TIME OF DEFORMATION IN THE ANDES- WILL THE REAL INCAIC AND QUECHUA EVENTS PLEASE STAND UP?

August 26, 2017 SouthAmerica

Major deformation events in the Andes have long been recognized as being short events in geological time, pulses when the crust along the South American western margin were crumpled through the compressive forces of convergent plate tectonics. The biggest compressional deformation event in Peru was named the Incaic phase by Steinmann (1929) in his classic work “Geologie von Peru“. This regional folding was originally considered a Cenozoic feature. Subsequently the event was temporally constrained by Noble and others (1979), and in another study by Noble and others (1985), to something that must pre-date about 42 million years ago. Later, in Chile, Tomlinson and Blanco (1997) suggested the Incaic phase affected the rocks at the latitude near Calama. The timing of the Quechua events was first pinned by Megard and others (1984) from Ayacucho, Peru. In Peru, the timing relations were based on what is now obsolete method of potassium-argon dating. For the Incaic phase, the two below citations have new results from argon isotopic dating, a far superior dating method, improving the timing of deformation to be older than 46 million years ago.

Noble, D.C., and Wise, J.M., 2016, Timing of Incaic deformation and subsequent erosion, middle Eocene volcanism, and plutonism in central southern Peru with implications for mineralization: Sociedad Geológical del Perú, XVIII Congreso Nacional de Geológia, Resumes Extendidos, 4 p.

Noble, D.C., and Wise, J.M., 2016, Minimum age of the Incaic II compressional phase, a major deformational event in Peru, and its lack of relationship to magmatic activity: Geological Society of America Denver annual meeting, Paper No. 208-4.

A more comprehensive manuscript providing the sample context and more data was submitted to the Journal of South America Geological Sciences (JSAGS) in 2015, with the title of “The Incaic II phase in Peru: timing and nature of compressional deformation and of the subsequent erosion surface“. During the review process Reynaldo Charrier, a Chilean geologist and Professor, rejected the paper for rather provincial reasons. Recently, Andean Geology has digitally published an article by Herrera and others called “Cenozoic tectonostratigraphic evolution and architecture of the central Andes in northern Chile based on the Aquine region, Western Cordillera (19° – 19°30’)“, which shows the timing of Cenozoic deformation events in northern Chile, marking the “Incaic” event to be at about 41-42 Ma (their figure 10), citing the timing from work by Cornejo and others (2003), and Charrier and others (2007;2009). Both of these studies basically retell the relations from Tomlinson and Blanco (1997). In fact, the area in Chile that best marks lower Cenozoic deformation was dated by Hammerschmidt and others (1992) [not cited by Herrera] to be a very short event at around 38.5 million years ago. Herrera and others (2017) do not cite (called omission) the widely known about studies by Noble and others in the timing of the Incaic event, work that has been around for thirty-five years. In addition, the Herrera study omits the information I published in 2008 called “Quechua II contraction in the Ayacucho intermontane basin: Evidence for rapid episodic Neogene deformation in the Andes of central Perú”, instead it cites the no longer useful timing relations in Megard and others (1984).

One can see how specialty researchers may find it difficult to keep up in their field and miss two recent conference papers that have information directly related to the topic they are publishing on, but to ignore age constraints in neighbouring Peru, the place where the Incaic deformation was defined, is a major oversight. Of course, if the paper on this topic was not rejected by JSAGS in 2015 it would be more obvious to Herrera and others that we have a problem on the timing of the Incaic event, which is that it is older than 46 million years in Peru, and in Chile, from a singular location, it is dated to about 38.5 million years of age. The continued miss-citation of the 41-42 million year timing is not helping matters.

The cross sections shown in this recent 2017 paper by Herrera and others indicate little absolute percent shortening as compared by Peruvian standards. I suspect this is in part because the geology around Aquine, and coastal southern Peru and northern Chile fall into the structural domain of the Coastal batholith block, as we published in 2014 paper called “Structural and stratigraphic relations between the rigid Coastal Batholith Block and the Highland Mobile Belt and their relation through time to magmatism and caldera formation“. This is a very important point because the Aquine study is not very useful for examining the Incaic deformation phase because it is hardly deformed at all.

I would recommend that the Incaic term be used for the event named and dated in Peru, and that this younger event in Chile no longer be termed the Incaic because they encompass two separate things. More grievous is that Herrera and others equivocally wish to take the deformation events in the Andes and stretch them out into prolonged 20 to 15 million year long periods when in fact the studies from Peru indicate they are very brief periods of deformation. Such is the results of selective “science” and extrapolation from providing only a few new ages with which to pin the timing of the geology in an inappropriately located small study area. My study in Peru was very specific, that the Quechua 2 event happened at 8.7 million years ago, and had duration far less than half a million years. Work on the Quechua 1 events places it at 17.5 million years, and it too was very rapid in Peru. Ayacucho is the type area of the Quechua 2 deformation. This information is completely at odds with the 14 to 6 million year long period of continuous contraction proposed by Herrera and others. Their study equivocally uses temporally widely spaced ages of essentially instantaneous volcanic deposit to interpret a continuum of deformation. The slight apparent rotation of units could result from the volcanic units overlapping paleo-topography, be from slumping, or even tilting caused by transpression of large wrench-style fault blocks adjacent to the Atacama fault zone. In other words, the tectonic framework represented by Herrera and others (2017) is completely wrong.

The real Incaic event of Peru was the most significant folding and thrusting to happen in the Peruvian Andes, and this deformation pre-dates 46 million years, and it was a rather quick event, lasting about three million years or less. The Quechua events at 17.5 and 8.7 million years ago were also rapid. The 38.5 million year old event documented by Hammerschmidt and others (1992) near Calama is something else, the “not the Incaic event.” The durations of compression in Chile clearly still need lots of work, and this should begin by not omitting previous work on the topics being studied. In the meantime, take a look at this paper:

Wise, J.M., 2015, Deformation and domains of the Central Peruvian Andes: A spatial approach using surface data: Boletín de la Sociedad Geológica del Perú, v.110, p. 161-175.

Or see the image at the top of this post, because it was the intense regional folding of pre-Cenozoic rocks that perked Steinmann’s attention, not subtle slight discordances and spaced low-displacement faults in overall very flat-lying tuffs along the coast of northern Chile. The real Incaic phase of deformation was spectacular and obvious.