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Mantle and Geological Evidence for a Late Jurassic–Cretaceous Suture

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Mantle and Geological Evidence for a Late Jurassic–Cretaceous Suture Mantle and geological evidence for a Late Jurassic–Cretaceous suture spanning North America Mantle and geological evidence for a Late Jurassic–Cretaceous suture spanning North America Karin Sigloch1,† and Mitchell G. Mihalynuk2 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK 2British Columbia Geological Survey, P.O. Box Stn Prov Govt, Victoria, BC, V8W 9N3, Canada ABSTRACT of the archipelago, whereas North America ambient mantle, and seismic waves propagate converged on the archipelago by westward through it at slightly faster velocities. Joint sam- Crustal blocks accreted to North America subduction of an intervening, major ocean, pling of the subsurface by thousands to millions form two major belts that are separated by the Mezcalera-Angayucham Ocean. of crossing wave paths, generated by hundreds a tract of collapsed Jurassic–Cretaceous The most conspicuous geologic prediction or thousands of earthquakes, enables computa- basins extending from Alaska to Mexico. is that of an oceanic suture that must run tion of three-dimensional (3-D) tomographic Evidence of oceanic lithosphere that once along the entire western margin of North images of the whole-mantle structure. Some underlay these basins is rare at Earth’s sur- America. It formed diachronously between high-velocity domains connect upward to active face. Most of the lithosphere was subducted, ca. 155 Ma and ca. 50 Ma, analogous to subduction zones, providing a direct verification which accounts for the general difficulty of diachro nous suturing of southwest Pacific arcs of slab origin as cold, dense, and seismically fast reconstructing oceanic regions from sur- to the northward-migrating Australian conti- oceanic lithosphere (e.g., for North America, face evidence. However, this seafloor was nent today. We proceed to demonstrate that see Grand et al., 1997). not destroyed; it remains in the mantle be- this suture prediction fits the spatio-temporal It should be possible to reconcile the sub- neath North America and is visible to seis- evidence for the collapse of at least 11 Middle surface record of subduction, as imaged by mic tomography, revealing configurations Jurassic to Late Cretaceous basins wedged geophysics, with the geological record of ac- of arc-trench positions back to the breakup between the Intermontane and Insular-Guer- cretionary orogens—an integration we refer of Pangea. The double uncertainty of where rero superterranes, about half of which are to as “tomotectonic” analysis. For Jurassic– trenches ran and how subducting lithosphere known to contain mantle rocks. These rela- Cretaceous North America, the prevailing in- deformed while sinking in the mantle is sur- tively late suturing ages run counter to the terpretation of the land and marine surface rec- mountable, owing to the presence of a spe- Middle Jurassic or older timing required and ords is that of eastward subduction of a single cial-case slab geometry. Wall-like, linear slab asserted by the prevailing, Andean-analogue large oceanic plate, the Farallon plate, beneath belts exceeding 10,000 km in length appear to model for the North American Cordillera. We the western continental margin since at least trace out intra-oceanic subduction zones that show that the arguments against late sutur- 180 Ma (Hamilton, 1969; Ernst, 1970; Burch- were stationary over tens of millions of years, ing are controvertible, and we present mul- fiel and Davis, 1972, 1975; Monger et al., and beneath which lithosphere sank almost tiple lines of direct evidence for late suturing, 1972; Monger and Price, 1979). This scenario vertically. This hypothesis sets up an absolute consistent with geophysical observations. We is analogous to today’s Nazca plate subducting lower-mantle reference frame. Combined refer to our close integration of surface and beneath the Andean margin of South America. It with a complete Atlantic spreading record subsurface evidence from geology and geo- has been challenged by a minority of geologists that positions paleo–North America in this physics as “tomotectonic analysis.” This type who instead argued that Mesozoic North Amer- reference frame, the slab geometries per- of analysis provides a stringent test for cur- ica overrode and accreted a huge archipelago mit detailed predictions of where and when rently accepted tectonic models and offers a of intra-oceanic trenches and their arc terranes ocean basins at the leading edge of westward- blueprint for similar, continental-scale inves- (Moores, 1970, 1998; Schweickert and Cowan, drifting North America were subducted, how tigations in other accretionary orogens. 1975; Ingersoll and Schweickert, 1986; Inger- intra-oceanic subduction zones were over- soll, 2008; Dickinson, 2004, 2008), which im- ridden, and how their associated arcs and 1. INTRODUCTION plies eastward and westward subduction of more basement terranes were sutured to the con- than one plate. The controversy was revived by tinent. An unconventional paleogeography Subduction of ocean basins leaves two kinds Johnston (2001, 2008) and Hildebrand (2009, is predicted in which mid- to late Mesozoic of records. At the surface, it builds accretion- 2012), who focused on westward subduction arcs grew in a long-lived archipelago lo- ary orogens from crustal slivers that grew or beneath a ribbon continent that included peri- cated 2000–4000 km west of Pangean North fused above subduction zones, for example, cratonic terranes. America (while also consistent with the con- arcs, accretionary wedges, and ophiolites. In the Magnetic isochrons on the Pacific plate leave ventional view of a continental arc in early mantle, subducted oceanic lithosphere loses its no doubt that the Farallon plate existed and Mesozoic times). The Farallon Ocean sub- rigidity and becomes part of a viscously deform- grew to the (north-)east of the Pacific plate since ducted beneath the outboard (western) edge ing slab that sinks toward Earth’s core under the 180+ Ma (Engebretson et al., 1985; Atwater, pull of gravity. A sinking slab remains visible 1989; Seton et al., 2012). Quantitative plate †karin .sigloch@ earth .ox .ac.uk to seismic tomography because it is cooler than reconstructions (e.g., Engebretson et al., 1985; GSA Bulletin; November/December 2017; v. 129; no. 11/12; p. 1489–1520; doi: 10.1130/B31529.1; 7 figures; 2 tables; Data Repository item 2017185; published online 30 June 2017. Geological Society of America Bulletin, v. 129, no. 11/12 1489 © 2017 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Sigloch and Mihalynuk Seton et al., 2012) are based on these data, but Figure 1. Evolution of the north- A because isochrons reflect seafloor spreading ern Pacific basin as constrained rather than subduction, they cannot resolve the by seafloor magnetic isochron 60 ambiguity of Andean- versus archipelago-style data, modified from Enge- paleogeography. This is illustrated by Figure 1, bretson et al. (1985). (A) Early where preserved constraints on the Farallon-Pa- Cretaceous (140 Ma) recon- cific spreading ridge are consistent with either struction of continents and oce- FAR a margin-hugging Farallon trench (Fig. 1A) or anic plates, in a fixed hotspot an intra-oceanic Farallon trench far outboard in (i.e., lower-mantle) reference IZA the proto–Pacific basin (Fig. 1B), which would frame. Unfilled gray coastlines 0 have been overridden and “accreted” to the show present-day positions of PAC west coast late in its life. This ambiguity was the Americas for reference. On 180 270 clearly pointed out by early workers in marine the nascent Pacific plate (PAC), west-northwest–striking iso- geophysics (Engebretson et al., 1985; Atwater, B 1970, 1989), but they adopted the Andean-type chrons record the existence of trench configuration that has since prevailed in a spreading ridge and hence 60 the geologic community (Fig. 1A). of the adjacent Farallon plate N (FAR). The existence of the E The model of unchanging Farallon-beneath- G continent subduction has the appeal of continu- Izanagi (IZA) plate is similarly O ity and simplicity. The Farallon plate subducts recorded by ENE-striking iso- T FAR I eastward into the continental Cascadia trench chrons on PAC. Blank areas of A today (Fig. 1C), and there was also eastward seafloor have since subducted. IZA B Eastward-dipping trench (gray L subduction beneath the southwest coast of the 0 E United States prior to ca. 170 Ma, which built barbs) along the American west the so-called “native Triassic–Jurassic arc” on coast marks the eastern edge of PAC 180 270 cratonic basement (e.g., Asmerom et al., 1990; the Farallon plate, as implied Dickinson, 2008; Barth et al., 2011; Saleeby by the standard “Andean-ana- C and Dunne, 2015), also called “Nazas arc” in logue” model. Paleolocation of North America (and of its Mexico (Dickinson and Lawton, 2001). By 60 equating the eastward-spreading Farallon plate, subsequent westward drift) is which existed since at least 180 Ma, with the constrained by the fully pre- eastward subducting plate that generated the na- served isochron record of the FAR tive arc, the Andean-analogue model interpolates central Atlantic. (B) Same as that Farallon subduction beneath the continent in A, but with a more westerly, would also have operated at all intermediate intra-oceanic Farallon trench PAC times. For the Canadian segment of the Cordi - (black barbs). Isochron data 0 llera since ~175 m.y., this scenario was espoused are equally compatible with by van der Heyden (1992) and many followers, this scenario. The area labeled and recently reasserted in a review by Monger “NEGOTIABLE” remains un- 180 270 (2014). For the U.S. segment, Andean-style constrained by marine surface observations and hosted the “archipelago” of island arcs that subduction has been asserted in a large body of we interpret from observed geometries of subducted seafloor. (C) Pacific basin at present day literature (e.g., Hamilton, 1969; Burchfiel et al., (0 Ma), showing all PAC-FAR and PAC-IZA isochrons preserved on the PAC plate.
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