Where all good things go to bad.
For immediate throwing
What's so special about the Ottawa Valley?
You may not believe this, Virginia, but the Ottawa Valley and associated structures are some of the most significant and recent signs of rock destruction in Eastern North America (ENA). Sure those big megathrusts under Ontario are something, but that happened a billion years ago! So Yesterday, in geologic speak. What we have here is a giant tear through the entire crust, only some odd hundred millions years old.
It all starts with Wilson Cycles. This is the tendency for the earth to smash up all the continents together in one big pile-up, and then split them all apart again. It has to do with the fact that oceanic crust only wants to stay on top when it is young and hot. After a wild life of an odd hundred million years or so, it gets tired and wants to die, so it dives back into the earth.
Well on the cycle before the last one, the continents were all happy together, when they decided to split up. Wasn't really their decision alone, since they were responsible for true global warming right under their feet. They had blanketed a huge section of the globe with their bodies, which did not conduct heat well, and things started to warm up.
This upwelling of heat lifted and stretched the continents, ready to split. In the Ottawa area, a giant zit started to form, that extended out in a three-way. One big crack went up the Ottawa Valley, another down the St. Lawrence, and a third going down Lake Champlain. Later geologists would call it the Ottawa-Bonnechere Graben.
In cross-section it looks even more ferocious.
Anyway, all this action was for naught, the continents decided to separate along different lines, and all this split rock was called a failed rift.
But it's not a failure for us! After the continents split, things cooled down, and the rift valleys tightened up. The continents then started to smash together again, and we had the Appalachian Mountains. All would be peaceful for Ottawa, except for the fact that when things started to heat up again, a 'hotspot' formed somewhere north of Ottawa. As the continent moved and traveled over this hotspot, it had the effect of a key on a Rolls Royce, except the key was coming from underneath!
This mushed up the rock all over again, and gave us nice scenic spots such as Mount Royal in Montreal. Of course, the glaciation added an immense weight which crunched things up some more.
We now have the most wonderfully cracked up rock in ENA! The Ottawa Graben forms a boundary of the extremely seismically active Western Quebec Zone. As well, it is the terminator, of the Ontario megathrusts.
We can expect the most marvellous huge earthquakes at the intersections of these two fault systems, just like the 1935 Timiskaming earthquake. All in all, a most wonderful bad site! What makes it absolutely terrible, is discussed in the next 'flyer'.