A 4-Km Cross Section of Venusian Crust Exposed by Normal Faults in Dali Chasma, SW of Atla Regio

Dali Chasma is a 4000 km long rift system extending SW from Atla Regio. Dali Chasma together with its continuation to the SW, Diana Chasma, extends 7,500 km and connects Thetis Regio in the west with Atla Regio in the east. Mapping of volcanic and tectonic features of Dali Chasma (within Stanton quadrangle (V-38) is being done at 1:500,000 scale. Within our map area we note a structural feature of interest that is the focus of the remainder of this abstract. This feature is interpreted to represent an up to 4 km thick section of Venusian crust exposed through normal faulting associated with asymmetric rifting. The unusual feature (500 km long and 20-70 km wide) is centered at 190°E / 12°S and trends NE, parallel to the overall trend of the Chasmata. It stands out from the rest of the Chasmata by exhibiting a distinct decrease in lineaments. A cross section shows that this portion of Dali Chasma represents an asymmetric rift bordered to the north-west by major normal faults and by many less extended normal faults to the south-east. These faults expose a 4 km section through the crust with a radar brighter ~3 km thick section overlying a radar darker ~1 km thick section. The radar brighter section locally exhibits parallel lineations which can be interpreted as horizontal layering, potentially reflecting a flood basalt sequence (or sequence of sills). The radar dark section beneath could represent an older unit (basement?) of basaltic rocks. We interpret irregular-shaped areas with mottled radar texture to represent talus accumulated through mass wasting at the base of the exposed escarpment.