Shallow magma sources during continental rifting and breakup of the South Atlantic

Dieter Franke, Soenke Neben, Stefan Ladage, Bernd Schreckenberger & Karl Hinz

Federal Institute for Geosciences and Natural Resources, Hannover, Germany (BGR)

Dieter.Franke@bgr.de; Soenke.Neben@bgr.de; Stefan.Ladage@bgr.de; Bernd.Schreckenberger@bgr.de; geohinzhannover@aol.com

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Introduction

The precursor of the plate tectonic theory, the continental drift hypothesis (Wegener, 1912), was initially inspired by the astonishing geometrical fit of the shelf edges of the South Atlantic. Consequentially, the southern South Atlantic was the target of the first reassembly to be done by sequentially fitting pairs of continents after determining their best fitting poles of rotation (Bullard et al., 1965). In particular the region between the Falkland-Agulhas Fracture Zone and the Rio Grande Rise/Walvis Ridge is undisturbed by major jumps of the spreading axis and there are no complex oceanic features such as volcanic ridges or volcanic plateaus (Figure 1). The opening of the South Atlantic occurred diachronously from south to north (e.g., Rabinowitz, 1976; Rabinowitz & Labrecque, 1979; Austin & Uchupi, 1982; Sibuet et al., 1984; Uchupi, 1989) and may be described as a northward-migrating sequence of unzipping of rift zones (e.g., Nürnberg & Müller, 1991; Jackson et al., 2000).

Figure 1: Overview map showing the western South Atlantic and the study area between the Falkland-Agulhas Fracture Zone and the Rio Grande Rise/Walvis Ridge.

The South Atlantic margins are of the rifted volcanic margin type (Hinz, 1981), like the majority of passive margins worldwide. The Early Cretaceous South Atlantic continental break-up and initial sea-floor spreading were accompanied by extensive transient volcanism and magmatism recorded as inferred sill intrusions, flood basalt sequences, voluminous volcanic wedges, and magmatic underplating. In seismic reflection data the voluminous extrusives show up as huge wedges of seaward-dipping reflectors (SDRs) on both sides of the southern South Atlantic (Gladczenko et al., 1997; Hinz et al., 1999; Franke et al., 2007).

In this webpage we summarise the results of a detailed investigation of the Argentine and Uruguay outer margin segments based on a set of about 25.000 line-kilometres of 2D multichannel seismic data that were acquired by the Federal Institute for Geosciences and Natural Resources (BGR) over the past 10 years (Figure 2). The synthesis of these data show that the SDR formations vary extensively but systematically in architecture, extent and thickness along the strike of the margin. We conclude that the emplacement of the now deeply buried, 60-120-km-wide SDRs occurred episodically. The overall northward propagation of the South Atlantic rift took place in huge (400-km scale), but distinct along-margin segments bounded by transfer zones. Each segment reveals internally the same trend of northward decreasing volume of emplaced extrusives.

Figure 2: Study area in the western South Atlantic and location of multichannel seismic reflection lines (yellow lines) of BGR cruise 1987, R/V SONNE cruise SO-85 1993, BGR cruise 1998 and BGR cruise 2004. The location of the example seismic lines shown in Figure 4 is indicated as a thick yellow solid line. Satellite-derived gravity field (Sandwell & Smith, 1997) is shown for the offshore area. Proposed transfer zones as interpreted in Figure 3 are indicated as dashed green lines. Oceanic fracture zones in the deep Argentine Basin may correlate with the suggested transfer zones.

Margin segmentation and style of the emplaced volcanics

Four major transfer zones on the volcanic Argentine/Uruguayan margin, South Atlantic, have been identified (Franke et al., 2007; Figure 3). It is suggested that the margin can be divided, at least, in four compartments (Segment I to IV) bounded by the Falkland Fracture Zone/Falkland transfer, the Colorado transfer, the Ventana transfer and the Salado transfer. Criteria for mapping and extending transfer zones across the margin were:

Figure 3: Structural map showing the distribution of extensive volcanics manifested by thick wedges of seaward dipping reflector sequences (SDRs), additional volcanic/magmatic features and oceanic basement depressions. Transfer zones and margin segmentation is interpreted from variations in the margin volcano-tectonic suites, as well as post-rift sediment distribution, potential field data and earlier studies.

According to our interpretation the transfer zones probably represent old zones of weakness controlling the onset of Upper Cretaceous seafloor spreading and may be linked to, and continuations of, recent oceanic fracture zones (Figure 2).

Our data confirm other studies that found considerable variations in the seismic character of SDRs (Franke et al., 2007). Distinct unconformities with low frequency seismic patterns separating the SDR wedges are concentrated in margin segments II and III (Figures 3 & 4). The individual flows are spatially separated in the south while the main SDR wedge 1 becomes increasingly buried beneath the main SDR wedge 2 tot he north. In the northern part of segment II one broad wedge is present with an arcuate, high-frequency internal pattern. This is again replaced by three spatially separated SDR wedges where the main wedge is offset sinistrally by 20 to 30 km at the Ventana transfer. In segment IV we observe again spatially separated individual flows bounded by strong unconformities. These variations are the expression of an apparently general trend in the breadth and thickness of the SDRs. The largest volumes of volcanics are systematically emplaced in the southern parts of the segments, just north of the transfer zones. The breadths and thicknesses of the SDRs decrease to the north up to the next transfer zone. Another wide and thick wedge occurs at the southern edge of the next segment to the north.

Figure 4: Profile BGR98-07 in segment II. Multiple SDR wedges are separated by strong unconformities. The top of the oceanic crust is well defined by a flat, low-frequency reflector band. Click here or on figure for enlargement.

Where was the melt generated?

The production of large volumes of basaltic magma as manifested by the SDR sequences in volcanic rifted margins is certainly spatially and temporally related to continental breakup. However, the mechanism responsible for the emplacement of the basaltic flows is controversial (e.g., Menzies et al., 2002). Among other factors melts are produced by variations in pressure and temperature. Changes in these parameters can be achieved by either lithospheric thinning or plumes or, to use a more neutral expression, thermal anomalies.

The Paraná-Etendeka continental flood basalt provinces in Brazil and Namibia, respectively, were emplaced mainly between 129 and 133 Ma (e.g., Renne et al., 1992; Stewart et al., 1996; Peate, 1997; Menzies et al., 2002). Newer studies from the western margin of the Paraná province reveal that melt generation occurred in two major phases; at 145 Ma and 127.5 Ma, i.e. before and at the end of the 139–127.5 Ma Paraná-Etendeka flood-basalt eruptions (Gibson et al., 2006). These authors conclude that the Paraná-Etendeka large igneous province, presumed to be associated with the impact of the Tristan plume, was long lived and immediately predated continental break-up.

Although these findings may point towards anomalously high mantle temperature influencing the evolution of the volcanic margin, the magmatic architecture of the margin and the structures identified in our data can hardly be explained by a simple plume model originating from the deep mantle.

The seismic data demonstrate that the offshore SDRs, which are more than 10 km thick, are much thicker than the average flood basalt (only 0.7 km) in the onshore Paraná province (Franke et al., 2007). More important, however, is the observation that the rate of lava production decreases from south to north within the individual volcanic margin segments II and III, bounded by the Colorado, Ventana and Salado transfer zones (Figures 2 & 5). The major part of the volcanic extrusives more or less terminates to the South at the Colorado transfer. Some 150 km south of this location there is another SDR wedge located beneath the slope. From these findings we suggest a link between margin segmentation and volume, architecture and breadth of the volcanics for the western South Atlantic margin. The four (at least) transfer zones offset the Lower Cretaceous rift and are associated with changes in distribution and volume of emplaced volcanic material. They mark changes in structural pattern and margin subsidence.

Figure 5A: Sketch illustrating the evolution of the southern South Atlantic rift and plate reconstructions for 133 Ma (modified from Jokat et al., 2003 and Macdonald et al., 2003). Time scale according to Gradstein et al. (2004). Margin segment I between the Falkland transfer (FT) and the Colorado transfer (CT) is dominated by strike-slip movements, which probably prevent the generation of large volumes of melt. North of the Colorado transfer (CT) a longer section (margin segment II) opens with initially a narrow SDR wedge. At the future transfer zone about 400 km to the north (Ventana Transfer, VT), rifting is interrupted resulting in heat accumulation in the upper mantle. This enhances convection in the asthenosphere and the subsequent emplacement of multiple SDR wedges. The breadth of the SDR wedges decreases northwards.

Figure 5B: Sketch illustrating the evolution of the southern South Atlantic rift and plate reconstructions for 128 Ma (modified from Jokat et al., 2003 and Macdonald et al., 2003). Time scale according to Gradstein et al. (2004). When the next segment opened (margin segment III) north of the Ventana transfer (VT) initially a narrow SDR wedge was emplaced with an offset in a sinistral sense. Another major transfer (ST - Salado transfer), again about 400 km to the north, interrupts the rifting process. The same heat accumulation and enhanced mantle convection is proposed as for margin segment II, resulting in the emplacement of the multiple SDR wedges in this margin segment. At this stage we suggest an overprint of the southern segment resulting in the formation of volcanic outer highs and SDRs.

Thus our findings seem not to be consistent with the current deep mantle plume model. Instead, we propose that the South Atlantic rift evolved more likely by instantaneous break-up of longer sections of about 400 kilometres rather than by continuous propagating rifting (Figures 5a & 5b). The transfer zones may have acted as rift propagation barriers. If so, the rift opened in distinct segments with episodic emplacement of the huge volume of volcanic material within short time periods in each segment. The emplacement of the SDR wedges would be mainly controlled in terms of excess melting by decompression. This in our view explains the observed discontinuities and the systematically decreasing breadth of the SDR wedges from south to north within the individual margin segments better than mantle-plume influences.

Lithosphere rupturing the in form of fast-propagating rift zones has been inferred as a mechanism for transient excess melting by decompression for the diachronous emplacement of the SDRs along the Argentina margin (Hinz et al., 1999). The new data suggests that the observed transfer zones are important features for the emplacement of the SDRs.

Interruption of the rifting process (transfer zone) will lead to heat accumulation beneath the thinned and stretched crust in the next segment. This may result in enhanced mantle convection (e.g., Mutter et al., 1988; Saunders et al., 1997), eventually leading to a mature rifting stage with the emplacement of huge amounts of extrusives. When the next segment was disrupted far field stresses may have affected the adjacent segment to the south resulting in the emplacement of the flat lying flows and the outer high, outer SDR wedges (Figure 5b). The mantle temperature may or may not have been elevated above that of normal asthenosphere before breakup but local melt generation by adiabatic decompression in our view explains better the distinct variations in the architecture of the volcanic margin. [Ed: See also Jolante page]

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