New Mexico Geological Society Annual Spring Meeting — Abstracts

Models for the deformational history of deep- to middle-crustal rocks rely heavily on the documentation of rock structures which are not always detectable in the field. Anisotropy of magnetic susceptibility (AMS) analysis allows for the evaluation of non-visible petrofabrics as it can detect structural anisotropies of less than 1% in rock samples. We conducted anisotropy of magnetic susceptibility (AMS) analysis on Proterozoic basement rocks west of Las Vegas, NM, in an attempt to better document and interpret deformation features. We studied medium-grade gneisses outcropping along County Road 65 in the Gallinas Canyon, which dissects a portion of the Hermit’s Peak batholithic complex. Rock types include quartzofeldspathic gneisses, biotite schists, and laminated amphibolites. The gneisses show intense penetrative deformation defined by a strong steeply dipping northeast trending axial planar foliation. Minor macroscopic linear structures, including isoclinal fold hinges and prismatic mineral alignments, plunge moderately to the southwest. Oriented AMS samples, typically eight to twelve samples per site, were collected from twelve sites throughout the canyon. All samples were analyzed on an AGICO static KLY-4S Magnetic Susceptibility/Anisotropy System at the University of New Mexico Rock Magnetism Laboratory in order to characterize the magnetic mineralogy and magnetic fabric of the rocks. Preliminary rock magnetic data indicate that the dominant magnetic phase in most specimens is a ferri/ferromagnetic oxide (magnetite, maghemite) as demonstrated by an average bulk susceptibility of 8.5x10⁻³ SI. Additional rock magnetic experiments are being conducted to further assess the magnetic mineralogy. The AMS fabric data are consistent with the macroscopic structural features, particularly the lineations, which were visible at only a fraction of the study sites. We propose that our petrofabric and rock magnetic data reflect the dominance of northwest-southeast contractional deformation and southwest-northeast extension in the assembly history of the continental lithosphere during the Proterozoic. Our study shows that AMS petrofabric analysis is a simple yet powerful tool for obtaining high quality orientation data from crystalline rocks for which visible rock structures are lacking or tenuous.