THE QUECHUA GEOLOGICAL MONSTER
I feel like I am beating on drum that nobody cares about enough to listen to and make changes to produce better quality research. Most people who are not interested in science have already moved on after that first sentence. I am talking about incomplete research, failure to document previous work, and have a full understanding of the topic that researchers are PUBLISHING about. This failure runs deep in our geological sciences today, going from the graduate student advisor, the journal reviewers, to the individual conducting the studies. Why care? In part, this is because my own research is not being credited during the discourse on the regional geology of Peru. That makes it rather personal. It warrants a personal response.
The screen capture shown below is from a 2017 paper that is one of the more recent examples of omission and miss-reporting:
(red boxes drawn from time boundaries on tectonic events discussed below)
This study, like the ones I will list afterwards, does not go all the way- it cites old work and misses that these areas were re-evaluated and updated with new results that are significantly improved. How so? The old paper cited is by Mégard and other (1984) in the Ayacucho intermontane basin that used K-Ar ages to date structural relations in the type area of the Quechua phases of deformation in the Andes. All modern researchers that understand isotope geochemistry know that K-Ar is unreliable for various reasons and that these ages have relatively broad errors. Our new study in the same area uses 40Ar/39Ar method that significantly is more accurate. The old results had errors approximately plus or minus 0.3 to 1.5 million years; the new results we published on have errors that are plus or minus 40,000 years, or 0.04 million years. On the figure above from Margirier et al. (2017) I have edited to show the new intervals permissible for the tectonic events to have happened. This is night and day in terms of geological processes.
The Quechua II event in this figure appears rather constrained by the relatively thin gray bar, but the time scale is not appropriate to really show the difference between the range they mark and the updated very brief period in which it really happened. ONLY the Quechua I event is immediately present near the Cordillera Blanca, the Quechua II and III may not have continued this far north from central Peru. In detail, Margirier et al. (2017) list the three Quechua phases of deformation as being Q1 20-12.5 Ma, Q2 9.5-8.2 Ma, and Q3 ~6 Ma, citing McKee and Noble (1982) and Mégard et al. (1984). Those are pretty broad ranges from limited samples and numbers made by the K-Ar method of dating. The Q1 event was miss-cited. McKee and Noble (1982) placed brackets on this event from the folded volcanic section west of Huancavelica; the 12.5 Ma age was an upper limit, it was not implied that the deformation continued all the way up to this time. In Mégard et al. (1984) the ages defining deformation timing was not clearly articulated, but they did give the Q2 as 9.5 to 8.5 Ma (not 8.2 Ma as reported in Margirier et al., 2017) and loosely called it ~9 Ma. Furthermore, they had the Q3 event at 6.5 to 4.5 Ma and loosely summarized it at ~6 Ma.
Our work on the Quechua II at Ayacucho (Wise et al., 2008) placed the event soundly at 8.7 Ma; lower limit at 8.76 +/- 0.05 Ma and an upper limit of 8.64 +/- 0.05 Ma. This gives a maximum range of deformation as being 300 k.y., and probable duration being less than 150 k.y. for the folding that deformed the basin fill. In addition, the Quechua 3 timing in central Peru and the Huancayo intermontane basin was updated by Wise (2007) as being between 5.4 Ma and 4.8 Ma.
It is remarkable how many published geology papers dealing with age constraints miss-quote the original work, and treat the numbers in a sloppy manner while at the same time report new values and use the façade of being quantative.
Of course this paleostress study Margirier et al. (2017) has greater concerns than the regional context; in particular can one take structural measurements and believe they represent past stress fields while having no geochronology on the measurements? There are probably a lot of apples and oranges making this geological juice story. None their measurements can directly be compared to any of the Quechua events.
The kicker to the entire omission of the more recent work that I published on is that my co-author, my Ph.D. advisor Donald Noble, was also a co-author on the Mégard et al. (1984) and that he did all of the geochronological work for that paper. It was Dr. Noble’s passion for the geology around Ayacucho that brought his recommendation that I study this area and improve upon the dating. It is even more dumbfounding that Margirier et al. (2017) paper was published in the Journal of South American Earth Sciences, the exact same journal where our updated geochronology of Ayacucho was published in 2008!
This is not a one off occurrence. Below I list other studies in the last ten years that cite the old study, and omit the new one with far better results. It used to be in the pre-computing days we would spend days and weeks going over hard copy papers in the library, searching out the previous work on a topic. Now large databases and simple Google searches provide quick and very easy assembly of existing studies. So what excuse is there for missing key studies on the topic that is being investigated? Has the scientific process become lazy? Sloppy? Uncaring? You think about it, the Margirier et al. study had four co-authors, most that likely hold Ph.D.’s, and the paper would have been screening by a reviewer with a Ph.D., and an editor with a Ph.D.- that is some 7 Ph.D.s making crap. And I am just harping about one aspect of the paper they got wrong; how many other issues have been skipped through?
Other studies that pretend to discuss Cenozoic tectonism, all of them referring to the Quechua phase of tectonism in the Andes, and through partial investigation come to incomplete to incorrect findings by not citing the paper that best characterizes and describes this feature in its type area, include:
Margirier, A., Audin, L., Pêcher, R.A., and Schwartz, S., 2017, Stress field evolution above the Peruvian flat-slab (Cordillera Blanca, northern Peru): Journal of South American Earth Sciences, v. 77, p. 58-69.
Michalak, M.J., Hall, S.R., Farber, D.L., Audin, L., and Hourigan, J.K., 2015, (U-Th)/He thermochronology records late Miocene acceleratd cooling in the north-central Peruvian Andes: Lithosphere, v. 8, p. 103-115.
Scherrenberg, A.F., Kohn, B.P, Holcombe, R.J., and Rosenbaum, G., 2016, Thermotectonic history of the Maranon fold-thrust belt, Peru: insights into mineralization in an evolving orogeny: Tectonophysics, v. 667, p. 16-36.
Others work on Andean deformation and uplift have no idea of the names of major rock deforming events in Peru.
Gunnell, Y., Thouret, J-C., Brichau, S., Carter, A., and Gallagher, K., 2010, Low-temperature thermochronology in the Peruvian Central Andes: implications for long-term continental denudation, timing of plateau uplift, canyon incision and lithosphere dynamics: Journal of the Geological Society, London, V. 167, 2010, p. 803-815.
Espurt, N., Brusset, S., Baby, P., Henry, P., Vega, M., Calderon, Y., Ramirez, L., and Saillard, M., 2017, Deciphering the late Cretaceous-Cenozoic structural evolution of the north Peruvian forearc system: Tectonics, 10.1002/2017TC004536, 32 p.
Perez, N.D., and Horton, B.K., 2014, Oligocene-Miocene deformational and depositional history of the Andean hinterland basin in the northern Altiplano plateau, southern Peru: Tectonics, 10.1002/2014TC003647.
van Zalinge, M.E, Sparks, R.S.J., Evenstar, L.A., Cooper, F.J., Aslin, J., and Condon, D.J., Using Ignimbrites to quantify structural relief growth and understand deformation processes: Implications for development of the Western Andean Slop, northernmost Chile: Lithosphere v. 9, p. 29-45.
If a study is going to discuss regional tectonic events, it better spend time studying them. Similarly, if one is to report upon sample results, the location of the samples should be reported: for example see, It’s not science without reproducible location.
Finally, the studies that provide the current age constraints on deformation events in the Peruvian Andes are:
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.
Wise, J.M., Noble, D.C., Spell, T.L., and Zanetti, K.A., 2008, Quechua II contraction in the Ayacucho intermontane basin: Evidence for rapid episodic Neogene deformation in the Andes of central Perú: Journal of South American Earth Sciences, v. 26, p. 383-396.
Wise, J.M., 2007, Tectono-stratigraphic history of the Huancayo intermontane basin, central Peru: Boletín de la Sociedad Geológica del Perú, v. 102, p. 63-78.
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