The Swiss Alps are both magnificient and impressive, luring lots of tourists every year. Apart from drawing tourists, this fairly new mountain range also attracts many geologists. A distinct line (as seen in Figure 5) seperating two rock layers outcrops at numerous locations in the Swiss Alps; this is known as the Glarus Overthrust Fault. The Glarus Thrust is located in the eastern part of the Alps of Switzerland. The thrust is exposed over a large area in the Canton of Glarus, St. Gallen and Graubünden regions.  

A thrust fault is simply a low-angle reverse fault, especially if that fault extends for several to hundreds of kilometers such as the Glarus Thrust (Fossen, 2010). Since a thrust fault dips at a low angle, it is possible for compression to push older rock sequences above younger ones. According to Herwegh et al., the Glarus Thrust dates back to the Oligocene-Miocene age, roughly 35 to 5 millions years ago. The fault separates the Glarus nappe from its footwall with a total displacement of at least 30 to 40 km (2006). As shown in Figure 3, the top of the Hausstock peak constitutes of Verrucano rocks, which form the base of the Helvetic Nappes. These rocks consists of rhyolitic and spilitic layered volcanic rocks, as well as red beds and conglomerates, dating back to the Permian (299 to 251 Ma)  (Raumer, Neubauer, 1993). The Verrucano beds can be seen at the base of the Helvetic nappes in some areas but in others, can overlie folds and younger rock strata (Herwegh et al., 2006). The Helvetic Nappes and Subhelvetic Units, overthrust the Wildflysh Nappe and the North Helvetic Flysch (NHF), hence known as Infrahelvetic complex. The NHF consists mainly of turbiditic sandstones and slates dating back to the Early Oligocene (roughly 34 Ma) (Gasser, Brok, 2008). The Glarus fault is an important feature in the geological world because it is shows a very clear example of a thrust fault where the nappe contact is extremely clear and is also visible in three-dimensions. Furthermore is has played a role in the history of geology, as it was not always clear that thrusting was the answer to this phenomenon (Buckingham et al., 2013). 

In the geological map of the Glarus Overthrust (Figure 4), the thrust line seems to somewhat follow the topography (circles around the peaks, observable even though the geological map does not include contour lines). This is caused by the fault dipping at a low angle. When the rock beddings are horizontal or nearly horizontal, their contacts will also be horizontal. Therefore, on a topographic map, the bedding contacts will run parallel to the contour lines (UOregon, 2010). This is also true for faults, explaining why the fault line seems to follow topography. On this map, the dark line with arrows is the Glarus fault line. The darkest layer is the Permian Verrucano rocks, while the white layer is the younger, Oligocene Flysch units. The Flysch Units in this map refer to the NHF as well as the Wildflysch Nappe. The flysch units contain many folds and other deformations. The map shows that the Helvetic Nappe, which contains the Verrucano rocks as well as Mesozoic-Cenezoic sediments, overlay the Infrahelvetic complex, which inclides the Flysch units, a Mesozoic cover and a crystalline basement. The older Helvetic Nappe overlays the Infrahelvetic complex along the thrust line, showing that the thrust fault was responsible for this displacement. The numbers on this map are irrelevant for the purpose of this text, they simply correspond to stops visited by Herwegh et al. in 2006. 

The deformation that took place in the Infrahelvetic Complex occured in three phases. During the first phase, the Sardona and Blattengrat Nappes slid onto the NHF. The Sardona and Blattengrat Nappes consist of Late Cretaceous and Eocene limestones, marls and marine sandstones. This phase could have occured by gravity sliding. In the second phase, the Infrahelvetic Complex was subjected to folding. The third phase is the development of the Glarus Thrust. The Glarus thrust is an out-of-sequence thrust, where a minimum displacement of 35 km which formed a steep crenulation cleavage below the thrust. Another theory is that the Sardona and Blattengrat Nappes were not caused by gravity sliding but by compressional tectonics. This could have thrusted and folded the NHF Unit. Also this theory suggests that the Glarus thrust was first displaced between 25 to 30 km and was later followed by another 5 to 10 km displacement. The Infrahelvetic complex would have been folded during the first displacement and would have developed its steep crenulation cleavage during the second displacement (Gasser, Brok, 2008). Figure 3 shows an example of this folding in the NHF. Figure 7 shows the Verrucano rock layer thrusted over the Quinten Formation (Qu). Underneath the Ofen (middle), the Qu displays a large syncline fold with parasticic folds which have been destroyed by the Glarus Thrust. On the left of the picture, there is a large recumbent fold which consists of Mesozoic and Cretaceous sediments. There is also an alteration zone between some of the Verrucano and Flysh Units contacts that can be observed. This alteration zone was caused by the friction heat released when the thrust was displaced (Herwegh et al., 2008). 

According to Buckingham et al., the stress which caused the Glarus Oversthrust, was the formation of Alps. The Alpine Orogeny is thought to have been formed by two different orogenies; one during the Cretaceous and one during the Tertiary. The formation of the Alps was caused by the collison of the European continent and the Adriatic continent, after subducting the ocean basins in between, like the Tethys Ocean. There is still shortening in the eastern part of the Alps today (Froitzheim, 2012). The Glarus Thrust was formed during the second part of the orogeny. This collision caused enough stress to push the older rock layers (Verrucano), along the low-angle thrust fault, above the younger sequences. Figure 8 shows an example of how overthrusts are formed, causing older rocks to lie on top of younger ones. The large white arrows show the direction of compression. As the figure illustrates, the fault has to be dipping at a low-angle. If it was not, the compression would simple cause a reverse fault and this inversion in rock layers could not be observed.


References:

Buckingham, Thomas, Walker Simon, Jurg Meyer, and Harry Keel. "Swiss Tectonic Arena Sardona." Unesco World Heritage, v3.0. 7 Aug. 2013. Print.

Fossen, Haakon. Structural Geology. Cambridge: Cambridge UP, 2010. Print.

Froitzheim, Nikolaus. "Geology of the Alps Part 1: General remarks; Austroalpine nappes." Structural Geology. University of Bonn, Germany. 27 Mar. 2012. PDF.  
<http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-the-alps-part-1-general-remarks-austroalpine-nappes>.



Gasser, Deta, and Bas Brok. "Tectonic Evolution of the Engi Slates, Glarus Alps, Switzerland." Swiss Journal of Geosciences 101 (2008): 311-22. Print.

Herwegh, Marco, Jean-Pierre Hürzeler, O. Adrian Pfiffner, Stefan M. Schmid, Rainer Abart, and Andreas Ebert. "The Glarus Thrust: Excursion Guide and Report of a Field Trip of the Swiss Tectonic Studies Group (Swiss Geological Society, 14.–16. 09. 2006)." Swiss Journal of Geosciences: 323-40. Print.

Imper, David. "Elm-Linthal: Crossing the Richetlipass." Federal Office of Topography Swisstopo. Swisstopo, 15 Feb. 2011. Web. 30 Mar. 2015. <http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/geology/viageoalpina/VGA_Sardona.parsys.000101.downloadList.80281.DownloadFile.tmp/stage0610elmlinthalen.pdf>.

Raumer, J. F. Von. Pre-Mesozoic Geology in the Alps. Berlin: Springer-Verlag, 1993. Print.

University of Oregon. "Outcrop Patterns." Structural Geology. 2010. Web. 29 Mar. 2015. <http://pages.uoregon.edu/millerm/Srpatterns.html>.

Figure 1. San Rafael Swell Specific Delimitations http://files.geology.utah.gov/maps/geomap/30x60/pdf/ofr-404.pdf

Figure 2. Portion of San Rafael Swell, "The Reef" Section (Johnson & Johnson 2000)

Figure 3. Geologic Map and Cross-Section of San Rafael Swell http://written-in-stone-seen-through-my-lens.blogspot.ca/2011/08/flight-plan-part-i-geology-of-san.html

Figure 4. Cross-Section of San Rafael Swell http://www.lemkeclimbs.com/san-rafael-swell.html

Bibliography

Delaney, Paul T. and Anne E. Gartner. 1997. "Physical Processes of Shallow Mafic Dike Emplacement Near the San Rafael Swell, Utah." Geological Society of America Bulletin 109 (9): 1177-1192. http://gsabulletin.gsapubs.org/content/109/9/1177.short

Hawley, C.C., R. C. Robeck, and H. B. Dyer. 1968. "Geology, Altered Rocks And Ore Deposits of The San Rafael Swell Emery County, Utah" U.S. Department of the Interior, U.S. Geological Survey http://pubs.usgs.gov/bul/1239/report.pdf

Jeffery, D. L., J. L. Bertog, and J. R. Bishop. 2011. "Sequence Stratigraphy of Dinosaur Lake: Small Scale Fluvio-Deltaic Stratal Relationships of a Dinosaur Accumulation at the Aaron Scott Quarry, Morrison Formation, San Rafael Swell, Utah." Palaios 26 (5): 275-283. http://palaios.sepmonline.org/content/26/5/275.short

Johnson, K. M. and A. M. Johnson. 2000. "Localization of Layer-Parallel Faults in San Rafael Swell, Utah and Other Monoclinal Folds." Journal of Structural Geology 22 (10): 1455-1468. http://www.sciencedirect.com/science/article/pii/S0191814100000468

Bartsch-Winkler, Susan, Robert P. Dickerson, Harlan N. Barton, Anne E. McCafferty, V.J.S. Grauch, Hayati Koyuncu, Keenan Lee, Joseph S. Duval, Steven R. Munts, David A. Bemjamin, Terry J. Cclose, David A. Lipton, Terry R. Neumann, and Spencee Willett. 1990. "Mineral Resources of the San Rafael Swell Wilderness Study Areas, Including Muddy Creek, Crack Canyon, San Rafael Reef, Mexican Mountain, and Sids Mountain Wilderness Study Areas, Emery County, Utah." U.S. Geological Survey, U.S. Bureau of Mines http://pubs.usgs.gov/bul/1752/report.pdf

Gilluly, James. 1928. "Geology and Oil and Gas Prospects of Part of the San Rafael Swell, Utah." Contributions to Economic Geology http://pubs.usgs.gov/bul/0806c/report.pdf

Freeman, Michael L., David L. Naftz, Terry Snyder, and Greg Johnson. 2008. "Assessment of Nonpoint Source Chemical Loading Potential to Watersheds Containing Uranium Waste Dumps Associated with Uranium Exploration and Mining, San Rafael Swell, Utah." U.S. Department of the Interior, U.S. Geological Survey http://pubs.usgs.gov/sir/2008/5110/pdf/sir20085110.pdf

                                     Figure 1.  Geological map of the Muddy Mountain thrust (Fossen, 2015) 

        Figure 2.  Cross-sectional view of the Muddy Mountain overthrust

                                                 (Brock & Engelder, 1977)

Works cited
Brock, William G., and Terry Engelder. "Deformation associated with the movement of the Muddy Mountain overthrust in the Buffington window, southeastern Nevada." Geological Society of America Bulletin 88.11 (1977): 1667-1677.

Fossen, H., Zuluaga, L.F., Ballas, G., Soliva, R., Rotevatn, A., Contractional
deformation of porous sandstone: insights from the Aztec Sandstone, SE Nevada, USA, Journal of Structural Geology (2015), doi: 10.1016/j.jsg.2015.02.014.

Soliva, R., Schultz, R.A., Ballas, G., Taboada, A., Wibberley, C., Saillet, E., Benedicto, A., 2013. A model of strain localization in porous sandstone as a function of tectonic setting, burial and material properties; new insight from Provence (southern France). Journal of Structural Geology 49, 50-63.

The Himalayan mountain range is one of the biggest mountain ranges of the world. It spans 2900 km across Nepal and India. Notably, Mount Everest (shown by a white triangle), with a peak above 9 km in altitude, the highest point on the Earth, is in the Himalayan mountain Range.

The Himalayan Frontal Thrust, also known as Himalayan Frontal Fault, is the youngest and most active fault of the Himalayas. The three main Himalayan faults are the Himalayan frontal thrust, the Main Boundary Thrust and the Main Central Thrust (not shown).

This immense mountain range has a long history. To understand how it came to be, we have to travel back to Pangea, the supercontinent.

About 225 Ma, the landmass that we know today as India was still attached to what is today known as Africa. Over the next 25 million years, the Indian tectonic plate slowly broke off of Africa and headed north to Asia. Fast forward to 80 Ma; the Indian land mass, moving at a speed varying from 15 to 20 cm/yr, is nearly six and a half thousand kilometer away from the Eurasian tectonic plate. Over the next 70 million years, the Indian plate finished its voyage across the ocean and 10 Ma, collided with the Eurasian plate.

For a more in depth animation of the formation of the Himalayan Mountains, consult: http://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Convergent/Continental-Collision

The initial collision of the two tectonic plates occurred while there was still a sea separating the Asian and Indian land masses. Once these two tectonic plates collided, the velocity of the Indian plate decreased significantly to 5 cm/yr. As the two land masses approached each other, the sea floor sediments were compressed and brought to the surface with the thickening of the crust. The two land masses officially collided when the sea was completely drained. Since the initial collision, several enormous earthquakes occurred to relieve this stress. The main central thrust was formed upon this initial collision and over the next few million years the main boundary thrust was created. “The initiation age of the [Himalayan Frontal Thrust] is constrained between 500 and 100 Ka.” (3) The Himalayan Frontal Thrust is the youngest and thus the most active out of the Himalayan Thrusts. “The [Main Central Thrust] is mainly inactive […] and the [Main Boundary Fault] in localized areas exhibits neotectonic acticity.” (3)

We know The Himalayan Frontal Thrust itself is a NW-SE trending thrust fault where the hanging wall is pushed up the footwall. The Frontal Thrust is the boundary between the Sub-Himalaya, often called Siwalik layer, and the north Indian plains. “A slip rate of ≥ 13.8 ± 3.6 mm/yr on the [Himalayan Frontal Thrust] has been estimated on the basis of a radiocarbon date, […] assuming an average fault dip of NE 30°” (3). This is thrusting the Siwalik layer above and onto the Ganges plains. The Siwalik layer is made up of Cenozoic sediments and consists mostly of animal fossils, elephants and horses, to name a few, that made up the wildlife at the time. The northern plains consists of the youngest sediments and the youngest layer of the Himalayan region.

The Himalayan Frontal thrust was originally thought to be a blind fault. An apparent anticline could be seen following a certain line, with shortening in a direction perpendicular to said line. An age disconformity could also be seen, with the older layer on top. With these indications, geologists explained the deformation by a blind thrust. However, recently, researchers from the Center of Neotectonic Studies of the University of Nevada, the Department of Geological Sciences of San Diego State University and the Wadia Institute of Himalayan Geology of Dehra Dun have shown evidence for traces of the exposed Himalayan Frontal Thrust. These researchers have found that at many sites along the Himalayan Frontal Thrust, namely, Rampur Ganda, Lal Dhang and Ramnagar, “trench exposures and vertical separations of 9-13 m are interpreted to indicate about 13 – 26 m of coseismic slip during the last earthquake.” (5) By analyzing these locations, these geologists have been able to date the simultaneous fracture to 1413 ± 9 A.D5. This indicates that a powerful earthquake caused more slip along the fault at this date. A few earthquakes have occurred in the last century, “the 1905 Kangra earthquake, 1934 Bihar-Nepal earthquake, and 1950 Assam earthquake.” (5) However, little is recorded of the surface rupture that happened during these earthquakes. The aforementioned geologists believe “that the earthquake recorded in the trenches is larger than historical earthquakes and indicate a potential for sections of the [Himalayan Frontal Thrust to rupture simultaneously along lengths of the [Himalayan Frontal Thrust]” (5)

                This is quite an important observation since the fault is still active, geologists and geophysicists will be better suited to study the Himalayan Frontal Thrust. They will then be able to make predictions about potential seismic activity in the area. Similarly, with a better understanding of the geology of the fault scientists, could make better estimations of what the Himalayas will look like in the near and distant future.

Works Cited

http://pubs.usgs.gov/gip/dynamic/himalaya.html (2)

http://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Convergent/Continental-Collision (1)

http://www.iisc.ernet.in/currsci/jun102004/1554.pdf (3)

https://www.himalayanclub.org/hj/66/9/geologic-formation-of-the-himalaya/ (4)

http://cires1.colorado.edu/~bilham/HimalayanEarthquakes/Himalayan_thrust.pdf  (5)

http://suvratk.blogspot.ca/2010/10/going-hiking-in-lesser-himalayas.html  (6)

Geologists have long been puzzled over certain structures and stratigraphic profiles associated with the Champlain Thrust, and more generally the Taconic Thrust Belt. West-central Vermont is not exactly Structural Geologist Heaven - there is not an abundance of outcrops and it is often hard to differentiate sedimentary facies. Hayman and Kidd (2002) proposed that there has been reactivation of pre-thrusting, flexure-induced normal faults, as well as one post-thrust normal fault (the Mettawee fault) that obscures the original thrust map pattern. The Mettawee fault reduces the map width of the Champlain Fault and decreases the apparent differences in structural level between the Champlain fault and nearby thrust fault systems. The exact age of the Mettawee fault is unknown, although it is believed to have formed some time after the Champlain Thrust but still during the Taconic orogeny event (Hayman and Kidd, 2002).

The Champlain Thrust is a visible reminder of the ancient orogeny that created the Green Mountains. The next time I find myself in Burlington, VT, I think I will make a pilgrimage to Lone Rock Point to admire the dolostone and shale, run my hand along an ancient fault and maybe catch a glimpse of Champ.

References


Doll, C.G., Cady, W.M., Thompson, J.B. and Billings, M.P. (1961). Centennial geologic map of Vermont Montpelier, Vermont Geologic Survey, scale 1:250,000.

Fossen, H. (2010). Structural Geology. Cambridge: Cambridge University Press.

Gale M. (1998). “Geology of Vermont: Champlain Thrust Fault at Lone Rock Point, Burlington VT”. Vermont Geological Survey. Accessed March 17, 2015. http://www.anr.state.vt.us/DEC/geo/chthrust.htm

Hayman, N. W., & Kidd, W. S. F. (January 01, 2002). Reactivation of prethrusting, synconvergence normal faults as ramps within the Ordovician Champlain-Taconic thrust system. Geological Society of America Bulletin, 114, 476-489.

Johnson, C.W. 1998. The Nature of Vermont: Introduction and Guide to a New England Environment. UPNE, 2nd Edition. ISBN: 0874518563

Rowley, D. B. (August 01, 1982). “New methods for estimating displacements of thrust faults affecting Atlantic-type shelf sequences: With an application to the Champlain Thrust, Vermont.” Tectonics, 1, 4, 369-388.

Stanley, R.S., 1987, “The Champlain thrust fault, Lone Rock Point, Burlington, Vermont”.  Geological Society of America Centennial Field Guide - Northeastern Section.

University of Vermont. “Geologic History of the Champlain Valley” UVM.edu. Accessed March 17, 2015. http://www.uvm.edu/shelburnelandscape/nature/geology.html

The Hudson Valley fold belt is a west-verging fold thrust belt located in Upstate New York. This feature has been heavily studied by local geology students, as road cuts along Route 23 near the town of Catskill give a clear cross section of the entire fold belt. It is bounded by the Catskill mountains to the west and the Hudson river to the east. The belt itself is rather narrow, being on average less than 4 kilometers wide, and is often described in literature as a miniature valley and ridge province. It is difficult to define the westernmost limit of this structure, as west directed displacement can be found a few kilometers away from the westernmost fold in the form of cleavage duplexes. There is some deformed strata the same age as the belt east of the river, which suggests the belt was once wider than its present dimensions. It extends for roughly 80 kilometers between the cities of Kingston to the south and Albany to the north, where it meets the Mohawk River valley.

Figure 1. Google map of the region containing the Hudson Valley fold belt, which lies between the cities of Albany and Kingston.

Figure 2: Structures associated with the Hudson Valley fold belt in New Paltz, New York.
http://www.newpaltz.edu/geology/nysga.html

Figure 3: Sedimentary layers of the Hudson Valley fold belt (Marshak, 1986).

The stratigraphic units found in the fold belt represent sediment deposited by turbidity currents on the continental margin of Laurentia, which would later become North America. The lowest exposed unit is a flysch from the middle Ordovician, which was deformed into tight folds with east dipping axial planes before the younger units of the formation were deposited. This creates what is referred to as the Taconic unconformity, which formed as a result of the Taconic orogeny 440 million years ago. The next two groups are from the lower Devonian and are followed by middle Devonian limestone. Distinguishing rock units of the Helderberg group which follow the Taconic unconformity are the Manlius formation, a laminated micrite deposited in a tidal flat, the Coeymans formation, a lime grainstone from a beach environment, the Kalkberg and New Scotland formations, both lime wakestones, and the Becraft formation, which is also a lime grainstone. Following the Helderberg group is the Tristates group, with both groups originating from a shallow sea that advanced and retreated repeatedly during the lower Devonian.The youngest layers exhibiting deformation are the Bakoven shale and the Mount Marian formation. Based on the coloration of conodont fossils contained in the rocks they appear to have been exposed to a maximum temperature of 200 to 240°C, though it is not known if this occurred during or after the deformation. Most of these rock units are carbonates, with the only significant non-carbonate being the Esopus shale. The largest stratigraphic throw of the Hudson Valley belt is near Kingston, where the Esopus formation was thrusted over the Onondaga limestone.

Figure 4: The Taconic angular unconformity.
http://hudsonvalleygeologist.blogspot.ca/2012/05/day-6-hudson-valley-fold-thrust-belt.html

Figure 5: Geological map of the region northwest of Catskill, New York along Route 23 that gives a clear west to east transect of the feature. (Marshak, 1989).

Three categories of faults are found in the belt: bedding parallel faults, cross strata faults, and strike-slip and normal faults. Bedding parallel faults are exactly what their name implies: their slip surfaces are parallel to the bedding of both the hanging wall and the footwall strata. No breccia or gouge is present, so these faults must have moved by crack seal extension instead of sliding. Instead, calcite fibers and spar often appear on these fault surfaces. The cross strata faults are also what their name suggests. These faults cut across the bedding planes and displace stratigraphic markers, and are also associated with calcite fibers. Many of these faults form ramp structures typical of thin-skinned deformation. It is thought that many of these faults formed in the later stages of deformation, because they seem to follow the folds that formed early on. The normal faults are represented on the map above as dashed lines and are oblique to the other belts of the fold belt. Their age and mode of formation is not known. On all faults, the hanging wall rocks moved westward relative to the footwall rocks.

There are two scales of folds in the Hudson Valley fold belt: megascopic and mesoscopic. There are ten main megascopic folds in this formation, with amplitudes between 50 and 120 meters and wavelengths of 200 to 800 meters. The synclines of these folds are wider and more symmetrical than the anticlines, and the northwestern limbs are either steep or overturned. The hinges of this series of folds are not always parallel. These folds were formed by the movement of rock over fault bends, regional flexure, and the formation of ramps. The mesoscopic folds (represented on the map) are far smaller, with amplitudes and wavelengths ranging from 1 centimeter to 10 meters. The exact type of fold appears to depend on the rock unit in which they are found, forming chevron folds in the Kalkberg formation and crenulations in the Esopus shale.

North of Kingston, the structures trend north/south to N10°E, while south of the town they change to N30°E. Because of the changes in their structural trend, structures southwest of Kingston are not considered to be part of the belt. This is an example of an orocline, a secondary bend where the curve is caused by the reorientation of preexisting structures. In this case, the older Hudson Valley fold belt structures were reoriented by the Appalachian fold thrust belt to the south.

Figure 6: Thrust fault duplex along the Hudson Valley fold belt exposed in a quarry. Curved horses can be seen in the photo. The floor thrust is known as the Rondout detachment.
http://web.cortland.edu/gleasong/fdcamp.html

The Hudson Valley fold thrust belt is a good example of thin-skinned deformation, which occurs at a convergent margin where thrust faults appear only in surface rock layers and not in the deeper basement rocks as they do in thick skinned deformation. The deformation history of this feature is believed to be as follows: layer parallel shortening happened before or concurrent with the initial folding and thrusting of the rocks. Ramps began to form through the more competent layers, while detachments formed in the weaker ones. While this was occurring, non-layer parallel shortening occurred as bending and interlayer slip. As the thrusting continued, the rocks were exposed to both pure and simple shear strain. Finally, frictional coupling caused the faults to lock, and instead the strain caused bulk shortening of the rocks. Overall, the strain in this feature expressed itself through faulting, folding, and bedding parallel slip.  
Shortening in the Hudson Valley fold belt occurred above two major detachment faults: the Austin Glen detachment and the Rondout detachment. The Austin Glen detachment is 500 meters below the surface and is not exposed anywhere along the fold belt. The Rondout detachment is stratigraphically higher, and in places where it is exposed it shows west verging mesoscopic folding. It crops out at or right above the post Taconic unconformity, and forms the floor thrust of a duplex. Horses stacked in duplexes are associated with the cross-strata faults, and occur in a variety of sizes.

Figure 7. Three possible relationships between the Rondout detachment (white) and the Austin Glen detachment (grey). (Marshak, 1986).

In model A, internal shortening occurred in the Austin Glen formation, which lead to the folding of the overlying Rondout detachment. Here, faults do not extend below the Rondout detachment. Model B shows the Rondout detachment as a roof thrust and the Austin Glen detachment as a floor thrust of a duplex system. In model C, ramp faults of the system run all the way through both detachments. In this case, the Rondout detachment is not the basal detachment. 

Video: explanation of the Acadian orogeny. https://www.youtube.com/watch?v=zMtuXPJnhOI

Video explaining the Alleghanian orogeny. https://www.youtube.com/watch?v=fJZy_BCKrIU

Fold belts can appear in a variety of tectonic settings, so not many conclusions can be drawn about the environment of formation. The westward vergence of the HVB could relate it to the westward movement of part of New England relative to the rest of North America. Several theories exist as to how and when this region deformed. Its location between the Acadian foreland basin and the New England Acadian orogeny suggests that it may have formed in the middle Devonian as part of the Acadian orogeny. This mountain building event is associated with the creation of the Laurussian supercontinent and occurred over the Middle to Late Devonian, 380 to 350 million years ago. The Avalon microcontinent traveled along an east-dipping subduction zone and collided with what would later become North America at an oblique angle, forming a mountain chain that stretched from southern Virginia to Newfoundland. Another theory is that it formed as part of the Alleghanian orogeny 320 to 250 million years ago due to its continuity with structures in the central Appalachian mountains. In this orogeny, northwest Africa (at that time part of the Gondwanaland supercontinent) collided with eastern North America and closed the ocean between them, forming Pangaea. By this time, the mountains formed in the Acadian orogeny had been significantly weathered down. In this case movement would have been due either the presence of a transform system that existed on the eastern margin of North America at the time, or the collision between Africa and North America. 

Works Cited:

Burmeister, Kurtis, Marshak, Steven. 2006. Along-strike changes in fold-thrust belt architecture: Examples from the Hudson Valley, New York. Geological Society of America: Field Guide 8.
http://www.friendsofwilliamslake.org/pdfs/burmeister.pdf

Fichter, Lynn S. "The Devonian Acadian Orogeny And Catskill Clastic Wedge." The Geological Evolution of Virginia and the Mid Atlantic Region. James Madison University, 13 Sept. 2000. Web. 28 Mar. 2015.

Fichter, Lynn S. "The Late Paleozoic Alleghanian Orogeny." The Geological Evolution of Virginia and the Mid Atlantic Region. James Madison University, 13 Sept. 2000. Web. 28 Mar. 2015.

Harris, John, Van der Pluijm, Ben. 1997. "Relative timing of calcite twinning strain and fold-thrust belt development; Hudson Valley fold-thrust belt, New York, U.S.A." Journal of Structural Geology, 20 (1): 21-31.
http://www.sciencedirect.com/science/article/pii/S019181419700093X#

Marshak, Steven, Engelder, Terry. 1984. "Development of cleavage in limestone of a fold-thrust belt in eastern New York." Journal of Structural Geology, 7: 345-359
http://www.sciencedirect.com/science/article/pii/0191814185900409

Marshak, Steven. 1986. "Structure and tectonics of the Hudson Valley fold-thrust belt, eastern New York State." Geological Society of America Bulletin, 354-368.
http://gsabulletin.gsapubs.org/content/97/3/354.abstract

Marshak, Steven, Tabor, John. 1989. "Structure of the Kingston orocline in the Appalachian fold-thrust belt, New York." Geological Society of America Bulletin, 683-703
http://gsabulletin.gsapubs.org/content/101/5/683.short

Majerczyk, Chris. 2011. Geology of the Roberts Hill Area in the Hudson Valley Fold-Thrust Belt, Greene County, Eastern New York. Submitted Thesis.
https://www.ideals.illinois.edu/bitstream/handle/2142/29626/majerczyk_chris.pdf?sequence=1

1. General Description of the Feature
The Seattle Fault Zone is located in the Seattle metropolitan area in the state of Washington in the United States of America. The approximate location is shown in the embedded Google Map below. The zone is shown within the two dashed lines in Figure 1.1. and is composed of four south dipping thrust faults (Johnson et al., 1994). It is composed of one major fault of a length reaching 45 km - shown in Figure 2.3. - and additional smaller ones. A thrust fault is a type of contractional fault with a low dip angle where the hanging wall relative to the footwall is going up (Fossen, 2010). In this particular case, the Seattle Fault has a high dip angle close to the surface and a low dip value as the fault goes deeper. It falls into the thrust fault cathegory of contractional faults. The faults in the zone trend east-west and are believed to have caused a major earthquake around 900 A.D. which had a magnitude over 7 on the Richter scale (Calvert et al., 2001). Because of this event and the fact that the faults have been active in the past 30 years - according to recent studies - the zone is believed to represent a major seismic hazard (Johnson et al., 1994). The fault zone has been heavily studied in the last 40 years as it goes directly under the City of Seattle - which is the biggest city in the State of Washington  (Information Please, 2007). The exact model describing the relation and motion between the various features is still debated among geologists today (Nelson 2014).

Figure 2.2. shows the four faults along with their projection underground. The different layers are also visible on the image. The faults are listric, and the master ramp is along the northernmost fault - labelled 3 (Jonhson et al., 1994).

The Seattle Fault Zone contains one major fault that is shown in Figure 2.3. below. The dips and displacement measured along the fault are shown in Table 2.1. The Seattle Fault and Tacoma Fault have a particular relationship as they would be connected and would be forming the boundaries of the Seattle uplift. The Tacoma Fault is composed of multiple faults and folds that are north dipping and east-west striking (Nelson, 2014). If the suggested model where the Tacoma and Seattle faults are connected, and form the Seattle uplift boundaries is right, they would do so at a distance of at least 10 km underground (Nelson 2014).

4. Graphics and Media
2 Minute Geology video about the Seattle Fault. It discuss the basic tectonic activities along with the tsunami believed to have ravaged the Seattle area in 900 A.D.

Short video explaining the tectonic configuration around the Juan de Fuca plate and its movement. It is relevant as it explains the motion of the Juan de Fuca plate relative to the North American Plate (where the Cascade arc resides) - along with the cause of the motion.

The Linville Falls Fault is a thrust fault located near the border between North Carolina and Tennessee, along the towns Linville Falls, Banner Elk, and Boone, in the Blue Ridge Province of the Appalachian Mountains.

Figure 1) Approximate location of the Linville Falls Fault in North Carolina, USA

A thrust fault is a fault which has pushed rocks of lower stratigraphic position (usually meaning older rocks) up and over higher (or younger) rocks. Seeing older rock layers on top of younger rock layers is usually a giveaway that a thrust fault has been involved. The Linville Falls Fault is a top-to-northwest thrust fault. Here one billion year old granite of the Grenville Province has been pushed over 700 million year old metamorphosed sandstone. The fault was created during the formation of the Appalachian Mountains about 300 million years ago in the Alleghenian Orogeny when Gondwana collided with Euramerica. (7)

Figure 2) A map of the geological features surrounding and including the Linville Falls Fault. All of the faults formed in the Alleghenian orogeny except the Burnsville Fault. The four thrust sheets of the Blue Ridge thrust complex are shown. (4)

During the formation of the thrust fault, collisional forces pushed a rock layer called the Blue Ridge thrust complex over 100km to its current location covering Grandfather Mountain. This rock layer forms the hanging wall of the Linville Falls fault. It is composed of alkali feldspar granite overlain by shear zone mylonites, and cranberry gneiss (also part of the Grenville Province). (8) The footwall of the fault consists of autochronous quartzite, which is part of the Grandfather Mountain formation, and quartz-sericite mylonite which was created through shearing. (4) At least 50 km of movement has occurred along the fault. (1) On one edge of the fault there is a thick ductile top-to-northwest shear zone called the Linville Falls Shear Zone, approximately one km wide near Banner Elk and consisting of mylonitic and ultramylonitic rocks with zones that have been cataclastically deformed. (4)

Figure 3) Relative location of the Linville Falls shear zone (4)

The Linville Falls Fault forms the border of the Grandfather Mountain Window. (figure 2) A window is formed when a layer of rock erodes away in one location to expose a patch of different rocks underneath. (figure 4)  Here, after the Blue Ridge Thrust complex was pushed over Grandfather Mountain by the fault, the peaks of the formation eroded away and in one place a gap eroded all the way through the layer, uncovering the younger rocks underneath. The other faults, such as the Brevard Fault, were formed in the same orogeny (Alleghenian) as the one that created the Linville Falls Fault.  The nearby Linville Falls Gorge arose when the Linville River eroded the softer quartzite out from under the granite and caused the falls to collapse. (7)

Figure 4) A basic diagram of a window formation

The Blue Ridge Thrust complex, in the hanging wall of the Linville Falls Fault and overlying the Grandfather Mountain Window, consists of stacks of crystalline thrust sheets which can be distinguished from one another by their metamorphic history. They are (in order from structurally lowest to highest) the Pardee Point thrust sheet, the Beech Mountain thrust sheet, the Pumpkin Patch thrust sheet, and the Spruce Pine thrust sheet. (4) A zone of metamorphosed rocks near the sole of the thrust sheet displays many small isoclinal folds with northwest-trending axial planes parallel to foliation in the thrust sheet. (3)

Figure 5) A geological map showing the orientations and elevations of features surrounding the Linville Falls Fault (4)

The faults in the Blue Ridge Mountains were formed in the Alleghenian Orogeny 320 to 260 million years ago during the late Carboniferous period. The Alleghenian Orogeny is one of the six orogenies making up the formation of the Appalachian Mountains. The mountain building occurred as Pangaea was coalescing, when Africa (at the time part of the supercontinent Gondwanaland) collided with the supercontinent Euramerica. The impact compressed rocks between the two landmasses and forced them up into a mountain chain which at the time was located in the center of the supercontinent. When Pangaea broke up millions of years later, the mountain range split in two. Today we have the Appalachians in North America and the Atlas Mountains in North Africa. The buckling from the collision also created a series of sedimentary folds, perpendicular to the force, bounded by thrust faults which shortened the land by as much as 320 km. The Linville Falls Thrust Fault formed due to the compressional forces in this orogeny. (8) The folds in the Blue Ridge Thrust Sheet are thought to have formed from flattening and passive rotation of earlier folds which originally formed perpendicular to the direction of movement. (3)

Figure 6) An animation showing the break up of Pangaea and the migration of the continents to their current positions. At the beginning of the video it can be seen where North America was connected to Africa. This is where the mountains were originally.

Sources:

(1)    CAROLINA GEOLOGICAL SOCIETY 1997 FIELD TRIP AND ANNUAL MEETING ROAD LOG AND STOP DESCRIPTIONS 
http://carolinageologicalsociety.org/CGS/1990s_files/gb%201997.pdf#page=27

(2)    Significance of lineation and minor folds near major thrust faults in the southern Appalachians and the British and Norwegian Caledonides   Bruce Bryant and John C. Reed Jr.
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805

(3)    PALEOZOIC STRUCTURAL EVOLUTION OF THE BLUE RIDGE THRUST COMPLEX, WESTERN NORTH CAROLINA  Kevin G. Stewart, Mark G. Adams , and Charles H.Trupe
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805

(4)    STRUCTURAL RELATIONSHIPS IN THE LINVILLE FALLS SHEAR ZONE, BLUE RIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA   Charles H. Trupe
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805

(5)    Geology of the Linville Falls Quadrangle North Carolina  GEOLOGICAL SURVEY BULLETIN 1161-B
http://pubs.usgs.gov/bul/1161b/report.pdf

(6)    Linville Gorge 
http://www.unc.edu/courses/2008spring/geog/006d/001/Class_Readings/Class%20%2313%20-%20April%2021%20(WNC%20-%20Team%20Presentations%20&%20Papers)/Linville%20Gorge_Class%20Paper.pdf

(7)    USGS Valley and Ridge Province
http://web.archive.org/web/20110722154205/http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm

(8)    CONDITIONS AND TIMING OF METAMORPHISM IN THE BLUE RIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA AND EASTERN TENNESSEE  Mark G. Adams and Charles H. Trupe http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4629596&fileId=S0016756800058805

(9) Image of window: http://upload.wikimedia.org/wikipedia/commons/b/b1/Thrust_system_en.jpg

(10) Video of Pangaea: https://www.youtube.com/watch?v=WaUk94AdXPA

The Aleutian Trench is part of a subducting zone in the Pacific Ocean connecting Alaska to Kamchatka. The trench is formed where the Pacific plate is being subducted under the North American plate. The boundary is visibly defined on map, see the embedded link below.  

Modelling the large-scale motion of the lithosphere is a fundamental topic in structural geology, plate tectonics is the current thinking to this end. The theory states that the lithosphere is broken up into tectonic plates, the boundary between plates is well defined in an appropriate geological framework [1]. We will investigate the Aleutian Trench, a convergent boundary between the Pacific plate and the North American plate. A map of the earth's major (and some minor) plates is presented in Figure 1. 

In a recent journal, Zahirovic, Sabin, et al analyse a plate motion model and extract topologies in 1 Myr intervals. The model uses paleomagnetic data sampled over the last 200 Myr, present day plate velocities from eight plates, and post-Pangea plate tectonic reconstructions [2]. The embedded link below models the break-up of Pangea using data from Zahirovic, Sabin, et al; the video is hosted by AAAS via Earthbyte.org. The model projects the Pacific plate forming around 190 Myr ago and the Aleutian Trench forming around 40 Myr ago.

When a tectonic plate is subducted, stress builds in the boundary between the two plates. At some critical value of stress, the plates slip against each other and energy is relieved. This large scale displacement (and the mechanical waves which follow) is an earthquake. Figure 2 presents earthquake foci recorded over a ten year period around the Aleutian Islands, the foci are recorded on both plates along the boundary. 

During 1980 and 1981, the U.S. Geological Survey completed a reconnaissance along the Aleutian Arc and the Aleutian Trench [3]. The locations investigated are presented in Figure 3, for our investigation we examine the (analysed) section L9-12.

McCARTHY et al. analyse the time and depth section from the seismic survey and build a structural model, these are presented in Figure 4. Relative to the two plates, the contact is a thrust fault over the oceanic crust. An accretion layer of off-scraped sediment forms as the oceanic crust subducts [4]. The faults formed on the oceanic crust suggest that a principle stress is in the direction of general vergence (Anderson fault theory). This is to say that the oceanic crust is extending and active. A master thrust fault runs along the plate boundary, a duplex fault system is forming on the hanging wall. The Pacific and North American plate form a convergent boundary.

The seismic data was processed at the U.S. Geological Survey marine seismic processing facility in Menlo Parktime [4]. To generate such data sets, elastic waves are introduced to the crust. Bedded surfaces act well as reflectors, sensors are run along the cross section.  Fault planes act as pseudo reflectors, in the depth section they can be seen cutting across the general vergence of the bedding. The time section assumes mild lateral velocity variations and so cannot resolve the faults.

The Aleutian Trench is an active convergent boundary between the Pacific and North American plates. The subduction is continuous, but the internal forces are high in the lithosphere so the displacement is expressed in discrete intervals. The North American plate is thrusting towards the Pacific plate, and the Pacific plate is extending towards the North American plate. This is apparent from the normal faults along the extending oceanic crust and the duplex system on the hanging wall. Relative to each other, the North American and Pacific plates form a contractional margin.

1. Wikipedia contributors. "Plate tectonics." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 22 Mar. 2015. Web. 22 Mar. 2015 http://en.wikipedia.org/wiki/Plate_tectonics.
2. Zahirovic, Sabin, et al. "Tectonic speed limits from plate kinematic reconstructions." Earth and Planetary Science Letters 418 (2015): 40-52.
   
3. Stauder, William. "Tensional character of earthquake foci beneath the Aleutian Trench with relation to sea‐floor spreading." Journal of Geophysical Research73.24 (1968): 7693-7701
4. McCARTHY, J. I. L. L., and David W. Scholl. "Mechanisms of subduction accretion along the central Aleutian Trench." Geological Society of America Bulletin 96.6 (1985): 691-701