The Okahandja Lineament and its significance for Damaran tectonics in Namibia

Abstract

The Okahandja Lineament is considered to represent the southern margin of a magmatie arc produced by the northerly subduction of the Damran Ocean between 750 and 520 Ma ago. Sediments of the Orogen and granitoid intrusives produced by subduetion and crustal melting, underwent shear deformation between 675 and 575 Ma in a low angle zone of sinistral sense shear.The Okahandja Lineament represents a zone of differential movement between the Central and Southern Zones of the Orogen during this simple shear deformation. Both overriding and overriden plates were deformed at about 550 and 520 Ma. Production of open S.E. verging structures on the northern plate was accompanied by more intense thrust and fold nappe production on the southern margin of the Damaran Orogen. During this last deformation event, the Central magmatic Zone was downfolded under the southern zone. This fold is the present expression of the Okahandja Lineament. Two tectonic models are proposed for the structures identified within the Central and Southern Zones of the Orogen, combined with evidence from the northern coastal arm of the Orogen. R~sum6Le lin6ament de Okahandja est consider6 comme la marge bordant au sud un << arc magmatique >> r~sultant de la subduction vers le nord de l'Oc6an damaranique durant l'intervalle 750--520 M.a. Le lineament de Okahandja repr~sente, pour tes .d6formations cisaillantes simples une zone de d~placements partiels entre la zone centrale et la zone m~ridionale de l'orog~ne. Les deux plaques, celle du toit et celle du tour, de ce chevauchement furent d~fonn~es entre approximativement 550 et 520 M. a. Ces structures ouvertes, d6vers6es vers le nord, se forrn~rent dans la plaque septentrionale, pendant que des nappes en plis naissaient simultan6ment, sous l'influence d'une forte so sollicitation, dans la marge m~ridionale de l'orog~ne damaranique, ce qui entratna Ie ploiement de la zone magmatique centrale sous la zone m6ridionale, Ce pli est la forme seus laquelle appara~t aujourd'hui le lin6arnent de Okahandja. --Deux mod~les tectoniques sent propos6s pour expliqeur les structures ainsi reconnues dans les zones centrale et m6ridionale de l'orog~ne, sur la base du fair d'un bras le long de la cbte septentrionale de l'orog6ne. JI~nea~eaT Okahandja npeigcwaBxaeT co6ofi son~i y~epeHHoro Z~Bm~eu~, np~-ae~ ~e>~y ~eHTpaJn, nof~ H tOJts 3ona/dH oporeHa ameba MeCTO ~edpopraanina c~o~>~e~aa, l'~m5a ~pos~a, ~a~ r~Sa no~ozaBSX ~peTepnezn neqbop~a~ro r~eac~y 550 r~ 520 r~OHO~ ne~ ~orey ~aza~. Ha ce~eprro_~ r~:6e o6pasosa~ac~ oTKpl~ITaa cTpy~ypa onpo~rmyraa Ha ~0POBOCTOK, B TO spe~a, rca~ n pesyJmTaTe ~anpa~eana BOOnIZ~a cI~Jm/]i~I~ noKpoBa. IIpH 3T0~ ~4eHTpa~bHa~ MaI'MaTH-9 ~eei~a~ 3oHa no~ to.Holt aacTr~m or:asaJ~ac~ CMaTO~. PacnoJIo~eHr~e OTI~X cI~a~oI~ oTpa~aeT cero~H~mr~ee pacno~o~eHHe ~Hr~eaMeHTa.~IH 05%~CHeHH~I BO3HHHHOBeHHH DTHX eTpyETypH~X 3J~e~IeHTOB B ~eHTpa~-no~ H ro~Ho~ s0Hax oporeHa npe~aramw j~Be TaKTOHHqecKHe Mo~eJIH. B KOT0p~x ygTeH~ H Crpy~Typn~e 3JIeMeHT~I ceBepHoro 5eperosoro ~pa~ oporeHa.

Role of crustal contribution in the early stage of the Damara Orogen, Namibia: New constraints from combined U–Pb and Lu–Hf isotopes from the Goas Magmatic Complex

“…A prolonged intrusive activity in the Central Zone core of the Damara Belt, approximately between 580 Ma (Allsopp et al, 1983, unconstrained U-Pb zircon age) and 470-460 Ma Veevers, 2007), was synchronous with intense polyphase deformation (Barnes and Sawyer, 1980;Downing and Coward, 1981;Kröner, 1982;Kasch, 1983;Nex, 1997;Poli, 1997) and high-T low-P metamorphism (Puhan, 1983;Masberg et al, 1992;Nex et al, 2001;, with increasing temperature along strike to the SW (Goscombe et al, 2004). The Pan-African plutonism, exposed over one third of the entire area of the inland branch, resulted in more than 300 syn-and post-tectonic intrusions and thousands of granitic and pegmatitic dykes (Miller, 2008).…”

Flow perturbation folding in shear zones

“…The effects of differential shear on fold hinges within the more internal portions of orogenic belts were also discussed by Downing & Coward (1981) in the Damaran belt of Namibia. Following the proposals made by Elliott (1976), Coward & Potts (1983) demonstrated that intense shear strains (71) may develop at the frontal tips to thrusts and shear zones, where layer-parallel shortening will lead to folding at a high angle to transport ( Fig.…”

Comparative Stratigraphic and Geochronological Evolution of the Northern Damara Supergroup in Namibia and the Katanga Supergroup in the Lufilian Arc of Central Africa

“…Northeast-directed subduction of this ocean floor beneath the leading edge of the Congo Craton, referred to as the Okahandja Lineament (Miller 1979;Downing and Coward 1981; Fig. 2), resulted in uplift and erosion of the frontal region of the Congo Craton and the Central Zone of the Damara Belt (Fig.…”