Michel Fornari

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

Abstract Integrated studies and revisions of sedimentary basins and associated magmatism in Peru and Bolivia (8–22°S) show that this part of western Gondwana underwent rifting during the Late Permian–Middle Jurassic interval. Rifting started in central Peru in the Late Permian and propagated southwards into Bolivia until the Liassic/Dogger, along an axis that coincides with the present Eastern Cordillera. Southwest of this region, lithospheric thinning developed in the Early Jurassic and culminated in the Middle Jurassic, producing considerable subsidence in the Arequipa basin of southern Peru. This ∼110-Ma-long interval of lithospheric thinning ended ∼160 Ma with the onset of Malm–earliest Cretaceous partial rift inversion in the Eastern Cordillera area. The lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and/or transpressional deformations. In particular, the Altiplano plateau corresponds to a paleotectonic domain of “normal” lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere. The high Eastern Cordillera of Peru and Bolivia results from Late Oligocene–Neogene intense inversion of the easternmost thinned area.

40Ar/39Ar dating of the Jurassic volcanic province of Patagonia: migrating magmatism related to Gondwana break-up and subduction

The Mesozoic large igneous province (LIP) of Patagonia (southern South America), which is one of the largest silicic provinces on Earth has been investigated by the 40Ar/39Ar method. Twenty-seven ages considered as valid, including twenty plateau ages, show that the volcanic activity, ranging from 187 to 144 Ma, occurred between and contemporaneously with the initial break-up of Gondwana (starting with the Karoo-Antarctic-Tasmanian (KAT) flood basalt province) in the east, and a subduction in the west. The data display a regular decreasing of ages from the ENE (187 Ma) to the WSW (144 Ma) along about 650 km, apparently related to the tectonic structure in half-grabens oriented NNW-SSE. The good fitting of this trend with the opening of the Rocas Verdes-Sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the SSW in a continental disruption behind the magmatic arc. The observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction. The spreading and thermal effect of the KAT plume may have an additional effect and also could account for the unusually large volume of magma.

Dating of paleolakes in the central Altiplano of Bolivia

Abstract The deposits of the extensive paleolakes of the Bolivian Altiplano correspond to an alternation of lacustrine episodes and dry periods, a phenomenon commonly explained by climatic changes. The lack of precise dating of the deposits has prevented detailed reconstruction of the evolution of paleolakes that would allow a determination of the relative influence of interconnections between neighboring basins and local climatic conditions. Among the five paleolakes of the northern Altiplano (Titicaca basin) and the three of the central Altiplano (Poopo, Coipasa and Uyuni basins) only the two most recent paleolakes have been dated. A 121xa0m long sediment core recovered from the Salar de Uyuni contained 11 lacustrine layers (L1–L11) separated by 12 salt crusts. Radiocarbon dating shows that the younger layers L1 and L2 are related to the Tauca lacustrine phase (12u2008000–16u2008000xa0calxa0yrxa0 bp ) and layers L3 and L4 to the Minchin lacustrine phase (30u2008000–73u2008000xa0calxa0yrxa0 bp ). Layer L5, located at 46xa0m below the surface, contains a volcanic ash with well-preserved biotites is here dated by the 40 Ar/ 39 Ar method. The measured age of 191u2008000±5000xa0y is the first date ever obtained from a pre-Minchin paleolake (Lake Escara). This result combined with 14 C and U/Th data obtained on lakes Tauca and Minchin from various authors suggests that the duration of the lacustrine events increased regularly during the Pleistocene. In contrast, paleolake levels in the northern Altiplano decreased from the oldest known (Lake Mataro, 3950xa0m) to the present Titicaca level (3806xa0m). A progressive erosion of the threshold between the northern and central Altiplano may have allowed important volumes of water to overflow from the northern basin into the central Altiplano. The existence of large paleolakes in the central Altiplano was thus not only dependent on local climatic conditions but also on additional inputs from the north, where climatic conditions could have been quite different from those prevailing in the south.

Geology of El Misti volcano near the city of Arequipa, Peru

Approximately 750u2009000 people live at risk in the city of Arequipa, whose center lies 17 km from the summit (5820 masl [meters above sea level]) of the active El Misti volcano. The composite edifice comprises a stratovolcano designated Misti 1 (ca. 833– 112 ka), partially overlapped by two stratocones designated Misti 2 and Misti 3 (112 ka and younger), and a summit cone Misti 4 (11 ka and younger). nEight groups of lava flows and pyroclastic deposits indicate the following volcanic history. (1) Three cones have been built up since ca. 112 ka at an average eruptive rate of 0.63 km3/k.y. (2) Several episodes of growth and destruction of andesitic and dacitic domes triggered dome-collapse avalanches and block-and-ash-flows. Deposition of these flows alternated with explosive events, which produced pyroclastic-flow deposits and tephra-fall and surge deposits. (3) Nonwelded, dacitic ignimbrites may reflect the formation of a 6 × 5 km incremental caldera collapse on Misti 2 (ca. 50u2009000 and 40u2009000 yr B.P.) and a 2 × 1.5 km summit caldera on Misti 3 (ca. 13u2009700 to 11u2009300 yr B.P.). (4) Tens of pyroclastic flows and at least 20 tephra falls were produced by Vulcanian and sub-Plinian eruptions since ca. 50 ka. On average, ash falls have occurred every 500 to 1500 yr, and pumice falls, every 2000 to 4000 yr. (5) Misti erupted relatively homogeneous andesites and dacites with a few rhyolites, but Misti 4 reveals a distinct mineral suite. Less evolved andesites prevail in scoriaceous products of group 4–1 including historical ash falls. Scoriae of Misti 4 and the ca. 2300–2050 yr B.P. banded pumice commonly show heterogeneous textures of andesite and rhyolite composition. This heterogeneity may reflect changes in physical conditions and magma mixing in the reservoir. (6) Deposits emplaced during the Vulcanian A.D. 1440– 1470 event and the sub-Plinian eruption(s) at ca. 2050 yr B.P. are portrayed on one map. The extent and volume of these deposits indicate that future eruptions of El Misti, even if moderate in magnitude, will entail considerable hazards to the densely populated area of Arequipa.

40 AR/ 39 AR dating of synkinematic white mica; insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps

Abstract This paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40Ar/39Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew at the expense of feldspar during Alpine deformation and its Ar spectra do not show any excess 40Ar. On the SE side of Mont Blanc, ages of shear zone phengites have a narrow range of 15.8–16.0±0.2 Ma, which is in the same age range as 40Ar/39Ar ages of minerals from kinematically related veins. The top-to-SE sense of shear is consistent with initiation of a Mont Blanc flower-structure within a dextral transpressional system by 16 Ma. On the NW side, mini-plateaux ages of 14.5±0.3 and 23.4±0.4 Ma are preserved in the same sample, suggesting the possibility of two phases of deformation. This is also supported by partly preserved ages of 18–36.6 Ma in biotites and phlogopites. Ages between 36 and 18 Ma might reflect ongoing top-to-NW thrusting, following Penninic Front activation, in a context of nappe stacking and crustal thickening. NW-directed thrusting on the NW side of Mont Blanc continued after 18 Ma, synchronous with SE-directed thrusting on the SE side of the massif. These divergent movements produced the overall pop-up geometry of the Mont Blanc Massif, which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending SW from the Rhone valley into the Mont Blanc area.

Jurassic to Early Cretaceous subduction-related magmatism in the Coastal Cordillera of northern Chile (18°30'-24°S): geochemistry and petrogenesis

Jurassic to Early Cretaceous magmatism in the Coastal Cordillera of northern Chile is represented by thick sequences of mostly basaltic-andesitic to andesitic lava flows and minor sedimentary rocks. The volcanic sucession was intruded by large plutonio bodies and smaller stocks and dikes. New geochemical data, including major and trace elements for a suite of Middle to Upper Jurassic volcanic and plutonio rocks from six localities in the Coastal Cordillera (18°30-24°S), are presented here. The volcanic rocks are characterized by their petrological and chemical homogeneity; they are highly porphyritic basaltic-andesites and andesites with calc-alkaline to high-K calc-alkaline affinities, higher LILE than HFSE abundances, negative Nb and Ti anomalies, and LREE/HREEfractionation, which are the typical compositional features of subduction-related igneous rocks. No significant differences are observed in rocks from different areas or ages, but the plutonio rocks show subparallel, less and more enriched patterns respectively compared to volcanic rocks. The evolution and differentiation of the parental magmas is mainly due to fractional crystallization dominated by plagioclase, olivine and clinopyroxene. Assimilation of the continental crust was not important, although Th and La contents would indicate increasing sediment contribution or crustal contamination of the magmas with time. The magma source is likely to be a depleted mantle metasomatized by fluids, which originated from dehydration of the subducted oceanic crust. No evidence of slab melting was found in the studied rocks. The extensional tectonic setting that dominated the evolution of the Jurassic-Early Cretaceous arc in northern Chile would have favoured the extrusion of huge amounts of volcanic rocks during a relatively short period of time, avoiding thus a mayor interaction with the continental crust.

Tectonic rotations and transcurrent deformation south of the Abancay deflection in the Andes of southern Peru

We report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from Nazca to Cuzco where the general structures of the Peruvian Andes are strongly offset across the Abancay deflection. Nine new 39Ar/40Ar ages better constrain the timing of volcanism along the western edge of the Western Cordillera at the latitude of Nazca. A mean paleomagnetic result from 22 sites in the lower Miocene volcanics does not show significant rotation (R = −2.3° ± 7.7°) of the western margin of the Central Andean Plateau since the early Miocene. Within the Western Cordillera we sampled three structural blocks bounded to the north by the Abancay fault system. In the westernmost block, a large counterclockwise rotation (R = −65.0° ± 11.1°) is found in Mesozoic limestones and Paleocene-Eocene red beds. Magnitude of rotation decreases toward the east from (R = −35.6° ± 12.8°) in the central block to (R = −4.5° ± 8.4°) south of the town of Cuzco. The anisotropy of magnetic susceptibility (AMS) recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation. We show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations. The present study confirms results from the Peruvian fore arc, showing that rotations are not older than circa 40 Ma and likely not younger than circa 20 Ma. The spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the Abancay deflection concomitant with a broad late Eocene-Oligocene oroclinal deformation in southern Peru.