HINTERLAND BASIN GEODYNAMICS: IMPLICATIONS OF LU-HF AND U-PB ANALYSES OF ZIRCON FROM THE TINCOPALCA BASIN, PERU

Geodynamic processes responsible for the formation of high topography in the Central Andean Plateau (CAP) are debated. Specifically, the relationship between crustal thickening and surface uplift in the CAP, and in Cordilleran orogens in general, is poorly understood. End member hypotheses include (1) coupled pure shear crustal thickening and concomitant slow (i.e., km/10s Myr) surface uplift; and (2) decoupled crustal thickening and rapid (i.e., km/Myr) surface uplift driven by foundering of mantle lithosphere. We address these hypotheses with new laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) paired U-Pb and Lu-Hf isotopic analysis of Oligocene-Miocene zircons from ignimbrite deposits in the Western Cordillera of southern Peru, collected and interpreted in the context of Oligocene – Miocene hinterland basin stratigraphy. Lu-Hf results calculated for the timing of crystallization, εHft, range from -0.4 to -16.8, and are consistent magma generation in thickened continental crust. Results show systematic increases in εHft from 22–18 Ma and 14–10 Ma, followed by a return to median pre-shift εHft values of ~-8. Increases in εHft between -10 and 0 are interpreted as juvenile magmatic input into thickened, partially recycled continental crust. The timing of pulsed juvenile magmatic input is in sync with a transition in hinterland basin deposition from dominantly fluvial to evaporitic lacustrine, and with recently reported rapid surface uplift between 22 and 10 Ma. Further, decreases in εHft do not correspond to episodes of surface uplift. Although quantitative analysis is still needed to test our preliminary interpretations, our qualitative analysis of LA-ICPMS U-Pb – εHft data tentatively supports a decoupling between crustal thickening and surface uplift in the northern CAP during the late Cenozoic, pointing to significant mantle involvement driving surface uplift.