GEOPHYSICAL IMAGING OF THE SAN JUAN MOUNTAINS BATHOLITH COMPLEX, SOUTHWESTERN COLORADO

The San Juan volcanic field (SJVF) of southwestern Colorado is the largest erosional remnant of a widespread mid-Cenozoic volcanic field that spanned much of the southern Rocky Mountains. One of the largest gravity lows over North America lies over the San Juan Mountains and largely coincident SJVF. A low-density silicic batholith complex related to the volcanic field has been the accepted interpretation of the source of the gravity low for decades. Previous interpretations of the batholith complex were plagued by inadequate processing of gravity data in the San Juan Mountains and thus were mainly qualitative. Here, we use more sophisticated processing of gravity data and modeling with seismic data that image low velocities in the upper crust to develop a quantitative view of the batholith complex. Assuming that the source of the gravity low is the same as the source of the low seismic velocities, modeling supports the interpretation of a batholith complex and defines its dimensions and density contrast. Models show that the thickness of the batholith complex varies laterally to a large degree, with the greatest thickness (~20 km) under the western SJVF, and lesser thicknesses (< 10 km) under the eastern SJVF. The largest group of nested calderas of the SJVF, the central caldera cluster, is not correlated with the thickest part of the batholith complex. This result is consistent with petrologic interpretations that the batholith complex continued to be modified after cessation of volcanism and therefore is not necessarily representative of synvolcanic magma chambers. The older calderas in the northeast and southeast parts of the SJVF lie outside the area of the interpreted batholith complex extent, suggesting that batholith formation was more closely associated with the younger calderas of the central and western SJVF. The total volume of the batholith complex is estimated to be 82,000 to 130,000 km³. The formation of such a large complex would inevitably involve production of a considerably greater volume of residuum, which may be present in the lower crust or uppermost mantle, an area not well imaged. The interpreted density contrast (-60 to -110 kg/m³), density (2590-2640 kg/m³), and seismic expression of the batholith complex are consistent with results of geophysical studies of other large batholiths in the western U.S.