Stress variations in space and time within the mantle section of an oceanic transform zone: Evidence for the seismic cycle

Geology ◽  
2020 ◽  
Vol 48 (6) ◽  
pp. 569-573 ◽  
Author(s):  
Vasileios Chatzaras ◽  
Basil Tikoff ◽  
Seth C. Kruckenberg ◽  
Sarah J. Titus ◽  
Christian Teyssier ◽  
...  
Keyword(s):  
New Caledonia ◽  
Stress Gradient ◽  
Oceanic Lithosphere ◽  
Temporal Variations ◽  
Ductile Deformation ◽  
Seismic Cycle ◽  
Fine Grained ◽  
Transform Zone ◽  
Stress Variations ◽  
Mantle Section

Abstract The Bogota Peninsula shear zone in New Caledonia (southwest Pacific Ocean) is the exhumed mantle section of an oceanic transform zone. Ductile fabrics in this zone formed at temperatures >820 °C, and differential stresses estimated from microstructures vary spatially and temporally. Along a transform-perpendicular transect, stresses increase toward the high-strain areas. We attribute this stress gradient to an increase in strain rate caused by imposed rather than intrinsic strain localization. Temporal stress variations are indicated by the formation of fine-grained microdeformation zones (MDZs) that truncate and offset coarser grains. We interpret the MDZs to result from zones of brittle deformation caused by earthquake fracture propagation downward in the upper mantle, which are in turn overprinted by ductile deformation at stresses 2–6 times higher (22–81 MPa) than their surrounding steady-state fabrics. We interpret the spatial and temporal variations in microstructures and stresses as reflecting different stages of the seismic cycle in oceanic lithosphere.

Imposed strain localization in the mantle section of an oceanic transform zone revealed by microstructural and stress variations

2020 ◽  
Author(s):  
Vasileios Chatzaras ◽  
Basil Tikoff ◽  
Seth C. Kruckenberg ◽  
Sarah J. Titus ◽  
Christian Teyssier ◽  
...  
Keyword(s):  
Shear Zone ◽  
Upper Mantle ◽  
Temporal Variations ◽  
Coarse Grained ◽  
Transform Fault ◽  
Differential Stress ◽  
Transform Faults ◽  
Fine Grained ◽  
Micro Deformation ◽  
Mantle Section

<p>Mantle earthquakes that occur deeper than the 600 °C isotherm in oceanic transform faults indicate seismic rupturing at conditions where viscous deformation (bulk ductile behavior) is dominant.  However, direct geological evidence of earthquake-related deformation at ambient upper mantle conditions is rare, impeding our understanding of earthquake dynamics in plate-boundary fault systems.  The Bogota Peninsula Shear Zone (BPSZ), New Caledonia, is an ancient oceanic transform fault exhumed from upper mantle depths.  Ductile structures in the BPSZ formed at temperatures > 800 °C and microstructures indicate that differential stress varies spatially and temporally.  Spatial variation is observed as an increase in differential stress with strain toward localized zones of high strain; stress increases from 6–14 MPa in coarse grained tectonites to 11–22 MPa within 1–2 km wide mylonite zones.  Temporal stress variation is observed by the formation of micro-deformation zones that seem to have brittle precursors, are filled with fine-grained recrystallized olivine grains and crosscut the background fabrics in the harzburgites that host them.  The micro-deformation zones are not restricted to the mylonite zones, but rather are located throughout the BPSZ, having affected the protomylonites and the coarse grained tectonites.  The micro-deformation zones record stresses of 22–81 MPa that are 2–6 times higher than the background, steady-state stresses in the surrounding mantle rocks.  We interpret the observed spatial and temporal variations in microstructures and stresses in the upper mantle to demonstrate the influence of seismic events in the upper part of the oceanic transform fault system.  We attribute the increase in stress with strain to be the result of imposed localization induced by downward propagation of the seismic rupture into the underlying mantle.  The micro-deformation zones could result from brittle fractures caused by earthquake-related deformation in the mantle section of the transform fault, which are in turn overprinted by ductile deformation.</p><p> </p><p>Synthesizing the spatial and temporal variations in stresses and microstructures in the Bogota Peninsula Shear Zone we propose a conceptual model where brittle fracturing and shearing take place during coseismic rupture at increased stress, ductile flow at decaying stress is concentrated in the micro-deformation zones during postseismic relaxation, and uniformly distributed creep at low stress occurs in the host-rocks of the micro-deformation zones during interseismic deformation.  The critical result from the studied paleotransform zone is that the fine-grained micro-deformation zones and the mylonites do not represent weak zones.  Instead, they form by dislocation creep at transient high-stress deformation during the seismic cycle.  The spatial distribution of the micro-deformation zones also suggests that repeated stress cycles in oceanic transform faults may not localize strain in pre-existing shear zones but disperse strain across the structure.</p>

Use of pyrite microfabric as a key to tectono-thermal evolution of massive sulphide deposits – an example from Deri, southern Rajasthan, India

1998 ◽  
Vol 62 (2) ◽  
pp. 197-212 ◽  
Author(s):  
Anju Tiwary ◽  
Mihir Deb ◽  
Nigel J. Cook
Keyword(s):  
Thermal Evolution ◽  
Regional Metamorphism ◽  
Massive Sulphide ◽  
Ductile Deformation ◽  
Sulphide Deposit ◽  
Fine Grained ◽  
Thermal Metamorphism ◽  
Sulphide Ores ◽  
History Of ◽  
South Delhi Fold Belt

AbstractPyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle–ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet–biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite–ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

scholarly journals Patterns of fish utilisation in a tropical Indo-Pacific mangrove-coral seascape, New Caledonia

2018 ◽  
Author(s):  
A Dubuc ◽  
N. Waltham ◽  
R. Baker ◽  
C. Marchand ◽  
M. Sheaves
Keyword(s):  
Fish Assemblages ◽  
New Caledonia ◽  
Lunar Cycle ◽  
Forest Edge ◽  
Temporal Variations ◽  
Conservation Strategies ◽  
Small Scale ◽  
Mangrove Forests ◽  
Species Specific ◽  
Main Component

AbstractMangrove forests are important habitats for fish. However, their utilisation by fish, and the specific values they confer, are still not fully understood. This study describes how fish use mangrove forests in an Indo-Pacific mangrove-coral reef seascape. Sampling was conducted using underwater video cameras (UVCs) to describe spatial and temporal variations in fish assemblages across a small-scale (~ 2.5 km2) system, and over the tidal and lunar cycle. UVCs were deployed in the two main component habitats of mangrove forests: at the mangrove forest edge, and inside the forest (5 m from the forest edge), to establish patterns of utilisation of fish across the tidal and lunar cycle. Proximity to coral reefs had a strong influence on the mangrove fish community, as most fish recorded were reef-associated. Juveniles of 12 reef species were observed, including two species classified as vulnerable on the IUCN list, and one endemic species. Fish assemblages on the mangrove edge differed significantly from those inside the forest. Most fish utilised the forest edge, with few species making regular use of in-forest habitats, supporting the contention that most fish species remain on the edge and potentially retreat into the forest for opportunistic feeding, or when threatened by larger predators. Species-specific patterns of utilisation varied across the tidal and lunar cycle. Small differences in depth profiles and substrate across the small-scale system had a significant effect on fish assemblages, highlighting the importance of accounting for spatial heterogeneity in these factors. These data provide important information for managers to implement adequate conservation strategies that include broader interconnected habitat mosaics.

Internal lithospheric rotation at the initiation of intra-oceanic rift-drift: An example of proto-transform tectonics from the Vourinos Ophiolite, Greece

2021 ◽  
Author(s):  
Anne Ewing Rassios ◽  
Dina Ghikas ◽  
Anna Batsi ◽  
Petros Koutsovitis ◽  
Evangelos Tzamos ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Oceanic Lithosphere ◽  
Ductile Deformation ◽  
New Paradigm ◽  
Crustal Rocks ◽  
Mantle Boundary ◽  
Ridge Crest ◽  
Transition Area ◽  
The 1960S

ABSTRACT The “petrological Moho” recognized in the Jurassic Vourinos Ophiolite (northern Greece) was the first “crust-mantle” boundary described within a fossil oceanic lithosphere. Early observations suggested a Cenozoic brittle-field block rotation of the petrological Moho transition area resulting in an oblique clockwise rotation of ∼100°, but a brittle fault system responsible for the mechanism of this rotation was never located. A modern interpretation of research dating from the 1960s to the present documents the occurrence of a diverse set of ductile structures overprinting this primary intra-oceanic feature. The following observations from our original “Moho” studies in the Vourinos complex are still pertinent: the contact between the upper mantle units and the magmatic crustal sequence is in situ and intrusional in nature; high-temperature intragranular ductile deformation (mantle creep at temperatures from around 1200 °C down to ∼900 °C) fabrics terminate at the crust-mantle boundary; the overlying oceanic crustal rocks display geochemical fractionation patterns analogous to crustal rocks in the in situ oceanic lithosphere. Since these original studies, however, understanding the mechanisms of ductile deformation and ridge crest processes have advanced, and hence we can now interpret the older data and recent observations in a new paradigm of oceanic lithosphere formation. Our major interpretational breakthrough includes the following phenomena: lower temperature, intergranular deformation of ∼900 °C to 700 °C extends from the upper mantle tectonites up into the lower crustal cumulate section; the origin of mineral lineations within adcumulate crustal rocks as remnants of ductile deformation during early phases of magmatic crystallization; syn-magmatic folding and rotation of the cumulate section; the tectonic significance of flaser gabbro and late gabbroic intrusions in the crustal sequence; and the relevance and significance of a cumulate troctolite unit within the crustal sequence. These observations collectively point to an important process of a ductile-field, syn-magmatic rotation of the Moho transition area. The most plausible mechanism explaining such a rotation is proto-transform faulting deformation near the ridge crest. By recognizing and distinguishing structures that resulted from such initial rotational deformation in the upper mantle peridotites of ophiolites, future field-based structural, petrographic, and petrological studies can better document the mode of the initiation of oceanic transform faults.

Spatio-temporal variations in the composition of organic matter in surface sediments of a mangrove receiving shrimp farm effluents (New Caledonia)

2015 ◽  
Vol 512-513 ◽  
pp. 296-307 ◽  
Author(s):  
Adélaïde Aschenbroich ◽  
Cyril Marchand ◽  
Nathalie Molnar ◽  
Jonathan Deborde ◽  
Cédric Hubas ◽  
...  
Keyword(s):  
Organic Matter ◽  
Surface Sediments ◽  
New Caledonia ◽  
Temporal Variations ◽  
Shrimp Farm ◽  
Spatio Temporal

scholarly journals Multimodal Transformer Networks for Pedestrian Trajectory Prediction

2021 ◽  
Author(s):  
Ziyi Yin ◽  
Ruijin Liu ◽  
Zhiliang Xiong ◽  
Zejian Yuan
Keyword(s):  
Optical Flow ◽  
Temporal Variations ◽  
Coarse Grained ◽  
Vehicle Speed ◽  
Temporal Consistency ◽  
Moving Vehicle ◽  
Dynamic Motion ◽  
Fine Grained ◽  
High Dynamics

We consider the problem of forecasting the future locations of pedestrians in an ego-centric view of a moving vehicle. Current CNNs or RNNs are flawed in capturing the high dynamics of motion between pedestrians and the ego-vehicle, and suffer from the massive parameter usages due to the inefficiency of learning long-term temporal dependencies. To address these issues, we propose an efficient multimodal transformer network that aggregates the trajectory and ego-vehicle speed variations at a coarse granularity and interacts with the optical flow in a fine-grained level to fill the vacancy of highly dynamic motion. Specifically, a coarse-grained fusion stage fuses the information between trajectory and ego-vehicle speed modalities to capture the general temporal consistency. Meanwhile, a fine-grained fusion stage merges the optical flow in the center area and pedestrian area, which compensates the highly dynamic motion of ego-vehicle and target pedestrian. Besides, the whole network is only attention-based that can efficiently model long-term sequences for better capturing the temporal variations. Our multimodal transformer is validated on the PIE and JAAD datasets and achieves state-of-the-art performance with the most light-weight model size. The codes are available at https://github.com/ericyinyzy/MTN_trajectory.

Kosmochlor and chromian jadeite aggregates from the Myanmar jadeitite area

2005 ◽  
Vol 69 (6) ◽  
pp. 1059-1075 ◽  
Author(s):  
G. H. Shi ◽  
B. Stöckhert ◽  
W. Y. Cui
Keyword(s):  
Ductile Deformation ◽  
Coarse Grained ◽  
Variable Composition ◽  
Dislocation Creep ◽  
Minimum Pressure ◽  
Fine Grained ◽  
Terrestrial Rock ◽  
Miscibility Gaps ◽  
Creep Regime

AbstractFour distinct textures and related compositions of kosmochlor (Ko) and chromian jadeite in rocks from the Myanmar jadeitite area are described: (1) spheroidal or ellipsoidal aggregates with a corona texture surrounding relict chromite; (2) spheroidal or ellipsoidal aggregates with a core of jadeite; (3) granoblastic textures in undeformed coarse-grained clinopyroxene rocks; and (4) recrystallized fine-grained aggregates in deformed jadeitite. Nearly pure kosmochlor (97 mol.% NaCrSi2O6) was found in type 2 textures, closest to the end-member reported so far from a terrestrial rock. Sharp compositional boundaries between kosmochlor and chromian jadeite of variable composition are interpreted to be related to progressive crystallization or replacement at differing conditions. The compositions analysed plot along the kosmochlor-jadeite join. In contrast to conclusions of previous studies on the Myanmar clinopyroxenes there is no unequivocal evidence for miscibility gaps. The preservation of relict chromite in the centre of coronitic spheroidal or ellipsoidal aggregates of kosmochlor and jadeite indicates a metasomatic origin from a peridotite protolith at an inferred minimum pressure of 1.0 GPa and temperatures of 250—370°C. Recrystallization during later ductile deformation of the clinopyroxene rocks in the dislocation creep regime leads to fine-grained aggregates of chromian jadeite, which are of particular gemmological interest.

Direct observations of a dynamic earthquake rupture in the lower crust

2020 ◽  
Author(s):  
Arianne Petley-Ragan ◽  
Yehuda Ben-Zion ◽  
Håkon Austrheim ◽  
Benoit Ildefonse ◽  
Francois Renard
Keyword(s):  
Lower Crust ◽  
Damage Zone ◽  
Granulite Facies ◽  
Wall Rock ◽  
Ductile Deformation ◽  
Fine Grained ◽  
Seismic Slip ◽  
Sequence Of Events ◽  
Shear Rupture ◽  
Brittle Faulting

<p>A significant number of studies in recent years have demonstrated that earthquakes in the lower crust are more abundant than previously thought. Specifically in continental collision zones, earthquakes are suggested to play a crucial role in permitting fluid infiltration and driving metamorphic transformation processes in crustal portions that are typically considered dry and metastable. However, the mechanisms that trigger brittle failure in the lower crust remain debated and the sequence of events that ultimately lead to seismic slip is unclear. To further understand this process we performed field and microstructural observations on an amphibolite facies fault (0.9-1 GPa) in granulite facies anorthosite from the Bergen Arcs, Western Norway. The fault preserves an exceptional record of brittle deformation and frictional melting that allows us to constrain the temporal sequence of deformation events. Most notably, the fault is flanked on one side by a damage zone where wall rock minerals are fragmented with little to no shear strain (pulverization). The fault core consists of a zoned pseudotachylyte bound on both sides by fine-grained cataclasites. Spatial relationships between these structures reveal that asymmetric pulverization of the wall rock and comminution preceded the seismic slip required to produce melting. These observations are consistent with the propagation of a dynamic shear rupture. Our study implies that high differential stress levels may exist within the dry lower crust of orogens, causing brittle faulting and earthquakes in a portion of the crust that has long been assumed to be characterized by ductile deformation.</p>

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