Permo‐Triassic Accretionary Subduction Complex, southwestern Klamath Mountains, northern California

Abstract

The western Paleozoic and Triassic subprovince of the southwestern Klamath Mountains has previously been subdivided into three relatively steeply dipping, fault‐bounded, litho‐tectonic terranes. In descending structural order from east to west, these terranes are (1) the North Fork terrane, (2) the Hayfork terrane, and (3) the Rattlesnake Creek terrane. In a general fashion each of these terranes exhibits its own particular set of lithologic, structural, and temporal relationships that sets it apart from adjacent terranes. This threefold subdivision of the southwestern Klamath Mountains needs modification only as far as the Hayfork terrane is concerned. The Hayfork terrane is actually a composite terrane consisting of a structurally lower assemblage of predominantly basaltic metavolcaniclastic rocks and a structurally higher assemblage of chaotically disrupted metasedimentary rocks, juxtaposed along a previously unrecognized major regional thrust. These two lithologically and structurally distinct assemblages have been elevated to the status of a terrane in this report and are herein referred to as the western Hayfork terrane (structurally lower metavolcaniclastic assemblage) and the eastern Hayfork terrane (structurally overlying metasedimentary assemblage), and the fault which juxtaposes these two terranes is herein referred to as the Wilson Point thrust. This paper will focus on the eastern Hayfork and North Fork terranes and their relationship to other roughly coeval tectonic elements of the Klamath Mountain province. The eastern Hayfork terrane is best described as a tectono‐stratigraphic unit consisting of a complex chert, argillite, and quartzose sandstone melange and broken formation that contains radiolarian cherts as young as Late Triassic in age. Scattered throughout this terrane are various types of blocks, some of which are clearly exotic. Metachert is by far the most common type of metamorphic block, and these blocks appear to have been derived via sedimentary processes from the Paleozoic central metamorphic belt and/or the Mesozoic Stuart Fork Formation. Shallow water Permian limestone blocks are also plentiful and are interpreted as reefal accumulations which flanked seamounts that became incorporated into the melange and broken formation as proto‐Pacific ocean floor was subducted beneath the western margin of North America during at least the Permo‐Triassic to Early Jurassic. The North Fork terrane has at its base basaltic rocks tentatively interpreted as an oceanic crustal remnant generated within an open‐ocean ridge setting that was removed from any appreciable source of continental or volcanic arc derived detritus. Overlying the basaltic rocks is a sequence of pelagic, hemipelagic, terrigenous, and tuffaceous metasedimentary rocks that have yielded a Late Triassic and Early or Middle Jurassic radiolarian fauna. The metasedimentary rocks of the North Fork terrane are lithologically very similar to those of the eastern Hayfork terrane; both terranes consist predominantly of chert and argillite, contain blocks of metamorphic rock and Permian limestone, and are, at least in part, of overlapping age. However, the metasedimentary rocks of the North Fork terrane are structurally coherent, unlike the chaotic melange and broken formation of the eastern Hayfork terrane. Taken together, the North Fork and eastern Hayfork terranes bear remarkable similarities to both modern and ancient rock assemblages that occupy sites within zones of lithospheric plate convergence and are interpreted as tectonic elements of an underthrust plate margin, developed oceanward, of a volcanic arc represented by the Permo‐Triassic to Middle Jurassic volcanic strata of the eastern Klamath belt.