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The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U-Pb ion microprobe analysis of detritai zircons and Sm-Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. [T.sub.DM] ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic (c. 1.8-1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Rio de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa-Antofalla massif.

The origin of pisoids in a diversity of environments has been recently discussed. While traditionally considered physico‐chemical sedimentary products developed in turbulent environments, or even grown in situ, new results show that microbes may influence their development. In the Terma Los Hornos travertine system, two types of actively forming pisoids were recognized: (i) laminated pisoids with a concentrically laminated crust surrounding a nucleus; and (ii) dendriform radial pisoids with no distinct nucleus. Laminated pisoids form in turbulent pools at the base of waterfalls, while dendritic pisoids develop within small depressions between terracettes, and they can occur superposed as a result of episodic transport and recycling within the travertine system (for example, dendritic textures as nucleus of laminated pisoids). The laminated pisoids are characterized by laterally constant laminae thickness with high inheritance and low amounts of randomly distributed organic matter and microbial remains, suggesting a predominantly abiogenic origin. In contrast, the dendritic pisoids exhibit an intimate relationship with diatoms and filamentous bacteria suggesting a stronger biotic influence in their formation. Thus, both abiogenic and biogenic carbonate structures can coexist in the same hydrothermal system. Pisoids found in the stratigraphic record of Terma Los Hornos allowed to test the preservation potential of the primary signals and reconstructing early diagenetic overprints. [ABSTRACT FROM AUTHOR]

[1] In the distal region of the modern flat-slab segment in the southern Central Andes, an unusual stack of middle Miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west. These features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system. Sedimentary thickness farther east and far from the Cordilleran tectonic loads suggests accommodation and preservation driven by 'nonisostatic' dynamic subsidence. Regional overlapping relationships and basin modeling suggest that the Modern broken foreland (present Sierras Pampeanas) can be interpreted as a reactivation of a formerly partitioned broad forebulge. Citation: Davila, F. M., R. A. Astini, T. E. Jordan, G. Gehrels, and M. Ezpeleta (2007), Miocene forebulge development previous to broken foreland partitioning in the southern Central Andes, west-central Argentina, Tectonics, 26, TC5016, doi:l 0.1029/ 2007TC002118.

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenic conglomerate facies provide a unique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic elastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intrabasinai sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation. A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ~2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene-Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas. Keywords: Central Andes, Sierras Pampeanas, broken foreland, basin analysis, provenance, conglomerates.

Ages from U-Pb and [sup.207]Pb/[sup.206]Pb analyses of detrital zircons from synrift sandstone in the Lower Cambrian Cerro Totora Formation of the Argentine Precordillera have strong similarities to those from late synrift sandstones in the Lower Cambrian Rome Formation of southern Laurentia (Alabama). Ages of detrital zircons from the Cerro Totora sample cluster at 1160-970 Ma (60% of analyzed zircons), 1490-1300 Ma (24%), and 1890-1640 Ma (16%). Ages from two Rome samples cluster at 1240-970 Ma (32% of analyzed zircons), 1540-1270 Ma (31%), 1840-1610 Ma (14%), 1970-1890 Ma (5%), and 2930-2310 Ma (18%). The ages of detrital zircons from the Rome and Cerro Totora sandstones are consistent with sediment supply from the Grenville and older Proterozoic Laurentian provinces, and the older cluster in the Rome sandstones corresponds in age to the Laurentian Archean Superior province. Neither the Rome nor Cerro Totora samples include components younger than Grenville, and the lack of zircons from distinctly Gondwanan provinces is consistent with a Laurentian provenance. The detrital-zircon ages support previous interpretations that the Precordillera was rifted from the Ouachita embayment of Laurentia during Early Cambrian time and subsequently was transferred to Gondwana. Keywords: Argentine Precordillera, continental rifting, detrital-zircon geochronology, Laurentia, terrane transfer.

[1] The Famatina belt, southern central Andes, records a circa 470 m.y. shortening episode (Ocloyic orogeny) affecting the peri-Gondwanan back arc basin in response to the accretion of the Precordillera terrane. Collision created distinct features across and along the margin, some of which persisted into the present Andean structure. From east to west, the Ocloyic deformation is recorded across the telescoped region of Famatina, represented by (1) an east vergent fold-thrust belt (EVFTB) involving a major angular unconformity between Early and Middle Ordovician rocks, (2) a highly deformed volcano-sedimentary belt, and (3) an east vergent ductile shear zone affecting Famatinian granites. Axis attitudes of Ordovician refolded anticlines in the EVFTB show west-east tectonic transport, in agreement with kinematics of Ocloyic high-strain belts, indicating shortening orthogonal to the present Pacific margin. Eastward displacement of the Ordovician volcanic arc at ~27[degrees]S latitude together with a documented sinistral slip zone across the proto-Andean margin suggests tectonic indentation. INDEX TERMS: 4267 Oceanography: General: Paleoceanography: 4279 Oceanography: General Upwelling and convergences: 3030 Marine Geology and Geophysics: Micropaleontology; 1050 Geochemistry: Marine geochemistry (4835, 4850); 4516 Oceanography: Physical: Eastern boundary currents; KEYWORDS: Ordovician, Ocloyic deformation, back arc foreland, Famatina belt, Precordillera terrane, indentation tectonics. Citation: Astini. R. A., and F. M. Davila (2004). Ordovician back arc foreland and Ocloyic thrust belt development on the western Gondwana margin as a response to Precordillera terrane accretion, Tectonics, 23, TC4008, doi: 10.1029/2003TC001620.

In the Famatina Ranges, Central Andes of western Argentina, 470 m.y. of protracted shortening is recorded through episodic coaxial-coplanar fold amplification. Ordovician anticlines within the east-vergent thrust belt were refolded and tightened during later Paleozoic and Cenozoic shortening. Folding episodes are recorded in four major unconformities that can be measured on both flanks of one of these map-scale anticlines. Superposed folding (Type 0), interpreted from mapping and bedding plots, shows similar axial orientation in older and younger rocks throughout the different stratigraphic layers. Of the shortening, 30% is pre-Cenozoic and allows the recording of episodic deformation during continuous plate convergence at the Gondwana-South American margin. Keywords: Type 0 folds, unconformities, protracted shortening, Central Andes, Argentina

Conclusions concerning the relation between the early Paleozoic biostratigraphy of the Precordillera of northwest Argentina and the Laurentian craton are discussed. Generally, there is consistency in the relationship although two aspects of the geodynamic model do not agree with the Argentine Precordillera rifting and drifting hypothesis. A detailed analysis of this geodynamic model is presented.