JOINTING IN THE SAGE HEN FLAT AND COTTONWOOD PLUTONS, WHITE MOUNTAINS, CALIFORNIA: A FIELD AND REMOTE SENSING STUDY

Plutons are commonly highly jointed, but the origin of the joints is sometimes unclear. Joints shape the landscape, channel groundwater flow, and enhance erosion. The Sage Hen Flat (SHF) pluton and nearby Cottonwood pluton are highly jointed and intrude similar Late Proterozoic sedimentary rocks in the White Mountains. We determined joint orientations in these plutons, using both field data and measurements obtained using Google Earth, in order to better understand their origin.

The SHF contains three joint sets: steep NE (dominant) and NW sets and a gently dipping set. Rose diagrams of the SHF joint azimuths measured in the field are strikingly similar to those created with measurements from Google Earth. Google Earth orientations of steep joints in the Cottonwood pluton are somewhat more variable, but the dominant NE and NW joint sets found in the SHF occur; however, they are rotated 10° clockwise. In the field, we distinguished two types of gently dipping joints: faults with slickensides containing local epidote and muscovite mineralization, and joints lacking secondary mineralization. SEM examination revealed the presence of desert varnish and thin coatings of chlorite near the surface of joints. Alteration along fault surfaces is different, showing chlorite, muscovite, epidote, albite, magnetite, and calcite within a few mm of the surface. Platy minerals along fault surfaces are typically oriented with plates parallel to the surface. Micas in fault surfaces also showed kink bands, deformation which was not present along joint surfaces.

The prominent, straight, NE-striking joint set in the SHF pluton traverses the entire pluton; a similar pattern occurs in the Cottonwood pluton, although it is locally curved. These patterns suggest that the joints in these plutons were caused by regional stresses rather than thermal stresses. Cenozoic right-lateral strike-slip faults on either side of the White Mountains could be responsible for this joint formation. In such a shear zone, the joints could have formed along Riedel or related secondary shears. The lack of noticeable R₁ fractures could be due to pluton rotation after the formation of the joints, deviating what could have been R₁ fractures to their present-day orientations.