Abstract

The outer margin of the Pan-African Gariep Belt, East of Rosh Pinah in southern Namibia, displays a transpressional deformational history with at least three recognizable fabric forming phases. D1 involved northwest – southeast-directed shortening and the development of mesoscopic, open west-southwest – east-northeast trending z-folds, particularly in the arkose dominated sequence of the Rosh Pinah Formation. Tight F1 minor folds with an accompanying S1 axial planar cleavage formed in lithologies such as thinly-interbedded carbonate-sandstone units of the Lekkersing Formation. D2 involved west - east shortening and later west-southwest – east-northeast shortening, and resulted in strain being partitioned across the outer margin due to rheological heterogeneity and zone boundary orientations. A western domain was dominated by dip-slip non-coaxial shear and flattening strains producing northwest – southeast trending, southwest-verging, asymmetric F2 folds and a moderately-dipping S2 cleavage. An eastern domain was characterized by a greater component of pure shear resulting in northwest – southeast trending upright folds and a subvertical S2 cleavage. D1 structures were overprinted by D2 structures, such that Fj folds were transposed into the S2 axial plane, and Sj cleavage was deformed. Strain markers were rotated to moderately plunging inclinations (10 to 45°) within the S2 foliation, as a result of triclinic strain produced by inclined transpression. The azimuth for the average facing direction of the most curvilinear fold hinges corresponds well with the plunge of strain markers, and confirms that the non-cylindrical nature of F2 folds is also a result of triclinic strain during D2. D2 shortening was accompanied by north-northeast - south-southwest stretching and sinistral shear on layers, particularly in the eastern domain, resulting in localized asymmetric boudinage of dolomitic layers on the limb of the McMillan’s East Anticline. Late-D2, strain was further partitioned, particularly in the eastern domain, where a component of simple shear was accommodated along oblique sinistral faults that utilized S2, while further amplification of folds occurred in the pure-shear dominated intervening zones. Subsequently, strain axes became more inclined during D3, resulting in localized deformation of S2 by crenulation cleavages that formed in quartz-sericite schist zones, kink bands which deformed S2 in arkosic rocks, and rare buckling of S2 in argillite. Following the transpressional event a switch in far-field stresses led to the formation of normal-dextral oblique-slip faults that utilized S2 cleavage.

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