Age and evolution of diachronous erosion surfaces in the Amazon: Combining (U-Th)/He and cosmogenic 3He records

Abstract

 (U-Th)/He geochronology of goethites in stepped landscapes shows that older surfaces are higher and younger surfaces are lower.  Cosmogenic 3 He in hematites from the Carajás Plateau indicate extremely low rates of erosion.  (U-Th)/He geochronology of goethites reveals that low-lying surfaces are diachronous, younging towards receding escarpments surrounding older plateaus. 2 ABSTRACT (U-Th)/He geochronology of two weathered plateaus in the Carajás Mountains, Pará, Brazil, reveals a history of weathering spanning from ca. 80 Ma to the present for this high elevation (~720 m) land surface. Cosmogenic 3 He measurements of hematite pebbles and blocks cemented onto the plateaus at two sites, N1 and S11D, yield erosion rates as low as 0.09 and 0.08 m.Ma-1 , respectively. Thus, these results confirm that the plateau surfaces are nearly immune to physical erosion for tens of millions of years. (U-Th)/He geochronology of ferruginous duricrusts blanketing the low elevation (250-100 m) plains surrounding the Carajás Mountains yield results consistently younger than ~10 Ma. The geochronology results also reveal that the low elevation plain is diachronous, becoming progressively younger towards the receding plateaus. The spatial distribution of (U-Th)/He ages permits reconstruction of the history of scarp retreat for the Carajás landscape, showing that scarp retreat along major river valleys may have been as fast as 20 km.Ma-1 during tectonically active and humid periods in the Cenozoic. The cessation of scarp retreat at some sites suggests that metamorphosed banded iron-formations and quartzites provide effective barriers to retreating escarpments, helping to preserve some of the oldest continuously exposed land surfaces on Earth.

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…Depth scale is approximate, as weathering profile thickness is variable and can be >100 m. Insets (b, c) show metallic, Fe oxide enrichment and a scanning electron microscope image of a goethite concretion (Allard et al, ), respectively. (d) Frequency of supergene goethite He and hematite He dates from northern Brazil (Allard et al, ; Monteiro, Vasconcelos, & Farley, ; Monteiro, Vasconcelos, Farley, & Lopes, ; Shuster et al, , ). (e) Temperature variations based on δ 18 O isotopic composition over the Cenozoic (Zachos et al, ).…”

“…Figure 10 depicts a typical lateritic profile and its mineralogical compartments, capped by a strong Fe-enriched weathering horizon, referred to as a duricrust horizon. Mesozoic to Cenozoic laterite chronology has enhanced our understanding of soil formation and opens new research (Allard et al, 2018;Monteiro, Vasconcelos, Farley, & Lopes, 2018;Shuster et al, , 2012. (e) Temperature variations based on δ 18 O isotopic composition over the Cenozoic (Zachos et al, 2001).…”

“…Tectonics avenues as laterites also record past climatic conditions (e.g., Vasconcelos et al, 2015) or landscape evolution (e.g., Bonnet et al, 2016). 40 Ar-39 Ar analysis of Mn oxides, and more recently, hematite and goethite He geochronology from laterites inform the timing, tempo, and geochemical processes of laterite formation via past climate change (Allard et al, 2018;Deng et al, 2017;Monteiro et al, 2014;Monteiro, Vasconcelos, Farley, & Lopes, 2018;Shuster, Vasconcelos, et al, 2005;Vasconcelos et al, 1994;Vasconcelos et al, 2013). For example, Figure 10d presents a compilation of published supergene goethite and hematite (U-Th)/He dates from northern Brazilian laterites.…”

Numerical Modeling of Weathering, Erosion, Sedimentation, and Uplift in a Triple Junction Divergent Margin

“…Weathering geochronology shows that continuously exposed weathering profiles at the Earth's surface can date as far back as ∼70 Ma and that older weathering profiles preferentially cap higher elevation parts of the landscape (Vasconcelos, , ; Monteiro et al, , ; Vasconcelos & Carmo, ). This age versus elevation relationship suggests that induration by weathering may promote greater resistance to erosion.…”

Weathering history and landscape evolution of Western Ghats (India) from 40 Ar/ 39 Ar dating of supergene K–Mn oxides

“…Much work was done for dating weathering profiles on most continents of the tropical belt, using 40 Ar/ 39 Ar dating of K-rich Mn oxides (e.g., Vasconcelos, 1999;Vasconcelos et Conroy, 2003;Beauvais et al, 2008;Bonnet et al, 2016;Deng et al, 2016;Li et al, 2007;Riffel et al, 2015), or (U-Th)/He dating of iron oxy-hydroxides (e.g., Shuster et al, 2005;Vasconcelos et al, 2013;Monteiro et al, 2018;Allard et al, 2018). For instance, ages of cryptomelane formed in situ in lateritic regolith of West Africa and Brazil allowed calibrating continental denudation over geological time scales (Beauvais and Chardon, 2013;Vasconcelos and Carmo, 2018).…”