Open AccessCompositional and metamorphic controls on velocity and reflectivity in the continental crust: An example from the Grenville Province of eastern Québec
Author(s) -
Hurich C. A.,
Deemer S. J.,
Indares A.,
Salisbury M.
Publication year - 2001
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000jb900244
Subject(s) - eclogite , geology , metamorphism , metamorphic facies , metamorphic rock , crust , mafic , geochemistry , continental crust , gabbro , protolith , eclogitization , granulite , petrology , facies , igneous rock , oceanic crust , subduction , tectonics , geomorphology , seismology , structural basin
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.