moho and lower crustal reflectivity beneath a young rift basin: results from a two-ship,...

Geophys. 1. Inl. (1994) 118, 159-180

Moho and lower crustal reflectivity beneath a young rift basin: results from a two-ship, wide-aperture seismic-reflection experiment in the Valencia Trough (western Mediterranean)

J. S. Collier,' P. Buhl,2 M. Tome3 and A. B. Watts1 'Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, U K Lamont-Doherty Geological Observatory, Palisades, NY 10964, USA Institute of Earth Sciences (Jaume Almera), Consejo Superior de Investigaciones Cientifcas, 08028 Barcelona, Spain

Accepted 1994 January 12. Received 1994 January 10; in original form 1993 April 21

S U M M A R Y We present new images of the lower crust and Moho beneath the Valencia Trough-a young rift basin in the western Mediterranean. These images were obtained from a two-ship, wide-aperture reflection experiment and show several features not distinguishable on previously available conventional single-ship reflection profiles.

The Moho, which was previously only seen intermittently, can now be confidently traced throughout the basin. We have constructed a present-day depth-to-Moho map and estimated the degree of crustal thinning for the whole basin. Crustal thinning is at a maximum in the centre of the basin, where p values reach 3.15 f 0.25. At the margins of the basin the p value decreases to 1.5 f 0.1.

The reflective character of the lower crust and Moho is different beneath different parts of the basin. We have been able to correlate these differences with the amount of stretching. We therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent (Neogene) stretching event that opened the Valencia Trough. Along the Iberian margin there is well-developed lower crustal reflectivity consisting of 1-2 s two-way time (TWT) of 1-4 km long, near-horizontal reflectors underlain by a more continuous, although not significantly stronger, reflector interpreted to be the reflection Moho. Offshore, this lower crustal reflective unit thins rapidly, such that it is undetectable 40-50 km from the coastline where the crust has been stretched by a factor of 1.7 f 0.1. As the lower crustal reflectivity becomes undetectable the reflection Moho becomes a robust , continuous event. Where /3 exceeds 2.4 f 0.2, however, the Moho is a weak event and difficult to trace. We infer that either the extension itself o r associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the Moho where extension was moderate.

The Balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the Valencia Trough. The lower crust shows evidence of weak, but locally variable lower crustal reflectivity. It is possible that the lower crustal reflectivity was preserved simply because the Moho was flexed downward and so decompression, and hence melting, of the upper mantle was restricted. This suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification.

Key words: lower crustal reflectivity, stretched continental crust, Valencia Trough.

159

at CSIC

on August 3, 2015

http://gji.oxfordjournals.org/D

ownloaded from

http://gji.oxfordjournals.org/

160 J . S. Collier et al.

1 INTRODUCTION

The nature of the continental lower crust and Moho beneath regions that have been recently tectonically active has been a subject of great interest since the proliferation of deep seismic reflection profiles in the 1980s. It has been suggested that beneath Tertiary rifts in western Europe the lower crust and Moho are comparatively young features that were jeformed during or after the main rifting phase (Bois 1992). Beneath rifts such as the Rhine Graben (Brun et al. 1991; Bois & ECORS 1991) and the Gulf of Lion (Bois & ECORS 1991) the reflective lower crust has been shown to be significantly thinned relative to the upper crust, and the seismic reflection Moho greatly elevated. The observation of high P-wave velocities at the base of the crust in these areas has been interpreted as evidence for intrusion of mantle material into the lower crust and hence the re-positioning of the Moho to shallower levels. In this paper we present evidence that this process appears to have occurred beneath the Valencia Trough.

The Valencia Trough is a small basin between the north-eastern border of the Iberian Peninsula and the

Baleric Islands (Fig. 1). It is though to be a rift-type basin that formed during an extensional event that began during the Late Oligocene/Early Miocene. It is one of a number of small basins in the western Mediterranean, lying within the zone of convergence between the European and African plates. Early refraction experiments showed that the basin was underlain by thinned continental crust (with a thickness of only 12-14 km at its centre) and anomalously low P-wave velocity (V,) mantle (7.2-7.8 km s-'; Hinz 1972; Banda et al. 1980). Widespread volcanism, which has been shown to be coeval with the extension of the trough, has been reported from both the commerical drill holes on the Ebro Platform and from DSDP sites 122 and 123 in the centre of the basin (Ryan et al. 1973; Riviere, Bellon & Bonnot-Courtois 1981; Lanaja 1987; Marti et al. 1990). The basin shows fundamental differences in structural style at its margins. Its north-western margin (Catalan-Valencian domain) is characterized by extensional block faulting that was active from Late Oligocene or Early Miocene and persisted during the whole of the Neogene; whilst its south-eastern margin (Betic-Balearic domain), which is thought to be part of the External Betic Belt (Fig. l), shows

Figure 1. Tectonic summary map of the western Mediterranean region based on various sources (e.g. Dewey er al. 1989). The Valencia Trough is bounded by the north-eastern coast of the Iberian Peninsula and the Balearic Islands. The Iberian margin has experienced tectonic extension since the Early Miocene. The Balearic Islands are thought to be an extension of External Betic thrust zone of southern Spain. The box indicates the area shown in Fig. 2.

at CSIC

on August 3, 2015


ownloaded from


https://www.researchgate.net/publication/242140185_Kinematics_of_the_Western_Mediterranean_In?el=1_x_8&enrichId=rgreq-eb8589a3-fa47-4eaf-b10b-5acb954c9eb8&enrichSource=Y292ZXJQYWdlOzIyOTg5MTg5MTtBUzoyNTgzNDY1NzgwODM4NDJAMTQzODYwNjAzMjY2Nw==

Moho and lower crustal reflectivity 161

folding and thrust faulting during the Early to Mid Miocene and a subsequent extension mode (e.g. Fontbot6 et al. 1990; Jenkyns, Sellwood & Pomar 1990; Roca & Desegaulx 1993). This asymmetry has also been determined for the basin’s deep structure by Watts & Torn6 (1992a). Using seismic, gravity and geoid data, together with flexural backstripping of sediments, Watts & Torn6 (1992a) found that the seismic reflection Moho beneath the Island of Mallorca was unex- pectedly deep. They attributed this deepening to be due to the flexural response to crustal loading via thrusting and folding associated with the Betic province. Therefore they con- cluded that, despite the basin’s apparent symmetry on bathy- metric maps (Fig. 2), the Balearic margin is not the conjugate to the Iberian margin, but that it must lie to the south.

The data presented here were collected in November/December 1988 as part of a two-ship seismic experiment known as project VALSIS-2 (Fig. 2) conducted jointly by the Lamont Doherty Geological University of Columbia University, the University of Paris and the Institute FranGais du Petrole. The experiment involved the research vessels R/V Robert D. Conrad, and MJV Jean Charcot and resulted in the collection of seven conventional, single-ship, multichannel reflection profiles (hereafter referred to as CDPs: common-depth-point profiles), eight two-ship, wide-aperture multichannel reflection profiles (COPs: constant-offset profiles) and six two-ship refraction/ wide-angle reflection profiles (ESPs: expanded-spread profiles). In addition, recordings of the airgun shots were made by seismographs operated on land (Gallart et al. 1990). The results of the CDP work can be found in Watts et af. 1990; Mauffret et al. (1992) and Maillard et al. (1992). The results of the ESP work can be found in Torn6 et al. (1992) and Pascal et al. (1992). This paper presents the results of the COP work.

Figure 3 shows crustal sections across the axis of the Velencia Trough from the ESP work of Torn6 et af . (1992) and a complimentary digital ocean-bottom seismometer experiment conducted in the basin by Dafiobeitia et al. (1992). These two refraction experiments confirmed that the basin is underlain by thinned continental crust. Beneath the flanks of the basin both surveys showed that the Moho consists of a sharp velocity contrast, underlying a lower crust with a fairly constant velocity of about 6.4-6.6 km s-’ (ESPs 3,5,6 and 7). Beneath the centre of the trough the results of the two surveys differ slightly. Torn6 et af. (1992) suggests that the lower crustal unit is thinned out all together (ESP 4: a result which they show is consistent with gravity data; Fig. 3b) but Daiiobeitia et al. (1992) suggest that instead it is replaced by a zone with a moderate velocity gradient (of about 0.1s-’; Fig. 3a). In the north-eastern part of the basin (ESP 2) Torn6 et al. (1992) showed the crust to be transitional to oceanic crust. North-east of the VALSIS study area there is oceanic crust (Le Douaran, Burrus & Avedik 1984; Pascal, Mauffret & Patriat 1993).

2 D A T A ACQUISITION A N D PROCESSING

Two-ship, wide-aperture experiments were first described by Buhl, Diebold & Stoffa (1982). During our 1988 experiment navigation was via GPS and Transit, with ship-to-ship ranges being measured using a Trisponder system. Both ships were equipped with a 96 channel, 2.4km long hydrophone

streamer, the Conrad with a digital steamer and the Charcot an analogue streamer. The Conrad fired a 10 gun, total volume 5861 in.” (1 in.’= 16.39cm”) airgun array at a pressure of 2000 psi (1 psi = 703.07 kg m-2) every 30 s. The shooting and recording time of the two vessels were synchronized by checking the ship clocks at the start of each COP. For the period of the survey the difference between the two was found to be constant to within a few milliseconds. During collection of the COP profiles, the two vessels steamed along the same track (Charcot leading) at 4.7 knots (1 knot = 1.85 km hr-I), maintaining a constant separation of 5.4 km (Fig. 2). This separation was chosen to give an unbroken shot-receiver range of 0.3-5.4 km, with ranges of 0.3-2.7km being recorded by the Conrad streamer and the 2.7-5.4 km ranges being recorded by the Charcot streamer. The Charcot steamer was towed at a depth of 20m in order to give constructive interference between the primary and sea-surface ghost within the frequency band 5-20 Hz required for imaging the lower crust and Moho.

The source-receiver offsets for the Charcot records were initially computed from the Trisponder ship-to-ship ranges and the bearings of the two ship courses. The logged ship course bearings were checked by comparing with optical sightings of the Charcot made from the Conrad. It was not possible during the experiment to record the bearing of the tail of the Charcot streamer from the Conrad so, as a first approach, it was assumed that the path of the Charcot and its streamer were in alignment. Traveltime-reduced water wave plots of the nearest-offset channel were then checked, and corrections for actual streamer feathering incorporated into the calculation of range.

The advantage of wide-aperture reflection data is the greater differential normal moveout (NMO) between the deep-water multiples and the primary deep events, which gives a better chance of their suppression and hence a potentially clearer image of the deeper structure. Various pre-stack velocity filtering schemes (four in all) were tested to assess their success of multiple suppression. The reader is referred to Hardy & Hobbs (1991) for a review of these techniques. These schemes were applied to the shot gathers (trace spacing, Ax=25m) rather than CDP gathers (Ax = 50 m) to reduce the problem of spatial aliasing. Our chosen velocity-filtering method was to design an fk fan filter and apply it to the raw (no NMO correction applied) shot gathers in the fk domain. In the raw gathers there was very little energy with negative wavenumbers so it was possible to analyse just positive wavenumbers, giving us up to twice the spatial Nyquist (80 km-’ in our case). As a start we generated test stacks using only the Charcot traces. The preferred filtering scheme consisted of a frequency bandpass filter from 0 to 30 Hz followed by an fk filter of 0-0.1 s km-’. The cost of this multiple-suppression scheme was the near-total obliteration of the upper part of the section. This was not a concern, however, as the upper crustal and sedimentary section was already well imaged in the CDP sections collected by the Conrad. Following this filtering scheme it was noticed on some of the COPs that there was some high-amplitude energy remaining at the nearer offset ranges of the Charcot streamer. Test stacks using all the Charcot traces showed these to be remnant multiples, and that better stacks were produced by omitting some of the

at CSIC

on August 3, 2015


ownloaded from


162 J. S. Collier e t al.

Location of seismic profiles

..I .vv . . . . Recording aperture 0.3-2.7 km Recording aperture 0.3-5.4 km

ESP Profiles 1

I I Figure 2. Top: location of seismic profiles collected during the VALSIS-2 experiment in the Valencia Trough. Profiles illustrated in this paper are marked with bold, continuous lines. Contours are of bathymetry in metres. Bottom: sketches of the various ship configurations used during the experiment. This paper presents the results of the COP data.

nearest offset traces from the stack. Choice of the number of the stack. Tests were also made for COPS for which it was traces to omit was done by trial and error; a clear decided to include all the Charcot traces to determine relationship with, for example, water depth did not emerge whether anything could be gained by including some of the on which to base the decision for subsequent stacks. We also far-offset Conrad recorded traces as well. In general, no experimented with more sophisticated weighting schemes, perceivable improvement was gained by including any of the but these did not result in any appreciable improvement to Conrad traces and, given that inspection of the raw gathers

at CSIC

on August 3, 2015


ownloaded from


Moho and lower crustal reflectiviy 163

(4 Danobeitia et al. (1 992) NW PROFILE I SE

Iberla Mallorca s11 W J 2 08.54 SI I

0

20

4 0 100 200 300 0

SI I s11 W J 2 08.54

1 0

~ Upper Cruet

4 0 200 300

(b) Torne et al. (1 992) (i) ESP results

ESP 5 ESP 4 ESP 3

.......................

Upper Mantle

15 km 80 km

(ii) Gravity results

SE

ESPS(L6 ESP 4 ESP 3

0 j 2100

t / 2350 Upper Mantle

32M)

0 40 80 120 160 200 240 2uo 320

v.e. x 2.5 Distance (km)

Figure 3. Previous models of the across-axis crustal structure of the Valencia Trough. Both cross-sections approximately coincide with line 821 of our study. (a) Cross-section of Datiobeitia et al. (1992) determined from DOBS-airgun data. P-wave velocities are in kms-'. The shaded part of the lower crust beneath the centre of the trough marks an area with a high-velocity gradient. (b) Cross-section of Torn6 et al. (1992) determined from (i) ESPs (P-wave velocities in kms- ') and (ii) gravity data (densities in kgm-'). In this model the lower crust is absent beneath the centre of the trough.

at CSIC

on August 3, 2015


ownloaded from



showed that the two streamers had a very different response, it was decided not to risk distortion of the arrivals by combining the two in a single stack. All the final COP stacks therefore consist of a traveltime merge of the Conrad-to-Conrad stack (from 0 s to the time of the sea-bottom multiple) and the Conrad-to-Charcot stack (from the time of the sea-bottom multiple to 10 s TWT) with a 0.5s TWT linear ramp between the two. Stacking velocities for the sub-basement were taken from the ESP results. Because of the low ray-parameters of the deep events (due to the great water depths over most of the study area and high interval velocities within the crust), the stacks were insensitive to variations of the NMO velocity function within reasonable bounds for continental crust. The same processing scheme (and in particular the number of traces used) was applied to the whole of each COP irrespective of changing water depth. Post-stack processing consisted simply of bandpass filtering (5-20 Hz). Time migration of features of particular interest was attempted but was generally unsuccessful in producing realistic geometries of deep events. All the seismic sections shown in this paper are unmigrated and the implications of this on the geometry of dipping events should be remembered.

3 DATA QUALITY

In the following section we will make observations about the variability of the reflectivity of the lower crust beneath the flanks and centre of the rift. It is therefore essential for us to assess the success of the COP technique at suppressing the deepwater multiples in order to get some measure of the confidence with which we can attribute the observations to geology rather than imaging technique in variable water depths. In general, all the COP profiles show an improvement in signal-to-noise of the deep section but to varying degrees depending on water depth.

Figure 4(a) shows a typical example of the single-ship, conventional CDP data collected during the experiment. In this profile structures of the upper part of the crust are well imaged, and include a well-developed post-rift Miocene- Quaternary sedimentary sequence overlying an eroded, pre-rift Mesozoic carbonate platform. Processing of this section included a pre-stack velocity filter and exclusion of near-offset traces for times greater than the first water-bottom multiple (i.e. in a similar manner to that described above for the processing of the COP data) as an attempt to suppress deep-water multiples. As can be seen, however, this was basically unsuccessful and only limited information as to the nature of the deeper structure can be extracted. In comparison, the COP stack of this line (Fig. 4b) shows a marked improvement in the image of the lower crust, and in particular that of an event around 7 s TWT. We interpret this event as being the reflection Moho on the basis of its arrival time when compared with the ESP results (Fig. 4c). This section, however, despite its significant improvement compared to the CDP stack, still contains remnant multiples. Indeed, in places the Moho itself has such a strong impedance contrast that its first-order sea-floor multiple is clearly visible (between 20 km and 50 km along profile). We therefore had to adopt a strategy to identify remnant multiples to ensure that we interpreted only real events. Three methods of multiple/real event discrimination

were possible-namely traveltime, amplitude and semblance velocity (Taner & Koehler 1969). The use of arrival time was straightforward and involved the recognition of the strongest near-surface reflectors (generally the sea-bed, Messinian unconformity and Mesozoic basement) and the computation of two-way traveltimes for simple first- and higher-order multiples. Obviously this misses complex peg-leg multiples. A further aid was therefore to look at the true amplitude plots of the data and determine the background multiple amplitude decay with time. Events with amplitudes above this level were likely to be real. The use of semblance velocity was a little more subjective but proved a useful method particularly in deep water. The technique consisted of computing standard hyperbolic semblance velocity scans from the Conrad-to-Charcot recordings (a merge of the Conrad and Charcot data was not used because of the problem of the different response of the two ship’s streamers). An example of one of these scans from the centre of the trough is given in Fig. 5. In this example, the arrivals observed between 5.3-6.5 s TWT have too low a semblance velocity to be real reflectors. However, the weaker, less-coherent arrivals between 7 and 8 s TWT have semblance velocities consistent with real, lower crustal events. Notice, however, the poor resolution of the scan for the deep data, and hence its inability to provide any estimates of interval velocities.

Of the eight COPS collected during this experiment, line 818 described above, which has intermediate water depths (500-1500 m), gave the best improvement in the image of the Moho compared to its equivalent CDP profile. As the water depth increases, the success of the COP technique to suppress multiples, and hence improve the image of the deep structure, generally deteriorated. Figs 6(a) and (b) show the CDP and COP stacks from the deepest part of the basin, where water depths exceed 2000m. Comparison of these two profiles, however, shows that, even for this line, there has been some improvement in the image of the base of the crust by the COP. By carefully studying arrival times, amplitudes and semblance velocity it was possible to tentatively recognize some reflectors that were genuine lower crustal returns. The interpretation of this section, as far as was possible, is given in Fig. 6(c). In addition to a problem with the imaging technique, there may also be real (geological) reasons for the poorer Moho image here. Modelling of ESP 2 implied that the Moho in this part of the basin constitutes a velocity gradient zone rather than a sharp velocity discontinuity as inferred from ESP 7 in the south-west part of the basin (Torn6 et al. 1992). Because of the problem of poor signal-to-noise ratio in the deep-water parts of the basin, we will not present any results here of variations in the nature of the lower crust and Moho during the transition from continental to oceanic crust along the axis of the trough (which occurs between ESPs 4 and 2).

Figure 7(a) shows the CDP profile shot along the shallow-water (depths 5500 m) Iberian margin of the basin. The image of the lower crust is good because multiples are not a serious problem and they have sufficient differential moveout to be suppressed in the single-ship stack. The corresponding COP is shown in Fig. 7(b). This section has been included as an illustration of how when working in shallow water the COP experiment design may give a degraded image of the lower crust and Moho. This

at CSIC

on August 3, 2015


ownloaded from



........... ....................... ...... . .................. ... ."%. <.. _.. ........ ....... ....... No Vanical Exaggeration

....... ..---.

-10

- 1 6

- 20

h

6 %

7

8

9 10

0

1

2

3

4 - i

-53 g L

7

8

9

at CSIC

on August 3, 2015


ownloaded from



Discrimination between Multiple and Real Reflectors

C-CH STACKED DATA ~ . " I Line 805 -3-

-4-

,5- v)

v

k

L- -7-

-8-

SEMBLANCE VELOCll

I -Y

- -- - 2.0 3.0 4.0 5.0

CDP Velocity (km/s)

Figure 5. Discrimination between multiple and real reflectors by semblance velocity. The left panel shows a piece of COP line 805 (see Fig. 2 for location). The right panel shows an averaged semblance velocity scan of CDP 6500-6520 gathers. Note how the arrivals between 7 and 8 s TWT have too high a semblance velocity to be multiples generated in the upper part of the section.

degradation of the wide-aperture stack is probably due to contamination by the inclusion of near-surface reflected/refracted arrivals that have non-hyperbolic traveltime curves and hence are not suppressed by the conventional NMO correction applied here. The difference between the CDP and COP stack is shown in more detail in Fig. 8(a). The COP stack shows the same overall features as the CDP but they appear 'smoothed out' and it has lower horizontal resolution. We generated synthetic stacks to simulate the CDP and COP stacks using the reflectivity method of Fuchs & Miiller (1971) and Kennett (1974, 1975). The results are shown in Fig. 8(b). Note that during this modelling no attempt was made to simulate the magnitude of the lower crustal reflectivity seen on CDP 819 but rather we were aiming to draw general conclusions about the imaging process. It is interesting to note, however, that our synthetic CDP 819 stack contains fairly prominent reflections generated at the small velocity steps in the lower crust determined from ESP 6 which will obviously contribute to the observed lower crustal reflectivity. Comparison of the synthetic CDP stack of line 819 with the data, however, implies that additional structure would be required to generate the complexity of the lower crustal unit. This could have been achieved by the addition of interlayered high and low velocities as previously suggested in similar studies (McCarthy & Thompson 1988). The general conclusion that we draw from our modelling is that the poorer resolution of the lower crustal reflective unit by the COP stack of line 819 can be explained by the

amplitude-with-offset characteristics of the crustal model. However, for the other seismic profile shot in shallow water, line 822 (Fig. 9), there seems to be an improvement in the image of the lower crust obtained from the COP compared to the CDP profile. In the CDP image there is no evidence of lower crustal events except for a narrow band of events at 8 s TWT interpreted to be the reflection Moho. In comparison, the COP suggests that there may be reflectors between 7 and 8 s TWT. This is shown in more detail in Fig. 8(c). Again we performed reflectivity modelling to investigate possible causes of the differences between the shallow-water images of lines 819 and 822. One possible explanation for the poor CDP 822 image of the lower crust was the near-surface structure. Offshore Mallorca, the thick post-rift sedimentary sequence of the Iberian margin is absent, and instead the sea-bed has a high P-wave velocity (possibly exposed Mesozoic carbonates; Torn6 et af. 1992). Note that in the two models used to generate the synthetics of Fig. 8 the lower crust has an identical velocity structure, and again we were not attempting to fit the observed data in detail. The synthetics for COP 819 and COP 822 are very similar (as is true for the data) but the synthetic CDP stacks are very different, with none of the reflectivity generated for CDP 819 being seen on CDP 822. The only possible explanation for this is the difference in the overlying crustal structure. For this reason we will use the CDP profiles to interpret the deep structure along the Iberian margin and the COP profiles to interpret the deep structure along the Balearic margin.

at CSIC

on August 3, 2015


ownloaded from



N

(a) CDP 808 sQ

N (b) COP 8 0 8

I

2

1

0 10 20 30 40 50 60 70 90 100 Distance along profile (krn)

Figure 6. Unmigrated seismic profiles of the line 808. The location of this section is given in Fig. 2. (a) Single-ship CDP. Note how below 5.5 s TWT the section is dominated by first and higher order multiples. Vertical exaggeration is x 1 at 4.3 km s-’ . (b) Two-ship COP. Note how although there has been some multiple suppression compared to the section shown in (a), it has not been totally successful. (c) Interpretation of COP with velocity-TWT solution from ESP 2. The multiple events were identified from traveltime and amplitude considerations and semblance velocity scans. Me = Messinian; UC = upper crust; M = Moho. The deeper structure of this section was the most difficult to interpret because of the relatively poor multiple suppression resulting from the deep water.

at CSIC

on August 3, 2015


ownloaded from



10

0 10 20 30 40 50 60 70 80 90 1M) Distance along profile (km)

Figure 7. Seismic profiles of line 819 that runs parallel to the Spanish mainland. (a) Migrated single-ship CDP. Vertical exaggeration is X 1 at 4.0 km s C ' . An enlargement of part of this section is given in Fig. 13. (b) Unmigrated two-ship COP with ESP 6 solution taken from Torn6 ef al. (1992). This profile illustrates the lower and narrow frequency range of the Charcot streamer, resulting in the lower crustal reflectors being much less sharply defined in the COP stack compared to CDP (Conrad streamer recorded) stack. This is an important consideration in the interpretation of the deep structure imaged by the COPS. The differences between the two stacks are shown in more detail in Fig. 8.

4 RESULTS

4.1 Moho topography and the amount of stretching

Reflectors are observed, albeit with variable clarity, on all COP lines between 7 and 8 s TWT. The approximate correspondence of the observed times with estimates of the two-way traveltime to the Moho obtained from the ESP data suggests that these reflections are from the Moho.

Figures 4(d) and 10(c) show line drawings of two profiles across the basin that have been depth converted and plotted without any vertical exaggeration. These sections were constructed by depth-converting reflector picks assuming vertical two-way travel paths and using a velocity field comprised of seismic stacking (NMO) and well-log sonic velocities for the near-surface structure and ESP velocities for the deeper structure. The main source of error in this process was uncertainty in the knowledge of the velocity field. For the Moho picks depth errors were estimated to be

f l km, equivalent to f0.3 s TWT. Note that no account of possible deep lateral velocity variations were made (for line 821 we used a linear interpolation between ESP solutions), but such variations are likely to be much smaller than shallow lateral velocity variations. For consistency the Moho was picked as the first negative (white) pulse. Picking the Moho one cycle too high or too low would contribute an error of f O . l s TWT (for a 10 Hz wavelet). equivalent to 0.3 km. In Fig. 10(c) there appear to be minor, vertical offsets (51 km) in the reflection Moho. These minor offsets are almost certainly due to uncorrected near-surface lateral velocity variations, in particular due to the 1-2 km vertical relief of the Mesozoic basement. In comparison, the Moho shown in Fig. 4(d) is much smoother, probably as a direct result of the smoother overlying basement.

Figure l l(a) shows the present-day depth to Moho constructed from the combined results of this study and previous refraction experiments (Banda et al. 1980; Zeyen et al. 1985; Suriiiach 1988; Gallart et al. 1990; Daiiobeitia et al.

at CSIC

on August 3, 2015


ownloaded from



5.0

CDP 819 COP 81 9 (a) (b) 6.0 VP ( k W

5.0 5.0

% -20 6.0 0 g-lF 7.0 TOP Lower 6.0

CNSt- 8.0

9.0

-30

7'0 3 8.0 z Moho- 7.0 3

8.0

10.0

9.0 9.0 (02.7km) (2 7-5.4km)

10.0 10.0

CONSTANT VELOCITY STACKS

Moho-

CDP 822

5.0

6 0

"O z 8.0 5

9 0 - 10.0

(d) COP 822

5.0

6.0

7.0

8.0

9.0

10.0

3 h

v v)

5.0

6.0

7.0

8.0

9.0

10.0 Conrad Charcot (02.7km) (2.7-5.4km)

Real Synthetic Figure 8. Investigation of the effect of upper crustal structure and offset range on the images obtained in shallow water. (a) Comparison of CDP and COP stacks collected along the Iberian margin (line 819). (b) Synthetic stacks of line 819. The synthetics were calculated using the reflectivity method of Fuchs & Miiller (1971) and include a phase velocity range from 0.35 km s- ' to infinity and include all p-p reflections. The velocity depth model is taken from ESP 6 (Torn6 er al. 1992). The differences in the nature of the image of the lower crust and Moho seen in (a) can be explained by the different source-receiver geometry of the two stacks. (c) Comparison of the CDP and COP stacks collected along the Balearic Mallorcan margin (line 822). (d) Synthetic stacks of line 822. The velocity depth model consists of the ESP 3 solution for the upper structure and the ESP 6 solution for the deep structure, i.e. it is identical to the model used to generate the synthetics in (b). Note how the COP stacks for lines 819 and 822 are similar but how different the CDP stacks are. This difference can only be due to the different upper crustal structure.

1992; Mauffret et al. 1992; Torn6 et al. 1992). Despite the differences in the types of data and methods of analysis, the determined depths to Moho are remarkably consistent; certainly given a reasonable error estimate of f l km on its determination. Our results confirm that the Moho has a minimum depth of 12-13 km below sea-level. Un- fortunately, the available data do not document well the geometry of the Moho in the region of transition from stretched continental to near-oceanic crust which occurs somewhere to the east of 2.5"E (between ESPs 4 and 2; Tom6 et al. 1992). The contours of Fig. l l (a ) give the impression that rather than rising and falling with a constant gradient, the Moho dips more gently in the centre of the basin than at its margins. This is an artefact of the present-day bathymetry and geometry of post-rift Neogene sediments, both of which have steep gradients at the basin margins. A more accurate idea of the geometry of the Moho at the time of the Late Oligocene/Early Miocene rifting

event can be gained from Fig. l l (b) in which a correction for bathymetry and Neogene sediments has been made. In this figure the resulting post-rift crustal thickness has been converted to stretching factor (p ) by assuming an initial (pre-rift) crustal thickness. In the literature the initial crustal thickness beneath the basin has been estimated to be between 30 and 35 km. We have chosen to illustrate the distribution of p factors assuming an initial crustal thickness of 35 km (the value determined for the Iberian mainland; Zeyen et al. 1985). Note, however, that this gives maximum /3 estimates. For a 35 km thick initial crust the resulting /3 values range from a maximum of 3.4 in the centre of the basin to a minimum of 1.6 along the margins. For a 30 km thick initial crust the computed stretching factors would be reduced by 86 per cent, such that the equivalent @ values would have been 2.9 and 1.4. Note that in general the contours of Fig. l l (b) are parallel to the coastline of the basin except for the region offshore Mallorca. Here the crust

at CSIC

on August 3, 2015


ownloaded from



4.2 The seismic character of the lower crust and Moho

The seismic character of the lower crust and Moho varies throughout the basin. These variations, which are illustrated in Fig. 12, are interpreted as being due to the different amounts of stretching experienced throughout the basin.

(4 LINE 822 CDP NE

(b) LINE 822 COP Vdkrnlsl

0

1

2

3

0 , s * .. .- . . . . . . . . .

4-4 s 5 4

6%

7

8

9

10

h

0 10 10 30 40 50 60 Distance along profile (km)

Figure 9. Unmigrated seismic profiles of line 822 which runs along the Mallorcan margin. (a) Single-ship CDP. Vertical exaggeration is x l at 4krns- ' . (b) Two-ship COP together with the velocity solution of ESP 3 taken from Torn6 et al. (1992). Me = Messinian; Mz = Mesozoic; UC = upper crust; LC = lower crust; M = Moho. Reflectivity modelling suggests that the strong reflectors that approximately parallel the sea-floor between 3 and 4 s TWT are S-wave arrivals generated at the near-surface.

is unusually thick, a result previously determined by Watts & Torn6 (1922a, b).

In the computation of the p values shown in Fig. l l (b ) we have made several assumptions that may have resulted in systematic errors. First, we made no account for syn-rift volcanic rocks within the trough. Aeromagnetic (Galdeano & Rossignol 1977) and seismic-reflection data (Marti et al. 1992) have shown the presence of extrusive volcanics in the eastern part of the basin. Unfortunately, however, they could not be corrected for, because their thickness is unknown (there being no identifiable reflector at their base). Failure to correct for these rocks would result in an underestimation of p. A second important assumption made was that the Moho was a passive marker throughout the rifting event. If in reality it has reformed since the extension we will have either over- or underestimated p, depending on the reforming mechanism.

4.2.1 North-west (Iberian) margin

On CDP 819 collected along the southern part of the Iberian margin there is a very distinctive lower crust. It is characterized by well-developed reflectivity consisting of individual, 1-4 km long near-horizontal reflectors (Fig. 13c), which are terminated at its base by a laterally more coherent event (on average 5-10km long) interpreted to be the reflection Moho (Fig. 7a). The Moho itself consists of two positive returns that is probably a result of the source wavelet; these data have had no source signature deconvolution applied. The amplitude of the first reflector of the reflective unit is greater that those that follow (including the Moho itself). The Moho, however, generally has an amplitude slightly higher than the events immediately above it (Fig. 13a). For convenience we will refer to this lower crustal pattern as Type I. We searched for evidence of this distinctive lower crustal signature beneath the centre of the trough by computing true amplitude envelopes for COPS 818 and 821. Fig. 14 shows the results for COP 818, which benefited from relatively good signal to noise as a result of being shot in only moderate water depths. CDP3800 ( p = 1.9 for a 35 km thick initial crust), at the north-western end of the profile has a very similar amplitude envelope to that of Fig. 13(a). To the south-east, CDP 3000, despite being noisier, also seems to have a similar (Type I) lower crustal reflectivity pattern. In contrast, CDP 2400 (j3 = 2.6 for a 35 km thick initial crust) shows a transition to the pattern exemplified by CDP 1800 ( p = 2.8 for a 35 km thick initial crust) in which the Moho is of high amplitude (three times that of CDP 3800) and there does not appear to be overlying reflectivity. We will refer to this pattern as Type I1 lower crustal reflectivity. Of course a single CDP is subject to noise introduced by near-surface multiples in this part of the profile, but this distinctive reflectivity pattern was consistently determined on neighbouring CDPs where the upper crustal structure was different and so expected to generate a different multiple train. We cannot therefore rule out the possibility of weak lower crustal reflectivity, but it is certainly not present with a comparable reflective character to that observed along the Iberian margin. The results for COP 821 are similar but the transitions between the reflectivity types occur at smaller /3 factors (Fig. 15). By CDP 1600 ( p = 2.4 for a 35 km thick initial crust) we have a Type I1 pattern. We conclude that the lower crustal reflective unit probably has disappeared by 20 km along profile and definitely disappeared by 30 km along profile where p exceeds 2.4 (for a 35 km thick initial crust). Both COP 818 and COP 821 therefore show that the lower crustal reflectivity rapidly terminates away from the margin, to be replaced by a second, and equally distinctive, signature. It is unrealistic, given the variable signal-to-noise ratio of the seismic data across the profile, to perform evaluations of reflection coefficients. The fact that the Moho beneath the north-west margin of the trough is two to three times

at CSIC

on August 3, 2015


ownloaded from



-

-

la\ N S

4 - - a E

Y - 1 2 5 Top ,ranr,,an., Mono7

Q MDh"

Vp (km/s) (b) oz

..-. ..- - __ -. ... -----------.. ..*-....-- -. - r,, .?- --- -- No venical exaggeration

Vp(krn/s) 0 2 4 6 8

- 1 6 0" ~ 20

24 Ak

Vp (kmls) 8 6 4 2 0

Figure 10. (a) Unmigrated COP 821. See Fig. 2 for location. An enlargement of part of this profile is given in Fig. 16. Vertical exaggeration is X l at 3.8 km s - ' , (b) Interpretated COP with ESP solutions. Me = Messinian; Mz = Mesozoic; UC = upper crust; LC = lower crust. (c) Depth-converted line drawing plotted without any vertical exaggeration. The details are as Fig. 4. The triangles indicate depth to Moho determined from ESPs 3.4 and 5.

weaker than that beneath the centre may simply be a consequence of it being overlain by 2 s TWT of highly reflective material.

Along the northern part of the Iberian margin (north of about 40"40'N) the lower crust is locally more variable and generally thinner than to the south (Fig. 12). Here, the Type I pattern typical of the region to the south (Fig. 7) is only seen in places, e.g. C2 and C5. In other places there is little reflectivity (C3) or it is significantly disrupted (C4). The reason for the difference in the reflectivity pattern could be most easily explained in terms of an increased stretching factor along the north-eastern part of the margin. This is not shown, however, by our stretching factor map of Fig. l l(b). It is possible that we underestimated the stretching factor in this part of the basin because we did not account for syn-rift volcanics and possible intrusions within the crust.

4.2.2 Centre of the trough

Beneath the centre of the trough the lower crust commonly exhibits the Type I1 pattern described above; that is, it consists of a pair of bright reflectors which are laterally continuous for 5-10 km and little reflectivity above or below

(Al, A2, A3 and A4; Fig. 12). On COP821, however, in the centre of the trough, where B exceeds 2.6 (for a 35 km thick initial crust), the pattern is different (Fig. 16). On this section a second, distinct reflector (labelled 'X') is observed about 1 s TWT earlier than the arrival interpreted as the reflection Moho (labelled 'M'). The amplitude of this event is low, and well with the noise level, so it is likely that we detected it merely because it has a dip and coherency which enabled us to distinguish it from generally flat-lying multiples. We were unable to confidently detect it on the crossing line, COP806, that runs down the centre of the trough. We conclude that it is from an interface striking parallel to the trough although we are unable to determine its true strike and dip. Fig. 16(c) shows the relationship between the events picked from our COP and previous refraction results. There is an inconsistency with reflector X, rather than our reflector M, coinciding with the base of the crust determined by Dafiobeitia et al. (1992) and Torn6 et al. (1992). However, we note that both reflectors X and Y are in the region where Torn6 et al. (1992) show the lower crustal unit to be pinched out. It is possible that X represents the top of a transitional Moho formed by heavy mixing of crustal and mantle material. The fact that

at CSIC

on August 3, 2015


ownloaded from



(.)Depth to Moho (km)

42'

41'

40'

39'

(6tretching factor (beta) (assuming 35 km o rig. thickness)

12"

41"

40"

39"

0" 1" 2" 3" 0" 1" 2" 3" Figure 11. (a) Map of the present-day depth to Moho beneath the Valencia Trough based on all available data. Depths are in kilometres below sea-level. Symbols mark individual datum. The data were averaged and gridded onto a 0.1 degree grid. (b) Approximate stretching factor ( p ) beneath the Valencia Trough. To estimate the crustal thickness immediately following the Late Oligocene/Early Miocene rifting event we subtracted the present-day depth to the base of Neogene sediments (Watts & Torn6 1992a) from the present-day depth to Moho given in (a). The result was converted to stretching factor by assuming an initial (pre-rift) crustal thickness of 35 km (the value determined for the Iberian mainland; Zeyen et al. 1985). As estimates of the initial crustal thickness range between 30 and 35 km, the p values shown are maximum estimates. If we had used an initial thickness of 30 km /I values would be 86 per cent lower and fall in the range of 1.4-2.9.

Daiiobeitia et al. (1992) found the lower crust here to have a high-velocity gradient is additional evidence for the intrusion of mantle material into the base of the crust here. Unfortunately ESP4 was shot along a large tilted basement ridge which made its inversion into a velocity-depth profile difficult as it violated the 1-D assumption of the ESP technique. It was not therefore possible to determine the detailed velocity-depth structure in this region and hence validate that the second of the reflector pair was coincident with a jump to velocities appropriate for the upper mantle. The amplitudes of our Moho picks shown in Figs 16(a) and (b) are too low for us to draw absolute conclusions, but we believe that they may represent the true crust-mantle boundary that has now been modified by magmatic or tectonic processes. For ease of reference we will call this reflective pattern Type 111.

4.2.3 South-east (Balearic) margin

Profiles collected close to the Balearic margin of the basin show yet another type of lower crustal reflectivity (which we will refer to as Type IV). This is exemplified by CDP800 (/3 = 2.2 for a 35 km thick initial crust) in Fig. 14, CDPs 5600 (/3 = 2.2 for a 35 km thick initial crust) and 6400 ( p = 1.6 for

a 35 km thick initial crust) in Fig. 15, and CDP900 (/3 = 1.6 for a 35 km thick initial crust) in Fig. 17. It consists of about 1 s TWT of high-amplitude reflectivity at the base of the crust. The Moho is interpreted to be at the base of the unit in a similar manner to that seen along the Iberian margin. However, compared to the Iberian margin profiles, the lower crustal reflectors themselves cannot be easily traced along the profile, but rather they appear fragmented (e.g. B2, B3 and B4 compared to C1, C2 and C5 of Fig. 12; Fig. 17 compared to Fig. 13). Given the shallow water and lack of near-surface low-velocity layers we believe that the disruption of the lower crustal reflectivity is genuine and not simply the product of interference from multiples.

5 DISCUSSION

5.1 Evaluation of the COP technique

Synthetic wide-aperture experiments are not common, and the only previously reported data sets known to us are those of the LASE group off the US East Coast Passive Margin (LASE Study Group 1986), the NAT group of oceanic crust in the Western North Atlantic (NAT Study Group 1985),

at CSIC

on August 3, 2015


ownloaded from


Trou

gh A

xis

Iber

ian

Mar

gin

1'

Bal

eari

c M

argi

n 5

km

B4

cs

?

* *

* E

* %

* *

*

* 4

%

* :-

2 $

62

* * * *

AS

*

2 *

* * *

* * * 4 *

4

2.

* L

Figu

re 1

2. E

xam

ples

of t

he r

efle

ctio

n M

oho

imag

ed b

enea

th v

ario

us p

arts

of

the

Val

enci

a T

roug

h. T

he d

ata

show

n ar

e un

mig

rate

d C

OP

stac

ks. T

he a

rrow

s po

int

to th

e ev

ent i

nter

pret

ed

as th

e re

flect

ion

Moh

o. T

he s

tars

mar

k re

gion

s of

low

er c

rust

al r

efle

ctiv

ity.

Not

e th

e co

ntra

st i

n th

e ch

arac

ter

of th

e M

oho

bene

ath

the

cent

re o

f th

e Tr

ough

. al

ong

the

Iber

ian

mar

gin

and

alon

g th

e B

alea

ric

Mar

gin.

The

loca

tions

of t

he i

llust

rate

d se

ism

ic s

ectio

ns a

re p

lotte

d on

a b

ase

map

of

fi st

retc

hing

fac

tor

show

n in

mor

e de

tail

in F

ig.

Il(h

).

+

4

W

at CSIC on August 3, 2015http://gji.oxfordjournals.org/Downloaded from



In - v I

E Y

a, C

v) - .-

5: Llr

h

v V

rn

0 N

3 n 4 ,

u

N N

m - 3

00

h e

at CSIC

on August 3, 2015


ownloaded from



10 km COP PROFILE 8 18

.EE I \ , , < I \ r. I , , lP 11 *. rVpd I NW' (a) --

CDP 1800 CDP 2400 CUP 3000 CDP 3800

CDP 800 CDP 1800 CDP 2400 CUP 300U CDP 3800 M2 = 2nd Serhlrom Uulr~ple. M3 = 3 d Scabuirom Mulfiplc ctc MM = Moho-Seabonom Muluple. n C =Top reflective lower cnrsr

TRUE AMPLITUDE

Figure 14. Recognition of lower crustal reflectivity patterns for COP818. (a) Stacked section showing identified patterns (see text for details); (b) true amplitude envelopes of individual CDPs used to determine patterns.

A I A A A A 1 1 A " CDP4M) IWO IW 2400 3200 WXl s w o 5600 6400

(b)

CDP 400 CDP loul CDP 1600 CDP 24x1 CDP 3200 CDP 4oM) CDP 5NO CDP 56W CDP 64W

Unlabelkd picks = seabottom multiplur M=Maho TZC=Topicflcci,vclowciciurf

TRUE AMPLITUDE

?@re 15. Recognition of lower crustal reflectivity patterns for 30P 821. (a) Stacked section showing identified patterns (see text or details); (b) true amplitude envelopes of individual CDPs used o determine patterns.

the BIRPS group off the north coast of Scotland (Peddy 1990), the Cambridge Marine Group off Rockall Trough (Joppen & White 1990) and the Lamont group around the Hawaiian-Emperor Seamount Chain (Watts et af. 1985; ten Brink & Brocher 1987). The COP technique has two principal advantages over conventional CDP data. First, it presents the opportunity to improve the relatively weak, deep reflectors through the provision of high CDP fold for statistical signal-to-noise ratio enhancement, and second, it provides sufficient offset to distinguish between primary and multiple arrivals. In the analysis presented here we have only exploited the latter possibility, which resulted in significant improvements in the image of the reflection Moho in particular. We did not exploit the former capability because of the different responses of the two streamers used in the experiment and uncertainties in the range determinations of the Charcot streamer (due to the inability to take navigational fixes of its tail from the Conrad). A further limitation of the experiment was the lack of a seismic source aboard Charcot. If Charcot had been equipped with a sufficient source then, with the same ship configuration during acquisition, the additional source-receiver range of 5.4-8.0 km would have been recorded by Conrad's streamer. In theory the larger range offsets would have enabled a more detailed analysis of the deep reflectors, such as improved resolution of stacking (NMO) velocities and AVO (amplitude versus offset) studies. However, ex- perience of the UK BIRPS group off the north coast of Scotland, where they collected a 16 km range synthetic- aperture reflection profile (SLAVE) is somewhat discourag- ing of the potential use of such experiments to deduce the physical properties of the lower continental crust. A detailed investigation of the AVO characteristics of possible physical models of the lower continental crust by Peddy (1990) suggests that angles of incidence in excess of 20" are required to infer anything meaningful. In our experiment design, maximum angles of incidence of Moho reflections are of the order of 5-10". An available offset range up to 8km would extend this maximum angle of incidence to about Is", i.e. still short of the required maximum. The potential improvement of the stack with the inclusion of Charcot-to-Conrad traces therefore had to be traded off -against the disadvantages of having both ships firing. This would have required an increased firing interval and hence compromised the fold of data for the conventional single-ship CDP stack required for the investigation of the shallow structure.

5.2 The seismic character of the Moho and lower crust

We have determined there to be a difference between the seismic signature of the lower crust and Moho beneath the margins of the Valencia Trough and its centre. Our observations are summarized in Fig. 18. We have found a correlation between the form of the reflectivity and the amount of extension. We therefore conclude that the most recent (Late Oligocene/Early Miocene) stretching event that opened the Valencia Trough was responsible for modifications to the lower crust.

Excluding for a moment the pattern observed along the Balearic margin, there appears to be a transition through

at CSIC

on August 3, 2015


ownloaded from



Distance along profile (km)

80 100 120

Distance along profile (km)

60 100 120

20 40 60 80 100 120 140 160 (c) I I I I I I I 12

- 13 Danobeitia et al. (1992) Torne et al (1992)

12

13-

- - - * COP 821 Picks

0 20 40 60 80 100 I20 140 160 Distance along profile (km)

Figure 16. Detail of C O P 821 (unmigrated) in the centre of the trough. (a) Amplitude-equalized, variable-area plot. (b) True amplitude wiggle plot with events interpreted as real marked. Events labelled X and Y are thought to be within the crust. The events labelled M are thought to be the Moho. Despite the low signal-to-noise ratio of all these events, we interpret them as real because of their traveltimes, coherency and semblance velocity. (c) Comparison of depth-converted picks shown above with crustal structures determined from two previous refraction studies. Note how the Moho of both refraction models coincides with event X.

at CSIC

on August 3, 2015


ownloaded from



COP PROFILE

(a) knplilude envelope 0 0 0

VI

U

3 3

W

(D

A

0 Stacked CDPs 900-903

822

Amplitude

- CDP900 901 902 903

TRUE AMPLITUDE Low pass filtered 15 Hz

Distance along profile (km) ( 4 20 30

5.0

6.0

7.0

h cn v

8.0

9.0

10.0

Figure 17. Detail of COP822 showing the character of the lower crustal reflectivity along the Balearic margin (Type IV reflectivity pattern). (a) Amplitude envelope; (b) individual CDPs plotted to true scale; (c) stacked section plotted with equalized amplitudes. The data are unmigrated and have been bandpass filtered from 3 to 20 Hz.

three basic types of pattern (Types I, I1 and 111). In general the transition occurs at lower p values in the north-eastern qart of the basin. We believe that this is evidence that we lave underestimated the stretching factor here because of he addition of the melt to the crust via extrusion and

intrusion of the lower crust and Moho region. Underestima- tion of p in the north-eastern part of the basin would also explain the disruption in the lower crustal reflectivity observed along the Iberian margin.

The conclusion that the extensional event that opened the Valencia Trough was responsible for the reflectivity pattern today, raises several questions that should be addressed. First, was the reflective layering observed along the Iberain margin (Type 1) added during this extension or was it weakenedlremoved from the other parts of the basin where the stretching was more severe? Lower crustal reflectivity similar to that seen on CDP 819 has been observed on many other deep seismic profiles in western Europe and several authors have speculated as to its origin. On the whole, previous studies have inferred that the reflectors originate by some sort of ductile flow or magmatic intrusion during rifting (e.g. Matthews & Cheadle 1986; Meissner & Kusznir

1987; Matthews & Smith 1987; Reston 1988). However, if the reflectors in the Valencia Trough were produced in some way by extension, we would expect, following the arguments of Peddy et al. (1989), the intensity of the reflectors to increase towards the trough axis. This clearly is not the case here, and so we believe that i t is probable that the reflective layering was present prior to the Late Oligocene/Early Miocene stretching event (see Watts et al. 1990, for a fuller discussion of this point). It is possible, however, that the reflectivity was introduced during an earlier extensional event that affected the whole of north-west Europe but it was the magnitude and/or extension rate in the Valencia Trough which subsequently significantly weakened or even destroyed it.

The second question to be addressed is therefore what process weakened or destroyed the lower crustal reflectivity (whilst maintaining or possibly enhancing the reflection Moho) in the centre of the trough? Two possible mechanisms are shearing and melting. If shearing were the dominant mechanism then mylonite shear zones would have had to be developed at the base of the crust to account for the reflectivity of the Moho. However, we prefer the

at CSIC

on August 3, 2015


ownloaded from



Along axis TypeI TypeII Type 111

Top of lower crust enhanced

Lower crustal reflecuvity lower crustal weakened reflectivity weakeneddestroyed

Moho enhanced Moho weakened/desWoyed

- . -

sw NE

Across axis

NW SE

s m h melt

volumes volumes

: C N S I

TypeI TypeII Type 111 Type IV Top of lower crust enhanced

Lower crustal Lower crustal reflectlvity lower crustal reflectlvity weakened reflectivity weakened! preserved

Moho deseoyed

enhanced Moho weakened/ - - - - - -

- - I

volumes volumes

Pre-Oligocene Post-Oligocene Figure 18. Diagram of the observations and possible processes affecting the lower crust beneath the Valencia Trough.

explanation that melting was the dominant mechanism. In this model the transition from Type I to Type I1 lower crustal reflectivity patterns occurs when stretching was sufficient to initiate melting in the upper mantle. It i s possible that there has been underplating at the base of the crust which has enhanced the reflection Moho.

The third observation that needs to be explained is the nature of the crust in the centre of the basin where the extension was greatest. We suggest that large volumes of melt were generated here and significant amounts of mantle material were injected into the base of the crust. This process has been well documented on volcanic margins of the north-east Atlantic (White 1987). Is it possibie that the material between our reflectors X and M (which reaches a maximum thickness of about 2.5 km) is underplated material? According to McKenzie & Bickle (1988), 2.5 km of melt could have been produced if the mantle temperature exceeded 1380 “C.

The final observation that we need to reconcile is the observed reflectivity pattern offshore the Balearic Islands. If we assume that the Balearic margin is not the conjugate to the Iberian margin and that extension was much greater than that calculated from the present-day Moho depth, how did the lower crustal reflectivity survive? The easiest explanation is that the Moho was flexed downward at the same time as the extension and so mantle decompression and hence melting was significantly restricted. If this

inference is correct it implies that it was the melting/igneous injection rather than the tectonic extension itself that weakenedldestroyed the lower crustal reflectivity in the centre of the basin.

6 CONCLUSIONS

We conclude that in the Valencia Trough the most recent (Neogene) extensional event modified the reflective character of the lower crust and Moho. The extension (or associated melting), where moderate (2.2 I f i z 2.9), significantly weakened or even destroyed previously existing lower crustal reflectivity whilst at the same time enhancing the reflection Moho. Where the extension was more severe (/3 2 2.9) both the lower crustal reflectivity and the Moho have been weakenedldestroyed.

ACKNOWLEDGMENTS

The data used in this study were obtained as part of a join, project between Lamont-Doherty Geological Observatory, the University of Paris and the Institut Franqais du Petrole. We would like to thank the officers and staff of the R/V Robert D. Conrad and the M/V Jean Charcot for their help at sea. We would also like to thank Joyce Alsop fc assistance with the processing of the COP data and Ala

at CSIC

on August 3, 2015


ownloaded from



Mauffret for dicussions on seismic stratigraphy. The original manuscript was improved by comments made by Tim Minshull and two anonymous reviewers. Principal funding for this work came from NSF grant OCE-8214363 and CRNS grant 1774.

REFERENCES

Banda, E., Ansorge, J., Boloix, M. & Cbrdoba, D., 1980. Structure of the crust and upper mantle beneath the Balearic Islands (Western Mediterranean), Earth planet. Sci. Lett., 49,

Bois, C., 1992. The evolution of the layered lower crust and Moho through Geological time in Western Europe: contribution of deep seismic reflection profiles, Terra Nova, 4, 99-108.

Bois, C. & ECORS Scientific Party, 1991. Post orogenic evolution of the European crust studied from ECORS deep seismic profiles, Am. geophys. Un. Ser., 22, 59-68.

Brun, J.P., Wenzel, F. & ECORS-DEKORP Team et al., 1991. Crustal scale structure of the Southern Rhine graben from ECORS-DEKORP seismic reflection data, Geology, 19,

Buhl, P., Diebold, J.B. & Stoffa, P.L., 1982. Array length magnification through the use of multiple sources and receiving arrays, Geophysics, 47, 311-315.

Daiiobeitia, J.J., Arguedas, M., Gallart, J . , Banda, E. & Makris, J., 1992. Deep crustal configuration of the Valencia trough and its Iberian and Balearic borders from extensive refraction and wide-angle reflection seismic profiling, Tectonophysics, 302, 37-55.

Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H. W. & Knott, S.D., 1989. Kinematics of the Western Mediterranean, in Alpine Tectonics, pp. 265-283, eds Coward, M.P., Detrich, D. & Park, R.G., Geological Society Special Publication No. 45, London.

FontbotC. J.M., Guimeri, J., Roca, E., Sibat, F., Santanach, P. & FernBndez-Ortigosa, F., 1990. The Cenozoic geodynamic evolution of the Valencia Trough (Western Mediterranean), Rev. SOC. Geol. ESP., 3, 249-259.

Fuchs, K. & Miiller, G., 1971. Computation of synthetic seismograms with the reflectivity method and comparison with observation, Geophys. J.R. astr. SOC., 23, 417-433.

Galdeano, A. & Rossignol, J.C., 1977. Contribution de I’aeromagnetisme 6 I’btude du Golf de Valence (MCditerranCe Occidentale), Earth planet. Sci. Lett., 34, 85-99.

Gallart, J., Rojas, H., Diaz, J . & Daiiobeitia, J . J . , 1990. Features of deep crustal structure and the onshore-offshore transition at the Iberian flank of the Valencia Trough (Western Mediter- ranean), In Geophysics of the Mediterranean Basin, eds Daiiobeitia, J.J. & Pinet, B. J . Geodyn., 12, 233-252.

Hardy, R.J.J. & Hobbs, R.W., 1991. A strategy for multiple suppression, First Break, 9, 139-144.

Hinz, K., 1972. Crustal structure of the Balearic Sea, Tectonophysics, 20, 295-302.

Jenkyns, H.C., Sellwood, B.W. & Pomar, L., 1990. A Field Guide to the Island of Mallorca, The Geologist’s Association, London.

Joppen, M. & White, R.S., 1990. The structure and subsidence of Rockall Trough from 2-ship seismic experiments, J . geophys. Res., 95, 19 821-19 837.

Kennett, B.L.N., 1974. Reflections, rays and reverberations, Bull. seism. SOC. Am. , 64, 1685-1696.

Kennett, B.L.N., 1975. Theoretical seismogram calculation for laterally varying crustal structures, Geophys. J. R. astr. SOC.,

2 19-230.

758-762.

42,579-589.

Lanaja, J.M., 1987. Contribucion de la exploracion petrolifera al concocimiento de la geologfa de Espaiia, ICME, Seru. Publ. Min. Indus. Energ., Madrid.

LASE Study Group, 1986. Deep structure of the US East Coast passive margin from large aperture seismic experiments (LASE), Mar. Petrol. Geol., 3, 234-242.

Le Douaran, S., Burrus, J. & Avedik, F., 1984. Deep structure of the north-western Mediterranean Basin: Results of a two-ship seismic survey, Mar. Geol., 55, 325-345.

Maillard, A., Mauffret, A., Watts, A.B., Torne, M., Pascal, G., Buhl, P. & Pinet, B., 1992. Tertiary sedimentary history and structure of the Valencia Trough Western Mediterranean), In Geology and Geophysics of the Valencia Trough, Western Mediterranean, Eds Banda, E. & Santanach, P., Tecionophysics, 203, 57-75.

Marti, J., Grachev, A , , Mitjavila, J., Aparicio, A. & Roca, E., 1990. Cenozoic magmatism in the Valencia Trough, Terra Abstr., 2, 6.

Marti, J., Mitjavila, J., Roca, E. & Aparicio, A, , 1992. Cenozoic magmatism of the Valencia Trough (Western Mediterranean): Relationship between structural evolution and volcanism, in Geology and Geophysics of the Valencia Trough, Western Mediterranean, eds Banda, E. & Santanach, P., Tectonophysics, 203, 145- 165.

Martin, P. & Surinach, E., 1988. Estructura de la corteza en la zona entre Ibiza y Castell6n. Primeros resultados, in Xarxes sismiques instrumentacid i aplicacid a la sismotectijnica, pp.

Matthews, D.H. & Cheadle, M.J., 1986. Deep reflections from the Caledonides and Variscides west of Britain and comparison with the Himalayas, in Reflect. Seism. Geodyn. Ser., Vol. 13, pp. 5-20, Am. geophys. Un., Washington, DC.

Matthews, D.H. & Smith, C., 1987. Deep seismic reflection profiling of the continental lithosphere, Geophys. J . R. astr.

Mauffret, A, , Maillard, A., Pascal, G., TornC, M., Buhl, P. & Pinet, B., 1992. Long-listening Multichannel seismic profiles in the Valencia Trough (VALSIS-2) and the Gulf of Lions (ECORS): a comparison, in Geology and Geophysics of the Valencia trough, Wesiern Mediterranean, in Banda, E. & Santanach, P., Tecionophysics, 203, 285-304.

McCarthy, J. & Thompson, G.A., 1988. Seismic imaging of extended crust with emphasis on the western United States, Geol. SOC. Am. Bull., 100, 1361-1374.

McKenzie, D.P. & Bickle, M.J., 1988. The volume and composition of melt generated by extension of the lithosphere, J . Petrol., 29, 625-679.

Meissner, R. & Kusznir, N. J., 1987. Crustal viscosity and the reflectivity of the lower crust, Ann. Geophys., 5, 365-374.

NAT Study Group, 1985. North Atlantic Transect: A wide-aperture two-ship multichannel seismic investigation of the oceanic crust, 1. geophys. Res., 90, 10 321-10 341.

Pascal, G., Tomb, M., Buhl, P., Watts, A.B. & Mauffret, A., 1992. Crustal and velocity structure of the Valencia Trough (Western Mediterranean). Part 11: Detailed interpretation of five expanded spread profiles, in Geology and Geophysics of the Valencia Trough, Western Mediterranean, eds Banda, E. & Santanach, P., Tectonophysics, 203, 21-35.

Pascal, G.P., Mauffret, A. & Patriat, P., 1993. The ocean- continental boundary in the Gulf of Lion from analysis of expanding spread profiles and gravity modelling, Geophys. J . Int., 1l3, 701-726.

Peddy, C., 1990. Synthetic aperture and near vertical deep reflection studies of the continental crust, PhD thesis, University of Cambridge, Cambridge.

Peddy, C., Pinet, B., Masson, D., Scrutton, R., Sibuet, J.C., Warner, M.R., Lefort, J.P. & Shroeder, I.J., 1989. Crustal structure of the Goban Spur continental margin, Northeast

521-537, Ed. CIRIT.

SOC., 89, 209-215.

at CSIC

on August 3, 2015


ownloaded from



Atlantic from deep seismic reflection profiling, J. geol. SOC. Lond., 146, 427-437.

Reston, T.J., 1988. Evidence for shear zones in the lower crust offshore Britain, Tectonics, 7, 929-945.

Riviere, M., Bellon, H. & Bonnot-Courtois, C., 1981. Aspects giochemiques et gCochronologiques du volcanisme pyroclas- tique fore dans le Golfe de Valence: site 123 DSDP, (Leg 13 (Espagne). ConsCquences gtodynamiques, Mar. Geol., 41,

Roca, E . & Desegaulx, P., 1993. Geological evolution and vertical movement analysis of the Valencia Trough area (western Mediterranean), Mar. Petrol. Geol., 9, 167-185.

Ryan, W.B.F., Hsu, K.I. et al., 1973. Initial Reports of the Deep Sea Drilling Project, Vol. 13, U S Government Printing Office, Washington, DC.

Taner, M.T. & Koehler, F., 1969. Velocity spectra-digital computer derivation and applications of velocity functions, Geophysics,

ten Brink, U.S. & Brocher, T.M., 1987. Multichannel seismic evidence for a subcrustal intrusive complex under Oahu and a model for Hawaiian volcanism, J. geophys. Res., 92,

TornC, M., Pascal, G., Buhl, P., Watts, A.B. & Mauffret, A, , 1992. Crustal structure of the Valencia Trough (Western Mediter-

295-307.

34, 859-881.

13 687-13 707.

ranean), Part I. A combined refraction/wise-angle reflection and near-vertical reflection study, in Geology and Geophysics of the Valencia Trough, Western Mediterranean, eds Banda, E. & Santanach, P., Tecfonophysics, 203, 1-20.

Watts, A.B. & TornC, M., 1992a. Subsidence history, crustal structure and thermal evolution of the Valencia Trough: A young extensional basin in the Western Mediterranean, J geophys. Res., 9, 20 021-20 041.

Watts, A.B. & TornC, M., 1992b. Crustal structure and the mechanical properties of extended continental lighosphere in the Valencia Trough (western Mediterranean), J. geol. SOC. Lond., 149, 813-827.

Watts, A.B., ten Brink, U.S., Buhl, P. & Brocher, T.M., 1985. A multichannel seismic study of lithospheric flexure across the Hawaii-Emperor seamount chain, Nature, 315, 105-1 11.

Watts, A.B., TornC, M., Buhl, P., Mauffret, A., Pascal, G. & Pinet, B., 1990. Evidence for reflectors in the lower continental crust before rifting in the Valencia trough, Nature, 348, 631 -63.5.

White, R.S., 1987. When continents rift, Nature, 327, 191. Zeyen, H.J., Banda, E., Gallart, J . & Ansorge, J., 1985. A wide

angle seismic reconnaissance survey of the crust and upper mantle in the Celtiberian Chain of eastern Spain, Earth planet. Sci. Lett., 75, 393-402.

at CSIC

on August 3, 2015


ownloaded from


moho and lower crustal reflectivity beneath a young rift basin: results from a two-ship,...

Documents