Anne M. Grunow

Changing magmatic and tectonic styles along the paleo‐Pacific margin of Gondwana and the onset of early Paleozoic magmatism in Antarctica

Basement rocks of the Transantarctic Mountains are believed to record a change in the paleo-Pacific margin of Gondwana from a passive to a tectonically active margin. Widespread emplacement of calc-alkaline batholiths (Granite Harbor intrusives) occurred during the active margin phase. We present new concordant zircon and titanite U-Pb ages for these magmatic rocks in southern Victoria Land and the Scott Glacier area. Most magmatic rocks previously associated with a pre-late Early Cambrian (>530 Ma) deformational event(s) (Beardmore orogeny) have yielded younger crystallization ages. The lack of definite arc magmatism prior to ∼530 Ma suggests that deformation may have been associated with a strike or oblique-slip regime, although shallow subduction without significant arc magmatism cannot be ruled out. Local transpressional and transtensional domains may account for compressional deformation and rare alkaline and carbonatite magmatism during this early period. The oldest and most voluminous magmatic rocks were emplaced after ∼530 Ma. This magmatism has been associated with active subduction, and suggests a fundamental change in the plate boundary at ∼530 Ma. Ductile shearing of plutons and contractional deformation of supracrustal rocks after ∼530 Ma (Ross orogeny) may have been due to transpressional tectonics in an oblique subduction setting and/or a collision. Compressional deformation associated with the Ross orogeny may have ceased by ∼500 Ma along the southern Victoria Land-Scott Glacier segment of the Antarctic margin, as indicated by undeformed magmatic rocks of this age, although magmatic activity continued to at least ∼485 Ma.

Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: New data from the Antarctic Peninsula and southernmost Andes

Subduction complexes along the Andean margin in central and southern Chile yield mid-Paleozoic to lower Mesozoic ages, yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length. The scarcity of uplifted subduction-complex rocks younger than mid-Mesozoic along the Chilean margin and in parts of the Scotia Arc suggests to us that these old, crystalline rocks, uplifted in the Triassic and Jurassic, represent a boundary in the forearc beyond which tectonic erosion does not easily occur. Greenschist-, blueschist-, and amphibolite-facies subduction-complex rocks from the Scotia Arc were originally thought to be a simple continuation of the subduction complexes in Chile. Based on new 40 Ar/ 39 Ar ages, the Scotia Arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in Chile. Structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the Scotia Arc occurred at some depth in a subduction zone or zones, certainly below the brittle-ductile transition, and in some cases under blueschist-facies conditions. We believe that the early subduction-related deformation and metamorphism in the greenschist- and blueschist-facies rocks of the Scotia Arc to be overprinted by mid-Cretaceous transpression along the South America-Antarctica plate boundary in the case of Elephant Island, transpression and subsequent localized transtension in the earliest Cenozoic in the case of the Darwin Complex, a mid-Cenozoic spreading-rate change in the case of Smith Island, and early Neogene initiation of Drake Passage opening/Shackleton Fracture Zone formation in the case of the Gibbs Island subgroup.

Late Gondwanide tectonic rotations within Gondwanaland

Geologic and paleomagnetic evidence from the Ellsworth-Whitmore mountains crustal block of West Antarctica, and from the Falkland Islands of South America, indicates that tectonic displacement of major portions of the “Samfrau geosyncline” occurred after the early Mesozoic Gondwanide folding but prior to seafloor spreading. Comparison with continental deformation in the collisional Alpine-Himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the Gondwanian orogeny resulted from collision of a fore-arc and magmatic arc terrane with the Panthalassic margin of the Gondwana craton during closure of a marginal basin. The displacements had major consequences for the ensuing evolution of the South Atlantic-Weddell Sea-Ross Sea region.

The Pre‐Cenozoic magmatic history of the Thurston Island Crustal Block, west Antarctica

New Rb-Sr and K-Ar geochronological data are presented for the majority of known pre-Cenozoic outcrops in Thurston Island, the Jones Mountains, and the western Eights Coast, which collectively represent the basement geology of the Thurston Island crustal block of West Antarctica. Almost all are of calc-alkaline igneous or metaigneous rocks, and indicate long-standing proximity to a magmatic arc. The observable history began with Late Carboniferous (309±5 Ma) emplacement of mantle-derived orthogneiss precursors in eastern Thurston Island. Nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200–1400 Ma throughout the area. A variety of cumulate gabbros was emplaced soon after gneiss formation, followed by crust-contaminated diorites that have Triassic mineral cooling dates of 240–220 Ma. In the nearby Jones Mountains, the oldest exposed rock is a muscovite-bearing granite with an Early Jurassic age of 198±2 Ma; its initial 87Sr/86Sr ratio of 0.710 and ϵNdt values of −5 to −7 indicate either anatexis or, at least, a high degree of crustal input during magma genesis. This belongs to a suite of such granites known throughout the Antarctic Peninsula and related to earliest rifting of the Gondwana supercontinent. The subsequent evolution of the Thurston Island area was dominated by I-type magmatism, apparently in two major episodes at 152–142 Ma (Late Jurassic granites) and 125–110 Ma (Early Cretaceous bimodal suite). Most of these magmas had initial 87Sr/86Sr ratios of 0.705–0.706 and ϵNdt values of +2 to −4 and were derived from slightly enriched mantle or from juvenile lower crust. They are thought to signify subduction of Pacific Ocean floor as in the adjacent parts of West Antarctica, although the Late Jurassic episode was of greater intensity in Thurston Island than elsewhere. The Cretaceous magmatism was intense and of Andean-type. Between 100 and 90 Ma, volcanism in the Jones Mountains became predominantly silicic, with increasing incorporation of crustal components (initial 87Sr/86Sr ratios of 0.706–0.709 and ϵNdt values of −3 to −6), as subduction-related magmatism ceased in this part of the margin.

Extensional tectonics and the fragmentation of Gondwanaland

Summary Evidence of widespread lithospheric extension, bimodal volcanism, and uplift and erosion, accompanying the fragmentation of Gondwanaland, is reviewed. One regime of extensional phenomena appears to be intimately associated with a Pacific-margin convergent-plate regime. A second appears to bear no direct geometrical relationship to the margin, but may reflect a thermal anomaly that resulted in the breakup of Gondwanaland. Possible causes of this anomaly are discussed. The two regimes overlap in space and time in the South Atlantic-Weddell Sea area. A causal relationship between the two is possible but remains non-proven.

A U‐Pb Age for the Cambrian Taylor Formation, Antarctica: Implications for the Cambrian Time Scale

Isotopic ages for the Middle Cambrian–Late Cambrian are few because formations of the appropriate age combining diagnostic fossils and rocks datable by precise isotopic techniques are not common. The Taylor Formation in Antarctica includes carbonate units that host trilobites, indicating an early late Middle Cambrian age. Volcanics below and above the trilobite‐bearing horizon yielded zircons that date the horizon at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Late Sinistral Shearing along Gondwana's Paleo-Pacific Margin in the Ross Orogen, Antarctica: New Structure and Age Data from the O'Brien Peak Area

505.1\pm 1.3

Correlation and Late-Stage Deformation of Liv Group Volcanics in the Ross-Delamerian Orogen, Antarctica, from New U-Pb Ages

\end{document} Ma. This age suggests that the Middle‐Late Cambrian boundary is less than 505 Ma, consistent with a ∼500 Ma boundary and a duration of ∼10 m.yr. each for the Middle and Late Cambrian.