Decollement Layer Research Articles

Numerical models of the inversion of half‐graben basins

We investigate the dynamic evolution of fold and thrust structures which form by compression and inversion of a sequence of half‐graben basins. The choice for a half‐graben geometry is motivated by seismic studies and reconstructions of preinversion geometry of inverted regions, which show that rifting often leads to a series of half‐grabens. To examine the deformational structures which result from basin inversion, we use a two‐dimensional, viscous‐plastic numerical model and start our experiments from a preexisting extensional geometry. We find that synrift and postrift sediments are uplifted in the initial stages of basin inversion. This uplift is accompanied by rotation of the basement blocks beneath the basins. In the postrift sequence new shear zones form which are a continuation of basin‐bounding faults. With continuing shortening, further inversion is more difficult owing to relative strengthening of the half‐graben region. Significant surface erosion facilitates inversion. Back thrusts mainly develop in association with listric basin‐bounding faults and less for planar (domino array) faults. Weak sediments (such as salt or shales) at the base of the basins promote the development of basement shortcut faults. The presence of a postrift decollement layer tends to decouple deformation of the postrift sediments from the material below it. In our model, preexisting weak basin‐bounding shear zones are a requirement for substantial amounts of basin inversion to occur. Our numerical model results display many first order characteristics of examples from nature and analogue studies.

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seismic reflection profiling

Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north–south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en échelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean–Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area.

Growth fault/rollover systems: Birth, growth, and decay

Growth fault/rollover systems are extremely common structures of thin‐skinned extensional systems resulting from gravity gliding above salt. Their development is studied using laboratory experiments based on brittle‐ductile models where the sedimentary cover and synkinematic sediments are simulated using brittle sand and the underlying decollement layer by viscous silicone putty. The experiments are compared to natural examples from seismic lines. The experimental results are used to present the evolution of growth fault rollover systems, these being the result of a finite amplitude instability with three distinct stages: birth, growth, and decay. Each of these stages is identified with typical structures which can be recognized on seismic sections. A discussion is presented on the interactions in time and space between synthetic and antithetic listric faults parallel and transverse to the direction of displacement.

Recent tectonic activity in the South Barbados prism Deep-towed side-scan sonar imagery

Data collected during the DIAPISAR Cruise (Nov. 1990) using the French deep-towed side-scan-sonar (S.A.R.) are presented. These data provide clear evidence of recent tectonic activity in the southern part of the Barbados accretionary prism. Many markers as linear lineaments or ‘sigmoidal features’ which are observed on diagenetically indurated sea-floor are the surficial traces of faults or folds and networks of conjugate fractures where fluids are released through the structures and provide very active venting. These markers can serve as a basis for detailed mapping of some of the tectonic structures and for an analysis of their kinematic evolution. Their geometry suggests the occurrence of reworked folds and shear zones. This domain is affected by abundant clay-diapiric tectonics due to the interaction between: (1) the sedimentological context characterized by the important terrigenous supplies coming from the Orinoco River and building a wide prodelta in which levels of clay supersaturated in water can act as decollement layers, and (2) the geodynamic context of the area situated in the connection zone of three different types of margins. The data presented show that, on the eastern end of the South Caribbean wrench fault system, shear stresses can rework either ‘en echelon’ folds associated with this system or compressional structures due to subduction. The most recent structures seem largely due to the influence of shearing and correspond to transpressive or transtensive deformations rather than pure compressional structures.

Hinterland-Vergent Tectonic Wedge Below the Riwat Thrust, Himalayan Foreland, Pakistan: Implications for Hydrocarbon Exploration

The Riwat thrust, with a surface trace of over 50 km, is one of the major faults in the footwall of the main boundary thrust in the Himalayan foreland of Pakistan. Surface geology shows that the Riwat thrust is a foreland-vergent thrust along which lower to middle Siwalik molasse strata are thrust southward over upper Siwalik strata. Seismic reflection interpretation shows that the Riwat thrust developed as a roof thrust of a hinterland-vergent tectonic wedge (triangle zone) underlain by evaporites. The Riwat thrust propagates upsection from a depth of about 4 km at the base of the Siwalik Group. At this depth, it merges into a hinterland-vergent blind thrust that propagates upsection as a ramp from Eocambrian evaporites covering the basement at a depth of about 6 km. Bounded between this set of conjugate faults, a tectonic wedge of Eocambrian (evaporites) to Neogene strata is thrust toward the hinterland to form a triangle zone. The roof thrusts of triangle zones have been widely mapped as backthrusts in deformed mountain fronts. Hinterland motion of tectonic wedges as in the Riwat thrust triangle zone may be a feature of the fold-and-thrust belts underlain by evaporites acting as an extremely weak decollement layer. Their recognition, with a trap-forming geometry below a thrust, is important for interpreting particular fold belts and for hydrocarbon exploration. These structures could be predicted by the surface geology data where hinterland vergence of a fold below a thrust is apparent; however, seismic reflection data appear to be critical in recognizing these structures.

Influence of mechanical stratigraphy on folding vs faulting

The competing processes of folding and faulting during shortening of a layered sequence are investigated using elastic-plastic models, in order to determine the influence of mechanical stratigraphy on the development of fold belts vs thrust belts. The models consist of a strong middle layer and weaker décollement and cover layers. The mechanical behaviour of each layer is determined by its plastic hardening modulus h and its elastic shear modulus G. With bonded contacts, fold amplification at low shortening strain (relatively large h) is greatest for a thin cover layer and a thick décollement layer. At higher shortening strain ( h ⪡ G), the middle layer may reach its peak strength before the other layers, causing it to become relatively weaker in shortening and causing fold amplification to be greatest for a thick middle layer. Free-slip contacts between the layers enhance folding and fold amplification is greatest if the slip surfaces are roughly equally distributed throughout the sequence. A relatively weak décollement layer enhances folding, consistent with the fact that fold belts usually include a salt décollement layer. Dominant wavelengths increase with increasing strength contrasts between the layers and with increasing thicknesses of the décollement and strong layers.

Inversion of the Broad Fourteens Basin (offshore Netherlands), a small-scale model investigation

The southern North Sea exhibits spectacular examples of graben inversion which have been extensively documented by oil industry exploration. The Broad Fourteens Basin, which is located at the southern limit of the Southern Permian Salt Basin, displays numerous structural patterns of positive inversion, which are related to the inversion, which are related to the presence and thickness of Zechstein salts. In the northern part of the Broad Fourteens Basin, the Zechstein salt is rather thick (> 500 m), allowing decollement of the Mesozoic series during both extension and compression. Salt is missing to the southern part of this graben, and both Mesozoic series and Palaeozoic ‘basement’ display similar patterns of deformatiqn. The mechanical significance of inversion patterns was investigated through laboratory experiments on brittleductile models. Models were built with sand and silicone putty that represent, respectively, the frictional behaviour of the Mesozoic cover as well as the Palaeozoic basement, and the viscous behaviour of a decollement layer, mainly composed of Zechstein salts, which is intercalated between them. The models were scaled to fit natural configurations observed in the Broad Fourteens Basin. All small-scale experiments were carried out in two steps: (1) graben formation with synkinematic sedimentation, and (2) compression at various angles of obliquity to the graben. Experimental results show that the structures generated by or reactivated during inversion are strongly dependent on: (a) the strength of the decollement layer, which is a function of the layer thickness and of the displacement velocity applied at the model boundaries; and (b) the strength of the sedimentary overburden, which is a function only of its thickness (for a given lithology). Reactivation of normal faults occurs when the angle between the compression and the direction of the normal faults is smaller than 45. The ductile layer, which can be a source for diapirs during extension, plays during compression an important rô1e in the thrusting of the graben fill onto the adjacent platforms.

Seismic Modeling of a Strongly Emergent Thrust Front, West Spitsbergen Fold Belt, Svalbard

To improve our understanding of the seismic signature of contractional structures in foreland fold-and-thrust belts, seismic modeling of a strongly emergent thrust front was carried out. The modeled structures are a set of well-exposed imbricated fault-bend folds with steep forelimbs and moderately dipping backlimbs, occurring in silicified Upper Permian carbonates and schists. These structures, present at the frontal part of the West Spitsbergen fold-and-thrust belt, are developed above a decollement layer in Permian evaporites. To simulate a synthetic migrated seismic section, image ray modeling was carried out. The acoustic impedances used in the modeling were obtained from laboratory measurements of rock samples. A significant apparent structural (time high), a well as non-real folds and faults, were created by the seismic reflection technique. Without the excellent field control, these structures would have been interpreted as real, indicating deformational features also structurally below the sole thrust in the area. The time high would most certainly survive a traditional depth conversion procedure, and could easily have been misinterpreted as a hydrocarbon trap. High-quality seismic reflection profiles in Isfjorden, paralleling the contraction direction and along strike from the modeled onshore section, show many of the same structural features as in the synthetic model. The seismic modeling demonstrates that care should be exercised when interpreting seismic sections in areas with contractional structures, and suggests that modeling of xposed structural analog is a necessary supplement in detailed seismic studies of such complex contractional geometries.

Laboratory Modeling of Graben Inversion with Application to Broad Fourteens Basin, Netherlands Offshore

The southern North Sea presents spectacular examples of basin inversion, which have been documented by numerous projects of the oil industry. Some basic inversion patterns identified through wells and seismic data were used to prepare a laboratory modeling investigation. Models are built with sand and silicone putty, respectively, which represent the frictional behavior of Mesozoic cover and Paleozoic basement and the viscous behavior of the decollement layer, mainly Permian salt, between them. They are scaled to fit natural configurations observed in the Broad Fourteens basin. All experiments are done in two steps: (1) graben formation with synkinematic sedimentation and (2) compression oblique to the graben. The experiments show that structures generated by or reactivated during inversion are strongly dependent on the strength of the decollement layer at the base of the sedimentary cover, which is itself dependent on the silicone viscosity, the layer thickness, and the displacement velocity applied at model boundaries; and the strength of the sedimentary cover, which is solely dependent on its thickness. This work is in progress. Preliminary results will be compared with examples from the Broad Fourteens basin on the basis of both seismic data and structural maps.

Fluid flow along a decollement layer: A Model applied to the 16°N section of the Barbados Accretionary Wedge

A seismic section across the whole Barbados accretionary complex at 16°N recently published by Westbrook et al. (1988) shows that the reflection marking the decollement fades between 40 and 60 km arcward of the deformation front but is present again further inland. This interruption appears to be related to a thrust cutting through the wedge. One‐dimensional steady state models of water flow along the decollement suggest a mechanism responsible for decollement disruption: a local increase of the slope either of the wedge, or of the decollement, causes the subducted layer to compact, and locally increases the friction along the decollement. If the thrust is used as a conduit for fluids to escape from the decollement, the friction along the decollement toeward of the thrust is further increased. The model also shows that the permeability of the decollement should be of the order of 10−14 m2, but may be higher near the deformation front for short periods if fluid expulsion mainly occurs in transient bursts associated with hydraulic fracturing. As a consequence of the non‐linearity of the compaction equation, pore pressure at depth in the decollement depends on the water flux, on the lithostatic pressure gradient along the decollement and on the parameters from which the physical properties of the sediment are calculated. However, it is almost independent on the pore pressure below the toe.

Seismic modeling of the decollement zone at the base of the Barbados Ridge Accretionary Complex

Wide‐aperture seismic reflection profiling of the northern Barbados Ridge accretionary complex at 16°12′N has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection. Synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2.5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity‐density properties along the decollement thrust zone. Modeling requires a thin, low‐velocity layer to develop landward along the decollement beneath the complex, while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex. Two‐dimensional modeling that incorporated lateral variations in topography, velocity, and density was performed on a 7‐km section of the decollement 20 km landward of the deformation front. The best fitting model includes a 20‐m‐thick low‐velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0.2 km/s. Changes in the reflection amplitude along this 7‐km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m. The shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness. The low‐velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure. High fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex.

A model of glacial tectonics, applied to the ice­pushed ridges in the Central Netherlands

During a detailed structural geological and geomorphological survey of ice-pushed ridges around the Gel­derse Vallei (centre of the Netherlands) several questions arose about the origin of these landforms. The Gelderse Vallei is a Saalian glacial basin filled with younger sediments, 40 km long and up to 20 km wide, running NNW-SSE. Thrust sheets which build up the ice-pushed ridges on either side of the valley were transported away from the centre of the basin. They are up to 25 m thick and are pushed to a level about 100 m above the decollement. The thrust sheets, mainly consisting of coarse sand layers, moved as rigid masses, while only the finegrained basal layers deformed by heterogeneous simple shear. Pore water pressure plays an important role by greatly reducing the sliding friction in the decollement layer. Once a glacial thrust sheet is formed, the weight of the upward moving frontal part (the toe) offers a resistance to movement, which can not be overcome by the basal shear stress of the glacier alone. Basal shear stresses in most glaciers are in the order of 0.1 MPa (1 bar) and appear to be incapable of lifting a toe of the size found in most ice-pushed ridges in the Netherlands. The article presents a model of a system ice lobe - substratum, which is to a large degree controlled by gravitational forces. The dilemma may be solved by the concept of the gradient stress field under the marginal area of an ice lobe. This concept was first formulated by Rotnicki (1976). Because ice thickness decreases towards the margin, the substratum is subjected to a decreasing load in the same direction. If the increments of stress difference under a slab of ice in the marginal zone are summed, a gradient stress is arrived at which is di­rected towards the ice edge. An estimate of the forces involved in the static equilibrium around the glacier margin indicates that the contribution by the gradient stress field is sufficient to move the toe. In the model proposed in the article, the energy to move and imbicate glacial thrust sheets around the margins of an ice lobe, is supplied by the continuous flow of ice into the ice lobe. The dimensions of ice­pushed ridges are thus to a high degree functions of the thickness of the ice lobe or icecap. The Gelderse Vallei ice lobe will have been at least 250 m thick to be able to form the ridges. The basal shear stress contributes less to the tectonic transport, the larger the thrust sheets and the high­er the ice-pushed ridges.