Año 2001, Boletín IFG vol. 71, números 1 - 2
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Ítem Acceso Abierto The Middle Tithonian (Upper Jurassic) Ammonoid fauna of Cañadón de los Alazanes, southern Neuquén-Mendoza Basin, Argentina(Instituto de Fisiografía y Geología, 2001) Parent, HoracioThe Middle Tithonian rock and ammonite successions of Cañadón de los Alazanes (southern Neuquén-Mendoza Basin, Argentina), sampled for the first time, correlate almost exactly with those of the close localities Mallín de los Caballos and Mallín Quemado, but showing a comparatively reduced thickness. The studied fauna comprises: Pseudolissoceras zitteli (Burckhardt) [M&m], Neochetoceras? sp., Pseudhimalayites subpretiosus (Uhlig) [M, = P. steinmanni (Haupt)], Torquatisphinctes proximus (Steuer) transient αand transient β, and Corongoceras cf. alternans (Gerth) [M]. This fauna shows strong Tethyan affinities enabling close time-correlation with the European standard chronostratigraphic scale. The type specimens of P. zitteli, P. planiusculum (Zittel), P. rasile (Zittel), C. alternans and C. lotenoense (Spath) are figured. The Haploceratid genus Pseudolissoceras Spath most probably originated in Early Tithonian Hybonotum Zone populations of Haploceras carachtheis of Crussol (France), widely expanded its biogeographic distribution in the Semiforme Zone in the form of P. zitteli, prior to the diachronous disappearence of local adaptations in the Neuquén-Mendoza basin [P. pseudoolithicum (Haupt)] and northern Central Tethys (P. concorsi Donze and Enay = P. bavaricum Barthel). P. subpretiosus occurs in the lower Proximus Biozone (approximately upper Semiforme-lower Fallauxi Zones), although it appears to range, with its local microconch Simocosmoceras adversum andinum Leanza and Olóriz, through the upper Zitteli and lower Proximus Biozones in other localities of the basin. T. proximus occurs abundantly, differentiated in transients α and β, the latter giving gradually origin to the early Andean himalayitids [e.g., Windhauseniceras internispinosum (Krantz)] via Torquatisphinctes? windhauseni (Weaver sensu Leanza), a form close to "Burckhardticeras" peroni (Roman). Both transients of T. proximus are mainly confined to the Proximus Biozone and their sexual dimorphism remains unrecognized.Ítem Acceso Abierto Anomalous upper mantle beneath the Central Andes : isostasy and Andean uplift(Instituto de Fisiografía y Geología, 2001) Introcaso, AntonioFour theoretical models isostatically compensated were prepared and compared between themselves in order to analyze either the isostatic equilibrium or the probable mechanisms for justifying the Central Andean uplift. They are: Model 1, or Airy Model: in this classical model the isostatic compensation would take place at the maximum Moho depth's level: 61.7 km. Model 2: this model justifies the Andean elevation by means of a combination of lithospherical thermal root and crustal root. In this case, the last one is diminished 5.3 km respect to crustal root's thickness of Model 1. Isostatic compensation takes place here at the bottom of thermal lithosphere: 140 km deep. Model 3: this model explains the elevation by means of a combination of the subsidence that the subducted Nazca Plate could produce, and the crustal root effect, that is 6 km thicker than the first model's root. The isostatic compensation could take place at a depth of 300 km. Model 4: this model involves Models 1, 2 and 3 mechanisms; the isostatic compensation takes place at a depth of 300 km. Crustal shortenings S are sensitive to the upper mantle's heterogeneity, varying from 17% to +18%, h as it is shown by the following values: S = 278 km (Model 1), S = 230 km (Model 2), S = 338 km (Model 3) and h h h S = 288 km (Model 4). h We also demonstrate that the model selected for evaluating the Andean isostatic equilibrium in this zone of the anomalous upper mantle is not critical. The analysis of the two EW gravity sections at 22°S and 24.5°S latitudes favors Model 2 as the most likely, since the crustal thickness found from it is clearly consistent with seismic data available at 24.5°S. Nevertheless, we can admit other gravimetric models.Ítem Acceso Abierto Lower and Middle Tithonian marine gastropods from the Neuquén-Mendoza basin, Argentina(Instituto de Fisiografía y Geología, 2001) Gründel, Joachim; Parent, HoracioSamples of marine Lower and Middle Tithonian gastropods from Picún Leufú, Cañadón de los Alazanes, Cerro La Parva (Neuquén) and Casa Pincheira (Mendoza) have provided three undescribed species. These are: Exelissa? arcuatoconcava nov. sp. (Mendozanus Biozone), Sinuarbullina melehuensis nov. sp. (Zitteli Biozone) and Dicroloma? sp. (Mendozanus Biozone).