Global Tectonic Asymmetries

Carlo Doglioni, University of Roma La Sapienza, Roma, Italy

Persistent asymmetries occur between orogens associated to W-directed and E-NE-directed subduction zones. They respectively present shallow and high structural and topographic elevation, shallow and deep rocks. Moreover W-directed subduction zones are associated with backarc spreading, single E-facing vergence of the accretionary prism, and one single foredeep where the highest subsidence rates occur (>1 mm/yr). The opposed E-NE-directed subduction zones-related orogens have double vergence, and two foredeeps with lower subsidence (<0.3 mm/yr). Asymmetries associated to the geographic polarity also occur along rift zones where the eastern flank is in average 100-300 m shallower both in oceanic and continental extensional settings. All of this indicates a geographically controlled global geodynamic signature, which can reconcile with the W-ward drift of the lithosphere relative to the mantle. This process can be the result of the tidal drag operating on Earth which is slowing the planet's spin, and the mantle and core convection where heavier material sink, opposingly producing an acceleration of the Earth's rotation. In this view, absolute plates motion, which describes a sinusoidal mainstream, is westerly polarized.

Assuming the kinematics imposed by the relative "E-ward" mantle flow, this can explain the fast E-ward retreat of W-directed subduction zones such as the Apennines and the Carpathians, as well as most of the western Pacific subductions, the Barbados, Sandwich and Banda arcs. Sedimentary rocks off-scraped from the hinge of the downgoing lithosphere mainly compose the related accretionary prisms. On the other hand, subduction zones, which follow the mantle flow such as the Alps and the Dinarides-Hellenides-Taurides exhibit different metamorphic evolution and the outcrop of deeper basement rocks. It is inferred that W-directed subduction zones nucleated along the retrobelt of pre-existing E-NE-directed subduction zones, when oceanic or thinned continental lithosphere was occurring to the "east" of the retrobelt. For example, the Apennines developed along the retrobelt of the Alps that were originally connected to the southwest to the Betics cordillera. The Carpathians developed along the retrobelt of the Dinarides, i.e. the Balkans. In both cases oceanic or thinned lithosphere of the Tethys-Ionian or Dacide basins were present in the foreland of the Alps and Dinarides retrobelt. The Atlantic analogues are the Barbados and Sandwich arcs, which developed along the retrobelt of the Cordillera where Atlantic oceanic lithosphere was present. In extensional settings, the asthenospheric mantle, once depleted under rift zones should shift "E-ward" determining a density deficit in the eastern side. This could explain the lower subsidence in the eastern flank of the oceanic basins and the higher topography in the eastern side of continental rifts. The lighter mantle should eventually transit underneath continents, producing continental uplift as it is observed in Africa after the Atlantic opening, or in Western Europe with the uplift of the Paris basin.