ALBERT

Description: Late Cretaceous–early Tertiary subduction-related fore-arc volcanic rocks are exposed in a north–south linear belt along the length of Alexander Island. The age and tectonic setting of these rocks is well understood; they are not considered to represent “normal” arc magmas but were generated in the fore-arc as a result of ridge subduction. Due to their distinct composition and mode of formation, they are no longer considered to be genetically related to the Antarctic Peninsula magmatic arc. They are therefore removed from the Antarctic Peninsula Volcanic Group and placed in a newly defined Alexander Island Volcanic Group. The group is made up of the Monteverdi, Staccato, Walton, Colbert, Elgar and Finlandia formations, which vary widely in lithology, facies and age. The Colbert and Elgar formations are subdivided into nine and three members respectively. Type localities, representative lithologies and age of each of the formations are discussed. The Staccato and Colbert Magmatic complexes are defined to include volcanic and plutonic rocks that are considered to be coeval. The Rouen Intrusive complex combines the plutonic rocks from the Rouen Mountains and Rothschild Island on the basis of age and chemistry.

Description: Fore-arc magmatic sequences associated with high Mg number andesite lavas unconformably overlie LeMay Group accretionary complex in Alexander Island. High-resolution 40Ar/39Ar, U–Pb zircon, fission track and K–Ar ages demonstrate that subduction-related fore-arc magmatism migrated northwards along the length of Alexander Island between c. 80 Ma and c. 46 Ma. The magmatic rocks represent a third of the western margin of the Antarctic Peninsula magmatic arc and are critical to the understanding of the final phase of subduction along the southern Antarctic Peninsula margin. The onset of late Cretaceous magmatism is recorded by poorly exposed volcanic rocks on Monteverdi Peninsula (79.7±2.5 Ma). In central and northern Alexander Island, the magmatic rocks can be distinguished by the proportion, range and types of lithofacies present, and by the periods of magmatism represented. The volcanic rocks of the Colbert Mountains range in age from c. 69–62 Ma and are dominated by large volume dacitic and rhyolitic crystal-rich ignimbrites interpreted as caldera-fill deposits. Elgar Uplands sequences range in age from c. 55–50 Ma, and contain approximately equal proportions of pyroclastic deposits and less evolved (basaltic-andesite and andesite) lavas including high Mg number andesite lavas near the base of three sequences. The volcanic rocks of Finlandia Foothills probably represent the youngest calc-alkaline units on Alexander Island (48±2 Ma). The sequence is lithologically similar to the Elgar Uplands and also contains high Mg number andesite lavas, but it is dominated by polymict conglomerates, with minor lavas, which were deposited in a graben associated with regional extension. Plutonic rocks exposed in the Rouen Mountains, adjacent to the Elgar Uplands, yielded a U–Pb age of 56±3 Ma which is in discordance with a previously published Rb–Sr age (46±3 Ma), probably due to hydrothermal perturbation of the Rb–Sr system. Northwards migration of magmatism was caused by the progressive collision and subduction of three ridge segments prior to the previously reported ridge crest–trench collisions that occurred c. 20–30 Ma later and following which subduction ceased.

Description: The collision of India and Asia can be defined as a process that started with the closing of the Tethyan ocean that, during Mesozoic and early Tertiary times, separated the two continental plates. Following initial contact of Indian and Asian continental crust, the Indian plate continued its northward drift into Asia, a process which continues to this day. In the Ladakh–Zanskar Himalaya the youngest marine sediments, both in the Indus suture zone and along the northern continental margin of India, are lowermost Eocene Nummulitic limestones dated at ∼54 Ma. Along the north Indian shelf margin, southwest-facing folded Palaeocene–Lower Eocene shallow-marine limestones unconformably overlie highly deformed Mesozoic shelf carbonates and allochthonous Upper Cretaceous shales, indicating an initial deformation event during the latest Cretaceous–early Palaeocene, corresponding with the timing of obduction of the Spontang ophiolite onto the Indian margin. It is suggested here that all the ophiolites from Oman, along western Pakistan (Bela, Muslim Bagh, Zhob and Waziristan) to the Spontang and Amlang-la ophiolites in the Himalaya were obducted during the late Cretaceous and earliest Palaeocene, prior to the closing of Tethys.The major phase of crustal shortening followed the India–Asia collision producing spectacular folds and thrusts across the Zanskar range. A new structural profile across the Indian continental margin along the Zanskar River gorge is presented here. Four main units are separated by major detachments including both normal faults (e.g. Zanskar, Karsha Detachments), southwest-directed thrusts reactivated as northeast-directed normal faults (e.g. Zangla Detachment), breakback thrusts (e.g. Photoksar Thrust) and late Tertiary backthrusts (e.g. Zanskar Backthrust). The normal faults place younger rocks onto older and separate two units, both showing compressional tectonics, but have no net crustal extension across them. Rather, they are related to rapid exhumation of the structurally lower, middle and deep crustal metamorphic rocks of the High Himalaya along the footwall of the Zanskar Detachment. The backthrusting affects the northern margin of the Zanskar shelf and the entire Indus suture zone, including the mid-Eocene–Miocene post-collisional fluvial and lacustrine molasse sediments (Indus Group), and therefore must be Pliocene–Pleistocene in age. Minimum amounts of crustal shortening across the Indian continental margin are 150–170 km although extreme ductile folding makes any balancing exercise questionable.