Fold and thrust belts are frequent structures that form under a compressive regime. They record a variety of deformation that guides earthquake ruptures and controls seismic hazard. The Naukluft mountains of Namibia with their extraordinary exposure, present a unique opportunity to study carbonate fault rocks. Through linked field investigations and microstructural analysis, we aim to map how changes in fault strength contributed to larger scale architectural evolution of a fold and thrust belt. Mineralogical assemblage of the stacked nappes show units of carbonate and shale in the lower to upper greenschist facies, correlating to the seismological depth for carbonate rocks. Cross cutting relations prove that the Naukluft Nappe Complex (NNC), previously thought to form through gravity sliding, is a foreword-propagating fold and thrust belt, stacking Neo Proterozoic sedimentary nappes on top of each other, and on top of the cambrian Nama fore-rift group. Most thrusts separating the stacked nappes are made of 20-150 meters of calc mylonites, except for the youngest two brittle thrust faults, including the basal thrust. Brittle deformation is recorded by localized brecciation and the development of discrete faults. The brittle faults follow the same orientation as the overall transport direction of the nappe complex. Ductile shearing is facilitated by crystal plastic deformation, leading to grain size reduction, and grain boundary sliding. Both ductile mylonite units and the younger brittle faults are also overprinted by later brittle faults, and between the ductile and brittle behavior, we map a unit of a block-in-matrix. We found no clear correlation between the temporal evolution of the nappe complex and the thickness of ductile shear zones. We show evidence for embrittlement through exhumation of a carbonate terrane, and suggest that it may be important to consider the effects of deeper ductile behavior in “Coulomb wedge” descriptions of thin-skinned thrust belts.

How to cite: Boianju, I., Faber, C., and Rowe, C.: Deformation in carbonates and its contribution to fold & thrust belt architecture, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12156, https://doi.org/10.5194/egusphere-egu23-12156, 2023.

Deformation in sandstones often takes the form of tabular strain localization structures known as deformation bands. To date, only a few localities have been documented with naturally occurring deformation bands that primarily localize compactant failure i.e. pure compaction bands (PCBs).

The PCBs are found in the Oligocene flysch sequence of the (Lower) Krosno beds within the Otryt sandstones of the Silesian Nappe, the Outer Carpathians (SE Poland). The PCBs are perpendicular to the bedding planes and occur within the folded strata. The PCBs were formed in very fine-grained to very coarse-grained sandstones of moderate or poor sorting. The spacing of PCBs is on average 7 mm and the average thickness of a band is 0.7 mm. The PCBs’ microstructures originated due to a range of mechanisms which include pore collapse resulting in compact grain packing and disaggregation, including kinking and cataclasis. The type of resultant microstructure shows a strong relationship with the textural parameters of the host rock. The frequency of cataclasis increases with grain size, whereas kinking is related to the moderate sorting of the host rock. The structural restoration of beds to the primary horizontal position indicates that the PCBs formed prior to the folding and recorded the SW-NE directed shortening which is consistent with the subsequent folding. The results of the mechanical modelling and stratigraphic constraints suggest that the PCBs formed under shallow burial depths <1 km.

The occurrence of hydrocarbons and calcite veins within some fractured PCBs shows that the PCBs might have played an important role in fluid transport during the subsequent evolution of the fold-and-thrust belt of the Outer Carpathians.

Acknowledgements: This research was funded by National Science Centre, Poland (grant number: 2018/31/N/ST10/02486).

How to cite: Strzelecki, P. and Świerczewska, A.: Pure compaction bands in the naturally deformed flysch sandstones of the Silesian Nappe (SE Poland): early markers of tectonic shortening, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14083, https://doi.org/10.5194/egusphere-egu23-14083, 2023.

We investigated Mesozoic sequences of the Franconian Platform (SE Germany) according to their deformation history. Based on field observations we performed fault-slip and stylolite stress inversion. We could distinguish two stress cycles in an intraplate compressional setting starting with (i) normal faulting, (ii) thrusting and the development of folds and tectonic stylolites (iii) and strike-slip regimes. The first cycle is caused by the Europe-Iberia-Africa collision in the Cretaceous, while the second cycle is induced by the Cenozoic Alpine orogeny. We can zoom into the transition between the thrusting and strike-slip regime of the first cycle by the preservation of a stress field with none principal stress being in the vertical axis. The combination of field and microscopic observations shows a rather complex chronology of this relatively short time span with alternating sedimentary and tectonic stylolites.

With our work we contribute to the understanding of stress development in intraplate compressional settings.

How to cite: Köhler, S., Koehn, D., Stollhofen, H., Fazlikhani, H., Stephan, T., and Duschl, F.: Zooming into the transition between thrusting and strike-slip in an intra-continental compressional setting, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13457, https://doi.org/10.5194/egusphere-egu23-13457, 2023.

In most idealised models of thrust fault formation thrusts form along a weaker basal detachment and then propagate upwards through strata, producing a hang-wall anticline and developing in a piggy-back sequence. This idealised thrust fault geometry commonly fails to match actual outcrop structures. Conversely, in a stiff layer thrusting model, the thrust originates and localises in mechanically competent beams as ramps and the thrust tip propagates both up and down slowly to create a linked fault system. This model has had few tests at outcrop level and the lack of field examples has hindered adoption of this model. 

In the UK, thrust structures in the Old Red Sandstone are ideal test outcrops as they contain a multilayer stratigraphy of competent sandstone beds encased in cleaved mudrocks. Field and photogrammetric mapping has been applied to the St Brides Haven outcrop on the west coast of Pembrokeshire in SW Wales, the 10m high outcrop exposes an open fold pair cut by thrusts typical of Variscan deformation. The outcrop has an abrupt rheological change within the multilayer with a 1m thick sandstone bed within mudstones. Thrust faults are confined to the strong sandstone beam, with a fault spacing along the competent beam, with cleavage development in the mudstone above and below. The structural style is controlled by variations in the multi-layer rheology, and fault localisation in the competent sandstone beams appears to be balanced by distributed deformation in the form of cleavage development in the mudstones. The outcrop matches key features of the stiff layer thrusting model.

Observations from this outcrop show that the observed thrust fault geometry evolved from soft to hard linked across the multilayer. Drag faults have developed in both the hanging and footwalls, showing thrusts initiate in the stiff layers. Cleavage developed in the mudstones and along rheological boundaries the cleavage and thrust trajectories connect. The outcrop matches key features of the stiff layer thrusting model. This work contributes to a series of studies on fold-thrust outcrops to expand the range of widely used models and avoid bias in interpretation of fold-thrust belts when basing work on theoretical models.

How to cite: Sleath, P., Butler, R., and Bond, C.: Strata-dependent thrust localisation in multilayers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13527, https://doi.org/10.5194/egusphere-egu23-13527, 2023.

The Jura, on the outer margin of the NW Alps, is classically interpreted as a “thin-skinned” fold-thrust belt, detached along Triassic evaporites. It includes the Opalinus Clay (Toarcian-Aalenian), the designated host formation for a radioactive waste repository in Switzerland. We develop a case study to illustrate how methods and approaches in cross-section balancing and restoration, based on seismic imagery, can be applied to assess risks and uncertainties for the integrity of the Opalinus Clay. We focus on the Nördlich Lägern area, announced in 2022 as the preferred site for nuclear waste disposal in Switzerland.  Between northern and southern fold-thrust structures, 3D seismic mapping defines a largely unfaulted domain within which there is no seismic indication of deformation in the Opalinus Clay. It is however seismically transparent. Bounded to the north and south by domains of strongly-faulted Mesozoic strata the low-strain zone has been translated tectonically and is underlain by a seismically resolved Permo-Carboniferous basin.

Here we consider multiple interpretations of key section lines with varying degrees of structural linkage and several conceptual models for the timing of the contractional deformation structures. By exploring multiple interpretations, we consider the key uncertainties in structural understanding and risks that might compromise the structural integrity of the waste repository site. Key questions include: were the thin-skinned thrust structures influenced by deep basement structures (that might reactivate in the future)? Has the Opalinus acted as an intraformational detachment that connects kinematically the northern and southern fold-thrust structures? Section balancing and restoration are used to assess whether strain in the two deformation domains can be matched on both sides of the Opalinus Clay. If not, then there is an increased risk of the Opalinus having acted as a detachment passing through the low strain domain, the candidate repository site. A combination of line-length and formation area balancing have been applied to the adjacent, upper and lower, units to the Opalinus Clay, evoking different interpretations of fault trajectories in the bounding fold-thrust domains. In all these interpretations, a balance in tectonic contraction can be achieved without involving distributed strain or requiring detachment within the Opalinus. On this basis, the structural integrity of the prospective repository site is not compromised; an inference consistent with borehole evidence. No significant layer-parallel fault zones have been identified in the four borehole penetrations in the Nördlich Lägern area. 

The work highlights the importance of structural interpretation and restoration techniques in assessing risks for energy and waste storage projects in contractional settings, even when searching for low strain zones.

How to cite: Bond, C., Robledo, F., and Butler, R.: The Jura Fold-Thrust Belt, Switzerland – a contractional deformation setting for nuclear waste disposal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9905, https://doi.org/10.5194/egusphere-egu23-9905, 2023.

The Beaufort-Mackenzie fold and thrust belt in Arctic Canada corresponds to the offshore part of the Cordilleran orogen in Yukon and Alaska. The main folds are associated with thrusts that root in a basal décollement at depths of ca. 12-15 km. Deformation mainly occurred prior to the deposition of the Richards sequence (late Eocene) and subsequent folding that progressed basinward and continued episodically to the present accounts for < 10% of the total shortening. In the shelf region, normal faults dipping either seaward or landward are located on the apex of anticlines. Detailed analysis on seismic sections of the variations of throw (T) and expansion index (Ei) with depth (z) indicates that despite the clear spatial relationship with anticlines, normal faults initiated after fold development. Normal faulting migrated seaward during the Oligocene to Pliocene (and possibly later) along with the shelf edge that resulted from the progradation of thick sedimentary wedges of clastic sediments. This strongly suggests that normal faulting is not related to tectonics (i.e., linked with the building of the Cordillera), but to gravity spreading due to high sedimentary loading in the Mackenzie delta; i.e. main depocenter for sediments originating from the northeastern Cordillera since the latest Cretaceous. The superimposition of extensional structures on an existing fold and thrust belt shows the complex interplay in time and space between far-field stresses linked with plate-dynamics and stresses associated with sedimentary loading in a delta setting.

How to cite: Pinet, N., Duchesne, M. J., and Brake, V.: Cenozoic tectonic evolution of the Beaufort-Mackenzie fold and thrust belt (Arctic Canada): from orogenic shortening to gravity spreading, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2766, https://doi.org/10.5194/egusphere-egu23-2766, 2023.

How to cite: Hemelsdaël, R., Averbuch, O., Beccaletto, L., Izart, A., Capar, L., Marc, S., and Michels, R.: A deformed wedge-top basin inverted during the collapse of the Variscan belt: the Permo-Carboniferous Lorraine Basin (NE France), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5404, https://doi.org/10.5194/egusphere-egu23-5404, 2023.

Foreland fold and thrust belts (FFTB) are the locus of strong interactions between tectonics and sedimentation during both mountain building then dismantling processes. Especially, the nature and rheology of decollement layers within the basin pre and/or syn-orogenic sequence are susceptible to influence the kinematic evolution and geometry of FFTBs as well as the growth of orogenic wedges.

In the South Pyrenean Zone, the Jaca basin forms the main pro-foreland basin filled by marine to continental siliciclastic sediments during Eocene and Lower Oligocene times. From Bartonian until Rupelian at least, several alluvial fans were successively deposited from east to west along its northern margin in the footwall of emergent thrusts, where they were accompanied by the syn-sedimentary growth of shale-cored anticlines downstream.

Field surveys and “bed-to-bed” mapping of syn-orogenic formations using high-resolution DEMs were used with re-interpreted published seismic lines to build two cross-sections that highlight the structural style and evolution of the Gavarnie-Jaca FFTB. This illustrates the tectono-sedimentary evolution of a transitional, shallow-marine to alluvial foreland margin.

The Eocene-Oligocene syn-orogenic deposits are folded as a wide synclinorium that is thrusted southward over the Ebro foreland along to the Triassic detachment level. In the northern limb, the late Eocene Orosia-Cancias conglomerates exhibit a short-wavelength and small-scale fold detached within the late Middle Eocene shallow marine marls. Beneath this decoupling level, the Lower and Middle Eocene turbidites form a gently foreland ward-dipping imbricate composed of several small-scales thrust-sheets detached at the base of Upper Cretaceous. During end-Bartonian and early Priabonian, the deposition of the Orosia alluvial fan was coeval with the early growth of the Basa anticline downstream. From Priabonian, the anticline was subsequently welded and forward-tilted coevally with the development of the turbidite imbricate beneath, in response to the Gavarnie thrust-sheet emplacement. At this time, the second Cancias conglomerate unit was deposited as a large fluvial fan that progressively filled the foreland basin northern margin.

The tectono-sedimentary evolution of the north-eastern Jaca foreland margin illustrate the interactions between the regional convergence, syn-orogenic shales tectonics and alluvial sedimentation within a foreland fold and thrust belt. The South Pyrenean foreland documents a first example of a gliding alluvial fan on gravity-driven mobile shales in a convergent margin setting.

How to cite: Menzer, L., Bonnel, C., Hoareau, G., Aubourg, C., and Callot, J.-P.: Role of syn-orogenic sedimentation in the evolution of a foreland fold and thrust belt: an example from the Jaca basin, South Pyrenean Zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14064, https://doi.org/10.5194/egusphere-egu23-14064, 2023.

Fold and thrust belts are a notoriously challenging environment when it comes to providing structural models for the subsurface, and the Romanian Carpathians are no exception. Hosting the largest onshore oil fields in Romania, this is a highly mature hydrocarbon area, with most of the fields producing since the late nineteenth century.

The structural style of this fold and thrust belt is influenced by a number of parameters which includes multiple detachments (including salt), combined with multiphase tectonic event and reactivation of basement structures which adds to the complexity and variability of the structural style.

As a consequence, the reservoirs, especially the Oligocene - lower Miocene (sub-salt), thought very prolific are structurally complex, heterogeneous, and compartmentalized. It is a constant struggle for geologists to create structural maps of these reservoirs due to complex deformation, and insufficient or ambiguous geological/geophysical data. Some of the most significant issues are related to scattering dip data and the overall difficulties in correlating well logs. In some cases, even the logs of the side-track well do not correlate with the initial log.

Single-method approaches (e.g. only seismic interpretation) often lead to uncertainties or contrasting models regarding the structural style. In this study, we adopted a multiscale/multimethod approach such as forward-modeled regional cross-sections that rely on seismic reflection and well data, UAV-based digital outcrop models, fieldwork, microstructure analysis, and scaled analogue modelling.

This holistic approach enhanced our understanding of fold and thrust belts and provided better constraints on the subsurface structural style in the Romanian Carpathians, as well as explaining complexities that have hitherto been ignored. These new models can reduce subsurface uncertainties regarding structural style, and unlock the full potential of the area which will significantly enhance future exploration programs.

How to cite: Tamas, A., Tamas, D. M., Copot, B., Tocariu, I. S. M., Dohan, D., Schléder, Z., and Krézsek, C.: Challenges in the subsurface interpretation of a fold and thrust belt and how to get the best insights: examples from the Romanian Carpathians, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6425, https://doi.org/10.5194/egusphere-egu23-6425, 2023.

The Mexican Fold-and-Trust Belt is a winding belt that formed after a series of protracted tectonic events, which began with the onset of sedimentation in the basins formed during the break-up of Pangea and during the roll-back of the oceanic Kula plate in Jurassic times. Later, the continued subduction of Kula-Farallon constituent plates at the western margin of Mexico triggered arc formation, thrusting, basin inversion, and folding. The kinematics of the fold-and-thrust belt curvature at regional and local scales is still debated. Different hypotheses have been explored to explain the trace of the orogen: 1) curvature is mainly and primarily controlled by the basin architecture; 2) Curvature is progressive and was at least partially acquired during deformation being controlled by the basement geometry, lithologic heterogeneities, and/or the shortening direction, and 3) The curvature is, at least partially, postdating the main orogenic deformation structures being formed as the youngest of the deformation phases. Several regional-scale studies have been performed (Eguiluz et al., 2010; Fitz-Diaz et al., 2017). But most of the previous paleomagnetic studies were focused on studying the configuration of the desegregated fragments of Pangea (Molina-Garza et al., 1992). In this study, we investigate the nature of the curvature in the north and central-eastern part of the Mexican Fold and Trust Belt using paleomagnetic data obtained from the Jurassic rocks of the Nazas Formation, this formation is composed of a volcanic and volcano-sedimentary succession of andesites and dacite flows interbedded with tuff deposited in an intra-arc or back-arc setting. Our results show remagnetizations, some potential primary magnetizations, and significant counterclockwise rotations. This implies a potential oroclinal bending origin for at least part of the Mexican fold-and-thrust-belt curvature. This contribution will discuss the potential mechanisms causing the curvature and the implications for the kinematics, tectonic and geodynamic evolution of the central-western Pacific subduction during the Mesozoic-Paleogene in northeastern Mexico. This work is a posthumous contribution of Dr. Roberto Stanley Molina-Garza and a tribute to his huge contribution to the understanding of the tectonic history of Mexico.

Keywords: Paleomagnetism; Mexican Fold and Trust Belt; Anticlockwise rotation; Remagnetization; Jurassic; Nazas Arc.

How to cite: Guerra Roel, R., Pastor Galán, D., Chávez Cabello, G., Ramírez-Peña, C. F., Aranda Gómez, J. J., Patiño Méndez, G., Rodríguez-Parra, A., Nova Rodríguez, E. G., and Molina Garza, R. S.: Another bend in the orogen? Kinematics of the winding Mexican Fold-and-Trust Belt: Paleomagnetism of the Nazas arc., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-324, https://doi.org/10.5194/egusphere-egu23-324, 2023.

Seismic interpretation, cross-section balancing and sequential restoration unravel how inherited (rifted continental margin-related) and syn-kinematic stratigraphy, structural features and rheology played different roles at various times and locations along the fold and thrust belt to produce major changes in structural style in the outer Albanides. Different processes appear to have concurred to slowdown and/or arrest detachment-dominated thrusting of the sedimentary cover during a late-stage (<5 Ma) switch from thin-skinned to thick-skinned thrusting. In the northern outer Albanides, the limited thickness of Triassic evaporites inhibited thrust belt propagation within the Ionian basin succession, favoring further advancement of the detached carbonate platform (Kruja) units in its hanging wall. The rapid accumulation of an up to 10 km thick succession of syn-tectonic strata in the Peri-Adriatic Depression likely had a twofold effect of: (i) stopping detachment-dominated thrust belt propagation into the foredeep strata; and (ii) providing a burial that substantially contributed to thermal weakening of the crust. On the other hand, in the southern outer Albanides the Ionian basin carbonate succession, here overlying thick Triassic evaporites, was intensely shortened. Folding and detachment-dominated thrusting proceeded efficiently up to the western margin of the basin. There, normal faults controlling the platform-to-basin transition and the reduced thickness of the Triassic evaporites at the base of the Sazani (Apulia) carbonate platform succession hindered thrust belt propagation into the thick shallow-water carbonate succession. The latter was later broadly folded and significantly uplifted above regional by deformation associated with the crustal thrust ramp controlling the present-day blind thrust front in this southern sector of the belt. In this region, tectonic burial – produced by the imbricated and thickened sedimentary cover – rather than sedimentary burial likely contributed to thermal weakening of the crust. 3D structural modelling effectively shows the role of major, inherited transverse structures in compartmentalizing fold and thrust belt architecture and tectonic evolution.

How to cite: Basilici, M., Mazzoli, S., Spina, V., Pierantoni, P. P., and Tondi, E.: Along-strike variations of fold and thrust belt architecture and tectonic evolution revealed by 3D structural modelling of the outer Albanides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1487, https://doi.org/10.5194/egusphere-egu23-1487, 2023.

The Hellenides of Western Greece is a salt detached fold and thrust belt which resulted from the inversion of the previous southern Neo-Tethys margin. Before the Paleogene and Neogene contractional deformation, the Ionian basin was formed during the Triassic to early Jurassic rifting, bounded by the Apulian and Gabrovo platforms to the west and to the east respectively. The structural style of Western Greece is largely controlled by the presence of Triassic evaporites flooring the sedimentary succession of the Ionian basin, while previous models invoked the existence of extensional faults affecting the Triassic to Jurassic succession as one of the main structural controls. Understanding the present contractional structure mainly depends on unraveling the geometry of the Mesozoic extensional system, the distribution of the salt horizon and the presence and geometry of salt structures preceding the contractional deformation. Based on an extensive field study carried out along the Ionian fold and thrust belt, we present several regional and local balanced cross-sections and define several characteristic structural templates resulting from of this area.

Despite inversion tectonics and reactivated salt structures are common features in many salt-detached fold and thrust belts but no evidence for the presence of inherited (and inverted) Jurassic extensional faults was found. However, the deformational style was strongly controlled by significant changes in thicknesses involving the pre-contractional stratigraphic package, and suggesting the presence of pre-existing salt structures. The pre-shortening configuration was dominated by salt pillows and salt plateaus formed during the Triassic to early Jurassic rifting. As a result, we observe a significative change in the deformation style, from remarkable detachment folds, box-folds or inclined folds, to more wide and far-travelled thrust sheets formed in the areas with a thicker stratigraphy. Despite the stratigraphic thickness is reduced to few hundreds of meters in some cases,  diapirism is just related with the contractional stage, as a result of the squeezing of former salt pillows, and no evidence of precursor passive diapirs has been found.

How to cite: Snidero, M., Muñoz De La Fuente, J. A., Santolaria Otin, P., Martinez Granado, P., and Likakis, N.: Structural style variations along the Ionian fold and thrust belt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16544, https://doi.org/10.5194/egusphere-egu23-16544, 2023.

In this work, we experimentally explore the role played by salt in contractional systems: from simple layer-cake, salt-floored settings to inverted salt-bearing rifted-margins, the inherent low-strength rheology of salt and its distribution determine particular structural styles and kinematics. Salt-detached contractional systems exhibit low taper angles, with no dominant structural vergence, and are comparatively wider in cross section than their non-salt-involved equivalents. The salt-sediment thickness controls structural spacing while the presence of pre-existing isolated salt bodies breaks the mechanical homogeneity. If salt bodies are connected through a framework of salt walls, contractional deformation concentrates in them and structural trends are no longer perpendicular to shortening but determined by the inherited orientation of salt bodies.

Finally, we assess the inversion of a salt-bearing rifted margin, where late syn-rift to early post-rift salt undergoes differential loading and gravity gliding leading to a salt-sediment architecture consisting of a salt wall-minibasin province, and a distal raft system. Upon shortening, inherited salt bodies localize contractional deformation and the salt-sediment architecture determines the structural configuration of the contractional system. In the experiments, a large-transport thrust detached along allochthonous salt accumulated in the distal raft system, and the squeezing of salt walls together with the tilting of minibasins, accounted for most of the shortening in the salt wall-minibasin province. During shortening, about 75% of the original salt evacuates and is eroded. Due to the low salt-sediment ratio found in fold-and-thrust belts, the role played by salt tectonics could be underestimated. So, the best practice approach for understanding structural style and kinematics of salt-influenced fold-and-thrust belt resides in the rifted margin stratigraphy involved, and not in the salt itself, since salt is generally poorly preserved and strongly deformed.

How to cite: Santolaria, P., Granado, P., Muñoz, J. A., Ferrer, O., Wilson, E. P., De Matteis, M., Snidero, M., Gratacós, O., and Roca, E.: Salt-influenced fold-and-thrust belts: insights from experimental analog models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8370, https://doi.org/10.5194/egusphere-egu23-8370, 2023.

Constraining the timing of tectonic events is of prime importance for the in-depth understanding of the complex evolution of orogenic deformation, particularly in the case of fold-and-thrust belts. In this work, we combined U-Pb dating of tectonic carbonates and K-Ar dating of fault gouges of selected key outcrops along two main thrusts of the Paleogene-Neogene Eastern Southern Alps (ESA), Italy. The ESA are the south-verging fold-and-thrust retrobelt of the Alpine orogen, offering spectacular exposures to study the details of past tectonic processes. However, despite a few published papers regarding the deformation mechanisms of a major thrust in the ESA (the Belluno Thrust), modern, multiscale structural and radiometric studies of fault zones in the ESA are missing, such that detailed reconstructions of the local and regional tectonic evolution through space and time remain only loosely constrained. We focused on the (i) Valsugana Thrust, which is a first-order thrust separating the Dolomites s.s. to the north from the Venetian Pre-Alps to the south, and (ii) the more external Belluno Thrust. We coupled U-Pb dating of tectonic carbonates and X-ray diffraction and K-Ar dating of clay minerals in fault gouges with structural analysis and microtextural characterization. We show that the Valsugana Thrust represents an inherited pre-Alpine structure that (i) registered far field deformation during the Early Cretaceous (K-Ar gouge age of 140 ± 32 Ma), (ii) strongly influenced the geometry and kinematics associated with deformation structures during the Alpine orogenesis and (iii) recorded multiple reactivations in Late Cretaceous (K-Ar age of 79.2 ± 8.4 Ma and 76.2 ± 1.4 Ma), late Miocene (U-Pb age of 9.1 ± 0.8 Ma), and Miocene-Pliocene (U-Pb age of 5.3 ± 1.6 Ma) times. Radiometric constraints from the Valsugana Thrust attest to remarkable out-of-sequence compressional movements in the inner ESA after the orogenic wave had progressed farther south to the more external Belluno Thrust, whose activity is constrained to the Oligocene by a 30.6 ± 5.8 Ma K-Ar gouge age and a 23 ± 14 Ma U-Pb syn-tectonic vein age.

How to cite: Vignaroli, G., Curzi, M., Zuccari, C., Aldega, L., Billi, A., Carminati, E., Van der Lelij, R., Kylander-Clark, A., and Viola, G.: Long-term tectonic evolution of the Eastern Southern Alps (Italy): a reappraisal from new structural and radiometric constraints, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16650, https://doi.org/10.5194/egusphere-egu23-16650, 2023.

Thermochronology has seen widespread application in the Eastern Alps. Tracking upper crustal cooling has focused mainly on the Tauern Window, the core of the collisional orogen, where exhumation has been most prominent. Further to the east, mostly fission track work is concentrated along fault zones and thermochronometers with lower closure temperatures, such as apatite (U-Th)/He dating, have hardly been applied to higher elements of the nappe pile. Due to the scarcity of the dataset and preferential application of fission track dating uppermost crustal cooling below ca. 80 °C remains undetected.

In this contribution, we present new low-T thermochronological ages from the easternmost Eastern Alps from the vicinity of the Vienna basin. We carried out apatite (U-Th)/He dating on clastic units, i.e. Gosau Group, Rhenodanubian Flysch and Lunz Formation sandstone. Additional apatite fission track analysis was performed on a smaller subset of these samples. A compilation of existing as well as new vitrinite reflectance data was used for estimating burial paleotemperatures. These served as criteria for sample selection, as sites with temperatures sufficient to reset at least the apatite (U-Th)/He system (> ca. 80 °C) and potentially the apatite fission track system (> ca. 110 °C) were preferentially targeted.

We find reset AHe and subordinately reset AFT ages, that monitor a so far un(der)appreciated phase of prominent cooling between ca. 18 to 25 Ma. For flysch sandstones from the Wienerwald both thermochronometers yield similar ages, implying an exhumation phase, which removed 4-6 km of overburden. Similar results were found for Lunz sandstone samples from the area around Lilienfeld. Apatite (U-Th)/He ages from Gosau sandstones along the western border of the Vienna basin were mostly reset with single grain ages clustering around 20 Ma.

Our new results are difficult to reconcile with geodynamic models that imply tectonic quiescence during large-scale subsidence and widespread deposition of Augenstein clastics. Interestingly, the sedimentary archive of the eastern part of the Molasse basin records a change in the sedimentation pattern and onset of rapid basin infill at ca. 19 Ma, too.

We discuss our findings in the light of postcollisional thrust wedge evolution and potential impact of margin architecture and the Bohemian Spur. This promontory acted as a buttress for foreland-propagating thrusting, intensifying exhumation above it. Slab detachment beneath the Eastern Alps has recently been proposed based on results from the AlpArray initiative. We discuss, to what extent the newly detected cooling pulse may constitute the surficial expression of this slab break-off.

How to cite: Heberer, B., Bernhard, S., István, D., Reinhard, S., Tari, G., Wagreich, M., von Hagke, C., and Wessely, G.: Rapid Oligocene to Miocene cooling in the easternmost Alps driven by thrusting onto the Bohemian promontory and/or deep mantle processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16083, https://doi.org/10.5194/egusphere-egu23-16083, 2023.

The structure of Iberian Atlantic margins resulted from multiple Mesozoic rift events and subsequent contractional deformation occurring from the Upper Cretaceous to Cenozoic during the Alpine Orogeny. Along the southern Bay of Biscay, the North Iberian margin shows various styles of contractional deformation, ranging from mild reactivation of pre-existing extensional structures, halokynetic-related processes, to wedging and underthrusting. The Biscay accretionary wedge developed as the major structure at the base of the continental slope in the central and western parts of the North Iberian margin, which are part of the western branch of the Pyrenean-Cantabrian Orogen, together with the Cantabrian Mountains onshore. The wedge is interpreted to continue from the western North Iberian margin, where incipient subduction has been proposed, to the Galicia Margin further to the southwest. Along the West Iberian margin, thrusting and related folding and halokynetic-related processes focused contractional deformation.

In this work, we describe the seismo-stratigraphy, and we map contractional structures along the North Iberian and West Iberian margins based on the interpretation of 2D seismic reflection profiles. We identify and describe structural domains along the extinct subduction zone along the North Iberian margin, describe the structure of the fossil Biscay accretionary wedge, and identify and map different styles of Alpine contractional deformation along the North Iberian and West Iberian margins. We also describe the pre-existing Mesozoic rift structure in order to analyse the overprint between different rift architectures and contractional styles of deformation. The overall goal is to define different styles and stages of Alpine contractional deformation along Iberian Atlantic margins during the first phases of the convergent cycle preceding or leading to subduction.

How to cite: Cadenas Martínez, P., Welford, J. K., C. Duarte, J., Fernández-Viejo, G., and Somoza, L.: Alpine subduction and intraplate deformation along the Iberian Atlantic margins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6654, https://doi.org/10.5194/egusphere-egu23-6654, 2023.

The subsurface structural geometries of the United Arab Emirates (UAE) fold-and-thrust belt (FTB) and foreland basins are interpreted from seismic, well data, and gravity and magnetic data, integrated with surface geology. We also determined the basement depths, as well as the relation between shallow sedimentary structures and deep basement features. Miocene to Lower Jurassic sequences were interpreted and mapped. Additionally, we outlined subsurface extent of Sumeini and Hawasina allochthonous nappes. The tectonic subsidence curves suggest that the final major passive margin rifting event occurred in the early Aalenian and lasted till Oxfordian. Loading of the Semail ophiolite thrust sheet and accompanying allochthonous thrust sheets resulted in uplift at ca. 95 Ma and rapid subsidence at ca. 83 Ma, indicating the transition of the Arabian margin from a rifted passive margin to a foreland basin. The region witnessed an accelerated subsidence during the late Oligocene-Miocene, attributed to the initial collision of the Central Iran and Arabian plates. The Permian-Jurassic NW-SE oriented rift faults were reactivated as thrust faults during the Late Cretaceous ophiolite obduction and late Oligocene-Miocene continental collision. Two different tectonic regimes are identified in the FTB. Based on the seismic reflection profiles and derivative maps of gravity and magnetic data, the northern regime has NNW–SSE striking thrusts, backthrusts and folds. It is characterized by major inversion of the rift faults with up to 3,700 m thrusting throw. Four major west-verging and east-dipping thrusts, which cross the northern area, form fault-propagation folds and dissect the entire stratigraphy. Whereas the southern regime around Jabal Hafit is dominated by the Tarabat backthrust, which cuts across the Upper Cretaceous and Cenozoic successions. Moreover, the Mesozoic platform carbonates are dissected by inverted faults (pop-up structure) in this regime. The Hawasina décollement, together with the inverted basement structures formed the Jabal Hafit anticline as a backthrust structure. On the other hand, results of the 3D gravity inversion indicate that the basement depths range from 11.5 km along uplifted areas of the FTB to 18.8 km within the deeper parts of the foreland basin. Significant northward increases in both basement depth and the thickness of the late Oligocene-early Miocene successions indicate a northward increase in the severity of thrusting and crustal loading along the main Khusub thrust. The basement uplifts are found to be correlated with shallower anticlinal features, indicating a direct relationship between basement uplifts and shallow sedimentary structures and hence indicating reactivation of basement structures during the Zagros collision.

How to cite: Abdelmaksoud, A., Ali, M., Geng, M., and Searle, M.: Fold-and-thrust belt and foreland basin of the United Arab Emirates: Sedimentary structures, basement morphology, and tectono-stratigraphic evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4943, https://doi.org/10.5194/egusphere-egu23-4943, 2023.

Determining the sequencing of different deformation mechanisms and styles is crucial to understanding how the Earth’s crust responds to tectonic stress.  The sequence of near surface compressional deformation in peripheral orogenic forelands typically starts with an early stage of bulk homogenous shortening, i.e. layer parallel shortening (LPS) followed by folding and contractional faulting. The temporal and spatial distribution of strain is also very much influenced by the inherited basement structure and pre-collisional basin architecture in such foreland settings, in particular inherited extensional fault systems. Tectonic contraction in such settings tends to promote the positive inversion of basins through LPS, folding and reverse faulting by the exploitation of pre-existing bounding structures and facilitated by the lower bulk density and strength of sedimentary fill sequences when compared to the surrounding basement.

This study will combine conventional structural field studies from the Variscides of SW Ireland with the spatial characterisation of finite strain using clast shape analysis and anisotropy of magnetic susceptibility (AMS) studies on deformed sandstones as a means of elucidating the temporal and spatial evolution of deformation in an orogenic foreland that is heavily influenced by pre-existing basement architecture and structures. AMS is increasingly seen to be a particularly sensitive proxy for tectonic strain signals in low to medium strain settings on the periphery of orogens.

The Variscan of SW Ireland offers an excellent opportunity to investigate the full foreland strain cycle from early bedding parallel oblate fabrics to more prolate intermediate fabrics and finally cleavage parallel oblate fabrics in an overall setting that incorporates marked basement control of the spatial distribution of finite strain. The adopted multi-technique approach will also allow for insights into the relative timing of deformation events.  

How to cite: Meere, P. A. and Parker, C. R.: The role of basement architecture in the spatial and temporal development of tectonic deformation in an orogenic foreland – An example from the Variscides of SW Ireland., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9481, https://doi.org/10.5194/egusphere-egu23-9481, 2023.

     We report studies of an ancient, ~10 km wide fold/thrust nappe belt developed as one zone of a 150 km wide, Archean collisional orogen in the North China Craton. Principles of cross-section construction and balancing were utilized to derive the first-reported, detailed map and crustal-scale profile of the Archean fold/thrust nappe belt. Combined with stratigraphy and geometric restoration, we demonstrate that the fold/thrust nappe pile consists of four major 2.69-2.52 Ga sub-horizontal nappes stacked upon each other, and ductile shear zones below each of them, emplaced onto a 2.56-2.51 Ga passive margin-like shallow marine platformal para-autochthon at 2.51-2.50 Ga. Four individual nappes were derived from different tectonic belts of an ancient paleo-ocean before they were emplaced. They are characterized by distinctive kinematic and thermal evolution.

    Geometric, statistical analysis of structural-fabrics and kinematic indicators illustrate that the fold/thrust nappes of forearc and oceanic floor sequences have been reworked by folding and thrusting with stratigraphic repetition during three kinematic phases, and the final emplacement was accomplished by a combination of rigid body translation along shear zones between nappes and the para-authochthon as well as internal strain by ductile deformation. The para-autochthon only records the predominant D₂ event, related to the fold/thrust nappes emplacement, but lacks the records of oldest kinematic phase.

    Thermal characteristic inferred by metamorphic studies reveal an inverted thermal structure in the fold/thrust nappes, indicated by the observation of a decrease in metamorphic grade from amphibolite facies in the nappes to greenschist facies in the underlying schistes lustrés forming the basal décollement; but it shows a downward temperature-increase in the metamorphic field gradient in the passive margin para-authochthon with relatively stable pressure records.

    This work explicitly unravels the first-order geometry and the tectono-structural evolution of the 2.5 Ga fold/thrust nappe belt by constraints on the scale, structural sequence, and metamorphic grade of the different tectonic units. This provides a quantitative basis for drawing analogies between Archean and present lithospheric deformation and discussing the style of tectonism during Archean orogenesis.

How to cite: Zhong, Y., Kusky, T., Wang, L., and Stüwe, K.: Structural and thermal evolution of an Archean fold/thrust nappe belt based on quantitative structural profile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6785, https://doi.org/10.5194/egusphere-egu23-6785, 2023.