A lithospheric investigation of the Southern Rio Grande Rift

Matthew George Averill, University of Texas at El Paso

Abstract

The crustal structure of the Rio Grande Rift is an important link to the understanding of mantle, crustal and surface processes associated with continental rift environments. We present a model for the crustal velocity structure of the rift derived from the analysis of seismic refraction/wide-angle reflection data acquired during the Potrillo Volcanic Field (PVF) experiment in May 2003. The 205 km long profile, consisting of 8 shots and 793 receivers across southern New Mexico and Far West Texas, was designed as a detailed seismic investigation of the structure and composition of the Southern Rio Grande Rift (SRGR) and the Potrillo Volcanic field, a very recent and well-known xenolith locale. Our results provide several new insights into the structure of the SRGR, including: (1) A new detailed cross-section of the basins and range structure across southern New Mexico into Far West Texas; (2) evidence for slightly thicker crust (∼30--31 km) in the SRGR than previously modeled with little overall topography on the Moho; (3) a highly complex upper-crustal velocity structure underlying the PVF, that includes several high velocity bodies between 5 and 10 km depth; (4) increased seismic reflectivity within the crust and at the Moho interface concentrated below the PVF and; (5) a dramatic step in a mid-crustal interface from west to east indicating a thickening of the mid-crust below the PVF. Interpretation of our velocity results is complimented by use gravity modeling, magnetic and heat flow data and xenolith studies. Furthermore, the resolution of our model has been analyzed using a new approach to determine velocity uncertainty in tomographic modeling. ^ Along the profile, the velocity structure of the upper 3--5 km reflects the basins and ranges of this recently extended area. Basin fill ranges in velocity from 2.5 to 4.5 km/s. In the ranges, velocities are 4.7 to 5.3 km/s and reflect uplifted Paleozoic sedimentary rock. A middle crust interface marks the transition from upper to middle crust at depths of 11--15 km. This layer steps up abruptly below the PVF and indicates a thickening of the middle crust associated with the PVF. Velocities range from 6.28 to 6.4 km/s below this transition to velocities of 6.7 to 7.1 km/s at the base of the crust. While near-vertical incidence records show a complicated reflectivity pattern at the Moho, velocity modeling does not suggest a pronounced lower crust transitional layer. Crustal thickness varies from 35 km at the western end of the profile to as little as ∼30 km beneath the El Paso area. Upper mantle velocities range from 7.75 to 7.9 km/s, which is consistent with a warm upper mantle and high heat flow values of 85 to 125 mWm2 associated with the SRGR. An assessment of the velocity model shows uncertainty values ranging from 0.015 km/s near the shot points and in the upper 5 to 10 km beneath the PVF to as high as 0.3 km/s at 20 to 30 km depth in the center of the model and beneath the El Paso urban area. The gravity result shows lateral density changes in the mid-crust and upper mantle beneath the PVF region, consistent with lateral changes in the velocity model. Densities in the mid-crust vary from 2740 to 2700 kg/m3 with a mid-crustal “welt” of density 2880 kg/m3. Upper-mantle density decreases from 3280 to 3250 kg/m3 beneath the PVF and El Paso region.^ High velocity bodies and complex lateral velocity variations at 5 to 10 km depth beneath the PVF region show the signature of Laramide basement uplifts. A thickening of the mid-crust and a distinct step in velocity at 13 km depth relates to the presence an Oligocene age (ca. 27 Ma) plutonic complex previously defined by xenolith data from Potrillo Maar. Deformation textures observed in xenolith data and velocities calculated from modal mineralogies agree with an interpreted Laramide detachment at 9 to 10 km depth and syn-plutonic deformation at temperatures of 550° to 400°C. The velocity structure, seismic reflectivity characteristics, and crustal compositions show strong similarities to core-complex structures in SW Arizona. ^

Subject Area

Geology|Geophysics

Recommended Citation

Averill, Matthew George, "A lithospheric investigation of the Southern Rio Grande Rift" (2007). ETD Collection for University of Texas, El Paso. AAI3273992.
http://digitalcommons.utep.edu/dissertations/AAI3273992

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