2.2.2. Identification via Outcrop

The Siljan reflections are relatively unique in terms of being tested by drilling. However, outcrop correlations have also been used to infer mafic sills as being responsible for similar reflection sequences. Surveys by the Consortium for Continental Reflection Profiling (COCORP) in central Arizona revealed a relatively thick suite of strong reflections in the upper crust (A in Figure 5) very similar to the Siljan seismic images (Figures 1 and 5 [57]). The seismic modeling of mafic sills exposed in basement outcrops approximately 20 km southeast of Line 3 was found to closely reproduce the character of the reflections seen on the seismic sections, bolstering their interpretation as arising from the subsurface extension of the outcropping cold diabase intrusions. This correlation is significant because the tectonic setting in Arizona would also be consistent with the presence of fluid magma in the subsurface. The deeper reflection C might be a candidate for hot magma, a suggestion based primarily on its discordance with sequence A and similarity in depth to the Socorro Magma body.

**Figure 5.** Prominent layered reflections (A) traced by the COCORP seismic reflection profiles in central Arizona [57]. Seismic modeling of mafic intrusions exposed nearby, together with the similarity in reflection character to the Siljan Ring reflectors, supports the interpretation of this sequence as also due to mafic intrusion [58]. The deeper reflections (C, B) may mark still-fluid magma. M indicates possible Moho reflections.

### 2.2.3. Extensive Sills in the Canadian Craton: Relicts of a Proterozoic Plume?

Among the most distinctive sill-like reflections, at least in terms of their observed extent, are a series of reflectors traced by the seismic surveys collected by the LITHOPROBE deep seismic program in Alberta and Saskatchewan, northwest Canada (Figures 1 and 6). The Winagami Reflection Sequence, first reported by Ross and Eaton [59], is a set of distinctive reflections that bears clear similarities to the Siljan reflections. Cross-cutting relationships between the Winagami reflections and weaker dipping reflections associated with dated tectonic events support an intrusive origin and sugges<sup>t</sup> that the Winagami sequence was emplaced by a thermal event between 1.760 and 1.890 GA [59]. Originally estimated to extend beneath 120,000 km<sup>2</sup> of Paleoproterozoic basement, subsequent profiling reported by Mandler and Clowes [60] traced comparable reflections (the Head-Smashed In sequence) over an additional 6000 km<sup>2</sup> in similar tectonic terrane further south

**Figure 6.** (**Top**) the Winagami reflection sequence revealed by LITHOPROBE reflection profiling in Alberta, NW Canada [59]. (**Bottom**) presumably correlative northward extension of the Winagami reflectors mapped by LITHOPROBE 3D seismic profiling [61].

Even more distinctive than the Winagami Sequence is the Wollaston Lake reflector (Figures 1 and 7 [62]), traced by the LITHOPROBE reflection profiles in the Trans-Hudson hinterland of Saskatchewan, approximately 500 km northeast of the Winagami surveys. The Wollaston Lake Reflector presents as a distinct narrow band of reflections that can be traced for over 160 km. Mandler and Clowes [62] associate the Wollaston Lake reflector with the 1.265 Ga McKenzie thermal event, which is associated with well-known outcroppings of diabase dykes [63]. This is substantially younger than the inferred age of the Winagami sequence and thus implies two distinct thermal events, both with extensive plutonic injections. While the long-distance lateral transport of magma has been documented in outcropping sills and dikes [7,63,64] the continuity of individual sills like the Wollaston Lake over such large distances is made most evident by these seismic images.

**Figure 7.** The Wollaston Lake reflector, imaged by LITHOPROBE seismic reflection profiling in Saskatchewan, NW Canada [62]. This feature can be traced as a distinct narrow band of reflections for 160 km within the Early Proterozoic Trans-Hudson hinterland. Based on its reflection character and tectonic context, it has been interpreted as a diabase sill associated with the 1.27 Ga McKenzie igneous event.

### 2.2.4. Basement Layering across the Central US: Fingerprints from the Keweenawan Plume?

The very first COCORP profiles were carried out in Hardeman County, northern Texas (Figure 1 [65]. The most notable discovery of those surveys was a distinctive sequence of subhorizontal reflections in the uppermost crystalline basement. These reflections were traced by subsequent COCORP surveys well into southern Oklahoma, where they are abruptly truncated by the Wichita Uplift (Figure 8). The extensive, layered nature of these reflections was initially interpreted to sugges<sup>t</sup> a Proterozoic sedimentary or metasedimentary origin [66]. A supracrustal interpretation was reinforced by the observation of similarly layered reflections on COCORP seismic reflection profiling in Indiana, Illinois, and Ohio (Figures 1 and 9 [67]) and oil industry data in eastern New Mexico (Figures 1 and 10 [68,69]). The vast extent implied by correlating these layers from eastern New Mexico to central Ohio (Figure 11) is consistent with a depositional origin, although the apparent spatial correlation of these layers with the 1.5 MYA Granite-Rhyolite province [70] sugges<sup>t</sup> the possibility that volcanic material (rhyolite?) rather than sedimentary rocks is involved [67]. However, the similarity of the Texas reflections to the Siljan images (compare Figure 8 with Figure 5) has continued to raise the question as to whether this midcontinent basement layering is actually a series of diabase sills.

Kim and Brown [69] revisited this issue in their interpretation of basement layering imaged by the reprocessing of 3D oil exploration seismic data in eastern New Mexico. Strong intrabasement reflections were reported from previous work by Adams and Miller [68] on 2D oil industry seismic data located nearby (Figure 10). Both of these papers referenced observations from an oil industry drillhole in southwest Texas that encountered layered ultramafic rocks at depths that seem to correspond to the basement layering in New Mexico (Figure 10 [72]). The ultramafic rocks recovered from the borehole were found to be of Keweenawan age (1.1635 Ga; [71]). Ernst and Buchan [73] describes links between layered ultramafic bodies and large-scale sill/dike intrusions. Kim and Brown ([69]) sugges<sup>t</sup> that the southwest Texas borehole "calibration", the similarity in appearance of the upper basement layering on COCORP seismic data from New Mexico to Ohio, together with their similarity to the Siljan results support their interpretation as mafic sills associated with the Keweenawan plume. If correct, this interpretation indicates lateral injection of Keweenawan magma in the upper crust of the central US on a continental scale (Figure 11). Kim and Brown [69] point out this extent is comparable to the spatial extent of the McKenzie dikes in NW Canada [62] which are presumably from the same plume source as the Wollaston Lake reflector (Figure 11).

**Figure 8.** Layered reflectors traced by COCORP seismic profiles in northern Texas and southern Oklahoma [66]. P-C indicates the top of Precambrian basement from local boreholes. Arrows mark the reflective sequence (A, B, and C), here interpreted as mafic sills. Dashed line indicates the apparent truncation of basement reflections along the Burch fault of the Wichita Mountains.

**Figure 9.** Layered basement reflections from the COCORP reflection profiles in western Ohio [67].

**Figure 10.** (**Left** and **Right**) layered basement reflections from the reprocessing of oil exploration seismic data from east central New Mexico [68,69]. (**Center**) oil industry drilling has identified correlative layered reflectors in southwest Texas as being from layered ultramafic rocks [71]. After Kim and Brown [69].

**Figure 11.** Comparison of the extent of sills, represented by LITHOPROBE seismic lines, and exposed dikes in northwest Canada associated with the Proterozoic McKenzie event with the extent of layered reflectors that appear to be correlative beneath much of the US midcontinent sedimentary rock cover, as represented by COCORP surveys. The lightly shaded area indicates the inferred extent of the Proterozoic Granite-Rhyolite provide [70]. After Kim and Brown [69].

2.2.5. Deep Sills and Ore Deposits—The Iberian Massif

The extensive nature of the aforementioned examples of known and possible sills sugges<sup>t</sup> thermal events of a substantial nature. Both heat and fluid transfer attendant on their emplacement may have been a significant factor in ore development. A possible link between deep sill emplacement and ore deposits is perhaps most strongly suggested by a deep seismic profile in southwestern Spain (Figures 1 and 12). The IBERSEIS seismic profile across the Iberian massif [74,75] revealed a prominent subhorizontal band of strong reflectivity at midcrustal depths (Figure 12). The interpretation of this 140 km-long feature, known as the Iberian Reflective Body (IRB), as a mafic sill is argued from its relatively high reflection amplitudes (20% higher than reflections immediately above and below), its geological setting (exposing late Carboniferous mafic intrusions in an early collision zone), and its correspondence with a relatively high conductivity indicated by MT and high density inferred from gravity data [76]. The IRB also appears to have served as a rheological decollement for structures both above and below [74,76].

**Figure 12.** IBERSEIS deep seismic reflection profile tracing a prominent midcrustal reflection sequence beneath the Iberian Massif [74]. Top: time migrated seismic section. Bottom: line drawing.

The correlation with high conductivity would seem to sugges<sup>t</sup> that fluids (partial melts?) are still present, but the lack of modern magmatism in the geological record argues against such an inference. Carbonell et al. [76] suggests instead that the conductivity correlation is due to the enhancement of connectivity between graphite deposits in the overlying Ossa-Morena Zone during the emplacement of the IRB.

The IRB underlies the Ni-Fe deposits of the Aguablanca Ni-Fe deposits [77] and terminates beneath the massive sulfide deposits of the Iberian Pyrite belt [78] near Rio Tinto. Both of these Late Variscan mineralizations imply a substantial heat source at depth, most likely from mantle-derived mafic intrusions. That the IRB represents the remains of mafic magmatism that provided both the heat and fluids to generate the overlying ore deposits may be speculative, but it is certainly plausible [76]. The tectonic origin of this magmatism is unclear, though Carbonell et al. [76] sugges<sup>t</sup> that is linked to a Carboniferous mantle plume which impacted a large part of northwestern Europe.
