Transcontinental retroarc sediment routing controlled by subduction geometry and climate change (Central and Southern Andes, Argentina)

Abstract

Central Argentina from the Pampean flat-slab segment to northern Patagonia (27°-41°S) represents a classic example of a broken retroarc basin with strong tectonic and climatic control on fluvial sediment transport. Combined with previous research focused on coastal sediments, this actualistic provenance study uses framework petrography and heavy-mineral data to trace multistep dispersal of volcaniclastic detritus first eastwards across central Argentina for up to ca. 1,500 km and next northwards for another 760 km along the Atlantic coast. Although detritus generated in the Andes is largely derived from mesosilicic volcanic rocks of the cordillera, its compositional signatures reflect different tectono-stratigraphic levels of the orogen uplifted along strike in response to varying subduction geometry as well as different character and crystallization condition of arc magmas through time and space. River sand, thus, changes from feldspatho-litho-quartzose or litho-feldspatho-quartzose in the north, where sedimentary detritus is more common, to mostly quartzo-feldspatho-lithic in the centre and to feldspatho-lithic in the south, where volcanic detritus is dominant. The transparent-heavy-mineral suite changes markedly from amphibole ≫ clinopyroxene > orthopyroxene in the north, to amphibole ≈ clinopyroxene ≈ orthopyroxene in the centre and to orthopyroxene ≥ clinopyroxene ≫ amphibole in the south. In the presently dry climate, fluvial discharge is drastically reduced to the point that even the Desaguadero trunk river has become endorheic and orogenic detritus is dumped in the retroarc basin, reworked by winds and temporarily accumulated in dune fields. During the Quaternary, instead, much larger amounts of water were released by melting of the Cordilleran ice sheet or during pluvial events. The sediment-laden waters of the Desaguadero and Colorado rivers then rushed from the tract of the Andes with greatest topographic and structural elevation, fostering alluvial fans inland and flowing in much larger valleys than today towards the Atlantic Ocean. Sand and gravel supply to the coast was high

From Patagonia to Río de la Plata: Multistep long‐distance littoral transport of Andean volcaniclastic sand along the Argentine passive margin

“…Thirty-nine sediment samples were collected in February 2018 from rivers, beaches and aeolian dunes along the ca 2170 km long stretch of the Argentine coast from south of the R ıo Chubut mouth (44°44 0 S) to Buenos Aires (34°29 0 S). Another 40 samples from all headwater branches of the Negro, Colorado and Desaguadero rivers represent the focus of the companion article (Garzanti et al, 2021b). Fluvial samples were collected from exposed sandbars, beach samples from backshore berms, and aeolian samples from the crest of backshore dunes.…”

“…The tHM suites of Río Salado mud and beach sand in the La Plata coastal province are distinctly enriched in amphibole relative to passive‐margin sand to the south (Fig. 6), in proportions comparable only with detritus generated in the Desaguadero catchment north of ca 34°S (Garzanti et al ., 2021b). This is fully consistent with the long‐held hypothesis that a palaeo‐Desaguadero with greatly augmented water and sediment discharge once flowed to the north of, or across the then subdued La Pampa High towards the Pampa Deprimida (Malagnino, 1988), influencing sedimentation through most of the Buenos Aires Province (Fig.…”

“…It is thus relatively strong mechanically, although labile chemically. Evidence of mechanical loss of olivine during long-distance transport in Patagonian rivers is lacking, because olivine percentages in the R ıo Colorado and R ıo Negro sands collected near their mouths roughly correspond to those averaged among major headwater branches in each river system (own data in Garzanti et al, 2021b). Olivine, more common in R ıo Negro (ca 6% tHM) than in R ıo Colorado (ca 3% tHM) sands, occurs in similar proportions in beaches of the Bah ıa San Blas (3.1 AE 0.2% tHM) and southern Bah ıa Blanca coastal provinces (2.3 AE 0.7% tHM) but it is detected only locally in the Pared del Golfo coastal province (Fig.…”

“…7) include the clinopyroxene/orthopyroxene ratio (Cpx/Opx), amphibole content and the Amphibole Color Index (ACI), which reflect the different character and crystallization conditions (for example, felsic versus mafic, lower versus higher temperature or pressure, and alkali or water content) of subduction‐related magmas emplaced in different tracts of the Andean orogen and adjacent retroarc basin (Deruelle, 1982; Kay et al ., 2005; Folguera & Ramos, 2011). Whereas hypersthene‐dominated tHM suites characterize diverse headwater branches of the Río Negro (Garzanti et al ., 2021b), draining andesites and dacites of the Cordillera Principal (Ramos, 2005), the tHM suite of Río Colorado is richer in clinopyroxene supplied from the Quaternary Payenia basaltic fields, representing the largest retroarc mafic volcanic province on Earth (Ramos & Folguera, 2011). Río Desaguadero sand is richer in amphibole, with an ACI lower than Río Colorado and higher than Río Negro sand (Table 1).…”

“…Detritus produced in elevated mountain areas is conveyed through the drainage network of large rivers that flow for thousands of kilometres across continents and deliver their load to the coast (Potter, 1978; Milliman & Farnsworth, 2011; Ashworth & Lewin, 2012). Travel at sea may continue through gravity flows following transverse or more complex pathways towards the abyssal plains (Graham et al ., 1975; Zuffa et al ., 2000; Shapiro et al ., 2007; Fierens et al ., 2020; Garzanti et al ., 2020, 2021a). Gravity, however, is not necessarily the leading actor, and sediment trajectories may not invariably follow the topographic gradient from source to sink.…”