**4. Tectonic Implications**

The direct implications of sills within sedimentary basins have been addressed in recent papers by Magee et al. [29], Schofield [106], as well as elsewhere in this volume [107,108]. The influence of deeper intrusives may be less obvious. Haxby et al. [109] modeled the e ffect of a high-density intrusion into the lower crust as a driver for basin formation, though this involved massive underplating rather than intrabasement sill emplacement. The deep sills cited in this paper, which appear as relatively thin layers, may seem volumetrically small compared to other plutonic manifestations (e.g., batholiths), however the more extensive examples, such as the Winagami reflector or the COCORP midcontinent basement layering, sugges<sup>t</sup> thermal perturbation over very large areas. The potential link of these two examples with the McKenzie and Keweenawan events, respectively, implies that they may serve as fingerprints of distant mantle plumes in a manner similar to the better known exposed dyke swarms [110].

The form of sill reflectivity—e.g., distinct reflections or reflection sequences vs. finely laminated zones—may be a proxy for crustal rheology and an indicator of the stress regime at the time of their emplacement [91,111,112].. The presentation of basement sills as saucers versus planar geometries is another clue to the emplacement mechanics [27]. The observation that many of these reflectors appear largely undeformed by subsequent tectonic events places constraints on the post-intrusive evolution of the crust—e.g., the lack of substantial deformation. The limited and relatively biased sampling of the crust represented by modern deep reflection profiling undermines any substantive generalization about the age distribution of observed sills, although many of the 'frozen" examples shown here either occur in Proterozoic crust or are believed to be Proterozoic in age.
