Abstract
West Africa was subjected to deformation and exhumation in response to Gondwana break-up. The timing and extent of these events are recorded in the thermal history of the margin. This study reports new apatite fission track (AFT) data from Palaeoproterozoic basement along the primary NE-SW structural trend of the Bole-Nangodi shear zone in northwestern Ghana. The results display bimodality in AFT age (populations of ~210-180 Ma and ~115-105 Ma) and length distributions (populations of 12.2 ± 1.6 and 13.1 ± 1.4 µm), supported by differences in apatite chemistry (U concentrations). The bimodal AFT results and associated QTQt thermal history models provide evidence for multiple cooling phases. Late Triassic – Early Jurassic cooling is interpreted to be related with thermal relaxation after the emplacement of the Central Atlantic Magmatic Province (CAMP). Early to middle Cretaceous cooling is thought to be associated with exhumation during the Cretaceous onset of rifting between West Africa and Brazil. Late Cretaceous – Cenozoic cooling can be related with exhumation of the Ivory Coast – Ghana margin and NNW-SSE shortening through western Africa. Furthermore, our data record differential exhumation of the crust with respect to the Bole-Nangodi shear zone, preserving older (CAMP) cooling ages to the south and younger (rifting) cooling ages to the north of the shear zone, respectively. This suggests that the Palaeoproterozoic BN shear zone was reactivated during the Cretaceous as a result of deformation in the Equatorial Atlantic region of Africa.
Highlights
The NE-SW striking Bole-Nangodi (BN) shear zone in northwestern Ghana represents a Palaeoproterozoic crustal-scale shear zone within the West African Craton (WAC)[1,2] (Fig. 1) that preserves evidence for a deformation overprint that coincides with the Palaeoproterozoic Eburnean Orogeny[3]
The time period between the end of the Triassic and the early Cretaceous is characterised by two stages of rifting and related magmatism that precede the opening of the Equatorial Atlantic Rift System: (1) break-up within the Central Atlantic transpired at ca. 200 Ma, contemporaneous with the emplacement of the Central Atlantic Magmatic Province (CAMP)[14,15] and (2) break-up within the South Atlantic occurred synchronously or marginally preceded the emplacement of the Paraná – Etendeka Large Igneous Province (LIP) at ca. 137 Ma as dated by 40Ar-39Ar methods[14,15,16]
We propose that the Late Triassic – Early Jurassic cooling of the Ghanese upper crust is related to thermal relaxation after emplacement of the Central Atlantic Magmatic Province (CAMP) (Fig. 5a)
Summary
The NE-SW striking Bole-Nangodi (BN) shear zone in northwestern Ghana represents a Palaeoproterozoic crustal-scale shear zone within the West African Craton (WAC)[1,2] (Fig. 1) that preserves evidence for a deformation overprint that coincides with the Palaeoproterozoic Eburnean Orogeny[3]. Magmatism in the Benue Trough between 147 and 106 Ma (as dated by 40Ar-39Ar methods) preceded the final stages of break-up in in the equatorial Atlantic Ocean and overlaps in time with Paraná – Etendeka magmatism[14,17] Rocks related to those igneous events are limited within Ghana and are mostly constrained to Nigeria, Guinea, Mali, Morocco, Mauritania and Liberia within the African continent[18,19]. Crossing the Atlantic from the Ghana study area, the Maranhão and Parnaíba Basins in north-eastern Brazil (Fig. 1a) yield abundant volcaniclastic evidence of both CAMP and Paraná – Etendeka magmatism (based on geochemical associations)[16] Both the African and Brazilian margins, contain evidence for rifting of the Atlantic Ocean over a 70–80 myr. Both the African and Brazilian margins, contain evidence for rifting of the Atlantic Ocean over a 70–80 myr. period[17,20,21,22]
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