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

https://doc.rero.ch/record/4946/files/1_mosar_sop.pdf

Subduction and obduction processes in the Swiss Alps

lithospheric mantle. Overall, the magmatic succession is more Extensional collapse of the Betic–Alboran Domain in the Miocene consistent with a geodynamic model involving convective removal of was accompanied by several discrete episodes of magmatism which the majority of the lithospheric mantle than with models invoking provide insights into the relationships between the thermal and wholesale delamination at the Moho, slab detachment or slab rollmechanical state of the lithosphere during late orogenesis. The back. tholeiitic Malaga dykes, emplaced at ~30–27 Ma, have flat rare earth element (REE) patterns and low Sr/Sr and high Nd/ Nd indicative of shallow decompression melting (5–15%) within the asthenosphere following removal of lithospheric mantle. Calcalkaline magmas, erupted between 15 and 6 Ma, have negative

Magmatism Associated with Orogenic Collapse of the Betic–Alboran Domain, SE Spain

Surface and subsurface data are combined to construct a crustal-scale cross section of the western central Pyrenees (France and Spain) at the boundary between the European and Iberian plates. The position of Moho reflections in the ECORS-Arzacq reflection profile suggests a tectonic wedge of European crust and upper mantle had indented the Iberian plate at lower crustal levels. The European wedge is overlain by an upper, Iberian wedge thus constituting a double (stacked) wedge geometry. The upper wedge was delaminated and deformed giving rise to the Pyrenean orogenic prism, manifested as a bivergent fan in upper crustal levels. The underthrust lower Iberian plate has been imaged to depths of 55–60 km, but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km. Total orogenic contraction calculated from surface structures is about 75–80 km, which was accomplished between the latest Cretaceous and the early Miocene at an averaged rate of 1.2 mm/yr. Consistent with these moderate values, exhumation of the orogen is much less than in the more shortened eastern parts of the range. Paleozoic basement and preorogenic Mesozoic rocks are little eroded, and much of the exhumation involved cannibalization of early foreland basins, which once covered the entire, poorly emergent orogen at this transect. This resulted in a continuous process of sediment recycling and, coupled with a considerable lateral arrival of material, a bulk negative erosion-sedimentation budget.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98TC00561

Crustal structure and orogenic material budget in the west central Pyrenees

The European Geotraverse (EGT) has been planned as a major geoscience project that will run for five to seven years involving the collaborative efforts of geophysicists, geologists, petrologists, geodesists and other geoscientists from different European countries. A first draft proposal was elaborated in 1981 by a Working Group of the European Science Research Councils (ESRC), a standing committee of the European Science Foundation (ESF); it was subsequently modified following the amendments requested by the ESRC and was finally approved by the General Assembly of the ESF on 9 November 1982. The broad aim of the Geotraverse is to secure an understanding of how the continental lithosphere formed and reacted to changing physical and geometrical conditions through time. One of the best locations for such a study is Europe, because it is made up of a number of tectonic provinces ranging in succession from the oldest Precambrian aress of Scandinavia to the currently active area of the Mediterranean. The concept of a geotraverse has been chosen in order to provide a continuous, integrated study of sufficient scale to cross this whole region as a swathe that will give new information about the dynamics and vertical and lateral variations of the lithosphere, both within and between adjacent provinces.

E. Banda

Papers

The European Geotraverse

Crustal structure beneath Spain from deep seismic sounding experiments

Explosion Seismology Studies of the Canary Islands. First Results

Regional Variations in the Crustal Structure of the Iberian Peninsula