Moho in the Archean Minnesota gneiss terrane; fossil, alteration, or layered intrusion?

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 da...

Full description

Saved in:
Bibliographic Details
Published in:Geology (Boulder) 1993-12, Vol.21 (12), p.1131-1134
Main Authors: Boyd, Nicholas K, Smithson, Scott B
Format: Article
Language:English
Subjects:
Archean
continental crust
crust
discontinuities
eclogite
facies
Geology
geophysical methods
geophysical surveys
gneisses
granulite facies
igneous and metamorphic rocks
intrusions
layered intrusions
lower crust
metamorphic rocks
Minnesota
Mohorovicic discontinuity
Petrology
Plate tectonics
Precambrian
reflection
seismic methods
Structural geology
surveys
terranes
thickness
United States
velocity structure
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary: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.
ISSN:0091-7613
1943-2682
DOI:10.1130/0091-7613(1993)021<1131:MITAMG>2.3.CO;2