Abstract To expand our evaluation of the Mesozoic Sierran arc, we need to consider the volcanic section that overlies the intrusive parts of the arc seen in Days 1–4. While doing so, we can discuss evidence for links between the magmatic and volcanic components of the arc, the nature of regional sedimentation and tectonism that accompanied active volcanism, and the challenges of estimating volume rates of volcanic activity in ancient arc systems. The most complete exposures of the volcanic section of the Mesozoic arc in the central Sierra occur in several pendants near the eastern topographic divide of the Sierra Nevada Mountains. These volcanic sections are exposed in the Saddlebag and Ritter Range pendants. We have completed new 1:24,000–1:10,000 scale mapping in four areas within these eastern pendants, which from north to south are: (1) Eagle Creek pendant around Twin Lakes, (2) the Virginia Canyon area, (3) the Saddlebag Lake pendant, and (4) the northern Ritter Range pendant…

ABSTRACT The Late Cretaceous Tuolumne Intrusive Suite in the Sierra Nevada batholith (California, USA) has been well studied for magmatic processes. The superb alpine exposures also allow one to investigate the development of magmatic fractures in these systems. Utilizing a cognitive framework of mental models, this field trip highlights and explores three fracture types that are rarely described in the literature and appear limited to magmatic systems. The first type is tabular fracture clusters (TFCs), which are a series of closely (sub-cm) spaced, sub-parallel fractures. Most TFCs in the Tuolumne Intrusive Suite strike ~020, but in some locations there is an orthogonal set that strikes ~110. Both sets have a sub-vertical dip. The second fracture type is magma-healed fractures. These fractures offset igneous features (e.g., layering), but there is no extant fault with grain-size reduction or evidence of cataclasis. Our interpretation is that these features formed while the system contained melt, and that the melt healed any brittle deformation features that may have formed. The third fracture type is primary en echelon P-fractures, recognized by the formation of epidote-filled, pull-apart zones. There are both right-lateral and left-lateral P-fracture arrays, which are spatially associated with the right-lateral Cascade Lake shear zone (part of the Sierra Crest shear zone system). These three types of fracture systems do not fit into any preexisting fault/fracture categories. These observations suggest that disciplinary categories (e.g., fault systems) may need to be expanded to form more categories (splitting). This category reorganization may be facilitated by taking a perspective that addresses both the Earth processes and mental processes that reason about them. During this two-day field trip, we will visit the tabular fracture clusters and healed magmatic fractures in Yosemite National Park, and the P-fracture arrays in the Saddlebag Lake area.