Geologic evolution of the Akna Montes-Atropos Tessera region, Venus

The investigated area comprises an arcuate mountain belt, Akna Montes, in W estern Ishtar Terra, associated with an outboard plateau, Atropos Tessera, to the west and a volcanic plateau, Lakshmi Planum, to the east. Eight geol ogic units have been recognized on the basis of their geomorphic and struct ural characteristics as they appear on Magellan radar images. Our stratigra phic analysis shows that the geological evolution of the study area can be explained by four main steps: (1) formation of the older substrata of Atrop os Tessera and Lakshmi, (2) extensive plains emplacement, (3) an orogenic p hase including the formation of Akna Montes, and (4) local emplacement of y ounger plains. The tectonic evolution shows a deformational sequence charac terized by contraction, shear, and topographic relaxation. This sequence is interpreted to be a consequence of the variation of crustal stresses and c rustal thickening during orogenic events as observed for terrestrial high p lateaus associated with a mountain belt (i.e., Himalaya and Tibet, Andes an d Altiplano). In order to estimate the amount of crustal shortening associa ted with the Akna Montes, we considered two end-members for structural styl e of the mountain belt: a symmetric fold model and a fault-bend fold model. The models are theoretical because terrestrial orogenic belts are often fo rmed by a combination of different compressional structures. However, symme tric and fault-bend faults represent the minimum and maximum crustal shorte ning, respectively, and thus they do place bounds on the amount of strain r ecorded bq Akna Monies. The first model yields a shortening value less than 1%, whereas a range of 17-34% is derived for the second model. The large d ifference between these values underscores the importance of fold geometrie s for estimating strain and to place constraints on geodynamic models for m ountain holt formation. On the basis of our study we think that a combinati on of mantle downwelling and horizontal convergence may provide a good expl anation of the geology and tectonics we observed in the Akna Montes-Atropos Tessera region.