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.

Anomalous Late Jurassic motion of the Pacific Plate with implications for true polar wander

“…Furthermore, this rate of net rotation agrees with expectations from the power of toroidal motion at higher spherical harmonic degrees, supporting the idea that the low observed rate is a natural consequence of mantle dynamics (O'Connell and Hager, 1990). Finally, numerical simulations of mantle convection show that net rotations faster than ∼0.2 • My −1 are difficult to produce without extremely deep continental keels (Zhong, 2001). In summary, if the net motion of the entire lithosphere or a large majority thereof is known, the motion of the entire crust-mantle frame, and hence the occurrence of TPW, can be quantified with uncertainty of order 0.…”

“…We can therefore compute the sense of shear between the Farallon and North American Plates using our paleolatitudes of the Pacific Plate, an estimate of the spreading rate between the Pacific and Farallon Plates, and published APW paths for North America. The results of this analysis can then be compared to measurements of rock fabric in the Blue Mountains, Oregon, which suggest sinistral, orthogonal, and dextral motion between the two plates at 160.2 ± 1.1, 146.7 ± 1.2, and 140.2 ± 0.7 Ma, respectively (1σ ; Johnson et al, 2015;Zak et al, 2016).…”

“…Due to the north-south strike of the Farallon-North American subduction zone, the sense of shear at this interface depends Notes: The three chosen ages reflect times at which the relative motion between the Farallon and North American Plates is constrained by petrographic observations (Zak et al, 2016). The inferred sense of shear at the Farallon-North American Plate interface is sinistral, orthogonal, and dextral at 160.2, 146.7, and 140.2 Ma, respectively.…”

“…Lower and Upper bounds of Farallon Plate motion are computed by assuming a 7.8 • clockwise and 4 • counterclockwise rotation of the Pacific Plate, respectively, which correspond to the 1σ range in vertical axis rotation of the Pacific Plate as constrained by the declinations of our H magnetizations. Latitudinal drift of North America is computed for Sacajawea Peak in Oregon, USA near the localities for which the sense of shear was inferred by Zak et al (2016).…”

Role of inherited structure on granite emplacement: An example from the Late Jurassic Shibei pluton in the Wuyishan area (South China) and its tectonic implications

“…Understanding how magma is emplaced (such as by sills, dykes or diapirs) is crucial for characterizing pluton construction processes and for deciphering the interplay between magmatism and crustal structures and thus regional tectonic context (e.g., Pitcher, 1979;Iyer, 1984;Parterson et al, 1989;Borradaile and Kehlenbeck, 1996;Ferré et al, 1997;Maes et al, 2007;Žák et al, 2017). Through detailed studies on plutons and their country rocks, including petrology, petro-fabrics, geochronology, geometry and numerical modelling studies, several emplacement hypotheses were proposed, e.g., diapirism (Roberts, 1970;Dixon, 1975;Cruden, 1990; Paterson and Vernon, 1995;Polyansky et al, 2010), ballooning (Bateman, 1984;Brown and McClelland, 2000;Schoene et al, 2012), dyke and sill models (Rubin, 1993;Weinberg, 1999;Burchardt, 2008;Mathieu et al, 2008;Gudmundsson, 2011;Schofield et al, 2012), laccolith and lopolith (Corry, 1988;Scaillet et al, 1995;Morgan and McGovern, 2005;de Saint-Blanquat et al, 2006;Roman et al, 2016), stoping (Daly, 1903;Glazner, 2006Glazner, , 2007Paterson, 2008;Huber et al, 2011) and syn-tectonic emplacement (Pitcher, 1979;Hutton, 1988;Tikoff et al, 1999;Allibon et al, 2011;de Saint-Blanquat et al, 2011;…”