The Moon Has Its Faults
At Moon Zoo, they have asked for your help in identifying linear features on the Moon, some of which are likely to be faults. So, what are faults? Faults are fractures in a planet or moon’s crust that accommodate some movement along them. If the movement along a fault is great enough, a quake can be felt.
On Earth, the majority of faults occurs at plate boundaries, like the San Andreas fault which separates the North American and Pacific plates (Figure 1) or along the boundary between the Indian and Eurasian plates which forms the Himalaya Mountains.
Figure 1. San Andreas fault in California. A) Aerial view of the Carizzo Plain. Note the offset drainages. B) Annotated image showing the fault trace (black line) and the arrows denote the direction each side of this strike-slip fault moved. Image A: IPGP, Paris.
However, a planet or moon does not need to have plate tectonics (as on the Earth) in order to have faults. Faults can form anywhere where there is enough stress (the amount of force distributed over an area) in the crust to make it break – or fault.
There are several ways in which faults form. On the Moon, the source of stress is most commonly associated with impact basins, so most of the faults are found in and around the large impact basins on the lunar nearside.
There are two types of faults found on the Moon. Normal faults occur where there is tension (stretching) of the crust. When this occurs, the crust in that area gets thinner and longer, which means that one side of the fault must move down relative to the other (Figure. 2). This happens when the weight of the mare in the basins pulls the crust apart as the middle of the basin subsides.
Figure 2. Cartoon showing cross-sectional and map views of a pair of inward dipping normal faults, resulting in the formation of a graben. The red arrows show that the crust is being pulled apart and white arrows show the movement directions along the faults. Image: Sue Selkirk/ASU.
Normal Faults: on the Moon, there are several types of normal faults: straight rilles normal faults and thrust faults.
1. Straight rilles are long, narrow troughs and they exhibit two fault-related surface morphologies: linear (Figure 3) and arcuate. These are formed by faults and are found in the highlands near mare basins as well as near their margins. They have steep sides, which are interpreted to be the normal faults, and flat floors. These structures with two inward sloping walls are called graben (meaning trench in German) (Figure 3).
Figure 3. Linear rilles or graben of Rima Ariadaeus is about 300 km in length and is located between Mare Tranquillitatis and Mare Vaporum. Image: WAC M116222025MC NASA/GSFC/Arizona State University.
2. Isolated normal faults also exist, but are rare compared to rilles (graben). Like some normal faults on Earth, these tend to be very linear as shown by the Rupes Recta (“Straight Wall or Cliff”) in Mare Nubium (Figure 4).
Figure 4. The Rupes Recta normal fault (shown by black arrows) is ~110 km long and is located in the southeastern part of Mare Nubium on the nearside of the Moon. The fault scarp is a few km high and is in shadow. Image: JAXA/NHK.
3. Thrust Faults: When stress acts in an opposite sense, compression of the crust occurs and thrust faults are formed. In this case, the crust gets thicker and one side of the fault moves upward over the other (Figure 5).
Figure 5. Cartoon showing a thrust fault. U: up, D: down. Large black arrows show compression direction and small half arrows show movement directions along the faults. The surface trace of the fault is what geologists would map. Image: American River College.
On the Moon, this happens most often in impact basin interiors where the weight of the mare causes the basin to subside, or bend downwards, causing compression in the mare. Thrust faults on the Moon have two morphologies: wrinkle ridges and lobate scarps. Wrinkle ridges (Figure 6) are found in the mare in the basins and are very complicated structures. They consist of a broad arch (the ‘ridge’ in wrinkle ridges) with narrow, often sinuous ridges superimposed (the ‘wrinkle’). Lobate scarps, which are much simpler in their morphology (Figure 7), are almost exclusively found in the lunar farside highlands far from the influence of basin subsidence. These structures are instead thought to have formed from the slow cooling of the lunar interior that led to a small amount of global contraction.
Figure 6. Wrinkle ridges in Mare Serenitatis. Note also that normal faults are present at the basin margin in the lower corners. Both wrinkle ridges and normal faults are primarily concentric to the basin. Image width is ~280 km. Image: mosaic of Apollo Metric camera frames AS17-450 and AS17-454. NASA.
Figure 7. Lee-Lincoln scarp in the Taurus-Littrow valley (Apollo 17 landing site). Image is 1.6 km across. Image: NAC frame M104318871LC. NASA/GSFC/Arizona State University
Why are faults important to lunar and planetary scientists? On the Moon, since most faults form in response to stresses from the lunar basins, they can tell us about the geologic and tectonic evolution of these basins, particularly the mare-filled ones on the lunar nearside. Wrinkle ridges and rilles can be used to determine the structure of the basin and in some cases how thick the mare are. The fact that lobate scarps are found primarily in the lunar highlands and wrinkle ridges are found only in the lunar mare suggests that these materials behave differently under stress, which is important when trying to puzzle out the geologic history.
The tectonic evolution of a planet or moon can also give us important information about what the interior is like since directly accessing the deep interior is nearly impossible. For example, the lobate scarps (along with the presence of mare) tell us that the Moon was initially hot and that it cooled some finite amount over its lifetime. This has been determined from the offsets along the faults to be about 70 m.
This post was written for Moon Zoo by by Amanda Nahm at the Lunar and Planetary Institute.