THE GLOBALLY FRAGMENTED, MOBILE LITHOSPHERE OF VENUS MAY RESEMBLE THE PERMOBILE TECTONIC REGIME OF ARCHEAN EARTH

The pioneering work by Kevin Burke to understand surface–interior processes on this world extended as well to other planets, including Venus. In contrast to modern plate tectonics on Earth, the standard view of at least the last ~500 Myr of geological history on Venus is that the lithosphere has been coupled to a highly viscous mantle and inhibited from major lateral motions. Here, 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; these belts also commonly display evidence for coeval lateral shear. 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. This tectonic pattern is seen predominantly at low elevations and within geoid lows, where large 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 detachments that are probably widespread 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. Importantly, because subductionless continental drift has been proposed as a means to account for Archean horizontal tectonics during the “permobile regime” proposed by Burke and John Dewey in 1973—a regime in which some mobility of the lithosphere was possible but before the full establishment of subduction and seafloor spreading—the finding of a fragmented but mobile lithosphere for Venus may provide new insight into tectonic processes in the early history of our own planet.