TECTONIC SIGNIFICANCE OF PALEOMAGNETISM OF THE EOCENE GOLDEN HORN BATHOLITH

Study of the Golden Horn batholith with modern equipment and techniques has overcome the “paleomagnetic frustration” of Whitney (1975). Fine-increment stepwise thermal demagnetization and measurement with a cryogenic magnetometer have revealed a two-component remanence. The second removed component of 36 specimens from seven sites typically has a paleomagnetic direction that is steeply inclined and reverse polarity. The distribution of directions is streaked in a NW-SE direction with a preliminary mean declination 216.6°, mean inclination 87.9°, a95=7.8 and k=9.5. Unblocking temperatures above 500C and preliminary ⁴⁰Ar/³⁹Ar arvfedsonite and biotite ages of 46.97 ± 0.23 Ma and 46.92 ± 0.04 Ma, respectively, support Eocene primary magnetization. Discordance from the Eocene expected direction for North America (declination 348.2°, inclination 68.0°; Diehl et al., 1983) is significant.

The simplest explanation of the observed paleomagnetic discordance is down to the north horizontal axis tilt of 23° associated with post-Eocene deformation of the North Cascades along the Ross Lake fault zone. Paleomagnetic evidence that Baja-B.C. terrane transport relative to North America essentially stopped by Eocene time (Irving et al., 1996) precludes latitudinal complication of this interpretation. This result will allow restoration of the structures of the Golden Horn batholith and the adjacent Late Cretaceous Black Peak batholith to Eocene orientation. Interpretation of structural patterns in restored orientation may allow interpretation of the Eocene and Late Cretaceous sense of movement and therefore the direction of terrane transport along the Ross Lake fault zone.

Estimation of magmatic petrofabrics using anisotropy of magnetic susceptibility (AMS) is in progress but has been completed for the southwest part of the Golden Horn batholith and for a transect across the central part of the Black Peak batholith. Golden Horn and Black Peak lineations (k_max) are generally N to N-NE trending and shallow plunging. Golden Horn foliations (from k_min) are generally N striking and moderately east dipping. Black Peak foliations (from k_min) are generally S-SW striking and steeply W-NW dipping. The formation of these fabrics, which are oblique to the North Cascades orogen and the Ross Lake fault zone, is as yet unexplained.