Moho Structure Across the Backarc‐Craton Transition in the Northern U.S. Cordillera

Abstract

In southern Canada and the northern United States, the present-day western edge of the North American craton (Laurentia) is marked by a profound change in lithospheric thickness, from <100 km within a hot backarc setting beneath the Cordillera to >200 km beneath the craton. A nearly uniformly flat Moho in the backarc region of southern Canada has been interpreted as a young feature that was produced by lower crustal channel flow. The tectonic environment of the southern Canadian Cordillera extends southward to the Lewis and Clark shear zone in the northern United States, where previously deployed EarthScope Transportable Array stations provide coverage on a ≈70 km grid. Here, data from Transportable Array stations are used to investigate teleseismic P to S receiver functions of crustal structure across the craton-backarc transition, coupled with lithospheric structure derived from a recent global tomographic model. These images suggest that, north of the Lewis and Clark shear zone, the shallower Moho beneath the Cordillera has encroached on the craton, creating a narrow complex transition zone. To the south, a step-like change in crustal thickness is observed, and the complex Moho transition zone is absent. Taken together with other geological and geophysical data, these observations suggest that a young Moho has progressed eastward accompanied by thermal and/or mechanical modification of the Laurentian lithosphere. Plain Language Summary We observe an interesting deep crustal and lithospheric feature in the north western United States. In the crust it appears like a complex or double layer at the crust and mantle interface. Deeper in the mantle lithosphere, there appears to be a lobate west dipping anomaly. Since this feature aligns with heat flow indicators suggesting it occurs at the edge of a Cordilleran wide high heat flow, we propose the formation of this feature is related to the high heat flow. We propose this crust and lithospheric feature is relatively young and that it is the relict North American lithosphere being eroded back by a combination of mantle flow and heat diffusion at the edge of the ancient Northern American margin.

Wavefield Migration Imaging of Moho Geometry and Upper Mantle Structure Beneath Southern New England

“…A crustal thickness difference between Laurentia and the Appalachian accreted terranes has been observed in several large‐scale seismic studies in the central and northern Appalachians (e.g., Li et al., 2018; Shen & Ritzwoller, 2016). Distinct Moho offsets or steps have also been suggested in other tectonic settings, such as the Northern US Cordillera (DiCaprio et al., 2020), the Tibetan Plateau (Zhu & Helmberger, 1998), and New Zealand (Salmon et al., 2011), and attributed to various causes. A natural explanation for the observed Moho offset is that it results from the Paleozoic juxtaposition of crustal blocks (Laurentian Grenville crust to the west and Appalachian crust to the east) with preexisting crustal thickness differences.…”

The Structure and Dynamics of the Uppermost Mantle of Southwestern Canada From a Joint Analysis of Geophysical Observations

“…Lithosphere instability in this region has been suggested to occur through both the progressive erosion of the craton margin (Hardebol et al, 2012) and through episodic delamination (Bao et al, 2014). Ongoing erosion and thinning of the lithosphere has been proposed to be occurring today immediately south of our study area (DiCaprio et al, 2020). Geodynamic models that link mantle dynamics with geophysical and magma observations are needed to assess the implications for the cordillera-craton boundary evolution in southwestern Canada.…”

The Thermal Regime of NW Canada and Alaska, and Tectonic and Seismicity Consequences

“…In the well‐studied example of western USA, the crust is mostly thin, average ∼33 km, in the high elevation Cordillera, in contrast to thicker crust averaging ∼40 km, in the low elevation stable continent to the east (e.g., Chen et al., 2018; DiCaprio et al., 2020; Hyndman, 2015a; Hyndman & Lewis, 1999). There is no thick crust Cordillera mountain root as expected for simple Airy isostasy (e.g., Chulick & Mooney, 2002; Fischer, 2002; Schmandt et al., 2015).…”