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The structure of the Alboran Sea: an interpretation from seismological and geological data
Tectonophysics 338 (2001) 79±95
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The structure of the Alboran Sea: an interpretation from seismological and geological data C. LoÂpez Casado a,*, C. Sanz de Galdeano b, S. Molina Palacios c, J. Henares Romero a b
a Departamento de FõÂsica TeoÂrica y del Cosmos, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, CSIC-Universidad de Granada, 18071 Granada, Spain c Departamento de Ciencias de la Tierra y del Medio Ambiente, Facultad de Ciencias, 03690 Alicante, Spain
Received 17 May 2000; accepted 22 February 2001
Abstract Almost all the earthquakes included in the catalog for the Iberian±Maghrebian area up to 2000 have been used in order to know the structure of the Alboran Sea area after the veri®cation of the aleatory nature of the errors in their spatial localization. A new focal mechanisms catalog has also been used as well as many of the available geological data. During the Mesozoic and till the Oligocene, the Betic±Rif Internal Zone was situated further E, but with the opening of the Algero-ProvencËal basin in the early Miocene, the Betic±Rif Internal Zone moved to the W. Contemporary, the Alboran Sea was created as the western prolongation of the Algero-ProvencËal basin. The Betic±Rif Internal Zone overthrused part of the Iberian and African plates, producing the partial sinking of both plates and being responsible of the intermediate seismicity existing in the western sector of the Alboran Sea. The intermediate earthquakes in the Atlas towards the NE and WSW are not related to lithospheric sinking but to signi®cant deep faults limiting a subplate in NW Africa. In the Atlantic, the intermediate earthquakes between the Gorringe sector and Gibraltar are produced in the contact between the Iberian and African plates and by the important faults crossing it. The existence of four very deep earthquakes is related to the previous sinking of the lithosphere originally associated to the domain in which the Betic±Rif Internal Zone was situated. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Alboran Sea; Betic Cordillera; Seismicity; Intermediate earthquakes; Subduction; Deep faults
1. Introduction The Alboran Sea, located between the Betic Cordillera and the Rif (N of Morocco) has a continental crust thinned during the early and middle Miocene and its geological setting is complex. To the N, the Betic Cordillera (Figs. 1 and 2) is divided into Internal and External zones with the Guadalquivir foreland * Corresponding author. Fax: 134-58-274-258. E-mail addresses: [email protected] (C. LoÂpez Casado), [email protected] (C. Sanz de Galdeano), [email protected] (S. Molina Palacios).
basin separating it from the Iberian Massif. The External Zone, divided into the Prebetic and Subbetic formed originally the south and southeast Mesozoic and Tertiary sedimentary cover of the Iberian shield and is arranged in many tectonic units. The Internal Zone basically consists of four tectonically superimposed complexes that, from bottom to top, are the Nevado-Filabride, the Alpujarride, the Malaguide and the Dorsal Complex, the last partially associated to the Malaguide. The ®rst two complexes present a welldeveloped Alpine metamorphism. The units of the Campo de Gibraltar came from the former Flysch basin, originally located to the S of the Internal Zone.
0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0040-195 1(01)00059-2
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Fig. 1. Main geological domains in the Iberian±Maghrebian region (from Buforn et al. (1995)).
To the S of Alboran, the Rif is also divided into Internal and External zones. The Internal Zone is common with the Betic Internal Zone, but without the presence of the Nevado-Filabride complex. For this reason it can be cited as the Betic±Rif Internal Zone. The External Zone, formed by Mesozoic and Tertiary sedimentary series, is different to the Betic External Zone. Further S, the Gharb foreland basin separates the Rif from the Moroccan Meseta. This last is in large part equivalent to the Iberian massif. South and east of the Meseta is the Atlas, a cordillera which has developed onto a band of old crustal weakness since the end of the Palaeozoic. In the Atlantic, the boundary of the Iberian and African plates extending from the Azores to the Gulf of Cadiz is relatively well de®ned in its westernmost part up to the sector between the Gloria and
Gorringe faults. Eastwards, this contact is strongly affected by transversal faults. Antecedents. The Neogene structuring of the Betic±Rif area and of the Alboran Sea has been the object of numerous hypotheses. Some explain the present position of the Betic±Rif Internal Zone due to its signi®cant westward displacement (Andrieux et al., 1971; Biju-Duval et al., 1976, 1977; DurandDelga, 1980; Rehault et al., 1984) linked to the opening of the Algero-ProvencËal basin (Sanz de Galdeano, 1990). Lonergan and White (1997) support the interpretation of a westward displacement of the Betic±Rif Internal Zone, related to E-dipping subduction. A subduction of Africa under Iberia, with the Alboran Sea being a back-arc basin was proposed by Morley (1993) while Royden (1993) locates a subduction dipping E ahead of the Gibraltar arc in the Atlantic, in the contact zone between the Iberian
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Fig. 2. General geological map of the Betic Cordillera, Rif and Alboran Sea. Modi®ed from Sanz de Galdeano (1997), Polyak et al. (1996) and Soto et al. (1996). Note how the area within the 4000 m isobath in the W Alboran coincides with the position of the arc of intermediate seismicity (Fig. 5a).
and African plates. Zeck (1996, 1997) assumed the existence of a broken subducted sheet producing extensional tectonics on the surface. Morales et al. (1999) proposed the existence of an active continental subduction of the Iberian plate beneath the Betic Cordillera and the Alboran Sea. Other hypotheses consider the formation of a large dome in the Alboran Sea and later extension and thinning of the crust linked to convection processes in the mantle (Weijermars, 1985, 1987; Doblas and Oyarzun, 1989a,b; Platt and Vissers, 1989). Vissers et al. (1995) suggested that the lithospheric thinning in the Alboran Sea took place through a convection process. Houseman (1996), without discarding other models,
sustains that the convection model provides the best explanation. The possible action of a process of lithospheric delamination in the Alboran Sea was indicated by GarcõÂa-DuenÄas et al. (1992), Watts et al. (1993) and Docherty and Banda (1995). Seber et al. (1996a,b) reported the existence of a lithospheric body rooted 350 km deep under part of N Morocco, the Alboran Sea and the Betic Cordillera. Possibly its sinking was due to a delamination process. Mezcua and Rueda (1997) relocated the recorded earthquakes from 1950 to 1995 in the Alboran Sea, Gulf of Cadiz, Betic Cordillera and Rif, concluding that a delamination process would better explain the intermediate
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and very deep seismicity of the region. Buforn et al. (1997) suggested the existence of a vertical sheet located between Malaga and Granada and the W Alboran Sea, extending from around 50 km to about 150 km depth which probably formed in a process of lithospheric delamination. Morales et al. (1997) discussed the origin of the intermediate earthquakes in the W Alboran Sea and proposed the existence of a delamination process which spread westwards. In this article, we provide an interpretation of the current structure of the Alboran area, especially from seismological data, but taking also into account the geological evolution of the western Mediterranean since at least the Oligocene times, taking into account the evolution of the Betic and Rif cordilleras at the same time. This is necessary because the Alboran Sea developed with the Algero-ProvencËal basin (the Alboran Sea is the western end of this basin). In fact, the basement of the Alboran Sea is formed by rocks of the Betic±Rif Internal Zone, covered by Miocene sediments and volcanic rocks. 2. Seismicity in the Iberian±Maghrebian area The seismicity in the western border of the African and European plates is characterized by two main features. First, it is not linearly distributed extending rather across a wide area embracing to the north a sector of the Atlantic Ocean and the Iberian Peninsula and to the south, almost all Morocco, north of Algeria and Tunisia. Second, there are intermediate earthquakes (30 , h , 180 km) under the south of the Iberian peninsula, the W and E of the Alboran Sea and the centre and the north of Morocco. Although much less frequent, intermediate earthquakes have also taken place in other regions of Iberia, such as the Pyrenees and the southeastern coast. We should mention also the existence of deep earthquakes (h . 600 km) located approximately south of Granada. 2.1. Quality analysis of the seismological information The data for the earthquakes have been taken from the IGN (Instituto Geogra®co Nacional) database up to 1999 which has been subjected to a recent relocation of the hypocentral coordinates. The hypocentral locations in the catalog are subjected, however, to errors that nonetheless, as shown below, do not affect
signi®cantly our overall descriptions of Iberian± Maghrebian seismicity. Several tests have been used to determine the accuracy of the hypocentral locations. We ®rst analysed the standard deviation in depth with time and space distribution. We divided the earthquakes into shallow (h , 30 km) and intermediate (h $ 30 km). It was observed that: (a) there is a drastic decrease on the errors after 1981 from several dozens of kilometres to no more than 15 km, (b) the standard deviation of the intermediate earthquakes is smaller than for the shallow ones, and (c) in the period 1982±2000 only a small nucleus, SW of San Vicente cape, reaches a 20 km error. Next, we compared the hypocentral locations of several earthquake databases in our study area. Thus, in addition to the IGN date base, we used a version of the original ®le without the latest relocations and the USGS database from 1973 to 1994. In both comparisons (IGN versus original ®le and IGNUSGS) discrepancies in latitude, longitude and depth are distributed around zero value and with a standard deviation below 10 km. Finally, we compared the spatial distributions of earthquake locations according to the IGN database for the periods 1400±1930, 1931±1981 and 1982± 1999 for shallow earthquakes and for the periods 1950±1980 and 1981±1999 for the intermediate ones. For both cases, locations from the different periods did not differ too much. Moreover, in the case of intermediate earthquakes, it was observed that their locations in the period with the largest errors (1952±1981) were about the same than those found in the period with the smallest errors (1982±1999). We therefore concluded that due to the fundamentally random nature of the location errors, grouped hypocentral locations do not constitute any mislocation problem when used to assign a seismicity model to a region. The focal mechanisms were selected from a new catalog of the Iberian±Maghrebian region (Henares et al., 2000). From 481 homogeneus focal mechanisms compiled in this catalog, we selected only 114, 61 with mb $ 5.0, 49 with 30 # h # 200 km and 3 with h $ 600 km. They are those we considered to be related directly with the area analysed in this study. These earthquakes are considered either as single events or as a set of events using the right-dihedra method (Angelier and Mechler, 1977).
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2.2. Shallow earthquakes Fig. 3a plot the epicentral distribution of the shallow earthquakes in this area (mb $ 0) for the period 1910±1999. Fig. 3b plot the epicentral distribution of the shallow earthquakes in this area (mb $ 5) and b-values of the Gutemberg±Ricther law for the period 1910±1999. Although at ®rst sight it seems to be randomly distributed, a detailed analysis reveals certain alignments. Thus, from west to east, according to the main seismic alignments, the value of parameter b, the activity rate and the larger earthquakes, the main characteristics can be described as follows: (a) There is a clear WNW±ESE seismic alignment from 188W to 108W, located at the boundary of the Iberian and African plates, which may continue to the Gulf of Cadiz and also turn towards Morocco. The maximum registered magnitude is 8.2. This area is characterized by a release of its seismic energy through large-magnitude characteristic earthquakes. Most of the focal mechanisms in this area are strike-slip or have a strike-slip component. The main direction of the fault-plane solutions of these mechanisms is WNW±ESE. The most common orientation of the pressure axes is NW± SE. Using the right-dihedra method, we ®nd a similar orientation of the state of stress. (b) Starting from 108W, the above alignment changes from E±W to ENE±WSW up to 38W, crossing the Gulf of Cadiz and part of the Betic Cordillera. The area of change, located around 6830 0 W, is perfectly de®ned by the lowest seismic activity. Although the seismic activity increases considerably in relation to case (a), the size of the earthquakes falls substantially, with the highest registered magnitude 5.2. Focal mechanisms in this area are variable, consisting of strike-slip to reverse and normal faults. The fault-plane solutions and the pressure axis have very different directions. The right-dihedra method does not give either a clear solution. (c) From 38W, at the end part of the above alignment, there is a NNE±SSW alignment, going from 388N to 308N. That is, it follows the SE coast of Spain, crosses the E Alboran Sea and continues down along the Atlas Mountain Chain. Seismic
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activity is similar to that of the above alignment, with two earthquakes of magnitude 5.7 in the central part of the alignment (in the southern Alboran Sea). Areas of very low seismic activity are detected. As in the above area, the focal mechanisms of this sector are quite variable, ranging from strike-slip to reverse and normal faults. The fault-plane solutions have different orientations, although the most abundant component of the pressure axes is N±S. The right-dihedra method, considering only earthquakes from the northern part of Morocco, gives aproximately an E±W orientation for the compression axis. (d) Last, starting from 1830 0 W, after a clear seismic gap, a new E±W to ENE±WSW seismic alignment is observed, reaching 108E and crossing the northern coasts of Algeria and Tunisia. Seismic activity is lower than in the last two alignments. However, the size of the earthquakes is much greater, although falling short of the values in the ®rst alignment. In the central area, there are two earthquakes of magnitude 6.7. In the western zone, clearly separated from the central sector, an earthquake of 5.7 magnitude have been registered. In the eastern area, with the lowest seismic activity, two earthquakes have been registered, reaching magnitude 5.6. Many of the focal mechanisms in this area have a reverse fault character, although strike-slip solutions are also abundant, while normal fault solutions are less signi®cant. Most of the fault-plane solutions are ENE±WSW and the pressure axes are predominantly NW±SE oriented. Similar results are obtained with the right-dihedra method.
2.3. Intermediate earthquakes The above alignments are also observed in the distribution of the intermediate earthquakes (Fig. 3c). The maximum depth for the intermediate seismicity is in the Gulf of Cadiz, where nine earthquakes exceed 140 km, the deepest reaching 180 km. The following groupings or alignements can now be de®ned based on the locations of their epicentral coordinates: (a) the High Atlas, (b) the E Alboran and the prolongation towards SE Spain and the Middle Atlas, (c) the W Alboran Sea and the southern part
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Fig. 3. General distribution of seismicity in the study area for the period 1910±1999: (a) mb . 0; (b) mb . 5 and b-value; (c) intermediate and deep earthquakes for the period 1952±1999, showing the group used in this work.
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of Malaga province, (d) the Gorringe Ð Gulf of Cadiz and (e) the Gorringe Highs Ð coast of Morocco. The projection of the hypocentres of these earthquakes in vertical planes of N±S, E±W, NW±SE and SE±NW direction (Fig. 4) suggests the impossibility of associating all this seismicity with a single subduction zone. Seismic depths seem to be dispersed, mainly in the western sectors, with substantial seismic gaps. However, in the area of Malaga and in the W Alboran Sea, there is a progressive increase in depth towards the south. The characteristics of these groups are: (a) The Atlas group (Fig. 4a) can be divided into four subgroups, the second from the SW position, being the denser and comprising the deeper earthquakes. The accuracy of the depth of these earthquakes was evaluated by Hatzfeld and Frogneux (1981). The sole focal mechanism present has a strike-slip character with a WNW±ESE pressure axis. (b) In the group of the E of Alboran (Fig. 4b), the depth distribution varies considerably. The NW± SE cross-section suggests a certain increase in depth from NW to SE with an almost vertical ending. We use two focal mechanisms, one has a fault normal character with a NW±SE pressure axis, and the other has a strike-slip character with the compression axis in the NE±SW direction. (c) In the group of the W Alboran Sea and S of Malaga provinces, there is a tendency to increase the depth towards the NW, with a dip of about 558 (Fig. 4c). The alignment N±S on the surface seems to be separated into three parts. The southern section corresponds to the deepest earthquakes. (Note the not existence of an aseismic zone at depths between 20 and 60 km.) The focal mechanisms in this area are varied in nature and although the pressure axes have different directions, most are situated in the NW±SE quadrants, ranging from N±S to E±W orientations. Two of them, located to the south, are NNE±SSW oriented. Using rightdihedra method we obtain at the north, a N±S tension with a vertical compression, at the centre, a NW± SE compression with a NE±SW tension, and at the south, a N±S compression with a WNW±ESE tension, acording to the increase of depth. (d) The group in the Gulf of Cadiz contains the earthquakes with the greatest depths. The distribution
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in depth seems to indicate an increase in such depth to the N (Fig. 4d). The focal mechanisms of this area have pressure axes preferably oriented NNW± SSE. Similar results are obtained with the rightdihedra method. (e) The alignment of the Gorringe Highs towards Morocco is characterized by their disperse spatial distribution (Fig. 4e).
2.3.1. 3-D view of distribution of the intermediate earthquakes in the Alboran Sea A 3-D view of the affected lithosphere by the intermediate earthquakes in the Alboran Sea (Fig. 5c) was obtained as follows: The area was divided into 10 £ 10 km cells, assigning the coordinates at the centre of the square to the deepest earthquake found in that cell. Then a smoothed contour and a 3-D surface map (Fig. 5a,b) were generated, using the inverse distance method. It can be inferred from these ®gures: (a) in the southern part of the Iberian Peninsula, the depth of the earthquakes increases from NE to SW, (b) in the Alboran Sea, the direction changes to N±S, (c) the increase in depth in the western Alboran Sea is very sharp, resembling a vertical wall, (d) the above alignment (b) is divided into two parts, the southern one being deeper, (e) the deep earthquake areas in N Morocco and the W Alboran Sea are separated from the previous ones and they have a clear NE±SW alignment. A similar view of the deep seismicity of Greece was carried out by Makropoulos and Burton (1984). 2.4. Deep earthquakes Up to the present, four deep earthquakes have been registered (Fig. 5a), all with depths between 620 and 670 km. They are quite closely located at the S of Granada city. The 1954 earthquake has been assigned a magnitude of 7.0 and the other ones 4.0, 4.8 and 3.9, respectively. One of the focal mechanisms of these deep earthquakes has a normal fault component with an E±W pressure axis. Two of these focal mechanisms present near vertical fault plane solutions, and the directions of the pressure axes have an approximately E±W to NW±SE horizontal component. Right-dihedra method did not give clear results.
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Fig. 4. SW±NE, NW±SE, W±E and N±S cross-sections of the hypocentral earthquakes corresponding to the groups: (a) High Atlas, (b) Dorsal of Alboran, (c) West Alboran, (d) Gorringe Ð Gulf of Cadiz and (e) Gorringe-High Atlas.
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Fig. 5. (a) Intermediate and deep earthquakes in the Alboran Sea, (b) smoothed contour map of the distribution of the intermediate earthquakes, (c) 3-D view.
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3. Geological evolution of the Alboran Sea and its environment The crustal thinning of the Alboran Sea is quite well documented (Cloetingh et al., 1992; Gallart et al., 1995; Galindo-ZaldõÂvar et al., 1998; etc.). The seismic pro®les show remarkable crustal thinning in the Alboran Sea, with only about 13 km thickness at some points (Fig. 6). Likewise, the gravimetric map of the Betic±Rif area shows positive anomalies in the sectors of the Alboran Sea with the more extensively thinned continental crust. In contrast, in the central sector of the Betic Cordillera and in that of the Rif, negative anomalies exist, coinciding with the maximum crustal thicknesses. Polyak et al. (1996) measured thermal ¯ow in the Alboran Sea, noticing a considerable decrease from E to W. The thickness of the sediments in its western part (Fig. 2) stands out, as they are more than 4000 m thick in the northern part and more than 8000 m thick in the southern part of an arc from W of Alhoceima to S of Malaga (De la Linde et al., 1996; Soto et al., 1996). These data are congruent with the interpretation that the Alboran area formed, thanks to a considerable extensional process in which the mantle was an active agent. Nevertheless, the possibility that the nappes of the Betic±Rif area formed only as a result of superposition due to the mantle swelling in the present Alboran area does not seem to have much basis. For instance, in the hypothesis of Weijermars (1985, 1987), the diapiric ascent of the Alboran mantle (from the Oligocene to the Burdigalian, 30±20 ma ago) produced the formation of the Betic±Rif Internal Zone nappes. According to this interpretation, the radial expansion of these nappes must have affected the Betic and Rif External zones almost immediately. However, there exists a remarkable diachrony between the most important deformations of the Internal and External zones. The Betic External zones were specially deformed from the middle Burdigalian (17±18 ma ago), while sediments from the late Aquitanian±early Burdigalian (about 22 ma) fossilized the contacts between the Malaguide and Alpujarride complexes of the Internal Zone. This indicates that the fundamental superposition of the Internal Zone nappes took place well before the deformation and structuring of the External Zone,
implying a considerable time lapse between these two events. In addition, the formation of the Betic and Rif tectonic nappes from the area now occupied by the Alboran Sea presents insoluble geometric problems, especially of space. If only the Betic Internal Zone is considered, its complexes repeated twice a tectonic overthursting of more metamorphic rocks overlying less metamorphic ones. In addition, the essentially unmetamorphized Malaguide units occur above (considering the complexes in general, without going into details within them, where more repetitions exist). This structuring is dif®cult to account for by a simple and only gravitational superposition of nappes. Moreover, if these different complexes as well as the units contained in them are extended, a much larger area than that occupied by the Alboran Sea is required. If we also consider the nappes of the Betic and Rif External zones, always according to the pattern of radial opening from the Alboran Sea, they alone would cover nearly the entire area of this sea and still more if the Flysch units were also considered. That is to say, the Betic Internal Zone alone would not ®t in the current space of the Alboran Sea and much less the Betic and Rif External zones and the Flysch units as well. In conclusion, the Betic Internal Zone could not have initially been in the area, now occupied by the Alboran Sea. In this sense, the directions of ductile deformation inside the Nevado-Filabride Complex rocks show a westward displacement of the whole Betic±Rif Internal Zone (Frizon de Lamotte et al., 1991). These structures are interpreted by GalindoZaldõÂvar et al. (1989) as a progressive westwards extension of the tectonically higher units. Other fragile deformations (GarcõÂa-DuenÄas et al., 1986) indicate movements towards the W or WSW too. These movements ocurred during the early and middle Miocene. In the Betic External Zone, the Subbetic was strongly deformed from the end of the early Burdigalian, but it was not equally disorganized in all sectors. It was rather progressively more deformed westwards, where it constitutes an immense olistostromic mass (Sanz de Galdeano and Vera, 1992; PeÂrez-LoÂpez and Sanz de Galdeano, 1994). This is congruent with a greater degree of tectonic transport of the
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Fig. 6. Schematic cross-section of the Betics, Alboran Sea and Rif showing the different crustal thicknesses in this area. (1) Neogene deposits in the Alboran Sea (with volcanic rocks) and in the foreland basins. (2) Betic±Rif Internal Zone, A±M: Alpujarride and Malaguide complexes, N-F: Nevado-Filabride complex. (3) Crust of the Betic and Rif External zones and their forelands. (4) Contact between the Betic Internal and External zones. The cross and the circle indicate west and eastward displacements, respectively. (5) Possible faults separating the Betic±Rif region from the forelands. Note the asymmetry of the Betic±Rif Internal Zone on both sides of the Alboran Sea. Inspired from Durand-Delga (1980).
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Subbetic in its westernmost sector, while in the eastern part it almost completely disappears, and the Internal Zone is very near the Prebetic. Existing data on block rotation in the Subbetic show clockwise rotation from about 30 to 608 (Allerton et al., 1993; Osete et al., 1989; Platzman and Lowrie, 1992; among others), in accordance with the displacements towards the W and N. Sanz de Galdeano (1996) shows that N±S folds and other structures in the central and eastern sectors of the Betic External Zone, situated in the contact or not far the contact with the Internal Zone, can be easily explained by the westward advance of this last zone. These data are congruent with the postulated westward advance of the Internal Zone from a more easterly area than that of the Alboran Sea. The displacement also has very important effects on the initial Flysch basin, which was entirely disorganized from the Burdigalian on and whose units have been considerably displaced to the W in the Gulf of Cadiz and Campo de Gibraltar and to the SSW in the Rif. In summary, the Betic±Rif Internal Zone moved westwards from an area approximately located to the south of Sardinia and the Balearic (Fig. 7). The collision with the Betic External Zone occurred starting from the lattermost early Burdigalian and then the tremendous disorganization of this area began. Coetaneous with the advance of the Betic±Rif Zone, the Algerian basin opened (of which the Alboran basin constitutes its westernmost end) undergoing expansion and crustal thinning. The Betic±Rif Internal Zone, containing the Alboran basin, is therefore a lithospheric segment that moved westwards, becoming emplaced between the Iberian and African plates. 4. The relationship of seismicity with geological structures The existence of four deep earthquakes with approximate epicentral locations S of Granada city leads to the assumption of a deeply subducted sheet located under the central and eastern areas of the Betic Cordillera and partly under the Alboran, between 200 and 700 km deep (Blanco and Spackman, 1993). We relate this sunken lithospheric body with the geologic and metamorphic evolution during the
Tertiary approximately till the Aquitanian of the microplate in which the Betic±Rif Internal Zone formed originally (described as Alkapeca Domain by Bouillin et al., 1986, or as South-Sardinian by Sanz de Galdeano, 1990). Immediately after, occurred the opening of the Algero-ProvencËal basin and the westward displacement of the Betic±Rif Internal Zone. In favour to its relationship with the Betic±Rif Internal Zone, we propose two arguments. (a) If the position occupied by this sunken sheet is compared (as done by Zeck, 1996, 1997) with the Betic±Rif Internal Zone, the coincidence is nearly absolute. Only in the northern part, the sheet underlies (partially) the Betic External Zone. This coincidence can hardly be accidental. (b) The probable original size of the deformed area affecting the Betic±Rif Internal Zone also coincides. The signi®cance of the intermediate earthquakes in the Betic±Rif region has been studied by Hatzfeld and Frogneux (1981) and Sanz de Galdeano and LoÂpez Casado (1990) and Buforn et al. (1991, 1995, 1997) albeit all the interpretations given present drawbacks. In the interpretation relating them to faults, the dif®culty lies in accepting the possibility of the continuation of faults down to 140 km and the rocks being fragile there. On the other hand, interpretations supporting only possible incipient subduction or a process of lithospheric delamination in the vicinities of Malaga and Granada have the disadvantage of not being able to explain many intermediate earthquakes outside that area. In the western part of the Alboran Sea, the position of the arc of intermediate earthquakes coincides exactly with the sector of maximum sediment thickness. The subsidence in that area is congruent with the approximately vertical stress reported for the earthquake focal mechanisms in the sector by Buforn et al. (1997). The convexity towards the W of the aforementioned arc seems to indicate that the Betic±Rif Internal Zone, being pushed westwards, progressively superimposed the southernmost part of the Iberian plate, which sank. A similar phenomenon may have occurred with a small sector of the African plate. Thus, Iberian and perhaps African lithospheric segments sank towards the E and are responsible for the seismic arc from Granada across to Malaga to the W Alboran and the N Rif.
C. LoÂpez Casado et al. / Tectonophysics 338 (2001) 79±95 Fig. 7. Geological evolution from the Eocene to the early Miocene in the W Mediterranean, especially the Betic and Rif Cordilleras and the Alboran Sea. The arrows indicate direction of displacement. White arrows show the growth of new oceanic crust. A: Alpujarride, M: Malaguide, N-F: Nevado-Filabride, Pb: Prebetic, Sb: Subbetic. Black and white triangles indicate active and inactive subductions, respectively, in the diagram of the Early Burdigalian; they also refer to tectonic windows. Taken from Sanz de Galdeano (1997). 91
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In accordance with the focal mechanisms presented by Buforn et al. (1997), there seems to be a certain WNW±ESE compression dipping slightly eastwards, in addition to a perpendicular one, almost vertical stress. Although this circumstance is inconclusive since the data are somewhat dispersed and there are only 11 focal mechanisms, it does support the existence in the W Alboran of a certain degree of sinking of a part of the lithosphere of Iberia and N of Africa. The intermediate seismicity of the Atlas, although locally separated from the shallow seismicity, is related to lithospheric fracturing, which was already taking place during the Triassic. Over time, these faults allowed the effusion of volcanic rocks, the most recent during the Quaternary. This seems to be the only commendable interpretation and consequently we deduce that this sector of NW Africa, almost all Morocco and part of the Atlantic are almost individualized as a subplate of the African plate. The intermediate seismicity in the Gulf of Cadiz is not speci®cally located in the contact between the Iberian and African plates. This plate boundary is quite clear in the sector between the Azores and the Gloria fault (Fig. 3a). Further E, in the sectors of Gorringe and Ampere, the interaction of faults striking predominantly NE±SW and NNE±SSW masks the contact. This sector coincides with the area where clear compression exists (see Olivet, 1996, Fig. 3). Therefore, from the sector of Gorringe to Algeria, the contact between the plates of Iberia±Europe and Africa becomes obscured. Interpretations for this sector are therefore diverse: Azevedo (1988) locate an incipient subduction in W Portugal that extends southwards in the Gorringe. UdõÂas and Buforn (1991), based on the earthquake mechanisms, believe in an initial subduction in which Africa begins to sink under Iberia. Royden (1993) proposes the possibility of a subduction that sank towards the E. In any case, this contact between the plates is very diffuse, so that, as indicated, its limit is not clear and it is necessary to assign part of the shallow and intermediate seismicity to the ENE±WSW faults extending from the Betic Cordillera and to the above-mentioned NE±SW and NW±SE faults. The seismicity of both the Atlas and of the Gulf of Cadiz therefore appear as different phenomena from those in the Alboran, although all these events need to be integrated in order to understand the overall
behaviour of the region. Fig. 8 presents the main aspects of the tectonic interpretation discussed here, summarily expressed in the conclusions. 5. Conclusions (a) During the Oligocene and the early Miocene, the progressive opening of the Algerian basin, with formation of new oceanic crust, caused the thinning of the continental crust of the Alboran Sea to the West. Meanwhile, in its front, the Betic±Rif Internal Zone (itself also affected by the extension) was coetaneously displaced westwards, causing in its advance the disorganization of the Betic and Rif External Zones during the middle Burdigalian. (b) The subducted sheet situated under the Betic± Rif Internal Zone probably corresponds to the original lithospheric bottom of this domain sunk during the Oligocene±early Miocene and also displaced westwards. The very deep earthquakes are related with this sheet, but there is no explanation for the lack of other earthquakes at depths between about 200 km and those at more than 600 km. Since only four earthquakes are known at more than 600 km depth, they are evidently scarce, while at the same time, the record is obviously incomplete as well. (c) The westward advance of the Betic±Rif Internal Zone caused its partial overthrusting on the Iberian plate and even on the NW African plate. Part of the Iberian plate (and perhaps of the African plate) sank and reached a depth of about 140 km. This would explain the intermediate seismicity in the western sector of the Alboran Sea, especially between Malaga and Granada and in the N of the Rif. Equally congruent with this interpretation is the signi®cant subsidence in the western part of the Alboran Sea, tracing the same arc as the intermediate seismicity. There seems to be a seismic gap at a depth of about 50 km in this arc, probably signifying the breakup of the small subducted sheet. (d) The large fractures marking the Atlas chain, with associated intermediate seismicity, extending towards the eastern sector of Alboran and towards the Canary Islands, are signi®cant enough to be able to distinguish a subplate inside the African plate. This subplate encompasses a large part of Morocco, extending from Agadir to the Canary
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Fig. 8. Tectonic interpretation of the Iberian±Moroccan area. (1) Faults. (2) Probable faults. (3) Present position of the former contact between the African plate and the South-Sardinian domain. (4) Present boundary of the Betic±Rif Internal Zone. (5) Betic±Rif Internal Zone. (6) Approximate contact between the Iberian and African plates. (7) Present possible prolongation to the east of the contact between the African and Iberian plates. (8) Position of the arc produced by the sinking of the Iberian and African lithospheres in the W Alboran Sea. (9) Arrows indicating direction of crustal sinking in the W Alboran Sea. (10) General direction of displacement of the Betic±Rif Internal Zone. (11) Position, according to Blanco and Spackman (1993) of the sunken lithospheric sheet in the Betic±Rif area. (12) Approximate limit of the new oceanic crust in the Algero-ProvencËal basin. (13) New oceanic crust in the Algero-ProvencËal basin.
Islands. From there, it continues towards the Middle Atlantic Ridge. The intermediate earthquakes between Gibraltar and the Gorringe sector correspond also to a situation unrelated to the sinking processes of lithospheric sheets, where only very incipient sinking is suspected. The contact between the African and the European plates is
barely outlined there because it is strongly affected by faults cross-cutting it and producing earthquakes. Acknowledgements We are grateful to anonymous reviewers whose
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comments considerably improved the manuscript. This work has been ®nanced by projects PB97-1267C03-01, PB96-0327 (DGICYT), AMB97-0975-C0201 (CICYT) and group no. RNM 0217 of the Junta de AndalucõÂa. References Allerton, S., Lonergan, L., Platt, J.P., Platzmann, E.S., McClelland, E., 1993. Palaeomagnetic rotation in the eastern Betic Cordillera, southern Spain. Earth Planet. Sci. Lett. 119, 225± 241. Andrieux, J., FontboteÂ, J.M., Mattauer, M., 1971. Sur un modeÁle explicatif de l'Arc de Gibraltar. Earth Planet. Sci. Lett. 12, 191± 198. Angelier, J., Mechler, P., 1977. Sur une meÂthode graphique de recherche des contraintes principales eÂgelement utilisable en tectonique et en seÂismologie: la meÂthode des dieÁdres droits. Bull. Soc. GeÂol. Fr. (7) 19 (6), 1309±1318. Azevedo, P., 1988. Portugal Alto risco Sismico. Futuro, CieÃnciasNovas Tecnologias-InovacËao, Lisboa. Ano II-No. 17-Juhno, 13±18. Biju-Duval, B., Dercourt, J., Le Pichon, X., 1976. La GeneÁse de la MeÂditerraneÂe. La Recherche 71 (7), 811±822. Biju-Duval, B., Dercourt, J., Le Pichon, X., 1977. From the Tethys to the Mediterranean seas: a plate tectonic model of the evolution of the Western Alpine System. In: Biju-Duval, B., Montadert, L. (Eds.). Structural History of the Mediterranean Basin. Technip, Paris, pp. 143±164. Blanco, M.J., Spackman, W., 1993. The P-velocity structure of the mantle below the Iberian Peninsula: evidence for subducted lithosphere below southern Spain. Tectonophysics 221, 13±34. Bouillin, J., Durand Delga, M., Olivier, P., 1986. Betic±Rif and Tyrrhenian distinctive features, genesis and development stages. In: Wezel, F.C. (Ed.). The Origin of Arcs. Elsevier, Amsterdam, pp. 281±304. Buforn, E., UdõÂas, A., Madariaga, R., 1991. Intermediate and deep earthquakes in Spain. Pure Appl. Geophys. 136 (4), 375±393. Buforn, E., Sanz de Galdeano, C., UdõÂas, A., 1995. Seismotectonics of the Ibero-Maghrebian Region. Tectonophysics 248, 247± 261. Buforn, E., Coca, P., UdõÂas, A., Lasa, C., 1997. Source mechanims of intermediate and deep earthquakes in South Spain. J. Seismol. 1, 113±130. Cloetingh, S., Van der Beek, P.A., Van Rees, D., Roep, Th.B., Biermann, C., Stephenson, R.A., 1992. Flexural interaction and the dynamics of Neogene extensional Basin Formation in the Alboran-Betic Region. Geo. Mar. Lett. 12 (2/3), 66±75. De la Linde, J., Comas, M.C., Soto, J.I., 1996. MorfologõÂa del Basamento en el Noroeste del mar de AlboraÂn. Geogaceta 20 (2), 355±358. Doblas, M., Oyarzun, R., 1989a. Neogene extensional collapse in the western Mediterranean (Betic±Rif Alpine orogenic belt): implications for the genesis of the Gibraltar Arc and magmatic activity. Geology 17, 430±433.
Doblas, M., Oyarzun, R., 1989b. Mantle core complexes and Neogene extensional detachment tectonics in the western Betic Cordilleras, Spain: an alternative model for the emplacement of the Ronda peridotite. Earth Planet. Sci. Lett. 93, 76±84. Docherty, C., Banda, E., 1995. Evidence for the eastward migration of the Alboran Sea based on regional subsidence analysis: a case for basin formation by delamination of the subcrustal lithosphere?. Tectonics 14 (4), 804±818. Durand-Delga, M., 1980. La MeÂditerraneÂe occidentale: eÂtape de sa geneÁse et probleÁmes structuraux lieÂs aÁ celle-ci. Livre Jubilaire Soc. GeÂol. Fr. 1830±1980 10, 203±224 Mem. h. seÂr. S.G.F.. Frizon de Lamotte, D., Andrieux, J., GueÂzou, J.C., 1991. CineÂmatique des chevauchements neÂogeÁnes dans l'Arc BeÂticoRifain: discussion sur les modeÁles geÂodynamiques. Bull. Soc. GeÂol. Fr. 162 (4), 611±626. Galindo-ZaldõÂvar, J., GonzaÂlez-Lodeiro, F., Jabaloy, A., 1989. Progressive extensional shear structures in a detachment contact in the Western Sierra Nevada (Betic Cordilleras, Spain). Geodinamica Acta 3, 73±85. Galindo-ZaldõÂvar, J., GonzaÂlez-Lodeiro, F., Jabaloy, A., Maldonado, A., Schreider, A.A., 1998. Models of magnetic and Bouguer gravity anomalies for the deep structure of the central Alboran Sea basin. Geo. Mar. Lett. 18, 10±18. Gallart, J., DõÂaz, J., Vidal, N., DanÄobeitia, J.J., 1995. The base of the crust at the Betic-AlboraÂn Sea transition: evidence for an abrupt structural variation from wide-angle ESCI data. Rev. Soc. Geol. EspanÄa 8 (4), 519±527. GarcõÂa-DuenÄas, V., MartõÂnez MartõÂnez, J.M., Navarro VilaÂ, F., 1986. La zona de falla de Torres Cartas, conjunto de fallas normales de bajo aÂngulo entre Nevado-FilaÂbrides y AlpujaÂrrides (Sierra Alhamilla, BeÂticas Orientales). Geogaceta 1, 17±19. GarcõÂa-DuenÄas, V., BalanyaÂ, J.C., MartõÂnez MartõÂnez, J.M., 1992. Miocene extensional detachments in the outcropping basement of the northern Alboran basin (Betics) and their implications. Geo. Mar. Lett. 12 (2/3), 88±95. Hatzfeld, D., Frogneux, M., 1981. Intermediate depth seismicity in the western Mediterranean unrelated to subduction of oceanic lithosphere. Nature 292, 443±445. Henares, J., Lopez Casado, C., Delgado, J., 2000. Catalog of focal mechanisms of the Iberian±Moghrebian region. 2 a Asamblea Hispano Portuguesa de Geodesia y Geo®sica, 8±12 Febrero, 2000. Lagos, Portugal. Houseman, G., 1996. From Mountains to basin. Nature 379, 771± 772. Lonergan, L., White, N., 1997. Origin of the Betic±Rif mountain belt. Tectonics 16 (3), 504±522. Makropoulos, K., Burton, P.W., 1984. Greek tectonics and seismicity. Tectonophysics 106, 275±304. Mezcua, J., Rueda, J., 1997. Seismological evidence for a delamination process in the lithosphere under the Alboran Sea. Geophys. J. Int. 129, 1±8. Morales, J., Serrano, I., Vidal, F., Torcal, F., 1997. The depth of the earthquake activity in the Central Betic (Southern Spain). Geophys. Res. Lett. 24, 3289±3292. Morales, J., Serrano, I., Jabaloy, A., Galindo-ZaldõÂvar, J., Zhao, D., Torcal, F., Vidal, F., GonzaÂlez-Lodeiro, F., 1999. Active
C. LoÂpez Casado et al. / Tectonophysics 338 (2001) 79±95 continental subduction beneath the Betic Cordillera and the AlboraÂn Sea. Geology 27, 735±738. Morley, C.K., 1993. Discussion of origins of hinterland basins to the Rif±Betic Cordillera and Carpathians. Tectonophysics 226, 359±376. Olivet, J.L., 1996. La cineÂmatique de la plaque IbeÂrique. Elf Aquitaine-IFREMER 18, 131±195. Osete, M.L., Freeman, R., Vegas, R., 1989. Palaeomagnetic evidence for block rotations and distributed deformation of the Iberian±African Plate boudary. In: Kissel, C., Laj, C. (Eds.). Paleomagnetic Rotations and Continental Deformation. Kluwer, pp. 381±391. Platt, J.P., Vissers, R.L.M., 1989. Extensional collapse of thickened continental lithosphere: a working hypothesis for the Alboran Sea and Gibraltar arc. Geology 17, 540±543. Platzman, E., Lowrie, W., 1992. Paleomagnetic evidence for rotation of the Iberian Peninsula and the external Betic Cordillera, Southern Spain. Earth Planet. Sci. Lett. 108, 45±60. Polyak, B.G., FernaÂndez, M., Khutorskoy, M.D., Soto, J.I., Basov, I.A., Comas, M.C., Khain, V.Y., Alonso, B., Agapova, G.V., Mazurova, I.S., Negredo, A., Tochitsky, V.O., de la Linde, J., Bogdanov, N.A., Banda, E., 1996. Heat ¯ow in the Alboran Sea, western Mediterranean. Tectonophysics 263, 191±218. PeÂrez-LoÂpez, A., Sanz de Galdeano, C., 1994. TectoÂnica de los materiales triaÂsicos en el sector central de la Zona SubbeÂtica (Cordillera BeÂtica). Rev. Soc. Geol. EspanÄa 7 (1±2), 141±153. Rehault, J.P., Boillot, G., Mauffret, A., 1984. The Western Mediterranean Basin geological evolution. Mar. Geol. 55, 447477. Royden, L.H., 1993. Evolution of retreating subduction boundaries formed during continental collision. Tectonics 12 (3), 629±638. Sanz de Galdeano, C., 1990. Geologic evolution of the Betic Cordilleras in the Western Mediterranean, Miocene to the present. Tectonophysics 172, 107±119. Sanz de Galdeano, C., 1996. The E±W segments of the contact between the external and internal zones of the Betic and Rif Cordilleras and the E±W corridors of the internal zone (a combined explanation). Estudios GeoloÂgicos 52, 123±136. Sanz de Galdeano, C., 1997. La Zona Interna BeÂtico±RifenÄa (Antecedentes, unidades tectoÂnicas, correlaciones y bosquejo de reconstruccioÂn paleogeogra®ca). Monogra®ca Tierras del Sur. Univ. de Granada, p. 316.
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Sanz de Galdeano, C., LoÂpez Casado, C., 1990. Earthquakes with a focal depth of between 40±180 km and Tectonics in the South of Spain and Northwest Africa. In: Oliveira, C.S. (Ed.), ECE/ UN Seminar on Prediction of Earthquakes (1988). Lisbon, vol. 2, pp. 885±895. Sanz de Galdeano, C., Vera, J.A., 1992. Stratigraphic record and palaeogeographical context of the Neogene basins in the Betic Cordillera, Spain. Basin Res. 4, 21±36. Seber, D., Barazangi, M., Ibenbrahim, A., Demnati, A., 1996a. Geophysical evidence for lithospheric delamination beneath the Alboran Sea and Rif±Betic mountains. Nature 379, 785± 790. Seber, D., Barazangi, M., Tadili, B.A., Ramdani, M., Ben Sari, D., 1996b. Three-dimensional upper mantle structure beneath the intraplate Atlas and interplate Rif mountains of Morocco. J. Geophys. Res. 101 (87), 3125±3138. Soto, J.I., Comas, M.C., de la Linde, J., 1996. Espesor de sedimentos en la cuenca de AlboraÂn mediante una conversioÂn sõÂsmica corregida. Geogaceta 20 (2), 382±385. UdõÂas, A., Buforn, E., 1991. Regional stresses along the Eurasia± Africa plate boundary derived from focal mechamism of large earthquakes. Pure Appl. Geophys. 136 (4), 433±448. Vissers, R.L.M., Platt, J.P., Van der Wal, D., 1995. Late orogenic extension of the Betic Cordillera and the Alboran Domain: a lithospheric view. Tectonics 14 (4), 786±803. Watts, A.B., Platt, J.P., Buhl, P., 1993. Tectonic evolution of the Alboran Sea basin. Basin Res. 5, 153±177. Weijermars, R., 1985. Uplift and subsidence history of the Alboran Basin and a pro®le of the Alboran Diapir (Western Mediterranean). Geol. Mijnbouw 64, 349±356. Weijemars, R., 1987. The Palomares brittle ductile shear zone of Southern Spain. J. Struct. Geol. 9, 139±157. Zeck, H.P., 1996. Betic-Rif orogeny: subduction of Mesozoic Tethys lithosphere under eastward drifting Iberia, slab detachment shortly before 22 Ma, and subsequent uplift and extensional tectonics. Tectonophysics, 254, 1±16. Zeck, H.P., 1997. Mantle peridotites outlining the Gibraltar Arc Ð centrifugal extensional allochthons derived from the earlier Alpine, wesward subducte nappe pile. Tectonophysics 281, 195±207.
www.elsevier.com/locate/tecto
The structure of the Alboran Sea: an interpretation from seismological and geological data C. LoÂpez Casado a,*, C. Sanz de Galdeano b, S. Molina Palacios c, J. Henares Romero a b
a Departamento de FõÂsica TeoÂrica y del Cosmos, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, CSIC-Universidad de Granada, 18071 Granada, Spain c Departamento de Ciencias de la Tierra y del Medio Ambiente, Facultad de Ciencias, 03690 Alicante, Spain
Received 17 May 2000; accepted 22 February 2001
Abstract Almost all the earthquakes included in the catalog for the Iberian±Maghrebian area up to 2000 have been used in order to know the structure of the Alboran Sea area after the veri®cation of the aleatory nature of the errors in their spatial localization. A new focal mechanisms catalog has also been used as well as many of the available geological data. During the Mesozoic and till the Oligocene, the Betic±Rif Internal Zone was situated further E, but with the opening of the Algero-ProvencËal basin in the early Miocene, the Betic±Rif Internal Zone moved to the W. Contemporary, the Alboran Sea was created as the western prolongation of the Algero-ProvencËal basin. The Betic±Rif Internal Zone overthrused part of the Iberian and African plates, producing the partial sinking of both plates and being responsible of the intermediate seismicity existing in the western sector of the Alboran Sea. The intermediate earthquakes in the Atlas towards the NE and WSW are not related to lithospheric sinking but to signi®cant deep faults limiting a subplate in NW Africa. In the Atlantic, the intermediate earthquakes between the Gorringe sector and Gibraltar are produced in the contact between the Iberian and African plates and by the important faults crossing it. The existence of four very deep earthquakes is related to the previous sinking of the lithosphere originally associated to the domain in which the Betic±Rif Internal Zone was situated. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Alboran Sea; Betic Cordillera; Seismicity; Intermediate earthquakes; Subduction; Deep faults
1. Introduction The Alboran Sea, located between the Betic Cordillera and the Rif (N of Morocco) has a continental crust thinned during the early and middle Miocene and its geological setting is complex. To the N, the Betic Cordillera (Figs. 1 and 2) is divided into Internal and External zones with the Guadalquivir foreland * Corresponding author. Fax: 134-58-274-258. E-mail addresses: [email protected] (C. LoÂpez Casado), [email protected] (C. Sanz de Galdeano), [email protected] (S. Molina Palacios).
basin separating it from the Iberian Massif. The External Zone, divided into the Prebetic and Subbetic formed originally the south and southeast Mesozoic and Tertiary sedimentary cover of the Iberian shield and is arranged in many tectonic units. The Internal Zone basically consists of four tectonically superimposed complexes that, from bottom to top, are the Nevado-Filabride, the Alpujarride, the Malaguide and the Dorsal Complex, the last partially associated to the Malaguide. The ®rst two complexes present a welldeveloped Alpine metamorphism. The units of the Campo de Gibraltar came from the former Flysch basin, originally located to the S of the Internal Zone.
0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0040-195 1(01)00059-2
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Fig. 1. Main geological domains in the Iberian±Maghrebian region (from Buforn et al. (1995)).
To the S of Alboran, the Rif is also divided into Internal and External zones. The Internal Zone is common with the Betic Internal Zone, but without the presence of the Nevado-Filabride complex. For this reason it can be cited as the Betic±Rif Internal Zone. The External Zone, formed by Mesozoic and Tertiary sedimentary series, is different to the Betic External Zone. Further S, the Gharb foreland basin separates the Rif from the Moroccan Meseta. This last is in large part equivalent to the Iberian massif. South and east of the Meseta is the Atlas, a cordillera which has developed onto a band of old crustal weakness since the end of the Palaeozoic. In the Atlantic, the boundary of the Iberian and African plates extending from the Azores to the Gulf of Cadiz is relatively well de®ned in its westernmost part up to the sector between the Gloria and
Gorringe faults. Eastwards, this contact is strongly affected by transversal faults. Antecedents. The Neogene structuring of the Betic±Rif area and of the Alboran Sea has been the object of numerous hypotheses. Some explain the present position of the Betic±Rif Internal Zone due to its signi®cant westward displacement (Andrieux et al., 1971; Biju-Duval et al., 1976, 1977; DurandDelga, 1980; Rehault et al., 1984) linked to the opening of the Algero-ProvencËal basin (Sanz de Galdeano, 1990). Lonergan and White (1997) support the interpretation of a westward displacement of the Betic±Rif Internal Zone, related to E-dipping subduction. A subduction of Africa under Iberia, with the Alboran Sea being a back-arc basin was proposed by Morley (1993) while Royden (1993) locates a subduction dipping E ahead of the Gibraltar arc in the Atlantic, in the contact zone between the Iberian
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Fig. 2. General geological map of the Betic Cordillera, Rif and Alboran Sea. Modi®ed from Sanz de Galdeano (1997), Polyak et al. (1996) and Soto et al. (1996). Note how the area within the 4000 m isobath in the W Alboran coincides with the position of the arc of intermediate seismicity (Fig. 5a).
and African plates. Zeck (1996, 1997) assumed the existence of a broken subducted sheet producing extensional tectonics on the surface. Morales et al. (1999) proposed the existence of an active continental subduction of the Iberian plate beneath the Betic Cordillera and the Alboran Sea. Other hypotheses consider the formation of a large dome in the Alboran Sea and later extension and thinning of the crust linked to convection processes in the mantle (Weijermars, 1985, 1987; Doblas and Oyarzun, 1989a,b; Platt and Vissers, 1989). Vissers et al. (1995) suggested that the lithospheric thinning in the Alboran Sea took place through a convection process. Houseman (1996), without discarding other models,
sustains that the convection model provides the best explanation. The possible action of a process of lithospheric delamination in the Alboran Sea was indicated by GarcõÂa-DuenÄas et al. (1992), Watts et al. (1993) and Docherty and Banda (1995). Seber et al. (1996a,b) reported the existence of a lithospheric body rooted 350 km deep under part of N Morocco, the Alboran Sea and the Betic Cordillera. Possibly its sinking was due to a delamination process. Mezcua and Rueda (1997) relocated the recorded earthquakes from 1950 to 1995 in the Alboran Sea, Gulf of Cadiz, Betic Cordillera and Rif, concluding that a delamination process would better explain the intermediate
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and very deep seismicity of the region. Buforn et al. (1997) suggested the existence of a vertical sheet located between Malaga and Granada and the W Alboran Sea, extending from around 50 km to about 150 km depth which probably formed in a process of lithospheric delamination. Morales et al. (1997) discussed the origin of the intermediate earthquakes in the W Alboran Sea and proposed the existence of a delamination process which spread westwards. In this article, we provide an interpretation of the current structure of the Alboran area, especially from seismological data, but taking also into account the geological evolution of the western Mediterranean since at least the Oligocene times, taking into account the evolution of the Betic and Rif cordilleras at the same time. This is necessary because the Alboran Sea developed with the Algero-ProvencËal basin (the Alboran Sea is the western end of this basin). In fact, the basement of the Alboran Sea is formed by rocks of the Betic±Rif Internal Zone, covered by Miocene sediments and volcanic rocks. 2. Seismicity in the Iberian±Maghrebian area The seismicity in the western border of the African and European plates is characterized by two main features. First, it is not linearly distributed extending rather across a wide area embracing to the north a sector of the Atlantic Ocean and the Iberian Peninsula and to the south, almost all Morocco, north of Algeria and Tunisia. Second, there are intermediate earthquakes (30 , h , 180 km) under the south of the Iberian peninsula, the W and E of the Alboran Sea and the centre and the north of Morocco. Although much less frequent, intermediate earthquakes have also taken place in other regions of Iberia, such as the Pyrenees and the southeastern coast. We should mention also the existence of deep earthquakes (h . 600 km) located approximately south of Granada. 2.1. Quality analysis of the seismological information The data for the earthquakes have been taken from the IGN (Instituto Geogra®co Nacional) database up to 1999 which has been subjected to a recent relocation of the hypocentral coordinates. The hypocentral locations in the catalog are subjected, however, to errors that nonetheless, as shown below, do not affect
signi®cantly our overall descriptions of Iberian± Maghrebian seismicity. Several tests have been used to determine the accuracy of the hypocentral locations. We ®rst analysed the standard deviation in depth with time and space distribution. We divided the earthquakes into shallow (h , 30 km) and intermediate (h $ 30 km). It was observed that: (a) there is a drastic decrease on the errors after 1981 from several dozens of kilometres to no more than 15 km, (b) the standard deviation of the intermediate earthquakes is smaller than for the shallow ones, and (c) in the period 1982±2000 only a small nucleus, SW of San Vicente cape, reaches a 20 km error. Next, we compared the hypocentral locations of several earthquake databases in our study area. Thus, in addition to the IGN date base, we used a version of the original ®le without the latest relocations and the USGS database from 1973 to 1994. In both comparisons (IGN versus original ®le and IGNUSGS) discrepancies in latitude, longitude and depth are distributed around zero value and with a standard deviation below 10 km. Finally, we compared the spatial distributions of earthquake locations according to the IGN database for the periods 1400±1930, 1931±1981 and 1982± 1999 for shallow earthquakes and for the periods 1950±1980 and 1981±1999 for the intermediate ones. For both cases, locations from the different periods did not differ too much. Moreover, in the case of intermediate earthquakes, it was observed that their locations in the period with the largest errors (1952±1981) were about the same than those found in the period with the smallest errors (1982±1999). We therefore concluded that due to the fundamentally random nature of the location errors, grouped hypocentral locations do not constitute any mislocation problem when used to assign a seismicity model to a region. The focal mechanisms were selected from a new catalog of the Iberian±Maghrebian region (Henares et al., 2000). From 481 homogeneus focal mechanisms compiled in this catalog, we selected only 114, 61 with mb $ 5.0, 49 with 30 # h # 200 km and 3 with h $ 600 km. They are those we considered to be related directly with the area analysed in this study. These earthquakes are considered either as single events or as a set of events using the right-dihedra method (Angelier and Mechler, 1977).
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2.2. Shallow earthquakes Fig. 3a plot the epicentral distribution of the shallow earthquakes in this area (mb $ 0) for the period 1910±1999. Fig. 3b plot the epicentral distribution of the shallow earthquakes in this area (mb $ 5) and b-values of the Gutemberg±Ricther law for the period 1910±1999. Although at ®rst sight it seems to be randomly distributed, a detailed analysis reveals certain alignments. Thus, from west to east, according to the main seismic alignments, the value of parameter b, the activity rate and the larger earthquakes, the main characteristics can be described as follows: (a) There is a clear WNW±ESE seismic alignment from 188W to 108W, located at the boundary of the Iberian and African plates, which may continue to the Gulf of Cadiz and also turn towards Morocco. The maximum registered magnitude is 8.2. This area is characterized by a release of its seismic energy through large-magnitude characteristic earthquakes. Most of the focal mechanisms in this area are strike-slip or have a strike-slip component. The main direction of the fault-plane solutions of these mechanisms is WNW±ESE. The most common orientation of the pressure axes is NW± SE. Using the right-dihedra method, we ®nd a similar orientation of the state of stress. (b) Starting from 108W, the above alignment changes from E±W to ENE±WSW up to 38W, crossing the Gulf of Cadiz and part of the Betic Cordillera. The area of change, located around 6830 0 W, is perfectly de®ned by the lowest seismic activity. Although the seismic activity increases considerably in relation to case (a), the size of the earthquakes falls substantially, with the highest registered magnitude 5.2. Focal mechanisms in this area are variable, consisting of strike-slip to reverse and normal faults. The fault-plane solutions and the pressure axis have very different directions. The right-dihedra method does not give either a clear solution. (c) From 38W, at the end part of the above alignment, there is a NNE±SSW alignment, going from 388N to 308N. That is, it follows the SE coast of Spain, crosses the E Alboran Sea and continues down along the Atlas Mountain Chain. Seismic
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activity is similar to that of the above alignment, with two earthquakes of magnitude 5.7 in the central part of the alignment (in the southern Alboran Sea). Areas of very low seismic activity are detected. As in the above area, the focal mechanisms of this sector are quite variable, ranging from strike-slip to reverse and normal faults. The fault-plane solutions have different orientations, although the most abundant component of the pressure axes is N±S. The right-dihedra method, considering only earthquakes from the northern part of Morocco, gives aproximately an E±W orientation for the compression axis. (d) Last, starting from 1830 0 W, after a clear seismic gap, a new E±W to ENE±WSW seismic alignment is observed, reaching 108E and crossing the northern coasts of Algeria and Tunisia. Seismic activity is lower than in the last two alignments. However, the size of the earthquakes is much greater, although falling short of the values in the ®rst alignment. In the central area, there are two earthquakes of magnitude 6.7. In the western zone, clearly separated from the central sector, an earthquake of 5.7 magnitude have been registered. In the eastern area, with the lowest seismic activity, two earthquakes have been registered, reaching magnitude 5.6. Many of the focal mechanisms in this area have a reverse fault character, although strike-slip solutions are also abundant, while normal fault solutions are less signi®cant. Most of the fault-plane solutions are ENE±WSW and the pressure axes are predominantly NW±SE oriented. Similar results are obtained with the right-dihedra method.
2.3. Intermediate earthquakes The above alignments are also observed in the distribution of the intermediate earthquakes (Fig. 3c). The maximum depth for the intermediate seismicity is in the Gulf of Cadiz, where nine earthquakes exceed 140 km, the deepest reaching 180 km. The following groupings or alignements can now be de®ned based on the locations of their epicentral coordinates: (a) the High Atlas, (b) the E Alboran and the prolongation towards SE Spain and the Middle Atlas, (c) the W Alboran Sea and the southern part
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Fig. 3. General distribution of seismicity in the study area for the period 1910±1999: (a) mb . 0; (b) mb . 5 and b-value; (c) intermediate and deep earthquakes for the period 1952±1999, showing the group used in this work.
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of Malaga province, (d) the Gorringe Ð Gulf of Cadiz and (e) the Gorringe Highs Ð coast of Morocco. The projection of the hypocentres of these earthquakes in vertical planes of N±S, E±W, NW±SE and SE±NW direction (Fig. 4) suggests the impossibility of associating all this seismicity with a single subduction zone. Seismic depths seem to be dispersed, mainly in the western sectors, with substantial seismic gaps. However, in the area of Malaga and in the W Alboran Sea, there is a progressive increase in depth towards the south. The characteristics of these groups are: (a) The Atlas group (Fig. 4a) can be divided into four subgroups, the second from the SW position, being the denser and comprising the deeper earthquakes. The accuracy of the depth of these earthquakes was evaluated by Hatzfeld and Frogneux (1981). The sole focal mechanism present has a strike-slip character with a WNW±ESE pressure axis. (b) In the group of the E of Alboran (Fig. 4b), the depth distribution varies considerably. The NW± SE cross-section suggests a certain increase in depth from NW to SE with an almost vertical ending. We use two focal mechanisms, one has a fault normal character with a NW±SE pressure axis, and the other has a strike-slip character with the compression axis in the NE±SW direction. (c) In the group of the W Alboran Sea and S of Malaga provinces, there is a tendency to increase the depth towards the NW, with a dip of about 558 (Fig. 4c). The alignment N±S on the surface seems to be separated into three parts. The southern section corresponds to the deepest earthquakes. (Note the not existence of an aseismic zone at depths between 20 and 60 km.) The focal mechanisms in this area are varied in nature and although the pressure axes have different directions, most are situated in the NW±SE quadrants, ranging from N±S to E±W orientations. Two of them, located to the south, are NNE±SSW oriented. Using rightdihedra method we obtain at the north, a N±S tension with a vertical compression, at the centre, a NW± SE compression with a NE±SW tension, and at the south, a N±S compression with a WNW±ESE tension, acording to the increase of depth. (d) The group in the Gulf of Cadiz contains the earthquakes with the greatest depths. The distribution
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in depth seems to indicate an increase in such depth to the N (Fig. 4d). The focal mechanisms of this area have pressure axes preferably oriented NNW± SSE. Similar results are obtained with the rightdihedra method. (e) The alignment of the Gorringe Highs towards Morocco is characterized by their disperse spatial distribution (Fig. 4e).
2.3.1. 3-D view of distribution of the intermediate earthquakes in the Alboran Sea A 3-D view of the affected lithosphere by the intermediate earthquakes in the Alboran Sea (Fig. 5c) was obtained as follows: The area was divided into 10 £ 10 km cells, assigning the coordinates at the centre of the square to the deepest earthquake found in that cell. Then a smoothed contour and a 3-D surface map (Fig. 5a,b) were generated, using the inverse distance method. It can be inferred from these ®gures: (a) in the southern part of the Iberian Peninsula, the depth of the earthquakes increases from NE to SW, (b) in the Alboran Sea, the direction changes to N±S, (c) the increase in depth in the western Alboran Sea is very sharp, resembling a vertical wall, (d) the above alignment (b) is divided into two parts, the southern one being deeper, (e) the deep earthquake areas in N Morocco and the W Alboran Sea are separated from the previous ones and they have a clear NE±SW alignment. A similar view of the deep seismicity of Greece was carried out by Makropoulos and Burton (1984). 2.4. Deep earthquakes Up to the present, four deep earthquakes have been registered (Fig. 5a), all with depths between 620 and 670 km. They are quite closely located at the S of Granada city. The 1954 earthquake has been assigned a magnitude of 7.0 and the other ones 4.0, 4.8 and 3.9, respectively. One of the focal mechanisms of these deep earthquakes has a normal fault component with an E±W pressure axis. Two of these focal mechanisms present near vertical fault plane solutions, and the directions of the pressure axes have an approximately E±W to NW±SE horizontal component. Right-dihedra method did not give clear results.
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Fig. 4. SW±NE, NW±SE, W±E and N±S cross-sections of the hypocentral earthquakes corresponding to the groups: (a) High Atlas, (b) Dorsal of Alboran, (c) West Alboran, (d) Gorringe Ð Gulf of Cadiz and (e) Gorringe-High Atlas.
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Fig. 5. (a) Intermediate and deep earthquakes in the Alboran Sea, (b) smoothed contour map of the distribution of the intermediate earthquakes, (c) 3-D view.
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3. Geological evolution of the Alboran Sea and its environment The crustal thinning of the Alboran Sea is quite well documented (Cloetingh et al., 1992; Gallart et al., 1995; Galindo-ZaldõÂvar et al., 1998; etc.). The seismic pro®les show remarkable crustal thinning in the Alboran Sea, with only about 13 km thickness at some points (Fig. 6). Likewise, the gravimetric map of the Betic±Rif area shows positive anomalies in the sectors of the Alboran Sea with the more extensively thinned continental crust. In contrast, in the central sector of the Betic Cordillera and in that of the Rif, negative anomalies exist, coinciding with the maximum crustal thicknesses. Polyak et al. (1996) measured thermal ¯ow in the Alboran Sea, noticing a considerable decrease from E to W. The thickness of the sediments in its western part (Fig. 2) stands out, as they are more than 4000 m thick in the northern part and more than 8000 m thick in the southern part of an arc from W of Alhoceima to S of Malaga (De la Linde et al., 1996; Soto et al., 1996). These data are congruent with the interpretation that the Alboran area formed, thanks to a considerable extensional process in which the mantle was an active agent. Nevertheless, the possibility that the nappes of the Betic±Rif area formed only as a result of superposition due to the mantle swelling in the present Alboran area does not seem to have much basis. For instance, in the hypothesis of Weijermars (1985, 1987), the diapiric ascent of the Alboran mantle (from the Oligocene to the Burdigalian, 30±20 ma ago) produced the formation of the Betic±Rif Internal Zone nappes. According to this interpretation, the radial expansion of these nappes must have affected the Betic and Rif External zones almost immediately. However, there exists a remarkable diachrony between the most important deformations of the Internal and External zones. The Betic External zones were specially deformed from the middle Burdigalian (17±18 ma ago), while sediments from the late Aquitanian±early Burdigalian (about 22 ma) fossilized the contacts between the Malaguide and Alpujarride complexes of the Internal Zone. This indicates that the fundamental superposition of the Internal Zone nappes took place well before the deformation and structuring of the External Zone,
implying a considerable time lapse between these two events. In addition, the formation of the Betic and Rif tectonic nappes from the area now occupied by the Alboran Sea presents insoluble geometric problems, especially of space. If only the Betic Internal Zone is considered, its complexes repeated twice a tectonic overthursting of more metamorphic rocks overlying less metamorphic ones. In addition, the essentially unmetamorphized Malaguide units occur above (considering the complexes in general, without going into details within them, where more repetitions exist). This structuring is dif®cult to account for by a simple and only gravitational superposition of nappes. Moreover, if these different complexes as well as the units contained in them are extended, a much larger area than that occupied by the Alboran Sea is required. If we also consider the nappes of the Betic and Rif External zones, always according to the pattern of radial opening from the Alboran Sea, they alone would cover nearly the entire area of this sea and still more if the Flysch units were also considered. That is to say, the Betic Internal Zone alone would not ®t in the current space of the Alboran Sea and much less the Betic and Rif External zones and the Flysch units as well. In conclusion, the Betic Internal Zone could not have initially been in the area, now occupied by the Alboran Sea. In this sense, the directions of ductile deformation inside the Nevado-Filabride Complex rocks show a westward displacement of the whole Betic±Rif Internal Zone (Frizon de Lamotte et al., 1991). These structures are interpreted by GalindoZaldõÂvar et al. (1989) as a progressive westwards extension of the tectonically higher units. Other fragile deformations (GarcõÂa-DuenÄas et al., 1986) indicate movements towards the W or WSW too. These movements ocurred during the early and middle Miocene. In the Betic External Zone, the Subbetic was strongly deformed from the end of the early Burdigalian, but it was not equally disorganized in all sectors. It was rather progressively more deformed westwards, where it constitutes an immense olistostromic mass (Sanz de Galdeano and Vera, 1992; PeÂrez-LoÂpez and Sanz de Galdeano, 1994). This is congruent with a greater degree of tectonic transport of the
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Fig. 6. Schematic cross-section of the Betics, Alboran Sea and Rif showing the different crustal thicknesses in this area. (1) Neogene deposits in the Alboran Sea (with volcanic rocks) and in the foreland basins. (2) Betic±Rif Internal Zone, A±M: Alpujarride and Malaguide complexes, N-F: Nevado-Filabride complex. (3) Crust of the Betic and Rif External zones and their forelands. (4) Contact between the Betic Internal and External zones. The cross and the circle indicate west and eastward displacements, respectively. (5) Possible faults separating the Betic±Rif region from the forelands. Note the asymmetry of the Betic±Rif Internal Zone on both sides of the Alboran Sea. Inspired from Durand-Delga (1980).
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Subbetic in its westernmost sector, while in the eastern part it almost completely disappears, and the Internal Zone is very near the Prebetic. Existing data on block rotation in the Subbetic show clockwise rotation from about 30 to 608 (Allerton et al., 1993; Osete et al., 1989; Platzman and Lowrie, 1992; among others), in accordance with the displacements towards the W and N. Sanz de Galdeano (1996) shows that N±S folds and other structures in the central and eastern sectors of the Betic External Zone, situated in the contact or not far the contact with the Internal Zone, can be easily explained by the westward advance of this last zone. These data are congruent with the postulated westward advance of the Internal Zone from a more easterly area than that of the Alboran Sea. The displacement also has very important effects on the initial Flysch basin, which was entirely disorganized from the Burdigalian on and whose units have been considerably displaced to the W in the Gulf of Cadiz and Campo de Gibraltar and to the SSW in the Rif. In summary, the Betic±Rif Internal Zone moved westwards from an area approximately located to the south of Sardinia and the Balearic (Fig. 7). The collision with the Betic External Zone occurred starting from the lattermost early Burdigalian and then the tremendous disorganization of this area began. Coetaneous with the advance of the Betic±Rif Zone, the Algerian basin opened (of which the Alboran basin constitutes its westernmost end) undergoing expansion and crustal thinning. The Betic±Rif Internal Zone, containing the Alboran basin, is therefore a lithospheric segment that moved westwards, becoming emplaced between the Iberian and African plates. 4. The relationship of seismicity with geological structures The existence of four deep earthquakes with approximate epicentral locations S of Granada city leads to the assumption of a deeply subducted sheet located under the central and eastern areas of the Betic Cordillera and partly under the Alboran, between 200 and 700 km deep (Blanco and Spackman, 1993). We relate this sunken lithospheric body with the geologic and metamorphic evolution during the
Tertiary approximately till the Aquitanian of the microplate in which the Betic±Rif Internal Zone formed originally (described as Alkapeca Domain by Bouillin et al., 1986, or as South-Sardinian by Sanz de Galdeano, 1990). Immediately after, occurred the opening of the Algero-ProvencËal basin and the westward displacement of the Betic±Rif Internal Zone. In favour to its relationship with the Betic±Rif Internal Zone, we propose two arguments. (a) If the position occupied by this sunken sheet is compared (as done by Zeck, 1996, 1997) with the Betic±Rif Internal Zone, the coincidence is nearly absolute. Only in the northern part, the sheet underlies (partially) the Betic External Zone. This coincidence can hardly be accidental. (b) The probable original size of the deformed area affecting the Betic±Rif Internal Zone also coincides. The signi®cance of the intermediate earthquakes in the Betic±Rif region has been studied by Hatzfeld and Frogneux (1981) and Sanz de Galdeano and LoÂpez Casado (1990) and Buforn et al. (1991, 1995, 1997) albeit all the interpretations given present drawbacks. In the interpretation relating them to faults, the dif®culty lies in accepting the possibility of the continuation of faults down to 140 km and the rocks being fragile there. On the other hand, interpretations supporting only possible incipient subduction or a process of lithospheric delamination in the vicinities of Malaga and Granada have the disadvantage of not being able to explain many intermediate earthquakes outside that area. In the western part of the Alboran Sea, the position of the arc of intermediate earthquakes coincides exactly with the sector of maximum sediment thickness. The subsidence in that area is congruent with the approximately vertical stress reported for the earthquake focal mechanisms in the sector by Buforn et al. (1997). The convexity towards the W of the aforementioned arc seems to indicate that the Betic±Rif Internal Zone, being pushed westwards, progressively superimposed the southernmost part of the Iberian plate, which sank. A similar phenomenon may have occurred with a small sector of the African plate. Thus, Iberian and perhaps African lithospheric segments sank towards the E and are responsible for the seismic arc from Granada across to Malaga to the W Alboran and the N Rif.
C. LoÂpez Casado et al. / Tectonophysics 338 (2001) 79±95 Fig. 7. Geological evolution from the Eocene to the early Miocene in the W Mediterranean, especially the Betic and Rif Cordilleras and the Alboran Sea. The arrows indicate direction of displacement. White arrows show the growth of new oceanic crust. A: Alpujarride, M: Malaguide, N-F: Nevado-Filabride, Pb: Prebetic, Sb: Subbetic. Black and white triangles indicate active and inactive subductions, respectively, in the diagram of the Early Burdigalian; they also refer to tectonic windows. Taken from Sanz de Galdeano (1997). 91
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In accordance with the focal mechanisms presented by Buforn et al. (1997), there seems to be a certain WNW±ESE compression dipping slightly eastwards, in addition to a perpendicular one, almost vertical stress. Although this circumstance is inconclusive since the data are somewhat dispersed and there are only 11 focal mechanisms, it does support the existence in the W Alboran of a certain degree of sinking of a part of the lithosphere of Iberia and N of Africa. The intermediate seismicity of the Atlas, although locally separated from the shallow seismicity, is related to lithospheric fracturing, which was already taking place during the Triassic. Over time, these faults allowed the effusion of volcanic rocks, the most recent during the Quaternary. This seems to be the only commendable interpretation and consequently we deduce that this sector of NW Africa, almost all Morocco and part of the Atlantic are almost individualized as a subplate of the African plate. The intermediate seismicity in the Gulf of Cadiz is not speci®cally located in the contact between the Iberian and African plates. This plate boundary is quite clear in the sector between the Azores and the Gloria fault (Fig. 3a). Further E, in the sectors of Gorringe and Ampere, the interaction of faults striking predominantly NE±SW and NNE±SSW masks the contact. This sector coincides with the area where clear compression exists (see Olivet, 1996, Fig. 3). Therefore, from the sector of Gorringe to Algeria, the contact between the plates of Iberia±Europe and Africa becomes obscured. Interpretations for this sector are therefore diverse: Azevedo (1988) locate an incipient subduction in W Portugal that extends southwards in the Gorringe. UdõÂas and Buforn (1991), based on the earthquake mechanisms, believe in an initial subduction in which Africa begins to sink under Iberia. Royden (1993) proposes the possibility of a subduction that sank towards the E. In any case, this contact between the plates is very diffuse, so that, as indicated, its limit is not clear and it is necessary to assign part of the shallow and intermediate seismicity to the ENE±WSW faults extending from the Betic Cordillera and to the above-mentioned NE±SW and NW±SE faults. The seismicity of both the Atlas and of the Gulf of Cadiz therefore appear as different phenomena from those in the Alboran, although all these events need to be integrated in order to understand the overall
behaviour of the region. Fig. 8 presents the main aspects of the tectonic interpretation discussed here, summarily expressed in the conclusions. 5. Conclusions (a) During the Oligocene and the early Miocene, the progressive opening of the Algerian basin, with formation of new oceanic crust, caused the thinning of the continental crust of the Alboran Sea to the West. Meanwhile, in its front, the Betic±Rif Internal Zone (itself also affected by the extension) was coetaneously displaced westwards, causing in its advance the disorganization of the Betic and Rif External Zones during the middle Burdigalian. (b) The subducted sheet situated under the Betic± Rif Internal Zone probably corresponds to the original lithospheric bottom of this domain sunk during the Oligocene±early Miocene and also displaced westwards. The very deep earthquakes are related with this sheet, but there is no explanation for the lack of other earthquakes at depths between about 200 km and those at more than 600 km. Since only four earthquakes are known at more than 600 km depth, they are evidently scarce, while at the same time, the record is obviously incomplete as well. (c) The westward advance of the Betic±Rif Internal Zone caused its partial overthrusting on the Iberian plate and even on the NW African plate. Part of the Iberian plate (and perhaps of the African plate) sank and reached a depth of about 140 km. This would explain the intermediate seismicity in the western sector of the Alboran Sea, especially between Malaga and Granada and in the N of the Rif. Equally congruent with this interpretation is the signi®cant subsidence in the western part of the Alboran Sea, tracing the same arc as the intermediate seismicity. There seems to be a seismic gap at a depth of about 50 km in this arc, probably signifying the breakup of the small subducted sheet. (d) The large fractures marking the Atlas chain, with associated intermediate seismicity, extending towards the eastern sector of Alboran and towards the Canary Islands, are signi®cant enough to be able to distinguish a subplate inside the African plate. This subplate encompasses a large part of Morocco, extending from Agadir to the Canary
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Fig. 8. Tectonic interpretation of the Iberian±Moroccan area. (1) Faults. (2) Probable faults. (3) Present position of the former contact between the African plate and the South-Sardinian domain. (4) Present boundary of the Betic±Rif Internal Zone. (5) Betic±Rif Internal Zone. (6) Approximate contact between the Iberian and African plates. (7) Present possible prolongation to the east of the contact between the African and Iberian plates. (8) Position of the arc produced by the sinking of the Iberian and African lithospheres in the W Alboran Sea. (9) Arrows indicating direction of crustal sinking in the W Alboran Sea. (10) General direction of displacement of the Betic±Rif Internal Zone. (11) Position, according to Blanco and Spackman (1993) of the sunken lithospheric sheet in the Betic±Rif area. (12) Approximate limit of the new oceanic crust in the Algero-ProvencËal basin. (13) New oceanic crust in the Algero-ProvencËal basin.
Islands. From there, it continues towards the Middle Atlantic Ridge. The intermediate earthquakes between Gibraltar and the Gorringe sector correspond also to a situation unrelated to the sinking processes of lithospheric sheets, where only very incipient sinking is suspected. The contact between the African and the European plates is
barely outlined there because it is strongly affected by faults cross-cutting it and producing earthquakes. Acknowledgements We are grateful to anonymous reviewers whose
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