Thesis (MSc (Earth Sciences))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The NE-trending South Central Zone of the Pan-African (ca. 550-500) Damara orogen in central
Namibia exposes deeply eroded mid-crustal rocks, thought to represent the magmatic arc of the
Damara orogen. Above average exposure of outcrop left unmodified by subsequent post-orogenic
processes made it possible to study the internal architecture of a ca. 50km traverse, stretching from
the continental suture-zone (between the Congo craton in the NW and the underplating Kalahari
craton in the SE) at the Okahandja Lineament Zone, well into the leading edge of the Congo craton
and into the magmatic-arc, the South Central Zone. This study considers and characterises the
change in structural styles and strain intensities in rocks of the Damara Supergroup and intrusions,
across the traverse between the towns of Otjimbingwe in the SE and Karibib in the NW.
In the SE of the traverse in the Okahandja Lineament Zone, steep, upright, tightly folded D2 fabrics in
meta-turbidites of the Tinkas and Kuiseb Formations record bulk NW-SE shortening and steep SW
extrusion of rocks. Penetrative non-coaxial fabrics imply a high-angle collsion between the Congo
and Kalahari cratons. This is in contrast to oblique collision described by a number of previous
authors (e.g. Blaine (1977), Stanistreet et al. (1991), Tack & Bowden, 1999). A marked decrease in D2
strain, and the presence of the silisiclastic basal Nosib group suggests the presence of the underlying
basement rocks and thus the leading edge of the Congo craton only a few km NW of the Okahandja
lineament. 8km NW of the Okahandja lineament is a km-scale NW verging F1 nappe, cored by
basement gneisses and refolded into a series of bi-vergent, doubly-plunging F2 folds, the Audawib
fold complex. The nappe is interpreted to have formed along a retroshear during early continental
collision (syn-D1, early-D2). Tectonically overprinted basement rocks are indicative of thermal
weakening, that resulted in the development of thick-skinned tectonics. Intruded mainly to the NW
of and around the aforementioned nappe are the areally extensive syn-D2 Salem-type granites.
Salem-type granites are shallowly intruded below the nappe and have likely detached the F1 nappe
from its root. NW of the Salem-type granites lies a basement window of ca. 15km2 surrounded by
the lower formations of the Damara Supergroup. Sheared marbles and D1 (early D2) diorites along
the basement contact indicate a shallow sheared detachment occurring just above the basement.
Basement rocks (1) unaffected by Damaran (D1-D2) tectonism and (2) unconformably overlain by
the Damara Supergroup are indicative of thin skin tectonics in this part of the South Central Zone,
some 30km NW of the Okahandja Lineament zone. Intrusive rocks across the South Central Zone
suggest that deformation in the NW ceased by 540 Ma, while deformation along the Okahandja
Lineament continued until at least 520 Ma.
iii
Along the Okahandja lineament, high angle continental collision resulted in tight, co-axial folding and
lateral extrusion of rocks along the continental backstop. The introduction of numerous late-D2
granites around the Okahandja Lineament Zone (such as the massive Donkerhuk granite) resulted in
thermal weakening of the crust, helping to accommodate lateral extrusion. Thermal weakening of
the basement allowed the development of thick-skinned tectonics and the formation of the Audwib
nappe. In the NW, cooler, more rigid crust deformed very differently to those in the SE, through
shallow shearing, thin skinned tectonics. Diachronous timing of the deformation in rocks in the NW
and SE of the traverse is due in part to the rheologic difference between cooler rocks in the NW that
had locked up to deformation, much earlier than thermally weakened ones in the SE at the plate
collision margin, where tectonic stresses where greater. / AFRIKAANSE OPSOMMING: Die NE-strekkende Suid Sentral Sone van die Pan-Afikaanse (ca. 550-500) Damara gordel in sentraal
Namibië stel diep gëerodeerde gesteentes van die middelkors blood wat die magmaties boog van
die Damara orogeen verteenwoordig. Goeie dagsome, ongemodifiseer deur subsekwente naorogenises
prosesse het dit moontlik gemaak om ‘n studie aan te pak van die interne argitektuur van
‘n omenby 50km opname wat strek van die kontinentale skeidings sone (tussen die Congo kraton in
die NW en die onderplatende Kalahari kraton in die SE) by die Okahandja Lineament Sone, tot ver
oor die die voorste punt van die Congo kraton in die magmatiese-boog, die Suid Sentral Sone.
Hierdie studie neem in ag en karakteriseer die verandering in struktuur styl en drukvervormings
(strain) intensiteit in klippe van die Damara Supergroep, tussen die dorpies Otjimbingwe in die SE en
Karibib in die NW.
In die SE van die traverse in die Okahandja lineament zone vind ’n mens styl, regop, styf gevoude D2
maaksels in die Tinkas en Kuiseb Formasies, wat bulk NW-SE verkortende en styl SW ekstrusie van
rotse aandui. Deurdringende nie-coaksiale maaksels impliseer n hoë-hoek botsing tussen die Congo
en Kalahari kratons. Dit is in teenstryding met skeefhoekige botsing wat voorgestel is deur verskeie
vorige outeurs (e.g. Blaine (1977), Stanistreet et al. (1991), Tack & Bowden, 1999). ‘n Vermindering
in D2 drukvervorming (strain) en die teenwoordigheid van van die silisiklastiese basale Nosib groep,
stel die verteenwoordigehid van die onderliggende vloergesteentes voor en sodoende, dat die
voorste punt van die onderliggende Congo kraton net ‘n paar kilomeer NW van die Okahandja
Lineament onwikkel is. 8km NW van die Okahandja Lineamnet is daar ‘n km-skaalse NW neigende F1
dekbladvou gekern deur gneis van die vloer gesteentes en hervou is tot ‘n klomp, bi-neigende,
iv
dubbel duikende F2 plooie in wat vernoem word die Audawib vou kompleks. Die dekbladvou word
geinterpriteer om te gevorm het vooraan a retro-verkuiwing, gedurende vreeë kontinentale botsing
(syn-D1, vroeë-D2). Oorverskuifde en herververvormde vloer gesteentes is ‘n aanduideing van
termale verswakking in die aarkors, wat gelei het tot “thick-skinned” tektoniek. Intrusiewe
gesteenste om, en na die NW van die Audawib dekblad vou is die’ weidverspreide Salem-tipe graniet.
Syn-D2 Salem-tipe graniete is vlak intrusief to onder die dekblad vou en het waarskynlik die F1 vou
van sy wortel sone ontkoppel. NW van die Salem tipe graniete lê ‘n 15km2 groote venster in die vloer
gesteentes in omring deur die onderste formasies van die Damara Supergroep. Verskuifde marmer
sowel en D1 (vroee-D2) dioriet lae op die vloer gesteente se boonste kontak dui daarop dat hierdie
boonste kontak verskeurings losmaakpunt is, ‘n tektoniese kontak. Vloergesteentes (1)
ongeaffekteerde deur Damara (D1-D2) tektoniek en (2) onkonformeerbare kontak met oorliggende
klippe van die Damara Supergroep is ‘n aanduding van “thin-skinned” tektoniek in die gedeelte van
die Suid Sentral Sone 30km NW van die Okahandja Lineament Sone. Intrusiewe gesteentes gee
aanduidings dat deformasie in die NW reeds ge-eeindig teen 540 Ma, terwyl vervorming in die
Okahandja Lineament Sone nog aktief was tot minstens 520 Ma.
Oor die Okahandja Lineament het die hoe-hoekige kontinentale botsing gelei tot stywe ko-aksiale
plooing en laterale ekstrusie van gesteentes langs die kontinentale “backstop”. Die intrusie van
verskeie laat-D2 grantiete (soos die masiewe Donkerhuk graniet) in die Okahandja lineament sone
het gelei tot termale verswaking van die kors wat gehelp het om laterale ekstrusie van klippe te
akkomodeer. Termale verswakking van vloer gesteentes het gelei tot die onwikkeling van “thickskinned”
tektoniek en die vorming van die Audawib dekblad vou. In die NW, het koeler, stewiger
gesteentes anders vervorm as daardies verder suid, deur vlak skeur-verkuiwings, “thin-skinned”
tectonics. Nie-samelopende vervorming in gesteentes in die NW en SE van die opname is die gevolg
van die rheologiese verskil tuseen die koeler gesteentes in die NW wat vroeer bestand geraak het
teen vervorm as warmer gesteentes in die SE teenaan die botsings sone, waar tektoniese druk
boonop groter was.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4172 |
| Date | 03 1900 |
| Creators | Anthonissen, Christoffel Jasper |
| Contributors | Kisters, Alex, Stevens, Gary, University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 138 p. : ill., maps |
| Rights | University of Stellenbosch |
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