Marine sedimentary rocks drilled on the southeastern margin of the South Orkney microcontinent in... more Marine sedimentary rocks drilled on the southeastern margin of the South Orkney microcontinent in Antarctica (Ocean Drilling Program Leg 113 Site 696) were deposited between ∼36.5 Ma to 33.6 Ma, across the Eocene–Oligocene climate transition. The recovered rocks contain abundant grains exhibiting mechanical features diagnostic of iceberg-rafted debris. Sand provenance based on a multi-proxy approach that included petrographic analysis of over 275,000 grains, detrital zircon geochronology and apatite thermochronometry rule out local sources (Antarctic Peninsula or the South Orkney Islands) for the material. Instead the ice-transported grains show a clear provenance from the southern Weddell Sea region, extending from the Ellsworth–Whitmore Mountains of West Antarctica to the coastal region of Dronning Maud Land in East Antarctica. This study provides the first evidence for a continuity of widespread glacier calving along the coastline of the southern Weddell Sea embayment at least 2.5 million yrs before the prominent oxygen isotope event at 34–33.5 Ma that is considered to mark the onset of widespread glaciation of the Antarctic continent.
Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and K... more Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe–Mg)ol–liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminium-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine–melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600 °C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine–spinel pairs indicate crystallisation temperatures from 1391 ± 42 °C to 1481 ± 35 °C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253 ± 64 °C to 1303 ± 40 °C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100 °C lower than the previously presented estimates, they are still ~ 200 °C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.
U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo ... more U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded ...
... at least 3 km thick, and facies analysis indicates deposition as submarine fans along a conti... more ... at least 3 km thick, and facies analysis indicates deposition as submarine fans along a continental margin (Smellie, Roberts & Hirons, 1996 ... 5.e. Jason Peninsula 5.e.1 Occurrence Jason Peninsula, with an area of approximately 3000 km 2 , extends eastward from Graham Land ...
The island of South Georgia exposes remnants of a Late Jurassic to Early Cretaceous Andean magmat... more The island of South Georgia exposes remnants of a Late Jurassic to Early Cretaceous Andean magmatic arc and marginal basin system that was compressively deformed during the mid-Cretaceous main Andean Orogeny forming widespread NW-SE trending folds and a coaxial penetrative cleavage displaying a predominantly NE-SW stretching lineation.Detailed structural studies of the Cooper Bay to Cape Vahsel area of South Georgia reveal that intense, mid-Cretaceous, polyphase deformation was strongly influenced by sinistral strike-slip shear parallel to the NW-SE regional structural grain, and along a major pre-existing fault, which we interpret as the partitioned wrench component of bulk transpressional deformation. The relationship between fold axial plane orientation and interlimb angle of widely distributed mesoscale folds is consistent with counter-clockwise rotation and fold appression as a result of sinistral simple shear deformation, suggesting kinematic strain partitioning of the wrench component was on the whole highly efficient. Locally, the modification of steep tectonic anisotropies to shallow inclinations during D2 deformation induced imperfect or inefficient partitioning with fold arrays exhibiting fold appression characteristic of a transpressional deformation path.Our partitioned transpression model for main Andean deformation of South Georgia fits well with tectonic interpretations of the Cordillera Darwin, Patagonia.
... 1). Precise UPb dating of zircons from the Mount Poster Formation have yielded ages of 188 ±... more ... 1). Precise UPb dating of zircons from the Mount Poster Formation have yielded ages of 188 ± 3 Ma, and 189 ± 3 Ma from Mount Peterson and the Sweeney Mountains (Fig. ... The Latady Formation The Latady Formation of the Orville Coast, eastern Ellsworth Land (Fig. ...
The Mesozoic dyke swarms of Western Dronning Maud Land, Antarctica, form a minor intrusive compon... more The Mesozoic dyke swarms of Western Dronning Maud Land, Antarctica, form a minor intrusive component of the Karoo large igneous province. Five-hundred and sixty one dykes were recorded intruding Neoproterozoic gneisses and Middle Jurassic syenite plutons. 40Ar/39Ar geochronology data reveal two temporally distinct components: the 178–175 Ma, alkaline, Straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton; and the 206–204 Ma, tholeiitic, Jutulrøra dyke swarm found throughout the study area. The Straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction, interpreted to be the result of high magma pressure equal to the maximum principal stress. The Jutulrøra swarm displays a fan of dyke trends, with dyke thickness and spacing increasing away from the inferred point of fan convergence. Anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the Straumsvola area, with the southern/outer exposures of the Jutulrøra swarm exhibiting lateral magma transport. Although associated with a long-lived, local igneous centre comparison of palaeostress estimates for the Straumsvola dyke swarm and contemporaneous dykes in Ahlmannryggen and Vestfjella, indicates the presence of a regional scale radial stress system in western Dronning Maud Land between 178–175 Ma, supporting a mantle plume origin for the Karoo large igneous province.
Marine sedimentary rocks drilled on the southeastern margin of the South Orkney microcontinent in... more Marine sedimentary rocks drilled on the southeastern margin of the South Orkney microcontinent in Antarctica (Ocean Drilling Program Leg 113 Site 696) were deposited between ∼36.5 Ma to 33.6 Ma, across the Eocene–Oligocene climate transition. The recovered rocks contain abundant grains exhibiting mechanical features diagnostic of iceberg-rafted debris. Sand provenance based on a multi-proxy approach that included petrographic analysis of over 275,000 grains, detrital zircon geochronology and apatite thermochronometry rule out local sources (Antarctic Peninsula or the South Orkney Islands) for the material. Instead the ice-transported grains show a clear provenance from the southern Weddell Sea region, extending from the Ellsworth–Whitmore Mountains of West Antarctica to the coastal region of Dronning Maud Land in East Antarctica. This study provides the first evidence for a continuity of widespread glacier calving along the coastline of the southern Weddell Sea embayment at least 2.5 million yrs before the prominent oxygen isotope event at 34–33.5 Ma that is considered to mark the onset of widespread glaciation of the Antarctic continent.
Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and K... more Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe–Mg)ol–liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminium-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine–melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600 °C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine–spinel pairs indicate crystallisation temperatures from 1391 ± 42 °C to 1481 ± 35 °C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253 ± 64 °C to 1303 ± 40 °C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100 °C lower than the previously presented estimates, they are still ~ 200 °C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.
U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo ... more U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded ...
... at least 3 km thick, and facies analysis indicates deposition as submarine fans along a conti... more ... at least 3 km thick, and facies analysis indicates deposition as submarine fans along a continental margin (Smellie, Roberts & Hirons, 1996 ... 5.e. Jason Peninsula 5.e.1 Occurrence Jason Peninsula, with an area of approximately 3000 km 2 , extends eastward from Graham Land ...
The island of South Georgia exposes remnants of a Late Jurassic to Early Cretaceous Andean magmat... more The island of South Georgia exposes remnants of a Late Jurassic to Early Cretaceous Andean magmatic arc and marginal basin system that was compressively deformed during the mid-Cretaceous main Andean Orogeny forming widespread NW-SE trending folds and a coaxial penetrative cleavage displaying a predominantly NE-SW stretching lineation.Detailed structural studies of the Cooper Bay to Cape Vahsel area of South Georgia reveal that intense, mid-Cretaceous, polyphase deformation was strongly influenced by sinistral strike-slip shear parallel to the NW-SE regional structural grain, and along a major pre-existing fault, which we interpret as the partitioned wrench component of bulk transpressional deformation. The relationship between fold axial plane orientation and interlimb angle of widely distributed mesoscale folds is consistent with counter-clockwise rotation and fold appression as a result of sinistral simple shear deformation, suggesting kinematic strain partitioning of the wrench component was on the whole highly efficient. Locally, the modification of steep tectonic anisotropies to shallow inclinations during D2 deformation induced imperfect or inefficient partitioning with fold arrays exhibiting fold appression characteristic of a transpressional deformation path.Our partitioned transpression model for main Andean deformation of South Georgia fits well with tectonic interpretations of the Cordillera Darwin, Patagonia.
... 1). Precise UPb dating of zircons from the Mount Poster Formation have yielded ages of 188 ±... more ... 1). Precise UPb dating of zircons from the Mount Poster Formation have yielded ages of 188 ± 3 Ma, and 189 ± 3 Ma from Mount Peterson and the Sweeney Mountains (Fig. ... The Latady Formation The Latady Formation of the Orville Coast, eastern Ellsworth Land (Fig. ...
The Mesozoic dyke swarms of Western Dronning Maud Land, Antarctica, form a minor intrusive compon... more The Mesozoic dyke swarms of Western Dronning Maud Land, Antarctica, form a minor intrusive component of the Karoo large igneous province. Five-hundred and sixty one dykes were recorded intruding Neoproterozoic gneisses and Middle Jurassic syenite plutons. 40Ar/39Ar geochronology data reveal two temporally distinct components: the 178–175 Ma, alkaline, Straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton; and the 206–204 Ma, tholeiitic, Jutulrøra dyke swarm found throughout the study area. The Straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction, interpreted to be the result of high magma pressure equal to the maximum principal stress. The Jutulrøra swarm displays a fan of dyke trends, with dyke thickness and spacing increasing away from the inferred point of fan convergence. Anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the Straumsvola area, with the southern/outer exposures of the Jutulrøra swarm exhibiting lateral magma transport. Although associated with a long-lived, local igneous centre comparison of palaeostress estimates for the Straumsvola dyke swarm and contemporaneous dykes in Ahlmannryggen and Vestfjella, indicates the presence of a regional scale radial stress system in western Dronning Maud Land between 178–175 Ma, supporting a mantle plume origin for the Karoo large igneous province.
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