the valencia trough (western mediterranean): an overview

Tectonophysics, 208 (1992)183-202

Elsevier Science Publishers B.V., Amsterdam 183

The Valencia trough (western Mediterranean) : an overview

E. Banda a and P. Santanach b a Institute of Earth Sciences “Jaume Almera”, CSIC, Marti i Fran&%, s / n, 08028 Barcelona, Spain

b Departament de Geologia Dincimica, GeojXca i Paleontologia, Facultat de Geologia, Campus Pedralbes, 08071 Barcelona, Spain

(Received August 6, 1991; revised version accepted January 8, 1992)

ABSTRACT

Banda, E. and Santanach, P., 1992. The Valencia trough (western Mediterranean): an overview. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume I. Case History Studies on Rifts: Europe and Asia. Tectonophysics, 208: 183.-202.

The Valencia trough, located between the Spanish mainland and the Balearic islands, corresponds to a late Oligocene to recent sedimentary basin that is characterized by a highly attenuated continental crust. Its northwestern part evolved in response to a late Oligocene to early Miocene rifting phase that was followed by a period of post-rift subsidence and a late Miocene-Pleistocene phase of renewed crustal extension and magmatism. Its southeastern part is characterized by late Oligocene to mid-Miocene compressional deformation and subsequent extension. In the axial parts of the basin the crust has been thinned by a factor of 3 whereby the lower crust is either absent or very thin. Upper crustal extension appears to be considerably smaller than indicated by the crustal configuration of the basin. A major problem in understanding the evolution of the Valencia trough is the lack of knowledge about the physical state and structure of the lithosphere before late Oligocene extension. Evolution of the northwestern part of the basin appears to be linked with the development of the oceanic Provengal Basin. Middle Miocene compressional deformation of the Balearic fold belt, forming the southeastern margin of the Valencia trough, appears to have interfered with the tensional evolution of the latter, which only resumed after crustal shortening in the Balearic domain had given way to extensional tectonics during the late Miocene.

Introduction

The Valencia trough is a NE-SW striking Cenozoic basin, some 200 km wide and 400 km long. It is located in the western Mediterranean between the Iberian Peninsula and the Balearic islands. The basin is closed to the southwest and opens up to the northeast into the oceanic Provensal Basin (Figs. 1 and 2). In the axial parts of the Valencia trough water-depths exceed 1000 m; its Cenozoic sedimentary fill locally attains thicknesses of up to 5 km.

The Valencia trough can be considered as forming the southwestern prolongation of the

Correspondence to: E. Banda, Institute of Earth Sciences “Jaume Almera”, CSIC, Marti i Franquts, s/n, 08028 Barcelona, Spain.

Cenozoic grabens of the RhBne Valley and the Gulf of Lions, and, as such, forms part of the Cenozoic rift system of Western and Central Eu- rope (Ziegler, 1992). However, unlike the Rhine, Bresse, Limagne and SaBne graben, which are located in the Alpine foreland, the Gulf of Li- ons-Valencia rift crosscuts the Pyrenean fold belt, the Catalan Coastal Ranges and the Iberian chain (Fig. 1). Its southeastern margin is formed by the Betic-Balearic thrust belt. Late Aquitanian-early Burdigalian opening of the oceanic Provensal Basin, entailing a counter-clockwise rotation of the Corsica-Sardinia Block (Rehault et al., 19851, was preceded by a short rifting phase during which also the Valencia trough started its development. Following crustal separation in the Provengal Basin, rifting activity in tti Valencia trough decreased rapidly. Compressional deformation of the Betic-Balearic thrust belt is dated

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as latest Oligocene-mid-Miocene. During late Mioccnc to Recent times the Valencia trough was extcnsionally reactivated.

The analysis of the Valencia trough, presented in this paper, is based on industry-type reflection seismic lines, calibrated by numerous wells along the Ebro platform, deep regional reflection seismic lines, refraction/wide-angle reflection and gravity data.

Geological studies of the Valencia trough and its margins have been carried out during several decades. During the last 20 years, its western margin (the Ebro platform) has been extensively explored for hydrocarbons. Oil accumulations, containing ultimate recoverable reserves of some 200 million bbls, are mainly contained in karstified Mesozoic carbonates underlying the Ceno- zoic sedimentary fill of the Valencia trough. Traps are provided by block-faulted burried-hill struc-

tures. Hydrocarbons are dcrivcd from martnc Mesozoic series and lacustrine early syn-rift Jc- posits that reached maturity for oil generation during the Neogene as a consequence of out building of the Ebro delta (Hark and Schoneich. 1977; Bouvier et al., 1990).

Crustal thinning across the Valencia trough was first recognized by Hinz (1972) and Gobert et al. (1972). The internal structure of its deeper crust and upper mantle was investigated during the eighties (Banda et al., 1980; Zeyen et al..

1985; Daiiobeitia et al., 1990). More recently the multinational VAUIS project (Watts et al., 1990; Foucher et al., 1992) has provided important new seismic, heat flow and gravity data. The newly available seismic data include a suite of multi- channel seismic reflection profiles and 6 Expand- ing Spread Profiles crossing the trough and following the axis. For a more detailed discussion of

Fig. 1 Simplified geological map of the Western Mediterranean. C.C. R. = Catalan Coastal Ranges.

VALENCIA TROUGH (WESTERN MEDITERRANEAN) 185

these data the reader is referred to Banda and Santanach (1992) where regional geological studies can also be found.

M~tectonic setting

The Valencia trough is superimposed on the Alpine megasuture between the African and

Eurasian plates. The Late Cretaceous and Ceno-

zoic convergence of these plates involved also movements of relatively autonomous micro-plates

and blocks, such as Iberia and the Corsica- Sardinia blocks. These blocks were located between the African and European cratons (Der- court et al., 1986; Ziegler, 1988; Dewey et al., 1989).

Fig. 2. Geologica map of the Valencia trough and surrounding areas. 1 = Basement; 2 = Mesozoic cover; 3 = Paleogepe; 4 = Early Miocene extensional faults and graben fill; 5 = extensional active faults since middle Miocene; 6 = Betic-Balenric domain; 7 = alkaline volcanism (Middle Miocene-Recent); 8 = talc-alkaline volcanism (Early-Middle Miocene); 9 = volcanism of unknown

volcanic character.

During the Mesozoic breakup of Pangca. Africa and Europe moved apart under a sinistral. transtensional setting. The Hercynian crust, occu- pying the present Mediterranean area, was ex- tended, leading to basin formation and the accu- mulation of thick sedimentary sequences in rifted basins. Following Middle Jurassic crustal separation between Europe and Africa, passive margins developed; however sinistral motions between Europe and Africa persisted. Early Cretaceous northward propagation of the Central Atlantic seafloor spreading axis culminated in Middle Cretaceous times in crustal separation between Iberia and Europe and the isolation of the Iberian plate, comprising the area of the Cenozoic Valen- cia trough. During the Late Cretaceous and Cenozoic Africa and Europe converged. This en- tailed compressional deformation and inversion of the Mesozoic basins and passive margins and the development of the Mediterranean Alpine chains. At the same time a number of new basins developed within the Alpine megasuture domain. According to their position relative to the evolving fold belts, these basins can be classified into compression-dominated foreland basins (e.g., Molasse, Ebro and Adriatic basins), back-arc extensional basins (e.g., Alboran, Tyrrhenian, Pan- nonian and Aegean basins), and rifts that apparently propagated from the foreland into, and partly across, Alpine fold belts (e.g., Pelagian Shelf grabens and Rh6ne-Valencia grabens). Some of these extensional basins progressed to the opening of oceanic basins, such as the Tyrrhenian (Wezel, 1985) and Provenqal basins (Burrus, 1984; Rehault et al., 1985). The geodynamic processes governing the evolution of these extensional basins, which are all superimposed on the Alpine megasuture, are unclear and appear to differ from one basin to the other (Boccaletti and Guazzone, 1974; Horvhth and Berckhemer, 1982; Royden and Horvhth, 1988; Comas et al., 1992).

In the westernmost Mediterranean domain, Late Cretaceous-mid-Oligocene convergence of Africa and Europe was mainly taken up along the collision zone between the northern margin of the Iberian block and the southern margin of France. This convergence lead to the formation of the Pyrenean fold belt that extends from

Provenqe (France) to the Kings trough (North Atlantic) (Srivastava and Tapscott, 1086; Boillot and Malod. 1988; Muiioz. lOY2). From middle Oligocene times onward. collisional strain was increasingly taken up along the southern margin of Iberia in the evolving Betic-Maghrcbian arc, whereas compressional deformation of the Pyre- nean system gradually decreased and ceased during the Miocene. During its accretion to the southern margin of Europe, the Iberian block was internally deformed, whereby Mesozoic rifted basins were inverted, now forming the Iberian chain and the Catalan Coastal Ranges. These intraplate deformations lasted from the earliest Tertiary until late Oligocene to early Miocene times (Vaillard, 1973; Alvaro et al., 1979; Ziegler, 1988; Guimera and Alvaro. 1990).

During the late Oligocene, rifting propagated apparently southward from the RhGne Valley through into the domain that is now occupied by the Provensal Basin and the compressionaly deformed area of the Valencia trough. After a rifting stage of some 8 Ma, crustal separation was achieved during the middle Aquitanian between the Corsica-Sardinia Block and southern France-Iberia and between the Balearic-Al- boran and Kabylian block. Opening of the Al- gero-Provensal oceanic basin during late Aqui- tanian-early Burdigalian was coupled with a counterclockwise rotation of the Corsica-Sardinia Block (Montigny et al., 1981; Burrus, 1984; Re- hault et al., 1985; Ziegler, 1988, 1992).

Subsidence of the Valencia trough commenced during late Oligocene under a tensional regime; during the Burdigalian rifting activity decreased and the Valencia trough entered a post-rifting stage. On the other hand, compressional deformation of the External Betic-Balearic zone, which forms the southeastern margin of the Va- lencia trough, lasted from late Oligocene until mid-Miocene times (FontbotC et al., 1990). Thus, the late Oligocene-early Miocene rifting stage of the Valencia trough was contemporaneous with crustal shortening in the Betic-Balearic thrust belt. Upon termination of erogenic activity in the latter, crustal extension resumed, particularly in the northwestern, coastal parts of the Valencia trough and persisted till the present.


In cross-sections the Valencia trough shows a

clear asymmetry that is also evident in its crustal structure. This can be related to fundamental

differences in structural style that characterize its northwestern and southeastern parts. The former (Catalan-Valencian domain) are characterized by extensional block faulting, partly affecting also late Miocene and Pliocene series, whereas the latter (Betic-Balearic domain) is dominated by folding and thrust faulting, affecting Middle Miocene and older strata (Fig. 2; FontbotC et al., 1990). The limit between these tectonically different domains is located in the southwestern parts of the Valencia trough near its axis (Soler et al.,

1983), but it is uncertain further to the Northeast (Maillard et al., 1992).

Various geodynamic models have been proposed for the evolution of the Valencia trough. These range from some sort of back-arc extensional mechanism, implying northwestward subduction of oceanic lithosphere (Boccaletti and Guazzone, 1974; Banda and Channell, 1979; Co- hen, 1980; Horvath and Berckhemer, 1982) to intra-continental rifting, related to large horizontal movements of crustal blocks and seafloor spreading (Auzende et al., 1973; Rehault et al., 1985) or the propagation of rifts from the Alpine foreland into and across the Alpine megasuture (Ziegler, 1988, 1990, 1992).

Pre-late Oligocene basin evolution

On the eastern part of the Iberian plate, Per- mian and Mesozoic series overlay, with a pro- found unconformity, Hercynian deformed Palaeozoic sediments and metamorphic rocks. Palaeozoic series outcrop in the Catalan Coastal Ranges, in the Iberian Chain, on the island of Menorca and in a few localities on the island of Mallorca.

Triassic strata are developed in Germanic fa- ties and consist of a basal continental elastic sequence that is overlain by carbonates and evaporites. Jurassic and Cretaceous series consist mainly of shallow marine carbonates. Rapid lateral thickness variations, partly accompanied by facies changes, indicate that these Mesozoic sequences accumulated in tensional basins. In such

basins up to 8 km of mainly late Jurassic and early Cretaceous sediments accumulated (Rota and Guimerh, 1992). Examples are the Cameros

Basin in the northwest of the Iberian Chain (Guiraud and Seguret, 1985), the Maestrat Basin, which is located near the Mediterranean coast within the Iberian Chain (Salas, 198711, and the offshore basins which are controlled by data of the petroleum industry (Fontbote et al., 1990; Rota and Guimerl, 1992) and by the results of the recent VALSIS-2 cruise (Maillard et al., 1992). The extensional character of these basins is docu- mented by the occurrence of syn-depositional normal faults (Salas, 1987; Casas-Sainz, 1990; Rota and Guimera, 1992) and associated alkaline volcanic rocks (Gomez et al., 1976; Alvaro et al., 1987; Lago et al., 1988). On the islands of Mal- lorca and Eivissa, most of the middle Jtrrassic-late Cretaceous series is developed in a slope and base of slope facies, reflecting their location along the Tethyan passive margin.

During the latest Cretaceous and Paleogene, compressional deformation of the Iberian plate resulted in the inversion of the Mesozoic rifted basins. Uplift of the Iberian chain involved the development of NW-SE and E-W striking thrust faults (Guimera, 1984; GuimerP and Alvaro, 1990) whereas the Catalan Coastal Ranges evolved in response to convergent wrench movements along a set of NE-SW striking faults (Ariadon et al., 1985). This phase of intraplate compression was also responsible for the uplift of the area that is now occupied by the offshore parts of the Valen- cia trough (Stoeckinger, 1976). During much of Paleocene to Oligocene times, this area was elevated above sea level whereby Mesozoic carbonates were karstified, thus leading to the development of porous reservoir rocks (Bouvier et al., 1990). In this elevated area, sedimenlation took place only locally (FonbotC et al., 1989; Ramos- Guerrero et al., 1989).

During the inversion of the Mesozoic extensional basins, throws along their normal faults were reversed, recovering part or all of their former hade, and in some instances even exceed- ing it (Casas-Sainz, 1990). For the area near Castellon, Rota and Guimera (19921 estimated that pre-Barremian crustal stretchingi amounted

IXh

to about 34%. Beneath the inverted Maestrat Basin, to the northwest of CastelIon (Fig. 2), the crust has now a thickness of 25-30 km; this is slightly thinner than the unstretched Hercynian crust of the surrounding lberian Massif. This suggests that during the Palaeogene inversion the Mesozoic crustal extension was not completely recovered.

Evolution of the Valencia trough

The sedimentary fill of the Valencia trough can be subdivided into four depositional sequences. These are separated by sequence boundaries corresponding in part to major uncon- formities (Fig. 3; Garcia-Sifieriz et al., 1979; Soler et al., 1983; Anaddn et al.. 1989; Johns et al.,

1989).

Lower Neogene

During the late Oligocene the tectonic setting of the Catalan Coastal Ranges, the southeastern

parts of the Iberian Charn and the area corrc- sponding to the future Valencia trough changed fundamentally. Surface geological data (Mois- senet, 1989) and reflection seismic data show that crustal extension commenced at this time in the western part of the Valencia trough, as evident by the development of a block-faulted relief (Fig. 4). Initially, alluvial elastics, containing some evaporites, lacustrine shales and scree deposits, derived from hanging-wall blocks, accumulated in fault- bounded, shallow basins; this series is covered by the early Miocene marine shales and carbonates of the Alcanar Group. This sequence, which reflects a rapid deepening of the basin, onlaps the block-faulted relief of the evolving Valencia rift (Fig. 3). Activity along tensional faults, accompanied by minor volcanism, ceased gradually during the Burdigalian (Stoeckinger, 1976; Johns et at.. 1989; Anadon et al, 1989; Bartrina et at., 1992). Close correlation between the different offshore and onshore lithostratigraphic units that were deposited during the rifting stage of the Valencia trough is difficult to establish. The synrift to early

ONSHORE OFFSHORE CATALAN-VMENCIAN SHELF

BALEARIC PROMONTORY

Fig. 3. Simplified chronostratigraphy across the Valencia trough from the Catalan Coastal Ranges to Mallorca. Mainly from Bartrina et al. (1992).


CATALAN-VALENCIAN DOMAP -z c BETtC-BALEARC DOMAIN NW-SE

CATALAN COASTAL MALLORCA

A RAN@ES A’ B B c

NW-SE m Upper Oligocene

Nsogene and Qurternary

A A m Eocme and LOWeI Oligomm

s.L B Mesozoic

m Bassment

NW-SE B

I:: ~;~_-&

lzl Cr~1acaous

Upper Jurassk _S.L.

Lower Jurassic

Fig. 4. Upper panel: cross-section of the Valencia trough from the Catalan Coastal Ranges to Mallorca. A-A’ and B-B’ indicate the cross-sections shown in the middle and lower panel. Middle panel: cross-section of the Catalan margin. Lower panel: cross-section through the Serra de Tramontana in Mallorca. S.L. = sea level. (From Rota and Guimera, 1992; Gelabert et al.,

1992.) For location see Fig. 6.

postrift sequence is thought to span Aquitanian- Langhian times, and probably extends downward into the Chattian, as indicated by the occurrence of mammals and charophyta in continental deposits (Agusti et al., 1985) and foraminifera in marine deposits (Soler et al., 1983; Bartrina et al., 1992) (Fig. 3).

Contemporaneous with this rifting event, northwest directed thrust systems developed in the Betic-Balearic domain, as evident on the Balearic Islands. This resulted in an individual- ization of the two flanks of the Valencia trough that is still evident today (Fig. 4). In Mallorca, Chattian to Aquitanian sediments, consisting of shallow-water carbonate and elastic rocks (Rodriguez-Perea, 1984; Anglada and Serra-Kiel, 1986), seal the most internal thrusts (S&bat, 1986). Burdigalian and Langhian series consist of

pelagic, calcareous turbidites, including olis- tostromes, which were deposited in a rapidly deepening foreland trough developing in front of the advancing nappe systems (Rodriguez-Perea, 1984; Ramos-Guerrero et al., 1989; Ferrus, 1990).

Middle Neogene

From Serravallian to early Messinian times (Soler et al., 1983; Agusti et al., 1985) deposition took place under a tectonically quiescent regime, characterized by post-rift thermal subsidence and deepening of the Valencia trough. This sequence, referred to as the Castellon Group, consists of a thick elastic wedge which prograded from the Spanish mainland into the Valencia trough. This sequence is made up of basinal clays which rest conformably on the Alcanar Group, foresetting

slope clays and silts and a vertically aggrading topset sequence of sands, silts and clays. This reflects the Tortonian high-stand in sea level (Johns et al., 1989; Garcia-Sifieriz et al., 1989). This sequence onlapped and progressively over- stepped the syn-rift structural relief that is up- held by Mesozoic strata and seals the thrusts of the Betic-Balearic fold belt (Fig. 4).

Serravallian to Tortonian shelf carbonates, capped by reefs, were deposited on the Balearic promontory under a tensional setting. As will be discussed later, this may be related to rifting in the Algerian and Alboran basins (Pomar et al., 1983a; Fornbs, 1987; Esteban, 1979/1980X

minor volcanic activity. However, similar cxten- sional faults are not evident in the axial parts of the Valencia trough that apparently continued to subside in response to thermal relaxation of the

lithosphere and its sedimentary loading. Quanti- tative subsidence analyses indicate that this pro- cess was disturbed by a renewed thermal impulse. This may be related to the resumption of crustal extension in the coastal areas, and possibly also by the buildup of compressional stresses causing accelerated subsidence as evident in the Gulf of Lions (Burrus, 1984; Burrus et al., 1987; Cloet- ingh and Kooi, 1992).

Magmatism Messinian

The Messinian sea-level drop, which is evident in the entire Mediterranean basin, lead to the development of a major, high relief unconfordy in the shallower parts of the Vabncia trough. However, in its central and northeastern parts a considerable thickness of halites and minor sui- phates accumulated (Mulder, 1973; Montadert et al., 1978). In the area of the Ebro delta, minor continental red beds were deposited in valley systems that were deeply incised into the Castel- Ion series (Stampfli and H(icker, 1989; Escutia and Maldonado, 1992).

Upper Neogene and Quaternary

The Ebro Group, comprising Pliocene and Quaternary sediments, attains thicknesses of up to 2.5 km. It records the Pliocene rise in sea level at the end of the Messinian salinity crises and the renewed progradation of deltaic systems from the Spanish coast into the Valencia trough. Gravita- tional growth faulting gives evidence for instabil- ity of the basinward prograding elastic wedge (Johns et al., 1989; Stampfli and H&ker, 1989; Nelson and Maldonado, 1989). On the Baicaric Promontory, deposition of shelf carbonates resumed during the Pliocene (Pomar et al., 1983a; Alonso et al., 1988).

Along the mainland coast, accumuh&ion of the Plio-Quatemary series was accompanied by a renewed, albeit mild phase of crustal extension and

Evidence for volcanic activity in the area of the Valencia trough comes from outcrops on the mainland and on the Balearic islands and from exploration wells chilled offshore and DSDP Site 123 (Ryan et al., 1972; Lanaja, 1987). The inven- tory of volcanic occurrences is supplemented by reflection seismic data (Ma&fret, 1977) and aero- magnetic data (Galdeano and Rossignol, 19771. The vast majority of high intensity magnetic anomalies correlate with areas for which seismic interpretations suggest the presence of volcanic buildups (Fig. 2; Maillard et al., 1992; Marti et al., 1992). The evolution of the Valencia trough is characterized by an .early Miocene and a late Miocene-Recent volcanic cycle; these are separated by a non-volcanic period (Marti et al., 1992).

The first cycle, which spans late Chattian-early Burdigalian times, is mainly characterized by talc-alkaline andesitic and silicic pyroclastic rocks. Such rocks have been sampled in outcrops on Mallorca (Alvaro et al., 1987) and at DSDP Site 123. They yield radiometric ages in the range 24-18.6 Ma (Ryan et al., 1972; Riviere et al., 1981; Mitjavila et al., 1990). Volcanic rocks en- countered in exploration wells have not yet been studied in detail; however, their stratigraphic position and petrographic characteristics appear to be similar. The first volcanic cycie is essentially coeval with the initial, main phase of rifting of the Valencia trough. Restoring the Corsica- Sardinia Block to its early Miocene position (see Ziegler, 19921, the Valencia trough volcanic


province aligns with the contemporary calc-al-

kaline province of Sardinia. Correspondingly, the first cycle of rifting and volcanism in the Valencia

trough may be interpreted as being related to a phase of back-arc extension above a northwest dipping subduction zone (Marti et al., 1992). This agrees with the talc-alkaline composition of ex- truded magmas.

The second volcanic cycle spans Tortonian to Recent times, as indicated by radiometric dates ranging between 10 and 0.3 Ma. Extrusive centres are clustered in the offshore of Caste116 in the area of the Columbretes islands (Parga Pondal, 1935; Alonso-Mantilla, 1985) and at the norteast- ern limit of the Valencia trough (Mauffret, 1977). They occur in onshore areas to the west of Valen- cia (Ancochea et al., 1984) and in the Catalan volcanic zone, which is located in the northeastern corner of the Iberian Penisula (Sole Sabaris, 1962; Araiia et al., 1983; Marti and Mallarach, 1987). The second volcanic cycle is characterized by poorly differentiated alkaline basalts. Their composition is compatible with intraplate crustal extension as evident by normal faulting during deposition of the late Neogene and Quaternary series. Geochemical differences between rocks from the different extrusive centres may be related to their position relative to #the zone of crustal extension (Marti et al., 1992).

Structural configuration and stress regime

The following discussion of the structural configuration of the Valencia trough addresses first the Catalan-Valencian domain, then the Betic- Balearic domain and lastly their interference zone along the axis of the basin.

The structure of the Catalan-Valencian do-

main is characterized by the presence of a complex system of extensional faults that affect the Mesozoic and Cenozoic sequences (Fig. 4; Rota and Guimera, 1992). In the north, faults strike ENE-WSW and thus parallel the compressive structures of the Betic-Balearic domain; in the south, the faults trend N-S and interfere with the Betic-Balearic structures (Fig. 2). Major faults occur mainly in the northwestern parts of this domain that is characterized by a well developed

system of horsts and grabens (Bartrina et al., 1992). The western limit of the area affected by Cenozoic tensional tectonics is well defined in the

northern parts of the basin, where an en-echelon array of fault systems constitutes the master faults of the Catalan Coastal Ranges (Anadon et al., 1985). To the south, the basin margin is not as well defined and faulting reaches further inland.

Offshore, detailed seismic control indicates the presence of NW-SE structures that have been interpreted as transfer zones (Maillard et al., 1992).

Age and duration of the late Oligocene-early Burdigalian rifting phase is stratigraphically well constrained (Stoeckinger, 1976; Soler et al., 1983; Johns et al., 1989; Moissenet; 1989; Bartrina et

al., 1992). The post-rift stage, commencing during the late Burdigalian, persists till the present, though late Miocene reactivation of rifting is evident by the resumption of volcanic activity and latest Miocene-Pleistocene faulting, particularly in coastal and near-shore areas. Crustal stretching from the Betic-Balearic front to the most landward faults, as derived from fault offsets, is estimated to amount to about 35 km (Rota and Guimera, 1992).

Palaeostress analyses carried out along the Iberian margin are in good agreement with its macrostructure. During the Paleogene and Neo- gene, the maximum horizontal stress (a., or ~~1 is roughly N-S oriented whereas the direction of minimum stress is E-W, according to the convergence of Africa and Europe (FonbotC el al., 1985; Guimera, 1988; Simon Gomez, 1989; Guimera and Alvaro, 1990). This condition persisted, presumably with little variation, till the present as indicated by the few reliable focal mechanisms available from the Pyrenean area and the Valen- cia trough, which indicate a N-S compression (Olivera et al., 1992).

Paleostress data obtained from Neogene rocks in the Valles-Penedes graben, located to the northwest of Barcelona, show a persistently E-W to NW-SE oriented horizontal (TV, while the maximum horizontal stress can be either U, or u2 (Guimera, 1988; Bartrina et al., 1992). At meso- scopic scale, episodic variations in stress field from extensional to strike-slip regime :are minor

compared with the constant (r3; they could be

related to northeast propagation of stresses ema-

nating from the Betic-Balearic orogen (Bartrina

et al., 1992).

The structure of the Betic-Bafearic domain corresponds to a west-northwest verging fold and thrust belt (Fig. 4) that was active during late Oligocene to mid-Miocene times (Fallot, 1922: Darder, 1925; Pierson d’Autrey, 1987; Ott d’Estevou et al., 1988; Ramos-Guerrero et al., 1989). Its evolution is thus contemporaneous with the main rifting stage of the Catalan-Valencian domain. The external part of the Betic-Balearic fold belt is thin skinned with thrusts soling out in the evaporitic Late Triassic Keuper series (Fig. 4). However, on Mallorca, Late Palaeozoic rocks are also involved in thrust sheets (Ramos-Guer- rero and Rodriguez-Perea, 1985; Ramos-Guer- rero et al., 1989; Gelabert et al., 1992).

Superimposed on this fold belt, an extensional system developed that is characterized by listric normal faults, dipping southeast; these sole out at the same level as the thrust faults (Rota and Guimera, 1992). The phase of extension began in the Balearic Islands at the transition from the Langhian to the Serravallian (Pomar et al., 1983a), whereas in the eastern Betics it started during the Tortonian. Although development of this extensional system slightly preceded the second rifting stage of the Catalan-Valencian area, it is essentially coeval with it.

Palaeostress fields, inferred from mesostruc- tures, are consistent with the above described evolution of the Betic-Balearic domain (Rota and Verges, 1989; Gelabert et al., 1992). The stress regime changed during the Neogene from WNW-ESE to E-W compression, via a dextral strike-slip regime, to NW-SE extension (Pomar et al., 1983b; FontbotC et al., 1989). The position of the Betic-Balearic domain within the framework of the Betic-Maghrebian arc, which is dominated by E-W material transport (Ziegler, 1988; Garcia-Duefias et al., 19921, implies the development of important E-W oriented dextral faults during the emplacement of thrust sheets.

The interference zone between the Catalan- Valencian domain and the Betic-Balearic fold and thrust belt can be studied in different basins.

The extensional basins of the Catalan-Valcncian

domain display an orientation (N-S) practically

perpendicular to the compressional structures of

.he Bctic-Balearic domain. Offshore. extensional

and compressional structures become parallel and therefore interference structures arc not found. The limit between the Catalan-Valencian and Betic-Balearic domains can be recognized west of Eivissa, while to the north its location remains uncertain.

The Alcoi basin, located in the external parts of the Betic fold belt to the south of Valencia, subsided during the Aquitanian-early Burdi- galian along a set of NNW-SSE trending extensional faults when Betic folding had already started to affect areas located further south. Dur- ing the middle Miocene, the extensional structures of the Alcoi basin were already overprinted by the Betic folding and thrusting whereby movement on some of the early Miocene extensional faults was completely reversed (Pierson d’Autrey, 1987).

Paleostress data from the study of mesostruc- tures are coherent with the evolution described above for the Alcoi basin. In the Aquitanian- Burdigalian, a3 had an ENE-WSW orientation while U, was NNW-SSE oriented. Later on, during the middle Miocene, 97 became vertical while cr, remained NNW-SSE (Pierson d’Autrey, 1987).

Paleomryprctie results

Paleomagnetic investigations covering parts of the Catalan Coastal Ranges and the Balearic islands were carried out during the last decades and results have been recently summarized by Pares et al. (1992). Paleomagnetic results from the Balearic islands differ from those of the Cata- lan Coastal Ranges and Sardinia. These authors conclude that early-middle Miocene clockwise rotations of about 20” in the islands of Mallorca and Menorca relative to the Catalan Coastal Ranges are associated with the emplacement of the Betic-Balearic thrust sheets. A late Mio- cene-Pliocene further rotation of up to 20”, as described by Par& et al. (19921, may be reiated to displacements along extensional faults. However, it is not excluded that part of the total amount of

rotation is due to whole lithospheric movements. Parts et al. (1992) point out that clockwise

rotations in the Balearic Islands are in disagree-

ment with most of the proposed models of the western Mediterranean evolution and the total amount is too large to be accounted for by crustal extension across the Valencia trough.

Crustal structure

The crustal configuration of the Valencia trough is summarized in the cross-section given in Fig. 5 and the crustal thickness map given in Fig. 6. The cross-section is based on one of the three refraction/wide angle reflection lines that cross the basin (Dafiobeitia et al., 1992). The map, however, integrates the results of these lines with earlier data, including gravity data (Banda et al., 1980; Zeyen et al., 1985; Torn6 and Banda, 1988; Watts et al., 1990). From these figures it is evident that the crust/mantle boundary rises gradually from a depth of 25-30 km under the Iberian mainland and Catalan margin to 12-13 km under the axial parts of the Valencia trough, and then descends more rapidly to 20-25 km under the Balearic Promontory.

The velocity/depth distribution observed in Fig. 5 is characteristic for attenuated continental crust, that is overlain by a variable sedimentary cover. Sedimentary velocities vary from 2.2 km/s for Cenozoic elastic series to close to 6 km/s for Mesozoic carbonates. The thickness of sediments is highly variable and ranges from 1 km to more

NW

Iberia

193

than 6 km, for instance in the Valencia Basin. For a depth map of the base of the Tertiary unconformity the reader is referred to Maillard et

al. (1992). Velocities of the Hercynian basement, out-

cropping in the Catalan Coastal Ranges, and of the upper crust down to a depth of about 12 km are of the order of 6.0-6.2 km/s, both under the basin margins and its axis (Banda et al., 1980; Gallart et al., 1990). Reflection seismic data indicate that the upper crust is non-reflective (TornC et al., 1992). The continent/ocean transition zone between the Valencia trough and the Proveqal Basin is poorly controlled and is only seen on the expanding spread profile 2 of Pascal et al. (19921, shot about 120 km north of Mallorca. The lower crust is characterized by average velocities of 6.4 km/s and varies from l-2 km thickness under the axial parts of the basin to lo-12 km under the margins of the Valencia trough (Dafiobeitia et al., 1992). Expanding spread profiles, combined with near vertical reflection data, indicate lower crustal velocities close to 6.6 km/s, except in the centre of the trough where 6.4 km/s material overlies the upper mantle (Pascal et al., 1992; Tome et al., 1992). In the southern part of the Valencia trough, which is characterized by an anomalously high heat flow, the lower crust appears to be characterized by a fairly stirong velocity gradient increasing from 6.4 to 6.8 km/s (Dafiobeitia et al.. 1992; Pascal et al., 1992). Re- flection seismic data show that the lower crust is distinctly layered beneath the Catalan-Valencian

SE

401 I I 1 I 0 100 200 300

MST- (km) Fig. 5. Representative cross-section of crustal structure following the results of Daiiobeitia et al. (1992) betweeb the Catalan Coastal Ranges and Mallorca. Velocities are VP in km/s. Shaded area indicates a strong velocity gradient. For location see Fig. 6.

42

41

40

3!l

t 42 N

1w 0 1 2 3 4 5E

Fig. 6. Thickness of the crust in the area of the Valencia trough. Mainly from Banda et al. (1980), Daiiobeitia et al. (1992), Gallart et al. (1990), To& et al. (1992). Pascal et al. (1992) and &yen et al. (1985). Lines indicate the location of cross suctions 8ho~11 in

Fig. 4 (dashed) and Fig. 5 (solid).

domain (Watts et al., 1990) and beneath the Mallorca platform. The reflective lower crust thins towards the trough axis where it appears to be either missing or to be reduced to a l-2 km thick layer (Dafiobeitia et al., 1992; Tome et al., 1992).

In the area of the Valencia trough the crust/ mantle boundary is well defined by seismic data. There is good correspondence between seismic and gravity data (Torn6 and Banda, 1988; Watts et al., 1990; TornC et al., 1992) which both have been used in the construction of the regional Moho depth map given in Fig. 6. In the axial parts of the Valencia trough the crystalline crust has been thinned to 9-10 km and is overlain by 3 km thick sediments. Possibly an even thinner crust (3-6 km?) may be present in the southern, axial parts of the trough were the Moho is well defined by reflection seismic data. The crustal thicknesses increase more rapidly toward the Balearic Promontory than toward the Spanish mainland.

Within the area of the Valencia trough, velocities of the subcrustal mantle range from 7.7 to 7.9

whereas under the lberian Peninsula 8.1 velocities are observed. Beneath its axial upper mantle velocities are in the range

7.7-7.8 km/s; towards the mainland velocities increase to 7.9 km/s whereas towards the Balearic margin they remain in the 7.7-7.8 km/s range (Banda et al., 1980, 1983; Pascal et al., 1992). Gravity modelling suggests an upper mantk den- sity of 3200 kg/m3.

Heat flow

Until recently studies on the thermal regime of the Valencia trough were limited to the determi- nation of temperature gradients in onshore water wells and offshore exploration wells. The results indicated large variations in temperature gradients, which presumably are related to water circu- lation (Femandez and Banda, 1989; Banda et al., 1991).

In the Ebro Basin, located 50 km to the west of Barcelona, a heat flow of 35-80 mW/m’ has been estimated on the basis of temperature gradi-


42

41

40

39

0 50 100 km

IW 0 I 2 3 4

51 3

‘42 N

.41

-40

‘39

I

SE

Fig. 7. Heat flow in the Valencia trough area. Heat flow values offshore correspond to transects of Foucher et al. (1992). The heat flow value onshore corresponds to the Osona basin (Fernkdez et al., 1989). Heat flow values on the islands of Mallorca and

Menorca are from Fern&de2 and Cabal (1992).

ent mapping and indirect conductivity measurements (Fig. 7; Fernandez et al., 1989). During the VALSIS cruise, 109 heat flow measurements were made along four profiles crossing the Valencia trough (Foucher et al., 1992). The results, assuming no blanketing effects of sediments, are summarized in Fig. 7. This figure illustrates that heat flow in the axial zone of the trough varies from 88 mW/m* in the southwest to 65 mW/m* at its transition to the Provensal Basin. Heat flow values decrease towards Eivissa, but vary considerably on Mallorca and Menorca (Fernandez and Cabal, 1992).

Vertical movements

Backstripped subidence curves of wells located in the Catalan-Valencian domain (wells 299, 419, 466 and 473) and in the Balearic-Betic domain (well 404) give evidence for exponential subsidence of both areas (Fig. 8). It is difficult, however, to separate the late Oligocene-early

Miocene syn-rift from the postrift subsidence as already pointed out by Watts et al. (1990), Bart- rina et al. (1992) and Rota and Desegaulx (1992). Analyzing the spatial distribution of stretching by flexurally backstripping a cross-regional transect (Fig. S), it is observed that the trough is associated with a broad region of subsidence slightly asymmetric towards the Balearic margin and tectonic uplift in its flanks. As shown in Fig. 8, the total tectonic subsidence increases gradually from the mainland margin to the center of the basin and reaches a second peak near the Balearic islands. This regional trend, however, is superimposed to a more local short wave-length compo- nent that could be attributed either to tilted fault blocks or volcanic activity. The observed assym- metry is also recognized by Rota and Desegaulx (1992). These authors observed that the Miocene synrift subsidence phase is followed by a slower, more uniform postrift subsidence phase.

The total tectonic subsidence along the transect varies from 1.4-1.6 km beneath tlhe Catalan margin to about 3 km at the centre of the basin.

The observed tectonic subsidence has been cx-

plained by Watts et al. (1990) and Watts and Torn6 (in press) using a non-uniform finite rifting model spanning g-16 Ma. The model gives stretching factors that increase from about 1.4 beneath the Catalan margin to about 2.8 at the centre of the basin. Although this model explains the observed tectonic subsidence along the transect there are difficulties in applying the model to other parts of the trough. The problem is particularly acute in the southern part of the trough where the observed heat flow values are consis- tently higher and the tectonic subsidence less than expected (Watts and Torn& in press).

A major problem, however, arises when trying to fit the observed topography along the Catalan

margin. In the onshore parts of the Valcnci;~

trough, Palaeogene fan-delta and shelf scdimcnts occurring in the Catalan Coastal Kangcs ;ue located at an elevation of 1200 m above sea level (Anadon et al., 19SS). Within the limits 01’ the city of Barcelona, Serravallian and Tortonian sediments occur at 190 m above sea level. South of Barcelona early Pliocene terrigenous, littoral sediments are located 120 m above sea level (Hartrina ct al., 1992). Uplift of these areas may be attributed to an edge effect of crustal thinning along the mainland margin of the Valencia trough (Morgan and Fernandez, 1992) or to a broad region of mantle stretching which extends beyond the region of crustal stretching (Watts and Torn& in press). Alternatively, it may be related to the

41

41’

40

39

-24 -21 -26 .I6 .16 -14 -11 -16 4 4 4 .a 0

Age mm

-m 1 catalMcoaslal Wench ‘Rwgh Mallorca f

woJ . 1 -150 -100 -50 0 SO 100

NW Diatmce (ktll) SE

Fig. 8. Subsidence and uplift of the Valencia trough. Upper left panel: location of the ba&st&xd wells (tria@es) and seismic line

821 (thick line). Lower left panel: tectonic subsidence based on backstripping of wells 299,419,466 and 473 (thin and dashed lines)

and 404 (thick line). Dashed and thin lines sbow the upper and lower limits of the tectonic subsidence considering uncertainties in

paltobathymetry. Upper right panel shows the observed stratigraphy based on line 821. Lower right panel: tectonic subsi&nce/up-

lift obtained by flexurally backstripping of the sedimentary units (modified from Watts and Tomt, 1992). M.I. = Ma&ma Island.

I. P. = lberian Peninsula. U/M = Upper/Middle. L/M - Lower/Middle. T, = Elastic thickness.

VALENCIATROUGH(WESTERNMEDITERRANEAN) 197

thermal perturbation of the lithosphere in conjunction with Plio-Pleistocene resumption of crustal extension and volcanic activity.

On the islands of Mallorca and Menorca, late Miocene and Pliocene marine deposits occur at 200 and 150 m above sea level, respectively (Bourrouilh and Mange, 1963; Cuerda et al., 1969; Pomar, 1979). Mechanisms of this uplift are uncertain and could be either thermal in nature or related to footwall uplift in conjunction with late Miocene rifting in the Algerian basin (Rota and

Desegaulx, 1992).

Discussion

The Valencia trough corresponds to a zone of strongly attenuated continental crust that is un- derlain by an anomalous low velocity upper mantle. There is no information on the configuration of the asthenosphere/lithosphere boundary in this basin that formed in response to late Oligocene-early Miocene crustal extension, followed by a period of tectonic quiescence and the resumption of rifting activity, accompanied by volcanism during late Miocene to recent times.

The continental crust underlying the Valencia trough was consolidated during the Hercynian orogeny. During the Mesozoic breakup of Pangea, leading to the middle Cretaceous isolation of the Iberian plate, this crust underwent repeated extensional events, controlling the subsidence of Mesozoic graben structures. During the late Cre- taceous and Paleogene suturing of Iberia to the southern margin of Europe, intraplate compressional stresses resulted in the inversion of the Mesozoic Iberian Chain, the Catalan Coastal Ranges and their offshore equivalents, whereby crustal thinning achieved during their Mesozoic subsidence was apparently largely recovered.

A special feature of the Valencia trough is that its late Oligocene to early Miocene main rifting stage is contemporaneous with significant crustal shortening in the Betic-Balearic fold belt, forming its southeastern margin, Crustal extension in the Valencia trough ceased during the late folding phases of the Betic-Balearic fold belt, possibly in response to the buildup of foreland compressional stresses. Resumption of crustal exten-

sion in the Valencia trough coincides with the

termination of compressional deformation in the Betic-Balearic fold belt and the onset of its ten-

sional partial collapse. The amount of late Miocene to Recent crustal stretching across the Valencia trough is small compared with that of the late Oligocene-early Miocene rifting event.

Total upper crustal extension, derived from geological observations, is estimated to amount to some 35 km across the Valencia trough. Crustal thinning, involving an almost complete attenua- tion of the lower crust under the axial parts of the basin suggests a significantly greater amount of crustal extension. However, recent magmatism and a possible young age of the Moho make it difficult to calculate the actual amount of extension. In addition, on the Balearic margin, the Early Neogene extension was obliterated by compression.

Basin modelling is quite successful in imaging the northern part of the basin, but fails for its southern part even when assuming a variety of initial lithospheric conditions. The principal rea- son for this is the observed inverse relationship between heat flow and depth of the seafloor. Although superimposing a recent rifting event on an earlier one, and incorporating melting according to McKenzie and Bickle (19881, this misfit is reduced, a more complex crustal deformation pattern must be invoked than advocated by the uniform stretching model (Foucher et al., 1992). The anomalous upper mantle velocity observed under the Valencia trough could be explained by the presence of partial melt which may have interacted with the lower crust, causing an up- ward displacement of the geophysically defined crust/mantle boundary (Moretti and Pinet, 1987; Ziegler, 1992). In these terms, the anomalous upper mantle may have to be regarded as ‘anomalous lower crust and upper mantle’, thus modifying our understanding of the seismic Moho. In this respect, however, the presence of a seismi- cally laminated lower crust, practically restricted to the Catalan-Valencian domain, does not seem to be related to the Neogene extension. It may possibly be related to the lamination found in Iberia, in particular in the Ebro basin (Watts et al., 1990; ECORS Pyrenees Team, 19881.

I 0s

C’lcarly, models for the Valencia trough arc still far from satisfactory and further detailed studies are required to unravel the complexity of its structural evolution. The development of the Catalan-Valencian domain is reasonably under- stood and appears to be closely related with the evolution of the Provenc;al Basin. In contrast, the Balearic Promontory has been the site of a suc- cession of tectonic events, some of which probably obliterated the record of preceding ones. In this context it is particularly relevant to obtain a better understanding of the nature and location of the boundary between the Catalan-Valencian and the Betic-Balearic domains. Moreover, the accommodation of left lateral movements along NE-SW trending faults in the eastern Betics may be of particular relevance for the southern part of the Valencia trough.

Acknowledgements

We are grateful to Peter Ziegler who not only triggered our work but also spent much of his time in revising our original manuscript. We have benefitted from ideas and results published in a special issue of Tectonophysics (Vol. 203) on the Valencia trough. Montserrat Tom6 was helpful in revising an earlier version of the manuscript and providing Fig. 8. This work has been partially financed by the CICYT project PB87-0362.

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