A Globally Fragmented and Mobile Lithosphere on Venus

The standard view of at least the last 500 Myr of Venus' geological history is that the lithosphere has been coupled to a highly viscous mantle and inhibited from major lateral motions. But that view is changing. With global radar image and topographic data, we identify a tectonic pattern across Venus that has previously been largely unrecognized but resembles that of the interiors of actively deforming continents on Earth. This pattern is defined by intersecting belts of shortening and extensional structures that demarcate low-lying areas infilled with smooth plains, interpreted as among the youngest units on the planet; these belts also commonly display evidence for coeval lateral shear. Together, our observations signify that the plains-filled lows correspond to discrete, mechanically coherent blocks that have moved relative to one another in a manner similar to jostling pack ice. Notably, tectonic structures frequently emerge from the bounding belts to deform, and thus presumably postdate the emplacement of, the plains infill. This tectonic pattern is seen predominantly at low elevations and within geoid lows, where high apparent depths of compensation likely indicate areas of downwelling mantle. Such downwelling could produce tractions at the base of the lithosphere sufficient to drive minor lateral motion, especially if facilitated by the relatively shallow (i.e., 10-15-km deep) crustal detachment that probably exists within the Venus lithosphere because of the high surface temperature. Indeed, the stresses calculated to be associated with gravitationally inferred mantle flow match the likely yield strength of the shallow lithosphere in almost all areas where these crustal blocks have been identified—demonstrating that this mechanism provides a basis for the transfer of some interior motion to the surface. The limited but widespread lithospheric mobility of Venus, in marked contrast to the static tectonic styles of Mercury, the Moon, or Mars, could characterize interior-surface coupling on other Earth-mass planets in orbit about other stars. Importantly, this type of lithospheric behavior may also have similarities to the "permobile regime" proposed for horizontal tectonics on the Archean Earth before the full onset of plate tectonics.