or isolated reflections at varying depths; (3) continuous, sub-horizontal reflections tens of kilometers in extent, often interpreted as the ‘base of the lithosphere’ in spite of a wide variation in depth and; (4) very deep reflections in the mantle lithosphere (> 125 km). The first category of mantle reflections is most commonly observed in proximity to crustal sutures, lending credence to their interpretation as fossil subduction zones. Isolated, discontinuous mantle reflections are by far the most common and, though observed in both ancient and modern collisional environments, are difficult to interpret due to both their limited extent and a corresponding lack of three-dimensional control. Sub-horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide. Of special note are the various reports of specific reflections at ∼80–100 km (22–24 s twt) in the North Sea, eastern Europe, and the Urals which suggest a more ubiquitous mantle horizon. The deepest events imaged thus far in a controlled source CMP survey are the ‘super-deep’ mantle reflections of the URSEIS project that have been identified at ∼135–165 km (35–43 s) and ∼225 km (56 s). Of the many reported mantle reflections, these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo-mechanical transition should be reflective at high frequencies (10–20 Hz).

Seismic lower crustal reflectivity and signal penetration in the Siljan Ring area, Central Sweden

During the period between 1984 and 1990, several reflection seismic profiles, with a total length of ∼100 km. were shot in the area of the Siljan impact structure, Central Sweden. The aim was to study the Proterozoic crystalline crust, prior to the drilling of a deep well. Parts of this data have been re-processed, addressing questions concerning signal penetration and lower crustal/upper mantle reflectivity, as strong lower crustal reflectivity is often absent on land data from the Baltic Shield. Earlier processing showed clear reflectors at upper crustal levels, some of which have been attributed to dolerites, through the use of borehole logging. At lower crustal levels, the picture was less clear with only diffuse bands of reflectors. We have tried to improve the image of the deeper parts, mainly by using standard processing techniques for correcting static shifts and for spectral balancing. The standard stacked sections show, as before, strong reflectors in the upper crust and some discontinuous events at lower crustal levels, primarily in the time window between 11 and 14 s TWT. From this we place the base of the crust at 13–14 s TWT (43–47 km at 6.6–6.7 km/s) in this area. There are strong lateral variations in this apparent reflectivity. These variations could, in part, be caused by differences in charge sizes. The depth of signal penetration, estimated through prestack amplitude decay, was not fully sufficient for the smallest charges (<10 kg). This is supported by coincident stacked sections, where data collected with smaller charges contain less coherent events at lower crustal levels. Some variations in apparent reflectivity are not related to the charge size. The effects of lateral variations in signal penetration have been studied, for 10-kg charges, by calculating amplitudes at fixed-offset channels for different shot positions. Although there are some variations in these amplitudes, we argue that this is not the only explanation for variations in apparent deep reflectivity and that the lower crustal reflectivity is weaker below the Transscandinavian Granite Porphyry Belt than below the Southern Svecofennian Province. From earlier studies, the meteor impact that has occurred in this area is interpreted not to have affected the lower crust.

Structure of the lithospheric mantle beneath the Trans-Hudson Orogen, Canada

Refraction and wide-angle reflection seismic data collected in northern Saskatchewan and Manitoba in 1993 by LITHOPROBE provide new glimpses of the deep structure of the lithospheric mantle beneath the Trans-Hudson Orogen. Velocity models obtained through ray-tracing modelling techniques revealed the presence of complicated mantle structures down to a depth of 160 km beneath the orogen. Like the crust, the lithospheric mantle appears to show a seismically transparent upper and a highly reflective lower part, suggesting that the rheological environment responsible for development of reflections in the lithospheric mantle and of that in the crust are similar. This appears to be consistent with the observation that the strong and brittle upper 30 km of the mantle contain remnants of former tectonic processes in a form of velocity anisotropy, and the lower, more ductile zone seems highly reflective and isotropic. The suspected beginning of the lithosphere-asthenosphere transition zone at a depth of 160 km, imaged as gradual disappearance of mantle reflections, coincides well with models established using other geophysical methods.

New reflections on the structure and evolution of the Makkovikian ‐ Ketilidian Orogen in Labrador and southern Greenland

Marine seismic reflection profiles across the Early Proterozoic Makkovikian ‐ Ketilidian Orogen in the Labrador Sea region suggest that it is a doubly vergent, asymmetric orogenic belt, comparable in width to younger collisional orogens. A southeast dipping reflector package is correlated with on‐land shear zones that mark the southeastern limit of exposed reworked Archean crust and is also associated with a cryptic isotopic boundary in granites, which documents a transition from “ancient” to “juvenile” basement. This boundary is interpreted as a suture, along which juvenile Proterozoic crust has been juxtaposed over (thrust over ?) the Archean craton. A major synorogenic to postorogenic plutonic terrane to the southeast has a poorly reflective upper crust but shows strong subhorizontal reflectivity in the lower crust and Moho regions. The southeastern part of the profile correlates with metasedimentary terranes of the Ketilidian Belt in Greenland and contains low‐angle, northwest dipping, reflector packages suggestive of large‐scale crustal imbrication by thrusting. At least two broad zones of reflectivity at mantle depths (up to 16 s) are also recognized. One dips northwest below the Archean craton, but the most widespread mantle reflectivity is southeast dipping in opposition to the dominant fabric in the overlying crust. These contrasting crustal and subcrustal reflectivity patterns define a geometric “focus” beneath the orogenic belt and may provide information about subduction polarity during its development. The doubly vergent reflectivity pattern resembles images from possibly correlative Precambrian orogenic belts such as the Penokean and Svecofennian and also younger “small collisional orogens” such as the Appalachian and Pyrenean belts. It also broadly resembles some results of geodynamic crustal deformation models based on detachment and underthrusting of mantle lithosphere following collision and the squeezing of “weak” zones between rigid bounding blocks. A speculative multistage model for the belt incorporates initial northward subduction beneath the craton, shifting to later southward subduction, followed by oblique accretion of a composite arc terrane and juvenile (?) continental block. The accreted, hotter, juvenile, Proterozoic crust behaved differently than the stable, cool, Archean crust and experienced subhorizontal shearing in lower crustal and Moho regions, associated with southeast directed imbrication of the middle and upper crust by thrusting. However, the present reflectivity pattern of the orogen may also include elements related to post collisional extensional collapse.

Seismic structure of the upper mantle beneath the southern Kenya Rift from wide-angle data

In February 1994, the Kenya Rift International Seismic Project carried out two wide-angle reflection and refraction seismic profiles between Lake Victoria and Mombasa across southern Kenya. Our investigation of the data has revealed evidence for the presence of two upper mantle reflectors beneath southwestern Kenya, sometimes at short range, from seven shotpoints. Two-dimensional forward modelling of these reflectors using a pre-existing two-dimensional velocity-depth model for the crust [Birt, C.S., Maguire, P.H.K., Khan, M.A., Thybo, H., Keller, G.R., Patel, J., 1997. The influence of pre-existing structures on the evolution of the Southern Kenya Rift Valley — evidence from seismic and gravity studies. Tectonophysics 278, 211–242], has shown them to lie at depths of approximately 51 and 63 km. The upper reflector, denoted d 1, shallows by about 5–10 km in the area beneath Lake Magadi, situated in the rift itself. Correlations for the deeper reflector, denoted d 2, are sparse and more difficult to determine, so it was not possible to define any shallowing corresponding to the surface expression of the rift. Only limited control exists over the upper mantle velocities used in the modelling. Immediately beneath the Moho we use a value of P n calculated from the crustal model, and constraints from previous refraction, teleseismic and gravity studies, to determine the velocity at depth. At the d 1 reflector a reasonable velocity contrast was introduced to produce a reflector for modelling purposes. Beneath the d 1 reflector the velocity decreases to the average value over 3 km. Beneath the rift the velocity also rises across d 1 and again, decreases to the average value over the next 3 km. At the d 2 reflector a similar model is used. This model accounts for the presence of the mantle reflectors seen in the data by using layers of thin higher velocity in a lower background velocity. Due to the uncertainty in the velocities the absolute position of both d 1 and d 2 could vary, but the relative upwelling beneath the rift is reasonably well constrained and data from four different shotpoints which indicate the shallowing show good agreement. A significant result of this study is that the continuity of the d 1 reflector indicates that the sub-Moho lithosphere has not been substantially disrupted by mantle upwelling, even though probably thinned and stretched.

A link between deformation history and the orientation of reflective structures in the 2.68–2.83 Ga Opatica belt of the Canadian Superior Province

True three‐dimensional reflector orientations can be derived from prestack seismic reflection data where a seismic profile is particularly crooked. This is accomplished by estimating a measure of coherency along travel time trajectories defined by the azimuth, dip, and depth of a reflector and the medium velocity. Results from Lithoprobe line 48, located in the Opatica belt of the Archean Superior Province, differentiate reflectors with two distinct orientations, which coincide with the attitudes of two deformational fabrics mapped at surface. Assuming a connection between reflectivity and strain induced by tectonic processes, the reflectors with NNE strikes and shallow dips toward the east are correlated with surface evidence for early west vergent thrusting in the Opatica belt. Other reflectors, which strike ENE‐WSW and dip shallowly to the north and to the south, indicate that most of the reflectors in the southern Opatica and beneath the Abitibi greenstone belt at middle and lower crustal levels formed during a later, approximately N‐S shortening event. Mantle reflections previously interpreted as a relict suture of an Archean subduction zone dip to the north at around 30°–45° and are also associated with this N‐S event. The distribution of reflector orientations estimated at the crooked parts of line 48 indicates that much of the Opatica crust was reworked during the N‐S shortening event, although a region of the middle and lower crust, characterized by the earlier D1 reflectors, is preserved in the central part of the belt.