Constraints on the thermal history of the Dead-Sea Graben as revealed by coal ranks in deep boreholes

Coal ranks have been studied by vitrinite reflectance measurements in three boreholes; Zuk Tamrur 1 and Zohar 8 on the Judea Desert plateau (adjacent to the graben margin) and Amiaz 1 on a down-faulted block at the southwest Dead-Sea Graben. In the plateau boreholes, time-temperature index (TTI) calculations reveal that the coalification profile probably evolved under a Late Cretaceous thermal event with a thermal gradient of 35°-38°C/km followed by gradual decay to the present level c. 20°C/km by Miocene time. In Amiaz 1, the highest coal rank encountered, 0.5% Ro, is marginal for independent TTI calculations, however, some constraints on thermal history can be inferred. Stratigraphic and structural relationships between the 0.5% Ro isoreflectance in Amiaz 1 and the plateau boreholes reveals that most of the coalification measured in Amiaz 1 pre dates the formation of the graben and that an additional 3.4 km of Miocene-Holocene graben fill barely affected the coalification. Cessation of coalification despite increasing burial indicates that the post-Miocene thermal gradient in the Amiaz block could not have exceeded 20°-23°C/km. This finding is consistent with present day heat flow and other geophysical information. Tectonic models for the evolution of the Dead Sea Graben requiring a high thermal regime are inconsistent with the presented data. This raises questions as to the mechanism involved in the formation of rhomb-shaped grabens in general, on the one hand and for the hypothesis of the "leaky" nature of the Dead Sea transform north of the Gulf of Elat, on the other.