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Arequipa, Peru

Roperch P.,IRD | Roperch P.,University of Rennes 1 | Carlotto V.,INGEMMET | Ruffet G.,French National Center for Scientific Research | And 2 more authors.
Tectonics | Year: 2011

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. Copyright 2011 by the American Geophysical Union. Source

Reimann C.R.,University of Munster | Bahlburg H.,University of Munster | Kooijman E.,University of Munster | Berndt J.,University of Munster | And 3 more authors.
Gondwana Research | Year: 2010

We present results of a combined study of in situ U-Pb and Lu-Hf analyses on detrital zircons of Ordovician to Devonian sandstone successions of the Eastern Cordillera of Peru and Bolivia, as well as of the Altiplano and Coastal Cordillera of Peru (14°-17°S). We use our data to constrain the provenance and tectonic evolution of this part of the Gondwana margin in the early Paleozoic. The zircon-age composition is very variable in the different locations. Sandstones of the Eastern Cordillera have a dominant input of Brazilian-age zircons (0.7-0.5Ga) with two prominent peaks at around 0.52- 0.58Ga and 0.61-0.67Ga. A prevailing eastern source (Brazilian Shield, Amazonian Craton) is inferred. In contrast, sandstones from the Coastal Cordillera and Altiplano have major inputs of Famatinian (0.5-0.4Ga), Grenvillian (1.2-0.9Ga) and occasionally zircons of 1.85-1.75Ga crystallisation ages. Here, a dominant provenance from the Arequipa Massif is likely. Zircons preserving a juvenile component with εHf(t) values range from about +12 to +6 are limited to crystallisation ages between 1.45 and 1.0 Ga. All younger grains have lower εHf(t) values consistent with recycling of old crust without juvenile additions during the Neoproterozoic. Brazilian-age zircons of an Amazonian craton provenance and Famatinian-age zircons of an Arequipa Massif provenance have similar Hf model ages suggesting their derivation from the same evolved crust. © 2010 International Association for Gondwana Research. Source

Lacroix P.,French National Center for Scientific Research | Berthier E.,LEGOS | Maquerhua E.T.,INGEMMET
Remote Sensing of Environment | Year: 2015

Major earthquakes in mountainous areas often trigger rapid landslides. Some observations also suggest that earthquakes can damage landslide prone areas or cause slow-moving landslides to accelerate, with a risk of evolution to rapid landslides in the following months after the earthquake. Here, we use optical images from the Pléiades satellites to detect slow-moving landslides and quantify the effect of earthquakes on the landslide motion. We process multi-temporal Pléiades images acquired in March, April, and July 2013 over an area of 210km2 in the Colca valley (South Peru), to obtain two Digital Elevation Models (DEM) and three displacement fields of the area. The processed DEMs have an uncertainty of 0.6m (1σ), an order of magnitude better than two global and freely available DEMs (GDEM-v2 and SRTM), whereas the displacement fields have an uncertainty of between 0.11 and 0.18m (1σ) in both horizontal directions. Using these data, we detect 9 slow-moving landslides and compare their velocities during the March-April and April-July periods. We find that landslide velocities are highest during the wet season, which suggests a strong groundwater control, and we also highlight a landslide acceleration caused by a regional Mw 6.0 earthquake. The major parameters controlling the acceleration of the slow-moving landslides are the rock type and the distance to the source, suggesting that friction at the basal interface in the weeks after the earthquake is dependent on the shaking intensity. © 2015 Elsevier Inc.. Source

Roperch P.,IRD | Carlotto V.,INGEMMET | Chauvin A.,Gosciences Rennes
Tectonics | Year: 2010

We report a combined study of anisotropy of low field magnetic susceptibility (AMS) and paleomagnetism from 16 sites in a sedimentary sequence of Eocene-early Oligocene red beds in southern Peru. Incipient tectonic strain is recorded during the early stages of deformation. Nonhorizontal magnetic lineation in geographic coordinate suggests either noncylindrical folding and/or interference of two phases of compressive deformation and tectonic rotation. Applying the classic tilt correction results in significant dispersion in paleomagnetic declinations and apparent clockwise and counterclockwise relative tectonic rotations. A dispersion in the orientation of the magnetic lineation also arises from a simple classic tilt correction inducing apparent local rotation in paleostress determination. The magnetic lineation is a good proxy to detect a complex history of folding when the finite strain is not large enough to reset the magnetic fabric acquired during the early stages of deformation and when detailed geological field mapping is not available or not possible. In the present study, a double correction rotating first the lineation to the horizontal reduces significantly the dispersion of the paleomagnetic data with respect to conventional tilt correction (Fisher parameter k increases from 14 to 35). The interest of this double correction must obviously be evaluated for each study according to the complexity of the folding and the intensity of the deformation. Assuming a mean age of 40 Ma for the sedimentary sequence, no significant rotation (-4.5 8.4) is observed in this area of the Peruvian Andes. © 2010 by the American Geophysical Union. Source

Lacroix P.,French National Center for Scientific Research | Zavala B.,INGEMMET | Berthier E.,French National Center for Scientific Research | Audin L.,French National Center for Scientific Research
Remote Sensing | Year: 2013

Earthquake is one of the dominant triggering factors of landslides. Given the wide areas covered by mega earthquake-triggered landslides, their inventory requires development of automatic or semi-automatic methods applied to satellite imagery. A detection method is here proposed for this purpose, to fit with simple datasets; SPOT5 panchromatic images of 5 m resolution coupled with a freely and globally available DEM. The method takes advantage of multi-temporal images to detect changes based on radiometric variations after precise coregistration/orthorectification. Removal of false alarms is then undertaken using shape, orientation and radiometric properties of connected pixels defining objects. 80% of the landslides and 93% of the landslide area are detected indicating small omission errors but 50% of false alarms remain. They are removed using expert based analysis of the inventory. The method is applied to realize the first comprehensive inventory of landslides triggered by the Pisco earthquake (Peru, 15/08/2007, Mw 8.0) over an area of 27,000 km2. 866 landslides larger than 100 m2 are detected covering a total area of 1.29 km2. The area/number distribution follows a power-law with an exponent of 1.63, showing a very particular regime of triggering in this arid environment compared to other areas in the world. This specific triggering can be explained by the little soil cover in the coastal and forearc regions of Peru. Analysis of this database finally shows a major control of the topography (both orientation and inclination) on the repartition of the Pisco-triggered landslides. © 2013 by the authors. Source

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