Staff Scientist: Paul Wallace
Co-Chief Scientists: Marie-Odile Beslier & R.B. Whitmarsh
Abstract
Ocean Drilling Program (ODP) Leg 149 defined landward and oceanward limits to the ocean continent transition (OCT) in the Iberia Abyssal Plain; however, only one hole penetrated basement in a region probably typical of such a transition. Leg 173 (Fig. 1) is a sequel to Leg 149, and it will enable (1) drilling of a well-imaged major detachment fault (an analogue to the S-reflector), (2) recovery of more rift-related igneous material (e.g., gabbro) and its host rock (continental or slow spreading crust?), (3) testing of the nature of the high between the rift-gabbros and the most landward known serpentinite complex (detached continental outlier, a volcanic mound, or serpentinite?), and finally (4) sampling the oldest oceanic crust that seems to have a continuous volcanic cover ("normal" crust). These kinds of observations, together with the improved quality and quantity of seismic images, will allow us to address the timing and nature of melt generation from the mantle during breakup and to determine the earliest generation of "normal" oceanic crust. The planned drilling will also add to our knowledge of the early sedimentary history of the rifted margin.
Rifted margins contain the principal record of the break up that follows continental rifting and the onset of seafloor spreading, both of which are first-order plate tectonic processes. Such margins exhibit a wide spectrum of characteristics, far greater than the characteristics observed in continental rifts, probably in response to different combinations of astheno spheric temperature, lithospheric rheology, strain rate, and stress. The rifting process, through the indirect effects of concurrent subaerial volcanism as well as greater sedimentation and heat flow, can also have important environmental and resource implications. Drilling commonly affords the only means of directly characterizing the nature, age, and emplacement conditions of igneous, metamorphic, and/or sedimentary rocks formed, deposited, or tectonically exposed during margin formation.
Nonvolcanic margins in particular provide opportunities to investigate and understand the tectonic aspects of rifting for two reasons. First, faults that penetrate deep into the crust and uppermost mantle are often evident on seismic profiles and, as was demonstrated by Leg 149, allow rocks from deeper lithospheric levels to be exposed at the top of acoustic basement. Second, voluminous intrusives/extrusives, which can obscure crustal tectonics, are limited and commonly appear to be absent. Conjugate rifted margins often exhibit some asymmetry in structural style. This asymmetry may be related to the mode of lithospheric rifting, e.g., pure or simple shear, or simply to the location of the original break in the continental crust. The west Iberia margin is an excellent example of a nonvolcanic rifted margin. The Galicia Bank and Iberia Abyssal Plain segments of the margin were cored during ODP Legs 103 and 149, and have been studied extensively by geophysical methods.
Iberia separated from the Newfoundland margin of the Grand Banks in the Early Cretaceous after prolonged rifting that began in the late Triassic and is well documented on both sides of the Atlantic (Wilson et al., 1989; Welsink et al., 1989). The subsequent plate tectonic and seafloor-spreading history of this part of the North Atlantic is well constrained by seafloor spreading magnetic anomalies and demonstrates that Iberia drifted away from North America along roughly E-W fracture zones (e.g., Klitgord and Schouten, 1986).
During the short-lived Late Cretaceous opening of the Bay of Biscay a ridge-ridge-ridge triple-point existed off NW Spain (Sibuet and Collette, 1991). However, throughout its post rift history the west Iberia margin has remained an essentially undisturbed rifted margin that has experienced only minor compression in the north in Eocene time (Pyrenean phase, short lived subduction of Bay of Biscay crust under northern Spain) and in the south and center in the middle Miocene (Rif-Betic phase, gentle folding of abyssal plain sediments).
Offshore, the west Iberia continental margin has been studied extensively by geophysical techniques and to a lesser extent by geological sampling (e.g., Beslier et al., 1993; Boillot et al., 1987, 1988; Hoffman and Reston, 1992; Sawyer et al., 1994; Whitmarsh et al., 1990, 1993; Whitmarsh and Miles, 1995; Whitmarsh et al., in press). It exhibits tilted continental fault blocks and an apparent lack of synrift volcanism, which are characteristic of a nonvolcanic rifted margin. Significant syn-rift volcanism is equally absent on shore.
The first drilling of the OCT off the west Iberia margin was carried out by ODP Leg 103 in 1985 (Boillot, Winterer, Meyer, et al., 1987); this leg drilled a short transect of holes west of Galicia Bank (Sites 637-641, Fig. 1). In 1991 the recommendations of the North Atlantic Rifted Margin Detailed Planning Group were accepted by JOIDES Planning Committee, which programmed two drilling legs in the North Atlantic during 1993. One of these, Leg 149, drilled a transect of holes into acoustic basement across the ocean-continent transition in the southern Iberia Abyssal Plain (Sawyer, Whitmarsh, Klaus, et al., 1994; Whitmarsh, Sawyer, Klaus, and Masson, in press; Sites 897-901, Fig. 1).
Results of Leg 149
Leg 149 drilled a west-to-east transect of five sites. Three sites (Sites 897, 899, and 900) reached acoustic basement (Figs.1 and 2). A fourth site (Site 901) enabled a firm prediction to be made that the underlying basement is continental crust. The sites were chosen in the context of a conceptual model of the location of the OCT previously defined by gravity, magnetic, and seismic velocity modelling and by seismic reflection profiles. The results obtained during the leg broadly confirmed this model but also produced some surprises.
The results of Leg 149 proved the existence of a peridotite ridge at the inferred landward edge of the oceanic crust formed by seafloor spreading. They also showed that between this ridge and Site 901, which is situated on a fault block of almost unequivocal continental crust, there exists a 130-km-wide region that is probably underlain mostly by a heterogeneous transitional crust. One indication of the transitional nature of this crust may be the MORB-like gabbro at Site 900. Other indications are the transitional to alkaline mafic clasts in mass wasting deposits at Sites 897 and 899. The magnetic and seismic reflection character and velocity structure of the crust provide additional evidence. Whether the Site 900 gabbro formed by pre- or synrift metamorphism, the original granulite metamorphic grade and the 40Ar/39Ar age of the cores imply that the gabbro was exhumed by important synrift faulting that accompanied lithospheric extension. Although Site 899 sampled a serpentinite breccia and an underlying serpentinized peridotite mass-flow deposit, magnetic evidence suggests that the basement of this site may be atypical of the rest of the Iberia Abyssal Plain OCT, and it may not be correct to infer continuity of peridotite basement between Sites 897 and 899. Leg 149 succeeded in defining the western boundary of the OCT precisely, and it also limited the eastern boundary without providing more than a single site that sampled the basement itself at a typical location within the transitional zone. Models that explain these observations are presented below. To test these models, drilling is planned to focus on the nature of the basement itself within this zone and on the possible detachment faults identified there.
Review of the Galicia Bank and Iberia Abyssal Plain Transects
The transects of Legs 103 and 149 and their associated research contributed to a number of aspects of the rifted west Iberia margin. These are lithospheric detachment faults, block faulting of the crust, the emplacement and exposure of mantle rocks, synrift magmatism, and the characterization of the OCT. The two margin segments, about 200 km apart, exhibit both similarities and differences. The Galicia Bank margin has a narrow OCT (~30 km), an unusually clear subhorizontal detachment fault (the S reflector), clear crustal fault blocks and extensive margin-parallel peridotite outcrops (possibly accentuated by post-rift uplift), and possible synrift magmatism (SchŠrer et al., 1995). The southern Iberia Abyssal Plain (IAP) margin has an unusually wide OCT (up to 130 km), clear crustal block faults, peridotite basement (exposed both along a narrow ridge and on an isolated high) and, at present, tentative evidence of syn-rift magmatism. Here too, there is apparently no obvious deep and extensive sub-horizontal detachment like S, although a possible detachment, dissected by higher-angle normal faulting, has been recognized. The results of Leg 149 have highlighted the need for more basement drilling, principally within the OCT, in order to understand the rift-to-drift tectonic and magmatic processes at this excellent example of a non-volcanic rifted margin. Further, independent geophysical work since Leg 149 has led to revised tectonic and magmatic models for the rifting and initial seafloor spreading at this margin (Whitmarsh and Miles, 1995; Krawczyk et al., in press; Whitmarsh and Sawyer, in press), which can now be tested by further drilling to basement.
Problems that need further investigation and can be resolved by drilling along the southern IAP transect include:
The above data provide evidence both for emplacement of gabbro in the transition zone, possibly immediately prior to break up, and for detachment tectonics west of Site 901, at least as far as Site 900. An integrated, relatively detailed model incorporating all available data is shown in Figure 5. Here, extension at crustal levels was originally controlled by simple-shear detachment faulting, which may have exhumed lower crust at Site 900 and even peridotite to the west in the broad northwest-southeast basement low east of Site 898. Lithospheric extension at depth, possibly by a pure-shear necking mechanism, and accompanying asthenospheric upwelling led to adiabatic decompression melting and intrusion of the lower crust. Intrusion may have occurred both during detachment faulting and during the subsequent block-faulting (in which the fault geometry was probably controlled by pre-existing fabric landward of about Site 901), which led to final break up. The model predicts the occurrence of a detachment fault, dissected by subsequent block faults, the exposure of progressively deeper lithospheric levels to the west of Site 901 and synrift intrusion into the lower crust or uppermost mantle west of Site 901. Additional drilling during this leg will determine if the model in Figure 5 is correct by testing the detachment hypothesis, by investigating the nature of the basement in the transition zone, and by seeking evidence of synrift magmatism.
As outlined above, Leg 149 largely succeeded in determining the oceanward and landward bounds of the OCT in the southern Iberia Abyssal Plain. The principal problem now is to investigate the nature of the basement within the OCT itself to determine its intrinsic relevance to the general problem of rift-to-drift processes and to test aspects of our best models for these tectonic and igneous processes at the west Iberia nonvolcanic margin. Leg 173 will attempt to achieve the following objectives:
There will be time on Leg 173 to drill no more than three sites. The first priority sites are Iberia-7A (to test the simple-shear-extension model for the upper lithosphere); Iberia-9A or -9B (to test the simple-shear-extension model and assess the lateral extent of the possibly synrift gabbro basement rocks sampled at Site 900); and Sites 901 or Iberia-8A (to confirm the continental nature of the basement, and its approximate original level in the crust, to thereby limit the landward edge of the OCT).
Leg 173 proposes to (1) drill a small number of holes to basement in order to characterize the OCT and a low-angle detachment fault, (2) test the pure and simple shear lithospheric extension models, (3) determine the extent of synrift magmatism and the nature of the earliest oceanic crust, and (4) investigate the mechanisms of mantle unroofing (related to the presence of a peridotite ridge).
The tectonic and petrologic objectives of this rifted-margin study should particularly benefit from logging data. We recommend that standard geophysical, geochemical, and Formation MicroScanner (FMS) logs be run on each hole. Downhole measurements will contribute to (1) acoustic characterization of penetrated structures, (2) site-to-site comparison of chemical signature in basement rocks within the OCT, and (3) detailed description of tectonic features. Standard geophysical and geochemical logs should be run in each of the proposed holes to meet the first two objectives. While the recording of physical properties data is essential to core-log integration studies, electric images allow a clear identification and centimeter-scale description of the succession of basement units. The FMS electrical images will give the necessary high resolution for accurate description of tectonic features, in terms of lithologic boundaries, bedding attitude (dip and strike), presence of fractures and faults and their spatial orientation, and degree of alteration of basement features. In summary, geochemical and geophysical logs and FMS high-resolution electrical images should be acquired in all holes.
Site Iberia-7A
The seismic crossing line confirms the N-S trend of the basement high on which the site is situated. Further processing will probably be required to image the possible detachment fault below the high seen on adjacent profile LG-12. No shift in the site is suggested on the basis of the new data.
Site Iberia-8A
The N-S crossing of this site plus a further E-W crossing of the broad high on which it lies reveal that the high has a trend somewhat west of north and shoals quite rapidly northward; the high could conceivably represent the southward continuation of the Vasco da Gama Seamount below the Iberia Abyssal Plain. There is a strong suggestion of tilted reflectors (pre-rift sediments?) on the east side of the high and of a diffractive 'ledge' on the west side (at CDP 3100) which may represent the top of the basement that underlies the prerift sediment. This reinforces the view that the site has features that make it an attractive alternative to re-drilling Site 901, also situated on a tilted fault block. The shallower basement on the E-W profile might, with further processing, suggest a better location more easily reached with the drill (possible new site here called Iberia-8B).
Sites Iberia-9A and Iberia-9B
The N-S seismic data crossing of Site Iberia-9B confirms the weak N-S trend of the basement high. The profile does not image the east-dipping detachment fault underlying the site that is visible on a prestack depth migrated profile; the new profile will require further processing. No shift in the site is suggested on the basis of the new data.
The following priorities have been assigned to the sites.
Priority 1. Site Iberia-7A, the most oceanward site within the OCT, will sample basement on a topographic high, which is apparently bounded to the west by a major normal fault. This will test the simple-shear extension model for the upper lithosphere and may even reveal an upper mantle exposure. Sites Iberia-9A/9B (either 9A or 9B, but not both, will be drilled) will transect a crustal detachment fault, and thereby test the simple-shear lithospheric extension model, and assess the lateral extent of the possibly synrift gabbro basement rocks sampled at Site 900. Site 901 or Site Iberia-8A/8B (one site only) will be drilled to confirm the continental nature of the basement, and its approximate original level in the crust, to thereby limit the landward edge of the OCT. Final choice of site will depend on results of further seismic processing and estimated drill times. All the above sites will potentially provide evidence to assess the contribution and lateral extent of synrift magmatism.
Priority 2. Site Iberia-10A will be drilled to demonstrate the existence, age, and chemical nature of the early-formed oceanic crust 20 km oceanward of the peridotite ridge.
To Leg 173 Proposed Site Information