Abstract

Zircon and apatite fission-track data from the LeMay Group accretionary complex and Fossil Bluff Group fore-arc basin sequence on Alexander Island (Antarctica) record a common regional Cretaceous and Cenozoic thermal and denudational history. With the exception of zircon data from samples closest to the trench, the apatite and zircon central ages are substantially less than known and, inferred stratigraphic ages. Thermal modelling of the data indicate cooling from maximum palaeotemperatures in the range 180–350°C at c. 100 Ma. A younger period of accelerated cooling occurred between 40 and 35 Ma with final cooling to surface temperatures taking place at reduced rates through the Tertiary. The start of cooling was close in time to the end of deposition within the fore-arc basin and is consistent with structural evidence for Cretaceous deformation in a strike-slip setting. The accelerated, Early Tertiary cooling episode was broadly coeval with, and may have been caused by ridge–trench collisions and cessation of subduction off-shore Alexander Island.

Zircon and apatite age data, from magmatic rocks emplaced in the fore-arc region in Late Cretaceous and Tertiary times, are close to their age of crystallization. This indicates rapid cooling of both fission-track systems from temperatures >350°C to <c. 60°C at high crustal levels of emplacement. The formation of these magmatic rocks, together with the cooling history of the accretionary prism, are related to changes in subduction zone parameters during Cretaceous and Cenozoic times. Slowing of subduction rates and role-back of the slab prior to cessation of subduction by ridge–trench collision may have been responsible for migration of the magmatic focus into the fore-arc region, uplift of the prism and formation of the high mountain ranges, and the extensional graben that separates Alexander Island from the Antarctic Peninsula.

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