Results of DEKORP 2-S and other reflection profiles through the Variscides

Summary. The first DEKORP profile, DEKORP 2-S, a 250 km long line perpendicular to the Variscan strike direction, has provided evidence of major crustal shortening during the Variscan orogeny. Sporadic dipping events in a generally transparent upper crust are interpreted as thrust faults, while the highly reflective lower crust fits into the general picture of Palaeozoic provinces. Correlations are established between certain reflectivity patterns and rheology. Moho depths and reflecting lamellae are considered to be post-Variscan.     

Deep seismic reflection studies in Israel - an update

Summary. The results of three deep crustal reflection lines are presently available from Israel. A 90 km line from near the Dead Sea rift to the Mediterranean coast was carried out for deep study. Two other lines in the Mediterranean coastal area were derived by recorrelation of oil exploration lines. The data shows a division between continental inner Israel and the coastal plain. In the first area a reflective lower crust is apparent with transparent upper crust and almost transparent upper mantle. Near the coast, in an area which was previously suggested as underlain by an ancient fossil oceanic crust, strong reflections characterize the uppermost mantle. Comparison between the reflection pattern and previous deep refraction and MT data indicates some agreement away from the coast and lack of correlation in the area of possible fossil oceanic crust near the coast.     

A deep seismic profile of 800 km length recorded in southern Queensland, Australia

Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.     

Pre-drilling reflection survey of the Black Forest, SW Germany

Summary. The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection meaasurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25–27 km above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is' also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.     

Crustal thickening under the palaeo-meso-Proterozoic Delhi Fold Belt in northwestern India: evidence from deep reflection profiling

The results of deep reflection profiling studies carried out across the palaeo-meso-Proterozoic Delhi Fold Belt (DFB) and the Archaean Bhilwara Gneissic Complex (BGC) in the northwest Indian platform are discussed in this paper. This region is a zone of Proterozoic collision. The collision appears to be responsible for listric faults in the upper crust, which represent the boundaries of the Delhi exposures. In these blocks the lower crust appears to lie NW of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks. A fairly reflective lower crust northwest of the DFB exposures appears to be the downward continuation of the DFB upper crust. The poorly reflective lower crust under the exposed DFB may be the westward extension of the BGC upper crust at depth. Thus, the lower crust in this region can be divided into the fairly reflective Marwar Basin (MB)-DFB crust and a poorly reflective BGC crust. Vertically oriented igneous intrusions may have disturbed the lamellar lower-crustal structure of the BGC, resulting in a dome-shaped poorly reflective lower crust whose base, not traceable in the reflection data, may have a maximum depth of about 50 km, as indicated by the gravity modelling.
The DFB appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. This has resulted in three Moho reflection bands, two of which are dipping SE from 12.5 to 15.0 s two-way time (TWT) and from 14.5 to 16.0 s TWT. Another band of subhorizontal Moho reflections, at ≈ 12.5 s TWT, may have developed during the crustal perturbations related to a post-Delhi tectonic orogeny. The signatures of the Proterozoic collision, in the form of strong SE-dipping reflections in the lower crust and Moho, have been preserved in the DFB, indicating that the crust here has not undergone any significant ductile deformation since at least after the Delhi rifting event.     

Results from regional vibroseis profiling: Appalachian ultra-deep core hole site study

Summary. In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians.
The major points in the interpretation are: 1) The Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) The Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures ("duplex tuning wedges") connected by other thrust faults that duplicate shelf strata to a thickness of 4–5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged.
The ADCOH data were originally recorded with 14–56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments.     

Ivrea seismic array: a study of continental crust and upper mantle

A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P - and S -wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation.
Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times.
We have obtained a three-layer crustal seismic model. The P -wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s⁻¹, 6.5±0.3 km s⁻¹ and 7.8±0.3 km s⁻¹ respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a V _P/ V _S ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station.     

Crustal Structure and Origin of Peake and Freen Deeps, NE Atlantic

Summary Peake and Freen Deeps are elongate structures some 30 nautical miles long by 7 miles wide situated near 43° N 20° W on the lower flanks of the Mid-Atlantic Ridge. Seismic reflection records show that underneath about 400 fm of layered sediment the bedrock lies at a depth greater than 3600 fm in Peake Deep and 3300 fm in Freen Deep; the surrounding seafloor is at about 2100 fm. Freen Deep is the eastern end of King's Trough, a flat floored feature some 400 fms deeper than the adjacent seafloor. The Trough extends 220 miles west-north-westwards towards the crest of the Mid-Atlantic Ridge. The area is aseismic and heat flow is normal; there is no displacement of the crest of the mid-ocean ridge on the projected line of King's Trough. Gravity and magnetic surveys have been made. With minor exceptions, magnetic anomalies are not due to bodies elongated parallel with the structure, which, therefore, cannot be a volcanic collapse caldera. Seismic refraction results in the Peake-Freen area show that the crust is not thinned under the deeps although the Moho may be depressed by 2 km. Bouguer anomalies also suggest that the Moho is flat and does not rise to compensate the deeps. Models consistent with gravity and seismic information suggest there is a dense block in the upper mantle under the area. Since no reason to ascribe the origin of the structure to tear faulting has yet been acquired, it is interpreted in terms of over thrusting perpendicular to the deeps, followed by inversion of the lower part of the thickened basaltic crust to eclogite, and its subsequent sinking into the mantle.     

COCORP: new perspectives on the deep crust

Summary. Relict sutures from colliding continents, regions characterized by a "young" Mono, layering and faulting throughout the crust, mid-crustal magma traps, and seismic "bright spots" which suggest deep crustal fluids are among recent COCORP findings. In addition, new studies of signal penetration, noise mitigation, recording geometry, and coherency filtering have yielded better understanding of, and substantial improvements in, data quality. Amplitude anomalies, or "bright spots", in the Basin and Range may be due to magma at mid-crustal levels; in one case, a normal fault appears to link the deep magma with young surface volcanics. Another bright spot. 15 km deep in southeastern Georgia, has a flat geometry that suggests a gas/liquid interface, perhaps within fluids underthrust along an Appalachian suture. The Mono continues to appear relatively undisturbed in many regions of past deformation, suggesting that its formation post-dates these major tectonic episodes. The diversity of reflection patterns from the U.S. Cordillera casts further doubt on the generality of the common model of a reflective, layered lower crust underlying a transparent upper crust.     

Coincident seismic reflection/refraction studies of the continental lithosphere: a global review

Summary. Vertical-incidence reflection profiling has identified several characteristic features of the continental Iithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling.     

Ccte d'Ivoire-Ghana margin: seismic imaging of passive rifted crust adjacent to a transform continental margin

During May 1990 and January-February 1991, an extensive geophysical data set was collected over the Côte d'Ivoire-Ghana continental margin, located along the equatorial coast of West Africa. The Ghana margin is a transform continental margin running subparallel to the Romanche Fracture Zone and its associated marginal ridge—the Côte d'Ivoire-Ghana Ridge. From this data set, an explosive refraction line running ∼ 150 km, ENE-WSW between 3°55'N, 3°21'W and 4°23'N, 2°4'W, has been modelled together with wide-angle airgun profiles, and seismic reflection and gravity data. This study is centred on the Côte d'Ivoire Basin located just to the north of the Côte d'Ivoire-Ghana Ridge, where bathymetric data suggest that a component of normal rifting occurred, rather than the transform motion observed along the majority of the equatorial West African margin.
Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data.     

Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex

Summary. LITHOPROBE has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern Canadian Cordillera, These reflection profiles, obtained during the Fall of 1985, extend from the Rocky Mountain thrust and fold belt, across the Rocky Mountain Trench, Purcell anticlinorium, Kootenay Arc, Nelson batholith and Valhalla gneiss complex. North American basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the Kootenay Arc. The Purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous North American basement. Proterozoic supracrustal rocks, thickened by folding and thrusting, occupy the core of the anticlinorium. Steeply dipping surface structures of the western Purcell anticlinorium and Kootenay Arc appear to be truncated at 3 - 4 s (9-12 km) by a gently east-dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below. Beneath the Kootenay Arc, at a travel time of 9–10 s (27–30 km), the North American basement seems to be truncated by the major east-dipping Slocan Lake fault zone, which can be traced from its surface exposure at the east edge of the Valhalla gneiss complex eastward to near the base of the crust. A high amplitude, west-dipping reflection underlies the Valhalla complex and may be related to a major compressional shear zone.     

Crustal reflection seismics: the contributions of oblique, low frequency and shear wave illuminations

Summary. Continuous vertical seismic reflection profiling, the use of which has been extended by COCORP, BIRPS, ECORS, DEKORP and other programmes (Barazangi & Brown 1986) from stratified sedimentary basins to the entire crust, provides a high resolution cross section of reflectivity. A similar extension of oblique or variable offset sounding would be expected only to provide complementary velocity information. However, combined use of the two methods at crustal scale is new, and we show that refractions and wide angle reflections do contribute original and relevant information, but generally in areas other than velocities - the reason being that interfaces and layers may have a different nature in the crystalline crust from that in sediments.     

Upper crustal shear velocity models from higher mode Rayleigh wave dispersion in Scotland

Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M _L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s⁻¹ in the upper 2 km, with a slow increase to around 3.8 km s⁻¹. P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.     

Deep events in U.K. South Western Approaches

Summary. Apparently planar dipping events are observed in seismic data off south-west Britain within otherwise essentially transparent upper and middle crust. These are believed to represent Variscan thrusts, some of which were re-activated during the post-Carboniferous phase of extension that affected the southern U.K. They can be seen in two extensive commercial seismic surveys recorded to 6 s two-way-time (TWT) and, where laterally persistent, they have been mapped to reveal their essentially planar nature. Commonly these dipping events are associated with deeper, near-horizontal, or gently convex upwards events with which they often appear to converge. Where 'real', these are thought to indicate a complex fault system or possibly the top of the reflective lower crust. The thrusts are seen over the whole area except where granite is known to occur, and commonly exert a major control on the position and subsequent deformation of overlying sedimentary basins.     

The Central Australian seismic experiment, 1985: preliminary results

Summary. The major objective of the Central Australian seismic experiment is to investigate the structural evolution of the Arunta Block and the Ngalia and Amadeus Basins. A regional north-south reflection line of 420 km length from the Northern Arunta Province to the southern part of the Amadeus Basin was recorded in 1985. The most significant basement features are prominent bands of reflectors from beneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s that dip towards the north. Deep crustal features south of the Ngalia Basin are less clear except in the Redbank Zone. Bands of deep reflectors similar to those observed in the north occur at times of between 5 and 10 s beneath the southern part of the Amadeus Basin. Additional seismic profiling included a reflection line of 40 km length recorded across the northern margin of the Redbank Zone, three expanding spread reflection profiles and a tomographic experiment. An east-west seismic refraction profile of 400 km length was recorded within the Arunta Block, and suggests an average crustal thickness of 55 km.     

Wide-angle vibroseis test across the Rhine graben

Summary. As a joint operation of ECORS and DEKORP, a deep wide-angle seismic experiment using vibrators was carried out in the autumn of 1984. The object was to get information on the deep crust under the Rhine graben without crossing through sedimentary layers. Offsets were in the range of 50 to 89 km. In the first phase, two vibration points were executed in the Vosges mountains. A signal was received in the Black Forest from solely the farthest VP. In the second phase, fourteen VPs were executed in the Black Forest. No stacking or correlation was performed in the field in France. The quality of the results is good only if an equalization is applied before vertical stacking and correlation in the computing centre.     

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian–Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.     

Crust and upper mantle structure of the central Iberian Meseta (Spain)

Summary. Quarry blasts recorded along three lines on the central Iberian Meseta are used in an attempt to interpret the crustal structure. The results of the interpretation of the data, together with published surface wave and earthquake data, suggest a layered structure of the crust having the following features: the basement, in some areas covered by up to 4 km of sediments, has a P -velocity of 6.1 km s⁻¹; a low-velocity layer, between 7 and 11 km depth, seems to exist on the basis of both P and S interpretation of seismic data; a thick middle crust of 12 km has a P -velocity of 6.4 km s⁻¹ and overlies a lower crust with a mean P -velocity of 6.9 km s⁻¹ and a possible slight negative gradient; the mean v _p/ v _s ratio for the crust is about 1.75; the Moho is reached at about 31 km depth and consists of a transition zone at least 1.5 km thick. The P -velocity of the upper mantle is close to 8.1 km s⁻¹ and the S -velocity about 4.5 km s⁻¹, which gives a v _p /v _s ratio of 1.8 for the uppermost mantle. A tentative petrological interpretation of the velocities and composition of the layers is given.     

The transition from subduction to continental collision: crustal structure in the North Canterbury region, New Zealand

The North Canterbury region marks the transition from Pacific plate subduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure. The velocity structure is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust, with the middle part of the crust being relatively aseismic. Seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited.
The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43S. To the south-west it is subhorizontal, while to the north-east it dips north-west at about 10. Fault-plane solutions for these earthquakes also change near 43S. For events to the south, P -axes trend approximately normal to the plate boundary (reflecting continental collision), while for events to the north, T -axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithospheric subduction is continuous across the transition, it is not clear whether the lower crust near 43S is flexed or torn.