A Fault Zone of Mars

The Science

This piece is based on a HiRISE image of northern Meridiani Planum on Mars, which shows a distinct fault zone of layered deposits. But how much can we tell from a photo? Using remote imaging on Mars, we observe the topography through stereo imaging (giving a three-dimensional view) and analyze it by applying principles of how rocks are deposited on Earth.

Geologists use cross-cutting relationships in order to determine a sequence of events. A fault crossing a set of layered deposits must be newer, as the rock layers were deposited before they were displaced by the fault. Working backwards, a geologist can construct a relative timeline to explain what they are seeing. In this image, the layered formations were first deposited, then compressed. This compression created folds upward (anticlines) and downward (synclines). These folded layers were weathered down to expose the layers, and then faulted. 

Using orbital photography on Mars, the topography can be visualized through stereo imaging, giving a three-dimensional view. On the left side of the fault zone, the layered deposits were domed (a circular anticline) before weathering down, causing the elongated but concentric rings of light and dark layers. On the right side are possible basins (circular synclines), again causing concentric rings of deposits. On Earth, these layers would be measured by a field geologist at ground level to determine their angle of tilting (dip), and whether they are dipping away from the center (indicating an anticline) or towards it (syncline).  

Faults observed on Earth are most simply a fracture between two blocks of rock. This fracture can be vertical, horizontal, or an angle in between. The angle of the fracture (dip) and direction of movement (slip) is how faults are categorized. Imagine standing on one block. If the other block moves downward and away from you, or upward and towards you, this is “dip-slip” motion. Downward motion is called a normal fault, caused by extension in the crust (example: the Basin and Range area of the United States.) Upward motion is a reverse (or thrust) fault, caused by compression in the crust. Horizontal motion is a strike-slip fault (example: the San Andreas fault in California.)  

This fault zone appears to be a mix of strike-slip faults and normal faults. Some of these faults show sharp contacts, indicating a clean break between blocks. Others seem to smear the layers in the direction of movement, showing “plastic deformation” (the block’s layers were soft enough to deform as the fault moved.) At the center of the image, there are possibly two fault wedges, small parts of the parent block, torn away and rotated or distorted by the fault bending over time. 

Reference photo credit: NASA/JPL-Caltech/UArizona

Reference Photo Link