4.1. Previously Proposed Models
Recent publications have converged to two competing but not necessarily mutually exclusive hypotheses, one proposing a progressive rolling or peeling back of lithosphere, the other proposing detachment of lithosphere. Within these broad categories, there is also debate about the involvement of crust in any recycling of lithosphere. Figure 14 shows a simple categorization and schematic cartoons of the most current models taken from their original publications.
4.1.1. Alboran block and retreating subduction. The predominant westerly transport direction
in the Betics and Rif lead to the proposal of the Alboran block model . The Internal zones were considered to have
behaved as rigid block that escaped to the west during N-S convergence. Subsequent observations that the Internal
zones underwent extension during westerly migration of the Alboran block necessitated modification of the model.
Migration of a collapsing Alboran block were proposed to result from roll-back to the west of an east dipping subducting
slab, with the Alboran block extending in a back arc setting . Royden suggested that arc migration may have proceeded
past Gibraltar. No evidence of such arc migration through the Gulf of Cadiz was found by recent marine surveys
. More recent models propose that subduction stopped retreating in the Gibraltar region .
4.1.2. Slab break off. An alternative subduction related model was initially proposed by Blanco and Spakman based tomography results and further modified by Zeck . Zeck suggested that a NW dipping slab was subducted beneath SE Iberia during the eastward movement of Africa and Iberia relative to Eurasia during the Cretaceous and Paleogene. This subducted slab was proposed to have broken off during the earliest Miocene initiating uplift, extension, and peripheral thrusting and now is oriented vertically beneath southern Spain. This hypothesis is attractive as it provides a mechanism for considerable lithospheric thickening and/or subduction of significant quantities of lithosphere beneath Spain, as the significant length of the imaged high-velocity anomaly (~450-500 km) is hard to explain given the limited convergence between Africa and Iberia. However, it should be noted that most of the eastward motion of Africa and Iberia probably occurred before the Mid Cretaceous , requiring any subducted lithosphere to remain hanging in the mantle for 60 million years before finally detaching and descending into the mantle.
4.1.3. Convective removal. P-T paths determined for rocks in the Betics and Alboran
as they were exhumed to the surface lead Platt and Vissers to propose convective removal of mantle lithosphere
as a possible mechanism. They proposed that lithospheric thickening as a result of convergence had generated a
collisional ridge with a substantial upper mantle root by Oligocene time. In their model this gravitationally unstable
lithospheric root was convectively removed in the Late Oligocene-Early Miocene and replaced by hot low-density
asthenospheric material, resulting in uplift and extension of the overlying crust. Thermal modeling of additional
P-T paths derived from data from Ocean Drilling Program (ODP) drill holes in the Alboran Sea have recently lead
Platt et al. to propose that lithospheric material was removed at approximately 60 km depth, very close
to the base of the overthickened crust. Platt et al. comment that removal of lithosphere at such a shallow
depth is more in keeping with a delamination (in the sense of) model but may still be modeled by a convective removal
process given specific mantle rheology and temperature boundary conditions.
4.1.4. Delamination. The apparent inability of a convective removal model to explain the predominant westward migration of deformation lead García-Dueñas et al. to hypothesize that a delamination model might be applicable to the Alboran. Docherty and Banda and Tandon et al. proposed a SE migrating delamination model based on marine seismic reflection profiles and wells. Observations of the 3-D pattern of earthquake hypocenters, gravity modeling, and regional wave propagation characteristics also lead Seber et al. to propose a delamination model. The N-S line of intermediate-depth seismicity apparently overlain by an aseismic, attenuating, low-velocity mantle and underlain by a region of high-velocity mantle was considered evidence that Iberian and African lithosphere peeled back from the east to west perhaps initiated by detachment of a gravitationally unstable thickened lithospheric root beneath the Internal zones. Mezcua and Rueda and Morales et al. supported this hypothesis based on their earthquake relocations. Seber et al. further suggested that the delamination process is still active today in the western Alboran.
4.2. Implications of Tomography Results
The most striking feature of the velocity model we obtained is the significant positive anomaly that appears to extend from lithospheric depths beneath the Strait of Gibraltar and southern Spain to depths of about 350-400 km beneath the Alboran Sea. We interpret this anomaly as a lithospheric body that has descended into the upper mantle. Although a low-velocity anomaly is present above this body beneath the southeastern coast of Spain, the body appears to be attached to Iberia to the west and possibly north (Figures 11 and 12) which is further supported by the continuation of intermediate-depth seismicity from the top of this body into the Iberian crust (Figure 4). Slab break off or convective removal models with complete detachment of a lone lithospheric body in the Early Miocene and no further mantle processes are probably not sufficient to explain such a geometry. The tomography results alone do not allow discrimination between retreating subduction and delamination processes, as both would have very similar velocity signatures. However, these processes would have produced a different thermal structure in the Alboran lithosphere during extension in the Late Oligocene and Early Miocene . A retreating subduction model would predict stretching of the overriding plate in a back arc setting, whereas delamination (or detachment) models would predict a sudden heating at the base of a thinned lithosphere associated with upwelling of asthenosphere to replace the sinking body. Platt et al. present strong evidence that lithosphere was removed from near the base of an overthickened crust and explicitly state that a retreating subduction mechanism cannot explain the P-T path followed by rocks presently beneath the western Alboran Sea. In our view, the above process of elimination leaves a delamination-based model as the simplest way to explain these observations.
The above argument is probably an oversimplification, as it does not allow more than one active process during the Neogene. The peridotites in the western Betics and Rif (Figure 2) show evidence of cooling in the mantle at 70 km depth before emplacement at the base of the crust . This cooling signature is interpreted by Vissers et al. to result from the location of these peridotites in the hanging wall of a subduction zone some time before the Neogene, suggesting that subduction probably played a role in the formation of the collisional belt. The existence of significant high-velocity anomalies at depths of 550-650 km, coincident with the few deep earthquakes, raises the possibility that some lithosphere may have become detached completely during the Neogene. It is also possible that the pull of this hanging lithosphere may be allowing some subduction of Iberian continental crust as Africa and Iberia continue to converge .
4.3. Synthesized Model
Figure 15 shows our modified version of the working hypothesis model proposed by Platt and Vissers (compare with Figure 14). Lithospheric thickening occurred during Paleogene convergence and probably involved limited subduction. At the end of Oligocene and Early Miocene the gravitationally unstable upper mantle root partially detached beneath the thickened Alboran crust and peeled back to the west with a portion of the lithosphere perhaps detaching completely and descending into the upper mantle. Crust above the inflowing asthenospheric material was heated, uplifted, and extended. Today, although this peeling back has significantly reduced in speed or perhaps terminated, a large lithospheric body remains beneath the Alboran Sea attached to Iberia beneath the western Betics and possibly the Rif. The intermediate depth earthquakes appear to be occurring near the top surface of this hanging body, whereas the deep earthquakes may be located in detached pieces of lithosphere, perhaps as they undergo phase transitions at the base of the upper mantle.