Monday, June 1, 2009

Day 346: Basin. Fault. Range.

one of my many takes on the slab basin and range formation

"Most mountain rangers around the world are a result of compression, of segments of the earth's crust being brought together, bent, mashed, thrust and folded, squeezed up into the sky. The Himalaya, the Appalachians, the Alps, the Urals, the Andes. The ranges of the Basin and Range came up another way. The crust- in this region between the Rockies and the Sierra- is spreading out, being stretched, being thinned, being literally pulled to pieces. The sites of Reno and Salt Lake City, on opposite sides of the province, have moved sixty miles apart. The crust of the Great Basin has broken into blocks. The blocks are not, except for simplicities sake, analogous to dominoes. They are irregular in shape. They more truly suggest stretch marks. Which they are. They trend nearly north south because the direction of the stretching is roughly east-west. The breaks, or faults, between them are not vertical but dive into the earth at angles that average sixty degrees, and this, from the outset, affected the centers of gravity of the blocks in a way that caused them to tilt. Classically, the high edge of one touched the low edge of another and formed a trough, or basin. The high-end sculpted, eroded, serrated by weather- turned into mountains The detritus of the mountains rolled into the basin. The basin filled with water- at first, it was fresh blue water- and accepted layer upon layer of sediment from the mountains, accumulating weight, and thus, unbalancing the block even further. Its tilt become more pronounced. In the manner of the seesaw, the high, mountain side of the block went higher and the low, basin side went lower until the block as a whole reached a state of precarious and temporary truce with God, physics, and mechanical and chemical erosion, not to mention, far below, the agitated mantle, which was running a temperature hotter than normal, as was, almost surely, controlling the action. Basin and Range. Integral fault blocks: low side the basin, high side the range. For 500 miles they nudged one another across the province of the Basin and Range. With extra faulting, and what not, they took care of their own irregularities. Some had their high sides on the west, some on the east. The escarpment of the Wasatch Mountains-easternmost expression of this immense suite of mountains -faced west. The Sierra-the westernmost, the highest, the predominant range, with Donner Pass only halfway up it-presented its escarpment to the east."
- Basin and Range, John McPhee

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