Deep Faults in Acheron Fossae
This image was taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express during orbit 37. It shows the Acheron Fossae region, an area of intensive tectonic (continental 'plate') activity in the past. The Acheron Fossae is situated at approximately 35°-40° North and 220°-230° East, about 1000 kilometres north of the large Olympus Mons volcano. North is to the right.
In Greek mythology, Acheron is the river entering the underworld, the Hades, and 'fossa' is the Latin word for trough.
Acheron Fossae marks the northern edge of the Tharsis plateau. It is part of a network of extensional fractures that radiates outward from their central focus in the Tharsis 'bulge', a huge area of regional uplift where intensive volcanic activity occurred. These curved 'faults' were caused in the process of this uplift: cracks in the crust formed when the hot material rising from deep in the mantle of Mars pushed the overlying 'elastic' lithosphere (surface layers of rock) upward. When the distorting tensions became too strong, the brittle crust on top of the lithosphere broke along zones of weakness.
The image is dominated by these curved features, showing a highly fractured, faulted and deformed area in the central part of the Acheron Fossae. In geological terms, this is called a 'horst and graben' system. When several parallel faults form, the block of crust between them drops down, forming a 'graben'. At Acheron, an almost classical example of parallel fault-bounded grabens has formed, dissected by remnants of the pre-existing topographical heights, the 'horsts'. The curved depressions have opened the surface up to 1700 metres deep.
For practical use on the internet, the image has been reduced in resolution - the data originally obtained from orbit at an altitude of 765 kilometres has a resolution of 30 metres per pixel.
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