130-5 Evidence for transpression in the heart of the Picuris Mountains

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part II

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Mesoproterozoic magmatism, deformation, and metamorphism have recently been synthesized into the framework of the trans-Laurentian Picuris-Baraboo-Pinware (PBP) orogeny. Oblique convergence and subduction zone segmentation along the 1.5-1.35 Ga active Laurentian margin have been proposed to explain many unusual features of the PBP belt, especially the intraplate location of significant PBP deformation. Here we investigate the strain and kinematics of deformation in the Picuris segment of the PBP belt as recorded in the Marqueñas Formation metaconglomerate (Picuris Mountains, New Mexico) in order to test the hypothesis that intraplate PBP deformation was facilitated by and is an expression of transpression. The Picuris Mountains expose Paleoproterozoic and Mesoproterozoic metasedimentary rocks that experienced three phases of deformation between ~1.45-1.38 Ga. The main D₂ phase produced the most prominent structures and fabrics, including ~E-W striking foliation, overturned folds, and shear zones that record overall north-directed tectonic transport. The Marqueñas Formation is a competent, shear zone-bounded wedge of quartzite and metaconglomerate within an otherwise schist-dominated package of metamorphic tectonites occurring along the southern margin of the Picuris Mountains. Deformed quartzite clasts in the metaconglomerate show flattening strain at the outcrop and map scale. Quartz fabrics within the clasts record a combination of flattening and non-coaxial shear. Kinematic vorticity axes, determined by crystallographic vorticity analysis, align with the principal flattening (XY) plane and with the field-measured S₂ foliation, indicating that the main D₂ phase in the Picuris Mountains was a progressive 3-D general shear deformation. Quartz c-axis opening angle thermometry indicates clast deformation at amphibolite-facies metamorphic conditions, further tying the development of flattening strain to the D₂ event. Structural relations suggest that the shear zone bounding the Marqueñas Formation experienced oblique slip with components of top-to-the north thrust and right-lateral shear. Flattening strain, 3-D general shear deformation, and oblique slip shear zones are consistent with transpression across the Picuris segment of the larger PBP orogen. Regional transpression (NW-SE shortening at an oblique angle to the Laurentian margin) during the Mesoproterozoic has previously been inferred from sheared margins of ferroan granitoid plutons (formerly "A-type granites") and from structures exposed in the Colorado Front Ranges, but this is the first study to document evidence of transpression in the type locality of Picuris deformation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10573

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