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Thrust Faults and the Appalachian Mountains
Lecture Goals : A) Thrust Faults B) The Appalachian Mountains
A) Thrust Faults, Several lectures ago, we discussed the major types of faults. Two of these were dip slip f aults where movement was along the dip direction of the fault plane. At the time, I hin ted that th ere was a special class of revers e dip slip fault associated with m ountain building. They are low angle reverse faults called thrust faults. Thrust faults occur when widespread com pression, usually associated with convergent plate boundaries, starts to affect continental rocks.
They are particularly prominent in th ick sequences of sedimentary rock. To m ake a long story short, when sedimentary rock sequences are compressed, a series of thrust faults that dip in the direction that the stress originated from start to develop. Any rock layers that lay above the thrust fault are pushed over top the layers that lay below the thrust f ault. This does two things. First of all, it " thrusts" the overlying layers up above the surface of the Earth forming linear trending mountain belts (amazingly called thrust
fault mountains). Seconding, it shortens the original width of the sedimentary succession. If you recall our initial lecture on defor mation, shortening is a natural consequence of compressive stress, so the latera l shortening that occurs when thrust faults develop is exactly as p redicted. It is, however, staggering just how much com pression can occur. There are thrust faults in the Cordilleran Mountains where more than 100 km of lateral displacement has occurred.
Very shortly, you will see a review film on thrust faults and mountain building that will very graphically demonstrate how this com pressive process occurs. You will also learn that thrust faults are s eldom isolated. They usually occur in sets. As com pression continues, successive thrust faults develop. Many b ranch off from earlier ones. Eventually, you can get very wide mountain belts com posed of nothing but thrust faults and the rocks that were f aulted. The block diagrams above are from your lab manual and demonstrate just how c omplex the geology (and the resulting geological m ap) can be when thrust faulting is extensive.
There are two m ore terms that need to beintroduced at this tim e. If you look at the previous two diagrams (and if you pay attention to the thru st fault film you will sh ortly see), you will hear the term s ramp and flat applied to thrust faults. The ramp is defined as the port of a thrust fault that is relatively steep that cuts up and through sedimentary
Mountains^1. We would see m ountains as high, andpossibly higher, than the Him alayas. They were also m uch more extensive. They extended from Alabama through w hat is today the Maritim e Provinces of Canada, and into Greenland and S candinavia. Three hundred million years of erosion can really take a lot out of you!
The origin of the Appalachian Mountains is one of the most interesting Earth history stories, at least as far as North Americaand Alabama are concerned. In order to under stand the events that occurred to form the Appalachians, we need to get back in our tim e machine and travel further back than 300 Ma. We need to go back to the Cambrian, some 550 million years in to the past.
(^1) We would also not be i n Mobile anymore. In fact , we would probably be st anding on part of what is
today, South America. Plate tectonics – you gotta love it!
Image from the Paleomap Project website (http://www.scotese.com/earth.htm)
The world at this time was very d ifferentthan the world of today. North Am erica^2 was tilted about 60 degrees from its present orientation and lay across the equator. The continent was drifting more toward the east than toward the northwest as it is today. Were we to step out of our tim e machine in the present location of Mobile, we would find ourselves in very deep water. The shoreline lay 100's of km to our north. Near Huntsville, the water was shallower. W e would see tropical biochemical sediment just about everywhere we looked. Ooids and reefs^3 were wide spread all along the eastern coastline. At the coas tline, we would find quartz sand along the beaches, mud in estu aries, lithic sand in delta channels. In other words, the coast of North America 550 Million years ago was very much like it is today, except that th e weather was better and the beasties were different.
Well off to our east was another land m ass.Today we would call it Europe. Like N orth America, its coastline was surrounded by largely tropical sediment deposits, beaches etc.
(^2) Technically speaki ng, North America did not existin the Cambrian. Nor d id Europe, Asia, Au stralia,
Africa, South America or Antarctic. These are names that we apply to the continents as they are configured today. Five hundred million years ago, the continents were different, so we give them different names. North America was called Laurentia , Europe was called Baltica , South America, Africa, Antarctica and Australia were all stu ck together into a larg e continent called Gondwanna. Asia was fragmented. Some present day parts of Asia were isolated continents(e.g., Siberia) where as ot hers were stuck to other continents (e.g., India was pa rt of Gondwanna). The oceans were also different. You will have to know all of these names for GY 112 as this is an important component of Earth history, but you are off the hook as far as this nomenclature is concerned for GY 111. (^3) This far in the past it is imp ortant to remember that the beasties that comprised reefs were co mpletely
different from the ones we see today. Evolution has induced a lot of changes in 500 Ma
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the Aleutians are p retty small with some consisting of little more than a volcanic vent. Island arc volcanoes tend to erupt interm ediate lavas (e.g., Andesite, P umice, Tuff) and can be violently explosive.
So, the first phase of m ountain buildingin what would becom e the Appalachian Mountains occurred about 500 m illion years ago when an i sland arc complex hit North America. Some people regard it as more ofa “docking“ o r an accretionary event. The island arc was sim ply “glued” to the outsid e margin of the contine nt. But with this docking came uplift, thrust faulting, and limited (localized) metamorphism. Many plutons
burned their way up through the continental crust at this time too. It m ay have been a relatively small collision, but it was accom panied by all of the geologic al consequences that we have come to expect from plate tectonics.
The second phase of m ountain building was m uch more impressive. It occurred during the early Devonian Period (around 400 Ma) and was a foll ow-up collision to the is land arc that hit us during the Taconic Orogeny. This event was a continent-continent collision between North America and Europe and it com pletely closed the northern portion of the Iapetus Ocean, re-deform ed some of the rocks previously affected by the Tac onic Orogeny, and would have caused a mountain belt the height of the Hi malayas in what is today Newfoundland, Nova Scotia, Maine, New York, England, Ireland, Scandinavia and Greenland. In North America, we call this event the Acadian Orogeny (see next cartoon).
The southern Appalachian Mountains that crop out from Virginia to Alabam a resulted from the third and last of the great m ountain building events to affect eastern North America during the Paleozoic. TheAlleghenian Orogeny occurred during the Pennsylvanian Period a nd resulted from eth collision of Africa/South America (then called Gondwanna) with North Am erica. This w as the final straw as far as the Iapetus Ocean was concerned. When the Alleghen ianOrogeny was finished, the Iapetus Ocean was gone and the continents were assem d bleinto the m ost recent of the great supercontinents. Pangaea had just formed.
As you know, Pangaea would eventually break up in the Mesozoic. Th e Atlantic Ocean began to form about 200 m illion years ago as North America started drifting to the west. The Cordilleran Moun tains would start their major phase o f growth about 150 m illion years ago as land m asses began to "dock" with North America. The separation of North America, Europe and Gondwanna occurred almost along the sam e " suture line " where these continents came together in the Paleo zoic. I think it's because the su ture line represented a weak surf ace. When the tectonic forces rev ersed from compression to tension, the continents slipped along the suture line, but it wasn' t perfect. Some parts of Gondwanna stuck to North America (e.g., southern Nova Scotia, parts of South Carolina) whereas parts of North Am erica stuck to Africa or Europe. The last cartoons in today' s lecture summarize the m ajor mountain building events responsible for the Appalachian
