Schwehr, K.; Driscoll, N.; Tauxe, L.; Lee, H. J.
Exploration of the Humboldt Slide using Anisotropy of
Magnetic Susceptibility Comparison with the Gaviota Slide
American Geophysical Union, Fall Meeting 2003, abstract #OS22A-1150
12/2003

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 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. Based on examining cores from the center and top of
the Humboldt Slide structure, we find no evidence for deformation. The
cores from the top are in an area that is clearly free from drape and
thus we can be sure that we are sampling the structure seen in chirp
seismics. We recently sampled a known slide for comparison to the
Humboldt Slide. We used 5 gravity cores from the USGS V1-81-SC cruise
which sampled the Gaviota Slide. The Gaviota Slide in on the northern
slope of the Santa Barbara Basin and is just to the west of the much
larger Golita Slide. The AMS measurements from the Gaviota Slide show
a clearly deformed fabric that we expect from a slide. From these
analyses we found that the fabric is dominantly oblate for the
Humboldt Slide while the Gaviota Slide is predominantly
triaxial. Based on these results, we re-emphasize our conclusion that
the Humboldt Slide is not a slide. We now know that the ridges and
swales of the Humboldt Slide appear to have grown as sediment waves on
an old slide scar and our chirp seismics show thickening on the
upslope side of these structures and continuous reflectors across
regions where others have mapped normal faults.