The Olympic-Wallowa lineament: A new look at an old controversy

Abstract

Abstract The Olympic-Wallowa lineament (OWL) is an alignment of geologic structures extending nearly 650 km across the Pacific Northwest (PNW) and has been a controversial feature since it was first described nearly 80 years ago. It extends from the Olympic Peninsula–Puget Sound area of western Washington to the Wallowa Mountains of northeast Oregon, crossing through the Columbia Basin and forming the southern boundary of the Pasco and Walla Walla Basins. Within the Columbia Basin, the OWL is a wide zone that aligns with >250 km of folds and faults that are part of the Yakima fold and thrust belt. Although the OWL is recognized to be active with deformation continuing from the Miocene to Recent, there are two competing end-member interpretations on the style of deformation: that of a major through-going, dextral strike-slip or oblique-slip fault and that of a series of thrusted anticlines. We focus on that portion of the OWL in the central Columbia Basin that extends from the Walla Walla River in northeast Oregon to Rattlesnake Mountain in central Washington. East of Wallula Gap, the OWL is expressed as the Wallula fault zone along the north flank of the Horse Heaven Hills anticline and the south fork of the Walla Walla River, while west of Wallula Gap, there are three structural alignments: the Rattlesnake-Wallula structural alignment, the Horse Heaven Hills–Badlands structural alignment, and the Horn Rapids–Badger Coulee structural alignment. Except for the Horse Heaven Hills anticline, the structural trends west of Wallula Gap mainly consist of a series of doubly plunging, northeast-verging anticlines. We demonstrate that the OWL consists of north-verging, thrusted anticlines whose orientation and structure are controlled by a basement fault without any evidence of strike-slip or significant oblique-slip fault movement since at least the middle Miocene. The difference in structural deformation of the portion of the OWL to the east of Wallula Gap versus that to the west appears to be controlled by the thickness of pre-basalt sediment within the central Columbia Basin and structures therein.