**5. Conclusions**

Crustal seismic reflection profiles have revealed anomalously strong reflectors at numerous sites around the world. Seismic "bright spots" in tectonically active regions have been frequently attributed to fluid magma at depth, most often associated with modern extensional regimes. Some of the most distinctive reflections have been found within Precambrian basement and presumed to be "frozen" magma, most likely mafic since silicic intrusions are unlikely to provide a su fficient reflection contrast with host rock. The drilling of prominent basement reflectors near Siljan, Sweden, has confirmed them to be Proterozoic diabase sills. The correlation of similar strong reflection sequences in the southwestern US with adjacent outcrops of mafic intrusions strongly corroborates their interpretation as mafic sills as well. Extensive prominent reflections in northwestern Canada are likely to be buried manifestations of the McKenzie Dike swarm. More speculatively, an extensive layered sequence of strong reflections in the upper crust on COCORP seismic profiles that stretch from eastern New Mexico to central Ohio may be related to a distant Keweenawan mantle plume based on correlation with ultramafic rocks encountered in an oil industry borehole in west Texas. Both of these examples detail the long distance lateral transport of magma in brittle regimes. As "fingerprints" of major thermal events (plumes?), indicators of crustal rheology and stress during emplacement, and markers for post-emplacement deformation, sill reflections in the continental basement o ffer new constraints for models of crust and basin evolution.

**Author Contributions:** D.K. contributed the results for the US mid-continent and their analogy to the Canadian observations. L.D.B. provided the examples of magma bright spots and the colds sills outside North America as well as the concluding discussion. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
