Vertical-axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata within the Bolivian Andes

Thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization (ChRM) directions from 256 sites at 22 localities in Mesozoic and Cenozoic sedimentary strata of the Bolivian Altiplano and Eastern Cordillera. An inclination-only fold test of site-mean ChRM directions from Cenozoic units (principally the Santa Lucía Formation) indicates optimum unfolding at 97.1% unfolding, consistent with a primary origin for the ChRM. For Mesozoic strata, optimum unfolding occurred at 89.2%, perhaps indicating secondary remagnetization at some locations. For Cenozoic units, comparison of locality-mean directions with expected paleomagnetic directions indicates vertical-axis rotations from 33° counterclockwise to 24° clockwise. Euler pole analysis of along-strike variation in crustal shortening within the Subandean and Interandean zones indicates 18° clockwise rotation south of the axis of curvature of the Bolivian Andes and 6° counterclockwise rotation northwest of the axis during the past 10 m.y. Along-strike variation of shortening within the Eastern Cordillera indicates 8° clockwise rotation south of the axis and 8° counterclockwise rotation northwest of the axis from 35 to 10 Ma. These vertical-axis rotations produced by along-strike variations in crustal shortening during development of the Bolivian fold-thrust belt agree well with observed rotations determined from paleomagnetism of Cenozoic rocks in the Eastern Cordillera and in the Subandean and Interandean zones. However, local rotations are required to account for complex rotations in the Cochabamba Basin and within the Altiplano. The curvature of the Bolivian Andes has been progressively enhanced during Cenozoic fold-thrust belt deformation

The Late Paleozoic - Early Mesozoic Chocolate Formation of southern Peru: new data and interpretations

The Chocolate Formation is exposed in southern Peru between the cities of Nazca and Tacna, mostly in the Pacifie slopes of the Cordillera Occidental and along the coast. This lithostratigraphic unit was initially described near Arequipa by Jenk s (1948; "Volcànicos Chocolate"), who assigned a Jurassic age to it. It consists of a succession of volcanic rock s, sandstones, and con glomerates. Near the top of the unit, a Sinemurian ammonite was found in a fossiliferous limestone intercalation. In Tacna a similar unit was mapped as "Junera ta Formation" by Wilson & Gard a (1962) and later equated with the Chocolate Formation (Monge & Cerv antes, 2000). Recent studies have suggested that the base of this unit might be as old as Late Carboniferous (Pino et a l., 2004; Sempere et al., 2004). Because the Chocolate volcanism is likely to have been active in the middle Permian, it partly represents a coastal equivalent of the Mitu volcanism known in the Eas tern Cordillera

Oligocene-Neogene tectonics and sedimentation in the forearc of southern Peru, Tacna area (17.5°-18.5°5)

Fault systems observed in the southernmost Peruvian forearc (Tacna area) provide information on the structure of the inner Bolivian Orocline (Arica elbow). Here we present the characteristics of the main fault systems observed in the Tacna area, as well as evidences of structural control on the deposition of Upper Cenozoic units and regional geoforms. We propose a synthetic chronology of deformation

Extension of the Late Triassic salt into western Peru: lmplications for Andean tectonics and mineral exploration

A better understanding of large- and small-scale Andean geological features and timeline can obviously make a difference in exploration. Knowledge of the Andean evolution continues however to be plagued by geological myths and obsolete concepts (such as "compressional tectonic phases• and "ocean-continent collision"), while major key features of this evolution remain underrated. One of these key features is the "Mitu interval", during which the Peruvian margin was submitted to intense extension starting -240 Ma. These extensional conditions caused intense back-arc rifting along the margin, and a related thick accumulation of sediments (and subordinate volcanic rocks), ranging from terrestrial clastics (the Mitu Group, 240~2 1 5 Ma) to shallow-marine carbonates (the Pucara Group, -21 O-s170 Ma). This rift basin was also the locus of thick salt accumulation approximately during the -21 ~21 0 Ma interval (the Pareni Formation, or Pareni Salt), making that western Peru was occupied by a giant salt basin during part of the Late Triassic (Sempere & Cotrina, 2018; Berrospi et al., 2018). Previous attributions of this major salt unit to the Permian are wrong (Sempere & Cotrina, 2018). Most of the western rim of Amazonian Peru is indeed characterized by abundant salt and other evaporites, as documented both by numerous outcropping diapirs and in seismic information (Sempere & Cotrina, 2018). In the high Andean plateau, where no Permian strata occur, the San Bias salt dome pierces the Pucara limestones southwest of Lake Junin, and gypsum occurs between the Mitu and Pucara groups, being mined at a number of places, such as -16 km WNW of the city of Tarma. Elsewhere in central Peru, effects of salt tectonics can be detected at least along the Cordillera Oriental (Berrospi et al., 2018). Sempere & Cotrina (2018) showed that this salt and related other evaporites must have originally represented a considerable volume (>1 00,000 km3 as a minimum). Halokinesis vigorously developed during the Jurassic and continued through later times, generating a variety of salt-tectonic deformations (which were generally misinterpreted as resulting from thrust tectonics due to influence of the mainstream compressional paradigm). In this preliminary paper, we address the issue of the western extension of this giant salt basin, and briefly review the implications of the existence of voluminous salt and other evaporites for Andean tectonics and mineral exploration

Evolución tectónica y metalogénesis del Perú

Para hacer que la exploración minera sea más eficiente en el Perú, es importante interconectar su historia geológica con los conocimientos de la metalogénesis. En este trabajo emprendemos una síntesis preliminar de la evolución geológica y tectónica que se puede reconstruir actualmente en base a las investigaciones en curso, junto con el análisis de datos metalogenéticos disponibles a la fecha. Se hace énfasis en el magmatismo dado que la gran mayoría de los depósitos minerales se relacionan con fenómenos magmáticos ocurridos en un arco de subducción

Evolución tectónica y metalogénesis del Perú

[EN] Improving the chances of success of mineral exploration requires to interconnect metallogenetic analyses with the geological evolution of western Peru that can be reconstructed from current knowledge and ongoing investigations. This communication presents a preliminary stage in this research.[ES] Para hacer que la exploración minera sea más eficiente en el Perú, es importante interconectar su historia geológica con los conocimientos de la metalogénesis. En este trabajo emprendemos una síntesis preliminar de la evolución geológica y tectónica que se puede reconstruir actualmente en base a las investigaciones en curso, junto con el análisis de datos metalogenéticos disponibles a la fecha. Se hace énfasis en el magmatismo dado que la gran mayoría de los depósitos minerales se relacionan con fenómenos magmáticos ocurridos en un arco de subducción

Unraveling the evolution of southernmost Peru between 100 and 50 Ma through U-Pb geochronology of the Toquepala Group: Implications for exploration of large porphyry copper deposits

Integration of the zircon U-Pb ages available on the Toquepala Group, La Caldera batholith (s.l.), and porphyry Cu systems, in combination with geochemical data, unravels the evolution of the magmatic arc of southernmost Peru during the Late Cretaceous and Early Paleogene. Arc magmatic production increased starting ~90 Ma, and culminated between ~74 and ~62 Ma through a flare-up characterized by large plutonic and volcanic (especially ignimbritic) volumes. This long process resulted in the thickening of the arc crust, to a state of overthickening that triggered its extensional collapse, starting diachronously 61–59 Ma, during which giant porphyry Cu systems were emplaced between ~60 and ~53 Ma. Our empirical reconstruction of this protracted evolution provides simple guides for exploration of giant porphyry Cu deposits in southern Peru, as well as a magmatotectonic theoretical framework

Algunos aportes e intentos de correlación sobre el Jurásico y Cretácico inferior en el límite oriental de la cuenca de Arequipa

Los afloramientos del Jurásico- Cretácico inferior en el borde oriental de la cuenca de Arequipa conforman 4 sectores, que se agrupan en las áreas de Mañazo-Lagunillas y de Río Blanco-Loripongo-Yunga. En ambas zonas no aflora el contacto inferior con otras unidades más antiguas

Progresos en el estudio de la Formación Ayabacas

La Formación Ayabacas (~Turoniano) es una unidad resedimentada que se observa sobre un área superior a 50000 km2 en el Altiplano y la Cordillera Oriental del sur del Perú (Sempere et al., 2000). Su génesis fue explicada de maneras muy diferentes: fallamiento de bloques y erosión intensa (Heim, 1947), tectónica con pliegues y cabalgamientos (Newell, 1949; Chanove et al., 1969), deformación disarmónica y/o polifásica, fracturación causada por karstificación y/o diapirismo de yesos, intrusiones hipovolcánicas (Audebaud, 1971), caos producido por deslizamientos subaéreos (De Jong, 1974) o submarinos (Audebaud, 1967; Sempere et al., 2000). El estudio en curso soporta esta última interpretación, describiendo la Fm Ayabacas como una megabrecha (u olistostromo), es decir el resultado de deslizamientos submarinos de gran amplitud (Spence and Tucker, 1997). Aunque las interpretaciones son diferentes, la mayoría de los autores hacen descripciones similares, al menos en las zonas estudiadas por ellos: un caos de bloques grandes (50-500 m) que aparentemente “flotan” dentro de una matriz más blanda. Estos bloques, a menudo plegados y en cada posición imaginable, son principalmente de calizas cretáceas (Fm Arcurquina), pero también de otras formaciones anteriores (Fm Huancané, Fm Muni, Fm Sipin, Grupo Mitu, Paleozoico). La matriz es una brecha con clastos grandes y pequeños de calizas y areniscas fracturadas dentro de pelitas multicolores (generalmente rojas) y areniscas. Sin embargo, un estudio más exhaustivo muestra que la Fm Ayabacas no es uniforme en cuanto a facies de deslizamiento

Spatial and temporal evolution of Liassic to Paleocene arc activity in southern Peru unraveled by zircon U-Pb and Hf in-situ data on plutonic rocks

International audienceCordilleran-type batholiths are built by prolonged arc activity along active continental margins and provide detailed magmatic records of the subduction system evolution. They complement the stratigraphic record from the associated forearcs and backarcs. We performed in-situ U-Pb geochronology and Hf isotope measurements on zircon grains from a large set of plutonic rocks from the Coastal Batholith in southern Peru. This batholith emplaced into the Precambrian basement and the Mesozoic sedimentary cover. We identify two major periods of voluminous arc activity, during the Jurassic (200-175 Ma) and the Late Cretaceous-Paleocene (90-60 Ma). Jurassic arc magmatism mainly resulted in the emplacement of a dominantly mafic suite with εHf values ranging from − 9.5 to + 0.1. Published ages south of the Arequipa area suggest that the arc migrated southwestward out of the study area during the Middle Jurassic. After a magmatic gap of 85 Ma, arc activity abruptly resumed 90 Ma ago in Arequipa. Intrusive bodies emplaced into both basement and older Jurassic intrusions and strata. This activity culminated between 70 and 60 Ma with the emplacement of very large volumes of dominantly quartz-dioritic magmas. This last episode may be considered as a flare-up event, characterized by intense magmatic transfers into the crust and rapid relief creation. The Late Cretaceous-Paleocene initial εHf are shifted toward positive values (up to + 3.3 and + 2.6) compared to the Jurassic ones, indicating either a larger input of juvenile magmas, a lesser interaction with the ancient crust, or an increase of re-melting of young mantle-derived mafic lower crust. These magmatic fluxes with juvenile component are coeval with the onset of the crustal thickening at 90 Ma and represent a significant contribution to the formation of the continental crust in this area