Blue Mountains, Domain 6

Abstract

The Blue Mountains domain is mostly in northeastern Oregon. It is the name that we and others (Orr and Orr 1996) have adopted for the Central Highlands subprovince of the Columbia Intermountain province (Freeman et al. 1945). Small areas of Blue Mountains ultramafic rocks are exposed in an arcuate trend from central Oregon through northeastern Oregon into western Idaho. They are in the Baker and Wallowa terranes (Vallier and Brooks 1995). These terranes with the ultramafic rocks are covered or surrounded by Tertiary volcanic flows, largely Columbia River basalt. The ultramafic rocks are exposed in the Canyon Mountain and Sparta complexes and in smaller areas from the edge of the Idaho Batholith near Riggins in Idaho south–southwest across northeastern Oregon to the Aldrich Mountains south of Dayville. The Snake River has cut a deep gorge through the Blue Mountains domain. At Hells Canyon it is >2000 m deep. Strawberry Mountain southeast of John Day rises to 2755 m. Ultramafic rocks are exposed from about 975 m at the foot of the Strawberry Range, near Canyon City, to 2243 m on Baldy Mountain in the Strawberry Range and a bit higher on Vinegar Hill, which is about 45 km northeast of the Strawberry Range, although the summit of Vinegar Hill (2478 m above sea level) is not composed of ultramafic rocks. Summers are hot and dry and winters are cold, with snow that persists through winters at the higher elevations. Mean annual temperatures are mostly in the 3°C–9oC range, and mean annual precipitation ranges from 25 to 100 cm. The frost-free period is about 150 days at lower elevations and <60 days at higher elevations. The ultramafic rocks were exposed by late Tertiary uplift and erosion of the overlying volcanic sequence. The older rocks are composed of a volcanic island arc complex that contains marine sediments interlayered with mafic volcanic flows. Deep erosion of this area has exposed the roots of the volcanic arc. The roots contain gabbro and peridotite–serpentine at their lowest levels. Seven-thousand-year-old volcanic ash from Mt.