Subcategoría andamiaje a partir de los requerimientos de los estudiantes

The processes that have contributed to the evolution of the Betic-Rif arc have been debated in various recent papers (e.g. Lonergan & White, 1997; Platt et al, 1998; Martinez-Martinez & Azanon, 1997). Recent seismic studies suggest the presence of up-welled asthenosphere (Seber et al, 1996) and isolated bodies of lithospheric mantle in the region (Calvert et al,

2000; Blanco & Spakman, 1993).

Various mechanisms which may cause a body of lithospheric material to become separated from the base of the lithosphere have been suggested. Proposed mechanisms include removal of the lithospheric mantle by convection, (Platt & Vissers, 1989), slab detachment / de-lamination (de Jong, 1991; Zeck et al, 1992) and a combination of these two processes acting together (Seber et al, 1996). Lonergan & White (1997) suggest that the Neogene evolution of the Betic-Rif area is the consequence of a westward-retreating subduction zone that generated rapidly extended continental crust as a consequence of its retreat.

The feasibility of geodynamic models proposed to explain the evolution of the Betic-Rif arc have been explored using the results from thermobarometry, geochronology and studies of mineral phase relations. Each model of the lithospheric response to the Africa-Iberian collision will produce a characteristic pattern of thermal evolution in the overlying crustal rocks. Platt et al (1998) produced thermal models of different tectonic scenarios by altering the values of variable parameters including thickness and thermal gradient of post-orogenic lithosphere; depth to which lithospheric mantle was removed and rate of extension. Results that produce pressure-temperature conditions comparable to those evidenced by

metamorphic assemblages in rocks from the ODP Site 976 are achieved when parameters defining a significant post-contractional pause, prior to removal of the lithosphere to a depth of 62.5 km, are defined. Delayed onset of lithosphere removal is predcted by numerical models of the behaviour of a conductive thermal boundary layer developed in a fluid with a non-linear vscous behavour (Molnar et al, 1998).

The evolution of mineral assemblages deduced from petrological studies of rocks taken from the Alboran Sea core (Site 976) forms the basis of a study by Soto & Platt (1999). These workers concluded that viable models of crustal evolution need to provide an

external source of heat (aside from that produced from radiogenic heat production) in order to generate P-T paths similar to those suggested by the petrology of these rocks. This constraint appears to challenge models involving trench rollback in the absence of lithosphere removal, (e.g. Lonergan & White, 1997) as these models do not permit a sufficient amount of heat to pass into the crust. Models in which post-collisional

radiogenic heating was followed by the removal of the lithospheric mantle below the orogen and rapid extension appear to most closely emulate the P-T paths recorded in the rocks (Soto & Platt, 1999).

Turner et al (1999) present a study of the geochronology and isotopic signatures of magmatic rocks within the Betic Cordillera (section 1.1.2). The feasibility of competing hypotheses recently proposed to account for the evolution of the Betic Cordillera are tested according to whether they predict the goechemical signatures and geometrical distribution of magmatic rocks discussed in this work. Models that suggest the orogen developed as the consequence of a retreating subduction zone, detached slab of lithospheric mantle or

delamination of the lithospheric mantle are discounted; these mechanisms would be expected to generate a progressive, systematically migrating sequence of volcanism, not concordant with the pattern seen in the Betics (Turner et al, 1999; figure 1.3). Turner et al

(1999) propose that a model involving convective removal of the lithosphere most readily explains the diffuse spatial distribution and temporal migration of activity directed

outwards from the Alboran Sea.

1.6

Aims of the project

As discussed in sections 1.3 - 1.5 above, numerous models have been suggested to explain the present-day configuration of the Betic Cordillera: it is evident from the preceeding discussions that ambiguities as to the evolution of the orogen remain.

In the broadest sense, the principal aim of this research is to provide a new set of structural and metamorphic data, collected from the Alpujarride Complex from within the Internal Zone of the orogen, in order to investigate the nature of the deformation that accompanied the decompression and exhumation of the Complex and to explore the relationship

between the deformational history and the thermal evolution experienced by the rocks. From this data, it is the intention that the hypothesis that exhumation was followed by large-scale thrusting and regional nappe-formation (i.e. Balanya et al, 1997) may be able to be tested. To achieve this, the zone around the towns of Almunecar, Salobrena and

LosGuajares has been elected for the following reasons:

1 Within this area, it has been recorded by previous workers that high grade metamorphic rocks (e.g. sillimanite and kyanite bearing assemblages) located predominately near to the coast pass structurally upwards into chlorite-bearing assemblages over a horizontal distance of approximately 8 km. As the dip of the main foliation in this area is recorded as being gently dipping, this indicates that the

transition from high grade to low grade rocks in this zone occurs through a reduced vertical thickness of crust. A study of this area will enable the macro-scale

structural configuration of the area to be determined, and analysis of the

microstructures and metamorphic mineral assemblages present in these rocks should permit the structural and metamorphic history of the rocks to be elucidated.

In the north of the area (see figure 1.6), near to the town of Los Guajares, exposures of rocks that have experienced high temperatures crop out immediately adjacent to rocks of considerably lower grade (IGME map of Motril (1972); Azanon et al,

1994). One common interpretation of the structure in the existing literature is that the juxtaposition of grades in this area is the consequence of a thrust, which places the higher grade rocks structurally above the low grade rocks (Azanon et al, 1994; Simancas & Campos, 1989). This area provides a useful ‘testing ground’ , then, for the hypothesis that the Internal Zones of the Betic Cordillera have experienced alternating periods of extension and compression during their evolution (i.e. Balanya et al, 1997) or whether they have experienced continual extension (i.e. Argles et al, 1999).

Numerous well-exposed north-south trending transects through this area,

predominately within road-cuttings, provide a source of relatively fresh samples for analysis of metamorphic minerals and microstructure, and the possibility that structural trends will be able to be traced through the area, and perhaps correlated between adjacent sections. It is intended that these sources of information will be synthesised in order to generate a model that improves the understanding of the metamorphic and structural evolution of the rocks in this area.