Hauling field gear up Gothic Mountain…
The San Luis Basin (SLB) of the northern Rio Grande rift (RGR) is a spectacular intermontane basin in south-central Colorado. The SLB is an east-dipping, half graben basin that formed in response to extension beginning ca. 26 Ma in the Cordillera. Oligocene ash-flow tuffs exposed in the eastern San Juan Mountains and along the western margin of the SLB provide an excellent opportunity to study the kinematic history of the northern Rio Grande rift using paleomagnetism. Outflow deposits of ash-flow tuffs are generally excellent recorders of the geomagnetic field and can be used to study the vertical-axis rotation component of extension related to the opening of the SLB. Four ash-flow tuffs (Carpenter Ridge, Fish Canyon, Chiquito Peak and Saguache Creek Tuffs) that were erupted between ca. 32 and 27 Ma were sampled at a total of 84 sites for paleomagnetic, rock magnetic, and magnetic fabric data. Paleomagnetic data indicate some complexities with using large-volume, regionally extensive outflow deposits of ash-flow tuffs, specifically that relatively thick outflow deposits (with thicknesses exceeding 100 m) may record paleosecular variation of the geomagnetic field. Even with these complications, the four targeted San Juan ash-flow tuffs may still be evaluated for regional domains of potential vertical-axis rotation. Overall, we observe a lack of vertical-axis rotation along the western margin of the SLB. Shear and fault linkage in the hanging wall of the major normal fault of the SLB does not appear to play a role in the evolution of slightly extended terranes. Distal outflow deposits of ash-flow tuffs may also be difficult, if not impossible, to tell apart and paleomagnetism is used to distinguish two units that have, until presently, been grouped together as the Chiquito Peak Tuff. The magnetic fabric of these Oligocene ash-flow tuffs was also measured and analyzed for complexities due to paleotopography and single-domain magnetic carriers. Preliminary results from a comparison between two magnetic fabric techniques are also presented and analyzed. Magnetic fabrics measured for all tuffs in the northeastern San Juan Mountains confirm that their flow was channelized through an Oligocene paleovalley that existed west of Saguache.
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