Andean deformation and uplift: a comparison between geological and geochronological constraints with fission-track exhumation method
Uplift of the Andes is a central question on Andean geology. Many studies have addressed uplift as a primary justification for field investigation, but there remains a significant disconnect between information from structures mapped in the field and constrained by isotopic dating and cross cutting relationships that indicates one certain set of facts about the mountain chain development whereas thermal modelling of fission track data on apatite and zircon infers another set of conditions on mountain formation. In particular, geological studies provide limits on the timing of deformation whereas the fission-track studies of rock cooling and exhumation derive statements on Andean uplift that do not match the deformation events. This summary in outline form provides the observations for comparison between geology and fission-track studies.
Geology and deformation timing
The Andean structures in Peru, such as folds and faults, formed during distinct short periods, including:
G1. Incaic deformation at ~50 to 46 Ma (Noble et al., 1979a; Noble and Wise, 2016), account for at least half of the rock strain (Wise, 2015). Most of this deformation happened east of the coastal batholith (Wise et al., 2014). Noble et al. (1979a) clearly stated that deformation was before 40-41 Ma (even emphasized this in the paper’s abstract); too many to count subsequent studies have used this number as the ACTUAL RANGE of the Incaic II deformation, which is completely incorrect.
G2. Quechua I event at about 17.9 Ma; note that volcanic formations are mainly parallel to one another from Morococha to Castrovirreyna, indicating that rotation/folding had not happened during the 42 Ma to ~24 Ma period in central Peru. In the southern Cordillera Negra a ca. 18 Ma ash flow tuff is folded. In central Peru the Quechua I event post-dates ca. 21 to 18 Ma ash flow tuffs at several locations.
G3. Quechua II event is narrowly bracketed at 8.7 +/- 0.2 Ma from a major angular unconformity in the Ayacucho intermontane basin (Wise et al., 2008). Major rock deformation is focused along the intermontane depressions and only locally with the western Cordillera of Peru.
G4. Quechua III event formed at about 5 Ma (Wise et al., 2007); as best marked by folded late Pliocene formations in the Huancayo intermontane basin.
G5. Time periods between the three Quechua events either represent neutral to slightly extensional conditions (Wise et al., 2008). This is well-marked by the Pliocene NS-striking dikes at Huancavelica: these cannot have formed during compression at this level in the crust. Modern seismicity marks both reverse and normal fault slips in the focal mechanisms (Suárez et al., 1983) and recent normal and reverse faults scraps are present in the Peruvian Andes (Blanc et al., 1983; Cabrera, J., 1989; Wise and Noble, 2003). Over geological time the sense of movement on major faults have changed, such as documented on the Chonta fault of central Peru (Wise and Noble, 2001). Extensional conditions are also marked by basaltic volcanism in central Peru (Noble et al., 1999).
G6. Of the three Quechua events number 1 was more pervasive and of a higher degree of strain (personal field observation). The Quechua II and III events were more restricted to the intermontane basins (remarked on by Mégard et al., 1984, seems to hold true).
Erosion/unconformity timing
Regional erosion surfaces and paleotopography is locally preserved that generally formed in the periods between the major compressional events outlined above.
G7. A post-Incaic erosion surface that is found throughout much of Peru.
G8. The Coastal batholith of Peru was mostly exhumed to surface before ~42 Ma (Noble et al., 1978). The rock uplift and erosion likely occurred during and immediately after the Incaic II compressional event (an example of G7).
G9. Some of the Pacific flank of the Peruvian Andes has polyphased erosion surfaces that represent preserved Post-Incaic erosion, reactivated similar surfaces, and early Miocene pediplanation (Noble et al., 1979b, Tosdal et al., 1984, Quang et al., 2005).
G10. Locally early Miocene paleo-canyons formed in central Peru that had up to 1600 meters of cross-canyon relief (Noble et al., 2009a).
G11. In the middle Miocene the Puna erosion surface formed from about 16 to 9 million years ago.
G12. Late Miocene canyon formation was widespread throughout Peru, and north of Lima, at Rio Forteleza (Myers, 1976), mark that the Andes had a canyon longitudinal paleo-relief of at least 2450 meters by about 7 Ma.
G13. Widespread coastal plains formation of late Miocene conglomerate deposits and mark significant erosion, transport, and deposition before 7 Ma (Noble et al., 2009b).
Paleo-elevation
G14. Oxygen isotope data from the Casapalca base-metal district of central Peru, a mining camp that sits at a current elevation of ~4500 m, was interpreted by Rye and Sawkins (1974) to have possibly been from source waters in the Miocene that represented higher elevations than the modern Andes.
G15. Paleo-elevation from faunal studies (Gregory-Wodzicki, 2000) provide some relative timing on high elevations in the Andes, although these indicators are too broadly dated to allow for meaningful correlations between deformation events or periods of cooling or heating defined by fission track studies.
Finally, some geological studies that are widely cited to build descriptive tectonic framework for the Peruvian Andes have been revised and updated (e.g., Mégard et al., 1984) while others that are widely cited are demonstrably invalid (Carlotto, 2013).
Fission track data and interpretation
Several fission-track studies on cooling of apatite and zircons have reported on a variety of locations throughout Peru over the last twenty-five years. The cooling history modelled in these studies made the following interpretations about the exhumation and uplift of the Andes.
F1. Naeser et al. (1991) reported zircon fission-track results from several Tertiary volcanic units within the sedimentary sequence in the Bagua syncline. These were straight ages not considering elevation and the study did not do any thermal modelling. They placed the ages with respect to the tectonic framework of Noble et al. (1989) but the method used does not give any direct measurement of the above outlined tectonic and erosion events.
F2. Two intrusions in the Cordillera Oriental of southern Peru were studied by Laubacher and Naeser (1994). The first pluton (Huachon granite) was emplaced in the Permian, and records zircon cooling at 160 Ma. Apatites from the same body yield Neogene ages. This suite of samples did not mark influence from the Incaic compression (G1). The second intrusion (Quiparaca granite) interpreted as being emplaced at 30 Ma.
F3. Garver et al. (2005) study of the Cordillera Huayhuash goes through very detailed fission-track method considering zircon reset conditions and couple it with (U-Th)/He single zircon grain dating. They report on 10 samples of detrital zircons from Cretaceous quartzites (Carhuaz, Oyon, and Chimu Formations), units that are appropriately old enough to record any thermal changes from the earliest Cenozoic tectonic events. Garver et al. (2005) samples mark resetting between 57 and 68 Ma (mean reset age at 63 Ma) for which they relate to structural imbrication (deformation) of the Cretaceous Formations. They discount a correlation with the Incaic event, but they did not have an understanding of a proposed duo or set of sub-Incaic events called for by Noble et al. (1985) nor have the revised upper limit to the Incaic II event at ~46 Ma (G1, Noble and Wise, 2016). A younger resetting condition marks an interval between 32 to 24 Ma (peak at 27 Ma) that Garver et al. (2005) correlated to reheating by the overlying Tscara Volcanics. A late Miocene reset and cooling event from 15 to 7 Ma, likely related to nearby 11 Ma granitic intrusions. This study shows no indication of the 17 Ma Quechua 1 event. Late Miocene exhumation is compared to the history of the Cordillera Blanca and local canyon incision, however, numerous other regional events are in the background (G7 through G13).
F4. Wipf (2006) studied the effects of the subduction of the Nazca ridge along the Peruvian margin by dating apatites using the (U-Th)/He method. This study is rather unsatisfactory in the application and conclusions even while it did take the effort to outline the tectonic events and discuss the ages of the units dated. For example, while going over the geomorphic effect of the Nazca ridge subduction, the study misses the most significant indicator of major erosion along the margin, which is to the north of the ridge the Pacific flank covering ash-flow sheets are mostly absent, whereas to the south units like the Nazca tuff and the Pocoto tuff drape over large gently tilted pediments. That Wipf (2006) does not mention the Pocoto tuff at all indicates incomplete research on the geology. The thermal models presented in this study result in very broad age ranges that are too noisy for interpretation.
F5. Ruiz et al. (2009) calls for steady exhumation rate of 0.17 km Ma-1 from 38 to 14 Ma from a profile take across a single pluton along the northern margin of the Andahuaylas-Yauri batholith. Their sample location is younger than the timing of the Incaic events (G1, G8), overlaps in timing of the multiple west Andean flank erosion surface (G9), and does not pick up any disturbance from the Quechua I compressional event (G2).
F6. Schildgen et al. (2007) and Thouret et al. (2007) gave a thermal model of cooling attributed to canyon erosion at 15 degrees south.
F7. Gunnell et al. (2010) low-temperature thermochronology study of the Cotahuasi-Ocoña River determined plateau erosion formed before 24 Ma, and canyon incision advanced from 14 to 9 Ma. This study pays close attention to other contributing factors on the geologic evolution, including crustal thickness, a flexural deformation style, absence of compressional deformation, and plate dynamics. In their figure caption 6 Gunnel et al. (2010) remark that the sampled rocks were near surface since 50 Ma, and that Cenozoic denudation since then has been limited, which is compatible with the noted batholith exhumation in central Peru (G8). Gunnell et al. (2010) note the importance of Oligocene to early Miocene erosion surfaces, remarking on marine deposits that are locally found beneath the Nazca Formation and the Moquegua Formation (G9).
F8. Michalak et al. (2013; 2015) increased cooling modelled from northern Peru starting at 14 to 10 Ma, suggested to mark climate change and increased exhumation rates.
F9. Margirier et al. (2015) sampled two drainages in the Cordillera Negra for FT analysis; their thermal modelling marked cooling between 30 to 23 Ma, reheating from 23 to 15 Ma, followed by cooling from 15 to 0 Ma. Margirier et al. (2015) suggested the reheating event coincides with the prolonged volcanic arc of the Calipuy volcanics, although these volcanic rocks pre-date the earlier part of their oldest sampled material, and the 23 to 15 Ma very loosely or broadly brackets the ~17.9 Ma Quechua 1 contraction (G2), which caused regional tilting and local folding of the Calipuy volcanics that are as young as 18 Ma. The role of the subducting Nazca ridge influencing the above thermal patterns appears completely speculative or not required.
F10. Scherrenberg et al. (2016) makes several FT based assertions on the timing of events in Peru. The summary graph in their fig. 5 suggests that thin-skinned deformation began at 45 Ma, whereas data by Noble and Wise (2016) demonstrated for the Incaic event it was finished before 46 Ma. Was Scherrenberg referring to areas only in the eastern foothills where no hard geological and dated cross cutting relationships can be used to sound out the concept? They place magmatism/exhumation in the 16-14 Ma period, exhumation at least may be delayed from the 17.9 Ma Quechua I shortening, but magmatism? In general this period represents a magmatic lull in Peru. Their marked period of 12-9 Ma magmatism and 7-0 Ma “uplift-exhumation-erosion” does not really fit with the 8.7 Ma Quechua II event, nor do they discuss the timing of this event from the type area of Ayacucho. The range shown for the Incaic I and Incaic II folding events in their fig. 7 is so sloppy that it is not worth repeating here.
F11. Benavente (2014; 2017) sample traverses at Rios Cañete and Nazca yield a couple of periods of cooling ages. The Nazca profile gave two cooling periods that are dependent on the distance from the coast. Near the coast at Nazca a period from 90 to 60 Ma cooling is evident in Jurassic aged plutons. Whereas in the headwaters cooling was from 60 to 40 Ma. Benavente (2017) provides yet another example of miss-citing information as explained under G1; see page 163 of his dissertation where he uses the 40-41 Ma upper limit on the Incaic event as the timing of the event. At the latitude of Cañete the samples gave a cooling period from 11 to 5 Ma [it should also be noted that only 4 samples were used to make this calculation]. Benavente (2017) also generalizes that both profiles record cooling after 9 Ma, but then on page 160 acknowledges the thermal inversions did not model both the AFT and AHE data on the Cañete profile. More troubling is during all of the tectonic summaries made the Quechua deformation event (s) in Peru is only mentioned once in his entire dissertation, for which we must award him the “Quechua Geological monster award” for highly incomplete research citations. I would not rank this study to even have master’s degree quality work from universities in the U.S.A., instead it is more like an extended term paper and it begs the question what is going on at French universities?
DISCUSSION
How does rock strain during regional compressional events relate to mountain building? It is generally believed that during folding crustal thickening happens, and during these events mountain building happens. How can rock deformation and strain be cross correlated to fission track studies that model uplift and then compared to tectonic events? This is a complex question that depends on the actual local context of rock during orogenesis. For example, during deformation high-levels in the crust may undergo uplift and erosion, yielding a cooling thermal history, whereas simultaneous other portions of the crust, such as beneath a thrust sheet, may undergo loading and burial, resulting in mineral resetting, which in thermal model space are considered reheating. The locational context is particularly important with respect to the rigid Coastal batholith block, which since the latest Cretaceous underwent uplift, and then during the Cenozoic remained stable while deformation was focused in domains to the east. Because the thermal history is complex, and the geological constraints widespread and local, plotting the various lines of evidence outlined above helps illustrate the problem (see the above graph).
We have very little, if any, evidence of cooling during the Incaic II (G1; possibly measured in F3) in the fission track studies done to date. This in part reflects the selected sample profiles with the studies perhaps having been more focused on Neogene uplift over resolving older deformation events. From the graph it is easy to appreciate that the duration of modelled cooling events are significantly longer than the geologically constrained periods of rock deformation. Paleo-relief data indicate that rock uplift and erosion were operative. We know this with observations from paleocanyon fills (G10, G12), and thermal models in certain areas (F6). Several fission-track studies give a broad period of Miocene cooling, which does not correlate well with the measured timing of the Quechua I compressive event (G2) or the Quechua II event (G3).
CONCLUSIONS
Thermal modelling of zircon and apatite fission track data in many areas has more than one or two geological processes or events in Peru to consider. It is too simplistic to focus on features such as the Nazca ridge, or to make conclusions about uplift without reference to a complete inventory of tectonic and paleo-erosion events. Sample location selection across the known structural domains is of great important for the testing of the above listed geological constraints. Incomplete research on the salient factors and structural domains of the Peruvian Andes may yield miss-directed conclusions. Part of the apparent spread in results derives from inconsistent reporting; some papers give cooling times, others focus on resetting or reheating, some state exhumation rates, and a few will estimate the amount of rock eroded. Plotting all of the above modelled times of erosion, exhumation, magmatism, folding, etc., results in overlapping ranges with no agreement between deformation phases and uplift timing. Exhumation can take place during compression, quiescence, and extension. Periods of higher erosion rates do not necessarily mean surface elevation increases. While uplift of the Andes is the extolled Holy Grail for many studies, what drives it (deformation) is the bigger question. Deflections, cooling, and local incisions are manifestation and the aftermath of deformation. Rapid thickening of the crust may be followed by a period of time for isostatic equilibrium to be reached. The issue of Andean uplift spans many geological parameters such that incomplete researched studies will only yield miss-interpretations. Finally, there is perhaps a general assumption that because the Andes are relatively high the present geomorphology is the culmination to a singular conclusion and does not consider during the geological past the surface elevation may have been greater than it is today. In any event, reaching a consensus between geological controls on Andean formation and thermal histories will require at some point the thermal methods to become more fully aware of the geology and conduct more comprehensive research.
James M. Wise, August, 2018
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Wise, J.M., and Noble, D.C., 2001, La Falla Chonta del Peru central- una falla inversa con reactivación de rumbo sinestral respondiendo a un cambio de la oblicuidad relativa de convergencia de las placas tectónicas: Boletín de la Sociedad Geológica del Perú, v. 92, p. 29-41.
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