Retreating subduction boundaries, formed where the rate of subduction exceeds the rate of overall plate convergence, appear to be commonly developed features within regions of early or incomplete continent-continent collision. They are characterized by regional extension within the overriding plate and, at their leading edge, by thin-skinned arcuate thrust belts that are concave towards the overriding plate. As is illustrated by examples from the Mediterranean region, the formation of retreating subduction boundaries is intimately related to the process of continental collision. During the early stages of collision, retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary. Retreating plate boundaries can also form before the main collision, and the associated thrust belts emplaced as precollisional accretionary assemblages. Because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab (slab pull), subduction usually ceases when, and only when, thick buoyant continental crust enters the subduction zone. Thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted. In some cases, retreating subduction boundaries may “escape” into the open ocean, where they form nearly isolated, local tectonic systems. In these systems the rate of subduction is approximately compensated by the rate of upper plate extension, and migration of the system across the oceanic region may be very rapid. For example, the Horseshoe Seamounts, located about 800 km offshore in the eastern North Atlantic, may be the active expression of an east dipping, westwardly migrating retreating subduction boundary that has evolved from the Betic Cordillera-Rif system active in Miocene time and may now be progressing across the Atlantic at approximately 50 mm/yr. An analogous situation may be represented by the Scotia Arc system, a westward dipping retreating subduction system located between the South American and Antarctic plates, which may have “escaped” into the South Atlantic ocean from a zone of crustal shortening in the Andes and is now progressing across the Atlantic at a rate of about 80 mm/yr.

Evolution of retreating subduction boundaries formed during continental collision

One - Printed Volume.- 1 The Mediterranean Area and the Surrounding Regions: Active Processes, Remnants of Former Tethyan Oceans and Related Thrustbelts.- Abstract.- 1.1 Introduction.- 1.2 Mediterranean Fold-and-thrust Belts.- 1.3 Mediterranean Marine Basins.- 1.4 Geological-geophysical Baseline.- 1.4.1 Heat Flow.- 1.4.2 Crustal and Lithospheric Structure.- 1.4.3 Gravity.- 1.4.4 Magnetic Field.- 1.4.5 Seismicity.- 1.4.6 Geodetic Data.- 1.4.7 Stress Field.- 1.5 Global Dynamics and Active Processes Exemplified in the Mediterranean.- 1.5.1 Subduction of the Eastern Mediterranean Lithosphere beneath the Calabrian and Aegean.- 1.5.2 Rifting and Passive Margin Development in Back-arc Regions and Other MediterranRelated to Tectonic Wedges, Tilted Blocks and Sedimentary Loadingean Domains.- 1.5.3 Mud and Salt Diapirism (Eastern Mediterranean Ridge, Alboran Sea, Nile Delta).- 1.5.4 Sea-level Changes, Salinity Crisis, Flooding (Messinian Mediterranean versus Pleistocene Black Sea).- 1.6 Record of Ancient Dynamics of the Tethyan Oceans, Ophiolitic Sutures, Mantle Tomography versus Paleogeography of the Mediterranean Realm.- 1.6.1 Collisional vs. Intracontinental Thrust Belts and Oceanic Sutures.- 1.6.2 Plate Dynamics and Palinspastic Restorations: Demise of the Concept of a Single Tethys.- 1.6.3 Cenozoic Magmatism in the Mediterranean Region.- 1.7 Conclusions.- Acknowledgements.- 2 A Tomographic View on Western Mediterranean Geodynamics.- Abstract.- 2.1 Introduction.- 2.2 The Global Tomography Model BS2000.- 2.3 Interpretation of Model BS2000 for the Western Mediterranean Mantle.- 2.3.1 Alps, Apennines, and the Western Mediterranean.- 2.3.2 The Betic-Rif and Alboran Region.- 2.4 Analysis: the Geodynamic Evolution of the Western Mediterranean.- 2.4.1 Tomographic Evidence for Slab Roll-back.- 2.4.2 Northern Apennines and Alpine-Tethys Subduction.- 2.4.3 Slab Detachment beneath the Central-southern Apennines.- 2.4.4 Calabria Subduction.- 2.4.5 The North African Margin.- 2.4.6 Betic-Rif and Alboran Region: I. Subduction and Roll-back of Predominantly Oceanic Lithosphere.- 2.4.7 Betic-Rif and Alboran Region: II. Development of Arc Geometry and Subduction Roll-back.- 2.4.8 Synthesis of Tomographic Constraints on the Geodynamic Evolution of the Western Mediterranean Region.- 2.5 Summary.- Acknowledgements.- Appendix 1 (CD-ROM).- Appendix 2 (CD-ROM).- 3 The TRANSMED Transects in Space and Time: Constraints on the Paleotectonic Evolution of the Mediterranean Domain.- Abstract.- 3.1 Introduction.- 3.2 The Western Tethys Main Plate Tectonic Constraints.- 3.2.1 The East Mediterranean-Neotethys Connection.- 3.2.2 The Apulia-Adria Problem.- 3.3 The Geodynamic Evolution of Greater Apulia and Surrounding Regions.- 3.3.1 Paleotethys Evolution (Figs. 3.2-3.6).- 3.3.2 Cimmerian Events and Triassic Marginal Oceans (Figs. 3.6-3.9).- 3.3.3 The Jurassic Oceans: Alpine Tethys, Central Atlantic and Vardar (Figs. 3.8-3.11).- 3.3.4 The Cretaceous Oceans: North Atlantic and the Pyrenean Domain (Figs. 3.10-3.14).- 3.4 The TRANSMED Transects in Space and Time.- 3.4.1 Transects I-II-III West.- 3.4.2 Transects IV-V-VI.- 3.4.3 Transects III East, VII and VIII.- 3.5 Conclusions.- Acknowledgements.- Appendix 3 (CD-ROM).- References: Preface, Chapters 1, 2 and 3.- References: CD-ROM.- Transect I: Iberian Meseta - Guadalquivir Basin - Betic Cordillera - Alboran Sea - Rif - Moroccan Meseta - High Atlas - Sahara Domain.- Transect II: Aquitaine Basin - Pyrenees - Ebro Basin - Catalan Coastal Ranges - Valencia Trough - Balearic Promontory - Algerian Basin - Kabylies - Atlas - Saharan Domain.- Transect III: Massif Central - Provence - Gulf of Lion - Provencal Basin - Sardinia - Tyrrhenian Basin - Southern Apennines - Apulia - Adriatic Sea - Albanian Dinarides - Balkans - Moesian Platform.- Transects IV, V and VI: The Alps and Their Forelands.- Transect VII: East European Craton - Scythian Platform - Dobrogea - Balkanides - Rhodope Massif - Hellenides - East Mediterranean - Cyrenaica.- Transect VIII: Eastern European Craton - Crimea - Black Sea - Anatolia C2014 Cyprus - Levant Sea - Sinai - Red Sea.

The transmed atlas : the mediterranean region from crust to mantle : geological and geophysical framework of the mediterranean and the surrounding areas

Magmatic evolution of the Alboran region: The role of subduction in forming the western Mediterranean and causing the Messinian Salinity Crisis

Orogenic architecture of the Mediterranean region and kinematic reconstruction of its tectonic evolution since the Triassic

The western Mediterranean recorded subduction rollback, slab segmentation and separation. Here we address the questions of what caused Oligocene rollback initiation, and how its subsequent evolution split up an originally coherent fore arc into circum-southwest Mediterranean segments. We kinematically reconstruct western Mediterranean geology from subduction initiation to present, using Atlantic plate reconstructions as boundary condition. We test possible reconstructions against remnants of subducted lithosphere imaged by seismic tomography. Transform motion between Africa and Iberia (including the Baleares) between ~120 and 85 Ma was followed by up to 150 km convergence until 30 Ma. Subduction likely initiated along the transform fault that accommodated pre-85 Ma translation. By the ~30 Ma inception of rollback, up to 150 km of convergence had formed a small slab below the Baleares. Iberia was disconnected from Sardinia/Calabria through the North Balearic Transform Zone (NBTZ). Subduction below Sardinia/Calabria was slightly faster than below the Baleares, the difference being accommodated in the Pyrenees. A moving triple junction at the trench-NBTZ intersection formed a subduction transform edge propagator fault between the Baleares and Calabria slab segments. Calabria rolled back eastward, whereas the Baleares slab underwent radial (SW-S-SE) rollback. After Kabylides-Africa collision, the western slab segment retreated toward Gibraltar, here reconstructed as the maximum rollback end-member model, and a Kabylides slab detached from Africa. Opening of a slab window below the NBTZ allowed asthenospheric rise to the base of the fore arc creating high-temperature metamorphism. Western Mediterranean rollback commenced only after sufficient slab-pull was created from 100 to 150 km of slow, forced subduction before ~30 Ma.

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Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation

Homologies geologiques entre les deux rives du detroit de Gibraltar

At both ends of the Maghrebides belt, in the Rif Mountains and Sicily, Middle to Upper Jurassic slices of basic rocks with an E-MORB character are associated with various tectonic units of the Maghrebian flysch zone. This zone, which was located between the internal zones, originally linked to the European plate and the African external zones of this Alpine belt, was therefore oceanized, at least partially, and acted as a transform fault between the mid Atlantic and the Ligurian ocean, both of them being open at that time.

Situation structurale et nature ophiolitique de roches basiques jurassiques associées aux flyschs maghrébins du Rif (Maroc) et de Sicile (Italie)

Resume La serie ophiolitique jurassique de Balagne, actuellement allochtone, appartenait a une partie de l'ocean ligure situee au voisinage relatif du continent ouest-europeen, car : a) des calcaires du Malm supra-ophiolitique renferment en abondance des galets des differents termes connus dans le socle de Corse occidentale ; b) un niveau de sables quartzeux est intercale dans ces laves, et les zircons qu'il renferme sont identiques a ceux des granites calco-alcalins du meme socle; c) les basaltes oceaniques de Balagne ont une composition comparable a celle des basaltes enrichis des rides medio-oceaniques (E-MORB), formes a un stade embryonnaire de l'oceanisation. La nappe de Balagne a du etre obductee sur le socle europeen a l'Eocene superieur, pendant que les ≪schistes lustres≫ ligures etaient soumis a un metamorphisme par subduction, vers l'ouest, sous ce meme socle.

Michel Durand-Delga

Papers

Homologies geologiques entre les deux rives du detroit de Gibraltar

Situation structurale et nature ophiolitique de roches basiques jurassiques associées aux flyschs maghrébins du Rif (Maroc) et de Sicile (Italie)

Arguments en faveur de la position, au Jurassique, des ophiolites de Balagne (Haute-Corse, France) au voisinage de la marge continentale européenne

Datations des radiolarites post-ophiolitiques de Corse alpine à l'aide des Radiolaires

Les formations burdigaliennes de Corse orientale dans leur cadre géodynamique