*Article* **Extensive Sills in the Continental Basement from Deep Seismic Reflection Profiling**

**Larry D. Brown 1,\* and Doyeon Kim 2**

1 Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA

2 Department of Geology, University of Maryland, College Park, MD 20742, USA; dk696@cornell.edu

**\*** Correspondence: ldb7@cornell.edu

Received: 28 July 2020; Accepted: 21 October 2020; Published: 10 November 2020

**Abstract:** Crustal seismic reflection profiling has revealed the presence of extensive, coherent reflections with anomalously high amplitudes in the crystalline crust at a number of locations around the world. In areas of active tectonic activity, these seismic "bright spots" have often been interpreted as fluid magma at depth. The focus in this report is high-amplitude reflections that have been identified or inferred to mark interfaces between solid mafic intrusions and felsic to intermediate country rock. These "frozen sills" most commonly appear as thin, subhorizontal sheets at middle to upper crustal depths, several of which can be traced for tens to hundreds of kilometers. Their frequency among seismic profiles sugges<sup>t</sup> that they may be more common than widely realized. These intrusions constrain crustal rheology at the time of their emplacement, represent a significant mode of transfer of mantle material and heat into the crust, and some may constitute fingerprints of distant mantle plumes. These sills may have played important roles in overlying basin evolution and ore deposition.

**Keywords:** crustal sills; hydrocarbon and mineral resources; sedimentary basins
