INVESTIGATIONS OF EFFECT OF SHOCK ON NATURAL REMANENT MAGNETISM

At Meteor Crater, Arizona, shock pressures generated by impact apparently had no effect on the RM of samples of Moenkopi red beds collected at four sites around the rim of the crater. The mean direction (D = 346°, I = 21°, K = 6) after correction for bedding attitude and thermal demagnetization to 400°C is close to that of Moenkopi samples collected outside the crater and the value reported by other workers elsewhere in Arizona. At the Nevada Test Site of the Atomic Energy Commission: (1) the orientation of NRM of samples of Rainier tuff from around the site of an underground nuclear explosion is close to that of the local earth's field, suggesting remagnetization due to the shock; (2) the bulk susceptibility of the Hardhat granodiorite has been reduced by shock; an effect on remanence is detectable in the most intensely shocked samples, but uncertainties in core-orientation and intrinsic instability of the rock preclude effective analysis of remanence directions. Intensity of RM of basalt subject to 10- to 100-kb pressure by artificial impact at Ames Research Center is reduced to ½ or ¼, with moderate change in direction. The intensity (and probably the orientation) of a viscous component acquired during shock is a function of the ambient field; it is largely erased by 50-oe af demagnetization. The intensity and orientation of RM in shocked and unshocked samples converge after 500-oe af demagnetization. Copyright 1969 by the American Geophysical Union.