This presentation challenges the conventional theory that thrust systems necessarily grow by the upward propagation of tip-lines of faults away from a basal detachment. The model underpins mechanical considerations of thrust belt dynamics, explanations of thrust-related folding and commensurate forecasting of distributed deformation in the surrounding strata. Yet there have been few tests of the paradigm at the cross-section scale – not least because the quality of seismic imagery in most foreland thrust belts is insufficient to resolve structural geometries. Rather, the widespread application of the upward propagation (conventional) theory results from a variety of cognitive biases, especially restrictive citation of published literature.

Here we present interpretations of exceptional 3D seismic imagery from the eastern, lateral termination of the Jura fold-thrust belt of Switzerland. The area (the Baden-Irchel-Herdern Zone, referred to here as the BIHZ - broadly overlying the eponymous basement lineament) shows relatively little horizontal shortening and therefore preserves structures that would otherwise be overprinted by more extensive thrusting.  Contractional structures are developed in a multilayer of Mesozoic strata that comprise competent carbonates and incompetent mudrocks and marlstones. Interpretations based on 2D seismic profiles display thrusts sweeping through the Mesozoic strata as simple, continuous ramps, splaying from a basal detachment in Triassic evaporites. However, the higher-quality 3D seismic imagery does not support this type of interpretation. Within the BIHZ, statal reflectors are offset by small-offset (<50m) thrust segments. These do not map out as continuous fault surfaces, either in depth or in map-view. Some thrusts dip forelandward, others towards the hinterland. Collectively, these thrusts are layer-confined and, in plan-view, have highly sinuous forms. These geometries are indicative of having formed by linkage of originally distinct fault segments. Collectively this zone of segmented/low-displacement thrusting represents a “bead” of distributed deformation, restricted to the BIHZ. Presumably, if the area then evolved with greater contraction, part of the thrust array will become fully-linked and develop as a continuous structure – perhaps as elsewhere along the Jura arc. Those fault segments not incorporated into the now-continuous thrust would remain, forming a broad halo of distributed faulting in the surrounding rock. In this spatial context, the swathe of distributed deformation might be designated as a “damage zone” with respect to the main thrust but in fact, the distributed faulting would have no causative relationship to this main thrust.

Our interpretation of thrust system evolution derives from the “ramps first” model of Eisenstadt & DePaor (1987) – a structural concept that has received substantially less attention that the “conventional theory”. We do not wish to imply that there is a single mechanism by which thrusts nucleate and grow – the “conventional” theory may well apply in some situations. So too, may others. We do however emphasise the importance of considering a range of different behaviours and their resultant structural geometries when interpreting thrust systems. Some of these may yet to be described!

How to cite: Butler, R., Robledo, F., Sleath, P., and Bond, C.: Thrust system growth by segment linkage in sedimentary multilayers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3200, https://doi.org/10.5194/egusphere-egu24-3200, 2024.

Tectonic mélanges, penetrative mixes of sediments and basalts, are often interpreted as fossil subduction plate interfaces. These formations, marked by intense deformation, contain witnesses of past earthquakes and commonly include remnants of the oceanic crust of the subducting plate. The original process that led to penetrative mixing of sediments and basalts is often controversial, and classically relies either on tectonic slicing of the downgoing plateduring subduction, or on pre-subduction, olistostrome-forming sedimentary mixing. In addition, magma emplacement in sediments of the downgoing plate may also explain the mixed lithologies of mélanges. This latter case should leave an aureole of contact metamorphism in sediments near basalts. In this work, we applied Raman Spectroscopy of Carbonaceous Material in the modern seafloor sediments where magmatism is reported, in order to evaluate the thermal influence from basalts onto carbonaceous material in the contacting sediments. Then, to check the potential contact metamorphism in mélanges, we employed the same methodology in several examples of sediment-basalt mélanges at (sub)-greenschist-facies conditions (Kodiak complex, Alaska; Shimanto Belt, Japan).

In modern ocean-floor settings, magmas intruding, and to a lesser extent, flowing onto sediments, resulted in higher crystallinity of carbonaceous material in a cm- to dm-thick contact aureole. In four of five studied mélanges, the crystallinity of carbonaceous material in metasediments increases toward basalts, indicating a ~1 cm-thick contact metamorphism aureole. Thus, we propose that for the studied mélanges the mixing likely occurred prior to subduction, with the preservation of contact metamorphism despite syn-subduction, low-temperature metamorphism.

As a consequence, the block-in-matrix structure observed in mélanges, as well as the occurrence of mafic bodies at seismogenic depths in accretionary prisms, is in many instances the result of pre-subduction structure, rather than tectonic slicing and step-down of the decollement into the oceanic crust. In particular, tectonic mélanges such as Mugi in Japan and Ghost Rocks in Alaska do not reflect simply the structure and thickness of the plate subduction interface, but a complex combination of pre-subduction geometry and tectonic processes during underplating and within the accretionary wedge.

Furthermore, in two studied paleo-accretionary complexes, deposition ages of the trench sediments forming the matrix of three examined mélanges overlap ages of magmatism within uncertainty. Considering that the basalts from these mélanges exhibit MORB signatures, this age overlap suggests that the mélanges possibly formed at the trench just before ridge subduction. We thus conclude that basalt-sediment mélanges stand as potential records documenting ancient ridge subduction events.

How to cite: Rajič, K., Raimbourg, H., Famin, V., and Moris-Muttoni, B.: The origin of tectonic mélanges and implication for the subduction interface processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8453, https://doi.org/10.5194/egusphere-egu24-8453, 2024.

The Jura fold and thrust belt is characterised by dominant thin-skinned thrusting of the Mesozoic-Cenozoic sedimentary cover and variable interaction with inherited structures and deep-seated (i.e., sub-detachment) faults. In its central parts, this fold-and-thrust belt is detached from an underlying mechanically stiffer basement, along Triassic evaporites and displaced up to 30 km. The eastern termination of the fold and thrust belt is characterised by (i) a much lower amount of shortening (on the order of the hundreds of meters), (ii) a significant thickness reduction of evaporites of the Muschelkalk Group, and (iii) interaction of the décollement with steps associated with faults bounding an underlying Permo-Carboniferous graben (Constance-Frick Trough; CFT).

Using recently processed seismic data (Nördlich Lägern 3D), we document the role exerted by the inherited structures rooted in the Permo-Carboniferous basin fill and basement in the development of the different styles of deformation and structures in an area located northwest of the city of Zürich. The ENE-striking master normal fault bounding the CFT to the south displays evidence of reactivation, during both extensional and compressional episodes, illustrated by apparent normal steps, alignments with no apparent displacement, and gentle folding of the Permo-Carboniferous basin fill. The investigated seismic volume indicates that contractional deformation is concentrated in two major ENE trending fold-thrust zones involving the Triassic-Jurassic epicontinental platform succession and the stratigraphically overlying Cenozoic Molasse Basin deposits. The northern fold-thrust zone (NFTZ), manifested at the surface by the Siglisdorf anticline, consists of an up to 2 km wide pop-up structure linked to thrusting in the detached cover series. The structure is located in correspondence with steps in the topography of the base Mesozoic unconformity produced by south dipping ENE striking minor faults rooted in the Permo-Carboniferous sequence without affecting the detachment integrity. The southern fold-thrust zone (SFTZ), corresponding to the Baden-Irchel-Herdern Lineament, is located above a north dipping major fault of the Late Paleozoic half-graben. The overlying deformation structures in the SFTZ, comprising multiple thrusts and backthrusts, that form fish-tail structures in a narrow, steep zone involving the Mid-Triassic-Jurassic series, are interpreted as the result of layer-parallel shortening associated with buckling of the Mesozoic multilayer. In contrast, the Mesozoic succession is characterized by low deformation and absence of major faults in the area over the central part of the graben (comprised between the Weiach – Glattfelden – Eglisau Lineament – WGEL – to the north and the south fold-thrust zone). The WGEL separates a sub-horizontal to gently folded Mesozoic rock panel to the south from a gently dipping panel to the north. This suggests that fault reactivation may have been accompanied by mild basin shortening and inversion.

In conclusion, different modes of interaction between the Mesozoic multilayer (including the weak evaporite level at its base) and underlying Upper Paleozoic basin fill with inherited basement faults have produced a marked contrast in structural style among the detachment-dominated NFTZ, the buckling-dominated SFTZ, and a low deformation central area.

How to cite: Zambrano, M., Mazzoli, S., and Mosar, J.: Influence of Inherited Structures on Deformation Patterns in the Eastern Jura Fold and Thrust Belt: Insights from 3D Seismic Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13025, https://doi.org/10.5194/egusphere-egu24-13025, 2024.

The Aragüés thrust system, in the Southern Pyrenees, is a superb and well-exposed sequence of thrust imbricates to study the deformation and development of fault-propagating folds. The mechanisms of folding and thrusting, including the importance of material contrast during the development of the thrust sequence, are well-studied in this area. However, insights into the distribution and magnitude of penetrative strain remain unknown. Therefore, analysis of anisotropy of magnetic susceptibility (AMS) is used to reveal penetrative deformation within the sedimentary rocks of the Aragüés thrust system. Rock samples from 66 locations are collected across the Aragüés thrust imbricates with a focus on the layered limestone unit. A total of 489 cylinders were retrieved from the oriented hand samples and their AMS signal was measured. Thermomagnetic and magnetic remanence measurements show that the magnetic susceptibility of the samples is dominated by paramagnetic minerals and the magnetic lineation (axis of maximum susceptibility) is mainly parallel to subparallel to bedding. However, the more strongly magnetic samples (kmean: 3-5 x 10-4SI) show a magnetic lineation parallel to the general NNE-SSW shortening direction at the back- and forelimbs of fault-propagation folds. In contrast, the samples with a lower magnetic susceptibility (kmean: 1-3 x 10-4 SI), which are also closer to fold hinges, reveal a magnetic lineation perpendicular to the main shortening direction. We interpret the differences in fabric alignment and magnetic susceptibility to mineral composition and structural evolution. For example, the magnetic lineation parallel to the shortening direction is a consequence of flexural slip and flow along bedding surfaces or incompetent beds of the layered limestone units in the fold limbs. Additionally, a relationship between the magnetic lineation and the development of cleavage and stylolites within the limbs can be identified, but is not consistent. The magnetic lineation at the fold hinge is perpendicular to the main shortening direction and needs further investigation for strain accommodation prior to folding, an effect of buckling and/or development as intersection lineation. These first interpretations of the magnetic fabric provide additional insights into the deformation of the Aragüés thrust system and the development of fault-propagating folds. Moreover, the AMS data adds new information on penetrative strain in this region of the Southern Pyrenees and reveals a general tectonic overprint on grain-scale within sedimentary rocks during deformation.

How to cite: Schöfisch, T., Koyi, H., Teixell, A., and Almqvist, B.: Penetrative strain of the Aragüés fault-propagating folds in the Southern Pyrenees revealed by magnetic fabric analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9604, https://doi.org/10.5194/egusphere-egu24-9604, 2024.

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-wedge and foreland of the southern Pyrenees and the retro-wedge and foreland of the northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this immature orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. These records are best preserved in the North Pyrenean Zone (NPZ), the retrowedge thrust belt and its adjacent syn-orogenic basin. This paper reviews along strike variations in structural and stratigraphic characteristics of the NPZ and adjacent basin evolution and explores the changing role in space and time of rheological, thermal and structural Inheritance and local and plate scale drivers.

How to cite: Ford, M.: Lateral and temporal variations in thrust belt behaviour in a low convergence retrowedge, north Pyrenees. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4040, https://doi.org/10.5194/egusphere-egu24-4040, 2024.

We present a new cross-section across the Jaca basin in the southern Pyrenees, aiming to analyze the temporal and spatial distribution of deformation in a fold-and-thrust belt. In this study, we have integrated all available subsurface data, including seismic sections and well data, with surface data to construct a balanced cross-section and partial restored cross-sections that illustrate the evolution of the south-Pyrenean fold-and-thrust belt.

Contractional deformation, initiating the development of the Pyrenean Mountain belt, started during Late Cretaceous times due to the reactivation of the Early Cretaceous hyperextended margin. Subsequently, deformation progressed during the Paleogene within the unstretched Iberian plate. In this region, the kinematics of the thrust wedge was controlled by inherited extensional faults and the unevenly distribution of Triassic evaporites above the basement.

Although deformation progressed toward the foreland, synchronous thrusting characterized the internal deformation of the thrust wedge. The localization and switching of deformation along the fold-and-thrust belt resulted from the interaction of unevenly distributed weak layers, including salt, and syntectonic sedimentation.

This work illustrates how detailed fieldwork, when combined with subsurface data and news concepts and methodologies (such as the use of drone imagery and analogue and numerical modelling), can significantly improve the understanding of the structure and evolution of fold-and-thrust belts.

How to cite: Muñoz, J., Izquierdo, E., Toro, R., Santolaria, P., Granado, P., Pueyo, E., and Casas, A.: Cover-basement relationships in a fold-and-thrust belt with inherited uneven weak horizons distribution: example from the southern Pyrenees across the Jaca basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7287, https://doi.org/10.5194/egusphere-egu24-7287, 2024.

The Apennines fold-and-thrust belt builds part of the Africa-Eurasia convergent plate boundary. It developed due to the Neogene subduction of the Alpine Tethys underneath Europe and to the subsequent involvement of the Adria microplate rifted margin into the collisional process. Since the Miocene, eastward retreat of the slab caused extensional deformation of the thrust pile, eventually leading to the opening of the Tyrrhenian back-arc basin. Multiple schools of thought exist about the structural style of the Apennines, each of which proposes rather irreconcilable models. The amount of shortening, the involvement of the crystalline basement, the architecture of the inherited rifted system with its degree of reactivation during convergence, and the role played by compressive inheritance during back-arc extension, are still under discussion.

In this contribution we focus on the crustal structure of the central Apennines, along a transect that stretches from the Sardinian shelf (West) to the center of the Adriatic Sea (Est). Our aim is to critically evaluate prior models and contextually to illuminate the deeper part of the belt, which is still seldom considered in modern-day Central Apennines solutions. Based on the most recent ideas on rift inheritance and thrust tectonics, we present a brand-new crustal balanced cross-section. We employ well-constrained surficial geological data from available public maps, as well as the most recent deep-reaching geophysical data (especially, tomography and seismological data), which are supplemented by thermochronological, biostratigraphic, and Sr-isotope datings. On top of that, data are encompassed in a coherent geodynamic framework that is supported by a geometrically balanced and consistent kinematic model. This way, we provide a comprehensive explanation for the regionally observed coupled forward migration of compressional and extensional domains, which is related to the slab pull/trench retreat system.

Our results point to a predominantly thin-skinned style for the orogen with a secondary basement involvement in the later collisional stage. Inherited extensional faults, developed during the Mesozoic rifting of Adria, were partially reactivated during convergence, and influence spacing and geometry of the compressional features. The main thrusts of the area are characterized by significant displacements, ranging from 10 to >50 km, and sole out into a basal décollement located at the base of the post-Variscan sedimentary sequence. Finally, post-thrusting back-arc extension is accommodated by faults that either displace the compressional décollement levels or reactivate them with an opposing kinematics.

How to cite: Maresca, A., Granado, P., Muñoz, J. A., Manatschal, G., Ogata, K., and Tavani, S.: Rift inheritance in a fold-and-thrust belt evolution: Central Apennine crustal balanced cross-section, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5837, https://doi.org/10.5194/egusphere-egu24-5837, 2024.

The central Southern Alps (cSA) form a complex south-verging polyphase fold-and-thrust belt developed from the Late Cretaceous onward due to the Alpine convergence (Schönborn, 1992; Zanchetta et al., 2015). The oldest ages related to thrust stacking have been obtained from pseudotachylythes along the Orobic Thrust exposed in the northern part of the belt (Zanchetta et al., 2011), stacking the Variscan basement onto the Permian-Triassic cover.

Aim of this contribution is to present new U-Pb radiometric ages of calcite tectonites obtained in the central and southern portion of the belt, where no precise time constrains of thrust stacking are available. E-W trending Eocene dike swarms crosscutting thrust planes are the only indirect evidence of a pre-Eocene age of the oldest stages of the Alpine contraction (D’Adda et al., 2011).

The central part of the cSA shows a thick pile of thrust sheets deforming the Lower to Middle Triassic carbonate successions. Our new U-Pb calcite ages obtained on growth fibers along fault planes, veins and calc-mylonites sampled along some of the most important regional thrust planes mainly result in Late Cretaceous ages. We obtained similar ages also within the southern portion of the belt, where the Norian “Dolomia Principale” thrust sheets, override the Rhaetian Riva di Solto Shale immediately to the north of the frontal portion of the belt. Younger ages resulted from the Paleogene units which are involved in the exposed frontal part of the belt, which is mostly buried under the recent infilling of the Po Plain forming the Milan Belt.

These data confirm previous interpretations of the central Southern Alps as part of a Late Cretaceous doubly vergent pre-collisional belt (Zanchetta et al., 2012). Research supported by the FAST PRIN Project (2021-NAZ-0299, Italian MUR).

D'Adda, P., Zanchi A., Bergomi M. A., Berra F., Malusà M. G., Tunesi A., & Zanchetta S. (2011) - Polyphase thrusting and dyke emplacement in the central Southern Alps (Northern Italy), International Journal of Earth Sciences, 100, 1095–1113.

Schönborn, G. (1992) - Alpine tectonics and kinematic models of the central Southern Alps. Memorie di Scienze Geologiche, 44, 229-393.

Zanchetta S., D'Adda P., Zanchi A., Barberini V. & Villa I.M. (2011) - Cretaceous-Eocene compressions in the central Southern Alps (N Italy) inferred from 40Ar/39Ar dating of pseudotachylytes along regional thrust faults. Journal of Geodynamics, 51, 245-263.

Zanchetta, S., Garzanti, E., Doglioni, C., and Zanchi, A. (2012). The Alps in the Cretaceous: a doubly vergent pre-collisional orogen, Terra Nova, 24, 351–356, 2012.

Zanchetta S., Malusà M. & Zanchi A. (2015) - Precollisional development and Cenozoic evolution of the Southalpine retrobelt (European Alps). Lithosphere, 7, 662-681.

How to cite: Zanchi, A., Zanchetta, S., Rocca, M., Montemagni, C., Aldega, L., Kylander-Clark, A., Fiorini, A., and Carminati, E.: U-Pb calcite geochronology attests Late Cretaceous S-verging thrusting in the central Southern Alps, Italy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10919, https://doi.org/10.5194/egusphere-egu24-10919, 2024.

Fold-and-thrust belts are usually deformed by major transverse zones oriented orthogonally to the main thrust faults. These structural discontinuities represent ancestral fault zones originated mainly during rifting and subsequently reactivated during orogenic shortening (Zanchi et al., 2012). Sectors displaying thicknesses and sedimentary facies changes within the syn-rifting successions are juxtaposed along transverse zones (Thomas, 1990). This variability results in diverse deformation and shortening styles, as thrusts exhibiting flat (detachment levels) or ramp (more competent lithologies) geometry occur at different depths.

Limited data exist on the origin of fluids circulating along transverse zones and how they control fluid flow at the regional scale. In this study, we analyse transverse zones in the Lecco area of the Central Southern Alps (Lombardy, Italy). This belt features N-S trending, km-scale transverse zones initiated as normal faults during Ladinian, Norian-Rhaetian and Early Jurassic rifting phases and subsequently reactivated as strike-slip faults and/or lateral ramps during the Alpine orogeny (Schönborn, 1992). Two transverse zones are foci of this work: the Lecco Line to the west, running along the southeastern branch of Como Lake, and the Faggio Line to the east.

Geological mapping and mesostructural analysis were conducted and geological cross sections were built to constrain the geometry and kinematics of such transverse zones. Inorganic thermal indicators were used to constrain the eroded overburden and the exhumation depth of the analysed fault zones. U-Pb radiometric dating on syntectonic calcite mineralizations allowed us to constrain the age of tectonic activity. Syntectonic calcite veins precipitated either during the Early Jurassic rifting phase or younger Alpine orogenic phases, testifying the complex and long-lasting tectonic history of these transverse zones. C and O stable isotopes analysis allowed us to assess the origin of fluids circulating within transverse zones and their degree of interaction with host rocks. Two significant findings emerged from these analyses: 1) calcite mineralizations with variable δ¹³C and δ¹⁸O values characterize these fault zones, pointing out different degrees of fluid-rock interactions and/or different origin of fluids circulating during rifting and orogenic shortening; 2) transverse zones and related structures bound sectors where mineralizations occasionally exhibit values within narrow ranges of δ¹³C and δ¹⁸O; conversely, in other sectors, isotopic values display significantly wider ranges. In conclusion, transverse zones in the Southern Alps are structural features with a complex tectonic and fluid flow history, as testified by the large variability of age and C-O stable isotopes values. Furthermore, they possibly exert a partial compartmentalization of fluid flow at the regional scale.

References:

- Schönborn, G. (1992). Alpine tectonics and kinematic models of the central Southern Alps. Memorie di Scienze Geologiche, 44, 229–393.

- Thomas, W. A. (1990). Controls on locations of transverse zones in thrust belts. Eclogae Geologicae Helvetiae, 83(3), 727-744.

- Zanchi, A., D’Adda, P., Zanchetta, S., & Berra, F. (2012). Syn-thrust deformation across a transverse zone: the Grem–Vedra fault system (central Southern Alps, N Italy). Swiss Journal of Geosciences, 105(1), 19-38.

How to cite: Fiorini, A., Aldega, L., Dallai, L., Di Marcantonio, E., Rocca, M., Tavani, S., Zanchetta, S., Zanchi, A., and Carminati, E.: Multiple tectonic phases and fluids variability along transverse zones of the Central Southern Alps (Lombardy, Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4663, https://doi.org/10.5194/egusphere-egu24-4663, 2024.

In the Nurra region (NW Sardinia), along the coast that stretches from Cala Viola to Cala del Turco bays, a stratigraphic sequence of continental sediments ranging from the upper Paleozoic to the lower Mesozoic is wonderfully exposed.

In the last twenty years this area has been a subject of renewed studies due to its exceptional stratigraphic and sedimentological features and thanks also to an exceptional paleontological record, encompassing trace fossils and body fossils, which provided key chronological data. The most recent investigations have been focused on stratigraphic and structural geology aspects, allowing to reconstruct the tectonic evolutionary history of Sardinia, from the middle Permian up to the Middle Triassic.

The redefinition of the stratigraphic and structural framework of the area became possible through integrating traditional geological surveys (mainly, field mapping, facies and structural analyses) with analyses conducted on 3D digital models of outcrops (DOMs) obtained via drone-based aerophotogrammetry. Investigations, both in the field and computer-based, have enabled the identification of the arrangement of cross-cutting relationships among fault systems.

Despite the absence of absolute geochronological constraints (e.g., dating of syn-kinematic mineralizations), it has nevertheless been possible to identify at least six to eight deformational events. These analyses have revealed the likely history of deformational processes in this sector of the Island, highlighting the tectonic events that have occurred from the Permian until at least the Pliocene, despite Pleistocene-Holocene tectonics cannot be excluded. The events can be summarized as follows:

• Event 1: extensional tectonics related to the angular unconformity between Permian and Triassic deposits (age post early middle Permian to Early Triassic)
• Event 2: extensional tectonics related to the rifting of the Ligurian-Piedmont Ocean and subsequent carbonates deposition (Middle Jurassic: Bajocian-Bathonian)
• Event 3: extensional event related to the early Pyrenaic deformation and represented by an extensive angular unconformity/hiatus in carbonate platform (Early Cretaceous: Aptian-Albian)
• Event 4: transpressive and compressive event related to deformative phase called “Laramian” tectonics (Late Cretaceous)
• Event 5: compressive and consequent transcurrent tectonics related to the “Pyrenaic phase” (Eocene to Aquitanian age)
• Event 6: extensional tectonics related to the opening of the Balearic basin and the rotation of the Sardinia-Corsica block (Burdigalian age)
• Event 7: extensional tectonic phases occurred during two different sub-events (Serravalian age and the Pliocene age)
• Event 8: extensional event occurred during the Pleistocene-Holocene

How to cite: Scarani, R., Cipriani, A., Menegoni, N., Stori, L., Citton, P., Romano, M., Nicosia, U., and Ronchi, A.: The Cala Viola-Torre del Porticciolo coastal area: classical geological survey and 3D digital outcrop models (DOMs) analysis to unravel the polyphase tectonics in NW Sardinia (Italy)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12571, https://doi.org/10.5194/egusphere-egu24-12571, 2024.

The overall SW-vergent and in-sequence structural architecture of the Dinarides fold and thrust belt resulted from collision of the Adriatic Microplate with Eurasia in the Late Cretaceous. Throughout Eo-Oligocene times, the deformation front extended outward, causing substantial crustal shortening within the External Dinarides. This part of the orogen is predominantly composed of Mesozoic carbonate platform rocks, originally deposited in an external passive margin setting. Analysis of fault kinematics and two balanced cross-sections suggests a Cenozoic deformation characterized by along-strike contraction. The 250 km long dextral transgressive Split-Karlovac Fault acts as a boundary, separating a southern, SW-vergent nappe stack forethrust dominated domain (as observed in the Split cross-section) from a northern western NE-vergent backthrust dominated segment (as observed in the southern Velebit cross-section). To understand the reasons for the contrasting along-strike deformation, a reevaluation of the temporal and spatial distribution of Paleo-Mesozoic lithofacies along- and across-strike on both sides of the Split-Karlovac Fault was conducted. Additionally, an assessment of the impact of mechanical stratigraphy on deformation styles in this section of the fold-thrust belt was undertaken.

The best-fit kinematic forward model for the central Velebit Mtn. portrays a 75 km wide triangle zone, which took up at least 47 km of Eo-Oligocene shortening. The triangle structure comprises a SW-vergent thick-skinned duplex system detached in the lower Paleozoic Adriatic basement and five thin-skinned backthrusts detached in the upper Paleozoic basement. These backthrusts nucleated at lateral facies boundaries, related to extensional half grabens that formed due to passive margin extension in Middle Triassic and Late Jurassic times. During Cenozoic folding and thrusting these inherited Mesozoic half graben boundary faults were selectively inverted into the NE-vergent backthrusts. This process contributed to the observed along-strike variations in the deformation style of the External Dinarides.

An analysis of instrumentally recorded earthquakes within the northwestern structural domain unveils contrasting seismic activity along the central and southern Velebit transects. In the central Velebit Mountain, the triangle structure currently predominantly undergoes strike-slip motion, with reverse faulting predominantly occurring to the east of the Split-Karlovac Fault. Conversely, seismic activity along the southern Velebit cross-section is concentrated in the structurally lowermost parts of the triangle zone and the foreland, while its structurally higher sections exhibit lower seismic activity. The prevalence of reverse faulting along this transect suggests the ongoing accommodation of shortening in this region.

How to cite: Balling, P., Tomljenović, B., Herak, M., and Ustaszewski, K.: Exploring mechanical stratigraphy's influence on deformation and seismicity: A 2D kinematic modelling study in the central External Dinarides, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12309, https://doi.org/10.5194/egusphere-egu24-12309, 2024.

Studying the complex subsurface structures in fold-and-thrust belts, such as the Eastern Carpathian Bend Zone, remains challenging due to poor seismic data quality caused by steep and intricate structures alongside multiple detachment levels.

To address this, we have conducted eight analogue modelling experiments at the Structural Modelling Laboratory of Babes-Bolyai University with mechanical stratigraphy scaled to the area of interest. We used coloured quartz sand, glass microspheres and silicone to simulate brittle rocks, ductile shales, and salt, respectively. The materials were layered in a 120 cm long deformation box, which was shortened at a constant rate of one centimetre per hour. The experiments were monitored using timelapse photography and particle image velocimetry. The models were consolidated, serially sectioned, and photographed. MOVE software (Petroleum Experts) was used for importing the section images and interpreting the 3D structural style for the models.

The structural style of the experiments varied based on the change in parameters, such as layer thickness, materials, shortening amounts, as well as the timing of erosion and salt deposition. The following description will refer to the most common deformation features observed in the experiments but will use the equivalent formation names and ages. The most noteworthy features are the post-salt lower Miocene's decoupling from the pre-salt section, the lower Miocene upper Kliwa formation decoupling from the Oligocene lower Kliwa formation along the lower Miocene Podu Morii shales, and the distinct wavelengths for the post-salt, upper Kliwa, and pre-upper Kliwa structures.

Considering well data, outcrop data, analogue modelling observations, and the 3D model, we performed seismic interpretation. This guided seismic data interpretation revealed short-wavelength upper Kliwa disharmonic folding, Oligocene–Cretaceous thrust-related hanging-wall anticlines, and large thrust fault displacements leading to hanging-wall erosion and out-of-sequence reactivation of the thrust faults. The results of these experiments proved to be effective tools for comprehending the controlling factors of deformation in the Eastern Carpathian Bend Zone and can aid in the interpretation of subsurface data, especially in areas with poor seismic data quality.

How to cite: Tocariu, I. S. M., Tamas, A., Tamas, D. M., Dohan, D., Lapadat, A., Schleder, Z., and Krezsek, C.: Analogue Modelling to Support Seismic Interpretation in the Eastern Carpathian Bend Zone, Romania, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-771, https://doi.org/10.5194/egusphere-egu24-771, 2024.

Deep earth geodynamic processes shape surface geology, topography, and the earth’s crust evolution with consequences on erosion, deposition, climate, and biogeography. This research investigates the early-stage growth of the NW Zagros belt in the Kurdistan region of Iraq after the early Oligocene Arabia-Eurasia collision and the geologic signatures of potential SE-ward propagating Neotethys slab tearing. We will test two end-member hypotheses: (i) the slab has already detached, causing subsidence before slab tearing, followed by rapid regional uplift in the vicinity of the suture zone afterward, or (ii) the slab is still attached, causing no pulse of significant hinterland uplift but continuous regional lower plate subsidence. In the NW Zagros belt, where the Arabian and Eurasian plates are sutured along the Main Zagros fault, allochthonous thrust sheets of ophiolitic and arc-related terranes were emplaced onto post-collisional autochthonous units of clastic and lower-middle Miocene shallow marine carbonate rocks, now at ~1-1.5 km elevation. The allochthonous thrust sheets host pre-collisional acidic and mafic intrusions that provide exhumation rate constraints throughout collision, whereas the post-collisional marine carbonate rocks attest to a significant suture zone subsidence and uplift. Thermal history modeling of bedrock samples using zircon and apatite (U-Th)/He thermochronometers suggests an early uplift during ~30-25 Ma, with most apatite (U-Th)/He samples being reset during ~15-10 Ma. To quantitatively determine the depositional age of the lower-middle Miocene marine carbonate rocks, ⁸⁷Sr/⁸⁶Sr isotope chronstratigraphy will be conducted. Furthermore, the isopach maps of the middle and upper Miocene foreland deposits show a notable shift in the depocenter axis and an enhanced subsidence toward the SE, concurrent with the arrival of the Afar plume to the suture zone in the NW. These preliminary results argue for the formation of the NW Zagros orogenic wedge as early as ~30-25 Ma, followed by suture zone subsidence during the early-middle Miocene and then uplift again during ~15-10 Ma, possibly due to initiation of Neotethys slab-tearing and its subsequent propagation along the suture zone from the NW to the SE. These findings have implications for investigating the role of the Arabia-Eurasia land bridge formation and deformation on vertebrate distribution and paleoclimate response to orographic barrier development since the late Miocene in the Middle East.

How to cite: Koshnaw, R., Kley, J., Dunkl, I., Ameen, F., and Vasilyan, D.: Tectonothermal constraints on the early-stage orogenic wedge formation along the Arabia-Eurasia suture zone in the NW Zagros orogenic belt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3749, https://doi.org/10.5194/egusphere-egu24-3749, 2024.

Fold-thrust belts are complex tectonic domains formed in response to near- or far-field compressional stress fields in the Earth’s crust. The complexity of structural style in theses belts is controlled by multiple factors. The presence of mechanically weak layers, defined by rocks that exhibit lower strength than their surroundings, play a significant role in the formation and evolution of fold-and-thrust belts. In this research, a two-dimensional numerical finite difference model incorporating a visco-elasto-plastic/brittle rheology is utilized to investigate the structural evolution of the Fars arc in the southeast of the Zagros fold-thrust belt. The modeling includes the tectonic inversion process, from Permian-Triassic rifting of the Neo-Tethys Ocean to the Oligocene-to-present continental collision between the Arabian and Eurasian plates. The Eulerian grid dimensions were considered as 500 km in length and 60 km in thickness, respectively, comprising 1101×121 nodes. Each cell contains 16 Lagrangian markers that carry the information and properties of each layer. According to the stratigraphic column of the Fars arc, there are 30 km of basement overlain by a 2-km-thick salt layer with a 3-kilometer-thick Palaeozoic sedimentary sequence above the salt layer. Furthermore, to replicate the interplay between Earth's internal and surface dynamics, a 25-kilometer-thick layer representing air is incorporated at the uppermost part of the model to approximate a free surface. The experiments include three inherited basement faults. The role of pre-existing weak zones in extensional tectonics is the shaping of rift basin geometry, especially the formation of half grabens. In the convergence phase, fold-thrust-belts with a basal detachment layer and pre-existing faults produce folding at two distinct scales. The faulting in the basement develops long wavelength folds in the sedimentary cover, while the presence of a salt layer shapes the smaller wavelength folds and thin-skinned thrust faults that extend from the detachment layer to the surface. The reactivated faults play an important role in stress transfer, leading to the emergence of new faults and seismic events. The results indicate that deformed listric faults show a meaningful correlation with the depth distribution of earthquakes throughout the Fars arc. The outcome associated with the presence of a thick basal salt layer and involvement of the basement exhibit a correlation with the structural style of the Fars arc.

How to cite: Gomar, F., Ruh, J. B., Najafi, M., and Sobouti, F.: Numerical modeling of basement inheritance and salt decoupling effects on the structural evolution of the Zagros Fold-Thrust Belt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3163, https://doi.org/10.5194/egusphere-egu24-3163, 2024.

The Fars Arc constitutes the southeastern segment of a tectonic recessions and salients array that characterizes the Zagros orogen. The delineation of the Fars Arc's western boundary is attributed to the prominent Kazerun fault system. However, further to the west, the southwestern periphery of the Arc (i.e., the Bushehr area) remains relatively unexplored, lacking documented evidence of tectonically active structures, as well as significant historical and instrumental seismic events. We combined tectonic geomorphology, remote sensing, and Quaternary geochronology to comprehensively study and identify tectonically active structures within the onshore area. Investigating the offshore area, we utilized 2D seismic-reflection data to interpret and image active subsurface structures in the Persian Gulf. We mapped and introduced several active faults in the Bushehr area, some of which are closely associated with active Quaternary anticlines, e.g., the Bushehr and Abtavil anticlines. Two generations of uplifted marine terraces, Terrace-I and Terrace-II across the Bushehr anticline, reveal a local uplift rate of approximately 0.8 mm/yr across the Bushehr Peninsula. The offshore 2D seismic-reflection data indicate ongoing deformation in the study area and show evidence of a recent sedimentary depocenter offshore of the Bushehr Peninsula. These observations define the present-day active deforming structures along the Zagros orogenic front in the Persian Gulf. The 2D seismic-reflection interpretations have characterized the active folding of the Bushehr anticline, indicating a minimum age of approximately 600 ka BP. The integration of on- and offshore geological analysis has revealed contemporaneous incipient deformation and syn-kinematic sedimentation, which significantly contributes to our understanding of geodynamics of the Zagros deformation front.

How to cite: Fathian, A., Koyi, H., Back, S., Nazari, H., H. Shugar, D., Shokri, M. A., and Reicherter, K.: Active tectonics within the southwestern edge of the Fars Arc, SW Iran, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6848, https://doi.org/10.5194/egusphere-egu24-6848, 2024.

The Paleogene West Spitsbergen Fold-and-Thrust Belt, WSFTB, is present in Svalbard (Norwegian High Arctic archipelago; 74-81°N, 15-35°E). The WSFTB’s contraction reflects the Eurekan Orogeny and transpressional continental breakup along a transform fault zone, followed by opening of a seaway between the North Atlantic and Arctic oceans. The overall timing of the contraction is not well constrained, and many authors link it to the development of a deep and narrow foreland basin system filled with Uppermost Paleocene and Eocene to Oligocene(?) deposits, while the Lower Paleocene deposits are considered to be deposited in a regional subsidence prior to Eurekan deformation. The WSFTB divides into a thick-skinned, basement-involved fold-thrust complex in the west, passing into a central zone of thin-skinned, fold-thrust units with associated detachments developed in Permian evaporites and Mesozoic organic-rich mudstones, separated by a prominent duplex system that reflects a thrust-ramp transferring movements from Permian to Mesozoic detachments eastwards. Stress transfer also reactivated the steep, basement rooted Billefjorden and Lomfjorden fault zones farther east, showing up to 200 m of reverse stratigraphic offsets. These fault zones are long-lived structural elements exposing multiple reactivation events since the Late Palaeozoic.

Our study targets the Paleogene reactivation and especially the sequence of deformation of the Billefjorden Fault Zone, based in detailed interpretations of onshore seismic lines, and field mapping supported by acquired and interpreted mountain-scale digital outcrop models. Results suggest initial shortening by reactivation of deep-rooted extensional faults, seen as reverse offsets of stratigraphic units and formation of fault-propagation monoclines as anticlines-synclines pairs. Subsequent deformation involved the regional-scale (tens of km long) decollement zones hosted by the Mesozoic mudstones. West of the Billefjorden Fault Zone, the decollement is seen in the Jurassic Agardhfjellet Formation. Crossing the Billefjorden Fault Zone, this decollement is seen truncating the reverse fault (digital outcrop data) and fault propagation fold (seismic data) and displacing them 2 km to the east, where the decollement continuous in the uplifted mid-Triassic mudstones of the Botneheia Formation. This sequence of deformation, with the reactivated deep-rooted faults truncated by the decollement, contradicts previously published models that advocate for reactivation of deep-rooted faults taking place late during the deformation, synchronised with an uplift of thick-skinned basement-involved thrusts.

The timing of the deep-rooted faults reactivation is poorly constrained. In this study, we hypothesise that it can be as early as the early to middle Paleocene, corresponding to phase of regional compression at 61 Ma indicated, by the recent studies, for the Lower Paleocene Firkanten Formation. This contraction could also be determined from the westward thickening and locally NE and E sourcing deposits of the Firkanten Formation and overlying Basilica and Grumantbyen formations, which indicate an existing topographic high or a foreland bulge as suggested by some of the authors. The timing of the second stage is consistent with the main shortening phase in the Eocene.

How to cite: Smyrak-Sikora, A., Braathen, A., Osmundsen, P. T., and Senger, K.: Sequential reactivation of the Billefjorden Fault Zone during evolvement of the West Spitsbergen Fold and Thrust Belt, Svalbard., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12111, https://doi.org/10.5194/egusphere-egu24-12111, 2024.