ABSTRACT Marine sandstones of the Lower-Middle Miocene Bear River beds, Upper Miocene-Lower Pleistocene Wildcat Group and Pliocene lower Falor Formation contain an average of 6 % K-spar. 40 Ar/ 39 Ar laser probe analyses of Lower Pleistocene Wildcat Group micas yield ages of approximately. 52-57 Ma. Analyses of Pliocene Wildcat Group micas yield three groups of ages: approximately 66-75, 128.5, and 299-303 Ma. A logical source for the three younger groups are Cretaceous-Eocene plutonic rocks in Idaho, a source proposed for subjacent Eocene Yager Formation sandstones in the Coast Ranges by Underwood & Bachman (1986). The source of the older micas remains enigmatic, but may be reset Devonian Abrams Schist of the centxal Klamath Mountains. Fluvial conglomerates and sandstones of the Upper Oligocene-Lower Miocene Weaverville Formation and Lower Pliocene Wimer Formation in the Klamath Mountains, the Upper Pleistocene upper Falor and Carlotta Formations in the Coast Ranges, and marine sandstones of the Lower Pliocene Wimer and St. George Formations in the Klamaths and Coast Ranges are derived largely from nearby basement rocks. Imbrication of westem Weaverville Formation fluvial clasts indicates westerly flow of a major braided river. However, the deltaic facies of this formation is missing, due either to Miocene erosional stripping or removal by faulting. The eastern Weaverville Formation accumulated syntectonically in upper plate grabens above the regional LaGrange detachment fault. Initiation of LaGrange detachment faulting, Weaverville sedimentation and, possibly, Klamaths uplift is coeval with increased oblique convergence of oceanic and continental plates ca. 30 Ma, and with alkalic intrusive activity in the southwest Oregon and northwest California Coast Ranges. The intrusions may reflect an unrecognized rifting event which might have resulted from clockwise rotation of the Klamath Mountains and Coast Ranges provinces. The Bear River beds accumulated at bathyal depths. Basal Wildcat Group sediments are fluvial to littoral. However, rapid submergence of the Coast Ranges region to bathyal depths (2-3 km) occurred following the initiation of Wildcat sedimentation. This abrupt bathymetric changes may relate to greater normal plate convergence ca. 9 Ma. Ca. 5 Ma the northwest Klamath Mountains were at sea level, as is indicated by presence of Wimer marine sediments on the present-day elevated erosion surface. Poorly dated, coarse fluvial conglomerates of the eastern Wimer facies may reflect local uplift and warping of this surface about a northwesttrending axis. Sediment shoaling, and onset of fluvial sedimentation in the Coast Ranges, may relate to the decreased plate convergence rate, lessening of angle of subduction and increased normal convergence ca. 1 Ma.

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.