The significance of the Coast shear zone (CSZ) on the development of
the western North American cordillera was the subject of a theme session at
the annual G.S.A. meeting in Salt Lake City, chaired by John Diebold
(Lamont Doherty Earth Observatory, Columbia University) and Lincoln
Hollister (Princeton University). The CSZ can be traced for at least 800 km
through southeast Alaska and northern British Columbia, and it may be over
2000 km long.  There were eighteen contributed papers for the theme
session*.

     Highlights of the theme session included the presentation of:

- newly processed seismic data showing that the CSZ is truly crustal-scale
and is nearly vertical from the surface to the Moho;

- new paleomagnetic results sparking much interest because of their high
quality but different possible interpretations;

- recognition of patterns of deformation based on mapping and kinematic
analysis indicating a transpressional regime dominated from late Cretaceous
to early Tertiary, which was followed by a terminal regime of early
Tertiary extension.  U/Pb ages on melts which are affected by different
rock fabrics have been particularly useful for defining the structural
history of the CSZ

     Because the CSZ is a relatively recently recognized major tectonic
feature, its definition continues to be in flux. We attempt here a
definition which is very likely to evolve as more work is done on it:

The Coast shear zone is a near-vertical, crustal-scale shear zone that
formed at high temperatures (>600oC).  The early Tertiary history of the
CSZ includes east side up followed by west side up relative movement. This
displacement history followed a period when the CSZ represented the western
boundary of a transpressive shear zone, now mainly occupied by late
Cretaceous to early Tertiary plutons.  Plutons in this age range occur up
to the CSZ along its eastern side, and they do not occur west of it.

Questions that were addressed in the papers presented included :

     What is the width and length of the CSZ?  Is it confined to the region
from near Juneau to Prince Rupert, or does it extend for the full length
(>2000 kms?) of the Coast Mountains, from Juneau to Washington state? Does
it continue to the western Idaho shear?  Does the Denali fault of Alaska
continue into the CSZ?  If so, how and where is the displacement on this
fault system accomodated? Is its width limited to the approximate 5 km zone
of high strain, or does it extend to the east where it would be largely
obscured by melting and later plutons?

     What was its history and kinematics?  Did its pre early Tertiary
history include a substantial (over 1000 km) component of strike slip
motion?  Is it part of a transform plate boundary?  Is it a terrane
boundary?  Why is there a sharp western boundary of late Cretaceous to
early Tertiary plutonic rocks that appears to coincide with the location of
the CSZ where it has been defined?   Even where the CSZ has not been
defined, there is a sharp western limit for late Cretaceous to early
Tertiary plutons which extends for the full 2000 km of the Coast Mountains.
 

     What is the relation of the CSZ to the proposed late Cretaceous Baja
BC fault system?  This fault system was defined by Cowan et al (1997) to
account for possibly thousands of km of right lateral transport with
respect to North America during the Late Cretaceous. Anomalous
paleomagnetic poles have been reported from tectonostratigraphic terranes
located outboard of the CSZ and have been used to support the
interpretation of large scale northward translation of these terranes.  Is
there an eastern bounding line for these anomalous paleopoles and, if so,
what is its relation to the CSZ?  New paleomagnetic data obtained as part
of the ACCRETE project imply that tilting is a likely explanation for the
observed patterns of paleopoles, but the timing and mechanism of the
tilting have yet to be resolved.

     The CSZ is one target of a large multidisciplinary study that formally
began with a 1993 controlled source seismic experiment along a 200 km fjord
at the Alaska - British Columbia border.  This study, which is called
ACCRETE**, continues with collaborative geophysical, geochemical, and
geological studies along the line of the seismic transect, which crosses
the CSZ.

     The new seismic data from ACCRETE indicate that mid-Cretaceous,
west-vergent thrust fabrics located west of the CSZ can be traced down-dip
to the east where they appear truncated under the surface exposure of the
CSZ.  These data also show that the Moho increases in depth to the east
across the CSZ by about 5 km over a horizontal distance of 15 kms.  East of
the CSZ,  horizontal (in section) deep crustal reflectors underlie late
Cretaceous to early Tertiary plutons of the Coast Plutonic Complex.  This
crust has a higher than normal average crustal velocity and a keel of high
velocity material not found to the west of the CSZ.  Another result of the
seismic study was the discovery of west (in section) dipping reflecting
horizons that shallow into the lower crust above Moho arches and terminate
in the upper crust under late Miocene grabens; these data indicate that the
region was extended during the late Tertiary.

     All in all, the papers presented collectively show that
multidisciplinary collaboration on specific geological and geophysical
problems yields results difficult to obtain using individual techniques.

*Presenters, in order, were David Brew, George Gehrels, Cathy Manduca,
Carol Evenchick, Robert Butler, John Diebold, Scott Smithson, Keith
Klepeis, Bill McClelland, Maria Luisa Crawford, Chris Andronicos, Lincoln
Hollister, Jay Thomas, Krishna Sinha, Dominique Chardon, Susie Gareau,
Harold Stowell, and Jim Metcalf.

**Scientists involved in ACCRETE, and who are funded from the Continental
Dynamics program of NSF, are at Beloit College; Bryn Mawr College; Columbia
University; Princeton University; Universities of Arizona, Wisconsin, and
Wyoming; and Virginia Polytechnic Institute.

Lincoln S. Hollister, Dept. of Geosciences, Princeton University, Princeton
NJ 08544
Keith Klepeis, Dept. of Geology and Geophysics, University of Sydney, NSW
2006 AUSTRALIA