CompuServe Messages

Doug, Disclaimer: I'm referring mainly to a basic geology/paleontology text dated 1989, so new info like your SA article may have changed some of this. But here's what this ref. says about the Appalachians: 1) In the Ordovician Period, Laurentia and Baltica were moving together, with a subduction zone in between. Sometime in the mid-Ordovician, that subduction zone was the site of a meeting between Laurentia and an island arc something like Japan or the Solomons. When they came together, the island arc sutured to Laurentia. That triggered the first major orogeny, the Taconic. The Taconic Mountains formed as the island arc's crust overrode Laurentia's crust. Rock formations seem to indicate that the Taconics were a high, volcanic type of mountain range similar to the Andes. Erosion had apparently worn down the Taconics by Mid-Silurian times, because about then is when the deposition of eroded sediments to the west stops. 2) A new subduction zone formed east of the old island arc, again drawing Laurentia and Baltica together. The Acadian orogeny resulted from the collision of Laurentia and Baltica, and their combination into the ORS continent. The Acadian orogeny started in the north (Greenland/Scandinavia) in the very late Silurian, and continued in irregular bursts all through the Devonian and into the Pennsylvanian. We know it was the collision of Laurentia and Baltica because what European geologists call the "Caledonian" orogeny is recognizably the same event as the Acadian, from the same time period, sharing similar faunas and sedimentary rock formations. A little later, well south of the Laurentia/Baltica collision region, another volcanic island arc was apparently sutured to Laurentia; that's now called the Avalon terrane. 3) In the upper Mississippian, another major orogeny occurred as a result of the ORS continent's collision with Africa to form Pangaea. This one is called the Alleghenian orogeny. It involved a lot of uplifting, folding, metamorphing, thrust faulting, and a number of igneous intrusions. The Alleghenian mountains were probably a lot like the Himalayas in their characteristics. The Alleghenian orogeny is also known from Europe and northern Africa. In Europe it's called the Hercynian orogeny; in Africa, it's the Mauritanides orogeny. By the late Mesozoic, even the Alleghenian mountains were gone, leveled by erosion. The modern Appalachians are the result of slow uplifting during the Cenozoic, possibly mixed with isostatic adjustment. Jon W.