GY 111 Lecture Notes D Haywick 2007 08 1 GY 111 Lecture Notes Thrust Faults and the Appalachian Mountains Lecture Goals A Thrust Faults B The Appalachian Mountains Reference Press et al 2004 Chapters 2 6 20 Grotzinger et al 2007 Chapters xxx xxx A Thrust Faults Several lectures ago we discussed the major types of faults Two of these were dip slip faults where movement was along the dip direction of the fault plane At the time I hinted that there was a special class of reverse dip slip fault associated with mountain building They are low angle reverse faults called thrust faults Thrust faults occur when widespread compression usually associated with convergent plate boundaries starts to affect continental rocks They are particularly prominent in thick sequences of sedimentary rock To make 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 fault 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 GY 111 Lecture Notes D Haywick 2007 08 2 fault mountains Seconding it shortens the original width of the sedimentary succession If you recall our initial lecture on deformation shortening is a natural consequence of compressive stress so the lateral shortening that occurs when thrust faults develop is exactly as predicted It is however staggering just how much compression 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 compressive process occurs You will also learn that thrust faults are seldom isolated They usually occur in sets As compression continues successive thrust faults develop Many branch off from earlier ones Eventually you can get very wide mountain belts composed of nothing but thrust faults and the rocks that were faulted The block diagrams above are from your lab manual and demonstrate just how complex the geology and the resulting geological map can be when thrust faulting is extensive There are two more terms that need to be introduced at this time If you look at the previous two diagrams and if you pay attention to the thrust fault film you will shortly see you will hear the terms 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 GY 111 Lecture Notes D Haywick 2007 08 3 layers The flat is the nearly horizontal part of the thrust fault where rock layers slide over top of other rock layers Flats are frequently lost from thrust fault mountains because of erosion You will also notice a new geological symbol on the geological block diagrams on the previous page This symbol indicates both the position of a thrust fault and which side of the fault is the overthrown side The triangles are always placed on the side of the fault that was overthrown The same symbol is also used for convergent plate boundaries In the cartoon at the top of the next page the map symbol indicates that the North American plate overlays the Pacific Plate B The Appalachian Mountains Up until now we have been using western North America as an example of thrust fault mountains This is understandable considering that these mountains especially the Canadian Rockies contain some of the best examples of thrust faults anywhere in the world But what about the other mountain range in North America What about the Appalachians For the rest of today I d like for you to consider the mountain belt in our own back yard True the Appalachians are not all that impressive compared to the Cordilleran Mountains They are less rugged less high and hence less intimidating But remember they are old Where we to get in a time machine and travel back to the present location of Mobile some 300 Million years ago we would be amazed by the Appalachian GY 111 Lecture Notes D Haywick 2007 08 4 Mountains1 We would see mountains as high and possibly higher than the Himalayas They were also much more extensive They extended from Alabama through what is today the Maritime Provinces of Canada and into Greenland and Scandinavia 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 America and Alabama are concerned In order to under stand the events that occurred to form the Appalachians we need to get back in our time 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 in Mobile anymore In fact we would probably be standing on part of what is today South America Plate tectonics you gotta love it GY 111 Lecture Notes D Haywick 2007 08 5 Image from the Paleomap Project website http www scotese com earth htm The world at this time was very different than the world of today North America2 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 time 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 We would see tropical biochemical sediment just about everywhere we looked Ooids and reefs3 were wide spread all along the eastern coastline At the coastline we would find quartz sand along the beaches mud in estuaries 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 the weather was better and the beasties were different Well off to our east was another land mass Today we would call it Europe Like North America its coastline was surrounded by largely tropical sediment deposits beaches etc 2 Technically speaking North America did not exist in the Cambrian Nor did Europe Asia Australia 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