Lithospheric Scale Research Articles

Tectonic delamination and subcrustal imbrication of the Precambrian lithosphere in northwestern Canada mapped by LITHOPROBE

Research Article| September 01, 1998 Tectonic delamination and subcrustal imbrication of the Precambrian lithosphere in northwestern Canada mapped by LITHOPROBE F. A. Cook; F. A. Cook 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar A. J. van der Velden; A. J. van der Velden 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar K. W. Hall; K. W. Hall 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar B. J. Roberts B. J. Roberts 2Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information F. A. Cook 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada A. J. van der Velden 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada K. W. Hall 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada B. J. Roberts 2Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1998) 26 (9): 839–842. https://doi.org/10.1130/0091-7613(1998)026<0839:TDASIO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F. A. Cook, A. J. van der Velden, K. W. Hall, B. J. Roberts; Tectonic delamination and subcrustal imbrication of the Precambrian lithosphere in northwestern Canada mapped by LITHOPROBE. Geology 1998;; 26 (9): 839–842. doi: https://doi.org/10.1130/0091-7613(1998)026<0839:TDASIO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Seismic reflection profiling of the Precambrian lithosphere in northwestern Canada has produced images of delamination structures (tectonic wedges) at crustal scale (to about 30 km depth) and, for the first time, at lithospheric scale (to about 100 km depth), and has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere. Coupled with observations of similar structures in thrust and fold belts, delamination and lower plate imbrication are persistent features at many scales. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Le bassin Stephanien de St-Etienne ne résulte pas d'une extension tardi-hercynienne généralisée: c'est un bassin pull-apart en relation avec un décrochement dextre

The St-Etienne area (Stephanian coal-basin and Mt Pilat metamorphic massif) has been taken as one of the examples of late orogenic extension in the Variscan belt. The key structure was the Mt Pilat ductile normal fault which was considered as the southern boundary of the St-Etienne coal basin. We show with precise field observations and taking into account the old subsurface data obtained from coal exploitation that this structural interpretation must be modified. The Stephanian coal-basin is a pull apart basin on a NE-SW strike-slip fault, which is distinct from the Mt Pilat detachment. We compare the latter to the North Himalayan normal fault and we discuss the evolution of this part of the Hercynian belt from the Monts du Lyonnais to the Cévennes al lithospheric scale.

Possible upper mantle reflection fabric on seismic profiles from the Tethyan Himalaya: Identification and tectonic interpretation

Upper mantle structures of the Himalaya collision system are not well‐defined, yet are integral to our understanding of the lithospheric scale processes which have built the Tibetan plateau. A 100‐km‐long deep seismic reflection profile (Tib‐1) and a short cross line (Tib‐2) were recorded within the Tethyan Himalaya to help constrain the geometry of the crust and mantle. At long travel times, the returned seismic energy is especially weak compared to the noise; therefore the data were reprocessed with signal‐to‐noise enhancing techniques to identify coherent events at mantle travel times. Visual comparisons of shot gathers, recordings of the ambient noise, common midpoint stacks, and synthetic sections suggest that a dipping reflection “fabric” is present in the upper mantle. On the basis of geometrical and velocity constraints, the fabric is not likely related to sideswipe, P‐to‐S phase conversions, multiples, or surface waves. The fabric is most likely within the underthrust Indian lithosphere beneath the peaks of the High Himalaya. The fabric strikes ∼N63°E, significantly oblique to the ∼E‐W strike of crustal deformation; however, the fabric dip direction (∼44°, N27°W) is essentially the same as the fast direction of polarization inferred from a previous SKS shear wave splitting study. The crust versus mantle strike discordance suggests that the fabric may predate the collision of India with Tibet and mark previous deformation of the Indian lithosphere (e.g., rifting episodes that formed the Tethyan ocean). Conversely, if the fabric is the result of collision, the discordance would suggest that mantle deformation is decoupled from crustal deformation within the Himalayas.

Do the Billecocha normal faults (Ecuador) reveal extension due to lithospheric body forces in the northern Andes?

Active compressional and transcurrent structures are widely reported for the entire northern Andes, whereas there is no clear evidence of extensional tectonics in that region. In contrast, both active compressional and extensional structures occur extensively in the Central Andes. Extensional tectonism in the central Andes occurs mainly in high plateaus of large wavelength (≥ 100 km), and is interpreted to have resulted from the effect of compensated high topography at the lithospheric scale. The observation of Holocene normal faults on the low wavelength (≤ 15 km) Billecocha high plateau in the northern Ecuadorian Andes raises a problem. Could the lithospheric body forces be the cause of the normal faulting in a chain with a mean altitude of ≤ 2800 m? Compared to the central Andes, where normal faulting is widely distributed across the chain, normal faulting on the Billecocha plateau is concentrated in a restricted 4-km-wide zone. In addition, normal faulting does not extend further than a few kilometres eastward, where compressive structures have been observed. Considering the local character of the extension, body forces at the lithospheric scale cannot be responsible for this normal faulting, and thus we can preclude a significant lowering of the magnitude of σH max in the northern Andes with respect to the central Andes. Analysis of these normal faults shows that normal faulting occurred between 10,000 yr and 6000 yr B.P. In addition, the relationship between the geometrical parameters (vertical throw and rupture length) of the major normal fault differs significantly from that given empirically by methods based on scale laws. In such a case, the Billecocha normal faults are unlikely to have a tectonic origin. Various local processes (e.g., gravity gliding, “sackung”) that may be possible are contemplated for triggering the normal faulting. In addition, a theoretical simple 2-D model is proposed in where local conditions peculiar to the plateau (structure, glaciation) and the surrounding areas are taken into account. In this model, normal faulting occurs by shearing along the planes of weakness (bedding planes) in response to the elastic rebound of the fluid-saturated plateau, which prevailed after the last glaciation.

Spacing of consecutive normal faulting in the lithosphere: A dynamic model for rift axis jumping (Tyrrhenian Sea)

We propose that the flexure of a broken elastic plate can determine the spacing of consecutive and localized zones of shearing on a lithospheric scale. The model provides a dynamic explanation for rift axis migration and, as a consequence, for a ‘wide rift’ mode of extension. Calculated values for rift axis shifting are consistent with the available observations and kinematic modelling results in the Tyrrhenian Basin. The quantitative relation obtained between lithospheric fault spacing and the effective elastic thickness (T e) of the lithosphere can be used in an inverse modelling approach, in which paleo T e values are calculated for a basin where the diachronous and spaced nature of rifting is known.

Mantle exhumation at passive margins

Laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent-ocean boundary. Four-layer models were constructed with sand and silicone putty, to represent the brittle and ductile layers, respectively, of both crust and mantle. It is shown that necking of the whole lithosphere model is nearly symmetrical (pure shear) but that asymmetrical structures (simple shear) develop internally, due to heterogeneous boudinage and/or faulting of brittle layers. Relative movements between brittle layers is accommodated by layer-parallel shear in the ductile layers defining a set of conjugate shear zones. With increasing stretching, lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer. The experimental results are used to propose a general model of passive margin formation leading to mantle exhumation. They are also compared to the situation observed on the west Iberia margin. The exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale. Shearing of exhumed mantle rocks does not correspond to detachment faults cross-cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high-strength sub-Moho mantle.

A Lithospheric Cross-Section Through the Swiss Alps-I. Thermokinematic Modelling of the Neoalpine Orogeny

In this paper we develop a forward 2-D thermokinematic model to investigate the Neoalpine 35-0 Ma phase of orogeny along the European Geotraverse (EGT) through the Swiss Alps on a crustal and lithospheric scale. Using a divergence-free kinematic model (div v=0), we define mass displacements, which subsequently serve as input to a transient thermal model. the thermal model uses critically assessed material prorameters and accounts for the depth dependence of the thermal properties in processes such as crustal thickening and mantle-lithospheric subduction. Based on the presentday density pattern of the deep seismic image and estimated exhumation and shortening rates, we derive, in a first modelling step, a mass-displacement field describing the Neoalpine orogeny as a uniform process in time. In a second—thermal—modelling step, this kinematic scenario is further refined by modelling the non-uniform cooling histories of the southern Lepontine in the Penninic domain. For that purpose we adopt lithospheric shortening rates—and consequently exhumation rates—to agree with total Neoalpine shortening, while keeping the geometry of the kinematic model fixed. the resultant thermokinematic model reflects the main characteristics of Neoalpine tectonics, and shows a good overall agreement with combined geological and geophysical data. the asymmetric feature of the present-day tectonic structure along the profile is strongly reflected in the thermal structure of the lithosphere. This demonstrates the need for a kinematic model to investigate the deep-temperature field in active tectonic provinces. For further refinement of the model, the amounts of shortening have to be more precisely estimated, and a higher spatial density in geochronological and metamorphic data is required. Furthermore, surface heat-flow values are, up to now, too uncertain to constrain the predicted surface heat flow. In summary, our results show that we need, in particular, data constraining the horizontal component of the tectonic and thermal evolution. the results of the Neoalpine orogeny modelling demonstrate that the presented thermokinematic procedure yields a good first-order approximation to investigate crustal-scale and lithospheric processes. We conclude. therefore, that the approach presented provides the potential for application not only to continent-continent collision zones, but also to any active tectonic province.

Distribution of volcanoes in active margins

We report a comprehensive study of the distribution of volcanoes in 16 active plate margins, corresponding to a total of 479 volcanic systems. The active volcanic arcs are found to have a ribbon geometry with an average length / width ratio of around 10. The shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the other. The projection direction agrees with the direction of plate convergence, which shows that plate motion is the main factor constraining magma generation on the lithospheric scale. There is no characteristic spacing between volcanoes. In each arc, the distribution of volcano spacing is best represented by a Gamma distribution which corresponds to randomly generated points in the same geometrical conditions. In order to explain how such distributions may be generated, we have investigated in the laboratory the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid. Different dynamic regimes are reached for different values of the viscosity ratio between the two fluids. For low viscosity contrasts, there is only one initial instability event, with buoyant plumes organized in a periodic pattern. These plumes are fed continuously by the source, even at large times. For high viscosity contrasts, which is relevant to the geological problem, plumes are produced intermittently by individual instability events. They take the form of “cavity” plumes fed by narrow tails. Once the plumes have reached the surface, they continue to be fed from below by material rising through their shrinking tail. In successive instability events, plume generation is not repeated at the same locations which leads to a complex plan‐form. With time, and hence with increasing number of instability events, the cumulative distribution of plume spacing changes from periodic to random. Material from a single plume is able to sustain volcanic eruptions for a period longer than the time between two instability events, and hence the distribution of active volcanic centers reflects the cumulative distribution of several instability events. This is consistent with the observed distributions.

Extension of stable continental lithosphere and the initiation of lithospheric scale faults

We address the physical conditions which control the style of continental extension. Geological evidence suggests that once lithospheric scale zones of localized deformation have been formed, they strongly affect continental deformation. It is the purpose of this paper to investigate mechanisms which may cause lithospheric scale faults to initiate in stable continental lithosphere which is laterally fairly homogeneous. Faults and shear zones cutting strong layers in the lithosphere will have a very significant influence on the evolution during extension. Based upon experimental flow laws, a strength maximum can be expected in the mantle directly beneath the Moho. Strain localization in the shallow upper mantle is therefore expected to have a very pronounced effect on the evolution of the extending lithosphere. Low viscosities in the lower crust decouple the crust mechanically from the upper mantle. Therefore causes for strain localization in the sub‐Moho mantle must be found in the mantle itself. Two potential causes satisfying this requirement are boudinageing and strain weakening. We use thermal‐mechanical finite element models which incorporate the elastic, visco‐plastic, and viscous response of lithospheric rocks. The results of our model experiments suggest that boudins do not evolve during extension of continental lithosphere in most situations; only when the extension rate is fast relative to thermal reequilibration may homogeneous boudinage result. By its very nature, however, this type of boudinage cannot produce lithospheric scale faults. Our model results suggest that shear zones may evolve after strain weakening. However, the style of extension on the scale of the lithosphere is pure shear like because the shear deformation in localized zones is balanced; in most cases the shear zones occur in conjugate pairs. Initiation of lithospheric scale faults is concluded to be unlikely in stable and homogeneous lithosphere in interior parts of continental plates.

Project conducts seismic reflection profiling in the Ural Mountains

Geophysicists from east and west are flocking to the Uralide orogen of Russia to acquire modern, multichannel deep seismic reflection profiles over targets in the Ural Mountains. Seismic reflection profiling promises to help answer geological and geophysical questions about the structure of the mountain belt at crustal and lithospheric scales, the development and evolution of a crustal root, the drift of the continents involved in the collision and tectonic evolution of the orogen, and the recent uplift history.EUROPROBE, a European Science Foundation (ESF) scientific program that is seeking to unravel key aspects of the tectonic evolution of Europe, involving both eastern and western researchers, is leading the effort to acquire profiles. Going a step further, EUROPROBE is collaborating with Consortium for Continental Reflection Profiling (COCORP) scientists in the United States to develop new, integrated geological and geophysical studies of the Urals. Priority is being given to investigating the relationships between the upper and lower crustal structure.

Localization of Martian ridges by impact craters: Mechanical and chronological implications

The eastern flank of the Tharsis dome (Mars) is affected by compressional ridges. Two conceptual models exist for these structures: (1) the ridges are the expression of a superficial shortening and are rooted on a rheological boundary located in the megaregolith or (2) the ridges result from a global shortening on a lithospheric scale and are rooted on the deep thermal brittle‐ductile transition inside the lithosphere. It is shown that the position of the ridges is strongly influenced by the existence of impact craters. The distribution of crater‐ridge distances has been measured in the Coprates area. This distribution shows that the intersection of ridges and impact craters with a diameter larger than 4 km is four times as frequent as a homogeneous and random distribution of craters relative to what ridges would produce. Very detailed geometric studies of relationships between intersecting ridges and craters show that these craters are older than the ridges. This localization implies that (1) ridges are superficial structures rooted at shallow depths in the megaregolith, and (2) the ridging phase occurred some time after the deposit of the ridged plain unit material, probably during the upper Hesperian. Detailed geometrical studies of ridge edges and graben/ridge width relationships suggest that ridges are bounded by two reverse faults with different offsets, and that the rheological boundary on which ridges and graben are rooted is around 1.5 km deep. Three ways are possible for an impact crater to localize strain in a regional uniaxial compressional stress field: (1) the strength of impacted material is lower than the strength of the surrounding nonaffected rocks, (2) the lithostatic pressure underneath the crater is lower than the lithostatic pressure at the same depth in the surroundings due to the existence of the crater cavity, and (3) the stress is concentrated on the crater borders if craters can be assimilated to holes in a shortened elastic plate. In all three cases, the initiation of the ridges occurs in the area where a low‐strength and/or stress concentration envelope around the crater is nearest to the low‐strength decollement level.

Miocene emplacement and deformation of the Konga Shan granite (Xianshui He fault zone, west Sichuan, China): Geodynamic implications

The presently active sinistral Xianshui He strike-slip fault (XSH) is a lithospheric scale strike-slip fault in the eastern Himalaya. In the study area this fault affects the eastern edge of the Konga Shan granitic massif, where it has caused both brittle and ductile deformation. A Rb Sr isochron and Nd and Pb isotope study of three samples, and a U Pb zircon study of a single sample, were completed on the granite. U Pb data indicate a granite emplacement age of 12.8 ± 1.4 Ma. The Rb Sr isochrons show that the granite emplacement and the deformational event were synchronous, at around 12-10 Ma (minimum age for deformation). The Nd and Pb isotope compositions of whole rocks and K-feldspars indicate the involvement of Proterozoic continental crust, which is confirmed by U Pb systematics indicating inherited zircons. Sinistral faulting along the XSH began at the latest at 12 Ma and marks the extrusion toward the east of the West Sichuan and South China blocks, following their extrusion along the Red River fault zone between 50 and 21 Ma.

An arbitrary Lagrangian-Eulerian formulation for creeping flows and its application in tectonic models

SUMMARY This paper presents and analyses numerical techniques developed to investigate viscoplastic Stokes flows within a model of lithospheric deformation. In particular, the techniques are related to a subduction model of compressional orogens. The driving mechanism in the model corresponds to the near-rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust. The numerical techniques use the arbitrary Eulerian-Lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving Eulerian finite-element grid that conforms to the material domain. A regridding algorithm allows the associated Lagrangian motion and fields to be followed, and, in addition, coupled back to the Eulerian calculation of the flow. Mass-flux boundary conditions are used so that the effects of erosion and deposition by surface processes, and mass loss by subduction can be included in the model calculation. The evolving model crustal layer is flexurally compensated using a general elastic beam formulation. The applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed. Simple flows, a linear viscous subduction model, a whirl flow, and a quasi-convection model are used to show that the mass conservation, regridding and surface tracking errors are small. The broader applicability of the modelling techniques is reviewed.

The Ronda peridotites (Los Reales nappe): An example of the relationship between lithospheric thickening by oblique tectonics and late extensional deformation within the Betic Cordillera (Spain)

The Internal (or metamorphic) Zone of the Betic Cordillera (southern Spain) is a 400 km long and ENE-elongated tectonic domain characterized by a widespread record of alternating compressional and extensional processes. In the westernmost part of the Internal Zone several orogenic Iherzolite massifs—the Ronda peridotites—were also involved in such processes. The Ronda peridotites form the lower portion of the Los Reales nappe and are overlain by a metamorphic sequence which varies from granulite- to greenschist-facies conditions. The tectono-metamorphic history of the Los Reales nappe involves two main synmetamorphic steps. A first extensional event at lithospheric scale is preserved within the attenuated continental crust which overlies the Ronda peridotites. It was related to the upwelling of an asthenospheric diapir below the thinned lithosphere, and led to the development of low-pressure-high-temperature metamorphism and of ductile shear zones with N110°E-trending stretching lineations. A subsequent compressional event is represented by the hot thrusting of the Ronda peridotites from the west-southwest to the east-northeast. Finally, late extensional deformations in a dominant N-S direction developed in response to the lithospheric thickening achieved during the former compressional event. The kinematics of the synmetamorphic deformations can be integrated into a model of sinistrally oblique tectonics compatible with the overall strike-slip motion of the African and Iberian plates from Jurassic to Tertiary times, whereas the later deformations should reflect the convergence between Africa and Iberia from Tertiary time onwards.

Rifting, erosion, and uplift history of the Recôncavo‐Tucano‐Jatobá Rift, northeast Brazil

The Recôncavo‐Tucano‐Jatobá (RTJ) Rift and many other smaller sedimentary basins in northeast Brazil formed during South Atlantic rifting and were subsequently uplifted and exhumed so that Albian marine sediments are now located up to 800 m above sea level some 400 km inland from the Atlantic margin. Local erosion caused by footwall uplift and regional erosion, probably resulting from magmatic underplating and uplift, has removed a large part of the thermal sag phase sediments from the RTJ Rift. The flexural cantilever model, incorporating the flexural isostatic response to simple‐shear faulting in the upper crust and pure‐shear necking of the lower crust and upper mantle, can explain the main geometrical features observed in the RTJ Rift without resorting to lithosphere scale detachments. The model has also been used to estimate the amount of uplift and erosion of the faulted rift flanks. Rift flank erosion can produce uplift and erosion across the whole RTJ Rift, thus explaining the postrift unconformity which preceded deposition of the Aptian age Marizal Formation. The largest uplift is predicted to occur at the edges of the basin and may explain the anomalously shallow depth to onset of oil generation at the basin margins deduced from vitrinite reflectance data. The model also predicts that maximum burial of much of the basin fill in the RTJ Rift occurred at the end of rifting and not during the postrift infill, as is usually the case. The amount of observed coarse conglomeratic detritus in the Recôncavo subbasin suggests that about 25% of the eroded footwall detritus was deposited and preserved in the adjacent hanging wall half‐graben, with the rest being transported more distally as finer material.

Relationship between deeper lithospheric processes and near-surface tectonics of sedimentary basins

Interactions between upper lithosphere deformation, as manifested in the structure and evolution of sedimentary basins, and lower lithospheric deformation presumably reflect large-scale regional or global tectonic events. We review the current state of knowledge regarding these interactions and suggest areas of research which may lead to an improvement in this knowledge. Understanding the development of sedimentary basins requires insight into the processes responsible both for basin subsidence and also for distribution and preservation of sediments within the basins. While some classes of basins are better understood than others, there remain significant gaps both in our ability to account for available data and also in the data available to test current models and develop new ones. Subsidence in some classes of basins can be related to lithospheric scale processes occurring as part of the overall plate tectonic paradigm. For example, it is widely accepted that passive margin basins subside as a result of lithospheric extension (McKenzie, 1978) and that foreland basins are flexural consequences of collision between a continent and outboard terranes (Beaumont, 1981). On the other hand, models for the origin of intracratonic, forearc and trans-tensional basins are essentially still at cartoon level, involving processes that are difficult to quantify and evaluate. In the following we review a number of fundamental questions of vital interest for understanding of the relationship between deeper lithospheric processes and near-surface tectonics.

Stretching and subsidence: Rifting of conjugate margins in the North Atlantic Region

Subsidence on rifted conjugate continental margins around the North Atlantic is analyzed to derive the amount and areal distribution of stretching in the crust and in the lower lithosphere during continental rifting. Study areas are the Grand Banks and Orphan Basin regions of the eastern Canadian continental margin and the Goban Spur and Galicia Bank regions off western Europe. In all areas, maps of synrift and postrift sediment thickness and bathymetry were used to derive maps of post‐ and synrift subsidence. A two‐layer lithospheric stretching model with independent amounts of stretching in the crust and in the lower lithosphere was assumed to be applicable, with the rifting history approximated by several instantaneous episodes of extension. This model was used to derive estimates of stretching at all points on a 0.05° geographical grid, where subsidence values were available within the study regions. The models are constrained with seismic measurements of crustal thickness. The results imply that pure shear stretching predominates at a lithospheric scale, while simple shear is more localized laterally and confined to the crust. In places there is significant decoupling between crustal and mantle stretching. Near the continent‐ocean boundary, final continental breakup may be localized on one side of the rift between conjugate margin pairs, rather than symmetrically located. Total extension of the margins is compatible with that estimated from normal fault geometries and indicates that the continent‐ocean boundary has been extended up to 350 km seaward of its original position, which should be considered in plate kinematic reconstructions.

Development of the North Viking Graben: inferences from laboratory modelling

The North Viking Graben is a classic sedimentary basin in the North Sea that has been thoroughly studied over the last decade. Despite the amount and quality of seismic and well data now available, the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate. We present the results of small-scale laboratory modelling. The models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale. The experiments were designed to study the effects of: (a) the degree of coupling between brittle and ductile layers; (b) the obliquity of extension; and (c) the reactivation of inherited crustal-scale structures. The results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales, as well as the relationship between sedimentation and deformation. It is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive. A model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level. In addition, this model accounts for the nearly horizontal stratification of Lower Cretaceous sediments, the basal Cretaceous unconformity which caps many of the normal faults, and the localization of deformation to the west of the graben during the Lower Cretaceous.

Stress transfer and seismic instabilities in the upper crust: example of the western Pyrenees

When the regional state of stress is close to the condition for earthquakes to occur, small stress and strain changes could trigger seismic instabilities. Stress diffusion ( Δσ = 1–2 MPa) has been suggested to explain, at lithospheric scale, space and time correlated earthquakes and surface deformations in subduction zones (100–200 km, 10–40 years). We test the possible regional mechanisms, at the upper crustal scale, in the western Pyrenees, where major regional events (M max = 5.7), locally induced events and subsurface fluid pressure fluctuations are temporally and spatially correlated. In a first phase, fluid withdrawal (decrease of pore pressure of the aquifer at the gas-water interface) affects the natural regional seismicity rate. The locked fault inhibits small events for a time, during which tectonic stresses accumulate. Major regional events then occur on the Pyrenean Fault, when the energy of deformation is large enough to allow seismic instabilities to occur. These major events (M 1 > 5) trigger seismic instabilities (M 1 = 4), in the vicinity of the gas extraction area, 30 km away. As a result of gas extraction, the Lacq field zone has local weakness characteristics similar to the neighbouring main active fault. The Pyrenean earthquakes lead the gas field events by 2 years. This correlation is consistent with a viscoelastic model, where salt rock acts as a viscoelastic channel, below an elastic bed. The possible connections proposed here may give a new insight worldwide in understanding the role of fluids in seismic instabilities and seismic risk assessment in the neighbouring of man-made fluid pressure fluctuations, when natural seismic activity is high.

Three-dimensional SeaMARC II, gravity, and magnetics study of large-offset rift propagation at the Pito Rift, Easter microplate

We present results from a SeaMARC II bathymetry, gravity, and magnetics survey of the northern end of the large-offset propagating East Rift of the Easter microplate. The East Rift is offset by more than 300 km from the East Pacific Rise and its northern end has rifted into approximately 3 Ma lithosphere of the Nazca Plate forming a broad (70–100 km) zone of high (up to 4 km) relief referred to as the Pito Rift. This region appears to have undergone distributed and asymmetric extension that has been primarily accommodated tectonically, by block faulting and tilting, and to a lesser degree by seafloor spreading on a more recently developed magmatic accretionary axis. The larger fault blocks have dimensions of 10–15 km and have up to several km of throw between adjacent blocks suggesting that isostatic adjustments occur on the scale of the individual blocks. Three-dimensional terrain corrected Bouguer anomalies, a three-dimensional magnetic inversion, and SeaMARC II backscatter data locate the recently developed magmatic axis in an asymmetric position in the western part of the rift. The zone of magmatic accretion is characterized by an axis of negative Bouguer gravity anomalies, a band of positive magnetizations, and a high amplitude magnetization zone locating its tip approximately 10 km south of the Pito Deep, the deepest point in the rift area. Positive Bouguer gravity anomalies and negative magnetizations characterize the faulted area to the east of the spreading axis supporting the interpretation that this area consists primarily of pre-existing Nazca plate that has been block faulted and stretched, and that no substantial new accretion has occurred there. The wide zone of deformation in the Pito Rift area and the changing trend of the fault blocks from nearly N-S in the east to NW-SE in the west may be a result of the rapidly changing kinematics of the Easter microplate and/or may result from ridge-transform like shear stresses developed at the termination of the East Rift against the Nazca plate. The broad zone of deformation developed at the Pito Rift and its apparent continuation some distance south along the East Rift has important implications for microplate mechanics and kinematic reconstructions since it suggests that initial microplate boundaries may consist in part of broad zones of deformation characterized by the formation of lithospheric scale fault blocks, and that what appear to be pseudofaults may actually be the outer boundaries of tectonized zones enclosing significant amounts of stretched pre-existing lithosphere.