Deformation in an accretionary melange, Alexander Island, Antarctica

Alexander Island contains several belts of melange in a wide accretionary complex. One melange belt in the northwest of the island incorporates both oceanic and trench-fill material. It evolved by many different deformation mechanisms: dispersed independent particulate flow (IPF) and limited cataclasis at shallow levels; and diffusion mass transfer (DMT) and limited crystal plastic processes at deeper levels. Fluid pressures may have risen due to the subduction of young hot oceanic crust, which probably affected the structural evolution of the region by controlling the strength of the decollement and hence the taper of the accretionary prism

Role of strike-slip faulting in the tectonic evolution of the Antarctic Peninsula

The Antarctic Peninsula Mesozoic magmatic arc has had a long history of dextral, strike-slip deformation. The deformation was initially associated with the development of a wide accretionary complex, by the migration of fore-arc slivers, and the formation and inversion of a thick fore-arc basin succession. It also formed an important component within major shear zones in the arc, and may have controlled the formation of sedimentary basins in the back-arc region. Although some transcurrent motion within the fore-arc region was related to a component of oblique subduction, the main movement occurred during the breakup of Gondwanaland and the formation of a major transtensional rift system. A new reconstruction for this part of Gondwanaland is presented taking this transcurrent motion into consideration

Mesozoic radiolarian faunas from the Antarctic Peninsula: age, tectonic and palaeoceanographic significance

New assemblages of Radiolaria, including some of the few occurrences of high southern latitude Jurassic and Cretaceous radiolarian faunas, show that several localities in the LeMay Group of Alexander Island range in age from latest Jurassic–earliest Cretaceous to at least Albian. By demonstrating that sedimentation and deformation in the LeMay Group was diachronous, younging oceanwards to the northwest, these new age assessments support the model of the LeMay Group as an accretionary complex. The polarity of subduction beneath Alexander Island was not affected by arc collisions from at least the Lower Jurassic to the Oligocene, and such a long period of continuous accretion appears to be unusual. Deposition of the LeMay Group spans the Kimmeridgian to Albian sedimentation in the Fossil Bluff Group fore-arc basin, thus making the earlier concept of the LeMay Group as pre-Jurassic ‘basement’ untenable. Allochthonous latest Jurassic–earliest Cretaceous radiolarian assemblages with some supposed Tethyan affinities are present in the LeMay Group. In contrast, an in situ latest Jurassic assemblage from the Nordenskjöld Formation of the back-arc basin and a further Jurassic assemblage from a probable trench-slope basin have characteristics believed diagnostic of high latitudes. The biogeographic affinities of radiolarians from cherts in the LeMay Group accretionary complex suggest that both these cherts, and associated basalts, are far-travelled slices of the Phoenix plate. Rocks from the probable trench-slope basin, formerly assigned to the younger Fossil Bluff Group fore-arc basin sequence, now appear to be part of a new, previously unrecognized formation

Field report on combined Anglo–Bulgarian geological studies in northern Alexander Island

The structural basement of Alexander Island consists of a wide belt of accretionary complex rocks, the LeMay Group (Bum 1984). This is overlain by ?Cretaceous-Tertiary calcalkalinevolcanic rocks (Bum 1981), and intruded by several slightly younger, but related, plutons (Care 1983). The Rouen Mountains batholith is a large group of these plutons which occurs in the northern part of the island

Sedimentology and structure of the trench-slope to forearc basin transition in the Mesozoic of Alexander Island, Antarctica

The Mesozoic forearc of Alexander Island, Antarctica, is one of the few places in the world where the original stratigraphic relationship between a forearc basin and an accretionary complex is exposed. Newlydiscovered sedimentary rocks exposed at the western edge of the forearc basin fill (the Kimmeridgian–Albian Fossil Bluff Group) record the events associated with the basin formation. These strata are assigned to the newly defined Selene Nunatak Formation (?Bathonian) and Atoll Nunataks Formation (?Bathonian-Tithonian) within the Fossil Bluff Group.The Selene Nunatak Formation contains variable thicknesses of conglomeratesand sandstones, predominantly derived from the LeMay Group accretionary complex upon which it is unconformable. The formation marks emergence and subsequent erosion of the inner forearc area. It is conformably overlain by the1 km thick Atoll Nunataks Formation, characterized by thinly-bedded mudstones and silty mudstones representing a marine transgression followed by trench-slope deposition. The Atoll Nunataks Formation marks a phase of subsidence, possibly in response to tectonic events in the accretionary prism that are known to have occurred at about the same time.The Atoll Nunataks Formation is conformably overlain by the Himalia Ridge Formation, a thick sequence of basin-wide arc-derived conglomerates. This transition from fine- to coarse-grained deposition suggests that a well-developed depositional trough (and hence trench-slope break) had formed by that time. The Atoll Nunataks Formation therefore spans the formation of the forearc basin, and marks the transition from trench-slope to forearc basin deposition

Hanging wall fault kinematics and footwall collapse in listric growth fault systems

We describe the structure of a listric growth fault system from SE Asia, using high-resolution, 3-D seismic data. The fault system shows systematic changes in geometry and kinematics that are sympathetic with along-strike changes in the structure of the bounding fault. Where the position of the bounding fault remained fixed, there is an overall landward decrease in the age of the hanging wall growth faults. Along strike, three phases of footwall collapse caused by the active bounding fault stepping back into the footwall block were responsible for the punctuated, stepwise, landward migration of the rollover hinge and associated hanging wall growth faults during extension. The migration of these hanging wall structures is similar to that predicted by simple analogue models with fixed detachment surfaces: care should therefore be taken in defining kinematic models in areas where the geometry of the bounding fault is either poorly defined or unknown