Steady-state vs. flare-up magmatism revealed by magmatic addition rates in the Central Andean arc

Magmatic additions to the crust during flare-ups are at least one order of magnitude higher than long-term steady-state magmatism, according to an analysis of the volumes erupted over the last 35 Myr in the Central Andes based on a new volcanic database
Steady-state vs. flare-up magmatism revealed by magmatic addition rates in the Central Andean arc

The characteristic secular trend of subduction-related magmatism in Cordilleran arcs is one of long periods of relatively calm activity, punctuated by relatively short, intense magmatic episodes. The former condition is characterized as steady state, while the latter, flare-up. The steady-state condition builds volcanic arcs characterized by composite volcanoes and other smaller-scale volcanic landforms. In contrast, flare-ups manifest themselves as periods of prodigious explosive volcanism, where hundreds to thousands of cubic kilometres of magma are individually erupted from large calderas, critically altering the landscape. Compared to the steady-state, flare-ups are hypothesized to be ignited by major geodynamic perturbations in the subduction system, causing elevated rates of magma addition to the crust. The difference in flux has implications for the construction and growth of upper crustal plutonic complexes, crustal architecture, and growth and evolution of the continental crust. However, magmatic fluxes and the precise timing between the start of the geodynamic perturbation and the onset of the flare-up are hard to constrain particularly due to the lack of high-resolution data on erupted volumes, eruption timing, and spatial distribution of volcanism over time, as well as major uncertainties in both crust-mantle mass balances and plutonic-to-volcanic ratios. The 22.5-29°S segment of the Central Andes (Fig. 1), straddling the Chilean-Argentinian border, records part of a Neogene flare-up that tracks the southward subduction of an oceanic ridge, long proposed as its major geodynamic driver. This region hosts a large number of volcanoes, several beautifully preserved due to the extreme aridity of the area, which in addition to recent, extensive geological mapping and new radiometric ages provides an ideal place where to investigate the questions outlined above.

Fig 1. Location map of the Central Andes
Fig. 1: Location map of the Central Andes.

We have computed new estimates of erupted volumes for the 22.5-29°S segment of the Central Andes for the last 35 Myr at a 10-kyr resolution. Our estimates are based on a new volcanic geospatial database, which compiles information from 53 geological maps, 2,029 radiometric ages, 217 scientific articles, targeted remote sensing studies, as well as fieldwork to ground truth data in areas of poor mapping or satellite coverage. 2,057 mapped volcanic deposits were assigned a volume and age range, and then integrated into a model to track the volume evolution over time for three segments of latitude by considering ignimbrites and other smaller-scale volcanic products both individually and combined. By calculating the overall magmatic addition to the crust (i.e. new crustal growth rates), we find that the average contribution during the ephemeral Neogene Central Andean flare-up exceeds that of the long-term steady-state stage by at least an order of magnitude (Fig. 2).

Fig 2. Crustal growth rate (in km3/km of arc/10 kyr) versus time (in Ma). Blue lines depict six different moving averages
Fig. 2: Crustal growth rate (in km3/km of arc/10 kyr) versus time (in Ma). Blue lines depict six different moving averages.

We estimate a time-lag  of ~8-12 Myr between the passage of the oceanic ridge and the onset of the flare-up at the surface, thus refining the 5-10 Myr lag time calculated by others using a lower temporal resolution. The Central Andean flare-up is time-transgressive, with the onset of the flare-up becoming younger as we move south. Should the lag time calculated here continue to the south of our study area, the 26-29°S segment is likely to experience a flare-up in the next few Myr. In this latter segment, some large-scale crustal anomalies detected at mid-crustal depths, interpreted as partial melt bodies, might be an early signal of this future flare-up.

Our study demonstrates the use of comprehensive volcanic databases as tools to unravel the tectono-magmatic history of continental arc segments. We believe our research provides an innovative and systematic approach to investigate the steady-state and flare-up magmatic stages for long-term active volcanic regions at detailed time scales. The methodology developed in our study can be tested and replicated elsewhere. The files required to do so are publicly available here.

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