Quantitative Subsidence Analysis of the Mesozoic Evolution of the Lusitanian Basin (Western Iberian Margin)

TECTONOPHYSICS

ELSEVIER Tectonophysics 266 (1996) 493-507

Quantitative subsidence analysis of the Mesozoic evolution of the Lusitanian basin (western Iberian margin)

a* Gerco Stapel ' , Sierd Cloetingh a, Bert Pronkb

o Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HVAmsterdam, Netherlands b Consultant to Gabinete para a Pesquisa e Exploraf~o de Petr61eo (GPEP), Rua do Vale de Pereiro 4, 1200 Lisboa Portugal Received 20 March 1995; accepted 23 November 1995

Abstract

Quantitative subsidence analysis of 26 wells in the Lusitanian basin provides new constraints on the western Iberian Mesozoic passive margin development. Backstripped tectonic subsidence curves show a three-fold subdivision of vertical motions from Late Triassic onward. Continental rifting was initiated during the Late Triassic and Early Jurassic. From Middle Jurassic onward a distinct different behaviour is expressed in the subsidence curves for the North and the South Lusitanian basin. During the Middle Jurassic the South Lusitanian basin records a stretching episode with stretching factors of about 1.08, while the North Lusitanian basin typically has a Middle Jurassic hiatus. This different development is also expressed in the Late Jurassic and Early Cretaceous sedimentary sequence when both the North and South Lusitanian basin are subjected to another stretching episode, with a more pronounced development of the southern than of the northern part of the basin. The stretching factors for this last phase are about 1.03 for the northern part and 1.08 for the southern part of the area. This north-south difference during the Middle Jurassic to Early Cretaceous, for which the transition roughly coincides with the location of the Nazar6 fault zone, is probably a result of differences in pre-rift crustal composition or thickness. Late Cretaceous sediments are mostly absent in the analysed wells. In the southern part of the basin the absence of the Cretaceous record is a consequence of erosion due to Cenozoic inversion of the basin. The generally low estimates for stretching factors suggest that the analysed eastern part of the Lusitanian basin forms the distal part of the mid-Cretaceous continental breakup. A comparison with subsidence curves of neighbouring basins of Iberia reflects general patterns in Mesozoic basin development and confirms the generally held view that the extension leading to continental breakup migrated from south to north during the Middle Jurassic to Early Cretaceous in West Iberia.

Keywords: Basin subsidence; Passive margins; Mesozoic; Iberia; Tectonophysics

1. Introduction tinents bordering the North Atlantic. This part of the North Atlantic was defined during the Mesozoic The Lusitanian basin, situated in the western by two major fracture zones: the Charlie Gibbs and part of the Iberian Peninsula, records the opening the Newfoundland-Gibraltar fracture zones (Verhoef of the North Atlantic Ocean during the Mesozoic. and Srivastava, 1989). For the western Iberian mar- Fig. 1 shows the pre-drift configuration of the con- gin and its conjugate, the Grand Banks of Newfound- land, the first indications of Mesozoic continental * Corresponding author. rifting are of Late Triassic age. During the Paleozoic

Po u ! ii i i:i TM of the Iberian Peninsula towards WNW-ESE for the western Iberian margin. Fig. 2 shows a detailed overview of the geology of the Lusitanian basin and Fig. 3 shows a cross-section d~rc /j/:J Polo-::t~ -""~ab.an(2) \1 through the basin, combining onshore and offshore Whale ...... Galici~i~ ~ )1 seismic sections. The basin is one of the few basins around the North Atlantic with synrift sediments ::/ usi('~n&4S::i::: Iberia ~n(5) exposed onshore. Mesozoic sediments are outcropping in the NNE-SSW-oriented inverted zone and in a small zone along the Variscan basement at the Africa eastern border of the basin. The onshore geology 0 200 Km of the Lusitanian basin has been extensively studied Fig. 1. Schematic overview of the rift basins of the Iberian (Ribeiro et al., 1979; Gudry et al., 1986; and refer- continent and its conjugate margin in pre-drift situation. The ences therein). In their study Wilson et al. (1989) directly neighbouring basins of the Lusitanian basin discussed in this paper are indicated by numbers that correspond with Fig. 9. emphasize the importance of the Nazar6 fault zone (Modified from Tankard and Balkwill, 1989.) (Fig. 2). It is evident from the stratigraphic well reports that the Lusitanian basin can from Middle Jurassic onward best be separated in two distinct the Variscan fold belt had developed in the North areas, separated by this Nazar6 fault zone. In the Atlantic region and during the Late Paleozoic the following they will be called the North and the South development of wrench faults marked a pronounced Lusitanian basin. So far, only Wilson et al. (1989) change in stress system (Ziegler, 1989). In the Mid- supplemented their studies with limited subsurface dle Jurassic, breakup was established by separation data. We present the results of a comparison of of Africa and North America. From then on the Cen- an extensive set of well data together with stretch- tral Atlantic seafloor spreading axis was propagating ing estimates as well as a comparison of Mesozoic to the north. During the Late Jurassic this probably subsidence for the different basins on the Iberian resulted in the separation of South Iberia and North Peninsula. America (Srivastava et al., 1990), followed by the Aptian separation of Galicia Bank and North Amer- 2. Stratigraphy ica (Boillot et al., 1989) and the mid-Cretaceous separation of Europe and North America. The crustal For this study 26 wells were analysed for which separation during the Aptian led the western Iberian the locations are given in Fig. 2. The wells were margin into a phase of relative tectonic quiescence, drilled in the period 1949-1990 and 13 of the wells while tensional stresses that had been active since the are situated onshore and 13 offshore. Fig. 4 sum- Late Triassic were relaxed. During the Late Creta- marizes the stratigraphic history of the study area ceous and Early Tertiary, a change of stress patterns together with mean thicknesses derived from a com- occurred associated with the collision of Europe and parison of the 26 wells. Five of the analysed wells Iberia during the Early Tertiary and the collision of reached basement. Four of these wells have Triassic Africa and Iberia in the mid-Tertiary. On the Iberian sandstones lying unconformably on Paleozoic and continent this stress regime resulted in the inversion Precambrian rocks. In the following a brief sum- of Mesozoic grabens. In the Lusitanian basin com- mary of the Mesozoic lithologies of the wells will pression resulted in an inversion of a part of the basin be given. The summary is based on GPEP (1986), and the associated development of two small-scale extended with new observations. basins to the WNW and ESE of the inverted area Mesozoic sedimentation started in an extensional (Ribeiro et al., 1979). The present-day stress field regime that came into existence during the Late Tri- documented by Cabral (1989) shows curved trajec- assic. The oldest dated sediments are of Carnian to tories for the maximum compressive principal stress Norian age (Palain, 1977) and consist mainly of red axis that turn from NNW-SSE in the extreme south continental sandstones and mudstones, deposited in G. Stapel et al./Tectonophysics 266 (1996) 493-507 495

ca o

ca .,,,,

I 0 5() km [~ Quaternary ~ J Tertiary ~ 1 Cretaceous ~ E F~ Upper Cret. intrusive

10~W 8°W Fig. 2. Geological setting of the Lusitanian basin and location of the 26 wells analysed in this study. The upper left corners of the boxes give the exact locations of the wells. N = Nazar6 fault zone. The dotted line gives the location of the cross-section shown in Fig. 3. small basins. The thicknesses in the three wells that The evaporites, consisting to a large extent of halite reach basement containing this unit range from 160 and gypsum alternated with shales, have consider- to 388 m. The exact upper age of the continen- able changes in thickness. The thicknesses are much tal red beds and the starting point of the subsequent more homogeneous in the southern than in the north- salt accumulations are badly constrained (see Sopefia ern wells, where the evaporites are clearly associated et al. (1988) for a discussion). The analysed wells with the presence of younger fault structures. show that sedimentation of clastics proceeded on The Hettangian typically ends with a thin lime- the margins while evaporites were deposited in the stone member in the northern part of the area central areas of the locally developing basins. As deposited under tidal flat conditions. During the in none of the wells the evaporites are dated, we Sinemurian shallow shelf carbonates were deposited assume for the subsidence analysis that evaporites in the north that to the central part of the basin were deposited during the Hettangian and that conti- laterally were exchanged for deeper shelf shales. nental sandstones represent the Upper Triassic. Thus, From Pliensbachian to Aalenian laminated shales the first homogeneous occurrences of evaporites are and lime/mudstones locally alternating with small given an age of 208 Ma, and represent in a number of amounts of marl were deposited in a large part of the wells the oldest stratigraphic marker in the analysis. area. To the extreme south and southeast of the basin 496 G. Stapel et al./Tectonophysics 266 (1996) 493-507

W El NW SE [ WNW ESE

Sea level 0.0- ...... ~.- ..... -/" ======i ,Jiiiili 2.0 iiiii!~!~.. :i:!

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~i-ertiary - Quaternary ~. -~" ~ Cretaceous 4.0 ~ Jurassic ~ and Lower Jurassm salt ~.~ r--] Pre-Mesozoic Dasernem I I 0 50 100 Distance (Km) Fig. 3. A cross-section through the western Iberian margin obtained from a combination of seismic lines. Seismic interpretation was done by the Geological Survey of Denmark and Greenland (GEUS). N -- Nazard fault zone. For the coastline area, where no data are available, and for the area underneath the Nazar6 fault zone the probable location of the basement is indicated with question-marks. Uncertain horizons are dashed. Location of the seismic sections in Fig. 2. these deeper shelf deposits graded into shallow shelf of Middle Jurassic deposits and leading to differ- carbonates. entiated sediment accumulation rates in the basin. During the Bajocian and Bathonian shallow ma- In the analysed wells large accumulations of salt rine conditions resulting in carbonate platforms be- are accompanied by small sedimentation rates dur- came present in the region. These platforms ex- ing the Late Jurassic and vice versa, indicating a tended into the basin from the eastern and western strong control of salt movement on sediment accu- basin margins. In the central part of the basin deeper mulation rates. During the Kimmeridgian sediment shelf limestones and marls were deposited during accumulation rates considerably increased especially the Bajocian followed by shallow marine carbonate in the South Lusitanian basin. The northern part of sediments during the Bathonian. During the Upper the basin was subjected to terrestrial (fluvial) condi- Callovian overall non-deposition conditions were es- tions during this time and marine influence increased tablished and part of the limestones and marls de- towards the south. Shallow marine and coastal con- posited during the early Callovian were removed. ditions were present in the central part of the basin This resulted in an overall absence of Callovian sedi- and outer shelf sandstones and marls were deposited ments in the northern part of the basin and about 150 in the southeast. During the Portlandian these en- m thickness of the unit in the southern part. The non- vironments persisted, although the shallow shelf deposition conditions extended into the Oxfordian carbonates were predominant then in the southern resulting in locally developed karst surfaces. part. The late Oxfordian begins with the deposition From the Portlandian onward sediment accumula- of lacustrine carbonates in the central part of the tion rates decrease, and during the Early Cretaceous basin which grade into open marine and deeper ma- there is almost no deposition on the margin. For rine conditions towards the south. The distinction the first time since the Early Jurassic sedimentation between north and south becomes remarkably clear rates are quite homogeneous for the complete basin. during this time span. Mean thicknesses for late During the Early Cretaceous continental and shal- Oxfordian sediments are about 300 m in the south low marine conditions prevail in the area, associated while the northern part was almost sediment-starved. with a number of unconformities that are, however, During the Late Jurassic an important phase of salt badly constrained in time. During the Cenomanian diapirism took place, resulting locally in erosion and Turonian an influx of shallow marine limestones G. Stapel et al./ Tectonophysics 266 (1996) 493-507 497

North Lusitanian Basin: South Lusitanian Basin:

Water depth (m): Water depth (m): 0 200 0 , 2o0

~..,4. Marine influx 0 . . ~ Terrestrial, shallow marine

Terrestrial, shallow marine ',- ','

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~..:.~.~..-=f..:~ i ii Deeper marine '.-., ~- a-vav-a ~ Brackish lagoonal

--'~ fiat Tidal Shallow marine

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Sandstones

Fig. 4. Stratigraphic columns compiled from the well reports. Mean thicknesses were taken for the north and south part separately in order to show the different accumulation rates and depositional environments• The transition between these two areas is discussed in the text. Water depths estimated from paleo-environment maps (GPEP, 1986) together with uncertainty ranges are also indicated. is recorded in a number of the wells. In most of the Fig. 4 shows the minimum and maximum inter- wells, however, the Cretaceous is not represented. preted palaeobathymetry for the North and South Lusitanian basin during the Mesozoic. It also indi- 3. Subsidence analysis cates the mean values that we adopted for our analysis. The depths are to a large extent constrained by The wells from the Lusitanian basin have been wells 13C-1, 16A-l, 20B-1 and Bf-1. For these wells backstripped (Watts and Ryan, 1976; Steckler and biostratigraphical well sheets exist indicating that de- Watts, 1978) adopting local isostasy to correct for positional depths were never extremely deep for the the effect of sediment loading. Compaction corrected basin. Water depths were interpreted for the whole sedimentary records of the wells were used with ap- study area with the aid of a compilation of palaeo- plication of the porosity-depth relationships accord- environments (GPEP, 1986). The depth estimations ing to Bond and Kominz (1984), constrained by sonic for the different environments are according to Ingle logs. Difference has been made for two depths, in- (1980). The palaeobathymetry curves show that af- cluding a depth where the low-depth relationship ter the continental environments of Late Triassic and takes over from the high-depth relationship. Further- Hettangian water depth is increasing during the Early more, the timescale of Harland et al. (1990) was used. Jurassic. This is followed by decreased water depths 498 G. Stapel et al./ Tectonophysics 266 (1996) 493-507 during the Middle Jurassic, during which carbonate subsidence curves are displayed in the proper geo- platforms developed in the South Lusitanian basin. graphical context. This means that the left side of The Late Jurassic shows considerable local changes the figure indicates the westernmost wells and the in water depth which are, however, never exceeding right side displays the easternmost wells. Within 150 m. From Early Cretaceous onward the water each column the wells are arranged from north at the depths are in general quite shallow. top to south at the bottom. The chronostratigraphic Fig. 2 shows that the analysed wells are all situ- horizons for each well are indicated by crosses. ated in a zone 80 km wide, parallel to the coastline. Almost all the wells record the Early Jurassic The results that will be discussed are representative continental rifting phase. The wells that contain the for this eastern (largely onshore situated) part of the Hettangian evaporites show that their initial tectonic basin only. The ocean-continent transition is situ- subsidence is to a large extent controlled by the ated at about 200 km offshore (Whitmarsh et al., large variations in thickness of this unit. However, 1993). Fig. 5 illustrates the effect of uncertainties while most of the wells have the evaporites as their in palaeobathymetry and porosity assumptions. The lowermost unit, the top of these deposits is the low- uncertainties in water depths are obviously negligi- ermost chronostratigraphic horizon in the analysis ble compared to the porosity uncertainties. Although and thickness variations are not taken into account not all the wells reach the Triassic sandstones or in the tectonic subsidence. The better defined wells underlying basement, most of the wells reach the (for example 17C-1 and 13C-1)indicate stretching Upper Triassic-Hettangian salt deposits. Our analy- during the Sinemurian and Pliensbachian, with a sis, therefore, is restricted to the subsidence of the subsequent thermal subsidence of the basin. A re- basin since the beginning of the Jurassic. Another markable deviation is present in the southernmost restriction of the data set is that a number of wells wells, where a hiatus represents the uppermost Early are located on local basement highs. Within these Jurassic. While only 50 km to the north the sed- limitations, however, the analysed wells offer clear iments indicate deep marine environments, a local constraints on the development of the Lusitanian uplift and associated erosion must have taken place basin. Fig. 6 gives an overview of the tectonic subsi- before the earliest Middle Jurassic. dence curves for all the analysed wells. The tectonic During the Middle Jurassic differentiated vertical

-1

ectonic ...... ~~{

soo Total

=1ooo Jurassic I Cre.taceous I Jurassic I Cretaceous I [_Early I M. IL.I Early.~ I Late~ [_Early M. L.I Earl~ I Late J I i i I ; i 200 150 100 Ma 200 1 0 100 Ma Fig. 5. Tectonic and total subsidence curves for two wells indicating the uncertainty in paleobathymetry (grey area) and in porosity estimates (error bars). G. Stapel et al./ Tectonophysics 266 (1996) 493-507 499 motions take place in the Lusitanian basin, and dis- phase of tectonically induced vertical motions. The tinct differences between the northern and southern movements during this time are obviously small part of the basin become evident. In the North Lusi- compared to the two preceding phases that recorded tanian basin wells only record small tectonic sub- the rifting and thermal development of the western sidence rates, or are represented by hiatuses during Iberian margin. this time. This is in contrast to the South Lusita- nian basin where considerable tectonic subsidence is 4. Stretching factors recorded. The boundary between these two different subsidence domains coincides with the Nazar6 Stretching factors have been estimated for the 26 fault zone, and our analysis suggests that from Mid- wells where possible. To this aim, we have exam- dle Jurassic onward this Nazar6 zone had a strong ined constraints on the thermo-mechanical structure control on the Lusitanian basin development. of the pre-stretched lithosphere. The Iberian Massif Overall rifting of the area was established during outcropping to the east of the Lusitanian basin is the Late Jurassic and Early Cretaceous, with vertical generally subdivided into a number of tectonometa- motions again being more pronounced to the south morphic zones (Julivert et al., 1972) of which the of the Nazar6 zone. The tectonic subsidence curves Central Iberian Zone and the Ossa-Morena Zone are of Fig. 6 show basin subsidence in the northern- of primary interest to this study (Fig. 8). The con- and southernmost wells during this time span. In the tact between these two zones consists of a major middle part of the basin, to the south of the Nazar6 fault zone (the Coimbra-Cordrba shear zone (Burg zone, the wells do not contain the complete subsi- et al., 1981)) which may represent either a Cadomian dence record of this Late Jurassic-Early Cretaceous or a Variscan suture (see Azor et al., 1994, for a phase. During the latest Cretaceous and Tertiary the discussion). When Mesozoic extension started, the basin became subjected to compressive forces and Variscan basement consisted of differently structured as a consequence the basin became uplifted with lithospheric blocks within which the Variscan fault the pronounced development of an inverted zone. zones were subsequently reactivated. The present- The Nazar6 zone was reactivated as a major reverse day Iberian crust seems to have a thickness similar fault during this deformation phase (Ribeiro et al., to mean European Variscan crust as described in 1990a). The erosion associated with this uplift has Cloetingh and Burov (1996). The ILIHA DSS Group removed for a large number of well locations the (1993) report crustal thicknesses of 28 km directly Cretaceous and part of the Late Jurassic record. A to the north of the Lusitanian basin and 30 km complicating factor for the Early Cretaceous is that near Lisbon. Crustal thicknesses for the Iberian plate the deposits during this time slice consist of terres- decrease to values below 10 km towards the ocean- trial sandstones that are extremely difficult to date. continent transition to the west (Whitmarsh et al., Most of the wells that contain Early Cretaceous 1993) and increase to values of about 32 km beneath sands are constrained only by the stratigraphic mark- the Iberian Massif to the east (ILIHA DSS Group, ers at the top and the bottom of this unit, and as a 1993). The pre-rift crustal thickness is assumed by consequence the information is very incomplete. The us to correspond roughly to the present-day value overall vertical motions, however, appear to be quite of the crustal thickness of Variscan Iberia and is homogeneous for the Early Cretaceous, suggesting taken to be 32 km here (following assumptions in that somewhere during this time span the transition Salas and Casas (1993) and Van Wees and Stephen- from synrift to postrift takes place. This observation son (1995)). The pre-stretched Variscan lithosphere is in close agreement with the generally held view has a thermo-tectonic age of about 170 Ma at the that separation of Galicia and the Grand Banks of onset of Lusitanian basin formation during the Late Newfoundland was established at 118 Ma (Boillot et Triassic (Ribeiro et al., 1990b; Beetsma, 1995). For al., 1989). this thermo-tectonic age an estimate of about 125 km The Late Cretaceous is only documented in the thickness for the pre-rift lithosphere is obtained. wells to the north of the Nazar6 zone and one Fig. 7 displays for three wells the fitting of well (Ms-I) in the south. The wells indicate a third stretching factors to the subsidence curves. A num- 500 G. Stapel et al./ Tectonophysics 266 (1996) 493-507

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Fig. 6. Tectonic subsidence curves for the 26 analysed wells in the eastern part of the Lusitanian basin. The curves are arranged horizontally from west to east and vertically from north to south. The chronostratigraphic horizons are indicated by crosses and different time units are shaded to simplify comparison of the curves. G. Stapel et al. / Tectonophysics 266 (1996) 493-507 501

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iiii?!ii!i!i!ii!!i~!!i~ i!~ !!iiii!iii!!ii!i!~ii!i~i~!!ii~!!ill ~i ~ i~i ~i ~ iI ~~t~o !! 2~o ii!i~!!ii~ i ii~ iiLiiiii~i!i~i~ !!!!~!~ 5O0 "500 L~ Jurassic Cretaceous I I Jurassic I Cretaceous ] arly I M. I L. I Early I Late / Earl M. L. I Earl Late i , 200 150 100 Ma 200 150 100 Ma Fig. 6. Continued. 502 G. Stapel et al. / Tectonophysics 266 (1996) 493-507

I 13C-1 + ; Tectonic subsidence o----o Modelled subsidence ~~'~"

• Sb-I

B25O°m

==500 Jurassic Cretaceous _J LE Jurassic ~arly M.L. Early I La. te arly I M tErn E.Cretaceous arly I L ateJ I J t I i i 200 150 100 Ma 200 150 100 Ma Fig. 7. Three examples of the fitting of stretching estimates to the tectonic subsidence curves (see text for discussion). ber of wells, especially in the North Lusitanian basin, mal subsidence or are represented by unconformities can be fitted by two stretching events, similar to during the Middle Jurassic. Well Sb-1 is situated in well 13C-1. The first stretching phase, which has the inverted zone and had probably, like 17C-1, its a stretching factor of 1.17 for well 13C-1, has a Cretaceous stratigraphic record removed by erosion. period of rapid stretching during the Early Jurassic Fig. 8 summarizes the estimated stretching fac- followed by thermal subsidence until the beginning tors for the analysed part of the basin for the Early of the Late Jurassic. During the Late Jurassic and Jurassic and the Late Jurassic to Early Cretaceous Early Cretaceous a second stretching phase of 1.02 stretching events. The continental rifting phase dur- was fitted to the data. Well 17C-1 and Sb-l in Fig. 7 ing the Early Jurassic (Fig. 8) is characterized by the are indicative for a number of wells that deviate same order of stretching factors for the whole area, from this general pattern. Well 17C-1 is situated although some large local deviations do exist. How- in the zone that became inverted during the Late ever, not all the wells reached basement and the dif- Cretaceous and Tertiary. Erosion removed the up- ferent lowermost chronostratigraphic horizons have permost Jurassic and Cretaceous sedimentary record, to be taken into account for this. Furthermore, the and the subsidence curve can be explained by a sin- stretching estimates are influenced by the Late Juras- gle Early Jurassic event with a stretching factor of sic salt movement, which scatters the Early Jurassic 1.22 and subsequent thermal subsidence. Well Sb-1 picture. The stretching factors during the Middle is representative for most wells in the South Lusita- Jurassic are quite homogeneous for the South Lusi- nian basin that cannot be explained reasonably with tanian basin with a small deviation around 1.08. The only two stretching phases. After the Early Juras- North Lusitanian basin did hardly experience tec- sic event with a stretching factor 1.06, well Sb-1 tonic subsidence during the Middle Jurassic phase. records a second stretching event during the Middle For the Late Jurassic to Early Cretaceous phase, Jurassic with a factor 1.10 followed by a third event which is recorded on a basinwide scale again, a dif- also of 1.10 during the Late Jurassic. Inspection of ference in estimated stretching factors is visible for Fig. 6 demonstrates that almost all the wells to the the northern and the southern part of the area (Fig. 8, south of the Nazar6 zone record this Middle Jurassic right). The northern part of the basin has stretching stretching phase, while northern wells (like 13C-1, factors of about 1.03 and the southern part of the for example) can reasonably be explained by ther- area has stretching factors of about 1.08. For wells G. Stapel et al. / Tectonophysics 266 (1996) 493-507 503

N ~N

N N

9°W 8°W 9Ow 8°W Fig. 8. Estimated stretching factors for two of the three distinguished phases together with main aspects of the Lusitanian basin tectonic setting. CIZ = Central Iberian Zone; OMZ = Ossa-Morena Zone; SPZ = South Portuguese Zone; C = Coimbra-Cord6ba shear zone; N = Nazar6 zone: T = Tagus fault. Major diapiric structures are indicated in black. Left: Early Jurassic continental rifting phase. Right: Late Jurassic-Early Cretaceous synrift phase. that are situated in the zone that was inverted during generally assumed that the basement directly to the the Cenozoic the factors only represent minimum east of the study area did not experience significant estimates, while erosion has removed part of the sed- extensional basin development. While the area under imentary record. As mentioned above, the difference discussion is situated quite near these basement out- between the north and south part of the basin roughly crops and relatively far from the present-day ocean- coincides with the Nazar6 zone. The north-south dif- continent transition, the estimates will probably ap- ferences during the Middle Jurassic to Early Creta- proximate the real stretching factors reasonably well. ceous could be explained by differences in thickness The low stretching estimates thus support the notion or density structures of the underlying crust, with that the eastern part of the Lusitanian basin forms thinner continental crust or a higher crustal density the distal part from the area where the continental for the area to the south of the transition zone. In this breakup with Grand Banks was established. context the Nazar6 zone could very well represent an inherited Variscan weakness zone. 5. Comparison with subsidence characteristics of The inferred stretching factors are generally low. other Mesozoic basins of Iberia However, in the framework of the opening of the North Atlantic the estimates are quite reasonable. The subsidence history of the Lusitanian basin Srivastava and Verhoef (1992) estimate from their clearly is controlled by the opening of the Atlantic studies of the restoration of plates a 30% space re- Ocean. In Fig. 9 our results are summarized by duction for the Lusitanian basin. Furthermore, it is backstripping curves for the synthetic stratigraphic 504 G. Stapel et al./Tectonophysics 266 (1996) 493 507 1. ~- GaliciaBasin . 0m 500 t1000

4. i T ~

6. ~c basin 0m

+ 250 B500 Jurassic L. Cretaceous J LTriassic / EarlyJurassic I M]L:~ Cretaceous I Early ] M. ] Early [ Late I Late .[ E._..arly.~. I L ate_~ i r r p 2~ 150 1~ Ma 2(~X3 150 100 Ma Fig. 9. Comparison of two subsidence curves based on the synthetic columns of Fig. 4 (curves 3 and 4) with subsidence curves from neighbouring basins of Iberia. The curve for the Cantabrian basin (2) is taken from Hiscott et al. (1990), the Iberian basin curve (5) is from Van Wees and Stephenson (1995) and the Prebetic basin (6) is from De Ruig (1992). The vertical scale is as indicated in the middle of the figure except for the Cantabrian basin which has the scale displayed to the right of the curve. Different time units are shaded to simplify comparison of the curves.

columns of Fig. 4, which visualize the subsidence the different basins of Iberia it is clear that the curves history of the area under discussion quite well. These do reflect the regional tectonic setting. In the follow- curves can be placed in a more regional context by ing, the timing of the Triassic phase of continental correlating the Mesozoic tectonic pulses with the rifting, the overall present Callovian to Oxfordian neighbouring basins of the Iberian Peninsula (Fig. 1 ). hiatus, the south to north shift of synrift motions and Inspection of Fig. 9 suggests that regional subsidence the response to the opening of the Bay of Biscay will pulses do not directly stand out with respect to devi- be discussed. ations due to local factors. However, from studies of The timing of the Triassic continental rifting G. Stapel et al./Tectonophysics 266 (1996) 493-507 505 phase is badly constrained in all the Mesozoic basins rapidly compared to the previous motions and was of Iberia. Sopefia et al. (1988) describe a system of associated with fast changing depositional environ- pull-apart basins that originated in Late Permian and ments and a number of unconformities (Hiscott et Early Triassic with the Iberian and Cantabrian basins al., 1990). containing the oldest sediments unconformable on From Fig. 9 a shift in time of the synrift motions Variscan basement. In the Prebetic the basin devel- can be deduced for the western Iberian margin from opment started probably a little later during the Early Middle Jurassic to Early Cretaceous. The Porto- or Middle Triassic (Sopefia et al., 1988). In the west- Galicia basin is subjected to extensional stresses ern part of the Iberian Peninsula the oldest sediments more or less at the Jurassic~:~retaceous boundary in lying unconformably on Variscan basement are of contrast with the Lusitanian basin which is subsid- Late Triassic or even Early Jurassic age, but were ing already during the Kimmeridgian and has to the never dated in detail. The same starting point for south of the Nazar6 zone also considerable subsi- sedimentation is also described by Jansa et al. (1980) dence during the Middle Jurassic. The Cantabrian for the Grand Banks. The lack of information on this basin shows a timing similar to the Porto-Galicia first phase of extensional motions, that lasted more basin and in the Prebetic basin Late Jurassic sub- or less until the Sinemurian, hampers any regional sidence represents the main rift phase (Peper and interpretation that goes farther than the probable shift Cloetingh, 1992). In general it seems that the synrift in time of the Triassic initial motions from east to motions were recorded earlier in the south of the west. In general the motions during this time span Iberian continent than in the north. For the Iberian are thought to be related to the progression of the basin this synrift phase is difficult to trace while Tethys rift system (Ziegler, 1989). The timing and vertical motions increased more slowly. kind of subsidence from Sinemurian to Bathonian During the Early Cretaceous, the Porto-Galicia also differs from basin to basin. In the foregoing and Cantabrian basins are under influence of the it has been pointed out that vertical motions in the opening of the Bay of Biscay. This opening resulted South Lusitanian basin indicate a stretching phase especially for the Cantabrian basin in fast subsidence during the Middle Jurassic. Salas and Casas (1993) rates. The other basins on the Iberian continent are describe a Middle Jurassic postrift stage for the subjected to the thermal component of the Middle Iberian basin and De Ruig (1992) describes a phase and Late Jurassic stretching, except for the Prebetic of rifting during the Pliensbachian for the Prebetic basin, which also records a phase of subsidence from basin. From Fig. 9 it appears that the Porto-Galicia Aptian onward which is probably related to motions basin has a hiatus during the Middle Jurassic, similar in the Mediterranean (De Ruig, 1992). to the non-deposition conditions that were described for the northernmost Lusitanian wells. 6. Discussion and conclusions Homogeneous vertical motions begin to take place from Callovian onward on the Iberian con- Subsidence analysis of the Lusitanian basin shows tinent. During the Callovian and earliest Oxfordian a that Mesozoic sedimentation is to a large extent in- hiatus is present in all the Mesozoic basins of Iberia fluenced by a three-fold subdivision of vertical mo- that is associated with karstified surfaces. This hiatus tions. The first episode resulted in continental rifting represents an overall change in depositional envi- of the Variscan basement from Late Triassic until the ronments (Hiscott et al., 1990). During the Middle Hettangian. The sediments were deposited in locally Jurassic carbonate deposition was established in all developing grabens of which the exact orientation the basins of the Iberian continent. The sedimenta- is difficult to trace. Several authors (e.g., Wilson tion during the Late Jurassic involves a substantial et al., 1989; Ribeiro et al., 1990a) have stated that supply in clastic sediments on the western and east- Variscan weakness zones exerted a major control on ern side of the Iberian Massif (Ziegler, 1989; Salas the location of the continental basins. The conditions and Casas, 1993). Ziegler (1989) relates this to an of the pre-rift crust are important controls on the important uplift and eastward tilting of Iberia dur- location of the basin development that are however ing this time span. In general subsidence took place extremely difficult to qualify. On the regional scale 506 G. Stapel et al./Tectonophysics 266 (1996) 493 507 of the Iberian continent the continental rifting started than the crust to the north of it. Thinner crust to earlier and is more pronounced to the east than to the the south also offers a reasonable explanation for the west. Late Cretaceous-Early Tertiary volcanism and em- The second and third episodes of rifting would placement of granites that only occurs to the south of ultimately lead to crustal separation between Iberia the Nazard zone. This alternative explanation implies and Grand Banks. These motions are expressed in that the Nazard zone was not necessarily reactivated a well pronounced different behaviour of the South as a zone with left-lateral displacement only, but and North Lusitanian basin from Middle Jurassic could have accommodated significant extensional to Early Cretaceous. During the Middle Jurassic movement during the Mesozoic as well. tectonic subsidence took place in the South Lusi- tanian basin, while the northern part was uplifted Acknowledgements and was probably subjected to erosion. During the Late Jurassic and Early Cretaceous this north-south This research is part of the MILUPOBAS Project difference is expressed by larger stretching factors sponsored by the E.C.C. (contract number JOU2- to the south. On a regional scale, subsidence of the CT94-0348) and the State of Portugal. Gabinete different Iberian basins supports a south to north mi- para a Pesquisa e Explora~5,o de Petr61eo (GPEP) is gration of the synrift extensional phase, that would kindly thanked for project coordination and permis- be followed during the Cretaceous by a south to sion to publish. Prof. A. Ribeiro and Prof. A. Arche north migration of the final breakup. Wilson et al. are thanked for constructive reviews of the paper. (1989) relate the existence of the Nazar6 weak zone This is contribution no. 950709 of the Netherlands to both the different timing and different subsidence Research School of Sedimentary Geology (N.S.G.). histories of the North and South Lusitanian basin. Our analysis suggests that the control of the weak References zone on differences in vertical motions is much more important than its control on the timing of breakup. Azor, A., Gonzfilez Lodeiro, E and Simancas, J.E, 1994. Tec- tonic evolution of the boundary between the Central Iberian The exact behaviour of the Nazar6 zone during and Ossa-Morena zones (Variscan belt, Southwest Spare). Tec- the Mesozoic in association with the observed dif- tonics. I3: 45-61. ferences in subsidence rates is of special interest. Beetsma, J.J., 1995. The Late Proterozoic/Paleozoic and Her- Wilson et al. (1989) suggest that the zone behaved cynian crustal evolution of the Iberian Massif, N Portugal as a major transfer fault during the Mesozoic and as traced by geochemistry and Sr-Nd-Pb isotope systematics of Pre-Hercynian terrigenous sediments and Hercynian grani- relate the difference in subsidence rates to different toids. Ph.D. thesis, Vrije UniversiteitAmsterdam, 223 pp. fault configurations of the two sections. J.C. Kull- Boillot, G.. Mougenot, D., Girardeau, J. and Winterer, E.L., berg (pers. commun., 1995) concluded from detailed 1989. Rifting processes on the West Galicia margin, Spain. tectonic studies on the Nazar6 fault zone that the In: A.J. Tankard and H.R. Balkwill (Editors), Extensional sense of movement during the opening of the Lusi- Tectonics and Stratigraphy of the North Atlantic Margins. Am. Assoc. Pet. Geol. Mem., 46:363 377. tanian basin had an oblique slip direction with both Bond. G.C. and Kominz, M.A., 1984. Construction of tectonic a left-lateral and an extensional component. This is subsidence curves for the early Paleozoic miogeocline, south- in agreement with the general reconstruction pic- ern Canadian Rocky mountains: implications for subsidence ture presented by Srivastava and Verhoef (1992) in mechanisms, age of breakup, and crustal throning. Geol. Soc, which they observe that at North Atlantic closure Am. Bull., 95: 155-173. Burg, J.R, Iglesias, M., Laurent, R, Matte, R and Ribeiro, A.. time the Nazard zone was co-linear with the norlh- 1981. Variscan intracontinental deformation: the Coimbra- eastern edge of the Bonavista platform that behaved Corddba shear zone (SW Iberian peninsula). Tectonophysics, as a major extensional fault during the Mesozoic. In 78:161 177. the foregoing we have suggested a different compo- Cabral, J., 1989. An example of intraplate neotectonic activity, sition or thickness of crustal blocks as an alternative Vilari~a basin, northeast Portugal. Tectonics, 8:285 303. Cloetingh, S. and Burov, E.B., 1996. Thermomechanical struc- hypothesis to explain the different subsidence rates. lure of European continental lithosphere: constraints from rhe- According to this new explanation, the crust to the ological profiles and EET estimates. Geophys, J. Int., 124: south of the fault zone probably would be thinner 695 723. G. Stapel et al./Tectonophysics 266 (1996) 493-507 507

De Ruig, M.J., 1992. Tectono-sedimentary evolution of the Pre- Salas, R. and Casas, A., 1993. Mesozoic extensional tectonics, betic fold belt of Alicante (SE Spain). Ph.D. thesis, Vrije stratigraphy and crustal evolution during the Alpine cycle of Universiteit Amsterdam, 207 pp. the eastern Iberian basin. Tectonophysics, 228: 33-55. GPEE 1986. The petroleum potential of Portugal, 62 pp. Sopefia, A., L6pez-G6mez, J., Arche, A., P~rez-Arlucea, M., Gu~ry, F., Montenat, C. and Vachard, D., 1986. Evolution Ramos, A., Virgili, C. and Hernendo, S., 1988. Permian and tectono-s6dimentaire du bassin Portugais au M~sozoique suiv- Triassic rift basins of the Iberian peninsula. In: W. Manspeizer ant la transversale de Peniche (Estr6madure). Bull. Centr. (Editor), Triassic-Jurassic Rifting. Developments in Geotec- R6ch. Explor. Prod. Elf Aquitaine, 10: 83-94. tonics, 22B, Elsevier, Amsterdam, pp. 757-786. Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, LE., Smith, Srivastava, S.E and Verhoef, J., 1992. Evolution of Mesozoic A.G. and Smith, D.G., 1990. A Geologic Time Scale 1989. sedimentary basins around the North Central Atlantic: a Cambridge University Press, New York, 265 pp. preliminary plate kinematic solution. In: J. Parnell (Editor), Hiscott, R.N., Wilson, R.C.L., Gradstein, EM., Pujalte, V., Basins on the Atlantic Seaboard: Petroleum Geology, Sedi- Garcfa-Mondejar, J., Boudreau, R.R. and Wishart, H.A., 1990. mentology and Basin Evolution. Geol. Soc. Spec. Publ., 62: Comparative stratigraphy and subsidence history of Mesozoic 397-420. rift basins of North Atlantic. Bull. Am. Assoc. Pet. Geol., 74: Srivastava, S.E, Roest, W.R., Kovacs, L.C., Oakey, G., L6vesque, 60-76. S., Verhoef, J. and Macnab, R., 1990. Motion of Iberia since ILIHA DSS Group, 1993. A deep seismic sounding investiga- the Late Jurassic: results from detailed aeromagnetic measure- tion of lithospheric heterogeneity and anisotropy beneath the ments in the Newfoundland basin. In: G. Boillot and J.M. Iberian Peninsula. In: J. Badal, J. Gallart and H. Paulssen Fontbot6 (Editors), Alpine Evolution of Iberia and its Conti- (Editors), Seismic Studies of the Iberian Peninsula. Tectono- nental Margins. Tectonophysics, 184: 229-260. physics, 221 : 35-51. Steckler, M.S. and Watts, A.B., 1978. Subsidence of the Atlantic- Ingle, J.C., Jr., 1980. Cenozoic paleobathymetry and depositional type continental margin off New York. Earth Planet. Sci. Lett., history of selected sequences within the southern California 41: 1-13. continental borderland. Cushman Found. Spec. Publ., 19: 163- Tankard, A.J. and Balkwill, H.R., 1989. Extensional tectonics 195. and stratigraphy of the North Atlantic margins. Am. Assoc. Jansa, L.F., Bujak, J.P. and Williams, G.L., 1980. Upper Triassic Pet. Geol. Mem., 46, 641 pp. salt deposits of the western North Atlantic. Can. J. Earth Sci., Van Wees, J.D. and Stephenson, R.A., 1995. Quantitative mod- 17: 547-559. elling of basin and rheological evolution of the Iberian basin Julivert, M., FontbotE, J.M., Ribeiro, A. and Nabais Conde, (Central Spain): implications for lithosphere dynamics of in- L.E., 1972. Mapa tect6uico de la penfnsula Ib6rica y Baleares, traplate extension and inversion. Tectonophysics, 252: 163- escala 1:1.000.000. Inst. Geol. Min. de Espafia, Madrid. 178. Palain, C., 1977. Age et pal6og6ograpbique de la base du M6so- Verhoef, J. and Srivastava, S.P., 1989. Correlation of sedimentary zoique (s6rie des gr~s de Silves) de l'Algarve-Portugal merid- basins across the North Atlantic as obtained from gravity and ional. Cuad. Geol. Ib6r., 4: 259-268. magnetic data, and its relation to the early evolution of the Peper, T. and Cloetingh, S., 1992. Lithosphere dynamics and North Atlantic. In: A.J. Tankard and H.R. Balkwill (Editors), tectono-stratigrapbic evolution of the Mesozoic Betic rifted Extensional Tectonics and Stratigraphy of the North Atlantic margin (southeastern Spain). In: E. Banda and E Santanach Margins. Am. Assoc. Pet. Geol. Mem., 46: 131-148. (Editors), Geology and Geophysics of the Valencia Trough, Watts, A.B. and Ryan, W.B.E, 1976. Flexure of the lithosphere Western Mediterranean. Tectonophysics, 203: 345-361. and continental margin basins. Tectonophysics, 36: 25-44. Ribeiro, A., Antunes, M.T., Ferreira, M.E, Rocha, R.B., Soares, Whitmarsh, R.B., Pinheiro, L.M., Miles, ER., Recq, M. and A.-E, Zbyszewski, G., Almeida, F.M., Carvalho, D. and Mon- Sibuet, J.-C., 1993. Thin crust at the western Iberia ocean- teiro, J.H., 1979. introduction h la G6ologie g6n6rale du Por- continent transition and ophiolites. Tectonics, 12:1230-1239. tugal. Serv. Geol. Port., Lisbon, 114 pp. Wilson, R.C.L., Hiscott, R.N., Willis, M.G. and Gradstein, F.M., Ribeiro, A., Kullberg, M.C., Kullberg, J.C., Manupella, G. and 1989. The Lusitanian basin of West Central Portugal: Meso- Phipps, S., 1990a. A review of Alpine tectonics in Portugal: zoic and Tertiary tectonic, stratigraphic and subsidence history. foreland detachment in basement and cover rocks. In: G. In: A.J. Tankard and H.R. Balkwill (Editors), Extensional Tec- Boillot and J.M. Fontbot6 (Editors), Alpine Evolution of Iberia tonics and Stratigraphy of the North Atlantic Margins. Am. and its Continental Margins. Tectonophysics, 184: 357-366. Assoc. Pet. Geol. Mem., 46: 341-361. Ribeiro, A., Quesada, C. and Dallmeyer, R.D., 1990b. Geody- Ziegler, P.A., 1989. Evolution of the North Atlantic -- an namic evolution of the Iberian Massif. In: R.D. Dallmeyer and overview. In: A.J. Tankard and H.R. Balkwill (Editors), Ex- E. Martinez Garcia (Editors), Pre-Mesozoic Geology of Iberia, tensional Tectonics and Stratigraphy of the North Atlantic pp. 399-409. Margins. Am. Assoc. Pet. Geol. Mere., 46:111-129.