Thermal history of the eastern Gulf of Suez, II. Reconstruction from apatite fission track and ⁴⁰Ar/³⁹Ar K-feldspar measurements

Apatite fission track (AFT) measurements from Miocene graben-fill sediments of the eastern Gulf of Suez in three deep boreholes (Belayim 113 M-2/6, Ras Garra M-1 and Alma-2) yield a wide range of ages (from 125 to 320 Ma) and mean track-lengths (∼ 10.1-12.7 μm). This range is similar to that recorded from Precambrian crystalline basement flanking the graben which is the major source for the rift fill. Since the AFT ages exceed the age of the host strata, which are presently at their highest post-depositional temperatures, the rift-related thermal regime is one of only moderate heating. Downhole AFT data in the Belayim 113 M-2/6 borehole attest to a higher rift temperature than in the Ras Garra M-1 borehole, some 85 km to the south, where little or no thermal overprinting is evident. These findings are consistent with previously reported bottom hole temperatures and vitrinite reflectance data in the study area. Despite the higher syn-rift thermal regime indicated for Belayim 113 M-2/6, apatite provenance ages in Ras Garra M-1 are considerably younger. Thus, the AFT data in the Ras Garra M-1 area do not record significant rift-related thermal effects, but rather, they mainly retain a pre-rift provenance signature which reflects the order and depth of erosion at the uplifted flanks. The younger AFT ages in Ras Garra M-1, despite the weaker rift-related thermal effect, suggest a deeper level proportional to an additional ∼5-10°C of exhumation of the uplifted crystalline basement southwards along the eastern rift flank by Mid-Miocene time. This result is consistent with earlier findings which indicate both increased extension and heat flow southwards in the Gulf of Suez, and earlier exhumation of the rift flank. Immediately preceding extension and opening of the Gulf of Suez, the most deeply exhumed basement rocks presently exposed on the rift flanks were heated to temperatures ≥110°C (total annealing of apatite), but <∼170°-200°C as constrained by ⁴⁰Ar/³⁹Ar data and non-resetting of zircon FT clocks in Sinai. ⁴⁰Ar/³⁹Ar data from granite penetrated in graben drilling at ∼3.89 km further corroborates pre-rift palaeotemperatures <∼170°C for crystalline basement underlying the Gulf of Suez.