Rinus Wortel

Zagros orogeny: a subduction-dominated process

This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ~12–8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.

Stress in the Indo-Australian plate

Cloetingh, S. and Wortel, R., 1986. Stress in the Ind~Austr~ian plate. In: B. Johnson and A.W. Bally (Editors), Intraplate Deformation: Characteristics, Processes and Causes. Tectonophysics, 132: 49-67 We modelled the state of stress in the Indo-Australian plate in order to investigate quantitatively variations observed in tectonic style. The numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere. Estimates are presented for the average net resistive forces at the Himalayan collision zone, the suction force acting on the overriding Indo-Australian plate segment at the Tonga-Kermadec trench and the drag at the base of the lithosphere. Our modelhng shows a concen~ation of compressive stresses of the order of 3-5 lcbar in the Ninetyeast Ridge area; the effects of the compressive resistance associated with Himalayan collision and subduction of young lithosphere off the northern part of the Sunda arc are focused in this region. The stress field as calculated gives a consistent explanation for the observed concentration of seismic activity (Stein and Okal, 1978) and significant deformation in the oceanic crust (Weissel et al., 1980; McAdoo and Sandwell, 1985) in the area. The calculated stress field in the area adjacent to the Southeast and Central India ridges is characterized by tension parallel to the spreading axis. This explains the concentration of near-ridge normal faulting seismicity (with T-axes subparaIle1 to the spreading ridge) in the Indian Ocean as recently observed by Bergman et al. (1984) and Wiens and Stein (1984). The regional stress field along the strike of the Sunda arc varies from compression seaward of and parallel to the Sumatra trench segment, to tension perpendicular to the Java-Flores segment. This explains the selective occurrence of well developed grabens seaward off the Java-Flores segment of the trench, observed by Hilde (1983). Our modelling shows that the observed rotation of the stress field (Denham et al., 1979) in the Australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down-going slab in each of these. The state of compression in west and central Australia is induced by the action of resistive forces at the Himalayan and Banda arc collision zones. The joint occurrence of high levels of compression in the plate’s interior and normal faulting seismicity in the near-ridge areas, is a transient feature unique to the present dynamic situation of the Indo-Australian plate.

A Tomographic View on Western Mediterranean Geodynamics

During the Cenozoic, the Western Mediterranean region has experienced a complex subduction history which involved the destruction of the Late Triassic/Jurassic Ligurian ocean and the West Alpine-Tethys. Lithosphere remnants of this evolution have been detected in the upper mantle by seismic tomography imaging. However, no general consensus exists on the interpretation of these remnants/slabs in the context of Ligurian ocean and West Alpine-Tethys subduction. In this paper we search for subduction remnants of the entire Cenozoic evolution in the recent global tomography model of Bijwaard and Spakman (2000) and compare these tomography results and our interpretations with those obtained in previous studies. Next, we present an analysis of imaged mantle structure in the context of the tectonic evolution of the Western Mediterranean during the Cenozoic. Our analysis leads to the following main results:

Nappe stacking resulting from subduction of oceanic and continental lithosphere below Greece

We quantitatively investigate the relation between nappe stacking and subduction in the Aegean region. If nappe stacking is the result of the decoupling of upper-crustal parts (5-10 km thick) from subducting lithosphere, then the amount of convergence estimated from balancing the nappe stack provides a lower limit to the amount of convergence accommodated by subduction. The balanced nappe stack combined with the estimated amount of completely subducted lithosphere indicates 700 km of Jurassic and 2400 km of post-Jurassic convergence. From seismic tomographic images of the underlying mantle, we estimate 2100-2500 km of post-Jurassic convergence. We conclude that (1) the imaged slab represents the subducted lithosphere that originally underlay the nappes, (2) since the Early Cretaceous, subduction in the Aegean has occurred in one single subduction zone, and (3) the composition of the original basement of the nappes indicates that at least 900 km of sub-upper-crust continental lithosphere subducted in the Aegean.

Passive margin earthquakes, stresses and rheology

The occurrence of large earthquakes on passive continental margins poses the challenge of explaining the causes of earthquakes not directly related to plate boundary processes. Here, we suggest several features of models for passive margin seismicity based on inferences from the distribution and mechanisms of seismicity in the zone along the Canadian Atlantic coast The concentration of earthquakes along the margin suggests that they are related to the reactivation of faults remaining from the continental rifting. The variation in focal mechanisms across the margin suggests a spatially varying stress field. Although the later feature may not be a general characteristic, the empiricism that passive margin seismicity seems most evident on recently deglaciated margins suggests that deglaciation is at least partially responsible for the earthquakes.

On the initiation of subduction zones

Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin.Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas.Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.

Some consequences of the subduction of young slabs

The negative buoyancy force exerted by a subducting oceanic slab depends on its descent velocity, and strongly on its age. For lithosphere close to thermal equilibrium, this force dominates by a large margin the resisting forces arising from friction on the plate boundary and compositional buoyancy. This may result in oceanward migration of the trench, with associated back-arc spreading. However, the strong age dependence of this force, and of the ridge push mean that a horizontal compressive stress is required to continue subduction if changing plate geometry should bring young lithosphere to the trench. Estimates can be made of the slab age, as a function of descent velocity, at which the driving forces are no longer sufficient to overcome a given resisting force. The transition corresponding to a resisting force of 8 × 1012 N/m divides regions displaying back-arc extensional tectonics from those displaying compressional tectonics. This is in good agreement with other estimates of the forces resisting slab motion. It is suggested that an increase in the width of — or the shear stress on — the plate boundary, associated with the subduction of lithosphere to the buoyant side of this transition, can result in a compressional stress on the overriding plate which is great enough to account for cordilleran tectonics. The proposed reduction in the one of driving forces of plate motion is still consistent with observations, being compensated by the greaterrelative importance of the push from the ridges.

On the origin of the Cocos-Nazca spreading center

To investigate the mechanism underlying the break-up of the Farallon plate into the Cocos plate and Nazca plate, we analyze the state of stress in the Farallon plate at about 30 m.y. B.P., just prior to the fragmentation. To this purpose we use finite element methods and a reconstruction of regional plate boundaries appropriate for 30 m.y. B.P. A key role in the model is played by the dependence of two important plate-tectonics forces (slab pull and ridge push) on the age of oceanic lithosphere. The results show a highly tensional stress field, with maximum principal stresses of 5 to 6 kbar. North-south tension in the vicinity of present-day Panama is proposed to have been the cause for the fragmentation of the Farallon plate and the inception of spreading along the new Cocos-Nazca plate boundary. Because the kind of plate interaction that gave rise to the high level of tensional stresses in the Farallon plate is not restricted to the area of this study, the proposed mechanism seems to shed light on the problem of fragmentation of oceanic plates in general.

Thermo-mechanical modelling of arc-trench regions

Abstract The thermal structure of the shallow part of a subduction zone, i.e. the region between the trench and the volcanic line, is calculated with a finite difference method. Published heat flow measurements, in combination with rheological arguments and the distribution of interplate thrust earthquakes, are used to constrain the thermal structure of this region and the magnitude of shear stresses acting on the plate contact. A pressure and temperature dependent rheology is used to model shear stresses. From our thermo-mechanical modelling it follows that temperatures at the plate contact and within the upper plate are determined by the subduction of cold material and by frictional heating. For models that satisfy the constraints the average shear stress at the plate contact (between the trench and the volcanic line) varies from about 10 to about 40 MPa and shear stresses during brittle deformation range from 2.5 to 7.5% of the lithostatic pressure. For a wide range of convergence velocities (4–12 cm/yr) and ages of the subducting oceanic lithosphere (30–150 Ma), shear stresses and temperatures at and above the plate contact are essentially independent of these parameters. Temperatures at the upper surface of the slab for the preferred thermal models are in good agreement with pressure-temperature conditions during high-pressure metamorphism inferred from mineral assemblages in the Franciscan Complex of California. Frictional heating has a large influence on the conditions for high-pressure metamorphism.

Continental Collision and the STEP-wise Evolution of Convergent Plate Boundaries: From Structure to Dynamics

Particularly interesting stages in the evolution of subduction zones are the two main transient stages: initiation and termination. In this contribution the focus is on the second of these: terminal stage subduction, often triggered by continental collision or arc-continent collision. The landlocked basin setting of the Mediterranean region, in particular the western-central Mediterranean, provides unique opportunities to study terminal stage subduction and its consequences