Exhumation Of The Peruvian Andes; Insights From Mineral Chronometers

Author(s): Michalak, Melanie | Advisor(s): Farber, Daniel | Abstract: The Andes of South America display the classic plate tectonic setting of oceanic-continental convergence. Subduction since the Mesozoic of the Nazca plate beneath the South American plate has produced the Andes, which are characterized by the second-largest plateau in the world, major deflections, flat-slab sections of subduction, and varying widths of high topography along-strike. While the general tectonic setting of the Andes is known, significant questions remain with respect to the timing and magnitude of the uplift of the Andes, the mechanisms responsible for uplift and exhumation, and how the tectonically segmented nature of the orogen affects its formation. Here, we use bedrock apatite and zircon (U-Th)/He thermochronology and bedrock and U-Pb zircon geochonology to determine rates and timings of crustal cooling due to exhumation, plutonic emplacement ages, and provenance of detritus, in order to investigate the evolution of topography and regional exhumation in the Peruvian Andes from 5 and 12°S. Six sample sites lie within different tectonomorphic and geologic regions of Peru, from the Mesozoic Coastal Batholith of the Western Cordillera, to the Cordillera Blanca Batholith region to the Maranon corridor of the Eastern Cordillera. In the Maranon Corridor of the Eastern Cordillera, we use both apatite and zircon (U-Th)He thermochronology to show that rock exhumation rates increased by roughly an order of magnitude from the Mesozoic by Late Miocene time, from ~0.04mm/yr to ~0.25mm/yr. U-Pb detrital zircon provenance from the Cretaceous aged-sandstones and quartzite samples from the Maranon sample sites shows that the ZHe cooling ages are not reflecting unreset cooling ages, rather, partial retention of He near the closure depth of Zr. The detrital U-Pb zircon ages from the sedimentary units show that during deposition in the Cretaceous, provenance sources changed spatially and temporally, from a dominant Paleozoic basement source, to Proterozoic cratonic sources from the Amazon shelf. In the Coastal Batholith of the Western Cordillera in Peru, a Miocene rock cooling signal from both zircon and apatite (U-Th)/He thermochronologic systems characterizes three along-valley sample sites at 5, 8 and 12°S, while U-Pb zircon geochonologic ages on the same samples vary from late-Cretaceous to Oligocene. In the site at 12°S, along the Rio Rimac, the age spectra displays younging orogen-ward, indicating a spatial gradient in exhumation, where faster rock exhumation occurred in the internal part of the orogen, compared to the coast. In summary, the results presented here constrain general rock uplift and exhumation timings of the Peruvian Andes. By the late Cretaceous, the Coastal Batholith was being emplaced, while a back-arc basin lie to the east, fed by Amazon cratonic units. Slow rock exhumation and possibly little relief growth affected the Western and Eastern Cordilleras until the early Miocene, where we show rock exhumation rates accelerated. Secondly, we find that the general timings of regional rock exhumation in the north-central Peruvian Andes are similar to the timings of uplift and generation of modern topography documented in the Central Andean segment. Despite the tectonomorphic differences between the two regions, perhaps a unifying mechanism might exert a control on formation of the Andes. We suggest orogenesis and along-strike relief growth may be accomplished by a strong coupling of tectonic and climatic forces since Miocene time.