MAARS OF THE SOUTHERN RIO GRANDE RIFT: TIMING AND TRAITS

The Quaternary Potrillo volcanic field (PVF) is along the southern axis of the Rio Grande rift, New Mexico. The alkalic mafic volcanism resulted in several hundred cones, flows and maars over approximately 4,600 km² during the past 1 Ma to possibly as recently as 8 ka ago. This maar study is part of larger project documenting geochemistry, spatial and temporal distributions of volcanic and fault features, and modeling of episodicity and magmatic history. Maars evaluated here include Riley (RM), Malpais (MM), Potrillo (PM), Kilbourne Hole (KH), and Hunt's Hole (HH). RM, PM and KH have brought mantle and crustal xenoliths to the surface, whereas only crustal xenoliths are found at HH and MM. Magmas forming the syn-maar deposits display increasing e_Nd and Pb isotopes and decreasing ⁸⁷Sr/⁸⁶Sr (designated Group I); HH is an exception, having opposite trends (Group II). For these melts, going from Group I to Group II corresponds to overall "younging" of deposits, decreasing incompatible element concentrations, and increasing degrees of partial melt.

Syn-maar eruption timing is: KH 30 ± 15 ka ago, HH 16 ± 8 ka, PM (a Bruhnes event) before ~ 70 ka ago, RM ~ 640 ka, and MM between 610 ka to 510 ka ago. Five eruption phases are recognized for the PVF as a result of the geochronology study that included using field relationships, ³He surface exposure and ⁴⁰Ar/³⁹Ar dating, and paleomagnetic results. Phases II through V are best documented for deposits of the down-dropped block along the East Robledo (ER) fault. KH and HH are the most recent (V) of phases II -V on that block whereas PM post-maar deposits define phase IV. Resulting eruption episodicity estimates for this eastern sector are ~ 50 ka. Notably, the most recent activity for all eruption styles shifted eastward with time. Further, PM syn-maar deposits are faulted (ER or splay?). The timing of this structural event relative to the post-maar episode at ~ 70 ka ago is yet unresolved and the focus of ongoing study. In summary, volcanic activity was not continuous in the PVF and possibly occurred as recently as the Holocene. The punctuated volcanic activity, presence of both mono- and polygenetic centers, and evidence for shifting eruption foci across 30 km lateral distances during the 10⁶ year history may be reflective of crack formation-coalescence due to local structural control.