APPLICATIONS OF PALEOMAGNETISM IN UNDERSTANDING SHEAR ZONE KINEMATICS: THE GOOD, THE BAD, AND UGLY

Numerous paleomagnetic investigations have demonstrated the power of the approach to understanding rigid body deformation, notably vertical axis to sub-vertical axis rotation of volumes of the earth’s crust. Such volumes are typically referred to as blocks, and regions of blocks with similar magnitudes (and senses) of rotation through motion along sets of shear zones are typically referred to as rotation domains. Models for block rotations show that orientation of strike-slip faults/shear zones relative to the attending stress state, as well as the geometry of the rotating blocks, will have a major control on the sense and magnitude of rotation. Rotation estimates include either absolute (requiring an adequate, long-term sampling of the geomagnetic field, ideally at each locality within a block) or relative (requiring sampling a single datum (e.g., lava flow), or a few datum across several blocks) approaches. Ideally, besides quantifying and placing realistic error estimates on such rotation, studies should be able to define the boundaries of rotated blocks, but this requires adequate sampling distribution, which may be precluded by field relations. A set of well-determined results that provide an adequate long-term sampling of the geomagnetic field, at a series of localities, is required to estimate absolute magnitude of rotation, and its error, yet is limited in its use without information on the location and geometry of the shear zones responsible for rigid deformation. In settings where shear zone location and geometry are known, sufficient rotation data can be used to ratify particular models and refute others. To emphasize these concerns, specific studies involving paleomagnetic rotation estimates are critically evaluated, on the basis of data, sampling distribution, and knowledge of regional/local structure. For example, near Hoover Dam, southern Nevada, late Cenozoic left-slip offset along the NE-SW trending Lake Mead fault system (LMFS) has resulted in the apparent rotation of structures. The Miocene Tuff of Hoover Dam (THD)(over 90 sites) yields a well-grouped magnetization that reveals about 35^o of counterclockwise rotation (R=-35.1^o, delR=12.4) from south to north across the dam site. This rotation may in part reflect differential extension northwest of the LMFS.