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

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.

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.

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 [...]

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 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.

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 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.

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.