North Cascades - Geology

Geology

The bulk of the North Cascades consists of "deformed and metamorphosed, structurally complex pre-Tertiary rocks". These originated in diverse locations around the globe: the area is built of several ("perhaps ten or more") different terranes of different ages and origins. These terranes are separated by a series of ancient faults, the most significant being the Straight Creek Fault, which runs north-south from north of Yale, British Columbia, through Hope, Marblemount, Washington, and down to Kachess Lake near Snoqualmie Pass. There is evidence of significant strike-slip movement on this fault in the past, with similar rocks on either side of the fault separated by dozens of miles. This is thought to be related to northward tectonic movement of the West Coast relative to the rest of North America.

Since about 35 million years ago, oceanic crust from the Pacific Ocean has been subducting under the continental margin, which has formed the current volcanoes as well as a number of igneous intrusions composed of diorite and gabbro. The current uplift of the Cascade Range began around 8 million years ago.

Rocks similar to those in the North Cascades continue north to the vicinity of Mount Meager in the Coast Mountains, where they abut the Stikinia Terrane of the Omineca-Intermontane Province that dominates the Interior Plateau of British Columbia. This geologic similarity between the North Cascades and Coast Mountains results in a fairly arbitrary boundary between the two.

In British Columbia, the western geologic boundary of the North Cascades is defined as the Fraser River as it follows the Straight Creek Fault, while in the United States the western boundary is defined by the Puget Lowlands in the west, although there are significant westward extensions of rocks similar in origin to those in the North Cascades found in the San Juan Islands.

The eastern geologic boundary of the North Cascades may be marked by the Chewack-Pasayten Fault. This fault separates the easternmost portion of the North Cascades, the Methow Terrane, from the Quesnellia Terrane, one of the Omineca and Intermontane Belts. The fault also separates the Methow River valley, part of the Methow Terrane, from the Okanagan Range, part of the Quesnellia Terrane. The Columbia River Basalt Group bounds the North Cascades to the southeast.

The southern limit of what is geologically considered the "North Cascades" may variously be defined as being the southern limit of eposure of igneous and metamorphic terranes which is generally north of Snoqualmie Pass, Snoqualmie Pass itself, or Naches Pass at the White River Fault Zone.