| Sedimentary provenance techniques can track changes in sediment source areas resulting from exposure or isolation of distinct sediment sources during the structural evolution of an orogen and thereby provide evidence for the timing, location, and style of deformation. Along the Andean convergent margin, higher and more orthogonal convergence rates between the Nazca and South American plates due to the breakup of the subducting Farallon plate ca. 27 Ma led to structural shortening and basinward propagation of the Patagonian fold-thrust belt. Shortening continued through early Miocene time and was inferred to have been accompanied by contemporaneous topographic growth and migration of the topographic crest within the orogen interior as well as orogenic growth above frontal thrusts resulting from activation of basement structures by previous researchers. Here I present an integrated provenance analysis of new sedimentology, sandstone and conglomerate compositions, and detrital zircon U-Pb geochronology from 340 m of measured section of the latest Chattian-Aquitanian Rio Guillermo Formation of the Magallanes-Austral Basin, Argentina. These data provide compelling evidence that sedimentary stacking patterns at that time were dominantly controlled by tectonic influences, and reveal a coeval shift in sediment composition reflective of emergent sediment sources in the hinterland of the thrust belt. Sandstone and conglomerate intervals are compositionally immature and dominated by mafic volcanic signatures. I suggest that this result indicates that sediment was derived dominantly from orogen interior and westernmost thrust-belt sources, and also reflects topographic growth within the orogen interior. This provenance argues against physical isolation of the hinterland by frontal topographic growth. Furthermore, the limited outcrop extent of the dominant sediment sources constrains the locations of active uplift and the paleotopographic crest ∼10-15 km west of the modern topographic crest at ca. 23-21 Ma. This finding is compatible with previously suggested orogen scale drainage divide migration, but would likely preclude major additional migration at that time or thereafter. Instead I suggest that a large magnitude (up to 100 km) of eastward divide migration was accomplished between ca. 26-21 Ma. This notion is consistent with previously reported paleoclimate and exhumation data, but contrasts with existing models that suggest a delayed orographic migration and weak subduction zone coupling. |
