The mid-crustal architecture of a continental arc - a transect through the South Central Zone of the Pan-African Damara Belt, Namibia

Anthonissen, Christoffel Jasper (Stellenbosch : University of Stellenbosch, 2010-03)

Thesis (MSc (Earth Sciences))--University of Stellenbosch, 2010.

Thesis

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.

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