Insights into the Tectonic and Provenance History of the Neotethyan Margin from Detrital Zircon (U-Th)/He from a Borehole in the Western Desert, Egypt
The subsurface of the Western Desert of Egypt contains multiple stacked sedimentary basin deposits separated by unconformities reflecting the long-lived tectonic evolution of the Neotethyan continental margin of NE Africa. Zircon (U-Th)/He (ZHe) data were collected from cuttings from a borehole that penetrated Tertiary and Cretaceous strata above a major unconformity juxtaposing Cretaceous and Cambro-Ordovician strata and supplemented by Jurassic cuttings from an offset. The 235 single-grain ZHe ages from 65 samples, analyzed to elucidate thermal evolution and detrital provenance of the drilled strata, can be subdivided into two major groups: (1) ZHe ages from post-Jurassic strata are significantly older than corresponding depositional age, preserving non-reset detrital ZHe ages. (2) ZHe ages from Cambro-Ordovician strata below the major unconformity yield ~320 Ma ages that are younger than the depositional age, implying resetting of ZHe ages (>180C) during the Hercynian orogeny. In detail, detrital ZHe ages form Mesozoic and Cenozoic strata show the following trends: (1) Detrital ZHe ages from Miocene strata mainly exhibit 280 and 83 Ma ages, (2) ZHe ages from Aptian-Early Cenomanian strata are characterized by age peaks of 450, 280, and 120 Ma, (3) samples from L. Hauterivian-Barremian strata show two detrital ZHe age peaks at 450 and 350 Ma, while
4 samples from Bathonian-E. Hauterivian strata exhibit dominant ZHe age components of 350, 170, and 210 Ma. These different detrital ZHe age populations record much of the tectonic evolution of NE Africa documenting the erosional unroofing of the Arabian-Nubian Shield, Hercynian terranes, Triassic and Jurassic fault blocks of the Neotethyan rifted margin, as well as Santonian inversion structures. Detrital ZHe age distributions show that Jurassic-Triassic and then Hercynian source input is progressively replaced by detritus from the Arabian-Nubian starting in the middle Cretaceous, marking the establishment of a stable passive continental margin in the Western Desert. In contrast, ZHe ages from Miocene strata document renewed arrival of detritus from a Hercynian source terrane and unroofing of Santonian inversion structures. This case study illustrates the power of ZHe dating as a detrital provenance tool elucidating the cooling and exhumation history of the source terrane, sediment dispersal patterns, and the tectonic/geological environment of the basin deposits.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009