Tectonic History of Anatolia from Adjoint Tomography
0
Citation
0
Reference
20
Related Paper
Abstract:
The Anatolian plate consists of several continental fragments that are connected to each other along suture zones and have gone through intricate geological processes over time. Its current tectonic and geodynamic processes mainly developed under the extensional regime caused by subduction along the Hellenic Arc and the northward collision of the Arabian plate on the eastern side. However, the complex subduction history of the region (e.g., closure of the Tethys Ocean, slab roll-back along the Hellenic arc, possible slab detachments, etc.) is still debated. Higher resolution tomographic models of the crust and the upper-mantle are crucial to produce detailed images of subducted slabs and old slab remnants and for interpretation of geodynamic processes. In this study, we discuss the past and present tectonic and geodynamic settings, in particular the slab history of the Anatolian plate and the surrounding area based on adjoint tomography of the region within the broader continental scale of Europe. Initial results reveal the major features of the Hellenic and Cyprus Arcs and slab remnants of the Tethys Ocean underneath Anatolia. The inversion procedure is based on spectral-element simulations of full wave propagation in 3D background models, which facilitates the use of full three-component seismograms, resulting in better data coverage and constraints on Earth structure. The tomographic image of the area is a result of more than 25 iterations using hundreds of thousands of frequency-dependent body- and surface-wave traveltime measurements. Broadband seismic waveforms were obtained from ORFEUS, IRIS (both from permanent and PASSCAL networks) and the Kandilli Observatory of Turkey, which we gratefully acknowledge for free data access.Keywords:
Seismic Tomography
Slab
Forearc
Volcanic arc
Seismogram
Tethys Ocean
Cite
The locus of continual active tearing at the lateral termination of a subduction zone is dubbed a STEP, and the surface trace left in the wake of a propagating STEP is called the STEP fault. STEP faults have played a major role in the (geologically) fast reorganization of plate boundaries in the Mediterranean, where present-day surface deformation is distributed. This thesis focuses on the effect of passive margins on the propagation of STEPs and lithosphere dynamics in the Mediterranean, studied through physical computer models. Many STEP settings observed on Earth today display a geometry in which the trench is oriented approximately perpendicularly to the STEP fault. Model geometries include a passive margin that is oriented at some angle with respect to the trench. Passive margins steer STEPs in case of favorable orientation with respect to the strike of the trench. In other cases, STEPs will continue to propagate in their original direction, that is, straight into an oceanic basin or a passive margin. Models show that the orthogonal setup of STEP fault and subduction trench is one that STEP systems will evolve towards. Subsequently, Nijholt tailors model setups to resemble the geometry of the African continental margin in the south-central Mediterranean: the domain of the Calabrian subduction zone. Model predictions indicate that the SW STEP of the Calabrian trench propagated eastward along the African (Sicilian) passive margin and then propagated into the Ionian basin. The geodynamic context of deformation in the Ionian basin, offshore Calabria and Sicily, constitutes two factors: STEP fault activity and a wide, lithospheric, dextral shear corridor. Nijholt then focuses on present-day kinematic observations in this same area through an interplay of known tectonic forces. The geodetically measured velocity field, seismicity and sense of slip on regional faults in the south-central Mediterranean can be reproduced by force-based models. These show that the regional imprint of tectonic forces is driven by Africa-Eurasia plate convergence, lateral variations in gravitational potential energy and slab pull. The magnitude of the resistance to fault slip on regional fault zones turns out to be variable. Whereas high resistance on faults that hosted major historical earthquakes confirms the high probability of future natural disasters, the Calabrian subduction interface is unlikely to host a future, major earthquake. In the westernmost Mediterranean, the formation of the Gibraltar arc was also dominated by subduction and STEP activity. However, subduction activity has faded and present-day deformation is distributed. Lastly, Nijholt employs a grid search methodology and finds that it is possible to constrain the rheology, resistance to slip on major fault systems, and slab pull and trench suction forces for the Gibraltar arc domain in light of the available kinematic observations. The observed surface velocity field, seismicity and sense of slip on regional faults in the Gibraltar arc appear to result mainly from Africa-Europe plate convergence and lateral variations in gravitational potential energy. Slab pull from the Gibraltar slab is very likely transmitted poorly into the surface plate.
Passive margin
Transform fault
Cite
Citations (1)
Abstract Taiwan is widely considered to be a typical example of an arc‐continent collision surrounded by two opposite dipping subduction zones. The manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood. Various hypotheses have been proposed, but the geodynamic feasibility of those remains to be tested. Here we present 3‐D thermomechanical models to study the geodynamic evolution process of a Taiwan‐like setting after an initial transform fault was consumed. In our model setup, the boundary between the Eurasian plate and the South China Sea is northeast trending. The results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small‐scale mountain belt with high topography and crustal exhumation occurs in most cases. The Eurasian continental crust is exhumed in a dome‐like manner exposing higher‐grade metamorphic rocks, facilitated by high erosion rates and a weak continental lower crust rheology, but inhibited by the presence of a weak arc. A high topography within the orogen, as well as continental slab detachment, can develop for the convergence direction of N307° and large convergence rates. Our modeling results are thus generally consistent with the Eurasian slab‐tearing model proposed for Taiwan based on seismic tomographic studies, and we suggest that the main characteristic features in Taiwan can be explained by the combined effects of fast erosion, a weak lower crust, fast convergence, and a small convergence azimuth.
Slab
Convergent boundary
Collision zone
geodynamics
Cite
Citations (8)
The motion of oceanic and continental lithosphere, volcanic activity, earthquakes, and other tectonic activities are expressions of processes in the Earth's mantle. Nowadays, it is widely accepted that these phenomena are related to convective flow in the mantle, which forms the mechanism to turn heat from the Earth's interior into mechanical work. One example of that mechanism is the creation of oceanic lithosphere at the mid-ocean ridges, which are zones of upwelling hot mantle material. At boundaries where two plates converge one of the plates, preferebly of oceanic type, is consumed by subduction into the mantle. Most of the information about the Earth's structure has been obtained by seismology. Seismological observations have demonstrated that there is a great variety in subduction zone geometries. Subducted slabs with dip angles varying from very small (e.g. Peru) to nearly 90° (e.g. Mariana) have been observed. Slabs can either stagnate at the upper-lower mantle boundary (e.g. Izu-Bonin) or can penetrate into the lower mantle (e.g. Sunda arc). Initiation or cessation of subduction are manifestations of the time-dependence of the dynamical processes in the mantle. Images of the seismic velocity distribution of the mantle interior provide an instantaneous view of time-dependent structures which must be interpreted in a dynamical context. Understanding the long-term evolution of the subduction process is the primary motivation for this study. More specifically, the aim is to investigate the sensitivity of subduction zone geometry to various parameters, as, for example, plate velocities and viscosity structure of the mantle. For that purpose we have chosen to use a computational method. Numerical studies have the advantage that parameters can be varied over a great range. Plate motions and mantle flow can be included as boundary conditions easily, particularly compared to laboratory experiments. It must be emphasized that the model used here includes several shortcomings. The major simplification is probably the neglect of the third dimension. Over the last two decades numerous studies concerning subduction zones have been performed. In Chapter 2 a summary of some influential work and recent studies from different fields of geophysical disciplines is presented. Constraints on the structure and evolution of subducting slabs obtained from seismology, geoid data, plate tectonic analysis, and numerical and experimental studies are discussed. This information forms the basis for the research presented in this thesis.
Volcanic arc
Hotspot (geology)
Slab window
Mountain formation
Crustal recycling
Cite
Citations (8)
The fastest modern-day tectonic block rotations on Earth (up to 9 degrees/Myr) occur in the forearcs of convergent plate margins where a transition from collision of a bathymetric high to subduction of normal oceanic crust occurs. GPS techniques have enabled accurate documentation of the kinematics of these rotations, leading us to develop a conceptual model where the change from collision to subduction exerts a torque on microplates within the plate boundary zone, causing them to spin rapidly about an axis at the collision point. We have investigated geophysical and geological data from several active plate boundaries (from the western Pacific and Mediterranean regions) to document a compelling spatial and temporal relationship between the transition from collision to subduction, plate boundary curvature, and rapid tectonic block rotations. In some cases, these microplate rotations can initiate back-arc rifting. We also present numerical modelling results supporting our conceptual model for block rotations at collision/subduction transition. Our results suggest that the rate of microplate rotation depends on the incoming indentor velocity, and can be greatly enhanced by: (1) extensional stresses acting at the subduction interface (possibly due to slab roll back), and (2) a low-viscosity back-arc. Where viscosity of the back-arc is low, forearc microplate rotation dominates. In contrast, tectonic escape of strike-slip fault-bounded microplates is predicted in areas where the back-arc viscosity is high. Previous workers have suggested that the kinematics of the Anatolian block and back-arc rifting in the Aegean are influenced by some combination of forces associated with Arabia/Eurasia collision, and/or subduction (including slab rollback) at the Hellenic trench. Based on previous work from active western Pacific arcs, we propose that the collision of two separate indentors (Arabian promontory in the east, Apulian platform in the west), is a fundamental tectonic mechanism for large-scale anticlockwise tectonic rotation of Anatolia and the opposing clockwise rotation of western Greece documented from paleomagnetic studies. The recognition of several global analogues for Mediterranean active tectonics may lead to new insights into the dominant forces behind tectonic processes there.
Forearc
Cite
Citations (9)
GPS satellite observations indicate that in the tectonically complex eastern Mediterranean and east African regions microplates rotate counterclockwise with respect to the neighboring African plate. Using 3D numerical models, Glerum relates these observations of crustal deformation to the dynamics of the lithosphere and the underlying mantle that may cause this deformation. Glerum first describes her additions to the ASPECT software necessary for numerically modeling the upper mantle and lithosphere dynamics of convergent and divergent plate boundaries. These additions include the tracking of multiple materials with different physical properties and nonlinear viscous as well as viscoplastic rheologies. The implementations of complex, multi-material rheologies are verified with well-known 2D benchmarks and multi-material viscoplasticity is applied in 3D time-dependent thermomechanical models of oceanic subduction. Subsequently, Glerum uses ASPECT to investigate the sensitivity of horizontal surface motions to individual geodynamic processes in the eastern Mediterranean. Identification of all mantle drivers that should participate in modeling attempts to explain observations of crustal flow is essential to fully exploit the information contained by surface motions about their driving processes. Glerum therefore employs 3D data-driven instantaneous dynamics models of compressible flow including a complete set of possible mantle drivers of surface deformation. The reference instantaneous flow model results indicate that mantle processes can explain a large part of the crustal motion of the Aegean-Anatolian microplate. Subsequent systematic perturbations of model properties with respect to this reference model help estimate the individual contributions of tectonic plate motions, slab pull and trench suction, and density-induced mantle flow interacting with the slab and overlying plates while moderated by the mantle’s bulk viscosity. In order of regional importance, the predicted crustal flow of the Aegean-Anatolian region is most sensitive to slab pull, followed by slab-mantle interaction and basal drag, mantle rheology, and the absolute plate motion reference frame. Lastly, Glerum demonstrates a possible mechanism for the counterclockwise rotation of the Victoria microplate in the East African Rift System, which is in striking contrast to the clockwise motion of the surrounding plates. 3D models of the divergent system show that Victoria’s rotation can be caused by the drag of the African and Somalian plates along the strong edges of the microplate, while the rift segments along inherited lithospheric weaknesses facilitate Victoria’s rotation. The amount of rotation is therefore primarily controlled by the distribution of preexisting stronger regions and the weaker Precambrian mobile belts that surround Victoria. The induced counterclockwise rotation of the microplate leads to a clockwise shift of the local extension direction from E-W to more WNW-ESE along the overlapping rift branches. Comparison of the resulting predicted stress field and tectonic regimes to observations helps to elucidate the interpretation of local stress and strain indicators and to reconcile different opening models used to interpret the East African Rift System.
geodynamics
Convergent boundary
Slab window
Cite
Citations (0)
This thesis is focused on various aspects of the evolution of the western Mediterranean region. Particularly, on modeling of the 3-D subduction evolution of the Rif-Gibraltar-Betic slab in the western Mediterranean region by means of 3D thermo-mechanical modeling and investigation its dependence on factors influencing the subduction dynamics. This region underwent a long and complicated history of slab rollback and lithosphere tearing, for which the only direct observations come from its geological history and present day mantle structure inferred by tomographic data. Different tectonic reconstructions were proposed for this region based on these observations. Despite the general agreement on the initiation of the subduction process around 35 Ma and slab buoyancy being a major driving force which forms the present day slab under Gibraltar these reconstructions propose distinctly different initial parameters of the subduction zone, as well as temporal evolution of the region during the last 35 My. To investigate the basic numerical and rheological settings, which allows for the development of a free rollback process this research had started with 2D numerical experiments of the evolution of the subduction process. This first phase also included investigation of influence of the side boundary conditions. The use of open side boundaries proved extremely important for reaching the overall goal of 3D numerical experiments. It was demonstrated that implementation of the open side boundary conditions allows to significantly reduce lateral domain size and computational time without any considerable impact on the flow pattern inside the modeling domain. A wide range of parameters for composite rheology was tested, which was later used for the 3D numerical modeling. In the second part of the thesis we extended the experiments to 3D numerical modeling domain to simulate the evolution of the subduction process in western Mediterranean since 35 Ma till present. Three distinctly different kinematic reconstruction scenarios were tested, which all claimed to reproduce the present day mantle structure as imaged by tomography studies. It was shown that these tectonic reconstructions after 35 My of subduction evolution led to completely different slab position and shape. As a result, different mantle structure was observed. For obtaining the most realistic model in terms of fitting present day mantle structure and timing of the evolution of the subduction process introduction of a second small subduction system to the east proved key and resulted in predicting the observed cold anomaly in this region interpreted as the Kabylides slab. The fit with two major temporal constraints: slab arrival to the African margin in Middle Miocene and slab stalling under the Gibraltar region since the Tortonian were significantly improved by including the Kabylides slab. On the basis of a successful model for the evolution of the western Mediterranean region, the research was continued with investigation of the influence of different absolute plate motion reference frames on the dynamics of the subduction process. On the basis of the relative Africa-Iberia convergence [van Hinsbergen et al., 2014] three different absolute plate motion reference frames were implemented and tested against prescribed absolute plate motions based on Doubrovine et al. [2012]. This demonstrates that for modeling of the evolution of complex real-earth subduction, on scales and style comparable to the western Mediterranean subduction, it is crucial to constrain numerical models by absolute plate motions. 1. Doubrovine, P. V., B. Steinberger, and T.H. Torsvik (2012), Absolute plate motions in a reference frame defined by moving hotspots in the Pacific, Atlantic and Indian oceans. Journal of Geophysical Research, 117, B09101, doi: 10.1029/2011JB009072. 2. van Hinsbergen, D.J.J., R.L.M. Vissers, and W. Spakman (2014), Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation, Tectonics.
Slab
Eclogitization
Slab window
Cite
Citations (0)
My thesis focuses on evolving and (geologically) short-lived plate boundary segments, their segmentation processes and geological imprints in the eastern Mediterranean. In Chapter 2, I investigate the nature/type of the plate boundary between the eastern Aegean region and the Africa plate. The work involves an integrative analysis of geological and geophysical information. I conclude that these surface observations document that the “Pliny-Strabo trench” is a predominantly strike-slip plate boundary. My interpretation is that this plate boundary represents an expression of slab tearing related to an active STEP (c.f. Figure 1.1). The paper represents the first detailed account of surface deformation related to a STEP fault, and constitutes a novel contribution to the understanding of the relation between deep processes and (near-) surface deformation, a key topic in geodynamic research. In Chapter 3, I investigate the location and nature of currently active plate boundaries and other major faults in the southern Anatolia-Aegean region, in the transition region from the Hellenic Arc to the Cyprus Arc. The question is particularly relevant for accessing earthquake hazard. I use mechanical models based on the finite element method. I explore various options for these faults, most of them proposed in the scientific literature, to explore how they would affect the deformation at locations where there are actual observations. The (mis)fit between model predictions and observations allows us to conclude that the active plate boundary is located offshore. The research question that I address in Chapter 4 is what the cause is of deformation in one of the seismically most active fault zones in Europe, the Kefalonia Transform Fault. I present results from a recent full-waveform tomographic model which particularly improves our understanding of the structure of the upper few hundred kilometers of the Earth. The cause of the deformation along the Kefalonia Transform Fault is likely rooted in a fragmented slab that we image for the first time. The geometry of the slab fragment leads me to conclude that it became disconnected from the larger Hellenic slab around 5 Ma, at about the time of opening of the Gulf of Corinth in the overriding plate, which suggests a highly interesting causal relation.
Slab
Cite
Citations (1)
Because of the close similarity of some Italian and Mediterranean tectonic situations to the East Asia tectonics - arcs, trenches, Wadati-Benioff zones, volcanic and seismic activities, and a typical horizontal bending ofthe alleged lithospheric slab -, many clues are examined in search of new interpretations of the Mediterranean geological and observational evidence, with the aim of finding solutions that are exportable to the problems of the circumpacific arc-trench zones. The facts coming from surface geology, magmatism, geochemistry, different method tomographies, etc., are at variance with the alleged Africa-Eurasia convergence. The clues for rifting prevail over those for compression, and many tectonic situations previously interpreted as due to plate collisions, are associated to or mixed to rifting evidence. The proposal is put forward that uprising of mantle material wedges between two separating lithospheric plates could be a new working hypothesis. On an expanding Earth the region interposed between Eurasia and Africa has always had a smaller latitudinal extension with respect to the large Paleo Tethys and Neo Tethys appearing on constant-radius paleogeographical reconstructions. It is then possible, in the expanding Earth view, also to identify as phases of opening the Paleo Tethys and Neo Tethys currently alleged 'closure', which has added to the Proterozoic nuclei the Variscan and Alpine terra lies respectively. These phases and their orogens have to be considered as extensional phases, and the added terranes of African provenance (e.g. the Adriatic fragment) should be regarded as fragments left behind as continental Africa moved away. In this sense, considering the ongoing process of opening as having Proterozoic origin, it is possible to speak of the Mediterranean as a slowly nascent ocean, but also - paradoxically - as a very old ocean.
Mountain formation
Extensional tectonics
Tethys Ocean
Cite
Citations (14)
A striking feature of the Indian Ocean is a distinct geoid low south of India, pointing to a regionally anomalous mantle density structure. Equally prominent are rapid plate convergence rate variations between India and SE Asia, particularly in Late Cretaceous/Paleocene times. Both observations are linked to the central Neo-Tethys Ocean subduction history, for which competing scenarios have been proposed. Here we evaluate three alternative reconstructions by assimilating their associated time-dependent velocity fields in global high-resolution geodynamic Earth models, allowing us to predict the resulting seismic mantle heterogeneity and geoid signal. Our analysis reveals that a geoid low similar to the one observed develops naturally when a long-lived back-arc basin south of Eurasia's paleo-margin is assumed. A quantitative comparison to seismic tomography further supports this model. In contrast, reconstructions assuming a single northward dipping subduction zone along Eurasia's margin or models incorporating a temporary southward dipping intra-oceanic subduction zone cannot sufficiently reproduce geoid and seismic observations.
Tethys Ocean
Ocean surface topography
Seismic Tomography
geodynamics
Cite
Citations (0)
We analyzed the structure and evolution of the external Calabrian Arc (CA) subduction complex through an integrated geophysical approach involving multichannel and single‐channel seismic data at different scales. Pre‐stack depth migrated crustal‐scale seismic profiles have been used to reconstruct the overall geometry of the subduction complex, i.e., depth of the basal detachment, geometry and structural style of different tectonic domains, and location and geometry of major faults. High‐resolution multichannel seismic (MCS) and sub‐bottom CHIRP profiles acquired in key areas during a recent cruise, as well as multibeam data, integrate deep data and constrain the fine structure of the accretionary wedge as well as the activity of individual fault strands. We identified four main morpho‐structural domains in the subduction complex: 1) the post‐Messinian accretionary wedge; 2) a slope terrace; 3) the pre‐Messinian accretionary wedge and 4) the inner plateau. Variation of structural style and seafloor morphology in these domains are related to different tectonic processes, such as frontal accretion, out‐of‐sequence thrusting, underplating and complex faulting. The CA subduction complex is segmented longitudinally into two different lobes characterized by different structural style, deformation rates and basal detachment depths. They are delimited by a NW/SE deformation zone that accommodates differential movements of the Calabrian and the Peloritan portions of CA and represent a recent phase of plate re‐organization in the central Mediterranean. Although shallow thrust‐type seismicity along the CA is lacking, we identified active deformation of the shallowest sedimentary units at the wedge front and in the inner portions of the subduction complex. This implies that subduction could be active but aseismic or with a locked fault plane. On the other hand, if underthrusting of the African plate has stopped recently, active shortening may be accommodated through more distributed deformation. Our findings have consequences on seismic hazard, since we identified tectonic structures likely to have caused large earthquakes in the past and to be the source regions for future events.
Accretionary wedge
Underplating
Décollement
Seafloor Spreading
Cite
Citations (175)