a scientist observes rock masses that have moved past eachother in opposite horizontal directions. Which feature scientist observe?

Strike-slip fault feature the scientist observes.

Explanation:

+When the blocks moves in a horizontal direction the fractures that occurs vertically refers to the Strike-slip faults. These are the fractures that are almost vertical in direction. These types of faults are due to the forces called shearing forces. When the block that is present on the far side of the fault moves in the direction of left then such faults are referred as left-lateral.

The faults such as airplane faults, dead sea transform are some of the examples of this type of faults. Here the scientist observes that the movement of the masses of rock follows horizontal direction and also they are opposite to each other. This acts as an example of Strike-slip fault.