Western Margin Of South America Research Articles

Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru

Research Article| June 01, 2015 Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru Angela M. Hessler; Angela M. Hessler 1The Deep Time Institute, 8711 Bluegrass Drive, Austin, Texas 78759, U.S.A. e-mail: ahessler@deeptimeinstitute.org Search for other works by this author on: GSW Google Scholar Andrea Fildani Andrea Fildani 2Statoil Research, Development and Innovation Center, 6300 Bridge Point Parkway, Suite 2–500 Austin, Texas 78730, U.S.A. Search for other works by this author on: GSW Google Scholar Journal of Sedimentary Research (2015) 85 (6): 646–659. https://doi.org/10.2110/jsr.2015.45 Article history received: 07 Oct 2014 accepted: 24 Mar 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Angela M. Hessler, Andrea Fildani; Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru. Journal of Sedimentary Research 2015;; 85 (6): 646–659. doi: https://doi.org/10.2110/jsr.2015.45 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Sedimentary Research Search Advanced Search Abstract: Eocene strata of the Talara forearc basin of northwest Peru contain detrital-zircon (DZ) populations that, when integrated with other provenance indicators, record paleogeographic shifts in the Andean forearc that can be linked to larger-scale tectonic drivers such as subduction erosion, plate convergence rate, and regional accretion events. Because forearc basins along the western margin of South America are particularly difficult to preserve, the new DZ results provide rare insight into the trench–arc dynamics associated with the Andean orogenic cycle. The Talara basin tapped into both the Andean volcanic arc and older basement, and the DZ populations subsequently reflect long-lived plate interactions and progressive crustal growth along the northwestern edge of Amazonia, Gondwana, and South America. Dominant U–Pb zircon age populations include: 36–110 Ma (Andes convergence); 215–285 Ma (western Gondwana extension); 460–625 Ma (Rodinia extension and western Gondwana convergence); 950–1250 Ma (Grenville orogeny); and > 1250 Ma (Amazonia assembly). The data corroborate previous geochronology on pre-Andean basement and support the existence of a Famatinian arc (∼ 480 Ma) and Carboniferous–Triassic magmatism along northwestern Gondwana. Pre-Andean and Amazonian DZ likely was recycled from Paleozoic metasedimentary rocks of the Andean Cordillera and Amotape terrane, although the data do not preclude delivery of Amazonian detritus directly into the forearc via a large-scale, westward drainage. The Andean DZ population constrains the timing of arc activity and quiescence, with two peaks at ∼ 75 Ma and ∼ 50 Ma, and a gap at ∼ 70 Ma, linked to accretion of the Caribbean Oceanic Plateau against northwest South America. Arc volcanism was concurrent with Eocene forearc deposition, and syndepositional DZ is used to revise maximum depositional age for the upper Eocene Helico Member and Verdun Formation. Progressive subsidence and arc shut-down in the late Eocene coincide with arc collision to the north, uplift of the Amotape block, subduction erosion, and slab shallowing. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Symmetry and tendency judgment of Ms ≥ 8.0 strong earthquakes in Chile

Abstract: The M s ≥ 8.0 strong earthquakes occurring in Chile since 1800 were analyzed using the ternary, quaternary, and quinary commensurability methods and the butterfly structure diagram, and it was believed that the earthquake signal in Chile in 2014 is relatively strong, a large earthquake is likely to occur in Chile in 2014. An analysis of spatial epicenter migrations showed that the longitudinal and latitudinal epicenter migrations have symmetry and synchronism, and there were five obvious northward migrations and four southward migrations. The symmetry axis of the longitudinal migrations is at about 71. 7° W and that of the latitudinal migrations is at about 30° S; these spatial symmetry axes are located at the subduction zone on the western margin of South America, where two major plates (the Nazca Plate and the South American Plate) converge.

Mesozoic–Cenozoic Evolution of the Western Margin of South America: Case Study of the Peruvian Andes

Based on the structural style and physiographic criteria, the Central Andes of Peru can be divided into segments running parallel to the Pacific coast. The westernmost segment, the Coastal Belt, consists of a Late Jurassic–Cretaceous volcanic arc sequence that was accreted to the South American craton in Cretaceous times. The Mesozoic strata of the adjacent Western Cordillera represent an ENE-vergent fold-and-thrust belt that formed in Eocene times. Tight upright folds developed above a shallow detachment horizon in the West, while more open folds formed above a deeper detachment horizon towards the East and in the neighboring Central Highlands. A completely different style with steeply dipping reverse faults and open folds affecting the Neoproterozoic crystalline basement is typical for the Eastern Cordillera. The Subandean Zone is characterized by mainly NE-vergent imbricate thrusting which occurred in Neogene times. A quantitative estimate of the shortening of the orogen obtained from balanced cross-sections indicates a total shortening of 120–150 km (24%–27%). This shortening was coevel with the Neogene westward drift of South America, occurred at rates between 3 and 4.7 mm/year and was responsible for the high elevation of the Peruvian Andes.

A new Late Triasssic phytogeographical scenario in westernmost Gondwana

Floral provincialism within the Southern Hemisphere during the Late Triassic (230 Ma) is characterized by the Ipswich and Onslow provinces, recognized originally in eastern Gondwana. However, new palynological assemblages from the Ischigualasto Formation, northwestern Argentina (231-225 Ma), change the phytogeographic interpretation for the Carnian-Norian in the westernmost Gondwana, which was previously considered part of the southern floral Ipswich province. Here we show the presence of diagnostic Euramerican species within assemblages dominated by Gondwanan taxa that allows us to refer the palynofloras to the Onslow province. Our new data extend the Onslow floral belt, previously recognized from the western edge of Tethys to Timor, to the western margin of South America. This has implications for palaeophytogeography, palaeoclimate reconstructions and the palaeoecology of a Triassic ecosystem, which has yielded significant vertebrate remains and is regarded important in the early evolution of groups such as the Dinosauria.

A paleoclimatic review of southern South America during the late Paleozoic: A record from icehouse to extreme greenhouse conditions

This paper provides a review of the Late Mississippian to Permian paleoclimatic history for southern South America based on lithologic indicators, biostratigraphic information, and chronostratigraphic data. The region is divided into three major types of basins: 1. Eastern intraplate basins (e.g., Paraná Basin), 2. Western retroarc basins (e.g., Paganzo Basin) and 3. Western arc-related basins (e.g., Río Blanco Basin). Four major types of paleoclimatic stages are recognized in these basins: 1. glacial (late Visean–early Bashkirian), 2. terminal glacial (Bashkirian–earliest Cisuralian) 3. postglacial (Cisuralian–early Guadalupian), and 4. semiarid–arid (late Guadalupian–Lopingian). The glacial stage began in the late Visean and continued until the latest Serpukhovian or early Bashkirian in almost all of the basins in southern South America. During the Bashkirian–earliest Cisuralian (terminal glacial stage), glacial deposits disappeared almost completely in the western retroarc basins (e.g., Paganzo Basin) but glaciation persisted in the eastern basins (e.g., Paraná and Sauce Grande Basins). A gradual climatic amelioration (postglacial stage) began to occur during the earliest Permian when glacial deposits completely disappeared across all of South America. During this interval, glacial diamictites were replaced by thick coal beds in the Paraná Basin while north–south climatic belts began to be delineated in the western basins, which were likely controlled by the distribution of mountain belts along the Panthalassan Margin of South America. Towards the late Permian, climatic belts became less evident and semiarid or arid conditions dominated in the southern South America basins. Eolian dunes, playa lake deposits, and mixed eolian–fluvial sequences occur in the Paraná Basin and in the western retroarc basins. Volcanism and volcaniclastic sedimentation dominated along the western margin of South America at that time. The stratigraphic record obtained in southern South America supports a long duration transition from icehouse to extreme greenhouse conditions.

Assessing the effect of sea-level change and human activities on a major delta on the Pacific coast of northern South America: The Patía River

This paper presents the main physical and human-induced stresses that have shaped the recent evolution of the Patía River delta, the largest and best-developed delta on the western margin of South America. During the Holocene, the Patía Delta moved southward and the northern part became an estuarine system characterized by large extensions of mangrove ecosystems. However, a major human-induced water diversion, starting in 1972, diverted the Patía flow to the Sanguianga River, and shifted the active delta plain back to its former Holocene location. This discharge diversion has led to sediment starvation of the southern delta lobe and changed the northern estuarine system into an active delta plain. In addition, coastal areas of the Patía delta subsided as a result of a devastating tsunami in 1979. Morphological changes along the delta coast are evidenced by: (1) coastal retreat along the whole delta front during the period 1986–2001; (2) coastal retreat along the abandoned delta lobe for the period 2001–2008; 56% of the southern delta shoreline is retreating and only 4% of the coast shows signs of accretion; (3) progradation of the northern delta region during the period 2001–2008; the discharge diversion of the Patía River to the Sanquianga has apparently balanced the observed trends in coastal erosion and sea-level rise (5.1mmyr−1 for the period 1984–2006, after the 1979 tsunami); (4) formation of transgressive barrier islands with exposed peat soils in the surf zone; and (5) abandonment of former active distributaries in the southern delta plain with associated inlet closure. In the northern delta lobe, major geomorphic changes include: (1) distributary channel accretion by morphological processes such as sedimentation (also in crevasses), overbank flow, increasing width of levees, inter-distributary channel fill, and colonization of pioneer mangrove; (2) freshening conditions in the Sanguianga distributary channel, a hydrologic change that has shifted the upper estuarine region (salinity<1psu) downstream; and (3) changes in vegetation succession; approximately 30% of mangrove forests in the current delta apex have been replaced by freshwater vegetation. Overall, the recent evolution of the Patía has been controlled by the interplay of (1) high basin-wide sediment load; (2) low discharge variability (Qmax/Qmin); (3) spatial switch of delta distributaries related to tectonic movements and subsidence; (4) a relative sea-level rise of 5.1mmyr−1 after the occurrence of the 1979 tsunami; (5) episodes of sea-level rise associated with the ENSO cycle; and (6) human-induced discharge diversion. The information presented here is valuable evidence for understanding the role of extreme events versus ‘normal’ conditions in creating and shaping deltas.

Episodic burial and exhumation in NE Brazil after opening of the South Atlantic

Research Article| May 01, 2012 Episodic burial and exhumation in NE Brazil after opening of the South Atlantic Peter Japsen; Peter Japsen † 1Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark †E-mail: pj@geus.dk Search for other works by this author on: GSW Google Scholar Johan M. Bonow; Johan M. Bonow 1Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar Paul F. Green; Paul F. Green 2Geotrack International, 37 Melville Road, Brunswick West, Victoria 3055, Australia Search for other works by this author on: GSW Google Scholar Peter R. Cobbold; Peter R. Cobbold 3Géosciences (UMR6118), Centre National de la Recherche Scientifique (CNRS) et Université de Rennes1, 35042 Rennes, France Search for other works by this author on: GSW Google Scholar Dario Chiossi; Dario Chiossi 4Statoil do Brasil, Praia de Botafoga 300, 22250-040 Rio de Janeiro, Brazil Search for other works by this author on: GSW Google Scholar Ragnhild Lilletveit; Ragnhild Lilletveit 5Statoil Angola Team, Grenseveien 21, 4035 Stavanger, Norway Search for other works by this author on: GSW Google Scholar Luciano P. Magnavita; Luciano P. Magnavita 6Petrobras, Avenida República do Chile 330, Rio de Janeiro, 20031-170, Brazil Search for other works by this author on: GSW Google Scholar Augusto Pedreira Augusto Pedreira 7Geological Survey of Brazil (CPRM), Avenida Ulysses Guimaraes 2862, Salvador, 41213-000, Brazil Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (5-6): 800–816. https://doi.org/10.1130/B30515.1 Article history received: 25 Mar 2011 rev-recd: 29 Jun 2011 accepted: 13 Jul 2011 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Japsen, Johan M. Bonow, Paul F. Green, Peter R. Cobbold, Dario Chiossi, Ragnhild Lilletveit, Luciano P. Magnavita, Augusto Pedreira; Episodic burial and exhumation in NE Brazil after opening of the South Atlantic. GSA Bulletin 2012;; 124 (5-6): 800–816. doi: https://doi.org/10.1130/B30515.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has undergone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceous breakup, the margin under went burial beneath a thick sedimentary cover; (2) uplift episodes in the Campanian and Eocene led to almost complete removal of these deposits; (3) the resulting large-scale, low-relief erosion surface (peneplain) was deeply weathered and finally reburied at the Oligocene-Miocene transition; and (4) Miocene uplift and erosion produced a new, lower-level peneplain by incision of the uplifted and re-exposed Paleogene peneplain. Previous studies have identified aspects of this interpretation, but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed Early Cretaceous rifting and Eocene–Oligocene peneplanation. We suggest that a late sedimentary cover protected Paleogene weathering profiles until the present day. The uplift phases in Brazil are synchronous with uplift phases in Africa and the Andes. The Andean phases coincided with rapid convergence on the western margin of South America, and the Campanian uplift coincided with a decline in spreading rate at the Mid-Atlantic Ridge. Consequently, we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause, which is lateral resistance to plate motion. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Discharge diversion in the Patía River delta, the Colombian Pacific: Geomorphic and ecological consequences for mangrove ecosystems

In the Patía River delta, the best-developed delta on the western margin of South America, a major water diversion started in 1972. The diversion of the Patía flow to the Sanquianga River, the latter a small stream draining internal lakes from the Pacific lowlands, shifted the active delta plain from the south to the north and changed the northern estuarine system into an active delta plain. The Sanquianga Mangrove National Park, a mangrove reserve measuring 800km2, lies in this former estuary, where major hydrologic and sedimentation changes are occurring. Overall, major environmental consequences of this discharge diversion in terms of geomorphic changes along distributary channels and ecological impacts on mangrove ecosystems are evidenced by: (1) distributary channel accretion by operating processes such as sedimentation, overbank flow, increasing width of levees, sedimentation in crevasses, interdistributary channel fill, and colonization of pioneer mangrove; (2) freshening conditions in the Sanquianga distributary channel, a hydrologic change that has shifted the upper estuarine region (salinity <1%) downstream; (3) downstream advance of freshwater vegetation, which is invading channel banks in the lower and mixing estuarine zones; (4) die-off of approximately 5200ha of mangrove near the delta apex at Bocas de Satinga, where the highest sediment accumulation rates occur; and (5) recurrent periods of mangrove defoliation due to a worm plague. Further analyses indicate strong mangrove erosion along transgressive barrier islands on the former delta plain. Here tectonic-induced subsidence, relative sea-level rise, and sediment starving conditions due to the channel diversion, are the main causes of the observed retreating conditions of mangrove communities. Our data also indicate that the Patía River has the highest sediment load (27×106tyr−1) and basin-wide sediment yield (1500tkm−2yr−1) on the west coast of South America. Erosion rates from the Patía catchment have been more pronounced during the decades of 1970–1980 and 1990–2000, as a result of land degradation and deforestation. The high sediment and freshwater inputs into the mangrove ecosystem create additional stress (both at ongoing background levels and, occasionally, at dramatic levels), which may periodically push local environmental parameters beyond the thresholds for mangrove survival. The future environmental state of the Sanquianga Mangrove National Reserve deserves more scientific and governmental attention.

Volcanic Stratigraphy and Geochronology of the Cretaceous Lancones Basin, Northwestern Peru: Position and Timing of Giant VMS Deposits

A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith. The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario.

Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic–Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520–420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200–1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syn depositional magmatic sources (420–380 Ma), and distal sources of chiefl y Mesoproterozoic basement (1650–900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic–Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous–Permian (310–250 Ma) and Late Triassic–Early Jurassic (220–180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased infl uence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050–950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850–1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous–Paleocene (90–55 Ma), Jurassic (190–150 Ma), and limited Permian–Triassic (280–220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene–Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180– 40 Ma) and shield-derived sediments (mostly 1850–950 Ma). Late Miocene–Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic– Cenozoic zircons and increased proportions of 1650–900 Ma zircons emblematic of Cretaceous strata.

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

Abstract 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 Hf isotopic compositions of selected dated zircons at the time of their crystallization (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 (Hf TDM) 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 − 8 and − 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 of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved 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.

Warm-water mollusc assemblages from northern Chile (Mejillones Peninsula): new evidence for permanent El Niño-like conditions during Pliocene warmth?

Although results have been controversial, understanding the tropical Pacific climatic state during the Pliocene warm interval ( c . 4.5–3.0 Ma) is crucial if insight is to be gained into the dynamic processes of present and future global warming. In the multi-proxy effort to reconstruct ancient climates, a critical role can be played by palaeoclimatic evidence provided by the spatial and temporal distribution of temperature-sensitive marine molluscs. Shallow-water strata of the Mejillones Peninsula, northern Chile (23°S), contain dense faunal assemblages in which molluscs exclusive to, or characteristic of, Pliocene deposits ( Chlamys simpsoni , Chlamys vidali , Chorus blainvillei , Concholepas nodosa , Fusinus remondi , Herminespina mirabilis ) coexist with surprisingly abundant and varied populations of extant warm-water species ( Bulla punctulata , Cerithium stercusmuscarum , Olivella sp., Turbo cf. fluctuosus , Anomia peruviana , Argopecten ventricosus , Donax peruvianus , Dosinia ponderosa , Mexicardia procera , Undulostrea megodon ), most of which have their current southern zoogeographical limit at 6°S. These tropical elements are reliable indicators of nearshore marine conditions and their abundant occurrence implies that sea surface temperatures (SST) along the northern Chile coast were at least 2 °C warmer in the mid-Pliocene than at present, and that these very different conditions lasted long enough to allow stable colonization of the area. Such a significantly warmer SST pattern strongly resembles general climatic conditions accompanying modern El Niño events, when warm tropical waters propagate southward along the western margin of South America; this supports the existence in this area of persistently El Niño-like conditions during the mid-Pliocene.

The Geology of Chile.: TERESA MORENO and WES GIBBONS, Editors. 424 Pp. Geological Society of London. 2007. Hardcover. ISBN 1-86239-219-6. Hardcover Price Members, 42.50. Other Societies, 51.00, Nonmembers, 85.00. Paperback Price Members, 27.50. Other Societies, 27.50, Nonmembers, 35.00.

More than 70 authors contributed to this English language volume, which handles the very broad geologic settings of Chile in able fashion, with approximately one-half of the text dedicated to the tectonostratigraphic and chronologic development of a country more than 4,000-km long, yet no more than about 200-km wide. Beginning with discussions of Chilean basement complexes and tectonostratigraphy, this book provides a very good general understanding of the geologic setting of the western margin of South America and gives a background on which essentially all other discussions of Chilean geology follow. The Andean orogen is highlighted by substantial detail and recent investigations concerning Andean magmatism (chapter 4). The authors subdivide Chile’s Andean magmatism into four distinct segments, in addition to Quaternary volcanism (chapter 5). The treatment of both the tectonic and geographic differences in Chilean magmatic activity gives readers an excellent …

Ages and cooling history of the Early Cretaceous Caleu pluton: testimony of a switch from a rifted to a compressional continental margin in central Chile

The Caleu pluton, in the Coastal Range of central Chile, represents the last magmatic event related to the Early Cretaceous rifting along the western margin of South America. The pluton was emplaced into a c . 10 km thick pile of mainly basalts and basaltic andesites deposited in an Early Cretaceous subsiding basin, and affected by very low-grade metamorphism. The cooling history of the pluton is documented on the basis of U–Pb, 40 Ar/ 39 Ar step-heating and fission-track dating. The U–Pb date suggests an age of emplacement in the interval 94.2–97.3 Ma. Rapid subsolidus cooling between 550–500 °C and 250 °C is documented by 40 Ar/ 39 Ar plateau ages on amphibole, biotite and plagioclase between 94.9 ± 1.8 and 93.2 ± 1.1 Ma. Slower subsolidus cooling to c . 100 °C is identified at the 94–90 Ma interval by the fission-track thermal model. The geochronological data show that the emplacement of the pluton is coeval with the very low-grade metamorphism of the host rocks. Therefore, this metamorphism is probably not the result simply of burial, but also of a regional thermal gradient related to the plutonism. Exhumation of the pluton started coevally with its emplacement and continued to about 90 Ma, being associated with the closure of the Early Cretaceous rifting. The Caleu plutonism represents an asthenospheric-derived event during maximum extension, and marks a turning point between extensional- and compressional-related magmatism.

The early Palaeozoic Orogen in the Central Andes: a non-collisional orogen comparable to the Cenozoic high plateau?

Abstract The subduction orogeny of the Central Andes, which created the Cenozoic Altiplano-Puna high plateau, shares many geological features with the early Palaeozoic Orogen at the western margin of South America. The presently available datasets for both orogens are compared. The similarities are a large-scale high temperature metamorphism, which was active in the Palaeozoic Orogen over a geological long period of time in the order of 100 Ma and which is active now in the crust of the Cenozoic plateau. It produced abundant granitoid melts from the crust during the Palaeozoic as well as during the Andean Orogen. The main contribution to granitoid magmatism is recycling of felsic crustal material with only minor additions from the mantle. Transport of deep parts of the crust into the erosion level did not occur in both orogens and, in both orogens, large-scale nappe tectonics typical for collision orogens are absent. Based on the similarities of the two orogens it is argued that the early Palaeozoic Orogen is a non-collisional orogen. Indications for terrane accretion are absent in the development of the high-grade metamorphic and igneous basement. The early Palaeozoic Orogen is an analogue for the presently active continental margin and, thus, allows the extrapolation of features which cannot be observed in the Andean Orogen.

Andean uplift and climate change

Sedimentological data indicate that a semi-arid/arid climate prevailed across the Central Andes from 15 Ma to 4 Ma. Between 4 and 3 Ma a switch to hyperaridity occurred along the western margin of South America. Palaeoaltitude data suggest that a substantial proto-Central Andean mountain range was in place between 15 and 9 Ma. These data support the idea that the Andean rain shadow existed by 15 Ma and that it reinforced the pre-existing climatic regime rather than changing it. The change to hyperaridity in western South America is attributed to a combination of global climate cooling and enhanced upwelling of the Humboldt current generated by closure of the Central American Seaway between 3.5 and 3 Ma, and not to the Andean rain shadow.

Initial plate geometry, shortening variations, and evolution of the Bolivian orocline

Comparisons of newly published cross sections across the Bolivian Andes with existing cross sections through Argentina emphasize significant along-strike changes in crustal shortening. A sharp decrease in the magnitude of crustal shortening from ∼530 km to ∼150 km (north to south) occurs at ∼23°S. A 20–40 m.y. difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening. Extending the western margin of South America to account for 530 km of shortening in the Bolivian Andes and 150 km of shortening in Argentina produces a central Andean salient that is perpendicular to the Nazca plate shortening direction from 60 to 26 Ma. During this same time interval, the Chilean coast south of 23°S was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike-slip offset and backarc extension. Deformation within the Andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of ∼23°S inhibited mountain building, whereas north of ∼23°S, normal convergence to a central Andean salient facilitated contractional deformation. The magnitude of deformation north of ∼23°S is a consequence of both plate-convergence direction, providing a longer period of contractional deformation (from ca. 70 Ma to the present), and a thick Phanerozoic sedimentary package that permitted large magnitudes of thin-skinned deformation. Significant along-strike changes in the shape of the South American margin—allowing for convergence to change from compression to extension along the strike of the orogen—may help explain the dramatic differences in timing, amount, and style of deformation in the Andes.

Earth's glacial record and its tectonic setting

Earth's glacial record and its tectonic setting

Compositional variations of Cretaceous pumpellyites along the western margin of South America and their relation to an extensional geodynamic setting

Abstract Pumpellyites in pervasively altered basic to intermediate rocks from three Cretaceous Andean volcanic sequences, the Colombian Diabasic Group, the Ecuadorian‐Peruvian Celica Formation/Casma Group and the Chilean Ocoite Group, were studied to test their compositional behaviour in relation to changes in geodynamic setting. They occur mostly in assemblages of the prehnite‐pumpellyite facies filling amygdules and inside plagioclase phenocrysts.(a) Pumpellyites from the three geodynamic settings define three distinct compositional fields in AFM space: (i) those in the Diabasic Group plot closer to the Fe corner; (ii) those in the Ocoite Group plot closer to the Al corner; (iii) those in the Celica Formation/Casma group plot between, and partly overlapping, fields (i) and (ii).(b) Pumpellyites in group (i) formed in T‐MORB like, K‐poor tholeiites (high Fe/Al), affected by ocean‐floor metamorphism in an oceanic back‐arc basin. Pumpellyites in group (ii) generated in K‐rich, calc‐alkaline (low Fe/Al) to shoshonitic metabasites affected by burial metamorphism in an ensialic, aborted, marginal basin with moderate attenuation of the continental crust. Pumpellyites in group (iii) formed in basic and intermediate, calc‐alkaline to tholeiitic rocks, metamorphosed in ensialic marginal basins with various degrees of continental crust thinning, from splitting to moderate attenuation. A correlation between pumpellyite and host‐rock composition is suggested by these characteristics.(c) Pumpellyites in prehnite‐pumpellyite facies assemblages of some of the units studied plot inside higher (and lower) grade reference fields (AFM space) corresponding to different geodynamic settings elsewhere. This anomaly is attributed to the changing characteristics of the marginal basins at the South American margin and emphasizes the need to compare equal facies referred to equal settings.

Mesozoic and Cenozoic Tectonic Evolution of the Maranon Basin in Southeastern Colombia, Eastern Ecuador and Northeastern Peru

The Late Triassic to Early Jurassic in the Maranon was characterized by tectonic quiescence and carbonate shelf deposition. During Middle to Late Jurassic, a northeast-southwest extensional event occurred which is documented by the presence of northwest oriented grabens filled with red beds and volcaniclastic rocks. Cretaceous deposition commenced during the Aptian and continued to the Early Campanian within the vast South America Cretaceous Seaway (SACS) that extended from Venezuela to Central Peru. These strata comprised of shallow marine clastics sources from the Brazilian and Guyana cratons to the east. Retreat of the SACS resulted from the Late Cretaceous (Peruvian) phase of the Andean Orogeny. Deposition became largely continental with sediments derived from the west. The deformation was comtemporaneous with oblique collision and accretion of an allochthonous terrain present in Colombia and Ecuador, as well as uplift of the Putumayo, Napo, Cutucu and Cenepa (PNCC) Mountains, westward erosion of the Napo/Chonta Formations, widespread deposition of red beds, volcanic activity in the foreland and the subtle inversion of half grabens. The Middle Eocene (Inca) phase of the Andean Orogeny, correlated to a relative increase in convergence rates along the western margin of South America (SA). This orogeny was characterized by the developmentmore » of folds and reverse faults within a narrow and elongated belt, the reactivation of the PNCC Uplifts, the deposition of varicolored fluviatile deposits, the renewed inversion of half grabens, and volcanic activity close to the hinterland. The three main pulses of the Late Miocene to Pliocene phase of the Andean Orogeny correlate with high rates of convergence along the SA margin. This orogenic phase was characterized by thick fluviatile deposition, reactivation of the PNCC uplifts, eastward propagation of the fold and thrust belt, renewed inversion of the half grabens and alkaline volcanism in the foreland.« less