Dating brittle tectonic movements with cleft monazite: Fluid-rock interaction and formation of REE minerals

[1] Two millimeter-sized hydrothermal monazites from an open fissure (cleft) that developed late during a dextral transpressional deformation event in the Aar Massif, Switzerland, have been investigated using electron microprobe and ion probe. The monazites are characterized by high Th/U ratios typical of other hydrothermal monazites. Deformation events in the area have been subdivided into three phases: (D1) main thrusting including formation of a new schistosity, (D2) dextral transpression, and (D3) local crenulation including development of a new schistosity. The two younger deformational structures are related to a subvertically oriented intermediate stress axis, which is characteristic for strike slip deformation. The inferred stress environment is consistent with observed kinematics and the opening of such clefts. Therefore, the investigated monazite-bearing cleft formed at the end of D2 and/or D3, and during dextral movements along NNW dipping planes. Interaction of cleft-filling hydrothermal fluid with wall rock results in rare earth element (REE) mineral formation and alteration of the wall rock. The main newly formed REE minerals are Y-Si, Y-Nb-Ti minerals, and monazite. Despite these mineralogical changes, the bulk chemistry of the system remains constant and thus these mineralogical changes require redistribution of elements via a fluid over short distances (centimeter). Low-grade alteration enables local redistribution of REE, related to the stability of the accessory phases. This allows high precision isotope dating of cleft monazite. 232Th/208Pb ages are not affected by excess Pb and yield growth domain ages between 8.03 ± 0.22 and 6.25 ± 0.60 Ma. Monazite crystallization in brittle structures is coeval or younger than 8 Ma zircon fission track data and hence occurred below 280°C.

The origin of oriented lakes: evidence from the Bolivian Amazon

The presence of hundreds of rectangular and oriented lakes is one of the most striking characteristics of the Llanos de Moxos (LM) landscape in the Bolivian Amazon. Oriented lakes also occur in the Arctic coastal plains of Russia, Alaska and Canada and along the Atlantic Coastal Plain from northeast Florida to southeast New Jersey and along the coast of northeast Brazil. Many different mechanisms have been proposed for their formation. In the LM, Plafker's (1964) tectonic model, in which subsidence results from the propagation of bedrock faults through the foreland sediments, is the most accepted. However, this model has not been verified. Here, we present new results from stratigraphic transects across the borders of three rectangular and oriented lakes in the LM. A paleosol buried under mid-Holocene sediments is used as a stratigraphic marker to assess the vertical displacement of sediments on both sides of the alleged faults. Our results show that there is no vertical displacement and, therefore, that Plafker's model can be ruled out. We suggest that, among all the proposed mechanisms behind lake formation, the combined action of wind and waves is the most likely. The evidence from the LM provides new hints for the formation of oriented lakes worldwide.

Landscape Evolution of the Dry Valleys, Transantarctic Mountains: Tectonic Implications

There are different views about the amount and timing of surface uplift in the Transantarctic Mountains and the geophysical mechanisms involved. Our new interpretation of the landscape evolution and tectonic history of the Dry Valleys area of the Transantarctic Mountains is based on geomorphic mapping of an area of 10,000 km(2). The landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution. Following a lowering of base level about 55 m.y. ago, there was a phase of rapid denudation associated with planation and escarpment retreat, probably under semiarid conditions. Eventually, downcutting by rivers, aided in places by glaciers, graded valleys to near present sea level. The main valleys were flooded by the sea in the Miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast. There has been remarkably little landform change under the stable, cold, polar conditions of the last 15 m.y. It is difficult to explain the Sirius Group deposits, which occur at high elevations in the area, if they are Pliocene in age. Overall, denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland, which is in good agreement with the results of existing apatite fission track analyses. It is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy. Most crustal uplift (2-4 km) is inferred to have occurred in the early Cenozoic with 400 m of subsidence in the Miocene followed by 300 m of uplift in the Pliocene.

Geomorphology and evolution of the late Pleistocene to Holocene fluvial system in the south-eastern Llanos de Moxos, Bolivian Amazon

In the Bolivian Amazon several paleochannel generations are preserved. Their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the Andean foreland. Therefore, in this study we mapped geomorphological characteristics of paleochannels, and applied radiocarbon and optically stimulated luminescence dating. Seven paleochannel generations are identified. Significant changes in sinuosity, channel widths and river pattern are observed for the successive paleochannel generations. Our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late Pleistocene. Changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms: (i) spatial changes and re-organizations of the drainage network in the upper catchment, and/or (ii) climate changes with their associated local to catchment-scale modifications in vegetation cover, and changes in discharge, inundation frequencies and magnitudes, which have likely affected the evolution of the fluvial system in the Llanos de Moxos. In summary, our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late Pleistocene to recent fluvial system in a large portion of Amazonia.

The geomorphology and morphometry of the banded terrain in Hellas basin, Mars

Hellas basin is a large impact basin situated in the southern highlands of Mars. The north-western part of the basin has the lowest elevation (-7.5 km) on the planet and contains a possibly unique terrain type, which we informally call “banded terrain”. The banded terrain is made up of smooth-looking banded deposits that display signs of viscous behavior and a paucity of superimposed impact craters. In this study, we use newly acquired high spatial resolution images from the High Resolution Imaging Science Experiment (HiRISE) in addition to existing datasets to characterize the geomorphology, the morphometry and the architecture of the banded terrain. The banded terrain is generally confined to the NW edge of the Alpheus Colles plateau. The individual bands are ~3–15 km-long, ~0.3 km-wide and are separated by narrow inter-band depressions, which are ~65 m-wide and ~10 m-deep. The bands display several morphologies that vary from linear to concentric forms. Morphometric analysis reveals that the slopes along a given linear or lobate band ranges from 0.5° to 15° (average~6°), whereas the concentric bands are located on flatter terrain (average slope~2–3°). Crater-size frequency analysis yields an Amazonian-Hesperian boundary crater retention age for the terrain (~3 Gyr), which together, with the presence of very few degraded craters, either implies a recent emplacement, resurfacing, or intense erosion. The apparent sensitivity to local topography and preference for concentrating in localized depressions is compatible with deformation as a viscous fluid. In addition, the bands display clear signs of degradation and slumping at their margins along with a suite of other features that include fractured mounds, polygonal cracks at variable size-scales, and knobby/hummocky textures. Together, these features suggest an ice-rich composition for at least the upper layers of the terrain, which is currently being heavily modified through loss of ice and intense weathering, possibly by wind.

Structural, metamorphic and geochronological relations between the Zanskar Shear Zone and the Miyar Shear Zone (NW Indian Himalaya): Evidence for two distinct tectonic structures and implications for the evolution of the High Himalayan Crystalline of Zanskar

The geologic structures and metamorphic zonation of the northwestern Indian Himalaya contrast significantly with those in the central and eastern parts of the range, where the high-grade metamorphic rocks of the High Himalayan Crystalline (HHC) thrust southward over the weakly metamorphosed sediments of the Lesser Himalaya along the Main Central Thrust (MCT). Indeed, the hanging wall of the MCT in the NW Himalaya mainly consists of the greenschist facies metasediments of the Chamba zone, whereas HHC high-grade rocks are exposed more internally in the range as a large-scale dome called the Gianbul dome. This Gianbul dome is bounded by two oppositely directed shear zones, the NE-dipping Zanskar Shear Zone (ZSZ) on the northern flank and the SW-dipping Miyar Shear Zone (MSZ) on the southern limb. Current models for the emplacement of the HHC in NW India as a dome structure differ mainly in terms of the roles played by both the ZSZ and the MSZ during the tectonothermal evolution of the HHC. In both the channel flow model and wedge extrusion model, the ZSZ acts as a backstop normal fault along which the high-grade metamorphic rocks of the HHC of Zanskar are exhumed. In contrast, the recently proposed tectonic wedging model argues that the ZSZ and the MSZ correspond to one single detachment system that operates as a subhorizontal backthrust off of the MCT. Thus, the kinematic evolution of the two shear zones, the ZSZ and the MSZ, and their structural, metamorphic and chronological relations appear to be diagnostic features for discriminating the different models. In this paper, structural, metamorphic and geochronological data demonstrate that the MSZ and the ZSZ experienced two distinct kinematic evolutions. As such, the data presented in this paper rule out the hypothesis that the MSZ and the ZSZ constitute one single detachment system, as postulated by the tectonic wedging model. Structural, metamorphic and geochronological data are used to present an alternative tectonic model for the large-scale doming in the NW Indian Himalaya involving early NE-directed tectonics, weakness in the upper crust, reduced erosion at the orogenic front and rapid exhumation along both the ZSZ and the MSZ.

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The postnappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent precollisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.

Geomorphology of the Imhotep region on comet 67P/Churyumov-Gerasimenko from OSIRIS observations

Context. Since August 2014, the OSIRIS Narrow Angle Camera (NAC) onboard the Rosetta spacecraft has acquired high spatial resolution images of the nucleus of comet 67P/Churyumov-Gerasimenko, down to the decimeter scale. This paper focuses on the Imhotep region, located on the largest lobe of the nucleus, near the equator. Aims. We map, inventory, and describe the geomorphology of the Imhotep region. We propose and discuss some processes to explain the formation and ongoing evolution of this region. Methods. We used OSIRIS NAC images, gravitational heights and slopes, and digital terrain models to map and measure the morphologies of Imhotep. Results. The Imhotep region presents a wide variety of terrains and morphologies: smooth and rocky terrains, bright areas, linear features, roundish features, and boulders. Gravity processes such as mass wasting and collapse play a significant role in the geomorphological evolution of this region. Cometary processes initiate erosion and are responsible for the formation of degassing conduits that are revealed by elevated roundish features on the surface. We also propose a scenario for the formation and evolution of the Imhotep region; this implies the presence of large primordial voids inside the nucleus, resulting from its formation process.

Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations

Aims. We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths). Methods. We computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface (Imhotep, Ash, Seth, Hathor, and Agilkia), using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, and Philae's footprint) and mechanical considerations. Results. The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0-20 degrees) are covered by a fine material and contain a few large (>10 m) and isolated boulders; ii) intermediate-slope terrains (20-45 degrees) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from <1m to 10m for the majority of them; and iii) high-slope terrains (45-90 degrees) are cliffs that expose a consolidated material and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3-15 Pa (upper limit of 150 Pa), 4-30 Pa for the shear strength of fine surface materials and boulders, and 30-150 Pa for the compressive strength of overhangs (upper limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.

Geomorphology and spectrophotometry of Philae's landing site on comet 67P/Churyumov-Gerasimenko

Context. On 12 November 2014 the European mission Rosetta succeeded in delivering a lander, named Philae, on the surface of one of the smallest, low-gravity and most primitive bodies of the solar system, the comet 67P/Churyumov-Gerasimenko (67P). Aims. The aim of this paper is to provide a comprehensive geomorphological and spectrophotometric analysis of Philae's landing site (Agilkia) to give an essential framework for the interpretation of its in situ measurements. Methods. OSIRIS images, coupled with gravitational slopes derived from the 3D shape model based on stereo-photogrammetry were used to interpret the geomorphology of the site. We adopted the Hapke model, using previously derived parameters, to photometrically correct the images in orange filter (649.2 nm). The best approximation to the Hapke model, given by the Akimov parameter-less function, was used to correct the reflectance for the effects of viewing and illumination conditions in the other filters. Spectral analyses on coregistered color cubes were used to retrieve spectrophotometric properties. Results. The landing site shows an average normal albedo of 6.7% in the orange filter with variations of similar to 15% and a global featureless spectrum with an average red spectral slope of 15.2%/100 nm between 480.7 nm (blue filter) and 882.1 nm (near-IR filter). The spatial analysis shows a well-established correlation between the geomorphological units and the photometric characteristics of the surface. In particular, smooth deposits have the highest reflectance a bluer spectrum than the outcropping material across the area. Conclusions. The featureless spectrum and the redness of the material are compatible with the results by other instruments that have suggested an organic composition. The observed small spectral variegation could be due to grain size effects. However, the combination of photometric and spectral variegation suggests that a compositional differentiation is more likely. This might be tentatively interpreted as the effect of the efficient dust-transport processes acting on 67P. High-activity regions might be the original sources for smooth fine-grained materials that then covered Agilkia as a consequence of airfall of residual material. More observations performed by OSIRIS as the comet approaches the Sun would help interpreting the processes that work at shaping the landing site and the overall nucleus.

Geological and hydrological histories of the Argyre province, Mars

The geologic history of the multi-ringed Argyre impact basin and surroundings has been reconstructed on the basis of geologic mapping and relative-age dating of rock materials and structures. The impact formed a primary basin, rim materials, and a complex basement structural fabric including faults and valleys that are radial and concentric about the primary basin, as well as structurally-controlled local basins. Since its formation, the basin has been a regional catchment for volatiles and sedimentary materials as well as a dominant influence on the flow of surface ice, debris flows, and groundwater through and over its basement structures. The basin is interpreted to have been occupied by lakes, including a possible Mediterranean-sized sea that formed in the aftermath of the Argyre impact event The hypothesized lakes froze and diminished through time, though liquid water may have remained beneath the ice cover and sedimentation may have continued for some time. At its deepest, the main Argyre lake may have taken more than a hundred thousand years to freeze to the bottom even absent any heat source besides the Sun, but with impact-induced hydrothermal heat, geothermal heat flow due to long-lived radioactivities in early martian history, and concentration of solutes in sub-ice brine, liquid water may have persisted beneath thick ice for many millions of years. Existence of an ice-covered sea perhaps was long enough for life to originate and evolve with gradually colder and more hypersaline conditions. The Argyre rock materials, diverse in origin and emplacement mechanisms, have been modified by impact, magmatic, eolian, fluvial, lacustrine, glacial, periglacial, alluvial, colluvial, and tectonic processes. Post-impact adjustment of part of the impact-generated basement structural fabric such as concentric faults is apparent. Distinct basin-stratigraphic units are interpreted to be linked to large-scale geologic activity far from the basin, including growth of the Tharsis magmatic-tectonic complex and the growth into southern middle latitudes of south polar ice sheets. Along with the migration of surface and sub-surface volatiles towards the central part of the primaiy basin, the substantial difference in elevation with respect to the surrounding highlands and Tharsis and the Thaumasia highlands result in the trapping of atmospheric volatiles within the basin in the form of fog and regional or local precipitation, even today. In addition, the impact event caused long-term (millions of years) hydrothermal activity, as well as deep-seated basement structures that have tapped the internal heat of Mars, as conduits, for far greater time, possibly even today. This possibility is raised by the observation of putative open-system pingos and nearby gullies that occur in linear depressions with accompanying systems of faults and fractures. Long-term water and heat energy enrichment, complemented by the interaction of the nutrient-enriched primordial crustal and mantle materials favorable to life excavated to the surface and near-surface environs through the Argyre impact event, has not only resulted in distinct geomorphology, but also makes the Argyre basin a potential site of exceptional astrobiological significance. (C) 2015 Elsevier Inc. All rights reserved.

Earthquake- induced Geomorphology: A Neotectonic Study at the Front of the Alps (Lake Thun & Aare Valley, Switzerland)

We present a multi-disciplinary two-step approach to assess the potential for seismic hazard of the Aare valley and perialpine Lake Thun (Switzerland). High-resolution seismic images and multibeam-bathymetric data, complemented by field observations represent the tools to identify potentially active seismogenic fault structures. Several second-order earthquake effects such as subaqueous mass movements, seismites and liquefaction structures have been observed in Lake Thun and ultimately document the seismic activity of the study area. A first investigation of possibly first-order active structures is presented in the scope of this study. Recently acquired bathymetric data in Lake Thun reveal significant morphologic depressions aligning with an observed lineament on land. Furthermore, high-resolution seismic images indicate potential fault structures in Lake Thun. However, their continuation with depth has to be verified with a multichannel seismic campaign, scheduled for March 2015.