EVIDENCE FOR A MAJOR IMPACT STRUCTURE IN THE SANGRE DE CRISTO MOUNTAINS, NEAR SANTA FE, NEW MEXICO

In 2005, shatter cones were discovered in road cuts on NM Highway 475 and in natural exposures, between the Santa Fe city limits and Hyde State Park, on the east side of the Sangre de Cristo Mountains. In the foothills, Mississippian and Pennsylvanian carbonates lie unconformably on fault blocks of Proterozoic metaplutonic rocks. Since their deposition, the region has been disturbed by movements associated with the Ancestral Rockies, the Laramide orogeny, and the late Tertiary uplift of the present mountains relative to the Rio Grande Rift to the west

In the vicinity of NM 475, the following features are exposed over a distance of 6 km, at progressively deeper structural levels: (1) Up to 10 m breccia blanket between fractured Proterozoic and intact Paleozoic rocks. (2) Several vertical dike-like mesobreccia bodies of Proterozoic rocks. (3) A megabreccia zone of deformed clast-supported Proterozoic rocks, and (4) an in situ zone of faulted Proterozoic rocks with shatter cones from cm-to-m size. The first of these is the target of a search for planar deformation features (PDF's) in quartz.

At its best exposure, the breccia blanket rests on a monomict crackle breccia of Proterozoic granite. In its lower part, randomly oriented m-size subangular blocks of granite are supported by argillaceous matrix. Toward the top, subangular to subrounded carbonate clasts appear. A tongue of polymict breccia, ~ 10 meters wide, invades the overlying Mississippian (?) carbonate beds. Within the tongue, carbonate blocks are traversed by microfractures, ~ 1 mm wide, injected with granite clasts. Quartz grains in these clasts, and in situ Paleozoic rocks, show up to 3 sets of planar microstructures (PM's). A petrofabric analysis of PM's in 23 quartz grains yielded, 55 PM angles measured, of which 11% do not fit standard stereographic projection of crystallographic planes with low Miller indices within plus or minus 5 degree margin of error. The resulting histogram indicates that the planar features are PDF's and is analogous to those of known impact structures, but the results are not confirmed.

At this early stage of the investigation, significance of various breccias remains uncertain, but PDF's (if confirmed) and shatter cones indicate that impact played a role. The maximum age of the event is Mississippian; actual age and extent are undetermined.