Most of the Earth’s continental material has accumulated since the end of the Hadean Eon, around 4 billion years ago. Its crudely speaking granitic composition most likely arose through a two stage magmatic process, first by oceanic lithosphere forming directly by partial melting of mantle rocks followed by subduction, dehydration of descending oceanic crust and water-induced partial melting of mantle above the subduction zone. Basalt to andesite magmas produced by the last process rose to partly crystallise in deep-crustal chambers, the residual magma of more silica-rich composition being added to the middle and upper crust. In a nutshell, that is what is currently happening in the continental magmatic arc of the Andes and also in the many volcanic island arcs that comprise much of the rest of the circum-Pacific ‘ring of fire’.
Examination of the deeper parts of much older continental crust, such as the 800 to 600 Ma old Pan African orogenic belt of NE Africa and Arabia reveals the fate of oceanic island arcs. That huge crustal sector comprises a great many linear belts of oceanic-arc volcanic, sedimentary and plutonic rocks that now lie side by side, sometimes separated by fragments of oceanic lithosphere caught up between them as they were rammed together around 650 Ma ago. The arc terranes are sufficiently different from one another to suppose that they formed in different places far apart. Their assembly can best be explained by accretion of low-density arc crust as the intervening oceanic lithosphere disappeared down numerous subduction zones. Structures in some of the terranes and at their shared contacts with others suggests that some accretion was oblique, in the manner of large ships docking, while other assemblages met side-to-side. Similar tectonics characterise the great mid- to late-Palaeozoic orogens of eastern North America and western Europe. But one of the key areas for unravelling the range of tectonic processes involved in the assembly of continents lies in the Western Cordillera of North America, again made up of dozens of slivers of mutually exotic terranes of different kinds. The difference is that their upper parts largely remain intact and dateable using fossils or radiometric dating and through assiduous palaeomagnetic research it is sometimes possible to chart their motions over time to see the manner in which they approached and collided with one another. It is now possible to link such a complex process with underlying tectonics, not by inference but through direct observation of the remains of subducted slabs in the mantle deep beneath (Sigloch, K. & Mihalynuk, M.G. 2013. Intre-oceanic subduction shaped the assembly of Cordilleran North America. Nature, v. 496, p. 50-56).
The key to achieving this breakthrough is seismic tomography of the mantle beneath North America produced from global records of seismic wave paths through the mantle and new results from a dense array of seismographs that has been ‘marched’ across the continent; the Earthscope USArray. What is emerging are several almost vertical ‘walls’ of cool rock with high P-wave speeds that record the fates of at least three major subduction systems. Aligned roughly N-S they have been overridden by the North America Plate as it progressively moves westwards, driven by sea-floor spreading along the mid-Atlantic ridge. So the easternmost ‘wall’ is the oldest relic of subduction and that in the west from the more recent subduction that still continues beneath the NW US states and western Canada and Alaska. Sigloch and Mihalynuk explain the walls as crumpled descending slabs, ‘nipped’ off once each subduction system is overridden by the continent and breaks; rather like a scarf dropped vertically. So they are a great deal thicker than the originally descending oceanic lithosphere. The depth to the top of a wall is related to the age when subduction stopped. Each marks the former site of a trench and the authors have correlated each with major arc accretion events for which there is plenty of data from field geology. The outcome is a much more intricate tectonic story during the Mesozoic to early Tertiary than previously imagined. It resulted from the vagaries of the now vanished eastern complement to the Pacific Plate to the west of the East Pacific Rise, which seems to have been similar to the multiple system of island arcs now seen in the West pacific basin. There must have been flips of subduction direction as well as initiation and death of destructive margins.
Tanya Atwater, now retired, first visualised the vanished tectonics of the eastern Pacific and North America purely from on-shore geology and magnetic stripe patterns of the ocean floor more than 40 years ago in one of the most celebrated analyses of early plate theory. One can imagine how thrilled she must be to see her vision fulfilled and amplified.
- Geology: How the West Was Formed (blogs.discovermagazine.com)
- Sigloch, K. 2011. Mantle provinces under North America from multi-frequency P-wave tomography. Geochemistry Geophysics Geosystems, v. 12, doi:10.1029/2010GC003421.
- Goes, S. 2013. Western North America’s jigsaw. Nature, v. 496, p.35-37
- Geology do-over: How the West was born (mnn.com)