Seismic properties of North America and the surrounding ocean basins

Gary Chulick, Walter D. Mooney and Shane Detweiler - U.S. Geological Survey, Menlo Park, CA

We present new maps of the seismic properties of North America and the surrounding ocean basins. Figure 1 shows the location of data that has been collected for this study. Our crustal thickness map for North America and the surrounding ocean basins was constructed using over 1700 data points (dots), of which more than 1300 are designated continental crust. Figure 2 shows the types of geologic provinces found in North America. These may be compared to the underlying crustal thickness map also shown in Figure 2. Figure 2 was derived from a comprehensive compilation of seismic refraction and reflection data, earthquake studies, and surface wave analyses. One feature of this map is an extension of the thin crust of the Basin and Range Province into Western Canada. The contour interval is 10 km; in addition, we include the 45 km contour (dashed line).

Figure 1 shows the location of data that has been collected for this study. Data file containing point locations Explanantion to the data file

Many well-known features are reproduced here, such as the thin crust of the Basin and Range and the thick crust of the US Great Plains and Midcontinent. Our map also reveals a number of new features. We find the thickest crust beneath Late Proterozoic regions, not the oldest crust (e.g., Superior Province, central and eastern Canada). The map shows 40+ km thick crust beneath the US Cascades, while thin crust is imaged to the north beneath the Canadian Cordillera.

These features were only hinted at in previously published contour maps (Soller et al., 1982, and Allenby and Schnetzler, 1983). In addition, the extent of the continental shelves along the Pacific and Atlantic coasts are more faithfully reproduced in terms of crustal thickness. The average thickness of the North American continental crust, including margins, is 36.5 km ranges from 14 km to 60 km.

Figure 2 Curstal Thickness in kilometers, derived from a comprehensive compilation of seismic refraction and reflection data, earthquake studies, and surface wave analyses. One feature of this map is an extension of the thin crust of the Basin and Range Province into Western Canada. The contour interval is 10 km; in addition, we include the 45 km contour (dashed line).

Figure 3 presents a map of sub-Moho p-wave velocity (Pn). The average crustal p-wave velocity (Pc) of North America including the sediments is shown in Figure 4. Figure 5 presents the average p-wave velocity of the consolidated or crystalline crust (Pcc) without sediments in North America.

Figure 3 is a map of sub-Moho p-wave velocity.

Figure 4 shows the average crustal p-wave velocity (Pc) of North America including the sediments.

Figure 5 shows the average p-wave velocity of the consolidated or crystalline crust (Pcc) without sediments in North America.

We find a close correspondence between high average crustal velocity and accretionary and magmatic orogenies (e.g., Trans-Hudsonian, Grenvillian, Acadian, Appalachian, the Cascades, and the Alaskan Range). In contrast, there are a number of interesting anomalously low average crustal velocities in the Basin and Range Province, the Snake River Region, and the Northwest Territories of Canada. Many of these are regions of active or recent extension or hotspot activity.