NEOTECTONIC BLOCK ROTATION ADJACENT TO THE WESTERN IDAHO SHEAR ZONE, IDAHO

The western Idaho shear zone near McCall, Idaho, is a late Cretaceous wrench zone that is currently undergoing neotectonic normal faulting due to Basin and Range extension. Normal fault blocks within the shear zone show no evidence for vertical axis rotation, while a previous study suggests that the Fish Lake Fault Block, located west of the shear zone, may be rotating counter-clockwise. This change in the fault block behavior suggests an abrupt change in the modern plate motion vectors across the shear zone. This sharp change may be attributed to the reactivation of the western Idaho shear zone. The evidence for this change in relative plate motions is based primarily on data from the Fish Lake Fault Block. Paleomagnetic data from an additional fault block located to the north of the Fish Lake Block might better constrain the kinematics of extension in this area. The Imnaha member of the Columbia River Basalt Group was deposited prior to the onset of normal faulting in the middle Miocene near this normal fault block and can serve as the paleomagnetic marker. Cores were taken from near McCall, Idaho from three different flows of the Imnaha member. The hypothesis is that this normal fault block will have experienced counter-clockwise rotation if the plate motions do in fact change across the shear zone. Preliminary data gathered using an alternating-field demagnetizer and a spinning magnetometer indicate two components of magnetism in the basalt. One is weak, probably a secondary weathering component, and the other is strong and interpreted as being primary. The primary component is south-trending, suggesting that the basalt flows were deposited during a reverse polarity interval. The preliminary paleomagnetic data does not support a change in the orientation of relative plate motion across the western Idaho shear zone.