EXAMINING DRIVERS FOR INTRAPLATE EXTENSION: INSIGHTS FROM THE RESERVE GRABEN IN THE COLORADO PLATEAU-RIO GRANDE RIFT-BASIN AND RANGE TRANSITION ZONE, NEW MEXICO

New geochronology from basin fill and fault-kinematic observations from the Reserve graben, New Mexico, USA, help inform the history of late Cenozoic extension and its causes far from the plate boundary of southwestern North America. The graben trends NE, different than most regional extensional basins, and lies within the transition zone between the southeastern Colorado Plateau and the diffuse junction of the Rio Grande rift (RGR) and southern Basin and Range province (BRP), ~600 km inboard of the southern San Andreas Fault. Its development constrains relative crustal motions among these provinces and suggests control due to plate-boundary changes. Plate-boundary forces commonly are considered a major control on extension in the BRP, but this is less well-established for the RGR further inland.

⁴⁰Ar/³⁹Ar analysis of basalts within earliest syntectonic basin fill indicates fault-related subsidence began ~16.5 Ma, significantly later than onset of extension in the region. Detrital sanidine ages of young basin fill and a groundmass age from a post-tectonic basalt bracket final faulting between ~7.5 and 1.9 Ma. Fault-slip data from the master fault system suggest a transition from early oblique dextral-normal to normal dip-slip sometime after ~16 Ma. Sanidine ages from a rhyolite unit that is cut by the master fault but not offset laterally indicate mainly normal dip-slip after ~15.2 Ma. With additional slip data from intrabasinal faults, our results suggest a change from NE-SW to NW-SE extension early in the graben’s development (between ~16 and 15 Ma), consistent with a middle Miocene change in extension direction over much of southwestern North America.

Onset of faulting in the Reserve graben coincides with accelerated rifting in the central BRP and RGR, and with episodic southward lengthening of the dextral plate boundary by oceanic microplate capture events by the Pacific plate and southward “jumps” of the Rivera triple junction. Localized mantle-related geodynamic events have been proposed to influence RGR development at various times, but none are recognized during the middle Miocene in this region. Thus, plate-boundary motions probably influenced extension at least as far inland as the study area.