Antiformal Stack Research Articles

The Impact of the Bohemian Spur on the Cooling and Exhumation Pattern of the Eastern Alpine Wedge of the European Alps

AbstractFold and thrust belt architecture may be influenced by basement geometry of the downgoing plate. This influence is notoriously difficult to assess due to a common lack of subsurface constraints and low resolution of exhumation estimates in space and time. The Bohemian Spur is a basement high at the transition from the Alps to the Carpathians. It coincides with narrowing of the foreland basin and an orogen‐scale change of strike. Its location in one of the best‐studied orogens in the world makes it an ideal case for understanding how basement topography influences fold and thrust belt tectonics. However, since thermochronological studies were mainly focused on the core of the Alps, timing and amount of exhumation remain poorly constrained in these peripheral parts of the orogen. We present new apatite (U‐Th)/He and fission track data from the wedge above the Bohemian Spur. Thermally reset ages monitor a so far un(der)appreciated phase of prominent Late Oligocene to Miocene cooling, associated with crustal thickening, uplift and erosion during wedge propagation. Pronounced exhumation on the order of 3–4.5 km can be related to basement steps beneath the advancing wedge. The spur acted as a buttress for foreland‐propagating thrusting, pinning deformation and nucleating antiformal stacking and duplexing and thus exhumation above it. We illustrate how along‐ and across‐strike changes of sub‐detachment topography impact wedge propagation and control fold and thrust belt geometries. The buttressing effect accounts for most of the exhumation, while deep‐seated slab dynamics are of subordinate importance for wedge uplift.

Impact of mechanical stratigraphy on deformation style and distribution of seismicity in the central External Dinarides: a 2D forward kinematic modelling study

The External Dinarides fold-thrust belt formed during Mid-Eocene–Oligocene times by SW-propagating thrusting from the Internal Dinarides towards the Adriatic foreland. Although previously considered as structurally quite uniform, recent work reported along-strike contrasting deformation styles in two structural domains within this fold-thrust belt. The two areas with very contrasting deformation styles are separated by the N–S-striking dextral Split-Karlovac Fault, a 250 km long, transpressive transfer fault. The southeastern domain is characterized by a thin-skinned SW-vergent nappe stack in contrast to the northwestern domain, where a set of blind, thick-skinned top-SW thrust duplexes prevail underneath the passive NE-vergent backthrusts. To better understand why the External Dinarides underwent such contrasting along-strike deformation, we reconsidered a temporal and spatial along- and across-strike distribution of Paleo-Mesozoic lithofacies to both sides of the Split-Karlovac Fault and estimated the role of mechanical stratigraphy on deformation styles in this part of the fold-thrust belt. Therefore, we constructed a new 2D kinematic forward model in the western backthrust-dominated domain. Our best-fit forward-modelled balanced cross section across the central Velebit Mtn. portrays a 75 km wide triangle zone. This zone took up at least 47 km of shortening during Eo-Oligocene times. It comprises a set of thin-skinned NE-vergent backthrusts detached in the upper Paleozoic atop a SW-vergent thick-skinned antiformal stack detached in the lower Paleozoic Adriatic basement. The NE-vergent backthrusts likely nucleated at lateral facies boundaries related to extensional half grabens that locally formed during Middle Triassic and Late Jurassic passive margin extension. During the Eo-Oligocene folding and thrusting, the selective inversion of inherited Mesozoic half grabens boundary faults into the NE-vergent backthrusts in the northwestern domain led to the observed along-strike changes in the deformation style of the External Dinarides. A seismotectonic analysis of instrumentally recorded earthquakes suggests contrasting seismic behaviour along the central and southern Velebit transects within the northwestern structural domain. The central Velebit Mtn. triangle structure appears to currently accommodate dominantly strike-slip motion, with reverse faulting being confined to east of the Split-Karlovac Fault. In contrast, seismicity along the southern Velebit cross section appears to be confined to the structurally lowermost parts of the triangle zone and the foreland, while it´s structurally higher parts are less seismically active. Also, a predominance of reverse faulting along this transect suggests ongoing accommodation of shortening in this part. Our results indicate that both the variations in the mechanical stratigraphy and the pre-orogenic structural inheritance obtained during rifting and passive margin stages exert control on contractional structures within the External Dinarides, including the distribution of present-day seismicity.

Crustal structure of the Western Azuero Peninsula, Panama: Insights into the structure of accretionary complexes and forearc ophiolites

ABSTRACT Detailed geologic mapping (639 field stations in ~700 km2) and a ~50 km-long gravity survey (142 stations) in the western Azuero Peninsula revealed two faulted and folded slivers of oceanic crust attached to the trailing edge of the Caribbean Large Igneous Plateau (CLIP). Our new data, along with published geochronology, allowed us to reconstruct the Cretaceous forearc configuration of the trailing edge of the CLIP prior to seamount collision, ophiolite accretion, and whole-margin deformation. The ophiolite in western Azuero is composed of two tectonic slivers arranged in south-verging, imbricated thrust faults that stack a ~73 Ma pillow, flow, and picritic basalt and black chert, together with a ~ 89–93 Ma and older basalt flows and capping red chert sequences. Accretion of these slivers to form a supra-subduction zone ophiolite resulted from the middle Eocene collision and accretion of Galapagos seamounts against the trailing edge of the CLIP. Accreted seamounts are arranged in a north-verging antiformal stack duplex, and below the thrust sheets. Change in kinematics after fission of the Cocos-Nazca Plate during early Miocene times prompted the propagation of the Azuero-Sona fault zone flower structure, favouring the preservation of these slivers of oceanic crust.

Pre‐Subduction Architecture Controls Coherent Underplating During Subduction and Exhumation (Nevado‐Filábride Complex, Southern Spain)

AbstractThe interplay between structural and metamorphic processes operating along the deep plate interface in subduction zones remains elusive as much of the geologic record is recycled into the mantle. In some cases, metamorphosed subducted rocks are underplated and exhumed to the surface, providing critical constraints on structural processes and the rheological evolution of subduction interfaces at convergent margins. One such exhumed high‐pressure/low‐temperature subduction complex is the Cenozoic Nevado‐Filábride Complex (NFC) in Southern Spain. This study presents new data from the NFC that elucidate the syn‐metamorphic deformation, stacking, and underplating of continental slivers along the subduction interface. The structurally lowest NFC dominantly comprises lithologically monotonous Paleozoic metamorphic basement rocks recorded by apatite U‐Pb ages and shows no evidence for large‐scale internal duplications suggesting it behaved as a coherent basement succession during subduction. In contrast, structurally higher levels of the NFC are characterized by the stacking of older‐on younger coherent slices and distinctly different metamorphic ages. These relationships document syn‐subduction structural repetitions and tectonic stacking of imbricate thin slivers (∼100s m) during subduction underplating. Structurally higher levels of the NFC exhibit both Eocene and Miocene metamorphic zircon rims and apatite ages, along with microstructures indicative of relatively higher temperature metamorphism. Large‐scale underplating and antiformal stacking of slivers in the subduction channel can provide buoyancy forces to underplate and assist exhumation. We demonstrate that the presubduction stratigraphic architecture is a key control on the style and timing of deformation and metamorphism, facilitating coherent subduction underplating.

Fault Gouge Dating in the Spanish Pyrenees: Fault Ages, Thrust Propagation Sequence, Wall‐Rock Provenance, and Thermal Constraints

AbstractDating brittle faults by the growth of authigenic illite in fault gouge allows for direct testing of kinematic models of orogenic evolution, such as in‐sequence versus out‐of‐sequence thrusting, and the presence of orogenic “pulses” of deformation on multiple thrusts simultaneously. We present illite gouge ages from five thrust faults along the Ter and Freser rivers in the south‐eastern Pyrenees fold‐and‐thrust belt dated by Ar‐Ar methods. The ages show in‐sequence thrusting for the south‐eastern Pyrenees fold‐thrust belt during the Eocene and early Oligocene, ranging from 49.5 + 4.4 Ma for the inboard Freser Antiformal Stack to 31.9 ± 3.9 Ma for the frontal L’Escala Thrust. Foreland progression of ages is preserved, although overlapping of ages of the frontal thrusts permits the interpretation of a period of more rapid fault displacement around 35 Ma. Shortening rates for the Ter‐Freser section range from 1.2–2.7 km/my, with strain rates ranging from 7.0 × 10−16 s−1 to 1.6 × 10−15 s−1 The latest Paleozoic ages of the cataclastically derived component inferred from the gouge dating methodology are shown to be pooled 2M1 illite/mica ages of wall rock sediments and have geological meaning. These inferred mica ages of the thrust wallrocks in the southeastern Pyrenees indicate Hercynian‐age micas were deposited in the Southern Pyrenean foreland basin as early as the late Cretaceous. Previous thermochronometer and Δ47 temperatures from syn‐faulting calcite veins constrain temperature of clay gouge formation in the Ter‐Freser section at 50–210°C, with most likely temperatures of 50–160°C, consistent with previous studies.

Genesis of the gneissic core complexes in the Arabian-Nubian Shield and its tectonic implications: A regional overview

Metamorphic core complexes (MCCs) are a domed structures cored by high grade gneiss overlain by low grade supracrustal rocks. They are characterized by some common features such as extensional fabrics, ultra high-grade metamorphic facies, low angle normal fault (detachment fault), strike-slip shear zones surrounded the core complexes and ductile mylontitic shear zone (thrust zone) that separate the overlying low-grade rocks from the lower high-grade rocks. There is a debate about the presence or absence of metamorphic core complexes in the Arabian-Nubian Shield (ANS) especially in its northern part. The gneissic complexes in the ANS are considered as strike-slip core complexes like the Qazaz and Krish Domes whereas, those in the Egyptian Nubian Shield (ENS) are interpreted as antiformal stacks (e.g. Meatiq and Hafafit) formed during thrusting, or core complexes formed during orogen-parallel crustal extension. Some metamorphic core complexes are domes or contain gneiss domes within them, but not all gneiss domes possess the essential elements of a true metamorphic core complex. The most important points that negate the existence of MCCs in the ANS are absence of ultra-high grade metamorphic facies, absence of real low-angle normal faults, not all the gneisses have a domal structures, adjacent syn-extensional basins have fill that is older than the gneissic complexes, and models of ANS core complex exhumation include strike-slip faults with slip senses recently found to be inconsistent with the models. The gneissic complexes in the ANS differ from the Cordilleran-type or Aegean-type metamorphic core complexes. The origin of gneiss domes in the ANS is controversial, and many of them are presumably produced by mechanisms other than horizontal extension. During the oblique convergence of East and West Gondwana, the gneissic complexes in the ENS evolved from pure shear to simple shear-dominated transpression due to oblique convergence between East and West Gondwana along the Mozambique belt.

Деформації зони насуву між скибами Зелем’янка і Парашка Скибового покриву Українських Карпат (Гребенівський кар’єр)

In the article, based on own field observations, the deformation structures into the thrust zone of the Zelemianka Skyba (=thrust-sheet) onto the Parashka Skyba (the Ukrainian Carpathian Skyba Nappe) are described. They are exposed in the Hrebeniv quarry located in the Opir River basin (Lviv Region, Skole district). The tectonically disintegrated Stryi Formation (Upper-Cretaceous–Paleocene flysch) characterized by the rigid sandstone blocks (formed mainly as a result of boudinage) placed in a ductile clay matrix is represented in the Hrebeniv quarry. This formation is proposed to be classified as a “broken formation”, which is strongly tectonized, but retain their lithological and stratigraphic identity. It is a transitional element between the weakly deformed strata and tectonic mélange. Their characteristic features are: linearity of the distribution zone; significant monomictic composition and absence of exotic formations; the presence of an intensively tectonized matrix with clastolites (blocks) of less tectonized rocks of the same lithostratum (formation, series); the presence of newly formed hydrothermal minerals in clastolite cracks; the upper and lower contacts limiting the broken formation have a tectonic nature. An intensely tectonized matrix and a weakly tectonized rigid blocks belong to the same stratigraphic unit in a broken formation. The structural features suggest a brittle deformations in the studied rocks. Tectonic processes occurred in the completely lithified deposits. Thrust processes were accompanied by the formation of the duplexes, including antiformal stack duplexes, which are well expressed in the quarry. Duplexes are observed in different parts of the quarry, and the most representative ones are developed in its central section. Horses in the duplexes are characterized by a size of 1 to 3 meters here. They are generally subparallel, which is consistent with monoclinal bedding, however, they sometimes acquire an antiform appearance as a result of tectonic thrusting.

Dinaric up-thrusts in the Pliocene evolution of the Central Apennines thrust belt of Italy: the Montagna dei Fiori structure

AbstractSeveral orogenic belts exhibit regional-scale anticlines characterized by prominent faults in their crestal/forelimb zone. These faults are also a common feature in the Neogene fold-and-thrust belt of the Apennines, where they have been contrastingly interpreted as younger-on-older thrust faults, large-scale strike-slip faults, and pre- or syn-thrusting normal faults. In this study, we analysed a NW–SE-trending fault (Montagna dei Fiori Fault) that affects the hinge-zone/forelimb of the Montagna dei Fiori Anticline. This fold is the outermost exposed contractional structure within the Pliocene–Quaternary antiformal stack of the outer Central Apennines. The integration of stratigraphic and structural data collected during a field geological survey enabled us to reconstruct a multiphase reactivation and deformation along the Montagna dei Fiori Fault. From the novel field data, a different interpretation for the evolution of the Montagna dei Fiori Fault is proposed. The fault originated as a Late Cretaceous – middle Miocene, NE-dipping, Dinaric up-thrust and was later reactivated, displaced and rotated during Pliocene Apennine thrusting and related folding, until assuming a present-day SW-dipping attitude with an apparent normal fault character. This newly proposed Dinaric origin of the Montagna dei Fiori structure is compared with an analogous subsurface example of a Palaeogene–Quaternary structure imaged by seismic reflection profile in the Adriatic foreland. The outcome of this combined field and subsurface investigation provides new elements to unravel the complex evolution of the Apennine thrust belt that developed at the expense of a previously deformed foreland, ahead of the advancing Dinaric chain.

Structural framework, crustal shortening and Cenozoic exhumation in the eastern Himalaya: A review

Significant variations (along strike) in deformation style, rate of crustal shortening, erosion and exhumation are observed in the eastern Himalayan orogen. We review the literature to understand the crustal shortening due to the collision of the Indian and Asian plates accommodated through major thrusts and shear zones. The Main Central Thrust (MCT) is a major thrust system along which hundreds of kilometres of crustal shortening have been estimated in eastern Himalaya. Recesses of MCT in the form of culmination zones are formed in Sikkim-Darjeeling, Bhutan and Arunachal Himalayas. Antiformal stacks in the hanging wall of MCT indicate faster exhumation compared to slow exhumation in the synform at the orogenic front. The folded nappes in eastern Himalaya depict the interplay of erosion and exhumation. We observe that the lateral variation in the orogen may also be attributed to transverse structures that divide the mountain range into sectors with dissimilar structural geometry. The strain variation reflected through crustal shortening along the orogen is accommodated by transverse structures like strike-slip faults. Many of these transverse structures are loci of major earthquakes that struck the region in the past. In the Sub-Himalaya, signatures such as changes of the river courses, upliftment of the terraces and active fault scarps indicate that the south directed post collisional compression is accommodated through the slip along the Himalayan Frontal Thrust (HFT) and its splays.

Paleomagnetism from multi-orogenic terranes is “not a simple game”: Pyrenees' Paleozoic warning

Summary Paleomagnetism is a versatile tool in the Earth sciences: it provides critical input to geological time scales and plate tectonic reconstructions. Despite its undeniable perks, paleomagnetism is not without complications. Remagnetizations overprinting the original magnetic signature of rocks are frequent, especially in orogens which tend to be the areas with better rock exposure. Unraveling the magnetic history of the rocks is a complicated task, especially in areas that underwent several orogenic pulses. In turn, constraining the timing of remagnetization represents an opportunity to solve post-magnetization structural and tectonic kinematics. Here, we evaluate the magnetization history of Silurian-Devonian carbonates from the Axial Zone of the Pyrenees. The Pyrenees are a multi-orogenic mountain belt where Silurian-Devonian rocks have seen the Variscan collision (late Paleozoic), the opening of the Atlantic/Bay of Biscay (early Cretaceous) and the Alpine orogeny (late Cretaceous to Miocene). Our results show widespread remagnetization(s) carried by magnetite and pyrrhotite in the Silurian-Devonian series of the Pyrenees. The majority of the samples show a post-folding but pre-alpine tilting magnetization. Considering the equatorial inclinations found in such samples, we suggest that they likely acquired their magnetization during the late Carboniferous and early Permian times. Two of the studied sites (located at the western Axial Zone) were subsequently remagnetized at the end of the Alpine orogeny. The paleomagnetic results constrained that the Variscan orogeny was responsible for the main folding event affecting Paleozoic rocks in the Axial Zone, whereas the Alpine orogeny produced the large-scale thrusting and antiformal stacking of these units. In addition, we observed a general clockwise rotational pattern which could be related with the formation of the Cantabrian Orocline and/or rotations associated with the Alpine orogeny. The Silurian-Devonian carbonates are thus useful to understand the tectonic evolution of the Pyrenean mountain range after a systematic combination of paleomagnetism with structural and petrological observations. In contrast, the secondary character of magnetization and complications associated with the Variscan tectonics indicate that a reassessment of Siluro-Devonian poles from the Variscan elsewhere in Europe might be appropriate.

Spatio-temporal patterns of Pyrenean exhumation revealed by inverse thermo-kinematic modeling of a large thermochronologic data set

Abstract Large thermochronologic data sets enable orogen-scale investigations into spatio-temporal patterns of erosion and deformation. We present the results of a thermo-kinematic modeling study that examines large-scale controls on spatio-temporal variations in exhumation as recorded by multiple low-temperature thermochronometers in the Pyrenees mountains (France/Spain). Using 264 compiled cooling ages spanning ∼200 km of the orogen, a recent model for its topographic evolution, and the thermo-kinematic modeling code Pecube, we evaluated two models for Axial Zone (AZ) exhumation: (1) thrust sheet–controlled (north-south) exhumation, and (2) along-strike (east-west) variable exhumation. We also measured the degree to which spatially variable post-orogenic erosion influenced the cooling ages. We found the best fit for a model of along-strike variable exhumation. In the eastern AZ, rock uplift rates peak at ≥1 mm/yr between 40 and 30 Ma, whereas in the western AZ, they peak between 30 and 20 Ma. The amount of post-orogenic (<20 Ma) erosion increases from <1.0 km in the eastern Pyrenees to >2.5 km in the west. The data reveal a pattern of exhumation that is primarily controlled by structural inheritance, with ancillary patterns reflecting growth and erosion of the antiformal stack and post-orogenic surface processes.

Stratigraphy and Geological Structures of the Miura Group in the Ubajima Islands, Northernmost Sagami Bay: Multiple Duplexes and Fault-propagation Folds Based on Tracing Tephra Key Beds

Mapping was carried out of well exposed folded and faulted strata in the Ubajima Islands off Chigasaki City, Kanagawa Prefecture, central Japan of the northernmost part of Sagami Bay, where the present Philippine Sea plate subduction boundary extends beneath the central part of the North America plate. The strata were dated with radiolarian fossils as middle to late Miocene, and correlated with part of the Misaki Formation of the Miura Group of the southern Miura Peninsula. The strata are composed of dominant basaltic pyroclastic or volcaniclastic sedimentary rocks with sporadic felsic tephra layers and rare silty sedimentary rocks, and were defined as the Ubajima Formation. Detailed field mapping of the Ubajima Formation verified that the strata are deformed in a complicated way into duplex structures having two stages. The first stage is characterized by bedding parallel to subparallel thrust faults in the SW direction of the horizontal reference frame. This might be of the antiformal stack type known by many parallel repetitions of the same horizons. At the second stage, the same parts are further duplicated into other antiformal stack-type duplex structures, forming multi-duplex structures. Finally those structures are folded and faulted by a fault-propagation fold system that has strong shear zones at propagation fault zones. The Ubajima Formation is thought to be deposited on the forearc area of the Izu volcanic arc, then accreted to the Honshu side by accretionary prism formation. These structures represent good examples of accretionary prism toes along subduction zones.

The Cenozoic Malaguide Basin from Sierra Espuña (Murcia, S Spain): An Example of Geological Heritage

The Cenozoic Malaguide Basin from Sierra Espuña (Internal Betic Zone, S Spain) due to the quality of outcropping, areal representation, and continuity in the sedimentation can be considered a key-basin. In the last 30 years, a large number of studies with very different methodological approaches have been done in the area. Models indicate an evolution from passive margin to wedge-top basin from Late Cretaceous to Early Miocene. Sedimentation changes from limestone platforms with scarce terrigenous inputs, during the Paleocene to Early Oligocene, to the deep basin with huge supplies of turbidite sandstones and conglomerates during the Late Oligocene to Early Miocene. The area now appears structured as an antiformal stack with evidence of synsedimentary tectonics. The Cenozoic tectono-sedimentary basin evolution is related to three phases: (1) flexural tectonics during most of the Paleogene times to create the basin; (2) fault and fold compartmentation of the basin with the creation of structural highs and subsiding areas related to blind-fault-propagation folds, deforming the basin from south to north during Late Oligocene to Early Aquitanian times; (3) thin-skin thrusting tectonics when the basin began to be eroded during the Late Aquitanian-Burdigalian. In recent times some works on the geological heritage of the area have been performed trying to diffuse different geological aspects of the sector to the general public. A review of the studies performed and the revisiting of the area allow proposing different key-outcrops to follow the tectono-sedimentary evolution of the Cenozoic basin from this area. Eight sites of geological interest have been selected (Cretaceous-Cenozoic boundary, Paleocene Mula Fm, Lower Eocene Espuña-Valdelaparra Fms, Middle Eocene Malvariche-Cánovas Fms, Lowermost Oligocene As Fm, Upper Oligocene-Lower Aquitanian Bosque Fm, Upper Oligocene-Aquitanian Río Pliego Fm, Burdigalian El Niño Fm) and an evaluation has been performed to obtain four parameters: the scientific value, the educational and touristic potential, and the degradation risk. The firsts three parameters obtained values above 50 being considered of “high” or “very high” interest (“very high” in most of the cases). The last parameter shows always values below 50 indicating a “moderate” or “low” risk of degradation. The obtained values allow us considering the tectono-sedimentary evolution of this basin worthy of being proposed as a geological heritage.

Basement-involved thrusting, salt migration and intramontane conglomerates: a case from the Southern Pyrenees

The northern margin of the Organyà basin (Southern Pyrenees) has a complex structure in which syn-rift Lower Cretaceous carbonates flank a wide Keuper evaporite province, featuring the leading edges of the basement-involved thrust sheets of the Pyrenean antiformal stack. Recent studies show that Keuper diapirs and salt walls grew during the Cretaceous extensional episode, conditioning the development of differentiated depocenters and minibasins. The role of salt tectonics during the Pyrenean orogeny has not been addressed in previous structural studies, but present-day cross-sections indicate a Keuper evaporite-bearing vertical thickness of up to 3000 m in the Senterada-Gerri de la Sal area. We infer that salt migration was a determinant mechanism in triggering a gentle northward tilting of the Organyà basin during the Eocene-Oligocene, recorded in the La Pobla de Segur and Gurp syn-tectonic conglomerates in a large north-directed onlap, opposite to the main sedimentary influx direction. Contemporaneously, we interpret that salt migration, promoted by conglomerate differential loading, enabled the sinking and rotation of the unrooted Nogueres thrust units (têtes plongeantes). We use new and published structural data for the Lower Cretaceous margin of the Organyà basin, combined with structural and clast provenance data from the Cenozoic alluvial fan conglomerates of La Pobla and Gurp, to understand the Lutetian to late Oligocene evolution of the northern margin of the Central South-Pyrenean Unit. The tectono-sedimentary evolution of this area and the salt evacuation patterns are closely related to the exhumation history of the stacked Paleozoic thrust sheets of the Pyrenean hinterland to the north. In this study, we correlate the movements over a mobile substratum and the paleogeographic changes of conglomeratic basins at the toe of an exhuming orogenic interior.

Numerical modeling of tectonic underplating in accretionary wedge systems

Abstract Many fossil and active accretionary wedge systems show signs of tectonic underplating, which denotes accretion of underthrust material to the base of the wedge. Underplating is a viable process for thickening of the rear part of accretionary wedges, for example as a response to horizontal growth perpendicular to strike. Here, numerical experiments with a visco-elasto-plastic rheology are applied to test the importance of backstop geometry, flexural rigidity, décollement strength, and surface erosion on the structural evolution of accretionary wedges undergoing different modes of sediment accretion, where underplating is introduced by the implementation of two, a basal and an intermediate, décollement levels. Results demonstrate that intense erosion and a strong lower plate hamper thickening of a wedge at the rear, enhancing localized underplating, antiformal stacking, and subsequent exhumation to sustain its critical taper. Furthermore, large strength contrasts between basal and intermediate décollements have an important morphological impact on wedge growth due to different resulting critical taper angles. Presented numerical experiments are compared to natural examples of accretionary wedges and are able to recreate first-order structural observations related to underplating.

Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak Décollements

AbstractOrogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two‐dimensional, visco‐elasto‐plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold‐and‐thrust belts dominated by out‐of‐sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold‐and‐thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise.

Validating Structural Styles in the Flysch Basin Northern Rif (Morocco) by Means of Thermal Modeling

Vitrinite reflectance and a micro-Raman spectroscopy parameters data set have been acquired on dispersed organic matter of the Maghrebian flysch basin and the Tangiers unit across a NE-SW section in the north-western Rif belt (North Morocco). Thermal maturity shows increasing values from the hinterland to the external unit (from NE to SW). Paleo-thermal indicators show that the internal flysch basin (i.e., the Mauretanian unit) is less mature than the external one, (i.e., the Massylian unit), with Ro% and Ro eq. Raman values ranging from 0.64% to 1.02% (from early mature to late mature stages of hydrocarbon generation). 1D thermal modeling estimates the overburden now totally eroded ranging from 3.1 km to 6.0 km, and has been used as constraint to reconstruct the complete thrust wedge geometry in Miocene times. The reconstructed geometry accounts for high shortening (about 63%) due to the development of an antiformal stack in the frontal part of the wedge made up by the flysch succession. This stacking is interpreted as a consequence of the western translation of the Alboran Domain in the core of the Betic-Rif orogenic system.

Kinematic deformation model of the south-central subandean zone, Bolivia

The Bolivian Subandean region is characterized as a thin-skinned fold-thrust belt with structures persisting for 50–100 km along strike in a broadly North-South direction. The evolution of the Subandean structures is related to shortening and mass transfer. The stratigraphy comprises ~12 km of Ordovician to Recent sediments, which are dominated by the Paleozoic section including Devonian source and reservoir horizons capped by the Middle Devonian Los Monos regional seal. Detachments occur at top of basement, dipping 2° to the west, within the Silurian and two Devonian levels, including the Los Monos shale.The fold-belt contains the Mega Gas Fields of Bolivia. This study focused on an exploration target in the Igüembe structure, located nearby the Margarita and Incahuasi Gas Fields. In addition to the standard methodology of integrating seismic lines, surface geology and well data, this study also utilized magnetotelluric (MT) analysis to create regional structural sections that were balanced and restored, recording ~40% shortening between 12Ma and the present. The compression was propagated from west to east along a basal detachment along top basement at ~12 km depth, creating four different and superimposed structural units. The Ordovician to Middle Devonian section deformed as antiformal stacks at ~15 km spacing, with the highly ductile Los Monos horizon overlying it and acting as a major detachment and seal. The Upper Devonian to Cretaceous section was buttressed by over 5 km of overlying Neogene sequence; in such a way, that it was forced to propagate to the surface directly above the antiformal imbricates in a sequence of triangle zones above and along the Los Monos detachment. The 5+ km thick Neogene section is preserved as parallel bedded open synclines between the triangle zone culminations. The Middle Devonian source and reservoir may have been buried 6–8 km prior to deformation, which has reduced porosity, but also generated hydrocarbons. It is plausible that during the Siluro-Devonian Cordilleran compressional phase, gentle regional anticlines or inversion structures might have been formed, perhaps with ~15 km spacing. These may have trapped the early-formed hydrocarbons and preserved porosity. These anticlines would be the nuclei for subsequent contractional deformation in the Neogene, with hydrocarbons re-migration and some hydrocarbons remnants trapped in the original structures.

Application of remote-sensing techniques in geological and structural mapping of Atalla Shear Zone and Environs, Central Eastern Desert, Egypt

The Atalla Shear Zone (ASZ) is one of the crucial megashears which are geometrically and kinematically akin to the Najd-Shear Corridor in the central Eastern Desert of Egypt. The Landsat-8 and ASTER-based mapping techniques integrated with extensive petrographic and structural-field investigations enhanced the geological and structural maps of the study area. The Neoproterozoic basement complex in the study area discriminated into ditinctive rock suites: Meatiq Group, ophiolitic melange, arc assemblage, syn- to late-tectonic granitoids, Dokhan Volcanics, Hammamat Sediments, post-Hammamat Felsites and post-tectonic granitoids. The true color composites (TCC) (4 3 2 for Landsat-8; 3 2 1 for ASTER), false color composites (FCC) (1 6 4, 1 6 7 and 7 5 1 for Landsat-8; 5 4 3 and 9 6 4 for ASTER), principal component analysis (PCA) (1 2 3, 3 2 1 and 6 4 2 for Landsat-8; 1 2 3 and 5 4 2 for ASTER), minimum noise fraction (MNF) (1 2 3 and 3 5 1 for Landsat-8; 1 2 4 and 3 2 1 for ASTER) and band ratios (BR) (6/2, 6/7, 6/5*4/3 and 7/6, 7/5, 5/3 for Landsat-8; 2/6, 7/5, 7/6 and 4/7, 3/4, 2/1 for ASTER besides the new created ratio 2/6, 7/5, 7/6) are the best combinations that demonstrate efficiency in discrimination of lithologic contacts and structural elements, using the spectral signature of different rock units. Moreover, the BR b7/b5, b5/b7 and b6/b4 of grey scale for Landsat-8 are used as a tool for mineral detection along with the spectral indices created to detect muscovite (b7/b6), ferrous silicates b5/b4 (biotite, chlorite and amphiboles) and ferrous iron b5/b3 (mafic minerals). The automatic lineament extraction utilizing the SRTM data advocated the role of NE- to NNE-trending lineaments in the tectonic framework of the study area. The structural fabric of the study area is evaluated in three main structural domains: Meatiq (MD), Atalla Shear Zone (ASZD) and Wadi Hammamat (WHD). The pronounced structural implication is the transposition of NE-oriented fabric in MD into NW-oriented penetrative shear-related fabric, fully in the ASZD and partly in WHD. The combination of transcurrent shearing with ENE-directed shortening along the ASZ that was resulted in noteworthy transpressive structures resemble in many respects those observed in the Ajjaj Shear Belt in Western Arabia. Among these structures are shear zone-related folds, imbricated thrust sheets, antiformal stacks and thrust duplexes. The deformation history of the investigated area proposed four phases of deformation (D1–D4). It depicts a long lasted deformational event covering the main accretion-collision phases concurrent with the E-W assembly of Gondwana, and the subsequent post-collision Najd-related transpressional phase which resulted in several major left-lateral transcurrent shear zones along with the exhumation of gneissic core complexes in the Arabian-Nubian Shield.

Closure of a hyperextended system in an orogenic lithospheric pop‐up, Western Pyrenees: The role of mantle buttressing and rift structural inheritance

AbstractThe Early Cretaceous hyperextended Mauléon rift is localized in the north‐western Pyrenean orogen. We infer the Tertiary evolution of the Mauléon basin through the restoration of a 153‐km‐long crustal‐scale balanced cross‐section of the Pyrenean belt, which documents at least 67 km (31%) of orogenic shortening in the Western Pyrenees. Initial shortening, accommodated through inversion of inherited crustal structures, led to formation of a pop‐up structure, in which the opposite edges underwent similar shortening with different tectonic reactivation styles, localized versus. distributed. Underthrusting of the Iberian margin accommodated further convergence, forming the Axial Zone antiformal stack of crustal nappes within a lithospheric pop‐up. Thin‐skinned and thick‐skinned structures propagated outward from the heart of this pop‐up, a block of strong mantle acting as a buttress inhibiting complete inversion of the Mauléon rift basin.