by Damien Knight
Today I will talk about faults and folds in the field. When we go to the Mojave, we will see many faults and folds. These features are a type of rock deformation due to stress. Rocks deform in three ways, elastic, brittle and ductile based on the stress. You can have confining pressure, shear stress, compression and tension.
Faults are a brittle deformation whereas folds are a ductile deformation. There are three types of faults, dip-slip faults, strike-slip faults, and oblique-slip faults. Strike-Slip faults are due to shear stress on the rocks. The dominant displacement on these faults is horizontal and parallel to the strike of the fault’s surface. A few sub categories of the strike-slip faults are right and left-lateral strike-slip faults and transform faults.
Left and right lateral strike-slip faults are faults that have crustal blocks on the opposite of the fault to either move right or to the left. The San-Andreas is a famous right lateral strike-slip fault. In the Mojave the Garlock Fault is a left-lateral strike-slip fault. Transform faults are strike-slips that cut through the earth typically at tectonic plate boundaries.
The most common fault type we will see in the Mojave are dip-slip faults. Many old mine shafts are found along dip-slip faults. Dip-slip fault blocks are identified by their position relative to the fault plane. The block above the fault plane is the hanging wall and below it the foot wall. Dip-slip faults encompass normal faults, reverse faults, and thrust faults. The fault block that drops during normal faulting are called grabens. The blocks above are horsts. We will see plenty of horsts and grabens out in the field. Below is a horst and graben at Mosaic Canyon.
Oblique slip faults are faults that have slip features of both prior fault types. To be classified an oblique-slip, the two slip types must be measurable.
While the Mojave is covered in brittle deformation, especially since Death Valley is a graben, it also has fantastic ductile formations. These formations are folds, basins and domes. With folds you have anticlines, synclines, monoclines and plunging folds.
Anticlines and synclines are folded arches and troughs created through compression stress. The arch folds are anticlines and the trough folds synclines. These folds are described as symmetrical when limbs are mirrored or asymmetrical when they are not. Asymmetrical folds are considered overturned if their limbs are tilted beyond vertical.
We will see spectacular folds at Mosaic Canyon. Below is a photo of an overturned fold in Mosaic Canyon.
Before I move to measuring strikes and dip I will talk about basins and domes. Mojave dessert has several basin areas. A basin is bowl like shape of down-warped crust. Domes are formed from up-warped crusts. The youngest strata are at the edges and the oldest in the center. In basins the youngest is in the center and the oldest strata is at the edges.
Finding and measuring these in the field is useful for several reasons. Taking measurement can help us envision the rock layers below. Knowing the structures of the beds help us understand the geological history of the rocks. Finding inactive faults can be useful for finding economic resources. Particularly miners sought inactive faults for gold mineralized into the fractures.
While in the field geologists use a compass with a clinometer to make these measurements. The strike is a direction of trend, dip is the angle of inclination. These are always 90 degrees of each-other.
This is how to take a strike and dip measure. First you need a compass with a clinometer. Our directions will assume we are using a Brunton Compass. First, we take the strike. First put the edge of your compass flat against the plane to be measured. Adjust orientation, then read compass needle for strike value.
After taking strike, rotate compass 90 degrees. Then place the side flat against the plane. Adjust until it is level and the clinometer air bubble is centred. Read the dip from the compass. After this you can measure trend and plunge.
Trend you have the “Fold-out pointer” Point down plunge. Hold compass above plane horizontally until the bubble is centered. Look through slot in “Fold-out pointer.” Rotate the compass until the slot is parallel. The line should run straight down slot. Read the North end for value of the trend. Plunge you place the edge of compass on the line. Make sure it is vertical. Adjust lever on back until the Clinometer is centered. Read plunge from the scale on compass.
I appreciate you taking the time to listen to me talk today. If you have any questions, please ask. I will do my best to answer.
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