Subhorizontal Reflections Research Articles

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.

Origin and nature of crystal reflections: Results from integrated seismic measurements at the KTB superdeep drilling site

For almost 10 years the KTB superdeep drilling project has offered an excellent field laboratory for adapting seismic techniques to crystalline environments and for testing new ideas for interpreting seismic reflections in terms of lithological or textural properties of metamorphic rock units. The seismic investigations culminated in a three‐dimensional (3‐D) reflection survey on a 19×19 km area with the drill site at its center. Interpretation of these data resulted in a detailed, structural model of the German Continental Deep Drilling Program (KTB) location with dominant, steep faults in the upper crust. The 3‐D reflection survey was part of a suite of seismic experiments, ranging from wide‐angle reflection and refraction profiles to standard vertical seismic profiles (VSP) and more sophisticated surface‐to‐borehole observations. It was predicted that the drill bit would meet the most prominent, steeply dipping, crustal reflector at a depth of about 6500–7000 m, and indeed, the borehole penetrated a major fault zone in the depth interval between 6850 and 7300 m. This reflector offered the rare opportunity to relate logging results, reflective properties, and geology to observed and modeled data. Post‐Variscan thrusting caused cataclastic deformation, with partial, strong alterations within a steeply dipping reverse fault zone. This process generated impedance contrasts within the fault zone on a lateral scale large enough to cause seismic reflections. This was confirmed by borehole measurements along the whole 9.1 km deep KTB profile. The strongest, reflected signals originated from fluid‐filled fractures and cataclastic fracture zones rather than from lithological boundaries (i.e., first‐order discontinuities between different rock types) or from texture‐ and/or foliation‐induced anisotropy. During the interpretation of seismic data at KTB several lessons were learned: Conventional processing of two‐dimensional (2‐D) reflection data from a presite survey showed predominantly subhorizontal layering in the upper crust with reflectivity striking in the Variscan direction. Drilling, however, revealed that all rock units are steeply dipping. This confirms that surface common depth point (CDP) seismics strongly enhances subhorizontal reflectivity and may thus produce a very misleading crustal image. Although this was shown for synthetic examples earlier, the KTB provides the experimental proof of how crucial this insight can be.

Structure crustale du bloc cadomien de Bretagne Nord (France): sismique réflexion verticale et sondage magnétotellurique (projet Géofrance 3D — Armor)

A new deep seismic profile has investigated the Upper Proterozoic terrains assembled during the Cadomian orogeny in the Northern Armorican Massif. This 60 km land profile prolongs the marine Swat 10 profile of the ECORS-BIRPS programme. Although the upper crust is poorly reflective on the stack section, several NW dipping reflectors have been observed and modelled from single shot records. They can be correlated with outcropping strike-slip and overthusting major contacts. The reflectivity of the lower crust, below 5 s, varies from one domain to another but it is relatively high and characterized by subhorizontal reflectors. The whole upper crustal Cadomian structures are interpreted as a SE verging overthusting prism the sole of which could be the top of the reflective lower crust.

Interpretation of reflections from the central part of the Siljan Ring impact structure based on results from the Stenberg-1 borehole

In 1984, 1985 and 1990, several multichannel seismic reflection profiles were shot over the Siljan Ring, a meteorite impact structure believed to have been formed approximately 360 Ma ago. The bedrock of the area consists mainly of gneisses and granites and the ring itself of Palaeozoic sedimentary rocks lying on top of granites. Dolerite dikes of different age and orientation have been mapped in the area. The existence of dolerite intrusions at depth has been verified through the drilling of two deep boreholes, Gravberg-1 and Stenberg-1. Interpretation of seismic data and borehole data from the Gravberg-1 borehole showed a strong correlation between high-amplitude subhorizontal reflections and dolerite sills. Geophysical and geological well-logging in the Stenberg-1 borehole showed the occurrence of dolerites in the borehole. Profile 4, running E-W across the borehole, has been reprocessed in order to improve the seismic image. Several thick dolerites below 5.7 km in the borehole correlate with high-amplitude reflectors on the seismic section. Both the logging data and the seismic interpretation suggest that these intrusions are subhorizontal and laterally continuous. Above 5.7 km in the borehole, the dolerites are thinner and are in some cases associated with fracture zones. On the seismic section at these depths there is a complex system of weaker dipping reflectors. Some of these dipping reflectors correlate with either fracture zones, thin dolerites or a combination of both.

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.

Winagami reflection sequence: Seismic evidence for postcollisional magmatism in the Proterozoic of western Canada

A recent Lithoprobe seismic-reflection transect in the Peace River arch region of the Western Canada sedimentary basin traverses continental crust that was assembled in a series of magmatic and collisional events during the interval 2.0–1.9 Ga. The seismic data reveal panels of dipping reflectivity at tectonic domain boundaries, but they are otherwise dominated by prominent, subhorizontal reflections that occur between 3.5 and 18.5 km depth over an area of at least 120 000 km 2 . These reflections, here named the Winagami reflection sequence, overlap several tectonic domains and are comparable in geometry, depth, and areal distribution to sills and discordant intrusive sheets imaged by previous seismic profiling in the Baltic shield and the Basin and Range province of the western United States. As many as five distinct reflections are present, some of which can be traced for more than 100 km. Contrasting reflection character in adjacent tectonic domains suggests an intrusive geometry controlled partly by the mechanical properties of the host rocks. Seismic modeling indicates that observed waveforms are consistent with reflections from tabular mafic bodies ∼70 m thick. The Winagami reflection sequence crosscuts the collisional fabric (ca. 1890 Ma), but the sequence is offset by a brittle fault with an estimated age of 1760 Ma. These timing constraints suggest that the Winagami reflection sequence is a previously unknown component of a regional magmatic event emplaced during a period of brittle indentation of the western Canadian shield by the Slave province.

Reflective oceanic crust formed at a fast-spreading center in the Pacific

We present new seismic images of Cretaceous crust formed at a fast-spreading center in the Pacific. The high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast-spreading centers are not seismically detectable. Subhorizontal reflections can be traced at 600–800 ms two-way time below the top of basement for tens of kilometres, suggesting the presence of a widespread seismic boundary, possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes. Lower-crustal reflections, dipping dominantly toward the paleo–spreading center, may represent mafic-ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections.

Deep penetrating MCS imaging of the rift-to-drift transition, offshore Douala and North Gabon basins, West Africa

We describe a regional grid of West African PROBE Study (WAPS) deep penetrating multi-channel seismic reflection (MCS) and potential field data crossing the transition from rifted continental crust (RCC) to normal oceanic crust (OC) in the offshore Douala and North Gabon Basins. In profiles oriented sub-parallel to oceanic fracture zones RCC is shown to terminate seaward into a very different form of crust interpreted to be composed of highly attenuated blocks of RCC, wedges of seaward-dipping reflectors representing intermixed mafic volcanics and sediments, mafic lower crust, and possibly exhumed mantle rocks, all of which form a complex faulted terrain. This heterogenous type of crust is referred to here as ‘proto-oceanic crust’ (POC). POC grades seaward into normal to thick OC. In the Douala Basin, sedimentary units deposited in an Early Cretaceous continental rift now form marginal basin highs overlain by < 1 km younger of sediment beneath the shelf. RCC here has undergone little attenuation, as more highly attenuated RCC is believed to occur below the conjugate Sergipe-Alagoas Basin in eastern Brazil. In the North Gabon Basin, RCC has been markedly thinned and resulting rift structures are deeply buried by post-rift deposits. Reflective lower crust within RCC forms discontinuous bands, where normal dip-slip faults sole into it and transform related faults crossthrough it. Interpreted Moho reflections atthe base of RCC form a discontinuous horizon that is sometimes offset at crustal deformation zones; elsewhere it transects the reflective grain of crustal deformation zones. Interpreted WAPS MCS data show that the margin is composed of rift-units separated at ca 40 km intervals by NE trending fracture zones and transfer fault zones of variable length, causing the margin to step progressively southwestward by alternating between normal and transformfaulted segments. Correlation of rift structures between these West African basins and the conjugate Sergipe-Alagoas Basin show that the two margins formed a connected rift-branch that underwent NE-SW oriented extension, ca 35° oblique to the rift axis. This orientation favoured occupation of NE-SW trending preexisting shear fabrics situated in the Late Proterozoic Pan-African/Braziliano mobile belt. At the RCC-POC boundary, wedges of seaward-dipping reflectors abut against RCC and overlie non-reflective lower crust and reflection Moho. They correlate with pronounced negative magnetic anomalies (−200 to −600 nT), probably caused by the juxtaposition of weakly magnetised RCC and more magnetic, mafic volcanic rocks of the POC. Steeply-dipping faults are interpreted to occur at transform-faulted segments, where they separate blocks of RCC from POC, or in some places OC. These NE trending transfer fault zones propagate into oceanic fracture zones (FZs) that compartmentalise and isolate both continental and oceanic rift-units. For example, the Ascension FZ is shown to intersect the African continent south of Bata, Equatorial Guinea, where it forms a 200 km long transform-faulted boundary between RCC and POC. The thickness of POC from the top of the seaward-dipping reflectors to reflection Moho ranges between 2–5 s two-way travel time (twtt). This POC probably formed by volcanic outpouring of partial melt at the time of continental rupture, and then during initial seafloor spreading. Phases of magmatic and amagmatic extension formed within rift-unit spreading cells, owing to variable widths of POC (20–200 km wide) trending parallel to flow lines before evolving seaward into normal OC. Aptial salt extends over RCC and POC, and in some places, salt has been mobilised farther seaward (<10 km) as horizontal sills emanating from salt diapirs. The northernmost known occurrence of this Aptial salt lies 100 km south of Douala, Cameroon. There is no evidence in the MCS data that salt was deposited behind structural highs rimming the margin of the salt basin. Instead, salt overlies sub-horizontal reflectors (0.2–1 s twtt thick) believed to represent sedimentary rocks that drape POC and OC of presumably Aptial age. This interpretation implies that salt in the part of West Africa was deposited in nearshore to deeper basin margin environments without restriction from central basin waters.

Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin

Wide‐angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (&lt;300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at 15–18 km depth is marked by a strong wide‐angle reflector, beneath which lies an 8‐ to 10‐km‐thick lower crust with an average velocity of 6.75 ± 0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide‐angle reflections define two layers within the lower crust, with velocities of 6.4–6.6 km/s and 6.9–7.3 km/s. Prominent subhorizontal reflectivity observed at near‐vertical incidence resides principally in the lowermost layer, the top of which corresponds to the “6‐s reflector” of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6‐s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide‐angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40–50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower‐crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15‐km‐thick layer of velocity 6.0–6.2 km/s overlying a 5‐km‐thick, high‐velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc‐derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike‐slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma.

Results of seismic reflection profiling in the Turanian Platform

Experimental studies of the Turanian Platform, which were carried out for more than 15 years by seismic reflection profiling, outline the main characteristics of its crust. The basins of the Turanian Platform are characterized by substantial changes of the crustal reflection character and a complex structure of the Moho boundary, which is marked primarily by subhorizontal reflectors. Reflectors are clearly concentrated in the upper crust of the Amu-Darya basin, in contrast to the Ustyurt and the Aral Sea basins where the major reflectivity is concentrated in the lower crust. The predominance of subhorizontal reflections is the most general feature of the crust beneath the basins. This picture differs greatly from that of the Central Kazakhstan Massif, where the major part of the crust is highly reflective with abundant inclined reflections and interspersed local ‘transparent’ zones which may indicate the existence of intrusive bodies. These differences are the result of important tectonic and geological processes. In contrast to the Central Kazakhstan Massif which was formed as the result of Palaeozoic overthrusting of thin-skinned nappes, the crust of the basins consists of monolithic massifs of ancient cratonic fragments (micro-continents) separated by narrow suture zones.

Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL Line 1

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

3-D ground‐penetrating radar applied to fracture imaging in gneiss

Three‐dimensional, ground‐penetrating radar (georadar) techniques suitable for geological engineering applications have been developed and tested. Initial experiments were conducted on the floor of a quarry in southern Switzerland from which ornamental gneissic rock is extracted. During a brief two‐day period, constant‐offset georadar data were recorded over a [Formula: see text] area with a grid cell size of 0.1 m × 0.2 m. Georadar velocities were estimated from the results of expanding spread surveys. All georadar data and associated geometry files were recorded automatically in seismic industry formats. The experimental georadar data set was processed, image‐enhanced, and interpreted using [Formula: see text] seismic reflection software operating on a workstation. Arbitrary vertical sections, time slices, [Formula: see text] images, and animated movies in which the observer “travels” through the entire data volume were constructed from the resultant migrated georadar data. Semi‐automatic tracking routines allowed continuous subhorizontal reflections to a maximum depth of 30 m to be mapped through the rock mass. These reflections, which are characterized by negative polarity onsets, are probably caused by a system of ubiquitous water‐filled fractures, 2–4 cm thick. Volumes of rock bounded by the subhorizontal fractures were estimated from isopach maps and rock quality was assessed on the basis of root‐mean‐square (rms) amplitudes of reflections. An extension of a steep‐dipping fault exposed on a nearby quarry wall was best delineated on maps representing the horizontal gradients of reflection times. To synthesize in a single figure the principal geological results of the study, picked reflection times were presented in the form of shaded relief surfaces, in which remarkably vivid structural details of the subhorizontal fractures and intersecting near‐vertical fault could be discerned. It is concluded that 3-D georadar methods have the potential to resolve a wide range of engineering and environmental problems.

Shallow seismic reflection profiling over a Mylonitic Shear Zone, Ruby Mountains‐East Humboldt Range Metamorphic Core Complex, NE Nevada

Seismic reflection profiling carried out with a sledgehammer source has imaged Tertiary extensional structures over a depth range of 45–500 m within lower plate rocks of the Ruby Mountains‐East Humboldt Range metamorphic core complex. The 400‐m CMP profile straddles an exposed contact between tectonic slices of dolomitic marble and metaquartzite emplaced by low‐angle ductile‐brittle normal faulting. Subhorizontal reflections from layering within the tectonic slices give way at 160 ms (160–220 m depth) to reflections that dip 15–45° to the east, in contrast with dips indicated in a poorly imaged segment of a coincident regional seismic line but in agreement with dips of foliation mapped for nearby up‐plunge exposures of a late Proterozoic ‐ early Cambrian sequence of metaquartzites, marbles, schists, and granitic rocks that forms the bulk of the underlying shear zone. Differences with the regional profile are attributed to the higher frequencies (30–100 Hz) generated by the smaller hammer source and the enhanced lateral resolution provided by the straighter profile and much smaller shot‐receiver offsets (46–157 m) contributing to the stack for each CMP. The results suggest that the near‐surface, east‐dipping component of the anastomozing shear zone extends at least 2 km farther east than previously interpreted. Rough estimates of interval velocities (1500–4500 m/s) inferred from stacking velocities are consistent with velocities of mylonitic rocks measured perpendicular to foliation at low confining pressures when the effects of macroscopic fractures and joints are taken into account. Peaks in amplitude spectra of stacked traces suggest long‐wavelength components of layering resolved at scales from 5–8 m (depth: 50 m) to 15–25 m (depth: 500 m).

Seismic reflection profiles across the "Mine Series" in the Noranda camp of the Abitibi belt, eastern Canada

The Abitibi–Grenville Lithoprobe project completed a regional (line 21) and a high-resolution (line 21-1) seismic survey in the Noranda Central Volcanic Complex of the Blake River Group, Abitibi, Quebec. Line 21 provides a regional framework in which the Archean crust is divided into three layers, two of which are discussed here: the uppermost layer, which corresponds to the Blake River Group, is the least reflective, and lies above 4 s (12 km), and the mid-crustal layer, which is composed of a complex pattern of generally east-northeast-dipping reflectors and lies between 4 and 8 s. Within the regional data, the Mine Series of the Central Volcanic Complex is imaged as a semitransparent series of reflectors overlying a highly reflective east-facing structure interpreted as the subvolcanic Flavrian pluton. The high-resolution data (line 21-1) were collected in the vicinity of the Ansil mine. The seismic images in this region can be controlled by surface geology and extensive drill-hole data, and the project was designed to test the applicability of seismic reflection profiling in providing structural and stratigraphic information for use in mineral exploration: shallow-dipping reflectors correlate well with lithological variations or contacts in the volcanic sequence; strong subhorizontal reflectors correspond to diorite and gabbro dykes and sills; several abrupt lateral changes in the reflectivity coincide with known intrusive contacts such as the Lac Dufault pluton.

A seismic-reflection-based regional cross section of the southern Abitibi greenstone belt

Seismic reflection profiles from the southern Abitibi greenstone belt reveal four first-order subdivisions: (1) Between 0 and ~4.5 s, the upper crust is weakly reflective, with prominent local to laterally extensive reflections. (2) Between ~4 and ~9 s, the crust is strongly and heterogeneously reflective with laterally continuous reflections. (3) From ~9 to ~13 s, the crust is more homogeneously reflective and displays downward decreasing reflectivity. (4) Below ~13 s (Moho?) the upper mantle is weakly reflective. The upper layer may correspond to subgreenschist–greenschist-facies supracrustal rocks cut by low-angle shear zones and intruded by regional tabular batholiths; the middle layer, to ductiley deformed amphibolite-facies gneisses, granitoids, and (or) metasediments; and the lower layer, to more homogeneously deformed granulite-facies rocks. North-dipping, low-angle reflections extending beneath both diverse supracrustal assemblages and regional batholiths may represent structural detachments upon which both the supracrustal assemblages and batholiths were imbricated and translated southward. However, the preservation of regional low-pressure metamorphic rocks and the common para-autochthonous relationships between assemblages suggest that thrust-related vertical separations and the magnitude of crustal thickening were not large. Steeply dipping regional shear zones within the greenstone belt appear to disrupt subhorizontal reflections down to ~15 km and may represent late-tectonic strains, which were progressively concentrated into linear zones during continued north–south shortening. The crustal-scale structure determined from the seismic reflection profiles, combined with surface geology, is compatible with post-2.70 Ga north–south shortening accommodated by south-directed(?) thrusting in a thermally softened mid crust and by upright folding in the upper crust. This scenario is comparable to recently proposed models for the Paleozoic, high-temperature, low-pressure Lachlan fold belt of Australia.

Emergent, long-lived Gondwanaland vs. submergent, short-lived Laurasia: Supercontinental and Pan-African heat imparts long-term buoyancy by mafic underplating

Gondwanaland was buoyant, as indicated by nonmarine facies, and Laurasia was depressed, as indicated by marine facies. As a supercontinent, Gondwanaland lasted much longer than Laurasia and was hotter from internal heat. Moreover, the Pan-African orogenic cycle, confined to Gondwanaland, augmented the heat supply, which possibly generated a permanently buoyant lower crust by underplating. A crustal layer in the Australian Proterozoic shield with subhorizontal reflectors and velocity ( Vp ) >7.5 km/s is interpreted as mafic underplating beneath latest Neoproterozoic flood basalt. The Pan-African terrane in East Africa also contains evidence of mafic underplating, and most of Gondwanaland (but not Laurasia) was affected by terminal Pan-African (0.5 Ga) uplift and cooling.

Upper-mantle reflectivity beneath the Williston basin, phase-change Moho, and the origin of intracratonic basins

COCORP (Consortium for Continental Reflection Profiling) deep-reflection profiles crossing the northern Williston basin show (1) dipping reflections within the crust of the underlying Trans-Hudson orogen that extend downward into the upper mantle and (2) no clearly defined reflection Moho. In contrast, Lithoprobe (Canada9s national geoscience project) profiles crossing the Trans-Hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defined subhorizontal reflection Moho—a reflection character typical of the interior of collapsed Phanerozoic orogens. We suggest that a crustal root produced during Hudsonian collision was incompletely removed by “normal” postorogenic extension in the region now underlain by the Williston basin. This remnant crustal keel much later underwent eclogite-facies metamorphism, which overprinted a new nonreflective Moho across the root and induced the subsidence of the Williston basin.

Velocity structure of a gas hydrate reflector at Ocean Drilling Program site 889 from a global seismic waveform inversion

Strong subhorizontal reflectors in the upper few hundred meters of oceanic sediments have been imaged by seismic reflection profiling adjacent to many continental margins. These reflectors are known as “bottom simulating reflectors” (BSRs) as they run roughly parallel to the seabed. BSRs generally have been interpreted as representing the base of a methane gas hydrate stability field. A series of boreholes was drilled in October–November 1992 during leg 146 of the Ocean Drilling Program to examine fluid discharge and its relationship to the BSR, in the Cascadia accretionary wedge. Independently, we performed a global waveform inversion of seismic reflection data collected near site 889. We use a multistep strategy for global waveform inversion of seismic reflection data. The strategy is based on the Bayesian inference theory, in which the a posteriori probability distribution is determined by the a priori probability distribution and the likelihood function relating the data with the model. Three different likelihood functions have been defined relating different parts of the data with different wavelengths of model parameter variation. First, global nonlinear search algorithms are used to estimate the large‐scale features of one‐dimensional velocity variations; then a nonlinear iterative search is used to retrieve the velocity structure in fine detail. At each stage, bounds on the model parameters are estimated. The strategy has been implemented in the slowness and intercept‐time domain. We find that the BSR corresponds to a 15–20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s, suggesting the presence of free methane gas in the low‐velocity zone, and a 15–20 m transition zone where the velocity gradually increases to a velocity of the surrounding sediments. Thus the total thickness of the low‐velocity zone is about 30–40 m. A small or negligible velocity increase just above the BSR, suggests that the hydrate fills less than 10% of the pore space in the sediments. Our result was confirmed by the drilling. The velocity derived using a vertical seismic profile and sonic log from the drill hole agree closely with our result. Waveform inversion can recover detailed and well‐constrained velocity variations from a high‐amplitude target such as a BSR; combined with the detailed interpretation of these velocity variations which the results from site 889 will provide, the inversion approach is a powerful tool for analysis of BSRs elsewhere.

Seismic reflection profiles across the central Kapuskasing uplift

The central Superior Province is transected by the intracratonic Kapuskasing uplift, which contains rocks exhumed from 30 to 35 km paleodepth. As part of the Lithoprobe Kapuskasing transect, approximately 52 km of 16 s seismic reflection data were collected in the central segment of the uplift along three profiles that traverse the northern Groundhog River block, the bounding Saganash Lake fault, and the eastern Val Rita block. The seismic sections have the following characteristics in common: (i) a complexly reflective uppermost portion (&lt; 1 s) limiting correlation of reflective zones and surface features; (ii) numerous subhorizontal, east- and west-dipping reflection zones; and (iii) a significant reduction in reflectivity beyond the refraction-defined Moho (~ 14 s). Beneath the Groundhog River block a series of straight, west-dipping (~ 20°) reflection zones between 2 and 10 s is underlain by subhorizontal reflections in the lower crust. Across the Saganash Lake fault, the Val Rita block is characterized by a maze of discontinuous, curvilinear reflections with general easterly dip down to 8 – 10 s, below which west-dipping events are prominent. A north–south cross profile reveals a highly reflective crust with dominantly horizontal reflection geometry below the Saganash Lake metavolcanic belt, and a steep truncation of reflection zones down to at least 7 s, which correlates with the surface trace of the Nansen Creek fault. This fault resembles well-known strike-slip faults in intraplate settings. The Saganash Lake fault, variably interpreted as a west-side-down normal fault with up to 15 km of throw or a major strike-slip zone, may be visible as a west-dipping, weakly reflective zone steeply truncating east-dipping reflections and becoming listric at depth. This interpretation accords with surface geological observations and gravity models for the structural geometry of the region in which the Groundhog River block is a thin thrust sheet of granulite perched on Abitibi belt rocks and truncated on the west by the crustal-scale Saganash Lake fault. Alternatively, the fault could be a seismically unresolved major transcurrent structure juxtaposing blocks with disparate reflection patterns in the upper 8 s. Limited amounts of late strike-slip motion have been inferred from various geophysical studies.

Deep seismic reflections near the SG4 borehole, central Urals

Abstract A 7.5 km long seismic reflection profile across the Urals SG4 deep borehole in the Tagil Synform, processed to 17.5 seconds two way traveltime (TWT), provides evidence of a highly reflecting crust at several levels, particuarly in the upper 1–4 s and at 8–10 s. In general, the reflectors dip at low to moderate angles eastwards. Below 10s, reflections are less well defined, but nevertheless exist down to the end of the processed section. This lower part (c. 20 km thick) corresponds to the deep root of the anomalous lower crust reported by previous authors on the basis of refraction velocities (vp c. 7.2–7.8 km/s) and gravity data. The moderately east dipping reflections in the upper crust can readily be correlated with lithologies of the Tagil Synform consisting of volcanic and sedimentary rocks of high contrast in acoustic impedance. The sub-horizontal reflectors at 8–10s are superficially similar to those recorded in Variscan western Europe and interpreted there to relate to the Moho. Thus, the results of this short profile indicate that future CDP profiling across the Uralide Orogen will contribute significantly to an understanding of the structure and tectonics of the orogen.