Discordant muscovite and biotite K-Ar dates of samples taken in six transects through the Cordillera Oriental of southeast Peru and northwest Bolivia, combined with 40Ar/39Ar age spectra, reveal a northwest-trending, structurally cryptic, tectonothermal zone of late Eocene age (ca. 38 Ma) that overprints Triassic and older igneous and metamorphic rocks. The zone is at least 450 km long but is less than 25 km wide; temperatures along its northeast boundary are estimated to have reached 350 °C. No Paleogene magmatism has been recognized that could account for the thermal resetting of the K-Ar isotopic system. Uplift, erosion, and southwest-verging thrusting accompanied the development of this domain, which defined the foreland boundary of the orogen prior to inception of northeast-verging Andean thrusting in Miocene time. It is proposed that this tectonothermal zone, the Zongo-San Gabán zone (ZSGZ), which involves thrusting and basement shortening, is the southerly manifestation of the Incaic compressional deformation, which produced the thin-skinned Marañón thrust and fold belt (MTFB) north of the Abancay deflection of central Peru.

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