The lithostratigraphy and structural components of the Eureka Shear Zone, southern Namibia
Date
2016-12
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The newly recognised Eureka Shear Zone (ESZ) in southern Namibia is a Late-Mesoproterozoic to Early
Neoproterozoic high strain zone in the Namaqua sector of the Namaqua Natal Metamorphic Province
(NNMP). Its current outcrop trace is closely aligned with the previously identified terrane boundary
between the Paleoproterozoic Richtersveld Subprovince and the Mesoproterozoic Gordonia Subprovince
(South Africa) or Grünau Terrane (Namibia). This has given rise to the question of what this high strain
zone represents in terms of its lithostratigraphy and tectonic history. A joint mapping program between
the Geological Survey of Namibia and the Council for Geoscience in South Africa has redefined and
clarified aspects of the regional geology and the relationship between different crustal blocks within the
Namaqua sector. Chief amongst these are the redefinition of: (1) the Paleoproterozoic Richtersveld
Subprovince into the Richtersveld Magmatic Arc (comprised of the greenschist-facies Vioolsdrif Domain
and the amphibolite-facies Pella Domain) in both South Africa and Namibia; (2) the Mesoproterozoic
Grünau Terrane into the Kakamas Domain in Namibia; (3) the SW-vergent, subhorizontal Lower Fish
River-Onseepkans Thrust (LFROT) that juxtaposes the granulite-facies Kakamas Domain on top of the
Pella Domain and (4) a zone of highly deformed and imbricated units on top of the LFROT that reworks
the Kakamas Domain, referred to as the Lower-Fish-River-Onseepkans Thrust Zone (LFROTZ) and
which also includes unique units not found in the Kakamas or Pella domains. In this study, detailed field
mapping, petrography, structural analysis and U-Pb zircon dating on the ESZ was undertaken to
understand its relationship to the LFROTZ and the nearby Marshall Rocks-Pofadder Shear Zone
(MRPSZ) with a view to understanding the evolution of the ESZ in the broader context of the Namaqua
sector and NNMP. The ESZ is a ~2 to 5 km wide, 50 km long and WNW-trending zone of intense
shearing. It is dominantly composed of two distinct lithostratigraphic packages: (1) the Eureka Complex
(EC) in the north, a heterogeneous sheared garnet-bearing gneiss with isolated m-scale and lens-shaped
remnants of pelitic granulite whose metamorphic grade and detrital zircon patterns correlate it to the
Kakamas Domain; and (2) fault/thrust-bounded packages of interlayered quartz-feldspar gneiss and
amphibolite (MQ), that occur as sigmoid-shaped mega-lenses within the EC and which are correlated to
units considered to be unique to the LFROTZ. South of the EC and MQ units, the ESZ shears migmatitic volcanoclastic rocks of the Pella Domain. Petrographic studies indicate that the rocks of the ESZ were
affected by retrograde metamorphism at mid to upper greenschist facies during shearing. Fault rock
textures in the ESZ vary from phyllonite to cataclasite dominant, with rarer mylonitic textures. This implies
a complex interplay between rheological contrast, slip rate and crustal depth under shearing conditions
where potassium feldspar was brittle and quartz was plastic. The orientation of the shear fabrics varies
with the ESZ but maintains a dominant WNW strike steeply dipping shear foliation accompanied by a NE
trending and moderately to steeply plunging lineation. The ESZ was intruded by a significant volume of
foliation sub-parallel, sheet-like, pegmatite and leucocratic equigranular granite dykes which date to
~1000 Ma and constrain the age of the shear zone. The geometry, age and strain regime of ESZ is
similar to the adjacent MRPSZ, but its lithostratigraphy reveal the presence of rocks found in the LFROTZ
making the ESZ a younger shear zone exploiting the older terrane boundary between the Kakamas and
Pella domain.
AFRIKAANS ABSTRACT: Geen opsomming beskikbaar
AFRIKAANS ABSTRACT: Geen opsomming beskikbaar
Description
Thesis (MSc)--Stellenbosch University, 2016.
Keywords
Lithostratigraphy -- Eureka Shear Zone --Southern Namibia, Sedimentology, Geology, Stratigraphic, UCTD