New Mexico Geological Society Annual Spring Meeting — Abstracts

Our working hypothesis is that Ancestral Rocky Mountain (320-290 Ma) and Laramide (70- 50 Ma) faults in New Mexico and Colorado reactivated zones of crustal weakness that formed during the Precambrian. We are testing this with ⁴⁰Ar/³⁹Ar K-feldspar thermochronology analyses. In Arizona, the Grand Canyon Supergroup sedimentary package is well exposed and provides an unambiguous example of development of a Laramide monocline due to reverse slip reactivation of a Neoproterozoic normal fault. However, because there are limited Precambrian sediments exposed or remaining in the more deeply exhumed Rocky Mountains it is difficult to directly observe reactivation of Precambrian structures. Because ⁴⁰Ar/³⁹Ar K-feldspar age data record the time when rocks cooled from about 300 to 150°C it is the only available thermochronometric system that allows quantitative evaluation of Neoproterozoic or Ancestral Rocky Mountain basement exhumation histories. The basement of New Mexico and Colorado is defined by a complex polygonal network of ~10 km scale blocks that were differentially exhumed between ~1.4 Ga to 0.5 Ga. Emerging K-feldspar data show that different blocks had distinct and variable cooling histories. Thermal histories of Precambrian basement rocks across specific Phanerozoic structures are revealing divergent exhumation histories that began during the culmination of the Grenville orogeny at about 1.1 Ga. These data support our hypothesis that many young fault systems are reactivating older structural weaknesses. Thermal history analysis also records a period of regional cooling (exhumation) between 850 to 750 Ma that coincides with the onset of Neoproterozoic rifting of the Rodinia supercontinent. Additionally, in Colorado the K-feldspar data appear to record cooling related to basement removal during the Ancestral Rocky Mountain orogeny. Continued work across distinct faults, with different orientations (NW versus NS) may reveal which segments of the Paleozoic to Laramide fault network were active at different times in the Proterozoic and hence may help decipher the geometry and kinematics of intracratonic Proterozoic fault systems.