The TOPO-IBERIA Research Initiative

Southern Region: Betic-Rif orogen.

Motivation

This area constitutes a natural lab which includes a particularly controvertial geodynamic scenario. This includes the Betic-Rif orogen, with a so called accretionary prism of the Gulf of Cadiz and the basins and margins of the Alboran Sea and southern Argelian-Balearic basins. This area is a broad zone of contact between the Northern African and Iberian plates which has been actively evolving for the last 30 Ma (since Neogene). The subcrustal litosphere has a complex evolution within a scenario of relatively oblique convergence between two continental plates. This area is the most seismically active one of the Peninsula, thus it is a natural laboratory to study the intense seismic activity and how it affects the present-day evolution of the relief.

This area has been the topic of interest for national and international Earth scientists for at least the last 20 years and a large number of conceptual tectonic models have been proposed and designed to explain the complex tectonic scenario. These conceptual models include: crustal/lithospheric recycling, delamination, subcrustal mantle erosion, etc. These processes correspond to key steps in the evolution stages of orogenic montain ranges.

Therefore this geodynamic scenario constitutes a unique natural laboratory in which such dynamic processes and their implications can be studied. As well as the implicationsinterrelation of these deep rooted processes and the surface evolution, for example relief changes in natural coast line configuration. This area is and has been affected by relatively important vertical movements for the last 10 My, which resulted in climate changes. The international scientific interest over this area has promoted a demonstration project in the area under the acronym of PICASSO (Program to Investigate Convective Alboran Sea System Overturn). It is supported by the ILP (Internacional Lithospheric Program)and is part of the TopoEurope- EuroArray research initiative. It is important to note that the extent of the processes is not limited to the Alboran basin. DTM data shows a symmetry from the Central Iberian Orogenic System in the Iberian Peninsula, to the Atlas mountain range in Northern Africa. Therefore, the area of study extends toward Morocco. The PICASSO initiative includes international grups (from UK, USA, Ireland, etc), that have already submitted proposals to their own funding agencies to contribute to this initiative, covering different aspects of the multidisciplinary data acquisition effort.

The Gibraltar Arc incorporates two slightly thickened regions under compression, the southern margin of Iberia (Betic Cordillera) and the northern margin of Morocco (Rif Cordillera). The internal part of the arc corresponds to a thinned crust under extension (Alboran Basin) overlying an anomalous mantle. The external part of the arc is characterized by two foreland basins flanking the respective cordilleras (Guadalquivir-Betics, Rharb-Rif) and a set of structures forming the accretionary prism of the Gulf of Cadiz. An extremely important and peculiar aspect is the abrupt change in stress regime between 24 Ma and 8 Ma. During this period, a rapid shift of structures in E-W direction is superimposed to the regional NNW-SSE field associated to the convergence between Africa and Iberia. Most E-W tectonic activity ends during Upper Tortonian, although some authors suggest it continues until present. To explain this configuration, a variety of models have been proposed based on the concepts of slab break-off, subduction, delamination, and convective mantle removal of a crustal root. These geodynamic conceptual models aim to explain the genesis of the system adopting different lithospheric evolutions: extensional collapse or convective removal of the negative-buoyant (cold and dense) lithospheric root, subduction of an Atlantic oceanic crust, or asymmetric delamination of a continental lithosphere. In this way, some models are related to plate tectonics, whereas others result from a previously thickened mountain belt.

The present lithospheric structure is nowadays a subject of debate. Here, the main scientific open question marks are to discriminate, based on solid evidence, the “geodynamic engine” running the system from Neogene to present, and to establish the processes leading to the recycling of a previously thickened continental lithosphere into the convective mantle. During the last decade, many numerical and analogue models have popped-up showing that such processes can occur under a wide variety of conditions, and the results are therefore not conclusive. Likewise, existing tomographic models of the region (seismic velocity distribution in depth), can be interpreted within both model types.

Though existing data are up to now insufficient to discriminate between models, there are evidences showing that the lithospheric architecture influences significantly on the processes of delamination and mantle convection, as much in terms of lithosphere recycling as in shaping landscape and relief. Such evidences can be summarized as follows:

• Seismic activity extends from the base of the crust to the lithospheric-asthenospheric boundary.
• Seismic tomography suggests the presence of a piece of detached lithosphere under the Betic-Rif Cordillera and the Alborán Basin, below 300 km depth.
• The observation of low seismic velocities at the base of the crust, suggests an asthenospheric upwelling which is replacing delaminated lithosphere and leading to decompressional melting and basaltic vulcanism.

Previous work accounting a wide participation of member of our Team indicates that both crust and lithospheric mantle show a notorious thinning from the emerged areas (Gibraltar Arc) towards the Alborán Basin, and also within this basin towards the East, towards the contact with the oceanic crust of the Algerian-Balear Basin. However, the style of thinning changes between segments of the south Iberian margin. The present crustal structure and lithospheric mantle and its lateral variations, strongly control the relief and morphology of the Alborán Basin margins, where a clear correlation is observed between abrupt thinning and the presence of a narrow platform with abrupt continental slopes. Similarly, the existence of recent volcanism and the uplift of the margins during Plioquaternary times, also changing according to the segment, seem to be intimately related to lithospheric anomalies.

Results from tomographic experiments and integrated numerical models support an important lithospheric thickening under the central and western Betics, Gulf of Cadiz and Rharb Basin. Overall, there is a strain partitioning between crustal and lithospheric mantle deformation, suggesting a strong decoupling between both. It is remarkable that the Gulf of Cadiz constitutes the continuation of the Betic-Rif system in the Atlantic, extending, in a broad sense, until the abyssal plains, where the plate boundary is less diffuse. The Gulf of Cadiz concentrates a strong seismic activity with shallow and intermediate earthquakes (60-120 km depth) and has recently been the scenario of important events of geological hazard (submarine slides, tsunamis, etc.).

The Atlas cordillera can be understood as the southernmost product of the Iberian-Africa plate boundary. A remarkable lithospheric thinning has also been described below these mountains, although here not linked to crustal thickening but to mantle convective dynamics, relatively independent of plate tectonics. The uplift of the Atlas and the adjacent plateaus has been caused both by crustal shortening and mantle upraise, although the chronology of both mechanisms is still poorly constrained.

Lithospheric transects across the contact between the Mediterranean Sea and the Atlantic Ocean. These models result from integrating a variety of regional data.

The present knowledge of the lithospheric structure is very uneven in the Betic-Rif/Alborán system. Seismic refraction experiments starting on the 70’s, together with available gravimetric and magnetic data provide only the first-order regional features of crustal thickness and its structure. During the 90’s, the ESCI profiles contributed to improve the knowledge of the crustal structures in the eastern Betic Cordillera and Alboran Basin. Available geophysical data are clearly insufficient to determine the lithospheric architecture in the region.

Quantitative information on the exhumation rate of the mountain belts is available based on Zircon and Apatite fission track studies, mostly concentrated on the study of the last stages of the extensional collapse of the orogen. Few studies have tried to find a relationship between the present lithospheric geometry, the localization of active structures, and the formation of relief and landscape evolution. There is a need to better understand the role of thermal anomalies on the localization of recent deformation. It is also necessary to evaluate the source-to-sink sediment budget between the Betic-Rifean mountains and the adjacent basins since Neogene, as well as the possible paleoenvironments and their chronology, and to establish the tectonosedimentary relationships that are compatible with erosion/sedimentation rates, and with paleogeographic and tectonic reconstructions.

Goals and tasks

The objectives of the research are based on an important set of hypothesis, results and existing data, a relevant part of which have been obtained from previous results of the solicitants who are now promoting an appropriate environment to perform integrated geological and geophysical research and make a qualitative step forward in understanding the geodynamics of the study areas.

The main specific objectives and corresponding tasks are:

• To establish the lithospheric structure of south Iberia and north Africa, and localize the active structures in relation with the plate boundary through integrated analysis of geological and geophysical data of continental and marine areas.
• To determine continuity of structural units of the Betic-Rifean system towards the Gulf of Cadiz, and the nature of the crust and localization of the continent-ocean boundary in the Atlantic margins of south Iberia and north Morocco, and in the Alborán and Algeria basins.
• To evaluate the role of crustal and mantle processes in the evolution of landscape and relief.
• To map cinematic indicators including the strain patterns and its temporal and spatial variations (Middle Miocene to present). Such map will allow testing a variety of numerical and analogue models addressing the style of deformation of the Gibraltar Arc along its different segments.
• To quantify the processes of uplift and subsidence in the Betic-Rif and Atlas cordilleras, the Alborán Basin, and the Gulf of Cadiz by establishing the thermal histories from Zircon and Apatite Fission Tracks data and accounting for constrains from other geochronometers (U-Th/He; 39Ar- 40Ar, U-Pb).
• To study the Quaternary evolution of relief in the Betic-Rif and Atlas cordilleras from the calculation of geomorphological indexes, fluvial-network incision rates and Alborán seafloor sediment transfer.
• To establish relationships between relief development and climate evolution in the southern Iberian Peninsula and northern Morocco since Miocene to present.
• To relate active structures to instrumental seismicity through the localization of seismic loci and inversion of the moment tensor, using the historical seismicity, the paleoseismicity information, with the aim of obtaining seismic parameters of those structures
• To analyze the evolution of relief associated to the mass movement produced by seismic activity.
• To date the quaternary surfaces and calculate denudation rates with in-situ cosmogenic isotopes.
• To determine the present velocity field of plate tectonic motions using GPS techniques to constrain the characteristics of present deformation at regional scale in the Betic-Rif-Atlas area, and to establish the areas of larger tectonic strain, and their effect on relief development.
• To contribute new data for the assessment of geological hazard (earthquakes, tsunamis, landslides) in the region in relation to tectonically-active structures (detachments, folds, faults).