GEOCHEMICAL EVIDENCE OF MIOCENE LITHOSPHERIC THINNING IN SOUTHERN NEW MEXICO: IMPLICATIONS FOR THE INITIATION OF RIFTING OF THE RIO GRANDE RIFT

The Mogollon-Datil volcanic field (MDVF) in southern New Mexico is the result of punctuated volcanism and the transition between arc and rift magmatism. The distinct style of arc volcanism in the MDVF contrasts significantly with the “normal” steady-state model, in which volcanism occurred far inland beneath a flat subducting slab that rolled back to a steeper dip. During the mid-Tertiary, large-scale MASH zones to developed in the lower to upper crust of western North America, induced by upwelling of basaltic magma. This fueled what is known as the “ignimbrite flare-up” at ~36 Ma in the MDVF. During mid- to late-Oligocene, the tectonic stress regime relaxed, initiating intercontinental extension, resulting in a rapid shift in the style and composition of volcanism to a more basaltic composition effusive volcanic field.

We present new geochemical data from volcanic rocks to assess the evolution of crustal thickness during the arc-rift transition. Geochemical variations suggest a change in crustal thickness correlates to changes Nb/Ta, (La/Yb)_N, Sr/Y and (Dy/Yb)_N ratios. Maximum thickness is suggested to occur during an ignimbrite flare-up between 36-24 Ma followed a crustal thinning event around 28 Ma ultimately leading to crustal extension and the initiation of the Rio Grande Rift in southern New Mexico. Trace-element data support a predominately lithospheric mantle source for most of the ignimbrite flare-up and associated intermediate lava flows with a transition to a more asthenosphere-derived composition during later Rio Grande rift-related basaltic volcanism.