Potrillo Volcanic Field - Geophysics

Geophysics

Recently, velocity data was acquired using both reflected and refracted arrivals. This data has shown that the upper 5 km of the crust beneath the Portillo volcanic field (PVF) is characterized by alternating regions of low and high velocities of 2.5-3.7 km/s and 3.5-4.3 km/s, respectively. The velocities increase with depth to approximately 6.0 km/s at 10 km, 6.5 km/s at 16 km, and they increase sharply from 6.9 to 8.0 km/s between 28 and 38 km.

The low crust of the PVF consists of a single density of 2880 kg/m3. The upper to middle crust (between 5 km and 20 km) of the PVF has a block with a density of 2740 kg/m3. The density at 11 to 15 kilometres (6.8 to 9.3 mi) of depth is 2880 kg/m3. This body is a mid-crustal “welt” and velocities within this region increase up to and greater than 6.35 km/s. This greater density creates a lateral density contrast and in turn generates long-wavelength gravity high. The upper mantle densities at the PVF are decreased from 3280 to 3250 kg/m3. Evidence of this can be supported by the west to east decrease in velocities from approximately 7.96 to 7.70 km/s.

Coyote Hill and the West Portillo mountains make up the western portion of the PVF, with region velocities in this uplift from 4.5 km/s to over 6.0 km/s at the depth of 2.5 km. To the east is the Malpais basin and velocities within this basin range from 1.68 km/s to 4.86 km/s at a depth of approximately 3 km. The Mesilla basin is an asymmetric basin that adjoins the eastern edge of the PVF and extends on to the western flank of the Franklin mountain uplift. At a depth of about 1.5 km the velocities in the western area range from approximately 2.4 km/s to 4.0 km/s at a depth of about 1.5 km.

There is increased seismic reflectivity within the crust and at the Moho interface concentrated below the PVF. The mid-crustal shows reflectivity increases between 40 and 70 kilometres (25 and 43 mi) offset; mid-crustal reflectivity is present between 4 and 6 seconds, reduced time.