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In the central Klamath Mountains, the Eastern Hayfork terrane (EHF) accreted directly seaward of the North Fork ophiolitic arc. The EHF trench mélange consists of very fine-grained, quartzose meta-argillite + boudinaged metachert layers as well as scattered blocks derived from both oceanic and inboard arc-margin sources. Among the exotic blocks are coarse-grained feldspathic metasandstones. Detrital zircon SHRIMP U-Pb ages ranging from the latest Archean to the early Proterozoic were reported by Scherer and colleagues in 2010 for five metasandstone blocks from the central and southern Klamaths; Phanerozoic zircons were totally lacking. It was concluded that the blocks were olistostromal and that erosion of a nearby thrust sheet outlier of the Proterozoic Antelope Mountain Quartzite of the Eastern Klamath belt, Yreka subterrane, was a likely source. To determine the provenance and time of deposition of the EHF muddy matrix host of the metasandstone blocks, we separated zircons from three very fine-grained, intimately interstratified, feldspar-poor meta-argillites from the Sawyers Bar map area. New laser ablation multicollector inductively coupled plasma mass spectrometry zircon U-Pb data indicate maximum sedimentary ages for the meta-argillites as ∼272, ∼211, and ∼201 Ma. Thus, the muddy strata were deposited near the end of Triassic time. The meta-argillites contain scattered Proterozoic and Paleozoic–early Mesozoic detrital zircons, indicating local inboard Klamath sources, including the Antelope Mountain Quartzite thrust sheet. St. Clair Creek distal turbidites in the landward North Fork terrane contain Paleozoic and Triassic–Early Jurassic (∼188 Ma) zircons as well as scattered Precambrian grains similar to those in the EHF. The EHF surficially docked against the North Fork terrane by ∼174–173 Ma because the amalgamated but unmetamorphosed terrane contact was invaded by the English Peak plutonic complex beginning at ∼172 Ma. Regional metamorphism and deformation took place throughout the central Klamath Mountains during the Siskiyou transpressive accretionary event at ∼170–168 Ma.

The addition of oceanic material through accretion-underplating was an important process in construction of the convergent/transpressive Cordilleran margin of North America. According to radiolarian ages, the Late Triassic or younger Eastern Hayfork terrane represents trench deposits that accreted outboard of the coeval North Fork immature island arc. It consists chiefly of chert and argillite but contains tectonic and olistostromal blocks of preexisting rock units derived from exotic sources that include inboard Klamath terranes. Fine-grained terrigenous debris was shed from these eastern terranes, but the Eastern Hayfork melange also contains blocks of quartzofeldspathic metasandstone of uncertain provenance. New detrital zircon U-Pb ages from five blocks of immature, variably metamorphosed, arkosic to subarkosic sandstone range from Late Proterozoic to Late Archean, with modes of ∼1804-1843, ∼1970, ∼2300, and ∼2550-2585 Ma. The adjacent, inboard North Fork terrane contains zircons with Phanerozoic ages, indicating derivation from eastern Klamath lithotectonic belts; however, no zircon grains with Phanerozoic ages were identified in the investigated Eastern Hayfork metasandstones. Petrographic analysis indicates that sand grains of the Eastern Hayfork blocks were initially derived from a transitional continental province whose textural immaturity precludes long sedimentary-transport distances. Thus, the metasandstone blocks were most likely tectonically incorporated into the oceanic melange from a local protolith and were not deposited in the trench as sand. Single-grain detrital zircon age spectra for the Eastern Hayfork metasandstone blocks are similar to those for several Cordilleran terranes, precluding clear discrimination among them regarding provenance. Although no currently exposed source is identical to the melange blocks, the Eastern Hayfork zircons show similarities to Laurentian passive-margin clastics in terms of Paleoproterozoic and Neoarchean ages. The presence of an outlier of the Antelope Mountain Quartzite of the Yreka terrane near the Eastern Hayfork terrane, however, indicates that this was a likely source of the exotic blocks. Evidently, this preexisting unit cropped out along the leading edge of the Cordilleran continental margin during Early Mesozoic melange formation, but its areal extent has been diminished by subsequent erosion, deposition, and/or deformation.