**3. Discussion**

The preceding description of prominent, relative extensive reflections and reflection sequences has emphasized their interpretation as mafic sills. The basis for these interpretations ranges from unequivocal (e.g., Siljan reflections—drilled) to likely (Arizona—outcrop) to speculative (central U.S. seismic character). There are a number of caveats that must be considered in evaluating the interpretations of these and other reported images of deep intrusions. For example, the seismic sections shown in this paper are all 2D. Without 3D control, apparently subhorizontal reflections could just as well be along-strike images of features that are actually dipping at right angles to the line of the section. However, most of the examples here are based on surveys that did include 3D control, either in the form of a local grid of surveys (e.g., Siljan [54], North Texas [65], Great Britain [81]) or as formal three D seismic arrays (e.g., the Winagami reflections sequence [61], layered reflections in eastern New Mexico [69]).

Another consideration is that igneous intrusions may be manifest with different seismic characteristics than that exemplified by the Siljan sequence—i.e., narrow bands of strong reflectivity separated by larger bands of non-reflectivity. For example, Figure 14 (Upper left) shows a sample of BIRPS marine deep seismic reflection profiling data from northwest Britain [88] that indicates a lower crust that is highly layered and strongly reflective, a pattern commonly referred to as Layered Lower Crust (LLC). However, the layering in this case is more of a lamination, without the distinctive separation between reflective units that has been used to associate many of the sill examples in this paper with Siljan. This laminated appearance is characteristic of a number of seismic profiles in western Europe, particularly in areas affected by post-Variscan extension [89]. Based on the strong amplitudes of reflections making up this lamination, Warner [88] argues that they are most likely due to igneous intrusions or the juxtaposition of contrasting metamorphic compositions by pervasive shearing, although this remains a matter of debate [18]. Meissner et al. [90] argue from seismic anisotropy that the LLC develops by ductile processes (extension?) within warm, low-viscosity felsic lower crust with intercalations of mafic intrusions. Numerical modeling by Gerya and Berg [91] illustrates how crustal rheology can control the geometry of mafic intrusion, with a warm lower crust resulting in the lateral spread of magma with coeval viscous deformation. Thus, the different presentations of sills like Siljan/Wollaston vs. the LLC may be due to a contrast in crustal rheology—i.e., hot and ductile for the LLC vs. cold and brittle for the Siljan and its analogous reflectors—modulated by the stress field at the time of emplacement. However, the seismic data from the extended Norwegian continental margin [4,92] sugges<sup>t</sup> that this maybe an over-simplification, as they show relatively continuous, distinct lower crustal sill reflectors (e.g., Figure 14; upper right) that resemble Siljan more than the LLC around Britain.

**Figure 14.** (**Left**) laminated reflections in the lower crust of NW Britain from the BIRPS marine reflection profiling [88]. (**Right**) distinct reflections interpreted as sills in the extended lower crust of the Norwegian continental margin [4]. T is interpreted as a reflection from the top of a high velocity body (HVB; mafic underplating?).

A number of mafic sills in both outcrop and seismic sections from basins exhibit a characteristic "saucer" shape which has fueled recent discussion about intrusion mechanics in sedimentary sequences [4,27,29,93,94]. This distinctive saucer shape has also been reported for intrabasement reflections on a several deep seismic profiles [48,95,96]. Figure 15 shows one particular distinctive example, the Surrency Bright Spot (SBS) encountered during a COCORP survey of the inferred suture zone between Laurasia and Gondwanaland buried beneath the coastal plain sedimentary rocks of southeastern Georgia. The strong amplitude of this spatially limited reflection was originally interpreted to indicate fluid involvement [95] though magma was ruled out due to its location on a long inactive passive continental margin. The saucer shape of the SBS (Figure 15) is now recognized as identical to that documented not only in oil industry 3D seismic data for various sedimentary basins but in notable outcrops of mafic intrusions [27]. The presence of saucer-shaped reflectors deep in the continental basement provides new context to evaluate mechanical models proposed to explain this geometry within sedimentary strata [4,38,93]. Thus the SBS (Surrency Bright Saucer?) is most likely a mafic intrusion, perhaps emplaced during the rifting of the Atlantic margin.

**Figure 15.** (**Left**) Unmigrated COCORP true amplitude section showing the Surrency Bright Spot [95]. (**Upper Right**) Blow up of migrated image of the Surrency Bright Spot [95]). (**Lower Right**) depth contour of the Surrency Bright Spot from the COCORP 3D seismic survey [97]. The "saucer" shape of this basement reflector is comparable to those associated with mafic intrusions imaged by 3D oil industry data and in outcrop [27].

Moho reflections on many deep seismic surveys also exhibit strong amplitudes (e.g., Figure 2; [98]) and/or a layered character [99]. These characteristics have been interpreted to represent "underplating", which may be another aspect of accreting/injecting mafic sills at the base of the crust [47,100].

Not every extensive prominent reflection or band of reflections is necessarily a sill. Setting aside the obvious example of sedimentary units (e.g., the finely layered, upper few seconds of the seismic sections shown in Figures 6 and 8–10). Other processes can produce distinct reflection bands traceable over large distance. Figure 16 (right) show seismic data from west central Sweden not too distant from Siljan [101]. Lacking any other constraints, it would be tempting to interpret these as sills, perhaps related to the Siljan reflections. However, the tectonic setting, outcropping geology, and—most definitively—drill holes make clear that these reflections are actually thrust faults, part of a crustal scale nappe complex emplaced during the Caledonian orogeny [101]. Regional detachment faults have also appeared on crustal reflections surveys as strong, subhorizontal reflections of regional extent. The southern Appalachian detachment traced on COCORP surveys in Tennessee and Georgia [102] and the Main Himalayan Thrust imaged by the INDEPTH surveys in southern Tibet [103] are prime examples.

**Figure 16.** High amplitude, subhorizontal, and west dipping layered reflections from deep seismic surveys in west-central Sweden. Drilling and outcrop correlations identify these as thrust structures rather than intrusion [101].

Implicit in our discussion of strong reflections is the presumption that the magmas involved—whether hot or frozen—represent emplacement from below. An alternative to consider is that the reflections arise from in situ melting. This was originally one interpretation of the seismic bright spots imaged by INDEPTH in Tibet [48,104]. A fluid magma at depth, whether a planar intrusion or an in-situ melt, should give rise to a prominent reflection (e.g., Table 1) regardless of its composition. It is unlikely that reflection data, or any geophysical observations, could distinguish between these two possibilities However, if a reflector under consideration is a frozen product of in situ melt, it is unlikely to have a significant reflection contrast with its surrounding country rock (of the same composition) unless substantial fractional crystallization is involved. On the other hand, a saucer-like reflection geometry in a strong indication that mechanical intrusion is involved. Of course, intrusions may well represent crustal as well as mantle melts, though the silicic composition of the former is less likely to generate a strong reflection unless it is still molten.

The seismic character of a reflector on a seismic section depends heavily on both details of acquisition and processing. This is particularly true for reflection amplitudes. For example, the upwardly convex geometry of saucer shaped intrusions implies some degree of amplification of the associated reflection due to focusing, which in turn is ameliorated by seismic migration [105]. Therefore, caution is warranted when basing interpretation on superficial similarities in appearance between reflections on seismic sections collected and processed by di fferent groups and individuals.
