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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.

The subtribe Espeletiinae (Asteraceae, Heliantheae) comprises morphologically and ecologically diverse plants endemic to the tropical montane paramos of the Andes of Venezuela, Colombia, and Ecuador. Though the ecophysiology and ecology of this adaptive radiation have been well studied, relationships among taxa in the subtribe and between the subtribe and other taxa in the Heliantheae are poorly known. In this study, sequences from the internal transcribed spacer (ITS) region of nuclear ribosomal DNA are used to test previous hypotheses about the phylogenetic position of the Espeletiinae within the Heliantheae and to determine which taxa are the subtribe's closest relatives. Gene phylogenies based on maximum parsimony analyses reveal that the Espeletiinae clade is nested well within the subtribe Melampodiinae and thus should be considered a monophyletic complex of species, not a separate subtribe. The most parsimonious gene trees suggest that the genus Ichthyothere may be the sister taxon to the Espeletia complex and that the genus Smallanthus and a species of Rumfordia are likely among the complex's other closest living relatives. These data offer preliminary insights into the origins of this adaptive radiation and the broader phylogenetic context in which it occurred. Key words: adaptive radiation; Asteraceae; Espeletia; Espeletiinae; Ichthyothere; ITS; Melampodiinae; paramo.

The present paper reviews Tertiary volcanic and sedimentary formations in the Inter-Andean region of southern Ecuador (between 2 [degrees] S and 4 [degrees] 20'S) in order to develop a geodynamic model of the region. The formations occur in the southern shallow prolongation of the Inter-Andean Valley between the Cordillera Real to the east, and the Cordillera Occidental and Amotape-Tahuin Provinces to the west. One hundred fifty zircon fission-track analyses has established a detailed chronostratigraphy for the sedimentary and volcanic formations and several small intrusions. The Paleogene to early Miocene formations are dominated by intermediate and acidic volcanic and pyroclastic rocks. In addition, relics of Eocene continental sedimentary series have been identified. The Neogene sedimentary series lie unconformably on deformed and eroded metamorphic, sedimentary and volcanic formations. They were deposited in two stages, which are separated by a major unconformity dated at [approximately equal to] 10-9 Ma. (1) During the middle and early late Miocene ([approximately equal to] 15-10 Ma) marginal marine deltaic, lagoonal, lacustrine and fluvial environments prevailed, which we group under the heading 'Pacific Coastal sequences'. They presumably covered a greater surface area in southern Ecuador than their present occurrence in small topographic depressions. We suggest that they were deposited in the shallow marine Cuenca and Loja Embayments. Deposition in a marginal marine environment is also supported by the occurrence of brackish water ostracods and other fauna. (2) Above the regional (angular) unconformity, the coastal facies are overlain by late Miocene ([approximately equal to] 9-5 Ma) continental alluvial fan and fluvial facies which are in turn covered by mainly airborne volcanic material. They represent the 'Intermontane sequences' of the basins of Cuenca, Giron-Santa Isabel, Nabon, Loja and Malacatos-Vilcabamba. Sedimentologic and stratigraphic results are used to discuss the tectonic setting of Neogene sedimentation in the forearc and arc domain of the Ecuadorian subduction system. During the Pacific Coastal stage, northward displacement of the coastal forearc block along the Calacali-Pallatanga fault zone has driven crustal collapse in the Inter-Andean region. As a result, extensional subsidence drove the eastward ingression of shallow seas into the Cuenca and Loja Embayments from the Manabi and Progreso Basins to the west. Tectonic inversion in the forearc area during the early late Miocene (at [approximately equal to] 9.5 Ma) reflects the initiation of W-E oriented compression and uplift in the Inter-Andean region and the establishment of smaller Intermontane stage basins, which host the continental sequences. Coeval topographic rise of the Cordillera Occidental is indicated by the onset of clastic input from the west. The small Intermontane Basin of Nabon ([approximately equal to] 8.5-7.9 Ma) formed during the period of maximum compression. The present data prove that the Neogene Andean forearc and arc area in southern Ecuador was a site of important but variable tectonic activity, which was presumably driven by the collision and coupling of the Carnegie Ridge with the Ecuadorian margin since [approximately equal to] 15-9 Ma. Keywords: sedimentary facies; volcanics; fission-track chronostratigraphy; Neogene tectonics

The tectonic inheritance, crustal architecture and contrasting structural styles in the Venezuela Andes were investigated using surface and subsurface data images. The results suggested that the Venezuela Andes comprised a Neogene transpressional feature. The tectonic inheritance of older structures were proposed to explain the contrasting structural styles along the strike of the North Andean flank and the middle of the northern, central and southern segments of the orogen. The results further suggested that the observed asymmetry was due to preexisting Paleozoic thrusts.

U-Pb data obtained from granitoids and metamorphosed gabbros from the Cordillera Real and the El Oro metamorphic complex in Ecuador elucidate the pre-Cretaceous magmatic history and subsequent metamorphic evolution of the northern Andean basement. Two major tectonothermal episodes and later thermal overprinting have affected the region, obscuring the timing of plutonic and metamorphic events as constrained by K-Ar and Rb-Sr geochronology. U-Pb zircon and monazite ages show that S-type granitoids in the Cordillera Real and El Oro complex, and mafic intrusions in El Oro, were emplaced during the Triassic. The Piedras mafic intrusions, important components of the El Oro complex that were previously considered to be Precambrian. were intruded at 221 +18/-16 Ma. The S-type Marcabeli pluton and the Limon Playa intrusion yield ages of 227.5 [+ or -] 0.8 Ma and 200 [+ or -] 30 Ma, and in the Cordillera Real, the large S-type Tres Lagunas batholith was intruded at 227.3 [+ or -] 2.2 Ma. The Marcabeli and Tres Lagunas data show that S-type granite emplacement was contemporaneous in the two belts, supporting field and geochemical evidence that the Cordillera Real and El Oro complex were contiguous in the Triassic and then separated. The granitoids constrain the age of regional shearing, metamorphism, and anatexis (Moromoro tectonothermal event) related to Gondwana-North America separation. After the Moromoro event, subduction-related calc-alkaline granite batholiths intruded the Cordillera Real. Cessation of this magmatic episode is probably constrained by the 143 [+ or -] 1 Ma Azafran batholith. Metamorphic U-Pb ages are only recorded by 78-82 Ma monazite ages from the Limon Playa granite, which are similar to many reset K-Ar ages from elsewhere in both the Cordillera Real and El Oro complex. Recycling of material originally derived from the Amazonian craton, together with younger Cambrian-Neoproterozoic crust, is suggested by single grain inherited zircon ages of 546 Ma, 2220 Ma, and 2876 Ma from the Marcabeli pluton, together with Neoproterozoic and Mesoproterozoic upper intercept ages for Tres Lagunas and Limon Playa zircons. The zircon data are supported by 1400-1600 Ma Nd-depleted mantle model ages ([T.sub.DM]) for both the Tres Lagunas granite and Paleozoic migmatitic gneisses from El Oro and Cordillera Real. Plate tectonic reconstructions supported by paleontological and isotope data indicate that paleo-Mexico and the Colombian segment of the Andes were joined prior to the breakup of Pangea. Additional evidence supporting this scenario is given by the overlap of [T.sub.DM] ages for granitoids and metasedimentary rocks in the Acatlan complex of Mexico and the Cordillera Real and El Oro complex; these rocks serve as a suitable proxy for related lithologies in Colombia, the Nd [T.sub.DM] ages of which are unknown.

The structure and kinematic evolution of the eastern Cordillera Oriental and Interandean Zone are analyzed. Their association to the Subandean Ranges based on three balanced cross sections in the Bolivian-Argentinian border region are investigated. Gravimetric data suggesting thin lower Paleozoic strata in the footwall of the Interandean thrust indicate that the Interandean thrust's main basement ramp follows an early Paleozoic synsedimentary normal fault.

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 [...]

An archeological investigation of sandstone and conglomerate beds in the Andes desert sediments of northern Chile reveal the presence of bones of pterosaur. The sediments display indications of a flood which could have singularly destroyed a vast population of pterosaurs in the Andes. Mass destruction of these flying animals suggests the absence of adequate food for the whole population and also implies that the pterosaurs were not totally incapable of sustaining themselves on land.

Isotopic data for Quaternary volcanic centers from lat 17.5 degrees to 22 degrees S in northern Chile provide evidence for crustal contributions to arc magma genesis in the central Andes and delineate basement domains. Crustal thickness, distance from trench, height of volcanic edifices above the seismically active subduction zone, and sediment supply to the trench are all constant along this segment of the arc. The only significant variable is crustal age (Paleozoic in the south, Proterozoic in the north). Pb isotopic compositions of Quaternary lavas in the northern sector are lower (206Pb/204Pb = 17.89-18.28) and Sr and O isotope ratios are less variable (87Sr/86Sr = 0.7058-0.7077, delta-18-O = +6.5 0/00-+8.4 0/00) than in the southern sector (206Pb/204Pb = 18.59-18.79, 87Sr/86Sr = 0.7053-0.7083, delta-18-O = +6.4 0/00-+9.3 0/00) for rocks similar in bulk composition, from 53 to 74 wt% SiO2. Together, these features are interpreted to reflect interaction of magmas with crustal contaminants of different isotopic character, rather than changes in the mantle source of central Andean arc magmas resulting from either subduction erosion or involvement of old enriched lithosphere.

Subduction complexes along the Andean margin in central and southern Chile yield mid-Paleozoic to lower Mesozoic ages, yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length. The scarcity of uplifted subduction-complex rocks younger than mid-Mesozoic along the Chilean margin and in parts of the Scotia Arc suggests to us that these old, crystalline rocks, uplifted in the Triassic and Jurassic, represent a boundary in the forearc beyond which tectonic erosion does not easily occur. Greenschist-, blueschist-, and amphibolite-facies subduction-complex rocks from the Scotia Arc were originally thought to be a simple continuation of the subduction complexes in Chile. Based on new 40Ar/39Ar ages, the Scotia Arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in Chile. Structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the Scotia Arc occurred at some depth in a subduction zone or zones, certainly below the brittle-ductile transition, and in some cases under blueschist-facies conditions. We believe that the early subduction-related deformation and metamorphism in the greenschist- and blueschist-facies rocks of the Scotia Arc to be overprinted by mid-Cretaceous transpression along the South America-Antarctica plate boundary in the case of Elephant Island, transpression and subsequent localized transtension in the earliest Cenozoic in the case of the Darwin Complex, a mid-Cenozoic spreading-rate change in the case of Smith Island, and early Neogene initiation of Drake Passage opening/Shackleton Fracture Zone formation in the case of the Gibbs Island subgroup.

Subduction erosion, or scraping off of the continental crust by subducting oceanic lithosphere, is an important tectonic process along parts of the Andean convergent plate boundary. Estimated rates of subduction erosion increase northward north of the southern Andes, due to decreasing subduction angle and decreasing sediment supply to the trench. West of the central Andes subduction erosion may remove as much as 535 [km.sup.3] of crust per 1 km of trench per 1 m.y., or 7 vol% of the subducting oceanic lithosphere. The higher [sup.87.Sr/.sup.86.Sr] ratios of mafic magmas erupted north of the southern Andes may be explained by the higher rates of subduction erosion and increased amounts of contamination of the sub-arc mantle magma source.

Previous work has suggested that the floras of the separate regions of the high Andes are the products of disparate geographical, ecologica, and geological factors. Among the important factors that differ among the regions are distances from source areas of propagules, climates, and differential effects of Pleistocene glaciations. To ascertain the relative importances of these factors in shaping the modern floras, and to test hypotheses about the causes of observed generic diversity patterns, models of abundances of species per genus were tested against data from three samples representing the major vegetation zones of the high Andes (paramo, puna, and austral alpine). While originally proposed for intraspecific phenomena, abundance models (broken stick, log normal, geometric series, and log series) appear to be applicable to supraspecific taxa and were used to assess alternative hypotheses. Potential sources of genera now present in the sample areas were compared, and coefficients of similarity were computed between these samples and between each of them and two North American high-elevation floras. Propagule types in the three Andean samples were also scored to see if methods of fruit dispersal differed. Results showed that all areas fit the geometric series model if abundance plots of ranked genera were compared, but these results were heavily influenced by the large number of genera represented by only one species. Both the puna and austral alpine samples fit the log series models, indicating that the habitats of these areas are relatively uniform and that arrival of propagules was random over time. The lack of a fit of the Colombian data to the log series or log normal models is due primarily to a large number of species-rich general relative to other areas. These findings support the hypothesis that there has been a pronounced level of autochthonous speciation in the Colombian Andes relative to other areas. Coefficients of similarity at the generic level show that a given flora is most similar to that of the nearest sample area within the same continent. However, the floristic similarity between Tierra del Fuego and the high mountains of Colorado is as high as that between the Tierra del Fuego sample and the neighboring Argentina sample, indicating that similarities in climate can be as important as distance in determining the composition of some Andean floras.

Balanced cross section across the Cordillera Oriental and Subandean zone of the central Bolivian Andes indicate the crustal shortening probably has played the dominant role in orogeny in this convergent margin setting. A minimum amount of shortening, 210 km, is documented, which can account for two-thirds of the present-day crustal cross-sectional area along a transect spanning the entire mountain range. Substantial crustal shortening may also require loss of the lower lithosphere to the asthenosphere. A large, minimum amount of crustal shortening in the Bolivian Andes shows, contrary to common assumptions about orogeny, that (1) magmatic addition may be volumetrically less important in orogeny in Andean-type margins and (2) crustal shortening is not uniquely associated with continental or island-arc collision.

An integrated floodwater and canal irrigation system in Tarata village in the Bolivian Andes was in use in AD 719 and apparently existed for thousands of years. An analysis of the sedimentology and geomorphology in the area indicates that a hybrid-style irrigation system is responsible for the layer of silt on the Tarata fan. Long term irrigation restricts the deposition of the coarse sediments to the canalized main channel but the fine sediment is dispersed to other parts of the fan surface.

[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.

New cosmogenic nuclide exposure ages of rock-avalanche deposits in front of Sierra Laguna Blanca in the arid Argentine Puna Plateau demonstrate the effects of westward propagation of reverse faulting and the influence of active faulting on slope oversteepening and subsequent gravitational collapse. The [sup.21]Ne exposure ages of 8 superposed landslides with volumes [is greater than] 0.3 [km.sup.3] are between ca. 150 and 430 ka, and indicate an average recurrence interval of large gravitational mountain-front failures of ~27.5 k.y. for the ultimate 7 landslides. Landsliding did not occur after 150 ka along this sector of the Sierra Laguna Blanca. The termination of landsliding activity correlates with reverse-fault propagation into the piedmont area of the Sierra Laguna Blanca. The exposure age of ca. 130 ka of a tectonically uplifted terrace postdates initiation of fault activity in the piedmont region, and the fault scarp height of 120 m indicates important continued uplift in the piedmont; no further offsets occurred at the mountain front. Due to this lack of tectonic activity at the mountain front, no further collapse has occurred, indicating that in this environment catastrophic mountain-front failure is mainly controlled by tectonically driven slope oversteepening. Keywords: landslides, rock avalanches, paleoearthquake magnitude, cosmogenic nuclide dating.

In Andean forests, Cyathea caracasana grows across a range of successional habitats. This study documents variation in several measures of plant performance (stem growth, leaf production, leaf longevity, and spore production) in C. caracasana growing in open habitat, low-canopy secondary forest, and high-canopy secondary forest, based on 33 mo of observation. In open habitat, C. caracasana displayed significantly higher growth rates, leaf production rates, and leaf turnover than in either of the forested habitats. The highest rates of spore production were also observed in open-habitat individuals, with only one plant in the forest understory producing spores during the study. Despite low growth and no reproduction, I observed no mortality among ferns in the forest understory. These data show that C. caracasana performs best under conditions of full sun but can persist under the closed canopy. This suggests a life history in which periods of rapid growth, spore production, and recruitment in forest gaps alternate with low growth rate and persistence in the understory. A phylogenetic perspective suggests that the habitat flexibility, which might conventionally mark C. caracasana as a habitat generalist, is better interpreted as specialization for the dynamic forest in which it grows. Key words: cloud forest; Cyathea; Cyatheaceae; generalist; habitat; specialist; succession; tree fern.

All members of the genus Lysipomia (Campanulaceae) are endemic to high elevations (~3,000 m) of the Andean mountain chain of South America, making this genus a unique member of the bellflower family. Lysipomia sphagnophila along with a small number of additional species appeared to form a monophyletic group within this genus in a preliminary phylogenetic analysis based on ITS sequence data. Due to a high degree of morphological similarity, species and subspecies delimitations within this complex remain tentative. nrITS sequence data may aid in clarifying species and subspecies relationships as well as in determining the possible monophyly of this group. ITS data may also be useful in a biogeographical interpretation of this complex including aspects such as species origin and subsequent radiation. Sequences for the entire ITS and accompanying 5.8S region of nrDNA were obtained for all members of this complex by direct double-stranded sequencing of PCR amplified DNA. Results of this phylogenetic analysis will be presented.

The Pliensbachian-Toarcian bivalve mass extinction in the Andean Basin of South America is characterized by a sharp drop in species diversity, caused mainly by extinction of endemics, and an extended lag phase. Whilst analysis of community attributes such as guild diversity, number of associations, and species richness of samples constituting associations did not yield any significant changes across the critical interval, rates of origination and extinction of species differed markedly between the various substages of the Early Jurassic. The most prominent feature is the drastic increase in the rate of origination of endemics culminating in the Early Pliensbachian. The time span between the peaks of rate of origination and extinction (5.7 Ma) closely corresponds to the longevity peak of Jurassic endemic bivalves (4-6 Ma). In addition to physico-chemical changes, we therefore explain the drop in species diversity as a result of the evolutionary dynamics of endemics, characterized by a preceding increase in the origination rate. Global mass extinctions are usually explained by one or more physico-chemical causes. We suggest that evolutionary factors, in particular species longevities, may substantially contribute to the phenomenon. Keywords: Jurassic, mass extinctions, communities, species diversity, evolution.

Research establishing data pertaining to the structural evolution and the geometry of the Andean backthrust belt of Bolivia is presented. Particular attention is given to the geological characteristics of the Central Andean plateau and the basement thrusts.

The locality of the Chiar Kkollu was found to be of early Miocene age based on evidences provided by radioactive dating, field observations and paleomagnetic proofs. The discovery, which was the result of the study made on the locality's crust, crustal contamination and the magma genesis of the volcanic rocks, also provided evidences to support the tectonic evolution of the Bolivian Andes. Further studies on the chemistry and isotopic composition of Chiar Kkollu need to be undertaken to determine the locality's central volcanic zone processes.

[1] The Santa Barbara System (SBS) of northern Argentina is a 400 km long segment of the Subandean foreland thrust belt. It is characterized by predominantly west verging, relatively high-angle thrust faults. Many of these faults are reactivated normal faults from one branch of a complex Cretaceous to Paleogene rift system. Heteroaxial folding in parts of the SBS is probably due to a slight component of range-parallel dextral strike-slip motion acting on preexisting faults striking north and NE during inversion. Regional balanced cross sections along two transects across the northern SBS indicate that the major faults flatten into a detachment in the basement at about 10 km depth. Neogene E-W contraction in the SBS is of the order of 21-26 km. Rift extension is not very well constrained but was probably less than 10 km. The structural style of the SBS differs from the thin-skinned Subandean thrust belt to the north and from the large-wavelength Sierras Pampeanas basement uplifts to the south. The changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland, suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; 8102 Tectonophysics: Continental contractional orogenic belts; 9360 Information Related to Geographic Region: South America; KEYWORDS: central Andes, Subandean zone, Salta rift, fault reactivation, flat subduction. Citation: Kley, J., and C. R. Monaldi, Tectonic inversion in the Santa Barbara System of the central Andean foreland thrust belt, northwestern Argentina.

The tropical high-altitude tundra-like communities of the Andes harbor approximately 35 endemics species of Lysipomia (Campanulaceae). Two of the species, L. laciniata and L. sphagnophila, are distributed at both the northern and southern ends of the range while the majority of the remaining species occur in extremely limited areas on either side of a major geologic division of the Andes, the Huancabamba Depression. We set out to test the hypothesis that the species on opposites sides of this barrier should be more closely related. In analyzing sequences of nuclear ribosomal internal transcribed spacer (nrITS) regions in 27 of the approximately 35 species, we have found that the genus diverges into two primary clades. The Depression appears to have been a semi-permeable barrier during the evolution of Lysipomia. The nrITS region phylogeny is congruent in most portions with one produced using morphological, anatomical and cytological evidence. We were also able to determine the relationships among the different populations of L. sphagnophila and L. laciniata.