The Ahlmannryggen group, western Dronning Maud Land, Antarctica.

Abstract

The Mesoproterozoic Ritscherflya Supergroup forms an extensive volcano-sedimentary cover succession on the Archaean Grunehogna Province of western Dromring Maud Land, Antarctica. The oldest:, predominantly sedimentary deposits of this cover succession are exposed across the Borgmassivet and southern Ahlmannryggen mountain ranges, and are collectively assigned to the Ahlmannryggen Group. A revised lithostratigraphy places exposures from these two regions in separate subdivisions, with three formations being recognised in the Ahlmannryggen (Pyramiden, Schumacherfjellet and Grunehogna Formations) and four formations being defined for the Borgmassivet (Veten, Framryggen, HogfOlma and Brapiggen Formations). Deposition of these successions occurred in a combination of fluvial braid-plain and braid-delta plain environments, with exposures in the Ahlmannryggen and Borgmassivet regions representing contemporaneous sedimentation in different portions of the same basin, under similar conditions. The development of the Ahlmannryggen Group basin is attributed to flexing associated with continental collision during the assembly of Rodinia. Collision and accretion of a continental island arc terrain (the Maudheim Province) along the southern margin of the Grunehogna Province is considered responsible for flexural snbsidence and the development of a peripheral foreland basin. The Ahlmannryggen Group represents 'molasse' stage infilling of this basin, with sedimentation being dominated by a combination of transverse and longitudinal drainage systems entering a depo-centre located to the east/southeast of the presently exposed succession. SAMANTIIA PERRlTT Detritus entering the basin was sourced either directly or indirectly from at least seven different terrains, aged ca. 1135Ma, ca. 1335Ma, ca. 1600-1700Ma, ca. 2000-2100, ca. 2645Ma, ca. 2400-2900Ma and ca 2900-3300Ma, according to UlPb detrital zircon SHRIMP analysis. The source terrains included the Maudheim Province, basement granites of the Grunehogna Province, an older sedimentary terrain dominated by a banded ironstone association, at least two further magmatic provinces and two metamorphic terrains. Of these source terrains, only the Maudheim Province and Grunehogna Province basement granites are presently exposed in western Dromring Maud Land The subsequent development of large-scale buckle folds and extensive brittle deformation within the Grunehogna Province cover rocks is attributed to the formation of a regionally extensive sinistral strike-slip system during NNW-SSE Pan-African compression, and can be correlated to structures exposed in the Maudheim Province and northern Mozambique. It is proposed that this strike-slip system developed in response to escape tectonics operating during a late stage of Gondwana amalgamation, as a result of the Ross Orogeny, and the suturing of East and West Antarctica