Love's Lookout EarthCache
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Love's Lookout, a scenic ridge two miles north of Jacksonville in north central Cherokee County, has an elevation of 720 feet above sea level and rises 240 feet above the surrounding terrain. The ridge is part of a long, flat-topped hill that extends for nine miles. The ridge itself is called the Mt. Enterprise Fault Line.
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.
Approximately 240 million years ago, the major geologic events in Texas shifted from West Texas to East and Southeast Texas. The European and African–South American plates, which had collided with the North American plate to form the Ouachita Mountains, began to separate from North America.
Fault types : Geologists categorize faults into three main groups based on the sense of slip:
• a fault where the relative movement (or slip) on the fault plane is approximately vertical is known as a dip-slip fault
• where the slip is approximately horizontal, the fault is known as a transcurrent, wrench or strike-slip fault
• an oblique-slip fault has non-zero components of both strike and dip slip. Dip-slip faults can occur either as "reverse" or as "normal" faults. A normal fault occurs when the crust is extended. Alternately such a fault can be called an extensional fault. The hanging wall moved downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is called a graben. An up thrown block between two normal faults dipping away from each other is called a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults. A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall.
Reverse faults indicate compressive shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°.
A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging wall and foot wall) folds. Thrust faults form nappes and klippen in the large thrust belts.
Subduction zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes. The fault plane is the plane that represents the fracture surface of a fault. Flat segments of thrust fault planes are known as flats, and inclined sections of the thrust are known as ramps. Typically, thrust faults move within formations by forming flats, and climb up section with ramps. Fault-bend folds are formed by movement of the hanging wall over a non-planar fault surface and are found associated with both extensional and thrust faults.
Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). A normal fault may therefore become a reverse fault and vice versa. The fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion.
Strike-slip faults with left-lateral motion are also known as sinistral faults. Those with right-lateral motion are also known as dextral faults.Each is defined by the direction of movement of the ground on the opposite side of the fault from an observer. A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault.
Nearly all faults will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.
A listric fault is a type of fault in which the fault plane is curved. The dip of the fault plane becomes shallower with increased depth and may flatten into a sub-horizontal décollement. Ring faults are faults that occur within collapsed volcanic calderas and the sites of bolide strikes, such as the Chesapeake Bay impact crater. Ring faults may be filled by ring dikes. Synthetic and antithetic faults cut through sedimentary layers and mostly accompany growth faults. Synthetic fault dips at the same direction as the growth fault while the antithetic dips in the opposite direction. Those faults may be accompanied by rollover anticline. Since the growth fault dips mostly toward the basin, then synthetic faults dips basinward and antithetic fault dips landward.
Answer these questions to earn your smiley
1. Based on what you can see at the overlook, how deep do you think the ridge is?
2. In 1981 an earthquake shook Jacksonville & damaged the site. What evidence are you able to see of that Earthquake?
3. What type of fault is the Mt. Enterprise Fault?
4. * Optional* Take a picture of yourself at the Overlook.
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