There are few landscapes quite as inspiring as mountains. Gazing across a vista of peaks, it’s easy to feel lost in the scenery, to feel small, and temporary. It’s also tempting to consider the Earth’s surface as immutable and unchanging. However, geoscientists have long recognized that Earth’s surface is far from static; over geologic timescales, mountains slowly rise and fall through the interplay of plate tectonics and surface processes like erosion. In our article “Cycles of Andean Mountain Building archived in the Amazon Fan”, we provide a deep-time perspective on large-scale and repeated tectonic processes of magmatism (creation of granite crust and volcanoes) and mountain building that affected western South America and show there have been cycles of mountain building occurring over timescales of approximately 60 – 90 Myr over the past half billion years.
Link to published article: https://www.nature.com/articles/s41467-022-34561-6.pdf
Studies of cordilleran systems, or mountain building in North America and western South America have revealed an episodic nature of magmatism and growth of the mountain belt. In other words, there are times of mountains growing — high magmatic activity and greater rates of surface uplift and movement of crust along faults — separated by times of relative inactivity. Some researchers have hypothesized a cyclic pace of these large-scale processes, with various estimates for the rhythm of these cycles, ranging from approximately 20 – 70 million years, for several locations in the central and northern Andes. However, remote and rugged terrain of the Andes complicates access to physical rock samples along the range, challenging the study of processes at such large scales. There is a way to collect samples of sediment from enormous portions of continents; by sampling river mouths and submarine fans we can integrate a sample across large-scale landscapes.
Large river systems, in particular the Amazon, have tributaries that carry sediment from mountain sources to the coast and ultimately to the deep sea, where thick accumulations of terrigenous sediment are preserved for millions of years. These deep-sea fans contain valuable information about the histories of the continents they are linked to, and in the case of the Amazon Fan, we speculated there would be information about the mountain-building history of the Andes.
My co-authors and I analyzed many individual sand-sized zircon crystals found in Pleistocene-aged sediment recovered offshore of eastern South America in the Amazon Fan. We applied U-Pb geochronology and U-Th/He thermochronology to detrital zircons; where generally, U-Pb ages represent the timing of crystallization from a magmatic source, and U-Th/He ages provide an estimate for the time since the zircon cooled through the helium closure temperature of about 200 degrees °C. Application of both dating techniques to the same zircon crystal, commonly called ‘double dating’, can be used to reconstruct ancient sediment source areas, tectonic configurations, and the timing of widespread or large-magnitude crustal cooling events often associated with mountain-building processes. In other words, we determined when a zircon crystal formed deep in the Earth and when it was brought to the surface to become sediment.
Fortunately, in the 1990s the Ocean Drilling Program (ODP, now International Ocean Discovery Program; IODP) collected dozens of sediment cores from the Amazon Fan, and that material is stored in a temperature-controlled facility at MARUM Core Repository, in Bremen Germany. In Bremen we sampled sediments deposited on the deep Atlantic sea floor, brought that material back to Texas to isolate the zircons, and applied U-Pb and U-Th/He double dating to 114 individual crystals.
We combined this new data set with our existing database of over 1,300 individual U-Pb ages from that same material, so we had a good idea of the distribution of samples that would capture the typical U-Pb age signature of the Amazon River-Fan system, and thus the typical helium cooling age distribution of the Amazon River.
We expected many young U-Th/He ages, representing recent erosion of rocks that make up the Andes (the lower-elevation parts of the continent erodes slowly, thus cooling ages are much older). Indeed, almost 40% of the zircons we analyzed had relatively young cooling ages indicative of Andean sources 1000s of km from the Atlantic coast. Some detrital zircons had older, craton U-Pb crystallization ages, and very young U-Th/He cooling ages, suggesting complex processes of sediment transport from the craton to the west to Andean basins, followed by burial heating of sediment deep in basins, and later re-exhumation and transfer to the east through the Amazon River—an interesting story in and of itself.
We focused also on populations of zircons with relatively old helium cooling ages, suggesting that they were affected by tectonic processes long before the formation of the modern Andes. Age peaks in the record spanning the entire Phanerozoic seem to have an episodic or periodic pattern. To further investigate what appeared like cyclic patterns, our collaborator Dr. Molly Patterson applied a technique of spectral analysis to the data from the Amazon Fan to quantify potential periodicity in the dataset. Somewhat to our surprise, we found statistically significant periods in age modes from detrital zircon U-Pb and helium data, with analyses suggesting a magmatic periodicity of 72 Myr, and crustal cooling events with two periods of 57 Myr and 91 Myr.
While our dataset doesn’t necessarily clarify the driving mechanisms of repeated and cyclic orogenesis, it constrains the timing of processes affecting magmatism and crustal cooling over an enormous length of the modern Andes. Our dataset suggests a long history, pre-dating the modern Andes, of plate coupling, magmatism, and deformation along the western margin of South America (or Gondwana).
This dataset and our interpretations represent an advance in our knowledge of the large-scale and deep-time tectonic behavior of the South American cordilleran system. We hope you enjoy our article!