ABSTRACT This fi eld trip guide explores the interactions among the geologic evolution, hydrology, and fluvial geomorphology of the central Oregon Cascade Range. Key topics include the geologic control of hydrologic regimes on both the wet and dry sides of the Cascade Range crest, groundwater dynamics and interaction between surface and groundwater in young volcanic arcs, and interactions between rivers and lava flows. As we trace the Willamette and McKenzie Rivers back to source springs high in the young volcanic rocks of the Cascade Range, there is abundant evidence for the large permeability of young lava flows, as manifested in streams that dewater into lava flows, lava-dammed lakes in closed basins, and rivers that emerge from single springs. These dynamics contrast sharply with the older, lower permeability Western Cascades terrane and associated runoff-dominated fluvial systems. On the east side of the Cascades we encounter similar hydrologic characteristics resulting in complex interactions between surface water and groundwater as we follow the Deschutes River downstream to its confluence with the Crooked River. Here, deep canyons have cut through most of the permeable part of the geologic section, have been invaded by multiple large intracanyon lava flows, and are the locus of substantial regional groundwater discharge. The groundwater and surface-water interaction in the Deschutes Basin is further complicated by surface-water diversions and an extensive network of leaking irrigation canals. Our west-to-east transect offers an unparalleled opportunity to examine the co-evolution of the geology and hydrology of an active volcanic arc.

A well-exposed, 1-km-long section of the La Grange fault, a major detachment fault in the southern Klamath Mountains, California, is examined for the dual purposes of analyzing the processes that operated during faulting and evaluating the tectonic significance of this fault. Black foliated fault-related rocks form a 25-cm-thick layer that caps the fault surface and consists of finely interlayered ultracataclasite and cataclasite. Features such as parallel slickenside striations on ultracataclasite layers, veins perpendicular to slickenside striations, clasts of ultracataclasite in cataclasite, and clasts of vein material in cataclasite record prolonged, brittle, extensional deformation along the La Grange fault. This faulting resulted in both south-southeastward transport of hanging-wall rocks and uplift and exhumation of footwall rocks. The presence of Yreka terrane units in the Oregon Mountain klippe suggests on the order of 60 km of southward displacement of the hanging wall of the La Grange fault. This faulting post-dates the assembly of accreted terranes at a long-lived accretionary margin, for which the Klamath Mountains province is renowned, and the overprint of extensional faulting both influences the map pattern and explains anomalies in extent and distribution of some Klamath terranes. At the La Grange fault, the lithologic contrast between amphibolite in the foot-wall and siltstone, sandstone, chert, and mica schist in the hanging wall permits assessment of the relative contributions of footwall and hanging-wall rocks to the ultracataclasite. The ultracataclasite is composed of <10% single mineral grains ≤100 μm diameter in an ultra-fine-grained (<<1 μm) matrix. Single mineral grains are predominantly quartz, but also include calcite, pyrite or pyrrhotite, sphene, rutile, apatite, zircon, and barite. Matrix composition of the ultracataclasite is distinctly different from that of larger grains. Comparison of footwall and hanging-wall rock compositions indicates that most of the larger grains in the ultracataclasite are single crystals of mechanically resistant minerals, and most of these originated in the hanging wall. The less resistant grains in the cataclastic rocks (calcite and barite) are minerals that occur in veins; these were most likely introduced into the fault-related rocks relatively late in the faulting process. The preferential preservation of specific minerals as larger grains within the ultracataclasite shows that the mechanical properties of individual minerals play an important role in comminution processes. Micro-scale textures provide information about the processes that have operated during faulting and about the conditions and duration of faulting. The ultracataclasite is composed of rounded to subangular grains (1–100 μm diameter) in an extremely fine-grained (<<1 μm) matrix. The cataclasite contains ultracataclasite clasts (up to 500 μm) and angular to subrounded individual mineral grains (5–100 μm) in a fine-grained matrix. Texture appears to vary with depth in the fault-related rocks: the abundance of larger single-mineral grains increases upward, with larger grains occupying 3.3 ± 1.1 area% at 2 cm, 2.8 ± 1.1% at 17 cm, and 7.0 ± 1.3% at 24 cm above the base of the foliated ultracataclasite. Extremely small grain size in the ultracataclasite records extreme grain crushing, milling, and sustained cataclastic deformation. This extensive comminution is consistent with a very high degree of strain localization along the fault.