Records of Magmatism and Erosion Found in Wedge-Top Strata of the Eocene Anta Formation in the Central Andes of Southern Peru

Manta R.; Sundell K.E.; Kenyon C.; Anderson R.B.; Pearson D.; Carlotto V. · Tectonics (2025) · DOI: 10.1029/2025tc008994

Resumen

Wedge-top depozones of foreland basin systems are uniquely informative for orogenic processes active before and during their formation because they form on the actively deforming thrust wedge at the interface between the orogenic hinterland and retroarc foreland basin system. However, these systems are by nature rarely preserved due to coeval rock uplift, thrust belt propagation, and sedimentary reworking. We characterized 2,300 m of strata interpreted as wedge-top deposits exposed by the deeply incised Apurimac River valley near Coyabamba, Peru, through lithofacies analysis and detrital zircon U-Pb geochronology. Newly measured strata are dominantly pebble to boulder conglomerate with rarer sandstones and mudstones formed in an alluvial-fluvial depositional environment. Detrital zircon maximum depositional ages bracket measured strata between 40 and 36 Ma, and exposures of previously documented Anta Formation strata between 40 and 30 Ma. Sedimentation rates vary spatially and temporally between 300 and 900 m/Myr and are interpreted to be driven by combined flexural and fault-controlled subsidence. Compositional analysis and detrital zircon age distributions are consistent with sediment sourcing from transitional to dissected arc terranes through unroofing and sediment recycling of the Western Cordillera, preceding the establishment of the inboard migration of the Andahuaylas-Yauri Batholith and Anta Volcanic Arc. Collectively, the Anta Formation represents a classic example of wedge-top deposition as documented and interpreted in orogenic systems globally, typified by significant variability in along- and across-strike stratigraphic thickness, grain size, textural maturity, sediment provenance, accumulation rate, basin longevity, and depositional environment due to basin segmentation and variable interconnectivity.

Instituciones: Universidad Nacional de San Antonio Abad del Cusco