IV- Seismic Examples (testing)
Plate 024- Four major Large Intrusion Provinces (LIPs) were recognized during the Pangea supercontinent : (i) Siberia Traps ; (ii) Central Atlantic Magmatic Province, which is better known by its shortened form CAMP ; (iii) Karoo lavas and Deccan Traps. These provinces, certainly, precursors of rifting and breakup of the Pangea lithosphere, provide the geological conditions required to develop volcanic divergent margin.
Plate 025- As illustrated, the Atlantic-type volcanic margins are known all along of the Pangea's breakup zones. The West Iberia margin and its Canadian conjugate are, probably, exceptions. Such margins are quite particular and probably can not be considered anymore as Atlantic-type (Bally's sense), particularly the Galicia offshore. In the areas where the margin develops, more or less, parallel to major fracture zones (e.g. Gulf of Guinea, Pernambuco offshore, etc.), the SDRs are present (see later). Their recognition requires a good knowledge of the tectonics of the area. In the East Australia, we do not have seismic lines enough and particularly regional lines to decide if the margin is volcanic or not.
Plate 026- Be that as it may, geoscientist build up two geological models to explained the divergent margins' observations : (i) Volcanic margins and Non-Volcanic margins. In the volcanic margins, the COB (Continental Oceanic Boundary) is not abrupt as in the non-volcanic margin, since an intermediate sub-aerial crust (SDRs) is developed between the continental and oceanic crust. Such intermediate sub-aerial crust does not exits in a non-volcanic margin. In a volcanic margin, the thinning and lengthening of the lithosphere is not so accentuated as in a non-volcanic margin. In volcanic margins, the rift-type basins are in filled by non-marine and/or shallow marine sediments, while in non-volcanic margins (Galicia and Flemish Cap), they are, often, starved with deepwater sediments.
The next seismic lines (uninterpreted and tentatively interpreted, in geological terms) can have been used to test, i.e., to refute to Total's hypothesis for Atlantic-type divergent margins associated with the breakup of the Pangea supercontinent. Put differently, if you disagree with a proposed tentative geological interpretation, as a geoscientist, you are scientifically obliged to propose another tentative interpretation trying to refute it and send it for discussion (carlos.cramez@bluewin.ch) in order to make progress (petroleum exploration progress by trial and error and not by no falsifiable interpretations). However, don't forget that "The knowledge that we get from our senses and foolishly believe in, is inevitable observer-relative" (Heraclitus, circa 500 B.C.)
Plate 026- On this Gall-Peter projection (rectangular equal-area map projection that distorts most shapes, but the latitudes 45° north and south where there is almost not distortion) are indicated a roughly the location of the seismic lines used to test the volcanic or non-volcanic nature of the Atlantic-type divergent margins.
Plate A1.01- On this seismic line, it is, easy, to recognize the basement (probably a Precambrian or Paleozoic fold-belt) in the lower eastern part and quite nice seaward dipping reflectors in the central and lower western part. Above the basement and the SDRs a post-Pangea sedimentary prism can be subdivided in a lower retogradational (transgressive) and an upper progradational (regressive) interval. An exploration well drilled not far from this seismic line allows a long distance geological calibration.
Plate A1.02- The geological results of the exploration well drilled not far from the previous seismic line are here summarized. On the subject discussed on these notes, it is interested to notice the well drilled around 500 meters of SDRs, i.e., of sub-aerial lava-flows, which before drilling were considered by a lot of geoscientists as reflectors induced by rift type basin sediments forming a potential generating petroleum sub-system (source-rocks).
Plate A1.03- In the 90’s, in this area, several major oil companies drilled exploration wells looking for hydrocarbons generated by potential lacustrine source-rocks deposited in rift-type basins developed in the Gondwana continental crust. The reflectors in the lower western part of this line, which are tilted and looking seaward, were interpreted as induced by sedimentary interfaces within Lower Cretaceous rift-type basins. Even before drilling, several geoscientists tentatively interpreted such reflectors as lava-flows and, strongly, suggest their management to give up or not participate in the Namibia offshore exploration. The results of all wells drilled in the area corroborate the sub-aerial lava-flows interpretation proposed by the so called "poperian" geoscientists.
Plate A1.04- On this seismic line, the seaward tilting of the sub-aerial lava-flows create a half-grabens with opposite vergence that should not be taken as rift-type basins. The rift-type basins are induced by a regional lengthening of the continental predating the breakdown of the lithosphere and not by local lengthening creating the sinking of the expansion centers. As we will see later, the presence of this kind of graben is well documented in other volcanic margins. An exploration well was located and drilled on this seismic line.
Plate A1.05- The results of the exploratory well drilled on the previous seismic line corroborate the presence of sub-aerial lava-flows as the infrastructure of the margin. The wildcat recognized more than 300 meters of volcanic material.
Plate A1.06- On this line the SDRs are quite obvious. The seaward gliding of the margin sediments created a local compressional tectonic regime with the development of real listric faults, as suggested on the tentative interpretation. The regional lengthening of the Pangea continental crust ended with the breakup unconformity. The emplacement of the sub-aerial lava-flows (expression a little bit redundant: lava-flows can just develop outside water, because under water the volcanic material froze) post-date the breakup and accrete (growing by accumulation or coalescence) the new formed continent. When the spreading centers sinking and become, completely, covered by water the extruded volcanic material froze forming the oceanic crust (pillow-lava). The oceanic crust is the, mainly, responsible of the oceanization and enlargement of the margin.
Plate A1.07- As suggest on this plate the transition from SDRs (sub-aerial axis, high extrusion rate and large outflow lengths) to oceanic crust (normal axis depth and extrusion rate, small flow lengths, often pillow-lava) is a consequence of the sinking and immersion of the spreading centers due to the loading of the successive lava flows. The subhorizontal geometry of the SDRs over the feeder dykes (sheeted dykes) explain why the magnetic anomalies are subdued, which is not the case in the oceanic crust (quite small or inexistent flow). On the seismic lines, SDRs induce, more or less, continuous and dipping seaward reflections, while the reflections associated with the oceanic crust are hummocky or mounded geometry.
Plate A2.01- This seismic line from Argentina offshore illustrates, clearly, the difference between rift-type basins, which predate the breakup of the lithosphere and SDRs, which post-date the breakup). A small rift-type basin is recognized in the western part of the line, while SDRs are obvious in the eastern part. It is not clear if SDRs cover the rift-type basin (the thickness of the volcanic material covering the rift-type basin can be under seismic resolution). Contrariwise to the geoscientists who proposed exploration wells in the Namibia offshore, the Argentine geoscientists taking into account the K. Hinze’s works, considered this offshore as very poor, in terms of hydrocarbon potential. In addition, as depicted on this line, the thickness of the margin sediments is to small to develop a generating petroleum sub-system. On the tentative interpretation there are not lava flows over the small rift-type basin developed in the continental crust during the lengthening of the Pangea (Gondwana supercontinent) before the breakup of the lithosphere. The unconformity underlying the breakup (BUU) lies at the base of the SDRs, near the axis of rupture, and at the top of the lengthened Pangea continental crust (rift-type basins appertain to this crust) far from the axis, where the continental crust is not covered by volcanic material. The thickness of Pangea continental crust is quite important even near the axis of the rupture. In fact according different geoscientists since the thickness of the continental crust reaches 10-15 km, it cannot anymore be lengthening by normal faults: it is stretched and injected by volcanic material. if such a conjecture is true, just the more proximal rift-type basins (in relation to the associated underlying mantle plume) can, eventually, be covered by lava-flows.
Plate A2.02- Different generations of SDRs can, often, be recognized on seismic lines as it the case on this line. Distinct disconformities with a low-frequency seismic pattern, separate the SDR wedges, which seem to be the expression of an apparently general trend in the breadth and thickness of the SDR sequences. In this area, 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. The transition from SDRs to oceanic crust is subtle. The oceanic crust is, probably, at the end of the line (SE). The margin is quite starved. Just the over-bank sediments associated with the Amaghino canyon have a significant thickness.
Plate A2.03- On this N-S seismic line (strike line in relation to the COB), southward dipping reflectors,, recognized at the bottom of the margin, northward of the A-subduction zone (south of the Malvinas) are, probably, seaward dipping reflectors (SDRs), that is to say, sub-aerial lava-flows. Such a conjecture seems seems to be corroborated by the seismic lines shot in north Malvinas offshore, as illustrated in the next plate.
Plate A2.04- Tentatively, in the southern area of this line, from the bottom to top, one can recognize the breakup unconformity (top of the Pangea continental, BUU), a SDRs interval a, the SU unconformity (at the top of the SDRs) and the margin sediments with a middle interval, strongly, prograding northward. Obviously, these SDRs are not typically, but one must take into account that the North-South orientation of these seismic lines does favor their recognition.
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Last updated: July, 2014
