INVESTIGATING THE TRANSITION FROM EOCENE SUBDUCTION TO OLIGOCENE EXTENSION IN SOUTHERN NEW MEXICO, USA

Rift-related magmatism and the formation of rift basins are hallmarks of continental extension. Determining the onset of extension can be difficult, particularly when there is a prior history of extension, shortening, and subduction-related magmatism, as there is in the Basin and Range (B&R) province and Rio Grande rift (RGR) of southern New Mexico (NM). The southern RGR is thought to have formed starting at ~ 36 Ma based on ages and thickness variations of ignimbrites in half-grabens and the transition from subduction-related intermediate volcanism to silicic ignimbrites. Here we use new geochronology and geochemistry of volcanic rocks and thermochronology to investigate the timing of extension in southern NM.

Widespread mid-Eocene intermediate magmatism in southern NM is related to shallow east-dipping subduction of the Farallon plate. This intermediate volcanism is followed by late-Eocene–Oligocene voluminous silicic tuffs. A slab breakoff hypothesis explains the compositional shift in magmatism but does not require that extension began concurrently. To investigate this transition, we have studied mafic and silicic rocks in the RGR and intermediate -silicic volcanics in the Schoolhouse Mtn. caldera (SMC) of SW NM. New dates on the oldest SMC andesites indicate eruption at ~35 Ma, only slightly older the overlying silicic tuff (34.9 Ma). Also, the SMC andesite and a newly dated 36 Ma basalt from southern NM have geochemistry consistent with a mantle source modified by subduction contributions. Given these new insights, does the transition from andesite to rhyolite signal the onset of widespread extension in southern NM? Our low-T thermochronology data indicates rapid exhumation beginning in the B&R starting around 35 Ma.

In the southern RGR, voluminous basaltic magmatism began with the Uvas basaltic andesite at ~28 Ma. Could extension have progressed sufficiently to allow mantle-derived magmatism at this time? A key indicator of the onset of rapid extension comes from our application of low-T thermochronology to uplifted fault blocks across the B&R/RGR transition. Our data from the southern RGR indicate rapid exhumation began ~ 25 Ma, consistent with the timing of the basalts. We suggest that RGR extension began ~28 Ma and accelerated by 25 Ma with extension slowing by 5 Ma and continuing to the present.