Middle Miocene vertebrates from the Amazonian Madre de Dios Subandean Zone, Perú

A new middle Miocene vertebrate fauna from Peruvian Amazonia is described. It yields the marsupials Sipalocyon sp. (Hathliacynidae) and Marmosa (Micoureus) cf. laventica (Didelphidae), as well as an unidentified glyptodontine xenarthran and the rodents Guiomys sp. (Caviidae), “Scleromys” sp., cf. quadrangulatus-schurmanni-colombianus (Dinomyidae), an unidentified acaremyid, and cf. Microsteiromys sp. (Erethizontidae). Apatite Fission Track provides a detrital age (17.1 +- 2.4 Ma) for the locality, slightly older than its inferred biochronological age (Colloncuran-early Laventan South American Land Mammal Ages: ~15.6e13.0 Ma). Put together, both the mammalian assemblage and lithology of the fossil-bearing level point to a mixture of tropical rainforest environment and more open habitats under a monsoonal-like tropical climate. The fully fluvial origin of the concerned sedimentary sequence suggests that the Amazonian Madre de Dios Subandean Zone was not part of the Pebas mega-wetland System by middle Miocene times. This new assemblage seems to reveal a previously undocumented “spatiotemporal transition” between the late early Miocene assemblages from high latitudes (Patagonia and Southern Chile) and the late middle Miocene faunas of low latitudes (Colombia, Perú, Venezuela, and ?Brazil)

Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?

Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He, 40Ar/39Ar hornblende, and U-Pb zircon ages from the granites of Tinos Island in the Aegean Sea, Greece, suggest, together with published ZFT data, that there are three extensional detachments on Tinos. The Tinos granites crosscut the Tinos detachment. Cooling of the granites was controlled by the Livadi detachment, which occurs structurally above the Tinos detachment. Our U-Pb zircon age is 14.6 ± 0.2 Ma and two 40Ar/39Ar hornblende ages are 14.4 ± 0.4 and 13.7 ± 0.4 Ma. ZFT and AFT ages go from 14.4 ± 1.2 to 12.2 ± 1.0 Ma and 12.8 ± 2.4 to 11.9 ± 2.0 Ma. (U-Th)/He ages are from 10.4 ± 0.2 to 9.9 ± 0.2 Ma (zircon) and 11.9 ± 0.5 to 10.0 ± 0.3 Ma (apatite). All ages decrease northeastward in the direction of hanging wall transport on the Livadi detachment and age-distance relationships yield a slip rate of 2.6 (+3.3 / −1.0) km Ma−1. This rate is smaller than a published slip rate of 6.5 km Ma−1 for the Vari detachment, which is another detachment structurally above the Tinos detachment. Because of the different rates and because published ZFT ages from the footwall of the Vari detachment are ∼10 Ma, we propose that the Vari detachment has to be distinguished from the older Livadi detachment. We discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the Vari detachment cut down into the footwall of the Livadi detachment successively exhuming deeper structural units. The thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation

First in situ U-Pb dating of supergene copper mineralization: case study from Atacama desert, Chile

International audienceDirect dating of copper mineralization remains challenging, yet an important objective for both metallogenic studies and prospecting purposes. Here, we present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from the Chuquicamata copper mine (Chile).The Mina Sur exotic copper deposit is located in northern Chile, within the Chuquicamata mining district, about 6 km south of the world class Chuquicamata porphyry copper deposit. It was discovered in 1957 from drilling in sub-alluvial channels and copper extraction began in 1969. The copper mineralization consists mainly of chrysocolla, Mn-rich chrysocolla (also known as copper-pitch or black chrysocolla), pseudomalachite bedding and atacamite emplaced mainly as cement in gravels but also filling fractures in the bedrock. Geomorphological, mineralogical and structural studies (Mortimer et al., 1977; Munchmeyer, 1996) have shown that the Mina Sur deposit is derived from the lateral migration of copper-rich solutions from the Chuquicamata porphyry deposit. The latter was formed by the 33-31 Ma old hydrothermal alteration (Ar/Ar dating of K-feldspar and sericite; Reynolds et al., 1998) of the 35-33 Ma old porphyry (U/Pb dating on zircon; Ballard et al., 2001). However, the age of the Mina Sur exotic deposit is unknown.A suite of Mn-rich chrysocolla clasts surrounded by pseudomalachite layers have been characterized and dated by in-situ U-Th-Pb LA-ICP-MS analyses. The data obtained on pseudomalachite define a concordant date of 19.45 0.21 Ma. In the Mn-rich chrysocolla clasts, the data yield concordant to sub-concordant dates between 12 and 7-6 Ma. These dates are therefore not consistent with the date obtained on the pseudomalachite layers that coat them. This could reflect either Pb loss linked to late fluids that affected specifically these clasts or an analytical bias due to the fact that there is no chrysocolla standard available for in-situ U-Pb dating. Assuming a recent lead loss, the chrysocolla data yield an intercept date of 23 ± 15 Ma, which is compatible with the younger U-Pb date obtained on the pseudomalachite.This preliminary study demonstrates, for the first time, that supergene copper mineralization can be dated by the U-Th-Pb method. Furthermore, the age obtained on pseudomalachite indicate that Mina Sur deposition took place at ca. 20 Ma, about 10 Ma after the unroofing and hydrothermal alteration at Chuquicamata, a result that is consistent with the supergene ages already known supergene in the Atacama Desert

Integrated study of supergene copper deposits from Atacama Desert, Northern Chile: coupled petro-geochemical approach and U-Pb LA-ICP-MS in situ dating

International audienceSupergene copper mineralization (SCM) are nowadays the economic viability of many porphyry copper deposits worldwide. These mineralization are derived from supergene processes, defined by Ransome (1912) as sulfide oxidation and leaching of ore deposits in the weathering environment, and any attendant secondary sulfide enrichment. For supergene copper mineralization to form, favorable tectonics, climate and geomorphologic conditions are required. Tectonics control the uplift needed to induce groundwater lowering and leaching of sulphides from a porphyry copper deposit. Climate controls copper leaching in the supergene environment and groundwater circulation towards the locus where supergene copper-bearing minerals precipitate. Two types of SCM have been recognized: 1) in-situ SCM, which are products of descending aqueous solutions and 2) exotic SCM, which are the products of lateral migration of supergene copper solutions from a parental porphyry copper deposit (Sillitoe, 2005).In the Atacama Desert, such deposits seem to take place during specific Tertiary climatic periods and relief formation. But many uncertainties remain regarding the genesis and the exact timing for their formation. In this study, a coupled approach combining a petro-geochemical study and LA-ICP-MS U-Pb dating were applied to four mining copper deposits (e.g. Mina Sur, Damiana, El Cobre, Zaldivar) from hyperarid Atacama Desert of Northern Chile. Textural features are the same in all the deposits with chrysocolla as the abundant mineral, followed by black chrysocolla, pseudomalachite and minor atacamite and copper wad. Their geochemical compositions (i.e. major, traces and rare Earth elements) also show homogeneous results suggesting similar process in their genesis. U-Pb dating were performed on black chrysocolla, chrysocolla and pseudomalachite from all the deposits. Apart from Mina Sur deposit, all the mines mentioned above showed high common lead content. To try to extract in these deposit an U-Pb age, complementary analyses to quantify accurately common lead concentration are ongoing, using MC-ICPMS. At Mina Sur, U-Pb dating performed on pseudomalachite bands yields a crystallisation age of 18.4 ± 1.0 Ma. For the black chrysocolla clasts, the 206Pb/238U apparent ages are ranging from 19.7 ± 5.0 Ma down to 6.1 ± 0.3 Ma, a spreading that we interpret as the result of uranium and lead mobility linked to fluid circulation following crystallization. Isotopic analyses, i.e. Cu and O isotopes, are in progress to better constrain the source and nature of these fluids. This study demonstrates, for the first time, that supergene copper mineralization presents a chronological potential and can be dated, at least in some case, by the U-Th-Pb method. Furthermore, the age obtained on pseudomalachite indicates that Mina Sur deposition took place as early as 19 Ma, a result that is in agreement with geological constraints in the mining district and the supergene ages already known in the Atacama Desert. These promising results represent a new tool to understand the physico-chemical, climatic and geological conditions that prevailed during the formation of supergene copper deposits and a proxy for their prospection around the world and maybe date climatic variation

Timing and distribution of exhumation in the Ebro basin reveal a plate-scale 10 Ma geodynamic event

International audienceThe interplays between deep geodynamic processes and Earth-surface erosion processes control the elevation and distribution of land masses, which in turn control environmental changes. These interactions can be best evaluated in post-orogenic landscapes defined by decaying crustal deformation. This is the case of the Iberia peninsula where a growing body of geological evidence suggests the existence of a post-orogenic erosion around 10 Ma, well documented across the Pyrenees. Different mechanisms have been proposed, including lithospheric and sublithospheric processes, and Earth-surface processes triggered by base-level change and/or climate forcing on erosion. Here, we focus on the Ebro basin, which was a closed basin that opened towards the Mediterranean in the late Miocene. The distribution and exact timing of erosion in the Ebro basin are, however, only partially known, and do not yet allow resolving the post-orogenic mechanisms involved. We present new (U-Th-Sm)/He dates on apatite obtained in sandstones distributed from the edges of the Ebro basin to its center. The results of the thermal modelling confirm that the exhumation of the Ebro Basin took place between 12 and 6 Ma, or around 10 Ma when accounting for model uncertainties. Reconstruction of the sediment infill reveals that the late Miocene post-orogenic deposits were the thickest, between 1 and 1.6 km, near the topographic front of adjacent mountain ranges and were the thinnest, approximately 300 m, in the center. The exhumation in the Ebro basin occurred after shortening has stopped, and before the Messinian Salinity Crisis. It is consistent with the widespread 10 Ma-exhumation event documented throughout Iberia, from the Aquitaine basin to the Betic Cordillera, and provides evidence of large-scale lithospheric and sublithospheric processes that coincide with the cessation of slab retreat in the West Mediterranean. We propose that a plate-scale uplift driven by deep-seated processes is the main cause of the Ebro basin incision. The response of the landscape to this geodynamically controlled erosion event led to a complex tectonic reorganization, e.g. along the strike of the Pyrenees, environmental changes in the Mediterranean, leading to the Messinian Salinity Crisis, and faunal turnover over Iberia

First in situ U-Pb dating of supergene copper mineralization: case study from Atacama Desert, Chile

International audienceDirect dating of copper mineralization remains challenging, yet an important objective for both metallogenic studies and prospectingpurposes. Here we present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from theChuquicamata copper mine (Chile). The Mina Sur exotic copper deposit is located in northern Chile, within the Chuquicamatamining district about 6 km south of the world class Chuquicamata porphyry copper deposit. It was discovered in 1957, fromdrilling in sub-alluvial channels and copper extraction began in 1969 (Mortimer et al., 1977). The copper mineralization consistsmainly of chrysocolla, Mn-rich chrysocolla (also known as copper-pitch or black chrysocolla), and pseudomalachite bedding andatacamite emplaced mainly as cement in gravels but also filling fractures in the bedrock. Geomorphological, mineralogical andstructural studies, have shown that the Mina Sur deposit is derived from the lateral migration of copper-rich solutions from theChuquicamata porphyry deposit. The latter was formed by the 33-31 Ma old hydrothermal alteration (Ar-Ar dating of K-feldsparand sericite) of the 35-33 Ma old porphyry (U-Pb dating). However, the age of the Mina Sur exotic deposit is unknown. A suite ofMn-rich chrysocolla clasts surrounded by pseudomalachite layers have been characterized and dated by in-situ U-Th-Pb LAICP-MS analyses. The data obtained on pseudomalachite define a concordant date of 19.45 ± 0.21 Ma. In the Mn-rich chrysocollaclasts, the data yield concordant to sub-concordant dates between 12 and 7-6 Ma. These dates are therefore not consistent withthe date obtained on the pseudomalachite layers that coat them. This could reflect either Pb loss linked to late fluids that affectedspecifically these clasts or to an analytical bias due to the fact that there is no chrysocolla standard available for in-situ U-Pbdating. Assuming a recent lead loss, the chrysocolla data yield an upper intercept date of 23 ± 15 Ma, which is compatible withthe younger U-Pb date obtained on the pseudomalachite. This preliminary study demonstrates, for the first time, that supergenecopper mineralization can be dated by the U-Th-Pb method. Furthermore, the age obtained on pseudomalachite indicate that MinaSur deposition took place at ca. 20 Ma, about 10 Ma after the unroofing and hydrothermal alteration at Chuquicamata, a result thatis consistent with the supergene ages already known in the Atacama Desert

Tectono‐Stratigraphic and Thermal Evolution of the Western Betic Flysch: Implications for the Geodynamics of South Iberian Margin and Alboran Domain

International audienceThe Betic-Rif orogen is a key region to understand the evolution of the plate boundary between Africa and Iberia/Europe. This study focuses on the Flysch Complex, which is considered the sedimentary cover of a domain originally positioned between the Iberian and Alboran margins. Based on stratigraphic and depositional evolution constraints, evidence for salt tectonics, combined with new apatite fission-tracks (AFT) and (U-Th-Sm)/He ages from the Flysch Complex and the Subbetic Zone, we propose a geodynamic interpretation for the formation of the Betic Cordillera, accounting for moderate N-directed transport of the Flysch Complex and synchronous exhumation between External and Internal Zones of the Betic. Early contraction between Africa and Iberia/Europe is reflected in the Cretaceous Flysch basin by a prolonged period of residence in the partial annealing zone for AFT and onset of foreland subsidence at 50 Ma. This stage lasted until the Early to Middle Miocene (20-15 Ma), marked by the rapid succession, in less than 5 Ma, of the deposition of Cenozoic flysch and their rapid exhumation. This event is interpreted to reflect the W-directed retreating mantle delamination between Africa and Iberia margins at the origin of the collapse of the proto-Betic orogenic domain and formation of the Alboran domain

Controls on timing of exhumation and deformation in the northern Peruvian eastern Andean wedge as inferred from low-temperature thermochronology and balanced cross section

In northern Peru, a 500km long regional balanced section has been constructed across the eastern Andean wedge, using fieldwork, industrial seismic sections, and wells. The structure is characterized by a thin-skinned thrust system involving the Eastern Cordillera (EC), the sub-Andean zone (SAZ), and the Maranon foredeep. In the SAZ and the easternmost foredeep the development of the thrust system has been driven by the combination of two structural events. Permian thrust faults had been reactivated to form a basement duplex underlying the SAZ and the foredeep. At the same time a Triassic-Jurassic extensional basin has been transported as a crustal ramp anticline on to the duplex roof fault, giving rise to the EC. The impingement of the EC was responsible for the deformation of the SAZ and the propagation of the thrust wedge. The minimum shortening calculated is 142km, representing a shortening strain of similar to-28%. A sequential restoration calibrated by (U-Th)/He and Fission Track dating on apatites and vitrinite reflectance values shows that shortening rates vary from 7.1mmyr(-1) between 17 and 8Ma to 3.6mmyr(-1) between 8Ma and today and suggests that the thrust wedge commenced propagation between 30 and 24 Ma. When compared with other Andean thrust wedges, we suggest that the timing of the thrust wedge propagation is not a simple function of the distance to the hinge of the Bolivian orocline and the propagation is not controlled by the precipitation regime. We rather suggest that reactivated basement faults favored thrust wedge propagation