C. Ervin Brown T. P. Thayer 1973 <p><i>Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.</i></p><p>Ironside Mountain is a 6- by 10-km block of folded rhyolite, andesite, and basalt flows of the Strawberry Volcanics bounded by a horse-shoe-shaped reverse fault and surrounded by Mesozoic rocks. The toe of the horseshoe points northeast; the fault dips outward at angles of 45° to 90°, has a minimum displacement of 600 m at the toe, and dies out or is buried at the open southwest end. Flows are folded to vertical in a crosswise anticline at the northeast end, and in a synclinal structure along the southwest edge. Local unconformities and thickening of flows in a small basin show that some deformation accompanied volcanism, which has been dated as late Miocene and early Pliocene. No centers of eruption have been identified within the Horseshoe fault.</p><p>The Ironside Mountain block seems to be a unique expression of cross folding in the region. The northwest-striking Mine Ridge–Ironside anticline follows the main regional trend, and is faulted where two large down-warps strike into it from the northeast. The Ironside Mountain structure is interpreted as being a transverse compressional feature in which opposing reverse faults took the place of anticlinal folding, directly on strike with a syncline to the northeast. A horst of pre-Tertiary rocks, which has been raised at least 600 m, crosses the axes of the abutting structures; the nose of the Horseshoe fault bounds one side of it, and a range-border fault bounds the other. Cross faults, of which we found only one, are believed to have facilitated arching and longitudinal shortening within the horst.</p> application/pdf 10.1130/0016-7606(1973)84<489:IMOALT>2.0.CO;2 en Geological Society of America Ironside Mountain, Oregon: A late Tertiary volcanic and structural enigma article