The tectonic significance of early Paleozoic convergent-margin rocks of the Alexander and Sierran-Klamath terranes is poorly understood. New phengite 40 Ar/ 39 Ar and Rb-Sr results from the schist of Skookum Gulch of the Yreka subterrane in the Klamath Mountains (454 ± 10 Ma) confirm that blueschists are the oldest known subduction-zone rocks of the western North American Cordillera. The blueschists are juxtaposed with kilometer-scale tectonic blocks of ca. 565 Ma tonalite. Detrital zircons from the blueschists require close proximity to a diverse source of cratonal or derivative supracrustal rocks and preclude formation within an isolated intra-oceanic setting. This strong cratonal provenance (mostly 1.0–2.0 Ga, with resolved concentrations of 1.49–1.61 Ga zircon) is also exhibited by adjacent Early Devonian lower greenschist units of the Yreka subterrane (Duzel phyllite and Moffett Creek Formation). Additional results from temporally equivalent arc-derived sedimentary units (Sissel Gulch graywacke and Gazelle Formation) yield strongly unimodal zircon age distributions of early Paleozoic zircon. The results indicate that the Yreka subterrane formed at an Ordovician–Silurian–Early Devonian convergent margin near a Mesoproterozoic-Paleoproterozoic craton and Ediacaran crust. Appreciable 1.49–1.61 Ga zircon within the Yreka subterrane is compatible with a recent biogeographic analysis that indicates a non-Laurentian origin for the eastern Klamath terrane. Additional new data reveal that key early Paleozoic convergent-margin rocks within the northern Sierran-Klamath and Alexander terranes share similar arc and cratonal provenance, including 1.49–1.61 Ga zircon. We hypothesize that the rocks from all three areas are dispersed tectonic fragments that were derived from the same convergent margin and were independently transported to western North America. Of the orogenic source regions indicated by previous paleomagnetic and biogeographic analysis, the detrital zircon provenance favors western Baltica over eastern Australia.

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.

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.

The Klamath Mountains province of northwestern California and southwestern Oregon is a classic example of a mountain belt that developed by the tectonic accretion of rock assemblages of oceanic affinity during progressive crustal growth along an active continental margin. Consequently, the Klamath Mountains province has served as an important model for the definition and application of the terrane concept as applied to the evolution of Phanerozoic orogenic belts. Early regional studies divided the Klamath Mountains province into four arcuate lithic belts of contrasting age (from east to west): the eastern Klamath, central metamorphic, western Paleozoic and Triassic, and western Jurassic belts. The lithic belts are bounded by regional thrust faults that commonly include ophiolitic assemblages in the hanging-wall block. The age of thrusting is a complex problem because of structural overprinting, but generally the age of regional thrust faulting is older in eastern parts of the province and younger to the west. The lithic belts were subsequently subdivided into many tectono-stratigraphic terranes, and these lithotectonic units are always fault-bounded. Few of the regional faults are fossil subduction zones, but multiple episodes of high pressure–low temperature (blueschist-facies) metamorphism are recognized in the Klamath Mountains province. The tectonostratigraphic terranes of the Klamath Mountains province are intruded by many composite, mafic to felsic, arc-related plutons, some of which reach batholithic dimensions. Many of these plutonic bodies were emplaced during the Jurassic; however, radiometric dates ranging from Neoproterozoic through Early Cretaceous have been determined from (meta)plutonic rocks of the Klamath Mountains province. The orogenic evolution of the province apparently involved the alternation of contraction and extension, as exemplified by the Jurassic history of the province. Widespread Middle Jurassic plutonism and metamorphism is associated with a poorly understood contractional history followed by the development of the Preston Peak–Josephine ophiolite and Upper Jurassic Galice Formation in a probable transtensional inter-arc basin. During the Late Jurassic Nevadan orogeny, this basin collapsed, and rocks of the Galice Formation were thrust beneath the Rattlesnake Creek terrane along the Orleans fault. During this regional deformation, the Galice Formation experienced polyphase deformation and was metamorphosed under lower greenschist-facies conditions. Immediately following thrusting, the hanging-wall and footwall blocks of the Orleans fault were intruded by a suite of composite, mafic to felsic plutons (i.e., western Klamath plutonic suite) that have oceanic-arc geochemical and isotopic characteristics, indicating a subduction-zone petrogenesis for the magmas. The western boundary of the Klamath Mountains province is a regional thrust fault that emplaced the rocks of the province above Early Cretaceous blueschist-facies rocks (South Fork Mountain Schist) of the Franciscan Complex. Neogene structural doming is manifested in the north-central Klamath Mountains by the Condrey Mountain window, which exposes the high pressure–low temperature Condrey Mountain Schist framed by chiefly amphibolite-facies metamorphic rocks of the Rattlesnake Creek terrane.

1. ABSTRACT The geology of the northern Sierra Nevada of California records >400 million years of active plate margin tectonic events as a part of the North American Cordilleran orogenic belt. This field-trip guide provides geologic background and description of field-trip stops for a two-day field trip of the 2023 Geological Society of America Cordilleran Section Meeting based in Reno, Nevada. In two days, we cannot sample the complete geologic record of the northern Sierra Nevada, so this guide does not provide an exhaustive review of this geology. We will focus on certain aspects of the geology that have been the subject of recent research and present some previously unpublished observations and interpretations including: (1) distinguishing between subduction complexes and deformed assemblages that overlay subduction zones; (2) evidence for subduction initiation, recorded in high-pressure (P), high-temperature (T) amphibolites and possibly greenschist facies rocks structurally beneath them; (3) finding of high- P , high- T amphibolite blocks in mélange zones in subduction complex units accreted structurally beneath intact high- P , high- T amphibolite horizons; (4) differences in stream profiles between southern Cascade and northern Sierra drainages, suggesting different forcing mechanisms for stream erosion in those regions; and (5) complex relationships between stream incision, volcanic deposition, and Late Cenozoic faulting. • DEDICATION • This field–trip guide is dedicated to Eldridge Moores (1938–2018) and Jason Saleeby (1948–2023) who were giants in Sierra Nevada geologic research. Eldridge passed away in October 2018 while leading a field trip in this area on which the leaders of the current field trip (JW, DS) were participants. Jason passed away during the writing of this guide. As will be clear from reading this guide, the saying “standing on the shoulders of giants” applies.

Abstract Detrital gold fulfils the criteria of chemical inertia and physical durability required by indicator minerals but it has not found wide application in this role because it may be formed in different deposit types. This problem is soluble, because the generic compositional features of hydrothermal gold differ according to mineralization environment. The wide distribution of gold as a minor component of mineralization where other commodities are the principal exploration target extends the potential of an indicator methodology based on detrital gold to beyond the search for gold itself. Here we highlight how distinctive gold compositional signatures derived from alloy composition and deposit- specific suites of mineral inclusions could contribute to exploration for Cu–Au porphyries, redox-controlled uranium mineralization and ultramafic-hosted PGE mineralization. Future refinement this approach will focus on establishing the spatial distribution of elements at trace levels within gold particle sections using ToF-LA-ICP-MS and application of Exploratory Data Analysis to the resulting datasets. This approach is in its infancy, but aims to develop a classification algorithm useful to researchers irrespective of their previous experience. A pilot study has shown that random forests provide the best approach to establishing gold particle origins.

The Upper Jurassic Galice Formation of the Klamath Mountains, Oregon-California, overlies the ca. 162-Ma Josephine ophiolite and the slightly younger Rogue–Chetco volcano-plutonic arc complex. The Galice Formation that overlies the Josephine ophiolite consists of a siliceous hemipelagic sequence, which grades upward into a thick turbidite sequence. Bedded hemipelagic rocks and scarce sandstone, however, also occur at several localities within the Josephine ophiolite pillow basalts. Corrected paleoflow current data suggest that the Galice Formation was derived predominantly from the east and north. Detrital modes of sandstones from the Galice Formation indicate an arc source as well as a predominantly chert-argillite source with minor metamorphic rocks. A sandstone located ∼20 m below the top of the Josephine ophiolite has detrital modes and heavy mineral suites similar to the turbidite sandstones. Detrital Cr-spinel compositions from the turbidite and intra-pillow lava sandstones are also similar, indicating supra-subduction zone mantle peridotite and volcanic sources. Published detrital zircon data from a turbidite sandstone chiefly give a bimodal age distribution of 153 Ma and ca. 227 Ma but with a minor Proterozoic component. Whole-rock geochemistry from intra-pillow lava sedimentary rocks, the hemipelagic sequence, and the turbidites suggest a mixture between mafic and cratonic sources. It is suggested that the source area for the intra-pillow lava sedimentary rocks, hemipelagic sequence, and turbidites resulted from the mixing of arc and accreted terranes. These data indicate that the source areas for the Galice Formation were already established by ca. 162 Ma, probably during a Middle Jurassic orogeny that predated formation of the Josephine basin.