Paleomagnetism of the Oligocene Kalamazoo Tuff: implications for middle Tertiary extension in east central Nevada

The Oligocene Kalamazoo Tuff (≃35 Ma) was sampled for paleomagnetic analysis across a 100-km-wide zone of highly extended crust in east central Nevada to estimate between-site vertical axis rotations and thus the relative importance of strike-slip faulting to the mechanism of extension. Subordinate sampling was also undertaken in a younger hornblende dacite intrusion (≃35 Ma) and the overlying tuff of North Creek (≃35 Ma). In general, ash flow tuffs cool and are magnetized within a short period of time relative to geomagnetic secular variation and their eutaxitic structures typically indicate the paleohorizontal. These attributes of ash flow tuffs, in addition to their generally widespread occurrence, make them excellent candidates for combined paleomagnetic and structural studies of regional deformation. However, in this study a number of problems associated with the paleomagnetism of the Kalamazoo Tuff are documented; they include topography-related variations of the eutaxitic structures, secular variation during slow cooling, postblocking rheomorphic flow, and local remagnetization of the tuff by younger igneous activity. The tilt-corrected data, with these sources of error reduced or eliminated, exhibit a 28° ± 12° clockwise rotation of the Schell Creek Range relative to the Kern Mountains region. This rotation implies differential extension accommodated by strike-slip faulting or N-S shortening. The paleomagnetic results also suggest that large changes in strike of layered units near faults with presumed strike-slip movement need not be the result of oroclinal bending, but could result from superimposed sets of orthogonal normal faults.