Search

The shortening history of the Andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of Cenozoic climate. However, the timing of uplift in the northern Andes is poorly constrained, with estimates ranging from Cretaceous to Pliocene. Detrital zircon U-Pb ages from the Middle Magdalena Valley Basin in Colombia reveal two provenance shifts during Cenozoic time. The first shift occurs between early and late Paleocene strata, where U-Pb results show a switch from Proterozoic-dominated to Phanerozoic-dominated age spectra. We attribute this change to uplift-related exhumation of the Central Cordillera. The second shift occurs between middle-late Eocene and late Oligocene strata, where increased Grenville ages and diminished Mesozoic ages can be linked to uplift of the Eastern Cordillera. Our results show that significant pre-Neogene deformation affected the northern Andes, underscoring the potential importance of Andean uplift on the dynamics of Paleogene climate. doi: 10.1130/G30775.1

Decaying logs on the forest floor can act as 'nurse logs' for new seedlings, helping with the regeneration of the vegetation. Fossil evidence of this ecological strategy is exceptionally well preserved in the Argentinean Andes, where an ~300 m.y. old permineralized forest was found at 3000 m elevation in San Juan Province. The fossil trunks, some of them in life position, are intercalated between volcanic rocks and sediments deposited in flooded environments (probably coastal lagoons). More than 100 specimens studied allow us to suggest the dominance of only one tree species in the forest. The first issue of the research was to determine the means used by the vegetation to survive in such adverse environmental conditions. Fossil evidence supports the hypothesis of regeneration via nurse logs. Little rootlets preserved inside the wood of several specimens indicate that seedlings developed on these logs. Important additional information provided by the fossils is the presence of aerenchymatic tissue in the rootlets. Aerenchyma tissue is a common feature developed in plants living in flooded environments; therefore its recognition in the fossil forest helps in the ecological interpretation. doi: 10.1130/G30557.1

We use digital elevation models ana satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the Central Cordillera of Patagonia between 46[degrees]S and 48[degrees]S, in a region overlying subducted segments of the Chile Spreading Ridge. This region shows a number of peculiar features, not expected in a normal subduction-related belt located away from a buried spreading center. First, it is characterized by overall short-wavelength topography and highly contrasted reliefs, typical of deeply incised young mountain belts, but surprisingly, also by an average very low elevation of ~900 m. Second, the Cordillera lacks central high reliefs; instead, there is a central, depressed domain with average low slope gradients including the deepest point of the study area (300 m below sea level in the bottom of Lake General Carrera/Buenos Aires). This depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips. To the east, the central Patagonian Cordillera is now separated from the flat plains of the Pampean domain (representing its former piedmont) and several elevated basaltic plateaus (mesetas) by 160[degrees]N-180[degrees]N trending morphotectonic corridors and the western border scarps of the mesetas. These scarps correspond to neotectonic features controlled by recent vertical faults, mostly active after 3 Ma. Remnants of a former peneplain, the Avellanos surface, are widespread within the entire Cordilleran region. From geological evidence, this peneplain formed between 15 and 4 Ma and was connected originally with the mesetas domain and now is dissected by these corridors. It is shown that a negative inversion related to extensional and strike-slip faulting occurred in the frontal region of the Cordillera at 3 Ma synchronously with the emplacement of magmatic products. Timing of the extensional deformation in central Patagonia is concomitant with the migration at depth of different segments of the South Chile Ridge, which entered the trench at 6 and 3 Ma, respectively, and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions. Finally, migration of segments of the Chile Spreading Ridge since 6 Ma triggered regional uplift of the Cordillera followed by limited extension in relation with increased thermal weakening of the crust. doi: 10.1029/2009TC002453

[1] The importance of west verging structures at the western flank of the Andes, parallel to the subduction zone, appears currently minimized. This hampers our understanding of the Andes-Altiplano, one of the most significant mountain belts on Earth. We analyze a key tectonic section of the Andes at latitude 33.5[degrees]S, where the belt is in an early stage of its evolution, with the aim of resolving the primary architecture of the orogen. We focus on the active fault propagation-fold system in the Andean cover behind the San Ramon Fault, which is critical for the seismic hazard in the city of Santiago and crucial to decipher the structure of the West Andean Thrust (WAT). The San Ramon Fault is a thrust ramp at the front of a basal detachment with average slip rate of ~0.4 mm/yr. Young scarps at various scales imply plausible seismic events up to [M.sub.w] 7.4. The WAT steps down eastward from the San Ramon Fault, crossing 12 km of Andean cover to root beneath the Frontal Cordillera basement anticline, a range ~5 km high and >700 km long. We propose a first-order tectonic model of the Andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations. The stage of primary westward vergence with dominance of the WAT at 33.5[degrees]S is evolving into a doubly vergent configuration. A growth model for the WAT-Altiplano similar to the Himalaya-Tibet is deduced. We suggest that the intracontinental subduction at the WAT is a mechanical substitute of a collision zone, rendering the Andean orogeny paradigm obsolete. doi: 10.1029/2008TC002427

Compositional variations of Central Andean subduction-related igneous rocks reflect the plate-tectonic evolution of this active continental margin through time and space. In order to address the effect on magmatism of changing subduction geometry and crustal evolution of the upper continental plate during the Andean orogeny, we compiled more than 1500 major- and trace-element data points, and 650 Sr-, 610 Nd-, and 570 Pb-isotopic analyses of Mesozoic-Cenozoic (190-0 Ma) magmatic rocks in southern Peru and northern Chile (Central Andean orocline), mostly from new data and the literature. This data set documents compositional variations of magmas since Jurassic time, with a focus on the Neogene period, when major crustal thickening developed and its influence on magma composition was most pronounced. We relate the observed variatious in Sr/Y, La/Yb, La/Sm, Sm/Yb, and Dy/Yb ratios, as well as in Sr-, Nd-, and Pb-isotopic ratios, to the crustal structure and evolution of the Central Andean orocline. In particular, the evolution of Dy/Yb and Sm/Yb ratios, which track the presence of the higher-pressure minerals amphibole and garnet, respectively, in the lower crust, documents that crustal thickness has grown through time. Spatial variations in trace elements and isotopic ratios further suggest that crustal domains of distinct composition and age have influenced magma composition through some assimilation. The crustal input in Quaternary magmas is quantified to have been between 7% and 18% by simple two-components mixing. When comparing our geochemical data set to the geological record of uplift and crustal thickening, we observe a correlation between the composition of magmatic rocks and the progression of Andean orogeny. In particular, our results support the interpretation that major crustal thickening and uplift were initiated in the mid-Oligocene (30 Ma) and that crustal thickness has kept increasing until present day. Our data do not support delamination as a general cause for major late Miocene uplift in the Central Andes and instead favor continued crustal thickening. doi: 10.1130/B26538.1

Along-strike geological segmentation in the Andean chain has been recognized at various scales and is usually attributed to changes in plate motion vectors, as well as the upper-plate expression of differing subducted slab age, strength and composition. We present new multi-phase [sup.40]Ar/[sup.39] Ar, apatite fission-track and zircon and apatite (U-Th)/He data from a north-south-oriented traverse between 35 and 38[degrees]S along the Principal Andean Cordillera of Chile that reveal (1) rapid cooling at 18 and 15 Ma, which can be attributed to both thermal relaxation following magmatic intrusion and regional-scale exhumation, and (2) along-strike differences in the extent of exhumation since 7.5 Ma that may be a consequence of the subduction of the Juan Fernandez Ridge above the flat-slab segment at 32[degrees], since 10 Ma. A comparison of the response of the South American Plate to the collision of the Carnegie, Nazca and Juan Fernandez ridges suggests that slab flattening is not the dominant driving force that exhumes the upper plate in these settings. Rather, the extent of exhumation is controlled by pre-existing structural weaknesses, the time duration of the dynamically supported topography, and climate.

[1] The Neogene evolution of the Ene and southern Ucayali basins of the Subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles. The lower thrust wedge is formed by deep-seated decollements within the basement related to thick-skinned foreland structures inherited from an Early Carboniferous thrust system. Seismic reflection data show that this Paleozoic compressional system has been eroded and unconformably covered by Late Carboniferous clastic sediments. It generated an irregular Paleozoic sedimentary architecture controlling the Neogene thrust propagation. The upper thin-skinned thrust wedge developed within this Paleozoic sedimentary series and constitutes the Subandean zone. Cross-section balancing shows an along-strike homogenous horizontal shortening of ~56 km (~30%) across the Ene--southern Ucayali thrust system. This amount of shortening was vertically partitioned onto the two stacked thrust wedges. The N-S thickness variations of the Paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge. The southern thickening of the Paleozoic series generated major decollements and the shortening excess is of 7 km (16%) in comparison to the north. Consequently, the northern lack of shortening onto the upper thrust wedge was transferred to the Early Carboniferous compressional structures of the lower thrust wedge. We suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp: the Tambo transfer zone. This geometrical analysis of the Ene--southern Ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region. Citation: Espurt, N., S. Brusset, P. Baby, W. Hermoza, R. Bolanos, D. Uyen, and J. Deramond (2008), Paleozoic structural controls on shortening transfer in the Subandean foreland thrust system, Ene and southern Ucayali basins, Peru, Tectonics, 27, TC3009, doi:10.1029/2007TC002238.

[1] Latitudinal changes in topography, climate, and thrust belt geometry in the central Andes have led to conflicting hypotheses that climate or tectonics exert a first-order control on orogen evolution. The relative roles of climate and tectonics in the evolution of the Andean orogen are difficult to quantify because of a lack of detailed observations for both the long-term deformation and erosion history of the Andean foldthrust belt. We contribute to the resolution of this problem by presenting a sequentially restored, balanced cross section based on new mapping across the northern Bolivia portion of the thrust belt (1517[degrees]S). The timing and magnitude of exhumation across the cross section are determined by synthesizing 10 new and ~70 previously published mineral cooling ages. Once balanced and restored, the section was sequentially forward modeled using stratigraphic and cooling age constraints. Results indicate the Eastern Cordillera (EC) records the highest magnitudes of shortening (123 km or 55%). The Interandean zone (IA) has shortened 48 km or 30%. In both the EC and IA individual thrust sheets are tightly folded and have minor offsets of 1-5 km. The Subandes (SA) has multiple levels of detachments allowing for thrust sheets with relatively large offsets (6-17 km). Total shortening in the SA is 66 km or 40%. Total magnitude of shortening for the entire fold-thrust belt in this region is 276 km (40%). New apatite and zircon fission track cooling ages in conjunction with published ages indicate two phases of rapid exhumation; an earlier phase from ~40 to 25 Ma in the EC and one prior to ~25 Ma in the IA, followed by distributed exhumation of the entire fold-thrust belt from ~15-0 Ma. Combined exhumation estimates from the balanced cross section and thermochronology suggest ~911 km of exhumation in the EC, ~5-9 km in the IA, and ~3-4 km in the SA. Long-term shortening rates are 7 mm/a for the EC and IA and 4-8 mm/a for the SA. The SA shortening rates are based on a ~15-0 Ma or 8-0 Ma deformation window. By linking cooling ages to location and magnitude of shortening, we suggest an ~10-17 Ma pause or a dramatic deceleration in the rate of deformation and propagation of the fold-thrust belt between 25 and ~15 or 8 Ma. Citation: McQuarrie, N.,J.B. Barnes, and T.A. Ehlers (2008), Geometric, kinematic, and erosional history of the central Andean Plateau, Bolivia (15-17[degrees]S), Tectonics, 27, TC3007, doi:10.1029/2006TC002054.

[1] Quantifying the timing, magnitude, and rates of exhumation and deformation across the central Andes is a prerequisite for understanding the history of plateau rise. We present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire (~500 km) Andean fold-thrust belt at ~19.5[degrees]S in Bolivia. Exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data, regional stratigraphy, geothermal gradients, and mass deficits inferred from a balanced section. Results suggest the following: (1) Initial exhumation of the Eastern Cordillera (EC) fore-thrust and back-thrust belts began in the late Eocene to early Oligocene (2736 Ma) and continued in a distributed manner in the late Oligocene to early Miocene (19-25 Ma). Interandean zone (IA) exhumation began 19-22 Ma, followed by a third pulse of exhumation (11-16 Ma) in the EC back-thrust belt and initial cooling in the westernmost Subandes (SA) 8-20 Ma. Finally, exhumation propagated eastward across the SA during the late Mio-Pliocene (2-8 Ma). (2) Exhumation magnitudes are spatially variable and range from maximums of

The force balance in plate tectonics is fundamentally important but poorly known. Here, we show that two prominent and seemingly unrelated observations--trench-parallel gravity anomalies along the Nazca-South America margin that coincide with the rupture zones of great earthquakes, and a rapid slowdown of Nazca-South America convergence over the past 10 m.y.--provide key insights. Both result from rapid Miocene-Pliocene uplift of the Andes and provide quantitative measures of the magnitude and distribution of plate coupling along the Nazca-South America margin. We compute the plate-tectonic force budget using global models of the faulted lithosphere coupled to high-resolution mantle circulation models and find that Andean-related plate-margin forces are comparable to plate-driving forces from the mantle, and they have sufficient magnitude to account for pronounced bathymetry variations along the trench. Our results suggest that plate coupling, gravity anomalies, and bathymetry variations along a given trench are all controlled by long-term stress variations in the upper portion of plate boundaries and that an explicit budget of driving and resisting forces in plate tectonics can be obtained. For the convergent margin considered here, spatial variations in the effective coefficient of friction associated with the distribution of lubricating sediments entering the trench are, by comparison, of minor importance. Keywords: Andean uplift, plate coupling, gravity anomalies.

Multiple geochronological methods using different metamorphic minerals were combined to date the regional, very low-grade metamorphism affecting Upper Jurassic-Lower Cretaceous volcano-sedimentary successions in the Andes of central Chile. Early Late Cretaceous metamorphic ages (between 82 and 108 Ma) were obtained by the K-Ar and U-Pb methods for celadonite and titanite. A much younger thermal event is responsible for actinolite formation at 8 Ma, most probably related to the intrusion of proximal Miocene granitoids. Previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist-facies event. The combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low-grade metamorphism associated with the Mesozoic extensional regime developed within the Andes.

Magmatic and detrital zircon ages from the Rocas Verdes basin, a tectonically juxtaposed remnant of sea floor in the Magallanes fold and thrust belt (southern Patagonia, South America), indicates that a rifting phase of the Rocas Verdes basin occurred between 152 and 142 Ma, and was accompanied by bimodal magmatism. A dacite dyke cross-cutting pillow-basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal Sanniento Ophiolite Complex contain 150 Ma zircon crystals, indicating that Inafic submarine volcanism had started prior to or during the Late Jurassic, 10-15 Ma earlier than previously thought. The silicic pyroclastic rocks of the Toblfera Formation, with two samples dated at 148 and 142 Ma, were heralded by synrift sedimentation along fault-bounded grabens within Palaeozoic metasediments. No evidence for an active volcanic arc during the early formation of the Rocas Verdes basin was detected in detrital zircon grains of the lower sedimentary member of the Tobifera Formation. A minimum of 25 Ma of continuous sedimentation in the Rocas Verdes basin is suggested by detrital zircon grains in the upper member of the Zapata Formation. The Rocas Verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution.

This article develops a methodology for revealing to an outsider the unelaborated cultural connotations of translated native words, using a case study of early intercultural communication in an Andean colonial setting. It focuses not on the general translation process from a native language into a European one, but rather on the reverse. 'Back translation' of language dealing with native paramount lords opens to cultural outsiders the spiritual dimension of native rulership.

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 quartzarenitic (during Impora and Cayara deposition) to sublitharenitic (during Camargo and Suticolio 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. 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. Keywords: Andes, foreland basins, foldthrust belts, Tertiary, sedimentology, sedimentary petrology.

Key words.--Andes, area of endemism, Aves, Choco, historical biogeography, Pleistocene refugia, protein electrophoresis, systematics, vicariance. Received May 17, 1994. Accepted October 19, 1995. The tropical rain forests of South America [...]

Few studies have quantified geographic variation in widely distributed Andean birds despite the fact that the linearity of their distributions provides unique opportunity to assess latitudinal geographic variation. We examined geographic variation of morphometric and plumage characters in populations currently treated as a single species, the Sepia-brown Wren (Cinnycerthia peruana), that inhabits humid montane forests from northern Colombia to central Bolivia. Our analysis supports the recognition of three biological species (olivascens, peruana, and fulva) based on discrete morphometric differences as well as marked plumage differences. Size variation within populations is inconsistent with the predictions of Bergmann's Rule, whereas variation across species runs counter to the predictions, with the smallest species occurring farthest from the Equator. Received 20 June 1995, accepted 10 Dec. 1995., Before hypotheses concerning the origin and maintenance of geographic variation in bird species can be formulated, the patterns of variation must be well-described (Zink and Remsen 1986). Although the humid [...]

[1] In this paper we present the results of the analysis of borehole breakouts from 115 wells drilled within Neuquen Basin in the Andean retroarc between 34[degrees] and 39[degrees]S (Argentina), The first-order present-day stress orientation in the Andean retroarc is expected to be mainly controlled by the plate boundary forces (azimuth 80[degrees]) and the topographic forces (E-W). The obtained maximum horizontal stress ([SH.sub.max]) has a preferred trend with a resultant direction of azimuth 88.7[degrees] and a 95% confidence interval of 13.3[degrees] consistent with the expected trend. The horizontal stress trajectory map achieved for this region shows that the [SH.sub.max] along the study area is not completely uniform. To the north of Colorado River, the [SH.sub.max] shows an ESE tendency interpreted as significant influenced by the topographic forces. To the south of Colorado River, [SH.sub.max] has an ENE trend similar to the expected based on plate boundary forces. To the southeast of the region, a NE direction was found, probably showing a basement structural control in the stress field geometry. The stress orientations obtained for the whole region show that plate boundary forces, drag basal, and topographic forces are strongly controlling the stress direction distribution. Citation: Guzman, C., E. Cristallini, and G. Bottesi (2007), Contemporary stress orientations in the Andean retroarc between 34[degrees]S and 39[degrees]S from borehole breakout analysis, Tectonics, 26, TC3016, doi: 10.1029/2006TC001958.