Seismic Evidence for Lithospheric Thinning and Heat in the northern Canadian Cordillera

Abstract

Geophysical and geological data indicate that the lithosphere in the northern Canadian Cordillera (NCC) is thin and hot. However, lack of direct constraints on mantle structure is fueling debates on the origin and nature of this lithosphere. Here we image the crust and uppermost mantle beneath the NCC and resolve the Moho at ~35‐km depth, the lithosphere‐asthenosphere boundary ~15 km deeper, and a westward dipping structure in the mantle below the eastern NCC. We examine end‐member tectonic models where the thin lithosphere is either long‐lived or has been rejuvenated by gravitational removal. In the long‐lived model, steady state thermal calculations indicate a Moho at ~1,000 °C; alternatively, time‐dependent calculations suggest that a removal event 5–25 Myr ago reproduces the seismic constraints. Combined with geologic evidence, these results suggest that the mantle lithosphere beneath the NCC is either exotic or the result of progressive thickening and cooling since the Miocene.

Hygrometric Control on the Lithosphere‐Asthenosphere Boundary: A 28 Million Year Record From the Canadian Cordillera

“…Shallow depths of ∼45–50 km are from Summit Lake in eastern BC (the oldest xenolith‐bearing lava at 28 Ma) and in far younger lavas (<0.2 Ma) in Yukon. The latter are consistent with a LAB of 50 ± 5 km estimated in Yukon by a seismic receiver function survey (Audet et al., 2019), giving credence to our aSiO 2 barometry method.…”

“…Most mantle lithosphere as evidenced by xenoliths in the CC and many other hot back arcs or mobile belts is notably fertile (Canil, 2004;Pearson et al, 2005, Supporting Information). Nevertheless, depleted harzburgite xenolith populations are significant at a few centers in the CC, notably in the same regions of Yukon having lower lava equilibration depths (Figure 2b) and a shallower seismic LAB at <55 km (Audet et al, 2019). This observation for CC peridotite xenolith suites has been noted previously (Francis et al, 2010;Shi et al, 1998) and we surmise some portion of harzburgitic lithosphere concentrated beneath these locations, could under a given Tp and H 2 O content, promote shallower melting and LAB by the H 2 O storage capacity limitation described above.…”

The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography

“…Another regional teleseismic body wave tomography study imaged the same high-velocity anomaly in northern British Columbia beneath the eastern branch of the CANOE network at 100-km depth (see Figures 8 and 9, Mercier et al, 2009), which was interpreted as cratonic lithosphere. This is also shown in Audet et al, (2019), where the thick fast lithosphere of the Canadian Shield extends beneath the northern Rocky Mountains.…”

“…A variety of geophysical studies indicate that the NCC is underlain by a flat (Cook et al, 2004;Cook & Erdmer, 2005) and shallow Moho at 32 ±2-km depth (Tarayoun et al, 2017) and crust with a similarly shallow Curie depth (Gaudreau et al, 2019). The region has high heat flow (≥100 mW/m 2 ; Lewis et al, 2003), high temperatures at the Moho (∼900 • C) in contrast with ∼400 • C in the Canadian Shield (Audet et al, 2019;Lewis et al, 2003), and <20-km effective elastic thickness (Audet et al, 2007;Flück et al, 2003). Several magnetotelluric (MT) studies have been conducted over the NCC, in particular spanning the Tintina Fault (i.e., Ledo et al, 2002Ledo et al, , 2004Jones et al, 2005) that reveal complex resistivity variations throughout the NCC.…”

Geophysical and Geochemical Constraints on Neogene‐Recent Volcanism in the North American Cordillera

“…However, on several profiles, there are thinner receiver function LAB depths in the northern Canadian Cordillera, of the Yukon/Northwest Territories/northern British Columbia. There the profiles are interpreted to indicate surprisingly shallow, 50 ± 5 km LAB depths that extend across the whole Cordillera to near the eastern mountain thrust front (Audet et al., 2019; Tarayoun et al., 2017). The shallow LAB is consistent with the heat flow in the northern Cordillera which is unusually high (Flück et al., 2003; Lewis, Hyndman, & Flück, 2003).…”