Deep Reflection Profiles Research Articles

Structural Observations of the Northern North Sea: Insights into Rift Failure Dynamics

Lithospheric extension leads to rift formation and may continue to the point of breakup, with oceanic ridge initiation and the formation of two conjugate rifted margins. In some settings, extension can cease, and the rift may be abandoned. These so-called failed rifts archive snapshots of early phases of deformation, with geometries that may help better constrain the parameters that can prevent a rift from reaching breakup, such as lithospheric rheology, thermal state, rift opening direction and rate, inheritance. This contribution summarizes a study of the Norwegian Continental Shelf which includes the North Sea Rift and the Møre and Vøring rifted margins. We proceeded to the interpretation of a new dataset of deep penetrating seismic reflection profiles and worked at the regional scale, deliberately ignoring local particularities, to focus on the large-scale structural picture. The aim is to list architectural similarities and differences between the failed rift and the successful rifted margins. Our mapping shows that the North Sea structural geometries and basement seismic facies are very similar to the observations listed for the adjacent Møre and Vøring rifted margins. Various types of tectonic structures are observed, from thick anastomosing shear zones possibly evolving into core-complex geometries, to composite large-scale detachment faults and standard high-angle normal faults. These are categorized into five classes and interpreted as exemplifying the rift tectonic evolution through distinct generations of deformation structures that can activate, de-activate and re-activate. Based on these observations, rift failure dynamics are discussed, and it is proposed that the North Sea rift abandonment may not be related to pre-rift local conditions but rather to the ability to initiate specific tectonic structures such distal breakaway complexes.

Shallow shear wave velocity structure of the Dongshan sag area using surface wave data in a deep reflection profile of the Yuanmou area of Yunnan province, China

Deep reflection seismic data contain abundant surface wave information for which the velocity structure in the near-surface shallow underground can be obtained using multichannel analysis of surface waves (MASW). This paper uses the MASW method to process deep reflection data in the Yuanmou area in Yunnan province, China. The MASW method uses the phase velocity spectral transformation of Rayleigh waves to obtain its dispersion curve. The initial model for different lithology classifications is established to invert the underground velocity structure. This study obtains a two-dimensional shear wave velocity structure profile to investigate the shear wave velocity structure. Generally, the study results have shown that the shear wave velocity structure obtained by the MASW method corresponds well with the surface conditions and accurately reflects faults' changes, folds, and lithology. Moreover, the formation sand content calculation shows a large difference in Dongshan sag's formation sand content. The sand contents of Cretaceous and Middle Jurassic strata are over 30% similar, and the sandstone content of underlying Early Jurassic strata is mostly 30% lower. The stratum buried depth of Early Jurassic strata is between 0– 800 m. The dip angle of LZJF at this location is 90°. The west side of the LZJF is based on Proterozoic uplift, and the Quaternary sedimentary thickness exceeds 300 m above the uplift stratum. Whereas on the east side of the LZJF, the stratum of the Dongshan sag section is mainly a synclinal structure.

Geophysical Constraints on the Crustal Architecture of the Transtensional Warm Springs Valley Fault Zone, Northern Walker Lane, Western Nevada, USA

AbstractThe Walker Lane is a zone of distributed transtension where normal faults are overprinted by strike‐slip motion. We use two newly acquired, high‐resolution seismic reflection profiles and a reprocessed Consortium for Continental Reflection Profiling (COCORP) deep crustal reflection profile to assess the subsurface geometry of the Holocene‐active, transtensional Warm Springs Valley fault zone (WSVFZ) near Reno, Nevada, USA. Our multiscale observations extend to 12 km depth and suggest that the WSVFZ is more complex in the subsurface than implied by late Pleistocene surface fault traces. Two 4‐km‐long high‐resolution profiles image to a depth of ∼2 km and reveal moderately dipping reflections and truncations, some of which project to mapped scarps formed in late Pleistocene surfaces. The shallow lines are collocated with COCORP profile NV 08 along ∼40°N latitude. Reanalysis of the COCORP data reveals previously unidentified coherent reflections to a depth of ∼12 km and a previously mapped ∼30° west‐dipping fault at 8–12 km. From these seismic profiles, the WSVFZ is not a simple, subvertical fault zone extending through the entire seismogenic crust. Instead, the reflections are consistent with a zone of steeply and moderately dipping faults that simplify and steepen with depth before intersecting a mid‐crustal, low‐angle (∼25–30°) fault. The complex fault geometry of the WSVFZ implies that crustal shear is accommodated by a mix of dipping and subvertical faults in the transtensional northern Walker Lane. If so, transtensional fault zones may present challenges to paleoseismic and geodetic studies and require careful treatment when included in seismic hazard analyses.

Seismotectonics of the 2013 Lushan Mw 6.7 earthquake: Inversion tectonics in the eastern margin of the Tibetan Plateau

AbstractOn 20 April 2013, an unexpected Mw 6.7 earthquake occurred in Lushan County at the southern Longmen Shan, the eastern margin of the Tibetan Plateau. A high‐resolution seismic reflection profile was combined with near‐surface geological data, earthquake relocation and geodetic measurements, and a recent deep artificial seismic reflection profile to identify the active fault and seismotectonics of this earthquake. Three‐dimensional imaging of the aftershocks was used to identify two planar faults that together form a y shape (f1 and f2). Seismic interpretations suggest that fault f1 did not break through the overlying Mesozoic and Cenozoic rocks and is a typical blind fault. Geodetic measurements suggest that the coseismic deformation is consistent with the geometry and kinematics of shear fault‐bend folding. Deep seismic data indicate the syndepositional nature of fault f1 a preexisting normal fault older than the Triassic, which underwent positive inversion tectonics during the Late Cenozoic. A thrust fault f3 converges with f1 at a depth of approximately 12 km with an accumulated slip 3.6 km. This 2013 Lushan earthquake triggered by blind faults is a hidden earthquake. Blind and reactivated faults increase the potential risk and uncertainty related to earthquakes in the eastern margin of the Tibetan Plateau.

Micro-seismicity in the Gulf of Cadiz: Is there a link between micro-seismicity, high magnitude earthquakes and active faults?

The Gulf of Cadiz seismicity is characterized by persistent low to intermediate magnitude earthquakes, occasionally punctuated by high magnitude events such as the M~8.7 1755 Great Lisbon earthquake and the M=7.9 event of February 28th, 1969. Micro-seismicity was recorded during 11months by a temporary network of 25 ocean bottom seismometers (OBSs) in an area of high seismic activity, encompassing the potential source areas of the mentioned large magnitude earthquakes. We combined micro-seismicity analysis with processing and interpretation of deep crustal seismic reflection profiles and available refraction data to investigate the possible tectonic control of the seismicity in the Gulf of Cadiz area. Three controlling mechanisms are explored: i) active tectonic structures, ii) transitions between different lithospheric domains and inherited Mesozoic structures, and iii) fault weakening mechanisms. Our results show that micro-seismicity is mostly located in the upper mantle and is associated with tectonic inversion of extensional rift structures and to the transition between different lithospheric/rheological domains. Even though the crustal structure is well imaged in the seismic profiles and in the bathymetry, crustal faults show low to negligible seismic activity. A possible explanation for this is that the crustal thrusts are thin-skinned structures rooting in relatively shallow sub-horizontal décollements associated with (aseismic) serpentinization levels at the top of the lithospheric mantle. Therefore, co-seismic slip along crustal thrusts may only occur during large magnitude events, while for most of the inter-seismic cycle these thrusts remain locked, or slip aseismically. We further speculate that high magnitude earthquake's ruptures may only nucleate in the lithospheric mantle and then propagate into the crust across the serpentinized layers.

SINOPROBE deep reflection profile reveals a Neo-Proterozoic subduction zone beneath Sichuan Basin

A new multichannel seismic reflection profile collected across the Sichuan Basin in southern China by SINOPROBE images prominent reflectors that originate within the lower crust and penetrate well into the underlying mantle. The geometry of these mantle reflectors is very similar to those observed on other deep reflection profiles that have been interpreted as relicts of ancient subduction. Considering the geological history of the basement beneath and surrounding Sichuan Basin and ages of granites encountered in nearby wells, we propose that these newly revealed reflectors are the remnants of Neo-Proterozoic subduction that occurred along the NW margin of the Yangtze Craton. This interpretation is consistent with geochemical studies from a gneissic complex at the west margin of the Sichuan Basin. Moreover, preservation of these reflectors supports the idea that the Sichuan lithosphere served as a consolidated tectonic buttress against which the Tibetan Plateau has impinged to produce the Longmenshan orogenic belt.

Deep reflection seismic imaging in SE Poland using extended correlation method applied to PolandSPAN™ data

Here I apply the extended correlation technique to a 156-km long regional seismic profile, PL1-5100, from the PolandSPAN™ survey located in SE Poland. This method allows the extension of the nominal record length of the survey (12s in this case) to much longer times (22s tested here), given that in the field the raw uncorrelated data were stored and a broadband sweep was used. Processing of the ground force correlated data compared to pilot sweep correlated one produced qualitatively similar results. Based on the consistency of the ground force measured along the profile, data were correlated with the mean of the ground force estimates. Results of the processing were validated against the coincident wide-angle reflection/refraction (WARR) profile (CELEBRATION 2000 CEL05 profile) and a deep reflection profile located further to the SE (POLCRUST-01 profile). I found a good correspondence between the wide-angle Moho signature and the near-vertical incidence (NVI) Moho reflectivity, both in terms of the depth and the reflection strength, with the exception of the area of the crustal keel (Moho >50km) interpreted from the WARR data, where the NVI reflectivity is observed at depth ca. 47km. Similarly to POLCRUST-01, line PL1-5100 is portraying the attenuated crust of the East European Platform margin with the dipping crystalline basement down to ca. 20km depth and the reflective lower crust. Two important fault zones: Izbica-Zamość Fault and Wilczopole Fault Zone are interpreted as the deeply-rooted crustal features, but there is no evidence for the Teisseyre–Tornquist Zone itself.

Unravelling the along-strike variability of the Angola–Gabon rifted margin: a mapping approach

Abstract We summarize here observations from the South Atlantic Angola–Gabon rifted margin. Our study was based on the interpretation of a selection of deep penetration depth-migrated seismic reflection profiles. We describe here the large-scale dip architecture of the margin under five structural domains (proximal, necking, distal, outer and oceanic) and list their characteristics. We investigated the necking domain further and discuss the architecture of the distal domain as a combination of hyper-extended crust and possible exhumed mantle. The mapping and characterization of these domains, at the margin-scale, allow us to illustrate the along-strike structural and stratigraphic variability of the margin. We interpret this variability as the result of a shift from an upper plate setting to a lower plate setting. This shift is either sharp, typified by a major regional normal fault on the northern flank of a residual hanging-wall block, identified offshore Cabinda–Zaire, or more diffuse to the south. First-order screening of conjugate profiles confirmed the segmentation and structural characteristics of the transfer zones. The dataset studied also allowed the identification of key sections that can be considered as type examples of upper plate and lower plate margins and which allow us to discuss the characteristics of these end-member settings.

The palaeo-bathymetry of base Aptian salt deposition on the northern Angolan rifted margin: constraints from flexural back-stripping and reverse post-break-up thermal subsidence modelling

The bathymetric datum with respect to global sea level for Aptian salt deposition in the South Atlantic is hotly debated. Some models propose that the salt was deposited in an isolated ocean basin in which local sea level was between 2 and 3 km below the global level. In this study, we use reverse post-break-up subsidence modelling to determine the palaeo-bathymetry of base Aptian salt deposition on the Angolan rifted continental margin. The reverse post-break-up subsidence modelling consists of the sequential flexural isostatic back-stripping of the post-break-up sedimentary sequences, decompaction of remaining sedimentary units and reverse modelling of post-break-up lithosphere thermal subsidence. The reverse modelling of post-break-up lithosphere thermal subsidence is carried out in 2D and requires knowledge of the continental lithosphere stretching factor (β), which is determined from gravity anomaly inversion. The analysis has been applied to the ION-GXT CS1-2400 deep long-offset seismic reflection profile, and two seismic cross-sections (P3 and P7+11) from offshore northern Angola. Reverse post-break-up subsidence modelling restores the proximal autochthonous base salt to between 0.2 and 0.6 km below global sea level at the time of break-up. In contrast, the predicted water-loaded bathymetries of the more distal base salt restored to break-up time are much greater between 2 and 3 km. The predicted bathymetries of the first unequivocal oceanic crust at break-up are approximately 2.5 km, as expected for newly formed oceanic crust of ‘normal’ thickness. Several interpretations of these results are possible. Our preferred interpretation is that all Aptian salt on the northern Angola rifted continental margin was deposited between 0.2 and 0.6 km beneath global sea level, and that the proximal salt subsided by post-rift (post-tectonic) thermal subsidence alone; while the distal salt formed during late syn-rift, when the underlying crust was actively thinning, resulting in additional tectonic subsidence (followed by post-rift thermal subsidence). An alternative interpretation is that the distal salt is para-autochthonous and moved downslope into much deeper water during and just after break-up. We do not believe that a deep isolated ocean basin, with a local sea level 2–3 km beneath that of the global sea level, as has been proposed, is required to explain the Aptian salt deposition on the northern Angolan rifted continental margin.

Stacking autocorrelograms to map Moho depth with high spatial resolution in southeastern Australia

AbstractCurrent estimates of Moho depth in southeastern Australia are based on sparse sampling. The results are augmented with 180 new Moho estimates constructed from spatial stacks of crustal P wave reflectivity derived from autocorrelograms at over 750 stations. The spatial stacks of reflectivity are constructed using a Gaussian with half width 0.5°. Picks of the base of crustal reflectivity are made with the aid of the previous Moho model, based on the sparser data, and knowledge of the variation in the character of the crust‐mantle boundary across the region. Good ties can be made to previous results from deep reflection profiling. The new information fills in many holes in coverage and provides a Moho map with closer ties to geological provinces. The procedure exploits the continuous records at the stations and just the vertical components and so can be applied to older data for which receiver function techniques cannot be used.

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south-central British Columbia, including across much of the Nechako–Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11–12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P-wave velocities of 6.45–6.98 km/s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle-derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east–west, consistent with north-northwest-trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks.

A mega-splay fault system and tsunami hazard in the southern Ryukyu subduction zone

In April 1771, a subduction earthquake generated a great tsunami that struck the south Ryukyu islands and killed ∼12,000 people, whereas its mechanism is still enigmatic (Nakata and Kawana, 1995; Nakamura, 2006; Matsumoto et al., 2009). In this paper, we show its probable source on a mega-splay fault system existing along the southern Ryukyu forearc. Analyses of deep multi-channel seismic reflection profiles indicate that the mega-splay fault system is rising from the summit of a ∼1km high ridge situated at a ∼5° landward dipping plate interface. An outer ridge marks the seafloor outcrop of the splay fault system and separates the landward inner wedge and the oceanward outer wedge. The inner wedge is uplifting and exhibits widespread normal faulting while the outer wedge shows folded structures. The mega-splay fault system is parallel to the Ryukyu Trench east of 125.5°E and is estimated to be ∼450km long. The origin of this south Ryukyu mega-splay fault system is ascribed to a resistant subduction of the elevated transverse ridges associated with the subducting portion of the trench-parallel Luzon–Okinawa Fracture Zone. In contrast, no similar splay fault is found west of 125.5°E where the oblique subduction has produced large shear zones along the south Ryukyu forearc. We infer that a thrust earthquake linked to the mega-splay fault system is responsible for the south Ryukyu tsunami. However, another possible scenario of generating a large tsunami affecting the south Ryukyu islands is that the subducted ridge in the western end of the mega-splay fault system nucleated a large earthquake and simultaneously triggered the ∼100km long E–W trending strike-slip fault west of 125.5°E and induced a southward-dipping tsunami-genic subsidence. In any case, after a quiescence of ∼241yr, a large earthquake and tsunami is anticipated in the south Ryukyu forearc in the near future.

Chapter 33 Stratigraphic and tectonic framework of the US Chukchi Shelf: exploration insights from a new regional deep-seismic reflection survey

Abstract The US Chukchi Shelf is a proven petroleum province similar to the prolific hydrocarbon-bearing region in the adjacent North Slope of Alaska. This shelf is part of the Arctic Alaska plate that was presumably connected to the Canadian Arctic Islands until the opening of the Canada Basin. A new data set (3130 km) of deep 2D seismic reflection profiles, acquired in 2006 and depth processed to 40 km, provides excellent images of the drillable ( c . 8 km depth) stratigraphy as well as of deeper structures. The data also permit regional mapping of the tops of crystalline basement and the Moho discontinuity. The area has a tectonic history of multiple phases of rifting: a pre-Late Devonian phase that culminated in inversion and thrusting; a Late Devonian–Mississippian phase that accommodated Ellesmerian sedimentary deposits as a sag sequence; and a Jurassic to Early Cretaceous phase that is contemporaneous with the opening of the Canada Basin. Hanna Trough is recognized as a Palaeozoic basin with a multistage history while the North Chukchi Basin formed in early Cretaceous as an extensional basin. Only five wells have been drilled in the area; hence numerous opportunities for further exploration exist in the Chukchi Shelf.

Crustal P‐Wave Velocity Structure and Layering Beneath Zhujiangkou–Qiongdongnan Basins in the Northern Continental Margin of South China Sea

AbstractBased on the stacking velocity spectrum data of 14 near‐vertical deep reflection profiles in the deep waters of Zhujiangkou–Qiongdongnan basins along the northern continental margin of South China Sea, we use the Dix formula to transform the stacking velocities into corresponding crust interval velocities (in TWT domain) and construct the crustal velocity structure in depth domain by time‐depth transformation. Integrating all the velocity profiles we analyze the spatial variation of P wave velocity at different depths and the stratified geometry of crust in the Zhujiangkou and Qiongdongnan basins in the continental margin of northern South China Sea. The result indicates that the crust of Qiongdongnan basin can be divided into a 4~8 km thick sediment layer (Vp is 1.7~4.7 km/s), a 4~10 km thick upper crust layer (Vp is 5.2~6.3 km/s), a lower‐crust layer about 5 km thick (Vp is 6.4~7.0 km/s), and a high‐velocity lower‐crust bottom layer about 2~6 km thick (Vp > 7.0 km/s). The existence of the high‐velocity lower crust layer can be considered as the seismological signature of underplating at the lower crust bottom induced by lithosphere extension in South China and its continental margin, or the residual mafic layer of the original Cathaysia lower crust. Synthesizing the available geophysical sounding results we construct the images of Moho and the lithosphere bottom across the continent of South China and the continental slope of South China Sea, which reveals the lithosphere thinning in continental South China and the northern continental margin of South China Sea.

The underestimated Proterozoic component of the Canadian Cordillera accretionary margin

Abstract Analysis of several types of seismic and potential field geophysical data consistently indicate that the majority of the crust underlying the Canadian Cordillera and much of western Canada was originally Proterozoic sedimentary rocks shed off the Canadian Shield into rift or basin structures between 1.84 and 0.54 Ga. These variably metamorphosed strata were primarily quartz- and limestone‐rich sediments and thus have distinctive geophysical signatures because of their lower density, lower magnetization, and lower Poisson's ratio compared with more mafic rocks. The sediments formed a prograding wedge that has a distinctive, internally reflective, seismic stratigraphy. In the east, these Proterozoic sedimentary rocks thicken at a ‘hinge line’ defined by the margin of the pre-1.84 Ga crystalline basement of the Canadian Shield; previous work mapped this hinge line locally using deep reflection profiles and regionally using distinctive gravity gradients. Here we assemble previously published results of several geophysical methods to define the overall shape of the wedge along the margin and westward to where it pinches out at the modern Moho beneath the crustal collage of exotic and suspect terranes accreted onto North America during the Mesozoic. The volume of crust occupied by this wedge limits the thickness of most accreted terranes to several kilometres and suggests that deeper portions of the accreted blocks detached or underthrust the wedge during accretion and are no longer contiguous to crust exposed at the surface. This type Cenozoic accretionary orogen thus spent most of its prior geological history as a passive or extensional margin punctuated by only a few, brief convergent or accretionary events.

Seismic tomographic cross-sections of the Bowser Basin in northwest British Columbia, Canada

Abstract First-break travel time picks along about 250 km of the Lithoprobe SNORCLE deep reflection profile were inverted using a 2-D crossing-ray tomographic method to estimate bulk P-wave speed within the uppermost 2 km of crust beneath the Bowser Basin in northwest British Columbia. The purpose was to assess the usefulness of resulting wave-speed models in defining local stratigraphy and structure and in petroleum exploration more generally. Two areas were investigated in detail. Near the Oweegee Dome, deeper levels of exposure at the surface and stratigraphic correlations with a nearby drill hole suggest that a marked regional increase in wave speed occurs at the contact between the lowermost Bowser Lake Group strata and lava flows of the underlying Hazelton Group. Near the northwest corner of the Bowser Basin, a triangle zone inferred from surface structure does not show clearly in the seismic wave-speed model but does contain distinctive horizontal layers of higher wave speed that represent thrust duplexes, igneous intrusions, or mineralization.

Developments in passive seismic techniques through the ANSIR National Research Facility

Since 1997 the ANSIR Major National Research Facility has significantly enhanced Australia’s national capacity for recording regional and distant earthquakes using seismic recorders distributed across the continent. Both broad-band and short-period recorders have been deployed in innovative experimental designs that have made major contributions to the understanding of the 3D structure in the Earth’s crust and upper mantle beneath the Australian region. The Facility continues its national role with investment in equipment suitable for both seismic and electromagnetic sounding.Deployments of recorders with broad-band seismometers have made a substantial contribution to surface wave tomography, particularly with the gathering of extensive data sets from Western Australia and a deployment bracketing the Tasman Line. The significant increase in the number of portable stations provides constraints on the character of the crust and the crust-mantle interface across the continent, via the analysis of receiver functions from distant earthquakes. Anisotropy beneath the continent is now better characterised but remains enigmatic. New analysis methods are likely to make more extensive use of continuous seismic data.Short-period instrument deployments have been mostly directed towards delay-time tomography studies in south-eastern Australia, and have employed new methods for event picking and tomographic inversion. Enhancements to the instruments mean that servicing of experiments will be simpler in future and allow new applications with three-component recording at higher sampling rates.Passive seismic recording offers a cost-effective way of obtaining structural information across substantial areas of the continent. The results are valuable in their own right, but can also provide important constraints on seismological structure that are valuable in planning more expensive deep crustal reflection profiles.

Developments in passive seismic techniques through the ANSIR National Research Facility

Since 1997 the ANSIR Major National Research Facility has significantly enhanced Australia's national capacity for recording regional and distant earthquakes using seismic recorders distributed across the continent. Both broad-band and short-period recorders have been deployed in innovative experimental designs that have made major contributions to the understanding of the 3D structure in the Earth's crust and upper mantle beneath the Australian region. The Facility continues its national role with investment in equipment suitable for both seismic and electromagnetic sounding. Deployments of recorders with broad-band seismometers have made a substantial contribution to surface wave tomography, particularly with the gathering of extensive data sets from Westem Australia and a deployment bracketing the Tasman Line. The significant increase in the number of portable stations provides constraints on the character of the crust and the crust-mantle interface across the continent, via the analysis of receiver functions from distant earthquakes. Anisotropy beneath the continent is now better characterised but remains enigmatic. New analysis methods are likely to make more extensive use of continuous seismic data. Short-period instrument deployments have been mostly directed towards delay-time tomography studies in south-eastem Australia, and have employed new methods for event picking and tomographic inversion. Enhancements to the instruments mean that servicing of experiments will be simpler in future and allow new applications with three-component recording at higher sampling rates. Passive seismic recording offers a cost-effective way of obtaining structural information across substantial areas of the continent. The results are valuable in their own right, but can also provide important constraints on seismological structure that are valuable in planning more expensive deep crustal reflection profiles.

Crustal structure of the central Nova Scotia margin off Eastern Canada

SUMMARY The central Nova Scotia margin off Eastern Canada is located at a transition from a volcanic margin in the south to a non-volcanic margin in the north. In order to study this transition, a wide-angle refraction seismic line with dense airgun shots was acquired across the central Nova Scotia margin. The 500-km-long transect is coincident with previous deep reflection profiles across the Lahave Platform and extending into the Sohm Abyssal Plain. A P-wave velocity model was developed from forward and inverse modelling of the wide-angle data from 21 ocean bottom seismometers and coincident normal-incidence reflection profiles. The velocity model shows that the continental crust is divided into three layers with velocities of 5.5‐6.9 km s −1 . The maximum thickness is 36 km. A minor amount (∼5 km) of thinning occurs beneath the outer shelf, while the major thinning to a thickness of 8 km occurs over the slope region. The seaward limit of the continental crust consists of 5-km-thick highly faulted basement. There is no evidence for magmatic underplating beneath the continental crust. On the contrary, a 4-km-thick layer of partially serpentinized mantle (7.6‐7.95 km s −1 ) begins beneath the highly faulted continental crust, and extends ∼200 km seawards, forming the lower part of the oceancontinent transition zone. The upper part of the transition zone consists of the highly faulted continental crust and 4- to 5-km-thick initial oceanic crust. The continent‐ocean boundary is moved ∼50 km farther seawards compared to an earlier interpretation based only on reflection seismic data. The oceanic crust in the transition zone consists of layer 2 and a high-velocity lower crustal layer. Layer 2 is 1‐3 km thick with velocities of 5.6‐6.0 km s −1 . The high-velocity lower crustal layer is 1‐2 km thick with velocities of 7.25‐7.4 km s −1 , suggesting a composite layer of serpentinized peridotite and gabbroic layer 3. Oceanic crust with normal thickness of 5‐7 km and more typical layer 3 with velocities of 6.95‐7.3 km s −1 is observed at the seaward end of the profile.

Geophysical methods applied to fault characterization and earthquake potential assessment in the Lower Tagus Valley, Portugal

The study region is located in the Lower Tagus Valley, central Portugal, and includes a large portion of the densely populated area of Lisbon. It is characterized by a moderate seismicity with a diffuse pattern, with historical earthquakes causing many casualties, serious damage and economic losses. Occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick Cenozoic sedimentary cover. The hidden character of many of the faults in the Lower Tagus Valley requires the use of indirect methodologies for their study. This paper focuses on the application of high-resolution seismic reflection method for the detection of near-surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the Tagus Valley, and that have been recognized on deep oil-industry seismic reflection profiles and/or inferred from the surface geology. These are a WNW–ESE-trending fault zone located within the Lower Tagus Cenozoic basin, across the Tagus River estuary (Porto Alto fault), and a NNE–SSW-trending reverse fault zone that borders the Cenozoic Basin at the W (Vila Franca de Xira–Lisbon fault). Vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out. According to the interpretation of the collected data, a complex fault pattern disrupts the near surface (first 400 m) at Porto Alto, affecting the Upper Neogene and (at least for one fault) the Quaternary, with a normal offset component. The consistency with the previous oil-industry profiles interpretation supports the location and geometry of this fault zone. Concerning the second structure, two major faults were detected north of Vila Franca de Xira, supporting the extension of the Vila Franca de Xira–Lisbon fault zone northwards. One of these faults presents a reverse geometry apparently displacing Holocene alluvium. Vertical offsets of the Holocene sediments detected in the studied geophysical data of Porto Alto and Vila Franca de Xira–Lisbon faults imply minimum slip rates of 0.15–0.30 mm/year, three times larger than previously inferred for active faults in the Lower Tagus Valley and maximum estimates of average return periods of 2000–5000 years for M 6.5–7 co-seismic ruptures.