In thrust fault systems, a fenster is defined as:

In thrust fault systems, a fenster is defined as:

• A. An eroded area exposing younger rocks beneath the thrust
• B. An eroded area exposing older rocks beneath the thrust
• C. A fault gouge zone within a thrust sheet
• D. A window of intact rock above the thrust

Answer: (A)

A fenster is a window through a thrust sheet revealed by erosion, showing rocks that lie beneath the thrust. In thrust fault systems, erosion can remove part of the overlying sheet, exposing the footwall rocks that were originally underneath. The rocks revealed in a fenster are the rocks beneath the thrust, and in this context they are described as younger than the rocks in the thrust sheet, which is why the correct choice emphasizes younger rocks beneath the thrust. This feature is not a fault gouge zone, nor a window of intact rock above the thrust.

A fenster is a window through a thrust sheet revealed by erosion, showing rocks that lie beneath the thrust. In thrust fault systems, erosion can remove part of the overlying sheet, exposing the footwall rocks that were originally underneath. The rocks revealed in a fenster are the rocks beneath the thrust, and in this context they are described as younger than the rocks in the thrust sheet, which is why the correct choice emphasizes younger rocks beneath the thrust. This feature is not a fault gouge zone, nor a window of intact rock above the thrust.