Structural geology of the Usakos Dome in the Damara Belt, Namibia

[摘要] ENGLISH ABSTRACT:The northeast-trending south Central Zone (sCZ) of the Pan-African Damara belt incentral Namibia is structurally characterized by kilometer-scale, northeast-trending domestructures developed in Neoproterozoic rocks of the Damara Sequence. A number ofdifferent structural models have been proposed for the formation of these domes in theliterature. This study describes the structural geology of the Usakos dome. The studydiscusses the structural evolution of the dome within the regional framework of the cSZthat represents the high-grade metamorphic axis of the Damara Belt, characterized byvoluminous Pan-African granitoids.The northeastern part of the Usakos dome is developed as an upright- to northwestverginganticlinorium containing a steep southeasterly-dipping axial planar foliation. Thenortheast fold trend persists into the southwestern parts of the Usakos dome. However,this southwestern core of the dome is inundated by synkinematic granitic sheets. Distinctmarker horizons of the Damara Sequence outcrop as screens within the granite,preserving a ghost stratigraphy. These screens illustrate the position and orientation ofsecond-order folds. Significantly, most of the stratigraphy of the Damara Sequence isoverturned in these folds. For example, some second-order anticlines developed in thenortheastern parts of the Usakos dome can be followed along their axial traces into thesouthwestern hinge of the dome, where they appear as synformal anticlines, i.e.synformal structures cored by older strata, plunging towards the northeast. The invertedstratigraphy and northeasterly fold plunges suggest the northeast-trending folds arerefolded by second-generation, northwest-trending folds, thus, forming kilometer-scaleType-2 interference folds. The resulting fold geometries are strongly non-cylindrical,approaching southwest-closing sheath folds indicating a top-to-the-southwest materialtransport. 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-dippingschistosity.The close spatial and temporal relationship between granite intrusion and the formationof the southwest-vergent, sheath-type folds, radial distribution of fold plunges and thesubhorizontal foliation confined to the southwestern hinge of the Usakos dome areinterpreted to signify the rheological weakening and ensuing collapse of the developingfirst-order Usakos dome immediately above the synkinematic granite intrusions. Orogenparallel,southwest-vergent sheath folds and top-to-the southwest extrusion of thesouthwestern parts of the Usakos dome and northwest-vergent folding and thrustingcharacterizing the northeastern extent of the Usakos dome are both responses to thenorthwest-southeast- directed contractional tectonics recorded during the main collisionalphase in the Damara belt. On a regional scale, the Usakos dome represents the linkbetween 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 becaused by the localized and transient rheological weakening of the crust during plutonicactivity.

[发布机构] Stellenbosch University