Abstract

A seismic survey in the Archean Minnesota gneiss terrane (3.6-3.8 Ga) imaged a seismically variable, discontinuous Moho at both vertical incidence and wide angles. The wide-angle data show the gneiss terrane crust to be thick (49 km) with a high average velocity (6.8 km/s). The vertical-incidence data show a sharp, 10-km-long Moho-depth (Fresnel zone radius = 2.8 km) reflection that disappears laterally, and data analysis suggests a 1 km Moho zone with 100-m-thick internal layering. Changes in the signal/noise ratio across the survey or energy losses from reflectors within the crust do not affect the variability in the reflection Moho; therefore, the reflection discontinuity is a geologic feature, and lateral structural variation of the Moho occurs over short (tens of kilometres) distances. Possible interpretations of the layered structure are (1) a layered intrusion, (2) localized alteration to eclogite by intrusion of fluids along shear zones or zones of weakness, or (3) fossil granulite-facies structures preserved within an eclogitized lower crust. A thin gradient, or thin layered zone with small impedance contrasts, or an abrupt velocity change are possible models for the regional Moho. The Moho shows considerable lateral structural variation within an early Archean high-grade terrane.

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