112lect37 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Cenozoic Tectonics Lecture Goals : A) Western North America; more accretion, more sedimentation B) Tectonic provinces of the western USA C) Plateaus and canyons Textbook reference: Levin 7 th edition (2003) Chapter 13; Levin 8 th edition (2006), Chapter 15 A) Western North America When last we visited western North America (Mesozoic tectonics), we learned that the region was dominated by impact events caused by the collision of several island arcs and other land masses. The reason for these collisions was pretty straightforward. The tectonic plate that contained North America was drifting toward the west-northwest and the plate that contained the Pacific Ocean in this part of the planet was moving toward the east. The two plates met in a classic convergent plate boundary. The oceanic plate was subducted below the continental plate. Compression produced the Cordilleran Mountains on the overriding North America Plate. Accretionary tectonics welded island arcs and other land masses to the west coast expanding the area of North America. Several orogenies occurred all along the west coast during the Mesozoic and the same style of tectonics continued well into the earliest part of the Cenozoic. I’m including a few cartoon cross sections across the area highlighting the effects of some of the important orogenies. The Sonoma Orogeny is the top right image, the Sevier the both right (both are from http://jan.ucc.nau.edu/~rcb7/ NAM_Orogenies.html). The net result is that we got a lot of deformation, particularly thrust faulting. A “typical” cross-section across the Rockies shows the effects of the faulting rather nicely:
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GY 112 Lecture Notes D. Haywick (2006) 2 By the Eocene, however, tectonics started to quiet down in parts of the west. Collisional tectonics that had been causing thrust faulting in the Rocky Mountains of Canada (the eastern most part of the Cordilleran Mountains) ceased. This is not to say that mountain building entirely stopped in western Canada, but it certainly was not as intense as earlier. Fewer and fewer island arcs and other land masses were being accreted elsewhere in the west as well. The most impressive Cenozoic mountain building occurred in the southern parts of the Cordilleran ranges in California, Nevada, New Mexico, Arizona, Colorado, Utah and Wyoming. The reason for the shift in tectonics had to due with the structure of the Pacific Ocean. When you look at the Pacific Ocean today, you will see that it is being subducted along almost every side. New oceanic crust is being produced at a divergent plate boundary in the east called the East Pacific Rise . It is rather like the other divergent plate boundary that we have discussed in this class, the Mid Atlantic Ridge with the exception that it has much higher spreading rates (10 cm/year ompared to 2.5 cm/year). To the west of the East Pacific rise, plate movement is westward. East of the rise, it is eastward. Throughout the Mesozoic
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112lect37 - GY 112 Lecture Notes D. Haywick (2006) 1 GY 112...

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