Constitution of the Lower Continental Crust Based on Experimental Studies of Seismic Velocities in Granulite

Research Article| February 01, 1975 Constitution of the Lower Continental Crust Based on Experimental Studies of Seismic Velocities in Granulite NIKOLAS I. CHRISTENSEN; NIKOLAS I. CHRISTENSEN 1Department of Geological Sciences and Graduate Program in Geophysics, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar DAVID M. FOUNTAIN DAVID M. FOUNTAIN 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1975) 86 (2): 227–236. https://doi.org/10.1130/0016-7606(1975)86<227:COTLCC>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation NIKOLAS I. CHRISTENSEN, DAVID M. FOUNTAIN; Constitution of the Lower Continental Crust Based on Experimental Studies of Seismic Velocities in Granulite. GSA Bulletin 1975;; 86 (2): 227–236. doi: https://doi.org/10.1130/0016-7606(1975)86<227:COTLCC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rocks of the granulite facies have been proposed as major constituents of the lower continental crust. To evaluate this possibility, compressional and shear wave velocities have been determined to pressures of 10 kb for 10 granulite samples, thus enabling comparisons of seismic data for the lower crust with the velocities and elastic properties of granulite rocks. The samples selected for this study range in composition from granitic to basaltic, with bulk densities of 2.68 to 3.09 g/cm3. At 6 kb, compressional (Vp) and shear (Vs) wave velocities range from 6.39 to 7.49 km/sec and from 3.36 to 4.25 km/sec, respectively. Velocities in granulite rocks are shown to vary systematically with variations in mineralogical constitution. Both Vp and Vs, increase with increasing pyroxene, amphibole, and garnet. Velocities increase with an increasing ratio of pyroxene to amphibole in hornblende-granulite subfacies rocks of approximately equivalent chemical compositions. Decreasing quartz content in granulite rocks produces an increase in Vp and an accompanying decrease in Vs, thereby significantly changing Poisson's ratio. The range of velocities measured for the granulite samples is similar to the range of seismic velocities reported for the lower continental crust; thus, the hypothesis that granulite rocks are major lower crustal constituents is further strengthened. Furthermore, it is shown that lower crustal composition is extremely variable, and therefore valid discussions of composition must be limited to specific regions where seismic velocities are well known. The use of seismic velocities in estimating lower crustal composition is illustrated for the Canadian Shield in Ontario and Manitoba. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.