P. Th. Meijer

Present-day dynamics of the Aegean region: A model analysis of the horizontal pattern of stress and deformation

In order to gain quantitative insight into the forces that control the present-day stress field and pattern of horizontal motions in the Aegean region, we adopt a forward model approach. Mechanisms that have been proposed to explain the present-day tectonics are represented, to first approximation, in terms of the expected force distributions. On the basis of a thin elastic shell representation of the Aegean lithosphere we then calculate the stress field and displacements associated with these force sets and compare the results with observations of the state of stress based on geological studies of fault kinematics, strain as expressed by earthquake focal mechanisms, and horizontal motions measured by means of satellite geodesy. The two most commonly cited mechanisms suggested to control the Aegean tectonics, (1) the push exerted by the westward moving Anatolian block and (2) forces associated with the Hellenic subduction zone, are first analyzed separately. It is found that the westward Anatolian push alone does not explain the observed prevalence of tensional stress. This is also expressed in a mismatch between the displacements computed to result from the westward push and the observed horizontal velocities. Regarding the forces on the Aegean overriding margin, we find that a model with outward pulling forces of uniform magnitude, acting normal to the arc from the SW Peloponnese to Rhodes, yields a stress field that matches the observed pattern of tension to a large extent. This distribution of forces is consistent with the notion of gravitational spreading of the Aegean lithosphere. The pattern of seismic strain of the overriding margin may evidence the occurrence of a small additional resistive force. Although the prevalence of tension thus appears due mainly to the subduction-related forces, other aspects of the stress field and also the pattern of horizontal motions indicate that these forces act in combination with westward push. Our model results allow us to address the relation between the forces experienced by the upper plate and the kinematics of subduction, not just in terms of a vertical cross section but also by taking into account the along-arc dimension.

Neogene evolution of the Aegean arc: paleomagnetic and geodetic evidence for a rapid and young rotation phase

New paleomagnetic data of the entire Aegean outer-arc are presented. The results indicate a young Pleistocene and rapid clockwise rotation phase in the western Aegean arc, covering at least Zakynthos and the Peloponessos. The eastern Aegean arc, incorporating Kassos, Karpathos and Rhodos, also experienced Pleistocene anticlockwise rotations. The anisotropies of the magnetic susceptibility (AMS) data are in agreement with arc-parallel extension in the south and south-eastern Aegean arc and arc-normal compression in the north-west, in agreement with structural and geodetic observations. We compare the paleomagnetic results with the present-day pattern of rotation as computed from geodetic data, and we find good agreement. The onset of the Pleistocene rotations coincides with the beginning of uplift and a change in the stress pattern of extension. We compare our findings with existing models for the Aegean area. fl 2000 Elsevier Science B.V. All rights reserved.

Late Miocene to Recent tectonic evolution of Crete (Greece): geological observations and model analysis

Abstract Using a numerical model, we focus on the late Middle Miocene to Recent kinematic evolution of the Cretan segment of the Hellenic Arc. Geological observations of Crete are given a quantitative interpretation in terms of tectonic mechanisms controlling the evolution of the active arc boundary. This was achieved by calculating intra-plate stress fields for various possible distributions of forces and comparing the models with observations. The models address specific questions concerning important changes in the observed horizontal stress patterns. We deal with the question of what caused extension to initiate. Modelling results of the collapse of an earlier formed topography and extensional forces acting on the plate boundary are compared with a newly reconstructed tectonostratigraphy of Crete. Our results suggest that arc-normal pull is the dominant force that generates the Late Miocene extension in the Cretan segment of the overriding plate, although arc-normal pull in combination with intra-plate spreading forces cannot be excluded. The observed transform motions in the Pliny and Strabo trenches led us to incorporate experiments with additional resistance on the eastern (Levantine) segment of the Hellenic Arc. The models performed with a transform resistance along the trenches are in agreement with the Cretan deformation for the Pliocene to Recent period. The stress fields for the short duration compressional periods around the Middle–Late Miocene and the Miocene–Pliocene boundary are modelled by assuming resistive instead of tensional forces acting at the overriding margin.

THE TWO-STAGE OPENING OF THE WESTERN-CENTRAL MEDITERRANEAN BASINS : A FORWARD MODELING TEST TO A NEW EVOLUTIONARY MODEL

Abstract The Algero–Provencal and the Tyrrhenian extensional basins developed in two stages in a geodynamic setting characterized by the nearly N–S convergence between Africa and Eurasia. The spreading of the Provencal basin occurred in the early Miocene following a long period of rifting in the western Mediterranean area. A dramatic eastward shift of the active extensional deformation resulted in the Tortonian to Quaternary opening of the Tyrrhenian basin. In a companion paper, Carminati et al. propose that: a trench retreat process in a geodynamic setting locked by the continental collisions in the Alps and in the Betic chain is a viable mechanism for the late Oligocene–early Langhian opening of the western Mediterranean; a Langhian slab detachment episode along the north African margin is likely to have caused the end of the trench retreat along this part of the boundary determining the end of active expansion in the western Mediterranean and the beginning of active extension in the Tyrrhenian basin. The objective of the present paper is to quantitatively test this proposed scenario. We calculate, by means of a thin shell model, the effects of these plate boundary reorganizations on the European stress and strain field. We show that the two-stage opening of the western and central Mediterranean can be explained by the evolution proposed by Carminati et al. and that, in particular, the eastward shift of the active extension which produced the termination of the first opening stage and the beginning of the second is likely to have been triggered by the slab detachment episode along the north African margin.

Temporal variation in the stress field of the Aegean Region

We use a forward numerical model to analyse the intra-plate stress field in an area of active extension of continental lithosphere : the Aegean region. Recent tomographic images of the Hellenic subduction zone offer the unique opportunity to address the relation between the evolving Aegean stress field and temporal variations in the subduction process. To this extent we apply our model to both the present-day and the Late Pliocene situation. Regarding the current situation it is found that subduction-related forces largely explain the observed pattern of tension. Results obtained for the Pliocene indicate that detachment of the slab subducted below western Greece may have controlled the observed temporal variation in the pattern of tension.

Dynamics of the lithosphere and the intraplate stress field

We outline the methodology of our numerical studies aimed at increasing the understanding of the relation between dynamics and stress field of the lithosphere with particular reference to oceanic lithosphere. The ridge-push force is modelled as a pressure gradient integrated over all contributing parts of the lithosphere. The slab-pull force is modelled as being dependent on the age of the subducting lithosphere. We parametrize the resistive forces and determine the unknown parameters by requiring the total torque of all forces acting on the plate to vanish. We illustrate the approach by the presentation and discussion of new modelling results for the Pacific plate.

Forces controlling the present-day state of stress in the Andes

The present-day state of stress in the Andes is expected to be controlled primarily by two different types of forces: (1) the resistive force exerted on the western overriding margin of the South American plate, and (2) forces that arise from the thickened crust of the Andes (i.e., the effect of topography and its compensating crustal root). We have studied these forces on the basis of a model for the dynamics of the entire South American plate in which the Andes are embedded. In this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish. A thin elastic shell representation is used to calculate the intra-plate stress field associated with the various force distributions. We define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin (Fpcr, plate contact resistance) and does not include the effects of topography. Subsequently, we investigate the effect of lateral variation in the magnitude of Fpcr and add the topography-related forces. The main results are: (1) A uniform magnitude of Fpcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin. (2) The amount of horizontal compression across the Andes, found in the case that ridge push is considered to be the only force driving the South American plate, is small compared to the value required to “sustain” the Andes.

Precessional control of Sr ratios in marginal basins during the Messinian Salinity Crisis

Based on 87Sr/86Sr data of the Primary Lower Gypsum (PLG) deposits in the Vena del Gesso basin—a marginal basin of the Mediterranean during the Messinian Salinity Crisis—a correlation between 87Sr/86Sr values and precessional forcing has recently been proposed but not yet confirmed. In this study, a box model is set up to represent the Miocene Mediterranean deep basin and a connected marginal basin. Measurements of 87Sr/86Sr in the Vena del Gesso and estimated salinity extrema are used to constrain model results. In an extensive analysis with this model, we assess whether coeval 87Sr/86Sr and salinity fluctuations could have been forced by precession-driven changes in the fresh water budget. A comprehensive set of the controlling parameters is examined to assess the conditions under which precession-driven 87Sr/86Sr variations occur and to determine the most likely setting for PLG formation. Model results show that precession-driven 87Sr/86Sr and salinity fluctuations in marginal basins are produced in settings within a large range of marginal basin sizes, riverine strontium characteristics, amplitudes of precessional fresh water budget variation, and average fresh water budgets of both the marginal and deep basin. PLG deposition most likely occurred when the Atlantic-Mediterranean connection was restricted, and the average fresh water budget in the Mediterranean was significantly less negative than at present day. Considering the large range of settings in which salinities and 87Sr/86Sr fluctuate on a precessional timescale, 87Sr/86Sr variations are expected to be a common feature in PLG deposits in marginal basins of the Mediterranean.