THE SANTA FE IMPACT STRUCTURE: CLUES TO DECADES-OLD GEOLOGIC PUZZLES IN THE SOUTHERN SANGRE DE CRISTO MOUNTAINS AND REGIONAL PROTEROZOIC TECTONICS OF NEW MEXICO, USA

Impact structures on the Earth are generally thought of as superficial blemishes unrelated to the overall geologic history of the terrestrial crust. We present a case where an early impact may have influenced the subsequent regional Proterozoic tectonics of the crust before the deposition of Paleozoic sediments. The unusual character of the rocks of the mountain ranges near Santa Fe, New Mexico, with extensive brittle fracturing and brecciation not reported elsewhere in the basement rocks of New Mexico has puzzled generations of geologists. In addition, the geologic history of the southern Sangre de Cristo Mountains is a subject of ongoing debate in regard to the timing, amount, and direction of movement along a system of north-trending faults. The discovery of shatter cones in Proterozoic rocks by Tim McElvain indicating the presence of an early Santa Fe impact structure prompted investigations aimed at estimating the original size of the impact crater, the depth of burial from which the shatter cones were exhumed, the timing of the event, and its influence on subsequent regional geology and inheritance. Field investigations have focused on the extent of the shatter cones and the field relations to zones of anomalous brecciation with clasts of the same lithologies as the shatter cones. Other categories of Proterozoic exposures have been attributed to tectonics, but may also be associated with an impact event, such as “pervasively fractured” outcrops. In addition, regional data collection and analysis has shown distinctive attributes of each of the major tectonic events in the vicinity of the Santa Fe impact structure (Caine et al., 2007). Our studies indicate that it is possible to unravel individual and multiple tectonic events and distinguish them from impact related structures. For example, superposition of structures in Santa Fe are represented by samples of breccia-in-breccia reminiscent of lunar breccias (Wright et al., 2010). Continuing work is needed to define how the impact event fits into the geochronology of the region, and its relationship to the tectonic history of the southern Sangre de Cristo Mountains and numerous breccia occurrences in the region.