Deciphering the origin of the Humboldt Slide using Anisotropy of Magnetic
Susceptibility

Kurt Schwehr - Scripps Institution of Oceanography, UCSD, La Jolla, CA.
Neal Driscoll - Scripps Institution of Oceanography, UCSD, La Jolla, CA.
Lisa Tauxe - Scripps Institution of Oceanography, UCSD, La Jolla, CA.


The origin of the "so-called" Humboldt Slide has raised much controversy.
Some researchers argue that it is a slide deposit, while others interpret the
deposit as a depositional feature mantling an old slide scar formed by
down-slope gravity flows.  We recently adapted the standard paleomagnetic tool
of Anisotropy of Magnetic Susceptibility (AMS) in order to detect minor
post-depositional deformation (i.e., "crypto-slumps") in sedimentary
successions and applied it to the Humboldt Slide controversy.

Crypto-slumps are slumps that are not easily observed in outcrop or core
samples. Undeformed sediments show a typical oblate fabric while even slightly
deformed sediments develop a triaxial fabric. In the winter of 2001, we
acquired 5 large diameter piston cores from the Humboldt Slide, Northern
California based on a previous chirp seismic survey collected as part of the
STRATAFORM project. The cores were acquired in two groups with the first being
located in the center of the Humboldt Slide covering the upslope and downslope
limb of one of the structures with one core located between the two
structures.  The second group of cores is located upslope near the top of the
so-called Humboldt complex. Both groups are located in the extensional regime
if this deposit is indeed a slump in an area morphologically described as the
ridges and swales by Gardner et al. (1999).  We measured the AMS on a total of
293 samples down-core and from these analyses we found that fabric was
dominantly oblate except in regions that had experienced flow-in from the
piston coring process.  Based on these results, we conclude that the Humboldt
Slide is not a slide.  The ridges and swales appear to have grown as sediment
waves on an old slide scar.  We have corroborated this model with further
analysis of our chirp seismics which show thickening on the upslope side of
these structures and continuous reflectors across regions where others have
mapped normal faults.

Gardner, J.V., Prior, D.B., Field, M.E., 1999, Humboldt Slide - a large
shear-dominated retrogressive slope failure, Marine Geology, v. 154, 323-338.