Progress of research in South Peru (and use of previously obtained credits)

The geological research I have developed in Peru since 2011, and in particular after my assignment in this country in 2012, has been conducted in parallel with the supervision of Mélanie Noury’s thesis (on a ministerial grant), and in collaboration with Matthias Bernet (the other supervisor of this thesis) and various Peruvian partners (Arequipa University, in particular).

The credits used were obtained following the internal ISTerre call for tenders (which redistributes credits paid by the IRD) for a total amount of 34,000 euros over 3 years (2011-2013, an average of 11,333 euros per year), as well as a financing by the INSU of 5100 euros (in 2013) which was specifically dedicated to Mélanie Noury’s thesis. I was the bearer of the various corresponding projects.

The sums thus obtained were used as follows :

• Various field campaigns, some of which are for teaching in the framework of local partnerships (contractual obligations of the IRD).
• Transport of samples.
• Processing of these samples for analysis.
• Dates by traces of fission (zircons ; apatites), U-Pb (on zircon), and Ar-Ar (especially on muscovites).
• Participation in the 2012 Peruvian Geological Congress.
• Purchases of equipment in Peru.

The partial use of credits from the IRD for expenses related to a thesis on a "South" subject corresponded to a practice that was common until then for me, especially since the conduct of this thesis was fully in line with the course of my own research on southern Peru.

It is obviously the costs related to the many dates obtained that were the most expensive, followed quite far by field campaigns.

The results obtained, which mainly consist of these dates, are very rich. On the one hand, they make it possible to complete, or even oblige us to revise, our knowledge of the geological processes that have been at the origin of Andean evolution, both in its early phase (Upper Cretaceous-Paleogenic) and in the Neogene, and to refine their chronology. On the other hand, they make it possible to propose an interpretation that is significantly different from what has been accepted until now. We thus highlight a period of very high magmatic flow at the subduction arc from ca. 73 to ca. 62 Ma. These very particular conditions seem to have led to a cruel thickening, since this period was immediately followed by a gravitational collapse of the arc from ca. 60 Ma, which gave rise to the formation of basins in extension (many normal sealed faults) where various sediments (conglomerates, sandstone, pelites, evaporites) of a dominant red colour (Lower Moquegua Group) accumulated from the upper Paleocene to the lower Oligocene.

Publications are being prepared, partly due to the late date of data collection (end 2013, for the most part). A first manuscript is well advanced and should be submitted in April. A second one, probably in June, concerning the exhumation of the Arequipa region, where the data are the most dense. A third will cover the whole of southern coastal Peru. Given the richness of the results obtained, it is likely that other publications will follow in the coming years.