Dipping Reflectors Research Articles

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.

Early Tertiary magmatism in the offshore NW UK margin and surrounds

The Iceland Plume, activated perhaps as early as Late Cretaceous time and extending through the Tertiary, was responsible for the generation of large volumes of igneous rocks, covering an area of roughly 2000 km in diameter, encompassing Greenland. the Vøring Basin, the Faeroes, the Faeroe–Shetland Basin, the Rockall Trough and NW Scotland. Within this large area, magmatism is mainly manifested as lava fields, sediment–sill complexes, and central or strato/shield type volcano complexes. Towards the continental margin of NW Europe, extrusive rocks occur m two distinct geographical areas. SW of the Munkagrunnur Ridge of the Faeroe Platform, lavas are mainly extruded radially from strato/shield volcano complexes. To the NE, in contrast, seismic evidence suggests that the tipper part of the Faeroe Plateau Lava Group extrusive originate from the same source that formed the Seaward Dipping Reflector Sequences NW of the Faeroes. Towards the SE of the Faeroes, the distribution of the Faeroe Plateau Lava Group (Upper. Middle and Lower Series) lavas is controlled by pre-existing structure and sea-level fluctuations, Seismic and well evidence confirms that, generally, the Upper Series extends furthest into the Faeroe–Shetland Basin while basaltic progradational units form the SE limit of the Lower Series. Similar structures of possibly the same age have been identified within the NE Rockall Trough. There appear to have been two major phases or pulses of Early Tertiary magmatism within the North Atlantic Igneous Province; an initial outburst at around 62 Ma and a later phase at around 55 Ma.

Velocity structure, composition and discrimination of crustal provinces in the former Soviet Union

Significant progress had been made over the last two decades in seismic refraction/wide-angle reflection investigations of continental Russia. Highlights of this progress include the determination of crustal thickness, lithospheric body-wave velocities, compressional and shear wave attenuation, and existence of deeply penetrating dipping reflectors. These new multiwave Deep Seismic Sounding (DSS) data, recorded along geotraverses crossing Precambrian cratons (PC), provinces of Cenozoic basin deposition (CB), and Palaeozoic fold belts (PB), provide a basis for the study of Eurasian continental structure and tectonics. Interpretation of DSS data has shown that geological provinces of different age and tectonic history can be distinguished by the relationship between V p/ V s and the crystalline crustal thickness ( h). There are two primary relationships in the V p/ V s vs. h function. In the first (characteristics of PC), V p/ V s is directly proportional to crustal thickness. In the second relationship, observed in CB and PB, V p/ V s is inversely proportional to crustal thickness. Province boundaries are imaged as dipping reflectors through analysis of the wave field of three component records using special velocity filtering. Dynamic features of these reflections suggest that these boundaries are detachment faults separating relatively thin layers with complicated fine structure. The composition of the crystalline crust, determined from body wave velocities, varies significantly over the transition zone from one geological province to another. Mapping of regions according to the V p/ V s vs. h function, shows a correlation of the known mineral, oil, and gas deposits with province boundaries.

Large‐scale structure of Calamus River deposits (Nebraska, USA) revealed using ground‐penetrating radar

The large‐scale (i.e. bar‐scale) structure of channel deposits of the braided, low‐sinuosity Calamus River, Nebraska, is described using ground‐penetrating radar (GPR) profiles combined with vibracores. Basal erosion surfaces are generally overlain by medium‐scale, trough‐cross‐stratified (sets 3–25 cm thick), very coarse to medium sands, that are associated with relatively high amplitude, discontinuous GPR reflectors. Overlying deposits are bioturbated, small‐scale cross‐stratified (sets <3 cm thick) and vegetation‐rich, fine to very‐fine sands, that are associated with low‐amplitude discontinuous reflectors. Near‐surface peat and turf have no associated GPR reflectors. In along‐stream profiles through braid and point bars, most GPR reflectors dip downstream at up to 2° relative to the basal erosion surface, but some reflectors in the upstream parts of bars are parallel to the basal erosion surface or dip upstream. In cross‐stream profiles through bars, GPR reflectors are either approximately parallel to bar surfaces or have low‐angle inclinations (up to 6°) towards cut banks of adjacent curved channels. Basal erosion surfaces become deeper towards cut banks of curved channels. These structures can be explained by lateral and downstream growth of bars combined with vertical accretion. Convex upwards forms up to 0·5 m high, several metres across and tens of metres long represent episodic accretion of unit bars (scroll bars and bar heads). Stratal patterns in channel fills record a complicated history of erosion and deposition during filling, including migration of relatively small bars. A revised facies model for this type of sandy, braided river has been constructed based on this new information on large‐scale bedding structure.

A simple seismic imaging exercise

After years of teaching courses on seismic techniques, from introductory to advanced, I have observed two things: (1) we learn best when we can visualize a concept, and (2) we learn by doing, not by observing what others do (Mike Graul taught me this). I have let these two principles guide my development of this tutorial on imaging, which I have used many times in a course titled “Advances in seismic processing.” In this exercise I use a simple dipping reflector to develop the concepts of NMO, prestack time migration, DMO, and poststack migration, and show how they are closely interrelated. The specific geometry was suggested to me years ago by Joong Chun and, although I never saw his development of the full exercise, I’m sure it would have gone something like this. By the time you get to the last figure, you will all say: “But that is so obvious!” What you won’t appreciate is the blood, sweat, and tears I have put in to make it so obvious!

Deep crustal reflectors under Reydarfjördur, eastern Iceland:crustal accretion above the Iceland mantle plume

Summary Deep seismic reflection profiling on the Faroe–Iceland Ridge Experiment (FIRE) imaged a sequence of dipping events in the upper to |-crust beneath the Tertiary lava pile in Reydarfjordur, eastern Iceland. These events dip towards the spreading centre at which the crust was formed, and continue down to ∼ 6 s two-way time, corresponding to depths down to 15–18 km. Similar crustal reflections have been observed previously within the crust in other areas of Iceland, although the reflective events located by previous surveys have not been interpreted rigorously. In this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava πle. The synthesized lava pile sections are based on field measurements made in the Reydarfjordur area: the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions, and some intercalated tuffaceous clastic horizons. We compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas. We conclude that the upper and mid-crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface, with the main reflections resulting from packets of flows of similar thicknesses. The reflectors that we have imaged beneath Iceland are similar to those defining the seaward-dipping reflectors on the volcanic continental margins, and we therefore suggest that the seaward-dipping reflections commonly result from packets of lavas with similar distributions of thicknesses. The occurrence of lavas down to 15–18 km beneath Reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust, although the volcanic processes forming the crust in Iceland generally lead to advection of extrusive material to mid-crustal levels. The production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity. The advection of frozen lava flows to mid-crustal levels contributes to the maintenance of the subsolidus crust that is observed on Iceland, and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques.

A deep seismic crustal transect from the NE Iberian Peninsula to the western Mediterranean

The crustal structure of the transition from the NE Iberian Peninsula to the western Mediterranean is characterized by large‐scale variations related to its successive compressional and extensional Cenozoic tectonics. The Spanish ESCI (Spanish acronym for seismic studies of the Iberian crust) Program recorded 450 km of deep vertical reflection profiles on land and at sea, to complete a first crustal transect from the Pyrenees to the South Balearic basin. Highly energetic, north dipping reflections imaged by combining the Etude Continentale et Océanique par Reflexion et Refraction Sismique (ECORS) and ESCI land profiles mark a crustal thickening toward the Pyrenees. The reflective band becomes horizontal beneath the Catalan coastal ranges, and the Moho is found at 11s. No evidence of crustal thinning affecting the Iberian mainland is observed. The transition to the Mediterranean basin is constrained by complementary onshore‐offshore recordings. Forward modeling combined with multichannel processing of the wide‐angle data set shows that a strong thinning of the continental crust by a factor of 2 over a distance of 50 km affects both the Catalan and Balearic flanks of the Valencia trough. This thinning affects mainly the deep crust, characterized by low velocities of 6.4–6.5 km/s, suggesting a removal of part of the lower crust during the rifting processes. Velocities of 7.8–7.9 km/s are interpreted in the uppermost mantle with no evidence of underplating. The Moho appears as a marked discontinuity throughout the transect. The NW part of the Balearic promontory is characterized by a thick and reflective crust from 12 to 30 km depth. An intensely fracturated evaporitic sequence is seen for 100 km along the south Balearic basin. The seismic images of the continent‐ocean transition are disturbed by these prominent sediments. However, the deep reflectivity suggests a crustal thinning (Moho at 6–7 s) to be related to the Middle Miocene‐present extensional tectonics.

Seismostratigraphy of the western flanks of El Hierro and La Palma (Canary Islands): a record of Canary Islands volcanism

A cruise carried out aboard the Spanish R/V Hespérides using a 255-inch 3 airgun allowed to identify three main seismostratigraphic units in the western submarine flanks of the islands of La Palma and El Hierro, Canary Islands. The time structure and thickness maps of each unit, together with the interpretation of the seismic facies, demonstrate that the evolution of the Canary Islands is recorded in the marine basin. The structure maps show that the older reflectors dip to the NW in contrast to the present-day sea floor which dips to the WNW. The thickness maps also show that the depocentre moved from a position parallel to the Northwest African margin to a position now occupied by the island of La Palma. Despite the high sedimentation rates of hemipelagic background sedimentation in the study area, the three seismic units can be interpreted in terms of their relation to the Canarian volcanism. Unit I was deposited before the initiation of volcanism and the arrival of volcaniclastic flows and is thus mainly, if not entirely, composed of hemipelagic sediments. Unit II seems to reflect the shield phases of the easternmost edifices, and as such would be constituted by the distal products of erosion and mass wasting of these islands. Unit III, in contrast, results mainly from the shield phase of the two westernmost islands (La Palma and El Hierro), and as such reflects more proximal conditions, as demonstrated by the presence of lateral collapse deposits from these islands.

Study of the crystalline crust from a two-ship normal-incidence and wide-angle experiment

In 1992, British Institutions' Reflection Profiling Syndicate conducted a two-ship coincident near-vertical and wide-angle experiment to determine the detailed velocity structure of the continental crust in the Central North Sea. The results show a near-surface high velocity gradient layer (P-wave velocity reaches 6.0 km/s at ∼5 km depth), a high-velocity zone (6.5 km/s) at 10–15 km depth, a 19-km-thick upper crust and the Moho at a depth of 32–34 km along the 135-km profile. The travel-time and amplitude of wide-angle data require the presence of a dipping reflector originating at the Moho (32 km depth) and continuing down to 38 km in the mantle. The velocity between 19 and 24 km depth is slightly lower (6.1 km/s) than that in the upper crust. The waveform modelling of the combined normal-incidence and wide-angle data requires a number of alternating high- and low-velocity layers in the lower crust (19–33 km). The velocity contrast in the lower part of the lower crust (29–33 km) is large, of the order of 1 km/s (6.5–7.5 km/s). There are a few alternating high- (8.2 km/s) and low-velocity (7.5 km/s) layers in the upper part of the mantle (33–36 km). The high velocity (6.5 km/s) in the upper crust may be due to a high grade of metamorphism, and it may have been formed during the subduction process. The low-velocity (6.1 km/s) in the upper part of the lower crust might be caused by the presence of meta-sediments. The large velocity variations (6.5–7.5 km/s) in the lower part of the lower crust suggest the presence of mafic materials, and may result from under-plating of mafic materials from the mantle. The fractionation of melt can also produce the layering in the upper part of the mantle that is observed from seismic analyses.

Continental margin off Western India and Deccan Large Igneous Province

Massive, transient late syn-rift-to-breakup volcanism during separation between the Seychelles microcontinent and India formed the Deccan continental flood basalts and their equivalents on the Seychelles-Mascarene Plateau and on the conjugate continental margins, i.e. the Deccan Large Igneous Province. We estimate an original extrusive area of at least 1.8×106 km2, and a volume >1.8×106 km3, and suggest a plate tectonic model comprising: (1) development of the Seychelles microplate by fan-shaped spreading in the Mascarene Basin, and continental extension followed by fan-shaped spreading between India and the Seychelles during A29-27 time. (2) Cessation of fan-shaped spreading just after A27 time, followed by spreading along the India-Seychelles plate boundary. (3) Margin subsidence, modified south of Goa by the persistent, time-transgressive effects along the plume trail. The margin is divided into three regional provinces by the prolongation of regional transforms which formed the east and west boundaries of the Seychelles microplate during breakup and early sea floor spreading. In some aspects, the conjugate margins are different from other volcanic margins; e.g. regional wedges of seaward dipping reflectors along the continent-ocean transition have not yet been reported. We ascribe this to the eruption of the most voluminous lavas during chron 29r, i.e on continental lithosphere in a late syn-rift setting. The enigmatic Laxmi Ridge is a complex marginal high comprised of both continental and oceanic crust. It was probably created during breakup, but may have experienced later magmatic and/or tectonic deformation.

Abstract: Holocene Tidal Deltas of the Trinity Incised Valley: Analogs for Exploration and Production

ABSTRACT The facies architecture and evolution of modern and Holocene tidal-delta deposits were examined using high-resolution seismic (chirper) data, sediment cores, and boring descriptions. During the last transgression (18,000 yr. BP to present) the Trinity/Sabine incised valley backfilled with continuous fluvial and bay-head delta facies and discontinuous middle-bay and tidal delta facies. The tidal delta facies preserved within the valley extend up to 22 km in dip-direction and are up to 12 m thick. The modern Bolivar Roads tidal delta complex is similar in size to those preserved within the offshore portion of the valley, and is a mud-dominated system. The overall size of the tidal delta complex decreased around 1,500 yr. BP as the peninsula accreted and the inlet grew narrower. Prior to this time the tidal delta complex was two times larger. The tidal delta seismic facies is characterized by stacked landward and seaward dipping reflectors overlain by a ravinement surface. Lithofacies have been described using sediment cores for the Holocene tidal delta complexes. Isopach maps of these tidal deltas provide reservoir analogs for subsurface exploration and production.

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.

Mountain building and active deformation studied in New Zealand

Strong, eastward dipping (∼40°) reflectors found 23 km below the central South Island were recorded by a joint U.S.‐New Zealand geophysical project during austral summer 1995–1996. When projected to the surface, the reflectors define a plane that coincides with the surface trace of the Alpine Fault (Figure 1).Data processing is still at an early stage, but preliminary results, reported at a workshop held at Victoria University of Wellington February 17–19, 1997, give an overall picture of asymmetric crustal thickening and active processes in the mid to lower crust. A jump in crustal thickness of about 20 km is inferred across the Alpine Fault, and the locus of thickening does not occur beneath the highest mountains, but is imaged at least 20 km farther to the east. Low seismic velocities and high electrical conductivity roughly coincide with the strong eastward‐dipping reflections in the mid to lower crust. Excess fluids and high pore pressure probably explain both the low seismic velocities and high electrical conductivity.

The crustal structure of the Canary Basin: Accretion processes at slow spreading centers

Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower‐crustal reflectivity is almost absent and reflections from the crust‐mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust‐mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid‐Atlantic Ridge.

Reflections from a mylonitized zone in central Sweden

Dipping reflections imaged on a deep seismic reflection profile acquired within the Baltic Shield can be indirectly linked to strain‐induced acoustic impedance contrasts within a regional brittle‐ductile shear zone: the Singö shear zone. Measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis. Velocity measurements include some made on cored field samples at confining pressures of ≤ 0.4 GPa in a laboratory (10 MHz sensor frequencies) and some made in situ using ultrasonic (54 kHz) sensors on exposures within the shear zone. Laboratory measurements show a 10% change in seismic anisotropy and a contrast of >0.5 km s−1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite. The in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes, including zones where strain and associated volume loss of silica remain elevated without mylonitization. Synthetic seismograms generated using a one‐dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6–25 Hz frequencies. Ray trace, two‐dimensional forward modeling of a high‐density, wide‐angle reflection section demonstrates that deep dipping reflections could result from strain‐induced acoustic impedance contrasts within the Singö or similar, nearby shear zones.

Crustal structure of Edoras Bank continental margin and mantle thermal anomalies beneath the North Atlantic

We discuss the structure of the continent‐ocean transition of the Edoras Bank area of the NE Atlantic margin and the influence of the Iceland mantle plume during continental breakup. Edoras Bank lies close to the present‐day limit of influence of the Iceland plume. Sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well‐developed over much of the margin. They form aerially extensive, dipping sheets, with individual reflectors imaged on margin‐parallel profiles as laterally continuous and subhorizontal features. We interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup. A crustal velocity model for the margin was developed by travel time modeling of wide‐angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling. Oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities. The crustal thickness increases to the SE reaching 17 km beneath the continental fragment of Edoras Bank. The continent‐ocean transition is dominated in the lower crust by a lens up to 8 km thick of high‐velocity material (7.2–7.5 km s−1), which exhibits a very low velocity gradient. It extends over a 60 km wide region beneath the seaward dipping reflectors. The high velocities and densities of the lower crust are probably caused by igneous rock with a high MgO content that underplated the margin or heavily intruded pre‐existing crustal material during continental breakup. Both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by Deep Sea Drilling Project (DSDP) boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup. Comparison of the structure and subsidence history of the Edoras Bank margin with other areas of the NW European margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the Iceland mantle plume during and following continental breakup. We find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle ∼150°C hotter than normal lay beneath the 2000 km long rift, extending from the Lofoten margin off Norway in the north to the Edoras Bank margin in the south. Over a period of 5–10 m.y. following the onset of rifting, the mantle thermal anomaly decayed rapidly to only 50°C or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism.

MONA LISA — Deep seismic investigations of the lithosphere in the southeastern North Sea

The MONA LISA collaborative project has collected 1112 km of seismic normal-incidence reflection data (recorded to 26 s) and wide-angle data from 26 onshore and 2 offshore locations along 4 profiles in the southeastern North Sea. The seismic data clearly image structures in the crust and uppermost mantle that may be related to Caledonian collision and Late Palaeozoic to Mesozoic rifting and basin formation. For the first time, dipping and subhorizontal reflections from the mantle have been observed to 24 s twt in seismic normal-incidence reflection sections. Strong sub-horizontal reflections are observed on two perpendicular profiles at c. 21 s twt, which is near the base of the lithosphere. These deep reflections are sharper than expected from a thermal–rheological transition suggesting that they are generated at lithological or tectonic interfaces. The Moho is reflective along all four profiles. Particularly strong and sharp normal-incidence and wide-angle reflections were recorded from the Moho south of the Caledonian Deformation Front. The normal-incidence reflection sections show unusually low intra-crustal reflectivity. Lower crustal, subhorizontal reflectivity is only observed west of the Central Graben and in a short profile segment at the base of the interpreted Caledonian Deformation Front. The crustal suture between Avalonia and Baltica is interpreted to be south- to west-dipping. SSW- and N-dipping reflections to 20 s twt are observed from the uppermost mantle. Two models of Tornquist Sea subduction may explain these events: either southward subduction and later extensional reactivation or northward subduction with later extension along a SSW-dipping shear zone. The crust is c. 5 km thinner below the Central Graben than below the surrounding parts of the Mid North Sea–Ringkøbing-Fyn High. The lower crust is reflective on the western flank of the Central Graben. East-dipping reflections from the lower crust and the upper mantle to the east of the graben may indicate a component of simple shear in the evolution of the rift structure.

A relic proterozoic subduction zone in western Canada: New evidence from seismic reflection and receiver function data

Lithoprobe seismic‐reflection profiles in the Western Canada Sedimentary Basin reveal a prominent band of dipping reflections that appear to extend into the upper mantle. The NNE strike of these reflections, determined from intersecting profiles, is parallel to the trend of the ca. 1.8 Ga Rimbey domain, an overlying magmatic belt. Receiver‐function analysis of teleseismic data recorded at a nearby site shows evidence for a low‐velocity zone (LVZ) beneath the Rimbey domain between 13 and 16 km depth. The unusually low S‐wave velocity of the LVZ (2.9±0.2 km/s) suggests a composition similar to serpentinized peridotites that occur in ophiolites. Taken together, our data support a model of Proterozoic subduction beneath the Hearne province. We interpret the dipping reflections observed on the seismic profiles as a shear zone that developed during subduction of a SE‐dipping slab, and the LVZ as a fragment of oceanic lithosphere that was obducted to the overriding plate.

Possible upper mantle reflection fabric on seismic profiles from the Tethyan Himalaya: Identification and tectonic interpretation

Upper mantle structures of the Himalaya collision system are not well‐defined, yet are integral to our understanding of the lithospheric scale processes which have built the Tibetan plateau. A 100‐km‐long deep seismic reflection profile (Tib‐1) and a short cross line (Tib‐2) were recorded within the Tethyan Himalaya to help constrain the geometry of the crust and mantle. At long travel times, the returned seismic energy is especially weak compared to the noise; therefore the data were reprocessed with signal‐to‐noise enhancing techniques to identify coherent events at mantle travel times. Visual comparisons of shot gathers, recordings of the ambient noise, common midpoint stacks, and synthetic sections suggest that a dipping reflection “fabric” is present in the upper mantle. On the basis of geometrical and velocity constraints, the fabric is not likely related to sideswipe, P‐to‐S phase conversions, multiples, or surface waves. The fabric is most likely within the underthrust Indian lithosphere beneath the peaks of the High Himalaya. The fabric strikes ∼N63°E, significantly oblique to the ∼E‐W strike of crustal deformation; however, the fabric dip direction (∼44°, N27°W) is essentially the same as the fast direction of polarization inferred from a previous SKS shear wave splitting study. The crust versus mantle strike discordance suggests that the fabric may predate the collision of India with Tibet and mark previous deformation of the Indian lithosphere (e.g., rifting episodes that formed the Tethyan ocean). Conversely, if the fabric is the result of collision, the discordance would suggest that mantle deformation is decoupled from crustal deformation within the Himalayas.

LITHOPROBE reflection studies of Archean and Proterozoic crust in Canada

LITHOPROBE, Canada's national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the North American continent, is a multidisciplinary program spearheaded by seismic reflection studies. Five recently recorded seismic lines, discussed in this paper, are located in Precambrian regions: the Mesoproterozoic Grenville Province, the Paleoproterozoic Trans-Hudson orogen (THO), the Paleoproterozoic/Archean basement of Alberta, and the Archean Superior Province. Data acquired across the Grenville orogen in eastern Quebec show strong reflectivity throughout the crust; upper crustal reflections can be correlated with exposed structural elements, including extensional shear zones and packages of deformed high-pressure rocks (eclogites). In a marine survey across the Grenville orogen off southeastern Labrador, seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly. Data acquired across central Alberta show crustal-scale thrust stacking and imbrication of the Archean Hearne craton. To the east across the Trans-Hudson orogen, images of similar collisional features are observed. Geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1.8 Ga, suggesting that collisional tectonic activity was coeval over a broad crustal region, ca. 1000 km across strike. In the Superior Province, seismic data across a collision zone involving the northern Abitibi greenstone belt and the arc-related Opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s (∼30 km) into the mantle. The latter feature is interpreted as representing a relict 2.69-Ga-old suture associated with subduction, providing the first direct evidence that plate tectonics was active in the late Archean. These five examples, supported by other LITHOPROBE results, refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies, combined with other geoscience studies, can lead to a better understanding of Precambrian tectonics. Reflectivity persists throughout the crust; there is no general separation into a poorly reflective upper crust and a reflective lower crust. Crustal reflectivity in Archean and Proterozoic regions is as pervasive as that in areas of more recent tectonism. The Precambrian reflection Moho is generally well defined but shows a range of characteristics. Relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity.