THE ROLE OF RIFTING IN THE GENERATION OF MELT - IMPLICATIONS FOR THE ORIGIN AND EVOLUTION OF THE LADA TERRA-LAVINIA PLANITIA REGION OF VENUS

A rift zone over 6000 km in total length runs along the border of Lada Terra, a highland in the southern hemisphere of Venus, and Lavinia Pl anitia, a basin that has been interpreted as a site of early-stage man tle downwelling. Along the length of the rift are a number of volcanic centers of widely varying morphology and volcanic output. These inclu de coronae, radially fractured domes, and large flow fields similar in scale to terrestrial flood basalts. We develop a model for the origin of extension related to passive rifting in response to stresses creat ed by the adjacent downwelling. Volcanism and extension at other rifts on Venus, such as Devana Chasma, have been attributed to deep-seated mantle plume activity. In contrast, we interpret the origin of extensi on and volcanism along the Lada rift to be linked to upwelling and dec ompression melting of mantle material due to rifting and, possibly, to counterflow associated with downwelling. Extension occurred generally prior to the formation of volcanic centers and the eruption of large- scale flow fields, although most of the volcanic centers have been fra ctured by continued extension along the rift. Current debate over the formation of terrestrial flood basalts centers on the necessity of pre existing extension and stretched and thinned lithosphere to produce en hanced decompression melting within a large plume head or mantle therm al anomaly. Our studies of large-scale flow fields associated with the Lada rift and coronae on Venus indicate that extension is a prerequis ite for the formation of the majority of large-scale flow units on Ven us.