STRESS HISTORY OF THE CRUST IN SOUTHEAST ATLA REGIO, VENUS: A TEST OF COLLAPSE CALDERA ELLIPTICITY FOR DETERMINING CRUSTAL STRESS

Bosworth et al. (2000) proposed a new technique for determining crustal stress by applying the concept of borehole breakouts to the development of magma chambers in shield volcanoes. They proposed that magma chambers and collapse calderas of shield volcanoes form elliptically in areas where the crust is under differential stress, with the long axis perpendicular to maximum horizontal compressive stress (Sh_max). Bosworth et al. tested the technique only in the East African Rift and Iceland.

Southeast Atla Regio on Venus displays a complex set of structural features and shield volcanoes with collapse calderas. The area is ideal for comparing stress fields determined by collapse caldera ellipticity and those determined by traditional stress field indicators, such as grabens, fractures, pit crater chains and eruptive fissures. In Southeast Atla Regio, Sh_max as determined using collapse caldera ellipticity corresponds closely to Sh_max determined from traditional indicators only in the youngest zone of active rifting in the main Atla Rift. During the time period when active rifting appears to have been very limited, Sh_max determined by the two methods do not correspond. Furthermore, in Hawaii and the Galapagos, the orientation of Sh_max suggested by eruptive fissures and grabens is nearly perpendicular to the orientation of Sh_max suggested by collapse caldera ellipticity. In areas of limited or no rifting, collapse caldera shape must be governed dominantly by processes other than those that control bore hole shape. Caldera ellipticity as a stress field indicator as proposed by Bosworth et al. (2000) has limited utility and should only be used with extreme caution.