Are you driving through “terrain” or “terrane”?  If you are looking at a topographic map, the answer is “terrain.”  If you are looking at a geologic map of western BC, southwestern Yukon, or southern Alaska, the answer may well be “terrane.”  Confused yet?  “Terrain” is the “lay of the land”—e.g., hilly terrain, mountainous terrain, etc.—and generally refers to differences in elevation or “topographic relief.” 

“Terrane” is a more complicated geologic term for a specific area of land that contains rocks formed by a particular geologic process, as in a volcanic terrane or a limestone terrane.  However, since the acceptance of plate tectonic theory in geology 30 years ago, “terrane” has taken on a narrower definition of broken-off pieces of continental plates, island arcs, or even oceanic crust that have crashed into (called “accreting” by geologists) a drifting continental plate instead of being “subducted” beneath plate.  These terranes then become an “exotic” part of the plate.  Dr. David G. Howell of the U.S. Geological Survey (USGS) in Menlo Park, CA, a specialist in terrane analysis, compares the drifting of continents to the motion of geologic “bumper cars"!

The western edge of the North American (NA) Plate has accumulated many long, narrow, flattened-out exotic terranes, some as big as Japan, or even California, over the past 200 million years.  The figure below, from the USGS website http://pubs.usgs.gov/gip/dynamic/Pangaea.html, shows a few of those terranes.  But this is greatly simplified from the newest geologic terrane maps of the West Coast of BC and Alaska. 

Of interest during our Alaska Highway trip is a particular terrane that is still in the process of bumping into the NA Plate.  Although in a remote area not near any highway, the Yakutat Block terrane covers a large area—360 miles long and 120 miles wide.  It is located on the coast between Glacier Bay and Cordova.  This Block has been moving onto the NA Plate at a rate of about 2.5 inches per year, or 4 miles during the 100,000 years since it first contacted the NA Plate.  The Yakutat Block can be seen on the map above as the light-colored "submarine deposits" area on the west (left) side of the long fault attached to the east end of the "Aleutian Trench" (just left of the ". . llia" in "Wrangellia Terrane").

Based on the rock types and fossils found in parts of the Yakutat Block, it apparently broke off the NA Plate at about the location of Prince Rupert and moved 330 miles north along one fault system at the same time 540 miles of relative movement occurred between the Block and the Pacific Plate, making it 870 miles northwest of its starting point after only 25 million years.  So, the NA Plate is now in the process of readopting one of its own children!

Once we get on the road and reach Washington State, we will be driving north along and across several of these accreted terranes on our way through Kamloops and Prince George, then up the Cassiar Highway (I hope, since it is now closed due to flooding!).  So, in the way of a hint about future posts, the north-south trending Okanogan-Okanagan valley is one of those “exotic terranes.”

Next time: Like crazy foxes, lousy weather, earthquakes, and atomic bombs?  Then you would love the Aleutian Islands!