Discussion on ‘Geological offsets and age constraints along the northern Dead Sea fault, Syria'

Abstract

Searle et al. (2010) present significant new evidence regarding the eruption chronology of the Homs Basalt, which adjoins the Misyaf Fault, part of the northern Dead Sea Fault Zone in western Syria. Their Ar–Ar date of 3.7 ± 0.1 Ma for the youngest basalt east of this fault, at Jebel Barshin, significantly post-dates any previously published; hitherto, the youngest numerical date for any part of the Homs Basalt has been 4.8 ± 0.2 Ma (Sharkov et al. 1994). Searle et al. (2010) also report a 4.0 ± 0.1 Ma Ar–Ar date for a basalt outlier west of the Misyaf Fault, near the village of Kafroun. This is mapped (Ponikarov et al. 1963 a ) as a proximal part of one of the most westerly Homs Basalt flow-units, which cascaded southwestward to the Mediterranean palaeo-coastline around the town of Safsafeh, where it overlies Early Pliocene marine sediments (e.g. Ponikarov et al. 1963 a , b , 1966, 1967). Notwithstanding the vagaries and inconsistencies in the site location information provided by Mouty et al. (1992), the new 4.0 ± 0.1 Ma Ar–Ar date is evidently from the same flow-unit and possibly even from the same outlier as yielded their 8.1 ± 0.2 Ma whole-rock K–Ar date. The revision now evident to the numerical age of this flow-unit brings it into line with the age-constraint from the underlying sediments. Moreover, this revision calls into question all extant dates for the Homs Basalt, from Mouty et al. (1992), Sharkov et al. (1994) and Butler & Spencer (1999); all are whole-rock K–Ar dates, and so may well be systematically old, as a result of the effect of excess argon in phenocrysts (see Kelley 2002). This conclusion is supported by other new Ar–Ar dates of 6.4 ± 0.1 Ma and 5.8 ± …