Region 5: The Basin and Range

Only a small portion of the Basin and Range is found in the Northwest Central US, located in the southeastern corner of Idaho. The entire Basin and Range region stretches from Idaho through all of Nevada, southeastern California, and southeastern Oregon, and reaches as far as western Texas.

The Basin and Range is characterized by rapid changes in elevation alternating from flat and dry basins to narrow and faulted mountains. This pattern of many parallel, north-south mountain ranges found throughout the region inspired geologist Clarence Dutton to famously observe that the topography of the Basin and Range appeared “like an army of caterpillars crawling northward. ”The formation of this topography is directly related to tectonic forces that led to crustal extension (pulling of the crust in opposite directions). After the Laramide Orogeny ended in the Paleogene, tectonic processes stretched and broke the crust, and the upward movement of magma weakened the lithosphere from underneath. Around 20 million years ago, the crust along the Basin and Range stretched, thinned, and faulted into some 400 mountain blocks. The pressure of the mantlebelow uplifted some blocks, creating elongated peaks and leaving the lower blocks below to form down-dropped valleys. The boundaries between the mountains and valleys are very sharp, both because of the straight faults between them and because many of those faults are still active.

These peaks and valleys are also called horst and graben landscapes (Figure 4.33). Such landscapes frequently appear in areas where crustal extension occurs, and the Basin and Range is often cited as a classic example thereof. In the Basin and Range, the crust has been stretched by up to 100% of its original width. As a result of this extension, the average crustal thickness of the Basin and Range region is 30 - 35 kilometers (19 - 22 miles), compared with a worldwide average of around 40 kilometers (25 miles).

Figure 4.33: A horst and graben landscape occurs when the crust stretches, creating blocks of lithosphere that are uplifted at angled fault lines.

In Idaho, the Basin and Range encompasses long, parallel mountain ranges, including the Bannock and Portneuf ranges (Figure 4.34). The crustal extension of the Basin and Range has increased strain and tension throughout the region, leading to a dynamic variety of active fault zones that create an abundance of earthquakes.

Figure 4.34: The Portneuf Range, Bannock County, Idaho.