Intra-oceanic subduction shaped the assembly of Cordilleran North America

Abstract

The western quarter of North America consists of accreted terranes--crustal blocks added over the past 200 million years--but the reason for this is unclear. The widely accepted explanation posits that the oceanic Farallon plate acted as a conveyor belt, sweeping terranes into the continental margin while subducting under it. Here we show that this hypothesis, which fails to explain many terrane complexities, is also inconsistent with new tomographic images of lower-mantle slabs, and with their locations relative to plate reconstructions. We offer a reinterpretation of North American palaeogeography and test it quantitatively: collision events are clearly recorded by slab geometry, and can be time calibrated and reconciled with plate reconstructions and surface geology. The seas west of Cretaceous North America must have resembled today's western Pacific, strung with island arcs. All proto-Pacific plates initially subducted into almost stationary, intra-oceanic trenches, and accumulated below as massive vertical slab walls. Above the slabs, long-lived volcanic archipelagos and subduction complexes grew. Crustal accretion occurred when North America overrode the archipelagos, causing major episodes of Cordilleran mountain building.

Geology, Mantle Tomography, and Inclination Corrected Paleogeographic Trajectories Support Westward Subduction During Cretaceous Orogenesis in the North American Cordillera

“…extensive eastern slab forms a vertical wall within the mantle from about 800 to at least 1800 km depth (Grand et al 1997;Sigloch and Mihalynuk 2013). During the opening of the Atlantic Ocean, at least during the JurassicEarly Cretaceous periods, North America was moving westward from a more or less fixed Africa (Coney 1971;Torsvik et al 2008a;Steinberger and Torsvik 2008;Seton et al 2012).…”

“…As pointed out by Sigloch and Mihalynuk (2013), a vertical slab wall effectively fixes the position of the trench when active, and because its upward truncation represents the collision and related slab failure, the correct paleogeographic model should match the collision zone with the slab wall at the appropriate time as deduced from geological arguments. One can use Gplates (Boyden et al 2011;Gurnis et al 2012;Williams et al 2012) as did Sigloch and Mihalynuk (2013) to compare the location of the eastern vertical slab with the predicted paleogeographic trajectories for North America from 140 Ma to present as deduced from the five separate models of Shephard et al (2012): (1) a hybrid hotspot model, which uses moving hotspots in both the Atlantic and Indian oceans from 100 Ma to the present (O'Neill et al 2005), and fixed hotspots for older ages (Müller et al 1993); (2) a fixed hotspot reference frame model (Müller et al 1993); (3) framework that also assumes no longitudinal motion of Africa (Steinberger and Torsvik 2008;Seton et al 2012); and (5) an entirely different type of model that matches subducted slabs with subduction zones (van der Meer et al 2010).…”

“…The largest of these anomalies is a steeply inclined slab wall in the transition zone and lower mantle that extends for over 40º of latitude in eastern North America (Sigloch 2011;Sigloch and Mihalynuk 2013). Another 'slab' farther west and as deep as 1800 km rises westward to connect with the Cascadia subduction and thus represents the Farallon slab at depth, so the separate eastern slab cannot be the Farallon slab (Sigloch et T a n T T a n a n a a n n a a n a n a a e S e (Hart et al 2004a, b;Rasmussen 2013) interpreted to result from failure of the west-dipping North American oceanic plate.…”

“…With improvements in computing power and high-resolution seismic arrays, such as USArray (Williams et al 2010), these advances have now reached a level where they can be integrated with geological models based on surface geology (Sigloch 2011;Sigloch and Mihalynuk 2013). Moreover, the seismic tomography can be used in consort with recently developed kinematic models for plate motions in deep-mantle reference frames based on seafloor-spreading history, hotspot migration, paleomagnetism, and moving continents (Müller et al 1993;O'Neill et al 2005;Torsvik et al 2008a, b;Doubrovine et al 2012;Shephard et al 2012;Seton et al 2012) to constrain the tectonic development of orogenic belts.…”

“…One can use Gplates (Boyden et al 2011;Gurnis et al 2012;Williams et al 2012) as did Sigloch and Mihalynuk (2013) to compare the location of the eastern vertical slab with the predicted paleogeographic trajectories for North America from 140 Ma to present as deduced from the five separate models of Shephard et al (2012): (1) a hybrid hotspot model, which uses moving hotspots in both the Atlantic and Indian oceans from 100 Ma to the present (O'Neill et al 2005), and fixed hotspots for older ages (Müller et al 1993); (2) a fixed hotspot reference frame model (Müller et al 1993); (3) framework that also assumes no longitudinal motion of Africa (Steinberger and Torsvik 2008;Seton et al 2012); and (5) an entirely different type of model that matches subducted slabs with subduction zones (van der Meer et al 2010). Uncertainty pervades nearly every aspect of the reconstructions, including possible errors in properly locating and dating hotspots, the shape of the North American margin, error envelopes in paleomagnetic pole locations and ages, errors in assigning movement to hotspots, and in the case of the subduction model, mistakes in matching mantle anomalies with subduction zones once the subducting slab has broken.…”