A Primer on Appalachian Structural Geology

A Primer on Appalachian Structural Geology 
Appalachian structure exhibits large thrust faults. Thrust faults are horizontal breaks along which one sheet of rock moves over top of another sheet of rock.  Typically the sheets of rock mountain-sized blocks, hundreds to thousands of feet thick It is possible to have a single, isolated thrust fault, but more commonly dozens of individual faults (sets) form. Thrust fault movement is complex and more detail is included here: Example.
~Processes of Thrust Fault Development~
Thrust faults develop when one block of earth, the hinterland, collides with and compresses another block, the foreland. The force of the hinterland and its movement creates horizontal stresses in the foreland rocks causing them to thrust fault and move. Similarly, stresses in the hinterland also lead to thrust faulting. 
The drawing to the right illustrates a hinterland moving from left to right over the edge of a foreland; a modern day example is Asia sliding up and over northern India to create the Himalayan Mountains. As the hinterland moves, its force and movement sets up stresses in the underlying foreland rocks, causing them to shear from left to right. Or, as the hinterland pushes inland, deformation of the foreland rocks also moves inland, for example, on the drawing from ramp fault number 1, to 2, to 3, etc.
Thrust fault deformation comes in two varieties, "decollement and imbricated splays", and "ramp and flat". These are not distinct from each other, but are end members in a spectrum. Most of the complexity of thrust-belt systems are generally combinations of these two faulting variations. 

Decollement and Imbricate Splays: horizontal fault-plane (the decollement) with upward curving splay faults. The splays develop in sequence, normally beginning closest to the source of stress, and migrating inward toward the foreland (foreland progression). Each drawing below represents the next stage of development. The splays are numbered in the drawing in order of their appearance. Arrows show the direction of movement.

Alternatively, there are occasions where the splays develop behind, and after, the frontal splay. That is, they develop back toward the hinterland rather than toward the foreland. Still, these splay upward in the direction of the foreland.

Finally, there are occasions where the splays not only develop behind, and after, the frontal splay, but the splays also splay back toward the hinterland; a hinterland progression.

These series of splay thrusts are said to be "imbricate"; stacked together something like shingles laid against a wall. This kind of thrust faulting is very common. 

Ramp and Flat: horizontal fault (glide) plane (the flat) ramps up through the rocks until it reaches another glide horizon, then begins to move horizontally (another flat) on the upper glide surface of the underlying rock. It is not unusual for the ramp to cut through thousands of feet of rock, and for the thrust sheet to move tens of miles along the upper flat. At the edge of the flat the moving rocks may get stuck and curl over to form an overturned anticline.
In some cases the rocks do not move long distances along the flat, but curl over and stop moving, forming a "ramp and overturned anticline". 
These overturned anticlines are surprisingly common in thrust belt systems, and we have shown several already; Blue Ridge Overthrust (Main Page discussion), Allegheny Front, or Example 1.
It is not unusual for the overturned anticline to be associated with a major thrust sheet, such as with the Blue Ridge anticlinorium, or the overturned anticline just east of the Allegheny Front, seen, for example, at Germany Valley. In other places, however, after the overturned anticline is formed, the rocks continue with deformation to produce an even more complex structure (Giles and Eagle Rock examples below). 

This simple introduction alone will help you recognize fault structures in thrust belt systems such as the Appalachians. If you want to understand a little about why the rocks behave this way, or want to see some examples of just how complex all this can become, read on.
~Generation of decollement and imbricate thrust faulting~
Due to horizontal stresses created by the impact of a hinterland pushing against a foreland, the rocks begin to fault, close to the point of collision where the forces are greatest, and then migrate inland toward the foreland interior (see above Cross Section). We illustrate the generation of a single decollement and ramp with the use of the block diagrams to the right.
Block 1: Imagine a block of the earth undergoing horizontal stress. The large red arrow numbered 1 is the stress. The purple arrow is the actual fault, where the rock has already broken and begun to move. The dashed black line is an incipient fault; the rock in front of the fault is stressed but has not broken yet. But as the stresses migrate inland, faults continue to develop as shown in Block 2 and Block 3. 

Block 2: As the rock strength fails, the overlying layer begins to slide over the underlying layer, indicated in the drawing by the top half of the yellow block beginning to move. This is the beginning of a horizontal thrust fault and is called a decollement (basal thrust fault). As the length of the decollement increases, however, the friction along the fault increases, and it requires more stress to continue movement. Also, as the fault grows, the more rock mass there is to move so stress begin to pile up.
Early in an orogeny there is adequate stress to overcome any resistance, but over time, stress seeks out the path of least resistance. Movement in the downward or forward directions requires the most energy, so the rocks overcome their cohesive strength and break upward to relieve the stress - Ramp 1; Block 3. 

Block 3: The formation and stress release along a ramp is only a short term solution because it can become locked when upward stresses can no longer overcome downward directed weight. At this point, the easiest way for stress to be released is forward again toward the foreland. The decollement reactivates and a horizontal fault begins to extend itself forward again, leading to the formation of Ramp 2. 
This process of decollement and subsequent ramp formation continues creating ramp after ramp splaying off a basal thrust fault as it migrates toward the foreland interior (Example). 
~Ramp and flat thrust faulting (and complications)~ 
The second style of thrust faulting begins in a similar way to the decollement and imbricate splay system discussed above, but progresses differently (Block 4). The decollement develops (called the flat), followed by the ramp, but the thrust sheet does not stop moving. Once it has broken through to the top, the ramp (fault) turns flat again, and the thrust sheet just keeps on moving across the top, as in Block 5. 

This effectively doubles the thickness of the pile of rocks. Drilling a hole down through the top sheet would go through the entire thickness, pass through the thrust fault, and then go through the same rocks in the lower sheet. In some cases, the thrust sheet becomes immobile at the top of the ramp to form an anticline (Block 6).

Rocks are not uniform in strength, however, and stresses do not act smoothly or continuously on these rocks. Stresses are transmitted in very complex ways. Every time a new flat, or ramp, or anticline develops - that is, every time the system becomes more complicated - the more difficult it becomes to predict what will happen. All of these processes occur below the surface, under conditions very different from today.
Typically, complications occur because thrust systems are mixed.  Some examples of possible scenarios are presented below:

Case One - A ramp and basal or floor thrust system that develops a decollement and imbricate ramp system. 
Imagine a basal thrust where a unit of strata ramps up to double the thickness of the strata. After this process, the bottom or basal thrust continues to extend forward forming a decollement. From this, extended decollement and a second ramp develops, only this time it ramps through the doubled strata causing it to quadruple. 

Case Two - The Duplex.
A duplex is a structural feature with a floor thrust and a roof thrust (i.e. two thrusts running parallel to each other separated by thousands of feet of rock), and in between the floor and roof thrusts are a series of imbricated splay faults. The imbricated splays cause the rock column to shorten and thicken, typically causing the entire pile to overturn.
Duplexes usually form as a result of two or three separate deformational episodes, but this is not unusual in a thrust system. One set of forces typically starts to deform the rocks, but as the orogeny continues, different sets of forces can act on the rock. 

A duplex typically begins with a decollement (floor thrust) and imbricate splay system, where a dozen splays may have formed. In the figure above, observe the imbricate splay thrusts in red rising off a bottom decollement. The blue thrust is the roof thrust. The green lines represent sedimentary rock layers several thousand feet thick. This drawing was loosely sketched from an example in the Valley and Ridge of southern Virginia. The section line runs NW to SE. Also notice that some of the splay faults connect together. 

In the cross section below, the whole system of ramp thrusts has been folded into an overturned anticline; this is the duplex.  Observe the land surface; thousands of feet of this structure has been eroded. 
Also observe that where rocks moved up along the ramp thrusts, after the folding, they appear to move downward into the earth. We have not included the strata in the folded version, but you can see in the first drawing it is sliced into sections, and will then be displaced by the folding. An example of this is seen at Eagle Rock

~Two Examples of Complex Appalachian Structure in Virginia~
1.  Imbricated ramp and flat in Giles County, Va
In the examples above, ramp and flat systems were kept simple, but in this real world example, complex ramps, faults, and folds are predominant.  

Ramps: Notice that there is a very clear basal ramp (numbered 1) and basal flat. Notice also that the horizontal strata below the basal flat was an immovable block (it is not deformed at all), so as thrust sheet after thrust sheet came from the southeast, they ramped up and sheared off at its southeastern end (Ramps numbered 1 - 4). As each new ramp developed it sheared off part of the previous ramp.
Also notice that ramps like 1 and 2 seemed to get hung up and form anticlines, but that 3, and probably 4, arched over the top of ramps 1 and 2 and kept right on going. As a result the blue fault has two ramps on it, number 3 and and the farthest one to the NW. 
Faults: The blue fault is all the same fault; notice how it became folded as it moved up over the ramp and down into the flat toward the NW. 
Also notice how faults connect together and then diverge again. It is hard to decide which fault is which, and which moved when. Clearly some of these faults were active for a while, became dormant, and then reactivated at a later time, sometimes only in parts, and maybe for an entirely different reason.
Folds: Many anticlinal and synclinal folds exist in this cross sections, some of them miles in dimensions, but each formed in response to movement along a fault. At the surface these folds represent big, prominent structures, but they are simply responses to imbricated ramp and flat thrust faulting.

2.  Eagle Rock, Va
The structural geology in this locale is complex and is based on an exposure of rocks along the James River near Fincastle (junction of Rts. 43 and 685) known as Eagle Rock. What we see in the outcrop in only a fraction of the structure.
In the drawing below, the blue horizontal line is road level, the faint green line is the top of the exposed outcrop, as seen in the photograph. The tallest part of the outcrop on the right side of the photo lies between the two far left faults in the drawing below.

Bartholomew, et. al., 1974, NE-SW GSA '82 Field Trip Guidebook

Each of the red lines is a thrust fault; they are folded and some of them merge together. Observe how some of the faults are discontinuous, cut by other faults so that they seem to have no origin and unlike most thrust faults that ramp upward, these move downward into the ground.
To understand an outcrop such as this usually requires some well developed model based on better data. The interpretation of this outcrop is based on the duplex structure discussed Above, that is, there are a minimum of two stages of deformation. The first is an imbricated ramp system and the second stage brought about folding (overturned anticline). In this process all the upward directed ramps became folded, with some of them now plunging down into the ground. 
Contributed by Lynn Fichter 
Wednesday, August 13, 2014
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