Evolution of the continental margin of southern Spain and the Alboran Sea

Marine Geology, 36 (1980) 205--226 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

EVOLUTION OF THE CONTINENTAL SPAIN AND THE ALBORAN SEA

205

MARGIN OF SOUTHERN

WILLIAM P. DILLON 1, JAMES M. ROBB', H. GARY GREENE 2 and JUAN CARLOS LUCENA 3

U.S. Geological Survey, Woods Hole, Mass. 02543 (U.S.A.) U.S. Geological Survey, Menlo Park, Calif. 94025 (U.S.A.) 3Empresa Nacional ADARO de Investigaciones Mineras, S.A., Madrid (Spain) (Received March 20, 1979; revised and accepted August 21, 1979)

ABSTRACT

Dillon, W.P., Robb, J.M., Greene, H.G. and Lucena, J.C., 1980. Evolution of the continental margin of southern Spain and the Alboran Sea. Mar. Geol., 36 : 205--226. Seismic reflection profiles and magnetic intensity measurements were collected across the southern continental margin of Spain and the Alboran basin between Spain and Africa. Correlation of the distinct seismic stratigraphy observed in the profiles to stratigraphic information obtained from cores at Deep Sea Drilling Project site 121 allows effective dating of tectonic events. The Alboran Sea basin occupies a zone of motion between the African and Iberian lithospheric plates that probably began to form by extension in late Miocene time (Tortonian). At the end of Miocene time (end of Messinian) profiles show that an angular unconformity was cut, and then the strata were block faulted before subsequent deposition. The erosion of the unconformity probably resulted from lowering of Mediterranean sea level by evaporation when the previous channel between the Mediterranean and Atlantic was closed. Continued extension probably caused the block faulting and, eventually the opening of the present channel to the Atlantic through the Strait of Gibraltar and the reflooding of the Mediterranean. Minor tectonic movemer/ts at the end of Calabrian time (early Pleistocene) apparently resulted in minor faulting, extensive transgression in southeastern Spain, and major changes in the sedimentary environment of the Alboran basin. Active faulting observed at five locations on seismic profiles seems to form a NNE zone of transcurrent movement across the Alboran Sea. This inferred fault trend is coincident with some bathymetric, magnetic and seismicity trends and colinear with active faults that have been mapped onshore in Morocco and Spain. The faults were probably caused by stresses related to plate movements, and their direction was modified by inherited fractures in the lithosphere that floors the Alboran Sea. INTRODUCTION

T h e A l b o r a n Sea, t h e w e s t e r n m o s t b a s i n o f t h e M e d i t e r r a n e a n , lies b e t w e e n Spain a n d Africa, just east of Gibraltar. It occupies an u n u s u a l t e c t o n i c situat i o n as a s m a l l sea b a s i n c a u g h t b e t w e e n t w o m a j o r p l a t e s t h a t are t h o u g h t t o be colliding, a l t h o u g h seismic r e f l e c t i o n profiles described here and o t h e r p u b l i s h e d w o r k d o n o t s h o w d i s t i n c t c o m p r e s s i o n a l f e a t u r e s i n t h e b a s i n sedi-

206 ments. Data were gathered using a 27 kJ sparker seismic reflection profiling system, a U n i b o o m high resolution profiler, and a proton precession magnetometer (Fig.l). The sparker profiles, on which this paper is mainly based, provide penetration to basement as well as better resolution than has been available in most previously published profiling studies in the area. The seismic stratigraphy derived from this improved resolution allows us to correlate sedimentary units to tectonic events. This geophysical survey was conducted during April and May 1974 by the U.S. Geological Survey and Empresa Nacional Adaro de Investigaciones Mineras aboard the Spanish Navy hydrographic survey vessel "B.H. Pollux". The data discussed here were collected to investigate the general structure of the continental margin of southern Spain and the central Alboran Sea. A much more closely spaced grid of profiles obtained south of Almeria supports interpretations presented here ( R o b b et al., 1976; Greene et al., 1977). Other seismic reflection profiles taken in the Alboran Sea have been discussed in many studies (Glangeaud et al., 1967; Giermann et al., 1968; Auzende et al., 1971; Le Pichon et al., 1972; Olivet et al., 1973a, b; Mulder, 1973; Ryan et al., 1973), and much of this wor~ was summarized by Auzende et al. (1975). Seismic profiles in the adjacent area of the Balearic Basin to the east have also been reported (Watson and Johnson, 1968; Mauffret, 1969; Montadert et al., 1970; Auzende, 1972b; Mauffret et al., 1972). Refraction

Fig.1. Map of the Alboran Sea, showing tracklines,bathymetry and proposed faultzone. Bathymetry adapted from Allan and Morelli (1971).

207

data have been collected in the Alboran Sea (Fahlquist and Hersey, 1969, 1970; Ritsema, 1969), and gravity and magnetics data have been obtained (Allan and Morelli, 1971; Vogt et al., 1971; Bonini et al., 1973; Galdeano et al., 1974). The approach in this paper will be first to examine the geological setting, including the tectonics that have affected deposition in the Alboran basin. Then we propose to identify seismic-stratigraphic units related to these regional events. GEOLOGICAL SETTING

The Alboran Sea is bordered to the north and south by mountain ranges that have somewhat similar lithologies and folded structures (Fig.2). These ranges, the Betic in Spain and the Rif in North Africa, trend parallel to the shores and were developed by compression during the Tertiary Alpine orogenies. Folding occurred mostly in the Eocene but its effects extended into Miocene time (Andrieux et al., 1971). Both the Betic (Glangeaud, 1962; Egeler, 1964; Rutten, 1969; Fernex and Magn~, 1969; Faure-Muret and

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Fig.2. Generalized geologic map of regions around the Alboran Sea. Adapted from UNESCO (1971, p.16).

208 Choubert, 1971) and the Rif (Durand Delga et al., 1962; Dillon and Sougy, 1974) are divided into an external zone, that part farthest from the Alboran Sea, and an internal zone, closer to the sea. In southern Spain, the internal zone of the Betic range (Fig.2) consists of a central, ENE trending core of highly metamorphosed rocks. Overlying these rocks and surrounding their outcrop are low-grade metamorphic rocks that apparently were emplaced as a pile of nappes. Bordering the internal zone to the north and scattered through the low-grade metamorphic rocks are remnants of mainly nonmetamorphosed Paleozoic and Mesozoic sedimentary rocks, also formed as a series of imbricate nappes. The northern part of the external zone in Spain is known as the Prebetic (Fig.2). It is characterized by shallow-water platform deposits, that are undisturbed to the north b u t become progressively more folded southward above a detachment plane. The southern part of the external zone is dominated by a great olistostrome of Mesozoic and Tertiary pelagic deposits, that became detached on Triassic shale, silt and gypsum and slid north from the position of the present internal zone and Alboran Sea (Faure-Muret and Choubert, 1971). Similar nappe development occurred contemporaneously in the Rif zone (Dubordieu, 1960; Suter, 1965). Timing of Neogene tectonic events must be examined very carefully to understand the geological evolution. The internal zones of the Betic and Rif ranges underwent a compressive phase in the Miocene, when steep imbricate faults were formed (Egeler, 1964; Egeler and Bodenhausen, 1964; Suter, 1965; Bousquet and Philip, 1976). Prior to deposition of the late Miocene basinal sediments the Alboran Sea area was elevated, along with most of the internal zone of the Betic Range, which probably was emergent (Fernex and Magn~, 1969). Sediment transport was in a landward direction, away from the present coasts; drainage patterns were directed inland; and the nappes moved away from the present sea (Ritsema, 1970). The gravity gliding nappes of the Subbetic (and probably Ultrabetic -- Fig.2) slid northward in Spain in a Miocene sea. Based on the age of subjacent sediments, these movements occurred in the early--middle Tortonian and were completed by the middle-late Tortonian as indicated by age of marls deposited on t o p of the nappes (Faure-Muret and Choubert, 1971). A very rapid subsidence of the Alboran basin seems indicated in order to allow it to accumulate younger late Miocene deposits. This abrupt reversal in relief in southern Spain, in which formerly positive areas subsided to become basins, has been referred to as the "Mediterranean revolution" (Pannekoek, 1969) or "revolution pontienne" (Andrieux et al., 1971). The rather confusing idea that this subsidence took place simultaneously with compression in the area has persisted. However, recent work suggests that a period of extension occurred from the late Miocene (approximately 10 m.y. ago) into the Pleistocene (approximately 1 m.y. ago). (Bousquet and Philip, 1976; Bousquet et al., 1976), and a somewhat similar episode apparently occurred in Algeria; there it was interrupted by an episode of orogeny at the end of the Miocene (Burollet et al., 1978). It seems simplest to propose that the Alboran basin formed as a result of the extension.

209 Models involving lithospheric extension have been suggested by Andrieux et al. (1971) and Arafia and Vegas (1974), but the complications in the geology of the area have led to a host of other models of Alboran basin formation (Glangeaud et al., 1967; Van Bemmelen, 1969; Ritsema, 1969, 1970; Glangeaud et al., 1970; Andrieux et al., 1971; Hsfi, 1971; Le Pichon, et al., 1971; Ryan et al., 1971; Smith, 1971; Auzende et al., 1972b; Le Pichon et al., 1972; Nesteroff, 1973; Olivet et al., 1973a; Arafia and Vegas, 1974; Auzende and Olivet, 1974; Galdeano et al., 1974; Stanley et al., 1974; Boccaletti, 1975. Loomis, 1975; Alvarez, 1976; Greene et al., 1977; Biju-Duval et al., 1978). Extension associated with plate collision occurs in back-arc spreading and could be suggested for the Alboran basin (Boccaletti, 1975), but the geological setting of the basin was, and is, so different from present-day examples of back-arc spreading (Chase, 1978) that this model seems inappropriate. A change from tension to renewed compression has been proposed for the period since approximately one million years ago (Bousquet et al., 1975b; Bousquet and Philip, 1976; Bousquet et al., 1976). Large uplifts have occurred in this period as indicated by the presence of marine upper Pliocene deposits found in basins of the Betics a kilometer above sea level (Pannekoek, 1969). Deposition in the Alboran basin has been affected, not 0nly by the major tectonic episodes, but also by large fluctuations in ocean surface level within the Mediterranean. These fluctuations were probably produced by restriction of the narrow entrance and attendant loss of water by evaporation, by responses of the Mediterranean to worldwide sea-level fluctuations (Vail et al., 1977), and by subsidence of the Mediterranean basins due to lithospheric heat loss and changes in loading. The major affect on the basin probably occurred during the Messinian (terminal Miocene) when large areas of the deep basin were covered by evaporite deposits and an unconformity was cut. The Messinian restriction of the Mediterranean was probably caused by a combination of the effects of a major and prolonged sea level fall (Vail et al., 1977: third order cycles TM 3.2 and 3.3) and tectonic movements causing the closure of a channel that probably had been located north of the Betics (Van Couvering et al., 1976). Major evaporite deposition occurred in the Mediterranean as a result (Le Pichon et al., 1971; Hs~, 1972; Mauffret et al., 1972; Biju-Duval et al., 1974; Van Couvering et al., 1976; Hsfi, et al., 1978b, c; Fabricius et al., 1978). This deposition also may have occurred in the Alboran basin (Auzende et al., 1975) although the presence of Messinian salt there is disputed by A. Mauffret (written communication, 1977). The question of whether evaporites formed in a deep or shallow basin has been hotly debated in a series of papers edited by Drooger (1973), a series edited by Hsii et al. (1978a), a series in an issue of Marine Geology edited by W.B.F. Ryan and M.B. Cita {1978, volume 27, no.3/4), papers by Stanley et al. (1974) and Stanley (1977), etc. Clearly, geodynamic principles require that depth of basins depended on their lithospheric thickness, composition and temperature. The basins were completely formed when evaporites were deposited, but were rather young and therefore the lithosphere was hot.

210 Thus there is no d o u b t that cooling (and loading) must have resulted in basin subsidence since Messinian, although the subsidence versus age curve derived at mid-ocean ridges probably is n o t applicable in this complex area, as pointed o u t by Montadert et al. (1978). The Alboran basin, because of its thicker (17-25 km), non-oceanic crust (Bonini et al., 1973; Loomis, 1975; Hatzfeld and Boloix, 1976; Udias et ah, 1976) probably has always been shallower than oceanic basins to the east. The Mediterranean was re-opened to the Atlantic at the end of Messinian (end of Miocene) and rapidly flooded (Benson, 1972; Benson and Ruggieri, 1974; Berggren and Hollister, 1974). SEISMIC UNITS AND THEIR CORRELATION TO STRATIGRAPHIC UNITS The sparker profiles in the Alboran basin (Figs.3, 4 and 5) show a very distinctive seismic stratigraphy. A surface layer consists of draped, strong continuous reflections interrupted in a few locations by evidence of slumping. Below a slight angular unconformity lies a unit of weak discontinuous reflections. Under this unit a strong reflector marks a faulted angular unconformity over a group of extremely weak reflections (see particularly profile 2, Fig.4, km 20 to 50) that in turn overlies a rough acoustic basement. Profile 3 (Fig.3) passed 2.5 km from Deep Sea Drilling Project (DSDP) drill site 121 where excellent correlation was reported between stratigraphy determined from drilling results and units recognized in seismic profiles (Christofferson and Fisk, 1973; Ryan and Gustafson, 1973; Ryan et al., 1973). The surface group of distinct continuous reflections, referred to as the "uniformly stratified carpet" (Ryan and others, 1973), consists of a Quaternary hemipelagic deposit of marl ooze (Nesteroff et al., 1973). The section of relatively poorly defined reflections below is referred to as the "gravity ponded facies" (Ryan et al., 1973) and is correlated with a section of marls and turbidite sands (Nesteroff et al., 1973) of early Pleistocene (Calabrian) to early Pliocene age. Below the marked unconformity lie reflections correlated with Tortonian (lower upper Miocene) marls that include a few sand layers. Ryan et al. (1973) indicated that Messinian (upper upper Miocene) and lowermost Pliocene deposits are missing at the DSDP site. Controversy has arisen regarding the location of the Miocene--Pliocene boundary in DSDP 121 and the age of the Miocene deposits that were sampled (Auzende et al., 1975; Montenat, 1975; Ryan and Cita, 1978). According to Montenat et al. (1975), paleontologic studies indicate that uppermost Miocene deposits are not Tortonian, but rather Messinian and that the Miocene--Pliocene boundary occurs 20--30 m deeper than that chosen by Ryan et al. (1973). Such a small difference in depth is unresolvable in our profiles because it represents only a b o u t 2% of the travel time at 2.2 sec time-depth, and average velocity cannot be determined to that accuracy. As there seems to be a significant faunal break between Miocene and Pliocene deposits according to all workers, it is reasonable to correlate the angular unconformity to that break.

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214

Auzende et al. (1975) also believe that the DSDP site 121 was inaccurately positioned with respect to the site survey profiles, and therefore the drilling did not penetrate an evaporite layer similar to the thin Messinian gypsum layer on land near Almeria (Addicott et al., 1978). Thus, in the opinion of Auzende et al. (1975), our Miocene reflectors in Figs.3, 4 and 5 would represent mainly Messinian evaporite layers (not halite) rather than the Tortonian marl and sand reported initially from DSDP 121. Messinian salt is thought to occur at depths greater than 3 sec (Auzende et al., 1975). Thus the broad diapir-like feature on profile 8 (Fig.3, at km 59) actually might be a diapir of Messinian salt. Acoustic basement near DSDP site 121 is correlated to samples of metamorphic and igneous rocks (schist, gneiss and granite) recovered from the b o t t o m of the hole. These samples all give evidence of crystallization under amphibolite facies conditions with retrograde metamorphism to greenschist facies, dated at 16 + 1 million years (Steiger and Frick, 1973), and they are considered to represent the metamorphic and crystalline basement characteristic of this area (Hs[i and others, 1973; Hsil and Ryan, 1973). Such petrographic associations are characteristic of the Betic--Rifean metamorphic basement (Kornprobst, 1973; Loomis, 1975). In general, acoustic basement probably consists largely of metamorphic rocks similar to those of the Betic cordillera. Estimates of depth to magnetic basement on profile 2 (Fig.4), where the line crosses shallow acoustic basement (km 76--85), show that magnetic basement and acoustic basement coincide at that location, and that basement is probably metamorphic or igneous rock. The acoustic basement may also include Triassic dolomite, which we observed on shore west of Almeria, and contains volcanic rocks as reported from dredge hauls on several banks (Giermann et al., 1968). STRUCTURE

Structural trends associated with basement

The structure of the continental margin of southern Spain, in the study area, generally is dominated by a broad continental rise basin b o u n d e d on the south b y a ridge of rock which forms acoustic basement (Figs.3 and 4). A similar pattern of dammed sediments appears in the southern Alboran Sea, where the Alboran Ridge and another acoustic basement ridge south of it have trapped sediment shed from Africa (Fig.5). In comparison to the continental rise, the continental shelf and slope of southern Spain are minor features; in profiles 3 and 8 (Fig.3) the shelf appears as a small beveled progradational wedge less than 10 km wide. The dominant structural trend in the eastern half of the Alboran basin is ENE; this trend is shown b y the b a t h y m e t r y (Fig.l) and basement structure (Fig.6) and is generally recognized in the literature (Bourcart, 1962; Giermann et al., 1968; Le Pichon et al., 1972; Olivet, Auzende and Bonnin, 1973a, b; Arafia and Vegas, 1974; Auzende et al., 1975). The ENE trend is

215

best displayed b y the Alboran Ridge as shown on Fig.5; in this figure profiles have been arranged to stack crossings of the Ridge. Rocks forming the acoustic basement are presumed to underlie the Ridge although they are identified only on profile 8. Basement returns have been obscured by multiples beneath the flat top of the ridge and by hyperbolic "diffractions" beneath the sides. A possibly parallel ENE basement feature appears south of the Ridge on profiles 12 and 14 (Fig.5). A parallel structural trend north of the ridge is related to the abrupt thickening of Pliocene deposits on profile 8 at km 52 (Fig.3) and the scarp on profile 6 at km 83 (Fig.3). A zone of rough shallow basement appears in the central parts of profiles 3 and 4 (Fig.3). It probably is part of a shallow basement block that is present southeast of Malaga and forms the southwestern boundary of a sedimentary basin under the continental rise of south central Spain (Fig.6).

Structure associated with inferred Miocene deposits The well-developed angular unconformity at the top of Miocene strata is evidence that the Alboran basin floor probably was emergent near the end of the Miocene. This unconformity is widespread in the Mediterranean. An episode of fracturing and tilting of blocks affected the Miocene deposits after the unconformity was cut, b u t before subsequent deposition. Evidence for this episode is seen most clearly in fault offsets that appear in the western part of profile 2 (Fig.4) and in tilted Miocene blocks at the south ends of profiles 6 and 8 (Fig.3). -5o 37'

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216 The anticlinal structure in Miocene deposits observed between the Spanish margin and the Alboran Ridge (Fig.3, profile 8, km 59) may have formed as a fold because of compression between the African and European plates. However, although compressive features have been proposed for the North African margin (Auzende et al., 1972a, b), this idea has been disputed (BijuDuval et al., 1974) and most profiles do n o t show compressional effects in the strata. Auzende et al. (1975) consider the structure on profile 8 to be within the area of salt deposition and, therefore, it may have resulted from salt flow. We interpret another small, diapir-like feature on profile 8 at km 31 (Fig.3) to have been formed of Miocene salt, and much of the irregularity of the top of Miocene in this area might be due to flow.

Structures associated with inferred Pliocene and lowermost Pleistocene ( Calabrian ) deposits A minor unconformity appears in the profiles at the top of Calabrian deposits. The unconformity is slightly angular and a few faults are seen that break the unconformity but do n o t propagate upward (Fig.5, profile 14, faults marked f, and Fig.3, profile 6, km 8). This might imply some early Pleistocene tectonism.

Structure of Quaternary (post-Calabrian ) deposits The surface layer observed in the sparker profiles consists mostly of a uniform undisturbed blanket, although some structural complications exist. Slumps disturb the smooth acoustic layering near the Spanish continental rise (left ends of profiles 7 and 6) and near the steep slopes of the Alboran Ridge. The flat-topped ridge presumably was beveled to its depth of 105 to 140 m during Pleistocene sea-level lowerings (Milliman et al., 1972). A few scattered faults break the Quaternary deposits, which is not surprising in this tectonically active region. In addition, however, three zones of faulting appear in the profiles: in profile 7 (Fig.4) at km 30--37, in profile 2 (Fig.4) at km 91--105, and in profile 6 (Fig.3) at km 90--94. Enlarged sections of the fault-zone interpretations for profiles 7 and 2 (Fig.7) and a photograph of profile 2 (Fig.8) show faults and sea floor irregularities associated with the faults, indicating that faulting in these zones has been active recently. The random tilting of blocks, increases of dip with depth in some blocks and decreases in others, and apparent thinning of layers across faults on the downdropped block suggest a style of faulting perhaps most c o m m o n l y seen with transcurrent movement. Positions of active zones of possible transcurrent faulting on profiles 2,6, and 7, as discussed above, appear to fall along a linear NNE trend. Because an extension of the trend crosses t h e p r o f i l e that connects tracks 4 and 6 (Fig.l), that profile (not shown) was examined and similar faulting was found in the predicted position. Faulting also seems to be present along this trend on the CHAIN 43 line, location shown in Fig.1 (courtesy of Woods Hole Oceano-

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218

graphic Institution Data Library). On the basis of these five crossings, the position of a possible fault zone has been inferred and is shown in Fig.1. If this lineation truly represents a zone of faulting, other sorts of data in this region would be expected to show a similar NNE alignment, oblique to the main ENE structural grain. Bathymetric features do follow the proposed fault zone, as shown in Fig.1 {after Allan and Morelli, 1971); the southeast side of Djibouti Bank parallels the trend, and the western part of the Alboran Ridge is offset along it. A more recently published bathymetric map {Auzende et al., 1975) shows similar relationships at Djibouti Bank and at the Alboran Ridge, and also indicates a pronounced saddle in the ridge where the projected fault zone would cross. Evidence that the NNE trend may be a reflection of deep basement features is shown in Fig.6, where the d o m i n a n t ENE basement trends of the eastern Alboran Sea appear to terminate against the proposed NNE fault zone. To the west, trends are more poorly defined and more varied. Distinct offsets of magnetic anomalies along the proposed fault zone are apparent in an aeromagnetic map published by Galdeano et al. (1974), and suggest a relationship of the zone to major basement features. The profiles show that faults offset the sediment surface. Therefore they probably are active, and earthquake patterns might be expected to follow the trend. Most of the seismic events in the Alboran Sea {Fig.9) appear to be concentrated in a group that crosses the central part of the sea in a NNE direction. The proposed fault zone {shown by a hatched pattern in Fig.9) is parallel to this band of epicenters and seems to form its western boundary, although epicenters have n o t been relocated and u n d o u b t e d l y are poorly positioned. The NNE trending zone of earthquakes, if extended into Africa, also seems to delimit the seismicity of the Rif zone; very few events took place east of it on land in this area. Seismicity resumes farther east in North Africa (McKenzie, 1972). If the fault zone extends from the Alboran basin onto land, active faults should be observable in field mapping. Such a NNE fault set has been mapped in Africa (Caire et al., 1974) about 20 km east of the center of the Alboran Basin fault zone trend, and these faults occur in the zone of most active seismicity shown in Fig.9. Three main phases of faulting were identified: in the late middle Miocene, in the late Miocene, and at the end of Pliocene. Weak Quaternary movements also were noted. Faults of Holocene age in southern Spain are reported trending NNE on the extension of the offshore fault zone {Jose Baena, oral communication, 1977), and examination of satellite photographs shows very weak NNE lineaments near the coast on the fault trend {photographs obtained from EROS Data Center, Sioux Falls, South Dakota; photograph identification numbers G30A0311 420000 and G30A0311 430000). A parallel set of very recently active faults has been well mapped east of Almeria (Bousquet et al., 1975a, b; Bousquet and Philip, 1976).

219

SPAIN

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o

~

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~

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Fig.9. Epicenters of earthquakes that have occurred in the area shown between January 1900 and June 1974. Data provided by National Geophysical and Solar Terrestrial Data Center, NOAA. Zone of active faulting, which was determined from seismic reflection profiling, is shown by hatched pattern. EVOLUTION OF THE ALBORAN BASIN

The Neogene--Quaternary history of the Alboran basin began with Alpine compression and faulting in Miocene, that probably continued until the early late Miocene (early Tortonian). In the early to middle Tortonian (perhaps 9--10 m.y. ago) compression probably gave way to extension, causing the block faulting of basement observed in our seismic records, and resulting in the cessation of nappe m o v e m e n t and changes in fault movement proposed b y Bousquet and Philip (1976) and Bousquet et al. (1976). Presumably, the Alboran Basin was formed by this extensional movement in which the lithosphere was stretched and thinned by fracture and intruded with mafic material; the thinner, denser lithosphere would have subsided to form the basin. The latest Miocene (Messinian) was a time of partial or total restriction of circulation between the Mediterranean and the world ocean, which resulted in the Messinian "salinity crisis" from a b o u t 7 or 6.5 m.y. ago to a b o u t 5 m.y. ago (Berggren and Van Couvering, 1974; Van Couvering et al., 1976). The previous channel, that allowed free circulation, was north of the Betics (Van Couvering et al., 1976). During this extensional episode, rotation and

220

translation of lithospheric fragments (microplates) apparently caused the closing of the old channel. Evaporation lowered the water level during the brief (1.5--2.0 m.y.) period of partial or complete restriction of the Mediterranean basin from the world ocean. An unconformity was cut, as seen at the top of the Miocene deposits, and salt was deposited in the deeper part of the basin. The amount of salt requires an inflow of sea water. The salt, apparently present in the deepest part of the eastern Alboran basin, subsequently flowed and disturbed younger sediments. The block-faulted appearance of the post Miocene unconformity, as well as basement, suggests that the extensional movement was fairly active at least to the end of Miocene time. The continual extension resulted in a new opening to the Atlantic Ocean, through the Alboran Sea and Strait of Gibraltar, at about 5 m.y. ago. Extension must have been much slower after that time, as younger sediments (Pliocene and lowermost Quaternary), deposited from ~ 5 to ~ 1 m.y. ago, are only slightly disturbed. However, the basin probably continued to subside due to heat loss. A distinct u n c o n f o r m i t y and change in sedimentation at the end of Calabrian (about 1 m.y. ago) is apparent in the seismic profiles. It appears at DSDP 121 as a termination of marl and turbidite deposition and onset of hemipelagic ooze accumulation. An u n c o n f o r m i t y is present of approximately the same age in southern Spain (Addicott et al., 1978). Its erosion was followed by a rapid transgression to at least 8--10 km north of the present coast near Almeria (Parke D. Snavely Jr., personal communication, 1977). Such a rapid transgression could account for termination of turbidite deposition because it would flood continental shelves and estuaries and cause trapping of land-derived sediments at those locations. The transgression might be due, in part, to the generally higher sea level of the late Pleistocene-Holocene (Vail et al., 1977). However, the slightly angular nature of the unconformity, and faults possibly associated with it, suggest possible tectonic displacements. A change from extension to compression for the region has been proposed for a b o u t this time Cone m.y. ago) (Bousquet and Philip, 1976; Bousquet et al., 1976). Possibly, the transgression and other affects are related to this tectonic change. The change in sedimentation was accompanied by a change in style of faulting. The previous pattern of block faulting was consistent with extension whereas the appearance of subsequent faulting suggests a zone of transcurrent shear. The present direction of movement of the African plate with respect to Spain is generally considered to be somewhat west of north (McKenzie, 1972; Pitman and Talwani 1972; Dewey et al., 1973; Boccaletti 1975; Minster and Jordan, 1978). McKenzie (1970, 1972) showed earthquake fault plane solutions which suggest a N--S compression. The NNE shear (?) zone comes very close to matching one of the principal planes of shear in a strain ellipsoid oriented with its compressional axis parallel to this proposed compressional direction. The crust in the Alboran Sea is far from homogeneous. It is 17--25 km thick (Bonini et al., 1973; Loomis, 1975; Hatzfeld and Bolois, 1976) and

221 u n d o u b t e d l y broken by deep fractures produced as the sea was formed (Udias et al., 1976). Therefore, recent faulting probably represents movement along some ancient fractures that are oriented fairly close to the direction of shear produced by the present stress field. CONCLUSIONS The seismic stratigraphy in the Alboran basin is clear and correlation to a DSDP drillsite is good, although controversy has developed regarding presence or absence of Messinian beds. Each of the four units recognized in the profiles -- basement, Miocene, Pliocene-Calabrian and post-Calabrian Quaternary - is bounded by unconformities and appears to record a distinct set of tectonic events. The Alboran basin probably began to form about 9--10 million years ago (early Tortonian), at the onset of an episode of extension that probably continued until about one million years ago. The basin may have subsided enough to have been flooded by the late Tortonian, about 8 million years ago (Van Couvering et al., 1976), when Miocene deposits observed in our profiles began to accumulate. Eventually the basin was cut off from the Atlantic, as was the rest of the Mediterranean, by closing of the narrow passage north of the Betic range. The resultant desiccation of the Mediterranean is indicated by the eroded, faulted, angular u n c o n f o r m i t y at the top of the Miocene section in our profiles. At the end of the Messinian {end of Miocene) about 5 million years ago the new connection to the Atlantic -- the Strait of Gilbraltar -- was opened through the Betic--Rif Range. This may have been the result of the continuation of extension that had already created the Alboran basin. At the end of Calabrian time, about 1 million years ago, flooding of some of the basin margins and a major change in basin sedimentation were accompanied by minor faulting which offset strata up to the top of the Calabrian units. This may be related to a change from extension to compression, a compression which activated a new set of NNE trending strike-slip faults that probably follow an old zone of weakness in this broad region of deformation and fracture between major plates. ACKNOWLEDGEMENTS Parke D. Snavely Jr. was instrumental in organizing our studies in Spain. Without his efforts in planning and organization this work could n o t have been done. We wish to thank all the members of the Empresa Nacional ADARD de Investigaciones Mineras, who collaborated with us in Spain and made our work very pleasant, in particular, Adriano Garcia Loygorri, Jaime Rodrigues and Manuel del Campo. The officers and crew of "B.H. Pollux" were of great help. The manuscript was reviewed by Kim D. Klitgord and Mahlon M. Ball, U.S. Geological Survey, and Daniel J. Stanley, National Museum of Natural History -- Smithsonian Institution, who offered excellent suggestions. It should n o t be assumed that those mentioned above, or all authors, necessarily agree with the conclusions presented in the paper.

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