The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsas-Grenville orogen after ∼1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic ...

/pdf/the-basement-of-the-central-andes-the-arequipa-and-related-58rb8wwpch.pdf

The Basement of the Central Andes: The Arequipa and Related Terranes

The Arequipa-Antofalla Basement, a Proterozoic crustal block exposed along the central Andean margin, provides a key to interpreting the pre-Andean history of South America. New U/Pb geochronology and whole-rock Pb and Nd isotope geochemistry from the Arequipa-Antofalla Basement refine the tectonic history and delineate three distinct crustal domains that decrease in age from north to south. The northern domain of southern Peru and western Bolivia contains juvenile Paleoproterozoic 2.02–1.79 Ga intrusions that were metamorphosed at 1.82–1.79 Ga. The Mesoproterozoic central domain in northernmost Chile contains a significant Mesoproterozoic juvenile component that incorporates Paleoproterozoic crust from the northern domain. Rock units from both the northern and central domains were metamorphosed between 1.20 and 0.94 Ga, with coeval magmatism occurring only in the central domain. The southern domain in northern Chile and northwestern Argentina comprises Ordovician rocks, derived from a mix of juvenile material and older crust. Similar Ordovician magmatism (476–440 Ma) also occurred in the northern and central domains followed by metamorphism at ca. 440 Ma.

Based on this refined geologic and tec tonic characterization of the Arequipa- Antofalla Basement and comparison with that of Amazonia, we conclude that: (1) the isolated exposures of the Arequipa-Antofalla Basement comprise a single basement block with multiple domains, (2) the Arequipa-Antofalla Basement was not derived from Amazonia, and (3) the Arequipa-Antofalla Basement accreted onto Amazonia during the 1.0 Ga Sunsas Orogeny.

An orphaned basement block: The Arequipa-Antofalla Basement of the central Andean margin of South America

A >2.5-km-thick succession of Tertiary strata in the Eastern Cordillera of southern central Bolivia consists of predominantly fluvial and lacustrine deposits. The age of the base of the succession is middle Paleocene, and its upper part (which is erosionally truncated) is probably late Oligocene-early Miocene. The lower 50-130 m of the succession consists of interbedded fluvial sandstone and mudstone (including minor paleosols) of the Santa Lucia Formation. These strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the Santa Lucia Formation and lower part of the Impora Formation that may represent much of late Paleocene-Eocene time. Above the paleosol zone is a thin zone (∼10 m) of lacustrine carbonate and clastic rocks in the uppermost Impora Formation, and this in turn is overlain by a >2000-m-thick, upward-coarsening succession of clastic fluvial deposits in the Cayara, Camargo, and Suticollo Formations. Paleocurrent data indicate that fluvial channels that deposited the Santa Lucia, Impora, and Cayara Formations flowed mainly westward, whereas channels responsible for the Camargo and Suticollo Formations flowed generally eastward. Modal sandstone petrographic data show a long-term evolution from subarkosic (during Santa Lucia deposition) to quartz-arenitic (during Impora and Cayara deposition) to sublitharenitic (during Camargo and Suticollo deposition) compositions. We argue that the lithofacies and sediment-accumulation history of this succession are most consistent with deposition in an eastward-migrating foreland basin system. E -mail: decelles@geo.arizona.edu. Critical to this interpretation is the zone of extreme stratigraphic condensation in the lower Impora Formation, the inferred upper Paleocene-Eocene part of the succession. The abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous, postrift thermal subsidence resulted in a gradual, continuous decay of early Tertiary sediment-accumulation rates. Stratigraphic condensation is, however, consistent with passage of the forebulge through the region, an interpretation that also can be reconciled with the record of foreland basin development in the Altiplano to the west and gross sediment accumulation patterns in the Subandean zone and modern foreland basin to the east. Simple flexural modeling and palinspastic reconstruction of the complete Cenozoic migration history of the foreland basin system suggest that ∼1000 km of foreland lithosphere has been underthrust westward beneath the Andean orogenic belt at the latitude of southern Bolivia. The amount of middle and lower crust that would have been added to the Andean infrastructure is sufficient to explain the present crustal thicknesses in Bolivia.

Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia

Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America

article i nfo Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U-Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2-0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52-0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hfisotopiccompositionsofselecteddatedzirconsatthetimeoftheircrystallization (eHf(T);T=3.3-0.25 Ga) vary between −18 and +11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive eHf(T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (HfTDM) between 1.55 and 0.8 Ga, the time of the Rondonia-San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock eNd(T) values fot these same rocks are between − 8a nd−3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd TDM ⁎ ) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode ofaccretionaryorogens.Incontrast,thecentralAndeansegmentof theTerraAustralisOrogenevolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution.

R Rössling

Papers

Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano)