Magnetic fabrics of arc plutons reveal a significant Late Jurassic to Early Cretaceous change in the relative plate motions of the Pacific Ocean basin and North America

Abstract

Contrasting magnetic fabrics in five successively emplaced syntectonic plutons reveal temporal and spatial variations in tectonic strain in the oceanic terranes of the Blue Mountains province, northeastern Oregon, during the Late Jurassic to Early Cretaceous. The inferred strain regimes changed from: (1) thrusting and sinistral shearing at ca. 160 Ma, to (2) horizontal stretching at ca. 147 Ma (in the forearc-accretionary wedge Baker terrane), to (3) dextral transpression that started from ca. 140 Ma onward and was associated with progressive anticlockwise rotation of the principal horizontal shortening direction from ca. 130 Ma to ca. 126 Ma (in the Wallowa oceanic arc terrane). These progressive strain reorientations are interpreted in terms of an outboard Wallowa-Baker terrane collision, lateral extrusion, docking of the amalgamated Blue Mountains superterrane into a continental-margin reentrant, and onset of oroclinal bending, respectively. The changes in crustal strains are then interpreted as recording a progressive change in relative motions between the Pacific Ocean basin and North America and suggest a transition from Late Jurassic sinistral deformation to Early Cretaceous dextral terrane translations along the paleo-Pacific margin. We speculate that these events may have been linked along large portions of the North American Cordillera, from central California to Blue Mountains, and may have culminated in the onset of accretion in the Franciscan complex and voluminous plutonism in the Sierra Nevada magmatic arc. A similar plate-kinematic change is inferred to have occurred in British Columbia several tens of millions of years later (at ca. 100 Ma), implying that these kinematic transitions may have varied in space and time along the length of the Cordilleran orogen.