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

Dieter Franke, Karl Hinz, Stefan Ladage, Soenke Neben*, Michael Schnabel, Bernd Schreckenberger

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

*Our research partner, kind and valued friend Soenke Neben passed away suddenly and unforeseen in his forty-seventh year on November 13th, 2009

Dieter.Franke@bgr.de ; geohinzhannover@aol.com ; Stefan.Ladage@bgr.de ; Michael.Schnabel@bgr.de ; Bernd.Schreckenberger@bgr.de

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This webpage is a synopsis of the papers:

• Franke, D., Neben, S., Ladage, S., Schreckenberger, B., Hinz, K., 2007. Margin segmentation and volcano-tectonic architecture along the volcanic margin off Argentina/Uruguay, South Atlantic, Marine Geology, 244/1-4, pp. 46-67. doi:10.1016/j.margeo.2007.06.009
• Franke, D., S. Ladage, M. Schnabel, B. Schreckenberger, C. Reichert, K. Hinz, M. Paterlini, J. de Abelleyra, and M. Siciliano, 2010. Birth of a volcanic margin off Argentina, South Atlantic, Geochem. Geophys. Geosyst., 11, Q0AB04, doi:10.1029/2009GC002715.

Introduction

The precursor of the plate tectonic theory, the continental drift hypothesis [Wegener, 1912], was initially pushed by the astonishing geometrical fit of shelf edges of the South Atlantic. Consequentially, the southern South Atlantic allowed a first reassembly accomplished 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 regional 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, rejuvenating 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 progressive northward 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], as are 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 are manifested by 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; Franke et al., 2010].

Here we report a detailed investigation of the Argentina and Uruguay outer margin segments based on a set of about 25.000 line-kilometres of 2D multichannel seismic data that were acquired by BGR during the past 20 years (Figure 2). Synthesis of these data show that the SDR formations vary extensively, yet 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 that are 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 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 into at least four compartments (Segments 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 as interpreted from variations in the margins volcano-tectonic suite, 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 the Upper Cretaceous seafloor spreading and may be linked and extending to 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 a low-frequency seismic pattern, 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 main SDR wedge 2 in the northern part of these segments. In the northern part of margin segment II one broad wedge is present with an arcuate, high frequency internal pattern. This is replaced by three spatially separated wedges of SDRs where the main wedge is offset in a sinistral sense by 20 to 30 km at the Ventana transfer. In margin 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 SDR sequences: The largest volumes of volcanics are systematically emplaced along the southern edges of the margin segments, just north of the transfer zones. The breadth and thickness of the SDRs decrease towards the north up to the next transfer zone. Another wide and thick wedge was emplaced at the southern edge of the segment adjacent to the north.

Figure 4: Profile BGR98-07, situated in margin segment II. Multiple SDRs 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.

Distribution of the volcanics along one margin segment

In the following we report on a detailed investigation of the volcanic/magmatic edifices across the margin and their variations along one margin segment [Franke et al., 2010]. We concentrate on a segment between the Colorado and the Ventana transfer zones, between 41°S and 44°S.

Within the margin segment SDRs are continuously present along strike. However, as Table 1 shows, the SDRs vary significantly in extent, thickness and volume. Although large uncertainties exist in the volume calculation, a considerable northward decrease of the volume of eruptives is obvious.

Table 1: Extent of the Inner SDRs in margin segment II. The position is given for the intersection of the strike lines with the margin-parallel line BGR98-26. Line BGR98-43 did not reach the seaward end of the SDRs.

Multiple Inner SDR wedges are distinct in the southern part of the segment. In the south the Inner SDR domain is up to 100 km wide in the east–west direction. The vertical thickness increases from 1.8 s (TWT) to a maximum value of 2.6 s (TWT) where multiple wedges show maximum overlap, at the northern limit of the multiple SDR occurrence. Further north, one broad, 30 to 53 km wide SDR wedge is present with an arcuate, high-frequency internal pattern. Here, the SDR wedge can be traced down almost to Moho depth where it is not masked by multiples and appears to terminate against horizontal reflections, about 1 s (TWT) above the Moho.

The fact that the multiple Inner SDR wedges off Argentina were emplaced rather side by side and overlap to a certain degree provides strong evidence for multiple phases of volcanism. This is similar to the Norwegian volcanic margin, where Eldholm et al. [1986] suggested that overlapping successions of SDRs were emplaced by consecutive phases of volcanism. Other indicators for episodic volcanic activity off Argentina are certain characteristics of the magnetic anomalies. The magnetic signature in the area of the SDRs changes both along and across the margin. North of the Colorado transfer zone, in the area of the multiple SDR wedges, several magnetic anomalies and polarity reversals are observed, indicating that the individual flows were emplaced with different polarities.

According to our interpretation, the SDRs furthest west and now furthest landward were emplaced first. Eventually one or several intrusions reached the surface and resulted in this volcanic flow unit. During a period of magmatic stagnation erosion and weathering affected the top of this flow resulting in an unconformity. Continuing extension resulted in thinning of the crust but a stable magma chamber did not yet develop. Before the next volcanic pulse was initiated, the injection center migrated east and the second wedge was emplaced next to the first. As before, this flow unit was exposed and eroded before the next flow was emplaced from an injection center located even farther east. In the south, this second flow partly covers the first wedge, but the main part of this wedge is located seaward of the first SDR flows. Towards the north, the main SDR wedge 1 decreases in width and becomes increasingly buried beneath main SDR wedge 2. SDR wedge 2, in contrast, shows a continuous or slightly increasing width and wedge 3 is irregularly distributed.

Evolution of the volcanic rifted margin

At the northern edge of the magma-poor margin segment, at ~44°S, an irregular crustal reflection pattern is present, which may be related to an evolving feeder dyke system. There may be an interbedding of volcanoclastics, tuff and ashes between this reflection and the basement reflection, but no distinct SDRs developed. Across the Colorado transfer zone, a transition from magma-poor to magma-rich rifting takes place within about 10 kilometers. Remarkably, the widest SDR wedges are found close to this transition, at the northern edge of the transfer zone. This abrupt change in emplaced magmatic volume leads us to consider alternative scenarios to the hypothesis that gradual along-margin variations in the thermal regime of the lithosphere and sublithospheric mantle (the traditional plume-driven model) are solely responsible for the transition from magma-poor to magma-rich volcanic margins. Gradual changes of mantle properties and dynamics would be expected to generate a smooth transition from magma-starved to volcanic rifting over at least a hundred or a few hundreds of kilometers [Franke et al., 2010].

Additional findings that do not support this hypothesis (or its application to the Argentine margin), include the episodic emplacement of the SDRs and the proposed seaward migrating injection center for the SDRs during rifting. If the region were underlain by a stable, long-lasting thermal anomaly driving the extension, why would multiple phases of volcanism alternate with periods of magmatic stagnation? Such periods of stagnation are the best explanation for the presence of unconformities on top of the SDRs and the varying magnetic signals of the SDR wedges. Finally, northward-decreasing volumes and production rates of melts as manifested by the SDR units (Table 1) are difficult to reconcile with the idea that the Tristan da Cunha plume caused the volcanic/magmatic edifices off Argentina. This plume hypothesis would predict a decrease of volcanism and magmatism with increasing distance from the plume head.

Where was the melt generated?

The production of the 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 quite 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, more generally expressed, 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 [Gibson et al., 2006]. These authors conclude that the Paraná-Etendeka large igneous province was associated with the impact of the Tristan plume, it was long-lived, and it immediately predated continental break-up.

Although these findings may be consistent with anomalous 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:

• Why should the rift start in the south at about 48°S when the plume was centered at about 30°S?
• If the South Atlantic opened like a zipper from south to north, how does this fit with the plume model?
• Most striking is the question of the spatial distribution of melts manifested by the SDR sequences. Within the plume hypothesis a continuous decrease (or, at least, a continuous amount) of volcanism and magmatism with increasing distance from the plume is expected.

The seismic data demonstrate that the offshore SDRs, which are in excess of 10 kilometres thick, are much thicker than the average flood basalt thickness in the onshore Paraná province, which is only 0.7 km [Franke et al., 2007]. More important, however, is the observation that the rate of volcanic rock 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 (minimum of) four 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 SDRs 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 SDR sequences.

As pointed out by Meyer et al. [2007] there are unexplained aspects in the mantle plume concept and alternative models need to be developed. We suggest that the mode of opening of the South Atlantic substantially influenced the varying emplacement of the volcanic extrusives. Breakup by a successive northward unzipping of rift zones [e.g., Nürnberg and Müller, 1991; Jackson et al., 2000] with a triangle-shaped opening of the ~ 400-km-long margin segment as suggested by Franke et al. [2007] is expected to result in differential stretching along the strike of the margin. This scissor-like opening explains the fact that the injection center of the multiple SDR wedges migrated contemporaneously with ongoing extension in a seaward direction in the south, while it was more fixed in the northern part of this segment. As a consequence, in the southern part of a margin segment the Inner SDRs were emplaced rather side by side, whereas in their northern part they were stacked on top of on another. Likewise the transitional zone between the Inner and Outer SDRs narrows in a northern direction. In this scenario phases of volcanism correspond to high extension rates while phases of stagnation reflect small extension rates or temporary stagnation in the rifting process.

Lithosphere rupturing in the form of fast propagating rift zones has been inferred as the 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 suggest 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 SDRs 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 better explains the distinct variations in the architecture of the volcanic margin. [Ed: See also Rifting decompression melting page].

Conclusions

• The architecture, style and extent of the seaward dipping reflector sequences (SDRs) vary extensively and systematically. A general trend is that the largest volumes are emplaced close to the mapped transfer zones and the breadth of the SDR wedges decreases northward within the individual margin segments.
• Plume-driven models are the traditional explanation for the formation of volcanic rifted margins. However, the following observations along the studied margin segment are difficult to explain by this model:
  
  

  • The transition from magma-poor to magma-rich rifting takes place within only about 10 kilometers.
  • The SDRs were emplaced episodically and the injection center for the multiple Inner SDRs likely migrated seaward during rifting.
  • The volumes of melts as manifested by the Inner SDR units decrease towards the Tristan da Cunha plume.
    
    
• A scissor-like opening of the margin segment under study explains both the migrating injection center of the multiple Inner SDR wedges in the south and the stacked SDRs in the north. We speculate that the varying amount of melts that were emplaced along the strike of the margin segment is related to the mode of opening of the South Atlantic. Presuming that the transfer zones formed prominent lithospheric discontinuities at the onset of rifting, these are expected to have strongly influenced the generation of melts. An axial-symmetrical small-scale mantle convection system may have developed all along the rifted margin segment [van Wijk et al., 2004] with the transfer zones having acted as rift propagation barriers. In this model decreasing extension rates toward the north along the margin segment under study result in decreasing volumes of melts. We do not exclude elevated mantle temperatures during rifting. However, we propose that a vast amount of the volcanic/magmatic structures found at this particular margin can be sufficiently explained if we consider that passive rifting processes controlled, at least partly, the production rates of melts.

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