Tag Archives: Scientific publishing

Why there is no Saharan Atlantic Ocean

Our paper “Oblique rifting along the Equatorial Atlantic ocean: Why there is no Saharan Atlantic Ocean” (Doi: 10.1130/G35082.1) is now available online, I believe a pre-issue publication listing will follow this week. We use plate kinematic and 3D numerical modelling to explain why the Equatorial Atlantic ocean formed in the Early Cretaceous time (around 120-100 Million years ago). Here’s a summary of the paper in simple terms:

Every schoolchild can recognise continents or parts of them based on their shape. But why does Italy look like a boot, why is Australia an island-like continent and what sculpted Africa’s margins? In this study we address the underlying processes that shaped Earth’s continental plates when the last era of supercontinents came to an end, between about 150 and 100 My years ago.

At the time when dinosaur evolution peaked, the southern continents were still united in the supercontinent Gondwana. However, vast continental rift systems comparable to the present East African rift, extended between present-day South America and Africa as well as within the African continent. These rifts are preserved as deep sedimentary basins in the subsurface of the African continent and along continental margins and document processes where continental crust is stretched like chewing gum. The so-called South Atlantic and West African rift systems were about to split the African-South American part of Gondwana North-South into nearly equal halfs, generating a South Atlantic and a Saharan Atlantic Ocean (see Image). In a dramatic plate tectonic twist, however, a competing rift along the present-day South American and African Equatorial Atlantic margins, won over the West African rift, causing it to become extinct, avoiding the break up of the African continent and the formation of a Saharan Atlantic ocean.

Our work elucidates the reasons behind the success and failure of these rift systems by coupling plate tectonic and advanced 3D numerical models of continental lithosphere deformation. We find that rift obliquity acts as a selector between successful and aborted rift systems, explaining why the South and Equatorial Atlantic Ocean basins formed and other rifts became aborted. Our modelling also sheds lights on the dynamics of rifting, suggesting that feedback loops caused a ten-fold acceleration in the velocities of the South American plate once the Equatorial Rift System had sufficiently weakened the last remaining continental bridge between both plates.

One hundred years after the German scientist Alfred Wegener developed first ideas of continental drift, this study provides a new keystone in understanding the rules which govern continental extension and tectonic plate motion ultimately sculpting Earth’s continents into the shapes as we recognise them today.

The  abstract of the paper:

Rifting between large continental plates results in either continental breakup and the formation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. The nonlinear interaction between key parameters such as plate boundary configuration, lithospheric architecture, and extension geometry determines the dynamics of rift evolution and ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa and South America that was preceded by ∼20–30 m.y. of extensive intracontinental rifting prior to the final separation between the two plates. While the South Atlantic and Equatorial Atlantic conjugate passive margins continued into seafloor-spreading mode, forming the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon after South America broke away from Africa. We investigate the spatiotemporal dynamics of rifting in these domains through a joint plate kinematic and three-dimensional forward numerical modeling approach, addressing (1) the dynamic competition of Atlantic and African extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the acceleration of the South America plate prior to final breakup. Oblique rifts are mechanically favored because they require both less strain and less force in order to reach the plastic yield limit. This implies that rift obliquity can act as selector between successful ocean basin formation and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of the last continental connection between Africa and South America produced a severe strength-velocity feedback responsible for the observed increase in South America plate velocity.

The associated data for the plate kinematic model is available full and for free as open data from the Datahub.org pages (http://datahub.io/dataset/southatlanticrift) of my earlier South Atlantic paper in Solid-Earth.net. More detailed explanations and animations will follow later.

Lastly, here’s our guess for how the world might look liked like if a Saharan Atlantic ocean had formed:

The world as it might have looked like if the West African Rift system had been "successful" in forming a "Saharan Atlantic Ocean basin". We explain in our paper why this did not happen.

The world as it might have looked like if the West African Rift system had been “successful” in forming a “Saharan Atlantic Ocean basin”. We explain in our paper why this did not happen. Made with GPlates and image manipulation.

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Geological colors

Making maps with GMT is fun (well, some might argue…), what is not fun is to find color palettes for GMT (*.cpt files) which have the “official” colors for geological time periods coded up against absolute ages as proposed by various geological time scales. The International Commission on Stratigraphy has all their time scales on their webpage, downloadable as PDF files. The different stages are assigned color values according to what apparently the Commission for the Geological Map of the World (CGMW) has proposed. At some stage they even produced a PDF which had both, RGB and CMYK color values for the different subdivisions of the time scale (could not find this anymore when I recently searched for it), but now they only have a giant image on their website which specifies the color assignment.

Such a color assignment is very helpful if you want to make maps in vectorgraphics illustrations by hand and you only have a few polygons to color in. It becomes a massive pain, however, if you want to automate map making, for example, when generating a sequence of paleo-tectonic maps using GMT. So what is needed here is to have that color information not locked up in an image file or as unstructured text, but rather as data file where an RGB value is assigned to a stage/period/eon (as categorical color palette) so that one can use this information in other applications – such as GMT or GPlates, but potentially also QGIS or ArcGIS or Inkscape/GIMP/Photoshop.

The same goes for information about magnetic polarity chrons which are used in plate tectonic reconstructions – a simple assignment of normal/reverse magnetisation to either white or black colors using a certain geomagnetic polarity timescale.

Here are some examples of how the color scales look like when plotted with GMT’s psscale:

GTS 2012 - Eons

GTS 2012 – Eons

GTS 2012 - Eras

GTS 2012 – Eras

GTS 2012 - stages

GTS 2012 – stages

And the geomag timescale plotted is here:

Snapshot of Gee and Kent (2007) magnetic polarities plotted using the GeeK07.cpt file

Snapshot of Gee and Kent (2007) magnetic polarities plotted using the GeeK07.cpt file

I have made a set of CPT files (available as Git repository on my BitBucket site) for the GTS 2012, GTS 2004, and Exxon 88 geological time scales as well as the Gee and Kent (2007) geomagnetic polarity time scale. The files are available under a Creative Commons Share-Alike Attribution License. A zip archive of the data can be downloaded directly through this link. The files should also be incorporated into the fantastic CPT-CITY website over the next weeks.

If you feel like contributing to this work, just fork the Git repository or send me text snippets for the color scales by email.

The joys of extracting data from geoscientific papers

I am in the process of revising a recent discussion paper I have published on the tectonic evolution of the South Atlantic rift system. So I started to collect some information from various papers to back up and supplement an alternative plate tectonic scenario. Unfortunately there only one or two key papers on this remote offshore region on the Argentine margin (not a prolific basin to drill for hydrocarbons), so what I usually do in that case is to take a screenshot of the relevant maps in that paper and georeference them. Once this is done, one can add extra information into the files I am using in GPlates for my reconstructions.

As geoscientists sometime do excel in trying to even make published data hard to use (I mean if you are mapping geospatial features, is there ANY reason to disguise your work in a low-resolution raster graphics depicting a map in a very odd projection without any information about the projection or its location so that no one can really USE it – apart from reading the paper?). Sometimes (oftentimes) there is, apparently, as I am about to find out. So in this case, we have an overview map with a set of offshore seismic lines indicated. While this overview map has national and international boundaries and a coastline, it misses a graticule, but through the coastline and the international boundaries it still can be georeferenced adequately. Here is the georeferenced image of the overview map:

Georeferenced overview map. Scale dimension are not too far off: 128 km measured in the GIS vs. 125 km long scale bar.

Georeferenced overview map. Scale dimension are not too far off: 128 km measured in the GIS vs. 125 km long scale bar.

Even the latitudinal position looks ok (mouse position not visible but the reading was taken at the right hand margin at 48˚S)

Even the latitudinal position looks ok (mouse position not visible but the reading was taken at the right hand margin at 48˚S)

The map I am interested in covers the offshore seismic grid around the SJ.es-1 well, and   shows the tectonic inventory of the San Julian Basin offshore Argentina. It is in a different projection than the overview map (going by the map frame annotation) and has no geographical features which can be used for georeferencing apart from the well and the 2 seismic line locations SL2 and SL4. Easy, I hear you say, two beautifully straight lines, and a point, what more do you need?  Have a look at the map scale from the first image above. Based on that image, the lines are about 75 km long and about 70 km apart, measured on the seismic grid:

Seismic line dimensions - 75 km long. So far so good.

Seismic line dimensions – 75 km long. So far so good.

Seismic line spacing between SL4 and SL2 is about 75 km according to the georeferenced image.

Seismic line spacing between SL4 and SL2 is about 70 km according to the georeferenced image.

Now, we have a look at the structural map a bit more in detail – different projection most likely going by the frame annotation (no information given in figure caption), no other georeferencable features such as coastlines or boundaries. But was we also see is that the seismic lines are spaced about  40 km apart at their closest distance, not really parallel and latitudinally offset. There are a few reasons why this could be – two I can think of right away: the different projection compared to the overview map or the actual lines shown could be a subset of the full lines. Here is the image:

The structural map (modified from original). Seismic line spacing around 40-50km (not 70 km like in the overview map) and the lines are slightly rotated relative to each other and not parallel (like in the overview map).

The structural map (modified from original). Seismic line spacing around 40-50km (not 70 km like in the overview map) and the lines are slightly rotated relative to each other and not parallel (like in the overview map).

Now we’re going to georeference the structural map with the information contained in both maps, namely the two seismic lines and the well. Even though the projections might differ this should not be too hard:

Georeferenced structural map based on the seismic lines and the well location as provided in the overview map.

Georeferenced structural map (40% transparent) based on the seismic lines and the well location as provided in the overview map placed on top of the overview map (non-transparent).

When the seismic line end points are used the map is scaled and rotated. While seismic line 4 seems to match reasonably well and we also do get a relatively good match with line 2, we can see that the map scale is still close to double the stated scale (49 km in the georeferenced version vs. 25 km stated on map). Ok, next try:

Scaled and rotated structural map with a best fit to the overview map. Note the mismatch not only in the way the lines are rotated (could be due to the different projections) but also the distance between the the two seismic lines in the overview map (more than 70 km) and in the structural map.

Scaled and rotated structural map (transparent, on top of overview map) with a best fit to the overview map. Note the mismatch not only in the way the lines are rotated (could be due to the different projections) but also the distance between the the two seismic lines in the overview map (more than 70 km) and in the structural map.

This time, I scaled the map to match the length scale (25.9km vs 25 km stated in the map) and then rotated the image with the SJ.es-1 well as control point. So it seems that the overview map does not show the correct information – either the lines are wrongly indicated (ie not full length, not the right lines), or the line locations on the structural map are wrong. Simply,  there is no (easy and straightforward) way to get the line locations in the overview map to match those in the strutural map which is a basic breakdown of scientific reproducibility… Sadly this means that the information in the structural map cannot be utilised by other people (like me) who try use it.  I can understand -to a degree- that geoscientists have a tendency to obscure their data by chosing map projections which make it harder to reverse engineer the information contained in the maps. But there is a difference between publishing a “hard to reverse-engineer” map and a plainly wrong map.

Experiences with the EGU open access publication Solid Earth

My paper on the evolution of the South Atlantic rift is now online in the open discussion of the new open-access journal of the EGU “Solid Earth“.  From submission (05. Jan 2013) to the manuscript being online for the discussion and sent out for  review, it took 11 days (the paper appeared online 16. Jan). We had similar experiences with a paper on the GPlates Information Model and Markup language in  Geoscientific Instrumentation, Methods and Data Systems (GI), another open-access journal of the EGU.

The format of the publication still somehow feels a bit awkward as all documents are only available as PDFs, meaning that one has to download the files instead of being able to take a first instant glance at the paper and figures on a simple webpage in HTML. Downloading such a PDF article and supplements has so far been not too fast altough the journal homepage FAQ say:

To ensure continuous online accessibility of SE and SED, the website contents are updated daily on several independent internet servers at different locations throughout the world (mirror sites).

I think  it would also be of benefit to be able to browse through the reviews and comments of papers without the necessity of downloading PDFs, similar to discussions and comments on major news websites. I guess this is done for the reason of archival but I couldn’t find any clear statement on the EGU or journal website on why this particular form of PDF and only HTML abstract has been chosen. The speed of publication, the arXiv-Style pre-publication (ie Discussion) availability of the research, favorable author rights, and making the reviews available online certainly is very promising step forward for peer-reviewed geoscience.

Here’s the abstract of the South Atlantic paper:

The South Atlantic rift basin evolved as branch of a large Jurassic-Cretaceous intraplate rift zone between the African and South American plates during the final breakup of western Gondwana. While the relative motions between South America and Africa for post-breakup times are well resolved, many issues pertaining to the fit reconstruction and particular the relation between kinematics and lithosphere dynamics during pre-breakup remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-breakup evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African rift zone, we have been able to indirectly construct the kinematic history of the pre-breakup evolution of the conjugate West African-Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic Pre-salt sag basin and the São Paulo High. We model an initial E–W directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities until Upper Hauterivian times (≈126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ≈17 Myr-long stretching episode the Pre-salt basin width on the conjugate Brazilian and West African margins is generated. An intermediate stage between 126.57 Ma and Base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE–SW. From Base Aptian onwards diachronous lithospheric breakup occurred along the central South Atlantic rift, first in the Sergipe-Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final breakup between South America and Africa occurred in the conjugate Santos–Benguela margin segment at around 113 Ma and in the Equatorial Atlantic domain between the Ghanaian Ridge and the Piauí-Ceará margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of this conjugate passive margins systems and can explain the first order tectonic structures along the South Atlantic and possibly other passive margins.