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

[摘要] ENGLISH ABSTRACT: The NE-trending South Central Zone of the Pan-African (ca. 550-500) Damara orogen in centralNamibia exposes deeply eroded mid-crustal rocks, thought to represent the magmatic arc of theDamara orogen. Above average exposure of outcrop left unmodified by subsequent post-orogenicprocesses made it possible to study the internal architecture of a ca. 50km traverse, stretching fromthe continental suture-zone (between the Congo craton in the NW and the underplating Kalaharicraton in the SE) at the Okahandja Lineament Zone, well into the leading edge of the Congo cratonand into the magmatic-arc, the South Central Zone. This study considers and characterises thechange 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 inmeta-turbidites of the Tinkas and Kuiseb Formations record bulk NW-SE shortening and steep SWextrusion of rocks. Penetrative non-coaxial fabrics imply a high-angle collsion between the Congoand Kalahari cratons. This is in contrast to oblique collision described by a number of previousauthors (e.g. Blaine (1977), Stanistreet et al. (1991), Tack & Bowden, 1999). A marked decrease in D2strain, and the presence of the silisiclastic basal Nosib group suggests the presence of the underlyingbasement rocks and thus the leading edge of the Congo craton only a few km NW of the Okahandjalineament. 8km NW of the Okahandja lineament is a km-scale NW verging F1 nappe, cored bybasement gneisses and refolded into a series of bi-vergent, doubly-plunging F2 folds, the Audawibfold complex. The nappe is interpreted to have formed along a retroshear during early continentalcollision (syn-D1, early-D2). Tectonically overprinted basement rocks are indicative of thermalweakening, that resulted in the development of thick-skinned tectonics. Intruded mainly to the NWof 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 nappefrom its root. NW of the Salem-type granites lies a basement window of ca. 15km2 surrounded bythe lower formations of the Damara Supergroup. Sheared marbles and D1 (early D2) diorites alongthe 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 bythe 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 Zonesuggest that deformation in the NW ceased by 540 Ma, while deformation along the OkahandjaLineament continued until at least 520 Ma.iiiAlong the Okahandja lineament, high angle continental collision resulted in tight, co-axial folding andlateral extrusion of rocks along the continental backstop. The introduction of numerous late-D2granites around the Okahandja Lineament Zone (such as the massive Donkerhuk granite) resulted inthermal weakening of the crust, helping to accommodate lateral extrusion. Thermal weakening ofthe basement allowed the development of thick-skinned tectonics and the formation of the Audwibnappe. In the NW, cooler, more rigid crust deformed very differently to those in the SE, throughshallow shearing, thin skinned tectonics. Diachronous timing of the deformation in rocks in the NWand SE of the traverse is due in part to the rheologic difference between cooler rocks in the NW thathad locked up to deformation, much earlier than thermally weakened ones in the SE at the platecollision margin, where tectonic stresses where greater.

[发布机构] Stellenbosch University