Kinematic evolution of the Miller Range Shear Zone, Central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to Early Paleozoic tectonics of the East Antarctic Margin of Gondwana

John W. Goodge, Vicki L. Hansen, Simon M. Peacock, Brad K. Smith, Nicholas W. Walker

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Abstract

High‐grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12–15 km. Shear‐sense indicators record consistent top‐to‐the‐SE, or left‐lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal‐plane slip, affirming both the consistency of shear sense and high‐grade syn‐kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal‐plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape‐preferred orientation, and (3) the presence of oriented muscovite “fish” included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left‐lateral (top‐to‐the‐SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen‐parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen‐normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left‐oblique convergence in which strain within the orogen was partitioned into deep‐level strike slip and shallow‐level contraction.

Original languageEnglish (US)
Pages (from-to)1460-1478
Number of pages19
JournalTectonics
Volume12
Issue number6
DOIs
StatePublished - Dec 1993
Externally publishedYes

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