A problem presented in the last
report was the explanation of
anomalous paleomagnetic
declinations (mostly deflected
clockwise, some over 90
) in
the Transverse Ranges of
California. In a major revision
of his previous model for Miocene
rotations in California,
Luyendyk [1991] now proposes
that locally extreme extension
occurred between rotating blocks,
adding the degrees of freedom to
make rotation possible.
Nicholson et al. [1994]
reconstructed the probable cause
of this transtensional rotation
event: the subducting Monterey
plate froze onto the Pacific
plate 20 Ma ago, suddenly
changing the direction of shear
tractions on the base of the
margin of North America from
northeastward to northwestward.
In Oregon today, the remarkable discovery of seafloor fracture zones cutting upward through the forearc wedge on the North America plate [ Goldfinger et al., 1992] suggest that the subduction zone is locked, and that clockwise rotation must be occurring in a dextral transpressive setting.
In the future, the interpretation of paleomagnetic data will be much more open to all earth scientists, thanks to the creation of a searchable database [ Harbert, 1993].