Proterozoic geological evolution of the northern Vestfold Hills, Antarctica

Abstract

The presence of polyphase shear zones transected by several suites of dolerite dykes in Archaean basement of the Vestfold Hills, East Antarctica, allows a detailed reconstruction of the local structural evolution. Archaean and early Proterozoic deformation at granulite facies conditions was followed by two phases of dolerite intrusion and mylonite generation in strike-slip zones at amphibolite facies conditions. A subsequent middle Proterozoic phase of brittle normal faulting led to the development of pseudotachylite, predating intrusion of the major swarm of dolerite dykes around 1250 Ma. During the later stages and following this event, pseudotachylite veins were reactivated as ductile, mylonitic thrusts under prograde conditions, culminating in amphibolite facies metamorphism around 1000–1100 Ma. This is possibly part of a large-scale tectonic event during which the Vestfold block was overthrust from the south. In a final phase of strike-slip deformation, several pulses of pseudotachylite-generating brittle faulting alternated with ductile reactivation of pseudotachylite.

Early Neoproterozoic granulite facies metamorphism of mafic dykes from the Vestfold Block, east Antarctica

“…This indicates that at least the southwestern margin of the Vestfold Block underwent tectonic burial of ~20 km to lower crustal levels (~30–35 km) during the Rayner orogeny. On the other hand, although mafic dykes from the northeastern Vestfold Block appear to be undeformed and unmetamorphosed, some ductile mylonites transecting major mafic dykes have amphibolite facies minerals in their fine‐grained matrix (Kuehner, ; Passchier et al ., ). These metamorphic conditions have been estimated to be 550–600 °C and 6 kbar (Kuehner, ).…”

“…What factors prevented and delayed kinetic reactions amongst minerals during metamorphism of the mafic dykes? Structural studies suggest that only limited compressional deformation characterized by the sharply bound, ductile, <15 cm wide mylonite zones occurred in the Vestfold Block during the late Mesoproterozoic to early Neoproterozoic (Passchier et al ., ; Hoek et al ., ; Dirks et al ., ). Apart from the development of millimetre‐sized garnet‐bearing zones along a few fractures in some mafic dykes, most of the dykes were unaffected by this deformation event, suggesting that the Vestfold Block was relatively rigid during metamorphism.…”

“…Pressure estimates from the igneous mineral paragenesis of the 2240 Ma mafic dykes and metamorphic mineral assemblages of sapphirine‐bearing granulite xenoliths trapped in the dykes suggest that the Vestfold Block basement was exhumed to within 8–11 km of the surface during the early Palaeoproterozoic (Seitz, ; Harley & Christy, ). Subsequently, the Vestfold Block resided at shallow crustal levels (10–13 km) until the middle Mesoproterozoic, as evidenced by the development of shear zones, brittle faults and pseudotachylites during emplacement of the mafic dyke swarms (Passchier et al ., ; Hoek et al ., ; Dirks et al ., ). This indicates that at least the southwestern margin of the Vestfold Block underwent tectonic burial of ~20 km to lower crustal levels (~30–35 km) during the Rayner orogeny.…”

“…In addition, alkaline and ultramafic lamprophyric dykes were emplaced before the intrusion of Group II or III mafic dykes. In the northern part of the Vestfold Block, the dykes are un‐deformed and display chilled margins and baked contacts, but in the southeastern part of the Vestfold Block, some dykes are metamorphosed and locally deformed (Collerson & Sheraton, ; Passchier et al ., ). Garnet is developed along dyke margins and in fracture zones within and across the dykes, and such features have been interpreted as the result of recrystallization during a Neoproterozoic or younger (1000–500 Ma) amphibolite facies metamorphism (600–660 °C and 5.6–7.5 kbar) (Kuehner, ; Kuehner & Green, ; Hoek et al ., ; Snape et al ., ; Zulbati & Harley, ).…”

“…One of the most important features of the Vestfold Block is the occurrence of several mafic dyke swarms of Proterozoic age (Black et al ., ; Lanyon et al ., ). Furthermore, some of these mafic dykes experienced amphibolite facies recrystallization and deformation (Collerson & Sheraton, ; Kuehner & Green, ; Passchier et al ., ), although the metamorphic age remains unconstrained. However, careful petrographic observations indicate that mafic dykes from the southwestern Vestfold Block underwent heterogeneous granulite facies metamorphism (Liu et al ., ).…”