SHATTER CONE OCCURRENCES INDICATE A POSSIBLE IMPACT STRUCTURE NEAR SANTA FE, NEW MEXICO

Shatter cone-like features have been documented ~5km northeast of Santa Fe, New Mexico, where they are exposed along and laterally adjacent to Hyde Park Road. Several continuous exposures of nested, sub-conical, planar to slightly curved multiply striated fracture surfaces occur within Proterozoic rocks including granitic gneiss, amphibolite, mica schist, and quartzite. We interpret these features to be shatter cones, indicating the remnant of an old, eroded impact structure. The striated surfaces are best developed within equigranular, potassium feldspar-rich granitic gneiss. Mapping to date has identified the cone structures only within Proterozoic metamorphic and crystalline rocks; the structures apparently are not present in nearby outcrops of the unconformably overlying, Carboniferous Madera Group sedimentary rocks. If impact evidence is proven to be lacking within the Paleozoic sequence, this would constrain initially the age of the event to post-Mesoproterozoic and pre-Early Carboniferous. The shatter cones, well exposed for more than 1 km along and north of Hyde Park Road, are individually up to 1 m long and display a general NE trend of their cone axes, which plunge to the SW. The trend of the cone axes cuts the foliation of the host rock, which strikes S to SE. An average apical angle of ~65° was measured for the master cones and the minor cones conjunctively. Megabreccia contained within Precambrian rock units has also been observed in relative proximity to the probable shatter cones, approximately 1 km west of the western-most shatter cone outcrop along Hyde Park Road. Although attributed traditionally to the regionally important Picuris-Pecos Fault, these megabreccias are anomalous in their volume, geometry, and geographic extent. In light of shatter cone evidence for an impact event, they must be re-examined. Ongoing research is further documenting the structure texture, and petrography of the shatter cones and the detailed structural fabric of their host rocks, mapping the areal and stratigraphic extent of the cone structures, and analyzing and mapping the megabreccia. If the features are confirmed as shatter cones, a deeply eroded impact structure exists near Santa Fe.