The Neogene structure of the eastern Iberian margin: Structural constraints on the crustal evolution of the Valencia trough (western Mediterranean)

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283

The Neogene structure of the eastern Iberian margin: structural constraints on the crustal evolution

of the Valencia trough (western Mediterranean)

Eduard Rota and Joan Guimer2 Llgartament de Geologia Dinrlmica, Geofwica i Paleontologia, Uniuersitat a2 Barcelona, 080714larceIonq Spain

(Received October 151990; revised version accepted March 22,1991)

ABSTRACT

Rota, E. and Guimerl, J., 1992. The Neogene structure of the eastern Iberian margin: structural constraints on the crustal evolution of the Valencia trough (western Mediterranean). In: E. Banda and P. Santanach (Editors), Geology and Geophysics of the Valencia Trough, Western Mediterranean. Tectonophysics, 203: 203-218.

The presence of a thinned crust beneath the Vafencia trough has been recognized for a long time. However, to date no attempt to link this thinning, inferred from geophysical data, with the superficial structures has been made. A quantification of the Neogerte extensional processes in the erustat evolution of the Valencia trough is presented. As a ~ns~uen~ of the complexity of the Betie-Batearic domain, the analysis has been done for the Catalan-Va~encian domain fnorthwestern areas of the trough). Crustal and superficial data in this area show that Neogene extension was accommodated by a listric normal fault system detached in a deep upper crustal level (13-15 km) in the marginal zone of the domain. Quantitative analysis of the Neogene structure shows the incohemnce between the upper crustal thinning inferred from the geometric analysis of the superficial structures (@ factor 1.4-1.5) and from geophysical data (1.8). Although the latter value agrees with that deduced from the analysis of Neogene subsidence, this incoherence suggests that the present thinned crust would be partially inherited from the Mesozoic extension. In this way, Mesozoic subsidence analysis and the recognized Mesozoic structure show that the Mesozoic extension was almost as important as the Neogene one.

lntfodu~ian

The Valencia trough is located in the western Mediterranean between the Iberian Peninsula and the Ralearic Islands (Fig. 1). Since the seven- ties, the presence of a thinned crust beneath the Valeneia trough has been recognized from deep seismic reflection data (Hint, 1972, Martin and Surifiach, 1988; Gallart et al., 1990; Watts et al,, 1990; Daiiobeitia et al., 1992-this volume), gravity field (Morelli et al., 1975; Haxby, 1983) and heat flow measurements (Albert, 1979; FernGndez et al., 1990, Foucher et al., 1992-this volume). Nev- ertheless, to date no attempt has been made to

Correspo&ence to: E. Roea, Departament de Geologia Dinsmica, Geoflsba i Paleontologia, Universitat de Barcelona, 08071-Barcelona, Spain.

link this thinning with the superficial structure. The vast majority of previous papers has explained the present crustal features with one Neogene extensive phase, applying a uniform pure-shear model (Vegas et al., 1980; Diaz de1 Rio, 1986; Watts et al., 1990). This simplistic model, however, does not take into account the Mesozoic extension and the surface structural data fsee Font~t~ et al,, 1989, 1990 for references>. In fact, the Neogene evolution of the trough is the result of superposition and interac- tion between extensional and compressional processes.

In this paper, we attempt to quantify the contribution of the Neogene extensional processes to the crustal evolution of the Vale&a trough. A geometrical analysis of Neogene superficial deformation has been carried out in order to de- duce the Neogene extensional model and the amount of upper crustal thinning. Later on, these

8@m-1951/92/$%%0 6 1992 - Elsevier Science Publishers B.V. Ail rights reserved

204 t. ROCA AND J. CjUlMERk

inferred values are compared with those deduced from geophysical and subsidence data. Moreover, a first attempt of quantification of the pre- Neogene processes has been compiled to deter- mine their possible contribution to the present crustal thinning.

It is important to remark that these quantifica- tions are only valid for the northwestern part of the Valencia trough (Catalan-Valencian domain), since the southeastern parts (Betic-Balearic domain) were affected after these extensions by the Betic compressive and extensive structures (Fontbote et al., 1990).

i

fJg!Jj 2,

ISERIAN mz ,/

PENINSULA 0 3 <-.._ @yJ4

I

Geological setting

The eastern Iberian margin is characterized by the superposition of several Cenozoic structural units developed on a Hercynian basement and a sedimentary cover made up of deposits ranging from Permian to Upper Oligocene.

Unconformably overlying the Hercynian basement and the Permian rocks, the Mesozoic series are fairly complete. They include a Triassic made up of terrigenous siliciclastic, carbonate and evaporitic deposits, and a Jurassic and Creta- ceous characterized by shallow-marine carbon-

.5

6 7

1 ‘.i~~:i:~:, 1 i ,,‘- .::i”““::::.:.:+:..

..::’ ::.:A’ .::::..

5 f (: i

x::: J : ::i:y ’ /’ . . .

L _:t. I ’ ’ ’ ,’ ,’ ,’ IP i

0, 90 100 km 1

1’ 2”

Fig. 1. Simplified geological map of the emerged areas of the Valencia trough: 1 = onshore Neogene deposits; 2 = Ebro basin infilling (Palaeogene-Miocene); 3 = Mesozoic; 4 = Palaeozoic; 5 = Palaeogene thrusts and faults; 6 = Neogene thrusts and faults;

7 = bathymetry (after Canals et al., 1982).

THE NEOGENE STRUCTURE OF THE EASTERN IBERIAN MARGIN 205

ates. The thickness of these deposits is very vari- able, generally depending on Late Jurassic-Early Cretaceous palaeogeography. While in most areas of eastern Iberia the Mesozoic cover is about 2.5-3.5 km thick, in individual basins where Late Jurassic to Early Cretaceous deposits are well developed it is up to 8 km thick: Los Cameros (NW Iberian chain), Maestrat and Columbrets offshore basins (Fig. 2; Guiraud and SCguret, 1985; Salas, 1987; Rota and Desegaulx, 1992).

This sedimentation took place in extensional intracontinental basins (Alvaro et al., 1979), as corroborated by the presence of synsedimentary normal faults (Salas, 1987; Casas Sainz, 1990), alkali volcanism (Triassic and Lias-Dogger in age: Gomez et al., 1976; Alvaro et al., 1979; Lago et al., 19881, and low-grade metamorphism (in Los Cameros basin: Guiraud and SCguret, 1985).

During the Palaeogene and earliest Miocene, a compressive event took place, giving rise to the Iberian chain thrust belt and the Catalan coastal chain strike-slip belt (Guimera, 1984; Guimerl and Alvaro, 1990), separated from the Pyrenees by the Ebro basin. NW-SE- and E-W-striking

Fig. 2. Main Late Jurassic-Early Cretaceous basins of eastern Iberia. I = Los Cameros basin; 2 = Maestrat basin; 3=

Columbrets basin. I = areas of no deposition or containing less than 500 m thick Late Jurassic-Early Cretaceous rocks; II = areas containing more than 500 m thick Late Jurassic- Early Cretaceous rocks; III = areas where Late Jurassic-Early Cretaceous rocks are supposed to be more than 500 m thick; Iv= areas where Late Jurassic-Early Cretaceous rocks had been eroded but they are supposed to be originally more than

500 m thick. B = Barcelona, V = Valencia, Z = Saragossa.

thrust systems in the Iberian chain and NE-SW sinistral convergent strike-slip faults in the Cata-

lan coastal chain developed by tectonic inversion of Mesozoic normal faults, affecting both the Hercynian basement and the Mesozoic cover. NE-SW shortening produced by thrusting in the Iberian chain is at least 40 km (Guimera and Alvaro, 1990). This tectonic inversion gave way in most cases to the recovery of the Mesozoic normal slip; e.g., the E-W thrust-bounding Los Cameros basin in the north, which has a thrusting displacement of 30 km, just inverting the Meso- zoic normal slip (Casas Sainz, 1990).

During the Miocene, both extension and compression developed in the eastern Iberian plate. Neogene sedimentation took place in areas containing thick Mesozoic rocks, in the southwest and the marginal northwest areas of the trough (Fig. 2). Two domains can be distinguished (Fontbote et al., 1990):

(a) The Catalan-Vulenciun domain, which includes the northwestern half of the Valencia trough and the easternmost Iberian Peninsula (Fig. 3). It is h c aracterized by extensional tectonics that, from Late Oligocene or Early Miocene, persisted during the whole Neogene. The generated normal faults overprinted most of the former compressional structures and resulted (at least in the Catalan coastal range) from the inversion of the strike-slip Palaeogene faults. For in- stance, the Neogene Vallbs-Penedb normal fault overprinted the Palaeogene transpressive strike- slip fault, which produced NW-directed basement slices (Fontbote, 1954; Anadon et al., 1985).

(b) The Befic-Buleuric domain is located southeast of the former domain. It includes the southeastern half of the Valencia trough, the Balearic Islands and the onshore Eastern Betics. Its structure consists of the superposition of two main Neogene deformational events (FontbotC et al., 1990). (1) A west-northwest vergent fold-and- thrust belt mainly Early-Middle Miocene in age (Pierson d’Autrey, 1987; Ott d’Estevou et al., 1988; Ramos-Guerrero et al., 1989). Its sole thrust is located within the upper Palaeozoic rocks on Mallorca (Ramos-Guerrero et al., 1989; Gelabert et al., 1992-this volume) and in the Upper Trias- sic (Keuper) evaporitic beds in the Prebetic areas

206 b.. ROC‘A ANI) .I (;lJIM!3Ki

THE NEOGENE STRUCTURE OF THE EASTERN IBERIAN MARGIN

c - C’

+ NNE SSW-+NNW SSE-+

207

a b c :alb 1 m NEOGENE

,000 PALAEOGENE

UPPER

cl CRETACEOUS LOWER

I

Fig. 4. Balanced cross-section through the frontal Eastern Betics showing the superposition of the extensive structures over the Betic compressive ones. This post-compressive extension generates the reactivation of some Betic thrusts as normal faults (a) and

the development of diapiric structures (b). See Fig. 3 for location.

(Figs. 3,4). (2) An extensional system, developed from the Langhian-Serravallian transition on- ward (in the Balearic Islands: Pomar et al., 1983; Alvaro et al., 1984) and Tortonian (in the Eastern Betics: Bousquet, 1979; Santisteban et al., 1987), characterized by normal SE-dipping listric faults whose sole fault coincides with the former sole thrust. This is inferred from the fact that most major faults were the result of the inversion of previous Betic thrusts (Fig. 4). The extensional situation was also reflected by the development in the external Eastern Betics of a diapirism of Keuper evaporitic rocks (Fig. 4). Both the normal faulting and the diapirism gave rise to small basins (e.g., Bicorb-Quesa and Central Mallorca basins), Middle to Late Miocene in age (Pomar et al., 1983; Moissenet, 1985).

Neogene extensional structures of the Catalan- Valencian domain

The Neogene structure in this domain is characterized by the development of a complex extensional fault system in which the orientation of main normal faults varies from ENE-WSW in the north to NNE-SSW to N-S in the south (Fig. 3). Major faults are located in the western bound-

ary of the domain, which is characterized by a widespread horst and graben system. In the central areas, extensional structure is characterized by less important faulting and wide troughs.

The western boundary of the extensional area is defined in the northern areas of the domain by an array of faults, located near the coast, that displays an “en echelon” disposition: the Vallbs- Penedes, El Camp and Baix Ebre faults; in the southern areas, this boundary is less defined by the faults limiting the Teruel graben, farther into the continent (Fig. 3).

Extension in the Valencia trough probably started in the latest Oligocene or Early Miocene (Stoeckinger, 1976; Soler et al., 1983; Moissenet, 1989; Bartrina et al., 1992-this volume). From geological data and subsidence analysis (Daiiobe- itia et al., 1990; Rota et al., 1990; Bartrina et al., 1992-this volume), two main stages can be distinguished in the evolution of the Catalan-Valen- cian domain: (1) a synrift stage, Late Oligocene- Burdigalian in age, and (2) a postrift stage which lasts up to the Present (Diaz de1 Rio et al., 1986) and was characterized by weak tectonic activity.

In order to recognize the main features of the Catalan-Valencian domain, two geological cross- sections across the northwestern Valencia trough

and the Iberian margin have been made up from the interpretation of offshore seismic reflection profiles and field data collected onshore (Fig. 5). The Moho and upper/lower crust depths are constructed from seismic refraction and reflection data complemented by gravimetric modelling (Hinz, 1972; Zeyen et al., 1985; Martin and Suriiiach, 1988; Choukroune and ECORS team, 1989; Watts et al., 1990; Pascal et al., 1992-this volume; Desegaulx and Rota, 1992).

Northern section A-A’

This NW-SE section crosses the entire Cata- lan-Valencian domain, from the Ebro basin to the Valencia trough axis (Fig. 5). Located far from the known limits of the Betic units, the A-A’ cross-section shows the well preserved Neo- gene extensional structure of the Valencia trough.

A-A’

-NW MARGINAL AREAS

j”P.G ED-1 BE-1

I I (proi) (py3l.)

Two well differentiated zones are observed: (1) A northwestern marginal zone, located

around the margin of the continent where the main superficial extension in the cross-section is concentrated. It is characterized by a SE-dipping normal fault system with kilometric slip; i.e., the Valles-Penedes fault (western boundary of the extended area, with 4 km of normal slip; Bartrina et al., 1992-this volume) and the Barcelona fault (with 6 km of normal slip). A large-scale Oligocene-Early Miocene roll-over structure, observed in the hanging wall of the Barcelona fault (Fig. 6), demonstrates the listric geometry of these master faults. The Barcelona fault also forms the boundary of a thick Mesozoic cover-mainly Lower Cretaceous-present beneath the Barce- lona graben (Lanaja, 1987). This and the lack of Jurassic and Cretaceous sedimentation in the emerged area (Anadon et al., 19791, on the foot-

SE-

CENTRAL AREAS

B-B’ SE-

Fig. 5. Regional depth sections through the Catalan-Valencian domain (A-A’ and B-B’ in Fig. 3): 2 = lower crust; 2 = upper

crust; 3 = Mesozoic-Palaeogene cover; 4 = Ebro basin infilling; 5 = Neogene sediments; 6 = Neogene volcanics; 7 = water;

8 = main Neogene normal faults. RMG. = Rubielos de Mora graben; T.G. = Teruel graben; W?G. = Vall&s-Penedbs graben.

Location of section I-I’ in Fig. 8 is shown.

THE NEOGENE STRUCTURE OF THE EASTERN IBERIAN MARGIN

wall of this fault, show its intra-Mesozoic motion.

(2) A southeastern central zone with a thicker Mesozoic cover (up to 4 km) slightly extended by Neogene NW-dipping normal faults. Volcanic rocks, supposed to be Late Miocene to Pleis- tocene in age (Mauffret, 1976) are present in the southeastern edge of the section.

The detachment depth of these faults can be deduced from the geometry of roll-over structures and using geometrical techniques. Applying a modified Chevron construction (Williams and Vann, 19871, a detachment of about 13-15 km

BC-1 n

209

has been predicted in all large Neogene faults.

This suggests that the faults sole out in a basal detachment. The Vallbs-Penedb fault is the northwestern emersion of this sole fault.

Southern section B-B’

This section runs across the southern part of the Catalan-Valencian domain, from the trough axis, close to the Betic frontal structures, to the

Teruel graben system (Fig. 3). The eastern areas of the section are affected by the Betic structures.

-SE

BC-1

6

Fig. 6. Unmigrated conventional reflection seismic line on the Barcelona graben. The graben is located close to a roil-over structure generated as a result of the normal motion of its NW bounding basin fault. I = Upper Oligocene(?)-Lower Miocene; II = Lower

Miocene-Upper Miocene; ZZI = Pliocene-Present. For location, see Fig. 3.

210 I:. ROCA AND J GlIIM~R/i

Thus, a weak crustal bending is recognized in of kilometric-slip normal faults to such an extent these areas (Rota and Desegaulx, 1992). that it becomes difficult to differentiate a marginal

This cross-section shows the existence of a and a central zone and to locate the western widespread Mesozoic basin in the offshore area, emersion of the sole fault of the extensional containing up to 7 km of Mesozoic rocks (Fig. 7). system. Its Neogene structure is characterized by a lack The geometry of the Rubielos de Mora graben

NW-

0~

-SE

Sea level CO

14 Cl

2

s

6

4 I 1

2

4

6

0 1 2 3 4 5 Km 1 t

Fig. 7. Migrated conventional reflection seismic line through the Mesozoic Valencia basin (SW Valencia trough). P = Hercynian basement; T = Triassic; .I = Jurassic; C = Cretaceous + lower Palaeogene(?); I-II = upper Palaeogene(?)-Upper Miocene; III =

Pliocene-Present. For location, see Fig. 3.


I NW SE I’

B

C

0 I 2 3km

/17J6 Hs +-ho -51

0 km

Fig. 8. (A) Geological map of the Desert de les Palmes study area. (B) Balanced cross-section (see Fig. 5 for location). The listric detachment horizon has been calculated from a modified Chevron technique (Williams and Vann, 1987). CC) Restored version of this cross-section in Barremian times. 1 = Neogene; 2 = upper Aptian; 3 = Barremian-lower Aptian; 4 = Jurassic; 5 = Middle Triassic (Muschelkalk); 6 = Lower Triassic (Buntsandstein); 7 = Hercynian basement; 8 = normal fault; 9 = anticline; 10 = syncline;

I I = dip of bedding.

212 E. ROCA AND .I. GlllMEKk

(Fig. 5, B-B’; Guimera, 1990) and the size (about 100 km length) of the Teruel graben system suggests the existence of a deep sole detachment of the extensive system. The westernmost recognized extensional structure of Early Miocene age are the faults bounding the Teruel graben; therefore they could be the western emersion of sole fault of this extensional system. The depth of this sole fault can not be deduced from surface data, thus we have assumed a similar depth as in the northern cross-section. Nevertheless, shallower detachment levels are recognized, located in the Mesozoic cover (i.e. Upper Triassic evaporitic and Albian detritic beds) and in the basement.

The sole fault of most cover normal faults (having a hectometric to kilometric fault trace) is located in Middle and Upper Triassic evaporitic beds. These faults are high-angle in upper strati-

graphic units and low-angle beneath Upper Trias- sic beds, and roll-over structures are common, showing the listric geometry of those faults. How- ever, decametric domino-style normal faults have also been recognized (Guimera, 1988).

The presence of a shallow basement detachment has been deduced from field data in the area of the Desert de les Palmes (Fig. 81, where basement tilted blocks and roll-over structures are found. From the geometrical construction, using the modified chevron construction, a 2.5- 3.0~km-deep detachment is deduced. In this area, most of the extensional deformation was Meso- zoic in age (Fig. 8): roll-over related tilting of Mesozoic beds is observed previous to Jurassic and Barremian in NW-dipping listric normal faults, giving rise to two generations of angular unconformities. More to the east, a SE-dipping listric normal fault gave way to an asymmetrical basin containing up to 1000 m of late Aptian carbonates (Salas, 1987) showing a wedge disposition. This Mesozoic extensional deformation probably also exists west of the Desert de les Palmes; however, the level of erosion does not allow us to observe it; so we have not represented it in cross-section B-B’ (Fig. 5).

Extensional model

From the above-mentioned data we can con- clude that the Neogene extension in the

Catalan-Valencian domain took place according to the following scheme:

(1) Extension was accommodated by a normal fault system detached at 13-15 km depth. That depth coincides with the Moho depth in the central areas of the Valencia trough, although in the marginal areas it is located in a deep upper crustal position.

(2) Locally, shallower detachments have been observed, both in the Mesozoic cover and in the Hercynian basement. Geometrical analysis shows that most normal faults are listric.

(3) Whereas in the northern area (cross-section A-A’ in Fig. 5) most superficial deformation is concentrated around the margin of the emerged areas (Catalan coastal rifts), in the southern area (cross-section B-B’ in Fig. 5) deformation spreads more or less uniformly over the whole area.

(4) Extensional deformation followed, at least in the northern area, a simple-shear model in the upper crust. This is deduced from the fact that while the main surface stretching is concentrated around the western boundary of the domain, the maximum thinning ratio is located in the central

Valencia trough. (5) Most of the main Neogene normal faults

involving the basement are inherited from Meso- zoic times: the Neogene basin is located in the areas where the Mesozoic is thicker (Fig. 2).

Estimation of extension ratios

The latest studies carried out in the Vaiencia trough (FontbotC et al., 1989; Rota et al., 1990) reveal the importance of knowing the role of the different Mesozoic and Cenozoic tectonic processes to explain its present lithospheric pattern. We have tried to quantify the thinning generated in the Catalan-Valencian domain by the Neo- gene and Mesozoic extensional processes with the purpose of recognizing their contribution to the geophysically inferred crustal thinning. The quantification has been carried out from the extensional Catalan-Valencian domain cross-sections and the existing subsidence data (Salas, 1987; Rota et al., 1990, Watts et al., 1990; Roca and Desegaulx, 1991). However, the exclusion of part


of the Betic-Balearic domain from the analysis

provides only a minimum estimate of the total extension.

Neogene extension

The Neogene stretching estimations have been calculated from the cross-sections of Figure 5. The method adopted to estimate the stretching factor (p) was developed by Chenet et al. (19831, which quantifies the average factors of extension dividing the cross-section length by the initial length of the strata previous to extension. Apply- ing this method, a p of approximately 1.1 has been estimated for the cross-sections A-A’ and B-B’ (Fig. 5). However, because most faults with stratigraphic offsets smaller than about 100 m are neglected in such geological sections, the total amount of extension should be significantly larger. Accurate field data show that minor faults can provide values of p larger than 1.2. To solve this problem, we have recalculated the stretching adopting the Faure and Chermette (1989) method. This method takes into account the internal deformation generated by the listric faults in their hanging-wall block. According to this method, the stretching value, perpendicular to the trend of the main normal faults, ranges from 1.2 (36 km) in cross-section B-B’ to 1.3 (36 km) in cross-section A-A’, whereas the stretching parallel to the

TABLE 1

Comparison between the upper crustal thinning ratios deduced from geophysical methods and the deduced ones from

the geometrical analysis of the superficial structures

-lFQERcRusTAL~ GECWiWCAL DATA lSUPEFt!=lCW SlRlCTUR.

I I ,

The Mesozoic calculated value (*) has only a local significance and could not be extrapolated to whole area studied.

main fault trend (NNE-SSW to ENE-WSW)

ranges from 1.05 to 1.15. The stretching calculated in this way implies

an approximate thinning in the upper crust of 1.4-1.5 (30-35%) in the A-A’ cross-section. This thinning was calculated using a mass conservation model and taking into account both perpendicular stretching values. These thinning values are lower than those inferred from the refraction seismic data analysis carried out in the Valencia trough (Table 1 and Fig. 9).

The seismic refraction data (Hinz, 1972; Gal- lart et al., 1990) and the E.S.P. (Pascal et al., 1992-this volume) indicate an average upper

A-A’

-NW

-v B 5.16 --- --------l

m Upper crust I Lower crust / Moho

Fig. 9. Rate of thinning from the crustal thickness deduced from geophysical methods along the A-A’ (Fig. 5) crustal cross-section

assuming an extension model in which crustal volume remains constant.

214 E. KOCA AND J. (;lllMEKk

crustal thickness of 13.5 km in the section A-A’ (Fig. 8). Assuming that the total crustal volume remains constant during extension, and that the initial thickness of the upper crust before extension was 24 km (present value of the upper crust in the neighbouring and undeformed Ebro basin: Roure et al., 19891, it implies an average thinning factor of 1.77 (44%).

From the above data, a contradictory relation- ship arises between the observed upper crustal thinning and that deduced from Neogene surface structures. This contradiction could be explained in three different ways.

The first considers that the measured stretching values in the superficial structures are under- estimated and therefore the calculated thinning value (1.4-1.5) is smaller than that really generated by the Neogene extension. Nevertheless, field data observations do not seem to support the large amount of extension necessary to create the present thinned crust (although it is unknown how much superficial extension occurred in the region now occupied by the Betic-Balearic domain). To explain it, a radial stretching value of 1.33 would be necessary, but the calculated value from Neogene structures does not exceed 1.20- this value is the radial stretching which produces the same amount of thinning as the observed stretching (1.30 and 1.10 in two perpendicular directions).

In contrast, the second hypothesis considers that the stretching value calculated using Neo- gene surface structures is more or less correct and that the thinning ratio deduced from the refraction seismic data is overestimated-the initial upper crust was thinner than 24 km. In this case, the present thinned crust would be partially inherited from pre-Neogene extensive processes.

Supporting this hypothesis, the subsidence studies carried out in the Provensal basin (Bessis, 1986; Bessis and Burrus, 1986) show (applying a non-uniform stretching model; Royden and Keen, 1980; Royden, 1986) that the crustal thinning inferred from the subsidence analysis is greater than the observed one. Taking into account that the Valencia trough corresponds to the SW pro- longation of the ProvenSal basin (Fontbote et al., 1990) and that both basins were surely generated

TABLE 2

Comparison between the crustal thinning values deduced from

geophysical methods and the inferred ones from the subsi-

dence analysis applying different methods

The calculated values of Mesozoic thin-

ning are from Salas (1987).

by the same geodynamic processes, we can sup- pose that the thinning derived from the Valencia trough subsidence anaIysis is also larger than that inferred from superficial structures. However, the seismic crustal thinning values agree with those inferred by Rota and Desegaulx (1991) from the study of Neogene subsidence in the Valencia trough (Table 2), applying the same calculation method. Therefore, the crustal thinning ratio would be lower than the one inferred from seismic data, since it is equal to the one derived from subsidence analysis.

Finally, the third hypothesis assumes a non- conservative mass model for the upper crust, so that the present upper-lower crust boundary does not correspond to the initial one. It implies that other mechanisms different from stretching would take part in the upper crustal thinning. According to this hypothesis, the Moho and the brittle- ductile transition boundary moved upwards during the crustal thinning due to stretching and to lower crustal metamorphic processes. In such a way, the thinning derived from the upper crustal mass balance computation is larger than that deduced from superficial structures.

Mesozoic extension

Previously to the Cenozoic structure of the western Mediterranean, an important Mesozoic extension phase took place that generated

THE NEOGENE STRUCTURE OF THE EASTERN IBERIAN MARGIN 21.5

widespread subsident basins. This extension, re-

sulting from the sinistral motion between Africa

and Eurasia (Smith, 1971; Dewey et al., 19731, was reflected in the Valencia trough area by the deposition of thick Mesozoic carbonate series. Subsidence analysis and the tectonic structure show that the Mesozoic extension was almost as important as the Neogene.

In this sense, subsidence analyses are very significant. Using a pure-shear uniform stretching model (McKenzie, 1978) and from the tectonic

subsidence (St) data, Salas (1987) deduced a Mesozoic crustal thinning in the Maestrat basin of 1.52. This value was calculated applying both the tables of Hardenbol et al. (1981) and the empirical formula propounded by Le Pichon (1980):

St = 7.5( 1 - l/B) (km)

where B = thinning ratio. The importance of this Mesozoic thinning ra-

tio is reflected in the fact that it is greater than the average B value calculated from the Neogene subsidence in the Iberian margin of the Valencia trough (1.191, applying the same methodology (Table 2).

A similar result can be inferred from geometric analysis of the superficial tectonic structure (Table 1). We have geometrically analyzed a sec- tor of the extensional Catalan-Valencian domain where superposition of different Mesozoic extensive phases has been recognized. In this area (Desert de les Palmes, Fig. 8), from a cross-section perpendicular to the major normal faults, we have calculated the stretching values previous and posterior to the Barremian. According to the Faure and Chermette (1989) method, the pre- Barremian stretching value (p = 1.34) is much higher than that calculated for the post-Bar- remian extension (/3 = 1.07). It is important to remark that the second stretching value does not correspond only to the Neogene extension, because since the Barremian this area was already affected by Cretaceous extensive tectonic phases (Salas, 1987). Although this proportional relation- ship can not be extrapolated to the whole basin, it points out the importance of the Mesozoic extension in the Valencia trough area.

The magnitude of the thickening produced by

the Palaeogene compressive processes is unknown in the Valencia trough, but would be

significant in the Iberian chain, since it was able to recover the crust thickness previous to the Mesozoic extension. In opposition to the thinning values deduced from subsidence analysis and extensional structures, the refraction seismic data show that beneath the Mesozoic basins not affected-or only slightly affected-by the Neo-

gene extension, the crustal thickness is normal or only slightly thinner. Thus, Moho depths of 30-32 km and 25-35 km have been observed beneath the Iberian chain (Banda, 1988) and Maestrat basin (Zeyen et al., 1985), respectively. This

post-Mesozoic crustal thickening in the emerged areas could be related to the building of the Palaeogene thrust systems that generated the inversion of the Mesozoic basins in the upper crust (see section Geological Setting), but these compressional structures have not been recognized offshore, in the Valencia trough.

Conclusions

The Neogene evolution of the western part of the Valencia trough (Catalan-Valencian domain) during Late Oligocene-Early Miocene is characterized by the development of extensional processes that generated a widespread horst and graben system and a noticeable crustal thinning.

This structure was overprinted in the south- easternmost areas by Betic structures. They con- sist mainly of a Middle Miocene fold and thrust system (Eastern Betics), which has been reacti- vated since the Late Miocene as a normal fault system.

Crustal and superficial structural data show that Neogene extension was carried out by a listric normal fault system detached at 15 km in depth, which coincides with the Moho depth in the central areas of the Valencia trough. In the marginal areas, however, it is located in a deep upper crustal position. From its geometry it can be deduced that above this detachment the extension was accomplished following a simple-shear model. This is evident in the northern areas of the Catalan-Valencian domain, where the super-

216 I: KO<‘A AND .I (illlMk.Kk

ficial extension is concentrated in the western marginal basin areas.

This extensional structure is developed in an area previously affected by Mesozoic extensional and Palaeogene compressional tectonics. There- fore, the present crustal configuration would be the result of the superposition of the crustal changes produced by several tectonic events.

The quantitative analysis of the Neogene structure shows the incoherence between the upper crustal thinning inferred from the geometric analysis of the superficial structures and from seismic data analyses. Whereas an approximate thinning of 1.4-1.5 has been calculated from the superficial perpendicular stretching values (1.3 and 1.11, the upper-crust thinning ratio deduced from seismic data is approximately 1.8. Although other mechanisms could be invoked to explain the incoherence between the different calculated thinning values, the most reasonable hypothesis seems to be the one proposing that the present thinned crust would be partially inherited from the Mesozoic extension. In this sense, subsidence analysis and the recognized Mesozoic tectonic structure shows that the Mesozoic extension was almost as important as the Neogene.

Acknowledgements

We wish to thank the REPSOL and SHELL ESPANA oil companies for their generous re- lease of data. We also thank P. Santanach, J. Verges, M. SCguret and two anonymous reviewers for their helpful comments. This work is a contribution to the project “Evolution neogena de la cuenca catalano-balear”, financed by the Comi- sion Interministerial de Ciencia y Tecnologia (CICYT project GE089-0831). One of the au- thors (E.R.) benefitted from financial support by a CIRIT grant (“Ajut per joves investigadors AR89”).

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The Neogene structure of the eastern Iberian margin: Structural constraints on the crustal evolution of the Valencia trough (western Mediterranean)