THERMAL HISTORY OF THE EASTERN GULF OF SUEZ, II - RECONSTRUCTION FROMAPATITE FISSION-TRACK AND AR-40 AR-39 K-FELDSPAR MEASUREMENTS/

Apatite fission track (AFT) measurements from Miocene graben-fill sedi ments 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 (similar to 10.1-12.7 mu m) This ra nge 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 a t their highest post-depositional temperatures, the rift-related therm al regime is one of only moderate heating. Downhole AFT data in the Be layim 113 M-2/6 borehole attest to a higher rift temperature than in t he Ras Garra M-1 borehole, some 85 km to the south, where Little or no thermal overprinting is evident. These findings are consistent with p reviously reported bottom hole temperatures and vitrinite reflectance data in the study area. Despite the higher syn-rift thermal regime ind icated for Belayim 113 M-2/6, apatite provenance ages in Ras Garra M-l are considerably younger. Thus, the AFT data in the Ras Garra M-1 are a 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 age s in Ras Garra M-1, despite the weaker rift-related thermal effect, su ggest a deeper level proportional to an additional similar to 5-10 deg rees C of exhumation of the uplifted crystalline basement southwards a long the eastern rift flank by Mid-Miocene time. This result is consis tent with earlier findings which indicate both increased extension and heat how southwards in the Gulf of Suez, and earlier exhumation of th e rift flank. immediately preceding extension and opening of the Gulf of Suez, the most deeply exhumed basement rocks presently exposed on t he rift flanks were heated to temperatures greater than or equal to 11 0 degrees C (total annealing of apatite), but <similar to 170 degrees- 200 degrees C as constrained by Ar-40/Ar-39 data and non-resetting of zircon FT clocks in Sinai. Ar-40/Ar-39 data from granite penetrated in graben drilling at similar to 3.89 km further corroborates pre-rift p alaeotemperatures (<similar to 170 degrees C for crystalline basement underlying the Gulf of Suez.