Deep Seismic Reflection Line Research Articles

Testing the limits of virtual deep seismic sounding via new crustal thickness estimates of the Australian continent

We apply virtual deep seismic sounding (VDSS) to data collected from both permanent and temporary seismic stations in Australia with the goal of examining (i) the resilience of the method to the presence of complex lithospheric structure, and (ii) the effectiveness of different approaches for estimating bulk crustal properties (namely thickness and Vp). Data from the permanent WRAB in the Northern Territory station is ideal for benchmarking VDSS (large number and favourable distribution of recorded earthquakes), with the results from several approaches agreeing on a thickness of 40-42 km. Application of VDSS to data from the temporary BILBY array, a linear distribution of broadband stations that traverses central Australia, shows that strong Moho reflections can be retrieved with as few as two earthquakes even at the transition between crustal blocks of different character and in the presence of thick sedimentary basins. Crustal thickness varies between 36-54 km and compare well with the reflectivity character of nearby deep seismic reflection lines. Furthermore, we find that off-line estimates of crustal thickness, calculated by binning the source regions according to back-azimuth, produce estimates of crustal thickness that are consistent with the regional geology. Overall, we find that VDSS is a powerful technique for estimating crustal thickness and velocity due to its insensitivity to complex short-wavelength structure and requirement of a small number earthquakes to produce a stable result. However, not all schemes tested for extracting bulk crustal properties appear to be robust and stringent data quality checking is still required during implementation.

The lithospheric transition between the Delamerian and Lachlan orogens in western Victoria: new insights from 3D magnetotelluric imaging

ABSTRACTThe magnetotelluric (MT) method was used to image the crust and upper mantle beneath the Delamerian and Lachlan orogens in western Victoria, Australia. During the Cambrian time period, this region changed from being the extended passive margin of Proterozoic Australia into an Andean-style convergent margin that progressively began to accrete younger oceanic terranes. Several broadband MT transects, which were collected in stages along coincident deep (full crust imaging) seismic reflection lines, have now been combined to create a continuous 500 km east–west transect over the Delamerian–Lachlan transition region in the Stawell Zone. We present the electrical resistivity structure of the lithosphere using both 3D and 2D inversion methods. Additionally, 1D inversions of long-period AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) MT data on a 55 km regionally spaced grid were used to provide starting constraints for the 3D inversion of the 2D profile. The Delamerian to Lachlan Orogen transition region coincides with the Mortlake Discontinuity, which marks an isotopic discontinuity in Cenozoic basalts, with higher strontium isotope enrichment ratios in the Lachlan Orogen relative to the Delamerian Orogen. Phase tensor ellipses of the MT data reveal a distinct change in electrical resistivity structure near the location of the Mortlake Discontinuity, and results of 3D and 2D inversions along the MT profile image a more conductive lower crust and upper mantle beneath the Lachlan Orogen than the Delamerian Orogen. Increased conductivity is commonly ascribed to mantle enrichment and thus supports the notion that the isotope enrichment of the Cenozoic basalts at least partially reflects an enriched mantle source rather than crustal contamination. Fault slivers of the lower crust from the more conductive Lachlan region expose Cambrian boninites and island arc andesites indicative of subduction, a process that can enrich the mantle isotopically, and also electrically, by introducing carbon (graphite) and water (hydrogen).

Constraining the VanDieland microcontinent at the edge of East Gondwana, Australia

Using airborne magnetic and marine gravity data, the geological subdivisions of western Tasmania have been interpreted north across Bass Strait into Victoria. The three westernmost Tasmanian zones, the King Island, Rocky Cape and Burnie zones, are inferred to form the largely concealed Selwyn Block in Victoria. The Eastern Tasmania Zone correlates with the Victorian Tabberabbera Zone. Thus the Tasmanian Tamar Fracture Zone corresponds with the Victorian Governor Fault. The Victorian Ceres Gabbro is correlated with magnetic rocks west of King Island that are tentatively considered to be Neoproterozoic. Most of the Cambrian felsic volcanic rocks of the Tasmanian Mount Read Volcanics lie above the Burnie Zone, as do the similar rocks exposed in the Jamieson, Licola and Glen Creek windows in central Victoria. Reinterpretation of a Victorian deep seismic reflection line indicates Burnie Zone equivalent rocks were thrust south-west over Rocky Cape Zone equivalents.A link between western Tasmania and central Victoria is evident from Upper Devonian granites intruded into the Selwyn Block region. The eastern end of the Upper Devonian Cobaw Complex and the Warburton Granodiorite contain calcsilicate enclaves interpreted to be derived from a northern equivalent to the Smithton Basin. The Mount Disappointment Granodiorite has high Ni and Cr contents and pseudomorphs after orthopyroxenes, consistent with having been partly sourced from the underlying basaltic rocks like those on the eastern margin of the King Island Zone. The magnetic responses under the Strathbogie Complex, the Cerberean Caldera and the Lysterfield Granodiorite are attributed to metamorphism of part of an extension of the Smithton Basin, probably equivalents to the 580Ma Spinks Creek Volcanics. Quartzite cobbles in a Devonian conglomerate in the south-eastern Melbourne Zone may be derived from Rocky Cape Group equivalents. When integrated with the geological interpretation of Tasmania, we provide a stratotectonic map of the VanDieland micro-continent.

Lithospheric architecture and deformation of NE Tibet: New insights on the interplay of regional tectonic processes

GPS measurements indicate rapid lateral extrusion of the NE Tibetan Plateau, which causes active NE-directed crustal shortening and has initiated oblique shearing along the margins of NE Tibet. However, the Tibetan highlands terminate around 103°E longitude and topographic relief disappears to the northeast. The exact reasons for this drop in elevation remain obscure due to widespread Tertiary sediments and Quaternary loess, which obscure details of the lithospheric structure. This study describes a new 310 km-long deep seismic reflection line striking NE-SW across the interior of NE Tibet. Integrating its data with a previously described 165 km-long deep seismic profile of the Tibet–Ordos transition zone together, these datasets provide a complete picture of the crustal architecture of the north-easternmost Tibetan Plateau. Gravity anomaly and previous geological evidence also help constrain complex deformation pattern in the region. Interpretations of these patterns indicate the importance of the large-scale sinistral Haiyuan fault zone and inherited vertical variation in mechanical properties of the lithosphere in the overall tectonic evolution of the NE Tibetan Plateau. The overall crustal architecture obtained in this study provides spatial context for the neotectonic evolution of NE Tibet and helps constrain the interplay of geologic and geodynamic processes affecting NE Tibet and adjacent regions.

Forward modelling along the Southern Carnarvon deep seismic reflection line - Using gravity data to investigate seismic interpretations

Forward modelling was undertaken to test the architecture of the Southern Carnarvon deep seismic reflection survey (11GA-SC1) interpretation against gravity data. The seismic data extends to ~60 km depth and images the crust-mantle boundary (MohoroviA?iA? discontinuity) allowing it to be incorporated in the forward modelling. Using average density values for the upper mantle, granulite to amphibolite facies mid to lower crustal rocks, and upper crustal felsic and sedimentary rocks, a model was generated which explains the observed gravity anomalies and is consistent with the seismic interpretation. This work highlighted areas where the seismically inferred models were inconsistent with the gravity data and the importance of understanding regional trends, such as those generated by the crust-mantle boundary. The modelling undertaken in this study reflects the most up to date understanding of the regional geology in this area.

Key features of new deep seismic reflection lines across frontier sedimentary basins in central Australia: the Arckaringa, Officer, Amadeus and Georgina Basins

The Onshore Energy Security Program, funded by the Australian Government and conducted by Geoscience Australia, in conjunction with State and Territory geological surveys, has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. Here, we present the key aspects of the stratigraphy and structural architecture identified using seismic data from the Arckaringa, Officer, Amadeus and Georgina basins on seismic lines 09GA-GA1 and 08GAOM1.

Crustal structures in the geothermal areas of southern Tuscany (Italy): Insights from the CROP 18 deep seismic reflection lines

Two deep seismic reflection lines, named CROP 18A and 18B and roughly NNW–SSE-oriented, were acquired to investigate the crust between the Larderello and the Mt. Amiata geothermal areas, both located in southern Tuscany. Since the Early–Middle Miocene, southern Tuscany has been affected by extensional tectonics, and since the Pliocene, by widespread magmatism. Presently, extension is demonstrated by NE-dipping normal faults and brittle shear zones which affect the crust down to the brittle/ductile transition. In the Larderello area, these shear zones are seismically evidenced by NE-dipping weak reflections and by the loss of the K-horizon reflectivity, a regional mid-crustal high amplitude reflector, located at the top of the brittle/ductile transition. The CROP 18A and 18B lines were reprocessed down to 10 s TWT, down to the crust/mantle boundary. Their reflectivity results are closely related to the direction of the survey: a clear decrease in reflectivity occurs when the survey plane intersects the crustal shear zones along strike. Nevertheless, both lines display segments with low reflectivity and homogeneous acoustic impedance which indicate intrusive magmatic bodies at different crustal levels. A wide, partially crystallised granitoid may be sited at about 7–8 km depth underneath the Larderello geothermal field. In the lower part of the crust, the CROP 18A and 18B transects show group of bright reflections, mainly related to the occurrence of pressurised fluids and/or lithological differences. The base of the crust is located at about 20 km depth.

Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80–100 km long transcrustal ramps, the southward dipping ‘Sub-Tauern-Ramp’ and the northward-dipping ‘Sub-Dolomites-Ramp’. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The ‘crocodile’-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic–African continents.

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s −1) P-wave velocities, while the eastern has higher (6.3–6.9 km s −1) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s −1 is significantly lower than the high (7 km s −1) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.

Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting

A wide‐angle seismic profile was acquired across the northeast Newfoundland margin, seaward to Orphan Knoll. The profile is complemented by existing multichannel reflection data, two online deep wells, and other deep seismic reflection and refraction lines. Modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the ∼400‐km‐wide stretched continental crust on this margin. Prerift metasedimentary rocks have a topographic relief of up to 5 km, with a velocity of ∼5.2 km/s increasing to 5.9 km/s. Unaltered crystalline crust below the metasedimentary section has a velocity range of 6.1–6.5 km/s. Most of the lower crust landward of Orphan Knoll has a typical continental lower crustal velocity (6.8–7.0 km/s), and the lower crust shallows significantly beneath Orphan Knoll. Our model shows no evidence for the 5‐km‐thick, 7.35 km/s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating, implying nonvolcanic rifting for the NE Newfoundland margin. We suggest that continental stretching persisted from ∼180 Ma to Chron 34N time (∼101 Ma) when the final breakup between Canada and western Europe took place, leaving a 400‐km‐wide zone of thinned continental crust underneath the shelf and deep water area of the Orphan Basin. On the landward side of the basin, gravity modeling indicates a zone of very thin (6–8 km) crust, possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between Canada and Europe. A possible sub‐Moho reflector suggests generally uniform stretching of the crust and upper mantle, and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the Grand Banks.

Flexural expression of European continental lithosphere under the western outer Carpathians

We present the results of an analysis of the flexural down bending of the European lithosphere under the western outer Carpathians for a grid of five profiles with a lateral spacing of about 100 km crossing the Carpathians and its foreland. Seismic sections, including two deep seismic reflection lines, and numerous wells allowed us to quantify the flexural curvature of the lithosphere. In addition, gravity data are used to obtain independent constraints. Two evolutionary scenarios involving subduction/underthrusting of passive margin and a phase change in the lower crust are investigated in a context of relatively flat Moho under the area. Furthermore, recent uplift, expressing itself in erosional features observed along the distal margin of the foredeep, is quantified. The generally modest Bouguer gravity anomaly points to nonisostatic processes causing this uplift. Steeply dipping basement under the Carpathians to about 10 km depth at the location of the Pieniny Klippen Belt is explained by relatively weak lithosphere, which is expressed in the predicted low values of effective elastic thickness of 16 km decreasing locally to 3 km. The low strength of the European lithosphere in this region is probably caused by mechanical weakening controlled by partial yielding due to high flexural curvature. The weakest parts of the lithosphere along the profiles, corresponding to low effective elastic thicknesses, are concentrated in the areas of maximum bending. Predicted flexural bending stresses as a function of bending moment and effective elastic thickness increase dramatically under the outer zone of the Carpathian belt, explaining reactivations of older basement structures in these regions.

Pre-Zechstein structures around the MONA LISA deep seismic lines in the southern Horn Graben area

Seismic reflection data from the Horn Graben area in the southeastern part of the North Sea, off-shore Denmark, have been interpreted to illustrate the upper crustal structures around the MONA LISA deep seismic lines. The study area comprises the southern Horn Graben area and the eastern part of East North Sea High, where the Caledonian collision suture between Baltica and Eastern Avalonia bends such that the strike direction changes from ESE in the south to NNW in the north. Integrated interpretation of normal-incidence reflection data and wide-angle refraction data reveals substantial occurrences of lower and upper Palaeozoic strata in the area, thickest below the Horn Graben. This may indicate that Horn Graben developed as a graben structure during late Palaeozoic in the former Caledonian foredeep. On the northern and eastern parts of the MONA LISA deep seismic reflection lines 1 and 3, the main E- dipping boundary fault of the southern Horn Graben segment appears to be listric at depth with a sub-horizon-tal detachment at the top of the reflective lower crust. We have mapped the lateral extent of the lower Permian, volcanic Rotliegend reflector in the study area on the basis of seismic lines from the RTD-81 survey. Dipping reflections observed in the sedimentary strata below the Rotliegend reflector are interpreted as Cal-edonian structures generated by folding and deformation in Lower Palaeozoic Baltica shelf sediments in the Caledonian foreland basin. A sequence of S- and W-dipping reflections above 4 s twt are interpreted as preserved Caledonian thrusts in the upper crustal frontal part of the SW-dipping Caledonian Deformation Front.

A deep seimic reflection line in the Danish Central Graben

Results are presented from a deep seismic reflection line, recorded to 16 seconds trave1 time, across the Coffee Soil Fault in the southern part of the Danish Central Graben. Data processing has been focused on the deeper parts of the sedimentary sequences, the crystalline crust and the crust-mantle boundary. Depth migration is performed down to 15 km depth, and time migration is performed on the entire section. The Coffee Soil Fault is clearly imaged as a normal fault dipping about 45" from about 2.5 to 9 km depth. Interpretation of the sedimentary sequences is constrained by well data and shows an almost uniform 2 km thick unit of post-chalk deposits. The thickness of the Mezosoic sequences is to a large extent controlled by faulting and highly influenced by movements of Zechstein salt. They exceed 4 km of thickness in the halfgraben immediately west of the Coffee Soil Fault. Large block-faulted pre-Zechstein units of more than 2 km thickness are seen indicating a total depth of at least 8 km to the crystalline basement. The crystalline crust is generally non-reflective. Around 11 seconds two-way trave1 time (about 28 km depth) reflectivity interpreted as the crust-mantle boundary (Moho) is observed. Comparison with other deep seismic profiles across the Central Graben is taken to indicate a local crustal thinning by a factor of 1.5 associated with graben formation by extension.

Erratum

In the article by Klinkby et al. ‘A deep seismic reflection line in the Danish Central Graben’ (Bulletin of the Geological Society of Denmark, Vol. 44(2), pp. 151–159, 1998), Tables 1 and 2 were unfortunately omitted. These are printed below.

Restricted rifting and its coexistence with compressional structures: results from the CROP 3 traverse (Northern Apennines, Italy)

A geological interpretation of the deep seismic reflection line CROP 3 (Italian program CROsta Profonda: deep crust, profile no. 3), which crosscuts the Northern Apennines from the Tyhrrenian to the Adriatic coast, is presented. The profile images the lithosphere up to 15 s TWT and highlights several peculiarities: (a) the lower crust is bedded by discontinuous sub parallel reflective markers which terminate at ≈ 7 s beneath the western side of the profile; (b) in the same area a notable reflection is recognizable at 10 s and is interpreted as the top of the asthenosphere; (c) east‐plunging shear zones are recognizable throughout the crust. By contrast, the Adriatic (outer) side of this line shows: (a) reflections deepening westward, from 13 to 15 s TWT, related to the base of the crust (33–38 km depth); (b) the absence of thick bedding of reflective markers within the lower crust; and (c) tectonic structures affecting the basement which are different from those which deform the cover of the Northern Apennines. Geological interpretation is based on the eastward migration of ductile shear zones which have been recognized on the seismic line. The bending of the outer zone crust is considered to be a consequence of the rifting process with the application of pushing forces against the Adriatic lithosphere which cannot escape toward east.

The OCEAN study area: Tectonic history from magnetic anomaly data and seismic reflectivity

The OCEAN experiment is a detailed geophysical study of a region of the Cape Verde basin. A dense network of new magnetic and gravity profiles has enabled us to constrain the spreading rate history of the region and the location of fracture zones. The main features on the gravity profiles are lineated perpendicular to the seafloor spreading magnetic anomaly lineations. Significant along‐axis variability in spreading history suggests that the Mid‐Atlantic Ridge behaved as a series of loosely coupled segments within which spreading was fundamentally asymmetric. Such variability is associated with a minor jump in the ridge axis which changes the offset and expression of one of the fracture zones. Deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations; seismic reflections occur at all levels within the crust, decreasing in amplitude and coherence below the level of the Moho. Analysis of the subbasement reflectivity provides compelling evidence that at least two major sets of dipping structure are present and are imaged separately on the two perpendicular sets of seismic profiles. Dipping reflections on flow line (“dip”) profiles, which are interpreted as faults due to their association with offsets in the basement surface, appear to strike parallel to the paleoridge axis. The majority of reflections that may be identified as faults dip toward the west, and although basement topography suggests that east dipping faults are also present, no reflections may be interpreted unambiguously as such. East dipping reflections observed only in the middle to lower crust have a more obscure origin. Dipping reflections seen on isochron (“strike”) profiles show clear contrasts in strength, lateral coherence, depth, and dip population; a number of these strike parallel to flow lines. Comparing reflection and refraction data shows that both the layer 2/layer 3 boundary and the Moho are marked by a change in the character of reflections and suggests that they may represent important structural, as well as seismological, boundaries within the oceanic lithosphere.

Mesozoic and Cenozoic stratigraphy, eastern Scotian Shelf: new interpretations

A 320 km long, deep seismic reflection line across the central part of the Scotian Basin provides new insight on the deep stratigraphy of the basin, confirms the existence of up to 18 km of Mesozoic and Cenozoic sedimentary strata, and provides additional control on the relationship of proximal and distal stratigraphic units within the basin. Detailed biostratigraphic and seismic correlation from three outer shelf–upper slope wells yields important new information on Late Cretaceous and Tertiary unconformities and identifies a number of third-order sequence boundaries. The dating of prominent unconformities and correlation with the oceanic seismic markers ACand AUresults in the reidentification of these horizons in the Sohm Abyssal Plain.

Crustal observations beneath the southern North Sea and their tectonic and geological implications

In 1991, a deep seismic reflection line, MPNI-9101, was acquired in the southern North Sea from the Mesozoic Broad Fourteens Basin, across the West Netherlands Basin onto the London-Brabant Massif (LBM). The resultant section shows a strongly reflective lower crust beneath the area of Mesozoic basin development. This lower crustal reflectivity continues to be strong beneath the LBM. The travel time to the base of the reflective zone increases from approximately 11.0 s beneath the Mesozoic basins to 12.5 s beneath the LBM, suggesting a southward thickening of the crust (Rijkers et al., 1993). Based on these travel times and information from deep wells and refraction surveys. Moho depth is estimated to increase from about 31 km beneath the Mesozoic basins to about 38 km beneath the LBM. This difference in depth to the Moho can partly be explained by coaxial stretching of the crust beneath the Mesozoic basins. In comparison with the Mesozoic basins, the crust beneath the LBM was thickened during the Caledonian and Variscan orogenies.

NIZUSE: a deep seismic reflection line in north-eastern Germany

Out of a dense network of seismic reflection lines for hydrocarbon exploration in the North-east German Basin, several lines were recorded to 12 s TWT to obtain information about the structure of the crust and the crust-mantle transition. One of these profiles is presented here. This stretches for 110 km in a NNE direction between Neustrelitz and the island of Usedom. It reaches from the External Variscides in the south across the North German Massif into the Rugen-Pomorze Terrane in the Baltic Sea. Below Cenozoic-Mesozoic-Paleozoic cover with clear reflections down to base Zechstein, the reflectivity varies considerably with depth and also laterally. The Paleozoic and Precambrian sediments and basement are generally void of reflections, but the lower crust and the Moho show strong reflections. To the north the reflectivity decreases, and the Moho depth increases to beyond the bottom of the record section at 12 s. There are no direct indications for deep-reaching faults such as the Trans-European Fault in the north. The North German Massif acted as a ramp towards the Variscan Orogeny, similar to the London-Brabant Massif further west.

The deep structure of the Grenville Front: a new perspective from western Quebec

The Grenville Front is a major tectonic boundary exposed on the Canadian Shield. The front is defined as the northwestern limit of Grenvillian deformation, on the basis of geochronological and metamorphic data. This boundary is also evident in some geophysical data sets. The Lithoprobe Abitibi–Grenville transect crosses the Grenville Front near Lac Témiscamingue in western Quebec. A new 114 km long deep seismic reflection line shows a crustal structure quite different from that seen on previous surveys across the Grenville Front. The Archean foreland (Pontiac Subprovince) has a pattern of reflectivity similar to that seen in most of the Superior Province. This pattern continues for some 30 km south of the surface exposure of the Grenville Front. There is no evidence for a band of dipping reflectors truncating the horizontal Pontiac reflectors; in fact, the leading edge of the Grenville Province is difficult to identify on the seismic section. The Moho is well defined and reveals that the crust thins under the Grenville Front. The magnetotelluric survey shows that the upper crust is resistive across the entire profile, but the resistivity is higher within a Grenvillian allochthonous terrane at the southern end of the profile. The mid-crustal low-resistivity layer and the upper mantle electrical anisotropy are also continuous across the Grenville Front. The Grenville Front is highly variable in its character along the Grenville Orogen, and this character may be strongly controlled by the nature of the foreland to the northwest.