Abstract

An orthogonal system of tube-bearing joints constitutes the oldest fractures in the Tiva Canyon Tuff at Yucca Mountain, Nevada. The joints formed within a month of ignimbrite deposition, prior to major degassing. The system consists of (1) narrow, persistent, northeast-striking joint swarms with trace lengths typically greater than 5 m and between-joint spacings of less than 20 cm and (2) northwest-striking swarms that have a more en echelon geometry and greater between-joint spacings compared to the northeast-striking swarms. Between-swarm spacing for both trends is ∼50 m. Questions concerning the joints include the following: (1) What was the origin of driving stress(es) for formation of the joints, particularly as their orientations were not consistent with the regional stress geometry at the time of their formation? (2) What mechanism caused the horizontal principal stresses to be reoriented so as to yield an orthogonal geometry? (3) What insights can be developed for predicting joint geometry in unexposed rock volumes by understanding joint origin? These questions are important because the joints and other fractures may affect the performance of a proposed high-level nuclear waste repository within Yucca Mountain.

To address these questions, we use new and existing field data about joint geometry and the relationships of joints to degassing structures, numerical modeling of fault behavior, and work by previous authors. Our interpretation begins with the initial ignimbrite eruption and deposition during caldera collapse. The ignimbrites were deposited over a preexisting topography that possibly included a shallow northwest-trending basin in the Yucca Mountain area. During initial ignimbrite cooling, joint swarms formed as elements of orthogonal fumarolic ridge systems where degassing was associated with vertical dilation. Both joint sets have unusual tubes that are interpreted to have formed during dilation and segmentation of joint faces resulting from lithophysae inflation in the cooling ignimbrite deposit. Modeling supports the interpretation that a combination of regional stresses and stress related to slip on local normal faults controlled the orientation of the joint swarms and favored the formation of the northeast-striking joints first. The faults might have moved in a stress field already perturbed by caldera collapse. Formation of the northwest-striking joints occurred after a local 90° switch of horizontal principal stress directions due to the presence of the northeast-striking swarms, possibly aided by differential compaction across the northwest-trending basin. Tube- bearing joints occur in all lithophysae-bearing lithostratigraphic units of the Topopah Spring Tuff, which is the stratigraphic interval for the proposed nuclear waste repository within Yucca Mountain. We conclude that the tube-bearing joints formed in the same manner and share similar geometric characteristics, adding a persistent joint population to the overall fracture system that influences hydrological and mechanical properties.

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