WHY DO THE YREKA TERRANE (KLAMATH MOUNTAINS, CA) AND ALEXANDER TERRANE (PRINCE OF WALES ISLAND, AK) HAVE SIMILAR ORDOVICIAN-SILURIAN FOSSILS WHEN THEIR STRATIGRAPHY AND TECTONIC HISTORY ARE DIFFERENT?

The Alexander terrane (AT) on Prince of Wales Island, AK, and the Yreka terrane (YT) of northern California formed on different tectonic plates with differing sedimentary environments and tectonic histories, but carbonate olistoliths and tectonic slices in YT mélanges have some Ordovician and Silurian fossils conspecific or similar to those in thick carbonate platform sequences of the AT. Faunal links and regional geology suggest migration along connecting island chains. YT mélange blocks may have been derived from AT carbonate platforms and transferred across convergent or transform boundaries from the AT into YT mélanges. Lower Paleozoic turbidites from both terranes that overlap in age may contain debris from closely related volcanic arcs.

Compare the two terranes. AT consists of: 1) Proterozoic-lower Paleozoic Wales Group: deep marine, arc-derived trench turbidites more intensely deformed and metamorphosed than most YT rocks; 2) Ordovician-Silurian Descon Formation: volcaniclastic trench turbidites; 3) a thick Ordovician-Silurian carbonate reef complex that transitions up into Devonian Karheen Formation fluvial redbeds. YT consists of: 1) Vendian Antelope Mountain Quartzite: shallow marine quartz arenites and argillites derived from a non-Laurentian continent, with Ediacaran cyclomedusoid fossils; it is thrust over undated metamorphosed Schulmeyer Gulch mélange; 2) a Late Ordovician (447 Ma) subduction complex, the Skookum Gulch blueschist mélange; 3) several unmetamorphosed mélanges with deep marine cherts, volcaniclastic graywackes, mid-Ordovician to Early Devonian fossils in carbonates, and debris eroded from the ophiolitic Trinity terrane (TT); 4) Siluro-Devonian turbidites: metamorphosed arc-derived trench deposits. Mid-Devonian subduction and terrane collision collapsed the YT elements into an accretionary complex thrust over polygenetic oceanic basement of the TT. Mid-Devonian to Late Jurassic deposition over the juxtaposed YT and TT was all marine with no regression recorded until Late Jurassic, when the Klamath superterrane accreted to North America. These dissimilar tectonic histories seem to disagree with the close biogeographic links.