212-1 A mudstone record of arc-continent collision: Chronostratigraphy and provenance of the upper Zapata Formation, southern Chile

Session: Reconstructing Earth Surface Processes in Orogenic Systems

Presenting Author:

Fernando Rey

Authors:

Rey, Fernando M¹, Malkowski, Matthew A², Dobbs, Steve C³, Clevenger, Griffin⁴, Sickmann, Zachary⁵, Gross Almonte, Nicholas⁶

(1) Department of Earth and Planetary Sciences, University of Texas at Austin - Jackson School of Geosciences, Austin, Texas, USA; FlagRock LLC, San Antonio, Texas, USA, (2) Department of Earth and Planetary Sciences, University of Texas at Austin - Jackson School of Geosciences, Austin, Texas, USA, (3) Stanford University, Stanford, California, USA, (4) University of Texas at Austin - Jackson School of Geosciences, Austin, TX, USA, (5) The University of Texas at Dallas, Dallas, Texas, USA, (6) Geosyntec Consultants, Austin, Texas, USA,

Abstract:

Arc–continent collisions are key drivers of continental growth and global climate regulation. These events are often preceded by the closure of ocean basins, which involves deformation, uplift, and eventual collision. Reconstructing these transitions is challenging due to erosion and structural overprinting that obscure early retroarc records. Traditionally, arc–continent collisions are recognized in the sedimentary record by the appearance of coarse clastic deposits. However, fine-grained basin sediments can preserve earlier signals of provenance shifts and uplift timing. In the Southern Patagonian Andes, the Early Cretaceous closure of the Rocas Verdes Basin initiated arc–continent collision. While coarse-grained turbidites of the Punta Barrosa Formation have been linked to this event, other studies suggest an earlier onset recorded in the underlying Upper Zapata mudstones. To test this, we analyzed the sedimentology, geochronology, and geochemistry of the Upper Zapata Formation at Cerro Tenerife, Chile. The ~600 m-thick Upper Zapata Formation consists mainly of mudstones interbedded with siltstones, sandstones, and tuffs—interpreted as hemipelagic sediments interrupted by low-density turbidity currents. Turbidites increase in frequency (from every ~5 to ~2 m) and thickness (~5 to ~15 cm) above 300 m, culminating in meter-scale beds marking the transition to the Punta Barrosa Formation. This upward coarsening reflects progradation from an abyssal plain to a distal submarine fan. Accumulation rates, based on five U–Pb zircon ash ages, three detrital zircon maximum depositional ages, and Bayesian modeling, show slow sedimentation in the lower section (0–225 m, ~35 m/My) and a marked increase in the upper section (225–610 m, ~140 m/My). Detrital zircon age spectra at 225 m are dominated by magmatic arc (75–144 Ma, 61%) and basement (>200 Ma, 29%) sources. Upper samples (548–610 m) show increased contributions (20–30%) from Early Jurassic rift-related volcanics (178–200 Ma), sourced over 500 km to the northeast. XRF geochemical data from 106 shale samples show low weathering and intermediate provenance in the lower section, shifting to felsic sources and higher weathering intensities above 225–300 m. We interpret these changes as the early onset of arc–continent collision, uplift, and drainage expansion, beginning ~4 Myr before coarse clastic deposition. Upper Zapata Formation offers a rare, continuous record of collision-related foreland evolution, highlighting the utility of integrating provenance and geochemical analysis in fine-grained successions.

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

doi: 10.1130/abs/2025AM-9551

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