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 suggest 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.
Highlights
The geometry, composition, and mechanics of crustal intrusions, including sills, have been subject of numerous field and lab investigations; the associated literature is extensive [1,2,3,4,5,6]
Sills are often observed in surface geological outcrops and available for direct sampling [7,8], their subhorizontal geometry makes them less likely to outcrop than near-vertical dykes, especially if buried beneath later sedimentary basins
The deep sills cited in this paper, which appear as relatively thin layers, may seem volumetrically small compared to other plutonic manifestations, the more extensive examples, such as the Winagami reflector or the COCORP midcontinent basement layering, suggest thermal perturbation over very large areas
Summary
The geometry, composition, and mechanics of crustal intrusions, including sills, have been subject of numerous field and lab investigations; the associated literature is extensive [1,2,3,4,5,6]. Gravity is notoriously non-unique [15], as are the various electrical methodologies [16,17] Both offer relatively limited resolution at depth, especially if the target is a thin planar structure. Seismic methods using both artificial and earthquake sources have been widely used to define crustal structure in general and magmatic additions both hot and cold. The primary geophysical method used for oil and gas exploration, has become a highly sophisticated tool for imaging the subsurface in both 2D and 3D at depths ranging from the near surface to the upper mantle. We focus on published examples of identified or inferred sills in the continental basement hidden beneath the sedimentary cover, with special attention to frozen sills that may be fingerprints of ancient thermal processes and the large-distance lateral transport of magma in the crystalline crust
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