|
canoebase.com:
Wilderness Canoe Tripping
Yukon Territory - North/Black/Liard River Canoe Trip, Tintina Fault and TrenchProbably the single biggest geological feature of the North/Black/Liard River trip is that, for the Black and Liard River portions, we were paddling along the Tintina Fault. The fault is a big one and otherwise significant in that it crosses the western continental divide. The orientation of the fault is southeast to northwest. It starts southeast of Watson Lake, Yukon, and goes up past Dawson, Yukon in a fairly straight line to Fort Yukon, Alaska. Actually, it's a continuation of the Rocky Mountain(?) Fault which starts way south at about the US/Canada border, so the fault system is in excess of 2,000 miles long (3,200 km). The Tintina Fault is a transform fault, a right lateral slip-strike fault similar to the San Andreas fault down in California. A transform fault is the kind where the land on one side of the fault slides sideways along that of the other side, all driven by currents of molten rock deep in the earth's interior. Sometimes there will be a drop or rise in elevation during an event (earthquake associated with fault movement), but for the most part, the ground on both sides just moves sideways past each other. In this case the continental land to the northeast of the fault is relatively stable, and the main movement happens in the attached terranes to the southwest sliding somewhat as a unit northeast, towards Alaska. Geologic evidence suggests that there has been, at minimum, 450 km (280 miles) of displacement in that northwest direction. Some data suggests that it could be as much as 1200 km (745 mi.) of slip. All of this movement occurred in the last 200 million years, because, before that time, there simply wasn't any land southest of the fault to move. (If you got here from somewhere else, see North/Black Geology for some background info.) It is usually very hard to see a fault, but sometimes the effects of a fault are quite obvious. The Tintina Fault is the direct cause of the Tintina Trench, which is easy to see on most any Yukon map with rivers marked on it. Small scale maps show an almost ruler-straight line running up the Liard from just below Watson Lake (Lower Post, BC), then up the Black River, over a pass (the western continental divide) to the Hoole River, and into the Pelly River past the towns of Ross River and Faro. After that, the rivers wander around a bit on a wide, flat plain, but the Pelly flows into the Yukon River, which the fault picks up at the town of Dawson. From there, the fault and Yukon River both continue on, straight into Alaska, past the town of Eagle, to Fort Yukon. The reason the Tintina Fault causes the trench is because the fault action grinds and breaks up rocks along its contact, the point where the two sides of the fault touch. This grinding action increases erosion in a long, narrow area, which slowly gets wider. In this case, because of the easy erosion, coupled with the fact that minimal differential ground uplift happens here, a long linear trench formed after several million years. The easier erosion along the fault contact let the rivers cut deeper, and slowly wider, and slowly forced area rivers to flow towards, into, and then along the fault. Continued erosion created the now very noticeable trench. In fact, it is very difficult for rivers to cross or even leave the trench. Some do, but not very many. The Liard leaves the trench at the south end, and the Yukon leaves it at the north end, but I don't see any rivers on the map that really cross the trench. The Pelly River wanders away near Pelly Crossing, but comes back in via the Yukon when that river is picked up at Dawson. The Stewart River looks like it enters the trench from the northeast at the town of Stewart Crossing, travels northwest along it, then ultimately crosses as it heads southwest, leaving the fault contact area. However, this area, where the Pelly and Stewart do funny things, is simply a very eroded, wide, flat, level plain, and the Stewart, like the Pelly, does come back in again at Dawson with the Yukon River. Think of the trench as being very wide here, and those rivers don't leave it, they just meander around a bit more in the wide spots than the other rivers are allowed to do where the trench is very narrow. On our North/Black/Liard trip, we started not far from the ultimate headwaters of the North River. It flows into the Black River, which flows in the Tintina Trench for its whole length. The North River is a case of a feeder stream working downhill into the Liard River portion of the Tintina Trench. Both the North and Black Rivers originate at the continental divide. The drainage to the west of the North River feeds the Pelly River, which ends up in the Yukon River end of the Tintina Trench. The drainage north of the North River feeds the Frances River, which we met via the Liard River, downstream on our run. That means it goes into the trench, too. The drainage east of the North River also feeds the Frances system. The drainage to the south is the Liard and other feeder streams. Everything in the area dumps into the Tintina Trench. One direction or the other. While we were on the Black River, we probably paddled either over, or alongside, the actual Tintina Fault contact, maybe several times. May even have stepped on it in the Black River portion of the trip. We didn't know it, though, as I hadn't researched this stuff thoroughly enough to know to look for it. An excuse to go back again and see. The heavy vegetation would probably have made it a difficult search. The fault contact continues on down the Liard. Again, we were probably paddling over, on, and beside it, but the Liard Plain is wide and flat with lots of river and lake sediments hiding the bedrock below. We would never have found it without getting our feet quite wet. At minimum.
Okay, so how do rivers leave the trench? Find out in
Bibliography: (so far) Yukon Geology website Back to previous page or North/Black Geology
| North/Black Intro |
North/Black Contents List |
Copyright © 1996 - 2002, and legal fine print |