Earth and Planetao, Science Letters, 22 (1974) 157.162 [-"71 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands DATING FAULTS BY FISSION TRACK DATING OF EPIDOTES - AN ATTEMPT M. BAR*, Y. KOLODNY and Y.K. BENTOR Department of Geology, The Hebrew University, Jerusalem [Israel} Received January 8, 1974 Revised version received February 14, 1974 An attempt was made to determine the absolute age of faulting by measuring fission track ages of epidotes from mineralization veins along fault and joint planes. The method was applied to epidotes from along faults and fissures in the Sinai Peninsula, which cut Precambrian rocks. Most samples yield Tertiary ages (32-11 m.y.) An Early Cretaceous event is also recorded (120-103 m.y.). Samples from country rocks (igneous and metamorphic) give ages older than 325 m.y. The results are compatible with previous geological information, and suggest that the method can be potentially useful for dating faults. 1. Introduction Geochronological procedures are usually applied to determine ages of crystallization or thermal events. The dating of tectonic events, however, such as fault- ing, is usually accomplished by stratigraphic reasoning- placing upper and lower limits on the sought age. Lyons and Snellenburg [ 1] applied K-Ar dating to illites from fault gauges which they interpreted as dates of the last motion along three faults in Western New Hampshire. Wagner and Reimer [2] used fission tracks in apatite to determine rates and time limits of tectonic uplift in the Swiss and Italian Alps. Faults, because they are discontinuities in rock sequences are often avenues for moving solutions. Min- eralization along faults is typical [3]. Epidotization along fault and fissure planes is common. Epidotes form in several ways: (1) by deuteric ac- tion during a late phase of magmatic crystallization; (2) by low-grade regional metamorphism, and (3) by hydrothermal activity, i.e., mineralizing solutions per- colating through and reacting with earlier rocks at rel- atively low temperatures, generally less than 300 °- 500°C [4, p.207, 5]. In the absence of later thermal events, the age of the first type of epidotes will indi- cate the date of final cooling of a plutonic body; that * Present address: Geological Survey of Israel, Jerusalem. of the second type the age of metamorphism; finally, if it is assumed that hydrothermal epidotes were formed during or shortly after the opening of the faults and fissures, along which the solutions travelled, their data will approximate the age of this tectonic event. Naeser et al. [6] showed that epidotes are well-suited for fission track dating; they are readily etched and are the most refractory mineral in which fission tracks re- main stable, probably up to the temperatures at which epidotes themselves become unstable. The present work is an attempt at dating tectonic movements along fault planes by determining the age of epidotes which crystallized along them. In order to check the reliability of the method three tests were performed: (1) Different grains from the same epidote sample have been dated separately. Their ages should be iden- tical if the sample age is to be meaningful. (2) Two samples of epidote were selected from along the same fault line, about 100 m apart (S-3624 and S-3623). If a geologically significant event is to be dated the two samples should yield an identical date within the limits of our analytical error. (3) Our samples were selected in such a way that for some of them an estimate of age could be given on geological grounds (see below). Agreement with such an estimate serves as further support for the method.