Orogenic-Orographic Feedback and the Rise of the Central Andes

Evenstar, L., Hartley, A. J. & Mather, A. E., 15 Dec 2022, In: Earth and Planetary Science Letters. 602, 117931 

The rise of large mountain ranges is considered to be driven by tectonics potentially coupled with climate driven-erosion, although the role of this coupling remains uncertain. The arid climate of the Central Andes allows us to strengthen our understanding of the relative roles of these processes in mountain range development globally. Here we compile estimates of exhumation, sedimentation, aridity and surface uplift across the Central Andes for the last 50 Ma. We aim to place constraints on the relative timing of rock uplift (displacement of rocks with respect to the geoid), exhumation (displacement of rocks with respect to the surface) and surface uplift (displacement of the earth's surface with respect to the geoid). We show that initial rock uplift of the Andes extends back at least 50 Myr. This rock uplift generated orographically driven precipitation on windward slopes leading to increased exhumation but limited preservation of surface uplift. Eastward propagation of the mountain range resulted in increasingly extreme orographic effects on the leeward side amplifying aridity, reducing exhumation and increasing preservation of surface uplift. Essentially, surface uplift shows a ~5-10 Myr lag behind initial rock uplift as the Andes grow asymmetrically through time. We suggest that an eastward propagating pattern of exhumation, aridity and surface uplift with time, reconciles previous contradictory models of Andean uplift.

Geomorphology on geologic timescales: Evolution of the late Cenozoic Pacific paleosurface in Northern Chile and Southern Peru

Evenstar, L. A., Mather, A. E., Hartley, A. J., Stuart, F. M., Sparks, R. S. J. & Cooper, F. J., 8 Apr 2017, In: Earth-Science Reviews. 171, p. 1-27 27 p. 

The Atacama Desert on the western margin of the Central Andes is one of the driest and oldest deserts in the world. It is defined by a distinct and ancient surface, known as the Pacific Paleosurface (PPS) or Atacama Paleosurface. The age of this surface is determined as the time at which sediment deposition ceased, and the surface was effectively abandoned. Early studies suggested that this abandonment took place between 14 and 10 Ma, and was related to both the uplift of the Andes and the onset of hyperaridity in the region. Here we provide a regional re-examination of the PPS, compiling existing work on the underlying geology, sedimentology, surface exposure dating, and seismic profiling. We also present new multispectral satellite maps of the PPS and 45 new cosmogenic 3He and 21Ne surface exposure ages in order to constrain the formation age, and the preservation and incision history of the paleosurface. We conclude that the PPS is not a single paleosurface, but instead is a mosaic of smaller surfaces that were formed by aggradational and degradational processes over 19 million years (or more) and should be termed collectively as the Pacific Paleosurfaces. The time at which individual paleosurfaces formed is related to regional climate, where the location of each is controlled by regional tectonic activity. Cosmogenic surface exposure ages suggest that the surfaces are a record of regional scale climate events.

Using spatial patterns of fluvial incision to constrain continental-scale uplift in the Andes

Evenstar, L., Mather, A. E. & Hartley, A., 11 Jan 2020, In: Global and Planetary Change. 186, 103119. 

Geomorphic archives, particularly longitudinal river profiles, are increasingly used as a proxy to reconstruct uplift rates in mountainous regions. Within the Atacama Desert, Northern Chile, slow, long-term erosion creates exceptional preservation of fluvial and alluvial surfaces. This enables river incision patterns to be used on a continental-scale (>250 km) along the western margin of the Andes (18°00’S to 20°15’S) and over a time frame from Miocene to Present day. The data show marked compartmentalisation of fluvial system behaviour with changes in incision rates from south to north creating 3 distinctly different regions. Within these different sectors, incision rates are broadly consistent between rivers suggesting a regional rather than a river specific control on rates. In Sector 1 (18°05’S to 19°20’S) the fluvial systems are exorheic with a terminal base level (the lowest base level to which the river system can erode) in the Pacific Ocean and span the Coastal Cordillera, Longitudinal Valley, Precordillera and western edge of the Western Cordillera. This constrains the total uplift over these regions to a minimum of 1200 m in 11 Myr with incision rates of ~200-120 m/Myr consistent with rapid but sustained uplift of the Andes in the Late Miocene. In Sector 2 (19°20’S to 19°50’S), to the immediate south, the rivers are shorter and terminate in the Longitudinal Valley, spanning only the Longitudinal Valley, Precordillera and the western edge of the Western Cordillera with lower incision rates of 100-50 m/Myr. Comparison of incision rates between Sector 1 and 2 can constrain the uplift of the Coastal Cordillera to 60m/Myr which is in keeping with previous studies from the region. In southernmost Sector 3 (19°50’S to 20°10’S), the fluvial systems terminate in the Longitudinal Valley and span only the Longitudinal Valley and eastern part of the Precordillera with low incision rates of 50 to 25 m/Myr. Differences between Sectors 2 and 3 are attributable to drainage loss by tectonic beheading of catchments through uplift of the Cordillera de Domeyko fault system, placing a minimum constrain on uplift in this region of 50 to 25 m/Myr. This study demonstrates the applicability of large-scale fluvial archives to access not just the timing of uplift on a continental scale, but also the relative uplift of individual tectonic provinces..

A Rusty Record of Weathering and Groundwater Movement in the Hyperarid Central Andes

Shaw, J., Evenstar, L., Cooper, F., Adams, B., Boyce, A., Hofmann, F. & Farley, K., 20 Jul 2021, In: Geochemistry, Geophysics, Geosystems. 22, 8, 

The Atacama Desert, on the western margin of the Central Andes, hosts some of the world's largest porphyry copper deposits (PCDs). Despite a hyperarid climate, many of these PCDs have undergone secondary “supergene” enrichment, whereby copper has been concentrated via groundwater-driven leaching and reprecipitation, yielding supergene profiles containing valuable records of weathering and landscape evolution. We combine hematite (U-Th-Sm)/He geochronology and oxygen isotope analysis to compare the weathering histories of two Andean PCDs and test the relative importance of climate and tectonics in controlling both enrichment and water table movement. At Cerro Colorado, in the Precordillera, hematite precipitation records prolonged weathering from ∼31 to ∼2 Ma, tracking water table descent following aridity-induced canyon incision from the late Miocene onward. By contrast, hematite at Spence, within the Central Depression, is mostly younger than ∼10.5 Ma, suggesting exhumation ended much later. A heavy oxygen isotopic signature for Spence hematite suggests that upwelling formation water has been an important source of groundwater, accounting for a high modern water table despite persistent hyperaridity, whereas isotopically light hematite at Cerro Colorado formed in the presence of meteoric water. Compared with published paleo-environmental and sedimentological records, our data show that weathering can persist beneath appreciable post-exhumation cover, under hyperarid conditions unconducive to enrichment. The susceptibility of each deposit to aridity-induced water table descent, canyon incision and deep weathering has been controlled by recharge characteristics and morphotectonic setting. Erosional exhumation, rather than aridity-induced water table decay, appears to be more important for the development of supergene enrichment.

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