A geodynamic model for the Paleoproterozoic (ca. 2.27–1.96 Ga) Birimian Orogen of the southern West African Craton – Insights into an evolving accretionary-collisional orogenic system

Abstract

This study presents an updated geodynamic model for the 2270–1960 Ma Birimian Orogen in the southern West African Craton (sWAC), which form part of a large accretionary-collisional orogenic system that extends into the Reguibat shields in northern WAC and southwards into equivalent crust in the Amazon Craton. The early evolution of the orogen was characterized by deposition of volcanic-volcanosedimentary successions and emplacement of limited intrusives, with a significant contribution from juvenile sources. This was followed by a collisional phase, associated with the deposition of large volcano-sedimentary and siliciclastic successions in both deep and shallow water conditions, along with ongoing volcanic activity and widespread emplacement of compositionally diverse intrusives. This occurred contemporaneously with deformation, regional greenschist-amphibolite facies metamorphism and juxtaposition of high and low-grade crustal domains, during what is referred to as the Eburnean Orogeny. A key aspect of the collisional phase is its diachronous nature, with the main period of magmatism, sedimentation and tectonothermal activity occurring around ca. 2160–2100 Ma in eastern sWAC, and between ca. 2120–2070 Ma in the west.The model envisions that the crust related to the accretionary phase formed in an elongate archipelago-style volcanic arc system, where juvenile crustal domains were juxtaposed with older continental fragments. The diachronous collisional phase is explained by largely continuous NW-directed (present-day orientation) indentation that affected the Birimian orogenic system during convergence, and which is inferred to reflect regional plate motions. The onset of the collisional phase in eastern sWAC is constrained to ca. 2200 Ma, which is marked by a gradual increase in the abundance of intrusives. However, its peak was not reached until 2160–2130 Ma, at which point the Birimian crust was affected by high-grade metamorphism. Continued NW-directed indentation during regional convergence drove the progressive westward shift in orogenic activity. A significant aspect of the model is the interplay between compression and extension, within the context of the overall NW-directed indentation that affected the system. Compression in response to convergence led deformation of the Birimian crust, but also triggered multiple periods of regional transtension, which led to the establishment of major structures, juxtaposition of high and low grade terranes, creation of basins in which supracrustal sequences were deposited, and establishment of magmatic and hydrothermal systems. These were subsequently overprinted and reactivated by deformation related to continued compression and convergence. The alternation between compression and extension within the context of regional convergence was a key factor in driving the evolution of the Birimian Orogen, and establishing the architecture of the crust exposed in sWAC.The end of orogenic activity in sWAC was marked by a progressive and diachronous decrease in magmatism, sedimentation and tectonothermal activity between 2100 and 2050 Ma and ArAr cooling ages on amphibole and mica between ca. 2050–1960 Ma, which coincided with the migration of orogenic activity outboard to the crust preserved in sWAC, including domains in the Reguibat Shield and the Amazon Craton. It is argued that the evolution of the Birimian Orogen in sWAC occurred in a paleogeographic setting comparable to that of the modern day Philippine archipelago in SE Asia, where the current N-NNE directed motion of the Indo-Australian plates would be comparable to the proposed NW-directed convergence proposed here for the Birimian orogenic system. Such a setting provides an environment for the formation of large volumes of arc-related volcanic and volcanosedimentary successions in a range of depositional environments, as well as the potential for significant input of juvenile material. On a regional to global scale, plate tectonics may thus have operated in a manner comparable to today. However, the apparent absence of blueschists, boninites and complete ophiolitic sections in the Birimian Orogen, together with the presence of rare komatiites, indicate the effect of secular changes on a local scale.