scholarly journals Gravity constraints on structure of the East-West Antarctic lithospheric transition zone

2021 ◽  
Author(s):  
Sam Treweek
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Geophysical Methods ◽  
East Antarctica ◽  
Gravity Models ◽  
West Antarctica ◽  
Asthenospheric Mantle ◽  
Surface Uplift ◽  
Mantle Viscosity ◽  
Taylor Valley

<p><b>The differing structural evolution of cratonic East Antarctica and younger West Antarctica has resulted in contrasting lithospheric and asthenospheric mantle viscosities between the two regions. Combined with poor constraints on the upper mantle viscosity structure of the continent, estimates of surface uplift in Antarctica predicted from models of glacial isostatic adjustment (GIA) and observed by Global Satellite Navigation System (GNSS) contain large misfits. This thesis presents a gravity study ofthe lithospheric transition zone beneath the Taylor Valley, Antarctica, conducted to constrain the variation in lithological parameters such as viscosity and density of the upper mantle across this region.</b></p> <p>During this study 119 new gravity observations were collected in the ice-free regions of the Taylor Valley and amalgamated with 154 existing land-based gravity observations, analysed alongside aerogravity measurements of southern Victoria Land. Gravity data are used to construct 2D gravity models of the subsurface beneath this region. An eastward gradient in Bouguer anomalies of ~- 1.6 mGal/km is observed within the Taylor Valley. Models reveal thickening of the Moho from 23±5 km beneath the Ross Sea to 35±5 km in the Polar Plateau (dipping at 24.5±7.2°), and lithospheric mantle 100 km thicker in East Antarctica (~200±30 km) than West Antarctica (~90±30 km). </p> <p>Models of predicted surface uplift history are used to estimate an asthenospheric mantle viscosity of 2.1x1020 Pa.s at full surface recovery beneath the Ross Embayment, differing by ~14% from the viscosity at 50% recovery. The temperature contrast between lithospheric and asthenospheric mantle is estimated as ~400°C, equivalent to a viscosity that decreases by a factor of about 30 over the mantle boundary.</p> <p>Results demonstrate that the history of surface uplift in the study area may be complicated, resulting in observations of uplift, or subsidence, at GNSS stations. Future work should incorporate additional geophysical methods, such as seismicity and electrical resistivity, improving constraints on gravity models. A better understanding of the surface uplift (or subsidence) history in the Transantarctic Mountains is critical, with implications in reducing uncertainty in GIA models.</p>

scholarly journals Gravity constraints on structure of the East-West Antarctic lithospheric transition zone

2021 ◽  
Author(s):  
Sam Treweek
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Geophysical Methods ◽  
East Antarctica ◽  
Gravity Models ◽  
West Antarctica ◽  
Asthenospheric Mantle ◽  
Surface Uplift ◽  
Mantle Viscosity ◽  
Taylor Valley

<p><b>The differing structural evolution of cratonic East Antarctica and younger West Antarctica has resulted in contrasting lithospheric and asthenospheric mantle viscosities between the two regions. Combined with poor constraints on the upper mantle viscosity structure of the continent, estimates of surface uplift in Antarctica predicted from models of glacial isostatic adjustment (GIA) and observed by Global Satellite Navigation System (GNSS) contain large misfits. This thesis presents a gravity study ofthe lithospheric transition zone beneath the Taylor Valley, Antarctica, conducted to constrain the variation in lithological parameters such as viscosity and density of the upper mantle across this region.</b></p> <p>During this study 119 new gravity observations were collected in the ice-free regions of the Taylor Valley and amalgamated with 154 existing land-based gravity observations, analysed alongside aerogravity measurements of southern Victoria Land. Gravity data are used to construct 2D gravity models of the subsurface beneath this region. An eastward gradient in Bouguer anomalies of ~- 1.6 mGal/km is observed within the Taylor Valley. Models reveal thickening of the Moho from 23±5 km beneath the Ross Sea to 35±5 km in the Polar Plateau (dipping at 24.5±7.2°), and lithospheric mantle 100 km thicker in East Antarctica (~200±30 km) than West Antarctica (~90±30 km). </p> <p>Models of predicted surface uplift history are used to estimate an asthenospheric mantle viscosity of 2.1x1020 Pa.s at full surface recovery beneath the Ross Embayment, differing by ~14% from the viscosity at 50% recovery. The temperature contrast between lithospheric and asthenospheric mantle is estimated as ~400°C, equivalent to a viscosity that decreases by a factor of about 30 over the mantle boundary.</p> <p>Results demonstrate that the history of surface uplift in the study area may be complicated, resulting in observations of uplift, or subsidence, at GNSS stations. Future work should incorporate additional geophysical methods, such as seismicity and electrical resistivity, improving constraints on gravity models. A better understanding of the surface uplift (or subsidence) history in the Transantarctic Mountains is critical, with implications in reducing uncertainty in GIA models.</p>

scholarly journals Mass balance of the Antarctic ice sheet 1992–2016: reconciling results from GRACE gravimetry with ICESat, ERS1/2 and Envisat altimetry

2021 ◽  
pp. 1-27
Author(s):  
H. Jay Zwally ◽  
John W. Robbins ◽  
Scott B. Luthcke ◽  
Bryant D. Loomis ◽  
Frédérique Rémy
Keyword(s):  
Mass Balance ◽  
Antarctic Peninsula ◽  
East Antarctica ◽  
Mantle Material ◽  
West Antarctica ◽  
Mantle Viscosity ◽  
Grounding Line ◽  
The Antarctic ◽  
Total Gain

Abstract GRACE and ICESat Antarctic mass-balance differences are resolved utilizing their dependencies on corrections for changes in mass and volume of the same underlying mantle material forced by ice-loading changes. Modeled gravimetry corrections are 5.22 times altimetry corrections over East Antarctica (EA) and 4.51 times over West Antarctica (WA), with inferred mantle densities 4.75 and 4.11 g cm−3. Derived sensitivities (Sg, Sa) to bedrock motion enable calculation of motion (δB0) needed to equalize GRACE and ICESat mass changes during 2003–08. For EA, δB0 is −2.2 mm a−1 subsidence with mass matching at 150 Gt a−1, inland WA is −3.5 mm a−1 at 66 Gt a−1, and coastal WA is only −0.35 mm a−1 at −95 Gt a−1. WA subsidence is attributed to low mantle viscosity with faster responses to post-LGM deglaciation and to ice growth during Holocene grounding-line readvance. EA subsidence is attributed to Holocene dynamic thickening. With Antarctic Peninsula loss of −26 Gt a−1, the Antarctic total gain is 95 ± 25 Gt a−1 during 2003–08, compared to 144 ± 61 Gt a−1 from ERS1/2 during 1992–2001. Beginning in 2009, large increases in coastal WA dynamic losses overcame long-term EA and inland WA gains bringing Antarctica close to balance at −12 ± 64 Gt a−1 by 2012–16.

Upper mantle viscosity structure and lithospheric thickness of Antarctica inferred from recent seismic models

2020 ◽  
Author(s):  
Douglas Wiens ◽  
Andrew Lloyd ◽  
Weisen Shen ◽  
Andrew Nyblade ◽  
Richard Aster ◽  
...  
Keyword(s):  
Antarctic Peninsula ◽  
Upper Mantle ◽  
Ice Sheet ◽  
West Antarctica ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Seismic Models ◽  
The Antarctic

&lt;p&gt;Upper mantle viscosity structure and lithospheric thickness control the solid Earth response to variations in ice sheet loading. These parameters vary significantly across Antarctica, leading to strong regional differences in the timescale of glacial isostatic adjustment (GIA), with important implications for ice sheet models. &amp;#160;We estimate upper mantle viscosity structure and lithospheric thickness using two new seismic models for Antarctica, which take advantage of temporary broadband seismic stations deployed across Antarctica over the past 18 years. Shen et al. [2018] use receiver functions and Rayleigh wave velocities from earthquakes and ambient noise to develop a higher resolution model for the upper 200 km beneath Central and West Antarctica, where most of the seismic stations have been deployed. Lloyd et al [2019] use full waveform adjoint tomography to invert three-component earthquake seismograms for a radially anisotropic model covering Antarctica and adjacent oceanic regions to 800 km depth. We estimate the mantle viscosity structure from seismic structure using laboratory-derived relationships between seismic velocity, temperature, and rheology. Choice of parameters for this mapping is guided in part by recent regional estimates of mantle viscosity from geodetic measurements. We also describe and compare several different methods of estimating lithospheric thickness from seismic constraints.&lt;/p&gt;&lt;p&gt;The mantle viscosity estimates indicate regional variations of several orders of magnitude, with extremely low viscosity (&lt; 10&lt;sup&gt;19&lt;/sup&gt; Pa s) beneath the Amundsen Sea Embayment (ASE) and the Antarctic Peninsula, consistent with estimates from GIA models constrained by GPS data. &amp;#160;Lithospheric thickness is also highly variable, ranging from around 60 km in parts of West Antarctica to greater than 200 km beneath central East Antarctica. In East Antarctica, several prominent regions such as Dronning Maude Land and the Lambert Graben show much thinner lithosphere, consistent with Phanerozoic tectonic activity and lithospheric disruption. Thin lithosphere and low viscosity between the ASE and the Antarctic Peninsula likely result from the thermal effects of the slab window as the Phoenix-Antarctic plate boundary migrated northward during the Cenozoic. Low viscosity regions beneath the ASE and Marie Byrd Land coast connect to an offshore anomaly at depths of ~ 250 km, suggesting larger-scale thermal and geodynamic processes that may be linked to the initial Cretaceous rifting of New Zealand and Antarctica. Low mantle viscosity results in a characteristic GIA time scale on the order of several hundred years, such that isostatic adjustment occurs on the same time scale as grounding line retreat.&amp;#160; Thus the associated rebound may lessen the effect of the marine ice sheet instability proposed for the ASE region.&amp;#160;&lt;/p&gt;

scholarly journals Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

2021 ◽  
Author(s):  
◽  
Richard J Wysoczanski
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Mineral Assemblage ◽  
Geophysical Methods ◽  
Crustal Material ◽  
West Antarctica ◽  
Rock Composition ◽  
Temperature Oxidation ◽  
Crustal Xenoliths ◽  
Lower Crustal

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

GIA effects of Holocene rapid ice thinning on the observed geodetic signals along the coast of Lützow-Holm Bay in East Antarctica

2021 ◽  
Author(s):  
Jun'ichi Okuno ◽  
Akihisa Hattori ◽  
Takeshige Ishiwa ◽  
Yoshiya Irie ◽  
Koichiro Doi
Keyword(s):  
Mass Balance ◽  
Elastic Deformation ◽  
Upper Mantle ◽  
East Antarctica ◽  
Crustal Movement ◽  
Crustal Motion ◽  
Mantle Viscosity ◽  
Uplift Rates ◽  
Current Mass ◽  
The Antarctic

&lt;p&gt;Geodetic and geomorphological observations in the Antarctic coastal area generally indicate the uplift trend associated with the Antarctic Ice Sheet (AIS) change since the Last Glacial Maximum (LGM). The melting models of AIS derived from the comparisons between sea-level and geodetic observations and glacial isostatic adjustment (GIA) modeling show the monotonous retreat through the Holocene era (e.g., Whitehouse et al., 2012,&amp;#160;&lt;em&gt;QSR&lt;/em&gt;; Stuhne and Peltier, 2015,&amp;#160;&lt;em&gt;JGR&lt;/em&gt;). However, the observed crustal motion by GNSS in some regions of Antarctica cannot be explained as the deformation rates by only glacial rebound due to the last deglaciation of AIS (e.g., Bradley et al., 2015,&amp;#160;&lt;em&gt;EPSL&lt;/em&gt;). One reason for this mismatch is considered as the control of the uplift induced by the re-advance of AIS following a post-LGM maximum retreat, which was recently reported as the West AIS re-advance in the Ross and the Weddell Sea sectors (e.g., Kingslake et al., 2018,&amp;#160;&lt;em&gt;Nature&lt;/em&gt;).&lt;/p&gt;&lt;p&gt;On the other hand, the current crustal motion includes the elastic GIA component due to the present-day surface mass balance of AIS. To reveal the secular crustal movement induced by GIA, the separation of the elastic deformation induced by the current mass balance using GRACE data is essential. In the L&amp;#252;tzow-Holm Bay, East Antarctica, GNSS observations have been carried out at several sites on the outcrop rocks since the 1990s to monitor recent crustal movements. Hattori et al. (2019, &lt;em&gt;SCAR&lt;/em&gt;) precisely analyzed the GNSS data obtained from this area, which revealed the secular crustal movement by correcting the elastic deformation due to current mass balance. The results indicated the mismatch between secular current crustal motion and GIA calculations based on the previously published ice and viscosity models. Consequently, to represent the observed crustal deformation rates based on the GIA modeling, we must carefully investigate the numerical dependencies of various parameters such as local and global ice history in the AIS.&lt;/p&gt;&lt;p&gt;Recently, the study of glacial geomorphology and surface exposure dating (Kawamata et al., 2020,&amp;#160;&lt;em&gt;QSR&lt;/em&gt;) has suggested that the abrupt ice thinning and retreat occurred in Skarvsnes, located at the middle of the L&amp;#252;tzow-Holm Bay, during 9 to 6 ka. We obtained the preliminary results related to the GIA effects induced by the abrupt thinning on the geodetic observations in this area. The numerical simulations that we examined are employed for a simple ice model with the thickness change by 400 m during 9 to 6 ka in this area based on the IJ05_R2 model grids (Ivins et al., 2013,&amp;#160;&lt;em&gt;JGR&lt;/em&gt;). The predictions based on the high-viscosity upper mantle (5x10&lt;sup&gt;20&lt;/sup&gt; Pa s) show high uplift rates (~ +4.0 mm/yr), whereas the calculated uplift rates for the weaker viscosity (2x10&lt;sup&gt;20&lt;/sup&gt; Pa s) show low value (~ +1.0 mm/yr). These results suggest that the viscoelastic relaxation due to the abrupt ice thinning in the mid-to-late Holocene may influence the current crustal motion and highly depend on the upper mantle viscosity profile. We will discuss the influences on the GIA-calculated crustal movement by AIS retreat history and mantle viscosity structure.&lt;/p&gt;

scholarly journals Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

2021 ◽  
Author(s):  
◽  
Richard J Wysoczanski
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Mineral Assemblage ◽  
Geophysical Methods ◽  
Crustal Material ◽  
West Antarctica ◽  
Rock Composition ◽  
Temperature Oxidation ◽  
Crustal Xenoliths ◽  
Lower Crustal

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

scholarly journals Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

2020 ◽  
Vol 224 (2) ◽  
pp. 961-972
Author(s):  
A G Semple ◽  
A Lenardic
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
Mantle Dynamics ◽  
Plate Motions ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Flow Feature ◽  
Mantle Rheology

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

scholarly journals Dislocation interactions in olivine control postseismic creep of the upper mantle

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
David Wallis ◽  
Lars N. Hansen ◽  
Angus J. Wilkinson ◽  
Ricardo A. Lebensohn
Keyword(s):  
Upper Mantle ◽  
Stress Transfer ◽  
Transient Creep ◽  
Slip Systems ◽  
Stress Distributions ◽  
Correlation Lengths ◽  
Mantle Viscosity ◽  
Dislocation Interactions ◽  
Different Temperatures ◽  
New Generation

AbstractChanges in stress applied to mantle rocks, such as those imposed by earthquakes, commonly induce a period of transient creep, which is often modelled based on stress transfer among slip systems due to grain interactions. However, recent experiments have demonstrated that the accumulation of stresses among dislocations is the dominant cause of strain hardening in olivine at temperatures ≤600 °C, raising the question of whether the same process contributes to transient creep at higher temperatures. Here, we demonstrate that olivine samples deformed at 25 °C or 1150–1250 °C both preserve stress heterogeneities of ~1 GPa that are imparted by dislocations and have correlation lengths of ~1 μm. The similar stress distributions formed at these different temperatures indicate that accumulation of stresses among dislocations also provides a contribution to transient creep at high temperatures. The results motivate a new generation of models that capture these intragranular processes and may refine predictions of evolving mantle viscosity over the earthquake cycle.

scholarly journals An Assessment of ERA5 Reanalysis for Antarctic Near-Surface Air Temperature

Atmosphere ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 217
Author(s):  
Jiangping Zhu ◽  
Aihong Xie ◽  
Xiang Qin ◽  
Yetang Wang ◽  
Bing Xu ◽  
...  
Keyword(s):  
Linear Relationship ◽  
Antarctic Peninsula ◽  
Austral Summer ◽  
Correlation Coefficients ◽  
East Antarctica ◽  
West Antarctica ◽  
Austral Spring ◽  
Reanalysis Dataset ◽  
Near Surface ◽  
The Antarctic

The European Center for Medium-Range Weather Forecasts (ECMWF) released its latest reanalysis dataset named ERA5 in 2017. To assess the performance of ERA5 in Antarctica, we compare the near-surface temperature data from ERA5 and ERA-Interim with the measured data from 41 weather stations. ERA5 has a strong linear relationship with monthly observations, and the statistical significant correlation coefficients (p < 0.05) are higher than 0.95 at all stations selected. The performance of ERA5 shows regional differences, and the correlations are high in West Antarctica and low in East Antarctica. Compared with ERA5, ERA-Interim has a slightly higher linear relationship with observations in the Antarctic Peninsula. ERA5 agrees well with the temperature observations in austral spring, with significant correlation coefficients higher than 0.90 and bias lower than 0.70 °C. The temperature trend from ERA5 is consistent with that from observations, in which a cooling trend dominates East Antarctica and West Antarctica, while a warming trend exists in the Antarctic Peninsula except during austral summer. Generally, ERA5 can effectively represent the temperature changes in Antarctica and its three subregions. Although ERA5 has bias, ERA5 can play an important role as a powerful tool to explore the climate change in Antarctica with sparse in situ observations.

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