Geophysical Studies of Southwestern Part of the North American Craton
The present study, based on combined geophysical, geochronological and isotopic data identifies a cratonic block which we call the Western Chihuahua-Mesa Central megablock (WCMB) in northern Mexico. New Bouguer and isostatic gravity anomaly maps revealed a conspicuously distinct, spatially-extensive gravity minima over a region extending from north western Chihuahua to as far south as the Mesa Central (MC). This region can be differentiated from the surrounding mobile belts with significantly higher gravity anomaly values. Gravity modeling combined with sparse seismically determined crustal thickness estimates indicate that the proposed WCMB region has crustal thicknesses ranging from 38 to 42 km. The regions surrounding WCMB are the spatially constrained mobile belts that are characterized by basinal and trough-like structures and are associated with A-type granites and bimodal volcanism. Spatial distribution of enriched initial Sr isotopic data (> 0.706) confirms the sialic character of the crust associated with the gravity minimum at least in western Chihuahua. The mobile belts surrounding the WCMB contain geologic records of the events since 1.8-0.9 Ga. These ages from within and just adjacent to the block correlate well with the Rodinia Neoproterozoic supercontinent that was assembled 1.3-0.9 Ga. We propose that the northwest-southeast directed WCMB is a cratonic region underlain by Proterozoic crust. The WCMB is most likely separated from the deformed and active southwestern part of the North American Proterozoic craton by repeated thermotectonic activities within the surrounding mobile belts. These events have helped to dislocate and transport the block to its present location through processes of repeated compression, extension, and transform faulting. The southeast shoulder of the Rio Grande Rift is located in southeastern New Mexico and west Texas in US and northern Chihuahua in Mexico. Noted mineral resources in the region are enriched in rare earth elements increasing the interest of research; specially around a block sitting in the east shoulder of the rift. These resources are related to tertiary volcanism. The magmas of this volcanism have similar composition of that of the oceanic island basalts suggesting that they were created from asthenosphere derived magmas from the basement of the North American Craton. That basement, in the area of interest, constitutes the Mazatzal and Grenville Proterozoic provinces of the Proterozoic Laurentia plate. This study is intended to contribute to the general understanding of the basement features of the region. In order to understand the structure of the crust and upper mantle we create a Rayleigh surface wave group velocity model of the southeast flank (or shoulder) of the Rio Grande Rift. Rayleigh wave group velocities were calculated using data from EarthScope’s TA and Flex arrays. The periods of the model range from 10 s to 160 s. The kernels of the model are taken from the joint inversions made for LA RISTRA, from where the depths corresponding to periods between 10 s to 160 s should be approximately between 10 km and 350 km of depth. The results show the anisotropy of the region and difficulties faced using the Rayleigh wave cross correlation. Some structures like the Delaware basin are complicated and sensitive to seismic radiation directions and patterns. In general, structures are better resolved when these radiation directions are perpendicular to the structure boundaries. The subtraction of the Rayleigh wave velocities traveling southwest from the ones traveling northwest show high correlation with previously published results of shear wave splitting in the region.
Magallanes, Luis Martin Sandoval, "Geophysical Studies of Southwestern Part of the North American Craton" (2018). ETD Collection for University of Texas, El Paso. AAI10790984.