Linear rille in Mare Tranquillitatis, the result of extensional stresses. What caused the offset in the rille on the east wall? LROC NAC M146858595LE, image width is 700 m [NASA/GSFC/Arizona State University].
Linear rilles are so named because of their nearly-straight morphology and surface expression. Unlike sinuous rilles, which are volcanic, linear rilles are tectonic in nature. Similar features on Earth are termed graben, and are created when two normal faults border a block of rock which has been depressed, producing a valley. Since normal faults are understood to be the products of extensional stresses (see yesterday's Featured Image post), we can assume this region of the Moon was "pulled apart" - creating these normal faults, dropping the middle blocks, and producing the linear rilles. So a linear rille is the lunar analog of a graben on Earth!
Context image of today's Featured Image. Several linear rilles are located in the area, and even cross-cut a secondary crater chain to the south. Image width is 100 km [NASA/GSFC/Arizona State University].
In today's Featured Image, two normal faults appear to be offset. What are we seeing here?
It is probably an en echelon step between the two normal faults making up the east wall of the rille. When two faults are near to each other, they can interact and create an en echelon step that helps to even out the displacement and forces that created the faults. En echelon steps are common, and are seen in other tectonicfeatures on the Moon.