Brittle tectonic and stress field evolution in the Pan-African Lufilian arc and its foreland (Katanga, DRC): from orogenic compression to extensional collapse, transpressional inversion and transition to rifting

摘要

Since the first and paroxysmal deformation stages of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly known although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computedfor a database of 1889 fault-slip data by interactive stress tensor inversion and data subset separation. They have been assembled and correlated into 8 major brittle events, their relative succession established primarily from field-based criteria and interpreted in function of the regional tectonic context. The first brittle structures observed were formed during the Lufilian compressional climax, after the transition from ductile to brittle deformation (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt (stage 2). Unfolding the stress directions allows to reconstruct a well-defined N-S to NNE-SSW direction of compression, consistent with the stress directions recorded outside the belt. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending. After the bending, the Lufilian arc was affected by a NE-SW transpression of regionalsignificance (stage 3), inducing strike-slip reactivations dominantly sinistral in the Lufilian arc and dextral in the Kundelungu foreland. The next two stages were recorded only in the Lufilian arc. Late-orogenic extension was induced by σ1–σ3 stress axis permutation in a more trans-tensional regime (stages 4). Arc-parallel extension (stage 5) marks the final extensional collapse of the Lufilian orogeny. In early Mesozoic, NW-SE transpressional inversion felt regionally (stage 6) was induced by far-field stresses generated at the southern active margin of Gondwana. Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero-Upemba trend).