Geology And Geophysics

A look at the Dynamics of a Thrust Fault



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"A look at the Dynamics of a Thrust Fault"
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The term “fault,” used in the context of geological events, describes a place where underground forces have pushed rock strata out of alignment with one another, and sometimes the resultant formation can be viewed above ground level.  There are different ways in which these disordered strata may be moved, whether up and down or sideways; “reverse faults are steeply dipping (more near vertical), thrust faults are closer to horizontal.”

Shaping the surface

Thrust faults display a low angle of attack between the hanging wall, which is that section of the strata that now appears above the fault, and the footwall, the strata that remain underneath the event.  The forces that press upon the two sides may be local or regional, and the resultant formations exhibit different shapes as a consequence.

In one instance where strata, perhaps for thousands of feet down, are pushed upward, the footwall may be many miles long.  The resultant formation’s upward motion therefore pushes upward until its own weight forces it into a horizontal track, while subsequent forced movements push up materials located farther along the same track.  When the two sides stop moving against one another, the outcome may resemble wooden shakes (shingles) pushed against one another and seemingly stacked side by side.

Examples of thrust faults

“…[A] modern day example is Asia sliding up over northern India to create the Himalayan mountains.”  This movement certainly illustrates in apt fashion the regional, even continental, size of the geophysical forces involved.  Forces of this magnitude help to explain why the height of Mount Everest will not be the same next year as it is this year.  The movement of the stratigraphic faces has not yet stopped.

“The modern Blue Ridge is an overturned anticline.”  The rocks were thrust-faulted past one another as if the bottom were a ramp upon which the upthrust strata traveled.  At certain points along that ramp structure, something got stuck, and a downward slope seemed to get locked into place (a structure known as an “anticline”).  The territory identified with the original surface features was shifted for many miles west as a result of these fault movements, changing flat land into mountainous land and building in folded features of strata, even doubled strata, in which core samples would show the same type of rock strata repeated at least once.

Though changed, the forces remain

In the beginning of its development as a planet, Earth’s surface temperatures were hot enough to keep rocks and metals in liquid form, that is, molten.  As the surface has cooled over the past four-plus billion years, water vapor has condensed, continental surfaces have cooled, and the molten materials are more deeply hidden, in and under the crust.  However, the continental plates still move on the hot surface of those molten materials.  Their temperature is lower than it was billions of years past, but they still have the power to push continental bodies a few inches a year.  This energy can and does cause volcanism, and the crust will still move in smaller regions as a consequence.  Someday that may not be the case, after hundreds of millions more years of cooling.  Today, all life on the surface of the good Earth is subject to occasional catastrophic shifting, and thrust faults can still destroy huge swaths of once-familiar real estate.

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