This project uses continent-wide mapping of a lower crustal layer with high seismic velocity (termed "7.x layer" based on P velocities greater than 7.0 km/s) to answer fundamental questions about the growth of continental crust: 1. Is mafic underplating more common in Proterozoic than stable Archean crust, as previously postulated? 2. Is it associated with rifting and plume tracks? 3. Does magmatic differentiation within the crust generate a mafic lower crust? 4. Does underplating occur during continental collision? The combination of receiver function mapping of a 'double Moho' across the Transportable Array, joint inversions of receiver functions and ambient noise surface waves, previously collected refraction data, lower crustal xenoliths, and spatial and temporal relationships of 7.x layer extent to tectonic provinces is used to map the 7.x layer across the continental U.S. and relate its presence to the time sequence of continental assembly and subsequent tectonic processes.
Nontechnical explanation / broader significance:
The assembly and growth of the North American continental crust is a sequence of episodes of collisions between continental blocks, crustal thickening and extension, and widespread melting. Continental crust is usually composed of material that has much slower seismic wavespeeds (i.e., the velocity with which earthquake waves travel through the material) than the underlying mantle. However, several of the continent-building processes lead to the formation of a lower crustal layer with higher seismic velocities that lie in between typical mantle and crustal values. This project maps the geographic extent of this high-velocity lower crustal layer using seismic data from the continent-wide EarthScope Transportable Array. The extent of the layer relative to geological provinces mapped on the surface and known areas of continental collision, breakup, and crustal melting provides information on the timing and mechanism of emplacement of the layer. Additional information comes from lower crustal rocks that were transported to the surface during volcanic eruptions and from seismic explosion data that were recorded in the past. EarthScope offers the first opportunity to collate all these data across the entire width of a continent in order to answer fundamental questions about how continental crust is built.
