Eight paleomagnetic poles are considered to be reliable Jurassic reference poles for cratonic North America. These poles form a consistent chronological progression defining two arcuate tracks of apparent polar wander (APW) from Sinemurian through Tithonian time (203–145 Ma). Combined with reliable Triassic and Cretaceous reference poles, the resulting path is well modeled by paleomagnetic Euler pole (PEP) analysis and is significantly different from previous APW compilations. These differences reflect differences in original data sets, modes of analysis, and geologic time scales and translate into substantial and important differences in paleolatitude estimates for cratonic North America. PEP analysis reveals two cusps, or changes in the direction of APW: one in the Late Triassic to Early Jurassic (Jl) and one in the Late Jurassic (J2). The Jl cusp represents the change in North American absolute plate motion associated with rifting of the central Atlantic and Gulf of Mexico, while the J2 cusp correlates temporally with the marine magnetic anomaly M21 plate reorganization and to various North American intraplate tectonomagmatic events (e.g., Nevadan Orogeny). Analysis of pole progression along the Jl to J2 and J2 to Cretaceous APW tracks indicates constant angular plate velocity of 0.6°–0.7°/m.y. from 203 to 150 Ma followed by significantly higher velocity from 150 to 130? Ma. Late Triassic‐Jurassic reference poles indicate more southerly paleolatitudes for cratonic North America than have previous compilations requiring modification of displacement scenarios for suspect terranes along the western Cordillera.

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, sub sidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, infl uence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra . Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America.The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1-TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, fi rst-order patterns within the detrital zircon age distributions.TSA1 represents a Paleozoic-Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600-1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic-Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0-1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, refl ecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7-1.8 Ga, which probably refl ects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 refl ects the phase of basin fi ll associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this a...

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Paleomagnetic study of Middle Jurassic volcanic and volcaniclastic rocks in southeastern Arizona yields a paleomagnetic pole located at 61.8øN, 116.0øE (k=49.6,•95=6.2ø).