SW Portugal Domain: pretty foreland structures

Gently deformed Carboniferous strata. Note anticline.

Gently deformed Carboniferous strata. Note central anticline.

The most amazing folds and accompanying thrust faults can be studied along the coast southward from somewhat south of Sines to the SW tip of Portugal near Sagres and Villa do Bispo, in a part of Portugal belonging to the SW Portugal Domain. I had to return to some of these exposures this summer, and with the good help of Portuguese structural geology professors Rui Dias and Fernando Ornelas Marques I got to explore some really nice sections along this stretch.

Folds are very nicely exposed along the coast

Folds are very nicely exposed along the coast, hear near Almograve. Strain is clearly higher in this picture than in the previous one. Some of the quartz veining is synchronous with the folding, and some is older.

This is a classical thin-skinned foreland system (to the Variscan orogen) where the marine sediments experienced progressive folding and thrusting with a push from the NE. The metamorphic grade is very low, and the rocks are turbidites with repeated sandstone-shale layers of contrasting mechanical contrast.

Competent layers control the folding, and the competent layers are so closely spaced that we have classical multilayer folding. Hence the fold wavelength is much longer than what we would expect from single-layer buckling (the layer thickness is typically at the decimeter to meter scale).

Nice

Weak shale layer (black in drawing) flowed ductility during folding together with flexural slip along limbs. Prof. Fernando Marques for scale. Almograve.

The weak shale layers are more mobile than the sandstone layers in the sense that they easily slip and flow. This is particularly obvious in the fold hinges, where shale extrude into the hinge zone (hinge collapse). Flexural flow and flexural slip are important during the formation of these folds, and non-cataclastic granular flow is the main deformation mechanism.

Nice

Folded turbidites separated by thrusts. Thrusting followed initial folding and further modified the folds. Geologist is Fernando Marques.

After a certain amount of folding (asymmetric with overall SE-directed vergence), thrusts initiate along weak shalely layers, connecting across more competent units. As a result, folds soon find themselves caught or sandwiched between thrusts, and some of the structures show intricate fold-thrust-cleavage relationships.

You can find a wealth of structures in this area that are hardly mentioned here: Cleavage refraction, intricate vein systems, boudins, refolding, complicated thrust systems and more. And nice beaches and sunsets!

Advertisements

About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
This entry was posted in Uncategorized and tagged , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s