A globally fragmented and mobile lithosphere on Venus.

Venus has been thought to possess a globally continuous lithosphere, in contrast to the mosaic of mobile tectonic plates that characterizes Earth. However, the Venus surface has been extensively deformed, and convection of the underlying mantle, possibly acting in concert with a low-strength lower crust, has been suggested as a source of some surface horizontal strains. The extent of surface mobility on Venus driven by mantle convection, however, and the style and scale of its tectonic expression have been unclear. We report a globally distributed set of crustal blocks in the Venus lowlands that show evidence for having rotated and/or moved laterally relative to one another, akin to jostling pack ice. At least some of this deformation on Venus postdates the emplacement of the locally youngest plains materials. Lithospheric stresses calculated from interior viscous flow models consistent with long-wavelength gravity and topography are sufficient to drive brittle failure in the upper Venus crust in all areas where these blocks are present, confirming that interior convective motion can provide a mechanism for driving deformation at the surface. The limited but widespread lithospheric mobility of Venus, in marked contrast to the tectonic styles indicative of a static lithosphere on Mercury, the Moon, and Mars, may offer parallels to interior-surface coupling on the early Earth, when global heat flux was substantially higher, and the lithosphere generally thinner, than today.

Reconstructing orogens without biostratigraphy: The Saharides and continental growth during the final assembly of Gondwana-Land.

A hitherto unknown Neoproterozoic orogenic system, the Saharides, is described in North Africa. It formed during the 900-500-Ma interval. The Saharides involved large subduction accretion complexes occupying almost the entire Arabian Shield and much of Egypt and parts of the small Precambrian inliers in the Sahara including the Ahaggar mountains. These complexes consist of, at least by half, juvenile material forming some 5 million km2 new continental crust. Contrary to conventional wisdom in the areas they occupy, evolution of the Saharides involved no continental collisions until the end of their development. They formed by subduction and strike-slip stacking of arc material mostly by precollisional coastwise transport of arc fragments rifted from the Congo/Tanzania cratonic nucleus in a manner very similar to the development of the Nipponides in east Asia, parts of the North American Cordillera and the Altaids. The Sahara appears to be underlain by a double orocline similar to the Hercynian double orocline in western Europe and northwestern Africa and not by an hypothetical "Saharan Metacraton." The method we develop here may be useful to reconstruct the structure of some of the Precambrian orogenic belts before biostratigraphy became possible.

A scale of greatness and causal classification of mass extinctions: implications for mechanisms.

A quantitative scale for measuring greatness, G, of mass extinctions is proposed on the basis of rate of biodiversity diminution expressed as the product of the loss of biodiversity, called magnitude (M), and the inverse of time in which that loss occurs, designated as intensity (I). On this scale, the catastrophic Cretaceous-Tertiary (K-T) extinction appears as the greatest since the Ordovician and the only one with a probable extraterrestrial cause. The end-Permian extinction was less great but with a large magnitude (M) and smaller intensity (I); only some of its individual episodes involved some semblance of catastrophe. Other extinctions during the Phanerozoic, with the possible exception of the end-Silurian diversity plunge, were parts of a forced oscillatory phenomenon and seem caused by marine- and land-habitat destruction during continental assemblies that led to elimination of shelves and (after the Devonian) rain forests and enlargement of deserts. Glaciations and orogenies that shortened and thickened the continental crust only exacerbated these effects. During the Mesozoic and Cainozoic, the evolution of life was linearly progressive, interrupted catastrophically only at the K-T boundary. The end-Triassic extinction was more like the Paleozoic extinctions in nature and probably also in its cause. By contrast, the current extinction resembles none of the earlier ones and may end up being the greatest of all.