Introduction

Syntheses of geologically complex regions over large amounts of time are challenging endeavors. Such studies must attempt to honor diverse geological and geophysical data and incorporate a wide range of complex and commonly differing interpretations of these data. In this study we present a regional synthesis of the tectonic assembly of Western North America from Permian to Early Tertiary, with emphasis on Jurassic and Cretaceous events. We attempt to track all major terranes and tectonic elements through a series of closely spaced paleotectonic and paleogeographic time slices. The time slices were originally assembled at 5-10 m. y. intervals and then presented at the intervals shown. By originally preparing such tight intervals, we were able to track individual elements within the constraints of available data and to check these events with respect to logical sequences of geologic events and plate-tectonic interactions at reasonable tectonic speeds.

When attempting this reconstruction, several major decisions must be made with respect to the origin of several important and controversial terranes and groups of terranes. We have followed a relatively conservative view concerning the origins of many of the terrane elements that comprise Western North America. Most terrane complexes have strong affinities to Western North America - from Permian through the Mesozoic, most were in proximity to the Cordilleran margin. The two major terranes that we acknowledge as exotic to North America are Greater Wrangellia (Insular Superterrane) and the Guerrero Superterrane. Many smaller terranes and blocks also suggest exotic origin but they were incorporated into larger elements with North American affinities. The Cordilleran margin was built during the late stages of the assembly of Pangaea and continued throughout the Mesozoic and Cenozoic. Our reconstructions show the following broad events: 1) After the Devonian-Mississippian Antler orogeny, the Mc Cloud arc was built on Western North America during the late Paleozoic. The Mc Cloud arc became dismembered and the various parts accreted to North America during the Triassic Sonoman orogeny. 2) During the Late Triassic and Early Jurassic, a series of fore arc and interarc elements formed west of and between elements of the dismembered Mc Cloud arc; some exotic material was incorporated into these terranes. The Cordilleran margin consisted of a southern continental arc and a northern marine arc. In some areas, the marine arc comprised two or more oceanic arcs. 3) In Middle and Late Jurassic, the exotic Wrangellia and Guerrero terranes approached the Cordilleran margin and the former collide with North America along its southern margin. The intervening Nutotzin Ocean closed from north to south and ophiolites accreted along the Cordilleran arc. As Wrangellia accreted, it was transposed southward along sinistral transform faults; parts of the northwest Cordilleran margin was captured by the south-moving block, the composite terrane referred to as Baja BC. 4) Baja BC moved southward during the Early Cretaceous and into the Late Cretaceous until about 100 Ma. The Nutotzin Ocean closed and a large transform separated Baja BC from the composite Intermontane terrane. 5) During Late Cretaceous and Early Tertiary, both Baja BC and Intermontane terranes continued northward dextral translation, though the former at a greater rate. By Late Eocene, both were in their approximate present latitude with respect to North America.

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