LOCALIZED ROTATION DURING PALEOGENE EXTENSION IN EAST CENTRAL IDAHO - PALEOMAGNETIC AND GEOLOGIC EVIDENCE

We collected paleomagnetic data from 137 sites in middle Eocene volcanic rocks of the northeastern Basin and Range province in order to characterize the three-dimensional kinematics of continental extension in the Lost River and Lemhi ranges, Idaho, and to test previous block rotation models. Statistically significant flattening of directions at three of 10 localities reflects either insufficient averaging of paleosecular variation or erroneous tilt corrections. Five of the seven remaining locality means are statistically indistinguishable from the middle Eocene reference direction. This indicates that vertical axis rotation did not accompany post-50 Ma normal faulting. Discordant declinations from two localities in the northern Lost River Range indicate 30-degrees-40-degrees of counterclockwise rotation of a large domain of NNW striking volcanic rocks. Inclination flattening at one of the two rotated localities is moderate but statistically significant (F +/- delta-F = 9.9-degrees +/- 6.5-degrees). In total, the paleomagnetic data conflict with earlier predictions of uniform clockwise rotation in response to right lateral transtension adjacent to the eastern Snake River Plain. There is a close correspondence between counterclockwise rotated localities and domains of NNW striking volcanic rocks and between unrotated localities and domains of north striking volcanic rocks. New mapping shows that counterclockwise rotated localities are all in the hanging wall of a major west dipping low-angle normal fault, the Donkey fault (named for the first time). This fault loses displacement southwards, was active in Eocene-Oligocene time and is probably a tilted high angle fault. The spatial distribution of rotated and unrotated localities is best explained by counterclockwise rotation of the hanging wall of the Donkey fault during opening of a southward tapering half graben. According to this hanging wall rotation model the footwall of the Donkey fault remained fixed relative to its rotating hanging wall. Our proposed kinematic model is generally applicable to extending continental lithosphere and provides a mechanism for coeval crustal thinning and vertical axis rotation. Thus vertical axis rotation may occur in the absence of strike-slip faults in extensional orogens.