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The Ashgillian (Upper Ordovician) Montgomery Limestone occurs as slide blocks in melange of the Shoo Fly Complex, northern Sierra Nevada, northern California. Brachiopods and sphinctozoan sponges from the Montgomery Limestone have closest biogeographic ties to coeval faunas of the eastern Klamath Mountains (Yreka terrane), and in the case of the brachiopods, to east-central Alaska (Jones Ridge). The latter was part of North America in the Ordovician. A small collection of Montgomery rugose corals yielded one species that is known elsewhere only in the Yreka terrane and in northern Maine. Montgomery tabulate corals have affinities with contemporaneous faunas of the Yreka terrane, northern Europe/Asia, Australia, and eastern North America. The apparent absence of similar tabulate taxa in western North America may be an artifact of incomplete collecting. As a whole, the biogeographic data indicate that the Montgomery Limestone was deposited close enough to Ordovician North America for faunal interchange to occur, and during its deposition was probably relatively near that continent. A comparison of lithologic units of the Shoo Fly Complex with those of the Yreka terrane indicates that some units in each area have no counterparts in the other (e.g., schist of Skookum Gulch in the Yreka terrane), and other units have general similarities in age (where known) and lithology, but differ in detail. The Yreka terrane has been interpreted as the remnants of an Early Cambrian arc and Ordovician-Devonian arc–fore-arc–accretionary prism, and the Shoo Fly Complex as a fragment of a Devonian or older accretionary wedge. Available biogeographic and stratigraphic data can be reasonably explained, as has been done by earlier authors, by a paleogeography in which the Yreka terrane and Shoo Fly Complex were parts of the same arc-trench system but were situated at different points along the strike of the arc. Lateral changes along strike in tectonic conditions and source areas could account for the observed disparities.

Newly recognized fossil cyclomedusoids from the Yreka terrane include Ediacaria sp. and Beltanella sp. They are typical of the Ediacaran fossil assemblage, range from 640 to 575 Ma, and thus are latest Neoproterozoic (Vendian) in age. The Yreka terrane structurally overlies the Trinity terrane, which also includes Vendian rocks. The Yreka terrane is a polygenetic stack of sedimentary and metasedimentary thrust sheets consisting of the Vendian Antelope Mountain Quartzite, Siluro-Devonian turbidites (trench fill), lower Paleozoic mélanges (accretionary complexes), and the Lower Devonian Gazelle Formation (trench-slope basin deposits). The Trinity terrane is a polygenetic mafic-ultramafic complex consisting of multiple mantle tectonite blocks and two ophiolitic crustal sequences, one Vendian and one Siluro-Devonian. Multistage textures and structures within the Trinity terrane indicate Vendian or Cambrian ductile deformation in the mantle blocks, followed by pre-Early Ordovician amalgamation, then regional uplift and brittle deformation. The Siluro-Devonian crustal sequence developed on this polygenetic composite basement in a supra-subduction zone setting. The Trinity and Yreka terranes formed close together, with some Yreka terrane components receiving Trinity terrane detritus. The Lower Devonian Gregg Ranch Complex was the active accretionary wedge on which the Gazelle Formation trench-slope basin formed, accompanied by minor near-trench volcanism. Kinematic analysis of the Gregg Ranch Complex indicates convergence directed from the Yreka terrane toward the Trinity terrane, combined with a strike-slip component, probably during Early Devonian collision of outboard terranes. The Yreka terrane–Trinity terrane composite terrane was then stitched together by Middle Devonian dike swarms that fed overlapping lava flows. Because of the close spatial and temporal proximity between the Yreka and Trinity terranes, paleopoles from the Trinity terrane can be used to suggest paleolatitudes where Yreka terrane biota may have originated, and biogeography of Yreka terrane fossils limits the paleogeographic setting of both terranes.

The eastern Klamath belt contains the fault-bounded Yreka, Trinity, and eastern Klamath terranes. The Yreka terrane comprises Lower Cambrian to Middle Devonian or younger igneous, metamorphic, and sedimentary rocks. The Trinity terrane consists of the Trinity ultramafic-mafic complex of Ordovician to Silurian age and an amphibolitic gabbro unit of Early Cambrian age. The eastern Klamath terrane bears igneous and sedimentary rocks of Early Devonian to Middle Jurassic age. Stratigraphic and intrusive relations imply that the Yreka and Trinity terranes were amalgamated by Early to Middle? Devonian time and that the Trinity terrane was the basement on which the eastern Klamath terrane formed. The lower Paleozoic rocks of the Yreka and eastern Klamath terranes are interpreted to represent the remnants of an Early Cambrian arc overlain by part of an Ordovician to Devonian arc-trench complex that faced west to northwest (present coordinates) in Late Ordovician and Early to Middle Devonian time. The Trinity complex may have formed in a marginal or back-arc basin northeast to southeast of the Lovers Leap–Gregg Ranch portion of the arc in Ordovician to Silurian? time (prior to existence of the eastern Klamath terrane). The biogeographic affinities of eight groups of early Paleozoic fossils, taken as a whole, demonstrate that the eastern Klamath belt was close enough to North America in the Middle Ordovician to Middle Devonian for faunal communication to occur, in some cases at the species level. This evidence and the presence in the belt of some coarse-grained strata possibly derived from a continent indicate that the belt may have been relatively near to North America in Silurian or Devonian time, yet its location is obscure.

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.

Newly recognized cyclomedusoid fossils in the Antelope Mountain Quartzite confirm that it is latest Neoproterozoic (Ediacaran) in age. Biogeographic affinities of the cyclomedusoid fossils suggest that the Yreka subterrane and its close associate, the Trinity subterrane, formed after the breakup of Rodinia in an ocean basin bordering Australia, northern Canada, Siberia, and Baltica. Reevaluating biogeographic, geological, and paleomagnetic evidence in the context of this starting point, the Yreka subterrane and Trinity subterrane may have been located at either 7°N or 7°S latitude ca. 580–570 Ma, but were not necessarily close to Laurentia. Continental detrital zircons (3.2–1.3 Ga) in the Antelope Mountain Quartzite most likely came from Australia or Siberia rather than Laurentia. The Yreka subterrane and Trinity subterrane record ∼180 m.y. of active margin events somewhere in Panthalassa (Proto-Pacific Ocean). Paleozoic biogeographic data, paleomagnetism, and regional relationships indicate that Yreka subterrane and Trinity subterrane were located throughout the early Paleozoic in the part of Panthalassa surrounded by Australia, NW Laurentia, Siberia, China, Baltica, and the Uralian terranes. By the mid-Devonian they were located at 31°N or 31°S in a somewhat isolated location, probably in a Northern Hemisphere oceanic plateau or island chain well outboard of other tectonic elements, and by the Permian they were almost completely isolated from other tectonic elements. The Yreka subterrane, as part of the Klamath superterrane, was not native to North America and did not accrete to it until the Early Cretaceous.