Category Archives: Geoscience

The Palaeozoic Variscan Oceans. An outstanding geoscience research contribution

 

The Variscan area of Europe with palinspastic movement of Iberia in relation to Central Europe to take account of the Bay of Biscay opening. Franke et al 2017

Gondwana Research 48 (2017) 257–284: Read Here
Wolfgang Franke, L. Robin M. Cocks, Trond H. Torsvik

The Variscan and related North American orogenies which now total 6,000 kilometres of strike, were caused by the opening and closing of the Rheic Ocean over a 100 million year period from 440MA to 320MA. This period saw the creation of several minor oceans and seaways, repeated periods of rifting and subduction and ultimately with the amalgamation of Laurussia and Gondwana, formation of the Pangea super-continent. Wolfgang Franks and colleagues have undertaken a comprehensive review and re-interpretation of the oceanic history of the Variscan domain. They attribute the complex geology to the opening and closing of 5 oceans or seaways, rifting and repeated subduction events. As a consequence of this complex tectonism, Variscan Europe is well endowed with mineral deposits although few are in production. Indeed this is where the industrial exploitation of Cu, Pb, Zn, Ag and Au commenced in post Roman times. The proposed complexity is very similar to that observed along the margins of the Tethyan Ocean to the east during a later period. This paper is well worth a detailed review.

In the comments below we summarize the Franks et al paper to produce a history of the Variscan.

A Brief History of the Variscan

Early Palaeozoic

  • From at least Cambrian times the Armorican Terrane Assemblage (ATA) appears to have formed a promontory at the edge of the Gondwana Craton near NW Africa
  • In the early Ordovician (~490MA) along the eastern side of Iapetus Ocean a rift developed along the NW flank of Gondwana forming the Rheic Ocean. A rifted Gondwana fragment Avalonia moved westwards towards Laurussia as the Rheic Ocean expanded at the expense of the Iapetus;
  • The Rheic Ocean became very wide;
  • Towards the end of the Ordovician Avalonia merged with Laurussia with much strike-slip faulting;

Silurian

  • During the Late Silurian and Early Devonian NE subduction of the Rheic Ocean lead to back-arc spreading and sedimentation in what is now in part the Rheno-Hercynian belt
  • Additional rifting in the Silurian (~440MA) along the NW margin of Africa resulted in the formation of the Saxo-Thuringian and the Galicia-Moldanubian seaways and the separation of the ATA elements from each other and ATA from Gondwana (with Palaeo-Adria to the immediate east).

Devonian

  • The Saxo-Thuringian Ocean and ultimately, collision of the Thuringia and Franconia elements of the ATA with Avalonia previously accreted onto Laurussia (Baltica) occurred at ~ 400MA with the final closure of the NW extent of the Rheic Ocean;
  • During the Devonian there was widespread strike-slip movement between ATA and Palaeo-Adria to the east possibly as consequence of east verging oblique subduction of the Saxo-Thuringian Ocean;
  • In the Early Emsian, the Rheic mid-ocean ridge was subducted southwards under the northernmost part of the ATA (Franconia), creat­ing the short-lived Baja California-type Rheno-Hercynian Ocean which incorporated the former back-arc basin sediments
  • The northward and lateral movements of Gondwana saw the successive closure of the Galicia-Moldanubian, Saxo-Thuringian and Rheic Oceans from south to north, over the period from about 380Ma through the Early carboniferous

Carboniferous and Permian

  • Laurussia and Gondwana finally collided at around 320MA to form the super-continent Pangea
  • Prior to this collision there was significant dextral strike-slip movement between Laurussia and Gondwana
  • Post collision, the Amorican terranes returned to roughly the same location prior to their separation more than 100 million years earlier;
  • This collision produced a very extensive orogen extending from the Ouachita and the Alleghanian Orogenies in North America through the Variscan of Western Europe;
  • Continued shortening into the Late Carboniferous saw dextral strike-slip faulting along the SW margin of Baltica and clockwise rotation of the Bohemian Arc into its current location
  • This orogenesis extended from the Carboniferous into the Middle Permian with collisional shortening of more than 1,000 km.

Abstract

Geological evidence, supported by biogeographical data and in accord with palaeomagnetic constraints, indicates that “one ocean” models for the Variscides should be discarded, and confirms, instead, the existence of three Gondwana-derived microcontinents which were involved in the Variscan collision: Avalonia, North Armorica (Franconia and Thuringia subdivided by a failed Vesser Rift), and South Armorica (Central Iberia/Armorica/ Bohemia), all divided by small oceans. In addition, parts of south-eastern Europe, including Adria and Apulia, are combined here under the new name of Palaeo-Adria, which was also Peri-Gondwanan in the Early Palaeozoic. Oceanic separations were formed by the break-up of the northern Gondwana margin from the Late Cambrian onwards. Most of the oceans or seaways remained narrow, but – much like the Alpine Cenozoic oceans – gave birth to orogenic belts with HP-UHP metamorphism and extensive allochthons: the Saxo-Thuringian Ocean be­tween North and South Armorica and the Galicia-Moldanubian Ocean between South Armorica and Palaeo-Adria. Only the Rheic Ocean between Avalonia and peri-Gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism, and its north-western arm closed before or during the Emsian in Europe. Ridge subduction under the northernmost part of Armorica in the Emsian created the narrow and short-lived Rheno-Hercynian Ocean. It is that ocean (and not the Rheic) whose opening and closure controlled the evolution of the Rheno-Hercynian fold-belt in south-west Iberia, south-west England, Germany, and Moravia (Czech Republic). Devonian magmatism and sedimentation set within belts of Early Variscan deformation and metamor­phism are probably strike-slip-related. The first arrival of flysch on the forelands and/or the age of deformation of foreland sequences constrains the sequential closure of the Variscan seaways (Galicia-Moldanubian in the Givetian; Saxo-Thuringian in the Early Famennian; Rheno-Hercynian in the Tournaisian). Additional Mid- to Late Devonian and (partly) Early Carboniferous magmatism and extension in the Rheno-Hercynian, Saxo-Thuringian and Galicia-Moldanubian basins overlapped with Variscan geodynamics as strictly defined. The Early Carboniferous episode was the start of episodic anorogenic heating which lasted until the Permian and probably relates to Tethys rifting.

Gondwana Research 48 (2017) 257–284: Read Here

A Planet of Plates

Chris Harrison from Department of Marine Geosciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami  in a paper in Earth,  Planets and Space has sought to resolve the question of the number plates that make up the surface of the earth.

Major Plate Boundaries. By USGS – http://pubs.usgs.gov/publications/text/slabs.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=535201

The number of tectonic plates on Earth described in the literature has expanded greatly since the start of the plate tectonic era, when only about a dozen plates were considered in global models of present-day plate motions.  With an ever-increasing number of earthquake monitoring sites, improving  ocean bathymetry using swath mapping, and the use of space based geodetic techniques, there has been a huge growth in the number of plates thought to exist.  In 2003 the total was thought to be 52 delineated on the basis of earthquake epicentre data.

Chris now proposes a total of 159 plates (with some additional smaller plates yet to be mapped).

The largest plate (Pacific) is about 20 % of the Earth’s area or 104 million km2, and the smallest plate is only 273 km2. The Earth is continuously evolving with the continuous creation of new oceanic crust and its destruction in subduction zones.  This has a very significant impact on the distribution of continents, movement of water within the worlds oceans (over time) and plays an important part in the evolution of life on this planet.

REF: The present-day number of tectonic plates, Earth, Planets and Space 2016, 68:37.  Download PDF

Abstract

The number of tectonic plates on Earth described in the literature has expanded greatly since the start of the plate tectonic era, when only about a dozen plates were considered in global models of present-day plate motions. With new techniques of more accurate earthquake epicenter locations, modern ways of measuring ocean bathymetry using swath mapping, and the use of space based geodetic techniques, there has been a huge growth in the number of plates thought to exist. The study by Bird (2003) proposed 52 plates, many of which were delineated on the basis of earthquake locations. Because of the pattern of areas of these plates, he suggested that there should be more small plates than he could identify. In this paper, I gather together publications that have proposed a total of 107 new plates, giving 159 plates in all. The largest plate (Pacific) is about 20 % of the Earth’s area or 104 Mm2, and the smallest of which (Plate number 5 from Hammond et al. 2011) is only 273 km2 in area. Sorting the plates by size allows us to investigate how size varies as a function of order. There are several changes of slope in the plots of plate number organized by size against plate size order which are discussed. The sizes of the largest seven plates is constrained by the area of the Earth. A middle set of 73 plates down to an area of 97,563 km2 (the Danakil plate at number 80, is the plate of median size) follows a fairly regular pattern of plate size as a function of plate number. For smaller plates, there is a break in the slope of the plate size/plate number plot and the next 32 plates follow a pattern of plate size proposed by the models of Koehn et al. (2008) down to an area of 11,638 km2(West Mojave plate # 112). Smaller plates do not follow any regular pattern of area as a function of plate number, probably because we have not sampled enough of these very small plates to reveal any clear pattern.