Lab 7: Geologic Structure - Faults - Normal


Normal faults are common in areas of the Earth's crust where thick piles of sedimentary layers have accumulated. Such sedimentary basins sag by nature -- that's why the area "accommodated" deposition in the first place. This sagging can result in stretching around the edges of the basin. Geologists call the stretching force by the term extension. Extensional tectonics, such as has affected the prominent Basin and Range province of the American West, forms block-faulted mountains and valleys, bounded by normal faults. The blocks that stay up, forming mountain ranges, are called horsts, and those that go down, forming valleys, are called grabens. In the following animation, and in the others in this section, one side of the fault moves down and the other side stays put, or moves up.



Normal faults are the result of extensional force, or, as geologists like to call it, stress. Extension is just stretching, so you can expect that as rocks are stretched, they are thinned, and eventually, if stretched enough, they will tear. This animation shows how there is a horizontal component of movement along a normal fault:



As shown here, the motion along the fault is exclusively along the dip direction of the fault plane. This is called dip-slip motion, and is the main component of motion of most normal faults.


Normal faults, because they form "rips" or "tears" in strata, can be troublesome in some situations for exploration of oil and gas. But in other situations, normal faults are the "trap" that keeps oil in a reservoir and available for extraction. Here an animation is presented to show the special zone along a normal fault where a given stratum could be absent or greatly thinned, because the drill location lies at the spot where the stratum was "torn," or separated, by the fault:



Oil wells drilled anywhere but in this narrow zone would encounter the stratum shown in the middle of the block diagram. A well drilled here would just miss this stratum altogether.


This animation shows arrows indicating the extensional stress stretching the rocks: