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Jurassic-Cretaceous Paleogeography, Terrane Accretion, and Tectonic Evolution of Western North America, Ronald C. Blakey and Paul J. Umhoefer, Department of Geology, Northern Arizona University, Flagstaff, AZ 86011

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.

Tectonic History Shown on Maps

Early to Middle Triassic (240 Ma)
  • McCloud arc fragmented and accreted to North America resulting in Sonoman orogeny
  • Caborca terrane moved SE along truncated continental margin
  • Subsequent Cordilleran arc was continental to south and marine to north

    Early to Middle Jurassic (180 Ma)

  • Fringing Cordilleran arc accreted to western North America including Bridge River, North Cascades, western parts of Eastern Oregon terranes, and western terranes of Klamaths and Sierra Nevada
  • Older Mesozoic back arc basins closed

    Middle to Late Jurassic (160 Ma)

  • Major arc magmatism
  • Initial collision between southern Wrangellia and Cordilleran margin at approximately the latitude of Klamaths and Sierras
  • Fringing arcs south of collision zone
  • Ophiolites obducted in collision zone and in inter arc region between Cordilleran arc and fringing arcs
  • Foreland basin in Utah and thrusting in Nevada
  • Rift basin in southwestern North America related to opening Gulf of Mexico

    Latest Jurassic (145 Ma)

  • Major plate reorganization -- change to sinistral motion of Farallon plate relative to North America causes Wrangellia to move southward
  • Complex series of events at SW margin of North America: Chortis and related terranes shift SE as Atlantic and Proto-Caribbean expand; transform fault and oblique rift systems developed along waning continental arc (McCoy-Bisbee; Mojave-Sonoran megashear); early Franciscan mélange formed farther outboard
  • Nutzotin Ocean between northern Wrangellia and North America remained open
  • Oblique convergence shut down much of Cordilleran arc

    Early Cretaceous (125 Ma)

  • Guerrero arc collided and subduction zone rebuilt on west face
  • South-moving Wrangellia linked to North Cascades, Tyaughton-Methow, and adjacent terranes to form Baja BC
  • Tectonic escape of Intermontane terrane and parts of California northward
  • Great Valley forearc basin and Franciscan subduction complex built on reorganized Cordilleran margin; as Baja BC moved southward, northern Great Valley a transpressive basin (?)
  • North Slope terrane rotated CCW as Canadian basin opened … Early Sevier thrust belt and foreland basin formed

    late Early Cretaceous (105 Ma)

  • Two large volume magmatic belts formed (Peninsular, Sierra Nevada, Idaho; Coast Plutonic complex built on Baja BC); plutonic complexes may have been fed by volatiles from under-thrusting of North American miogeocline
  • South-moving Baja BC approached maximum S position at latitude of southern Arizona
  • Baja BC west of classic Great Valley and Franciscan complexes; Great Valley as interarc basin(?)
  • Rotated North Slope terrane collided with Yukon-Tanana and related terranes
  • Sevier orogeny continued and foreland basin expanded eastward

    Late Cretaceous (85 Ma)

  • Plate reorganization as Kula separated from Farallon Plate; dextral transpression with North America; Kula-Farallon ridge moved northward along coast
  • Baja BC transported northward with obliquely converging Kula plate
  • Major magmatic activity in two belts (110-85 Ma) followed by strong decline and null in magmatic activity (80-40 Ma)
  • Andean-style segmentation of Farallon arc from Central America to Southern California
  • Sevier thrusting continued and foreland basin shifted locus of subsidence southward in response to shallow Laramide subduction

    Latest Cretaceous - Early Tertiary (65 Ma)

  • Triple junction and Baja BC moved rapidly northward
  • Intermontane terranes translated to north
  • Forearc sedimentation (Franciscan-Great Valley) continued along southern margin
  • uplift of Coast plutonic complex generated large deep-sea fan deposits (Chugach flysch)
  • Strong Sevier thrusting and foreland basin
  • Laramide orogeny: Farallon Plate shallow subduction caused expansion NE of foreland uplifts and basins

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