Sirenum Fossae

Today's VIS image shows a small portion of Sirenum Fossae. The linear depression is a tectonic graben. Graben are formed by extension of the crust and faulting. When large amounts of pressure or tension are applied to rocks on timescales that are fast enough that the rock cannot respond by deforming, the rock breaks along faults. In the case of a graben, two parallel faults are formed by extension of the crust and the rock in between the faults drops downward into the space created by the extension. The graben is this image is trending from north-northeast to south-southwest. Because the faults defining the graben are formed perpendicular to the direction of the applied stress, we know that extensional forces were pulling the crust apart in the west-northwest/east-southeast direction. The Sirenum Fossae graben are 2735km (1700 miles) long and stretch from eastern Terra Sirenum into western Daedalia Planum. The graben in this image cuts across the rim of a crater, which shows the tectonic forces post-date the impact that created the crater.