Cenozoic reactivation of the Great Glen Fault, Scotland: additional evidence and possible causes

International audienceThe Great Glen Fault trends NNE-SSW across northern Scotland. According to previous studies, the Great Glen Fault developed as a left-lateral strike-slip fault during the Caledonian Orogeny (Ordovician to Early Devonian). However, it then reactivated right-laterally in the Tertiary. We discuss additional evidence for this later phase. At Eathie and Shandwick, minor folds and faults in fossiliferous Jurassic marine strata indicate post-depositional right-lateral slip. In Jurassic shale, we have found bedding-parallel calcite veins ('beef' and 'cone-in-cone') that may provide evidence for overpressure development and maturation of organic matter at significant depth. Thus, the Jurassic strata at Eathie and Shandwick accumulated deeper offshore in the Moray Firth and were subject to Cenozoic exhumation during right-lateral displacement along the Great Glen Fault, as suggested by previous researchers. Differential sea-floor spreading along the NE Atlantic ridge system generated left-lateral transpressional displacements along the Faroe Fracture Zone from the Early Eocene to the Late Oligocene (c. 47-26 Ma), a period of uplift and exhumation in Scotland. We suggest that such differential spreading was responsible for reactivation of the Great Glen Fault. Indeed, leftlateral slip along the Faroe Fracture Zone is compatible with right-lateral reactivation of the Great Glen Fault

Variations in amount and direction of seafloor spreading along the northeast Atlantic Ocean and resulting deformation of the continental margin of northwest Europe

International audienceThe NE Atlantic Ocean opened progressively between Greenland and NW Europe during the Cenozoic. Seafloor spreading occurred along three ridge systems: the Reykjanes Ridge south of Iceland, the Mohns Ridge north of the Jan Mayen Fracture Zone (JMFZ), and the Aegir and Kolbeinsey Ridges between Iceland and the JMFZ. At the same time, compressional structures developed along the continental margin of NW Europe. We investigate how these compressional structures may have resulted from variations in the amount and direction of seafloor spreading along the ridge system. Assuming that Greenland is rigid and stationary, we have used a least squares method of palinspastic restoration to calculate differences in direction and rate of spreading along the Reykjanes, Kolbeinsey/Aegir and Mohns Ridges. The restoration generates relative rotations and displacements between the oceanic segments and predicts two main periods of left-lateral strike slip along the main oceanic fracture zones: (1) early Eocene to late Oligocene, along the Faeroe Fracture Zone and (2) late Eocene to early Oligocene and during the Miocene, along the JMFZ. Such left-lateral motion and relative rotation between the oceanic segments are compatible with the development of inversion structures on the Faeroe-Rockall Plateau and Norwegian Margin at those times and probably with the initiation of the Fugløy Ridge in the Faeroe-Shetland Basin during the Eocene and Oligocene. The Iceland Mantle Plume appears to have been in a position to generate differential seafloor spreading along the NE Atlantic and resulting deformation of the European margin

A new 4D model of Alpine orogenesis based on AlpArray

Wholesale slab breakoff or detachment in the Alps in late Paleogene time has been invoked to explain Periadriatic calc-alkaline magmatism (43-29 Ma), rapid exhumation of HP metamorphics, as well as clastic infill of proximal parts of the Alpine Molasse basin (30-28 Ma). However, the 14 My timespan of these events exceeds the duration of slab detachment estimated from thermomechanical modelling (2-8 My) and from foreland depocenter migration (~5 My) along equivalent lengths of neighboring Alpine orogens with torn slabs (Carpathians, Apennines). Moreover, wholesale slab detachment does not explain major E-W differences in Neogene crustal structure, basin evolution, erosion and indentation in the Alps. Teleseismic Vp tomography from AlpArray suggests that the slab segment beneath the Central Alps comprises European lithosphere, is attached to its orogenic lithosphere and extends down to ~250 km depth, in parts possibly even to the Mantle Transition Zone (Fig. 1). This marks a first phase of partial slab detachment, probably in late Paleogene time based on comparing slab length with shortening in the C. Alps and of Adria-Europe convergence since 35 Ma. In contrast, the slab segment beneath the Eastern Alps is detached between 80-150 km depth. The age of this second phase of slab detachment is bracketed at 23-19 Ma by criteria below and by comparing vertical detachment distance with global slab sink rates. We propose a new model of Alpine mountain-building that features the northward motion of subduction singularities above delaminating and detaching Alpine slab segments, respectively in the C. and E. Alps (Fig. 2), to explain the aforementioned E-W differences in Oligo-Miocene structure, magmatism, and foreland sedimentation. Mountain-building began at ~35 Ma with a decrease in Adria-Europe convergence to <1cm/yr collision, causing the European slab to steepen and detach beneath both the Central and Eastern Alps. Periadriatic magmatism may have initiated prior to slab detachment due to fluxing of the cold mantle wedge by fluids from devolatilizing crust along the steepened Alpine slab. Thereafter, the Central and Eastern Alps evolved separately (Fig. 2). Northward motion of the singularity during slab delamination in the Central Alps increased both horizontal shortening and the taper angle of the orogenic wedge, with rapid exhumation and denudation in the retro-wedge. Slab steepening and delamination are inferred to have been more pronounced in the Eastern Alps, possibly due to the greater negative buoyancy of the slab in the absence of Brianconnais continental lithosphere in the eastern part of Alpine Tethys. The delaminating slab in the east drove subsidence and continued marine sedimentation in the E. Molasse basin from 29-19 Ma, while the western part of the basin in the C. Alps filled with terrigeneous sediments. Slab detachment beneath the E. Alps at ~20 Ma coincided broadly with several dramatic events within only 5 Ma (23-17 Ma): (1) a switch from advance of the northern thrust front to indentation of the E. Alps by the eastern S. Alps along the sinistral Giudicarie Fault; (2) rapid exhumation of Penninic nappes in the core of the orogen (Tauern Window) and orogen-parallel escape of orogenic crust toward the Pannonian Basin; (3) rapid filling of the E. Molasse basin. These events are attributed to a northward and upward shift of the singularity to within the orogenic crust during Adriatic indentation (Fig. 2). The eastward propagation of the uplifting depocenter in the E. Molasse basin is interpreted to reflect propagation of a subhorizontal slab tear beneath the E. Alps which is imaged by Vp teleseismic tomography. This slab tearing ultimately accompanied Miocene rollback subduction in the Carpathians, as inferred from the migrating depocenter around the orogenic foredeep

Investigating the plate motion of the Adriatic microplate by 3D thermomechanical modelling

Mantle dynamics in the Alpine-Mediterranean area provides a complex geodynamic picture and is still subject of ongoing debate. The Adriatic microplate represents the central part of the Mediterranean and is affected by various subduction zones, like the Hellenic slab, the Calabrian slab or the Apenninic slab. These different processes pose challenges in making qualitative assumptions about the unique impact factors influencing the plate motion. In this study, we conduct 3D thermomechanical forward simulations of the Alpine-Mediterranean area using the LaMEM (Kaus et al., 2016). Our simulations incorporate a viscoelastoplastic rheology and an internal free surface, enabling us to investigate both internal dynamics and surface response. The initial setup for the simulations is based on the kinematic reconstructions of Le Breton et al. (2021) at 35 Ma. Our objective is to determine the main driving forces behind the plate motion of the Adriatic microplate by examining the effects of different model parameters, such as the thermal structure, slab geometry, mantle viscosity, and brittle parameters of the crust. Although these forward simulations do not yet precisely reproduce the present-day tectonic setting, they provide valuable insights into the parameters that influence the plate dynamics. Based on our findings, we have identified two distinct stages of plate motion affecting Adria over the past 35 million years. The initial phase is dominated by the northwards moving African plate, which pushes Adria to the north. However, as the Hellenic slab advances from the east and the Calabrian and Apenninic slabs propagate from the west, the Adriatic microplate is decoupling from the African plate which induces an anticlockwise rotation of Adria. The extent and the thermal structure of the Ionian oceanic lithosphere are significant parameters that influence the retreat of the Hellenic and Calabrian slab and therefore the rotation of Adria. Simultaneously, the northwards motion of Adria during the rotation is caused by the retreat of the Western Alpine slab

Investigating the post-collisional reorganisation of the Eastern Alps using a 4D reconstruction

In Neogene time, the Eastern Alps underwent a profound post-collisional tectonic reorganisation. This featured indentation of the Alpine orogenic wedge by the Adriatic upper plate, eastward lateral extrusion between conjugate strike-slip faults, and a shift from thrust propagation on the European lower plate to the Adriatic upper plate, accreting the eastern South Alps fold-thrust belt. The triggers and driving forces of this tectonic reorganisation remain hotly debated. We present new sequentially restored orogen-scale cross sections along the TRANSALP (12°E) and EASI (13.3°E) transects, plus an E-W orogen-parallel section (46.5°E) to investigate the kinematic evolution of the Neogene tectonic reorganisation in 4D. These transects were affected by eastward lateral extrusion, and so we used a map-view reconstruction to restore out-of-section transport of rock at the onset of rapid extrusion (23 Ma), and the onset of thick-skinned thrusting in the eastern South Alps fold-thrust belt (14 Ma). We then compared our results with Vp LET and teleseismic models of the crust and upper mantle. The geologic record reveals two phases of indentation in the Tauern Window: (Phase 1, 23-14 Ma) The Adriatic crust acted as a coherent indenter, with northward motion relative to Europe accommodated by shortening within the Eastern Alps orogenic wedge as well as sinistral motion along the Giudicarie Fault. Initially, upright folding of Penninic units, including the Venediger nappes, in the Tauern Window accommodated most shortening, but by middle Miocene time, eastward lateral extrusion of the entire metamorphic edifice and NCA was the primary mechanism accommodating N-S shortening. This shortening required ongoing subduction of the European lithosphere, ruling out previous models involving north-dipping Adriatic subduction. A purported detachment below the Venediger Duplex is inferred to have served as the base of the laterally extruding wedge, which comprised the previously subducted and exhumed European crust. (Phase 2, 14 Ma-Present): Since the middle Miocene, the leading edge of the Adriatic indenter has been deforming, forming the thick-skinned South Alps fold-thrust belt. The onset of S-directed shortening is recorded by Langhian-Serravallian rocks beneath the Valsugana Thrust. In contrast, the Adriatic lower crust of the fold-thrust belt was decoupled and transported northwards into the orogenic wedge. In the TRANSALP section, the European lithospheric mantle currently extends beneath the orogenic wedge, whereas in the EASI section the subducted European lithosphere has detached. The Adriatic lower crust indented the deeply buried equivalents of the European Venediger rocks exposed in the Tauern Window. A high-velocity (6.8-7.25 km/s) bulge in LET models of the TRANSALP section images this indenter, and possibly includes accreted European lower crust. We find that when the European slab detached beneath the Eastern Alps, shortening, exhumation, and lateral extrusion of the Eastern Alps orogenic wedge became increasingly important in accommodating Adria-Europe convergence. This culminated in the accretion of the South Alps which now forms the southern part of the orogenic wedge and primarily accommodates ongoing convergence. We note that in the E-W orogen-parallel section, a vertical gap within the slab anomaly, interpreted as a horizontal slab detachment, occurs east of the western boundary of the Tauern Window and the north projection of the Giudicarie Fault. Slab detachment (Handy et al., this volume) is an appealing explanation for the Neogene evolution by eliminating slab pull and redirecting the shortening into the south part of the orogenic wedge

3D geodynamic modelling of the present-day and long-term deformation of the Alps and Adria

Linking geophysical data with geological constraints to understand the dynamics of Alpine Mountain building was one of the main goals of the 4D-MB SPP project. Whereas seismic tomography inversions give a snapshot of how the seismic velocity structure may look like today, geological constraints give (incomplete) pieces of information of how it may have evolved over time. Linking such information with the physics of the lithosphere requires geodynamic numerical models and due to the geometric complexity of the region, 3D is a must. A problem with geodynamic models is that the driving forces are usually the density differences of subducting plates with the surrounding mantle (or far-field forces), whereas the (uncertain) rheology of mantle and crustal rocks plays a crucial role as well. As such, there are quite a few uncertainties in the input parameters, even if the geometry is well-constrained. During the first phase of the 4D-MB project, we focused on present-day models of the Alpine system. Whereas it is possible to invert for the rheology of the lithosphere in mountain belts using probabilistic Bayesian methods (Baumann and Kaus, 2015), this method requires a large number of forward simulations and thus remains infeasible in 3D. An alternative approach is to employ gradient-based inversion methods, in which the adjoint method is employed as a particularly efficient method to compute the gradient of the misfit of the model and data (usually GPS data) versus model parameters (Reuber et al., 2020; Reuber, 2021). Since the adjoint gradient method is computationally cheap (compared to a forward simulation), it can also be used to quickly determine the key model parameters of a particular simulation (Reuber et al., 2018b) or can be combined with gravity and seismic inversions (Reuber and Simons, 2020). We had initially applied this to a case where the starting forward model setup was already giving a reasonably good fit to the uplift data, in which case the method rapidly converged (Reuber et al., 2018a). It thus seemed straightforward to do the same for the Alps. Yet, several issues were encountered in the process: a) we need to consider a much wider region than just the Alps to avoid issues with the lateral boundaries; b) even with high-resolution P-wave tomographic models at hand, one still needs to interpret the seismic velocity anomalies to create an initial model setup, which is a highly non-unique step and results in various possible interpretations; c) coming up with an initial forward model that gives a reasonably good fit to the GPS data turned out to be a significant challenge. Despite running well over 350 forward simulations, we failed to obtain forward simulations that provided a well-enough fit of the velocity in Adria, and without a good starting model, gradient based geodynamic inversions do not converge to a meaningful solution (Reuber, 2020). More recently, we made another attempt in which seismic velocity was directly translated to density and viscosity anomalies using a simple, linear, scaling law, while also prescribing the far-field velocities at the model boundaries (such as that of the N. Anatolian plate). Results give a better fit in Adria (Fig. 1A), but also show that the details of the slab geometry underneath the Alps do not have much impact on the model results, while the model fit within the Alps remains unsatisfying (perhaps because of the small velocities there). Instead of just focusing at the present day structure of the Alps, it is also interesting to see how the system evolved over the last 20-30 million years, which was the focus of our project during the 2nd phase of 4D-MB. The idea was to start with a plate tectonic reconstruction (Le Breton et al., 2021) and let the model evolve forward in time. As for the present-day models, there are many uncertainties in the plate tectonic reconstructions, such as: What was the slab dip? What were the lengths of the slabs? Were they laterally broken or not? What was the thermal and rheological structure of the plates? Given the difficulties with the present-day models, and the increased computational demands of time-dependent simulations, it is unreasonable to expect model results that magically fit all available constraints. Yet, after performing many hundreds of forward simulations, we do get some consistent results and in some of the simulations Adria moves northward and rotates anticlockwise relative to Europe by about the correct amount. The Gibraltar slab arrives at the correct place (Fig. 1B), and the models clearly show that the northward motion of Africa has little impact on the dynamics of Adria, which is instead mostly driven by the interaction of the Hellenic and Calabrian slabs while being pulled northwards by the retreating Western Alpine slab. The size and thermal structure of the Ionian oceanic lithosphere is important as well. Figure 1: A) Example of present-day geodynamic models, B) Snapshot of a forward geodynamic simulation that started at 30 Ma. We also made various technical advances, which includes the Julia package GeophysicalModelGenerator.jl to create complicated 3D geodynamic model setups from geophysical/geological data, DataPicker.jl which provides a GUI for GMG, and LaMEM.jl which is the Julia interface to LaMEM and allows installing and running LaMEM in parallel (either directly from Julia or via Jupyter or Pluto notebooks). We also extended LaMEM to include a continuous integration, adjoint inversion (Reuber et al., 2020; Reuber, 2021) and sensitivity testing (Reuber et al., 2018b)

The Numidian formation and its Lateral Successions (Central-Western Mediterranean): a review

The widely debated late Oligocene-middle Miocene Numidian Fm (NF) consists of supermature quartzose sediments deposited in the Maghrebian Flysch Basin (MFB) outcropping from the Betic Cordillera to the Southern Apennine passing by the Maghrebian Chain. The NF is commonly composed of three lithostratigraphic members and is characterized by two vertical successions (Type A and Type B) corresponding to different sedimentation areas in the MFB. It is noteworthy the occurrence of widespread lateral successions of the NF (Types C, D and E) indicating in some cases an interference of the Numidian sedimentation with other different depositional systems and supplies. The Type C ‘Mixed Successions’, deposited in depocentre areas, are composed of supermature Numidian supply interfingering with immature siliciclastic materials, coming from the internal portion of the MFB. The Type D consists of supermature Numidian materials supplied from the Africa Margin (external sub-domains) deposited in sub-basins on the Africa-Adria margins, outside the typical Numidian depositional area. The Type E, which stratigraphically overlies both the South Iberian Margin (SIM) and the Mesomediterranean Microplate (MM), represents the migration of the Numidian depositional system to reach the opposite margins of the MFB. The occurrence at a regional scale of all the above-mentioned lateral successions reveals a great evolutionary complexity resulting also from further constraints, which must be considered for palaeogeographic and palaeotectonic reconstructions. Another important point deals with the diachronism of the top of the NF, observed eastward from the Betic-Rifian Arc and the Algerian-Tunisian Tell (Burdigalian p.p.) to Sicily (Langhian p.p.) and up to the Southern Apennine (at least Langhian/Serravallian boundary) which can be related with eastwards delay in the MFB closure. The palaeogeographic reconstruction of the Numidian depositional area presented in this paper, which is also included into a global kinematic model, represents a first attempt to use the software GPlates for this subject

Constraints on Crustal Structure in the Eastern and eastern Southern Alps

In the course of this study, an extensive seismological dataset from both the temporary SWATH-D network (Heit et al., 2021) and selected stations of the AlpArray Seismic Network (Hetényi et al., 2018) was analyzed. The primary aim of this endeavor was to gain comprehensive insights into the crustal structure of the southern and eastern Alps. The small inter-station spacing (average of ∼15 km within the SWATH-D network) allowed for depicting crustal structure at unprecedented resolution across a key part of the Alps. The methodological approach employed in this study entailed a sequential series of analyses to unveil the underlying features. The preliminary step encompassed the determination of the arrival times of both P and S seismic waves. Subsequently, a Markov chain Monte Carlo inversion technique was deployed to simultaneously calculate robust hypocenters, a 1-D velocity model, and station corrections (Jozi Najafabadi et al., 2021). This data was then utilized for calculation of 3-D VP and VP/VS models (Jozi Najafabadi et al., 2022). In addition, the path-averaged attenuation values were obtained by a spectral inversion of the waveform data of selected earthquakes. The attenuation structure (1/QP model) is then calculated using damped least square inversion of the path-averaged attenuation values (Jozi Najafabadi et al., 2023). These analyses resulted in a multidimensional depiction of the subsurface. The derived models for QP, VP and VP/VS indicate subsurface anomalies that can be attributed to rock’s physical parameters, presence of fluids within rocks and their motion in pores and fractures, temperature, and partial melting. The findings reflect head-on convergence of the Adriatic indenter (the part of the Adriatic Plate that has modified the Alpine orogenic edifice) with the Alpine orogenic crust. Furthermore, a highly heterogeneous crustal structure within the study area was unveiled. The velocity model illuminated decoupling of the lower crust from both its mantle substratum and upper crust. The Moho, taken to be the iso-velocity contour of Vp = 7.25 km/s, provided insights into the southward subduction of the European lithosphere, a phenomenon previously investigated in the Eastern and eastern Southern Alps (e.g., Kummerow et al., 2004 and Diehl et al., 2009). The most pronounced high-attenuation (low QP) anomaly is found to be closely correlated with the high density of faults and fractures in the Friuli-Venetian region, as well as the presence of fluid-filled sediments within the Venetian-Friuli Basin. Furthermore, the northwestern edge of the Dolomites Sub-Indenter (NWDI) corresponds to a low attenuation (high QP) anomaly which is interpreted as a reflection of the NWDI's stronger rocks compared to the surrounding areas

Kinematics and Convergent Tectonics of the Northwestern South American Plate During the Cenozoic

The interaction of the northern Nazca and southwestern Caribbean oceanic plates with northwestern South America (NWSA) and the collision of the Panama-Choco arc (PCA) have significant implications on the evolution of the northern Andes. Based on a quantitative kinematic reconstruction of the Caribbean and Farallon/Farallon-derived plates, we reconstructed the subducting geometries beneath NWSA and the PCA accretion to the continent. The persistent northeastward migration of the Caribbean plate relative to NWSA in Cenozoic time caused the continuous northward advance of the Farallon-Caribbean plate boundary, which in turn resulted in its progressive concave trench bending against NWSA. The increasing complexity during the Paleogene included the onset of Caribbean shallow subduction, the PCA approaching the continent, and the forced shallow Farallon subduction that ended in the fragmentation of the Farallon Plate into the Nazca and Cocos plates and the Coiba and Malpelo microplates by the late Oligocene. The convergence tectonics after late Oligocene comprised the accretional process of the PCA to NWSA, which evolved from subduction erosion of the forearc to collisional tectonics by the middle Miocene, as well as changes of convergence angle and slab dip of the Farallon-derived plates, and the attachment of the Coiba and Malpelo microplates to the Nazca plate around 9 Ma, resulting in a change of convergence directions. During the Pliocene, the Nazca slab broke at 5.5°N, shaping the modern configuration. Overall, the proposed reconstruction is supported by geophysical data and is well correlated with the magmatic and deformation history of the northern Andes

The preserved plume of the Caribbean Large Igneous Plateau revealed by 3D data-integrative models

Remnants of the Caribbean Large Igneous Plateau (C-LIP) are found as thicker than normal oceanic crust in the Caribbean Sea that formed during rapid pulses of magmatic activity at similar to 91-88 and similar to 76 Ma. Strong geochemical evidence supports the hypothesis that the C-LIP formed due to melting of the plume head of the Galapagos hotspot, which interacted with the Farallon (Proto-Caribbean) plate in the eastern Pacific. Considering plate tectonics theory, it is expected that the lithospheric portion of the plume-related material migrated within the Proto-Caribbean plate in a north-north-eastward direction, developing the present-day Caribbean plate. In this research, we used 3D lithospheric-scale, data-integrative models of the current Caribbean plate setting to reveal, for the first time, the presence of positive density anomalies in the uppermost lithospheric mantle. These models are based on the integration of up-to-date geophysical datasets from the Earth's surface down to 200 km depth, which are validated using high-resolution free-air gravity measurements. Based on the gravity residuals (modelled minus observed gravity), we derive density heterogeneities both in the crystalline crust and the uppermost oceanic mantle (<50 km). Our results reveal the presence of two positive mantle density anomalies beneath the Colombian and the Venezuelan basins, interpreted as the preserved fossil plume conduits associated with the C-LIP formation. Such mantle bodies have never been identified before, but a positive density trend is also indicated by S-wave tomography, at least down to 75 km depth. The interpreted plume conduits spatially correlate with the thinner crustal regions present in both basins; therefore, we propose a modification to the commonly accepted tectonic model of the Caribbean, suggesting that the thinner domains correspond to the centres of uplift due to the inflow of the hot, buoyant plume head. Finally, using six different kinematic models, we test the hypothesis that the C-LIP originated above the Galapagos hotspot; however, misfits of up to similar to 3000 km are found between the present-day hotspot location and the mantle anomalies, reconstructed back to 90 Ma. Therefore, we shed light on possible sources of error responsible for this offset and discuss two possible interpretations: (1) the Galapagos hotspot migrated (similar to 1200-3000 km) westward while the Caribbean plate moved to the north, or (2) the C-LIP was formed by a different plume, which - if considered fixed - would be nowadays located below the South American continent