Abstract
AbstractThe configuration of mid-ocean ridges subducted below North America prior to Oligocene time is unconstrained by seafloor isochrons and has been primarily inferred from upper-plate geology, including near-trench magmatism. However, many tectonic models are permitted from these constraints. We present a fully kinematic, plate tectonic reconstruction of the NW Cordillera since 60 Ma built by structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface. We map in three-dimensions the attached Alaska and Cascadia slabs, and a detached slab below western Yukon (Canada) at 400–600 km depth that we call the “Yukon Slab.” Our restoration of these lower plates within a global plate model indicates the Alaska slab accounts for Pacific-Kula subduction since ca. 60 Ma below the Aleutian Islands whereas the Cascadia slab accounts for Farallon subduction since at least ca. 75 Ma below southern California, USA. However, intermediate areas show two reconstruction gaps that persist until 40 Ma. We show that these reconstruction gaps correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that bound an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate. Our reconstructions support a “northern option” Farallon ridge geometry and allow up to ∼1200 km Chugach terrane translation since Paleocene time, providing a new “tomographic piercing point” for the Baja-British Columbia debate.
Highlights
Subduction of mid-ocean ridges is an important control on magmatic patterns, arc geochemistry, uplift, and deformation along the North American Cordillera (Dickinson and Snyder, 1979; Thorkelson and Taylor, 1989; Severinghaus and Atwater, 1990; Sisson et al, 2003b; Thorkelson et al, 2011; Eddy et al, 2016)
We address these topics by presenting a new, fully kinematic unfolded-slab plate tectonic reconstruction of the Paleocene to present NW Cordillera that constrains the positions of these cryptic mid-ocean ridges
The orientation and number of mid-ocean ridges subducted below North America prior to Oligocene time has been obscured by subduction
Summary
Subduction of mid-ocean ridges is an important control on magmatic patterns, arc geochemistry, uplift, and deformation along the North American Cordillera (Dickinson and Snyder, 1979; Thorkelson and Taylor, 1989; Severinghaus and Atwater, 1990; Sisson et al, 2003b; Thorkelson et al, 2011; Eddy et al, 2016). Possible ridge-trench intersections in these models have been inferred from preserved Pacific plate seafloor magnetic anomalies (Engebretson et al, 1985), near-trench magmatism (Bradley et al, 2003; Haeussler et al, 2003; Madsen et al, 2006), inboard magmatism (Breitsprecher et al, 2003; Cole et al, 2006; Ickert et al, 2009), and deformation (Eddy et al, 2016) (Fig. 1) Improved constraints on these ancient ridge-trench intersections is important for North American tectonics and for reconstructing past geometries and kinematics of the partially subducted Kula and Farallon oceanic plates
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