Seismic structure of the continental crust based on rock velocity measurements from the Kapuskasing Uplift

Abstract

The Michipicoten greenstone belt (MGB), Wawa gneiss terrane (WGT), and Kapuskasing structural zone (KSZ) in Ontario, Canada, are regarded as a 25‐km‐thick partial cross section through the crust of the Superior Province. We measured compressional wave velocities (Vp) at confining pressures up to 600 MPa for representative rock samples from these terranes. Vp for MGB is variable (6.1–7 km/s) and depends on composition and anisotropy. Compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the WGT and the KSZ show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range, a trend corroborated by data for other high‐grade rocks. A model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics. The shallowest levels (MGB) are seismically heterogeneous with high average velocities (6.6 km/s at 100 MPa) because of the dominance of metabasalts. The balance of the upper crust (WGT) is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6.35 km/s at 100 MPa and 6.6 km/s at 600 MPa. The uppermost lower crust (KSZ) is layered, seismically heterogeneous and transversely isotropic. The average velocity at 600 MPa is 6.84 km/s. The boundary between WGT and KSZ is marked by a velocity increase of between 0.2 and 0.3 km/s, an increase similar to that commonly associated with midcrustal discontinuities (i.e., Conrad) in shield areas. Here, this boundary corresponds to the amphibolite‐granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust. Refraction results show an anomalous high‐velocity zone in the upper crust under the KSZ that, on the basis of laboratory velocity data, can be correlated with KSZ lithologies. These data indicate that KSZ rocks can be traced to the west to depths as great as 20 km. Levels deeper than 20 km, not exposed at the surface in the KSZ, are characterized by velocities greater than 7.0 km/s and may be dominated by mafic and anorthositic lithologies or by more garnet‐rich rocks. The velocity model of the Superior Province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the MGB from refraction experiments. The velocity structure of this model is also generally similar to the velocity structure of shield areas.

Eclogite-facies shear zones—deep crustal reflectors?

“…Compressional-wave velocities were measured to confining pressures of 600 MPa for three mutually orthogonal cores (normal to foliation, perpendicular to lineation, and parallel to lineation) cut from each sample using the equipment and methods described in Fountain et al (1990). Densities (p) were determined for each core based on its mass and its volume (determined with a helium pycnometer).…”

Heat flow and crustal thermal structure in the Late Archaean Closepet Granite batholith, south India

“…Examples include the Canadian, Baltic, south African, and southern Indian shields (e.g., Weaver and Tarney 1984;Ashwal et al 1987;Fountain et al 1987Fountain et al , 1990Pinet and Jaupart 1987;Pinet et al 1991;Mareschal et al 2000a;Nicolayson et al 1981;Jones 1988;Roy and Rao 2003;Ray et al 2003;Rao et al 2003), the Ivrea-Verbano and Strona-Ceneri zones, southern Alps, Italy (e.g., Galson 1983), the Arizona Basin and Range (Sass et al 1994;Ketcham 1996) and the Sierra Nevada batholith (Brady et al 2006) in North America. A common occurrence in several of these crustal segments is the systematic metamorphic (structural) gradation from greenschist facies through amphibolite and granulite facies (e.g., Percival et al 1992), and the associated depletion in crustal radiogenic heat production with increase in metamorphic grade (e.g., Rudnick and Fountain 1995).…”

Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts

“…[11][12][13][14][15]). Vanorio et al [16] have previously reported P-and S-wave velocity data for volcanic rocks, including some from Etna.…”