Somuncura Plateau Research Articles

Mantle wedge deformation by subducting and rotating slab and its possible implication

Abstract We have constructed a simple model of the deformation of the mantle wedge caused by the subducting and rotating slab based on corner flow model. By applying this model to the geological settings of the Somuncura plateau volcanic region, northern Patagonia, which is located far from the volcanic front, we constrain the mechanical aspect of the hypothesis that the volcanisms of the Somuncura region are triggered by the dehydration-induced melting of the up-warped transition zone which may contain more water than other regions do. Assuming that the water concentrates in the transition zone under the Somuncura plateau, we find that the above scenario may be possible, if the speed of the subducting slab is less than ∼2 cm/yr or the dip angle changes significantly (∼50 degrees within ∼8 Myr).

A First Record of Phlebotominae from Argentinean Patagonia (Diptera: Psychodidae: Phlebotominae)

A species of the Lutzomyia oswaldoi group is recorded from the Valcheta stream basin, Somuncura plateau, Patagonia. It represents the World southermost record of a species of Phlebotominae, apparently supporting the hypothesis about the relictual character of several components of the Somuncura plateau biota, particularly for those species belonging to the Paranaense lineages.

Aggressive Behaviour Between Gymnocharacinus Bergi (Pisces, Characidae) And Other Neotropical Fishes From A Thermal Stream In Patagonia

Gymnocharacinus bergi is a stenothermal endangered fish only found in the headwaters of a thermal stream within the temperate Somuncura Plateau (Patagonia, Argentina). In spite of the presence of other fish species in the Valcheta Stream basin, none of these has been found at the headwaters. Temperature and salinity cannot account for the absence of extremely eurytopic Cnesterodon and Jenynsia. In addition, there is no evidence of diet overlapping or territory defence between these species and G. bergi. Therefore, we hypothesise that agonistic behaviour might be related to G. bergi's isolation. Experiments were performed in order to analyse interspecific interaction between G. bergi and individuals of the other two species. Cnesterodon decemmaculatus showed a submissive behaviour while Jenynsia multidentata was clearly aggressive. G. bergi decreased its aggressive behaviour towards C. decemmaculatus throughout the experiment. However, when placed with J. multidentata, its aggressive behaviour did not diminish. We suggest that the characteristics of the agonistic behaviour displayed by J. multidentata, opposite from those displayed by C. decemmaculatus, might account for the absence of J. multidentata in G. bergi's present habitat. With respect to C. decemmaculatus, other factors such as its low counter current swimming ability might play a major role in this phenomenon.

The northern Patagonia Somuncura plateau basalts: a product of slab‐induced, shallow asthenospheric upwelling?

The Meseta de Somuncura forms the largest basaltic plateau (20 000 km2) of southern Argentina (extra‐Andean domain). Most of these tholeiitic to alkaline rocks were extruded at ˜ 25 Ma (late Oligocene). The absence of rifting–thinning processes, plume activity, or slab‐window phenomena leaves only one major possibility for the generation of Somuncura: asthenospheric (‘OIB‐like’) corner flow leading to a transient thermal anomaly above the subducting plate. It is suggested herein that the intake of hot asthenosphere was forced into a favourable topography (concave‐up) of the subducting plate, when a major plate reorganization event (Farallon to Nazca) was taking place in late Oligocene to early Miocene time. The fast and vigorous intake of asthenosphere would have been induced by slab roll‐back, leading to decoupling of the subducting plate. The Somuncura volcanic episode can be regarded as a marker of the passage from the extremely oblique subduction of Farallon, to the birth of the Nazca plate and roughly perpendicular convergence between South America and Nazca.

Threatened Fishes of the World: Gymnocharacinus bergi Steindachner, 1903 (Characidae)

Common names: Mojarra desnuda, mojarra bronceada (spanish), naked characin (E). Conservation status: ‘Endangered’, Red Data Book (IUCN 1996). Identification: Robust and small characin (up to 100 mm TL) that belongs to a monotypic subfamily Gymnocharacininae. The fin rays are D 12, A 19, the caudal fin (P14-M1-R17(9/8)M1-P12) is forked with rounded lobes and lacks the adipose fin. The mouth is terminal with a slight superior prognatism. In adults, the lips do not cover the external series of pentacuspidal teeth. Each premaxillae and dentaries bear two rows of teeth. The maxillae bear two pentacuspidal teeth. The continuous lateral line is straight. Juveniles between 33–36.3 mm TL begin to reabsorb the scales and adults greater than 50 mm TL only show scales on the lateral line and posterior to the opercula. The colour is dark brown, greenish, the belly is creamish and present a band copper-coloured on the flanks, beneath the lateral line. The opercular zone shows golden reflections (Ringuelet et al. 1967, Miquelarena & Aramburu 1983, Ortubay 1998). Drawing by Ortubay (1998). Distribution: Endemic to the Somuncura Plateau at the NE of Argentinean Patagonia, restricted to the thermal headwaters of the two branches, ‘Frio’ and ‘Caliente’, of the Valcheta Stream (500–900 m altitude), an endorheic system of 100 km which disappears in the patagonian esteppe (Luling 1978). Abundance: Fish are scarce at the Frio branch and abundant at the Caliente branch (Ortubay 1998). Habitat and ecology: water temperature has a range of 21.5–26◦C. Ortubay (1998) summarises water chemistry and habitat description. Gymnocharacinus bergi has gregarian habits. Larvae feed on ostracods, chironomid larvae and acari. Juveniles and adults, with a grinding feeding apparatus, feed on periphitic algae. Reproduction: Maturity is reached at 37 mm TL in males and 38 mm TL in females, the breeding season occurs during later winter and early spring (August–October). Females present up to 200 oocytes (1.4-mm diameter) rich in yolk. Threats: In 1941, Oncorhynchus mykiss and Salvelinus fontinalis were introduced in the stream; in the colder Frio branch headwater, O. mykiss preys upon G. bergi and in winter months trout ascend the Caliente branch (Ortubay 1998). Nevertheless, the thermal barrier protects G. bergi at the warmer headwaters of Valcheta Stream. There, the mayor problems are the goat and cattle, pollution and catches for illegal sale of this species to collectors. The Frio branch is easily accessible by road. A single catastrophic event could eliminate the entire subfamily. Conservation actions: The Somuncura Plateau, of 15 000 km, was declared a Reserve Area in 1986 because of the presence of numerous endemics. Up to date, the limits are not established and there is no effective protection. In 1994, G. bergi was declared Natural Monument and catalogued as ‘Endangered’ (IUCN 1996). Conservation recommendations: Resettle two humans from the Caliente branch, educate the local people from the Frio branch, control the use of waters for cultivation and bathing, eradicate the salmonids from the headwaters, and construct a definitive barrier at the confluence of the two branches where the temperatures are extreme and out of the thermal tolerance for G. bergi (Ortubay et al. 1997, Ortubay 1998).

On the habitat and isolation of Gymnocharacinus bergi (Osteichthyes: Characidae)

Gymnocharacinus bergi is a rare Paranensean fish which is the only characiform almost lacking scales in the adult. It is endemic and the only species in a peculiar spot — a tributary of the Valcheta creek — in the Somuncura plateau in northern Patagonia, Argentina, over 300 km from the nearest place with a paranensean fish fauna. Besides its geographical isolation, G. bergi occurs within an area with climatic features drastically different from those currently associated with fishes from Neotropical temperate zones. We tested the assumption that water temperature in the naked characin habitat do not agree with the northern Patagonia climate. We also considered the isolation of G. bergi within the framework of an increasing inpoverishment of the paranensean ichthyofauna along a NE-SW axis in the Buenos Aires province. For this we applied a decrement equation used in island biogeography. Our findings demonstrate that the existence of G. bergi in its isolated habitat is possible because of the thermal traits of the water at the sources of the creeks, its temperature being independent of the climate of the area. The chemical composition of water was found to be within the range of common environments in the Buenos Aires ‘pampas’ inhabited by several species of Paranensean fishes. Geographically, G. bergi lives in the last of a series of habitats which show a decreasing number of species correlated with the increasing distance from the La Plata River. Conservation status of the species is briefly discussed.

Two New Subspecies of the Liolaemus fitzingeri Complex from Argentina

Liolaemus fitzingeri cuyanus n. subsp. from sandy regions north of the Rio Colorado (Argentina) and L. fitzingeri xanthoviridis n. subsp. from coastal range of Chubut (Argentina) are described. The former is easily distinguishable from other known forms of the group by a lower number of dorsal scales, lack of ventral melanism and the presence of a distinct black antehumeral band. L. fitzingeri xanthoviridis is distinguishable from its nearest relative L. fitzingeri canqueli by shorter limbs, more distinctly keeled dorsal scales, anterior margin of ear with smaller non-proyecting scales, and different color pattern. Psammophilous lizards of the L. fitzingeri complex are widespread Argentine iguanids, living in arid and semiarid regions, south of 280S. The nomenclatural history of the species has been noted previously (Cei, 1973, 1975) but no attempt has yet been made to fix definitively the identity of the syntypes of Proctotretus fitzingeri Dumeril and Bibron (1841), whose locality is obviously erroneous. These syntypes shall be discussed elsewhere, but we have determined conclusively that they do not belong to either of the subspecies here described. Although Koslowsky (1895) cited the species for Catamarca, all subsequent authors regarded it as a characteristic Patagonian lizard and emphasized its remarkable population variation. Eulaemus affinis Girard 1858 and Liolaemus melanops Burmeister 1888 have been considered to be synonyms of Liolaemus fitzingeri (Boulenger 1885; Peters and Donoso Barros 1970), although the name melanops was later applied to an isolated population from Meseta Canquel, Chubut, south of Chubut River [L. fitzingeri melanops (Cei, 1973)]. The name was later properly applied to the populations north of Chubut River, found in xerophilous environments from Somuncura plateau (Rio Negro) to Valdes peninsula (Cei, 1975; Cei and Scolaro, 1977a; Scolaro and Cei, 1977) and the population formerly called Liolaemus fitzingeri melanops from meseta Canquel was described as a new geographical form (L. fitzingeri canqueli Cei 1975). The nominate form L. fitzingeri fitzingeri was there referred to the southernmost Patagonian populations, in accordance with Donoso Barros's redescription (1966:293: terra typica, Chile Chico, Prov. Aysen (sic)) and Peters and Donoso Barros's Catalogue of the Neotropical Squamata (1970). Further morphological and immunological studies carried out on polymorphic populations from Valdes Peninsula and Puerto Madryn, Chubut, allowed us to place the lizard described as Liolaemus goetschi by Muller and Hellmich (1938) in the synonmy of Liolaemus fitzingeri melanops. It was based upon northern Patagonian populations from Rio Negro, close to the Rio Colorado (Cei and Scolaro, 1977a). More recently field research by the senior author in San Juan and La Rioja province (western central Argentina) has revealed a new distinct northern population of the fitzingeri group. Another different population was discovered by the second author south of Chubut River, along the Atlantic coast. Both of these taxa are described herein and their relation to other geographical races of Liolaemus fitzingeri is examined with reference to ecological characteristics. Specimens cited are in the Museo Zoologico, Florence, Italy (MF), and in the Instituto Biologia Animal, Universidad Nacional de Cuyo, Mendoza, Argentina (IBA-UNC). *Present Address: Rancho Somuncuri, Rua F. de Figueiredo, Birre, 2750 Cascais, Portugal.

Further Observations on Endemic Telmatobiid Frogs from the Patagonian Somuncura Plateau (Rio Negro, Argentina)

Males and tadpoles of Telmatobius somuncurensis, living in thermal springs (20 - 22?C) are described. Preliminary observations on the spermatogenic cycle of the species are dis- cussed. Ecological observations on the Somuncura plateau, 700-800 m, are reported.