140-5 Devonian Deformation Recorded in Shear Zones Throughout the Western Sierras Pampeanas, Argentina

Session: Building the South American Cordillera, Paleozoic to Recent: Insights from Geo-Thermochronology

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Authors:

Abstract:

The early Paleozoic evolution of the proto-Andean margin of southwest Gondwana was characterized by accretion of numerous Laurentian-derived terranes following the breakup of the supercontinent Rodinia. The location of suture zones between these “ribbon” terranes and the timing of accretion are debated. The Cuyania terrane is estimated to have accreted to Gondwana in the middle to late Ordovician, coincident with the end of the Famatinian Orogeny. This event was followed by accretion of the Chilenia terrane along the western margin of the newly accreted Cuyania terrane. Proposed Cuyania terrane boundaries include (1) an eastern suture zone along the Sierra de Valle Fertil crustal discontinuity or (2) a western suture along the Sierra Pie de Palo and associated Cerro Islas exposed along-strike. We aim to resolve competing models that imply Cuyania shear zones record a punctuated late Ordovician deformation event along the Sierra de Valle Fertil suture versus protracted Ordovician-Devonian deformation across Sierra Pie de Palo and the Cerro Islas. We integrate field mapping, zircon and apatite U-Pb geochronology, and microstructural analysis to constrain the timing and kinematics of shear zone deformation across the western Sierras Pampeanas. The detrital zircon U-Pb record reflects a shared Grenville-Sunsas provenance with the western Neoproterozoic-Cambrian metasediments for western facies while eastern facies reflect an eastern Brasiliano source. Apatite U-Pb data from across Sierra Pie de Palo and the western Cerro Islas yield ages that span the Silurian to early Devonian, concurrent with prolonged accretion events. Ages are largely Devonian with some older, potentially un-reset grains that overlap with Famatinian and Pampean deformation. Apatite U-Pb ages are consistent with published ⁴⁰Ar/³⁹Ar data that record Ordovician-Devonian deformation and exhumation. Microstructural observations and electron backscatter diffraction (EBSD) analysis of quartz and feldspar from shear zone samples show evidence of dynamic recrystallization through dominant dislocation creep mechanisms. Quartz EBSD displays either rhomb <a> or prism <a> slip with strong lattice preferred orientations (LPO) while apatite EBSD shows grain rotation and a strong uniform LPO. Reset apatite U-Pb ages paired with microstructural analyses record deformation related to Famatinian and Chanic accretion of exotic Laurentian terranes and favor a Cuyania suture that lies to the west of Sierra Pie de Palo and the Cerro Islas.

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

doi: 10.1130/abs/2025AM-11018

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