Compositional and metamorphic controls on velocity and reflectivity in the continental crust: An example from the Grenville Province of eastern Québec

Abstract

Abstract. We present velocity and density data for a suite of gabbros and metagabbros over a range of metamorphic grades, from subgreenschist to the eclogite facies. Most of these rocks have a mesocratic protolith with overlapping bulk chemistry ranges providing an opportunity to investigate the systematics of mineralogy and physical properties across a wide range of metamorphic grades with minimal compositional variation. In addition, more mafic gabbros in the eclogite facies were also investigated in order to take into account variations in the extent of transformation of gabbro to eclogite controlled by bulk chemistry. The physical properties data are used as the basis for a first-order crustal model composed of granitic, anorthositic, and gabbroic components. The objective of constructing the crustal model is to evaluate the relative contribution of compositional variation and metamorphism to the velocity, density, and reflectivity structure of the continental crust. The model suggests that in the upper crust, if fracture porosity is neglected, composition dominates the velocity and density structure, while in the middle and lower crust, composition and metamorphism play equally important roles. Simulations designed to predict potential reflection coefficient size distributions for the various bulk chemistries and mineralogies in the crustal model indicate step increases in the potential for large reflection coefficients that are dominantly controlled by metamorphic grade.Step increases in reflectivity are predicted between the lower and upper amphibolite facies and between incompletely and completely eclogitized metagabbros in the eclogite facies. We also consider several alternative mixing models which account for the effects of one of the three lithologic components occurring as isolated bodies rather than perfectly mixed with the other two components. These models indicate a reduction of the potential for large reflection coefficients by as much as 50%, emphasizing the importance of the degree of mixing for crustal reflectivity.

Seismic properties of the Voisey’s Bay massive sulfide deposit: Insights into approaches to seismic imaging

“…Analysis of the probability distribution of potential acoustic impedance contrasts; and thus, the probability distribution of reflection coefficients leads to an estimate of potential reflectivity (Hurich et al, 2001), which can then aid in the prediction of observable reflections from potential lithologic contrasts. Troctolite (trace-5%)/massive sulfide −33-50 þ5 Figure 5.…”

The 3-D crustal structure in the Manicouagan region: new seismic and gravity constraints

“…This list was compiled from different sources, and the rock identification used by the respective authors reflects the purpose for which the density data were acquired and the tectonic framework that existed at the time. For example, Hurich et al (1997) sampled rocks from Gagnon terrane, adjacent to the Molson Lake terrane (Rivers et al 1993), and high-pressure terranes near the Manicouagan Reservoir (Indares et al 1998), to determine the effects of metamorphic grade on velocity and density trends in the Parautochthonous Belt. Regional sampling reported by Sweeney (1978) was designed to constrain gravity models of the Manicouagan impact structure, but rock classifications for units outside the central island (not shown in Table 3) are difficult to reconcile with present-day tectonic subdivisions.…”

“…Finally, it is worth noting that gabbroic rocks from the Parautochthonous Belt exhibit a trend of increasing density with increasing metamorphic grade. High-pressure granulite-facies gabbros, an apparent anomaly in this trend, are a subcategory used by Hurich et al (1997) to classify leucocratic units that preserve some residual plagioclase, despite unusually high pressures of metamorphism.…”

Variations in the thickness of the crust of the Kaapvaal craton, and mantle structure below southern Africa

“…This underplated material is probably mafic granulite. The seismic properties of mafic rocks can vary quite considerably with composition and metamorphic grade under lower crustal conditions (Hurich et al, 2001). A boundary between intermediate granulite and plagioclase-rich mafic granulite will result in a small change in S wavespeed and a relatively large change in density.…”

“…8 are taken from Table 1. However, all other P and S wavespeeds and densities are estimates based on laboratory estimates on rocks from other parts of the world published by Rudnick and Fountain (1995) and Hurich et al (2001). The changes in S wavespeed and density, β and ρ are approximate, since a broad range of possibilities exist for each rock type (Fig.…”