Thesis (MSc)--Stellenbosch University, 2005. / ENGLISH ABSTRACT: The northeast-trending south Central Zone (sCZ) of the Pan-African Damara belt in
central Namibia is structurally characterized by kilometer-scale, northeast-trending dome
structures developed in Neoproterozoic rocks of the Damara Sequence. A number of
different structural models have been proposed for the formation of these domes in the
literature. This study describes the structural geology of the Usakos dome. The study
discusses the structural evolution of the dome within the regional framework of the cSZ
that represents the high-grade metamorphic axis of the Damara Belt, characterized by
voluminous Pan-African granitoids.
The northeastern part of the Usakos dome is developed as an upright- to northwestverging
anticlinorium containing a steep southeasterly-dipping axial planar foliation. The
northeast fold trend persists into the southwestern parts of the Usakos dome. However,
this southwestern core of the dome is inundated by synkinematic granitic sheets. Distinct
marker horizons of the Damara Sequence outcrop as screens within the granite,
preserving a ghost stratigraphy. These screens illustrate the position and orientation of
second-order folds. Significantly, most of the stratigraphy of the Damara Sequence is
overturned in these folds. For example, some second-order anticlines developed in the
northeastern parts of the Usakos dome can be followed along their axial traces into the
southwestern hinge of the dome, where they appear as synformal anticlines, i.e.
synformal structures cored by older strata, plunging towards the northeast. The inverted
stratigraphy and northeasterly fold plunges suggest the northeast-trending folds are
refolded by second-generation, northwest-trending folds, thus, forming kilometer-scale
Type-2 interference folds. The resulting fold geometries are strongly non-cylindrical,
approaching southwest-closing sheath folds indicating a top-to-the-southwest material
transport. Lower-order folds in this overturned domain show radial fold plunges,
plunging away from the centre of the dome core, as well as a shallowly-dipping
schistosity. The close spatial and temporal relationship between granite intrusion and the formation
of the southwest-vergent, sheath-type folds, radial distribution of fold plunges and the
subhorizontal foliation confined to the southwestern hinge of the Usakos dome are
interpreted to signify the rheological weakening and ensuing collapse of the developing
first-order Usakos dome immediately above the synkinematic granite intrusions. Orogenparallel,
southwest-vergent sheath folds and top-to-the southwest extrusion of the
southwestern parts of the Usakos dome and northwest-vergent folding and thrusting
characterizing the northeastern extent of the Usakos dome are both responses to the
northwest-southeast- directed contractional tectonics recorded during the main collisional
phase in the Damara belt. On a regional scale, the Usakos dome represents the link
between the foreland-vergent northeastern part of the sCZ and the southwest-vergent,
high-grade southwestern parts of the sCZ.
The results of this study illustrate how dramatic variations in structural styles may be
caused by the localized and transient rheological weakening of the crust during plutonic
activity. / AFRIKAANSE OPSOMMING: Die noordoos-strekkende, suidelike Sentrale Sone (sSS) van die Pan-Afrikaanse Damara
gordel in sentraal Namibië word karakteriseer deur kilometer-skaal, noordoosstrekkende
koepel strukture, ontwikkel in die Neoproterozoïkum gesteentes van die
Damara Opeenvolging. 'n Aantal verskillende struktuur modelle is voorgestel in die
literatuur vir die vorming van hierdie koepels. Hierdie ondersoek beskryf die struktuur
geologie van die Usakos koepel. Die ondersoek bespreek die strukturele ontwikkeling van
die koepel in die regionale konteks van die sSS, wat die hoë graadse metamorfe
magmatiese as van die Damara Gordel verteenwoordig, en karakteriseer word deur
omvangryke Pan-Afrikaanse granitoïede.
Die noordoostelike gedeelte van die Usakos koepel is ontwikkel as 'n antiklinorium met
'n vertikale- tot noordwestelike kantelrigting. wat 'n steil hellende, suidoostelike asvlak
planêre foliasie bevat. Die noordoos-strekkende plooiing kom voor tot in die
suidwestelike kern van die Usakos wat ingedring is deur sinkinematiese granitiese plate.
Die posisie en oriëntasie van tweede-orde plooie is afgebeeld in die graniete deur 'n
skimstratigrafie wat preserveer is deur duidelike merker horisonne van die Damara
Opeenvolging. Die stratigrafie van die Damara Opeenvolging is opmerklik meestal
omgekeer in hierdie plooie. Byvoorbeeld, tweede-orde antikliene ontwikkel in die
noordoostelike gedeelte van die Usakos koepel kan gevolg word langs hul asvlakspore tot
in die suidwestelike skarnier van die koepel, waar dit voorkom as sinforme antikliene,
d.w.s. sinforme strukture met ouer strata in die kern wat na die noordooste duik. Die
omgekeerde stratigrafie en noordoostelike plooi duiking impliseer dat die noordoosstrekkende
plooie weer geplooi is deur tweede-generasie, noordwes-strekkende plooie,
wat dus aanleiding gegee het tot die vorming van kilometer-skaal, tipe-2 interferensie
plooie. Die gevolglike plooi geometrieë is uitdruklik nie-silindries, en toon 'n oorgang na
skede plooie met 'n sluiting na die suidweste, wat dui op 'n bokant-na-die-suidweste
materiaal vervoer. Laer-orde plooie in die omgekeerde domein vertoon radiale duiking
van die plooie, weg van die middelpunt van die koepel kern, sowel as 'n vlak hellende
skistositeit. Die noue ruimtelike en temporele verwantskap tussen graniet intrusie en die vorming van
skede-tipe plooie met 'n kantelrigting na die suidweste, die radiale verspreiding van
plooi duiking, en die subhorisontale foliasie wat beperk is tot die suidwestelike skarnier
van die Usakos koepel, word interpreteer as 'n aanduiding van die reologiese
verswakking en die gevolglike ineenstorting van die ontwikkelende eerste-orde Usakos
koepel, onmiddellik aan die bokant van die sinkinematiese graniet intrusies. Die orogeenparalleie
skede plooie met kantelrigting na die suidweste en bokant-na-die-suidweste
ekstrusie van die suidwestelike gedeelte van die Usakos koepel, en plooiing met
kantelrigting na die noordweste en stootverskuiwing wat kenmerkend is van die
noordoostelike gedeelte van die Usakos koepel, is beide 'n reaksie op die noordwessuidoos-
gerigte vernouings tektoniek opgeteken gedurende die hoof botsings fase in die
Damara gordel. Op 'n regionale skaal verteenwoordig die Usakos koepel die verbinding
tussen die noordoostelike gedeelte van die sSS met 'n voorland kantelrigting. en die hoë
graad suidwestelike gedeelte van die sSS met 'n kantelrigting na die suidweste.
Die resultate van hierdie ondersoek toon aan hoe dramatiese variasies in struktuur style
veroorsaak kan word deur die gelokaliseerde en kortstondige reologiese verswakking van
die kors gedurende plutoniese aktiwiteit.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/50457 |
| Date | 12 1900 |
| Creators | Johnson, Shannon D. |
| Contributors | Kisters, A., 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 | 159 p. : ill. |
| Rights | Stellenbosch University |
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