MINIMUM AGE OF THE INCAIC II COMPRESSIONAL PHASE, A MAJOR DEFORMATIONAL EVENT IN PERU, AND ITS LACK OF RELATIONSHIP TO MAGMATIC ACTIVITY

The Incaic phase of the Andean orogeny, accounting for upwards of 50% of the total crustal shortening in Peru, involved folding and thrusting of Mesozoic platform sedimentary rocks and latest Cretaceous-Paleocene continental red beds of the Casapalca Formation, producing the Incaic fold and thrust belt. (e.g., Benavides Cáceres, 1999). Incaic orogenesis may consist of two distinct events: Incaic I and Incaic II. Isotopic dating of volcanic rocks unconformably overlying the surface produced by erosion of the resultant topography placed an upper bracket of 40-41 Ma on Incaic II deformation (Noble et al., 1979). The Coastal Batholith of Peru near Lima also was unroofed and is overlain by tuff dated at 41.3±0.5 Ma (K-Ar; Noble et al., 1978). New ⁴⁰Ar/³⁹Ar ages of 44.81±0.29 and 45.51±0.19 Ma on rocks overlying the post-deformation erosion surface at two locations within the Huancavelica Department plus a ⁴⁰Ar/³⁹Ar age of 45.78±0.31 Ma from the classic angular unconformity locality (12.441°S, 75.678°W) 14.5 km SSE of Yauricocha increase the upper limit of tectonism to 45-46 Ma. Various workers have incorrectly inferred or assumed that the minimum age of the post-Incaic II surface effectively dates the Incaic II compressive event; in fact deformation may well have ceased millions of years before surface formation. The lower limit of Incaic tectonism remains imprecise because of a scarcity of datable material below the unconformity. Episodes of compressive tectonism have been conflated with periods of increased magmatic activity. Such a relationship does not hold for the Incaic II phase; middle Eocene magmatic activity throughout Peru peaked between ca. 40 and 35 Ma, whereas tectonism had ceased by 45-46 Ma. The previous pulse of igneous activity in central Peru, defined by voluminous ash-flow units, plutons and basalt erupted between ca. 67 to 52 Ma may set a lower limit on Incaic tectonism. Tectonic interpretations and models, fission track and other exhumation studies, etc. should honor these time limits on tectonic and magmatic activity. For example, the new age constraints show that the major Incaic orogenic event, which extended from at least northernmost Peru to possibly south of Santiago, Chile, coincided with the ca. 50 Ma beginning of change in plate vectors tracked by the Hawaiian-Emperor chain (Wright et al., 2015).