Rift-Parallel Magmatic Dikes beneath the Midcontinent Rift: Evidence from Shear-Wave Splitting Analysis
Upper mantle anisotropy beneath the Mesoproterozoic Midcontinent Rift (MCR) and adjacent areas is investigated using splitting of P-to-S converted phases (SKS, SKKS and PKS) at the core-mantle boundary. Available data from all the portable and permanent broadband seismic stations in the area of 38-46N and 92-102W are used in the study. Well-defined measurements were obtained at about 20 stations, with a mean splitting time of about 1 s which corresponds to a layers of anisotropy of about 110 km thick (4% anisotropy). The most interesting feature in the spatial distribution of the splitting parameters (fast polarization direction PHI and splitting time DT) is that outside the 1.1 Ga MCR, the PHI observations are mostly parallel to the absolute plate motion (APM) direction, while in the vicinity of the rift, the majority of the PHI values are consistent with the strike of the rift. The APM direction in the study area is about 250 degrees clockwise from the north, while the strike of the MCR is 215-220 degrees. The most likely cause for the intriguing observation is azimuthal anisotropy caused by sub-vertical magmatic dykes along the strike of the rift. Although the dykes have long become inactive, the slight difference in seismic velocity between the dykes and the surrounding lithospheric rocks can lead to the observed anisotropy. This interpretation is consistent with the observation that igneous activities were pervasive while the rift was active, leading to a layer of basaltic rocks of up to 20 km thick in the rift valley.
M. Moidaki et al., "Rift-Parallel Magmatic Dikes beneath the Midcontinent Rift: Evidence from Shear-Wave Splitting Analysis,", vol. 89, no. 23 American Geophysical Union (AGU), May 2008.
AGU Joint Assembly (2008: May 27-30, Fort Lauderdale, FL)
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
Body waves; Mantle; Lithosphere
Article - Conference proceedings
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