[meteorite-list] Mars Express Images: Deep Faults and Disrupted Crater at Acheron Fossae

[Ron Baalke]

http://www.esa.int/export/SPECIALS/Mars_Express/SEMKRR77ESD_0.html

Mars Express
European Space Agency
May 7, 2004

Deep faults and disrupted crater at Acheron Fossae
 
These images were taken by the High Resolution Stereo Camera 
(HRSC) on board ESA's Mars Express of the Acheron Fossae region, 
an area of intensive tectonic (continental "plate") activity in 
the past.
 
The images show traces of enormous stress and corresponding 
strain in the crust of the Red Planet. The HRSC was pointed 
twice at this interesting geological feature in the Acheron 
Fossae mountain range, during orbits 37 and
143.  
 
The feature is situated at approximately 35º-40º North and
220º-230º East, about 1000 kilometres north of the large 
Olympus Mons volcano. 

For practical use on the internet, the images have been reduced 
in their resolution - the data originally obtained from orbit at 
an altitude of 765 kilometres (orbit 37) and 1240 kilometres 
(orbit 143) have a resolution of 30 metres and 50 metres per 
pixel respectively. 
 
The images in colour, high-resolution and 3D show spectacular curved
depressions that have opened the surface, up to 1700 metres deep,
through faulting in the Acheron mountain ranges. 

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. 

Images 1-3, from orbit 37, are 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". 
 
 
Images 4-6, with the large crater, 55
kilometres in diameter, were taken about 250
kilometres west of images 1-3. 

They show how the rifting crosses the older
impact crater with at least three alternating
horsts and grabens. 

The Acheron Fossae region can be compared to
rift zones on Earth, where continental plates
spread apart, as is known from the Kenyan Rift
Valley in eastern Africa.
 
The 3D capability of the HRSC instrument
allows geologists to investigate in great detail
these tectonic structures on Mars that could be
similar to continental rifts on Earth. 

>From the edge of a horst in Acheron Fossae to
the bottom of a graben, the digital elevation
data from the HRSC reveal height differences of
more than 1700 metres. 

The large graben in the centre of the image is
about 15 kilometres wide. 
 
 
By viewing the 3D (anaglyph) images through
stereoscopic glasses, you can see the different
topographic levels from which material has been
removed and then flowed to lower levels of terrain.
Lobe-shaped features are indicative of viscous flow. 

Erosional processes later transported material from the
outside the area into the crater and resurfaced its floor,
erasing the tectonic features inside the crater. The
depth of the crater from rim to bottom is 2000 metres. 
 

Perspective view of Acheron Fossae
 
The colour and black and white images show the view looking 
straight down from the spacecraft; north is to the right. The 
perspective view shows the same region including some adjacent 
areas to the south without vertical exaggeration.

The 3D images require stereoscopic glasses to view.