Abstract Title: Mid-Tertiary buoyancy modification and its relation to rock 
exhumation, cooling, and subsequent extension at the eastern margin of the 
Colorado Plateau

Abstract Author(s): Roy, Mousumi and Kelley, Shari  

Abstract: In the Southern Rocky Mountains (SRM) and Rio Grande Rift (RGR) 
regions, rock cooling from apatite fission track (AFT) data and net exhumation 
from rock preservation patterns spatially correlate with areas of voluminous 
mid-Tertiary caldera complex magmatism, the Òignimbrite flare-upÓ. These 
observations, together with gravity data are used to constrain calculations of 
the isostatic effects of mid-Tertiary caldera-complex magmatism and 
accompanying lithospheric modification. Simple isostatic models predict that 
mid-Tertiary magmatism drove spatially-variable rock uplift and thermal 
perturbations that, coupled with variable exhumation, can explain the AFT 
cooling and rock preservation patterns. During isostatic response to magmatism, 
if rock uplift exceeds exhumation then the resulting surface uplift combined 
with thermal weakening of the lithosphere can influence subsequent extension. 
Lithospheric modification during caldera complex magmatism should, therefore, 
be considered an important mechanism for profoundly influencing lithospheric 
dynamics. The arguments in this abstract focus on the eastern margin of the 
Colorado Plateau, exploiting the relatively minor extensional dismemberment of 
mid-Tertiary ignimbrite centers such as the San Juan volcanic field in southern 
Colorado. However, the general geodynamic processes of interest here, 
voluminous magmatism setting lithospheric conditions for subsequent extension 
may have broad relevance to the whole western US, including the Great Basin and 
its margins.