PREFERRED FRACTURE ORIENTATIONS RESULTING FROM SECONDARY DEFORMATION OF A RHYOLITIC ASH FLOW TUFF: 2 MA HUCKLEBERRY RIDGE TUFF, SOUTHEAST IDAHO

We analyze structures caused by a unique secondary deformational event that affected an ash flow tuff immediately post-deposition. The structures include plastically sheared cooling fractures, en-echelon vapor phase-filled extension fractures, horizontal shear zones, cooling joints, and domes.

The 2 Ma Huckleberry Ridge Tuff erupted from the Big Bend Ridge caldera segment of the Yellowstone Plateau Volcanic field. In the Eastern Snake River Plain 60 miles to the southwest, >100 m of welded tuff was deposited. Immediately after deposition, underlying lacustrine sediments and alluvial gravels formed sedimentary diapirs. These diapirs are best seen at the site of the failed Teton Dam in Teton Canyon near Rexburg, Idaho. As a result of diapir formation, the overlying tuff was involved in local gravity sliding and secondary deformation manifest by an extensional rift, shear zones, domes, and joints. Vitrophyres formed at the base of the tuff and around the edges of the diapirs. Diapiric domes in the tuff are locally overturned, joints are locally opened up to a foot, and cooling fractures are locally plastically sheared.

Although joint intensity and orientations vary vertically within the tuff, a NW-SE orientation is pervasive and is parallel to the Hog Hollow rift NE of the canyon. Anticlines in the tuff are broad to overturned and display NW-SE trending fold axes. A series of moderately dipping joints show evidence of shearing along zones parallel to foliation. In a zone approximately 100 feet above the base of the tuff, s-shaped fractures are common, and without exception are associated with zones of concentrated vapor phase mineralization. The ends of the fractures are parallel to foliation, and the central sections cross the vapor phase zones at an angle of 30-50 degrees, suggesting a rheological control during cooling and deformation of the tuff.

Failure of the Teton Dam in 1976 was, along with other contributing factors, a direct result of the structures formed during the diapiric deformation, particularly the opened joints. The features formed during this deformation provide insights into the post-depositional cooling, devitrification, formation of vapor phase minerals, and development of vitrophyres in ash flow tuffs.