scholarly journals Deformation along the roof of a fossil subduction interface in the transition zone below seismogenic coupling: The Austroalpine case and new insights from the Malenco Massif (Central Alps)

Geosphere ◽  
2020 ◽  
Vol 16 (2) ◽  
pp. 510-532
Author(s):  
Paraskevi Io Ioannidi ◽  
Samuel Angiboust ◽  
Onno Oncken ◽  
Philippe Agard ◽  
Johannes Glodny ◽  
...  
Keyword(s):  
Transition Zone ◽  
Shear Zones ◽  
White Mica ◽  
Seismogenic Zone ◽  
Central Alps ◽  
Massif Central ◽  
Brittle Ductile Transition ◽  
Wide Range ◽  
Deformation Processes ◽  
Transition Field

Abstract A network of fossil subduction plate interfaces preserved in the Central Alps (Val Malenco, N Italy) is herein used as a proxy to study deformation processes related to subduction and subsequent underplating of continental slices (in particular the Margna and Sella nappes) at depths reported to in the former brittle-ductile transition. Field observations, microfabrics, and mapping revealed a network of shear zones comprising mostly mylonites and schists but also rare foliated cataclasites. These shear zones are either located at the contacts of the two nappes or within the boundaries of the Sella unit. Microprobe results point to two different white mica generations, with higher-pressure (Si-rich) phengites rimming lower-pressure (Si-poor) phengites. Garnet is locally observed overgrowing resorbed pre-Alpine cores. Pressure-temperature estimates based on pseudosection modeling point to peak burial deformation conditions of ∼0.9 GPa and 350–400 °C, at ∼30 km depth. Rb/Sr geochronology on marbles deformed during the Alpine event yields an age of 48.9 ± 0.9 Ma, whereas due to incomplete recrystallization, a wide range of both Rb/Sr and 40Ar/39Ar apparent ages is obtained from deformed orthogneisses and micaschists embracing 87–44 Ma. Based on our pressure-temperature, structural and geochronological observations, the studied shear zones last equilibrated at depths downdip of the seismogenic zone in an active subduction zone setting. We integrate these new results in the frame of previous studies on other segments of the same Alpine paleosubduction interface, and we propose that this system of shear zones represents deformation conditions along the subduction interface(s) in the transition zone below the seismogenic zone during active subduction.

scholarly journals Deep decoupling in subduction zones: Observations and temperature limits

Geosphere ◽  
2020 ◽  
Vol 16 (6) ◽  
pp. 1408-1424 ◽  
Author(s):  
Geoffrey A. Abers ◽  
Peter E. van Keken ◽  
Cian R. Wilson
Keyword(s):  
Subduction Zone ◽  
Shear Zone ◽  
Subduction Zones ◽  
Thermal Structure ◽  
Shear Zones ◽  
Seismic Attenuation ◽  
Plate Interface ◽  
Brittle Ductile Transition ◽  
Wide Range ◽  
Shear Heating

Abstract The plate interface undergoes two transitions between seismogenic depths and subarc depths. A brittle-ductile transition at 20–50 km depth is followed by a transition to full viscous coupling to the overlying mantle wedge at ∼80 km depth. We review evidence for both transitions, focusing on heat-flow and seismic-attenuation constraints on the deeper transition. The intervening ductile shear zone likely weakens considerably as temperature increases, such that its rheology exerts a stronger control on subduction-zone thermal structure than does frictional shear heating. We evaluate its role through analytic approximations and two-dimensional finite-element models for both idealized subduction geometries and those resembling real subduction zones. We show that a temperature-buffering process exists in the shear zone that results in temperatures being tightly controlled by the rheological strength of that shear zone’s material for a wide range of shear-heating behaviors of the shallower brittle region. Higher temperatures result in weaker shear zones and hence less heat generation, so temperatures stop increasing and shear zones stop weakening. The net result for many rheologies are temperatures limited to ≤350–420 °C along the plate interface below the cold forearc of most subduction zones until the hot coupled mantle is approached. Very young incoming plates are the exception. This rheological buffering desensitizes subduction-zone thermal structure to many parameters and may help explain the global constancy of the 80 km coupling limit. We recalculate water fluxes to the forearc wedge and deep mantle and find that shear heating has little effect on global water circulation.

greater the probability that the particles will miss the high shear zones on a single pass through the device. For this reason the discharge of many rotor/stator devices is re-stricted with a series of holes or bars, or with a grid, which performs two separate tasks. First, a discharge grid absolutely limits the maximum particle size that can exit the mixer. If the holes on the discharge grid are 2 mm, it is virtually impossible for par-ticles larger than that diameter to pass through the machine. Rotor/stator in-line mix-ers are made with restricting grids as small as 0.5 mm, and a wide range of larger sizes up to 50 mm or even unrestricted outlets. A sample of such grids is presented in Fig. 29. But even the smallest usable discharge grid does not necessarily provide abso-lute control of the required size distributions in the range of fine dispersion. This is the second reason for the restriction on the discharge. By putting a more limiting dis-charge on the outlet, the flow rate through the device for a given set of pumping cir-cumstances (viscosity, density, system suction, and discharge head) is reduced and the residence time in the machine for a given particle or droplet is increased. The longer residence time increases the probability that a particle has to travel through the highest shearing zones and, thereby, be reduced to the smallest size that the machine is capable of producing. • • • • " ' ; • " * * «L £ J- •-

1998 ◽  
pp. 358-360
Keyword(s):  
Particle Size ◽  
Residence Time ◽  
Flow Rate ◽  
Shear Zones ◽  
High Shear ◽  
Single Pass ◽  
Fine Dispersion ◽  
Wide Range ◽  
Maximum Particle ◽  
Pass Through

French and Belgian Uplands

2005 ◽  
Author(s):  
Bernard Etlicher
Keyword(s):  
Continental Crust ◽  
Rift Valley ◽  
Sedimentary Basins ◽  
Shear Zones ◽  
Metamorphic Complex ◽  
French Massif Central ◽  
Massif Central ◽  
Montagne Noire ◽  
Oceanic And Continental Crust ◽  
Moderate Elevation

The French Uplands were built by the Hercynian orogenesis. The French Massif Central occupies one-sixth of the area of France and shows various landscapes. It is the highest upland, 1,886 m at the Sancy, and the most complex. The Vosges massif is a small massif, quite similar to the Schwarzwald in Germany, from which it is separated by the Rhine Rift Valley. Near the border of France, Belgium, and Germany, the Ardennes upland has a very moderate elevation. The largest part of this massif lies in Belgium. Though Brittany is partly made up of igneous and metamorphic rocks, it cannot be truly considered as an upland; in the main parts of Brittany, altitudes are lower than in the Parisian basin. Similarities of the landscape in the French and Belgian Uplands derive from two major events: the Oligocene rifting event and the Alpine tectonic phase. The Vosges and the Massif Central are located on the collision zone of the Variscan orogen. In contrast, the Ardennes is in a marginal position where primary sediments cover the igneous basement. Four main periods are defined during the Hercynian orogenesis (Bard et al. 1980; Autran 1984; Ledru et al. 1989; Faure et al. 1997). The early Variscan period corresponds to a subduction of oceanic and continental crust and a highpressure metamorphism (450–400 Ma) The medio- Variscan period corresponds to a continent–continent collision of the chain (400–340 Ma). Metamorphism under middle pressure conditions took place and controlled the formation of many granite plutons: e.g. red granites (granites rouges), porphyroid granite, and granodiorite incorporated in a metamorphic complex basement of various rocks. The neo-Variscan period (340–320 Ma) is characterized by a strong folding event: transcurrent shear zones affected the units of the previous periods and the first sedimentary basins appeared. At the end of this period, late-Variscan (330–280 Ma), autochthonous granites crystallized under low-pressure conditions related to a post-collision thinning of the crust. Velay and Montagne Noire granites are the main massifs generated by this event. Sediment deposition in tectonic basins during Carboniferous and Permian times occurred in the Massif Central and the Vosges: facies are sandstone (Vosges), shale, coal, and sandstone in several Stephanian basins of the Massif Central, with red shale and clay ‘Rougier’ in the south-western part of the Massif Central.

scholarly journals Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard

Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 125 ◽  
Author(s):  
Christopher J. Barnes ◽  
Katarzyna Walczak ◽  
Emilie Janots ◽  
David Schneider ◽  
Jarosław Majka
Keyword(s):  
High Pressure ◽  
Tectonic Evolution ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Structural Studies ◽  
Metamorphic Overprint ◽  
Subaerial Exposure ◽  
Facies Metamorphism

The Vestgӧtabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar/39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 ± 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 ± 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430–400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the Lower Unit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365–344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgӧtabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsäkkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgӧtabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

scholarly journals Hydrous oceanic crust hosts megathrust creep at low shear stresses

2020 ◽  
Vol 6 (22) ◽  
pp. eaba1529
Author(s):  
Christopher J. Tulley ◽  
Åke Fagereng ◽  
Kohtaro Ujiie
Keyword(s):  
Oceanic Crust ◽  
Shear Zones ◽  
Interface Strength ◽  
Shear Stresses ◽  
Plate Interface ◽  
Strength And Deformation ◽  
Transition Field ◽  
Flow Laws ◽  
Sw Japan ◽  
Low Shear

The rheology of the metamorphosed oceanic crust may be a critical control on megathrust strength and deformation style. However, little is known about the strength and deformation style of metamorphosed basalt. Exhumed megathrust shear zones exposed on Kyushu, SW Japan, contain hydrous metabasalts deformed at temperatures between ~300° and ~500°C, spanning the inferred temperature-controlled seismic-aseismic transition. Field and microstructural observations of these shear zones, combined with quartz grain-size piezometry, indicate that metabasalts creep at shear stresses <100 MPa at ~370°C and at shear stresses <30 MPa at ~500°C. These values are much lower than those suggested by viscous flow laws for basalt. The implication is that relatively weak, hydrous, metamorphosed oceanic crust can creep at low viscosities over a wide shear zone and have a critical influence on plate interface strength and deformation style around the seismic-aseismic transition.

scholarly journals Soil, snow, weather, and sub-surface storage data from a mountain catchment in the rain–snow transition zone

2013 ◽  
Vol 6 (2) ◽  
pp. 811-835 ◽  
Author(s):  
P. R. Kormos ◽  
D. Marks ◽  
C. J. Williams ◽  
H. P. Marshall ◽  
P. Aishlin ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Transition Zone ◽  
Soil Texture ◽  
Soil Depth ◽  
Meteorological Data ◽  
Dew Point ◽  
Soil Profiles ◽  
Data Set ◽  
Hydrologic Condition ◽  
Wide Range

Abstract. A comprehensive hydroclimatic data set is presented for the 2011 water year to improve understanding of hydrologic processes in the rain-snow transition zone. This type of dataset is extremely rare in scientific literature because of the quality and quantity of soil depth, soil texture, soil moisture, and soil temperature data. Standard meteorological and snow cover data for the entire 2011 water year are included, which include several rain-on-snow events. Surface soil textures and soil depths from 57 points are presented as well as soil texture profiles from 14 points. Meteorological data include continuous hourly shielded, unshielded, and wind corrected precipitation, wind speed, air temperature, relative humidity, dew point temperature, and incoming solar and thermal radiation data. Sub-surface data included are hourly soil moisture data from multiple depths from 7 soil profiles within the catchment, and soil temperatures from multiple depths from 2 soil profiles. Hydrologic response data include hourly stream discharge from the catchment outlet weir, continuous snow depths from one location, intermittent snow depths from 5 locations, and snow depth and density data from ten weekly snow surveys. Though it represents only a single water year, the presentation of both above and below ground hydrologic condition makes it one of the most detailed and complete hydro-climatic datasets from the climatically sensitive rain-snow transition zone for a wide range of modeling and descriptive studies. Data are available at doi:10.1594/PANGAEA.819837.

scholarly journals In Situ and Step‐Heating 40 Ar/ 39 Ar Dating of White Mica in Low‐Temperature Shear Zones (Tenda Massif, Alpine Corsica, France)

Tectonics ◽  
2020 ◽  
Vol 39 (12) ◽  
Author(s):  
Alexandre Beaudoin ◽  
Stéphane Scaillet ◽  
Nicolas Mora ◽  
Laurent Jolivet ◽  
Romain Augier
Keyword(s):  
Low Temperature ◽  
Shear Zones ◽  
White Mica ◽  
Step Heating ◽  
Temperature Shear

scholarly journals Multidisciplinary characterisation of sedimentary processes in a recent maar lake (Lake Pavin, French Massif Central) and implication for natural hazards

2010 ◽  
Vol 10 (9) ◽  
pp. 1815-1827 ◽  
Author(s):  
E. Chapron ◽  
P. Albéric ◽  
D. Jézéquel ◽  
W. Versteeg ◽  
J.-L. Bourdier ◽  
...  
Keyword(s):  
Lacustrine Sediments ◽  
French Massif Central ◽  
Mass Wasting ◽  
Data Set ◽  
Massif Central ◽  
Volcanic Material ◽  
Deep Waters ◽  
Wide Range ◽  
Maar Lake ◽  
Lake Pavin

Abstract. Sedimentation processes occurring in the most recent maar lake of the French Massif Central (Lake Pavin) are documented for the first time based on high resolution seismic reflection and multibeam bathymetric surveys and by piston coring and radiocarbon dating on a sediment depocentre developed on a narrow sub aquatic plateau. This new data set confirms the mid Holocene age of maar lake Pavin formation at 6970±60 yrs cal BP and highlights a wide range of gravity reworking phenomena affecting the basin. In particular, a slump deposit dated between AD 580–640 remoulded both mid-Holocene lacustrine sediments, terrestrial plant debris and some volcanic material from the northern crater inner walls. Between AD 1200 and AD 1300, a large slide scar mapped at 50 m depth also affected the southern edge of the sub aquatic plateau, suggesting that these gas-rich biogenic sediments (laminated diatomite) are poorly stable. Although several triggering mechanisms can be proposed for these prehistoric sub-aquatic mass wasting deposits in Lake Pavin, we argue that such large remobilisation of gas-rich sediments may affect the gas stability in deep waters of meromictic maar lakes. This study highlights the need to further document mass wasting processes in maar lakes and their impacts on the generation of waves, favouring the development of dangerous (and potentially deadly) limnic eruptions.

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