In this section we focus on an overview of the relevance of scientific results from Leg 161 with regard to the geological and geophysical background of the Alboran region, and also on the major questions left incompletely resolved by drilling in the Alboran Basin. In our opinion, one of the main contributions of Leg 161 derives from the results at Site 976 because those results have provided essential clues about the origin of the basin. Nonetheless, drilling results in the East and South Alboran Basins (Sites 977, 978, and 979) helped to better constrain the timing of later tectonic stages in basin evolution and, therefore, to discern the reasons for the present structural pattern in the westernmost Mediterranean region (Fig. 13, Fig. 14). New findings revealed by drilling improve current knowledge on the tectonic evolution of the basin (Fig. 15).
The two crustal sections in Figure 14 illustrate east-west (Fig. 14A) and north-south (Fig. 14B) true-scale structures across the Alboran Basin itself and the peripheral arcuate thrust-belt in the Gibraltar region and Betic and Rif Chains. These sections accommodate shallow structures shown in the map in Figure 13, and illustrate the areal distribution and organization of the distinct crustal domains, as well as the abrupt crustal thinning that characterizes the transition from the Betics and Rif to the Alboran Sea. The subcrustal structure for the whole region is quite debatable, as available geophysical data are not sufficient to confidently depict the lithospheric structure yet.
Effective constraints of extension-related exhumation history at Site 976 are given by the cooling history of the basement (Hurford et al., Chap. 21, this volume; Platt and Kelley, Chap. 22, this volume; Platt et al., in press). As detailed in the previous sections, thermal modeling of the P-T path indicates that exhumation of the basement, occurred very rapidly, at an average rate of more than 4 km/m.y. Because the required total time for exhumation of metamorphic rocks from 40 km depth to the surface is about 9 m.y., Platt et al. (in press) propose that the extensional denudation and thinning of the Alboran Crustal Domain started during the late Oligocene (at about 27 Ma); in other words, somewhat before the marine transgression that created the Alboran Sea (Fig. 15). Results from drilling therefore support previous suggestions, based on analysis of the subsidence history, that part of the crustal thinning in the Alboran Domain occurred above sea level (Watts et al., 1993).
It is worth remarking that the P-T path of the high-grade schist at Site 976 contrasts with the published P-T paths of the different units in the Alpujárride Complex of the Alboran Domain in the western Betic Cordillera, particularly with regard to the occurrence of an increase in temperature during the extensional decompression path (Fig. 11). The current P-T evolution of these Alpujárride Complex units considers a conspicuous pressure drop (with an estimated -
P >10 kbar) under isothermal conditions (e.g., García-Casco and Torres-Roldán, 1996; Balanyá et al., 1997; Azañón et al., 1998). After the decompression, the different Alpujárride units show quick cooling histories (100°-350°C/m.y.; Zeck et al., 1992; Monié et al., 1994), comparable to that at Site 976.
The lithologic association at Site 976 broadly resembles some that are found within the Alpujárride Complex. Sánchez-Gómez et al. (Chap. 23, this volume) suggest that rocks sampled at Site 976 correlate with a specific Alpujárride unit—the Ojén Unit—underlying the Ronda-peridotite slab in the western Betic Cordillera (30 km west-southwest of Málaga; Fig. 4). Like the basement samples at Site 976, this unit is formed of marbles, high-grade schist, amphibolite, and gneiss, with abundant migmatite and leucogranitic rocks (granite and granodiorite). Leucogranite occurs as late dikes cutting most of the ductile structures and as large sheet-like bodies associated with a shear zone that comprises the lower peridotite boundary (Sánchez-Gómez et al., Chap. 23, this volume). The P-T path presented for the Ojén Unit (Fig. 11B) shows a significant decompression path, from HP-conditions recorded in relict eclogite (P > 15 kbar at T = 720°-790°C; Tubía and Gil Ibarguchi, 1991) to low-P conditions (P 
3-4 kbar at T = 650°-700°C) occurring at the end of the development of the main foliation (Sp) (Westerhof, 1977; Tubía et al., 1997; Sánchez-Gómez et al., Chap. 23, this volume). Furthermore, leucogranitic rocks formed at low-P conditions (5.5-4.2 kbar at 650°-680°C) during the latest stages of decompression and the initiation of the cooling path (Fig. 11). Therefore, in contrast with Site 976, the metamorphic evolution of the Ojén Unit does not record an increase in temperature during decompression.
The discrepancies in the metamorphic history make the correlation of the basement rocks at the Site 976 High and the Ojén Unit somewhat unclear, because lithologic similarities among the Alpujárride units, even if clear, should not be the only criteria for correlation. Further attempts at offshore-onshore basement correlation should consider that the low-P metamorphism and associated heating at Site 976 could be a particular feature of the basement beneath the Alboran Sea, where the crust is extremely thin. Identical metamorphic evolution would therefore not have occurred in other regions of the Alboran Domain where the crust is thicker (Fig. 14).
The crustal section in Figure 14A is based on a synthesis of structural data from the Gibraltar region and MCS profiles in the Alboran Sea, and 3D modeling, considering gravity, surface heat-flow, and elevation data (Torné et al., unpubl. data). The crustal thickness beneath the Alboran Sea estimated from this modeling is, for the most part, in agreement with available seismically derived estimates of the Moho depth. Seismic and modeled data suggest that the continental crust is about 18 km thick in the West Alboran Basin, thinning eastward to the East Alboran Basin (to less than 12 km), progressively giving way toward the east to an anomalous crust, with extensive volcanism, and then to a probable oceanic crust (basement of 2 s TWT in thickness) in the western South Balearic Basin. The eastern end of the crustal section in Fig. 14A is based on profile ESCI-Alb2, where isolated deep reflections distinguished between 6.5 and 7 s TWT underneath the Messinian salt diapirs have been interpreted as associated with the reflective Moho (Comas et al., 1997). The crustal section in Figure 14B is based on a synthesis of structural data from Spain and Morocco, on Conrad seismic lines, and on a density model obtained by comparing observed and calculated Bouguer anomaly data (Watts et al., 1993). The density model proposes a thicker crust (up to 22 km; Fig. 14B) beneath the Alboran Sea than that obtained from the 3D model (up to18 km, Fig. 14A). A possible reason for the discrepancy in the modeled crustal-thickness values could be that the 3D model (which gives a lower crustal thickness) considers the gravity effect of the asthenospheric mantle, deduced from the heat-flow density values in the region. The lack of refraction and wide-angle reflection seismic data, for example, makes it difficult to present a conclusive value for the crustal thickness in the region; although both crustal sections confirm that the continental crust beneath the Alboran Basin has been substantially thinned. Further geophysical constraints are needed to determine the true amount of crustal thinning.
Much of the crustal stretching during the Miocene rifting can be related to intracrustal low-angle extensional detachments seen in the deep MCS reflection profiles, implying these detachments drove and accommodated the shallow extensional structures observed in the Alboran Basin (Fig. 14). Intracrustal reflectors (ICR; Fig. 16, Fig. 18), interpreted as probable extensional shear-zones (mylonitic zones), are imaged in the Conrad lines between 4 and 7 s TWT in the northern Alboran Sea margin, with an apparent dip to the south (Watts et al., 1993). In the ESCI-Alboran 1 line, located at 3ºW meridian in the Spanish shelf and striking N20ºE across the northern Alboran Sea, similar ICR can be seen, with an apparent dip to the south-southwest, between 6 and 8 s TWT in depth (Comas et al., 1997). The ICR observed in ESCI-Alboran 1 line, probably represent the continuation of a near-flat prominent reflective band, recognized at about 6 s depth in the ESCI-Béticas 2 line (striking N30ºE across the Alboran Domain near the coastline at 3ºW meridian), also interpreted as an extensional shear-zone or a mid-crustal decoupling level (García-Dueñas et al., 1994; Martínez-Martínez et al., 1997b). Reflectors interpreted at the top of the reflective lower crust (TLRC; Fig. 16, Fig. 18), imaged at about 6-8 s TWT in depth in the Conrad lines (Watts et al., 1993), can also be observed as a band of highly coherent reflectivity between 6-9 s TWT in depth in the ESCI-Alboran 1 line (Comas et al., 1997). The bottom of this beam of reflections (TLRC) might be attributed to the Moho boundary discontinuity, which is in agreement with results from stacked wide-angle data in the ESCI-Betics 2 line (Carbonell et al., 1997). Intracrustal seismic images, and consistency between the timing of extensional episodes reported from offshore and onshore regions, suggest that the crustal thinning beneath the northern Alboran Sea basin is linked to the extensional detachment-systems in the Betic Chain.
Drilling data demonstrated that the Site 976 High was exposed to the seafloor by the late Serravallian, but near-site MCS profiles (Fig. 16, Fig. 17) suggest that the basement high has been submarine, or occasionally emergent, relief since the early Miocene. Major unconformities within the early to middle Miocene sedimentary cover (reflectors R5 and R4; Fig. 8, Fig. 15) are probably a result of pulses of fault activity during the rifting processes. The R3 reflector (Fig. 3 and Fig. 8), appears to mark the end of Miocene rifting in the Alboran Basin (Fig. 15). On the whole, seismic interpretations support the existence of low-angle, fault-related continuous extension in the basin from the Aquitanian?-Burdigalian (the age of the marine transgression) to the early-late Miocene (Fig. 15), in agreement with subsidence data from the Andalucía-A1 well (Fig. 6). Different directions of local Miocene extension (south-southeast, southwest, west-southwest, and northeast) have been reported in the Alboran Basin; nonetheless, a resulting direction, with the top to the southwest or south-southwest, of maximum extension appears to dominate the Miocene rifting (Fig. 4). Changes in direction of extension are probably caused by the interplay of transfer-faults within the middle-to-late Miocene extensional system, which may in turn have been conditioned by previous early Miocene extensional faults.
Drilling results also provided information of certain events of the postrift contractive deformation of the Alboran Basin (Fig. 15). Directions of compression are indicated by east-northeast-west-southwest trending folds and sinistral northeast-southwest and north-northeast-south-southwest, and dextral west-northwest-east-southeast strike-slip conjugate fault systems (120°-130° angles between the two strands; Fig. 13). The contractional deformation was dominated by post-Messinian wrench tectonics (Fig. 16, Fig. 17, Fig. 18, Fig.19); the conjugate strike-slip faults give way to both transtensive conditions that originated new depocenters (e.g., the Alboran Trough and the Yusuf Basin) and a transpressive situation producing local uplifting (positive flower-structures) by oblique-reverse strike-slip faults and folds (e.g., the Alboran Ridge and its prolongation at Xauen Bank). Most of these faults are active presently, leading to distributed seismicity in the region. The focal mechanisms of earthquakes and moderate teleseisms document the present-day state of stress in the basin, which is quite variable from one site to another, in accordance with the distinct types of deformation now occurring in the region (Morel and Meghraoui, 1996; Calvet et al., 1997; Mezcua and Rueda, 1997). De Larouzière et al. Chap. 24, (this volume) confirm a predominant southwest-northeast extensional (or transtensional) present-day situation at Site 976, in agreement with positive subsidence in the West Alboran Basin since the late Pliocene (Fig. 6). Significant morphological lineaments in the Alboran Sea basin and in its transition to the South Balearic Basin (e.g., the Palomares and Carboneras strike-slip shear zones, Fig. 4 and Fig. 13; Weijermars, 1987; Keller et al., 1995) formed as a result of this compression. Contractional structures are congruent with those known in areas surrounding the Alboran Sea (de Larouzière et al., 1988; Montenat, 1990). The post-Messinian deformation of the basin is consistent with the north-northwest/north current convergence of Iberia and Africa.
Subsidence analysis at the Site 976 High and commercial wells provide new data on the subsidence/uplift history of the marine basin (Fig. 6; Rodríguez-Fernández et al., Chap. 5, this volume). Subsidence data at this site reveal significant events in basin evolution: rapid tectonic subsidence (at rates of 3 km/m.y.) from the late Serravallian till the early Tortonian, gentle subsidence till the earliest Pliocene, relative uplift of the structural high during the early Pliocene, and an active subsidence-pulse by the earliest Pleistocene. New data from the Andalucía-A1 well denote three episodes of rift-related subsidence at 15.5-14.5 Ma (Langhian-early Serravallian), at 13-10.7 Ma (late Serravallian), and at 9.2-8.5 Ma (Tortonian), separated by episodes of uplifting. These results are somewhat different from previous subsidence estimates in the Andalucía-A1 well (Watts et al., 1993; Docherty and Banda, 1992, 1995), probably because the backstripping technique used to obtain the subsidence trend presented here first includes paleobathymetric corrections. Subsidence analyses across the Alboran Basin indicate a conspicuous lateral variation of rift-related subsidence phases, suggesting migration of locus of extension, and probably also changes in extensional-transport direction during Miocene rifting. Differences of subsidence amount and rate during contemporaneous rifting-phases were probably related to different basement tilting or rotation. Early Pliocene uplift and earliest Pleistocene (at about 1.7 Ma) rapid subsidence at Site 976 could represent tectonic events during the post-Messinian contractive reorganization of the basin; the rapid subsidence probably being related to episodic transtensional conditions in the West Alboran Basin. The almost general gentle phase of subsidence that affected the entire basin during post-Messinian times, contemporaneous to its contractive reorganization, has been interpreted either as thermal (Watts et al., 1993) or as flexural subsidence (Docherty and Banda, 1995). From the latest Tortonian onward, the overall negative trend of the subsidence in the onshore Neogene Basins (e.g., Granada Basin, Fig. 6) indicates general uplift at the basin margins, in agreement with the changeover from marine to continental deposits since the early Pliocene in these intramontane basins (Sanz de Galdeano and Vera, 1992).
Although Leg 161 did not drill or sample any in-situ volcanic rocks, geochemical studies, and Ar/Ar age-dating of reworked volcanic pebbles recovered at shallow sedimentary levels in the Eastern Alboran Basin yield additional information on the characteristics and ages of the middle and late Miocene magmatism in the region (Fig. 15). The Neogene volcanism in the Alboran Sea is better developed in its central and eastern parts, thus suggesting more vigorous magma production on this side of the basin (Fig. 13). In southern Spain, volcanic rocks lie within the Neogene Betic basins and also in the Alboran Domain of the Betic Chain, with a few located in the Betic External Zones. Magmatic rocks in the central-western Betics include basic dikes of tholeiitic affinity of about 23-22 Ma (K/Ar dating; Torres-Roldán et al., 1986) and leucogranites at about 18-20 Ma (Ar/Ar dating; Zeck et al., 1989, 1992; Monié et al., 1994). Magmatic products in the Cabo de Gata region, the most ample volcanic province (about 400 km2) in the Betic Chain (Fig. 4 and Fig. 13), belong to calc-alkaline series containing all the petrological types, from basic to acid volcanism (e.g., Fernández-Soler, 1992; 1996). In the Almería and Murcia region, scattered shows of magmatic rocks belong to a peraluminous (S-type) volcanic province and to an ultrapotassic (lamproitic) suite, giving way to the northeast to Pliocene alkaline basalts in the area of Cartagena. In the Rif, magmatic manifestations include calc-alkaline andesitic and rhyolitic suites, as well as shoshonitic and alkaline-sodic suites of late Miocene and Pliocene age; similar rocks appear farther east in the Oran area (Araña and Vegas, 1974; Bellon et al., 1983; de Larouzière, 1985; Hernandez et al., 1987; Louni-Hacini et al., 1995). Ages (mostly K/Ar dating) of typical calc-alkaline series in the eastern Betics and Rif range from 15 Ma to 8 Ma, the shoshonitic to lamproitic lavas occurred at about 9-4 Ma, and the alkaline basalts erupted from 6 to 1.7 Ma (Bellon et al., 1983; Hernandez and Bellon, 1985; Di Battistini et al.,1987; Hernandez et al., 1987).
In the marine realm, the Alborán Island is formed of low-K basalts and andesites and reported radiometric-age data are imprecise and quite discordant (9-9.2 Ma, Hernandez et al., 1987; or 7 Ma and 18 Ma, Aparicio et al., 1991). Volcanic rocks from Alboran seafloor scarpments have been obtained recently by diving with the submersible Cyana on the northern flank of Alborán Island, at the Yusuf Fault scarpment, and at Al-Mansour Seamount (Fig. 4). Samples recovered by diving are mainly composed of rocks of calc-alkaline rhyolite and low K-basalt to andesite series, yielding K/Ar radiometric ages of 10.1 ± 0.8 Ma (andesite, from the whole rock) and 9.6 ± 0.3 Ma (rhyolite, from biotite) in the Yusuf scarpment and the Alborán Island, respectively (Fernández-Soler and Comas, unpubl. data).
The Ar/Ar age dating in the volcanic pebbles from both Sites 977 and 978 presented by Hoernle et al. (Chap. 27, this volume) register the magmatic activity to a time span of about 6 m.y. (from 12 Ma, Serravallian, to 6 Ma, Messinian). The authors emphasize that the volcanic pebbles are older in age downcore at Site 978; therefore, they suggest the volcanic clasts to have been deposited shortly after their eruption. However, this conclusion is not consistent with drilling data, which indicates that cored pebbles, being thinner in diameter than the core liner (maximum diameter of volcanic pebbles is ~6 cm, minimum ~1 cm), are mixed when logged along the core liner. Biostratigraphic ages recorded in fine-grained marine facies overlying and underlying the gravel interval at Site 978 (Iaccarino and Bossio, Chap. 42, this volume; de Kaenel and Siesser, Chap. 16, this volume) imply that volcanic pebbles are reworked, mixed with sedimentary and metamorphic clasts, and deposited all together in a sedimentary gravel bed by the latest Messinian or shortly afterward (i.e., after 5.6 Ma). Hoernle et al. (Chap. 27, this volume) also declare that the volcanic pebbles proceed from volcanic areas adjacent to the drilling sites, which would not be entirely plausible. As pointed out by the Shipboard Scientific Party (1996c, 1996d), because the gravel interval has been found at similar stratigraphic positions—corresponding exactly to the M-reflector—at sites located 37 km apart, the gravel beds likely occupy latest-Messinian or lowermost-Pliocene submarine paleo-valleys; the volcanic pebbles could therefore mainly correspond to detritus from the Almería and Cabo de Gata region that moved out toward the South Balearic Basin along the valley bottoms.
The hypotheses currently most widely debated to explain the formation of the Alboran Sea basin can be summarized as follows:
The principal tectonic objective of ODP Leg 161 was to obtain information that could constrain these and other models for the origin of the Alboran Sea. All three models summarized above are based on ideas and assumptions about the mechanical behavior of the lithosphere in convergent plate settings and are difficult to test directly. It is therefore appropriate to consider what predictions these models make that could be tested by near-surface observations. The most obvious differences among them are in the predictions they make about (a) the thermal structure of the lithosphere and (b) the history of surface elevation; and (c) the nature and distribution of the magmatism.
The rollback model attributes extension primarily to forces applied at a distance. Extension is therefore "passive" in that convective or advective motion of material and heat is not the primary cause of lithospheric deformation. The thermal structure of the lithosphere during extension should therefore be simply predictable as a result of horizontal stretching. This causes the thermal gradient to increase, but individual material points within the lithospheric column will either decompress isothermally (if extension is rapid compared to the characteristic time for conductive cooling to the surface) or they will cool (Ruppel et al., 1988), unless there is significant heat input from crustal radioactivity or from magmatism. The upper surface of the lithosphere should on average subside throughout this process (McKenzie, 1978) as a result of crustal thinning, cooling of the stretched lithosphere, and sediment loading. Local uplift may occur in the footwalls of major faults during rifting. Furthermore, contemporaneous magmatism is predicted to have a subduction component signature. Passive extension is unlikely to be associated with significant magmatism unless the mantle potential temperature is abnormally high, or the amount of extension is very large (McKenzie and Bickle, 1988). Any contemporaneous magmatism is therefore likely to be related to subduction at the plate boundary and should show an appropriate geochemical signature.
Delamination, as originally proposed by Bird (1978), on the other hand, involves the wholesale removal of the lithospheric mantle, bringing asthenospheric material to the base of the crust. This would cause a dramatic step in the geotherm, which would decay, producing substantial temporary heating of the crust. This heating may accompany decompression during extension, producing distinctive P-T paths. Substantial melting of the lower part of the crust may occur as a result of this heating, depending on its precise composition, and melting would be further stimulated by the rise of basaltic magma produced by decompression melting in the underlying asthenosphere. At the same time, the removal of the negatively buoyant mantle lithosphere could result in as much as 3000 m of surface uplift, depending on the initial thickness and composition of the lithosphere. This would then be followed by rift-related subsidence as extension gets under way, and then by thermal subsidence. Both types of subsidence should proceed at a higher rate initially than in the case of passive rifting because of the absence of the negatively buoyant and thermally insulating mantle lithosphere, but the total subsidence, once the lithosphere has reached full thermal equilibrium, would eventually be the same for a given amount of extension.
The concept of convective removal of the lower part of a lithospheric root as the driving force for extension in orogenic settings lies somewhere between the two other hypotheses in terms of its implications for the thermal evolution and subsidence history. Removal of part of the lithosphere should cause a step in the geotherm, and hence some degree of heating of the overlying crust and the remaining lithospheric mantle (Platt and England, 1994), partial melting within the lithosphere, and also some initial uplift (possibly 2-3 km; England and Houseman, 1989). The amounts of heating and uplift would be less than in the case of complete delamination, depending on how much of the lithosphere is removed.
It follows from the above that both the thermal evolution of the extended crust, the magmatism, and the uplift and subsidence history of the region should contain information constraining these hypotheses. The seismic and drill-hole information obtained during Leg 161 should therefore make a significant contribution toward resolving this question for the Alboran Sea.
The character of the Alboran Basin magmatism is an important factor discerning among competing models for basin formation; however, proposed magmagenetic models for the calc-alkaline rocks of the Alboran Domain remain controversial. Magma generation has been related to the melting of one (e.g., Araña and Vegas, 1974) or two (e.g.,Torres-Roldán et al., 1986) subducted oceanic crusts, in an early Miocene subduction; to melting of an ancient subducting, rolled-back or detached, lithosphere slab (e.g., de Jong, 1991; Zeck, 1996; Lonergan and White, 1997); associated with a trans-Alboran crustal shear zone (de Larouzière et al., 1988); or related to lithosphere mantle removal or delamination (Fernández-Soler, 1992, 1997).
Hoernle et al. (Chap. 27, this volume) affirm that the calc-alkaline affinities and the incompatible element systematics in volcanic pebbles from Sites 977 and 978 are characteristic of subduction-zone volcanism, indicating that their geochemical signature, and certain geophysical evidences, support a northwest-dipping oceanic-subduction zone beneath the eastern Alboran from 6 to at least 12 Ma; in addition, these authors propose that a Pliocene "slab detachment and lithosphere delamination" followed the late Miocene subduction. However, given that many subduction-related magmas obtain their geochemical signature by the addition of subducted sediment to shallow mantle-derived melts (e.g., Tatsumi and Eggins, 1995), the use of geochemistry may be problematic in distinguishing them from crustally contaminated asthenospheric melts (Kay and Kay, 1993).
Geochemical work on the very similar suites of volcanic rocks exposed onshore in the eastern Betic Cordillera, however, suggest that alternative interpretations of the geochemistry are possible (S. Turner, pers. comm., 1998). A calc-alkaline suite exposed in the Cabo de Gata region, with an age range comparable to the pebbles found at Sites 977 and 978, show enrichment of the more incompatible trace-elements, such as Rb, Th, and U, and are markedly LREE-enriched with a variable negative Eu anomaly. They have prominent negative Ba, Ta-Nb, and Ti anomalies and a large positive Pb anomaly, relative to primitive mantle. Modeling of trace-element ratio-isotope arrays indicates that these magmas can be interpreted as representing variable degrees of crustal contamination (20%-50%) of MORB-type melts produced by shallow melting within asthenospheric mantle. Similar conclusions have been reported from extensive geochemical data (trace-element/isotopic modeling) carried out in numerous rock-types and wide age spectrum of the calc-alkaline and shoshonitic volcanism in the Cabo de Gata region (Fernández Soler, 1992, 1996). The geochemical characteristics of the Alboran-Yusuf low-K mafic suite sampled by diving suggest that these rocks are less evolved than the Cabo de Gata calk-alkaline series, probably because of the very thinned crust in this marine region. Incompatible/immobile element data from these rocks indicate a depleted MORB source area (flat to LREE-depleted patterns), and the presence of an "arc component" signature (lithophile [LIL] element enrichment, relative depletion of Nb and Ta). Their 87Sr/86Sr contents are higher than typical mantle values and much higher than depleted mantle values, thus indicating the presence of a metasomatizing sialic (crustal) components in the magmas. Modeling of trace-element ratio-isotope arrays also suggests that these magmas could derive from a depleted mantle source, to which the "crustal" or "slab" component has been introduced from partial melting of both crust and heterogeneous mantle by upwelling of hot asthenospheric mantle (Fernández-Soler and Comas, unpubl. data).
It is worth noting that all three hypotheses discussed here are subject to variations in terms of the geometry and kinematics of the process, so that observations on the geometry and kinematics of extension in the basin may not be definitive in distinguishing among them. An example of this is that two recent proponents of the trench rollback hypothesis (Royden, 1993; Lonergan and White, 1997) suggest quite different end positions for the subduction zone. Similarly, recent advocates of mantle delamination have proposed that propagation in different directions (García-Dueñas et al., 1992; Docherty and Banda, 1995; Shipboard Scientific Party 1996a [Fig. 3]; Seber et al., 1996). The concept of convective removal of mantle can be similarly modified by the introduction of an arbitrary velocity for the extending Alboran Domain relative to the African and Iberian plates (e.g., Platzman, 1992).
The main conclusion of the thermal calculations from the basement at Site 976 is that, in the absence of evidence for substantial magmatic heat input, the substantial heating (by about 75°C) of the high-grade schist during decompression to very low pressures can only be explained by the removal of the mantle lithosphere below about 60 km. Removal of lithosphere at depths significantly greater than 62.5 km cannot explain the combination of high temperatures reached by the Site 976 high-grade schist and the shallow depth at which they attained maximum temperature. Given that this corresponds approximately to the thickness of the pre-extensional orogenic crust in the region, this is equivalent to delamination in the sense of Bird (1978).
Tectonic models for the formation of the Alboran Sea basin that involve lithospheric stretching in response to plate-boundary forces, without any removal of lithosphere, nor partial convective removal of the mantle lithosphere, cannot explain the late onset of heating and the high temperatures reached by Site 976 basement cores. It can be argued that one possible heat source to account for the observed increase in temperature during the latest Oligocene(?)-early Miocene exhumation of the Alboran Sea basement could be provided by contemporaneous magmatic activity. As stated above, in the western Betic Chain, the intrusive rocks are of early Miocene age (18-20 Ma for the leucogranite dikes and 22-23 Ma for the basaltic dikes) and have a limited extension. Volcanism is particularly important east of the 3°W meridian (Fig. 13), but it is younger in age (mainly developed from 15 Ma onward [Fig. 15]). In consequence, the magmatic activity in the Alboran Sea appears to have occurred after the exhumation of the Alboran Sea basement and during the latest stages of its extensional history. Furthermore, at Site 976, the relative volume of the leucogranite dikes to the overall metapelite volume is too small to have acted as an additional heat source. In accordance with all of these data, there is little evidence for significant coeval magmatic activity near Site 976 or in the surrounding Alboran Domain that could have produced the considerable heating observed in the thermal evolution of the metamorphic rocks.
From the competing genetic hypotheses tested by drilling in the Alboran Sea basin (Shipboard Scientific Party, 1996a), we believe that cogent evidence from basement rocks at the Site 976 High favors models invoking wholesale removal of mantle lithosphere beneath the basin. This hypothesis appears to be clearly supported by extensive structural data from the whole region—the Betic and Rif Chains and the Alboran Basin—and is in accordance with geophysical constraints such as seismicity distribution, tomography models, gravity, and heat-flow data; even the geochemical signature of the Alboran magmatism does not contradict this proposed scenario. A northwest-dipping middle-to-latest Miocene (from 6 to at least 12 Ma; Hoernle et al., Chap. 27, this volume) oceanic-crust subduction beneath the eastern Alboran Basin, clearly confront with kinematic and structural data from the region. No evidence of coeval oceanic crust are recognized in the region, neither from geophysical nor geological data. Several geophysical data, however, might conform to a subduction model, as proposed by previous authors (see above). To verify a genetic model based solely on the nature of the magmatism, it is important to determine the areal distribution of the distinct geochemical signature in the entire volcanic region.
In conclusion, although some of the results presented here may be controversial, it is worth noting that hereafter any reliable genetic model for the origin of the Alboran Basin should be contrasted with the thermal data characterizing the metamorphic basement at Site 976. In this regard, we believe that further geological and geophysical data of the westernmost Mediterranean region are needed to validate the Leg 161 insights presented in this synthesis. to complete a definitive, fully satisfying model for the Neogene evolution of the lithosphere beneath the Alboran Sea.
