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[email protected]. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Geological Society of America Special Paper 408 2006 Chapter 2.3 Fractures in welded tuff Kenneth Wohletz Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA INTRODUCTION Rare surface exposures of fracture sets show a polygonal, often rhombohedral, pattern, though in places the pattern is nearly A notable feature and, in many cases, a distinguishing orthogonal or hexahedral. Most fractures are steeply dipping aspect of welded tuff is the occurrence of numerous fractures. (~80°), but nearly horizontal fractures are also evident. They dis- These fractures, generally oriented in a nearly vertical fashion, play both planar (constant aperture) and sinuous (variable aper- are rather equally spaced, and show little or no displacement.