Dispersed remnants of a northeast Pacific fringing arc: Upper Paleozoic terranes of Permian McCloud Faunal affinity, western U.S.

Abstract

Two fragmentary, subparallel belts of terranes within the western North American Cordillera contain upper Paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting Permian faunal affinity. These two belts are (A) volcanic‐arc related successions of Permian McCloud faunal affinity (McCloud belt) and (B) subduction‐related accretionary complexes of Permian Tethy an faunal affinity (Cache Creek belt). This paper supports the hypothesis that the fragmentary terranes of the McCloud belt once constituted parts of a northeast Pacific fringing‐arc system and, in constrast to some earlier interpretations, concludes that the volcanic arc evolved above an eastward dipping subduction zone. The absolute distance between this arc and western North America during the late Paleozoic cannot be constrained, however, there is little evidence to suggest closure of a major (>10³ km) ocean basin or protracted periods of westward dipping subduction. Parts of Devonian to Permian volcanic island arc sequences of the western U.S. Cordillera are represented in the northern Sierra, eastern Klamath, Bilk Creek, Grindstone, and Chilliwack terranes. These scattered volcanic arc remnants share several fundamental characteristics: (1) The sequences were constructed across continental‐affinity basement assemblages. (2) They underwent similar tectonic evolution during late Paleozoic time, such as coeval pulses in volcanism and related depositional histories. (3) They contain Early Permian McCloud‐type fauna, of distinctive biogeographic affinity. (4) McCloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the Cache Creek belt which contain fragments of Upper Triassic blueschist and Permian limestone blocks bearing Tethyan Permian fusulinids and corals. Based on the presence of distinctive Early Permian McCloud fauna, the island arc remnants discussed in this paper are referred to as the McCloud belt. The similarities between McCloud belt terranes suggest that they were once parts of a long‐lived late Paleozoic volcanic arc complex. The stratigraphy of these terranes records the tectonic evolution of the volcanic arc. Volcanism within the Paleozoic arc began as early as the Early Devonian, and was succeeded or accompanied by carbonate and epiclastic sedimentation during the Middle and Late Devonian. An influx of detritus derived from sedimentary and metasedimentary rocks during the Late Devonian and Mississippian records the uplift and erosion of arc basement sequences, an event that was, at least locally, accompanied by ongoing volcanism. A pulse in volcanism during the Pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and, locally, subaerial exposure which persisted into the early Early Permian. Elsewhere during the Early Permian, shallow‐water carbonate deposition was established, with only minor influx of volcanogenic debris. During the Late Permian, and locally as early as late Early Permian, a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas. Most of these sequences are overlain by Upper Triassic carbonate platformal sequences. Arc volcanism was recorded, at least locally, until the Middle Jurassic. Paleomagnetic data are used to constrain a schematic pre‐Cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages. The inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone, with a long‐lived paleogeographic relation to North America. The current structural position of these terranes is largely the result of Mesozoic and locally younger deformation.