Seismic Reflection Profiles Research Articles

Structure of an accreted intra-oceanic arc: potential-field model of a crustal cross-section through the Macquarie Arc, Lachlan Orogen, southeastern Australia

Construction of a 3D geological model for the eastern Lachlan Orogen provided a new reference model for a combined magnetic and gravity model of the Macquarie Arc, an intra-oceanic arc accreted to Gondwana during the Late Ordovician to early Silurian Benambran Orogeny. Geologically constrained, iterative 2.5 D forward modelling represented the entire thickness of the crust over a west-to-east profile extending for 230 km, approximately coinciding with seismic reflection profiles 99AGSL1 and 99AGSL2. The output model, referred to here as the Forbes model, confirms west-dipping thrust stacks of the Junee-Narromine and Molong volcanic belts of the Macquarie Arc, in packages that also comprise underlying arc basement, overlying Ordovician turbidites, and overlying Silurian–Devonian volcanic and sedimentary units. Ordovician rocks in the Parkes Fault Zone are distributed in wedges within a flower-fault structure. These structural elements are broadly similar to an earlier interpretation of the seismic profiles and corresponding potential-field models but differ in the nature of the substrate to the Macquarie Arc. The arc substrate is best fitted with physical properties broadly that of an intermediate calc-alkaline rock, rather than mid-ocean ridge basalt (MORB) as previously inferred. Highly magnetic metamafic rocks of the Jindalee Group appear to occupy relatively narrow fault-bounded slivers and may correspond to mafic volcanic rocks in the forearc rather than fragments of MORB basement. The thick intermediate-composition crustal profile required by the Forbes model cannot distinguish between an Izu-Bonin-like thickened arc or a compound of an Ordovician arc developed on a pre-existing Cambrian arc, but the simple MORB substrate previously assumed for the Macquarie Arc can be excluded, as can obduction of the Macquarie Arc as a nappe over the Wagga Belt. KEY POINTS A 3D model for the Lachlan Orogen provides geological constraints on joint magnetic and gravity model across the accreted intra-oceanic Macquarie Arc. The substrate of the arc is best fitted with magnetic susceptibility and density typical of an intermediate composition, in contrast with previous interpretations that the substrate preserves an oceanic crust basement. Western and central belts of the Macquarie Arc are stacked by west-dipping thrusts, inconsistent with emplacement as an allochthonous nappe.

3‐D Structure of the Variscan Thrust Front in Northern France: New Insights From Seismic Reflection Profiles

AbstractIn NW Europe, the late Carboniferous Variscan collision between the Laurussia and the Armorica‐Gondwana continental blocks led to the development of a crustal‐scale north‐verging thrust system along the southern Laurussian margin. In northern France, the 3‐D geometry and kinematics of the Variscan deformation front have been investigated on the basis of reprocessing and interpreting 532 km of industry‐level seismic reflection profiles. This extensive seismic imaging provides new constraints on the structural and kinematic features of the orogenic front. It particularly emphasizes the localization of displacement along the main frontal thrust zone that accommodated more than 50 km of total displacement of the allochthonous units above the foreland. It also highlights the induced large underthrusting of the foreland basin below the frontal thrust zone, and its truncation in a general out‐of‐sequence mode of thrust propagation. We built structural maps that led to better delineating a major NW‐SE lateral ramp along the main frontal thrust. The Mid‐Upper Devonian series within the flexured foreland were deformed at depth by N060–080° trending and N110–130° trending syn‐sedimentary normal faults that led to their south‐to southwestward thickening. These pre‐existing structures along the margin have exerted a primary control on the ensuing dynamics and geometry of the Northern Variscan Front by localizing both the frontal and lateral ramps during thrust wedge growth.

Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the 2011 Tohoku Earthquake Rupture Area

Multi-channel seismic reflection and sub-bottom profiling data reveal landward-dipping normal faults as potential conduits for mantle-derived fluids in the coseismic slip area of the 2011 Tohoku earthquake (Mw9.0). Normal faults below the helium isotope anomaly sites appear to develop through the forearc crust (i.e., the seafloor sedimentary section and Cretaceous basement) and to evolve to lower dip angles as extension progresses deeper, potentially extending down to the mantle wedge, despite their intermittently continuous reflections. The faults are characterized by high-amplitude, reverse-polarity reflections within the Cretaceous basement. Moreover, deep extension of the faults connecting to a low-velocity region spreading from the Cretaceous basement into the mantle wedge across the forearc Moho suggests that the faults are overpressured by local filling with mantle-derived fluids. The locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show normal-faulting focal mechanisms. The 2011 Tohoku mainshock and subsequent aftershocks can lead the pre-existing normal faults to be reactive and more permeable so that locally trapped mantle fluids can migrate up to the seafloor through fault fracture zones. The reactivated normal faults may be an indicator of shallow coseismic slip to the trench. Locally elevated fluid pressures can decrease the effective normal stress for the fault plane, facilitating easier slip along the fault and local tsunami. The landward-dipping normal faults developing from the seafloor down into the Cretaceous basement are predominant in the middle slope region of the forearc. A possible shear zone with high-amplitude, reverse-polarity reflections above the plate interface, which is almost localized to the middle slope region, suggests more intense basal erosion of the overlying plate in that region.

A Rapid Numerical Method to Constrain 2D Focused Fluid Flow Rates Along Convergent Margins Using Dense BSR‐Based Temperature Field Data

AbstractEstimates of the sub‐seabed fluid flow rates are important for understanding hydrological budgets, biogeochemical cycles, and physical properties of the sediments. Fluid flow rates and directions, however, are difficult to measure, particularly beneath the seafloor. We developed a rapid method to estimate regional fluid migration rates using an extensive database of seismic reflection profiles taken offshore SW Taiwan. We observe bottom‐simulating reflector (BSR) that deflects toward the seafloor near thrust faults that indicate localized heat flow variations. At these sites, advecting warm pore fluids transport heat to shallower depths and force the BSR shallower. Our 2D steady‐state numerical method quantifies the fluid flow rates required to cause such thermal anomalies. We found that fluid flow rates near the trench of the accretionary wedge range between 0.1 and 16 m3 yr−1 m−1, with slower and faster rates generally associated with slope basin discontinuities and faults, respectively. To evaluate the fluid pattern evolution from subduction to collision, we studied three transects: one along the Manila subduction zone in the south and two in Taiwan's initial collision zone in the north. We quantified the fluid budget and partitioning of fluid flow between focused discharge through faults and diffusive flow through the wedge. Faults in Taiwan's accretionary wedge capture on average 25% of the total dewatering flux in the younger subduction zone and 38.5% in the tectonically mature collision zone. Our method provides estimates of fluid migration rates along convergent plate boundaries, and contributes to our understanding of focused fluid flow processes in many other regions.

Estimation of lateral correlation length from deep seismic reflection profile based on stochastic model

Estimation of lateral correlation length from deep seismic reflection profile based on stochastic model

Reflection seismic imaging to unravel subsurface geological structures of the Zinkgruvan mining area, central Sweden

The Zinkgruvan mining area is located on the south-eastern part of the Bergslagen district, one of the three major mineral producing regions in Sweden. In this study, we present the results from three (P1, P2 and P8) reflection seismic profiles each approximately 3000 m-long crossing the Zinkgruvan Zn-Pb-Ag-(Cu) mining area. P1 was acquired using cabled geophones with 10 m receiver and source interval and crossed major geological features. The other two profiles (P2 and P8) were acquired by wireless recorders with 20 m receiver and 10 m source interval and ran perpendicular to P1. Through a special data processing workflow adapted to this dataset, good quality seismic sections were obtained along these profiles, although a high noise level due to high voltage electric power lines was present. The interpretations were constrained by (1) seismic P-wave velocity and density data from a series of downhole logging measurements, (2) 3D forward reflection traveltime modelling in both pre- and post-stack domains, and (3) other geophysical and geological observations available from the site. Despite the notably complex geology, the processed seismic sections clearly reveal a series of horizontal to gently dipping reflections associated with known geological formations. Results indicate that most structures and lithological contacts dip or plunge to the northeast, including the targeted Zinkgruvan Formation. The results from this seismic survey are encouraging regarding the potential of the seismic method for base-metal exploration in Sweden and in particular in the Bergslagen district. It shows the high resolving power of the reflection seismic methods for imaging complex geological structures in a cost-effective and environmentally-friendly way.

Permian rifting processes in the NW Junggar Basin, China: Implications for the post-accretionary successor basins

The Junggar Basin, bounded by the West Junggar along late Carboniferous strike-slip faults in the west, is a post- orogenic successor basin within the Neoproterozoic to Paleozoic Central Asian Orogenic Belt (CAOB). The Junggar Basin is a key region for understanding the Permian rifting processes since it is located in an extensional step-over zone between the Irtysh and North Tianshan strike-slip fault zones that were most active in the Permian. Based on new seismic reflection profiles, detrital petrology and zircon U-Pb ages, together with regional geological data and a refraction seismic profile, this study investigates the tectonic evolution and the geodynamic mechanics of the basin. The early Permian phase of syn-rift mechanical subsidence in the western Junggar Basin formed half grabens. Volcanic detritus is abundant, and detrital zircon U-Pb ages show sediment supply was entirely local. During the middle Permian post-rift phase with thermal subsidence, sediment supply exceeded subsidence, and there are increasingly distant sources of sediment. The Permian regional extensional stress field between major post- accretionary strike-slip fault zones triggered the reactivation of the pre-existing late Carboniferous strike-slip faults in West Junggar and produced thinning of the crust beneath the Junggar Basin, with a β crustal stretching factor of 1.74. Seismic refraction data reveals asthenospheric upwelling resulted in underplated mafic lower crust (6–10 km). The location of initial rift axis in the Junggar Basin was controlled by the juxtaposition of thin and weak crust beneath the basin and relatively strengthened thick crust in the West Junggar attributed to the cooling of A-type granites, resulting from localized strain. Similar location of post-orogenic rifting in southwestern CAOB, in areas of high heat flow following collision, suggests that Junggar Basin is an example of a more general process.

Outer Cilicia Basin – A piggy back basin developed in an intramontane setting following the partitioning of a large ancestral Miocene basin across the northeastern Mediterranean

Multichannel seismic reflection profiles show that the Outer Cilicia Basin evolved mainly during the uppermost Messinian–Quaternary as an asymmetric piggyback basin on the hanging wall of the large south-verging Kyrenia thrust culmination, following the partitioning of the much larger Miocene basin. A major north-verging backthrust and several similarly-verging smaller splays developed during the deposition of the uppermost Messinian–Quaternary succession, elevating the southern sector of the Outer Cilicia Basin ~250–500 ms above the central portion of the basin. An E–W trending zone characterized by 6–9 positive flower structures which are cored by the Messinian evaporite successions developed immediately north of the backthrust. Two prominent broadly E–W striking extensional fault zones define the tectonic framework of the north and south margins of the basin: (a) the Zafer–Kormakiti zone in the south with 5–6 internally parallel and invariably north-dipping large listric extensional faults developed immediately north of the Kyrenia Range and (b) the Anamur–Silifke zone in the north with 4–8 internally parallel and steeply north- and south-dipping planar faults defining a series of shore-parallel horst and graben structures developed immediately south of the Central Taurus Mountains. The Zafer–Kormakiti and the Anamur–Silifke fault zones are shown to be the westward extensions of the uppermost Messinian–Quaternary Misis–Kyrenia and Kozan fault zones, respectively. The tectonic framework of the Miocene successions across the Mut Basin delineated using >310 strike and dip measurements extracted from 1:25,000–1:100,000 maps supplemented by >1200 strike and dip determinations using the rule of Vs and construction of structure contours. These data revealed two distinct regional folds, where the anticlinal and synclinal axes are oriented WNW–ESE and NE–SW directions. Geological cross sections constructed suggest that the folds are likely cored by broadly southerly verging deep-seated blind thrusts.

Inversion structures across the crest of the Larnaka Ridge associated with strike-slip faulting during the uppermost Messinian–Quaternary, eastern Mediterranean

The structural evolution of the Larnaka Ridge is studied using high-resolution multichannel seismic reflection profiles and multibeam mosaics of the seafloor. Multibeam maps revealed the presence of two families of en-échelon tension cracks (young and old) on the seafloor north of the ridge. Rose diagram on the young tension cracks revealed that most lineations are oriented NW–SE, where the instantaneous tensile σ3 is oriented NE–SW. The old tension cracks are oriented N-S, here the instantaneous tensile σ3 is oriented E–W. In addition, the seafloor in the Latakia Basin and over the crest of the Latakia Ridge includes by several prominent ridges and their intervening valleys, forming four groups of seafloor lineations, each having a distinctive orientation, including ENE–WSW, ESE–WNW, NE–SW and NNW–SSE.The multichannel seismic reflection profiles revealed that (a) the ridges and valleys are developed exclusively within the uppermost Messinian–Quaternary Unit 1, bounded at their tops by the seafloor and bases by the top erosional surface (TES), (b) there is a reciprocal relationship between the seafloor and the TES, such that when the seafloor goes up, the TES goes down, (c) there is a prominent mid–Unit 1 unconformity (γ-reflector), which separates the convex-up reflectors of the upper subunit 1a from the convex-down reflectors of the lower subunit 1b. These sedimentary structures are developed as a series of positive and negative flower structures along the Amanos–Larnaka fault zone, a prominent sinistral strike-slip system in the eastern Mediterranean. The data show evidence of a switch during the Quaternary of the direction of the principal maximum compressive stress from N-S to NW–SE, probably associated with the docking of the Eratosthenes Seamount which must have fixed the northwestern margin of the Sinai Block, allowing the northeastern sector to freely rotate counterclockwise.

Superimposed Pattern of the Southern Sichuan Basin Revealed by Seismic Reflection Profiles Across Lushan–Chishui, China

Abstract The Sichuan Basin is a typical intracraton superimposed basin. It is rich in oil and gas resources in the different sets of sedimentary sequences. It underwent multistage tectonic evolution, which resulted in different types of prototype basins. However, there are still many different opinions on the types and superimposed patterns of the Sichuan Basin in different geologic periods, which largely affect the understanding of the mechanism of effective oil and gas accumulation and preservation. This paper aims to re-recognize several prototype types of the Sichuan Basin by discussing the prototype basins and their superimposed models to deepen the significance of superimposed basin evolution for hydrocarbon accumulation. The regional geological and drilling data are used for a detailed interpretation of seismic reflection profiles across Lushan–Chishui. Then, five regional unconformities are identified with the equilibrium profiles technique which is used to flatten the formation interface in different geologic periods. Based on the unconformities, the southern Sichuan Basin is divided into six tectonic layers, each of which is regarded as a prototype basin: a pre-Sinian crystalline basement (AnZ), a marine rift cratonic basin (Z–S), a marine intracratonic sag basin (P2l–T2l), a marine–continental downfaulted basin (T3x1–T3x3), a continental depressed basin (T3x4–J), and a foreland basin (K–Q). The different prototype basins are vertically superimposed to form a “layered block” geologic structure of the multicycle basins. Affected by the late-stage tectonic transformation, the geologic structure of vertical stratification underwent a strong transformation, which had a profound impact on oil and gas accumulation with the characteristics of early accumulation and late adjustment.

Seismic imaging of dolerite sills and volcanic vents in the Central Karoo, South Africa: implications for shale gas potential

Abstract The Karoo Basin of South Africa covers an area of 700 000 km2 and has been identified as a possible shale gas reserve. Any evaluation of the shale gas potential of the basin must consider the widespread dolerite dykes and sills. These intrusions were emplaced into the Karoo Supergroup and are well dated at around 183 Ma. Their intrusion triggered the explosive releases of gas in the basin, marked on surface by breccia pipes and hydrothermal vents. This outpouring of gas has been proposed as a significant contributor to global climate change. Research into the three-dimensional interconnected structure of these dolerite sills and dykes and their interaction with the hydrocarbon rich layers in the lower part of the Karoo Supergroup has been limited to localized observations of outcrop, magnetic data, legacy seismic data (from the 1970s) and well core. Here we present an interpreted 65 km long higher-resolution 2D seismic reflection profile across the Karoo Basin, approximately 100 km southeast of Trompsburg. These data were collected in the 1990s and at the time deeper structures along the line interpreted. In this study we focus on the top 0.6 to 2 seconds TWT of the data. The seismic line images the interconnected and cross cutting nature of the dolerite dykes and sills along the profile. We also report possible evidence of a gas escape structure (approximately 2.5 km in diameter at surface) emerging near the edge of a dolerite sill in close proximity to the Whitehill Formation, which is the main target for shale gas exploration. This suggests that gas vents in the eastern Karoo Basin close to Lesotho are due to the release of gas from the carbonaceous shales of the Ecca Group. This is similar to breccia pipes mapped on surface in the western part of the Karoo Basin. This seismic section highlights why dolerite sills and dykes must be considered when evaluating the shale gas potential of the Karoo Basin. We propose that better characterization of the Karoo Basin subsurface by seismic and magnetic studies is necessary prior to any efforts to calculate shale gas reserves.

Probabilistic Assessment of Slip Rates and Their Variability Over Time of Offshore Buried Thrusts: A Case Study in the Northern Adriatic Sea

When sedimentation rates overtake tectonic rates, the detection of ongoing tectonic deformation signatures becomes particularly challenging. The Northern Apennines orogen is one such case where a thick Plio-Pleistocene foredeep sedimentary cover blankets the fold-and-thrust belt, straddling from onshore (Po Plain) to offshore (Adriatic Sea), leading to subtle or null topo-bathymetric expression of the buried structures. The seismic activity historically recorded in the region is moderate; nonetheless, seismic sequences nearing magnitude 6 punctuated the last century, and even some small tsunamis were reported in the coastal locations following the occurrence of offshore earthquakes. In this work, we tackled the problem of assessing the potential activity of buried thrusts by analyzing a rich dataset of 2D seismic reflection profiles and wells in a sector of the Northern Apennines chain located in the near-offshore of the Adriatic Sea. This analysis enabled us to reconstruct the 3D geometry of eleven buried thrusts. We then documented the last 4 Myr slip history of four of such thrusts intersected by two high-quality regional cross-sections that were depth converted and restored. Based on eight stratigraphic horizons with well-constrained age determinations (Zanclean to Middle Pleistocene), we determined the slip and slip rates necessary to recover the observed horizon deformation. The slip rates are presented through probability density functions that consider the uncertainties derived from the horizon ages and the restoration process. Our results show that the thrust activation proceeds from the inner to the outer position in the chain. The slip history reveals an exponential reduction over time, implying decelerating slip-rates spanning three orders of magnitudes (from a few millimeters to a few hundredths of millimeters per year) with a major slip-rate change around 1.5 Ma. In agreement with previous works, these findings confirm the slip rate deceleration as a widespread behavior of the Northern Apennines thrust faults.

Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

KineDyn: Thermomechanical forward method for validation of seismic interpretations and investigation of dynamics of rifts and rifted margins

Physical processes interact during rifting to produce a range of crustal and sedimentary architectures and subsidence histories. Methodologies to analyze these processes combine techniques such as cross-section restoration, backstripping and numerical simulations. Despite the valuable outcomes of each method, intrinsic limitations related to particular interactions between kinematics and thermo-mechanics leave open questions on how seismic interpretation is consistent in terms of thermomechanical aspects of lithospheric extension. In this work, we present a new modelling technique, KineDyn, to simulate basement architecture, heat-flow, subsidence and sedimentation patterns along a given basin-scale seismic reflection profile in 2D. KineDyn is the fusion of dynamic thermomechanical approach and kinematic forward modelling of faulting and excludes drawbacks of conventional methods while preserving their valuable outcomes. In our approach, faults are initially controlled by prescribed initial locations, slips and timings, while the rest of the model is resolved in a fully dynamic mode, with non-linear visco-elasto-plastic rheology. KineDyn allows dynamic simulation of the response of the lower crust and mantle to different patterns of faulting in time and space and direct comparison to seismic observations. Thus, it gives, in effect, the same results as existing section restoration techniques (i.e. the potential history of faulting) and forward modelling techniques (i.e. the likely history of sedimentation, thinning, heat flow and subsidence), while simultaneously taking into account non-linear interactions between processes occurring during rifting. In addition, comparison of the model outcomes to lithospheric-scale seismic observations gives insight into the relationship between shallow crustal structures and deeper geodynamic processes in the lithosphere-asthenosphere system, which shape rifts.

Structural and seismographic characteristics of a sector of the Magdalena Shelf, western margin of Baja California, Mexico, from 2D seismic reflection profiles

The Magdalena Shelf, located on the Pacific margin of Baja California, Mexico, is crosscut by the Tosco-Abreojos transform fault system, while the Santa Margarita and San Lazaro faults are sub-parallel and located east of the shear zone that cuts the continental slope longitudinally. Processing and interpretation 2D multichannel marine reflection seismic data show evidence for the structural deformation and seismostratigraphic characteristics of a sector of the shelf. This study allowed mapping unknown faults, of which the two main ones are named Iray and Magdalena Faults. San Lazaro fault has a strike of N 15° W and length of 70 km, with an apparent dip of ~ 42° to the NE. The San Lazaro fault controls the western flank of the basin of the same name forming a semi-graben of 20 km wide and its sedimentary fill of ~ 4.5 km in thickness. The Iray-Magdalena basin has a depth of ~ 4.0 km. We identified six seismic stratigraphic sequences, and two main discordances, which are considered to be regional discordances. One of which is the Cretaceous-Paleogene discordance, and the other corresponds to the Miocene discordance, which has been considered an important geological marker of the tectonic evolution of region. Acoustic basement defines a structural high parallel to the continental slope and aligned with outcrops of the ophiolite complex. The zone of active deformation of the Tosco-Abreojos fault system is between 15 and 18 km wide. This region may contain previously unknown structures and can represent a geological/seismic risk, which can have not only scientific implications, but also social impact in the communities of the region.

Assessing the accuracy of combined DEM-based lineament mapping and the normalised SL-index as a tool for active fault mapping

Satellite image-based lineament mapping in combination with the normalised Stream Length Gradient Index (SL/K-index) is one of the most widely used methods to identify active faults in areas where other verification methods, such as faulted strata, geophysical subsurface data, or seismic activity in the upper crust, are unavailable, and to classify their degree of activity. The accuracy of this approach, however, has not yet been assessed rigorously. We assess its accuracy by comparing the results against 27 thrust fault segments that were identified from six seismic reflection profiles with a total length of 340 km. Over 106,000 SL/K points were calculated from 815 river profiles on basis of the SRTM v.3.0 DEM. Standard deviations (SD) of < -2SD (class 1) to > +2SD (class 5) were calculated from the lognormalised SL/K data and five SL/K classes were made and assigned to the nearby lineaments. A comparison with the fault segments from the seismic profiles showed a good coincidence between the presence of knickpoints, SL/K classes 4 and 5 and to a lesser extent class 3. Lineaments with SL/K classes 1 and 2 never corresponded to actual faults. We identified seven master thrusts of which three are very active to highly active, likely older, structures with important amounts of accumulated slip. The other thrusts are probably younger faults with relatively little accumulated slip, yet they are also active to highly active faults. Minor strike-slip faults were identified as very active to highly active structures. Published slip rates shows that at least some of the SL/K class 4 faults experience slip rates of ~1 to ~4 mm a−1 and the SL/K class 5 strike-slip faults may experience slip rates as high as one metre over the course of several months. Most active faults occur where the basal evaporite layer is abnormally thick.

Seismic imaging across fault systems in the Abitibi greenstone belt – an analysis of pre- and post-stack migration approaches in the Chibougamau area, Quebec, Canada

Abstract. Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada, help understand historic volcanically hosted massive sulfide (VMS) deposits and hydrothermal Cu–Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with a known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTMs) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to the DMO and PSTM methods and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0–9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper-crustal rocks near the fault zones along both profiles. In the northern seismic reflection section, the key geological structures identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids.

Effects of basement structures and Carboniferous basin configuration on evaporite distribution and the development of salt structures in Nordkapp Basin, Barents Sea—Part I

AbstractPotential field data, reprocessed regional 2D seismic reflection profiles and 3D seismic tied to wells were used to study the late Devonian‐Permian development of the Nordkapp Basin in Barents Sea. The composite basin can be subdivided into the northeastern, central and southwestern segments that developed above a basement that contains elements of the Timanian and Caledonian regimes. The transition between the Timanian and Caledonian structures is positioned below the central basin segment. The rheological properties, locations, orientations and interaction of inherited structures together with two subsequent extensional phases, defined the presalt rift architecture and gave rise to seven subbasins within Nordkapp Basin. During the late Devonian‐early Carboniferous NE‐SW oriented extension, the basin consisted of two regional half‐grabens (northern and southern) separated by an interbasin ridge. During the late Carboniferous, the shift of extension direction to NW‐SE orientation reshaped the two regional half‐grabens. In particular, an interbasin transfer zone divided the northern regional half‐graben by separating its hinged‐margin portion (incipient northeastern segment) from the deeper part (incipient central segment). At the same time, the elevated interbasin ridge separated the incipient central and southwestern segments. Internally within the seven subbasins, the evolving structural elements including cross‐cutting master faults and structural highs have influenced the deposition and character of the Pennsylvanian‐lower Permian layered evaporite sequence and the distribution of subsequent salt structures. We suggest that the synrift to early postrift processes, relative depth of each subbasin, arrangement of the structural highs and depositional palaeo‐environment all controlled the thickness and facies of the layered evaporite sequence. The latter in combination with the presalt architecture comprised a laterally varying in thickness and character substratum that during the earliest Triassic influenced the sediment routings and deposition of the progradational system arriving in Nordkapp Basin. The study outcomes are pertinent and applicable to other salt‐influenced rift basins worldwide.

Test on the Reliability of the Subsurface Fault Geometry Estimated by Deformed River Terraces Along the Bailang River, North Front of the Qilian Shan (North West China)

The subsurface fault geometry is the base for understanding a process of crust deformation and mountain building. Based on kinematic models for fault-related folds, a geomorphic method is recently applied to estimate the subsurface fault geometry, while the validation on its reliability is lacking. In this study, we surveyed a suit of river terrace surfaces across an active fold at the north front of the Qilian Shan. According to the deformation geometry of the terraces, the fold deformation is interpreted by a listric fault fold model, and based on this kinematic model, the fault geometry underlying the fold is estimated. In comparison between the estimated fault geometry and a seismic reflection profile, we found that the decollement depth and the back thrust are highly consistent with each other. Although some small fault bends or internal shearing cannot be estimated solely by the terrace deformation, the overall fault geometry is successfully revealed by the terrace deformation. Using this fault geometry and the terrace dating results, the region deformation kinematics are re-evaluated, which suggest that the dip slip (in a rate of 1.8 ± 0.4 mm/a) along the decollement is mainly accommodated by two structures, one is the blind-back-thrust fault within the piggy basin in a dip-slip rate of 0.9 ± 0.3 mm/a and another is the thrust and fold at the west portion of the Yumu Shan range.

Decoding a complex record of anthropogenic and natural impacts in the Lake of Cavazzo sediments, NE Italy

The Lake of Cavazzo is a natural, lacustrine basin in the Friuli-Venezia Giulia region (NE Italy), which occupies a fluvio-glacial suspended valley of the Tagliamento river, at the southern front of the Alpine chain. The lake formed after the melting of major glacial tongues at the end of the Last Glacial Maximum, and has been affected in recent years by natural and anthropogenic events, with economic, cultural and environmental consequences. According to local witnesses, the lake environment has changed dramatically after the 1950s, when the basin was connected to a hydroelectric power plant collecting waters from a wider catchment area, as it served as its final discharge basin.In this work, based on a densely-spaced grid of high-resolution seismic reflection profiles complemented by sediment cores, we analysed the uppermost lake stratigraphy, highlighting major changes occurring at a decadal scale. Our main purpose is to verify whether and how the lake sediments record the transition from pristine/natural to artificial conditions, as well as the effects of multiple natural impacts including the 1976–77 Friuli earthquake sequence. The results of our analysis suggest varying environmental conditions of the lake after the 1950s indicated by changing sediment mineralogy, increased deposition of allochthonous clastic sediments, and recurrent episodes of anoxic conditions at the lake's floor, likely triggered by hyperpycnal flows derived from the hydroelectric power plant discharges. We also observe that the effects of the 1976–77 seismic sequence are recorded in the lake as resedimented levels, likely due to shaking, in situ deformation, landslides, and turbidity currents. Our results stress the importance of lacustrine environments as efficient recorders of anthropogenic and natural events.