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Basement dykes in Panafrican crust, Sinai Peninsula

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The Saharan Atlantic ocean makes ripples (edited)

Our “Saharan Atlantic ocean” paper has just been featured in GEOLOGY's “Research Focus” article in the March issue. The focus article is entitled "Roadmap to continental rupture: Is obliquity the route to success?" is written by Cythia Ebinger and Jolante van Wijk and is available as open access. This is fantastic news!

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2020/12/29 23:20 · christian · 0 Comments

Why there is no Saharan Atlantic Ocean

Our Geology paper “Oblique rifting along the Equatorial Atlantic ocean: Why there is no Saharan Atlantic Ocean” 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.

Here's our guess for how the world might look liked like if a Saharan Atlantic ocean had formed (Image licensed under a Creative Commons Share-Alike Attribution License): 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.

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. More detailed explanations and animations will follow later.

2020/12/29 23:13 · christian · 0 Comments

Working with plate velocities in GPlates

GPlates v1.3 can display and extract plate velocities. Depending on your work, you might have different requirements for these domains, plus there are a few pitfalls on the way. Currently, there are two three  ways to create velocity domains in GPlates:

  1. Create a global set of points at regular spacing which stay fixed absolute and the plates move across them. This method will report velocities of whatever plate will be on top of these points. This method is generally used when working with global models and when one wants to export boundary conditions for global geodynamic modelling (the CitcomS and Terra mesh generation options). Use Features → Generate Velocity Point Domains to create a velocity domain setup according to your needs. Note that this also allows to create a regular spaced lon/lat grid of distributed points. The features are gpml:MeshNode features in the GPlates Geological Information Model.Adding a global layer of velocity domain points using GPlates' built-in function
  2. Alternatively, you can just go and create a set of points wherever you require them - you might have a few small plates which fall through the cracks when creating the global velocity domains. So here, you just go and create a few point features, assigning them the right properties in the 'Create Feature' dialogue. Create and save the layer after you are done. You can automatically assign PlateIDs using GPlates' cookie-cutting tool afterwards which saves you typing in PlateIDs manually.Manually creating a point feature collection where velocities are to be extracted
  3. Use QGIS and the Vector toolbox to create a regular spaced point grid in OGR Vector format (ESRI Shapefile, OGR GMT etc) and load that into GPlates, cookie-cut - or rather - assign plate ids and there is a regional, regular-spaced point set which you can use as velocity domains.Creating a regular spaced point grid to be used as velocity domain, using QGIS.

One important thing to know is that GPlates utilises layers for different types of data, so here's a little digression and some background info. This layer business is much like GIS software has vector and raster layers, and other layers which are the result of some computations/combination of other layers - or like image manipulation applications like GIMP or Photoshop. The following layer types exist in GPlates (n.b. ideally a detailed overview of the layers with examples should follow here but currently there is no time, I suppose this might be found in the GPlates documentation somewhere):

  • ReconstructionTree: Rotation files/models are automagically assigned to the yellow layer.
  • ReconstructedGeometries: Essentially all feature data one works with gets stuffed into the green layer type. Shapefiles, standard GPML files (not topologies), OGR GMT files all belong to this layer type.
  • Co-registration: Features related to data mining and association checks are combined in this layer type. This is one example where a layer does not correspond directly to a feature collection (ie a single file on disk). Instead, the user selects a set of seed points (a feature collection) and target geometries/rasters (another feature collection/raster) to generate a new data type.
  • Calculated velocity fields: Another one of of those layers where the layer=feature collection equation breaks down. Here we load a point feature collection and combine it with other data such as topological polygons or static polygons to compute velocities.
  • Resolved topologial networks:   This (brown) layer type is similar to the topological geometries but creates triangulated networks used for deformation.
  • Resolved topological geometries: Topologically closed polygons are created from the combination of a rotation file (ReconstructionTree) and a feature layer (ReconstructedGeometries).
  • Reconstructed Raster: Raster data gets loaded into the red layer type.
  • 3D scalar field: Volume-rendering of scalar fields as, for example, from seismic tomography

Now that this is sorted, let's generate some velocity fields.

After completing either of those steps above you should have a point layer from which you can now extract velocities. Depending on the way you created these points, the steps to display velocities might differ a bit.

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2020/12/29 22:32 · christian · 0 Comments

The world in (geological) colors

The color palettes I described in my recent post can also be used with GPlates to color loaded features. Here are three examples which use the static polygon dataset which comes with GPlates. The first is using the GTS2012 chronostratigraphic time scale to color the static polygons by epoch and fill the polygons:

The world colored by geological epoch, filling the static polygons.

The second is just using the same data set and color the line features by geological era:

Coloring the polygons by era in GTS2012 colors.

And the last is a bit Bauhaus-y by coloring everything in black and white according to the Gee & Kent geomagnetic polarity time scale (not that this would make a lot of sense, but as we can do it…):

The world in normal polarity and reverse polarity according to the Gee & Kent geomagnetic polarity timescale (using the filled static polygons).

That planet looks way to boring - hemispherical view of the Earth with the static polygons colored according to a normal (white) and reverse (black) geomagnetic polarity. Again using the Gee & Kent 2007 timescale.

In order to color loaded features by age (and timescale) just add the color palettes to the “Draw style” settings (Features → Manage colouring) like this:

The geological age color palettes can be added to the Draw style (Manage colouring).

Once the new colour palettes are available, they can be assigned to the individual layers either through the layer window or through “Features → “Manage colouring”.

2020/12/29 22:12 · christian · 0 Comments

Shapefile of reconstructable graticule lines

5-Degree spaced graticule lines using the Seton et al plate polygons, reconstructed to 100 Ma.

I have generated a shapefile with reconstructable, global graticule lines (5 degree spacing) for the Seton et al. plate model continental polygons. Download the file here. This file is released under a Creative Commons Attribution Share-Alike 3.0 license and was created with CreateGraticuleLines.py (Link to BitBucket site).

2020/12/29 21:58 · christian · 0 Comments

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