THERMAL HISTORY AND EXHUMATION OF THE WESTERN GONDWANAN FAMATINIAN OROGEN, SIERRAS PAMPEANAS, ARGENTINA: A MULTI-GEO-THERMOCHRONOMETRIC APPROACH

The proto-western Gondwanan margin (present Sierras Pampeanas region, ARG) underwent convergent tectonism, magmatism, and lithospheric growth during the early Paleozoic Famatinian orogeny. The orogeny’s main phase (490-465 Ma) was associated with the Famatinian magmatic arc (FMA) and subsequent collision of the Laurentia-derived Cuyania continental plate, with a late orogenic phase transitioning to extensional collapse (440-360 Ma). The Cuyania-FMA suture created a broad zone of rock uplift and deformation, which has affected regional sedimentation and deformation patterns since the Ordovician, including the exhumation of deeply buried (>12 km) Cuyania and FMA rock. Although this exhumation is crucial to understanding the spatiotemporal distribution of Famatinian phases and paleo-topographic highs that constrained regional basins through time, the timing and extent of exhumation remain unclear.

We present a multi-geo-thermochronometer study to investigate tectonic causes of deformation and exhumation across the Cuyania-FMA suture zone using zircon/monazite U-Th-Pb, zircon/monazite/apatite (U-Th)/He, and zircon fission-track (ZFT) geo-thermochronology. Samples were taken from FMA granitoids and Cuyania metamorphic units at 11 locations spanning ~130 km across the suture zone at ~29-30 °S.

Cuyania monazite Th-Pb ages are restricted to ca. 500-420 Ma and are interpreted to record crystallization during Famatinian deformation phases. FMA zircon/monazite U-Th-Pb ages record arc formation (488-468 Ma), with some monazite ages (440-400 Ma) indicating regionally restricted late phase deformation. Zircon (U-Th)/He results from Cuyania and FMA samples show a spread in dates from ca. 400-250 Ma, while ZFT ages yield a narrower range from ca. 330-300 Ma. Thermal history modeling suggests that samples underwent post-300 Ma reheating, interpreted to be from Permo-Triassic burial and heating during Mesozoic rift basin formation. Preliminary stepped-heating He diffusion experiments and (U-Th)/He thermochronology of Th-Pb dated monazites (closure temperature ~250-300 °C) show linear Arrhenius behavior and potential for further constraining Cuyania and FMA thermal histories. Although the pre-Permian cooling history remains poorly constrained, thermochronology results indicate that Cuyania and FMA were exhumed by the upper Carboniferous.