Not Written in Stone

3 Mar 2018

Link provided by Robertus Maximus. It is the February 2018 newsletter of the AAPG (the Explorer). The piece is written by Keith James a consultant geologist and fellow of the Institute of Geophysics and Earth Sciences at Aberystywyth University in Wales. He has worked with Shell all over the world - and later with Conoco. The piece begins by saying that published Plate Tectonics (PT) teaching is complacent. It should adapt to emerging data, including multiple working hypotheses, and enable students to think. The 2017 Geological Society 'William Smith' meeting in London was self congratulatory as it marked 50 years of PT. William Smith was responsible for the early geology maps of the UK. PT is in effect a paradigm that is rarely questioned but James says there are a number of discoveries that should put the theory under scrutiny. Supercontinent Pangaea began to fragment in the Jurassic. PT goes on to claim that from the Jurassic onwards mid ocean ridge formations have generated new sea floor that gradually separated the continents. Here, magnetic stripes are said to record reversals of Earth's magnetic field and spreading progress. This sea floor crust is consumed by the subduction process at the continental boundaries  and creates blue schists in the process. Subduction of one plate beneath another is said to cause mountain uplift. However, magnetic data suggest an intriguing alternative to coastlines/bathymetric contours for Pangaeanb reconstruction - whilst there exist large areas below the South Atlantic where there is no evidence of spreading during a large part of the Cretaceous (some 40 million years of sedimentation). Further, a similar signature actually suggests they are foundered parts of adjacent continents. Likewise, in the orth Atlantic there are similar signatures where sea floor fractures continue onshore to older Appalachian offsets and indicate ancient continental origins.

In modern PT paleontology takes a back seat - but was part and parcel of the Wagener 'continental drift' story. A new book published by the Geological Society of London 'Crustal Evolution of India and Antarctic' relates India to the Antarctic using geochronological data and petrology. Fossil data relate India to neighbouring Euro/Asia. Dinosaurs, freshwater snails, catfish, cichlid fish, angiosperms, flightless birds and manatees are evidence of communication between supposedly long separated areas. Monkeys and rodents are said to have travelled 2600km from Africa to South America following the early Cretaceous separation - at some point in the Oligocene. Mammals migrated between Africa and Madagascar as recently as the Miocene - in spite of supposedly separating in the Jurassic. Explanations offered include swimming, rafting or island hopping. Foundered continental areas are an interesting alternative.

PT maintains that subduction slab pull is the major driver of spreading. This seems to be a non-starter he says. Another major pillar of PT is that sea floor magnetic stripes, often attributed to magnetic field reversals, or to high versus low intensity magnetic fields, are a record of the spreading process. Trouble is they also occur in continental rifts. Moving offshore  seismic surveys reveal lightly stretched continental crust followed by thinning from 30km to less than 10km, and thence to highly extended crust, presumed ocean-continent boundaries (of shelf areas). The crust appears to be subject to a stretching process which would rule out the ridges as a spreading driver.

Extended crust (or continental shelf systems) also carry asymmetric basins between 60 and 200km wide and up to 25km deep, where reflections, some with sedimentary architecture, dip towards boundary faults. Here magma rises to intrude sills and extrude basalt. Stretching also results in serpentinisation (with magnetite) of exhumed periodotite, generating magnetic anomalies unrelated to spreading. Could they explain the sea floor magnetic stripes he asks?

Deep sea drilling aimed at calibrating increasing age of oceanic crust encountered basalts assumed to be basement but some contained sediment clasts and some deeper basalts were interbedded with sediments. Perhaps oceanic crust inlcudes extended continent basins far offshored. Is there any evdience? PT actually overlooks numerous sample of continental material dredged from ocean floors, some with trilobites and graptolites. Rocks as old as 2 billion years occur on the Mid Atlantic Ridge, at Peter and Pauls islands. Continental isotopes are widespread in Indian Ocean basalts. Proterozoic zircons in lavas on Mauritius suggest ancient basement below the island while magnetic data indicate a large submerged continental area east of Mauritius. In the South Atlantic granites was discovered in 2013, on the outward edge of South America's magentic extension. Basalts on the Walvis Ridge also display continental features and it would seem both areas are associated with large extensions of nearby continents. PreCambrian zircons have been found in lavas of the Galapagos Islands, another supposed volcanic hot spot 1000km west of South America. Mesozoic zircons occur on Iceland while in 2017 ZXealandia was nominated as a newly recognised continent. This fragmented and largely submerged area between Australia and New Zealand is the size of India. It explains plant and animal distribution in the South Pacific. However, magentic data also seems to show major extensions in the north and east.

Proliferating models of Pangaea and its break-up and dispersal show increasing complexity, recognising ever more small terranes and their migrations - but they also overlook important data. Is this another failure of modelling.  Reconstructions show continental crust east of Japan and New Zealand and west of South America. Japan received sediments from the east in the Palaeozoic-Paleogene and a million cubic km of Devonian sediments occur in Bolivia and Argentina came from the west. Geological, geophysical and dredging data show evidence of PreCambrian younger continental crust under NW Pacific abyssal plains. It subsided below deep sea at the end of the Jurassic but you can still see it on the magnetic map.

PT reconstructions use constant size earth. There are 75,000 km of spreading ridges and only 30,500 km of trenches and 9000 km of collision zones. Production of more crust than consumption via subduction suggests the earth is expanding, he says. Space d=geodetic data shows the solid earth expanded about 0.24 mm annually in recent decades. Growth increments on fossil corals and brachiopods show that days per years declined from 424 to 365 today - which suggests the earth is growing, and slowing. Does radial growth contribute continental separation, extension and subsidence?

Intra oceanic volcanic arcs are  characterised by high silica andesite (named after the Andes). This cannot derive from subducting slab low silica basalt so we have an andesite problem. In PT the rock reflects new continental crust formed by complex partial melting of sediments, the slab, the mantle, or the mantle wedge (or their combinations) in what are virtually subduction factories. Recently discovered Precambrian and Palaeozoic zircons produced by the volcanoes of and continental seismic voleocities below island arcs show they are underpinned by original continent. There is no andesite problem and subduction factories are not required.

Blue Schists are seen as classic indicators of fossil subduction processes and zones, involving the descent of material into the Mantle and unexplained resurrection of the same material over millions of years. However, some radiometric data suggest metamorphism only slightly younger than predecessor rocks. There are no blue schists in the central American and Lesser Antilles subduction arcs. In the northern and south Caribbean margins such rocks occur close to strike slip faults and some are even interbedded with sedimentary equivalents. While normal faults might involve 10s of km of displacement and thrusts up to  several hundreds of km, which show strike slip displacement can be very high. Such premordial faults lie conjugate NW and NE and have a global pattern. Transgressions within this generates secondary north trending extension and east trending compression. Such polygonal blocks are repeatedly shuffled within the global fabric.

Another favourite of PT is that the African plate collided with the Euroasian plate and caused the uplift of the Alps. Yet there is no uplift along the same boundary to the west out in the Atlantic. The Alps and Carpathians carry European brachiopods. There is also nothing African present in Europe. Meanwhile, India is supposed to have migrated 7500 km across the Indian Ocean to push up the Himalayas but fossils relate India to Eurasia. Head on collision of far travelled sections of plate is not indicated - and perhaps strike slip plays a role, he suggests. He then looks at South America and the Caribbean but finds no evidence of uplift as a result of collision.

The Pacific is markedly assymetric - the East Pacific Rise approaches South America and converges with North America. Magnetic data indicate lrge areas of extended subsided continent west of the rise and none east of it - yet large amounts of conglomerates and sandstones in the Andes came from the Pacific. Crustal thicknesses here up to 70km speak of a merger. The trenches and strike slip zones have occurred along North America from Alaska to Mexico and the Cordillera appear to be distal/oceanic thrust sheets detached from subducted Pacific crust to overly the Precambrian and Mesozoic shelf sequences.  Extended and thinned crust is perhaps easily thrust onto continental margins. Paleozoic and Mesozoic troughs in Peru, with steep western boundaries, and gental eastern slopes, contain 10 to 12 km thick prisms of shallow deepwater deposits. Bounding growth faults acted as volcanic conduits. They resonate with the asymmetric basins seen on deepwater seismic data. Vanished continents once linked North America, SE Asia, Australia and South America. They explain plant and animal fossil distribution. He ends by saying PT is the consensus theory and as such it doesn't even bother to keep up with new discoveries - yet alone try and explain them. He says it should adapt to emerging data and encourage multiple working hypotheses.