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Onshore and o¡shore compressional tectonics in the eastern Betic Cordillera (SE Spain) P. Alfaro , J. Delgado, A. Este ¤vez, J.M. Soria, A. Ye ¤benes Departamento de Ciencias de la Tierra y del Medio Ambiente, Universidad Alicante, Apdo. 99, E-03080 Alicante, Spain Received 27 April 2001; accepted 2 May 2002 Abstract This study concerns the northern terminal splay of the Eastern Betic Shear Zone (Betic Cordillera, SE Spain), where the Bajo Segura Basin lies, and its offshore prolongation along the western Mediterranean margin. We have integrated seismic reflection profiles with gravimetry, seismicity, wells and outcrop data from the Bajo Segura Basin and the Alicante shelf in order to determine the current geodynamic setting of the area. The results indicate that the same compressional structures in Upper Miocene^Quaternary rocks are observed both on- and offshore. In the onshore Bajo Segura Basin, there are ENE^WSW growth folds related to reverse faults in the basement. In the Alicante shelf, the main structure is an ENE^WSW anticlinorium which deforms Upper Miocene^Quaternary syntectonic deposits. These compressional structures are still active at present, as shown by the offshore seismicity. From the structural analysis and focal mechanisms of the earthquakes we conclude that the Bajo Segura Basin and its adjacent shelf have been subject to NNW^SSE compression since Late Miocene until the Present. Folding and reverse faulting of the Upper Miocene^Quaternary sedimentary cover and of its basement have accommodated this compression. The main active structures onshore, located in the Bajo Segura Basin, extend eastwards into the Mediterranean Sea. This fact is interpreted as the Eastern Betic Shear Zone continues offshore to the east. ȣ 2002 Elsevier Science B.V. All rights reserved. Keywords: neotectonics; focal mechanisms; growth folds; reverse faults; Eastern Betic Shear Zone; SE Spain 1. Introduction The study area is located in the north-eastern sector of the Eastern Betic Shear Zone (Silva et al., 1993), also known as the Betic segment of the Trans-Alboran Shear Zone (De Larouzie 're et al., 1988; Doblas et al., 1991) (Fig. 1). This is one of the main structural features related to the recent evolution of the Eastern Betic Cordillera. The Bajo Segura Basin and the Alicante shelf are lo- cated at the northern terminal splay of this shear zone. This onshore terminal segment has worked as a transpressive zone since, at least, Plio^Pleis- tocene times (Silva et al., 1993). There have been several detailed studies of the geodynamic evolution of the Bajo Segura Basin (e.g. Montenat, 1977; Somoza, 1993; Alfaro, 1995). From the Late Miocene to the Quaternary, this basin underwent a compressional stress ¢eld, with a maximum horizontal axis which varies be- 0025-3227 / 02 / $ ^ see front matter ȣ 2002 Elsevier Science B.V. All rights reserved. PII:S0025-3227(02)00336-5 * Corresponding author. Tel./Fax: +34-96-590-3552. E-mail address: [email protected] (P. Alfaro). Marine Geology 186 (2002) 337^349 www.elsevier.com/locate/margeo

Onshore and offshore compressional tectonics in the eastern Betic Cordillera (SE Spain)

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Page 1: Onshore and offshore compressional tectonics in the eastern Betic Cordillera (SE Spain)

Onshore and o¡shore compressional tectonics inthe eastern Betic Cordillera (SE Spain)

P. Alfaro �, J. Delgado, A. Este¤vez, J.M. Soria, A. Ye¤benesDepartamento de Ciencias de la Tierra y del Medio Ambiente, Universidad Alicante, Apdo. 99, E-03080 Alicante, Spain

Received 27 April 2001; accepted 2 May 2002

Abstract

This study concerns the northern terminal splay of the Eastern Betic Shear Zone (Betic Cordillera, SE Spain),where the Bajo Segura Basin lies, and its offshore prolongation along the western Mediterranean margin. We haveintegrated seismic reflection profiles with gravimetry, seismicity, wells and outcrop data from the Bajo Segura Basinand the Alicante shelf in order to determine the current geodynamic setting of the area. The results indicate that thesame compressional structures in Upper Miocene^Quaternary rocks are observed both on- and offshore. In theonshore Bajo Segura Basin, there are ENE^WSW growth folds related to reverse faults in the basement. In theAlicante shelf, the main structure is an ENE^WSW anticlinorium which deforms Upper Miocene^Quaternarysyntectonic deposits. These compressional structures are still active at present, as shown by the offshore seismicity.From the structural analysis and focal mechanisms of the earthquakes we conclude that the Bajo Segura Basin and itsadjacent shelf have been subject to NNW^SSE compression since Late Miocene until the Present. Folding and reversefaulting of the Upper Miocene^Quaternary sedimentary cover and of its basement have accommodated thiscompression. The main active structures onshore, located in the Bajo Segura Basin, extend eastwards into theMediterranean Sea. This fact is interpreted as the Eastern Betic Shear Zone continues offshore to the east. = 2002Elsevier Science B.V. All rights reserved.

Keywords: neotectonics; focal mechanisms; growth folds; reverse faults; Eastern Betic Shear Zone; SE Spain

1. Introduction

The study area is located in the north-easternsector of the Eastern Betic Shear Zone (Silva etal., 1993), also known as the Betic segment of theTrans-Alboran Shear Zone (De Larouzie're et al.,1988; Doblas et al., 1991) (Fig. 1). This is one ofthe main structural features related to the recent

evolution of the Eastern Betic Cordillera. TheBajo Segura Basin and the Alicante shelf are lo-cated at the northern terminal splay of this shearzone. This onshore terminal segment has workedas a transpressive zone since, at least, Plio^Pleis-tocene times (Silva et al., 1993).

There have been several detailed studies of thegeodynamic evolution of the Bajo Segura Basin(e.g. Montenat, 1977; Somoza, 1993; Alfaro,1995). From the Late Miocene to the Quaternary,this basin underwent a compressional stress ¢eld,with a maximum horizontal axis which varies be-

0025-3227 / 02 / $ ^ see front matter = 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 3 3 6 - 5

* Corresponding author. Tel./Fax: +34-96-590-3552.E-mail address: [email protected] (P. Alfaro).

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tween NW^SE and N^S (Alfaro, 1995). This localtectonic regime is consistent with various regionalstudies : in the eastern Betic Cordillera (Montenatet al., 1990), in the whole Betic Orogen (GalindoZald|¤var et al., 1993) and in the Iberian Peninsula(Herraiz et al., 2000). In addition, studies of thepresent-day stress ¢eld, based on focal mecha-nisms, show a compressional setting characterizedby a NNW^SSE stress ¢eld (Alfaro et al., 1999;Herraiz et al., 2000). This compressional setting isrelated to the collision of the African and Eur-asian plates, which are converging at a rate ofapproximately 5 mm/yr according to Argus etal. (1989) and the NUVEL 1A model (De Metset al., 1994).

Nevertheless, the part of the Alicante shelf thatcorresponds to the prolongation of the Bajo Se-gura Basin towards the Mediterranean has beenscarcely documented from a tectonic point ofview. General tectonic features have been in-cluded in regional studies such as those describedby Mau¡ret et al. (1992) and the ITGE (1994)o¡shore geological map. Both studies highlightedthe extensional tectonics working in this o¡shoresector of the Betic orogen. According to Mau¡retet al. (1992) the structural framework of this ma-rine area is completely di¡erent from the adjacentonshore Betic Cordillera. The western Balearicmargin underwent extension, as shown by the for-

mation of E^W horsts and half-grabens (e.g. theTorrevieja graben, the Tabarca horst). Accordingto these authors, the o¡shore structures formedbefore the Messinian, in an E^W to WNW^ESEstress regime.

In order to study the contradictory geodynamicsettings onshore and o¡shore, we analyzed severalseismic re£ection pro¢les and wells in the Alicanteshelf. The data from these seismic pro¢les andwells were complemented by surface geologicaland gravity data, to understand the recent activityof the study area. We integrated these data withonshore and o¡shore seismic data and we alsodiscussed several focal mechanisms of recent on-shore earthquakes to calculate the present-daystress ¢eld.

2. Onshore geology

The Crevillente and Bajo Segura Faults, run-ning ENE^WSW and E^W respectively, areprominent features of the study zone (Fig. 2).Both are active faults that delimit, to the northand south respectively, the Pliocene^Quaternarybasin of the Bajo Segura.

2.1. Stratigraphy of the Bajo Segura Basin

The sedimentary record in the eastern end ofthe Bajo Segura Basin comprises four main strati-graphic units, which can be established from out-crops as well as from subsurface data (Fig. 3).These units range in age from the Late Mioceneto the Quaternary. All are bounded by disconti-nuities related to major tectonic or eustatic events(Montenat et al., 1990; Ferna¤ndez et al., 1998;Soria et al., 2001). The lowest unit, T-II (UpperTortonian), rests directly on the basement of thebasin. In the most complete successions (Fig. 3,wells 1, 2, 3 and 4), unit T-II de¢nes a transgres-sive deepening-upward megasequence, which be-gins with shallow marine facies (conglomeratesand sandstones) and ¢nishes with slope and pe-lagic basin facies (marls and turbidites). The unitT-M, Upper Tortonian^Messinian, forms a re-gressive shallowing-upward megasequence. It iscomposed of slope and pelagic basin facies at

Fig. 1. Regional geological map of the Eastern Betic ShearZone, showing the location of the study area.

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the base, and by shallow marine facies (evapo-rites, sandstones and coral reefs with stromato-lites) at the top.

The P unit is composed of four lithologicalunits de¢ned as formations (Montenat et al.,1990). These formations are well-represented inthe stratigraphic successions that crop out in thesouthern part of the Segura river (Fig. 3). In as-cending order, these formations are: HurchilloMarls Fm (pelagic marine facies), Rojales Sand-stone Fm (coastal and shallow marine platformfacies), Variegated Lutite Fm (river £oodplain fa-cies) and Segura Conglomerates Fm (£uvial chan-nelized facies). These are organized in a regressivemegasequence resulting from the progradation ofcontinental facies over coastal and marine facies.Based on existing biostratigraphic data, the age ofthe Hurchillo Marls Fm has been established byits planktonic foraminifera as Early Pliocene,

G. crassaformis zone (Montenat et al., 1990; Soriaet al., 1996). The Variegated Lutite Fm, in itsvertical transition zone with the Segura Conglom-erates Fm, has been dated using micromammalsas Early Pliocene (base of the biozone MN15;Soria et al., 1996), which leads us to suggestthat the base of the Segura Conglomerate Fmalso corresponds to the Early Pliocene. It is inter-esting to note that the uppermost part of the Se-gura Conglomerate Fm does not outcrop at anypoint within the basin, since this formation is cov-ered by the Q unit. This upper part has beenrecognized by means of boreholes in the axialzone of the basin (Soria et al., 1999), gradingvertically with the Variegated Lutite Fm. Un-fortunately, there are no data of its age. Forthis reason, we cannot reject the possibility thatthe top of the Segura Conglomerates Fm may beLate Pliocene or Pleistocene, as indicated by

Fig. 2. Detailed geological map of the Bajo Segura Basin. Figure shows the location of deep wells and seismic pro¢les undertak-en within the study area, as well as the location of seismic pro¢les on the Alicante shelf.

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Montenat et al. (1990) and Bardaj|¤ et al. (1995).All these aspects related to the chronology of theP unit are important because the dating of therecent tectonics of the Bajo Segura Basin islargely based on the deformation of the forma-tions that make up this unit, especially the SeguraConglomerates Fm (e.g. Taboada et al., 1993;Goy et al., 1989, 1990; Alfaro, 1995).

The Q unit has its principal outcrop along thecentral axis of the Bajo Segura Basin, to the northof the Segura river (Figs. 2 and 3). The £at to-pography of this area, currently undergoing sed-imentation, prevents the exposure of sediments ofthis unit, and so the lithological survey has beendone using boreholes (Soria et al., 1999). Accord-ing to these authors, the upper part of the Q unitis comprised of 30 m of unconsolidated sedimentsdeposited in a £uvial, lagoon and coastal environ-ment, arranged in a similar paleogeographic pat-

tern to that found in the present-day Bajo SeguraBasin; its age has been established using 14C dat-ing, as between 14 570 yr BP and the Present, forthe upper 20 m. Other materials found within theQ unit correspond both to alluvial fans, situatedto the south of the Crevillente fault, and to beachdeposits raised by neotectonics, situated close tothe present coastline (Goy and Zazo, 1988, 1989).With respect to the beach deposits, the oldest ra-diometric dating gives an age of more than 250kyr for raised beaches at 40 m asl (Dumas, 1977),whilst the most recent are 2000 yr old for raisedbeaches 2 m asl (Goza¤lvez, 1985).

The Upper Miocene^Quaternary rocks were de-posited over a basement made up of materialsfrom the External Zone (to the north), and fromthe Internal Zone (to the south). The ExternalZone basement crops out in the Sierra of Crevil-lente. The Internal Zone basement crops out in

Fig. 3. Stratigraphic series observed in the Bajo Segura Basin. The discontinuous line represents the location of the cross-sectionpresented in Fig. 4.

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the Sierras of Orihuela and Callosa, and also inthe Tabarca island. Oil exploration wells drilledwithin the study area penetrated materials even asfar as the basement at 740 m depth at Torrevieja(well 1, Fig. 2), at 760 m in the La Mata sector(well 2), at 918 m to the west of the La Marina(well 3), at 1215 m in the Rojales sector (well 4),at 1524 m to the south of Beneju¤zar (well 5) andat 1110 m depth close to San Miguel de Salinas(well 6) (Gauyau, 1977).

From the Late Miocene until the Early Pliocene(base of the Segura Conglomerates Fm), the studyarea has been considered a unique sedimentarybasin, the Bajo Segura Basin. It would haveevolved since the Pliocene from pelagic marineenvironments (Hurchillo marls Fm), coastal andshallow marine environments (Rojales sandstoneFm), and ¢nishing with continental environments(Variegated Lutite Fm and Segura ConglomeratesFm); this evolution is recorded as a characteristicregressive megasequence, which is visible through-out the basin. As a result of the Plio^Pleistoceneactivity of the Bajo Segura, Torrevieja and SanMiguel Faults, structural highs divide the areain the three present independent basins: Bajo Se-gura, Torrevieja and La Mata basins (Somoza,1993; Silva et al., 1993).

2.2. Tectonics of the Bajo Segura Basin

The Upper Miocene^Quaternary sedimentarycover of the Bajo Segura Basin shows active fold-ing and faulting. As a consequence, positive reliefshave been generated (e.g. Santa Pola, La Marina,Guardamar) as well as subsident areas (e.g. ElPinet, Segura river), which control the distribu-tion of the marine and continental deposits, andde¢ne the current morphology of the basin. Mon-tenat et al. (1990) and Alfaro (1995) deduce aNW^SE/N^S compression that has occurred inthe study area since the Late Miocene until thepresent day and has been accommodated by fold-ing, and by reverse and right/left-lateral faultingof the sedimentary cover and the basin basement.According to Goy and Zazo (1988, 1989), duringthe Quaternary the Bajo Segura Basin has under-gone tectonic activity with E^W trending anticli-nal folding and N^S normal faulting.

The folds a¡ect the basement of the Internaland External Zones of the Betic Cordillera, aswell as the Upper Miocene^Quaternary sedimen-tary cover of the Bajo Segura Basin (Fig. 2). Fold-ing is not synchronous over the whole basin ascan be deduced from the analysis of the progres-sive unconformities developed on the fold limbs.It is deduced that one set of folds started duringthe Late Miocene while others started during theEarly Pliocene or even during the Quaternary (Al-faro et al., 2000).

In the northern sector of the basin (ExternalZone) the folds form a long anticline along thetrace of the Crevillente Fault, which progressivelydeform Upper Miocene^Quaternary units. In thesouthern sector of the basin, there are severalfolds; one of the main folds is the Torremendoanticline which is more than 30 km long and has aset of associated folds of smaller size in bothlimbs. It also progressively deforms Upper Mio-cene units.

There are other anticlinal folds that started dur-ing the Early Pliocene (Alfaro et al., 2000). Someof them are situated along the southern edge ofthe Segura river valley. They are kilometric in sizeand have an average E^W strike. In addition, tothe north of the Segura river and close to thecoastline, there are other anticlines such as thoseof La Marina and Santa Pola, as well as an anti-clinorium composed of small, hectometric second-ary folds, situated in the vicinity of El Saladar.

Synclines have developed between these anti-clines, sharing the same strike. They were ¢lledsyntectonically with sediments that can rangefrom Late Miocene up to Quaternary. These in-clude the synclines of the Bajo Segura, SantaPola-Pinet and El Saladar.

Besides these E^W folds, other oblique foldsappear running NW^SE. To the east of the Tor-remendo anticline these run parallel to the NW^SE faults that deform the Pliocene and Quater-nary sediments in this sector, giving rise to theTorrevieja and La Mata synclines (Fig. 2).

Some of these folds are related to active faults.These include the aforementioned Crevillente andBajo Segura faults (Montenat, 1977; Bousquet,1979; Lo¤pez Casado et al., 1987; Somoza, 1993;Taboada et al., 1993; Alfaro, 1995), as well as

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other NW^SE right-lateral faults (Guardamar,Torrevieja and San Miguel faults) (Fig. 2).

Vertical movements also occurred in the studyarea from the Late Miocene until the Present,which are related to the NW^SE shortening.The uplifted areas correspond to growth anti-clines, whilst the subsident areas are related togrowth synclines. The largest uplift correspondsto the Sierra of Crevillente, where reliefs of closeto 1000 m high are found (Fig. 4). This fault isalso responsible for the subsidence of the south-ernmost sectors, where the Bajo Segura Basin islocated. Geophysical studies (gravity, seismic pro-¢les, and deep vertical electrical soundings) placethe Triassic basement at more than 1000 m depthin Elche, near the Crevillente fault (Gauyau,1977; Gauyau et al., 1977). From these data,the resulting vertical uplift associated with the ac-tivity of this fault exceeds 2000 m from Late Tor-tonian until the present day. Also in the south,important movements have been detected within

the Alpujarride basement. Here seismic pro¢lesindicate that the Alpujarride basement has beenuplifted by more than 500 m compared to theblock located to the north of the Bajo Segurafault. This uplift of the basement is clearly re-£ected by the gravity studies (Gauyau, 1977).Thus, in the southern, uplifted block Bouguer re-sidual anomalies of 39 mGal are recorded, whilein the subsident northern block larger anomaliesof 312 mGal are observed (Fig. 4). In the studyarea, Zazo et al. (1993) established mean rates ofuplift and subsidence for the last 100 kyr varyingbetween +5.2 and 33.0 cm/kyr.

The activity of most of the described structures,especially the Crevillente, Bajo Segura, San Mi-guel and Torrevieja faults (Lo¤pez Casado et al.,1992; Alfaro, 1995; Sanz de Galdeano et al.,1995), is responsible for the seismicity recordedin the study area (Fig. 5), which is one of themost seismically active areas of the Iberian Pen-insula. Small-magnitude earthquakes (mb6 5.0)

Fig. 4. Onshore geological cross-section devised from surface observations together with interpretation of seismic lines.

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characterize the instrumental seismic activitydocumented in the region (Lo¤pez Casado et al.,1987), although there have been occasional histor-ic, moderate- to high-magnitude earthquakes. The1829 Torrevieja earthquake stands out amongthese historic earthquakes: this seismic eventreached MSK intensity X at its epicenter (Mun‹ozet al., 1984). More recently, several authors (Mu-

n‹oz and Ud|¤as, 1991; Delgado et al., 1993) haveestimated its Ms magnitude as between 6.3 and6.9. Other historic earthquakes, with a lower in-tensity at the epicenter (VII^VIII), have been ex-perienced in the southern part of the study zone:at Guardamar del Segura (1523), Rojales (1746),Torrevieja (1802, 1828, 1829, 1837, 1867, and1909) and Jacarilla-Beneju¤zar (1919).

Fig. 5. Seismicity and focal mechanisms calculated within the study area. Focal mechanisms: 1, 3 and 4 from Alfaro et al.(1999), 2 from Coca and Buforn (1994), and 5 from Buforn and Ud|¤as (1991).

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Fig. 6. Seismic pro¢les on the continental shelf (see location in Fig. 2). (A) Seismic pro¢le perpendicular to the Tabarca anticlinal ridge. (B) Prograding geometryof the re£ectors in the Pliocene^Quaternary unit over the Upper Miocene unit. (C) Detail of divergent re£ectors in the Upper Miocene unit. (D) High-resolutionpro¢le (uniboom) showing the syntectonic in¢ll of a trough during the Late Miocene^Quaternary.

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3. O¡shore geology

3.1. Stratigraphy

Two seismic units can be recognized from theseismic lines on the Alicante shelf, which havebeen correlated with wells 8 and 9. Both unitsare characterized by di¡erent seismic facies (Fig.6). The lower seismic unit, dating from the LateMiocene, correlates with the T-II and T-M strati-graphic units onshore according to lithologicaland paleontological criteria. The top of this unitis represented by a conspicuous re£ection causedby the lithological contrast between the evaporitesand/or sandstones at the top of the T-M unit, andthe marly sediments at the base of the P unit. Thissharp re£ection masks information below, espe-cially to the south of the study zone, where it isdi⁄cult to identify the position of the base of thisseismic unit. The interior of the unit is character-ized by divergent re£ectors as a result of a syntec-tonic sedimentation.

The upper seismic unit, which is Pliocene^Qua-ternary, correlates with the P and Q stratigraphicunits onshore. It is characterized by clear, internalre£ections (Fig. 6). Along the seismic line S81B2,which runs perpendicular to the coast, one can seehow the internal geometry of the upper seismicunit shows prograding sigmoidal clinoforms,with downlapping terminations on the top of low-er seismic unit. Along the S81B9 seismic line, par-allel to the coastline, the geometry of this unitis characterized by re£ections, locally divergent,which ¢nish in an onlap against the margins ofthe troughs developed during the deformation ofthe lower seismic unit (e.g. La Mata and Alicantetroughs, Fig. 6).

The lithological survey of the Upper Mioceneand Pliocene^Quaternary seismic units was under-taken using two oil exploration wells (Fig. 2). Inwell 8 (drilled in 1978 by ENIEPSA and calledTorrevieja Marino C-1) two lithological unitsare recognized: a basal one that corresponds tothe top of the Upper Miocene seismic unit, and anupper one that coincides with the Pliocene^Qua-ternary seismic unit. The ¢rst is composed of veryporous dolomitic limestone and bioclastic lime-stone and has been interpreted as a Messinian

reef (Mart|¤nez del Olmo and Serrano, 2000); thesecond is dominated by clays rich in foraminifera,with thin sand layers. In well 9 (drilled in 1984 byEsso Exploration and called Alicante A-1) threelithological units have been drilled: the lower two,which overlie the carbonate basement rock of theExternal Zone, correspond to the top of theUpper Miocene seismic unit, while the upperunit corresponds to the Pliocene^Quaternary seis-mic unit. The lower two begin with an evaporitesuite composed of alternating clays, anhydrite,dolomite and gypsum; they ¢nish with a lime-stone packstone rich in foraminifera, bryozoansand echinoids. The upper lithological unit is rep-resented by clays with abundant foraminifera andfragments of bivalves, gastropods, bryozoans, redalgae and echinoids, within which there are fre-quent, quartz-rich sand layers. According to theoriginal report for well 9 (Alicante A-1), the ap-pearance of Hyalinea baltica sp. in the upper partof the clay sequence indicates the base of the Qua-ternary (ITGE, 1994). Well 9 has been correlatedwith other wells in the western Mediterranean inthe frame of the Deep Sea Drilling Project ; spe-ci¢cally sites 122, 124 and 132 of Leg 13 (Hsu« etal., 1973) and sites 371 and 372 of Leg 42A (Hsu«et al., 1978), which exhibit a sequence of Messi-nian evaporites followed by pelagic clays withlayers of sand that have been dated as Early Plio-cene and Quaternary.

3.2. Structure of the Alicante shelf

Seismic re£ection pro¢les in the Alicante shelfshow how the Upper Miocene^Quaternary sedi-mentary cover is folded (Fig. 6). The main struc-ture is an ENE^WSW anticlinorium, called theTabarca anticlinal ridge, which is the o¡shore pro-longation of the same fold structure observed inthe Bajo Segura Basin. This anticlinorium is 20^30km wide and extends at least 80 km from the coastin an ENE^WSW strike. The size is deduced fromthe seismic lines available for this zone and fromthe signi¢cant, positive free-air gravity anomaly(40 mGal) observed in this zone (Fig. 7). Thisregional ENE^WSW anticlinorium shows smalleranticlines and synclines. One of these secondarysynclines is the Segura river trough.

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On both sides of the anticlinorium there aretwo growth synclines: the Torrevieja syncline tothe south, and the Alicante syncline to the north.Since the Late Miocene, when the folding began,these synclines have operated as subsidenttroughs. The sediments in the study zone showsigni¢cant lateral variations in thickness, con-trolled by these compressional structures.

Syntectonic strata form the limbs of the anti-clinorium. In the o¡shore seismic pro¢les (Fig. 6),the boundary between pre-growth strata andgrowth strata has been placed in the Late Mio-cene, the period in which the folding and reverse

faults started. During the folding, which is stillactive, the secondary growth synclines were ¢lledwith syntectonic sediments, some of them LateMiocene^Quaternary in age, and others, such asthose of the Bajo Segura, Lower Pliocene^Quater-nary in age.

In the Alicante shelf, the deformation is ob-scured by a sedimentary unit of variable thick-ness, probably deposited during the Late Pleisto-cene^Holocene. This unit can be correlated, onthe basis of its structural continuity onshore,with the Q unit deposited in the axial area ofthe Bajo Segura Basin (Soria et al., 1999). These

Fig. 7. Geodynamic map showing uplifted (+) and subsident (3) areas, both on- and o¡shore. (A) Map of the free-air anomalyon the shelf (Sandwell and Smith, 1997) and of the Bouguer anomaly on land (Gauyau, 1977). (B) Situation of the Tabarca anti-clinal ridge (shaded zone); contour levels show the topography and bathymetry in meters.

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recent sediments are mainly attributable to thelast eustatic rise (Holocene). This unit is not oldenough to have been appreciably deformed. Con-sequently, the recent sediments are practicallyhorizontal in the seismic pro¢les. Nevertheless,high-resolution seismic pro¢les indicate some de-formation even for these sediments (Fig. 6), andonly the most super¢cial re£ectors show no defor-mation.

3.3. Seismotectonics of the Alicante shelf

These folds are commonly related to reversefaults at depth which are active, as can be de-duced from their seismicity.

The occurrence of o¡shore earthquakes is sim-ilar to that onshore, and is concentrated in a wideband trending ENE^WSW, within the eastwardsprolongation of the Eastern Betic Shear Zone. AnENE^WSW alignment of earthquakes can be de-termined, just in the eastern prolongation of theBajo Segura fault (Fig. 5). The seismicity recordedalong the Alicante shelf is characterized by shal-low, small-magnitude earthquakes (mb6 5.0).Most of them show magnitudes not exceeding 3.5.

Only three focal mechanisms are available forthe Alicante shelf (Fig. 5). The highest-magnitudeseismic event (mb= 4.9) corresponds to the 1981Alicante earthquake, located at 5 km depth (event5 in Fig. 5). This earthquake had a focal mecha-nism of reverse faulting on the ENE^WSW faultplane (Buforn and Ud|¤as, 1991). Its location ¢tswell into the north-east extension of the Crevil-lente Fault.

Another focal mechanism solution of reversefaulting is related to the 1993 Santa Pola earth-quake (event 4 in Fig. 5), located at 4 km depth(Alfaro et al., 1999). This 3.8-mb-magnitude eventhas a main fault plane solution with an ENE^WSW strike.

The third focal mechanism solution corre-sponds to the 1993 Guardamar earthquake (event3 in Fig. 5), located at 7 km depth (Alfaro et al.,1999). The seismic event was located close to theintersection of the Bajo Segura reverse fault andthe NW^SE strike-slip Guardamar Fault. The so-lution planes are compatible with these activefaults. The NNW^SSE solution plane is a pre-

dominantly right-lateral strike-slip fault with a re-verse component, whilst the ENE^WSW solutionplane is a reverse fault with a left-lateral compo-nent.

A NNW^SSE compressional stress ¢eld alongthe Alicante shelf is consistent with these threefocal mechanism solutions. This agrees with re-gional determinations of the present-day stress¢eld deduced for the eastern Betic Cordillera (Bu-forn et al., 1995; Herraiz et al., 2000).

4. Discussion and conclusions

In the seismic re£ection pro¢les studied in theAlicante shelf we can recognize the folding andthrusting of the Upper Miocene^Quaternary sedi-mentary cover and the basement of the easternBetic Cordillera. The most important structuresare the ENE^WSW Tabarca anticlinorium andtwo growth synclines to either side: the Torreviejasyncline to the south and the Alicante syncline tothe north. Deformation began o¡shore during theLate Miocene, when the major synclines were syn-tectonically ¢lled. Recently, the Tabarca anticli-norium and the Torrevieja and Alicante synclineshave not produced signi¢cant structural highs andlows, because of the low tectonic rates. On theother hand, the highs may not be well developedbecause the sedimentary accumulation on the Ali-cante shelf is greater than the uplift.

These compressional structures are also presentonshore at the Bajo Segura Basin. From the struc-tural analysis of the folded strata we can deducethat the onshore folding and thrusting is not syn-chronous. From the analysis of the progressiveunconformities developed on the fold limbs it isdeduced that some set of folds started during theLate Miocene while others started during theEarly Pliocene or even during the Quaternary.

Thus, we observe good agreement between theonshore and o¡shore geology with regard to theirtectonic character. The o¡shore seismic activity isalso compatible with the seismicity onshore. Twoof the focal mechanisms have reverse solutionswith ENE^WSW fault planes, and a third has adextral solution with a NW^SE fault plane or areverse solution with a ENE^WSW fault plane.

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The link between the on- and o¡shore seismicityindicates that co-seismic folding related to ENE^WSW reverse faults such as the onshore Crevil-lente and Bajo Segura faults, is likely to occur.

As a result of the structural and seismotectonicanalysis, we can conclude that the Eastern BeticShear Zone extends eastwards into the Mediterra-nean Sea. In this geodynamic context, the EasternBetic Shear Zone has operated since at least theLate Miocene as a transpressive zone, folding andthrusting the sedimentary cover and the basement,as indicated previously by Silva et al. (1993). Thesubaerial and marine structures, as well asthe focal mechanisms, indicate a compressionalNNW^SSE stress ¢eld.

Finally, the active uplift of the Alicante shelf,associated with its co-seismic folding, will lead tothe future emersion of the area. This emersion willvary spatially and temporally because of the dis-tribution of anticlines and synclines and, ofcourse, because of eustatic oscillations. This activeuplift of the Alicante shelf agrees with the lowrates of subsidence established by Zazo et al.(1993) in the study area in comparison withmore subsident adjacent areas.

Acknowledgements

This paper bene¢ted from the constructive com-ments made by Drs. P.G. Silva and A. Mau¡ret.This study was ¢nanced from Project BTE2000-0339 of the Direccio¤n General de Ensen‹anzaSuperior e Investigaciones Cient|¤¢cas, and Re-search Group GR00-22 of the Generalitat Va-lenciana. Seismic re£ection and well data wereused with permission of the Servicio de Hidrocar-buros of the Ministerio de Econom|¤a of Spain.

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