Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The close spatial and temporal relationship of shear-zones and magmas is commonly interpreted to
indicate positive feedback between magma migration, granitic emplacement and shear-zone-associated
deformation. Emplacement geometries and structural fabrics are however rarely preserved, hampering the
study of shear-zones and granitic magmas interactions. This study focuses on an area around the Pofadder
Shear Zone (PSZ) in Namibia and South Africa as a case study for granitic bodies, mainly as pegmatite sills
and dykes, and their spatial and temporal relationships to a crustal-scale shear-zone.
The PSZ is a NW-SE trending, dextral, Mesoproterozoic-Neoproterozoic transpressional shear-zone in
Namaqualand, interpreted to have accommodated late-stage lateral escape of the Namaqua Metamorphic
Complex in response to southward indentation of the Kaapvaal Craton around 1030 – 1080 Ma. In this
study it is shown that the shear-core records an asymmetrical strain variation across the PSZ. This is
indicated by pervasively banded ultramylonites, mylonites and the significant development of pervasive
phyllonites at the southern margin, defining the internal ductile to brittle-ductile fabrics of the shear,
during a progressive deformational evolution. Mapping of the PSZ fabrics and associated pegmatites
documents how pegmatites are emplaced in structurally distinctive sites within, and adjacent to the PSZ.
New U-Pb monazite ages derived from this study, show how pegmatite emplacement has occurred at
different times of shear-zone development. The pegmatites are emplaced into earlier ductile to later
brittle-retrograde fabrics that accompanied the ca. 45 Ma shear-zone exhumation. Pegmatites
concentrated along the northern PSZ-margin are interpreted to be controlled by anisotropies developed
axial planar to large km-scale and parasitic folds during the initial, predominately strike-slip stages of shearzone
deformation that occurred as early as 1005 ± 5 Ma. Within the PSZ core, pegmatite emplacement is
controlled by the syn-kinematic development of (a) subvertical, mylonitic and phyllonitic foliations and (b)
fracture permeabilities created by synthetic Riedel shears and dextral dilatant jogs. The most significant
pegmatite development around the PSZ is the Skimmelberg Pegmatite Stockwork (SPS) which forms an
extensive interconnecting network of concurrent, foliation-parallel sills and thick (> 50 m) discordant dykes
within the southern footwall of the PSZ. The dykes intrude as late as 958 ± 5 Ma into feather-shaped N-S
extensional fractures (mode I) that developed due to episodic stick-slip at the boundary between the PSZ
core and footwall rocks during periods of late-stage transpression. The SPS forms a steeply dipping fracture
network that not only creates space needed for emplacement but effectively acts as a conduit for magma
transport along the margin of the PSZ. The large extensional fractures of SPS create the necessary hydraulic
gradients to tap the magma source of a regional trending pegmatite belt and form a sheeted complex
adjacent to the PSZ. Therefore, this study documents how, during the progressive exhumation of a largescale
transcurrent shear-zone, magma emplacement is not only concentrated within the highly permeable,
high-strain domains (cores) of shear-zones but may be concentrated in diachronous, structurally controlled
sites along the shear-zone margins. / AFRIKAANSE OPSOMMING: Die noue ruimte-tydsverband tussen skuifskeure en magmas word algemeen geïnterpreteer as ‘n
aanduiding van positiewe terugkoppeling tussen magma migrasie, graniet-inplasing en skuifskeurgeassosieerde
vervorming. Geometrie en struktuurmaaksels van inplasings word egter selde bewaar en
belemmer die studie van interaksies tussen skuifskeure en graniet-magmas. Die studie fokus op ‘n area
rondom die Pofadder Skuifskeur (PSS) in Namibië en Suid-Afrika as ‘n gevallestudie vir graniet-liggame,
hoofsaaklik as pegmatiet plate en gange, asook voorafgenoemde se ruimte-tydsverband met ‘n
grootskaalse skuifskeur.
Die PSS is ‘n NW-SO-waarts strekkende, regs-laterale, Mesoproterosoïse-Neoproterosoïse transpressieskuifskeur
in Namakwaland, wat geïnterpreteer word om die latere-stadium laterale ontsnapping van die
Namakwa Metamorfiese Kompleks te akkomodeer in reaksie op die suidwaartse indrukking van die
Kaapvaal Kraton omstreeks 1030-1080 Ma. In hierdie studie word getoon dat asimmetriese
vervormingsvariasie deur die skuifskeurkern aangeteken word. Hierdie word aangetoon deur gebande
ultramilioniete, milioniete en die noemenswaardige ontwikkeling van filoniete wat die suidelike rand
deurtrek en definiëer die interne plastiese- tot bros-plastiese maaksels van die skuifskeur gedurende ‘n
progressiewe vervormingsevolusie. Kartering van die PSS maaksels en geassosieerde pegmatiete
dokumenteer hoe pegmatiete, aangrensend en binne die PSS, in eiesoortige strukturele terreine
binnedring. Nuwe U-Pb monasiet ouderdomme, afgelei vanuit hierdie studie, toon aan hoe inplasing
plaasgevind het gedurende verskillende tye van skuifskeurontwikkeling. Pegmatiete het vroeëre plastiesetot
latere bros-retrogressiewe maaksels binnegedring wat die herontbloting van die ca. 45 Ma skuifskeur
meegaan. Pegmatiete, gekonsentreerd langs die noordelike rand van die PSS, word geïnterpreteer as
beheer deur anisotrope wat parallel aan die asvlak van groot km-skaalse en ondergeskikte plooie ontwikkel
gedurende die aanvanklike, hoofsaaklik strekkingwaarste, stadiums van skuifskeurontwikkeling wat so
vroeg as 1005 ±5 Ma plaasgevind het. Binne die kern van die PSS word die inplasing van pegmatiete beheer
deur die sinkenimatiese ontwikkeling van (a) subvertikale, milionitiese- en filonitiese foliasies en (b) breukdeurdringbaarheid
wat gevorm is deur sintetiese riedelskuifskeure en regslaterale uitsettende “jogs”. Die
mees noemenswaardige pegmatiet ontwikkeling rondom die PSS is die Skimmelberg Pegmatiet Stokwerk
(SPS) wat ‘n intensiewe netwerk vorm van intergekonnekteerde konkurrente plate, parallel aan die foliasie,
en dik (>50m) diskordante gange binne die suidelike vloer van die PSS. Die gange dring in so laat as 958 ± 5
Ma binne-in veervormige N-S uitbreidende breuke (modus1) wat ontwikkel het as gevolg van die episodiese
hak-en-glip op die grens tussen die PSS kern- en vloergesteentes gedurende periodes van laat-stadium
transpressie. Die SPS vorm ‘n styl hellende breuk-netwerk wat nie net spasie maak vir indringing nie, maar
dien ook effektief as ‘n geleidingsweg vir die vervoer van magma langs die rand van die PSS. Die groot
uitbreidende breuke van die SPS skep die nodige hidroliese gradiënt om die magma bron van ‘n regionale
pegmatiet gordel te tap en vorm ‘n bladvormige kompleks aangrensend tot die PSS. Gevolglik dokumenteer die studie hoe, gedurende die progressiewe ontbloting van ‘n grootskaalse
torsieskuifskeur, magma inplasing nie net gekonsentreer is binne die hoogs deurdringbare,
hoogsvervormde areas (kerne) van skuifskeure nie, maar ook hoe magma kan konsentreer in diachroniese,
struktuur beheerde gebiede teen die rande van skuifskeure.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/85682 |
| Date | 12 1900 |
| Creators | Lambert, Christopher William |
| Contributors | Kisters, Alexander F. M., Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 128 p. : ill., maps |
| Rights | Stellenbosch University |
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