The teleseismic signature of fossil subduction: Northwestern Canada

Abstract

broadband, three-component seismometers were deployed along the MacKenzie-Liard Highway in Canada's Northwest Territories as part of the joint Lithoprobe-IRIS Canada Northwest Experiment (CANOE). These stations traverse a paleo-Proterozoic suture and subduction zone that has been previously documented to mantle depths using seismic reflection profiling. Teleseismic receiver functions computed from $250 earthquakes clearly reveal the response of the ancient subduction zone. On the radial component, the suture is evident as a direct conversion from the Moho, the depth of which increases from $30 km to $50 km over a horizontal distance of $70 km before its signature disappears. The structure is still better defined on the transverse component where the Moho appears as the upper boundary of a 10 km thick layer of anisotropy that can be traced from 30 km to at least 90 km depth. The seismic response of this layer is characterized by a frequency dependence that can be modeled by upper and lower boundaries that are discontinuous in material properties and their gradients, respectively. Anisotropy can be characterized by a ±5% variation in shear velocity and hexagonal symmetry with a fast axis that plunges at an oblique angle to the subduction plane. The identification of this structure provides an unambiguous connection between fossil subduction and fine-scale, anisotropic mantle layering. Previous documentation of similar layering below the adjacent Slave province and from a range of Precambrian terranes across the globe provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction.

A Paleoproterozoic Andean-type iron oxide copper-gold environment, the Great Bear magmatic zone, Northwest Canada

“…These include deep-seismic reflection profiling (Cook et al, 1999;Cook and Erdmer, 2005;Oueity and Clowes, 2010), magnetotelluric experiments (Wu et al, 2005;Spratt et al, 2009), and teleseismic data acquisition (Bostock, 1998;Mercier et al, 2008;Snyder et al, 2014). The 2D geophysical data in those studies collectively support that there is a stacked east-dipping lithospheric mantle beneath this region, related to subducted and frozen oceanic crust (e.g., Cook et al, 1999).…”

“…The double stacked, north and north-northwest striking surfaces beneath the Great Bear and Slave craton are best interpreted as stalled subducted slabs (e.g., Cook et al, 1999;Cook and Erdmer, 2005;Wu et al, 2005;Mercier et al, 2008;Oueity and Clowes, 2010;Snyder et al, 2014). We interpret the upper surface as a relic related to the older Hottah arc and the lower related to Great Bear magmatism; both are interpreted to be derived from the cryptic Nahanni terrane oceanic crust .…”

CrustalVSstructure in northwestern Canada: Imaging the Cordillera-craton transition with ambient noise tomography

“…Recent seismic images of the upper mantle across northwestern Canada have been obtained from teleseismic body waves [e.g., Mercier et al, 2008Mercier et al, , 2009] and surface waves [Frederiksen et al, 2001; van der Lee and Frederiksen, 2005;Nettles and Dziewonski, 2008]. These studies suggest that the upper mantle structure is dominated by a substantial temperature contrast between the high-temperature, tectonically young Cordillera and the lower-temperature ancient craton; the boundary between these upper mantle domains roughly corresponds to the deformation front (Figure 15).…”

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

“…MLDs are continuous with neighbouring, younger tectonic areas (Western Canada, USA, Australia; Mercier et al, 2008;Ford et al, 2010;Foster et al, 2014), whereas in others, they are truncated, indicating translithospheric boundaries that depict terrane sutures at depth (Courtier et al, 2010) (Fig. 2C).…”